In the hopes of making batteries that not only perform better than those currently used in EVs, but also are made from readily available materials, a group of Drexel University chemical engineers have found a way to introduce sulphur into lithium-ion batteries – Lithium-Sulphur batteries – with astounding results.
A recent breakthrough by researchers in Drexel’s College of Engineering, published in the journal ‘Communications Chemistry’, has provided a way to sidestep the obstacles that have subdued Li-S batteries in the past, finally pulling the sought-after technology within commercial reach.
Their discovery is a new way of producing and stabilizing a rare form of sulphur that functions in carbonate electrolyte – the energy-transport liquid used in commercial Li-ion batteries. This development would not only make sulphur batteries commercially viable, but they would have three times the capacity of Li-ion batteries and last more than 4,000 recharges – the equivalent of ten years of use, which is also a substantial improvement.
“Sulphur has been highly desirable for use in batteries for a number of years because it is earth-abundant and can be collected in a way that is safe and environmentally friendly, and as we have now demonstrated, it also has the potential to improve the performance of batteries in electric vehicles and mobile devices in a commercially viable way,” said Drexel’s Vibha Kalra, PhD, George B. Francis Chair professor in the College’s Department of Chemical and Biological Engineering, who led the research.
The challenge of introducing sulphur into a lithium battery with a commercially friendly carbonate electrolyte has been an irreversible chemical reaction between intermediate sulphur products, called polysulphides, and the carbonate electrolyte. Because of this adverse reaction, previous attempts to use a sulphur cathode in a battery with a carbonate electrolyte solution resulted in nearly immediate shutdown and a complete failure of the battery after just one cycle.
“In the past decade, the majority of Li-S field adopted ether electrolytes to avoid the adverse reactions with carbonate,” Kalra said. “Then over the years, the researchers deep-dived into enhancing performances in ether-based sulphur batteries by mitigating what is known as polysulphide shuttle/diffusion — but the field completely overlooked the fact that the ether electrolyte itself is a problem. In our work, the primary objective was to replace ether with carbonate, but in doing so we also eliminated polysulphides, which also meant no shuttling, so the battery could perform exceptionally well through thousands of cycles.”
Previous research by Kalra’s team also approached the problem in this way – producing a carbon nanofiber cathode that slowed the shuttle effect in ether-based Li-S batteries by curtailing the movement of intermediate polysulphides. But to improve the commercial path of the cathodes, the group realised it needed to make them function with a commercially viable electrolyte.
“Having a cathode that works with the carbonate electrolyte that they are already using is the path of least resistance for commercial manufacturers,” Kalra said. “So rather than pushing for the industry adoption of a new electrolyte, our goal was to make a cathode that could work in the pre-existing Li-ion electrolyte system.”
So, in hopes of eliminating polysulfide formation to avoid adverse reactions, the team attempted to confine sulphur in the carbon nanofiber cathode substrate using a vapor deposition technique. While this process did not succeed in embedding the sulphur within the nanofiber mesh, it did something extraordinary, which revealed itself when the team began to test the cathode.
“As we began the test, it started running beautifully – something we did not expect. In fact, we tested it over and over again – more than 100 times – to ensure we were really seeing what we thought we were seeing,” Kalra said. “The sulphur cathode, which we suspected would cause the reaction to grind to a halt, actually performed amazingly well and it did so again and again without causing shuttling.”
After more than a year of testing, the sulphur cathode remains stable and, as the team reported, its performance has not degraded in 4,000 charge-discharge cycles, which is equivalent to 10 years of regular use. And, as predicted, the battery’s capacity is more than three-fold that of a Li-ion battery.
“While we are still working to understand the exact mechanism behind the creation of this stable monoclinic sulphur at room temperature, this remains an exciting discovery and one that could open a number of doors for developing more sustainable and affordable battery technology,” Kalra said.
Replacing the cathode in Li-ion batteries with a sulphur one would alleviate the need for sourcing cobalt, nickel and manganese. Supplies of these raw materials are limited and not easily extracted without causing health and environmental hazards. Sulphur, on the other hand, is found everywhere in the world and exists in vast quantities in the United States because it is a waste product of petroleum production.
Kalra suggests that having a stable sulphur cathode, that functions in carbonate electrolyte, will also allow researchers to move forward in examining replacements for the lithium anode – which could include more earth-abundant options, like sodium.
“Getting away from a dependence on lithium and other materials that are expensive and difficult to extract from the earth is a vital step for the development of batteries and expanding our ability to use renewable energy sources,” Kalra said. “Developing viable Lithium-Sulphur batteries opens a number of pathways to replacing these materials.”