Preparation And Optimization Of Lithium Iron Phosphate

Lithium-ion batteries have become a strong competitor for new-energy vehicle cells due to their advantages of high energy density,large output power and environmental protection.At present,lithium iron phosphate has been one of the commercialized cathode materials.The Feelement contained in lithium iron phosphate（Li FePO₄）comes from a wide range of sources and is low in price.Meanwhile,its stable structure in charge and discharge makes its cycling performance and safety stability excellent.Therefore,Li FePO₄ has become a strong competitor of cathode materials for lithium ion batteries.However,the arrangement characteristics of Fe,O and P atoms in the bulk phase structure limit the transmission of Li⁺,and the capacity of the battery decreases rapidly under large current and the multiplier performance is poor.Therefore,how to improve the diffusion rate of Li⁺in Li FePO₄ so as to improve the capacity of the battery during charge and discharge becomes the key.Nanoscale,ion doping and surface modification are mainly used to improve the diffusion rate of Li⁺.Throughout the market development of Li FePO₄ battery,the price of Li FePO₄ is from 100,000/ton in 2015 to about 30,000/ton in2017.Therefore,production cost control has become the core link of Li FePO₄ enterprise competition.The main industrial production route of Li FePO₄ takes iron phosphate（FePO₄）and lithium salt as raw materials,and the route is simple and controllable.The research focus of this paper is to prepare Li FePO₄ battery materials with excellent performance by FePO₄ route combined with carbon coating and nano modification ideas according to the market production and research modification status of Li FePO₄ materials.At the same time,based on existing Li FePO₄ in the market,the study of surface structure reconstruction is carried out to further improve its electrochemical performance.The specific content is as follows:（1）after a large number of literature,research evaluation to ferric chloride（FeCl₃）and ammonium dihydrogen phosphate（NH₄H₂PO₄）as raw material through co-precipitation preparation FePO₄,furfuryl alcohol monomer（FA）as coated source in poly(furfuryl alcohol FePO₄ particle surface in situ polymerization（PFA）layer,prevent particles together to form a uniform particle size distribution of the size of about 50 nm FePO₄/PFA,and explored the reaction temperature,p H value and the morphology of furfuryl alcohol consumption of materials,performance and the influence of yield.It was found that the FePO₄/PFA prepared was amorphous and had poor crystallinity.With the increase of reaction temperature,the particle arrangement became compact,while the particles appeared obvious agglomeration at too high temperature,and the precipitation yield gradually decreased.By comparison,it can be seen that under the conditions of reaction temperature of 90℃,p H of about 1 and furol alcohol dosage of 0.8 m L,the morphology distribution of the precursor is more uniform and the yield is more appropriate.（2）Carbon-coated lithium iron phosphate was prepared by liquid-phase and solid-phase lithium mixing of FePO₄/PFA precursors respectively,and the effects of sintering processes such as temperature and sintering time on the two paths were investigated.With the increase of sintering temperature,the crystallinity of particles increased,and the particles melted together at too high temperature.The suitable sintering temperature is about 650℃.With the prolongation of sintering time,the crystallinity of the material increases,and the excessively long sintering time at high temperature leads to the particle ripening and agglomeration.The suitable sintering time is 6 h.At the same time,after comparing the two paths,it was found that the LFP/C obtained by FePO₄/PFA precursor at about 50 nm through the liquid phase for H-Li exchange with lithium salt and then sintering and carbonization has excellent electrochemical performance.The discharge specific capacity of 0.2C is 153 m Ah/g,the capacity retention rate of 10C is 63.8%,and the capacity loss of 1C constant current is only 2.7%after 500 cycles.The electrochemical performance of the material is better than that of direct sintering and carbonization of solid phase with lithium salt（0.1C discharge capacity is 120 m Ah/g）.（3）For the lithium iron phosphate with very little carbon coating on the market,after grinding and firing with polypyrrole,more Li can be stored by the interaction between the newly formed carbon-containing layer and Li FePO₄ to improve the electrochemical performance of the material.Under the condition that the capacity loss is minimized when the current is low,the specific capacity of the material is still 110 m Ah/g at high current of 10 C,which is higher than that of the material before modification（55 m Ah/g）.