Electrolyte and lithium ion battery

Abstract

The invention relates to the technical field of electrochemistry, in particular to an electrolyte and a lithium ion battery. The invention provides an electrolyte and a lithium ion battery, the electrolyte comprises an organic solvent, a lithium salt and an additive, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is A, the mass percentage content of lithium bis (fluorosulfonyl) imide is B, A / B is greater than or equal to 0.8 and less than or equal to 1.2, and A + B is greater than or equal to 10% and less than or equal to 16%; the additive comprises a first additive, the first additive is selected from lithium 2, 2, 2-trifluoroethyl sulfate, based on the mass of the electrolyte, the mass percentage content of the lithium 2, 2, 2-trifluoroethyl sulfate is C, and C is larger than or equal to 0.1% and smaller than or equal to 3%. Through the arrangement, the corrosion phenomenon of the aluminum foil is improved, the oxidation resistance of the electrolyte is improved, and the cycle performance and the rate capability of the lithium ion battery are improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemistry technology, and particularly to an electrolyte and a lithium-ion battery. Background Art

[0002] Lithium-ion batteries are widely used in fields such as smart phones, wearable devices, consumer drones, and electric vehicles due to their advantages of high energy density, long cycle life, and no memory effect. With the wide application of lithium-ion batteries in the above fields, the market has higher and higher requirements for the performance of lithium-ion batteries. As the ion conduction carrier between the positive and negative electrodes of a lithium-ion battery, the electrolyte is very crucial for the performance of the lithium-ion battery. The novel lithium salt lithium bis(fluorosulfonyl)imide (LiFSI) has good thermal stability, high conductivity, and good hydrolysis resistance, and shows excellent electrochemical performance in lithium-ion batteries. Compared with lithium hexafluorophosphate (LiPF6), it is more suitable for the development needs of long endurance of lithium-ion batteries. However, LiFSI can corrode the positive current collector aluminum foil, thus affecting the cycle performance of the lithium-ion battery. Therefore, it is urgent to develop an electrolyte to improve the corrosion phenomenon of aluminum foil, improve the oxidation resistance of the electrolyte, and improve the cycle performance and rate performance of the lithium-ion battery. Summary of the Invention

[0003] The purpose of the present application is to provide an electrolyte and a lithium-ion battery to improve the corrosion phenomenon of aluminum foil, improve the oxidation resistance of the electrolyte, and improve the cycle performance and rate performance of the lithium-ion battery. The specific technical solutions are as follows:

[0004] In the first aspect of the present application, an electrolyte is provided. The electrolyte includes an organic solvent, a lithium salt, and an additive. Among them, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is A, the mass percentage content of lithium bis(fluorosulfonyl)imide is B, 0.8 ≤ A / B ≤ 1.2, and 10% ≤ A + B ≤ 16%; the additive includes a first additive, and the first additive is selected from lithium 2,2,2-trifluoroethyl sulfate. Based on the mass of the electrolyte, the mass percentage content of lithium 2,2,2-trifluoroethyl sulfate is C, and 0.1% ≤ C ≤ 3%.

[0005] In some embodiments of the present application, the mass ratio A / B of lithium hexafluorophosphate to lithium bis(fluorosulfonyl)imide is 1.

[0006] In some embodiments of the present application, 0.5% ≤ C ≤ 1.5%.

[0007] In some embodiments of the present application, the additive further includes a second additive, and the second additive is selected from at least one of fluoroethylene carbonate and difluoroethylene carbonate; based on the mass of the electrolyte, the mass percentage content of the second additive is D, and 0.5% ≤ D ≤ 10%.

[0008] In some embodiments of the present application, the organic solvent is selected from at least one of carbonate compounds and ether compounds; the carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate; the ether compounds are selected from at least one of dimethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,3-dioxolane.

[0009] The second aspect of the present application provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte provided by the first aspect of the present application.

[0010] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector is selected from aluminum foil, the thickness of the aluminum foil is t μm, and the roughness of the aluminum foil is Ra μm; the lithium-ion battery satisfies the relational expression I: 7.96 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 14.

[0011] In some embodiments of the present application, the thickness of the aluminum foil is t μm, 8 ≤ t ≤ 20, preferably, 10 ≤ t ≤ 14.

[0012] In some embodiments of the present application, the roughness of the aluminum foil is Ra μm, 0.1 ≤ Ra ≤ 0.5, preferably, 0.25 ≤ Ra ≤ 0.4.

[0013] In some embodiments of the present application, the lithium-ion battery satisfies the relational expression II: 9.83 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 13.83.

