Electrolyte for lithium ion capacitor and preparation method thereof

Abstract

The invention relates to the technical field of electrolyte, in particular to electrolyte for a lithium ion capacitor and a preparation method of the electrolyte. The electrolyte for the lithium ion capacitor comprises the following preparation raw materials: 50 to 55 parts of ethylene carbonate, 30 to 35 parts of methyl cyclohexylformate, 8 to 10 parts of siloxane solvent, 20 to 25 parts of lithium salt, 0.5 to 0.8 part of bis (trifluoromethanesulfonyl) imine, 2 to 5 parts of quaternary ammonium salt and 2 to 2.5 parts of functional additive. The electrolyte for the lithium ion capacitor prepared by the invention has excellent low-temperature performance, high power characteristic, wide temperature range adaptability, long cycle life and safety by optimizing the solvent system, the lithium salt ratio and the synergistic effect of the multifunctional additive, and is suitable for the high-performance lithium ion capacitor.

Description

Technical Field

[0001] This application relates to the field of electrolyte technology, and in particular to an electrolyte for lithium-ion capacitors and a method for preparing the same. Background Technology

[0002] Lithium-ion capacitors combine the high power of supercapacitors with the high specific energy of lithium-ion batteries. However, significant kinetic differences exist between traditional battery-type and double-layer electrode materials, greatly limiting the long-cycle performance of lithium-ion capacitors under high-rate conditions. As a crucial component of lithium-ion capacitors, the electrolyte not only provides the cations and anions required for charging and discharging but also provides a liquid environment for their migration. Hard carbon (HC) possesses abundant defect sites, nanopores, and short-range graphitization domains, which can shorten the lithium-ion diffusion path and reduce local current density. Choosing hard carbon as the anode enables high-rate characteristics in lithium-ion capacitors. At the hard carbon anode interface, the electrolyte participates in the evolution of the solid electrolyte interphase (SEI) film, forming a stable and robust SEI film. Differences in SEI film quality directly affect the long-cycle performance of lithium-ion capacitors. Therefore, developing a high-performance electrolyte compatible with hard carbon anodes is an effective approach to achieving ultra-long cycles in lithium-ion capacitors under dynamic conditions.

[0003] To better leverage the characteristics of lithium-ion capacitors in high and low temperatures and high-rate, long-cycle operation, modifying the electrolyte composition and structure is undoubtedly a simple and efficient modification method. The solvent composition significantly influences the rate of lithium-ion transport and the operating temperature range, while trace amounts of functional additives affect the formation of the solid electrolyte film at the negative electrode. Therefore, the selection of the electrolyte and functional additives is crucial. Summary of the Invention

[0004] The purpose of this application is to address the shortcomings of the current technology by providing an electrolyte for lithium-ion capacitors and a method for preparing the same. The electrolyte for lithium-ion capacitors prepared in this application has high ionic conductivity and oxidation potential at low and room temperatures, and has good compatibility with hard carbon anodes. It is suitable for lithium-ion capacitors and improves the high-rate charge-discharge and long-cycle characteristics of batteries.

[0005] In a first aspect, this application provides an electrolyte for lithium-ion capacitors, employing the following technical solution: An electrolyte for lithium-ion capacitors, comprising, by mass parts, the following raw materials: 50-55 parts ethylene carbonate, 30-35 parts methyl cyclohexylcarboxylate, 8-10 parts siloxane solvent, 20-25 parts lithium salt, 0.5-0.8 parts bis(trifluoromethanesulfonyl)imide, 2-5 parts quaternary ammonium salt, and 2-2.5 parts functional additives.

