The separator is an inactive material that does not participate in electrochemical reactions. They are necessary components for the operation of lithium batteries, providing pathways for ion transfer while physically separating the positive and negative electrodes. Like the anode, cathode, and electrolyte, separators play an important role in determining battery performance and safety. To better understand the electrolyte properties, we should examine the function, structure and properties of the separator.
1. The function of the battery separator
Lithium rechargeable battery separators are microporous polymer membranes with pore sizes ranging from nanometers to micrometers. The most commonly used diaphragm materials are polyolefins, such as polyethylene PE and polypropylene PP, which offer several advantages, including good mechanical properties, chemical stability, and low cost. Commercialized separators have pores of 0.03–1 μm, porosity of 30%–50%, and low closed-cell temperature (PE: ~135 °C, PP: ~165 °C). If an internal short circuit raises the temperature, the melted separator closes the pores, restricting the movement of ions, thereby improving the safety of the battery by hindering thermal reactions. When manufacturing batteries, thin separators are used to maximize the battery’s capacity. Especially in high-capacity cylindrical cells (18650 class, 3Ah) 16 μm thick separators are used.
Figure 1 shows the surface of the PE separator prepared by the dry method. The PE crystals formed a network structure (bright areas), and the pores were interconnected to form micrometer-sized pores (dark areas). Due to the small size of the pores, the separator can prevent the contact between the electrodes and hinder the migration of substances released by the electrodes. Ionic conduction can be achieved by filling the micropores with a liquid electrolyte.
2. Basic properties of battery separators
The fundamental properties of battery separators should be optimized to improve battery safety and prevent mechanical strength issues that may arise during production.
The basic properties of battery separators can be divided into the following categories.
1) Thickness: Since the ionic conductivity of liquid organic electrolytes is 100 times lower than that of aqueous electrolytes, it is important to reduce the distance between electrodes while maximizing electrode surface area to achieve maximum output and energy density. Therefore, the thickness of the film should not exceed 25 μm, and the film thickness of the most commonly used separators is generally 20, 16 or 10 μm. The thin separator increases the discharge capacity of the electrode by increasing the concentration of the liquid electrolyte around the electrode and facilitating the migration of species. However, thin septa can develop pinholes and tear easily. The risk of short circuit between electrodes increases, and at the same time, the safety of the battery is reduced.
2) MacMullin number: The MacMullin number is the impedance of the diaphragm filled with an electrolyte divided by the impedance of the electrolyte itself, generally as high as 10~12.
3) Resistance: As an insulator, the diaphragm should have lower resistance when filled with electrolyte. Higher resistance can affect battery performance, including discharge capacity.
4) Permeability: Permeability, described in Gurley units, is a measure of the time it takes for air to penetrate under the same conditions (same pressure, same area, etc.). It is also one of the separator properties that affects battery performance.
5) Pore size and porosity: The porosity is generally about 40%. The pore size should be under tens of microns, which is smaller than the particle size to prevent dendrite growth and internal short circuits caused by impurities.
6) Breakdown strength: Internal short circuits may be caused by impurities released from the electrodes, surface states of the negative and positive electrodes, and dendrite growth of lithium. Breakdown strength represents the resistance of the diaphragm to these hazards and can be measured by squeezing the diaphragm with a probe. The larger the value, the less likely the diaphragm will cause an internal short circuit.
7) Heat shrinkage: The heat shrinkage of the separators of different manufacturers is different, and the shrinkage should be less than 5% after drying under vacuum at 90 ℃ for 1h.
8) Tensile strength: Like in the winding process, tensile strength is a property that has an important impact on the production process. The separator has high tensile strength in the direction of elongation. The tensile strength in the longitudinal direction of the separator with a thickness of 25 μm is above 1000 kgf/cm². When uniaxially stretched, the tensile strength in the transverse direction is only one tenth of the longitudinal tensile strength; when biaxially stretched, the transverse tensile strength is basically the same as the longitudinal direction.
9) Closed cell: Closed cell is a safety function that blocks the circuit by closing the micropore when overcurrent is caused by an internal or external short circuit. Because the temperature rise can be prevented by closing the pores during early short circuit, PE separators are often used in lithium rechargeable batteries.
10) Melting stability: Melting stability is a property of maintaining the diaphragm structure for a long time when the temperature is higher than the melting temperature. Like the closed-cell temperature, it is an important factor for battery safety performance requirements.
11) Wetability: The separator is required to have a fast wetting rate and sufficient wettability.
