Insights into the Li Intercalation and SEI Formation on LiSi Nanoclusters

Abstract

We report a first-principles atomic level assessment of the lithiation and reactivity of pre-lithiated Si clusters. Density functional theory formation energy calculations reveal that the pre-lithiated Li<sub>16</sub>Si<sub>16</sub> cluster exposed to two different Li fluxes can store Li between the concentrations of Li<sub>2.5</sub>Si and Li<sub>3.5</sub>Si. This increase in storage capacity is attributed to the start of an amorphization process in the cluster, and more importantly these results show that the intercalation reaction can be controlled by the flux of the Li-ions. However, in a real battery, the lithiation of the anode occurs simultaneously to the electrode-electrolyte reactions. Here we simulate the solid-electrolyte interphase (SEI) formation and simultaneous lithiation of a Li<sub>16</sub>Si<sub>16</sub> cluster in contact with two different electrolyte solutions: one with pure ethylene carbonate (EC), and another with a 1 M solution of LiPF<sub>6</sub> in EC. Our ab initio molecular dynamics simulations show that the solvent and salt are decomposed leading to the initial stages of the SEI layer formation and large part of the added Li becomes part of the SEI. Interestingly, the pure EC solution results in lower storage capacity and higher reactivity, whereas the presence of the salt causes the opposite effect: higher lithiation and reduced reactivity.