Capacitor ionic liquid electrolytes have advantages in thermal stability in large-scale secondary electrochemical energy storage devices, which can extend the operating temperature of these devices to a wide temperature range (up to 1508°C). The properties of ionic liquids can be changed according to application requirements by changing the combination of cations and anions. The synthesis of capacitor ionic liquid electrolytes is mainly carried out with lithium salts. According to the requirements for electrolyte performance, a suitable ionic liquid is selected as the electrolyte solvent and mixed with lithium salts. With the continuous deepening of research on electrolytes and ionic liquids, researchers have conducted in-depth research on capacitor ionic liquid electrolytes and ionic liquid composite solid electrolytes and have made good progress.
1 Design, preparation and physical property study of capacitor ionic liquid electrolytes
1 Selection and preparation of electrolyte salt
The specific application of capacitor ionic liquid electrolytes in lithium secondary battery is introduced. As a component of lithium secondary battery electrolyte, ionic liquid has the following application modes: ① ionic liquid + lithium salt; ② ionic liquid + lithium salt + organic additive; ③ organic solvent + lithium salt + ionic liquid (low ionic liquid content, as additive); ④ organic solvent + lithium salt + ionic liquid (high ionic liquid content, as flame retardant). Lithium secondary battery is a complex system composed of positive electrode material, negative electrode material, diaphragm and electrolyte, and the application mode of ionic liquid in lithium secondary battery is determined by the properties of ionic liquid itself and other battery materials. Lithium salt is an indispensable component of capacitor ionic liquid electrolyte, so its selection is very important and must meet the following conditions: wide electrochemical window, high ionic conductivity; good thermal stability, no decomposition in a wide temperature range; good chemical stability, no reaction with other battery components; easy to dissolve in corresponding solvents; easy to embed into positive and negative electrode materials; from the application point of view, the cost should be as low as possible.
At present, the lithium salts that are studied more include two categories: one is inorganic lithium salts, such as LiClO4, LiBF4, LiPF6, LiAsF6; the other is organic lithium salts, such as lithium trifluoromethanesulfonate LiCF3SO3, bis(trifluoromethylsulfonyl)imide lithium LiN(CF3SO2)2, bis(polyfluoroalkoxysulfonyl)imide lithium LiN(RFOSO2)2, tri(trifluoromethylsulfonyl)methyl lithium LiC(SO2CF3)3, etc. Due to the defects of poor thermal stability, low flash point, high volatility and flammability of organic carbonate-based solvents, coupled with LiPF6. Salts are extremely sensitive to humidity and will produce corrosive HF, which limits the use of lithium-ion batteries at operating temperatures above 80°C. In order to solve this problem, measures such as adding flame retardants to the electrolyte or using high flash point co-solvents can be taken. In addition, organic ionic liquids can be used to partially replace carbonate solvents to improve the safety of electrochemical devices. In the preparation process, lithium salts that are consistent with the anions of the ionic liquid used are usually selected.
The preparation process of the capacitor ionic liquid electrolyte, which is liquid at room temperature, is similar to that of the traditional electrolyte, that is, according to the required concentration, a certain molar ratio of electrolyte salt, additive and ionic liquid is mixed (most of the capacitor ionic liquid electrolytes currently used are mixed with some organic solvents), and stirred evenly to obtain the required capacitor ionic liquid electrolyte. The preparation process of the electrolyte has extremely high requirements for water and atmosphere. Therefore, the ionic liquid and electrolyte salt required for the preparation of the capacitor ionic liquid electrolyte need to be fully dried in advance and placed in a glove box with an inert atmosphere for standby use.
Compared with the original ionic liquid, the capacitor ionic liquid electrolyte usually has a higher viscosity. This is because the addition of electrolyte salt will increase the viscosity of the electrolyte system, which will further lead to a decrease in the conductivity of the electrolyte. Therefore, the conductivity of the capacitor ionic liquid electrolyte is usually lower than that of the original ionic liquid system. The addition of electrolyte salt may also cause a decrease in the electrochemical window, but it is usually still above 1 V. Therefore, the battery system based on ionic liquid electrolyte is best to use a material with a voltage platform higher than 1V as the negative electrode.
2 Design and preparation of capacitor ionic liquid electrolytes
1 Preparation of Imidazolium Ionic Liquid Electrolyte
The synthesis and purification of ionic liquids require standard synthetic methods to ensure their consistency. The yield of large-scale synthesis of ILs for industrial applications as well as their toxicity and environmental friendliness have become the focus of attention.
Imidazole ionic liquids are the earliest ionic liquids studied. The abbreviations and structures of the cations and anions in common imidazole ionic liquids are shown in Table 1. 1-Ethyl-3-methylimidazolium cation [Emim] ionic liquids have become a hot topic of research due to their amphoteric properties. Among them, [Emim]AlCl4 ionic liquids were first used in lithium-ion batteries. However, the proton on C2 of the imidazole ring of this ionic liquid has a strong reducing property and a high reduction potential, about 1 V (vs. Li/Li+). This voltage is higher than the deposition potential of lithium, which limits its application. For this reason, anionic groups such as TFSI–, PF–6, and BF–4 are often used to replace AlCl–4. Halogen ions have hygroscopic properties, and ionic liquids containing halogens are rarely used in practice, but they can be used as precursors of corresponding tetrafluoroboric acid or hexafluorophosphate ionic liquids. The ionic liquid prepared with alkyl imidazole as cation has incomparable advantages over other ionic liquids, such as stability to air and water, a wide liquid temperature range and low coordination ability.
