Electrolyte and secondary battery containing same

Abstract

The invention provides an electrolyte and a secondary battery containing the same, and belongs to the technical field of secondary batteries. The electrolyte comprises unsaturated boric acid ester, electrolyte salt and a non-aqueous solvent. According to the electrolyte provided by the invention, unsaturated boric acid ester is added and can form an interfacial film with a good protection effect on the surface of the positive electrode of the battery, so that the side reaction of the positive electrode and the electrolyte is effectively inhibited, and meanwhile, the interface impedance is reduced, so that the battery has good cycle performance, high-temperature storage performance and low gas production performance; the internal resistance is relatively low, and the power performance of the battery is improved.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of secondary batteries, and particularly relates to an electrolyte and a secondary battery comprising the same. BACKGROUND

[0002] In recent years, new energy vehicles develop well, and electric vehicles are rapidly popularized, but the market has never lowered the requirements for the improvement of the energy density of lithium ion batteries. From the perspective of battery research and development, increasing the working voltage of ternary lithium batteries or using positive electrode materials with higher nickel content has become the preferred energy density improvement scheme in the industry. However, both of them will increase the positive active material, leading to the intensification of the positive electrode-electrolyte side reaction, causing the battery to produce gas, and the storage and cycle performance to deteriorate.

[0003] In order to alleviate the performance loss brought by the improvement of the energy density of the battery, electrolyte researchers have developed a large number of positive electrode protection additives to inhibit the positive electrode-electrolyte side reaction, among which unsaturated phosphates and unsaturated siloxanes have better effects, which can significantly reduce the gas production of the battery and improve the storage and cycle performance of the battery. However, the introduction of such additives will increase the internal resistance of the battery, significantly deteriorating the power performance and charging capacity of the battery. At present, there is no good solution, and the improvement of the comprehensive performance of the battery is limited by balancing the improvement of the storage and cycle performance of the battery and the deterioration of the power performance through adjusting the concentration. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide an electrolyte and a secondary battery comprising the same. The electrolyte can reduce the internal resistance of the battery while making the battery have good cycle performance, high-temperature storage performance and low gas production performance, and improve the power performance of the battery.

[0005] To achieve this purpose, the present application adopts the following technical solutions:

[0006] The present application provides an electrolyte, which comprises an unsaturated borate, an electrolyte salt and a non-aqueous solvent.

[0007] In some embodiments of the present application, the unsaturated borate is selected from at least one of the compounds represented by formula I, formula II and formula III;

[0008]

[0009] wherein R1, R2 and R3 are each independently selected from one of a halogen atom, a C1-C10 alkyl group, a C1-C10 halogenated alkyl group, a C6-C12 aryl group, a C1-C10 alkenyl group and a C1-C10 alkynyl group;

[0010] R4, R5, and R6 are each independently selected from one of the following: C1-C10 alkyl, C1-C10 haloalkyl, C6-C12 aryl, C2-C10 alkenyl, and C2-C10 alkynyl.

[0011] Furthermore, R4 in Formula I, at least one of R4 and R5 in Formula II, and at least one of R4, R5 and R6 in Formula III are C2-C10 alkenyl or C2-C10 alkynyl.

[0012] In some embodiments of the present invention, R1, R2, and R3 are each independently selected from one of halogen atoms, C1-C3 alkyl groups, C1-C3 haloalkyl groups, phenyl groups, C2-C3 alkenyl groups, and C2-C3 alkynyl groups;

[0013] R4, R5, and R6 are each independently selected from one of the following: C1-C3 alkyl, C1-C3 haloalkyl, phenyl, C2-C3 alkenyl, and C2-C3 alkynyl.

[0014] Furthermore, R4 in Formula I, at least one of R4 and R5 in Formula II, and at least one of R4, R5 and R6 in Formula III are C2-C3 alkenyl or C2-C3 alkynyl.

[0015] In some embodiments of the present invention, the unsaturated borate ester is selected from at least one of the following compounds:

[0016]

[0017] In some embodiments of the present invention, the unsaturated borate ester in the electrolyte has a mass percentage content of 0.01-3%, preferably 0.1-1%.

