Abstract
Stirred media milling is a common method for the efficient production of nanoparticles. Here the grinding of semi-metallic silicon nanoparticles is presented, which are of special interest as anode material for next generation lithium-ion batteries. Ground silicon particles show an enormous reactivity in water due to particle etching but only surficial oxidization in alcoholic solvents, which inhibits further particle etching. Therefore, the grinding process was realized in ethanol as a solvent in order to avoid particle etching but allow good integrity to a water-based anode production later on. From the application point of view the colloidal stability of silicon nanoparticle suspensions is of great importance, to realize anode coating structures with fine disperse silicon nanoparticles. Hence, this study is focusing on the electrostatic stabilization of the silicon nanoparticles in ethanol, which was characterized by zeta potential and the agglomerate size measurements. These results corresponding to electrostatical interactions are also in good accordance with rheological characterization of the suspensions and theoretical calculations. Additionally, the final metallic silicon content was also of high interest for the application, so that a thermogravimetric analysis procedure was established and evaluated by a chemical pulping procedure according to DIN EN ISO 21068-2. Furthermore the impact of the chosen stabilization additive and the solvent purity on the silicon content are discussed. Finally the process is realized by a pre-grinding step in a planetary ball mill and a fine grinding step within a stirred media mill. With this setup a production route for suspensions with a median primary particle size of less than 150 nm and metallic silicon content above 80 wt.% of the particles is presented. The ground nanoparticles show a surficial oxidized shell with a silicon core and a flake-like shape with crystalline and amorphous regions.
Highlights
The application of nanoparticles with customized properties is common in a variety of industries [1,2,3,4]
For the quantification of the metallic silicon content a thermogravimetric analysis (TGA) method was developed and evaluated with a chemical pulping procedure according to DIN EN ISO 21068-2
For aqueous systems the electrostatic stabilization of the ground particles due to the analysis and adjustment of the zeta potential ζ is a commonly known technique [27]. The latter can be adapted to ethanolic systems by manipulating the pH* value of the suspension [24]
Summary
The application of nanoparticles with customized properties is common in a variety of industries [1,2,3,4]. The combination of nanoscale and material dependent characteristics is often the basis for innovations and enhanced product performances. Concerning this matter silicon offers several interesting opportunities. In the present study the production of silicon nanoparticles in a two-step grinding procedure is established for the later. Zhang [19] and Liu [20] showed that reversible lithiation of silicon particles can be reached with a particle size below 150 nm. This particle size was adapted as a target size of the grinding process
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