Diaphragm is an important part of lithium-ion battery, lithium-ion battery plays a role in preventing short-circuit between the positive and negative, and conduction ions.Preparation of the current diaphragm has two broad categories: 1) dry stretch process ; 2) wet process, dry process is currently the most widely used membrane preparation process, but there is obvious anisotropy of the membrane prepared by the dry process, for example, in the longitudinal MD direction, the tension of the diaphragm The strength of up to 120MPa above, but in the transverse TD and diagonal DD direction tensile strength is only slightly higher than 20MPa, this feature of the separator also led to lithium-ion batteries in the external deformation, in all directions There is a big difference in the strains that can be sustained.
At present, the research on the mechanical properties of the diaphragm is mainly horizontal and vertical tensile strength, the research on the compressive properties of the diaphragm is relatively small, mainly because the diaphragm thickness is very thin, the traditional squeeze test method is affected by the resolution, often Need tens of layers, or even hundreds of layers of membrane stack together, the different layers of membrane in the direction of the consistency and the air between the diaphragm will have a great impact on the accuracy of the test results, so the traditional extrusion test method is very Difficult to obtain accurate diaphragm compressive strength data.
Recently, Shutian Yan et al. Of Michigan State University studied the compressive properties of Celgard 2400 membrane in air and DMC using capacitive principle. The following figure shows the 3D reconstruction model of Celgard 2500, which can be seen from the model There is a clear difference in the structure of the separator in the TD, MD and TTD, which is also the fundamental reason for the significant difference in the mechanical strength of the separator in all three directions.
The principle of the capacitance test method designed by Shutian Yan is shown in the figure below: The experimental equipment is mainly composed of two smooth glass disks, and the center of the glass wafer is coated with a 5-nm thick metal by magnetron sputtering Tungsten coating as the two electrodes of the capacitor during the test, the diaphragm needs to be made into a 3mm disc, the three diaphragm samples were placed in the three positions shown in Figure a, and then on the two glass sheets Pressure is applied to deform the diaphragm causing the distance between the two tungsten electrodes to change, causing a change in capacitance, which can be calculated by measuring the change in capacitance between the two tungsten electrodes Calculate the diaphragm strain.
The test uses a two-layer diaphragm, making the total thickness of the diaphragm about 50um, in order to reduce the test error.A below is the curve of the capacitance and applied external force, you can see in the beginning, a short period of range, The membrane is subject to very little force and the capacitance is much greater (indicating a much smaller membrane thickness). This may be due to the rough surface of the membrane or the burr on the rim, causing the upper and lower glass plates to not initially be in contact with the diaphragm at the beginning With the increase of applied pressure, the contact area between the diaphragm and the glass sheet also gradually increases, and the compressive capacity of the diaphragm also increases significantly.
A careful analysis of the stress and strain curves (Figure d) shows that the stress-strain curve can be divided into four zones: 1) the initial contact zone 2) the linear zone 3) the yield zone 4) the compacted zone where zone 1 is the same As we have analyzed above, because of the incomplete contact between the diaphragm and the glass sheet, a greater deformation occurs under a small pressure. Region 2 is mainly a region where the diaphragm linearly deforms. Region 3 indicates that the diaphragm has reached Yield Limit, while Zone 4 is a common compaction zone for porous materials.
Shutian Yan calculated the modulus of elasticity of the separator using Zone 2 and Zone 4, the modulus of elasticity of the Celgard 2400 diaphragm in air at 0.191 ± 0.020 GPa for Zone 2, and the elastic modulus of the diaphragm in DMC Modulus is 0.165 ± 0.020GPa.The elastic modulus of the separator in the thickness direction is very close to the tensile strength of the separator in the transverse direction TD, which is related to the structure of the separator. The single-layer PP separator Celgard 2400 adopts the dry-drawing process , The parallel sheets originally stacked together in the separator are pulled apart during stretching, and the stretched separator is mainly composed of amorphous nanofibers between the semi-crystalline sheets and the sheets, as shown in the first sheet As shown in the picture, the longitudinal MD direction mainly absorbs tensile force of the amorphous nanofibers, while in the longitudinal direction TD and the thickness direction TTD, the tensile force is given by the semi-crystalline sheet. Since the diaphragms have similar TD and TTD directions Structure, the elastic modulus in the thickness direction of the separator and the tensile strength in the transverse direction TD are relatively close to each other.
The Celgard 2400 membrane has a modulus of elasticity in air of 0.270 ± 0.004 GPa and a modulus of elasticity in DMC of 0.386 ± 0.035 GPa when the membrane strain reaches Region 4, resulting in an increase in the compressive capacity of the membrane in DMC It is possible that part of the DMC trapped in the pores inside the membrane after the membrane was compressed into the compacted area played the role of a membrane.
In order to simulate the squeezing of the separator in a real battery, Shutian Yan also conducted a squeeze elastic modulus analysis on the NMC positive electrode and the three-layer composite structure of PP separator / NMC positive electrode / PP separator. The test results show that the positive electrode of NMC (1.084 ± 0.029GPa). The modulus of elasticity in DMC is 0.892 ± 0.033GPa, which is significantly higher than that of LCO cathode (0.232GPa) and LiNiCoAlO2 cathode (0.610GPa).
The compressive elastic modulus of the PP membrane / NMC positive electrode / PP membrane composite in air was 0.362GPa, and the elastic modulus was 0.336MPa in DMC.According to the principle of mixing elastic modulus of elasticity The elastic modulus of the layer composite structure can be calculated by the following formula, but the actual test results are not completely consistent with the theoretical result prediction, indicating that there is an interaction between the NMC positive electrode and the separator.As is well known, the surface roughness of the NMC positive electrode is far higher On the surface of the glass, the protruding position of the NMC positive surface will be squeezed to the diaphragm first when squeezed. During the subsequent extrusion deformation, the NMC positive electrode rough surface will also limit the diaphragm, This will also affect the elastic modulus of the diaphragm, which also leads to a certain deviation of the test results from the predicted ones.
In order to simulate the squeezing of the separator in a real battery, Shutian Yan also conducted a squeeze elastic modulus analysis on the NMC positive electrode and the three-layer composite structure of PP separator / NMC positive electrode / PP separator. The test results show that the positive electrode of NMC (1.084 ± 0.029GPa). The modulus of elasticity in DMC is 0.892 ± 0.033GPa, which is significantly higher than that of LCO cathode (0.232GPa) and LiNiCoAlO2 cathode (0.610GPa).
The compressive elastic modulus of the PP membrane / NMC positive electrode / PP membrane composite in air was 0.362GPa, and the elastic modulus was 0.336MPa in DMC.According to the principle of mixing elastic modulus of elasticity The elastic modulus of the layer composite structure can be calculated by the following formula, but the actual test results are not completely consistent with the theoretical result prediction, indicating that there is an interaction between the NMC positive electrode and the separator.As is well known, the surface roughness of the NMC positive electrode is far higher On the surface of the glass, the protruding position of the NMC positive surface will be squeezed to the diaphragm first when squeezed. During the subsequent extrusion deformation, the NMC positive electrode rough surface will also limit the diaphragm, This will also affect the elastic modulus of the diaphragm, which also leads to a certain deviation of the test results from the predicted ones.
