Coating a Na₃V₂(PO₄)₃ cathode material with carbon to improve its sodium storage

A sodium superionic conductor (NASICON)-type Na₃V₂(PO₄)₃ (NVP) with a 3D framework is a promising cathode material for sodium ion batteries. We used citric and oxalic acids as carbon sources to prepare carbon-coated NVP/C cathode materials by a sol-gel method. Their effect on the crystal structure, morphology and electrochemical performance of the coated NVP were investigated. Results indicate that compared with the NVP/C prepared from oxalic acid, NVP/C using citric acid as the carbon source has larger unit cell parameters of NVP, a smaller particle size, a thinner carbon coating layer, wider channels and shortened paths for Na⁺ migration, and superior kinetic characteristics. It had a high capacity of 112.3 mAh g⁻¹ at 0.1 C and an excellent rate capability with reversible capacities of 90.0 and 89.1 mAh g⁻¹ at 2 and 5 C, respectively. It also had an excellent cycling stability with capacity retention rates of nearly 100%, 92.7% and 90.0% after cycling 200 times at 1, 2 and 5 C, respectively. It is therefore a promising cathode material for practical use.