K_xVPO₄F (x∼0): A New High-Voltage and Low-Stain Cathode Material for Ultrastable Calcium Rechargeable Batteries

The utilization of high-voltage intercalation cathodes in calcium-ion batteries (CIBs) is impeded by the substantial size and divalent character of Ca²⁺ ions, which result in pronounced volume alterations and sluggish ion mobility, consequently causing inferior reversibility and low energy/power densities. To tackle these issues, polyanionic K-vacant K_xVPO₄F (x∼0, designated as K₀VPF) is proposed as high-voltage and ultra-stable cathode material in CIBs. The K₀VPF demonstrates a decent calcium storage capacity of 75 mAh g⁻¹ at 10 mA g⁻¹ and remarkable capacity retention of 84.2% over 1000 cycles. The average working voltage of the K₀VPF is 3.85 V versus Ca²⁺/Ca, representing the highest value reported for CIB cathodes to date. The combined experimental and theoretical investigations revealed that the low volume changes and hopping diffusion barriers contribute to the extraordinary stability and high-power capabilities, respectively, of K₀VPF. The distribution of Ca ions into polyanionic frameworks with pronounced spatial separation effectively attenuates the Ca²⁺–Ca²⁺ repulsive force and thus augmenting the Ca migration kinetics. The high voltage of K₀VPF is attributed to the inductive effect from the largely electronegative fluorine. In conjunction with a calcium metal anode and a compatible electrolyte, Ca metal full cells featured a record-high energy density of ≈300 Wh kg⁻¹.