Impedance IIIa - mmrc.caltech.edu Echem/lierature...What have we learned ? 41/42 •The reaction...
Transcript of Impedance IIIa - mmrc.caltech.edu Echem/lierature...What have we learned ? 41/42 •The reaction...
Impedance IIIa
Study of the insertion reaction by impedance
Application to the characterization of batteries
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Objective
Understand to which mechanisms correspond the impedance graphs obtained on batteries.
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Outline
1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Outline
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1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Electrolyte
1. Initial state (charged) : Ecell = EOC
M
M
+ -
M
Ecell = Cell voltage = potential difference between the positive electrode and the negative electrode.
M
5
The Li battery
Introduction
Current Collector
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e-
M M+ M M
6 Introduction
The Li battery
Cathode/Reduction (Insertion)
e-
M+ M
M
Anode/Oxidation (Disinsertion)
M
Electrolyte
+ -
2. Discharge: Ecell < EOC (charged) (Spontaneous reactions)
Ex: LiFePO4 Ex: LixC
Current Collector
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Cathode/Reduction (Insertion)
e-
M+ M
M
Anode/Oxidation (Disinsertion)
M
7 Introduction
The Li battery
Electrolyte
+ -
3. Charge : Ecell > EOC (discharged) (Forced reactions)
Ex: LiFePO4 Ex: LixC
Current Collector
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Outline
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1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Electrolyte
Host material MnO2
Nb2O5
W2O3
LiCoO2
…
Substrate (current collector) Pt Graphite…
e- e-
M+ <M>
0 L x
<M>
No inserted species can flow in the substrate. At the host/substrate interface, J<M>(L,t) = 0
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Mechanism
The insertion reaction with restricted diffusion
Let us consider one electrode of the battery.
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f = F/(RT) Symmetry factors : i + d = 1
i = if(t) + ic(t) = -Fv(t) + CdcdE/dt
10 The insertion reaction with restricted diffusion
Mechanism
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Electrolyte Host material Substrate
0 L x
X
M+*
<M> (0,t) <M> (L,t)
11 The insertion reaction with restricted diffusion
Mechanism
Non-steady state regime
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In the steady-state regime, the applied potential E is an equilibrium potential if = -Fv = 0
Electrolyte Host material Substrate
0 L x
X
M+*
<M>
12 The insertion reaction with restricted diffusion
Mechanism
Steady-state regime
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Outline
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1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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The Taylor development of the Faradaic current followed by its Laplace transformation leads to an expression of the Faradaic impedance :
Rct is the charge transfer resistance = lim Zf()
0 Z<M> is the impedance related to the concentration of the inserted species <M>.
14 The insertion reaction with restricted diffusion
Faradaic Impedance
The Faradaic current is expressed as : if(t) = -Fv(t)
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Z<M>(p) can be written :
This impedance is equivalent to an electrical circuit. The impedance of such a circuit can be displayed in a Nyquist diagram. In this case, p = jω = j2πf with f the frequency (Hz). d = L2/D
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Faradaic Impedance
The insertion reaction with restricted diffusion /42
Outline
The insertion reaction with restricted diffusion 16
1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Rct
Z<M>
Faradaic impedance Faradaic impedance
W
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Equivalent Circuit
The insertion reaction with restricted diffusion /42
Rct
Z<M>
Cdl
Faradaic impedance
Double layer capacitance
W
Electrode impedance
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Equivalent Circuit
The insertion reaction with restricted diffusion /42
RΩR
Rct
Z<M>
Cdl
Faradaic impedance
Electrolyte resistance
Total measured impedance
Faradaic impedance
Electrode impedance
Double layer capacitance
W
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Equivalent Circuit
The insertion reaction with restricted diffusion /42
Simulated impedance graph using ZSim
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Inductance component L (due to cell connections, wires…)
Outline
The insertion reaction with restricted diffusion
Q
M
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Outline
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1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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H2 insertion
From J.S. Chen, J. –P. Diard, R. Durand, C. Montella, Electrochemistry and Materials Science of Hydrogen Absorption and Adsorption, Electrochemistry Society Meeting, B.E. Conway, G. Jerkiewicz (ed.), San Francisco, (1994) p. 207
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Experimental Data
The insertion reaction with restricted diffusion /42
From P. Millet, R. Ngameni, Electrochim. Acta 56 (2011) p. 7907
Obtained on a Pd electrode with PH2 = 12 mbar
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Experimental Data
The insertion reaction with restricted diffusion
H2 insertion
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LiFePO4 battery : the use of ZFit
RΩ
Rct
W
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Experimental Data
The insertion reaction with restricted diffusion
Inductance
Q
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LiFePO4 battery : the use of ZFit
RΩ
Rct1 Rct2
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Experimental Data
The insertion reaction with restricted diffusion
Inductance
Qdl1 Qdl2
Which EC should be chosen ?
