Functional electrolyte, preparation method thereof and application of functional electrolyte in lithium ion capacitor

Abstract

The invention discloses a functional electrolyte in the technical field of electrolytes, and a preparation process of the functional electrolyte comprises the following steps: fully and uniformly mixing a carboxylic ester solvent and a fluorinated organic solvent, then adding lithium hexafluorophosphate and one or more lithium salts, heating and stirring for dissolving, then adding a multifunctional additive with a certain mass ratio, and stirring for dissolving to obtain the functional electrolyte. And stirring until the additive is completely and uniformly dissolved to obtain the functional electrolyte of the technical scheme. The functional electrolyte disclosed by the invention has higher oxidation potential and ionic conductivity at high and low temperatures, and the lithium ion capacitor assembled by the functional electrolyte has excellent high and low temperature discharge performance and can meet discharge in an environment of-40 DEG C to 60 DEG C. And meanwhile, high-rate (10C charging / 20C discharging) long cycle performance and high power characteristic can be realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrochemical energy storage, and in particular relates to a functional electrolyte and a preparation method thereof and application in a lithium ion capacitor. Background Art

[0002] With the proposal of carbon neutrality and carbon peak targets, new energy power generation and electrochemical energy storage have broad development prospects. As an important electrochemical energy storage device, lithium-ion capacitors have developed rapidly in recent years due to their high power density, long cycle life and fast response. As the most important component of lithium-ion capacitors or lithium-ion batteries, the electrolyte can not only continuously transport anions and cations for the charging and discharging of energy storage devices, but also provide the necessary liquid environment for the migration of ions between the positive and negative electrodes. It plays the role of transporting and conducting current between the positive and negative electrodes, and directly affects the charge and discharge performance of electrochemical devices, such as rate performance, cycle performance, high and low temperature performance, as well as safety and cost. Therefore, designing an electrolyte system that matches the electrode material is the key to improving the performance of lithium-ion capacitors.

[0003] In the prior art, there is an electrolyte containing propylene carbonate, lithium hexafluorophosphate, a film-forming stabilizer and a high-temperature stabilizer. The electrolyte improves the high and low temperature discharge characteristics of the energy storage device, but the cycle performance and high power characteristics at high voltage (>4.2V) and high rate (>5C charge and discharge) need to be improved. For example, announcement number CN108807012B discloses a lithium ion capacitor electrolyte, which includes lithium salt, carbonate solvent and film-forming additive, high voltage additive and ammonium salt additive. The electrolyte enables the lithium ion capacitor to have excellent high and low temperature storage performance and cycle performance, and has a long cycle life under high temperature and high pressure. However, like most electrolytes in the prior art, there is a common problem that the working voltage at room temperature cannot reach more than 5V, and the working voltage at a high temperature of 60°C cannot reach more than 4V, so there is a low working voltage, which cannot meet the requirements of high voltage (>4V) and high rate cycle discharge. That is, the electrolyte in the prior art has not yet achieved the technical effect of improving high and low temperature discharge characteristics, high power characteristics and high rate cycle performance. Summary of the invention

[0004] The present invention aims to provide an electrolyte that is resistant to high voltage and has high conductivity, and can be applied to lithium ion capacitors to achieve high voltage, high rate cycle performance, high and low temperature discharge performance and high power performance.

[0005] The first aspect of the present invention provides a method for preparing a functional electrolyte, which is carried out in an environment with a humidity of less than 1% according to the following steps:

[0006] S1. Take one or more carboxylic acid ester solvents and fluorinated organic solvents respectively, mix them thoroughly and evenly, and then prepare an organic mixed electrolyte solvent; the volume ratio of the carboxylic acid ester solvent to the fluorinated organic solvent is 75-95:5-25;

[0007] S2, adding lithium hexafluorophosphate and one or more lithium salts to the organic mixed electrolyte solvent in S1, and then heating and stirring to dissolve;

[0008] S3. After all the lithium salt in S2 is dissolved, add a certain mass ratio of a multifunctional additive and stir until the additive is completely and evenly dissolved, thereby obtaining the functional electrolyte of the present technical solution.

