Basics of EQCM-D

1. In situ hydrodynamic spectroscopy for
structure characterization of porous energy
storage electrodes
Saurav Chandra Sarma
Under the guidance of
DR. SEBASTIAN C. PETER
SOLID STATE AND INORGANIC CHEMISTRY LAB
NCU, JNCASR 1

2. In situ hydrodynamic spectroscopy for structure
characterization of porous energy storage electrodes
2
Netanel Shpigel, Mikhael D. Levi, Sergey Sigalov, Olga Girshevitz, Doron Aurbach, Leonid
Daikhin, Piret Pikma, Margus Marandi, Alar Jänes, Enn Lust, Nicolas Jäckel & Volker
Presser

3. Introduction
Novelty of the Paper
EQCM technique
Some common terms
Factors affecting resonance frequency and resonance width
Experimental Section
Results and Discussions
Conclusion
3
Sketch of my talkSketch of my Talk

4. Introduction
4

5. Lithium insertion/deinsertion in Li Ion battery
Repeated ion insertion/deinsertion affects the electrode porous structure, dimensions
and mechanical integrity, and is a major cause of deterioration in the long-term cycling
performance.
5Source: http://www.eco-aesc-lb.com/en/about_liion/

6. Electrode deterioration during cycling
6Source: Song, B. et al., J. Mat. Chem. A 3, 18171-18179 (2015)

7. Novelty of the paper
7Source: Wang, C. M. et al., Nano Letters 11, 1874-1880 (2011)

8. Disadvantage of conventional technique
The 2D microstructure in (a) could be generated by aligned ellipses (b), by non-aligned
cylinders (c), or by myriad other 3D structures. Although additional 2D sections can rule out
some possibilities, they cannot resolve all ambiguities.
8
It has a high vertical strain resolution (<10 nm) but it is less informative for investigating
lateral changes at mesoscopic scale.
Atomic Force Microscopy (AFM)

9. Novelty of the paper
• In this paper they introduced a new comprehensive methodology, to track
the in situ electrode changes during charging/discharging process.
• It uses EQCM technique to probe the change in electrode morphology.
• It is a non-destructible technique.
• Both vertical and lateral changes can be probed using this technique.
• Rigidity, roughness, porosity, specific capacity, dimensions of the electrode
material can be easily tracked using this technique.
9

10. EQCM is an acronym for Electrochemical Quartz Crystal Microbalance
It is a extremely sensitive label-free technique that is capable of measuring
the mass in the nanogram to sub-monolayer range while simultaneously probing
the electrical properties of the system
EQCM was first introduced by Kanazawa et. al. in 1990
This technique makes use of the piezoelectric effect of quartz to induce
an oscillation at the fundamental frequency (4-10 MHz) of the quartz crystal
EQCM - An analytical method
10

11. Quartz is used in clocks and watches to keep time. If a
charge is passed through a special shaped quartz crystal,
it will vibrate at an exact number of time per second
(around 33000). These vibrations remain constant provide
the crystal is connected to a battery
The circuit inside a timepiece detects vibration from
the crystal and will know how many times it should
vibrate within a second. This means that everytime the
circuit detects a fixed number of vibrations, it will send
an electric pulse to add 1 second to a digital or
analogue counter.
Quartz as a Piezoelectric material
11

12. Quartz crystal cut and Vibration mode
12

13. The electrochemical quartz crystal microbalance (EQCM) can be used to study a variety of
interfacial phenomena.
These phenomena can include:
• Li+ Intercalation
• Electrodeposition
• Corrosion Studies
• Electropolymerization
• Ion/Solvent Adsorption and Transport
• Binding Events
All of these processes result in mass changes to the surface being studied. The EQCM utilizes quartz
crystals that resonate at specific frequencies upon the application of an AC signal. These resonant
frequencies will change as mass is lost or added to the electrode on the face of the quartz crystal.
Applications of EQCM
13

14. EQCM Schematic
14

15. Piezoelectric effect: a mechanical deformation (on the order of 10-100nm) is produced
when an electric field is applied [or vice versa]
This oscillation creates an acoustic shear wave that penetrations into the media above
the crystal. This wave is sensitive to changes in the viscosity of the media or physical
adsorption of materials onto the crystal, which is typically shown by a decrease in
frequency
~
Eac
Mechanism of EQCM
15
Source: King. W. H., Anal. Chem. 36, 1735-39 (1964)
Q. Xie, Q. et. al., J. Chem. Ed. 84, 681-4 (2007)

