What are the performance requirements of 18650 lithium battery diaphragm?
It mainly includes thickness, permeability, wettability, chemical stability, pore size, puncture strength, thermal stability, thermal closing temperature and porosity. These factors directly affect the quality of lithium battery products. The diaphragm of 18650 lithium battery mainly plays two roles:
1: The insulating layer can effectively prevent the internal short circuit caused by the contact between the positive and negative electrodes
2.: semi permeable layer, mainly in liquid batteries such as 18650 lithium battery, can organize the passage of large molecules and only allow the passage of small charged ions, which can improve the concentration difference near the positive and negative electrodes, facilitate the diffusion of lithium ions, and improve the storage efficiency of lithium battery.
Performance requirements for 18650 lithium battery diaphragm:
For capacity 18650 lithium batteries, 25 micron diaphragms have gradually become the standard, but now the increasing use of portable products requires lighter, larger and thinner diaphragms, such as 20 micron, 18 micron, 16 micron and even thinner diaphragms. However, for the rate battery, due to the mechanical requirements of the assembly process, a thicker diaphragm is required, and the safety is also very important. The safety performance of lithium battery with thicker diaphragm is better.
2. Air permeability
Macmullin number: the ratio between the resistivity of the diaphragm containing electrolyte and the resistivity of the electrolyte itself. The smaller the value, the better. The value of capacity 18650 lithium battery is close to 8.
Gurley number: the time required for a certain volume of gas to pass through a certain area of diaphragm under certain pressure is directly proportional to the internal resistance of the battery equipped with diaphragm, that is, the greater the value, the greater the internal resistance. For example, it is meaningless to compare Gurley numbers of different diaphragms, because the microstructure of diaphragms is completely different; However, the Gurley number of the same diaphragm can well reflect the size of internal resistance, because the microstructure of the same diaphragm is relatively the same or comparable.
3. Degree of infiltration
In order to lower the internal resistance of lithium battery, it is required that the diaphragm can be completely soaked by the electrolyte used by lithium battery, which is related to the diaphragm material itself and the surface and internal microstructure of the diaphragm.
Judgment method a: take a typical electrolyte (e.g. EC: DMC = 1:1, 1m LiPF) and drop it on the surface of the diaphragm to see if the droplet will disappear quickly and be absorbed by the diaphragm.
Judgment method B: use the camera to record the process from droplet contact with the diaphragm to droplet disappearance, calculate the time, and compare the wettability of the two diaphragms through the length of time.
4. Chemical stability
The diaphragm is required to be inert in electrochemical reaction, inactive to strong reduction and strong oxidation, no attenuation of mechanical strength and no impurities. It is generally believed that the current diaphragm material PE or PP can meet the requirements of chemical inertness.
5. Diaphragm aperture
Prevent electrode particles from directly passing through the diaphragm. The pore diameter of the diaphragm is required to be 0.01-0.1um. When it is less than 0.01um, lithium ions can penetrate
The capacity is too small. When it is greater than 0.1um, the battery is easy to short circuit when dendrites are generated in the battery. The electrode particles are generally in the order of 10 microns, while the conductive additives used are in the order of 10 nanometers, but fortunately, the general carbon black particles tend to agglomerate to form large particles. Generally speaking, the diaphragm with submicron pore size is enough to prevent the direct passage of electrode particles. Of course, it does not rule out some conditions such as micro short circuit caused by poor electrode surface treatment and more dust.
6. Diaphragm puncture strength
Puncture strength: at a certain speed (3-5 meters per minute), let a needle with a diameter of 1mm without sharp edge to the annular fixed diaphragm, and the maximum force exerted on the needle to penetrate the diaphragm. Because the methods used in the test are very different in the actual battery, it is not particularly reasonable to directly compare the puncture strength of the two diaphragms, but when the microstructure is certain, the puncture strength is relatively high and the assembly defect rate is low. However, the simple pursuit of high puncture strength will inevitably lead to the decline of other properties of the diaphragm.
7. Thermal stability
The diaphragm needs to be thermally stable within the temperature range of the battery (- 20 ℃ ~ 60 ℃). Generally speaking, PE or PP materials currently used for diaphragm can meet the above requirements. Generally, under vacuum conditions, the diaphragm shall be kept at 90 ℃ for 60 minutes, and the transverse and longitudinal shrinkage shall be less than 5%.
8. Diaphragm thermal closing temperature
The simulated battery is heated, and the temperature when the internal resistance increases by three orders of magnitude; Closed cell temperature: the temperature at which the micropores of the diaphragm are blocked by the heat generated during external short circuit or abnormal high current. Melting rupture temperature: the temperature at which the diaphragm is heated to rupture when the temperature exceeds the melting point of the sample.
9. Diaphragm porosity
The porosity of most lithium ion battery diaphragms is between 30% - 50%. There is a certain relationship between porosity and internal resistance, but the absolute value of porosity between different diaphragms cannot be directly compared. 18650 lithium battery diaphragm performance requirements are mainly the above points, for reference only!