Imidazole-containing electrolyte additive, electrolyte and lithium-ion battery

Abstract

The present invention relates to the technical field of lithium-ion batteries, and specifically to an electrolyte additive containing an imidazole structure, an electrolyte thereof, and a lithium-ion battery. The additive structure is as shown in Formula I or Formula II: X and Y are independently selected from oxygen atoms or are absent; R1 and R2 are the same or different; R1 and R2 are independently selected from one of methyl, ethyl, cyanoethyl, trifluoroethyl, difluoroethyl, allyl, propargyl, trimethylsilyl, and dimethylvinylsilyl; R1 and R2 can also be connected to form a five-membered ring or a six-membered ring; R3 to R8 are the same or different; R3 to R8 are independently selected from one of hydrogen, C1-C4 alkyl, C1-C4 fluorinated alkyl, cyano, phenyl, fluorophenyl, alkoxy-containing phenyl, and alkylphenyl. The electrolyte additive can improve electrolyte stability and inhibit changes in acid value and color.

Description

Technical Field

[0001] The invention relates to an electrolyte additive containing an imidazole structure, an electrolyte thereof and a lithium ion battery, belonging to the technical field of lithium ion batteries. Background Art

[0002] To meet the needs of large mobile electrical equipment, the development of high-capacity batteries is urgent. Since cathode materials have always been a shortcoming in the development of lithium-ion batteries, currently increasing the specific capacity of lithium-ion batteries means increasing the specific capacity of the cathode. There are generally two ways to increase the specific capacity of cathode materials. The first is to develop new high-capacity cathode materials for lithium batteries; the second is to increase the voltage of lithium-ion batteries. However, all high-voltage cathode materials face a common problem - the decomposition of the electrolyte under high voltage. How to solve the oxidative decomposition reaction of the electrolyte on the surface of high-voltage cathode materials is one of the core issues facing current high-voltage electrolyte research.

[0003] Currently, in order to utilize nickel-based positive electrode active materials suitable for high capacity while simultaneously addressing the issue of high-temperature battery stability, a solution has been proposed to improve battery lifespan and high-temperature stability by adding well-known electrolyte additives such as vinylene carbonate and vinyl ethylene carbonate to form the SEI film. However, conventional electrolytes prepared using these additives exhibit good battery characteristics at voltages below 4.2V, but increase the operating voltage above 4.2V and the battery's internal resistance and lifespan decrease dramatically.

[0004] Solving the electrolyte stability issue at high voltage is crucial for the promotion and application of high-voltage cathode materials. Therefore, developing a lithium-ion battery electrolyte that can be used in high-voltage batteries and reliably guarantees cycle performance, storage performance, and safety is a top priority.

[0005] Patent application CN113054258A discloses a ternary high-nickel electrolyte. By adding the additive silicon-based phosphate to the electrolyte, the high-temperature cycling performance and high-temperature storage life of lithium-ion batteries can be improved. This type of silicon-based phosphate significantly reduces the interfacial impedance of the positive electrode, which is beneficial to the migration of lithium ions at the positive electrode interface, and effectively reduces the oxidative activity of the high-nickel positive electrode material on the electrolyte. In particular, its oxidation of the electrolyte under high temperature conditions can inhibit the reduction reaction and dissolution of transition metals such as nickel and cobalt caused by changes in the structure of the high-nickel positive electrode material, thereby improving the high-temperature cycling performance and high-temperature storage life of lithium-ion batteries.

[0006] Patent application CN104300174A discloses a high-voltage non-aqueous electrolyte for lithium-ion batteries. By adding phosphate compounds and sulfone compounds containing unsaturated bonds as film-forming additives, a uniform and dense protective film can be formed on the electrode surface, reducing the oxidation reaction of the electrolyte on the surface of the positive electrode material, so that the battery has excellent high-temperature performance and cycle performance.

