A fluorine-free electrolyte for calcium metal batteries

Calcium metal batteries (CMBs) have emerged as a promising alternative to lithium(Li)-based technologies due to calcium's (Ca) low reduction potential, high volumetric capacity, and abundance. However, challenges such as poor Ca²⁺ transport across passivation layers and limited electrolyte compatibility hinder practical implementation. Here, we report a fluorine-free electrolyte formulation based on calcium bis(methanesulfonimide) (Ca(MSI)₂) in dimethylacetamide (DMAc). This system offers a safer, more sustainable, and environmentally benign alternative to conventional fluorinated electrolytes. Comparative physicochemical and electrochemical evaluations with the benchmark Ca(TFSI)₂/DMAc electrolyte reveal that Ca(MSI)₂ enables stable calcium plating/stripping over 1600 hours with lower overpotential and improved rate performance. In fact, spectroscopic analyses confirm the formation of a more uniform, fluorine-free interphase that supports better Ca²⁺ transport throughout the passivation layer. These findings highlight the potential of fluorine-free salts for enabling reversible room temperature calcium metal cycling and advance the development of safer, high-performance multivalent batteries.