Current Developments in MXene-Based Energy Storage Systems

Abstract

The demand for high-performance energy storage systems with enhanced energy and power density is growing alongside the renewable energy, mobile devices, and electric vehicle sectors. MXenes, a class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, have emerged as promising electrode materials for next-generation energy storage systems owing to their high electrical conductivity, hydrophilicity, and tunable surface chemistry. This review provides a comprehensive analysis of recent progress in MXene-based energy storage systems, focusing on MXene synthesis routes, their performance in energy storage applications, associated challenges, and future research directions. It discusses the advantages and disadvantages of various MXene synthesis routes and MXene-based composites, defect engineering, and MXene oxidation, which are crucial for energy storage applications, including rechargeable batteries and supercapacitors. The review also explores the challenges and prospects of scaling up MXenes and their composites for energy storage applications and the existing obstacles to integrating these materials into energy storage systems, with the aim of developing next-generation energy storage systems.

1. Introduction

As energy consumption increases, traditional power plants emit more carbon dioxide into the atmosphere, which also leads to the faster depletion of natural energy resources. Limited fuel resources and their increased consumption for generating electric and thermal energy to meet growing demand encourage the implementation of various measures, such as energy saving at the energy transmission and consumption stages, as well as the introduction of generating capacities based on renewable energy converters into the power system. The adoption of renewable energy converters into electrical grids is gaining growing support globally. Renewable energy sources (RES) are crucial for enhancing environmental sustainability. The global adoption of renewable energy converters may reduce CO₂ emissions and align with the UN General Assembly’s Sustainable Development Goals [1].

However, renewable energy converters can introduce fluctuations into the electric power system (EPS), primarily due to variations in weather and environmental conditions, as well as the time of day and season. This results in deviations from nominal generation capacity and causes fluctuations in voltage and frequency, negatively affecting EPS stability and reliability. With the growth of integrated capacities of renewable energy-based power plants in the energy system, traditional reserve capacities may become insufficient to balance them [2]. In such cases, system operators impose restrictions and only grant permission to connect new generating capacities alongside necessary balancing capacities, which incorporate energy storage systems. Several studies and initiatives have been conducted to address this issue and improve the efficiency of renewable energy converters [3,4], including the implementation of MXene-based materials in wind turbines [5]. Studies indicate that for every 10% contribution from wind power plants, an additional 2–4% balancing capacity is required to maintain stable operation [6].

Energy storage systems mitigate these fluctuations by storing excess energy during low-demand or high-generation periods and releasing it during peak demand or low generation from renewable energy converters [7]. This contributes to maintaining the balance between electrical energy production and consumption, as well as reducing deviations in electrical energy parameters to ensure high power quality.

Energy storage systems are generally classified as electrochemical, mechanical, or electrical [8]. The selection of an appropriate storage device depends on performance, cost, and system requirements, since each technology offers unique benefits for different energy management needs. Electrochemical energy storage systems include lithium-ion and sodium-ion batteries, vanadium flow batteries, and hybrid supercapacitors, which have properties of both batteries and capacitors. There are also fuel cells that directly convert the chemical energy of a stored fuel, such as hydrogen, into electrical energy with high efficiency and low emissions. Lithium-ion and sodium-ion batteries operate based on chemical oxidation-reduction reactions. During charging, lithium ions accumulate at the anode as a result of intercalation into the active electrode material, and during discharge, they return to the cathode [9]. Examples of mechanical energy storage include pumped-storage hydroelectric power plants, compressed air energy storage, and flywheels [10]. Electrical energy storage systems include superconducting magnetic energy storage and electric double-layer capacitors [8,11].

Demand for high-performance energy storage systems with enhanced energy and power density has led to research into advanced materials for electrochemical energy storage system components, including electrodes and electrolytes. Various nanomaterials and composites have been explored, developed, and adapted to batteries and supercapacitors. Among these, two-dimensional transition metal carbides and nitrides, known as MXenes (pronounced “maxenes”), with the chemical formula M_n+1X_nT_x, where “M” is an early transition metal, “X” is carbon or nitrogen, and “T_x” denotes the surface terminations (O, OH, F, or Cl), have received great attention across a range of fields (Figure 1) [12].

A literature-search methodology was employed to ensure comprehensive coverage of MXene-based energy storage studies. Relevant publications were retrieved from Web of Science, Scopus, and Google Scholar using a combination of keywords such as “MXene”, “electrochemical energy storage”, “battery”, and “supercapacitor”. This review synthesizes findings from 128 studies selected from an initial pool of 955 records identified across multiple databases. The search was limited to the period 2011–2025 to capture both foundational and recent developments. Studies were included if they reported MXene synthesis methods, electrochemical characterization, and MXene-based composites. Works focused on non-electrochemical applications and studies with insufficient electrochemical characterization were excluded. Figure 2 depicts the temporal distribution of MXene energy storage research, highlighting trends in publication volume over time.

Most existing reviews focus on isolated aspects, such as synthesis techniques, electrochemical performance in specific devices, or fundamental material properties, without integrating these perspectives into a unified framework (

Table 1. Comparative analysis of existing MXene review articles and the contribution of this work.

Review Focus	Typical Scope in Previous Reviews	Limitations of Previous Reviews	Contribution of This Work
MXene synthesis	Detailed description of etching methods (HF, MILD, molten salts)	Focus mainly on synthesis without linking to electrochemical performance	Systematic correlation between synthesis route and electrochemical behavior (batteries and supercapacitors)
Electrochemical performance	Reporting capacitance and capacity values for specific systems	Lack of cross-comparison across studies and systems	Comparative analysis across different energy storage systems with identification of performance trends
Surface chemistry & terminations	Discussion of –O, –OH, –F functional groups	Limited connection to ion transport and stability mechanisms	Integrated analysis of how surface terminations influence conductivity, ion diffusion, and cycling stability
MXene composites	Description of MXene/carbon, MXene/oxide, MXene/polymer systems	Mostly descriptive; limited discussion of trade-offs	Identification of structure–performance trade-offs (capacity vs. stability, conductivity vs. expansion)
Energy storage applications	Separate discussions of batteries or supercapacitors	Lack of unified framework across different devices	Unified perspective linking material design principles across batteries and supercapacitors
Critical analysis	Limited or absent in many reviews	Narrative summaries dominate over analytical comparison	Explicit discussion of discrepancies between studies and their origins (synthesis conditions, testing parameters)
Scalability & challenges	Mentioned briefly	No systematic evaluation	Dedicated section on scalability, oxidation, and standardization challenges
Overall approach	Fragmented (topic-specific reviews)	Lack of integration	Holistic framework: synthesis → structure → properties → performance

). In particular, there is a lack of systematic analysis linking synthesis routes to surface chemistry, defect structures, and interlayer spacing and how these parameters collectively influence ion transport, capacity, and cycling stability [12,13,14]. Furthermore, limited attention has been given to comparative evaluation across different energy storage systems and to the practical challenges associated with scalability and environmental impact. This paper reviews MXene-based energy storage systems, discussing their advantages, limitations, and efficiency, and highlights how MXenes and their composites enhance performance by linking structure, properties, and electrochemical behavior while identifying key research gaps across studies. The research question addressed in this review is: How do MXene synthesis, composite engineering approaches, and oxidation rates influence the electrochemical performance and stability of energy storage systems?

Figure 1. Periodic table showing compositions of MXenes. MXene components are highlight-coded. At the bottom are schematics for four typical MXene structures. Reprinted with permission from [14].

Figure 1. Periodic table showing compositions of MXenes. MXene components are highlight-coded. At the bottom are schematics for four typical MXene structures. Reprinted with permission from [14].

Figure 2. Temporal distribution of MXene energy storage research.

Figure 2. Temporal distribution of MXene energy storage research.

Therefore, this review aims to bridge these gaps by providing an integrated perspective on MXene materials, focusing on synthesis, structure, performance correlations, and composite design strategies. The review is summarized in Figure 3.