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Outline
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1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
5. The Solid Electrolyte Interphase
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Other more complicated insertion reactions are possible
1. Insertion with preliminary electrosorption
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Other mechanisms
The insertion reaction with restricted diffusion /42
Faradaic impedance Zf() = Rct + Z().
Z() is the impedance related to the electroadsorbed species.
Electrode Impedance
RctR Rad
Cad
Cdl
Faradaic impedance
Z W
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Electrosorption + Insertion
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Adsorption :
Insertion :
2. Insertion with preliminary adsorption
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Other mechanisms
The insertion reaction with restricted diffusion /42
Faradaic impedance = Zf() = Rct + Zad() + Z<M>()
Rad
Cdl
RctR
Cad
Zad
Z<M>
Faradaic impedance
Electrode Impedance
W
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Adsorption + Insertion
The insertion reaction with restricted diffusion /42
Simulated Nyquist Plot
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Adsorption + Insertion
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Experimental Data From M.D. Levi et al, J. Electroanal. Chem, 569, 2004
Mg-ion insertion into the Chevrel electrode (MxMo6S8, 0 < x < 2 for Mg and 0 < x < 4 for Li)
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Adsorption + Insertion
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Outline
1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Electrolyte Host material
0 L
X
M+*
<M>
The case for which the length of the host material is very large is a limit case of the direct insertion with restricted diffusion where either L is very large or D<M> is very small.
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Mechanism
The insertion reaction with semi-infinite diffusion /42
Zf() = Rct + Zw(j)
The impedance component of a diffusion in a semi-infinite media is called a Warburg component. It is symbolized by the letter W.
Zw() = (1-j)/(j)1/2
Rct
Zw
Cdl
Faradaic impedance Faradaic impedance
Electrode impedance
Double layer capacitance
W
Such a circuit is called a Randles circuit
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Equivalent Circuit
The insertion reaction with semi-infinite diffusion /42
The vertical component on the Nyquist diagram is shifted to the much lower Frequencies, the time constant τd1 (= L2/D<M>) is much larger.
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Rd1 = 0,2418 Ω instead of 0,02418 Ω, τd1 = 100,17 s instead of 67,17 s
Nyquist Plot
The insertion reaction with semi-infinite diffusion
W
W
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-45°
Randles circuit + electrolyte resistance
Simulated impedance graph using ZSim
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Nyquist Plot
The insertion reaction with semi-infinite diffusion /42
Outline
1. Introduction : the Li battery
2. The insertion reaction with restricted diffusion 1. Mechanism 2. Expression of the Faradaic impedance 3. Equivalent circuit 4. Experimental data 5. Other mechanisms
3. The insertion reaction with semi-infinite diffusion
4. The insertion reaction with bounded diffusion
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Zf (j) = Rct + Zw (j)
The impedance component of a bounded diffusion is a W component.
Zw(j) = Rd th(dj)1/2/(dj)1/2
Rct
Zw
Cdl
Faradaic impedance Faradaic impedance
Electrode impedance
Double layer capacitance
W
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Equivalent Circuit
The insertion reaction with bounded diffusion /42
Simulated impedance graph using ZSim
¼ lemniscate
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Nyquist Plot
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What have we learned ?
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• The reaction that takes place in a battery undergoing a charge or discharge is an insertion.
• The insertion involves a diffusion of the inserted species in three
different conditions : restricted, semi-infinite, bounded.
• The insertion mechanism can be direct or involve a preliminary electrosorption or adsorption.
• For all these conditions, we now know : o The expression of the Faradaic impedance o The equivalent circuit o The Nyquist diagrams of the impedance
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Now what ?
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• Having this knowledge, we now how to interpret impedance data obtained on batteries.
• The next tutorial Impedance IIIb will show what useful
characteristics of the batteries can be obtained from the impedance data.
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References
Bio-Logic Website : www.bio-logic.info http://www.bio-logic.info/potentiostat/notesifil.html Faradaic Impedances Mathematica Player files : Direct Insertion http://www.bio-logic.info/potentiostat/notes/20080131%20-%20Insertion1-ZmmaP.nbp Electrosorption + Insertion http://www.bio-logic.info/potentiostat/notes/20080307%20-%20IndirectInsertion1-mmaP.nbp Adsorption + Insertion http://www.bio-logic.info/potentiostat/notes/20080312%20-%20IndirectInsertion2-mmaP.nbp Equivalent Circuits http://www.bio-logic.info/potentiostat/iecl/20101004%20-%20RandlesCircuit-mmaP.nbp