Engineering Science Research

Engineering Science Research Paper

Low Temperature Performance of LiFePO4/C Cathode Materials for Lithium Batteries

Title Low Temperature Performance of LiFePO4/C Cathode Materials for Lithium Batteries
Abstract

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.

Category Electric Power
Subject LiFePO4/C, Lithium-ion batteries, low temperature performance, PPy coating, slight Mn substitution,
FileType PDF
Pages 156
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