packaged inkjet-printed flexible supercapacitors

U.S. Army Research, Development and Engineering Command

Matthew Ervin (ARL), Linh Le (SIT), and Woo Lee (SIT)

Packaged Inkjet-Printed Flexible Supercapacitors

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Flexible Supercapacitors for Munitions

Description of Application:•Flexible printed supercapacitors for storing energy to power flexible munition electronics

POC: Brian Fuchs & Jim ZuninoEmail address: [email protected] [email protected]

Important Specifications:•Stores >3 mJ at 3 V•Survive 50+ kGs, 1000 rpm•Flexible•Printable

Benefits Anticipated:•Enable flexible circuits•Reduced cost•Manufacture on demand•Reduced Obsolescence•Improved volume utilization/increased leathality•Rapid prototyping/Mission tailored

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Thin-film Supercapacitors for Integration with Uniforms and Equipment

Description of Application:Thin-film supercapacitors employing ionogel electrolytes to be integrated in parallel with batteries in uniforms and equipment

POCs: Stephanie Flores Zopf Natalie Pomerantz

Email addresses: [email protected] [email protected] Specifications:•Energy density > 0.5 kJ/m2, > 0.5 kJ/kg•Conductivity = 1 mS/cm•Capacitance = 1µF/cm2

•Electrochemical window = 2.5 V•Long term stability over charge/discharge cycles•Mechanically flexible and conformable•Lightweight

Benefits Anticipated:

•Size, weight and power savings

•Environmentally safer than electrolytic supercapacitors

•Increased reliability over current electrolytic capacitors

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Energy Storage Why Supercapacitors?

Supercapacitors store charge by the adsorption of ions onto the electrodes using an electric field. Since there is no dielectric, the voltage must remain low enough that there is no charge transfer or electrochemical breakdown of the electrolyte. Capacitance is proportional to accessible surface area.

Advantages:Stable performanceHigher specific power (~100x batteries)Millions of charge/discharge cyclesRapid charge and discharge timesEfficiencies (98%)Perform well at extreme temperaturesSafetyShelf-life

Challenges:Lower energy densities than batteriesLimited voltage rating on individual cells:

~1 V for aqueous electrolytes and ~3 V for organic electrolytes.

Voltage varies with chargeRigid PackagingSlow response <1Hz vs other capacitor typesSelf-discharge

Supercapacitor vs. Electrolytic

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Capacitor Types

Ta 50VAl 500V

Highest power/frequencyLowest energyThicker dielectric yields higher

voltage, but volumetric energy density unchanged

Dielectric Electrolytic Electrochemical Double Layer

‘

Al foil Al foil

anode

dielectric

separator

cathode

electrolyte

permittivity

Dielectric strength

Lower power/frequencyMore energy

Ragone Plot of Electrochemical Devices

Lowest power/frequencyHighest energy

Aqueous 1VOrganic 2.7VIonic liquid >3.5V

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Graphene SupercapacitorRationale

Rationale:

Graphene has the highest surface area which correlates to capacitance.

2630 m2/g, external surface area (20uF/cm2 yields 550F/g theoretical)

Graphene is highly conductive which improves power performance.

Carbon has a very good electrochemical window.

The mechanical properties of graphene will enable flexible/conformal

supercapacitors.

Graphene oxide makes good solutions, and it is readily reduced.

<$50/kg anticipated in 3 years for graphene.

Goal:

To increase power and energy density of supercapacitors using graphene

CNT/G

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Flexible Device Component Choices and Issues

• Current Collector• Graphene Ink/Printing• Binder(less)• Separator(less)• Electrolyte• Substrate/Package

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Substrate/Packaging Material

Kapton:

Stable to 400oC – facilitates metal ink sintering

Good dielectric properties

Low outgassing

But…

Permeable to water, oxygen – electrolyte degradation

Use metallization to improve hermetic sealing

Not directly heat sealable

FEP:

Enables heat sealing – flows during sealing (350oC)

Chemically inert

But…

Adhesion of printed features?

