Polymer/Ionic Liquid Electrolytes and Their Potential in Lithium Batteries

National Aeronautics and Space Administration!

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Recent Research in Lithium Batteries and Fuel Cells

Dean Tigelaar Polymers Branch

NASA Glenn Research Center

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Polymer/Ionic Liquid Electrolytes and Their Potential in Lithium Batteries

Allyson Palker, Dean Tigelaar Polymers Branch William Bennett

Electrochemistry Branch NASA Glenn Research Center

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Lithium Polymer/Ionic Liquid Batteries

 Motivated by PERS program   Polymer Energy Rechargeable System.

 Advantages  Safety

 Commercial batteries contain flammable solvents.

 Li metal anodes

 Disadvantages  Lithium ion conductivity

 Maximum conductivity ~10-4 S/cm

*Gaston Narada International Ltd *

Our Objective

 Prepare polymer separator that has:  High lithium ion conductivity (~10-3 S/cm)  No volatile components  High long term stability with lithium metal

electrodes

 Strategy: Polymer gel electrolyte that contains ionic liquids  Nonvolatile, nonflammable, wide ESW.

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GRC Polymer Electrolyte

 Rod segment provides mechanical strength.

 PEO coil segment helps conduct lithium ions.

 High degree of crosslinking.

 Can hold large amounts of liquid additives (>400%).

Variables:

A.  Amount of Room Temperature Ionic Liquid (RTIL)

~ 200, 300, 400%

B.  Concentration of Lithium Bis(trifluoromethane) sulfonimide (LiTFSi)

~ .5, .75, 1.0 mol/kg

C.  Addition of Alumina (Al203)

~ 0, 5, 10, 15%

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Cycling Data Experiment 1: Amount of IL added

200% IL with .5 mol/kg 300% IL with .5 mol/kg

400% IL with .5 mol/kg

400% IL is the most compatible with the Lithium electrodes at a current density of .25 mA/cm2, 60°C

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Experiment 2: Concentration of LiTFSi

400% IL with .5 mol/kg 400% IL with .75 mol/kg

400% IL with 1.0 mol/kg

The concentration of Lithium salt that was the most compatible with the Lithium electrodes was the 1.0 mol/kg.

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Experiment 3: Addition of Alumina

400% IL with 1.0 mol/kg and 0% Alumina 400% IL with 1.0 mol/kg and 10% Alumina

The addition of 5% Alumina caused the Voltage to decrease five fold showing there is less resistance and better stability in comparison to the sample without Alumina.

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Impedance Data

•  Addition of alumina results in a significant decrease in interfacial resistance •  More stable interfacial layer.

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Summary

  Made electrolytes by varying: 1.  Amount of RTIL 2.  Concentration of Li salt 3.  Addition of Alumina

  Symmetric coin cells made with the polymer electrolytes

  Improved cycling stability in coin cells from <3 hrs to >1000 hrs at 0.25 mA/cm2 current density

  400% IL with 1.0 mol/kg and 10% Alumina was the most compatible with the Lithium electrodes

•  Tigelaar, D. M.; Palker, A. P.; Meador, M. A. B.; Bennett, W. R., J. Electrochem. Soc., 2008, 155, A768.


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Progress of Proton Exchange Membrane (PEM) Fuel Cells

Dean Tigelaar, Allison Palker Polymers Branch

NASA Glenn Research Center

Huan He, Christine Jackson, Kellina Anderson, Tyler Peter, Jesse Wainright,

Robert Savinell Case Western Reserve University

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Potential Uses •  Propulsion

–  Automotive, zero emission aircraft

•  Stationary –  Power supply (Gemini V)

•  Portable –  Astronaut equipment

•  Regenerative –  Coupled with photovoltaic systems for energy storage –  Hydrolysis of water back into H2 and O2

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Advantanges •  Efficient energy conversion (up to 70%) •  High energy density •  Generates water in exhaust •  No recharge needed


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Proton Exchange Membrane must: •  Have high proton conductivity. •  Have low electrical conductivity. •  Be mechanically robust in the wet and dry state. •  Processable into thin film. •  Be stable to a high temperature, high humidity, highly acidic

environment for thousands of hours.


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Nafion-State of the art membrane

poly(perfluorosulfonic acid) “Nafion”

Advantages: • Excellent proton conductivity

(0.1 S/cm ) • Good mechanical and chemical properties • Long-term stability

Disadvantages: • Expensive • Limited operation temperature

(≤80°C) •High methanol permeability.


