Modification Of LiFePO₄ Precursor And Theory Of High Temperature Synthesis Of LiFePO₄

FePO₄ is an important precursor for LiFePO₄,and its microstructure is of great significance to the preparation of lithium-ion batteries with excellent performance.Firstly,ferrite source and phosphorus sources have an effect on the morphology of iron phosphate.Secondly,iron phosphate with different morphology and structure can be prepared by different preparation methods.In addition,the morphology of iron phosphate can be modified by surfactant.However,there are many kinds of surfactants,which require a lot of experiments to explore.The main contents and results of this paper are as follows:Firstly,Iron phosphate was synthesized by hydrothermal method using Fe?NO₃?₃·9H₂O?NH₄H₂PO₄ as raw material,and the synthesis of ferric phosphate at120?temperature and reaction time was compared.The crystal structure and morphology of the prepared iron phosphate were investigated by XRD,SEM,TG-DTA et al.The results show that the morphology of iron phosphate prepared by hydrothermal reaction at 120?for 6 h is better.Secondly,FeSO₄·7H₂O?H₃PO₄?H₂O₂ and FeCl₃?H₃PO₄ were used as raw materials for 6 h of hot reaction at 120?,and compared with iron phosphate prepared from Fe?NO₃?₃·9H₂O?NH₄H₂PO₄.The results showed that Fe?NO₃?₃·9H₂O and NH₄H₂PO₄ were used as raw materials to prepare iron phosphate with better morphology and structure.Thirdly,Fe?NO₃?₃·9H₂O?NH₄H₂PO₄ as raw material,iron phosphate was prepared by coprecipitation method and compared with that prepared by hydrothermal method.The results show that the iron phosphate prepared by co-precipitation method generally does not contain crystalline water,but the particle size is irregular.And the particle size of iron phosphate prepared by hydrothermal method is small,but it is easy to be heterogeneous and the dispersion is not good.In addition,the iron phosphate prepared by hydrothermal method is FePO₄·2H₂O.The iron phosphate prepared by the two methods has different crystal structures,while the iron phosphate prepared by hydrothermal method belongs to the Monoclini crystal system and the space group P2₁/n?14?,while the iron phosphate prepared by the coprecipitation method belongs to the Hexagonal crystal system and the space group P321?150?.Forthly,the morphology of iron phosphate prepared by hydrothermal method was modified,that is to say,the surfactant was added to the reactants.In chapter 4,the addition of surfactant is described in detail.Four kinds of surfactants?cationic surfactant,anionic surfactant,nonionic surfactant,polymer surfactant?were added.The results showed that the anionic surface surfactant could promote the formation of spherical iron phosphate,but had no obvious effect on dispersing particles and inhibiting agglomeration.The optimal addition of salicylic acid was 1.5%³%,the optimum addition of ascorbic acid was 3%,and that of citric acid was 5%;The cationic surfactant?CTAB?can form a spherical shape of iron phosphate,and a small amount of addition can also disperse the particles and inhibit agglomeration.The optimal addition of CTAB is 1.5%;Nonionic surfactant?dimethyl succinate,Tween?can reduce the particle size and improve the morphology,but it can promote the agglomeration of the particles.The addition of a small amount of polymer surfactant?PEG,PVA,PVP?contributes to the formation of spherical particles.Excessive addition of polymer surfactants can inhibit the formation of spherical particles and aggravate the agglomeration between particles.The optimal addition of PEG is 1.5%,the optimum amount of PVA is 1.5%,the optimum addition of PVP is 1.5%.Last,the kinetics of the synthesis of lithium ferric phosphate at high temperature was studied.Firstly,the reaction process was investigated by TG-DSC.The kinetic parameters of the reaction were obtained by linear fitting of Kissinger method and Ozawa method,that is,activation energy E,frequency factor A.By comparing the kinetic parameters obtained by the mode function method,the coats-redfern formula,the Kissinger method and the Ozawa method,the suitable most probable mechanism function is determined.Finally,the non-isothermal kinetic equation of the reaction is obtained.The results show that the reaction model accords with the Avrami-Erofeev?n=2/3?model,G???=[-ln?1-??]^2/3,f???=4?1-??[-ln?1-??]^3/4.And the non isothermal kinetic equation is:???...