Pressure induced structural phase of lithium disulfide with a close to intermediate product character of lithium-sulfur battery

Recently spotlighting on lithium disulfide (Li₂S₂) as an intermediate product of a promising high capacity lithium–sulfur (Li–S) battery, a structure of P4₂/mnm had been proposed for understanding the existence of Li₂S₂ in Li–S battery. Here using first-principles unbiased CALYPSO structure searching calculations, we uncovered a new energetically favored structural phase (P2₁/c) of Li₂S₂, being more stable than the P4₂/mnm structure at above 2.78 GPa. This predicted P2₁/c structure possess a similar quasi-molecular S₂ and Li atoms in the interstice of S₂ with P4₂/mnm structure, but having a denser polyhedral packing (LiS₆) with Li atoms six-fold coordination in contract to the four-fold coordinated LiS₄ polyhedron of P4₂/mnm. Further, a calculated discharge voltage of 2.34 V was obtained for 2Li⁺ + 2e⁻ + Li₂S₂ →2Li₂S and comparable to that of P4₂/mnm. Adopting CI-NEB method, we calculated the diffusion barrier and diffusion coefficient of Li ions with a high diffusion coefficient of 5.98 × 10⁻²/cm² s⁻¹. This character reflects that it can acts as an intermediate product like as P4₂/mnm in a Li–S battery. This work has a value in exposing the pressure–induced phase of Li₂S₂ and enriching the views for understanding in Li–S batteries.