Enhancing the Stability of Solid Electrolyte Interphase with Ionic Liquids for Long-Lasting Lithium–Metal Batteries

Lithium (Li) metal batteries are considered promising candidates for next-generation energy storage due to their high energy density. However, their practical application is hindered by the unstable solid electrolyte interphase (SEI), which limits battery lifespan and raises significant safety concerns. Therefore, a stable SEI is essential for overcoming these battery challenges. Here, we stabilize the SEI by introducing 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim][BF4]) into an ether-based electrolyte. The incorporation of [Bmim][BF4] not only serves as an immediate source of lithium fluoride (LiF), which is known to enhance interfacial stability, but also facilitates the formation of a stable and ion-conductive SEI. This modification reduces the initial interfacial resistance from 171 to 142 Ω and lowers the polarization voltage drop from 115 to 47 mV. Consequently, the lifespan of a Cu//Li cell extends from fewer than 30 to over 100 cycles, and the LiCoO2//Li cell demonstrates enhanced capacity retention, improving from 20 to 100 mAh·g–1 after 95 cycles. These findings highlight a simple yet effective approach to stabilize SEI and enhance the lifespan of Li metal batteries.