Ultra-thin flexible screen printed rechargeable polymer battery for wearable electronic applications

Ultra-thin flexible screen printed rechargeable polymer battery for wearable electronic applications

Organic Electronics 26 (2015) 386–394 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel ...

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Organic Electronics 26 (2015) 386–394

Contents lists available at ScienceDirect

Organic Electronics journal homepage: www.elsevier.com/locate/orgel

Ultra-thin flexible screen printed rechargeable polymer battery for wearable electronic applications Z. Tehrani a,⇑, T. Korochkina a, S. Govindarajan a, D.J. Thomas a, J. O’Mahony b, J. Kettle c, T.C. Claypole a, D.T. Gethin a a b c

Welsh Centre for Printing and Coating, College of Engineering, Swansea University, Singleton Park, Swansea SA2 8PP, United Kingdom Waterford Institute of Technology, Cork Road, Waterford City, Ireland School of Electronic Engineering, Bangor University, Bangor, Gwynedd LL57 1UT, Wales, United Kingdom

a r t i c l e

i n f o

Article history: Received 7 July 2015 Accepted 9 August 2015

Keywords: Printed rechargeable battery Flexible PEDOT: PSS Gel electrolyte

a b s t r a c t This research has demonstrated how an ultra-thin rechargeable battery technology has been fabricated using screen printing technology. The screen printing process enabled the sequential deposition of current collector, electrode and separator/electrolyte materials onto a polyethylene terephthalate (PET) substrate in order to form both flexible and rechargeable electrodes for a battery application. The anode and cathode fabricated were based on the conducting poly (3,4-ethylenedioxythiophen): poly (styrene sulfonate) (PEDOT: PSS) and polyethyleneimine (PEI) which were combined to form the electrodes. The difference in the oxidation level between the two electrodes produced an open circuit voltage of 0.60 V and displayed a practical specific capacity of 5.5 mAh g1. The battery developed had an active surface area of 400 mm2 and a device thickness of 440 lm. The chemistry developed during this study displayed longterm cycling potential and proves the stability of the cells for continued usage. This technology has direct uses in future personal wearable electronic devices. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction There is a wide range of wearable electronic applications which require direct energy supplies to allow for self-contained operations. These applications demand a combination of; very low cost fabrication, low volume, light weight, flexibility, environmental friendliness and robust performance. Typical examples of power needs for such optimised components are 10 lW for a low power RFID chip, 17 lW at 1.4–1.6 V for an operational amplifier, 20 lW at 0.9 V for a GMR-switch or 5 lW cm2 for the update of an e-Ink display [1]. Printed batteries are reported to have lower durability, failure due to defects and non-uniform surfaces, attributed to difficulty in obtaining optimum materials and their associated challenges for formulation into an ink [2]. However, a completely printable battery is attractive because it gives freedom in form for integration with wearable devices that can only be commercially delivered through the use of printing technology. Energy source requirements for wireless sensor node applications for primary and rechargeable printable battery are low cost, high safety and small size [3]. However, despite these possibilities, the applications ⇑ Corresponding author. E-mail address: [email protected] (Z. Tehrani). http://dx.doi.org/10.1016/j.orgel.2015.08.007 1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

of such thin-film batteries for different products are currently limited by their cost of production [1]. With the miniaturisation of electronics in mind, the design of batteries has changed over the past decades, and different types of prismatic or other thin batteries have been developed [3]. Several attempts have been made to produce printed batteries commercially. Thin primary batteries using the zinc–manganese chemistry have been manufactured by means of printing and are currently in use [4–8]. These technologies are still at an early stage, and the ability to print these will create new avenues for manufacturing novel miniaturized devices. Large area rechargeable printable battery technology, as would be required for wearable technology is still in its early phase of development. The physical and electrical contact within a flat thin film battery cell as well as working duration, have been reported as limitations in design. These may be addressed through the design of anode and cathode components, through sealing film layers, or by adding binders or adhesives to the various components of the system [9]. As an example, an inkjet printed zinc–silver 3D battery has been reported having an energy density of 3.95 mWh cm2 [10]. This system is formed by electroplating zinc from a ZnO solution, but is limited by the challenge of dissolving ZnO in the alkaline electrolyte. In order to avoid this issue, there has been the development of an alkaline gel electrolyte to form a planar printed

