Electrolyte for improving large-rate long-cycle performance of lithium ion capacitor and preparation method and application thereof

Abstract

The invention discloses an electrolyte for improving the large multiplying power and long circulation of a lithium ion capacitor in the field of capacitors, the electrolyte comprises a mixed organic solvent, a conductive lithium salt, a quaternary ammonium salt and a functional additive, and the functional additive comprises a film-forming additive and fluorite with the purity of 99.9%. According to the electrolyte, synergistic solvation among ions is achieved by adjusting the components of the solvent, the salt and the additive, a trace amount of high-purity fluorite is introduced as a novel functional additive for the first time to participate in formation of an SEI film, and decomposition of the electrolyte is inhibited. The electrolyte has relatively high ionic conductivity and oxidation potential at normal temperature, has good compatibility with a hard carbon negative electrode, is suitable for a lithium ion capacitor, and improves the high-rate charge-discharge long cycle characteristic of a battery.

Description

Technical Field

[0001] The invention belongs to the technical field of electrolytes, and in particular relates to an electrolyte capable of improving the high-rate and long-cycle of lithium-ion capacitors, and a preparation method and application thereof. Background Art

[0002] Lithium-ion capacitors can combine the high power of supercapacitors with the high specific energy of lithium-ion batteries. However, there are huge kinetic differences between traditional battery-type and double-layer electrode materials, which largely limit the long cycle characteristics of lithium-ion capacitors under high-rate conditions. As an important component of lithium-ion capacitors, the electrolyte not only provides the anions and cations required for device charging and discharging, but also provides a liquid environment for the migration of anions and cations. Hard carbon (HC) has abundant defect sites, nanopores and short-range graphitization domains, which can shorten the diffusion of lithium ions and reduce the local current density. Choosing hard carbon as the negative electrode can achieve the high-rate characteristics of lithium-ion capacitors. At the interface of the hard carbon negative electrode, the electrolyte participates in the evolution of the solid electrolyte interface film (SEI) to form a stable and tough SEI film. The difference in the quality of the SEI film directly affects the long cycle performance of lithium-ion capacitors. Therefore, to achieve ultra-long cycles of lithium-ion capacitors under dynamic conditions, it is an effective way to develop an electrolyte that is compatible with hard carbon negative electrodes and has high performance.

[0003] CN116247302A discloses a hard carbon / graphite-based lithium-ion battery electrolyte and its application, which has a relatively high low-temperature conductivity, and the battery significantly improves the low-temperature performance without reducing the high-temperature performance. However, the battery does not have a high-rate long cycle characteristic, i.e., more than 5,000 cycles at 10C charge / 20C discharge.

[0004] In order to better utilize the characteristics of lithium-ion capacitors in high-rate and long-cycle applications, the regulation of electrolyte components and structures is undoubtedly a simple and efficient modification method. The composition of the solvent greatly affects the speed of lithium ion transmission. At the same time, trace functional additives affect the formation of the negative electrode solid electrolyte membrane. Therefore, the selection of the electrolyte and functional additives is crucial. Summary of the invention

[0005] The present invention is intended to provide an electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors, thereby solving the problem that the electrolyte in the prior art does not have high-rate and long-cycle characteristics and is not suitable for lithium-ion capacitors.

[0006] An electrolyte for improving the high-rate and long-cycle of lithium-ion capacitors in this solution is characterized by comprising a mixed organic solvent, a conductive lithium salt, a quaternary ammonium salt and a functional additive, wherein the functional additive comprises a film-forming additive and fluorite with a purity of 99.9%.

[0007] Furthermore, the mixed organic solvent includes carbonate and carboxylate organic solvents, and the mass ratio of carbonate to carboxylate is 20-80:80-20.

[0008] Furthermore, the carbonate organic solvent is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the carboxylate organic solvent is one or more of ethyl acetate, methyl acetate, propyl propionate, and butyl butyrate.

[0009] Furthermore, the added amount of the fluorite is 0.5-1% of the mass of the mixed organic solvent.

