Improved electrochemical performance of the Na3V2(PO4)3 cathode by B-doping of the carbon coating layer for sodium-ion batteries

Abstract

Boron-doped carbon coating is initially used to improve the electrochemical performance of Na₃V₂(PO₄)₃ cathode materials of sodium-ion batteries. Based on the precise analysis of Raman spectroscopy, electrochemical impedance spectroscopy and X-ray photoemission spectroscopy, it is found that there are four different B-doping species (B₄C, BC₃, BC₂O and BCO₂) in various boron-doped carbon coated Na₃V₂(PO₄)₃ samples; moreover, different B-doping species in the carbon coated layer have different impacts on the improvement of the electrochemical performance of Na₃V₂(PO₄)₃. Compared to lithium-ion batteries, the mechanism of B-doped carbon coating for the improvement of the electrochemical performance in sodium-ion batteries is different. Due to the introduction of the O atom in the carbon coated layer, BC₂O and BCO₂ significantly increase numerous extrinsic defects and active sites, which could significantly accelerate Na⁺ transport in the carbon layer. Therefore, it is unexpectedly demonstrated that Na₃V₂(PO₄)₃/C + B, which consists of the largest total amount of BC₂O + BCO₂, exhibits the best electrochemical performance, particularly high-rate performance and cycling stability. As an example, when the discharging rate increases from 0.2 C to 5 C, it delivers 95.8 mA h g⁻¹ to 90.3 mA h g⁻¹ and an amazing capacity retention of 94% is achieved.