[0006] By employing the above technical solutions, ethylene carbonate, as a high dielectric constant solvent, provides excellent lithium salt dissociation capability and forms a gradient solvation structure with weakly solvated siloxane solvents, balancing ion dissociation and migration rates. Methyl cyclohexylcarbamate, as a low-viscosity co-solvent, improves low-temperature fluidity and forms a low-melting-point eutectic system with carbonates, broadening the operating temperature range. Siloxane solvents (such as 3-isocyanate-propyltriethoxysilane and aniline-methyltriethoxysilane) are used. 3-Isocyanate-propyltriethoxysilane contains active isocyanate groups and can self-assemble into a film on the electrode surface; aniline-methyltriethoxysilane enhances interfacial electron transport through a benzene ring conjugated system. The hydrolysis of silicon-oxygen bonds generates a SiO₂-x inorganic phase, constructing a gradient SEI structure, and the weak solvation effect reduces the lithium-ion desolventization activation energy. Lithium salts (such as those composed of lithium hexafluorophosphate and organic lithium salts, where lithium hexafluorophosphate provides high dissociation degree Li₂) are used. + The source, with anion radii matching the pore structure of the hard carbon anode, and organic lithium salts containing large-volume organic anions, facilitates competitive adsorption of anions: PF6- preferentially adsorbs at the positive electrode, while organic anions accumulate at the anode interface, reducing the activation energy for lithium-ion transport. Quaternary ammonium salts improve the conductivity of the system and enhance the cycle life of lithium-ion capacitors. Bis(trifluoromethanesulfonyl)imide inhibits electrolyte decomposition and enhances thermal stability and corrosion resistance. Functional additives (such as magnesium fluoride, lithium bis(oxalato)borate, and vinylene carbonate) are used. Magnesium fluoride is introduced for the first time as a novel functional additive. On one hand, it can form a fluorinated solvent, adjusting the viscosity and interfacial compatibility of the electrolyte; on the other hand, the strong electron-withdrawing properties of fluorine atoms can affect the solvation ability of the solvent and participate in the formation of the SEI film, thereby inhibiting electrolyte decomposition. This is beneficial for improving the oxidation potential of the electrolyte and the cycle performance of lithium-ion capacitors under high-rate charge-discharge conditions. Lithium bis(oxalate-borate) inhibits the oxidative decomposition of the electrolyte under high voltage, enhances thermal stability, and has a more significant effect on improving low-temperature performance, making it suitable for wide-temperature-range applications. Ethylene carbonate forms a stable protective film (SEI / CEI) on the electrode surface, reducing metal corrosion and improving cycle performance. In summary, this system achieves a triple effect of solvation structure regulation, interface engineering optimization, and enhanced transport kinetics through the synergistic function of multiple components, making it particularly suitable for wide-temperature-range, high-power lithium-ion capacitor applications.

[0007] Preferably, the siloxane solvent is composed of 3-isocyanate-propyltriethoxysilane and aniline-methyltriethoxysilane in a mass ratio of 3:2.

[0008] By employing the above technical solutions, the active isocyanate group (-NCO) of 3-isocyanate-propyltriethoxysilane exhibits high reactivity, capable of undergoing condensation reactions with hydroxyl (-OH) or amino (-NH2) groups on the electrode surface to form an organic-inorganic composite interfacial film in situ. The triethoxysilane structure, upon hydrolysis, generates a silica (SiO2) precursor, participating in the construction of an inorganic-rich SEI (solid electrolyte interphase) layer. It self-assembles on the hard carbon surface of the negative electrode to form a cross-linked network structure, enhancing the interfacial mechanical strength. Chemical bonding seals the active sites on the electrode surface, reducing electrolyte decomposition. The aniline conjugated system of aniline-methyltriethoxysilane provides electron transport channels through its π-electron delocalization structure, enhancing interfacial charge transfer capability. The methyltriethoxysilane group, while generating SiO2 through hydrolysis, imparts hydrophobicity to the interfacial layer, inhibiting moisture penetration. The benzene ring conjugated structure improves interfacial electronic conductivity. The flexible aniline group alleviates volumetric strain during charge and discharge. The aromatic ring structure increases the decomposition temperature to >300℃, suppressing high-temperature interfacial failure. The synergistic mechanism of a 3:2 mass ratio results in film density: the cross-linking effect of isocyanate groups (3 parts) dominates the formation of a dense underlayer, while aniline groups (2 parts) construct a conductive network on the surface, forming a gradient interface structure. The amino groups of isocyanate and aniline undergo partial urea bond (-NH-CO-NH-) cross-linking, enhancing the interfacial bonding strength. Desolvation is accelerated: the weak solvation effect of isocyanate groups synergistically works with the electron transport of aniline, shortening the Li... + Desolvation time. Ion transport channels: The SiO2 nanoparticles generated by hydrolysis and the aniline conductive network jointly construct a three-dimensional ion transport path, enhancing the interface Li... + The diffusion coefficient is high. The dual-barrier mechanism: the cross-linked network formed by isocyanate inhibits organic solvent penetration, while the conjugated structure of aniline blocks the migration of high-valence metal ions, synergistically enhancing the oxidation potential. Self-healing properties: at a 3:2 ratio, unreacted siloxane groups can dynamically repair interface defects during cycling. This specific ratio, through precise control of the chemical composition and physical structure of the interface layer, achieves synergistic optimization of mechanical strength, conductivity, and kinetic performance, and is one of the core design features for achieving wide temperature range and high power characteristics in this system.