[0014] Advantages of the present application:

[0015] The present application provides an electrolyte and a lithium-ion battery. The electrolyte includes an organic solvent, a lithium salt, and an additive. Based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is A, the mass percentage content of lithium bis(fluorosulfonyl)imide is B, 0.8 ≤ A / B ≤ 1.2, 10% ≤ A + B ≤ 16%; the additive includes a first additive, and the first additive is selected from lithium 2,2,2-trifluoroethyl sulfate. Based on the mass of the electrolyte, the mass percentage content of lithium 2,2,2-trifluoroethyl sulfate is C, 0.1% ≤ C ≤ 3%. Through the above settings, it is beneficial to improve the corrosion phenomenon of the aluminum foil, improve the oxidation resistance of the electrolyte, and improve the cycle performance and rate performance of the lithium-ion battery.

[0016] Of course, it is not necessary to achieve all the above advantages simultaneously when implementing any product or method of the present application. Detailed implementation manners

[0017] The technical solution in this application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of this application, not all of them. All other embodiments obtained by those skilled in the art based on this application belong to the scope of protection of this application.

[0018] A first aspect of the present application provides an electrolyte, the electrolyte comprising an organic solvent, a lithium salt and an additive, wherein the lithium salt comprises lithium hexafluorophosphate and lithium bisfluorosulfonyl imide, based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, the mass percentage of lithium bisfluorosulfonyl imide is B, 0.8≤A / B≤1.2, 10%≤A+B≤16%; preferably, A / B=1; the additive comprises a first additive, the first additive is selected from 2,2,2-trifluoroethyl lithium sulfate, based on the mass of the electrolyte, the mass percentage of 2,2,2-trifluoroethyl lithium sulfate is C, 0.1%≤C≤3%, preferably, 0.5%≤C≤1.5%. For example, the mass ratio A / B of lithium hexafluorophosphate to lithium bisfluorosulfonyl imide can be 0.8, 0.9, 1, 1.1, 1.2 or a range consisting of any two of the values therein; the total mass percentage A+B of lithium hexafluorophosphate and lithium bisfluorosulfonyl imide can be 10%, 11%, 12%, 13%, 14%, 15%, 16% or a range consisting of any two of the values therein; the mass percentage C of lithium 2,2,2-trifluoroethyl sulfate can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 2.8%, 3% or a range consisting of any two of the values therein.

[0019] LiFSI has good thermal stability and its electrolyte has high ionic conductivity, which is beneficial to improve the rate performance and high temperature performance of secondary batteries. However, as one of the main lithium salts in the electrolyte, when an external voltage is applied to the lithium-ion battery, the Al2O3 protective layer naturally formed on the surface of the positive electrode current collector aluminum foil becomes unstable under the positive electrode polarization and decomposes to release Al 3+ The aluminum metal exposed by the positive electrode current collector aluminum foil is active and easily oxidized to produce Al 3+ The generated Al 3+ It is a strong Lewis acid and tends to react with anions in the electrolyte, i.e., FSI -It coordinates to form unstable Al(FSI)3, or undergoes a solvation effect with the solvent to form a complex and easily soluble complex. The complex dissolves and diffuses into the electrolyte, and continuous dissolution and diffusion cause corrosion of the aluminum foil, thereby affecting the cycling performance of the lithium-ion battery. In addition, the aluminum-containing compounds formed by the corrosion of the aluminum foil may dissolve and deposit on the surface of the negative electrode, which may cause serious electrolyte decomposition and lithium dendrite growth, leading to serious battery failures and safety problems. Based on this, in this application, LiPF6 is used as another main lithium salt of the electrolyte, and the mass ratio of LiPF6 to LiFSI is adjusted within the scope of this application, which is beneficial to form a stable AlF3 component on the aluminum foil of the positive current collector, thereby inhibiting the corrosion of the aluminum foil.

[0020] At the same time, lithium 2,2,2-trifluoroethyl sulfate is introduced into the electrolyte, and the mass percentage content of lithium 2,2,2-trifluoroethyl sulfate is adjusted within the scope of this application. Lithium 2,2,2-trifluoroethyl sulfate has a fluorine-substituted alkyl group and a strong coordinating sulfate group, which can effectively increase the oxidation potential of the electrolyte system. And lithium 2,2,2-trifluoroethyl sulfate can decompose preferentially on the aluminum foil surface over Al2O3 / AlF3 under positive electrode polarization and capture the dissolved Al 3+ , to form a stable organic-inorganic composite passivation film containing aluminum fluoride and sulfur-oxygen bonds on the aluminum foil surface, thereby inhibiting the further dissolution of the Al2O3 / AlF3 passivation layer on the aluminum foil surface and enhancing the corrosion resistance of the aluminum foil. The electrolyte of this application introduces lithium 2,2,2-trifluoroethyl sulfate on the basis of using LiPF6 and LiFSI as the main lithium salts, and adjusts the values of A / B, A + B, and C within the scope of this application, which is beneficial to improving the corrosion phenomenon of the aluminum foil, enhancing the oxidation resistance of the electrolyte, and improving the cycling performance and rate performance of the lithium-ion battery.

