Electrolyte and preparation method and application thereof

Abstract

The invention relates to the technical field of lithium batteries, and discloses an electrolyte and a preparation method and application thereof. The electrolyte comprises the following components: a lithium salt, an additive and an organic solvent, wherein the additives are lithium difluoro (oxalato) borate and lithium difluorophosphate; the content of the lithium difluoro (oxalato) borate in the electrolyte is 0.05 to 0.2 mol / L; and the content of the lithium difluorophosphate in the electrolyte is 0.5 wt%-2wt%. According to the electrolyte provided by the invention, the lithium difluoro (oxalato) borate and the lithium difluorophosphate composite additive are added, and the lithium difluoro (oxalato) borate and the lithium difluorophosphate composite additive can interact on the surface of the positive electrode material to form an organic-inorganic composite CEI film, so that the oxygenolysis of the electrolyte is reduced, the lithium ion transmission efficiency is improved, the internal resistance of the battery is reduced, and the cycling stability of the battery is improved; the electrolyte is used for assembling a lithium ion battery, the interface morphology of a lithium cobalt oxide electrode and a graphite electrode can be improved, the integrity of lithium cobalt oxide particles is high, and a solid electrolyte interface layer is more uniform and smoother.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium batteries, and particularly relates to an electrolyte and its preparation method and application. Background Art

[0002] Lithium-ion batteries are widely used in 3C products, new energy vehicles and energy storage devices. The market's requirements for the energy density of lithium-ion batteries are also increasing day by day. Increasing the charging cut-off voltage is an important direction for the development of high energy density lithium-ion batteries. How to make lithium-ion batteries work stably at high voltages remains a hot and difficult issue in current research.

[0003] The main materials of lithium-ion batteries include the positive electrode, negative electrode, electrolyte and separator. Among them, lithium cobaltate and graphite are still the most widely used positive electrode material and negative electrode material at present. The method of improving the battery energy density by developing new electrode materials requires a large amount of time and funds. As the "blood" of lithium-ion batteries, the electrolyte plays a role in conducting lithium ions. Improving the working voltage of the battery by improving the electrolyte system is a simple and effective method compared with developing new electrode materials.

[0004] However, the main component of the current commercial lithium-ion battery electrolyte is carbonate, which has weak antioxidant properties. When the battery working voltage reaches above 4.5V (vs Li / Li + ), serious oxidative decomposition will occur, and products such as CO2 and H2O will be generated, resulting in problems such as an increase in electrolyte acidity, transition metal dissolution, unstable electrode interface, and an increase in battery internal resistance. Eventually, the battery capacity will rapidly decrease and the cycle life will decay. Therefore, developing an electrolyte that can be applied to high voltages is particularly important for the development of high energy density lithium-ion batteries. Summary of the Invention

[0005] The present invention aims to solve at least one of the above technical problems in the prior art. For this reason, one of the purposes of the present invention is to provide an electrolyte; the second purpose of the present invention is to provide a preparation method of this electrolyte; the third purpose of the present invention is to provide a lithium-ion battery; the fourth purpose of the present invention is to provide an application of this lithium-ion battery.

[0006] In order to achieve the above purposes, the technical solutions adopted by the present invention are as follows:

[0007] The first aspect of the present invention provides an electrolyte, comprising the following components: a lithium salt, an additive and an organic solvent;

[0008] Wherein, the additive is lithium difluorooxalate borate and lithium difluorophosphate; the content of lithium difluorooxalate borate in the electrolyte is 0.05 - 0.2 mol / L; the content of lithium difluorophosphate in the electrolyte is 0.5 wt% - 2 wt%.

[0009] In some embodiments of the present invention, in the electrolyte, the content of lithium difluoro(oxalato)borate is 0.08 - 0.15 mol / L.

[0010] In some specific embodiments of the present invention, in the electrolyte, the content of lithium difluoro(oxalato)borate is 0.08 - 0.12 mol / L.

[0011] In some embodiments of the present invention, in the electrolyte, the content of lithium difluorophosphate is 0.8 wt% - 1.5 wt%.

[0012] In some specific embodiments of the present invention, in the electrolyte, the content of lithium difluorophosphate is 0.8 wt% - 1.2 wt%.

[0013] The basic principle of the present invention is described as follows:

[0014] 1) Lithium difluoro(oxalato)borate (LiDFOB) has a relatively high highest occupied molecular orbital and a relatively low lowest unoccupied molecular orbital. It can participate in the formation of an interfacial film on both the positive electrode and the negative electrode, and the formed interfacial film has the characteristics of low impedance and high conductivity. However, the interfacial film (CEI film) formed by lithium difluoro(oxalato)borate on the positive electrode is mainly composed of oligomers. Under harsh conditions such as long-term cycling or high temperature and high voltage, the CEI film will decompose severely, resulting in instability at the interface between the electrolyte and the positive electrode material, and causing rapid attenuation of battery performance. Therefore, it is necessary to add rigid components (such as phosphides, nitrides, or carbon nanotubes, etc.) to the interfacial film of the designed electrode to improve the stability of the CEI film, reduce the decomposition of the electrolyte, and ensure stable operation of the electrode during long-term cycling.

