Lithium ion battery and battery pack applying same

Abstract

The invention provides a lithium ion battery and a battery pack applying the lithium ion battery, the lithium ion battery comprises a positive electrode, a negative electrode and an electrolyte, an active material of the positive electrode comprises a nickel-cobalt compound LiNixCoyM1-x-yO2, x is greater than or equal to 0.8, M comprises at least one of Mn and Al, an active material of the negative electrode comprises at least one of graphite and silicon carbon, and the electrolyte comprises 2, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri (3, 4, 6-tri The invention discloses a lithium ion battery electrolyte, which comprises lithium hexafluorophosphate, 2, 3, 5-trifluorobenzene boroxane, fluoroethylene carbonate, lithium salt and a solvent, wherein the lithium salt comprises lithium hexafluorophosphate; on the basis of the total mass of the electrolyte, the content of the 2, 4, 6-tri (3, 4, 5-trifluorobenzene) boroxane is 0.2-3wt%, and the mass ratio of the 2, 4, 6-tri (3, 4, 5-trifluorobenzene) boroxane to the lithium hexafluorophosphate is 1: (2.6-26). According to the lithium ion battery, the CEI membrane and the SEI membrane which are stable in structure and good in thermal stability are constructed, so that the situation that the lithium ion battery generates heat in a large scale due to unexpected reaction is reduced, the short circuit of a positive electrode and a negative electrode caused by shrinkage, melting and collapse of a diaphragm due to heat production is avoided, and the overcharge resistance, the safety and the cycling stability of a battery device are improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, in particular to a lithium ion battery and a battery pack using the same. Background Art

[0002] Overcharging means that the battery is charged to a level that exceeds its designed capacity and then continues to be charged. This will cause the battery's internal pressure to increase, the battery to deform, leak, and the battery's performance to deteriorate. Especially for large-capacity battery packs, the overcharging problem will be more common, prominent, and difficult to overcome due to the imbalance in the charging state of each internal single cell. When overcharging occurs, the voltage inside the battery will experience an abnormal increase, and this voltage runaway is a precursor to thermal runaway. Thermal runaway is a chain reaction. Once triggered, the heat inside the battery will accumulate rapidly, the temperature will rise sharply, and then cause irreversible changes in the structure of the electrode active material and the decomposition of the electrolyte, generating more heat, forming a vicious cycle. Eventually, it will cause safety hazards such as fire and explosion.

[0003] In order to prevent overcharging of batteries, the common method used by industry insiders is to improve the process structure outside the battery shell and add anti-overcharging devices, such as installing a current interrupt device (CID), an explosion-proof safety valve (Vent), and a PTC polymer switch in the battery's safety cap. However, before the anti-overcharging device cuts off the battery current, the temperature inside the battery is already high. At this time, the diaphragm inside the battery usually shrinks, melts, collapses, etc., causing the positive and negative electrodes to directly contact and short-circuit, which in turn causes the battery temperature to rise sharply and triggers thermal runaway of the battery. Summary of the invention

[0004] The object of the present invention is to provide a lithium-ion battery and a battery pack using the same. The lithium-ion battery reduces the situation where the battery generates a lot of heat due to unexpected reactions by constructing a CEI membrane and a SEI membrane with stable structure and good thermal stability, thereby improving the overcharge resistance of the battery device and enhancing the safety and cycle stability of the battery device.

[0005] According to one aspect of the present invention, a lithium ion battery is provided, comprising a positive electrode, a negative electrode and an electrolyte, wherein the active material of the positive electrode comprises a nickel-cobalt compound LiNi x Co y M 1-x-y O 2, wherein x≥0.8, M includes at least one of Mn and Al, the active material of the negative electrode includes at least one of graphite and silicon carbon, the electrolyte includes 2,4,6-tris(3,4,5-trifluorophenyl)boroxine, fluoroethylene carbonate and a lithium salt, and the lithium salt includes lithium hexafluorophosphate; based on the total mass of the electrolyte, the content of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 0.2-3wt%, and the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to lithium hexafluorophosphate is 1:2.6-26.