[0007] Although the above patent reports can improve the high-temperature cycling and high-temperature storage performance of batteries under high voltage, in order to promote the development of high-voltage electrolytes, more effective technical solutions need to be proposed; in addition, the above patents do not effectively solve the problem that can easily lead to fission of battery performance, that is, they do not study the increase in acidity and chromaticity of the electrolyte. Summary of the Invention

[0008] The present invention aims to address the deficiencies in the prior art and provides an electrolyte additive containing an imidazole structure, an electrolyte thereof, and a lithium ion battery. The electrolyte additive can improve electrolyte stability and inhibit changes in acid value and chromaticity.

[0009] The technical solution of the present invention to solve the above technical problems is as follows: an electrolyte additive containing an imidazole structure, wherein the structure of the additive is as shown in Formula I or Formula II:

[0010]

[0011] Wherein, X and Y are independently selected from oxygen atoms or do not exist. When X does not exist, the phosphorus element connected to X is an unsaturated phosphine. When Y does not exist, the phosphorus element connected to Y is an unsaturated phosphine.

[0012] R1 and R2 are the same or different, and R1 and R2 are independently selected from one of methyl, ethyl, cyanoethyl, trifluoroethyl, difluoroethyl, allyl, propargyl, trimethylsilyl, and dimethylvinylsilyl. R1 and R2 can also be connected to form a five-membered ring or a six-membered ring;

[0013] R3 to R8 are the same or different, and are independently selected from one of hydrogen, C1 to C4 alkyl, C1 to C4 fluorinated alkyl, cyano, phenyl, fluorophenyl, alkoxy-containing phenyl, and alkyl-containing phenyl.

[0014] Furthermore, the additive is selected from any one or a combination of the following structural formulas:

[0015]

[0016]

[0017]

[0018]

[0019] The invention also discloses an electrolyte, which comprises a solvent, an electrolyte lithium salt and the electrolyte additive containing the imidazole structure of the invention.

[0020] Furthermore, based on the total mass of the electrolyte, the mass content of the electrolyte additive containing the imidazole structure is 0.005-2 wt %.

[0021] Furthermore, the electrolyte also includes auxiliary additives; the auxiliary additives are at least one of 1,3-propane sultone, 1,4-butane sultone, propenyl-1,3-sultone, vinyl sulfate, methylene disulfonic acid methylene ester, vinyl carbonate, fluoroethylene carbonate, lithium bis(fluorosulfonyl)imide (LiFSI), lithium difluorooxalatoborate (LiODFB), lithium difluorooxalatophosphate, lithium difluorophosphate, and lithium tetrafluoroborate (LiBF4).

[0022] Furthermore, based on the total mass of the electrolyte, the mass content of the auxiliary additive is 0.1 to 3.0 wt%.

[0023] Furthermore, the electrolyte lithium salt is selected from one or a combination of LiPF6, LiBF4, LiODFB, LiTDI, LiTFSI and LiFSI.

[0024] Furthermore, based on the total mass of the electrolyte, the content of the electrolyte lithium salt is 10-20 wt%.

[0025] Furthermore, the solvent is selected from one or more of cyclopentyl methyl ether, cyclopentyl trifluoroethyl ether, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylpropynyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, difluoroethanol acetate, ethyl propionate, propyl propionate or ethyl butyrate.

[0026] The present invention also discloses a lithium ion battery, which comprises a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and the electrolyte of the present invention.

[0027] The beneficial effects of the present invention are:

[0028] (1) The electrolyte additive containing an imidazole structure of the present invention contains a nitrogen atom with a lone electron pair, and exhibits weak alkalinity in the electrolyte. It can remove HF produced by the decomposition of LiPF6, thereby effectively inhibiting the increase in the acidity of the electrolyte, and can improve the thermal stability and storage stability of the LiPF6 solution, thereby improving the stability of the electrolyte and inhibiting changes in acid value and color.

