Revealing Charge Transport Mechanisms in LiS for Li-Sulfur Batteries.

Besides lithium sulfide (LiS), lithium persulfide (LiS) is another solid discharge product in lithium-sulfur (Li-S) batteries. Revealing the charge transport mechanism in the discharge products is important for developing an effective strategy to improve the performance of Li-S batteries. LiS cannot transport free electrons due to its wide bandgap between the valence band maximum (VBM) and conduction band minimum (CBM). However, electron polarons (p) and hole polarons (p) can appear in solid LiS due to the unique molecular orbital structure of the S anion. The thermodynamic and kinetic properties of native defects are investigated. It is found that negatively charged Li vacancies (V) and p are the main native defects with a low formation energy of 0.77 eV. The predominant charge carrier is p because p has a high mobility. The electronic conductivity related to p diffusion is dependent on temperature, and high temperatures are preferred to increase the conductivity.