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primary 2D silver–zinc battery to obtain an area energy density of 4.1 ± 0.3 mWh cm2 [11]. A recently developed flexible zinc–air battery has been produced by screen-printing a zinc/carbon/ polymer composite anode, and a vapour polymerised PEDOT cathode onto two sides of a photo quality paper [12]. The lithium chloride and lithium hydroxide electrolyte was inkjet printed in eight layers to be absorbed within the papers cross-section and therefore, between the two electrodes [12]. Substantial work has been undertaken to develop secondary thin-film batteries. For example, a thin-film Li-ion microbattery has been developed using sputtered electrodes on a glass surface and a micro-injected sol–gel electrolyte [13]. A screen printed nickel metal hydride (Ni-MH) rechargeable battery on thin, flexible roll-fed plastic materials has been fabricated and a capacity of 32 mAh was reported [5]. A flexographically printed rechargeable Zn–MnO2 battery with MnO2 cathode, zinc anode and ionic gel electrolyte consisting of a 1:1 mixture of poly(vinylidene fluoride-co-hexafluoropropene) (PVDF-HFP) and 0.5 M solution of zinc trifluoromethanesulfonate (Zn + Tf) salt dissolved in 1-butyl3-methylimidazolium trifluoromethanesulfonate (BMIM + Tf) ionic liquid has been developed [14]. This yielded a first cycle capacity of 0.13 mAh cm2 with a 72% drop in capacity for successive cycles, with a reported reversible capacity of 0.05 mAh cm2 [14]. The decrease in capacity demonstrates a significant loss of active material after a few cycles, thereby reducing its use in practical applications. Some of the reported disadvantages of the Zn–MnO2 batteries are rapidly decreasing capacity with cycling and depth of discharge, limited cycle life and higher internal resistances when compared with NiMH [15]. A flexible Ag–Zn rechargeable battery has been developed using screen-printing on tattoo paper. This uses an alkaline gel electrolyte [16] and has the advantage of being thin, light weight, flexible, inexpensive and biocompatible. This configuration provided a maximum discharge capacity of 2.1 mAh cm2, but exhibited limited cycling capability and capacity loss due to high internal resistance [16]. Various other configurations of batteries include microfabricated alkaline Ni–Zn [17] and laser direct-writing of Zn–AgO micro-batteries [18], Li-ion electrode microarrays [13] and thin-film lithium batteries have been developed by several groups in recent years [4] and [19–22], but none of these have been printed on flexible substrates. Based on the preceding discussion the limitations of the thinfilm technologies can be broadly summarized as having lower capacities, recharge cycle limitations, being not fully printable and exhibiting challenges relating to their assembly and temporal stability. This leads to an interest in developing an all polymer based battery technology. In previous work, a battery concept comprising PEDOT: PSS and PEDOT: PSS with PEI electrodes covered with a PSSNa electrolyte demonstrated low open circuit voltage (OCV) of 0.5 V for a co-planar polymer battery fabricated using casting [23]. In this configuration, the two electrodes are deposited by casting, leaving a gap between them and the electrolyte was then deposited over this gap again by casting. This design led to an uneven current density distribution across the electrodes, resulting in increased internal resistance and lower performance. In this work, we use the concept set out in [23] as a starting point. However we differentiate through the development of a layered architecture in which electrodes that have a large area in contact with the electrolyte are fabricated by screen printing and a separator that can also be fabricated by screen printing is placed between the electrodes to form the battery. The separator has to serve two purposes, the first is to facilitate ionic conduction while at the same time providing electrical insulation. This will be referred to as an electrolyte-separator in the following text.

