These results are encouraging, since the large overpotential observed within Li-O2 cells was found to be related to side reactions on the carbon electrode surface6. The voltage gap in K-O2 cells was lower than 100 mV, and round trip efficiency, as high as 98%. The cycleability of the cells was found to be limited by side reactions between the potassium metal anode and ether-based electrolyte in the presence of dioxygen reduced species, rather than side reactions within the carbon cathode. The main components of this insulating anode surface layer were K2CO3, KOH and KO2. Similar anode surface layer composition was found for different combinations of ether electrolytes and potassium salts. However, while using super-concentrated electrolytes (3M and 5 M KTFSI in dimethoxyethane), O2 crossover was inhibited, and no KO2 was detected on the potassium metal anode surface. As a result, these cells were able to be cycled over 50 times under shallow cycling of 50 mAh/g. The superconcentrated electrolytes had no effect on the cathode reaction product, and the battery cycled through KO2 formation and oxidation. Capacity fade eventually occurred, as repetitive stripping/plating of K metal anode caused the disruption of the protective layer. This work highlights that once a suitable electrolyte has been selected, a metal-O2 cell can indeed be cycled numerous times. The challenge is to increase the cycleable capacity to more appealing values, whilst maintaining cell stability.
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