Electrolyte additive, electrolyte and secondary battery

Abstract

The present invention belongs to the technical field of secondary batteries, and in particular, relates to an electrolyte additive, an electrolyte, and a secondary battery, wherein the electrolyte additive comprises a first additive of formula I and a second additive of formula II, wherein R1 to R4 are independently selected from a linear or cyclic alkyl or alkoxy group, a linear or cyclic alkenyl or alkenyloxy group, a linear or cyclic alkynyl or alkynyloxy group, a halogen group, an aryl group, a hydrogen group, a silane group, or a combination of one or more of the siloxane groups; wherein R5 is independently selected from an alkenyl group, an alkynyl group, a hydrogen group, a nitrile group, an ether group, a halogen group, an aryl group, or a combination of one or more of the silane groups, and R5 has a carbon-carbon double bond and / or a carbon-carbon triple bond. The electrolyte additive of the present invention can improve the stability of the SEI film, protect the electrode interface film, reduce side reactions, and bond to the surface of the positive and negative electrode materials through chemical bonds to inhibit their expansion under high temperature and high pressure, thereby improving high temperature storage performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and in particular relates to an electrolyte additive, an electrolyte and a secondary battery. Background Art

[0002] Lithium-ion batteries are widely used in smart wearables, computers, smart phones, electric vehicles and other fields due to their high energy density, long cycle life, high operating voltage and low self-discharge rate. Regardless of the application field, with the advancement of science and technology, higher requirements are placed on lithium-ion battery technology in this field, requiring better storage performance at high temperatures. Existing electrolytes can easily cause side reactions in the SEI film of the pole piece under high temperature conditions, resulting in damage to the SEI film, causing the battery to produce gas and expand, thus affecting the battery quality. Summary of the invention

[0003] One of the purposes of the present invention is to provide an electrolyte additive to address the deficiencies of the prior art, which can improve the stability of the SEI film, protect the electrode interface film, reduce side reactions, and can be chemically bonded to the surface of the positive and negative electrode materials to inhibit their expansion under high temperature and high pressure, thereby improving the high temperature storage performance of the battery.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] An electrolyte additive comprises a first additive of formula I and a second additive of formula II, wherein the structural formula of formula I is:

[0006]

[0007] Wherein, R1 to R4 are independently selected from one or a combination of linear or cyclic alkyl or alkoxy, linear or cyclic alkenyl or alkenyloxy, linear or cyclic alkynyl or alkynyloxy, halogen, aryl, hydrogen, silane, siloxane, and the total number of carbon-carbon double bonds and / or carbon-carbon triple bonds in R1 to R4 is ≥2; the structural formula of Formula II is:

[0008]

[0009] Wherein, R5 is independently selected from one or a combination of alkenyl, alkynyl, hydrogen, nitrile, ether, halogen, and aryl, and R5 has a carbon-carbon double bond and / or a carbon-carbon triple bond.

[0010] Preferably, the structural formula of the first additive includes one or more of the following:

[0011]

[0012] Preferably, the structural formula of the second additive includes one or more of the following:

[0013]

[0014] The second purpose of the present invention is to provide an electrolyte with high temperature resistance, improve the stability of the electrode interface film, reduce side reactions, and improve battery performance in order to address the deficiencies of the prior art.

[0015] In order to achieve the above object, the present invention adopts the following technical solutions:

[0016] An electrolyte comprises the above electrolyte additive.

[0017] Preferably, the first additive accounts for 0.1 wt% to 5 wt% of the total mass of the electrolyte; and the second additive accounts for 0.1 wt% to 10 wt% of the total mass of the electrolyte.

[0018] Preferably, the electrolyte further comprises an organic solvent, and the organic solvent accounts for 50 wt% to 90 wt% of the electrolyte.

[0019] Preferably, the electrolyte further comprises a lithium salt, and the lithium salt accounts for 8wt% to 20wt% of the mass of the electrolyte.