Table 1 Abbreviations and structural formulas of cations and anions in common imidazole ionic liquids
Compared with traditional organic electrolytes, ionic liquids have the characteristics of wide liquid range, non-volatility and non-flammability. Applying them to electrolytes is expected to solve the safety problems of lithium-ion batteries. Based on imidazole ionic liquids, this paper introduces the experimental synthesis and characterization process of two capacitor ionic liquid electrolytes, [Emim]BF4 and [Emim]TFSI.
The chemicals required for the experiment are: 1-ethyl-3-methylimidazolium bromide ([Emim]Br), lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI), dichloromethane, deionized water, silver nitrate, [Emim] Cl, NH4BF, acetone, alumina, and lithium salt.
1) Preparation of [Emim] BF4 capacitor ionic liquid electrolyte
Emim]Cl needs to be stored in a glove box with an argon environment. After weighing a certain amount of [Emim] Cl in the glove box, add 100mL of acetone. After that, add an equimolar amount of NH4BF to the acetone, stir for three days, and filter out the impurity precipitate. In order to remove a small amount of organic impurities, add an excess of neutral alumina to the filtrate, then filter the alumina, and remove the acetone by rotary evaporation. Finally, colorless and odorless [Emim]BF4 is obtained, which is dried at 120°C in a vacuum drying oven for 6h. The preparation of capacitor ionic liquid electrolyte needs to be carried out in a glove box with an argon environment. Add a certain amount of lithium salt to the [Emim]BF4 ionic liquid, stir at room temperature for 3~4 h, until it is completely dissolved, and the [Emim]BF4 capacitor ionic liquid electrolyte is obtained.
2) Preparation of [Emim] TFSI capacitor ionic liquid electrolyte
Take [Emim] Br and Li-TFSI in a molar ratio of 1:1, dissolve them in deionized water respectively, mix them and stir them at room temperature for 12 hours. Add a certain amount of dichloromethane to the mixed liquid to extract the lower oily liquid. Wash the lower oily liquid with deionized water until the upper aqueous solution does not become turbid when tested with silver nitrate. Stir and heat the collected lower oily liquid to evaporate the dichloromethane, and put the product into an oven to dry for one day to finally obtain [Emim] TFSI ionic liquid. Add a certain amount of lithium salt to the [Emim] TFSI ionic liquid and stir it at room temperature for 3~4 hours until it is completely dissolved to obtain [Emim] TFSI capacitor ionic liquid electrolyte.
2 Preparation of piperidine and pyrrole capacitor ionic liquid electrolytes
Compared with other types of ionic liquids, piperidine and pyrrole ionic liquids have the advantages of low viscosity and high conductivity, and are the current focus of research on the application of ionic liquids in lithium-ion batteries.
In the preparation process of piperidine ionic liquids, temperature is the main factor affecting the progress of the reaction. If the temperature is too low, the reaction rate is too slow, and if the temperature is too high, the reactants are easily decomposed, which may lead to reduced product purity. The feeding method has a certain degree of influence on the reaction, and the dropwise addition method has a relatively weak effect on improving the yield of ionic liquids. Compared with ionic liquids synthesized in a constant temperature water bath, ionic liquids synthesized by ultrasonic bath reflux method are more suitable as electrolytes.
Pyrrole ionic liquids usually have a large density and are quaternary ammonium ionic liquids whose cations contain five-membered rings and six-membered ring structures. They have similar electrochemical stability to quaternary ammonium ionic liquids and quaternary phosphine ionic liquids. In 2005, Sakaebe et al.2 first attempted to apply LiTFSI/PP13TFSI electrolytes in Li/LiCoO2 batteries and obtained a high capacity retention rate. Pyr13TFSI has a short cationic alkyl side chain length, which results in a relatively low viscosity, and a high dielectric constant, which results in a higher solubility of discharge products. These properties can effectively improve the performance of the battery. The application of Pyr13TFSI electrolyte in lithium polymer batteries effectively improves the cycle reversibility of the battery.
This section takes PP13DFOB and Pyr13TFSI as representatives of piperidine and pyrrole capacitor ionic liquid electrolytes, respectively, and introduces their synthesis processes.