16. 2
2 o
q q
f
f m
A
f C m
 
   
   
Sauerbrey Equation:
off
m
where is the change in frequency, is the fundamental frequency of
quartz crystal, , , are the area, density and shear modulus of the
quartz crystal, respectively, and is the change in mass on the crystal
surface
A q q
0 100 200 300 400 500
0.00
-1.25
-2.50
-3.75
-5.00
-6.25
-7.50
0
25
50
75
100
125
150
m/ng
f/Hz
Time / s
Mechanism of EQCM
16

17. Some common terminologies
17

18. Resonance frequency and resonance width
Resonance frequency, f, corresponds to the conductance peak, whereas the full-
width at half-maximum (W) characterizes the dissipation of oscillation energy.
18
Admittance= 1/ Impedance= X + iY

19. Factors affecting Resonance Freq. and Resonance Width
Liquids trapped in the narrow pores contribute only to the frequency shift,
whereas movement of liquid in wider pores results in shifts in both the frequency
and resonance width
19
• Pore Size

20. 20
It shows the case of metal deposition. The QCM act as
a true microbalance. The resonance frequency is
shifted to lower values with increasing mass, but the
shape of the spectrum remains altered.
The resonance frequency is shifted to lower values
with increasing viscosity, and the resonance width
changes dramatically.
Resonance frequency is shifted to lower values with
increasing electrode roughness, and the resonance
width changes dramatically.
Factors affecting Resonance Freq. and Resonance Width
• Mass Deposition
• Density and viscosity of electrolyte medium
• Roughness of the electrode surface
Source: Q. Xie, Q. et. al., J. Chem. Ed. 84, 681-4 (2007)

21. Penetration depth as hydrodynamic probe
Penetration depth is a characteristic depth of decaying of the transverse wave coming
from the oscillating quartz crystal surface towards liquid environment.
The equation shows that the penetration depth decreases with the frequency of the wave
(that is, with n) and the density of the liquid, ρ, but increases with its dynamic viscosity, η.
Ex-situ measurement: Changing η and ρ
In-situ measurement: Changing n
21

22. Experiment 1:
22
Purpose:
1) To compare the in situ and ex situ hydrodynamic
spectroscopy
2) To characterize the nature and roughness of the electrode
surface

23. Validation of hydrodynamic model for multiple harmonics
23
Plane Surface Rough Surface
Au covered quartz crystal Lithographically fabricated polymeric
photoresist
Two limiting cases

24. Validation of hydrodynamic model for multiple harmonics
Ideally flat gold covered quartz crystal immersed in water and water-glycerol
mixtures. 24

25. Equations for hydrodynamic modelling
25
q= fraction of crystal surface coverage by electrodes,
h= average thickness of the hydrodynamic porous boundary layer,
ξ= permeability length of hydrodynamic porous layer,
r= average radius of non-porous aggregates,
m = surface density

26. Validation of hydrodynamic model for multiple harmonics
Ideally flat gold covered quartz crystal immered in water and water-glycerol
mixtures. 26

27. Validation of hydrodynamic model for multiple harmonics
Artificially rough surface composed of lithographically fabricated polymeric
photoresist semi-spheres in contact with water.
27
Average radius of semi-spheres= 1.78 μm
Surface density occupation= 0.002 μm-2

28. Conclusion from Experiment 1
• Ex-situ technique can now be extended to in-situ technique and hence lot more
information on electrode changes can be obtained.
• Fitting experimental patterns through hydrodynamic equation, the extent of
roughness can easily be inferred.
• Output parameters such as average radius of bumps on electrodes and their
surface density can be obtained.
28

29. Experiment 2:
29
Purpose:
1) To know the effect of loading of electrode material of
different amount.

30. Fabrication of LiMn2O4 coatings on quartz crystal
Spray Pyrolysis Technique:
0.025 M CH3COOLi + 0.05 M Mn(NO3)2 in Ethanol  Spray Pyrolysed at 300 oC
30