[0009] Furthermore, the carboxylate solvent is one or more of ethyl acetate, methyl acetate, propyl propionate, and butyl butyrate.

[0010] Furthermore, the fluorinated solvent is one of fluorinated organic carbonate and fluorinated ether, or a mixture of the two.

[0011] Furthermore, the lithium salt is one or more of lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium bis(oxalatoborate), and lithium bis(fluorosulfonyl)imide.

[0012] Furthermore, the concentration of the lithium hexafluorophosphate in the organic mixed electrolyte solvent is 0.5-3.0M, and the concentration of the lithium salt in the organic mixed electrolyte solvent is 0.1-0.5M.

[0013] Furthermore, the multifunctional additive is one or more of VC (vinyl carbonate), FEC (fluoroethylene carbonate), PS (1,3-propane sultone), MMDS (methylene disulfonate), DTD (vinyl sulfate) and PST (propenyl-1,3-sulfur sultone); the added amount of the multifunctional additive is 1% to 10% of the mass percentage of the organic mixed electrolyte solvent.

[0014] The second aspect of the present invention provides a functional electrolyte prepared by the method described in the first aspect.

[0015] The third aspect of the present invention provides a functional electrolyte solution according to the second aspect for use in preparing a lithium ion capacitor or a lithium ion battery.

[0016] Furthermore, the lithium ion capacitor is a lithium cobalt oxide / hard carbon system.

[0017] The beneficial technical effect of the present invention is that the functional electrolyte of the present invention has a high oxidation potential and ion conductivity at high and low temperatures, and the lithium ion capacitor assembled therefrom has excellent high and low temperature discharge performance and can meet the discharge in an environment of -40°C to 60°C. At the same time, it can achieve high rate (10C charge / 20C discharge) long cycle performance and high power characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 This is a low temperature capacity test diagram after the functional electrolyte of the embodiment and the electrolyte of the comparative example are assembled into a 0.6Ah lithium ion capacitor;

[0019] Figure 2 This is a high temperature capacity test diagram of a 0.6Ah lithium ion capacitor assembled with the functional electrolyte of the embodiment and the electrolyte of the comparative example;

[0020] Figure 3 The graph is a cycle performance test diagram of a 0.6Ah lithium ion capacitor assembled with the functional electrolyte of the embodiment and the electrolyte of the comparative example. DETAILED DESCRIPTION

[0021] The following is further described in detail through specific implementation methods:

[0022] Embodiment 1: A method for preparing a functional electrolyte, comprising the following steps:

[0023] Step 1: In a glove box with humidity <1%, 100 mL of ethyl acetate and fluoroether organic solvent are measured and placed in a beaker with a volume ratio of 95:5. After being fully mixed, an organic mixed electrolyte solvent is prepared;

[0024] Step 2: Add lithium hexafluorophosphate (LiPF6) and a novel lithium salt lithium difluorooxalate borate (LiODFB) to the organic mixed electrolyte solvent in step 1, respectively, wherein the concentrations of lithium hexafluorophosphate (LiPF6) and lithium difluorooxalate borate (LiODFB) in the organic mixed electrolyte solvent are 1M and 0.1M, respectively, and place on a magnetic stirrer and heat to dissolve at 300rpm and 40°C;

[0025] Step 3: After the lithium salt is completely dissolved, add VC and FEC additives, the addition amount of the two additives is 1% of the mass of the organic mixed electrolyte solvent, place it on a magnetic stirrer and stir until the additives are completely and evenly dissolved, and the functional electrolyte of the present technical solution is obtained.

