Characterisation of the lithium-solid electrolyte interface in solid state batteries

Thesis

All-solid-state batteries (ASSBs) with a Li-metal negative electrode have the potential to drastically improve on state-of-the-art energy densities. Improved understanding failure mechanisms is needed to reach commercialisation, but physical characterisation is challenging. In this thesis, I will investigate the use of electron microscopy as a tool to better understand degradation mechanisms at the interfaces between solid-state electrolytes (SSEs) and Li. First, plasma focused-ion beam (PFIB) and scanning electron microscopy (SEM) are used to access and image the interface between Li-based electrodes and a Li₆PS₅Cl electrolyte with minimal artifact introduction. Li alloys are found to be more stable to ion milling than pure Li, allowing Li-Mg to be used as a characterisation proxy for Li metal, and the microstructure of the Li-In reference electrode to be interpreted. Cryo-PFIB is then used to investigate the Li deposition structure with more complex SSEs; a composite, thin SSE and a SSE combined with a C-Ag interlayer. To investigate the morphology of the thin (approx. 10 - 100s nm), air- and electron-beam- sensitive solid electrolyte interphase (SEI) layer, a sample preparation technique is developed to reliably produce electron-transparent samples incorporating the SSE-SEI-Li interface. This is combined with low-dose 4D-scanning transmission electron microscopy, which enables spatio-compositional mapping of the interface without beam damage. Analysis reveals information on the crystallite size and distribution across the SEI. Statistical analysis of the SEI component spatial relationships demonstrates a complex structure with component intermixing. This is also shown to be the case with Li_5.5PS_4.5Cl_0.75Br_0.75, despite the differing morphology leading to smaller SEI component domain sizes. This information helps better understand the degradation processes at ASSB interfaces and, in particular, enables improved characterisation of the SEI.

Authors: Chart, Y

• DOI: 10.5287/ora-qamqgp4va
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: focused ion beam; solid electrolyte interphase; electron microscopy; solid-state battery; next-generation battery; lithium metal; battery
• Subjects: Microscopy; Battery industry