Holistic characterisation of cylindrical lithium ion cells by thermal ramp experiments and simulation of failure scenarios
A method for characterising the over-temperature behaviour of lithium ion cells has been established. Single cells of the 18650 format are submitted to a heating ramp and their respective thermal behaviour, i.e. occurring exothermic and endothermic effects, is recorded. The events of gas release are monitored by ex-situ analysis using gas chromatography. The thermal response is evaluated by comparison of the trigger temperatures for degradative effects and by calculation of the respective energy. Approx. 30 kJ of heat and 6000 cm³ of gas containing more than 70% of H2 and CO have been detected. Experimental data is used to establish a 2D model, taking in account the rotational symmetry of the cylindrical cells. The simulation framework then is used for prediction of the thermal response at various heating rates. Evaluation of the safety relevant factors from experiment and simulation allows for the postulation of safety characteristics regarding different types of cell.