Toward a Molecular Understanding of Energetics in Li-S Batteries Using Nonaqueous Electrolytes: A High-Level Quantum Chemical Study

The Li-S battery (secondary cell or redox flow) technology is a promising future alternative to the present lithium intercalation-based energy storage, and, therefore, a molecular level understanding of the chemical processes and properties such as stability of intermediates, reactivity of polysulfides, and reactivity toward the nonaqueous electrolytes in the Li-S batteries is of great interest. In this paper, quantum chemical methods (G4MP2, MP2, and B3LYP) were utilized to compute reduction potentials of lithium polysulfides and polysulfide molecular clusters, energetics of disproportionation and association reactions of likely intermediates, and their reactions with ether-based electrolytes. Based on the computed reaction energetics in solution, a probable mechanism during the discharge process for polysulfide anions and lithium polysulfides in solution is proposed and likely intermediates such as S-4(2-), S-3(2-), S-2(2-), and S-3(1-) radical were identified. Additionally, the stability and reactivity of propylene carbonate and tetraglyme solvent molecules were assessed against the above-mentioned intermediates and other reactive species by computing the reaction energetics required to initiate the solvent decomposition reactions in solution. Calculations suggest that the propylene carbonate molecule is unstable against the polysulfide anions such as S-2(2-), S-3(2-), and S-4(2-) (Delta H-dagger < 0.8 eV) and highly reactive toward Li2S2 and Li2S3. Even though the tetraglyme solvent molecule exhibits : increased stability toward polysulfide anions compared to propylene carbonate, this molecule too is vulnerable to nucleophilic attack from Li2S2 and Li2S3 species in solutions. Hence, long-term stability of the ether molecules is unlikely if a high concentration of these reactive intermediates is present in the Li-S energy storage systems.