Carrier effective mass, phonon behavior and vibration characteristics of Li2X and Li2X2 (X = O, S) from first-principles calculations

Lithium–sulfur (Li–S) batteries are considered one of the most promising energy storage systems due to their high theoretical energy density. However, their practical applications are hindered by the shuttle effect caused by polysulfide dissolution. To address this challenge, we have systematically investigated the carrier effective mass, phonon behavior and vibrational characteristics of Li₂X and Li₂X₂ (X = O, S) using First-principles calculations. Effective mass calculations indicated higher carrier mobility in cubic Li₂S and hexagonal Li₂S₂. Its broader optical absorption also suggests potential in light-assisted suppression of reaction barriers. Due to different electronic transitions between S-p and Li-s states, hexagonal Li₂S₂ showed stronger thermally excited lattice vibrations compared to tetragonal Li₂S₂. Phonon behavior and lattice vibrational analyses suggest that the S-S bond vibrations can be optically modulated. It promotes the production of Li₂S rather than the polysulfides in the reduction, thus suppressing the extent of the shuttle effect. These findings offer valuable insights for the rational design of high-performance Li-S batteries with reduced shuttle effects and enhanced energy capacity.