Structural relaxation of the

electrical double layer in ionic liquid

probed by ESPR:

Effect of Li salts addition

Takeru Arai」

Department of Energy and Hydrocarbon Chemistry,

Graduate School of Engineering

Background: Ionic Liquids

Liquids composed of only cations and anions at ambient temperature

Ionic Liquids(ILs)

Ionic Liquids: liquid Melting point: < 100K [2] Ex) C4mimTFSA, Pyr13BF4

• Delocalization of charge

• High steric hindrance • Large ionic radius

Organic Cations and Anions

General salts: solid Melting point: > 800K [1] Ex) NaCl, CaCO3, CaSO4

[1]http://www.shiojigyo.com/siohyakka/about/data/condition.html

[2]https://www.sigmaaldrich.com/content/dam/sigma-aldrich/docs/SAJ/Brochure/1/j_cf05-06.pdf 1/12

Introduction: Application of ILs

Li ion batteries

Properties

2/12

Past studies: Interface

[4] N. Nishi et al., Phys. Chem. Chem. Phys., 15 (2013) 11615.

[3] S. Makino et al., Electrochem,. Commun., 13 (2011) 1365.

・Structural relaxation time against the potential steps is over a minute[3,4] ・Ionic layers are formed at the interface between metal|IL.

3/12

Purpose

Samples used in the present study

ILs

Li salts

[5] M. Yamagata et al., Electrochim. Acta,110(2013)181-190.

Same anion Different anion

4/12

AC impedance method

•R : Liquid resistance

•C : Capacitance

Charging time at the interface assuming interfacial

structure to be the same as bulk structure

tRC =RC (RC time constant)

Experimental plot for C4mimTFSA and fitting curve.

Equivalent circuit

CPE：Constant Phase Element

(Capacitance dependent on frequency)

R: Liquid resistance

Rwe: Charge transfer resistance

5/12

AC impedance method results

Same anion

・R increased

・C decreased (almost to the half amount)

・R almost unchanged

・C decreased slightly

0

1

2

3

4

-0.2 -0.1 0.0 0.1 0.2

0

10

20

30

40

R /

kΩ

C /

μF

cm

-2

E / V

●:C4mimTFSA

▲:C4mimTFSA+LiTFSA

0

1

2

3

4

-0.2 -0.1 0.0 0.1 0.2

0

10

20

30

40 R

/ kΩ

C /

μF

cm

-2

E / V

●:C 4mimFSA

▲:C 4mimFSA+LiFSA

0

1

2

3

4

-0.2 -0.1 0.0 0.1 0.2

0

10

20

30

40

R /

kΩ

C /

μF

cm

-2

E / V.

●:C 4mimTFSA

▲:C 4mimTFSA+LiFSA

Different anion

Different anion : C only changed slightly

Promissing?? 6/12

Refractive index change at Au electrode surface

due to cation/anion rearrangement

against the potential steps

Principle

Reflectivity change at interface

with different refractive index of adsorption layer.

ESPR Measurement : Principle

・Incident laser excites surface plasmon wave at Au thin film.

・Resonance angle(i.e. incident angle at minimum reflectivity)

changes (Dθ)

・Resonance angle correlates with refractive index

at the interface of Au thin film.

7/12

Refractive index change at Au electrode surface

due to cation/anion rearrangement

against the potential steps

Principle

Reflectivity change at interface

with different refractive index of adsorption layer.

ESPR Measurement : Principle

8/12

ESPR measurement: Fitting

5

10

15

20

25

30

35

40

45

0 20 40 60 80 100 120-3

-2

-1

0

1

2

3

t/s

:Experimental

single

double

triple

D/

md

eg

DD/

md

eg

single

double

triple

From ESPR (fitting) (1) relaxation time for ionic rearrangement against potential steps (2) The index related to the refractive index of the adsorption layer

9/12

Different anion Same anion

Different anion: Highly dense ionic layer

Huge decrease in D0

•Half C ≒ Half rearranging ions •Refractive index (n): TFSA,FSA> Li complex anion

