Electrolyte Regulation Strategies for Improving the Electrochemical Performance of Aqueous Zinc-ion Battery Cathodes

Abstract: The ever-worsening world-wide energy crisis and environmental issues are encouraging the development of green and renewable energy. Thus, rechargeable batteries are being developed and employed for energy storage and conversion in various electronic equipment. When compared with metal lithium batteries, aqueous rechargeable batteries have gained significant attention due to their advantages of high safety, low cost, and environmental friendliness. Among the various known rechargeable batteries (Li+, Na+, K+, NH4+, Mg2+, Ca2+, and Al3+), aqueous zinc-ion batteries (ZIBs) are considered as promising energy storage devices because the zinc electrode exhibits high capacity (820 mAh∙g-1) and low potential (-0.76 V vs. Standard hydrogen electrode (SHE)). To date, various ZIBs cathode materials with excellent performance have been developed, such as manganese- and vanadium-based oxides, Prussian blue and its analogues, and organic compounds. Unfortunately, some of these materials, especially manganese- and vanadium-based oxides, suffer from critical structural collapse, dissolution, and cathode/electrolyte interfacial side reactions, which lead to low Coulombic efficiency and poor cycle performance. The poor cycle performance is one of the main obstacles hindering the large-scale application of manganese- and vanadium-based oxides. Therefore, the structural design of cathodes and electrolyte regulation strategies have been extensively investigated to solve these problems and improve electrochemical performance. In comparison, electrolyte regulation is an important and effective strategy for improving the performance of ZIBs cathodes. It is well known that a strong interaction force exists between Zn2+ and H2O, therefore, Zn2+ can coordinate with six H2O molecules to form[Zn(H2O)6]2+ in the dilute aqueous electrolyte, while forming numerous hydrogen bonds between the H2O molecules. The Zn2+-solvation structure and hydrogen bonds can be destructed and restructured by changing the anion, and using highly concentrated electrolyte and/or organic solvent, thereby decreasing the number of H2O molecules in the solvated structure and the activity of free water. Furthermore, additives can change the pH value of the aqueous electrolyte and build a dissolution equilibrium between the cathode and electrolyte. Hence, an appropriate electrolyte regulation strategy can broaden the electrochemical stability window of electrolytes, improve the working potential, suppress the occurrence of interfacial side reactions, and prevent the dissolution of the active materials, thereby improving the electrochemical performance of ZIBs. Herein, we review the possible electrolyte regulation strategies for enhancing the electrochemical performance of ZIBs cathodes and classify regulation strategy into two main categories:1) Solute (including different zinc salts, additive, and water-in-salt) and 2) Solvent (composite of organic/inorganic hybrid electrolytes). We then discuss the advantages and challenges of each strategy, and finally predict the possible future direction of electrolyte development.