[0010] Furthermore, the conductive lithium salt is one or more of lithium hexafluorophosphate, lithium bis(difluorosulfonyl imide), lithium bis(trifluoromethylsulfonyl imide), lithium tetrafluoroborate, lithium bis(oxalatoborate) and lithium difluorooxalatoborate, and the concentration of the conductive lithium salt in the mixed organic solvent is 0.3M to 2.5M.

[0011] Furthermore, the quaternary ammonium salt is one or more of N,N-dimethylpyrrolidinium tetrafluoroborate, 5,5-spirobispyrrolium tetrafluoroborate, and 1-ethyl-3-methylimidazolium bistrifluoromethanesulfonyl imide salt, and the added amount of the quaternary ammonium salt is 1% to 5% of the mass of the mixed organic solvent.

[0012] Furthermore, the film-forming additive is vinylene carbonate, fluoroethylene carbonate, vinyl carbonate, and 1,3-propane sultone, and the film-forming additive accounts for 1% to 2% of the mass of the mixed organic solvent.

[0013] The method for preparing an electrolyte for improving the high-rate and long-cycle performance of a lithium-ion capacitor according to the present invention comprises the following steps:

[0014] S1. The electrolyte is prepared in a dry room with a dew point of ≤-40°C and a temperature of 5°C to 20°C. One or more carbonate or carboxylate organic solvents are placed in a beaker and stirred thoroughly to form a mixed organic solvent.

[0015] S2, weigh fluorite and place it in a mortar, and add the mixed organic solvent in S1 dropwise to grind it and stir it thoroughly;

[0016] S3, adding conductive lithium salt and quaternary ammonium salt to the mixed solvent in S2, heating and stirring to dissolve, and preparing a mixed solution;

[0017] S4: adding a film-forming additive to the mixed solution obtained in S3, and stirring until the film-forming additive is completely and evenly dissolved to obtain the electrolyte.

[0018] The invention discloses an application of an electrolyte for improving the high-rate and long-cycle performance of a lithium ion capacitor in the preparation of a lithium ion capacitor. The positive electrode of the lithium ion capacitor comprises high-voltage lithium cobalt oxide and activated carbon, the negative electrode is hard carbon, and the diaphragm is a functional ceramic diaphragm.

[0019] The beneficial technical effect of the present invention is that the electrolyte of the present invention realizes the synergistic solvation between ions by adjusting the organic solvent, salt and additive components, and introduces a trace amount of high-purity fluorite as a new functional additive for the first time. On the one hand, the trace amount of fluorite (CaF2) can form a fluorinated solvent to adjust the viscosity and interface compatibility of the electrolyte; on the other hand, the strong electron-withdrawing property of fluorine atoms can affect the solvation ability of the solvent and participate in the formation of the SEI film, thereby inhibiting the decomposition of the electrolyte. The electrolyte has high ionic conductivity and oxidation potential at room temperature, and has good compatibility with the hard carbon negative electrode, is suitable for lithium ion capacitors and improves the high-rate charge and discharge long cycle characteristics of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The figures are the cycle test results of the electrolytes of the embodiments of the present invention and the comparative examples under the condition of 10C / 20 high rate.

[0021] Figure 2 Surface image of hard carbon negative electrode sheet after cycling of lithium ion capacitor prepared with electrolytes obtained in Examples and Comparative Examples.

[0022] Figure 3 The scanning electron microscope images of lithium ion capacitors prepared with the electrolytes obtained in the examples and comparative examples after cycling. DETAILED DESCRIPTION

[0023] The following is further described in detail through specific implementation methods:

[0024] Example 1: Preparation of an electrolyte for improving the high rate and long cycle of a lithium ion capacitor, comprising the following steps:

[0025] Step 1: The electrolyte is prepared in a dry room with a dew point of ≤-40°C and a temperature of 5°C to 20°C. Dimethyl carbonate and ethyl acetate organic solvents are weighed in a mass ratio of 40:60 and mixed in a beaker. The mixture is placed on a magnetic stirrer at room temperature at 300 rpm to form a mixed organic solvent.

