Supervalent doping and its effect on the thermal, structural and electrochemical properties of Li₇La₃Zr₂O₁₂ solid electrolytes

Supervalent doping is one of the most common methods used to stabilize the highly conductive cubic phase of Li₇La₃Zr₂O₁₂ (LLZO) solid electrolytes. While several different doping elements have shown that they are capable of stabilizing the cubic LLZO structure, there is still no clear consensus as to an optimal doping strategy. In this study we present one of the most extensive comparative analyses on supervalent doping of LLZO done to date. Herein, we compare the effects of eight different doping elements (Al, Ga, Fe, Ta, Nb, Sb, W and Mo) on the synthesis, crystal structure, morphology, and electrochemical properties of LLZO. We also propose a new guideline that would allow for the quick and easy identification of doping elements in LLZO and the estimation of their concentration using Raman spectroscopy. Our results show how Ga doping leads to exceptionally high ionic conductivities (1.30 × 10⁻³ S cm⁻¹) and low activation energies (0.26 eV) due to changes in the crystal symmetry of LLZO. On top of that, Ga doping also significantly lowers the required synthesis temperatures and increases the relative density of the LLZO structure, making Ga the most suitable element for LLZO doping. On the other hand, Nb doping shows the lowest ionic conductivity (1.91 × 10⁻⁴ S cm⁻¹) and a high activation energy (0.44 eV) of the investigated dopants, due to a poor sintering performance.