[0015] 2) Lithium difluorophosphate (LiDFP) can decompose in the positive electrode to generate an inorganic CEI film mainly composed of lithium phosphate (Li3PO4) and lithium fluoride (LiF), which can inhibit the reconstruction of the electrode surface under high voltage, prevent the overflow of transition ions, and avoid the decomposition of the electrolyte.

[0016] 3) The present invention adds lithium difluoro(oxalato)borate and lithium difluorophosphate as composite additives to the electrolyte. They interact with each other on the surface of the positive electrode material to form an organic-inorganic composite CEI film. The composite film can reduce the oxidative decomposition of the electrolyte on the electrode surface, extend the life of the electrolyte, and the combination of organic and inorganic components can also provide a more stable interface, reduce side reactions at the interface, thereby improving the lithium ion transport efficiency, reducing the internal resistance of the battery, and enabling the battery to maintain stable performance during long-term cycling.

[0017] In some embodiments of the present invention, the content of the lithium salt in the electrolyte is 0.5 - 2 mol / L.

[0018] In some specific embodiments of the present invention, the content of the lithium salt in the electrolyte is 0.8 - 1.2 mol / L.

[0019] In some embodiments of the present invention, the lithium salt is selected from at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0020] In some embodiments of the present invention, the organic solvent is selected from at least one of cyclic carbonates and linear carbonates.

[0021] In some embodiments of the present invention, the organic solvent is a mixed solvent of cyclic carbonate and linear carbonate with a mass ratio of 1:(1 - 5).

[0022] In some specific embodiments of the present invention, the organic solvent is a mixed solvent of cyclic carbonate and linear carbonate with a mass ratio of 1:(1 - 3).

[0023] In some embodiments of the present invention, the cyclic carbonate is selected from one of ethylene carbonate (EC) and propylene carbonate (PC).

[0024] In some embodiments of the present invention, the linear carbonate is selected from at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC).

[0025] The second aspect of the present invention provides a method for preparing the electrolyte described in the first aspect of the present invention, including the following steps:

[0026] Dissolve the lithium salt and the additive in the organic solvent to obtain the electrolyte.

[0027] In some embodiments of the present invention, the method for preparing the electrolyte specifically includes the following steps:

[0028] S1. Mix the cyclic carbonate and the linear carbonate, and let it stand for more than 24 h to obtain a mixed solvent;

[0029] S2. Add the lithium salt and lithium difluoro(oxalato)borate to the mixed solvent, and let it stand for more than 24 h to obtain a basic electrolyte;

[0030] S3. Add lithium difluorophosphate to the basic electrolyte, and stir at 30 - 35 °C for 10 - 15 h to obtain the electrolyte.

[0031] The third aspect of the present invention provides a lithium-ion battery, including the electrolyte described in the first aspect of the present invention, as well as a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode.

[0032] In some embodiments of the present invention, the positive electrode active material on the positive electrode is selected from at least one of lithium cobaltate, lithium manganate, nickel cobalt manganese ternary material, and nickel cobalt aluminum ternary material.

[0033] In some specific embodiments of the present invention, the positive electrode active material on the positive electrode is lithium cobaltate.

[0034] In some embodiments of the present invention, the negative electrode active material on the negative electrode is selected from at least one of graphite, silicon, silicon alloy, silicon carbon, and silicon oxide.

[0035] In some specific embodiments of the present invention, the negative electrode active material on the negative electrode is graphite.

[0036] In some embodiments of the present invention, the separator includes a polypropylene film.

[0037] The fourth aspect of the present invention provides the application of the lithium-ion battery described in the third aspect of the present invention in 3C products, new energy vehicles, or energy storage devices.

[0038] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0039] 1) The electrolyte provided by the present invention simultaneously adds lithium difluorooxalate borate and lithium difluorophosphate as additives. The two can interact with each other on the surface of the positive electrode material to form an organic-inorganic composite CEI film, thereby reducing the oxidative decomposition of the electrolyte, improving the lithium ion transmission efficiency, reducing the internal resistance of the battery, and improving the cycle stability of the battery.

[0040] 2) The preparation method of the electrolyte provided by the present invention has simple steps and is suitable for industrial application.

