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

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)(2)O-12 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 (Li5+xLa3Nb2-xPrxO12) up to 0.56 mS cm(-1) (at 50 degrees 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.