Electrolyte additive, lithium ion battery electrolyte and lithium ion battery

Abstract

The application discloses an electrolyte additive, a lithium ion battery electrolyte and a lithium ion battery. The electrolyte additive comprises a compound I with the structure of formula (1) and a compound II with the structure of formula (2); wherein R1, R2, R3, R4, R5, R6, R7 and R8 are independently selected from one of H, a halogen atom, an alkyl group with 1-10 carbon atoms, an unsaturated hydrocarbon group with 2-10 carbon atoms, an alkoxy group with 1-10 carbon atoms, an alkanoyl group with 2-10 carbon atoms and a phenyl group; and H in the alkyl group, the unsaturated hydrocarbon group, the alkoxy group and the alkanoyl group can be partially or totally substituted by one or more of a halogen atom, a cyano group, a carboxyl group and a sulfonic acid group. The electrolyte additive of the application combines the compound I with the specific structure and the compound II, which can not only effectively inhibit the peeling of graphite, but also inhibit the dissolution of transition metals in the positive electrode material, and the two synergistically improve the storage performance and the cycle performance of the battery under the high-voltage system.

Description

Technical Field

[0001] This invention relates to the field of liquid electrolyte technology, and more specifically, to an electrolyte additive, a lithium-ion battery electrolyte, and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries are widely used in 3C electronic products, power tools, and energy storage power stations due to their high energy density, wide operating temperature range, and long cycle life. Furthermore, with the development of new energy vehicles and the increasing demand for portable mobile devices, the specific capacity requirements for lithium-ion batteries are also rising. Current technologies generally use high-voltage cathode materials to improve the specific capacity of lithium-ion batteries. However, even high-voltage cathode materials (such as lithium cobalt oxide) suffer structural instability when the operating voltage exceeds 4.5V. This causes transition metals in the cathode material to dissolve and be reduced and deposited on the anode surface, resulting in poor battery storage performance and decreased cycle stability. Moreover, conventional electrolytes (carbonate electrolytes) are prone to decomposition and gas generation under high voltage and / or high temperature conditions, which not only further reduces the battery's cycle stability but also causes battery expansion and may even lead to safety issues.

[0003] To address the aforementioned issues, functional additives are typically added to the electrolyte. For example, existing technologies disclose an electrolyte additive containing a phosphoric anhydride structure, which can form a protective film on the surface of high-voltage ternary cathode materials, thereby improving the battery's high-temperature storage performance. Another example is an additive containing double bonds and cyano groups, which can also form a protective substance on the cathode surface to improve the battery's high-temperature storage performance. Alternatively, high-voltage film-forming additives can be added to improve the battery's cycle performance. However, improvements in battery storage performance are often accompanied by increased impedance, leading to a decrease in cycle performance. In other words, existing conventional electrolyte functional additives cannot simultaneously improve both storage and cycle performance. Summary of the Invention

[0004] The purpose of this invention is to address the shortcomings and deficiencies of existing electrolyte functional additives that cannot simultaneously improve battery storage performance and cycle performance, and to provide an electrolyte additive.

[0005] Another object of the present invention is to provide a lithium-ion battery electrolyte.

[0006] Another object of the present invention is to provide a lithium-ion battery.

[0007] The above-mentioned objective of this invention is achieved through the following technical solution:

[0008] This invention protects an electrolyte additive, comprising additive a, wherein additive a comprises compound I with the structure of formula (1) and compound II with the structure of formula (2);

[0009]

[0010] R1, R2, R3, R4, R5, R6, R7 and R8 are each independently selected from one of H, a halogen atom, an alkane group with 1 to 10 carbon atoms, an unsaturated hydrocarbon group with 2 to 10 carbon atoms, an alkoxy group with 1 to 10 carbon atoms, an alkanoyl group with 2 to 10 carbon atoms, and a phenyl group.

[0011] Furthermore, the H atoms in the alkane group, the unsaturated hydrocarbon group, the alkoxy group, and the alkanoyl group may be partially or completely substituted by one or more of the halogen atom, cyano group, carboxyl group, and sulfonic acid group.

