Safety issue of lithium-ion batteries (LIBs) is always a concern. We have studied the inhabitation on thermal runaway (TR) and propagation of 18,650 LIBs in an enclosed space systematically. LIBs at 70% state of charge are chosen for testing. Four fire extinguishing agents are applied on LIB arrays for 20 s, and the inhibiting effects are different. The cooling efficiency varies with the surface temperatures of LIBs. Water spray has the highest cooling efficiency and inhibits the TR propagation among LIB arrays successfully. Three LIBs undergo TR for the releasing of ABC ultrafine dry powder. BC ultrafine dry powder and Novec 1230 are failed to inhibit the TR propagation. Nevertheless, Novec 1230 shows the best on inhibiting fire occurring and the generation of toxic gas. Generally, this study provides valuable information for the choice of fire extinguishing agents.

Flowability greatly affects the application of ultrafine dry powder fire extinguishing systems, while hydrophobicity and acute inhalation toxicity are concerns for fire extinguishing agents. In the present study, we examined the impact of hydrophobic fumed silica on the hydrophobicity and flow properties of ammonium dihydrogen phosphate as the base. Our findings revealed that incorporating 6 wt.% of hydrophobic fumed silica resulted in optimal flowability, accompanied by a hydrophobicity angle of 126.48°. The excessive inclusion of hydrophobic fumed silica impeded powder flow within the ammonium dihydrogen phosphate particles. Furthermore, the investigations indicated that the incorporation of a small quantity of bentonite (0.5 wt.%) amongst the three functional additives—bentonite, magnesium stearate, and perlite—offered further enhancements in powder flowability. In fire extinguishing experiments’ total flooding conditions (1 m³), the designed UDPA exhibited a minimum required extinguishing concentration of merely 41.5 g/m³, which is better than the publicly reported value. Moreover, observations on the well-being of mice subjected to nearly three times the extinguishing concentration at 60 s, 10 min, and 3 days, respectively, demonstrated the absence of acute inhalation toxicity associated with the designed UDPA. Collectively, the developed ultrafine dry powder fire extinguishing agent displayed promising performance and possesses broad applicability.