Recent Progress on Printed Flexible Batteries: Mechanical ...

DOI: 10.1002/ente.201402182

Recent Progress on Printed Flexible Batteries: MechanicalChallenges, Printing Technologies, and Future ProspectsAbhinav M. Gaikwad,*[a] Ana Claudia Arias,[a] and Daniel A. Steingart[b]

1. Introduction

Advances in wireless technologies, low-power microelectron-ic devices, and easy access to the internet have enabled theability to interconnect electronic objects, so that the objectscan communicate with each other, make decisions, and pro-vide users information that helps to improve their life.[1,2] To-wards such an ecosystem, low-cost, wearable, flexible elec-tronics in the form of smart watches, sensors embedded inclothing, activity trackers, and health-monitoring tags wouldbe widely adopted. Future generations of these deviceswould be thin, conformable to the human body, ubiquitous,and almost imperceptible to the user.[3–12] Powering these de-vices while retaining their mechanical properties will bea challenge.[13–16]

Batteries are essential for powering portable electronic de-vices. A battery is a closed system in which energy is storedin chemical form, and it is converted to electrical energy byconnecting the battery to an external load to complete thecircuit, causing electric current to flow between the anodeand cathode.[17–19] Electronics have traditionally been de-signed around commercial batteries—prismatic, cylindrical,and coin cells—which are bulky, rigid, and non-flexible,making them unsuitable for powering flexible electronics.[19]

A power source for a flexible electronic device should bethin, bendable, and mechanically compliant.[20–43] Flexibleelectronics are fabricated by patterning traditional inorganiccomponents in ultra-thin form[4,44–46] or by depositing solu-tion-processed organic/inorganic semiconductors and conduc-tive inks on flexible substrates.[12,47–54] Due to the thinness ofactive layers and conducting electrodes, these devices can beflexed to low bending radii without reaching their fracturelimit (strain�1.0 %).[44, 46] To fabricate flexible batteries, allof the rigid components of a battery are replaced by flexiblecounter parts. This includes replacing rigid packaging withflexible pouches, the use of flexible current collectors, anddesigning the active layers and interfaces to the current col-

lectors such that cracking and delamination are prevent-ed.[31, 55–57]

An electrochemical cell consists of active layers supportedon conductive substrates (current collectors) to form theanode and cathode of the battery. The electrolyte providesionic contact between the electrodes and helps to completethe redox reactions within the cell. Printing processes such asscreen, stencil, and blade printing are well established andthey can be used to deposit battery components by designingprintable inks for the active layers, current collectors, andelectrolyte. Batteries fabricated using printing processes havethe advantage of low cost, flexible form factor, ease of pro-duction, and integration with electronic devices.[52,58–60] Theterm “printed battery” is used to describe a battery forwhich at least one of the components is solution processedand deposited using a printer.[61–69] The non-printed compo-nent (current collector or separator) serves as a support forthe printed components. The mechanical characteristics ofa printed battery depend on its architecture and design. Overthe past couple of years, there has been significant progresstowards using printing-based processes to fabricate powersources. Researchers have demonstrated full-cells using con-

Traditional printing methods offer the advantage of well-ma-tured technology, high accuracy of depositing inks over flexi-ble substrates at high web speeds, and low cost of fabrication.The components of a battery—the current collectors, activelayers, and separator—can all be deposited using convention-al printing techniques by designing suitable inks. A combina-tion of low thickness of printed electrodes, flexible packag-

ing, battery architecture, and material properties makesprinted batteries flexible. In this paper, we will discuss mate-rial challenges and mechanical limits of flexible printed bat-teries. We will review several printing techniques and presentexamples of batteries printed using these methods. In addi-tion, we will briefly discuss other novel non-printed compli-ant batteries that have unique mechanical form.

[a] Dr. A. M. Gaikwad, Prof. A. C. AriasElectrical Engineering and Computer Sciences DepartmentUniversity of California BerkeleyCory HallBerkeley, CA 94720 (USA)E-mail: [email protected]

[b] Prof. D. A. SteingartDepartment of Mechanical and Aerospace Engineering andAndlinger Center for Energy and the EnvironmentPrinceton UniversityD428 Engineering QuadranglePrinceton, NJ 08544 (USA)

This manuscript is part of a Special Issue on printed energy technolo-gies. A link to the issue’s Table of Contents will appear here. This text willbe updated once the Special Issue is assembled.

Energy Technol. 2000, 00, 1 – 25 � 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim &1&

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ventional printing methods and provided protocols for de-signing inks. Novel designs have enabled batteries that canflex and stretch without any significant changes in capacity.This Review provides a summary of the progress in printedcompliant power sources, including mechanical considera-tions and printing methods, and the future challenges forcompliant batteries are also discussed.

2. Batteries

Batteries have played an important role in the pervasive useof portable electronic devices in today�s society.[18,19] A bat-tery is a device that converts the chemical energy stored inthe active materials into electrical energy by way of redox re-actions. The term “battery” is used to describe a system con-sisting of multiple electrochemical cells connected in seriesto increase the voltage, or in parallel to increase the capacity.A battery consists of five basic components, which are illus-trated in Figure 1—the anode (negative pole), cathode (posi-

tive pole), electrolyte, and the current collectors for theanode and cathode. The anode is typically a strong reducingagent (e.g., lithium, zinc) and it easily gives up its electron,which travels through the external circuit and oxidizes thecathode. The cathode is an electron acceptor (e.g., lithiumcobalt oxide, manganese oxide, lead oxide). It accepts elec-trons from the anode and gets oxidized. The electrolyte, anionic conductor, provides ionic contact between the anodeand cathode and prevents electrical contact between them.Battery electrolytes are generally in the form of a liquid andthey can be divided into aqueous, non-aqueous, and polymerelectrolytes. An aqueous electrolyte consists of salts ofstrong acids and bases dissolved in water. Due to the high di-electric constant of water and its high solvation power, thesalts dissolve favorably in water thereby enabling conductivi-ties as high as 1 Scm�1. Lithium-ion batteries use non-aque-ous electrolytes consisting of lithium salts dissolved in carbo-nated organic solvents. The conductivity of non-aqueouselectrolytes is low (0.01–0.001 Scm�1) due to the low dielec-tric constant of the organic solvents, which limits the solubili-ty of lithium salts. Water-based electrolytes are thermody-namically stable only to a voltage window of approximately1.3 V, making them unsuitable for batteries with high voltag-es.[70, 71] Organic electrolytes are stable up to a potential ofapproximately 4.6 V.[72] The stability of the electrolyte de-pends on the proximity of its lowest unoccupied molecularorbital (LUMO) and highest occupied molecular orbital

Dr. Abhinav Gaikwad is Postdoctoral Re-

search Fellow in the Electrical Engineering

and Computer Sciences Department at

the University of California in Berkeley.

He received his Ph.D. degree in chemical

engineering from City College of New

York, where he focused on printed compli-

ant electrochemical devices. Prior to that

he received his bachelor’s degree in chem-

ical engineering from the University Insti-

tute of Chemical Technology, Mumbai

University. His research interests include

printed organic electronics, compliant power sources for wearable appli-

cation, and mechanics of battery structures.

Dr. Ana Claudia Arias is an Associate Pro-

fessor in the Electrical Engineering and

Computer Sciences Department at the

University of California in Berkeley and

a faculty director at the Berkeley Wireless

Research Center (BWRC). Prior to joining

the University of California she was the

Manager of the Printed Electronic Devices

Area and a Member of Research Staff at

PARC, a Xerox Company, Palo Alto, CA.

She received her Ph.D. on semiconduct-

ing polymer blends for photovoltaic devi-

ces from the Physics Department at the University of Cambridge, UK.

Prior to that, she received her master and bachelor degrees in Physics

from the Federal University of Paran� in Curitiba, Brazil. Her research

focuses on devices based on solution-processed materials and applica-

tions development for flexible sensors and electronic systems.

Dr. Dan Steingart is an assistant professor

in Mechanical and Aerospace engineering

and the Andlinger Center for Energy and

the Environment at Princeton University.

He has a Sc.B. in engineering from

Brown University and M.S. and Ph.D. de-

grees in materials science from the Uni-

versity of California at Berkeley. His re-

search is focused on battery engineering

at the intersection of materials science, di-

agnostics, and system design. Prior to his

current appointment he was an Assistant

Professor in the Department of Chemical Engineering at the City Col-

lege of New York, and a co-founder of Wireless Industrial Technologies.

Figure 1. Block diagram of a battery cell. During discharging, electrons flowfrom the anode to the cathode through an external load. During recharging,the flow of electrons is reversed.



(HOMO) levels and the chemical potentials of the anodeand cathode. Depending on the energy levels, the electrodecan reduce or oxidize the electrolyte. A polymer electrolyteconsists of conducting salts dissolved in a polymer matrix.Due to the absence of liquid solvents, they are inherentlysafer.[73,74] The current collectors in a battery are highly con-ductive substrates in the form of foils, meshes, or foams,which enable efficient charge insertion and removal from thebattery, and also serve as a support for the active layers.[75]

A conventional battery fabrication process starts with pre-paring slurries of the active materials, which consists of a mix-ture of electrochemically active particles, conductive addi-tives such as carbon black and graphite (to improve conduc-tivity), binder (to hold the electrode together), and a suitablesolvent to dissolve the binder. The slurries are cast onto con-ductive supports to form the anode and cathode of the bat-tery. Then the anode and cathode are stacked together witha separator. The areal capacity of a single stack of anode/sep-arator/cathode is approximately 1–2 mAh cm�2. The areal ca-pacity of a battery is improved by folding/rolling the stackand inserting them into rigid casing to form prismatic or cy-lindrical cells (Figure 2). In a battery, the separator, conduc-

tive additives, binder, current collectors, and packaging areinactive components, which reduce the energy density of thebattery. In the battery community there is a drive towards re-ducing the weight and thickness of inactive components. Thisemphasis is particularly important for flexible battery sys-

tems, where the packaging material and current collectorsdominate the thickness of the battery.