1) Preparation of PP13DFOB capacitor ionic liquid electrolyte
PP13DFOB ionic liquid is prepared by a two-step synthesis method. First, the intermediate product PP13Br is synthesized. Weigh 1-bromopropane and N-methylpyrrolidine in equal molar ratios and an appropriate amount of ethyl acetate solvent, place them in a three-necked flask for alkylation reaction, and stir them magnetically at 40°C overnight. At this time, a white egg-like solid is formed in the flask, and then washed with ethyl acetate and filtered several times to obtain the brominated N-methyl-N-propylpiperidinium salt intermediate product, which is then dried at 90°C in a vacuum oven for 10 hours and finally transferred to a dryer for use. In the second step, PP13DFOB is synthesized. The intermediate product PP13Br and LiDFOB are dissolved in an appropriate amount of acetonitrile at a molar ratio of 0.95:1. The content of LiDFOB is slightly excessive to ensure that PP13Br can react completely and avoid the introduction of bromide ion impurities. Then the acetonitrile solution of LiDFOB is slowly added dropwise to PP13Br, and LiBr precipitate can be seen at this time. After the addition is completed, react at 60°C for 12 hours. The byproduct LiBr was filtered and suspended to obtain the crude product PP13DFOB. The purification of the ionic liquid was the same as that of PP14TFSI, and PP13DFOB ionic liquid was finally obtained. After vacuum drying, it was transferred to a glove box filled with argon for storage. A certain amount of lithium salt was added to the PP13DFOB ionic liquid and stirred at room temperature for 3-4 hours until it was completely dissolved to obtain the PP13DFOB capacitor ionic liquid electrolytes.
2) Preparation of Pyr13TFSI capacitor ionic liquid electrolyte
Pyr13I was prepared by reacting 1-methylpyrrolidine with the corresponding stoichiometric amount of 1-iodopropane in ethyl acetate. The obtained Pyr13I salt was washed several times with ethyl acetate, and then the salt was dissolved/melted in hot acetone and recrystallized by adding ethyl acetate to obtain a pure white salt. Pyr13I and LiTFSI (1:1 molar ratio) were mixed in deionized water to obtain Pyr13TFSI (the aqueous layer containing LiI and ILs were separated). The aqueous phase was removed and the salt was washed five times with deionized water to remove the residual Lil. The last aqueous layer was removed and Pyr13TFSI was heated on a hot plate to remove excess solvent. Activated carbon was added and the hot mixture was stirred on a hot plate overnight. Acetone was added and the mixture was filtered through an activated alumina column. The acetone was removed by heating and Pyr13TFSI was dried in vacuum at 100°C overnight and then at 120°C for 6h. The obtained Pyr13TFSI was a transparent colorless liquid at room temperature. The ionic liquid was stored in a dry environment (0.2% relative humidity, 20°C) until use.
3 Preparation of inorganic salt capacitor ionic liquid electrolytes
Inorganic salt ionic liquids mainly include quaternary ammonium salt ionic liquids and quaternary salt ionic liquids. Compared with other ionic liquids, quaternary ammonium salt ionic liquids have cheap and easy-to-obtain raw materials and simple synthesis routes. Quaternary phosphine salt ionic liquids have strong nucleophilic anions and are more stable than quaternary ammonium salt ionic liquids. In addition, quaternary salt ionic liquids have higher thermal stability and viscosity than imidazole ionic liquids containing the same anions. The following introduces the synthesis processes of benzyltriethylamine hydrogen sulfate, tributyltetradecylphosphine hydrogen sulfate and new perchlorate counter ionic liquids.
1) Preparation of Benzyltriethylamine Hydrogen Sulfate Capacitor Ionic Liquid Electrolyte
Place 10 mmol of benzyltriethylamine chloride salt and 30 mL of dichloromethane in a three-necked flask equipped with a reflux condenser and stir evenly. The reflux condenser is equipped with a drying tube. The three-necked flask is placed in an ice-water bath, and an equal amount of concentrated sulfuric acid (mass fraction 98%) is added dropwise. After the addition is complete, react for 8 hours, and transfer the three-necked flask to a 50°C oil bath. After the reaction is completed, the solvent is rotary evaporated to obtain a slightly yellow viscous liquid, namely, benzyltriethylamine hydrogen sulfate ionic liquid. Add a certain amount of lithium salt to the benzyltriethylamine hydrogen sulfate ionic liquid, stir at room temperature for 3~4 hours until it is completely dissolved, and obtain the benzyltriethylamine hydrogen sulfate capacitor ionic liquid electrolytes.
2) Preparation of tributyl tetradecyl hydrogen sulfate capacitor ionic liquid electrolyte
At room temperature, 147.2 g of tributyltetradecylphosphonium chloride (TTPC) was added to a 500 mL reaction bottle with stirring, a thermometer and a reflux condenser, and 35.5 g of concentrated sulfuric acid was added dropwise under stirring. After the addition was completed, the temperature was raised to 95-100 ° C and the reaction was carried out for 20 hours. After the reaction was completed, the product was washed three times with a saturated sodium bisulfate aqueous solution to remove excess sulfuric acid and generated hydrogen chloride. The washed organic layer was dehydrated by vacuum distillation, and then cooled, filtered and other operations were performed to remove a small amount of precipitated salt to obtain the final target product tributyltetradecylphosphonium hydrogen sulfate ionic liquid. A certain amount of lithium salt was added to the tributyltetradecylphosphonium hydrogen sulfate ionic liquid, stirred at room temperature for 3-4 hours, and the tributyltetradecylphosphonium hydrogen sulfate capacitor ionic liquid electrolyte was obtained after complete dissolution.
