Modeling Discharge Pathways in Li-O2 Batteries to Optimize Capacity

Li-O2 batteries offer the possibility of storing twice the gravimetric energy density of Li-ion batteries. Li-O2 batteries operate by reacting oxygen with lithium ions in a non-aqueous solvent to form Li2O2 on a conductive cathode material. However, Li2O2 has poor electronic conductivity and passivates the electrode area. Achiev- ing high capacity requires careful attention to Li-O2 dis- charge mechanisms in order to optimize cathode void space filling by Li2O2.

Li-O2 discharge occurs by two competing mechanistic pathways which are responsible for two possible morphologies of Li2O2 discharge product. The surface pathway involves two consecutive electron transfers to form a ~10 nm thin film of Li2O2. The solvent pathway involves the solvation of the reaction intermediate Li+-O2-, which then reacts in solution to form ~100 nm in diameter toroids of Li2O2. Since toroids allow for greater volumes of Li2O2 to form with less electrode area coverage, toroids are preferable to maximize capacity. However, the exact dependence of each pathway on different discharge conditions and olvent properties to promote toroid formation is not fully understood.

Rotating ring-disk electrode (RRDE) experiments were performed to understand these pathway trends. A rotating rod creates convection currents that sweep reactants to the central disk electrode. Li2O2 film and soluble Li+-O2- are formed at the disk. Soluble Li+-O2- is swept to the ring electrode and oxidized, providing a measure of the relative size of the solvent pathway. By comparing ring and disk currents, the separate contribution of each discharge pathway can be determined.

We then developed a model based on nucleation and growth of the Li2O2 film to explain potentiostatic discharge curves collected from RRDE experiments under different discharge conditions, such as varying solvent water content (Figure 2). The model demonstrates that high Li+-O2- solvent solubility inhibits the surface pathway and that this effect is primarily responsible for toroid promotion.