How to understand the electrochemical impedance spectroscopy of lithium ion batteries

Lithium-ion batteries are secondary batteries that use intercalation compounds that can reversibly insert and extract lithium ions as the positive and negative electrodes: during charging, the lithium ions in the positive electrode are extracted from the positive electrode active material and embedded in the negative electrode active material.

During discharge, lithium ions are extracted from the negative active material and inserted into the positive active material. Therefore, the charge and discharge capacity, cycle stability, charge and discharge rate, and high and low temperature charge and discharge performance of lithium-ion batteries are all related to the extraction and insertion of lithium ions in the electrode active material, the diffusion in the electrolyte, and the passage of lithium ions. The process of solid electrolyte interface (SEI) film on the surface of the negative electrode active material particles is closely related.

Studying the relevant kinetic parameters of the above process is of great significance for in-depth understanding of the comprehensive electrochemical performance of the corresponding lithium-ion battery.

As an important method to study the electrochemical interface process, electrochemical impedance spectroscopy (EIS) is widely used to study the intercalation and extraction of lithium ions in carbon materials and transition metal oxidation.

The so-called EIS is an AC test method in which sine wave AC signals with a certain amplitude and different frequencies are applied to the electrochemical system to obtain corresponding electrical signal feedback in the frequency domain. The following will combine the specific physical and chemical processes during charging and discharging to analyze the EIS spectrum of a typical Li+ battery.

During the charging process of the lithium ion battery, the extraction and insertion process of lithium ions in the chimera electrode includes the following five steps, as shown in Figure 2 (Barsoukov et al. proposed a modification process):

Step ①: electrons are transferred to the surface of the active material and between active material particles, and lithium ions are transferred in the electrolyte;

Step ②: Lithium ions diffuse and migrate through the SEI film on the surface of the active material particles;

Step ③: The charge transfer process (or reaction combination process) at the conductive junction of electrons and lithium ions;

Step ④: The diffusion process of lithium ions in the solid particles inside the active material particles;

Step ⑤: Accumulation and consumption of lithium ions in the active material, and the resultant changes in the crystal structure of the active material particles or the formation of new phases (due to the test frequency limitation, this part of the common EIS spectrum is not tested).

Using an electrochemical workstation, perform AC impedance scanning between 10KHz and 50μHz to obtain the EIS spectrum as shown in Figure 3, which is specifically disassembled and analyzed,