Multivalent Metal-Ion Batteries: Unlocking the Future of Post-Lithium Energy Storage

Abstract

The increasing demand for sustainable and high-performance energy storage underscores the limitations of lithium-ion batteries (LIBs), notably in terms of finite resources, safety issues, and rising costs. Multivalent metal-ion batteries (MMIBs)—employing Zn²⁺, Mg²⁺, Ca²⁺, and Al³⁺ ions—represent promising alternatives, as their multivalent charge carriers facilitate higher energy densities and greater electron transfer per ion. The widespread availability, lower cost, and favorable safety profiles of these metals further enhance MMIB suitability for large-scale deployment. However, MMIBs encounter significant obstacles, including slow ion diffusion, strong Coulombic interactions, electrolyte instability, and challenging interfacial compatibility. This review provides a systematic overview of recent advancements in MMIB research. Key developments are discussed for each system: electrode synthesis and flexible architectures for zinc-ion batteries; anode and cathode innovation alongside electrolyte optimization for magnesium-ion systems; improvements in anode engineering and solvation strategies for calcium-ion batteries; and progress in electrolyte formulation and cathode design for aluminum-ion batteries. The review concludes by identifying persistent challenges and future directions, with particular attention to material innovation, electrolyte chemistry, interfacial engineering, and the adoption of data-driven approaches, thereby informing the advancement of next-generation MMIB technologies.

1. Introduction

In recent decades, LIBs have become fundamental to modern energy storage, supporting applications from portable electronics to electric vehicles and facilitating renewable energy integration [1,2]. Their adoption is driven by high energy density, long cycle life, and established manufacturing processes [3]. Despite these advantages, LIB technology faces increasing scrutiny regarding resource sustainability, environmental impact, and rising costs associated with lithium extraction and supply chain constraints [4]. As global demand for energy storage grows, challenges such as escalating raw material costs, limited lithium availability, and safety concerns including dendrite formation have become more pronounced. These factors highlight the urgent need for sustainable, cost-effective, and high-performance next-generation energy storage solutions [5]. A promising alternative is MMIBs, which use cations with charges greater than +1, such as zinc (Zn²⁺), magnesium (Mg²⁺), calcium (Ca²⁺), and aluminum (Al³⁺) (Figure 1a) [6]. These multivalent ions can transfer two or more electrons per redox event, potentially enabling much higher volumetric energy densities than lithium. This advantage makes MMIBs strong candidates for high-capacity energy storage, particularly in large-scale applications where energy density and cost efficiency are essential. For instance, aluminum offers a theoretical volumetric capacity of about ~8040 mAh cm⁻³, significantly exceeding lithium (~2060 mAh cm⁻³) (Figure 1b) [7,8,9]. These metrics highlight the significant potential of MMIB technologies for grid-scale storage and transportation.

In addition to their superior theoretical energy densities, MMIBs present notable advantages concerning resource abundance and cost-effectiveness [10]. Elements such as magnesium, calcium, and aluminum are considerably more prevalent in the Earth’s crust than lithium; magnesium and calcium are among the most common elements, while aluminum is the most abundant metal overall. This widespread availability corresponds to lower raw material costs, thereby alleviating economic constraints as global demands for energy storage continue to rise [11]. The economic viability of these metals positions MMIBs as promising candidates for large-scale energy storage applications, particularly in facilitating the integration of renewable energy sources and ensuring grid stability.

A further significant benefit of MMIBs, particularly in the case of zinc-ion batteries (ZIBs), lies in their intrinsic safety features. Unlike traditional LIBs, which typically employ flammable organic electrolytes, ZIBs often utilize aqueous electrolytes, thereby mitigating the risk of thermal runaway and catastrophic failure [12]. In addition, the low toxicity and the ease of handling associated with zinc-based systems enhance their suitability for stationary storage and commercial uses [13]. The unique electrochemical behaviors of multivalent ions also contribute to operational safety, as they tend to suppress dendritic growth—a prominent issue in lithium-based systems [14]. Nevertheless, the realization of MMIBs’ full practical potential remains hindered by several challenges [15]. Most concerning among them is the inherently sluggish solid-state diffusion of multivalent ions within electrode host materials [16]. Owing to their higher charge densities and stronger Coulombic interactions with the crystal lattice, multivalent cations exhibit slower kinetics and reduced intercalation capacities compared to lithium ions [17]. This limitation narrows the selection of viable cathode materials and often results in inferior rate performance relative to LIBs.

The development of suitable electrolytes constitutes another major challenge. Achieving efficient and reversible deposition and stripping of multivalent metals such as magnesium and calcium is complicated by the formation of passivation layers and incompatibility with conventional electrolyte systems [18]. It is therefore essential to identify electrolytes that offer both high ionic conductivity and electrochemical stability within appropriate voltage ranges to meet practical performance requirements [19]. Recent advancements in characterization methodologies and computational modeling have significantly expanded the understanding of ion transport and interfacial processes within MMIBs. Techniques such as in situ spectroscopy, synchrotron-based analyses, and atomistic simulations have elucidated key aspects of diffusion mechanisms, interphase evolution, and degradation pathways. When combined with emerging machine learning and data-driven design strategies, these insights are expediting the discovery of optimized materials and electrolyte chemistries.

Performance assessment of MMIBs typically involves evaluating parameters such as specific capacity, operating voltage, Coulombic efficiency, and rate capability. While ZIBs are distinguished by high reversibility and safety, magnesium- [20] and calcium-ion [21] systems offer prospects for enhanced energy densities, contingent upon resolving kinetic and interfacial barriers. Notably, calcium-ion batteries have demonstrated the potential for high-voltage operation and competitive specific capacities, though the lack of suitable electrode materials continues to impede commercialization efforts. Similarly, magnesium-ion batteries have shown progress via innovative cathode design, but substantial advancements in electrolyte formulation are still required to address stability and kinetic challenges.

Therefore, the advancement of MMIBs toward widespread practical adoption necessitates concerted efforts across multiple fronts. Material development must focus on creating host structures that facilitate rapid multivalent ion diffusion while maintaining structural robustness. Electrolyte research should prioritize the formulation of non-corrosive, highly conductive systems capable of supporting reversible multivalent metal deposition [22]. A deeper understanding of interfacial phenomena, particularly solid-electrolyte interphase (SEI) formation [23], is critical for achieving long-term stability and high Coulombic efficiency. Integrating advanced experimental techniques with computational and data-driven approaches—such as predictive modeling, high-throughput material screening, and machine learning—will be instrumental in accelerating the identification of optimal electrode and electrolyte combinations. Such a holistic strategy is essential for transitioning MMIB technologies from laboratory-scale studies to practical, real-world applications [24].

This review presents an in-depth analysis of recent developments in MMIBs, with a particular emphasis on material innovations and engineering approaches designed to overcome prevailing performance limitations. The discussion is systematically structured into several key sections. Section 2 examines ZIBs, discussing novel synthesis methods for advanced electrode materials [25,26], strategies for interface engineering [27], the optimization of electrolyte additives [28], and progress in the development of flexible [29], self-powered battery configurations. Section 3 is dedicated to Magnesium-Ion Batteries (MIBs), highlighting advancements in anode stabilization, cathode structural engineering, electrolyte formulation [30], and the optimization of electrode-electrolyte interfaces to improve battery performance [31]. Section 4 focuses on Calcium-Ion Batteries (CIBs), summarizing achievements in anode design, polyanionic and framework-based cathodes, solvation engineering for electrolyte systems, and the integration of organic and hybrid electrode materials [32]. Section 5 addresses Aluminum-Ion Batteries (AIBs), reviewing efforts in optimizing electrolytes, developing anode-free configurations, engineering advanced cathode architectures, employing molecular design for organic cathodes, and exploring sulfur- and molten-salt-based systems for high-capacity applications [7]. The final section, Section 6, discusses outstanding challenges and future directions, emphasizing the importance of continued material innovation, electrolyte engineering, interfacial optimization, and device architectural improvements, as well as the transformative potential of data-driven discovery techniques. Through this organized framework, the review aims to provide a comprehensive perspective on MMIB technologies, elucidating both current progress and the prospective evolution of post-lithium energy storage systems.

2. Zinc-Ion Batteries (ZIBs)

ZIBs have garnered considerable attention as competitive alternatives to LIBs, offering a safer, more sustainable, and economically advantageous route for both large-scale energy storage and consumer electronics applications [13]. Several intrinsic features underpin the appeal of ZIBs, most notably the utilization of Earth-abundant, low-cost zinc, operation with non-flammable aqueous electrolytes, and a pronounced safety profile that substantially reduces fire and explosion hazards [23]. These properties position ZIBs as particularly suitable for scenarios where safety, sustainability, and economic efficiency are prioritized. This section reviews recent progress in the field, focusing on advancements in material synthesis, anode interface engineering via electrolyte additives [24], innovative cathode modification strategies, the development of high-performance organic and polymeric cathodes, and the design of flexible, self-powered aqueous ZIBs. Collectively, these topics provide an integrated overview of the technological advancements that are propelling ZIBs toward next-generation performance and commercial viability.

2.1. Emerging Synthesis Strategies for High-Performance ZIB Materials

A central advantage of ZIB technology is its compatibility with non-flammable, thermally stable aqueous electrolytes, thereby significantly reducing the probability of fire or explosion—persistent concerns in LIBs [33]. The abundance and affordability of zinc further amplify the attractiveness of ZIBs for deployment in grid-scale applications. Additionally, the elimination of toxic hazards and the reduced dependence on critical metals, such as lithium and cobalt, further reinforce the environmental sustainability of ZIB systems. These inherent strengths have motivated a range of innovative synthesis approaches to enhance the safety, cost-effectiveness, and electrochemical performance of ZIBs.

Within this context, transition metal oxides—particularly manganese oxides—are notable ZIB cathode materials due to their elevated theoretical capacities and low cost [25]. However, they are often limited by poor structural reversibility and sluggish kinetics. To address these issues, a microstructure strain modulation strategy has been employed for ZnMn₂O₄ (ZMO) through partial nickel substitution at the tetrahedral sites, producing ZNxMO with expanded crystal planes, strengthened Mn–O bonds, and an increased density of oxygen vacancies. These structural refinements facilitate Zn²⁺ migration and improve lattice stability. For example, ZN₀.₅MO supported on nitrogen-doped carbon nanotubes (Figure 2a) achieves a specific capacity of 239.2 mAh g⁻¹ at 0.1 A g⁻¹ (Figure 2b) and demonstrates robust cycling stability over 3000 cycles at 1.0 A g⁻¹ [34]. These enhancements are attributed to the asymmetric MnO₆ channel structures and the reversible nature of Zn²⁺ intercalation, establishing an effective avenue for optimizing Mn-based electrodes.

In a parallel effort, a plasma-assisted hydrothermal (PAHT) synthesis method has been introduced for the fabrication of water-intercalated vanadium oxide (WiVO) nanosheets in both two- and three-dimensional morphologies [35]. The PAHT process integrates plasma-induced oxidation in aqueous media with the thermal and pressure effects of hydrothermal synthesis, enabling the rapid nucleation of V₂O₅ nanosheets with precise control over water intercalation at temperatures below 80 °C (Figure 2c). Subsequent post-synthetic annealing transforms the 2D nanosheets into robust 3D clusters, which display enhanced cycling stability and improved capacity retention. In comparison to conventional synthesis techniques that require high temperatures and extended durations, the PAHT process proceeds at atmospheric pressure in under 80 min and facilitates in situ monitoring. The resultant WiVO 3D nanosheet clusters exhibit superior electrochemical characteristics relative to both commercial V₂O₅ and non-clustered WiVO, thus demonstrating the versatility and promise of this method for advanced aqueous ZIB electrode development [35].

Similarly, calcium vanadate (CaV₄O₉) microflowers produced through hydrothermal synthesis have been integrated with carbon nanotubes to yield free-standing composite membranes. Ex situ structural characterization has shown that cycling above 1.4 V causes irreversible Ca²⁺ extraction, resulting in a phase transition to amorphous V₂O₅.nH₂O [36]. This amorphous phase effectively accommodates mechanical strain during Zn²⁺ insertion and extraction processes while providing numerous active sites, thereby supporting high areal capacities of 10.5 mAh cm⁻² at mass loadings of approximately 50 mg cm⁻². Flexible Zn//CaVO/CNT batteries based on these composites maintain stable electrochemical performance even under mechanical deformation, indicating their promise for high-loading, flexible ZIB applications.

2.2. Anode Interface Engineering and Electrolyte Additives

Zinc metal anodes have established themselves as critical components in aqueous ZIBs, owing to their high reversibility and robust electrochemical stability, attributes that have been refined through extensive research into their redox mechanisms [37]. These anodes are capable of sustaining elevated current densities while consistently enabling efficient plating and stripping, both of which are indispensable for achieving high operational efficiency in practical devices [38,39]. The technological maturity of zinc deposition and dissolution processes underpins the scalability and commercial feasibility of ZIBs, making the optimization of anode performance a fundamental step toward extending battery lifespan and reliability. Despite their inherent advantages, practical implementation of aqueous ZIBs is frequently challenged by the formation of zinc dendrites and parasitic side reactions, phenomena that can severely compromise cycle life and operational safety [2]. Zinc dendrite growth, in particular, poses one of the most significant barriers to long-term anode stability, as it can lead to internal short circuits and rapid capacity fading [40]. Effective mitigation of dendrite formation requires precise regulation of zinc deposition dynamics. Recent work has focused on surface modification and alloying strategies to promote uniform zinc plating [3]. The application of nanostructured coatings—such as graphene, carbon nanotubes, and MXenes—serves to homogenize the electric field at the electrode surface, thereby reducing the propensity for dendritic protrusions and enhancing the efficiency of both plating and stripping processes. These approaches not only bolster cycling stability but also contribute to an improved safety profile for ZIBs.

