Ultrastable and High Energy Calcium Rechargeable Batteries Enabled by Calcium Intercalation in a NASICON Cathode

Ca-ion batteries (CIBs) have been considered a promising candidate for the next-generation energy storage technology owing to the abundant calcium element and the low reduction potential of Ca²⁺/Ca. However, the large size and divalent nature of Ca²⁺ induce significant volume change and sluggish ion mobility in intercalation cathodes, leading to poor reversibly and low energy/power densities for CIBs. Herein, a polyanionic Na superionic conduction (NASICON)-typed Na-vacant Na₁V₂(PO₄)₂F₃ (N₁PVF₃) with sufficient interstitial spaces is reported as ultra-stable and high-energy Ca ion cathodes. The N₁PVF₃ delivers exceptionally high Ca storage capacities of 110 and 65 mAh g^-1 at 10 and 500 mA g^–1, respectively, and a record-long cyclability of 2000 cycles. More interestingly, by tailoring the fluorine content in N₁PVF_x (1 ≤ x ≤ 3), the high working potential of 3.5 V versus Ca²⁺/Ca is achievable. In conjunction with Ca metal anode and a compatible electrolyte, Ca metal batteries with N₁VPF₃ cathodes are constructed, which deliver an initial energy density of 342 W h kg^-1, representing one of the highest values thus far reported for CIBs. Origins of the uncommonly stable and high-power capabilities for N₁PVF₃ are elucidated as the small volume changes and low cation diffusion barriers among the cathodes.