Evaluation of the effect of site substitution of Pr doping in the lithium garnet system Li5La3Nb2O12

Abstract

Li ion conducting garnets have been attracting considerable interest for use as the electrolyte in all solid-state batteries, due to their high ionic conductivity and wide electrochemical stability window. Consequently, there have been a number of doping studies aimed at optimising the conductivity, focusing on both doping in Li₇La₃Zr₂O₁₂ and Li₅La₃(Nb/Ta)₂O₁₂ systems. In this paper, we report a detailed study of Pr doping in Li₅La₃Nb₂O₁₂, and show that this is a rare example of an ambi-site dopant, being able to be doped onto either the La or Nb site. Interestingly the resultant Pr oxidation state is determined by the site substitution, with oxidation states of 3+ for the La site, and 4+ for the Nb site. While the conductivity is essentially unchanged for the La site substitution, Pr⁴⁺ substitution on the Nb site leads to a large increase in the conductivity associated with the increase in Li content (Li_5+xLa₃Nb_2−xPr_xO₁₂) up to 0.56 mS cm⁻¹ (at 50 °C) for x = 0.8. Overall, this work highlights the flexibility of these garnet materials to doping, and suggests that further consideration of site substitution be considered for other dopants.