the role of the electrolyte in lithium ion batteries€¦ · the role of the electrolyte in lithium...

The Role of the Electrolyte in

Lithium Ion Batteries

15.05.2009 | Andrea Balducci Page | 1

Lithium Ion Batteries

Drive-E Akademie

09.03.2010

Andrea Balducci

Institute of Physical Chemistry, Westfälische Wilhelms-University Münster,

Corrensstraße 28/30, 48149 Münster

1

102

103

104

kW

kg

-1)

Capacitors

Energy (kWh/g): the capacity to do workPower (kW/kg): how fast the energy is delivered

Energy vs. Power


10-2

10-1

100

101

102

103

10-3

10-2

10-1

100

101

Sp

ec

ific

Po

we

r /

(kW

kg

Specific Energy / (Wh kg-1)

Supercapacitors

Batteries FuelCells

Page 1

Batteries

LITHIUM-ION


LITHIUM-ION

TODAY

MEDIUM

HIGH

LOW

Lithium-ion battery

TOMORROW


Energy SafetyLife CostPower

LOW

TOMORROW: Green & High Performance Batteries

10.07.2009 5Page 5

Lithium-ion batteries

Lithium-ion batteries in automotive industry

NEW APPLICATIONS POSSIBLE


TODAY


MEDIUM

HIGH

LOW

Battery of tomorrow

TOMORROW

15.05.2009 | Andrea Balducci Page | 1Page 4

How to improve the performance?

10.07.2009 5Page 5

Electrolyte Components

Materials (Active, Inactive)

Outlines

• Electrolyte in lithium-ion batteries: general aspect

• Electrolyte and ionic liquids

• Solid polymer electrolytes & ILs

15.05.2009 | Andrea Balducci Page | 110.07.2009 7

• Conclusions

Lithium-ion batteries

Electrodes

Electrolyte


Electrodes

Colle

cto

r(C

u) P

os. C

urre

nt

Sep

ara

tor

Lithium-ion Battery


Neg

. C

urr

ent

Colle

cto

r Cu

rren

tC

olle

cto

r(A

l)

Sep

ara

tor

Anode CathodeElectrolyte

Graphite, Li4Ti5O12

Si, Si/CLiCoO2, LiMn2O4, LiFePO4

10.07.2009 2Page 2

e-

e-N

eg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)

Lithium Ion Battery: Charge


Se

pa

rato

r

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


10.07.2009 Page 3

e-

e- e-

e-

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)

Lithium Ion Battery: Discharge


Se

pa

rato

r

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


10.07.2009 4Page 4

• Liquid

• Liquid organic solvent based electrolytes

• Liquid inorganic solvent based electrolytes

• Molten salts (low temperature = ionic liquids)

• "Solid"

Electrolyte materials


• "Solid"

• Solid polymer electrolytes

• Ceramic electrolytes

• Glassy electrolytes

• Composites

• Gel electrolytes

Outlines

• Electrolyte: general aspect



15.05.2009 | Andrea Balducci Page | 110.07.2009 7Page 7

• Conclusions

Electrolyte in General

Electrolyte: electrolytic solution-type consisting of salts („electrolyte solutes“)

dissolved in solvents

Dissociation due to thermodynamic interactions between solvent and solute

molecules = solvation

Function: medium for the transfer of charge in form of ions

between the electrodes

Requirements for electrochemical devices:


Requirements for electrochemical devices:

high ionic conductivity

low melting and high boiling points

chemical and electrochemical stabilities

safety

SEI Film forming ability!!!

