Investigation of the Effect of Electrolyte Composition on the Performance of Pre-Structured Porous Silicon Anode Materials
Authors: Cavers Heather , Nöhren Sandra , Straub Benedikt , Polonskyi Oleksandr , Adelung Rainer 
Published: February 2020

Crystalline silicon is an ideal anode candidate for lithium ion batteries as it offers a high Li+ ion storage capacity, direct current pathway and is a stable and earth abundant element. One of the limitations of silicon as electrode material is its mechanical instability during repeated cycling of the battery. The incorporation of lithium ions into the silicon crystal structure causes extreme volume expansion (400 %) of the anode material. The high volume expansion during cycling can lead to crack formation and pulverization of the silicon anode. Consequently, the silicon can react further with the electrolyte, resulting in a build-up of the solid electrolyte interphase (SEI) layer, eventually hindering Li+ ion diffusion. Porous silicon, characterized by bulk silicon crystallites separated by pores, can compensate to some extent for the volume expansion without unwanted mass increase of the electrode (as with paste electrodes), while still maintaining lower costs than with silicon microwires due to the scalability of the porous silicon etching process[1]. In addition, porous silicon offers shorter diffusion pathways for Li+ due to the incorporation of the electrolyte into the pore structure.

Porous silicon still possesses some limitations regarding the amount of volume expansion it can undergo, as well as the diffusion of the electrolyte into the pore structure. To further compensate for the mechanical stress during cycling, by creating pre-defined breakage points in the silicon surface, and open up the pore structure to improve the electrolyte diffusion, pre-structuring of the porous silicon was performed via reactive ion etching (RIE) techniques using a flow ratio of 2 SF6/6 Ar (sccm/sccm), power level of 200 W and etch times varying from 30 seconds to 8 minutes. The resulting textured surface possessed microstructures with trench depths ranging from 1-2 µm and widths of around 1 µm. Improved cycling stability was seen for samples subjected to lower etch times (30 seconds to 2 minutes).

In order to have long term cycling stability, it is required to have an SEI layer that is thin enough to allow faster uptake of Li+ into the Si structure, and stable enough that it will not continuously react with active silicon. The composition of this SEI layer can be tailored by modifying the electrolyte composition. The type and concentration of lithium salt (LiTFSA, LiPF6, LiBOB etc.) and solvent molecule (ethylene carbonate, dimethyl ether, doluene, propylene carbonate etc.) can be altered in order to improve the SEI layer. In this work, we investigate the effect of varying electrolyte compositions on the performance of porous silicon anodes in a half-cell as well as on the SEI layer. The change in composition of the SEI layer is investigated via ex situ Raman and X-ray photoelectron spectroscopy techniques.

Electrolytes with high viscosity (such as LiPF6 containing electrolytes) experience limited infiltration into the silicon pore structure of the anode. The viscosity of the electrolyte can be tailored by modifying the salt concentration to ensure easy infiltration into the pores to allow maximum charging of the porous silicon anode. Improved electrolyte infiltration into the pores was seen with the RIE treated porous silicon samples for all electrolytes, including those with high viscosity. Reduction of the organic solvent molecules after repeated cycling lead to higher carbon content in the SEI after repeated cycling. Electrolytes with flourine containing lithium salts lead to the formation of LiF compounds, reducing the amount of active lithium.

[1] Hansen, S., Schütt, A., Carstensen, J. and Adelung, R., J. Electrochem. Soc.,
2016, 163(14), A3036-A3045.