Revealing the Hidden Dopant Effect on the Driving Force of Li Conduction and Dendrite Formation Mechanism in Cubic LLZO Solid Electrolytes

Garnet-type oxide cubic LLZO (c-LLZO) with doping of super-valent cations is a promising solid electrolyte because of the enhanced ionic conductivity of around 10^-3 S/cm. Different dopants have been proposed, such as Al, Ga, and Ce. However, the ionic conductivity is still limited and is not simply determined by the charge of dopants or the Li vacancy concentrations. The interaction between the Li and dopants and the complicated local environment due to various configurations of Li is still unclear. To comprehensively reveal the dopant effect in c-LLZO, we have investigated the c-LLZO bulk and surfaces with different dopants (Al, Ga, and Ce) via First-principle-based Density Function Theory (DFT) calculations. For screening out the energetic-favorable atomic structures, we have excluded the electrostatically-unfavorable Li configurations. It is found that the dopant atom tends to occupy the empty 24d sites (<i>tetrahedral</i>) in c-LLZO and results in three Li vacancies at the nearby 96h sites (<i>octahedral</i>) after doping. To further understand the dopant effect on the driving force of Li conduction, we calculated the vacancy formation energy of all the Li atoms as an indication of the Li chemical potential. The Li chemical potential increases and decreases by around 0.05 eV after doping with Ga and Al, respectively. It indicates that the Li atoms are thermodynamically more “mobile” when Ga is introduced, which agrees well with experimental results. Moreover, by calculating the Density of States (DOS) and Bader charge, it appears that Ga attracts more electrons than Al, resulting in a “more positive” environment for Li atoms that enhance Li conduction. Besides, the effect of different dopants on the surface electronic properties has been investigated, and the “trapping electron” tendency is evaluated to connect to the Li dendrite formation. We have also proposed a possible dopants/replacement that may be able to enhance the Li conduction and surpass the Li dendrite formation altogether.