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The battery can be twisted, stretched and bent without interrupting the current flow.
The battery can be twisted, stretched and bent without interrupting the current flow.
( Photo: Gruppe Niederberger, ETH Zürich)

flexible Li-ion battery Totally twisted: ETH team builds Li-ion battery from flexible components

| Author / Editor: Peter Rüegg, ETH / Jochen Schwab

ETH researchers are also currently working on a concept for a flexible lithium-ion battery that can be twisted, bent and stretched. The researchers use only flexible components.

The electronics industry is increasingly relying on computers or smartphones with folding or rolling screens. In intelligent clothing, portable miniature devices or sensors are used to monitor body functions, for example. However, all these devices need an energy source, and this is usually a lithium-ion battery. However, such batteries are heavy and rigid and therefore unsuitable in principle for applications in flexible electronic devices or textiles.

Markus Niederberger, Professor of Multifunctional Materials at ETH Zurich, and his team are now finding a remedy for this problem. The researchers have developed a prototype of a flexible thin-film battery. The battery can be bent, stretched or even twisted without interrupting the power supply.

The heart of this new battery is the electrolyte, i.e. the part of a battery through which the lithium ions have to move when the battery is discharged or charged. The electrolyte was developed by ETH doctor Xi Chen, first author of a study just published in the journal "Advanced Materials"

Consistently flexible components used

The sandwich-like structure of the new battery is based on commercial rechargeable batteries. For the first time, however, the researchers used exclusively flexible components to keep the battery as a whole flexible and stretchable. "So far, nobody has used flexible components exclusively to produce a lithium-ion battery as consistently as we have," says Niederberger.

The two current collectors for the anode and the cathode are made of an expandable plastic containing electrically conductive carbon. This is also the outer shell. The researchers applied a thin layer of tiny silver flakes to the inside of the plastic.

Due to the roof tile-like arrangement of the silver flakes, they do not lose contact with each other even when the plastic is strongly stretched. This guarantees the conductivity of the current collector even if it is strongly stretched. If the silver flakes nevertheless lose contact with each other, the electric current flows through the carbon-containing plastic, albeit to a lesser extent.

Using a mask, the researchers then sprayed anode or cathode powder onto the silver layer in a precisely limited area. The cathode powder contains lithium manganese oxide, the anode vanadium oxide.

Water-based gel electrolyte

Separated by a separating layer resembling a picture frame, the scientists finally placed the two current collectors with the electrodes on top of each other and filled the gap in the frame with electrolyte gel.

This gel is more environmentally friendly than previous ones, as Niederberger emphasizes. "Electrolyte fluids in today's batteries are toxic and flammable. The one developed by his doctoral student Xi Chen, on the other hand, is based on water. The gel contains a high concentration of lithium salt, which not only allows lithium ions to migrate between the cathode and anode during charging and discharging, but also prevents electrochemical decomposition of the water.

For their prototype, the scientists used adhesive to join the various components together. "If we want to commercialize the battery, we have to find another method to ensure that it remains leak-proof in the long term," says Niederberger.

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Numerous application possibilities

There are more and more applications for such a battery. Well-known mobile phone manufacturers are currently outbidding each other with flexible screens for their devices. Rollable displays of computers, smartwatches and tablets are also conceivable; flexible power suppliers are also needed in functional textiles containing flexible electronics. "One could, for example, sew such a battery into clothing," says Niederberger. It is important that in the event that the battery leaks, the leaking liquid does not cause any damage, which is why its electrolyte offers advantages.

Niederberger emphasizes, however, that further research is needed to optimize the flexible battery and to think about commercialization. Above all, they would have to increase the charge with electrode material.

Original publication:

Chen X, Huang H, Pan L, Liu T, Niederberger M. Fully Integrated Design of a Stretchable Solid‐State Lithium‐Ion Full Battery. Adv.Mater.2019, first published: 06 September 2019 doi: 10.1002/adma.201904648.

This article was first published in German by Elektropraxis.

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