Electrochemical Characterisation Flashcards
In which direction do lithium ions move during discharging in a LIB?
Anode to cathode
What is the mechanism called when lithium ions are inserted into the graphite layers during charging?
Intercalation
What is insertion?
The Li ions in the cathode host material are found in unoccupied sites. For layered anode host materials such as graphite, lithium ions are inserted in between the layers, known as intercalation.
True or false: New layers are formed during insertion.
False
Does insertion or conversion have a larger capacity?
Conversion
Does insertion or conversion have a more significant volume and structural changes?
Conversion
What is conversion?
A mechanism comprised of reconstruction reactions where new phases are formed and grow, while other phases shrink and disappear. These can be formation or displacement reactions.
Why is conversion less popular?
It has a higher degree of volume and structural change.
Write the equation for the reaction that happens at a graphite anode in a LIB.
LiC6 <–> Li+ + C6 + e-
Write the equation for the reaction that happens at an LFP cathode in a LIB.
FePO4 + Li+ + e- <–> LiFePO4
During charging of a LIB, which species is being oxidised and which species is being reduced?
The cathodic species is being oxidised, losing an electron and a Li ion and the anodic species is being reduced, gaining an electron and a Li ion.
Write the equation for the reaction that happens at an LCO cathode in a LIB.
CoO2 + Li+ + e- <–> LiCoO2
What mode of operation/type of control is used for CV and EIS?
Potentiostatic control - potential is set as a function of time and the current is measured.
What is galvanostatic control?
The value of current is set and the potential of the working electrode is measured.
What does the capacity of a battery describe?
The maximum amount of charge that can be stored and then drawn on in a cycle.
Why is the calculated theoretical capacity often not achieved?
Because in cathode materials, the lithium atoms supports the structure therefore, if 100% of the Li was extracted/inserted, the structure would collapse.
How is the theoretical capacity increased?
1) Lower molecular weight
2) Reaction that releases more electrons
What does cyclic voltammetry allow you to determine?
1) the reversibility of a redox couple
2) the number of electrons transferred in an oxidation or reduction event
3) information about the rate constants, mechanisms and diffusion coefficients
What does CV measure and how does it work?
CV measures current.
A voltage ramp is applied by scanning at a constant speed until a defined voltage is reached. During this time, a potentiostat measure the current resulting from the applied voltage. The scanning speed is then reversed to complete the full cycle.
What is CV used for in electrolytes?
To determine the electrochemical stability window where no redox activity occurs.
Draw a CV graph for an electrolyte
Attach image
For a CV graph using IUPAC convention, is the reduction peak at the top or bottom?
Bottom
For reversible electrode kinetics, the peak position is constant and the peak height is a function of ______ ______.
Scan rate
True or false: Electron transfer limits the current rate at the electrode surface.
False - mass transfer by diffusion limits the current rate at the electrode surface due to the concentration polarisation.
What are the two maximum values obtained at the oxidation and reduction peaks of a CV graph?
Anodic peak potential and peak current for anodic oxidation.
and
Cathodic peak potential and peak current for cathodic reduction.
How is reversibility shown on a CV graph?
Better overlap of peaks on subsequent cycles.
What is impedance?
A complex and frequency (time) dependent resistance that measures the ability to resist the flow of current without limitations of Ohm’s law.
Does impedance use DC or AC theory?
AC
True or false: In AC circuits, resistors, capacitors and inductors impede the flow of electrons
True
How is electrochemical impedance spectroscopy carried out?
By applying low amplitude AC potential to a cell and measuring current.
What type of circuit is used to model reaction interfaces/fit EIS data? Why do we use this?
Resistor-Capacitor (RC) circuits are used to fit the semi-circle.
This is because reactions are not instant and have time dependence.
What aspect of the reaction interface represents a capacitor of an RC circuit? Where does this show up on a Nyquist plot?
The accumulation of charged species that sit at the interface “waiting” to complete the reaction (i.e. the double layer).
This shows up on the imaginary axis.
What aspect of the reaction interface represents a resistor of an RC circuit? Where does this show up on a Nyquist plot?
The resistance that the ions experience to reach their destination.
This shows up on the real axis.
What aspect of the reaction interface represents the characteristic frequency (omega)?
The average time it takes for the reaction to occur.
What are the 3 main resistances you can determine from a Nyquist plot?
Ohmic resistance (from electrolyte), charge-transfer (kinetic) resistance (from cell) and Warburg impedance shows slow diffusion/migration of ions in low frequency region.
What is the name for the standard equivalent circuit made up of a resistor and an RC circuit?
Randles
What circuit component does a solid state electrolyte act as?
Resistor
How is an equivalent circuit fitted for a Nyquist plot of a solid state electrolyte?
Using a constant phase element (CPE) instead of a capacitor to model the double layer capacitance. This accounts for non-ideal behaviour.
What are the main 2 observations on a Nyquist plot for a solid state electrolyte?
1) Semi-circle at high frequencies due to resistance towards ionic diffusion.
2) Low frequency tail confirming ionic conduction contribution to the impedance
How is ionic conductivity calculated from a Nyquist plot for a solid state electrolyte?
Using Pouillet’s law
How do you know from a Nyquist plot whether there is any convection?
Presence of linear spike and 45 degree angle is the Warburg resistance - indicating undisturbed diffusion.
Draw and label a quasi-reversible, reversible and irreversible cyclic voltammogram.
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Draw and label a Nyquist plot for a RC equivalent circuit and a resistor and RC equivalent circuit
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