“Li-stuffed” garnet electrolytes can deliver the adequate Li-conductivity, safety and cycle life required for the commercialisation of high-energy density batteries (i.e. high voltage Li-ion and Li-metal based batteries). However, these garnet electrolytes, if they are not processed properly, suffer from severe moisture-sensitivity that leads to drastic degradation of their transport and microstructural properties – a problem that has not been universally recognised in the field and that is impeding the achievement of the optimum performance of garnet electrolytes in Li-batteries. This is partially due to the Li-H Exchange taking place during the atmospheric degradation process that not only leads to a decrease in the Li transport number but also to the formation of insulating secondary phases (i.e. Li₂CO₃, LiOH) in the grain boundaries and interfaces. In fact, the integration of garnet electrolytes in Li battery devices has been delayed due to the high interfacial resistances at the lithium metal anode side which limits the performance to low current density cycling[1]. Again, this high interfacial resistance has been related to the formation of a Li₂CO₃ layer between the garnet and the Li-metal when not enough care is taken with the control of moisture during processing of the ceramics[2].

We have developed a unique setup that allows a strict control of the moisture during the processing and characterization of the garnets. The synthesis and processing of Li_7‑nxA_x(n‑1)xLa₃Zr₂O₁₂ (A= Ga(III), Ge(IV), = lithium vacancy, V_Liꞌ) garnets have been performed in a Ar-filled glove box with a high temperature furnace coupled to it. This has result in a three-fold enhancement in the total lithium-ion conductivity, up to 1.3 mS/cm at 24 °C[3] and the performance of garnets with highly controlled microstructure with densities up to 98%. The intrinsic Li-mobility in garnets has been characterised by means of impedance spectroscopy and ⁶Li-isotopic labelling. The effect of the moisture-degradation on the charge carriers concentration and mobilities has also been analysed using a controlled exchange in D₂O. To achieve this, a focussed-ion beam, secondary ion spectrometer has been used to detect all elements and their isotopes and yields the distribution of species in three dimensions on a nanoscale resolution, allowing the evaluation of the independent contributions of each isotope (⁶Li⁺, ⁷Li⁺, H⁺, D⁺) to the conduction process.