Revealing the Anion-Solvent Interaction for Ultralow Temperature Lithium Metal Batteries

Anion solvation in electrolytes can largely change the electrochemical performance of the electrolytes, yet has been rarely investigated. Herein, three anions of bis(trifluoromethanesulfonyl)imide (TFSI), bis(fluorosulfonyl)imide (FSI), and derived asymmetric (fluorosulfonyl)(trifluoro-methanesulfonyl)imide (FTFSI) are systematically examined in a weakly Li⁺ cation solvating solvent of bis(3-fluoropropyl)ether (BFPE). In-situ liquid secondary ion mass spectrometry demonstrates that FTFSI^- and FSI^- anions are associated with BFPE solvent, while weak TFSI^- /BFPE cluster signals are detected. Molecular modeling further reveals that the anion-solvent interaction is accompanied by the formation of H-bonding-like interactions. Anion solvation enhances the Li⁺ cation transfer number and reduces the organic component in solid electrolyte interphase, which enhances the Li plating/stripping Coulombic efficiency at a low temperature of -30 °C from 42.4% in TFSI-based electrolytes to 98.7% in 1.5 m LiFTFSI and 97.9% in LiFSI-BFPE electrolytes. The anion-solvent interactions, especially asymmetric anion solvation also accelerate the Li⁺ desolvation kinetics. The 1.5 m LiFTFSI-BFPE electrolyte with strong anion-solvent interaction enables LiNi_0.8 Mn_0.1 Co_0.1 O₂ (NMC811)||Li (20 µm) full cell with stable cyclability even under -40 °C, retaining over 92% of initial capacity (115 mAh g^-1 , after 100 cycles). The anion-solvent interactions insights allow to rational design the electrolyte for lithium metal batteries and beyond to achieve high performance.