[0026] Step 2: Weigh 0.5% of the mass of the functional additive fluorite to the mass of the mixed organic solvent, put it in a mortar and grind it thoroughly, and add the mixed organic solvent in step 1 dropwise to grind it until the fluorite is evenly ground and mixed with the organic solvent in step 1, and place it on a magnetic stirrer at room temperature at 300 rpm to stir it thoroughly.

[0027] Step 3: Add lithium hexafluorophosphate and lithium bis(difluorosulfonyl)imide to the mixed solvent in step 2, respectively, and the concentrations of the two lithium salts in the mixed organic solvent are 0.3M and 1M, respectively; at the same time, add N,N-dimethylpyrrolidinium tetrafluoroborate accounting for 2% of the mass of the mixed organic solvent, place it on a magnetic stirrer at 45°C, heat it at 300rpm, and stir and dissolve it to prepare a mixed solution of a certain concentration.

[0028] Step 4: Add 1% of vinylene carbonate and 1% of fluoroethylene carbonate as film-forming additives to the mixed solution of step 3, and stir at 300 rpm on a magnetic stirrer until the additives are completely and evenly dissolved.

[0029] Example 2

[0030] Example 2 is carried out under the conditions of Example 1, except that in step 2, the amount of fluorite added as the functional additive is 1%; the amounts of other components added and the experimental steps are the same as those in Example 1.

[0031] Comparative Example 1

[0032] Comparative Example 1 is the same as Example 1 except that the functional additive fluorite is not added, and the addition amounts of other components and the experimental steps are the same as those of Example 1.

[0033] Comparative Example 2

[0034] Comparative Example 2 is the same as Example 1, except that no film-forming additive and functional additive fluorite are added, and the addition amounts of other components and the experimental steps are the same as those of Example 1.

[0035] test:

[0036] 1. The electrolytes of Example 1, Example 2 and Comparative Example 1, Comparative Example 2 were subjected to LSV oxidation potential test and ion conductivity test. The results are shown in Table 1 and Table 2.

[0037] Table 1 Electrolyte oxidation potential values

[0038]

[0039] The test results in Table 1 show that trace amounts of functional additives such as fluorite and film-forming additives can increase the oxidation potential of the electrolyte.

[0040] Table 2 Electrolyte conductivity

[0041]

[0042] The test results in Table 2 show that trace amounts of functional additives such as fluorite and film-forming additives will reduce the conductivity of the electrolyte to a certain extent, but the difference is not large.

[0043] 2. The electrolytes prepared in Example 1, Example 2 and Comparative Example 1, Comparative Example 2 are respectively used to prepare lithium ion capacitors, wherein the positive electrode of the lithium ion capacitor comprises high-voltage lithium cobalt oxide and activated carbon, the negative electrode is hard carbon, and the diaphragm is a functional ceramic diaphragm. The lithium ion capacitor is subjected to a high-rate 10C charge / 20C discharge long cycle performance test and the negative electrode sheet after the test is subjected to a scanning electron microscope test.

[0044] like Figure 1 As shown, the cycle performance was tested under high-rate 10C charging / 20C discharging and full voltage 2.5-4.2V. After the lithium ion capacitor in Example 1 cycled 5500 times, its capacity retention rate was 76.93%, and after the lithium ion capacitor in Example 2 cycled 5600 times, its capacity retention rate was 79.80%. After the lithium ion capacitor in Comparative Example 1 cycled 2850 times, the capacity retention rate was only 78.85%, and after the lithium ion capacitor in Comparative Example 2 cycled 2100 times, the capacity retention rate decayed to 71.05%. The results show that film-forming additives can improve the cycle performance of lithium ion capacitors under high-rate charge and discharge, while trace functional additives such as fluorite have a significant improvement in the high-rate cycle performance of lithium ion capacitors.

