Electrolyte and lithium ion battery

Abstract

The application relates to the technical field of electrolyte, in particular to an electrolyte and a lithium ion battery. The electrolyte comprises an organic solvent, a lithium salt, a diluent and a cosolvent. The organic solvent comprises a carbonate solvent, the lithium salt is dissolved in the organic solvent, the diluent does not participate in the solvation reaction of lithium ions in the organic solvent and has flame retardance; the cosolvent is an aromatic compound, and the polarity of the cosolvent is between that of the diluent and that of the organic solvent. The mass fractions of the carbonate solvent, the diluent and the cosolvent in the electrolyte are w1, w2 and w3 respectively, and 30% <= w1 <= 65%, 15% <= w2 <= 50% and 5% <= w3 <= 30%. The electrolyte in the application can balance the fast-charging performance and the thermal safety performance of the lithium ion battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrolyte, in particular to an electrolyte and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high specific energy, high battery voltage, wide working temperature range, long storage life, etc., and have become one of the most popular charging technologies at present, and are widely used in electric vehicles, aerospace and portable devices, etc.

[0003] With the continuous expansion of application scenarios, lithium ion batteries are not only required to work normally in normal and high temperature environments, but also need to have good fast charging capacity, so an electrolyte with good fast charging capacity is needed. In order to improve the fast charging capacity of the battery, the electrolyte needs to maintain high ionic conductivity and low interfacial impedance, so low-viscosity linear carbonate solvents are usually added, and high-impedance positive and negative film-forming additives are minimized or avoided to avoid increasing the impedance of lithium transmission at the electrode / electrolyte interface. However, such fast charging electrolyte will have the problem of poor thermal safety performance. SUMMARY

[0004] In view of the above problems, the present application provides an electrolyte and a lithium ion battery. By adding a suitable low-viscosity cosolvent to the electrolyte in a solvent system using a thermally stable diluent, the fast charging performance of the lithium ion battery can be improved, while its thermal safety performance is also considered.

[0005] One aspect of the present application provides an electrolyte, comprising an organic solvent, a lithium salt, a diluent and a cosolvent. The organic solvent comprises a carbonate solvent, and the mass fraction of the carbonate solvent in the electrolyte is w1, 30%≤w1≤65%; the lithium salt is dissolved in the organic solvent; the diluent does not participate in the solvation reaction of lithium ions in the organic solvent, and has flame retardancy, and the mass fraction of the diluent in the electrolyte is w2, 15%≤w2≤50%; the cosolvent is an aromatic compound, and its polarity is between that of the diluent and the organic solvent, and the mass fraction of the cosolvent in the electrolyte is w3, 5%≤w3≤30%.

[0006] Optionally, the mass ratio of the cosolvent to the diluent is n, n=w3 / w2, n satisfies the following relationship: 1 / 4≤n≤2.

[0007] Optionally, the diluent is a C3-C8 perfluoro or polyfluoro-substituted ether diluent.

[0008] Optionally, the diluent is any one or a combination of at least two of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether, perfluorobutyl ethyl ether; and the co-solvent is any one or a combination of at least two of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, methylbenzene, ethylbenzene, trifluoromethylbenzene.

[0009] Optionally, the carbonate-based solvent is composed of a cyclic carbonate-based solvent and a linear carbonate-based solvent, and the mass fraction of the cyclic carbonate-based solvent in the carbonate-based solvent is 5%-20%.

[0010] Optionally, the cyclic carbonate-based solvent is ethylene carbonate and / or propylene carbonate.

[0011] Optionally, the lithium salt includes any one or a combination of at least two of lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, and the mass fraction of the lithium salt in the electrolyte is 10%-20%.

[0012] Optionally, the electrolyte includes an additive, and the additive includes a cyclic carbonate additive and a lithium salt additive, wherein the cyclic carbonate additive includes any one or a combination of at least two of vinylene carbonate, fluorinated vinylene carbonate, vinyl ethylene carbonate; the lithium salt additive includes any one or a combination of at least two of lithium difluorophosphate, lithium bisoxalate borate, lithium tetrafluoroborate, lithium difluorobisoxalate borate, lithium difluorobisoxalate phosphate; and the mass fraction of the additive in the electrolyte is 0.2%-5.5%.

[0013] Optionally, the lithium salt additive is one or both of lithium difluorobisoxalate borate and lithium difluorobisoxalate phosphate, and the mass fraction of the lithium difluorobisoxalate borate and / or lithium difluorobisoxalate phosphate in the electrolyte is 0.1%-0.8%.

[0014] Optionally, the electrolyte includes an additive, and the additive includes a cyclic carbonate additive and a methanedisulfonate. The cyclic carbonate additive includes any one or a combination of at least two of vinylene carbonate, fluorinated vinylene carbonate, vinyl ethylene carbonate, and the mass fraction of the methanedisulfonate in the electrolyte is 0.1%-0.8%.