[0021] In some embodiments of this application, based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is A, and the mass percentage content of lithium bis(fluorosulfonyl)imide is B, 4.5% ≤ A ≤ 8.7%, 4.6% ≤ B ≤ 8.8%. For example, the value of the mass percentage content A of lithium hexafluorophosphate can be 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 8.7% or a range composed of any two of these values; the value of the mass percentage content B of lithium bis(fluorosulfonyl)imide can be 4.6%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 8.8% or a range composed of any two of these values.

[0022] In some embodiments of the present application, the additive further includes a second additive, and the second additive is selected from at least one of fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC); based on the mass of the electrolyte, the mass percentage of the second additive is D, 0.5% ≤ D ≤ 10%, preferably, 0.8% ≤ D ≤ 5%. For example, the value of the mass percentage D of the second additive can be 0.5%, 0.8%, 1%, 3%, 5%, 8%, 10% or a range composed of any two of these numerical values. By further introducing the second additive into the electrolyte, the second additive changes the solvation structure on the surface of the positive electrode and the aluminum foil. The second additive decomposes to release fluoride ions, which can quickly combine with the aluminum ions dissolved from the aluminum foil to form an inorganic layer structure rich in aluminum fluoride with higher mechanical strength on the surface of the aluminum foil. When used simultaneously with the first additive lithium 2,2,2-trifluoroethyl sulfate, it can synergistically generate an organic-inorganic composite passivation layer with both mechanical strength and flexibility, effectively inhibiting the corrosion of the aluminum foil and being beneficial to improving the electron conduction ability. The first additive and the second additive in the present application act synergistically, thereby further improving the cycle performance and rate performance of the lithium-ion battery.

[0023] In some embodiments of the present application, the organic solvent is selected from at least one of carbonate compounds and ether compounds; the carbonate compounds are selected from at least one of ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC); the ether compounds are selected from at least one of dimethyl ether (DME), ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,3-dioxolane. The present application has no particular limitation on the content of the organic solvent in the electrolyte, as long as the purpose of the present application can be achieved. For example, based on the mass of the electrolyte, the mass percentage of the organic solvent is 71% to 89%. The organic solvents within the above range have a relatively low dielectric constant, which is beneficial to reducing the solvation energy of Al 3+ , being beneficial to inhibiting the dissolution of aluminum-containing substances, and the organic solvents within the above range have good compatibility with the aluminum foil, being beneficial to keeping the aluminum foil in a relatively stable passivated state. The electrolyte of the present application includes the organic solvent within the above range, which is beneficial to further improving the corrosion phenomenon of the aluminum foil, thereby improving the cycle performance and rate performance of the lithium-ion battery.

[0024] The second aspect of the present application provides a lithium-ion battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte provided in the first aspect of the present application.

[0025] In some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector is selected from aluminum foils, the thickness of the aluminum foil is t μm, and the roughness of the aluminum foil is Ra μm; the lithium-ion battery satisfies the relationship I: 7.96 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 14; preferably, 9.83 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 13.83. For example, the value of the relationship I can be 7.96, 8, 8.5, 9, 9.5, 9.83, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 13.83, 14 or a range composed of any two of these values. It has been found that by adjusting the values of t, Ra, A / B, and C to satisfy the above relationship, a good compatibility can be achieved between the aluminum foil of the positive electrode current collector and the electrolyte, which is beneficial to further improving the corrosion phenomenon of the aluminum foil, thereby improving the cycle performance and rate performance of the lithium-ion battery.

[0026] In some embodiments of the present application, the thickness of the aluminum foil is t μm, 8 ≤ t ≤ 20, preferably, 10 ≤ t ≤ 14. For example, the thickness of the aluminum foil can be 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm or a range composed of any two of these values. By adjusting the thickness of the aluminum foil within the range of the present application, the aluminum foil can have high mechanical strength and strong corrosion resistance, and at the same time, the lithium-ion battery can also have high energy density.