[0018] In some embodiments of the present invention, the electrolyte salt is a lithium salt.

[0019] In some embodiments of the present invention, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium difluorosulfonylimide.

[0020] In some embodiments of the present invention, the content of the electrolyte salt in the electrolyte is 0.5-1.5 mol / L.

[0021] In some embodiments of the present invention, the non-aqueous solvent includes at least one cyclic carbonate and at least one chain carbonate.

[0022] In some embodiments of the present invention, the cyclic carbonate includes at least one of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and butene carbonate.

[0023] In some embodiments of the present application, the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate.

[0024] In some embodiments of the present application, the electrolyte further includes vinyl sulfate, and the mass percentage of the vinyl sulfate in the electrolyte is 0.2-3%, preferably 0.5-2%.

[0025] In some embodiments of the present application, the electrolyte further includes at least one of the following additives:

[0026] vinylene carbonate, 1,3-propane sultone, 1-propenyl-1,3-sultone, methyl bisulfite, vinyl ethylene carbonate and tris(trimethylsilyl) phosphate.

[0027] In a second aspect, the present application provides a secondary battery including the electrolyte of the first aspect.

[0028] Compared with the prior art, the present application has the following beneficial effects:

[0029] The electrolyte provided by the present application adds unsaturated borate, which can form an interface film with good protection effect on the surface of the positive electrode, effectively inhibiting the side reaction of the positive electrode and the electrolyte. In addition, the three-coordinated boron in the unsaturated borate molecule can react with the active metal fluoride MF (M is an active metal element, such as lithium, sodium, etc.) component on the surface of the positive electrode to form a four-coordinated M3BO3, which on the one hand reduces the content of the active metal fluoride on the positive electrode-electrolyte interface, and on the other hand, the formed M3BO3 component has good compatibility with the electrolyte, and the four-coordinated boron anion contained in M3BO3 can act as an active metal ion channel to support the transmission of active metal ions at the interface, which synergistically enhances the permeability of active metal ions at the interface and reduces the interface impedance. Therefore, the electrolyte provided by the present application can not only make the battery have good cycle performance, high-temperature storage performance and low gas production performance, but also reduce the internal resistance of the battery and improve the power performance of the battery. DETAILED DESCRIPTION

[0030] In order to more clearly understand the above-mentioned purposes, features and advantages of the present application, the schemes of the present application will be further described below. It should be noted that the embodiments of the present application and the features in the embodiments can be combined with each other without conflict.

[0031] In the following description, many specific details are set forth in order to provide a thorough understanding of the present application, but the present application can also be implemented in other different ways from those described herein; obviously, the embodiments in the description are only some embodiments of the present application, not all embodiments.

[0032] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0033] In the description of the embodiments of the present application, for a chemical group, "Ca~Cb" means that the number of carbon atoms in the chemical group is a~b.

[0034] In the description of the embodiments of the present application, the term "alkyl" refers to a saturated aliphatic hydrocarbon group including branched or straight chains with a specified number of carbon atoms. For example, a C3 alkyl group can be n-propyl or isopropyl, and a C4 alkyl group can be n-butyl, sec-butyl, isobutyl or t-butyl.

[0035] In the description of the embodiments of the present application, the term "haloalkyl" refers to a group in which at least one hydrogen atom in an alkyl group is replaced by a halogen atom.

[0036] In the description of the embodiments of the present application, the term "aryl" refers to a monocyclic, fused ring or polycyclic aromatic group (ring atoms are only carbon atoms), and at least one ring has a conjugated π electron system. For example, a C6-C12 aryl group includes, but is not limited to, phenyl, naphthyl, biphenyl, etc.

[0037] In the description of the embodiments of the present application, the term "alkenyl" refers to a branched or straight chain aliphatic hydrocarbon group containing at least one carbon-carbon double bond. The term "alkynyl" refers to a branched or straight chain aliphatic hydrocarbon group containing at least one carbon-carbon triple bond.

