Multifunctional Batteries: Flexible, Transient, and Transparent

Abstract

The primary task of a battery is to store energy and to power electronic devices. This has hardly changed over the years despite all the progress made in improving their electrochemical performance. In comparison to batteries, electronic devices are continuously equipped with new functions, and they also change their physical appearance, becoming flexible, rollable, stretchable, or maybe transparent or even transient or degradable. Mechanical flexibility makes them attractive for wearable electronics or for electronic paper; transparency is desired for transparent screens or smart windows, and degradability or transient properties have the potential to reduce electronic waste. For fully integrated and self-sufficient systems, these devices have to be powered by batteries with similar physical characteristics. To make the currently used rigid and heavy batteries flexible, transparent, and degradable, the whole battery architecture including active materials, current collectors, electrolyte/separator, and packaging has to be redesigned. This requires a fundamental paradigm change in battery research, moving away from exclusively addressing the electrochemical aspects toward an interdisciplinary approach involving chemists, materials scientists, and engineers. This Outlook provides an overview of the different activities in the field of flexible, transient, and transparent batteries with a focus on the challenges that have to be faced toward the development of such multifunctional energy storage devices.

Figure 1

Various architectures reported to be highly stress accommodating and advantageous for flexible and stretchable electrochemical energy storage: (a) Fiber mats. Used with permission from ref (22). Copyright 2019 Royal Society of Chemistry. (b) Fabrics. Used with permission from ref (28). Copyright 2018 Springer Nature. (c–e) Stress accommodating interfaces and interlayers through: (c) Sliding contacts. Used with permission from ref (42). Copyright 2018 John Wiley and Sons. (d) Patterning processes. Used with permission from ref (18). Copyright 2018 John Wiley and Sons. (e) Hydrogel interlayer. Used with permission from ref (68). Copyright 2019 John Wiley and Sons.

Figure 2

Performance comparison of bending tests with flexible cablelike (outlined) and two-dimensional (filled) electrochemical cells. The graph shows their initial capacity vs number of bending cycles as well as their capacity retention after the specified bending cycles (indicated by symbol size). Data are colored according to battery chemistries with LIBs (green) and Li/S (orange) and Zn (blue) batteries.

Figure 3

Transient behavior in batteries. (a) Schematic illustration and optical image of a biodegradable primary Mg–Mo battery in a stacked configuration with four cells in series. Used with permission from ref (88). Copyright 2014 WILEY-VCH. (b) Optical images demonstrating the biodegradation profile of a Mg–air battery in buffered protease solution at 37 °C. Used with permission from ref (90). Copyright 2017 American Chemical Society. (c) Schematic illustrations of a degradable rechargeable LIB and its triggered dissolution process: upon contact with water, the encapsulation and separator of the battery dissolved first (I), followed by the reaction of Li metal with water to produce LiOH (II). The generated base reacted with V₂O₅ and Al metal leading to the complete dissolution of the battery (III). Used with permission from ref (101). Copyright 2015 American Chemical Society.

Figure 4

(a) Schematic illustration of an encapsulated thin-film Mg–air battery, (b) fabrication procedure and a digital image of the gel electrolyte (choline nitrate (ionic liquid) embedded in silk fibroin) used in the battery. (c) Open circuit voltage changes of the encapsulated battery without and with an additional crystallized silk protection layer in phosphate buffer solution (PBS) and in air. Used with permission from ref (90). Copyright 2017 American Chemical Society. (d) Schematic illustration of a biodegradable Mg-MoO₃ battery, (e) powering an LED in PBS for over 16 h. (f) Optical images at various stages of battery degradation in PBS. Used with permission from ref (11). Copyright 2018 WILEY-VCH.

Figure 5

(a) Photograph and (b) cross-section of a colorless Li₄Ti₅O₁₂ thin-film electrode. (c) Photograph and (d, e) cross-section of a brownish LiMn₂O₄ thin-film electrode. Used with permission from ref (123). Copyright 2016 Elsevier B.V. (f) Transparent SWNT anode and V₂O₅ nanowire cathode. Used with permission from ref (124). Copyright 2015 American Chemical Society. (g) Thin-film battery with a grid-structured design of LiCoO₂/LiPON/Si on glass substrates. Used with permission from ref (126). Copyright 2019 American Chemical Society. (h) Photograph of a transparent and flexible battery electrode, (i) magnified optical image, and (j) scanning electron microscopy image and UV–vis spectrum of the gel electrolyte, a single electrode, and the full battery. Used with permission from ref (125). Copyright 2011 National Academy of Sciences of the United States of America.