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

Traditional polyolefin membrane under the condition of the battery overcharge or improper use, the battery internal or external to make batteries overheat temperature exceeds 160 ℃, polyolefin diaphragm will shrink or fusing, lead to the battery positive and negative electrode contact and short circuit, are at risk of battery from the combustion or explosion, a serious threat to the life safety of the user. Therefore, the power lithium battery requires that the diaphragm used in addition to the basic performance of the ordinary diaphragm should also have more excellent high temperature resistance, and many power lithium battery manufacturers require the diaphragm to have a high temperature heat shrinkage performance of 150 ° C. In the conventional polyolefin separator, the polyethylene separator has a melting point of 130 ° C, and the separator is melted beyond the melting point; while the melting point of the polypropylene is 163 ° C, when the temperature reaches 150 ° C, the separator shrinks by more than 30%. Therefore, the conventional polyolefin separator cannot meet the requirements of the power lithium battery, and the conventional polyolefin diaphragm 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, its long-term use temperature can be as high as 300 ° C, is the best comprehensive film insulation material today. Compared with polyolefin separators, PI has a good lithium ion electrolyte affinity because of its polar group, so it is considered as a next-generation lithium ion battery separator material.

1 polyimide in the diaphragm material

There are two main applications of PI in battery separators. One is to modify the separator of other substrates to prepare composite membranes by PI, to improve the thermal stability of the substrate separator, and the other is to prepare PI membranes by using PI alone. The following is a brief introduction to the research of these two methods in the field of separators.

1.1.1 PI surface modified composite diaphragm

Conventional polyolefin separators have poor thermal dimensional stability, and shrinkage or even melting occurs when the battery temperature is high, causing the battery to short-circuit due to positive and negative contact, thereby causing fire or explosion. Therefore, researchers have improved the thermal stability of polyolefin separators by coating ceramic surface or composite PI on the polyolefin surface. There are two main methods for improving the thermal dimensional stability of the substrate separator by using PI. One is to modify the substrate film with PI solution, and the other is to modify the substrate film by means of PI porous film. The following two methods are introduced one by one.

1.1.1.1 PI solution surface modified composite film

When the surface of the separator having poor thermal dimensional stability is surface-modified with a PI solution, the composite of PI with such a separator includes coating, electrospinning, and the like. The introduction form of PI may be a polyamic acid or a polyimide. Since PI needs to be imidized at a high temperature, the manner of introduction depends on the thermal stability of the composite membrane. Xuyu Hu dissolved the self-made polyimide in N-methylpyrrolidone, and added the nano-SiO2 particles to obtain the PI coating liquid. The coating solution was coated on both sides of the PP membrane to prepare nano-SiO2/PI. Coating modified polypropylene diaphragm. The PP film modified by PI coating has a heat shrinkage rate reduced from the original 27% to 1.8% at 150 ° C, the dimensional stability is significantly improved, and the safety of the battery is improved; and the film is in the same charge and discharge condition. Next, its first discharge specific volume has also increased from the original 138mAh / g to 140mAh / g. Shoushou Huang electrospin the prepared polyimide acid solution, using a PET nonwoven fabric with a high melting point as a substrate, and finally incubated at 220-250 ° C for 1-3 h to prepare a PI/PET composite membrane. The composite membrane has the characteristics of high mechanical strength, high porosity, strong liquid absorption and liquid retention ability, and good thermal stability. Shuqi Wu prepared the PI dissolved in DMAc, using a uniaxially stretched polypropylene separator as the receiving substrate, and making the transverse direction of the polypropylene separator coincide with the rotating direction of the drum, and preparing by electrospinning. The PI/PP composite separator improves the transverse tensile strength and overall puncture strength of the uniaxially stretched PP separator, and improves the thermal stability and safety of the polypropylene separator. This method can improve the thermal stability of the conventional diaphragm, but increases the thickness of the diaphragm. The increase in the thickness of the diaphragm affects the charge and discharge rate and cycle performance of the battery.

1.1.1.2 PI porous membrane modified composite membrane

When PI is used to improve the substrate membrane with poor thermal dimensional stability, the porous PI film can also be modified [16-19]. The composite of PI porous film and polyolefin can be bonded by adhesive in the form of film, or another film can be coated in the form of solution before the film is formed. Weiguo Yang on PI porous membrane, such as coating binder containing pore forming material, after removing the pore forming material, and polyolefin porous membrane was prepared by means of hot pressing and PI/polyolefin composite membrane, the composite diaphragm aperture to an average of 68 ~ 290 nm, has good air permeability and mechanical strength, in 500 after the charge and discharge, the remaining power of 78% ~ 78%, under 150 ~ 180 ℃, no short circuit and explosion phenomenon after processing significantly improves the security of the battery. Technology co., LTD. Ningbo changyang county public a kind of preparation methods of preparation of PI/polyolefin composite membrane, the method is to polyamide acid solution containing pore forming material coated on the glass plate, after the imine get PI film containing pore forming material, and then melting of polyolefin masterbatch containing pore forming material coated on PI film containing pore forming material, eliminate pore forming material and get PI porous membrane with polyolefin film composite membrane, only two of the composite diaphragm structure, strong adhesive force between layer and layer, fall off not easily, the composite of the diaphragm aperture is 60 ~ 250 nm, porosity of 30% ~ 30%, After 500 times charging and discharging cycle, the remaining power is 87% ~ 90%, and will charge the battery in dealing with 30 min at 400 ℃ oven, no explosion happened.

