Implication of Mechanical Properties of Li-S Binary Compounds Obtained from the First-Principles Study
Qing Guo, Kah Chun Lau, Ravindra Pandey
Abstract
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 Li2S. 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.