Lithiation and Delithiation Processes in Lithium-Sulfur Batteries from Ab lnitio Molecular Dynamics Simulations

Lithium-sulfur (Li-S) batteries are a promising alternative to the Li-ion technology due to their high theoretical capacity and low cost. Unlike intercalation compounds, the sulfur cathode undergoes a series of complex electrochemical reactions that give rise to substantial structural and morphological changes. Here, we report ab initio molecular dynamics simulations of the lithiation and delithiation reactions that are important in Li-S batteries. The lithiation is studied on two low-energy surfaces, (100) and (001), of sulfur (SO, whereas delithiation is studied on the (111) surface of lithium sulfide (Li2S). The effect of electrolyte is included by constructing interfacial systems between these surfaces and dimethoxyethane, a widely used liquid electrolyte. During both lithiation and delithiation, a layer by-layer reaction pattern is revealed. The evolution of atomistic structure and reaction voltage during lithiation and delithiation is studied, and the microscopic reaction mechanisms are analyzed. Dissolution of lithium polysulfides into the electrolyte is also observed in our simulations, which is attributed to the strong interaction between lithium polysulfides and electrolyte molecules in the form of lithium bonds. Studies of the delithiation process in Li2S confirm that the experimentally observed initial charge barrier is of kinetic origin.