Corrosion of Calcium Metal in Ca(TFSI) ₂ /DMAc Electrolyte and its Solution via Alloy Interface and Competitive Solvation

Abstract Calcium metal batteries (CMBs) are promising for large‐scale energy storage due to calcium‘s abundance and high theoretical capacity, but hindered by poor compatibility between electrolyte and calcium metal. Here, we unveil two corrosion types of calcium in calcium bis(trifluoromethanesulfonimide)/dimethylacetamide [Ca(TFSI) 2 /DMAc] electrolytes: native passivation from solvent reduction during aging and electrochemical corrosion from anion decomposition. The native passivation layer aggravates the electrochemical corrosion by the decomposition of TFSI − to the large grain Ca 2+ ‐insulator CaF 2 , leading to the calcium metal failure. We introduce a bifunctional SnI 2 electrolyte additive to simultaneously mitigate both corrosions. Sn 2+ facilitate the spontaneous galvanic replacement reaction to form the Ca‐Sn alloy, inhibiting native passivation, while I − replace TFSI − to participate in the Ca 2+ primary solvation shell and prevent the electrochemical TFSI − decomposition. By taking advantage of those features above, over 500 h of stable cycling with a reduced overpotential of 0.33 V at 0.1 mA cm −2 and 0.1 mAh cm −2 was achieved, outperforming state‐of‐the‐art Ca(TFSI) 2 ‐based electrolytes. Our work provides a fundamental understanding of corrosion at calcium metal surface and will guide suitable electrolyte design for anti‐corrosion in metal batteries.