The Test Technology and Related Application of Polar Film Resistance in Lithium-ion Battery

During the charging and discharging process of a lithium ion battery, lithium ions and electrons are transported inside the battery pole piece, wherein lithium ions are transported through the electrolyte filled in the pores of the electrode, and the electrons are mainly transmitted through a three-dimensional network composed of solid particles, particularly a conductive agent. The active material particle/electrolyte interface participates in the electrode reaction. The conduction characteristics of electrons have a great influence on the performance of the battery, which mainly affects the rate performance of the battery. In the battery pole piece, the main factors affecting the conductivity include the bonding interface between the foil substrate and the coating, the distribution state of the conductive agent, and the contact state between the particles. By measuring the resistance of the pole piece, the uniformity of the microstructure in the pole piece, the formulation characteristics of the pole piece, the material properties, and the performance of the battery can be predicted. The article "Lithium battery pole piece conductivity test method and its influencing factors" briefly summarizes the conductivity test method of the battery pole piece, and enumerates some influencing factors of the pole piece conductivity.

In the process of lithium-ion battery process development and quality monitoring, the electrode resistance measurement technology also plays an important role. Lithium-ion battery raw materials are the basis. The quality of the material directly determines the performance of the battery. The conductivity of the raw material plays a decisive role in the internal resistance and impedance of the final battery. At the same time, the process parameters of the electrode and core process will also have an important effect on battery performance. Therefore, Polar film resistance becomes the bond between material, process, and performance.

In the development and evaluation of active material powder materials, the development and optimization of electrode formulas, the monitoring of production processes, and the analysis of failure, Polar film resistance testing can play an important role, such as:

1. Comprehensive assessment of the stability of the slurry in the process of stirring to coating, and identification of abnormal reunification of conductive agents;

2. Evaluation of the stability of the electrode(the stability of the resistance of the electrode) in the production process;

3. Uniform anomaly identification for mixed electrode such as Silicon negative electrode;

4. Electronic conductivity assessment for different main materials and formulations;

5. Electronic conductivity evaluation for different conductive agents and formulations;

6. Electron conductivity assessment of different crosslinking agents and formulations;

7. Electron conductivity assessment of fluid-gathering functional bottom coatings;

8. Polar failure analysis for electronic conductive networks;

9. Contact resistance analysis for the interface layer of positive and negative electrode materials.

Here are some practical examples of the application of the pole piece film resistance test.

Application Case 1: Preparation and Optimization of Polar Tablets(Determination of Electroconductivity Abnormality)

The dispersion of conductive agents in the development of the electrode process is affected by many complex process control parameters such as raw material formulas, mixing conditions, coating conditions, and drying conditions. The dispersion of conductive agents will greatly deteriorate the kinetic performance of the core. It is difficult to find through monitoring methods such as polar adhesion, Chromaticity, and appearance and is often overlooked, resulting in irreparable economic losses. The polar film resistance test can assess the distribution status of the conductive agent. As shown in Figure 1, the ideal conductive agent distribution should be the aggregate sufficiently evenly dispersed and coated on the surface of the active substance particles to ensure that electrons can be transferred to the electrode. / Electrolytic interface everywhere, Participate in the electrode reaction. After testing the polar film resistance data in the polar film production, after accumulating a certain amount of data through the normal process, the control range of the membrane resistance can be determined. From Figure 1, it can be seen that the electrode resistance increases significantly when the conductive agent is reunited. When there is an abnormal dispersion of conductive agent in a batch, it can be easily identified by the film resistance and remove the bad product.

Application Case 2: Assessment of Development of Coating Foils Process

It is a breakthrough technological innovation to use functional coating for surface treatment of battery conductive substrate. Coating carbon aluminum foil is to evenly and delicately coat aluminum foil with scattered nano conductive graphite and carbon coated particles. It can provide excellent static conductivity and collect microcurrents of active substances, which can greatly reduce the contact resistance between the material and the aggregate and can increase the adhesion between the two. The use of binders can be reduced and the stability of the interface can be significantly improved. The stability of the long cycle of lithium-ion battery is enhanced, and the overall performance of the battery is greatly improved. The collector fluid and the corresponding polar resistance of the coating can be effectively tested and the resistors of each part can be distinguished to provide a strong guarantee for the development of the technology. The electrode resistance test can effectively measure and accurately distinguish the differences caused by the micro-coating design. As shown in Figure 2, when the liquid bottom coating is applied, the polar film resistance corresponding to different coating processes is different. These data can effectively assess the formulation and process of the foil coating and evaluate the performance of the functional coating.

Application Case 3: Assessing the Reliability of Polar Storage

For nickel-based materials, spontaneous reactions occur on the surface of the particles, Ni3 + is converted to Ni2 +, and O2-is released. When materials with high nickel content(NMC 622, NMC811, NCA, etc.) are exposed to air, It is easier to absorb carbon dioxide and water in the air, and the reaction forms the Li2CO3 and LiOH layers on the surface of the particles. The high proportion of Ni in the material and the higher PH value are, while Li2CO3 and LiOH consume Li in the material and do not have electrochemical activity. Therefore, the capacity will be attenuated, and the dense surface of Li2CO 3 will hinder Li's diffusion and affect battery performance. LiOH also reacts with PVDF and LiPF6, adversely affecting battery technology and performance. The reaction of materials with air will be carried out in the entire process of raw material preservation, electrode preparation, Polar storage, etc. Therefore, for high-nickel materials, strict environmental control is required from raw materials to the entire battery production process. By studying the film resistance value after the electrode is stored in different humidity, the data support can be produced to determine the process control time. Figure 3 is an example of the evolution of the storage membrane resistance of the NMC 811 electrode in different environments. From this, it can be seen that the lower the storage humidity of the 811 electrodes, the more stable the film resistance changes. Therefore, the production control humidity of the 811 system should be as low as possible.

Application Case 4: Abnormal Assessment of Raw Material Incoming Batch

Polar-film resistance import quality management, as an incoming material detection method, can reduce production risk, improve production efficiency and product reliability. Figure 4 is an actual case in which a production batch begins to increase the core DCR(70 % SOC) anomaly. In response to this anomaly, different batches of positive electrode materials(including normal and abnormal groups) are extracted for electrode resistance measurement. The electrode resistance of the abnormal incoming material group was found to be significantly higher than that of the normal group.

Application 5: Estimation of conductivity of polar coatings

Based on the diaphragm resistance value, the real conductivity of the material can be calculated by linear fitting to provide a technical guarantee for product development. As shown in the actual case in Figure 5, it can be seen that:(1) There is a clear linear relationship between the resistance of the LCO positive diaphragm and the coating weight(thickness) of the electrode; (2) Through the linear fitting of the resistance at the same test pressure, the conductivity of the cathode active substance can be calculated as 2.73 S/m.

Application Case 6: Analysis of Polar Resistance and Pressure Sensitivity in Different Life Cycle

When using two probe methods to test the overall resistance of the electrode, the test loading pressure will have a certain impact on the results. In general, the test loading pressure increases, the electrode membrane resistance decreases, and after reaching a certain value, the test results are independent of the pressure. After roller pressure, fresh electrode after assembling the battery, and polar plate that undergoes different cycles, they show different resistance values and test loading pressure sensitivity differences during different life cycles, and the pressure sensitivity differences through the diaphragm. The variation of expansion thickness of different life cycle Polaroids can be calculated, which provides a new measurement and characterization method for the evaluation of Polaroids and lithium batteries.

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