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Development andcharacterisation of

lithographically printedvoltaic cells

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Citation: SOUTHEE, D.J. ... et al, 2006. Development and characterisationof lithographically printed voltaic cells. IN: Proceedings of 1st. Electronics Sys-temintegration Technology Conference, Dresden, Germany, 5th-7th September,pp. 1286-1291.

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Development and Characterisation of Lithographically Printed Voltaic Cells

Darren Southee*, Gareth I. Hay, Peter S. A. Evans and David J. HarrisonCleaner Electronics Research GroupSchool of Engineering and Design

Brunel UniversityUxbridge, Middlesex, UB8 3PH, U.K.

* Darren.Southeegbrunel.ac.uk, +44 (0) 1895 274000

AbstractThis paper reports progress in thefabrication ofvoltaic cells and batteries via offset lithographic printing.

Successful design and manufacture oflithographically printed voltaic cells wouldfacilitate the integration ofprintedpassive components, interconnects and display elementsfor disposable electronics in low-volume,low weight circuits and systems. The Conductive Lithographic Film (CLF) printingprocess was developedby Brunel University tofabricate circuit interconnect and various passive components at high speed and low

cost. A feasibility study, investigatingprinted cells based on Zinc-Carbon electrochemistry, is described.This has resulted in the production ofa printed battery able to power a microcontroller-driven LED display

system for more than three hours. Further work seeks to improve cell capacity, address manufacturingprocess issues, and characterise the structures in more detail.

IntroductionConventional "Printed" circuit boards comprise a

copper foil bonded to a substrate medium, the circuitimage being formed by photo-resist and subtractiveetching processes. There are numerousenvironmental problems associated with the photo-resist and etching processes used in conventional"printed" wiring board manufacture. These include:

* Spent etching solutions that areenvironmental pollutants and incur extracosts in disposal;

* Complex multiple processes are required;. Toxic materials and waste-products;* Conventional wiring boards contain lead

(Pb) and various fire retardants andfungicides. End of life disposal is thereforeenvironmentally problematic.

Screen-printed conductive films were first widelyused for electrical interconnect in the 1940's. Thefilms formed the basis of a thick film technology forproducing electronic circuit modules. Thick filmtechniques developed over the following 60 years, toprovide a mature technology with many applicationsin specialised electronic circuits [1].

The Conductive Lithographic Film (CLF)process was originally developed as an alternativefor etched resin-laminate circuit boards. Thetechnique utilises standard offset lithographicprinting technology used in the mass production ofbooks and magazines. The CLF process possesses a

number of key advantages over more traditionalforms of electronic circuit board fabrication:

* High production speed (6,000impressions/hour)

10,000

* Good resolution of image (80 - 100 micron trackwith 60 micron gap achievable)

* Low cost (low ink volume determining thatsubstrate material proves the largest expense)

* Ability to produce flexible electronic circuits andsystems

* Reduced environmental impact (less energy,reduction in material use, easier disposal, toxicheavy metals (e.g. lead) eliminated)

Electrically conductive ink films can be printedon a wide range of flexible substrates includingpaper and polymer films. A range of passivecomponents and sensor structures have beenmanufactured by the CLF process by designingstructures that exploit the electrical properties of theprinted ink films.

In the past few years trends towards greaterintegration in electronics, combined with reductionsin power requirements, and demand for mobility,have led to increasing interest in using additiveprinting technologies for the production ofelectronics. The printing of a "disposable" mobilephone, using polymer thick film techniques,received much publicity [2]. Another development isthe printing of batteries, using silk-screentechnologies to produce zinc anode and manganesedioxide cathode layers fabricated from ink-like

2006 Electronics Systemintegration Technology ConferenceDresden, Germany1-4244-0553-1/06/$20.00 (D2OO6 IEEE 1286

materials [3]. Developments in thin-film batterytechnology have been reported by 'Power Paper' -

an Israeli company who have Patented processes forscreen-printing various types of thick-film voltaiccell [4] [5]. Previous research conducted by ourgroup has however established that screen-printedelectronic components are both slower tomanufacture and significantly more expensive thansimilar components fabricated by offset lithography.A key feature of the Power Paper screen-printedbattery process is a deliquescent electrolyte thatattracts sufficient water from the atmosphere tocompensate for evaporation losses. Voltaic cellsfabricated by the Power Paper process cannottherefore be sealed against leakage or externalcontamination.

Developments of lithographically printable inksfrom which anode and cathode electrode structurescan be fabricated are presented. The electrode inkswhich have been developed consist of fineparticulates suspended in an alkyd resin containingdilitants to modify the rheological properties, andcan be compared to similar inks formed for themanufacture of polymer thick film electronics.