A notable innovation in anode protection involves the creation of a fluoride-based interphase by coating zinc anodes with a dense CaF₂ protective layer [38]. This insulating and corrosion-resistant coating exhibits low polarization, which enables efficient Zn²⁺ ion transport while simultaneously inhibiting hydrogen evolution—one of the primary parasitic reactions in aqueous ZIBs (Figure 3a). Theoretical calculations based on density functional theory (DFT) indicate that the high electronegativity of fluorine atoms in the CaF₂ layer facilitates robust Zn–F interactions, effectively reducing the nucleation overpotentials and energy barriers for Zn²⁺ deposition. Experimentally, the Zn@CaF₂ anode demonstrates exceptional stability, achieving over 4000 h of cycling in symmetric cells and maintaining 70% of its capacity after 1000 cycles at 2 A g⁻¹ in full-cell configurations with V₁₀O₂₄ cathodes. This method thus presents a promising pathway toward dendrite-free, long-lasting zinc anodes for high-performance aqueous ZIBs [38]. Further advancements in anode interface engineering have been realized by employing zinc trifluoromethanesulfonate (Zn(OTf)₂)-assisted low-temperature cyclization of polyacrylonitrile, resulting in a pyridine nitrogen-rich, Zn²⁺-conductive coating known as CPANZ. This pre-zincified interfacial layer adheres strongly to the zinc surface, significantly suppressing hydrogen evolution and corrosion while ensuring uniform Zn²⁺ transport. Notably, CPANZ-coated Zn anodes provide stable cycling for more than 6000 h at 0.5 mA cm⁻², with nearly 100% Coulombic efficiency sustained over thousands of cycles in both full-cell and hybrid capacitor arrangements [41]. This strategy offers a broadly applicable approach to designing interfacial layers capable of substantially extending the operational life of aqueous ZIBs.

Electrolyte engineering is equally critical to the advancement of ZIB technology. In ZIBs, early primary and dry cell technologies utilized aqueous electrolytes such as ammonium chloride (NH₄Cl) or combinations of NH₄Cl and zinc chloride (ZnCl₂) in mildly acidic environments [42]. While conventional aqueous zinc sulfate (ZnSO₄) has been widely used, alternative electrolyte systems—such as concentrated water-in-salt formulations and gel polymer matrices—have been shown to effectively suppress dendrite formation and hydrogen evolution, both of which are principal contributors to capacity degradation [43]. Gel-based electrolytes, in particular, enhance mechanical integrity and safety, and the ability to fine-tune electrolyte composition expands the operational windows of ZIBs. Innovative electrolyte additives [44,45] such as graphene quantum dots (GQDs) have also been explored. Synthesized via a single-step hydrothermal process from graphite powder, GQDs act as stabilizers for zinc anodes [46]. Both DFT calculations and experimental evidence indicate that GQDs lower nucleation energy barriers for Zn deposition and promote uniform Zn²⁺ distribution during cycling. The presence of oxygen-containing functional groups on GQDs enhances Zn²⁺ adsorption, which mitigates dendrite formation and reduces overpotentials. Additionally, GQDs reinforce hydrogen bonding between water molecules, thus suppressing corrosion and undesirable byproduct formation. As a result, Zn anodes with GQD additives exhibit prolonged cycling lifetimes—up to 2200 h at 0.8 mA cm⁻² with minimal voltage hysteresis. In full-cell configurations, NaxV₂O₅.nH₂O cathodes paired with GQD-modified electrolytes retain 164.3 mAh g⁻¹ after 600 cycles at 1 A g⁻¹, demonstrating clear performance benefits [46].

Moreover, the deployment of a Zn₃(PO₄)₂.4H₂O (ZP) surface coating on zinc anodes has shown efficacy in mitigating dendrite formation [47]. Operando synchrotron tomography reveals that the ZP coating, upon cycling, decomposes into P₂O₅, Zn⁰, and Zn(OH)₂ (Figure 3b) [47]. P₂O₅ subsequently reacts with water to generate H₃PO₄, which further interacts with metallic zinc to regenerate Zn₃(PO₄)₂.4H₂O, establishing a cyclic self-healing process that consistently prevents dendrite growth—even at elevated current densities (Figure 3c). Long-term cycling tests in NaV₃O₈‖ZP-coated Zn cells have demonstrated stable performance over 1000 cycles, highlighting the potential applicability of this approach across both aqueous and nonaqueous ZIB platforms.

Figure 3. (a) Schematic illustration of a fluoride-based interphase formed by CaF₂ coating on a Zn anode, suppressing corrosion and hydrogen evolution (reprinted with permission from Ref. [38]; copyright 2022, Royal Society of Chemistry). (b) SEM images of NaV₃O₈ electrode surfaces (i) after full discharge and (ii) charge using the ZP-coated Zn anode. (c) Potential vs. time plots for Zn||Zn symmetric cells with bare and ZP-coated Zn anodes at 2.5 (upper panel) and 5 (lower panel) mAh cm⁻² (reprinted with permission from Ref. [47]; copyright 2022, John Wiley & Sons, Inc.).

Figure 3. (a) Schematic illustration of a fluoride-based interphase formed by CaF₂ coating on a Zn anode, suppressing corrosion and hydrogen evolution (reprinted with permission from Ref. [38]; copyright 2022, Royal Society of Chemistry). (b) SEM images of NaV₃O₈ electrode surfaces (i) after full discharge and (ii) charge using the ZP-coated Zn anode. (c) Potential vs. time plots for Zn||Zn symmetric cells with bare and ZP-coated Zn anodes at 2.5 (upper panel) and 5 (lower panel) mAh cm⁻² (reprinted with permission from Ref. [47]; copyright 2022, John Wiley & Sons, Inc.).

2.3. Inorganic Cathode Modification Strategies

Recent progress in the engineering of inorganic cathodes has markedly advanced the electrochemical performance of ZIBs, resulting in cathodes that deliver both high specific capacities and remarkable cycling stability [48]. Transition metal oxides, particularly vanadium pentoxide (V₂O₅) and various polymorphs of manganese dioxide (MnO₂), have become leading candidates owing to their substantial interlayer spacing and pronounced redox activity, which collectively facilitate the efficient intercalation of Zn²⁺ ions and enable durable energy storage.

Defect engineering—most notably through heteroatom doping—has been widely adopted to modify both the chemical environment and the crystal structure of these metal oxides. For example, α-MnO₂ has been doped with a range of elements, such as K, Ag, Cu, Al, Ce, and V, although its application as a ZIB cathode in aqueous systems remains underexplored. A notable breakthrough involves the doping of α-MnO₂ with bismuth, a strategy that has led to significant improvements in the material’s electrochemical behavior. Bi-doped α-MnO₂ (BMO), synthesized via a redox process at ambient temperature followed by annealing, exhibits enhanced electrical conductivity and Zn²⁺ storage capability [49]. Insights from DFT calculations and experimental characterization reveal that the introduction of high-charge-density Bi ions stabilizes the structure via Bi–O bond formation, narrows the band gap to improve electron conduction, and weakens the Zn–O bond, thereby facilitating Zn²⁺ diffusion. The electron cloud distortion on oxygen atoms further increases the electrochemically active surface area. The optimized BMO-6 cathode demonstrates stable phase retention through Zn²⁺/H⁺ co-insertion cycles, delivering a specific capacity of 365 mAh g⁻¹ at 0.1 A g⁻¹, with 93% capacity preserved after 10,000 cycles and an energy density of 486 Wh kg⁻¹. Importantly, flexible BMO-6 electrodes supported by bacterial cellulose maintain high performance under mechanical stress, successfully powering miniature electronics such as light-emitting diodes and fans and thus highlighting their promise for wearable energy storage [49].

Another innovative cathode material is the layered magnetic topological insulator MnBi₂Te₄, previously renowned for its quantum and spintronic properties and now investigated for battery applications [50]. Theoretical analyses suggest that MnBi₂Te₄ conductive surface states and edge channels enable rapid Zn²⁺ storage and transport, making it an attractive ZIB cathode. Experimental evaluations confirm these predictions; MnBi₂Te₄ cathodes achieve an average discharge capacity of 264.8 mAh g⁻¹ at 0.40 A g⁻¹, with 88.6% capacity retention after 400 cycles at 4.00 A g⁻¹, and outstanding rate performance, maintaining 95.1% of the original capacity as current density increases from 0.40 to 8.00 A g⁻¹. Quasi-solid-state MnBi₂Te₄/Zn batteries exhibit 79.9% retention after 1000 cycles at 4.00 A g⁻¹ and stable charge-discharge profiles across a broad temperature range (0 °C to 75 °C), underscoring the robust performance and broad applicability of this unconventional cathode material [50].

Vanadium dioxide (VO₂), distinguished by its tunnel-like crystal structure and high theoretical capacity, has also emerged as a promising cathode for aqueous ZIBs. However, its practical deployment has been constrained by limited reaction kinetics and structural deterioration caused by strong electrostatic interactions between Zn²⁺ ions and the VO₂ lattice. The introduction of oxygen vacancies—a form of defect engineering—can alleviate these issues by improving both ion and electron transport as well as structural integrity. Nevertheless, achieving optimal oxygen vacancy concentration remains technically demanding. A recent advancement involves in situ one-step hydrothermal growth of VO₂ on carbon nanofibers, producing VO₂@CNF with precisely tuned oxygen vacancy content [51]. This method tailors the adsorption and migration energy barriers for Zn²⁺, enabling enhanced interaction and ion mobility. The VO₂@CNF cathodes deliver high specific capacities exceeding 450 mAh g⁻¹ at 0.1 A g⁻¹, with 318 mAh g⁻¹ retained after 2000 cycles at 5 A g⁻¹, corresponding to 85% capacity retention. This approach exemplifies the potential of rational defect modulation for advancing ZIB cathode design. Potassium vanadate (KVO) nanobelts have also been identified as high-performance cathodes for ZIBs, offering a specific discharge capacity of 461 mAh g⁻¹ at 0.2 A g⁻¹ and exceptional cycling durability, with 96.2% capacity retention over 4000 cycles at 10 A g⁻¹ [52]. Notably, the energy efficiency of aqueous ZIBs is significantly enhanced by pairing KVO with an acetylene-black-modified Zn foil anode (AB-Zn; Figure 4a), as evidenced by an increase in efficiency from 47.5% to 66.5% at 10 A g⁻¹ (Figure 4b). The AB-Zn//KVO configuration thus combines high capacity, long lifespan, and improved energy efficiency, rendering it a promising candidate for grid-scale energy storage [52].

Expanding the interlayer spacing of vanadium oxide has been shown to further accelerate Zn²⁺ storage kinetics. A two-step method has been developed to insert polypyrrole (PPy) into the lamellar structure of hydrated vanadium oxide (V₂O₅.nH₂O), resulting in an interlayer distance of 14.0 Å [53]. This expanded spacing allows for rapid Zn²⁺ (de)intercalation, yielding a specific capacity of 383 mAh g⁻¹ at 0.1 A g⁻¹, and stable long-term performance in Zn‖PPy/VOH cells. This method highlights the utility of conductive polymer intercalation as a structural engineering approach for high-rate, high-capacity ZIB cathodes [53]. The stabilization of vanadium oxide cathodes has also been achieved through Zn²⁺ intercalation during in situ electrochemical oxidation, producing Zn_δV₂O₅.nH₂O (ZVO) phases with tunable stoichiometry. Zn–O polyhedra formed between VO layers act as “pillars”, reinforcing the structure without hindering ion mobility [54]. At an optimal δ of 0.36, ZVO-2 achieves a maximum specific capacity of 508.3 mAh g⁻¹ at 0.5 A g⁻¹, with 95% retention after 2000 cycles and 80% after 5000 cycles (Figure 4c). DFT calculations confirm that increasing the Zn/V ratio enhances stability, and the lower crystallinity resulting from electrochemical oxidation improves activity compared to hydrothermal analogues [54]. MXene-based composites have recently drawn attention as next-generation ZIB cathodes given their excellent conductivity, hydrophilicity, and tunable structure [55]. A novel hybrid composed of MXene and bismuth (MXene/Bi) has been developed to overcome the challenges faced by pure Bi, such as volume expansion and aggregation during cycling [56]. The dispersed MXene nanosheets enhance electronic transport and mechanical robustness while maintaining electrochemical activity. Electrodes fabricated from this composite exhibit high capacity, prolonged cycle stability, and efficient Zn²⁺ ion transport. Quasi-solid-state ZIBs based on MXene/Bi cathodes not only demonstrate outstanding flexibility and heat resistance but also operate as self-powered pressure sensors—capable of real-time pressure monitoring via current response—thereby illustrating the multifunctional potential of MXene/Bi systems for advanced ZIBs [56].

Figure 4. (a) SEM image of an acetylene-black-modified Zn anode after plating/stripping, and (b) plot of energy efficiency versus current density for AB-Zn||KVO compared to bare Zn (reprinted with permission from Ref. [52]; copyright 2021, American Chemical Society). (c) Specific capacity vs. cycle number for Zn0.36V₂O₅.nH₂O (ZVO) at 0.5 A g⁻¹ (reprinted from Ref. [54]; copyright 2022, Elsevier).

Figure 4. (a) SEM image of an acetylene-black-modified Zn anode after plating/stripping, and (b) plot of energy efficiency versus current density for AB-Zn||KVO compared to bare Zn (reprinted with permission from Ref. [52]; copyright 2021, American Chemical Society). (c) Specific capacity vs. cycle number for Zn0.36V₂O₅.nH₂O (ZVO) at 0.5 A g⁻¹ (reprinted from Ref. [54]; copyright 2022, Elsevier).

2.4. Advanced Organic/Polymer Cathodes, Flexible and Self-Powered Architectures for ZIBs

Recent years have witnessed a growing interest in organic and polymer cathodes as promising alternatives to traditional inorganic materials for ZIBs, owing to their inherent structural versatility, lightweight nature, and environmental compatibility [57,58]. The molecular design flexibility of these organic materials enables precise tuning of redox-active centers, which in turn enhances both Zn²⁺ storage capability and cycling stability. Conjugated polymers, carbonyl-rich molecules, and metal-organic frameworks exhibit high theoretical capacities, rapid redox kinetics, and notable mechanical flexibility, making them attractive for next-generation ZIBs [59]. Quinone-based organics, for instance, have demonstrated superior capacity retention through proton-coupled electron transfer mechanisms, effectively overcoming the structural decay commonly encountered in inorganic hosts.

