Electrolyte for lithium battery and application thereof

Abstract

The application provides electrolyte of lithium battery and application thereof, and the electrolyte comprises at least the following components: organic solvent, lithium salt and additive, wherein the additive comprises component A and vinylene carbonate, and the structural formula of the component A is as follows: wherein R1 and R2 are respectively alkyl or alkenyl with 0-5 carbon atoms, alkyl or alkenyl with 0-4 fluorine atoms, ester group with 2-4 carbon atoms or fluorine atom, R3 is alkyl with 1-5 carbon atoms or alkenyl with 1-5 carbon atoms, which are substituted by 0-7 fluorine atoms. The electrolyte of the lithium battery and the application thereof can improve the cycle performance of the lithium battery.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and more specifically to an electrolyte for lithium batteries and its application. Background Technology

[0002] In recent years, with the widespread use of lithium batteries in people's daily lives, such as cameras, laptops and electric vehicles, reducing size, lightening weight, increasing energy density and extending service life have become the development trends and requirements of the battery product industry.

[0003] In low-temperature environments, the wetting ability between the electrolyte and the electrodes and separator in lithium-ion batteries deteriorates, and the diffusion coefficient of lithium ions inside the active material decreases. This leads to lithium plating on the negative electrode of the battery cell and an increase in the thickness of the solid electrolyte interface (SEI). Consequently, the discharge capacity of lithium-ion batteries deteriorates in low-temperature environments, which greatly limits the popularization of lithium-ion batteries in cold regions. Summary of the Invention

[0004] This invention proposes an electrolyte for lithium batteries and its application, which can improve the low-temperature cycle performance of lithium batteries and extend their high-temperature cycle life.

[0005] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution.

[0006] This invention proposes an electrolyte for lithium batteries, comprising at least the following components:

[0007] Organic solvents;

[0008] Lithium salts; and

[0009] The additive comprises component A and vinylene carbonate, wherein component A has the following structural formula: R1 and R2 are each an alkyl or alkenyl group with 0-5 carbon atoms, an alkyl or alkenyl group with 0-4 fluorine atoms, an ester group or a fluorine atom with 2-4 carbon atoms, and R3 is an alkyl group with 1-5 carbon atoms substituted with 0-7 fluorine atoms or an alkenyl group with 1-5 carbon atoms substituted with 0-7 fluorine atoms.

[0010] In one embodiment of the present invention, component A comprises compound 1: Compound 2: And compound 3: One or more of them.

[0011] In one embodiment of the present invention, the mass content of the vinylene carbonate in the electrolyte is 1.5wt%-3.5wt%.

[0012] In one embodiment of the present invention, the mass content of component A in the electrolyte is 0.5wt%-15wt%.

[0013] In one embodiment of the present invention, the viscosity of the electrolyte is 2.5 mm. 2 / s~4mm 2 / s.

[0014] In one embodiment of the present invention, the lithium salt includes one or more of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2 or CF3SO3Li.

[0015] In one embodiment of the present invention, the concentration of the lithium salt is 0.1 mol / L to 2 mol / L.

[0016] In one embodiment of the present invention, the organic solvent includes one or more combinations of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, or propyl propionate.

[0017] In one embodiment of the present invention, the organic solvent in the electrolyte has a mass content of 60wt% to 85wt%.

[0018] The present invention also proposes a lithium battery comprising the electrolyte described above.

[0019] The present invention also proposes an electrochemical device comprising the aforementioned lithium battery.

[0020] In summary, this invention proposes an electrolyte for lithium batteries and its application, which can reduce the low-temperature viscosity of the electrolyte, thereby ensuring excellent cycle performance of the lithium battery at low temperatures. It can also effectively coat the negative electrode interface, further improving the stability of the SEI film and extending the high-temperature cycle life of the lithium battery. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the structure of a lithium-ion battery according to the present invention.

[0023] Labeling explanation: 10, positive electrode; 20, negative electrode; 30, separator; 40, electrolyte. Detailed Implementation

[0024] 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.

[0025] 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.

[0026] The technical solution of the present invention will be further described in detail below with reference to several embodiments and accompanying drawings. 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.

