Fracture processes and toughening mechanisms in solid-state electrolytes

Thesis

Climate change is a worldwide problem and has made clean energy a global priority. As the shift to clean energy progresses, the demands for improved energy storage continue to grow. The current energy storage solution of choice is the Li-ion battery; however, to meet the energy storage demands of the future, the All-Solid-State-Battery is the most promising next generation technology. The ASSB contains a solid electrolyte which enables the use of lithium metal anodes to achieve greater performance. Unfortunately, this has generated several as of yet unsolved and poorly understood issues all stemming from fracture in the solid electrolyte. Principally, cell failure due to lithium dendrites which grow through and crack the solid electrolyte. This thesis utilises a mechanics based approach to understand the fracture behaviour of solid electrolytes which has until now been poorly characterised.

As the most electrochemically promising solid electrolytes are highly air-sensitive, work is initially carried out to develop novel testing techniques to enable mechanical characterisation of these solid electrolytes; providing the first look at their experimentally determined properties. Firstly, an investigation was carried out into the effect of density on both the mechanical and electrochemical properties of these solid electrolytes. This highlighted that more highly densified solid electrolytes achieved better performance and identified Li₆PS₅Cl as the choice solid electrolyte. Densified Li₆PS₅Cl electrolyte was mechanically tested to determine the dendrite initiation model using local fracture strength, and the dendrite propagation model using fracture toughness. To improve fracture toughness, the effect on mechanical performance of a Li₆PS₅Cl composite using metastabilised zirconia particles was also investigated. To replicate the effect of repeated cycling and mechanical fatigue on two solid electrolytes, Li₆PS₅Cl and Li_6.4La₃Zr_1.4Ta_0.6O₁₂, is investigated by a novel micro cantilever fatigue test; this demonstrated that crack growth can occur below the single cycle failure stress.

Authors: Perera, J

• DOI: 10.5287/ora-veypbyw6b
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: mechanical characterisation; nanoindentation; solid electrolytes; electric vehicles; solid-state batteries; electrochemistry; batteries; air-sensitive testing
• Subjects: Solid state batteries; Materials science; Electrochemistry; Solid state chemistry