  We study the oxo-hexametallate Li$_7$TaO$_6$ with first-principles and
classical molecular dynamics simulations, obtaining a low activation barrier
for diffusion of $\sim$0.29 eV and a high ionic conductivity of $5.7 \times
10^{-4}$ S cm$^{-1}$ at room temperature (300 K). We find evidence for a wide
electrochemical stability window from both calculations and experiments,
suggesting its viable use as a solid-state electrolyte in next-generation
solid-state Li-ion batteries. To assess its applicability in an electrochemical
energy storage system, we performed electrochemical impedance spectroscopy
measurements on multicrystalline pellets, finding substantial ionic
conductivity, if below the values predicted from simulation. We further
elucidate the relationship between synthesis conditions and the observed ionic
conductivity using X-ray diffraction, inductively coupled plasma optical
emission spectrometry, and X-ray photoelectron spectroscopy, and study the
effects of Zr and Mo doping.
