High-temperature electrolyte, preparation method thereof and application thereof in lithium ion battery

Abstract

The application discloses preparation and application of a functional electrolyte containing a special additive, and belongs to the technical field of lithium ion batteries, in particular to a high-temperature electrolyte and a preparation method thereof and application thereof in lithium ion batteries. The additive functional electrolyte can be obtained by adding the additive into a common electrolyte. The application uses a special additive as an interface film-forming additive in the electrolyte of the lithium ion battery, and a uniform and low-impedance CEI protective film can be formed on the surface of the positive electrode after three cycles of activation. The interface film between the optimized positive electrode and the electrolyte inhibits the oxidative decomposition of the electrolyte on the surface of the electrode and inhibits the dissolution of the transition metal. This can significantly improve the cycle performance and high-temperature aging resistance of the lithium ion battery containing the electrolyte additive at 3-4.5 V.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, and particularly relates to a high-temperature electrolyte, its preparation method, and its application in lithium-ion batteries. Background Technology

[0002] A lithium-ion battery is a rechargeable battery in which lithium ions can be inserted and extracted between the positive and negative electrodes. With the advent of mobile electronic devices such as smartphones and laptops, the demand for portable energy sources has increased dramatically. Lithium-ion batteries, due to their ultra-high energy density, have become a focus of academic and industrial attention. The electrolyte in a lithium-ion battery is responsible for lithium-ion transport. Currently, most commercially available lithium-ion battery electrolytes consist of a carbonate solution combined with lithium hexafluorophosphate and electrolyte additives. These additives can improve the stability and safety of lithium-ion batteries through their own electrochemical decomposition.

[0003] Theoretical studies indicate that qualified interface modification additives should possess a narrower electrochemical window. This allows them to preferentially decompose before the electrolyte solvent or lithium salt during battery activation, participating in the modification of the solid electrolyte interface (CEI) on the electrode surface. Continuous electrolyte decomposition and structural damage to the cathode material are significant factors affecting the high-voltage performance and high-temperature aging of lithium-ion batteries. Although commercially available carbonate-based electrolytes exhibit a wide electrochemical window in redox tests of inert electrodes, the catalytic effect of transition metals in the cathode material on the electrolyte must be considered in practical lithium-ion battery applications. This makes it impossible for currently used carbonate-based electrolytes to sustain lithium-ion batteries in high-voltage or high-temperature environments for extended periods. Furthermore, the CEI formed by currently used commercially available carbonate-based electrolytes is very fragile in high-voltage or high-temperature environments, unable to withstand corrosion from HF generated by electrolyte decomposition. When this fragile CEI layer is damaged, the electrolyte erodes the positive electrode. Transition metal ions with catalytic properties detach from the positive electrode material and migrate to the negative electrode side, disrupting the interface and causing continuous electrolyte decomposition, producing more HF. Battery gas production and increased interfacial impedance also arise due to electrolyte decomposition, severely impacting the safety performance of lithium-ion batteries. Summary of the Invention

[0004] To address the shortcomings and deficiencies of existing technologies, this invention provides a series of electrolyte additives, electrolytes containing these additives, and lithium-ion batteries. These electrolyte additives can undergo oxidative decomposition during lithium-ion battery formation, polymerizing on the surface of the positive electrode material to form a stable CEI (Chemical Electrolyte Injection). The CEI formed by these electrolyte additives can effectively improve the high-temperature, high-voltage aging problems of lithium-ion batteries and address safety issues such as gas generation.

[0005] The specific technical solution of this invention is as follows:

[0006] An electrolyte comprising a lithium salt, an organic solvent, and an electrolyte additive, characterized in that the electrolyte additive has the following structure:

[0007]

[0008] Where X is any one of the following cases:

[0009] Straight-chain or branched alkyl groups having 1 to 5 carbon atoms;

[0010] Straight-chain or branched alkoxy groups with 1 to 5 carbon atoms;

[0011] Straight-chain or branched alkyl groups in which some or all of the hydrogen atoms are replaced by halogen atoms;

[0012] Aldehyde group or aldehyde group substituted with halogen atoms;

[0013] Carbonate or carbonate groups substituted with halogen atoms;

[0014] Halogen atom;

[0015] Hydrogen atom.

