Investigation of the Solid Electrolyte Interphase of Silicon Wafers Using a Fluorine-Free Electrolyte

Abstract

Lithium-ion (Li-ion) batteries have become an important solution for energy storage owing to their high gravimetric and volumetric energy density. The electrolyte solution of the battery has a large effect on the stability and performance of the cell. During the first cycles of a battery, the solid electrolyte interphase (SEI) layer is formed due to the degradation of the electrolyte. Hence the composition of this layer, and the stability, are dependent on the electrolyte composition.The most commonly used electrolyte for Li-ion batteries is the salt lithium hexafluorophosphate LiPF6 dissolved in organic solvents together with additives, both containing fluorinated compounds.1 However these fluorinated compounds are susceptible to release HF which is toxic and corrosive which could be detrimental for the cell performance but also hinder the recycling process. Therefore, fluorine-free electrolytes could be a promising alternative. While this type of electrolytes has been investigated with graphite anodes, when moving to higher energy densities, such as silicon, their performance and degradation mechanism is not yet clear.2 Previous research has shown increased stability for Li-ion batteries using a fluorine-free electrolyte based on the salt lithium bis(oxalato) borate (LiBOB).2 Furthermore, operando X-ray reflectometry (XRR) measurements on silicon wafers half-cells with the same electrolyte have shown the build-up of multiple layers on the anode, providing the thicknesses and densities of the different layers. This work presents the chemical composition of these different layers formed on the silicon wafers at different potentials when using a fluorine-free electrolyte based on LiBOB in ethylene carbonate and methyl ethyl carbonate. These results are also compared with the effect of adding vinylene carbonate VC as an additive. The different components in these layers are correlated to the previous XRR data to explain the SEI-formation mechanism with fluorine-free electrolytes. Xu, K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem Rev 104, (2004).Hernández, G. et al. Elimination of fluorination: The influence of fluorine-free electrolytes on the performance of LiNi1/3Mn1/3Co1/3O2/silicon-graphite li-ion battery cells. ACS Sustain Chem Eng 8, (2020).