Study On The Composite Of Lithium Iron Phosphate With Graphene By Spray-Thermochemical Reaction As The Cathode Of Lithium Ion Battery

In recent years,lithium ion battery,as one of the most important parts of electric vehicles is being widely studied,since the rapid development of electric vehicles all over the world.Among a variety of lithium ion battery cathode materials,polyanionic lithium iron phosphate cathode material is considered to be the most promising material by majority reserachers,because of a series of advantages such as high safety performance,good thermal stability,good electrochemical performance,rich raw material and low cost.However,the lithium iron phosphate material has poor conductivity,the complex process of producing,and the high cost of preparation in the current market somehow limit its wide application.In this work,we used the cheap iron oxide as the iron source to prepare the composite of lithium iron phosphate with grapheme?G/LiFePO₄?materials by spray thermochemical reaction,in order to overcome these shortcomings,and some key influence factors on the electrochemical properties of materials was systematic investigated as well.In this paper,we used lithium carbonate?Li₂CO₃?,phosphate?H₃PO₄?,iron oxide?Fe₂O₃?and liquid acrylonitrile oligomer?LPAN?as raw materials,dispersing in deionized water,to prepare the precursor of G/LiFePO₄ by ordinary milling or nano grinding combined with spray drying technology.Then,the final product was obtained by calcining precursor at high temperature.The physical and chemical properties of G/LiFePO₄ materials prepared under different conditions were characterized,such as morphology,particle size,conductivity,tap density,crystal phase structure,electrochemical properties,cyclic voltammetry,impedance and the likes.At the same time,the relationship between material structure and electrochemical properties was studied.The main research contents and results are as follows:1.Using Li₂CO₃,H₃PO₄ and Fe₂O₃ as raw materials,LPAN as carbon source,LiFePO₄ cathode material was synthesized by the ordinary milling method.The feasibility of the scheme was proved and the effects of molar ratio of Li/Fe on the physicochemical properties of the products were discussed as well.In our research,the LiFePO₄ cathode material with uniform particle size distribution and better electrochemical performance is obtained when Li/Fe molar ratio is 0.94.Under the current density of 0.1 C and 1 C,the discharge specific capacity can reach to 120 m Ah/g,81 m Ah/g,respectively.2.G/LiFePO₄ was prepared by spray—thermochemical reaction.Meanwhile,effects of different synthesis processes on the physical and chemical properties of the materials were investigated.We get the best experimental program as follows: firstly,ferric oxide and phosphoric acid is mixed and then added with lithium carbonate and LPAN.Discharge specific capacity and cycle stability of the G/LiFePO₄ cathode material synthesized by this method are the best.3.The process conditions of preparing G/LiFePO₄ by spray thermochemical reaction were further optimized.Based on the ratio of raw materials,the effects of on the the materials prepared when nLi/Fe=0.96¹.10 were analyzed by XRD,particle size,SEM and so on.Our experimental results showed that when the molar ratio of lithium to iron is 1.02,the surface morphology of the material we obtained has the most smooth,the even particle size distribution and the best electrochemical performance.Subsequently,we investigated the effects of LPAN content and calcination temperature on the particle size,morphology,phase and electrochemical propertie.4.We prepared lithium iron phosphate with the best performance under a certain condition.In particular,the precursor was calcined at 700 oC for 6 h,which included Li₂CO₃,H₃PO₄ and Fe₂O₃,as raw materials with molar ratio Li:Fe:P=1.02:1:1,and LPAN,occupied 20% of mass of other three materials mentioned before.The discharge specific capacity of the material reaches 162 m Ah/g at 0.1 C and 129 m Ah/g at 1 C.In addition,after 50 cycles,the capacity retention rate can maintain at more than 99%.The results showed that the specific discharge capacity is 114.8 m Ah/g,106 m Ah/g,and 85 m Ah/g at 3 C,5 C and 10 C,respectively,which indicates that it has excellent high rate of chargedischarge performance.Under the optimum conditions,the particle size of the material we obtained is D50=4.7 ?m,and its compaction density is 1.32 g/cm3.We further confirmed the existence of graphene composite layer by raman spectroscopy and transmission electron microscopy.The cyclic voltammetry showed that the charge and discharge of the material are reversible and the polarization is very low.The electrochemical measurements showed that the discharge capacity is close to 100 m Ah/g at a high current density of 10 C.Through a series of studies,the composite of lithium iron phosphate with graphene prepared by this research has excellent physical and chemical properties.Moreover,the process is simple,strong operability and low energy consumption,which meets the requirements of green low-cost industrialization.In general,our experiments has provided a new way to produce G/LiFePO₄ with low cost and excellent performance.