Lithium-ion batteries are usually made with graphite, which serves as the negative electrode in the electrochemical cell. There is interest in replacing some or all of the graphite with silicon, since the lithium storage capacity of silicon is about ten times that of graphite. However, large volume changes that occur when lithium is inserted into, or removed from, silicon can fracture the particles. These fractures increase reactions with the electrolyte and rapidly degrade battery performance.
Modifying the surface of silicon particles has been proposed as a way to minimize reactions with the battery electrolyte. Haasch et al. introduce a series of papers in a special collection about X-ray photoelectron spectroscopy (XPS) data from surface-modified silicon powders.
The silicon powders were produced by milling and coated with a thin layer of carbon-containing material. Additional coatings were also applied: polyethylene glycol, polyvinylene difluoride (PVdF) and perfluorooctene.
The investigators gathered XPS data for surface-modified silicon powders. The XPS data showed that while the spectra of the raw powders have similar peak shapes, there were variations in intensities in certain spectral regions for different coatings. In particular, the absence of one specific peak for the PVdF-coated particles suggested that this coating had decomposed during the surface modification process.
The XPS data also yielded elemental composition of the particles. Varying contents of silicon, fluorine, oxygen and carbon were observed for the samples.
Finally, the investigators made electrodes from the surface-modified powders and placed them into 2032-type coin cell batteries to study their electrochemical performance. All cells containing 70Si/C electrodes displayed good performance, indicating that surface-modified silicon can enhance the performance of lithium-ion batteries.
Source: “Si powders and electrodes for high-energy lithium-ion cells,” by Richard T. Haasch, Stephen E. Trask, Marco-T. F. Rodrigues and Daniel P. Abraham, Surface Science Spectra (2020) The article can be accessed at https://doi.org/10.1116/1.5130764.
