Oct 27 2016
Inspired by the structure of cells that line the walls of human intestine, researchers from the University of Cambridge have produced a prototype of the next-generation lithium- sulfur batteries. Upon commercial development, the batteries can achieve energy density that is nearly five times that of the lithium-ion batteries that are used in smartphones and various electronics.
Computer visualization of villi-like battery material. ( Credit: Teng Zhao)
The new model overcomes battery degradation caused due to the loss of constituent material, which is one of the main technical drawbacks that hampers the commercial success of lithium-sulfur batteries. The outcomes of this research have been published in the journal Advanced Functional Materials.
The Cambridge researchers constituting Dr Vasant Kumar’s team from the Cambridge’s Department of Materials Science & Metallurgy worked with collaborators from the Beijing Institute of Technology to develop and test a lightweight nanostructure material resembling villi. Villi are the finger-like protrusions lining the walls of the small intestine in humans. They absorb the products of the digestive process by increasing the surface area over which the process occurs.
Similarly, a material layer with villi-like formation made of tiny zinc oxide wires is positioned on the surface of one of the electrodes of the new lithium-sulfur battery. This layer can trap the active material fragments that break off, thus holding them electrochemically accessible and enabling the reuse of the material.
It’s a tiny thing, this layer, but it’s important.” He further added that “This gets us a long way through the bottleneck which is preventing the development of better batteries.
Dr Paul Coxon, study co-author, from the Department of Materials Science & Metallurgy
The three separate components found in a usual lithium-ion battery are a negative electrode (anode), an electrolyte in the middle, and a positive electrode (cathode). Materials with layered structures such as lithium cobalt oxide and graphite are the commonly used materials for the cathode and the anode, respectively. Lithium ions with positive charge move to and fro from the cathode into the anode, through the electrolyte.
The amount of energy that can be compressed into the battery is determined by the crystal structure of the electrode materials. For instance, the atomic structure of carbon in which each carbon atom can hold six lithium ions, limits the maximum potential of the battery.
Lithium and sulfur react in different ways through a multi-electron transfer mechanism, that is to say, elemental sulfur offers greater theoretical potential, thus offering greater energy density to a lithium-sulfur battery. Yet when the battery gets discharged, the sulfur and lithium react with each other, resulting in the transformation of ring-like sulfur molecules into chain-like structures called poly-sulfides. Over numerous charge-discharge cycles, small portions of poly-sulfides enter into the electrolyte, resulting in the gradual loss of the active material over time.
A functional layer developed by the Cambridge research team consists of minute, one-dimensional zinc oxide nanowires formed on a scaffold and is placed on top of the cathode. The layer secures the active material to a conductive framework, thus enabling reuse of the active material. Commercially accessible nickel foam was used as a support for testing the prototype. Upon attaining the desired outcomes, a lightweight carbon fiber mat was selected to replace the foam in order to minimize overall weight of the battery.
Changing from stiff nickel foam to flexible carbon fibre mat makes the layer mimic the way small intestine works even further.
Dr Yingjun Liu, study co-author
Similar to the intestinal villi, the functional layer has a very high surface area. The stronger chemical bond between the material and the poly-sulfides enables long-term usage of the active material, thus significantly increasing the battery’s working life.
Teng Zhao, a PhD student from the Department of Materials Science & Metallurgy as well as the study’s lead author, said that “This is the first time a chemically functional layer with a well-organised nano-architecture has been proposed to trap and reuse the dissolved active materials during battery charging and discharging.” He further stated that “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.”
At present, the device is only a proof of concept; hence the commercial development of lithium-sulfur batteries can take some more years. Moreover, although the number of charge and discharge cycles of the battery has been increased, it is still far from realizing the number of charge cycles achieved by a lithium-ion battery. Nevertheless, as there is no need to charge a lithium-sulfur battery as frequently as a lithium-ion battery, there is a probability that the overall lower number of charge-discharge cycles are canceled out by the increase in the energy density.
This is a way of getting around one of those awkward little problems that affects all of us.” He further added that “We’re all tied in to our electronic devices—ultimately, we’re just trying to make those devices work better, hopefully making our lives a little bit nicer.
Dr Coxon