Mechanism investigations of EC/EMC electrolyte combustion suppression by phosphorus-iron agents via ReaxFF MD

To elucidate the influence of the phosphorus-to-iron (P/Fe) ratio on the combustion suppression mechanism of phosphorus-iron-based fire-extinguishing agents during lithium-ion battery thermal runaway, ReaxFF molecular dynamics simulations were performed on ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte systems with an approximately 1:1 mass ratio, containing additives with different P/Fe molar ratios (1:3 to 3:1) in an oxygen environment. Under identical conditions, the evolution of reactant consumption, gaseous products, reaction pathways, activation energies, and radical species was systematically analyzed. The results showed that the P/Fe ratio did not change the primary decomposition pathways of EC and EMC but significantly affected radical generation and secondary reaction channels. The 1:1 system exhibited the highest EC and EMC residues (28.3% and 28.4%) and the lowest CO2 and H2O formation. It also presented the highest activation energies for EC and EMC decomposition. Radical analysis indicated that H· generation was lowest in the P/Fe = 1:1 system (174) and increased toward both Fe-rich and Fe-poor compositions, reaching its maximum in the P/Fe = 3:1 system (433), whereas systems with high P/Fe ratios generated more phosphorus-containing species such as PHO2· and PH2O3·. In contrast, the P/Fe = 1:1 system exhibited relatively low occurrence frequencies of key radical-generating pathways while maintaining radical-consumption reactions involving extinguishing-agent-derived species, resulting in the strongest inhibition of chain reactions. Mechanistically, phosphorus-containing species participated in the consumption of H·, OH·, and O· radicals, while variation in the P/Fe ratio regulated the decomposition behavior of the extinguishing agent and the resulting radical reaction network. Within the investigated composition range, the P/Fe = 1:1 system exhibited the strongest overall combustion-suppression performance.