1 Capacitor Ionic Liquid Polymer Electrolyte

Capacitor Ionic Liquid Polymer Lithium-ion batteries based on traditional liquid electrolytes will form irregular metal lithium electrodeposition layers during repeated charge and discharge cycles, leading to lithium dendrites and causing safety issues. This will not only reduce the battery’s Coulomb efficiency, but also accelerate the consumption of electrolytes and even increase the risk of short circuits, posing a serious risk of thermal runaway.

Capacitor ionic liquid polymer is a new type of polymer material containing ionic liquid structure in the repeating unit. Its polymer chain contains at least one ion center. The repeating unit is similar to the common ionic liquid structure and has special properties. Compared with conventional polymers, it has many excellent properties, such as good film-forming properties, non-flammable, high thermal stability and electrochemical stability, and good compatibility with ionic liquids. In addition, it also has the advantages of high conductivity, good mechanical properties, and no solvent volatilization and leakage.

Whether capacitor ionic liquid polymer electrolytes can be used in the next generation of solid lithium-ion batteries has attracted great attention. Capacitor ionic liquid polymer electrolytes composed of ionic liquids and polymers combine the advantages of ionic liquids and polymers, have the characteristics of strong stability, high conductivity, and no risk of leakage, and have attracted widespread attention from scientists in recent years. Capacitor ionic liquid polymer electrolytes can be roughly divided into two categories: one is that there is no chemical reaction between the ionic liquid and the polymer, that is, the polymer contains ionic liquid, and the ionic liquid actually exists in the solid polymer in the form of a solution; the other is to introduce ionic liquid structure into the polymer molecule to obtain capacitor ionic liquid polymer electrolytes. According to the charge distribution on the polymer chain, capacitor ionic liquid polymers can be divided into polycationic type, polyanionic type and anionic-cationic copolymer type, as shown in Figure 1. There are three main methods for preparing capacitor ionic liquid polymer electrolytes: ① directly blending ionic liquid and polymer into a film; ② immersing ionic liquid into the inside of the polymer that has already formed a film; ③ directly polymerizing polymer monomers in ionic liquids. The capacitor ionic liquid polymers currently studied are mainly polycationic capacitor ionic liquid polymers. This may be because the polymerization of perfluorinated anions in ionic liquids is difficult, and the interaction between the polyanion skeleton and lithium ions will also reduce the mobility of lithium ions. At the same time, the anions of capacitor ionic liquid polymers have a significant effect on their glass transition temperature and ionic conductivity. According to the number of cationic centers connected to the skeleton chain in the repeating unit of polycationic capacitor ionic liquid polymers, it can be divided into single-center cationic ionic liquid polymers and double-center cationic capacitor ionic liquid polymers.

Figure 1 Classification of capacitor ionic liquid polymers

In recent years, the capacitor ionic liquid polymer electrolytes that have been studied more include gel electrolytes, ionic plastic crystal electrolytes, mixed electrolytes and single ion conductive electrolytes.

1. Capacitor Ionic Liquid Polymer-Gel electrolyte

Organic polymer solid electrolytes belong to a new type of electrolyte category. They are supported by macromolecular substances as a matrix, and ions can migrate inside to have the characteristics of conductivity. Because the gel has a special structure between solid and liquid, the gel polymer has both the diffusion and conductivity of the liquid and the cohesion of the solid. Using the polymer as a matrix, the organic electrolyte or lithium salt-ionic liquid solution is fixed in it to obtain a self-supporting polymer electrolyte membrane, which can perfectly combine the mechanical flexibility of the polymer and the electrochemical properties of the electrolyte. The safety performance of polymer electrolytes based on organic electrolytes is still not ideal due to the presence of flammable and volatile organic solvents. The capacitor ionic liquid polymer electrolyte is obtained by introducing ionic liquids into polymers, which not only retains the flexibility of the polymer and many excellent electrochemical properties of ionic liquid electrolytes, but also obtains battery performance similar to that of ionic liquid electrolytes. However, the ionic liquid-based gel polymer electrolyte prepared by the composite of conventional polymer matrix and ionic liquid will have the problem of poor compatibility, such as the electrolyte system obtained by fixing the lithium salt-ionic liquid solution in the polymer matrix. To solve this problem, capacitor ionic liquid polymers can be prepared with ionic liquids as repeating units of the main chain and applied to ion gel systems.

The β-phase crystal in PVDF has a large polarity, which makes it have a high dielectric constant, which is conducive to the dissociation of lithium salts. At the same time, it enhances the affinity with the electrolyte, and PVDF has good mechanical strength and electrochemical stability. Therefore, polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) is considered to be a potential matrix for gel electrolytes due to its excellent electrochemical stability, easy film formation and high stability . Que et al. reported a flexible thin film ionic liquid-based gel electrolyte composed of PVDF-HFP and room temperature ionic liquid [EmimlTFSI. Figure 2 is a schematic diagram of the structure of the gel electrolyte and 3P (MPBIm-TFSI). The prepared gel electrolyte shows good thermal stability, no weight loss at up to 370°C, and has a relatively high ionic conductivity. The battery based on this electrolyte shows a stable energy delivery capability. Jia et al. pointed out that the factors affecting the ionic conductivity of PVDF-HFP-based GPE are: the amount of liquid electrolyte absorbed, the interaction between the polymer matrix and the liquid electrolyte, and the inherent ionic conductivity of the liquid electrolyte.

