Researchers have taken inspiration from the structure of cells within the gut that allow us to absorb nutrients, engineering a new lithium-sulphur battery which could have five times the energy density of a typical lithium-ion battery.
The researchers from the University of Cambridge designed the next-generation lithium-sulphur battery to overcome one of the key technical problems hindering the commercial development of lithium-sulphur batteries, which has thus far been vulnerable to degradation caused by the loss of material within it.
Typical lithium-ion batteries consist of three separate components: an anode, a cathode, and an electrolyte in the middle. The anode and cathode are commonly made of graphite and lithium cobalt oxide respectively, which both have layered structures. Positively-charged lithium ions move back and forth from the cathode, through the electrolyte and into the anode.
The energy density of the battery is dependent on the crystal structure of the electrode materials.
In lithium-sulphur batteries, the reaction is different, thanks to a multi-electron transfer mechanism. This means that elemental sulphur can offer a much higher theoretical capacity, and thus the battery can provide higher energy density.
However, when the battery discharges, the lithium and sulphur interact and the sulphur molecules transform into chain-like structures, known as a poly-sulphides. Over multiple charge-discharge cycles, the poly-sulphide leaks into the electrolyte, causing the battery to gradually lose its active material.
The researchers from the University of Cambridge's Department of Materials Science and Metallurgy collaborated with a team at the Beijing Institute of Technology, developing and testing a lightweight nanostructured material which resembles the villi, which are finger-like protrusions that line the small intestine. Villi are used to absorb the products of digestion, and are structured to 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 zinc oxide nanowires, is placed on the surface of one of the battery’s electrodes.
This functional layer has a very high surface area, and the material also has a very strong chemical bond with the poly-sulphides. Thus it effectively traps fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused.
With this approach being still a proof of principle, commercially-available lithium-sulphur batteries are still some years away. The structure does improve the number of times the battery can be charged and discharged , but it is still not able to go through as many charge cycles as a lithium-ion battery.