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The strategic aim of this design is to increase battery capacity and to use a fabrication route that will facilitate large scale manufacturing by printing. In a stacked configuration (Fig. 1), the electrodes, electrolyte and an insulator/separator are fabricated as a series layers. The stacked design is more suited for high duty applications due to a better energy density. The very short, vertical ion path through the consistent thickness electrolyte-separator layer to the electrode layers leads to much higher charge and discharge currents and uniform performance across the electrodes. Therefore, the shape or configuration of the cell influences the battery capacity as it affects factors such as internal resistance due to ion mobility and heat dissipation [15]. To prevent short circuits between the electrodes, a pinhole-free separator now becomes very important. A major manufacturing problem is the handling of the electrolyte/separator and a seal against water vapour is necessary in order to avoid dehydration and complete breakdown of the battery [5]. Presently, there are major hurdles in formulating such a printable electrolyte: giving a formula that remains wet and stable over the battery life. Currently, no printable solution has been formulated, which prevents production of batteries on an industrial scale. With regard to performance, within this work we also demonstrate for the first time the cyclic charge–discharge behavior of this battery design. Thus in this research work, we address have overcome some of the challenges plaguing the area of flexible printable wearable batteries to demonstrate an approach that facilitates integration into a roll-to-roll (R2R) process. Once integrated into a R2R process, thinfilm batteries have the potential to become one of the next generation of power sources for portable electronic applications due to the unique possibility for low cost, high-volume production on flexible substrates. 2. Experimental methods 2.1. Materials and reagents A 175 lm thick heat stabilized SU320 polyester film from HiFi Industrial films was used due to its low surface roughness and ability to withstand temperatures up to 150 °C without deformation. This transparent film also allows visual examination of the printed layers for pinholes. Silver polymer ink (C2080415P2) and carbon ink (C2050503P1) were purchased from Gwent Electronic Materials (GEM, Gwent, UK). The anode and the cathode comprise silver and carbon layers. The silver ink has been chosen to provide a high level of conductivity, as this is used as a current collector and the highly conductive carbon ink chosen provides better adhesion to the underlying silver layer in an attempt to maintain lower internal resistance of the battery and to provide protection for the silver layer from undergoing redox reactions. The PEDOT: PSS (EL-P-5015) ink was purchased from Agfa. PEDOT: PSS is one of the most explored organic conducting polymers in electrochemical devices due to its higher conductivity and stability [23]. It is regarded as a promising material for electronic organic devices as it enables the fabrication of cost-effective, flexible devices through mass production [23–24]. Therefore, both the anode and the cathode were made of PEDOT: PSS. Additionally, on the anode, a polyethyleneimine (PEI) layer was applied in order retains the air stability of the electrode and the PEDOT: PSS layer underneath [23]. PEI, xanthan gum and hydroxypropyl cellulose (HPC) were obtained from Sigma– Aldrich. Using polymer electrolytes instead of aqueous electrolytes in batteries reduces complications significantly as it simplifies encapsulation and hence increases shelf life and the range of

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Fig. 1. The schematic diagram of the cross section of the polymer battery.

operating temperatures [25]. The solid electrolyte poly (styrene sulfonate sodium) (PSSNa) was purchased from Alfa Aesar. The PSSNa electrolyte in the battery provides mobility to the sodium cations (Na+) that dissociate from their polymer counter-ions to migrate to the negatively charged electrode in order to complete the redox reaction at the cathode. The polyanions (PSS) are regarded as being immobile. 2.2. Battery fabrication 2.2.1. Fabrication of electrodes by screen-printing A battery was fabricated with electrodes, each measuring 400 mm2. The components of the battery were printed using a DEK 248 screen printing press under ambient air conditions. The substrate was cleaned using isopropyl alcohol (IPA) before printing the bottom and top electrodes of the battery in a stack configuration. The cathode is composed of three layers that include PEDOT: PSS printed on a current collector comprising two layers: a first silver layer that is then covered with a layer of carbon. The anode is composed of the three layers used for the cathode with the addition of a PEI layer. The ink formulation and mesh selection, and hence the ink film thickness for each layer that formed the anode and cathode, were chosen or developed based on the requirements of the amount of deposited active material, electrical properties, electrode stability and electrolyte penetration (Table 1). There are many process settings that can influence the deposition thickness, uniformity and topography of the printed layer along with print to print consistency. Through a comprehensive experimental programme, this was optimised so that the print processing time for each layer was identical, thus assuring a path to full scale manufacturing. The dry ink film thickness is listed in Table 1 and the relevant process parameters are listed in Tables 2 and 3. When these were established they were found to deposit layers having consistent thickness and sufficiently smooth topography (typically 395 nm Ra). The inks were touch dried at different temperatures on a belt dryer and then cured finally at different temperatures in an oven where they were heated for a fixed time in order to remove residual solvent in the printed layers. The settings are listed in Table 3. Once the silver, carbon and PEDOT: PSS layers were deposited, PEI was also screen printed onto the anode only. Following drying and curing, the color of the anode will change from transparent to dark blue as shown in Fig. 2, due to the heat catalyzed reduction reaction occurring between the PEDOT: PSS and PEI forming