Batteries are divided into three classes—primary, secon-dary, and specialty batteries. Primary batteries are dischargedonce, and discarded after use. The inactive phases formedafter discharge makes them irreversible. Examples of pri-mary batteries include carbon–zinc, MnO2–zinc, and lithium–metal batteries.[20,55] Secondary batteries are rechargeable.They can be restored to their original state by passing cur-rent in the reverse direction. Examples of secondary batter-ies include lithium-ion, nickel–metal hydride, nickel–cadmi-um, and silver–zinc batteries.[31,76, 77] Specialty batteries aredesigned for a particular applications (e.g., medical device,military). These batteries have long cycle lives but they areexpensive and often contain environmentally unfriendly ma-terials.[78,79] Examples of specialty batteries for medical appli-cations include Li/CFx and Li/iodine.

The theoretical capacity of a battery is based on the equiv-alent weights of the active materials. Theoretically 1 gramequivalent of active material will deliver 26.8 Ah. The gram-equivalent weight is the molecular weight of the active mate-rial in grams divided by the number of electrons that takepart in the reaction. For example, zinc with a molecularweight of 65.4 g and with two electrons taking part in the re-action has a theoretical capacity of 820 mAh g�1

[26800 mAh/(65.4 g/2)]. Battery systems are generally char-acterized by their energy density, expressed per unit ofweight (Whkg�1) or volume (WhL�1). The power is a productof voltage (V) and ampere–hour capacity (Ah) of the bat-tery, which depends on the battery chemistry.[19] The practicalenergy density of a battery is 35–50 % of its theoretical valuedue to the weight and volume of the inactive components.The approximate practical energy densities of alkaline bat-tery systems such as Zn–MnO2 and Zn–AgO are 145 and135 Whkg�1, respectively, and the traditional lithium-ion bat-tery with a carbon anode and metal oxide cathode is approxi-mately 150 Whkg�1. The capacities of thin-film batteries arereported in terms of mAh g�1 or mAh cm�2, after the capacityis normalized with the weight of the limiting electrode or thefootprint of the battery, respectively.

The electrode design and architecture of a battery plays animportant role in forming an efficient battery. Lithium-ionbatteries generally use a sandwich architecture, where theanode and cathode of the battery are stacked together (Fig-ure 3 a). This design helps to reduce the footprint of the bat-tery and decrease the distance that the lithium ions have totravel upon moving between the anode and cathode, therebylowering the internal resistance. This is particularly impor-tant in lithium-ion battery systems due to the low conductivi-ties of non-aqueous electrolytes. Batteries with aqueous elec-trolytes can be fabricated in sandwich or parallel architecturedue to the high conductivity of the electrolytes. In parallel ar-chitecture the anode and cathode of the battery are placedside-by-side (Figure 3 b). Most reports on stretchable batter-ies use parallel architecture.[77,80,81] The fabrication process issimplified and there is minimal risk of shorting during bat-tery stretching.

Figure 2. Schematic of cylindrical (a) and prismatic (b) cells, respectively. Re-printed with permission.[17] Copyright (2001), Macmillan Publishers Ltd.



Batteries are tested by electrochemically cycling themwithin set voltage limits. The upper and lower voltage limitsof a battery depend on the chemistry of the system. The volt-age limit prevents the formation of inactive phases and deg-radation of the electrolyte. A battery with a sound designhas high rate capability and excellent capacity retention (lowdrop in capacity with cycle number). The charge/dischargerate of the battery is given in terms of “C-rate” (C/x rate, forwhich x is the time taken in hours for charging or dischargingthe battery). Batteries can be charged in two modes, constantcurrent (CC) or constant current–constant voltage (CC–CV).[19] In CC mode, the battery is charged at constant cur-rent till it reaches a set voltage limit. In CC–CV mode thebattery is charged at a high constant current (compared toCC mode) till it reaches a set voltage limit and then the bat-tery is kept at that voltage till the current drops below a cer-tain limit. In CC charging mode, any changes in the batterydue to formation of inactive phases, side reactions or me-chanical degradation in the battery will lead to polarization,causing the battery to reach the voltage limit before beingcompletely charged. In such a case, the drop in capacity ofthe battery can be related to electro-mechanical changeswithin the battery. Commercial chargers use CC–CV charg-ing procedure. The CC stage enables quick charging at highrate till it reaches the set potential limit. At this point thebattery is typically 65–75 % charged. The CV stage ensuresthe battery is completely charged without exceeding the volt-age limit.

Currently there are no standard procedures for testingcompliant batteries. In early studies, the electrochemical per-formance of batteries were compared upon flexing to differ-ent bending radii. In these studies, the effect of fatigue dueto repeated flexing was not considered.[21,57] In later studies,the batteries were flexed for a certain number of cycles (50–100) before comparing their performances.[20,55] In recentstudies, flexible batteries were tested by electrochemicallycycling them during continuously flexing.[26] The differencesin battery designs/architectures and variations in electrodethicknesses make it difficult to compare the performances ofbatteries demonstrated in the literature. Factors such asspeed of flexing and the number of flexing cycles have a sig-nificant effect on the mechanical integrity of the electrodes,but they are seldom considered in testing the electromechan-

ical properties of batteries. Having a standardized protocolfor testing compliant batteries will make it easy to comparebatteries with different designs.

3. Mechanical Considerations for Compliant Bat-teries

For fabricating compliant batteries it is important to under-stand the mechanical limitations of conventional batterycomponents and architectures. Such understanding will helptowards modifying standard battery designs and materials tomake them more compliant. In the following section we willdiscuss the mechanical requirements of components for com-pliant batteries, which include the current collectors, activelayers, electrolyte, and packaging.

Current collectors are the most essential part of a batteryas they serve as a support for the active layers and providepathways for efficient charge insertion and removal from theactive layers. The current collectors must be chemically com-patible with the active layers and have good adhesion prop-erties to prevent delamination during flexing and calendering(the process of compressing the electrodes to reduce porosityand improve particle-to-particle contact). The current collec-tor should also be stable within the operating potential ofthe battery to prevent corrosion.[75,82] This is especially im-portant in lithium-ion battery systems where the current col-lectors experience large oxidizing and reducing potentials.Current collectors in lithium-ion batteries are in the form ofthin foils (15–20 mm) of aluminum (cathode) and copper(anode).[17–19] These foils are flexible to a certain extent dueto their thinness, but they crack after repeatedly flexing tolow bending radii (<10 mm), which limits their use in flexi-ble batteries.[26] In other battery systems such as nickel–metalhydride (NiMH), nickel mesh and foam are used as currentcollectors for the anode and cathode, respectively, but theyare rigid and not suitable for flexible batteries. Most of theearly efforts in the field of flexible batteries were directed to-wards finding suitable flexible current collectors. Towardsthis goal, there have been numerous reports of flexible bat-teries with current collectors based on printable conductiveinks (Figure 4 a),[55,67] carbon nanotubes (CNTs) embedded infabric/paper (Figure 4 b),[31,83, 84] evaporated metals supportedon flexible supports,[69,85, 86] and conductive fabrics (Fig-ure 4 c,d).[26,81, 87,88] Gaikwad et al. stencil printed carbon andsilver ink on top of reinforced zinc and MnO2 electrodes, re-spectively, to form the current collectors (Figure 4 a). Thesheet resistances of the carbon and silver electrodes were ap-proximately 30 W/& and 0.015 W/&, respectively.[24] LiangbingHu et al. demonstrated the use of CNTs to serve as currentcollectors for lithium-ion batteries (Figure 4 b). The ink wasprinted on “Xerox” paper using the Mayer rod coating pro-cess to form a conductive layer with a sheet resistance of ap-proximately 10 W/&[31,83] Thin films of evaporated metalssuch as nickel, gold, aluminum, and copper were also used ascurrent collectors. The evaporated metal can be patterned byusing standard lithography techniques or by evaporatingthrough a shadow mask.[69,85] In another approach, silver- and

Figure 3. Schematic of a battery with sandwich (a) and parallel (b) architec-tures, respectively.



nickel-coated fabrics were used as current collectors for flexi-ble and stretchable batteries (Figure 4 c,d).[26, 81,87] The con-ductivities of current collectors based on conductive inks andcarbon nanotubes are orders of magnitude lower than con-ventional metallic current collectors. The low conductivityleads to an ohmic potential drop at high C-rates. Conductivefabrics have a high conductivity but their large thickness re-duces the volumetric energy density of the battery. At thistime, metal foils and metals evaporated on thin flexible sup-port are the best choice for current collectors in flexible bat-teries. They are highly flexible and have high electrical con-ductivity.

The active layers of a battery are cast in the form of wetslurries on metal foils or in form of a thick paste, which isimpregnated into metal meshes and foams. The foil, mesh orfoam serves as the current collector and provides a physicalsupport for the active layers. In a flexible battery, the activelayers should maintain their mechanical integrity upon beingflexed. From a materials standpoint, the active materials andconductive additives used in battery electrodes are inherentlynon-flexible and rigid. If used in a composite structure witha polymeric binder, the electrodes are flexible to a certainextent due the porous architecture. In early reports on flexi-ble batteries, thin layers of battery slurries were cast on flexi-ble current collector substrates.[20,62] The areal capacity ofthese batteries was low and they had limited flexibility. Itsoon became clear that it is necessary to increase the arealloading of the active materials and use architectures that im-prove the mechanical flexibility of the electrodes. Since then,there have been numerous design innovations in flexible bat-tery designs.[26, 27,55,56, 81,89–91] Standard battery slurries wereembedded within supports such as meshes, membranes, andfabrics to improve the flexibility and capacity of the battery.The support absorbed the stresses generated in the batteryduring flexing and prevented mechanical failures.[21,26,55, 57,81, 89]

In another approach, binder-free flexible electrodes wereprepared by supporting the active materials within ultra-longCNTs and a graphene mat. The interconnected network ofCNTs/graphene provides a physical support for the activeparticles and the percolative conductive network improvesthe electrochemical characteristics of the electrodes.[89, 92–100]

In another process, the active materials were synthesized di-rectly on flexible conductive substrates such as CNT mats,carbon fibers, and graphene foams to form the anode andcathode of the battery.[57, 91,101–104] The thinness of the activematerials and flexible nature of the support make the overallbattery highly flexible. From a prospective of commercializ-ing flexible batteries, the high cost of ultra-long CNTs andgraphene foam will limit their use in flexible electrodes untilthe cost is significantly reduced. Direct synthesis of activematerials on flexible conductive substrates is a batch processand designing a continuous fabrication process is difficult atthis moment. Electrodes based on direct printing of slurrieson conductive porous supports are suitable for large-scalefabrication of flexible batteries.

