Anomalous Inferior Zn Anode in High-Concentration Electrolyte: Leveraging Solid-Electrolyte-Interface for Stabilized Cycling of Aqueous Zn-Metal Batteries

Aqueous Zn-metal batteries (AZMBs) are promising large-scale energy storage devices for their high safety and theoretical capacity. However, unstable Zn-electrolyte interface and severe side reactions have excluded AZMBs from long cycling required by practically reversible energy storage. Traditional high-concentration electrolyte is an effective strategy to suppress dendrites growth and resolve the poor electrochemical stability and reversibility of Zn-metal anodes, yet how scientifically universal such strategy is for hybrid electrolyte of different concentrations remains unclear. Herein, we studied the electrochemical behaviors of AZMBs comprising a ZnCl2-based DMSO/H2O electrolyte of two distinct concentrations (1 m vs. 7 m). The electrochemical stability/reversibility of Zn anodes in both symmetric and asymmetric cells with high-concentration electrolytes are unusually inferior to the ones with low-concentration electrolyte. It was found that more DMSO components in the solvation sheath of low-concentration electrolyte exist at the Zn-electrolyte interface than in high-concentration counterpart, enabling higher organic compositions in solid-electrolyte-interface (SEI). The rigid inorganic and flexible organic compositions of SEI decomposed from the low-concentration electrolyte is accounted for improved cycling and reversibility of Zn metal anodes and the respective batteries. This work reveals the critical role of SEI than the high concentration itself in delivering stable electrochemical cycling in AZMBs.