A noteworthy innovation in this field is the development of an aza-fused π-conjugated microporous polymer (Aza-CMP), which possesses a theoretical capacity of 602 mAh g⁻¹. When hybridized with graphene, the resulting composite (G-Aza-CMP) displays significantly enhanced electrochemical properties [60]. The incorporation of graphene increases interlayer spacing, disperses the polymer domains, and introduces additional electroactive sites, while strong π–π stacking interactions reinforce stability and facilitate charge transfer. This engineering shifts the charge storage mechanism from a mixed Zn²⁺/H⁺ co-insertion in pristine Aza-CMP to a predominantly H⁺ insertion mechanism in the graphene hybrid, thereby enabling ultrafast reaction kinetics (Figure 5a) [60]. The Zn‖G-Aza-CMP cell achieves a high specific capacity of 456 mAh g⁻¹ at 0.05 A g⁻¹ and retains 91.2% of its capacity even after 9700 cycles at 10 A g⁻¹. DFT analysis indicates that Zn²⁺ contributes 29.6% and H⁺ 70.4% to the overall storage, highlighting the composite as a high-performance ZIB cathode. Another recent advance involves the design of poly(phenazine-alt-pyromellitic anhydride; PPPA), synthesized through condensation polymerization (Figure 5b(i)) [61]. Structural and electrochemical characterizations confirm a reversible Zn²⁺ coordination mechanism in PPPA. The extended π-conjugated backbone of the polymer yields an exceptional Zn²⁺ diffusion coefficient of 1.2 × 10⁻⁷ cm² s⁻¹, the highest among currently reported organic cathodes for ZIBs, and supports a specific capacity above 200 mAh g⁻¹ (Figure 5b(ii)). Theoretical analysis attributes these enhancements to a minimized bandgap in the planar polymer configuration, accelerating intramolecular electron transfer and promoting high-rate Zn²⁺ transport [61].

Flexible electrode architectures are increasingly being pursued for wearable and portable electronic devices [62]. These systems combine mechanical flexibility with structural adaptability and, in some cases, integrated energy-harvesting functionalities, allowing for continuous device operation without the need for external charging. Progress in material science, device engineering, and hybridization with other energy technologies is thus driving the practical realization of flexible energy storage platforms. In the context of self-powered wearable systems and the Internet of Things (IoT), integrating energy storage with sensing functionality is highly desirable [63]. One such innovation is the zinc-ion battery pressure sensor, constructed as a solid-state device with an isolation layer that modulates internal resistance in response to mechanical pressure [64]. The sensor exhibits excellent mechanical resilience, with response and recovery times of 76 ms and 88 ms, respectively, and maintains stable operation over more than 100,000 cycles. Its ability to accurately detect physiological signals, such as pulse and limb movement, underscores its potential for compact, multifunctional wearable systems [64]. Additive manufacturing—specifically, three-dimensional printing [65]—has been harnessed to fabricate microplanar flexible ZIBs utilizing CNT@MnO₂ composites. Carbon nanotube incorporation enhances electrical conductivity, establishes a robust 3D conductive framework, and improves charge-transfer kinetics, resulting in a substantial increase in specific capacity (by approximately 100 mAh g⁻¹) [66]. Replacing traditional zinc foil with micron-scale zinc powder ink leads to a highly flexible, separator-free device architecture that mitigates dendrite formation and optimizes ion transport pathways. These 3D-printed systems deliver 63 μAh cm⁻² at 0.4 mA cm⁻², with only 2.72% capacity loss after multiple bending cycles (Figure 5c,d) [66]. Such advances affirm the potential of additive manufacturing for producing adaptable, high-performance energy storage solutions.

The concept of self-powered systems, designed for operation in resource-constrained environments without the need for external energy sources, is gaining increasing relevance [67]. For example, a flexible, coin-type aqueous ZIB utilizing hexacyanohexaazatrinaphthylene (HCHATN) as the cathode has been realized, achieving a high volumetric energy density (7.81 mWh cm^–3), a gravimetric energy density of 120.9 Wh kg^–1, and robust cycling performance via Zn²⁺ and H⁺ co-insertion/extraction processes [68]. Notably, this flexible Zn‖HCHATN battery showcases a self-charging capability, enabled by a spontaneous redox reaction between the discharged cathode and atmospheric oxygen—facilitating chemical recharging in ambient air. Following 15 h of air exposure, the device delivers a discharge capacity of 225 mAh g⁻¹ at 0.5 A g⁻¹ and retains 220 mAh g⁻¹ at 20 A g⁻¹, evidencing both high rate capability and long-term reusability in diverse charging regimes [68].

Advances in transient and biodegradable ZIBs have also been reported. Substituting conventional separators with a cross-linked agarose/carboxymethyl cellulose hydrogel electrolyte enables complete biodegradability while maintaining high electrochemical performance [69]. Such hydrogel-based electrolytes have extended Zn anode lifespans to over 4000 h, overcoming previous stability barriers without the need for hazardous additives. The use of a polydopamine organic cathode—renowned for its redox activity and biocompatibility—further enhances device performance, with careful structural optimization ensuring both substantial capacity and longevity. A thin agarose shell serves to protect internal components during operation and enables rapid degradation under composting conditions, leading to the realization of fully transient, high-performance ZIBs [69].

Figure 5. (a) Schematic of storage mechanism evolution from Zn²⁺/H⁺ co-insertion in Aza-CMP to dominant H⁺ insertion in G-Aza-CMP (reprinted from Ref. [60]; copyright 2022, Elsevier). (b) (i) Synthesis of poly(phenazine-alt-pyromellitic anhydride; PPPA) and (ii) specific capacity comparison vs. voltage with other organic electrodes (reprinted with permission from Ref. [61]; copyright 2022, John Wiley & Sons, Inc). (c) Galvanostatic charge-discharge curves (inset of Figure 5c shows the 3D-printed ZIB with parallel connection) and (d) photographs of 3D-printed flexible ZIB of CNT@MnO₂ during various bending states (1–4) (reprinted with permission from Ref. [66]; copyright 2022, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

Figure 5. (a) Schematic of storage mechanism evolution from Zn²⁺/H⁺ co-insertion in Aza-CMP to dominant H⁺ insertion in G-Aza-CMP (reprinted from Ref. [60]; copyright 2022, Elsevier). (b) (i) Synthesis of poly(phenazine-alt-pyromellitic anhydride; PPPA) and (ii) specific capacity comparison vs. voltage with other organic electrodes (reprinted with permission from Ref. [61]; copyright 2022, John Wiley & Sons, Inc). (c) Galvanostatic charge-discharge curves (inset of Figure 5c shows the 3D-printed ZIB with parallel connection) and (d) photographs of 3D-printed flexible ZIB of CNT@MnO₂ during various bending states (1–4) (reprinted with permission from Ref. [66]; copyright 2022, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

3. Magnesium-Ion Batteries (MIBs)

Magnesium-ion batteries (MIBs) have emerged as increasingly attractive alternatives to LIBs, largely due to their inherent safety, abundance of magnesium resources, and potential for high energy density [70]. The growing emphasis on environmentally responsible energy storage is in part a response to the ecological impacts associated with lithium extraction, persistent safety risks such as the formation of lithium dendrites, and the price volatility of lithium and cobalt [24]. In this context, MIBs are recognized as promising candidates for large-scale stationary storage and electric vehicle (EV) applications, where both operational safety and high performance are of critical importance.

One of the principal advantages of magnesium is its exceptional volumetric capacity—around 3833 mAh cm⁻³—substantially greater than that of lithium (about 2062 mAh cm⁻³) [20]. This high volumetric energy density is particularly advantageous for compact, high-capacity storage systems. Notably, magnesium metal does not typically form hazardous dendrites under conventional electrochemical conditions, thereby addressing a key limitation of LIBs, where dendritic growth may cause internal short circuits and catastrophic failure. In addition to its electrochemical strengths, magnesium is the fifth most prevalent element in the Earth’s crust, is economically accessible, and remains chemically stable under ambient conditions [71]. As a result, MIBs offer the prospect of safer and more resilient energy storage while also reducing the supply chain constraints associated with lithium and cobalt. These attributes collectively position MIBs as a sustainable solution for next-generation energy storage technologies.

3.1. Anode Innovations and Protective Strategies

Despite the appeal, the development of high-performance magnesium anodes has been hindered by surface passivation, sluggish ionic diffusion, and limited compatibility with conventional electrolytes [72,73]. To overcome these obstacles, researchers have implemented a range of advanced structural and chemical strategies designed to enhance the stability, reversibility, and electrochemical performance of magnesium anodes [31,74]. These approaches encompass compositional modifications, nanostructuring, and the adoption of innovative synthesis and alloying techniques, all of which are instrumental in bringing MIBs closer to commercial viability.

A representative example is the assembly of a full-cell device pairing Na₃V₂(PO₄)₃@C composite cathodes with Bi nanowire anodes, utilizing a Mg(TFSI)₂/AN electrolyte [75]. The uniform carbon coating on Na₃V₂(PO₄)₃ improved electrical conductivity, while the nanowire morphology of the Bi anode enabled efficient Mg²⁺ storage and alleviated mechanical stress during cycling. This configuration exhibited high-voltage discharge plateaus at 2.7 V and 2.2 V, delivering an initial capacity of 76 mAh g⁻¹ and maintaining 62 mAh g⁻¹ after 100 cycles (82% retention), with robust rate capability of 45 mAh g⁻¹ at 500 mA g⁻¹. These outcomes underscore the importance of electrode architecture in achieving prolonged cycle life and fast charge-discharge characteristics, advancing the practicalization of MIBs [75]. Further progress has been achieved with the development of tin-containing silicon oxycarbide (SiOC/Sn) nanobeads, synthesized with varying Sn and carbon concentrations (Figure 6a,b) [76]. Electrochemical tests revealed an initial discharge capacity of 198.2 mAh g⁻¹, stabilizing at 144.5 mAh g⁻¹ after 100 cycles at 500 mA g⁻¹. The electrodes demonstrated notable rate performance, maintaining 85.2% efficiency across different current densities (Figure 6c) [76]. Mechanistic studies attributed these improvements to the high surface area, which increased active Mg²⁺ storage sites, and to the presence of Sn, which enhanced both conductivity and storage through reversible alloying-dealloying with Mg–Sn phases. The SiOC matrix further prevented structural collapse and contributed additional capacitive storage, highlighting the benefits of compositional design and nanoscale engineering for MIB anodes.

Innovative synthesis techniques have also been explored to improve sustainability and safety. One such strategy involves using crystal water in precursors as a reaction medium, which eliminates the need for hazardous organic solvents. Through this approach, cotton-like NiS₂ structures were synthesized in a NaCl-confined system [77]. The resulting material demonstrated high connectivity and plentiful electroactive sites, supporting rapid charge transfer and favorable kinetics. This solvent-free methodology not only minimizes impurity incorporation and solvent consumption but also provides a scalable, environmentally friendly route for producing transition metal sulfides with improved electrochemical properties [77]. In addition to solid anode materials, the exploration of liquid alloy anodes represents a transformative advance for addressing dendrite-related challenges. While solid Mg alloys can limit dendrite growth, they often undergo structural damage due to significant volume changes during cycling. To address this, researchers have proposed self-healing eutectic Ga–In liquid alloy anodes, which undergo reversible liquid-solid transitions during (de)magnetization. This adaptive mechanism absorbs volume fluctuations, prevents cracking, and eliminates dendrite nucleation. The system offered a theoretical capacity of 246 mAh g⁻¹, with experimental results showing 225 mAh g⁻¹ and 91% retention after 2000 cycles at 1C and 25 °C [78]. In situ wide-angle X-ray scattering and cryo-scanning electron microscopic analyses confirmed the reversibility of morphological transformations, validating the self-healing effect. Such liquid alloy electrodes represent a promising pathway toward durable, dendrite-free MIBs.

In brief, these advancements demonstrate the diverse and synergistic strategies implemented to optimize magnesium anodes. From carbon-coated phosphate and Sn-based nanocomposites to solvent-free synthesis and adaptive liquid alloy architectures, each approach addresses specific challenges in stability, conductivity, and cycling life. The integration of structural design, compositional tuning, and unconventional concepts is rapidly advancing MIB technology, laying the groundwork for its large-scale adoption in sustainable energy storage systems.

3.2. Advanced Cathode Materials and Structural Engineering

While the optimization of magnesium anodes is crucial, the engineering of cathode materials has also posed significant challenges for the advancement of MIBs [79]. The divalent character of Mg²⁺, while beneficial for high charge storage, results in strong electrostatic interactions with the host lattice [80]. This characteristic imposes limitations on diffusion kinetics and reduces electrochemical reversibility, making cathode innovation a central focus for the realization of practical MIBs [81]. Over the past decade, research efforts have spanned a wide spectrum of material classes—from traditional inorganic compounds to advanced hybrid and nanostructured frameworks—to address the inherent sluggishness of Mg²⁺ migration and improve overall battery performance [82].

Chevrel-phase compounds, such as Mo₆S₈ [83], were among the earliest cathode materials studied for MIBs [84]. Their three-dimensional framework structures allow for reversible Mg²⁺ insertion and extraction, providing moderate capacities with relatively low diffusion barriers and high structural stability over cycling. These compounds have served as foundational model systems, guiding the development of subsequent cathode architectures. In parallel, organic cathode frameworks have emerged as promising candidates, offering tunable redox sites, structural flexibility, and, when engineered appropriately, enhanced electronic conductivity [85]. The diversity of organic materials opens new avenues for overcoming the rigidity and diffusion constraints of inorganic hosts. Transition-metal sulfides and oxides have also shown promise as high-capacity cathodes, though their compatibility with magnesium electrolytes often requires further optimization [86]. To further elevate cathode performance, strategic doping methods have been implemented to modify lattice structure and enhance electrochemical characteristics. For instance, a template-free solvothermal route has enabled the synthesis of TeO₃²⁻-doped CoS/Te composites with varied molar ratios [87]. The resulting skeleton-like morphologies increase the active surface area and facilitate electrolyte interaction, while the expanded lattice spacing reduces Mg²⁺ diffusion barriers. Te doping, in particular, lowers the potential barrier of conversion reaction, thereby accelerating Mg²⁺ storage kinetics and improving cycling stability. The doped cathodes demonstrate robust cycling, maintaining a stable capacity of 30 mAh g⁻¹ for 400 cycles at 200 mA g⁻¹, and exhibit reversible rate performance even at high current densities, validating the effectiveness of anion doping in advanced sulfide cathodes [87].