The electrolyte in a battery carries charges in the form ofions between the anode and cathode and completes the elec-trochemical reactions within the battery. Most flexible batter-ies use a liquid electrolyte, which is drop cast and soakedwithin the battery.[72,105,106] In the batteries using a liquid elec-trolyte, a sheet of battery separator is used to physically sep-arate the anode and cathode. The separator consists of mi-cropores that enable the easy flow of ions between the elec-trodes while preventing electrical contact between them.[107]

The separators are designed based on the chemistry of thesystem and the nature of the electrolyte. The battery separa-tors are highly flexible, and they can be folded/flexed withoutany physical changes. The liquid nature of the electrolyte en-sures continuous ionic contact between the anode and cath-ode upon being flexed. In recent years, there has been an in-terest towards replacing liquid electrolytes with polymer-based electrolytes.[73,74, 108–112] Liquid electrolytes are toxic,unsafe, and there is always a danger of leakage if the batteryis misused or physically damaged. Electrolytes in the form ofpolymer gel electrolytes and solid-state polymer electrodeshave been used in flexible lithium-ion batteries.[113–115] The re-duced content of solvent (or no solvent in some cases)makes them inherently safe. The conductivity of the polymerelectrolyte is an order of magnitude lower than for a liquidelectrolyte, which can reduce the rate capability of the bat-tery.

Finding a suitable packaging material for sealing flexiblebatteries is a challenge, and this is a problem that is oftenoverlooked in the flexible battery community.[116] In the liter-ature, the packaging used for flexible batteries includes alu-minum-laminated pouches (Figure 5 a),[21,26,117] plastics suchas polyethylene, polypropylene, polyester and PVC (Fig-ure 5 b),[55,67] thin sheets of polydimethylsiloxane (PDMS)(Figure 5 c),[31,57,85] and elastomers (Figure 5 d).[56, 80,81] Thepackaging material should be flexible, sealable by applicationof heat or glue, have air, moisture, and electrolyte barrier ca-pabilities, and be chemically resistant to aqueous and organic

Figure 4. a) Silver-flake-based ink stencil printed on a reinforced MnO2 elec-trode. Reprinted with permission.[24] Copyright (2013), Wiley. b) Carbon nano-tubes Mayer rod coated on commercial paper. Reprinted with permission.[31]

Copyright (2010), American Chemical Society. c) Nickel-coated polyesterfabric. Reprinted with permission.[26] Copyright (2013), American ChemicalSociety. d) Conductive, stretchable fabric with silver-coated nylon threadsweaved through a rubber network. Reprinted with permission.[81] Copyright(2012), Wiley.



solvents/chemical products formed in the battery during cy-cling. The main concern in battery packaging is the slowegress of electrolyte through the surface of packaging materi-al and the sealant edge. Similarly, moisture and air couldenter the battery through the packaging, which can degradethe electrolyte.[116] Battery pouches based on PDMS andplastic sheets are highly flexible but they are permeable toair and moisture, and the electrolyte can egress from thepouch leading to drying of the electrolyte. Such packagingworks well for sealing the battery for a short period of time.For a battery with long shelf life, packaging based on alumi-num-laminated foils is preferred, as it provides all the de-sired properties required for a packaging pouch. The lami-nated foil consists of thin metal film (usually aluminum),which is laminated on both sides with a thin layer of plastic.The side in contact with the battery electrode is laminatedusing a heat sealable plastic such as polypropylene/polyethy-lene and the exterior side of the pouch is laminated witha scratch-resistant polymer such as nylon or PET. Alumi-num-laminated pouches are thick (100–150 mm), stiff andthey dominate the thickness of the batteries. The thickness ofthe aluminum foil within the laminated pouch is sized de-pending on the pressure that the pouch will experience dueto gas generation during initial formation cycles and uponbattery overcharging.[116] In thin-film batteries, the gas forma-tion is reduced due to low loading of the active materials.Hence it is possible to use very thin foils, which are moreflexible, and it will help reduce the overall thickness of thepouch. In another approach, it may be possible to use heat-sealable plastics coated with a thin layer of inorganic layer(moisture barrier) to seal the battery.[118,119] Such inorganic

coatings are being currently developed in the flexible elec-tronics industry for packaging solar cells and OLEDs.

Even with the impressive advances in the field of flexiblebatteries, there is a lack of fundamental understanding ofmechanical limits of flexible batteries, and the factors affect-ing their performances.[21] The mechanical flexibility of a bat-tery depends on the electrode architecture, material proper-ties, and the binder used to hold the electrode together.Upon flexing a battery, there is a neutral plane, which passesthrough the center of the battery stack (generally the batteryseparator) and the anode and cathode of the battery wouldbe in states of compressive and tensile stresses, respectively(Figure 6).[21,24, 27] A battery can fail due to delamination ofthe active layer or cracking of the active layers during flex-ing, which can lead to loss in electrochemical capacity of thebattery. The adhesion of the active layers to the current col-lectors depends on the property of the binder and its chemi-cal stability towards the electrolyte. The adhesion of theactive layers can be improved by designing better binders orby engineering the interfaces to prevent delamination. Thedegree and tendency of cracking is related to the amount ofstrain (expansion) that the electrode can absorb. Electrodesusing a standard composite battery design compared againsta binder-free electrode with ultra-long CNTs can absorbstrains of 0.4–1.0 % and 2.0–3.5 %, respectively.[89,120] Usinga simple analysis, a conventional composite battery electrodeat a distance of 100 mm from the neutral plane with a strainlimit of 0.5 % has a flexing limit of 20 mm bending radius(strain = thickness from neutral plane� 100/bending radius,0.1 mm �100/20 mm= 0.5 %). Beyond this point, the elec-trode will form cracks and start to disintegrate with as thenumber of flexing cycles increases. To prevent mechanicaldegradation during flexing, the thicknesses of the activelayers are engineered based on the minimum bending radiusthat the battery will experience, ensuring that the maximumstress is always below its critical fracture limit.[27] Repeatedflexing can lead to fatigue of the electrode, which can de-grade the electrochemical performance of the battery. Acombination of mechanical modeling and empirical in situstudies of battery flexing will clarify the relation betweenflexing-induced mechanical stresses and the electrochemicalperformance of the battery.

4. Printing Techniques

Traditionally printing processes have been used to depositinks over substrates to create text and images. In recentyears, printing has been used as a fabrication tool for depos-iting functional inks such as inorganic/organic semiconduc-tors and conductive inks on flexible surfaces to form a newclass of electronics that is generally referred to as “printedelectronics”. Printing processes offer the advantage of fabri-cating electronic components over large areas inexpensivelyas compared to traditional microfabrication techniques.Printing processes are attractive from a production stand-point, because battery components can be deposited withlow cost and high throughput. The fabrication process for

Figure 5. a) Flexible battery sealed within an aluminum-laminated pouch. Re-printed with permission.[91] Copyright (2012), American Chemical Society.b) Cable battery sealed within heat-shrinkable tubing. Reprinted with permis-sion.[90] Copyright (2012), Wiley. c) Solid-state lithium-ion battery sealedwithin PDMS sheets. Reprinted with permission.[27] Copyright (2012), Ameri-can Chemical Society. d) Zn–MnO2 stretchable battery sealed within a stretch-able elastomer. Reprinted with permission.[81] Copyright (2012), Wiley.



a printed battery starts by selecting the printing tool, fol-lowed by tailoring the rheological properties (viscosity) forthe inks used to print active layers, current collectors, andelectrolyte. The non-printed components of the battery serveas supports for the printed components. The literature listsnumerous reports on batteries fabricated using printingmethods. In this section we will briefly review printing tech-niques and provide examples of batteries fabricated usingthese methods.

4.1. Blade coating

Blade coating is the most commonly used printing techniquefor depositing battery slurries in large-scale manufacturing(Figure 7).[31,55, 121] The technique uses a blade, and the verti-cal distance separating the blade and the substrate controlsthe thickness of the wet ink. In a laboratory setup, the tech-nique works by placing a doctor blade on the substrate witha pool of ink on one side of the substrate. The gap betweenthe blade and substrate is kept in the range of 50–200 mm.The blade is moved over the substrate at a constant speed of15–30 cm min�1 to uniformly spread the ink.[122–124] Then thesubstrate is heated in an oven to remove the residual solvent.In industrial setups, the blade is kept stationary, and the sub-strate, in form of a roll, is moved at a constant speed under-neath the blade.[121] In some setups, the substrate is heatedduring the coating process to enhance the drying process.Blade coating is a blanket-coating process, so it cannot be

used to pattern inks over substrate. The thickness of thedried electrode depends on the concentration and the viscos-ity of the ink. Inks with large particles tend to shrink afterdrying whereas inks with nanosized particles tend to formmicropores during the drying process, which can degrade itsmechanical integrity. After drying, the electrodes are calen-dered to reduce the porosity within the electrodes. The po-rosity of the electrode is adjusted to increase the particle-to-particle contact and improve wetting of the electrolyte.Zheng et al. studied the effect of porosity on the mechanicaland electrochemical performance of Li[Ni1/3Mn1/3Co1/3]O2

electrode containing 8 % polyvinylidene fluoride (PVDF)and 7 % acetylene black.[120] The mechanical properties ofthe composite electrode depended strongly on the electrodeporosity whereas the electrical conductivity is independent ofcalendering. Electrodes with porosities between 30–40 %

Figure 6. Illustration of the stress profile generated in a battery stack during flexing and SEM micrographs of cracking and delamination of the active layer dueto mechanical failure.

