Solids with Two Mobile Ions: Proton H+ Self-Diffusion in Li-H Exchanged Garnet-Type Li6La3ZrTaO7 as Seen by Solid-State 1H NMR Relaxation

The development of ceramic proton conductors is currentlyattractinggreat attention, as they might be useful to construct new energy storagesystems. Li6La3ZrTaO12 (LLZTO) isknown for its rapid Li+ diffusivity as has been directlyrevealed by Li-7 NMR measurements. Exchanging parts of thehighly mobile Li+ ions by protons through treatment ofa single crystal in water or glacial acetic acid yields a mixed proton-lithiumionic conductor. Here, H+ proton diffusivity and Li+ diffusivity have separately been studied with element-specific H-1 and Li-7 NMR spectroscopy. While long-range Li-7 diffusion is noticeably slowed in Li-H exchangedLLZTO, we directly observe rather high H+ diffusivity,which is, however, significantly slower than Li+ dynamics.With the help of spin-lattice relaxation measurements we wereable to measure local (and long-range) energy barriers (0.20(1) eVvs 0.45(3) eV) as well as the self-diffusion coefficient D (H) of H+ dynamics (1.2 x 10(-15) m(2) s(-1) at 125 degrees C). These encouragingresults are assumed to open new directories in designing ceramicsoffering fast transport pathways for protons.

Automated summary

The development of ceramic proton conductors is of great interest due to their potential use in energy storage systems. Li6La3ZrTaO12 (LLZTO) has been found to exhibit rapid Li+ diffusivity. By treating a single crystal in water or glacial acetic acid, mobile Li+ ions can be exchanged with protons, resulting in a mixed proton-lithium ionic conductor. In this study, the diffusion of H+ protons and Li+ ions in the exchanged LLZTO has been investigated using element-specific H-1 and Li-7 NMR spectroscopy. The results show slower long-range Li-7 diffusion but relatively high H+ diffusivity, albeit slower than Li+ dynamics. By measuring spin-lattice relaxation and self-diffusion coefficient D(H) of H+ dynamics, energy barriers and transport properties have been determined, suggesting the potential of designing fast transport pathways for protons in ceramics.