A new high-voltage calcium intercalation host for ultra-stable and high-power calcium rechargeable batteries

Rechargeable calcium batteries have attracted increasing attention as promising multivalent ion battery systems due to the high abundance of calcium. However, the development has been hampered by the lack of suitable cathodes to accommodate the large and divalent Ca²⁺ ions at a high redox potential with sufficiently fast ionic conduction. Herein, we report a new intercalation host which presents 500 cycles with a capacity retention of 90% and a remarkable power capability at ~3.2 V (vs. Ca/Ca²⁺) in a calcium battery. The cathode material derived from Na_0.5VPO_4.8F_0.7 is demonstrated to reversibly accommodate a large amount of Ca²⁺ ions, forming a series of Ca_xNa_0.5VPO_4.8F_0.7 (0 < x < 0.5) phases without any noticeable structural degradation. The robust framework enables one of the smallest volume changes (1.4%) and the lowest diffusion barriers for Ca²⁺ among the cathodes reported to date, offering the basis for the outstanding cycle life and power capability.