From Lithium to Proton Mobility in Garnet Electrolytes: An NMR and Conductivity Study of H5.2Li1.3La3Zr1.5Ta0.5O12

Abstract

Garnet-type oxides are among the most promising solid-state electrolytes owing to their high chemical stability and comparatively high lithium-ion conductivity. They may also provide a basis for proton conductors through Li⁺/H⁺ exchange. In a recent ¹H and ⁷Li NMR study, we showed that minor proton incorporation in single-crystalline garnets exerts only a weak influence on Li-ion diffusion. Here, we investigate the polycrystalline hydrogarnet H_5.2Li_1.3La₃Zr_1.5Ta_0.5O₁₂ (HLZTO), obtained by aqueous Li⁺/H⁺ exchange from Li_6.5La₃Zr_1.5Ta_0.5O₁₂ (LLZTO). The precursor, synthesised at the remarkably low temperature of 600 °C, exhibits a large surface area enabling an efficient exchange process. X-ray diffraction reveals a slight lattice expansion upon protonation, while 6Li high-resolution MAS NMR and Raman spectroscopy indicate a pronounced alteration of the local Li environment. Thermogravimetric analysis shows a 5.5 wt% loss between 200 and 600 °C, consistent with the release of H2O. The total ionic conductivity of LLZTO is constrained by poor interparticle contact, whereas in HLZTO it drops sharply upon heating as protons are removed. Variable-temperature 1H NMR relaxation reveals a diffusion-induced maximum between 125 and 180 °C, resulting in a self-diffusion coefficient D ≈ 1 × 10⁻¹⁵ m² s⁻¹ (ca. 150 °C). As seen by 7Li NMR, the remaining Li ions in HLZTO are almost immobile on the NMR time scale, revealing that proton motion dominates charge transport in HLZTO. While defects in LLZO promote localized motions, their healing appears to facilitate the establishment of long-range Li⁺ transport pathways.