[0020] Preferably, the electrolyte further comprises a third additive, and the third additive accounts for 0.5 wt% to 20 wt% of the electrolyte.

[0021] The third object of the present invention is to provide a secondary battery with good high temperature resistance and cycle stability in view of the deficiencies in the prior art.

[0022] In order to achieve the above object, the present invention adopts the following technical solutions:

[0023] A secondary battery comprises the above electrolyte.

[0024] Preferably, the secondary battery also includes a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer arranged on at least one surface of the negative electrode collector, and the positive electrode active material layer or the negative electrode active material layer includes polyvinylidene fluoride.

[0025] Preferably, the content of polyvinylidene fluoride in the positive electrode sheet is 0.1-3%, the content of polyvinylidene fluoride in the negative electrode sheet is 0.1-3%, and the relative molecular mass of the polyvinylidene fluoride is 100000-10000000.

[0026] The first additive and polyvinylidene fluoride satisfy the following relationship: the number of molecules of the first additive is K1, the number of molecules of polyvinylidene fluoride is K2, and (K1:K2)>5.

[0027] Compared with the prior art, the beneficial effects of the present invention are: an electrolyte additive of the present invention can improve the stability of the SEI film, and can be bonded to the surface of the positive and negative electrode materials through chemical bonds, protect the electrode interface film, reduce side reactions, inhibit its expansion under high temperature and high pressure, and improve the high temperature storage performance of the battery. DETAILED DESCRIPTION

[0028] The present invention will be described in further detail below in conjunction with specific implementation modes, but the implementation modes of the present invention are not limited thereto.

[0029] 1. An electrolyte additive, comprising a first additive of formula I and a second additive of formula II, wherein the structural formula of formula I is:

[0030]

[0031] Wherein, R1 to R4 are independently selected from one or a combination of linear or cyclic alkyl or alkoxy, linear or cyclic alkenyl or alkenyloxy, linear or cyclic alkynyl or alkynyloxy, halogen, aryl, hydrogen, silane, siloxane, and the total number of carbon-carbon double bonds and / or carbon-carbon triple bonds in R1 to R4 is ≥2; the structural formula of Formula II is:

[0032]

[0033] Wherein, R5 is independently selected from one or a combination of alkenyl, alkynyl, hydrogen, nitrile, ether, halogen, and aryl, and R5 has a carbon-carbon double bond and / or a carbon-carbon triple bond.

[0034] An electrolyte additive of the present invention can improve the stability of the SEI film, and can be bonded to the surface of the positive and negative electrode materials through chemical bonds, protect the electrode interface film, reduce side reactions, inhibit its expansion under high temperature and high pressure, and improve the high temperature storage performance of the battery. Specifically, R1 to R4 of the first additive of formula I described in the present invention have carbon-carbon unsaturated bonds, including carbon-carbon double bonds and / or carbon-carbon triple bonds, which can be reduced to form a film on the surface of the electrode, and SEI is formed on the surface of the positive and negative electrodes. The silane structure itself has good stability, which can improve the stability of the generated SEI, protect the positive and negative electrodes, and reduce side reactions with the electrolyte; at the same time, R5 in the second additive of formula II has a carbon-carbon unsaturated bond, PVDF can decompose and release HF at high temperature to produce a small amount of carbon-carbon unsaturated bonds, and the unsaturated bonds can be added to form a network. The first additive can fix the second additive of formula II and PVDF on the surface of the positive and negative active materials. The second additive of formula II contains nitrile functional groups, which can complex the transition metal ions dissolved from the positive electrode to prevent them from destroying the SEI on the surface of the negative electrode, improve the stability of the SEI, protect the negative electrode, and also consume the acid generated at high temperature to prevent gas production. PVDF can be chemically bonded to the surface of the positive and negative electrode materials, stabilize the network structure of the positive and negative electrode material particles, and inhibit their expansion under high temperature and high pressure. The two types of additives can play a synergistic role in the protection of the positive and negative electrodes and improve the high-temperature storage expansion rate of the battery. Preferably, the number of nitrile groups in the second additive is greater than or equal to 2.