[0027] In some embodiments of the present application, the roughness of the aluminum foil is Ra μm, 0.1 ≤ Ra ≤ 0.5, preferably, 0.25 ≤ Ra ≤ 0.4. For example, the roughness of the aluminum foil can be 0.1μm, 0.15μm, 0.2μm, 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, 0.5μm or a range composed of any two of these values. By adjusting the roughness of the aluminum foil within the range of the present application, it is beneficial for the positive electrode material layer to stably adhere to the surface of the aluminum foil, and it can also make the contact area between the aluminum foil and the electrolyte appropriate, reducing the corrosion rate of the aluminum foil.

[0028] In the present application, the aluminum foil of the positive electrode current collector can be obtained by purchase, and tested in combination with the test methods of "testing the thickness of the aluminum foil and testing the roughness of the aluminum foil" provided in the present application, and the aluminum foil with the required thickness and roughness is selected as the positive electrode current collector.

[0029] In the present application, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the aluminum foil of the positive electrode current collector. The above-mentioned "positive electrode material layer provided on at least one surface of the aluminum foil of the positive electrode current collector" means that the positive electrode material layer can be provided on one surface of the aluminum foil of the positive electrode current collector along its own thickness direction, or can be provided on both surfaces of the aluminum foil of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the positive electrode current collector or a partial area of the surface of the positive electrode current collector. There is no special limitation in the present application as long as the purpose of the present application can be achieved. The present application has no special limitation on the thickness of the positive electrode material layer as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided positive electrode material layer is 50 μm to 250 μm.

[0030] The positive electrode material layer includes a positive electrode active material. The present application has no special limitation on the positive electrode active material as long as the purpose of the present application can be achieved. For example, the positive electrode active material can include but is not limited to lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate or lithium titanate, etc.

[0031] The positive electrode material layer further includes a positive electrode conductive agent and a positive electrode binder. The present application has no special limitation on the types of the positive electrode conductive agent and the positive electrode binder as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent can include but is not limited to at least one of Super P, acetylene black, Ketjen black, carbon nanotubes, graphene or carbon fiber. For example, the positive electrode binder can include but is not limited to at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer or fluorinated acrylate resin. The present application has no special limitation on the mass ratio of the positive electrode active material, the conductive agent and the binder in the positive electrode material layer, and those skilled in the art can select according to actual needs as long as the purpose of the present application can be achieved.

[0032] In the present application, there is no special limitation on the preparation method of the positive electrode plate as long as the purpose of the present application can be achieved. For example, it can be prepared by the following method: Mix the positive electrode active material, the positive electrode conductive agent and the positive electrode binder, add N-methylpyrrolidone (NMP) and stir evenly to obtain a positive electrode slurry with a solid content of 50 wt% to 85 wt%. Coat the positive electrode slurry evenly on one surface of the positive electrode current collector, and after drying, a positive electrode plate with a single-sided coated positive electrode material layer is obtained. Then repeat the above coating step on the other surface of the positive electrode current collector, and after drying, a positive electrode plate with a double-sided coated positive electrode material layer is obtained. After coating is completed, it is cold-pressed and cut to obtain the positive electrode plate.

[0033] In the present application, the negative electrode tab includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The statement "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its thickness direction, or can be disposed on both surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or a partial area of the surface of the negative electrode current collector. There is no particular limitation in the present application, as long as the object of the present application can be achieved. There is no particular limitation on the negative electrode current collector in the present application, as long as the object of the present application can be achieved. For example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam or copper foam, aluminum foil or a composite negative electrode current collector. The above composite negative electrode current collector can be a polymer material base layer and a metal layer formed on at least one surface of the polymer material substrate. The material of the above polymer material base layer can include, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the above metal layer can include, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. There is no particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector in the present application, as long as the object of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 50 μm to 180 μm, and the thickness of the negative electrode current collector is 3 μm to 10 μm.

[0034] The negative electrode material layer includes a negative electrode active material. There is no particular limitation on the type of the negative electrode active material in the present application, as long as the object of the present application can be achieved. For example, the negative electrode active material can include, but is not limited to, graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Graphite can include, but is not limited to, at least one of natural graphite or artificial graphite; the above silicon-based materials can include, but is not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the above tin-based materials can include elemental tin, tin oxide compounds, or tin alloys.