2. Synthesis of MXenes for Energy Storage Systems

MXenes are a rapidly expanding class of two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides. A single layer of an M₂X MXene structure consists of an X atom layer sandwiched between two M atom layers, forming an M-X-M arrangement (Figure 4). MXenes have emerged as promising electrode materials for next-generation energy storage systems owing to their high electrical conductivity, hydrophilicity, and tunable surface chemistry [15]. A comparison of MXene synthesis strategies reveals significant trade-offs between structural quality, surface chemistry, and scalability. HF-based etching produces highly conductive MXenes but introduces fluorine terminations and raises safety concerns. In contrast, MILD methods improve structural integrity and reduce defects, although residual fluorine remains an issue. Fluorine-free approaches, such as alkali or molten salt etching, provide environmentally friendly alternatives but often suffer from lower yield or require high processing temperatures.

These synthesis-dependent variations directly affect electrochemical performance. For instance, O-terminated MXenes generally exhibit higher capacitance and improved ion transport compared to F-terminated counterparts, while enlarged interlayer spacing enhances ion diffusion but may reduce volumetric energy density. Furthermore, defect engineering can improve capacity but may compromise long-term stability if not properly controlled.

MXene synthesis relies on two approaches: top-down and bottom-up [16]. The top-down approach involves removing the “A” layer via chemical or electrochemical etching of the parent MAX phase [17], where “MAX” is simplified from the general formula M_n+1AX_n. In this formula, “M” is an early transition metal, “A” denotes interleaving elements generally from groups 13 and 14, and “X” denotes non-metallic atoms (carbon or nitrogen) [18,19]. The bottom-up approach involves growing or depositing precursors onto a substrate [20]. The incorporated synthesis route determines the morphology, surface terminations, defect density, and other properties of MXenes, which in turn influence the electrochemical characteristics of various ion batteries and supercapacitors [21,22]. While top-down methods are dominant, bottom-up synthesis offers atomic-scale precision. This section addresses the various synthesis routes for MXenes, including top-down and bottom-up synthesis approaches, as well as composite formation strategies that are essential for optimizing the electrochemical performance of energy storage systems.

Figure 4. A typical M₂X MXene structure without the terminations. Reprinted from ref. [23].

Figure 4. A typical M₂X MXene structure without the terminations. Reprinted from ref. [23].

2.1. Top-Down Synthesis Approaches

The top-down synthesis approach incorporates well-known and widely adopted etching methods to produce MXenes intended for application in energy storage systems. This approach to MXene synthesis from MAX phase precursors includes the use of highly concentrated hydrofluoric acid (HF or HFA) for the selective chemical etching process of the parent Ti₃AlC₂ MAX phase to prepare Ti₃C₂T_x MXene, as demonstrated by Naguib et al. in 2011 [24], and more than 70% of all MXene research has focused on the Ti₃C₂T_x composition [12]. This type of etchant effectively removes A-layers from the parent MAX phase and preserves multilayered transition metal carbides or nitrides, which also need an additional MXene flake delamination step [25]. During the etching process, bonds with A-layer atoms are replaced by outer-end bonds, creating terminal groups such as –F (fluoride), –OH (hydroxyl), or others [26]. Terminal groups of as-synthesized MXenes depend on the type of etchant, and their composition depends on etchant concentration and other synthesis parameters. The created terminal groups significantly impact the electrical conductivity, hydrophilicity, and specific capacity of the synthesized MXene materials [27]. For example, optimized protocols for the laboratory-scale synthesis of Ti₃C₂T_x MXene have been reported [28]. Key parameters affecting MXene quality include pre-etch treatment of the Ti₃AlC₂ MAX phase precursor, selective etching conditions, and delamination parameters. Pre-washing the MAX phase, which was obtained via the excess-metal route in a 1.25Ti:2.2Al:2TiC ratio, in HCl for 18 h at room temperature effectively removes intermetallic impurities, while selective HF-HCl at 35 °C for 24 h ensures complete Al removal. Immediate delamination after etching, using LiCl at 65 °C for 1 h under an inert atmosphere, yields single- to few-layer flakes with high lateral dimensions and high electrical conductivity, about 19,560 S/cm (Figure 5 and Figure 6).

Other etching methods are also involved, such as fluoride salt, molten salt, alkali, and electrochemical etching. Fluoride salt etching and the minimally intensive layer delamination (MILD) method combine hydrochloric acid (HCl) and fluoride salts like LiF, NaF, or FeF₃ to produce HF in situ in order to etch A layers from the MAX phase [29]. Fluoride ions with A-layer atoms create soluble fluoride complexes, which are dissolved in acid and can be removed during the washing process [30]. Following that, the intercalation of Li+ ions into the MXene layers is promoted simultaneously, which increases interlayer spacing and facilitates delamination [31]. The MILD synthesis method enables MXene flakes with larger lateral dimensions and fewer -F surface terminations and defects to be obtained compared to pure concentrated HF etchant [32]. This method is simple and less hazardous due to using a lower concentration of HF. Furthermore, varying the reaction conditions allows for accurate control over the properties of MXene, broadening its potential applications in energy storage systems [32]. The MILD method is a single-step process that combines etching and delamination, eliminating the need for additional chemicals. However, this method also requires safety precautions during etching due to the presence of hazardous HF. Also, it is difficult to eliminate residual fluoride ions in termination groups after the etching and delamination procedures. For instance, Bera et al. [33] used a combination of density functional theory (DFT), kinetic Monte Carlo (KMC), and ab initio molecular dynamics (AIMD) to study the effect of varying fluorine and oxygen terminations on lithium-ion diffusion and specific capacity in Ti₃C₂O_2(1−x)F_2x MXene. The study results revealed that O-rich mixed-terminated surfaces from various MXene compositions exhibit higher specific capacity than their F-rich counterparts. Fagerli et al. [34] incorporated hydrolysis at 300 °C with a continuous gas flow to remove fluoride terminations from the multilayered V₂CT_x. The cycling results of V₂CT_x electrodes in lithium-ion battery (LiB) half cells demonstrated that the rate capability of MXene with reduced F content improved significantly compared to its F-rich counterparts.

The molten salt etching method is more environmentally friendly because of the absence of HF. This method incorporates high-temperature treatment (typically above 500 °C) of the MAX phase in the presence of Lewis-acid molten salts (LAMS), such as CuCl₂, CuBr₂, CuI, ZnCl₂, CoCl₂, CdCl₂, CdBr₂, NiCl₂, NiBr₂, and AgCl, during the etching process [35]. In this case, the molten salt selectively removes the A layer in the MAX phase, creating intermediates like A-layer atoms with Cl, which can be removed during the washing step [36]. This method enables the production of high-quality MXene flakes with fewer defects and precise control over surface terminations [37].

The alkali etching method is also an environmentally friendly and fluorine-free top-down synthesis approach, which incorporates alkalis during the etching of the MAX phase’s A layer [38]. During the etching process, the A layer is removed, and soluble hydroxide complexes are created. Frequently, MXenes produced using this method have –OH and –O terminal groups depending on etching parameters and no –F because of the absence of fluorides in the etching process, which leads to enhanced pseudocapacitive behavior in supercapacitor applications [21,39]. Nevertheless, this method requires a longer period of time for etching and has lower output. To lower these obstacles, improvements like adding catalysts are made [40,41].

The electrochemical etching method incorporates an electrical potential during the etching process, enabling fine control of selectively removing the A layer from the parent MAX phase, which is submerged in an electrolyte [42]. Changing the electrolyte composition and electrical potential can modify the terminal groups of synthesized MXene and prevent over-etching [23].

