AsianScientist (Aug. 1, 2017) – In a study published in Science, a research team from the Korea Advanced Institute of Science and Technology (KAIST) have created high-capacity silicon anodes by integrating molecular pulleys into the electrode binders of lithium ion batteries.
Silicon anodes are receiving a great deal of attention from the battery community. They can deliver three- to five-times higher capacities compared with the graphite anodes used in lithium ion batteries today. A higher capacity means longer battery use per charge, which is particularly critical in extending the driving mileage of all-electric vehicles.
Although silicon is abundant and cheap, silicon anodes have a limited charge-discharge cycle number and can tolerate recharging less than 100 times. Their volume expands enormously during each charge-discharge cycle, leading to fractures of the electrode particles or delamination of the electrode film which also decays its capacity.
The KAIST team integrated molecular pulleys, called polyrotaxanes, into a battery electrode binder which is a polymer included in battery electrodes to attach the electrodes onto metallic substrates. In a polyrotaxane, rings are threaded into a polymer backbone and can freely move along the backbone.
The free moving of the rings in polyrotaxanes can follow the changes in volume of the silicon particles. Importantly, the rings’ sliding motion can efficiently hold silicon particles without disintegrating, even as the volume of the silicon particles continues to change. Even pulverized silicon particles can remain coalesced because of the high elasticity of the polyrotaxane binder.
The functionality of the new binders is in sharp contrast with existing binders (usually simple linear polymers) with limited elasticity since existing binders are not capable of holding pulverized particles firmly. Previous binders allowed pulverized particles to scatter, and the silicon electrode thus degrades and loses its capacity.
“This is a good example of the importance of fundamental research. Scientists working on polyrotaxane received the Nobel Prize last year based on a newly identified concept called the mechanical bond,” said the authors. “The mechanical bond can be added to classical chemical bonds in chemistry, such as covalent, ionic, coordination, and metallic bonds. This fundamental knowledge is now being applied to multiple unexpected contexts in ways that address longstanding challenges in battery technology.”
The authors also mentioned that they are currently collaborating with a major battery maker to get their molecular pulleys integrated into real battery products.
“Mechanical bonds have come to the rescue for the first time in an energy storage context. The KAIST team’s ingenious use of mechanical bonds in slide-ring polyrotaxanes—based on polyethylene glycol threaded with functionalized alpha-cyclodextrin rings—marks a breakthrough in the performance of marketable lithium-ion batteries,” said Sir Fraser Stoddart of Northwestern University in the US who received the 2016 Nobel Prize in Chemistry for his work on the mechanical bond.
“This important technological advancement provides yet more evidence that when pulley-like polymers carrying mechanical bonds displace conventional materials based on chemical bonds alone, the unique influence of this physical bond on the properties of materials and the performance of devices can be profound and game-changing.”
Source: Korea Advanced Institute of Science and Technology; Photo: Shutterstock.
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