Effects of state of charge on the degradation of LiFePO4/graphite batteries during accelerated storage test

In this paper, the degradation of LiFePO4/graphite batteries during 10 months of storage under different temperatures and states of charge (SOCs) is studied. The effects of SOC during storage process are systematically investigated using electrochemical methods and post-mortem analysis. The results show that at elevated temperature of 55 °C, higher stored SOC results in more significant increase in bulk resistance (Rb) and charge-transfer resistance (Rct) of full battery, whereas the rate-discharge capability of stored battery is unchanged. The side reactions at the electrode/electrolyte interface caused by self-discharge are the main reasons for the performance fading during storage. For LiFePO4 cathode, long-time storage does not influence the framework structure under various SOCs. The existence of little irreversible capacity loss and impedance increase indicates that side reactions also occur at the positive electrode. For graphite anode, only a little capacity loss is found upon storage. There is a significant increase in impedance and a small amount of Fe deposition on graphite anode after storage at 100% SOC and 55 °C. The lithium ion loss arises from side reactions taking place at the graphite anode, which is responsible for the capacity degradation of battery during the storage process. XPS analysis confirms that a deposit layer composed of Li2CO3 and LiF is formed on the surface of anode.