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.     

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.     

Low-Cost Multi-Function Electrolyte Additive Enabling Highly Stable Interfacial Chemical Environment for Highly Reversible Aqueous Zinc Ion Batteries

The practicality of aqueous zinc ion batteries (AZIBs) for large-scale energy storage is hindered by challenges associated with zinc anodes. In this study, a low-cost and multi-function electrolyte additive, cetyltrimethyl ammonium bromide (CTAB), is presented to address these issues. CTAB adsorbs onto the zinc anode surface, regulating Zn²⁺ deposition orientation and inhibiting dendrite formation. It also modifies the solvation structure of Zn²⁺ to reduce water reactivity and minimize side reactions. Additionally, CTAB optimizes key physicochemical parameters of the electrolyte, enhancing the stability of the electrode/electrolyte interface and promoting reversibility in AZIBs. Theoretical simulations combined with operando synchrotron radiation-based in situ Fourier transform infrared spectra and in situ electrochemical impedance spectra further confirm the modified Zn²⁺ coordination environment and the adsorption effect of CTAB cations at the anode/electrolyte interface. As a result, the assembled Zn-MnO₂ battery demonstrates a remarkable specific capacity of 126.56 mAh g⁻¹ at a high current density of 4 A g⁻¹ after 1000 cycles. This work highlights the potential of CTAB as a promising solution for improving the performance and practicality of AZIBs for large-scale energy storage applications.     

Antifreezing Hydrogel with High Zinc Reversibility for Flexible and Durable Aqueous Batteries by Cooperative Hydrated Cations

Hydrogels are widely used in flexible aqueous batteries due to their liquid‐like ion transportation abilities and solid‐like mechanical properties. Their potential applications in flexible and wearable electronics introduce a fundamental challenge: how to lower the freezing point of hydrogels to preserve these merits without sacrificing hydrogels' basic advantages in low cost and high safety. Moreover, zinc as an ideal anode in aqueous batteries suffers from low reversibility because of the formation of insulative byproducts, which is mainly caused by hydrogen evolution via extensive hydration of zinc ions. This, in principle, requires the suppression of hydration, which induces an undesirable increase in the freezing point of hydrogels. Here, it is demonstrated that cooperatively hydrated cations, zinc and lithium ions in hydrogels, are very effective in addressing the above challenges. This simple but unique hydrogel not only enables a 98% capacity retention upon cooling down to ?20 °C from room temperature but also allows a near 100% capacity retention with >99.5% Coulombic efficiency over 500 cycles at ?20 °C. In addition, the strengthened mechanical properties of the hydrogel under subzero temperatures result in excellent durability under various harsh deformations after the freezing process.     

Metal Ions versus Protons: Tracking of Charge-Carrier Insertion into a Cathode Oxide in Aqueous Rechargeable Batteries

Protons in aqueous electrolytes can perform as an additional type of charge carrier for insertion/extraction in addition to the primary carrier cations in aqueous rechargeable batteries. Despite many diverse claims regarding the effect of protons, mutually conflicting experimental results and their interpretations without direct evidence have been reported over the last decade. Systematic examinations and analyses are thus imperative to clarify the conditions of proton insertion in aqueous rechargeable batteries. Utilizing V₂O₅ as a model cathode and beaker-type cells with a sufficient amount of ZnSO₄ aqueous electrolytes in this work, it is demonstrated that protons are inserted into the cathode prior to Zn-ions in low-pH conditions (pH ≤ 3.0). In stark contrast, the influence of protons on the discharge voltage and capacity is insignificant, when either the pH becomes higher (pH ≥ 4.0) or the electrolyte volume is considerably low in coin-type cells. Similar behavior of pH-dependent proton insertion is also verified in Na–, Mg–, and Al-ion electrolytes. Providing a resolution to the controversy regarding proton insertion, the present study emphasizes that the influence of protons substantially varies depending on the pH and relative volume of electrolytes in aqueous batteries.     

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.     