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 various preparation methods of PI membranes, electrospinning, template method, there are many methods to study, and the following methods are briefly introduced.

1.2.1 Electrospinning

Electrospinning is a novel technique in which a polymer solution or a melt is stretched into a microfiber at the top of a capillary Taylor under the action of an electric field force by a high-voltage electric field. Electrospinning is a well-known and simple method for preparing ultra-thin nanofiber membranes. [20] The fiber membrane prepared by electrospinning has the characteristics of small fiber diameter, large surface area, high porosity and consistent fineness. Since Reneker first proposed in 1996 that electrospinning technology can be applied to the preparation of PI nanofibers, many studies have been done on the preparation of PI membranes by electrospinning. Jinhui Zhou prepared PI nanofiber separator by electrospinning method with a porosity of up to 92%, and studied the electrochemical cycle performance of the separator under two common charge cut-off voltages (4.2V and 4.4V). The results show that the membrane's liquid absorption rate, capacity retention rate, specific capacity attenuation and other properties are significantly better than Celgard 2400 diaphragm, and the capacity retention rate at 2.8 ~ 4.4V is as high as 91.6%. Anping [25] prepared a PI lithium ion battery separator by electrospinning method, which prepared a separator with high porosity (>90%) and good electrolyte wettability and liquid retention. The separator has outstanding thermal dimensional stability compared to a conventional polyolefin separator having 20% shrinkage at 150 ° C, and its size does not change significantly at high temperatures up to 500 ° C. At the same time, the separator has excellent electrochemical performance, and maintains a discharge rate of 33.6% under high-rate discharge conditions up to 28.8C, while the discharge rate of the polyolefin separator under 16C discharge conditions is only 8.48%.

However, the conventional PI nanofiber nonwoven membrane is prone to expansion in the electrolyte, the swelling size of the membrane is difficult to control, and the mechanical strength of the nanofiber membrane is poor because there is no strong interaction between the fibers.  Therefore, an electrospun PI separator having a cross linked structure appeared. Cross-linking methods include thermal cross-linking, lyotropic cross-linking, lye etching, etc. Qiyan Huang prepared a PI nanofiber membrane with micro-cross linked structure by thermal cross-linking and lyotropic cross-linking method, enhanced the interaction between fibers, improved the loose overlap between fibers and opened the pore structure increases the tensile strength of the PI fiber membrane from the original 14.76MPa to 76.10MPa. Among the various methods for preparing PI separators, electrospinning is a commonly used one. The PI separators introduced by DuPont and Jiangxi Xiancai are all prepared by electrospinning. Although the electrospinning method has many advantages, the method has low yield and is relatively demanding on the temperature and humidity of the spinning solution and the environment.

1.2.2 Template method

The template method is a method that takes pore-forming agent with certain structure size and incompatible with polyamide acid as template, mixes polyamide acid with pore-forming agent, obtains the pore-forming agent/polyimide composite film after imidization, and then removes the pore-forming agent with template removing agent to prepare PI porous film. The porous may be a metal, a metal oxide, a non-metal oxide, a hydroxide, a carbonic acid compound, or the like. Xuyao Hu prepared a nano-SiO2-doped PI composite membrane, and then removed the nano-SiO2 by HF solution to obtain a PI porous membrane. Compared with the thermal shrinkage (40%) of the Celgard 2300 separator at 150 ° C, the PI porous membrane No significant shrinkage occurs below 180 °C. Siyu Huang pointed out that the PI porous membrane prepared by using the above porous as template is brittle and poor mechanical properties. Taking CaCO3 porous as an example, the PI porous membrane is more brittle when CaCO3 is used as porous. The reason for the infrared spectroscopy test shows that the addition of CaCO3 makes the degree of imidization of PI only 80%, which is the main reason for the poor mechanical properties of the porous membrane.

Pore-forming agents can also be substances with pyrolysis or high-temperature volatilization characteristics. PI porous membrane is obtained by the decomposition or volatilization of porogen in the process of thermal imination. Jiugui Liu prepared polyurethane/polyamide acid mixed solution by in-situ polymerization with polyurethane as the pore-forming agent, spread the polyurethane/polyamide acid membrane and conduct thermal imination treatment. In the imination process, polyurethane was degraded to prepare PI porous membrane with long strips of nanoparticles. However, this method is difficult to completely remove the pore-forming agent, resulting in uneven texture of PI porous film. The biggest advantage of the template method is that the structure and size of micropores can be controlled by changing the particle size of the pore-forming agent, but the mechanical properties of the prepared membrane may be poor due to incomplete removal of the pore-forming agent and influence of the degree of imidization.