12) Chemical stability: Chemical stability refers to the stability under redox conditions. The diaphragm is resistant to electrolyte corrosion at high temperatures.
13) Average molecular mass and mass distribution: This is an important factor determining the thermodynamic and mechanical properties of polyolefins. By increasing and decreasing, good mechanical properties and narrow melting temperature range can be achieved.
3. Influence of separator on battery assembly
The electrode active material is coated on the current collector, and the separator is inserted between them before winding. It is then inserted into a case, filled with electrolyte and sealed to form a lithium rechargeable battery. A reel is used during the winding process. In order to ensure the highest possible density without flaking of the active material and without distortion of the separator, it is necessary to use thin separators with good mechanical properties. Good tensile and tensile strengths are required in the machine direction to avoid wrap damage and narrowing. In addition to this, a high breakdown strength is also required to protect the cell from impurities and damage. These properties are determined by porosity and film thickness and affect the manufacturing process. Other requirements include winding cores, wettability when injecting electrolyte, and sampling inspection of the electrochemical stability of electrolyte.
4. Oxidation resistance of the diaphragm
The separator in the battery in contact with the negative and positive electrodes undergoes oxidation and reduction reactions on the surface of the electrodes. Oxidative decomposition occurs because the polyolefin separator is less resistant to oxidative reactions. This oxidative decomposition is more severe at high temperature, ultimately reducing the cycle performance of the battery. Oxidation resistance varies with the material, with PP separators being more resistant than PE. Some manufacturers make separators that include PP/PE/PP triple layers, which have better oxidation resistance than single-layer PE films. As the demand for high-capacity batteries increases, the oxidation resistance of the separator is even more important. This needs to be achieved by improving the oxidation resistance of the electrolyte and separator. However, the oxidation resistance of separators has not been extensively studied. One of the ways to improve the oxidation resistance of the separator is to coat the surface of the negative electrode with a layer of PVdF-based polymer. Figure 2 shows the results of Fourier transform infrared spectroscopy (FTIR) analysis of the disassembled cell held at 4.4 V, 90 °C for 4 h. Peaks of C=C double bonds were observed in the range of 700–1900 cm-1, which indicated that the oxidation reaction occurred on the surface of the PE separator. This is confirmed by the color change of the PE separator in the FTIR analysis results of Figure 2: a) Lithium battery and b) PVdF on the negative electrode surface. As shown in Figure 3, when the PVdF-based colloidal polymer electrolyte was coated on the cathode surface, the battery did not undergo any oxidation reaction at 4.4 V and maintained a good performance. This is because the PVdF-based gel electrolyte on the cathode surface using the PE separator prevents direct contact with the cathode.
In addition to the modification of electrode surfaces with polymer electrolytes, oxidation resistance can be further improved by introducing organic/inorganic compound films on the surfaces of electrodes and separators.
5. Thermal stability of the separator
When an external or internal short circuit causes a current overload, which in turn causes the temperature of the cell to rise, the reaction between the electrode and the electrolyte or the decomposition of the electrolyte can cause the release of gases and liquids and cause a fire. Here, the separator can improve the safety of the battery. As the temperature of the cell increases, the separator melts to close the micropores, limiting the ionic conductivity, thus delaying the combustion caused by thermal diffusion accumulated over time, and even the cell can stop reacting at the closed-pore temperature. As the temperature inside the battery continues to rise, the separator needs to have a higher fusing temperature. Like porosity and permeability, cell closing temperature and fusing temperature are important factors to ensure high safety performance of batteries. Considering the short-circuit and melting properties, some researchers have tried to improve the melting properties by using polyolefins with appropriate molecular mass and proportions above the melting temperature to suppress flowability. Another approach is to mix materials with different fusing temperatures, such as PE and PP, to obtain lower closed cell temperatures and better fusing properties.
1) PE diaphragm
The separator acts as an insulator by maintaining a low closed cell temperature and a high fusing temperature by using an ultra-high molecular weight polyethylene that has little fluidity above the fusing temperature.
2) PE/PP multilayer diaphragm
By superimposing PE and PP films with different fusing temperatures, the battery safety is promoted by the isolation in the middle core layer when the lithium rechargeable battery is short-circuited. For separators that can effectively maintain battery safety, mixing films with different fusing temperatures is a known method to make separators insulate over a wide temperature range. In the three-layer separator shown in Figure 4, we can see that a short circuit occurs at 130°C, and the insulating material remains stable at 180°C without any melting.