[0035] The negative electrode material layer further includes a negative electrode conductive agent and a negative electrode binder. There are no particular limitations on the types of the negative electrode conductive agent and the negative electrode binder in this application, as long as the objectives of this application can be achieved. For example, the negative electrode conductive agent may include, but is not limited to, at least one of superconducting carbon black (Super P), acetylene black, Ketjen black, carbon nanotubes, graphene, or carbon fiber. The above-mentioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fiber may include, but is not limited to, vapor-grown carbon fiber (VGCF) and / or nanofiber. For example, the negative electrode binder may include, but is not limited to, at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), sodium carboxymethyl cellulose (CMC), polymethacrylic acid (PMAA), or carboxymethyl chitosan (CMCS). There are no particular limitations on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer in this application, and those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved.

[0036] In this application, there are no particular limitations on the preparation method of the negative electrode plate, as long as the objectives of this application can be achieved. For example, it can be prepared by the following method: Add the negative electrode active material, negative electrode conductive agent, and negative electrode binder to deionized water and stir evenly to obtain a negative electrode slurry with a solid content of 45wt% to 70wt%. Coat the negative electrode slurry evenly on one surface of the negative electrode current collector, and after drying, a negative electrode plate with a single-sided coated negative electrode material layer is obtained. Then repeat the above coating steps on the other surface of the negative electrode current collector, and after drying, a negative electrode plate with a double-sided coated negative electrode material layer is obtained. After the coating is completed, it is cold-pressed and cut to obtain the negative electrode plate.

[0037] In this application, the lithium-ion battery may further include a separator. There are no particular limitations on the separator in this application, as long as the objectives of this application can be achieved. For example, the material of the separator may include, but is not limited to, at least one of polyethylene (PE), polypropylene (PP), glass fiber, polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA). The type of the separator may include, but is not limited to, at least one of a woven film, non-woven fabric, microporous film, composite film, rolled film, or spun film. In this application, there are no particular limitations on the thickness of the separator, as long as the objectives of this application can be achieved. For example, the thickness of the separator may be 4μm to 20μm.

[0038] In this application, the lithium-ion battery further includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the gel electrolyte, as well as other components known in the field of lithium-ion batteries. This application does not limit the above-mentioned other components. This application has no special restrictions on the housing and can be a housing well-known in the art as long as it can achieve the purpose of this application. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal. This application does not limit the type of metal, and a metal hard shell housing known in the art can be used as long as it can achieve the purpose of this application. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0039] The preparation process of the lithium-ion battery of this application is well-known to those skilled in the art. This application has no special restrictions. For example, the preparation process of the lithium-ion battery can include but is not limited to the following steps: stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into the housing, injecting the electrolyte into the housing and sealing it to obtain a lithium-ion battery. Or, stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fixing the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, placing the electrode assembly into the housing, injecting the electrolyte into the housing and sealing it to obtain a lithium-ion battery. In addition, an overcurrent protection element, a guide plate, etc. can be placed in the housing as needed to prevent the pressure inside the lithium-ion battery from rising and overcharging and discharging.

[0040] Examples

[0041] Hereinafter, examples and comparative examples are given to illustrate the embodiments of this application more specifically. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0042] Test methods and equipment:

[0043] Measurement of aluminum foil thickness

[0044] By weighing the mass of an aluminum foil sample with a known area S and density ρ, the thickness of the aluminum foil is calculated. Specifically: use an analytical balance with a precision of 0.1 mg, use a quantitative standard sampler with a punching area of 100 cm 2 After removing the wrinkled part on the outer layer of the aluminum foil roll, cut a sample from the foil roll, sandwich the sample with paper with an area larger than 100 cm 2 The paper used shall not cause defects in the sample. Punch a sample on the clamped aluminum foil with the sampler once, and the area deviation of the sample should not be more than ±0.35 cm 2Before the test, wipe the specimen with absolute ethanol to remove oil or dirt. Place the specimen with a clean and dry surface on an analytical balance and weigh it accurately to the fourth decimal place, recording its mass m. Calculate the thickness t of the aluminum foil through the formula t = m / (ρ × S), where the unit of t is μm.

[0045] Measurement of Aluminum Foil Roughness

[0046] Take 1 aluminum foil sample with a side length of 50 ± 0.2 mm and the same thickness at the center and both sides in the width direction of the aluminum foil, for a total of 3 samples. Calibrate with a roughness standard sample, and use a surface roughness measuring instrument with a probe to measure one data in both the longitudinal and transverse directions for each sample. A total of 6 data are obtained from the 3 samples, and take their arithmetic mean as the measurement result of this test, denoted as Ra.