[0006] Nickel-cobalt compound LiNi x Co y M 1-x-y O 2 Including lithium nickel cobalt aluminum oxide NCA material and lithium nickel cobalt manganese oxide NCM material, which not only have the advantages of high reversible specific capacity and low material cost, but also have high structural stability and safety, thereby improving the cycle stability of batteries using nickel cobalt compounds. Therefore, nickel cobalt compounds are one of the most popular materials currently studied in commercial positive electrode materials. With the increase of nickel content in the positive electrode active material, nickel cobalt compounds provide higher specific capacity, but also sacrifice capacity retention rate and thermal stability. The above-mentioned lithium-ion battery uses nickel cobalt compounds with high Ni content as the active material of the positive electrode, and nickel cobalt compounds with high Ni content will quickly react with the electrolyte under overcharge conditions. The present application uses 2,4,6-tris(3,4,5-trifluorophenyl) cycloboroxine (TTFPB) and fluoroethylene carbonate (FEC) as additives and lithium hexafluorophosphate as lithium salt to construct a CEI film and SEI film with stable structure and good thermal stability, thereby inhibiting the oxidative decomposition of the electrolyte under high pressure and high temperature conditions, and reducing the loss of positive electrode active materials and negative electrode active materials under overcharge conditions, thereby reducing the large amount of heat generated in the battery due to unexpected reactions, thereby reducing the probability of shrinkage, melting or collapse of the diaphragm due to overheating, thereby improving the safety performance of the battery device using the electrolyte, which is reflected in the fact that the battery device has an improved ability to withstand a voltage exceeding its normal charging upper limit during charging, can tolerate overcharging during charging to a certain extent, and reduce safety risks caused by improper charging.

[0007] On the one hand, 2,4,6-tris(3,4,5-trifluorophenyl)boroxine (TTFPB) and lithium hexafluorophosphate (LiPF 6 ) interact with each other and can participate in the formation of CEI membrane together. When the feed amount of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane and lithium hexafluorophosphate meets the above requirements, the anion PF of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane and lithium hexafluorophosphate 6 -It can quickly decompose at low oxidation potential to form a LiF-rich inner surface and a LiB-rich outer surface. x O y / LiF CEI film. This structure of the CEI film has excellent mechanical strength, thermal stability and uniformity, which helps to reduce the internal temperature rise of the battery, improve the cycle stability of the positive electrode material, and reduce the electrochemical unexpected reaction under high voltage. On the other hand, fluoroethylene carbonate (FEC) in the electrolyte can form a stable, LiF-rich SEI film, and fluoroethylene carbonate has little effect on 2,4,6-tris (3,4,5-trifluorophenyl) cycloboroxane and lithium hexafluorophosphate. At the same time, under overcharge conditions, lithium is precipitated from the graphite negative electrode to form lithium dendrites. The SEI film formed by fluoroethylene carbonate can effectively inhibit the growth of lithium dendrites, reduce the risk of lithium dendrites penetrating the diaphragm to cause a short circuit, and also reduce unexpected reactions with the electrolyte, thereby reducing heat generation. Therefore, by introducing the above-mentioned additives and lithium salts into the electrolyte, it is possible to promote the construction of a structurally stable SEI film and CEI film in the battery device using the electrolyte, reduce the situation where the battery generates a lot of heat due to unexpected reactions, improve the overcharge resistance of the battery device, and enhance the safety and cycle stability of the battery device.

[0008] Preferably, the nickel-cobalt compound comprises lithium nickel-cobalt-aluminate LiNi x Co y Al 1-x-y O 2 , where x ≥ 0.8.

[0009] Preferably, the nickel-cobalt compound comprises LiNi 0.8 Co 0.15 Al 0.05 O 2 .

[0010] Preferably, the lithium salt further comprises at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium bis(oxalatoborate), lithium tetracyanoborate, lithium difluorobismalonateborate, lithium bis(phenylborate), and lithium disalicylatediborate.

[0011] Preferably, the content of lithium salt is 8-15wt% based on the total weight of the electrolyte. When the content of lithium salt in the electrolyte is within the above range, the chemical stability and ion conductivity of the electrolyte can be improved, thereby improving the cycle stability of the battery device using the electrolyte.

[0012] Preferably, the solvent includes at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and carbonates synthesized from a straight-chain or branched aliphatic alcohol with 3 to 8 carbon atoms and carbonic acid. Among them, ethylene carbonate, propylene carbonate, γ-butyrolactone, and butylene carbonate are cyclic carbonates, and dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and carbonates synthesized from a straight-chain or branched aliphatic alcohol with 3 to 8 carbon atoms and carbonic acid are chain carbonates. In other words, the solvent selected for the electrolyte includes cyclic carbonates and / or chain carbonates.