Spinel oxides are another important class, but many suffer from lattice distortion and oxidation instability, limiting their long-term reliability. Recent advances have focused on inverted spinel oxides such as Mg(Al,Fe,Mn,R)₂O₄ (Figure 6d), synthesized from rare-earth-rich pelagic clay leachates using foamed sol–gel and calcination strategies [88]. The resulting materials achieved a high inversion index (i = 0.62) and large surface area (Figure 6e), facilitating electrochemical reactions. Electrochemical studies showed stable reversible capacities of 96.7 and 102.4 mAh g⁻¹ after 200 cycles (Figure 6f), and a significantly improved Mg²⁺ diffusion coefficient of 1.08 × 10⁻⁵ cm² s⁻¹. These results highlight the potential of using abundant pelagic clays as sustainable precursors for the scalable synthesis of advanced spinel cathodes.

Figure 6. (a,b) TEM images and (c) rate capability of SiOC/Sn nanobeads with different Sn and carbon contents (Reprinted from Ref. [76]. Copyright 2023, Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/). (d) Preparation and (e) TEM image of inverted spinel oxide Mg(Al,Fe,Mn,R)₂O₄. (f) Cycling stability of spinel oxide-based electrode for MIBs (Reprinted from Ref. [88]. Copyright 2024, American Chemical Society).

Figure 6. (a,b) TEM images and (c) rate capability of SiOC/Sn nanobeads with different Sn and carbon contents (Reprinted from Ref. [76]. Copyright 2023, Distributed under a Creative Commons Attribution License 4.0 (CC BY) https://creativecommons.org/licenses/by/4.0/). (d) Preparation and (e) TEM image of inverted spinel oxide Mg(Al,Fe,Mn,R)₂O₄. (f) Cycling stability of spinel oxide-based electrode for MIBs (Reprinted from Ref. [88]. Copyright 2024, American Chemical Society).

In addition to doping, the construction of heterostructures using metal-organic frameworks (MOFs) as templates has received considerable attention. A notable case is the synthesis of NiSe₂–CoSe₂@TiVCTx (NCSe@TiVC) heterostructures (Figure 7a) using MOF templates and co-assembly protocols [89]. By integrating TiVCT_x MXene with Ni–Co selenides, these heterostructures demonstrate enhanced electrical conductivity, structural durability, and rapid charge-transfer dynamics. Electrochemical evaluation shows a specific discharge capacity of 136 mAh g⁻¹ at 0.05 A g⁻¹, with stable cycling performance over 500 cycles. Integration into flexible pouch cells further underscores their suitability for wearable and self-powered energy systems. Mechanistic analyses (XRD, XPS, DFT) confirm the stability of Mg adsorption sites within the heterostructure, pointing to the promise of MXene-based designs for practical magnesium-ion storage. MXene-supported cathodes also address the challenge of strong Coulombic interactions between Mg²⁺ and the host, as hierarchical hybrid structures such as VS₄ nanosheets grown on carbon-coated Ti₃C₂ MXene (VS₄@Ti₃C₂/C) have shown [90]. V–C bond formation ensures strong anchoring of VS₄ to the MXene substrate, promoting accessibility to the electrolyte and facilitating rapid ion migration. These cathodes achieve discharge capacities of 492 mAh g⁻¹ at 50 mA g⁻¹ and remain stable over 900 cycles at 500 mA g⁻¹, demonstrating the transformative potential of MXene-supported hybrids for multivalent-ion batteries.

Morphological engineering provides another path for optimizing Mg²⁺ diffusion and structural robustness. An illustrative example is the development of ball cactus-like MgV₂O₄ cathodes [91], where the unique morphology acts as a structural buffer (Figure 7b,c), mitigating volume changes during cycling and ensuring stable ion diffusion pathways. The material maintains a capacity of 111.7 mAh g⁻¹ after 300 cycles and performs reliably in a wide temperature range (−5 °C to 45 °C), highlighting the importance of morphology control for “all-climate” battery operation. Spinel MgMn₂O₄, known for its high redox potential, remains a compelling cathode candidate, but issues such as phase transitions and instability have hindered its application. Researchers have addressed these issues through the use of a bimetallic Mg–Mn-MOF-74 precursor to confine MgMn₂O₄ nanoparticles (Figure 7d) within ordered porous frameworks [92]. These nanostructured cathodes deliver excellent rate capabilities and retain 91.7% of their capacity after 800 cycles. A self-activation process (Figure 7e) further stabilizes discharge plateaus and enhances long-term cycling, showcasing the effectiveness of MOF-assisted strategies in alleviating structural limitations.

Finally, titanium-based cathodes, such as TiO₂-B—a metastable form with open channels for multivalent ion accommodation—have shown promise. A solvothermal approach was used to dope TiO₂-B nanoflowers with Ni and coat them with reduced graphene oxide/carbon nanotube (RGO@CNT) composite [93]. Ni doping narrows the bandgap and accelerates Mg²⁺ diffusion, while the RGO@CNT shell enhances electrical conductivity and structural stability. The NT-2/RGO@CNT composite achieves 167.5 mAh g⁻¹ at 100 mA g⁻¹, more than doubling the capacity of undoped TiO₂-B, illustrating the efficacy of merging compositional and surface engineering for advanced cathode performance [93]. Thus, the field of cathode research for MIBs has advanced through multifaceted strategies—ranging from lattice and anion doping to hybridization, morphology control, and MOF-enabled synthesis. Each approach addresses specific barriers to Mg²⁺ diffusion, structural integrity, and electrochemical reversibility. The integration of advanced materials such as MXenes, conductive carbons, and innovative organic frameworks has significantly expanded the design landscape. As these approaches converge with scalable synthesis methods, they are poised to yield cathodes with the high capacity, durability, and practicality required propelling magnesium-ion batteries toward widespread commercial adoption.

3.3. Electrolyte and Ionic Transport

MIBs are increasingly recognized as promising contenders within the post-lithium energy storage domain, owing to the natural abundance of magnesium, its inherent safety characteristics, and its exceptional volumetric capacity [30]. For MIBs, initial developments centered around Grignard reagent-based electrolytes (RMgX in ether solvents) and organo-haloaluminate systems, such as RMgCl combined with AlCl₃ in tetrahydrofuran. These formulations enabled reversible magnesium plating and stripping, although they were constrained by drawbacks including low oxidative stability and high nucleophilicity [94,95]. Despite these advantages, the commercial viability of MIBs remains hampered by significant electrolyte-related challenges. The unique electrochemical behavior of magnesium, distinct from lithium, introduces complex obstacles in electrolyte selection and ionic transport that must be overcome to realize efficient and long-lived MIB devices [96]. Consequently, the design and optimization of electrolytes have become focal points of recent research, with notable advances in both nonaqueous and aqueous frameworks.

Unlike lithium, magnesium is prone to forming passivating surface films when exposed to typical nonaqueous electrolytes. These passivation layers impede reversible Mg²⁺ deposition and dissolution, severely restricting cycle life and Coulombic efficiency—one of the most persistent bottlenecks thwarting the commercial realization of MIBs [30]. Recognizing the limitations of nonaqueous electrolytes, researchers have increasingly explored hybrid and aqueous systems to enhance Mg²⁺ transport. One significant advancement is the development of an H-PG-Mg@Li electrolyte, wherein the incorporation of LiCl into a MgCl₂-based polyethylene glycol solvent-in-water matrix yields a hybrid system [97]. The presence of LiCl provides synergistic effects: lowering cell voltage, suppressing dendrite growth, and facilitating the dissolution of MgCl₂ deposits, thereby continually exposing fresh magnesium anode surfaces. The resulting electrolyte exhibits an expanded electrochemical stability window of 3.3 V—approximately 1.5 times broader than that of the LiCl-free system—and achieves an impressive ion transference number of 0.70. When paired with MgO–V₂O₅–P₂S₅ cathodes, this system retained 60% of its capacity after 1000 cycles, and full Mg cells demonstrated stable operation over 150 cycles [97]. Ex situ elemental mapping verified reversible Mg²⁺ accommodation and mitigated charge density effects, thus confirming the practical advantages of LiCl integration with SIW electrolytes for aqueous MIBs.

An alternative approach employs free-water-based electrolytes to further boost ion transport kinetics. In a notable study, nano-amorphous Mg–MnO₂ cathodes were coupled with VO₂ anodes in full MIB cells with unsaturated 2 mol L⁻¹ Mg(NO₃)₂ solutions plus trace MgCl₂ [98]. Upon overdischarge, the crystalline Mg–δ–MnO₂ underwent conversion to a porous amorphous phase, substantially increasing accessible surface area and generating new ion-conductive channels. Concurrently, the VO₂ anode acquired a conductive surface coating, which selectively dissolved and redeposited as a crystalline V₆O₁₃ layer at the interface during overcharge. Cl⁻ ions played a pivotal mechanistic role by facilitating electrode etching, promoting oxygen vacancy formation, and reducing Mg²⁺ desolvation barriers. These synergistic processes enabled accelerated ion transfer, yielding a V₆O₁₃@Mg–MnO₂‖VO₂/V₆O₁₃ full cell that exemplified the potential of free-water electrolytes for high-performance aqueous magnesium batteries. Expanding on aqueous electrolyte design, the “water-in-salt” (WIS) concept has been adapted to magnesium-lithium hybrid systems. In a representative example, a superconcentrated 20 M electrolyte with a fixed Mg²⁺:Li⁺ molar ratio of 1:10 was formulated [99]. With MnO₂/graphene oxide (GO) composites as cathodes and perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) as anodes, this WIS hybrid extended the electrochemical stability window and suppressed active material dissolution, thereby enhancing both energy density and device longevity. The system delivered a reversible capacity of 144.1 mAh g⁻¹ at 1 A g⁻¹, achieving an energy density of 135 Wh kg⁻¹ as well as strong rate performance [99]. Mechanistic studies, including DFT modeling, revealed a unique two-stage discharge: initial simultaneous Mg²⁺ and Li⁺ co-intercalation into the MnO₂/GO lattice, followed by progressive Li⁺ displacement by Mg²⁺ at lower potentials. At the anode, both cations engaged in concerted charge-discharge reactions. This dual-ion exchange was critical to the robust electrochemistry of system, suggesting that WIS hybrid electrolytes are highly promising for next-generation aqueous MIBs.

To integrate the benefits of high ionic conductivity and safety, quasi-solid-state Mg-ion batteries (QSMBs) have been introduced. In a recent example, a QSMB delivered an energy density of 264 Wh kg⁻¹, demonstrating the potential of quasi-solid-state electrolytes for advanced MIBs [100]. Drawing inspiration from lithium-ion battery designs, polyethylene oxide (PEO) was employed to immobilize aqueous Mg electrolytes, resulting in a cell architecture that combines high ionic mobility with a wide electrochemical window (Figure 8a) [100]. The Mg/CuHCF full cell maintained a stable voltage plateau of 2.6–2.0 V and stable cycling over 900 cycles (Figure 8b). In situ analyses and theoretical simulations established that the quasi-solid-state electrolyte suppressed unwanted proton insertion while promoting efficient Mg²⁺ intercalation and further enabled high-voltage (MgCl₃)⁻ anion intercalation/deintercalation, all contributing to improved electrochemical performance. These findings highlight the viability of quasi-solid-state electrolytes in addressing leakage and stability concerns found in liquid systems. A parallel direction involves hybrid sodium/magnesium systems leveraging environmentally friendly electrode materials. For instance, a PTCDA anode was paired with a manganese-based Prussian blue analog (Mn-PBA) cathode [101]. However, Mn dissolution in aqueous electrolytes remained problematic. To resolve this, a triple-electrolyte configuration was devised: initial use of Na₂SO₄-based electrolytes containing Mg²⁺ suppressed PTCDA dissolution and improved reversibility, followed by the addition of Mn²⁺ ions to stabilize the Mn-PBA cathode. PTCDA was chosen for its commercial availability, power capability, and long-term cycling stability [101]. Electrochemical results validated improved durability, establishing the triple-ion electrolyte as a simple yet highly effective stabilization method for aqueous sodium/magnesium batteries.

These developments in electrolyte chemistry—from non-nucleophilic and hybrid aqueous formulations to quasi-solid-state and multi-ion strategies—reflect the diverse tactics being pursued to surmount the fundamental barriers in MIBs. While nonaqueous electrolytes provide stability but suffer from slow kinetics, aqueous systems offer rapid ion transport but require stabilization for longevity. Quasi-solid-state and hybrid designs promise to combine the best attributes of both. The systematic evolution of electrolyte engineering is thus central to the advancement of magnesium-ion batteries, moving them closer to commercial adoption as sustainable, high-performance energy storage solutions for the post-lithium era.

3.4. Interfacial Engineering and Performance Enhancement

Achieving stable cycling of Mg metal anodes remains a pivotal challenge in the advancement of MIBs, principally due to the formation of passivating surface films that significantly hinder ion transport during repeated cycling [102]. One of the most promising approaches to address this limitation is the strategic engineering of SEI layers, which serve to lower interfacial resistance and facilitate efficient Mg deposition and stripping. Notably, early investigations demonstrated that magnesium iodide (MgI₂), formed in situ by introducing iodine additives into the electrolyte, reduced overpotentials and enhanced reversibility of Mg cycling [103]. Moving beyond in situ formation, ex situ fabricated MgI₂ coatings have also been assessed for their effects on electrochemical performance. While pure MgI₂ was only moderately effective for low-overpotential operation, the addition of small quantities of iodine or Bu₄NI₃ markedly improved results. Detailed electrochemical analyses indicated that triiodide (I₃⁻) acted as a solvating mediator, decreasing charge-transfer resistance and promoting uniform Mg²⁺ deposition and stripping. These findings underscore the potential of additive-enhanced SEI engineering for achieving reliable Mg cycling [103].

Artificial protective coatings represent an additional avenue for interfacial stabilization. Through electrodeposition, Mg–Sn@Mg and Mg–Bi@Mg protective layers have been fabricated, imparting rapid ion transport channels and enabling uniform Mg plating and stripping. Symmetric cells utilizing these modified electrodes demonstrated stable cycling for over 1000 h with low polarization, while maintaining excellent rate performance across a broad range of current densities (0.1–1 mA cm⁻²; Figure 8c) [104]. Even at elevated current densities, coated electrodes consistently outperformed their bare Mg counterparts, exhibiting enhanced stability and reduced polarization. When coupled with Mo₆S₈ cathodes in full cells, these protected anodes achieved cycle lifespans exceeding 5000 cycles at 10 C, highlighting the substantial improvements in reversibility and durability afforded by electrodeposited protective layers [104]. While much progress has been made in anode protection, the development of high-performance cathodes remains equally critical for MIB advancement. Conventional intercalation cathodes are hampered by slow Mg²⁺ diffusion, a consequence of the strong electrostatic interactions between Mg ions and host frameworks. To overcome these kinetic barriers, a conversion-type Mg∥Te battery was recently introduced, operating via a reversible two-step conversion mechanism (Te to MgTe₂/MgTe) [105]. This system delivered large discharge capacities of 387 mAh g⁻¹ and retained strong performance at high rates, maintaining 165 mAh g⁻¹ at 5 A g⁻¹. Importantly, the Mg²⁺ diffusivity reached 3.54 × 10⁻⁸ cm² s⁻¹, attributed to the high conductivity of tellurium and the abundance of conversion-active sites. Ab initio molecular dynamics simulations corroborated the rapid transport mechanism, positioning conversion cathodes as an effective solution to the diffusion limitations in traditional MIB cathodes [105].