[0015] Another aspect of the present application provides a lithium ion battery, which includes a positive electrode sheet, a negative electrode sheet, and the above-mentioned electrolyte.

[0016] Optionally, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material, and the positive electrode active material includes any one or a combination of at least two of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, and lithium nickel cobalt aluminum oxide.

[0017] Optionally, the negative electrode tab includes a negative current collector and a negative active material, wherein the negative active material includes any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide compounds, silicon carbon compounds, and lithium titanate.

[0018] The fast-charging electrolyte provided in some examples of the present application uses a low-viscosity diluent substituted with multiple fluorines or all fluorines, and reduces the amount of high-melting-point, high-viscosity cyclic carbonate solvents in the electrolyte organic solvent, so that the electrolyte forms a local high-concentration system, while ensuring the performance of the electrolyte, effectively reduces the viscosity of the electrolyte, and enables better diffusion and transmission of lithium ions in the electrolyte. Moreover, the diluent itself has a stable electrochemical window, effectively improving the stability of the electrolyte on the positive and negative electrode sides.

[0019] The diluent provided in some examples of the present application does not participate in the solvation reaction of lithium ions in the electrolyte. By adding the above diluent to the electrolyte while reducing the content of cyclic carbonate solvents with strong binding capacity, the diluent with weak force is dispersed in the anion and cation cluster in the solvation structure of lithium ions, which can accelerate the diffusion and transmission process of lithium ions in the electrolyte, thereby improving the fast-charging capacity of the lithium ion battery, and ensuring that the battery still has a lower impedance and a higher capacity retention rate under high-rate charging conditions.

[0020] In some examples of the present application, the diluent can self-extinguish when the battery cell of the lithium ion battery is in thermal runaway, so when such a diluent is added to the electrolyte, it can inhibit violent reactions when the lithium ion battery is in thermal runaway, thereby significantly improving the thermal runaway temperature of the lithium ion battery, and thereby significantly improving its thermal safety performance. On the other hand, since the electrolyte is added with a cosolvent having a polarity between the diluent and the organic solvent and a slightly strong intermolecular force, the cosolvent can also assist in dispersing anion and cation clusters to form easily migratory anion and cation pairs, so that the lithium ion battery has excellent thermal safety stability while still having good fast-charging capacity. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only some of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0022] Unless otherwise stated or contradictory, the terms or phrases used in this document have the following meanings:

[0023] Solid Electrolyte Interface (SEI): During the charging and discharging process of lithium ion battery, the electrode material and electrolyte react at the solid-liquid interface to form a passivation layer covering the surface of the electrode material.

[0024] Herein, in relation to a numerical range, if not specifically stated, the distribution of the optional numerical values in the numerical range is considered to be continuous, and includes both numerical end points of the numerical range (i.e. the minimum value and the maximum value), and every numerical value between the two numerical end points. When multiple numerical ranges are provided to describe a feature or a characteristic, these numerical ranges can be combined.

[0025] In the process of creating the present application, the inventors found that in the prior art, the linear carbonate-based solvent has poor chemical and electrochemical stability at high temperature with some electrode materials and lithium salts (especially lithium hexafluorophosphate), and further reacts with the by-products generated by the components that decompose at high temperature with lithium salts, resulting in reduction gas. The components of the cyclic carbonate-based solvent that help the dissociation of lithium salts are easily oxidized to generate carbon dioxide and other gases on the surface of the positive electrode (especially high-nickel positive electrode), accelerating the capacity decay of the battery. On the other hand, the amount of high-temperature additives added in the electrolyte is too small, which also affects the stability of the solid electrolyte interface film in a high-temperature environment. The above factors together cause the problem of poor thermal safety performance of the fast-charging electrolyte in the prior art.

[0026] The first aspect of the present application provides an electrolyte, comprising an organic solvent, a diluent, a cosolvent and a lithium salt. The embodiments of the present application select a carbonate-based solvent as the organic solvent of the electrolyte, which comprises a cyclic carbonate-based solvent and a linear carbonate-based solvent, specifically comprising any one or a combination of at least two of vinyl carbonate, propylene carbonate, dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, and the mass fraction of the carbonate-based solvent in the electrolyte is w1, 30%≤w1≤65%.

[0027] The diluent is a type of diluent that can be used in a local high-concentration electrolyte system, specifically a diluent with certain flame retardancy, which can improve the self-extinguishability of the lithium ion battery. Preferably, the diluent is a C3-C8 perfluoro or polyfluoro-substituted ether diluent. More preferably, the diluent comprises any one or a combination of at least two of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether and perfluorobutyl ethyl ether.

[0028] Since the diluent has small polarity and weak intermolecular force, it does not participate in the solvation reaction of lithium ions in the electrolyte. At the same time, due to the existence of hydrogen bond and other weak intermolecular forces between the diluent and lithium ions, the coordination between lithium ions and solvents is weakened, thereby accelerating the desolvation process of lithium ions. By adding the above diluent to the electrolyte and reducing the content of the cyclic carbonate solvent with strong binding capacity, the weak force diluent is dispersed in the solvation structure of lithium ions, which can accelerate the diffusion and transmission process of lithium ions in the electrolyte, thereby improving the fast charging capacity of the lithium ion battery, ensuring that the battery still has low impedance and high capacity retention rate under high-rate charging conditions.

