Strategies and Prospects for Engineering a Stable Zn Metal Battery: Cathode, Anode, and Electrolyte Perspectives

Abstract

Zinc metal batteries (ZMBs) appear to be promising candidates to replace lithium-ion batteries owing to their higher safety and lower cost. Moreover, natural reserves of Zn are abundant, being approximately 300 times greater than those of Li. However, there are some typical issues impeding the wide application of ZMBs. Traditional inorganic cathodes exhibit an unsatisfactory cycling lifetime because of structure collapse and active materials dissolution. Apart from inorganic cathodes, organic materials are now gaining extensive attention as ZMBs cathodes because of their sustainability, high environmental friendliness, and tunable molecule structure which make them usually exhibit superior cycling life. Nevertheless, due to the inferior conductivity of organic materials, their mass loading and volumetric energy density still cannot meet our demands. In addition, the specific working mechanism of inorganic/organic cathodes also needs further investigation, necessitating the use of advanced in situ characterization technologies. Reversibility of metallic Zn anodes is also crucial in determining the overall cell performances. Like Li and Na anodes, uncontrolled dendrite growth is also an annoying problem for Zn anodes, which may penetrate the separator and cause inner short circuit. In aqueous electrolyte, highly reactive H₂O molecules easily attack metallic Zn anode, leading to undesired Zn corrosion. Furthermore, during cell operation, hydrogen evolution reaction (HER) occurs, which leads to continuous consumption of electrolytes and formation of insulating byproducts on Zn anodes. Although strategies like novel Zn anode design and artificial SEI layer construction are proposed to inhibit dendrites growth and protect Zn anodes from active H₂O attack, the corresponding manufacturing process remains complex. Modifying electrolyte components is relatively simple to implement and effectively stabilizes Zn anodes. However, HER cannot be completely eliminated when H₂O exists in the modified electrolytes. Under such conditions, nonaqueous electrolytes appear to be a promising solution for ZMBs in the future due to their aprotic nature and high stability with the Zn anodes. However, the ionic conductivity of nonaqueous electrolytes is relatively low compared to that of aqueous electrolytes. Most of the previous reviews focus only on the individual components of ZMBs. A review of ZMBs from a higher perspective, focusing on advanced ZMBs system design, is currently lacking.

In this Account, we begin with a brief overview of ZMBs, highlighting their advantages and current challenges. Subsequently, we give a summary of the development of inorganic cathodes (such as MnO₂) for ZMBs. Specifically, development history and representative modification strategy of inorganic cathodes are illustrated. Following this, representative organic cathodes are discussed, along with introduction of novel modification strategies for organic cathodes. Afterward, Zn anode form design, additive selection and artificial solid electrolyte interface (SEI) layer are briefed for development of Zn anodes. Thereafter, formulation of electrolyte components is systematically discussed, highlighting potential future of nonaqueous electrolyte in ZMBs. Unlike other reviews giving very detailed information in one aspect, this Account offers an overview of current opportunities and challenges faced by ZMBs. We hope this Account can provide researchers with deeper insights into the evolution of ZMBs, encouraging them to devise effective and innovative strategies that will accelerate widespread application of ZMB technology.