The work has used Leclanche battery chemistryas the base from which lithographically printedvoltaic cell electrodes could be formed. The zinc -

manganese (IV) dioxide Leclanche cell is one of theoldest forms of battery chemistry available. Manyvariations on the formulation of such cells exist yetmuch of the details are proprietary to the individualcompanies involved in their fabrication. However,Leclanche cells are a proven technology and arecapable of developing up to 1.7V with reasonablecurrents for general domestic products [6].

The paper describes cell configurations andmanufacturing processes while also presentingresults demonstrating the performance of cells.These results are discussed and suggestions forimprovements in the manufacturing process areproposed.

Discussion 1 Electrode Ink DevelopmentFor a printable CLF ink the formulation must:

* be thixotropic (non-newtonian properties);* be hydrophobic;* remain fluid on press but dry quickly when

deposited on substrate.

Lithographic printing inks are generallycomposed from two phases. The first, a colourpigment, takes the form of fine particulates and the

second, an ink vehicle, acts as a carrier for thepigmentation through the printing press and providesa bonding mechanism for the pigmentation to thesubstrate material. Lithographically printableelectronic inks developed by Brunel University aresimilar in composition to commercially availablecolour inks, such that, formulations are composedfrom an active 'electronic' material in particulateform (approximately 1 - 3 ptm in size) dispersed inan organic resin vehicle.

The ink fabrication process follows closely thatof standard lithographic inks and includes;

* Polymeric ink vehicle mixed* Conductive particulate introduced* Three roll milling* Fineness of grind dispersion tested* Assessment of rheological properties

The polymer based vehicle portion of the inkconsists of three components; a polymeric resin,which constitutes the largest portion of the vehicle, anon - volatile dilitant solvent to adjust the viscosityand an anti oxidant agent to retard drying of the inkduring printing.

Each component of the ink is combined andagitated until a smooth uniform mixture is formed.To improve distribution of particulates and breakdown agglomerates the mixture is sheared on athree-roll mill. Without the process of milling it islikely agglomerates of active material will exist, thusreducing the likelihood of the ink attaining thecorrect rheological properties while also introducinguncertainties to the electrical characteristics of thecured ink film. Breaking down of the agglomeratescauses an increase in particulate surface area which,in turn, leads to a larger spread of vehicle over thesurface of the active material, causing an increase inviscosity.

To obtain the degree of particulate dispersion andensure no agglomerates of particulates are present,ink formulations are subjected to Fineness Of Grindtesting (FOG test) using a grindometer. Agrindometer is a precision fabricated block ofstainless steel with two graduated troughs milledinto the top surface. The troughs, running along thelength of the block, have a start depth of 25 mmwhich graduate to 0 mm as they reach the oppositeend of the block. FOG testing is conducted bydepositing two beads of ink in either trough at thedeepest end and drawing them down the grindometerusing a flat edge or blade. The presence of

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agglomerates is apparent should "streaking" beobserved in the drawn samples. The location of thestreaks along the grindometer gives an indication ofagglomerate size.

Ink rheological characteristics are obtained usinga cone and plate type viscometer, model HaakeVT550. It is important that lithographic printing inksattain the property of thixotropy (shear thinning).

Ink specimens are subjected to shear rates fromOs-' - 400s-1. During testing, measurements of shearstress are recorded and used to calculate theviscosity by the rule:

'7=2'

where l denotes viscosity (Pascalsecond, Pas), Xdenotes shear stress (Pa) and y denotes shear rate(sec-1). Ink running through the ink train of alithographic printing press is likely to be subjectedto shear rates in the region of 1O,OOOs-1, however, intest conditions these shear rates are difficult toreproduce. It is widely accepted that if an inkachieves a viscosity in the region of 7 - 12 Pas at400s-1, while exhibiting thixotropic behaviour, it islikely to perform well at increased shear rates.

Through a process of ink development, anodeand cathode inks based on zinc and graphiteparticulates respectively were fabricated. The inkformulations utilise active materials with meanparticulate distributions of approximately 3 ptm.Composition of each formulation is detailed in Table1.

Discussion 2 First iteration structure designInitial voltaic cells were fabricated by depositing

suitable electrode structures using anode andcathode ink formulations. These structures utilisingzinc - carbon chemistry were developed to provethat lithographically deposited electrode structuresexhibit an electrochemical potential.

Electrode structures were orientated as depictedin Figure 1, such that cells comprised; substrate,cathode layer, electrolyte saturated membraneseparator, anode layer, substrate.

