Current Status and Challenges in Printed Batteries: Toward Form


Current Status and Challenges in Printed Batteries: Toward Form...

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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 ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01086 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on December 16, 2017

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ACS Energy Letters

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

AUTHOR INFORMATION Corresponding Author *S.Y.L.: E-mail: [email protected] Tel: (+82)522172948. Website: http://syleek.unist.ac.kr

[†] The authors equally contributed to this work.

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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 perspective provides a 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.

TOC GRAPHICS

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ACS Energy Letters

Forthcoming 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, 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 pre-designed forms.7-8 Research directions on printed batteries is 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

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techniques. These research directions have the ultimate goal of eventually enabling the application of so-called “all-printed-batteries”. This perspective describes the current status and challenges of the 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.

CURRENT STATUS AND CHALLENGES. To successfully develop printed batteries, battery component inks with well-tuned rheological properties and dispersion state 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.

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

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ACS Energy Letters

rheological properties. However, these printing techniques often require the use of prepatterned masks, thus hampering the diversification of form factors.

Figure 2.

Screen or stencil printing5-6 is an on-contact process in which the ink is squeezed through pre-patterned 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 pre-patterned 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 pre-patterned masks (Figure 2b). Because the fine droplets are deposited via air, this technique allows the printing onto uneven/curved surfaces and the formation of thin layers over large areas. Additionally, for inks with low-vapor-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 non-flat 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 5 ACS Paragon Plus Environment

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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 m. 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 MnO 2 cathode inks for a flexographicprinted zinc (Zn)-based battery (Figure 2f and g).11 The MnO2 inks were repeatedly printed 10 times to reach sufficient thicknesses (20 m) 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 non-contact 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.

Figure 3.

Inkjet printing is a non-contact, additive-based, and high-precision printing method that does not require the use of pre-designed masks, and can create versatile patterns through programmable digital files. Inkjet printing requires low-viscosity inks with sufficiently low 6 ACS Paragon Plus Environment

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ACS Energy Letters

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 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 the 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 (< 2-4 μm), which limits its use for the fabrication of high-loading, thick battery electrodes. Meanwhile, inkjet printing is effective in printing microbatteries and thin-film batteries owing to its high-resolution scale. Durou et al. demonstrated an inkjet-printed micro-supercapacitor (Figure 3c and d).13 3D printing systems consist of ink syringes that deposit inks over substrates (Figure 3b).12 The ink is printed in the form of filaments or drops by modulating the pressure in the ink barrel. 17 Preparing inks for the dispenser printing is more difficult compared to preparations for other printing methods. Battery electrode inks are a mixture of electrode active particles, conductive additives, binders and suitable solvents. Due to the large particle-size distribution and the different densities between the particles and solvents, the inks may clog the needle during printing. The solvent fraction in the ink should be kept to a minimum to prevent sedimentation, and the active particles are ball milled to reduce the average particle size. Therefore, fine-tuning the rheological properties is important to ensure that the printed ink smoothly flows through the printing nozzles and obtains structural integrity after ejection from the nozzles. Driven by the non-contact nature of dispenser printing, the ink can be printed over uneven and irregular surfaces, which is difficult to achieve with the previously mentioned on-contact printing methods. 3D-printed Li-ion microbatteries14,18 have been prepared with high-aspect-ratio electrode arrays interdigitated on a sub-millimeter scale (Figure 3e-g). To secure facile printing and structural integrity after printing, a solvent system with graded volatility was applied. 7 ACS Paragon Plus Environment

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However, a high-temperature post treatment was required during fabrication to eliminate residual solvent from the 3D-printed electrodes, which remains a critical obstacle to practical applications

Preparation of battery component inks. Battery component inks should be rationally designed to secure process compatibility with printing technologies as well as electrochemical performance. Key requirements for battery component inks include: (i) component dispersion, (ii) rheological properties (i.e., viscosity and viscoelasticity) tailored for the specific printing process, (iii) structural/dimensional stability after printing (e.g., cohesion between particles, adhesion with the substrate and mechanical tolerance upon external stress) and (iv) electrochemical performance of the resulting printed battery. For thin-film printed batteries, battery component inks should have low viscosity and surface tension. Along with ensuring that the electrode inks possess long shelf life, electrode component aggregation should be prevented to circumvent clogging problems in the printing nozzles.19 A small amount of surfactant is often added to improve particle dispersion state and to tune the electrode ink surface tension.20 The surfactant molecules preferentially bind to particle surfaces and increase the steric repulsion between particles, thus suppressing particle coagulation in the battery component inks.20 In the fabrication of thick-film printed batteries and in extrusion-based 3D printing, concentrated battery component inks should be used. Two important issues should be addressed to formulate the concentrated inks. First, the viscoelastic properties should be controllable to ensure flow under the application of shear stress. Second, the mechanical stiffness and strength of inks should be sufficiently high to support the entire structure during the ink deposition and rapid solidification processes. These criteria require proper control of the ink formulations and rheological properties to generate a stable suspension that promotes a

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ACS Energy Letters

fluid-to-gel transition, which allows for the simultaneous retention of printed shapes and fusion with previously deposited layers. (i) Printed electrodes. To develop printed electrodes with various form factors and reliable physical/electrochemical properties, controlling the rheological properties of the electrode inks is an important prerequisite, along with the rational design of electrode active materials. Details on the theoretical understanding of the rheological behaviors of the electrode inks, along with their relationship with coating and printing techniques, have been described in previously reported review papers.6,17,21 Printed electrodes are typically composed of electrochemically active materials and electrically conductive additives in the presence of binders. Note that the materials, compositions, and rheological properties of electrode inks should be tailored to ensure process compatibility with the chosen printing process. Information on materials and formulations of various electrode inks is provided based on the battery system in Table 1.

Table 1.

Figure 4.

Conductive additives are used to facilitate electron transport in electrodes. Among the readily available conductive additives, carbonaceous substances, such as carbon nanotubes (CNTs) and graphenes, have been extensively explored due to their high electrical conductivities and affordable electrochemical capacitances, which are beneficial for electrical double-layer capacitors (EDLCs).20 A major challenge facing the use of carbonaceous substances is their dispersion in water and other polar solvents. In particular, for CNTs, strong inter-tube affinity and intrinsic hydrophobicity pose a stringent problem to securing good 9 ACS Paragon Plus Environment

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dispersion.7,20 In most studies, the dispersion of CNTs has been achieved with the assistance of a CNT-dispersing agent, such as sodium dodecylbenzene sulfonate (SDBS), or by CNT surface functionalization (Figure 4a and b).7,20,36,37 Note that these additional procedures for CNT dispersion should not impair the intrinsic electrical conductivity of CNTs. To electrochemically activate electrodes, ion and electron pathways in electrodes must be simultaneously guaranteed. As shown in Table 1, one interesting finding is that most electrode inks contain solely the electrically conductive component (represented by carbon-based additives) without an ionically conductive component. Consequently, the resulting printed electrodes should absorb liquid electrolytes after assembly with separator membranes and packaging substances. Recently, Lee et al. demonstrated a new electrode slurry that afforded mixed electron/ion conduction.9 The electrode slurries were composed of an electrochemically active powder, carbon black conductive additive and ionically conductive matrix (i.e., UVcrosslinkable triacrylate monomer and thermally tolerant liquid-state electrolyte). Compared to previously reported systems, these printed electrodes already contained the electrolyte and carbon conductive additive, resulting in the elimination of the liquid-electrolyte injection step and traditional processing solvents, such as water and N-methyl-2-pyrrolidone (NMP). The rheological properties (i.e., thixotropic fluid behavior) of the electrode inks were well tuned to allow a stencil printing process (Figure 4c). The self-standing, flexible printed electrodes were successfully fabricated through stencil printing followed by UV irradiation (Figure 4d). Current collectors, along with electrochemically active materials, are an important component to enable reliable electrochemical reactions in battery electrodes. To develop printed electrodes with design diversity, current collectors and other battery components should also be printable. The most commonly used current collectors in commercial electrodes are metallic foils; however, their predetermined shape and mechanical stiffness are impediments to diversifying form factors of printed batteries. Unfortunately, only a few works have reported 10 ACS Paragon Plus Environment

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ACS Energy Letters

printed current collectors. Cui et al. presented a CNT-coated conductive paper current collector (Figure 5a).38 Another approach was the utilization of a spray-coating method to produce a conductive CNT current collector (Figure 5b).10 In addition to CNT current collectors, a spraycoated metal current collector was prepared using copper (Cu) nanowire (NW) inks, which indicated the potential applicability of metallic inks as printed current collectors. Substrates for printing electrode components also play a vital role in fabricating high-precision printed batteries. For example, printing on so-called “wetting substrates”, such as conventional paper or textiles, tends to provoke random spreading of ink droplets due to the uncontrollable capillary force created by the inhomogeneously distributed/several-hundred-micrometer-sized pores of the substrates.39,40 Meanwhile, “non-wetting substrates”, such as non-porous polyethylene terephthalate (PET) films, often result in ring-like deposition (i.e., coffee-ring formation) of ink particles39 as well as poor adhesion between the printed products and the substrates.19 To resolve these substrate-triggered printing failures, previous works have been primarily devoted to tuning ink materials and formulations; however, these methods often limit the ink chemistry selection. As a facile and efficient approach to address this substrate issue, Lee et al. demonstrated an inkjet-printed cellulose nanofibril (CNF) layer on commercial A4 paper (Figure 5c).41 The CNF layer was designed to exhibit a nanoporous structure and uniform surface roughness, combining advantageous attributes of “wetting” and “non-wetting” substrates (Figure 5d). Consequently, the CNF layer contributed to fabricating high-resolution printed images on conventional A4 paper.