[0038] In a first aspect, the embodiments of the present application provide an electrolyte, which comprises an unsaturated borate ester, an electrolyte salt and a non-aqueous solvent.

[0039] The electrolyte provided by the present application adds an unsaturated borate ester, which can form an interface film with good protective effect on the surface of the positive electrode of the battery, effectively inhibiting the side reaction of the positive electrode and the electrolyte. In addition, the three-coordinated boron in the unsaturated borate ester molecule can react with the active metal fluoride MF (M is an active metal element, such as lithium, sodium, etc.) component on the surface of the positive electrode to form a four-coordinated M3BO3, which on the one hand reduces the content of the active metal fluoride on the positive electrode-electrolyte interface, and on the other hand, the formed M3BO3 component has good compatibility with the electrolyte, and the four-coordinated boron anion contained in M3BO3 can act as an active metal ion channel to support the transmission of active metal ions at the interface, which synergistically enhances the permeability of active metal ions at the interface and reduces the interface impedance.

[0040] Therefore, the electrolyte provided by the present application can not only make the battery have good cycle performance, high-temperature storage performance and low gas production performance, but also reduce the internal resistance of the battery and improve the power performance of the battery.

[0041] In some embodiments of the present application, the unsaturated borate ester is selected from at least one of the compounds shown in Formula I, Formula II, and Formula III;

[0042]

[0043] wherein R1, R2, R3are each independently selected from one of a halogen atom, a C1-C10 alkyl group, a C1-C10 haloalkyl group, a C6-C12 aryl group, a C2-C10 alkenyl group, and a C2-C10 alkynyl group;

[0044] R4, R5, R6are each independently selected from one of a C1-C10 alkyl group, a C1-C10 haloalkyl group, a C6-C12 aryl group, a C2-C10 alkenyl group, and a C2-C10 alkynyl group;

[0045] and at least one of R4in Formula I, R4and R5in Formula II, and R4, R5, and R6in Formula III is a C1-C10 alkenyl group or a C1-C10 alkynyl group.

[0046] wherein the halogen atom can be a F atom, a Cl atom, a Br atom, or an I atom.

[0047] The number of carbon atoms in the C1-C10 alkyl group can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0048] The number of carbon atoms in the C1-C10 haloalkyl group can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0049] The number of carbon atoms in the C6-C12 aryl group can be 6, 7, 8, 9, 10, 11, or 12, etc.

[0050] The number of carbon atoms in the C1-C10 alkenyl group can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0051] The number of carbon atoms in the C1-C10 alkynyl group can be 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10.

[0052] As a preferred technical solution, R1, R2, R3are each independently selected from one of a halogen atom, a C1-C3 alkyl group, a C1-C3 haloalkyl group, a phenyl group, a C2-C3 alkenyl group, and a C2-C3 alkynyl group;

[0053] R4, R5, R6are each independently selected from one of C1-C3alkyl, C1-C3haloalkyl, phenyl, C2-C3alkenyl and C2-C3alkynyl;

[0054] and at least one of R4in formula I, R4and R5in formula II, and R4, R5and R6in formula III is C2-C3alkenyl or C2-C3alkynyl.

[0055] In some embodiments of the present application, the unsaturated borate ester is selected from at least one of the following compounds:

[0056]

[0057] In the embodiments of the present application, with the increase of the number of carbon atoms in the unsaturated borate ester, the protection ability of the positive electrode surface interface film to the positive electrode will also decrease accordingly. By optimizing the type of the unsaturated borate ester, the cycle performance of the battery can be further improved, the gas production can be reduced, and the service life of the battery can be increased.

[0058] In some embodiments of the present application, the mass percentage content of the unsaturated borate ester in the electrolyte is 0.01-3%; for example, it can be 0.01%, 0.02%, 0.03%, 0.05%, 0.06%, 0.08%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.3%, 2.5%, 2.6%, 2.8% or 3%, etc. Preferably, it is 0.1-1%.

