Study On The Synthesis Methods And Electrochemical Properties Of Lithium Vanadium Phosphate As Electrode Materials

The monoclinic lithium vanadium phosphate?LVP?has high theoretical specific capacity(197 mAh?g^-1)and high energy density,making it a promising cathode material for high-voltage and high-capacity lithium-ion batteries.In recent years,intensive research has been focused on the development of synthesis methods for nanostructured or carbon coated LVP to improve the electrical conductivity,thereby enhancing the electrochemical performance of the material.For the same purpose,this thesis reports our studies on the synthesis methodology of LVP and electrolyte additives for the LVP battery system.?1?The effects of valence states of the vanadium precursor on the electrochemical performance of resulting LVP materials were studied.Under the same conditions,V?III?and V?IV?were used as vanadium sources to synthesize LVP[V?III?]and LVP[V?IV?]respectively,through a sol-gel method.In the initial cycling,LVP[V?III?]shows 11%higher specific capacity than LVP[V?IV?],as well as better rate performance.The improved electrochemical performance of the LVP[V?III?]material can be attributed to the narrower distribution of the grain size and denser and more uniform carbon coating.These more favorable microstructure features is probably due to the more straightforward and efficient pathway to form LVP solid phase from V?III?,without the reduction step complexing with the mass transform process that is involved in the reaction using V?IV?precursor.On the basis of the above result,we then optimized the pH value during the sol formation step and carbon source in the synthesis process to further improve the performance of LVP[V?III?]materials.Compared with the condition without adjusting pH value?pH=7?in the sol formation step,a LVP material with more uniform grain size and narrow distribution can be obtained at pH=9.In addition,using PVP as a single carbon source gives LVP with smaller electrochemical impedance?40?vs 140??than that synthesized from a combined carbon source of citric acid and PVP.At the discharge rate of 5 C,the former material shows a higher specific discharge capacity than the latter in the voltage ranges of 3.0-4.3 V and 3.0-4.8 V(95.0 mAh?g^-1 vs 36.9 mAh?g^-1 and 101mAh?g^-1 vs 22.26 mAh?g^-1).?2?The effect of electrolyte additives on the electrochemical performance of LVP[V?III?]battery system was also studied.The above-mentioned optimized LVP[V?III?]material was subject to the electrolyte study,and an electrolyte with composition of 1 M LiPF₆-EC:DEC?1:1?was used as the electrolyte.Biphenyl,dimethoxytetrafluorobiphenyl?1?and fluorophenyl-3-methoxyphenyl?2?were used as the electrolyte additives,repectively.Compared with the electrolyte without additive,the biphenyl additive degraded the cycling stability of LVP[V?III?],while the other two fluorine-containing additives can both improve performance electrochemical during cycling.Specifically,using additive 1 can significantly improve the rate performance of the materials in the voltage range of 3.0-4.3 V.The improvement of rate performance when using additive 2 is remarkable at the high voltage range of 3.0-4.8 V,and when cycling at 5 C,8 C and 10 C,the discharge specific capacities of the material reach 103.4mAh?g^-1,94.6 mAh?g^-1 and 88.8 mAh?g^-1,respectively.