Unraveling the Significance of Zinc Ratio in Water-in-Salt Electrolytes

Abstract

Water-in-salt electrolytes (WISEs) have been proposed as an effective approach to suppress the side reactions related to free water activity on zinc (Zn) metal anode, thereby enhancing its electrochemical performance. While most WISEs correlate total salt concentration with water content and Zn2+ solvation structure, the impact of Zn ratio has been largely overlooked. In this work, we prepared a range of WISEs with varying Zn ratios while ensuring low water content, and scrutinized their effects on Zn nucleation behavior, Zn plating/stripping overpotential, and overall Zn metal anode stability. Our results demonstrated that the increase of Zn content in the electrolyte promotes high transference number and the 3D Zn2+ diffusion process – thus enhancing the Zn anode stability and reversibility. Moreover, we identified an optimized Zn ratio of approximately 0.5, suggesting that beyond this threshold, the impact of Zn ratio diminishes. Our findings show that reaching the WISE region is insufficient to improve Zn metal performance; instead, an optimal concentration of Zn2+ ions is the determining factor. Consequently, we advocate for the optimization of Zn ratio in the future design of WISEs for high-performance Zn-ion batteries.