Understanding LiOH Chemistry in a Ruthenium-Catalyzed Li-O₂ Battery

Non-aqueous Li-O₂ batteries are promising for next-generation energy storage. New battery chemistries based on LiOH, rather than Li₂ O₂ , have been recently reported in systems with added water, one using a soluble additive LiI and the other using solid Ru catalysts. Here, the focus is on the mechanism of Ru-catalyzed LiOH chemistry. Using nuclear magnetic resonance, operando electrochemical pressure measurements, and mass spectrometry, it is shown that on discharging LiOH forms via a 4 e^- oxygen reduction reaction, the H in LiOH coming solely from added H₂ O and the O from both O₂ and H₂ O. On charging, quantitative LiOH oxidation occurs at 3.1 V, with O being trapped in a form of dimethyl sulfone in the electrolyte. Compared to Li₂ O₂ , LiOH formation over Ru incurs few side reactions, a critical advantage for developing a long-lived battery. An optimized metal-catalyst-electrolyte couple needs to be sought that aids LiOH oxidation and is stable towards attack by hydroxyl radicals.