[0029] (2) The electrolyte additive containing an imidazole structure of the present invention organically combines imidazole, phosphonate or phosphinate structural groups to form a new structure with a suitable redox potential. At the same time, the nitrogen-containing imidazole structure in the formed new compound structure is coordinated with the transition metal of the positive electrode to stabilize the positive electrode structure. The contained phosphate structure is electron-rich and is oxidized before the electrolyte solvent during the battery preparation and formation process, consuming the oxygen generated at the positive electrode interface to form CEI on the positive electrode surface, avoiding side reactions between the positive electrode interface and the electrolyte, improving the positive electrode stability, and preventing the decomposition of the electrolyte, thereby suppressing the generation of gas, improving the expansion phenomenon of the battery, and improving the high-temperature storage stability of the lithium secondary battery. DETAILED DESCRIPTION

[0030] The present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar improvements without violating the scope of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed.

[0031] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. The terms used are only for describing specific embodiments and are not intended to limit the present invention.

[0032] 1. Preparation of additive compounds

[0033] Synthesis example 1

[0034] Preparation of compound PN-01

[0035]

[0036] Dissolve 22.4g (0.10mol) of KTDI in 100g of anhydrous acetonitrile, heat to 75-80°C, and slowly add dropwise a mixture of 12.8g (0.101mol) of 2-chloro-1,3,2-dioxaphosphinate and 50g of acetonitrile. Stir the reaction for 12 hours. Then, switch to a vacuum distillation apparatus and remove the solvent until no fraction remains, yielding a brownish-yellow viscous product. Under nitrogen, add 100g of dichloromethane and stir at room temperature for slurrying. Filter under nitrogen pressure. The filter cake consists primarily of insoluble salts. The filtrate is purified by chromatography (diluted with n-heptane) to obtain 16.0g of PN-01, a yield of 57.97%.

[0037] GC-MS: 276, 1 H NMR (400 MHz): solvent: deuterated chloroform, δ (ppm): 3.69 (d, -OCH2); 13C NMR (100 MHz): solvent deuterated chloroform, δ (ppm): 144.8 (C-CF3), 115.9 ppm (-CN), 118.3 (-CF3), 109.3 (C=C), 65.6 (-OCH2),;

[0038] Synthesis example 2

[0039] Preparation of compound PN-22

[0040]

[0041] 7.0 g (0.103 mol) of imidazole was dissolved in 70 g of anhydrous cyclopentyl methyl ether. 4.4 g (0.11 mol, 60% content) of sodium hydride was added in batches while stirring. The temperature was raised to 100-105° C. and stirred for 1.0 hrs. A mixture of 16.7 g (0.107 mol) of 2-chloro-1,3,2-dioxaphosphate and 50 g of cyclopentyl methyl ether was slowly added. The mixture was stirred for 16.0 hrs, cooled to room temperature, washed with water, and the organic phase was desolvated under reduced pressure until no fraction was left. The product was then purified by chromatography using dichloromethane + n-heptane to obtain 11.5 g of PN-22 with a yield of 61.13%.

[0042] GC-MS: 188, 1 H NMR (400 MHz): solvent: deuterated chloroform, δ (ppm): 7.902 ppm (s, 1H), 7.271-7.289 ppm (d, 1H), 7.065-7.083 ppm (d, 1H), 4.481-4.610 ppm (m, 4H), 2.390-2.4891 (m, 1H), 1.832-1.874 (m, 1H); 13 C NMR (100 MHz): solvent: deuterated chloroform, δ (ppm): 135.3 ppm (NCHN), 128.4, 122.4 (NCH=CHN), 70.4 (-OCH2), 25.7 (C-CH2-C);

[0043] Synthesis example 3

[0044] Preparation of compound PN-28

[0045]

[0046] 6.4 g (0.02 mol) of perfluorophenylimidazole potassium salt was dissolved in 60 g of anhydrous acetonitrile, heated to 75-80°C, and a mixture of 3.2 g (0.025 mol) of dimethoxyphosphine chloride and 30 g of acetonitrile was slowly added dropwise. After completion of the addition, the reaction was stirred for 16.0 hrs. The reaction was further modified to a vacuum distillation apparatus, and the solvent was removed until no fraction remained. Under nitrogen protection, 200 g of dichloromethane was added and stirred at room temperature. The mixture was filtered under nitrogen pressure. The filter cake mainly contained undissolved salts. The filtrate was purified by chromatography (the filtrate was diluted with n-heptane) to obtain 4.8 g of PN-28, with a yield of 63.83%.