[0013] Preferably, the solvent includes ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC).

[0014] Preferably, the mass ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) is 2-2.5:5-5.5:2.5-3.

[0015] Preferably, the mass ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) is 2.3:5.1:2.6.

[0016] Preferably, the content of the solvent is 70 to 90 wt % based on the total mass of the electrolyte.

[0017] Preferably, the content of fluoroethylene carbonate is 0.2-9wt% based on the total mass of the electrolyte. Fluoroethylene carbonate within the above content range can promote the electrolyte to form a dense, flexible, low-impedance SEI film on the negative electrode surface without limiting the role of other components in the electrolyte, which is conducive to the rapid exchange reaction of charges on the electrode surface, improves the battery cycle performance, delays battery aging, and prolongs the battery cycle life.

[0018] Preferably, based on the total mass of the electrolyte, the content of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 2wt%, and the content of fluoroethylene carbonate is 7wt%. The lithium-ion battery has good overcharge resistance, can prevent battery failure and potential safety accidents, and also helps to maintain the health of the battery and maintain its long-term electrochemical performance, creating greater and more room for improvement in the development of the new energy field. Preferably, S1. After injecting the electrolyte into the lithium-ion battery, the injected lithium-ion battery is left to stand at 22 to 28°C for 20 to 28 hours, and then heated to I 1 The current size is constant current charging for 80 to 120 minutes, I 1≤0.1C, then let the lithium-ion battery stand at 50-70°C for 20-28 hours, and then let the lithium-ion battery stand at 22-28°C for 8-16 hours; S2. 2 The current size is constant current and constant voltage charging to the upper limit voltage U 1 , the cut-off current is 90~110mA, where 0.4C≤I 2 ≤0.6C, 4.0V≤U 1 ≤4.5V. The lithium-ion battery using the above electrolyte, using the above formation operation, can promote the electrolyte to form a CEI film and SEI film with good mechanical properties, good structural stability and good thermal stability. 1 The first charging process of the constant current charging adopts a lower current and is kept at high temperature; the purpose is to age the battery, promote the conversion of some organic components in the SEI film into inorganic components, and form a denser and more stable SEI film; at the same time, fluoroethylene carbonate (FEC) also participates in the film-forming reaction of the SEI film during this stage. 2 The second charging process of constant current and constant voltage charging is to charge the battery in constant current mode until the battery voltage reaches the upper limit voltage U 1 , and then switch to constant voltage mode until the charging current drops to the cut-off current to ensure that the battery is fully charged to ensure that 2,4,6-tris(3,4,5-trifluorophenyl)boroxine (TTFPB) and lithium hexafluorophosphate (LiPF6) form a structurally stable CEI film.

[0019] Preferably, after completing S2, the following operations are also included: S3. Releasing the lithium ion battery with I 3 The current size is constant current discharge to the lower limit voltage U 2 , 0.9C≤I 3 ≤1.1C, 2.0V≤U 2 ≤3.0 V. Through the discharge operation in S3, the lithium-ion battery is placed in a low voltage state, thereby reducing the side reactions of the lithium-ion battery and improving the storage stability of the lithium-ion battery.

[0020] Preferably, S3 also includes dividing the capacity of the lithium-ion batteries.

[0021] Preferably, S1 also includes: winding the positive electrode, the separator, and the negative electrode on a winding machine in sequence to obtain a winding core, placing the winding core in a cylindrical steel shell, and then injecting liquid into the cylindrical steel shell.

[0022] According to another aspect of the present invention, a battery pack is provided, comprising a plurality of batteries, wherein the batteries include the above-mentioned lithium-ion batteries. Since the voltage and stored energy of lithium-ion battery cells are limited, it is difficult to directly meet the needs of electrical appliances with higher voltage and higher power. Therefore, in most cases, lithium-ion batteries do not directly provide energy to electrical appliances by battery cells, but use a battery pack composed of several, dozens, hundreds or even thousands of single cells connected in series and parallel, etc., to obtain a sufficiently high voltage and energy to meet the power needs of different electrical appliances. According to the existing lithium-ion battery production process, the uniformity problem of lithium-ion batteries cannot be fundamentally solved, that is, there will be differences in capacity, internal resistance and other aspects among the single cells in the same battery pack. Therefore, when the battery pack is working, some lithium-ion batteries will be in abnormal working states such as overcharging and overdischarging. By applying the above-mentioned electrolyte, the overcharge resistance of lithium-ion batteries can be improved, thereby improving the safety performance of the battery pack, preventing thermal runaway, and even causing fire and explosion. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A schematic diagram of a 16A overcharge test curve of a battery pack composed of lithium-ion batteries provided in Example 1;

