The effect of sidewall rupture on the propensity for thermal runaway propagation in a small lithium-ion battery module

Six thermal runaway propagation tests were performed on small modules consisting of seven 21700 lithium-ion cells in a hexagonal configuration with 3 mm spacing between adjacent cells. One cell in the centre of the module was triggered into thermal runaway using an 8 mm diameter nail penetrated through the positive terminal of the cell. For half of the tests, sidewall rupture was initiated in the trigger cell using a 35 mm penetration depth. For the other half of the tests, sidewall rupture was not initiated in the trigger cell using a 10 mm penetration depth. In all tests where the trigger cell experienced sidewall rupture, there was thermal runaway propagation to the remaining six cells in the module; in all tests where the trigger cell did not experience sidewall rupture, there was no thermal runaway propagation to any other cells in the module. These results are explained by the directionality and magnitude of heat transfer for sidewall rupture failures relative to nominal failure. These results highlight the increased propensity for thermal runaway propagation when a sidewall rupture failure occurs in a battery module and emphasise the importance of methods to mitigate this failure in battery systems.