Eutectic Transition and Interfacial Modulation of Multifunctional Ionic Liquid Additives for Subzero‐Temperature Lithium‐Ion Batteries

Commercial lithium‐ion batteries (LIBs) suffer substantial performance degradation at subzero temperatures due to the increased viscosity of ethylene carbonate (EC)‐based electrolytes and a high energy barrier for lithium‐ion (Li⁺) desolvation at the graphite anode interface, posing critical challenges for applications in cold climates and extreme environments. To overcome this, a phosphonium‐based ionic liquid, allyl trimethyl phosphonium bis(trifluoromethane)sulfonimide (APT), is introduced as a multifunctional electrolyte additive. APT forms a eutectic mixture with EC, effectively lowering the freezing point and viscosity while enhancing ionic conductivity at low temperatures. Furthermore, APT weakens the Li⁺–EC interaction, facilitating more efficient Li⁺ desolvation at the graphite interface, and promotes the formation of a thin, uniform, LiF‐rich solid electrolyte interphase on the graphite anode, leading to the fast interfacial Li⁺ transfer kinetics. Pouch cells with high‐mass‐loading electrodes (NCM811||graphite, 4.9 mAh cm−2) and lean electrolyte (3 g Ah−1) containing 1 wt.% of APT retained 87.56% of their capacity after 100 cycles at −20 °C, significantly outperforming cells without the additive (64.60% retention). Therefore, this work provides a rational design strategy for multifunctional electrolyte additives that simultaneously optimize bulk transport properties and interfacial stability for reliable LIB operation under subzero conditions.