Figure 7. Schematic illustration of the blade-coating process in a batch androll-to-roll process forms.



showed the best electrochemical performance. In convention-al batteries with “jelly-roll” architecture, blade coating orslot–die printing techniques are used to deposit battery slur-ries on thin metal foils, which serve as the current collectorsfor the electrodes. Blade coating is used extensively in theprinted electronics industry for fabricating solar cells[122] andorganic light emitting diodes (OLEDs).[123]

Hu et al. used a blade coating process to sequentially printthe current collector and the active layer onto a stainless-steel support.[31] The current collector ink was based onCNTs dispersed in water with surfactants. Due to the pooradhesion of the CNTs on stainless steel, the CNT/activelayer delaminated from the stainless steel upon being dippedin a water bath. These free-standing lithium cobalt oxide(LCO) and lithium titanate oxide (LTO) electrodes withCNTs as the current collectors formed the cathode andanode of the battery, respectively. The electrodes were lami-nated on Xerox paper, which served as the separator anda support for the active layers (Figure 8 a). A thin layer of

PDMS was blade coated onto the active layers to seal thebattery. The total thickness of the battery was approximately300 mm. LiPF6 in ethylene carbonate/diethyl carbonateserved as the electrolyte for the battery. The full-cell wascycled between 1.5 and 2.7 V and yielded a specific capacityin the range of 110–120 mAh g�1 (Figure 8 b). The capacityretention of the battery after 20 electrochemical cycles was93 %, which could be further improved with better packag-ing. The battery was able to maintain its mechanical integrityafter flexing 50 times to a bending radius of 6 mm (Fig-ure 8 c).

4.2. Dispenser printing

A dispenser printing system consists of an ink syringe thatdeposits ink over the substrate. Inks with a wide range of vis-cosities can be printed using dispenser printing. The ink isprinted in the form of filaments or drops by modulating thepressure in the ink barrel (Figure 9).[125–127] The opening of

the needle can range from 0.5–400 mm. The larger-diameterneedles are made of stainless steel whereas smaller-diameterneedles are made of pulled glass capillaries. The amount ofpressure required to force the ink through the needle de-pends on the diameter of the needle and the viscoelastic be-havior of the ink. The shear thinning behavior of the ink ena-bles printing at considerably lower pressures. Preparing inksfor dispenser printing is more difficult than for other printingmethods. Battery slurries are a mixture of active particles,conductive additives, binder, and suitable solvent. Due to thedifference in the densities of the solid particles and the sol-vent and the large particle size distribution, the ink caneasily clog the needle during printing. The fraction of the sol-vent in the ink is kept to a minimum to prevent sedimenta-tion, and the active particles are ball milled to reduce theaverage particle size. Dispenser printing can be used to printinks over areas ranging from 100 mm2 to 1 m2 by drawing pat-terns in the form of repeated lines or drops. Dispenser print-ing large electrodes is considerably slower as compared toother printing methods but it is advantageous for printingelectrodes with a small footprint over a defined location.Due to the non-contact nature of dispenser printing, the inkcan be printed over uneven surfaces, which is not possiblewith other roll-to-roll printing methods. Dispenser printinghas been used to fabricate thermoelectric generator,[128–130]

batteries[61,63–65,128] and ceramic structures.[126,127] Reports onbatteries fabricated using dispenser printing have predomi-nately involved printing the active layers and polymer elec-trolyte on glass substrates with pre-patterned current collec-tors formed by lithography.

Sun et al. demonstrated a 3D lithium-ion battery with in-terdigitated electrodes.[64] A custom-built printer setup wasused to extrude concentrated inks of lithium iron phosphate

Figure 8. a) Free-standing CNT/LCO and CNT/LTO electrodes laminated onpaper using a rod and a thin layer of PVDF as the adhesive. b) Galvanostaticcharge/discharge curve of a LCO–LTO laminated paper battery. c) Paper bat-tery powering a red LED upon being flexed. Reprinted with permission.[31]

Copyright (2010) American Chemical Society.

Figure 9. Schematic illustration of the dispenser printing process. The ink isdeposited in form of continuous filaments or uniformly distributed drops toform the desired patterns.



(LFP) and LTO-based inks over lithographically patternedgold current collectors (Figure 10 a–d). Fine filaments of theconcentrated inks were formed by printing the ink througha glass needle with a diameter of 30 mm. The shear thinningbehavior of the inks enabled the flow of concentrated inksthrough small nozzles. A system of high-boiling-point solventand volatile solvent were used to control ink solidificationand adhesion during patterning. The evaporation of the vola-tile solvent during the printing process leads to partial solidi-fication of the printed filament, and the remaining high-boil-ing-point solvent served as a humectant to promote bondingbetween the individual layers (Figure 10 e, f). The areal ca-pacity of the battery was increased by using a layer-by-layerprinting process. The battery was enclosed inside a plasticcasing and the liquid electrolyte (1 m LiClO4 in 1:1 ratio ofethylene carbonate/dimethyl carbonate by volume) was usedto provide ionic contact to the anode and cathode. The arealcapacity of the battery with LFP and LTO as the cathodeand anode of the battery was approximately 1.5 mAh cm�2 at1 C rate and they had an energy density of 9.7 Jcm�2 ata power density of 2.7 mW cm�2. The full-cell demonstratedgood capacity retention after 30 electrochemical cycles (Fig-ure 10 g,k).

Ho et al. demonstrated a dispenser-printed rechargeableZn–MnO2 based battery with a solid polymer gel electrolytecontaining an ionic liquid.[63] The MnO2 ink, polymer separa-tor, and zinc ink were sequentially printed onto of a stain-less-steel foil (Figure 10 i). The polymer gel electrolyte wascomposed of 1:1 mixture of PVDF–HFP and 0.5 m solutionof zinc trifluoromethanesulfonate (Zn+Tf�) salt dissolved inBMIM+Tf� . The cells had a footprint of 0.25 cm2 and a totalthickness of 80–120 mm. The battery was cycled in the rangeof 1–2 V. After the initial activation cycles, the areal capacityof the battery was approximately 1 mAh cm�2 with negligibledrop in capacity after 75 electrochemical cycles (Figure 10 j).

In a modification to the conventional dispenser printingprocess where the ink is forced through the needle with com-pressed gas, Gaikwad et al. used a dispenser printer to printlow-viscosity nanoparticle silver ink by dragging a meniscusof the ink over a glass substrate.[65] Low vacuum was appliedinside the ink cartridge to control the meniscus of the ink.Once the silver ink was printed, the electrodes were an-nealed at 280 8C to remove the dispersing solvent and assistwith fusing of the nanoparticles. The Zn–AgO battery wasformed by electrodepositing Zn onto one electrode and oxi-dizing the other electrode (Figure 10 k). The battery had

Figure 10. Schematic of the 3D interdigitated microbattery architecture fabricated on a (a) gold current collector by printing (b) Li4Ti5O12 (LTO) and (c) LiFePO4

(LFP) inks through a 30 mm nozzle, followed by sintering and (d) packaging. e) Optical photographic image of the LFP ink deposition through a 30 mm nozzle.f) SEM micrograph of a printed and annealed 16-layer interdigitated LFP-LTO electrode. g) Galvanostatic discharge curve of the LFP–LTO full-cell with 8 layersof interdigitated electrodes. h) Areal capacity of the battery with 8 layers of interdigitated LFP–LTO electrodes. Reprinted with permission.[64] Copyright (2012),Wiley. i) Cross section of a dispenser-printed microbattery with a polymer gel electrolyte. j) Discharge capacity of the printed microbattery over 70 cycles. Re-printed with permission.[63] Copyright (2010), IOP Publishing Ltd. k) Optical micrographs of charging and discharging of a dispenser-printed Zn–AgO microbat-tery tested using a microfluidic cell. Reprinted with permission.[65] Copyright (2011), The Electrochemical Society.



a thickness of less than 1 mm and an areal capacity of0.7 mAh cm�2. The shear strength of the battery was studiedusing an electrochemical microfluidic cell. The flow of theelectrolyte over the electrode was used to replicate the shearstress experienced by the electrodes during flexing. Theshear strength of AgO was an order of magnitude higherthan Ag2O.

4.3. Stencil printing

Stencil printing is an “on-contact” process where a laser cutmetal or plastic stencil with desired pattern is placed in con-tact with the substrate (Figure 11). A squeegee is used todrag the ink over the mask. Once the squeegee is passedover the openings in the stencil, the stencil is moved up and

the image from the stencil is transferred onto the substrate.The concentration of the ink or the thickness of the maskcan be modified to change the thickness of the printed layer.The stencil can be reused after cleaning them. A stencil canbe used in combination with an airbrush to pattern spray-de-posited layers. Stencil printing is easy and it can be used inthe laboratory for rapid deposition of inks for studying theeffect of ink composition on the electrochemical per-formance.[55,67, 131,132] The solvent in stencil-printed inks iskept to the minimum to prevent oozing of the solvents fromthe edges of the mask.

Gaikwad et al. used stencil printing to deposit all thelayers of a 14 V flexible battery consisting of 10 Zn–MnO2

cells in series (Figure 12 a).[67] Zn and MnO2 inks were depos-ited onto a battery separator, which was followed by printingthe silver ink onto the active layers to form the current col-lectors and interconnects. A hydrophobic Teflon solution wasdispenser printed between the electrodes to isolate the indi-vidual cells. The battery was activated by printing a polymergel electrolyte onto the electrodes followed by folding thesubstrate to form an ionic contact between the MnO2 and Znelectrodes. The cross-sectional SEM micrographs of theMnO2 and Zn electrodes with the silver current collectorsdemonstrates the excellent interfacial contact (Figure 12 b, c).Due to the porous nature of the battery separator, a fractionof the MnO2 and Zn electrode was embedded within the sep-arator, providing mechanical adhesion to the active layers.The battery demonstrated a capacity of 0.8 mAh when dis-

Figure 11. Schematic illustration of the stencil printing process. A stencil withthe pattern is placed on the substrate and a squeegee is moved linearly overthe stencil to deposit the ink on the substrate through the openings in themask.

