Advancing cathode materials for next-generation Li/Na-ion batteries: transition-metal migration in disordered rock-salts and layered oxides

Thesis

The development of next-generation rechargeable batteries hinges on advanced cathode materials with high capacity, stability, and efficiency. This thesis systematically investigates the structural and redox mechanisms that dictate the electrochemical performance of layered oxides and disordered rock-salt (DRX) compounds for Li- and Na- ion batteries. Layered oxides remain the most industrially mature and energy-dense cathodes, while DRX materials offer a promising alternative with higher compositional flexibility, stronger structural robustness, and the potential to access multi-electron redox reactions beyond the limits of conventional layered frameworks.

Firstly, in representative layered systems, Na0.52Li0.2Mn0.8O2 and its Li-exchanged analogue Li0.56Li0.2Mn0.8O2, the structural and electrochemical implications of transition metal (TM) migration are systematically investigated. A novel electrochemical descriptor “ΔP”, is defined to investigate the relationship between electrochemical behavior, local structural transformations, and TM migration kinetics.

Secondly, in Mn-based DRX systems, Li2Mn0.75W0.25O2F is studied compared to Li2MnO2F, revealing that high-valent d0 cation W6+ simultaneously boost Mn redox activity with enhanced cycling stability and modulate local spinel ordering, at the cost of Li accessible sites and diffusion kinetics, evidenced by Monte Carlo simulations and spectroscopy.

Finally, a comparative study of Ni-based DRXs, Li1.2Ni0.4Nb0.4O2 and Li1.2Ni0.5W0.3O2, reveals that the choice of d0 cation significantly influences redox mechanisms and voltage hysteresis. LNNO supports cooperative Ni and O redox with minimal oxygen loss and low hysteresis, while LNWO exhibits limited Ni redox, higher voltage hysteresis, and irreversible O2 release. Band structure analysis highlights the importance of cation electronegativity and charge transfer energy in stabilizing oxygen redox.

These insights provide mechanistic guidelines for designing robust cathode materials, advancing the development of high-capacity, low-hysteresis Li/Na-ion batteries.

Authors: Xu, H

• DOI: 10.5287/ora-qr4e6xek6
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: cathdoes materials; stacking faults in layered materials; disordered rocksalts; lithium-ion battteries
• Subjects: Materials science