For lithium and lithium ion batteries:

10.07.2009 8Page 8

Se

pa

rato

r

Lithium Ion Battery: Exfoliation

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


Se

pa

rato

r

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


10.07.2009 9Page 9

Solid Electrolyte Interphase

- passivation layer

- prevent exfoliation

- formed within the first few cycles via

electrolyte decomposition

- minimum of irreversible material and

charge loss, minimum of side reaction

Properties:

- just permeable for Ions, high

ionic conductivity

- ideally electronically insulating

⇒ no further decomposition

- uniform morphology and

chemical composition

SEIe-

Li+

e-

Li+


1 Emanuel Peled Journal of the Electrochemical Society 1979, 126, 2047.

chemical composition

⇒ homogenious current

distribution

- good mechanical strength and

flexibility

⇒ allows expansion and

contraction of the graphene lattice

- low solubility in electrolytes

⇒ no dissolutionAnode CathodeElectrolyte

10.07.2009 10Page 10

Se

pa

rato

r

Lithium Ion Battery: SEI-Film

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


Se

pa

rato

r

SEI

Neg. C

urr

ent

Colle

cto

r (C

u) P

os. C

urre

nt C

olle

cto

r (Al)


10.07.2009 11Page 11

Electrolyte System

Multi Component System

Li Salt Solvents Additives


Design of the electrolyte components

Energy SafetyLifeCostPower

10.07.2009 13Page 13

Electrolyte System

ConductivityEnvironment - friendly

Temperature range of use

Electrolyte System

��

?

?


Vapor pressure Film forming ability

Electrochemical stability window

State of the art:

Organic electrolytes

��

?

?

10.07.2009 Page 14

Properties from best ⇒ to worst

Ion mobility LiBF4 LiClO4 LiPF6 LiAsF6 LiTf1) LiTFSI2)

Ion pair dissociation LiTFSI LiAsF6 LiPF6 LiClO4 LiBF4 LiTf

Solubility LiTFSI LiPF6 LiAsF6 LiBF4 LiTf

There is No Universally Superior Electrolyte Salt

Lithium salts


Thermal stability LiTFSI LiTf LiAsF6 LiBF4 LiPF6

Chemical inertness LiTf LiTFSI LiAsF6 LiBF4 LiPF6

SEI formation LiPF6 LiAsF6 LiTFSI LiBF4

Al corrosion LiAsF6 LiPF6 LiBF4 LiClO4 LiTf LiTFSI

1) LiTf lithium triflate 2) LiTFSI lithium bis(trifluoromethansulfonyl)imide

Nakajima, T.; Groult H. (eds.), Fluorinated Materials for Energy Conversion, Elsevier, Amsterdam, 2005

Properties from best ⇒ to worst

OO

O

EC

OO

O

PC

O O

GBL

O O

O

DEC

O O

O

DMC

O O

O

EMC

High dielectric solvents: HDS Low viscosity solvents: LVS

Electrolyte Solvents


Low melting point DEC EMC PC GBL DMC EC

High boiling point EC PC GBL DEC EMC DMC

High dielectric constant εεεε EC PC GBL DMC EMC DEC

Low viscosity ηηηη DMC EMC DEC GLB PC EC

High flash point EC PC GBL DEC DMC

SEI formation EC

O O

O

OO

DME

DMC

Salts

�LiPF6

�LiBF4

�LiAsF6

�LiClO4

High dielectricsolvents (HDS)

OO

O

OO

O

EC

Low viscositysolvents (LVS)

Electrolyteadditives

OO

O

O

O

O

VC

VAInsufficientOxidation Stability

Electrolytes components


O O

O

O O

O

DMC

DEC

EMC

4

�LiCF3SO3

�LiN(SO2CF3)2

�LiBOB

OO

PC

O O

GBL

OO

F

P

O

MeO OMe

OMe

FEC

TMP

BPUneffective SEI

Stability

Not Well-Conductiveor Toxic or

Explosive orCorrosive

- Salt:

- Solvents:

For conductivity reasons use of solvent mixtures of: OO

O

OO

O

LiPF6

P

F

F FF

FF

-Li+

SEI forming compound

+

State of the Art


- high dielectric solvents HDS:

- low viscosity solvents LVS:

EC

O O

O

DEC

O O

O

DMC

O O

O

EMC

PC

compound

State of the art: LiPF6

+ Instability ⇒ SEI forming agent, Al current collector protection

+ Conductivity

- Instability ⇒ Thermal and chemical

LiPF6 + H2O LiF + POF3 + 2HF Toxic and corrosiveLewis-acids,

State of the Art: lithium salt


LiPF6 LiF + PF5

+ LiPF6

∆T

OFF

LiCoO2

∆T

Lewis-acids, Catalysts for polymerization

Highly(!) toxicOO

O

Not ideal, but "best" among commercially available candidates⇒⇒⇒⇒ Alternative or at least partial replacement urgently needed