[0026] Embodiment 2: A method for preparing a functional electrolyte, comprising the following steps:

[0027] Step 1: In a glove box with humidity <1%, 100 mL of methyl acetate (MA) and fluoroether organic solvent are respectively placed in a beaker with a volume ratio of 90:10. After being fully mixed, an organic mixed electrolyte solvent is prepared;

[0028] Step 2: Add lithium hexafluorophosphate (LiPF6), a new lithium salt lithium difluorooxalatoborate (LiODFB) and lithium tetrafluoroborate (LiBF4) to the organic mixed electrolyte solvent in step 1, respectively, wherein the concentrations of lithium hexafluorophosphate (LiPF6) and the two new lithium salts in the organic mixed electrolyte solvent are 1M, 0.1M and 0.1M, respectively, and place on a magnetic stirrer and heat to dissolve at 300rpm and 40°C;

[0029] Step 3: After the lithium salt is completely dissolved, add VC, FEC and PS additives, the addition amount of the three additives is 1% of the mass of the organic mixed electrolyte solvent, place it on a magnetic stirrer and stir until the additives are completely and evenly dissolved, and the functional electrolyte of the present technical solution is obtained.

[0030] Embodiment 3: A method for preparing a functional electrolyte, comprising the following steps:

[0031] Step 1: In a glove box with humidity <1%, 100 mL of ethyl acetate and fluorinated organic carbonate organic solvent are respectively placed in a beaker with a volume ratio of 85:15. After being fully mixed, an organic mixed electrolyte solvent is prepared;

[0032] Step 2: Add lithium hexafluorophosphate (LiPF6), a new lithium salt lithium difluorooxalatoborate (LiODFB) and lithium bis(oxalatoborate) (LiBOB) to the organic mixed electrolyte solvent in step 1, respectively, wherein the concentrations of lithium hexafluorophosphate (LiPF6) and the two new lithium salts in the organic mixed electrolyte solvent are 1M, 0.1M and 0.1M, respectively, and place on a magnetic stirrer at 300rpm and 40°C to heat and dissolve;

[0033] Step 3: After the lithium salt is completely dissolved, add VC, FEC and DTD additives, the addition amount of the three additives is 1% of the mass of the organic mixed electrolyte solvent, place it on a magnetic stirrer and stir until the additives are completely and evenly dissolved, and the functional electrolyte of the present technical solution is obtained.

[0034] Embodiment 4: A method for preparing a functional electrolyte, comprising the following steps:

[0035] Step 1: In a glove box with humidity <1%, 100 mL of propyl propionate (PP) and fluorinated organic carbonate organic solvent are respectively placed in a batching beaker, with the volume ratio of the two organic solvents being 90:10. After being fully mixed, an organic mixed electrolyte solvent is prepared;

[0036] Step 2: Add lithium hexafluorophosphate (LiPF6), a new lithium salt lithium difluorooxalatoborate (LiODFB) and lithium bis(fluorosulfonyl)imide (LiFSI) to the organic mixed electrolyte solvent in step 1, respectively. The concentrations of lithium hexafluorophosphate (LiPF6) and the two new lithium salts in the organic mixed electrolyte solvent are 1M, 0.1M and 0.2M, respectively. Place on a magnetic stirrer and heat at 300rpm and 40°C to dissolve;

[0037] Step 3: After the lithium salt is completely dissolved, add VC, FEC and PST additives, wherein the addition amount of VC and FEC is 1% of the mass of the organic mixed electrolyte solvent, and the addition amount of PST is 2% of the mass of the organic mixed electrolyte solvent. Place it on a magnetic stirrer and stir until the additives are completely and evenly dissolved, so as to obtain the functional electrolyte of the present technical solution.

[0038] Comparative Example 1

[0039] The only difference from Example 1 is that no fluorinated organic solvent is added in step 1, and the solvent is only a carboxylic acid ester solvent. The remaining components, component proportions and preparation methods are the same as those in Example 1.

[0040] Comparative Example 2

[0041] The only difference from Example 1 is that the multifunctional additive is not added in step 3, and the other solvent and lithium salt components and contents and their preparation methods are the same as those in Example 1.

