Si-based materials have recently drawn a great deal of attentions for their great promise to replace graphite as the anodes of Li-ion batteries. However, their use as the anodes are still suffering from the problems of poor durability and cycling performance during the lithiation and delithiation processes. One of the main reasons can be attributed to the fact that it cannot form a stable solid-electrolyte-interphase (SEI) layer on the lithiated silicon surface, thereby causing huge capacity loss during the electrochemical processes. Accordingly, a detailed atomistic understanding of the formation of SEI layers, particularly the reduction mechanisms of the electrolyte molecules on the anode surfaces, is of critical importance to help accelerate the realization of the Si-based anodes in Li-ion batteries.

In this study, we employed first-principles density functional theory calculations and ab initio molecular dynamic (AIMD) simulations to investigate the reduction mechanisms of ethylene carbonate (EC) molecule on the amorphous lithiated surfaces of Si anodes in Li-ion batteries. We first generated the amorphous surface models of Si anodes within four different levels of lithiation (Li₄₀Si₈₈, Li₆₄Si₆₄, Li₈₄Si₄₄, and Li₁₀₀Si₂₈) via the MD “melt-and-quench” approach, and then examined the possible reduction pathways of EC molecules on these amorphous lithiated Si surfaces using ab initio molecular dynamic simulations.

Our AIMD simulations showed that EC molecules can be reduced on the Li_xSi surfaces via three different kinds of two-electron processes (two are simultaneous and one sequential), which appear to be highly dependent on the surface composition of the Si-based anodes. As the Li concentration on the anode surface was low (Li₄₀Si₈₈ and Li₆₄Si₆₄), our results showed that EC reduction was predominately initiated by the adsorption of a EC molecule onto the anode surface via the formation of Si-C bond followed by two simultaneous electron transfer leading to ring-opening,  which can be represented using the following formula:

EC + 2e^- → OCOC₂H₄O^2-          (1)

In this case, the interaction between the negatively charged surface Si atom and an EC molecule was found to be the main driving force to initiate this surface reduction reaction. However, on the highly lithiated Si surfaces (Li₈₄Si₄₄ and Li₁₀₀Si₂₈), the reduction of EC molecule was found to majorly proceed via another two simultaneous electron transfer process. In this reduction process, EC adsorption was driven by the electrostatic interaction between the C=O bond of an EC molecule and the positively-charged Li atoms on the anode surfaces.  Moreover, our results showed that the reduction rate of EC decomposition tends to increase with the Li content on the anode surfaces, and the increment of EC reduction can be attributed to the enhanced electron transfer from the anode surfaces to the EC molecules as revealed in our work function calculations. Our calculations further showed that EC molecules can even go through a four-electron transfer process on these highly lithiated Si anode surfaces, leading to the formation of CO^2- and O(C₂H₄)OCO^2- products.

Besides the mechanisms driven by adsorption, our AIMD simulations also revealed another reduction pathway on the highly lithiated Si surfaces via electron tunneling. In this case, the reduction of EC molecules can proceed via one sequential electron transfer process consisting of two steps:

EC + e^- → EC^-      (2)

EC^- + e- → CO₃^2- + C₂H₄     (3)

According to our prediction for the reaction energy, we found that this non-adsorption reaction was actually more energetically favorable than the reaction paths initiated by surface adsorption. However, it turns out to be an infrequent process on the lithiated Si surfaces due to its higher energy barriers to induce EC reduction. On the other hand, our calculations also showed that the reaction rate of EC reduction on the amorphous lithiated Si anode surfaces can be manipulated by some surface doping or chemical modification. These interesting new findings will also be addressed in this presentation.