(Invited) Density Functional Theory Modeling of Coupled Mechanical/Electrochemical Interfacial Processes during Li-Insertion into Silicon Anode

Silicon anodes, unlike graphite widely used in commercial batteries, experience large (up to 400%) volumetric expansion after Li insertion during charging.  This likely leads to unique SEI structures and formation mechanisms, especially at atomic lengthscales  not readily probed using existing experimental techniques.  We apply DFT modeling to model the incremental expansion of amorphous Li_xSi as the Li-content (“x”) increases; examine how such expansion stretches or even cracks the model passivating layers; and how such SEI evolution/degradation leads to additional electrolyte decomposition.  First we create a series of amorphous Li_xSi slabs by incrementally increasing the Li content.  The stoichiometry can range from LiSi to Li₁₃Si₄.  The slabs are coated by a thin passivating layer (e.g., LiF_z, which should form rapidly in the presence of FEC additives).  During expansion, the surface films evolve, become stretched, or even crack. AIMD simulations of liquid ethylene carbonate (EC) films are then conducted on expanded Li_xSi_y with stretched passivating LiF_x layers.  The instantaneous, electronic voltage, responsible for SEI formation, is periodically computed to ensure the modeling conditions conform to the experimental voltage window.  The effect of spatial inhomogeneity, e.g., holes in the passivating layer, on inducing voltage ”hot-spots” that can accelerate electrolyte decomposition, are highlighted.  To some extent, the modeling work represents very fast Li insertion, on the time scale of SEI formation.  Despite this, studying the stretching and potential breaking of model SEI layers can provide valuable insight concernin potentially unique SEI behavior on Si anode surfaces.

This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231, Subcontract No. 7060634 under the Batteries for Advanced Transportation Technologies (BATT) Program. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corpo ration, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.