What progress has been made in the research of polyimide in the field of lithium battery separators?

　　In the case of battery overcharging or improper use, the polyolefin separator will shrink or melt when the battery temperature exceeds 160°C due to overheating inside or outside the battery, causing the positive and negative electrodes of the battery to contact and short-circuit, causing the battery The danger of burning or explosion seriously threatens the lives of users.

　　In the case of battery overcharging or improper use, the polyolefin separator will shrink or melt when the battery temperature exceeds 160°C due to overheating inside or outside the battery, causing the positive and negative electrodes of the battery to contact and short-circuit, causing the battery The danger of burning or explosion seriously threatens the lives of users. Therefore, the power lithium battery requires that the diaphragm used in it not only has the basic performance of the ordinary diaphragm, but also has more excellent high temperature resistance. Many power lithium battery manufacturers require the diaphragm to have a high temperature heat shrinkage performance of 150 ℃. In conventional polyolefin membranes, the melting point of polyethylene membrane is 130°C, and the membrane will melt if it exceeds the melting point; while the melting point of polypropylene is 163°C, when the temperature reaches 150°C, the membrane will shrink by more than 30%. Therefore, the traditional polyolefin separator cannot meet the requirements of the power lithium battery, and the traditional polyolefin separator has poor liquid absorption and liquid retention, which increases the internal resistance of the battery.

　　Polyimide (PI) has good thermal stability, chemical stability and outstanding mechanical properties [3-5], its long-term use temperature can be as high as 300 ℃, it is the best comprehensive performance of the film-type insulating material [6] ]. Compared with polyolefin separators, PI has a better affinity for lithium ion electrolytes because of its polar groups, so it is regarded as the next generation of lithium ion battery separator materials [7-9].

　　1 Application of polyimide in diaphragm materials

　　There are two main applications of PI in battery separators. One is to use PI to modify the separators of other substrates to prepare composite separators to improve the thermal stability of the substrate separators, and the other is to use PI alone to prepare PI separators. , The following describes the research status of these two methods in the field of diaphragm.

　　1.1.1PI surface modified composite diaphragm

　　The traditional polyolefin separator has poor thermal dimensional stability. When the battery temperature is high, it will shrink or even melt, causing the battery to short-circuit due to the contact between the positive and negative electrodes, causing fire or explosion. Therefore, researchers have improved the thermal stability of polyolefin membranes by coating ceramics [10-12] or composite PI on the surface of polyolefins [13-15]. There are two main methods of using PI to improve the thermal dimensional stability of the substrate membrane. One is to modify the substrate membrane with PI solution, and the other is to modify the substrate membrane in the way of PI porous membrane. The following two methods are introduced one by one.

　　1.1.1.1PI solution surface modification composite membrane

　　"When the PI solution is used to modify the surface of the membrane with poor thermal dimensional stability, the composite method of PI and this type of membrane includes coating, electrospinning, etc. The introduction form of PI can be polyamic acid or polyimide. Because PI needs to be imidized at a high temperature, its introduction method needs to be determined according to the thermal stability of the composite membrane. Hu Xuyao et al. [13] dissolved self-made polyimide in N-methylpyrrolidone, and added nano-SiO2 particles to obtain a PI coating solution. The coating solution was coated on both sides of a PP diaphragm to prepare nano-SiO2/PI. Coated modified polypropylene diaphragm. The thermal shrinkage rate of the PP separator modified by PI coating is reduced from 27% to 1.8% at 150℃, the dimensional stability is obviously improved, and the safety of the battery is improved; and the film is under the same charging and discharging conditions. The specific capacity of its first discharge has also been increased from the original 138mAh/g to 140mAh/g. Huang Shuishou et al. [14] electrospinning the prepared polyimide acid solution, using a PET non-woven fabric with a high melting point as the substrate, and finally holding it at 220-250°C for 1 to 3 hours to prepare a PI/PET composite film. The composite membrane has the characteristics of high mechanical strength, high porosity, strong liquid absorption and liquid retention capacity, and good thermal stability. Wu Shuqiu et al. [15] dissolve the prepared PI in DMAc, use a uniaxially stretched polypropylene membrane as the receiving matrix, and make the transverse direction of the polypropylene membrane consistent with the rotation direction of the drum, and prepare it by electrospinning. The PI/PP composite diaphragm improves the transverse tensile strength and the overall puncture strength of the uniaxially stretched PP diaphragm, and improves the thermal stability and safety of the polypropylene diaphragm. This method can improve the thermal stability of the traditional diaphragm, but the thickness of the diaphragm is increased, and the increase in the thickness of the diaphragm will affect the charge-discharge rate and cycle performance of the battery.

