Instability of Lithium Garnets against Moisture. Structural Characterization and Dynamics of Li_7-xH_xLa₃Sn₂O₁₂ and Li_5-xH_xLa₃Nb₂O₁₂

We have recently demonstrated the instability of Li 7 La 3 Sn 2 O 12 garnet in humid atmosphere: a spontaneous and reversible ionic exchange Li + /H + occurs rapidly, leading to the protonated garnet Li 7- x H x La 3 Sn 2 O 12 . In the present article, we show that this instability cannot be generalized to all lithium garnets. We have tested different garnets with various cell parameters, lithium quantity, and lithium distribution and have observed that the Li + /H + exchange feasibility is directly connected to the lithium stoichiometry: if the concentration of Li + ions is greater than what can be accommodated on the tetrahedral site commonly occupied, meaning more than three lithium ions per formula, the garnet is sensitive to humidity. Structures determined by powder neutron diffraction are presented for two exchanged garnets: Li 7- x H x La 3 Sn 2 O 12 and Li 5- x H x La 3 Nb 2 O 12 obtained from ionic exchange in ethanol and benzoic acid. For the Sn one, the ionic Li + /H + exchange is associated with a transition from a tetragonal ( I 4 1 / acd ) to a cubic ( Ia 3̅ d ) cell, while for the Nb phase, the use of a noncentrosymmetric space group ( I 2 1 3), confirmed by second harmonic generation (SHG) test, is essential to describe the structure. The work is completed by 6 Li, 7 Li, and 119 Sn solid state Nuclear Magnetic Resonance (NMR) of the tin compounds. The impact of the Li + /H + exchange on the dynamics of the lithium ions has been investigated by 7 Li relaxation, and the dynamics of protons and lithium ions in the exchanged phase have been compared.