A Review on Design, Synthesis and Application of Composite Materials Based on MnO₂ for Energy Storage

Abstract

The design, synthesis, and application of composite materials based on manganese dioxide (MnO₂) for energy storage are pivotal in advancing efficient, sustainable, and high-performance energy storage systems. The MnO₂ is widely recognized for its abundance, low cost, environmental friendliness, and excellent electrochemical properties, making it a promising candidate for use in supercapacitors, batteries, fuel cells, and other energy storage systems. This study offers a comprehensive overview of how various materials influence the performance of MnO₂ as an energy storage medium. Specifically, the design of composite materials is examined with respect to morphological control, integration with conductive additives, doping strategies, and structural engineering, all of which impact the final material properties. Additionally, the influence of diverse synthetic techniques—including hydrothermal synthesis, electrochemical deposition, sol–gel processing, co-precipitation, and templating methods—is evaluated. The latest attempts through which the developed composites showcase improved structural stability, inherent conductivity, and electron mobility compared to the original first material are presented in this review article. The presented results have been quite promising for the synthesis of great-performing materials with improved electrochemical data compared to that of MnO₂ alone, competing with other significant energy storage materials. This review highlights future prospects for the development of state-of-the-art devices, large-scale production applications, and the use of environmentally friendly materials and methods. It is anticipated that this research will provide valuable insights to facilitate further improvements in performance and broaden the scope of practical applications in this rapidly evolving field of composite materials.

1. Introduction

As technology progresses, the environmental impact of traditional fossil fuels is driving a greater demand for efficient and sustainable energy storage solutions. Therefore, the industry requires constant improvements in the performance and quality of batteries (lithium-ion, sodium-ion, flow, aqueous, metal–air), supercapacitors, and hybrid energy storage devices. Lithium-ion batteries (LIBs), amongst all, currently lead the global market and are expected to maintain their dominant position, with lead–acid batteries continuing to hold a significant share thereafter. Rechargeable zinc–air batteries (ZABs) also present great characteristics such as energy efficiency, power density, and cycling durability, creating an appealing profile for applications in energy storage devices. The main focus of recent technology developments is to enhance performance metrics such as energy density, charging speed, lifespan, safety, and sustainability [1,2,3,4]. Supercapacitors have also gained popularity due to their rapid charge–discharge cycle and their excellent power density, characterized as a bridge between traditional capacitors and batteries. They are divided into pseudocapacitors and electric double-layer capacitors. Electric double-layer capacitors store energy through non-faradaic charge separation occurring at the interface between the electrode and the electrolyte. Pseudocapacitors, on the other hand, rely on faradaic redox reactions involving electroactive substances to store charge. Supercapacitors demonstrate good perspectives for future applications in new energy technologies, such as hybrid vehicles, portable mobile devices, micro-electronics, rockets, and smart and wearable electrical products [3,5]. However, they lack the energy density capability compared to batteries, urging researchers to seek better-performing materials for their construction [6]. Furthermore, another interesting approach combines the beneficial properties of both devices, resulting in a supercapacitor–battery hybrid with high energy density, provided by the battery character, and quick power delivery, provided by the supercapacitor. This technology, although promising, requires improvement to show the hybrids’ full potential and follow an eco-friendlier route [7].

The first report of MnO₂ as a component in an energy storage system was in the Leclanche Cell in 1866. Georges Leclanché constructed a zinc–carbon battery that utilized MnO₂ as a depolizer to prevent a decrease in voltage. It consisted of a zinc anode and a carbon cathode along with a saturated solution of ammonium chloride as the electrolyte. Electricity was generated through the oxidation of zinc and the reduction in manganese dioxide, with the ammonium chloride acting as a conducting medium. This was the case of the first practical and commercially viable batteries with applications in early telegraphy and doorbells [8].

Since then, the effect of many transition metal oxides in energy devices has been explored by researchers, including MnO₂, a material low in cost and toxicity, high in theoretical capacity (1370 F g⁻¹), and easily available [9]. However, the very low conductivity of MnO₂ (10⁻⁵–10⁻⁶ S cm⁻¹) significantly restricts its potential as an effective material for power storage. To overcome these limitations, researchers have focused on designing composite materials that integrate MnO₂ with other components to enhance its performance. The integration of MnO₂ with conductive materials like carbon-based nanomaterials (graphene, carbon nanotubes, activated carbon) [10,11], conductive polymers [11], and metallic nanostructures [12] has been proven to emphasize the synergistic effects achieved in electrochemical performance, stability, and overall energy storage capabilities. They also explored how morphological control—such as forming nanostructures through doping with elements like Co, Ni, and Fe, or by functionalizing the material with groups like –COOH and –OH, as well as through hybridization, affects the material’s properties and, in turn, its electrochemical performance [13,14,15]. The selection of specific MnO₂ polymorphs (α, β, γ, δ, ε) is also crucial to the properties of the final composite due to their different structural properties and effect on the electrochemical performance [16].

Various synthesis methods are referred to in this review, highlighting the importance of experimental synthesis to obtain the desired properties in each material. The solvothermal/hydrothermal method provides control over the morphology of the product and produces stable crystal structures. The growth of crystals evolves under high-pressure and high-temperature water conditions from substances that are insoluble in ordinary conditions (<100 °C, <1 atm). By monitoring the hydrolysis reaction rate and solubility, the particle size of the metal oxide can be controlled [17]. Electrochemical deposition is the ideal route for a more precise and simpler synthetic process, while co-precipitation produces uniformly combined composites. The sol–gel and template-assisted process is suitable for porous products and hollow nanostructures.

By examining recent advancements in the field, this article aims to provide insights into the rational design of high-performance MnO₂-based composite materials for advanced energy storage applications, including batteries, supercapacitors, and hybrid energy storage devices. This review explores the design, synthesis, and applications of MnO₂-based composite materials in energy storage systems, focusing on recent advancements and diverse applications (Figure 1).

2. Design of MnO₂-Based Composite Materials

The design of MnO₂-based composite materials is focused on leveraging the unique properties of manganese dioxide (MnO₂), such as high surface area, excellent redox activity, and environmental friendliness, while addressing its inherent limitations, like poor electrical conductivity and structural instability. The following considerations are central to designing MnO₂-based composites for various applications such as energy storage, catalysis, and water purification.

2.1. Morphological Control

Designing MnO₂ nanostructures (e.g., nanorods, nanowires, nanosheets) to maximize surface area and improve ion diffusion pathways. The MnO₂ exists in various polymorphs (α, β, γ, δ, ε), each with unique electrochemical characteristics (Figure 2). Selecting or combining phases can optimize performance:

• α-MnO₂: Tunnel-like structure, good for ion diffusion.
• β-MnO₂: Dense, more stable, but less active.
• γ-MnO₂: Mixed phase, often used in batteries.
• δ-MnO₂: Layered structure, suitable for ion intercalation.

The polymorph is chosen based on the application, such as the following:

• Energy storage (batteries, supercapacitors): α- or δ-MnO₂.
• Catalysis: γ-MnO₂ for mixed-phase activity.
• Water treatment: δ-MnO₂ for adsorption and ion exchange.

Electrochemical capacitors have gained significant attention for their potential in energy storage, with transition metal oxides like MnO₂ emerging as promising candidates due to their high capacitance, redox properties, and multiple valence states. In a study by the physics department of Vellore Institute of Technology, four phases of MnO₂ (λ, γ, δ, and Mn₂O₃) were synthesized using various methods: chemical reduction, coprecipitation, hydrothermal, and biological. These phases were combined individually with graphite and polyvinylidene fluoride (PVDF) to create composite electrodes, which were then applied to fluorine-doped tin oxide substrates, in order to study and compare their electrochemical properties. Among the phases tested, λ-MnO₂ exhibited the highest specific capacitance (326.4 F g⁻¹) and excellent electrothermal performance, including energy and power densities of 62.6 Wh kg⁻¹ and 493 W kg⁻¹, respectively. The superior performance of λ-MnO₂ is attributed to its cubic spinel structure, larger surface area, and higher porosity. In contrast, Mn₂O₃, γ-, and δ-MnO₂ demonstrated lower capacitance due to their smaller tunnel size and amorphous characteristics, with the sequence being completed as such [λ- > γ- > δ-MnO₂ > Mn₂O₃]. Though the results were promising, the team was not able to calculate the capacitance with short deviation from the theoretical values (theoretical specific capacitance: 358 F g⁻¹ at 1 A g⁻¹) and aims to address these shortcomings with the use of a superior working electrode [16].

Researchers Zhumei Sun et al. created an ε-MnO₂/MXene composite with outstanding electrochemical performance by using the ϵ polymorph of MnO₂. In this material, MXene (Ti₃C₂Tx) acts as a conductive network, enhancing charge transfer and activating electrochemical sites of ε-MnO₂. The structure features spherical ε-MnO₂ nanoparticles grown on MXene layers, resulting in exceptional desalination performance, while the use of ε-MnO₂ is attributed to its superior Na+ storage ability and its tunnel-like structure. The composite achieves a maximum deionization rate of 17.54 mg g⁻¹ min and a desalination capacity of 114.66 mg g⁻¹ in a 1000 mg/L NaCl solution at 1.2 V, surpassing the performance of pure ε-MnO₂ (74.93 mg g⁻¹) and MXene (29.26 mg/g). Additionally, it exhibits a specific capacitance of 337.2 F/g at 1 A/g, approximately 2.16 times higher than that of an ε-MnO₂ electrode. These results highlight the ε-MnO₂/MXene composite’s potential for practical CDI applications [17].

Exploiting the α-MnO₂ polymorph and active carbon derived from waste date palm leaves, a team from Khalifa University of Science and Technology synthesized an anode material for Na ion batteries (NIBs). The α-MnO₂/AC composite electrode was fabricated using a two-step hydrothermal process, resulting in a flexible and free-standing structure. The scanning electron microscopy (SEM) analysis revealed that nanostructured α-MnO₂ was embedded within the porous AC matrix, promoting electron transfer due to its tunnel-like structure. Electrochemical tests demonstrated excellent cycling stability, a consistent coulombic efficiency of 100% and a discharge capacity of 466 mAh g⁻¹, using a current density of 50 mA g⁻¹. This innovative approach not only offers a high-capacity and long-cycle solution for NIBs but also provides an environmentally friendly way to repurpose biomass waste in energy storage applications. By utilizing waste materials, this method contributes to a more sustainable energy future [18].

Another study by Saibabu et al. focused on synthesizing sphere-like β-MnO₂ as a cathode material for zinc-ion batteries (ZIBs). The β-MnO₂ structure, composed of MnO₆ octahedra single chains forming 1 × 1 tunnels, was characterized by thermodynamic stability and efficient charge storage. Electrochemical performance was tested in a three-electrode setup with 1 M ZnSO₄ electrolyte and further evaluated in pouch and coin cell configurations. The pouch-type cell, using a polyvinyl alcohol (PVA) polymer gel matrix with ZnSO₄ as an electrolyte, achieved a high specific capacity of 218.42 mAh g⁻¹ at 64 mA g⁻¹ and retained 81.11% of its capacity after 250 cycles at a 10 C rate with 99% coulombic efficiency. The coin cell, using a ZnSO₄-MnSO₄ electrolyte mixture, showed enhanced performance. These results demonstrated that β-MnO₂ microspheres exhibit excellent cycling stability, rate capability, and energy density, making them a promising cathode material for aqueous rechargeable ZIBs [19].

In an additional study comparing different MnO₂ polymorphs, researchers synthesized α-MnO₂, δ-MnO₂, and ε-MnO₂ cathodes utilizing a ball-milling treatment, used in the construction of aqueous ZIBs. More specifically, δ-MnO₂ exhibits a higher capacity, due to the significant contribution from the first discharge plateau and the greater interaction of Zn⁺ with the surface. The α-MnO₂ structure, with its narrow tunnels, requires the partial breaking of the H-H₂O bond to accommodate H⁺ storage, whereas the δ-MnO₂ structure, with its larger interlayer spacing, allows for more efficient H⁺ mobility. In contrast, ε-MnO₂ is a less stable phase, where Mn⁴⁺ ions occupy half of the positions in the lattice structure. The electrochemical tests showed that the δ-MnO₂ electrode maintained 93% of its capacity at 0.5 C after 500 cycles and exhibited 83 mAh g⁻¹ rate capability at 2 C, proving what was indicated by its structure [20].

2.2. Composite Integration

The MnO₂ can be integrated with materials such as graphene, carbon nanotubes, conductive polymers, or metal oxides to improve its structural stability, electrical conductivity, and overall electrochemical performance (Figure 3).

2.2.1. Incorporation with Conductive Materials

The enhancement of MnO₂’s poor conductivity can be conducted via its combination with carbon-based materials such as graphene, carbon nanotubes, and activated carbon. For instance, electrochemical redox deposition of MnO₂ onto carbon nanotubes (CNTs) and carbon nanofibers (CNFs) yields composite materials with enhanced electrical conductivity. In order to improve the mechanical properties, thermal stability, and electrical conductivity of CNTs, they have been electrospun and incorporated into polymer nanofibers. The constructed CNT-embedded CNF hybrid material resulted in uniform deposition of MnO₂, while the final material showed improved electrochemical performance of MnO₂ at high mass loading. The MnO₂ deposits incorporated onto the CNTs-embedded CNFs network by electrodeposition resulted in a capacitance of 374 F g⁻¹ and a rate capability of 53.4% in comparison to the specific capacitance of MnO₂/CNFs composite electrode at 329 F g⁻¹ [21].

An FTO/MnO₂–graphene composite was synthesized through a one-step electrochemical method using chronoamperometry (SnO₂ doped in Fluorine: FTO). This composite film revealed significant potential for use in supercapacitor electrodes and energy storage devices due to its improved electrochemical properties, low cost, availability, and simple application. In asymmetric supercapacitor tests, the FTO/MnO₂–graphene composite exhibited excellent reversible performance, with an energy density of 3 W h kg⁻¹ at a specific power of 25 W kg⁻¹. After 1000 cycles, the composite maintained 99% of its capacitance. The inclusion of graphene enhances ion diffusion and electron transport, improving the electrolyte–electrode interaction and therefore, the overall performance. Cyclic voltammetry tests demonstrated that the addition of graphene increased the capacitance of the thin films from 73.5 F g⁻¹ for pure MnO₂ to 192.3 F g⁻¹ for MnO₂–graphene composites. Additionally, electrochemical impedance spectroscopy (EIS) highlighted the positive impact of graphene on charge transfer and capacitance behavior. The diameter of the semicircles was found to decrease with increasing graphene content in the MnO₂ matrix. The charge transfer resistance values were 11.5 Ω·cm² for the FTO/MnO₂ sample and 6.57 Ω·cm² for the FTO/MnO₂-GR sample, highlighting the enhanced conductivity of the oxide upon graphene incorporation (Figure 4) [10].

Another hybrid material with a wide range of applications due to the synergistic effect of its components was a CeO₂-MnO₂/CNF composite, which was produced hydrothermally. The produced material consisted of fragmented CeO₂ grown with MnO₂ (crystalline phase) on the surface of carbon nanofibers. The specific nanocomposite was used in the manufacture of supercapacitors with maximum electrochemical capacitance of 1453 F g⁻¹ (scan rate—10 mV s⁻¹) and 1498 F g⁻¹ (at current density of 0.8 A g⁻¹). Due to the flexibility and porosity of CNF with CeO₂-MnO₂, the composite was also used as an H₂O₂ sensor with a high sensitivity of 612 μA cm⁻² and a detection limit of 0.1 μm (Figure 5) [22].