[0041] 3) For the lithium-ion battery provided by the present invention, when using an electrolyte containing a composite additive of lithium difluorooxalate borate and lithium difluorophosphate, and using lithium cobaltate as the positive electrode active material or graphite as the negative electrode active material, the electrolyte can improve the interfacial morphology of the lithium cobaltate electrode and the graphite electrode. The lithium cobaltate particles have high integrity, and the solid electrolyte interface layer is more uniform and smooth. Description of the Drawings

[0042] Figure 1 It is the constant current charge and discharge test result of lithium cobaltate / lithium half-cells 1-4 in the application example;

[0043] Figure 2 It is the constant current charge and discharge test result of lithium cobaltate / lithium half-cells 1 and lithium cobaltate / lithium half-cells 5-8 in the application example;

[0044] Figure 3 It is the rate performance test result of lithium cobaltate / lithium half-cells 1-4 in the application example;

[0045] Figure 4 The constant current charge and discharge test results of the graphite / lithium half-cells 1-4 in the application examples;

[0046] Figure 5 The rate performance test results of the graphite / lithium half-cells 1-4 in the application examples;

[0047] Figure 6 The impedance fitting data graphs of the lithium cobalt oxide / lithium half-cells 1-4 in the application examples after 3 and 150 charge and discharge cycles;

[0048] Figure 7 The electrochemical impedance graph and its corresponding fitting curve of the lithium cobalt oxide / lithium half-cells 1-4 in the application examples after 3 charge and discharge cycles;

[0049] Figure 8 The electrochemical impedance graph and its corresponding fitting curve of the lithium cobalt oxide / lithium half-cells 1-4 in the application examples after 150 charge and discharge cycles;

[0050] Figure 9 The equivalent circuit diagram for impedance fitting of the lithium cobalt oxide / lithium half-cells 1-4 in the application examples;

[0051] Figure 10 The scanning electron microscope image of the lithium cobalt oxide electrode sheet in the application examples;

[0052] Figure 11 The scanning electron microscope image of the graphite electrode sheet in the application examples. Detailed implementation manners

[0053] The content of the present invention will be further described in detail below through specific examples. The raw materials, reagents or devices used in the examples and comparative examples can be obtained from conventional commercial channels or by existing technical methods without special instructions. Unless otherwise specified, the test or measurement methods are conventional methods in the art.

[0054] Example 1

[0055] In this example, an electrolyte solution was prepared as follows:

[0056] S1. Ethylene carbonate and ethyl methyl carbonate were mixed at a mass ratio of 1:1 to completely dissolve the ethylene carbonate, and it was left for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0057] S2. Lithium hexafluorophosphate and lithium difluoro(oxalato)borate were added to the mixed solvent and left for more than one day to ensure complete dissolution, obtaining a basic electrolyte solution containing 0.1 mol / L lithium difluoro(oxalato)borate and 1 mol / L lithium hexafluorophosphate;

[0058] S3. Add lithium difluorophosphate to the basic electrolyte and stir it at 35 °C for 12 h until it is completely dissolved to obtain an electrolyte containing 1 wt% lithium difluorophosphate.

[0059] Example 2

[0060] An electrolyte is prepared in this example, and the steps are as follows:

[0061] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:2, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing to obtain a mixed solvent;

[0062] S2. Add lithium bis(fluorosulfonyl)imide and lithium difluoro(oxalato)borate to the mixed solvent, let it stand for more than one day to ensure complete dissolution, and obtain a basic electrolyte containing 0.12 mol / L lithium difluoro(oxalato)borate and 1.2 mol / L lithium hexafluorophosphate;

[0063] S3. Add lithium difluorophosphate to the basic electrolyte and stir it at 35 °C for 12 h until it is completely dissolved to obtain an electrolyte containing 1.2 wt% lithium difluorophosphate.

[0064] Example 3

[0065] An electrolyte is prepared in this example, and the steps are as follows:

[0066] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:3, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing to obtain a mixed solvent;

[0067] S2. Add lithium bis(trifluoromethanesulfonyl)imide and lithium difluoro(oxalato)borate to the mixed solvent, let it stand for more than one day to ensure complete dissolution, and obtain a basic electrolyte containing 0.08 mol / L lithium difluoro(oxalato)borate and 0.8 mol / L lithium hexafluorophosphate;

[0068] S3. Add lithium difluorophosphate to the basic electrolyte and stir it at 35 °C for 12 h until it is completely dissolved to obtain an electrolyte containing 0.8 wt% lithium difluorophosphate.

[0069] Example 4

[0070] An electrolyte is prepared in this example. The difference from Example 1 is that the addition amount of lithium difluoro(oxalato)borate is reduced by half. The preparation steps are as follows:

[0071] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing to obtain a mixed solvent;

[0072] S2. Add lithium hexafluorophosphate and lithium difluoro(oxalato)borate to the mixed solvent, and let it stand for more than one day to ensure complete dissolution, obtaining a basic electrolyte containing 0.05 mol / L lithium difluoro(oxalato)borate and 1 mol / L lithium hexafluorophosphate;

[0073] S3. Add lithium difluorophosphate to the basic electrolyte, and stir it at 35 °C for 12 h to completely dissolve it, obtaining an electrolyte containing 1 wt% lithium difluorophosphate.

[0074] Example 5

[0075] This example prepares an electrolyte, which is different from Example 1 in that the addition amount of lithium difluoro(oxalato)borate is increased to twice. The preparation steps are as follows:

[0076] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, completely dissolve ethylene carbonate, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0077] S2. Add lithium hexafluorophosphate and lithium difluoro(oxalato)borate to the mixed solvent, and let it stand for more than one day to ensure complete dissolution, obtaining a basic electrolyte containing 0.2 mol / L lithium difluoro(oxalato)borate and 1 mol / L lithium hexafluorophosphate;

[0078] S3. Add lithium difluorophosphate to the basic electrolyte, and stir it at 35 °C for 12 h to completely dissolve it, obtaining an electrolyte containing 1 wt% lithium difluorophosphate.

