Searching for lithium superionic conductors attracts a lot of attention over the past few decades, especially for the development of all-solid-state lithium batterie. So far, the only kinds of solid material with ionic conductivity over 10^-2 S/cm at room temperature are among sulfide materials (Li₁₀GeP₂S₁₂ and its derivative, Li₇P₃S₁₁ glass-ceramic, and Li_9.54Si_1.74P_1.44S_11.7Cl_0.3). However, sulfides generally suffer from high sensitive to moisture and poor stability of solid electrolyte/oxide cathode interfaces. Compared with sulfides, the oxide SEs have significantly better chemical and electrochemical stability, whereas the highest ionic conductivity achieved in them is still one order of magnitude lower than those of liquid electrolytes. To combine the advantages of sulfides and oxides, anion mixing is expected to be a promising strategy to design the lithium superionic conductors with improved stabilities and high ionic conductivity. To the best of our knowledge, there is still no discovery of crystalline oxysulfide for SEs used in lithium batteries.

In this work for the first time we identify a new layered oxysulfide LiAlSO in orthorhombic structure as a novel lithium superionic conductor through first-principles calculations and crystal structure prediction techniques. Two kinds of stacking sequences of AlS₂O₂ layer 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, making it a promising solid state electrolyte in lithium secondary batteries with fast ionic conductivity and wide electrochemical window. This is a first attempt that the lithium superionic conductors are designed by crystal structure prediction method and may help explore other mixed-anion battery materials.

Acknowledgment

We acknowledge the National Natural Science Foundation of China (Grant No. 11234013), ‘‘863’’ Project (Grant No. 2015AA034201), and the Beijing S&T Project (Grant No. D161100002416003) for financial support and the Shanghai Supercomputer Center for providing computing resources. We would like to express our thanks to Prof. Jiawang Hong (Beijing Institute of Technology) for his fruitful discussion.