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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 Na1 V2 (PO4 )2 F3 (N1 PVF3 ) with sufficient interstitial spaces is reported as ultra-stable and high-energy Ca ion cathodes. The N1 PVF3 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 N1 PVFx (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 N1 VPF3 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 N1 PVF3 are elucidated as the small volume changes and low cation diffusion barriers among the cathodes.