Interfacial Behavior of ElectrolytesJohn Kerr

Joon Ho Shin, Leanne Beer and Pratyay Basak

Lawrence Berkeley National Laboratory.

February 28, 2008.

This presentation does not contain any proprietary or confidential information

Outline• Purpose of work• Barriers • Previous Review Comments • Approach• Performance Measures and Accomplishments• Plans for Next Fiscal Year• Summary• Collaborations and Technology Transfer• Publications/Patents

Purpose of the Work• Investigate the Chemical and Electrochemical

reactivity of Li Ion electrolytes.– Effect of impurities and additives.– How are Calendar and Cycle Life affected?– Are 5V electrolytes at all feasible?

• How do the electrolyte components, reaction products and impurities affect the interfacial impedance at the electrodes.

Purpose of the Work.Impact of Interfacial vs. Bulk Impedance

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120

PEGDME250-liTFSI(20) + 10% R805 SiO2

Bulk1 Interfacial 1 Bulk2 Interfacial2

R/oh

m.c

m2

Temperature(0C)

50

100

150

0 0.002 0.004 0.006 0.008 0.01 0.012

Peak

Pow

er, W

/kg

Anode Film resistance - Ohm-cm2Ohm.m2

Experimental Measurementson Li/Li symmetrical cells

Model of effect of Interfacial Impedance on Peak Power.(Karen Thomas-Alyea)

0.01 ohm.m2 = 100 ohm.cm2

Barriers Addressed• Poor Calendar Life – 2-3 yrs vs. 15 yr goal.• Poor Cycle Life – 3,000 -5,000 cycle goal.• Increase Power Density to 750 W/kg• Increase Energy Density to 100 Wh/kg• Low temperature (-30oC) performance• Increased Abuse Tolerance.

Electrolyte chemistry and interfacial behavior impacts these barriers.

Previous Review Comments

Recommendation:Reevaluate work on SPEs and ionic liquids (IL), as they may not meet low T or power requirements.

DOE Response:• LBNL/DOE will refocus some of the PIs’ who

are currently doing support work in these areas (Kerr and Doeff)

Previous Review Comments • The work on radical polymer electrolytes and ILs is the most

interesting project of those listed in future plans. • IL should be tested further. • Systematically determine water content vs. degradation

(various types) is very useful to industry, very relevant to next generation systems as well as to ameliorate effect of water in present electrolytes.

• Need to dig deeper than just running an experiment to validate or reject a one liner statement.

Reviewer Recommendations for Additional work:• This work should be continued, need polymer exploration.• Need to look carefully at the stability of ionic liquids (ILs) at

negative potentials.

Approach• Continue Chemical and Electrochemical

studies of carbonate-based electrolytes.– Better understand the role of water and alcohols– Understand the roles of acid and base components,

impurities and side products.– Improve correlation of chemical measurements

with electrochemical observations • Bring Ionic Liquid Work to some conclusions.• Exploratory work on Polymer-electrode/IL

systems

Radical Polymer Electrodesand Ionic Liquids

Li PTMA/C

Current collectorLi+ TFSI-

N

R1 R2NO

OO

n

NO

OO

n

PTMA

t / s

0 200 400 600 800 1000

Cel

l vol

tage

/ V

3.0

3.2

3.4

3.6

3.8

4.0First discharge10th 20th 30th 40th 50th 70th100th

Justin Salminen, Kentaro Nakahara (NEC, Japan) H. NishideWaseda Univ.Science, 2008,319(Feb 8), 737;Interface, 2005, 14, 32.

Remove the Li+

Both Electrodes are Redox Polymers.Ionic Liquid Electrolyte increases Safety

-0.001

-0.0005

0

0.0005

0.001

-2.5 -2 -1.5 -1 -0.5 0 0.5 1 1.5

50 m V500 mV5000 mV

Cur

rent

(A)

Potential (V) vs. Ag+

CV: 5mM TEMPO, 0.1M TEAClO4 in acetonitrile on glassy carbon

NO

PTMA/CTFSI-N

R1 R2

PTMA/CNO

OO

n

NO

OO

n

NO

OO

n

NO

OO

n

TEMPO

Electrolyte water content 1000ppmCathode reaction not sensitive to water.Anode reaction sensitive andKinetically very slow.Replace anode polymer with polynitrostyrene(Miller, JACS 1980, 102, 3833)

NO

+NO

NO

+NO

+NONO

N

O

e N

O

Electrolyte System Studied

Ternary mixture electrolytes

PYR14TFSI + x LiTFSI + y PEGDME 250 (or 500)

x = LiTFSI mol/PYR14TFSI-kg, x = 0.5

y = PEGDME-kg/PYR14TFSI-kg, y = 1.0, 1.5 and 2.0

EO/Li ratio: 18 (y = 2.0), 13 (y = 1.5), 9 (y = 2.0)