[0027] This invention proposes an electrolyte for lithium batteries, comprising an organic solvent, a lithium salt, and additives. The additives include vinylene carbonate (VC) and component A, the structural formula of which is: In this electrolyte, R1 and R2 are each composed of 0-5 carbon alkyl or alkenyl groups, 0-4 fluorine atoms in an alkyl or alkenyl group, 2-4 carbon ester groups, or fluorine atoms. R3 is a 1-5 carbon alkyl group with 0-7 fluorine atoms substituted, or a 1-5 carbon alkenyl group with 0-7 fluorine atoms substituted. Component A, containing a carboxylic acid ester structure, is introduced into the electrolyte, resulting in lower viscosity at low temperatures, thus ensuring excellent cycle performance of the lithium battery at low temperatures. Component A also contains a fluorosulfonyl functional group, and the fluorine atoms have a strong affinity, resulting in strong wetting ability of the electrolyte on the separator and electrodes. Furthermore, fluorine atoms are easily reduced at the negative electrode interface to generate inorganic salts such as LiF, providing a good coating effect on the negative electrode interface. Furthermore, when component A is used in combination with vinylene carbonate (VC), the organic compound SEI film formed by VC can make good contact with the negative electrode interface, forming a structure with inorganic SEI on the outside and organic SEI on the inside, which further improves the stability of the SEI film and can extend the cycle life of lithium battery.

[0028] In one embodiment of the present invention, component A is, for example, compound 1: Compound 2: Or compound 3: One or more of the following are used: [List of components]. In one embodiment of the invention, the mass content of vinylene carbonate in the electrolyte is, for example, 1.5 wt% to 3.5 wt%, and the mass content of component A in the electrolyte is, for example, 0.5 wt% to 15 wt%. Further, the mass content of component A in the electrolyte is, for example, 1 wt% to 5 wt%. When the mass content of components A and VC is too low, the improvement on the low-temperature fast charging capability and cycle performance of the lithium battery is not significant. Conversely, if the mass content of components A and VC is too high, a thick coating layer will form at the negative electrode interface, blocking the effective transport of lithium ions, thereby leading to a decrease in the low-temperature fast charging capability and cycle performance of the lithium battery. In one embodiment of the invention, the viscosity of the electrolyte is, for example, 1.5 mm. 2 / s~3.5mm 2 / s.

[0029] In one embodiment of the present invention, the organic solvent includes, for example, any one or a combination of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), propylene carbonate (PC), or diethyl carbonate (DEC). In one embodiment of the present invention, the mass content of the organic solvent in the electrolyte is, for example, 60 wt% to 85 wt%. In this embodiment, the organic solvent is, for example, a mixture of ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC), and the mass ratio of ethylene carbonate, dimethyl carbonate, and ethyl methyl carbonate is, for example, 3:5:2.

[0030] In one embodiment of the present invention, the lithium salt includes, for example, one or more of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2, or CF3SO3Li. The concentration of the lithium salt in the electrolyte is, for example, 0.1 mol / L to 2 mol / L.

[0031] Please see Figure 1 As shown, the present invention also proposes a lithium battery, which includes a positive electrode 10, a separator 30, a negative electrode 20 and the electrolyte 40 described above. The separator 30 is located between the positive electrode 10 and the negative electrode 20, and the electrolyte 40 described above is filled between the positive electrode 10, the separator 30 and the negative electrode 20.

[0032] Please see Figure 1As shown, in one embodiment of the present invention, the positive electrode sheet 10 includes, for example, a positive electrode current collector and a positive electrode slurry coated on the surface of the positive electrode current collector. The positive electrode current collector is, for example, selected from aluminum foil. The positive electrode slurry includes a positive electrode material, a binder, a conductive agent, and a thickener, etc. The binder is, for example, selected from any one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), polyhexanefluoropropylene, or polymerized styrene-butadiene rubber (SBR). The conductive agent is, for example, selected from any one or more of conductive carbon black (Super P, SP), acetylene black, carbon nanotubes, and graphene. The thickener is selected from one or more of N-methylpyrrolidone (NMP), sodium carboxymethylcellulose (CMC-Na), or sodium alginate. The cathode material is selected from one or more of lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide, nickel-manganese-cobalt ternary materials, or nickel-cobalt-aluminum ternary materials.