[0059] In the embodiments of the present application, when the content of the unsaturated borate ester in the electrolyte is low, the integrity and stability of the interface film formed on the positive electrode surface of the battery are poor, the protection ability to the positive electrode is limited, the interface impedance is large, and thus the improvement effect on the cycle performance, high-temperature storage performance, gas production and internal resistance of the battery is weak. When the content of the unsaturated borate ester in the electrolyte is too high, the compactness of the interface film is large, which although is better for the protection of the positive electrode and can further reduce the gas production of the battery, but is not conducive to the permeation of active metal ions, leading to the increase of the interface impedance, the increase of the internal resistance of the battery, and the decrease of the cycle performance and high-temperature storage capacity retention rate. Therefore, in the embodiments of the present application, the content of the unsaturated borate ester in the electrolyte is preferably within the above range, which can ensure that the battery has good cycle performance, high-temperature storage performance, low gas production and low internal resistance.

[0060] In some embodiments of the present application, the electrolyte salt is a lithium salt. When the electrolyte provided by the embodiments of the present application is applied to a lithium ion battery, a lithium salt can be used as the electrolyte salt. However, the present application is not limited thereto, and the electrolyte can also be applied to a sodium ion battery or other secondary batteries, in which case a salt of a corresponding active metal can be selected as the electrolyte salt.

[0061] In some embodiments of the present application, the lithium salt includes at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium bisfluorosulfonimide. However, the present application is not limited thereto, and other conventional lithium salts in the art can also be applicable to the present application.

[0062] In some embodiments of the present application, the content of the electrolyte salt in the electrolyte is 0.5-1.5 mol / L; for example, it can be 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.3 mol / L, or 1.5 mol / L, etc.

[0063] If the content of the electrolyte salt in the electrolyte is too low, the conductivity of the electrolyte will be low, and the internal resistance of the battery will be large; if the content of the electrolyte salt is too high, the viscosity of the electrolyte will be large, and the conductivity of the electrolyte will also be low, and the internal resistance of the battery will be large.

[0064] In some embodiments of the present application, the non-aqueous solvent includes at least one cyclic carbonate and at least one chain carbonate.

[0065] Cyclic carbonates have a large polarity and a large solubility for lithium salts, but their viscosity is generally relatively large, while the viscosity of chain carbonates is relatively low. The combination of the two helps the electrolyte to have good solubility for lithium salts and good wettability.

[0066] In some embodiments of the present application, the cyclic carbonate includes at least one of ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and butylene carbonate. However, the present application is not limited thereto, and other conventional cyclic carbonates in the art can also be applicable to the present application.

[0067] In some embodiments of the present application, the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, and methyl ethyl carbonate. However, the present application is not limited thereto, and other conventional chain carbonates in the art can also be applicable to the present application.

[0068] In some embodiments of the present application, the electrolyte further comprises vinyl sulfate, and the mass percentage of the vinyl sulfate in the electrolyte is 0.2-3%, for example, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.8%, 1%, 1.2%, 1.3%, 1.5%, 1.6%, 1.8%, 2%, 2.2%, 2.3%, 2.5%, 2.6%, 2.8% or 3%, etc. Preferably, 0.5-2%.

[0069] The present application has found that when the unsaturated borate ester is used in combination with the vinyl sulfate, the two can play a synergistic effect, and further improve the cycle performance and high-temperature storage performance of the battery. Therefore, in the present application, the electrolyte preferably comprises both the unsaturated borate ester and the vinyl sulfate. If the content of the vinyl sulfate in the electrolyte is too low, the synergistic effect of the unsaturated borate ester and the vinyl sulfate is weak, and the improvement effect on the battery is not obvious. If the content of the vinyl sulfate is too high, the impedance of the battery will increase, and the cycle performance and high-temperature storage performance will decrease.

[0070] In some embodiments of the present application, the electrolyte further comprises at least one of the following additives:

[0071] Vinylene carbonate, 1,3-propane sultone, 1-propenyl-1,3-sultone, methylene methyl disulfonate, vinyl ethylene carbonate and tris(trimethylsilyl) phosphate.