[0079] Example 6

[0080] This example prepares an electrolyte, which is different from Example 1 in that the addition amount of lithium difluorophosphate is reduced by half. The preparation steps are as follows:

[0081] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, completely dissolve ethylene carbonate, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0082] S2. Add lithium hexafluorophosphate and lithium difluoro(oxalato)borate to the mixed solvent, and let it stand for more than one day to ensure complete dissolution, obtaining a basic electrolyte containing 0.1 mol / L lithium difluoro(oxalato)borate and 1 mol / L lithium hexafluorophosphate;

[0083] S3. Add lithium difluorophosphate to the basic electrolyte, and stir it at 35 °C for 12 h to completely dissolve it, obtaining an electrolyte containing 0.5 wt% lithium difluorophosphate.

[0084] Example 7

[0085] In this example, an electrolyte is prepared. The difference from Example 1 is that the addition amount of lithium difluoro(oxalato)borate is increased to twice, and the addition amount of lithium difluorophosphate is increased. The preparation steps are as follows:

[0086] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0087] S2. Add lithium hexafluorophosphate and lithium difluoro(oxalato)borate to the mixed solvent, let it stand for more than one day to ensure complete dissolution, obtaining a basic electrolyte containing 0.1 mol / L of lithium difluoro(oxalato)borate and 1 mol / L of lithium hexafluorophosphate;

[0088] S3. Add lithium difluorophosphate to the basic electrolyte, stir at 35 °C for 12 h to completely dissolve it, obtaining an electrolyte containing 2 wt% of lithium difluorophosphate.

[0089] Comparative Example 1

[0090] In this comparative example, an electrolyte is prepared. The difference from Example 1 is that lithium difluorophosphate is not added. The preparation steps are as follows:

[0091] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0092] S2. Add lithium hexafluorophosphate and lithium difluoro(oxalato)borate to the mixed solvent, let it stand for more than one day to ensure complete dissolution, obtaining an electrolyte containing 0.1 mol / L of lithium difluoro(oxalato)borate and 1 mol / L of lithium hexafluorophosphate.

[0093] Comparative Example 2

[0094] In this comparative example, an electrolyte is prepared. The difference from Example 1 is that lithium difluoro(oxalato)borate is not added. The preparation steps are as follows:

[0095] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1, dissolve the ethylene carbonate completely, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0096] S2. Add lithium hexafluorophosphate to the mixed solvent, let it stand for more than one day to ensure complete dissolution, obtaining an electrolyte containing 1 mol / L of lithium hexafluorophosphate;

[0097] S3. Add lithium difluorophosphate to the basic electrolyte, stir at 35 °C for 12 h to completely dissolve it, obtaining an electrolyte containing 1 wt% of lithium difluorophosphate.

[0098] Comparative Example 3

[0099] This comparative example prepares an electrolyte, which is different from that of Example 1 in that no additives are added. The preparation steps are as follows:

[0100] S1. Mix ethylene carbonate and ethyl methyl carbonate in a mass ratio of 1:1 to completely dissolve ethylene carbonate, and let it stand for more than one day to ensure uniform mixing, obtaining a mixed solvent;

[0101] S2. Add lithium hexafluorophosphate to the mixed solvent and let it stand for more than one day to ensure complete dissolution, obtaining an electrolyte containing 1 mol / L lithium hexafluorophosphate.

[0102] Application Example

[0103] The electrolytes in Example 1, Examples 4 - 7 and Comparative Examples 1 - 3 are used for the assembly of lithium cobaltate / lithium half-cells 1 - 8, and the electrolytes in Example 1 and Comparative Examples 1 - 3 are used for the assembly of graphite / lithium half-cells 1 - 4, where:

[0104] The lithium cobaltate / lithium half-cell uses lithium cobaltate as the positive electrode active material and a polypropylene film as the separator. The lithium cobaltate electrode is prepared by defoaming mechanism pulping. The binder used is polyvinylidene fluoride (PVDF), the conductive agent used is acetylene black, the solvent used is N-methylpyrrolidone (NMP), and the total defoaming mechanism pulping time is 60 min; the obtained lithium cobaltate slurries are respectively coated on aluminum foils with a coating thickness of 70 μm, first dried in a blast drying oven at 80 °C for 10 min, and then dried in a vacuum drying oven at 120 °C for 12 h to obtain lithium cobaltate electrodes.

[0105] The assembled lithium cobaltate / lithium half-cell uses a 2025-type battery case. The diameter of the lithium cobaltate electrode is 12 mm, the diameter of the lithium sheet is kept the same as that of the steel gasket (15.6 mm), the amount of the electrolyte used is 70 μL, and the separator used is a Celgard 2500 polypropylene film with a diameter of 16 mm.