In addition to conversion cathodes, structural design of intercalation-based materials has yielded notable advances. For instance, a lamellar Mg₀.₇₅V₁₀O₂₄.4H₂O (MVOH) cathode with an expanded interlayer spacing of 13.9 Å was synthesized, incorporating copious lattice water molecules to facilitate Mg²⁺ mobility and confer structural robustness [106]. The presence of mixed-valence V⁴⁺/V⁵⁺ ions enabled electron hopping and further elevated conductivity. Computational studies proposed a “waltz-like” shuttle mechanism in which lattice water dynamically coordinated with Mg²⁺, adjusting orientation to expedite ion migration and stabilize the lattice during cycling (Figure 8d) [106]. This mechanism effectively suppressed proton co-insertion and electrolyte breakdown, frequent issues in aqueous MIBs. Experimentally, the MVOH cathode achieved 350 mAh g⁻¹ at 0.05 A g⁻¹ and retained 70 mAh g⁻¹ at 4 A g⁻¹, while enabling ultralong cycle life of 5000 cycles and an energy density of 67 Wh kg⁻¹.

Figure 8. (a) Schematic representation showing the hydrogen bonding interactions among the ions and water molecules in the case of traditional and water-in-salt electrolyte systems. (b) Galvanostatic discharge profiles of quasi-solid-state Mg/CuHCF full cell (reprinted with permission from Ref. [100]; copyright 2023, American Association for the Advancement of Science). (c) A plot of rate capability of various symmetric cells of Mg-Sn/-Bi-based electrodes at different current densities (reprinted with permission from Ref. [104]; copyright 2024, Royal Society of Chemistry). (d) Pictorial representation of “waltz-like” shuttle pathway of Mg²⁺/H₂O electrolyte system in lamellar Mg₀.₇₅V₁₀O₂₄.4H₂O (MVOH) cathode during the charge-discharge processes (reprinted with permission from Ref. [106]; copyright 2024, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

Figure 8. (a) Schematic representation showing the hydrogen bonding interactions among the ions and water molecules in the case of traditional and water-in-salt electrolyte systems. (b) Galvanostatic discharge profiles of quasi-solid-state Mg/CuHCF full cell (reprinted with permission from Ref. [100]; copyright 2023, American Association for the Advancement of Science). (c) A plot of rate capability of various symmetric cells of Mg-Sn/-Bi-based electrodes at different current densities (reprinted with permission from Ref. [104]; copyright 2024, Royal Society of Chemistry). (d) Pictorial representation of “waltz-like” shuttle pathway of Mg²⁺/H₂O electrolyte system in lamellar Mg₀.₇₅V₁₀O₂₄.4H₂O (MVOH) cathode during the charge-discharge processes (reprinted with permission from Ref. [106]; copyright 2024, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

Hybrid battery architectures that pair magnesium with alkali metals (e.g., lithium, sodium) have also gained attention for their ability to combine the fast kinetics of monovalent ions with the dendrite-free and cost benefits of magnesium. Mg–Li and Mg–Na hybrid systems have demonstrated enhanced energy densities and improved cycling stability relative to pure Mg batteries [107,108]. Recent work has shown that such hybrid frameworks can further boost safety and energy density, leveraging the stable, dendrite-free nature of Mg anodes. In concise, the refinement of interfacial engineering and cathode design stands at the forefront of unlocking MIB performance. Additive-assisted SEI layers and artificial coatings have proven vital for stabilizing Mg anodes, while conversion and advanced intercalation cathodes offer solutions to kinetic constraints. Hybrid systems further expand the design landscape, but progress toward commercial viability will rely on integrated experimental and computational advances to achieve robust, safe, and long-lasting magnesium-ion batteries for large-scale energy storage.

4. Calcium-Ion Batteries (CIBs)

CIBs have recently attracted significant interest within the field of multivalent metal-ion batteries, primarily due to the abundance and low cost of calcium, as well as its advantageous electrochemical characteristics [32]. These features position CIBs as promising contenders for future energy storage technologies, spanning applications from portable devices to grid-scale renewable energy integration. As the energy sector increasingly prioritizes sustainability, the plentiful supply and economic advantages of calcium provide strategic benefits over lithium and cobalt [109].

A key advantage of calcium is its standard redox potential of approximately −2.87 V versus the standard hydrogen electrode, which is comparable to lithium (−3.04 V). This similarity suggests that calcium-based systems can achieve high-voltage operation. The divalent nature of Ca²⁺ allows for the transfer of two electrons per ion during redox reactions, theoretically offering higher energy densities than monovalent systems like lithium-ion or sodium-ion batteries [110]. Beyond electrochemical performance, calcium abundance—ranking among the most common elements of Earth—ensures cost-effective and secure supply chains, overcoming critical resource limitations associated with lithium technologies. Additionally, calcium metal is less reactive under ambient conditions than lithium, enhancing safety and compatibility with standard electrolytes and current collectors. This section reviews recent progress in CIBs, with a focus on design innovations, electrode development, electrolyte systems, and performance optimization.

4.1. Advances in Anode Design and Interface Engineering

The development of CIBs as viable post-lithium energy storage solutions is propelled by favorable redox potential and natural abundance of calcium [111]. However, the divalent charge and relatively large ionic radius of Ca²⁺ introduce challenges, including sluggish ion diffusion and a narrow selection of suitable electrode materials. Overcoming these hurdles requires advances in materials science and interface engineering to enable stable, high-performance anodes. One persistent challenge is the high interfacial resistance observed in calcium metal anodes, which leads to poor cycling stability and limited Coulombic efficiency [112]. To address this, researchers are designing artificial SEIs that act as barriers, facilitating uniform ion transport and suppressing unwanted side reactions [113]. These engineered SEIs decrease interfacial impedance and extend the operational life of calcium electrodes, making them a strong foundation for future CIB architectures.

Beyond SEI modification, alloying calcium with metals such as tin (Ca–Sn) or aluminum (Ca–Al) has proven effective in enhancing electrode stability [111]. Alloy formation improves mechanical robustness and mitigates volume changes during cycling, thereby preventing electrode pulverization and prolonging cycle life. Such alloy-based anodes provide a pathway to higher energy densities and robust performance compared to pure calcium systems. In addition to metal-based solutions, insertion-type anodes have displayed significant promise. For instance, the layered Na₂Ti₃O₇ (NTO) structure features abundant intercalation sites and rapid two-dimensional Ca²⁺ diffusion channels [114]. Electrochemical measurements revealed that NTO delivered a discharge capacity of 165 mAh g⁻¹ at 100 mA g⁻¹ (Figure 9a) and retained 80% capacity after 2000 cycles at 500 mA g⁻¹—figures that surpass many previously reported intercalation anodes. Upon initial discharge, NTO undergoes a transformation to a Ca-intercalated phase (Ca^VIINa^IXTi₃O₇, CNTO; Figure 9b), supporting subsequent reversible ion insertion/extraction. DFT calculations confirmed the energetics and kinetics of this process, underscoring the importance of layered oxides in CIB development [114].

Transition metal dichalcogenides (TMDs) like MoS₂ have also received attention as anode candidates, given their layered structures and high theoretical capacities. However, limited interlayer spacing can impede Ca²⁺ intercalation, necessitating structural engineering. Recent research on expanded-interlayer 1T-phase MoS₂ (ES-1T-MoS₂) has shown substantial improvements in capacity and rate capability [115]. The capacity increased from 29.4 to 91.2 mAh g⁻¹, with robust performance at 2 A g⁻¹ (76.1 mAh g⁻¹) and stability from −20 to 50 °C. Operando analysis and DFT studies attribute these enhancements to increased interlayer spacing and phase stability, facilitating more efficient ion transport. Combined with a Prussian blue analogue cathode, ES-1T-MoS₂ achieves promising full-cell CIB performance. Amorphous materials present another avenue for durable, high-capacity CIB anodes. A breakthrough example is a molybdenum-doped vanadium sulfide composite, synthesized via lattice disruption and in situ activation [116]. This amorphous anode demonstrated discharge capacities of 306.7 mAh g⁻¹ and 149.2 mAh g⁻¹ at 5 A g⁻¹ and 50 A g⁻¹, respectively, with remarkable 2000-cycle stability at 91.2% retention. Mechanistic studies revealed a storage process driven by partial phase transitions, offering new insights into Ca²⁺ transport in disordered sulfide frameworks (Figure 9c) [116].

Figure 9. (a) Comparison of specific capacity of Na₂Ti₃O₇, NTO at various current density with previously reported materials in the literature, and (b) Chemical structures of Na₂Ti₃O₇, NTO and Ca^VIINa^IXTi₃O₇, CNTO (reprinted with permission from Ref. [114]; copyright 2024, American Chemical Society). (c) Formation of Mo-induced amorphous structures of vanadium sulfide through in situ electrochemical activation (reprinted with permission from Ref. [116]; copyright 2024, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (d) Galvanostatic charge-discharge curves of α-MoO₃ along with the chemical structure (reprinted with permission from Ref. [117]; copyright 2022, American Chemical Society).

Figure 9. (a) Comparison of specific capacity of Na₂Ti₃O₇, NTO at various current density with previously reported materials in the literature, and (b) Chemical structures of Na₂Ti₃O₇, NTO and Ca^VIINa^IXTi₃O₇, CNTO (reprinted with permission from Ref. [114]; copyright 2024, American Chemical Society). (c) Formation of Mo-induced amorphous structures of vanadium sulfide through in situ electrochemical activation (reprinted with permission from Ref. [116]; copyright 2024, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (d) Galvanostatic charge-discharge curves of α-MoO₃ along with the chemical structure (reprinted with permission from Ref. [117]; copyright 2022, American Chemical Society).

4.2. Polyanionic and Framework-Based Cathodes

Progress in high-performance CIBs fundamentally depends on the discovery and optimization of effective cathode materials [118]. Unlike lithium-ion systems, CIBs face distinctive obstacles due to the relatively large ionic radius (1.00 Å) and strong Coulombic interactions of Ca²⁺, which hinder rapid diffusion and reversible intercalation. As a result, many cathode structures suitable for lithium are not readily compatible with calcium, prompting the search for alternatives with structural flexibility, voltage stability, and robust cycling behavior. A persistent limitation in CIB technology is the scarcity of cathode materials that enable reversible Ca²⁺ intercalation at high voltage without suffering structural breakdown during prolonged use [119]. Overcoming this challenge requires a deep understanding of ion-host interactions and the development of frameworks offering open channels, minimal migration barriers, and strong lattice integrity. This section reviews advances in polyanionic, Prussian blue analogue, layered oxide, and organic-modified cathodes for next-generation CIBs.

Polyanionic phosphate-based frameworks have emerged as attractive cathodes, valued for their inherent structural stability and facilitation of fast ion movement. Owing to the strong inductive effect of phosphate groups, these materials stabilize transition metal centers in high oxidation states, producing high operating voltages. For example, NaV₂O₂(PO₄)₂F displays a dynamic, electrochemically driven evolution: discrete nanoparticles transform into interconnected VOPO₄ nanosheets via gradual elimination of Na⁺ and F⁻ ions during cycling [120]. The resulting layered structure, with a lattice spacing of 0.68 nm, greatly enhances ion diffusion. This material achieved a diffusion coefficient of 3.19 × 10⁻⁹ cm² s⁻¹—the highest yet reported for inorganic CIB cathodes—together with a capacity near 100 mAh g⁻¹ and robust stability over 1000 cycles [120]. These results underscore the importance of structural adaptability for durable CIB cathodes. Complementing empirical insights, computational high-throughput screening of NaSICON-type frameworks has guided rational design [121]. Ca_XM₂(ZO₄)₃ compounds (M = Ti, V, Cr, Mn, Fe, Co, Ni; Z = Si, P, S) were systematically evaluated for stability and Ca²⁺ migration barriers. Among these, Ca_XV₂(PO₄)₃, Ca_XMn₂(SO₄)₃, and Ca_XFe₂(SO₄)₃ stood out for their balance of lattice robustness and favorable transport properties, whereas silicate-based frameworks proved unstable under CIB conditions [121]. Prussian blue analogues (PBAs) are also promising, given their three-dimensional open frameworks that can accommodate large Ca²⁺ ions without severe lattice strain. Recent advances include compositional tuning to improve durability and charge compensation. For instance, Fe²⁺-enriched copper hexacyanoferrate (CuHCF) cathodes demonstrated improved structural stability during Ca²⁺ cycling [122]. This adjustment minimized lattice distortion (0.13% change), enabled effective Fe²⁺/Fe³⁺ redox activity, and led to a reversible capacity of 54.5 mAh g⁻¹ with 90.43% retention over 1000 cycles—marking a step forward for PBA-based CIBs. These findings highlight the necessity of precise compositional control for enhancing cycling life.

