In this work, we reveal the impact of moisture-induced chemical degradation and proton-lithium exchange on the Li-ion dynamics in the bulk, the grain boundaries and at the interface with lithium metal in highly Li-conducting garnet electrolytes. A direct correlation between chemical changes as measured by depth-resolved secondary ion mass spectrometry and the change in transport properties of the electrolyte is provided. In order to probe the intrinsic effect of the exchange on the lithium kinetics within the garnet structure, isolated from secondary corrosion product contributions, controlled-atmosphere processing was first used to produce proton-free Li6.55Ga0.15La3Zr2O12 (Ga0.15-LLZO), followed by degradation steps in a H2O bath at 100 C, leading to the removal of LiOH secondary phases at the surface. The proton-exchanged region was analysed by focussed ion beam - secondary ion mass spectrometry (FIB-SIMS) and found to extend as far as 1.35 m into the Ga0.15-LLZO garnet pellet after 30 minutes in H2O. Impedance analysis in symmetrical cells with Li metal electrodes evidenced a greater reactivity in grain boundaries than in grains and a significantly detrimental effect on the Li transfer kinetics in the Li metal/garnet interface correlated to a threefold decrease in the Li mobility in the protonated garnet. This result evidences that the deterioration of Li charge transfer and diffusion kinetics in proton-containing garnet electrolytes is not only related to the presence of isolating secondary products but also to the detrimental mobility of the Li charge carriers in the garnet lattice and grain boundaries. This has fundamental implications for the optimisation of the processing of these materials for their further integration in commercial battery devices.

Garnet-type solid electrolytes have attracted great interest in solid state battery research thanks to their high ionic conductivity at room temperature (10⁻³ S cm⁻¹) and their electrochemical stability against lithium metal anodes. However, the formation of lithium dendrites following charge/discharge limits their applicability and commercialisation. Although widely investigated, no clear explanation of dendrite formation has been previously reported. In this work, we employ cubic Al- and Ga-doped Li₇La₃Zr₂O₁₂, which represent two of the solid electrolytes with higher technological importance, to investigate the formation and chemical composition of dendrites. For the first time, this study elucidates the role that the dopants play in determining the critical current density for dendrite formation and highlights the importance of controlling the dopant distribution in the garnet structure. We use a combination of techniques including Secondary Electron Microscopy and Secondary Ion Mass Spectrometry in order to analyse the microstructure and chemical composition of dendrites in Li₇La₃Zr₂O₁₂. We show that, following electrochemical cycling, Li_6.55Ga_0.15La₃Zr₂O₁₂ systematically displays a critical current density 60% higher than Li_6.55Al_0.15La₃Zr₂O₁₂. Chemical analysis revealed that in Li_6.55Al_0.15La₃Zr₂O₁₂ the dendritic features are composed of a mixture of Al and Li species, whereas in Li_6.55Ga_0.15La₃Zr₂O₁₂ they are uniquely composed of Li. We also show that only in pristine Li_6.55Al_0.15La₃Zr₂O₁₂, the dopant segregates at the grain boundaries suggesting that local chemical inhomogeneity can have a fundamental role in the nucleation and propagation of dendrites.

The intrinsically defective nature of garnet-type electrolytes is revealed, probing the chemistry of the surface, grains and grain boundaries.

Current lithium ion battery technology makes use of flammable liquid electrolytes and so the development of solid ceramic electrolytes for the next generation of all-solid-state batteries can offer a safer alternative. However, the lithium diffusion behaviour in these solid electrolytes is not yet well characterised, despite the importance of this information for optimising cell performance. Similarly, the transport properties at the metal anode interface are critically important, but not well understood. We propose a methodology for obtaining lithium diffusion coefficients of bulk solid ceramic garnet-type Li₇La₃Zr₂O₁₂ (LLZO) electrolytes by coupling dense pellets with isotopically labelled lithium metal, followed by analysis with focused-ion-beam secondary ion mass spectrometry. We report room temperature lithium diffusivities of 2–8 × 10⁻¹³ m² s⁻¹ for doped LLZO using an estimate of the lithium diffusion length in good agreement with electrochemical impedance spectroscopy. Simultaneous detection of positive and negative secondary ion species by SIMS enables correlation of layered interfaces consisting of metallic lithium, corrosion/surface degradation products and bulk LLZO during depth profiling. Charging of the ceramic during ion sputtering is investigated and shown to have a minimal effect on the obtained lithium isotopic fractions in the current setup. Additionally, the effect of the presence of corrosion products at the surface of garnets as a result of air-exposure is investigated. This method could be extended to any Li-metal stable solid electrolyte, or with a reactive solid electrolyte coupled with a stable interlayer. As such, this work sets the basis of a methodology for further quantitative diffusion analyses for Li-conducting solid ceramic electrolytes and their interfaces with electrodes, as used in both solid-state lithium batteries and hybrid systems coupling solid and liquid electrolytes.