Portable power: flexible batteries

Stretchy and flexible batteries are being developed to fuel the next generation of gadgets.

Wearable and flexible gadgets are poised to become the next major trend in electronics, according to many observers. Manufacturers such as Samsung have already demonstrated prototype smartphones with flexible screens for the next generation of smartphones. Wearable consumer products have debuted in the form of the Jawbone Up wristband activity monitor and the Google Glass head-mounted computer. There is intense speculation that Apple’s next product launch will be a smart wristwatch, the iWatch. Taking things a stage further, fashion house CuteCircuit has demonstrated a dress with a mobile phone integrated into it, embedded into clothing using conducting thread.

The snag with all these applications is that they need battery power. Batteries tend to be the opposite of flexible: solid and made in awkward shapes not readily suited to flexible devices. ‘There’s no physical reason that batteries can’t be flexible; it’s just a practical problem,’ said Will Stewart, chair of the IET communications quality panel and a visiting professor at Southampton University’s optoelectronics research centre. Batteries just need a container for the electrolyte but this is usually rigid. ‘The container could be made flexible, provided it retained its integrity,’ said Stewart. As an engineering challenge, he added, it should be readily soluble.

On the cards: US company Flexion is marketing batteries for the next generation of ‘intelligent’ credit cards

Reports suggest Apple’s iWatch may feature a flexible display that wraps around the user’s wrist, so speculation ramped up last month with the publication of a patent application from Apple for a flexible battery. In a wide-ranging list of potential applications, alongside calculators, tablet computers and music players, the patent application specifically mentions ‘wristwatches’.

Apple’s patent application says: ‘Electronic devices are ubiquitous in society… Many of these electronic devices require some type of portable power source [but] also have unique form factors. Because of this, the portable power source of any one electronic device may not fit within any other electronic device. Furthermore, these unique form factors often require flexible battery arrangements, whereas conventional battery packs are often too rigid to flexibly conform to these form factors.’

The application sets out to solve this, but not by changing the design of the cell itself. Instead it describes how a number of cells, which could be galvanic or photovoltaic, are encapsulated within a bottom and top laminate layer. Between any two cells the laminate layers are glued together to isolate the cells from each other and to seal them against moisture and other contaminants. At each of these seal points the laminate can flex, allowing the battery as a whole to be bent into a curved shape as required.

Meanwhile, a team led by John Rogers at Illinois University has developed a battery which can be stretched by a factor of three in any direction. A company, MC10, is commercialising the idea, aiming at applications in consumer electronics, remote monitoring for the health industry and implantable medical devices.

These applications use a 2D array of ultra-miniature batteries that are separately fabricated and packaged and then connected by ‘serpentine’ connections — wires in the form of a repeating zig-zag or ‘S’ shape, with the connection as a whole looped into
a larger ‘S’ shape. The cells and connections are embedded in a stretchable polymer, and as the polymer is stretched the connecting wires straighten.

A potentially significant breakthrough, as far as flexibility is concerned, was developed by a team at Leeds University, led by Prof Ian Ward, who has now licensed the technology to US company Polystor Energy Corporation, which is working on commercialisation. This was a polymer gel electrolyte that could replace the liquid electrolytes currently used in lithium cells used in a wide range of portable consumer devices such as laptops, digital cameras, phones and music players.

“It’s possible to imagine that, in future, any valuable item will have a sensor built in so that you can always find it.

Will Stewart, IET

Traditional lithium-ion batteries are based on cells filled with a liquid chemical forming the electrolyte. Anode and cathode, with a polymer separator between them, are formed of thin sheets rolled into a spiral within the container holding the electrolyte. If the separator is damaged allowing the positive and negative electrodes to form a short circuit, the battery heats up rapidly and can catch fire.

The polymer gel overcomes this problem, creating an electrolyte that is solid but flexible and eliminates the need for a rigid container. The Leeds team developed an automated extrusion/lamination manufacturing process that sandwiches the gel between an anode and a cathode at a rate of 10m/min to create a highly conductive strip that is just nanometres thick. The resultant film can be cut to any shape.

Prof Stewart said that, to some extent, product designers have been able to work around the shape limitations of current batteries because of advances in the efficiency of electronics, so that batteries can at least be small. ‘For example, smartphones have an enormous amount more performance than old mobiles without significantly reduced battery life, because processors are cleverer.’ For a handheld computer or smart watch, he said, ‘a flexible display is likely to be more significant’.

Stretching out: a 2D array of tiny batteries are separately fabricated and then connected by ‘serpentine’ connections into a battery that can be stretched in any direction

Where he believes new developments in small, flexible batteries are likely to be very important, however, is in the ‘internet of things’ — the use of tiny sensors and communication devices to identify objects and connect them to a network, from electronic transport tickets to medical implants.

The Flexion range of batteries developed by Solicore, based in Florida, is designed for this type of application. The batteries are based on the company’s patented solid polymer electrolyte technology, which allows manufacture of lithium polymer cells just 0.45mm thick. The company is marketing the batteries for applications such as the next generation of ‘intelligent’ credit cards and security cards, which will contain memory chips or microprocessors and require embedded battery power. The batteries can survive the hot lamination process used to make cards and are also suitable for radio-frequency identification tags and sensors, security and information devices and thin-film medical drug-delivery products that can be attached to the skin.

Solicore vice-president of worldwide sales David Eagleson said: ‘The key to our battery is the separator layer that in effect acts as a sponge-like material, which allows the battery to be bent and continue operating in an effective manner.’ However, he added: ‘We have an extremely good bend radius and [producing] a wristband would not be an issue.’

Blue Spark in Ohio also produces ultra-thin batteries, in this case conventional zinc-carbon batteries, but in printed form. Printed electronics uses standard graphics processes to print electronic circuits and components on media such as paper, plastic and textiles. The concept has been around for some time, but recent advances in conductive ink chemistry and flexible substrates promise to make possible a range of new markets and applications.

Blue Spark prints the active components on a PET substrate and adds electrolyte and separator layers; a top PET layer then seals the battery. One application is Sealed Air Corporation’s Turbo Tag radio-frequency identification time and temperature monitoring system, which features autonomous data logging. The tag is used for temperature-sensitive foods and pharmaceutical products and can track and record cold chain temperatures from the processing plant, in transit, and up to the point of delivery.

Printed batteries could help extend the internet of things to include almost any small but important everyday object. Stewart said: ‘It’s possible to imagine that, in future, any valuable item will have a sensor built in so that you can always find it.’