[0033] Please see Figure 1As shown, in one embodiment of the present invention, the negative electrode sheet 20 includes, for example, a negative electrode current collector and a negative electrode slurry coated on the surface of the negative electrode current collector. The negative electrode current collector is, for example, selected from copper foil. The negative electrode slurry includes a negative electrode material, a binder, a conductive agent, and a thickener. The negative electrode material is, for example, any one or more of natural graphite, artificial graphite, soft carbon, hard carbon, pure silicon, silicon oxide, silicon carbide, or lithium titanate. The binder is, for example, any one or more of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinyl ether, polymethyl methacrylate (PMMA), polyhexanefluoropropylene, or polymerized styrene-butadiene rubber (SBR). The conductive agent is selected from one or more of conductive carbon black (Super P, SP), acetylene black, carbon nanotubes, and graphene. The thickener is selected from one or more of N-methylpyrrolidone (NMP), sodium carboxymethylcellulose (CMC-Na), or sodium alginate.

[0034] Please see Figure 1 As shown, in one embodiment of the present invention, the separator 30 may be selected from one or more composite membranes of polyethylene (PE), polypropylene (PP), or polyvinylidene fluoride (PVDF). In one embodiment of the present invention, the positive electrode 10, the separator 30, and the negative electrode 20 are stacked sequentially, so that the separator 30 is located between the positive electrode 10 and the negative electrode 20 to serve as a separator.

[0035] 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.

[0036] Example 1

[0037] Preparation of electrolyte: Ethyl carbonate, dimethyl carbonate, and ethyl methyl carbonate in a mass ratio of 3:5:2 were mixed thoroughly. Compound 1, with a mass content of 8 wt%, was added to the mixed solvent. And 3 wt% vinylene carbonate (VC). Then, fully dried lithium hexafluorophosphate was added to the above organic solvent to make the concentration of lithium hexafluorophosphate 1 mol / L, and an electrolyte was prepared.

[0038] Preparation of the positive electrode: LiMn 0.6 Fe 0.4 PO4, SP, and PVDF are dissolved in NMP at a mass ratio of 95:3:2 to obtain a uniform positive electrode slurry. The positive electrode slurry is coated onto aluminum foil, dried, rolled, and cut to obtain a positive electrode sheet.

[0039] Preparation of negative electrode: Graphite, SP, SBR and CMC-Na are dissolved in deionized water in a mass ratio of 96:2:1:1 to obtain a uniform negative electrode slurry. The negative electrode slurry is coated on copper foil, dried, rolled and cut to obtain a negative electrode sheet.

[0040] Preparation of the secondary battery: A 9μm thick PE film is used as the base film, and a 3μm thick nano-alumina coating is coated on the base film to obtain the separator. The above-mentioned positive electrode, separator, and negative electrode are stacked in sequence to obtain the bare cell. The bare cell is placed in an aluminum-plastic film, and then baked at 80°C to remove water. After that, electrolyte is injected and the cell is sealed. Then, after processes such as standing, hot and cold pressing, formation, clamping, and capacity testing, the finished soft-pack lithium-ion secondary battery is obtained.

[0041] Example 2

[0042] The mass content of compound 1 was changed to 3 wt%, and other operations remained the same as in Example 1.

[0043] Example 3

[0044] The mass content of compound 1 was changed to 12 wt%, and other operations remained the same as in Example 1.

[0045] Example 4

[0046] The mass content of compound 1 was changed to 0.5 wt%, the mass content of VC was changed to 1.5 wt%, and other operations were kept the same as in Example 1.

[0047] Example 5

[0048] The mass content of compound 1 was changed to 2 wt%, the mass content of VC was changed to 2.5 wt%, and other operations were kept the same as in Example 1.

[0049] Example 6

[0050] The mass content of compound 1 was changed to 15 wt%, the mass content of VC was changed to 3.5 wt%, and other operations were kept the same as in Example 1.

[0051] Comparative Example 1

[0052] Compound 1 was not added, and all other operations remained the same as in Example 1.

[0053] Comparative Example 2

[0054] The mass content of compound 1 was changed to 5 wt%, and no vitamin C was added. Other operations were the same as in Example 1.

[0055] Comparative Example 3

[0056] The mass content of compound 1 was changed to 25 wt%, the mass content of VC was changed to 5 wt%, and other operations were kept the same as in Example 1.