[0035] In some embodiments, the structural formula of the first additive includes one or more of the following:

[0036] In some embodiments, the structural formula of the second additive is:

[0037]

[0038] 2. An electrolyte with high temperature resistance, which improves the stability of the electrode interface film, reduces side reactions, and improves battery performance.

[0039] An electrolyte comprises the above electrolyte additive.

[0040] In some embodiments, the first additive accounts for 0.1wt% to 5wt% of the total mass of the electrolyte. Preferably, the first additive accounts for 0.1wt% to 1wt%, 1wt% to 2wt%, 2wt% to 3wt%, 3wt% to 4wt%, 4wt% to 5wt% of the total mass of the electrolyte. Specifically, the first additive accounts for 0.1wt%, 0.8wt%, 1.2wt%, 1.5wt%, 1.8wt%, 2wt%, 2.5wt%, 3.5wt%, 3.8wt%, 4wt%, 4.6wt%, 4.8wt%, 5wt% of the total mass of the electrolyte.

[0041] In some embodiments, the second additive accounts for 0.1wt% to 10wt% of the total mass of the electrolyte. The second additive accounts for 0.1wt% to 2wt%, 2wt% to 4wt%, 4wt% to 6wt%, 6wt% to 8wt%, 8wt% to 10wt% of the total mass of the electrolyte. Specifically, the second additive accounts for 0.1wt%, 0.5wt%, 0.8wt%, 1.2wt%, 1.8wt%, 2.4wt%, 2.5wt%, 3.5wt%, 3.7wt%, 4.2wt%, 4.5wt%, 5wt%, 6wt%, 8wt%, 10wt% of the total mass of the electrolyte. When the weight ratio of the first additive to the second additive is 0.5 to 2: 2 to 4, the synergistic effect of the two can be better exerted to improve the performance of the pole piece.

[0042] In some embodiments, the electrolyte further comprises an organic solvent, and the organic solvent accounts for 50 wt% to 90 wt% of the electrolyte. Preferably, the organic solvent accounts for 50 wt% to 60 wt%, 60 wt% to 70 wt%, 70 wt% to 80 wt%, 80 wt% to 90 wt% of the electrolyte, and specifically, the organic solvent accounts for 50 wt%, 55 wt%, 60 wt%, 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt%, 90 wt% of the electrolyte.

[0043] In some embodiments, the electrolyte further comprises a lithium salt, and the lithium salt accounts for 8wt% to 20wt% of the electrolyte mass. Preferably, the lithium salt accounts for 8wt% to 10wt%, 10wt% to 14wt%, 14wt% to 16wt%, 16wt% to 20wt% of the electrolyte mass. Specifically, the lithium salt accounts for 8wt%, 10wt%, 12wt%, 15wt%, 18wt%, 20wt% of the electrolyte mass.

[0044] In some embodiments, the electrolyte further comprises a third additive, and the third additive accounts for 0.5wt% to 20wt% of the electrolyte mass. Preferably, the third additive accounts for 0.5wt% to 8wt%, 8wt% to 15wt%, 15wt% to 20wt% of the electrolyte mass, and specifically, the third additive accounts for 0.5wt%, 1.5wt%, 2.5wt%, 3wt%, 4.5wt%, 5wt%, 8wt%, 10wt%, 14wt%, 16wt%, 18wt%, 20wt% of the electrolyte mass.

[0045] 3. A secondary battery with good high temperature resistance and cycle stability.

[0046] A secondary battery comprises the above electrolyte.

[0047] In some embodiments, the secondary battery further includes a positive electrode sheet and a negative electrode sheet, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, and the positive electrode active material layer or the negative electrode active material layer includes polyvinylidene fluoride. PVDF exists in the positive and negative electrode sheets, and is physically mixed and contacted with the positive and negative electrode particles. Its contact force is small and it is easy to fall off. It is fixed to the surface of the material particles through chemical crosslinking of unsaturated bonds to prevent it from falling off. At the same time, the use of the above-mentioned electrolyte additive in combination with polyvinylidene fluoride can collaboratively improve the high temperature resistance of the battery.