[0016] Preferably, the amount of electrolyte additive used is 0.01~5% of the total electrolyte mass.

[0017] Preferably, the lithium salt is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(oxalate-borate), lithium fluorooxalate-borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate; and the organic solvent is one or more of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, and nitrile solvents.

[0018] Preferably, the carbonate solvent is one or more selected from ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and dipropyl carbonate; the carboxylic acid ester solvent is one or more selected from ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, and 1,4-butyrolactone; the sulfone solvent is one or more selected from dimethyl sulfoxide, sulfolane, diphenyl sulfoxide, thionyl chloride, and dipropyl sulfone; and the nitrile solvent is one or more selected from acetonitrile, propionitrile, succinic anhydride, and adiponitrile.

[0019] Preferably, the concentration of the lithium salt is 0.5~1.5M.

[0020] Preferably, the organic solvent is a mixed solution of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a mass ratio of 3:5:2.

[0021] The method for preparing the above-mentioned electrolyte is characterized by comprising the following steps:

[0022] (1) The organic solvent is purified and dehydrated using molecular sieves;

[0023] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) and stir until homogeneous to obtain a common electrolyte;

[0024] (3) Add electrolyte additives to the ordinary electrolyte prepared in step (2).

[0025] The application of the above electrolyte in lithium-ion batteries.

[0026] Preferably, the lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0027] Preferably, the positive electrode comprises a positive electrode active material, wherein the positive electrode active material is LiCoO2, LiNiO2, LiMn2O4, LiFePO4, or LiNi x Co y Mn z One or more of O2, wherein x+y+z=1; the negative electrode includes a negative electrode active material, which is one or more of natural graphite, artificial graphite, lithium titanate, silicon and silicate composites; the separator is one or more of polyethylene, polypropylene, polyimide, aramid, ceramic, and PVDF.

[0028] During charge-discharge cycles at high temperatures, the electrolyte in lithium-ion batteries undergoes continuous oxidative decomposition on the positive electrode surface. This decomposition leads to the generation of Lewis acids such as HF and gases such as CO2. Although commercially available carbonate-based electrolytes have high oxidation potentials on inert Pt electrodes, they are easily oxidized in lithium-ion battery applications. This is because the catalytic effect of the positive electrode cannot be ignored. Experiments have shown that the additive provided by this invention can preferentially form an inert protective layer on the positive electrode surface compared to other components in the battery. This inert protective layer isolates the electrolyte from the positive electrode, preventing further oxidative decomposition of the electrolyte. Furthermore, this inert interface protects the positive electrode from electrolyte corrosion, significantly mitigating the problem of transition metal dissolution. This results in lithium-ion batteries using the additive of this invention exhibiting more stable high-temperature performance.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] This invention utilizes a special additive as an interfacial film-forming additive in lithium-ion battery electrolytes. After three activation cycles, it can form a uniform and low-resistance CEI protective film on the surface of the positive electrode, and more easily form an inert polymer protective layer on the positive electrode surface.

[0031] (2) The optimized interface film between the positive electrode and the electrolyte can inhibit the oxidative decomposition of the electrolyte on the electrode surface and suppress the dissolution of transition metals. Therefore, it can better protect the positive electrode from electrolyte corrosion. This makes the cycle performance and high-temperature aging resistance of lithium-ion batteries containing this electrolyte additive significantly improved at 3-4.5V, thereby effectively improving the high-voltage cycle life and high-temperature cycle performance of the battery. Attached Figure Description

[0032] Figure 1 This is a circuit fitted for impedance testing. Rs represents the bulk resistance of the solution, Rf represents the resistance of the interfacial membrane, Rct represents the resistance of lithium ion transfer between the electrode and electrolyte, Wo represents the Weber impedance, CPE1 represents the capacitance of the interfacial membrane, and CPE2 represents the double-layer capacitance.