 

 

Figure 2 Schematic diagram of the structure of gel electrolyte and 3P (MPBIm-TFST)

Liu et al. added the solvent of PVDF-HFP to the acetone solution of ionic liquid 1-ethyl-3-methylimidazole dicyanamide (EMIM:DCA) and PVDF-HFP before the formation of ion gel, which could significantly change the PVDF – The microscopic solid network formed by HFP effectively improves the ionic conductivity of the ion gel. Chaudoy et al. cross-linked PVDF-HFP and polyethylene oxide (PEO) to form a semi-interpenetrating polymer network, which carried N-propyl-N-methylpyrrolidone bis(fluorosulfonyl)imide (P₁₃FSI ) and LiTFSL, which exhibit high ionic conductivity and excellent mechanical properties when applied to lithium metal batteries. Caimi et al. prepared pVDF-HFP-based GPE using deionized water as solvent and pointed out that excessive ionic liquid would reduce the uniformity and integrity of the membrane. Experiments show that GPE has the best performance when the ionic liquid content is 70%. Singh et al. also studied the effect of ionic liquid content on PVDF-HFP-based GPE and found that the flammability of the electrolyte decreased with increasing ionic liquid content. Similarly, when the ionic liquid content is 70% (mass fraction), the best GPE performance is obtained.

Sakaebe et al. studied the performance of Li/LiCoO₂ batteries based on ionic liquids containing three different anions. The asymmetric properties were introduced into the anions by using amides, which improved the viscosity and conductivity of RTILs. Appetecchi et al.reported a novel poly(diallyldimethylammonium)bis(trifluoromethanesulfonyl)imide polymer ionic liquid as the main component, with N-methyl-N-butylpyrrole bis(trifluoromethanesulfonyl)imide added. Electrochemical performance of solvent-free ternary polymer electrolyte based on [Pyra]TFSI ionic liquid and LiTFSI salt. The study found that the PILs-LiTFSI-Pyr₁₄TFSI electrolyte membrane still has excellent chemical stability even after long-term contact with the lithium anode, and has a thermal stability of up to 300°C. Lee et al. in situ polymerized an electrolyte solution containing vinyl ionic liquid monomers to obtain a gel polymer electrolyte. The polymerization of non-flammable ionic liquid monomers leads to a significant reduction in the flammability of the gel polymer electrolyte. The lithium polymer battery assembled with a stable gel polymer electrolyte has a discharge specific capacity of 134.3 mAh·g^-1 and exhibits good capacity retention at room temperature.

In lithium-ion batteries, the polymer electrolytes that have been studied more are polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO), and the rest are copolymers formed on the basis of these polymers. The typical polymer gel electrolyte series mainly includes copolymers of vinylidene fluoride and hexafluoropropylene [IP (VDF-HFP)] and copolymers of acrylonitrile-methyl methacrylate-styrene [P (AN-MMA-ST)].

1) PEO-based

The earliest and most widely studied gel electrolyte is PEO-based electrolyte. As early as 1973, researchers discovered that PEO and alkali metal salts have ionic conductivity, which was the beginning of the research on PEO-based electrolytes. In 1978, researchers proposed that PEO and alkali metal salt complexes can be used as ionic conductors for rechargeable batteries with alkali metal electrodes.

PEO-based electrolytes can form stable combinations with lithium salts in the absence of any organic plasticizers. However, PEO-based polymers are prone to crystallization, which is not conducive to the conductivity of gel electrolytes. In order to reduce the crystallinity of PEO, copolymerization with acrylamide (AAM) and methyl methacrylate (MMA) can improve the conductivity of the electrolyte. The copolymers formed are mainly P(EO-AAM) and P(EO-MMA). The addition of AAM and MMA increases the flexibility of the PEO chain, increases the movement amplitude of the free chain segments in the polyether amorphous phase, reduces the cross-linking degree of lithium ions in the polymer, and increases the speed of free movement of lithium ions. The high crystallinity of PEO at room temperature (i.e., low ionic conductivity) remains the main challenge for the commercialization of PEO-based polymer electrolytes. In addition, traditional PEO-based electrolytes are not compatible with high voltage during the charge and discharge process, which limits their application in ternary cathode materials (NCM).

2) PVDF-based

Now researchers are increasingly studying PVDF-based electrolytes. Polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP) as the matrix of the polymer electrolyte can not only withstand high voltage, but also has good liquid absorption and mechanical strength, which makes the battery have higher energy density, faster reaction kinetics and higher safety. Liu et al. used UV curing to prepare a semi-interpenetrating PVDF-HFP-based polymer electrolyte. The macromolecules were introduced into the monomer ethoxylated trimethylol triacrylate (ETPTA) under ultraviolet light to initiate polymerization to form a network structure, which together formed a semi-interpenetrating polymer electrolyte. The introduction of this network is beneficial to improve the mechanical strength of the polymer electrolyte and improve the performance of lithium metal batteries.

