Effect of Rb and Ta Doping on the Ionic Conductivity and Stability of the Garnet Li 7+2x--y (La 3--x Rb x)(Zr 2--y Ta y)O12 (0 <= x <= 0.375, 0 <= y <= 1) Superionic Conductor: A First Principles Investigation

In this work, we investigated the effect of Rb and Ta doping on the ionic conductivity and stability of the garnet Li7+2x-y(La3-xRbx)(Zr2-yTay)O12 (0{$\leq$}x{$\leq$}0.375, 0{$\leq$}y{$\leq$}1) superionic conductor using first principles calculations. Our results indicate that doping does not greatly alter the topology of the migration pathway, but instead acts primarily to change the lithium concentration. The structure with the lowest activation energy and highest room temperature conductivity is Li6.75La3Zr1.75Ta0.25O12 (Ea = 19 meV, {$\sigma$}300K = 1 x 10-2 S cm-1). All Ta-doped structures have significantly higher ionic conductivity than the undoped cubic Li7La3Zr2O12 (c-LLZO, Ea = 24 meV, {$\sigma$}300K = 2 x 10-3 S cm-1). The Rb-doped structure with composition Li7.25La2.875Rb0.125Zr2O12, has a lower activation energy than c-LLZO, but further Rb doping leads to a dramatic decrease in performance. We also examined the effect of changing the lattice parameter at fixed lithium concentration and found that a decrease in the lattice parameter leads to a rapid decline in Li+ conductivity, whereas an expanded lattice offers only marginal improvement. This result suggests that doping with larger cations will not provide a significant enhancement in performance. Our results find higher conductivity and lower activation energy than is typically reported in the experimental literature, which suggests that there is room for improving the total conductivity in these promising materials.