In Situ Characterization of Internal Short Circuit Caused Thermal Runaway in Lithium-ion Batteries: Effects of SOC, Cooling, and Electrolyte Solvents

Abstract In situ characterization is used to understand internal short circuit (ISC)-caused thermal runaway of 4-Ah Li-ion cells with graphite anode and LiNi0.8Mn0.1Co0.1O2 (NMC811) cathode under different state-of-charge (SOC), cooling conditions, and electrolyte solvents. While 75% SOC led to thermal runaway, 50% and 25% SOC did not, which can be attributed to the rapid decrease of ISC current. The decrease is due to insufficient energy storage in the 25% SOC cell and due to separator shutdown in the 50% SOC cell. With liquid water cooling, the cell did not experience obvious damage with cooling temperature of 5 °C or 23 °C, but experienced thermal runaway when the cooling temperature was 60 °C. The difference can be attributed to higher ISC current and ISC heating rate at higher operating/cooling temperatures. The ratio of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in the electrolyte influenced cell capacity and ISC behaviors. EC-free electrolyte led to higher capacity and postponed the onset of thermal runaway, but the resulting thermal runaway is more severe and reached higher maximum temperatures, as compared to electrolyte with EC. The differences can be attributed to higher reactivity, higher conductivity, and lower combustion enthalpy of EC as compared to EMC. These findings demonstrated that in situ characterization can enhance the understanding of ISC and thermal runaway behaviors of Li-ion cells by providing details of the transient and localized phenomena.