Mechanistic\nStudy on Oxygen Reduction Reaction in\nHigh-Concentrated Electrolytes for Aprotic Lithium–Oxygen Batteries

Highly concentrated electrolytes (HCEs) have energized\nthe development\nof high-energy-density lithium metal batteries by facilitating the\nformation of robust inorganic-derived solid electrolyte interfaces\non the lithium anode. However, the oxygen reduction reaction (ORR)\noccurring on the cathode side remains ambiguous in HCE-based lithium–oxygen\n(Li–O<sub>2</sub>) batteries. Herein, we investigate the ORR\nmechanism in a highly concentrated LiTFSI-CH<sub>3</sub>CN electrolyte\nusing ultra-microelectrode voltammetry coupled with in situ spectroscopies.\nIt is found that, compared to the dilute electrolyte, the HCE prolongs\nthe lifespan of superoxide intermediates and decelerates their migration\nrate to the bulk solution, resulting in a change in growth mode for\nthe discharge product of Li<sub>2</sub>O<sub>2</sub> from traditional\ntwo-dimensional film growth to surface three-dimensional expansion\ngrowth. This alteration reduces the cathode passivation and thus delivers\nthe enhanced discharge capacity. Additionally, the HCE also increases\nthe reaction energy barrier between superoxide and solvent molecules,\nthereby minimizing parasitic reactions and improving the cycle performance\nof Li–O<sub>2</sub> batteries. Our study reveals the intricate\ninterplay between electrolytes and oxygen intermediates and provides\nimportant insights into electrolyte chemistries for better Li–O<sub>2</sub> batteries.