Figure 5.

(ii) Printed electrolytes and separator membranes. Liquid electrolytes pose serious limitations in cell assembly because they require packaging materials to avoid leakage and 11 ACS Paragon Plus Environment

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separator membranes to prevent electrical contact between the electrodes, which subsequently limits the form factors and design flexibility of conventional batteries. Unfortunately, in most printed batteries reported to date, separator membranes and electrolytes have not been a center of attention compared to electrodes. Subramanian et al. developed a UV-cured gel-type separator membrane through a stencil printing process (Figure 6a),32 in which crosslinked poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) were incorporated to control the rheological properties of the membrane inks and improve the mechanical stability of the resulting separator membrane. Ajayan et al. fabricated a spray-printed separator membrane.10 The membrane ink was composed of poly(vinylidene fluoride-hexafluoropropylene) (PVdFHFP)/poly(methylmethacrylate) (PMMA) and fumed SiO2 powder in an acetone/N,Ndimethylformamide (DMF) solvent mixture. The resulting separator membrane possessed a fibrous morphology with high porosity, which was beneficial for absorbing liquid electrolyte. Durstock et al. reported a 3D-printed separator membrane using a phase inversion technique (Figure 6b).42 The ink, which consisted of PVdF and Al2O3 nanoparticles in a NMP (good solvent) and glycerol (weak nonsolvent) solvent mixture, was used to form the porous structure during the phase inversion. Lee et al. demonstrated an imprintable and shape-conformable gel polymer electrolyte.43 The printed solid-state electrolyte was composed of a UV-crosslinked polymer network as a mechanical framework, high-boiling point electrolyte and Al2O3 nanoparticles as a spacer/viscosity-controlling agent (Figure 6c). The high-fidelity, shapeconformable polymer electrolyte was achieved by constructing a maze-like 3D structured solidstate electrolyte via UV-assisted nanoimprint lithography (UV-NIL), in which the rheologically tuned electrolyte pastes were imprinted onto a PDMS stamp with maze patterns and solidified after UV irradiation exposure (Figure 6d). Similar approaches have been applied to the development of ionic-liquid-based solid-state electrolytes. An ionic liquid was mixed

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ACS Energy Letters

with a UV-curable acrylate monomer (Figure 6e) 41 or confined in a silica matrix via a sol-gel reaction (Figure 6f) 22, leading to printed solid-state electrolytes.

Figure 6.

Electrochemical performances of printed batteries. The electrochemical performances of printed batteries, which highly depend on their potential applications, should be quantitatively evaluated and tailored. In many previous studies, the electrochemical performances of printed batteries have been expressed based on the weight/volume of the electrode active materials (or electrodes). Other components of printed batteries have not been included, which often misleads the cell performance information. Generally, printed batteries tend to employ soft and lightweight packaging substances, which could be beneficial for the gravimetric (and volumetric) energy/power densities of the resulting batteries. Meanwhile, energy/power densities based on cell areas, which are affected by the cell architecture and mass loading of the electrode active materials, are also important electrochemical characteristics of the printed batteries. A Ragone plot, which represents the areal energy/power densities, of previously reported printed batteries is depicted in Figure 7. To deliver more comprehensive information, voltage profiles of some representative battery systems9,10,14,18,32,44-48 are depicted in Figure S1. Many previous works have not reached a satisfactory level of cell performance for practical application when compared to current state-of-the-art commercial batteries. Another important requirement for rechargeable batteries is their cycling stability, which eventually affects performance reliability/sustainability of battery-powered electronic devices. Cycling performance of the printed batteries strongly depends on electrochemical stability of cell components, internal 13 ACS Paragon Plus Environment

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ohmic resistance, and cell packaging. In particular, significant attention should be paid to the internal ohmic resistance, which is influenced by electronic conduction pathways of electrodes, interfacial contact resistance between electrode active materials and current collectors, and ionic resistance. Therefore, rational design of cell components and structure based on understanding of the internal ohmic resistance is highly required. In addition, cell packaging issue should be also resolved to enable wide application of the printed batteries. For example, printed batteries with complex shapes and multidimensional structure may be easily disrupted and lose their electrochemical performance upon exposure to mechanical deformation/thermal stresses, which thus demands robust packaging substances and sealing methods to resolve the aforementioned problems. Systematic and elaborate approaches should be pursued to further improve the electrochemical performances of printed batteries.

Figure 7.

Recent advances in printed battery systems and their applications. Printed batteries can be easily integrated with a variety of electronic devices. The potential application fields of printed batteries include sensors, radio frequency identification (RFID) tags, smart cards/toys and biomedical patches/devices. These new applications feature thin films, shape diversity, mechanical flexibility, and sometimes disposability, which are important development guidelines for printed batteries. In particular, for their potential application to flexible/wearable electronics, printed batteries should withstand serious mechanical deformations such as bending, winding, twisting, and folding. In addition to the materials and associated electrochemistry, the design and configuration of cell components are also crucial factors for the realization of high-performance printed batteries with aesthetic versatility. In addition to the vertically stacked configuration, which is commonly used in conventional 14 ACS Paragon Plus Environment

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ACS Energy Letters

batteries, an in-plane configuration can also be applied to batteries, in which the anode and the cathode are positioned side-by-side. This in-plane configuration is effective for widening the printing process window owing to the process simplicity and a minimal risk of short-circuit failure.6 Meanwhile, many previously reported printed batteries have utilized Zn-based electrochemistry.11,29-35,44-47,49,50 Other battery systems, such as supercapacitors,41,51-56 Li-based batteries (such as Li-ion,9,10,14,18,23-26,48 Li-S,27 and Li-O228 batteries) and polymer batteries57 for potential use as printed power sources have recently been explored. The types, components, printing techniques and electrochemical characteristics of printed battery systems are summarized in Table 2.

Table 2.

Below is an overview of various printed batteries in terms of the electrochemical cell system and design diversity. (i) Zn-based batteries. Zn-based batteries are known as low cost, safe power sources due to naturally abundant zinc metal and aqueous electrolytes. 58 In particular, the use of aqueous electrolytes allows the fabrication of printed Zn-based batteries in ambient conditions without concerns on dehumidification which is an essential prerequisite for preparation of Li-ion batteries. However, relatively low cell voltage (~ 1.5 V) of Zn-based batteries hinders their application to high voltage-operation electronic devices.59,60 Also, their charge/discharge cyclability does not yet reach a satisfactory level, as compared to other rechargeable batteries. A roll-to-roll (R2R) processable Zn/MnO2 battery was fabricated using a fluidic cathode paste (Figure 8a).49 The prototype Zn/MnO2 battery consisted of a Zn foil as an anode and current collector, a separator membrane soaked with electrolyte solution and a solution-processable MnO2 paste as a cathode. The MnO2 paste contained MnO2 active material, a conductive CNT 15 ACS Paragon Plus Environment

