Density functional theory assessment of the lithiation thermodynamics and phase evolution in si-based amorphous binary alloys

Abstract

Development of novel alloy-based anodes has the potential to increase the energy storage capacity of current Li-ion based energy storage technology. In particular, Si-based anodes are of interest due to their high theoretical capacity, but suffer from poor cycle and calendar life stemming from large volumetric expansion and a non-passivating solid-electrolyte interface. The addition of amorphous components to the Si anode has been shown to improve the mechanical and chemical stability during lithiation. In this study, we use density functional theory (DFT) to probe the thermodynamics of amorphous alloy formation in a range of binary Si-X alloy systems, where X constitutes any element from periodic table groups 1-17. The alloying elements are classified as active or inactive components based on the reactivity with Li, where active elements form stable binary compounds with Li and inactive elements do not. We find that when alloying inactive elements, most inactive components do not fully reduce and hence result in the extrusion of metallic phases. Formation of Si-X compounds with no reactivity to Li results in deactivation of Si and decreased capacity. Alloying with Li-inactive elements also bypasses early Si lithiation stages and decreases the onset potential for lithiation. Most of the Li-active elements do not form stable Si-X binaries or Li-Si-X ternaries, resulting in lithiation potentials composed of voltage steps matching those of the base elements (Li x X and Li x Si), while the others may not be of much practical use due to their high lithiation potentials or preciousness, but may buffer against volumetric expansion.