1.2.3 Phase conversion method

The phase inversion method refers to changing the thermodynamic state of the solution by a certain composition of the polymer solution, so that the homogeneous polymer solution is phase-separated and finally transformed into a three-dimensional macromolecular network gel structure. Specific methods for preparing PI porous membranes include thermal phase inversion, high humidity induced phase inversion, and impregnation precipitation phase conversion. Among them, the immersion precipitation phase inversion method is a commonly used method in which a polyamic acid solution or a PI solution is smeared on a support and then immersed in a non-solvent of the polyamic acid or PI to make a solvent and a non-solvent. The solvent is exchanged, and after a certain degree of liquid-solid phase separation, after removing the solvent, the space occupied by the non-solvent forms a pore of the PI film. Weiguo Yang prepared a PI separator with a porosity of 30% to 60% by immersion precipitation phase transformation method. The average pore diameter of the separator was 0.02-0.15μm, and there was no closed cell and good gas permeability (the gas permeability was 150 s/100 cc~ 300s/100cc), low heat shrinkage, good heat resistance, dimensional stability at 300 °C, greatly improved the heat resistance temperature of commercial separators, and improved battery safety. TNGUYEN et al. [46] prepared a kind of PI porous membrane by impregnation and precipitation phase transformation method, and polymerized perfluorosulfonic acid proton exchange membrane (Nafion) filled in the porous membrane to prepare PI/Nafion composite membrane that can be directly used in methanol fuel cell. 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 etc. [49] by controlling the solids by immersion precipitation phase catalysis was prepared in 47% ~ 87% porosity of PI diaphragm, the diaphragm glass transition temperature as high as 274 ℃, 200 ℃ after heat treatment under the heat shrinkage rate is only 1%. In addition, the surface of the PI septum is polar and outstanding for its absorbency, with a absorbance of up to 190% ~ 378%, while the absorbance of celagr2400 is only 116%. Under the same charging and discharging conditions, the diaphragm and Celgard diaphragm have similar discharge capacity (129 ~ 131mAh/g).

1.2.4 Other methods

Since PI separators are currently difficult to process and mass-produce, the common methods for preparing PI porous membranes are not practical, so scholars have also explored other methods for preparing PI porous membranes, such as sintering [50-51], irradiation etching, graft or copolymerization unstable segment method and the like. HMUNAKATA filtered silica gel crystals to obtain a film with silicon ion precipitated, and then sintered the film at a high temperature of 1100 ° C to obtain a template with regular arrangement of silicon ions, casting a polyamic acid solution between the silicon templates, and high temperature. After imidization, a Si/PI composite film was obtained, and silicon was etched with hydrofluoric acid to obtain a PI porous film. When the film was directly used in a methanol fuel cell, it was found that the permeation of methanol can be suppressed by changing the size of the pores, and the proton conductivity/methanol permeability is 1.2 x 105Scm - 3 s, which is an order of magnitude higher than that of the Nafion membrane. Qingchen Cui proposed a method for preparing PI porous membrane by irradiation etching. The process is based on PI film, irradiating PI surface with high energy heavy ions, and then using PI or solvent to irradiate PI film. The sensitized PI film was immersed in a NaOH-KMnO4 solution for chemical etching to obtain a porous PI film having a pore diameter of 0.01 to 3μm. KRCARTER et al. [54] introduced heat-labile polypropylene oxide on the PI chain of the perfluoro skeleton, imidized it in an inert atmosphere at 310 ° C, and then heat-treated in an aerobic environment at 250 ° C to heat The unstable polypropylene oxide segment was degraded to obtain a porous PI film, and the process is shown in FIG. However, in the actual application process, the preparation methods of the above several PI porous membranes lack corresponding supporting equipment, which affects the industrialization process.

2. Conclusion

With the development of electronic information and new energy industries, higher requirements are placed on the safety of lithium-ion batteries, especially for new energy vehicles. Therefore, the high temperature performance requirements 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. The PI diaphragm is regarded as a next-generation diaphragm material with a focus on development due to its excellent thermal stability and good electrolyte liquid retention. It provides better safety for power batteries. At present, although the research on PI diaphragm at home and abroad has achieved more stage results, the research results mostly stay 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 recommended that the corresponding research institutes, equipment processing enterprises, diaphragm manufacturers and diaphragm application enterprises carry out collaborative research through the methods of “production, learning, research and use”, focusing on PI diaphragm formulation and modification mechanism, supporting production equipment and Research work on technology and PI separator in lithium battery to shorten the PI diaphragm development cycle and accelerate the industrialization process of PI diaphragm.

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