Transparent electronic devices have recently attracted substantial attention. Various applications have been demonstrated, including displays, touch screens, and solar cells; however, transparent batteries, a key component in fully integrated transparent devices, have not yet been reported. As battery electrode materials are not transparent and have to be thick enough to store energy, the traditional approach of using thin films for transparent devices is not suitable.

Transparent BatteryStanford University researchers developed and demonstrated transparent battery with a grid-structured electrode to solve this dilemma, which is fabricated by a microfluidics-assisted method. The feature dimension in the electrode is below the resolution limit of human eyes, and, thus, the electrode appears transparent. Moreover, by aligning multiple electrodes together, the amount of energy stored increases readily without sacrificing the transparency. This results in a battery with energy density of 10 Wh∕L at a transparency of 60%. The device is also flexible, further broadening their potential applications.

Fabrication of Transparent Battery

Transparent Battery TestingFirst, a polydimethylsiloxane (PDMS) substrate with grid-like trenches is fabricated by spin coating PDMS precursor (Sylgard 184) onto a silicon mold patterned by photolithography. The PDMS film is cured at 80 °C and peeled off from the mold. After 100 nm gold is evaporated onto PDMS as the current collector, the substrate surface is made hydrophilic by air plasma. Then the electrode material solution is filled in by a microfluidics-assisted method. The electrode material is composed of 90% active materials (LiMn2O4 or Li4Ti5O12), 7% carbon black, and 3% aqueous binder (Pred materials). The concentration of solid materials in the solution is 18–20%.

To make a full cell, aluminum strip is wrapped onto the side of each electrode. Then, gel electrolyte is placed on the anode electrode first, and the transparent cathode is put on the anode under optical microscope at 20×. The full cell is sealed inside a highly transparent poly(vinyl chloride) (PVC) thermoplastic bag with metal current collector extending out. The cell configuration is very similar to a pouch cell except that all components are transparent.

Source: Stanford University