[0029] The above diluent contains more fluorine elements, and fluorine elements can self-extinguish when the battery cell of the lithium ion battery is in thermal runaway. Therefore, when the electrolyte is added with such a diluent, the violent reaction can be inhibited in the thermal runaway test of the lithium ion battery, thereby significantly improving the thermal runaway temperature of the lithium ion battery, and thereby significantly improving the thermal safety performance thereof.

[0030] The mass fraction of the diluent in the electrolyte is w2. In some embodiments of the present application, when the amount of the diluent added to the electrolyte is too small, it has a certain effect on optimizing the direct current impedance (DCR) of the lithium ion battery secondary cell, but the self-extinguishing function of the electrolyte is obviously reduced, thereby causing the thermal runaway temperature to decrease significantly. If too much diluent is added to the electrolyte, it will seriously cause the DCR to increase, thereby causing the fast charging capacity of the lithium ion battery to decrease. Therefore, by setting the mass fraction w2 of the diluent within a certain range, the fast charging and thermal safety performance of the lithium ion battery can be considered at the same time, so as to reach a relatively optimal level. In some embodiments of the present application, 15%≤w2≤50%, preferably 20%≤w2≤40%, more preferably 25%≤w2≤35%.

[0031] Since the polarity difference between the diluent and the organic solvent in some embodiments of the present application is large, they are difficult to be stably dispersed in the electrolyte. To solve this problem, in some examples of the present application, a polar solvent with a polarity between the diluent and the organic solvent is added to the electrolyte. Such a cosolvent can also help disperse the anion-cation cluster to form an anion-cation pair that is easy to migrate.

[0032] In some embodiments of the present application, the cosolvent can be selected from aromatic compounds, for example, can include any one or a combination of at least two of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, toluene, ethylbenzene, trifluoromethylbenzene, since such cosolvents have moderate polarity and low viscosity, which is helpful for the diffusion of lithium ions, so when such cosolvents are added to the electrolyte containing the above-mentioned diluent, it can help to form an anion-cation pair that is easy to migrate, thereby improving the overall kinetic reaction speed of the lithium ion battery.

[0033] The mass fraction of the cosolvent in the electrolyte is w3, in some embodiments of the present application, when the amount of cosolvent added to the electrolyte is too small, it will cause the DCR of the lithium ion battery to rise, resulting in a decrease in its fast-charging ability; while adding too much cosolvent will cause the mass fraction of the diluent in the electrolyte to decrease, thereby reducing the thermal safety performance of the lithium ion battery. By setting the mass fraction w3 of the cosolvent within a specified range, the fast-charging performance and thermal safety performance of the lithium ion battery can be simultaneously optimized. In some embodiments of the present application, 5%≤w3≤30%. Preferably, 7%≤w3≤15%; more preferably, 9%≤w3≤12%.

[0034] The mass ratio of the cosolvent to the diluent is denoted as n, i.e. n = w3 / w2, in order to make the lithium ion battery have strong fast-charging performance while taking into account its thermal safety performance, n satisfies the following relationship: 1 / 4≤n≤2. More preferably, 0.27≤n≤ / 0.4, or 0.3≤n≤ / 0.35.

[0035] The carbonate-based solvent is composed of a cyclic carbonate-based solvent and a linear carbonate-based solvent. The cyclic carbonate-based solvent has high dielectric constant, high ionic conductivity, and can form a stable solid electrolyte interface film (SEI film) on the negative electrode surface, but has high viscosity, and is commonly used as an organic solvent in electrolyte.

[0036] In the embodiments of the present application, any one or a combination of at least two of ethylene carbonate and propylene carbonate can be selected as the cyclic carbonate-based solvent, and any one or a combination of at least two of dimethyl carbonate, methyl ethyl carbonate, and diethyl carbonate can be selected as the linear carbonate-based solvent.

[0037] If the proportion of the cyclic carbonate solvent in the carbonate solvent is too low, the dissociation ability of the electrolyte to the lithium salt will be insufficient, which will increase the DCR of the lithium ion battery, thereby reducing its fast charging performance. Due to the insufficient stability of the cyclic organic solvent on the positive electrode surface, if the proportion of the cyclic carbonate solvent in the carbonate solvent is too high, the cyclic carbonate solvent will decompose and produce gas on the positive electrode surface, which will destroy the interface between the electrode and the electrolyte, and finally slightly reduce the thermal safety performance of the lithium ion battery. By comparing the test results of different groups of examples and comparative examples, it is shown that if the mass fraction of the cyclic carbonate solvent in the carbonate solvent is 5%-20%, and the remaining 80%-95% is a linear carbonate solvent, the DCR of the electrolyte and the interface between the electrode and the electrolyte can remain relatively stable, thereby ensuring the fast charging performance and thermal safety performance of the lithium ion battery.

