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Passive Mitigation of Thermal Runaway Propagation in Dense 18650 Lithium Ion Cell Assemblies

The dynamics, heating rates, gaseous emissions and energetics of thermally-induced thermal runaway propagation in dense arrays consisting of 12–15 fully charged 18650 lithium ion cells have been quantified to determine the effectiveness of several passive mitigation strategies. These strategies include implementing 5 mm gaps between select rows and columns in the array, and inserting physical barriers, such as double-layer stainless steel, intumescent material or ceramic fiber board into the gaps. All experiments were performed in a wind tunnel facility that allows tracking of thermal runaway propagation through the arrays with well-defined experimental conditions. None of the tested mitigation strategies completely prevented propagation. However, the physical barriers were found to be effective in slowing the propagation speed. Among the barriers, ceramic fiber board was found to be the most effective slowing down the propagation by more than a factor of 30. Additionally, contributions of different heat transfer processes driving the propagation were quantified. In air experiments on the arrays without gaps or barriers, 50% of heat flow to downstream (non-failed) cells was associated with flaming combustion of ejected battery materials, 20% was associated with direct cell-to-cell conduction and the rest was associated with convective and radiative heat transfer between cells.