Fire-Extinguishing Microcapsule Composite Separator for Chemical Safety Management in Lithium-Ion Battery System

Lithium-ion batteries (LIBs) have become essential to modern energy storage applications, from portable electronic devices to electric vehicles (EVs), owing to their high energy density(~275 Wh kg⁻¹), long lifespan, and high operating voltage(~ 4.3V vs Li⁺/Li). However, the increasing use of flammable organic electrolytes, typically composed of linear and cyclic carbonates, has raised serious safety concerns under abnormal conditions. Thermal runaway caused by exothermic reactions such as electrolyte decomposition, separator damage, and electrode degradation remains a critical safety issue that can lead to fire or explosion. Conventional fire extinguishing agents, including nitrogen-based and phosphate-based substances, have shown limited effectiveness in suppressing LIB fires. Phosphorus-based flame retardants, such as triphenyl phosphate (TPP), have been widely studied due to their ability to scavenge gas-phase radicals. However, self-extinguishing time (SET) testing results confirmed that most existing flame retardants failed to suppress LIB electrolyte fires and that their effectiveness was limited under actual operating conditions. In addition, many phosphorus and boron-based flame retardants exhibit high volatility or decomposition characteristics at LIB operating temperatures, making them unsuitable for long-term safety improvement. In contrast, condensed phase flame retardants, which directly suppress combustion within the electrolyte, are a more robust and effective safety measure. One of the most promising approaches involves halogen-based flame retardants, which exhibit superior radical scavenging efficiency. In our previous study, calcium bromide (CaBr₂) was introduced as a promising flame retardant for LIBs, and excellent fire suppression performance was demonstrated through the condensed-phase radical scavenging mechanism. Although CaBr₂ effectively suppresses combustion within the electrolyte, it causes undesirable side reactions and battery deterioration due to its high hygroscopicity and reactivity with the electrolyte. To solve these problems, a thermo-responsive microencapsulation method was developed in which CaBr₂ was encapsulated in a poly(methyl methacrylate-co-divinylbenzene) (poly(MMA-co-DVB)) shell. The microcapsules (CBMC) fabricated in this way block premature reaction with the electrolyte under normal conditions and release flame retardants in response to heat. In this study, it was demonstrated that CBMC not only provides effective fire suppression in LIBs but also maintains electrochemical stability. Our encapsulation approach significantly enhances fire suppression efficiency, as demonstrated by a nearly 90% reduction in self-extinguishing time, from 82 seconds to just 9 seconds, when CaBr₂ microcapsules (CBMCs) were introduced into the electrolyte. The improved fire suppression effect was also confirmed in the pouch cell explosion experiment, with the cell containing CBMC showing an ignition delay time of up to 80 s, which is almost twice that of the control cell at 46 s. In addition, the CBMC-coated polyethylene separator (CCPE) showed stable performance for more than 100 cycles, demonstrating high compatibility with the LIB system. These results show that microencapsulated halogen-based flame retardants are a practical strategy to improve the safety of next-generation high energy density LIBs. Figure 1