[0042] Example 5: Lithium Ion Capacitor Preparation

[0043] The functional electrolytes of Examples 1 to 4 and the electrolytes of Comparative Examples 1 and 2 were used to prepare lithium ion capacitors, respectively, and the operation process was as follows:

[0044] Positive electrode: Lithium cobalt oxide, activated carbon, and polytetrafluoroethylene were wet-mixed in a mass ratio of 92:4:4 and then coated on copper foil to prepare a surface density of 1.6 g / 100 cm 2 The positive electrode sheet is prepared by baking, rolling, die cutting and other processes.

[0045] Negative electrode sheet: Hard carbon, activated carbon, and polytetrafluoroethylene were wet-mixed in a mass ratio of 94:3:3 and then coated on aluminum foil to prepare a surface density of 0.54 g / 100 cm 2 The electrode sheet is prepared by baking, rolling, die cutting and other processes to obtain the negative electrode sheet.

[0046] The positive electrode sheet, the negative electrode sheet and the separator are stacked and assembled into a 0.6Ah soft-pack battery. After being packaged with an aluminum-plastic film, the functional electrolyte in the above embodiment and the electrolyte in the comparative example are respectively injected. After a series of measures such as aging, formation and vacuuming, the final six groups of lithium-ion capacitors are obtained.

[0047] Comprehensive performance test:

[0048] 1. Oxidation potential test (LSV): The functional electrolyte prepared in the above embodiment and the electrolyte obtained in the comparative example were respectively assembled into button cells consisting of stainless steel sheet / electrolyte / metal lithium sheet. The test voltage range was 0-6V. The decomposition voltage was tested at 0.2 mA / s at room temperature 25°C and high temperature 60°C. The test results are shown in Table 1.

[0049] Table 1 LSV data table

[0050]

[0051] In Table 1, at room temperature 25°C, the decomposition voltage of the electrolyte in the embodiment is between 5.45 and 5.72 V, while in the comparative example, it is 4.57 V and 4.89 V respectively; when the temperature is raised to 60°C, the decomposition voltage of the electrolyte decreases, and is 4.4 to 4.53 V in the embodiment, while it is 3.4 V and 3.82 V in the comparative example. The results show that the addition of the organic mixed electrolyte solvent and the multifunctional additive prepared in the present invention achieves a mutually promoting effect and synergistically improves the decomposition voltage of the electrolyte.

[0052] 2. Ionic conductivity test: The conductivity of the electrolyte at room temperature 25°C and high temperature 60°C was measured using a handheld DDS-307A conductivity meter. The test results are shown in Table 2.

[0053] Table 2 Ionic conductivity test values

[0054]

[0055] It can be seen from Table 2 that the addition of the organic mixed electrolyte solvent and the multifunctional additive prepared in the present invention is beneficial to improving the ionic conductivity of the electrolyte, thereby improving the ultimate specific power of the battery during high-rate discharge.

[0056] 3. High / low temperature capacity test: The functional electrolyte prepared in the above embodiment and the electrolyte obtained in the comparative example were assembled into a 0.6Ah lithium ion capacitor according to the method of Example 5. The capacity test was carried out at a low temperature of -40°C and a high temperature of 60°C. The voltage range was 2.5-4.2V, the charge and discharge current was set to 1C, and the program was constant current and constant voltage charging / constant current discharging. The high / low temperature performance test results are as follows: Figure 1 , Figure 2 And as shown in Table 3.

[0057] Table 3 High / low temperature capacity data table

[0058]

[0059] Figure 1 and Figure 2 Corresponding to the data in Table 3 and Table 4. In low temperature performance, the low temperature capacity recovery rate of the battery in the embodiment is about 6% higher than that in the comparative example, while in high temperature performance, the organic mixed electrolyte solvent and the multifunctional additive in the embodiment promote each other to improve the high temperature capacity recovery rate, reaching more than 91%.