31. Fabrication of LiMn2O4 coatings on quartz crystal
Spray Pyrolysis Technique:
0.025 M CH3COOLi + 0.05 M Mn(NO3)2 in Ethanol  Spray Pyrolysed at 300 oC
31
Loading 1
Δf=0.16 kHz
Loading 2
Δf=0.9 kHz
Loading 3
Δf=2.7 kHz
Loading 4
Δf=3.3 kHz

32. Behaviour of spray-pyrolysed LiMn2O4 electrodes
Electrochemical properties of spray-pyrolysed LiMn2O4 electrode coatings of different
loading masses
Slope of the straight line
gives the specific capacity of
144 mAhg-1 which matches
well with that of 148 mAhg-1
Integration of the differential capacity curve is proportional to the charge
Four electrode were coated with different mass of LiMn2O4
Δfn /n = −Cm Δm,
Sauerbrey’s equation
32

33. Complex porous electrode structures
Thin Dense Layer
Thicker porous layer
Non-porous asperities
• Increase in the thickness of layer 1 affects Δfn /n rather than ΔW/n
• Layer 2 having characteristic pore size d>> penetration depth contributes to both Δfn /n
and ΔW/n
• Layer 3 contributes significantly to the hydrodynamic spectroscopic curves at OCV,
appear electrochemically inactive, with no contribution to the potential-dependent
changes Δfn /n and ΔW/n. 33

34. Characterization of the electrodes in air
34
To make sure that there are no substantial
dissipation of oscillation energy inside these
electrodes, i.e. that they are rigid.
Ideally, in the case of rigid electrode, W /n for
all overtone orders should be exactly the same
as that on neat (uncovered crystal).
For, 3.3 kHz coating, W/n is not fixed as a
function of higher overtones. So, the coated
material is not rigid and thus neglected.

35. Conclusion from Experiment 2
• Amount of loading of dry mass can easily be obtained by using Sauerbrey’s
equation
• Total charge storage capability of this dry mass can be obtained from the
differential capacity curve
• Specific capacity of the material can be obtained quite accurately
• Nature of each of the layer can be understood from their electrochemical and
hydrodynamic interaction
• Deformation in the electrodes can be monitored and optimal loading for the
electrochemical process can be finalized
35

36. Experiment 3:
36
Purpose:
1) To know the effect on electrode surface at open circuit
voltage (OCV) and during charging/discharging process.

37. Equations for hydrodynamic modelling
37
q= fraction of crystal surface coverage by electrodes,
h= average thickness of the hydrodynamic porous boundary layer,
ξ= permeability length of hydrodynamic porous layer,
r= average radius of non-porous aggregates,
m = surface density

38. In situ hydrodynamic spectroscopy of LiMn2O4 electrodes
• A large deviation of Δfn /n and ΔW/n
from the straight lines of the ideally flat
surface is observed as a function of
higher electrode mass.
38
For 0.9 kHz coating,
q= 0.24
h= 100 nm
ξ= 53 nm
For 2.7 kHz coating,
q= 1
h= 240 nm
ξ= 80 nm
m= 0.0004 μm-2
r= 3.05 μm
For 0.16 kHz coating,
q= 0.24
h= 90 nm
ξ= 57 nm
At OCV

39. Raw EQCM data
39
For 0.9 kHz coating,
q= 0.24
h= 102.7 nm
ξ= 51.2 nm
For 2.7 kHz coating,
q= 1
h= 260 nm
ξ= 71 nm
m= 0.0004 μm-2
r= 3.05 μm
0.16 kHz coating is the
gravimetric case
During Charging/Discharging

40. Comparison of OCV data with charging/discharging
40
For 0.9 kHz coating,
q= 0.24
h= 102.7 nm
ξ= 51.2 nm
For 2.7 kHz coating,
q= 1
h= 260 nm
ξ= 71 nm
m= 0.0004 μm-2
r= 3.05 μm
For 0.9 kHz coating,
q= 0.24
h= 100 nm
ξ= 53 nm
For 2.7 kHz coating,
q= 1
h= 240 nm
ξ= 80 nm
m= 0.0004 μm-2
r= 3.05 μm
At OCV During Charging
h increases
Electrode layer swollen
Due to insertion of
solvent molecules
within the micropores
Li deintercalates
Pore Size
decreases
ξ decreases

