Unraveling the significance of the 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 anodes, thereby enhancing their electrochemical performance. While most WISEs correlate total salt concentration with water content and the Zn²⁺ solvation structure, the impact of the Zn ratio has been largely overlooked. In this work, we prepared a range of WISEs with varying Zn molar ratios while ensuring low water content, and scrutinized the impact on Zn nucleation behavior, Zn plating/stripping overpotential, and overall Zn metal anode stability. Our results demonstrated that the increase in Zn content in the electrolyte promotes a high transference number and the 3D Zn²⁺ diffusion process, – thus enhancing the Zn anode stability and reversibility. Moreover, we identified an optimized Zn molar ratio of approximately 0.5, suggesting that beyond this threshold, the impact 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 the Zn ratio in the future design of WISEs for high-performance Zn-ion batteries.