In order to improve synthesized MXenes’ properties like surface area, ion transport, and prevention of flake restacking, there are also various other modification procedures, including the intercalation of cations or molecules and freeze-drying [43]. Also, chemical modification of the MAX phase before etching may involve controlled defects in the produced MXene nanosheets, which can enhance its specific capacitance [44]. In previous work, [45] synthesized the defect-controlled Mo_2−□CT_z MXene with various vacancy concentrations from Mo₂Ga₂C MAX phase powder by regulating the etching duration in a mixed solution of HF and HCl. The Mo_2−□CT_z-10 electrode with a vacancy concentration of 2.43 mmol/g demonstrated enhanced electrochemical characteristics. Lui et al. [46] synthesized defect-engineered P-doped Ti₃C₂T_x MXene by using an etching solution of HCl and sodium fluoride in order to remove Al from the Ti₃AlC₂ MAX phase. The obtained Ti₃C₂T_x MXene was mixed with phosphorus powder and annealed for 1 h under an argon atmosphere in a tube furnace. As a result, Ti-P bonds and lattice defects formed on the Ti₃C₂T_x MXene surface, increasing its layer spacing up to 1.477 nm and its electrochemical performance in supercapacitor applications.

Safety and Environmental Considerations in MXene Synthesis

The synthesis of MXenes, particularly via HF-based etching of MAX phases, poses significant chemical hazards and raises environmental concerns. HF is highly corrosive and toxic, requiring strict handling protocols, such as the use of personal protective equipment and the proper neutralization of waste solution processes [47]. Improper disposal can pose serious risks to human health and the environment [48].

Alternative approaches, such as in situ HF generation using LiF/HCl solutions, have been explored to reduce these hazards while maintaining a high MXene yield. Furthermore, post-synthesis treatment and recycling of etchants can mitigate the environmental impact. Incorporating these safety and sustainability measures is essential for scaling up MXene production and promoting its responsible use in lab-scale and industrial applications.

2.2. Bottom-Up Synthesis Approaches

Based on the comparative analysis, several design principles can be proposed for high-performance MXene-based materials: (a) controlling surface terminations to favor oxygen-rich groups, (b) engineering interlayer spacing to facilitate ion transport, (c) introducing conductive spacers to prevent restacking, and (d) developing composite structures to combine high conductivity with high capacity. These strategies provide a pathway toward optimizing MXene performance in next-generation energy storage systems.

The bottom-up synthesis approach involves techniques related to directly growing or depositing precursors onto a substrate and enables the synthesis of ultrathin materials. This approach is more flexible in terms of composition and structural design. It can be produced by chemical vapor deposition (CVD), where transition metals and carbon or nitrogen atoms can be layered onto the substrate surface under controlled conditions by using highly pure, evaporative precursors [49]. Using template-assisted techniques enables precise control of MXene morphology and defect density, which is difficult to achieve using a top-down approach. The work by Xiao et al. [50] reported the successful synthesis of 2D molybdenum nitride (MoN) via a template-based approach. Initially, a molybdenum (Mo) precursor was annealed on NaCl in an argon (Ar) atmosphere at 280 °C to form MoO₃-coated NaCl (MoO₃@NaCl). The MoO₃@NaCl crystals were then ammoniated with NH₃ at 650 °C to produce MoN@NaCl powders. Removal of the NaCl using deionized water resulted in the formation of 2D MoN nanosheets (Figure 7). Furthermore, these techniques allow MXene compositions to be obtained that are difficult to obtain from existing MAX phases [51]. Vertically grown MXene layers on metal surfaces might be peeled off, which provides an opportunity for scaling up [49].

Nevertheless, the complete synthesis of stoichiometric MXenes remains difficult due to the thermodynamic stability of bulk phases. Currently, the bottom-up synthesis approach is largely confined to laboratory-scale studies with low productivity, and it is a slow process [52]. There is a need for continued progress, improvement, and optimization to enhance the scalability and efficiency of MXene synthesis [49,51]. Nevertheless, bottom-up methods offer significant potential for expanding compositional diversity and achieving precise, atomic-scale control.

Table 2. Comparison of top-down and bottom-up MXene synthesis approaches.

Aspect	Top-Down Synthesis Approaches	Bottom-Up Synthesis Approaches
Principle	Selective etching of the A-layer from MAX phases	Direct growing or depositing precursors onto a substrate
Advantages	High yield; well established; scalable	Better control over composition, thickness, and morphology
Limitations	Limited control over surface terminations (–F, –OH, –O); limited compositional diversity	Complex synthesis; lower yield; higher cost
Scalability	Higher (industrial potential demonstrated)	Currently limited (mainly lab-scale)
Safety considerations	Use of hazardous chemicals; requires strict handling protocols	Generally safer, but may involve high temperature

summarizes the comparison between top-down and bottom-up MXene synthesis approaches, including their advantages, limitations, scalability, and safety considerations.

2.3. MXene-Based Composite Synthesis

Despite their excellent intrinsic properties, pristine MXene flakes tend to restack due to van der Waals interactions. This reduces the active surface area and accessibility of ions. Also, oxidation is one of the major limitations affecting their electrochemical capacity and long-term stability. Studies have shown that Ti₃C₂T_x MXene stored in water can undergo significant oxidation within weeks, resulting in the formation of TiO₂ and a substantial decrease in electrical conductivity [53]. To address this challenge, several strategies have been proposed to improve MXene stability in energy storage systems. Forming a composite with polymers, carbon materials, and organic molecules can effectively protect MXene sheets from oxidation. In a recent study [54], incorporating tannic acid into Ti₃C₂T_x MXene films improved oxidation resistance while achieving a capacitance of 848 F/g at 1 A/g and 100% capacitance retention after 20,000 cycles, demonstrating electrochemical stability. Therefore, MXene-based composite synthesis is critical for advancing the potential of MXenes in energy storage systems.

MXene-based composites introduce a critical pathway to overcome intrinsic limitations such as restacking and limited capacity. Carbon-based materials improve electrical conductivity and structural stability, while silicon and metal oxides enhance capacity through conversion or alloying mechanisms. However, these improvements often involve trade-offs, such as volume expansion in silicon-based systems or reduced conductivity in oxide-based composites. Therefore, achieving optimal performance requires careful balancing of these competing factors.

2.3.1. MXene and Carbon Composites

Integrating MXenes with graphene, carbon nanotubes (CNTs), and activated carbon improves their electrochemical properties in the resulting composite. Carbon spacers between MXene layers enhance their surface area, ion accessibility, and specific capacitance by anchoring the MXene sheets and preventing them from restacking [55]. This leads to an increase in the performance and stability of energy storage systems. In recent work [56], sandwich-like MXene/CNT papers were fabricated by filtering 1 mL of multilayer Ti₃C₂T_x MXene dispersion, prepared by etching the MAX phase with 50 wt% aqueous HF, through a polypropylene membrane. Then, 1 mL multiwalled carbon nanotube (MWCNT) dispersion was filtered on the top of the filtered MXene layer. This alternate filtration was repeated several times. The obtained MXene/CNT composite demonstrated a high capacitance in supercapacitor applications compared to pure MXene. Obtaining the Ti₃C₂T_x MXene/carbon nanofiber composite is another route to enhancing the electrochemical properties of the active electrode mass [57]. MXene flakes and polyacrylonitrile (PAN) were combined at a weight ratio of 2:1 in an electrospinning solution to create fiber mats containing 35 wt% MXene. The fiber networks were then carbonized, yielding a Ti₃C₂T_x MXene/carbon nanofiber composite with enhanced properties for supercapacitor application. In a previous work [58], carbon fibers (CF) were coated with Ti₃C₂T_x MXene by functionalizing the CF surface with aryl diazonium salts and a poly (o-phenylenediamine). Prepared for supercapacitor applications, the MXene-coated CF composite exhibited an increased specific capacitance and interfacial adhesion in epoxy-based composites.

