Progress of Phosphate-based Polyanion Cathodes for Aqueous Rechargeable Zinc Batteries

Aqueous rechargeable zinc batteries (ARZBs) are recently prevailing devices that utilize the abundant Zn resources and the merits of aqueous electrolytes to become a competitive alternative for large-scale energy storage. Benefiting from the unique inductive effect and flexible structure, the past five years have experienced a diversiform of phosphate-based polyanion materials that are used as cathodes in ARZBs. In this review, the most recent advances in the Zn²⁺ storage mechanisms and electrolyte optimization of the phosphate-based cathodes of ARZBs, which mainly focus on vanadium/iron-based phosphates and their derivatives are presented. Furthermore, in addition to significant progress on polyanion phosphate-based cathode materials, the design strategies both for electrode materials and compatible electrolytes are also elaborated to improve the energy density and extend the cycling life of aqueous Zn/polyanion batteries.     

Hydrated Layered Vanadium Oxide as a Highly Reversible Cathode for Rechargeable Aqueous Zinc Batteries

Rechargeable aqueous zinc batteries have gained considerable attention for large‐scale energy storage systems because of their low cost and high safety, but they suffer from limitations in cycling stability and energy density with advanced cathode materials. Here, a high‐performance V₅O₁₂·6H₂O (VOH) nanobelt cathode uniformly located on a stainless‐steel substrate via a facile electrodeposition technique is reported. We show that the hydrated layered VOH cathode enables highly reversible and ultrafast Zn²⁺ cation (de)intercalation processes, as confirmed by various electrochemical, X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy analyses. It is demonstrated that the binder‐free VOH cathode can deliver a discharge capacity of 354.8 mAh g^?1 at 0.5 A g^?1 with a high initial Coulombic efficiency of 99.5%, a high energy density of 194 Wh kg^?1 at 2100 W kg^?1, and a long cycle life with a capacity retention of 94% over 1000 cycles. In addition, a flexible quasi‐solid‐state Zn–VOH battery is constructed, achieving a reversible capacity of ≈300 mAh g^?1 with a capacity retention of 96% after 50 cycles and displaying excellent electrochemical behaviors under different bending states. This work sheds light on the development of rechargeable aqueous zinc batteries for stationary grid storage applications or flexible energy storage devices.     

Sn Alloying to Inhibit Hydrogen Evolution of Zn Metal Anode in Rechargeable Aqueous Batteries

The detrimental hydrogen evolution side reaction is one of the major issues hindering the commercialization of Zn metal anode in high-safety and low-cost rechargeable aqueous batteries. Herein, the authors present a Sn alloying approach to effectively inhibit the hydrogen evolution and dendrite growth of the Zn metal anode. Through in situ monitoring of the hydrogen production during repeated plating/stripping tests, it is quantitatively demonstrated that the hydrogen evolution of alloy electrode with appropriate Sn amount is only half of that of pure Zn electrode. Furthermore, the Sn alloying allows for favorable Zn nucleation sites, lowering the Zn nucleation energy barrier and promoting more uniform Zn deposition. The Zn-Sn alloy electrode offers much-improved plating/stripping cycling, that is, over 240 h at 5 mA cm^?2 and 35.2% depth of discharge. This work provides a practically viable strategy to stabilize Zn metal electrode in rechargeable aqueous batteries.     

Cathode Design for Aqueous Rechargeable Multivalent Ion Batteries: Challenges and Opportunities

With the rapid growth in energy consumption, renewable energy is a promising solution. However, renewable energy (e.g., wind, solar, and tidal) is discontinuous and irregular by nature, which poses new challenges to the new generation of large-scale energy storage devices. Rechargeable batteries using aqueous electrolyte and multivalent ion charge are considered more suitable candidates compared to lithium-ion and lead-acid batteries, owing to their low cost, ease of manufacture, good safety, and environmentally benign characteristics. However, some substantial challenges hinder the development of aqueous rechargeable multivalent ion batteries (AMVIBs), including the narrow stable electrochemical window of water (≈1.23 V), sluggish ion diffusion kinetics, and stability issues of electrode materials. To address these challenges, a range of encouraging strategies has been developed in recent years, in the aspects of electrolyte optimization, material structure engineering and theoretical investigations. To inspire new research directions, this review focuses on the latest advances in cathode materials for aqueous batteries based on the multivalent ions (Zn²⁺, Mg²⁺, Ca²⁺, Al³⁺), their common challenges, and promising strategies for improvement. In addition, further suggestions for development directions and a comparison of the different AMVIBs are covered.     

