An Investigation on the Cycle Performance of LiFePO₄ Pouch Cells by a Combination of Synchrotron Based X-Ray Diffraction and Absorption Spectroscopy

The LiFePO4 cathode with a Fe/P ratio of 0.99 has the best cyclic performance at the charge/discharge rate of 2 C/5 C by using a laminated pouch type 1.0 Ah full cell. The cycle life curves of the pouch cell show that the number of cycles increases to 2269 cycles with the capacity retention of 80% as the Fe/P ratio increases from 0.93 to 0.99. To investigate the effect of Fe concentration of stoichiometric LiFePO4 on the aging mechanism of a pouch cell during long-term cycling tests, the structure of electrodes was characterized by a combination of electrochemical tests and synchrotron based X-ray diffraction and absorption spectroscopy. To better understand the moisture effect on LiFePO4/C cells under different humidity levels, we measured the water content, specific surface area and discharge capacity of commercial LiFePO4/C cells over various periods of time. The water content and specific surface area increase with time and the change in surface area is irreversible. The discharge capacity of coin cells does not change with time, but the discharge capacity of pouch cells decreases due to delithiation caused by moisture.