Implication Of Mechanical Properties Of Li-S Binary Compounds Obtained From The First-Principles Study

Lithium-sulfur (Li-S) batteries are one of the most promising substitutes for the current Li-ion battery system as a next-generation storage system because of their high theoretical energy density and low cost. However, challenges still exist for Li-S battery systems from being commercialized, one of which is the volume variation of the S-rich cathode during cycling. To provide an atomistic view of the factors causing the volume variation and subsequent change in the elastic properties, we present a systematic and detailed study of lithium polysulfides that are intermediate products at the S-rich cathode during cycling. The calculated results based on density functional theory find a small volume expansion of high-order lithium polysulfides, but a significantly large volume expansion of low-order polysulfides, which is consistent with the two-stage discharge process in the Li-S battery. During lithiation, the calculated elastic properties show an upward trend, which can essentially be attributed to the nature of the chemical bonding that changes from S-S covalent bonds to Li-S ionic bonds in going from Li2S8 to Li2S. Moreover, analysis of the electronic density of states shows smaller band gaps associated with the S-rich polysulfides, suggesting that they are easier to charge than the insulating Li,S. The results, therefore, provide a fundamental understanding of the changes that occurred at the cathode during lithiation and would be useful for the design of the next-generation cathode materials that can withstand significant stress and strain while enduring significant volume changes during the Li-S cell cycling.