Enhancing Cycle Life in Superoxide-Based Na–O2 Batteries by Reducing Interface Reactivity

Abstract

Sodium–oxygen (Na–O2) batteries are considered a promising energy storage alternative to current state-of-the-art technologies owing to their high theoretical energy density, along with the natural abundance and low price of Na metal. The chemistry of these batteries depends on sodium superoxide (NaO2) or peroxide (Na2O2) being formed/decomposed. Most Na–O2 batteries form NaO2, but reversibility is usually quite limited due to side reactions at interfaces. By using new materials, including a highly active catalyst based on vanadium phosphide (VP) nanoparticles, an ether/ionic liquid-based electrolyte, and an effective sodium bromide (NaBr) anode protection layer, the sources of interface reactivity can be reduced to achieve a Na–O2 battery cell that is rechargeable for 1070 cycles with a high energy efficiency of more than 83%. Density functional theory calculations, along with experimental characterization confirm the three factors leading to the long cycle life, including the effectiveness of the NaBr protective layer on the anode, a tetraglyme/EMIM-BF4 based electrolyte that prevents oxidation of the VP cathode catalyst surface, and the EMIM-BF4 ionic liquid aiding in avoiding electrolyte decomposition on NaO2.