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

Development of high specific energy Li-O2 batteries faces a problem of poor cycling due to passivation of positive electrode by both discharge product (Li2O2) and side products (Li2CO3 etc.) Side products are the result of oxidation of the electrode materials or electrolytes 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 electrolyte including high donor number solvents (MeCN>TMS≈DMA>DMSO). Moreover, such behavior is reproduced for three different positive electrode materials (glassy carbon (GC), TiC and TiN) that pinpoint the primary role of different ORR mechanisms (Li2O2 surface deposition or solution growth), which are implemented at low and high Li+ solvation energy, rather than chemistry of interaction between LiO2/Li2O2 and electrode/solvent. Additionally, we found that for the electrode made of GC and TiN in all electrolyte solutions the passivation by side products suppress oxygen reduction after certain number of cycles. In contrast, for TiC electrode after a number of cycles further passivation doesn't happen due to formation of thin and stable TiO2 layer in high donor number solvents.