Study On Process Modification And Co-doping Of Li₃V₂?PO₄?₃ As Cathode Material For Lithium Ion Batteries

Lithium ion battery has been widely concerned because of its application in portable mobile devices,hybrid electric vehicles?HEV?and electric vehicles?EV?.Li₃V₂?PO₄?₃ with NASICON?sodium ion conductor?structure has higher theoretical capacity?197 m Ah/g?and thermal stability.This 3D open structure,which provides a wider natural space for lithium ion transmission.Li₃V₂?PO₄?₃ has a high charge discharge voltage?3?4.8 V?.Three lithium ions can be completely reversible out and embedded.At this time,the theoretical capacity of the material can reach 197 m Ah/g.However,due to the separation of VO₆ octahedron and the polarization of V-O bond,the intrinsic electronic conductivity and ionic conductivity of Li₃V₂?PO₄?₃are low.In order to overcome this limitation and improve the electrochemical performance of cathode materials,many modification methods such as carbon coating,ion doping and particle size reduction have been studied.In this paper,solid state method,sol-gel method and hydrothermal method were used to synthesize cathode materials,and Bi³⁺was selected as metal ion dopant.The solid state method is made by ball mill grinding at 350 r/min speed for 4 h.The sol-gel method is to dry the mixture to the gel state in the magnetic stirrer.The hydrothermal method is to place the solution in the reactor,220?,12 h in the oven,and the three synthetic routes are all calcined in the tubular furnace.Three kinds of Li₃V_2-xBi_x?PO₄?₃?x=0,0.01,0.03,0.05?with different synthetic routes were studied in terms of object image?X-ray diffraction,XRD?,morphology?scanning electron microscope,SEM?and electrochemical performance?cyclic voltammetry,CV,electrochemical impedance spectroscopy,EIS?.The experimental results show that the sample prepared by the sol-gel method:Li₃V_1.97Bi_0.03?PO₄?₃,due to the regular spherical morphology,uniform distribution and small particle diameter,shortens the lithium ion transmission path during the charge and discharge process.The area of reaction with the electrolyte is increased,and the electrode polarization and the electrochemical transfer resistance of the material are effectively reduced,thereby exhibiting the most excellent electrochemical performance.The Li₃V_1.97Bi_0.03?PO₄?₃sample prepared by the sol-gel method shows discharge capacity of 172.06 m Ah/g at a rate of 0.1 C.Comparing the Li₃V_1.97Bi_0.03?PO₄?₃ sample prepared by the solid state method and the hydrothermal method discharge capacity:164.84 m Ah/g and 168.62 m Ah/g.The samples prepared by the sol-gel method showed the most excellent discharge capacity.After 300 cycles at 1 C,the capacity of the sol-gel sample Li₃V_1.97Bi_0.03?PO₄?₃ decreased from 161.88m Ah/g to 138.64 m Ah/g,and the capacity retention rate was 85.64%.In recent years,in addition to cation doping,anion doping has gradually attracted much attention.Based on previous research,Li₃V_1.97Bi_0.03?PO₄?₃ composite was selected as a Cl^-doped substitute.The discharge capacity of Li₃V_1.97Bi_0.03?PO₄?_2.95Cl_0.05/C composite material at 0.1 C rate is 188 m Ah/g,compared with the discharge capacity of Li₃V_1.97Bi_0.03?PO₄?₃ sample with single Bi³⁺doped composite material:172.06 m Ah/g.The Li₃V_1.97Bi_0.03?PO₄?_2.95Cl_0.05/C sample showed more excellent discharge capacity.After 300 cycles at 1 C,the capacity of Li₃V_1.97Bi_0.03?PO₄?_2.95Cl_0.05/C composite material was reduced from 175 m Ah/g to 146m Ah/g.Compared with the single-doped Bi³⁺sample Li₃V_1.97Bi_0.03?PO₄?₃:the capacity was reduced from 161.88 m Ah/g to 138.64 m Ah/g.The Li₃V_1.97Bi_0.03?PO₄?_2.95Cl_0.05/C sample showed better cycle performance.The effects of co-doping of different halogen anions and cations on the electrochemicalperformanceofmaterialswereinvestigated.Li₃V_1.97Bi_0.03?PO₄?_2.96F_0.04/C composite material has a first discharge capacity of175.18 m Ah/g at a rate of 0.1 C,300 cycles at 1 C,Li₃V_1.97Bi_0.03?PO₄?_2.96F_0.04/C The composite material capacity decreased from 164.86 m Ah/g to 135.5 m Ah/g,and the capacity retention rate was 82.19%.F^-and Cl^-belong to the same main group element,but the experimental results show that the Cl^-with larger ionic radius,composites synthesized with Bi³⁺show better electrochemical performance.