First Principles Study of the Li10GeP2S12 Lithium Super Ionic Conductor Material

The continued drive for high performance lithium batteries has imposed stricter requirements on the electrolyte materials. Solid electrolytes comprising lithium super ionic conductor materials exhibit good safety and stability and are promising to replace current organic liquid electrolytes. One major limitation in the application of Li-ion conductors is that their typical conductivity is less than 10−4 S/cm at room temperature. Recently, Kamaya et al. reported a new Li super ionic conductor Li10GeP2S12 (LGPS), which has the highest conductivity ever achieved among solid lithium electrolytes of 12 mS/cm at room temperature (comparable conductivity with liquid electrolytes) and outstanding electrochemical performance in Li batteries. The high conductivity in LGPS is attributed to the fast diffusion of Li in its crystal structural framework, which consists of (Ge0.5P0.5)S4 tetrahedra, PS4 tetrahedra, LiS6 octahedra, and LiS4 tetrahedra. Kamaya et al. proposed that diffusion in LGPS occurs along one dimension (1D) with diffusion pathways along the c axis. The authors also proposed that Li atoms in LiS4 tetrahedra enable fast diffusion along the c direction, while Li atoms in LiS6 octahedra are not active for diffusion. This hypothetical diffusion mechanism in LGPS has been inferred from the large anisotropic thermal factors and the Li disorder in the 1D channels but has not been directly proven. Understanding this LGPS material is important to improve its performance and may provide insight into designing new Li super ionic conductor materials. Using first principles modeling, we investigated the diffusivity, stability, and electrochemical window of LGPS. We provide a hypothesis for the observed wide electrochemical window of LGPS. We also identified the diffusion pathways and calculated the corresponding activation energies and diffusion coefficient. All calculations in this study were performed using the Vienna Ab initio Simulation Package (VASP) within the projector augmented-wave approach. Unless otherwise noted, all calculations were performed using the Perdew−Burke− Ernzerhof generalized-gradient approximation (GGA) to density functional theory (DFT). We assessed the phase stability of LGPS by constructing the quaternary Li−Ge−P−S phase diagram using all known Li− Ge−P−S compounds in the Inorganic Crystal Structure Database, all LixPySz compounds compiled by Holzwarth et al., and the calculated ground state of LGPS. As the refined structure has partial occupancies, we ordered the arrangement of Li, Ge, and P atoms in LGPS using an electrostatic energy criterion. Of the 10 orderings with the lowest electrostatic energy, the structure with the lowest calculated DFT energy was selected as the representative ground state. The calculation input parameters are based on those used in the Materials Project to leverage on the large set of computed data available in that database. Our calculated phase diagram predicts LGPS to be thermodynamically unstable at 0 K with respect to the following decomposition: