Orders in the disordered: the underlying electrochemical mechanisms of disordered rocksalt cathode materials

Thesis

For the electrification of transportation, it is crucial to develop batteries that offer high energy density while also being cost-effective and sustainable. Current Li-ion batteries for high-energy density applications use layered materials like LiCoO₂ and LiNi_xMn_yCo_zO₂ in cathodes. However, Co and Ni present environmental and cost concerns. LiFePO₄ is a more affordable and sustainable option, but it has a relatively low energy density.

Disordered rocksalt materials provide a promising high-energy cathode option, which can be made using cheap and abundant elements. However, they generally exhibit capacity and voltage fade upon cycling, which must be addressed for practical applications. Additionally, the understanding of their structure is limited. This includes both the pristine structure, which has been shown to exhibit short-range order, and the structural evolution over cycling. In this thesis, model disordered rocksalt materials are studied to understand their structure and degradation mechanisms.

Two representative lithium manganese oxyfluoride disordered rocksalt materials: Li₂MnO₂F (LMOF2121) and Li₃Mn₂O₃F₂ (LMOF3232) are synthesised and studied in parallel. They both show rocksalt to spinel phase transition over cycling. However, the phase transition is not directly linked to degradation. The oxygen redox in LMOF2121 is not stable, leading to increased surface reactions and reduced Mn oxidation states over cycling. Surface reactions, including decomposition of solvent and salt in the electrolyte, form a cathode-electrolyte interphase (CEI) layer that is resistive to electrons, contributing to capacity and voltage fade. Disordered rocksalt LiMnO₂ (LMO) is studied and compared to LMOF2121 and LMOF3232. It is found that the incorporation of fluorine suppresses the spinel phase transition significantly.

Finally, four disordered rocksalt oxide materials with varying d⁰ metals are synthesised and studied: Li_1.2Mn_0.4Ti_0.4O₂, Li_1.2Mn_0.6Ta_0.2O₂, Li_1.2Mn_0.6Nb_0.2O₂ and Li_1.2Mn_0.4Zr_0.4O₂. Their short-range order is controlled by the d⁰ metals, likely related to their ionic radii and valence charges. A correlation between short-range order and Li⁺ transport is established. Both short-range order and electronic conductivity play crucial roles in determining the electrochemical performance of these materials.

Authors: Pi, L

• DOI: 10.5287/ora-jn2q0rnz0
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: disordered rocksalt; energy; Li-ion batteries; energy storage; cathode; crystal structure; electrochemical performance; green technology
• Subjects: Materials; Crystallography; Energy transition; Chemistry