Molecular pulleys empower better batteries

20 July 2017

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Pulleys first revolutionised engineering over 2300 years ago. Those early adopters realised that they could reduce the effort needed to make things move by stringing together a system of ropes running around wheels. Now modern-day chemical engineers have achieved a similar feat at the molecular level to build better batteries.

As devices improve so does their hunger for power, and design aspirations are often curtailed by the practicalities of storing supplying enough energy.

The same constraints govern transport, and the reason that diesel and petrol still reign on the roads, despite the drawbacks of noise, inefficiency and pollution, is that battery evolution has stalled.

Sure, the development of technologies like lithium ion cells marked a step change, but it's still not enough to produce cars with a reasonable range, or a smartphone that can go more than a day without a charge.

So what's holding back the field? Part of the problem is that batteries work through an internal chemical reaction which releases surplus charges in the form of electrons which can be supplied to power a circuit.

In the case of lithium batteries, when they are charging, a lithium-rich material called the electrolyte surrenders some of its lithium. This migrates towards and soaks into the positive electrode, known as the anode, causing it to swell.

When the battery discharges again, the lithium ions are squeezed out from the anode, which shrinks back to its starting shape.

But this movement of lithium, with repeated cycles of charging and discharging, can cause the anode material to progressively fragment, limiting the lifetime and future capacity of the battery.

At the moment the anodes that show the most resilience to this shattering effect are made from carbon, but this is not the best material in terms of its ability to store charge.

Silicon, on the other hand, would be vastly superior from an electrical perspective. Battery manufacturers cannot use it, however, because it becomes grossly swollen when it soaks up lithium while taking a charge.

This makes it shatter prematurely, rendering the battery useless.

Now scientists in Korea have built batteries with internal molecular pulleys that can distribute stresses and stop silicon shattering. Writing in Science, Sunghun Choi and his colleagues used long string-like molecules called polyethylene glycols - which operated as the "ropes", and threaded onto them rings made from sugar molecules known as cyclodextrins; these were the pulleys.

This molecular "block and tackle" apparatus was embedded in a binding agent called polyacrylic acid.

The result was that the pulleys coupled up to the silicon electrode material and helped to spread the load as the silicon soaked up lithium during charging.

This enabled the silicon to stretch by 10 fold more than conventional silicon electrodes could accomplish. And when discharging occurred, the tension stored in the "ropes" pulled the silicon particles back together, preventing them from becoming electrically isolated. Even better, the material lost only about 10% of its performance after 150 charge and discharge cycles...

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