A reversible four-electron Sn metal aqueous battery

Sn is a promising metal anode for aqueous batteries, with up to four-electron redox available per atom (903 mAh g⁻¹_Sn). However, practically harnessing the four-electron Sn(OH)₆²⁻/Sn reversibility remains challenging due to limited mechanistic understanding. Here, we reveal a kinetically asymmetric redox pathway involving a successive four-electron plating and a stepwise 2 + 2 electron stripping through a Sn(OH)₃⁻ intermediate. The crossover of Sn(OH)₃⁻ induces a reversible self-discharge that reduces Coulombic efficiency but does not impact cyclability, demonstrated by four-electron Sn-Ni full cells that sustain >800 h of stable cycling. By tuning the ion selectivity of the separator to suppress Sn(OH)₃⁻ crossover while allowing OH⁻ transport, we further demonstrate high Sn utilization (67%) and high energy density (143.1 Wh L⁻¹_cell). The results provide key understandings of the tradeoffs in engineering reversible multi-electron metal anodes and define a new benchmark for practical energy density that exceeds any Sn-based aqueous batteries to date.