[0106] The graphite / lithium half-cell uses graphite as the negative electrode active material and a polypropylene film as the separator. The graphite / lithium half-cell uses a 2025-type battery case. The diameter of the graphite electrode is 12 mm, the diameter of the lithium sheet is kept the same as that of the steel gasket (15.6 mm), the amount of the electrolyte used is 70 μL, and the separator used is a Celgard 2500 polypropylene film with a diameter of 16 mm.

[0107] The corresponding relationship between each half-cell and the electrolyte is shown in Table 1:

[0108] Table 1 Corresponding relationship between half-cells and electrolytes in the application example

[0109]

[0110]

[0111] Performance Test

[0112] 1. After standing the lithium cobalt oxide / lithium half-cells 1-8 assembled in the application example for 12 h and the graphite / lithium half-cells 1-4 for 6 h, constant current charge and discharge tests were respectively carried out by a Blue Electric Tester to evaluate the cycle stability of the batteries assembled with different electrolytes, where:

[0113] 1) The voltage test range for the constant current charge and discharge test of the lithium cobalt oxide / lithium half-cell was 3-4.5 V. The first 3 cycles were activated with 0.2 C, and then long cycles were carried out with 1 C.

[0114] 2) The voltage test range for the constant current charge and discharge test of the graphite / lithium half-cell was 0.005-2.5 V. The first 5 cycles were activated with 0.1 C, then 5 cycles were carried out with 0.2 C, and finally long cycles were carried out with 1 C.

[0115] 2. After standing the lithium cobalt oxide / lithium half-cells 1-4 assembled in the application example for 12 h and the graphite / lithium half-cells 1-4 for 6 h, rate tests were respectively carried out by a Blue Electric Tester to evaluate the rate performance of the batteries assembled with different electrolytes, where:

[0116] 1) The voltage test range for the rate test of the lithium cobalt oxide / lithium half-cell was 3-4.5 V. After cycling 5 times at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C respectively, cycling was carried out 55 times at 1 C.

[0117] 2) The voltage test range for the rate test of the graphite / lithium half-cell was 0.005-2.5 V. After cycling 5 times at 0.2 C, 0.5 C, 1 C, 2 C, and 5 C respectively, cycling was carried out 55 times at 1 C.

[0118] 3. After standing the lithium cobalt oxide / lithium half-cells 1-4 assembled in the application example for 12 h, electrochemical impedance tests were carried out to evaluate the impedance change of the batteries assembled with different electrolytes, where:

[0119] The test voltage range was 3-4.5 V. The first 3 cycles were activated with 0.2 C, and then long cycles were carried out with 1 C. At the end of the 3rd and 150th cycles, 1 C charging was carried out to make the battery in a charged state. The frequency range used in the test was 100000 Hz - 0.01 Hz.

[0120] 4. After standing the assembled lithium cobalt oxide / lithium half-cells 1-4 in the application example for 12 h and the graphite / lithium half-cells 1-4 for 6 h, constant current charge-discharge cycles were carried out respectively. After the lithium cobalt oxide / lithium half-cells 1-4 and the graphite / lithium half-cells 1-4 that had been cycled a certain number of times were disassembled, the obtained lithium cobalt oxide electrodes and graphite electrodes were washed and soaked in ethylene carbonate dimethyl carbonate solvent to remove the residual salts and other impurities on the surface. After the ethylene carbonate dimethyl carbonate solvent completely volatilized, the dried lithium cobalt oxide electrodes and graphite electrodes were respectively placed in 10 mL centrifuge tubes for storage and standby; 1 / 4 area of the electrodes was taken, and their surface morphologies were observed by scanning electron microscopy and compared with the surface morphologies of the uncycled lithium cobalt oxide electrodes and graphite electrodes, where:

[0121] 1) The constant current charge-discharge cycle voltage range of the lithium cobalt oxide / lithium half-cell was 3-4.5 V. The first 3 cycles were activated at 0.2 C, and then long cycles were carried out at 1 C. The lithium cobalt oxide / lithium half-cell that had been cycled 150 times was disassembled to obtain the lithium cobalt oxide electrode.

[0122] 2) The constant current charge-discharge cycle voltage range of the graphite / lithium half-cell was 0.005-2.5 V. The first 5 cycles were activated at 0.1 C, then 5 cycles were carried out at 0.2 C, and finally long cycles were carried out at 1 C. The graphite / lithium half-cell that had been cycled 400 times was disassembled to obtain the graphite electrode.