Layered oxide frameworks are also gaining traction in CIB research due to their anisotropic structures and open channels. The layered oxide α-MoO₃, for instance, has been re-examined for its calcium intercalation properties. Recent work clarified the mechanism, reporting a reversible capacity of 165 mAh g⁻¹ at approximately 2.7 V vs. Ca²⁺/Ca, along with favorable rate characteristics and ambient stability (Figure 9d) [117]. These results reinforce the value of nanostructuring for improved insertion kinetics. Similarly, post-spinel CaMn₂O₄, prepared by solid-state synthesis exhibited reversible calcium intercalation at specific potentials and a plateau at −1.5 V. While its capacity (52 mAh g⁻¹ at C/33) is modest, DFT studies confirmed favorable insertion energetics and suggest further optimization is possible [119]. Achieving higher voltages is crucial for energy density, and a major advance was realized with zirconium-doped ammonium vanadium oxide (Zr–NH₄V₄O₁₀), which operates at ~3.0 V, delivers 78 mAh g⁻¹, and achieves 231 Wh kg⁻¹, while retaining 89% capacity over 500 cycles and exhibiting minimal fade [123]. Structural degradation and low conductivity persist as challenges for conventional inorganic hosts. To address this, organic molecule intercalation has been explored. Quinoline-modified V₂O₅ (QVO) expands the V₂O₅ interlayer to 1.25 nm, stabilized by hydrogen bonding [124]. This modification accelerates Ca²⁺ migration and curbs irreversible transitions. QVO achieves 168 mAh g⁻¹ at 1 A g⁻¹ and maintains 80% capacity over 500 cycles at 5 A g⁻¹, with X-ray analysis confirming reversible order-disorder transitions. Such organic-inorganic hybrids represent a promising route to surmounting the rigidity of traditional frameworks.

4.3. Electrolyte Design and Solvation Engineering

The progress of CIBs fundamentally depends on the development of electrolytes that provide stable and efficient Ca²⁺ transport while maintaining compatibility with electrode materials. Electrolyte design directly impacts key parameters such as interfacial stability, ionic conductivity, and the overall electrochemical performance of the battery [125]. In the context of CIBs, early electrolytes comprised high-temperature molten salt mixtures of metal chlorides—such as CaCl₂, KCl, and LiCl—employed in thermal or primary cells operating at temperatures at or above 400–450 °C [126]. Subsequent advancements explored nonaqueous organic electrolytes containing calcium salts like Ca(BF₄)₂ or Ca(ClO₄)₂ in solvents such as acetonitrile, tetrahydrofuran, or propylene carbonate. However, these systems were often hampered by the formation of passivation layers, which severely limited reversible calcium deposition. Recent developments have centered on innovative formulations and solvation engineering, which have addressed longstanding bottlenecks in the field [127]. Initial investigations focused on carbonate-based electrolytes, but these systems proved unstable upon contact with calcium metal, resulting in poor cycling efficiency and limited reversibility [128]. This limitation has driven a transition toward more advanced chemistries, including fluorinated borate-based salts and tailored ionic liquids, both of which offer superior oxidative stability and improved compatibility with calcium metal. These breakthroughs have enabled reversible Ca²⁺ cycling at room temperature, marking a critical step forward for CIB viability. Super-concentrated electrolyte systems have also been introduced to enhance conductivity and expand the voltage window [127]. By optimizing salt concentration and solvent interactions, these electrolytes reduce side reactions while preserving robust ion transport, thereby significantly improving cycling stability and rate capability—bringing CIBs closer to practical application.

Beyond traditional liquid electrolytes, gel polymer electrolyte materials have been developed to address leakage and safety issues that liquid systems often encounter. These gels combine mechanical flexibility with electrochemical stability, making them particularly well-suited for flexible and wearable energy storage applications [129]. The customizable nature of polymer matrices allows for further enhancements in ionic conductivity, supporting their integration into next-generation CIBs. Importantly, beyond the composition of the electrolyte itself, the solvation structure of Ca²⁺ ions is a critical determinant of transport properties. Conventional solvents typically form strong solvation shells around Ca²⁺, resulting in high desolvation energy barriers and sluggish intercalation kinetics. To overcome this, solvation engineering using donor number (DN)-based strategies has been explored. For instance, employing a low-DN solvent like propylene carbonate weakens Ca²⁺ solvation, thus reducing desolvation energy and facilitating faster ion transport across the electrode-electrolyte interface (Figure 10a) [130]. In Na₂V₆O₁₆·2H₂O cathodes, this approach yielded high capacities (376 mAh g⁻¹ at 0.3 A g⁻¹) and strong rate performance (151 mAh g⁻¹ at 5 A g⁻¹), with reversible structural changes verified by V–O bond analysis—demonstrating the power of solvation tuning for high-rate layered oxide cathodes.

Electrolyte engineering has also enabled advancement in dual-ion battery (DIB) systems, where reversible Ca²⁺ intercalation occurs at both electrodes. A notable example uses a highly concentrated calcium bis(fluorosulfonyl)imide (Ca(FSI)₂) electrolyte in carbonate solvents [131]. This configuration supports a Ca-DIB system that offers a discharge capacity of 75.4 mAh g⁻¹ at 100 mA g⁻¹ and maintains 84.7% capacity after 350 cycles, ranking among the most promising CIB systems to date. These results highlight the importance of electrolyte concentration in stabilizing electrode-electrolyte interfaces and enhancing multivalent ion transport. Sustainable aqueous CIB systems are also gaining traction, especially for grid-scale storage. Conventional aqueous alkali-ion batteries are prone to electrode degradation, particularly in PBAs and organic electrodes [132]. Although “water-in-salt” electrolytes improve stability, their reliance on costly fluorinated salts limits scalability. An alternative involves replacing monovalent K⁺ with divalent Ca²⁺ in the electrolyte, which greatly enhances structural integrity for both CuHCF cathodes and polyimide anodes. This adjustment enables rapid charging, extended cycle life, and improved cost-effectiveness, making aqueous Ca-ion batteries attractive for large-scale, budget-conscious applications [133].

In concise, electrolyte design and solvation engineering are central to the evolution of CIBs. Advances—ranging from highly concentrated and gel polymer electrolytes to donor number-based solvation modifications and improved aqueous systems—are overcoming intrinsic challenges in Ca²⁺ transport and interfacial stability. These efforts are broadening the scope of CIB applications, from grid storage to flexible and wearable electronics, and are steadily advancing CIBs toward commercial realization.

4.4. Organic and Hybrid Electrode Architectures

Organic and hybrid electrode systems have rapidly advanced as promising solutions for the persistent challenges faced by CIBs, including sluggish Ca²⁺ diffusion, limited electrolyte compatibility, and structural instability [134]. Organic compounds are especially attractive for both aqueous and nonaqueous CIBs due to their structural tunability, lightweight nature, and adaptable redox properties. Recent efforts have further strengthened these advantages by integrating conductive additives and combining organic-inorganic hybrids to significantly enhance electrochemical performance. This section provides an overview of key developments in organic and hybrid electrode strategies relevant to state-of-the-art CIBs.

Organic electrode materials exhibit notable adaptability in nonaqueous CIBs, primarily because their flexible molecular structures can accommodate large, multivalent Ca²⁺ ions. A prime example is the poly(anthraquinonyl sulfide) (PAQS)/CNT composite, which serves as a high-performance nonaqueous cathode [135]. During discharge, PAQS undergoes reversible carbonyl-based redox reactions, coordinating with both Ca²⁺ and Ca(TFSI)⁺ ions. The inclusion of CNTs markedly increases electrical conductivity and reduces voltage hysteresis, thus improving rate capability and cycling stability. In particular, the PAQS-34 composite (34 wt% CNTs) achieves a specific capacity of 116 mAh g⁻¹ at 0.05 A g⁻¹ and maintains 83% capacity after 500 cycles at 1 A g⁻¹ (Figure 10b) [135]. Computational studies confirm a feasible co-intercalation mechanism, providing valuable design guidance for future organic cathodes in calcium-based systems. Another promising nonaqueous system features 3,4,9,10-perylenetetracarboxylic diimide (PTCDI), a small-molecule organic compound, as a calcium-ion host [136]. In highly saturated electrolytes, PTCDI benefits from suppressed π–π stacking and hydrogen bonding, enabling the formation of robust films and accelerating the enolization-driven Ca²⁺ storage process. A full cell configuration, using PTCDI as the negative and a carbon-based material as the positive electrode, achieved a high power density above 3000 W kg⁻¹ and an energy density around 150 Wh kg⁻¹ [136]. The cell exhibited stable cycling over 4000 cycles and maintained operability at subzero temperatures (−10 °C), highlighting PTCDI suitability for high-power and low-temperature applications. PTCDI has also delivered exceptional results in aqueous CIBs, particularly when combined with water-in-salt electrolytes [137]. This configuration yielded a discharge capacity of 131.8 mAh g⁻¹ and sustained over 68,000 cycles (approximately 470 days) with 72.7% capacity retention. Notably, PTCDI electrodes continued to deliver 86.2 mAh g⁻¹ at 10,000 mA g⁻¹, and advanced characterization techniques (in situ ATR-FTIR and ex situ XPS) confirmed an enolization-driven Ca²⁺ storage mechanism. Computational modeling further highlighted the favorable ion diffusion pathways. When paired with a high-voltage Prussian blue analogue cathode, this system demonstrated reliable performance across a broad temperature range (−20 to 50 °C) in scalable pouch cell formats [137].

Other organic crystalline compounds have proven effective, such as 5,7,12,14-pentacenetetrone (PT), which acts as an efficient aqueous anode [138]. With its π–π stacked layers, PT combines robust structural integrity with rapid ion transport, yielding high capacities of 150.5 mAh g⁻¹ at 5 A g⁻¹ and impressive rate performance of 86.1 mAh g⁻¹ at 100 A g⁻¹ (Figure 10c). Proton-assisted Ca²⁺ insertion and preferential interstitial site occupation stabilize the redox process, while full-cell tests with high-voltage cathodes underscore its practical utility. Additionally, nitrogen-rich covalent organic frameworks (COFs) are being developed as advanced anodes. The TB-COF, abundant in carbonyl and imine groups, achieves a reversible capacity of 253 mAh g⁻¹ at 1 A g⁻¹ and exceptional cycling durability, with only 0.01% decay per cycle over 3000 cycles [139]. Mechanistic investigations reveal that Ca²⁺/H⁺ co-intercalation, facilitated by multiple functional sites including an unexpected C=C active center, is key to performance. Computational studies suggest the architecture can accommodate up to nine Ca²⁺ ions per unit cell, supporting the prospects of COFs in multivalent storage. Integrating organic and inorganic elements yields hybrid electrode solutions that address problems like structural collapse and low conductivity in traditional electrodes. A notable method involves pre-intercalating polyaniline (PANI) into V₂O₅, producing a PANI@V₂O₅ (PVO) cathode with greatly expanded interlayer spacing (13.8 Å; Figure 10d) [140]. This approach reduces electrostatic repulsion, increases electronic conductivity, and introduces supplementary redox-active sites via PANI. Electrochemical tests confirm significantly improved performance, with a capacity of 205 mAh g⁻¹ at 100 mA g⁻¹. DFT analysis corroborates the dual advantages of structural flexibility and enhanced conductivity, underscoring the effectiveness of hybridization in advanced CIB cathodes.

The rise in flexible electronics has also encouraged the development of fiber-shaped CIBs using robust electrolytes [141]. One recent innovation features a free-standing ZnHCF@CF cathode paired with a PANI@CF anode, supporting Ca²⁺/H⁺ co-insertion across a broad voltage window and delivering a volumetric energy density of 43.2 mWh cm⁻³. The device maintains stable operation under mechanical deformation and integrates with fiber-shaped strain sensors for real-time physiological monitoring, highlighting the potential for multifunctional energy storage in smart textiles [141]. Hence, the advancement of organic and hybrid electrode architectures is central to the future of both aqueous and nonaqueous CIBs. Materials such as PAQS, PTCDI, PT, and TB-COF stand out for their excellent rate capabilities, cycle life, and environmental sustainability, while hybrid strategies leveraging conductive polymers and inorganic hosts further improve stability and conductivity. Continued research should prioritize the optimization of electrode-electrolyte interfaces, exploration of multifunctional organic frameworks, and scalable hybrid design for practical deployment. Collectively, these innovations are propelling CIBs toward sustainable, high-performance alternatives to lithium-based batteries.

Figure 10. (a) Plot showing the donor number vs. solvation energy change for various solvents (Reprinted with permission from Ref. [130]. Copyright 2023, John Wiley & Sons, Inc). (b) Cyclic stability of poly(anthraquinonyl sulfide (PAQS) and carbon nanotube (CNT)-based composite electrodes with Coulombic efficiency (reprinted with permission from Ref. [135]; copyright 2022, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (c) Galvanostatic charge discharge curves of 5,7,12,14-pentacenetetrone (PT) at different current densities (reprinted with permission from Ref. [138]; copyright 2021, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (d) High-resolution TEM image of PANI@V₂O₅ (PVO)-based cathode (reprinted with permission from Ref. [140]; copyright 2024, Elsevier).

Figure 10. (a) Plot showing the donor number vs. solvation energy change for various solvents (Reprinted with permission from Ref. [130]. Copyright 2023, John Wiley & Sons, Inc). (b) Cyclic stability of poly(anthraquinonyl sulfide (PAQS) and carbon nanotube (CNT)-based composite electrodes with Coulombic efficiency (reprinted with permission from Ref. [135]; copyright 2022, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (c) Galvanostatic charge discharge curves of 5,7,12,14-pentacenetetrone (PT) at different current densities (reprinted with permission from Ref. [138]; copyright 2021, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (d) High-resolution TEM image of PANI@V₂O₅ (PVO)-based cathode (reprinted with permission from Ref. [140]; copyright 2024, Elsevier).

5. Aluminum-Ion Batteries (AIBs)

AIBs have rapidly emerged as a focal point within the class of multivalent metal-ion batteries, thanks to the distinctive attributes of Al [7]. The trivalent nature of aluminum (Al³⁺) is a particularly compelling advantage, enabling the transfer of three electrons per ion during redox reactions. This property endows AIBs with a theoretical capacity surpassing that of monovalent lithium systems and even divalent metal-ion batteries, positioning them as strong candidates for high-energy-density applications [142].

Aluminum as the third most abundant element in the Earth crust assures a secure and sustainable supply chain, facilitating the large-scale deployment of AIBs [143]. Unlike lithium, which is burdened by both supply chain vulnerabilities and environmental concerns, aluminum offers substantial stability and cost-effectiveness. Another important benefit is the chemical stability of aluminum, which means that AIBs are inherently safer—aluminum does not form dendritic structures during cycling. This feature dramatically reduces the risk of short circuits and thermal runaway, making AIBs especially attractive for electric vehicles, grid storage, and consumer electronics where safety is paramount.

