Electrochemomechanics of lithium metal and alloy anodes for solid-state batteries

Thesis

The fracture of ceramic solid electrolytes, driven by the plating of lithium within cracks, has been identified as one of the fundamental issues to successfully develop solid-state batteries. Understanding the mechanics of lithium at the nanoscale is therefore essential. In the first experimental chapter, the elastic and plastic properties of lithium are measured by nanoindentation within an electron microscope. Lithium metal samples are characterized by electron backscattered diffraction before and after indentation to understand the dependence of the mechanical properties on crystallographic orientation and determine the stiffness tensor components, moduli, and Poisson’s ratio using a method first proposed by Vlassak and Nix. The measured stiffness tensor components are C11 = 13.3, C12 = 11.2, and C44 = 8.8 GPa. Hardness measurements show a clear size effect with hardness in excess of 100 MPa observed for indent depths below 300 nm, which could contribute toward observed lithium filament propagation.

Lithium alloys have the potential to overcome anode-side challenges in solid state batteries. In the second experimental chapter, lithium-rich magnesium alloys are synthesised and characterised, quantifying the changes in mechanical properties, transport, and surface chemistry that impact electrochemical performance. Increases in hardness, stiffness, adhesion, and creep resistance are quantified by nanoindentation as a function of magnesium content. A decrease in diffusivity is quantified with 6Li pulsed field gradient nuclear magnetic resonance spectroscopy and chronopotentiometry, and an increase in interfacial impedance due to the presence of magnesium is identified with electrochemical impedance spectroscopy which is correlated with x-ray photoelectron spectroscopy data. Throughout, changes in properties are linked to electrochemical performance. This work provides a framework to investigate other lithium alloy systems.

The performance limitations of lithium metal are holding back the development of solid-state cathodes and lithium-free anodes. In the final experimental chapter, indium-lithium alloys are explored as a combined counter and reference electrode for all-solid-state batteries. They enable high current densities without shorting, fast discharge capacity, and high cycle stability. The performance is attributable to very fast diffusion kinetics in the InLi intermetallic, D298KLi =5.8×10−7 cm2s−1. The indium metal phase is essentially ion-blocking, so the performance is tied to the microstructure, which evolves during charge and discharge. A two-layer microstructure is proposed which offers good accessible capacity and good cycle life at high current densities.

Authors: Aspinall, J

• DOI: 10.5287/ora-bpq9xaz2k
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: battery; alloy; solid state; lithium; nanoindentation
• Subjects: Materials science; Solid state batteries; Lithium alloys