6. Development of diaphragm materials
1) Microporous polyolefin membrane
In the early days, separators were made of PE with micropores. At fusing temperatures above 120 °C, the movement of ions and organic solvents through the pores is restricted and the battery fails. Since PE flows at high temperatures, it can be difficult to separate the electrodes or continue to melt in the event of a fire. In order to overcome this shortcoming, PE and PP whose fusing temperature is at least 40 ℃ higher than it are used together. However, PE separators have been more studied because of the complexity of multilayer films, and the high manufacturing cost. Research is now underway to use ultra-high molecular weight PE to replace PP to solve the problem of fluidity above the melting temperature of PE.
2) Porous PVdF membrane
PVdF has been used as a binder in electrodes for lithium-ion batteries. Compared to polyolefins, fluoropolymers contain highly electronegative fluorine atoms in the backbone, and therefore have very strong resistance to liquid electrolytes due to their strong interactions with polar solvents. affinity. In the early 1990s, P(VdF-co-HFP) separators were used in plastic lithium-ion batteries developed by Bellcore. The crystallization temperature and crystallinity of PVdF are reduced by the copolymerization of VdF and HFP. Despite its high electrolyte intake and ionic conductivity, PVdF has too poor mechanical properties compared to polyolefins, so PVdF has not been used in Li-ion batteries. Other developments are composite-type separators, such as the fabrication of composite-type separators by combining high mechanical properties of polyolefins with high-quality PVdF polymers.
3) Inorganic coating of diaphragm
Due to the nature of the material and the stretching during the manufacturing process, the thermal shrinkage of the polyolefin separator is severe above 100°C. Due to the presence of metal particles and other impurities in the battery, they are easily broken. This is also the main cause of the known internal short circuit between the negative and positive electrodes. In order to overcome this drawback, there are now extensive researches on layers containing inorganic nanoparticles (SiO2, TiO2, Al2O3, ZrO2, etc.) mixed with binders (polymers or inorganics) and coated on polyolefins, non-woven fabrics or electrode surfaces the new diaphragm. In particular, advanced technological means have been applied to replace typical polyolefins in separators (see Figure 5) composed of polyester nonwovens and inorganic coatings.
The inorganic coating method improves the thermodynamic and mechanical properties on the basis of the polyolefin separator and improves the battery safety by suppressing the internal short circuit. Moreover, due to the high ionic conductivity obtained from the microscopic and structural control of inorganic nanoparticles, binder properties and inorganic coating, the battery performance of inorganic coating is better than that of gel polymer electrolytic and electrolyte. Okay. In the development of inorganic-coated separators, various approaches have been used by different manufacturers, and the results have not been published in detail. More specifically, basic research is underway on unsupported inorganic composite separators made of CaCO3 and PTFE and composite separators composed of alumina, PVdF binders.
7. Manufacturing process of diaphragm
1) Membrane technology
Film technologies include extrusion and stretching processes. Extrusion is generally accomplished by twin screw extruders, single screw extruders may be used when the manufacturing process for mixing polymers and solvents is not involved. The extruded sheet from the T-shaped extrusion die can be stretched uniaxially in the machine direction or biaxially stretched in the machine and transverse directions. The films made by biaxial stretching have higher strength and isotropy, so they are more suitable as separators. Another approach is to use a cylindrical die for extrusion followed by tubular stretching.
2) Hole making technology
Pore making techniques can be divided into dry and wet methods.
Dry method The dry method is to form pores by stretching the extruded film at low temperature to generate small cracks on the surface of the layered crystals. Figure 6a shows the SEM image of the microporous membrane prepared by this method.
Wet Process In the wet process, the polymer and plasticizer are homogeneously mixed together at high temperature and then phase separated by cooling. The plasticizer is then removed to create pores (see Figure 6b). It is also possible to add an inorganic powder and then remove it along with the plasticizer. The latter method enables the fabrication of large pore size and high porosity separators.
8. Prospects for diaphragms
As lithium rechargeable batteries become more compact, higher energy and power densities can be achieved, and the separator should also be thinner, stronger and less prone to shrinkage. Especially when PE-containing aluminum-plastic composite films are used as encapsulation materials, dimensional stability is critical, as external forces may cause the cells to bend or twist. The thinner the separator, the better the electrolyte injection, but may reduce the injection amount or electrolyte retention capacity. Therefore, there is an urgent need to improve the compatibility of separators and electrolytes. To meet the requirements of high energy density and high specific power characteristics, new methods to facilitate pore closure and fusing of separators need to be investigated.