Study On Synthesis And Electrochemistry Performance Of LiFePO₄ As Cathodic Materials Of Lithium-ion Battery

Olivine-type LiFePO₄ is considered as the most promising candidate for the next-generation cathode materials of Li-ion batteries because it has high theoretical specific capacity, is cheap and environmentally friendly, and also has good cycling characteristics and excellent safety. However, pure LiFePO₄ shows low reversible capacity and poor charge-discharge characteristics at high current density due to its poor electronic conductivity. Some measures such as reducing particle size, adding conductive additive, and cation ion doping were taken to improve the electrochemical performances of LiFePO₄, and some progresses have been achieved so far.The research aims at developing LiFePO4 that can reach a level of practical application. LiFePO₄, Li (Mn, Fe) PO₄, Li (Co, Fe) PO₄, Li (Ni, Fe) PO₄ have been synthesized by the solvothermal method and the corresponding carbon-coated composites were also prepared using high-temperature heat treatment route. LiFePCVC and (Li, Nb) FePO₄ /C composites have been synthesized by one-step solid phase reactions. The microstructures and morphologies of these composites were investigated by XRD, SEM, TEM, and Rietveld refinement. The electrochemical performances have been evaluated by galvanostatic charge-discharge cycling, cyclic voltammetry (CV) and electrochemical impedance spectra (EIS). The differences in performances between solvothermally and solid phase synthesized products have been studied. The effects of the synthesis methods and synthesis parameters on the electrochemical performances of LiFePO₄ have been systemically investigated.LiFePO₄ has been synthesized by the solvothermal method using FeSO₄, H₃PO₄ and LiOH as raw materials. The product shows a high purity, good crystallinity, small particle size and homogenous size distribution. The effects of synthesis temperature and time on the morphologies and electrochemical performances of LiFePO₄ have been analyzed. It was found that well-crystallized LiFePO₄ with small particle size shows good electrochemical performances. In the present case, LiFePO₄ synthesized at 150℃ for 15 h shows best electrochemical performances. The study on the formation mechanism of LiFePO₄ by the solvothermal method shows that in order to obtain single-phase LiFePO₄, LiOH and H₃PO₄ must be mixed first to form intermediate Li₃PO₄, and then Li₃PO₄ reacts with Fe²⁺ to form pure LiFePO₄. LiFePO₄/C composites are synthesized by carbon-coating the hydrothermally synthesized LiFePO₄ through pyrolyzing polypropylene at high temperature. The study suggested that the best coating temperature is 550℃. Under this condition, ball-likeLiFePCVC was obtained, which shows a first discharge capacity of 162 and 159 mA h g"1 at 0.05 C and 0.1 C, respectively, close to the theoretical capacity of LiFePC>4. However, the electrochemical performances of the materials at large current density are far from practical application.LiMxFei.xPO4 and their corresponding carbon-coated composites have been synthesized by the solvothermal method using Mn, Co and Ni as doping atoms and the corresponding heat treatment route. This research has explored the best synthesis technology and best doping content for solid solution doping. It was found that the electrochemical performances of LiFePO4 could be improved to some extent by Mn doping. LiMno.isFeo.ssPC^ prepared by hydrothermal route at 170°C for 3 h shows a 17% higher specific capacity than pure LiFePO4. After carbon coating, however, the charge-discharge performances at large current is not obviously improved. In this research, it is found that the optimized Ni doping content is 5%. LiNio.o5Feo.95P04 prepared by hydrothermal route at 160°C for 3 h shows a 33% higher specific capacity than pure LiFePO4. Under this condition, the capacity of LiNio.osFeo.gsPC^ is 33% higher than LiFePO4. After carbon coating, this material yields a discharge capacity of 130 mA h g"1 at 1 C. The cycling stability of LiFePCVC can be obviously improved after Ni doping. When cycled at 1 C for 100 times, this material exhibits capacity increase instead of decrease. However, Co doping cannot help improve the electrochemical performances of LiFePCVC whether it was undergone carbon coating or not.Carbon-coated LiFePC>4 has been developed by solid-state reaction using inexpensive Fe2C>3, NH4H2PO4 and LiOH as the raw materials and polypropylene as the reductive agent and the carbon source and the synthesis technology was explored. It was found that LiFePCVC composite prepared at 600°C for 10 h exhibits good electrochemical performances, yielding a discharge capacity of 136 mA h g"'at 1 C. Based on the above research, Lii-sjtNbxFePCVC has been synthesized by a one-step solid-state reaction through Li+ sites doping using Nb5+, whose radius is close to that of Li+. It was found that when the molar ratio of doping content is 0.008, the discharge capacity of this material reaches 148 mA h g"1 at 1 C and 130 mA h g"1 at 2 C, and the capacity fade is only 2.6% after 100 cycles at 1 C. Cyclic voltammetry tests of Lii.5lNb,FePO4/C indicate that polarization decreases obviously after doping. EIS tests show that the sum of/?ctand Rf shows a continuous decrease with increasing Nb doping content in Lii.sjcNbxFePCVC. When the Nb doping content increases from 0 to 0.01, the sum of Rct and R{ decreases from 300 to 65 Q. It indicates that Nb doping greatly improves the conductivity of LiFePCVThis research has analyzed the reason why the electrochemical performances of LiFePCVC prepared by the solvothermal method are not so good as that prepared by solid-state reactions. Rietveld refinement fitting shows that the length of Li-0 bond is larger of LiFePO4 by solid-state reactions compared with that prepared by hydrothermal route. It is suggested that binding force is weakened because of the large bond length, which facilitates diffusion of Li+ in solid phase. As a result, the LiFePCVC prepared by solid phase reactions exhibits better electrochemical performances.