Stable Aqueous Anode-Free Zinc Batteries Enabled by Interfacial Engineering

Anode-free zinc batteries (AFZBs) are proposed as promising energy storage systems due to their high energy density, inherent safety, low cost, and simplified fabrication process. However, rapid capacity fading caused by the side reactions between the in situ formed zinc metal anode and electrolyte hinders their practical applications. To address these issues, aqueous AFZBs enabled by electrolyte engineering to form a stable interphase are designed. By introducing a multifunctional zinc fluoride (ZnF₂) additive into the electrolyte, a stable F-rich interfacial layer is formed. This interfacial layer can not only regulate the growth orientation of zinc crystals, but also serve as an inert protection layer against side reactions such as H₂ generation. Based on these synergy effects, zinc deposition/dissolution with high reversibility (Coulombic efficiency > 99.87%) and stable cycling performance up to 600 h of are achieved in the electrolyte optimized by ZnF₂. With this electrolyte, the cycling life of AFZBs is significantly improved. The work may initiate the research of AFZBs and be useful for the design of high energy, high safety, and low-cost power sources.     

Zincophobic Electrolyte Achieves Highly Reversible Zinc-Ion Batteries

Zinc metal batteries show tremendous applications in wide-scale storages still impeded by aqueous electrolytes corrosion and interfacial water splitting reaction. Herein, a zincophobic electrolyte containing succinonitrile (SN) additive is proposed, the SN electrolyte shows a lower affinity for zinc but a stronger affinity for solid-state interphase (SEI). In the SN electrolyte, zinc hydroxide sulfate (ZHS) is more inclined to accumulate horizontally, forming a dense SEI protective layer on the surface of the Zn anode, effectively slowing down the corrosion of Zn and dendrite growth. The zincophobic SN electrolyte enables excellent performance: zinc plating/stripping Coulombic efficiency of 99.71% for an average of 400 cycles; stable cycles in a symmetric cell for 4000 h (0.9% zinc utilization) and 325 h (86.1% zinc utilization). The soft pack battery using limited zinc delivers maximum energy density of 57.0 Wh kg⁻¹ (based on mass loading of cathode materials and anode materials). Such a simple additive strategy provides a theoretical reference for zinc chemistry in a mild electrolyte environment in practical applications.     

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.     

An Extremely Stable,Highly Soluble Monosubstituted Anthraquinone for Aqueous Redox Flow Batteries

An extremely stable, energy-dense (53.6 Ah L⁻¹, 2 m transferrable electrons), low crossover (permeability of <1 × 10⁻¹³ cm² s⁻¹ using Nafion 212 (Nafion is a trademark polymer from DuPont)), and potentially inexpensive anthraquinone with 2-2-propionate ether anthraquinone structure (abbreviated 2-2PEAQ) is synthesized and extensively evaluated under practically relevant conditions for use in the negolyte of an aqueous redox flow battery. 2-2PEAQ shows a high stability with a fade rate of 0.03–0.05% per day at different applied current densities, cut-off voltage windows, and concentrations (0.1 and 1.0 m ) in both a full cell paired with a ferro/ferricyanide posolyte as well as a symmetric cell. 2-2PEAQ is further shown to have extreme long-term stability, losing only ≈0.01% per day when an electrochemical rejuvenation strategy is employed. From post-mortem analysis (nuclear magnetic resonance (NMR), liquid chromatography–mass spectrometry (LC-MS), and cyclic voltammetry (CV)) two degradation mechanisms are deduced: side chain loss and anthrone formation. 2-2PEAQ with the ether linkages attached on carbons non-adjacent to the central ring is found to have three times lower fade rate compared to its isomer with ether linkages on the carbon adjacent to the central quinone ring. The present study introduces a viable negolyte candidate for grid-scale aqueous organic redox flow batteries.     

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.     

