Study On The Synthesis And Electrochemical Performance Of Carbon Coated LiFePO₄as Cathode Materials

In recent years, facing a serious shortage of non-renewable energy and issue ofenvironmental pollution, all of us have seen a rapid development of breakthrough infacile but effective usage of reproducible energy and especially the new energy storagesystems such as Lithium ion batteries （LIBs）. LIBs have been on a stage of substitutingthe conventional energy storage devices like Ni-Cd, Ni-H, etc in fields of electricvehicles, medical instruments and communication devices. LiFePO₄has a series ofmerits: low-cost, nontoxic, environment friendly, high thermal stability and good cyclecapability, which has made it one of the most promising cathode materials for LIBs ofnext generation. However, it has several defects such as fussy fabrication process, lowconductivity and slow Lithium ion diffusion. Intending to overcome these defects andimprove performance of LiFePO₄, in this work, we prepared the LiFePO₄/C via a newprecipitation-carbothermal reduction method and made research on the choice of thecarbon sources and structure of the carbon coating on LiFePO₄.Firstly, we came up with a new synthesis method taking the advantages of thepast co-precipitation and carbothermal reduction methods, due to the need forindustrialization. Then, we optimized the temperature, time for the heat treatment, andthe optimum carbon content for the carbon coating, using FePO₄, Li₂CO₃and carbonsources as the reactants.Second, for the first time, have we studied the influence of the pyran-ringstructure in carbon sources on the performance of the LiFePO₄/C, trying to find theproper carbon source for carbon LiFePO₄/C, because the carbon sources have greatinfluence on the performance of LiFePO₄. It is concluded that LiFePO₄/C using starchwith many pyran rings as carbon sources exhibits the highest discharge capacity, ascompared to those using glucose with one pyran ring in the molecule and lauric acidwithout pyran-ring structure. The LiFePO₄/C using starch as carbon sources hassmaller particle size, uniform coating thickness, lower ID/IGratio, higher electricalconductivity, lower charge-transfer resistance and hence leads to the betterelectrochemical performance than glucose and lauric acid. The pyran-ring structurebenefits the formation of large conjugated aromatic structure at high temperature and improves the graphitized degree of pyrolysed carbon in LiFePO₄/C, accounting for thegood electrochemical performance.Last, the core-shell and network structured LiFePO₄/C was successfullyfabricated through a facile method to get the most of the electrochemical performanceof LiFePO₄/C. The core-shell and network structure was composed of LiFePO₄/Cparticle with a full, uniform carbon shell and continuous carbon film networkconnecting adjacent particles, which has high specific surface and some pore structure.The structure promotes the formation of conducting path of electrons and improves thecontacts between LiFePO₄/C and electrolyte. As a result, it exhibits a goodelectrochemical performance especially an exceptionally excellent high-rate cyclingperformance.