  Combining high-throughput first-principles calculations and experimental
measurements, we have identified a novel family of fast lithium-ion
chalcogenide conductors in Li$_2$BMQ$_4$ (2114, B = Ca, Sr and Ba; M = Si, Ge
and Sn; Q = O, S and Se) systems. Our calculations demonstrate that most of the
thermodynamically and kinetically stable sulfides and selenides in this new
system exhibit ultralow Li$^+$ ion migration activation energy (0.16 eV ~ 0.56
eV) and considerable bandgaps varying between ~ 2 eV and 3.5 eV. We have
successfully synthesized Li$_2$BaSnS$_4$ and Li$_2$SrSiS$_4$, and they exhibit
excellent moisture stability through H$_2$S gas measurements. Electrochemical
impedance measurements indicate 2114 systems show the typical features of solid
ionic conductors, with a room-temperature Li$^+$ conductivity close to
5$\times$10$^{-4}$ mS/cm aligning with our molecular dynamics simulations.
Furthermore, we have theoretically investigated the substitution of Cl$^-$ at
S$^{2-}$ site. The doped compounds display significantly higher conductivity,
with an increase of about three orders of magnitude (up to a maximum of 0.72
mS/cm) compared to the undoped compounds. These findings offer valuable
insights for the further exploration of potential chalcogenide solid
electrolyte materials with robust air stability and enhanced ionic conductivity
for practical applications in lithium-ion batteries.
