1. Overview of polyimide and introduction of application fields

1. What is polyimide (PI) Polyimide (PI) is a class of aromatic heterocyclic polymer compounds containing imide rings (-CO-N-CO-) on the backbone, which is one of the best heat-resistant varieties in engineering plastics, and is widely used in aviation, aerospace, microelectronics, nano, liquid crystal, laser and other fields. 2. The downstream application field of polyimide has been widely used in many basic industries and high-tech fields due to its good high and low temperature resistance, environmental stability, mechanical properties and excellent dielectric properties. Flexible circuit boards: Copper foil substrates (FCCL) for flexible circuit boards and protective layers for flexible circuit boards (FPCB) are the most common applications and the largest market. Insulation materials: electrical and electronic equipment insulation, high-temperature resistant wires and cables, electromagnetic wires, high-temperature resistant wires, insulating composite materials, etc. Electronics industry: printed circuit board motherboards, mobile phones, mobile phones, lithium batteries and other products. Generally speaking, PI films below 25 m are commonly used. Semiconductor applications: passivation layer and buffer inner coating of microelectronics, multi-layer metal interlayer dielectric materials, important substrates for optoelectronic printed circuit boards. In the field of amorphous silicon solar cells: transparent PI film can be used as a soft solar cell backplane. The ultra-thin PI film can be applied to solar sails (baresails).

2. Polyimide separator

1. Why polyimide can become a raw material for lithium battery separator First of all, PI material has outstanding high temperature resistance, and the long-term use temperature can reach 300 °C, which gives the separator good thermal dimensional stability and improves the safety of the battery at high temperature; Secondly, the PI molecular structure contains abundant polar groups, and the electrolyte has better wettability, which is helpful to improve the interface performance between the separator/electrolyte and the comprehensive performance of the battery. Finally, the PI material is flame retardant and self-extinguishing, providing a stronger safety guarantee for lithium-ion batteries. 2. The method for producing separators by polyimide (1) The template method is a method of preparing a PI porous membrane by mixing polyamic acid with a pore-forming agent with a certain structural size and incompatible with polyamic acid as a template, mixing polyamic acid with a pore-forming agent, and obtaining a pore-forming agent/polyimide composite film after imidization, and then removing the pore-forming agent with a template remover. Porous agents can be metals, metal oxides, non-metal oxides, hydroxides, carbonic acid compounds, etc. Pore-forming agents can also be substances with pyrolysis properties or high-temperature volatile properties. PI porous membranes are obtained by the decomposition or volatilization of pore-forming agents during thermal imidization. The biggest advantage of the template method is that the structure and size of the micropores can be controlled by changing the particle size of the pore-forming agent, but the mechanical properties of the prepared separator may be poor due to the incomplete removal of the pore-forming agent and the influence of the degree of imidization.

(2) Immersion precipitation methodThe immersion precipitation method is to scrape the polyamic acid (PAA) precursor solution or soluble PI solution on the carrier (such as glass, etc.), immerse it in a non-solvent, and use the polymer to phase separation in its solvent/non-solvent mixed solution. When the solvent is removed, the space occupied by the non-solvent forms a pores. By changing the casting solution formulation and process conditions, the pore structure of the porous membrane can be simply and effectively controlled.

(3) The basic principle of electrospinning technology is to apply high-voltage static electricity to the polymer solution, and when the charge repulsion on the surface of the liquid is greater than its surface tension, a Taylor cone is formed at the needle mouth. The polymer solution ejected at high speed is stretched, deformed, and split, and with the volatilization of the solution, the polymer solution jet solidifies, and finally deposits on the receiver to form a nanofiber membrane. Electrospinning technology has many advantages, such as simple device, wide variety of applicable substances, and macroscopic preparation, and has become one of the effective ways to prepare PI separators. The nanofiber membranes prepared by electrospinning technology have a 3D network structure and high porosity, which provide abundant channels for the rapid migration of lithium ions. Compared with traditional nonwovens, nanofiber membranes have finer fiber diameters (between a few nanometers and hundreds of nanometers) and smaller pore sizes, which is conducive to mitigating the self-discharge phenomenon of batteries. In addition, the researchers have also explored other film-forming methods, such as grafting or copolymerization of unstable chain segments, wet papermaking technology, and irradiation etching. (4) Other methodsBecause PI separators are difficult to process and mass-produce at present, the common methods for preparing PI porous membranes are not practicable, so scholars have also explored other methods for preparing PI porous membranes, such as sintering method, irradiation etching method, grafting or copolymerization unstable chain segment method, etc.

