Low Temperature Performance Of LiFePO₄/C Cathode Materials For Lithium Batteries

LiFePO₄/C cathode material was prepared by ball milling method, and the low temperature electrochemical performance in a quaternary carbonate-based electrolyte (1.0M LiPF6 / EC+DMC+DEC+EMC (1:1:1:3, v/v)) was studied. Electrochemical measurements indicate that the operating temperature has pronounced effects on the charge-discharge performance. The discharge capacities of the LiFePO₄/C cathode were about 134.5mAh/g (20℃), 114mAh/g (0℃), 90mAh/g (-20℃) and 69mAh/g (-40℃) using a 1C charge-discharge rate. The data show that the LiFePO₄/C cathode with this electrolyte could operate down to -40℃using 1C rate. A comparison of rate capability of the LiFePO₄/C cathode between -20℃and 20℃demonstrates that though LiFePO₄/C cathode exhibits appreciable rate performance at 20℃, its high rate performance is obviously hindered at -20℃. Cyclic voltammetry measurements show obviously sluggish of the lithium insertion-extraction process of the LiFePO₄/C cathode as the operation temperature falls below -20℃. Electrochemical impedance analyses demonstrate that the sluggish of charge-transfer reaction on the electrolyte/ LiFePO₄/C interface and the decrease of lithium diffusion ability in the bulk LiFePO₄ was the main performance limiting factors at low-temperature. The activation energies for charge transfer and for lithium diffusion in the LiFePO₄/C cathode were calculated to be 38.9 and 44.5 kJ/mol, respectively.LiFePO₄/C and LiFe1-xMnxPO4/C (x=0.02, 0.04, 0.06) cathode material were prepared by ball milling method. Since LiFe0.98Mn0.02PO4/C shows a highest discharge capacity which reaches 137.8 mAh/g. The low temperature performance of LiFe0.98Mn0.02PO4/C was investigated compared with LiFePO₄/C. When discharge at -20℃with 17 mA/g(0.1C), the capacity is 124.4 mAh/g and 120.5 mAh/g respectively. LiFe0.98Mn0.02PO4/C shows a better rate performance as the discharge current increases. The discharge capacity of LiFe0.98Mn0.02PO4/C is: 99.8mAh/g (1C), 80.7 mAh/g (2C) and 70 mAh/g (5C), while LiFePO₄/C is 90.7 mAh/g (1C), 70.4 mAh/g (2C) and 52.2mAh/g (5C). Cyclic voltammetry and AC impedance results show that Mn doping could improve the reaction reversibility and decrease the resistance of particle connection and charge transfer, which lead to the improvement of low temperature performance.PPy coated LiFePO₄/C cathode material was prepared by chemical oxidation method using TS-Fe as oxidizer. Charge-discharge result shows that PPy coating could improve the capacity under low temperature. When discharged at 2C under different temperature, the capacity of LiFePO₄/C-PPy is128.7mAh/g(20℃),109.3mAh/g(0℃),93.9mAh/g(-20℃),66.1mAh/g(-40℃), while the capacity of LiFePO₄/C is 132.6mAh/g(20℃), 113.2mAh/g(0℃), 87.7mAh/g(-20℃), 44.1mAh/g(-40℃). Cyclic voltammetry and AC impedance were used to investigate the low temperature performance of LiFePO₄/C-PPy and LiFePO₄/C. The result shows that the peak current of LiFePO₄/C-PPy is higher than those of LiFePO₄/C's, while the difference of peak voltage is smaller. The peak current of LiFePO₄/C-PPy under -20℃is twice as large as LiFePO₄/C's. The AC impedance results indicate the increasement of electrolyte and particle connection resistance of LiFePO₄/C-PPy and LiFePO₄/C is smaller than charge transfer resistance. The resistance of electrolyte, particle conection and charge transfer increase as temperature decreases, while the charge transfer resistance increases faster than the others.