Bio-mimicry presents itself in aerodynamics, from the emulation of soaring bird’s wing shapes on sailplane’s surfaces to owl-feather-like trailing edges on wind turbines. We don’t often think of biological equivalents in energy storage (your editor didn’t until now, at least). But researchers at Cambridge University in England and the Beijing Institute of Technology in China have turned to the small intestine for their breakthrough in battery development.
Tiny cells lining the human intestine inspired these researchers to develop a prototype of a lithium-sulfur battery that they claim could have five times the energy density of conventional lithium-ion batteries. Dr. Paul Coxon from Cambridge’s Department of Materials Science and Metallurgy says “This gets us a long way through the bottleneck which is preventing the development of better batteries.”
Is That You, Villi?
Villi in the gut help process food being digested, trapping nutrient particles in millions of tiny, “finger-like protrusions” which increase the absorbent surface area over which digestion takes place. In batteries, surface area enables greater electrochemical reaction between anodes, cathodes and electrolytes.
Dr. Vasant Kumar and his Cambridge team created and tested a lightweight nanostructured material with tiny zinc-oxide wires, which they placed on the battery’s electrodes. According to researchers, “This can trap fragments of the active material when they break off, keeping them electrochemically accessible and allowing the material to be reused.” This prevents loss of active materials and increases the life span of the battery.
How This Works in a Lithium-Sulfur Battery
Different materials determine how much energy can be squeezed into the battery. Carbon electrodes can take on six lithium ions per carbon atom. Sulfur can take on many more, making the battery theoretically more energy dense. At least five times more so, if we’re to believe the Cambridge/Beijing researchers. That’s where there the electronic villi come into action.
Mimicking Our Inner Workings
During discharge, when we get the stored energy back as work, “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.”
To emulate the work villi do in the body, 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 trialled using commercially-available nickel foam for support. After successful results, the foam was replaced by a lightweight carbon fiber mat to reduce the battery’s overall weight.”
Study co-author Dr. Yingjun Liu explained, “Changing from stiff nickel foam to flexible carbon fiber mat makes the layer mimic the way small intestine works even further.”The high surface area exposed on the fingers and its strong chemical bond with the polysulfides allows the material to last longer, and increases the lifespan of the battery.
“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,” said the study’s lead author Teng Zhao, a PhD student from the Department of Materials Science & Metallurgy. “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.”
Not So Fast
Alas, as with so many such developments, things done at laboratory scale sometimes take another decade to reach commercial reality. Talk of Manhattan Project levels of investment and urgency never seem to gain ascendancy for peace-time endeavors. Let’s hope this and similar efforts find willing investors.