2.3.2. MXene and Silica Composites

MXene–silica composite materials have demonstrated promising enhancements in the electrochemical performance of energy storage applications. In order to overcome restacking issues and increase the specific surface area of MXene, it is important to introduce other nanomaterials, such as silica, between its layers. Maughan et al. [59] synthesized a SiO₂-pillared Ti₃C₂ composite material using amines as copillars with tetraethylortho silicate (TEOS) as the silicon (Si) source. They achieved an interlayer spacing range of 0.75–3.2 nm. This amine-assisted pillaring methodology enables the successful intercalation of silica-based pillars between MXene layers, increasing the specific surface area to 235 m²/g. In a recent work [60], an MXene/silica composite was synthesized for supercapacitor applications. The nanoporous silica was introduced on the MXene flakes in order to prevent its restacking and increase the surface area, specific capacitance, and stability of the resulting composite. Ti₃C₂T_x MXene was obtained by the etching of Ti₃AlC₂ MAX phase powder in 24% HF solution. Then, TEOS solution (TEOS and ethanol) was poured into the MXene solution, placed in an autoclave, and heated at 180 °C to obtain the MXene/silica composite with various amounts of TEOS via the hydrothermal method. The resulting composite demonstrated an increased surface area compared to the Ti₃C₂T_x MXene, 20.73 m²/g and 8 m²/g, respectively. Duan et al. [61] prepared a carbon cloth (CC) MXene@SiO₂ epoxy resin monomer (EP) composite (CC/MXene@SiO₂-EP) in order to enhance the cycling stability, interfacial properties, and electrochemical performance of the MXene-based flexible electrodes in wearable technology. MXene@SiO₂ nanostructures were obtained via adding a tetraethyl silicate-containing alcohol solution to the multilayered Ti₃C₂T_x MXene in ethanol solution, which was titrated with ammonia water. Multilayered Ti₃C₂T_x MXene was obtained by the etching of the Ti₃AlC₂ MAX phase in a LiF and HCl solution. Epoxy monomers were added to the MXene@SiO₂ nanostructure acetone solution. Then, the CC was immersed in the mixed solution and heated for in situ polymerization. The prepared CC/MXene@SiO₂-EP composite improved the mechanical properties and stability of the asymmetric supercapacitor during the cycling process.

2.3.3. MXene and Metal Oxide Composites

MXene–metal oxide composites combine the high conductivity and mechanical strength of MXenes with the high theoretical capacity of metal oxides such as TiO₂, SnO₂, and MoS₂ [62]. The MXene matrix buffers volume expansion and facilitates electron transport in the resulting composite material, achieving enhanced electrochemical stability [63]. Wu et al. [64] compounded Ti₃C₂T_x MXene with Co₃O₄ nanowires to enhance the electrochemical performance of the resulting composite. Ti₃C₂T_x MXene was obtained by etching the Ti₃AlC₂ MAX phase in a LiF/HCl solution at 35 °C for 24 h. Then, Co₃O₄ was synthesized on nickel foam through the hydrothermal method and immersed in the reaction vessel with MXene for the hydrothermal reaction in order to obtain MXene/Co₃O₄. The obtained composite material incorporates the synergetic effect of Ti₃C₂T_x MXene and Co₃O₄ by enhancing its surface area, specific capacitance, and cycling stability. Zhu et al. [65] obtained a MnO₂-MXene composite in which MnO₂ was grown onto Ti₃C₂T_x MXene sheets. The Ti₃C₂T_x MXene was synthesized using the MILD etching method in a LiF/HCl solution. Then, to the mixture of MXene and MnCl_2, KMnO₄ was slowly added, and the resulting suspension was kept at a temperature of 120 °C for 6 h. After drying, an MnO₂eMXene composite containing a mixture of two different MnO₂ polymorphs in the form of nanosheets and fibers was prepared. Its interconnected structure boosts the electrical conductivity of MnO₂ and suppresses the restacking of MXene flakes. A recent study [66] presented a synthesis approach for nickel-intercalated MXene that improves its electrochemical performance. The Ni-intercalated Ti₃C₂T_x MXene achieved an energy density of 26 Wh/kg at a power density of 1872 W/kg. In a previous work [67], a hierarchical 1D/2D TiO₂/Ti₃C₂ MXene heterostructure electrode material was synthesized by an alkalized treatment for the in situ synthesis of TiO₂ nanowires on the surface of Ti₃C₂ MXene flakes. As a result, a composite material with enhanced ion diffusion and specific capacitance was developed. Monolayer Ti₃C₂ MXene was added to a KOH solution at 65 °C, stirred, washed, and freeze-dried to obtain TiO₂/Ti₃C₂ (Figure 8).

2.3.4. MXene and Polymer Composites

MXene–polymer composites are one approach to overcoming the restacking issues of MXene by adding polymer binders/spacers between its layers [68]. MXene–polymer composites have enhanced mechanical, electrical, and thermal properties compared to pristine polymers [69]. Luo et al. [70] prepared a flexible Ti₃C₂T_x/polyaniline (MXene/PANI) supercapacitor electrode composite material. Ti₃C₂T_x MXene was obtained via a MILD etching method using a LiF/HCl solution poured into a beaker with PANI nanofibers, which were prepared by dispersing the purified aniline monomer in HCl solution, followed by injecting an ammonium persulfate (APS)-containing HCl solution to initiate the polymerization reaction. The introduction of PANI increased the electrochemical performance and ion diffusion properties of the composite. Varghese et al. [71] intercalated PANI into Ti₃C₂T_x MXene to enhance its interlayer spacing and ion contact. Ti₃C₂T_x was dispersed in HCl, followed by adding aniline monomer and APS. The surface area of the MXene–PANI composite achieved 12.2 m²/g.

2.3.5. MXene and Hydrogel Composites

MXene–hydrogel composites have recently emerged as promising materials for electrochemical energy storage systems due to their three-dimensional structure, high ionic conductivity, and mechanical flexibility [72,73]. The hydrogel network can effectively prevent MXene restacking and enhance ion transport during electrochemical cycling. Various fabrication approaches, including in situ polymerization, physical crosslinking, and self-assembly, have been reported to construct MXene-based hydrogels [74]. These composites have shown improved electrochemical performance and cycling stability in supercapacitors and other energy storage systems. The work by Liu et al. [75] reported a method to prepare MXene-doped ionic conductive hydrogel electrolyte (MSAH) with a wide temperature tolerance from −25 to 50 °C. The MSAH demonstrated a wide working voltage from 0 to 2.2 V and an energy density of 141.06 Wh/kg at a power density of 1999.5 W/kg in a symmetric supercapacitor with carbon electrodes. In a previous work [76], MXene-enhanced polyvinyl alcohol/polyaniline (AMPH) hydrogel electrodes fabricated by ice templating-induced polymerization showed excellent mechanical elasticity and capacitance retention in flexible supercapacitors. The AMPH achieved 88.02 mF/cm² at a density of 2 mA/cm² in a wide temperature tolerance from −30 to 80 °C.

3. MXenes and Their Composites in Energy Storage Systems

Electrical energy storage systems, such as batteries and supercapacitors with various electrode materials, are being researched and developed through the discovery of new nanomaterials and the modification of existing materials. Such materials include two-dimensional transition metal carbides and nitrides. MXenes, since their discovery, have found applications in various fields of science because of their better electrical conductivity and tailored surface chemistry. This opens up the possibility of also using MXenes in electrical energy storage devices. Around 40 distinct MXenes have been successfully obtained [77]; however, Ti₃C₂T_x has a broader set of applications. Considerable attention has also been given to the use of 2D MXene materials for electrical energy storage systems [78,79,80], which have the potential to increase the performance of electrochemical energy storage devices [81,82]. MXenes’ multilayer structure, having a thickness of only a few atoms, results in excellent conductivity and a large cross-sectional area [83], making them appropriate for application in supercapacitors and batteries [84,85]. MXenes and related composites are of significant interest because they have a graphene-like shape and rapid ion transport capabilities, and they have the potential to be exploited in the development of sustainable, energy-efficient, and economically viable energy storage systems.

In this regard, MXenes are a type of 2D materials that combine metallic conductivity, a hydrophilic surface, and a high density of functional groups (–OH, –O, –F), which provides excellent compatibility with electrolytes and the possibility of functional modification [86]. This enables the incorporation of MXenes as active electrode materials in energy storage systems like batteries and supercapacitors to improve electrical conductivity, ion transport, and stability.

Ti₃C₂T_x is the first synthesized MXene material, with a high electrical conductivity of about 104 S/cm [87] and high mechanical strength. Other MXenes are also of considerable interest, namely Nb₂C, V₂C, Mo₂C, and Ti₂C [88,89,90]. Changing the transition metal type, number of layers, and outer termination groups allows electrical conductivity, ion transport mobility, and other properties to vary [91]. High electrical conductivity, mechanical strength and flexibility [92], rapid ion intercalation and transport ability [93], and modification ability by inserting various materials [94] of MXenes make them promising materials in electrochemical energy storage systems.