3) Preparation of novel perchlorate counter-ionic liquid
When 1,10-benzotriline reacts with 1,4-butanesulfonone at different molar ratios and thermal conditions, only one of the two nitrogen atoms is attacked, resulting in the 1,10-benzoquinone part containing only one butylsulfonic acid side chain, which may be related to the steric hindrance in the double bond structure or due to the interference of the enhanced positive charge on the aromaticity of the benzene ring. On the other hand, piperazine salts were prepared by changing the molar ratio of mono- and di-substituted 1,4-dimethyl and the reaction temperature. The monosubstituted piperazine salt was obtained after the reaction of 1,4-dimethylpiperazine with 1,4-butanesulfonone at an equivalent molar ratio for 4 h at room temperature, while the monosubstituted piperazine salt was obtained after the reaction at 60 ° C for 2 h when the molar ratio of 1,4-dimethylpiperazine to 1,4-butanesulfonone was 1:3. Phenytoin sodium or piperazine salt completely dissolved in water was reacted with HC1O4 at an equal molar ratio for 24 h at room temperature. Finally, the targeted BAILs ([PhBs1]CIO4, [PipBs1]C1O4 and [PipBs2](CIO4)2) were obtained after evaporation of the aqueous solvent. The synthetic process is shown in Figure 1
Fig.1 Flow chart of the preparation of [PipBs1]CIO4 and [PipBs2](C1O4)2
4 Preparation of other capacitor ionic liquid electrolytes
There are many types of ionic liquids. In addition to the above-mentioned major categories of capacitor ionic liquid electrolytes, the synthesis of some other types of capacitor ionic liquid electrolytes will be introduced below.
1) Preparation of poly(diallyldimethylammonium)bis(trifluoromethanesulfonyl)imide polymer ionic liquid
Polymeric ionic liquids can be synthesized by polymerization and anion exchange reaction. Compared with polymerization reaction, anion exchange reaction is fast, low cost and simple to operate. Poly(diallyldimethylammonium)bis(trifluoromethanesulfonyl)imide polymer ionic liquid can be synthesized by anion exchange reaction. A certain amount of lithium bis(trifluoromethanesulfonyl)imide salt and poly(diallyldimethylammonium) were dissolved in distilled water respectively. The polydimethyldiallylammonium chloride (DADMAC) solution was placed in a 250 mL round-bottom conical flask, and LiTFSI solution was added dropwise to mix with the poly(DADMAC) solution. Then, the mixed solution was homogenized by magnetic stirring at 25°C, and anion exchange reaction occurred after stirring for 10 minutes, and a white precipitate was produced at the bottom of the conical flask. The precipitate was separated by vacuum filtration, washed with distilled water several times to remove the residual LiCl, and then dried in a vacuum oven at 120°C for 24 hours to obtain a white poly(DADMATFSI) polymer ionic liquid powder.
The synthesized poly(DADMATFSI) was verified by AgNO3 precipitation titration. A small amount of AgNO3 and poly(DADMATFSI) were dissolved in acetone, and the results showed that there were no chloride ions in the polymeric ionic liquid, that is, the synthesized poly(DADMATFSI) did not contain impurities.
2) Preparation of ionic liquid electrolyte for PDEIm capacitor
PDEIm can be prepared based on the anion exchange reaction of poly(1.2-diethoxyethylimidazole) with acetate counterion (PDEImAcO for short). 1.2-Bis(2-aminoethoxy)ethane (4.85 g, 3.3 mmol), acetic acid (4 mL), glyoxal solution (4.85 mL), formaldehyde solution (2.5 mL) and deionized water (10 mL) were mixed in a 100 mL round-bottom flask under ice bath and then refluxed at 100 °C for 2 h. Subsequently, the precipitated product was collected by filtration. The obtained brown-yellow product was thoroughly washed with acetone (200 mL) and then dried under vacuum at 60 °C for 24 h to obtain PDEImAcO. In order to convert PDEImAcO into the desired PDEIm, PDEImAcO aqueous solution (10 mg·mL-1) was added dropwise to LiTFSI aqueous solution (0.1 mol·L-1) and then stirred for 3 h. The obtained product was thoroughly washed with deionized water and then dried under vacuum at 80 °C for 24 h to obtain the PDEIm ionic liquid.