[0047] Linear Sweep Voltammetry (LSV) Test of Electrolyte

[0048] Construct a two - electrode electrolytic cell for LSV test. Select a platinum electrode as the working electrode and a lithium sheet as the reference electrode. Add the electrolyte of each example or comparative example into the electrolytic cell, and the electrolyte just needs to cover the electrodes. Use an electrochemical workstation (Model: Shanghai Chenhua Electrochemical Workstation CHI604e) to conduct the LSV test. The scanning frequency is 1 mV / s, and the scanning voltage range is OCP~5.5 V (OCP refers to the open - circuit voltage value obtained by the open - circuit potential test of the electrolytic cell). Record the initial oxidation voltage of the electrolyte as V1. The larger the value of V1, the better the oxidation resistance of the electrolyte. Applying it to lithium - ion batteries is beneficial to improving the electrochemical stability of lithium - ion batteries.

[0049] 25°C Cycle Performance Test

[0050] Place the lithium - ion battery in a 25°C constant - temperature test chamber and let it stand for 30 minutes to reach a constant temperature. Charge it at a constant current of 1C to 4.35 V, then charge it at a constant voltage of 4.35 V until the cut - off current is 0.05C, and discharge it at a constant current of 1C to 3V. Record the first - discharge capacity as C0, and this is one charge - discharge cycle. Repeat the above charge - discharge cycle, and record the number of cycles when the discharge capacity decays to 80% of the first - discharge capacity. The larger the number of cycles, the better the cycle performance of the lithium - ion battery.

[0051] 45°C Cycle Performance Test

[0052] Place the lithium-ion battery in a constant-temperature test chamber at 45°C and let it stand for 30 minutes to reach a constant temperature. Charge it at a constant current of 1C to 4.35V, then charge it at a constant voltage of 4.35V until the cut-off current is 0.05C, and discharge it at a constant current of 1C to 3V. Record the first discharge capacity as Q0, and this is one charge-discharge cycle. Repeat the above charge-discharge cycle and record the number of cycles when the discharge capacity decays to 80% of the first discharge capacity. The larger the number of cycles, the better the cycle performance of the lithium-ion battery.

[0053] 25°C Rate Performance Test

[0054] Place the lithium-ion battery in a constant-temperature test chamber at 25°C and let it stand for 30 minutes to reach a constant temperature. Charge it at a constant current of 0.5C to 4.35V, then charge it at a constant voltage of 4.35V until the cut-off current is 0.05C, and discharge it at a constant current of 0.5C to 3V. Repeat the above charge-discharge steps 10 times and record the 10th discharge capacity as D1; charge it at a constant current of 1.0C to 4.35V, charge it at a constant voltage of 4.35V until the cut-off current is 0.05C, and then discharge it at a constant current of 1.0C to 3V. Repeat the above charge-discharge cycle 10 times and record the 10th discharge capacity as D2; charge it at a constant current of 2C to 4.35V, charge it at a constant voltage of 4.35V until the cut-off current is 0.05C, and then discharge it at a constant current of 2C to 3V. Repeat the above charge-discharge cycle 10 times and record the 10th discharge capacity as D3; charge it at a constant current of 3.0C to 4.35V, charge it at a constant voltage of 4.35V until the cut-off current is 0.05C, and then discharge it at a constant current of 3.0C to 3V. Repeat the above charge-discharge cycle 10 times and record the 10th discharge capacity as D4.

[0055] Capacity Retention Rate R (%) = D4 / D1 × 100%. The larger the value of the capacity retention rate R, the better the rate performance of the lithium-ion battery.

[0056] Example 1

[0057] <Preparation of Electrolyte>

[0058] In an inert gas atmosphere with a water content of less than 0.1 ppm and an oxygen content of less than 1 ppm, mix ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a mass ratio of 3:7 to obtain a basic organic solvent. Then add LiPF6 and LiFSI to the basic organic solvent and mix evenly, and then add the first additive lithium 2,2,2-trifluoroethyl sulfate and mix evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of LiPF6 is 6.25%, the mass percentage content of LiFSI is 6.25%, and the mass percentage content of lithium 2,2,2-trifluoroethyl sulfate is 1%, and the balance is the basic organic solvent.

[0059] <Preparation of Positive Electrode Sheet>

[0060] The positive electrode active material LiNi 0.6 Co 0.1 Mn 0.3 O2 (NCM613), the positive electrode conductive agent Super P, the conductive agent CNT, and the positive electrode binder polyvinylidene fluoride (PVDF) are mixed according to a mass ratio of 95:3:0.5:1.5, and N-methylpyrrolidone (NMP) is added as a solvent to prepare a slurry with a solid content of 65 wt%. After vacuum stirring evenly, the positive electrode slurry is obtained. The positive electrode slurry is evenly coated on both surfaces of a positive electrode current collector aluminum foil with a thickness t of 12 μm and a roughness Ra of 0.35 μm, dried at 85 °C and then cold-pressed to obtain a positive electrode plate with a double-sided coated positive electrode material layer. Among them, the thickness of the single-sided positive electrode material layer is 125 μm. Then, trimming, slicing, and slitting are carried out. After slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the electrode tabs are welded to obtain a positive electrode plate with a specification of 540 mm × 50 mm for standby.

