Generation of solid electrolyte interface (SEI) film on graphite negative electrode surface in calcium-ion secondary batteries and its dependence on electrolyte

Currently, secondary batteries utilizing divalent cations are gaining attention as high-energy-density devices. However, the optimization of electrolytes and materials used in these next-generation batteries has been largely unexplored. Thus, this study focused on calcium-ion batteries (CIBs), which use divalent cations, and investigated the impact of the solid electrolyte interface (SEI) formed by various electrolytes for CIBs on their charge–discharge behavior. Four types of solvents propylene carbonate (PC), dimethyl carbonate (DMC), dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF), were used, and Ca(ClO₄)₂was used as the electrolyte. CIBs using PC and DMC as solvents did not exhibit reversible redox reactions. However, the use of DMSO and DMF enabled reversible redox reactions and increased the charge–discharge capacity. Electrolytes demonstrating reversible charge–discharge contained an SEI with calcium, whereas those that hindered charge–discharge had an SEI with minimal calcium content.