Figure 12. a) Schematic and optical images of the high-voltage printed battery based on 10 Zn–MnO2 cells stencil printed on a battery separator and electricallyconnected to each other with silver interconnects. Cross-sectional SEM micrographs of the MnO2 (b) and Zn (c) electrode stencil printed on the battery separa-tor using silver current collector. d) Discharge curve of the battery through a 100 kW resistor. Reprinted with permission.[67] Copyright (2013), AIP PublishingLLC.



charged by connecting to a 100 kW resistor (Figure 12 d). Thebattery pack was used to power a 5-stage printed comple-mentary ring oscillator. The battery was able to continuouslypower the ring oscillator for 15 min without any noticeabledrop in the voltage of the battery.

4.4. Spray printing

Spray printing can be used to deposit inks with a wide rangeof viscosities over arbitrary surfaces (Figure 13).[68,133–137]

Spray printing is based on an airbrush in which a high-veloci-ty compressed carrier gas atomizes ink from a reservoir,which is subsequently directed towards the target substrate.The formation of the aerosol and leveling of the ink once itreaches the substrate is a complex process and is poorly un-derstood. For inks with low-vapor-pressure solvents, the sub-strate is heated during the spray-coating process to enhancethe drying process and prevent uneven drying of the ink.There are number of ways to control the thickness ofa sprayed layer, which includes opening the spray nozzle,pressure of the carrier gas, ink composition, and the numberof spray passes. Spray printing is particularly attractive fordepositing inks over non-flat surfaces such as foams andmeshes where standard roll-to-roll printing techniquescannot be used. Spray deposition can be used to print a com-plete battery or certain layers of the battery for cases that

spray deposition provides an advantage over printing meth-ods. Spray printing is attractive for sequentially printing mul-tiple inks that share a common solvent on top of each other.Due to the quick drying of spray-deposited inks, multilayerelectrodes can be printed without dissolving the previouslayer. Spray printing is well established in the automobile in-dustry. A similar setup can be envisioned to print batteriesover large surfaces. Spray printing has been used to depositactive layers and electrodes for solar cells[137, 138] and superca-pacitors.[133,134, 136]

Singh et al. demonstrated a lithium-ion battery in which allthe components including the current collectors, active layersand separator were deposited using spray deposition (Fig-ure 14 a). LCO and LTO were used as the cathode andanode for the battery, respectively.[68] The current collectorfor the cathode was formed by spray depositing a solution ofsingle-walled nanotubes (SWNTs) with 20 wt % of Super Pconductive additive dispersed in N-methyl-2-pyrrolidone(NMP). Commercially available copper ink was used to formthe current collector for the negative electrode. The batterywas fabricated by printing SWNT ink onto the supportingsubstrate, followed by sequentially spray printing the LCOink, polymer separator, LTO ink, and copper ink (Fig-ure 14 b). The full-cells had a nominal voltage of 2.5 V andcapacity of 30 mAh for an active area of 25 cm2. The cell re-tained approximately 90 % of its capacity after 60 cycles witha columbic efficiency of >98 %. Nine sprayed cells were con-nected in parallel to store a total energy of approximately0.65 Wh. A polycrystalline solar cell was used to charge thebattery pack, which was later discharged by connecting to40 red LEDs.

4.5. Inkjet printing

Inkjet printing is a form of digital printing using which thedesired image is formed by dropping ink droplets onto thesubstrate (Figure 15). Inkjet printing is a high-resolution pro-cess and can easily achieve up to 1200 dpi (drops per inch).The resolution of the pattern depends on the quality of inkand characteristics of the print head.[59,60, 139] The drops areformed by mechanically compressing the ink througha nozzle (piezoelectric head) or by heating the ink to in-

Figure 13. Schematic illustration of the spray printing process. Within the air-brush, compressed air atomizes the ink and the plume of ink is directed to-wards the substrate.

Figure 14. a) Schematic of a conventional cylindrical battery with “jellyroll” architecture and the process of sequentially spray depositing inks for battery fabrica-tion. b) Cross-sectional SEM micrograph of the spray-painted battery. Reprinted with permission.[68] Copyright (2012), Macmillan Publishers Ltd.



crease its pressure. The final thickness of the printed elec-trode depends on the number of drops, the volume of thedrop, the concentration of the ink and the footprint of theprinted area. Inks for inkjet printing have stringent require-ments. The ink should have viscosity of 5–25 cP and surfacetension of 25–29 mN m�1. Generally a combination of sol-vents is used to achieve the desired ink properties. The inkshould have a high shelf life and the segregation of the pig-ments/particles in the ink should be minimized to preventclogging at the print heads. A small amount of surfactant isadded to the ink to improve the dispersion of particleswithin the ink. The surfactants adsorb on the surface of theparticles and increase the steric repulsion between the parti-cles, preventing coagulation. Inkjet printing is slow in com-

parison to other printing techniques and the printed layersare relatively thin (<2–4 mm), which limits its use for print-ing thick composite battery electrodes. Due to its high reso-lution, inkjet printing could be used to print micro-batteriesfor microelectromechanical systems (MEMS) applications.Inkjet printing has been used to fabricate electronic compo-nents such as thin-film transistor (TFT) backplanes,[140,141]

solar cells,[142] OLEDs,[143] and conductive traces.[144]

Ho et al. demonstrated 3D Zn–AgO micro-batteries fabri-cated using a custom-built super inkjet printer (SIJP).[62] TheSIJP uses electrohydrodynamic actuation, enabling the abili-ty to print features with dimensions less than 1 mm. The dropsize generated by a SIJP is three orders of magnitude smaller(femtoliter) than conventional inkjet printers (picoliter). Dueto the small size of the resulting droplets, the ink driesduring its flight from the print head to the substrate. Asa result, successive drops can be printed accurately ona fixed location to create high-aspect-ratio features (Fig-ure 16 a–c). The setup was used to print silver electrodes withevenly distributed pillars to increase the areal capacity of thebattery. Once the electrodes were printed and baked, theywere dipped in a bath with a KOH/ZnO electrolyte. Zn waselectrodeposited onto one electrode and the silver onto thecounter electrode was oxidized, forming a Zn–AgO battery.The battery with the pillars gave a higher areal capacity (�2.4 mAh cm�2) than the batteries without pillars (�1.5 mAh cm�2). Due to the mechanical stresses generatedin the silver electrodes during oxidation to the Ag2O andAgO state, the battery was only electrochemically stable fora few cycles (Figure 16 d,e).

Figure 15. Schematic illustration of the inkjet printing process using a datapulse to generate droplets on demand using a pressure transducer.

Figure 16. a) Assembly of the super inkjet-printed 3D zinc–silver microbattery. b) Super inkjet-printed silver electrode with an array of pillars. c) Micrograph ofpillars. d) Galvanostatic discharge curves of the battery with increasing cycle number. e) Galvanostatic discharge curves of microbatteries with planar and 3Dpillar electrodes. The batteries were discharged at 1.1 mA cm�2. Reprinted with permission.[62] Copyright (2009), IOP Publishing Ltd.



Wang et al. demonstrated a traditional lithium-ion battery,where the active electrodes (LCO and LTO) were printed oncurrent collector foils using commercial inkjet printing.[145]

Inkjet printable inks were prepared by ball milling a mixtureof active particles, carbon black, and PVDF binder witha small fraction of surfactant (Tween-80/FC-4430) in NMP/propylene carbonate (1:1) at 1000 RPM for 24 h. The settlingrate of particles depends on its size. Ball milling the particleshelps to reduce the particle size and the surfactants reducethe coagulation rate by increasing the steric repulsion be-tween the particles. The inks were stable for a period of30 days without any visual settling of the particles. The LCOand LTO inks were printed on current collector foils usinga commercially available inkjet printer. The thicknesses ofthe active layers were approximately 4–5 mm. The inkjet-printed batteries demonstrated electrochemical characteris-tics similar to batteries printed using conventional methods.

4.6. Flexographic printing

Flexographic printing is a mature printing technology and itis used in the packaging industry to print labels for foodpackaging and corrugated shipping boxes (Figure 17). The

web speeds can reach as high as 600 mmin�1 and the lowcost of the printing plates and improvements in layer-to-layerregistration have increased its popularity in recent years.[146]

Flexographic printing is a high-speed web-based printing pro-cess that uses a relief plate to transfer images onto a sub-strate. Depending on the complexity of the image, multipleprinting steps are used to form the final image. A standardflexographic printing setup consists of four cylinders—thefountain roll, anilox roll, printing plate cylinder, and impres-sion cylinder. The fountain roll rotates in an ink reservoir.The purpose of the fountain roll is to pick up the ink andtransfer it to the anilox roll. The anilox roll is chrome platedand it contains micro-engraved cells throughout its surface.The purpose of the cells is to form a thin layer of ink on theanilox roll, which is then transferred to the image plate. Theanilox roll contains a doctor blade to wipe off the excess ofink from the roll. The printing plate cylinder consists ofa photopolymer plate with a relief pattern of the image to be

printing. The printing plates are fabricated by exposing thephotopolymer plate to UV light through a negative mask.The regions exposed to the UV light crosslink and the rest ofthe photopolymer can be removed by washing in a solvent.The flexographic plates are attached to the cylinder usingdouble-sided tape. The substrate to be printed is passed be-tween the printing plate cylinder and the impression cylinderthrough a web-based feed process. The impression cylindersupports the substrates and applies a constant force betweenthe substrate and the printing plates, which helps to transferthe ink from the printing plates to the substrate. In a flexo-graphic printing setup, the thickness of the printed image islow (1–3 mm). Standard battery electrodes have thicknessranging from 20 to 80 mm. A flexographic printing setup fordepositing battery slurries would require multiple print sta-tions to build up the thickness of the electrodes. The interestin smart packaging and “internet of things” makes flexo-graphic printing interesting. Smart sensors and power sourcescan be printed on conventional packaging using existingsetups.[66,147–151] Modification to conventional flexographicplates could help increase the quantity of the ink transferredduring a single print pass.

