Preparation And Properties Of Lithium Vanadium Phosphate And Its Modified Materials

As a kind of polyanion positive electrode material,which is most likely to become one of the newly generation of cathode materials for lithium ion batteries,monoclinic lithium vanadium phosphate?Li₃V₂?PO₄?₃?have the advantages of high working voltage,high theoretical capacity,good stability,good safety,and low price.However,V atoms are separated from each other because of the special spatial structure of monoclinic structure Li₃V₂?PO₄?₃,which limits the transmission of electrons,leads to a low electronic conductivity of the material and limits the large-scale commercial application.In order to improve the shortcomings of Li₃V₂?PO₄?₃,this paper made a modification research through optimizing the synthesis process,improving the electrochemical performance of materials and adopting ion doping method.This paper also tested the performance of materials,which include several kinds of metal cations with different valence states and different periods of halogen anion doping,and made a conclusion.This paper studied the synthesis of Li₃ V₂?PO₄?₃/C material by sol-gel method.The effect of different process conditions on the electrochemical performance of Li₃V₂?PO₄?₃ material was studied by controlling the amount of experimental reagents or experimental conditions and we obtained the optimum process parameters:pH=7,the amount of citric acid added was twice the amount of V₂O₅ material,the sintering temperature was 800°C,and the sintering time was 8h.The synthesized Li₃V₂?PO₄?₃/C material with optimized process has a first cycle discharge specific capacity of140.4mAh/g at a 0.2C rate in the voltage range of 3.0-4.8V,and the material's large rate performance needs further improvement.The effect of cation doping on the modification of materials was tested in different valences and positions.There are substitutions at the V position:Mn²⁺,Ba²⁺,Sb³⁺,Cr³⁺,and Li sites are substituted for Na⁺.The optimum doping ratio of divalent cations Mn²⁺and Ba²⁺is x=0.04,and the discharge specific capacity of the material with the best doping ratio at 5C is 92.4mAh/g,87.6mAh/g,and the trivalent metal cation Sb³⁺and Cr³⁺,respectively.The optimum doping ratios were x=0.04 and x=0.01,respectively,and the discharge specific capacities of the best doping ratio materials at 5C large rate were 69.6mAh/g and 98.7mAh/g,respectively.Both are much higher than the large rate performance of undoped materials.The results show that the smaller the ion radius in the cation doping with the same valence,the better the performance improvement of the material.The effect of different halogen anion doping on the modification of the material was studied.Anion replaces the position of PO₄ group,in which the optimal doping ratio of F^-is x=0.10,the optimal doping ratio of C l^-,Br^-is x=0.12,and the optimal doping ratio of the material is 0.2C rate.The discharge capacity of the first cycle was142.7mAh/g,147.4mAh/g,and 143.6mAh/g,respectively,and the discharge specific capacities at the 5C large rate were 86.1mAh/g,96.0mAh/g,and 98.7mAh/g,respectively.Doping with halogen elements can increase the capacity at low rate.The smaller the ion radius,the greater the charge and discharge performance of the material will be.C harge and discharge on high rate,the larger the ion radius is,the greater the charge and discharge specific capacity of the material is.