Table 1 Summary of the dry film thickness of deposited layers. Ink

Thickness of dry film

Carbon Silver PDOT: PSS PEI

27 lm 2 lm 395 nm NA

Table 2 Summary of the parameter settings used for the screen printing process (squeegee angle 45°). Parameters

Unit

Setting

Flood speed Print speed Snap off Squeegee load

mm/min mm/min mm kg

70 70 2 10

Table 3 Summary of the settings used for drying the printed layers. Ink

Temperature set on the belt dryer (°C)

Heat treatment time (min)

Heat treatment temperature (°C)

Carbon Ag PEDOT: PSS PEI

120 120 100 100

5 5 5 10

130 130 130 90

PEDOT and PEI: PSS [23] (Eq. (1)), indicating completeness of this process stage. As the capacity of a printed battery is defined by the amount of electrochemically active material that is used, for measuring the performance, the amount of anode and cathode materials must therefore be very carefully calculated [5]. The mass ratio between PEI–PEDOT vs. PEDOT in this battery is 5:4.5.

2.2.2. Fabrication of electrolyte-separator The key manufacturing challenge is the fabrication and handling of the electrolyte-separator. The separator in a conventional battery is an electrolyte polymer-membrane material which is placed upon the electrodes. The electrolyte must have good ionic but no electrical conductivity, as this will cause an internal short-circuit [15]. Therefore, for manufacturing of printed batteries

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a)

50mm

b)

50mm

Fig. 2. Micrographs of additions to the PEI to the PEDOT: PSS and showing the change in color from light to dark blue due to the reduction reaction that is catalyzed by heat. (a) Anode before PEI and 90 °C heat treatment and (b) anode PEI after 90 °C heat treatment.

in a stack configuration, a printable gel electrolyte-separator is an essential requirement and needs to be developed. During this research, a method was established to produce the electrolyte/liquid-gel solution in the form of a film. Extensive laboratory work based around a factorial approach on binder/solvent combinations and thickener agents resulted in the development of a film electrolyte-separator that eliminates the need for another separator layer. The resulting electrolyte gel was prepared as follows: 2–3 wt% poly(styrene sulfonic acid) sodium salt with a molecular weight 500,000 g mol1 (Alfa Aesar), was dissolved in a mixture of deionised water and isopropanol (>99.7%, Sigma–Aldrich). The solution was then put into an ultrasonic bath for an hour at room temperature in order to completely dissolve the solute. To this, 0.1% hydroxypropyl cellulose was added to increase the viscosity, and the resulting gel was placed on a flat surface and allowed to dry to form a film. A layer of electrolyte (sodium styrene sulfonate – PSSNa) in the form of a gel or a pre-cast film with thickness of about 3 lm was placed over the electrodes covering its entire surface area. 2.2.3. Battery assembly After the cathode and anode were printed and the film electrolyte was placed between these two electrodes, the battery was held together using a clip. No encapsulation was necessary as the electrolyte was not in solution and therefore did not warrant any lamination steps or an air-tight seal. The battery thus produced in a stack configuration was then ready for testing. 2.3. Battery testing Before testing the fully assembled battery, electrochemical investigations on each of the components that comprise the battery were performed, in order to determine their performance individually before assembly. Therefore, half-cell measurements on each of the electrodes were performed against a reference electrode with a known potential at specific conditions of measurement. The two electrodes used were a printed reference electrode (Ag/AgCl) and the printed working electrode. Cyclic voltammetry (CV) measurement was performed individually with the PEDOT: PSS, carbon and silver as the working electrode. CV measurements were conducted on solid electrolyte assays, the potential was scanned from 0.5 V to +0.5 V, at 50 mV/s and all assay tests were conducted in triplicate. Also Galvanostatic Cycling with Potential Limitation (GCPL), OCV and

