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Current Status and Challenges in Printed Batteries: Toward Form Factor-Free, Monolithic Integrated Power Sources Keun-Ho Choi,† David B. Ahn,† and Sang-Young Lee* Department of Energy Engineering, School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea S Supporting Information *
ABSTRACT: With the advent of the ubiquitous electronics era, high-performance power sources with aesthetic diversity are indispensably needed as a key-enabling technology. Printed batteries have recently emerged as a crispy energy storage system to address this issue. Printed batteries are fabricated through simple, low-cost, and scalable printing processes. Their salient features include various form factors, shape conformability, and monolithic integration with devices of interest. Research directions on printed batteries are currently focused on (i) the design of battery shapes and configurations, (ii) synthesis of battery component inks with tunable rheological properties and electrochemical performances, and (iii) adoption of suitable printing techniques. We describe the current status and challenges of printed batteries, with a particular focus on the form factors, battery component inks, printing techniques, cell performances, and integration with other systems. The development directions and outlook of printed batteries are also discussed along with their potential applications. This Review provides new insight into printed batteries and their opportunity as an efficient and versatile platform technology to enable shape-versatile/monolithic-integrated power sources with functionalities far beyond those of conventional batteries. orthcoming flexible/wearable electronics, the Internet of Things (IoT), and electric vehicles, which are expected to provide unforeseen ubiquitous connections via electronic transmission, have spurred the relentless pursuit of advanced power sources with reliable electrochemical performance and flexibility.1−3 Enormous emphasis should be paid to battery form factors and deformability to achieve this challenging goal, along with continuing efforts in the development of new electrochemically active materials. Meanwhile, from a battery architecture point of view, conventional batteries with fixed shapes and sizes are fabricated by winding or stacking cell components (such as anodes, cathodes, and separator membranes) and then packaging these components into (cylindrical-/rectangular-shaped) metallic canisters or pouch films, followed by liquid-electrolyte injection.4 Using conventional battery materials and assembly processes, the resulting batteries have limited form factors and mechanical flexibility, thus imposing formidable difficulties for their integration into complex-shaped electronic devices. “Printed power sources” have recently emerged as a new battery system to address the aforementioned issues on the design diversity and flexibility.5,6 The printed batteries are fabricated through simple, low-cost, and scalable printing processes. Their salient features include various form factors, shape conformability, and monolithic integration with devices,
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© 2017 American Chemical Society
which are difficult to attain with conventional battery technologies. Printing technology is widely used in a variety of application fields (in particular, printed electronics) due to its simple processability, reproducibility, and versatility, in which rationally designed inks (including pastes) are printed in predesigned forms.7,8 Research directions on printed batteries are currently focused on (i) the design of battery shapes and configurations, (ii) synthesis of battery component (i.e., electrodes, electrolytes, and separator membranes) inks with tunable rheological properties and electrochemical performances, and (iii) adoption of suitable printing techniques. These research directions have the ultimate goal of eventually enabling the application of so-called “all-printed-batteries”. This Review describes the current status and challenges of printed batteries, with a focus on form factors, battery component inks, printing techniques, cell performances, and integration with other systems. Noteworthy achievements in battery design and shape diversity are reviewed in terms of electrochemical systems and printability. The development directions and outlook of printed batteries are also provided along with their potential application fields. Received: November 4, 2017 Accepted: December 14, 2017 Published: December 14, 2017 220
DOI: 10.1021/acsenergylett.7b01086 ACS Energy Lett. 2018, 3, 220−236
Review
Cite This: ACS Energy Lett. 2018, 3, 220−236
ACS Energy Letters
Review
Screen or stencil printing5,6 is an on-contact process in which the ink is squeezed through prepatterned masks (screen or stencil) onto the substrates of interest (Figure 2a). After pressing the ink by dragging a squeegee across the mask, the mask is lifted, and the ink is transferred to the substrate in a desired pattern. Due to its simplicity and scalability, screen or stencil printing has been the most popular fabrication method for printed batteries. The technique, in particular, is favorable for thick layer (>20 μm) printing, which is suitable for typical battery components. However, the use of prepatterned masks results in low resolution of printed patterns and low throughput. Lee et al. fabricated a shape-conformable, printed Li-ion battery via sequential stencil printing of the anode, solid-state electrolyte, and cathode (Figure 2d).9 Spray printing5,6 is a technique in which micro/nanosized ink droplets are deposited onto a substrate through prepatterned masks (Figure 2b). Because the fine droplets are deposited via air, this technique allows printing onto uneven/curved surfaces and the formation of thin layers over large areas. Additionally, for inks with lowvapor-pressure solvents, the solvents immediately evaporate upon printing on the substrate, which enables