The electrolyte mixture was formed by dispersinga small quantity of polyethylene oxide in waterfollowed by the introduction of ammonium chloride.A small quantity of manganese dioxide, in fineparticulate form, was introduced to the formulationto act as a depolarising element.

Table 1. Anode and Cathode ink formulation

Component Zinc inkMean particulate size:

Particulate (w/w):XV1578 resin (w/w):M71A solvent (w/w):

Eugenol anti-oxidant (w/w):

3ptm75%90%9%1%

Graphite ink

3ptm33%77%22%10%

Both inks displayed shear thinning and viscosity values inthe region of 7 - 9 Pas at a shear rate of 400 sec-1

Substrate

ZincMembraneElectrolyte

GraphiteSubstrate

Figure 1. First iteration cell configuration

The principal operation of the membraneseparator is to contain the electrolyte preventingmigration of this phase through the cell. Cells weresealed using adhesive treated polymer film.

Discharge testing of these initial structuresproved that a voltage of 1.5V was achievable,however, the current capability of such devises waspoor. A peak short circuit current in the region of2nA was recorded and has been attributed to thehigh sheet resistance of the graphite and zincelectrode structures (approximately 1.5kQ/E and2MQ/ respectively).

Discussion 2 Second iteration structure designThe configuration of second generation cells,

Figure 2, followed closely that of initial cells, withkey differences to electrode structures and theaddition of a manganese (IV) dioxide paste.Graphite cathode and zinc anode structures weredeposited on top of lithographically printed currentcollectors formed using a silver loaded conductiveink. Introduction of the current collectors reducesanode and cathode sheet resistivity to approximately10 Q/ , resulting in reduced cell internal resistance.

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Substrate -Silver -

Meembrane -

MnO2 - C pasteGraphite

Sbil

Substriver

ate

Figure 2. Second iteration cell configuration

Ink films deposited by offset lithography have athickness of approximately 3 - 5 ptm, which is instark contrast to films deposited by screen printingprocesses (approximately 50 ptm) and the thicknessof foil material laminated to conventional resinlaminate circuit boards (approximately 30 - 75 ptm).

Capacity of a voltaic cell is proportional to thevolume of anode and cathode material available forthe electrochemical reaction. During discharge of aLeclanche cell, reduction of MnO2 is the cathodicreaction and oxidation of zinc is the correspondinganodic reaction. As such, increasing the volume ofcathode material sustains the electrochemicalreaction for longer.

Considering the thickness of lithographicallydeposited films, cell development has focused on theformulation of a 'stencil' deposited manganese (IV)dioxide paste.

As in conventional Leclanche cell design, MnO2pastes consist predominantly of manganese (IV)dioxide in particulate form mixed with water. Inaddition to this, carbon/graphite particulates areintroduced to the formulation. The addition ofgraphite to the cathodic paste is important for tworeasons. 1. The bulk resistivity of manganese (IV)dioxide is much higher than that of zinc(approximately 1.44 [iQm and 50 nQmrespectively), hence carbon/graphite is required toreduce the bulk resistivity of the mixture. 2. Whilstmanganese (IV) dioxide is an important element inthe electrochemical reaction, it also behaves as adepolarising element, converting hydrogen dissolvedin the electrolyte solution (produced by oxidation ofthe anode material) into water. High concentrationof aqueous hydrogen in the electrolyte causeshydrogen bubbles to form, resulting in increased cellinternal resistance, hence deteriorating performance.The carbon / graphite portion of the cathodic pasteacts as a transport mechanism promoting the ingressof hydrogen in solution into the manganese (IV)

paste, hence increasing the surface area of MnO2participating in the depolarising reaction.A cathodic paste constituting a 75:25 ratio of

MnO2 and C respectively was developed and isdetailed in Table 2. During cell fabrication, acathodic paste layer of approximately 300 ptmthickness was 'stencilled' over the graphite electrodestructure. A paper membrane separator(approximately 500 ptm thick) saturated with theammonium chloride electrolyte solution, detailed inTable 2, was positioned over the paste. Finally, thezinc anode was placed above the membranecompleting cell fabrication, as detailed in Figure 2.

Table 2. Electrolyte and MnO2 paste composition

Manganese (IV) dioxide Electrolyte Compositionpaste formulationMnO2: 42.9% NH4Cl: 25%

C: 14.2% Water: 75%Water: 42.9%

Second iteration structures were evaluated viaconstant discharge through a 5 kQ load for 105minutes. A typical discharge curve produced duringevaluation is depicted in Figure 3.

