A Reversible Four-electron Sn Metal Aqueous Battery

Sn is a promising metal anode for aqueous batteries due to its dendrite-free plating, large hydrogen evolution overpotential, and high theoretical capacity with up to four-electron redox per Sn atom. However, practically achieving the theoretical capacity for Sn remains challenging, with only limited cell energy densities demonstrated thus far. We validate a kinetically asymmetric [Sn(OH)6]2-/Sn redox pathway involving a direct four-electron plating and a stepwise 2+2 electron stripping through a [Sn(OH)3]- intermediate, which decreases the Coulombic efficiency (CE) by shuttling to the cathode and promoting chemical self-discharge. By using ion-selective membranes to suppress [Sn(OH)3]- crossover, we demonstrate Sn-Ni full cells with high round-trip efficiency (~80%) and energy density (143.1 Wh L-1). The results provide key understandings to the tradeoffs in engineering reversible multi-electron metal anodes and define a new benchmark for practical energy density that exceeds Sn-based aqueous batteries to date.