[0048] Preferably, the relative molecular mass of the polyvinylidene fluoride is 100000-10000000. The relative molecular mass of polyvinylidene fluoride is 100000-500000, 500000-800000, 800000-1000000, 1000000-5000000, 5000000-8000000, 8000000-10000000, specifically, the relative molecular mass of polyvinylidene fluoride is 500000, 800000, 1200000, 1600000, 10000000. When polyvinylidene fluoride is used as the binder of both the positive and negative electrodes, the above electrolyte additives can be used to improve the stability of the interface film of the electrode sheet, avoid damage to the interface film, and thus improve the performance of the electrode sheet and the battery.

[0049] Preferably, the content of polyvinylidene fluoride in the positive electrode sheet is 0.1-3%, and the content of polyvinylidene fluoride in the negative electrode sheet is 0.1-3%. The content of polyvinylidene fluoride in the positive electrode sheet is 0.1-3%, preferably, the content of polyvinylidene fluoride in the positive electrode sheet is 0.1-1%, 1-2%, 2-3%, specifically, the content of polyvinylidene fluoride in the positive electrode sheet is 0.1%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 1.9%, 2%, 2.2%, 2.4%, 2.5%, 2.8%, 3%.

[0050] Preferably, the first additive and polyvinylidene fluoride satisfy the following relationship: the number of molecules of the first additive is K1, the number of molecules of polyvinylidene fluoride is K2, and (K1:K2)>5.

[0051] Specifically, a secondary battery includes a positive electrode sheet, a separator, a negative electrode sheet, a shell and the above-mentioned electrolyte, wherein the separator is used to separate the positive electrode sheet from the negative electrode sheet, and the shell is used to encapsulate the positive electrode sheet, the separator, the negative electrode sheet and the electrolyte.

[0052] positive electrode

[0053] The positive electrode active material layer includes a positive electrode active material, which may be, but is not limited to, a chemical formula such as Li a Ni x Co y M z O 2-b N b (wherein 0.95≤a≤1.2, x≥0, y≥0, z≥0, and x+y+z=1, 0≤b≤1, M is selected from a combination of one or more of Mn, Al, and Ti, and N is selected from a combination of one or more of F, P, and S) The positive electrode active material may also be, but is not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive active material may also be modified. The method for modifying the positive active material should be known to those skilled in the art. For example, the positive active material may be modified by coating, doping, etc. The material used for the modification may include but is not limited to a combination of one or more of Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive current collector is usually a structure or part for collecting current. The positive current collector may be various materials suitable for use as a positive current collector for lithium-ion batteries in the art. For example, the positive current collector may include but is not limited to metal foil, etc., and more specifically may include but is not limited to aluminum foil, etc. The positive active material layer also includes a binder and a conductive agent. The binder is the above-mentioned polyvinylidene fluoride.

[0054] negative electrode

[0055] The negative electrode active material layer includes a negative electrode active material, which may include but is not limited to one or more of graphite, soft carbon, hard carbon, carbon fiber, mesophase carbon microspheres, silicon-based materials, tin-based materials, lithium titanate or other metals that can form alloys with lithium. Among them, the graphite can be selected from one or more of artificial graphite, natural graphite and modified graphite; the silicon-based material can be selected from one or more of elemental silicon, silicon oxide compounds, silicon carbon composites, and silicon alloys; the tin-based material can be selected from one or more of elemental tin, tin oxide compounds, and tin alloys. The negative electrode current collector is usually a structure or part for collecting current. The negative electrode current collector can be various materials suitable for use as a negative electrode current collector for lithium-ion batteries in the art. For example, the negative electrode current collector can include but is not limited to metal foil, and more specifically can include but is not limited to copper foil. The negative electrode active material layer also includes a binder and a conductive agent, and the binder is the above-mentioned polyvinylidene fluoride.

