Achieving a Stable Nonaqueous Air Cathode under True Ambient Air

Metal–air batteries promise high gravimetric energy and low material cost but suffer from poor cycle life due to severe cathode side reactions. Here, the reactivity of metal superoxide is demonstrated to play a decisive role in determining the cathode reversibility under true ambient air. The K + -based oxygen electrode is much more robust against H 2 O/CO 2 contamination because the stability of KO 2 is higher than those of LiO 2 and NaO 2 . As a result, ambient air batteries using 1.0 mA cm –2 at 0.25 mAh cm –2 (500 h, 1000 cycles) and at 1.0 mAh cm –2 (>800 h, >400 cycles) were demonstrated. Online electrochemical mass spectrometry and Fenton's tests reveal that the accumulation of carbonate-based side product is negligible even under true ambient air (i.e., 0.07 mol ‰ of KO 2 per cycle over 500 cycles). This work reveals key factors in achieving reversible nonaqueous air cathode reactions and demonstrates unprecedented long-life rechargeable ambient air batteries.