Electrochemical Quartz Crystal Microbalance

In situ hydrodynamic spectroscopy for structure characterization of porous energy

storage electrodes

Saurav Chandra Sarma

Under the guidance of

DR. SEBASTIAN C. PETERSOLID STATE AND INORGANIC CHEMISTRY LAB

NCU, JNCASR 1

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In situ hydrodynamic spectroscopy for structure characterization of porous energy storage electrodes

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

Introduction

Novelty of the Paper

EQCM technique

Some common terms

Factors affecting resonance frequency and resonance width

Experimental Section

Results and Discussions

Conclusion

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Sketch of my talkSketch of my Talk

Introduction

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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/

Electrode deterioration during cycling

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

Novelty of the paper

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

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.

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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)

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.

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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 inducean oscillation at the fundamental frequency (4-10 MHz) of the quartz crystal

EQCM - An analytical method

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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

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Quartz crystal cut and Vibration mode

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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

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EQCM Schematic

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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

15Source: King. W. H., Anal. Chem. 36, 1735-39 (1964)

Q. Xie, Q. et. al., J. Chem. Ed. 84, 681-4 (2007)

22 o

q q

ff mA

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 5000.00

-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

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Some common terminologies

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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.

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Admittance= 1/ Impedance= X + iY

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

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• Pore Size

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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)

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

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Experiment 1:

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Purpose:

1) To compare the in situ and ex situ hydrodynamic

spectroscopy

2) To characterize the nature and roughness of the electrode

surface

Validation of hydrodynamic model for multiple harmonics

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Plane Surface Rough Surface

Au covered quartz crystal Lithographically fabricated polymeric

photoresist

Two limiting cases

Validation of hydrodynamic model for multiple harmonics

Ideally flat gold covered quartz crystal immersed in water and water-glycerol

mixtures. 24

Equations for hydrodynamic modelling

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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

Validation of hydrodynamic model for multiple harmonics

Ideally flat gold covered quartz crystal immered in water and water-glycerol

mixtures. 26

Validation of hydrodynamic model for multiple harmonics

Artificially rough surface composed of lithographically fabricated polymeric

photoresist semi-spheres in contact with water.

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Average radius of semi-spheres= 1.78 μm

Surface density occupation= 0.002 μm-2

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.

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Experiment 2:

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Purpose:

1) To know the effect of loading of electrode material of

different amount.

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

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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

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Loading 1

Δf=0.16 kHz

Loading 2

Δf=0.9 kHz

Loading 3

Δf=2.7 kHz

Loading 4

Δf=3.3 kHz

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

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Complex porous electrode structures

Thin Dense LayerThicker 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

Characterization of the electrodes in air

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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.

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

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Experiment 3:

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Purpose:

1) To know the effect on electrode surface at open circuit

voltage (OCV) and during charging/discharging process.

Equations for hydrodynamic modelling

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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

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.

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For 0.9 kHz coating,q= 0.24h= 100 nmξ= 53 nm

For 2.7 kHz coating,q= 1h= 240 nmξ= 80 nmm= 0.0004 μm-2

r= 3.05 μm

For 0.16 kHz coating,q= 0.24h= 90 nmξ= 57 nm

At OCV

Raw EQCM data

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For 0.9 kHz coating,q= 0.24h= 102.7 nmξ= 51.2 nm

For 2.7 kHz coating,q= 1h= 260 nmξ= 71 nmm= 0.0004 μm-2

r= 3.05 μm

0.16 kHz coating is the gravimetric case

During Charging/Discharging

Comparison of OCV data with charging/discharging

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For 0.9 kHz coating,q= 0.24h= 102.7 nmξ= 51.2 nm

For 2.7 kHz coating,q= 1h= 260 nmξ= 71 nmm= 0.0004 μm-2

r= 3.05 μm

For 0.9 kHz coating,q= 0.24h= 100 nmξ= 53 nm

For 2.7 kHz coating,q= 1h= 240 nmξ= 80 nmm= 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

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.

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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.

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Series RLC resonance circuit

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Inductive Reactance, XL= Lω

Capacitive Reactance, Xc= 1/ Cω

Resonance Frequency =

So, resonance freq. is completely determined by ‘L’ and ‘C’

At Resonance Frequency,XL=XC

Total Impedance, ZZ=R

So. Total impedance or energy loss is completely determined by change in resistance

• 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

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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.

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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∙

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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).

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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:

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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.