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network and electrolyte solution, wherein the CNTs acted as a current collector and conductive agent. Steingart et al. reported printed, flexible Zn/MnO2 batteries based on Nylon meshcontaining electrodes (Figure 8b).45 The Nylon mesh-embedded electrodes were fabricated using a stencil printing process. Zn and MnO2 slurries were sequentially stencil-printed, followed by printing a silver (Ag) ink as a current collector. Finally, the battery was assembled by stacking the mesh-embedded electrodes and a polyacrylic acid gel polymer electrolytesoaked Nylon mesh separator (Figure 8c). The mesh in the electrodes played a viable role in the mechanical flexibility of the Zn/MnO2 battery. No loss in the cell performance was observed after repeated bending deformation. However, the aforementioned Zn/MnO2 batteries did not achieve reliable charge/discharge cyclability. Wright et al. reported a rechargeable Zn/MnO2 microbattery through a direct-write dispenser printing technique (Figure 8d and e).46 The shape change in the Zn electrode, dendrite formation and reaction product dissolution into the electrolyte remain formidable challenges in securing meaningful electrochemical rechargeability. To address these issues, ionic liquids were used as alternative electrolytes.50 Zn/Ag batteries have attracted considerable attention due to their high energy density and air stability.31 However, the high material cost of Ag poses a barrier to extending the applications of Zn/Ag batteries. Printing technology may be an efficient way to reduce the mass consumption of Ag, thereby boosting the development of Zn/Ag batteries as promising power source systems. Wang et al. reported a rechargeable, skin-worn Zn/Ag tattoo battery, which consisted of screen-printed electrodes, temporary tattoo paper, an alkaline gel electrolyte and a PDMS cover (Figure 8f).44 The application of the tattoo battery onto human skin as a shapeconformable power source was successfully achieved. Progress of stretchable electronics, which are beyond simple deformable devices, is often hindered by bulky and rigid power sources. Wang et al. reported an all-printed stretchable Zn/Ag rechargeable battery using a lowcost screen printing of highly elastic, conductive inks (Figure 8g).47 The novelty of this work 16 ACS Paragon Plus Environment

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ACS Energy Letters

hinged on the hyperelastic properties (1300% elongation) of the polystyrene-blockpolyisoprene-block-polystyrene (SIS) as a new elastic binder for stretchable batteries.

Figure 8.

(ii) Supercapacitors. Supercapacitors, specifically electrical double layer capacitors (EDLCs), are based on adsorption/desorption phenomena of charges in electrical double layers of high-surface-area electrode materials. Owing to the non-faradaic reaction mechanism, supercapacitors show high power density (1-10 kW kg-1), long cycle life, and high electrochemical efficiency. However, supercapacitors suffer from low energy density, low cell potential, and high self-discharge rate.61 Printed supercapacitors have been fabricated by various printing techniques, including screen printing51,62-64, gravure printing65, inkjet printing41,51,52,66-76, spraying54,55,77,78, and dispenser printing79-81. Cui et al. reported printed, thin-film supercapacitors based on single-walled carbon nanotube (SWCNT) networks (serving as both the electrodes and current collector) and a printed gel polymer electrolyte (Figure 9a and b).54 Gogotsi et al. demonstrated textile supercapacitors based on knitted carbon fibers and an activated carbon (AC) ink (Figure 9c).56 The textile supercapacitor maintained good electrochemical performance while subjected to bending and stretching. However, some loss of capacitance under the bent state (bending angle = 180°) was observed, indicating that further optimization of electrode materials for the textile structure is needed. Conductive polymers (such as polyaniline (PANI) and PEDOT:PSS) and 1D-/2Dcarbon materials (such as CNTs, graphenes, and graphene oxides) were used as a building block material for printed supercapacitors due to their high capacitance and compatibility with solution processes.65-78,80,81 Müllen et al. demonstrated a flexible micro-supercapacitor fabricated with exfoliated graphene/PEDOT:PSS hybrid inks (Figure 9d). 67 The PEDOT:PSS 17 ACS Paragon Plus Environment

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served as an electroconductive network and also a surfactant, thus enabling the hybrid inks to be used for spray-coating and inkjet printing. The resultant micro-supercapacitor showed high capacitance retention under repeated mechanical stress (bending radius = 5 mm, 1000 cycles). Furthermore, the micro-supercapacitor can be attached to a human finger or other body parts, demonstrating its exceptional flexibility and compatibility with wearable electronics. Even upon being subjected to twisting and folding, the deformed micro-supercapacitor presented a slight loss (~ 1%) of capacitance compared to the flat-state one. Li et al. presented 3D-printed graphene composite aerogels (3D-GCA) consisting of graphene oxide (GO) and graphene nanoplatelets (GNP) for supercapacitor electrodes (Figure 9e). 81 To enable the 3D printing process, hydrophilic fumed silica particles were added to the GO-GNP electrode ink to control rheological properties. The GO-GNP ink was extruded through a micronozzle to pattern 3D structures. Subsequently, the 3D structures were converted to aerogels for use as a supercapacitor electrode. Lee et al. demonstrated solid-state flexible supercapacitors directly fabricated on conventional A4 paper using a household desktop inkjet printer (Figure 9f).41 The novel supercapacitors appeared as typical inkjet-printed letters or patterns commonly found in office documents. The inkjet-printed supercapacitors were easily connected in series or in parallel without the extra aid of metallic interconnects, thereby enabling user-customized control of the cell voltage and capacitance. In addition, inkjet-printed supercapacitors with computer-designed artistic patterns and letters were aesthetically combined with other inkjetprinted art images and smart glass cups (Figure 9g-i).

Figure 9.

(iii) Li-ion batteries. Li-ion batteries are most widely used in various application fields due to their lightweight, high average cell potential (~ 3.7 V) and high energy densities (~ 260 18 ACS Paragon Plus Environment

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Wh kg-1, ~ 780 Wh L-1).82 Driven by this exceptional electrochemical performance, Li-ion batteries can be also suggested as a promising power source for flexible/wearable electronics. However, the use of flammable organic electrolytes causes unwanted problems such as safety failures and swelling issues. Furthermore, a stringent requirement for low-humidity manufacturing conditions hampers the adoption of diverse printing techniques. Ajayan et al. reported a painted battery using a spray printing technique.10 The battery components, including the electrodes, separator membranes and current collector, were prepared in the form of paint solutions and then sequentially spray-coated to construct the painted battery. The aforementioned battery component paints were sprayed through shadow masks with predetermined geometries. Additionally, the painted battery was integrated with a photovoltaic (PV) panel to develop an energy conversion/storage hybrid device for various outdoor applications (Figure 10a). The 3D battery based on the micro- and nanostructured electrodes could be an appealing power source for microscale devices. The 3D-printed microbatteries were prepared with high-aspect-ratio electrode arrays interdigitated on a sub-millimeter scale (Figure 10b and c).13,18 However, the relatively low power density, which is ascribed to the large interdigital distance needed to avoid short-circuit problems between the electrodes, remains a formidable obstacle to practical applications. Moreover, a high-temperature post treatment was required during fabrication of the 3D-printed electrodes, thus imposing serious restrictions for broadening the material design and formulation. Many of the abovementioned previous studies have required pre-designed masks, supplementary spatial alignments and liquid electrolytes as ionic media. Particularly, the use of liquid electrolytes results in unwanted problems, such as safety failure, an additional electrolyte injection step and packaging limitations. To address this long-standing issue, Lee et al. reported printed, solid-state rechargeable Li-ion batteries.9 A UV-cured solid-state composite electrolyte was incorporated as the printed separator membrane/electrolyte. 19 ACS Paragon Plus Environment

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Consequently, solvent drying processes and electrolyte injection steps were removed in the cell fabrication. The cell components were sequentially printed, followed by UV irradiation for solidification, resulting in the fabrication of all-solid-state printed batteries that can be seamlessly integrated with various objects, such as paper-made eyeglasses and glass cups (Figure 10d and e). The charge/discharge behavior of the letter-shaped, printed cells was measured under repeated bending deformation (bending radius = 5, 10, and 15 mm). No loss in the electrochemical performance was observed (Figure 10e). Additionally, the printed Liion batteries were seamlessly integrated with miniaturized Si PVs, resulting in monolithically integrated photorechargeable portable power sources (Figure 10f and g).48 The Si PV–Li-ion battery device exhibited considerable improvements in photocharging (rapid charging in less than 2 min), discharge rate capability, and photocharge/galvanostatic discharge cycling performance compared to conventional PVs or Li-ion batteries alone.

Figure 10.

FUTURE DEVELOPMENT DIRECTIONS AND OUTLOOK

Figure 11.

This perspective briefly reviewed some pioneering works of printed batteries reported to date. Based on the understanding of these previous studies, the future development directions and outlook of printed batteries are described below.