[0123] Figure 1 is the constant current charge-discharge test result of the lithium cobalt oxide / lithium half-cells 1-4 in the application example. It can be seen from Figure 1 that for the lithium cobalt oxide / lithium half-cell 4 using the electrolyte without any additives in Comparative Example 3, after about 30 charge-discharge cycles, the discharge specific capacity began to rapidly decay, the Coulomb efficiency was lower than 99%, and the fluctuation was severe. After 200 charge-discharge cycles, the discharge specific capacity was 35.7 mAh·g -1 , and its capacity retention rate was only 20.28% of the initial value; the lithium cobalt oxide / lithium half-cells 2 and 3 used the electrolytes in Comparative Example 1 and Comparative Example 2 respectively, in which lithium difluorooxalate borate and lithium difluorophosphate were added as electrolyte additives. The discharge specific capacities of the half-cells after 200 cycles were 107 mAh·g -1 and 117.1 mAh·g -1 respectively, and the capacity retention rates were 61.32% and 66.31% of the initial values respectively. The Coulomb efficiency was stable at about 99.5%. Compared with the lithium cobalt oxide / lithium half-cell 4, the cycle stability of the half-cell was improved to a certain extent, but the effect was average; while the lithium cobalt oxide / lithium half-cell 1 used the electrolyte in Example 1, which contained 0.1 mol / L lithium difluorooxalate borate and 1 wt% lithium difluorophosphate. The discharge specific capacity of the half-cell after 200 cycles was 160.1 mAh·g -1, the capacity retention rate was 91.24% of the initial value, the Coulomb efficiency was more stable, and compared with the lithium cobaltate / lithium half-cells 2-4, the cycle stability was significantly improved. It shows that lithium difluorooxalate borate and lithium difluorophosphate have a synergistic effect. After adding both additives to the electrolyte, the cycle stability of the lithium cobaltate / lithium half-cell at 3-4.5V can be significantly improved, and the Coulomb efficiency can be increased.

[0124] Figure 2 are the constant current charge and discharge test results of the lithium cobaltate / lithium half-cells 1 and 5-8 in the application examples. It can be seen from Figure 2 that compared with the lithium cobaltate / lithium half-cell 1, the difference is that the lithium cobaltate / lithium half-cells 5 and 6 use the electrolytes prepared in Examples 4 and 5 respectively. The content of lithium difluorophosphate is the same, but the content of lithium difluorooxalate borate is half and twice the content of lithium difluorooxalate borate in the electrolyte of Example 1 respectively. It can be seen that the cycle stability of the half-cell becomes worse. After 200 cycles, the capacity retention rates are 69.4% and 78.6% respectively. However, compared with the lithium cobaltate / lithium half-cell 4, the cycle stability is improved. It is speculated that it is related to the content of the organic components in the interface layer formed by the decomposition of lithium difluorooxalate borate. The higher the concentration of lithium difluorooxalate borate, the more organic components in the interface layer, which is beneficial to protecting the structural integrity of lithium cobaltate, but will increase the interface impedance and is not conducive to capacity utilization. Also, because the electrolyte contains lithium difluorophosphate at the same time, even a lower concentration of lithium difluorooxalate borate still has a synergistic effect with it. Therefore, the battery capacity retention rate of the lithium cobaltate / lithium half-cell 5 is still higher than that of the lithium cobaltate / lithium half-cell 3. Similarly, when the content of lithium difluorooxalate borate in the electrolyte is the same, but the content of lithium difluorophosphate is reduced to half or increased to twice respectively, the cycle stabilities of the lithium cobaltate / lithium half-cells 7 and 8 also become worse. After 200 cycles, the capacity retention rates are 57.7% and 76.4% respectively. It is speculated that the decomposition products of lithium difluorophosphate are mainly inorganic products, but some harmful substances are also produced at the same time. When the concentration of lithium difluorophosphate is low, on the one hand, the harmful products generated by its decomposition damage the organic components of the interface phase, and on the other hand, the interface layer composed of the inorganic products generated by its decomposition is not sufficient to protect lithium cobaltate during long-term charge and discharge cycles in a high-voltage environment, resulting in a lower capacity retention rate of the lithium cobaltate / lithium half-cell 7 than that of the lithium cobaltate / lithium half-cell 2; when the concentration of lithium difluorophosphate is high, excessive lithium difluorophosphate decomposes on the surface of lithium cobaltate particles to form a thicker interface layer rich in inorganic products, increasing the interface impedance and resulting in a worse cycle stability of the half-cell. It shows that when lithium difluorooxalate borate and lithium difluorophosphate are used in combination in the electrolyte, attention should be paid to the proportional relationship between the two to achieve the best battery performance improvement effect. And by comparing the lithium cobaltate / lithium half-cells 1 and 5-8, it is speculated that the inorganic products decomposed by lithium difluorophosphate may have a better protective effect on the lithium cobaltate interface than the organic products decomposed by lithium difluorooxalate borate.

[0125] Figure 3For the rate performance test results of the lithium cobalt oxide / lithium half-cells 1-4 in the application examples, it can be seen from Figure 3 that during the process of gradually switching from a low rate of 0.2C to a high rate of 5C, the discharge specific capacities of the lithium cobalt oxide / lithium half-cells 1-4 all gradually decrease. However, the discharge specific capacities of the lithium cobalt oxide / lithium half-cells 1-3 are always higher than those of the lithium cobalt oxide / lithium half-cell 4, and the discharge specific capacity of the lithium cobalt oxide / lithium half-cell 1 always remains the highest, indicating that adding lithium difluoro(oxalato)borate or lithium difluorophosphate to the electrolyte can improve the rate performance of the half-cell to a certain extent, and when both are added simultaneously, the effect is the best.

