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Battery usage has expanded from mobile phones and laptops to LED lamps, portable fans, toys, toothbrushes, and even automobiles. Now, the battery applications are expanding further as the Internet of Things, Industry 4.0, big data, mobile and cloud computing are introduced. People build systems to obtain, manage and utilise data to improve our lives. Objects become connected devices, and batteries are needed to provide the power.
There is also rapid development of flexible and wearable electronics is giving rise to an exciting range of applications, from smart watches and flexible displays — such as smart phones, tablets, and TV — to smart fabrics, smart glass, transdermal patches, sensors, and more. With this rise, demand has increased for high-performance flexible batteries.
For example, the Japanese government has announced to change the paradigm of healthcare by 2035 via the campaign called Japan Vision: Healthcare 2035. The government plans to create an ecosystem consisted of predictive, personalised and participatory healthcare. Technologies to leverage for this vision are IoT, wearables, smart phones, smart homes, robots, etc. With that, it is inevitable to look for a new battery solution right for these devices
Making a flexible battery to power these flexible and wearable systems is another challenge. While the lithium-polymer batteries used in smartphones today are somewhat flexible, they can’t survive being bent many times. When it is necessary to collect the data from human bodies or portable objects via wearables or connected devices, the rigid and thick batteries have challenges in developing user-friendly designs. In order to get sufficient amount of data, users have to feel comfortable having the device on their bodies during the required amount of time.
Jenax has developed flexible lithium-ion cell technology that can be a great solution for the IoT and wearable products. Lithium-ion batteries are the most widely adopted chemistry for energy storage, and Jenax’s battery – J.Flex – is an advanced version of lithium-ion batteries with flexibility, enhanced safety and fast charge capability. J.Flex is unlike conventional batteries and other thin film batteries because of the degree of flexibility it features. J.Flex can be bent, flexed and rolled to any direction.
Plus, our battery technology does not sacrifice performance for the flexibility: the flexible battery was tested by continuous 10,000 times of bending to 20mm in radius, and the capacity of the bent battery was the same as that of a non-bent battery. Also, J.Flex uses gel polymer electrolyte that provides protection against leakage when the battery is penetrated or cut, thereby minimising risk to users.
For now, the rise of Internet of Things and Wearables. Flexible battery is an enabling technology for these markets. For the foreseeable future, one of specific applications we are interested in is a flexible phone on a wrist where a flexible battery and flexible display shine as the main components of the device. Generally, as flexible battery can be implemented to a diverse range of devices that need energy storage, it can be used for wearable and connected devices in medical, sports, fashion, military and other industries. The applications are limitless.
Flexible battery technologies
When batteries are made very thin, they can be flexible. This approach usually involves new materials/chemistries or new manufacturing methods. PowerStream has been working with a team of engineers in China to develop the thinnest possible lithium polymer batteries. “We can now offer design engineers batteries of 2.0 mm to 0.5 mm thick.These cells should be useful for smart cards, RFID tags and other applications that require energy storage in a very thin battery.”
Panasonic Corporation has also announced that it has developed a Flexible Lithium-ion Battery with a thickness of only 0.55mm, or about 0.022 inches.Suitable for use in card-type and wearable devices, this rechargeable battery can retain its characteristics even after repeatedly bent into a radius of 25mm or twisted to an angle of 25 degrees.
However how much energy can be stored in a battery largely depends on the battery volume and existing energy density. Thin batteries have reduced thickness than ordinary batteries. Therefore, they tend to have less capacity and even less power compared with ordinary batteries with similar footprint.
Another approach is to add special features including flexibility to traditional lithium-ion batteries. That can be achieved by altering structural designs and improved materials. For those batteries, they usually have better performance than the first kind, but continuously bending and curving batteries may introduce cracks and damage the batteries, leading to shorter cycle life.
Flexible battery works when stretched and could power wearable devices
Nicholas Kotov at the University of Michigan and his colleagues have developed a conducting component for a lithium-ion battery that maintains its electrical conductivity even when stretched to a strain of more than 300 per cent. The conductor is made from multiple layers of polyurethane and gold nanoparticles. Polyurethane is a polymer used to make common objects such as foam sponges and garden hoses.
Layers of negatively charged nanoparticles are alternated with positively charged layers of the softer polymer. As the conductor is stretched, the gold nanoparticles self-organise into aligned pathways, allowing them to continue conducting electricity.
A major challenge of designing flexible batteries is finding a balance between stretchiness and electrical conductivity, says Kotov.
The team tested the performance of the conductor in a battery with a lithium electrolyte. The stretchable battery has a lower power density than regular lithium ion batteries, but after 1000 cycles, it retained 96 per cent of its capacity. This dropped significantly in tests in which the battery was always in its stretched state: under those conditions it retained only 72 per cent of its capacity after just 10 cycles. But once the strain is released, its capacity increases again.
Kotov envisages such batteries could be used in wearable or implantable devices, as well as used in soft robots with flexible legs or tentacles. He says the properties of the battery can be modified. “We can adapt to the specific mechanics and charge storage requirements that implantable devices or other devices might need,” he says.
Engineers develop flexible lithium battery for wearable electronics
Up to now, however, researchers have had difficulty obtaining both good flexibility and high energy density concurrently in lithium-ion batteries. A team led by Yuan Yang, assistant professor of materials science and engineering in the department of applied physics and mathematics at Columbia Engineering, has developed a prototype that addresses this challenge: a Li-on battery shaped like the human spine that allows remarkable flexibility, high energy density, and stable voltage no matter how it is flexed or twisted. The study is published today in Advanced Materials.
“The energy density of our prototype is one of the highest reported so far,” says Yang. “We’ve developed a simple and scalable approach to fabricate a flexible spine-like lithium ion battery that has excellent electrochemical and mechanical properties. Our design is a very promising candidate as the first-generation, flexible, commercial lithium-ion battery. We are now optimizing the design and improving its performance.”
Taking inspiration from human spine, his prototype has a thick, rigid segment that stores energy by winding the electrodes (“vertebrae”) around a thin, flexible part (“marrow”) that connects the vertebra-like stacks of electrodes together. His design provides excellent flexibility for the whole battery. “As the volume of the rigid electrode part is significantly larger than the flexible interconnection, the energy density of such a flexible battery battery can be greater than 85 percent of a battery in standard commercial packaging,” Yang explains. “Because of the high proportion of the active materials in the whole structure, our spine-like battery shows very high energy density — higher than any other reports we are aware of. The battery also successfully survived a harsh dynamic mechanical load test because of our rational bio-inspired design.”
The battery shows stable capacity upon cycling, as well as a stable voltage profile no matter how it is flexed or twisted. Even when the cell was continuously flexed and twisted during the whole discharge, the voltage profile remained. The battery in the flexed state was also cycled at higher current densities, and the capacity retention was quite high (84% at 3C, the charge in 1/3 of an hour). The battery also survived a continuous dynamic mechanical load test, rarely reported in earlier studies.