Another study by Guo and co-workers focused on enhancing the capacitance of textile-based electrodes by depositing MnO₂ onto graphene/polyester composite fabrics using a facile hydrothermal method. Adjusting the reaction time impacts the electrochemical performance by controlling the morphology of MnO₂. The composite electrode achieved a specific capacitance of 332 F g⁻¹ (scan rate of 2 mV s⁻¹), demonstrating excellent cycling stability. Notably, the material maintained stable electrochemical performance even under mechanical bending and stretching. The integration of graphene enhanced the conductivity and MnO₂ growth, providing a 3D conductive network that contributed to the superior capacitive properties, along with the synergistic effect of the materials combined. This approach offered a scalable method for fabricating flexible electrode materials for energy storage devices [23].

Furthermore, a study investigated the development of hybrid supercapacitor–battery devices using flexible MnO₂ nanoparticle-coated air-oxidized carbon nanotube (MnO₂/aCNT) electrodes, through a redox reaction of KMnO₄ and aCNTs. The MnO₂ nanoparticles (≈10 nm in diameter) are uniformly attached to the aCNTs through Mn–O–C linkages, which create strong chemical interactions. This structure allowed the aCNT network to mitigate the strain caused by MnO₂ volume changes during rapid charge/discharge cycles, maintaining electrode integrity. The network also ensured efficient electron flow and lithium-ion diffusion, thereby enhancing ultrafast, reversible lithium storage, a feature desired in electric vehicle applications. The MnO₂/aCNT electrodes showed impressive high-current performance, with capacities of 395.8 mAh g⁻¹ at a current density of 10 A g⁻¹ and 630.2 mAh g⁻¹ after 150 cycles at 2 A g⁻¹. Their low-capacity fading rates over 1000 cycles further highlight their potential for high-performance energy storage devices. This combination of high energy density and quick charge/discharge times positions MnO₂/aCNT electrodes as promising candidates for hybrid supercapacitor–battery applications [24]. The incorporation of MnO₂ onto carbon structures produces materials with improved structural, electrochemical, or mechanical properties. Carbon nanostructures such as CNTs and carbon incorporated onto synthetic fabrics (graphene/polyester fabric) provide flexibility of the end product, ideal for applications in wearable technology and smart devices. The porous, crystalline structure and synergistic oxide interactions of the CeO₂-MnO₂/CNF composite cause the most impressive maximum electrochemical capacitance of 1498 F g⁻¹. On the other hand, graphene, as a composite component, produced materials with better energy density retention (99% after 1000 cycles).

Amjad from the Polytechnic University of Tehran and his coworkers proposed a method to improve the conductivity of polyaniline (PAni) fibers with the assistance of MnO₂ during the polymerization. Using MnO₂ as seeds during interfacial polymerization results in a product with higher conductivity and greater capacity compared to PAni nanofibers, as observed by electrochemical capacitance measurements. The constructed composites made by rapid mixing, interfacial polymerization of Pani, and using MnO₂ as seeds were characterized by SEM, Fourier transform infrared (FT-IR) spectroscopy, and UV-vis spectroscopy, and demonstrated a uniform structure compared to other MnO₂-PAni nanofibers. The manufactured material was evaluated as a fitting electrode component in energy storage systems [25].

A different composite material of polyaniline (PAni-MnO₂) was synthesized through the in situ chemical oxidative polymerization of aniline in an acidic aqueous solution (MnO₂ sourced from the reaction of MnSO₄ with KMnO₄). Examination of the structure of PAni and PAni-MnO₂ composites using FT-IR, SEM, and thermogravimetric analysis (TGA) confirms the formation of these materials with spherical MnO₂ particles distributed within the PAni matrix. Supercapacitor performance was evaluated using cyclic voltammetry and chronopotentiometry in a 0.5 M Na₂SO₄ solution, with the PAni-MnO₂ showing a high specific capacitance (Cs) of 242 F g⁻¹, which remained stable over 1000 cycles (at a current density of 0.25 A g⁻¹). The specific capacitance of the constructed materials grows gradually from PAni to PM5 composite, ranging from 99 to 242 F g⁻¹ at 0.1 A g⁻¹. These results demonstrate that PAni-MnO₂ composites possess excellent electrochemical properties, making them promising candidates for supercapacitor applications [26].

Poly ortho-aminophenol (POAP) is also a conducting polymer, which is proven to improve its conductive performance with the presence of mesoporous MnO₂@Zeolite-Y (MOZ) synthesized by the ultrasonic method. Researchers used electro-polymerization to produce a POAP/MnO₂@Zeolite-Y (MOZ/POAP) composite on the zeolite electrode surface. The desired structure of the composite was evaluated through X-ray diffraction (XRD), SEM, Energy Dispersive X-ray elemental analysis (EDS), FT-IR, and Brunauer–Emmett-Teller (BET) analysis. The electrochemical analysis showed a specific capacitance of 422 F g⁻¹ at 1 A g⁻¹ and the best rate at 16 A g⁻¹ with 97.22% cyclic stability over 10,000 cycles. The astonishing results can be attributed to the presence of N and O in the matrix of POAP, as well as the synergistic effect of Mn (MnO₂@Zeolite-Y), enhancing the pseudocapacitance behavior. The synthesized (MOZ/POAP) shows great cycle life, high power density, and specific capacitance, making it an ideal component in energy storage applications [27].

Examining the synergistic effect that both conductive polymers and carbon structures have on MnO₂ is essential to this research. Researchers, led by A. Rehman, created a composite material from active carbon cloth (ACC) layered with 3,4-ethylene dioxythiophene (PEDOT) and MnO₂. The experimental synthesis of ACC@MnO₂@PEDOT composite initiated by the electrodeposition of MnO₂ on ACC and was followed by a second electrodeposition of the PEDOT layer on the prepared ACC@MnO₂. The produced electrode exhibited excellent capacitance performance, 1882.5 mF cm⁻², with a discharge current density of 1 mA cm⁻². When combined with a negative ACC electrode, the produced SC device shows an areal capacitance of 368.05 mF cm⁻² at a scan rate of 1 mV cm⁻² with a voltage window of 1.8 V and a cycling stability of 94.6% after 10,000 GCD cycles. These results demonstrate that the constructed material has great cyclic performance, making it a suitable application for high-performance devices [28].

Metals such as Ag, Ni, Co, Cu, and others also have conductive properties and can further enhance electron mobility when combined with conductive polymer materials. Researchers from the School of Chemistry and Chemical Engineering in Guangzhou developed an advanced asymmetric supercapacitor using hierarchical polypyrrole (PPy)-based composites as both the anode and cathode. The anode consists of ultrathin PPy and pedal-like MnO₂ coating onto 3D-Ni (3D snuzzle-like Ni@PPy@MnO₂ structure) with great charge storage capability. The cathode features a 3D cochleate-like Ni@MnO₂@PPy structure with enhanced energy storage and fast ion transport abilities. The unique 3D structures create storage chambers and efficient ion pathways. The cathode, in particular, offers excellent pseudocapacitance and contributes to a wide voltage window of 1.3–1.5 V, with high energy (59.8 Wh kg⁻¹) and power (7500 W kg⁻¹) densities. The supercapacitor demonstrates superior performance, including impressive cycling stability over 3000 cycles, and shows promise for future asymmetric supercapacitor designs. This work marks the first use of hierarchical PPy-based composites for both electrodes in asymmetric supercapacitors [29]. Both polyaniline composites showcase improved conductivity compared to plain PAni, but did not outperform the rest of the composite materials. Performance metrics indicate that the inclusion of secondary structures such as zeolites, carbon cloth, and 3D nickel substrates acted synergistically with the polymer materials and contributed significantly to enhanced ion/electron transport and mechanical stability.

In an attempt to overcome the dissolution problems and volume fluctuations of MnO₂ as a cathode material, Sho Li and Junsheng Zhu composed a material based on coconut activated carbon, MnO₂ nanorods, and cobalt. A simple one-step oil bath method results in uniform deposition of Co-doped MnO₂ onto the CAC surface, revealing their synergistic effect and enhancing the H⁺ and Zn²⁺ ion diffusion kinetics when used as a cathode in Zinc-ion batteries. The capacity of Co-βMnO₂/CAC composite increases from 224.7 mAh g⁻¹ to 436.7 mAh g⁻¹ after 120 cycles at 0.2 A g⁻¹, while also displaying a high reversible capacity of 233.2 mAh g⁻¹ after 900 cycles at 0.5 A g⁻¹. According to

Table 1. The fitted impedance parameters of β-MO, Co-β-MO, and Co-β-MO/CAC, reproduced with permission [12].

Cathode Materials	Rs (Ω)	Rf (Ω)	Rct (Ω)
β-MO	13.5	31.4	30.9
Co-β-MO	12.1	27.33	21.2
Co-β-MO/CAC	8.2	4.5	19.4

, which tabulates the fitted EIS parameters of the produced materials, it can be clearly seen that the charge transfer resistance (R_ct) of β-MO (30.9 Ω) is higher than that of Co-β-MO (21.2 Ω) and Co-β-MO/CAC (19.4 Ω), demonstrating that cobalt doping and CAC addition effectively lower the electrochemical reaction resistance in the Co-β-MO/CAC system. The electrochemical tests (EIS) and the galvanostatic intermittent titration technique) reveal that the end cathode material shows improved performance compared to a non-doped material due to the combined effect of the components [12].

Research conducted by the Department of Nanoscience and Engineering at Inje University focused on the development of nickel (Ni)-doped λ-MnO₂ used as anode materials for lithium-ion (Li-ion) batteries. The composite was synthesized using a facile and cost-effective method and has a hierarchical hollow porous structure, providing high surface area, loading capacity, and low density to the anode composite. The Ni-doping method was employed to tailor the physical and chemical properties of MnO₂, enhancing its electrochemical performance. The Ni-doped MnO₂ demonstrated excellent cycling stability, rate capability, and enhanced diffusion properties when used as an anode in Li-ion batteries. Additionally, the material was tested in a Li-ion hybrid capacitor, where it exhibited superior energy and power performance, achieving a specific capacity of 25 mAh g⁻¹ at a high current density of 5 A g⁻¹, with a power density of 29 W kg⁻¹ and an energy density of 30 Wh kg⁻¹. These results highlight Ni-doped MnO₂ as a promising candidate for energy storage applications, with potential for other metal oxide systems. The study opens up avenues for creating advanced nanostructured materials for various energy storage devices [30].

One additional doping method promising to improve the low conductivity and instability of δ-MnO₂ produces a copper-doped Cu_0.05K_0.11Mn_0.84O₂•0.54H₂O (Cu₂-KMO, copper-potassium manganese oxide) cathode. The cathode material is produced via a hydrothermal method and has a 3D pedal-shaped mesoporous structure, which promotes electron transfer and hastens reaction kinetics. Due to the double electron transfer between Mn⁴⁺ and Mn²⁺ in the Cu₂-KMO cathode, which converts into alkaline zinc sulphate nanosheets, the Zn//Cu₂-KMO battery displays high performance capacity. Partial capacity for the ZIBs is also provided by Mn²⁺ in the mixed electrolyte. After electrochemical tests, the battery with Cu₂-KMO used as the cathode material showed a specific capacity of 600 mAh g⁻¹ at 0.1 A g⁻¹ after 75 cycles, shedding light on the beneficial effect of Cu doping on manganese dioxide base materials [31].

Another composite nanostructure highlighting the synergistic effect of its components consists of 2D MXene, 1D tubular MnO₂, and Ag nanospheres, forming one electrode with a triple heterogeneous multi-level structure. The MXene nanosheets were produced using a water intercalation method, while the MnO₂ nanotubes were synthesized hydrothermally. The end composite was prepared through sonication and magnetic stirring of the components, resulting in an anode material with excellent properties. More specifically, the MXene/MnO₂/Ag anode achieved a specific capacitance of 880.8 F g⁻¹ at 2 A g⁻¹, while the assembled MXene/MnO₂/Ag//AC asymmetric supercapacitor displayed a specific capacitance of 112.5 F g⁻¹ at 3 A g⁻¹ and maintained 85% of its capacitance after 5000 cycles. These notable results can be attributed to the synergistic effect of the components, as the MXene sheets promote rapid electron transfer due to the expanded interlayer caused by the integration of MnO₂ and Ag between the nanosheets. This three-dimensional structure also shortens the electron transmission distance and increases the available active sites of the material, improving the overall electrochemical performance. In conclusion, this novel nanoarchitectural MXene/MnO₂/Ag anode material is promising to upgrade the technology of energy storage supercapacitors [32].

While metallic Li is considered to be an excellent anode material, the growth of Li dendrites and volume fluctuations pose an obstacle to its applications in modern-day electronics. With the hope of overcoming these difficulties, Derong Liu, along with a team of researchers, developed a carbon cloth (CC) modified with Ag-doped MnO₂ nanosheets as a 3D host for Li metal anode (LMA). The Ag-doped MnO₂ vertically aligned nanosheets decorate the CC through a hydrothermal process, producing the final anode (Ag/MnO₂@CC). The existing pathways in the 3D structure allow for sufficient spaces that can accommodate volume fluctuation, enhancing the electrochemical performance. The restriction of dendrite growth was attributed to the unique nanostructure, which homogenizes Li-ion flux and reduces local current density. Density functional theory (DFT) calculations confirmed that the MnO₂ and Ag presented better lithiophilicity (−4.31 and −2.34 eV) than C (−1.65 eV). Ag/MnO₂@CC electrodeposited with Li showcased a Coulombic efficiency of 99.9% (after 300 cycles), while the full cell using LiFePO₄ as a cathode, exhibited excellent capacity of 105.5 mAh g⁻¹ after 400 cycles (at 1 C). Therefore, the constructed 3D structure could play a promising role in lithium metal battery applications in modern technology [33]. As previously mentioned, multicomponent synergistic designs, such as Ag/MnO₂@CC and MXene/MnO₂/Ag, seem to maximize capacitance. The use of 3D porous layers in the structure also promotes electron transferability and is ideal for supercapacitor applications.

2.2.2. Structural Engineering

The deliberate design and manipulation of a material on a micro- or nano-scale is another challenge for researchers in the development of high-performing composites. The combination of materials with differences in morphology, porosity, and hierarchical organization results in products with enhanced electrochemical properties ideal for various energy storage applications. Some of the methods employed by researchers include the fabrication of core–shell architectures, hollow nanostructures, and layered frameworks, which improve mechanical resistance and electron mobility. A constructed material suggested by the Harbin University of Science and Technology is based on PVDF films doped with MnO₂@carbon core–shell particles. The process followed starts with the formation of the C-shell around the MnO₂ nanoparticles, yielding MnO₂@C, and ends with combining it with the polymer by solution bending. The existing C layer promotes electron transfer while enhancing interfacial polarization, while the produced composite film shows a resemblance to the dielectric properties of conductive fillers and complies with the percolation theory. Compared to the dielectric constant of PVDF at 100 Hz (7.68), the constructed MnO₂@carbon composite is 3 times more (24.4). With an energy storage capacity of PVDF/MnO₂@C, which is 4.95 J cm⁻³, the material can be used as a semiconductor and as a component of high-energy storage capacitors (Figure 6) [34].

The design of MnO₂@amorphous carbon core–shell using a hard template method was also studied as a proposed cathode material in aqueous ZIBs with improved electron transfer properties. It was proved that the nano dimensions of MnO₂ rods contribute to the insertion reactions of Zn in active sites, thereby shortening the ion diffusion path and therefore improving the electrochemical performance. The porous structure of MnO₂@C also limited the phenomenon of capacity degradation caused by MnO₂ distortion, as it allowed a necessary room for volume expansion. After a series of tests, the MnO₂@C displayed a capacity of 102 mAh g⁻¹ at 0.8 A g⁻¹, maintaining 89.5% of its capacity after 600 cycles at 0.8 A g⁻¹. This research offered an effective method for developing high-performance manganese-based cathode materials for zinc-ion batteries, while also showcasing a pathway to enhance electrode materials [35].

