The role of LiO2 solubility in O2 reduction in aprotic solvents and its consequences for Li–O2 batteries

Abstract

When lithium-oxygen batteries discharge, O2 is reduced at the cathode to form solid Li2O2. Understanding the fundamental mechanism of O2 reduction in aprotic solvents is therefore essential to realizing their technological potential. Two different models have been proposed for Li2O2 formation, involving either solution or electrode surface routes. Here, we describe a single unified mechanism, which, unlike previous models, can explain O2 reduction across the whole range of solvents and for which the two previous models are limiting cases. We observe that the solvent influences O2 reduction through its effect on the solubility of LiO2, or, more precisely, the free energy of the reaction LiO2(*) ⇌ Li(sol)(+) + O2(-)(sol) + ion pairs + higher aggregates (clusters). The unified mechanism shows that low-donor-number solvents are likely to lead to premature cell death, and that the future direction of research for lithium-oxygen batteries should focus on the search for new, stable, high-donor-number electrolytes, because they can support higher capacities and can better sustain discharge.

Realizing the Embedded Growth of Large Li2O2 Aggregations by Matching Different Metal Oxides for High‐Capacity and High‐Rate Lithium Oxygen Batteries

“…Simultaneously, in the bulk electrolyte, LiO 2sol transforms to Li 2 O 2 through disproportionation (Equation (S12) in Figure S8 of the Supporting information) 12. When Li 2 O 2 cluster reaches a supersaturated state, solid Li 2 O 2 deposits on the electrode surface as sol‐seeds (“sol‐seeds” represent the seeds formed through solution) 6, 19. Thus, for Co 3 O 4 material, the nucleation occurs simultaneously through surface and solution, which leads to further growth of Li 2 O 2 on the surface and in the electrolyte.…”

“…In a typical Li‐O 2 battery, during the discharging process, O 2 reacts with Li + to form Li 2 O 2 with insulating and insoluble characteristics that lead to the gradual increase of electrode impedance until the electron transport cannot match the current density 3, 4, 5. For this reason, if conformal Li 2 O 2 film or small Li 2 O 2 aggregation is formed, the electrode surface will be passivated early and thus yield a low capacity 6, 7. If large‐sized Li 2 O 2 aggregation is produced, the passivation of electrode surface will be prolonged and result in a relatively high capacity 8, 9, 10.…”

“…Therefore, the capacity of Li‐O 2 batteries is strongly related to the size of Li 2 O 2 aggregation. For instance, Bruce and co‐workers and Luntz and co‐workers found that the size of Li 2 O 2 toroids could be adjusted by changing the solvent or electrolyte additive, and the battery capacity could be increased with Li 2 O 2 sizes 6, 8. Han and co‐workers reported that Li 2 O 2 with large sheet‐like morphology provided a higher capacity than the small Li 2 O 2 nanoparticles 11.…”

Phenol‐Catalyzed Discharge in the Aprotic Lithium‐Oxygen Battery

“…Li 2 O 2 is an insoluble, insulating solid and therefore discharge by the surface mechanism results in low capacities, poor rates and early cell death 3a,3f, 5. Whereas, if Li 2 O 2 forms via the solution mechanism high capacities can be obtained and early cell death by surface passivation is avoided.…”

“…and written in the form of the surface mechanism which is expected to be dominant in glycol ethers with a LiTFSI salt 3a, 6a. The peak height is lower than in the equivalent CV containing TBATFSI even though a 2 e − product is now forming and this is due to passivation of the electrode surface by Li 2 O 2 —a consequence of the surface mechanism 6a.…”