Thermal behavior and failure mechanism of lithium ion cells during overcharge under adiabatic conditions

•The modified adiabatic method is used to measure the heat generation under overcharge.•Side reactions contribute 80% heat to thermal runaway in the cases with cycling rate below 1.0 C.•The inflection and maximum voltages increase linearly with the increasing current rates.•The decomposed products of cathode materials are soluble with that of SiOx.•Lithium plating on anode is due to changes of distance between the cathode and anode.

Cells in battery packs are easily overcharged when battery management system (BMS) is out of order, causing thermal runaway. However, the traditional calorimetry could not estimate dynamic overcharging heat release. In this study, commercial LiCoO2 + Li(Ni0.5Co0.2Mn0.3)O2/C + SiOx cells are employed to investigate the dynamic thermal behaviors during overcharge under adiabatic condition by combining a multi-channel battery cycler with an accelerating rate calorimeter. The results indicate that overcharging with galvanostatic - potentiostatic - galvanostatic regime is more dangerous than that with galvanostatic way. Side reactions contribute 80% heat to thermal runaway in cases below 1.0 C charging rate. To prevent the thermal runaway, the effective methods should be taken within 2 min to cool down the batteries as soon as the cells pass inflection point voltage. Hereinto, the inflection and maximum voltages increase linearly with the increasing current rates. By scanning electron microscope and energy dispersive spectrometer, the decomposed products of cathode materials are suspected to be soluble with SiOx. The overcharge induced decomposition reaction of Li(Ni0.5Co0.2Mn0.3)O2 is also proposed. These results can provide support for the safety designs of lithium ion batteries and BMS.