The work in this thesis is focused on the mechanistic investigations of all-solid-state battery degradation associated with a lithium metal anode, via the development of operando characterisation techniques, and ultimately enabling the reliable application of lithium metal anode in all-solid-state batteries. In Chapter 3, the interfacial degradation with the Li metal anode on Li stripping is explored. The void formation is recognised as a form of interfacial degradation that leads to voltage polarisation, local current flux enhancement, and ultimately failure of the cell. The evolution of voids in response to the passage of current is characterised, quantitatively analysed, and with the effects from stack pressure and cycling temperature determined. Chapter 4 explores the solid electrolyte cracking and dendrite formation as a response to Li plating towards the Li metal anode. The spallation is observed as a way of cracking in solid electrolyte associated with the transverse crack that propagates across the whole cell. The crack propagation and the distribution of lithium inside the crack is tracked as function of the charge passed. The propagation process of Li dendrite and its implication are discussed. In Chapter 5, models are proposed to explain the dendrite initiation and propagation in all-solid-state batteries based on experimental observations. The assumptions and predictions of the models are explored and discussed. Guided by the models, effective approaches are explored to improve the performance the all-solid-state batteries.