Rechargeable batteries, specifically lithium-ion batteries, utilize reversible electrochemical processes between the anode and cathode to offer high energy density. These batteries are often found in mobile devices and electric cars [95,96]. In contrast, supercapacitors store energy by forming an electrical double layer at the electrode–electrolyte interface or via pseudocapacitance processes. Supercapacitors can deliver high power with long cycle stability, making them perfect for applications demanding immediate energy release and great cycle stability [97,98].

This advantage allows supercapacitors to be used in regenerative braking systems and to damp rapidly changing high-frequency fluctuations in power systems where a significant proportion of generation comes from renewable energy converters. However, supercapacitors have a lower specific capacity than batteries. Batteries generally have a lower power density and a lower number of charge/discharge cycles than supercapacitors. To overcome these limitations, new materials and their composites, such as MXenes, are being researched and developed for use as active electrode materials and separators.

Studies have been conducted to understand fundamental energy storage mechanisms, including new methods for synthesizing MXene and studying its electrochemical properties [99]. Particular attention is currently being paid to creating electrode materials with excellent electrical conductivity, specific capacitance, and structural stability, since these parameters determine the efficiency and durability of electrochemical energy storage systems. Creating such materials based on nanostructured carbon, silicon, and MXene composites offers new possibilities for improving the performance of batteries and supercapacitors, as well as for implementing hybrid systems that combine the benefits of both systems [100].

3.1. MXenes in Battery Systems

Due to their metallic conductivity, layered two-dimensional structure, and tunable surface terminations, MXenes have attracted increasing attention as anode materials for lithium-ion batteries (LIBs). Pristine Ti₃C₂T_x MXene, synthesized via HF etching of the Ti₃AlC₂ MAX phases and intercalation with DMSO to delaminate the stacked Ti₃C₂(OH)_xO_yF_z layers, demonstrated reversible capacities of 410 mAh/g at a 1 C cycling rate and 110 mAh/g at 36 C [101]. Another pristine Ti₃C₂Tx MXene, synthesized via LiF/HCl etching of the Ti₃AlC₂ MAX phases, delivered moderate reversible capacities of 64–105 mAh/g at 10 mA/g [102]. Exposure of Ti₃C₂Tx MXene to water and air increased the capacity from 21 mAh/g to 106 mAh/g; however, longer exposure resulted in partial capacity fading during cycling. Restacking is likely to limit the oxidation of the internal nanosheets, leaving significant electrochemically inactive sites. Lithium storage in these systems originates from a combination of Li⁺ intercalation and surface redox reactions associated with –O, –OH, and –F terminations.

There is also another strategy based on defect engineering. By changing the etching duration, MXenes exhibit controlled defect densities. This enables a specific capacity of about 160 mAh/g at 1C [103]. Ball milling of the precursor of MXenes might produce defect-rich nanosheets, which enhance their specific capacity of 330 mAh/g at 100 mA/g up to 1000 cycles [104]. Applying a fluoride-free method of synthesis, MXenes, like alkali etching with a predominance of –O and –OH terminal groups and enlarged interlayer spacing, show 106.6 mAh/g after 250 cycles at 0.5 A/g [105].

The capacity of pristine MXenes may vary depending on the synthesis protocol, oxidation state, extent of restacking, and experimental conditions. However, the theoretical specific capacity of graphite anodes is 372 mAh/g, and in practice, it usually reaches around 300 mAh/g [106]. In contrast, pristine MXenes exhibit a similar or lower specific capacity; therefore, there is a need for MXene-based composite materials. Table 3 provides electrochemical performance data of pristine MXene-based battery systems.

MXene-based composite materials introduce synergetic electrical energy storage mechanisms. For instance, SnS/MXene pillared composites combine MXene’s high conductivity and mechanical strength, while the MXene matrix buffers volume expansion and maintains electrode integrity, delivering capacities as high as 866 mAh/g at a 500 mA/g current rate [107]. Sun et al. [108] combined Ti₃C₂Tx MXene with TiNb₂O₇ quantum dots and N-doped mesoporous carbon (NMC), where quantum dots were embedded into NMC, and obtained a novel TiNb₂O₇-NMC/MXene composite that achieved a reversible capacity of 363.4 mAh/g at 1 A/g after 500 cycles. Yoon et al. [109] integrated Au nanoparticles onto the Ti₃C₂Tx MXene surface with highly predominated –F terminal groups, which extended the cycle life to 600 cycles in a lithium metal battery by stabilizing lithium deposition and the SEI layer. In other work, 2D Ti₃C₂Tx MXene was synthesized using an LiF/HCl etchant mixture blended with commercial Li₄Ti₅O₁₂ and 0D Super-P particles to prepare the lithium titanate (LTO) anode [110]. Herein, the MXene-integrated LTO anode exhibited a capacity of 127.1 mAh/g at 20 C with a capacity retention of 93.4% over 10,000 cycles. Kim et al. [111] synthesized single-layer Ti₃C₂T_x MXene using an eco-friendly molten salt method and integrated MXene with nano-silicon followed by application of polydopamine coating and carbonization to obtain the resulting Si-MXene/PDA composite. The Si-MXene/PDA anode demonstrated a retention rate of 7.98% after 250 cycles, achieving a final capacity of ~300–400 mAh/g. Maughan et al. [59] synthesized a SiO₂-pillared Ti₃C₂ composite material using amines as copillars with tetraethylortho silicate (TEOS) as the silicon (Si) source. The obtained composite material was tested for Na-ion battery applications and showed a 131 mAh/g capacity by the 100th cycle at 20 mA/g, retaining 98.5% capacity between the 50th and 100th cycles. In a previous work [112], few-layer Ti₃C₂T_x-T(O) anode materials with rich –O functional groups on the surface were obtained by HF etching and low-temperature oxidation. The prepared Ti₃C₂T_x-T(O) anode achieved a higher reversible specific capacity of 174.4 mAh/g at a current density of 5 A/g compared to its F-rich counterparts.

Overall, these results demonstrate that defect engineering, interlayer and surface terminations control, and composite design are key factors governing the electrochemical performance of MXene-based battery anodes. Table 4 presents the electrochemical performance data of MXene-based composite materials used in battery systems.

Table 3. Electrochemical performance of pristine MXene-based battery systems.

Material System	Synthesis Route	Structural Characteristics	Performance	Ref.
Pristine Ti3C2Tx MXene (LIB anode)	Selective etching of Ti3AlC2 using HF	Layered 2D flakes prone to partial restacking	110 mAh/g at 36 C; 410 mAh/g at 1 C	[101]
Pristine Ti3C2Tx MXene (LIB anode)	Selective etching of Ti3AlC2 using LiF/HCl	Layered 2D flakes prone to partial restacking	64–105 mAh/g at 10 mA/g	[102]
Alkali-etched Ti3C2Tx (LIB anode)	Fluorine-free alkali etching	Enlarged interlayer spacing	~106.6 mAh/g after 250 cycles (0.5 A g−1)	[105]
Partially etched MXene (LIB anode)	Short-duration HF etching	Controlled defects	~160 mAh/g (1 C)	[103]
NanoMAX-derived MXene (LIB anode)	Ball milling MAX precursor → etching	Defect-rich nanosheets	~330 mAh/g (100 mA g−1), stable up to 1000 cycles	[104]
V2CTx (LIB anode)	HF etching of V2AlC	Layered 2D flakes	260 mAh/g at 1 C	[113]

Table 3. Electrochemical performance of pristine MXene-based battery systems.

