Electrolyte Design for High–Rate Performance Biphasic Membrane–Free Batteries

ABSTRACT Biphasic membrane–free (BMF) batteries hold great promise for energy storage applications. However, their commercialization is hindered by their limited rate performance. To address this issue, this study investigates each step during BMF battery reaction, and elucidates the role of supporting electrolyte salt ion transfer across liquid–liquid (L/L) interface—a unique feature of BMF systems. A thin–film electrode system is developed to evaluate ion transfer potential and rate. The findings reveal that ion transfer across the L/L interface is the rate–determining step. Based on this insight, supporting electrolyte salts are designed—particularly using PF 6 −— to enable stable charge–discharge current of 11 mA cm −2 (4 C) of BMF batteries (0.1 M ZnSO 4 /Zn, TEMPO/graphite), considerably surpassing the performance of previously reported BMF batteries. Notably, this study introduces a novel strategy utilizing thin–film electrodes to guide the design of supporting electrolyte salts for enhanced rate performance. Rapid screening of electrolyte salts is achieved by measuring ion transfer potentials, with lower potentials correlating with improved rate performance. Furthermore, square wave voltammetry is employed to analyze ion transfer rates and define the applicable rate range of supporting electrolyte salts, ultimately facilitating the development of high‐rate BMF batteries.