The vision of a high-energy, long-lasting and fast-charging flexible battery for use in increasingly popular foldable smartphones will require next-generation batteries with greater capabilities than current lithium-ion technologies. Lithium-sulfur (Li-S) batteries are a potential alternative that could offer high energy density and capacity, along with lower toxicity and cost, but sulfur’s low conductivity poses challenges in synthesizing effective cathode materials. Researchers at the Pohang University of Science & Technology (POSTECH) have demonstrated the synthesis of a new sulfur-based copolymer cathode material that provides great conductivity and mechanical integrity while taking less than 30 minutes to produce.
The cathode material combines sulfur and vinylphosphonic acid (VPA) copolymerized through inverse vulcanization, a solvent-free process involving an autocatalytic reaction between sulfur radicals and VPA. The process results in uniformly-sized, hierarchically-ordered sulfur particles composed of phase-separated α-sulfur and sulfur-VPA (SVPA) networks based on low-density sulfur allotropes. Long sulfur chains to which VPA is attached form at the beginning of the reaction, stabilizing the spherical shape of the SVPA particles without the use of a surfactant.
These particles were examined through Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS), confirming the formation of SVPA networks by covalent linking of VPA and sulfur. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to inspect the morphology of the particles, which revealed wrinkles and pores on the surface of the particles. The researchers found that these features facilitated electrolyte penetration into the cathode while also relieving mechanical stress applied to the electrode surface.
Additionally, the researchers were able to prevent elution of lithium polysulfide through the phosphonic acid functional groups on the surface of the active cathode material, and, being a crosslinked polymer, the material showed excellent elasticity. These qualities make the Li-S batteries a promising candidate for use in flexible and foldable electronics. This research was published in Nano Energy.
“In this study, a flexible lithium sulfur battery was developed by synthesizing an inverse vulcanized polymer with abundant phosphonic acid groups in a low-cost and eco-friendly way,” said Moon Jeong Park, who led the study. “The findings are significant in that it has chemically trapped lithium polysulfides, resolving the elution issue that has hampered commercialization, and has increased the potential use in wearable device batteries by adding flexible characteristics to the sulfur cathode, which has been difficult to implement so far.”
The abundance of sulfur in nature and as a by-product of processes such as oil refining can also make Li-S batteries a less expensive option.
Photo: Images of flexible lithium-sulfur battery testing. The left image shows the result with the battery in a flat position and the right image shows the result when the battery is folded. Credit: POSTECH