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Store five times more charge in your cell phone – thanks to its Lithium Sulfur battery!
Posted on December 19th, 2016 by Dr. Sina Ebnesajjad in New Materials & Applications
This post is about serious progress in battery technology, specifically capacity. The development of lithium sulfur batteries is making leaps and bounds forward in the race to replace the lithium ion battery that is approaching the end of its capability. The Li-S battery’s advantage lies in the high theoretical capacity of sulfur at 1,672 mAh/g, which is an astonishing 10 times higher than that of the conventional cathode materials of lithium ion batteries. Commercial achievement of even half the theoretical capacity would move the Li-S battery technology far ahead of the lithium ion battery.
More energy storage, less weight, lower cost and cleaner technology are among the advantages of Li-S over Li-ion battery. The prospects of significantly higher storage capacity for electric cars are simply tantalizing. Currently Tesla Model S is advertised to have a range of 265 miles (426 km). Just imagine expanding that range by a factor of three to 1,278 km; that is even better than internal combustion engines!
The main barrier to bringing the Li-S battery to market has been its limited “cycle life”. That problem appears to have been solved by the University of Cambridge (United Kingdom) researchers.
What is battery Cycle Life?
Massachusetts Institute of Technology defines cycle life as: “The number of discharge-charge cycles the battery can experience before it fails to meet specific performance criteria [e.g., 80-85%]. Cycle life is estimated for specific charge and discharge conditions. The actual operating life of the battery is affected by the rate and depth of cycles and by other conditions such as temperature and humidity. The higher the Depth of Discharge, the lower the cycle life” Source: http://web.mit.edu/evt/summary_battery_specifications.pdf.
The longstanding deficit of Li-S battery has been the deterioration of charge/discharge relatively quickly. Minute pieces of the material break off from the electrodes and become electrochemically unavailable thus reducing the life of the battery. In October 2016 researchers in Cambridge reported a new prototype of the lithium-sulfur battery that could have five times the energy density of a typical lithium-ion battery. Source: Next-generation smartphone battery inspired by the gut, ScienceDaily, www.sciencedaily.com/releases/2016/10/161026102701.htm, October 26, 2016. It overcomes the major hurdle that has been preventing commercial development of Li-S battery. The mechanism is based on mimicking the structure of the cells that allow us to absorb nutrients.
Working with collaborators at the Beijing Institute of Technology, the Cambridge researchers based in Dr. Vasant Kumar’s team in the Department of Materials Science and Metallurgy developed and tested a lightweight nanostructured material which resembles villi, the finger-like protrusions which line the small intestine. In the human body, villi (Figure 1) are used to absorb the products of digestion and increase the surface area over which this process can take place.
Figure 1 Three views of Villi’s finger-like projections in the small intestine – they help absorb food more efficiently Source: R. K. Pai, Seminars in Diagnostic Pathology, V 31, Issue 2, Elsevier, pp124–136, Mar 2014
In the new lithium-sulfur 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 (Figure 2). This can trap fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused. “It’s a tiny thing, this layer, but it’s important,” said study co-author Dr Paul Coxon from Cambridge’s Department of Material Science and Metallurgy. “This gets us a long way through the bottleneck which is preventing the development of better batteries.”
Figure 2 Computer visualisation of villi-like battery material. Courtesy: Teng Zhao, PhD student from the University of Cambridge, Department of Material Science & Metallurgy, October 2016
A typical lithium-ion battery is made of three separate components: an anode, a cathode and an electrolyte in the middle. The most common materials for the anode and cathode are 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 crystal structure of the electrode materials determines how much energy can be squeezed into the battery. For example, due to the atomic structure of carbon, each carbon atom can take on six lithium ions, limiting the maximum capacity of the battery.
Sulfur and lithium react differently via a multi-electron transfer mechanism meaning that elemental sulfur can offer a much higher theoretical capacity, resulting in a lithium-sulfur battery with much higher energy density. However, when the battery discharges, the lithium and sulfur interact and the ring-like sulfur molecules transform into chain-like structures, known as a polysulfides. As the battery undergoes several charge-discharge cycles, bits of the polysulfide can go into the electrolyte, so that over time the battery gradually loses active material.
The Cambridge researchers have created a functional layer that 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 tested using commercially available nickel foam for support. After successful results a lightweight carbon fiber mat, to reduce the battery’s overall weight, replaced the Ni foam.
“Changing from stiff nickel foam to flexible carbon fiber mat makes the layer mimic the way small intestine works even further,” said study co-author Dr Yingjun Liu. 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 polysulfides, allowing the active material to be used for longer, greatly increasing the lifespan of the battery.
“This is the first time a chemically functional layer with a well-organized nano-architecture has been proposed to trap and reuse the dissolved active materials during battery charging and discharging,” said the study’s lead author Teng Zhao, a PhD student from the Department of Material Science & Metallurgy. Ref: Advanced Lithium-Sulfur Batteries Enabled by a Bio-Inspired Polysulfide Adsorptive Brush. Teng Zhao, Yusheng Ye, Xiaoyu Peng, Giorgio Divitini, Hyun-Kyung Kim, Cheng-Yen Lao, Paul R. Coxon, Kai Xi, Yingjun Liu, Caterina Ducati, Renjie Chen, R. Vasant Kumar. Advanced Functional Materials, 2016; DOI: 10.1002/adfm.201604069 “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.”
For the time being, the device is only a proof of principle. Commercially available lithium-sulfur batteries are still some years away, although Oxis Energy is working hard on such a battery. Additionally, 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-sulfur 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.
This forward leap in Li-S battery technology may be foretelling a future when the boundaries between biological and technological begin to fade. In April 3rd, 2015 issue, Forbes magazine declared the Li-S the battery of the future. Maybe that description is deserved.
Principal source of the post: Next-generation smartphone battery inspired by the gut, ScienceDaily, www.sciencedaily.com/releases/2016/10/161026102701.htm, October 26, 2016.
All opinions shared in this post are the author’s own.
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Dr. Sina Ebnesajjad
President at FluoroConsultants Group, LLC
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