Anion-Regulated coordination and interfacial behavior in ionic liquid electrolytes for Li metal batteries

Tuning the coordination structure of electrolytes is an effective strategy for addressing the issues at Li metal surface such as dendrite growth and unstable solid electrolyte interphase formation. This study explores the fundamental insight into adding anions with different donor numbers, specifically [PF6](-) and [OTf](-) ([OTf](-) = trifluoromethanesulfonate) to [FSA](-)-based ([FSA](-) = bis(fluorosulfonyl)amide) ionic liquids on coordination structures and interfacial behavior. Addition of [PF6](-) and [OTf](-) anions alters the clustering and interaction of ionic components in the ionic liquid electrolytes, elucidating their correlations with interfacial and electrochemical properties. Both theoretical and experimental evidence revealed that a favorable coordination structure with low (de)coordination energy and facile electrochemical processes can be regulated by mixing anion species. This approach enables the formation of a robust solid electrolyte interphase, effectively enhancing the Li deposition/dissolution process and suppressing dendrite growth, resulting in improved cycle life and capacity retention of Li metal batteries using LiFePO4.