Synthesis and electrochemical performance of Ni doped Na3V2(PO4)3/C cathode materials for sodium ion batteries

To improve the electrochemical performances of Na3V2(PO4) cathode materials in sodium ion batteries, Na3V2-xNix(PO4)3/C (x = 0, 0.01, 0.03 and 0.05) materials were designed and prepared through a facile sol-gel route. The effect of Ni doping on the composition and crystal structure of Ni doped Na3V2(PO4)/C samples was investigated. The results confirmed that Ni atoms were successfully doped into Na3V2(PO4) crystal and substituted the V sites. It is shown that a carbon layer of about 2 nm was covered on the particle surface of the sample, which is beneficial for enhancing the electronic conductivity. The discharge capacities of all Ni doped samples were improved at high rates (2, 5 and 10 C). When Ni doping content was 3%, the sample exhibited the best rate performance and cycling capability. Na3V1.97Ni0.03(PO4)3/C sample delivered the specific capacity of 98.1 mAh·g−1 at 5 C, which is much higher than that of pristine sample, 64.3 mAh·g−1. The capacity retentions of Na3V1.97Ni0.03(PO4)3/C and pristine samples were 93.5% and 60.2%, after 50 cycles at 5 C, respectively. Electrochemical impedance spectroscopy and cyclic voltammetry performance analyses revealed that the polarization of the electrodes decreased and the diffusion coefficient of sodium ions increased by Ni doping.