Lithium salt addition to Ionic Liquids: SPR research

1. 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

2. 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

3. Introduction: Application of ILs
Li ion batteries
Properties
2/12

4. 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

5. 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

6. 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

7. 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/kΩC/μFcm-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
40R/kΩC/μFcm-2
E / V
●:C4mimFSA
▲:C4mimFSA+LiFSA
0
1
2
3
4
-0.2 -0.1 0.0 0.1 0.2
0
10
20
30
40
R/kΩC/μFcm-2
E / V.
●:C4mimTFSA
▲:C4mimTFSA+LiFSA
Different anion
Different anion
: C only changed slightly
Promissing?? 6/12

8. 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

9. 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

10. 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/mdegDD/mdeg
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

11. Different anionSame anion
Different anion: Highly dense ionic layer
Huge decrease in D0
•Half C ≒ Half rearranging ions
•Refractive index (n): TFSA,FSA> Li complex anion
Factors
Little change in D0
•C : slightly decrease
•n: TFSA,FSA> Li complex anion
-0.2
0.2
0 240 480 720
-50
0
50
-50
0
50
0.0
0.4
D/mdegD/mdeg
t / s
C4 mimFSA
C4 mimFSA+LiFSA
E/VE/V
-50
0
50
0 240 480 720
-50
0
50
-0.2
0.2
C4 mimTFSA+LiTFSA
C4 mimTFSA
D/mdegE/V
0 240 480 720
-50
0
50
-50
0
50
-0.2
0.2
C4 mimTFSA+LiFSA
C4 mimTFSA
D/mdegE/V
ESPR results： D0
10/12

12. Different anionSame anion
1.40
1.41
1.42
1.43
1.44
1.45
C4mimTFSA
Refractiveindex
C4mimTFSA
+LiTFSA
1.40
1.41
1.42
1.43
1.44
1.45
C4mimFSA
C4mimFSA
+LiFSA
Refractiveindex 1.40
1.41
1.42
1.43
1.44
1.45
C4mimTFSA
Refractiveindex
C4mimTFSA
+LiFSA
Refractive index (n): TFSA,FSA> Li complex anion
ESPR results： D0
11/12

13. Different anionSame anion
ESPR results：t
-3
-2
-1
0
1
2
log(t/s)
tp
tRC
tn
C4 mimTFSA
C4 mimTFSA
+LiFSA
-3
-2
-1
0
1
2
log(t/s)
tp
tRC
tn
C4 mimTFSA
C4 mimTFSA
+LiTFSA
-3
-2
-1
0
1
2
log(t/s)
(a) (b)
C4 mimFSA
C4 mimFSA
+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

14. Different anionSame anion
Discussion
Difference in the anion size is not
necessarily responsible for D0 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 layerSame anion: Low dense ionic layer 13/12

15. 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

16. 15/12

17. Melting point
Lattice Energy
E = 𝑘
𝑄1 𝑄2
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

18. 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

19. 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

20. 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