Harnessing oxygen redox to extend the energy density of li-ion batteries
Keywords: Chemistry.

The importance of energy storage in shifting humanity towards a more sustainable and efficient energy infrastructure is becoming increasingly clear. Electrochemical energy storage systems in particular promise to substantially reduce the financial and environmental cost of electricity -- through a range of grid services such as load shifting and frequency regulation -- and transportation -- by enabling the use of electric vehicles. Of these, lithium ion batteries have received substantial interest due to their low cost, high energy densities, long lifetimes, and ease of integration into existing energy infrastructures. In all of these next-generation applications, however, the performance and cost requirements for lithium ion batteries far exceed what is typical for technologies in which they currently find use, such as portable electronics. Most significantly, the energy density of the positive electrode is a major bottleneck. One of the most promising strategies to increase the positive electrode energy density is by increasing the voltage and useable atomic fraction of lithium in intercalation materials. These materials are designed to undergo reversible delithiation with a concomitant oxidation of the otherwise mostly rigid host lattice and generation of vacancies at the former lithium sites. Oxygen redox, wherein electrons are reversibly removed from lattice oxygen during delithiation, has been recently shown to be able to support deep delithiation from oxide intercalation materials with a high average voltage, making it extremely attractive for developing high energy density electrodes. However, oxygen redox almost invariably results in irreversible voltage fading with cycling, which drains energy density over time, as well as charge-discharge voltage hysteresis, which reduces the round-trip energy storage efficiency. The origin of these unfavourable electrochemical properties has remained mostly a mystery due to the lack of understanding of the nature of the oxidised oxygen species and the materials properties governing their stability and the reversibility of their formation. Consequently, oxygen redox is yet to find commercial application. In this thesis, by revealing the mechanism of oxygen redox, I establish the origin of its associated unfavourable electrochemical properties. I show that when oxygen is oxidised, it experiences a strong driving force to change its local bonding configuration in order to stabilise its higher oxidation state. Bonding changes that can stabilise oxidised oxygen include increasing the bond order with a neighbouring transition metal through a substantial contraction of their existing bond, or forming a new bond with another oxygen to form a short O--O dimer. Crucially, these bonding changes cannot typically be accommodated by minor distortions to the host crystal structure, and so structural defects form within which the desired bonding changes can take place. It is the generation of these structural defects -- most commonly observed as migration of transition metals into vacant lithium sites during delithiation -- that gives rise to the disordering of the host lattice that is at the root of the poor electrochemical properties of oxygen redox. This new understanding reveals a dilemma, in which the structural disordering that results in unfavourable electrochemical properties also allows for the bonding changes necessary to stabilise oxidised oxygen. I propose strategies that resolve this dilemma, which mainly aim at making structural and chemical modifications to the host lattice that promote the stabilising bonding changes while avoiding an overall disordering of the lattice during cycling. This work lays the foundation for the development of practical, high energy density intercalation electrodes employing oxygen redox.