CV measurements were performed using a Bio-logic Science Instruments’ VMP3 potentiostat. 3. Results and discussion 3.1. System characterisation The layers that formed the electrodes fabricated using screen printing were characterized for their thickness and topography using different techniques that include atomic force microscopy (AFM), white light interferometry (WLI) and scanning electron microscopy (SEM). These were used to perform measurement appropriately at the different stages of fabrication. In order to determine the thickness and roughness of a PEDOT: PSS layer, a single layer was screen-printed onto a plastic substrate and the film topography acquired using AFM in contact mode (data shown in Fig. 3). The edge of the layer was scanned for step measurement. The roughness (Ra), calculated as an average of nine individual measurements, and the thickness, an average of 20 individual measurements, were found to be about 90 nm and 3951 nm respectively, for a single layer of PEDOT: PSS. This shows that the printed PEDOT: PSS ink does not provide a very smooth surface. However, this may be advantageous because increasing the surface area of the electrode can lead to increased performance and efficiency [15]. It was found that a single layer of PEDOT: PSS provided very low battery capacities and therefore, in order to improve battery performance as determined by the target layer thickness, typically 3 layers of PEDOT: PSS was deposited. White light interferometry (WLI) was then used to perform surface characterization of the multiple PEDOT: PSS layers and to check for the reproducibility of the printed features. This work was performed using a Wyko NT9300 optical profiling system from Veeco Instruments Inc. This allowed a full three-dimensional surface profile to be captured, so that print thickness and local surface variations could be evaluated. Five times magnification was used, giving a measurement area of 1.25 mm by 0.94 mm (a resolution of 640  480 pixels with sampling at 1.9 lm intervals). An image from the scan is shown in Fig. 4. The wet layer thickness is typically 1 lm and on drying the solvent content that comprises 60% by volume is removed through evaporation. WLI may be used readily on opaque surfaces using relatively simple optical reflection properties, but not on transparent sur1 The wet layer thickness is typically about 1 lm and on drying the solvent content that comprises 60% by volume is removed through evaporation.

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Fig. 3. AFM topography images of PEDOT: PSS conductive ink, screen-printed onto a plastic substrate (a) surface roughness and height of features and (b) axonometric profile.

Fig. 4. Axonometric profiles of the surface with white light measurements.

faces. As the anode also consists of the additional PEI layer that is transparent, scanning electron microscopy (SEM) was used to gather surface and cross-sectional information of the layers comprising the anode and cathode, Fig. 5a and b. Fig. 5(a) shows the PEDOT: PSS cathode and Fig. 5(b) shows the anode with the additional PEI layer on top of the PEDOT: PSS. SEM images for cryofractured sections of the cathode and anode are shown in Fig. 6, illustrating different material layers. The thickness of the deposited layers on each substrate is about 39 lm. The crack shown in Fig. 6b is most likely to be due to the cryofracture process, as it was difficult to obtain a clean fracture due to the properties of the plastic substrate. It was not due to poor adhesion or deposition of the individual layers by screen printing. A clean fracture for the electrode could not be obtained while performing SEM due to the substrate used. Also, another SEM (Ultra-High Resolution FE-SEM S-4800, Hitachi) could not be used due to the presence of PEI, which would contaminate the system during out gassing under high vacuum. An electrolyte-separator film was then made as described earlier. An SEM image of this film is shown in Fig. 7 at 178 k magnification. A section of the film without defects was cut and sandwiched between the electrodes. After the application of the electrolyte-separator film upon the electrodes, the cell was assembled ready for testing using methods

that included galvanostatic cycling with the potential limitation method as defined for the Bio-Logic Science Instruments’ VMP3 potentiostat. 3.2. Characterisation of electrode performance The oxidation of the neutral PEDOT0 part by dioxygen and the reaction of PEDOT+ with PEI make PEI (Eq. (1)) and dioxygen (Eq. (4)), the two actual reactants of this polymer battery. PEDOT in the electrodes appear as an intermediate reactant and a product in the chains of reactions [23]. The suggested corresponding halfreactions upon discharge are: Anode:

PEDOTþ : PSS þ PEI ! PEDOT0 þ PEIþ : PSS

ð1Þ

PEDOT0 þ PSS : Naþ ! PEDOTþ : PSS þ Naþ þ e

ð2Þ

Cathode:

PEDOTþ : PSS þ Naþ þ e ! PEDOT0 þ PSS : Na

ð3Þ

PEDOT0 þ 1=4O2 þ 1=2H2 O þ PSS : Na ! PEDOTþ : PSS þ NaOH

ð4Þ

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Fig. 5. SEM images of screen printed electrode (a) cathode (b) anode (2 lm).

Fig. 6. Scanning electron micrographs of the cross-section of the dried electrode for measuring the thickness; (a) thickness of the cathode has been shown to be 39 lm; (b) shows a crack in between the layers of the anode, after cutting under liquid nitrogen.

Fig. 7. The SEM images illustrate the uniformity of the electrolytes (200 nm).

Subsequent to printing each layer for the electrodes (for both cathode and anode), electrical characterization of the electrode materials was carried out using cyclic voltammetry and a two electrodes setup as shown on Fig. 8. For each layer in the half-cell a CV was recorded at a scan rate of 50 mV s1 in the film electrolyte and the loops are shown in Fig. 9. First of all the CV for the silver electrode was run and the measurements which are shown by the red curve demonstrates oxidation and reduction. Then, the electrode was covered by carbon. The blue graph shows that in this test there were no signs of oxidation and

Fig. 8. Physical set up of two electrodes; printed Ag/AgCl acted as reference electrode and silver, carbon PEDOT: PSS – PEI as the working electrode respectively.

reduction. This provides clear evidence that the carbon covers the silver electrode completely and isolates it from the subsequent PEDOT: PSS layer and confirming that the silver layer can act only as a current collector. The Black graph shows the oxidation and

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Fig. 9. Cyclic voltammograms (representative red silver, blue carbon, black polymer electrode), respectively. In the cathodic process of the first cycle, an obvious peak located at about 0.1 V was observed, which shifted to lower potential in the subsequent reduction process 0.3 V. This can be attributed to the reduction of PEDOT.

Fig. 10. (a) Open circuit voltage (OCV) of a polymer battery for three hours; (b) charge and discharge of the polymer battery cell with constant current 0.6 lA for charge and discharge; and (c) OCVs for 6 different batteries showing over 0.6 V.

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separators. For this reason, the practical specific capacity was measured. Practical specific capacity means the capacity when operating the battery at a constant current until the battery reaches its cut-off potential (Vcutoff). These can be obtained from the discharge curves shown in Fig. 11. Practically, a constant current was applied on the battery and the potentiostat was used to measure tcutoff and Vcutoff. By using Eq. (5) [25] and knowing the amount of active material and the area A, the practical specific capacity can be calculated.

Practical specific capacity ¼ ðI  A  tcutoff Þ=ð3600  WÞmAhg1

Fig. 11. Cycling performance of a printed all polymer battery: capacity of charge and discharge of battery vs. cycle number.

