Limitations to the recharging of lithium-oxygen batteries

Thesis

There is an urgent need for rechargeable batteries with higher energy density. The lithium-oxygen battery has the highest theoretical energy density of any rechargeable cell, and it may be the only option for the practical electrification of consumer aviation. However, the battery suffers from extreme inefficiencies during cycling, which has restricted its adoption to within the lab. This thesis investigates the most significant of these inefficiencies, highlighting how future efforts should be directed to bring about a fully realised lithium-oxygen battery.

The first is the cell’s energy efficiency, as historically, lithium-oxygen cells require impractically more energy to fully charge than the amount generated during discharge. Soluble catalysts called redox mediators have contributed significantly to improving the energy efficiency of lithium-oxygen cells, but the limits of their efficacy are unclear. This study reveals that their ability to facilitate the charging reaction is thermodynamically limited. They are also hindered by the degradation products that form during lithium-oxygen cell cycling.

This degradation is the second inefficiency investigated and is arguably the greatest obstacle to achieving a long-cycle-life lithium-oxygen cell. In this work, a series of novel solvents have been tested for their stability during cell discharge. While no solvent is sufficiently stable towards the discharge reaction of lithium-oxygen cells, it has been demonstrated that solvent modification can lead to improved stability. Furthermore, a new, solvent-specific mechanism for the redox-mediated discharge has been identified.

Authors: Dewar, D

• DOI: 10.5287/ora-beaq6pbwd
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English