[0056] Isolation film

[0057] The isolation membrane can be any material suitable for lithium-ion battery separators in the art, for example, it can be a combination of one or more of polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester and natural fibers, etc., but not limited to.

[0058] The shell is made of aluminum-plastic film or stainless steel.

[0059] Example 1

[0060] 1. Prepare an electrolyte: In a glove box filled with argon, the moisture content is less than 5ppm, the oxygen content is less than 5ppm, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (DEC), propyl propionate (PP), and ethyl propionate (EP) are mixed in a mass ratio of 1:1:1:1:1 to obtain an organic solvent. The organic solvent is mixed with lithium hexafluorophosphate according to the prior art so that the weight percentage of lithium salt in the electrolyte is 13.70%, and a mixture of organic solvent and lithium hexafluorophosphate is obtained, and the compounds of formula I-1 and formula II-1 of the present invention are added, the total mass of formula I-1 accounting for 0.5wt% of the total mass of the electrolyte, the compound of formula II-2 accounting for 3wt% of the total mass of the electrolyte, and 3wt% 1,3-propane sultone (PS), 3wt% fluoroethylene carbonate (FEC), and 3wt% 1,3,6-hexane trinitrile are mixed uniformly to obtain an electrolyte.

[0061] 2. Preparation of positive electrode sheet: NCM811 positive electrode active material, conductive agent superconducting carbon and carbon tube, binder polyvinylidene fluoride (relative molecular weight 1200000) are mixed evenly at a mass ratio of 96:2.0:0.5:1.5 to form positive electrode slurry, the positive electrode slurry is coated on one surface of the current collector aluminum foil, and after drying and rolling at 85°C, the positive electrode slurry is coated and dried on the other side of the aluminum foil according to the above method, and then the prepared aluminum foil with a positive electrode active material layer on both sides is cold pressed; trimming, cutting, and stripping are performed, and after stripping, positive electrode sheets are made.

[0062] 3. Preparation of negative electrode sheets: The silicon-carbon negative electrode active material, the conductive agent superconducting carbon, and the binder polyvinylidene fluoride or styrene-butadiene rubber (SBR) with a relative molecular mass of 1600000 are prepared into a negative electrode slurry in a mass ratio of 96:1:3, coated on the current collector copper foil and dried and rolled at 85°C, and then the negative electrode slurry is coated and dried on the other side of the copper foil according to the above method, and then the prepared copper foil with a negative electrode active material layer on both sides is cold pressed; trimming, cutting, and stripping are performed to make negative electrode sheets after stripping.

[0063] 4. Isolation membrane: A polyethylene porous film with a thickness of 16 μm was selected as the isolation membrane.

[0064] 5. Preparation of battery: The positive electrode sheet, separator and negative electrode sheet are wound into a battery cell with a capacity of about 5Ah. The separator is located between the adjacent positive and negative electrode sheets, the positive electrode is spot welded with an aluminum tab, and the negative electrode is spot welded with a nickel tab; then the battery cell is placed in an aluminum-plastic packaging bag, baked, and injected with the above electrolyte, and finally made into a lithium-ion battery through packaging, formation, capacity division and other processes.

[0065] Examples 2-11 and Comparative Examples 1-4 were prepared with reference to the preparation method of Example 1. The specific components and contents of Examples 1-11 and Comparative Examples 1-4 are shown in Table 1.

