State estimation and aging mechanism of 2nd life lithium-ion batteries: Non-destructive and postmortem combined analysis

Lithium-ion batteries used in electric vehicles (EVs) are expected to reach end-of-life (EoL) after 10–15 years of operation. Consequently, it is urgent to devise repurposing strategies, which may range from recycling to a 2nd life in stationary energy storage applications. In this paper, aiming the 2nd life repurposing of the batteries, the state estimation and aging mechanisms of retired 32,650-type LiFePO4/graphite batteries were studied. The aging state of batteries was evaluated in terms of their capacity, resistance and self-discharge rate. Test results show that there is no strong correlation between battery capacity decay and resistance increase. To identify and quantify the aging mechanisms of the 2nd life batteries after being cycled at different rates (0.5C, 1C and 2C), non-destructive analyses (incremental capacity/differential voltage curves, modified equivalent circuit model, and electrochemical impedance spectroscopy) are combined with postmortem analysis. The results reveal that the aging of the batteries cycled at 0.5C is primarily due to the loss of active material (LAM), while the loss of lithium inventory (LLI) is rather insignificant. For the batteries cycled at 1C and 2C, in addition to LAM, LLI is also severe, eventually leading to an increase in resistance of the battery, which causes a rapid decline of the battery capacity. The results also indicate the non-destructive methods are reliable to identify ongoing aging mechanisms.