What is EIS (Electrochemical Impedance Spectroscopy) in battery?

Impedance measurements can be used to monitor and control the degradation of the performance of a battery during cycling. Electrochemical Impedance Spectroscopy (EIS) measurements are usually performed at higher frequencies to determine the internal resistance of an element. The impedance related to electron transfer at the electrode interface through the solid electrolyte interphase layer (SEI) can also be very important in battery optimization. In the case of batteries involving intercalation of species, at lower frequencies, impedance measurements can also prove to be useful, as in certain conditions it is possible to extract the diffusion coefficient of the intercalated species. This quantity is also of great importance in the processes for the optimization of battery materials and also monitoring.