One additional material used in zinc-ion batteries is a MnO₂@polyaniline core–shell nanowire film, produced by an in situ interfacial method. The polyaniline nanoshells coated the MnO₂ cores uniformly and formed the films using a simple vacuum filtration technique. This composite showcased high flexibility, increased reaction kinetics between electrons, and impressive stability due to the conducting PAni, increasing the battery’s cycle life (2000-cycling lifetime). The MnO₂@PAni core–shell nanowire films reach a maximum specific capacity of 342 mAh g⁻¹ (at 0.2 A g) and a high capacity of 100 mAh g⁻¹ at 3 A g⁻¹. One remarkable application of the constructed material consists of batteries used in portable and wearable electronic devices, leading the path for the development of advanced technology in this field [36].

Researchers Du Huang and his co-workers from the School of Materials Science and Engineering at South China University of Technology developed another more complex material, based on Ti foil, as an electrode component. The core–shell nanostructure consists of 3D TiO₂ nanoflowers (NFs) on activated Ti foil coated uniformly by Au film and MnO₂ nanowires. The process follows a three-step reaction method of hydrothermal treatment, gold-spraying, and electrodeposition. The structure of TiO₂ nanoflowers improves electron transfer and hastens reaction rate due to the larger surface area provided, while the Au film increases conductivity and adherence between the TiO₂ scaffold and MnO₂. The TiO₂ NFs@Au@MnO₂ based supercapacitor shows maximum specific capacitance of 223.75 F g⁻¹ (0.5 A g), an energy density of 19.87 Wh kg (0.2 Kw kg⁻¹), and a cycling stability of 85.8% after 2000 cycles. Therefore, this specific nanostructure, combined with other carbon-based materials, could be widely used in the manufacture of flexible asymmetric supercapacitors [37].

Another core–shell and spherical structured Fe₃O₄@MnO₂ nanostructure is proposed by Jieting Ding (and others) who followed an affordable two-step hydrothermal method via a facile. In order to form the core–shell structure, δ-MnO₂ is grown on the surface of Fe₃O₄ nanospheres uniformly. The BET surface area of Fe₃O₄@MnO₂, which is 81.17 m² g⁻¹, was twice the size of Fe₃O₄ due to the nanosized layer of δ-MnO₂. The constructed composite as an electrode shows a specific capacitance of 243.7 F g⁻¹ (with a current density of 0.1 A g) and a capacitance retention of nearly 100% after 3000 cycles. This cycling stability and affordability of the core–shell structured Fe₃O₄@MnO₂ material could make it ideal for practical applications in pseudo capacitors [38].

Through research, Jhankal and co-workers synthesized a nanocomposite bonding MnO₂ in nanoneedle form with reduced graphene oxide (rGO) by facile synthesis. The composed material acts as the positive electrode in an ion-quasi-solid-state asymmetric supercapacitor, showcasing great pseudo-capacitive electrochemical behavior with potential ranging from 0 to 1.8 V and a maximum capacity of 216 F g⁻¹ at 1 A g⁻¹ current density. The deposition of nanoneedles onto graphene oxide was executed uniformly, causing an interconnected structure that promotes ion exchange during electrochemical processes. After the execution of electrochemical tests, the asymmetric supercapacitor (rGO/MnO₂–GO) device demonstrated a maximum energy density of 24.5 Wh kg⁻¹, a power density of 900 W kg⁻¹, and a retention of 90% following 6000 cycles. This research paves the way for electrochemical investigations of the newly created nano-structured composite material combined with various polymer electrolytes, while also exploring differing composite morphologies for other energy storage uses [39].

Researchers from the Hunan University of Science and Engineering in China proposed the structural alignment of Chinese fir (pine tree) scraps for energy storage applications. The anode consisted of aligned carbon nanotube arrays chemically deposited in carbonized wood tracheids, while a manganese dioxide-activated wood carbon (MnO₂@AWC) acts as a cathode material. Achieving high energy density and cycle stability in electrodes was a challenge for the researchers, who used chemical vapor deposition to align CNTs within the tracheids of Chinese fir in order to boost conductivity and surface area. The electrochemical deposition of MnO₂ nanosheets onto the inner surface of the tracheids improved the specific capacitance to 652.0 F g⁻¹ at 10 mA cm⁻². The resulting all-solid-state asymmetric supercapacitor achieved an energy density of 48.6 Wh kg⁻¹ (twice that of supercapacitors made from other biomass materials) and maintained 93% capacitance retention after 10,000 cycles. This approach, combining CNTs and MnO₂, creates an electrode with excellent stability, high energy density, and long-term cycle performance, offering a promising path for future wood-based carbon electrode development in energy storage systems [40].

In order to improve the specific capacitance and enhance the environmental profile of MnO₂, researchers M. Mohsen and N. Moosavi propose the combination of MnO₂ with a 3D carbon substrate. Targeting the low conductivity of MnO₂, the researchers introduced metal ions into the structure, enhancing the electron transfer phenomenon with the deposition of Mg-, Zn-, and Mg-Zn-doped MnO₂ nanostructures onto CC hydrothermally. Amongst the three, Zn-MnO₂@CC showcased the best performance, while the fabricated flexible solid-state symmetric supercapacitor (with Zn-MnO₂@CC used both as an anode and cathode) showcased a maximum energy density of 0.197 mWh cm⁻² (at a current density of 2 mA cm⁻²) and a power density of 2378 MW cm⁻². In addition, the device maintained 83% of its capacitance following 4000 charge-discharge cycles. Finally, in another series of practical tests, the supercapacitor provided electricity for 3 LEDs and a fan motor. Therefore, according to the research team, the obtained results could be considered as a novel technology, providing a low-cost, high-performance material with useful applications in modern-day technology [41].

Utilizing a simple one-pot method, Jianghua Wu and co-workers synthesized potassium(K)-doped MnO₂ nanowires with promising qualities backed up by experimental tests. The addition of potassium causes the MnO₂ to change from its a- to its δ-form, increasing lattice defects and improving the material’s conductivity. There was a notably higher percentage of active sites in the composite, enhancing its capacitive performance. The composed K-doped MnO₂ showed enhanced morphology, electrochemical properties, and stability due to the presence of KOH and its ions (OH⁻ and K⁺) in the lattice structure. The K0.35MnO₂/CC electrode showcased a maximum capacity of 405 F g⁻¹ (with a current density of 1 A g⁻¹) while the asymmetric supercapacitor K0.35MnO₂/AC displayed a maximum energy density of 71.5 Wh kg⁻¹. After 8000 cycles, the device maintained 92.7% of its capacitance (at 20 A g⁻¹). Through this research, the electrode K0.35MnO₂/CC could be considered as a novel and defective material for various energy storage applications with remarkable performance [42].

Following a two-step method, Rahul S. Ingole, with a team of researchers, deposited rGO sheets onto unique 3D-hierarchical MnO₂ nanorods, using CC as a base. The process involveδ a hydrothermal deposition of MnO₂ nanorods onto CC, and afterwards, an ex-situ decoration with rGO. By modifying the concentration of GO each time, the researchers can note its impact on the physical and electrochemical properties of MnO₂-rGO electrodes. The addition of rGO sheets created a mesoporous structure, promoting the interfacial electron transfer phenomenon and improving the overall electrochemical performance. The rGO-decorated MnO₂ electrode (concentration of rGO was 20 mg/100 mL) achieved a specific capacitance of 1072.28 F g⁻¹ at 5mV sec⁻¹ in a 1 M Na₂SO₄ electrolyte and exhibited great cycle stability, as it retained 87% of its capacitance after 2000 cycles. This innovative material showed great morphological, electrochemical, and structural characteristics, ideal for wearable technology and other advanced applications [43]. Superior capacitive behavior is attributed to materials with controlled nanostructured morphology and porous design. More specifically, MnO₂-rGO decorated nanorods showcased outstanding specific capacity (1072.28 F g⁻¹) while Fe₃O₄@MnO₂ and MnO₂–nanoneedle/rGO had incredible cycle stability, 100% and 90%, respectively. Core–shell structures also seem to improve electron transfer and structural stability as the core–shell architectures in MnO₂@carbon, MnO₂@polyaniline, and Fe₃O₄@MnO₂ enhance conductivity, accommodate volume changes, and boost cycling stability.

2.2.3. Doping and Functionalization

Two pivotal strategies in advancing the performance of MnO₂-based composites for energy storage applications are doping and functionalization. Doping involves the incorporation of foreign atoms or ions into the matrix of the MnO₂ lattice, which can significantly enhance electronic conductivity, modify the crystal structure, and improve the overall electrochemical performance [44]. Functionalization, on the other hand, refers to the modification of MnO₂ surfaces or interfaces, which can strengthen interfacial interactions, facilitate charge transfer, and further optimize the material’s capacitive behavior; often through the integration of conductive materials or chemical groups (e.g., -OH, -COOH) [45,46]. Meena Rittiruam and researchers examined the effects of metal doping (Co, Ni, and Pd) on α-MnO₂ for enhancing oxygen reduction reaction (ORR) performance in metal–air batteries (MABs), utilizing the DFT and computational hydrogen electrode (CHE) calculations. After examination, it has been proven that the doping of metals modifies the adsorption strength of surface species during ORR by inducing electron accumulation at the Mn site and altering the d-band centre. More specifically, Co and Pd doping improve ORR activity, acting as promoters, while Ni doping negatively affects certain ORR steps, lowering activity. The obtained results revealed that enhanced catalytic performance occurred when Co and Pd were doped at interstitial sites, rather than substituting for Mn. These findings underscore the importance of controlling dopant concentration and atomic dispersion at interstitial sites to optimize electrocatalytic activity for MABs. Bulk calculations suggested that doping transitions α-MnO₂ from semiconductor to metallic behaviour, enhancing conductivity. The study provided valuable insights into how doping affects electronic properties and surface interactions, offering guidance for improving the performance of MAB cathodes (Figure 7) [14].

In order to improve the poor conductivity and overall performance of MnO₂, a suggested approach by Rathod, Vishal. T et al. involved doping the surface with Ni ions (or various metals), which enhances the charge storage capacity, especially in lithium-ion batteries. In this study, pristine and Ni-doped MnO₂ nanocrystals were synthesized using a triethanolamine (TEA)–ethoxylate-assisted hydrothermal method. The results of electrochemical testing on the final composite showed that the 4% Ni-doped MnO₂ electrodes exhibited the highest specific capacitance of 432.5 F g⁻¹ at a scan rate of 5 mV s⁻¹ and maintained 96.56% stability after 2000 cycles at a high current density. The Ni doping enhanced the overall electrical conductivity while lowering the electrolyte and charge-transfer resistance of MnO₂. Additionally, an asymmetric supercapacitor using 4% Ni-doped MnO₂ as the positive electrode and activated charcoal as the negative electrode achieved a maximum specific capacitance of 136.6 F g⁻¹. Based on the results, the researchers stated that Ni-doped MnO₂ can act as a high-performance electrode material for supercapacitors and advanced energy storage systems [47]. It should be mentioned that in the work of Rathod, Vishal. T et al., the incorporation of Ni positively influenced the final product, whereas in the work of M. Rittiruam et al., the addition of Ni negatively affected the final electrode.

Researchers from the Department of Physics of Annamalai University in India focused on the effects of cobalt doping on α-MnO₂ nanoparticles, which were synthesized using a co-precipitation technique. The electrochemical properties of the Co-doped α-MnO₂ electrode were assessed through cyclic voltammetry, galvanostatic charge–discharge measurements, and electrochemical impedance spectroscopy. The results revealed that the Co-doped α-MnO₂ electrode exhibited pseudocapacitive behavior and significantly higher capacitance values compared to pure α-MnO₂. Specifically, the Co-doped α-MnO₂ demonstrated a capacitance of 1015 F g⁻¹ at 10 mV s⁻¹, compared to 901 F g⁻¹ for pure α-MnO₂. Furthermore, charge–discharge measurements showed a capacitance of 996 F g⁻¹ at 1 A g⁻¹ for Co-doped α-MnO₂, outperforming pure α-MnO₂, which had a capacitance of 876 F g⁻¹. The Co-doped α-MnO₂ also exhibited a lower charge transfer resistance, indicating improved supercapacitor performance. Phase and morphological analyses indicated that the Co-doped α-MnO₂ nanoparticles have a tetragonal structure with smaller crystallite sizes and a rod-like shape due to the Co doping, as confirmed by SEM and TEM imaging [48].

According to the research of N.D. Raskar and co-workers, following a hydrothermal method, α-MnO₂ nanocomposites, rGO-based α-MnO₂ composites, and Fe-doped α-MnO₂ composites were fabricated. They proceeded to evaluate the applicability of the composites in supercapacitors and photocatalytic dye degradation devices through the examination of the phase transition from α-MnO₂ to the cubic phase of α-Mn₂O₃, using XRD analysis. Incorporating rGO and Fe doping significantly enhanced the materials’ electrochemical performance and photocatalytic activity. Specifically, the rGO-based 5% Fe-doped MnO₂ (GFMO₅) exhibited a specific capacitance of 683.35 F/g and achieved a 90% degradation of methylene blue dye under visible light. The XRD, SEM, and X-ray photoelectron spectroscopy (XPS) analyses confirmed the phase transition and morphology changes due to Fe substitution in the MnO₂ lattice, enhancing the material’s performance. The results showed that Fe doping and rGO incorporation improved the MnO₂ capabilities, making it suitable for both energy storage and environmental applications. The rGO-based 5% Fe-doped MnO₂ nanocomposite demonstrated the best overall performance, highlighting its potential as both a photocatalyst and a high-performance supercapacitor [49].

Researchers from the Wuhan Polytechnic University in Hubei synthesized mesoporous Ag-doped γ-MnO₂ nanowires at room temperature, for the first time, following an Ag⁺ activated persulfate process. This budget-friendly method results in nanopowder materials with uniformly distributed nanoparticles, aiming to enhance the performance of cathodes in aqueous ZIBs. The resulting Ag-doped γ-MnO₂ electrodes showed remarkable electrochemical performance, achieving a discharge capacity of 978 mAh g⁻¹ after 65 cycles at a battery current rate of 0.1 C, stabilizing at 804 mAh g⁻¹ after 150 cycles. The Ag@γ-MnO₂ electrode showcased excellent rate capability, as the retention of the specific capacity using differing C-rates was calculated to be more than 100% due to the predominant surface capacitive-controlled process. This Ag doping improved both oxygen defects and the stability of the material, contributing to superior electrochemical performance. The electrodes delivered a capacity of 320 mAh g⁻¹ at a current density of 1540 mAh g⁻¹ and exhibited outstanding cycling stability, highlighting the potential of Ag-doped γ-MnO₂ for high-performance aqueous ZIBs [50].

As mentioned, MnO₂ is a promising cathode material for aqueous zinc-ion batteries but faces challenges with cycling stability due to the instability of its structure and the dissolution of Mn²⁺ ions. To address this, amino-functionalized multi-wall carbon nanotubes (MWCNTs-NH₂) were introduced by the group of Huang Du in order to create a nanocomposite (MWCNTs-NH₂/γ-MnO₂) that significantly improved the stability of MnO₂. The MWCNTs-NH₂ showed enhanced structural integrity and conductivity due to their three-dimensional structure, which created pathways enabling electron movement. The integration of amino groups (groups with high nucleophilicity) helps in the storage of Zn²⁺/H⁺ ions in the material and the deposition of Mn²⁺ to the cathode, preventing the loss of active material. This leads to a high capacity of 417.9 mAh g⁻¹ (at a current density of 0.2 A g⁻¹) and excellent cycling stability, with 91.4% capacity retention after 3500 cycles. Therefore, it was proven that the presence of amino-doped carbon nanotubes increased the electrochemical performance of γ-MnO₂, offering a strategy to improve the long-term performance of MnO₂-based cathodes in zinc-ion batteries [13].

