Combined experimental and computational study of Ce-doped La₃Zr₂Li₇O₁₂ garnet solid state electrolyte

Li-containing garnet materials have been attracting considerable interest as potential solid-state electrolytes for Li ion batteries. In such Ln₃M₂Li_xO₁₂ (Ln = lanthanide, alkaline earth; M = Zr, Hf, Sn, Nb, Ta, Sb, Bi, Te), the best Li ion conductivity is observed for Li contents, x, just below the maximum 7.0. The decrease in conductivity for x = 7.0 systems is related to Li ordering (cell changes from cubic to tetragonal) to prevent too short Li–Li interactions. In this work, we report a combined experimental and modeling study of Ce⁴⁺ doping in La₃Zr₂Li₇O₁₂. We show for the first time that Ce⁴⁺ can be doped onto the Zr⁴⁺ site in this material. This doping strategy results in a reduction in the tetragonal distortion as well as a lowering of the temperature of the tetragonal–cubic phase transition, attributed to the increase in cell size reducing Li–Li interaction strain. Coupled with these changes, the conductivity shows a significant (1.5 orders of magnitude) improvement. Furthermore, the Ce doping also reduces the interfacial resistance (388 Ω cm² for Li₇La₃Z_1.75Ce_0.25O₁₂) in contact with Li metal, giving additional potential benefits to this doping strategy. The long-term cycling stability of a Li//garnet//Li symmetric cell over 190 h has been demonstrated.