Preparation of Polyimide Diaphragm

1. Why can polyimide be used as a raw material for lithium battery diaphragm?

  • First, PI material has outstanding high temperature resistance, and the long-term use temperature can reach 300℃, which gives the diaphragm good thermal dimensional stability and improves the safety of high temperature use of the battery;
  • Secondly, the PI molecular structure contains abundant polar groups, and the electrolyte wettability is better, which helps to improve the interface performance between the diaphragm/electrolyte and the comprehensive performance of the battery;
  • Finally, PI material is flame retardant and self-extinguishing, providing a stronger safety guarantee for lithium-ion batteries.

2. Method for producing diaphragms with polyimide

  • Template method
    The template method is a method of using a porogen with a certain structural size and incompatible with polyamic acid as a template, mixing polyamic acid with the porogen, and obtaining a porogen/polyimide composite membrane after imidization, and then removing the porogen with a template remover to prepare a PI porous membrane. The porogen can be a metal, a metal oxide, a non-metal oxide, a hydroxide, a carbonate compound, etc.
    The porogen can also be a substance with high-temperature decomposition characteristics or high-temperature volatilization characteristics. The PI porous membrane is obtained by decomposing or volatilizing the porogen during the thermal imidization process. The polyurethane/polyamic acid mixed solution is prepared by in-situ polymerization using polyurethane as the porogen. After the polyurethane/polyamic acid is laid, it is subjected to thermal imidization treatment. During the imidization process, the polyurethane is degraded to prepare a PI porous membrane with long strips of nanopores. However, it is difficult to completely remove the porogen by this method, resulting in uneven texture of the PI porous membrane. 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, but the mechanical properties of the prepared membrane may be poor due to incomplete removal of the porogen and the influence of the imidization degree.
  • Immersion precipitation method The immersion precipitation method is to scrape the polyamic acid (PAA) precursor solution or the soluble PI solution on the carrier (such as glass, etc.), immerse it in the non-solvent, and use the polymer to phase separate in the mixed solution of its solvent/non-solvent. After removing the solvent, the space occupied by the non-solvent forms a pore channel. By changing the casting solution formula and process conditions, the pore structure of the porous membrane can be simply and effectively regulated.
    The pore structure of the PI porous membrane was regulated by changing the addition amount of the porogen PEG400, and a submicron-level sponge-like pore structure was obtained. The prepared PI porous membrane exhibited excellent thermal dimensional stability: no obvious shrinkage occurred after heating at 180°C for 1h. In addition, the PI porous membrane has good electrolyte wettability, and the contact angle with the electrolyte is only 9.3°, which is much lower than the commercial PP diaphragm (64.8°).
  • Electrospinning method
    The basic principle of electrospinning technology is to apply high-voltage static electricity to the polymer solution. When the charge repulsion on the liquid surface is greater than its surface tension, a Taylor cone is formed at the needle nozzle. 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 membrane.
    Electrospinning technology has many advantages such as simple equipment, a wide variety of applicable materials, and macroscopic preparation. It has become one of the effective ways to prepare PI diaphragms. The nanofiber membrane prepared by electrospinning technology has a 3D network structure and high porosity, providing abundant channels for the rapid migration of lithium ions. Compared with traditional non-woven fabrics, the fiber diameter of nanofiber membranes is finer (between a few nanometers and hundreds of nanometers) and the pore size is smaller, which is conducive to alleviating the self-discharge phenomenon of batteries.
    In addition, researchers have also explored other membrane-forming methods, such as grafting or copolymerization of unstable chain segments, wet papermaking technology, and irradiation etching.
  • Other methods
    Since PI diaphragms 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, irradiation etching, grafting or copolymerization of unstable chain segments, etc.
    After filtering the silica gel crystals, a membrane with precipitated silicon ions was obtained. After sintering the membrane at 1100°C, a template with regular silicon ion arrangement was obtained. A polyamic acid solution was poured between the silicon templates, and a Si/PI composite membrane was obtained after high-temperature imidization. The silicon was etched with hydrofluoric acid to obtain a PI porous membrane.
    After the membrane was directly used in a methanol fuel cell, it was found that the permeation of methanol could be suppressed by changing the size of the pores. Its proton conductivity/methanol permeability was 1.2×105Scm-3s, which was an order of magnitude higher than that of the Nafion membrane.

Preparation of Polyimide Diaphragm - Polyimide Coatings

Ultrasonic spraying systems have been proven to be used in various applications that require uniform and repeatable photoresist or polyimide film coatings. The thickness control range is from submicron to more than 100 microns, and any shape or size can be coated.

Ultrasonic spraying technology is used for semiconductor photoresist coating. Compared with traditional coating processes such as spin coating and dip coating, it has the advantages of high uniformity, good encapsulation of microstructures, and controllable coating area. In the past 10 years, it has been fully demonstrated that the 3D microstructure surface photoresist coating using ultrasonic spraying technology, the prepared photoresist coating is significantly higher than the traditional spin coating in terms of microstructure wrapping and uniformity Craft.

The ultrasonic spraying system can precisely control the flow rate, coating speed and deposition volume. Low-speed spray shaping defines atomized spray as a precise and controllable pattern to avoid excessive spray when producing a very thin and uniform layer. The ultrasonic spray system can control the thickness from sub-micron to more than 100 microns, and can coat any shape or size.

About Cheersonic

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.

Our coating solutions are environmentally-friendly, efficient and highly reliable, and enable dramatic reductions in overspray, savings in raw material, water and energy usage and provide improved process repeatability, transfer efficiency, high uniformity and reduced emissions.

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