Electrospinning non-woven diaphragm

Electrospinning non-woven diaphragm

Electrospinning process is one of the more ideal and important methods for preparing nanofiber non-woven fabrics developed in recent years, and it is an innovative research on new diaphragm materials. The electrospinning process is also called the polymer electrostatic jet stretch spinning method (Figure 1). Under the action of a strong electric field force, the polymer solution or melt is added with high voltage static electricity, and the charged droplets form a Taylor cone. After overcoming the surface tension, a jet is formed. Finally, the solvent volatilizes or solidifies to form a mesh film stacked with uniform nanofibers.

Electrospinning non-woven diaphragm
Figure 1 – Schematic diagram of electrospinning process

The diameter of the fiber prepared by the method is in the nanometer level, and it has the characteristics of large specific surface area, high porosity, good liquid absorption, high ion conductivity, small and uniform pore size, and the like. The thermal stability and cycle performance of lithium-ion batteries assembled using electrospun nanofiber membranes have been significantly improved. The main materials used at this stage are polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polyimide (PI) and polyvinylidene fluoride (PVDF)

And its copolymers, etc., the electrostatic spinning device is mainly composed of high-voltage power supply, injector, spinneret and receiving plate. According to the placement method, it can be divided into two types: vertical and horizontal. The vertical type uses the gravity of the solution to perform electrospinning, while the horizontal type uses a propulsion pump to extrude the solution at a constant speed.

The main factors affecting the electrospinning process include the following points.

(1) The concentration and viscosity of the polymer play an important role in the shape, diameter and distribution of the fiber. As the concentration of the polymer solution increases, undesirable phenomena such as the unstable jet flow and the formation of liquid beads or fibers with uneven diameters will gradually disappear. When the critical concentration is reached, fibers of uniform diameter are formed, and as the concentration increases, the fiber diameter increases. But when the concentration is too high, the viscosity will increase further and the surface tension will increase, making it difficult to form fibers.

(2) With the increase of the voltage, the longer the jet is stretched, the thinner the fiber is formed; as the distance between the spinneret and the receiving plate increases, the fiber diameter decreases; the flow rate is proportional to the fiber diameter.

(3) The influence of environmental parameters (solution temperature, air temperature and humidity, air velocity, etc.). For example, the higher the temperature of the solution, the smaller the diameter of the spinning fiber. When the temperature is too high, the spinning fiber will quickly cool and solidify in the low-temperature air at the moment of ejection, and it is difficult to form a continuous fiber.

The PVDF or PVDF-HFP non-woven fabric membrane prepared by the electrostatic spinning method has the advantages of large specific surface area, high porosity, small pore size, and good wettability. In addition, two or more polymers can be dissolved in a co-solvent to form a homogeneous solution, and then spun to obtain a composite polymer fiber. Or adding inorganic materials to the fluoropolymer to form an inorganic composite diaphragm, which has good mechanical strength, thermal stability, electrolyte affinity and chemical stability. When used in liquid lithium-ion batteries, it can swell to form a gel electrolyte, which ensures the battery’s cycle performance and safety.

Fang et al. prepared PVDF polymer nanocomposite membranes modified with different contents of montmorillonite (MMT) by electrospinning. The results show that PVDF/5%MMT has the highest ionic conductivity (4.2mS/cm), the smallest interface impedance (97Ω), and the best electrochemical stability. The battery assembled with PVDF/5%MMT also showed higher capacity and more stable cycle performance.

The PVDF fiber membrane prepared by Liang et al. using the electrospinning and heat treatment process has high tensile strength, electrolyte wettability and thermal stability, and the room temperature ion conductivity can reach 1.35mS/cm. Compared with the Celgard 2400 separator, the heat-treated PVDF fiber membrane exhibits a higher electrochemical stability window and lower interface impedance, and the assembled LiFePO4/Li battery has better charge and discharge performance and stable cycle performance.