3) Preparation of acetic acid and lactic acid based ionic liquids
The ionic liquid is prepared by a two-step reaction. The first step is to obtain bromides, including N-methyl-N.propylpyrrolidine bromide ([MPPyrr][Brl), N-methyl-N-propylpiperidinium bromide ([MPPip][Brl), N,N-dimethyl-N-Z-ylpiperazine bromide (IDMEPpzl[Brl) and N.N’-dimethyl-N-propylpiperazine bromide ([DMPPpz][Brl). The second stage is anion exchange using an ion exchange resin through an acid-base reaction. Dowex® resin is loaded into a column (about 200 mL) rinsed with deionized water and treated with an excess of aqueous NaOH. Excess NaOH is washed with deionized water to remove all unreacted ions, and the effectiveness of OH-exchange is controlled by measuring the conductivity of the eluent. Then, a solution of pyrrolidine, piperidine or dimethylpiperazinyl bromide is added to the column to obtain the corresponding aqueous hydroxide solution, which is added dropwise to a stoichiometric acetic acid or lactic acid solution as an eluent. Figure 2 shows the reaction mode for obtaining pyrrolidine, piperidine or dimethylpiperazine-based ionic liquids. After evaporating water on a rotary vacuum desiccator, the ionic liquid was dried under vacuum at 333K (60°C) for 24 hours. The white precipitate obtained included [MPPyrr][Brl, [MPPip][Brl, [DMEPpz][Br] and [DMPPpz][Br] with a yield of up to 95%~98%. The colorless liquid obtained included [MPPyrr][Ac] and [MPyr][L]. The water content (mass fraction) in the synthesized ionic liquid was found to be less than 0.1% by Karl Fischer titration0.1%.
4) Preparation of tetraalkylammonium polymer ionic liquids
N, N-diethyl-N-methyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide (DEME-TFSI) as a plasticizer can be prepared by a two-step process. N-diethylmethylamine (1.0 g, 0.012 mol) and 2-bromoethyl methyl ether (2.09 g, 0.015 mol) were added to 40 mL of anhydrous acetone. The mixture was refluxed for 48 h to obtain a white solid DEME-Br and washed repeatedly with ether. Then, DEME-Br and LITFSI were mixed in deionized water and stirred at room temperature for 5 h. The ionic liquid was extracted with chloroform several times and dried over anhydrous magnesium sulfate. After removing the solvent, DEME-TFSI was obtained and vacuum dried at 110 ° C for 12 h to obtain a tetraalkylammonium polymer ionic liquid [DEME-TFSI.
5) Preparation of poly(dimethyldiallyl) capacitor ionic liquid electrolytes and poly(dimethyldiallyl) electrolyte film
Predetermined masses of bis(trifluoromethanesulfonyl)imide salt and poly(diallyldimethylammonium) chloride [poly(diallyldimethylammonium) sodium chloride] were dissolved in distilled water, respectively. The poly(DADMACI) solution was placed in a 250 mL round-bottom conical flask, and the LiTFSI solution was added dropwise to mix with the poly(DADMACI) solution. Then, the mixture was stirred with magnetic force at 25°C for 10 min before anion exchange reaction occurred, and a white precipitate was produced at the bottom of the conical flask. The precipitate was separated by vacuum filtration, washed with distilled water several times to remove the residual LiCl, and then dried in a vacuum oven at 120°C for 24h to obtain a white poly(DADMATFSI) polymer ionic liquid powder. The synthesized poly(DADMATFSI) was verified by AgNO3 precipitation titration. A small amount of AgNO3 and poly(DADMATFSI) were dissolved in acetone. The results showed that chloride ions did not exist in the polymeric ionic liquid, that is, the synthesized poly(DADMATFSI) did not contain impurities.
Polyacetone films were prepared by tape casting with polyacetone as solvent. First, poly(DADMATFSI) polymer ionic liquid was dissolved in acetone at 25°C by magnetic stirring, and then a predetermined amount of LiTFSI salt was added to the poly(DADMATFSI) solution under continuous stirring for 8 h to obtain a uniform mixture. Finally, the solution was poured into a clean glass Petri dish, the solvent was allowed to evaporate at room temperature, and then vacuum dried to form a polymer electrolyte membrane at the bottom of the Petri dish. The resulting polymer electrolyte membrane was placed in a vacuum oven at 40°C for 6 h to remove the residual solvent. In the LiTFSI-doped poly(DADMATFSI) polymer electrolyte film, LiTFSI releases ions for conduction, while poly(DADMATFSI) acts as a host matrix for mechanical stability through the entanglement of polymer chains, and also provides some TFSF ions for conduction. The thickness of the polymer electrolyte membrane was measured by a screw micrometer with an accuracy of 5 mm to be about 120 mm. Before characterization, the films were stored in a glove box to avoid environmental moisture contamination.
6) Preparation of electrolyte membrane by blending poly(aryletherketone)-polyimidazole copolymer with ionic liquid
As shown in Figure 3, BuMePyr-TFSI can be synthesized from mepiquat and ethylene glycol. A solution of 19g IB and 10mL THF was slowly dripped into a reactor containing 10g MePyr and 25mL ethyl acetate. A uniform solution was obtained by continuous stirring at room temperature for 2h. The reactor temperature was raised to 50°C and reacted for 6h until a white solid appeared. The white solid was filtered and washed with ethyl acetate several times, and then vacuum dried at room temperature for 1 day to obtain the BuMePyr-I product. The dried BuMePyr-I (9.5 g) and LiTFSI salt (10 g) were dissolved in 9 deionized water for ion exchange. The initial mixture phase separated after 6h. The bottom phase was collected and rinsed with cold water to remove excess salt. 0.7175g of LiTFSI salt was dissolved in 5mL of BuMePyr-TFST to prepare BuMePyr-TFSI ionic liquid containing 0.5mL LiTFSI.
