Lithium-ion battery anode material series four - the detection of related properties of graphite anode sheet

The last article mainly introduced some basic process data that need to be detected in the process of homogenization, coating and rolling of negative electrode materials. For lithium-ion battery enterprises, the fluctuation and change of these data can be monitored in the actual production process. , so as to detect and eliminate abnormalities as early as possible, so as to achieve the purpose of mass production smoothly. As for the rolled pole piece, its own production process has been completed, and the negative pole piece needs to exert its own electrical properties through a reasonable production process. What parameters need to be tested in this process? The fourth article in the series will take you to find the answer.

1. Peel strength:

For substances that are bonded together, it is the maximum force required to peel off the unit width from the contact surface. Generally, a tensile tester is used to test. There are two test results. One represents the peeling force, and the unit is Newton (N), Kilogram force (kgf), a characterization of peel strength, represents the force per unit length, in Newton/meter, (N/m) kilogram force/centimeter (kgf/cm), and now the industry standard GB2792 and ASTMD3330 have used 25mm as Standard width unit.

Fix the surface to be tested on a rigid support with double-sided tape, and stick the other side to the stainless steel plate, then fix the stainless steel plate and the current collector on the two fixtures of the equipment, and start the test, the equipment runs at a certain speed and load , the force when the current collector is completely peeled off is the peeling force. It should be pointed out that the peel strength of the front and back sides of a pole piece is often different. In the actual production process, it is necessary to pay attention to the peel strength of the two sides. surface phenomenon that affects the performance of the battery

2. Surface state:

The surface state is actually a very general concept. From a macro perspective, the state of the pole piece that can be seen by the naked eye can be called the surface state. A good negative surface state is smooth, free of particles and scratches, and it feels very Smooth; if there are defects visible to the naked eye, it means that there is a problem in the coating process, and relevant parameters need to be adjusted. Here we will focus on the microscopic properties of the pole pieces.

The distribution of the negative electrode material and the conductive agent is uneven, and the negative electrode material has obvious agglomeration. This distribution cannot be intuitively manifested on the macroscopic level. If the conductive agent is unevenly distributed, the impedance of the battery will increase during use, and the local current density will exceed Therefore, in practice, it is necessary to make the distribution of negative electrode materials and conductive agents more uniform as much as possible, which is more conducive to the formation of a conductive network and reduces the problem of excessive local polarization caused by high current. resulting problems. In addition, SEM can also be used to see whether the particles on the surface of the pole piece are broken, agglomerated, etc.; in addition, the longitudinal section of the negative pole piece can also be analyzed to analyze whether the negative electrode material is evenly distributed in the longitudinal direction, etc. The comparison of different materials can summarize relevant experience and bring corresponding guidance to subsequent mass production.

3. Porosity:

The porosity of the pole piece is related to the amount of subsequent electrolyte added, and is also related to the electrical properties. At present, the measurement is generally a mercury porosimeter test or a liquid filling test, and the porosity of the pole piece is obtained by subsequent calculation. Through this index, the physical properties of different anode materials can be initially distinguished. If the porosity is too large, it is necessary to consider increasing the compaction density of the material. If the porosity is too small, it is necessary to consider extending the battery during subsequent injection. rest time, etc.

4. Surface resistance:

Surface resistance is also called surface specific resistance. An important data characterizing the electrical properties of a dielectric or insulating material. It represents the resistance per square area of the dielectric surface to the surface leakage current between the opposite sides of the square. The unit is ohm. The size of the surface resistance is not only determined by the structure and composition of the dielectric, but also related to the voltage, temperature, surface condition of the material, processing conditions and ambient humidity. Ambient humidity has a great influence on the surface resistance of dielectrics. The greater the surface resistance, the better the insulation performance.

Of course, the smaller the test data, the better. Through the comparison of the surface resistivity of different materials and different conductive agents, some suitable ratios and processing techniques can be obtained, which can be used as an inspection parameter in the production process for batch inspection. and recorded data.

5. Liquid absorption performance:

It is related to the porosity of the pole piece and the surface state of the material. Generally, a small amount of electrolyte is added to the surface of the negative electrode in the drying room, and the time when the electrolyte completely disappears is recorded. Statistical laws, which are then used to guide the production process.

6. Contact angle:

Refers to the tangent line of the gas-liquid interface made at the intersection of gas, liquid and solid three phases, the angle θ between the tangent line on the liquid side and the solid-liquid boundary line is a measure of the degree of wetting, and the wetting process is related to interfacial tension of the system. When a drop of liquid falls on a horizontal solid surface, when equilibrium is reached, the contact angle formed and each interfacial tension conform to the following Young's formula:

γ = γ + γ×cosθ

1) When θ=0, complete wetting;
2) When θ<90°, partial wetting or wetting;
3) When θ=90°, it is the boundary line of wetting or not;
4) When θ>90°, no wetting;
5) When θ=180°, no wetting at all.

For the negative electrode material itself, the wetting condition is relatively good, and this value may not be measured, but when the porosity is relatively low, it is possible to measure the contact angle. This parameter can be used to compare different electrolyte materials and negative electrodes. Wetting performance is of certain significance.

7. Rebound performance:

As the energy density of ion batteries is getting higher and higher, the coating amount and compaction density of the negative electrode are also getting higher and higher. Therefore, from the completion of the production of the negative electrode piece to the completion of the finished battery, the negative electrode piece has a certain amount of Therefore, it is necessary to record the thickness of the negative electrode at different stages. Generally, the thickness after rolling, drying, and full electric dissection is recorded, so as to monitor whether the thickness of the negative electrode changes abnormally. Through batch differences Consider production consistency.

Summary: Through a series of tests of the negative pole piece, the basic physical properties of the negative pole piece can be obtained. Of course, these indicators need to be combined with the subsequent electrical properties to comprehensively describe the comprehensive performance of a negative electrode material, which needs to be explained. Yes, the cathode material is also suitable for these test techniques, so I won't go into details here. Forewarning, the next article will be the last article in this series, mainly to popularize the current production process of negative electrode raw materials, so stay tuned.