Na-ion batteries are promising in large-scale energy storage owing to the abundant raw material resources, low     cost, and high safety.

Sodium vanadium fluorophosphate (Na₃(VOPO₄)₂F) with a theoretical energy density of 480 Wh/Kg is regarded as a strong candidate among various cathode materials. However, the intrinsic low conductivity and high energy-consumption during synthesis process hinder its commercialization.

Researchers from the Institute of Process Engineering (IPE) and the Institute of Physics of the Chinese Academy of Sciences developed a one-step mechanochemical method to rapidly prepare the polyanionic compound sodium vanadium fluorophosphate as the cathode materials for Na-ion batteries, which exhibited excellent rate performance and cycle stability.

The prepared Na₃(VOPO₄)₂F/KB composite delivered a high discharge capacity of 142 mAh g-1 at 0.1C. The extra capacity beyond the theoretical specific capacity (130 mAh g-1) benefited from the interfacial charge storage.

Moreover, a specific capacity of 112 mAh g-1 can be obtained even at 20 C, which means this Na-ion battery could be fully charged/discharged in three minutes.

Superior cycle stability of this composite was demonstrated by ultrahigh cycling stability with 98% retention over 10,000 cycles.

High-resolution transmission electron microscopy revealed that the nanocrystallines of Na₃(VOPO₄)₂F about 30 nm were embedded in the carbon framework, which facilitated the rapid conduction of electrons and Na ions.

The reversible structural evolution and negligible volume change of Na₃(VOPO₄)₂F/KB composite during charging/discharging were also demonstrated by in situ X-ray diffraction and ²³Na nuclear magnetic resonance       spectrum.

“The method provides a feasible strategy to improve the rate performance and cycle performance of cathode materials. Besides, the kilogram-scale product indicates the mechanochemical method is suitable for rapid large-scale production electrode materials for Na-ion batteries,” said Prof. Zhao Junmei, a co-corresponding author of the study.