Lithium battery manufacturers analyze the introduction of lithium battery polarization

How does polarization come about? When the battery is discharged, all of this energy cannot be completely converted into electrical energy. Electrochemical reactions are always accompanied by energy loss. These energy losses include: 1) Activation polarization-it causes electrochemical reactions on the electrode surface 2) Concentration polarization --It is produced due to the difference in the concentration of reactants and products in the electrode surface and the bulk phase, and is the result of material transfer. The existence of polarization consumes part of the energy and releases it in the form of heat. Polarization (Polarization) batteries are polarized during charging and discharging. Generally, the polarization of lithium-ion batteries can be divided into ohmic polarization, electrochemical polarization and concentration polarization. The response speed of several types of polarization is different. There are many factors that affect the degree of polarization, but in general, the greater the charge and discharge current density, the greater the polarization. The polarization classification is explained below: (1) Ohmic polarization, as the name implies, is the polarization caused by the ohmic internal resistance of lithium-ion batteries, which is called ohmic polarization, which is also resistive polarization. The ohmic internal resistance (R) of a battery is composed of electrode material, electrolyte, diaphragm resistance, and contact resistance of various parts (some explanations also include membrane resistance). When a certain current is passed, its polarization potential can be calculated. Eu003dIR (Europe). Ohmic polarization occurs instantaneously. (2) Electrochemical polarization refers to the polarization caused by the fact that the electrochemical reaction speed on the positive and negative electrodes is slower than the speed of electron movement. Electrochemical polarization is generally considered to be microsecond level (3) Concentration polarization refers to polarization caused by the fact that the diffusion rate of lithium ions participating in the reaction in the solid phase is less than the electrochemical reaction rate. Concentration polarization is generally considered to be in seconds. In a broad sense, the composition of the internal resistance of a lithium battery is the same as the ohmic resistance (IR). Both activation polarization and concentration polarization can be understood as the components of the internal resistance of the battery, or as activation resistance and concentration resistance. The magnitude of activation polarization and concentration polarization needs to be calculated by the establishment of a complex mathematical model. The internal impedance is composed of the following parts: ☆ Ionic resistance ①The influence factors of the electrolyte inside the diaphragm, the electrolyte conductivity, diaphragm area, thickness, porosity, tortuosity coefficient (Gurley) ②The influence factors of the electrolyte inside the positive electrode, the electrolyte conductivity, positive electrode Thickness, thickness, porosity, tortuosity factor ③The influence factors of electrolyte inside the negative electrode, electrolyte conductivity, positive electrode thickness, thickness, porosity, tortuosity coefficient☆ Electronic resistance ①The active material of the two electrodes influence factors of electrode conductivity, thickness, Area ② Current collector (copper foil and aluminum foil) influencing factors, current collector thickness, width, length, number and position of the tabs ③ Leads (tabs, poles, internal conductive connecting elements) influencing factors, external dimensions, electrical conductivity ☆ active material and The contact resistance of the current collector, the electrochemical impedance of the positive electrode material and the aluminum foil negative material and the electrochemical impedance of the copper foil battery The electrode is an equivalent circuit diagram, which mainly consists of ohmic resistance Rb, electric double layer capacitance Cd, electrochemical reaction resistance Rct and diffusion resistance Rw. Generally speaking, during the lithium ion insertion and extraction cycles, the Rb value generally does not change much, while the changes in Cd and Rct are more obvious. The battery impedance is mainly reflected in the electrochemical reaction impedance, while the ohmic impedance is relatively small. It can be seen from the figure below. As the temperature drops, 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 contribution of the cathode impedance. 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 smaller than the impedance of the overall battery. As the charge and discharge cycle progresses, the impedance of the cathode increases, and the impedance of the battery also increases. . Therefore, special attention should be paid to the cathode resistance test during the electrode production process, and the pole piece resistivity is particularly important. The AC internal resistance and the DC internal resistance are measured by the AC method. In fact, companies often require the DC internal resistance (DCR) of the battery to be tested. The IEC standard also stipulates the test of the battery's DC impedance. IEC test method After the battery is fully charged, discharge at 0.2C for 10s, test voltage is U1, current I1. Then discharge at 1C for 1s, then voltage is U2, current I2. Then the current DC internal resistance is: then DC internal resistance and AC How much difference is there? The following is the test data. From the data point of view, the AC internal resistance of the battery is related to the DC internal resistance, which is basically in line with the linear relationship. According to the battery EIS test results, the AC internal resistance of the battery is similar to the ohmic resistance, but the DC internal resistance includes the ohmic internal resistance and the activation impedance. Therefore, in general, the measurement of the DC internal resistance is of great significance. Disclaimer: Some pictures and content of articles published on this site are from the Internet. If there is any infringement, please contact to delete. Previous post: Why does lithium battery have internal resistance and where does the internal resistance of the battery come from?