Regulating Li⁺ solvation structure of locally concentrated ionic liquid electrolytes with different fluorinated diluents for lithium metal batteries

Due to the low viscosity and weak polarity, fluorinated ethers and fluorinated aromatic compounds are widely used as fluorinated diluents in locally concentrated ionic liquid electrolytes (LCILEs) to improve the electrochemical performance of ionic liquid electrolytes (ILEs). However, the extent to which these two types of diluents regulate Li⁺ solvation structure remains unclear. In this work, two LCILEs were designed using 1,1,2,2-tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFETFE) and 1,2-difluorobenzene (2FB) as diluents, denoted as PPT and PPB, respectively. Raman spectroscopy reveals that both TFETFE and 2FB can strengthen the interaction between Li⁺ and anion, thereby promoting the formation of aggregate ion pairs (AGGs). Compared with TFETFE, 2FB exhibits a more pronounced ability to enhance Li⁺-anion interaction, increasing the proportion of AGGs in PPB electrolyte to 42%, which is significantly higher than that in PPT electrolyte (30%). The enriched AGGs facilitates the preferential reduction and deep decomposition of FSI⁻ at the electrode/electrolyte interphase, leading to the formation of inorganic-rich (LiF, Li₃N, Li₂S and Li₂O) solid electrolyte interphase (SEI) and cathode electrolyte interphase (CEI) with the compact and uniform morphologies. Benefiting from the favorable interfacial chemistry, Li||Cu half battery based on PPB electrolyte delivers a high Li plating/stripping Coulombic efficiency (CE) of 98.7% at 1 mA·cm⁻², exceeding that of PPT (98.0%). In addition, the Li||Li symmetric battery exhibits stable cycling for 160 h at 2 mA·cm⁻². The Li|PPB|LFP full battery further retains 83.6% of its initial capacity after 150 cycles with an average CE of 99.8%, demonstrating superior electrochemical performance. This work provides practical guidance for the rational selection and structural design of diluents for high performance lithium metal battery electrolytes.