Flexible batteries

Definition

Batteries whose shape can be altered without impairing functionality enable numerous new applications: smart product labels, clothing with integrated sensors or medical dressings that do more than just protect a wound. Researchers at universities and in companies are hard at work, looking into this issue.   

When designing flexible batteries, various physical properties of batteries come into conflict with flexible design. So materials must be found and new designs developed that allow the combination of energy storage and flexible form without compromising safety.  

Current applications and opportunities

Some medications are temperature-sensitive and lose their efficacy when stored at excessively high temperatures. Today, freight containers are equipped with thermometers to verify the efficacy of such medications. But it would be far more accurate and reliable not to measure the ambient temperature, but rather to equip the individual ampoules, blisters or boxes with temperature sensors so that the object itself could record the conditions it had been exposed to.  

When complete accountability can be provided regarding where a particular item – such as a box of medicine, a bottle of very expensive wine or an ampoule containing a vaccine – was at any given time and what conditions individual units were exposed or where an item has come from, this protects potential buyers, investors or users from counterfeits, loss of quality and loss of efficacy due to incorrect storage. 

Additionally, flexible batteries could be used in wound care. Nowadays, plasters protect against environmental influences. However, it is entirely conceivable that they might in future also provide information about healing progress or even supply wounds with active substances.  

Current batteries cannot be integrated into labels or plasters because they are neither bendable nor foldable. So energy storage devices whose shape can adapt to the surrounding area must be developed. Since it can be assumed that such batteries will only be short-lived, it is crucially important that the materials used in them are very inexpensive and easily recoverable and do not require any special disposal.  

Challenges

A battery is a chemical energy storage system. Think of it like a container. The larger it is, the more liquid it can hold. The more mass batteries of the same design have, the more energy they can store and the less flexible they become. Four key challenges that make building flexible batteries difficult are: safety and functionality versus flexibility; bending versus creasing; the hazardous nature of lithium-ion batteries; and new materials versus equivalent performance. 

Batteries work because they consist of two electrodes – the anode and the cathode. These two electrodes have different potentials. Electric current flows because this difference seeks equalisation. For current to flow through the external circuit, all the components must be pressed together without any gaps and as tightly as possible: the tighter the casing, the greater the pressure. But a solid casing and flexible design are two contradictory components.  

When something made up of different layers is bent, creases will form. Where the material folds, the two layers lose contact with one another. They delaminate, to use the technical term. Where there is no contact between the layers, current can no longer flow, and the battery loses performance. 

Today’s most powerful batteries work using lithium ions. For ions to move between components, batteries contain an electrolyte – a liquid enabling ion exchange. Unfortunately, this liquid is toxic and flammable, which is why there are strict safety requirements for battery construction. Since a flexible design is less secure and such batteries break more easily than those with rigid casings, current safety regulations prohibit the use of toxic and highly flammable substances. 

Researchers are looking for new materials that are safe and non-toxic. One promising alternative that has been researched at ETH Zurich is zinc ions. These batteries are operated with water as electrolyte, which is interesting because all the components of such batteries are non-toxic, unproblematic and relatively inexpensive. However, zinc-ion battery performance is not yet comparable to that of lithium-ion batteries. The main reason for this is that water decomposes into hydrogen and oxygen at approximately 2 volts. 

Focus on industry

There are no truly flexible, bendable and foldable batteries available on the market yet. But research into the development of such batteries is underway in many places, so it isn’t unreasonable to expect that products of this kind to launch on the market in the coming years.  

When flexible batteries can be produced cost-effectively, this will lead to applications at many stages of different value chains. So it is worthwhile even today to use this technology, which is still at the fundamental research stage, as an opportunity to consider how companies can digitally map and complement their business processes.  One example of this is the question of whether sensors could realistically be used in labels. 

International perspective

Most batteries today come from China, which is not only the hotspot in terms of quantity, but also sets the tone in terms of quality in the battery development and manufacturing.  

Nonetheless, Switzerland is in a good position. It has excellent fundamental research and education in all subjects that are important for battery development: chemistry, electrical engineering and materials sciences.  

Several companies capable of innovating battery technology are also based here. However, the budgets for research and development, not to mention the production of new battery types, are very small compared to those of large production locations. This severely limits options.  

Future applications

In future, flexible batteries could also be used in soft robotics. Soft robots are robots whose components are soft. They are used in applications requiring a high degree of robot-human interaction, such as healthcare and nursing for the elderly.  

Another goal of research is to develop batteries that are not only flexible, but also biodegradable. If this were to be achieved, it would enable the development of new implants that decompose in the body after fulfilling their function.  

The technology also promises new consumer goods applications: foldable and rollable displays, for example, which no not exist yet partly due to the lack of suitable power sources.