Ultra‐long cycle life and high rate performance subglobose Na 3 V 2 ( PO 4 ) 2 F 3 @C cathode and its regulation

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.