S2E4
Episode 4 (October 25, 2020)
Wei Yan
MIT
Doris Danninger
Johannes Kepler University
Advanced multi-material optoelectronic and electronic fiber devices
Dielectric separators for a new generation of stretchable batteries
Abstract:
Electronic systems that can offer performances of planar, rigid wafer-based devices but with the ability to be flexible, soft, stretchable, biocompatible and wearable are opening a breadth of unique applications in our everyday lives. The integration of a variety of electronic materials within thermal-drawn fibers has emerged as a versatile platform for the fabrication of advanced functional fiber electronic devices. This approach exploits the thermal drawing – the same technology used to fabricate optical fibers –of a macroscopic preform, where functional materials or prefabricated devices are arranged at a prescribed
position, yielding kilometers of electronic fibers with a sophisticated architecture and complex functionalities in a very simple and scalable manner. A single strand of fiber that incorporates materials with disparate electronic, optoelectronics, thermomechanical, rheological and acoustic properties can see objects, hear sound, sense stimuli, communicate, store and convert energy, modulate temperature, monitor health and dissect brains. Integrating these electronic fibers into fabrics, ancient yet largely underdeveloped forms, is setting a stage for fabrics to be the next frontier in computation and Artificial Intelligence. In this presentation, I will present the development of thermally drawn fiber electronics and highlight their unique opportunities in communications, sensing, energy, artificial muscles, 3-D printing, healthcare, smart wearables, robotics, neuroscience as well as in-fiber materials fundamental research in materials science and physics. I will conclude some perspectives for realizing an analogue of “Moore’s law” in fibers and fabrics and the remaining challenges for future research.
Abstract:
Powering soft and elastic forms of robots, machines, and electronic skins is a key issue that promotes the ongoing soft revolution in several fields, from consumer electronics and robotics to biomedical systems, sports and healthcare [1]. A battery as energy storage system is an obvious choice but must be rendered stretchable and soft to be fully compliant and imperceptible to soft robots and human beings, with a first cell configuration developed by us in 2010 [2]. To meet the performance of its rigid counterpart, soft batteries must improve in terms of energy density, capacity, and recharge-ability.
Seeking full compliance with soft bodies, imminent development includes extreme deformability and biocompatibility, when used as energy source for smart healthcare electronics on the human skin. Our focus lies at one key part of such an energy source, namely the separator. Its first and foremost function is to keep positive and negative electrodes apart to prevent electrical short circuits, yet at the same time it has to allow rapid transport of ionic charge carriers. Instead of the widely used hydrogels, we incorporate a porous polymer synthesized by UV-polymerization of a high-internal-phase emulsion (HIPE), with the internal phase being the electrolyte solution needed for the battery. Such a polyHIPE, a continuous polymer envelope surrounding the dispersed droplets of the internal phase, constituting up more than 74% of the volume, forms if only the continuous, external phase contains monomers [3]. The tunability of porosity and thus of conductivity as well is a key feature allowing optimization of the resulting battery characteristics.
Results and Discussion
We study the ion mobility as a function of porosity using electrochemical impedance spectroscopy over a wide frequency range (from mHz to MHz). With high-porosity polyHIPEs we managed to achieve free-electrolyte to polyHIPE conductivity ratios of below 2, whilst still retaining enough mechanical stability to allow use as a battery separator. With this new configuration of the battery cell we managed to achieve unprecedented low internal resistance of the primary cell [4].
Conclusions
With the polyHIPE separators we found a material featuring high stretchability, tunable porosity and fast ion transport, allowing a new configuration of stretchable batteries (Fig. 1). The careful study of the effects of varying porosity on the conductivity and system performance have been rendered possible by electrochemical impedance spectroscopy using a setup designed specially for our needs.
References
[1] S. Bauer, S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, R. Schwödiauer, „25th anniversary article: a soft future: from robots and sensor skin to energy harvesters”, Advanced Materials 26(1), 149-162, 2014
[2] M. Kaltenbrunner, G. Kettlgruber, C. Siket, R. Schwödiauer, S. Bauer, “Arrays of Ultracompliant Electrochemical Dry Gel Cells for Stretchable Electronics”, Advanced Materials, 22:2065-2067, 2010
[3] M.S. Silverstein and N.R. Cameron, „PolyHIPEs — Porous Polymers from High Internal Phase Emulsions“, Encyclopedia of Polymer Science and Technology, 2002
[4] D. Wirthl, R. Pichler, M. Drack, G. Kettlgruber, R. Moser, R. Gerstmayr, F. Hartmann, E. Bradt, R. Kaltseis, C. M. Siket, S. E. Schausberger, S. Hild, S. Bauer, M. Kaltenbrunner, „Instant tough bonding of hydrogels for soft machines and electronics”, Science Advances, 3(6), e1700053, 2017