Collaborative Research: Production of Solar Quality Silicon by Model-Driven Molten Salt Electrolysis

Worcester Polytechnic Institute, Worcester MA

Investigators

Abstract

Silicon is the dominant solar material because of its abundance, low cost, and high solar efficiency. But manufacturing high-purity silicon required for solar energy is very complex, hard to scale, and unsafe since it involves handling toxic flammable gases. This award lays the scientific foundations for a new solar silicon production technology based on molten salt electrolysis. This process uses high purity quartzite or silica as its raw material. By this method, the production cost of solar silicon is expected to fall from $10 per kg today to just $1-2 per kg. The new silicon process uses significantly less energy than current practice and eliminates all direct CO₂ and related emissions. These advances in solar energy production technology contribute to the economic competitiveness of the U.S. solar industry. The fundamental knowledge gained in this research can be used to devise similar novel environmentally sound and efficient production methods for high-melting refractory metals, as well as silicon carbide and other semiconducting materials. The project’s impact is also further broadened through published model data and code, educational modules developed from the research results, and student training in a collaborative and multidisciplinary environment which includes both computational and experimental work. This project establishes four integrated mathematical models of the process based on new experimental data which help in the understanding of silicon molten salt electrolysis and its scale-up. The first molten salt structure (MSS) model involves spectroscopic analysis coupled with atomistic models to understand the molecular structure of complex ions in the five-component molten salt. The second CALculation of PHAse Diagrams (CALPHAD) model to understand and predict thermodynamic and thermophysical properties of the molten salt including silica solubility, silicon compound volatility and ion mobility. These properties are experimentally validated and used in a third transport model to study the reaction and diffusion kinetics of silicon electrodeposition at the cathode-electrolyte interface to predict deposit structure and composition. A fourth phase field model uses these molten salt properties to study the transport of silicon to the cathode and oxygen to the anode to understand boundary layer structure and improve the deposition rate. Using these models, a new current wave-form switching system is developed to maintain stable dendrite-free silicon growth at high current density. The new low-cost single operation silicon molten salt electrolysis replaces multiple energy-intensive unit operations of carbothermic silicon production, acid digestion, trichlorosilane synthesis, distillation, and chemical vapor deposition used in the current Siemens process. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.