[0072] The above-mentioned additives are conventional electrolyte additives in the art, and the present application does not make special limitations on the content of the additives in the electrolyte, which can be selected conventionally by those skilled in the art.

[0073] In a second aspect, the present application provides a secondary battery comprising the electrolyte of the first aspect.

[0074] In order to make the technical problems solved by the present application, technical solutions and beneficial effects clearer, the present application will be further described in detail below in combination with embodiments. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative, but not as any limitation on the present application and its application. 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.

[0075] If a specific technology or condition is not specified in the embodiments, it is carried out according to the conventional technology or condition in the art, or the technology or condition described in the literature, or the product instruction. If the reagent or instrument used is not specified by the manufacturer, it is a conventional product that can be purchased on the market.

[0076] Example 1

[0077] The present embodiment provides an electrolyte, which comprises 0.01 wt% of trivinyl borate, 1 mol / L of LiPF6, and the rest is solvent. The preparation method of the electrolyte in the present embodiment is as follows:

[0078] In an argon atmosphere glove box with water oxygen content less than 1 ppm, ethylene carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) are mixed as electrolyte solvents in a mass ratio of 3:1:6, cooled to below -10°C, and LiPF6 is slowly added thereto under stirring, with the process temperature controlled to be not higher than 0°C. After LiPF6 is completely dissolved, trivinyl borate is added, and the electrolyte is obtained after uniform mixing.

[0079] Examples 2-17 and Comparative Examples 1-6

[0080] Examples 2-17 and Comparative Examples 1-6 each provide an electrolyte, which is prepared by referring to the preparation method of Example 1, and the solvent composition, lithium salt type, and lithium salt concentration are the same as those of Example 1, and other components are shown in Table 1.

[0081] Table 1

[0082]

[0083]

[0084]

[0085] Preparation of lithium ion secondary battery

[0086] Lithium ion batteries are prepared using the electrolytes provided in the above examples and comparative examples, respectively, and the preparation method is as follows:

[0087] (1) Preparation of positive electrode sheet:

[0088] LiNi 0.9 Co 0.03 Mn 0.07 O2 and conductive agent acetylene black are added to a stirring tank and mixed uniformly, and NMP (N-methyl pyrrolidone) solution of polyvinylidene fluoride is added thereto for continuous stirring. After uniform mixing, a black positive electrode slurry is obtained, wherein the mass ratio of positive electrode active material, conductive agent and polyvinylidene fluoride is 96:3:1. The positive electrode slurry is coated on both sides of an aluminum foil, and the desired positive electrode sheet is obtained through the steps of continuous baking, rolling, slitting and cutting.

[0089] (2) Preparation of negative electrode sheet:

[0090] The graphite and the conductive agent Super P (conductive carbon black) were added into a stirring tank and mixed uniformly, the binder SBR (styrene-butadiene rubber) and deionized water were added and stirred until mixed uniformly to obtain a black negative electrode slurry, wherein the mass ratio of graphite, conductive agent and SBR was 96:1.5:2.5. The negative electrode slurry was coated on both sides of the copper foil, and after continuous baking, rolling, slitting and cutting, the required negative electrode sheet was obtained.

[0091] (3) Assembly of the battery:

[0092] The prepared positive electrode sheet, negative electrode sheet and PE separator were stacked and wound in the order of positive electrode, separator and negative electrode, and after heat pressing, tab welding and other steps, a bare cell was obtained. The bare cell was then top-side sealed with an aluminum plastic film, and then placed in an oven at 95°C for 24 hours or more. The moisture content of the electrode sheet was tested, and the moisture content was confirmed to be qualified (less than 300 ppm) before injecting electrolyte. After vacuum packaging, standing, formation, shaping and other processes, a lithium ion secondary battery was obtained.

