Rational Design of Moderately Concentrated Electrolytes to Consolidate the Electrode‐Electrolyte Compatibility for Ultrahigh‐Efficiency and Harsh‐Condition‐Tolerant Aqueous Tin Metal Batteries

Aqueous tin (Sn) metal batteries have attracted growing attention for energy storage; however, the drastic Sn 2+ hydrolysis reaction has significantly hindered their development. Currently, strong alkaline and acidic solutions prevail as the leading method to suppress hydrolysis and stabilize Sn 2+ electrolytes. Nevertheless, this approach incurs critical issues, including the self‐corrosion of Sn metal, parasitic hydrogen evolution, and inferior efficiency under harsh conditions. Herein, we rationally designed an inexpensive and moderately concentrated electrolyte of 0.2 m SnSO 4 + 5 m (NH 4 ) 2 SO 4 , which overcomes the hydrolysis challenge by strengthening H 2 O molecules and regulating Sn 2+ /H 2 O interactions. Importantly, our electrolyte features a mild pH environment (pH = 3.2) and outperforms the aggressive alkaline (pH > 14) and acidic (pH<0) electrolytes. Consequently, exceptional electrode/electrolyte compatibility is achieved, leading to high‐efficiency Sn plating behavior (99.95% at 1 mA cm −2 ) and robustness against harsh conditions, including exceptionally low currents (0.1 mA cm −2 , 99.8%; 0.01 mA cm −2 , ≈98.6%), long storage (1 month, 93.8% retention), and extreme temperatures (−10–+80°C, >98.7%). We also demonstrated high‐capacity Sn‖S batteries (≈2300 mAh g −1 , sulfur‐based) and long‐cycling Sn‖KNiFe(CN) 6 batteries (7,500 cycles). This work offers new insights into advanced Sn 2+ electrolytes and Sn plating chemistry for energy storage.