Influence of Li+ and H+ Distribution on the Crystal Structure of Li7-xHxLa3Zr2O12 (0 ≤ x ≤ 5) Garnets

With appropriate doping or processing, Li7La3Zr2O12 (LLZO) is an excellent candidate to be used in Li batteries either as a solid electrolyte or as a separator between the Li anode and a liquid electrolyte. For both uses, the reactivity with water either from the air or in aqueous media is a matter of interest. We address here the structural changes undergone by LLZO as a result of H+/Li+ exchange and relate them with the amount of H content and atomic distribution. Neutron diffraction is performed to elucidate Li and H location. Two different cubic phases derive from LLZO through I (4) over bar 3d/Li exchange: Deep hydration up to 150 degrees C yields a noncentrosymmetric I43d phase in which octahedral Li ions are exchanged by H ions, tetrahedral Li ions split into two sites with very different occupancies, and H ions form O4H4 entities around the less occupied tetrahedral site. Annealing above 300 degrees C results in a centrosymmetric Ia (3) over bard phase with lower H content in which Li ions occupy the usual sites of the cubic garnets and H ions occupy a split pseudooctahedral site. The centrosymmetric or noncentrosymmetric character is determined by the temperature at which exchange is performed and the H content. Both factors are not independent: at low temperature, the high H content favors H ordering around the vacant tetrahedra, while low H content and higher mobility at 350 degrees C lead to a disordered configuration of Li and H ions. The deeply hydrated garnets are stable up to at least 300 degrees C and also upon aging at room temperature.