Study On Controllable Synthesis And Modification Of Lithium Iron Phosphate For Lithium Ion Battery Cathode Material

Lithium-ion battery(LIB)system is one of the most promising energy storage systems as the key point of the Internet of Energy,Smart Grid and new energy vehicle power system.Lithium iron phosphate(LiFePO4,LFP)has the advantages of good safety,outstanding theoretical specific capacity and excellent cycle performance,thus becomes a bright cathode material for lithium-ion battery.However,its poor electronic conductivity and insufficient Li-ion diffusion coefficient limit its further application.In this thesis,the LiFePO4 material was modified by means of particle morphology optimization and material surface modification.The main research contents are as follows:Hollow LiFePO4 microspheres were prepared by a two-step solvothermal method.The influence of different experimental parameters on the synthesis of LiFePO4 was investigated by single factor experiment.The parameters of controllable synthesis of LiFePO4 particles were obtained,synthesis time-4 h,solvent-EG,temperature-180 °C and the pH-7.The LiFePO4 material prepared by optimized parameter had pure-phase crystal structure and a morphology of spherical with hollow structure.The three-dimensional structure shortens the electron transport and Li+ diffusion distance in the bulk phase.The initial charge-discharge performance was 145.3 mA·h/g and 143.2 mA·h/g at 0.1 C rate,respectively.At 0.5 C,1 C,2 C and 5 C,the discharge capacities of the samples were 130.4,117.5,101.3 and 82.7 mA·h/g,respectively.In the 100-time 0.1C cyclic test,capacity loss was less than 5%.As a pure phase LiFePO4 material,LiFePO4 hollow microspheres exhibited improved chemical performance.LFP/ CNS and LFP/ C composites were prepared by carbon coating modification of LiFePO4 hollow microspheres with laboratory prepared carbon nanoscroll(CNS)and ascorbic acid(Vc).Both samples retained the microstructure of LiFePO4 hollow microspheres and achieved uniform carbon coating on the surface of spheres.The initial discharge capacities of LFP/ CNS and LFP/ C materials were 160.3 mA·h/g and 156.3 mA·h/g at 0.1 C,respectively,and the capacity retention rates after 60 cycles at diffrent C of LFP/ CNS and LFP/ C materials were 94.5% and 92.8%,respectively.Carbon coating improves the electrical conductivity of the material,which is much better than that of pure LiFePO4 hollow microspheres.In addition,CNS added to form a highly efficient conductive network between LiFePO4 particles,so that the overall performance of composite materials was even more significant.The free-standing flexible composite electrode materials were prepared by vacuum filtration method with LiFePO4,graphene with good electronic conductivity and nanofibrillated cellulose(NFC)with excellent mechanical strength and flexibility.The flexible composite electrode exhibited good mechanical performance.NFC and graphene formed a three-dimensional conductive frame in the composite,which provided highspeed electron conduction channel for LiFePO4 and reduced polarization of the electrode during charging and discharging.The electrochemical performance of LiFePO4/ graphene/ NFC electrodes was enhanced obviously,the initial discharge capacity was 150.6 mA·h/g at 0.1 C,the capacity retention rate was 95.0% after 60 cycles at higher current rate.