Thermodynamic and Mechanical Stability of Crystalline Phases of Li₂S₂

In lithium–sulfur (Li–S) battery, Li 2 S 2 is one of the key intermediate products which may exist as an insoluble solid in a battery system. In this article, the low-energy crystalline phases of Li 2 S 2 are investigated using density functional theory coupled with the van der Waals correction terms. The calculated results find that the tetragonal, triclinic, and hexagonal phases of Li 2 S 2 phases are thermodynamically stable with the preference for the hexagonal phase to be the ground state. The low-energy Li 2 S 2 phases also exhibit highly anisotropic elastic properties which can be attributed to the unique S–S bond orientations in their lattices. We believe that the elastic modulus in Li–S solids can be properly tuned if the S–S bonds orientation and distribution can be controlled during the synthesis process, which may be helpful for the development of functionalized cathode in the Li–S battery.