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

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Abstract 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.

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