• Johansen, Morten

Abstract

The transition towards more sustainable and green energy sources is a necessity.<br/>Sustainable energy sources are often intermittent and require energy storing technologies<br/>such as rechargeable batteries. Since its commercialization the Li-ion battery has been<br/>widely used for everything between small portable devices to large scale grid storage. Today<br/>most rechargeable batteries rely on scarce and/or toxic elements, such as Li and Co. In recent<br/>years, Na-ion batteries have proven attractive as an alternative to the current Li-ion<br/>technology. Na-ion technologies are often based on highly abundant transition metals such<br/>as iron and manganese. Na-ion technology has matured to a point where the first Na-ion<br/>battery powered electric vehicles enter the market later in 2024. Na-ion technologies offer a<br/>large variation in elemental composition and structural diversity. This thesis focuses on<br/>exploring the effects from varying the chemical composition of Na battery electrodes and<br/>the origin of intercalation-induced disorder in Li and Na-ion materials. Elucidating the<br/>structural behaviour has been done using powder X-ray diffraction, X-ray total scattering<br/>and X-ray absorption spectroscopy with the latter being an element-specific probe.<br/>Rutile VO2(M) initially undergoes a solid solution reaction when inserting lithium into<br/>the vacant octahedral site. When discharging beyond Li0.5VO2(M) a two-phase region is<br/>observed with the structure rearranging into a layered α-NaFeO2. Extensive broadening of<br/>the Bragg reflections is observed alongside the phase transition. It appears as the<br/>α-NaFeO2-like phase cycles reversibly between two highly disordered states upon<br/>subsequent cycling. Layered materials are often seen in Na-ion batteries crystallizing as O3<br/>or P2 structures. Different O3 structures have been investigated with a special focus on the<br/>effect from increasing Fe/Mn ratio or introducing 10% dopant in a structure with a Fe/Mn<br/>ratio of one. Samples with Fe/Mn ratio below 7:3 rapidly transform into an P3 structure after<br/>extracting about 0.1 Na-ions per formula unit. Upon further Na extraction the structure<br/>rearranges into an OP2 structure describing the introduction of O-type stacking faults in a<br/>P3 structure until charging ends. Discharge starts with a gradual contraction of the OP2 unit<br/>cell before a rapid rearrangement back into a O3 structure. When increasing the Fe/Mn ratio<br/>above 7:3 the electrode undergoes a solid solution reaction with no clear phase<br/>transformations.<br/>Operando experiments are also possible using laboratory sources and the DANOISE cell<br/>has been developed for laboratory based operando powder X-ray diffraction and X-ray<br/>absorption spectroscopy.<br/>The electrode performance is highly sensitive to the nature of the active ion and<br/>introducing small variation in the structure can lead to completely different structural<br/>behaviour. Understanding the mechanism of ion intercalation and effects from varying<br/>structure and stoichiometry is highly important in developing novel battery technologies.<br/>This thesis contributes to the understanding of intercalation-induced disordering of<br/>functional battery materials, giving an insight into the atomic level of intercalation type<br/>rechargeable battery electrodes. The presented results will help in future design of materials<br/>for rechargeable Li- and Na-ion batteries.

Topics

• impedance spectroscopy
• phase
• experiment
• extraction
• layered
• powder X-ray diffraction
• phase transition
• chemical composition
• Lithium
• iron
• Manganese
• x-ray absorption spectroscopy
• stacking fault