In another study, researchers Dipanwita Majumdar and Swapan Kumar Bhattacharya utilized hydroxyl groups to improve the electrochemical performance of a graphene–MnO₂ nanocomposite. The hydroxy-functionalized graphene/MnO₂ (hGO-MnO₂) material was synthesized through a one-pot sonochemical method in order to anchor MnO₂ nanosheets onto hGO, optimizing oxygen content on graphene, something not successfully achieved through other techniques. Both of the components’ optimum properties combine in this composite, exploiting their synergistic effect, resulting in electrodes with high specific capacitance (376.7 F g⁻¹) and stable cycling performance (91% after 1000 cycles, compared to 60% for the non-MnO₂–graphene composite). The (hGO-MnO₂) electrode material exhibits great electrochemical performance and uses a simple, low-cost, environmentally friendly method, making it a great candidate for energy storage applications [51].

Researchers from Daegu University in South Korea examined the effect of B₂O₃ on the structure of multilayer carbon nanofiber/MnO₂ composites. Utilizing sequential electrospinning, they composed a tri-layered boron-MnO₂/CNF composite (PPMnB) and studied its applicability as a supercapacitor material based on the characteristics. Each of the composite components acted individually to enhance the electrochemical performance as it exhibits a specific capacitance of 215 F g⁻¹ at 1 mA cm⁻², a rate capability of 83% capacity retention at 20 mA cm⁻², and a great cycling stability (6% loss after 10,000 cycles at 1 mA cm⁻²). The B-related functional groups exist independently between the nanosheets within the 3D interconnected CNF network while optimizing the pseudocapacitance of the material. Specifically, the first PAN-based CNF layer consists of micropores enhancing the materials’ capacitance. The second layer showed a quasi-capacitance behavior due to the MnO₂ particles and numerous pores, leading to a high-rate response and a stable cycle life. The third layer was more hydrophilic and had high electrical conductivity due to the B-related functional groups, promoting electron transfer. Therefore, the composed PPMnB material was mentioned as a promising component in high-power, high-energy, high-capacity supercapacitors [52].

Pseudocapacitive materials with ion exchange sites on the surface are popular for asymmetric capacitors due to their electrochemical properties. Using activated CC decorated with featherlike MnO₂ nanoparticles, researchers manage to compose oxygen-enriched MnO₂ with a sheetlike structure via electrochemical oxidation. Because of the abundant oxidative functional groups on the surface of the composite, great conductivity and a capacitance of 3160 mF cm⁻² at 1 mA cm⁻² were exhibited. The anode of the capacitor consisted of activated carbon with an organic framework via the hydrothermal method, which is additionally doped with N in the AC lattice using annealing. N-doping further increased the functional groups and disorders on the surface, improving the overall performance. The complete device showed a maximum energy density of 8723 mWh cm⁻³ at a power density of 14,248 mWh cm⁻³, with a capacitance of 95.5% after 10,000 cycles, while it maintained 4548 mWh cm⁻³ at 148.835 mW cm⁻³. This research proposes an excellent product with applications in modern-day manufacture of energy storage devices [53].

Therefore, based on these studies, it is indicated that Co and Pd doping significantly promote the oxygen reduction reaction (ORR) in metal–air batteries by modulating the d-band center and electronic density on Mn sites. Nickel, on the other hand, while it improves conductivity and capacitance in MnO₂ used in supercapacitors, it hinders ORR activity when introduced inappropriately, highlighting the importance of site-specific doping and application context. The most impressive specific capacitance results are exhibited by the Co-doped α-MnO₂ and the Ag-doped γ-MnO₂ composites at 1015 F g⁻¹ and 978 mAh g⁻¹ (capacity), selectively. In the cobalt composite, smaller crystallite size and morphological control improve electrochemical performance in most applications, while noble metals perform incredibly for AIZBs in specialized systems. Two of the carbon-functionalized products, hGO-MnO₂ and MWCNTs-NH₂/γ-MnO₂, exhibit remarkable cycling performance at ≈91% after 1000–3500 cycles.

2.2.4. Hybridization with Synergistic Materials

The synergistic interactions between MnO₂ and complementary materials such as conductive polymers, carbon-based materials, or other metal oxides are critical to overcome the poor electrical conductivity and slow reaction kinetics of MnO₂. Utilizing the synergistic effect of MnO₂ and Ti₃C₂ on MgH₂ can improve the performance of Mg/MgH₂ as a hydrogen storage material by constructing a MnO₂-doped Ti₃C₂ composite. By depositing MnO₂ onto Mxene, the original flat structure of the material was maintained while showing lower hydrogen absorption/desorption temperatures and faster reaction rates. The investigated composite exhibited numerous pathways for hydrogen atoms and promoted H diffusion due to the coexistence of Mg/Ti₃C₂, Mg/TiO₂, Mg/MnO₂, Mg/MnO, and Mg/Mn multiphase interfaces in the MgH₂+Ti₃C₂@MnO₂ system. The present study carves the path for novel materials useful in hydrogen storage applications, increasing in popularity in recent years (Figure 8) [54].

Neha Kanaujia and coworkers reported the synthesis of flower-like MoS₂, spherical MnO₂, and their MnO₂/MoS₂ nanocomposite using a hydrothermal method in order to bind the MnO₂ nanoparticles and MoS₂ nanosheets together. Specifically, MoS₂ demonstrated higher ionic conductivity than metal oxides, had a wide surface area, and exhibited high specific capacitance due to ion diffusion through its layers. The electrochemical properties of nanocomposites, with differing ratios, were evaluated using a three-electrode setup in a 2M KOH electrolyte at room temperature. The MnO₂@MoS₂ nanocomposite with a 3:1 molar ratio exhibited superior performance, with a specific capacitance of 352 F g⁻¹ at 1 A g⁻¹, 72% capacity retention after 2000 cycles at 3 A g⁻¹, and 88% coulombic efficiency. This sample also demonstrated enhanced ion transport with a smaller relaxation time constant (150 ms) and a higher diffusion coefficient. The high surface area, mesopore structure, and optimal pore size distribution contributed to its exceptional electrochemical properties, offering valuable insights for the development of high-performance energy storage devices [55].

Scientists from the National Center of Nanoscience and Technology and the Institute of Physics in Beijing fabricated a coral-like porous MnO₂ nanostructure film on stainless steel using a simple ZnO-nanorod-array-mediated hydrothermal method. The resulting electrode materials exhibited impressive electrochemical performance, with a specific capacitance of 221 F g⁻¹ at a current density of 0.5 A g⁻¹ and 86% capacitance retention after 3000 cycles at 5 A g⁻¹. The high performance was attributed to the porous, hierarchical MnO₂ nanostructure formed on the conductive stainless-steel substrate and the existence of ZnO, a semiconductive material that is easily dissolved and purchased. The ZnO nanowires served as a template, directing the formation of disordered birnessite-MnO₂ nanocrystals, enhancing the overall stability and efficiency of the supercapacitor. This unique structure offered great potential for use in low-cost, high-performance supercapacitor applications [56].

A study by Godlaveti and co-workers introduced a new binary composite of copper sulfide and manganese oxide designed to improve supercapacitor performance. Generally, introducing metal oxides, metal sulfides, and their derivatives as electrode material has been proven to enhance the device’s electrochemical behavior. The synthesized composite consisted of 80% CuS and 20% MnO_2, as this ratio shows optimized performance. The resulting high-purity nanoparticles and nanorods showed promising properties, with the CuS/MnO₂ composite demonstrating excellent charge storage capabilities. The material exhibited a specific capacitance of 451 F g⁻¹, low resistance, and remarkable cyclic stability (98.33% after 2000 cycles). Additionally, the composite achieved a high-power density of 1997 W kg⁻¹ and an energy density of 28.19 Wh kg⁻¹. Structural analysis confirmed the successful synthesis of CuS and MnO₂, and the composite outperformed bare CuS electrodes in electrochemical tests, highlighting the synergistic effects between CuS and MnO₂. These results suggested that CuS/MnO₂ composites could be promising for advanced energy storage applications [57].

A Sm-doped δ-MnO₂/Carbon aerogel (SDM/CA) composite was synthesized using a hydrothermal method to enhance the electrical conductivity and structural stability of δ-MnO₂ for supercapacitors. The carbon aerogel acted synergistically due to its 3d porous structure providing more redox reaction sites and electron transferability, while the presence of Sm³⁺ improved electron diffusion. The SDM/CA material, rich in oxygen vacancies, exhibited improved conductivity, reduced charge transfer resistance, and increased surface area compared to pure δ-MnO₂. As a result, it demonstrated excellent cycling stability with 94% retention after 10,000 cycles at 10 A g⁻¹. The assembled asymmetric supercapacitor using SDM/CA as the positive electrode showed a specific capacitance of 31.28 F g⁻¹ and 73.75% capacity retention after 10,000 cycles. Overall, the enhanced performance was attributed to the synergistic effect of Sm-doping and carbon aerogels, improving charge accumulation and reducing polarization [58]. Synergistic hybridization greatly improves performance, as demonstrated in all studies. The hybridization of MnO₂ with synergistic materials—such as MXenes (Ti₃C₂), MoS₂, carbon aerogels, metal sulfides, semiconductors, and templates—enhances capacitance, energy/power density, H₂ kinetics, and long-term cycling stability (

Table 2. Tabulation of MnO₂ composite materials stating their properties and applications.

Composite Material	Main Components	Key Properties	Applications
MnO2/Graphene	MnO2 + Graphene oxide/reduced GO	- High conductivity, large surface area, flexibility	- Supercapacitors, Li-ion batteries, sensors
MnO2/Carbon Nanotubes (CNTs)	MnO2 + CNTs	- Enhanced mechanical strength, fast electron transport	- Electrodes for capacitors and fuel cells
MnO2/Polyaniline (PANI)	MnO2 + Conductive polymer	- Good electrochemical stability, high pseudocapacitance	- Supercapacitors, electrochromic devices
MnO2/Fe3O4	MnO2 + Magnetic Fe3O4	- Magnetic properties, high redox activity	- Environmental remediation, sensors
MnO2/TiO2	MnO2 + Semiconductor TiO2	- Synergistic photocatalytic activity	- Photocatalysis, degradation of organic pollutants
MnO2/NiCo2O4	MnO2 + NiCo2O4 spinel	- High capacity, improved cycling performance	- Battery and supercapacitor electrodes
MnO2/Chitosan	MnO2 + Biopolymer chitosan	- Biocompatibility, ion exchange capability	- Biosensors, drug delivery systems
MnO2/Activated Carbon	MnO2 + Activated carbon	- High surface area, enhanced ion transport	- Hybrid capacitors, water purification
Doped MnO2	MnO2 + Fe-, Co-, Ni-, Cu	- Improved electrical conductivity via defect states; enhanced redox activity; better ion diffusion; structural stability	- Supercapacitors; Li-ion and Na-ion batteries; electrocatalysis (ORR, OER)
MnO2/ZnO, MnO2/TiO2, MnO2/Fe2O3(MnO2 Heterostructures)	MnO2 + ZnO, TiO2, Fe2O3	- Synergistic interfacial effects. Enhanced electron-hole separation; broadened light absorption (in photoactive systems)	- Photocatalysis (pollutant degradation, water splitting); gas sensors; hybrid capacitors
Yolk–Shell MnO2 Structures	MnO2@C, MnO2@MOF	- High surface area and hollow structure; strain accommodation and structural integrity; fast ion/electron transport	- Battery electrodes with long cycle life; electrochemical capacitors; drug delivery

).

The goal of functionalization and doping is to enhance the capacity performance of MnO₂ in composite materials, utilizing its synergistic effect with various components. Metal doping, for instance, promotes electron mobility while non-metal doping boosts pseudo-capacitance through surface redox reactions. The integration of conductive materials, such as graphene, CNTs, conductive polymers, with manganese oxide also boosts conductivity and leads to higher specific capacitance. Additionally, modifying composites at the nanoscale (nano structuring) utilizing nanomaterials for deposition and synthesis targets the low theoretical capacitance of such materials [21,59,60]. The phenomenon of structural breaking can be addressed by incorporating the metal oxide onto flexible substrates comprised of nanofibers or polymer binders with reduced mechanical stress. Core–shell structures can also enhance the durability of the material, maintaining its integrity during expansion/contraction [35,61,62].

3. Synthesis Strategies

Effective synthesis techniques are crucial for achieving desired properties in MnO₂-based composites. By carefully controlling parameters such as reaction time, temperature, pH, and precursor materials, researchers can affect the morphology, surface area, and crystallinity of the resulting composites. These factors directly impact the stability, electrochemical performance, and functionality of MnO₂ in several applications, making the synthesis method a key determinant of overall material performance (Figure 9).

3.1. Hydrothermal and Solvothermal Methods

This method provides control over morphology and crystallinity. According to this synthetic process, well-crystallized MnO₂ with tunable morphologies is fabricated. A flexible hydrogel-based supercapacitor made from hydro/solvothermally grown bimetallic ZnS/MnO₂ Metal–Organic Framework (MOF) was developed for all-solid-state energy storage devices. More specifically, the solvothermal method for the synthesis of ZnS/MnO₂-MOF was executed at 180 °C for 20 h, while its hydrogel was fabricated via freeze-drying. Through this synthesis method, the composite formed a cubic structure (confirmed from X-ray analysis) and demonstrated a uniform dispersion of ZnS/MnO₂-MOF nanosheets, which enhanced its electrochemical properties. The supercapacitor exhibited a specific capacitance of 112 F g at 1.5 A g⁻¹, with an energy density of 30.3 Wh kg⁻¹ and a power density of 1050 W kg⁻¹. The device showed excellent stability, retaining 63% of its capacitance after 20,000 cycles. The use of PVA-KOH as the electrolyte contributed to the high performance and flexibility, making the device ideal for wearable and medical applications. This technology presented a promising alternative to traditional supercapacitors with superior stability, safety, and usability [63].

Another approach by Muhammad Abdullah and researchers explored the synthesis of pure manganese oxide, zinc sulfide, and their nanocomposite (ZnS/MnO₂) for use in electrochemical supercapacitors. The materials were synthesized using a simple, eco-friendly hydrothermal method and characterized through various techniques to confirm their purity, defined crystallinity, and single-phase morphology. More specifically, the ZnS and MnO₂ were dispersed at a 1:1 ratio in 50 mL of DI water, followed by sonication (20 min), moved to a hydrothermal reactor at 180 °C for 4 h, and isolated after dehydration at 65 °C. The ZnS/MnO₂ nanocomposite showed excellent electrochemical performance, with a high specific capacitance of 1002 F g⁻¹ at 1.0 A g⁻¹ and improved discharge times compared to other rates. The composite exhibited a symmetric behavior with a specific capacitance of 373 F g⁻¹ and energy density of 15.45 Wh kg⁻¹. The structure of ZnS/MnO₂ showcases an expanded electric contact area of MnO₂, leading to improved ion transferability. This study highlights the potential of ZnS/MnO₂ composites for cost-effective, high-performance energy storage applications (Figure 10) [64].

In a study by Mohamed Racik and co-workers, α-Fe₂O₃/MnO₂ nanocomposites were synthesized using a hydrothermal method and characterized using various techniques, including XRD, SEM, TEM, FT-IR, and vibrating sample magnetometer. The facile hydrothermal reaction consisted of dissolving a-Fe₂O₃ in double-deionized water, followed by the addition of manganese (II) chloride tetrahydrate and urea into the dispersion. After 1h of stirring, the solution was heated up to 120 °C for 24 h, and the sediments were dehydrated at 80 °C for 12 h, giving the final product. Electrochemical analysis revealed that the α-Fe₂O₃/MnO₂ NCs exhibited enhanced specific capacitance of 216.35 F g⁻¹, significantly higher than pure α-Fe₂O₃ and MnO₂ nanoparticles. The obtained nanocomposites demonstrated a high energy density of 135.42 Wh kg⁻¹ and retained 89.2% of their initial capacitance after 10,000 cycles at 1 A g⁻¹, indicating excellent stability. The synergistic effect between α-Fe₂O₃ and MnO₂ improved the electrochemical performance, making the composite a promising candidate for high-efficiency hybrid supercapacitors [65].

