Stable cycling of β-VOPO4/NaVOPO4 cathodes for sodium-ion batteries

We have recently reported the reversible sodium intercalation in β-VOPO₄ that has a moderate-high discharge plateau at 3.3 V vs. Na/Na⁺ and a theoretical capacity of 166 mAh g⁻¹. However, the practical applications of the cathodes are still impeded by the low electrical and ionic conductivities as seen in many metal polyanion cathodes. Herein, VOPO₄/C and NaVOPO₄/C composites have been prepared by ball-milling and tested in sodium ion batteries. X-ray diffraction and scanning electron microscopy confirm the phase is maintained after the ball-milling process, while the particle size is obviously reduced. Electrochemical tests show much improved capacities of 85 mAh g⁻¹ (∼0.5 Na-ions per formula) at C/20. At a higher temperature of 45 °C, the discharge capacities are further increased to 130 mAh g⁻¹ and 115 mAh g⁻¹ for VOPO₄/C and NaVOPO₄/C (∼0.78 Na-ions per formula for either) along with long-term cycling. The preliminary results suggest β-VOPO₄ has the potential to exhibit good practical performance upon structural optimization. Thus, the exploration on the fabrication of nanoparticles and more efficient conductive coating will probably lead to more fruitful achievements.