[0066]

[0067]

[0068] High temperature storage test: Place the lithium-ion batteries in a 25℃ constant temperature chamber and let them stand for 30 minutes to allow the lithium-ion batteries to reach a constant temperature. Charge the lithium-ion batteries that have reached a constant temperature at a constant current of 0.5C to a voltage of 4.4V, then charge at a constant voltage of 4.4V to a current of 0.05C, then discharge at a constant current of 0.2C to a voltage of 3.0V, record the initial capacity C1, then charge at a constant voltage of 4.4V to a current of 0.05C, and record the battery thickness T1. Transfer the battery to an 85℃ incubator and let it stand for 24h. After the battery is taken out and cooled, test the battery thickness T2. Discharge at a constant current of 0.2C to a voltage of 3.0V, record the remaining capacity C2, charge at a constant current of 0.5C to a voltage of 4.4V, then charge at a constant voltage of 4.4V to a current of 0.05C, then discharge at a constant current of 0.2C to a voltage of 3.0V, and record the recovery capacity C3.

[0069] The performance test results of the secondary batteries of Examples 1-11 and Comparative Examples 1-4 are shown in Table 2.

[0070]

[0071]

[0072] By comparing the above Table 1 and Table 2, it can be obtained that the use of the electrolyte additive of the present invention can improve the thermal stability of the battery, reduce battery expansion, and improve the capacity retention rate of the battery. By comparing Examples 1-7 with Examples 8-10, it can be obtained that when PVDF is used in combination with the electrolyte additive of the present invention, relative to SBR in combination with the electrolyte additive of the present invention, when applied to a secondary battery, it has better performance, lower battery expansion rate, and the lowest expansion rate can be as low as 6.4%, and higher capacity recovery rate, which can reach 92.2%. By comparing Examples 1-3 with Examples 4-7, it can be obtained that when the electrolyte additive uses the first additive and the second additive in combination, the electrolyte obtained has a better effect, and the battery expansion rate, the battery remaining capacity ratio, and the capacity recovery rate are relatively high. By comparing Examples 1-3, it can be obtained that when the electrolyte additive is set to be Formula I-1 and Formula II-3 in a weight ratio of 2:3, the electrolyte additive obtained has a better effect, and the battery expansion rate, the battery remaining capacity ratio, and the capacity recovery rate are relatively high.

[0073] According to the disclosure and teaching of the above description, those skilled in the art to which the present invention belongs can also change and modify the above embodiment. Therefore, the present invention is not limited to the above specific embodiment, and any obvious improvement, replacement or modification made by those skilled in the art on the basis of the present invention belongs to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation to the present invention.

Claims

1. A secondary battery, characterized in that: Including electrolyte, positive electrode sheet and negative electrode sheet; The electrolyte includes additives, organic solvents and lithium salts, the additives include a first additive, a second additive and a third additive, the first additive is of formula I-1, the second additive is of formula II-1, and the third additive is 1,3-propane sultone, fluoroethylene carbonate and 1,3,6-hexane trinitrile; Formula I-1 Formula II-1 The first additive accounts for 0.1wt% to 5wt% of the total mass of the electrolyte, the second additive accounts for 0.1wt% to 10wt% of the total mass of the electrolyte, and the third additive accounts for 0.5wt% to 20wt% of the total mass of the electrolyte; The organic solvent includes ethylene carbonate, propylene carbonate, ethyl methyl carbonate, propyl propionate, and ethyl propionate, and the mass ratio of each substance is 1:1:1:1:1, and the lithium salt is lithium hexafluorophosphate; The positive electrode sheet includes a positive electrode collector and a positive electrode active material layer arranged on at least one surface of the positive electrode collector, and the negative electrode sheet includes a negative electrode collector and a negative electrode active material layer arranged on at least one surface of the negative electrode collector, the positive electrode active material layer and the negative electrode active material layer include polyvinylidene fluoride, and the content of polyvinylidene fluoride in the negative electrode sheet is 3%.

2. The secondary battery according to claim 1, characterized in that: The content of polyvinylidene fluoride in the positive electrode sheet is 0.1-3%, and the relative molecular mass of the polyvinylidene fluoride is 100000-10000000.

3. The secondary battery according to claim 1, characterized in that: The first additive and polyvinylidene fluoride satisfy the following relationship: the number of molecules of the first additive is K1, the number of molecules of polyvinylidene fluoride is K2, and (K1:K2)>5.

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