State of the art: EC (PC) + low viscosity solvent LVS (DEC, DMC, EMC,…)

+ Sufficient SEI film form ability

+ TR conductivity

- Low and high T behavior (conductivity, wetting, viscosity,…)

- Safety: flammability of LVS!

reactivity with electrodes, in particular at higher T

- Liquid: immobilization, leakage,…? ⇒ safety and performance concern

State of the Art: liquid solvents


- Liquid: immobilization, leakage,…? ⇒ safety and performance concern

2 Possibilities:

� Keep liquid organic electrolyte, but- substitute or add novel solvent components and electrolyte additives- immobilize liquid electrolyte (polymer matrix ⇒ hybrid or "gel" electrolyte)

� Substitute liquid organic electrolyte, e.g., by ceramic, solid polymer, IL,…electrolytes

Electrochemical stability requirements: Cation: stable vs. reduction, anion: stable vs. oxidation

Solubility/dissociation requirements:Small Li+ cation + large anion ⇒ small lattice energy

⇒ good dissociation, e.g., LiPF6, LiBOB

Conductivity: Salt Issuses


In general:

Conductivities are 2-3orders of magnitude lower than aqueousbattery electrolytes!

⇒Thin electrolyte filmsto keep resistance small!

Liquid organic electrolytes for lithium ion batteries are based on solvent

mixtures for conductivity reasons.

High dielectric solvent (HDS):Solvates ions, thus favors electrolyte salt dissociation.

But: (Too) high viscosity

Conductivity: Solvent Issues


Low viscosity solvent (LVS):Is a dilutant, thus lowers viscosity.

But: Poor ion solvation⇒ ion pair formation(= lack of free charge carriers)

Anode: graphite

Electrolyte: 1M LiClO4 in PC

Scan rate: 50 µV.s-1

Anode: graphite

Electrolyte: 1M LiClO4 in PC

Scan rate: 50 µV.s-1

+2 wt.% ethyl isocyanate (Et-NCO)

Importance of the SEI Film


C. Korepp et al. Journal of Power Sources 2007, 174, 628.


Li+

Li+Li+(solv)y

SEI Interface

Li+

Electrolyte Decomposition


Reductive electrolyte

decomposition mechanismOxidative electrolyte

decomposition mechanism

Reaction products: - directly react with electrodes- diffuse and then react with the electrode- redox-shuttle between the electrodes, etc.

Reactions depend on: - electrolyte composition- electrodes (bulk, surface)- potentials- temperature, etc.


What we know?

- exist and has its function

(SEI determines cell safety, life, etc.)

- consists of electrolyte decomposition

products and Li+

- not perfect (no true solid electrolyte)

- influenced by many parameters

- there is no universal SEI!

- SEI grows and ages during storage/cycling

Knowledge about SEI Film


- SEI grows and ages during storage/cycling

What we do not know ?

⇒ Only aspects are known about SEI formation, growth, aging, etc. No clear picture! ⇒ Composition of SEI is unclear: many contradictory reports!⇒ No rule for SEI formation procedure and for finding a good SEI forming agent.

Chemically very similar compounds show a totally different SEI behavior!

What is a good SEI? What is a bad SEI? Empirical Approach!

SEI

AssemblyElectrolyte, electrode,active and "inactive"

Formation charge, potential,change of chemistry side reactions, etc.

SEI: Formation


SEI

Battery propertieselectrochemistry

∆T behavioursafety, etc.