[0024] Figure 2 This is a schematic diagram of a 16A overcharge test curve of a battery pack composed of lithium-ion batteries provided in Comparative Example 1. DETAILED DESCRIPTION

[0025] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention and the drawings in the embodiments. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0026] Example 1

[0027] This embodiment provides a lithium-ion battery, including a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0028] (1) Electrolyte:

[0029] The composition of the electrolyte is shown in Table 1:

[0030] Table 1. Raw material composition for preparing electrolyte in Example 1

[0031]

[0032]

[0033] Note: The ratios in Table 1 are mass ratios, such as "ethylene carbonate: dimethyl carbonate: ethyl methyl carbonate = 2.3:5.1:2.6", that is, the mass ratio of ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate is 2.3:5.1:2.6, and they are mixed as a solvent.

[0034] Among them, the CAS number of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 223440-94-6. From the above table, it can be calculated that the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to lithium hexafluorophosphate is 1:5.

[0035] Prepare the materials according to the above raw material composition, and prepare the electrolyte according to the following steps: in a glove box filled with argon, first mix the solvent evenly, then add lithium salt and additives to the solvent, and mix evenly to obtain the electrolyte.

[0036] (2) Positive electrode:

[0037] The positive electrode includes a positive electrode current collector and a positive electrode active coating, wherein the positive electrode active coating includes a positive electrode active material lithium nickel cobalt aluminum oxide (LiNi 0.8 Co 0.15 Al 0.05 O 2 , NCA).

[0038] (3) Negative electrode:

[0039] The negative electrode includes a negative electrode current collector and a negative electrode active coating, and the negative electrode active coating includes negative electrode active materials such as graphite and silicon carbon.

[0040] (4) Diaphragm:

[0041] The diaphragm includes a substrate and an isolation coating. The substrate is a PE substrate and the isolation coating is an Al 2 O 3 Ceramic coating.

[0042] (5) Lithium-ion battery (cylindrical battery):

[0043] S1. Wind the positive electrode, separator and negative electrode on a winding machine in order to obtain a winding core, place the winding core in a cylindrical steel shell, inject the above electrolyte according to the electrolyte injection coefficient of 6.3g / 4Ah, vacuum seal, and let it stand at 25±3℃ for 24 hours. 1 The current size is constant current (CC) charging for 108 minutes, I 1 =0.1C (1C=4Ah). Next, the lithium ion battery was left at rest at 60°C for 24 hours, and then left at rest at 25±3°C for 12 hours.

[0044] S2. Then I 2 The current size is constant current and constant voltage charging to the upper limit voltage U 1 , I 2 =0.5C, U 1 =4.2V, cut-off current is 100mA.

[0045] S3. Then I 3 The current size is constant current discharge to the lower limit voltage U 2 ;I 3 =1C,U 2 = 2.5 V. The lithium-ion battery is divided into different capacities to obtain a lithium-ion battery.

[0046] Example 2

[0047] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that: in the process of preparing the electrolyte, the addition amount of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane, the solvent and the type of lithium salt are adjusted. Specifically, the content of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane is 0.5wt%. The lithium salt includes lithium hexafluorophosphate and lithium difluorooxalate borate, wherein the content of lithium hexafluorophosphate is 2.5wt%, and the content of lithium difluorooxalate borate is 7.5wt%. The remaining formula materials and preparation methods are strictly consistent with those in Example 1, especially the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane to lithium hexafluorophosphate is strictly consistent with that in Example 1.

[0048] Example 3

[0049] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that: in the process of preparing the electrolyte, the addition amount of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine, lithium salt and solvent is adjusted. Specifically, the content of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 3wt%, and the content of lithium hexafluorophosphate is 15wt%. The remaining formula materials and preparation method are strictly consistent with those in Example 1, especially the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to lithium hexafluorophosphate is strictly consistent with that in Example 1.

