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Electrochemical impedance spectroscopy: Applications to LixCoO2 electrodes

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Ohm's law defines resistance in terms of the ratio between voltage E and current ... Steep sloping line at lowest frequencies accumulation of intercalant (Li) into ... – PowerPoint PPT presentation

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Title: Electrochemical impedance spectroscopy: Applications to LixCoO2 electrodes


1
Electrochemical impedance spectroscopy
Applications to LixCoO2 electrodes
  • Literature
  • Sundeep Kumar
  • February 9, 2005

2
Impedance definition Concept of complex impedance
  • Concept of electrical resistance It is the
    ability of a circuit element to resist the flow
    of electrical current. Ohm's law defines
    resistance in terms of the ratio between voltage
    E and current I.
  • R E / I
  • it's use is limited to only one circuit element
    -- the ideal resistor.
  • An ideal resistor
  • It follows Ohm's Law at all current and voltage
    levels.
  • It's resistance value is independent of
    frequency.
  • AC current and voltage signals though a resistor
    are in phase with each other.
  • Like resistance, impedance is a measure of the
    ability of a circuit to resist the flow of
    electrical current. Unlike resistance, impedance
    is not limited by the simplifying properties
    listed above.
  • Electrochemical impedance is usually measured by
    applying an AC potential to an electrochemical
    cell and measuring the current through the cell.
    Assume that we apply a sinusoidal potential
    excitation. The response to this potential is an
    AC current signal.

3
Measurement of impedance
  • The excitation signal, expressed as a function of
    time, has the form
  • Et E0 sin(? t)
  • Et is the potential at time t, E0 is the
    amplitude of the signal, and ? is the radial
    frequency.
  • The response signal, It, is shifted in phase (f)
    and has a different amplitude, I0.
  • It I0 sin (? t f)
  • An expression analogous to Ohm's Law allows us to
    calculate the impedance of the system as
  • The impedance is therefore expressed in terms of
    a magnitude, Zo, and a phase shift, f.

4
Data Presentation
  • The expression for Z(?) is composed of a real and
    an imaginary part. If the real part is plotted on
    the Z axis and the imaginary part on the Y axis
    of a chart, we get a "Nyquist plot".
  • The y-axis is negative and that each point on the
    Nyquist plot is the impedance at one frequency.
  • On the Nyquist plot the impedance can be
    represented as an vector (arrow) of length Z.
    The angle between this vector and the x-axis is
    f.
  • The semicircle is characteristic of a single time
    constant corresponding to a physical process in
    the system
  • Impedance spectroscopy is used to extract the
    information on these physical processes

5
Electrical Circuit Elements
  • The impedance of a resistor is independent of
    frequency and has no imaginary component. Current
    stays in phase with the voltage across the
    resistor.
  • A capacitor's impedance decreases as the
    frequency is raised. Capacitors also have only an
    imaginary impedance component.

Z R
Z 1/j?C
6
Physical Electrochemistry and Equivalent Circuit
Elements
  • Electrolyte resistance and resistance from
    current collectors
  • Double layer capacitance
  • A electrical double layer exists on the interface
    between an electrode and its surrounding
    electrolyte. This double layer is formed as ions
    from the solution "stick on" the electrode
    surface. Charges in the electrode are separated
    from ions charges.
  • Charge transfer resistance
  • Charge transfer resistance corresponds to
    interfacial charge transfer of Li ion (related
    to ionic motion) and electronic conductivity of
    the electrode

7
Physical Electrochemistry and Equivalent Circuit
Elements
  • Diffusion
  • Diffusion also can create an impedance called the
    Warburg-impedance. At high frequencies the
    Warburg impedance is small since diffusing
    reactants don't have to move very far. At low
    frequencies the reactants have to diffuse
    farther, increasing the Warburg-impedance.
  • Constant Phase Element
  • Capacitors in EIS experiments often do not behave
    ideally. Instead they act like a constant phase
    element as defined below.
  • The impedance of a capacitor can be expressed
    as
  • where, A 1/C The inverse of the capacitance a
    An exponent which equals 1 for a capacitor
  • For a constant phase element, the exponent a is
    less than one. The "double layer capacitor" on
    real cells often behaves like a CPE, not a
    capacitor.

8
Example Simulation of impedance data from known
equivalent circuit
  • The parameters in this plot were calculated
    assuming a 1 cm2 electrode undergoing uniform
    corrosion at a rate of 1 mm/year.
  • RP 250 ?, Cdl 40 µF/cm2 and Rs20 ? were
    assumed to simulate the impedance plot
  • One can simulate the impedance data if one knows
    the equivalent circuit before hand
  • OR
  • One can fit the experimental impedance data to an
    equivalent circuit

9
EIS on LiCoO2 electrodes
  • Goodenough and co-workers _at_ Oxford University,
    England
  • Aurbach and co-workers _at_Bar-Ilan university,
    Isreal
  • Scrosati and co-workers _at_ Universita di Roma,
    Italy
  • M.G.S.R. Thomas, P.G. Bruce and J.B. Goodenough,
    J. Electrochem. Soc., 132 (1985) 1521
  • D. Aurbach et al., J. Electrochem. Soc., 145
    (1998) 3024
  • M.D. Levi et al., J. Electrochem. Soc., 146
    (1999) 1279
  • F. Nobil et al., J. Phys. Chem. B., 106 (2002)
    3909

10
Goodenough and co-workers
  • AC Impedance Analysis of Polycrystalline
    Insertion Electrodes Application to Li1-xCoO2
    M.G.S.R. Thomas, P.G. Bruce and J.B. Goodenough,
    J. Electrochem. Soc. 132 (1985) 1521 - 1528
  • In this paper, an equivalent circuit model is
    presented for interpreting the A.C. impedance of
    a pressed-powder insertion-compound electrode
    (Li1-xCoO2) in contact with a liquid electrolyte