sequential printing of multiple layers. Spray printing is particularly attractive for depositing inks over nonflat and curvilinear surfaces. However, aerosol formation and ink leveling on the substrates are difficult to achieve because the printing processability is affected by environmental humidity, temperature, and surface roughness of the substrate. Singh et al. demonstrated an all-spray-printed Li-ion battery (including the current collector, anode, cathode, and separator) (Figure 2e).10 Flexographic printing is commonly used to print labels for food packaging and corrugated shipping boxes (Figure 2c).6 Flexographic printing has an advantage over lithography due to its facile use of a wider range of inks (spanning from water-based inks to oil-based inks) and compatibility with various substrates such as plastic, foil, paper, and others. The printing plates are fabricated by exposing photopolymer plates to ultraviolet (UV) light through a negative mask. In the flexographic printing setup, the thickness of the printed image is generally low (1−3 μm). Traditional battery electrodes typically have a thickness range from 20 to 100 μm. Therefore, a flexographic printing setup for depositing battery inks would require multiple print stations to reach the desirable electrode thicknesses. Although the flexographic printing is a fast, adaptable technique suitable for continuous manufacturing processes, it suffers from high cost and long setup time. Recently, Wang et al. developed MnO2 cathode inks for a flexographic-printed zinc (Zn)-based battery (Figure 2f,g).11 The MnO2 inks were repeatedly printed 10 times to reach sufficient thicknesses (20 μm) of the resulting electrodes. The inks were prepared to show a shear-thinning behavior of viscosity, which is important for ink transfer from the flexographic printing pads to the substrates. Digital printing (including high-resolution jet and 3D printing), which is a noncontact and additive manufacturing process, can create complex-shaped objects.6 Note that this printing technique is effective in constructing multiple battery arrays or battery-integrated devices with microscale resolutions. Inkjet printing is a noncontact, additive-based, and highprecision printing method that does not require the use of predesigned masks and can create versatile patterns through programmable digital files. Inkjet printing requires low-viscosity inks with sufficiently low surface tension to facilitate the jetting process (Figure 3a).5,6,15,16 Colloidal suspensions, hydrogels, and polymer solutions are good candidates to allow continuous
Current Status and Challenges. To successfully develop printed batteries, battery component inks with well-tuned rheological
To successfully develop printed batteries, battery component inks with well-tuned rheological properties and dispersion states must be synthesized and then combined with suitable printing techniques, which should be conducted based on an in-depth consideration of the electrochemical performances and safety tolerances of the resulting batteries. properties and dispersion states must be synthesized and then combined with suitable printing techniques, which should be conducted based on an in-depth consideration of the electrochemical performances and safety tolerances of the resulting batteries. A research strategy for the development of printed batteries is conceptually provided in Figure 1.
Figure 1. Research strategy for development of printed batteries: (i) design of battery shape/configuration, (ii) synthesis of battery component inks, and (iii) adoption of suitable printing techniques. The key-enabling technologies are based on the rheology/ printability of the battery component inks and the electrochemical performance/safety tolerance of the resulting printed batteries.
Printing Techniques. Printing techniques exploited for the fabrication of printed batteries are highly dependent on the rheological properties of the battery component inks. Main advantages and disadvantages of various printing techniques presented herein are summarized in Table S1. Conventional printing methods (e.g., screen, stencil, flexographic, gravure, and spray printing), which are known as low-cost, high-throughput processes, have been commercially adopted in various application fields and can be readily combined with various inks with different rheological properties. However, these printing techniques often require the use of prepatterned masks, thus hampering the diversification of form factors. 221
DOI: 10.1021/acsenergylett.7b01086 ACS Energy Lett. 2018, 3, 220−236
ACS Energy Letters
Review
Figure 2. Printed batteries fabricated through stencil, spraying, and flexographic printing techniques. Schematic illustrations of (a) stencil, (b) spray, and (c) flexographic printing techniques. (Reprinted with permission from ref 6. Copyright 2015 Wiley.) (d) Stepwise stencil printing procedure for the fabrication of printed solid-state Li-ion batteries. (Reprinted with permission from ref 9. Copyright 2015 American Chemical Society.) (e) Direct fabrication of a Li-ion battery on the surface of interest by sequentially spraying a component paint stencil mask tailored to the desired geometry and surface. (Reprinted with permission from ref 10. Copyright 2012 Nature Publishing Group.) (f) Multistage flexographic printing process for the large-scale production of Zn-based batteries. (g) Printing quality of MnO2 cathode inks with different polymeric binder solutions on stainless steel foils. (Reprinted with permission from ref 11. Copyright 2014 Elsevier.)
extrusion of filaments due to their well-tailored viscoelasticities under external stimuli (i.e., pressure, heat, and electromagnetic field), leading to the formation of complex-structured power sources. A major challenge of inkjet printing lies in long-term durability because the narrow inkjet nozzles are prone to clogging. Moreover, inkjet printing is relatively slow compared to other printing techniques, and the printed layers are thin (