[0061] <Preparation of the negative electrode plate>

[0062] The negative electrode active material graphite (model: Zichen QC-6, manufacturer: Jiangxi Zichen Technology Co., Ltd.), the conductive agent Super P, the binder styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) are mixed according to a mass ratio of 95:1.5:2:1.5, and deionized water is added as a solvent to prepare a slurry with a solid content of 49 wt%. After being stirred evenly by a vacuum mixer, the negative electrode slurry is obtained. The negative electrode slurry is evenly coated on both surfaces of a negative electrode current collector copper foil with a thickness of 9 μm, dried at 85 °C and then cold-pressed to obtain a negative electrode plate with a double-sided coated negative electrode material layer. Among them, the thickness of the single-sided negative electrode material layer is 73 μm. Then, trimming, slicing, and slitting are carried out. After slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the electrode tabs are welded to obtain a negative electrode plate with a specification of 660 mm × 59 mm for standby.

[0063] <Preparation of the separator>

[0064] The separator uses a PE-coated ceramic separator, which is purchased from Shenzhen Xingyuan Materials Technology Co., Ltd., with a model of PE Ceramic 10 + 2, and the total thickness of the separator is 12 μm.

[0065] <Preparation of the lithium-ion battery>

[0066] The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in sequence, with the separator placed in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and then wound to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum baked at 85 °C for 48 h, and then the prepared electrolyte is injected. After vacuum packaging, standing, formation, shaping and other processes, a lithium-ion battery is obtained. Among them, the standing time is 12 hours, the formation current is 0.1C, the formation time is 6.5 h, and the formation temperature is 25 °C.

[0067] Examples 2 to 14

[0068] Except that in <Preparation of electrolyte>, the relevant parameters are adjusted according to Table 1, the rest are the same as in Example 1. Among them, when the mass percentage content of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, the first additive or the second additive in the electrolyte changes, the mass percentage content of the basic organic solvent changes accordingly.

[0069] Examples 15 to 22

[0070] Except that in <Preparation of positive electrode sheet>, the relevant parameters of the positive current collector aluminum foil are adjusted according to Table 1, the rest are the same as in Example 1.

[0071] Examples 23, 24

[0072] Except that in <Preparation of electrolyte> and <Preparation of positive electrode sheet>, the relevant parameters are adjusted according to Table 1, the rest are the same as in Example 1.

[0073] Example 25

[0074] Except that in <Preparation of electrolyte>, the basic organic solvent is prepared by mixing dimethyl ether (DME), ethylene carbonate (EC), and ethyl methyl carbonate (EMC) in a mass ratio of 2:3:5, the rest are the same as in Example 1.

[0075] Comparative Examples 1 to 6

[0076] Except that in <Preparation of electrolyte>, the relevant parameters are adjusted according to Table 1, the rest are the same as in Example 1. Among them, when the mass percentage content of lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, the first additive or the second additive in the electrolyte changes, the mass percentage content of the basic organic solvent changes accordingly.

[0077] The preparation parameters and performance parameters of each example and comparative example are shown in Table 1.

[0078] Table 1

[0079] Note: " / " in Table 1 indicates the absence of corresponding preparation parameters or substances.

[0080] It can be seen from Examples 1 to 25 and Comparative Examples 1 to 6 that for the lithium-ion batteries of each embodiment of the present application, the electrolyte includes lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, and the first additive lithium 2,2,2-trifluoroethyl sulfate, and the ranges of A / B, A+B, and C are regulated within the scope of the present application. The obtained electrolyte has a larger V1 value, and the obtained lithium-ion battery has a higher number of 25°C cycle test cycles, a higher number of 45°C cycle test cycles, and a higher capacity retention rate R, indicating that the lithium-ion battery has good cycle performance and rate performance.

[0081] The mass percentage content of the second additive will affect the cycle performance and rate performance of the lithium-ion battery. It can be seen from Example 1 and Examples 10 to 14 that by adding the second additive to the electrolyte and regulating the value of the mass percentage content D of the second additive within the range of 0.5% to 10%, the obtained electrolyte has a larger V1 value, and the obtained lithium-ion battery has a higher number of 25°C cycle test cycles, a higher number of 45°C cycle test cycles, and a higher capacity retention rate R, indicating that the cycle performance and rate performance of the lithium-ion battery are further improved.