11
Goodenough and co-workers
  • LiLiBF4 (in PC)LiCoO2
  • Galavanostatically charged up to Li0.65CoO2
  • Impedance measurements at 10mV-rms AC
    perturbation sweeping the frequency range 10kHz
    to 0.1 mHz.
  • Assumption The electronic conductivity of the
    insertion compound is high and that each particle
    is in contact with the aggregate across a
    solid-solid interface making an ohmic contact of
    low resistance to electron flow

12
Goodenough and co-workers
Faradic process at electrode
Solution resistance
Non-Faradic process
At least six circuit components 3 resistors,
two capacitors and a Warburg component are
required to produce the basic form of the response
13
Goodenough and co-workers
  • Two processes
  • Adsorption of Li ions or PC onto the surface of
    the electrode without charge transfer
  • And formation of an ionically conducting but
    electronically insulating surface layer at the
    electrode surface

14
Goodenough and co-workers
15
Goodenough and co-workers
  • Three separate types of experiments
  • The time dependence of the AC impedance response
  • The influence of premixing of the electrolyte
    with the cathode material
  • The variation of circuit parameters with applied
    voltage

16
Goodenough and co-workers
  • Initially a CdlgtCads is found, this is physically
    unreasonable situation
  • And it is most improbable that adsorption should
    cause Cdl to decrease with time, rather
    increasing electrolyte penetration with time
    should increase Cdl
  • Both the adsorption based model were discarded on
    similar basis

17
Goodenough and co-workers
  • According to SL model
  • Rsl ?(L/A)
  • CSL ?(A/L)
  • Hence RSL should increase and CSL should decrease
    with time
  • Therefore, a SL model contains equivalent circuit
    parameters that vary in self-consistent manner
    with the electrochemical processes they represent
    as the cell conditions are varied.

18
Goodenough and co-workers conclusions
  • Evidences of surface layer formation on
    electrode surface

19
Aurbach and co-workers
  • Solid-State Electrochemical Kinetics of Li-Ion
    Intercalation into Li1-xCoO2 Simultaneous
    application of Electroanalytical Techniques SSCV,
    PITT and EIS M.D. Levi et al., J. Electrochem.
    Soc., 146 (1999) 1279.
  • Li1M LiAsF6 in ECDMC(13) LiCoO2 (with carbon
    and binder)
  • The analysis of impedance spectra in terms of
    equivalent circuit
  • Impedance measurements were taken during charging

20
Aurbach and co-workers
  • Low solution resistance (25 ohms compared to
    60 ohms observed by Goodenough et al.
  • Semicircles are more resolved than reported by
    Goodenough and coworkers
  • Medium frequency semicircle becomes smaller on
    increasing the voltage

21
Aurbach and co-workers
  • High-frequency semicircle surface layer related
  • Medium-frequency semicircle Charge transfer
    resistance related to slow Li ion interfacial
    transfer, coupled with a capacitance at the
    surface film/Li1-xCoO2 particle interface
  • At low frequency, a narrow Warburg region
    solid-state diffusion of Li ions into the bulk
    cathode material
  • Steep sloping line at lowest frequencies
    accumulation of intercalant (Li) into the bulk

22
Aurbach and co-workers
  • Simulated and experimental impedance data

At E 4.07V Li0.50CoO2
23
Aurbach and co-workers
  • There is some correlation between decrease in Rct
    and increase in the LixCoO2 electrical
    conductivity with potential in this range.
  • However, Rct for such an electrode does not
    reflect only the electronic conductivity of the
    particles, but also interfacial charge transfer
    that relates to ionic transport.

24
Aurbach and co-workers conclusions
25
Scrosati and co-workers
  • An AC Impedance Spectroscopic Study of LixCoO2
    at Different Temperatures F. Nobili et al., J.
    Phys. Chem. B., 106 (2002) 3909.
  • The paper presents an EIS of LiCoO2 electrodes at
    various temperatures (0-30 C) (Temperature
    dependence)
  • Li1M LiClO4 (ECDMC- 11)LiCoO2 (Composite
    cathode with bonder and carbon)
  • 10mV perturbation and frequency sweep of 100kHz
    to 1mHz

26
Scrosati and co-workers
  • At any potential, a not well defined semicircle
    is present at high frequency limit
  • As potential increases, another semicircle
    develops at medium frequency
  • And at lowest frequency limit, Warburg branch
    appears

27
Scrosati and co-workers
  • The ill-defined semicircle present at 24C in
    both graphs splits progressively into two
    distinct semicircles that become fully developed
    at the lowest temperature

28
Scrosati and co-workers
  • A high frequency dispersion (gt1kHz) because of
    presence of passivating layer
  • An intermediate frequency dispersion (between
    10Hz and 1kHz) because of charge transfer
  • A low-frequency semicircle associated with the
    electronic properties of the material
  • Very low frequency spike of the ionic diffusion
  • The drop of the resistance associated with the
    low frequency semicircle occurs over the narrow
    x-range that corresponds to the insulator to
    metal transition
  • The growth of the additional semicircle in the
    middle frequency range becomes noticeable in
    correspondence of potential values at which the
    intercalation process takes place at an
    appreciable rate.

29
Scrosati and co-workers
CPE
  • Both the circuits give same impedance plots and
    they both are equivalent
  • However, the (b) is more close to the physics of
    the processes.

30
Scrosati and co-workers
  • Insulator to metal transition can be seen from
    activation
  • barriers

31
Scrosati and co-workers Conclusions
32
Equivalent circuits Conclusions
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