Insight into LIB Diffusion Phenomena Using Analytical Impedance Models

Abstract

Electrochemical impedance spectroscopy (EIS) is extensively used to characterize the transport properties in porous electrodes of lithium-ion batteries (LIB). In the low-frequency domain (f < 1 Hz), solid diffusion and electrolyte diffusion are competing over similar timescales, which explains why both phenomena are difficult to characterize independently. In which case can the solid diffusion coefficient be correctly extracted from experimental EIS? To answer this question, the linearized P2D-Newman model of a complete cell is solved using a Fourier transform to obtain a physics-based analytical impedance. The solution allows deriving the characteristic frequencies and non-dimensional numbers of the system, and gives the parameters range for which solid diffusion is overwhelming electrolyte transport in the impedance signal. A simple criterion is then given to discriminate electrodes that are properly designed for active materials electrochemical characterization.