Study on extinguishing performance and suppression mechanism of thermal runaway combustible gas flame of lithium battery by NH4H2PO4 water mist

As the proportion of fires caused by new energy vehicles in road traffic accidents continues to rise, research on the suppression of lithium battery fires has become particularly important. The flames resulting from thermal runaway of batteries primarily originate from the combustion of gases released during the thermal runaway process. Therefore, effectively suppressing the thermal runaway flames of lithium batteries not only requires reducing the battery temperature but also necessitates the effective suppression and extinguishment of combustive gases that escape during thermal runaway. Given that flammable gases such as H 2 and CO released during the thermal runaway of lithium iron phosphate batteries account for more than 95 % of total products, this study innovatively constructs a small-scale flame simulation system using an ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ) aqueous solution as a firefighting agent, focusing on exploring its extinguishing efficiency and reaction kinetics mechanism. Using a self-built cup burner experimental system, this study examines the minimum extinguishing concentration (MEC) of fogging agents at different concentration gradients (1 %, 2 %, 5 %, and saturation state). The saturated solution exhibits excellent performance with an MEC of 8.21 %, significantly outperforming traditional extinguishing agents such as nitrogen (11.04 %), carbon dioxide (10.37 %), and perfluorohexane (9.34 %). At the microscopic reaction mechanism level, a comprehensive kinetic model (Overall-Mech) containing 1120 elementary reactions was constructed using Chemkin software, identifying four key suppression pathways. The study reveals that phosphorus-containing active substances significantly deplete H/OH radical concentrations through the dual suppression cycle formation of HOPO↔PO 2 and HOPO 2 ↔PO 2 , reducing the overall reaction rate. In the macro extinguishing experiments, the saturated fine water mist system realized fire extinguishment within 3–5 s, and the battery box temperature decreased by 89 % during the subsequent 10 min of constant temperature control, demonstrating both rapid extinguishing and prolonged thermal control protective characteristics. This research provides a technically valuable solution with both theoretical depth and practical applicability for addressing thermal runaway fires in lithium batteries of new energy vehicles.