Recently, Wang et al. developed MnO2 cathode inks fora flexographic-printed zinc-based battery (Figure 18 a).[66] Inthis work, the authors studied the effect of the solvent,binder, and additives on the printability of a MnO2 ink. Theinks showed a shear-thinning behavior at high shear stress,which is important for efficient ink transfer from the flexo-printing pads to the substrate.[58,152] Another important pa-rameter is the critical shear stress of the ink, which repre-sents the transition point where the internal structure of theink breaks down and the ink transitions from solid-like toliquid-like behavior (Figure 18 b). Inks with low critical shearstress break down during the ink-transfer step and getsqueezed to the edge of the printed area. MnO2 inks withPSBR (polystyrene butadiene rubber) binder had a high crit-ical shear stress and showed excellent printability and prop-erties, similar to commercially available graphic inks. Theaverage thickness of the MnO2 electrodes formed usingflexographic printing was 1–3 mm, which is much lower thanthe thickness of standard battery electrodes (20–80 mm). Thedimensions of the cells on standard flexographic plates limitthe quantity of ink that can be transferred in a single printingpass. The authors printed the MnO2 inks 10 times to increasethe thickness of electrodes (20 mm) (Figure 18 c). With in-creasing print passes, the roughness of the electrode de-creased. The flexographic-printed MnO2 electrodes werecycled with zinc foils as the counter electrodes. The capacityof the battery after 20 electrochemical cycles was approxi-mately 0.05 mAh cm�2 (Figure 18 d).

4.7. Screen printing

Screen printing uses a screen of woven material (syntheticfiber or steel mesh) that is glued under tension to a support-ing frame (Figure 19). The desired pattern is formed in themesh by filling the screen with a photosensitive emulsion

Figure 17. Schematic illustration of the flexographic printing process. Thefountain roller fills the anilox roller. The anilox roller transfers inks onto theraised features on the printing cylinder, which is transferred by passing a webof substrate between the printing cylinder and the impression roll.



and then selectively crosslinking the emulsion and removingnon-cross-linked areas with a solvent to form openings in themesh, resembling the print pattern. The screen filled with theink is brought in proximity to the substrate and a squeegee isused to force the screen to touch the substrate; the squeegeeis moved linearly along the screen bringing the open areas inthe mesh into contact with the substrate. Screen-printableinks should have high viscosity and low vapor pressure. Thewet thickness of the screen-printed layer depends ona number of factors: the diameter of the mesh and the open-ing size, the emulsion thickness, the concentration of ink,and the fraction of ink transferred to the substrate. Screen-printing is used extensively in industry for printing text andimages on fabrics. Screen-printing is inherently a batch pro-cess but it can be made compatible with a roll-to-rollsetup.[153,154] A screen with the patterns can be attached ontoa rotary cylinder using a squeegee on the inside to press the

ink and a substrate feed on the outside supported on a roll.The rheology requirements of screen printing inks are similarto standard battery slurries. Screen printing is the mostpromising technique for printing batteries, as the printedelectrodes are thick. The ink can be patterned and the tech-nique is compatible with roll-to-roll processing.[153]

Kang et al. recently demonstrated a flexible thin-film lithi-um-ion battery by sequentially screen printing the currentcollectors, active layers, and the polymer gel electrolyte ontothe heat-sealable side of an aluminum-laminated pouch (Fig-ure 20 a, b).[69] The conducting carbon paste for the currentcollector was prepared by mixing carbon black and graphitewith hydroxypropyl cellulose as the binder. The current col-lector reached its saturation conductivity of 100 W/& ata thickness of 15 mm. The slurries for the cathode and anodewere based on LCO and natural graphite, respectively. Fora comparative study, evaporated layers of copper and alumi-num replaced the carbon current collectors for the anodeand cathode, respectively. In summary, the battery with theevaporated current collector demonstrated the best per-formance (Figure 20 c). Batteries using the carbon currentcollector showed an ohmic potential drop due to low conduc-tivity of the current collectors. The areal capacity of the bat-tery was 2.5 mAh cm�2 with a capacity retention of 84 %after 50 electrochemical cycles. The battery was able to main-tain its mechanical integrity after bending 200 times to a cur-vature of 20 mm. In other reports, Park et al. characterizedthe electrochemical performance of screen printed LCO-based cathodes for application in solid-state batteries.[155, 156]

Table 1 provides a comparison between the printing tech-niques discussed in this section. We compare parameters

Figure 18. a) Schematic of proposed multi-station flexographic printed process of large-scale battery production. b) Shear stress as a function of shear stressfor five inks. c) Cross-sectional SEM micrograph of flexographic-printed MnO2 electrode with PSBR as the binder on current collector foils. d) Discharge capaci-ty of flexographic-printed MnO2 electrode with gel electrolyte and zinc foil as the counter electrode, discharged at current density of 0.1 mA cm�2. Reprintedwith permission.[66] Copyright (2014), Elsevier.

Figure 19. Schematic illustration of the screen-printing process.



such as ink wastage, patternability, printing speed, ink prepa-ration, ink viscosity requirements, thickness of the driedelectrode, and possibility of using them in a roll-to-roll setup.The choice of printing process depends on the rheologyof the ink, desired thickness of the electrode, and patterna-bility.

5. Non-Printed FlexibleBatteries

In the previous section we re-viewed several printing tech-niques for depositing batterycomponents. Batteries fabricat-ed using printing processes areattractive because they can befabricated in large quantities atlow cost. Over the past coupleof years, there have been nu-merous reports of non-printedflexible batteries using nano-structured active materials andcarbon-based materials, suchcarbon nanotubes and gra-phene, to improve the flexibilityand electrochemical per-formance of the battery. Thesebatteries are divided into twogroups. The first group consistsof batteries without traditionalbinders, where the active parti-cles are supported within ultra-long CNTs, graphene, or micro-fibrillated cellulose, which serveas conductive additive and me-chanical support for the activeparticles.[16,89,92–97,99, 100,157–164]

Due to absence of binder, thevolumetric energy densities ofthese batteries are higher and

the carbon support improves the flexibility of theelectrodes. The second group includes batteryelectrodes where the active material is synthesizeddirectly on flexible carbon supports such ascarbon textiles, CNT mats, or graphenefoams.[57,91,92, 101–104,164–175] Due to intimate contact be-tween the active particles and the conductive sup-port, these batteries have high rate capabilities. Inthe section, we will briefly discuss the advantages ofthese batteries. We encourage the reader to refer torecent review papers on flexible lithium-ion batter-ies for detailed discussion on such batteries.[13–16]

The conductive additives, binder, and the currentcollectors in battery electrodes are inactive andthey increase the overall weight and reduce theenergy density of the battery.[18,176] Ultra-long CNTscan be intermixed with active particles to formhighly flexible battery electrodes.[177] The CNT net-

work helps to improve the electronic conductivity of theactive layer and, at the same time, serves as the current col-lector for the electrodes. The CNTs serve as a support forthe active particles and do not require additional binder. Thebinder in a conventional battery is non-conductive and itblocks the surfaces of active particles. The removal of binder

Figure 20. a) Photograph of the current collector (left), positive (middle), and negative (right) electrode screenprinted on an aluminum-laminated pouch. b) Photograph of the lithium-ion battery formed by screen printing theanode, cathode, and gel electrolyte on evaporated current collectors. c) Galvanostatic charge/discharge curve ofthe batteries with different gel electrolytes operated at C/5 rate. Reprinted with permission.[69] Copyright (2014),Elsevier.

Table 1. Comparison of printing techniques.

Technique InkWaste[a]

Pattern Speed Ink Prepa-ration

Ink Vis-cosity

Thickness[mm]

R2R Com-patible

blade 2 0 1–3 1 1–5 10–200 yesdispenser 1 4 1–2 2 1–4 0.5–50 yesstencil 2 3 1–3 1 2–5 10–200 nospray 2 2 1–2 2 1–3 5–200 yesinkjet 1 4 1–3 4 1 0.5–5 yesflexographic 2 2 3–5 3 1–3 0.5–5 yesscreen 2 2 1–4 2 3–5 10–100 yes

[a] Ink Waste: 1 (none), 2 (some), 3 (considerable). Pattern: 0 (0-dimentional), 1 (1-di-mentional), 2 (2-dimentional), 3 (pseudo, 3-dimentional), 4 (digital master). Speed:1 (very slow), 2 (slow, <1 m min�1), 3 (medium, 1–10 mmin�1), 4 (fast, 10–100 m min�1), 5 (very fast, 100–1000 mmin�1). Ink preparation: 1 (simple), 2 (moder-ate), 3 (demanding), 4 (difficult). Ink viscosity: 1 (very low, <10 cP), 2 (low, 10–100 cP), 3 (medium, 100–1000 cP), 4 (high, 1000–10000 cP), 5 (very high, 10000–100 000 cP).



from the electrodes improves the energy density of the bat-tery and reduces electrode polarization at high operatingrates. Due to the high aspect ratio of the carbon nanotubes,a small weight fraction of CNTs is sufficient to form a con-ductive matrix, and the high porosity of the electrodes helpsto improve the accessibility of the electrolyte. Luo et al.demonstrated binder-free LCO/carbon nanotube cathodesfor lithium-ion batteries (Figure 21 a, b).[84] The cathode inkwas prepared by dispersing LCO particles with super-alignedcarbon nanotubes (SACNTs) in ethanol. The dispersion washomogenized using an ultrasonication process. Due to thehigh density of the composite mix, the LCO–SACNT was co-deposited to the bottom of the container as soon as the ultra-sonication process was terminated (Figure 21 c,d). Due tohigh strength of the SACNT bundles, the LCO–SACNT com-posite electrode was strong and could be flexed without

cracking. The strength and Young’s modulus of the classicalLCO–Super P electrode were 3 % and 10 % of the binderfree electrodes with SACNTs, respectively (Figure 21 e). Thespecific capacity of binder free electrodes with 3 wt % ofSACNTs was 28.7 % higher than for standard electrodes with10 wt % of Super P conductive additive. The process can bescaled to a roll-to-roll process by making gel-like slurries ofthe composite and spreading them over supporting sub-strates. The authors demonstrated a large-scale compositeelectrode with diameter of more than 15 cm and thickness of24 mm, showing the possibility of fabricating such electrodesfor larger electrodes (Figure 21 f).