[0033] Figure 2 Impedance comparison diagram of lithium secondary batteries made with electrolytes prepared in Example 1 and Comparative Example 1.

[0034] Figure 3 Impedance comparison diagram of lithium secondary batteries made with electrolytes prepared in Example 3 and Comparative Example 1.

[0035] Figure 4 This is a cycle stability test of lithium secondary batteries made with electrolytes prepared in Examples 2, 3, 4, and 5 of this invention and Comparative Example 1.

[0036] Figure 5 This is a cycle stability test of commercial soft-pack graphite / LiCoO2 batteries prepared with the electrolytes prepared in Example 1 and Comparative Example 1 of this invention.

[0037] Figure 6 This is a high-temperature 60°C cycle stability test of lithium secondary batteries made with electrolytes prepared in Example 3 and Comparative Example 1 of the present invention.

[0038] Figure 7 This is a high-temperature 70°C cycle stability test of lithium secondary batteries made with electrolytes prepared in Example 3 and Comparative Example 1 of the present invention.

[0039] Figure 8The images show scanning electron microscope (SEM) images of the lithium cobalt oxide cathode and the lithium cobalt oxide cathode without battery cycling, obtained after a high-temperature (60°C) cycling stability test of the lithium secondary batteries prepared with the electrolytes prepared in Example 3 and Comparative Example 1 of this invention.

[0040] Figure 9 Transmission electron microscopy (TEM) images of lithium cobalt oxide cathodes and lithium cobalt oxide cathodes without battery cycling, obtained by testing the cycling stability of lithium secondary batteries made with the electrolytes prepared in Example 3 and Comparative Example 1 of this invention, using the electrolytes prepared in Example 3 and Comparative Example 1. Detailed Implementation

[0041] This invention can be implemented in many different forms and is not limited to the embodiments described herein. The invention will be further described below with reference to embodiments.

[0042] Example 1

[0043] (1) The cyclic carbonate solvent ethylene carbonate (EC) and the linear carbonate solvent ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0044] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0045] (3) The amount of additive added to the ordinary electrolyte prepared in step (2) is 2% of the mass of the ordinary electrolyte. The structure of the additive is as follows:

[0046]

[0047] Example 2

[0048] (1) The cyclic carbonate solvent ethylene carbonate (EC), the linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0049] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0050] (3) The amount of additive added to the ordinary electrolyte prepared in step (2) is 1% of the mass of the ordinary electrolyte. The structure of the additive is as follows:

[0051]

[0052] Example 3

[0053] (1) The cyclic carbonate solvent ethylene carbonate (EC), the linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0054] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0055] (3) The amount of additive added to the ordinary electrolyte prepared in step (2) is 1% of the mass of the ordinary electrolyte. The structure of the additive is as follows:

[0056]

[0057] Example 4

[0058] (1) The cyclic carbonate solvent ethylene carbonate (EC), the linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0059] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0060] (3) The amount of additive added to the ordinary electrolyte prepared in step (2) is 1% of the mass of the ordinary electrolyte. The structure of the additive is as follows:

[0061]

[0062] Example 5

[0063] (1) The cyclic carbonate solvent ethylene carbonate (EC), the linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0064] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0065] (3) The amount of additive added to the ordinary electrolyte prepared in step (2) is 1% of the mass of the ordinary electrolyte. The structure of the additive is as follows:

[0066]

[0067] The additives used in the above examples were all purchased from TCI.

[0068] Comparative Example 1

[0069] (1) The cyclic carbonate solvent ethylene carbonate (EC), the linear carbonate solvent ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:EMC:DEC=3:5:2, and purified by molecular sieve to remove impurities and water.

