Imidazole additive, electrolyte containing the additive and application thereof

Abstract

The present invention relates to the field of battery technology and discloses an imidazole additive, an electrolyte containing the additive, and applications thereof. When the imidazole compound acts as an electrolyte additive, it can form a highly stable protective film on the surface of the positive and negative electrodes, effectively inhibiting the decomposition of the electrolyte and the expansion of the silicon negative electrode. The imidazole compound contains silicon-oxygen bonds or silicon-nitrogen bonds, which can react with water and acid in the electrolyte, thereby maintaining stable acidity of the electrolyte at high temperatures, further reducing battery gas production, and improving the high-temperature cycle performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to an imidazole additive, an electrolyte containing the additive, and applications thereof. Background Art

[0002] With the rapid development of electric vehicles, low-altitude drones and other fields, people's demand for high-performance lithium-ion batteries is increasing. The theoretical capacity of lithium-ion battery silicon negative electrodes is as high as 4200mAh / g, far exceeding the theoretical capacity of graphite negative electrodes of 372mAh / g. However, lithium-ion batteries have severe volume expansion during the charge and discharge process (up to 300% or more), which leads to electrode structure rupture, repeated growth of the solid electrolyte interface (SEI) film, and a sharp decline in cycle performance.

[0003] At present, there are many solutions to the expansion of silicon negative electrodes, such as material modification, electrode structure design, electrolyte optimization and binder development. Material modification usually improves the expansion of silicon from the perspectives of nano-silicon particles, silicon-based composite materials, silicon-metal / oxide composites, etc.; the electrode structure usually designs the electrode porosity (30-50%), hollow structure or three-dimensional porous collector to reserve expansion space; high-performance polymer binders are developed to inhibit the expansion of silicon negative electrodes, but the most effective and convenient solution is electrolyte optimization, that is, adding additives that inhibit the expansion of silicon negative electrodes to the electrolyte to form a stable and flexible SEI film to inhibit silicon expansion.

[0004] A commonly used additive in electrolytes is fluoroethylene carbonate (FEC), which can effectively inhibit the expansion of silicon negative electrodes and improve the cycle performance of silicon-containing batteries. However, FEC easily produces gas during the cycle, resulting in a decrease in battery cycle performance. Therefore, it is necessary to develop an electrolyte additive to reduce the use of FEC, reduce the gas production of the battery, and improve the battery cycle performance. Summary of the Invention

[0005] In order to solve the technical problems existing in the background technology, the present invention provides an imidazole additive, which includes a compound represented by structural formula (I);

[0006]

[0007] Wherein, R1 is a group containing Si-N group or Si-O group; R2 is an unsaturated group; and n is an integer of 1-5.

[0008] The Si-N group-containing group is any one of structural formulas 1-3, and the Si-O group-containing group is any one of structural formulas 4-6;

[0009] .

[0010] The R2 is one of alkynyl, cyano or alkenyl.

[0011] The compound shown in structural formula I is

[0012] .

[0013] In the present invention, the imidazole ring of the imidazole additive can effectively inhibit the decomposition of the electrolyte, the unsaturated group undergoes electrochemically active film formation and the functional group orientation effect promotes the formation of a highly stable electrolyte interface film on the surface of the positive and negative electrode materials, and the group containing Si-N or Si-O group can undergo Si-N or Si-O rupture and react with the acid and water in the electrolyte, thereby maintaining the acidity of the electrolyte at high temperature, reducing flatulence and damage to the SEI film by the electrolyte.

[0014] The present invention also provides an electrolyte, which includes the above-mentioned imidazole additive.

[0015] The imidazole additive accounts for 0.1-5 wt % in the electrolyte.

[0016] The electrolyte further comprises a lithium salt and an organic solvent;

[0017] The lithium salt is at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate or lithium difluorooxalatoborate;

[0018] The organic solvent is at least one of organic esters, C1-C10 alkyl ethers, cyclic ethers, sulfones, dinitriles, and ionic liquids;

[0019] Preferably, the organic ester is at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate or ethyl butyrate;

[0020] Preferably, the C1-10 alkyl ether is at least one of dimethyl ether, diethyl ether or methyl ethyl ether;

[0021] Preferably, the dinitrile is at least one of adiponitrile, succinonitrile or glutaronitrile;

[0022] Preferably, the sulfone is at least one of dimethyl sulfoxide or sulfolane;

[0023] Preferably, the ionic liquid is at least one of an imidazole or pyrrole ionic liquid.

[0024] The lithium salt accounts for 8-26 wt% in the electrolyte; the organic solvent accounts for 72-90 wt% in the electrolyte;

[0025] Preferably, the lithium salt accounts for 10-16 wt% in the electrolyte; and the organic solvent accounts for 80-90 wt% in the electrolyte.

