Synthesis,Modification And Electrochemical Performance Of Na₃V₂（PO₄）₃/C Cathode Materials For Sodium-ion

In this paper,the Na3V2(PO4)3/C(NVP/C)cathode materials are prepared by sol-gel and modified solid phase reaction based on orthogonal optimization.Carbon coating,doping and Na-rich modification are carried out to obtain excellent electrochemical performance.NaOH,NH4H2PO4,NH4VO3,critic acid and anhydrous ethanol are ued as the raw materials to synthesis NVP/C cathode material by solid state reaction.The optimal technic parameters are obtained by orthogonal test method including molar ratio of NVP to C of 1:1.5,milling time of 12 h,calcination temperature of 850℃ and calcination time of 12 h.The NVP prepared under the optimized synthetic conditions shows no impurity and has smaller particles size(4-5 μm).The optimized sample delivers an initial discharge capacity of 123 mAh/g at 0.2 C rate.After 100 cycles.there is no capacity lossCritic acid-assisted sol-gel method is used to optimize the preparation of NVP/C cathode materials by orthogonal experiment.The factors affecting the initial discharge capacity of NVP/C cathode materials are as follows:calcination temperature>NVP:C>water bath temperature>calcination time.The Na3V2(PO4)3/C prepared at 800℃ delivers an initial discharge capacity of 117.6 mAh/g at 0.2 C rate.Dopamine is choosed as carbon source to secondary coat NVP/C by sol-gel method.Calcined dopamine can form a three-dimensional network structure,which can enhance the contact between the active material particles and between the active material and the electrolyte.After comparison,the NVP/C-2 prepared under dopamine concentration of 2 mg/mL dilivers the best electrochemical performance(87.1mAh/g,at 20 C).the dimension of residual carbon can be controlled by changing the order of additon of dopamine.Adding dopamine before and after NVP can form two-dimensional flaky carbon and three-dimensional conductive carbon networks,the three-dimensional network structure improves the electrochemical performance of NVP more significantlyNa3V2-xLax(PO4)3(x=0.01,0.02,0.03,0.04)cathode materials are synthesized by sol-gel method.A moderate La3+ doping can effectively widen the Na+ diffusion channel,and reduce the particles size of NVP/C.For La doping samples,when x=0.01,the cycle performance and rate capacity can reach the best.Na3V1.99La0.01(PO4)3 exhibits discharge capacity of 105 mAh/g at 0.2 C,after 100 cycles the capacity retention is as high as 97.1%.when charging and discharging at a higher rate of 20 C,the capacity still remain 66.1 mAh/gThe solid state methd is used to design and prepare a series of Na3+xV2(PO4)3/C(x=0.0.2,0.4,0.6)cathode materials.The electrochemical performance of Na3.4V2(PO4)3/C is the best,and the discharge capacity of 132.4 mAh/g is obtained at the current density of 20 mA/g.