A study of the main reactions occurring at the electrodes of Li-ion cells during the thermal runaway

A Li-ion cell is composed by a carbon-based negative electrode (NE), a porous polymeric membrane (usually high-density polypropylene and/or polyethylene) which keep the electrodes electrically apart allowing the passage of Li-ions, and a positive electrode (PE) made of lithium transition metal oxides (LiMO2, M=Co, Ni, Mn or Al). All is immersed in a mixture of organic solvents (dimethyl carbonate, DMC, ethyl methyl carbonate, EMC, ethyl carbonate, EC) in which a lithium salt (LiPF6) is melted. To produce the electrodes, metal current collectors (Al for the positive electrode and copper for the negative electrode) are coated with active material, polymeric binder (usually polyvinylidenefluoride, PVDF) and small amounts of carbon with a high surface area [1]. In recent years, Li-ion batteries have been widespread used in many applications from electric vehicles to energy storage systems, but they have also been the source of several serious accidents. In fact, under electrical, mechanical or thermal abuse conditions a thermal runaway can occur with a consequent uncontrollable increase in pressure and temperature which can lead to fires and explosions [2]. This work wants to analyze the mechanism of the reactions that take place during the thermal runaway occurring at the anode and the cathode, in order to individuate adequate solutions to avoid/mitigate these reactions. To this aim Panasonic’s NCR18650 lithium-ion cylindrical cells (LiNiCoAl oxide), previously charged at different state of charge (30-100%), are disassembled and each electrode of the cell is analyzed by Differential Scanning Calorimetry.