[0026] The electrolyte further comprises a non-imidazole film-forming additive, wherein the non-imidazole film-forming additive comprises fluoroethylene carbonate, and the proportion of the non-imidazole film-forming additive in the electrolyte is 0-20 wt %.

[0027] In the present invention, fluoroethylene carbonate is prone to gas production during the cycle, and the imidazole additive in the electrolyte reduces the usage of traditional electrolyte additives (non-imidazole film-forming additives), especially the usage of fluoroethylene carbonate. As a result, the gas production during battery cycling is further reduced, and the battery's cycling performance is improved.

[0028] The present invention also provides a lithium-ion battery, comprising the above-mentioned electrolyte;

[0029] Preferably, the lithium-ion battery further comprises a positive electrode, a negative electrode, a separator, and an aluminum-plastic film;

[0030] Further preferably, the positive electrode material is selected from one of lithium cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium iron phosphate or lithium manganese iron phosphate;

[0031] More preferably, the positive electrode material is lithium nickel cobalt manganese oxide;

[0032] Preferably, the active material of the negative electrode is selected from any one or a combination of graphite, silicon oxide, silicon carbon, lithium metal, and lithium titanate;

[0033] More preferably, the active material of the negative electrode is selected from a combination of silicon carbon and graphite;

[0034] Preferably, the diaphragm is selected from one of polypropylene (PP), polyethylene (PE) diaphragm or glass fiber diaphragm;

[0035] More preferably, the above-mentioned diaphragm is selected from polypropylene (PP) diaphragm.

[0036] Beneficial effects of the present invention:

[0037] (1) The present invention proposes an imidazole additive that can form a stable and tough protective film on the surface of the negative electrode, inhibiting the volume expansion of the silicon negative electrode during the charge and discharge process, thereby reducing the damage of the SEI film;

[0038] (2) This imidazole additive contains Si-N or Si-O groups, which have the effect of removing water and acid, further preventing the electrolyte from generating gas and causing flatulence due to reaction with water and acid;

[0039] (3) The electrolyte containing the above-mentioned imidazole additive proposed in the present invention has good compatibility with positive and negative electrode materials, can inhibit the expansion of the silicon negative electrode and the flatulence of the lithium-ion battery, and improve the battery cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 Schematic diagram of the synthesis of compound (II);

[0041] Figure 2 is the H NMR spectrum of compound (II);

[0042] Figure 3 Schematic diagram of the synthesis of compounds (I-II);

[0043] Figure 4 This is the H NMR spectrum of compound (I-II). DETAILED DESCRIPTION

[0044] To facilitate understanding of the present invention, the present invention will be described more fully below in conjunction with specific embodiments. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to provide a more thorough and comprehensive understanding of the disclosure of the present invention.

[0045] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art of the present invention. The terms used in the specification of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention.

[0046] The technical solution of the present invention is described more clearly and completely below with reference to specific embodiments and comparative examples.

[0047] Synthesis of compound (II)

[0048] Under nitrogen protection, 135.17 g of 1-but-3-ynyl imidazole-2-amine and 190 mL of trimethylsilyl chloride were dissolved in 1 L of anhydrous toluene, heated to 85°C, and refluxed with stirring for 6 hours to carry out a silylation reaction. The silylation reaction was monitored by TLC (developing solvent, ethyl acetate:n-hexane = 1:3). After the reaction of 1-but-3-ynyl imidazole-2-amine was complete, the reaction solution was cooled to 25°C, and toluene and excess trimethylsilyl chloride were removed by distillation under reduced pressure. The residue was then washed with 500 mL of n-hexane, and the insoluble matter was removed by filtration. The filtrate was concentrated to obtain compound (II).

[0049] Synthesis of compound (I-II)

[0050] (1) Cool 5 L of acetonitrile to 0 °C in an ice bath, dissolve 135.17 g of 1-but-3-ynyl imidazole-2-amine in the acetonitrile, maintain 0 °C, and slowly add 0.25 L dropwise under stirring to a concentration of 8 mol / L -1 After the HI aqueous solution was added dropwise, a uniform mixed solution was obtained. 82.8 g of Add the above mixed solution and stir for 1 hour to carry out diazotization reaction. Monitor the diazotization reaction by TLC (developing solvent, ethyl acetate: n-hexane = 1:3). After the reaction of 1-but-3-ynyl imidazole-2-amine is complete, add 3L saturated The solution was quenched, extracted with 5 L of ethyl acetate, dried, and concentrated to obtain the intermediate 1-(but-3-ynyl) 2-diazoiodiimidazole;

[0051] (2) Under nitrogen protection, 246.05 g of 1-(but-3-ynyl) 2-diazoiodimidazole and 330 mL of , 15g Ni catalyst and 130 g of Zn powder were added to anhydrous THF, heated to 60°C, and stirred for 12 hours to carry out a nucleophilic substitution reaction. The nucleophilic substitution reaction was monitored by TLC (developing solvent: n-hexane: ethyl acetate = 3:1). After the nucleophilic substitution reaction was completed, it was filtered, concentrated, and purified by column chromatography (initial solvent n-hexane, gradient solvent n-hexane: ethyl acetate = 4:1) to obtain compound (I-II).