Organic Diamine-Regulated Vanadium Oxides as Cathode Materials for High-Performance Sodium Ion Batteries

Pre-intercalating ions between V O layers is considered to be an effective strategy to modulate the interlayer spacing of 2D vanadium oxides. However, the rigid pre-intercalated ions hardly keep stable during repeated charging/discharging process and their sizes limit the extent of interlayer spacing expansion, which inevitably lead to poor rate capability and cycle stability. In this work, aliphatic diamines are adopted as pre-intercalated guests to elastically modulate the interlayer spacing of V O layers by tuning the chain length of the organic diamine molecules. Benefiting from the strong interaction between the terminal doubly protonated amine and the polar negative oxygen bridge of the V O layers, the aliphatic diamine molecules can act as a structural stabilizer between the layers and boost fast Na ion diffusion (10⁻⁸ to 10⁻¹⁰ cm² s⁻¹). The sodium ion battery based on the first synthesized 1,6-hexanediamine pre-intercalated vanadium oxide supported on nickel foam hybrid cathode achieves a large specific capacity of 597 mAh g⁻¹ at 0.09 A g⁻¹, as well as superior rate performance and cycling stability. This work provides a strategy to elastically modulate 2D layered materials with tunable interlayer spacing for batteries based on large-size-ions.     

Reconstruction of Electric Double Layer for Long-Life Aqueous Zinc Metal Batteries

Aqueous rechargeable Zn metal batteries (AZMBs) have attracted widespread attention due to their intrinsic high volumetric capacity and low cost. However, the unstable Zn/electrolyte interface causes Zn dendrite growth and side reactions, resulting in poor Coulombic efficiency and unsatisfactory lifespan. Herein, a SiO₂ reinforced-sodium alginate (SA) hybrid film is designed to regulate solid–liquid interaction energy and spatial distribution of all species in the electric double layer (EDL) near the Zn electrode. The unique interfacial layer gives rise to a uniform distribution of Zn²⁺ in the Helmholtz layer through solvation sheath modulation. Moreover, theoretical calculations show that the SO₄²⁻ anions and free-water are substantially reduced in the Helmholtz layer, effectively suppressing hydrogen evolution reaction and formation of by-products through strong charge repulsion and hydrogen bond fixing of free-water. The reconfigured EDL not only ensures homogenous and fast Zn²⁺ transport kinetics for dendrite-free Zn deposition, but also eliminates interface parasitic side reactions. The Zn@SiO₂-SA electrode enables excellent cycling stability of symmetrical cells and high-loading full AZMBs with a lifespan over 3000 h and an areal capacity of 2.05 mAh cm⁻², thus laying a solid basis for realizing practical AZMBs.     

High-Energy-Density Quinone-Based Electrodes with [Al(OTF)]2+ Storage Mechanism for Rechargeable Aqueous Aluminum Batteries

Rechargeable aqueous aluminum batteries (AABs) are potential candidates for future large-scale energy storage due to their large capacity and the high abundance of aluminum. However, AABs face the challenges of inferior rate capability and cycling life due to the high charge density of Al³⁺, which induces the sluggish intercalation/extraction dynamics and structure collapse of inorganic cathode materials during discharge–charge cycles. Here, the optimization of macrocyclic calix[4]quinone (C4Q) with a large cavity and multi-adjacent carbonyls structure from quinone compounds to become excellent cathode materials for high-energy-density AABs is reported. It exhibits a high capacity of 400 mAh g⁻¹, a high rate capability (300 mAh g⁻¹ at 800 mA g⁻¹), and an excellent low-temperature performance (224 mAh g⁻¹ at − 20  ° C). The combination of experiments and theoretical calculations proves that Al(OTF)²⁺ cations coordinate with the carbonyl groups of C4Q during the discharge process, which can reduce desolvation penalty. Moreover, the fabricated pouch-type Al-C4Q battery delivers an energy density of 93 Wh kg⁻¹_cell, showing great potential for large-scale applications. This work is expected to facilitate the application of organic cathode for AABs.     

