El efecto y la influencia del diafragma en la batería de litio | Diafragma Aro
The effect and influence of diaphragm on lithium battery
The thickness is related to the internal resistance, the thinner the smaller the internal resistance, so as to achieve high-power charging and discharging. As small as possible under a certain mechanical strength, the thicker the puncture strength, the better. For consumable lithium-ion batteries, 25μm is generally used as the standard for the thickness of the separator. However, in the form of increasing demand for portable products, diaphragms of 16μm and even thinner have begun to be used in a wide range of applications. For power batteries, the mechanical requirements in the assembly process make the required diaphragm thicker, and safety performance is very important for power batteries, and a thicker diaphragm means better safety.
The uniformity of the thickness of the diaphragm is a particularly important quality indicator, which directly affects the appearance quality and the internal performance of the diaphragm roll. It must be strictly controlled during the production process. In a highly automated diaphragm production line, the diaphragm thickness is automatically detected and controlled by a highly accurate online non-contact thickness gauge and a fast feedback control system. The thickness uniformity of the diaphragm includes longitudinal thickness uniformity and lateral thickness uniformity, of which the lateral thickness uniformity is particularly important, and it is generally required to be controlled within ±1 micron.
The lithium battery separator material itself has a microporous structure, and the distribution of micropores in the entire separator material should be uniform. The electrode particles currently used are generally on the order of 10 microns, and the pore diameter is generally 0.03-0.12um. Too small pore size will increase resistance, too large pore size will easily make the positive and negative poles contact or be pierced and short-circuited by dendrites. Generally speaking, the submicron pore size membrane is sufficient to prevent the direct passage of electrode particles. Of course, some problems such as micro short circuit caused by poor electrode surface treatment and more dust are not excluded.
Porosity is the volume percentage of pores in the volume of the monomer film, which is related to the density of the raw resin and the film. The size of the porosity has a certain relationship with the internal resistance, but the absolute value of the porosity between different kinds of membranes cannot be directly compared. The porosity of the existing lithium ion battery separator is between 40%-50%.
Theoretically speaking, the diaphragm is not a necessary part of the battery. It will be added to meet industrial production in the future. Therefore, the diaphragm needs to meet a very important performance: it cannot deteriorate the electrochemical performance of the battery, which is mainly manifested in the internal resistance. Two parameters are used to evaluate this performance:
MacMullin number: The ratio between the resistivity of the diaphragm containing the electrolyte and the resistivity of the electrolyte itself. The smaller the value, the better, and the value of a consumable lithium-ion battery is about 8.
Gurley number: The time required for a certain volume of gas to pass through a certain area of a diaphragm under certain pressure conditions. It is proportional to the internal resistance of the battery assembled with the diaphragm, that is, the larger the value, the greater the internal resistance. However, it is meaningless to simply compare the Gurley numbers of two different diaphragms, because their microstructures may be completely different, but the Gurley number of the same type of diaphragm can well reflect the size of the internal resistance.
Closed cell temperature: Exothermic reaction inside the battery self-heating, overcharging or external short circuit of the battery will generate a lot of heat, causing the micropores to close, thereby blocking the continued transmission of ions and forming an open circuit, which plays a role in protecting the battery , The temperature when the pores are closed is the closed pore temperature. But for small batteries, the effect of the thermal shutdown mechanism is limited. Generally, PE is 130-140℃, and PP is 150℃. It is better to have a lower closed cell temperature.
Membrane rupture temperature refers to the internal self-heating of the battery, and an external short circuit increases the internal temperature of the battery. After the closing temperature is exceeded, the micropores are blocked to block the flow of current, and the temperature of the hot melt performance further rises, causing the diaphragm to rupture and the battery short circuit. The temperature at the time of rupture is the rupture temperature. A higher rupture temperature is better.
At a certain speed (3-5 meters per minute), a needle with a diameter of 1mm without sharp edges is pierced against the ring-shaped fixed septum. The maximum force applied to the needle to penetrate the septum is called the puncture strength . Sufficient puncture strength can prevent lithium dendrites and pole piece burrs from piercing the diaphragm and causing short circuits. The puncture resistance value is generally 300-500g. However, the method used in the test is very different from the actual battery situation. It is not particularly reasonable to directly compare the puncture strength of the two types of separators.
Mechanical strength mainly refers to the tensile strength of the diaphragm, and sufficient tensile strength can prevent the diaphragm from deforming. The tensile strength of the diaphragm is related to the film making process. When uniaxial stretching is used, the strength of the diaphragm in the stretching direction is different from that in the vertical direction; when biaxial stretching is used, the consistency of the diaphragm in the two directions will be similar. Generally, the tensile strength mainly refers to that the longitudinal strength should reach more than 100MP, and the transverse strength should not be too large. Too much will lead to an increase in the transverse shrinkage rate. This shrinkage will increase the chance of lithium battery manufacturers contacting the positive and negative electrodes.
In order to ensure that the internal resistance of the battery is not too large, it is required that the diaphragm can be completely wetted by the electrolyte used in the battery. On the one hand, the degree of wettability is related to the diaphragm material itself, and on the other hand, the surface and internal microstructure of the diaphragm are closely related. Better wettability is conducive to improving the affinity of the diaphragm and the electrolyte, expanding the contact surface between the diaphragm and the electrolyte, thereby increasing the ion conductivity, and improving the charge and discharge performance and capacity of the battery. The wettability can be measured by measuring its liquid absorption rate and liquid holding rate.
Due to the difference in the preparation process, the consistency of the diaphragm may be quite different. Consistency includes self-characteristics such as closing temperature, as well as apparent consistency such as the consistency of the hole and thickness of the observation under the electron microscope. The higher the consistency of the diaphragm, the better the other aspects of performance.