Brimstone to fire batteries - sulphur-based polymer electrode for Li-S batteries
A process called inverse vulcanisation could lead to a sulphur-based plastic capable of powering the next generation of lithium-sulphur (Li-S) batteries. Most of the world’s sulphur stockpiles are the result of a chemical process known as hydrodesulphurisation, used in the fossil fuel industry to remove the element during the refining process and cut sulphur dioxide emissions. Despite having some industrial uses, the amount of sulphur generated from fuel refining currently far outstrips demand for the element.
By flipping Charles Goodyear’s vulcanisation process on its head, an international team of scientists from the University of Arizona, USA, South Korea’s Institute for Basic Science and the University of Hamburg, Germany, has developed a way to convert this waste sulphur into useful polymeric electrode materials.
Vulcanisation entails mixing 90% natural rubber with sulphur, producing a rubber that has increased stability, elasticity and usefulness. Professor Jeffrey Pyun from the University of Arizona explains that the team inverted the ratio and combined 90% sulphur with a small amount of organic co-monomer. After dissolving the sulphur, the researchers used the chemical reagent 1,3-diisopropenylbenzene (1,3-DIB) to help overcome the troublesome behaviour of the sulphur’s polymer chains. ‘When you heat sulphur, it turns into a polymer, but that polymer is chemically unstable and depolymerises,’ says Pyun.
‘It just so happens that sulphur hates most substances that dissolve,’ Pyun adds, explaining that he was prepared for a lengthy trial-and-error search for something that could stabilise the polymer. ‘We started with the conventional, inexpensive, standard monomers for free-radical polymerisation,’ he says, but they soon struck upon a series of substances that were compatible with sulphur and would add stability, settling on 1,3-DIB as the specific additive.
‘The additive exhibits all the desirable characteristics that we would want for this polymer and this polymerisation process – it is soluble and it has the right chemical functionality to react with liquid sulphur,’ says Pyun. He adds that the inexpensive, commercial availability of the 1,3-DIB compound, coupled with the abundant nature of sulphur, made the process easy to scale up.
The researchers used the process to turn waste sulphur into processable copolymer forms with tuneable thermomechanical properties, using imprint lithography to create micropatterned films rich in sulphur. Pyun is confident that these films could be used as the active material in next-generation Li-S batteries. ‘The plastic is an electrochemically active agent, but it’s not conductive,’ says Pyun, adding that conductive carbons would need to be added to improve the conductivity of the films. ‘It’s more like a composite, and these carbon particles would help get the electrons in and out.’
Pyun said that Li-S batteries will offer a different set of capabilities to lithium-ion (Li-ion) batteries. ‘They have a storage capacity that is at least four or five times higher than current Li-ion batteries. The downside is it has nowhere near the lifetime of Li-ion batteries in terms of cycle number.’
However, with more research, Pyun thinks that Li-S batteries could offer cheaper alternatives for use in electric and hybrid cars. ‘Li-ion technology just doesn’t have the energy density that you would really need to get a reliable electric vehicle. We need a whole new platform of chemistry to generate new types of chemical polymers with a variety of different properties that could lead to batteries with longer lifespans.’
Pyun says the sheer amount of sulphur available would make sulphur-based plastic a sustainable material source. ‘It’s not renewable because it comes from fossil fuels, but because we continue to use fossil fuels on such a large scale, there is no doubt about the sustainability of this kind of material.’