Study On Modification And Storage Energy Of LiVOPO₄ Cathode Materials

In all kinds of existing energy storage systems,LIBs as the main carrier of it,is considered as the optimal battery energy storage system.Cathode material is a vital component of LIBs,which determines the cost and electrochemical characteristics of the battery.Olivine LiFe PO₄occupies a superior position in the market due to its advantages of high working voltage and long cycle life.The voltage platform of LiVOPO₄is 0.3 V higher than that of LiFe PO₄.However,due to the instability of its crystal structure,LiVOPO₄material has poor ionic conductivity,so that its capacity in practical application is also greatly weakened.In view of the poor ionic conductivity of LiVOPO₄itself,currently the main methods are to change the particle size and morphology,ion doping,surface modification,material composite,and a small number of researchers to synthesize non-quantitative compounds to improve the performance of the material.In this paper,α-LiVOPO₄cathode material was successfully synthesized by ball milling method,and the infection of different reaction time（4 h,6 h,8 h）and carbon source（oxalic acid,citric acid,glucose）on the structure,morphology and electrochemical properties of the material were investigated.After cyclic testing,the results show that when oxalic acid is used as carbon source,the reaction time is 6 h.Its capacity retention rate is as high as92.5%.Although the cyclic properties of the materials synthesized by this method are excellent,the first-week discharge properties need to be improved.Therefore,α-Livopo4cathode material was synthesized by sol-gel method,and the material was explored from two aspects of calcination temperature（550℃,600℃,650℃）and time（8 h,10 h,12 h）.After cyclic testing,the results show that the initial discharge performance and cyclic stability of the material are satisfactory.The specific discharge capacity of the material sintered at 600℃for10 h is 141.3 m Ah g^-1at week 1 and 128.1 m Ah g^-1at week 50,and the capacity loss is only9.4%.By comparing the properties ofα-LiVOPO₄/C prepared by the two methods,it is found that the latter is not only easy to operate,but also improves the specific discharge capacity of the material.On this basis,a series of non-stoichiometricα-Li_xVOPO₄/C materials（00.9VOPO₄/C sintered at 600℃for 10 h were better after cyclic testing.On the basis of exploring the properties ofα-LiVOPO₄/C material in the previous two parts,the properties ofβ-LiVOPO₄/C material synthesized by sol-gel method were explored.After cyclic testing,the sintering temperature at 600℃for 10 h showed the best performance,and the discharge capacity at the first week was 139.7 m Ah g^-1（0.1 C,3-4.5 V）,and at 50 weeks it was 130.7m Ah g^-1,and the capacity loss is only 6.5%.On this basis,the electrochemical properties of x LiVOPO₄-（1-x）Li₃V₂（PO₄）₃/C（x=0.1,0.3,0.5,0.7,0.9）materials with different mixing ratios were explored.LiVOPO₄-Li₃V₂（PO₄）₃/C with a ratio of 0.5:0.5 had the best electrochemical properties.The specific discharge capacity was 143.2 m Ah g^-1（0.1 C,3-4.5 V）in the first week,and 135.6 m Ah g^-1after 50 weeks of circulation.The capacity retention rate was up to94.8%.The mixed materials coexist in three phases,LiVOPO₄and Li₃V₂（PO₄）₃in the center of the material,the outermost layer of about 2.5 nm amorphous carbon coating,quite jacking the electrochemical performance of the material.The introduction of Li₃V₂（PO₄）₃improves the diffusion rate of Li⁺in LiVOPO₄material to a certain extent,thus improving the discharge performance and cyclic stability of the material.