Pitch-Derived Soft-Carbon-Wrapped NaVPO4F Composite as a Potential Cathode Material for Sodium-Ion Batteries

Sodium vanadium fluorophosphate (NaVPO4F) is a potential electrode material for sodium-ion batteries due to its stable structural framework with a reasonable theoretical capacity of 143 mAh However, its poor electronic conductivity limits its C-rate performance and cycle stability. To solve the problem associated with poor electronic conductivity, in this work, pitch-derived soft-carbon-wrapped NaVPO4F particles with an interconnected carbon matrix is proposed. Transmission electron microscopy (TEM) and Raman studies suggest that the interlayer spacing of the soft carbon layer is around 0.47 nm, which facilitates faster kinetics upon cycling. The presence of 15% soft carbon in the composite samples results in higher Na-ion diffusion coefficient values, stable cycling, and C-rate performances. The composite cathode delivers an initial reversible capacity of 109 mAh with 95% capacity retention at 0.1C for 300 cycles. Besides, 77% capacity retention is observed after 1000 cycles at a 0.5C rate. Such a remarkable electrochemical performance is attributed to the appropriate amount of soft carbon wrapped onto NaVPO4F and the interconnected carbon matrix, which provides continuous electronic conduction, improved structural integrity, and faster kinetics of NaVPO4F particles. Finally, practical sodium-ion full cells using the NaVPO4F composite cathode and the pretreated hard carbon anode are realized. The cells show an operating voltage of 3.3 V with a capacity retention of similar to 87% at the end of the 50th cycle at 30 mA g(-1). This suggests that the NaVPO4F-based composite electrode materials are robust cathodes and can be potentially implemented in sodium-ion batteries.

Automated summary

This study proposed a method for preparing pitch-derived soft carbon-wrapped NaVPO4F particles to improve the sodium ion diffusion performance, stable cycling, and C-rate performance by increasing the soft carbon content. The fabrication of practical sodium-ion full cells was achieved, enhancing the electrochemical performance of sodium-ion batteries.