The lithium ion battery pole piece is a three-layer composite material composed of an electrode coating and a current collector foil, that is, a coating composed of particles, uniformly coated on both sides of the metal current collector, and mainly composed of four parts: 1) active material particles; (2) a conductive phase and a binder mixed with each other (carbon phase); (3) pores, filled with electrolyte; (4) metal foil current collector.
The mechanical stability of the pole piece has an important effect on the battery, especially like a silicon-based anode. When inserting and extracting lithium during the charge/discharge cycle, the volume change reaches 270%, and the cycle life is poor. This volume expansion causes the silicon particles to be pulverized. And the coating is separated from the copper current collector.
An important method for determining the expected service life and performance of an active coating is to examine the bond strength of the coating and the analysis of the failure of the coating. The failure of the coating includes the peeling of the coating from the substrate, which may be due to mechanical or thermal Stress, electrochemical stress, etc. The stripping of the coating material can be manifested in many different ways: cracking, delamination, spalling, chipping or plastic deformation. Check coating adhesion and coating failure analysis A reliable and practical method to quantify the adhesion strength between the coating and the substrate and characterize the failure mechanism is important information for preventing or stopping the adhesion failure. Understanding this knowledge can help improve the quality and performance of the overall coating.
The actual adhesion is the load that needs to be applied to separate the coating from the substrate. The actual adhesion may be affected by many factors such as coating thickness, roughness of the substrate, mechanical properties of the coating, and surface chemical results of the substrate. Impact. The measurement of actual adhesion may also be affected by the test method. The most common methods include peel test, bend test, scratch test and indentation test.
This article briefly summarizes the test methods for the mechanical properties of lithium battery pole pieces. Due to the limited personal level, the errors in the text are welcome to criticize and correct, and you are welcome to leave a message.
1, nanoindentation
Nanoindentation technology, also known as Depth-Sensing Indentation (DSI), is one of the simplest methods for testing the mechanical properties of materials. It can measure various mechanical properties of materials at the nanoscale, such as load-displacement curves. , elastic modulus, hardness, fracture toughness, strain hardening effect, viscoelastic or creep behavior, etc. The following video is the basic principle of nanoindentation.
Figure 1 (a) Schematic diagram of nanoindentation test; (b, c) Negative pole piece indentation scan photo
Figure 1 is a schematic diagram of the principle of nanoindentation test and a scan of the indentation of the negative electrode of a lithium ion battery. When testing, a load P is applied to the indenter, and the indenter is pressed into the sample, and the indentation is left on the surface of the sample after unloading. 2 is the typical load-displacement curve in the nanoindentation test. The first thing that occurs on the surface of the specimen during the loading process is elastic deformation. As the load is further increased, the plastic deformation begins to appear and gradually increases. The unloading process is mainly the elastic deformation recovery. The process, and the plastic deformation eventually causes the surface of the sample to form an indentation. In the figure, hc is the contact depth, ht is the displacement at the maximum load, ε is the instrument parameter related to the indenter. As can be seen from Figure 2, the load gradually increases from 0 Increase to the maximum load of 30mN, then the load basically decreases linearly. At this time, the slope of the straight line is the contact stiffness of the sample. S. The hardness H can be calculated by measuring the indentation load P, the indentation surface area A and the contact stiffness S. And elastic modulus E.
Figure 2 Typical load-displacement curve in the nanoindentation test
Figure 3 is the load-displacement curve of the multi-nanoindentation test for the lithium ion battery (a) positive electrode and (b) negative electrode, and the elastic modulus corresponding to (a) positive electrode and (b) negative electrode different indentation depth test. Studies have shown that The microstructure and internal stress inside the coating are the main reasons for the change of the elastic modulus of the coating when the coating thickness is different. When the coating is prepared, the thicker the coating, the higher the density and the greater the internal stress, resulting in the coating of the test. The greater the modulus of elasticity of the layer. When the depth of penetration is small, especially when the surface of the sample is rough, a significant surface effect is produced. This is mainly caused by the surface roughness, which is mainly when the test is started. The data is not real and scattered. In order to reduce the influence of surface roughness as much as possible, it is recommended that the indentation depth is not less than a certain length to ensure that the indentation depth caused by surface roughness is relatively small.
Figure 3 Lithium-ion battery (a) positive electrode and (b) negative electrode multiple nanoindentation test load-displacement curve, and (a) positive electrode and (b) negative electrode corresponding to different indentation depth test
2, tensile test
Tensile test is a test method for determining the material properties under axial tensile load. The data obtained by tensile test can determine the elastic limit of the material, elongation, elastic modulus, proportional limit, area reduction, pull Tensile strength, yield point, yield strength and other tensile properties.
Figure 4 is a tensile test sample size and simple tensile test fixture for lithium ion battery pole pieces.
Figure 4 Lithium-ion battery pole piece tensile test sample specifications and simple tensile test fixture
Figure 5 is a stress-strain curve of the negative electrode, positive electrode and aluminum foil tensile test of a lithium ion battery. Similar to the typical stress-strain curve of a metal material, it is generally divided into the following stages:
1) Elastic phase: The stress and strain are basically linear. After unloading, the original length can be restored. The curve is called the yield point at the point where the deformation reaches 0.2%, and the corresponding strength is the yield strength. At this time, the elastic modulus can be calculated. E, the slope of the curve.
