As the requirements placed on rechargeable batteries continue to expand, next-generation technologies beyond today’s Li-ion cells are being increasingly sought. Solid-state designs using lithium metal anodes may provide a leap in energy density, but their charging currents have been limited by the formation of Li dendrites which lead to catastrophic cell failure. In this thesis, an examination is made into both the origin of Li dendrites as well as the influence of the solid electrolyte’s microstructure and mechanical properties. Firstly, an investigation is made into underlying processes by which Li dendrites grow within argyrodite-type Li6PS5Cl solid electrolytes. Key characteristics are elucidated using operando X-ray tomographic imaging, from which a new mechanistic understanding is proposed and modelled using a wide selection of experimentally determined mechanical and microstructural inputs. Secondly, a range of differently densified Li6PS5Cl electrolytes are produced using field-assisted sintering. Following which, an attempt is made to answer the question to what extent realistic changes in the electrolyte’s microstructure can affect sustainable Li-anode charging rates. Finally, the effect of dispersing metastabilised zirconia particles within the microstructure of Li6PS5Cl is investigated. The effect on both the fracture toughness and on ionic conductivity is assessed, after which the question as to whether transformation toughening can stabilise propagating dendrite cracks is considered.