[0093] Performance test

[0094] The lithium ion secondary battery prepared above was subjected to normal temperature cycle, high temperature storage, high temperature gas production and direct current impedance test, and the specific test methods were as follows:

[0095] 1. Normal temperature cycle capacity retention rate test:

[0096] The lithium ion secondary battery was subjected to charge and discharge cycle test at 25°C with 1C / 1C charge and discharge rate in the range of 2.5-4.25V, and the first discharge capacity C0 and the discharge capacity C after each cycle were recorded. x The xth cycle capacity retention rate = C x / C0x100%.

[0097] 2. Storage capacity retention rate test:

[0098] The lithium ion secondary battery was subjected to constant current charging at 1C current to 4.25V at 25°C, and then constant voltage charging to the cutoff current 0.05C, standing for 10 min, and then constant current discharging at 1C current to 2.5V. The first discharge capacity C0 of each battery was recorded. The battery was subjected to constant current charging at 1C current to 4.25V after standing for 10 min, and then constant voltage charging to the cutoff current 0.05C. Then the battery was placed in a constant temperature oven at 60°C for heat storage. After 30 days, the battery was taken out and cooled to 25°C. The battery was subjected to constant current charging at 1C current to 4.25V, and then constant voltage charging to the cutoff current 0.05C. After standing for 10 min, the battery was subjected to constant current discharging at 1C current to 2.5V. The discharge capacity C of the battery was recorded. 30 The 30-day storage capacity retention rate = C 30 / C0x100%.

[0099] 3. High-temperature gas production test

[0100] The lithium-ion secondary batteries were left to stand at 25°C for 120 minutes, and the volume V0 of each battery was measured using the Archimedes displacement method. The batteries were then charged at a constant current of 1C to 4.25V, followed by constant voltage charging to a cutoff current of 0.05C. After standing for 10 minutes, they were placed in a 70°C constant temperature oven for 15 days of thermal storage. After 15 days, the batteries were removed, left to stand at 25°C for 120 minutes, and their volume V0 was measured using the Archimedes displacement method. 15 Then the battery volume growth rate = (V 15 -V0) / V0×100%.

[0101] 4. DC Impedance (DCR) Test:

[0102] The lithium-ion secondary battery was left to stand at 25°C for 120 minutes, then discharged at a constant current of 1C to 2.5V. After standing for 10 minutes, it was charged at a constant current of 1C to 4.25V, then charged at a constant voltage to the cutoff current of 0.05C. After standing for 10 minutes, it was discharged at a constant current of 1C to 2.5V, and the discharge capacity C0 was recorded. After standing for 10 minutes, it was charged at a constant current of 1C to 4.25V, then charged at a constant voltage to the cutoff current of 0.05C. After standing for 10 minutes, it was discharged at a constant current of 1C0 (note: here 1C0 refers to 1 times the rate at which the measured capacity C0 is measured) for 30 minutes, until the battery charge reached 50% SOC. The battery was then discharged at a constant current of 4C0 for 30 seconds, and the voltage U before discharge was recorded. 前 and the voltage U after discharge 后 , then DCR=(U 前 -U 后 ) / Discharge current.

[0103] The results of the above performance tests are shown in Table 2 below.

[0104] Table 2

[0105]

[0106]

[0107] As can be seen from the test results in Table 2, compared with Comparative Example 1, the lithium-ion battery using the electrolyte provided in this embodiment of the invention exhibits higher capacity retention at room temperature cycling, higher capacity retention at high temperature storage, lower volume growth rate at high temperature storage, and / or lower internal resistance. This indicates that adding unsaturated borate esters to the electrolyte can effectively reduce battery gas production; and by controlling the content of unsaturated borate esters within a suitable range, the battery's cycle performance and high-temperature storage performance can also be improved, and internal resistance reduced.

[0108] Comparing the experimental results of Comparative Example 2-4 and Example 4, it can be seen that, under the same content, the unsaturated borate ester has the same effect on the cycle performance, high-temperature storage performance, and high-temperature gas production of the battery as the tetra-vinyl silane and tri-allyl phosphate, and is superior to 1,3-propane sulfone lactone; however, compared with the tetra-vinyl silane and tri-allyl phosphate, the lithium ion battery using the electrolyte containing the unsaturated borate ester has a lower internal resistance, which helps to improve the power performance of the battery.

