Exploration of compounds in the LiBr-LiOH phase diagram for energy storage applications

Thesis

The sustainable energy transition demands advancements in energy storage technologies. This thesis investigates materials within the LiBr–LiOH binary system, with potential applications in solid-state batteries and thermal energy storage (TES). However, proposed phase diagrams for the LiBr-LiOH system are characterised by complex peritectic and peritectoid transitions, which may present challenges for the synthesis and performance of these materials. The work in this thesis provides new insights into the phase behaviour, structure, and lithium-ion conductivity of phases in the LiBr–LiOH system, offering indications of their potential suitability for energy storage applications.

The first experimental chapter, Chapter 3, looks to establish a stronger understanding of the LiBr-LiOH system and clarify the impacts of these findings on Li₄(OH)₃Br’s potential as a phase change material for TES. The previously uncharacterised high-temperature phase, Li₃(OH)₂Br, was identified and structurally characterised using a combination of experimental techniques and supported by theoretical modelling. X-ray diffraction studies indicate that the phase adopts a hexagonal structure in the P6₃/mmc space group. Promising lithium-ion dynamics are found in this phase, however it could not be retained to room temperature metastably. Investigations into Li₄(OH)₃Br revealed that earlier characterisations were compromised by air exposure. A revised Pmnm crystal structure is proposed for the anhydrous stable phase, and strategies to overcoming its non-equilibrium solidification explored. This different crystal structure to previous reports explains why experimental melting enthalpies are significantly lower than initially predicted.

The second experimental chapter, Chapter 4, focuses on two antiperovskites, Li₂OHBr and the lesser reported Li₅(OH)₂Br₃, which have been suggested as candidate electrolytes for solid-state batteries. The synthesis of these phases is approached with consideration of the LiBr-LiOH phase diagram, and the importance of cooling conditions on phase purity is demonstrated. Some success is achieved in designing a strategy to produce large single crystals of Li₂OHBr, albeit with significant porosity. Directional solidification is suggested as a route to improve the microstructure further and produce crystals suitable for fundamental studies.

Finally, vitrification was explored as a strategy to enhance ionic conductivity in Li₂OHBr. Amorphous Li₂OHX (X = Br, Cl) glasses were successfully synthesised using a melt-quenching technique, with solid-state NMR confirming improved lithium dynamics. However, these glasses were prone to crystallisation in initial attempts to press bulk electrolyte pellets, limiting their practical use.

Authors: Milan, E

• DOI: 10.5287/ora-e9vzzpq04
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: thermal energy storage; Li2OHBr; grain boundaries; Li4(OH)3Br; single crystal; Li3(OH)2Br; solid-state batteries; phase change material; phase diagram; solid electrolyte; novel glasses; antiperovskite
• Subjects: Chemistry, Inorganic; Batteries; Materials science