A Lithium-Ion Battery Electrolyte and Its Application

Abstract

The present invention discloses a lithium-ion battery electrolyte, which comprises a lithium salt, a pentafluorophenylimidazole-based ionic liquid, and an organic solvent, wherein the structural formula of the pentafluorophenylimidazole-based ionic liquid is as shown in formula (1). The present invention also discloses the application of the above lithium-ion battery electrolyte in a lithium-ion battery with a ternary cathode material. The electrolyte of the present invention contains a pentafluorophenylimidazole-based ionic liquid, which contains a large amount of F element, can improve the flame retardancy and thermal stability of the electrolyte, and has the characteristics of high conductivity, wide electrochemical window, and good chemical stability, can effectively improve the safety performance of the lithium-ion battery, extend the cycle life, and meet the requirements of high energy and high safety performance of the lithium-ion battery.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and particularly to a lithium-ion battery electrolyte and its application. Background Art

[0002] Lithium-ion batteries are widely used because of their high working voltage, high specific energy, long cycle life, and no memory effect. For example, currently lithium-ion batteries have been widely used in the field of 3C consumer electronics products. And with the development of new energy vehicles, lithium-ion batteries are also widely used in the fields of power and energy storage. However, with the continuous increase in people's demand for electric vehicles, the driving range of electric vehicles has become a key concern. Developing cars with a higher driving range requires a higher battery energy density. Therefore, it is urgent to develop a battery system with a higher energy density. Increasing the working voltage is an effective way to improve the energy density of lithium-ion batteries. Currently, the liquid electrolytes of commercial lithium-ion batteries are mainly carbonate-based electrolytes based on ethylene carbonate. However, when the system voltage is higher than 4.5V, the conventional carbonate-based electrolyte solvents will decompose, resulting in a decline in the performance of the entire battery. Since the birth of lithium-ion batteries, safety has always been an important issue restricting their usage scenarios. After lithium-ion batteries entered the consumer electronics field, large-scale recall programs have been carried out many times due to potential battery fires. In the field of electric transportation, the safety accidents of power batteries have gradually increased with the increase in the sales volume of new energy vehicles. With the increasingly expanding application scenarios of lithium-ion batteries, their safety has sparked extensive discussions and research in both the industrial and academic circles. To meet the requirements of high energy and high safety performance of lithium-ion batteries, it is very necessary to develop a new solvent system.

[0003] Ionic liquids are composed of two parts: cations and anions, and are salts that exist in liquid form at room temperature or near room temperature. Compared with traditional organic solvents, ionic liquids have the following excellent physical and chemical properties: extremely low vapor pressure, low volatility, non-flammability, thermal stability, wide liquid temperature range, high electrical conductivity, chemical stability, and good solubility. The cation structures in ionic liquids are diverse, with thousands of varying substituent groups. The variation space of anions is relatively much smaller, mostly being Cl - 、Br - 、I - 、BF4 - 、PF6 - and other inorganic anions, as well as CH3COO - 、CF3SO3 - 、C4H9SO3 - 、CF3COO - 、N(CF3SO2)2 - 、N(C2F5SO2)2 - 、N(C4F9SO2)2- , N[(CF3SO2)(C4F9SO2)] - , C(CF3SO2)3 - and other organic anions. Anions Cl - , Br - , I - have too low oxidation potential to be used in lithium batteries. Most ionic liquids with carboxylate and sulfonate anions are solid at room temperature and have relatively high melting points. Currently, the ionic liquids successfully applied in lithium batteries have anions such as BF4 - , PF6 - or sulfonylimide groups, etc. However, the existing electrolytes containing ionic liquids still have problems in simultaneously achieving flame retardancy, thermal stability and high-voltage resistance. In order to meet the requirements of high energy and high safety performance of lithium-ion batteries, further exploration and development are needed. SUMMARY OF THE INVENTION

[0004] Based on the technical problems existing in the background art, the present invention proposes a lithium-ion battery electrolyte and its application.