[0038] The lithium salt in the electrolyte can release a large number of active lithium ions after dissolving in the organic solvent, so that the electrolyte has good electrical conductivity. The lithium salt can be selected according to the conventional types in the art. In the currently marketable electrolyte with high selectivity, the lithium salt contained therein generally has low dissociation energy and high solubility. The low dissociation energy ensures that the electrolyte formed after the lithium salt is dissolved has high electrical conductivity, thereby realizing the high rate performance of the battery. The high solubility ensures that there are enough lithium ions in the electrolyte for transmission. At the same time, the lithium salt should have good stability, so that the lithium ion battery will not react with other components when working at high voltage and high temperature. And if the lithium salt has good SEI film forming performance, it can ensure that the electrolyte will not be continuously consumed in the subsequent cycle process. In the examples of the present application, the lithium salt includes any one or a combination of at least two of lithium bisfluorosulfonylimide, lithium bis-trifluoromethylsulfonylimide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, and lithium hexafluoroarsenate. The mass fraction of the lithium salt in the electrolyte can be selected according to the conventional proportion in the art. In the present application, the mass fraction of the lithium salt in the electrolyte is 10%-20%.

[0039] In the embodiment of the present application, in order to further improve the fast charging ability and thermal safety performance of the lithium ion battery, the electrolyte comprises an additive, the additive comprises a cyclic carbonate additive and a lithium salt additive, the cyclic carbonate additive can promote the formation of SEI film and effectively prevent the further decomposition of the electrolyte, and can also improve the low temperature performance of the electrolyte; the lithium salt additive generally has high electrochemical stability, can improve the conductivity of the non-aqueous electrolyte solution, can not only improve the low temperature output characteristics of the lithium ion battery, but also can inhibit the decomposition of the positive electrode surface and prevent the oxidation reaction of the electrolyte solution during high temperature cycling, thereby improving the output characteristics and swelling characteristics after high temperature storage. Among them, the cyclic carbonate additive comprises any one or a combination of at least two of vinylene carbonate, fluoroethylene carbonate, and vinyl ethylene carbonate, and is preferably fluoroethylene carbonate; the lithium salt additive comprises any one or a combination of at least two of lithium difluorophosphate, lithium bisoxalate borate, lithium tetrafluoroborate, lithium difluoro oxalate borate, and lithium difluoro oxalate phosphate; the mass fraction of the additive in the electrolyte can be set according to the conventional proportion of the additive in the electrolyte in the art, which is not limited herein, and in the embodiment of the present application, the mass fraction of the additive in the electrolyte is 0.2%-5.5%. Preferably, the mass fraction of the cyclic carbonate additive is 4.5%-5.5%, and the mass fraction of the lithium salt additive is 0.1%-0.8%.

[0040] It is found through the performance test results of the lithium ion battery in some embodiments of the present application that when the lithium salt additive is added as part of the additive to the electrolyte, the lithium salt additive is preferably one or both of lithium difluoro oxalate borate and lithium difluoro oxalate phosphate, and when the mass fraction of lithium difluoro oxalate borate and / or lithium difluoro oxalate phosphate in the electrolyte is 0.1%-0.8%, the fast charging cycle ability and thermal runaway stability of the lithium ion battery will be more obviously improved. This is because lithium difluoro oxalate borate and lithium difluoro oxalate phosphate can form a SEI film with good quality on the positive electrode side, which can be beneficial to the improvement of the cycle stability and thermal safety performance of the lithium ion battery.

[0041] In some embodiments of the present application, the additive in the electrolyte includes a cyclic carbonate additive and a methylene methanedisulfonate. The cyclic carbonate additive includes any one of vinylene carbonate, fluoroethylene carbonate, vinyl ethylene carbonate or a combination of at least two thereof, and preferably, the mass fraction of the cyclic carbonate additive is 4.5%-5.5%. The mass fraction of the methylene methanedisulfonate in the electrolyte is 0.1%-0.8%, and preferably 0.1%-0.3%. The above embodiments show good fast charging cycle performance and thermal safety performance in the performance test of the lithium ion battery. This is because after the methylene methanedisulfonate is added as an additive to the electrolyte, on the one hand, the lithium ion battery will first generate a uniform and dense SEI film on the positive electrode surface during formation and charging, and the SEI film as an interface layer has the characteristics of a solid electrolyte, is an electronic insulator but is a good conductor of lithium ions, and lithium ions can freely insert and extract through the SEI film; on the other hand, the electrolyte containing the above diluent will generate an SEI film mainly composed of inorganic components on the negative electrode side, and can inhibit the reaction of the active lithium in the negative electrode, the SEI film and the electrolyte at high temperature, improve the high-temperature storage performance of the lithium ion battery, and maintain the stability of the cycle.

