Ionic liquid-based electrolyte with all-fluorinated solvents enables rich aggregate-II solvation structure for lithium metal batteries

The unique anionic and cationic structures of ionic liquids (ILs) enable ionic liquid-based electrolytes (ILEs) to contain a quantity of aggregates (AGGs) solvation structure, improving electrochemical performance of lithium metal batteries (LMBs). The addition of highly polar organic solvents can reduce the viscosity of ILEs, but it alters the Li⁺ solvation structure. The strong electron absorption ability of F atom makes the fluorinated solvent show weak solvation ability, promoting the combination of Li⁺ and anions. Here in, this research provides a strategy to construct abundant aggregate-II (AGG-II) solvation structure in ILEs by adding weakly solvation all-fluorinated solvents. As the solvents fluorination degree of electrolyte increases, the content of AGG-II solvation structure increases, forming inorganic-rich solid electrolyte interphase (SEI). In the ionic liquid-based electrolyte (ILE) containing all-fluorinated solvents, the high coordination number (1.26) of Li⁺ to fluoroethylene carbonate (FEC) further encourages the reduction of FEC, generating more LiF in SEI. The Li||LiFePO₄ full battery shows exceptional cycling performance with a capacity retention of up to 79.7% after 200 cycles at 2.5–4.2 V and 0.5C. Such ILE with all-fluorinated solvents strategy promises to be enriched with AGG-II solvation structure, enabling outstanding electrochemical performance of LMBs.