[0057] The raw materials and proportions of the electrolytes in Examples 1-6 and Comparative Examples 1-3 of this invention were adjusted, and the raw material formulations in each example and comparative example were compiled into Table 1.

[0058] The electrolyte was then used to prepare lithium batteries using the above preparation method. The cycle performance and dielectric impedance (DCR) of the lithium batteries were then tested, and the test results are shown in Table 2.

[0059] The AC impedance DCR test at -20℃ was conducted at 25℃. The battery was charged at a constant current of 0.33C to 3.65V, then constant voltage at 3.65V to 0.05C. After resting for 30 minutes, it was discharged at a constant current of 0.33C to 2.5V. After confirming the cell capacity C0, it was charged again at a constant current of 0.33C to 3.65V, then constant voltage at 3.65V to 0.05C. After resting for 30 minutes, it was discharged at a constant current of 0.33C to 50% C0. After resting for 30 minutes, the ambient temperature was adjusted to -20℃, and the battery was allowed to equilibrate for 2 hours. The terminal voltage V1 was recorded. A constant current discharge of 1C was performed for 30 seconds, and the terminal voltage V2 and current I were recorded. At this point, DCR(-20℃) = (V1-V2) / I.

[0060] The capacity retention test at -20℃ involves testing the terminal voltage V2, then readjusting the temperature chamber to 25℃, equilibrating for 2 hours, charging at a constant current of 0.33C to 3.65V, then maintaining a constant voltage of 0.05C, letting it stand for 30 minutes, then adjusting the ambient temperature back to -20℃, discharging at 0.33C to 2.0V, and recording the cell capacity C1 at -20℃. The low-temperature capacity retention rate is then calculated as C1 / C0*100%.

[0061] The capacity retention test at 45℃ high-temperature cycling involves charging the lithium battery at a constant current of 0.5C to 3.65V at 45℃, then charging it at a constant voltage of 3.65V until the current is less than 0.05C. After resting for 30 minutes, it is discharged at a constant current of 1C to 2.8V. The discharge capacity of the lithium battery at this point is measured and is considered the discharge capacity of the first cycle. The battery is then subjected to multiple cycles under the above conditions, and the capacity retention rate after 1000 cycles is calculated. The capacity retention rate relative to the cycle is calculated using the following formula:

[0062] Capacity retention rate (%) = (Discharge capacity after 1000 cycles / Discharge capacity after the first cycle) × 100%.

[0063] Table 1. Composition ratios of each component in the electrolytes of Examples 1-6 and Comparative Examples 1-3

[0064]

[0065] Table 2. Performance of lithium batteries in Examples 1-6 and Comparative Examples 1-3

[0066]

[0067]

[0068] Please refer to Tables 1 and 2. Combined with Comparative Examples 1-2 and Examples 1-3, when component A or VC is added alone to the electrolyte, the lithium battery's DCR at -20℃ exceeds 945 mΩ, and the capacity retention at -20℃ is only about 16%, while the capacity retention after 500 cycles at 45℃ is only about 82%. However, when components A and VC are used simultaneously in the electrolyte, the high-temperature cycle performance of the lithium battery is significantly improved, and the low-temperature DCR is greatly reduced, effectively enhancing the low-temperature fast-charging capability and low-temperature cycle performance of the lithium battery. This is because component A contains a carboxylic acid ester structure, resulting in lower electrolyte viscosity at low temperatures, thus ensuring excellent cycle performance and fast-charging capability of the lithium battery at low temperatures. Furthermore, component A also contains fluorosulfonyl functional groups, and the fluorine atoms have a strong affinity, resulting in a strong wetting ability of the electrolyte on the separator and electrodes. On the other hand, fluorine atoms are easily reduced at the negative electrode interface to generate inorganic salts such as LiF, which provides a good coating effect on the negative electrode interface. Furthermore, the organic SEI film formed by VC can make good contact with the negative electrode interface, creating a structure with inorganic SEI on the outside and organic SEI on the inside. This improves the stability of the SEI film and ensures the high-temperature performance of the lithium battery.