N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR14TFSI)

Poly (ethylene glycol) dimethyl ether (PEGDME); n ~5 or 10

Ionic Liquid with Co-solvents Conductivity and Viscosity

10-5

10-4

10-3

10-2

10-1

2.6 2.8 3 3.2 3.4 3.6 3.8 4

PYR14TFSIy=0y=0.1y=0.3y=0.5y=1.0y=1.5y=2.0

Con

duct

ivity

(S/c

m)

1000/T (K-1)

0

20

40

60

80

100

120

20 30 40 50 60 70 80 90

Temperature (oC)

Visc

osity

(cP)

Pyr14T+0.5LiT+1.0P-500Pyr14T+0.5LiT+1.5P-500Pyr14T+0.5LiT+2.0P-500Pyr14T+0.5LiTPyr14T+0.5LiT+1.0P-250Pyr14T+0.5LiT+1.5P-250Pyr14T+0.5LiT+2.0P-250

Ionic conductivityPYR14TFSI + 0.5M LiTFSI

+ (y) PEGDME 250 mixtures.

ViscosityPYR14TFSI + 0.5M LiTFSI

+ PEGDME 250 and 500 mixtures.

PEGDME 250PEGDME 250

PEGDME 500PEGDME 500

Interfacial Impedance of Ionic Liquidswith co-solvents

Li/Li Graphite vs. Li reference

PEGDME 250PEGDME 250

PEGDME 500PEGDME 500

Exchange Current Density of Ionic Liquid Co-solvent Mixtures at Lithium and Graphite

Li/Li Graphite vs. Li Reference

Carbonate Based Electrolytes- Anode Formation Experiments

on Gen 2 Anode (ATD-ANL)

Schematic of three electrode cell

-15

-10

-5

0

5

10

0 0.5 1 1.5 2 2.5

CV_1mV_ECEMC+salt_RT_total

LiTFSILiPF6C

urre

nt (m

A/c

m2 )

Voltage (V)

Cyclic voltammetry of Li/EC/EMC (3/7 v/v) + 1.2M LiX/Graphite cell; 1mV/s, 25oC

0

100

200

300

400

500

600

0 100 200 300 400 500 600

Imp_ECEMC+salt_RT_before CV_total

LiTFSILiPF6LiTFSILiPF6

-ZIm

ag (O

hm c

m2 )

ZReal

(Ohm cm2)

before CV

after CV

LiPF6 soln.Water = 0 ppmbut pH low;LiTFSI soln.H2O = 30 ppm

Nyquist plot of ac impedance of Li/EC/EMC (3/7 v/v) + 1.2M LiX/Graphite cell at 25oC. 3-electrode measurement

Effect of Formation Potentialon Gen 2 Anode with LiTFSI

-4

-3

-2

-1

0

1

2

3

-0.5 0 0.5 1 1.5 2

LiC30_CV_RT_total

0.5mV/s,1st&2nd0.5mV/s,3rd&4th0.5mV/s,5th

Cur

rent

(mA

/cm

2 )

Voltage (V vs. Li/Li+)

0

200

400

600

800

1000

0 200 400 600 800 1000

LiC30_Imp_CV_0.5mV_RT_total

before CVafter 2nd scanafter 4th scanafter 5th scan

-ZIm

ag (O

hm c

m2 )

ZReal

(Ohm cm2)

Cyclic voltammograms(left) and Nyquist plots (right) of Li/EC/EMC(3/7v/v),1.2M LiTFSI/graphite, scan rate 0.5mV/s, 25oC

Nature of Surface Analysis of Products after formation cycling at Sputtered Mag-10 Anodes

Time/ min.0 10 20 30 40

Inte

nsity

a.u

.

(a)

DMC ECMeOH

*

*

(b)

Time/ min.0 10 20 30 40

Inte

nsity

a.u

.

(a)

DMC ECMeOH

*

*

(b)

Sputtered

Fresh

Gas Chromatography Analysis

O O

OO O O

OO

O

O O

O

O

O

O

OO

1 2 3 4

5 6

Mass Spectral analysis of GC peaks are consistent with structures 1-6 amongst othersincluding higher M.Wt. products (1500) that appear to contain fluorine. Compound 6 is consistent with base catalyzed ring opening of ECCE analysis also detects phosphates.

Ring-opening of EC initiated by Electrogenerated base.

O O

O

O O

OOR

O O

OOR

O O

O

O O

O

OO

O OR

RO OO O

O

O

O O

O

n

R'O

RO

n

PEO + CO2

• RO- anion is formed by reaction at the anode.•CO2 removed by reduction at anodes to form CO, carbonate, formate and Oxalate. Oxalate can act as a shuttle for reversible self-discharge or “Marching”

O O

OO

O

6 + CO2

Cathode Interfacial ImpedancePostulated that side reaction products

alter wetting characteristics.