Material System	Synthesis Route	Structural Characteristics	Performance	Ref.
Pristine Ti3C2Tx MXene (LIB anode)	Selective etching of Ti3AlC2 using HF	Layered 2D flakes prone to partial restacking	110 mAh/g at 36 C; 410 mAh/g at 1 C	[101]
Pristine Ti3C2Tx MXene (LIB anode)	Selective etching of Ti3AlC2 using LiF/HCl	Layered 2D flakes prone to partial restacking	64–105 mAh/g at 10 mA/g	[102]
Alkali-etched Ti3C2Tx (LIB anode)	Fluorine-free alkali etching	Enlarged interlayer spacing	~106.6 mAh/g after 250 cycles (0.5 A g−1)	[105]
Partially etched MXene (LIB anode)	Short-duration HF etching	Controlled defects	~160 mAh/g (1 C)	[103]
NanoMAX-derived MXene (LIB anode)	Ball milling MAX precursor → etching	Defect-rich nanosheets	~330 mAh/g (100 mA g−1), stable up to 1000 cycles	[104]
V2CTx (LIB anode)	HF etching of V2AlC	Layered 2D flakes	260 mAh/g at 1 C	[113]

Table 4. Reported electrochemical performance of MXene-based composite materials in battery systems.

Material System	Synthesis Route	Structural Characteristics	Performance	Ref.
SnS/MXene composite (LIB anode)	In situ oxide growth on MXene	0D–2D heterostructure	866 mAh/g (500 mA/g)	[107]
p-Ti3C2Tx/CNT (Li-ion half-cell working electrode)	Etching of Ti3C2Tx → p-Ti3C2Tx → introduce CNTs	Porous MXene and introduced CNTs structure	1250 mAh/g at 0.1 C; 330 mAh/g at 10 C	[114]
MoS2/Mo2TiC2Tx-500 (Li-ion half-cell working electrode)	Liquid mixing → heating	2D heterostructure	554 mAh/g at 100 mA/g	[115]

Table 4. Reported electrochemical performance of MXene-based composite materials in battery systems.

Material System	Synthesis Route	Structural Characteristics	Performance	Ref.
SnS/MXene composite (LIB anode)	In situ oxide growth on MXene	0D–2D heterostructure	866 mAh/g (500 mA/g)	[107]
p-Ti3C2Tx/CNT (Li-ion half-cell working electrode)	Etching of Ti3C2Tx → p-Ti3C2Tx → introduce CNTs	Porous MXene and introduced CNTs structure	1250 mAh/g at 0.1 C; 330 mAh/g at 10 C	[114]
MoS2/Mo2TiC2Tx-500 (Li-ion half-cell working electrode)	Liquid mixing → heating	2D heterostructure	554 mAh/g at 100 mA/g	[115]

3.2. MXenes in Supercapacitors

In contrast to battery applications, MXenes exhibit exceptional performance in supercapacitors, where charge storage is dominated by capacitive and pseudocapacitive processes rather than solid-state intercalation. Early studies of Ti₃C₂T_x MXene films revealed gravimetric capacitances of 200–350 F/g and volumetric capacitances of 700–1000 F/cm³, which are attributed to their metallic conductivity and densely packed layered structure [101,116,117]. The dominant storage mechanism involves fast surface redox reactions coupled with electric double-layer capacitance. Microwave-assisted hydrofluoric acid etching has recently been utilized for the synthesis of the Ti- and V-based carbide MXene, providing a rapid and efficient route compared to conventional etching methods [118]. The obtained Ti₃V₂C₃T_x MXene exhibits a well-defined layered structure with improved conductivity and an accessible surface area. Electrochemical evaluation shows that these materials deliver a high specific capacitance of 465 F/g at a scan rate of 1 mV/s and excellent cycling stability with 94% capacitance retention after 10,000 cycles, highlighting their strong potential for advanced energy storage applications. Additionally, ~900 F/cm³ volumetric capacitance with high cyclability was performed using MXene obtained through a top-down approach by using LiF and HCl [117]. Additive-free Ti₃C₂T_x ‘clay’ film electrodes were prepared via a roller mill at various film thicknesses (5, 30, and 75 µm) and tested within a relatively narrow voltage range in three-electrode Swagelok cells. MXene served as the testing electrode, over-capacitive activated carbon as the counter electrode, Ag/AgCl in 1 M KCl as the reference electrode, and Celgard membranes as the separator. The electrolyte was M H₂SO₄. Increasing the MXene film thickness and the scan rate from 2 mV/s to 100 mV/s resulted in a decrease in both the volumetric and gravimetric capacitance. For instance, the volumetric capacitances of the 5, 30, and 75 µm electrodes were 910, 534, and 355 F/cm³, respectively, while the corresponding gravimetric capacitances were 246, 182, and 161 F/g at a scan rate of 2 mV/s, highlighting the influence of electrode thickness and loading level on the electrochemical performance.

Composite electrodes provide an additional pathway to enhance gravimetric and volumetric performance while enabling practical capacitance and long-term cycling stability suitable for scalable devices. In previous work, [56] obtained sandwich-like MXene/CNT papers. The obtained MXene/CNT composite demonstrated a high capacitance in supercapacitor application compared to pure MXene and randomly mixed MXene/CNT. The sandwich-like MXene/SWCNT papers achieved a volumetric capacitance of 390 F/cm³ at a scan rate of 2 mV/s, and the sandwich-like MXene/CNT paper reached 350 F/cm³ at 5 A/g with no degradation after 10,000 cycles. Liang et al. [119] prepared a Ti₃C₂T_x–MWCNT composite as the negative electrode active mass in an asymmetric supercapacitor, where polypyrrole-coated MWCNT was used in the positive electrode. Cyclic voltammetry demonstrated that the Ti₃C₂T_x–MWCNT composite’s areal capacitance is 0.94 F/cm² in Na₂SO₄ electrolyte at 35 mg/cm² mass loading for the asymmetric supercapacitor in the 0–1.6 V window. Levit et al. prepared Ti₃C₂T_x MXene/carbon nanofiber composite supercapacitor electrodes using MXene flakes and PAN in electrospinning solution [57]. The obtained fiber mats were used as electrodes without binders and additives. The prepared Ti₃C₂T_x MXene/carbon nanofiber composite achieved an areal capacitance of up to 205 mF/cm² at 50 mV/s. Dharmasiri et al. [58] obtained a MXene-coated carbon fiber (CF) composite for supercapacitor application. The prepared MXene-coated CF composite achieved a specific capacitance of 157 F/g at 5 mV/s and 908 mF/g at 0.5 mA/g.

