Probing the mobility of lithium in LISICON: Li⁺/H⁺ exchange studies in Li₂ZnGeO₄ and Li_2+2xZn_1-xGeO₄

Abstract

We investigated Li⁺/H⁺ exchange in the lithium ion conductors (LISICONS) [Li_2+2xZn_1-xGeO₄; x = 0.5 (I) and x = 0.75 (II)] and their parent, γ-Li₂ZnGeO₄. Facile exchange of approximately 2x lithium ions per formula unit occurs with both the LISICONS in dilute acetic acid, while the parent material does not exhibit an obvious Li⁺/H⁺ exchange under the same conditions. The results can be understood in terms of lithium ion distribution in the crystal structures: the parent Li₂ZnGeO₄, where all the lithium ions form part of the tetrahedral framework structure, does not exhibit a ready Li⁺/H⁺ exchange; LISICONS, where lithium ions are distributed between framework (tetrahedral) and nonframework sites, undergo a facile Li⁺/H⁺ exchange of the nonframework site lithium ions. Accordingly, Li⁺/H⁺ exchange in dilute aqueous acetic acid provides a convenient probe to distinguish between the mobile and the immobile lithium ions in lithium ion conductors.