Synthesis And Electrochemical Performance Of LiFePO₄/C For Lithium-Ion Battery Cathode Material

The olivine LiFePO₄ is very hopeful material as positive electrode materials for the future lithium-ion batteries material because of its nontoxic, inexpensive, environmentally benign, high theoretical specific capacity, fine cycle performance and stable thermal nature. However, LiFePO₄ has a low electronic/ionic conductivity and its cycle ability at large charging/discharging current should be improved. This thesis focuses on olivine LiFePO₄ snythesized by high temperature solid state reaction and rheological phase method and its electrochemical performance, the samples are characterized by thermogravimetry and differential thermal analysis (TG/DTA), X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared (FTIR), laser Raman spectroscopy (RAMAN) and constant-current charge/discharge test system. The main aspects are as follows:(1) The LiFePO₄/C cathode materials are synthesized by high temperature solid state reaction with the resorcinal-formaldehyde (RF) used as addition materials. The effect of the RF content, calcination time and sintering temperature on purity and electrochemical performance of the products are investigated. The results show that the content of RF with 9 % and the products sintered at 700°C for 16 h possess excellent electrochemical performances.(2) The effect of different carbon sources on the electrochemical performance of the LiFePO₄/C cathode materials. LiFePO₄/C cathode materials are prepared using soluble starch, sucrose, citric acid and RF produced in situ esterification restriction method as carbon sources by rheological phase method. The results indicate that LiFePO₄/C cathode material is higher capacity and better cycle property that an in situ esterification restriction method to produce the RF as carbon source.(3) The effect of adding method of resin on the electrochemical performance of the LiFePO₄/C cathode materials. The content of resorcinol and formaldehyde and sintering temperature of the products are investigated in situ esterification restriction method. The results show that the optimal sample synthesized at 750°C for 12 h and with a carbon content of 5.1 %, delivers an initial discharge capacity of 138.4 mAh·g^-1 at 0.2 C and a good cycling property at 0.5 and 1.0 C rate, respectively. The LiFePO₄/C cathode material synthesized with the water soluble phenol-formaldehyde resin (PF) used as carbon source is also discussed. The results show that its cycle ability at large charging/discharging current is rather better than that of the cathode materials synthesized using PF produced an in situ esterification restriction method as carbon source.