Electrochemical performance of lithium iron phosphate cathodes at various temperatures

The delithiation-lithiation process of C-coated olivine LiFePO4 (C/LFP) cathodes with the temperature range between 25 and 55 °C has been investigated using cyclic voltammtery, charge-discharge cycling, and ac impedance spectroscopy. Using polyethylene glycol as carbon precursor, the C/LFP powders are synthesized by an efficient calcination/pyrolysis method at 650 °C. The experimental results reveal that the high-temperature operation of C/LFP cathodes shows an improved capacity at 0.1-5 C but negative effect on high-rate cyclic stability. The ac impedance spectroscopy incorporated with equivalent circuit indicates the decrease in equivalent series resistance and the increase in diffusion coefficient (DLi) with operating temperature. The DLi value achieved is as high as 1.49 × 10−12 cm2 s−1 at 55 °C according to the Randles plot, and the apparent activation energy for the ionic diffusion in the C/LFP cathode is approximately 110 kJ mol−1, determined from the Arrhenius plot. On the basis of the results, the improved performance is attributed to high ionic conductivity, high ionic migration rate in solid electrolyte interphase film, high electronic conductivity, and high diffusion rate.