Polymer electrolytes can be prepared in a variety of ways, depending on the properties of the material and the fabrication technique of the battery. For example, there are solution casting method, porous film infiltration method, in situ cross-linking method and thermal melting method []. The advantages and disadvantages of each method are described below.
1. Solution casting method
The earliest method for preparing polymer electrolytes is solution casting, which is to prepare thin films by evaporating the volatile solvent in the casting solution of polymer and lithium salt. Figure 1 is a process diagram of obtaining a polymer electrolyte membrane using a solution casting method. The polymer solution was poured onto a glass slide, and the thickness of the film was controlled by a spatula. The film thins as the solution evaporates and is then removed from the slide. Because the film peels easily, teflon can be used instead of glass. In addition to this, the evaporation rate should be adjusted by covering the tray to limit air circulation. This simple and low-cost method is widely used in the preparation of polymer electrolytes. When preparing gel polymer electrolytes, linear carbonate solvents such as DMC and DME cannot be used because they will run off during evaporation. Because of this, this method is limited to the preparation of organic electrolyte gel polymer electrolytes containing high-boiling solvents, such as EC and PC. Examples of polymer electrolytes prepared using solution casting are PEO and PMMA.
2. Porous membrane impregnation method
Hydrophobic polyolefin separators in gel polymer ion batteries can be obtained by dipping porous films in lithium secondary batteries into electrolytes. The microporous separators fabricated using this method have excellent affinity for liquid electrolytes and high polarity. sex. Typical polymer examples are P(VdF-co-HFP) and other PVdF copolymers. The preparation process of the gel polymer electrolyte impregnated with the porous separator using P(VdF-co-HFP) copolymer can be described as follows. First, the polymer, plasticizer, and filler-containing mixture is dissolved in an acetone solvent. High-boiling polar solvents such as dibutylphthalate (DBP) serve as suitable plasticizers and silica or alumina as fillers. These inorganic materials added here can not only enhance the mechanical properties of the porous membrane but also have more materials adsorbed into the membrane layer. After the polymer solution was cast to the appropriate thickness on the glass slide, the acetone was evaporated off. After the acetone was removed, the film contained a large amount of plasticizer. When the film is placed in a non-solvent such as water, methanol or aldehyde, the DBP plasticizer is removed from the membrane and micropores are then formed. The film is dried under vacuum and then immersed in an electrolyte to form a gel polymer electrolyte. The gel polymer electrolyte is formed by fabricating a cell consisting of positive electrode/porous separator/negative electrode before electrolyte injection, which is not a direct impregnation method. Because the electrolyte is added in the last step, it is easy to maintain a hydrophobic environment. An advantage over solution casting is that different types of electrolyte solutions can be used. However, aging is required in order to achieve uniform impregnation in gel polymer electrolytes.
3. In situ cross-linking method
As shown in Figure 2, the in-situ cross-linking method is to fill the cell composed of positive electrode/porous separator/negative electrode by using active oligomers, cross-linking agents, initiators and linear polymers dissolved in the electrolyte. Then heating or ultraviolet irradiation is performed to prepare a gel polymer electrolyte with a three-dimensional network structure. A frequently used crosslinking agent is PEGDMA containing two functional groups. The thermal and photoinitiators are azodiisobutyronitrile (AIBN) and aromatic ketones, respectively.
Since chemically cross-linked gel polymer electrolytes have a network structure, they hardly suffer structural changes compared to physically cross-linked electrolytes. In addition to this, the supported polyolefin membrane has good mechanical properties. However, since crosslinking requires exposure to high temperatures for a specified period of time, this process is not very productive. Another drawback is that it is difficult to remove inactive monomers from polymer precursors.
4. Hot melt method
The thermal melting method is a method of obtaining a gel film layer by directly dissolving the polymer into the electrolyte, followed by casting and cooling at room temperature. There is no loss in this method, and gel polymer electrolytes can be prepared using organic electrolytes and polymers without the use of additives. Because the polymer dissolves directly into the electrolyte at high temperatures, low-boiling organic solvents cannot be used. This thermal melting method is often used to prepare PAN-based polymer electrolytes using EC or PC as electrolytes.