[0126] Figure 4 For the constant current charge-discharge test results of the graphite / lithium half-cells 1-4 in the application examples, it can be seen from Figure 4 that the graphite / lithium half-cell 4 uses the electrolyte without any additives in Comparative Example 3. After 420 charge-discharge cycles, the discharge specific capacity is 211.3 mAh·g -1 , and its capacity retention rate is only 55.7% of the initial value; the graphite / lithium half-cells 2 and 3 use the electrolytes in Comparative Example 1 and Comparative Example 2 respectively, in which lithium difluoro(oxalato)borate and lithium difluorophosphate are added respectively. The discharge specific capacities of the half-cells after 420 cycles are 267.3 mAh·g -1 and 280.1 mAh·g -1 respectively, and the capacity retention rates are 70.6% and 72.5% of the initial values respectively. Compared with the graphite / lithium half-cell 4, the cycle stability of the half-cell has a certain improvement, but the effect is average; while the graphite / lithium half-cell 1 uses the electrolyte in Example 1, which contains 0.1 mol / L lithium difluoro(oxalato)borate and 1 wt% lithium difluorophosphate. The discharge specific capacity of the half-cell after 420 cycles is 327.5 mAh·g -1 , and the capacity retention rate is 87.7% of the initial value, and the cycle stability is significantly improved. It shows that lithium difluoro(oxalato)borate and lithium difluorophosphate have a synergistic effect, and adding the two additives to the electrolyte simultaneously can significantly improve the cycle stability of the graphite / lithium half-cell.

[0127] Figure 5 For the rate performance test results of the graphite / lithium half-cells 1-4 in the application examples, it can be seen from Figure 5 that during the process of gradually switching from a low rate of 0.1C to a high rate of 10C, the discharge specific capacities of the graphite / lithium half-cells 1-4 all gradually decrease. Among them, the graphite / lithium half-cell 4 needs to be charged and discharged for a certain number of cycles to achieve normal capacity performance, but the capacity performance of the graphite / lithium half-cells 1-3 is stable after switching the rate, indicating that after adding lithium difluoro(oxalato)borate or lithium difluorophosphate to the electrolyte, the interfacial phase resistance formed by their reduction on the graphite surface is relatively low, and the utilization rate of graphite is relatively high, which can significantly improve the rate performance of the graphite / lithium half-cell.

[0128] Figure 6 Impedance fitting data graphs of the lithium cobalt oxide / lithium half-cells 1-4 in the application example after 3 and 150 charge-discharge cycles Figure 7 Electrochemical impedance graph and its corresponding fitting curve of the lithium cobalt oxide / lithium half-cells 1-4 in the application example after 3 charge-discharge cycles Figure 8 Electrochemical impedance graph and its corresponding fitting curve of the lithium cobalt oxide / lithium half-cells 1-4 in the application example after 150 charge-discharge cycles Figure 9 Equivalent circuit diagram for impedance fitting of the lithium cobalt oxide / lithium half-cells 1-4 in the application example. As can be seen from Figures 6 - 9 It can be seen that after 150 charge-discharge cycles of the lithium cobalt oxide / lithium half-cell 4, the fitted film impedance decreases from 30 Ω to 16.1 Ω, but is still greater than that of the lithium cobalt oxide / lithium half-cells 1-3 (9.3 Ω, 10.8 Ω, and 8.2 Ω respectively); while the charge transfer impedance of the lithium cobalt oxide / lithium half-cell 4 is as high as 1747 Ω, much higher than that of the lithium cobalt oxide / lithium half-cells 1-3 (27.3 Ω, 114.4 Ω, and 88.4 Ω respectively), indicating that during the charge-discharge cycle, the electrolyte undergoes an oxidation reaction on the surface of the lithium cobalt oxide electrode, continuously constructing the electrode interface phase, and both lithium difluoro(oxalato)borate and lithium difluorophosphate have a positive effect on the construction of the electrode interface phase, and there is a synergistic effect between the two. When used together, it can significantly improve the charge transfer ability of the lithium cobalt oxide interface phase and effectively slow down the impedance growth of the lithium cobalt oxide / lithium half-cell