Application

SEI


Formation charge, potential,change of chemistry side reactions, etc. Formation

Mechanism

composition,

impurities, surface

monitoring,

characterization

in situ, on-line

characterization

ex situ

ANALYT

SEI Composition

Thinking

SEI: Formation ⇒ Characterization

⇒⇒⇒⇒ Understanding


SEI



Application

Mechanismex situTICS


Formation charge, potential,change of chemistry side reactions, etc. Formation

Mechanism

composition,

impurities, surface

monitoring,

characterization

in situ, on-line

characterization

ANALY

SEI Composition

Thinking

SEI: Formation ⇒⇒⇒⇒ Characterization

⇒⇒⇒⇒ Understanding ⇒⇒⇒⇒ Improvement


SEI

Application

Mechanismex situLYTICS

characterization in situ & ex situ

Application

Improved Performance

Understanding

Thinking



Multi Component Electrolyte ⇒⇒⇒⇒ Multi Component SEI

⇒ Electrolyte: solvents, salt(s), additive(s), impurities

⇒ Anode (material, surface)⇒ Many different SEI products

⇒ SEI may vary in lateral dimensions

⇒ Anode-near SEI parts must be stable against reduction potentials close to/equal to 0 V vs. Li/Li+, anode-far (electrolyte-near) parts

Characterize & Understand the SEI


"Heterogeneous", "mosaic", "complex" structure/composition

close to/equal to 0 V vs. Li/Li , anode-far (electrolyte-near) partsdo not have to

⇒ SEI composition varies in depth

⇒ Electrode formulation ("inactive components", etc.)

⇒ Formation conditions: charge procedures, current densities, etc.

⇒ SEI growth and aging during storage and cycling (temperature)

Multi Component SEI ⇒⇒⇒⇒ Locally Different SEI

The SEI is heterogeneously composed in depth and in lateral dimensions.⇒ Locally different SEI: Locally applied analytical method will give only local information of the SEIGlobally applied analytical method will give only global (average) information of the SEI



Locally Different Electrolyte Decomposition Products / SEI on Graphite

RelevantMethodology:

In situEx situ Li+(C

u)

Li+Electrolytedecomposition

products

Basal plane surface



Ex situ

AFMSTMSEMTEMXPSAugeretc.

Li+

Curr

ent

Co

llecto

r(C

u)

Li+

Electrolyte decomposition

products

Prismatic surface

Locally Different Electrolyte Decomposition Products on Basal Plane and Prismatic Surfaces of Graphite

XPS measurements of HOPG

Prismatic

(%)

Basal plane

(%)Elements



of HOPGafter one cycle in 1.2 M LiAsF6

in EC : DEC electrolyte

Use of diverse methods which allow to detect certain aspects of the SEI



Characterization of a "practical" electrodeafter electrochemical experiment (SEI formation)

- Removal from cell (under protective atmosphere)

- Rinsing and cleaning (under protective atmosphere) to remove the electrolyte: also parts of the SEI can be removed

Ex situ Approach



- Transport and transfer to the analytical chamber (under protective atmosphere)

- Often conversion or destruction of the SEI by the specific analytical experiment(Vacuum, Beam)

Characterization of a “non practical” electrodeduring electrochemical experiment (SEI formation)

Model electrodesInert metals, glassy carbon, carbon fibers, "binder-free“, instead of composite (binder/carbon) electrodes

In situ Approach



Model experimental conditionsSlow/fast electrochemical experimentsCell housing, Inactive materials (grids, etc.) different

Model electrolytesExcess of electrolyteModel electrolyte components

Ex situ

Battery Electrode

Practical “Battery”Electrochemical Conditions

but

In situ

Model Electrode

“Model“ ElectrochemicalConditions

but

In Situ vs. Ex Situ



Results of

Practical Electrode ?

butHandling & Analysis

Outside BatteryEnvironment

Results of

Model Electrode !

butHandling & AnalysisInside “Non-Battery”

Environment

Outlines





• Conclusions

TODAY


MEDIUM

HIGH

LOW

Battery of tomorrow

TOMORROW

15.05.2009 | Andrea Balducci Page | 1Page 410.07.2009 5Page 5

Ionic Liquids (ILs)

Room Temperature Ionic Liquids (ILs)Typically consist of organic cations and inorganic/organic anions. The low melting temperatures result from unfavourable crystal packing and ion flexibility.