Ultrafast Rechargeable Aqueous Zinc-Ion Batteries Based on Stable Radical Chemistry

Aqueous zinc-ion batteries (ZIBs) are a promising candidate for fast-charging energy-storage systems due to its attractive ionic conductivity of water-based electrolyte, high theoretical energy density, and low cost. Current strategies toward high-rate ZIBs mainly focus on the improvement of ionic or electron conductivity within cathodes. However, enhancing intrinsic electrochemical reaction kinetics of active materials to achieve fast Zn²⁺ storage has been greatly omitted. Herein, for the first time, stable radical intermediate generation is demonstrated in a typical organic electrode material (methylene blue [MB]), which effectively decreases the reaction energy barrier and enhances the intrinsic kinetics of MB cathode, enabling ultrafast Zn²⁺ storage. Meanwhile, anionic co-intercalation essentially avoids MB molecules rearranging their configuration and sharing Zn²⁺ with adjacent functional groups, thus keeps the structure stable. As a result, Zn–MB batteries exhibit an excellent rate capability up to 500C and ultralong life of 20 000 cycles with a negligible 0.07% capacity decay per cycle at 100C, which is superior to that of most reported aqueous ZIBs batteries. This work provides a novel strategy of stable radical chemistry for ultrafast-charging aqueous ZIBs, which can be introduced to other appropriate organic materials and multivalent ion battery systems.     

The Cathode Choice for Commercialization of Sodium‐Ion Batteries: Layered Transition Metal Oxides versus Prussian Blue Analogs

With the unprecedentedly increasing demand for renewable and clean energy sources, the sodium‐ion battery (SIB) is emerging as an alternative or complementary energy storage candidate to the present commercial lithium‐ion battery due to the abundance and low cost of sodium resources. Layered transition metal oxides and Prussian blue analogs are reviewed in terms of their commercial potential as cathode materials for SIBs. The recent progress in research on their half cells and full cells for the ultimate application in SIBs are summarized. In addition, their electrochemical performance, suitability for scaling up, cost, and environmental concerns are compared in detail with a brief outlook on future prospects. It is anticipated that this review will inspire further development of layered transition metal oxides and Prussian blue analogs for SIBs, especially for their emerging commercialization.     

Toward a Nanoscale-Defect-Free Ni-Rich Layered Oxide Cathode Through Regulated Pore Evolution for Long-Lifespan Li Rechargeable Batteries

Ni-rich layered oxides are envisioned as the most promising cathode materials for next-generation lithium-ion batteries; however, their practical adoption is plagued by fast capacity decay originating from chemo-mechanical degradation. The intrinsic chemical–mechanical instability, inherited from atomic- and nanoscale defects generated during synthesis, is not yet resolved. Here, atomic- and nanoscale structural evolution during solid-state synthesis of Ni-rich layered cathode, Li[Ni_0.92Co_0.03Mn_0.05]O₂, is investigated using combined X-ray/neutron scattering and electron/X-ray microscopy. The multiscale analyses demonstrate the intertwined correlation between phase transition and microstructural evolution, with atomic-scale defects derived from the decomposition of precursors leading to the creation of intra/inter-granular pores. The nucleation and coalescence mechanism of pore defects during the synthesis of Ni-rich layered cathodes are quantitatively revealed. Furthermore, a modified synthetic route is proposed to effectively circumvent the formation of nanoscale defects in Ni-rich layered cathodes by facilitating uniform synthetic reactions, resulting in superior electrochemical and microstructural stability.     

