Saluki SEC1106 series electrochemical workstation can be used for analytical battery electrochemical reaction applications.

When the AC impedance method is applied to an electrochemical system, it is also called electrochemical impedance spectroscopy (EIS). This method refers to controlling the current (or potential) passing through the electrode to change according to the sinusoidal law with time under the condition of small amplitude, and simultaneously measuring the changing law of the electrode potential (or current) as its response with time, or directly measuring the AC impedance of the electrode (or admittance). This method has become one of the most important methods to study electrode process kinetics and electrode surface phenomena.

After the impedance spectrum data is measured, it is necessary to carry out reasonable data processing. Usually, the equivalent circuit method is used, that is, each unit step in the electrode process is represented by the components in the equivalent circuit model. The equivalent circuit and its component parameters can determine the kinetic mechanism of the electrode process and the kinetic parameters of each unit step.

The electrode equivalent circuit of the hybrid control system is shown in Figure 1, where R represents the solution resistance Ru, C represents the double-layer capacitance Cd, Rct represents the charge transfer resistance, and W represents the Warburg impedance.

Figure 1  Electrode equivalent circuit

Electrochemical impedance spectroscopy (EIS) is one of the most powerful tools to study the electrochemical processes occurring at the electrode/electrolyte interface and is widely used to study the intercalation and deintercalation of lithium ions in the electrode active materials of Li-ion batteries. EIS can characterize each step of the electrochemical intercalation reaction in a wide frequency range according to the different relaxation time constants of each step of the electrochemical intercalation reaction.

The extraction and intercalation process of lithium ions in electrodes includes the following steps:

① The transport of electrons between active substances and the transport of lithium ions in the electrolyte;

② The diffusion and migration of lithium ions through the SEI film;

③ The charge transfer process at the electrode interface;

④ The solid diffusion process of lithium ions inside the active material;

⑤ The insertion and extraction of lithium ions lead to the change of the crystal structure of the electrode material or the generation of new phases.

Figure 2  Impedance spectrum and equivalent circuit of lithium battery measurement

According to this process, the EIS spectrum consists of 5 parts (as shown in Figure 2):

① Ultra-high frequency region (above 10 kHz): The ohmic resistance related to the mobile transport of lithium ions and electrons appears as a point on the EIS spectrum, represented by Rs.

② High-frequency region: It is the diffusion process of lithium ions penetrating the SEI film, which can be represented by an RSEI/CSEI parallel circuit.

③ Intermediate frequency region: for electrochemical charge transfer process control, represented by Rct/Cd parallel circuit.

④ Low-frequency region: It is the solid diffusion process of lithium ions inside the active material, which is shown as a diagonal line on the figure.

⑤ Extremely low-frequency region (< 0.01 Hz): The electrode structure changes or the formation of new phases is related to the process, which is shown as a vertical line.

Figure 3  Typical electrochemical impedance spectroscopy

According to Figure 1, a simulated electrolytic cell was constructed with capacitors and resistors, where Ru=16Ω, Cd=4.7nF, and Rct=499Ω. SEC1106 uses EIS to measure the real part and imaginary part of the equivalent resistance of the electrolytic cell, and the upper computer is represented by a Nyquist diagram, as shown in Figure 4 below.

Figure 4  Electrochemical impedance spectroscopy of simulated electrolyte cell model

In this experiment, the electrochemical impedance spectrum was measured and analyzed for the simulated electrolytic cell composed of resistance and capacitance, and then the electrochemical impedance spectrum was measured and analyzed for the lithium-ion battery.