ILs properties

• negligible vapor pressures • high ionic conductivities• wide electrochemical stability window• thermally stable


• thermally stable• easily dissolve lithium salt (doping)• “green electrolyte”

ILs electrolytes in lithium batteries

15Page 15

ILs Chemical Physical Properties

2,0-4 -3 -2 -1 0 1 2

Potential vs. Ag° / AgCF3SO

3 in PYR

14TFSI (V)

80 70 60 50 40 30 20 10

T / °C

Arrhenius Conductivity plot Electrochemical stability window at RT

Very high purity (>95%), H2O content < 1 ppm

N-methyl-N-buthylpyrrolidinium

bis(trifluoromethansulfonyl)imidePYR14TFSIN SS

O

O

O

O

CC

F

F

F

F

F

F

-

N

+

•

N-methyl-N-propyl pyrrolidinium bis(fluorosulfonyl)imide PYR13FSI

-

N

+

N SS

O

O

O

O

FF•


PYR14TFSI: better electrochemical stability

PYR13FSI: higher conductivity (even higher than PC-LiTFSI 1M)

0 1 2 3 4 5 6-2,0

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

2,0 PYR

14TFSI

PYR13

FSI

Cu

rren

t d

en

sit

y (

mA

cm

-2)

Potential vs. Li / Li+ (V)

2,8 2,9 3,0 3,1 3,2 3,3 3,4 3,51

10

PC - LiTFSI 1M

PYR14

TFSI

PYR13

FSI

σσ σσ

/ m

S c

m-1

103 T

-1 / K

-1

Page 16

SEI in graphite electrodes

The selection of

Cu Graphite + Li

SEI

sep

ara

tor

Solvent Li-salt

Electrolyte

Additive


The selection of film forming electrolytes additives and Li salt

is crucial in the case of organic liquid electrolytes

What is the importance of additives and Li-salt in ILs?

Page 17

0.3 M LiTFSI in PYR14TFSI

PYR14TFSI - LiTFSI as Electrolyte

0.3 M LiTFSI in PYR14TFSI + 5% wt. VC

-0,1

0,0

0,1

0,2 1

st cycle

2nd

cycle

3rd cycle

i / m

A m

g-1

O O

O

Selected graphite: KS6 (TIMCAL) very sensitive to the electrolyte properties

-0,05

0,00

0,05

0,10

0,15

i / m

A m

g-1

1st cycle

2nd

cycle

3rd cycle


Poor electrochemical performance

0 50 100 150 200 250 300 350 400-0,2

-0,1

E / mV

Better cyclability because of VC,

but low specific capacity

Efficiency (3rd cycle): 74,5%Specific capacity: 132 mAhg-1

0 250 500 750 1000-0,15

-0,10

E / mV

Page 18

PYR13FSI - LiTFSI as Electrolyte

0.3 M LiTFSI in PYR13FSI 0.3 M LiTFSI in PYR13FSI + 5% wt. VC

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

1st cycle

2nd

cycle

3rd cycle

i / m

A m

g-1

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

1st cycle

2nd

cycle

3rd cycle

i / m

A m

g-1

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

1st cycle

2nd

cycle

3rd cycle

i / m

A m

g-1


0 50 100 150 200 250 300 350 400-0,3

E / mV

VC improves the efficiency,

but not the specific capacity

Stability comparable

with PYR14TFSI + 5% VCLow specific capacity


Efficiency (3rd cycle): 89,8%Specific capacity: ca. 130 mAhg-1

0 200 400 600 800 1000 1200 1400-0,3

E / mV

0 50 100 150 200 250 300 350 400-0,3

E / mV

Page 19

In Situ FTIRS Measurements

CounterElectrode

ReferenceElectrode

Workingelectrode

Home-made IR cell, provided with an optical ZnSe window.