[0042] The second aspect of the present application also provides a lithium ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator and the electrolyte in the above embodiments. During the charging and discharging of the battery, lithium ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet, the separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation, and the electrolyte plays a role of ion conduction between the positive electrode sheet and the negative electrode sheet.

[0043] The positive electrode sheet of the lithium ion battery of the embodiments of the present application includes a positive electrode current collector and a positive electrode active material. The positive electrode current collector is usually made of a material with good electrical conductivity and mechanical strength, such as a copper foil, which is not limited herein. The positive electrode active material, as a lithium source in the lithium ion battery, needs to provide a high electrode potential for the lithium ion battery and maintain a stable voltage platform, and at the same time has a high ion and electron conductivity. The positive electrode active material can be selected according to the conventional types in the art, including any one of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide or a combination of at least two thereof. In the lithium ion battery of the present application, the positive electrode sheet is selected as a lithium nickel cobalt manganese oxide positive electrode sheet.

[0044] The negative electrode sheet of the lithium ion battery comprises a negative electrode current collector and a negative electrode active material. The negative electrode current collector can be made of a material with good electrical conductivity and mechanical strength, such as a copper foil, without limitation. In the field of lithium ion battery research, the negative electrode active material needs to maintain a stable voltage platform and have a relatively stable structure during the charging and discharging reaction process. The negative electrode active material can be selected according to the conventional types in the art, including any one or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, silicon oxide compounds, silicon carbon compounds, and lithium titanate, which can be selected by a person skilled in the art according to actual needs. In the lithium ion battery of the embodiment of the present application, the negative electrode sheet is selected to be a graphite-silicon composite negative electrode sheet.

[0045] The separator film is selected from conventional types in the art, for example, a PE or PP porous film can be selected as the separator film, the thickness of the separator film is 9 to 18 μm, the air permeability is 180 s / 100 mL to 380 s / 100 mL, and the porosity is 30% to 50%.

[0046] The technical solutions of the present application are described in detail below through specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art, and the instruments used in the examples are commercially available.

[0047] The electrolyte compositions of the examples and comparative examples in different groups are specifically referred to Table 1

[0048] Example 1

[0049] In an argon atmosphere glove box with a water content of <10 ppm, battery-grade ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) were mixed in a mass ratio of 1:1:8 to form an organic solvent. The lithium salt selected for use in the electrolyte in Table 1 is lithium hexafluorophosphate (LiPF6), which has a concentration of 1 mol / L in the electrolyte; the diluent selected is ethyl nonafluorobutyl ether, which has a mass fraction w2 of 30% in the electrolyte; the cosolvent selected is trifluoromethylbenzene, which has a mass fraction w3 of 10% in the electrolyte; n is 1 / 3; and the additive is fluoroethylene carbonate (FEC), which has a mass fraction of 5% in the electrolyte. The above components were combined to obtain the electrolyte. In Table 1, the content of each component other than the solvent is the weight percentage calculated based on the total weight of the electrolyte. The electrolytes in other examples are prepared in the same manner as in this example, except that the component proportions are specified in the table.

[0050] Example 2

[0051] The difference between this example and Example 1 is that perfluorobutyl methyl ether is selected as the diluent in the electrolyte, and the rest is the same as Example 1.

[0052] Example 3

[0053] This example is different from Example 1 in that w2 is 20%, n is 1 / 2, and the rest is consistent with Example 1.

[0054] Example 4

[0055] This example is different from Example 1 in that w2 is 15%, n is 2 / 3, and the rest is consistent with Example 1.

[0056] Example 5

[0057] This example is different from Example 1 in that w2 is 15%, w3 is 30%, n is 2, and the rest is consistent with Example 1.

[0058] Example 6

[0059] This example is different from Example 1 in that w2 is 20%, w3 is 5%, n is 1 / 4, and the rest is consistent with Example 1.

[0060] Example 7

[0061] This example is different from Example 1 in that the mass ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) in the organic solvent is 3:1:6, and the rest is consistent with Example 1.

[0062] Example 8

[0063] This example is different from Example 1 in that the mass ratio of ethylene carbonate (EC), dimethyl carbonate (DMC) and methyl ethyl carbonate (EMC) in the organic solvent is 0.3:1:8.7, and the rest is consistent with Example 1.

[0064] Example 9

[0065] This example is different from Example 1 in that, in addition to including fluoroethylene carbonate (FEC) in the additive, methanedisulfonate methylene (MMDS) is additionally added, and the mass fraction of MMDS based on the total mass of the electrolyte is 0.2%, and the rest is consistent with Example 1.

[0066] Example 10

[0067] This example is different from Example 1 in that, in addition to including fluoroethylene carbonate (FEC) in the additive, MMDS is additionally added, and the mass fraction of MMDS based on the total mass of the electrolyte is 0.5%, and the rest is consistent with Example 1.

[0068] Example 11

[0069] The difference between this example and Example 1 is that, in addition to including fluoroethylene carbonate (FEC) in the additive, lithium difluoro(oxalato)borate (LiDFOB) is additionally added, the mass fraction of LiDFOB being 0.5% based on the total mass of the electrolyte, and the rest being consistent with Example 1.

