Effects of High and Low Salt Concentration in Electrolytes at Lithium–Metal Anode Surfaces

The use of high-concentration salts in electrolyte solutions of lithium–sulfur (Li–S) batteries has been shown to be beneficial for mitigating some effects such as polysulfide shuttle and dendrite growth at the Li metal anode. Such complex solutions have structural-, dynamical-, and reactivity-associated issues that need to be analyzed for a better understanding of the reasons behind such beneficial effects. A passivation interfacial layer known as solid–electrolyte interphase (SEI) is generated during battery cycling as a result of electron transfer from the metal anode causing electrolyte decomposition. Here, using density functional theory and ab initio molecular dynamics simulations, we investigate the salt decomposition, solvation effects, interactions among intermediate products and other species, and potential components of the SEI layer as a function of chemical nature and concentration of the salt for lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) and lithium bis­(fluorosulfonyl)­imide (LiFSI) at 1 and 4 M concentrations in dimethoxyethane. It is found that LiTFSI undergoes a less complete reduction and facilitates charge transfer from the anode, whereas LiFSI shows a more complete decomposition forming LiF as one of the main SEI products. In addition, the specific decomposition mechanisms of each salt clearly point to the initial SEI components and the potential main products derived from them. Very complex networks are found among the salt and solvent molecules in their attempt to maximize Li ion solvation that is quantified through the determination of coordination numbers.