Working Electrode = Glassy Carbon (ø=12 mm) Counter electrode = LiReference electrode = Li

Glassy carbon (GC) has a goodcapability for IR beam reflection


IRcapability for IR beam reflection

SNIFTIR method (Subtractively Normalized Interfacial FTIRectroscopy)

Reference spectra at OCV (R0) Stepwise to 0.4 V vs Li/Li+ (Ry)

Page 20

PYR14TFSI DOES NOT DECOMPOSE

PYR14TFSI

4000 3500 3000 2500 2000 1500 1000 500

OCV 1000 mV 750 mV

500 mV

Tra

sm

issio

n

ν / cm-1

0.3 M LiTFSI in PYR14TFSI

0.3 M LiTFSI in PYR14TFSI + 5% wt VC

Tra

sm

issio

n

*

In Situ FTIR Measurements


PYR14TFSI DOES NOT DECOMPOSE

VC DECOMPOSES

The FSI- anionDECOMPOSES

4000 3500 3000 2500 2000 1500 1000 500

OCV 1000 mV

500 mV 400 mV

Tra

sm

issio

n

ν / cm-1

**

O O

O

O O

O

1820 1810 1800 1790 1780

1,000

1,005

1,010

1,015

1,020

1,025

1,030

1,035

RE / R

0

νννν / cm-1

0.3 M LiTFSI in PYR13FSI

1240 1220 1200 1180 1160 1140 1120 1100

1,000

1,005

1,010

1,015

1,020

1,025

νννν / cm-1

RE / R

0

N SS

O

O

O

O

FF N SS

O

O

O

O

FF

4000 3500 3000 2500 2000 1500 1000 500

OCV

1000 mV

750 mV

500 mV

ν / cm-1

Tra

sm

issio

n

Page 21

PYR13FSI

LOWER STABILITYBetter electrochemical

performance

Additives maybe not

necessary:FSI- source for the SEI layer

PYR14TFSI

HIGHER STABILITYPoor electrochemical

performance

Additives necessary:VC source for the SEI layer

ILs and Additive Electrolytes


Different SEI chemistry

Optimization of the SEI chemistry

Li salt

Page 22

0.3 M LiPF6 in PYR14TFSI 0.3 M LiPF6 in PYR14TFSI + 5% wt. VC

-0,1

0,0

0,1

0,2

0,3 1

st cycle

2nd

cycle

3rd cycle

i / m

A m

g-1

PYR14TFSI as Electrolyte with LiPF6

Role of Li salt

-0,1

0,0

0,1

0,2

0,3

i / m

A m

g-1

1st cycle

2nd

cycle

3rd cycle

LiPF6


Poor electrochemical performance

0 50 100 150 200 250 300 350 400-0,3

-0,2

E / mV

Higher specific capacity compared to

LiTFSI but lower cycling stability


0 50 100 150 200 250 300 350 400-0,3

-0,2

E / mV

Page 23

0.3 M LiPF6 in PYR13FSI 0.3 M LiPF6 in PYR13FSI + 5% wt. VC

0 50 100 150 200 250 300 350 400-0,5

-0,4

-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

0,4

0,5 1

st cycle

2nd

cycle

3rd cycle

i /

mA

mg

-1

PYR13FSI as Electrolyte with LiPF6

0 50 100 150 200 250 300 350 400-0,3

-0,2

-0,1

0,0

0,1

0,2

0,3

i / m

A m

g-1

1st cycle

2nd

cycle

3rd cycle


0 50 100 150 200 250 300 350 400

E / mV

The VC improve the efficiency,

but not the specific capacity

In PYR13FSI the use of LiPF6 increases

the specific capacity more than 2 times

BUT lower cycling stability

0 50 100 150 200 250 300 350 400

E / mV



Page 24

PYR14TFSI and PYR13FSI

PYR13FSI + LiPF6

Mixtures of

PYR14TFSI / PYR13FSI

How to obtain high specific capacity

and high cycling stability?

Specific

capacity /

mA

hg

-1

300

400

The selection of Li salt is critical also in ILs based electrolytes


� Wide electrochemical stability (from PYR14TFSI)� High conductivity (from PYR13FSI) � Intrinsic film form ability (from PYR13FSI)

PYR14TFSI+ LiTFSI

Cycling stability

Specific

capacity /

mA

hg

Low Medium High

100

200

0

Mixtures of PYR14TFSI / PYR13FSI

Page 25

Mix of ILs as electrolyte

� Several mixtures have been prepared:

(x)PYR14TFSI/(1-x)PYR13FSI/LiTFSI

(x)PYR14TFSI/(1-x)PYR13FSI/LiPF6

� Evaluation of electrochemical stability window and ionic conductivity(and cost..!)


