Lithium-ion batteries are indispensable in our mobile electronic devices. Although considered as promising next-generation storage systems for green energy, safety aspects are of top priority of current research. The root of these safety issues associated with lithium-ion batteries lies in the electrolyte materials used at present. Those are based on flammable non-aqueous liquid electrolytes. Irrespective of these risks they could not been replaced so far because of the lack of inherently safe inorganic solids which should show both very high ionic diffusivities^1,2 and a practicable electro­chemical stability. Garnet-type metal oxides,³ such as Li₇La₃Zr₂O₁₂ (LLZO), are being considered as suitable candidates as they fulfil many of the requirements to realize “beyond” Li-ion battery technologies. As an example, to be applicable as a water-protective layer for Li- metal electrodes, the solid electrolyte must be stable in aqueous solutions. In general, the majority of different LLZO types show a high stability in humid environments compared to other solid electrolytes. Upon exposure to aqueous solutions, a Li⁺/H⁺ exchange reaction is reported to occur in this class of materials. In certain compounds this exchange is believed to lead to structural changes and degradation of the Li⁺ conductivity.⁴

The present work is aimed at investigating degradation phenomena on single crystalline Li_6.4La₃ZrTaO₁₂ samples in aqueous solutions (distilled water, 0.1 M HCl, saturated LiCl, and 1 M LiOH) as well as the degradation in both humidity and air. While grazing-incidence XRD and SEM helped identify degradation products, broadband conductivity spectroscopy was used to characterize ionic transport parameters of the bulk electrical responses. Albeit the ionic conductivities stay nearly constant the activation energies show drastic changes. As an example, for Li_6.4La₃ZrTaO₁₂ immersed in distilled water we obtained an activation energy of 0.75 eV in contrast to 0.59 eV of the pristine sample at ambient conditions. This observation can be attributed to the change in the Arrhenius pre-factor caused by the reduction of the number density of Li-ion charge carriers in the garnet.

Financial support by the Austrian Federal Ministry of Science, Research and Economy, as well as the Austrian National Foundation for Research, Technology and Development is highly appreciated.

¹ V. Epp, O. Gün, H.-J. Deiseroth, M. Wilkening, J. Phys. Chem. Lett. 4 (2013) 2118.
² M. Uitz, V. Epp, P. Bottke, M. Wilkening, J. Electroceram. 38 (2017), 142.
³ Thangadurai et al., Chem. Soc. Rev. 43 (2014) 4714.
⁴ C. Ma, E. Rangasamy, C. Liang, J. Sakamoto, K. L. More, M. Chi, Angew. Chem. Int. 54(1) (2015), 129-133.