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

Abstract

The development of ceramic proton conductors is currently attracting great attention, as they might be useful to construct new energy storage systems. Li6La3ZrTaO12 (LLZTO) is known for its rapid Li+ diffusivity as has been directly revealed by 7Li NMR measurements. Exchanging parts of the highly mobile Li+ ions by protons through treatment of a single crystal in water or glacial acetic acid yields a mixed proton–lithium ionic conductor. Here, H+ proton diffusivity and Li+ diffusivity have separately been studied with element-specific 1H and 7Li NMR spectroscopy. While long-range 7Li diffusion is noticeably slowed in Li–H exchanged LLZTO, we directly observe rather high H+ diffusivity, which is, however, significantly slower than Li+ dynamics. With the help of spin–lattice relaxation measurements we were able to measure local (and long-range) energy barriers (0.20(1) eV vs 0.45(3) eV) as well as the self-diffusion coefficient DH of H+ dynamics (1.2 × 10–15 m2 s–1 at 125 °C). These encouraging results are assumed to open new directories in designing ceramics offering fast transport pathways for protons.