[0012] Compound I in the electrolyte additive a of this invention uses a highly active carbonyl group as the main chain backbone (C=O), while introducing active functional groups (such as F atoms, methoxy groups, and nitrogen-methyl groups), giving it not only good lithium-ion affinity but also generating a lower impedance and forming a stable negative electrode interface film on the surface of the negative electrode material to effectively suppress graphite exfoliation. Compound II in additive a introduces active functional groups nitrile groups (-CN), unsaturated double bonds (-C=C), and ethoxy groups into its lipid backbone. The nitrile groups and unsaturated double bonds can induce the formation of a robust and stable CEI film on the surface of the positive electrode material to suppress the dissolution of transition metals in the positive electrode material; while the ethoxy group can provide lithium-ion coordination, reduce the viscosity of the additive, and thus improve the rate and cycle performance of the battery. The combination of Compound I and Compound II can improve the storage and cycle performance of the battery under high voltage (4.53V) systems through the interaction of different specific active functional groups, especially significantly improving the battery's high-temperature storage performance, high-temperature cycle performance, and rate performance.

[0013] Preferably, in compound I, R1, R2, and R3 are halogen atoms, and R4 and R5 are alkane groups with 1 to 10 carbon atoms. More preferably, in compound I, R1, R2, and R3 are all fluorine atoms, and R4 and R5 are all methyl groups. When R1, R2, and R3 are all fluorine atoms, a LiF-rich negative electrode interface film can be formed on the negative electrode surface, which more effectively suppresses graphite negative electrode peeling and improves the battery's rate performance, high-temperature storage, and cycle performance.

[0014] Preferably, R6, R7, and R8 in compound II are alkane groups having 1 to 10 carbon atoms. More preferably, R6 in compound II is an ethyl group, and R7 and R8 are both methyl groups.

[0015] Specifically, compound I is

[0016] At least one of them; compound II is At least one of them.

[0017] Specifically, the mass ratio of compound I to compound II is (0.5-3):(0.5-2); preferably, the mass ratio of compound I to compound II is (1.5-2):1; more preferably, the mass ratio of compound I to compound II is 2:1.

[0018] Generally, the electrolyte additive further includes additive b, which is one or more selected from ethylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, 1,3-propanesulfonate lactone, propylene sulfite, ethylene sulfate, 4-methylethylene sulfate, succinic anhydride, succinic anhydride, adiponitrile, and 1,3,6-hexanetrionitrile. When additive b works synergistically with additive a, a more uniform and dense SEI film with lower impedance can be formed, which can further improve the battery's storage performance, cycle performance, and rate performance.

[0019] Specifically, the mass ratio of additive a to additive b is (1.5 to 4):15. Optionally, the mass ratio of additive a to additive b is 1.5:15, 2:15, 2.5:15, 3:15, 3.5:15, or 4:15, but it is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0020] The present invention also protects a lithium-ion battery electrolyte comprising a lithium salt, an organic solvent, and the above-mentioned electrolyte additives.

[0021] Specifically, the lithium salt can be at least one of lithium hexafluorophosphate (LiPF6), lithium perchlorate, lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiN(SO2CF3)2), lithium bis(oxalateborate)borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorooxalateborate (LiODFB), lithium difluorodioxalate phosphate (LiPF2(C2O4)2), and lithium bis(fluorosulfonyl)imide.

[0022] The organic solvent is preferably at least one of carbonate and carboxylic acid ester, wherein the carbonate is a cyclic carbonate or a chain carbonate, and optionally, the carbonate is one or more of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, and butyl carbonate; the carboxylic acid ester is one or more of butyl acetate, ethyl propionate, and propyl propionate.

[0023] Specifically, the electrolyte additive is present in a mass percentage of 16.5% to 19% relative to the lithium-ion battery electrolyte. Optionally, the mass percentage is 16.5%, 17%, 17.5%, 18%, 18.5%, or 19%, but is not limited to the listed values; other unlisted values ​​within this range are also applicable.

[0024] A lithium-ion battery, including a positive electrode, a negative electrode, a separator located between the positive and negative electrodes, and the aforementioned lithium-ion battery electrolyte, is also within the scope of protection of this invention.

[0025] Specifically, the positive electrode material in the positive electrode sheet is lithium cobalt oxide or other high-voltage positive electrode material; the negative electrode material in the negative electrode sheet is graphite, such as artificial graphite or natural graphite.

