Electrochemical impedance spectroscopy

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Charge transfer

The step in which an electron crosses the interface between electrode and electrolyte as an ion is inserted or removed. Its resistance appears as a semicircle in impedance spectra and grows as films thicken and the cell cools.

Impedance

The frequency-dependent generalisation of resistance, with a magnitude and a phase. A cell’s impedance separates ohmic, charge-transfer and diffusion contributions that a single DC resistance lumps together.

Nyquist plot

The usual display of an impedance spectrum: negative imaginary part against real part, one point per frequency. Reading left to right, the ohmic intercept, one or two semicircles for interfaces and a diffusion tail each tell a different story about the cell.

Randles circuit

The classic equivalent circuit for one electrode interface: electrolyte resistance in series with a double-layer capacitance that shunts the charge-transfer resistance and a Warburg diffusion element. It is the template for reading an impedance spectrum.

Warburg impedance

The impedance of diffusion, which appears as a 45° line at the low-frequency end of a Nyquist plot. Its slope and onset frequency give a route to the diffusion coefficient.

References & further reading

Further reading on this topic

These sources provide broader context; they are not presented as citations for every statement in this entry.

1. Battery Test Manual For Electric Vehicles, Revision 3

   Jon P. Christopherson · Idaho National Laboratory · 2015

   Topic background · Performance characterization, pulse resistance/power, life testing and reference performance tests; historical 2015 edition.