Soluble catalysts for aprotic Li-O2 batteries

Thesis

Aprotic lithium-air (O₂) batteries have attracted significant interest due to their high theoretical specific energy. In an aprotic Li-O₂ cell, O₂ is reduced to form Li₂O₂ on discharge and the process is reversed on charge. Li₂O₂ is an insulating and insoluble solid, leading ultimately to poor cycling rates, low capacities and early cell death if it formed on the electrode surface. This is exacerbated by formation of Li2CO3 due to the cathode degradation with the presence of Li₂O₂ surface film. It is therefore desirable to form Li₂O₂ particles in solution, typically in batteries with high donor number electrolyte solvents that can dissolve the LiO₂ intermediate. However, such solvents are more susceptible to nucleophilic attack by the reduced oxygen species, resulting in undesired side reactions.

This project focuses on tackling the dilemma by promoting solution phase formation of Li₂O₂ in relatively stable low donor number solvents. Phenol was investigated as a protic additive that acts as a phase-transfer catalyst, dissolving Li₂O₂, avoiding electrode passivation and forming large particles of Li₂O₂. Discharge mediators were studied to shift O₂ reduction to Li₂O₂ in solution via a new route that avoids the LiO₂ intermediate, so that it increases the discharge potential, suppresses the growth of the Li₂O₂ thin film on the cathode surface, and thus postpones the cell death, increases the discharge capacity 80-100 fold and permits high discharge capacities with relatively high rates.

However, Li₂O₂ particles growing via solution mechanism is disconnected from the electrode surface and therefore electronically isolated during charging, which requires redox mediators on charging. By combining discharge and charge mediators, the dual-mediator Li-O₂ cell in this project was able to sustain cycling with capacities of 2 mAh cm^-2_areal at a rate of 1 mA cm^-2_areal over 50 cycles. By forming/decomposing Li₂O₂ in solution and avoiding high charge potentials, the carbon instability is significantly mitigated.

Authors: Gao, X

• DOI: 10.5287/ora-zvvbwpbqr
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English