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

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.