[0026] Compared with the prior art, the beneficial effects of the present invention are:

[0027] The electrolyte additive of the present invention combines compound I and compound II with specific structures. Through the interaction of different specific active functional groups, it can not only form a stable negative electrode interface film on the surface of the negative electrode material to effectively inhibit graphite exfoliation, but also induce the formation of a strong and stable CEI film on the surface of the positive electrode material to inhibit the dissolution of transition metals in the positive electrode material. The synergistic effect of the two improves the storage performance and cycle performance of the battery under high voltage system. Detailed Implementation

[0028] The present invention will be further described below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise stated, the raw materials and reagents used in the embodiments of the present invention are conventionally purchased raw materials and reagents.

[0029] Conventional positive and negative electrode sheets in this field can be used in this invention. To facilitate the explanation of the technical effects of this invention, the following positive electrode sheets (positive electrode material is LiCoO2) and negative electrode sheets (negative electrode material is commercial graphite) were specifically selected for the experiment.

[0030] Example 1

[0031] A lithium-ion battery electrolyte is prepared by the following method:

[0032] S1. Prepare an organic solvent in a nitrogen-filled glove box (O2 < 2 ppm, H2O < 3 ppm). The organic solvent is composed of EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate) and PP (propyl propionate) in a weight ratio of 15:10:10:65.

[0033] S2. Slowly add LiPF6 to the organic solvent in S1 to prepare a lithium salt solution with a concentration of 1.2 mol / L;

[0034] S3. Mix the lithium salt solution in S2, the organic solvent in S1, compound I in additive a, compound II in additive a, and additive b in a weight ratio of 15:67:2:1:15 until homogeneous to obtain the lithium-ion battery electrolyte.

[0035] The structural formula of compound I is as follows: The structural formula of compound 2 is

[0036] Example 2

[0037] A lithium-ion battery electrolyte is prepared in a manner that is basically the same as in Example 1, except that the mass ratio of lithium salt solution, organic solvent, compound I in additive a, compound II in additive a, and additive b in step S3 is 15:68.5:0.5:1:15.

[0038] Example 3

[0039] A lithium-ion battery electrolyte is prepared in a manner that is basically the same as that in Example 1, except that the mass ratio of lithium salt solution, organic solvent, compound I in additive a, compound II in additive a, and additive b in step S3 is 15:68:1:1:15.

[0040] Example 4

[0041] A lithium-ion battery electrolyte is prepared in a manner that is basically the same as that in Example 1, except that the mass ratio of lithium salt solution, organic solvent, compound I in additive a, compound II in additive a, and additive b in step S3 is 15:66:3:1:15.

[0042] Example 5

[0043] A lithium-ion battery electrolyte is prepared in a manner that is basically the same as in Example 1, except that the mass ratio of lithium salt solution, organic solvent, compound I in additive a, compound II in additive a, and additive b in step S3 is 15:67.5:2:0.5:15.

[0044] Example 6

[0045] A lithium-ion battery electrolyte is prepared in a manner that is basically the same as that in Example 1, except that the mass ratio of lithium salt solution, organic solvent, compound I in additive a, compound II in additive a, and additive b in step S3 is 15:66:2:2:15.

[0046] Example 7

[0047] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound I in additive a in step S3 is...

[0048] Example 8

[0049] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound I in additive a in step S3 is...

[0050] Example 9

[0051] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound I in additive a in step S3 is...

[0052] Example 10

[0053] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound I in additive a in step S3 is...

[0054] Example 11

[0055] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound II in additive a in step S3 is...

[0056] Example 12

[0057] A lithium-ion battery electrolyte, the preparation method of which is basically the same as that in Example 1, the difference being: the structural formula of compound II in additive a in step S3 is...

[0058] Comparative Example 1

[0059] A lithium-ion battery electrolyte is prepared by the following method:

[0060] S1. Prepare an organic solvent in a nitrogen-filled glove box (O2 < 2 ppm, H2O < 3 ppm). The organic solvent is composed of EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate) and PP (propyl propionate) in a weight ratio of 15:10:10:65.

