Emerging applications in wearable devices require unique functionalities in order to efficiently implement sensor systems having the desired capability and form factors. As an example, integration of energy storage remains a key factor limiting the capability for many wearable applications-ranging from activity monitoring to personal health and remote healthcare. Furthermore, with new technology developments such as electrochromic films, “smart glass”, or low power displays, not only has power become a premium, but added functionality such as transparency may be required. The ability to controllably and selectively add or combine functionalities at the device and materials process level provides an enabling capability that will foster the innovation of new and unique technologies impacting wearable electronic devices.
Recently, Gittleson, et. al., reported on the development of transparent energy storage cells utilizing spin-spray layer-by-layer (SSLbL) assembly. Layer-by-layer assembly, previously reviewed by InterNano (Functionalized Carbon Nanotube Electrodes for Increased Power Density in Lithium Ion Batteries, Rapid Assembly of Functional Thin Films Using Spin-Spray Layer-by Layer Processing), is a solution-based deposition technique applicable to the development of functional thin-films. The method exploits electrostatic interactions between charged polyelectrolyte species to create thin-films in spontaneous and self-limiting fashion. The Taylor group (Rapid Assembly of Functional Thin Films Using Spin-Spray Layer-by Layer Processing) has previously reduced the time of deposition employing the spin-spray method enabling rapid drying of solvents thereby realizing practical deposition times. For this recent work, this versatile coating process has been used to control the function and properties of two separate materials to create a transparent anode/cathode configuration for integrated thin-film batteries.
To create the electrodes for the energy storage cell, SSLbL was utilized to optimize the conductivity and transparency of single walled carbon nanotubes (SWNT) for the cathode, and vanadium pentoxide (V2O5) nanowires for the anode. For each material, the authors investigated coatings consisting of 50-150 bilayers, with each bilayer having a nominal thickness of ~2 nm. As expected the coating thickness impacted both transparency of the cell as well as cell performance. Optimization of depositions for each electrode demonstrated >87% transparency while exhibiting 23 and 7 µA-hr/cm2 capacity for cathode and anode respectively at 5 µA/cm2. Overall cell capacity demonstrated 5 µA-hr/cm2 over 100 charging cycles. While these performance numbers may not be impressive compared to bulk or thick film batteries, it represents the ability to trickle charge and provide power for application requirements where there may be no alternative. More importantly, it illustrates the power of this nanomanufacturing process method to impart specific and controllable functionality at the nanomaterial level, providing an “atoms to product” or bottom-up methodology for device design.
Ultrathin Nanotube/Nanowire Electrodes by Spin–Spray Layer-by-Layer Assembly: A Concept for Transparent Energy Storage
Forrest S. Gittleson, Daniel Hwang, Won-Hee Ryu, Sara M. Hashmi, Jonathan Hwang, Tenghooi Goh, and André D. Taylor
ACS Nano 2015 9 (10), 10005-10017
Image reprinted with permission from Ultrathin Nanotube/Nanowire Electrodes by Spin–Spray Layer-by-Layer Assembly: A Concept for Transparent Energy Storage. Forrest S. Gittleson, Daniel Hwang, Won-Hee Ryu, Sara M. Hashmi, Jonathan Hwang, Tenghooi Goh, and André D. Taylor. ACS Nano 2015 9 (10), 10005-10017