Abstract:

The limited lithium reserves and the increasing cost of lithium sources have hampered extensive applications of lithium-ion batteries to large-scale electric energy storage. It is significantly urgent to develop alternative low-cost electric energy storage devices, during which rechargeable sodium-ion batteries (SIBs) have attracted extensive attention due to abundant sodium resources and similar electrochemical properties to lithium-ion batteries. Na₃V₂(PO₄)₂F₃ (NVPF) is considered as one of the most promising candidates, owing to the merits of super high ionic conductivity, high theoretical specific capacity, good thermal stability and small volume effect. In this work, through hydrothermal process and post calcination, nitrogen-doped carbon-coated NVPF (NVPF@C-N) composites were obtained by adding urea and citric acid during the hydrothermal process. Particularly, nitrogen doping could substantially enhance the pore structure and electrical conductivity of the carbon layer. When used as the SIB cathode, NVPF@C-N exhibited high reversible capacity and excellent rate capability. Under 1 C and 10 C rates, the NVPF@C-N cathode delivered initial discharge capacity of 121 mAh/g and 110 mAh/g, respectively. Even at the rate of 90 C, 66 mAh/g can be obtained. For the cycling stability of NVPF@C-N, the voltage plateau still could be well maintained even after 200 cycles at the rate of 1 C, and the electrode still retained a capacity of 111 mAh/g. Particularly, a retention of 87% was obtained after 1000 cycles at the rate of 10 C, and a retention of 54% was still maintained even after 6000 cycles.

Key words: sodium ion batteries, Na₃V₂(PO₄)₂F₃, carbon coating, nitrogen doping, high-rate performance