[0047] GC-MS: 376, 1 H NMR (400 MHz): Solvent: deuterated chloroform, δ (ppm): 4.053 ppm (s, 1H), 13 C NMR (100 MHz): solvent: deuterated chloroform, δ (ppm): 145.3, 143.9, 139.2, 137.6, 119.7, 112.3, 108.5, 58.8.

[0048] 2. Thermal Stability Example of Lithium Hexafluorophosphate

[0049] Example 1

[0050] In a nitrogen atmosphere glove box with a water content of less than 1 ppm, organic solvents of ethylene carbonate (EC), dimethyl carbonate (DMC), and methylpropyl carbonate (PMC) were mixed in proportion, LiPF6 was dissolved in the above organic solvents, and then an electrolyte additive was added to the organic solvent, completely dissolved, and mixed evenly to obtain an electrolyte.

[0051] Among them, EC has a low freezing point and needs to be preheated and melted at 50°C on an electric magnetic stirrer. The mass content of LiPF6 in the electrolyte is 13.5wt%. The weight ratio of ethylene carbonate: dimethyl carbonate: methylpropyl carbonate is EC: DMC: PMC = 20:50:30. The additive in this embodiment is PN01, and the amount of PN01 used is 0.5wt% of the total weight of the electrolyte.

[0052] The electrolyte containing the PN01 additive provided by the present invention was sampled in a glove box to test for acidity and color. Acidity was measured using a potentiometric titrator, with the acidity converted to HF in parts per million (ppm). Color was measured using a platinum-cobalt colorimeter with the colorimetric unit measured in Hazen. Furthermore, using 19F-NMR (model: JNM-ECZ400S, 400 MHz) using deuterated acetone (internal standard: trifluoromethylbenzene), the integrated ratio of the peak of the LiPF6 decomposition product relative to the LiPF6 was less than 0.09%.

[0053] The electrolyte was then heated at 80°C for 240 hours. After heating, the electrolyte acidity and color were measured. Furthermore, the peak of LiPF6 decomposition products measured by 19F-NMR showed an integrated ratio of 0.11% relative to the total amount of LiPF6 decomposition products. The results are shown in Table 1.

[0054] The electrolytes obtained in Examples 2-12 and Comparative Examples 1-3 were all tested for performance using the same method as in Example 1. The specific test data are shown in Table 1 below.

[0055] Table 1 Performance test data of electrolytes of Examples 1-12 and Comparative Examples 1-3

[0056]

[0057]

[0058] It can be seen from the data in Table 1 that the electrolyte additive containing an imidazole structure of the present invention can effectively inhibit the decomposition of lithium hexafluorophosphate. The imidazole structure contained in the compound structure of the present invention contains a nitrogen atom with a lone electron pair, and is weakly alkaline in the electrolyte. It can effectively neutralize the acidic substances in the electrolyte, thereby effectively inhibiting the decomposition of lithium hexafluorophosphate, and effectively improving the thermal stability and storage stability of the electrolyte containing LiPF6, thereby helping to improve the cycle stability and high-temperature performance of lithium secondary batteries.

[0059] 3. Preparation of high nickel ternary lithium-ion batteries

[0060] The coating density is determined based on the battery's designed capacity (2000mAh) and the capacity of the positive and negative electrode materials. The positive electrode material is sourced from Dangsheng Technology; the negative electrode material is artificial graphite from Shenzhen BTR; and the separator is a 20μm thick PE-coated ceramic separator from Xingyuan Materials.

