Ultra‐long cycle life and high rate performance subglobose Na3V2(PO4)2F3@C cathode and its regulation

Summary A nitrogen‐doped carbon coated subglobose Na3V2(PO4)2F3@C (NVPF) cathode for sodium‐ion batteries was synthesized by using hexadecyl trimethyl ammonium bromide (CTAB) as soft template and polyvinylidene fluoride (PVDF) as carbon source. CTAB plays a significant role on the formation of sphere micelles. Precursor ions are self‐assembled on the surface at appropriate concentration and its mechanism is investigated in subglobose NVPF@C‐4. CTAB also increases the conductivity of carbon layer as −(CH3)3N+ in CTAB is combined with residual carbon from PVDF to form partially N‐doped carbon. Meanwhile, the carbon source PVDF contributes to prevent the generation of impurity Na3V2(PO4)3 by compensating the evaporative fluorine. Generally, CTAB and PVDF play multifunctional roles in regulating Na3V2(PO4)2F3@C cathode with well‐developed crystallite, high rate performance, good conductivity, and ultra‐long cycle life. The specific capacity of NVPF@C‐4 cathode at 0.1 C and 10 C is as high as 121.5 mAh·g−1 and 99.2 mAh·g−1 with high capacity retention of 90.1% even after 1000 cycles at 10 C. The excellent rate performance is also attributed to the high diffusion coefficient of Na+ and high exchange current according to the kinetic analysis. The enhanced electrochemical performances reveal the special regulation in this paper is feasible to obtain excellent structural stability of NVPF materials. Spherical micelles are formed through the regular arrangement of hydrophobic long carbon chain (C16H33−) and hydrophilic trimethylamine (−(CH3)3N+) of CTAB. The hydrophobic PVDF are homogeneously dispersed in the aqueous solution under the dispersing role of CTAB. Through the self‐assembled of precursor anions (PO43−, F−) and cations (Na+, V3+) on the surface of micelle, subglobose Na3V2(PO4)3F3 cathode for sodium‐ion batteries with well‐developed crystallite, ultra‐long cycle life and high rate performance are constructed after the sintering process.