This year has already seen some big battery tech breakthroughs, from devices that charge via bacteria, to nanowire electrodes that can cope with hundreds of thousands of cycles. Now, researchers at the University of Cambridge have turned to biology – to cells that line the human intestine – for inspiration in designing next-generation batteries. It's a big step forward for lithium-sulphur batteries, but it'll likely still be years before the tech becomes commercially available.
Lithium-sulphur battery technology has a lot of potential – it could provide as much as five times the energy density of lithium-ion solutions used today. But batteries made using the materials tend to be short-lived, with active material being lost during the repeated charge-discharge cycle. A Cambridge team believes it's now solved the issue, by adding a thin layer of material to the setup.
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But taking a step back – what makes lithium-sulphur battery tech so appealing in the first place?
Well, a typical lithium-ion battery is made up of a negative electrode called an anode (often made of graphite), a positive electrode called a cathode (usually a lithium cobalt oxide), and an electrolyte placed between them. Charged ions moving between the two electrodes are what charge and discharge the battery, but it's the makeup of the anode and the cathode that puts a cap on the capacity of the device. The carbon atoms found in the graphite rod, for example, can take on a total of six lithium ions.
Lithium-sulphur battery design makes a number of improvements over that setup, in theory at least. In lithium-ion batteries, the graphite anode serves as little more than a storage space for lithium ions, before they pass through the electrolyte to the cathode. In a lithium sulphur battery, this is replaced by a small amount of pure lithium, which acts as both the supplier of lithium ions, and as the electrode itself. It shrinks as the battery discharges, and reforms during charging. Additionally, sulphur is used as a replacement for the metal oxide cathode, being both a cheaper and lighter alternative.
With smaller, lighter and cheaper components than lithium-ion batteries, the prototype tech could lead to alternatives that are not only more cost-effective, but that can also pack in significantly more energy density. Unfortunately, the technology is not without its problems.
When the battery discharges, the interaction between the lithium and sulphur elements gives rise to chain-like structures called poly-sulphides. The more the battery is charged and discharged, the more this occurs, and poly-sulphides can be lost in the electrolyte. This gradual loss of active material means that the more the battery is used, the lower its capacity gets.
To overcome the issue, the Cambridge team – working in collaboration with Beijing Institute of Technology researchers – took inspiration from finger-like protrusions on the lining of the human intestine, called villi. In biology, villi increase the surface area of the gut, and help in the absorption of the products of digestion.
Thinking along the same lines, the researchers created a layer of tiny zinc oxide wires that mimic the villi structure, and placed it on the surface of the lithium-sulphur battery's cathode. In testing, they found that the villi-like layer of wires, with its high surface area, formed strong chemical bonds with the poly-sulphides, trapping them, and allowing the material to be reused. This directly tackles the issue of the gradual loss of active materials, significantly increasing the lifespan of the battery.
There's still a lot of work that needs to be done before the technology becomes widely available – even in light of the improvements, lithium-suphur batteries still can't go through as many cycles as lithium-ion batteries. But as the researchers point out, they also don't need to be charged nearly as often, meaning that they're already looking like a good alternative.
"By taking our inspiration from the natural world, we were able to come up with a solution that we hope will accelerate the development of next-generation batteries," said the study's lead author, University of Cambridge PhD student, Teng Zhao.
Full details of the research are published in the journal Advanced Functional Materials.
Source: University of Cambridge