When charge is applied to Li-S cells, lithium ions migrate to the sulphur and lithium sulphide is produced.
A by-product of this reaction, ‘polysulphfide’, tend to cross back over to the lithium side and prevent the migration of lithium ions to sulphfur, according to Purdue – decreasing charge capacity and lifespan.
“The easiest way to block polysulfide is to place a physical barrier between lithium and sulfur,” said Purdue chemist Patrick Kim, and porous carbon-based barriers have been tried as the pores have potential to catch polysulfide.
Purdue’s barrier is based on low-density polythene (LDPE), heated with a sulphur-containing solvent to promote sulphonation and carbonisation of the plastic and induced a higher density of pores for catching polysulfide. In this case, the heating was in a microwave oven (see diagram)
“The plastic-derived carbon from this process includes a sulphonate group with a negative charge, which is also what polysulfide has,” said Kim.
According to the university, the result is “the ideal substance for increasing the lifespan of the forthcoming batteries to more than 200 charging-discharging cycles.”
The work is published in ACS Applied Materials and Interfaces.
Purdue university marketing the use of LDPE in the process as a way to reduced plastic bag waste.
There is a video and an ACS Applied Materials & Interfaces paper: “Toward high-performance lithium-sulfur batteries: Upcycling of LDPE plastic into sulfonated carbon scaffold via microwave-promoted sulfonation“.
“Microwave process not only boosts the sulphonation reaction of LDPE but also induces huge amounts of pores within the sulphonated LDPE plastic. When a PSC layer was utilised as an inter-layer in lithium–sulphur batteries, the sulphur cathode delivered an improved capacity of 776mAh/g at 0.5C and a cycle retention of 79% over 200 cycles,” according to the paper’s abstract.