Positive Electrode Passivation by Side Discharge Products in Li-O₂ Batteries

The development of high specific energy Li-O₂ batteries faces a problem of poor cycling as a result of passivation of the positive electrode by both the discharge product (Li₂O₂) and side products (Li₂CO₃, etc.). The latter are the result of oxidation of the electrode materials or electrolyte components primarily by discharge intermediate superoxide anions (O₂^-) and, in less degree, by Li₂O₂. We report cyclic voltammetry studies of the electrode passivation in different relatively stable solvents. We found that slower passivation is observed for the electrolytes based on high donor number solvents or solvents with high viscosity. Moreover, such behavior is reproduced for three different electrode materials [glassy carbon (GC), TiC, and TiN] that pinpoints the primary role of different oxygen reduction reaction mechanisms (Li₂O₂ surface deposition or solution growth) influenced by Li⁺ solvation energy and solvent viscosity. The chemistry of interaction between LiO₂/Li₂O₂ and the electrode/solvent turns out to be less important. Additionally, we found that, for the electrode made of GC and TiN in all electrolyte solutions, the passivation by side products suppresses oxygen reduction after a certain number of cycles. In contrast, for TiC after several cycles, further passivation does not happen as a result of the formation of a thin and stable TiO₂ layer in high donor number solvents.