Upcycling pure silicon to make lithium-ion batteries

Scientists from Nanyang Technological University Singapore (NTU Singapore) have developed an efficient method to recover high-purity silicon from expired solar panels to produce lithium-ion batteries that could help meet the global demand to power electric vehicles. High-purity silicon makes up the majority of solar cells, yet they are typically discarded at the end of the operational lifespan after 25 to 30 years. It is challenging to separate the silicon from other solar cell components such as aluminium, copper, silver, lead and plastic. Moreover, recycled silicon has impurities and defects, making it unsuitable for other silicon-based technologies.

Existing methods to recover high-purity silicon are energy-intensive and can involve toxic chemicals, limiting their widespread adoption among recyclers. The NTU researchers developed the new extraction method using phosphoric acid, a substance commonly used in the food and beverage industry. The NTU approach demonstrated a higher recovery rate and purity than present silicon recovery technologies. The process involves a single reagent (phosphoric acid), whereas conventional methods include at least two types of chemicals (highly acidic and highly alkaline).

Nripan Mathews, principal investigator of the study, said the researchers did not have to use multiple chemicals for silicon recovery, thereby reducing the time spent on post-treatment of the chemical wastes. “At the same time, we achieved a high recovery rate of pure silicon comparable to those produced by energy-intensive extraction techniques,” Mathews said.

The NTU researchers believe their silicon recovery method could solve the growing problem of solar panel waste by keeping resources in a loop. The study was published in the scientific journal Solar Energy Materials and Solar Cells.

To extract the silicon, the NTU researchers first soaked the expired solar cell in hot diluted phosphoric acid for 30 minutes to remove metals (aluminium and silver) from their surfaces. This process was repeated using fresh phosphoric acid to ensure complete removal of the metals, resulting in pure silicon wafer at the end of another 30 minutes. Using advanced spectroscopic analyses to evaluate the elemental content of the recovered wafer, researchers found the sample achieved a recovery rate of 98.9% with a purity of 99.2%. When the recovered silicon was upcycled into a lithium-ion battery anode and tested for efficiency, it performed similarly to newly purchased silicon.

Lead author of the study Dr Sim Ying said the comparable performance of the upcycled silicon-based lithium-ion battery proves that the NTU approach is feasible. “We envision our faster and cheaper silicon recovery method to be a positive boost for the development of EV batteries. Aside from EVs, there are also potential applications such as thermoelectric devices,” Ying said.

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