[0005] A lithium-ion battery electrolyte proposed by the present invention comprises a lithium salt, a pentafluorophenylimidazole-based ionic liquid and an organic solvent; wherein, based on the sum of the mass percentages of the lithium salt, the pentafluorophenylimidazole-based ionic liquid and the organic solvent being 100%, the mass percentage of the lithium salt is 5-15%, the mass percentage of the pentafluorophenylimidazole-based ionic liquid is 30-45%, and the mass percentage of the organic solvent is 40-65%;

[0006] The pentafluorophenylimidazole-based ionic liquid is composed of a cation and an anion, and its structural formula is shown as formula (1):

[0007]

[0008] wherein, R1 is selected from any one of H and CH3, R2 is selected from any one of H, CH3, CH3CH2 and PhCH2, and m is any integer from 1 to 5; Y - is selected from - PF6 - , TFSI - , FSI - , BF4 - any one of

[0009] Preferably, the structural formula of the cation of the pentafluorophenylimidazole-based ionic liquid is:

[0010]

[0011]

[0012] Preferably, the lithium salt is lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPF2O2), lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiDFOB), lithium bis(pentafluoroethylsulfonyl)imide (LiBETI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), or lithium trifluoromethanesulfonate (LiCF3SO3), and preferably at least one of LiPF6, LiFSI, LiDFOB, and LiPF2O2.

[0013] Preferably, the organic solvent is at least one of carbonate organic solvents, carboxylate organic solvents, phosphate organic solvents, ether organic solvents, nitrile organic solvents, and halogenated nitrile organic solvents, and preferably at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), methyl propyl carbonate (MPC), 1,4-butyrolactone (1,4-BC), methyl formate (MF), ethyl acetate (EA), methyl propionate (MP), ethyl propionate (EP), butyl propionate (BP), ethyl butyrate (EB), trimethyl phosphate (TMP), triethyl phosphate (TEP), and triphenyl phosphate (TPP).

[0014] Preferably, the electrolyte further includes an externally added additive, and the externally added additive is at least one of a film-forming additive, a high-temperature additive, a low-temperature additive, and a stability additive.

[0015] Preferably, the mass of the externally added additive is 0.5 - 10% of the sum of the masses of the lithium salt, the pentafluorophenylimidazole-based ionic liquid, and the organic solvent.

[0016] Preferably, the externally added additive is one or more of vinylene carbonate (VC), vinyl ethyl carbonate (VEC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), 1,3-propane sultone (1,3-PS), 1,4-butane sultone (1,4-BS), 1,3-propene sultone (PES), methylene methanedisulfonate (MMDS), hexamethyldisilazane (HMDS), tris(trimethylsilyl) borate (TMSP), tris(trimethylsilyl) phosphite (TMSPi), tris(pentafluorophenyl) boron (TPFPB), biphenyl (BP), fluorobenzene (FB), succinonitrile, and methylene methanedisulfonate.

[0017] Application of the described lithium-ion battery electrolyte in a lithium-ion battery with a ternary cathode material.

[0018] The beneficial effects of the present invention are as follows:

[0019] In the lithium-ion electrolyte of the present invention, a solvent system is composed of a pentafluorophenylimidazole ionic liquid and a conventional organic solvent, introducing a large amount of F elements and aromatic groups, which not only effectively improves the flame retardancy and thermal stability of the electrolyte, but also due to the high oxidation potential and wide electrochemical window of the pentafluorophenylimidazole ionic liquid, it can remain stable and does not undergo oxidation decomposition reactions under high voltages, avoiding the problem of gas swelling in lithium-ion batteries, and can significantly improve the electrochemical performance of lithium-ion batteries. The stability is significantly improved under high voltages (4.5 - 5.0V); the electrolyte of the present invention can effectively improve the safety performance of lithium-ion batteries, extend the cycle life, and can meet the needs of high energy and high safety performance of lithium-ion batteries. Description of the Drawings

[0020] Figure 1 It is the 1H NMR spectrum of the pentafluorophenylimidazole ionic liquid in Example 1 of the present invention.

[0021] Figure 2 It is the 13C NMR spectrum of the pentafluorophenylimidazole ionic liquid in Example 1 of the present invention.

[0022] Figure 3 It is the 1H NMR spectrum of the pentafluorophenylimidazole ionic liquid in Example 4 of the present invention.

[0023] Figure 4 It is the 13C NMR spectrum of the pentafluorophenylimidazole ionic liquid in Example 4 of the present invention. Detailed Description of the Invention

[0024] Next, the technical solution of the present invention will be described in detail through specific examples.

[0025] Example 1

[0026] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, a lithium salt LiPF6, a pentafluorophenylimidazole ionic liquid, and an organic solvent are mixed in a mass ratio of LiPF6:pentafluorophenylimidazole ionic liquid:organic solvent = 12.5:42.5:45. The organic solvent is composed of EC, DEC, and EMC in a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, pentafluorophenylimidazole ionic liquid, and organic solvent is added, and stirred and mixed evenly to obtain a lithium-ion battery electrolyte.

