Synthesis and Electrochemical Performance of Uniform Carbon-Coated Na₃V₂(PO₄)₂F₃ Using Tannic Acid as a Chelating Agent and Carbon Source

Carbon-coated Na3V2(PO4)2F3 (NVPF@C) composites are prepared using tannic acid as a chelating agent and carbon source via a sol–gel process-assisted solid-state reaction for the first time. When introducing tannic acid in two steps, a complete and uniform carbon coating layer can be successfully fabricated because the tannic acid added in the second step can repair the discontinuous and uneven coating obtained by the first coating. This uniform carbon layer can not only effectively improve the electronic conductivity of NVPF and accelerate Na+-ion diffusion but also prevent NVPF from being corroded by the electrolyte. As a result, the carbon-coated NVPF composite prepared by introducing tannic acid in two steps has a higher initial capacity (124.5 mAh g–1 at 0.2 C) and a better cycle stability (94.8% capacity retention ratio over 300 cycles at 10 C) compared to pristine NVPF and another carbon-coated NVPF composite prepared by introducing tannic acid in one step. Therefore, this work not only finds a cheap chelating agent for the sol–gel process but also develops an effective strategy for coating materials with poor electronic conductivity.