The redox chemistry of oxygen in cathode materials for rechargeable alkali-ion batteries

Thesis

There is a clear and urgent need for rechargeable batteries of higher energy density. Alkali-ion batteries based on Li-ions are the state-of-the-art, although Na-ion may offer a cheaper alternative. In both cases the cathode imposes the biggest limitation on overall energy density.

Alkali-rich cathode materials can store alkali-ions and electrons densely by invoking redox chemistry of both the transition metal (TM) and oxide ions in the lattice. However, O-redox is accompanied by voltage hysteresis, irreversible O-loss and complex structural changes. In this thesis, model Na-ion materials have been investigated to isolate and examine these phenomena.

Firstly, Na_0.67[Mg_0.28Mn_0.72]O₂, with Mg2+ ions occupying the TM layer rather than alkali-ions, is studied and shown to exhibit O-redox. This reveals that ionic interactions between the TM layer substituent and O are the necessary precondition for O-redox.

Secondly, by comparing the O-loss behaviours of Na_0.67[Mg_0.28Mn_0.72]O₂ and Na_0.75[Li_0.25Mn_0.75]O₂, the critical role of the TM layer substituent is revealed; when removed from the structure, the co-ordination number around O can drop below three and O-loss is triggered. Unlike Li+, Mg2+ is retained within the structure preventing O-loss.

Thirdly, O-redox materials Na_0.75[Li_0.25Mn_0.75]O₂ and Na_0.6[Li_0.2Mn_0.8]O₂ are studied in parallel to understand why the former exhibits voltage hysteresis and the latter not. The preservation of the TM layer ordering scheme is shown to prevent hysteresis maintaining stable electron holes on O₂- in Na_0.6[Li_0.2Mn0.8]O₂. For Na_0.75[Li_0.25Mn_0.75]O₂, severe in-plane TM migration destroys the ordering scheme, triggered by formation of molecular O₂ gas which is directly evidenced for the first time in the bulk. Reduction of O₂ occurs on discharge at low voltage giving rise to hysteresis.

Finally, a new Li-rich oxyfluoride, Li₂MnO₂F, with a stable disordered rocksalt structure is presented. Li₂MnO₂F stores charge utilising both Mn and O-redox with negligible O-loss demonstrating the potential of disordered rocksalts to deliver high energy densities.

Authors: House, R

• DOI: 10.5287/ora-wr7jpym54
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: Cathode Materials; Oxygen Redox; Anion Redox; Disordered Rosksalts; Na-ion Batteries; Resonant Inelastic X-ray Scattering; Li-ion Batteries
• Subjects: Diffraction; Chemistry; Synthesis; Chemistry, Inorganic; Solid state chemistry; X-ray spectroscopy; Materials; Energy storage