Curve graph -Effects of temperature on charge/discharge LiFePO4 cathode for Li-ion batteries

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Low temperature performance of LiFePO4 cathode is investigated by charge/discharge test and electrochemical impedance spectroscopy (EIS). The results show that the effect of charge temperature on charge and discharge capacity is very slight in range of −20 to 20 °C and the influence of discharge temperature on discharge capacity is more greater. At −20 °C, LiFePO4 cathode only retains less than 50% of the discharge capacity obtained at 20 °C. We can extend time durability of constant-voltage charge to improve the discharge capacity, especially below −20 °C. Charge/discharge dependence on temperature is studied by EIS and the results show that the charge-transfer resistance (Rct) becomes significantly higher than bulk resistance (Rb) and solid-state interface resistance (Rsei) as temperature decreases, no matter in charge or discharge state. The Rct value of charged state is also found to be much lower than that of discharge state. Charge-transfer activation energies of charge and discharge are calculated to be 33.80 and 40.90 kJ mol−1, indicating that the extra energy barrier is the main thermodynamic factor causing the sluggish discharge behavior at low temperature.







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