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Toward design versatility and performance improvements of printed batteries (Figure 11a). To enrich the form factors of printed batteries, all the battery components (ranging from the electrodes to packaging materials) should exhibit shape diversity and flexibility. However, most previously reported studies on printed batteries have not paid much attention to current collectors and packaging materials compared to electrodes, electrolytes and separator membranes. To reach the ultimate goal of so-called “all-printed-batteries”, printed current collectors and packaging substances should also be developed, which will eventually enable unprecedented advances in their form factors and aesthetic versatility. The rational design and synthesis of metal powder (or carbon-based) inks with decent electronic conductivities and tunable rheological properties can contribute to the realization of printed current collectors. Packaging or encapsulation introduces an additional set of requirements that are in conflict, including air/moisture barrier, chemical resistance, flexibility and fatigue life, 83 which should be resolved to produce printed packaging materials. The introduction of new printing techniques facilitates the design diversity of printed batteries. Among various printing processes, 3D printing holds great promise from the viewpoint of creating products with various form factors. 3D printing techniques include conventional stereolithography (SLA), selective laser sintering (SLS), fused deposition modeling

(FDM),

extrusion-based

direct-writing,

UV-assisted

dispensing,12,17

and

electrohydrodynamic (EHD) printing, which differ in their solidification methods or dispensing mechanisms of the printed inks. Despite their promising potential, the synthesis of multicomponent inks (fulfilling both rheological and electrochemical requirements) continues to pose a formidable challenge. We envision that a combination of advanced 3D printing techniques with well-designed inks will allow for the fabrication of 3D printed batteries featuring unprecedented shape diversity and micro/nanoscale dimensions, thus enabling a series of aesthetic breakthroughs for printed batteries. 21 ACS Paragon Plus Environment

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The electrochemical performances of printed batteries can be improved by incorporating new electrochemically active materials, such as high-capacity/high-voltage electrode powders and high-conductive/nonflammable electrolytes. To successfully achieve this goal, the dispersion and rheological properties of electrode/electrolyte inks should be secured as a prerequisite. Considering that the battery performance is basically governed by the combined transport of electrons and ions, the electrode/electrolyte inks should be designed to enable the construction of well-developed electron-/ion-transport channels in the resulting printed batteries. Colloidal chemistry can serve as a powerful tool in the design/synthesis of electrode/electrolyte inks. Controlling the ionic strength in a colloidal suspension leads to charge screening of colloidal particles, thus forming interconnected particle networks. 84-86 For example, the interconnected particle networks built by ionic strength control can act as electron conduction channels in printed electrodes, while the interstitial voids (after being filled with electrolyte) formed between the particles can allow for ion transport, 86,87 thereby providing 3D bicontinuous electron-/ion-conduction pathways.

Toward monolithic integration of printed batteries with electronic devices and energyharvesting systems (Figure 11b). The exceptional form factors and adaptability make printed batteries promising power sources for highly integrated, multifunctional applications. To achieve this goal, an in-depth consideration of both the integration techniques and battery specifications (e.g., capacity, voltage, current and others) is needed. Note that battery configuration selection (in series and in parallel) as well as proper electrochemistry selection should be considered to successfully operate the integrated devices. Here, sharing packaging substances and current collectors with electronic devices could be a distinct advantage of printed batteries, which could reduce the overall volume/weight and widen the design diversity of the resulting integrated devices. For example, printed batteries can be directly integrated into 22 ACS Paragon Plus Environment

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printed electronics without the use of external battery holders or interconnection wires, leading to the realization of a new class of power-source-embedded electronics. Meanwhile, in contrast to conventional brittle and rigid electronic devices, so-called soft electronics, such as implantable/patchable electronics, e-textiles, optics and paper-like displays, are often seamlessly integrated onto curvilinear substrates. To enable the monolithic integration of printed batteries with soft electronics, conformal printing techniques and the synthesis of battery component inks with well-tailored rheological properties are required. Another attractive application is the combination of printed batteries with energyharvesting systems, which could effectively address ever-increasing challenges related to high energy/power density and ubiquitous mobility. For example, a hybrid PV–printed battery system can continuously operate electronic devices under light illumination on demand, exhibiting promising potential as a sustainable energy source that can resolve both the energy density problems of batteries and energy storage concerns of PVs. 48,88 Printed batteries have garnered considerable attention as an innovative technology that breaks common beliefs on power sources from a cell design and architecture viewpoint. This perspective described an overview of printed batteries, with a focus on battery component inks, printing technologies, electrochemical performances, shape conformabilities/form factors and device integration. Future research on printed batteries should be mainly directed to the synthesis of new battery component inks (based on chemical/rheological considerations) and the adoption of newly emerging printing technologies, such as high-fidelity inkjet, EHD and 3D printing. We envision that printed batteries will open a new avenue toward form-factorfree, monolithic-integrated energy storage systems with object-tailored design versatility, which will play a vital role as promising power sources in upcoming flexible/wearable electronics, IoT and ubiquitous energy applications.

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AUTHOR INFORMATION Corresponding Author *S.Y.L.: E-mail: [email protected] Tel: (+82)522172948. Website: http://syleek.unist.ac.kr Notes The authors declare no competing financial interest. Supporting Information Available: Advantages/disadvantages of various printing techniques; Voltage profiles of representative battery systems.

Biographies Keun-Ho Choi is a Post Doc. in department of energy engineering at Ulsan National Institute of Science and Technology (UNIST). He received his Ph.D. in department of energy engineering at UNIST under supervision of Prof. Sang-Young Lee. His research interests are focused on printed power sources. David B. Ahn is a Ph.D. candidate in department of energy engineering at UNIST. His research interests lie in micro/nanoscale printed power sources fabricated through high-fidelity printing techniques. Sang-Young Lee is a professor in department of energy engineering at UNIST. His research interests include printed power sources, paper batteries, solid-state electrolytes, nanomatstructured electrodes and permselective separator membranes.

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ACKNOWLEDGMENT This work was supported by the Basic Science Research Program (2015R1A2A1A01003474) and Wearable Platform Materials Technology Center (2016R1A5A1009926) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning. This work was also supported by the Industry Technology Development Program (10080540) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea). This work was also supported by the Korea Forest Research Institute (Grant No. FP 04002016-01).

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(52) Ujjain, S. K.; Bhatia, R.; Ahuja, P.; Attri, P. Highly Conductive Aromatic Functionalized Multi-Walled Carbon Nanotube for Inkjet Printable High Performance Supercapacitor Electrodes. PLoS One 2015, 10, e0131475. (53) Wang, S. L.; Liu, N. S.; Tao, J. Y.; Yang, C. X.; Liu, W. J.; Shi, Y. L.; Wang, Y. M.; Su, J.; Li, L. Y.; Gao, Y. H. Inkjet Printing of Conductive Patterns and Supercapacitors Using a Multi-Walled Carbon Nanotube/Ag Nanoparticle Based Ink. J. Mater. Chem. A 2015, 3, 2407-2413. (54) Kaempgen, M.; Chan, C. K.; Ma, J.; Cui, Y.; Gruner, G. Printable Thin Film Supercapacitors Using Single-Walled Carbon Nanotubes. Nano Lett. 2009, 9, 18721876. (55) Huang, C.; Young, N. P.; Grant, P. S. Spray Processing of TiO2 Nanoparticle/Ionomer Coatings on Carbon Nanotube Scaffolds for Solid-State Supercapacitors. J. Mater. Chem. A 2014, 2, 11022-11028. (56) Jost, K.; Stenger, D.; Perez, C. R.; McDonough, J. K.; Lian, K.; Gogotsi, Y.; Dion, G. Knitted and Screen Printed Carbon-Fiber Supercapacitors for Applications in Wearable Electronics. Energy Environ. Sci. 2013, 6, 2698-2705. (57) Tehrani, Z.; Korochkina, T.; Govindarajan, S.; Thomas, D. J.; O'Mahony, J.; Kettle, J.; Claypole, T. C.; Gethin, D. T. Ultra-Thin Flexible Screen Printed Rechargeable Polymer Battery for Wearable Electronic Applications. Org. Electron. 2015, 26, 386-394. (58) Li, Y.; Dai, H. Recent Advances in Zinc-Air Batteries. Chem. Soc. Rev. 2014, 43, 52575275. (59) Sambandan, S.; Lujan, R.; Arias, A. C.; Newman, C. R.; Facchetti, A. Electrical Stability of Inkjet-Patterned Organic Complementary Inverters Measured in Ambient Conditions. Appl. Phys. Lett. 2009, 94, 233307.