3) PMMA-based

The most typical polyester polymer gel electrolyte is polymethyl methacrylate (PMMA). The carbonyl oxygen in the polymer has a strong affinity, which makes it effectively dissolve with the plasticizer and the electrolyte, and can accommodate a larger amount of electrolyte. At the same time, it has good interfacial stability to the lithium electrode, which can increase the chemical stability and safety of the battery. A serious disadvantage of using plasticizers is that the mechanical strength of the polymer electrolyte is reduced. In order to eliminate this problem, blending with PMMA is considered to be an effective way to enhance mechanical stability. In summary, PMMA-based electrolytes have the following advantages: ① There is a carboxyl side group in the MMA structural unit, which has a strong interaction with the oxygen in the plasticizer and can accommodate a large amount of liquid electrolyte; ② It has good interfacial stability to the electrode; ③ The interfacial impedance with the lithium electrode is low: ④ It has good compatibility with the electrolyte; ⑤ PMMA raw materials are cheap and easy to synthesize.

4) PAN-based

Polyacrylonitrile (PAN)-based polymer gel electrolyte is a type of electrolyte material with simple synthesis. Its advantages include high conductivity, high thermochemical stability, and wide electrochemical window. Its disadvantages are soft gel and low mechanical strength. In addition, CN has a strong electron-pulling effect, poor compatibility with liquid electrolytes, and severe passivation with lithium electrodes, which can easily cause instability of the interface film, making the battery prone to leakage and causing safety problems. Shah et al. studied a method for preparing a new type of polymer gel electrolyte, which uses efficient polyacrylonitrile-co-butyl acrylate (PAN-co-PBA) as a carrier for solid DSSC. The prepared PAN-co-PBA has high water absorption. Moreover, PAN-co-PBA gel electrolyte has high ionic conductivity. The DSSC prepared with 7% PAN-co-PBA has the highest total conversion efficiency, about 5.23%. The improvement in DSSC performance may be related to the good gelation and high ionic conductivity of PAN-co-PBA gel electrolyte.

2. Capacitor Ionic Liquid Polymer-Ionic plastic crystal electrolyte

Ionic plastic crystals are compounds composed of organic molecules that exhibit disorder while maintaining high diffusivity and plasticity. Solid-solid phase transitions below the melting point give organic plastic crystals good mechanical flexibility and plasticity. Therefore, under the action of external forces, they can be easily deformed without breaking.

Jin et al. reported a new solid electrolyte based on organic ionic plastic crystal triisobutyl (methyl) bis (fluorosulfonyl) imide (P1444FSI). Figure 3 shows the DSC spectrum of the prepared ionic plastic crystal electrolyte. The two phase transition temperatures (from 8°C to 24°C and from 24°C to 36°C) indicate that they are typical plastic crystals. The prepared electrolyte exhibits good conductivity (0.26×10-3S·cm-1) and high discharge capacity (160 mAh·g^-1) at 22°C.

Li et al. prepared a class of solid polymer electrolytes N-ethyl-N-methylpyrrolidine bis(fluorosulfonyl)imide (P₁₂FSI) by impregnating the matrix with organic ionic plastic crystals. The prepared PILs-P₁₂FSI-LiTFSI electrolyte showed flexible mechanical properties (see Figure 4), a wide operating temperature range (25~80℃) and the advantage of inhibiting lithium dendrite growth.

Figure 4. Photograph of solid-phase PILs-P1zFSI-LiTFSI electrolyte membrane

A1-Masri et al. reported an organic ionic plastic crystal electrolyte containing 90 mol% lithium bis(fluorosulfonyl)imide and 10 mol% bis(fluorosulfonyl)pyrrolidine imide. The resulting quasi-solid electrolyte showed a conductivity of 0.24×10³ S·cm^-1 at 30°C and a lithium ion transfer number of 0.68. This result provides a new way to design safer quasi-solid electrolytes with good electrochemical and transport properties.

3. Capacitor Ionic Liquid Polymer-Mixed electrolytes

Hybrid electrolytes based on polyionic liquids are those in which the ionic liquids are partially chemically bound to the inorganic particle substrate, resulting in beneficial synergistic effects on the physicochemical properties of the electrolyte. According to Wang and colleagues, the ion transport properties of polymer electrolytes based on PILs can be enhanced by introducing 2D silica nanofillers, which provide continuously interconnected ion transport pathways. The mixed electrolyte significantly improved the cycling performance and ionic conductivity of the cell (increased by 1130% at room temperature). Figure 5 (a) shows the anion exchange process of PDMA (TFS), and (b) shows the preparation process of PILs functionalized nanosheets.

Ma and colleagues reported a hybrid electrolyte based on PILs and Li_1.3Al_0.3Ti_1.7(PO₄)₃(LATP) inorganic particles (Figure 6). Polyethylene terephthalate (PET) nonwoven fabric was used as a skeleton to improve mechanical strength, and PILs-LiTFSI-LATP was used as an ion transport material. LiFePO4/lithium batteries using PILs-LiTFSI-LATP (10% LATP by mass) as a solid electrolyte exhibited excellent rate performance and high capacity retention (close to 97% after 250 cycles at 60°C).

Figure 5 (a) Anion exchange process of PDMA(TFSI); (b) Preparation process of PILs functionalized nanosheets

Figure 6 Preparation process of mixed electrolyte based on PILs

In addition, PILs-based electrolytes can also be used as interlayers of lithium-sulfur batteries. For example, when PPy@PILs-poly(vinyl cyanide) nanofibers prepared by electrospinning were used as interlayers of lithium-sulfur batteries, the PILs-based electrolytes exhibited a high capacity of 844 mAh·g^-1 at 0.1C and super strong adsorption for polysulfides. The chemical properties of most sulfides make carbonates unsuitable as electrolyte salts for lithium-sulfur batteries, and the low boiling point of ether solvents makes the system highly volatile and flammable. The low stability of ethers and the dissolution of polysulfides also make lithium-sulfur batteries difficult to commercialize. In addition, the high overpotential required for the delithiation of large-sized Li₂O₂ will inevitably lead to irreversible oxidation of carbonates or ether solvents.

