Battery Polarization Potential Analysis and Battery Internal Resistance Measurement Calculation

The ohmic resistance (R) of the battery consists of the electrode material, the electrolyte, the resistance of the diaphragm, and the contact resistance of the various parts (some explanations also include the film resistance). When a certain current is passed, the polarization potential can be calculated. E = IR (Europe).

1. resistance

The resistance represents the degree of hindrance of the current transfer by a circuit element. The unit is ohms.

So how did polarization come from?

All of this energy is not completely converted into electricity when the battery is discharged. The electrochemical reaction is always accompanied by a loss of energy. These energy losses include: 1) activation polarization - it causes an electrochemical reaction at the electrode surface 2) concentration polarization - It is due to the different concentrations of reactants and products in the electrode surface and body phase, which is the result of the material transfer. The presence of polarization consumes some of the energy and releases it in the form of heat.

2. Polarization

The battery is polarized during charge and discharge. Lithium-ion battery polarization can be generally divided into three categories: ohmic polarization, electrochemical polarization and concentration polarization. The response speeds of several types of polarization are also different. There are many factors that affect the degree of polarization, but in general, the greater the charge-discharge current density, the greater the polarization.

The following classification explains:

(1) Ohmic polarization As the name suggests, there is polarization caused by the ohmic resistance of a lithium ion battery, which is called ohmic polarization, and also becomes resistance polarization. The ohmic resistance (R) of the battery consists of the electrode material, the electrolyte, the resistance of the diaphragm, and the contact resistance of the various parts (some explanations also include the film resistance). When a certain current is passed, the polarization potential can be calculated. E = IR (Europe). Ohmic polarization occurs instantaneously.

(2) Electrochemical polarization

Refers to the polarization caused by the electrochemical reaction rate on the positive and negative electrodes being lower than the electron movement speed. Electrochemical polarization is generally considered microsecond

(3) Concentration polarization refers to the polarization caused by the diffusion rate of lithium ions participating in the reaction in the solid phase that is lower than the electrochemical reaction rate. Concentration polarization is generally considered second-order.

Composition of Lithium Battery internal resistance

In a broad sense, like ohmic resistance (IR), both active polarization and concentration polarization can be understood as the constituent factors of the internal resistance of the battery, or as the active impedance and the concentration impedance. The magnitude of activated polarization and concentration polarization requires the establishment of complex mathematical models to calculate.

The internal impedance consists of the following parts

☆ Ion resistance

1 electrolyte inside the diaphragm

Influencing factors: electrolyte conductivity, membrane area, thickness, porosity, zigzag coefficient (Gurley)

2 positive electrolyte inside

Influencing factors: electrolyte conductivity, cathode thickness, thickness, porosity, zigzag coefficient

3 negative electrode internal electrolyte

Influencing factors: electrolyte conductivity, cathode thickness, thickness, porosity, zigzag coefficient

☆ Electronic resistance

1 two electrode active materials

Influencing factors: electrode conductivity, thickness, area

2 current collector (copper foil and aluminum foil)

Influencing factors: collector thickness, width, length, number of tabs, position

3 leads (tabs, posts, internal conductive connecting elements)

Influencing factors: size, conductivity

Contact resistance between active material and current collector

Cathode material and aluminum foil, anode material and copper foil

Electrochemical resistance of the battery

The electrode is an equivalent circuit diagram, mainly composed of ohmic resistance Rb, double-layer capacitance Cd, electrochemical reaction resistance Rct, and diffusion resistance Rw. In general, the change of Rb value is generally small during the intercalation and deintercalation of lithium ions, but the change of Cd and Rct is more obvious.

The battery impedance is mainly reflected in the electrochemical reaction impedance, while the ohmic resistance is relatively small. It can be seen from the figure below. With the decrease of temperature, the battery impedance gradually rises, and after falling to 0°C, the impedance speed increases, and the cathode impedance shows a similar trend. From a numerical point of view, the battery impedance mainly comes from the cathode impedance contribution.

In fact, during the entire cycle of the battery, the impedance of the cathode occupies the main part of the impedance of the entire battery, which is only slightly lower than the impedance of the overall battery. With the charge-discharge cycle, the impedance of the cathode increases, and the impedance of the battery also increases. . Therefore, special attention should be paid to the test of cathode resistance in the process of electrode manufacturing, and the resistivity of the pole piece is particularly important.

AC internal resistance and DC resistance

The impedance of the battery is measured by the AC method. In fact, the DC resistance (DCR) of the test battery is often required in the enterprise. The IEC standard also specifies the test of the DC resistance of the battery.

IEC test method

After the battery is fully charged, it is discharged for 10 seconds at 0.2C, the test voltage is U1, and the current is I1. Then it is discharged for 1 second at 1C. At this time, the voltage is U2, and the current is I2. The current DC resistance is:

How much difference is there between DC resistance and AC?

The following are the test data. From the data point of view, the battery's AC resistance and DC resistance are related and basically accord with the linear relationship.

According to the EIS test results of the battery, the AC resistance of the battery is similar to the ohmic resistance, but the DC resistance includes the ohmic resistance and the activation resistance.