Researchers at the ETH Zurich around the professor for multifunctional materials Markus Niederberger have developed a battery that consists entirely of soft materials and can be bent, stretched, twisted and folded. It should form the basis for larger batteries in foldable smartphones and clothing.
Current lithium ion batteries are rigid and easily burn when damaged by excessive stress such as bending, rendering them largely unsuitable for use in garments and flexible electronic devices. The prototype of the flexible thin-film battery from ETH Zurich, on the other hand, can be bent, twisted and stretched without interrupting the power supply.
At the heart of this new battery is the hydrogel electrolyte, which is the part of a battery through which the lithium ions must move when the battery is being discharged or charged. The electrolyte was developed by ETH PhD student Xi Chen. The battery's sandwich design is based on commercial batteries, but for the first time researchers used only flexible components to keep the battery as a whole flexible and stretchable.
Silver flakes like roof tiles
The two current collectors for the anode and the cathode are made of a stretchable plastic containing electrically conductive carbon. This is at the same time outer shell. On the inside of the plastic, the scientists applied a thin layer of tiny silver flakes. Due to the roof-tile-like arrangement of the silver flakes, they do not lose contact with one another even if the plastic is greatly stretched. This guarantees the conductivity of the current collector even when it is stretched strongly. However, if the silver flakes lose contact with each other, the electric current, though weaker, flows through the carbonaceous plastic.
Using a mask, the researchers then sprayed anode or cathode powder in a precisely defined area on the silver layer. The cathode powder contains lithium manganese oxide, the anode vanadium oxide.
Water-based gel electrolyte
Separated by a separating layer similar to a picture frame, the scientists finally put the two current collectors together with the applied electrodes and filled the gap in the frame with electrolyte gel.
This gel is more environmentally friendly than previous, toxic and flammable electrolyte fluids, as Niederberger emphasizes. The one developed by PhD student Xi Chen, on the other hand, is based on water. The lithium gel is introduced in high concentration in the gel, which not only allows the migration of lithium ions between the cathode and anode during charging and discharging, but also prevents the electrochemical decomposition of the water.
In practice, there is still a lack of tightness
For their prototype, the scientists combined the various components with adhesive. In commercial practice, however, this technique can not be used because no long-term tightness can be guaranteed. For the commercialization of the battery, therefore, a method must be found that ensures this. The loading of electrode material would also need to be increased.