[0061] S2. Slowly add LiPF6 to the organic solvent in S1 to prepare a lithium salt solution with a concentration of 1.2 mol / L;

[0062] S3. Mix the lithium salt solution in S2, the organic solvent in S1, compound II in additive a, and additive b in a weight ratio of 15:69:1:15 until homogeneous to obtain the lithium-ion battery electrolyte.

[0063] Comparative Example 2

[0064] A lithium-ion battery electrolyte is prepared by the following method:

[0065] S1. Prepare an organic solvent in a nitrogen-filled glove box (O2 < 2 ppm, H2O < 3 ppm). The organic solvent is composed of EC (ethylene carbonate), PC (propylene carbonate), DEC (diethyl carbonate) and PP (propyl propionate) in a weight ratio of 15:10:10:65.

[0066] S2. Slowly add LiPF6 to the organic solvent in S1 to prepare a lithium salt solution with a concentration of 1.2 mol / L;

[0067] S3. Mix the lithium salt solution in S2, the organic solvent in S1, compound I in additive a, and additive b in a weight ratio of 15:68:2:15 until homogeneous to obtain the lithium-ion battery electrolyte.

[0068] Performance testing

[0069] The lithium-ion battery electrolytes in Examples 1-12 and Comparative Examples 1-2 were assembled with positive electrode sheets, separators, and negative electrode sheets to form lithium-ion batteries. The specific preparation method is as follows: the positive electrode sheets, separators, and negative electrode sheets were stacked and wound to obtain bare cells. After the bare cells were encapsulated with aluminum-plastic film and baked, the lithium-ion battery electrolytes were injected. Then, the cells were subjected to standing, formation, fixture shaping, secondary sealing, and capacity testing in sequence to obtain lithium-ion batteries.

[0070] The positive electrode sheet is prepared by the following method: the positive electrode material (4.53V high voltage LiCoO2), binder (polyvinylidene fluoride, PVDF), and conductive agent (CNT) are dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 98:1:1, and stirred under vacuum until stable and uniform. Then, it is uniformly coated on an aluminum foil with a thickness of 10μm, dried at room temperature, and then transferred to a forced-air oven at 120℃ for 1 hour. After cold pressing and die cutting, the positive electrode sheet is obtained.

[0071] The above-mentioned negative electrode sheet is prepared by the following method: the negative electrode material (commercial graphite), binder (polyvinylidene fluoride, PVDF), and conductive agent (CNT) are dispersed in N-methylpyrrolidone (NMP) at a mass ratio of 97:2:1, and stirred under vacuum until stable and uniform. Then, it is uniformly coated on a copper foil with a thickness of 8μm, dried at room temperature, and then transferred to a forced-air oven at 120℃ to dry for 1 hour. After cold pressing and die cutting, the negative electrode sheet is obtained.

[0072] The lithium-ion batteries prepared above were subjected to room temperature / high temperature cycling tests, rate performance tests, and 60℃ high temperature storage performance tests. The test results are shown in Table 1. The specific test methods are as follows:

[0073] (1) Room temperature (25℃) / high temperature (45℃) cycle test: The prepared lithium-ion battery was placed in a constant temperature chamber and left to stand for 4 hours. Then it was charged to 4.53V with a constant current and constant voltage of 0.5C / 0.05C. Then it was discharged to 3.0V with a constant current of 1C. This cycle was repeated, and the initial capacity of the lithium-ion battery and the discharge capacity of the last cycle (300 / 200th cycle) were recorded.

[0074] Capacity retention (%) = Discharge capacity in the last cycle (cycle 300 / 200) / Initial capacity × 100%.

[0075] (2) Rate performance test: The lithium-ion battery was charged to 4.53V at a constant current of 0.5C in an environment of 25℃, and then charged to 0.02C at a constant voltage. After that, it was discharged to 3.0V at a constant current of 0.2C / 1C / 2C / 3C / 5C respectively. The discharge capacity of the cell was recorded.

[0076] Capacity retention rate (%) at different discharge rates = discharge capacity at different discharge rates / initial capacity at 0.2C × 100%.

