Contributd by Zhiqian Wang and Somenath Mitra*, Department of Chemistry and Environmental Science, New Jersey Institute of Technology
The development of flexible electronics printed on polymer matrices has opened up the possibilities of mobile phones to be folded into the pockets and a variety of electronic products including flexible displays, smart surfaces (Figure 1) and sensors to be made into different bendable shapes, that can be rolled up, twisted or inserted in films. Thanks to flexible organic LEDs and solar cells, it is possible to have rolled up window shades that have solar cells on one side to generate electricity and TV screen on other side for watching the evening news. Flexible batteries rolled up as carpets can be used for energy storage in solar powered green buildings. Other applications of flexible electronics include the “internet of things” such as sensors that go on food packaging and smart tags. The market for flexible electronics is expected to go from 24 billion in 2014 to over 70 billion by 2024 (source IDTechEx.com). There is an additional requirement for this new electronic revolution to remain “flexible”, and that is the availability of flexible power sources such as batteries. Typical batteries in the market are AA, AAA, D type and Lithium ion batteries. At present, these batteries appear to be the source of inflexibility and researchers are scrambling to develop flexible batteries.
The basic structure of flexible battery being developed in Dr. Mitra’s group at New Jersey Institute of Technology is shown below (Figure 2 a). Like all other batteries, a flexible battery is composed of cathode, anode, separator between the electrodes, current collectors, and packaging materials; the only requirement is that each component must be flexible.
Figure 2. a) Schematic diagram of a flexible battery; b) a flexible electrode; c) flexible batteries powering LEDs.
One of the major challenges in the fabrication of such batteries is the flexible electrodes. Traditional electrodes crack and disintegrate on bending. In order to acquire desired performance and flexibility, polymeric binders along with conductive polymers are often added into the electrode to “glue” the electroactive particles together. The requirements can be stringent: the binder should not dissolve in the electrolyte, and the electrodes should have low electrical resistance, high mechanical strength and high reactivity. Thanks to nanotechnology, there can be many options. For example, when traditional conductive materials like graphite cannot meet the specifications, nanomaterials like carbon nanotubes provide an alternative. These nanocarbons are highly conductive, flexible with high mechanical strength, and are chemically inert. Due to their tiny dimensions, these nanotubes can fill the gaps among electroactive particles to form conductive networks more efficiently than graphite, and hence bring down the electrode resistance dramatically. Certain treatment of carbon nanotubes, like purification, may further improve the performance. Furthermore, composition of additives needs to be optimized to maintain a balance between the battery performance and flexibility.
Another consideration in the battery fabrication is the separator, which should be electrically insulating but allow ions to pass through. Besides the structure and packaging considerations, a separator needs to store electrolytes and has special requirements in the case of harsh environment such as in an alkaline cell where the electrolyte is a strong base (KOH). Finally, all components need to be packaged into a compact system with good electrical connections. In general the platform for flexible batteries is quite interesting. Various conventional batteries such as zinc-carbon, primary alkaline, rechargeable alkaline as well as lithium-ions are being fabricated. Conventional screen printing, inkjet printing as well as 3-D printing open the doors to inexpensive manufacture of a variety of these batteries to power electronics and harvest energy at home from flexible solar cells.
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