[0009] Preferably, the lithium salt is composed of lithium hexafluorophosphate and an organic lithium salt in a mass ratio of 5:1-2.

[0010] Preferably, the organolithium salt is at least one of lithium bis(trifluoromethanesulfonylimide) and lithium difluorooxalate borate.

[0011] Preferably, the quaternary ammonium salt is at least one selected from N,N-dimethylpyrrolidineonium tetrafluoroborate, tetrafluoroborate bispyrrolidine spirocyclic quaternary ammonium salt, and 5,5-spirobispyrrolidineonium tetrafluoroborate.

[0012] Preferably, the functional additive is at least one of magnesium fluoride, lithium bis(oxalato)borate, and vinylene carbonate.

[0013] Preferably, the functional additive is composed of magnesium fluoride, lithium bis(oxalate)borate, and vinylene carbonate in a mass ratio of 3:2:5.

[0014] By adopting the above technical solutions, magnesium fluoride (MgF2) forms a dense inorganic interface layer, improving the stability of the electrode / electrolyte interface, while reducing the viscosity of the electrolyte and promoting the rapid transport of lithium ions at low temperatures. Lithium bis(oxalato)borate (LiBOB) enhances the high-temperature and high-pressure stability of the electrolyte, reduces transition metal dissolution, and further improves the mechanical strength and ion selectivity of the interface. Vinylene carbonate (VC) forms a stable protective film on the electrode surface, reducing metal corrosion and improving cycle performance and overall battery life. The three components work synergistically: 1) Interface construction: MgF2 forms the bottom inorganic substrate, LiBOB provides the ion sieving function of the intermediate layer, and VC forms the flexible protective film on the surface, jointly constructing a gradient interface structure. 2) Solvation-transport synergy: The weak solvation effect of MgF2 and the viscosity-reducing effect of LiBOB work synergistically to improve low-temperature ionic conductivity; the film-forming effect of VC reduces the loss of active lithium, and works with the Li- ion transport of LiBOB... + The complementary effects create a dynamic equilibrium. 3) Electrochemical-thermodynamic coupling: The combination of MgF2 and LiBOB improves the electronic insulation of the SEI and reduces the self-discharge rate; the high thermal stability of VC and the antioxidant properties of LiBOB work synergistically, enabling the system to operate over a wide temperature range. In summary, these three additives, in a mass ratio of 3:2:5, work synergistically to significantly improve the performance of the electrolyte at low and normal temperatures, extend the cycle life of the battery, and enhance overall stability.

[0015] Secondly, this application provides a method for preparing an electrolyte for lithium-ion capacitors, employing the following technical solution: As a general technical concept, this application also provides a method for preparing the electrolyte for the above-mentioned lithium-ion capacitor, including the following steps: S81. According to the mass fractions, in a drying room with a dew point ≤ -40℃ and a temperature of 15-20℃, mix ethylene carbonate, methyl cyclohexyl carboxylate and siloxane solvent, stir thoroughly, and prepare a mixed solvent. S82. According to the mass fractions, lithium salt, bis(trifluoromethanesulfonyl)imide, quaternary ammonium salt and functional additives are added to the mixed solvent in sequence, stirred and mixed, and then heated to 30-35℃ and stirred to dissolve, so as to obtain the electrolyte for lithium-ion capacitors.