[0109] Comparing the experimental results of Comparative Example 5 and Example 4, it can be seen that the saturated borate ester has little effect on the overall performance of the battery, because the saturated borate ester cannot effectively form a film and has poor interface protection capability, and although the three-coordinated borate ester can improve the impedance to a certain extent, the poor interface film still makes the battery impedance large. However, the unsaturated borate ester can effectively form a film, significantly improves the cycle, storage, and gas production performance of the battery, and the three-coordinated boron element can reduce the lithium fluoride component in the interface film, and the four-coordinated boron-lithium component formed in situ in the interface film can form a lithium ion transmission channel, so that the internal resistance of the battery is significantly reduced.

[0110] Comparing the experimental results of Comparative Example 1 and Examples 1-7, it can be seen that, with the increase of the content of the tri-vinyl borate ester, the cycle and storage performance of the battery is first significantly improved, and the internal resistance is reduced, but when the content of the tri-vinyl borate ester is high, further increasing the concentration will gradually deteriorate the storage and cycle performance of the battery, and the impedance is increased. From the gas production data, with the increase of the concentration of the tri-vinyl borate ester, its improvement effect on the gas production of the battery is gradually enhanced. The reason is that when the content of the tri-vinyl borate ester is low, the positive electrode has insufficient film-forming capability; with the increase of the content, the integrity and stability of the positive electrode interface film gradually improve, so the cycle and storage performance of the battery is improved; but when the concentration of the tri-vinyl borate ester is too high, the positive electrode will be over-filmed, resulting in an increase in the polarization of the battery, and the compactness of the positive electrode interface film is improved, which is not conducive to the permeation of lithium ions, so the cycle and storage capacity retention rate of the battery gradually decreases, and the internal resistance gradually increases. However, the more compact interface film has a stronger protective effect on the positive electrode, and the electrode-electrolyte oxidation side reaction is less, so the gas production of the battery decreases with the increase of the content of the tri-vinyl borate ester. Therefore, from the overall results of the battery performance, taking the mass fraction of the electrolyte as 100%, the concentration of the unsaturated borate ester is preferably 0.01-3.0%, and more preferably, to achieve better battery performance, the concentration of the unsaturated borate ester is preferably 0.1-1.0%.

[0111] From the experimental results of Example 4 and Examples 8-15, it can be seen that the unsaturated borate ester containing an alkenyl group has basically the same effect on the improvement of the battery performance as the unsaturated borate ester containing an alkynyl group. From the number of unsaturated bonds in the molecule, with the decrease of the unsaturated bonds in the unsaturated borate ester, the improvement effect of the unsaturated borate ester on the cycle, storage, and gas production performance of the battery will decrease.

[0112] Comparing the experimental results of Comparative Example 6, Example 16 and Example 20, it can be seen that, when the unsaturated borate and the vinyl sulfate are added simultaneously in the electrolyte, the battery has higher room temperature cycle capacity retention rate and high temperature storage capacity retention rate, compared with the battery in which only the unsaturated borate (Example 16) or only the vinyl sulfate (Comparative Example 6) is added, indicating that the unsaturated borate and the vinyl sulfate have a synergistic effect, and can further improve the cycle performance and high temperature storage performance of the battery.

[0113] Comparing the experimental results of Example 4 and Example 17, it can be seen that, although the vinyl sulfate is added in Example 17, the content of the vinyl sulfate is low, and the room temperature cycle capacity retention rate, the high temperature storage capacity retention rate, the high temperature storage volume growth rate and the internal resistance of the battery have no significant difference with Example 4, indicating that when the content of the vinyl sulfate is too low, the synergistic effect of the unsaturated borate and the vinyl sulfate is weak, and the improvement effect on the battery is not obvious.