Pivotal Role of Organic Materials in Aqueous Zinc-Based Batteries: Regulating Cathode,Anode, Electrolyte,and Separator

Aqueous zinc-based batteries have garnered considerable interest as promising energy storage devices due to the low cost, remarkable energy density, high safety, and eco-friendliness. However, the mutual challenges of cathode dissolution, electrolyte parasitic reactions, disordered zinc dendrite growth, and easily punctured separator have significantly impeded the widespread commercialization of aqueous zinc-based batteries. Realizing high-performance zinc-based batteries becomes imperative yet remains extremely challenging. To address these concerns, great efforts have recently been made to design high-performance zinc-based batteries. Here the state-of-the-art in organic materials is critically reviewed for aqueous zinc-based batteries, covering main components of a battery. This review provides a comprehensive overview on the design strategies of organic materials for zinc-based batteries, encompassing cathode, anode, electrolyte, and separator. Furthermore, the challenges and prospective research directions are also discussed to provide a guideline for further development of highly stable zinc-based batteries.     

Systematic Modification of MoO3-Based Cathode by the Intercalation Engineering for High-Performance Aqueous Zinc-Ion Batteries

Aqueous Zn-ion batteries are attracting extensive attention, but their large-scale application is prevented by the poor electrochemical kinetics and terrible lifespan. Herein, a strategy of introducing the conductive poly(3,4-ethylenedioxythiophene) (PEDOT) into the interlayers of α-MoO₃ is reported to systematically overcome the above shortcomings. Through data analyses of the cyclic coltammetry, electrochemical impedance spectroscopy, and galvanostatic intermittent titration technique, the electrochemical kinetics of the PEDOT-intercalated MoO₃ (PEDOT-MoO₃) is proved to be significantly improved. The first-principles calculations microscopically disclose that the changed energy band and the lowered binding energy between Zn²⁺ and host O²⁻ boost electrochemical kinetics of PEDOT-MoO₃. Meanwhile, its decreased hydrophilicity and the suppressed dissolution of molybdenum stabilizes the repeated cycling processes. Interestingly, it is found that excellent electrochemical kinetics of cathode electrode can restrain the growth of zinc dendrite on the Zn anode, prolonging the lifespan of aqueous Zn-ion batteries. As a result, the PEDOT-MoO₃ exhibits the enhanced specific capacity (341.5 vs 146.7 mAh g⁻¹ at 0.1 A g⁻¹), high rate capacity (178.2 vs 19.4 mAh g⁻¹ at 30 A g⁻¹) and prolonged cycling stability (77.6% capacity retention over 500 cycles vs 2.3% capacity retention over 100 cycles at 30 A g⁻¹) compared with pristine MoO₃. Moreover, the PEDOT-MoO₃ as cathode of quasi-solid-state ZIBs also delivers an impressive electrochemical performance.     

Defect Modulation in Cobalt Manganese Oxide Sheets for Stable and High-Energy Aqueous Aluminum-Ion Batteries

Rechargeable aqueous Al-ion batteries (AIBs) are promising low-cost, safe, and high energy density systems for large-scale energy storage. However, the strong electrostatic interaction between the Al³⁺ and the host material, usually leads to sluggish Al³⁺ diffusion kinetics and severe structure collapse of the cathode material. Consequently, aqueous AIBs currently suffer from low energy density as well as inferior rate capability and cycling stability. Here, defective cobalt manganese oxide nanosheets are reported as cathode material for aqueous AIBs to improve both reaction kinetics and stability, delivering a record high energy density of 685 Wh kg⁻¹ (based on the masses of the cathode and anode) and a reversible capacity of 585 mAh g⁻¹ at 100 mA g⁻¹ with a retention of 78% after 300 cycles. The impressive energy density and cycling stability are due to a synergistic effect between the substituted cobalt atoms and the manganese vacancies, which improve the structural stability and promote both electron conductivity and ion diffusion. When applied in aqueous Zn-ion batteries, a high specific energy of 390 Wh kg⁻¹ at 100 mA g⁻¹ is realized while retaining 84% initial capacity over 1000 cycles. The study offers a new pathway to building next-generation high-energy aqueous rechargeable metal batteries.     

