Study On Carbon Coating For Hydrothermally Synthesized LiFePO₄

Carbon coating process is a very important step for improving the conductivityof bare LiFePO₄（LFP） prepared by hydrothermal method. Carbon source and coatingprocess have significant effect on physical and chemical performance of LiFePO₄/C（LFP/C）. Glucose, sucrose and PEG6000were selected as the carbon sources.The heat behaviors were analyzed by TG-DTA for individual carbon source and themixture of carbon sources and LiFePO₄powder. The effect of carbon source, carbonamount and calcination atmosphere on the electrochemical performance of LFP/C wascharacterized by X-ray diffraction, transmission electron microscope andelectrochemical test.Washing treatment is needed for the LFP obtained from hydrothermal synthesisbefore carbon coating. The effect of water consumption and wash times on the SO42-percentage in LFP and electrochemical performance was studied. The results showthat the optimal washing process conditions are mwater: mslurry=2:1, and two timeswashing.The heat behaviors are obviously different from individual carbon source andthat on the surface of LFP. The results show that the carbonization reaction iscatalyzed by LFP, which can lower the starting temperature more than8%. The massloss occurs mainly in the temperature less than400°C. The key point of calcinationtemperature was determined by TG-DTA curves. The effect of calcination temperature,temperature rising rate and holding time on the electrochemical performances of LFPwere investigated. The optimal calcination conditions were obtained: the mixture ofcarbon sources and LFP powder are heated to350°C in the rising rate of8°C/minthen holding for two hours before it heated to700°C in the same rising rate, the holding time is4hours at700°C and then cooling to ambient temperature under N2atmosphere. The calcination temperature is the key factor that influencing theelectrochemical performance of LFP/C by improving the conductivity of carbon andthe crystallinity of LFP.The effect of carbon amount on the electrochemical performances of LFP wasstudied. The results show that the optimum amount of carbon is2wt.%. PEG6000could lead to a more evenly distributed carbon on the surface of LFP and a betterelectrochemical performance such as discharge capacity, C-rate capability, cyclingand low temperature performance.The effect of reducing atmosphere resulting from moisture in the mixture on theperformance of LFP/C was studied by moisture variation in the samples andatmosphere. The results show that the reducing atmosphere has a certain effect on theelectrochemical performance of LFP/C. The reducibility can be enhanced by themoisture. Enhanced reducibility can lead to less evenly distributed carbon on thesurface of the LFP and lower the surface activity of the particles which resulting inthe poorer conductivity and electrochemical performance of LFP/C.In this work, the optimum carbon coating condition was concluded byexperimental optimizing and theoretical analysis. The LFP/C samples exhibit perfectelectrochemical performance: the discharge capacities are161.6,156.6and144.0mAh/g at0.2,1and3C and at room temperature, respectively. The low temperature（-20°C） discharge capacity is125mAh/g at0.2C. The capacity retention can reach78%compared to that at room temperature.