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(60) Ng, T. N.; Schwartz, D. E.; Lavery, L. L.; Whiting, G. L.; Russo, B.; Krusor, B.; Veres, J.; Broms, P.; Herlogsson, L.; Alam, N. et al. Scalable Printed Electronics: An Organic Decoder Addressing Ferroelectric Non-Volatile Memory. Sci. Rep. 2012, 2, 585. (61) Wang, G.; Zhang, L.; Zhang, J. A Review of Electrode Materials for Electrochemical Supercapacitors. Chem. Soc. Rev. 2012, 41, 797-828. (62) Keskinen, J.; Sivonen, E.; Jussila, S.; Bergelin, M.; Johansson, M.; Vaari, A.; Smolander, M. Printed Supercapacitors on Paperboard Substrate. Electrochim. Acta 2012, 85, 302306. (63) Pettersson, F.; Keskinen, J.; Remonen, T.; von Hertzen, L.; Jansson, E.; Tappura, K.; Zhang, Y.; Wilén, C. E.; Österbacka, R. Printed Environmentally Friendly Supercapacitors with Ionic Liquid Electrolytes on Paper. J. Power Sources 2014, 271, 298-304. (64) Zhang, H.; Qiao, Y.; Lu, Z. Fully Printed Ultraflexible Supercapacitor Supported by a Single-Textile Substrate. ACS Appl Mater Interfaces 2016, 8, 32317-32323. (65) Xiao, Y.; Huang, L.; Zhang, Q.; Xu, S.; Chen, Q.; Shi, W. Gravure Printing of Hybrid [email protected] Interdigitated Electrodes for Flexible Microsupercapacitors. Appl. Phys. Lett. 2015, 107, 013906. (66) Chen, P.; Chen, H.; Qiu, J.; Zhou, C. Inkjet Printing of Single-Walled Carbon Nanotube/RuO2 Nanowire Supercapacitors on Cloth Fabrics and Flexible Substrates. Nano Research 2010, 3, 594-603. (67) Liu, Z.; Wu, Z. S.; Yang, S.; Dong, R.; Feng, X.; Müllen, K. Ultraflexible in-Plane MicroSupercapacitors by Direct Printing of Solution-Processable Electrochemically Exfoliated Graphene. Adv. Mater. 2016, 28, 2217-2222. (68) Chi, K.; Zhang, Z.; Xi, J.; Huang, Y.; Xiao, F.; Wang, S.; Liu, Y. Freestanding Graphene Paper Supported Three-Dimensional Porous Graphene-Polyaniline Nanocomposite 32 ACS Paragon Plus Environment

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Synthesized by Inkjet Printing and in Flexible All-Solid-State Supercapacitor. ACS Appl Mater Interfaces 2014, 6, 16312-16319. (69) Xu, Y.; Hennig, I.; Freyberg, D.; James Strudwick, A.; Georg Schwab, M.; Weitz, T.; Chih-Pei Cha, K. Inkjet-Printed Energy Storage Device Using Graphene/Polyaniline Inks. J. Power Sources 2014, 248, 483-488. (70) Chiolerio, A.; Bocchini, S.; Porro, S. Inkjet Printed Negative Supercapacitors: Synthesis of Polyaniline-Based Inks, Doping Agent Effect, and Advanced Electronic Devices Applications. Adv. Funct. Mater. 2014, 24, 3375-3383. (71) Jung, H.; Ve Cheah, C.; Jeong, N.; Lee, J. Direct Printing and Reduction of Graphite Oxide for Flexible Supercapacitors. Appl. Phys. Lett. 2014, 105, 053902. (72) Li, J.; Mishukova, V.; Östling, M. All-Solid-State Micro-Supercapacitors Based on Inkjet Printed Graphene Electrodes. Appl. Phys. Lett. 2016, 109, 123901. (73) Cheng, T.; Zhang, Y.-Z.; Yi, J.-P.; Yang, L.; Zhang, J.-D.; Lai, W.-Y.; Huang, W. InkjetPrinted Flexible, Transparent and Aesthetic Energy Storage Devices Based on Pedot:Pss/Ag Grid Electrodes. J. Mater. Chem. A 2016, 4, 13754-13763. (74) Sundriyal, P.; Bhattacharya, S. Inkjet-Printed Electrodes on A4 Paper Substrates for Low-Cost, Disposable, and Flexible Asymmetric Supercapacitors. ACS Appl Mater Interfaces 2017, 9, 38507-38521. (75) Hyun, W. J.; Secor, E. B.; Kim, C.-H.; Hersam, M. C.; Francis, L. F.; Frisbie, C. D. Scalable, Self-Aligned Printing of Flexible Graphene Micro-Supercapacitors. Adv. Energy Mater. 2017, 7, 1700285. (76) Sollami Delekta, S.; Smith, A. D.; Li, J.; Ostling, M. Inkjet Printed Highly Transparent and Flexible Graphene Micro-Supercapacitors. Nanoscale 2017, 9, 6998-7005. (77) Wu, Z. S.; Liu, Z.; Parvez, K.; Feng, X.; Müllen, K. Ultrathin Printable Graphene Supercapacitors with AC Line-Filtering Performance. Adv. Mater. 2015, 27, 3669-3675. 33 ACS Paragon Plus Environment

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(78) Shi, X.; Wu, Z. S.; Qin, J.; Zheng, S.; Wang, S.; Zhou, F.; Sun, C.; Bao, X. GrapheneBased Linear Tandem Micro-Supercapacitors with Metal-Free Current Collectors and High-Voltage Output. Adv. Mater. 2017, 29, 1703034. (79) Ho, C. C.; Steingart, D.; Evans, J.; Wright, P. Tailoring Electrochemical Capacitor Energy Storage Using Direct Write Dispenser Printing. ECS Transactions 2008, 16, 3547. (80) Sun, G.; An, J.; Chua, C. K.; Pang, H.; Zhang, J.; Chen, P. Layer-by-Layer Printing of Laminated Graphene-Based Interdigitated Microelectrodes for Flexible Planar MicroSupercapacitors. Electrochem. Commun. 2015, 51, 33-36. (81) Zhu, C.; Liu, T.; Qian, F.; Han, T. Y.; Duoss, E. B.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A.; Li, Y. Supercapacitors Based on Three-Dimensional Hierarchical Graphene Aerogels with Periodic Macropores. Nano Lett. 2016, 16, 3448-3456. (82) Li, W.; Song, B.; Manthiram, A. High-Voltage Positive Electrode Materials for LithiumIon Batteries. Chem. Soc. Rev. 2017, 46, 3006-3059. (83) Cobb, C. L.; Ho, C. C. Additive Manufacturing: Rethinking Battery Design. Electrochem. Soc. Interface 2016, 25, 75-78. (84) Lewis, J. A. Colloidal Processing of Ceramics. J. Am. Ceram. Soc. 2000, 83, 2341-2359. (85) Youssry, M.; Madec, L.; Soudan, P.; Cerbelaud, M.; Guyomard, D.; Lestriez, B. Formulation of Flowable Anolyte for Redox Flow Batteries: Rheo-Electrical Study. J. Power Sources 2015, 274, 424-431. (86) Zhang, Y.; Narayanan, A.; Mugele, F.; Cohen Stuart, M. A.; Duits, M. H. G. Charge Inversion and Colloidal Stability of Carbon Black in Battery Electrolyte Solutions. Colloid Surface A 2016, 489, 461-468.

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(87) Duduta, M.; Ho, B.; Wood, V. C.; Limthongkul, P.; Brunini, V. E.; Carter, W. C.; Chiang, Y.-M. Semi-Solid Lithium Rechargeable Flow Battery. Adv. Energy Mater. 2011, 1, 511516. (88) Haight, R.; Haensch, W.; Friedman, D. Solar-Powering the Internet of Things. Science 2016, 353, 124-125.

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Figure Captions Figure 1. A 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.

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.)

Figure 3. Printed batteries fabricated through inkjet and 3D printing. Schematic representations of (a) inkjet (reprinted with permission from ref. 6. Copyright 2015 Wiley) and (b) 3D printing (direct ink writing) (reprinted with permission from ref. 12. Copyright 2016 Royal Society of Chemistry). (c) Schematic illustration of an inkjet-printed interdigital micro-supercapacitor. (d) Optical images of (left) a micro-supercapacitor and (right) the chips at low magnification. (Reprinted with permission from ref. 13. Copyright 2010 Elsevier.) (e) Schematic illustration 36 ACS Paragon Plus Environment

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of 3D-interdigitated microbattery architectures fabricated on a gold current collector by printing Li4Ti5O12 (LTO) and LiFePO4 (LFP) inks. (f) Scanning electron microscopy (SEM) images of printed/annealed 16-layer interdigitated LTO-LFP electrode architectures. (g) Areal capacity of a full cell containing 8-layer interdigitated LTO-LFP electrodes as a function of the charge/discharge cycle number. (Reprinted with permission from ref. 14. Copyright 2013 Wiley.)

