Studying the effect of battery impedance vs SOC using EIS

Electro-Impedance Spectroscopy (EIS) is a method of obtaining performance characteristics of electro-chemical cells. For galvanostatic EIS, the cell is exposed to an excitation current at a given frequency with the resulting voltage signal being measured. For a fully resistive load, the voltage would be completely in-phase with the current. When the load has capacitance, the voltage will experience a phase shift in proportion to the capacitive impedance. When the frequency of the excitation is varied, electro-chemical cells exhibit different impedance characteristics which can be used to analyse performance and durability.

Whilst EIS is usually performed as a sweep of sinusoidal frequencies, it can be performed with multi-sine signals consisting of multiple frequencies superimposed onto the same excitation signal as well as PRBS (pseudo-random-binary-sequence)

Impedance vs. State of charge

The graphs below show EIS data taken at CamMotive’s battery testing facility for a Panasonic 18650 cell at 0% and 100%.? The increase in impedance at lower frequencies suggests that the ohmic resistance and solid electrolyte interphase layer remain unchanged, whilst the resistance to charge transfer increases. The reduction in intercalated lithium available at the anode could be one reason for seeing impedance rise below ~1Hz.