41. Conclusion from Experiment 3
• Electrode deformation during charging/discharging can be monitored
using following output parameters
 Fraction of the total surface covered.
 Height of the porous layer
 Mean pore size diameter
 Radius of non-porous species
 Surface density of such non-porous species.
41

42. Conclusion
• Based on easy assessment of high overtone orders by EQCM, a new method have been
developed for in situ hydrodynamic spectroscopy of porous/rough battery electrodes
with penetration depth n as the hydrodynamic probe.
• It enables a much better and more comprehensive understanding of how ion
insertion/extraction affects the structure of the porous electrode in contact with
solutions on a mesoscopic scale of penetration depth.
• It has the potential to become a primary tool for selecting better electrode coating for
batteries and supercapacitors.
42

43. 43

44. 44

45. Series RLC resonance circuit
45
Inductive Reactance, XL= Lω
Capacitive Reactance, Xc= 1/ Cω
Resonance Frequency =
𝟏
𝑳𝑪
So, resonance freq. is completely
determined by ‘L’ and ‘C’
At Resonance Frequency,
XL=XC
Total Impedance, Z= 𝑹 𝟐 + 𝑿 𝑳 − 𝑿𝑪 𝟐
Z=R
So. Total impedance or energy loss is completely determined by change in
resistance

46. • The surface charge can affect adsorption of molecules (i.e. SiO2 layer carriers negative
charge inhibiting adsorption of anionic species)
• It can be difficult to model since we lack control outside of the sample preparation
(solutions cannot be to complicated)
Problems with QCM:
Solution – Couple with electrochemistry:
Mechanism of EQCM
• Thus, by adding electrochemical techniques, we can alter the
charge on the surface to adsorb/desorb a wide range of
molecules and we can two independent data sets (i.e.
frequency shifts with EC data) to allow for modeling of more
complex reactions
46

47. Shift/Damping of Frequency
The top diagram illustrates how the frequency
of the oscillating sensor crystal (quartz)
changes when the mass is increased by
addition of a molecular layer.
The bottom diagram illustrates the difference
in dissipation signal generated by a rigid (red)
and soft (green) molecular layer on the sensor
crystal.
47

48. Advantage of EQCM-D measurements
M.D. Levi et al. / Electrochemistry
Communications 67 (2016) 16–21
• It enables probing a wide range of penetration depths, δn
Two methods:
• In-situ gravimetric sensing:
a) One necessary condition to fulfill this requirement is that the electrode
coating is rigidly attached to the quartz crystal surface and retains its rigidity both in air
and in contact with solutions under open-circuit potential (OCP) and during polarization.
This excludes the appearance of dissipation inside the solid matrix of the electrode.
b) The next necessary condition is the absence of potential dependence of the
electrode volume/porosity changes since such dependences modify the hydrodynamic
solid–liquid interactions resulting in changes of the dissipation factor within the
boundary hydrodynamic layer.
When these two conditions are fulfilled, the electrodes loading can be considered as
entirely inertial. Consequently, the mass changes of electrodes in EQCM measurements
can be related to change in frequency, Δf, in accordance with the Sauerbrey's equation:
Δm = −C∙Δf/n, where C is the mass sensitivity constant and n is the overtone order
48

49. When an external polarization is applied to an electrode coating on a crystal surface, the
resonance peak may behave in two principally different modes.
The polarization results in changes of the electrode mass due to insertion/extraction or
adsorption/desorption of ions in a way that resonance peak shifts without change of its
width; thus the change of the electrode mass occurs without change in the dissipation
factor.
Significant changes of the dissipation factor occur in addition to the change of the resonance
frequency, which generally indicate that in addition to mass change, the electrode layer is
subjected to deformation (expansion/contraction). This should modify the solid-liquid
hydrodynamic interactions, and or accompanied by changes of the elastic properties of
composite electrodes (e.g. electrodes containing ions inserting particles and polymeric
binder).
49

50. Advantage of EQCM-D measurements
M.D. Levi et al. / Electrochemistry
Communications 67 (2016) 16–21
• It enables probing a wide range of penetration depths, δn
Two methods:
• In-situ hydrodynamic sensing:
50

51. 51

52. Although traditional in situ height-sensing techniques (atomic force microscopy or
electrochemical dilatometry) are able to sense electrode thickness changes at a nanometre
scale, they are much less informative concerning intercalation-induced changes of the
porous electrode structure at a mesoscopic scale.
52