The Study On Modification Of Lithium Ion Cathode Material Li₃V₂（PO₄）₃

For the shortcomings of lithium vanadium phosphate（LVP）cathode material,the Li_3-xNa_xV₂（PO₄）₃ composite cathode material was synthesized by introducing the sodium ions in the synthesis process on the basis of the composition of LVP,using the bio-chemical synthesis method and the yeast cells as the biological template in this paper.In this study,the effects on the phase,structure,chemical composition,electrochemical properties and lithium ion transport and conduction of the composites of sodium content,sintering temperature,holding time,carbon content and hydrothermal treatment time were systematically studied by using modern testing techniques.The results show that the addition of sodium source in synthetic process can result in the generation of rhombohedral LVP and sodium vanadium phosphate（NVP）in the prepared materials.By adjusting the molar ratio of lithium to sodium,the composite cathode material with different phase content of the monoclinic LVP and rhombohedral LVP and rhombohedral NVP can be obtained.With the increase of sodium content,the content of monoclinic LVP in the material decreased gradually,and the content of rhombus NVP increased gradually.The change of sodium content did not affect the structure of the material.However,with the increase of sodium content,the electrochemical performances of the material increased first and then decreased.When the molar ratio of lithium was 5:1,the electrochemical performances of the composite was highest.The higher the sintering temperature,the higher the content of rhombohedral LVP in the material.When the sintering temperature was 800^oC,the as prepared material had a small amount of spherical particles,the amorphous carbon was coated on the particle surface of the material with a mesoporous structure;The electrochemical performance of the composites increased first and then decreased with the increase of temperature,and the electrochemical performance was the highest at 800℃,and the discharge capacity was 118.55 mAh g^-1and 98.27 mAh g^-1respectively at 5 C and 10 C.The changes of holding time would not lead the change the phase composition of the material.The material prepares under holding time of 8h was coated witha uniform carbon layer;and the electrochemical performance of the material increases first and then decreases with the increase of the holding time.The electrochemical properties of the prepared material under the holding time of 8 h was highest.The discharge capacity of the composites was 119.7 mAh g^-1and 91.78mAh g^-1 respectively at 5 C and 10 C.The increase in carbon content does not change the phase composition of the material but leads to a slight decrease in the content of monoclinic LVP.When the carbon content increased,the electrochemical performance of the material increased first and then decreased.When the carbon content was6.24%,the electrochemical performance of the material was the highest.The longer the hydrothermal treatment time,the higher the content of monoclinic LVP in the material,the lower the content of rhombohedral LVP.The particle size of the synthesized material under 24 h hydrothermal treatment was about 200 nm,and the mesoporous structure with single pore size.With the increasing of hydrothermal time,the electrochemical performance of the material increased first and then decreased.The discharge capacity of the material synthesized through 24 h hydrothermal treatment was the highest,and the discharge capacity at 5C and 10C rate was 114.75and 92.63 mAh g^-1,respectively.The particle dispersion of the material synthesized by hydrothermal treatment was higher than that of the non-hydrothermal treatment,and its lithium ion transport performance is much higher than that of the latter.