4.Capacitor Ionic Liquid Polymer- Single ion conductive electrolyte

When anions are bound to the polymer backbone, cations become the only freely mobile ions in the electrolyte, forming a single-ion conducting electrolyte. Generally, a lower glass transition temperature and higher polymer segment mobility can improve the ionic conductivity of single-ion conductors. The high ionic conductivity and lithium ion transference number of single-ion conducting electrolytes can inhibit the growth of lithium dendrites on the electrode.

In 1984, Bannister reported that single-ion conducting electrolytes can be obtained by using anionic polymers, which accelerate the transfer of lithium ions by limiting the binding of anions to lithium ions. Porcarelli and colleagues reported a new PILs-based single-ion conducting electrolyte prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. The preparation process is shown in Figure 7(a). According to Figure 7(b), the single-ion conducting electrolyte can be used in solid-state lithium-ion batteries, showing a good conductivity of 2.3×10^-6S·cm^-1 at 25°C and a specific capacity of up to 130mAh·g^-1. The single-ion conducting electrolyte based on PILs achieves high ionic conductivity and high electrochemical stability by controlling the molecular structure through RAFT polymerization.

2 Capacitor Ionic Liquid Polymer-Composite solid electrolytes

1. Poly(dimethyldiallylammonium)bis(trifluoromethylsulfonyl)imide

The synthesis of pyrrolidone-based capacitor ionic liquid polymer electrolyte poly (dimethyldiallylammonium) bis (trifluoromethylsulfonyl) imide [P(DADMA-TFSI)1] is introduced as an example.

Preparation of capacitor ionic liquid polymer P(DADMA-TFSI) electrolyte membrane: First, a pyrrolidine polymer ionic liquid was synthesized. Poly(diallyldimethylammonium) bis(trifluoromethanesulfonyl)imide synthesized by anion exchange reaction and 20g (24.74mmol monomer unit) poly(diallyldimethylammonium) chloride aqueous solution (20wt%) were diluted with 100mL deionized water and then mixed with 8.52g (29.68mmol) of LiTFSI in 20mL of deionized water. After stirring at room temperature for 2h, a large amount of white solid precipitated from the solution. The white solid was collected by filtration and washed five times with deionized water. The solid was vacuum dried at 105℃ for 48h to obtain white TFSI. After that, a silver nitrate titration experiment was performed, that is, silver nitrate was added to the poly(DADMA)TFSI acetone solution to check whether chloride ions remained in the poly(DADMA)TFSI. The experimental results confirmed the absence of chloride ions because no silver chloride precipitation was produced.

The preparation process of poly(DADMA)TFSI-SN composite polymer electrolyte is as follows: the composite polymer electrolyte of poly(DADMA)TFSI-SN was prepared by dissolving poly(DADMA)TFSI, succinonitrile and LiTFSI in acetone. The mixture was cast on a polytetrafluoroethylene slide and the solvent was evaporated for 24 hours under an argon atmosphere. Then, the electrolyte membrane was vacuum dried at 30°C for 48 hours and moved into an argon-filled glove box (O₂<0.1 ppm, H₂O<0.1 ppm) for 24 hours to remove the residual solvent. The obtained membrane thickness was about 250~300 μm.

2. Poly(1-ethyl-3-vinylimidazole bis(trifluoromethylsulfonyl)imide)

The synthesis of imidazole-based capacitor ionic liquid polymer electrolyte poly(1-ethyl-3-vinylimidazole bis(trifluoromethylsulfonyl)imide) [P(EtVIm-TFSI)] is introduced as an example.

P(EtVIm-TFSI) was synthesized by a three-step method. In the first step, VIm was free radically polymerized in toluene with AIBN as the initiator to obtain PVIm homopolymer (monomer mass fraction of 0.5%) at 65°C for 8 h. Subsequently, the brominated intermediate P(EtVIm-Br) was synthesized by refluxing a mixture of PVIm (monomer unit is 1 equivalent) and ethyl bromide (3 equivalents) in a methanol solution at 50°C for 20 h. Finally, P(EtVIm-TFSI) was synthesized by anion exchange reaction of P(EtVIm-Br) (1 equivalent relative to the monomer unit) with LiTFSI (1.2 equivalents).

Surface morphology analysis: mainly studies the effect of capacitor ionic liquid polymer electrolyte on electrode sheets during the cycle process, that is, the surface morphology of the cycled and uncycled electrodes is observed. The cycled electrodes are disassembled in a glove box, and the removed electrodes are cleaned of the residual capacitor ionic liquid polymer electrolyte on the surface with DMC, and then placed in a glove box to allow the organic solvent to fully evaporate before being sealed in a bottle for later use.

Fourier transform infrared spectroscopy analysis: Analyze the molecular structure of capacitor ionic liquid polymer electrolytes and the interactions between molecules.