Das et al. [60] synthesized an MXene/silica composite designed for supercapacitor applications. Nanoporous silica was introduced onto MXene flakes to prevent restacking and increase the surface area, specific capacitance, and stability of the resulting composite. The nanoporous silica enhanced the specific capacitance of the MXene, which demonstrated 718 F/g at 1 A/g with a rate capability of 80%. In a recent work, [61] obtained a CC/MXene@SiO₂-EP composite in order to enhance cycling stability, interfacial properties, and electrochemical performance of the asymmetric supercapacitor. The obtained CC/MXene@SiO₂-EP composite demonstrated a specific capacitance of 481.4 F/g in the three-electrode system, with 94.07% capacitance retention after 8000 cycles. Also, the composite was prepared for use in an asymmetric supercapacitor with an activated carbon anode electrode (AC//CC/MXene@SiO₂-EP), which achieved an energy density of 39.74 Wh/kg at a power density of 400 W/kg. The capacitance retention was 96.7% after 5000 cycles at 900 reciprocal bending. The high cycling stability of the CC/MXene@SiO₂-EP electrode was provided by the synergetic effect of each material in the composite. During the charging and discharging processes, the MXene flakes prevent the agglomeration of the silica, while the introduced silica nanoparticles prevent the MXene flakes from restacking. Additionally, traces of epoxy resin improve structural stability. In a recent work, [64] prepared an enhanced MXene/Co₃O₄ composite material with a greater surface area and specific capacitance, as well as improved cycling stability. This was achieved by preventing the restacking of MXene flakes through the use of intercalated Co₃O₄ nanoparticles. The MXene/Co₃O₄ composite electrode demonstrated an area-specific capacitance of 6.456 F/cm² at 3 mA/cm². The obtained MXene/Co₃O₄ composite was also tested in an asymmetric supercapacitor (MXene/Co₃O₄//AC) using PVA/KOH as the electrolyte, demonstrating an energy density of 0.37 mWh/cm² at a power density of 6.41 mW/cm², and an energy density of 0.18 mWh/cm² at a power density of 24.92 mW/cm² with a capacitance retention of 82.3% after 5000 cycles at a current density of 50 mA/cm², demonstrating that the incorporated strategy directly contributes to long-term electrochemical stability. Microwave-assisted synthesis was applied to obtain Co₃O₄–MXene nanocomposites, enabling rapid formation of well-dispersed metal oxide nanoparticles on MXene sheets. The resulting composite exhibited enhanced electrochemical performance, delivering a high specific capacitance of 868 F/g at a current density of 1 A/g in a 2 M KOH electrolyte using a three-electrode setup. Also, the obtained Co₃O₄–Ti₃C₂T_x composite was utilized for the positive electrode in a solid-state asymmetric supercapacitor and exhibited an energy density of 10.41 Wh/kg at a power density of 2500 W/kg, along with high cycling stability, preserving 97.6% of its initial capacitance after 5000 cycles. An MnO₂eMXene composite containing a mixture of two different MnO₂ polymorphs in the form of nanosheets and fibers was prepared for a supercapacitor [65]. The composite demonstrated a capacitance of 272 F/g at 0.5 A/g and retained 93% of the capacitance after 5000 cycles at a current density of 5 A/g in a three-electrode setup using a KOH electrolyte. In an asymmetric device (AC//MnO₂eMXene-Na) with an Na₂SO₄ electrolyte, the synthesized composite achieved a 19.3 Wh/kg energy density at 500 W/kg of power density. A hierarchical 1D/2D TiO₂/Ti₃C₂ MXene heterostructure electrode material demonstrated a specific capacitance of 207 F/g at 10 mV/s with 100.9% specific capacitance retention in a three-electrode cell configuration using a 1 M NaCl electrolyte [67]. The capacitance slightly increased after long-term cycling, which may be attributed to improved interfacial contact between TiO₂ nanowires and Ti₃C₂ nanosheets induced by electroactivation during ion deintercalation, thereby enhancing charge transport and the overall electrochemical performance of the TiO₂/Ti₃C₂ composite. A flexible Ti₃C₂T_x/polyaniline (MXene/PANI) supercapacitor electrode composite material exhibited a specific capacitance of 272.5 F/g at 1 A/g and a capacitance retention rate of 71.4% after 4000 cycles at 2 A/g [70]. In a previous work [71], a Ti₃C₂T_x MXene–PANI composite achieved 430 F/g at 1 A/g and a 38 Wh/kg energy density at 800 W/kg power density in two-electrode configurations.

Beyond Ti-based MXenes, Mo_1.33CT_z–cellulose composite electrodes achieve a gravimetric capacitance of up to 440 F/g and a volumetric capacitance of up to 1178 F/cm³ and demonstrate outstanding cycling stability with 5.2 mg/cm² MXene loading [120]. These values are among the highest reported for MXene-based supercapacitor electrodes and demonstrate that MXenes are particularly well-suited to supercapacitors thanks to their ability to charge and discharge rapidly, their high current density, and their capacitance. In a recent work [45], Mo_2−□CT_z MXene was obtained with various vacancy concentrations from a Mo₂Ga₂C MAX-phase powder. The Mo_2−□CT_z-10 electrode, which was etched for 10 days, achieved a gravimetric capacitance of 117.5 F/g at 0.1 A/g and a rate performance of 39.4% in 1 M H₂SO₄ electrolyte. Table 5 provides performance data for MXene-based supercapacitors. The table below contains a combination of gravimetric, volumetric, and specific performance metrics. Values are presented in accordance with the data provided in the original sources. The electrochemical performance of MXene-based materials spans multiple metrics, including gravimetric, volumetric, and areal capacitance, which are therefore discussed separately due to their different physical bases. CC/MXene@SiO₂–EP and Ti₃C₂Tx quantum dot–based electrodes exhibit the highest gravimetric and volumetric capacitance, respectively, while MXene/Co₃O₄ and Ti₃C₂Tx–MWCNT devices show strong areal performance at the device level.

Table 5. MXene-based materials for supercapacitors.

Electrochemical Setup	Synthesis Route	Structural Characteristics	Capacitance	Cycling Stability	Ref.
Gravimetric capacitance
Ti3C2Tx MXene clay electrode in three-electrode Swagelok cells	Etching with LiF/HCl, filtering, and rolling into film	Few-layered structure	245 F/g at 2 mV/s	~100% (10k cycles)	[118]
Mo1.33CTz–cellulose composite in symmetric device with H2SO4	MXene–cellulose mix vacuum filtrated	Cellulose pillared between nanosheets	~97 F/g at 2 mV/s	~95% (35k cycles)	[120]
CC/MXene@SiO2–EP composite in the three-electrode system	LiF/HCl etching	Carbon cloth composite	481.4 F/g at 0.5 A/g	94.07% after 8000 cycles	[61]
Volumetric capacitance
RuO2@MXene in flexible micro-supercapacitors	MXene–RuCl3, AgNW mix into ink	RuO2 nanoparticles decorated on the nanosheets	864.2 F/cm3 at 1 mV/s	>90% (10k cycles)	[121]
Ti3C2Tx Quantum dots/L-Ti3C2Tx fiber electrode, F-MMT/PVA DHGE as electrolyte and separator	LiF/HCl etching, wet-spinning	Ti3C2Tx QDs pillared between nanosheets	1560 F/cm3 at 20 A/cm3	>79%	[122]
Areal capacitance
MXene/Co3O4 composite in an asymmetric supercapacitor and PVA/KOH as electrolyte	Co3O4 intercalated MXene	Co3O4 nanoparticles decorated on the nanosheets	0.49 F/cm2 at 30 mA/cm2	82.3% after 5000 cycles at 50 mA/cm2	[64]
Ti3C2Tx–MWCNT composite as negative electrode active mass in asymmetric supercapacitor	Co-dispersion of Ti3C2Tx and MCNT	Water-insoluble hydrophilic binders	0.94 F/cm2 in Na2SO4 electrolyte 0–1.6 V window	>90%	[119]

Table 5. MXene-based materials for supercapacitors.

Electrochemical Setup	Synthesis Route	Structural Characteristics	Capacitance	Cycling Stability	Ref.
Gravimetric capacitance
Ti3C2Tx MXene clay electrode in three-electrode Swagelok cells	Etching with LiF/HCl, filtering, and rolling into film	Few-layered structure	245 F/g at 2 mV/s	~100% (10k cycles)	[118]
Mo1.33CTz–cellulose composite in symmetric device with H2SO4	MXene–cellulose mix vacuum filtrated	Cellulose pillared between nanosheets	~97 F/g at 2 mV/s	~95% (35k cycles)	[120]
CC/MXene@SiO2–EP composite in the three-electrode system	LiF/HCl etching	Carbon cloth composite	481.4 F/g at 0.5 A/g	94.07% after 8000 cycles	[61]
Volumetric capacitance
RuO2@MXene in flexible micro-supercapacitors	MXene–RuCl3, AgNW mix into ink	RuO2 nanoparticles decorated on the nanosheets	864.2 F/cm3 at 1 mV/s	>90% (10k cycles)	[121]
Ti3C2Tx Quantum dots/L-Ti3C2Tx fiber electrode, F-MMT/PVA DHGE as electrolyte and separator	LiF/HCl etching, wet-spinning	Ti3C2Tx QDs pillared between nanosheets	1560 F/cm3 at 20 A/cm3	>79%	[122]
Areal capacitance
MXene/Co3O4 composite in an asymmetric supercapacitor and PVA/KOH as electrolyte	Co3O4 intercalated MXene	Co3O4 nanoparticles decorated on the nanosheets	0.49 F/cm2 at 30 mA/cm2	82.3% after 5000 cycles at 50 mA/cm2	[64]
Ti3C2Tx–MWCNT composite as negative electrode active mass in asymmetric supercapacitor	Co-dispersion of Ti3C2Tx and MCNT	Water-insoluble hydrophilic binders	0.94 F/cm2 in Na2SO4 electrolyte 0–1.6 V window	>90%	[119]