Figure 4. Printed electrode inks with a focus on dispersibility and rheological control. (a) Different chemical structures of modified graphene inks. (Reprinted with permission from ref. 36. Copyright 2011 IOP Publishing.) (b) Functionalization of MWCNTs with carboxyl, hydroxyl, and carbonyl groups to ensure their dispersibility in water for inkjet printing. (Reprinted with permission from ref. 37. Copyright 2006 Wiley.) (c) Rheological properties of printable LFP electrode inks with an ionically conductive matrix and photographs of stencilprinted, letter-shaped LFP electrode inks. (d) Photographs showing the mechanical flexibility of the resulting printed LFP electrodes. (Reprinted with permission from ref. 9. Copyright 2015 American Chemical Society.)

Figure 5. Printed current collectors and substrates. (a) Meyer rod coating of CNT or Ag NW ink on commercial Xerox paper. (Reprinted with permission from ref. 38. Copyright 2009 National Academy of Sciences.) (b) Cross-sectional SEM image of a spray-printed Li-ion battery full cell containing the spray-printed CNT and Cu current collectors (scale bar is 100 μm). (Reprinted with permission from ref. 10. Copyright 2012 Nature Publishing Group.) (c) Schematic illustration of inkjet-printed supercapacitors with CNF layers on commercial A4 paper. (d) Effect of the substrate on the printability of the inkjet printing process. (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.) 37 ACS Paragon Plus Environment

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Figure 6. Printed electrolytes and separator membranes. (a) UV-cured gel-type separator membranes fabricated through a stencil printing process. (Reprinted with permission from ref. 32. Copyright 2015 Wiley.) (b) Components and viscosity of the inks for the fabrication of 3Dprinted separator membranes (photograph) through a phase inversion technique. (Reprinted with permission from ref. 42. Copyright 2017 Wiley.) (c) Conceptual illustration of an imprintable and shape-conformable composite gel polymer electrolyte. (d) SEM images of the maze-patterned composite gel polymer electrolyte fabricated via a UV-NIL technique (inset cross-sectional image). (Reprinted with permission from ref. 43. Copyright 2013 Wiley.) (e) Viscosity of the inkjet-printable electrolyte ink (composed of ionic liquid and UV-curable acrylate monomer) as a function of the shear rate. (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.) (f) Scheme of the ink jet printing process of an ionogel and SEM image of the LFP composite electrode obtained through printing (followed by polycondensation) of the ionogel precursors. (Reprinted with permission from ref. 22. Copyright 2015 Elsevier.)

Figure 7. A Ragone plot (based on areal energy/power densities) of previously reported printed batteries.

Figure 8. Zn-based printed batteries. (a) Proof-of-concept device constructed with a SWCNTfilm charge collector and MnO2-SWCNT mixture. (Reprinted with permission from ref. 49. Copyright 2007 American Institute of Physics.) (b) Schematic diagram of the fabrication of the mesh-embedded, printed MnO2 cathode and Zn anode with a printed Ag current collector. (c) (left) Schematic representation of a Zn/MnO2 alkaline battery with sandwich-type architecture and (right) photograph of a flexible Zn/MnO2 battery laminated inside a polyethylene pouch. 38 ACS Paragon Plus Environment

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(Reprinted with permission from ref. 45. Copyright 2011 Wiley.) (d) Schematic illustration of the printing process for a Zn–metal-oxide stacked microbattery and (e) SEM image of the resulting printed microbattery. (Reprinted with permission from ref. 46. Copyright 2010 IOP Publishing.) (f) Schematic diagram illustrating the different steps involved in the fabrication of the Zn/Ag cell. Prototype of the Zn/Ag cell on a temporary transfer tattoo support (inset photograph). (Reprinted with permission from ref. 44. Copyright 2014 Royal Society of Chemistry.) (g) Stepwise screen printing of a Zn/Ag battery on a stretchable textile using a SIS binder. Photographs of the sealed battery while being 0% stretched, twisted, indented, 100% stretched and biaxial stretched. (Reprinted with permission from ref. 47. Copyright 2017 Wiley.)

Figure 9. Printed supercapacitors. (a) SEM image of as-deposited SWCNT networks. (b) Thinfilm supercapacitor using sprayed SWCNT films on a PET substrate as electrodes and a PVA/H3PO4-based polymer electrolyte as both the electrolyte and separator. (Reprinted with permission from ref. 54. Copyright 2009 American Chemical Society). (c) Schematic of the assembled flexible devices composed of a carbon fiber current collector (this geometry holds true for knitted or woven devices), porous PTFE separator, silicotungstic acid (SiWA) polymer electrolyte and screen-printed AC ink. (Reprinted with permission from ref. 56. Copyright 2013 Royal Society of Chemistry.) (d) Photographs of ultrathin and flexible micro-supercapacitors. The micro-supercapacitors are thinner than a human hair, wearable on human hands, and conformable to bent fingers. (Reprinted with permission from ref. 67. Copyright 2016 Wiley.) (e) Schematic illustration of fabrication process of 3D-printed graphene composite aerogels (3D-GCA). (Reprinted with permission from ref. 81. Copyright 2016 American Chemical Society) (f) Schematic illustration of the stepwise fabrication procedure of the inkjet-printed supercapacitors and a photograph of the utilized desktop inkjet printer. Photographs of (g) the 39 ACS Paragon Plus Environment

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inkjet-printed, letter (‘‘BATTERY’’)-shaped supercapacitors (marked by a red box) and (h) traditional Korean ‘‘Taegeuk’’ symbol-like supercapacitors (marked by a red circle) that were seamlessly connected with the inkjet-printed electrical circuits and a light-emitting diode (LED) lamp. The inset is a photograph of the supercapacitor after being placed on a hot plate (set at 150 ºC) for 0.5 h. (i) (left) Photograph depicting the operation of a blue LED lamp in the smart cup (for cold water (~ 10 ºC)), wherein the inset is a photograph of a temperature sensor. (right) Photograph depicting the operation of a red LED lamp in the smart cup (for hot water (~ 80 ºC)). (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.)

Figure 10. Printed Li-ion batteries. (a) Photographs of paintable Li-ion batteries fabricated on various substrates and their integration with PV panels and LEDs. (Reprinted with permission from ref. 10. Copyright 2012 Nature Publishing Group.) (b) Photograph of a 3D-interdigitated microbattery architecture composed of LTO-LFP electrodes after packaging. (Reprinted with permission from ref. 14. Copyright 2013 Wiley.) (c) Photograph of the 3D-interdigitated LFP/GO and LTO/GO electrodes. (Reprinted with permission from ref. 18. Copyright 2016 Wiley.) (d) (left) Photograph of a solid-state Li-ion battery directly printed on paper-made eyeglasses. (right) Photograph of a heart-shaped solid-state Li-ion battery printed on a transparent glass cup with a curvilinear surface. The printed Li-ion battery delivered normal charge/discharge behavior (inset image). (e) (left) Photograph of a “PRISS” letter-shaped solidstate Li-ion battery and (right) its charge/discharge profiles while being completely wound along rods with different diameters (5, 10 and 15 mm). (Reprinted with permission from ref. 9. Copyright 2015 American Chemical Society.) (f) (left) Schematic illustration of the printingbased stepwise fabrication of a solid-state bipolar Li-ion battery cell directly on a c-Si PV module. Cross-sectional SEM images of (upper right) a 2-stack bipolar Li-ion battery cell and 40 ACS Paragon Plus Environment

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(lower right) seamlessly combined interface between the bipolar Li-ion battery cell and c-Si PV module. (g) Photograph showing the operation of the Si PV–Li-ion battery-embedded smartcard. Inset shows the overall thickness of the resulting smartcard. (Reprinted with permission from ref. 48. Copyright 2017 Royal Society of Chemistry.)

Figure 11. Schematic representation describing the future development directions and outlook of printed batteries. (a) Toward design versatility and performance improvements of printed batteries. Conceptual illustrations of (left) advanced 3D printing techniques with well-designed inks enabling form factor-free fabrication of all-printed-batteries and (right) 3D bicontinuous electron-/ion-conduction pathways in the printed batteries. (b) Toward monolithic integration of printed batteries with electronic devices and energy-harvesting systems. Conceptual Illustration of (left) printed batteries embedded in printed electronic circuits and (right) major components of the resultant power source-integrated electronic devices.

