Instability of Lithium\nGarnets against Moisture. Structural\nCharacterization and Dynamics of Li_7‑<i>x</i>H_<i>x</i>La₃Sn₂O₁₂ and Li_5‑<i>x</i>H_<i>x</i>La₃Nb₂O₁₂

Abstract

We have recently demonstrated the instability of Li₇La₃Sn₂O₁₂ garnet in humid\natmosphere:\na spontaneous and reversible ionic exchange Li⁺/H⁺ occurs rapidly, leading to the protonated garnet Li_7‑<i>x</i>H_<i>x</i>La₃Sn₂O₁₂. In the present article, we show that this\ninstability cannot be generalized to all lithium garnets. We have\ntested different garnets with various cell parameters, lithium quantity,\nand lithium distribution and have observed that the Li⁺/H⁺ exchange feasibility is directly connected to the\nlithium stoichiometry: if the concentration of Li⁺ ions\nis greater than what can be accommodated on the tetrahedral site commonly\noccupied, meaning more than three lithium ions per formula, the garnet\nis sensitive to humidity. Structures determined by powder neutron\ndiffraction are presented for two exchanged garnets: Li_7‑<i>x</i>H_<i>x</i>La₃Sn₂O₁₂ and Li_5‑<i>x</i>H_<i>x</i>La₃Nb₂O₁₂ obtained from ionic exchange in ethanol and benzoic acid. For the\nSn one, the ionic Li⁺/H⁺ exchange is associated\nwith a transition from a tetragonal (<i>I</i>4₁/<i>acd</i>) to a cubic (<i>Ia</i>3̅<i>d</i>) cell, while for the Nb phase, the use of a noncentrosymmetric\nspace group (<i>I</i>2₁3), confirmed by second\nharmonic generation (SHG) test, is essential to describe the structure.\nThe work is completed by ⁶Li, ⁷Li, and ¹¹⁹Sn solid state Nuclear Magnetic Resonance (NMR) of the tin\ncompounds. The impact of the Li⁺/H⁺ exchange\non the dynamics of the lithium ions has been investigated by ⁷Li relaxation, and the dynamics of protons and lithium ions\nin the exchanged phase have been compared.