Revealing the Impact of Fast Charge Cycling on the Thermal Safety of Lithium-Ion Batteries

The safety of the degraded lithium-ion batteries has an essential impact on second life application. This study systematically investigates the thermal safety changes of lithium-ion batteries after deep aging under the fast charge aging path and reveals the degradation mechanisms caused by fast charge cycling. Lithium plating is the primary degradation mechanism, which thickens the solid electrolyte interface film, causes the loss of active lithium and electrolyte, and leads to a significant increase in impedance and a dramatic decrease in capacity. Therefore, compared with the fresh cells, the heat generation rate increases, while the total heat generation is reduced for aged cells. Besides, the thickened solid electrolyte interface film has lower thermal stability, decreasing the self-heating temperature for aged cells. Furthermore, thermal runaway results of partial cells prove that fast charge cycling reduces the thermal stability of the anode, which further proves that the thermal runaway-triggering temperature decrease is the result of the combination of the anode–electrolyte and anode–cathode reactions. Moreover, fast charge cycling reduces the lithium plating potential upon overcharging, which leads to the occurrence of side reactions in advance, creating the ratio of side reaction heat increase of aged cells for thermal runaway triggering. In addition, the loss of active materials reduces the maximum temperature and maximum temperature rise rate of the aged cell. The findings can provide references for battery safety management system optimization and safer battery screening.