Solar panels may be the poster child of the green energy revolution, but they’re on course to cause their own environmental faux pas. Spent solar panels at the end of their life will soon be stacking up sky high. However, with some creative recycling, we might yet avoid a massive solar cell scrap heap.

Scientists from Nanyang Technological University (NTU) in Singapore recently presented a new method for recycling high-purity silicon from discarded solar panels. Their solution could kill two birds with one stone, not just mitigating the problem of solar waste but also enabling the next generation of electric vehicle (EV) batteries.

A massive source of electronic waste

The typical operational life of solar cells is 25 to 30 years. With ongoing investments in solar energy and the relatively large size of solar panels, it is expected that end-of-life photovoltaic cells will be a significant source of electronic waste in the near future. The International Renewable Energy Agency (IRENA) estimated it will add up to 78 million tons by 2050.

Rather than go to waste, these old cells could be a good source of recycled silicon. The production of crystalline, solar-grade silicon is a highly energy-intensive and costly process. However, recycling silicon from solar cells is challenging because of the mix of other components such as aluminum, copper, lead, plastic, silver and a special anti-reflective coating used to improve the efficiency of solar cells.

Because of these complications, recovered silicon usually contains several impurities that make it unsuitable for many applications. Additionally, the current recycling methods require high energy and involve toxic chemicals that are not environmentally friendly.

A new method of solar panel recycling

The NTU researchers discovered a new silicon extraction method that could solve the current challenges. They presented their findings in an August 2023 paper titled Simplified silicon recovery from photovoltaic waste enables high performance, sustainable lithium-ion batteries.

The conventional silicon recovery method uses a minimum of two reagents that are highly acidic and highly alkaline. The new method uses a single reagent, the widely available and environmentally friendly phosphoric acid. The researchers claim the new process is more efficient with a higher silicon purity and recovery rate.

“We do not have to use multiple chemicals, 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,” said researcher Nripan Mathews in an NTU news release.

Phosphoric acid is generally used in various material recovery processes for the extraction and purification of metals and minerals. It has a relatively low environmental impact compared to other harsh chemicals. The new approach starts by removing metals from the solar panel surfaces by soaking the cells in hot, diluted phosphoric acid for 30 minutes. The process is repeated in fresh phosphoric acid for another 30 minutes to achieve higher silicon purity.

Using this approach, the researchers achieved a 98.9% recovery rate with 99.2% silicon purity. This is comparable with conventional methods while being faster, less expensive and more environmentally friendly.

Recovered silicon is pure enough to be used in batteries

Many researchers see silicon as one of the most promising alternative materials to traditional graphite anodes in lithium-ion batteries. Silicon can store considerably more lithium ions than graphite, enabling batteries to store more energy per unit mass. It’s also faster at shuttling lithium ions across the battery’s membrane. For electric vehicles, silicon has the potential to enable batteries with extended range and faster charging.

However, high silicon purity is essential to reduce the potential side reactions with lithium ions that could negatively impact the battery performance. The NTU researchers believe that their high purity silicon recovered from solar cells could solve the challenge of material availability for next generation batteries. The researchers developed a battery with an anode made of recovered silicon and said its performance was comparable to that of a similar battery made with new silicon.

Besides batteries, the recovered silicon could be reused to manufacture new solar cells or for a variety of other purposes in the electronics industry. The NTU researchers are currently searching for industry collaborators to explore market applications.

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