Factors

Little change in D0

•C : slightly decrease •n: TFSA,FSA> Li complex anion

-0.2

0.2

0 240 480 720

-50

0

50

-50

0

500.0

0.4

D/

md

eg

D/

md

eg

t / s

C4 mimFSA

C4 mimFSA+LiFSA

E/

V E

/ V

-50

0

50

0 240 480 720

-50

0

50

-0.2

0.2

C4mimTFSA+LiTFSA

C4mimTFSA

D/

md

eg

E/

V

0 240 480 720

-50

0

50

-50

0

50-0.2

0.2

C4mimTFSA+LiFSA

C4mimTFSA

D/

md

eg

E/

V

ESPR results： D0

10/12

Different anion Same anion

1.40

1.41

1.42

1.43

1.44

1.45

C4mim

TFSA

Re

fra

cti

ve

in

de

x

C4mim

TFSA

+LiTFSA

1.40

1.41

1.42

1.43

1.44

1.45

C4mim

FSA

C4mim

FSA

+LiFSA

Re

frac

tiv

e i

nd

ex

1.40

1.41

1.42

1.43

1.44

1.45

C4mim

TFSA

Re

fra

cti

ve

in

de

x

Re

fra

cti

ve

in

de

x

C4mim

TFSA

+LiFSA

Refractive index (n): TFSA,FSA> Li complex anion

ESPR results： D0

11/12

Different anion Same anion

ESPR results：t

-3

-2

-1

0

1

2

log

( t/s

)

tp

tRC

tn

C4mim

TFSA

C4mim

TFSA

+LiFSA

-3

-2

-1

0

1

2lo

g( t

/s)

tp

tRC

tn

C4mim

TFSA

C4mim

TFSA

+LiTFSA

-3

-2

-1

0

1

2

log

( t/s

)

(a) (b)

C4mim

FSA

C4mim

FSA

+LiFSA

tp

tRC

tn

•tp:Increased

•tn:Decreased

•tRC: Constant

Obvious asymmetry in t

Different anion: Complicated ionic layer with Li+

tp,tn,tRC :Increased

Symmetric change in t→similar to neat ILs

For both of the same anion systems…

12/12

Different anion Same anion

Discussion

Difference in the anion size is not

necessarily responsible for D0 and 𝜏 .

Li complex anions may form to

decelerate relaxation in EDL of TFSA

and FSA based samples.

Asymmetry of the relaxation time is

noticeable.

Multi-core complexes can slowly

make a structure through mass

transfer to the interface.

/ Strong adsorption

A part of bonds of multi-core

complexes break to leave from

the interface relatively quickly.

/ Weak adsorption

Negative step

Positive step

Different anion: Highly dense ionic layer Same anion: Low dense ionic layer 13/12

Conclusion

All the relaxation times increased.

Li complex anions may form to decelerate relaxation in EDL.

・Positive step: Multi-core complexes can slowly form a structure

through mass transfer

・Negative step: Multi-core complexes can leave

from the interface relatively quickly.

Same anion

Different anion

Lower capacitance

Similar capacitance

14/12

15/12

Melting point

Lattice Energy

E = 𝑘𝑄1𝑄2

𝑑

Lattice energy

Delocalization of electric charge

Ionic radius

Steric hindrance

Low melting point

High Steric hindrance

Large Ionic radius Low Delocalization of

electric charge

16/12

Discussion: Same anion system

All the relaxation times increased.

→Li complex anions may form to decelerate relaxation in EDL.

Lower capacitance ≒ Less ionic rearrangement

17/12

Discussion: Same anion system MD calculation：

C4mimFSA+LiFSA

Q. Li+ forms some Li-anion complex at the interface?

A. Yes, it does.

★Li+&FSA forming complex anion network（snapshots）

★@negative charged electrode

・ Li+&FSA: density in the first ionic layer decreases or peak shifted to the

bulk side ・C4mim＋: density in the first ionic layer increases (number density

distribution)

Snapshots & number density distribution

18/12

Asymmetry of the relaxation time is noticeable.

Multi-core complexes can slowly make a structure through

mass transfer to the interface. / Strong adsorption

A part of bonds of multi-core complexes break to leave

from the interface relatively quickly. / Weak adsorption

Negative step

Positive step

Discussion: Different anion system

Different anion: Highly dense ionic layer

19/12