Oxysulfide LiAlSO: A Lithium Superionic Conductor from First Principles

Through first-principles calculations and crystal structure prediction techniques, we identify a new layered oxysulfide LiAlSO in orthorhombic structure as a novel lithium superionic conductor. Two kinds of stacking sequences of layers of ${\mathrm{AlS}}_2{\mathrm{O}}_2$ are found in different temperature ranges. Phonon and molecular dynamics simulations verify their dynamic stabilities, and wide band gaps up to 5.6 eV are found by electronic structure calculations. The lithium migration energy barrier simulations reveal the collective interstitial-host ion “kick-off” hopping mode with barriers lower than 50 meV as the dominating conduction mechanism for LiAlSO, indicating it to be a promising solid-state electrolyte in lithium secondary batteries with fast ionic conductivity and a wide electrochemical window. This is a first attempt in which the lithium superionic conductors are designed by the crystal structure prediction method and may help explore other mixed-anion battery materials.

Figure 1

Crystal structure of $Pmc{2}_1\mathrm{LiAlSO}$ and its phonon dispersion. (a) A polyhedral view of the predicted structure and the views along the (b) $b$ and (c) $c$ crystal axis. (d) Calculated phonon dispersion curves of $Pmc{2}_1\mathrm{LiAlSO}$. The grey, yellow, and red spheres represent the Li, S, and O atoms, respectively. The dark blue tetrahedron stands for the ${\mathrm{AlS}}_2{\mathrm{O}}_2$ unit.

Figure 2

(a) Average structure of a pristine LiAlSO supercell from FPMD simulations at $\mathrm{T}=800\mathrm{K}$ for the last 10 ps. (b) The averaged MSDs for Al, S, and O atoms from FPMD simulations at 1500 K for last 10 ps. (c) The difference of Gibbs free energy between $Pmc{2}_1$ and $Cmc{2}_1\mathrm{LiAlSO}$ as a function of temperature.

Figure 3

The calculated kinetic properties of $Pmc{2}_1\mathrm{LiAlSO}$. (a) The continuous ${\mathrm{Li}}^{+}$ migration channels determined by the BV-based method, and the energy barriers simulated by the NEB method for (b) the kick-off mechanism along the $a$ direction, (c) the direct vacancy hopping along the $a$ axis, and (d) the interstitial ${\mathrm{Li}}^{+}$ hopping along the $c$ axis. The interstitial sites related to the ${\mathrm{Li}}^{+}$ migration are marked by dotted circles. The local atomic configurations of initial, transition, and final states calculated with the NEB method are shown as insets. The initial and final structures of each pathway are labeled on the horizontal axis as “ini_” and “end_”, respectively. The labels “dumbbell”, “vac,” and “inter” represent the dumbbell, vacancy, and interstitial configuration, respectively.

Figure 4

The calculated kinetic properties of $Cmc{2}_1\mathrm{LiAlSO}$. (a) The continuous ${\mathrm{Li}}^{+}$ migration channels determined by the BV-based method, and the energy barriers simulated by the NEB method for (b) the kick-off mechanism along the $a$ direction, (c) the direct vacancy hopping along the $a$ axis, and (d) the kick-off mechanism along the $c$ axis. Symbols and labels have the same meaning as Fig. 3.