[0052] Figure 1 Schematic diagram of the synthesis of compound (II), wherein ET3N is triethylamine and TOL is toluene.

[0053] Example 1

[0054] This embodiment provides an electrolyte, and its preparation method is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and compound (II) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the usage amounts of lithium hexafluorophosphate, mixed organic solvent, FEC and compound (II) are 13wt%, 84wt%, 2wt% and 1wt% of the total mass of the electrolyte, respectively.

[0055] Example 2

[0056] This embodiment provides an electrolyte, and a preparation method thereof is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and compound (I-II) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the usage amounts of lithium hexafluorophosphate, mixed organic solvent, FEC and compound (I-II) are 13wt%, 84wt%, 2wt% and 1wt% of the total mass of the electrolyte, respectively.

[0057] Example 3

[0058] This embodiment provides an electrolyte, and its preparation method is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and compound (II) are added, and the mixture is stirred to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, mixed organic solvent, FEC and compound (II) used are 13wt%, 84.5wt%, 2wt% and 0.5wt% of the total mass of the electrolyte, respectively.

[0059] Example 4

[0060] This embodiment provides an electrolyte, and a preparation method thereof is as follows: in an argon glove box with a water and oxygen content of ≤0.1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and compound (I-II) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the usage amounts of lithium hexafluorophosphate, mixed organic solvent, FEC and compound (I-II) are 13 wt%, 84.5 wt%, 2 wt% and 0.5 wt% of the total mass of the electrolyte, respectively.

[0061] Comparative Example 1

[0062] This comparative example proposes an electrolyte, the preparation method of which is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) is added and stirred to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, the mixed organic solvent and FEC used are 13wt%, 85wt% and 2wt% of the total mass of the electrolyte, respectively.

[0063] Comparative Example 2

[0064] This comparative example proposes an electrolyte, the preparation method of which is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and lithium difluorophosphate are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, mixed organic solvent, FEC and lithium difluorophosphate used are 13wt%, 84wt%, 2wt% and 1wt% of the total mass of the electrolyte, respectively.

[0065] Comparative Example 3

[0066] This comparative example provides an electrolyte, the preparation method of which is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and 1,3-propane sultone (PS) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, the mixed organic solvent, FEC, and 1,3-propane sultone (PS) used are 13wt%, 84wt%, 2wt%, and 1wt% of the total mass of the electrolyte, respectively.

[0067] Comparative Example 4

[0068] This comparative example proposes an electrolyte, the preparation method of which is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and vinylene carbonate (VC) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, the mixed organic solvent, FEC, and vinylene carbonate (VC) used are 13wt%, 84wt%, 2wt%, and 1wt% of the total mass of the electrolyte, respectively.

[0069] Comparative Example 5

[0070] This comparative example provides an electrolyte, the preparation method of which is as follows: in an argon glove box with a water and oxygen content of ≤0.1ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) are stirred evenly in a mass ratio of 3:7 to obtain a mixed organic solvent, and then lithium hexafluorophosphate (LiPF6) is slowly added to the mixed organic solvent. After complete dissolution, fluoroethylene carbonate (FEC) and N,N'-carbonyldiimidazole (CAS No. 530-62-1) are added, and the mixture is stirred evenly to obtain an electrolyte, wherein the amounts of lithium hexafluorophosphate, the mixed organic solvent, FEC, and N,N'-carbonyldiimidazole used are 13wt%, 84wt%, 2wt%, and 1wt% of the total mass of the electrolyte, respectively.

[0071] Soft pack battery preparation and performance testing

[0072] The electrolytes in Examples 1-4 and Comparative Examples 1-5 were used to prepare soft-pack batteries for room temperature cycle performance testing and volume expansion rate testing. Table 1 shows the performance test results of soft-pack batteries P1-P4 and DP1-DP5:

[0073] The specific steps for preparing soft pack batteries are as follows:

[0074] (1) The positive electrode material, lithium nickel cobalt manganese oxide (NCM90), the conductive agent, carbon black (SuperP), and the binder, polyvinylidene fluoride (PVDF), were mixed in a mass ratio of 94:3:3. After the mixing was completed, N-methylpyrrolidone was added. The mixture was homogenized using a vacuum defoamer to obtain a positive electrode slurry with a solid content of 55%. The positive electrode slurry was evenly coated on a 12 μm thick aluminum foil. After drying, rolling, and cutting, a positive electrode sheet was obtained.