Controlling Vanadate Nanofiber Interlayer via Intercalation with Conducting Polymers: Cathode Material Design for Rechargeable Aqueous Zinc Ion Batteries

Aqueous zinc ion batteries (ZIBs) are promising energy storage devices due to the high ionic conductivity of the aqueous electrolyte as well as the safety, eco-friendliness, and low cost. Vanadium oxide-based materials are attractive cathode materials for aqueous ZIBs because of their high capacity from their layered structure and multiple valences. However, it is difficult to achieve high cycle stability and rate capability due to the low electrical conductivity and trapping of diffused electrolyte cations within the crystal structure, limiting the commercialization of aqueous ZIBs. In this study, the authors propose a facile sonochemical method for controlling the interlayer of the vanadate nanofiber crystal structure using poly(3,4-ethylene dioxythiophene) (PEDOT) to overcome the shortcomings of vanadium oxide-based materials. In addition, the electrochemical correlation between the interplanar distance of the expanded vanadate layers by the insertion of PEDOT and the behavior of Zn²⁺ ions is investigated. As a result, the intercalation of the conducting polymer increases the electron pathway and extends the distance of the vanadate layers, which helps to increase the number of active sites inside the vanadate and accelerate the zinc ion intercalation/de-intercalation process. Their findings may guide research on the next generation of ZIBs that can replace lithium ion batteries.     

Synergistic Effect between S and Se Enhancing the Electrochemical Behavior of SexSy in Aqueous Zn Metal Batteries

Developing high-capacity conversional cathode materials for aqueous Zn batteries is promising to improve their energy densities but challenging as well. In this work, three kinds of selenium–sulfur solid solutions and their composites (denoted as SeS₁₄ @ 3D-NPCF, SeS_5.76 @ 3D-NPCF, and SeS_2.46 @ 3D-NPCF) are proposed and systematically investigated. Due to the introduction of Se and its synergistic effect with S, their physical and electrochemical properties are manipulated; in particular, by optimizing the Se content in these composites, SeS_5.76 @ 3D-NPCF shows a capacity of 1222 mAh g⁻¹ and flat plateau of 0.71 V at 0.2 A g⁻¹, reaching an ultrahigh energy density of 867.6 Wh kg⁻¹ (based on SeS_5.76), superior rate capacity of 713 mAh g⁻¹ at 5 A g⁻¹, and stable cycling of 75% capacity retention after 500 cycles. In addition, the Zn storage kinetics is determined by the discharge process, during which SeS_5.76 @ 3D-NPCF is converted into ZnSe and ZnS. More importantly, theoretical calculations reveal that Se can tailor the electron density difference, band structure, and reaction energy of S, which increase its conductivity and reactivity to facilitate the electrochemical reaction with Zn. This work explores high performance conversional cathode materials for aqueous Zn metal batteries and presents an effective strategy to modify their intrinsic properties.     

Understanding the Stability for Li‐Rich Layered Oxide Li2RuO3 Cathode

Lithium‐rich layered oxides are considered as promising cathode materials for Li‐ion batteries with high energy density due to their higher capacity as compared with the conventional LiMO₂ (e.g., LiCoO₂, LiNiO₂, and LiNi_1/3Co_1/3Mn_1/3O₂) layered oxides. However, why lithium‐rich layered oxides exhibit high capacities without undergoing a structural collapse for a certain number of cycles has attracted limited attention. Here, based on the model of Li₂RuO₃, it is uncovered that the mechanism responsible for the structural integrity shown by lithium‐rich layered oxides is realized by the flexible local structure due to the presence of lithium atoms in the transition metal layer, which favors the formation of O₂^2?‐like species, with the aid of in situ extended X‐ray absorption fine structure (EXAFS), in situ energy loss spectroscopy (EELS), and density functional theory (DFT) calculation. This finding will open new scope for the development of high‐capacity layered electrodes.     