� Selected Mixtures:

(80%) PYR14TFSI / (20%) PYR13FSI / LiTFSI (80/20/LiTFSI)(80%) PYR14TFSI / (20%) PYR13FSI / LiPF6 (80/20/LiPF6)(50%) PYR14TFSI / (50%) PYR13FSI / LiTFSI (50/50/LiTFSI)(50%) PYR14TFSI / (50%) PYR13FSI / LiPF6 (50/50/LiPF6)

VC was added to improve the efficiency

Page 26* G.B. Appetecchi et al., Journal of Power Sources,192,2,599-605

PYR13FSI + LiPF6

Specific

capacity /

mA

hg

-1

200

300

400

The mixtures of (x)PYR14TFSI/(1-x)PYR13FSI good strategy

80/20/LiTFSI

Mix of ILs as electrolyte

0

50

100

150

200

250

300

350

400

0

20

40

60

80

100

discharge capacity

charge capacity

cap

ac

ity

/ m

Ah

g-1

efficiency

effic

ien

cy / %

290 mAhg-1


PYR14TFSI+ LiTFSI

Cycling stability

Specific

capacity /

mA

hg

Low Medium High

100

0

� Wide electrochemical stability (from PYR14TFSI)� High conductivity (from PYR13FSI) � Intrinsic film form ability (from PYR13FSI)

0 10 20 30 40 500 0

cycle number

� High specific capacity � High cycling stability

Page 27* S.F. Lux et al., Journal of Power Sources,192,2,606-611

Outlines





• Conclusions

Solid polymer electrolytes & ILs

S/cmS/cm

1010--33

1010--55

aprotic liq.aprotic liq.

solid polymersolid polymer

(1979 (1979 ––2000)2000)

&&

ionic liquidsionic liquids

Solid polymer electrolyte based on PEO Poly(ethylene oxide)

Overcoming the conductivity drawback of PEO electrolytes


1010--88

PEO-LiX-IL electrolytes

� Polymer matrix (PEO) + 2 salts (LiX and IL) having the same anion(PEO-PYR14TFSI-LiTFSI)

Page 29

• Very low interactions between Cation and Anion of IL

• No interaction between Cation and PEO host (IL does

not interact with PEO)

• No interaction between Cation and Li+

• Strong interaction between Li+ and TFSI- lowers the

(Anion)-

Li+

Li+

(Anion)-

(Cation)+

(Anion)-

(Anion)-

PEO host

(Anion)-

Promoted lithium ion mobility



• Strong interaction between Li+ and TFSI- lowers the

strength of the Li+ - PEO coordination

PEO host

(Anion)-

(Cation)+Li+

(Anion)-

* M. Castriota et al., J. Phys. Chem. A 109 (2005) 92,* I. Nicotera et al., J. Phys. Chem. B 109 (2005) 22814.*J.H. Shin et al., J. Electrochem. Soc., 152 (2005) A978

� Conductivity enhancement

The IL-LiTFSI interaction prevents the formation of

the crystalline P(EO)6LiTFSI phase

Page 30

Problem for the incorporation of ILs

Over a certain content of IL the mechanical stability becomes poor

Crosslinking of PEO



Increase the conductivity of the filmwhile maintaining the mechanical stability

Crosslinking of PEO

Page 31

O

hv

O *

-CH2-CH2-O-

OH

-CH2-CH-O-+

CH2

CH

O

CH

O

H2CCH2

CH

O

HC

O

H2C

PEO is sensitive to β + γ radiation (fragmentation !)

Benzophenone acts

via H abstraction !

