As one of the core components of lithium ion battery, the diaphragm is mainly used to mechanically isolate the positive and negative electrodes of the battery, prevent them from short circuit due to direct contact, and allow the smooth migration of lithium ions while hindering the passage of electrons. Diaphragm is a material with high added value and high technical barrier. Although it does not directly participate in the electrochemical reaction, its structure and performance ultimately affect the actual working parameters of the battery, such as the use safety, cycle life and discharge capacity.

At present, the commercialized membranes are mainly polyolefins such as polyethylene (PE) and polypropylene (PP). However, there are still some problems with polyolefin membranes:

The low melting point of polyolefin and the poor thermal dimensional stability of the diaphragm result in a hidden danger to the safety of the battery at high temperature;

Polyolefin materials have low polarity, intrinsic hydrophobicity, and poor affinity with highly polar electrolyte;

The polyolefin materials are mainly made by dry or wet methods. The prepared membranes have low porosity (about 40%), which brings some resistance to the transmembrane transport of Li+.

Therefore, it is of great significance to develop high-quality high-temperature resistant battery separator.

Polyimide (abbreviated as PI) refers to a kind of imide ring (- CO-N-CO -) on the main chain, which is one of the best comprehensive properties. PI material is expected to replace traditional polyolefin material and become an ideal diaphragm material.

      

PI molecular formula

First of all, PI material has outstanding high temperature resistance. The long-term use temperature can reach 300 ℃, giving the diaphragm good thermal dimensional stability, and improving the high temperature use safety of the battery;

Secondly, PI molecular structure contains abundant polar groups, and the electrolyte has better wettability, which is helpful to improve the interface performance between the membrane and electrolyte and the overall performance of the battery;

Finally, PI material is flame-retardant and self extinguishing, providing a more powerful security guarantee for lithium ion batteries.

Traditional PI materials are insoluble in most organic solvents, and have very high melting temperature Tm and glass transition temperature Tg. This "insoluble and refractory" property greatly limits the film forming ability of PI materials. According to the literature, at present, the preparation methods of PI membrane mainly include template method, phase inversion method and electrospinning method.

1 Template method

The template method needs to first prepare the PI composite film containing porogen, and then use chemical etching, solvent dissolution or calcination to remove the porogen to obtain the PI porous film. Commonly used pore forming agents include metal oxides, hydroxides or non-metallic oxides.

He Xiangming's research group used nano SiO2 as porogen, and then used HF solution to remove it to obtain PI porous membrane. It was found that the PI porous membrane had excellent thermal dimensional stability, and no obvious shrinkage occurred at 180 ℃, while the commercial Celgard 2300 membrane had a shrinkage rate of 40% at 150 ℃.

2 Immersion sedimentation method

The immersion precipitation method is to scrape the polyamide acid (PAA) precursor solution or soluble PI solution onto the carrier (such as glass), immerse it in a non solvent, and use the polymer to separate the phase in its mixed solvent/non solvent solution. After the solvent is removed, the space occupied by the non solvent forms a pore. The pore structure of porous membrane can be simply and effectively controlled by changing the casting solution formula and process conditions.

The porous membrane of PI (PMDA-ODA) was prepared by the research group of Zhu Baoku, Zhejiang University, using polyamide acid (PAA) as precursor, combined with immersion phase precipitation method and thermal imidization method. The pore structure of PI porous membrane was adjusted by changing the amount of porogen PEG400, and the sponge like pore structure of submicron level was obtained. The prepared PI porous membrane showed excellent thermal dimensional stability: no obvious shrinkage occurred when heated at 180 ℃ for 1h. In addition, PI porous membrane has good electrolyte wettability, and the contact angle with electrolyte is only 9.3 °, which is far lower than that of commercial PP membrane (64.8 °).

Li et al. used the mixture of dibutyl phthalate (DBP) and glycerol (Gly) as the pore forming agent. Compared with mono porous agent, the PI porous membrane prepared by this method has more uniform pore structure and higher porosity. In particular, the battery assembled by it can still work normally after being heated at 140 ℃ for 1h.

3 Electrospinning

The basic principle of electrospinning technology is to apply high voltage static electricity to the polymer solution. When the charge repulsion force of the liquid surface is greater than its surface tension, Taylor cone is formed at the tip. The polymer solution ejected at high speed is stretched, deformed and split. With the volatilization of the solution, the polymer solution jet solidifies and finally deposits on the receiver to form a nanofiber film.

Electrospinning technology has many advantages, such as simple device, wide variety of applicable substances, macroscopic preparation, etc. It has become one of the effective ways to prepare PI membranes. The nanofiber membrane prepared by electrospinning technology has a 3D network structure and high porosity, providing a rich channel for the rapid migration of lithium ions in it. Compared with traditional non-woven fabrics, the fiber diameter of the nanofiber membrane is smaller (between a few nanometers and a few hundred nanometers) and the pore diameter is smaller, which is conducive to mitigating the self discharge phenomenon of the battery.

In addition, researchers also explored other film forming methods, such as grafting or copolymerization of unstable chain segments, wet paper making technology and irradiation etching method.

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

1 Surface coating modification method

Surface coating modification refers to the method of realizing modification by depositing or coating a functional layer on the surface of the base film. For example, mining

PI nanofiber membrane was coated with Al2O3 nanoparticles. The surface of Al2O3 nanoparticles contains rich polar groups, which is conducive to improving the affinity between PI nanofiber membrane and electrolyte and reducing the interface impedance of the battery.