5.1. Electrolyte Optimization and Anode-Free Strategies

Electrolyte optimization is central to the progress of AIBs, as it directly determines capacity, cycling stability, and overall battery efficiency [144]. The majority of conventional AIBs employ chloroaluminate-based ionic liquids, such as 1-ethyl-3-methylimidazolium tetrachloroaluminate (EMIm–AlCl₄), which enable the fundamental reversible deposition and dissolution of aluminum—a prerequisite for functional AIBs. While such electrolytes have enabled the first wave of AIB advancements, their corrosive nature and pronounced moisture sensitivity remain major barriers for scale-up. Corrosive electrolytes complicate cell fabrication and restrict the range of compatible materials, thereby limiting commercial translation.

To overcome these issues, current research is shifting toward chloride-free deep eutectic solvents and other alternative electrolytes that are less aggressive and more environmentally benign [145,146]. These newer systems have demonstrated promising compatibility with aluminum anodes and a variety of cathode materials, though long-term electrochemical stability is still under investigation. Parallel advances in hybrid AIB architectures—such as solid-state and quasi-solid-state formats—are also being pursued to enhance safety, reduce volatility, and improve mechanical robustness [146]. If challenges regarding ionic conductivity and interfacial contact can be resolved, these approaches offer a viable pathway for translating AIBs from laboratory prototypes to real-world products. A unique advantage of AIBs is their potential for anode-free operation. Studies using graphite cathodes with AlCl₃-based ionic liquid electrolytes have shown that a variety of current collectors can facilitate stable charge-discharge cycles without a pre-deposited aluminum anode [147]. This results from the reversible plating and stripping of aluminum directly onto the current collector during cycling. However, this performance is highly sensitive to the corrosion resistance of the substrate in chloride-rich environments. Comprehensive in situ and ex situ spectroscopic studies have validated these mechanisms, providing important guidelines for the design of simplified and efficient multivalent battery systems [147].

Recent AIB research highlights the interplay between electrolyte development and electrode engineering. For instance, customized carbon xerogels paired with a urea–AlCl₃ electrolyte have achieved stable capacities above 300 mAh g⁻¹ for more than 300 cycles, marking a meaningful advancement toward practical AIB implementation [148]. This success is attributed to the favorable physicochemical environment, which enhances ion transport and intercalation kinetics. Electrochemical analysis further indicates improved ionic diffusion and interfacial stability, supporting the scalability of this approach. The adoption of low-cost, non-corrosive components also elevates the economic and environmental sustainability of the technology, reinforcing its suitability for large-scale deployment. The nature of the electrolyte has a substantial influence on the electrochemical dynamics and cycle life of AIBs. Comparative analyses of TEA–AlCl₃ and EMIMCl–AlCl₃ electrolytes, for example, have demonstrated distinct performance profiles. In symmetric aluminum cells at 1 mA cm⁻², TEA–AlCl₃ displayed potentials 60–70 mV higher than EMIMCl–AlCl₃, a result of its superior electrochemical stability [149]. Full cells with TEA–AlCl₃ also achieved higher cut-off voltages (2.45 V versus 2.40 V for EMIMCl–AlCl₃), resulting in an increase in specific capacity from 60 to 94 mAh g⁻¹ after 1000 cycles—a 57% improvement. Structural examinations showed increased defect sites in graphite electrodes, underscoring the critical role of electrolyte-electrode interface in maintaining long-term battery function.

Current density is another key factor influencing aluminum deposition and dissolution. Studies of various AlCl₃-based electrolytes indicate a stability hierarchy: Urea:AlCl₃ > TEA:AlCl₃ > EMIMCl:AlCl₃. In Al–Al symmetric cells, current densities above 1 mA cm⁻² led to pronounced overpotentials, largely due to limited active surface area on the aluminum foil [150]. Full aluminum-graphite batteries exhibited similar behavior, with incomplete AlCl₄⁻ intercalation leading to capacity fade at high rates. To mitigate this, researchers have optimized upper cut-off voltages in accordance with current density and electrolyte stability. This adjustment produced significant gains: at 1 A g⁻¹, energy density increased by 10% in EMIMCl:AlCl₃, 48% in TEA:AlCl₃, and 250% in Urea:AlCl₃ systems [151]. Prolonged cycling under these optimized conditions produced stable performance with minimal degradation, further highlighting the importance of current-density and electrolyte management for achieving high-rate, durable AIBs.

The convergence of advanced electrolyte engineering and innovations in anode-free architectures is bringing aluminum-ion batteries closer to commercial adoption. The evolution from chloride-based and deep eutectic solvent systems to solid-state and quasi-solid-state designs addresses fundamental issues of corrosion, safety, and scalability. Simultaneously, optimizing electrochemical parameters and the synergy between electrode and electrolyte materials have resulted in record-setting capacities and cycle lives. Collectively, these advances chart a clear path toward sustainable, high-performance, and cost-effective aluminum-ion battery systems.

5.2. Advanced Cathode Architectures and Doping Strategies

The quest for high-performance cathode materials remains a primary challenge in the advancement of AIBs, largely due to the substantial size and high charge density of Al³⁺ ions [152]. These properties result in slow ion transport, intense Coulombic interactions, and a tendency toward structural degradation during cycling. Recent research has investigated a variety of solutions, including carbon-based cathodes, targeted doping, development of heterostructures, and the exploration of innovative conversion-type mechanisms [153].

Graphite is among the most thoroughly studied and widely used cathode materials for AIBs [154]. Its layered structure accommodates reversible intercalation of AlCl₄⁻ anions from chloroaluminate-based ionic liquid electrolytes, resulting in stable cycling [150]. Despite these advantages, graphite cathodes are constrained by the formation of graphite-intercalated compounds at full charge, which inherently limits theoretical capacity. In response, polycyclic aromatic hydrocarbons (PAHs) have attracted attention as alternative cathodes. Their extended conjugated π-systems enable strong π–anion interactions with AlCl₄⁻, bypassing the stage limitations typical of graphite. Computational studies have established a direct relationship between capacity and the number of fused benzene rings in PAHs, leading to experimental validation with anthracene. This PAH cathode displayed a specific capacity of 157 mAh g⁻¹ at 100 mA g⁻¹ and retained 130 mAh g⁻¹ after 800 cycles, thus demonstrating both high capacity and cycling durability [155]. This work exemplifies the successful translation of computational predictions into practical organic electrode design. Nonetheless, the inherent limitations of graphite —such as restricted capacity—have motivated the exploration of carbon architectures with greater accessible surface areas and improved conductivity.

A promising approach involves the development of porous carbons with hierarchical architectures. Activated carbon derived from coconut shell chars is illustrative, offering an extensive surface area (2686 m² g⁻¹), pronounced mesoporosity, and a high density of defect sites [156]. These features improve ion accessibility and intercalation capability, resulting in a discharge capacity of 150 mAh g⁻¹ at 0.1 A g⁻¹ and excellent cycling stability over 1500 cycles at 1 A g⁻¹. The scalability and cost-effectiveness of biomass-derived carbons further enhance their appeal for practical AIB deployment, highlighting the importance of high-surface-area carbons for overcoming the intrinsic drawbacks of graphite. Beyond traditional activated carbons, novel carbon allotropes such as graphyne (GY) have shown significant promise in theoretical studies [157]. Common cathode materials frequently encounter issues like substantial volume expansion, ambiguous thermodynamic stability, and high intercalation barriers. First-principles calculations identify α- and γ-graphyne as top candidates for AIB cathodes, primarily due to their expanded interlayer spacing that permits AlCl₄⁻ accommodation [158]. These materials exhibit theoretical capacities around 186 mAh g⁻¹ with open-circuit voltages of 2.18 V (α-GY) and 2.22 V (γ-GY). Notably, α-GY demonstrates a relatively low volumetric expansion (186%), the least among known AIB cathode hosts, and a minimal expansion energy (~0.003 eV Å⁻²), supporting structural reversibility. Nudged elastic band (NEB) calculations reveal low activation barriers—0.26 eV for monolayers and 0.06 eV for bilayers—indicating the potential for rapid ion diffusion compared to conventional hosts. These computational insights position graphyne-based materials as compelling, stable cathodes for advanced AIBs.

In addition to carbonaceous frameworks, heterostructured materials comprising multiple elements have broadened the design space for AIB cathodes. For example, sp-hybridized SiBN nanosheets have been proposed as advanced cathode hosts for their polar-covalent bonds and strong adsorption affinity for Al-Cl clusters [159]. Theoretical analysis predicts a specific capacity of 330 mAh g⁻¹—about six times higher than graphite—alongside low diffusion barriers (0.19–0.22 eV) conducive to fast ion transport. Furthermore, thermodynamic modeling shows negligible chlorine gas evolution, addressing key safety concerns. The structural robustness of SiBN nanosheets is further reinforced through bilayer adsorption-intercalation, making these materials promising for high-energy-density and fast-charging AIBs. Superlattice cathodes such as P-V₂O₅ represent another frontier, where fine-tuning interlayer interactions allows for the simultaneous co-(de)intercalation of anions (AlCl₄⁻) and cations (Al³⁺) (Figure 11b) [160]. This dual mechanism leverages quantum confinement and tunable van der Waals forces to activate both anion and additional O²⁻ redox reactions, enabling a dual redox process. The result is a marked increase in energy density, achieving 466 Wh kg⁻¹ at 107 W kg⁻¹ (Figure 11c) and demonstrating robust cycling (225 mAh g⁻¹ over 3000 cycles at 2.0 A g⁻¹) [160]. By exploiting both anion and cation participation, these advanced architectures overcome the inherent limitations of traditional rocking-chair designs and pave the way for next-generation multivalent storage. Inorganic cathodes are also gaining traction for AIBs. Hexagonal molybdenum oxide (h-MoO₃; Figure 11d), when paired with an AlCl₃–urea electrolyte, illustrates a scalable, cost-efficient system with reduced corrosion [161]. Both structural and electrochemical studies confirm that h-MoO₃ promotes effective insertion and extraction of redox-active species through well-regulated diffusion. The inclusion of carbon nanotubes further enhances capacitive behavior, yielding ~100 mAh g⁻¹ at 100 mA g⁻¹ and ~45 mAh g⁻¹ at 500 mA g⁻¹, with Coulombic efficiencies over 90%.

The field of cathode design for aluminum-ion batteries is rapidly evolving, propelled by innovations in carbon materials, heterostructures, inorganic oxides, and conversion-type electrodes. While graphite remains the benchmark, new candidates—porous carbons, graphyne, SiBN nanosheets, metal-organic frameworks, and superlattice architectures—promise superior energy densities and cycling stability. Conversion-type cathodes, especially those employing solution-mediated mechanisms, offer further potential for high capacity and rapid kinetics. By addressing existing challenges in ion transport, structural durability, and electrolyte compatibility, these strategies could enable aluminum-ion batteries to become a viable, commercial energy storage solution.

5.3. Organic Cathode Development and Molecular Engineering

Organic cathode materials are increasingly recognized as sustainable, cost-efficient alternatives to conventional inorganic electrodes for rechargeable AIBs [162]. Their molecular flexibility allows for tailored electronic structures, multiple redox-active sites, and compatibility with multivalent ion intercalation [163]. Despite these advantages, practical adoption has been hindered by challenges such as modest capacity, solubility in electrolytes, and instability in harsh electrochemical conditions. Recent progress in molecular design—incorporating electron-donating groups, extended conjugation and covalent organic frameworks—has made significant strides toward overcoming these barriers.

Historically, achieving true multivalent ion storage in aluminum-based batteries has been challenging, with most AIBs relying on redox activity involving monovalent complex ions—thus underutilizing the full capacity potential of aluminum. A notable breakthrough is the introduction of tetradiketone macrocycle cathodes capable of reversible divalent AlCl₂⁺ ion storage (Figure 12a) [164]. This material delivers a high capacity of 350 mAh g⁻¹ and retains function over 8000 cycles (Figure 12b), marking a significant advance in multivalent storage. The improved stability is attributed to resonance stabilization, which helps suppress degradation seen in monovalent systems. This discovery opens a new avenue for high-capacity organic cathode development by targeting specific ion valence interactions. Another approach utilizes PTCDA as a cathode with EMIm⁺ chloroaluminate electrolytes. Contrary to earlier assumptions about Al³⁺ intercalation, both experimental and DFT studies show that redox activity is driven by divalent AlCl₂⁺ ions, with a theoretical four-electron transfer per PTCDA molecule and a capacity of 273 mAh g⁻¹ [165]. However, PTCDA solubility in the electrolyte leads to poor reversibility and limited cycle life, despite the promising mechanism. Nonetheless, these insights are crucial for future optimization of organic cathodes for multivalent systems.

The adaptability of organic molecules also enables performance tuning via targeted molecular modifications. For instance, 1-aminopyrene (ANP), a pyrene derivative functionalized with electron-donating amino groups, outperforms traditional p-type organics (Figure 12c) [166]. ANP achieves a capacity of 212 mAh g⁻¹, exhibits excellent rate capability, and retains 106 mAh g⁻¹ after 12,000 cycles at 10 A g⁻¹. Mechanistic analysis attributes these improvements to optimized electron density from the amino group, which stabilizes oxidized states and enhances AlCl₄⁻ ion compatibility. This underscores the impact of strategic molecular engineering on electrochemical performance. To mitigate solubility and degradation, COFs introduce rigid, extended architectures with high chemical stability. Incorporating C–N and C=N functionalities into macrocyclic frameworks increases ion-binding capacity and resistance to aggressive electrolytes. Recent work demonstrates that COFs can reversibly accommodate AlCl₂⁺ ions, achieving 161.2 mAh g⁻¹ at 1 A g⁻¹ and maintaining nearly 100% Coulombic efficiency for over 3000 cycles [167]. Both theoretical and experimental studies confirm stable ion-framework interactions with minimal degradation, positioning COFs as a scalable solution for high-performance AIB cathodes.

Structural asymmetry in organic molecules has also emerged as an effective design strategy. Naphthoquinone (NQ) derivatives, particularly asymmetric isomers like 1,2-NQ, demonstrate stronger binding with AlCl₂⁺ ions than symmetric analogs, leading to improved electrochemical stability. Such materials deliver an initial capacity of 165.9 mAh g⁻¹ and retain 117.2 mAh g⁻¹ after 600 cycles, maintaining 98% Coulombic efficiency [168]. These results validate the concept of leveraging molecular asymmetry to enhance ion interaction and cycling stability. The rapid evolution of organic cathode research for AIBs highlights the power of molecular engineering to address key obstacles such as low capacity, instability, and solubility. Approaches including multivalent ion targeting, π-conjugation, electron-donating substitutions, COF architectures, and asymmetric designs collectively chart the course toward sustainable, high-performance aluminum-ion batteries.