[0027] The above pentafluorophenylimidazole ionic liquid is composed of a cation and an anion, and the structural formula of the cation is as follows:

[0028]

[0029] The anion is PF6 - .

[0030] The above-mentioned pentafluorophenylimidazole-based ionic liquids were characterized, and their NMR data are as follows:

[0031] 1H NMR(400MHz,CDCl3):δ3.30(1H,quint,J=7.6Hz),3.49(6H,s),3.83 - 4.09(8H,3.89(d,J=7.6Hz),3.89(d,J=7.6Hz),4.00(dt,J=14.4,6.2Hz),4.02(dt,J=14.4,6.2Hz)),4.10 - 4.30(4H,4.18(dt,J=16.0,6.2Hz),4.22(dt,J=16.0,6.2Hz)),8.19(2H,s). The corresponding spectrum is shown in Figure 1 .

[0032] 13C NMR(100MHz,CDCl3):δ38.6(2C,s),40.7(1C,s),49.1(2C,s),53.3(2C,s),55.5(2C,s),128.0(1C,s),137.9(2C,s),140.8(2C,s),141.5(1C,s),145.7(2C,s). The corresponding spectrum is shown in Figure 2 .

[0033] Example 2

[0034] The difference between Example 2 and Example 1 is only that: the anion of the pentafluorophenylimidazole-based ionic liquid is BF4 - .

[0035] Example 3

[0036] The difference between Example 3 and Example 1 is only that: the structural formula of the cation of the pentafluorophenylimidazole-based ionic liquid is as follows:

[0037]

[0038] Example 4

[0039] The difference between Example 4 and Example 1 is only that: the structural formula of the cation of the pentafluorophenylimidazole-based ionic liquid is as follows:

[0040]

[0041] The above-mentioned pentafluorophenylimidazole-based ionic liquids were characterized, and their NMR data are as follows:

[0042] 1H NMR (400 MHz, CDCl3): δ 3.27 (1H, quint, J = 8.4 Hz), 3.56 - 3.75 (10H, 3.61 (s), 3.70 (s), 3.70 (s)), 3.86 - 4.07 (8H, 3.92 (d, J = 8.4 Hz), 3.92 (d, J = 8.4 Hz), 3.99 (dt, J = 14.4, 6.2 Hz), 3.99 (dt, J = 14.4, 6.2 Hz)), 4.18 - 4.33 (4H, 4.25 (dt, J = 17.4, 6.2 Hz), 4.25 (dt, J = 17.4, 6.2 Hz)). The corresponding spectrum is shown in Figure 3 。

[0043] 13C NMR (100 MHz, CDCl3): δ 29.2 (2C, s), 38.6 (2C, s), 40.7 (1C, s), 49.1 (2C, s), 53.3 (2C, s), 55.5 (2C, s), 124.7 (2C, s), 128.0 (1C, s), 137.8 - 138.0 (6C, 137.9 (s), 137.9 (s)), 141.4 - 141.6 (3C, 141.5 (s), 141.5 (s)), 145.4 (4C, s), 145.7 (2C, s), 164.7 (2C, s). The corresponding spectrum is shown in Figure 4 。

[0044] Example 5

[0045] The difference between Example 5 and Example 1 is only that the structural formula of the cation of the pentafluorophenylimidazole - based ionic liquid is as follows:

[0046]

[0047] Example 6

[0048] The difference between Example 6 and Example 1 is only that the structural formula of the cation of the pentafluorophenylimidazole - based ionic liquid is as follows:

[0049]

[0050] Example 7

[0051] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, a lithium salt LiPF6, a pentafluorophenylimidazole-based ionic liquid, and an organic solvent are mixed at a mass ratio of LiPF6:pentafluorophenylimidazole-based ionic liquid:organic solvent = 12.5:30:57.5. The organic solvent consists of EC, DEC, and EMC in a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, the pentafluorophenylimidazole-based ionic liquid, and the organic solvent is added, and the mixture is stirred thoroughly until evenly mixed to obtain a lithium-ion battery electrolyte.

[0052] The pentafluorophenylimidazole-based ionic liquid is the same as that in Example 1.

[0053] Example 8

[0054] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, a lithium salt LiPF6, a pentafluorophenylimidazole-based ionic liquid, and an organic solvent are mixed at a mass ratio of LiPF6:pentafluorophenylimidazole-based ionic liquid:organic solvent = 12.5:45:42.5. The organic solvent consists of EC, DEC, and EMC in a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, the pentafluorophenylimidazole-based ionic liquid, and the organic solvent is added, and the mixture is stirred thoroughly until evenly mixed to obtain a lithium-ion battery electrolyte.