[0061] Application Example 1

[0062] (1) Preparation of positive electrode sheet

[0063] LiNi 0.6 Co 0.2 Mn 0.2 O2 is the positive electrode material: the positive electrode LiNi 0.6 Co 0.2 Mn 0.2 O2 powder, carbon black (particle size of 1000nm), polyvinylidene fluoride (PVDF) and N, N-dimethylpyrrolidone (NMP) are mixed to form a uniform positive electrode slurry. The positive electrode slurry is evenly coated on both sides of the aluminum foil, dried, rolled, and baked at 120℃ for 12 hours. After drying, the electrode sheet has LiNi 0.6 Co 0.2 Mn0.2 O2 accounts for 94% of the total weight of the coating, the binder (PVDF) accounts for 4% of the total weight of the coating, and the carbon black accounts for 2% of the total weight of the coating. After welding the aluminum positive electrode ear with an ultrasonic welder, a positive electrode sheet with a thickness of between 100 and 150 μm is obtained.

[0064] (2) Preparation of negative electrode sheet

[0065] Using artificial graphite as the negative electrode material: Artificial graphite, polyvinylidene fluoride (PVDF), and N-methylpyrrolidone (NMP) are mixed to form a uniform negative electrode slurry. This slurry is then coated on both sides of copper foil, dried, and rolled to produce a carbon negative electrode material. The material is then baked at 120°C for 12 hours. The dried electrode sheet contains 96.4% graphite and 3.6% PVDF by weight. Nickel negative tabs are then attached using an ultrasonic welder to create a negative electrode sheet with a thickness of 100 to 150 μm.

[0066] (3) Preparation of electrolyte

[0067] The electrolyte was prepared by the method of Example 1. The specific formula components of the electrolyte are shown in Table 2.

[0068] (4) Preparation of lithium-ion batteries

[0069] The positive electrode sheet, the negative electrode sheet and the PE ceramic diaphragm described in the above steps (1) and (2) are wound to obtain a battery cell, the battery cell is placed in an aluminum-plastic film package, dried, injected with electrolyte and sealed, and subjected to conventional battery preparation processes such as standing, formation, secondary sealing, and capacity division to obtain a lithium-ion battery.

[0070] Application Examples 2-17 and Comparative Application Examples 1-4 were also prepared using the method of Application Example 1 to prepare lithium-ion batteries. The specific formulations of the positive electrode materials and the corresponding electrolytes are shown in Table 2.

[0071] Table 2 Specific formula composition of positive electrode materials and corresponding electrolytes of Application Examples 1-17 and Comparative Examples 1-4

[0072]

[0073]

[0074]

[0075] 4. High-temperature cycle performance test of lithium-ion batteries

[0076] The prepared batteries were subjected to the following tests:

[0077] At 45°C, the lithium-ion battery was first charged at a constant current of 1C to a voltage of 4.35V, then charged at a constant voltage to a current of 0.1C, and then discharged at a constant current of 1C to 3.0V. A charge and discharge test was performed for 500 cycles, and the discharge capacity at the 500th cycle was detected.

[0078] Capacity retention rate after cycles=(discharge capacity after 500 cycles / discharge capacity at the first cycle)×100%.

[0079] Before cycling at 45°C, the lithium-ion battery was charged at a constant current of 1C to a voltage of 4.35V at room temperature, and then at a constant voltage of 4.35V to a current of 0.1C. At this time, the thickness of the lithium-ion battery was tested; after cycling at 45°C for 500 cycles, the battery was taken out and allowed to stand at room temperature for 6 hours before the thickness was tested.

[0080] Thickness change rate of lithium ion battery after high temperature storage (%) = (thickness of lithium ion battery after high temperature storage - thickness of lithium ion battery before high temperature storage) / thickness of lithium ion battery before high temperature storage × 100%.