[0016] In summary, the beneficial technical effects of this application are as follows: 1. Significantly improved low-temperature performance High ionic conductivity: Maintains high ionic conductivity at -40℃, supporting efficient operation under low-temperature conditions.

[0017] Rapid desolvation: Siloxane solvents reduce the desolvation energy of lithium ions, ensuring rapid transport at low temperatures.

[0018] 2. High power characteristics at room temperature High oxidation potential: Increases the oxidation potential of the electrolyte, supporting the stable operation of high-voltage cathode materials.

[0019] Low interface impedance: The formation of a dense SEI / CEI film reduces interface impedance and supports high-rate charge and discharge.

[0020] 3. Hard carbon anode compatibility optimization Dense SEI protective layer: forms a composite inorganic layer, improving the first coulombic efficiency of the hard carbon anode.

[0021] Suppressing negative electrode expansion: The flexible structure alleviates the volume change of hard carbon and improves cycle stability.

[0022] 4. Wide temperature range adaptability High-temperature stability: Inhibits the decomposition of electrolyte at high temperatures and improves thermal stability.

[0023] Low-temperature fluidity: Maintain the electrolyte in a liquid state at low temperatures to avoid performance degradation.

[0024] 5. Long cycle life Dynamic interface repair: Self-healing SEI membrane improves cycle life.

[0025] Metal corrosion inhibition: The protective film effectively blocks the electrolyte from contacting the current collector, reducing corrosion.

[0026] 6. Safety and economy Low gas production characteristics: Reduces side reactions, lowers gas production, and improves safety.

[0027] Cost control: Control costs through precise proportioning to improve cost-effectiveness. Detailed Implementation

[0028] The embodiments of this application will be described in detail below with reference to examples. However, those skilled in the art will understand that the following examples are for illustrative purposes only and should not be considered as limiting the scope of this application. Unless otherwise specified in the examples, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all commercially available conventional products.

[0029] In the following examples, 1 part means 1g.

[0030] Example 1 An electrolyte for lithium-ion capacitors, comprising, by mass parts, the following raw materials: 50 parts ethylene carbonate, 30 parts methyl cyclohexylcarboxylate, 8 parts siloxane solvent, 20 parts lithium salt, 0.5 parts bis(trifluoromethanesulfonyl)imide, 2 parts N,N-dimethylpyrrolidone ontium tetrafluoroborate, and 2 parts functional additives. The siloxane solvent is composed of 3-isocyanatopropyltriethoxysilane and anilinemethyltriethoxysilane in a mass ratio of 3:2. The lithium salt is composed of lithium hexafluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in a mass ratio of 5:1. The functional additive is ethylene carbonate. The preparation method of the electrolyte for lithium-ion capacitors mentioned above includes the following steps: S81. According to the mass fractions, in a drying room with a dew point ≤ -40℃ and a temperature of 15℃, mix ethylene carbonate, methyl cyclohexyl carboxylate and siloxane solvent, stir thoroughly, and prepare a mixed solvent. S82. According to the mass fractions, lithium salt, bis(trifluoromethanesulfonyl)imide, quaternary ammonium salt and functional additives are added to the mixed solvent in sequence, stirred and mixed, and then heated to 30°C and stirred to dissolve, so as to obtain the electrolyte for lithium-ion capacitors.