　　1.1.1.2PI porous membrane modified composite membrane

　　When PI is used to improve the substrate membrane with poor thermal dimensional stability, the modification method of PI porous membrane can also be used [16-19]. The composite of PI porous film and polyolefin can be bonded by an adhesive in the form of a film, or another film can be coated in the form of a solution before the film is formed. Yang Weiguo et al. [16] coated the PI porous membrane with a binder containing pore-forming materials, and after removing the pore-forming materials, a PI/polyolefin composite membrane was prepared by thermocompression bonding with the polyolefin porous membrane. The composite membrane The average pore diameter is 68～290nm, which has good air permeability and mechanical strength. After 500 times of charging and discharging, the remaining electricity reaches 78%～90%. There is no short circuit and explosion after treatment at 150～180℃. The safety of the battery is greatly improved. Ningbo Changyang Technology Co., Ltd. discloses a method for preparing PI/polyolefin composite diaphragm [17]. The method is to coat a polyamic acid solution containing pore-forming substances on a glass plate, and obtain a polyamide-containing compound after imidization. Then the polyolefin masterbatch containing the pore-forming material is melted and coated on the PI film containing the pore-forming material. After removing the pore-forming material, a composite diaphragm of the PI porous film and the polyolefin porous film is obtained. The composite diaphragm has only a two-layer structure, and the bonding force between the layers is strong, and it is not easy to fall off. The pore diameter of the composite diaphragm is 60～250nm, and the porosity is 30%～60%. After 500 charge and discharge cycles, the remaining power is 87%～90%, and put the charged battery in a 400℃ oven for 30min without explosion.

　　1.2PI single layer diaphragm

　　In addition to being used to modify polyolefin separators with poor thermal stability, PI can also be used alone to prepare lithium-ion battery separators. In the preparation methods of various PI separators, electrospinning, template method, and phase inversion are used. There are many researches on the method, and these methods are briefly introduced below.

　　1.2.1 Electrospinning method

　　Electrospinning is a new type of technology in which polymer solution or melt is stretched into ultra-fine fibers at the top of the capillary Taylor cone under the action of a high-voltage electric field. Electrospinning is a recognized simple and effective method for preparing ultra-thin nanofiber membranes [20]. The fiber membranes prepared by electrospinning have the characteristics of small fiber diameter, large surface area, high porosity, and consistent fineness. Since Reneker first proposed that the electrospinning technology can be applied to the preparation of PI nanofibers in 1996, people have done a lot of research on the preparation of PI membranes by electrospinning [21-31]. Zhou Jinhui et al. [24] prepared PI nanofiber membranes by electrospinning with a porosity as high as 92%, and studied the electrochemical cycling performance of the membranes under two commonly used charge cut-off voltages (4.2V and 4.4V). The results show that the performance of the membrane, such as liquid absorption rate, capacity retention rate, and specific capacity attenuation, are significantly better than Celgard2400 membrane, and the capacity retention rate at 2.8-4.4V is as high as 91.6%. Anping [25] used electrospinning to prepare a PI lithium-ion battery separator, which has a high porosity (>90%) and good electrolyte wettability and liquid retention. Compared with the traditional polyolefin membrane with a shrinkage rate of 20% at 150°C, the membrane has outstanding thermal dimensional stability, and its size does not change significantly at high temperatures up to 500°C. At the same time, the diaphragm has excellent electrochemical performance. It still maintains a discharge rate of 33.6% under high-rate discharge conditions as high as 28.8C, while the discharge rate of the polyolefin diaphragm under 16C discharge conditions is only 8.48%.

　　However, the traditional PI nanofiber nonwoven membrane is prone to swelling in the electrolyte, the swelling size of the membrane is difficult to control, and because there is no strong interaction between the fibers, the mechanical strength of the nanofiber membrane is poor [26-27] , Therefore, an electrospun PI membrane with a cross-linked structure appeared. Cross-linking methods include thermal cross-linking, lyotropic cross-linking, lye etching, etc. [27-29]. Huangsu grape [27] prepared PI nanofiber membranes with micro-crosslinked structure by thermally induced crosslinking and lyotropic crosslinking methods, which enhanced the interaction between the fibers and improved the loose overlap and openness of the fibers. The pore structure (as shown in Figure 1) makes the tensile strength of the PI fiber membrane increased from the original 14.76MPa to 76.10MPa. Among the many methods for preparing PI diaphragms, electrospinning is the more commonly used one. The PI diaphragms introduced by DuPont and Jiangxi Xiancai are all prepared by electrospinning [30-31]. Although the electrospinning method has many advantages, its output is low, and it has strict requirements on the temperature and humidity of the spinning solution and the environment.

　　1.2.2 Template method

　　The template method uses a porogen that has a certain structural size and is incompatible with polyamic acid as a template. After mixing the polyamic acid and the porogen, the porogen/polyimide composite film is obtained after imidization. , And then use the template remover to remove the porogen to prepare the PI porous membrane. The porogen can be metal [6], metal oxide [32], non-metal oxide [33-34], hydroxide [35], carbonic acid compound [7] and the like. Hu Xuyao et al. [36] prepared a nano-SiO2 doped PI composite film, and then used HF solution to remove the nano-SiO2 to obtain a PI porous film. Compared with the thermal shrinkage rate (40%) of the Celgard2300 diaphragm at 150 ℃, the PI porous film No significant shrinkage will occur below 180°C. Huang Siyu et al. [7] pointed out that the PI porous film prepared with the above porogen as a template is brittle and has poor mechanical properties. Taking the CaCO3 porogen as an example, they studied that the PI porous film is brittle when CaCO3 is used as the porogen. The infrared spectrum test results show that the addition of CaCO3 makes the imidization degree of PI only reach 80%, which is the main reason for the poor mechanical properties of the porous membrane.