[0070] (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) to a final solubility of 1.0 mol / L, stir evenly, and obtain a common electrolyte;

[0071] Effect comparison:

[0072] The functional electrolyte for lithium secondary batteries prepared in Example 1 and the ordinary electrolyte prepared in Comparative Example 1:

[0073] (1) Figure 2 The impedance comparison diagram shows the lithium secondary batteries prepared with the electrolytes prepared in Example 1 and Comparative Example 1. The test method was as follows: using a LiCoO2 electrode as the working electrode and a lithium sheet as the counter and reference electrode, 0.05 ml each of the additive functional electrolyte prepared in Example 1 and the ordinary electrolyte prepared in Comparative Example 1 were added to the separator, and a button-type lithium secondary battery was assembled and allowed to stand for 12 hours. The assembled battery was then charged and discharged twice at a rate of 0.3C using a blue electrode, and then charged to a full charge state of 4.4V for impedance testing. Figure 2 This indicates that the lithium secondary battery using the additive functional electrolyte prepared in Example 1 has a lower interfacial impedance.

[0074] (2) Table 1 shows the impedance test fitting results of Example 1 and Comparative Example 1. The fitted circuit is as follows: Figure 1 As shown.

[0075] Table 1 Impedance test fitting results of Example 1 and Comparative Example 1

[0076]

[0077] The fitting results show that the additive can form a low-impedance interface film at the positive electrode.

[0078] (3) Figure 3Impedance comparison diagrams are shown for lithium secondary batteries prepared with the electrolytes prepared in Example 3 and Comparative Example 1. The test method was as follows: using a LiCoO2 electrode as the working electrode and a lithium sheet as the counter and reference electrode, 0.05 ml each of the additive functional electrolyte prepared in Example 3 and the ordinary electrolyte prepared in Comparative Example 1 were added to the separator, and a button-type lithium secondary battery was assembled and allowed to stand for 12 hours. The assembled battery was then charged and discharged twice at a rate of 0.3C using a blue electrode, and then charged to a full charge state of 4.5V for impedance testing. Figure 3 This indicates that the lithium secondary battery using the additive functional electrolyte prepared in Example 3 has a lower interfacial impedance.

[0079] (4) Table 2 shows the impedance test fitting results of Example 3 and Comparative Example 1. The fitted circuit is as follows: Figure 1 As shown.

[0080] Table 2 Impedance test fitting results of Example 3 and Comparative Example 1

[0081]

[0082] The fitting results show that the additive can form a low-impedance interface film at the positive electrode.

[0083] (5) Figure 4 The figures show the cycle stability tests of lithium secondary batteries prepared with the electrolytes prepared in Examples 2, 3, 4, and 5 of this invention, and Comparative Example 1. The results show that the cycle stability of lithium secondary batteries was significantly and effectively improved in the functional electrolyte with added additives.

[0084] (6) Figure 5 This is a cycle stability test of commercial soft-pack graphite / LiCoO2 batteries prepared with the electrolytes prepared in Example 1 and Comparative Example 1 of this invention. The results in the figure show that the cycle stability of commercial soft-pack graphite / LiCoO2 batteries in the functional electrolyte with added additives is significantly and effectively improved.

[0085] (7) Figure 6 This is a high-temperature (60°C) cycle stability test of lithium secondary batteries prepared with the electrolytes prepared in Example 3 and Comparative Example 1 of this invention. The results in the figure show that the high-temperature aging resistance of the lithium secondary batteries is significantly and effectively improved in the functional electrolyte with added additives.

[0086] (8) Figure 7 This is a high-temperature (70°C) cycle stability test of lithium secondary batteries made with the electrolytes prepared in Example 3 and Comparative Example 1 of this invention. The results in the figure show that ordinary electrolytes cannot withstand the extremely high temperature of 70°C, while functional electrolytes with added additives can withstand such high temperatures.

[0087] (9) Figure 8 These are scanning electron microscope (SEM) images of lithium cobalt oxide cathodes and lithium cobalt oxide cathodes without battery cycling, obtained after a high-temperature (60°C) cycling stability test using the electrolytes prepared in Example 3 and Comparative Example 1 of this invention. The results show that a large amount of electrolyte decomposition products accumulated on the surface of the lithium cobalt oxide cathode after cycling in a conventional electrolyte. However, the lithium cobalt oxide cathode, after cycling in a functional electrolyte with added additives, remained in its uncycled state. This indicates that functional electrolytes with added additives are less prone to electrochemical decomposition.