reduction of the electrode after overprinting PEDOT: PSS–PEI on top of the carbon (Fig. 9). As layers are added, the curves differ considerably from the first (red), and the last (black) cathodic and anodic peaks shift to a lower potential, suggesting strong, PEI-driven, structural or textural modifications. 3.3. Operational electrical performance A secondary battery must demonstrate charge and discharge characteristics as well as discharge current and charge capacity, which are both important parameters. Fig. 10 shows the open circuit voltage (OCV) and through the use of the programmable potentiostat, the charge–discharge characteristics of the battery. The latter demonstrate a saw tooth form that is characteristic for a battery and furthermore clearly confirms the rechargeable nature of the device. The highest OCV achieved was 0.85 V, which is much higher than the previously reported [23] value of 0.5 V. Also, most of the batteries tested (35 out of 55), provided an OCV of over 0.6 V. Battery capacity is a measure of the charge stored by the battery, and is determined by the mass of active material contained in the battery [26]. The charge and discharge rate as determined for each battery also depends on the mass of active material and the impedance inherent to the battery. In practice, the theoretical battery capacity cannot be reached, due to the inclusion of nonreactive and non-conductive components such as binders and

ð5Þ

where I is the current density in Amperes/m2, A the area in m2, tcutoff is the time to reach the cut off potential (Vcutoff) in seconds and W is the weight of the active material in kg. By using this equation, the practical specific capacity was calculated to be 5.5 mAh g1. A battery performance of 5.5 mAh g1 has been achieved, which is comparable to the specific capacities obtained from conventional rechargeable lead-acid batteries [27]. However, as this is a thin polymer battery and the amount of active material is very small, this battery can only serve limited applications such as powering devices that are activated periodically for short time windows. The battery capacity was also explored over a number of charge–discharge cycles and the results are shown in Fig. 11. The curves of charge and discharge show similar trends and they are close to each other showing that the cyclic performance of the battery is good. Further, to establish manufacturing repeatability, three batteries were tested and they were run through a high frequency charge–discharge cycle (typically 3 min), the results from which are shown in Fig. 12. They were recharged electrically after discharge by passing a current of 0.6 lA as shown in Fig. 10b and which was calculated based on the amount of active material. This test shows the consistency that was achieved for the three batteries that were tested where each achieved an OCV of 0.61 V. The important feature of this all-polymer, all printable battery is its air stable anode, which shows a long almost constant opencircuit voltage. The polymer base PEI plays a dominant role to balance the reaction to oxygen in air and to keep the oxidation level of PEDOT low at the anode [23]. It is well-known that only outer active sites can sustain redox reactions completely at high scan rates. To further improve the battery performance requires consideration of two principal factors. The first concerns the improvement of electrode conductivity and the second concerns improvement of

Fig. 12. Charge–discharge cycling of three printed polymer batteries.

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contact with the electrolyte/separator. This will be the focus of future research effort. 4. Conclusions A printed rechargeable, flexible, all polymer battery with an almost constant open-circuit voltage and an active area of 400 mm2 that can charge and discharge with practical specific capacity of 5.5 mAh g1 and an overall thickness of about 440 lm has been developed. For the first time, a printable polymeric electrolyte gel has been formulated, demonstrating stability, while not affecting the layer quality. There are several advantages of the battery reported in this research. The materials are printable and non-toxic, with a single film functioning as both an electrolyte as well as an insulator/ separator. The battery also provides a reversible capacity during cycling with higher OCV and no sealing requirements. This battery, therefore, is highly suitable low-power consumption devices and can form an integral part of large area flexible electronics. Nomenclature AFM BCD CV ESM GCPL OCV SEM PET PEDOT PEI PSSNa WLI

atomic force microscopy battery capacity determination cyclic voltammetry electrochemical strain microscopy galvanostatic cycling with potential limitation open circuit voltage scanning electron microscope polyethylene terephthalate poly (3,4-ethylenedioxythiophene) polyethyleneimine poly (styrene sulfonate sodium) white light interferometry

Acknowledgements The authors wish to thank The European Regional Development Fund (ERDF) through the Ireland Wales Program INTERREG 4A, for financial funding of this project also Prof. Marta Gunde, Dr Manu Petal and Dr Jozˇe Moškon from National Institute of Chemistry, Ljubljana, Slovenia. References [1] J. Wusten, K. Potje-Kamloth, Organic thermogenerators for energy autarkic systems on flexible substrates, J. Phys. D Appl. Phys. 41 (13) (2008).

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