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.
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