What are the main performance requirements for lithium-ion battery diaphragms?
A lithium-ion battery consists of a positive and negative electrode material, an electrolyte, a diaphragm and a battery shell. As the third pole of the battery, the diaphragm is one of the key inner components of the lithium-ion battery, its performance determines the interface structure of the battery, internal resistance, etc., which directly affects the energy, cycle and safety characteristics of the battery, and the high quality diaphragm plays an important role in improving the comprehensive performance of the lithium battery. So, what are the main performance requirements of lithium-ion battery separator?
1, thickness: For consumable lithium-ion batteries, 25 micron diaphragm has gradually become the standard. However, due to the increasing use of portable products, thinner diaphragms, such as 20 microns, 18 microns, 16 microns, and even thinner diaphragms are starting to be widely used. For power batteries, due to the mechanical requirements of the assembly process, often need a thicker diaphragm, of course, for power with large batteries, safety is also very important, and thicker diaphragm often means better safety, EV/HEV use a thickness of about 40 microns diaphragm.
2, air permeability: In general, there will be a air permeability parameter in the diaphragm, and it is called the Gurley index. Gurley index refers to the time required for a certain volume of gas to pass through a certain area of diaphragm under a certain pressure condition, and the volume of gas is generally 50cc. The Gurley index is proportional to the internal resistance of the battery, and the greater the value, the greater the internal resistance. Because the microstructure of the same diaphragm is relatively the same or comparable, the Gurley number of the same diaphragm can well reflect the internal resistance.
3, automatic closing mechanism: at present, the diaphragm of lithium batteries is generally able to provide an additional function, that is, thermal closing. This feature can provide an additional safety protection for lithium-ion batteries. The main parameters of this function are closed cell temperature and film breaking temperature.
The obturator temperature is the temperature at which the micropore is closed, that is, the closing temperature. When there is an exothermic reaction, self-heating, overcharge or short circuit outside the battery, these conditions will generate a lot of heat. Due to the thermoplasticity of the polyene through the material, when the temperature is close to the melting point of the polymer, the micropore closure forms a thermal closure, blocking the continuous transmission of ions, forming a break, and protecting the battery. PE is 130-140℃, PP is 150℃.
4. Porosity: Porosity refers to the percentage of the pore volume in the bulk material and the total volume of the material in its natural state. Generally, the membrane porosity is between 35% and 60%. The size of porosity has a certain relationship with the internal resistance, but the absolute value of the porosity between different kinds of diaphragms cannot be directly compared.
5, thermal stability: the diaphragm needs to be stable in the temperature range of the battery (-20°C~60°C). At present, the PE (polyethylene) or PP (polypropylene) materials used in the diaphragm can meet the above requirements. Generally, under vacuum conditions, at a constant temperature of 90C for 60 minutes, the transverse and longitudinal contraction of the diaphragm should be less than 5%.
6, mechanical strength: The mechanical strength of the diaphragm can be measured by puncture resistance and tensile strength.
Puncture strength: refers to the mass applied to puncture the diaphragm sample at a given needle shape, indicating the tendency of the diaphragm to short circuit during assembly. In order to prevent short circuit, the diaphragm must have a certain puncture resistance strength. Puncture resistance strength is generally 300-500g.
Tensile strength: It is a parameter that reflects the dimensional stability of the diaphragm when it is subjected to external force during use. If the tensile strength is not enough, the diaphragm is not easy to restore the original size after deformation, which will lead to short circuit of the battery.
The tensile strength of the diaphragm must meet the bias yield of the diaphragm should be less than 2% when an external force of 1000 psi is applied.
7, chemical stability: in other words, the diaphragm is required to be inert in the electrochemical reaction. Chemical stability is generally assessed by corrosion resistance and expansion rate of electrolytes. After several years of industrial testing, it is generally believed that the current diaphragm material PE or PP meets the requirements of chemical inertness.