2) Yield stage: The stress remains basically the same, and the strain increases significantly.
3) Strengthening phase: This phase is the plastic hardening phase. This phase is not observed in the battery pole piece. The stress peak corresponding to the f point is the tensile strength.
4) Local deformation stage: At this time, the sample will be necked until it breaks.
The pole piece tensile fracture process is shown in Figure 6.
Figure 5 (a, b) negative electrode of lithium ion battery, (c) positive electrode and (d) aluminum foil tensile test stress-strain curve
Figure 6 Schematic diagram of the pole piece tensile fracture process
Figure 7 is a stress-strain curve of the (a) negative electrode and (c) positive electrode tensile test of a lithium ion battery. Based on these test data, the constitutive relationship of the lithium ion battery pole piece is inferred, and the fitting model of these pole pieces is applied. 2. In the simulation calculation of lithium ion battery, study the mechanical properties of the battery.
Fig. 7 (a) negative electrode and (c) positive electrode tensile stress-strain curve of lithium ion battery, and model fitting of pole piece constitutive relation
3, compression test
In the mechanical properties test of metal materials, the mechanical properties and the corresponding calculation formulas defined in the tensile test are basically applicable in the compression test. However, when a uniaxial compression load is applied to the specimen, the stress state is soft. The coefficient is significantly larger than the tensile state, so that some materials that exhibit brittle fracture in the tensile test (such as gray cast iron, ceramics, amorphous alloys, etc.) may show some plastic deformation in the compression test, or show a higher Strength. Therefore, in the study of the deformation and fracture behavior of brittle materials, compression tests are often used, and their strength and plasticity are measured.
In the study of the constitutive relationship model of the lithium ion battery pole piece, in order to more fully understand the mechanical properties of the pole piece, while stretching the pole piece, the pole piece is often subjected to a compression test, and FIG. 8 is a lithium ion battery. (a) Negative and (c) stress-strain curves for positive compression testing, and model fitting of pole piece constitutive relations. Construct a polar constitutive model based on the tensile and compression experimental test data of the pole piece, and then model It is applied to study the pole piece fracture behavior in the battery assembly process. The experimental and simulation comparison results are shown in Fig. 9.
Figure 8: Lithium-ion battery (a) negative and (c) positive compression test stress-strain curve, and model fit of pole piece constitutive relationship
Fig. 9 Experimental and simulation study on the fracture behavior of the pole piece in the battery assembly process
4, bending test
The bending test has the largest surface stress and can sensitively reflect the surface defects of the material. It is often used to study the surface strengthening process and surface properties. Figure 9 shows the loading and recording load deflection curves of the common three-point bending test. The stress value corresponding to the dotted line is the flexural strength or bend strength of the material.
Figure 10 Schematic diagram of the load deflection curve of the bending test loading and recording
5, peel test
The peel strength of the coating refers to the force required to peel off the coating per unit area between the coating and the substrate from the bonding surface of the substrate. It is an important indicator for detecting the performance of the coating. If the bonding strength is too small, it will be light. Causes the coating life to decrease, resulting in early failure, and the coating is partially peeled off, and the peeling cannot be used.
The tensile strength of the coating is the ultimate ability of the coating to withstand the normal tensile stress, which is the most important indicator for assessing the bonding strength of the coating. The test tool or equipment is used to subject the specimen to the tensile force perpendicular to the surface of the coating. , until the sample is pulled apart, that is, the coating is peeled off, the load at the time of the damage is recorded, and the tensile strength of the coating is obtained by dividing the load value of the sample by the sectional area.
In the general test method, the pole piece is divided, the pressure sensitive 3M-VHB double-sided tape is attached to the surface of the electrode, and the other side is attached to the stainless steel plate, and the stainless steel plate and the current collector are fixed on the two fixtures of the stretching device, and then The sample is stretched at a certain speed and subjected to a 180 degree peel test. When the aluminum current collector is completely peeled off, the force detected is the peeling force. The test principle is shown in Fig. 11.
Figure 11 Schematic diagram of coating peel strength test
1. The tensile test, compression test, peel test, tear test and shear and bending test can be carried out by using the microcomputer controlled electronic universal testing machine.
6, scratch test
Figure 12 Schematic diagram of the general operation of the scratch tester. During the scratch test, the stylus made of diamond or other hard material is linearly scribed along the surface of the coating while applying a constant or increasing load. , the needle is drawn into the coating, reaches the coating interface or passes through the coating to the substrate interface. The coating and substrate system will cause cohesion and adhesion failure. The inspection is obtained directly from the scratch test and microscopic analysis after scratching. The data gives useful information about the coating itself and the coating-substrate system.
Figure 13 is a scanning electron micrograph of a silicon-based anode of two different processes under different loads. By studying the experimental data of the scratch test, the mechanical stability of the negative pole piece can be compared, and the cycle life and performance of the battery are inferred.
Figure 12 Schematic diagram of the general operation of the scratch tester.
Figure 13 Scanning electron micrograph of scratches of silicon-based anodes with two different processes under different loads
