Carrageenan is a derivative of red seaweed and usually used to thicken food, but a team from the Lawrence Berkeley National Laboratory have discovered it also works well as a stabiliser in lithium-sulphur batteries. The finding has opened up a whole new way of thinking about battery chemistry say the scientists.
Lithium-sulphur batteries have great potential as a low-cost, high-energy source for use in vehicles and grid applications: they have more than twice the energy density of lithium ion batteries and are much more lightweight. But they suffer from significant capacity fading - a phenomenon observed in rechargeable batteries where the amount of charge a battery can deliver at the rated voltage decreases with use.
There’s a lot of demand for energy storage, but there’s very little chemistry that can meet the cost target. Sulphur is a very low-cost material - it’s practically free. And the energy capacity is much higher than that of lithium-ion. So lithium-sulphur is one chemistry that can potentially meet the target.
Gao Liu, Research Leader
Rechargeable lithium-sulphur batteries have some commercial applications – they provided night time power in the 2010 record-setting 14-day solar-powered flight of unmanned aircraft Zephyr – but they are limited. The critical issue with the chemistry is that the sulphur dissolves, creating the so-called polysulphide shuttling effect. In trying to solve this problem, Liu and his team were experimenting with binders – the substance which holds all the active materials in a battery together.
“A binder is like glue, and normally battery designers want a glue that is inert,” Liu said. “This binder we tried worked really well. We asked why, and we discovered its reacting - it reacted immediately with the polysulphide. It formed a covalent bonding structure.”
The sulphur was being prevented from dissolving by chemically reacting with the binder – a synthetic polymer in the first instance. Once the researchers determined this, they began searching for a naturally occurring material that would perform the same function. They landed upon carrageenan, a substance extracted from red seaweed and in the same functional group as their synthetic polymer.
“We looked for something that was economical and readily available,” Liu said. “It turns out carrageenan is used as a food thickener. And it actually worked just as well as the synthetic polymer - it worked as a glue and it immobilised the polysulphide, making a really stable electrode.”
Liu teamed with Jinhua Guo from Berkeley Lab’s Advanced Light Source, one of the world’s brightest sources of ultraviolet and soft x-ray beams to make this discovery.
The light source provides unique x-ray based tools. We want the tool to monitor the electrochemistry simultaneously while the battery is charging. In this case, we made a dedicated battery cell with the materials, then used x-rays to monitor the process under in situ conditions.
Jinhua Guo, Berkeley Lab’s Advanced Light Source
“You can’t do this kind of experiment anywhere else,” added Liu. “In this case we have a unique beamline to detect sulphur. It’s always a lot of effort to design the tool for in situ. Ex situ is easy, but in this case, ex situ didn’t give you the result. With the in situ cell, we were able to watch where the sulphur goes. Turns out, it doesn’t go anywhere. That was really cool.”
The result was confirmed by General Motors, industry research partner of Berkeley Lab’s Energy Storage & Distributed Resources Division. Liu said: “They independently tested it and saw the same effect we saw - in fact the stability was even better.”
Liu believes the results open up a whole new way of thinking about battery chemistry: “Scientifically, it’s a totally different concept, of a binder that is reactive rather than inert,” he said. “People don’t think that way. They think a binder’s function is to physically hold things together. We found, no, we need a way to chemically bind the polysulphide.”
The team – who have published their results in Nano Energy - now hope to improve the lifetime of lithium-sulphur batteries even further to obtain thousands of cycles and to understand the chemical reactions within the cell.
After this polymer binds with sulphur, what happens next? How does it react with sulphur, and is it reversible? Understanding that will allow us to be able to develop better ways to further improve the life of lithium-sulphur batteries.
Gao Liu, Research Leader
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