Table 1. Electrode ink materials/composition and relevant printing techniques for various printed batteries based on the battery system.

Table 2. Battery systems, components, printing techniques and electrochemical characteristics of various printed power sources.

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Figure 1. A 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.

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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.)

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Figure 3. Printed batteries fabricated through inkjet and 3D printing. Schematic representations of (a) inkjet (reprinted with permission from ref. 6. Copyright 2015 Wiley) and (b) 3D printing (direct ink writing) (reprinted with permission from ref. 12. Copyright 2016 Royal Society of Chemistry). (c) Schematic illustration of an inkjet-printed interdigital micro-supercapacitor. (d) Optical images of (left) a micro-supercapacitor and (right) the chips at low magnification. (Reprinted with permission from ref. 13. Copyright 2010 Elsevier.) (e) Schematic illustration of 3D-interdigitated microbattery architectures fabricated on a gold current collector by printing Li4Ti5O12 (LTO) and LiFePO4 (LFP) inks. (f) Scanning electron microscopy (SEM) images of printed/annealed 16-layer interdigitated LTO-LFP electrode architectures. (g) Areal capacity of a full cell containing 8-layer interdigitated LTO-LFP electrodes as a function of the charge/discharge cycle number. (Reprinted with permission from ref. 14. Copyright 2013 Wiley.)

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Figure 4. Printed electrode inks with a focus on dispersibility and rheological control. (a) Different chemical structures of modified graphene inks. (Reprinted with permission from ref. 36. Copyright 2011 IOP Publishing.) (b) Functionalization of MWCNTs with carboxyl, hydroxyl, and carbonyl groups to ensure their dispersibility in water for inkjet printing. (Reprinted with permission from ref. 37. Copyright 2006 Wiley.) (c) Rheological properties of printable LFP electrode inks with an ionically conductive matrix and photographs of stencilprinted, letter-shaped LFP electrode inks. (d) Photographs showing the mechanical flexibility of the resulting printed LFP electrodes. (Reprinted with permission from ref. 9. Copyright 2015 American Chemical Society.)

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Figure 5. Printed current collectors and substrates. (a) Meyer rod coating of CNT or Ag NW ink on commercial Xerox paper. (Reprinted with permission from ref. 38. Copyright 2009 National Academy of Sciences.) (b) Cross-sectional SEM image of a spray-printed Li-ion battery full cell containing the spray-printed CNT and Cu current collectors (scale bar is 100 μm). (Reprinted with permission from ref. 10. Copyright 2012 Nature Publishing Group.) (c) Schematic illustration of inkjet-printed supercapacitors with CNF layers on commercial A4 paper. (d) Effect of the substrate on the printability of the inkjet printing process. (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.)

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Figure 6. Printed electrolytes and separator membranes. (a) UV-cured gel-type separator membranes fabricated through a stencil printing process. (Reprinted with permission from ref. 32. Copyright 2015 Wiley.) (b) Components and viscosity of the inks for the fabrication of 3Dprinted separator membranes (photograph) through a phase inversion technique. (Reprinted with permission from ref. 42. Copyright 2017 Wiley.) (c) Conceptual illustration of an imprintable and shape-conformable composite gel polymer electrolyte. (d) SEM images of the maze-patterned composite gel polymer electrolyte fabricated via a UV-NIL technique (inset cross-sectional image). (Reprinted with permission from ref. 43. Copyright 2013 Wiley.) (e) Viscosity of the inkjet-printable electrolyte ink (composed of ionic liquid and UV-curable acrylate monomer) as a function of the shear rate. (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.) (f) Scheme of the ink jet printing process of an ionogel and SEM image of the LFP composite electrode obtained through printing (followed by polycondensation) of the ionogel precursors. (Reprinted with permission from ref. 22. Copyright 2015 Elsevier.)

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Figure 7. A Ragone plot (based on areal energy/power densities) of previously reported printed batteries.

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Figure 8. Zn-based printed batteries. (a) Proof-of-concept device constructed with a SWCNTfilm charge collector and MnO2-SWCNT mixture. (Reprinted with permission from ref. 49. Copyright 2007 American Institute of Physics.) (b) Schematic diagram of the fabrication of the mesh-embedded, printed MnO2 cathode and Zn anode with a printed Ag current collector. (c) (left) Schematic representation of a Zn/MnO2 alkaline battery with sandwich-type architecture and (right) photograph of a flexible Zn/MnO2 battery laminated inside a polyethylene pouch. (Reprinted with permission from ref. 45. Copyright 2011 Wiley.) (d) Schematic illustration of the printing process for a Zn–metal-oxide stacked microbattery and (e) SEM image of the resulting printed microbattery. (Reprinted with permission from ref. 46. Copyright 2010 IOP Publishing.) (f) Schematic diagram illustrating the different steps involved in the fabrication of the Zn/Ag cell. Prototype of the Zn/Ag cell on a temporary transfer tattoo support (inset photograph). (Reprinted with permission from ref. 44. Copyright 2014 Royal Society of Chemistry.) (g) Stepwise screen printing of a Zn/Ag battery on a stretchable textile using a SIS binder. Photographs of the sealed battery while being 0% stretched, twisted, indented, 100% stretched and biaxial stretched. (Reprinted with permission from ref. 47. Copyright 2017 Wiley.) 49 ACS Paragon Plus Environment

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Figure 9. Printed supercapacitors. (a) SEM image of as-deposited SWCNT networks. (b) Thinfilm supercapacitor using sprayed SWCNT films on a PET substrate as electrodes and a PVA/H3PO4-based polymer electrolyte as both the electrolyte and separator. (Reprinted with permission from ref. 54. Copyright 2009 American Chemical Society). (c) Schematic of the assembled flexible devices composed of a carbon fiber current collector (this geometry holds true for knitted or woven devices), porous PTFE separator, silicotungstic acid (SiWA) polymer electrolyte and screen-printed AC ink. (Reprinted with permission from ref. 56. Copyright 2013 Royal Society of Chemistry.) (d) Photographs of ultrathin and flexible micro-supercapacitors. The micro-supercapacitors are thinner than a human hair, wearable on human hands, and conformable to bent fingers. (Reprinted with permission from ref. 67. Copyright 2016 Wiley.) (e) Schematic illustration of fabrication process of 3D-printed graphene composite aerogels (3D-GCA). (Reprinted with permission from ref. 81. Copyright 2016 American Chemical Society) (f) Schematic illustration of the stepwise fabrication procedure of the inkjet-printed supercapacitors and a photograph of the utilized desktop inkjet printer. Photographs of (g) the inkjet-printed, letter (‘‘BATTERY’’)-shaped supercapacitors (marked by a red box) and (h) traditional Korean ‘‘Taegeuk’’ symbol-like supercapacitors (marked by a red circle) that were 50 ACS Paragon Plus Environment

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seamlessly connected with the inkjet-printed electrical circuits and a light-emitting diode (LED) lamp. The inset is a photograph of the supercapacitor after being placed on a hot plate (set at 150 ºC) for 0.5 h. (i) (left) Photograph depicting the operation of a blue LED lamp in the smart cup (for cold water (~ 10 ºC)), wherein the inset is a photograph of a temperature sensor. (right) Photograph depicting the operation of a red LED lamp in the smart cup (for hot water (~ 80 ºC)). (Reprinted with permission from ref. 41. Copyright 2016 Royal Society of Chemistry.)

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Figure 10. Printed Li-ion batteries. (a) Photographs of paintable Li-ion batteries fabricated on various substrates and their integration with PV panels and LEDs. (Reprinted with permission from ref. 10. Copyright 2012 Nature Publishing Group.) (b) Photograph of a 3D-interdigitated microbattery architecture composed of LTO-LFP electrodes after packaging. (Reprinted with permission from ref. 14. Copyright 2013 Wiley.) (c) Photograph of the 3D-interdigitated LFP/GO and LTO/GO electrodes. (Reprinted with permission from ref. 18. Copyright 2016 Wiley.) (d) (left) Photograph of a solid-state Li-ion battery directly printed on paper-made eyeglasses. (right) Photograph of a heart-shaped solid-state Li-ion battery printed on a transparent glass cup with a curvilinear surface. The printed Li-ion battery delivered normal charge/discharge behavior (inset image). (e) (left) Photograph of a “PRISS” letter-shaped solidstate Li-ion battery and (right) its charge/discharge profiles while being completely wound along rods with different diameters (5, 10 and 15 mm). (Reprinted with permission from ref. 9. Copyright 2015 American Chemical Society.) (f) (left) Schematic illustration of the printingbased stepwise fabrication of a solid-state bipolar Li-ion battery cell directly on a c-Si PV module. Cross-sectional SEM images of (upper right) a 2-stack bipolar Li-ion battery cell and (lower right) seamlessly combined interface between the bipolar Li-ion battery cell and c-Si PV module. (g) Photograph showing the operation of the Si PV–Li-ion battery-embedded smartcard. Inset shows the overall thickness of the resulting smartcard. (Reprinted with permission from ref. 48. Copyright 2017 Royal Society of Chemistry.)

