Lithium ion battery electrolyte and application thereof

Abstract

The application provides a lithium ion battery electrolyte and application thereof, and the electrolyte at least comprises a non-aqueous solvent, a lithium salt and an additive, wherein the additive comprises a compound with a structure of formula (I), and the general formula of the compound with the structure of formula (I) is shown in the description, wherein R1 is a C1-C3 alkyl, a methoxy group, an ethoxy group, a propoxy group or an amino group. The lithium ion battery electrolyte and application thereof can inhibit oxidation of the electrolyte and improve the cycle performance and high-temperature performance of the lithium ion battery.

Description

Technical Field

[0001] This invention relates to the field of electrochemical energy storage technology, specifically to a lithium-ion battery electrolyte and its application. Background Technology

[0002] Lithium-ion batteries are widely used in electric vehicles due to their high energy density and long cycle life. However, with the development of the electric vehicle industry, driving range has become a major limiting factor for the further development of lithium-ion batteries. Increasing the operating voltage of lithium-ion batteries or increasing the nickel content in ternary cathodes are common methods to improve driving range. However, these methods can lead to increased oxygen release from the cathode, significantly increasing the probability of electrolyte oxidation and thus limiting the development of lithium-ion batteries. Summary of the Invention

[0003] This invention proposes a lithium-ion battery electrolyte and its application, which can reduce electrolyte oxidation, lower DC impedance, improve capacity retention performance, and enhance the cycle performance and high-temperature performance of lithium-ion batteries.

[0004] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution.

[0005] This invention provides a lithium-ion battery electrolyte, comprising at least:

[0006] Non-aqueous solvents;

[0007] Lithium salts; and

[0008] Additives, said additives including compounds of formula (I), the general formula of said compounds of formula (I) is:

[0009]

[0010] Wherein, R1 is a C1 to C3 alkane group, methoxy group, ethoxy group, propoxy group, or amino group.

[0011] In one embodiment of the present invention, in the compound of formula (Ⅰ), R1 is a methyl group.

[0012] In one embodiment of the present invention, the mass content of the compound of formula (I) in the electrolyte is 0.05wt%-1wt%.

[0013] In one embodiment of the present invention, the non-aqueous solvent includes a cyclic carbonate, which is selected from any one or a combination of two of ethylene carbonate or propylene carbonate.

[0014] In one embodiment of the present invention, the cyclic carbonate in the non-aqueous solvent has a mass content of 10wt% to 40wt%.

[0015] In one embodiment of the present invention, the non-aqueous solvent includes a linear carbonate, which is selected from any one or at least a combination of two of dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

[0016] In one embodiment of the present invention, the lithium salt is selected from any one or at least a combination of two of lithium hexafluorophosphate, lithium bisfluorosulfonylimide, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorobis(oxalate)phosphate, lithium difluorooxalateborate, lithium difluorophosphate, or lithium trifluoromethanesulfonate.

[0017] In one embodiment of the present invention, the lithium salt in the electrolyte has a mass content of 8wt%-20wt%.

[0018] The present invention also provides a lithium-ion battery, comprising the lithium-ion battery electrolyte described above.

[0019] The present invention also provides an electrochemical device comprising the lithium-ion battery described above.

[0020] In summary, this invention proposes a lithium-ion battery electrolyte and its application. Through compounds with the structure of formula (Ⅰ), the DC impedance of the lithium-ion battery is effectively reduced, and the oxidation of the electrolyte by cathode oxygen release can be effectively suppressed from multiple aspects. This effectively inhibits the impact of cathode oxidation on lithium-ion battery performance, improves lithium-ion battery performance, and meets the requirements for the driving range of lithium-ion batteries. By controlling the content of additives, the cycle stability of the lithium-ion battery can be improved, and the formation quality of the SEI film can be further improved, effectively enhancing the cycle performance and high-temperature performance of the lithium-ion battery. Detailed Implementation

[0021] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0022] It should be understood that the invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0023] The technical solution of the present invention will be further described in detail below with reference to several embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention proposes a lithium-ion battery electrolyte, comprising at least a non-aqueous solvent, a lithium salt, and additives, wherein the additives include compounds with the structure of formula (I), and the general formula of the compounds with the structure of formula (I) is:

[0025] Wherein, R1 is a C1-C3 alkane group, methoxy group, ethoxy group, propoxy group, or amino group. In this embodiment, R1 is, for example, a methyl group, and the mass content of the compound of formula (I) in the electrolyte is, for example, 0.05wt%-1wt%, or, for example, 0.1wt%-0.5wt%. The compound of formula (I) contains multiple low-valent sulfur atoms, which can effectively absorb oxygen released from the cathode. The compound of formula (I) contains a cyano group, which can complex with high-valent metal ions in the cathode, reducing the oxidizing power of high-valent metal ions and acting as a positioning agent, allowing low-valent sulfur to be adsorbed on the cathode surface, thereby enhancing the ability of low-valent sulfur to absorb oxygen released from the cathode. The compound of formula (I) contains unsaturated bonds, which mainly increase the degree of polymerization. A high degree of polymerization ensures that the SEI film formed by this substance will not dissolve in the electrolyte, effectively reducing the oxidation of the electrolyte by the cathode during the regeneration process of SEI dissolution. Therefore, compounds with the structure of formula (Ⅰ) can improve the problem of oxygen release from the cathode from multiple perspectives, improve the problem of electrolyte oxidation, and meet the requirements of the driving range of lithium-ion batteries.

[0026] In one embodiment of the present invention, the lithium salt is selected from any one or a combination of at least two of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(oxalate borate) (LiBOB), lithium difluorobis(oxalate phosphate) (LiODFP), lithium difluorooxalate borate (LiODFB), lithium difluorophosphate (LiPO2F2), or lithium trifluoromethanesulfonate (LiCF3SO3). In this embodiment, the lithium salt is, for example, sodium hexafluorophosphate and lithium bis(fluorosulfonyl)imide, and the mass content of the lithium salt in the electrolyte is, for example, 8 wt%-20 wt%, or, for example, 12 wt%-18 wt%.

[0027] In one embodiment of the present invention, the non-aqueous solvent is selected from at least one of cyclic carbonates or linear carbonates. The cyclic carbonate is selected from at least one or a combination of two of ethylene carbonate (EC) and propylene carbonate (PC), and the content of the cyclic carbonate accounts for 10 wt%-40 wt% of the total non-aqueous solvent. The linear carbonate is selected from at least one or a combination of at least two of dimethyl carbonate (DMC), diethyl carbonate (DEC), or ethyl methyl carbonate (EMC). The non-aqueous solvent may also include carboxylic acid esters, selected from at least one or a combination of at least two of ethyl acetate (EA), propyl acetate (PA), ethyl propionate (EP), or propyl propionate (PP). In this embodiment, the content of linear carbonate accounts for 10wt%-80wt% of the total solvent by mass.

[0028] In one embodiment of the present invention, when preparing the electrolyte, the nitrogen content in the glove box is 99.999%, the actual oxygen content in the glove box is less than or equal to 0.1 ppm, and the moisture content is less than or equal to 0.1 ppm. After the non-aqueous solvent is mixed evenly according to the mass ratio, the fully dried lithium salt is added to the above-mentioned non-aqueous solvent. After the lithium salt is completely dissolved, the additives are added according to the ratio and stirred evenly to prepare the lithium-ion battery electrolyte.

[0029] This invention also proposes a lithium-ion battery, comprising a positive electrode, a separator, a negative electrode, and an electrolyte. The separator is located between the positive and negative electrodes, and the electrolyte is filled between the positive electrode, the separator, and the negative electrode. The electrolyte is the lithium-ion battery electrolyte described above. The lithium-ion battery can be, for example, a primary battery or a secondary battery. A secondary battery can be, for example, a pouch battery, a hard-case battery, or a cylindrical battery, etc. This invention does not impose specific limitations.

[0030] In one embodiment of the present invention, the positive electrode sheet includes a positive current collector, a positive active material, a binder, and a conductive agent. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or non-woven fabric. The thickness of the positive current collector is, for example, 8μm-15μm. In this embodiment, the positive current collector is, for example, aluminum foil, and the thickness of the aluminum foil is, for example, 13μm.

[0031] In one embodiment of the present invention, the positive electrode active material is selected from, for example, lithium iron phosphate (LiFePO4), lithium cobalt oxide (LCO), or LiNi. x Co y Mn z O2 (x+y+z=1) and other positive electrode active materials, or combinations of at least two of them. In other embodiments, other materials may be selected as the positive electrode active material, and the present invention does not impose specific limitations. The binder is selected from, for example, any one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), ethylene-propylene-diene terpolymer (EPDM), polyhexanefluoropropylene (Polyhexafluoropropylene), or polymerized styrene-butadiene rubber (SBR). The conductive agent is selected from, for example, any one or more of carbon black, acetylene black, carbon nanotubes, and graphene.

