Cu substrate as a bi-directional kinetic promoter for high-efficiency four-electron Sn aqueous batteries

Abstract

Aqueous batteries utilizing four-electron tin (Sn) anodes are promising candidates for grid-scale energy storage due to their intrinsic safety and high energy density. However, carbon-based anode substrates exhibit non-uniform Sn deposition and sluggish Sn(OH)₆²⁻/Sn kinetics, limiting voltage efficiency. Here, we employ a copper (Cu) anode substrate that delivers bi-directional kinetic enhancement through Cu–Sn interfacial chemistry. Surface-sensitive analyses unveil an in situ formation of a Cu₆Sn₅ alloy interphase during plating and a surface-bound Sn(OH)_x intermediate during oxidation. Benefiting from this strong Cu–Sn affinity, the Cu substrate eliminates the nucleation voltage spike on charge and reduces the second-step discharge overpotential by ∼300 mV at 1 mA cm⁻², yielding a near-single-plateau voltage profile for this four-electron redox. As a result, the round-trip efficiency of Sn–Ni full cells rises from 70% to 80%, sustained for >200 cycles with improved rate capability. This study underscores the importance of substrate engineering in achieving high efficiency and offers guiding principles for interface-driven optimization in multi-electron aqueous batteries toward practical, long-duration energy storage.