Li-ion batteries are present in almost all portable electronic devices and play a crucial role in modern society. There is, however, a rising demand for safer batteries with both higher energy density and longer lifetime. One possibility is to replace the current liquid electrolyte with a solid-state electrolyte (SSE). In this way, one can theoretically increase the energy density with the use of Li metal anodes by physically hindering Li dendrite formation and growth. Additionally, the use of SSEs will result in safer batteries due to the removal of the highly flammable liquid electrolyte, thus facilitating a wider range of operating temperatures. However, the implementation of SSEs has proven challenging as current materials suffering from low ionic conductivities, poor air and water stability, poor electrolyte-electrode interfaces, as well as dendrite growth across grains and cracks, in spite of the solid electrolyte. Good contact between the electrolyte and the electrodes is necessary to realise a well-functioning battery with longterm cyclability. Chemical expansion as well as differ- * benjamin.williamson@ntnu.no ent thermal expansion coefficients make good electrodeelectrolyte interfaces very challenging to prepare and to maintain throughout operation. In 2003 a group of materials with the garnet structure were suggested as potential SSEs. Early studies on garnet materials displayed high stability alongside Li-ion conductivities of about 10 -6 S cm -1 . To date, arguably the most well studied garnet SSE is Li 7 La 3 Zr 2 O 12 (LLZO) displaying relatively high Li-ion conductivities of ∼ 10 -6 -10 -3 S cm -1 depending on doping, synthesis technique and environment, and the structural phase of LLZO. At room temperature (RT), the structure is stable in its tetragonal form (space group: I4 1 /acd), whilst at higher temperatures it undergoes a phase transition into the disordered cubic phase (space group: Ia 3d) which possesses ionic conductivities around two orders of magnitude higher than the tetragonal phase. The wide electrochemical stability window of LLZO allows it to benignly contact with Li metal anodes. However, a very thin layer of tetragonal-like LLZO structure has been observed at the interface between cubic LLZO and Li metal potentially resulting in a bottleneck to the Li-ion conductivity at the electrolyte-anode interface. Such a phenomenon means that it is important to understand Li transport and defects within both tetragonal LLZO as well as cubic LLZO.