The researchers put ink-free plastic soaked in a solvent containing sulfur into a microwave, and used that as a carbon scaffold for a lithium sulfur battery cell.
Lithium-sulfur batteries are cheaper and more energy-dense at 2600 Wh/kg than lithium ion cells but struggle with limited lifetime of around 100 cycles. The process, published in ACS Applied Materials and Interfaces, shows that putting sulfur-soaked plastic in a microwave, including transparent plastic bags, transforms the material into the ideal substance for increasing the lifespan of the forthcoming batteries to more than 200 cycles.
“No matter how many times you recycle plastic, that plastic stays on the earth,” said Vilas Pol, associate professor in Purdue’s School of Chemical Engineering. “We’ve been thinking of ways to get rid of it for a long time, and this is a way to at least add value.”
Low-density polyethylene plastic, which is used for packaging and comprises a big portion of plastic waste, helps address the polysulfide shuttling effect that limits how long a battery can last between charges. When a current is applied to the cell, 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,” said 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. Instead, the researchers looked at how plastic might be incorporated into a carbon scaffold to suppress polysulfide shuttling in a battery.
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 carbonization 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. This produces a sulfur cathode with an improved capacity of 776 mAh/g at 0.5C charging and a charge retention of 79% over 200 cycles.
“This is the first step for improving the capacity retention of the battery,” said Pol. “The next step is fabricating a bigger-sized battery using this concept.”