A flexible thin-film solid state battery that can be bent, stretched and even twisted without interrupting the supply of power has been developed by researchers in Zurich. The team led by Markus Niederberger, Professor for Multifunctional Materials at ETH Zurich, used a new electrolyte that allows a battery to be built with flexible layers that can be stretched to three times its own length.
"To date, no one has employed exclusively flexible components as systematically as we have in creating a lithium-ion battery," said Niederberger. This could be used for wearable electronics as well as the next generation of foldable devices.
The two current collectors for the anode and the cathode consist of bendable polymer composite that contains electrically conductive carbon and that also serves as the outer shell. On the interior surface of the composite, the researchers applied a thin layer of micron-sized silver flakes. The flakes overlap like roof tiles so they maintain contact with one another when the elastomer is stretched. This guarantees the conductivity of the current collector even if it is subjected to extensive stretching. If the silver flakes do lose contact with each other, the electrical current can still flow through the carbon-containing composite, although this is a weaker connection.
With the help of a mask, the researchers then sprayed anode and cathode powder onto a precisely defined area of the silver layer. The cathode is composed of lithium manganese oxide and the anode is a vanadium oxide. In the final step, the scientists stacked the two current collectors with the applied electrodes on top of each other, separated by a barrier layer filled with the water-based electrolyte polyacrylamide gel.
Rather than today’s flammable electrolytes, the new material contains water with a high concentration of a lithium salt, which not only allows lithium ions to move between cathode and anode while the battery is charging or discharging, but also keeps the water from decomposing.