[0032] In one embodiment of the present invention, the positive electrode active material is, for example, LiNi. 0.8 Co 0.1 Mn 0.1 O2, a conductive agent such as acetylene black, and a binder such as polyvinylidene fluoride are used. The positive electrode active material, acetylene black, and polyvinylidene fluoride are mixed, for example, in a weight ratio of 95:3:2. An organic solvent is then added and stirred until the system is homogeneous to obtain a positive electrode slurry. The organic solvent is, for example, N-methylpyrrolidone (NMP). The positive electrode slurry is uniformly coated onto aluminum foil and dried. Then, through processes such as rolling and cutting, a positive electrode sheet is obtained.

[0033] In one embodiment of the present invention, the negative electrode sheet includes, for example, a negative electrode current collector, a negative electrode active material, a binder, and a conductive agent. The negative electrode current collector is selected from, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector, and the thickness of the negative electrode current collector is, for example, 8 μm-15 μm. In this embodiment, the negative electrode current collector is, for example, copper foil, and the thickness of the copper foil is, for example, 13 μm.

[0034] In one embodiment of the present invention, the negative electrode active material is selected from any one or a combination of at least two of the following: artificial graphite, natural graphite, soft carbon, hard carbon, pure silicon, silicon oxide compounds, or silicon carbide compounds. The binder is selected from any one or more of the following: polyvinylidene fluoride, polyamide, polypropylene, polyacrylate, polyethylene ether, sodium carboxymethyl cellulose, polymethyl methacrylate, polyhexane, or styrene-butadiene rubber. The conductive agent is selected from any one or more of the following: conductive carbon black, acetylene black, carbon nanotubes, and graphene.

[0035] In one embodiment of the present invention, the negative electrode active material is selected from artificial graphite, the conductive agent is selected from acetylene black, and the binder is selected from sodium carboxymethyl cellulose. In one embodiment of the present invention, the negative electrode active material, the conductive agent, and the binder are mixed in a mass ratio of 96:2:2, deionized water is added, and the mixture is stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on copper foil, and the negative electrode sheet is obtained through processes such as drying, rolling, and cutting.

[0036] In one embodiment of the present invention, the separator is, for example, a polyethylene (PE) film, a polypropylene (PP) film, a glass fiber film, a polyethylene film, or a composite film. The thickness of the separator is, for example, 9μm-15μm. In this embodiment, a polyethylene film is selected as the separator. In one embodiment of the present invention, the above-mentioned positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrode to act as a separator. An aluminum-plastic film is then inserted to obtain a dry cell. After baking at 80°C to remove water, electrolyte is injected and the cell is sealed. Following processes such as settling, hot and cold pressing, formation, clamping, and capacity testing, a finished soft-pack lithium-ion secondary battery is obtained.

[0037] The present invention will be explained in more detail below by referring to embodiments, which should not be construed as limiting. Appropriate modifications can be made within the scope of the present invention, and all such modifications fall within the technical scope of the present invention.

[0038] Example 1

[0039] Preparation of electrolyte: Under the conditions of 99.999% nitrogen content, 0.1 ppm actual oxygen content, and 0.1 ppm moisture content in a glove box, EC, EMC, and DEC were mixed in a mass ratio of 30:50:20. 12 wt% lithium hexafluorophosphate, 4 wt% lithium difluorosulfonylimide, and 0.05 wt% of the compound with the structure of formula (1) were mixed uniformly with a solvent to obtain the electrolyte.

[0040] Preparation of the positive electrode: The positive electrode active material LiNi... 0.8 Co 0.1 Mn 0.1O2, conductive agent acetylene black, and binder polyvinylidene fluoride are mixed in a mass ratio of 95:3:2, and N-methylpyrrolidone solvent is added. The mixture is stirred until it becomes homogeneous and transparent to obtain a positive electrode slurry. The positive electrode slurry is uniformly coated onto an aluminum foil current collector, and after drying, rolling, and cutting, a positive electrode sheet is obtained.