Manganese Dioxide nanorods anchored on graphene sheets were successfully synthesized using a solvothermal process for use as electrode materials in electrochemical capacitors. The synthesis involved integrating highly pristine graphene with MnO₂ through liquid-phase exfoliation, which uses environmentally friendly reactants and low temperatures to produce the crystalloid material. The XRD and Raman analysis confirmed the formation of a tetragonal α-MnO₂ structure, while SEM and TEM imaging showed MnO₂ nanorods anchored on graphene sheets. Electrochemical tests demonstrated a high specific capacitance of 380 F g⁻¹ at 5 mV s⁻¹, an energy density of 53 Wh kg⁻¹, and excellent cyclic stability. The superior performance is attributed to the pseudocapacitance of MnO₂ combined with the high conductivity of graphene. The study presents an efficient method for synthesizing graphene-MnO₂ composites, making them promising candidates for energy storage applications [66].

Hindavi D. Kirdat and co-workers synthesized MnO₂, Cr₂O₃, and their MnO₂-Cr₂O₃ composite, which was studied for its performance as an electrode in a solid-state asymmetric supercapacitor device. The two reactants for the hydrothermal synthesis were combined in a 1 MnO₂/2Cr₂O₃ ratio along with 50 mL of DI water. Afterward, the mixture was transferred into a hydrothermal apparatus, heated at 160 °C for 12 h, and dried at 80 °C. Detailed analyses of the composite’s crystallography, morphology, and electronic states were performed. Electrochemical tests revealed that the MnO₂-Cr₂O₃ composite achieved an impressive specific capacitance of 623 F g⁻¹, outperforming individual MnO₂ and Cr₂O₃ electrodes. The composite also demonstrated a power density of 600 W kg⁻¹, an energy density of 31.16 Wh kg⁻¹, and 77% capacitance retention after 10,000 cycles. The solid-state asymmetric supercapacitor device, using this composite as the positive electrode, showed a capacitance of 47 F g⁻¹ and 71% retention over 10,000 cycles. These results confirm the MnO₂-Cr₂O₃ composite as a promising material for advanced energy storage applications, offering high capacitance, stability, and performance [67].

3.2. Electrochemical Deposition

This method enables precise deposition of MnO₂ on conductive substrates. It directly deposits MnO₂ on conductive substrates for electrochemical applications. For example, thin films of FTO/MnO₂ composed of graphene were produced by submerging graphene (1 mg of 1 cm² surface) into an electrolyte bath (0.3 M MnSO₄ H₂O in a mixture of 1 M H₂SO₄ and 0.5 M Na₂SO₄), dispersing the solution due to weak agitation (60 rpm). During this process, the graphene particles were deposited onto the MnO₂ films under vigorous stirring. The electrochemical cell consisted of three attached electrodes controlled by a potentiostat/galvanostat. The deposition of FTO/MnO₂-GR films occurred at 1.2 V for 420 s while the graphene sheets were obtained by the electrochemical method [10].

Researchers Shuang Xi and co-workers composed a graphene-based substrate (a-LIG) utilizing direct laser writing and KOH activation and electrodeposited MnO₂ in order to produce a flexible a-LIG/MnO₂ thin-film electrode. The procedure used a 3-electrode system with the working electrode being a-LIG, the counter electrode a platinum sheet, and silver chloride as a reference electrode with a current density of 30 mA cm⁻². The end hybrid electrode showcased impressive electrochemical performance, achieving a real capacitance of 304.61 mF cm⁻² at 1 mA cm⁻² in a Na₂SO₄ aqueous electrolyte. The constructed flexible asymmetric supercapacitor, utilizing a-LIG/MnO₂ as the anode and a-LIG as the cathode with PVA/H₃PO₄ as the gel electrolyte, delivered an energy density of 2.61 µWh cm⁻², a power density of 260.28 µW cm⁻², while it retained 90.28% capacitance after 5000 cycles. The LIG technique simplifies device fabrication and enables scalability, providing a low-cost, high-precision approach for flexible energy storage solutions [68].

In an effort to tackle the challenges of poor constructability and conductivity associated with PAni, researchers from the Central South University of Changsha developed a novel, durable, and highly conductive PAni/MnO₂/graphene oxide (PMGO) composite cathode for seawater batteries. The PMGO composite was synthesized via an in situ electrochemical method on graphite paper, resulting in a 3-dimensional porous network with even dispersion of PAni, MnO₂, and graphene oxide. More precisely, the graphite’s surface was oxidized to multilayer GO before the PAni and MnO₂ were electrodeposited on top. The PMGO cathode showcases improved conductivity, a 1.46 V discharge voltage, and a 500–600 Wh kg⁻¹ energy discharge performance, compared to traditional PAni and AgCl electrodes. The composite’s unique morphology, with nanowires and a 3D network, reduced polarization and enhanced the electrode reaction. The PMGO/Mg seawater battery outperformed others with a high specific energy of 544 mWh g⁻¹, while its eco-friendly, cost-effective, and easy-to-produce profile made it suitable for large-scale production and practical applications (Figure 11) [69].

The study by Shaobo Liu and his co-workers demonstrated a Fe-doped MnO₂ (Fe-MnO₂) nanosheet that undergoes dynamic reconstruction during galvanostatic charge/discharge activation, optimizing its pseudocapacitive storage. After activation, the Fe-MnO₂ nanosheets converted to a composite structure of Fe-doped, oxygen-deficient nanosheets and nanowires. The activation occurred using a three-electrode system by an uninterrupted galvanostatic charge–discharge process at 10 A g⁻¹, causing the dissolution and redeposition of Mn. This reconstruction enhanced electron and ion transfer by increasing the electrochemically active surface area and improving the kinetics. It was theoretically proven that the combination of Fe doping and oxygen defects promoted charge delocalization, while the nanowires’ tip-enhanced electric field attracted more ions. Based on the electrochemical tests, the activated Fe-MnO₂ showcased a specific capacitance of 500.1 F g⁻¹, surpassing the 379.2 F g⁻¹ of the original material. These findings highlighted the importance of morphology and defect evolution in boosting the energy storage performance of Fe-MnO₂ electrodes, providing insight into designing high-performance electrode materials [70].

Researchers from the Jiangxi Science and Technology of Normal University constructed a porous MnO₂-poly(5-cyanoindole) (MnO₂/PCIn) composite using a two-step electrochemical method to enhance the performance of supercapacitors. To be more precise, the MnO₂ nanoparticles were deposited onto poly(5-cyanoindole) glassy carbon (PCln/GC) that was the host material, in a 5 mM KMnO₄ solution (precursor) at 0 V. The PCIn film acted as a three-dimensional matrix that scattered MnO₂ nanoparticles, preventing them from forming clusters. This composite electrode exhibited a specific capacitance of 322.5 F g⁻¹ at a current density of 8.3 A g⁻¹, outperforming MnO₂ deposited on a bare GC electrode, which had a capacitance of 247 F g⁻¹. The MnO₂/PCIn/GC composite also demonstrated a high energy density of 36.1 Wh kg⁻¹ at a power density of 12.5 kW kg and a capacitance loss of 28.3% after 1000 cycles. These results indicated that the PCIn modification significantly enhanced the specific capacitance and stability of MnO₂, making it a promising material for supercapacitor electrodes [71].

3.3. Co-Precipitation

The co-precipitation method is a widely used technique for the production of homogeneous mixed-metal compounds, especially nanoparticles. It includes the simultaneous precipitation of multiple ions from a solution, typically by adding a precipitating agent or adjusting pH. This method offers control over morphology, particle size, and composition. It is cost-effective, simple, and suitable for large-scale production.

A one-step co-precipitation method was used to synthesize MWCNT-decorated ε-MnO₂ nanoflowers (ε-MnO₂/MWCNTs) with a hexagonal structure. The experimental method started by incorporating a solution B (0.02 M KMnO₄) with a solution A (3mmol MnCl₂•4H₂O and 100 mg of acid-treated MWCNTs to 100 mL DI water) under constant stirring. After 6 h, the sediment was washed with DI water and dried at 90 °C for 12 h, resulting in the end product. The obtained material combined the high conductivity of MWCNTs and the nanostructured nature of MnO₂, resulting in remarkable zinc-ion storage performance. The ε-MnO₂/MWCNTs exhibited an impressive reversible capacity of 335.6 mAh g⁻¹ at 0.2 A g⁻¹ after 150 cycles and maintained a long-term capacity of 115.7 mAh g⁻¹ after 4000 cycles at 2.0 A g⁻¹. Carbon decoration of MnO₂, reduced polarization of MnO₂, improved the structural stability, and enhanced the reaction kinetics of MnO₂. This composite material, combining high capacity, stability, and rapid ion exchange, could be considered as an excellent potential to be used as a cathode material for eco-friendly, high-performance batteries, offering a promising approach for large-scale energy storage in aqueous metal-ion systems (Figure 12) [60].

Nanospheres of α-MnO₂ with a tunnel structure were synthesized using a one-step co-precipitation method. This was a beneficial experimental method, following understandable steps, with low cost, providing a uniform product. The α-MnO₂ nanospheres exhibited excellent zinc-ion storage performance, with a high specific capacity of 462 mAh g⁻¹ at a current density of 0.1 A g⁻¹ and strong rate capability of 160 mAh g⁻¹ at 1.0 A g⁻¹. The material also demonstrated remarkable cycle stability, retaining 75.7% of its capacity after 500 cycles at 1.0 A g⁻¹. Electrochemical analysis revealed that the energy storage mechanism incorporates H⁺ into α-MnO₂ nanospheres, causing more OH⁻ ions on the surface and formation of Zn₄SO₄(OH)₆•5H₂O. The following second discharge plateau was attributed to the decrease in H⁺ and incorporation of Zn²⁺. Diffusion and capacitance mechanisms enhanced the electrodes’ performance and cycle stability. These findings highlighted the potential of α-MnO₂ nanospheres as a promising cathode material for safe, efficient, and durable zinc-ion batteries, offering insights for the development of low-cost, highly reversible energy storage solutions [72].

Kun Li and co-workers utilized a one-step co-precipitation method to construct Cu²⁺-intercalated δ-MnO₂ nanoflowers, suitable for commercial manufacture. Incorporating Cu²⁺ into the matrix provided more space between the interlayers because of water molecules while also supporting the structure, improving conductivity, and increasing the rate capability due to the exchange mechanism of Zn²⁺/H⁺ ions. The resulting cathode material for ZIBs exhibited a high specific capacity of 300 mAh g⁻¹ at a current density of 0.1 A g⁻¹, excellent rate capability of 195 mAh g⁻¹ at 1 A g⁻¹, and remarkable long-term stability, retaining 89.9% of its capacity after 2000 cycles at 1 A g⁻¹. The Cu²⁺ insertion also improved the structure’s conductivity and provided additional active sites for Zn²⁺ adsorption, confirmed by DFT calculations. Ex situ characterization revealed the reversible Zn²⁺/H⁺ co-insertion mechanism. The study also demonstrated the commercial potential of flexible ZIBs, highlighting their stability under various conditions and offering valuable insights for advancing ZIB technology [73].

A microwave-assisted co-precipitation method was used to synthesize rGO-decorated δ-MnO₂ and Ni-doped δ-MnO₂ nanocomposites, through a basic, fast, and economic process. This method preserved the monoclinic crystal δ-MnO₂ structure, as proved by the x-ray diffraction results, while the addition of Ni and rGO transformed the MnO₂ morphology into perforated, porous nanoflakes with higher roughness in 3% Ni-doped MnO₂, caused by adsorption. Raman and XPS analyses revealed the presence of mixed Mn valencies (Mn²⁺, Mn³⁺, Mn⁴⁺) and Ni²⁺ valency, as well as surface defects from rGO that enhanced performance. The specific capacitance of pure δ-MnO₂ was 68 F g⁻¹, while the rGO-decorated 3% Ni-doped MnO₂ reached 694 F g⁻¹ at 10 mV s⁻¹. The EIS confirmed that this composite had the lowest resistance, indicating excellent conductivity (

Table 3. The specific capacitance of all as-synthesized nanocomposite samples, reproduced with permission [74].

Active Electrode	Specific Capacitance F g−1 at Scan Rate 10 mV/s	IR Drop (V)	Charge Transfer Resistance (Ω)
PMO	68.33	0.61	1.60
GMO	305.16	0.45	2.24
GNMO 1	216.79	0.55	3.40
GNMO 3	694.81	0.33	3.68
GNMO 5	158.02	0.57	12.86

). These results suggested that rGO-decorated 3% Ni-doped MnO₂ nanoflakes were promising for high-performance supercapacitors, with enhanced capacitance and stability due to surface defects and the porous structure [74].

A team of researchers from Shandong University of Science and Technology of China studied the effect of ferrous ion doping through a co-precipitation synthesis of δ-MnO₂ and Fe/δ-MnO₂-X (X = 0, 1, 2, 5) nanomaterials, as well as their performance in catalytic oxidation of formaldehyde at room temperature. The simple one-step water bath co-precipitation technique consisted of a 6:1 ratio of KMnO₄ and MnSO₄•H₂O with a 1:5 molar ratio of Fe to Mn. Various characterization techniques, such as XRD, SEM, TEM, BET, and XPS, were used to analyze the nanomaterials, revealing that iron doping and sulfuric acid modification increased specific surface area, promoted uniform particle distribution, and altered Mn distribution. The Mn³⁺ content was found to increase, promoting better catalytic activity. Iron doping improved the material’s surface properties, but its low-temperature oxidation performance was limited. Sulfuric acid treatment enhanced the oxidation capabilities, particularly through the formation of abundant active oxygen species. The Fe/δ-MnO₂-5 nanomaterial showed significantly improved performance, achieving a 99.4% conversion of 3.35 ppm formaldehyde within 8 h. Ultimately, the catalytic performance at room temperature was most affected by the surface Mn distribution, with Mn³⁺ being crucial for effective formaldehyde decomposition [75].

A novel co-precipitation technique was employed to synthesize manganese oxide (Mn₃O₄) nanoparticles with a spinel structure. This synthesis approach employed metal nitrates and NaOH under ultrasonic irradiation at low temperatures, aiming to influence key material properties such as phase composition, morphology, and cation arrangement—all crucial for electrochemical performance. Polyethylene glycol (PEG) played a dual role in the process, acting as both a particle size regulator and a stabilizing agent. A structural analysis using XRD and FTIR confirmed the formation of the tetragonal Mn₃O₄ phase. The average crystallite sizes were calculated to be around 36–40 nm. Electrochemical testing, including cyclic voltammetry, chronopotentiometry, and impedance spectroscopy, demonstrated a specific capacitance of 296 F/g and low resistance, indicating strong conductivity. The use of PEG and ultrasonic energy in the synthesis was shown to significantly enhance the electrochemical behavior of the Mn₃O₄ material, making it promising for supercapacitor applications. This specific application of the co-precipitation method has not been reported before compared to the other synthetic routes, increasing the interest around co-precipitation-based synthesis of novel materials [76].

3.4. Sol–Gel Processing

The sol–gel processing method is a versatile technique for the fabrication of metal oxides and ceramics through the transition of a system from a liquid “sol” into a solid “gel.” It contains hydrolysis and polycondensation of metal alkoxides or salts in solution. According to this method, precise control can be obtained over purity, composition, and microstructure at relatively low processing temperatures. It is widely used in the production of thin films, nanomaterials, and coatings.

The sol–gel method was considered suitable for the synthesis of thin films, as stated by S.S. Falahatgar et al., due to its cost-effective procedure, homogeneity of the product, and quality final result. In particular, they used a sol–gel method and a dip-coating process followed by annealing at 300 °C to synthesize MnO₂–ZnO thin films with different Zn/Mn molar ratios (8%, 16%, and 25%) and deposited them on glass and Indium Tin Oxide substrates. The XRD analysis showed that all samples were amorphous, while AFM and SEM images revealed increased roughness and fractal dimension as the Zn percentage increased. Electrochemical tests, including cyclic voltammetry, demonstrated that the film with 25% Zn showed the best performance, with higher anodic and cathodic charge densities at lower scan rates. The electrochemical performance of the films decreased over multiple cycles, and optical band gaps varied with Zn concentration [77].

