Computers or smartphones with folding screens, smart clothing, and wearable sensors all require an energy source, which is usually a lithium-ion battery. These are typically heavy and rigid, making them fundamentally unsuitable for applications in flexible electronics or textiles.
A prototype for a flexible, thin-film battery was developed that can be bent, stretched, and even twisted without interrupting the supply of power. The battery is built in layers like a sandwich and uses flexible components to keep the whole battery bendable and stretchable.
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, a thin layer of micron-sized silver flakes was applied. Due to the way the flakes overlap like roof tiles, they don’t lose 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. In the event that the silver flakes do lose contact with each other, the electrical current can still flow through the carbon-containing composite.
With the help of a mask, anode and cathode powder was sprayed 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 two current collectors were stacked with the applied electrodes on top of each other, separated by a barrier layer similar to a picture frame, while the gap in the frame was filled with the electrolyte gel.
The gel is more environmentally friendly than commercial electrolytes. It contains water with a high concentration of a lithium salt, which not only facilitates the flow of lithium ions between cathode and anode while the battery is charging or discharging, but also keeps the water from electrochemical decomposition.