Figure 3 Synthesis steps of BuMePyr-TFSI
Figure 4 is a synthesis scheme of PBuVIm-I terminated with hydroxyl groups. First, 32 g of VIm and 30 mL of tetrahydrofuran solution were slowly added to a reactor containing 57 g of IB and 70 mL of ethyl acetate to synthesize BuVIm-I. The product was washed into a transparent phase of ethyl acetate after heating at the bottom of the reactor for 24 h. The dried BuVIm-I product was obtained by freeze drying. After adding dried BuVIm-I (5 g) and 30 mL of chloroform to the reactor, AIBN (2%, mass fraction) and ethanol mercaptan were added. The polymerization reaction was carried out under continuous stirring at 75 °C for 2 days under a nitrogen environment. The initial homogeneous phase was transformed into a transparent liquid phase at the top and a high-viscosity liquid phase at the bottom. The high-viscosity liquid phase was collected, washed with chloroform, and vacuum dried to prepare PBuVIm-I terminated with hydroxyl groups.
Figure 4 Synthesis steps of hydroxyl-terminated PBuVIm-TFSI
As shown in FIG5 , PAEK-COOH was synthesized from VB (2.95 g) and DB (2.56 g) monomers in a mixture of dimethyl sulfoxide (45 g) and toluene (40 g) at 170° C. The synthesized PAEK-COOH (2.24 g) and DMF solvent (75 mL) were placed in a three-necked flask and stirred to obtain a uniform solution. PBuVIm-I (0.015 mol) and DMAP (1.5%, molar fraction) were added to the flask. After stirring for 2 h, 3 mL of THF solution containing 2% DCC was slowly added dropwise to the reactor and reacted at 70° C. for 24 h. The reaction product was filtered to remove a small amount of white solid impurities. The filtered solution was precipitated and washed into IPA solvent. The synthesized PAEK-g-[PBuVIm-I] was dried in a vacuum oven at 60° C. for 24 h. The dried PAEK-g-[PBuVIm-I] and 10 mL THF were mixed with another LiTFSI (10 g) salt solution in 16 mL deionized water. After stirring for 2 h, the solid product PAEK-g-[PBuVIm-TFSI was obtained by filtration. The product was dried in a vacuum oven and stored.
Figure 5 Synthesis steps of PAEK-g-[PBuVIm-TFSI]
PAEK-g-[PBuVIm-TFSII (0.5 g) was dissolved in 5 mL of N-methylpyrrolidone (NMP), and 40% of ILs (LiTFSI/BuMePyr-TFSI) was added to prepare a lithium-ion battery electrolyte membrane. After the uniform solution was cast on a polytetrafluoroethylene substrate, it was first dried at 60 ° C for 24 h and then dried under vacuum at 70 ° C for 24 h to completely remove NMP.
3 Study on the physical properties of capacitor ionic liquid electrolytes
In this section, the electrochemical characterization of all capacitor ionic liquid electrolytes is performed in the form of assembled button cells, and each electrolyte is matched with the Li4Ti5O12/LiCoO2 active material. Safety is an important indicator of the quality of secondary batteries, so the flame retardancy and thermal stability of the electrolyte are two important indicators for judging the performance of the electrolyte. Conductivity directly affects the impedance of the battery and is another important indicator for judging the performance of the electrolyte. Different active materials have different voltage ranges for battery charging and discharging. The electrochemical window of the electrolyte is also an important indicator for judging the electrolyte. In addition, the electrolyte will react with the active substances on the electrode sheet to form an SEI film on the surface of the electrode sheet, resulting in an increase in impedance and a decrease in battery capacity. The morphological analysis of the electrode sheet before and after the cycle can observe the amount of side reaction products on the electrode sheet, so as to judge the performance of the electrolyte in combination with the electrochemical performance.
1 Electrode Preparation and Cell Assembly
The active electrode material, conductive material and binder are mixed in a mass ratio of 8:1:1, and the consistency is adjusted with NMP to obtain a uniform slurry with moderate consistency. Use a scraper to evenly apply the mixed slurry to the clean copper foil or aluminum foil, and place it in a vacuum box at 60°C to dry overnight to allow the solvent to fully evaporate. Roll the copper foil or aluminum foil loaded with active substances on a roller press three times, and after compaction, use a slicer to punch out 11mm circular electrode sheets, weigh the mass of the electrode sheet and the copper foil or aluminum foil of the same size, and calculate the loading amount of the active electrode material on the electrode sheet.
The battery assembly process is carried out in a glove box with an Ar environment. The prepared electrode sheet is used as the working electrode, placed in the positive electrode shell, covered with a diaphragm, and an appropriate amount of capacitor ion liquid electrolyte is added. A metal lithium sheet is placed as the counter electrode, and a spring and a gasket are added. The negative electrode shell is covered and sealed with a sealing machine to complete the battery assembly. The assembled battery is left to stand for 12 hours before testing.
