(335d) First-Principles Modeling of Anode/Electrolyte Interfaces in Beyond Li-Ion Batteries

Lithium-ion (Li-ion) batteries have revolutionized consumer electronics and have begun to find their way into vehicles. However, the performance of Li-ion batteries is beginning to plateau. For batteries to revolutionize transportation, new storage technologies with capacities surpassing that of Li-ion batteries are highly desirable. One approach for improving batteries is to replace the graphite anode with a metal, such as lithium or magnesium; this has the potential to improve energy capacities by as much as an order of magnitude. Yet, metallic anodes introduce new challenges for battery operation, which are highly dependent on the interface that forms between the anode and the electrolyte. In this work, we employ first-principles computations to model interfacial phenomena at the anode/electrolyte interface in metallic magnesium and lithium batteries.

Magnesium batteries suffer from the formation of a passivation layer on the anode, hindering the transport of Mg²⁺ across the anode/electrolyte interface. One possible explanation for this behavior is the decomposition of species present in the electrolyte upon interaction with the anode surface. Using density functional theory, we assessed the energetics for decomposition of a common solvent, dimethoxyethane (DME), on plausible surface compositions of a Mg anode. The energetics of solvent decomposition were found to strongly depend on the composition of the anode surface. On the metallic surface, DME was found to readily decompose into ethylene gas and adsorbed fragments. Surface compositions consisting of films comprised of oxides and chlorides were predicted to be less susceptible to solvent decomposition, supporting previous literature advocating the presence of a Mg-Cl “enhancement layer” that improves battery performance.

In Li batteries, a solid-electrolyte interphase (SEI) layer is formed between the anode and the electrolyte in early charge cycles. Any battery employing a metallic Li anode will be limited by the properties of the SEI. Experimental studies on Li anodes have revealed that the native oxide that forms on Li metal is likely to constitute the most inner layer of this SEI. In this study, we modeled the metallic Li/native oxide interface by oxidizing a metallic Li slab with ab initio molecular dynamics (AIMD) and compared the resulting structure to a Li/Li₂O crystalline slab model. The AIMD procedure generated a physically realistic, oxidized slab, which showed many similarities with the crystalline model. Our results provide a more realistic picture of the atomistic structure of the interface between metallic Li and its native oxide, and demonstrate some of the fundamental constraints when constructing a battery with a metallic Li anode.