[0031] Example 2 An electrolyte for lithium-ion capacitors, comprising, by mass parts, the following raw materials: 55 parts ethylene carbonate, 35 parts methyl cyclohexylcarboxylate, 10 parts siloxane solvent, 25 parts lithium salt, 0.8 parts bis(trifluoromethanesulfonyl)imide, 5 parts bispyrrolidine spirocyclic ammonium tetrafluoroborate, and 2.5 parts functional additives. The siloxane solvent is composed of 3-isocyanatopropyltriethoxysilane and anilinemethyltriethoxysilane in a mass ratio of 3:2. The lithium salt is composed of lithium hexafluorophosphate and lithium difluorooxalate borate in a mass ratio of 5:2. The functional additive is lithium dioxalate borate. The preparation method of the electrolyte for lithium-ion capacitors mentioned above includes the following steps: S81. According to the mass fractions, in a drying room with a dew point ≤ -40℃ and a temperature of 20℃, mix ethylene carbonate, methyl cyclohexyl carboxylate and siloxane solvent, stir thoroughly, and prepare a mixed solvent. S82. According to the mass fractions, lithium salt, bis(trifluoromethanesulfonyl)imide, quaternary ammonium salt and functional additives are added to the mixed solvent in sequence, stirred and mixed, and then heated to 35°C and stirred to dissolve, so as to obtain the electrolyte for lithium-ion capacitors.

[0032] Example 3 An electrolyte for lithium-ion capacitors comprises, by mass parts, the following raw materials: 53 parts ethylene carbonate, 34 parts methyl cyclohexylcarboxylate, 9 parts siloxane solvent, 23 parts lithium salt, 0.7 parts bis(trifluoromethanesulfonyl)imide, 3 parts 5,5-spirobispyrroleon tetrafluoroborate, and 2.3 parts functional additives. The siloxane solvent is composed of 3-isocyanatopropyltriethoxysilane and anilinemethyltriethoxysilane in a mass ratio of 3:2. The lithium salt is composed of lithium hexafluorophosphate and lithium difluorooxalate borate in a mass ratio of 5:1.5. The functional additive is magnesium fluoride. The preparation method of the electrolyte for lithium-ion capacitors mentioned above includes the following steps: S81. According to the mass fractions, in a drying room with a dew point ≤ -40℃ and a temperature of 18℃, mix ethylene carbonate, methyl cyclohexyl carboxylate and siloxane solvent, stir thoroughly, and prepare a mixed solvent. S82. According to the mass fractions, lithium salt, bis(trifluoromethanesulfonyl)imide, quaternary ammonium salt and functional additives are added to the mixed solvent in sequence, stirred and mixed, and then heated to 33°C and stirred to dissolve, so as to obtain the electrolyte for lithium-ion capacitors.

[0033] Example 4 Similar to Example 3, except that the functional additive is composed of magnesium fluoride, lithium bis(oxalate)borate, and vinylene carbonate in a mass ratio of 3:2:5.

[0034] Example 5 Similar to Example 3, except that the functional additive is lithium bis(oxalato)borate.

[0035] Example 6 Similar to Example 3, except that the functional additive is vinylene carbonate.

[0036] Comparative Example 1 Similar to Example 4, except that the siloxane solvent is 3-isocyanate-propyltriethoxysilane.

[0037] Comparative Example 2 Similar to Example 4, except that the siloxane solvent is aniline methyltriethoxysilane.

[0038] Performance testing 1. The lithium-ion capacitors prepared in Examples 1-6 and Comparative Examples 1-2 were subjected to LSV oxidation potential and ionic conductivity tests using electrolytes. The results are shown in Table 1.

[0039] 2. The lithium-ion capacitors prepared in Examples 1 and 6 and Comparative Examples 1-2 were used to prepare lithium-ion capacitors using electrolytes. The positive electrode of the lithium-ion capacitor included high-voltage lithium cobalt oxide and activated carbon, the negative electrode was hard carbon, and the separator was a functional ceramic separator. The lithium-ion capacitors were subjected to high-rate 10C charging / 20C discharging. The capacitance retention rate of the capacitors after 5000 cycles was tested under the conditions of voltage 2.5-4.2V and temperature -40℃ and 25℃. The results are shown in Table 1.

[0040] Table 1 Performance Tests Analyzing the data in Table 1, we can see that: 1) The electrolytes for lithium-ion capacitors prepared in Examples 1-6 have high ionic conductivity and oxidation potential at low and room temperatures, and have good compatibility with hard carbon anodes. They are suitable for lithium-ion capacitors and improve the high-rate charge-discharge and long-cycle characteristics of batteries.

