Key Messages

Using first-principles simulations, we predict a high-performance solid electrolyte with composition Na10GeP2S12 for use in sodium–sulfur (Na–S) batteries. The thermodynamic stability of its structure is established through determination of decomposition reaction energies and phonons, while Na-ionic conductivity is obtained using ab initio molecular dynamics at elevated temperatures. Our estimate of the room-temperature (RT) conductivity is 4.7 × 10−3 S cm−1, which is slightly higher than those of other superionic solid electrolytes such as β″-alumina and Na3Zr2Si2PO12, currently used in practical high-temperature Na–S batteries.