[0077] (3) 60℃ high temperature storage test: The lithium-ion battery was charged at 0.5C constant current to 4.53V in an environment of 25℃, charged at constant voltage until the current dropped to 0.02C, and then discharged at 0.2C constant current to 3.0V. The cell thickness, internal resistance and initial capacity were recorded at this time. Then, the battery was charged at 0.5C constant current to 4.53V and then placed in an oven at 60℃ for 7 days. The cell thickness and internal resistance were measured again at this time. The capacity discharged at 0.2C current to 3.0V was recorded as the residual capacity. The stored cell was charged at 0.5C constant current to 4.53V, charged at constant voltage until the current dropped to 0.02C, and then discharged at 0.2C constant current to 3.0V. This was recorded as the recovered capacity.

[0078] Capacity retention rate = Remaining capacity / Initial capacity × 100%;

[0079] Capacity recovery rate = (Recovered capacity / Initial capacity) × 100%;

[0080] Thickness expansion rate = (Cell thickness after storage - Cell thickness before storage) / Cell thickness before storage × 100%.

[0081] Table 1. Performance of lithium-ion battery electrolytes in each embodiment and comparative example.

[0082]

[0083]

[0084] As shown in Table 1, the lithium-ion batteries with the electrolyte additive of this invention exhibit good capacity retention under both room temperature (25°C) and high temperature (45°C) conditions, as well as under different rate conditions, indicating excellent cycle performance. Furthermore, after 7 days of storage at 60°C, the capacity retention rate reaches over 75%, the capacity recovery rate exceeds 80%, and the thickness expansion rate is below 11%, demonstrating excellent cycle performance. Meanwhile, Examples 1, 1, and 2 show that only when Compound I and Compound II are used together as electrolyte additives can the high-temperature storage performance and cycle performance of the battery be effectively improved; using Compound I or Compound II alone is insufficient to improve the high-temperature storage performance and cycle performance of the battery.

[0085] The above embodiments of the present invention are merely examples for clearly illustrating the present invention and are not intended to limit the implementation of the present invention. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively describe all possible implementations here. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. An electrolyte additive, characterized in that, Including additive a, said additive a comprising compound I with the structure of formula (1) and compound II with the structure of formula (2); Equation (1); Equation (2); In compound II, R6 is an ethyl group, and R7 and R8 are each independently selected from one of H, a halogen atom, an alkane group with 1 to 10 carbon atoms, an unsaturated hydrocarbon group with 2 to 10 carbon atoms, an alkoxy group with 1 to 10 carbon atoms, an alkanoyl group with 2 to 10 carbon atoms, and a phenyl group. Furthermore, the H atoms in the alkane group, the unsaturated hydrocarbon group, the alkoxy group, and the alkanoyl group may be partially or completely substituted by one or more of the halogen atom, cyano group, carboxyl group, and sulfonic acid group.

2. The electrolyte additive according to claim 1, characterized in that, In compound II, R7 and R8 are alkane groups with 1 to 10 carbon atoms, respectively.

3. The electrolyte additive according to claim 1, characterized in that, In compound II, both R7 and R8 are methyl groups.

4. The electrolyte additive according to claim 1, characterized in that, The mass ratio of compound I to compound II is (0.5~3):(0.5~2).

5. The electrolyte additive according to any one of claims 1 to 4, characterized in that, The electrolyte additive further includes additive b, which is one or more of the following: ethylene carbonate, fluoroethylene carbonate, ethylene ethylene carbonate, 1,3-propanesulfonate lactone, propylene sulfite, ethylene sulfate, 4-methylethylene sulfate, succinic anhydride, succinic anhydride, adiponitrile, and 1,3,6-hexanetrionitrile.

6. The electrolyte additive according to claim 5, characterized in that, The mass ratio of additive a to additive b is (1.5~4):

15.

7. A lithium-ion battery electrolyte, comprising a lithium salt, an organic solvent, and additives, characterized in that, The additive is the electrolyte additive according to any one of claims 1 to 6.

8. The lithium-ion battery electrolyte according to claim 7, characterized in that, The additive has a mass percentage of 16.5% to 19% relative to the lithium-ion battery electrolyte.

9. A lithium-ion battery, comprising a positive electrode, a negative electrode, a separator located between the positive and negative electrode, and an electrolyte, characterized in that, The electrolyte is the lithium-ion battery electrolyte as described in claim 7 or 8.

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