[0060] 4. Cycle performance test: The functional electrolyte prepared in the above embodiment and the electrolyte obtained in the comparative example were assembled into a 0.6Ah lithium ion capacitor according to the method of Example 5. The capacity test was carried out at room temperature 25°C, the voltage range was 2.5-4.2V, and the charge and discharge current was set to 20C constant current constant voltage charging / 10C constant current discharge. The cycle test results are as follows: Figure 3 shown. Figure 3 The figure is a capacity retention rate diagram of the battery after 2000 cycles. The capacity retention rate of the battery in the embodiment is above 90% after 2000 cycles, while the capacity retention rates of the batteries in the two comparative examples are only 80.63% and 83.22%, with a difference of about 10%.

[0061] 5. Specific power test: The functional electrolyte prepared in the above embodiment and the electrolyte obtained in the comparative example were assembled into a 0.6Ah lithium ion capacitor according to the method of Example 5, and charged to 4.2V at a constant current of 0.20C at room temperature 25°C, then charged to a constant voltage current of 0.05C, left for 10 minutes, and discharged at 800A for 20ms. The specific power test results are shown in Table 4.

[0062] Table 4 Specific power discharge data table

[0063]

[0064] Table 4 shows that the electrolyte in the embodiment has more excellent ionic conductivity, is more conducive to large current discharge of the battery, and improves the specific power of the battery.

[0065] The above test results show that the organic mixed electrolyte solvent prepared in the present invention is combined with the added multifunctional additives to obtain the overall performance of the functional electrolyte. The addition of the two components has a good synergistic effect on the conductivity, cycle performance and other aspects of the functional electrolyte. Specifically, the functional electrolyte of the present invention has a higher oxidation potential and ion conductivity at high and low temperatures, and the assembled lithium ion capacitor has excellent high and low temperature discharge performance and can meet the discharge in an environment of -40℃ to 60℃. At the same time, it can achieve long cycle performance and high power characteristics at high rates (10C charge / 20C discharge).

Claims

1. A method for preparing a functional electrolyte, characterized in that The process includes the following steps and is performed in an environment with a humidity of <1%: S1. Take one or more carboxylic acid ester solvents and fluorinated organic solvents respectively, mix them thoroughly and evenly, and then prepare an organic mixed electrolyte solvent; the volume ratio of the carboxylic acid ester solvent to the fluorinated organic solvent is 75-95:5-25; S2, adding lithium hexafluorophosphate and one or more lithium salts to the organic mixed electrolyte solvent in S1, and then heating and stirring to dissolve; S3. After all the lithium salt in S2 is dissolved, add a certain mass ratio of a multifunctional additive and stir until the additive is completely and evenly dissolved, thereby obtaining the functional electrolyte of the present technical solution.

2. The method for preparing a functional electrolyte according to claim 1, characterized in that: The carboxylate solvent is one or more of ethyl acetate, methyl acetate, propyl propionate and butyl butyrate.

3. The method for preparing a functional electrolyte according to claim 2, characterized in that: The fluorinated solvent is one of fluorinated organic carbonate and fluorinated ether or a mixture of the two.

4. The method for preparing a functional electrolyte according to claim 3, characterized in that: The lithium salt is one or more of lithium difluorooxalatoborate, lithium tetrafluoroborate, lithium bis(oxalatoborate), and lithium bis(fluorosulfonyl)imide.

5. The method for preparing a functional electrolyte according to claim 4, characterized in that: The concentration of the lithium hexafluorophosphate in the organic mixed electrolyte solvent is 0.5-3.0M, and the concentration of the lithium salt in the organic mixed electrolyte solvent is 0.1-0.5M.

6. The method for preparing a functional electrolyte according to claim 5, characterized in that: The multifunctional additive is one or more of vinylene carbonate, fluoroethylene carbonate, 1,3-propane sultone, methylene disulfonate, vinyl sulfate and propenyl-1,3-sulfur sultone; the addition amount of the multifunctional additive is 1% to 10% of the mass percentage of the organic mixed electrolyte solvent.

7. Functional electrolyte, characterized in that Prepared according to the method according to any one of claims 1 to 6.

8. The functional electrolyte according to claim 7 is used to prepare lithium ion capacitors and / or lithium ion batteries.

9. The lithium ion capacitor according to claim 8 is a lithium cobalt oxide / hard carbon system.

Images