Insights into lithium ion deposition on lithium metal surfaces.

Lithium metal is among the most promising anodes for the next generation of batteries due to its high theoretical energy density and high capacity. Challenges such as extreme reactivity and lithium dendrite formation have kept lithium metal anodes away from practical applications. However, the underlying mechanisms of Li ion deposition from the electrolyte solution onto the anode surface are still poorly understood due to their inherent complexity. In this work, density functional theory calculations and thermodynamic integration via constrained molecular dynamics simulations are conducted to study the electron and ion transfer between lithium metal slab and the electrolyte in absence of an external field. We explore the effect of the solvent chemistry and structure, distance of the solvated complex from the surface, anion-cation separation, and concentration of Li-salts on the deposition of lithium ions from the electrolyte phase onto the surface. Ethylene carbonate (EC), 1,2-dimethoxyethane (DME), 1,3-dioxolane (DOL), and mixtures of them are used as solvents. These species compete with the salt anion and the Li cation for electron transfer from the surface. It is found that the structure and properties of the solvation shell around the lithium cation has a great influence on the ability of the cation to diffuse as well as on its surrounding electron environment. DME molecules allow easier motion of the lithium ion compared with EC and DOL molecules. The slow growth approach allows the study of energy barriers for the ion diffusion and desolvation during the deposition pathway. This method helps elucidating the underlying mechanisms on lithium-ion deposition and provides a better understanding of the early stages of Li nucleation.

Lithium-Ion Transport through Complex Interphases in Lithium Metal Batteries.

Lithium metal is one of the best anode candidates for next-generation batteries. However, there are still many unknowns regarding the structure and properties of the solid electrolyte interphase (SEI) formed due to electron transfer reactions between the Li metal surface and the electrolyte. In addition, because of the difficulties to study amorphous and dynamic phases and interphases, there are many questions about the ion diffusion mechanism through complex multicomponent materials and interphases. In this study, using first-principles theory and computation, we focus on developing a better understanding of the ion motion mechanisms in the vicinity of a SEI formed when a seed Li2O or LiOH cluster nucleates on the Li metal surface. We study the role of charge transfer at the interface between charged surfaces and the electrolyte, and we investigate the evolution of inhomogeneous Li metal deposits present in the neighborhood of the SEI nuclei, aiming to fundamentally understand how these events modify the ion transport through complex electrochemically active materials.

Solvation vs. surface charge transfer: an interfacial chemistry game drives cation motion.

Electrolyte structure and ion solvation dynamics determine ionic conductivities, and ion (de)solvation processes dominate interfacial chemistry and electrodeposition barriers. We elucidate electrolyte effects facilitating or impeding Li+ diffusion and deposition, and evaluate structural and energetic changes during the solvation complex pathway from the bulk to the anode surface.

Role of Inorganic Surface Layer on Solid Electrolyte Interphase Evolution at Li-Metal Anodes.

Lithium metal is an ideal anode for rechargeable lithium-battery technology. However, the extreme reactivity of Li metal with electrolytes leads to solid electrolyte interphase (SEI) layers that often impede Li+ transport across interfaces. The challenge is to predict the chemical, structural, and topographical heterogeneities of SEI layers arising from a multitude of interfacial constituents. Traditionally, the pathways and products of electrolyte decomposition processes were analyzed with the basic and simplifying presumption of an initial pristine Li-metal surface. However, ubiquitous inorganic passivation layers on Li metal can reduce electronic charge transfer to the electrolyte and significantly alter the SEI layer evolution. In this study, we analyzed the effect of nanometric Li2O, LiOH, and Li2CO3 as surface passivation layers on the interfacial reactivity of Li metal, using ab initio molecular dynamics (AIMD) calculations and X-ray photoelectron spectroscopy (XPS) measurements. These nanometric layers impede the electronic charge transfer to the electrolyte and thereby provide some degree of passivation (compared to pristine lithium metal) by altering the redox-based decomposition process. The Li2O, LiOH, and Li2CO3 layers admit varying levels of electron transfer from a Li-metal slab and subsequent storage of the electronic charges within their structures. As a result, their ability to transfer electrons to the electrolyte molecules, as well as the extent of decomposition of bis(trifluoromethanesulfonyl)imide anions, is significantly reduced compared to similar processes on pristine Li metal. The XPS experiments revealed that when Li2O is the major component on the altered surface, LiF phases formed to a greater extent. The presence of a dominant LiOH layer, however, results in enhanced sulfur decomposition processes. From AIMD studies, these observations can be explained based on the calculated quantities of electronic charge transfer found for each of the passivating films.