No Battery Disassembly Needed! EIS Unlocks the Internal “Resistance Code” of Lithium Batteries
Electrochemical Impedance Spectroscopy (EIS) is a testing method that obtains kinetic information regarding the electrochemical processes inside a battery by applying a small-amplitude sinusoidal voltage signal to the battery system and measuring the change in the ratio between the system’s response current and the excitation voltage. Its core advantage lies in its non-destructive testing property: the small-amplitude signal does not alter the internal thermodynamic equilibrium state of the battery, allowing it to truly reflect the intrinsic electrochemical behavior of the battery. Meanwhile, EIS enables wide-frequency-domain scanning ranging from a few microhertz to several megahertz, covering electrochemical processes of different time scales within the battery and making it possible to comprehensively analyze the battery’s reaction mechanism.
(I) Four Components of the Impedance Spectrum
A typical lithium-ion battery impedance spectrum presents a specific shape on the complex plane, with the real part of impedance (ZRe) as the abscissa and the imaginary part of impedance (ZIm) as the ordinate. It mainly consists of four parts, corresponding to distinct electrochemical processes:
Ultra-high Frequency Region (Several MHz)
The intersection of the impedance curve with the horizontal axis corresponds to ohmic resistance ($$R_b$$), which mainly stems from electrolyte resistance, bulk resistance of electrode materials, and contact resistance between current collectors and electrodes. This part of the impedance has the fastest response speed, is barely affected by factors such as State of Charge (SOC) and temperature, and represents the most fundamental resistive component inside the battery.
High Frequency Region
It appears as a semicircle, corresponding to the impedance of lithium ions passing through the Solid Electrolyte Interphase (SEI) film ($$R_{sei}$$). The SEI film is a passivation layer formed on the electrode surface during the initial charge and discharge cycles of the battery. Its thickness and structural integrity directly affect lithium-ion transport efficiency, and the diameter of the high-frequency semicircle directly reflects the transport resistance of the SEI film.
Middle Frequency Region
Another semicircle corresponds to charge-transfer resistance ($$R_{ct}$$), also known as electrode polarization resistance, which reflects the difficulty of electrochemical reactions occurring at the electrode/electrolyte interface. This process involves key steps such as intercalation and deintercalation of lithium ions and electron transfer, making it the core impedance component that determines the battery’s rate capability and charge-discharge efficiency. The larger the diameter of the middle-frequency semicircle, the greater the resistance in the charge-transfer process.
Low Frequency Region
It presents as a straight line forming a 45-degree angle with the real axis, corresponding to the diffusion impedance of lithium ions inside the electrode active material (W), also called concentration polarization impedance. This part of the impedance reflects the diffusion rate of lithium ions in the solid phase, which is closely related to the microstructure and particle size of electrode materials. The slope of the low-frequency straight line can be used to evaluate the value of the diffusion coefficient.
(II) Construction of Equivalent Circuit Models
To quantitatively analyze each component parameter in the impedance spectrum, the battery system needs to be simplified into an equivalent circuit model consisting of resistors, capacitors and other components. The commonly used equivalent circuit model for lithium-ion batteries corresponds one-to-one with each frequency component of the impedance spectrum:
– Ohmic resistance directly corresponds to the series resistance in the circuit;
– SEI film resistance is connected in series with SEI film capacitance to simulate the semicircular response in the high-frequency region;
– Charge-transfer resistance is connected in parallel with double-layer capacitance , corresponding to the semicircular feature in the middle-frequency region;
– Warburg impedance (W) is used to characterize the lithium-ion diffusion process in the low-frequency region.