[0041] Preparation of the negative electrode sheet: Artificial graphite (negative electrode active material), acetylene black (conductive agent), and sodium carboxymethyl cellulose (binder) are mixed in a mass ratio of 96:2:2. Deionized water is added, and the mixture is stirred thoroughly to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated onto copper foil, and after drying, rolling, and cutting, the negative electrode sheet is obtained.

[0042] Selection of diaphragm: 12μm polyethylene was selected as the diaphragm.

[0043] Battery preparation: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes to act as a separator. An aluminum-plastic film is then inserted to obtain a dry cell. After baking at 80°C to remove water, electrolyte is injected and the cell is sealed. After standing, hot and cold pressing, formation, clamping, and capacity testing, a lithium-ion secondary battery is obtained.

[0044] Example 2

[0045] The content of the compound with the structure of formula (1) was changed to 0.1 wt%, and the remaining steps were the same as in Example 1.

[0046] Example 3

[0047] The content of the compound with the structure of formula (1) was changed to 0.5 wt%, and the remaining steps were the same as in Example 1.

[0048] Example 4

[0049] The content of the compound with the structure of formula (1) was changed to 1 wt%, and the remaining steps were the same as in Example 1.

[0050] Comparative Example 1

[0051] In this comparative example, the electrolyte additive of formula (Ⅰ) in Example 1 was removed, and the other conditions were the same as in Example 1.

[0052] In this invention, lithium-ion batteries were prepared using different electrolytes in Examples 1-4 and Comparative Example 1, and the electrolyte formulations are shown in Table 1. The performance characteristics of the lithium-ion batteries, including DCR, cycle capacity retention, high-temperature storage, and high-temperature gas generation, were tested, and the test results are shown in Table 2.

[0053] In one embodiment of the present invention, the cycle test is performed at 25°C by charging the lithium-ion battery at a constant current of 1 / 3C to 4.35V, followed by constant voltage charging to a current of 0.05C. The battery is then discharged at a constant current of 1 / 3C to 2.5V. The capacity C0 is recorded. This charge-discharge cycle is repeated 800 times, and the discharge capacity C1 after 800 cycles is recorded. The battery capacity retention rate = C1 / C0 * 100%.

[0054] In one embodiment of the present invention, the DCR test is performed by charging a lithium-ion battery that has undergone 800 cycles at 25°C with a constant current of 1 / 3C to 4.35V, followed by constant voltage charging to a current of 0.05C. The battery is then discharged with a constant current of 1 / 3C to 2.5V. This charging process is repeated, and the charging capacity is recorded as C2. The battery is then discharged with a constant current of 1 / 3C to (50% * C2), and the initial voltage is recorded as V2. The battery is then discharged with a constant current of 1C2 for 30 seconds, and the final voltage is recorded as V3. Therefore, DCR = (V2 - V3) / (C2 * 1).

[0055] In one embodiment of the present invention, the high-temperature storage test involves storing the lithium-ion battery at 60°C for 27 days. The battery is then discharged at 25°C with a constant current of 1 / 3C to 2.5V, followed by charging at a constant current of 1 / 3C to 4.35V, and then charged at a constant voltage to a current of 0.05C. The battery is then discharged again at a constant current of 1 / 3C to 2.5V, and the discharge capacity is recorded as C3. The capacity recovery rate is (C3 / C2)*100%. The above charging steps are repeated, and the charging capacity is recorded as C4. The battery is then discharged at a constant current of 1 / 3C to (50%*C4), and the initial voltage is recorded as V4. The battery is then discharged at a constant current of 1C for 30 seconds, and the final voltage is recorded as V5. Therefore, the DCR after 27 days of storage is (V4-V5) / (C4*1). The DCR growth rate is (DCR after 27 days of storage - initial DCR) / initial DCR*100%.

[0056] In one embodiment of the present invention, the high-temperature gas generation test is performed by charging the lithium-ion battery at a constant current of 1 / 3C to 4.35V at 25°C, and then charging it at a constant voltage to a current of 0.05C. The thickness is measured using a thickness gauge, and the initial thickness T1 is recorded. The battery is then stored in a 60°C constant temperature chamber for 30 days. After being removed and cooled to 25°C, the cell thickness T2 is measured using a thickness gauge, and the thickness change rate is calculated as (T2-T1) / T1*100%.