3. Research on polyimide separator modification technology

The PI nanofiber membranes prepared by electrospinning technology have high porosity and good electrolyte wettability, but high porosity will also reduce the mechanical properties of the separator, which will put pressure on the assembly and use of batteries. On the other hand, the large pore size of the PI nanofiber membrane also brings the problem of self-discharge of the battery. In view of this, the researchers carried out a series of high-performance modification works on PI separators, especially PI nanofiber membranes.

1. Surface coating modification methodSurface coating modification refers to the method of deposition or coating a functional layer on the surface of the base film to achieve modification. For example, Al2O3 nanoparticles are used to coat and modify PI nanofiber membranes. The surface of Al2O3 nanoparticles is rich in polar groups, which is conducive to improving the affinity between the PI nanofiber membrane and the electrolyte and reducing the interfacial impedance of the battery. The coating modification method can realize the functional modification of the separator, but there are still some disadvantages: on the one hand, the introduction of the coating layer increases the quality of the separator and reduces the energy density of the battery; Secondly, the coating layer will bring a certain degree of pore plugging effect, which will increase the resistance of Li+ migration. Finally, when the interaction between the coating layer and the substrate is weak, the interfacial resistance is increased, and there is a risk of detachment during long-term use.

2. Blending modification method is also a simple and effective high-performance modification method, which only needs to introduce modifiers before or during film formation.

3. Gel filling methodThe gel filling method is to inject a gel polymer electrolyte into the internal pores of the PI separator to improve the liquid absorption and retention ability of the PI separator. For example, by combining the characteristic advantages of PI non-woven fabric and 2-acrylamide-2-methylpropanesulfonic acid (AMPS), the in-situ polymerization product of AMPS is used to modify the PI non-woven fabric by gel filling.

4. In the nanofiber membrane prepared by electrospinning by crosslinking modification method, due to the absence of interaction between fibers and fibers, the mechanical strength of the nanofiber membrane is low, which is difficult to meet the tension requirements of the separator in the battery assembly process. In order to improve the mechanical strength of nanofiber membranes, PI nanofiber membranes with cross-linked structures were prepared by thermoevogenous micro-cross-linking, lyso-induced micro-cross-cross-linking, lye etching and coaxial spinning.

Fourth, the future prospect of polyimide separator

With the development of electronic information and new energy industry, higher requirements have been put forward for the safety of lithium-ion batteries, especially power batteries for new energy vehicles. Although the separator does not actually participate in the energy conversion process of lithium-ion batteries, it still becomes one of the key materials that determine the performance of batteries. From the basic principle of electrochemical energy conversion, the electronic barrier effect of the separator is a necessary condition for the establishment of chemical power supply. From the perspective of practical application, the separator is an important link to prevent thermal runaway of the battery and determine the safety performance of the battery.

Therefore, the requirements for the high temperature resistance of the power lithium battery separator are also correspondingly improved, and many power lithium battery manufacturers require the separator to have a high temperature heat shrinkage performance of 150 °C. PI separator is regarded as the next generation of separator material due to its excellent thermal stability and good electrolyte liquid absorption and liquid retention, providing better safety guarantee for power batteries. At present, although the research on PI diaphragm at home and abroad has achieved more phased results, the research results are mostly in the laboratory research stage. At the same time, compared with the existing polyolefin separator, its mechanical properties are poor, the processing cost is high, and there are still many problems in the equipment and process required for mass production, so there is still a long distance from industrial production. It is suggested that the corresponding scientific research institutes, equipment processing enterprises, separator manufacturers and separator application enterprises should carry out cooperative research and research through the method of "production, learning, research and application", focusing on the formulation and modification mechanism of PI separator, supporting production equipment and technology, and the application of PI separator in lithium batteries, so as to shorten the development cycle of PI separator and accelerate the industrialization process of PI separator.