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

Reactivity at the Lithium–Metal Anode Surface of Lithium–Sulfur Batteries

Due to their high energy density and reduced cost, lithium–sulfur batteries are promising alternatives for applications such as electrical vehicles. However, a number of technical challenges need to be overcome in order to make them feasible for commercial uses. These challenges arise from the battery highly interconnected chemistry, which besides the electrochemical reactions includes side reactions at both electrodes and migration of soluble polysulfide (PS) species produced at the cathode to the anode side. The presence of such PS species alters the already complex reactivity of the Li anode. In this work, interfacial reactions occurring at the surface of Li metal anodes due to electrochemical instability of the electrolyte components and PS species are investigated with density functional theory and ab initio molecular dynamics methods. It is found that the bis­(trifluoromethane)­sulfonimide lithium salt reacts very fast when in contact with the Li surface, and anion decomposition precedes salt dissociation. The anion decomposition mechanisms are fully elucidated. Two of the typical solvents used in Li–S technology, 1,3-dioxolane and 1,2-dimethoxyethane, are found stable during the entire simulation length, in contrast with the case of ethylene carbonate that is rapidly decomposed by sequential 2- or 4-electron mechanisms. On the other hand, the fast reactivity of the soluble PS species alters the side reactions because the PS totally decomposes before any of the electrolyte components forming Li₂S on the anode surface

Anisotropic Electron–Phonon Coupling in Colloidal Layered TiS₂ Nanodiscs Observed via Coherent Acoustic Phonons

Atomically thin layered transition metal dichalcogenides with highly anisotropic structure exhibit strong anisotropy in various material properties. Here, we report the anisotropic coupling between the interband optical transition and coherent acoustic phonon excited by ultrashort optical excitation in a colloidal solution of multilayered TiS₂ nanodiscs. The transient absorption signal from the diameter- and thickness-controlled TiS₂ nanodiscs dispersed in solution exhibited an oscillatory feature, which is attributed to the modulation of the interband absorption peak by the intralayer breathing mode. However, the signature of the interlayer acoustic phonon was not observed, while it has been previously observed in noncolloidal exfoliated sheets of MoS₂. The dominance of the intralayer mode in modulating the interband optical transition was supported by the density functional theory (DFT) calculations of the optical absorption spectra of TiS₂, which showed the stronger sensitivity of the interband absorption peak in the visible region to the in-plane strain than to the out-of-plane strain