[0041] 2) A comparative analysis of the performance of the electrolytes for lithium-ion capacitors prepared in Examples 4 and 3, and Examples 5 and 6, shows that the functional additive, composed of magnesium fluoride, lithium bis(oxalate-borate), and vinylene carbonate in a mass ratio of 3:2:5, significantly improves the oxidation potential of the electrolyte at low and room temperatures and the cycle life of lithium-ion capacitors at high charge-discharge rates by utilizing their synergistic effect. The high oxidation potential originates from the synergistic antioxidant effect of magnesium fluoride and LiBOB, which, although slightly reducing conductivity, significantly improves interfacial stability.

[0042] 3) The performance comparison analysis of the electrolytes for lithium-ion capacitors prepared in Example 4 and Comparative Examples 1-2 shows that the use of siloxane solvent composed of 3-isocyanate-propyltriethoxysilane and aniline-methyltriethoxysilane in a mass ratio of 3:2 can effectively weaken the ion-dipole interaction and accelerate the desolvation process of lithium ions at low temperature by utilizing their synergistic effect. It can also construct an electrode / electrolyte interface rich in inorganic matter in situ, and realize rapid interfacial transport kinetics to achieve high rate performance.

[0043] The above embodiments are only used to explain the technical solutions of this application and are not intended to limit it. Although the above embodiments have provided specific descriptions of this application, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of this invention. Any modifications and equivalent substitutions that do not depart from the spirit and scope of this application should be covered within the protection scope of this application.

Claims

1. An electrolyte for lithium-ion capacitors, characterized in that, The preparation materials include the following raw materials by mass: 50-55 parts ethylene carbonate, 30-35 parts methyl cyclohexylcarboxylate, 8-10 parts siloxane solvent, 20-25 parts lithium salt, 0.5-0.8 parts bis(trifluoromethanesulfonyl)imide, 2-5 parts quaternary ammonium salt, and 2-2.5 parts functional additives.

2. The electrolyte for a lithium-ion capacitor according to claim 1, characterized in that, The siloxane solvent is composed of 3-isocyanate-propyltriethoxysilane and aniline-methyltriethoxysilane in a mass ratio of 3:

2.

3. The electrolyte for a lithium-ion capacitor according to claim 1, characterized in that, The lithium salt is composed of lithium hexafluorophosphate and an organic lithium salt in a mass ratio of 5:1-2.

4. The electrolyte for a lithium-ion capacitor according to claim 3, characterized in that, The organic lithium salt is at least one of lithium bis(trifluoromethanesulfonylimide) and lithium difluorooxalate borate.

5. The electrolyte for a lithium-ion capacitor according to claim 1, characterized in that, The quaternary ammonium salt is at least one of N,N-dimethylpyrrolidine ontium tetrafluoroborate, tetrafluoroborate bispyrrolidine spirocyclic quaternary ammonium salt, and 5,5-spirobispyrrolidine ontium tetrafluoroborate.

6. The electrolyte for a lithium-ion capacitor according to claim 1, characterized in that, The functional additive is at least one of magnesium fluoride, lithium bis(oxalato)borate, and vinylene carbonate.

7. The electrolyte for a lithium-ion capacitor according to claim 6, characterized in that, The functional additive is composed of magnesium fluoride, lithium bis(oxalate)borate, and vinylene carbonate in a mass ratio of 3:2:

5.

8. A method for preparing an electrolyte for a lithium-ion capacitor according to any one of claims 1-7, characterized in that, Includes the following steps: S81. According to the mass fractions, in a drying room with a dew point ≤ -40℃ and a temperature of 15-20℃, mix ethylene carbonate, methyl cyclohexyl carboxylate and siloxane solvent, stir thoroughly, and prepare a mixed solvent. S82. According to the mass fractions, lithium salt, bis(trifluoromethanesulfonyl)imide, quaternary ammonium salt and functional additives are added to the mixed solvent in sequence, stirred and mixed, and then heated to 30-35℃ and stirred to dissolve, so as to obtain the electrolyte for lithium-ion capacitors.

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