Dendrite-Free Sn Anode with High Reversibility for Aqueous Batteries Enabled by "Water-in-Salt" Electrolyte

Metallic tin (Sn) has always been considered as an anode candidate for nonaqueous lithium-ion batteries (LIBs). Nevertheless, there were hardly any attempts to use Sn as an anode material in aqueous batteries. Although Sn possesses relatively high specific capacity (451.54 mAh/g, two-electron transfer reaction), mild electrode potential (-0.1375 V vs the standard hydrogen electrode, Sn2+ + 2e(-) = Sn), high hydrogen overpotential, and environmental friendliness, it also suffers from bad reversibility and low Coulombic efficiency (CE) in common aqueous electrolytes. In this work, a dendrite-free Sn anode with highly improved reversibility in an aqueous electrolyte was realized using a type of "water-in-salt" electrolyte (1 m Sn(OTf)(2) + 20 m LiTFSI). The results of Fourier transform infrared (FTIR) spectra and density functional theory (DFT) calculations have verified the significant impact of highly concentrated LiTFSI salt on Sn2+ hydrolysis, which is the key factor in improving the reversibility of the Sn anode and suppressing the dendrite formation. A Sn/LiFePO4 cell applying this electrolyte was acquired with a discharge plateau at around 0.6 V. In the cyclability test, the Sn anode exhibited a stable discharge capacity of more than 107 mAh/g with a Coulombic efficiency (CE) of 95% for 35 cycles (0.2C rate).