[0050] Example 4

[0051] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that: in the process of preparing the electrolyte, the type of lithium salt is adjusted so that the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl) cycloboroxane to lithium hexafluorophosphate is 1:2.6. Specifically, the lithium salt includes lithium hexafluorophosphate and lithium difluorooxalate borate, the content of lithium hexafluorophosphate is 5.2wt%, and the content of lithium difluorooxalate borate is 4.8wt%. The remaining formula materials and preparation method are strictly consistent with Example 1.

[0052] In particular, in practical applications, at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium tetracyanoborate, lithium difluorobismalonateborate, lithium bisphenylborate, and lithium disalicylatediborate can be used to replace the lithium difluorooxalatoborate in this embodiment.

[0053] Example 5

[0054] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that: in the process of preparing the electrolyte, the content of 2,4,6-tris (3,4,5-trifluorophenyl) boroxane, lithium salt and solvent is adjusted so that the mass ratio of 2,4,6-tris (3,4,5-trifluorophenyl) boroxane to lithium hexafluorophosphate is 1:26. Specifically, the content of 2,4,6-tris (3,4,5-trifluorophenyl) boroxane is 0.4wt%, and the content of lithium hexafluorophosphate is 10.4wt%. The remaining formula materials and preparation method are strictly consistent with Example 1.

[0055] Example 6

[0056] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that in the formation process of preparing the lithium ion battery, I 2 The size is adjusted to 0.3C, and the rest of the formula materials and preparation method are strictly consistent with Example 1.

[0057] Example 7

[0058] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that in the formation process of preparing the lithium ion battery, I 2 The size was adjusted to 0.7C, and the rest of the formula materials and preparation method were strictly consistent with those in Example 1.

[0059] Example 8

[0060] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that in the formation process of preparing the lithium ion battery, I 3 The size is adjusted to 0.8C, and the rest of the formula materials and preparation method are strictly consistent with Example 1.

[0061] Example 9

[0062] This embodiment refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this embodiment and Example 1 is that in the formation process of preparing the lithium ion battery, I 3 The size of is adjusted to 1.2C, and the rest of the formula materials and preparation method are strictly consistent with Example 1.

[0063] Comparative Example 1

[0064] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that in the preparation of the electrolyte, an equal mass of lithium salt is used to replace the 2,4,6-tris(3,4,5-trifluorophenyl)boroxine and fluoroethylene carbonate in Example 1, and the remaining formula materials and preparation method are strictly consistent with Example 1.

[0065] Comparative Example 2

[0066] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that in the preparation of the electrolyte, an equal mass of lithium difluorooxalatoborate is used instead of the lithium hexafluorophosphate in Example 1, and the remaining formula materials and preparation method are strictly consistent with Example 1.

[0067] Comparative Example 3

[0068] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that: in the process of preparing the electrolyte, the addition amount of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane, the solvent and the type of lithium salt are adjusted. Specifically, the content of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane is 0.1wt%. The lithium salt includes lithium hexafluorophosphate and lithium difluorooxalate borate, wherein the content of lithium hexafluorophosphate is 0.5wt%, and the content of lithium difluorooxalate borate is 9.5wt%. The remaining formula materials and preparation methods are strictly consistent with those in Example 1, especially the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane to lithium hexafluorophosphate is strictly consistent with that in Example 1.

[0069] Comparative Example 4

[0070] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that: in the process of preparing the electrolyte, the addition amount of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine, lithium salt and solvent is adjusted. Specifically, the content of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 3.5wt%, and the content of lithium hexafluorophosphate is 17.5wt%. The remaining formula materials and preparation method are strictly consistent with those in Example 1, especially the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to lithium hexafluorophosphate is strictly consistent with that in Example 1.

[0071] Comparative Example 5

[0072] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that: in the process of preparing the electrolyte, the type of lithium salt is adjusted so that the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxane to lithium hexafluorophosphate is 1:2. Specifically, the lithium salt includes lithium hexafluorophosphate and lithium difluorooxalate borate, the content of lithium hexafluorophosphate is 4wt%, and the content of lithium difluorooxalate borate is 6wt%. The rest of the formula materials and preparation method are strictly consistent with Example 1.

[0073] In particular, in practical applications, at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium bis(oxalatoborate), lithium tetracyanoborate, lithium difluorobismalonateborate, lithium bis(phenylborate), and lithium disalicylateborate can be used to replace the lithium difluorooxalatoborate in this comparative example.

