High-Voltage Phosphate Cathodes for Rechargeable Ca-Ion Batteries

Calcium-ion batteries (CIBs) are under investigation as next-generation energy storage devices due to their theoretically high operating potentials and lower costs tied to the high natural abundance of calcium. However, the development of CIBs has been limited by the lack of available positive electrode materials. Here, for the first time, we report two functional polyanionic phosphate materials as high-voltage cathodes for CIBs at room temperature. NaV2(PO4)(3) electrodes were found to reversibly intercalate 0.6 mol of Ca2+ (81 mA h g(-1)) near 3.2 V (vs Ca2+/Ca) with stable cycling performance at a current density of 3.5 mA g(-1). The olivine framework material FePO4 reversibly intercalates 0.2 mol of Ca2+ (72 mA h g(-1)) near 2.9 V (vs Ca-2(+)/Ca) at a current density of 7.5 mA g(-1) in the first cycle. Structural, electronic, and compositional changes are consistent with reversible Ca2+ intercalation into these two materials.