[0070] Example 12

[0071] The difference between this example and Example 1 is that, in addition to including fluoroethylene carbonate (FEC) in the additive, lithium difluoro(oxalato)borate (LiDFOB) is additionally added, the mass fraction of LiDFOB being 0.5% based on the total mass of the electrolyte, and the rest being consistent with Example 1.

[0072] Example 13

[0073] The difference between this example and Example 1 is that, in addition to including fluoroethylene carbonate (FEC) in the additive, lithium difluoro(oxalato)borate (LiDFOB) is additionally added, the mass fraction of LiDFOB being 0.5% based on the total mass of the electrolyte, and the rest being consistent with Example 1.

[0074] Comparative Example 1

[0075] The difference between this example and Example 1 is that w2 is 60%, n is 1 / 6, and the rest is consistent with Example 1.

[0076] Comparative Example 2

[0077] The difference between this example and Example 1 is that w2 is 10%, n is 1, and the rest is consistent with Example 1.

[0078] Comparative Example 3

[0079] The difference between this example and Example 1 is that no diluent is added to the electrolyte, n>2, and the rest is consistent with Example 1.

[0080] Comparative Example 4

[0081] The difference between this example and Example 1 is that w2 is 20%, w3 is 50%, n is 2.5, and the rest is consistent with Example 1.

[0082] Comparative Example 5

[0083] The difference between this example and Example 1 is that w2 is 15%, no cosolvent is added to the electrolyte, and the rest is consistent with Example 1.

[0084] Table 1. Electrolyte composition of different groups of examples and comparative examples

[0085]

[0086]

[0087] The electrolyte of Examples 1 to 13 and Comparative Examples 1 to 5 is respectively used in the preparation of lithium ion batteries.

[0088] In the embodiments of the present application, the electrochemical device is a lithium ion battery, which is a primary lithium ion battery or a secondary lithium ion battery, comprising a positive electrode sheet, a negative electrode sheet, a separator between the positive electrode and the negative electrode, and an electrolyte. The preparation method of the secondary lithium ion battery of some embodiments of the present application is as follows:

[0089] (1) Preparation of lithium nickel cobalt manganese oxide positive electrode sheet

[0090] The positive electrode active material (LiNi 0.8 Co 0.1 Mn 0.1 O2), polyvinylidene fluoride as a binder, and Super P as a conductive agent are mixed in a weight ratio of 98:1:1, and then N-methyl pyrrolidone (NMP) is added to the mixture. The mixture is stirred by a vacuum stirrer until it becomes a uniform transparent mixture, i.e. a positive electrode slurry is obtained; the positive electrode slurry is uniformly coated on an aluminum foil; the aluminum foil is transferred to an oven for drying after being air-dried at room temperature, and then cold-pressed, cut to obtain a positive electrode sheet.

[0091] (2) Preparation of graphite-silicon composite negative electrode sheet:

[0092] The mixture of artificial graphite and silicon-carbon composite obtained by mixing them in a mass ratio of 9:1 is used as the negative electrode active material, Super P is used as the conductive agent, carboxymethyl cellulose sodium (CMC-Na) is used as the thickening agent, and styrene-butadiene rubber (SBR) is used as the binder; the above-mentioned negative electrode active material, conductive agent, thickening agent, and binder are mixed in a mass ratio of 96:1:1:2, deionized water is added, and the mixture is stirred uniformly by a vacuum stirrer to obtain a negative electrode slurry; the negative electrode slurry is uniformly coated on a negative electrode current collector copper foil; the copper foil is transferred to an oven for drying after being air-dried at room temperature, and finally cold-pressed, cut to obtain a negative electrode sheet.

[0093] (3) Preparation of electrolyte

[0094] In an argon atmosphere glove box with water content <10 ppm, battery grade ethylene carbonate (EC), dimethyl carbonate (DMC), and methyl ethyl carbonate (EMC) were mixed according to the mass ratio shown in Table 1 to form an organic solvent. The lithium salt selected for the electrolyte in Examples 1-13 and Comparative Examples 1-5 of the present application was lithium hexafluorophosphate, and the lithium salt concentration was 1 mol / L. The types and mass fractions of the diluent, cosolvent, and additive based on the total weight of the electrolyte were set as described above in Table 1 for Examples 1-13 and Comparative Examples 1-5. After mixing the above components, the electrolyte was obtained. In Table 1, the content of each component other than the organic solvent was the mass percentage calculated based on the total mass of the electrolyte; the electrolytes in different examples and comparative examples were prepared in the same way except that the component ratio was specified in the table.

[0095] (4) Preparation of the separator film

[0096] A 12 μm polypropylene film (PP) was selected as the separator film.

