Battery technology is advancing rapidly with new materials and new chemistries; however, materials stability determining battery lifetime and safety issues constitutes the main bottleneck. Electrolyte degradation processes triggered by electron transfer reactions taking place at electrode surfaces of rechargeable batteries result in multicomponent solid-electrolyte interphase (SEI) layers, recognized as the most crucial yet less well-understood phenomena impacting battery technology. Electrons flow via tunneling from the bare surface of negative electrodes during initial battery charge causing electrolyte reduction reactions that lead to SEI nucleation, but the mechanisms for further growth beyond tunneling-allowed distances are not known. Our first-principles computational studies demonstrate that radical species are responsible for the electron transfer that allows SEI layer growth once its thickness has evolved beyond the electron tunneling regime. In addition, the composition, structure, and properties of the SEI layer depend on the electrolyte, especially on the extent to which they are able to polymerize after reduction. Here we present a detailed study of polymerization mechanisms and propose mechanistic differences for electrolytes yielding a fast and a slow SEI growth. This new understanding leads to firm guidelines for rational electrolyte design.

Ab initio molecular dynamics simulations are used to identify mechanisms of reduction of ethylene carbonate on Si surfaces at various degrees of lithiation, where the low-coordinated surface Si atoms are saturated with O, OH, or H functional groups. The lowest Si content surfaces are represented by quasi-amorphous LiSi4 and LiSi2; intermediate lithiation is given by LiSi crystalline facets, and the highest Li content is studied through Li13Si4 surfaces. It is found that ethylene carbonate (EC) reduction mechanisms depend significantly on the degree of lithiation of the surface. On LiSi surfaces EC is reduced according to two different two-electron mechanisms (one simultaneous and one sequential), which are independent of specific surface functionalization or nature of exposed facets. On the less lithiated surfaces, the simultaneous two-electron reduction is found more frequently. In that mechanism, the EC reduction is initiated by the formation of a C-Si bond that allows adsorption of the intact molecule to the surface and is followed by electron transfer and ring-opening. Strongly lithiated Li13Si4 surfaces are found to be highly reactive. Reduction of adsorbed EC molecules occurs via a four-electron mechanism yielding as reduction products CO(2-) and O(C2H4)O(2-). Direct transfer of two electrons to EC molecules in liquid phase is also possible, resulting in the presence of O(C2H4)OCO(2-) anions in the liquid phase.

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 VC(2-) anion. At higher degrees of lithiation, such a species receives two extra electrons from the surface resulting in an adsorbed CO(2-)(ads) anion and a radical anion ˙OC2H2O(2-). Additionally, in agreement with experimental observations, it is shown that CO2 can be generated from reaction of VC with the CO3(2-)anion, a product of the reduction of the main solvent, ethylene carbonate (EC). On the other hand, FEC reduction on LixSiy surfaces is found to be independent of the degree of lithiation, and occurs through three mechanisms. One of them leads to an adsorbed VC(2-) 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 CO2(2-), F(-), and ˙CH2CHO(-) or CO(2-), 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.