In another fabrication process, the active materials can begrown directly on conductive substrates. Substrates such ascarbon nanotube mats, carbon-based fabrics, and graphenefoams are used as templates for the in situ synthesis of active

Figure 21. Schematic of the binder-free LiCoO2–SACNT cathode (a) and classical LiCoO2–Super P cathode (b), respectively. c) Different features of SACNT andCNT-1: SACNT network vs. CNT-1 suspension; LiCoO2–SACNT co-deposition vs. LiCoO2–CNT-1 suspension; flexible LiCoO2–SACNT composite vs. crackedLiCoO2–CNT-1 composite. d) Cross-sectional SEM image of LiCoO2-1 wt% SACNT composite electrode. e) Mechanical properties of the binder-free LiCoO2–SACNT composites and the LiCoO2–Super P composite. f) Free-standing LiCoO2–SACNT cathode made on a large scale. Reprinted with permission.[89] Copyright(2012), Wiley.



materials. Due to the thinness of the active materials, theclose proximity to the conductive network, and the porousnature of the electrodes, they can operate at much higher C-rates as compared to conventional battery electrodes. Theflexible nature of the supporting substrate makes these elec-trodes flexible, and the thinness of the active materials re-duces the stresses within the electrode at low bending radii.The fabrication process is currently a batch process. Envi-sioning a roll-to-roll fabrication process for such electrodes isnot clear. The high rate capabilities of these electrodes makethem attractive for future electronic applications for whichhigh power is necessary. Na et al. demonstrated graphene-based lithium-ion batteries with ultrafast charge and dis-charge rates (Figure 22 a).[57] The active materials were syn-thesized on a graphene foam that had a thickness of 100 mmand weight of approximately 0.1 mgcm�2. The porosity of thefoam was 99.7 % with a very high specific surface area. Thefoam could be bent into arbitrary shapes without breaking.LTO and LFP were synthesized on the graphene foam byusing hydrothermal synthesis (Figure 22 b–d). The full-cellwas fabricated by stacking the LTO/GF and LFP/GF electro-des together with a polypropylene separator and sealed be-tween 250 mm thick PDMS sheets (Figure 22 e). The total

thickness of the battery was less than 800 mm. The full cell at10 C yielded a capacity of 117 mAh g�1, approximately 88 %of the capacity at 1 C rate. The battery was able to retain itscapacity after flexing multiple times to a bending radius of5 mm (Figure 22 f, g).

6. Novel Battery Systems

Innovations in battery designs and packaging can enable thepossibility of powering electronics embedded within jewelry,tattoos, and clothing. In this section we will briefly reviewbatteries that have novel form factors and mechanical prop-erties that are not possible with conventional battery designs.

The shape and size of conventional batteries have limiteddesign innovations in consumer electronic devices. Flexibleand stretchable batteries are generally in the form of thinsquares, which limits the shape of the device. Kwon et al.demonstrated a cable-type lithium-ion battery, which couldretain its electrochemical capacity after bending to differentforms.[90] The battery design consisted of several electrodestrands coiled into a hollow spiral (anode) with cathodeslurry along the outer ring and PET separator to electricallyisolate the anode and cathode (Figure 23 a). A thin alumi-

Figure 22. a) Schematic of the flexible battery containing an anode and cathode based on 3D interconnected graphene foam. b) Photograph of free-standingflexible LTO/GF. SEM micrographs of LTO/GF at different magnifications (c,d). e) Photograph of the battery powering a red LED during flexing. f) Galvanostaticcharge/discharge curve of the battery in the flat state and after flexing. g) Cyclic capacity of the battery under flat and bend conditions. Reprinted with permis-sion.[57] Copyright (2012), PNAS.



num wire served as the current collector for the cathode. Thehollow structure enabled easy wetting of the electrolyte andit absorbed the stresses generated within the battery due toexternal mechanical deformation. Electrodeposited Sn–Ni ona copper wire and slurry of LCO served as the anode andcathode of the battery, respectively. The battery was electro-chemically cycled between 2.4 and 4.2 V and provided a ca-pacity of 1.0 mAh cm�1 during first few cycles (Figure 23 a).The cable battery is extremely versatile and can be woven indifferent shapes and forms (Figure 23 c,d). Since the demon-stration of the cable-type lithium-ion battery, there havebeen numerous reports on other batteries and supercapaci-tors that are in the form of a wire.[178–183] Fu et al. integrateda fiber-shaped dye-sensitized solar cell with a supercapacitor,which can convert solar energy into electrical energy andstore it within the supercapacitor. The overall energy conver-sion efficiency of the integrated system was approximately2.1 %.[178] Ren et al. recently demonstrated a wire-shapedlithium-ion battery that can be flexed and stretched. Theelectrodes were based on multiwalled carbon nanotubes em-bedded with LTO and lithium manganese oxide (LMO). Thebattery had a capacity retention of 97 % after 1000 flexingcycles and was stretchable to a strain of 100 % after wrappingthe electrodes around a stretchable substrate.[183]

Jiang and co-workers presented a design approach wherelithium-ion batteries were folded along predefined lines tocreate complex shapes that can be twisted and folded with-out significant mechanical damage (Figure 24 a, b).[117,184]

Folding improved the areal capacity (mAh cm�2) of the bat-tery by increasing the areal loading of the active materi-als.[26, 88] Laboratory Kimwipes embedded with carbon nano-tubes (CNT) formed the current collectors of the batteries.LCO and LTO inks were printed onto the current collectors

to form the cathode and anode of the battery. The areal ca-pacity of the flat cell increased from 0.2 to 2.0 mAh cm�2

after using origami folding.[117] Using similar approaches,Choi and co-workers demonstrated a large-area foldable bat-tery with conductive fabric as current collectors for the bat-tery.[26,88] The electrodes were stacked together with a separa-tor and creased along pre-defined locations to form a foldablebattery. In comparison to flexible batteries, foldable batteriesexperienced stresses only at the regions along the fold. Thehigh mechanical strength and fibrous nature of the conduc-tive fabric ensured continuous electrical contact within thecurrent collector and between the current collector and theactive particles. The battery retained approximately 88 % ofits capacity after 5500 deep folding-unfolding cycles.[26] Theauthors connected multiple large-area batteries to increasethe voltage of the battery pack and demonstrated designswhere the battery pack could be folded and rolled.[88] Folda-ble batteries could find application as energy storage mediafor lightweight, foldable solar cells, where the device wouldbe folded using origami techniques during transportation andthen opened during use.[26,88,117, 184–186]

In recent years, transparent devices such as displays, touchscreens, and solar cells have been demonstrated.[187–190] Forsystem-level integration of these devices, it would beneficialto have a power source that is transparent. Transparent elec-tronic components are fabricated by reducing the thicknessof the active materials below their optical absorptionlength.[191–194] Battery active materials are good absorbers oflight, even below thickness of 1 mm and reducing thickness ofthe active layers decreases the areal capacity of the bat-tery.[194] Hence it would be beneficial to use an electrode ar-

Figure 23. a) Optical cross-sectional image of the cable battery with hollowanode having an outer diameter of 1.2 mm. b) Galvanostatic charge/dis-charge curve of the cable battery with hollow and non-hollow anodes. c) Pho-tograph of a cable battery with a length of 25 cm used to power a red LEDscreen. d) Galvanostatic discharge curves of the cable battery under differentbending strains every 20 min. Reprinted with permission.[90] Copyright (2012),Wiley.

Figure 24. a) Photograph of the origami battery in the completely unfoldedstate and powering a green LED and after folding the battery to the complete-ly compressed state. Reprinted with permission.[117] Copyright (2014), Mac-millan Publishers Ltd. b) Photograph of a battery sealed with Parylene-C andschematic of Miura folding procedure for 5�5 cell pattern. Reprinted withpermission.[184] Copyright (2013), American Chemical Society.



chitecture that enables high areal loading. Yang and co-workers demonstrated an innovated approach where theactive layers were patterned in the form of grids with a fea-ture size lesser than detection limit of the human eye (Fig-ure 25 a–e).[85] The active inks were deposited onto a PDMSsubstrate with grid-like trenches by using a unique microflui-

dic-assisted method. Firstly, gold was evaporated onto thepatterned PDMS to form the current collectors for the elec-trode. The PDMS substrate was then plasma treated, anda thin layer of PDMS was placed on the grids. An aqueousslurry containing the active materials was dropped on oneside of the blocking PDMS and capillary action caused theink to flow inside the grids. The anode and cathode werestacked with a polymer separator to form the battery. Theenergy density of the full-cell, including the packaging, was10 WhL� and the transparency was 60 % (Figure 25 f, g). Theinitial capacity of the full-cell was 100 mAh g�1 and it drop-ped to 80 mAh g�1 after 15 cycles. From an applications per-spective, transparent batteries can be used to power transpar-ent epidermal sensors and displays with low power require-ments.