Figure 3 suggests that discharge characteristics oflithographically deposited voltaic cells are notdissimilar to that expected from conventionalLeclanche cell technology. Cell voltage is noticed tofall off at an increased rate during the first 80minutes of discharge, after which, the rate of voltagedrop decreases.

However, it is apparent that the expected cellpotential of 1.5 - 1.7V is not obtained from thesecond iteration cell design. This is attributed to theresistance of the membrane separator utilised for thisconfiguration.

Analysis has proven that the shelf life of seconditeration cells is poor. Under no load, cell potentialfalls below 80% of maximum within 3 hours. Onehypothesis to explain this characteristic concerns thesubstrate material employed for electrodefabrication. Teslin is a polyethylene based materialdesigned for the printing industry, and is favouredfor its high quality graphic reproduction, duepredominantly to the surface composition. Duringmanufacture, the extruded polyethylene isimpregnated with silica to create a highly poroussheet. As such, approximately 60% of the materialsvolume is air. While aiding ink adhesion, due toabsorption of the ink vehicle into the substratesurface, it is believed that the electrolyte phase of

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20 40 60 80 100

Time (minutes)Figure 3. Second iteration cell discharge characteristics

cells permeates the material and is drawn away fromthe cathodic and anodic materials, hence reducingcell capacity.

ConclusionsThis paper describes a process of using offset

lithographic printing to form electrode structuressuitable for voltaic cell fabrication. Anode andcathode inks have been developed using activematerial in a particulate form suspended in a

polymeric resin. Inks developed attain hydrophobicand thixotropic properties necessary to facilitatelithographic printing.

First iteration cells proved that suitable electrodematerials could be printed and realised the chemicalpotential of Leclanche chemistry. Second iterationstructures utilising silver current collectors achievedsuperior current capability compared to theirpredecessors.

Second iteration cell structures have beencombined in a battery to power a microcontollercircuit incorporating an LED array for over 3 hours,Figure 4.

Future workIt has been shown that the Conductive

Lithographic Film printing process can be used toform electrode structures suitable for voltaic cellfabrication. It is proposed that the short shelf life ofsuch cells is due to electrolyte absorption of thesubstrate material. Current work is concerned with

the deposition of electrode structures on non porousPET substrate materials. The successful utilisation ofsuch materials should extend cell shelf life bypreventing electrolyte migration away from theelectrochemically reactive materials.

Figure 4. Lithographically printed battery- poweredcircuit

Ongoing development of cathodic pastes willfurther increase cell capacity by optimising the ratioof manganese (IV) dioxide to graphite particulates.

While the predominant concern of this work isthe development of a 'clean' printed power source, itis widely accepted that the inclusion of zinc chloride

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1.6

1.4

1.2

1

0.8

0.6

0.4

0.2

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in the electrolyte phase increases the efficiency ofLeclanche chemistry. The inclusion of zinc chloridehas to date been discouraged due to the toxic natureof the material.

Development of a lithographically printablemanganese (IV) dioxide / graphite ink is underway.Problems associated with high sheet resistivity arecurrently being investigated. A lithographicallyprinted layer is desirable to replace the cathodicpaste. Due to the thin nature of lithographicallydeposited ink films, it is envisaged that a cathodicink layer will require over printing a number oftimes to achieve suitable ink volume. It is thereforenecessary to formulate a printable ink which, whileattaining a suitable sheet resistivity, will alsoachieve a low modulus of elasticity. This isnecessary to prevent cured ink film cracking duringover printing.

AcknowledgmentsThe authors would like to thank Narinder Bains

(Rohm and Haas Electronic Materials Europe Ltd.),Peter Herdman (Arjo Wiggins Fine Papers Ltd.) andBill MacDonald (Dupont Teijin Films Ltd.) for theirassistance in support of this work. Thanks also to theIeMRC for funding the feasibility study.

References1. GILLEO, K. Polymer Thick Film Technology, John

Wiley & Sons Inc., 1996.2. Altschul, R L. Disposable Wireless Telephone Method,

US Patent No. 5845218.3. http://www.powerpaper.com/3_technology/

batteryspecs.htm, [site visited 06/05/05]4. Baruch Levanon. Flexible Thin Open Electrochemical

Cell And Applications of Same, US Patent No.575190. December 20, 1995.

5. Power Paper Inc. Flexible Thin Open ElectrochemicalCell And Applications of Same, US Patent No.873868. June 12, 1997.

6. VINCENT, C. A., SCROSATI, B. Modern batteries:An introduction to electrochemical power sources. 2ndedition. Arnold, U.K., 1997.

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