From Bulk to Surface: A Raman Spectroscopic Analysis of Solvation Structures in Concentrated Acetonitrile Electrolytes for Li–O2 Batteries

Recent studies indicate that concentrated electrolyte solutions can enhance the stability of organic solvents during the charge/discharge processes in lithium–oxygen (Li–O₂) batteries. However, the effects of electrolyte concentration on the solvation structures of lithium ions (Li-ions) at the electrode surface and their implications for oxygen reduction and evolution reactions (ORR/OER) remain poorly understood. In this study, we investigate the solvation structures of Li-ions in bulk solutions and on a gold electrode surface at various concentrations of acetonitrile (CH₃CN) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolytes, using in situ Raman and surface-enhanced Raman spectroscopy. Our findings show that increasing electrolyte concentration decreases the number of free CH₃CN molecules, significantly altering solvation structures at the electrode surface. Decomposed CH₃CN species predominate the gold electrode surface, while the irreversible side reactions are suppressed in highly concentrated electrolytes. This research highlights the importance of electrolyte concentration in optimizing solvation structures and enhancing the electrolyte stability of Li–O₂ batteries.