Deterioration of lithium iron phosphate/graphite power batteries under high-rate discharge cycling

In this study, the deterioration of lithium iron phosphate (LiFePO4) /graphite batteries during cycling at different discharge rates and temperatures is examined, and the degradation under high-rate discharge (10C) cycling is extensively investigated using full batteries combining with post-mortem analysis. The results show that high discharge current results in an instability of electrode/electrolyte interface and unstable solid electrolyte interphase (SEI) layers are expected to form on the newly exposed graphite anode surface, which cause sustainable consumption of active lithium and further lead to the performance degradation of active materials. For LiFePO4 cathode, the initial capacity is largely recovered under low rate (0.1-0.2C), whereas a decline in the capability is observed at higher rates (0.5-3.0C). For graphite anode, half-cell study shows that considerable capacity loss occurs even at low rates. A small amount of Fe deposition is observed on graphite anode after cycling under 10C discharge at 55 °C. X-ray photoelectron spectroscopy (XPS) analysis confirms that a layer composed of lithium compounds is formed on the surface of anode, which can not participate in the reversible electrochemical reaction again. In addition, electrochemical impedance spectrum (EIS) measurements of half-cell indicate that the increased resistance of the positive electrode is suggested to be the root cause of power fading under high-rate discharge cycling, especially at high temperature.