[0129] Figure 10 Scanning electron microscope images of the lithium cobalt oxide electrode in the application example, where Figure 10 (a) is the scanning electron microscope image of the lithium cobalt oxide electrode that has not been cycled Figure 10 (b) is the scanning electron microscope image of the lithium cobalt oxide electrode disassembled from the lithium cobalt oxide / lithium half-cell 1 Figure 10 (c) is the scanning electron microscope image of the lithium cobalt oxide electrode disassembled from the lithium cobalt oxide / lithium half-cell 2 Figure 10 (d) is the scanning electron microscope image of the lithium cobalt oxide electrode disassembled from the lithium cobalt oxide / lithium half-cell 3 Figure 10 (e) is the scanning electron microscope image of the lithium cobalt oxide electrode disassembled from the lithium cobalt oxide / lithium half-cell 4. As can be seen from Figure 10It can be seen that, compared with the cobalt lithium oxide electrode sheet without cycling, the morphology of the cobalt lithium oxide particles on the cobalt lithium oxide electrode sheet disassembled from the cobalt lithium oxide / lithium half-cell 4 shows a state of fragmentation and tearing, and the interphase layer is not obvious; while the integrity of the cobalt lithium oxide particles on the cobalt lithium oxide electrode sheet disassembled from the cobalt lithium oxide / lithium half-cells 1-3 is relatively high, which is related to the solid electrolyte interphase on its surface; among them, on the cobalt lithium oxide electrode sheet disassembled from the cobalt lithium oxide / lithium half-cell 2, there is a relatively large amount of loose decomposition products accumulated on the surface of the cobalt lithium oxide particles; on the cobalt lithium oxide electrode sheet disassembled from the cobalt lithium oxide / lithium half-cell 3, the decomposition products are evenly distributed, but there are obvious granular and rough feelings; on the cobalt lithium oxide electrode sheet disassembled from the cobalt lithium oxide / lithium half-cell 1, the surface of the cobalt lithium oxide particles is smooth and flat, and the decomposition products are evenly distributed. It shows that adding lithium difluoro(oxalato)borate and lithium difluorophosphate to the electrolyte can form an organic-inorganic composite CEI film on the surface of the cobalt lithium oxide positive electrode and inhibit the oxidative decomposition of the electrolyte at high voltages.

[0130] Figure 11 FIG. is a scanning electron microscope image of the graphite electrode sheet in the application example, where Figure 11 (a) therein is a scanning electron microscope image of the graphite electrode sheet without cycling, Figure 11 (b) therein is a scanning electron microscope image of the graphite electrode sheet disassembled from the graphite / lithium half-cell 1, Figure 11 (c) therein is a scanning electron microscope image of the graphite electrode sheet disassembled from the graphite / lithium half-cell 2, Figure 11 (d) therein is a scanning electron microscope image of the graphite electrode sheet disassembled from the graphite / lithium half-cell 3, Figure 11 (e) therein is a scanning electron microscope image of the graphite electrode sheet disassembled from the graphite / lithium half-cell 4. It can be Figure 11 seen that, compared with the graphite electrode sheet without cycling, a very large amount of unevenly distributed decomposition products are deposited on the surface of the graphite / lithium half-cell 4, and the surface is rough, showing a sense of fragmentation; a thick layer of decomposition products is also deposited on the surface of the graphite / lithium half-cell 2, but the morphology remains intact; the surface of the graphite / lithium half-cell 3 is covered with pimply decomposition products; the surface of the graphite / lithium half-cell 1 is the smoothest and flattest. It shows that adding lithium difluoro(oxalato)borate and lithium difluorophosphate to the electrolyte can form a dense interfacial film on the graphite negative electrode.

Claims

1. An electrolyte, characterized in that, It comprises the following components: a lithium salt, an additive, and an organic solvent; Among them, the additive is lithium difluoro(oxalato)borate and lithium difluorophosphate; the content of lithium difluoro(oxalato)borate in the electrolyte is 0.05 - 0.2 mol / L; the content of lithium difluorophosphate in the electrolyte is 0.5 wt% - 2 wt%.

2. The electrolyte according to claim 1, characterized in that, In the electrolyte, the content of lithium difluoro(oxalato)borate is 0.08 - 0.15 mol / L; and / or, the content of lithium difluorophosphate is 0.8 wt% - 1.5 wt%.

3. The electrolyte according to claim 1, wherein The content of the lithium salt in the electrolyte is 0.5 - 2 mol / L.

4. The electrolyte according to claim 3, characterized in that, The lithium salt is selected from at least one of lithium hexafluorophosphate, lithium perchlorate, lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide.

5. The electrolyte according to claim 1, characterized in that, The organic solvent is selected from at least one of cyclic carbonates and linear carbonates.

6. The electrolyte according to claim 5, characterized in that, The organic solvent is a mixed solvent of cyclic carbonate and linear carbonate with a mass ratio of 1:(1 - 5).

7. The method for preparing the electrolyte according to any one of claims 1-6, characterized in that, It comprises the following steps: Dissolve the lithium salt and the additive in the organic solvent to obtain the electrolyte described above.

8. A lithium-ion battery, characterized in that, It comprises the electrolyte according to any one of claims 1 - 6, as well as a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode.

9. The lithium-ion battery according to claim 8, characterized in that, The positive electrode active material on the positive electrode is selected from at least one of lithium cobaltate, lithium manganate, nickel cobalt manganese ternary material, and nickel cobalt aluminum ternary material; and / or, the negative electrode active material on the negative electrode is selected from at least one of graphite, silicon, silicon alloy, silicon carbide, and silicon oxide.

10. Application of the lithium-ion battery according to claim 8 or 9 in 3C products, new energy vehicles, or energy storage devices.

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