Unlocking rapid and robust sodium storage of fluorophosphate cathode via multivalent anion substitution

Abstract

Polyanion-type fluorophosphate Na3V2(PO4)2O2F (NVPOF) is broadly regarded as a fascinating cathode candidate for high energy density and sustainable sodium-ion batteries (SIBs) due to its high operating potential, high capacity and robust sodium super-ionic conductor (NASICON) framework. However, the inferior rate property and service life caused by its intrinsically low electron conductivity seriously impede its electrochemical performance. Herein, an extraordinary Na+ storage performance in Na3V2(PO4)1.95(SiO4)0.05O2F (NVPOFSi0.05) is obtained through partially replacing PO43- in NVPOF with SiO44-. Theoretical calculation and experimental analysis indicate that this anion substitution optimizes the electronic conductivity and broadens ionic transport channels, leading to faster ion/charge diffusion kinetics. Meanwhile, SiO44--introduced reinforces the crystal structure to support reversible two-electron reaction, and enhances energy density with higher mean discharge voltage. Therefore, the NVPOFSi0.05 cathode exhibits a remarkable enhancement in high-rate capability (75.5 mA h g−1 at 30 C) and virtually no capacity loss during the long-term cycling at 10 C over 1000 cycles. The full cell coupled with the NVPOFSi0.05 cathode and hard carbon anode possesses high energy density (280 W h kg−1) and exceptional long-term cyclability (92.3% capacity retention after 300 cycles at 5 C). This anion substitution device opens new avenues toward the design of advanced cathode materials for SIBs.