Another great application of a sol–gel processing is the formation of durable solid–solid interfaces at a MnO₂ electrode/SiO₂–Nafion electrolyte composite, as suggested by Kazushi Shimamoto et al. The procedure was initiated with the addition of MnO₂ and acetylene black into a hydrolyzed tetraethoxysilane with Nafion gel, followed by solidification of the sol. All-solid-state hybrid capacitors were fabricated using the sol–gel composites or a hand-mixed powder mixture of MnO₂, acetylene black, and SiO₂–Nafion as positive electrodes, while the negative electrode was activated carbon, and the solid electrolyte used was phosphosilicate gel. The capacitors with the composite electrodes demonstrated higher capacitance (85 F g⁻¹ at a current density of 1 mA cm⁻²) and better rate performance compared to those made with hand-mixed electrodes (48 F g⁻¹). Additionally, the capacitors using composite electrodes performed well across a wide temperature range, from −30 °C to 60 °C. The researchers consider the sol–gel process preferable to the formation of electrode active material–solid electrolyte interfaces and all-solid-state capacitors with great performance [78].

Scientists from the University of Punjab, Kour et al., decorated the surface of activated carbon with MnO₂ utilizing a simple and cost-effective sol–gel technique at room temperature, to enhance its performance. This technique involved the dispersion of MnO₂ and activated carbon in double-distilled water separately, and afterwards mixing of both at 500 rpm for 3 h. The isolation of the end product involved drying at 60 °C. Activated carbon improved the electric conductivity of MnO₂ and provided a wide surface area for efficient ion interaction, thereby boosting the charge storage capacity of the composite. This composite demonstrated significantly improved capacitance (398.5 F g⁻¹ at current density 1 A g⁻¹) compared to pure MnO₂ (161.8 F g⁻¹) and excellent energy performance of 105.2 Wh kg⁻¹ at 2 kW kg⁻¹. Testing with symmetric supercapacitor cells showed that the composite could power a blue LED for about one minute and a red LED for approximately 12 min, indicating its promising potential as a supercapacitor electrode material [79].

In a different approach, δ-MnO₂ nanoparticles were synthesized using a sol–gel reaction between fumaric acid and KMnO₄ (in alcohol/aqueous solution), followed by modification with HCl and Fe²⁺ to create Fe/MnO₂ materials with distinct electrochemical properties. Researchers opted for a sol–gel procedure due to its simplicity and high performance of the composite. The Fe/MnO₂ sample demonstrated a high specific capacitance of 210 F g⁻¹ at 0.4 A g⁻¹, indicating its suitability for supercapacitor applications. After protonation, the δ-crystalline phase changes to a-type. The incorporation of Fe into the MnO₂ matrix enhanced its surface area at 83 m² g⁻¹, while it enhances the faradic reaction of Na⁺. Notably, the FeMO sample exhibited excellent cycling stability, retaining 100% efficiency after 1000 cycles, highlighting its potential for use in supercapacitors [80].

A valuable research study by Worku and co-workers explored different synthesis methods for differing MnO₂ nanoparticles as oxygen electrocatalysts in zinc–air batteries. Specifically for the sol–gel preparation method for the nanoparticles, they stated that it offered control over the shape and composition of the final product. This approach enabled the creation of a uniform distribution of binary oxides with great properties for MnO₂-based nanoparticles. Key parameters such as concentration, pH, temperature, and reaction time were crucial for the synthesis process. It was a low-pressure, low-temperature process used to develop nanostructured materials from small molecules, making it ideal for metal oxide development. Therefore, the sol–gel technique was effective in controlling the morphology and particle size of MnO₂-based nanoparticles for various applications [81].

3.5. Template-Assisted Methods

Template-assisted methods involve the use of pre-designed templates to direct the formation or deposition of materials into specific structures. These methods offer precise control over shape, size, and spatial arrangement at the micro- and nanoscale. Typical templates include porous membranes, structured surfaces, and self-assembled patterns. An easy and highly effective method suitable for the construction of hollow MnO₂ structures was template synthesis, as stated by Xiao and co-workers. The researchers used SiO₂ microspheres as colloids wrapped with a thin layer of graphene oxide, produced via electrostatic interaction, as a template to produce core–shell SiO₂@δ-MnO₂ microspheres. Afterward, they removed the inner SiO₂ core hydrothermally, resulting in the final mesoporous δ-MnO₂ hollow microspheres (δ-MnO₂ HMS). Using a three-electrode system, the composite showcased a specific capacitance of 216.4 F g⁻¹ (at current density 0.5 A g⁻¹) and a capacitance retention of 91.2% after 3000 cycles at 5 A g⁻¹. The constructed symmetric supercapacitor δ-MnO₂ HMS//δ-MnO₂ HMS also showed excellent specific capacitance (58.8 F g⁻¹), rate capability (86.4%), and energy density (8.2 Wh kg⁻¹) [82].

Another research proposed the fabrication of MnO₂-SiO₂ composite films using a potentiodynamic deposition process, with SiO₂ acting as a template. After the composite films were immersed in a KOH solution for activation, the hard SiO₂ template was removed, resulting in the formation of a porous nanostructured MnO₂ film. This synthetic technique led to the creation of highly porous and active metal/oxide films. In this method, MnO₂ was deposited at an anodic potential, while SiO₂ formed at a cathodic potential. Although the current efficiency decreases during this process, the structure and composition of MnO₂ were enhanced. These porous MnO₂ films demonstrated high electrochemical activity, making them suitable for use as supercapacitor materials [83].

An interesting environmentally friendly approach of template-synthesis used sawdust as a natural template in order to produce MnO₂ nanorods from the KMnO₄ and methanol redox reaction. The economic template material was sourced naturally, while the byproducts of its removal consisted of water, CO, and CO₂, making it a green-synthesis method. The procedure began with the dispersion of sawdust in a KMnO₄ solution with the addition of CH₃OH in drops, producing a MnO₂/sawdust composite. After centrifugation, the dried template was annealed at 500 °C. The fabricated nanorods showcased great stability, specific capacitance (78 F g⁻¹), and retained 93% of their capacitance after 1000 cycles. Therefore, this novelty method was suitable for future applications in supercapacitor devices and is also expected to be applied to other metal oxides (Figure 13) [84].

Another composite material utilizing a template synthesis method was MnO₂@polypyrrole (MnO₂@H-PPy), which is ideal for supercapacitor applications with a mesoporous hollow microsphere structure. This material was synthesized using polystyrene beads as a hard template; the MnO₂ covered the polystyrene surface hydrothermally, and afterward, the template was removed through calcination or dissolution. The material showed remarkable performance in a 1 M Na₂SO₄ solution, achieving a specific capacitance of 295 F g⁻¹ at a current density of 1 A g⁻¹ while it retained 100% of its capacitance after 20,000 charge/discharge cycles at a current density of 10 A g⁻¹. The MnO₂@H-PPy (1:4) composite exhibited the highest performance, demonstrating a specific capacitance of 63 F g⁻¹ in an asymmetric supercapacitor configuration, with an energy density of 42 Wh kg⁻¹ and a power density of 1100 W kg⁻¹. This made it a promising candidate for supercapacitor electrodes (Figure 14) [85].

The comparative analysis of all synthetic methods mentioned (

Table 4. Tabulation of the advantages and disadvantages of different synthesis strategies [86,87,88].

Synthesis Method	Materials	Benefits	Disadvantages
Hydrothermal/Solvothermal	ZnS/MnO2-MOF, ZnS/MnO2, α-Fe2O3/MnO2, MnO2-Cr2O3	- High crystallinity and phase purity; uniform particle size and controlled morphology - Commonly used for nanostructures - Enhanced stability	- Requires high temperature and pressure - Time-consuming - Can produce harmful byproducts
Template Method (Hard/Soft)	δ-MnO2 HMS, porous MnO2 films, MnO2 nanorods, MnO2@H-PPy	- Morphology control via template - High surface area, crystallinity, and stability - Produces hollow/shell structures - Hard templates eco-friendly when mild reactants and reusable	- Soft templates may use toxic surfactants - Template removal can be complex - Environmental concerns with surfactants and solvents - Limited scalability
Electrochemical Deposition	FTO/MnO2-GR, a-LIG/MnO2, PAni/MnO2/graphene oxide, Fe-doped MnO2 MnO2/PCIn	- High precision - Uniform and binder-free coating; simple, green, and low-temperature process	- Limited to conductive substrates - Not suitable for large-scale bulk powder production
Co-precipitation	ε-MnO2/MWCNTs, α-MnO2 nanospheres, Cu2+/Mno2, rGO/δ-MnO2 and Ni/δ-MnO2, Fe/δ-MnO2-X, Mn3O4	- Simple and cost-effective - Good control of composition - Scalable	- Poor morphology control - Requires further treatment (e.g., calcination) - Lower specific surface area than templated or hydrothermal products
Sol–Gel Method	MnO2–ZnO, MnO2/SiO2–Nafion, MnO2/@C, Fe/MnO2	- Homogeneous product with well-defined shape - Low-temperature and low-cost process	- Long gelation and drying times - Possible shrinkage and cracking - Requires organic solvents, which can have environmental impact
Biological Method	nanosized MnO2	- Green and sustainable; uses plant extracts or microbes; low cost and energy input; room temperature synthesis; also useful for wastewater remediation	- Limited control over crystal structure; reproducibility can vary; slower reaction kinetics compared to chemical methods
Microwave-Assisted	rGO/δ-MnO2 and Ni/δ-MnO2	- Rapid and energy-efficient; short reaction times; high product purity and uniformity; often solvent-free	- Requires specialized equipment; scale-up can be challenging; control of particle morphology can be less flexible than template methods

) allows for a deeper understanding of how each approach influences the morphology, crystallinity, and overall performance of the resulting materials. More specifically, hydrothermal/solvothermal techniques are ideal for creating well-defined crystalline structures and are utilized in most supercapacitor devices when there is a need for excellent control over nanostructure and porosity. Electrochemical deposition allows for precise control over composite growth on substrates and can be used for large-scale applications. It is a low-cost method because of the low amount of energy required, and is highly precise due to the control of electrochemical parameters. It mainly applies to conductive substrates and may result in thin layers of the deposited material. On the other hand, co-precipitation is an easily applicable method, suitable for scale-up applications, but does not provide as control over fine structure. Sol–gel methods are employed for the incorporation of nanoparticles into the matrix of the material, while they can form thin films with controlled thickness and uniformity. The sol–gel process requires more reaction time; thus, it is slower and more complex than precipitation. Templating techniques are employed for tailored architectures like core–shell or mesoporous structures, ideal for ion diffusion and structural integrity in long-cycle energy storage systems. One of the main disadvantages is the complex removal procedure of said template and the restriction to only laboratory applications.

4. Applications in Energy Storage

The MnO₂-based composites are widely utilized in energy storage devices due to their environmental friendliness, high theoretical capacitance, and low cost. They are effectively employed as electrode materials in supercapacitors and lithium-ion batteries, providing improved charge storage capabilities and cycle stability. The incorporation of conductive additives or nanostructured matrices further improves their electrochemical performance.

MnO₂ is a key component of energy storage devices through the years of technological development, participating in primal batteries as a versatile component in modern energy storage systems. The Leclanche cell, one of the primal cell batteries constructed in 1866, utilized a mixture of MnO₂ with powdered carbon as a depolizer, aiming to reduce the produced H₂ by the reaction (1):

H₂ + 2MnO₂ → H₂O + Mn₂O₃

(1)

Its key components consisted of Zn as the anode and ammonium chloride as the electrolyte. This revelation stimulated the rise of zinc–carbon battery technology as a power supplier in radios, recorders, remote controls, watches, calculators, and cameras. This type of primary battery rose in popularity in the 20th century, utilizing Zn as an anode and MnO₂ as a cathode material. During the 1960s, alkaline batteries gained popularity when the Union Carbide Corp produced alkaline zinc-manganese dioxide batteries with greater capacities and current capabilities than before [89]. Following this revelation, the rise of rechargeable batteries changed the perspective on energy storage, with the first being a lead–acid battery invented in 1859 by the French physicist Raymond Gaston Planté. The design of Li-Ion and Na-Ion rechargeable batteries was based on this prototype, utilizing 1D MnO₂ and Na_0.44MnO₂ as a cathode material. More specifically, around 1980, a distributor in Taridan (Israel) first mass-produced secondary batteries with Li_0.3MnO₂ as a cathode [8,90,91]. As the technology evolved, the demands of energy suppliers became higher, leading to the development of supercapacitors and hybrid capacitors as a key energy source for devices. Manganese dioxide participates in redox reactions and showcases good capacitive performances in aqueous electrolytes while being environmentally friendly, making it an ideal material for supercapacitors [92]. In modern technology, researchers use nanostructured MnO₂ as a component of complex supercapacitor materials to improve their electrochemical performance and use them in modern high-tech applications [93].

4.1. Supercapacitors

The MnO₂ is extensively used in supercapacitors due to its high theoretical capacitance, natural abundance, and environmental compatibility. Acting as a pseudocapacitive material, it supports fast and reversible redox reactions, facilitating efficient energy storage. Nanostructured variants of MnO₂ offer increased surface area and better ion transport, leading to enhanced capacitance and improved rate capability (Figure 15).

One interesting approach in supercapacitor design via a polymer binding technique, used δ-MnO₂ nanoflowers as electrodes, with an ion-conducting polymer, not conventional binders such as PVDF, improving the performance and ion mobility. The electrode composition, made from acetylene black, polymer (polyethylene oxide), salt (LiClO₄), and δ-MnO₂, was tested with liquid and solid polymer electrolytes. In a liquid electrolyte setup, using 1M of LiClO₄ solution, the specific capacitance reached about 385 F g⁻¹, with energy and power densities of 23 Wh kg⁻¹ and 341 W kg⁻¹, respectively. The devices demonstrated a pseudocapacitive charge storage process and high stability. All-solid-state supercapacitors made from Li⁺ ion membrane showed promising performance with 496 F g⁻¹ capacitance, 19 Wh kg⁻¹ energy, and 367 W kg⁻¹ power after 500 cycles. The study highlighted the importance of the polymer in enhancing performance and revealed that δ-MnO₂-based electrodes were effective for both liquid and solid electrolytes. Additionally, the use of an ion-conducting polymer instead of a binder in the electrodes improved the effective surface area, provided pathways for intercalation/deintercalation, overall enhancing the material’s performance [94].

Another novel advanced electrode material for supercapacitors was h-Boron nitride/β-MnO₂ nanocomposite, reported for the first time by researchers Liyang Lin et al. The composite was synthesized using a one-step hydrothermal method, where gauze-like h-BN nanosheets were doped onto β-MnO₂ nanobelts, creating a 3D structure beneficial to electron transfer. The incorporation of h-Boron nitride effectively inhibited agglomeration of the β-MnO₂ nanobelts and contributed to the improved electrochemical performance of the composite. Electrochemical tests presented that the h-Boron nitride/β-MnO₂ nanocomposite positive electrode showcased a high specific capacitance of 578.4 F g⁻¹ at a current density of 0.5 mA cm⁻². Furthermore, it demonstrated excellent cycling stability, retaining 97.1% of its capacitance after 5000 cycles [95].

