Computational Modelling of Next-Generation Lithium-Ion Cathode Materials

Abstract

Energy storage technologies of sufficient cost, mass and volume efficiency are required as part of the fight against climate change. Lithium-ion batteries are a promising form of energy storage that may be able to meet these criteria, providing that a step change in their performance can be achieved in good time. This thesis is concerned with specifically next-generation lithium-ion cathode materials, which are required to improve the energy density of lithium-ion batteries as a whole. Many novel lithium-ion cathodes have indeed achieved significantly higher energy densities in comparison to current standards, but for the most part these achievements have come at the cost of poor cyclability and other undesirable electrochemical behaviours. It is generally agreed that many of these performance problems arise from partially irreversible structural rearrangements occurring within the cathode as the cell is cycled. In many cases, these structural changes are themselves thought to result from oxygen redox: the involvement of lattice O2 – ions in charge compensation. Whilst this link between oxygen redox activity and performance inhibiting structural rearrangements is relatively well established, the precise “mechanism” of oxygen redox remains heavily debated. In other words, we do not know what the product of oxygen redox is (the identity of the oxidised oxygen species) and we do not know exactly how the formation of this species leads to problematic structural changes. This knowledge gap is highly problematic for the commercialisation of next-generation lithium-ion cathode materials, as we cannot mitigate the electrochemical problems associated with oxygen redox if we do not understand exactly how these problems arise on the atomic scale. The purpose of this thesis is to work towards addressing this knowledge gap, applying computational modelling to improve our understanding of the redox behaviours occurring in novel lithium-ion cathode materials. More specifically, we interrogate the redox processes in three different materials, each of which belongs to a distinct structural class: Li2FeSiO4 (a polyanion system), Li4.15Ni0.85WO6 (a d0-containing disordered rock-salt) and Li1.68Mn1.6O3.4F0.6 (a partially disordered spinel). In Li2FeSiO4, we find that charge compensation is achieved via a compound redox mechanism of many individual steps, the ultimate result of which is the formation of molecular O2 trapped in the bulk, mirroring observations previously made in other materials that have radically different structural features. As part of this redox sequence, at high states of charge, we observe the reduction of high-valent FeIV back to FeIII due to the further oxidation of oxygen, a process that has previously been described in less specific terms as a “ligand-to-metal charge transfer”. More generally, each step of the overall redox process in Li2FeSiO4 has been reported individually in other materials, but never as part of the same overall charge compensation mechanism. Similarly in Li4.15Ni0.85WO6, we observe another chain of redox processes with the same end result of bulk O2 formation, despite previous reports of the d0 tungsten species acting to stabilise peroxides against further oxidation. Much like FeIV in Li2FeSiO4, we find that NiIV acts as a metastable intermediate that is reduced at high states of charge by the further oxidation of oxygen. Overall, we have shown that, despite the ferocity of debate on the subject in recent times, there is significant overlap between the redox behaviours of novel lithium-ion cathode materials that may (on a surface level) appear very different from one another. We have also shown that many of the mechanisms of oxygen redox previously proposed in the literature are not mutually exclusive, but may operate together as part of a single compound process.

Date of Award	12 Nov 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Benjamin Morgan (Supervisor), Steve Parker (Supervisor) & M. Saiful Islam (Supervisor)