[0074] Comparative Example 6

[0075] This comparative example refers to the preparation method provided in Example 1 to prepare a lithium ion battery. The difference between this comparative example and Example 1 is that: in the process of preparing the electrolyte, the content of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane, lithium salt and solvent is adjusted so that the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane to lithium hexafluorophosphate is 1:27. Specifically, the content of 2,4,6-tris(3,4,5-trifluorophenyl) boroxane is 0.5wt%, and the content of lithium hexafluorophosphate is 13.5wt%. The remaining formula materials and preparation method are strictly consistent with Example 1.

[0076] Test Example 1

[0077] Test objects: lithium-ion batteries provided in Examples 1 to 9 and Comparative Examples 1 to 6.

[0078] Test items and test methods:

[0079] (1) Overcharge resistance: A single cell is discharged at 0.8A to 2.5V, and then the cells are welded with connecting pieces (copper-tin alloy, thickness 0.25mm) to form a battery pack (5 series and 2 parallel, 5S2P). The positive and negative electrodes are connected to the power supply and charged at 16A current and 60V voltage. The charging lasts for 7 hours or the voltage stops increasing. Record whether the battery pack has thermal runaway and the highest temperature of the battery pack during the overcharge process.

[0080] (2) Battery cycle performance:

[0081] At room temperature, the battery was discharged to 2.5V at 0.8A current, left for 10 minutes, then charged to 4.2V at 6A constant current, switched to constant voltage charging, and stopped charging when the charging current dropped to 0.1A. After charging, it was left for 10 minutes. After another hour, it was discharged to 2.5V at 0.8A current, and the initial discharge capacity was calculated (point recording: charging for 30s, left for 60s after charging, discharging for 10s, and left for 120s after discharging).

[0082] Then do the following:

[0083] a) At room temperature, the battery is discharged at 0.8A to 2.5V, left for 10min, then charged at 4A to 4.2V, then switched to constant voltage charging, and stopped charging when the charging current drops to 0.1A. After charging, leave for 5min, then discharged at 4A to 2.5V or 75℃, and left for 10min;

[0084] b) Repeat step a for 500 cycles;

[0085] c) Record the discharge capacity, discharge energy and process temperature every week (recording points: charge for 30 seconds, stand for 60 seconds after charging, discharge for 10 seconds, stand for 120 seconds after discharging).

[0086] Test results: The test results are shown in Table 2. In addition, the 16A overcharge test curve diagram of the battery pack composed of lithium-ion batteries provided in Example 1 is shown in Figure 1 As shown, the schematic diagram of the 16A overcharge test curve of the battery pack composed of lithium-ion batteries provided in Comparative Example 1 is as follows Figure 2 shown.

[0087] Table 2. Test data obtained in this test case

[0088]

[0089] Result analysis:

[0090] By comparing the test performance of Examples 1 to 9 with Comparative Examples 1 to 6 in Table 1, it can be found that compared with Comparative Examples 1 to 6, the lithium ion batteries provided by Examples 1 to 9 not only have better overcharge resistance, are less prone to thermal runaway, but also have good cycle stability. Among them, the lithium ion battery provided by Example 1 has the best overcharge resistance and cycle capacity retention rate.

[0091] By comparing the comprehensive performance of the lithium-ion batteries provided in Examples 1 to 9 with those in Comparative Examples 1 to 2, it can be found that the lithium-ion batteries provided in Examples 1 to 9 have better overcharge resistance than the lithium-ion batteries in Comparative Examples 1 to 2. This is because 2,4,6-tris(3,4,5-trifluorophenyl)boroxane (TTFPB) and lithium hexafluorophosphate (LiPF 6 ) interact with each other to form a CEI film with excellent mechanical properties and thermal stability.

[0092] Comparing the lithium ion batteries provided in Examples 1 to 3 with those provided in Comparative Examples 3 to 4, it can be found that the batteries provided in Examples 1 to 3 are superior to the batteries provided in Comparative Examples 3 to 4 in terms of overcharge resistance and cycle stability. This indicates that when the content of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 0.2 to 3 wt%, the lithium ion battery has better overcharge resistance and cycle stability.

[0093] By comparing the lithium ion batteries provided in Examples 1, 4-5 and Comparative Examples 5-6, it can be found that as 2,4,6-tris(3,4,5-trifluorophenyl)boroxine (TTFPB) and lithium hexafluorophosphate (LiPF 6 ) increases, the overcharge resistance and cycle stability of lithium-ion batteries show a trend of first rising and then falling. Among them, when the mass ratio of 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to lithium hexafluorophosphate is 1:2.6-26, it is beneficial for lithium-ion batteries to form a CEI film with stable structure and good thermal stability, which helps to reduce the internal temperature rise of the battery.