[0097] (5) Preparation of the secondary lithium ion battery

[0098] The above-prepared positive electrode sheet, separator film, and negative electrode sheet were sequentially laminated with the separator film between the positive and negative electrode sheets to play a separating role. Then, the aluminum plastic film was wrapped, and the battery was transferred to a vacuum oven for drying at 120°C. After 3.0 g / Ah of the above-prepared electrolyte was injected, the battery was sealed, and the electrolyte was formed. Finally, a soft-pack battery (i.e., lithium ion battery) with a capacity of 1 Ah was prepared.

[0099] The lithium ion batteries assembled with the electrolytes in Examples 1-13 and Comparative Examples 1-5 were tested for performance, and the test results are shown in Table 2. The test methods are as follows:

[0100] (1) DC resistance (DCR) test of the secondary lithium ion battery

[0101] At a specified temperature, when the secondary lithium ion battery was discharged at a 1C current to 50% SOC (state of charge, reflecting the remaining capacity of the battery), the current was increased to 4C and maintained for 30 s. The difference between the updated stable voltage and the original platform voltage was detected, and the ratio of the value to the 4C current value was the DC resistance of the battery. The DCR test result after the first full charge of the battery was the initial DCR of the battery.

[0102] (2) Cycle test of the secondary lithium ion battery

[0103] The lithium ion battery was charged at a current of 3C, discharged at a current of 1C, and cycled in a specified potential range in an oven at a specified temperature (room temperature 25°C or high temperature 45°C), and the discharge capacity of each cycle was recorded. When the battery capacity reached 80% of the first cycle capacity, the test was ended, and the number of battery cycles was recorded.

[0104] (3) Thermal oven test of secondary lithium ion battery

[0105] The secondary lithium ion battery was fully charged to 4.25V and subjected to a thermal oven test (130°C, 30min, followed by gradual temperature increase until thermal runaway).

[0106] For lithium nickel cobalt manganese oxide / silicon-graphite batteries, the charge and discharge cut-off voltage was 2.5-4.25V.

[0107] Table 2. Performance test results of example and comparative example lithium ion batteries in different groups

[0108]

[0109] From the above data, the following conclusions can be drawn:

[0110] As can be seen from Comparative Examples 1-2 and Comparative Example 3, the addition of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether and other diluents to the electrolyte can significantly increase the temperature of thermal runaway, because these diluents contain more fluorine elements, which can inhibit the intense heat release of the reaction during the thermal runaway test.

[0111] As can be seen from Comparative Examples 1, 3, 4 and Comparative Examples 1-2, when too little diluent is added to the electrolyte, the DCR is optimized to some extent, but the self-extinguishing function of the electrolyte is reduced, and the thermal runaway temperature of the lithium ion battery is significantly reduced. When too much diluent is added, the DCR is severely deteriorated, resulting in a decrease in the fast charging capacity of the lithium ion battery. Therefore, the amount of diluent should be controlled within a specified range to balance the fast charging and thermal safety performance of the lithium ion battery. Within the optimal dosage range, increasing the amount of diluent will slightly deteriorate the DCR and fast charging performance of the lithium ion battery, but will improve its thermal safety performance, because such diluents have high viscosity and no ability to dissociate lithium salts, resulting in a certain degree of reduction in the conductivity of the electrolyte.

[0112] As can be seen from Comparative Examples 1, 4-6 and Comparative Examples 4, 5, when the amount of the cosolvent added to the electrolyte containing the diluent is too small, the DCR of the lithium ion battery will greatly increase and the fast charging capacity thereof will decrease, because the electrolyte lacks the cosolvent with moderate polarity for assisting the formation of the ion pair of the anion and the cation easy to migrate, so that the kinetics of the whole battery is poor. When the amount of the cosolvent added to the electrolyte is too large, the proportion of the diluent will decrease, which will result in the deterioration of the thermal safety performance of the lithium ion battery. Within the optimal dosage range, increasing the amount of the cosolvent will slightly improve the DCR and the fast charging performance, because the viscosity of the cosolvent is low, which can help the diffusion of lithium ions.

[0113] As can be seen from Comparative Examples 1, 7 and 8, when the proportion of the cyclic carbonate solvent in the carbonate solvent is too low, the dissociation ability of the electrolyte for the lithium salt will be insufficient, which will result in the deterioration of the DCR of the lithium ion battery. When the proportion of the cyclic carbonate solvent in the carbonate solvent is too high, the cyclic carbonate organic solvent will decompose to produce gas due to the insufficient stability of the cyclic carbonate organic solvent on the surface of the positive electrode, which will deteriorate the interfacial stability of the electrode and the electrolyte, and finally the thermal safety performance of the lithium ion battery will slightly decrease. Preferably, the mass fraction of the cyclic carbonate solvent, such as ethylene carbonate and / or propylene carbonate, in the carbonate solvent is 5%-20%.