Researchers Shuang Xi and co-workers developed a flexible supercapacitor by using a one-step electrochemical deposition method to create a ternary PAni/MnO₂/CC composite electrode. The optimal process parameters were determined by applying different deposition durations and examining the electrochemical performance each time. The results depicted that the most effective deposition time was 300 s, resulting in a specific capacitance of 1694.25 mF cm⁻², outperforming PAni@CC, MnO₂@CC, and other PAni-based electrodes. The assembled asymmetric supercapacitor demonstrated an energy density of 25.58 μWh cm⁻², a power density of 4770.9 μW cm⁻², and retained 64.1% of its capacitance (after 2000 cycles). This simple composite electrode exhibited great performance and was suitable for use in flexible symmetric supercapacitors for energy storage devices [96].

In search of electrochemical capacitor materials, researchers from Deagu University of South Korea synthesized a composite electrode material through the incorporation of hierarchical porous carbon nanofibers (HPCNF) and graphene (G), which assisted in the uniform distribution of MnO₂ particles, preventing agglomeration. The produced MnO₂/HPCNF/G electrode was obtained through a one-step electrospinning and thermal process. The addition of 5 wt% graphene enhanced the specific surface area and electrical conductivity, enabling fast, reversible faradaic processes. The electrochemical tests presented 210 F g⁻¹ (at current density 1 mA cm⁻²) specific capacitance, 170 F g⁻¹ at 20 mA cm⁻² as rate capability, and 24–19 Wh kg⁻¹ as energy density at power densities in a range of 400 to 10,000 Wkg⁻¹. The double-layer capacitance of CNF/graphene acted synergistically with the general electrochemical behavior of MnO₂ to explain these results and composed an ideal material for supercapacitor applications [97].

4.2. Rechargeable Batteries

The MnO₂ is extensively explored in rechargeable batteries due to its high energy density and eco-friendly nature. Serving as a durable cathode material in systems like rechargeable alkaline and zinc-ion batteries, MnO₂ benefits from its multiple oxidation states, enabling effective charge and discharge processes. This research aims to enhance its structural stability to improve battery durability and overall performance (Figure 16).

In lithium-ion batteries, MnO₂ served as a cathode material with high theoretical capacity. ZOU and coworkers constructed a MnO₂/CNT nanocomposite using a soft template approach in deionized water at room temperature with Pluronic PI23 as a surfactant. Structurally, the poor crystalloid a-MnO₂ nanorods were fused together, forming spherical clusters that absorbed onto CNTs. The composite was intended as a cathode material in rechargeable lithium batteries due to its beneficial electrochemical properties, which were evaluated through galvanostatic charge–discharge and EIS (Figure 17). Specifically, the MnO₂/CNT composite showed a capacity of 257.3 mAh g⁻¹, a cyclic stability of 203.0 mAh g⁻¹ after 25 cycles, and a rate performance of 81.5%. Figure 17 depicts representative Nyquist plots of the MnO₂ and MnO₂/CNT electrodes recorded prior to the first discharge. As illustrated, the MnO₂/CNT electrode exhibits significantly lower charge transfer resistance compared to the MnO₂ electrode, indicating that the incorporation of CNTs enhances the electrical conductivity of the MnO₂ electrode. These results, compared to those of plain MnO₂, proved that the composed material had better performance in discharge capacity, cyclic stability, and rate capability, with great applications in rechargeable batteries [98].

Another novelty material based on a-MnO₂ nanoneedle microspheres coated with Pd nanoparticles was proposed as a catalyst in rechargeable Li-air batteries by Zhang and his researchers. The core–shell structure was formed by an electroless uniform deposition of Pd (with a Pd mass fraction of 8.88%) onto the hollow microsphere, composed of the a-MnO₂ nanoneedles. The a-MnO₂/Pd catalyst showcased improved cycling performance of the air electrode and higher energy conversion efficiency, as the initial specific discharge capacity was calculated at 1220 mAh g⁻¹ (at current density 0.1 mA cm⁻²) and the capacity retention rate at 47.3% (after 13 charge-discharge cycles). The conducted charge–discharge tests certify that the composed core–shell Pd-coated-MnO₂ catalyst was a promising material for lithium–air batteries [59].

Another multi-component composite can be constructed by depositing nanoplatelets of MnO₂ on graphene/TiO₂ via a facile water-bath reaction. The MnO₂@graphene/TiO₂ displayed a high specific area of 283.9 m² g⁻¹, providing a large area for electrochemical reactions between lithium ions, enabling more ion diffusion pathways. The experimental results showed that the reversible capacity of the composite remains 243 mAh g⁻¹ after 150 cycles with 83.5% capacity retention. Therefore, the plated MnO₂ graphene/TiO₂ composites demonstrated better cycling performance and rate capabilities because, even in low quantities, MnO₂ improved the reversible capacity of graphene/TiO₂. In conclusion, it was proved that surface decoration with MnO₂ enhanced the performance of anode materials in rechargeable lithium-ion batteries [99].

One additional rechargeable energy storage solution is rechargeable sea water batteries, which charge through oxygen evolution reaction (OER) and discharge through reduction reaction (ORR) on the cathode utilizing the Na⁺ transportation. Researchers Seohae Kim and co-workers investigated the use of MnO₂ as a bifunctional electrocatalyst to accelerate the OER/ORR reaction in energy storage systems. The amorphous structure of MnO₂ nanoparticles prepared via precipitation, enhanced pseudocapacitive behavior. The electrochemical performance of supercapacitor-based cells with MnO₂ showed significant improvements in capacity and cycling stability compared to cells without the catalyst. These results were attributed to the combination of pseudocapacitance and catalytic activity, making MnO₂ a suitable material for future battery applications [100].

After enduring rapid redox reactions between permanganate and glycine, researchers Ediga Umeshbabu and coworkers produced α-MnO₂ nanowires through a cost-effective and green method. The composed nanowires displayed consistent morphology and mesoporous structure with a high specific surface area (~181 m² g⁻¹), favorable to Li⁺ rechargeable battery applications. The reason behind this was that the materials’ structure promoted Li⁺ ion transport and enhanced electrode–electrolyte contact during charge/discharge cycles. The anode material demonstrated a high discharge capacity of 1373 mAh g⁻¹, good rate capability, and impressive cycling stability, retaining 735 mAh g⁻¹ after 100 cycles. Due to the simplicity, cost, and speed of the method, it can be applicable to various oxide nanostructures synthesis for different energy conversion, storage, and additional applications like sensors, catalysis, and microelectronics [101].

In a study by the University of New South Wales in Sydney, a new carbon fiber-reinforced Zn–MnO₂ composite battery is introduced, featuring a MnO₂ cathode and zinc ion anode. The carbon fibers enhanced both the battery’s current conduction and its mechanical strength. Following a simple vacuum consolidation process, the produced composite battery exhibited impressive properties, including a tensile strength of 293 MPa and a specific capacity of 145.9 mAh g⁻¹, while achieving a high energy density of 181.5 Wh kg⁻¹ and retaining 88.3% of its capacity after 100 charge/discharge cycles. Compared to traditional Li⁺ batteries, this new proposed composition presented higher energy density and capacity (50.2% of capacity after 500 cycles). Carbon fiber-reinforced Zn-MnO₂ batteries represent an innovative energy storage model, offering safer use, eco-friendliness, and versatility, and promise to lower the impact of electric-powered transport [102].

Taking advantage of the qualities of MnO₂ as an excellent cathode material, with the addition of coating, the researchers of Dalian Maritime University produced a composite cathode material with improved electrochemical performance. By conducting a basic in situ growth method at room temperature, they deposited uniformly Zeolite Imidazolium Framework-67 (ZIF-67) nanoparticles onto a-MnO₂ nanorods. The produced a-MnO₂@ZIF-67 composite displayed a high specific capacity (313.4 mAh g⁻¹ at 100 mA g⁻¹) and a long-life cycle with a capacity retention of 65% after 1500 cycles at 1000 Ma g⁻¹. These satisfactory results were attributed to the porous structure of the material and its beneficial role in electron/ion diffusion, making it a great candidate for a cathode material, specifically in rechargeable batteries [103].

The development of non-precious electrocatalysts was essential for improving the air electrodes in rechargeable zinc–air batteries. A study from the Bahir Dar University of Ethiopia introduced a co-precipitation method to create Ag-doped α-MnO₂ nanoparticles and examined their performance as cathode materials in zinc–air batteries. Between pure α-MnO₂ and Ag-doped α-MnO₂ nanoparticles, the Ag-doped material showcased better ORR performance, particularly at a doping level of 7.5 mmol, although higher doping (10 mmol) reduced the ORR efficiency. The Ag-doped α-MnO₂ nanoparticles exhibited improved electron transfer and lower charge-transfer resistance, indicating the successful incorporation of Ag into α-MnO₂. A zinc-air battery using Ag–MnO₂-7.5 demonstrated high performance, with excellent open circuit potential, low resistance, and a specific capacity of 795 mAh g⁻¹, corresponding to a high energy density of 875 Wh kg⁻¹ at 1 mA cm⁻² [104].

Researchers of Taizhou Vocational College of Science and Technology of China proposed another composite material aiming to overcome the difficulties of MnO₂ as a cathode material related to Mn²⁺ dissolution and its low conductivity. The synthesis involved electrodepositing ε-MnO₂ onto carbon fibers (ε-MnO₂/CFs), which assisted in reducing the dissolution of ε-MnO₂, enhanced its electrical conductivity, and displayed increased energy density due to the reduction in mass. The ε-MnO₂/CFs cathode demonstrated impressive cycling stability, maintaining a capacity of 256.3 mAh g⁻¹ after 100 cycles at 0.1 C (charge or discharge rate that would fully charge or discharge the battery in 10 h), and excellent rate performance with a capacity of 118.3 mAh g⁻¹ at 3.0 C (charged or discharged at three times its capacity per hour—it would be fully charged or discharged in 1/3 of an hour, or 20 min). This approach offered a strategy to enhance MnO₂-based cathodes for aqueous ZIBs, improving both energy density and performance without the need for additional conductive additives or binders [105].

The wide use of rechargeable ZIBs referenced in this section was attributed mostly to the low cost, safety, and easy supply of zinc, overshadowing the device’s deficiency in electrochemical performance. To overcome this issue, researchers Ning Pang and co-workers synthesized a graphene-oxide-modified MnO₂ composite electrode (MnO₂-GO/GF) using a hydrothermal method, producing an anode material with a 3D structure and enhanced conductivity due to the integration of graphene. Afterward, they embedded the anode into a flexible zinc-based battery with a polyacrylamide quasi-solid electrolyte, which improves ion distribution and mechanical properties of the battery, and promotes Zn dendrite formation. The battery demonstrated exceptional performance, maintaining a 100% retention rate after 2000 charge/discharge cycles and a charge/discharge duration of 13,200 s. It also exhibited high capacity (1250.4 mAh m⁻²) and stability (91.6%) after 5000 cycles. The results highlighted the promising potential of the MnO₂-GO/GF electrode for flexible, wearable devices powered by ZIBs [106].

4.3. Hybrid Devices

A hybrid supercapacitor combines various energy storage mechanisms into one, with a battery-type anode (Li/Na) and a capacitor-type cathode combining the benefits of each one (Figure 18) [107]. Researchers Jinhe Wei and co-workers developed a core–shell NiCo₂S₄/MnO₂ nanocomposite for applications in such hybrid supercapacitors using a facile NaBH₄ reduction process. The use of NiCo₂S₄ nanoneedles created a hierarchical core–shell structure, promoting electron transfer and providing a wide surface for the deposition of MnO₂. Calculations of DFT showed the effect of oxygen vacancies promoting active site reactivity and enabling easier migration of K⁺ onto the MnO₂ surface. The solid-state hybrid supercapacitor NiCO₂S₄/MnO₂-60 with AC electrodes showcased great cyclic stability, maintaining 92.1% after 10,000 cycles and an energy density of 43.16 WH kg⁻¹ when the power density reaches 384.21 W kg⁻¹. This study proposed a novel composite material for use in future applications of hybrid supercapacitors [108].

Another study by Israr Ahmad et al. presented the fabrication of a hybrid device using potassium-doped manganese oxide (K-δMnO₂) nano-spheres embedded in resorcinol formaldehyde aerogel (RF) and reduced graphene oxide (rGO). Therefore, a successful development of a high-performance electrode material, rGO/K-δMnO₂/RF, using a hydrothermal method followed by calcination was accomplished. The material exhibited impressive areal capacitance of 1037.30 mF cm⁻² and gravimetric capacitance of 671.90 F g⁻¹. It was used in an asymmetric hybrid aqueous supercapacitor (rGO/K-δMnO₂/RF//AC), with the fabricated material as the anode and activated carbon as the cathode. The enhanced electrode material featured a large specific surface area (676.314 m² g⁻¹), increased electroactive sites, and improved conductivity due to the combination of reduced graphene oxide and resorcinol formaldehyde aerogel. The supercapacitor device demonstrated remarkable performance with a real cell capacitance of 321.63 mF cm⁻², energy density of 132 Wh kg⁻¹, and power density of 533.33 W kg⁻¹. Additionally, it maintained 96.7% stability over 10,000 charge–discharge cycles, positioning it as a strong candidate for hybrid energy storage applications [15].

One team of researchers managed the synthesis of sandwich-type NiMn₂O₄@N-C@MnO₂ core–shell nanostructures, featuring a unique structure compared to previously reported core–shell materials. The nitrogen-doped carbon (N-C) in the material enhanced conductivity, promoting electron transport and serving as a buffer during charge-discharge cycles, improving the stability of the nanostructures. The addition of nitrogen increased active sites for redox reactions, further boosting performance. The best performing electrode was NiMn₂O₄@N-C@MnO₂-2 due to the unique preparation of NiMn₂O_4, which was bound along with the nickel foam substrate. The NiMn₂O₄@N-C@MnO₂-2//AC battery–supercapacitor hybrid device achieved an energy density of 34.29 Wh kg⁻¹ at a current density of 10 mA/cm², with a power density of 946.75 W kg⁻¹. After 30,000 cycles, the device retained 96.68% of its initial capacitance, outperforming most related studies. These outstanding electrochemical properties highlighted the potential of these nanostructures for energy storage and conversion applications [62].

A team from the Changchun University of Technology in China combined the benefits of both batteries and supercapacitors by constructing a composite anode for the construction aqueous Zn-ion hybrid supercapacitors. The anode consisted of nitrogen-doped MnO₂ (N-MnO₂) nanowalls, composed by in situ deposition, a material promoting ion transport and enhancing zinc ion extraction/insertion. The N-MnO₂ anode was paired with activated carbon cathodes to form a hybrid supercapacitor, achieving a high energy density of 712.5 μWh cm⁻² at a power density of 1000 μW cm⁻² (1 mA cm⁻²), with excellent cycle stability (92.9% retention after 25,000 cycles). The final hybrid product had a multilayer structure with improved flexibility suitable for wearable devices. These results highlighted the potential of N-MnO₂-based zinc-ion hybrid supercapacitor devices for future energy storage applications [109].

In another study by Kamran Khan et al., a two-dimensional hierarchical ZnO@MnO₂ hybrid material was developed as a hybrid supercapacitor component using a co-precipitation and hydrothermal method. The synergistic effect between ZnO and MnO₂ enhanced charge transport and rate performance for advanced supercapacitors. The 2D structure created by MnO₂ and the high conductivity of ZnO nanorods increased the composite’s surface, improving its capacity to 304 F g⁻¹. The hybrid supercapacitor (ZM-4//AC) demonstrated excellent performance, achieving a capacitance of 105.0 F g⁻¹ at a current density 1 A g⁻¹ and 69.0 F g⁻¹ at 10 A g⁻¹ while exhibiting a wide voltage range of 1.6 V in a KOH aqueous solution. The device reached a high energy density of 26 Wh kg⁻¹ at a power density of 1100 W kg⁻¹, with good retention (12 Wh kg⁻¹) at a maximum power density of 4790 W kg⁻¹ at 10 A g⁻¹. Additionally, it maintained 86% of its capacity after 8000 cycles, demonstrating high cycling stability, making ZnO/MnO₂ a suitable material for energy storage and conversion [110].

