A Low-Acidity Chloride Electrolyte Enables Exceptional Reversibility and Stability in Aqueous Tin Metal Batteries

Tin (Sn) metal has emerged as a promising anode for aqueous batteries, due to its high capacity, non-toxicity, and cost-effectiveness. However, Sn metal has often been coupled with strong and corrosive sulfuric acids (2-3 M), leading to severe electrode corrosion and hydrogen evolution issues. Although high efficiency and long cycling were reported, the results were achieved using high currents to kinetically mask electrode-electrolyte side reactions. Herein, we introduce a low-acidity tin chloride electrolyte (pH=1.09) as a more viable option, which eliminates the need of strong acids and enables a reversible dendrite-free Sn plating chemistry. Remarkably, the plating efficiency approaches unity (99.97 %) under standard testing conditions (1 mA cm^-2 for 1 mAh cm^-2), which maintains high at 99.23-99.93 % across various aggressive conditions, including low current (0.1-0.25 mA cm^-2), high capacity (5-10 mAh cm^-2), and extended resting time (24-72 hours). The battery calendar life is further prolonged to 3064 hours, significantly surpassing literature reports. Additionally, we presented an effective method to mitigate the potential Sn²⁺ oxidization issue on the cathode, demonstrating long-cycling Sn||LiMn₂O₄ hybrid batteries. This work offers critical insights for developing highly reversible Sn metal batteries.