Conductivity test: The capacitor ionic liquid polymer gel electrolyte is assembled into a simulated battery of “stainless steel sheet/electrolyte membrane/stainless steel sheet”, and the conductivity of the capacitor ionic liquid polymer gel electrolyte is tested by electrochemical impedance spectroscopy. The instrument used is an electrochemical workstation. The test frequency range is 1Hz~100kHz, and the AC amplitude is 5mV. During the entire test process, the ambient temperature of the sample is controlled by a constant temperature water bath. After the water bath reaches the set temperature, the temperature is kept constant for 1h before testing. The entire test is repeated three times, and the average value of the three conductivity values ​​is taken.

Lithium ion migration number test: The lithium ion migration number of the polymerized ionic liquid gel electrolyte membrane is tested by AC impedance method and chronoamperometry, and the instrument used is an electrochemical workstation. The battery is assembled into a simulated battery of “lithium sheet/electrolyte membrane/lithium sheet”, and the simulated battery is tested in the order of AC impedance, chronoamperometry, and AC impedance. Among them, the frequency range of AC impedance is 1Hz~100kHz, and the AC amplitude is 5mV. In the chronoamperometry, the polarization voltage is 10mV, the test time is 1000s, and the test temperature is 60°C

Electrochemical window test: Linear sweep voltammetry (LSV) was used to test the lithium ion migration number of the polymeric ionic liquid gel electrolyte membrane, and the instrument used was an electrochemical workstation. The battery was assembled into a simulated battery of “stainless steel sheet/electrolyte membrane/lithium sheet”, in which the stainless steel sheet was used as the working electrode and the lithium sheet was used as the reference electrode and the counter electrode. The test temperature was 60°C, the scan rate was 1mV·s^-1, and the voltage range was 2~6V.

3. Polypropylene carbonate/polymethyl methacrylate coated polyethylene polymer electrolyte

PPC and MMA with different mass ratios (5:5, 6:4, 7:3, 8:2, 9:1) were dissolved in acetone containing 10% BPO (related to the mass of MMA) and stirred vigorously under nitrogen flow at 80°C for 6 hours. Afterwards, the microporous polyethylene separator was immersed in the above homogeneous solution for 2 hours and heated and cured at 45°C for 12 hours. Finally, the thickness of the flexible translucent PPC/PMMA-coated Celgard PE film was about 40μm, which was immersed in a 1.0 mol· dm^-3LiPF₆ electrolyte solution dissolved in EC/DMC (volume ratio 1:1) for 1 hour to form a PPC/PMMA-coated Celgard PE-gel.

New GPEs were prepared based on PPC/PMMA coated Celgard-PE membrane. By introducing PPC into PPC/PMMA polymer, the wettability of the membrane, the absorption rate of electrolyte, the ionic conductivity of GPE and its compatibility with the anode were improved. PPC/PMMA coated Celgard-PE-GPE based on PPC:MMA=8:2 polymer has the highest ionic conductivity (1.71×10^-3 S·cm^-1 at room temperature). The Li/LiFePO₄ battery assembled with this PPC/PMMA modified Celgard-PE-GPE has a specific capacity of 93 mAh·g^-1 at 2C rate, and retains 94.8% of the initial capacity (154 mAh·g^-1) after 100 cycles. At 0.1C rate, it has better rate performance and cycle performance than batteries using liquid electrolytes and PE separators.

4. New gel polymer electrolyte (GPE)

New GPE can be synthesized by using cheap plant materials containing a large amount of cellulose, such as wheat straw and corn straw, as raw materials and grafting copolymerizing with acrylic acid (AA).

The resulting polymer film is immersed in a conventional liquid electrolyte for a period of time, such as a LiPF₆/EC+DMC+DEC solution. The film is then removed from the electrolyte and the excess solution on the surface of the film is absorbed with filter paper. The mass of the polymer film is recorded before and after absorbing the liquid electrolyte, and the amount of electrolyte absorbed is calculated using the following equation.

Where: A is the absorption rate of the polymer film, mo is the mass of the polymer film before absorbing the liquid electrolyte, and ma is the mass of the polymer film after absorbing the liquid electrolyte. The ionic conductivity of GPE is proportional to the absorption of the liquid electrolyte. The conventional GPE has a low absorption rate of liquid electrolyte, resulting in low ionic conductivity. To overcome this problem, polyvinyl alcohol is used as a plasticizer and a precursor copolymer because it has a higher absorption rate of organic solvents. In the conventional process for preparing GPE, the liquid electrolyte is heated together with the polymer host to form a gel. During this process, the thermal instability of the lithium salt (LiPF₆ or LiBF₄) and the volatility of the solvent (DMC, EMC, etc.) may cause the resulting GPE to deviate from the desired composition or even degrade. To solve this problem, the polymer host film) is immersed in a liquid electrolyte solution at room temperature and kept for a long time to maximize the absorption of the electrolyte. The ionic conductivity of the prepared GPE can reach 6.7×10^-3 S·cm^-1, which is close to the conductivity of the liquid electrolyte (~10^-3S·cm^-1). Therefore, lithium batteries containing the new GPE still have good charge and discharge efficiency at high current density.

