To do this, the engineers put ink-free plastic soaked in sulfur-containing solvent into a microwave, and then into batteries as a carbon scaffold.
Purdue researchers have found a way to increase the lifespan of lithium-sulfur batteries, which have been hailed as the next generation of batteries to replace lithium ion but only have a limited number of charging cycles.
In the process, they also discovered it has the added bonus of being a convenient way to recycle plastic. The technique shows that by putting sulfur-soaked plastic in a microwave, including transparent plastic bags, the material transforms into the ideal substance for increasing the lifespan of the forthcoming batteries to more than 200 charging-discharging cycles.
Low-density polyethylene plastic, which is used for packaging and comprises a big portion of plastic waste, helps address a long-standing issue with lithium-sulfur batteries - a phenomenon called the polysulfide shuttling effect that limits how long a battery can last between charges.
When a current is applied to lithium-sulfur batteries, lithium-ions migrate to the sulfur and a chemical reaction takes place to produce lithium sulfide. The byproduct of this reaction, polysulfide, tend to cross back over to the lithium side and prevent the migration of lithium ions to sulfur. This decreases the charge capacity of a battery as well as lifespan.
"The easiest way to block polysulfide is to place a physical barrier between lithium and sulfur," explains Patrick Kim, a Purdue postdoc research associate in chemical engineering.
Previous studies had attempted making this barrier out of biomass, such as banana peels and pistachio shells, because the pores in biomass-derived carbon had the potential to catch polysulfide.
"Every material has its own benefit, but biomass is good to keep and can be used for other purposes," says associate professor Vilas Pol of Purdue. "Waste plastic is really valueless and burdensome material."
Instead, the researchers thought of how plastic might be incorporated into a carbon scaffold to suppress polysulfide shuttling in a battery. Past research had shown that low-density polyethylene plastic yields carbon when combined with sulfonated groups.
The researchers soaked a plastic bag into sulfur-containing solvent and put it in a microwave to cheaply provide the quick boost in temperature needed for transformation into low-density polyethylene. The heat promoted the sulfonation and carbonisation of the plastic and induced a higher density of pores for catching polysulfide. The low-density polyethylene plastic could then be made into a carbon scaffold to divide the lithium and sulfur halves of a battery coin cell.
"The plastic-derived carbon from this process includes a sulfonate group with a negative charge, which is also what polysulfide has," Kim adds. Sulfonated low-density polyethylene made into a carbon scaffold, therefore suppressed polysulfide by having a similar chemical structure.
"This is the first step for improving the capacity retention of the battery," Pol ventures. "The next step is fabricating a bigger-sized battery utilising this concept."