[0075] (2) The negative electrode material composed of silicon carbon and graphite (with a gram capacity of about 650 mAh / g), the conductive agent SuperP, the thickener sodium carboxymethyl cellulose (CMC), and the binder styrene-butadiene rubber (SBR) were mixed in a mass ratio of 94:1.5:1.5:3. After the mixing was completed, deionized water was added and the mixture was homogenized using a vacuum defoamer to obtain a negative electrode slurry with a solid content of 52%. The negative electrode slurry was evenly coated on a copper foil with a thickness of 8 μm. After drying, rolling, and cutting, a negative electrode sheet was obtained.

[0076] (3) Soft-pack laminated batteries were prepared in an environment with a dew point temperature below -60°C. The positive electrode sheet, separator and negative electrode sheet were stacked in order to ensure that the separator completely separated the positive and negative electrode sheets. The battery moisture content was baked to below 200 ppm before injection. The electrolyte was injected and then sealed, formed and capacity-separated to obtain experimental batteries. Among them, the soft-pack batteries prepared using the electrolytes in Examples 1-4 were recorded as P1-P4, and the soft-pack batteries prepared using the electrolytes in Comparative Examples 1-5 were recorded as DP1-DP5.

[0077] Battery volume expansion rate test:

[0078] The prepared soft-pack batteries P1-P4 and DP1-DP5 were placed in a constant temperature room at an ambient temperature of 25°C, and the battery volume V0 was measured; after capacity separation, the battery volume V1 was measured; they were then charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.8V. This cycle was repeated for 100 cycles, and the battery volume V2 was measured; the volume expansion rate was calculated based on the values ​​of V0, V1, and V2 = (V2-V1) / V0×100%.

[0079] Battery room temperature cycle test:

[0080] In the test environment, the prepared soft-pack battery was placed in a constant temperature room at an ambient temperature of 25°C, charged at a constant current of 1C to a voltage of 4.2V, then charged at a constant voltage of 4.2V to a current of 0.05C, and then discharged at a constant current of 1C to a voltage of 2.8V. The cycle was repeated 300 times, and the capacity retention rate was recorded. The capacity retention rate of the nth cycle (%) = (discharge capacity of the nth cycle / discharge capacity of the first cycle) × 100%.

[0081] Grade expansion test:

[0082] In an environment with a dew point temperature below -40°C, take the soft-pack battery after the room temperature cycle test (25°C, 350 weeks) of the above-mentioned soft-pack battery, disassemble the fully charged battery cell (100% SOC) of the above-mentioned soft-pack battery, take out the middle negative electrode sheet, and measure the sheet thickness with a screw micrometer. The thicknesses measured at three points in the middle are T1, T2, and T3. The initial thickness of the negative electrode sheet is T0, and the sheet expansion rate = {[(T1+T2+T3) / 3-T0] / T0}×100%.

[0083]

[0084] As can be seen from Table 1 above, the volume expansion rate of P1-P4 is significantly reduced compared with DP1-DP5, indicating that compound (II) and compound (I-II) form a stable SEI film on the electrode surface, which inhibits gas generation during the battery cycle; the capacity retention rate of P1-P4 is significantly higher than that of DP1-DP5, indicating that the simple imidazole ring cannot effectively improve the cycle performance of the battery, while compound (II) or compound (I-II) can effectively improve the cycle performance of the battery; the data on the electrode expansion rate also show that the SEI film formed by compound (I-II) is better than compound (II) in inhibiting silicon expansion.

[0085] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. An imidazole additive, characterized in that The imidazole additive includes a compound shown in structural formula I; .

2. An electrolyte, characterized in that The electrolyte comprises the imidazole additive according to claim 1.

3. The electrolyte according to claim 2, characterized in that The imidazole additive accounts for 0.1-5 wt % in the electrolyte.

4. The electrolyte according to claim 2 or 3, characterized in that The electrolyte further comprises a lithium salt and an organic solvent; The lithium salt is at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorophosphate or lithium difluorooxalatoborate; The organic solvent is at least one of organic esters, C1-C10 alkyl ethers, cyclic ethers, sulfones, dinitriles, and ionic liquids.

5. The electrolyte according to claim 4, characterized in that The lithium salt accounts for 8-26 wt % in the electrolyte; and the organic solvent accounts for 72-90 wt % in the electrolyte.

6. The electrolyte according to claim 4, characterized in that The electrolyte further comprises a non-imidazole film-forming additive, wherein the non-imidazole film-forming additive comprises fluoroethylene carbonate, and the proportion of the non-imidazole film-forming additive in the electrolyte is 0-20 wt %.

7. A lithium-ion battery, characterized in that: The lithium-ion battery comprises the electrolyte according to any one of claims 2 to 6.

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