[0114] Comparing the experimental results of Example 20 and Example 23, it can be seen that, in Example 23, the content of the vinyl sulfate is high, and the room temperature cycle capacity retention rate and the high temperature storage capacity retention rate of the battery decrease, and the internal resistance increases, indicating that when the content of the vinyl sulfate is too high, it is not conducive to the synergistic effect of the unsaturated borate and the vinyl sulfate, and is not conducive to the improvement of the battery performance.

[0115] The above description is merely a specific implementation of the present disclosure, which enables those skilled in the art to understand or implement the present disclosure. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein can be implemented in other embodiments without departing from the spirit or scope of the present disclosure. Therefore, the present disclosure will not be limited to these embodiments described herein, but will conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. An electrolyte, characterized in that, The electrolyte comprises unsaturated borate esters, electrolyte salts, and non-aqueous solvents.

2. The electrolyte according to claim 1, characterized in that, The unsaturated borate ester is selected from at least one of the compounds shown in Formula I, Formula II and Formula III; R1, R2, and R3 are each independently selected from one of the following: halogen atom, C1-C10 alkyl, C1-C10 haloalkyl, C6-C12 aryl, C2-C10 alkenyl, and C2-C10 alkynyl; R4, R5, and R6 are each independently selected from one of the following: C1-C10 alkyl, C1-C10 haloalkyl, C6-C12 aryl, C2-C10 alkenyl, and C2-C10 alkynyl; and at least one of R4 in Formula I, at least one of R4 and R5 in Formula II, and at least one of R4, R5, and R6 in Formula III is a C1-C10 alkenyl or a C1-C10 alkynyl. Preferably, R1, R2, and R3 are each independently selected from one of a halogen atom, a C1-C3 alkyl group, a C1-C3 haloalkyl group, a phenyl group, a C2-C3 alkenyl group, and a C2-C3 alkynyl group; R4, R5, and R6 are each independently selected from one of a C1-C3 alkyl group, a C1-C3 haloalkyl group, a phenyl group, a C2-C3 alkenyl group, and a C2-C3 alkynyl group; and R4 in Formula I, at least one of R4 and R5 in Formula II, and at least one of R4, R5, and R6 in Formula III are either a C2-C3 alkenyl group or a C2-C3 alkynyl group.

3. The electrolyte according to claim 1 or 2, characterized in that, The unsaturated borate ester is selected from at least one of the following compounds:

4. The electrolyte according to any one of claims 1-3, characterized in that, The unsaturated borate ester has a mass percentage content of 0.01-3% in the electrolyte, preferably 0.1-1%.

5. The electrolyte according to any one of claims 1-4, characterized in that, The electrolyte salt is a lithium salt; Preferably, the lithium salt comprises at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium bis(oxalate)borate, lithium difluorooxalateborate, lithium hexafluoroarsenate, lithium difluorophosphate, lithium trifluoromethanesulfonate, lithium perchlorate, and lithium difluorosulfonylimide.

6. The electrolyte according to any one of claims 1-5, characterized in that, The content of the electrolyte salt in the electrolyte is 0.5-1.5 mol / L.

7. The electrolyte according to any one of claims 1-6, characterized in that, The non-aqueous solvent includes at least one cyclic carbonate and at least one chain carbonate; Preferably, the cyclic carbonate includes at least one selected from ethylene carbonate, fluoroethylene carbonate, propylene carbonate, and butene carbonate; Preferably, the chain carbonate includes at least one of dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate.

8. The electrolyte according to any one of claims 1-7, characterized in that, The electrolyte also includes vinyl sulfate, wherein the mass percentage of vinyl sulfate in the electrolyte is 0.2-3%, preferably 0.5-2%.

9. The electrolyte according to any one of claims 1-8, characterized in that, The electrolyte also includes at least one of the following additives: Ethylene carbonate, 1,3-propanesulfonate lactone, 1-propenyl-1,3-sulfonate lactone, methyl disulfonate, ethylene ethylene carbonate, and tris(trimethylsilyl)phosphate.

10. A secondary battery, characterized in that, Includes the electrolyte as described in any one of claims 1-9.

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