　　The porogen can also be a substance with high-temperature decomposition characteristics or high-temperature volatilization characteristics [37-39]. Through the decomposition or volatilization of the porogen in the thermal imidization process, the PI porous membrane is obtained. Liu Jiugui et al. [39] used polyurethane as a porogen to prepare a polyurethane/polyamic acid mixed solution by in-situ polymerization. After coating the polyurethane/polyamic acid film, it was subjected to thermal imidization treatment to degrade the polyurethane during the imidization process. A PI porous membrane with long strip-shaped nanopores was prepared. But this method is difficult to completely remove the porogen, resulting in uneven texture of the PI porous membrane [40]. 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 porogen. However, it is possible that the mechanical properties of the prepared diaphragm are poor due to the incomplete removal of the porogen and the influence of the degree of imidization.

　　1.2.3 Phase conversion method

　　"Phase inversion method" refers to a polymer solution of a certain composition, through physical methods to change the thermodynamic state of the solution, so that the homogeneous polymer solution is phase separated, and finally transformed into a three-dimensional macromolecular network type gel structure. Specifically, the preparation methods of PI porous membranes include thermally induced phase inversion method [41-43], high humidity induced phase inversion method [44-45], and immersion precipitation phase inversion method [46-49]. Among them, the immersion precipitation phase inversion method is a more commonly used method. The process is to scrape the polyamic acid solution or PI solution on the support, and then immerse the polyamic acid or PI non-solvent to make the solvent and non-solvent The solvent exchange occurs, and after reaching a certain degree, the liquid-solid phase separates. After the solvent is removed, the space occupied by the non-solvent forms the pores of the PI film. Yang Weiguo et al. [1] prepared a PI membrane with a porosity of 30% to 60% by the immersion precipitation phase inversion method. The membrane has an average pore diameter of 0.02～0.15μm, no closed pores, and good air permeability (air permeability of 150s/100cc～ 300s/100cc), low thermal shrinkage, good heat resistance, and dimensional stability can be maintained at 300°C, which greatly increases the heat resistance temperature of current commercial separators and improves the safety of batteries. TNGUYEN et al. [46] prepared a porous PI membrane by the immersion precipitation phase inversion method, and polymerized the porous membrane by filling the monomer of the perfluorosulfonic acid proton exchange membrane (Nafion), which can be directly used for methanol fuel. The PI/Nafion composite membrane of the battery. Compared with Nafion membrane, the composite membrane has higher tensile strength (4 times higher than Nafion membrane), lower methanol permeability (1/80 of Nafion membrane) and higher proton conductivity. WangHJ et al. [49] prepared a PI diaphragm with a porosity of 47% to 87% by controlling the solid content and adopting the impregnation precipitation phase inversion method. The glass transition temperature of the diaphragm is as high as 274℃. Is 1%. In addition, due to the polarity of the PI membrane surface, it has outstanding liquid absorption, with a liquid absorption rate of 190% to 378%, while the liquid absorption rate of Celagrd 2400 is only 116%. Under the same charge and discharge conditions, the separator and Celgard separator have similar discharge capacity (129-131mAh/g).

　　1.2.4 Other methods

　　Because PI diaphragms are currently difficult to process and mass-produce, and the common methods for preparing PI porous membranes are not practical, scholars have also explored other methods for preparing PI porous membranes, such as sintering [50-51] and radiation etching. [52], grafting or copolymerization of unstable segment method [53-55], etc. HMUNAKATA et al. [50] filtered the silica gel crystals to obtain a membrane with deposited silicon ions. After sintering the membrane at a high temperature at 1100°C, a template with a regular arrangement of silicon ions was obtained. The polyamic acid solution was poured between the silicon templates at high temperature. After imidization, a Si/PI composite film is obtained, and the silicon is etched with hydrofluoric acid to obtain a PI porous film. After the membrane was directly used in a methanol fuel cell, it was found that the permeation of methanol can be inhibited by changing the size of the pores. The proton conductivity/methanol permeability is 1.2×105Scm-3s, which is an order of magnitude higher than that of the Nafion membrane. Cui Qingchen et al. [52] proposed a radiation etching method to prepare PI porous membranes. The process uses PI thin films as substrates, irradiates the PI surface with high-energy heavy ions, and then uses ultraviolet rays or solvents to irradiate the PI thin films. For sensitization, the sensitized PI film is immersed in a NaOH-KMnO4 solution for chemical etching to obtain a porous PI film with a pore size of 0.01-3 μm. KRCARTER et al. [54] introduced thermally unstable polypropylene oxide into the PI chain of the perfluorinated skeleton, imidized it in an inert atmosphere at 310°C, and then subjected it to heat treatment in an oxygen environment at 250°C to make the heat Unstable aggregation