Ionic Liquid Additive Mitigating Lithium Loss and Aluminum Corrosion for High-Voltage Anode-Free Lithium Metal Batteries

Concentrated electrolytes based on lithium bis(fluorosulfonyl)imide (LiFSI) have been proposed as an effective Li-compatible electrolyte for anode-free lithium metal batteries (AFLMBs). However, these electrolytes suffer from severe aluminum corrosion at an elevated potential. To address this issue, we propose a binary ionic liquid (IL) electrolyte additive comprising the 1-methyl-1-butyl pyrrolidinium cation (Pyr₁₄⁺), difluoro(oxalate)borate anion (DFOB^-), and difluorophosphate (PO₂F₂^-) anion to mitigate the Li inventory loss and Al corrosion in 4 M LiFSI/DME electrolyte simultaneously. On the anode side, the IL additive facilitates the formation of a robust Li₃N- and LiF-rich solid electrolyte interphase, promoting highly reversible Li plating/stripping and uniform Li deposition. Additionally, the ILs alter the Li⁺ solvation structure, leading to enhanced t_Li⁺ and rapid Li⁺ desolvation kinetics. Concurrently, on the cathode side, the ILs aid in the generation of dense LiF- and AlF-rich passivation films against Al corrosion. By using the IL-added electrolyte, the Cu||LiMn_0.7Fe_0.3PO₄ cell operates stably at 4.5 V, and the Cu||NCM613 cell with a high loading of 4.0 mA h cm^-2 sustains 142 cycles until 80% capacity retention. This research contributes to a deeper understanding of the IL additive mechanism at the electrode-electrolyte interfaces and offers a straightforward approach to designing practical high-voltage AFLMB electrolytes.