Unstable substrate when sealing

Permeable to small molecules, e.g. CO2

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Kapton permeability2.xls

Packaging Permeability

AN/thick Kapton/FEP

IL/thick Kapton/FEP

PC/thick Kapton

H2O/thick Kapton/Al tape

H2O/thick Kapton/FEP

H2O/thin Kapton/FEPH2O/thin Kapton

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Inkjet Printing Graphene Oxide

N

D D

Print HeadIR Heat Lamp

Substrate

Dreyer el al., Chem. Soc. Rev., 2010, 39, 228-240

Suspension Stable, Hydrophilic Graphene Oxide (GO) in Water (2mg/ml), no surfactant

Inkjet Printing Attributes

•Micropatternable at 50 um resolution

•Additive, net-shape manufacturing with minimum nanomaterial use and waste

•Scale-up and integration readiness with rapidly emerging printed electronics

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Ink Preparation

– Concentration (2mg/ml)• Less aggregation and nozzle clogging, but requires more printing

– Solvent• Using water with graphene oxide, pvdf not soluble in water• Could use N-methyl pyrrolidone with graphene and pvdf binder (more

robust)– Graphene oxide functionalization/activation

• Introduces defects that can decrease conductivity.• Requires reduction step: photo/thermal/chemical • Functional groups can introduce pseudocapacitance which may or

may not be desirable. • Decomposition of functional groups/impurities can result in gas

liberation which can rupture the package.– Surfactants

• Generally nonconductive, must be removed– Sonication

• Aids in solubilization but may damage graphene– Inclusion of sacrificial porogens to tailor porosity– Inclusion of pseudocapacitive materials

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500 nm

100 Printed Layers: Cross-Section

Stacks of horizontal sheets of graphene

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Packaged Prototype Assembly

Graphene printed on evaporated Ti/Au on Kapton

Double-side FEP coated Kapton used for sealing

Device sealed on three sides

Separator inserted Electrolyte injected to wet the separator/electrodes

The final heat seal is made

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LL0114A03 3/25/14

LL0114B04 5/7/14

LL0114A03 3/25/14

LL0114A01 3/25/14

per rGO mass only

Prototype CV Results

Cyclic Voltamogram

Charge/Discharge1M H2SO4

0-1V

BMIMBF4

0-3V

Capacitance (F/g)

192 @ 20mV/s

73 @ 20mV/s

Energy Density (Wh/kg)

5.0 @ 0.25 A/g

5.5@ 0.25 A/g

Power Density (kW/kg)

10 @ 10 A/g

19@ 10 A/g

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LL0114A023-25-14

• Good capacitive behavior at low frequencies

Prototype EIS Results with H2SO4

0 50 100 150 200 Ohms

8.2 mF @ 10mHz

-79 deg @ 10mHz

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Bending expt 2 04 11 13.xls

Bending Test

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Flex tests 3 25 14.xlsm

Flex tests 3 25 14.xlsm

Bending Cycles

• 150FN019 packaging

• 1M H2SO4 electrolyte

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Flex cycle life tests 5 14.xls


• EIS at 0V shows only a loss of 20% capacitance

Cycle-Life Testing

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104 F/g

89 F/g

124 F/g

67 F/g

Cycle-Life Testing

140 F/g

153 F/g

• Dropcast, coin cells

• Inkjet printed, Flex Kapton cell

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Flex Ragone plots.xlsm

Bending Cycles

• With H2SO4: 3.3 Wh/kg rGO at 0.25 A/g, 6.8 kW/kg rGO at 10 A/g, 0-1V

• With BMIMBF4: 6.2 Wh/kg rGO at 0.25 A/g, 39.2 kW/kg rGO at 10 A/g, 0-3V

per rGO mass

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Flex Ragone plots.xlsm

Bending Cycles

• With H2SO4: 3.3 Wh/kg rGO at 0.25 A/g, 6.8 kW/kg rGO at 10 A/g,

• With BMIMBF4: 6.2 Wh/kg rGO at 0.25 A/g, 39.2 kW/kg rGO at 10 A/g

• BMIMBF4 packaged: 0.0010 Wh/kg pkg at 0.25 A/g, 0.063 kW/kg pkg at 10 A/g

per rGO mass

per package mass

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• Demonstrated Inkjet-printed, flexible packaged supercapacitors

• No need for binders

• 7 mF in 3 x 3 cm package of 0.23 g demonstrated with BMIMBF4. – Need to optimize: package, current collector, electrode thickness, electrolyte, etc.

• Ink development difficult, limited range of metal inks available for current collectors

• Slow Deposition Rates-dilute inks, thick electrodes difficult – need new printing methods

• Graphene activation, electrolyte optimization, or inclusion of pseudocapacitive materials could increase power or energy density.

• Need to investigate rGO cycle life in different electrolytes

• Shelf life needs to be investigated – water permeation into IL

Conclusions

packaged inkjet-printed flexible supercapacitors

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