2 Physical property testing and characterization of ionic liquid electrolytes
1) Electrolyte flammability test
The general method for testing the flammability of ionic liquid electrolytes is the self-extinguishing time test. During the test, a rectangular glass fiber of a certain size is used to completely soak it in the ionic liquid electrolyte to be tested, and the excess ionic liquid electrolyte is absorbed with filter paper. Use an iron stand to hang one end of the treated glass fiber soaked with the ionic liquid electrolyte to be tested, and the bottom end is about 50mm away from the wick of the alcohol lamp. The glass fiber is in contact with the fire source for about 10s, and the alcohol lamp is removed. Observe the burning of the glass fiber. If the glass fiber is not ignited throughout the process or automatically extinguishes after the alcohol lamp is removed, the ionic liquid electrolyte is considered to be non-flammable. Otherwise, the ionic liquid electrolyte is considered to be flammable. If the ionic liquid electrolyte is flammable, record the time when the glass fiber starts to burn and the time when the burning ends and automatically extinguishes, and calculate the burning time of the ionic liquid electrolyte. Repeat the test three times. If the results of the three tests are all non-flammable, the ionic liquid electrolyte is considered to be non-flammable. If it is flammable, calculate the average burning time of the three tests.
2) Thermal stability test of ionic liquid electrolyte
The thermal stability test of ionic liquid electrolyte is mainly carried out by thermogravimetric analysis of ionic liquid electrolyte. The ionic liquid electrolyte sample is placed in a thermogravimetric analyzer to observe the change of the sample mass with temperature and time, so as to study the stability and composition of the ionic liquid electrolyte.
3) Surface morphology analysis
The surface morphology analysis mainly studies the effect of ionic liquid electrolyte on the electrode sheet during the cycle process, that is, the surface morphology of the cycled electrode sheet and the uncycled electrode sheet is observed. The cycled electrode sheet is disassembled in the glove box, and the electrode sheet is taken out to clean the ionic liquid electrolyte and other residual on the surface with DMC, and then placed in the glove box to wait for the organic solvent to fully evaporate before being sealed in a bottle for use.
4) Fourier transform infrared spectroscopy analysis
Fourier transform infrared spectroscopy is used to identify the molecular structure in capacitor ionic liquid electrolytes and analyze the interactions between molecules.
3 Electrochemical performance test
1) Conductivity
The conductivity of the ionic liquid electrolyte was tested by the electrochemical impedance method, and the conductivity tester was used for measurement. During the entire test process, the ambient temperature of the sample was controlled by a constant temperature water bath. After the water bath reached the set temperature, the temperature was kept constant for 1 hour before testing. The entire test was repeated three times, and the average value of the three conductivity values was taken.
2) Cyclic voltammetry
The cyclic voltammetry test was performed using an electrochemical workstation to study the electrochemical reaction process of the electrode material and observe the electrochemical stability of the ionic liquid electrolyte.
3) Electrochemical impedance spectroscopy
The electrochemical impedance spectroscopy is an important means to study the process kinetics, electrochemical properties and interfacial reactions between the ionic liquid electrolyte and the electrode. The electrochemical impedance spectroscopy was measured using an electrochemical workstation to analyze the impedance during the reaction and the diffusion impedance of the electrode material. The test frequency was 10-2~105Hz.
4) Constant current charge and discharge
The constant current charge and discharge test is to charge and discharge the assembled battery through the battery test system according to the current density and voltage range set by the program, and the battery test system records the voltage-current change curve and the amount of electricity released during the charge and discharge process. The constant current charge and discharge test is the most intuitive method to study the electrochemical properties of ionic liquid electrolytes and electrode materials. According to the previously calculated active material load on the electrode, multiplied by the corresponding current density, the current value of charge and discharge is obtained. The voltage range is the same as the voltage range of the cyclic voltammetry test.
4 Specific example analysis
The following is a detailed analysis based on several examples from recent years.
1) Characterization of temperature-controlled bisimidazolium ionic liquids
Most ionic liquid two-phase catalytic reactions occur at the interface of the two phases or in the ionic liquid phase. Because general ionic liquids are immiscible with organic solvents, many reactions are hindered and the reaction rate is limited. Ionic liquids that are miscible with organic solvents are difficult to separate after the catalysis, which is not conducive to the recycling of catalysts and makes it difficult to purify the products. Based on the disadvantages of ordinary ionic liquids, researchers have further proposed temperature-controlled ionic liquids. The solubility of temperature-controlled variable phase ionic liquids (also known as temperature-controlled ionic liquids) can change significantly with temperature, and can achieve “high temperature homogeneity and low temperature separation”, that is, liquid catalysis at high temperature, and easy to form solid separation at low temperature. It is a new type of ionic liquid that combines high catalytic activity of homogeneous catalysts and easy separation and recovery of heterogeneous catalysts. This example is based on the literature on the synthesis of temperature-controlled ionic liquids and uses crystallographic data to conduct a relevant analysis of the structure of biimidazole-type temperature-controlled ionic liquids.