Manipulating Oxygen Vacancies to Spur Ion Kinetics in V2O5 Structures for Superior Aqueous Zinc-Ion Batteries

Vanadium-based intercalation materials have attracted considerable attention for aqueous zinc-ion batteries (ZIBs). However, the sluggish interlaminar diffusion of zinc ions due to the strong electrostatic interaction, severely restricts their practical application. Herein, oxygen vacancy-enriched V₂O₅ structures (Zn_0.125V₂O₅·0.95H₂O nanoflowers, O_v-ZVO) with expanded interlamellar space and excellent structural stability are prepared for superior ZIBs. In situ electron paramagnetic resonance (EPR) and X-ray diffraction (XRD) characterization revealed that numerous oxygen vacancies are generated at a relatively low reaction temperature because of partially escaped lattice water. In situ spectroscopy and density functional theory (DFT) calculations unraveled that the existence of oxygen vacancies lowered Zn²⁺ diffusion barriers in O_v-ZVO and weakened the interaction between Zn and O atoms, thus contributing to excellent electrochemical performance. The Zn||O_v-ZVO battery displayed a remarkable capacity of 402 mAh g⁻¹ at 0.1 A g⁻¹ and impressive energy output of 193 Wh kg⁻¹ at 2673 W kg⁻¹. As a proof of concept, the Zn||O_v-ZVO pouch cell can reach a high capacity of 350 mAh g⁻¹ at 0.5 A g⁻¹, demonstrating its enormous potential for practical application. This study provides fundamental insights into formation of oxygen-vacant nanostructures and generated oxygen vacancies improving electrochemical performance, directing new pathways toward defect-functionalized advanced materials.     

Highly Reversible Anion Redox of Manganese-Based Cathode Material Realized by Electrochemical Ion Exchange for Lithium-Ion Batteries

Anion energy storage provides the possibility to achieve higher specific capacity in lithium-ion battery cathode materials, but the problems of capacity attenuation, voltage degradation, and inconsistent redox behavior are still inevitable. In this paper, a novel O2-type manganese-based layered cathode material Li_x[Li_0.2Mn_0.8]O₂ with a ribbon superlattice structure is prepared by electrochemical ion exchange, which realizes the highly reversible redox of anions and excellent cycle performance. Through low-voltage pre-cycling treatment, the specific capacity of the material can reach 230 mAh g⁻¹ without obvious voltage attenuation. During the electrochemical ion exchange, the precursor with P2 structure transforms into Li_x[Li_0.2Mn_0.8]O₂ with O2 structure through the slippage and shrink of adjacent slabs, and the special superlattice structure in Mn slab is still retained. Simultaneously, a certain degree of lattice mismatch and reversible distortion of the MnO₆ octahedron occur. In addition, the anion redox catalyzes the formation of the solid electrolyte interface, stabilizing the electrode/electrolyte interface and inhibiting the dissolution of Mn. The mechanism of electrochemical ion exchange is systematically studied by comprehensive structural and electrochemical characterization, opening an attractive path for the realization of highly reversible anion redox.     

Ultralow-Salt-Concentration Electrolyte for High-Voltage Aqueous Zn Metal Batteries

Aqueous Zn metal batteries are regarded as a promising pathway for large-scale energy storage systems due to their green, low-cost, and intrinsically safe characteristics. However, they have long been suffered from narrow voltage windows and severe parasitic reactions (e.g., hydrogen evolution, corrosion, etc.), which hinder their further development. The above challenges are essentially related to the existence of hydrated ions (i.e., Zn(H₂O)_x²⁺ and SO₄²⁻·(H₂O)_x), which are highly reactive species. Herein, a counterintuitive ultralow-salt-concentration electrolyte strategy to solve the aforementioned problems by decreasing Zn salt concentration to reduce active hydrated ions is presented, so as to minimize water-induced side reactions and thus anomalously enlarge the electrolyte splitting voltage window. Additionally, the gap between the charge and discharge medium voltages of full cells is also narrowed due to the reduced polarization in the ultralow-salt-concentration electrolyte. By adopting this strategy, the Zn-Fe₄[Fe(CN)₆]₃ full cell stably works at a high-voltage of 1.40–2.30 V with a high cathode loading of ≈7 mg cm⁻² and the Zn-polyaniline full cell can stably work at 0.50–1.50 V with a high cathode loading of ≈11 mg cm⁻².     