[0055] The pentafluorophenylimidazole-based ionic liquid is the same as that in Example 1.

[0056] Comparative Example 1

[0057] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, a lithium salt LiPF6 and an organic solvent are mixed at a mass ratio of LiPF6:organic solvent = 12.5:87.5. The organic solvent consists of EC, DEC, and EMC in a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6 and the organic solvent is added, and the mixture is stirred thoroughly until evenly mixed to obtain a lithium-ion battery electrolyte.

[0058] Comparative Example 2

[0059] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, a lithium salt LiPF6, a pentafluorophenylimidazole-based ionic liquid, and an organic solvent are mixed at a mass ratio of LiPF6:pentafluorophenylimidazole-based ionic liquid:organic solvent = 12.5:57.5:30. The organic solvent consists of EC, DEC, and EMC in a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, the pentafluorophenylimidazole-based ionic liquid, and the organic solvent is added, and the mixture is stirred thoroughly until evenly mixed to obtain a lithium-ion battery electrolyte.

[0060] Among them, the pentafluorophenyl imidazole ionic liquid is the same as that in Example 1.

[0061] Comparative Example 3

[0062] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, the lithium salt LiPF6, pentafluorophenyl imidazole ionic liquid, and organic solvent were mixed at a mass ratio of LiPF6:pentafluorophenyl imidazole ionic liquid:organic solvent = 12.5:27.5:60. The organic solvent was composed of EC, DEC, and EMC at a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, pentafluorophenyl imidazole ionic liquid, and organic solvent was added, and the mixture was stirred thoroughly to obtain a lithium-ion battery electrolyte.

[0063] Among them, the pentafluorophenyl imidazole ionic liquid is the same as that in Example 1.

[0064] Comparative Example 4

[0065] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, the lithium salt LiPF6, pentafluorophenyl imidazole ionic liquid, and organic solvent were mixed at a mass ratio of LiPF6:pentafluorophenyl imidazole ionic liquid:organic solvent = 12.5:12.5:75. The organic solvent was composed of EC, DEC, and EMC at a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, pentafluorophenyl imidazole ionic liquid, and organic solvent was added, and the mixture was stirred thoroughly to obtain a lithium-ion battery electrolyte.

[0066] Among them, the pentafluorophenyl imidazole ionic liquid is the same as that in Example 1.

[0067] Comparative Example 5

[0068] In an inert atmosphere glove box with moisture less than 10 ppm and oxygen content less than 5 ppm, the lithium salt LiPF6, 1-ethyl-3-methylimidazolium tetrafluoroborate, and organic solvent were mixed at a mass ratio of LiPF6:1-ethyl-3-methylimidazolium tetrafluoroborate:organic solvent = 12.5:42.5:75. The organic solvent was composed of EC, DEC, and EMC at a mass ratio of 1:1:1. Then, vinylene carbonate equivalent to 2% of the total mass of LiPF6, 1-ethyl-3-methylimidazolium tetrafluoroborate, and organic solvent was added, and the mixture was stirred thoroughly to obtain a lithium-ion battery electrolyte.

[0069] The electrolytes of the above Examples 1-8 and Comparative Examples 1-5 were injected into 20 Ah NCM / C system soft-pack battery cells, and after formation and grading, they were subjected to 45°C high-temperature cycling (1C charge and discharge, 3.0 - 4.5V) tests, high-temperature storage performance tests, electrolyte flame retardancy tests, and needle penetration experiments.

[0070] Wherein:

[0071] High-temperature storage performance test steps: First, perform constant volume for 3 weeks at room temperature of 25°C (1C, 3.0 - 4.5V) and end with a full charge state; place it in an oven at 60°C for 7 days. After the placement, cool it at room temperature for 6 hours and then perform constant volume for 3 weeks at room temperature, and calculate the capacity retention rate and recovery rate.

[0072] Flame retardancy test of the electrolyte:

[0073] Use the self-extinguishing method to detect the flame retardancy of the electrolyte sample. The specific operation is as follows: Immerse a glass wool ball with a mass of m1 and a diameter of 0.3 mm in the lithium-ion battery electrolyte to be tested. After full infiltration, weigh its mass m2. Then place the glass wool ball in a wire loop, ignite it with an ignition device, record the time T from ignition to flame extinction, and use the self-extinguishing time t per unit mass of the electrolyte as the standard to measure the flame retardancy of the electrolyte. The calculation formula is: t = T / (m2 - m1), and the average value of three measurements is taken for each sample measurement result.

