Revealing Origins for Sluggish Reaction Kinetics in Li-O2 Battery Via Operando Liquid Electron Microscopy

Rechargeable lithium-oxygen (Li-O2) battery has the potential to replace conventional lithium ion battery for high-energy applications. However, this system is plagued by its sluggish kinetics during the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) with the mechanisms not fully understood. Here, we explored the discharge/charge (i.e. ORR/OER) fundamentals in a working Li-O2 battery using in situ liquid transmission electron microscopy. During ORR, Li2O2 is observed to nucleate at the carbon electrode/electrolyte interface, and its growth kinetics is controlled by Li+ diffusion in the electrolyte. Li2O2 nucleation and growth are also observed within the electrolyte where there is no direct contact with the carbon electrode, and its growth exhibits O2 - diffusion-limited kinetics, indicating the existence of non-Faradaic disproportionation reaction of intermediate LiO2 into Li2O2. Li2O2 at the carbon electrode surface and within the electrolyte are both actively decomposed during OER. For Li2O2 particles attached to the carbon electrode surface, their decomposition initiates at the electrode/ Li2O2 interface, indicating electron-conduction limited charge kinetics. Interestingly, for Li2O2 isolated within the electrolyte, we observed a side-to-side decomposition mode, which can be associated with the non-Faradaic formation of dissolvable O2 -, whose diffusion in the electrolyte controls the overall charge kinetics. The mechanisms are summarized in Figure 1. This work reveals the ORR/OER fundamentals in Li-O2 battery system in terms of the various limiting factors controlling the discharge and charge process. We expect findings here to guide both the electrode design and the electrolyte screening aiming for enhancement of the ORR and OER kinetics. Figure 1