Revealing Charge Transport Mechanisms in Li2S2 for Li–Sulfur Batteries

Besides lithium sulfide (Li2S), lithium persulfide (Li2S2) 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. Li2S2 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 Li2S2 due to the unique molecular orbital structure of the S22– anion. The thermodynamic and kinetic properties of native defects are investigated. It is found that negatively charged Li vacancies (VLi–) 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.