Si quality and milling technology parameters effect on resulting Si powder properties, for various applications with focus on Li-ion battery

There is a continuous technological development, and new applications with the potential use of silicon and silicon-based materials are emerging. Among scientists it is well known that silicon in Li-ion battery will contribute to a significant increase in the battery energy storage capacity. Thermoelectric materials is another promising application. Additive Manufacturing is a third and very interesting technology. New applications require new specifications for the products from Elkem ASA. Elkem ASA is one of the world?s leading producers of metallurgical silicon. Casting and solidification of silicon, with subsequent crushing to smaller factions are the last process steps in this production. The main markets for silicon today are production of silicones, silicon for solar cells, electronics, or aluminum. Some of these new potential applications will require silicon particle sizes in nano scale, which implies further downsizing of the current size of silicon products by use of dedicated milling technologies. The aim of this project is to increase the understanding of how to control the silicon powder properties in the size range of millimeter to nanometer by controlling the operation of the grinding process. The experimental work includes use of lab scale planetary ball mill and industrial scalable stirred media mill. Operation parameters of interest are rotational speed, grinding media size, time, and formulation. Existing Elkem metallurgical silicon qualities will be the feed material for the grinding process. The silicon powders produced will be characterized by use of analysis techniques for examination of shape, size, surface area, crystallinity, and level of contamination. Surface properties of the powder particles will attain additional focus due to its importance for the milling process control, post-milling handling, and characterization. In addition, required energy for production of various silicon powder qualities will be reported and to some extent optimized. This project will be conducted as an industrial Ph.D. study where an Elkem ASA employee completes the program within a three years period working at NTNU, Technische Universität Braunschweig, and in the company. The project will be partially funded by the Norwegian Research Council.

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Production of silicon powders is of great interest to Elkem AS because the area of application is expanding. New application areas are among others as anode material in Li-ion batteries, as thermoelectric material (TE materials) and in additive manufacturing (3D printing). There are several downsizing technologies, with grinding and milling as the most common ones. However, the specifications on particle size, morphology, and chemistry differs depending on the application area. Silicon is known to theoretically result in a great improvement of the Li-ion battery capacity by substituting/being combined with the carbon as the anode material in the state-of the art Li-ion battery. It is also a common understanding that Nano silicon (polysilicon), made by chemical vapor deposition or a similar technology, is the preferred silicon quality for battery application. This states the importance of silicon purity. However, such technologies/such materials are expensive and energy consuming. Moreover, there is a coming question regarding the importance of nano-size and purity level. This project will investigate how different silicon qualities currently produced at Elkem AS and possible waste source from production of other silicon qualities will respond to different types of mechanical milling and the final silicon powder properties. Implementation of the most promising milling technology/classification technology will be looked into. Several different characterization techniques will be used to define these properties. A selection of promising silicon powders for batteries will be tested as half-cell or full cells.