Reduction mechanisms of additives on Si anodes of Li-ion batteries

Abstract

Solid-electrolyte interphase (SEI) layers are films deposited on the surface of Li-ion battery electrodes during battery charge and discharge processes. They are due to electrochemical instability of the electrolyte which causes electron transfer from (to) the anode (cathode) surfaces. The films could have a protective passivating role and therefore understanding the detailed reduction (oxidation) processes is essential. Here density functional theory and ab initio molecular dynamics simulations are used to investigate the reduction mechanisms of vinylene carbonate (VC) and fluoroethylene carbonate (FEC) on lithiated silicon surfaces. These species are frequently used as “additives” to improve the SEI properties. It is found that on lithiated Si anodes (with low to intermediate degrees of lithiation) VC may be reduced via a 2e⁻ mechanism yielding an opened VC²⁻ anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO²⁻_(ads) anion and a radical anion ˙OC₂H₂O²⁻. Additionally, in agreement with experimental observations, it is shown that CO₂ can be generated from reaction of VC with the CO₃²⁻anion, a product of the reduction of the main solvent, ethylene carbonate (EC). On the other hand, FEC reduction on Li_xSi_y surfaces is found to be independent of the degree of lithiation, and occurs through three mechanisms. One of them leads to an adsorbed VC²⁻ anion upon release from the FEC molecule and adsorption on the surface of F⁻ and one H atom. Thus in some cases, the reduction of FEC may lead to the exact same reduction products as that of VC, which explains similarities in SEI layers formed in the presence of these additives. However, FEC may be reduced via two other multi-electron transfer mechanisms that result in formation of either CO₂²⁻, F⁻, and ˙CH₂CHO⁻ or CO²⁻, F⁻, and ˙OCH₂CHO⁻. These alternative reduction products may oligomerize and form SEI layers with different components than those formed in the presence of VC. In all cases, FEC reduction also leads to formation of LiF moieties on the anode surface, in agreement with reported experimental data. The crucial role of the surface in each of these mechanisms is thoroughly explained.