[0069] Please refer to Tables 1 and 2. Combining Comparative Example 3 and Examples 1-3, as the amount of component A gradually increases, the low-temperature DCR and low-temperature capacity retention of the lithium battery first increase and then decrease. Furthermore, when the amount of component A exceeds 15 wt%, there is no significant improvement in the performance of the lithium battery. This is because excessively high component A content reduces the resistance to reduction, leading to a decrease in lithium battery performance. When the mass content of component A in the electrolyte is in the range of 1 wt% to 5 wt%, the lithium battery exhibits optimal low-temperature fast charging capability and low-temperature cycling performance. Further, combining Comparative Example 3 and Examples 4-6, as the amount of VC gradually increases, the low-temperature DCR and low-temperature capacity retention of the lithium battery first increase and then decrease. Furthermore, when the amount of VC exceeds 3.5 wt%, there is no significant improvement in the performance of the lithium battery. This is because excessively high VC content will generate a thick coating layer at the negative electrode interface, thereby blocking the effective transport of lithium ions and leading to a decrease in lithium battery performance. When the VC content in the electrolyte is in the range of 1.5wt% to 3.5wt%, the lithium battery has both the best low-temperature fast charging capability and low-temperature cycling performance.

[0070] In summary, this invention proposes an electrolyte for lithium batteries and its application. By introducing component A into the electrolyte, which contains a carboxylic acid ester structure, the viscosity of the electrolyte is low at low temperatures, thus ensuring excellent fast-charging capability and cycle performance of the lithium battery at low temperatures. Furthermore, component A also contains fluorosulfonyl functional groups, and the fluorine atoms have a strong affinity, resulting in strong wetting ability of the electrolyte for the separator and electrodes. On the other hand, fluorine atoms are easily reduced at the negative electrode interface to generate inorganic salts such as LiF, which provides a good coating effect on the negative electrode interface. Simultaneously, vinylene carbonate (VC) is introduced into the electrolyte. The organic compound SEI film formed by VC can make good contact with the negative electrode interface, forming a structure with inorganic SEI on the outside and organic SEI on the inside, further improving the stability of the SEI film and extending the high-temperature cycle life of the lithium battery. By using component A and VC in combination, the lithium battery simultaneously possesses good low-temperature fast-charging capability and low-temperature cycle performance, thus extending the high-temperature cycle life of the lithium battery.

[0071] 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.

[0072] 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. An electrolyte for a lithium battery, characterized in that, It includes at least the following components: Organic solvents; Lithium salts; as well as The additive comprises component A and vinylene carbonate, wherein component A has the following structural formula: In this electrolyte, R1 and R2 are each a hydrogen atom, an alkyl or alkenyl group with 1-5 carbon atoms, an alkyl or alkenyl group with 1-4 fluorine atoms, an ester group with 2-4 carbon atoms, or a fluorine atom; R3 is an alkyl group with 1-5 carbon atoms substituted with 0-7 fluorine atoms or an alkenyl group with 1-5 carbon atoms substituted with 0-7 fluorine atoms; the vinylene carbonate content in the electrolyte is 1.5wt%-3.5wt%; the component A content in the electrolyte is 0.5wt%-15wt%; and the viscosity of the electrolyte is 2.5 mm. 2 / s~4mm 2 / s.

2. The electrolyte for a lithium battery according to claim 1, characterized in that, Component A includes compound 1: Compound 2: And compound 3: One or more of them.

3. The electrolyte for a lithium battery according to claim 1, characterized in that, The lithium salt includes one or more of LiPF6, LiBF4, LiFSI, LiTFSI, LiBOB, LiODFP, LiODFB, LiPO2F2, or CF3SO3Li.

4. The electrolyte for a lithium battery according to claim 1, characterized in that, The concentration of the lithium salt is 0.1 mol / L to 2 mol / L.

5. The electrolyte for a lithium battery according to claim 1, characterized in that, The organic solvent includes one or more combinations of ethylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, propylene carbonate, ethyl acetate, or propyl propionate.

6. The electrolyte for a lithium battery according to claim 1, characterized in that, The organic solvent has a mass content of 60wt% to 85wt% in the electrolyte.

7. A lithium battery, characterized in that, Includes the electrolyte as described in any one of claims 1-6.

8. An electrochemical device, characterized in that, Including the lithium battery as described in claim 7.

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