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Sulfonated Poly(arylene ether)s (McGrath)

•  High thermal and chemical stability •  Good film forming properties •  Several monomers and polymers are commercially available •  Controlled degree of sulfonation

–  Controls conductivity and mechanical properties –  30-40% sulfonated monomer


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Polybenzimidazole/H3PO4 (PBI) (CWRU)

•  Excellent thermal and oxidative stability. •  Less dependant on humidification. •  Operating temperatures up to 200oC. •  High H3PO4 uptake (~200 wt%). •  But: Difficult to process into strong film. •  Produced commercially by BASF.


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Our Strategy: Synthesize Novel Polymer

•  Fully Aromatic –  Thermo-oxidatively stable and mechanically strong.

•  Heterocyclic –  Coordination with H3PO4 by acid-base or H-bonding. –  Similar to PBI but easier to process.

•  Highly soluble in common organic solvents –  NMP, DMAc, CHCl3.


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Solution: Poly(arylene ether triazine)s

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•  Fully aromatic •  Soluble due to ether links and bulky pendant groups. •  Can be made conductive in 2 different ways. 1) Nitrogen groups capable of bonding with H3PO4 2) Can be sulfonated on exclusively on pendant groups


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Monomer Synthesis

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Polymer Synthesis

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•  High molecular weight (IV 0.6-1.0 dL/g). •  Thermo-oxidative stability (Td > 500°C in air). •  Rigid but soluble (Tg 150-290°C, soluble in CHCl3, NMP,

CF3CO2H). •  Good film forming properties.


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Glass Transition Temperature

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Polymer Sulfonation

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Conductivity of Sulfonated Films

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• Most conductive film is more conductive than Nafion 117. • This film is brittle in it’s dry state, but can be fixed by changing to a more flexible monomer. •  The most conductive polymer was the lowest water uptake and ion exchange capacity. Why? Tigelaar, D. M.; Palker, A. P.; Jackson, C. M.; Anderson, K. M.; Wainright, J. Savinell, R. F Macromolecules, 2009, 42, 1888.

0

0.02

0.04

0.06

0.08

0.1

0.12

0 10 20 30 40 50 60 70 80 90 100

Con

duct

ivity

/ S

cm-1

Temperature / oC

Nafion 115 DPA-Pket DPA-diket DPA-PS


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TEM Data

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DPA-PS IEC = 1.88 meq/g

Water uptake = 131% σ = 0.11 S/cm at 90°C

2-10 nm hydrophilic regions Dark background

DPA-pket IEC = 2.12 meq/g

Water uptake = 211% σ = 0.082 S/cm at 90°C

5-15 nm hydrophilic regions Well connected

Tigelaar, D. M.; Palker, A. P.; He, R.; Scheiman D. A.; Petek, T.; Savinell, R. F.; Yoonessi, M. J. Membrane Science, 2011, 369, 455.


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Phosphoric Acid Uptake of DPA-PS/PBI Blends

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0

100

200

300

400

500

600

700

800

900

0 20 40 60 80 100 120 140 160 180 200 220 240 260

Upt

ake

(wt %

)

Time (hr)

1:1 DPA-PS:PBI 3:1 DPA-PS:PBI 9:1 DPA-PS:PBI

50oC 90oC Room Temp

•  Uptake of PBI by this method is 200%. •  “As received” PBI can be used for 3:1, 9:1 blends.


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Phosphoric acid uptake

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85% H3PO4 90°C 22 days

100% 3:1 DPA-PS:PBI blend 7% polymer 93% H3PO4


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Conclusions •  Synthesized novel poly(arylene ether)s that are fully

aromatic, soluble, and with high molecular weight. •  Polymers have high H3PO4 uptake, but lose

dimensional stability as high temperatures. •  Most conductive sulfonated polymer has the same

conductivity as Nafion 115 at 100% RH. •  Most conductive polymer is brittle when dry.

–  This problem can be fixed by replacing sulfone with isophthaloyl group or using a comonomer.

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Acknowledgements • Dan Scheiman, Mitra Yoonessi • Robert Savinell, Jesse Wainright, Christine Jackson, and Kellina Anderson, Huan He.

Technology

Polymer/Ionic Liquid Electrolytes and Their Potential in Lithium Batteries