A High Capacity Calcium Primary Cell Based on the Ca–S System

Abstract Conversion reaction cells afford the ability to explore new energy storage paradigms that transcend the dogma of small, low‐charge cations essential to intercalative processes. Here we report the use of earth‐abundant and green calcium and sulfur in unprecedented conversion reaction Ca–S primary cells. Using S‐infiltrated mesoporous carbon (abbreviated S@ meso ‐C) cathodes, we achieve discharge capacities as high as 600 mAh g −1 (S basis) within the geometry Ca|Ca(ClO 4 ) 2 /CH 3 CN|S@ meso ‐C, at a discharge rate of C/3.5. The electrolyte system in the Ca–S battery is of paramount importance as the solid electrolyte interface (SEI) formed on the Ca anode limits the capacity and stability of the cell. We determine that 0.5 M Ca(ClO 4 ) 2 in CH 3 CN forms an SEI that advantageously breaks down under anodic bias to allow oxidation of the anode. This same SEI, however, exhibits high impedance which increases over time at open circuit limiting the shelf life of the cell.