Stretchable batteries have gained interest in recent years,as they offer superior mechanical properties to flexible bat-teries. Stretchable batteries can flex, stretch, twist, and con-form over curvilinear objects. Fabricating such batteries isnon-trivial and over the years there have only been handfulof demonstrations. The primary requirement of a stretchable

battery is a stretchable current collector, which can retain itselectrical conductivity after stretching. Once a suitablestretchable current collector is identified, the active layers ofthe battery should be designed such that there is no delami-nation or cracking during stretching/twisting. The packagingfor the batteries should be stretchable and impermeable to

moisture. Examples of stretcha-ble current collectors includescarbon black paste with siliconoil (Figure 26 a),[80] stretchableconductive fabric (Fig-ure 26 b, d),[81,87] a segmentedcurrent collector with stretcha-ble interconnects (Fig-ure 26 c),[56] and conductingnanowires embedded in PDMS(Figure 26 e).[77] The activelayers are printed onto thestretchable current collectorsusing conventional printingtechniques. The region betweenthe anode and cathode is filledwith a polymer gel electrolyteto provide ionic contact be-tween them.

Kaltenbrunner et al. demon-strated the first intrinsicallystretchable battery with Zn andMnO2 as the active materials inthe battery (Figure 26 a).[80] Acarbon black/silicon oil pastewas printed onto an elastomersupport to serve as the currentcollectors. Zn and MnO2 pasteswere printed on the current col-lectors to form the anode and

cathode of the battery, respectively, which was followed byprinting an electrolyte gel (NH4Cl, ZnCl2) to complete thebattery. The battery was laminated with another layer ofacrylic elastomer. To prevent the shorting of the batteryduring stretching, the anode and cathode of the battery wereplaced side-by-side with a gap of 3 mm between them. Thebattery was able to continuously power a green LED evenafter stretching to a strain of 100 %. The capacity of the bat-tery at 0 % strain was 7 mAh and it dropped to 3.1 mAh at50 % strain. The drop in capacity was due to cracking of theactive layers and the current collectors, which led to a dropin electrical conductivity and particle-to-particle contact.After the initial report by Kaltenbrunner et al. , there havebeen other reports on stretchable batteries with designs thatare more resilient to mechanical stretching.[56,77,81, 87,195–198]

Gaikwad et al. demonstrated a Zn–MnO2 based stretchablealkaline battery where the active materials were embeddedwithin fibers of a commercially available stretchable silverfabric (Figure 26 b).[81] The fabric consisted of silver-coatednylon threads, which were weaved through a rubber matrix.The fabric could be stretched repeatedly up to 100 % strain

Figure 25. a) Process flow diagram of fabricating a transparent lithium-ion battery. b) Photograph of the transpar-ent and flexible battery electrode with grid patterns. Magnified optical (c) and SEM (d) micrographsof the batteryelectrode. e) Transparent and flexible gel electrolyte. f) Transmittance of the gel electrolyte, single electrode andfull-cell. g) Transparent battery powering a red LED. Reprinted with permission.[85] Copyright (2011), PNAS.



without permanent deformation or drop in conductivity. Thefabric was embedded with the Zn and MnO2 slurry by usinga simple dip-coating process followed by a blotting step toremove the particles that were not supported within thefibers. The fabric embedded with the active materials under-went plastic deformations of 6 % and 8 % after stretching toa strain of 100 % for 200 and 1000 cycles, respectively. Thebattery was formed by sealing the fabric embedded with Znand MnO2 inks within an acrylic elastomer, with a gel elec-trolyte based on KOH and ZnO and polyacrylic acid (PAA)as the polymeric support. The capacity at 0 % strain was4 mAh cm�2 and the battery was able to retain its capacityeven under stretching to 50 % and 100 % strain. Two batter-ies connected in series were able to power a red LED contin-uously, even after stretching to 150 % strain and twisting by908. The supported electrode design ensured that there wasno delamination of the active materials during stretching. Xuet al. reported on the first stretchable battery based on lithi-um-ion chemistry (Figure 26 c).[56] The battery design wasbased on segmented active layers printed on rigid islandssupported on an elastomeric layer with stretchable serpen-tine interconnects. The cathode and the anode of the batterywere based on LCO and LTO and they were printed on thesegmented current collector by using a transfer printing tech-nique. The anode and cathode were stacked together with an

offset distance of 0.51 mm to prevent electrical shorting anda polymer gel electrolyte was injected between them to pro-vide ionic contact between them. The battery had an arealcapacity of approximately 1.1 mAh cm�2 at a nominal poten-tial of 2.35 V. The battery was able to retain its capacity afterstretching to 300 % strain and powered a red LED continu-ously, even after biaxial stretching to 300 %. The authorsdemonstrated a system for wireless charging of the battery.

7. Future Aspects and Outlook

In past couple of years there has been tremendous interest incompliant power sources for powering the next generation offlexible and wearable electronic devices. Numerous printedflexible and stretchable power sources have been reported inthe literature. The electrochemical capacities of these batter-ies are much lower as compared to traditional batteries. Forexample, the AA alkaline battery and silver-oxide-based coincells have capacities of approximately 2500 and 100 mAh, re-spectively. The capacity of a flexible battery with active areaof approximately 5 cm2 is in the range of 0.5–5 mAh (arealcapacity of 0.1–1.0 mAh cm�2) and they are flexible only toa bending radius of 20–40 mm. These batteries are only suita-ble for powering devices that have very low power require-ments. Powering the future generation of devices such ashealth tags, smart watches, flexible phones, and displays willrequire a significant increase in the capacity of compliantbatteries. Currently, the thinness of the active materials inflexible batteries reduces the areal capacity of the battery.Flexible batteries would benefit from the development ofnew active materials with high energy density.[199–209] Thiswould help improve the areal capacity and volumetricenergy density of the battery without increasing the overallthickness of the active layers. In conventional thin-film flexi-ble batteries, single layers of the anode, cathode, and the sep-arator are stacked together and sealed within a flexiblepouch. In commercial prismatic and cylindrical cells, the bat-tery stack is rolled/folded to increase its areal capacity. Itmay be possible to use a similar concept for increasing thecapacity of flexible batteries. Folding the battery stack morethan once makes it thicker and non-flexible. To maintain theflexibility of the battery, the individual battery stacks can beplaced on top of each other such that they are electricallyconnected with each other by flexible interconnects, but me-chanically isolated to ensure that the stacks can move freelyupon flexing the battery. Such design will help the batteryabsorb stresses during flexing and increase the areal capacityof the battery. In another possible design, the battery can bedivided into number of microcells with stretchable intercon-nects between them. The microcells would be rigid/non-flexi-ble, but the stretchable interconnect would provide the re-quired mechanical flexibility to the battery.

The commercialization of flexible batteries will requirebattery designs and fabrication processes that are easy totransfer to an industrial roll-to-roll manufacturing setup. Thecomponents making up the battery should be low cost andeasily available to keep the cost of the battery low. Printing

Figure 26. a) Zn–MnO2 stretchable battery based on carbon black/silicone oilas the current collector. Reprinted with permission.[80] Copyright (2010),Wiley. b) Zn–MnO2 stretchable battery with stretchable silver fabric as thecurrent collector. Reprinted with permission.[81] Copyright (2012), Wiley.c) Stretchable lithium-ion battery. Reprinted with permission.[56] Copyright(2013), Macmillan Publishers Ltd. d) Rechargeable Zn–Mn2 stretchable bat-tery with current collectors based on nickel fabric. Reprinted with permis-sion.[87] Copyright (2013), The Royal Society of Chemistry. e) Stretchable Zn–AgO battery. Reprinted with permission.[77] Copyright (2014), Wiley.



processes will play an important role in the industrial manu-facturing of flexible batteries. Printing processes are econom-ical and easily customizable. In addition, the seamless inte-gration of batteries with flexible electronics is important.Batteries can be customized based on the mechanical andpower requirements of the device. Batteries can be printeddirectly on top of devices or they can be fabricated and pack-aged separately and then integrated with the device. The in-tegration of rechargeable batteries into autonomous sensorsand devices would require a method to charge the batteriesafter it is completely discharged. Using an external chargingsystem for these devices would be too expensive. Thus, suchdevices would benefit from the coupling of energy sources(solar cells, thermoelectric modules), which can charge thebatteries after the discharge step.

Improvement in packaging is also equally important forthe success of compliant batteries. The development of thin-ner, flexible packaging that is impermeable to moisture andprevents the egress of electrolyte from the battery will helpto improve the volumetric energy density of the batteriesand make them more compliant. The commercially availablealuminum-laminated pouches used in academic studies arethick (�125 mm) and currently make up the bulk of thethickness of the battery. Standard battery pouches are de-signed and rated for thick prismatic cells. It would be benefi-cial if pouches were designed specifically for thin-film flexi-ble batteries. The packaging for stretchable batteries is notstraightforward. Pouches based on PDMS or stretchable elas-tomers have the required mechanical properties but they arepermeable to moisture. The development of stretchable inor-ganic coatings that can be deposited on PDMS or elastomerswould be helpful to reduce the permeation of moisture intothe cell.

From the perspective of a battery designer, it is importantto know the limits of mechanical flexibility of batteries fora given combination of electrode architecture and currentcollectors, and to know the relationship between structuralchanges within the battery and the electrochemical per-formance of the battery. Such understanding will help batterydesigners set a limit on the minimum bending radius of thebattery, design batteries taking into account the loss in ca-pacity with mechanical flexing, and make design changesthat reduce the structural changes due to flexing. The devel-opment of models to predict the electro-mechanical per-formance of flexible batteries will be beneficial for the futuredevelopment of flexible batteries.

Keywords: batteries · energy conversion · flexibleelectronics · mechanical properties · printing

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REVIEWS

A. M. Gaikwad,* A. C. Arias,D. A. Steingart

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Recent Progress on Printed FlexibleBatteries: Mechanical Challenges,Printing Technologies, and FutureProspects

Printed flexible batteries: Compliantpower sources are necessary for power-ing the next generation of flexible andwearable electronic devices. In thisReview, the material challenges andmechanical limits of flexible printedbatteries are discussed. The authorsreview several printing techniques, andpresent examples of batteries printedusing these methods, in addition to in-troducing other novel non-printedcompliant batteries that have uniquemechanical form.



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