|Fig. 1: shows the schematics of the flexible freestanding battery materials/CNT through a simple peeling process.|
Flexible devices are gaining immense interests among researchers and the industry. Imagine a world full with flexible electronics; cell phones that are as thin as a sheet of paper, roll-up touch screen display that wraps around the wrist ... Having features of flexibility and transparency, market of electronic devices will be unprecedented boosted. To fully extend the potential of flexible device in the electronic industry, a flexible (and transparent) power source is ideal. In this report, the concept of flexible batteries and supercapacitor would be explored.
Secondary Li-ion batteries is a key component in portable electronics due to high power and energy density and long cycle life. In these batteries, metal strips, mainly copper (~10 mg/cm2) and aluminum (5 mg/cm2), are used as current collectors.  However, those metal electrodes are not very flexible. Solution-processed carbon nanotube (CNT) thin films have been widely studied and applied as electrodes for optoelectronics due to their high conductivity and flexibility.  Researchers have explored a type of batteries to replace metal electrode with the use of CNT thin films on plastic substrates as current collectors for batteries or supercapacitor.
Hu et al. integrated all of the components of a Li-ion battery into a single sheet of paper through a simple lamination process.  CNT thin films were used as both anode and cathode. The double layer films were laminated onto commercial paper, and the paper functions as both the mechanical support and Li-ion battery and as separator.
Porous morphology of paper allows the electrolyte to diffuse efficiently into it, which allows the paper to be used effectively as a separator. The battery performance was tested and exhibited low sheet resistance (~5 Ω/sq) and a high energy density (108 mWh/g).
Sheet resistance is calculated as:
Where R is the sheet resistance, ρ is the resistivity of the electrolyte (100 ohm-cm for the standard EC/DEC solution), L is the thickness of the separator, A is transversal area (the area perpendicular to the axis of the electrode, τ is tortuosity (the ratio between the path length of the ions and the thickness of the electrode), and f is pore fraction (the ratio between the pore volume and the total geometrical volume of the electrode). The ratio τ/f is important for the separator, as it indicates how easy it is for the electrolyte to penetrate through.
Moreover, the entire battery is of lightweight (~0.2 mg/cm2) and with excellent flexibility, as determined by bending test. The battery functions well even when it is bent to a radius down to 6 mm. These physical and chemical properties make it a desirable material for providing power source to a flexible device.
|Fig. 2: shows the cartoon of the nanocup in its concave shape.|
Another collaboration group from Northeastern and Rice University presented their design of a flexible and transparent supercapacitor.  The technology is based on a nanomaterial with a coffee mug shape but in the nanoscale. This "nanocup" is able to contain other materials, such as electrolyte, in order to function as a supercapacitor. Jung et al. made this nanocup by etching nanoscopic divots into an aluminum film through oxidation. By changing the voltage and time of this process, researchers can further tune the size of the cups.
This morphology allows them to come into greater contact with the electrolyte. Fabricating the supercapacitor device by impregnation of this porous nanocup electrode in transparent polymer electrolyte films would give both transparency and mechanical flexibility properties into this thin film supercapacitor.
The energy density of flexible batteries remains low as there is limitation regarding the thickness of the materials and therefore the total electrolyte loading.  Mechanical flexibility heavily depends on the physical properties of the electrodes. Performance of flexible batteries or supercapacitor is also restricted by the parasitic chemical reactions and mechanical breakdown due to swelling of the electrodes or mechanical strain during charging and discharging.  It is still a very early stage in flexible batteries research, and a lot has to be done in optimizing the process and also looking for a more ideal materials.
We need a flexible battery to power a flexible tablet. The advent of a high performance flexible thin film battery will accelerate the development of next-generation fully flexible electronic systems in combination with existing flexible components such as display, memory, interactive user interfaces and LED.
© John To. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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