[0088] (10) Figure 9 These are transmission electron microscopy (TEM) images of lithium cobalt oxide cathodes and lithium cobalt oxide cathodes without battery cycling, obtained after a high-temperature (60°C) cycling stability test using the electrolytes prepared in Example 3 and Comparative Example 1 of this invention. The results show that the lithium cobalt oxide electrode without battery cycling has a smooth surface; the lithium cobalt oxide cathode surface after cycling in a conventional electrolyte has a thick and rough interface; and the lithium cobalt oxide cathode surface after cycling in a functional electrolyte with additives has a thin and uniform interface.

[0089] (11) Table 3 shows the content of transition metal Co on the surface of lithium sheet after 400 cycles of lithium secondary batteries made with the electrolytes prepared in Example 3 and Comparative Example 1 of the present invention. The results in the table show that the polymer interface formed by the additives can inhibit the dissolution of transition metals.

[0090] Table 3 shows the content of the transition metal Co on the surface of the lithium sheet after 400 cycles of lithium secondary batteries made with the electrolytes prepared in Example 3 and Comparative Example 1.

[0091]

[0092] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. An electrolyte comprising a lithium salt, an organic solvent, and an electrolyte additive, characterized in that, The structure of the electrolyte additive is as follows: 。 2. The electrolyte according to claim 1, characterized in that, The amount of the electrolyte additive is 0.01 to 5% of the total electrolyte mass.

3. The electrolyte according to claim 1, characterized in that, The lithium salt is one or more of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium methanesulfonate, lithium trifluoromethanesulfonate, lithium bis(oxalate)borate, lithium fluorooxalateborate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate; the organic solvent is one or more of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, and nitrile solvents.

4. The electrolyte according to claim 3, characterized in that, The carbonate solvent is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, and dipropyl carbonate; the carboxylic acid ester solvent is one or more of ethyl acetate, methyl acetate, propyl acetate, butyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, and 1,4-butyrolactone; the sulfone solvent is one or more of dimethyl sulfoxide, sulfolane, diphenyl sulfoxide, thionyl chloride, and dipropyl sulfone; and the nitrile solvent is one or more of acetonitrile, propionitrile, succinic anhydride, and adiponitrile.

5. The electrolyte according to claim 1, characterized in that, The concentration of the lithium salt is 0.5~1.5M.

6. The electrolyte according to claim 4, characterized in that, The organic solvent is a mixed solution of ethylene carbonate, methyl ethyl carbonate, and diethyl carbonate in a mass ratio of 3:5:

2.

7. A method for preparing an electrolyte as described in any one of claims 1-6, characterized in that, Includes the following steps: (1) The organic solvent is purified and dehydrated using molecular sieves; (2) At room temperature, dissolve the lithium ion-conducting salt LiPF6 in the solvent obtained in step (1) and stir until homogeneous to obtain a common electrolyte; (3) Add electrolyte additives to the ordinary electrolyte prepared in step (2).

8. The application of the electrolyte according to any one of claims 1-6 in lithium-ion batteries.

9. The application according to claim 8, characterized in that, The lithium-ion battery includes a positive electrode, a negative electrode, a separator, and an electrolyte as described in any one of claims 1 to 6.

10. The application according to claim 9, characterized in that, The positive electrode includes a positive electrode active material, which is LiCoO2, LiNiO2, LiMn2O4, LiFePO4, or LiNi x Co y Mn z One or more of O2, wherein x+y+z=1; the negative electrode includes a negative electrode active material, which is one or more of natural graphite, artificial graphite, lithium titanate, silicon and silicate composites; the separator is one or more of polyethylene, polypropylene, polyimide, aramid, ceramic, and PVDF.

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