Electrochemical Overview: A Summary of ACoxMnyNizO2 and Metal Oxides as Versatile Cathode Materials for Metal-Ion Batteries

Early LiCoO₂ research provided the basis for the tremendous commercial success of Li⁺ batteries since their invention in the early 1990s. Today, LiNiMnCoO₂ (Li-NMC) is one of the most widely used batteries in the rapidly evolving electronic vehicle industry. Li-NMC batteries continue to receive significant interest as research efforts aim to partially, or entirely, replace the use of scarcely available and toxic Co with elemental doping to form binary, ternary, and quaternary layered oxides. Furthermore, safety concerns and rising uncertainty for the future of Li supplies have resulted in growing curiosity toward non-Li⁺ rechargeable batteries such as Na⁺ and K⁺. Unfortunately, the success of Li⁺ host materials does not always directly transfer to Na⁺ and K⁺ batteries due to the difficulty of reversibly intercalating larger ions without irreparably distorting the host structure. Consequently, this report provides an overview of the Li-based materials surrounding the success of commercial Li-NMC and the subsequent progress of their lesser studied Na and K counterparts. The challenges for current cathode materials are highlighted, and the opportunities for progression are suggested. The summary presented in this review can be consulted to steer new and unique research avenues for layered oxide materials as metal-ion battery cathodes.     

A Molecular‐Cage Strategy Enabling Efficient Chemisorption–Electrocatalytic Interface in Nanostructured Li2S Cathode for Li Metal‐Free Rechargeable Cells with High Energy

Using high‐capacity and metallic Li‐free lithium sulfide (Li₂S) cathodes offers an alternative solution to address serious safety risks and performance decay caused by uncontrolled dendrite hazards of Li metal anodes in next‐generation Li metal batteries. Practical applications of such a cathode, however, still suffer from low redox activity, unaffordable cost, and poor processability of infusible and moisture‐sensitive Li₂S. Herein, these difficulties are addressed by developing a molecular cage–engaged strategy that enables low‐cost production and interfacial engineering of Li₂S cathodes for rechargeable Li₂S//Si cells. An efficient chemisorption–electrocatalytic interface is built in extremely nanostructured Li₂S cathodes by harnessing the confinement/separation effect of metal–organic molecular cages on ionic clusters of air‐stable, soluble, and low‐cost Li salt and their chemical transformation. It effectively boosts the redox activity toward Li₂S activation/dissociation and polysulfide chemisorption–conversion in Li‐S batteries, leading to low activation voltage barrier, stable cycle life of 1000 cycles, ultrafast current rate up to 8 C, and high areal capacities of Li₂S cathodes with high mass loading. Encouragingly, this highly active Li₂S cathode can be applied for constructing truly workable Li₂S//Si cells with a high specific energy of 673 Wh kg^?1 and stable performance for 200 cycles at high rates against hollow nanostructured Si anode.     

Application of 2D MXene in Organic Electrode Materials for Rechargeable Batteries: Recent Progress and Perspectives

Organic electrode materials (OEMs) are emerging green power because of the promising advantages such as environmental friendliness, abundant sources, easy recycling, and structural diversity. However, several inherent issues, including low electronic conductivity, dissolution of active materials, and particle pulverization restrict their practical application. MXene, as a novel 2D material has exhibited enormous potential to solve the issues of OEMs due to its high conductivity, unique structure, exceptional mechanical property, and abundant surface groups. Up to now, various effective strategies have been presented and achieved positive effects, such as constructing heterojunction structures, in situ assembly, dip-coating, preparing free-standing MXene paper, etc. Nonetheless, comprehensive review of the progress and status is rare. Herein, an overview of the application of MXene in organic electrode materials for rechargeable batteries is systematically put forward. Meanwhile, recent progress and future development directions are presented. This review can serve as a guide for future research.     