4. Current Challenges and Future Research Recommendations

Despite their significant advantages, MXenes have several limitations that hinder their use as battery anode materials on a large scale. One key drawback is their limited theoretical capacity: for the best-studied MXene, Ti₃C₂Tx, it is approximately 320 mAh g⁻¹, which is significantly lower than that of materials such as silicon (4200 mAh g⁻¹) or tin oxide (SnO₂, 782 mAh g⁻¹) [123]. In this case, there are numerous possible structures of various metal atoms with in-plane and out-of-plane ordering, including single or mixed surface terminations. This opens up a wide range of possibilities for tuning the properties of MXene by using different metal atoms with high theoretical capacity and applying efficient synthesis methods to regulate surface terminations in order to obtain 2D materials with enhanced specific capacity and stability. Systematic studies linking synthesis parameters to defect density, surface terminations and resulting electrochemical performance and stability are needed. Reported variations in the performance of similar MXene-based composites often arise from differences in the synthesis process, mass loading and voltage window. Therefore, it is essential to establish standardized electrochemical testing protocols that include consistent mass loading, voltage window, current density and electrolyte conditions, in order to enable meaningful comparisons across studies and advance the field.

MXenes are also prone to agglomeration and loss of active surface area during repeated charge–discharge cycles [124]. Over time, the individual layers of the material stick together, reducing the area available for ion diffusion and resulting in reduced capacity and poor cyclic stability. The assembly of hierarchical MXene-based composite materials prevents the stacking of MXene flakes and enhances the surface area by anchoring them with other materials [125]. This creates galleries between the MXene flakes, which have a high specific capacitance. It also increases ion diffusion and cyclic stability. In order to obtain various enhanced MXene-based composite materials, it is important to understand their self-assembly mechanism, the effect of synthesis conditions on gallery height, and the stability of the materials.

Another concern is MXene’s oxidative instability, especially under humid conditions or in the presence of oxygen [126]. This results in the formation of transition metal oxides, leading to a loss of electrical conductivity. Also, there is an issue with surface chemical stability, which reduces their properties and efficiency. Surface terminations of MXenes play a key role in obtaining composite materials based on them and their effective use in energy storage devices with high stability, which requires the development of an effective method for their modification. Studies have reported that the electrochemical performance of MXene is strongly influenced by exposure to air, indicating that prolonged contact with water and air leads to a gradual increase in capacity [102]. In contrast, oxidized MXene films showed a pseudocapacitance loss of almost half compared to fresh material, and this increased with partial oxidation [127]. Some reports suggest that partially oxidized Ti₃C₂T_x improves rate and cycling, but critical gaps remain in long-term degradation mechanisms, scalable and reproducible oxidation control, a quantitative understanding of –O/–F/–OH termination roles, and systematic testing with various electrolytes and hybrid architectures. To overcome these issues, it is important to establish a systematic protocol for synthesizing and testing that controls surface terminations and enhances their property correlations for energy storage applications.

Industrial-scale production of MXene remains challenging due to low productivity, high reagent costs, and quality control difficulties [128]. There is also the problem of maintaining structural integrity and properties when scaling up existing MXene synthesis methods. One such barrier is the use of hazardous materials, such as hydrofluoric acid, in the synthesis of MXenes. When scaling up MXene synthesis by this method, it is necessary to consider environmental and economic sustainability, as there is a question of disposal and environmental impact. The relatively high costs of synthesis and application reduce their competitiveness. Therefore, a more cost-effective, safe, efficient, and environmentally friendly synthesis method for large-scale production must be developed.

Reported MXene capacitances and cycling stabilities often vary widely across studies. These discrepancies are closely linked to differences in synthesis routes, surface terminations, defect densities, electrode mass loading, voltage windows and electrolyte conditions. A careful analysis reveals that such variations are not intrinsic to the material itself but rather arise from inconsistent preparation and testing protocols. This emphasizes the need for standardized synthesis and electrochemical characterization to enable meaningful comparisons.

5. Summary and Outlook

This review emphasizes that integrating energy storage systems into existing power systems is vital for advancing renewable energy development, enhancing efficiency and promoting sustainable energy solutions. It discusses the characteristics of various energy storage systems, including their specific capacity, charge/discharge density, storage efficiency and cycle life. The review also covers studies using MXenes and their composites in various energy storage systems, which have achieved excellent results.

However, there are still a number of limitations that need to be addressed for further improvement:

The relatively low theoretical capacity of MXenes remains a challenge. This restricts the energy density of MXene-based batteries and necessitates the development of composite systems to compensate for this limitation. To overcome these limitations, combining MXene-based composite materials with carbon nanomaterials (graphene, CNTs, activated carbon) or silicon components is actively being researched. Also, a composite approach is used: MXene is combined with active components (Si, Sn, Fe₂O₃, MoS₂, etc.) that have high theoretical capacity. These hybrid materials can combine the high conductivity of MXene and the capacity of nanoparticles, creating a synergistic effect. This could be achieved by carrying out systematic studies with standardized protocols that correlate synthesis parameters, defect density, and surface terminations.

The oxidative instability and surface chemistry stability of MXenes pose a challenge. Enhancing structural stability in MXene-based composites is important for the cycle life in energy storage applications. To increase the stability of MXene, surface modification can be used, including the application of protective coatings made of carbon, polymers (for example, PVDF, PAN) or metal oxides (Al₂O₃, TiO₂, SiO₂). These hybrid systems offer the potential to improve stability, increase specific capacity, and expand the functionality of MXene in next-generation electrochemical energy storage systems. The electrochemical performance of MXenes is affected by exposure to air and water, which gradually changes their capacity depending on the oxidation rate. Although partial oxidation can enhance capacity, gaps remain in understanding long-term degradation, quantitative roles of termination groups, and reproducible oxidation control. Standardized testing protocols are needed to enable reliable property correlations.

Industrial-scale production of MXene remains challenging due to low productivity. To improve processability, continuous and flow reactors can be used to ensure uniform etching and high productivity. Another promising direction is the development of MXene inks for printing technologies—inkjet, screen and 3D printing of electrodes. These methods open up opportunities for large-scale and flexible production of MXene components, including flexible energy storage devices.

The use of hazardous materials in the synthesis of MXenes remains challenging. Numerous etching methods use fluorine-based etchants. Therefore, it is important to develop a more efficient and scalable fluorine-free etching method.

The charge storage mechanism of MXene is strongly dependent on synthesis, terminations, defect density, electrolyte and composite architecture. To resolve the interplay between intercalation, surface redox and pseudocapacitance, standardized electrochemical protocols must be combined with multi-technique in situ characterization in order to establish reliable structure, property, and performance relationships.

The development of MXenes and their composites is intended to create sustainable, safe and cost-effective synthesis methods, as well as functional composites with controlled structures, high capacity and improved stability. Comprehensive implementation of these solutions will realize the potential of MXenes as key materials for next-generation, high-performance batteries, supercapacitors and hybrid energy devices.

The further development of MXenes and their composites aims to improve their functionality, stability and adaptability for use in next-generation energy storage systems. A key area of focus is heteroatomic doping, which involves introducing nitrogen, sulfur, or phosphorus atoms into the MXene structure. This alters the electron density, improves charge transfer and accelerates ion diffusion, while simultaneously increasing electrical conductivity and the number of active sites. This improves electrochemical activity and stability during long-term cycling.

Building on these advances, MXene is now being used in the development of next-generation solid-state and aqueous batteries. Thanks to its hydrophilicity, high ionic mobility and electrical conductivity, MXene can be used effectively as an electrode material and an ion-conducting interface. This is particularly important for ensuring the safety of systems with non-flammable, environmentally friendly electrolytes. MXene also enables complex modification, hybridization and integration into multifunctional structures that combine high conductivity, stability and activity. These approaches pave the way for the creation of environmentally friendly, flexible and highly efficient next-generation energy storage systems for a wide range of applications, including batteries and supercapacitors.