0

200

400

600

800

1000

0 200 400 600 800 1000

LiG15_Imp_RT_072407_total

before testafter 1st chargeafter 1st discharge

-ZIm

ag (O

hm c

m2 )

ZReal

(Ohm cm2)

0

20

40

60

80

100

0 20 40 60 80 100

LiG15_Imp_RT_072407_total

before testafter 1st chargeafter 1st discharge

-ZIm

ag (O

hm c

m2 )

ZReal

(Ohm cm2)

Impedance of Gen 2 Cathode before and after cycling

Kinetic Measurements of Electrolyte Reactions

-10

-8

-6

-4

-2

0

20.0027 0.0028 0.0029 0.003 0.0031 0.0032

1/T (K)

Ln( r

ate

/ mg

uL-1

d -1

)-1

Ln (dDEC / dt)

Ln (d -EC / dt)

0oC•Previous measurements of Electrolytereactivity carried out under poorly controlled conditions

– e.g. variable moisture content.•Prepare solutions under better controland add known amounts of moisture, acids,bases and additives (LiF).

•EC/EMC-LiPF6+ 0.2mM H2O gelled at 43oC!-followed by thinning and coloring.

•EC/EMC-LiTFSI – no reaction at 85oC for 2 years!

- addition of 5mM HTFSI at 43oC results in no visible reaction.EC/EMC- LiPF6 heated

in glass vials

60oC84oC 4

Polymer Exploration Single-ion Conductors (NASA funded)

Possess adequate bulk transport but enormous interfacial impedance. What is the source of this?

2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4-9

-8

-7

-6

-5

-4

-3

-2

-12.4 2.6 2.8 3.0 3.2 3.4 3.6 3.8 4.0 4.2 4.4

-9

-8

-7

-6

-5

-4

-3

-2

-1

Log σ

/ Scm

-1

1000 / T (K-1)

75 wt% PC/EMC (1/1, v/v) 50 wt% PC/EMC (1/1, v/v) 50 wt% PC/DMC (1/1, wt/wt) 50 wt% EC/DMC (1/1, wt/wt) Dry PAE8-g-E2SO3Li EO/Li = 118

25oC

0

5000

1 104

1.5 10 4

2 104

2.5 10 4

0 5000 1 104 1.5 10 4 2 104 2.5 10 4

Before cycling

After cycling

- Z"/Ω

Z'/Ω

Ionic Conductivity Interfacial Impedance at Lithium

Plans for Next Fiscal Year• Complete electrolyte degradation analysis under rigorously

controlled conditions– All operations in Glove-box or Schlenk-line.

• Continue to explore how electrolyte degradation affects impedance growth at both electrodes.

• Expand interfacial studies to cathodes.– Detailed analysis of cathode side reaction products and effects on

interfacial behavior– Study the effects of impurities (water, acids, additives) on interfacial

behavior.

• Use Single Ion Conductor polymers from DOE Hydrogen Program to investigate interfacial impedance.

Summary • Interfacial behavior of electrolytes is at least as

critical as bulk transport properties– Electrochemical kinetics.– Electrolyte mobility/wettability at surfaces.– Chemical/electrochemical degradation.

• Interfacial Impedance of Ionic Liquids precludes their use in high rate conventional cells.

• Ionic Liquids may find use in supercapacitors and hybrid polymer electrode batteries.

Collaborations and Technology Transfer

• ATD Program (Kostecki, Abraham)• BATT - Doeff, Battaglia, Kostecki, Creager,

HydroQuebec, Modeling Groups (Smith/Borodin, Newman/Srinivasan).

• CSIRO (Australia) Tony Hollenkamp. Adam Best

• Synergy with DOE Hydrogen program for Polymer materials.

• NASA

Publications/PatentsPapers:1. Saint, J., et al., Compatibility of LixTiyMn1-yO2 (y=0, 0.11) electrode

materials with pyrrolidinium-based ionic liquid electrolyte systems.Journal of the Electrochemical Society, 2008. 155(2): p. A172-A180.

2. Salminen, J., et al., Physicochemical properties and toxicities of hydrophobic piperidinium and pyrrolidinium ionic liquids. Fluid Phase Equilibria, 2007. 261(1-2): p. 421-426.

Patents: Shanger Wang, Jun Hou, Steve E. Sloop, Yong Bong Han and John B. Kerr.

Polymeric electrolytes based on hydrosilyation reactions USP 7,101,643, September 5, 2006.

Presentations:Joon Ho Shin, John B Kerr, Thermal Behavior and Interface Property of N-

methyl-n-butylpyrrolidinium Bis(trifluoromethanesulfonyl)imide-LiTFSI-PEGDME Mixtures. Presented at 212th ECS Meeting,Washington, DC, US October 7-12, 2007.