Qiong Zheng

Y-Doped Na3V2(PO4)2F3 compounds for sodium ion battery cathodes: electrochemical performance and analysis of kinetic properties

To improve the intrinsic electronic conductivity and Na ion mobility of Na3V2(PO4)2F3 (NVPF), Y(yttrium) atoms are introduced into the NVPF/C complex as a partial substitute for V(vanadium) through a sol–gel method. The effects of Y substitution on the crystal structure, morphology, electrochemical performance and kinetic properties of NVPF were investigated. Based on the battery performance comparison of the Na3V2−xYx(PO4)3/C (x = 0, 0.05, 0.1 and 0.2) samples, Na3V1.9Y0.1(PO4)3/C showed the best electrochemical performance and cycling stability. At a low rate of 0.5C, the 5 mol% Y-doped sample delivered a discharge capacity of 121.3 mA h g−1, which was very close to the theoretical specific capacity. And even at a high rate of 50C, the discharge capacity achieved was higher than 80 mA h g−1. After 200 cycles, the capacity retention of Na3V1.9Y0.1(PO4)3/C could still remain as high as 93.46% at 1C. From the morphology determination and analysis of kinetic properties, it was confirmed that the excellent electrochemical performance of Na3V1.9Y0.1(PO4)3/C was mainly due to the enhanced intrinsic electronic conductivity and Na ion mobility caused by introducing a moderate amount of Y to replace the V sites in the NVPF crystal structure. In order to get a better understanding of the relationship between the kinetic properties and the electrochemical performance in a sodium ion battery, a mass and electron transfer process model has been proposed for the first time in the present research.

Towards enhanced sodium storage by investigation of the Li ion doping and rearrangement mechanism in Na3V2(PO4)3 for sodium ion batteries

Metal ion doping is an effective way for improving the intrinsic properties of Na3V2(PO4)3. However, mechanistic exploration of metal ion doping in Na3V2(PO4)3, which is essential for the design and optimization of metal ion doping, is indistinct. To achieve an in-depth understanding of the mechanism of metal ion doping, Li ion doped Na3−xLixV2(PO4)3/C (x = 0, 0.01, 0.05, 0.1, 0.5, 0.7 and 1.0) compounds are synthesized. By utilizing DFT, NMR and Rietveld methods, the mechanism of Li/Na ion doping and rearrangement in the Na sites and Na ion insertion/extraction during electrochemical cycles are explored and identified. It is found that during the synthesis process, Li ions are inclined to enter mainly the Na2 sites in Na3V2(PO4)3 when the x value is low, but they will occupy the Na1 and Na2 sites simultaneously when x is high. Meanwhile, Li ions in the as-prepared Na3−xLixV2(PO4)3 compounds are reversibly replaced by Na ions during the idle state, due to the very small energy difference of the exchange between Li and Na ions. Furthermore, it is confirmed that there are more than two Na ions inserted/extracted during the charge–discharge process, resulting in the final extra specific capacity.

Facile construction of nanoscale laminated Na3V2(PO4)3 for a high-performance sodium ion battery cathode

A novel facile construction of nanoscale laminated Na3V2(PO4)3 for a high-performance sodium ion battery (SIB) cathode is proposed. In the synthesis process, a crystallized intermediate precursor with low-cost raw materials is prepared by introducing a high temperature molten-state NH3 thermal-reduction process, which acts as a reaction template to control the crystal growth and the final morphology of Na3V2(PO4)3. The as-synthesized nanoscale laminated Na3V2(PO4)3 possesses continuous Na+/electron pathways, large electrode/electrolyte contact area and sufficient carbon coating, resulting in fast Na+ extraction/insertion and electron transport during the electrochemical reaction process, which is shown to achieve excellent rate capability and decent cycling stability. At a low rate of 0.5C, the discharge specific capacity is approximately 117 mA h g−1, which is very close to its theoretical specific capacity, and there is only a very minor capacity fade after continuous 250 cycles at 2C. Even at 50C, the discharge specific capacity is as high as 80 mA h g−1 and the reversible capacity retention after 3000 cycles remains more than 78%. In addition, the X-NVP cathode shows stable cycling performance and acceptable rate performance with a reversible capacity of 110 mA h g−1 at 0.2C at a low temperature of −20 °C, which has rarely been reported previously in the SIB field. The intermediate precursor prepared by the high temperature molten-state thermal-reduction method, acting as the reaction template of the final product, provides a facile and economic solution for the synthesis of high-performance SIB cathode materials.