After 200 cycles, the interface impedance of the battery assembled by coating PI nanofiber membrane with Al2O3 was 45.8 Ω, which was lower than that of pure PI nanofiber membrane (51.1 Ω) and PP membrane (63.4 Ω). Under 10C high rate cycle, the discharge capacity retention rate of the battery assembled is 78.91%, which is higher than that of pure PI nanofiber membrane (68.65%) and commercial PP membrane (18.25%).

Shi et al. successfully constructed a PI/PE composite nanofiber membrane with thermal shutdown function by coating a layer of PE particles on the surface of the PI nanofiber membrane. The composite nanofiber membrane showed excellent thermal dimensional stability: when heated at 230 ℃ for 0.5h, the shrinkage was less than 10%. At the same time, when the battery temperature is close to the PE melting point, the PE part will melt and close the micropores, increasing the internal resistance of the battery, reducing the current passing through, and thus preventing further chemical reactions.

The coating modification method can realize the functional modification of the membrane, but there are still some shortcomings: on the one hand, the introduction of the coating layer increases the quality of the membrane and reduces the energy density of the battery; Secondly, the coating will bring a certain degree of hole plugging effect, increasing the resistance of Li+migration; Finally, when the interaction between the coating and the substrate is weak, the interface resistance is increased, and there is a risk of falling off during long-term use.

2 Blending modification method

Blending is also a simple and effective high-performance modification method, which only requires the introduction of modifiers before or during the film formation.

Shayapat et al. prepared PI hybrid nanofiber membrane by blending polyamidic acid ammonium salt (PAAS) with SiO2 and Al2O3 nanoparticles. Compared with the coating modification, the blending modification is based on a single fiber and retains the 3D network structure of the nanofiber membrane, avoiding the problem that the formation of a multi-layer structure leads to the reduction of the ionic conductivity of the membrane. It was found that the porosity and liquid absorption of PI hybrid nanofiber membrane were as high as 90% and 790%, respectively.

Chen et al. prepared PI/polyvinylidene fluoride hexafluoroisopropene copolymer (PVDF-HFP) composite nanofiber membrane by cross electrospinning with four nozzles. Among them, PI material provides good thermal dimensional stability and ensures the safety of battery at high temperature. PVDF-HFP material melts at a lower temperature to increase the adhesion between nanofibers and improve the mechanical properties of composite nanofiber membranes.

3 gel filling method

The gel filling method is to inject gel polymer electrolyte into the internal pores of the PI diaphragm to improve the liquid absorption and retention capacity of the PI diaphragm. For example, by combining the characteristics and advantages of PI non-woven fabric and 2-acrylamide-2-methylpropane sulfonic acid (AMPS), the PI non-woven fabric is filled with gel by using the in-situ polymerization product PAMPS of AMPS.

4 Crosslinking modification method

In the nanofiber membrane prepared by electrospinning, because there is no interaction between fibers, the mechanical strength of the nanofiber membrane is low, and it is difficult to meet the tension requirements of the membrane during battery assembly. In order to improve the mechanical strength of the nanofiber membrane, the researchers prepared cross-linked PI nanofiber membrane by means of thermal micro crosslinking, solution micro crosslinking, alkali etching and coaxial spinning.

Huang et al. designed and synthesized a series of PAA nanofiber membranes with flexible units in the main chain. In the process of thermal amination of such PAA nanofiber membranes, thermal micro crosslinking phenomenon can be generated between fibers to form PI nanofiber membranes with cross-linked structure. The degree of crosslinking can be effectively controlled by controlling the time and temperature of heat treatment. Due to the introduction of the bonding point, the strength of the nanofiber membrane is greatly improved.

At present, only DuPont and Jiangxi Xiancai have a small number of polyimide diaphragm related products on the market. In 2010, DuPont announced the development of Energain PI diaphragm and the construction of a factory in Virginia, USA. It is reported that its diaphragm can meet the use requirements of the battery at higher temperatures, increase the battery power by 15% - 30%, and extend the battery's endurance.

It is estimated that the cost of raw materials for DuPont to produce PI diaphragms is about 2.8 yuan/square meter, while the price of PI diaphragms produced by DuPont in South Korea is equivalent to 80 yuan/square meter, and sales to China are prohibited.

With its outstanding heat resistance and good electrolyte wettability, PI material has great competitive advantages, especially in ensuring the high temperature use safety of batteries. It is currently a more researched diaphragm material. However, the "insoluble and refractory" characteristics of PI materials limit their poor film forming processability.

The template method has the problems of incomplete removal of pore forming agent and low degree of imidization. The immersion precipitation method can avoid the defects caused in the stretching process, but the interaction between PAA precursor and solvent is often strong, the separation process takes a long time, and the industrial application prospect is limited. The nanofiber membrane prepared by electrospinning also has the problems of poor uniformity and low mechanical strength.

In addition, the spinning process is more demanding to the environment. Therefore, in order to improve the industrialization process of PI membrane, in addition to focusing on the research and development of the molecular structure design and modification mechanism of PI, the research and development of the film forming technology of PI membrane and the supporting production equipment and process also need to be strengthened. Secondly, expanding raw material capacity, optimizing process flow, improving processing efficiency and other ways to reduce the production cost of PI diaphragm are also key issues to realize the rapid promotion and application of PI diaphragm.