5. Capacitor Ionic Liquid Polymer-Hybrid Silicon Ion Gel Electrolyte

Ion gels are obtained by attaching ionic liquids to stable aerogel structures by various methods. From another perspective, ion gels are aerogels functionalized with ionic liquids. When preparing hybrid ion gels, in addition to the traditional precursor tetraethyl orthosilicate, an organically modified silane precursor methyltrimethoxysilane (MTMS) is added. CTAB is added as a cationic surfactant and mixed with MTMS. Then. In the presence of HCI catalyst, the hydrolysis reaction is promoted by adding a mixture of ethanol (EtOH) and deionized water to the silica precursor. LiTFSI salt is dissolved in [Bmim]TFSI, and the total mixture is added to the sol to improve ionic conductivity. NH₄OH is used as a base catalyst to accelerate the condensation reaction after the hydrolysis reaction. The sol consists of TEOS:MTMS:EtOH:H₂O:CTAB with a molar ratio of 0.011:0.026:0.021:0.942:8×10^-5. The molar ratio of [Bmim]TFSI to the total Si amount is maintained at 0.167. Varying the amount of LiTFSI salt in the ionic liquid (0.5~2 mol·L^-1) can reflect the effect of Li⁺ concentration in the ionic liquid on the physical, morphological, thermal stability and electrochemical properties of the resulting hybrid ion gels. These ion gels are abbreviated as M-IGE-X, where X represents the concentration of lithium ions in the ionic liquid (X = 0.5, 1, 1.5, 1.7 and 2)

Subsequently, the sol was transferred to a polypropylene mold for gelation, and the resulting gel was placed under ambient conditions for 24 h and then aged in ethanol at 25 °C for 24 h. The aged wet gel was then immersed in a mixture of TMCS and n-hexane with a volume fraction of 10% for surface modification. The modification was performed under ambient conditions for 24 h. Before and after the surface modification step, the solvent in the pores was exchanged with n-hexane to alleviate the evaporation of the solvent during drying. Each solvent exchange was performed under ambient conditions for 24 h. The prepared wet ion gel was dried at normal pressure by gradually increasing the temperature to 20 °C to dry the wet sample every 2 h to prevent the solvent from suddenly expanding in the pores.

The density of aerogel was calculated using the ratio of aerogel mass to volume as shown below:

The solid skeleton density of silica aerogel is 2.2 g·cm^-2; Va and Vg represent the volume (cm³) of the material after drying and before surface modification, respectively.

The chemical composition of the synthesized silicon ion gel was determined by Fourier transform infrared spectroscopy (FTIR) (PerkinElmer, Spectrum 100, ABD) in the wave number range of 500~4000 cm^-1. The microstructure of the ion gel was analyzed on a scanning electron microscope (SEM) (PHILIPS, XL30SFEG). The specific surface area, average pore volume and pore size of the silica ion gel were determined at 77K using Quanta Chrome Corporation physical adsorption of Nz. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) technique, and the average pore size of the sample was determined using the Barrett-Joyner-Halenda (BJH) technique. Before the measurement, the silica ion gel was degassed at 200°C overnight to remove residual moisture and adsorbed gases. The thermal stability of the synthesized silica ion gel was determined by thermogravimetric analysis (TAInstruments SDT Q600) at a heating rate of 5°C·min-‘ from room temperature to 600°C in a nitrogen environment. The conductivity of the synthesized ion gel at room temperature was determined by electrochemical impedance spectroscopy (EIS) with a frequency range of 0.1Hz~10MHz and an amplitude of 10mV. The electrochemical stability window of the ion gel was studied by cyclic voltammetry (CV). The synthesized ion gel was cut into 5 mm thick slices, placed between aluminum foil and copper foil with a diameter of 28mm, and then placed in a bracket for EIs and CV measurements. EIS and CV analyses were performed using a Parstat 4000 potentiostatic/galvanostatic/EIS analyzer.

6. Hybrid polymer electrolyte based on 1-methyl-1-propylpyrrolidine bis(trifluoromethanesulfonyl)imide

PVDF-HFP/LiTFSI solid electrolyte and PVDF HFP/ LiTFSI/PMPyrrTFSI-ILGPEs were prepared by standard solution casting process. A predetermined amount of PVDF-HFP and LiTFSI were added to THF and magnetically stirred for 12 h at 60 °C to obtain a uniform solution. The stirred solution was coated on aluminum foil by a coater, and the solvent was slowly evaporated at room temperature (25 °C) and then vacuum dried to obtain a free-standing solid polymer electrolyte membrane. The composition of PVDF-HFP/LiTFSI=70/30 (70PVDF-HFP:30LiTFSI) is the best. PVDF-HFP and LiTFSI are placed in THF by magnetic stirring at 70:30 (mass ratio) at 60°C to obtain a uniform mixture. Different amounts of PMPyrrTFSI are added to the prepared PVDF-HFP/LiTFSI solution (70PVDF-HFP/30LiTFSI) and stirred continuously at room temperature for 12h to obtain a transparent viscous solution. The solution is poured onto aluminum foil and the THF solvent is evaporated at room temperature to obtain a gel polymer electrolyte membrane. All processes are carried out in an Ar-filled glove box (H₂O<0.5ppm, O₂<0.5ppm). Various compositions of PMPyrrTFSI ionic liquid combined with ILGPE are represented as 70PVDF-HFP/30LiTFSI/xPMPyrrTFSI. Where x is the mass ratio of PVDFHFP and LiTFSI. Since the ILGPE film does not have the stable physical properties of a solid when x is greater than 60%, the thickness of the prepared ILGPE with different PMPyrrTFSI is controlled within 100um, and the 70PVDF-HFP/30LiTFSI/xpMPyrrTFSI (x=0%, 20%, 40% and 60%) film is transparent and has no cracking due to repeated bending, indicating that it has good flexibility and sufficient mechanical strength.

