Tailoring the Solid Electrolyte Interphase Composition on Lithium Metal Anodes by the Choice of Ionic Liquid during Mechanochemical Modification

Abstract Lithium metal batteries (LMBs) have a great potential to become widely commercialized. However, an improved solid electrolyte interphase (SEI) is needed to enable safe long‐term cycling. Here, further a mechanochemical modification method is developed, where lithium metal is roll‐pressed in contact with ionic liquids (ILs). The choice of IL allows tailoring the composition and thickness of the SEI, examined via X‐ray photoelectron spectroscopy and cryo transmission electronic microscopy, to tune its properties and enable low overvoltage, smooth deposit morphology, and cycling at high current densities. Among the examined ILs, N‐butyl‐N‐methylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₄FSI) provides the best results, facilitating stable cycling in a carbonate‐based electrolyte at current densities up to 10 mA cm⁻², which results from the suppression of dendrite formation and electrolyte consumption presumably due to a better lithium ion conductivity and homogeneity of the SEI. Furthermore, the modified lithium metal anodes show a good compatibility with NMC cathodes, which is crucial for high‐voltage LMB applications. Finally, modified lithium anodes are used in combination with a ternary solid polymer electrolyte, showing also in this context, a much‐improved cycling performance.