[0094] Furthermore, by comparing the lithium ion batteries provided in Example 1 with those provided in Examples 6 to 9, it can be found that during the formation process, as I 2 As the current increases, the overcharge resistance of the lithium-ion battery shows a trend of first rising and then falling. Among them, the lithium-ion batteries provided by Examples 1 and 7-8 have better overcharge resistance, because when I2 is in the range of 0.4C-0.6C, it is conducive to the formation of a CEI film with good mechanical properties by 2,4,6-tris(3,4,5-trifluorophenyl) cycloboroxane (TTFPB) and lithium hexafluorophosphate (LiPF6), thereby reducing the large amount of heat generated in the battery due to side reactions and improving the overcharge resistance of the lithium-ion battery.

[0095] The above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the protection scope of the present invention. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the essence and scope of the technical solution of the present invention.

Claims

1. A lithium ion battery, characterized in that: It includes a positive electrode, a negative electrode and an electrolyte. The active material of the positive electrode includes a nickel-cobalt compound LiNi x Co y M 1-x-y O2, wherein x≥0.8, M includes at least one of Mn and Al, and the active material of the negative electrode includes at least one of graphite and silicon carbon, The electrolyte includes 2,4,6-tris(3,4,5-trifluorophenyl)boroxine, fluoroethylene carbonate, a lithium salt and a solvent, wherein the lithium salt includes lithium hexafluorophosphate; Based on the total mass of the electrolyte, the content of the 2,4,6-tris(3,4,5-trifluorophenyl)boroxine is 0.2-3wt%, and the mass ratio of the 2,4,6-tris(3,4,5-trifluorophenyl)boroxine to the lithium hexafluorophosphate is 1:2.6-26.

2. The lithium-ion battery according to claim 1, characterized in that: The lithium salt further comprises at least one of lithium bis(trifluoromethylsulfonyl)imide, lithium bis(fluorosulfonyl)imide, lithium difluorooxalatoborate, lithium bis(oxalatoborate), lithium tetracyanoborate, lithium difluorobismalonateborate, lithium bis(phenylborate), and lithium disalicylatediborate.

3. The lithium ion battery according to claim 1, characterized in that: Based on the total mass of the electrolyte, the content of the lithium salt is 8-15wt%.

4. The lithium-ion battery according to claim 1, wherein: The solvent includes at least one of ethylene carbonate, propylene carbonate, γ-butyrolactone, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, and carbonates synthesized from linear or branched fatty monoalcohols with 3 to 8 carbon atoms and carbonic acid.

5. The lithium ion battery according to claim 4, characterized in that: The solvents include ethylene carbonate, dimethyl carbonate and ethyl methyl carbonate.

6. The lithium ion battery according to claim 4, characterized in that: Based on the total mass of the electrolyte, the content of the solvent is 70-90wt%.

7. The lithium ion battery according to claim 1, characterized in that: The content of the fluoroethylene carbonate is 0.2-9wt%.

8. The lithium ion battery according to any one of claims 1 to 7, characterized in that: The formation process of the lithium-ion battery includes the following operations: S1. After injecting the electrolyte into the lithium ion battery, the lithium ion battery after injection is allowed to stand at 22 to 28 ° C for 20 to 28 hours, and then charged at a constant current of I1 for 80 to 120 minutes, I1 ≤ 0.1C, and then the lithium ion battery is allowed to stand at 50 to 70 ° C for 20 to 28 hours, and then the lithium ion battery is allowed to stand at 22 to 28 ° C for 8 to 16 hours; S2. Charge the lithium-ion battery at a constant current and constant voltage of I2 to an upper limit voltage U1, with a cut-off current of 90-110 mA, wherein 0.4C≤I2≤0.6C, and 4.0V≤U1≤4.5V.

9. The lithium ion battery according to claim 8, characterized in that: After completing S2, the following operations are also included: S3. Discharge the lithium-ion battery at a constant current of I3 to a lower limit voltage U2, 0.9C≤I3≤1.1C, 2.0V≤U2≤3.0V.

10. A battery pack, characterized in that: The invention comprises a plurality of batteries, wherein the batteries include the lithium-ion battery according to any one of claims 1 to 9.

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