Lithium ion conductive polymer electrolytes based on PVDF-HFP, LiTFSI and PMPyrrTFSI were prepared by low-cost doctor blade technology, and their composition characteristics were studied. Based on the results of XRD, thermal stability and ionic conductivity, it was confirmed that 70PVDF-HFP/30LiTFSI/60PMPyrrTFSI is a gel polymer electrolyte with excellent performance. 70PVDF-HFP/30LiTFSI/60PMPyrrTFSI has the highest ionic conductivity (6.93×10^-44S·cm^-1), a high lithium ion transfer number (Z_Li+=0.38) and high electrochemical stability (up to 5.6Vvs.Li/Li⁺) at room temperature, which meets the basic performance of solid-state lithium secondary batteries. By using the optimized gel polymer electrolyte (70PVDF-HFP/30LiTFSI/60PMPyrrTFSI), the half-cell (Li/ILG PE/LiFePO₄) maintained 97.7% of the initial capacity and 99% Coulombic efficiency after 100 cycles (C/10). The XRD patterns of pure LiTFSI, original PVDF-HFP polymer membrane without LiTFSI, and PVDF-HFP/LiTFSI polymer membranes containing different mass ratios of LiTFSI are shown in Figure 7.

Figure 7 X-ray diffraction patterns of pure PVDF-HFP, LiTFSI salt and (100-x)PVDF-HFP/xLiTFSI

The thermal behavior of the polymer films [pristine PVDF-HFP and (100-x)PVDF-HFP/xLiTFSI, where x is the mass percentage of the polymer electrolyte] was studied by DSC in the temperature range of 30~250℃. As shown in Figure8, the pristine PVDF-HFP exhibited a strong endothermic peak at 141.9℃, which is related to the crystalline melting temperature (Tm) of PVDF-HFP (i.e., confirmation of the crystalline phase of PVDF-HFP). On the other hand, 90PVDF-HFP/10LiTFSI weakened and broadened the T endothermic peak and moved it to a lower temperature (131.8℃), indicating that not only the degree of crystalline phase of PVDF was reduced, but also the crystal form had changed. However, the interaction between PVDF-HFP chains and ions (Li⁺, TFSF^– ) interfered with the formation of PVDF-HFP crystalline regions and was therefore considered to reduce Tm. When the LiTFSIsalt concentration increased to 30% (70PVDF-HFP/30LiTFSI), the Tm endothermic peak gradually shifted to lower temperatures and the peak intensities decreased (124.7°C and 118.3°C for 80PVDF-HFP/20LiTFSI and 70PVDF-HFP/30Li TFSI, respectively).

Figure 8 Differential scanning calorimetry thermograms of pure PVDF-HFP and (100-x)PVDF-HFP/xLiTFSI

Synthesis of PUA/PMMA polymer electrolyte: Polyurethane acrylate (PUA), diethyl ether amide bis (trifluoromethylsulfonyl) imide (DEEYTFSI) ionic liquid were mixed in a reasonable mass ratio according to different experimental designs, with lithium bis (trimethylsulfonyl) imide (LiTFSI) and 5% ethylene glycol dimethacrylate (EGDMA) as crosslinkers to promote the formation of covalent bonds between polymer molecular chains. Then heated and stirred at 60~70℃ for 1h. In addition, 1.6g of methyl methacrylate (MMA) and 0.6% benzoyl peroxide (BPO) were mixed in a certain mass ratio and stirred for 20min. The two solutions were mixed together and then stirred in N-dimethylformamide (DMF) for 20min to form a uniform and transparent liquid. Finally, the liquid was poured onto a glass template and the electrolyte membrane was obtained after 6 min of SUNON ultraviolet (UV)-light irradiation (Sunonwealth Elec). The films were punched into discs with a diameter of 16 mm and kept in a glove box.Synthesis of PUA/PMMA polymer electrolyte: Polyurethane acrylate (PUA), diethyl ether amide bis (trifluoromethylsulfonyl) imide (DEEYTFSI) ionic liquid were mixed in a reasonable mass ratio according to different experimental designs, with lithium bis (trimethylsulfonyl) imide (LiTFSI) and 5% ethylene glycol dimethacrylate (EGDMA) as crosslinkers to promote the formation of covalent bonds between polymer molecular chains. Then heated and stirred at 60~70℃ for 1h. In addition, 1.6g of methyl methacrylate (MMA) and 0.6% benzoyl peroxide (BPO) were mixed in a certain mass ratio and stirred for 20min. The two solutions were mixed together and then stirred in N-dimethylformamide (DMF) for 20min to form a uniform and transparent liquid. Finally, the liquid was poured onto a glass template and the electrolyte membrane was obtained after 6 min of SUNON ultraviolet (UV)-light irradiation (Sunonwealth Elec). The films were punched into discs with a diameter of 16 mm and kept in a glove box.

Characterization of PUA/PMMA gel polymer electrolyte: The conductivity of the electrolyte membrane was tested by electrochemical impedance spectroscopy (EIS) at 20°C, 40°C, 60°C and 80°C using a SS/ge electrolyte/SS (SS is stainless steel) cell on a CHI-760E electrochemical workstation. The conductivity was calculated based on the volume resistivity (Ra) using the formula σ=d/RpS, where d is the thickness of the capacitor ionic liquid polymer electrolyte membrane (cm): R is the volume resistivity (U) of the capacitor ionic liquid polymer electrolyte membrane, and S is the electrode area of ​​the steel (cm²).

