Researchers have developed a prototype of a next-generation lithium-sulphur battery. The invention takes its inspiration in part from the cells lining the human intestine.
According to the researchers, the batteries, if commercially developed, would have 5 times the energy density of the lithium-ion batteries used in smartphones and other electronics.
The results are reported in the journal Advanced Functional Materials. Researchers from the University of Cambridge led the study.
They prevented the degradation of the battery caused by the loss of material within it, and hence overcame a key technical problem in the commercial development of lithium-sulphur batteries.
In the study, researchers developed and tested a lightweight nanostructured material, which resembles villi, the finger-like protrusions which line the small intestine.
In the human body, villi are used to absorb the products of digestion and increase the surface area over which this process can take place.
In the new lithium-sulphur battery, a layer of material with a villi-like structure, made from tiny zinc oxide wires, is placed on the surface of one of the battery’s electrodes.
This can trap fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused.
Sulphur and lithium react differently via a multi-electron transfer mechanism. This means elemental sulphur can offer a much higher theoretical capacity, resulting in a lithium-sulphur battery with much higher energy density.
However, when the battery discharges, the lithium and sulphur interact and the ring-like sulphur molecules transform into chain-like structures, known as a poly-sulphides.
As the battery undergoes several charge-discharge cycles, bits of the poly-sulphide can go into the electrolyte, so that over time the battery gradually loses active material.
The Cambridge researchers have created a functional layer, which lies on top of the cathode and fixes the active material to a conductive framework so the active material can be reused.
The layer is made up of tiny, one-dimensional zinc oxide nanowires grown on a scaffold. The concept was trialed using commercially available nickel foam for support.
After successful results, a lightweight carbon fiber mat replaced the foam to reduce the battery’s overall weight.
This functional layer, like the intestinal villi it resembles, has a very high surface area.
The material has a very strong chemical bond with the poly-sulphides, allowing the active material to be used for longer, greatly increasing the lifespan of the battery.
For the time being, the device is a proof of principle, so commercially available lithium-sulphur batteries are still some years away.
While the number of times the battery can be charged and discharged has been improved, it is still not able to go through as many charge cycles as a lithium-ion battery.
However, since a lithium-sulphur battery does not need to be charged as often as a lithium-ion battery, it may be the case that the increase in energy density cancels out the lower total number of charge-discharge cycles.
Citation: Zhao T, et al. (2016). Advanced Lithium–Sulfur Batteries Enabled by a Bio-Inspired Polysulfide Adsorptive Brush. Advanced Functional Materials, published online. DOI: 10.1002/adfm.201604069.
Figure legend: This Knowridge.com image is credited to Teng Zhao.