Determination of crystal structure: A white single crystal sample of size 0.20 mm×0.10 mm×0.10 mm was selected and diffraction data were collected at 296K using a CCD-X-ray SMART-1000 single crystal diffractometer. MoKa rays (r=0.71073) monochromated by a graphite monochromator were used to collect a total of 3517 diffraction data within the range of 2.6°<0<25.50° in ω scanning mode. The data were corrected by Lp factor and empirical absorption. All non-hydrogen atoms were found by direct method and several rounds of difference Fourier synthesis. The coordinates of all non-hydrogen atoms and their anisotropic temperature factors were refined by full matrix least squares method. All structural calculations were completed using SHELX-97 program.
The crystallographic data of DIM3 in Table 2 show that the DIM3 crystal presents a tetragonal system and is highly centrosymmetric in the unit cell stacking structure. The symmetry of this structure has certain application value in temperature-controlled separation and can improve the polarity of water. When it is used in a two-phase system mixed with toluene to catalyze the condensation reaction of amine aldehydes, the condensation reaction results are good, achieving the purpose of “high temperature homogeneous phase and low temperature phase separation” of the temperature-controlled ionic liquid, and it can be recovered and reused after the reaction.
Table 2 DIM3 crystallographic data
2) Preparation and characterization of novel polydiionic liquid (PGDIL)
As shown in Figure 6, a new type of polydiionic liquid (PGDIL) film was prepared by electrochemical polymerization. The morphology and structure of the PGDIL film were analyzed by SEM and FTIR, and the polymerization mechanism of GDIL on the gold electrode surface was determined to be three-dimensional instantaneous nucleation. FTIR confirmed the presence of polyaniline-like units in the PGDIL chain, indicating that the aniline group of GDIL was obtained by electropolymerization on the Au electrode surface. The electrochemical performance of PGDIL in phenol was studied by cyclic voltammetry, and it was determined that the PGDIL film electrode promoted the electrocatalysis of phenol.
Figure 6 Preparation of PGDIL
Figure 7(a) is a scanning electron microscope image of PGDIL powder. As shown in the figure, PGDIL powder consists of uniform, stacked spherical particles, which are similar in shape to most conductive polymers. In order to further understand the structure of PGDIL and GDIL, the IR spectra of the polymer powders were tested, as shown in Figure 7(b). In the FTIR spectrum of PGDIL, the bands at 3158cm-1 and 3106cm-1 are clearly visible, which are caused by the C-H ring stretching vibration of the aromatic ring (imidazole and benzene). As more hydrogen bonds are formed after polymerization, the C-H bond length increases, the bond force constant decreases, and the absorption wavenumber red-shifts. 3462cm-1 is the stretching vibration band of secondary amine, and the shoulder band 3342 cm-1 is the N-H double frequency peak of bending vibration. Compare the red curve (GDIL) and the black curve (PGDIL). The peak at 1312 cm-1 may be attributed to the C-N stretching vibration band, the peaks in the range of 1700~1400 cm-1 belong to the N-H deformation vibration (1609 cm-1) and the stretching vibration bands of C=C and C-C in the aromatic ring, the deformation of the imidazole ring at 924 cm-1, attributed to the PeF stretching vibration, and the peak at 830 cm-1 is attributed to the N-H out-of-plane bending vibration (broad peak). In the infrared spectrum of the GDIL monomer, the C-(CH2)2C skeleton vibration at 739 cm-1 and the C-H (1,3-disubstituted benzene ring) out-of-plane bending vibration at 682 cm-1 were observed. In the infrared spectrum of PGDIL, the peak at 1329cm-1 belongs to the C—N stretching vibration of the benzoquinone ring variant, the peak outside 1250cm-1 belongs to the conjugated C=N stretching vibration in the quinone ring structure, the peak at 1108cm-1 belongs to the stretching vibration of N=O=N, O=N+H=B and B—N+H—B, the imidazole ring is deformed at 920cm-1, and PeF has a stretching vibration (spike) at 835cm-1, which means that the -NH2 group participates in the polymerization reaction and is converted into -NH— or -N1/4 C—(CH2)2—C vibration is observed at 744cm-1, and C—H (13.5-trisubstituted benzene ring) out-of-plane bending vibration is observed at 652cm-1.
However, the infrared spectrum of PGDIL is very different from that of GDIL in the range of 1700 cm-1 to 1400 cm-1. This is because the structures of phenylenediamine and quinolinediamine formed by the polymerization of aniline groups in GDIL are similar to those of polyphenylene [Figure 7(c)
Figure 7 (a) SEM image of PGDIL; (b) IR spectra of GDIL and PGDIL; (c) Schematic diagram of the molecular structure of PGDIL
Capacitor ionic liquid electrolytes are suitable for secondary electrochemical energy storage devices over a wide temperature range due to their excellent thermal stability and adjustable physical properties. Different types of electrolytes can be obtained by selecting suitable ionic liquids and lithium salts in combination with specific preparation methods. Common ionic liquids include imidazoles, piperidines, pyrroles and inorganic salts, and their preparation requires strict control of moisture and atmosphere to ensure stable performance. These ionic liquid electrolytes have shown great potential in improving the safety, cycle life and electrochemical stability of lithium batteries, especially in applications under high temperature conditions.
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