First principle studies on stability of reactive products of Li-O and Li-S batteries

Materials Modelling Centre, University of Limpopo, Private Bag X1160, Sovenga, South Africa

^* Corresponding author: cliffton.masedi@ul.ac.za

Abstract

We investigated the phase stabilities of insoluble discharge products of oxygen and sulphur (Li₂O, Li₂S, Li₂O₂ and Li₂S₂ structures) formed in Li-O and Li-S batteries using density functional theory within the generalized gradient approximation. Their structural, electronic and mechanical properties were determined to show their stability trend. The lattice parameters were well reproduced and agree to within 1% with the available experimental data. We have found good correlation between the heats of formation, density of states, elastic constant and phonon dispersion curves. The heats of formation predict Li₂O to be the most stable structure whereas Li₂S₂ is the least stable. Our phonon dispersion calculations show that Li₂O, Li₂S and Li₂O₂ structures are mechanically stable consistent with the elastic constants. The Li₂S₂ structure display soft modes associated mainly with sulphur atoms vibrations in the a-b plane, suggesting that the structure is unstable.