[0081] The lithium-ion batteries prepared in Application Examples 1-17 correspond to battery numbers 1 to 17, and the lithium-ion batteries prepared in Application Comparative Examples 1-4 correspond to battery numbers 1# to 4#. The cycle performance test results of the batteries at 45°C are shown in Table 3 below.

[0082] Table 3 Cycling performance test results of batteries at 45°C

[0083]

[0084]

[0085] As can be seen from Table 3, Batteries 1 to 17 containing the additive of the present invention have much better cycle performance at 45°C than Batteries 1# to 4#, and can effectively suppress the increase in battery thickness. This is precisely because the novel additive formed by organically combining imidazole, phosphonate, or phosphinate structural groups in the present invention has the effect of inhibiting the decomposition of lithium hexafluorophosphate, thereby further suppressing the decomposition and gas production of the electrolyte. After application to batteries, the problem of battery thermal stability is solved, and it has good application prospects.

[0086] The technical features of the above-described embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are exhaustively listed. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0087] For those skilled in the art, several variations and improvements may be made without departing from the scope of the present invention, which all fall within the scope of protection of the present invention. The scope of protection of the present invention shall be based on the appended claims.

Claims

1. An electrolyte, characterized in that: The electrolyte comprises a solvent, an electrolyte lithium salt and an electrolyte additive containing an imidazole structure, wherein the electrolyte additive containing an imidazole structure has a structure as shown in Formula I or Formula II: Wherein, X and Y are independently selected from oxygen atoms or do not exist. When X does not exist, the phosphorus element connected to X is an unsaturated phosphine. When Y does not exist, the phosphorus element connected to Y is an unsaturated phosphine. R1 and R2 are the same or different, and R1 and R2 are independently selected from one of methyl, ethyl, cyanoethyl, trifluoroethyl, difluoroethyl, allyl, propargyl, trimethylsilyl, and dimethylvinylsilyl. R1 and R2 can also be connected to form a five-membered ring or a six-membered ring; R3 to R8 are the same or different, and are independently selected from one of hydrogen, C1 to C4 alkyl, C1 to C4 fluorinated alkyl, cyano, phenyl, fluorophenyl, alkoxy-containing phenyl, and alkyl-containing phenyl.

2. An electrolyte according to claim 1, characterized in that: The electrolyte additive containing an imidazole structure is selected from any one or a combination of the following structural formulas:

3. An electrolyte according to claim 1, characterized in that Based on the total mass of the electrolyte, the mass content of the electrolyte additive containing the imidazole structure is 0.005-2wt%.

4. An electrolyte according to claim 1, characterized in that The electrolyte also includes an auxiliary additive; the auxiliary additive is at least one of 1,3-propane sultone, 1,4-butane sultone, propenyl-1,3-sultone, vinyl sulfate, methylene disulfonic acid methylene ester, vinyl carbonate, fluoroethylene carbonate, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium difluorooxalatophosphate, lithium difluorophosphate, and lithium tetrafluoroborate.

5. An electrolyte according to claim 4, characterized in that: Based on the total mass of the electrolyte, the mass content of the auxiliary additive is 0.1-3.0 wt%.

6. An electrolyte according to claim 1, characterized in that The electrolyte lithium salt is selected from one or a combination of LiPF6, LiBF4, LiODFB, LiTDI, LiTFSI and LiFSI.

7. An electrolyte according to claim 6, characterized in that: Based on the total mass of the electrolyte, the content of the electrolyte lithium salt is 10-20wt%.

8. The electrolyte according to claim 1, characterized in that The solvent is selected from one or a combination of cyclopentyl methyl ether, cyclopentyl trifluoroethyl ether, ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methylpropynyl carbonate, 1,4-butyrolactone, methyl propionate, methyl butyrate, difluoroethanol acetate, ethyl propionate, propyl propionate or ethyl butyrate.

9. A lithium-ion battery, characterized in that: The lithium-ion battery comprises a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and the electrolyte according to any one of claims 1 to 8.

Images