Silicon is a promising material for anodes of Li-ion batteries due to its enhanced storage capacity, about one order of magnitude higher than that of graphite. The main constituents of liquid organic electrolytes are usually chemically unstable and become reduced in contact with the electrode surface, forming a polycrystalline solid film known as solid-electrolyte interphase (SEI) layer. In this work, we report results of density functional theory (DFT) analyses of decomposition of EC (solvent) and additives such as fluoroethylene carbonate (FEC) and vinylcarbonate (VC) on Si anodes. We utilize DFT and ab initio molecular dynamics simulations within two approaches: a cluster emulating ultra-low lithiated Si (001) surfaces, and periodic slabs simulating Si surfaces at various degrees of lithiation, where the low-coordinated surface Si atoms are saturated with various functional groups. The lowest Si content surfaces are represented by quasi-amorphous LiSi15H16, 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 reduction mechanisms (especially those of EC) depend significantly on the degree of lithiation of the surface.  On LiSi surfaces EC is reduced according to two two-electron mechanisms (one simultaneous and one sequential), independently of specific surface functionalization or nature of exposed facets.  On the less lithiated surfaces the simultaneous 2 e- reduction is found more frequently. In that mechanism, 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. FEC is reduced by multi-electron mechanisms in most of the studied surfaces. Detailed analyses of activation energies in the limit of very low lithiation yield information about the early stages of SEI formation on Si anodes.

Reduction mechanisms of organic solvents and additives on Si anodes: Effects of degree of lithiation and nature of exposed surface

247th National Meeting (2014)

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