Purdue University engineers discover a microwave oven can make lithium-sulfur batteries last longer.

Hailed as next-generation replacements for lithium-ion batteries, the lithium-sulfur variety is less costly and has greater energy density.

But the big knock on the commercial use of lithium-sulfur batteries to this point is they last only for about 100 charging cycles – a big drawback to their use in everything from electric vehicles to laptop computers.

Making lithium-sulfur batteries good enough for commercial use runs parallel with the need to reduce plastic in landfills, and Purdue researchers say they have figured out a way to do just that.

Their process, published in the journal Applied Materials and Interfaces, involves putting sulfur-soaked plastic – including transparent plastic bags – in a microwave, transforming the material into the ideal substance for doubling the battery lifespan to more than 200 charging-discharging cycles.

Vilas Pol, an associate professor in Purdue’s School of Chemical Engineering, says low-density polyethylene plastic, used for packaging and making up a big portion of plastic waste, helps address a longstanding issue with lithium-sulfur batteries: a phenomenon called the polysulfide shuttling effect, which 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.

Patrick Kim, a Purdue postdoctoral research associate in chemical engineering, says the byproduct of this reaction, polysulfide, tends to cross back over to the lithium side and prevent the migration of lithium ions to sulfur. This decreases both the battery’s charge capacity and its lifespan.

“The easiest way to block polysulfide is to place a physical barrier between lithium and sulfur,” Kim says.

Attempts to make this barrier out of biomass, including banana peels and pistachio shells, didn’t work because the pores in biomass-derived carbon had the potential to trap polysulfide. The Purdue researchers decided to see if plastic could be incorporated into a carbon scaffold to suppress polysulfide shuttling in a battery. Previous research showed low-density polyethylene plastic yields carbon when combined with sulfonated groups.

The researchers soaked a plastic bag in a sulfur-containing solvent and put it in a microwave to provide the quick, inexpensive boost in temperature needed for transformation into low-density polyethylene.

The heat promoted the sulfonation and carbonization of the plastic and induced a higher density of pores for catching polysulfide. The low-density polyethylene plastic then could be made into a carbon scaffold to divide the lithium and sulfur halves of a battery coin cell.

“This is the first step for improving the capacity retention of the battery,” Pol says. “The next step is fabricating a bigger-sized battery utilizing this concept.”