5.4. Sulfur-/Molton-Salts-Based Systems and Chalcogen-Aluminum-Ion Batteries

AIBs have emerged as attractive alternatives to lithium-ion systems, owing to natural abundance, low cost, and high volumetric capacity. AIBs in their early stages made use of aqueous acidic and alkaline electrolytes, including dilute sulfuric acid (H₂SO₄) and sodium hydroxide, as well as aluminum chloride in molten or ionic liquid forms [169]. Early aluminum anode cells—such as “Zn/Hg–Al” systems employed dilute H₂SO₄ or amalgamated aluminum in alkaline solutions as electrolytic media [146]. Within this domain, molten-salt and chalcogen-based systems have garnered particular interest for their potential in large-scale energy storage [170]. These approaches offer the advantages of sustainable materials, high thermal stability, and substantial energy density. Despite these benefits, commercialization has been hampered by challenges such as high operating temperatures, slow reaction kinetics, and safety concerns related to dendrite formation. Recent progress in electrolyte engineering, electrode architecture, and redox chemistry is expanding the operational boundaries of these technologies.

Traditional aluminum-sulfur (Al–S) batteries have required operation at elevated temperatures to maintain a molten state, complicating thermal management and system design. A notable recent advance introduced a quaternary alkali chloroaluminate electrolyte with a low melting point, supporting stable cycling at just 85 °C [171]. This electrolyte features electrochemically active Al–Cl clusters, which facilitate rapid Al³⁺ desolvation and promote high-rate performance even below boiling point. In tandem, a nitrogen-doped porous carbon cathode regulates sulfur electrochemistry and enhances cycle life. The system demonstrated 85.4% capacity retention after 1400 cycles at 1C, while operando X-ray absorption spectroscopy revealed an asymmetric redox pathway for sulfur involving polysulfide intermediates [171]. By reducing the operational temperature, this design streamlines system complexity and strengthens the commercial feasibility of molten-salt Al–S batteries for grid-level applications.

However, AIBs have long been limited by the sluggish kinetics and degradation of solid-to-solid conversion cathodes. A recent innovation leverages solution-to-solid conversion within molten-salt electrolytes, introducing controlled solubility and accelerated reaction kinetics [172]. For example, the use of an InCl/InCl₃ redox couple allows soluble InCl to reversibly convert into sparingly soluble InCl₃ (Figure 12d). This configuration achieves a reversible capacity of ~327 mAh g⁻¹ with an exceptionally low overpotential of 35 mV at 1C and 150 °C (Figure 12e). The cell also displays robust rate capability, retaining 100 mAh g⁻¹ at 50C, and remains stable for over 500 cycles at 20C. In parallel, a carbon-supported indium oxide (In₂O₃@C) cathode exploits a similar reversible solution-to-solid conversion route [173]. Here, In₂O₃ transitions to soluble InCl during discharge, followed by reversible cycling between InCl and InCl₃. The hollow carbon rod structure, decorated with nanoscale In₂O₃, ensures high electronic conductivity and ample reaction sites, supporting efficient precipitation and dissolution. This design achieves 335 mAh g⁻¹ with a low overpotential (50 mV) at 0.2 A g⁻¹ and notable rate performance at 5 A g⁻¹. The inherent self-healing effect of the solution phase further enhances system durability, presenting a promising pathway for advanced aluminum-ion batteries.

Figure 12. (a) Chemical structures and the redox reactions during the charge-discharge cycle, and (b) a plot of cyclic stability of tetradiketone macrocycle-based electrode (reprinted with permission from Ref. [164]; copyright 2021, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (c) Schematic representation showing the effect of electron-donating substituents of pyrene derivatives in the electron transfer capabilities during the redox reactions (reprinted with permission from Ref. [166]; copyright 2022, Elsevier). (d) Pictorial representation of solution-to-solid conversion-type indium chloride (InCl₃)-based cathode in molten salt electrolyte system and (e) the galvanostatic charge-discharge curves of Al||Mo/In full cell at different operating temperatures (reprinted with permission from Ref. [172]; copyright 2023, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

Figure 12. (a) Chemical structures and the redox reactions during the charge-discharge cycle, and (b) a plot of cyclic stability of tetradiketone macrocycle-based electrode (reprinted with permission from Ref. [164]; copyright 2021, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/). (c) Schematic representation showing the effect of electron-donating substituents of pyrene derivatives in the electron transfer capabilities during the redox reactions (reprinted with permission from Ref. [166]; copyright 2022, Elsevier). (d) Pictorial representation of solution-to-solid conversion-type indium chloride (InCl₃)-based cathode in molten salt electrolyte system and (e) the galvanostatic charge-discharge curves of Al||Mo/In full cell at different operating temperatures (reprinted with permission from Ref. [172]; copyright 2023, distributed under a Creative Commons Attribution License 4.0 (CC BY): https://creativecommons.org/licenses/by/4.0/).

Metallic aluminum anodes offer the promise of high energy density and simplified cell design, but dendrite growth has historically posed a major challenge. A recent development introduces a bidirectional aluminum-chalcogen battery utilizing a NaCl–KCl–AlCl₃ molten-salt electrolyte enriched with polymerized Al–Cl clusters, such as Al₂Cl₇⁻ and Al₄Cl₁₃⁻ [174]. These species promote rapid Al³⁺ desolvation and facilitate high faradaic exchange currents, enabling ultrafast charging rates up to 200C without dendrite formation. Unlike conventional systems reliant on low-capacity compound cathodes, this approach uses elemental sulfur, achieving superior energy density and enhanced operational safety. The projected cost of such Al–S batteries is less than one-sixth that of lithium-ion counterparts, while also offering advantages including recyclability, raw material abundance, and inherent fire resistance. Through innovations in electrolyte formulation, redox chemistry, and electrode design, molten-salt and chalcogen-based aluminum batteries are approaching commercial readiness. These advances directly address longstanding barriers of temperature sensitivity, kinetic sluggishness, and dendritic instability, providing a scalable, safe, and cost-effective solution for grid-scale storage in the future.

6. Challenges and Future Perspectives

The advancement of MMIBs—including those based on Zn²⁺, Mg²⁺, Ca²⁺, and Al³⁺—is pivotal for achieving sustainable, cost-effective, and high-performance energy storage [175]. While MMIBs offer inherent benefits such as high volumetric capacity (

Table 1. Characteristics of multivalent metal systems and their device performances [8,9].

Property	Li	Mg	Ca	Al	Zn
Gravimetric Capacity (mAh/g)	>3500	~2000	~1200	~2900	~820
Volumetric Capacity (mAh/cm3)	~2000	~3800	~2000	~8000	~5800
Cycle Life	500–2000 cycles	100–500 cycles	100–500 cycles	100–500 cycles	300–1000 cycles
Operating Temperature	−20 °C to 60 °C	−10 °C to 50 °C	−10 °C to 50 °C	−20 °C to 60 °C	−0 °C to 40 °C

) [8,9] and widespread material availability, their path to commercialization is impeded by substantial scientific and technological hurdles. Realizing their practical potential necessitates a cohesive research approach that addresses challenges in material innovation, electrolyte formulation, interfacial engineering, device architecture, and data-driven design. This section reviews these core issues and outlines the future directions needed to propel MMIB technologies toward widespread adoption.

6.1. Material Innovation: Discovering Optimal Host Structures

A central challenge in MMIB development lies in identifying electrode materials capable of reversibly accommodating multivalent ions. The strong electrostatic interactions and elevated charge density of ions such as Mg²⁺ and Al³⁺ impede solid-state diffusion and result in sluggish charge-discharge kinetics and poor reversibility, restricting overall energy efficiency. Overcoming these limitations requires the design of host structures that balance mechanical robustness with high ionic mobility.

Emerging classes of materials, such as MOFs and COFs, offer promising solutions due to their adjustable porosity, tunable functionality, and chemical versatility [176]. By modulating pore sizes, functional groups, and crystallinity, these frameworks can facilitate multivalent ion diffusion while reducing kinetic bottlenecks. Additionally, layered TMDs—including TiS₂ and MoS₂—exhibit interlayer spacings suitable for accommodating larger multivalent cations [177]. However, achieving a balance between structural flexibility (to enhance transport) and stability (to prevent collapse during cycling) remains a challenge. Future progress should focus on advanced structural engineering methods, such as heteroatom doping, vacancy engineering, and strain modulation, to produce robust hosts with high capacity and extended cycle life.

6.2. Electrolyte Engineering: Ensuring Stability and Compatibility

Electrolyte selection is a critical determinant of the operational stability, voltage window, and efficiency of MMIBs [178]. Aqueous MMIBs, particularly for Zn²⁺, have benefited from water-in-salt and gel polymer electrolytes, which broaden voltage stability, suppress dendrite growth, enhance safety, and extend battery lifespan. In contrast, nonaqueous systems are essential for Mg²⁺, Ca²⁺, and Al³⁺ batteries due to their wider electrochemical windows and ability to support reversible metal deposition and stripping. Recent innovations include borate-based salts, ionic liquids, and deep eutectic solvents, which offer lower nucleophilicity and greater chemical stability, while delivering adequate ionic conductivity. Despite this progress, persistent issues remain, including limited compatibility with metal anodes, electrolyte decomposition, and interfacial passivation.

Hybrid electrolytes, which combine the advantages of both liquid and solid-state systems, are gaining traction as a means to enhance safety without sacrificing performance. Moving forward, it will be vital to deepen our mechanistic understanding of ion solvation, transport, and decomposition at the molecular level—supported by in situ spectroscopy and advanced molecular simulations—to guide the rational design of next-generation electrolytes.

6.3. Interfacial Optimization: Enhancing Stability at the Electrolyte-Electrode Interface

The electrode-electrolyte interface is a decisive factor in MMIB performance and cycle life. For Mg and Ca metal anodes, uncontrolled surface reactions and formation of passivating layers result in low Coulombic efficiency and rapid capacity fade. Effective interfacial engineering is needed to create robust, ion-conductive, and chemically stable SEIs. Recent research has explored artificial SEI coatings, advanced surface treatments, and tailored electrolyte additives to minimize reactivity and improve mechanical properties [102]. Inorganic and polymer-based coatings, in particular, can reduce interfacial resistance and enhance durability.

A deeper understanding of ion transport across interfaces is essential. Recent findings show that coupled cation-anion migration mechanisms can lower diffusion barriers and accelerate charge transfer in solid-state batteries. The future of MMIBs will rely on interface-engineered electrodes analyzed with operando diagnostic tools [179], enabling real-time tracking of interfacial processes and guiding the optimization of interphase chemistry.

6.4. Innovative Device Architecture and Recyle: Bridging Lab and Real-World Applications

In addition to advancements at the material level, rethinking device architectures is critical to ensuring that MMIBs are suitable for a wide range of applications, from electric vehicles and portable electronics to grid-scale renewable integration. Safety, scalability, and mechanical flexibility are increasingly viewed as essential characteristics for next-generation batteries.

Solid-state and quasi-solid-state MMIBs offer superior safety by eliminating flammable liquid electrolytes and enhancing mechanical robustness [180]. However, these architectures often face challenges related to interfacial contact and reduced conductivity. Solutions may include composite solid electrolytes, pressure-assisted cell assembly, and hierarchical or graded electrode structures. Furthermore, commercial viability will depend on scalable, cost-effective manufacturing—such as additive manufacturing, roll-to-roll processing, and low-temperature synthesis—coupled with sustainable raw material sourcing.

Further, LIBs have reached a stage of industrial maturity, as evidenced by their large-scale production—global installed capacity now amounts to several hundreds of gigawatt-hours. Well-established manufacturing processes and significant reductions in production costs, from over US$1000 per kilowatt-hour in the early 2000s to approximately US$200/kWh today [181]. The widespread commercial adoption of LIBs in electric vehicles, grid-scale energy storage, and portable electronics is driven by their high energy density, robust cycle life, and satisfactory safety under operational conditions. In contrast, MIBs remain predominantly at the laboratory or prototype phase [182]. Their development is constrained by several critical challenges, including a scarcity of cathode materials that combine high stability with substantial capacity, persistent issues with electrolyte compatibility—particularly with respect to reversible magnesium plating and stripping or the prevention of electrode passivation—lower practical energy densities, and ongoing difficulties in scaling cell formats from laboratory-scale (e.g., coin cells) to larger, commercially relevant modules such as pouch cells.

Recycling spent multivalent-ion batteries (Zn, Mg, Ca, Al) holds significant environmental, economic, and technological value. Unlike lithium-ion systems [183,184] reliant on scarce resources, these batteries use Earth-abundant, low-cost metals, enabling sustainable material loops and reduced ecological impact. Recycling can recover electrodes, electrolytes, and components, lowering costs and improving supply security for large-scale storage. Technologically, it allows retrieval of high-purity metals for reuse in next-generation devices, addressing resource depletion. However, effective strategies must accommodate unique chemistries—strong Coulombic interactions, passivation layers, and corrosive electrolytes—to ensure recycling aligns with circular economy principles and supports the viability of multivalent battery technologies. These differences demand tailored recycling strategies distinct from those developed for LIBs.

6.5. Data-Driven Discovery: Accelerating Innovation Through Computational Methods

The integration of computational modeling and machine learning (ML) into MMIB research is revolutionizing the pace and precision of materials discovery [185]. High-throughput computational screening rapidly identifies promising electrode and electrolyte chemistries from vast chemical spaces, prioritizing candidates with optimal ion mobility, voltage range, and compatibility. ML models, trained on both experimental and simulated data, can elucidate complex relationships between material structure and electrochemical performance, enabling targeted material design. As databases grow, these predictive models will become increasingly accurate, helping to streamline the path from fundamental research to practical application. The synergy between ML, automated experimentation, and robotics holds the promise of dramatically reducing the time-to-market for new MMIB technologies.

7. Conclusions

The road to commercialization for multivalent batteries involves addressing a complex network of challenges spanning materials, electrolytes, interfaces, device engineering, and data science. Future advances will depend on interdisciplinary collaboration—bringing together electrochemists, materials scientists, engineers, and data specialists—to translate laboratory insights into scalable, market-ready solutions. By systematically overcoming these barriers, MMIBs are poised to become a key technology for next-generation energy storage, underpinning global efforts toward clean energy, decarbonization, and a more sustainable future.