[0082] The thickness of the positive current collector aluminum foil will affect the cycle performance and rate performance of the lithium-ion battery. It can be seen from Example 1 and Examples 15 to 18 that by regulating the thickness of the positive current collector aluminum foil within the range of 8 μm to 20 μm, the obtained lithium-ion battery has a higher number of 25°C cycle test cycles, a higher number of 45°C cycle test cycles, and a higher capacity retention rate R, indicating that the lithium-ion battery has good cycle performance and rate performance.

[0083] The surface roughness Ra of the positive current collector aluminum foil will affect the cycle performance and rate performance of the lithium-ion battery. It can be seen from Example 1 and Examples 19 to 22 that by regulating the surface roughness Ra of the positive current collector aluminum foil within the range of 0.1 μm to 0.5 μm, the obtained lithium-ion battery has a higher number of 25°C cycle test cycles, a higher number of 45°C cycle test cycles, and a higher capacity retention rate R, indicating that the lithium-ion battery has good cycle performance and rate performance.

[0084] The value of Relationship I will affect the cycle performance and rate performance of the lithium-ion battery. It can be seen from Examples 1 to 25 that by regulating the relationship between t, Ra, A / B, and C to satisfy Relationship I, the obtained lithium-ion battery has a higher number of 25°C cycle test cycles, a higher number of 45°C cycle test cycles, and a higher capacity retention rate R, indicating that the lithium-ion battery has good cycle performance and rate performance.

[0085] The type of organic solvent will affect the cycling performance and rate performance of lithium-ion batteries. As can be seen from Example 1 and Example 25, by using an organic solvent selected within the scope of this application, the obtained lithium-ion battery has a higher number of cycling test cycles at 25°C, a higher number of cycling test cycles at 45°C, and a higher capacity retention rate R, indicating that the lithium-ion battery has good cycling performance and rate performance.

[0086] The above are only the preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of this application shall be included within the scope of protection of this application.

Claims

1. An electrolyte, the electrolyte comprising an organic solvent, a lithium salt, and an additive, characterized in that, The lithium salt includes lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide. Based on the mass of the electrolyte, the mass percentage content of lithium hexafluorophosphate is A, and the mass percentage content of lithium bis(fluorosulfonyl)imide is B, where 0.8 ≤ A / B ≤ 1.2 and 10% ≤ A + B ≤ 16%. The additive includes a first additive selected from lithium 2,2,2-trifluoroethyl sulfate. Based on the mass of the electrolyte, the mass percentage content of lithium 2,2,2-trifluoroethyl sulfate is C, where 0.1% ≤ C ≤ 3%.

2. The electrolyte according to claim 1, characterized in that, A / B = 1.

3. The electrolyte according to claim 1, characterized in that, 0.5%≤C≤1.5%。 4. The electrolyte according to claim 1, wherein, The additive further includes a second additive selected from at least one of fluoroethylene carbonate and difluoroethylene carbonate; based on the mass of the electrolyte, the mass percentage content of the second additive is D, where 0.5% ≤ D ≤ 10%.

5. The electrolyte according to claim 1, characterized in that, The organic solvent is selected from at least one of carbonate compounds and ether compounds; the carbonate compounds are selected from at least one of ethylene carbonate, propylene carbonate, ethyl methyl carbonate, diethyl carbonate, and dimethyl carbonate; the ether compounds are selected from at least one of dimethyl ether, ethylene glycol dimethyl ether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, and 1,3-dioxolane.

6. A lithium-ion battery, characterized in that, The lithium ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, and the electrolyte according to any one of claims 1 to 5.

7. The lithium-ion battery according to claim 6, wherein The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The positive electrode current collector is selected from aluminum foil, the thickness of the aluminum foil is t μm, and the surface roughness of the aluminum foil is Ra μm. The lithium ion battery satisfies the relationship I: 7.96 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 14.

8. The lithium ion battery according to claim 7, characterized in that, 8≤t≤20。 9. The lithium ion battery according to claim 7, wherein, 10≤t≤14。 10. The lithium-ion battery according to claim 7, wherein, 0.1 ≤ Ra ≤ 0.

5.

11. The lithium ion battery according to claim 7, wherein 0.25 ≤ Ra ≤ 0.

4.

12. The lithium ion battery according to any one of claims 7 to 11, characterized in that, The lithium ion battery satisfies the relationship II: 9.83 ≤ t×(A / B) - 0.06 / (Ra×C×100) ≤ 13.83.

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