The threat of diaphragm closure to thermal runaway during battery charging

The widespread use of lithium-ion batteries has raised safety concerns, especially during charging, where thermal runaway incidents are more frequent. Accident investigations have revealed carbonization marks inside cells that suggest overcharging, despite no overcharging occurring. This has caused alarm among users. This study explored thermal abuse experiments on a 25 Ah LiFePO₄/graphite battery in a nitrogen environment during charging. Different charge currents (0.5C, 1C, 1.5C) were applied at various states of charge (SOC: 50 %, 75 %, and 100 %) to analyze voltage changes during thermal runaway and investigate its internal mechanisms. The study also examined the impact of diaphragm closed pore due to localized short circuits in the cell's micro-structure, which accumulate heat during charging. Results showed that thermal runaway did not immediately cause short circuits but led to diaphragm closed pore under continuous current, raising voltage and causing failure. The temperature gradient inside the battery varied with different SOCs, influencing diaphragm closed pore timing and the extent of thermal runaway. At 50 % SOC, diaphragm closed pore occurred before thermal runaway, causing voltage to rise to 5 V. At 75 % SOC, diaphragm closed pore followed partial thermal runaway, with voltage dropping twice. At 100 % SOC, diaphragm closed pore and thermal runaway occurred nearly simultaneously, amplifying the effect. The study concludes that diaphragm closed pore exacerbates thermal runaway, with its severity linked to initial SOC and charging rate. A 0.5C increase in charging rate requires a 6.66 % decrease in initial charge to balance thermal runaway risks.