Ellen Phiddian
Cosmos science journalist
Engineers have found a way to turn phosphorus from city wastewater into parts for lithium-ion batteries.
The Chinese researchers say that their method could be used to supply 35% of the phosphorus demand for their national lithium-ion battery industry.
They’ve published their findings in Engineering.
Phosphorus is a common component in the cathodes of lithium-ion batteries – specifically, lithium iron phosphate batteries, which represent about 60% of the lithium-ion market according to the researchers.
As an important ingredient in fertilisers and industrial chemicals, mineral phosphorus is in high demand. Mining stocks of phosphorus are expected to be depleted in the next 50-100 years.
But, point out the researchers, more than 250,000 tonnes of phosphorus pollutes Chinese wastewater every year, coming from food consumption and chemical waste. This is more phosphorus than the amount consumed each year to make batteries.
There are existing methods to extract phosphorus from sludge, but they’re both inefficient, and designed to extract it in a form that’s useful for fertilisers. Because fertilisers are cheap, this makes the extraction methods economically unviable.
The researchers, who are based at the Shenzhen Engineering Research Laboratory for Sludge and Food Waste Treatment and Resource Recovery, designed an environmentally conscious method to extract higher-value phosphorus from the sludge.
They used effluent from a Shenzhen wastewater treatment plant, taking it after it had been biologically treated. They mixed a small amount of iron chloride into the wastewater, which attracted the phosphorus to it and allowed it to coagulate into a sludge.
Then, they dehydrated the sludge in an oven for a day and sintered it for 2 hours at 600°C, before washing the results with hydrochloric acid and water.
This yielded a material that was rich in oxides of phosphorus and iron. The oxides could be used to substitute up to 35% of the iron phosphate normally used to make lithium iron phosphate batteries.
The researchers used their wastewater-derived mixture to build small lithium-ion batteries in the lab. These batteries could charge and discharge at the rates needed for electric vehicles and large-scale storage systems, and they kept 99.2% of their capacity after being charged and discharged 100 times.
Batteries made with higher doses of the wastewater material performed better than batteries made with lower doses. The researchers believe that impurities from the sludge helped to stabilise the batteries, allowing them to perform better.
“The amount of phosphorus recovered from municipal wastewater is projected to be sufficient to meet up to 35% of the phosphorus demand by the lithium-ion battery industry in China, enhancing the cost-effectiveness of phosphorus recovery and alleviating the global shortage of phosphorus resources to achieve both clean energy and sustainable development,” conclude the researchers in their paper.