A Synergistic System for Lithium–Oxygen Batteries in Humid Atmosphere Integrating a Composite Cathode and a Hydrophobic Ionic Liquid‐Based Electrolyte

Moisture in air is a major obstacle for realizing practical lithium‐air batteries. Here, we integrate a hydrophobic ionic liquid (IL)‐based electrolyte and a cathode composed of electrolytic manganese dioxide and ruthenium oxide supported on Super P (carbon black) to construct a promising system for Li‐O₂ battery that can be sustained in humid atmosphere (RH: 51%). A high discharge potential of 2.94 V and low charge potential of 3.34 V for 218 cycles are achieved. The outstanding performance is attributed to the synergistic effect of the unique hydrophobic IL‐based electrolyte and the composite cathode. This is the first time that such excellent performance is achieved in humid O₂ atmosphere and these results are believed to facilitate the realization of practical lithium‐air batteries.     

Potassium Prussian Blue Nanoparticles: A Low‐Cost Cathode Material for Potassium‐Ion Batteries

Potassium‐ion batteries (KIBs) in organic electrolytes hold great promise as an electrochemical energy storage technology owing to the abundance of potassium, close redox potential to lithium, and similar electrochemistry with lithium system. Although carbon materials have been studied as KIB anodes, investigations on KIB cathodes have been scarcely reported. A comprehensive study on potassium Prussian blue K_0.220Fe[Fe(CN)₆]_0.805?4.01H₂O nanoparticles as a potential cathode material is for the first time reported. The cathode exhibits a high discharge voltage of 3.1–3.4 V, a high reversible capacity of 73.2 mAh g^?1, and great cyclability at both low and high rates with a very small capacity decay rate of ≈0.09% per cycle. Electrochemical reaction mechanism analysis identifies the carbon‐coordinated Fe^III/Fe^II couple as redox‐active site and proves structural stability of the cathode during charge/discharge. Furthermore, for the first time, a KIB full‐cell is presented by coupling the nanoparticles with commercial carbon materials. The full‐cell delivers a capacity of 68.5 mAh g^?1 at 100 mA g^?1 and retains 93.4% of the capacity after 50 cycles. Considering the low cost and material sustainability as well as the great electrochemical performances, this work may pave the way toward more studies on KIB cathodes and trigger future attention on rechargeable KIBs.     

Achieving Synergetic Anion-Cation Redox Chemistry in Freestanding Amorphous Vanadium Oxysulfide Cathodes toward Ultrafast and Stable Aqueous Zinc-Ion Batteries

Flexible aqueous zinc-ion batteries (AZIBs) with high safety and low cost hold great promise for potential applications in wearable electronics, but the strong electrostatic interaction between Zn²⁺ and crystalline structures, and the traditional cathodes with single cationic redox center remain stumbling blocks to developing high-performance AZIBs. Herein, freestanding amorphous vanadium oxysulfide (AVSO) cathodes with abundant defects and auxiliary anionic redox centers are developed via in situ anodic oxidation strategy. The well-designed amorphous AVSO cathodes demonstrate numerous Zn²⁺ isotropic pathways and rapid reaction kinetics, performing a high reversible capacity of 538.7 mAhg^-1 and high-rate capability (237.8 mAhg^-1@40Ag^-1). Experimental results and theoretical simulations reveal that vanadium cations serve as the main redox centers while sulfur anions in AVSO cathode as the supporting redox centers to compensate local electron-transfer ability of active sites. Significantly, the amorphous structure with sulfur chemistry can tolerate volumetric change upon Zn²⁺/H⁺ insertion and weaken electrostatic interaction between Zn²⁺ and host materials. Consequently, the AVSO composites display alleviated structural degradation and exceptional long-term cyclability (89.8% retention after 20 000 cycles at 40 Ag^-1). This work can be generally extended to various freestanding amorphous cathode materials of multiple redox reactions, inspiring development of designing ultrafast and long-life wearable AZIBs.