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Figure 11. Schematic representation describing the future development directions and outlook of printed batteries. (a) Toward design versatility and performance improvements of printed batteries. Conceptual illustrations of (left) advanced 3D printing techniques with well-designed inks enabling form factor-free fabrication of all-printed-batteries and (right) 3D bicontinuous electron-/ion-conduction pathways in the printed batteries. (b) Toward monolithic integration of printed batteries with electronic devices and energy-harvesting systems. Conceptual Illustration of (left) printed batteries embedded in printed electronic circuits and (right) major components of the resultant power source-integrated electronic devices.

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Table 1. Materials/composition of electrode inks and relevant printing techniques for various printed batteries as a function of battery system. Battery system Supercapacitors

LIBs

Active materials Activated carbon (3 wt%)

Zn/MnO2

Printing technique

Ref.

Ethylene glycol

Triton X100

Inkjet printing

13

3D printing

14

Inkjet printing

22

Screen printing

23

Glycerol (20~27 wt%) Aqueous hydroxypropyl cellulose (HPC) (8~9 wt%) Aqueous hydroxyethyl cellulose (HEC) (1~2 wt%) HCl, NaOH, Triton X-100, Glycerin

Ethylene glycol (20-30 wt%) DI water (31.7~38.5 wt%)

LiFePO4

Carbon black

CMC

DI water

LiFePO4 (24.12 wt%)

Carbon black (3.015 wt%)

poly(vinylidene fluoride) (PVDF) (3.015 wt%)

N-methyl-2pyrrolidone (NMP) (69.85 wt%)

PVdF (5 wt%)

NMP (50 wt%)

-

Dispenser printing

24

PVdF,

NMP

-

Screen printing

25

PVdF

NMP

-

Doctor blade

26

-

-

DI water (92.691 wt%)

Isopropyl alcohol (7.286 wt%)

Dispenser printing

27

-

Nafion (2.6 wt%)

2-propanol (86.7 wt%)

-

Screen printing

28

Zn powders (69.3wt%)

-

Styrene-butadiene (1.6 wt%)

Ethylene glycol (10.9 wt%)

ZnO nanopowder (7.3 wt%), Bi2O3 (10.9 wt%)

Stencil printing

29

A: Zn powders (75~77wt%) C: MnO2 (48~53wt%)

C: Graphite (9~13 wt%)

A: PEO (1.3~1.5 wt%) C: PEO (1.4~1.8 wt%)

A: H2O (21~24 wt%) C: H2O (37~39 wt%)

-

Screen printing

30

PSBR (4 wt%)

DI water

-

Flexographic printing

11

NMP

-

Extrusion printing

31

DI water

-

Stencil printing

32

DI water n-tetradecane

-

Inkjet printing

33

Acetone

Flexographic printing

34

Screen printing

35

Li4Ti5O12 LiCoO2 Aligned multiwall carbon nanotubes (MWCNT) (0.023 wt%) Carbon black (10.7 wt%)

Zn:Ag2O (3~4:1)

Acetylene black (5 wt%) Carbon black Carbon black

Acetylene black (6 wt%) Acetylene black

Zn Ag2O

PVA (1 wt%), PVDF (1 wt%) Methylcellulose (5 wt%) PEO

Silver

Zn/Air

Additives

-

MnO2 powder (90 wt%)

Zn/Ag

Solvents

-

LiCoO2

Li-O2

-

Binding agents Polytetrafluoro ethylene (PTFE) (5 wt%)

Li4Ti5O12 (1.3 wt%), LiFePO4 (1.5 wt%)

Li4Ti5O12 (40 wt%)

Li-S

Conducting agents

Zn powders (88 wt%)

CNF (3 wt%), CB (1 wt%)

PVDF-HFP (8 wt%)

Co3O4 nanoparticles (1mg mL-1)

Carbon black

Anionic polymer binder (AS-4)

Acetone

Zn powder

Carbon powder

Polycarbonate (15 wt%)

Tetrahydrofuran

-

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Table 2. Battery systems, components, printing techniques and electrochemical characteristics of various printed power sources. Battery system

Cathode

Supercapacitors

Activated Carbon/SWNT

Polymer Battery

PEDOT:PSS

LIB

Anode

Separator and electrolyte

Capacity/voltage

Printing technique

Ref.

Inkjet printing

41

-2

LiFePO4 (LFP)

Activated Carbon/SWNT PEDOT:PSS / PEI Li4Ti5O12 (LTO)

LiFePO4 (LFP)

Li4Ti5O12 (LTO)

MnO2

Zn

MnO2

Zn

Ag

Zn

Ag2O

Zn

Ag2O

Zn

Ag

Zn

10M KOH

Co3O4

Zn

PVA-gelled electrolyte membrane (KOH/PVA=35/2)

2905 Wh L-1 /1.2~1.3 V

Doctor blade casting

34

PEDOT

Zn

8M LiCl/LiOH

0.5 mAh cm-2 /0.8 V

Screen printing

35

Zn/MnO2

Zn/Ag

Zn/Air

ETPTA/ [BMIM][BF4]

100 mF cm /2.0 V

PSSNa

5.5 mAh g-1

Screen printing

57

PVDF-co-HFP and Al2O3 ETPTA/ 1M LiPF6 in EC/PC (1:1 v/v) and Al2O3

100 mAh g-1 /2.5 V

3D printing

18

160 mAh g-1 /2.5 V

Stencil printing

9

0.8 mAh cm-2 /1.5 V

Stencil printing

29

1.0 mAh cm-2 /1.6~1.8 V

Flexographic printing

11

2.1 mAh cm-2 /1.5 V

Screen printing

44

3.95 mWh cm-2 /1.4 V

Extrusion printing

31

Stencil printing

32

Inkjet printing

33

PAA-based gel electrolyte 1:1=PVDF-HFP: 0.5 M solution of zinc trifluoromethanesulfonate (in BMIM + Tf-) PAA- based gel 6M KOH + 1M LiOH Methylcellulose 17M KOH (57:29:14=H2O:KOH:PEO) PAA-based gel electrolyte 8.4M KOH

5.4 mA h cm-2 /1.55 V 3.95 mWh cm-2 /1.5, 1.8V

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List of Quotes

Quote 1. “Printed power sources” have recently emerged as a new battery system to address the aforementioned issues on the design diversity and flexibility.”

Quote 2. To successfully develop printed batteries, battery component inks with well-tuned rheological properties and dispersion state 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.

Quote 3. Key requirements for battery component inks include: (i) component dispersion, (ii) rheological properties (i.e., viscosity and viscoelasticity) tailored for the specific printing process, (iii) structural/dimensional stability after printing (e.g., cohesion between particles, adhesion with the substrate and mechanical tolerance upon external stress) and (iv) electrochemical performance of the resulting printed battery.

Quote 4. The exceptional form factors and adaptability make printed batteries promising power sources for highly integrated, multifunctional applications. To achieve this goal, an in-depth consideration of both the integration techniques and battery specifications (e.g., capacity, voltage, current and others) is needed.

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50x48mm (300 x 300 DPI)

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Figure 1 81x77mm (300 x 300 DPI)

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Figure 2 176x117mm (300 x 300 DPI)

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Figure 3 159x125mm (300 x 300 DPI)

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Figure 4 178x103mm (300 x 300 DPI)

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Figure 5 176x112mm (300 x 300 DPI)

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Figure 6 130x119mm (300 x 300 DPI)

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Figure 7 78x55mm (300 x 300 DPI)

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Figure 8 161x155mm (300 x 300 DPI)

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Figure 9 173x171mm (300 x 300 DPI)

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Figure 10 160x107mm (300 x 300 DPI)

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Figure 11 170x106mm (300 x 300 DPI)

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