[0074] Pinprick performance detection:

[0075] Charge the experimental battery at a constant current of 0.33C to 4.2V, then switch to constant voltage charging until the current is less than 0.05C to reach 100% SOC. Let it stand for 30 min before the test, fix the battery in a pinprick test cabinet, and attach temperature sensor wires and voltage wires to the positive and negative electrode posts. Use a tungsten steel needle with a diameter of 5 mm to penetrate the battery at the geometric center at a speed of 25 ± 5 mm / s, and observe the experimental phenomena.

[0076] The test results are shown in Table 1 and Table 2:

[0077] Table 1 High-temperature storage and high-temperature cycling performance of the battery

[0078]

[0079]

[0080] Table 2 Flame retardancy performance and pinprick test results of the battery

[0081]

[0082]

[0083] As can be seen from the data in Table 1 and Table 2, adding an appropriate amount of pentafluorophenylimidazole ionic liquid to the electrolyte of the present invention can significantly improve the high-temperature storage performance and high-temperature cycling performance of lithium-ion batteries, and can pass the flammability test and the needle penetration test, proving that it has excellent flame retardant performance and high-voltage use safety. Therefore, it can meet the requirements of high energy and high safety performance of lithium-ion batteries.

[0084] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and should be covered by the protection scope of the present invention.

Claims

1. A lithium-ion battery electrolyte, characterized in that, It contains a lithium salt, a pentafluorophenylimidazole ionic liquid and an organic solvent; wherein, based on the sum of the mass percentages of the lithium salt, the pentafluorophenylimidazole ionic liquid and the organic solvent being 100%, the mass percentage of the lithium salt is 5-15%, the mass percentage of the pentafluorophenylimidazole ionic liquid is 30-45%, and the mass percentage of the organic solvent is 40-65%; The pentafluorophenylimidazole ionic liquid is composed of a cation and an anion, and its structural formula is shown in Formula (1): Among them, R1 is selected from any one of H and CH3, R2 is selected from any one of H, CH3, CH3CH2, and PhCH2, and m is any integer from 1 to 5; Y - is selected from PF6 - , TFSI - , FSI - , BF4 - , BOB - and any one of them.

2. The electrolyte of the lithium-ion battery according to claim 1, characterized in that The structural formula of the cation of the pentafluorophenylimidazole ionic liquid is:

3. The electrolyte for a lithium-ion battery according to claim 1, characterized in that, The lithium salt is at least one of lithium hexafluorophosphate, lithium perchlorate, lithium tetrafluoroborate, lithium difluorophosphate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium bis(pentafluoroethylsulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium trifluoromethanesulfonate.

4. The electrolyte for a lithium-ion battery according to claim 1, wherein, The lithium salt is at least one of lithium hexafluorophosphate, lithium difluorophosphate, lithium bis(trifluoromethylsulfonyl)imide, lithium bis(oxalato)borate.

5. The lithium-ion battery electrolyte according to claim 1, characterized in that, The organic solvent is at least one of carbonate organic solvents, carboxylate organic solvents, phosphate organic solvents, ether organic solvents, nitrile organic solvents, halogenated nitrile organic solvents.

6. The electrolyte for a lithium-ion battery according to claim 1, wherein, The organic solvent is at least one of ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, 1,4-butyrolactone, methyl formate, ethyl acetate, methyl propionate, ethyl propionate, butyl propionate, ethyl butyrate, trimethyl phosphate, triethyl phosphate, triphenyl phosphate.

7. The electrolyte for a lithium-ion battery according to claim 1, characterized in that, The electrolyte further contains an external additive, and the external additive is at least one of a film-forming additive, a high-temperature additive, a low-temperature additive, and a stability additive.

8. The lithium-ion battery electrolyte according to claim 7, wherein, The mass of the external additive is 0.5-10% of the sum of the masses of the lithium salt, the pentafluorophenylimidazole ionic liquid and the organic solvent.

9. The lithium-ion battery electrolyte according to claim 7, characterized in that, The external additive is one or more of vinylene carbonate, ethylene vinyl carbonate, fluoroethylene carbonate, ethylene sulfate, 1,3-propane sultone, 1,4-butane sultone, 1,3-propene sultone, methylene methanedisulfonate, hexamethyldisilazane, tris(trimethylsilyl) borate, tris(trimethylsilyl) phosphite, tris(pentafluorophenyl) boron, biphenyl, fluorobenzene, succinonitrile, methylene methanedisulfonate.

10. Application of a lithium-ion battery electrolyte as described in any one of claims 1-9 in a lithium-ion battery with a ternary cathode material.

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