  Recent theoretical and experimental evaluations demonstrated that KLi6TaO6 is
a good Li-ion conductor. In this study, the energetics and detailed mechanism
of Li-ion migration, relevant to the point defects of KLi6TaO6, were analyzed
by first-principles calculations. Defect formation energy analysis suggested
that it has limited chemical potential conditions for attaining Li-excess
conditions through doping (substituting tetravalent elements for Ta). The
formation of other native defects, such as Li vacancies, hinders the
stabilization of the dopant and compensates for the interstitial Li. When the
doping is successful, the interactions between the coexisting dopant and
interstitial Li can increase the migration energy barrier of the interstitial
Li. This phenomenon limits the factors responsible for achieving high Li-ion
conductivity in this material. Based on the results of the investigations on
KLi6TaO6, isostructural materials of the form ALi6XO6, with various
combinations of constituent elements A and X, were each screened on the basis
of high stability and low Li-ion migration energy. Twelve structures of the
form (A = K or Rb)Li6XO6 were suggested, of which X was pentavalent. They also
exhibited limited chemical potential conditions for achieving Li-excess
conditions through doping. Combinations of the suggested isostructural oxides
and dopants were identified to reduce the interactions between interstitial Li
and dopant. Some isostructural oxides were doped using Sn and were evaluated
using first-principles molecular dynamics; their Li-ion conductivities at room
temperature were found to be comparable with those of garnet-type Li-ion
conductors.
