Gas-phase pyrolysis and combustion characteristics of lithium-ion battery electrolyte components: difference from liquid-based fire hazard classification

Abstract

Fire safety has become a more serious concern for lithium-ion battery (LIB) applications with their market growth to various usage and operation conditions, especially in the energy and transportation sectors. A well-recognized fire hazard classification of LIB electrolyte solvents is determined based on the liquid phase that strongly reflects phase change properties. This study poses a difference in the potential fire hazard of LIB electrolyte solvents depending on liquid-/gas-phase properties considered by demonstrating fundamental gas-phase pyrolysis and combustion simulations. By assuming a LIB fire scenario where vaporized electrolyte solvents are present, pyrolysis in a perfectly stirred reactor, laminar flame speeds and ignition delay times were simulated for commercially used carbonate ester mixtures, ethylene carbonate (EC)/dimethyl carbonate (DMC), EC/ethyl methyl carbonate (EMC) and EC/diethyl carbonate (DEC), using a chemical kinetic model. Pyrolysis simulations indicated that at “low” temperatures (≲ 600 K), even after 1000 s, which could be attainable during a thermal runaway event, carbonate esters are still dominant components but not pyrolysis products, such as H₂/CO/CO₂/CH₄ for DMC and CO₂/C₂H₄/alcohol for EMC and DEC cases. Simulated laminar flame speeds of EC/DMC/air mixtures were lower than those of EC/EMC/air and EC/DEC/air mixtures at 500 K, having a good correlation with a transport parameter, i.e., thermal diffusivity. The relative fire hazard of LIB electrolyte solvents in terms of gas-phase combustion based on laminar flame speed is opposite to that of liquid-phase classification. A proper flammability classification and key combustion properties need to be considered depending on the expected LIB fire situation.