Investigation of fluorinated amides for solid-electrolyte interphase stabilization in Li-O₂ batteries using amide-based electrolytes

Solvent and electrode stability is critical for the successful development of the rechargeable, organic electrolyte Li-O₂ (air) battery. Straight-chain alkyl amides, such as N,N-dimethylacetamide (DMA), show superior stability at the O₂ cathode compared to organic carbonates and glymes, but these solvents do not form a stable solid-electrolyte interphase (SEI) to prevent a sustained reaction with Li metal. In this work, we use electrochemical impedance spectroscopy and cycling tests on a symmetric Li/electrolyte/Li cell to determine the ability of several fluorinated amide solvents to stabilize the lithium/electrolyte interface. The LiTFSI/N,N-dimethyltrifluoroacetamide (DMTFA) system shows the smallest interfacial impedance and the lowest polarization for Li dissolution and deposition. We present quantum chemical calculations indicating that α-fluorinated alkyl amides are reduced on Li to form insoluble LiF with no or little activation energy. XPS analysis confirms the presence of LiF in the SEI on Li metal exposed to DMTFA, which is likely to play an important role in stabilizing the lithium/electrolyte interface. The improved stability of a metallic Li anode in a rechargeable Li-O₂ battery with LiTFSI/DMA electrolyte is demonstrated using 2% DMTFA as the SEI-stabilizing additive.