Thin films of PEO (few µm) containing 5 %wt. of BPh showed an insoluble (gel) fraction W=80%

UV crosslinking with a photoinitiator is possible

Crosslinking of PEO


Thin films of PEO (few µm) containing 5 %wt. of BPh showed an insoluble (gel) fraction W=80%after photo-crosslinking at 70°C under inert gas condition

PEO/PYR14TFSI/Benzophenone

Film preparation:

• Mixing in solid state 57/43/5 (in weight)• Hot pressing between 2 mylar foils at 70°C for 5 min

• 150 µm thickness

• Punching (disc)

• UV curing (365 nm).

PEO/PYR14TFSI/Benzophenone

Page 32

200 250 300 350 400 450 5000,0

0,5

1,0

365nm

Ab

sorp

tio

n

Wavelength [nm]

PYR14

TFSI

Benzophenone

PEO

320nm

Crosslinking of PEO/IL/Li salt

There is no need to remove the Mylar foil for illumination: transparent >320 nm

At 365 nm ONLY the benzophenone adsorb


Wavelength [nm]

� The insoluble fraction of PEO is about

80% after 5 min of illumination

� PYR14TFSI DOES NOT participate in the photo-crosslinking reaction

0 100 200 300 400 500 600 700 800 900 10000

20

40

60

80

100

Ge

lfra

ctio

n [%

]

Irradiation time [s]

Fragmentation

of the PolymerCrosslinking of

the Polymer

� The addition of LiTFSI did not modify the crosslinking reaction

Page 33

365 nm

UV-curing

With non crosslinked composites, the limiting composition for mechanical stable film

is approx. 10/1/1 (with higher IL content sticky gels are obtained)

crosslinked PEO / PYR14TFSI / LiTFSI = 10:2:1 (mol)

Mechanical stability of PEO/IL/Li salt


365 nm

5 min per side

The transparency of the sample after curing indicates low crystallinity

With crosslinked PEO is possible to obtain significantly improve the mechanical stability

* B. Rupp et al., European Polymer Journal 9,44 (2008) 436Page 34

DSC & conductivity measurements

-100 -80 -60 -40 -20 0 20 40 60 80 100-0,9

-0,8

-0,7

-0,6

-0,5

-0,4

-0,3

-0,2

He

at flo

w [W

/g]

(1) uncured 1:1:10

(2) cured 1:1:10

(3) uncured 2:1:10

(4) cured 2:1:10

4

1

3

2

41°C

36°C

� The crossilinkg process reduce significantly the crystalline fraction

PYR14TFSI / LiTFSI / PEO

� No crystalline domain are discernible in the sample (from SEM)


-100 -80 -60 -40 -20 0 20 40 60 80 100-0,9

Temperatur [°C]

Half order of magnitude increase in the ionic conductivity:0.4 mS/cm @ 20°C 1 mS/cm @ 40°C

Page 35

Electrolyte Scale-up

Thin films are vacuum sealed Thin films are UV cured at 70°C for

PEO / LiTFSI / PYR14TFSI (10:1:2) + 5% (PEO wt.) Benzophenone

Mixtures are prepared through a solvent-free procedure:1. Benzophenone and LiTFSI are dissolved in PYR14TFSI @ 70°C2. PEO is added to the solution and mixed to form a paste3. The paste is stored in oven @ 120°C to homogenize 4. Thin films are made by hot-pressing the electrolyte mixtures @ 90°C


Thin films are vacuum sealed in polyethylene envelopes

Thin films are UV cured at 70°C for

different time (3, 5, 7, and 9 minutes)

Page 36

Mechanical properties

8 by 8 cm polymer electrolytes thin films

Fully amorphous &Highly adhesive


Highly adhesive

Very good mechanical properties

Elastomeric behavior

Page 37

Outlines



• Binders and lithium battery


• Binders and lithium battery

• Conclusions

Page 43

Summary

� The electrolyte plays a crucial role in lithium ion batteries

� The electrolyte is a multi-component system

� The electrolyte can (and need) be design

� The formation of the SEI is necessary


� Organic electrolyte are the state of the art

� Ionic liquids display promising properties

� Solid polymer electrolytes & ILs possible alternative

Acknowledgment

AK Winter


the role of the electrolyte in lithium ion batteries€¦ · the role of the electrolyte in lithium...

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