The PET nanofiber membrane prepared by Hao et al. using the electrospinning process has high tensile strength, thermal stability, electrochemical stability and ionic conductivity, etc., and is suitable for application in high-performance lithium-ion batteries.

The brittleness problem of PMMA can also be solved by blending with polyacrylonitrile (PAN). PAN has high solvation ability, and the interaction between its functional groups CN and CO and carbonate solvent molecules can enhance ionic conductivity. However, PAN has a low retention rate for the electrolyte, and leakage problems may occur when used alone, while PMMA has good compatibility with liquid electrolytes, and the two have complementary advantages, and are easy to prepare and low in cost. The PMMA/PAN nanofiber membranes with different components obtained by Mousavi et al. through electrospinning have higher porosity and electrolyte absorption. The excellent electrolyte absorption performance leads to higher ionic conductivity (7.02mS/cm).

Studies have shown that the fiber diameters of PAN nanofiber membranes, PVDF-PAN and PAN-PVC composite membranes prepared by electrospinning are uniform. They has rich pore structure and high mechanical strength, ion conductivity and electrolyte absorption rate, good thermal stability and electrochemical stability. Recently, Lee et al. used electrospinning method to conduct PAN at 230℃. Partial oxidation modification (oxy-PAN, Figure 2).

Electrospinning non-woven diaphragm
Figure 2 – Schematic diagram of the preparation process of oxy-PAN diaphragm

It is found that the modified oxy-PAN has high elastic strength, maintains thermal stability even under high electrolyte absorption, and has high ionic conductivity at room temperature. More importantly, it has good compatibility with lithium metal negative electrode and liquid electrolyte, which can extend the cycle life of the battery. However, in the pyrolysis state of PAN, its linear molecular chain undergoes dehydrogenation, denitrification and cyclization, forming a conductive trapezoidal ring structure and a chaotic graphite layer structure, which conducts both ions and electrons, so it cannot be used alone as a diaphragm.

The nano-level PI fiber membrane obtained by electrospinning technology has good porosity and electrolyte wettability. Qingdao Energy Storage Industry Technology Research Institute prepared uniaxial PI membrane and coaxial PI@PVDF-HP nano-fiber non-woven membrane membrane by electrostatic spinning method and imidization treatment, respectively. Figure 3 is the reaction equation of the prepared PI polymer and its precursor. Compared with polyolefin separators, the former two have excellent thermal stability and electrochemical performance.

Electrospinning non-woven diaphragm
Figure 3 – Reaction equation of PI and its precursor

P I polymer is also often used as a coating material to improve the performance of the base material to meet the requirements of battery separators. Studies have shown that the P I polymer is uniformly coated on the PE substrate to form a porous structure through a simple coating method, and the high temperature thermal stability of P I is used to effectively improve the thermal shrinkage of the PE non-woven fabric. In addition, the electrospinning process is used to uniformly coat PAA on the PET non-woven fabric, and then the PI/PET composite membrane formed by the imidization treatment has higher thermal stability and ionic conductivity. The PSA-SiO2 composite non-woven fabric and the coaxial PSA@PVDF-HFP core-shell structure nanofiber membrane prepared by Qingdao Energy Storage Industry Technology Research Institute using the electrostatic spinning process have excellent comprehensive properties such as thermal stability and interface stability .

At present, the basic research of laboratory electrospinning diaphragms mostly uses small electrospinning machines. A major problem that restricts the development of electrospinning diaphragms is the development of large-scale industrialized equipment. Internationally, the large-scale electrospinning equipment for nano spiders of Czech Elmarco company monopolizes the market. Qingdao Energy Storage Industry Technology Research Institute took the lead in developing large-scale electrospinning equipment prototypes in China, breaking foreign monopolies, and successfully achieving pilot production of polyimide electrospinning membranes, which will inject new ideas into my country’s electrospinning industry. Vitality.