Overcoming the rate-determining kinetics of the Na3V2O2(PO4)2F cathode for ultrafast sodium storage by heterostructured dual-carbon decoration

Abstract

Sodium-ion batteries have been intensely developed as cost-effective alternatives to current lithium-ion batteries for large-scale applications, especially in energy storage systems and low-speed electric transportations. Na₃V₂O₂(PO₄)₂F (NVOPF), as one particular derivative of sodium vanadium fluorophosphates with excellent ionic conductivity and robust polyanionic structure, has been attractive as a high-energy cathode candidate to exhibit superior sodium storage performance. However, the high-rate and cycling performance of the NVOPF cathode is far away from expectations, owing to its intrinsic poor electronic conductivity. Here, we report a dual-carbon decoration to address this essential issue. Specifically, an amorphous carbon (C) layer is homogeneously coated at the surface of NVOPF particles, coupled with an in situ wrapping of graphitized carbon nanofibers (CNFs) externally, resulting in a heterostructure of dual-carbon coated NVOPF@C/CNFs. Through a post-annealing (PA) treatment, the NVOPF@C/CNF-PA cathode reveals the desired rate capability and cycling stability even under elevated working temperatures. The material can deliver an impressive capacity of 86.7 mA h g⁻¹ at 50C and can also retain 82.5% capacity retention after prolonged 2500 cycles at 5C at 60 °C, which is superior to previously reported NVOPF-based cathode materials with different surface engineering. In situ galvanostatic intermittent titration technique (GITT) and electrochemical impedance spectroscopy (EIS) examinations show that the heterostructured dual-carbon decoration considerably improves the sodium storage kinetics during extraction of the first Na-ion, i.e., overcomes the rate-determining Na-ion diffusion for the NVOPF cathode, which accounts for the ultrafast sodium storage. This work highlights an effective surface engineering protocol that significantly boosts the rate capability of polyanion-type cathodes for fast-charging sodium-ion batteries.