[0114] As can be seen from Comparative Examples 1, 9 and 10, when MMDS is added to the electrolyte as a part of the additive, on the one hand, a uniform and dense SEI film will be first generated on the surface of the positive electrode during the formation and charging of the lithium ion battery, and on the other hand, the electrolyte containing the diluent will generate an SEI film mainly composed of inorganic components on the negative electrode side, which can inhibit the reaction of the negative active material, the SEI film and the electrolyte at high temperature, improve the high-temperature storage performance of the lithium ion battery and maintain the stability of the fast charging cycle, so that the lithium ion battery with MMDS has good performances in the fast charging cycle and the electrode and electrolyte interface of the positive and negative electrodes, which can improve the thermal safety performance of the lithium ion battery to a certain extent.

[0115] As can be seen from Comparative Examples 1, 11-13, compared with LiBF4, LiDFOB and LiDFOP can more obviously improve the fast charging cycle capacity and the thermal runaway temperature of the lithium ion battery. This is because the quality of the film formed by LiDFOB and LiDFOP on the positive electrode is better than that of LiBF4, which is beneficial to the improvement of the cycle stability and the thermal safety performance, so the lithium salt additive is preferably LiDFOB or LiDFOP.

[0116] The preferred embodiments of the present application have been described above with reference to the drawings, but the present application is not limited to the above examples. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall fall within the protection scope of the present application.

Claims

1. An electrolyte, characterized by, Comprise: an organic solvent, the organic solvent comprising a carbonate-based solvent, and a mass fraction of the carbonate-based solvent in the electrolyte being w1, 30%≤w1≤65%; a lithium salt, dissolved in the organic solvent; a diluent, not participating in the solvation reaction of lithium ions in the organic solvent, and having flame retardancy, a mass fraction of the diluent in the electrolyte being w2, 15%≤w2≤50%; a co-solvent, the co-solvent being an aromatic compound, a polarity of the co-solvent being between the diluent and the organic solvent, a mass fraction of the co-solvent in the electrolyte being w3, 5%≤w3≤30%; the diluent being any one or a combination of at least two of ethyl nonafluorobutyl ether, perfluorobutyl methyl ether, perfluorobutyl ethyl ether; the co-solvent being any one or a combination of at least two of o-difluorobenzene, m-difluorobenzene, p-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, trifluoromethylbenzene; the carbonate-based solvent being composed of a cyclic carbonate-based solvent and a linear carbonate-based solvent; the cyclic carbonate-based solvent being vinyl carbonate and / or propylene carbonate.

2. The electrolyte of claim 1, wherein a mass ratio of the co-solvent to the diluent being n, n=w3 / w2, n satisfying the following relationship: 1 / 4≤n≤2.

3. The electrolyte of claim 1, wherein the diluent being a C3-C8 perfluoro or polyfluoro-substituted ether diluent.

4. The electrolyte of claim 1, wherein a mass fraction of the cyclic carbonate-based solvent in the carbonate-based solvent being 5%-20%.

5. The electrolyte of claim 1, wherein the lithium salt comprising any one or a combination of at least two of lithium bisfluorosulfonylimide, lithium bistrifluoromethylsulfonylimide, lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, a mass fraction of the lithium salt in the electrolyte being 10%-20%.

6. The electrolyte of claim 1, wherein further comprising an additive, the additive comprising: a cyclic carbonate additive, the cyclic carbonate additive comprising any one or a combination of at least two of vinylene carbonate, fluorinated vinylene carbonate, vinyl ethylene carbonate; and a lithium salt additive, the lithium salt additive comprising any one or a combination of at least two of lithium difluorophosphate, lithium bisoxalate borate, lithium tetrafluoroborate, lithium difluorobisoxalate borate, lithium difluorobisoxalate phosphate; a mass fraction of the additive in the electrolyte being 0.2%-5.5%.

7. The electrolyte of claim 6, wherein the lithium salt additive being one or both of the lithium difluorobisoxalate borate and the lithium difluorobisoxalate phosphate, a mass fraction of the lithium difluorobisoxalate borate and / or the lithium difluorobisoxalate phosphate in the electrolyte being 0.1%-0.8%.

8. The electrolyte of claim 1, wherein further comprising an additive, the additive comprising: a cyclic carbonate additive, the cyclic carbonate additive comprising any one or a combination of at least two of vinylene carbonate, fluorinated vinylene carbonate, vinyl ethylene carbonate; and methylene methane disulfonate, a mass fraction of the methylene methane disulfonate in the electrolyte being 0.1%-0.8%.

9. A lithium-ion battery, characterized by the lithium ion battery comprising a positive electrode sheet, a negative electrode sheet, and the electrolyte as claimed in any one of claims 1-8.

10. The lithium-ion battery of claim 9, wherein, the positive electrode sheet comprising: a positive electrode current collector; and A positive electrode active material including any one of or a combination of at least two of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, or lithium nickel cobalt aluminum oxide.

11. The lithium-ion battery of claim 9, wherein the lithium-ion battery is a lithium-ion battery. The negative electrode tab includes: A negative electrode current collector; and A negative electrode active material including any one of or a combination of at least two of soft carbon, hard carbon, artificial graphite, natural graphite, silicon, a silicon oxide compound, a silicon carbon compound, lithium titanate.

Images