Stability of RbO2 As a Discharge Product of Metal–O2 Batteries
Authors: Tatara Ryoichi , Fujimoto Ryusei , Hosaka Tomooki , Igarashi Daisuke , Komaba Shinichi 
Published: December 2024

The cathode reaction in metal–oxygen batteries, also referred to as metal–air batteries, has been extensively studied for the electrochemical management of the oxygen reduction reaction (ORR) to harness it for energy conversion. Alkali metal–oxygen batteries utilizing non-aqueous electrolytes have garnered interest for their high theoretical energy density, which leverages atmospheric oxygen as the cathode active material. However, their practical application remains limited by the low reversibility of the ORR.
The Li–O2 battery, first introduced in 1996,[1] was followed by Na–O2 and K-O2 batteries.[2, 3] The cathode reaction in metal–oxygen batteries involves the reduction and reoxidation of oxygen. In non-aqueous electrolytes that contain alkali metal ions, the formation of alkali metal superoxide (MO2) occurs as a discharge product.[4] MO2 stability is empirically known to be influenced by the ionic radii of the alkali metal ion (M+) and the superoxide ion (O2
-).[5] Lithium superoxide (LiO2), which forms through a one-electron reduction process in lithium-based electrolytes, exhibits a significant ionic radius discrepancy between the lithium ion (Li+ 0.76 Å in octahedral coordination) and the relatively large O2
-. This discrepancy results in instability as an ion pair, allowing lithium superoxide to exist only as a transient reaction intermediate.[6, 7, 8] Consequently, the disproportionation reaction (2LiO2 → Li2O2 + O2) and/or subsequent reduction reaction lead to the formation of lithium peroxide (LiO2 + Li+ + e- → Li2O2).[9] The two-electron reduction product, Li2O2, poses challenges for re-oxidation, causing Li–O2 batteries to experience a significant charge overpotential (1–1.5 V). This overpotential greatly diminishes reversibility and energy efficiency.[10] Conversely, in sodium-based electrolytes (Na+: 1.02 Å), NaO2 can be isolated because of its larger ionic radius compared to Li+, offering different electrochemical properties. Unlike Li2O2, NaO2 can be charged at a relatively low overpotential (approximately 200 mV). However, NaO2 reacts with trace amounts of H2O and CO2 present in the cell, facilitating formation of Na2O2 and its hydrates.[11] In potassium-based electrolytes, which feature an even larger ionic radius (K+: 1.38 Å), KO2 can be isolated, and because of its relative stability, KO2 is commercially available as a chemical reagent.[12] Their high stability against moisture and CO2 has been demonstrated; these cells can undergo repeated charge–discharge cycles over extended periods without significant degradation.[13] Despite the high stability of KO2, it can still gradually contribute to the problems mentioned previously, thus degrading battery performance. In this study, we concentrated on enhancing the stability of MO2 against H2O and CO2. We systematically examined and compared ORR mechanism with those involving rubidium (Rb+: 1.52 Å) and cesium ions (Cs+: 1.67 Å), both of which possess even larger ionic radii than potassium.
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