[0045] Figure 2 and Figure 3 It is the macroscopic and microscopic morphology of the hard carbon negative electrode after the lithium ion capacitor prepared by the electrolyte obtained in the embodiment and the comparative example is cycled. It can be seen from the figure that the negative electrode of the lithium ion capacitor corresponding to embodiments 1 and 2 is clean after long-term cycling at a high rate. A large amount of white attachments appear on the surface of the lithium ion capacitor corresponding to comparative example 1, in the shape of a hundred flowers. Scanning electron microscopy observation shows that agglomerated products are generated on the surface of the hard carbon negative electrode; the electrode of the lithium ion capacitor corresponding to comparative example 2 shows that the white substance is in a spiral shape and a sky blue product appears in the center. Scanning electron microscopy observation shows that the hard carbon negative electrode surface contains a large number of agglomerated products. This result shows that the film-forming additive can generate SEI film to a certain extent, but the stability is not strong. The trace functional additive fluorite has a synergistic effect with the film-forming additive to form a stable and strong SEI film at the negative electrode interface, which satisfies the lithium ion capacitor to maintain excellent long-cycle performance under high-rate charge and discharge.

Claims

1. An electrolyte for improving the high-rate and long-cycle performance of a lithium-ion capacitor, characterized in that: The invention comprises a mixed organic solvent, a conductive lithium salt, a quaternary ammonium salt and a functional additive, wherein the functional additive comprises a film-forming additive and fluorite with a purity of 99.9%.

2. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 1, characterized in that: The mixed organic solvent comprises carbonate and carboxylate organic solvents, and the mass ratio of carbonate to carboxylate is 20-80:80-20.

3. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 2, characterized in that: The carbonate organic solvent is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; the carboxylate organic solvent is one or more of ethyl acetate, methyl acetate, propyl propionate, and butyl butyrate.

4. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 1, characterized in that: The added amount of the fluorite is 0.5-1% of the mass of the mixed organic solvent.

5. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 1, characterized in that: The conductive lithium salt is one or more of lithium hexafluorophosphate, lithium bis(difluorosulfonyl imide), lithium bis(trifluoromethylsulfonyl imide), lithium tetrafluoroborate, lithium bis(oxalatoborate) and lithium difluorooxalatoborate, and the concentration of the conductive lithium salt in the mixed organic solvent is 0.3M to 2.5M.

6. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 1, characterized in that: The quaternary ammonium salt is one or more of N,N-dimethylpyrrolidinium tetrafluoroborate, 5,5-spirobipyrrolium tetrafluoroborate, and 1-ethyl-3-methylimidazole bistrifluoromethanesulfonyl imide salt, and the addition amount of the quaternary ammonium salt is 1% to 5% of the mass of the mixed organic solvent.

7. The electrolyte for improving the high-rate and long-cycle performance of lithium-ion capacitors according to claim 1, characterized in that: The film-forming additive is vinylene carbonate, fluoroethylene carbonate, vinyl carbonate, and 1,3-propane sultone, and the content of the film-forming additive is 1% to 2% of the mass of the mixed organic solvent.

8. A method for preparing an electrolyte for improving high-rate and long-cycle performance of lithium-ion capacitors according to any one of claims 1 to 7, characterized in that: The following steps are involved: S1. The electrolyte is prepared in a dry room with a dew point of ≤-40°C and a temperature of 5°C to 20°C. One or more carbonate or carboxylate organic solvents are placed in a beaker and stirred thoroughly to form a mixed organic solvent. S2, weigh fluorite and place it in a mortar, and add the mixed organic solvent in S1 dropwise to grind it and stir it thoroughly; S3, adding conductive lithium salt and quaternary ammonium salt to the mixed solvent in S2, heating and stirring to dissolve, and preparing a mixed solution; S4: adding a film-forming additive to the mixed solution obtained in S3, and stirring until the film-forming additive is completely and evenly dissolved to obtain the electrolyte.

9. Use of an electrolyte for improving high-rate and long-cycle lithium ion capacitors according to any one of claims 1 to 7 in the preparation of lithium ion capacitors, characterized in that: The positive electrode of the lithium ion capacitor comprises high-voltage lithium cobalt oxide and activated carbon, the negative electrode is hard carbon, and the diaphragm is a functional ceramic diaphragm.

Images