Novel Polygonal Vanadium Oxide Nanoscrolls as Stable Cathode for Lithium Storage

Scroll‐shape structures with adjustable space provide interlayer sliding to accommodate the volume changes, which are promising candidates for increasing the stability of lithium batteries (LBs). In this work, for the first time, novel vanadium oxide polygonal nanoscrolls (PNSs) are synthesized in solution through self‐rolling, Ostwald ripening, and scroll‐by‐scroll processes. The PNSs are of various shapes (including triangle, quadrangle, pentagon, and so forth) and spiral‐wrapped multiwall. When evaluated as cathode for LB, the vanadium oxide PNSs cathode exhibits largely enhanced cycling stability (capacity retention of 91.7% after 150 cycles at 0.1 A g^–1 in 2.0–4.0 V) compared with those of nonscrolled nanobelts (40.0%) and nanowires (35.8%). Even at 1.0 A g^–1, the PNSs cathode delivers high‐rate long‐life performance with capacity retention of 80.6% after 500 cycles. The unique polygonal nanoscroll structure is favorable for improving the cyclability and rate capability in energy storage applications as demonstrated here, and it will be interesting and has great potential for other related applications.     

Gel Polymer Electrolyte toward Large-Scale Application of Aqueous Zinc Batteries

Aqueous zinc batteries are promising candidates for energy storage and conversion devices in the “post-lithium” era due to their high energy density, high safety, and low cost. The electrolyte plays an important role in zinc batteries by conducting and separating the positive and negative electrodes. However, the issues of zinc dendrites growth, corrosion, by-product formation, hydrogen evolution and leakage, and evaporation of the aqueous electrolytes affect the commercialization of the batteries. Moreover, the widely used aqueous electrolytes result in large battery sizes, which are not conducive to the emerging smart devices. The intrinsic properties of gel polymer electrolytes (GPEs) can solve the above problems. In order to promote the wider application of GPEs-based zinc batteries, in this review, the working principle and the current problems of zinc batteries are first introduced, andthe merits of GPEs compared to aqueous electrolytes are then summarized. Subsequently, a series of challenges and corresponding strategies faced by GPE is discussed, and an outlook for its future development is finally proposed.     

Phenyl Selenosulfides as Cathode Materials for Rechargeable Lithium Batteries

The Se? Se bond in an organo‐diselenide (RSeSeR, R is an organic group) can break in a 2e^? reduction reaction, but it has limited capacity as a cathode material for rechargeable lithium‐ion batteries. To increase its capacity, redox active species (e.g., sulfur) can be added in the middle of the selenium atoms. Herein, phenyl diselenide (PDSe, PhSeSePh) is mixed with sulfur to form two hybrid compounds with 1:1 and 1:2 molar ratios, which almost double and triple the capacity of PDSe, respectively. Theoretical calculations suggest that phenyl selenosulfide (PDSe‐S, PhSe‐S‐SePh) and phenyl selenodisulfide (PDSe‐S₂, PhSe‐SS‐SePh) can form via addition reactions, which is supported by mass spectrometry analysis. The hybrid materials exhibit three highly reversible redox plateaus and enhanced cycling stability due to the reduced solubility of the discharge products. PDSe‐S and PDSe‐S₂ show initial capacities of 252 and 330 mAh g^?1, respectively, followed by stable cycling performance with a capacity retention of >73% after 200 cycles at C/5 rate. In addition, they show steady rate capabilities. This study reports a novel strategy to increase the electrochemical performance of organo‐diselenide by addition of sulfur.