[0057] Table 1. Electrolyte formulations in Examples 1-4 and Comparative Example 1

[0058]

[0059] Table 2 shows the performance test results of the lithium-ion batteries in Examples 1-4 and Comparative Example 1.

[0060]

[0061] Please refer to Tables 1 and 2. Comparing Examples 1-4 and Comparative Example 1, it can be seen that adding a compound with structure (I) to the electrolyte can improve the cycle performance, high-temperature storage, and gas generation performance of lithium-ion batteries, reduce the DCR of lithium-ion batteries, and reduce the increase of DCR during use. This indicates that by using a compound with structure (I), the combined effect of sulfur atoms, cyano groups, and unsaturated bonds in the compound can effectively inhibit the oxidation of the electrolyte by oxygen release from the cathode, improve capacity retention, reduce DC impedance, effectively alleviate the problem of cathode oxygen release caused by increased voltage or other factors, improve the performance degradation of lithium-ion batteries, and enhance the overall performance of lithium-ion batteries.

[0062] Please refer to Tables 1 and 2. Comparing Examples 1-4, it is evident that increasing the amount of compound with structure (I) increases the performance of the lithium-ion battery. However, when the amount of compound with structure (I) exceeds 0.5 wt%, the performance of the lithium-ion battery decreases. This indicates that adding a small amount of compound with structure (I) to the electrolyte can improve the performance of the lithium-ion battery, and the amount of compound with structure (I) is limited to 0.05 wt%-1 wt%, or for example, 0.1 wt%-0.5 wt%, to ensure that the lithium-ion battery achieves excellent performance.

[0063] In summary, this invention proposes a lithium-ion battery electrolyte and its application. The compound with formula (Ⅰ) effectively reduces the DC impedance of the lithium-ion battery and effectively suppresses the oxidation of the electrolyte by oxygen release from the cathode. This effectively inhibits the impact of cathode oxidation on lithium-ion battery performance, improves battery performance, and meets the range requirements of lithium-ion batteries. By controlling the additive content, film formation on the negative electrode can be promoted, improving the cycle stability of the lithium-ion battery and further enhancing the quality of the SEI film formation, effectively improving the cycle performance and high-temperature performance of the lithium-ion battery.

[0064] The above description is merely a preferred embodiment of this application and an explanation of the technical principles used. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the inventive concept. For example, technical solutions formed by replacing the above-mentioned features with technical features with similar functions disclosed in this application (but not limited to) each other.

[0065] Apart from the technical features described in the specification, the other technical features are known to those skilled in the art. To highlight the innovative features of this invention, the other technical features will not be described in detail here.

Claims

1. A lithium-ion battery electrolyte, characterized in that, At least including: Non-aqueous solvents; Lithium salts; as well as Additives, said additives including compounds of formula (I), the general formula of said compounds of formula (I) is: Wherein, R1 is a C1 to C3 alkane group, methoxy group, ethoxy group, propoxy group, or amino group.

2. The lithium-ion battery electrolyte according to claim 1, characterized in that, In compounds with the structure of formula (Ⅰ), R1 is a methyl group.

3. The lithium-ion battery electrolyte according to claim 1, characterized in that, The compound of formula (Ⅰ) is present in the electrolyte at a mass content of 0.05 wt% to 1 wt%.

4. The lithium-ion battery electrolyte according to claim 1, characterized in that, The non-aqueous solvent includes cyclic carbonates selected from any one or a combination of two of ethylene carbonate and propylene carbonate.

5. The lithium-ion battery electrolyte according to claim 4, characterized in that, The cyclic carbonate has a mass content of 10 wt% to 40 wt% in the non-aqueous solvent.

6. The lithium-ion battery electrolyte according to claim 1, characterized in that, The non-aqueous solvent includes linear carbonates, which are selected from any one or at least a combination of two of dimethyl carbonate, diethyl carbonate, or methyl ethyl carbonate.

7. The lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt is selected from any one or at least a combination of two of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorobis(oxalate)phosphate, lithium difluorooxalateborate, lithium difluorophosphate, or lithium trifluoromethanesulfonate.

8. The lithium-ion battery electrolyte according to claim 1, characterized in that, The lithium salt has a mass content of 8wt%-20wt% in the electrolyte.

9. A lithium-ion battery, characterized in that, Includes the lithium-ion battery electrolyte as described in any one of claims 1-8.

10. An electrochemical device, characterized in that, Including the lithium-ion battery as described in claim 9.

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