Table 5. Different energy storage mechanisms, together with their synthesis strategies.

Energy Storage Mechanism	MnO2 Polymorph(s) Used	Composite/Material Type	Synthesis Strategy
Supercapacitors	λ-, γ-, δ-MnO2	MnO2/Graphite/PVDF	Chemical reduction, co-precipitation
Sodium-ion Batteries	α-MnO2	α-MnO2/Activated Carbon	Two-step hydrothermal
Zinc-ion Batteries	β-MnO2, δ-MnO2	β-MnO2 spheres, δ-MnO2 nanostructures, Ag-doped/γ-MnO2,Co-doped/α-MnO2	Hydrothermal, ball-milling, template, co-precipitation
Seawater Batteries	α-, γ-, δ-MnO2	PMGO (PAni/MnO2/GO)	In situ electrochemical
Lithium-ion Batteries	λ-MnO2	Ni-doped λ-MnO2, MnO2/Acnt, Ag-doped MnO2@CC	Hydrothermal, redox deposition
Hybrid devices	Various	MnO2/aCNT, MnO2/Graphene, MnO2/Polymers	Redox reaction, electrodeposition

tabulates different energy storage mechanisms together with their synthesis strategies.

In general, MnO₂ materials as supercapacitor components increase theoretical capacitance values, accelerate reaction kinetics (pseudocapacitance), and have overall a more environmentally friendly impact. Besides improved electrochemical performance, composites consisting of polymer and/or carbon nanostructured substrates showcase enhanced flexibility, making them ideal for portable and wearable devices. In all types of mentioned batteries [lithium-ion batteries (LIBs), sodium-ion batteries (NIBs), zinc-ion batteries (ZIBs), metal–air batteries (MABs)], doping, morphology control, and composite design of MnO₂ materials enhance the overall performance. These materials find applications in specific energy storage technologies where specific performance qualities are requested. In hybrid devices, MnO₂ modification combines battery-level energy density with supercapacitor-level power density (

Table 6. Performance metrics of mentioned composite materials.

Composite/Material	Specific Capacitance (F g−1)	Cycling Stability/Retention	Ref. No.
MnO2–rGO (20 mg/100 mL)	1072	87% after 2000 cycles	[43]
MnO2@Zeolite-Y–POAP	422	97.2% after 10,000 cycles	[27]
MnO2–Graphene (FTO substrate)	192.3	99% after 1000 cycles	[10]
MnO2–CNT/CNF hybrid	374	53.4% rate capability	[11]
CeO2–MnO2/CNF	1498	–	[22]
MnO2@AWC (activated wood carbon)	652	93% after 10,000 cycles	[40]
MnO2@polyaniline nanowire film	342	2000-cycle life	[36]
Fe3O4@MnO2 core–shell	243.7	~100% after 3000 cycles	[38]
hGO–MnO2	376.7	91% after 1000 cycles	[48]
Zn–MnO2@CC	–	83% after 4000 cycles	[41]
K-doped MnO2	405	92.7% after 8000 cycles	[42]
Ni-doped MnO2	432.5	96.6% after 2000 cycles	[44]
Co-doped α-MnO2	1015 (at 10 mV s−1)	–	[45]
Fe-doped MnO2 (post-activation)	500.1	Enhanced via cycling-induced evolution	[67]
MnO2/Graphene/Polyester textile	332	Stable under mechanical strain	[23]
ACC@MnO2@PEDOT	368.05	94.6% after 10,000 cycles	[28]
MnO2–PANI (in situ polymerized)	242	Stable over 1000 cycles	[26]
MnO2–GO–PAni (seawater battery)	–	500–600 Wh/kg equivalent	[66]
MnO2@C (core–shell)	102	89.5% after 600 cycles	[35]
MnO2/PCIn (poly(5-cyanoindole))	322.5	28.3% loss after 1000 cycles	[68]
MnO2–Cr2O3	623	77% after 10,000 cycles	[64]

and

Table 7. Performance metrics of other types of materials used for energy storage.

Material	Specific Capacitance/Energy Density	Cycling Stability/Retention	Ref. No
MXenes	300–500 F g−1	~95–100% over >10,000 cycles	[111]
COFs/MOFs	Up to ~100 Wh kg−1 via hybrid designs	80–85%	[112]
Conductive Polymers	400–600 F g−1	~80–95% over 1000–5000 cycles	[113]
graphene/CNT hybrid	166.8 mAh g−1 at 50 mA g−1	50 mAh g−1 after 300 cycles at 200 mA g−1	[11]
NiCo2O4, RuO2	600–800 F g−1	~90–95% over 5000–10,000 cycles	[112]
Black Phosphorus	501 mA h g−1	80.2%/500 cycles	[114]

).

5. Challenges and Future Directions

The MnO₂-based composites are expected to be key enablers of next-generation energy storage systems. Unlocking their full potential will require interdisciplinary efforts across materials science, chemistry, and engineering. Despite their promise, several challenges remain.

5.1. Cycling Stability

Cycling stability plays a vital role in assessing the long-term effectiveness of MnO₂-based composites in energy storage systems. Pristine MnO₂ tends to exhibit limited structural stability and may undergo dissolution over prolonged charge–discharge cycles, resulting in a decline in capacity. To mitigate these issues, integrating conductive substrates, polymers, or carbon materials can enhance both structural support and electrical conductivity. Moreover, the development of nanostructured designs aids in managing volume fluctuations and promoting efficient ion transport. Together, these approaches markedly enhance the cycling lifespan and reliability of MnO₂ composites in real-world applications. For example, the otherwise poor cycling stability of birnessite-type MnO₂ can be overlooked with the hydrothermal integration of Cu/Bi ions onto the MnO₂ material. The constructed cathode shows improved electrochemical properties with only 0.0042% capacity fading after each cycle at 3 A g⁻¹ [115]. Another novel approach involves the 3D printing of a hybrid electrode based on 3D thermoplastic polyurethane embedded polypyrrole–CuO@MnO₂. The constructed flexible supercapacitors can undergo cyclic loading without dysfunction and retain 98.8% cycling stability after 6000 cycles [116]. It has been reported that the preparation of electrode materials free of additives through the direct growth onto conductive porous substrates (such as nickel foam) enhances electrical conductivity, increases available active sites, and enhances the structural stability of the nanomaterials [117]. Therefore, the scientific field should focus on such next-generation porous substrates (titanium mesh, copper foam, flexible polymer-based conductors), which collectively contribute to prolonged cycling life and improved electrochemical performance.

5.2. Ion Diffusion

Ionic diffusion plays a crucial role in controlling the electrochemical behavior of MnO₂-based composites used in energy storage applications. The inherently low conductivity of MnO₂ can hinder ion mobility, particularly under high current conditions. To address this limitation, composite architectures are often engineered with porous structures and nanoscale features that reduce diffusion distances and expand the active surface area. Incorporating conductive materials like graphene, carbon nanotubes, or metal oxides into the matrix further promotes rapid ion transport. These design strategies collectively enhance rate performance and optimize charge storage efficiency. For example, the ion-beaming technique, rising recently in popularity, introduces materials into the crystal lattice, enabling more ion diffusion pathways. Therefore, the materials created via ion beam irradiation will hold a shorter ion diffusion path length, proposing this method for more applications in the future [118]. Another approach includes controlling the nanosized diffusion pathways utilizing atomic layer deposition. Through this method, the thickness of the developed materials is precisely controlled at the nano, Angstrom, and atomic levels, decreasing the resistance to ionic diffusion [119]. Therefore, integrating emerging techniques like ion beam irradiation and atomic layer deposition to precisely tailor diffusion pathways at the atomic scale could assist ion transferability in the future. These methods offer the potential to create MnO₂-based composites with significantly reduced ion transport resistance, paving the way for faster, more efficient energy storage systems under high-rate conditions.

5.3. Scalability

Scalability is a key factor in enabling the real-world application of MnO₂-based composite materials in commercial energy storage technologies. Although lab-scale methods often demonstrate excellent performance, scaling up to industrial levels presents challenges related to cost, process complexity, and uniformity of the final product. To address this, scalable synthesis approaches like hydrothermal treatment, electrodeposition, and sol–gel methods are under investigation for large-scale production. The use of readily available and cost-effective raw materials further enhances the commercial appeal of these composites. Achieving consistent performance and reproducibility at scale remains a central focus for ongoing research. For example, a synthesis method promising for a wide-scale production of portable and wearable devices utilizes a gas-phase spray drying method to produce 3D microflowers of MnO₂ nanowires. This spray drying strategy is environmentally friendly and ideal for industrial applications [120]. Additionally, another sonochemical technique was developed to produce MnO₂/graphene supercapacitor materials. This route can be expanded from laboratory application to large-scale manufacture for an economical, efficient production of high-performance supercapacitors [121]. Roll-to-roll processing could also be a promising method for the continuous production of flexible MnO₂ electrodes with uniform deposition on large areas and with the potential for high-throughput production of wearable and portable energy storage systems [122].

5.4. Nanostructure Optimization

Optimizing nanostructures is crucial for improving the performance of MnO₂-based composite materials in energy storage applications. By carefully adjusting the shape, size, and morphology of MnO₂ at the nanoscale, its electrochemical properties, such as ion diffusion, surface area, and electrical conductivity, can be significantly improved. Techniques like forming nanosheets, nanowires, or hollow structures improve ion accessibility and accelerate electron transport. Moreover, integrating conductive or polymer nanostructured carbon materials into the composite enhances both conductivity and mechanical stability. These structural enhancements are vital for achieving greater efficiency, faster charge/discharge cycles, and improved cycling stability. For example, utilizing pioneering simulation modelling programs to view the nanostructure of the composed materials and, moreover, predict the evolution of interfacial reactions is important to the development of improved electrode materials. The technology provided by microscopic and spectroscopic devices, combined with knowledge on the reaction kinetics and programmatic tools, can optimize the product’s properties [61]. Combining predictive simulation and advanced characterization techniques, real-time analysis will drive the development of MnO₂ composites with optimized electrochemical performance and long-term stability.

5.5. Multi-Functional Composites

The MnO₂-based multi-functional composites are increasingly recognized for their ability to combine a range of properties, boosting the performance of energy storage devices. When MnO₂ is integrated with materials like carbon-based nanomaterials, conductive polymers, or metal oxides, these composites exhibit synergistic advantages, including enhanced electrical conductivity, structural integrity, and improved electrochemical performance. For example, blending supercapacitive materials with MnO₂ can result in better charge storage and faster energy release. Moreover, these composites can offer additional benefits such as self-healing, superior thermal stability, and increased mechanical strength, enhancing their versatility for a variety of energy storage applications. As such, they hold significant potential for advanced, high-performance energy systems. One great example of such a dual-function material is Co₃O₄/MnO₂ nanocomposite, which can be used as an electrode in metal–air batteries and as a catalyst. Through its catalytic action, it enhances the reaction kinetics of oxygen reduction, a useful reaction for fuel cell and battery technology [123].

5.6. Green Synthesis Methods

Green synthesis methods for MnO₂-based composite materials are gaining increasing attention due to their eco-friendly and sustainable characteristics. These approaches often utilize natural, non-toxic solvents, plant extracts, or biological agents like bacteria or fungi, eliminating the need for harmful chemicals and reducing energy consumption. Green synthesis techniques also enable the production of well-defined nanostructures with enhanced properties, such as increased surface area and improved electrochemical performance. Furthermore, these methods tend to be cost-effective and environmentally conscious, making them suitable for large-scale, sustainable production of energy storage materials. With a focus on cleaner production, green synthesis presents a promising route for developing high-performance, environmentally friendly MnO₂ composites. For example, an interesting, environmentally friendly approach involves a one-pot hydrothermal method for the synthesis of MnO₂/activated carbon composite, with carbon sourced from coconut shells, using K₂CO₃ as an activator. The addition of AC to MnO₂ creates a material with excellent cycling and rate performances due to the material’s synergistic effect, while it also improves the zinc storage performance of MnO₂ [124].

Additional green research by Khani and co-workers utilizes eggshells as a hard template to synthesize δ-MnO₂ nanoplatelet spheres inside the eggshell pore, which is later removed with HCl. The eggshells were ground and screened into different mesh sizes for comparison with MnO₂ made by a traditional method. The MnO₂ derived from eggshells (mesh of 200–400) exhibited a large specific surface area and superior electrochemical performance. This method not only offers a novel approach to synthesizing δ-MnO₂ for LIBs but also contributes to recycling waste materials and improving the performance of other battery materials [125]. In general, green production of MnO₂ has been reported, evaluating biogenic wastes and plant extracts in order to isolate the nanoparticles. Mostly, these green synthesis methods can be modified for large-scale production of oxides from bioactive substances in food waste, agro-wastes, bacteria, fungi, and biopolymers [88,126]. Besides the environmentally friendly isolation of manganese oxides, there are also green synthesis methods using safer reactants in mild conditions that can be employed for the construction of the composite. These include hard template synthesis, which utilizes nontoxic organic solvents and is low in cost. Microwave-assisted methods have an eco-profile attribute to the fast reaction times and energy efficiency, while electrodeposition is another technique that stands out due to its nonpolluting nature and minimal waste. These low-cost characteristics and usage of renewable energy are qualities appealing to industrial production and can be scaled up, using a continuous flow reactor system and freeze-drying methods [87].

6. Conclusions

In conclusion, MnO₂ composite materials have been a staple in energy storage technologies through the years while being potential candidates in the next generation of devices due to their excellent electrochemical performance, environmental friendliness, and economic feasibility. This review highlights that morphological control, polymorph selection, and composite integration are key to enhancing the performance of MnO₂ materials. The synthetic procedure, such as the ultrasonic-assisted co-precipitation method, is what defines particle size, crystallinity, and phase purity, directly affecting the electrochemical performance of MnO₂-based electrodes. Different synthetic methods produce materials with special characteristics, and the choice of the compatible one is up to the researcher and the final application of the material. Advanced synthesis techniques, particularly hydrothermal and redox processes, allow precise control over morphology and composite integration, unlocking enhanced performance in both traditional and next-gen energy storage systems. Co-precipitation and ball milling are simpler methods but still yield competitive performance in supercapacitors and Zn-ion batteries.

The addition of surfactants during synthesis not only assists in achieving homogeneous morphology but also favors the electrochemical performance by preventing particle agglomeration and enhancing surface area. Additionally, through the utilization of synergistic effects with conductive materials such as graphene, carbon nanotubes, and metallic nanostructures, researchers have significantly improved MnO₂ composites’ conductivity, stability, and energy storage performance. These hybrid structures are able to address MnO₂’s inherent limitations, such as poor electrical conductivity and limited cycling stability. Advanced characterization techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) have facilitated the understanding of material structure, oxidation states, and morphology, parameters that are critical for optimizing performance.

Despite these advances, certain challenges, such as the achievement of theoretical capacitance values, prevention of structural degradation during long-term cycling, and encouraging uniform ion diffusion, remain critical challenges. Through this research, it has been noted that Li-ion batteries show lower capacity with λ-MnO₂ but improved power density, making them suitable for fast charging needs. In contrast, hybrid supercapacitors show very high capacity but high complexity for large-scale production. Future work needs to be aimed at maximizing the capacitance capabilities of the materials, utilizing eco-friendly synthesis routes, exploring new dopants and composite structures, and scale-up approaches for industrial-level applications. Another dimension of growing possibility is the integration into flexible and wearable electronics, particularly with the development of lightweight, mechanically flexible MnO₂-based supercapacitors. Early researches show that the combination of MnO₂ with polymeric substrates with increased flexibility showcases ideal properties as a material for smart wearable electronic applications.

Overall, MnO₂ composites represent a versatile and promising platform for energy storage devices in future batteries and supercapacitors that bridge the gap between high performance and sustainability. With ongoing advances in materials design, synthesis, and device integration, MnO₂ composites will be at the center of green and efficient energy storage technology to come.