Using Li/GPE/Li symmetric cells, the migration number of DEEYTFSI/LiTFSI/PUA/PMMA gel electrolyte was determined according to the formula [t,.=1(AV-IoR)/To(△V-IRo)]. In the formula, Ro (initial state) and Rss (steady state) are the interface resistances, including the charge transfer current (Rct) and SEI film resistance (Rrlm) before and after polarization; Io and Is are the currents before and after polarization, respectively; AV is the potential difference of 10mV.

7. Emm and very body-based electrolytes

Synthesis of [Emim]FSI ionic liquid-based polymer electrolytes: PEO + 20% LiFSI + x% [Emim]FSI (x = 0, 2.5, 5, 7.5, 10, 12.5, 15) films were prepared by solution casting. In this technique, an appropriate amount of PEO was dissolved in methanol and stirred continuously at 500 r/min for 2 h at 50 °C to obtain a clear and uniform solution. Then 20% (mass fraction) of LiFSI was added and the solution was stirred for 2 h. After that, different mass percentages of [Emim]FSI (i.e., 0, 2.5%, 5%, 7.5%, 10%, 12.5%, and 15%) were added to the above solution and stirred at 50 °C for 4 h until a highly viscous and uniform solution was obtained. Then, these viscous solutions were poured into polypropylene Petri dishes for drying to allow the methanol to evaporate under dry ambient conditions. After several days of solvent evaporation, a self-supporting film with a thickness of 100-150 μm was obtained, which was then vacuum dried at a pressure of 1-3 Torr for 3 days to completely evaporate the solvent. These dried samples were then stored in an argon-filled glove box for further study.

[Emim] Characterization of FSI ionic liquid-based gel polymer electrolytes: Differential scanning calorimetry (DSC) studies were performed in a nitrogen environment at a heating rate of 10°C min^-1 in the range of -120~100°C using a Mettler-Toledo differential scanning calorimeter (DSC-1). Thermogravimetric analysis was performed in a chlorine-filled environment at a heating rate of 10°C min-! in the range of 30~500°C using a Mettler-Toledo-TGA/DSC-1 system. The ionic conductivity of the GPE was studied using the complex impedance spectroscopy technique using a NOVO controlled impedance analyzer. The prepared GPE was sandwiched between two gold-plated barrier electrodes with a diameter of 20 mm and a 10 mV AC signal with a frequency range of 1 Hz~40 mHz was applied. The ionic conductivity was calculated using the formula σ=/(R×A). Where 1 and A are the thickness and cross-sectional area of ​​the sample, respectively, and Rn is the bulk resistance obtained from the complex impedance plot or Nyquist plot.

The total ion transfer number was determined by the DC polarization method. In this test, a 10mV DC voltage was applied to the GPE between two stainless steels, and the steel sealed electrodes and the corresponding current were recorded as a function of time. The total ion transfer number of the GPE was calculated as follows: tion = (it-ie) / ie where it and ie are the total current and residual current, respectively.

The cation transfer number (z) of gel electrophoresis was determined by combined AC and DC technology. The Li/GPE/Li battery was polarized for 6 h with a DC voltage of 10 mV, and the resultant current was recorded. The battery resistance before and after polarization was measured by the AC impedance method, and the lithium ion migration number was calculated according to the following formula: TLI+, =Iss(V-IoRo)/Io(V-IssRss)]. Among them, Io and Iss are the initial and steady-state current values, Ro and Rss are the resistance of the battery before and after polarization, respectively.

The electrochemical stability window of GPEs was determined in a three-electrode system using the LSV technique. A stainless steel barrier electrode was used as the working electrode, and metal lithium foil was used as the reference electrode and counter electrode. At room temperature, the voltage scan was performed between 0.0 and 5.0 V (vs. Li/Li⁺) at a scan rate of 0.1 mV·s^-1. At room temperature, CV measurements of Li/GPEs/LiFePO₄ were performed in the voltage range of 2.0 to 4.5 V with a scan rate of 0.1 mV·s^-1. The battery was subjected to constant current charge and discharge and cycle tests at room temperature using an AUTOLAB-PGSTAT-302N electrochemical analyzer.

Capacitor ionic liquid polymer electrolyte is a new type of material that combines the advantages of ionic liquids and polymers, with excellent thermal stability, electrochemical stability, high conductivity and good mechanical properties. Its structure includes polycationic type, polyanionic type and anionic-cationic copolymer type, which is mainly prepared by blending, impregnation or polymerization of ionic liquids and polymers. In recent years, this material has been widely used in lithium-ion batteries, especially gel electrolytes and ionic plastic crystal electrolytes. Gel electrolytes use PVDF, PEO, PMMA and other matrices to composite with ionic liquids, showing high ionic conductivity and safety, but the material compatibility needs to be optimized; ionic plastic crystal electrolytes show good flexibility and thermal stability due to their unique solid-solid phase transition. Both help to suppress lithium dendrites and improve battery performance, and are potential electrolyte materials for the next generation of solid-state lithium batteries.

If you want to know more about electrolytic capacitors, please click the link:https://capacitorsfilm.com/