Study On Synthesis Methods And Property Of The Cathode Material LiFePO₄ For Lithium Ion Battery

Olivine type LiMPO4 (M=Fe, Mn, Co, Ni) proposed by Padhi gruoup have been intensively studied as Li-ion secondary batteries due to its overwhelming advantages of low cost, low toxicity, excellent cycle life et al. However, its practical application is limited by its extremely low intrinsic electronic conductivity, Li-ion diffusion rate and pack density. Two approaches have been attempted to overcome this problem. One is to enhance its electronic-conductivity by coating an electron-conducting layer such as conductive carbon, nano-copper and nano-silver around the particles or by doping with guest cations. The other is to minimize the particle size by modifying synthesis conditions such as ball-milling synthesis, co-precipitation, sol-gel preparation and hydrothermal method.In this thesis, the synthetic technology of three different wet methods (electrolytic method, hydrothermal and coprecipitation) were explored to enhance the electrochemical performance of cathode material LiFePO4 for lithium ion battery, and the compositions of LiFePO4 precursor prepared by three wet methods were attempted to discuss. The distinguishing innovation in this paper is to successfully synthesize LiFePO4 by electrolytic method, and studied the preparation and electrochemical properties. Also the parameters and electrochemical properties of the other two wet synthesis of LiFePO4 were studied. The main contents and conclusions are as follows:1. The effect of different lithium sources and different amounts of CTAB additives as inducer on the LiFePO4 material crystal structure, morphology and electrochemical properties were investigated by hydrothermal method respectively. The results show that the material LiFePO4 synthesized under the conditions of the lithium source in alkaline and a certain amount of CTAB has simple pure olive-type and improved charge-discharge performance, probably because Differences of pH of the selected lithium source resulted the different composition of the precursor, which caused impurities in the synthesized materials LiFePO4 phase. In addition, Fe2+ was easily oxidized to Fe3+ in the calcination process without the additive amount of reduction.2. LiFePO4 cathode materials were synthesized by electrolytic method, and the influences of pH on the structure and the performance of the samples were discussed. The results showed that the annealed LiFePO4 has simple pure olive-type phase when the molar ratio of Li+ and PO43- reached to 3 or above and the pH was maintained at 7.0±0.5. In essence, the precursor prepared by electrolytic method is composed of mixture of Fe3 (PO4) 2·8H2O and Li3PO4 with its molar ratio of 1:1. It is critical process parameters to control pH and composition of the electrolyte for obtaining the pure phase.3. LiFePO4 cathode materials were synthesized by co-precipitation, and the influences of pH on the purity of the precursor were investigated in the water system. The results showed that the LiFePO4 prepared by co-precipitation have impure substance with Fe3(PO4)2 under acidic conditions; the LiFePO4 have impure substance with Li3PO4 under alkaline conditions. This may be because crystal LiFePO4 formed after calcination as following reaction:Fe3(PO4)2·8H2O+Li3PO4â†'3LiFePO4+8H2O and the precursor of the precipitation is the mixtures of Fe3(PO4)2·8H2O and Li3PO4. Li3PO4 is difficult to precipitate in acidic conditions, so the molar ratio of the system don't reached to 1:1, LiFePO4 have impure substance with Li3PO4 under acidic conditions and with Fe3(PO4)2·8H2O under alkaline conditions. To further confirm that the co-precipitation precursor mixture is Fe3(PO4)2·8H2O and Li3PO4, LiFePO4 of single phase was synthesized by ball milling under the conditions of Fe3(PO4)2·8H2O and Li3PO4 as raw materials with the molar ratio of 1:1 and the capacity of the material can reach to the capacity of the material prepared by coprecipitation.