Thermal runaway of large automotive Li-ion batteries

Abstract

Damaged or heavily over-heated Li-ion batteries in electric vehicles can transit into a thermal runaway reaction with further heat and gas release. The heat may cause a battery fire and fast gas release may damage the battery-pack casing. To characterise heat and gas release of large automotive Li-ion cells, a heavy duty test bench was developed and a test series was performed.

Fig. 1. Drawing of a hypothetical PHEV battery-pack to demonstrate the safety issues: (left) including complete casing, (right) with upper part of the casing removed. A possible hot spot is shown in red. Dimensions in mm.

Fig. 2. Tested cell before and after the thermal runaway experiment. On the fresh cell, the burst-plate of the cell is hidden below the white sticker. After a thermal runaway the cell is heavily damaged: the burst plate is open, the plastic insulation of the terminals is melted and the cell casing is deformed by the pressure inside the cell.

Fig. 3. Reactor in opened and closed state.

Fig. 4. View of the inside of the reactor, before and after a couple of thermal runaway experiments. The inside of the reactor became coated with anode and cathode particles which were vented by the cells during the experiments.

Fig. 5. Simplified sketch of the reactor in closed state, wiring not shown.

Fig. 6. Exploded view of the cell holder and the Li-ion cell.

Fig. 7. (left) Sketch of the amount of gas inside the reactor during a typical thermal ramp experiment. The cell can produce gas in two venting events: the first (minor) venting and the second (major) venting. (right) The method to calculate the characteristic venting rate ṅ_ch. The amount of gas n can not be used directly because n is calculated from gas temperature measurement, which is distorted by violent gas flows during the main venting event. Instead, it is assumed, that 50% of n_V2 is released during 1/2Δt and 1/2Δt is the timespan during which pressure rises by 1/2Δp. The pressure based method can characterise high venting rates, because it is less affected by gas flows.

Fig. 8. Characteristic temperatures associated with the events of cell failure (voltage drop), first and second gas release, reaching the critical temperature, reaching the maximal cell temperature during thermal runaway and the maximum vent-gas temperature.

Fig. 9. Sketch of the sequence of events as the temperature of the cell increases for experiment 1 and 3. In experiment 1 cell case swelling occurs at 120 °C, the cell fails at 176 °C, the burst disc opens and releases the overpressure from the cell into the reactor and some cell swelling is reversed at 203 °C then increasingly exothermic chemical reactions slowly evolve into a thermal runaway with a critical temperature of 248 °C and second major venting at 247 °C. The cell casing reaches a maximum temperature of 531 °C. In experiment 3 the cell also starts to swell at 120 °C and the cell fails at 160 °C. Then – at an average cell temperature of 170 °C and the hottest cell case sensor showing 192 °C – an internal short circuit triggers a sudden heat release which immediately causes venting and thermal runaway. The cell casing reaches a maximum temperature of 518 °C.

Fig. 10. (left) Gas emission. Each bar shows the minor venting n_V1 (if present) and the major venting n_V2 on top. (center) Rate of gas emission ṅ^ch_V2. (right) Mass of the cell after the thermal runaway experiment compared to mass of the fresh cell.

Fig. 11. Experiment no. 1 (heat ramp). (left) Temperatures and gas pressure during the whole duration of the experiment and (centre) during the main exothermic event. (right) Amount of released gas into the reactor and temperature rates of the cell case sensors.

Fig. 12. Experiment no. 3 (heat ramp), with an internal short circuit occurring at 171 °C. (left) Temperatures and gas pressure during the whole duration of the experiment and (centre) during the main exothermic event. (right) Amount of released gas into the reactor and temperature rates of the cell case sensors.

Fig. 13. Experiment no. 7 (one sided heating). (left) Temperatures and gas pressure during the whole duration of the experiment and (centre) during the main exothermic event. (right) Amount of released gas into the reactor and temperature rates of the cell case sensors. Here the one-sided heating method created a huge temperature difference between the heated and the non-heated surface of the cell. This divided the cell case temperature sensors into two groups: ones that measured the heated side of the cell and ones that measured the cooler side. Sensors of both groups reached above 400 °C during the thermal runaway of the cell. Both groups are also clearly seen in the rate plot. The thermal runaway started, when the hottest sensor on the heated side of the cell reached 281 °C. The thermal runaway propagated throughout the cell from the hot side to the cooler side. After it reached the cooler side it caused a steep temperature rate increase in the sensors which were only at 120 °C.

Fig. 14. Experiment no. 8 (stepwise heating), unfortunately only two cell temperature sensors remained intact in this experiment. (left) Temperatures and gas pressure during the whole duration of the experiment and (centre) during the main exothermic event. (right) Amount of released gas into the reactor and temperature rates of the cell case sensors.

Fig. 15. Experiment no. 10 (reactor heating), unfortunately only two cell temperature sensors remained intact in this experiment. (left) Temperatures and gas pressure during the whole duration of the experiment and (centre) during the main exothermic event. (right) Amount of released gas into the reactor and temperature rates of the cell case sensors.