An electrolyte and a lithium-ion battery

Abstract

This application relates to the field of electrochemical technology, and in particular to an electrolyte and a lithium-ion battery. This application provides an electrolyte and a lithium-ion battery. The electrolyte includes an organic solvent, a lithium salt, and additives. Based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, the mass percentage of lithium difluorosulfonylimide is B, 0.8 ≤ A / B ≤ 1.2, and 10% ≤ A+B ≤ 16%. The additives include a first additive selected from lithium 2,2,2-trifluoroethyl sulfate. Based on the mass of the electrolyte, the mass percentage of lithium 2,2,2-trifluoroethyl sulfate is C, and 0.1% ≤ C ≤ 3%. This configuration helps to improve the corrosion resistance of aluminum foil, enhance the oxidation resistance of the electrolyte, and improve the cycle performance and rate performance of the lithium-ion battery.

Description

Technical Field

[0001] This application relates to the field of electrochemical technology, and in particular to an electrolyte and a lithium-ion battery. Background Technology

[0002] Lithium-ion batteries, due to their advantages such as high energy density, long cycle life, and no memory effect, are widely used in smartphones, wearable devices, consumer drones, and electric vehicles. With the widespread application of lithium-ion batteries in these fields, market demands for their performance are increasing. The electrolyte, as the ion carrier between the positive and negative electrodes, is crucial to the performance of lithium-ion batteries. The novel lithium salt lithium bisfluorosulfonylimide (LiFSI) exhibits good thermal stability, high conductivity, and good hydrolysis resistance, demonstrating excellent electrochemical performance in lithium-ion batteries. Compared to lithium hexafluorophosphate (LiPF6), it is more suitable for the development needs of lithium-ion batteries, such as long battery life. However, LiFSI can corrode the aluminum foil of the positive electrode current collector, thus affecting the cycle performance of the lithium-ion battery. Therefore, it is urgent to develop an electrolyte to improve the corrosion of the aluminum foil, enhance the electrolyte's oxidation resistance, and improve the cycle performance and rate performance of the lithium-ion battery. Summary of the Invention

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

[0004] The first aspect of this application provides an electrolyte comprising an organic solvent, a lithium salt, and an additive, wherein, based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, the mass percentage of lithium difluorosulfonylimide is B, 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, and based on the mass of the electrolyte, the mass percentage of lithium 2,2,2-trifluoroethyl sulfate is C, and 0.1% ≤ C ≤ 3%.

[0005] In some embodiments of this application, the mass ratio A / B of lithium hexafluorophosphate to lithium difluorosulfonylimide is 1.

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

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

[0008] In some embodiments of this application, the organic solvent is selected from at least one of carbonate compounds and ether compounds; the carbonate compound is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate; the ether compound is 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 this application provides a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte provided in the first aspect of this application.

[0010] In some embodiments of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive 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 relationship I: 7.96≤t×(A / B)-0.06 / (Ra×C×100)≤14.

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

[0012] In some embodiments of this 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 this application, the lithium-ion battery satisfies relation II: 9.83≤t×(A / B)-0.06 / (Ra×C×100)≤13.83.

[0014] The beneficial effects of this application are:

[0015] This application provides an electrolyte and a lithium-ion battery. The electrolyte includes an organic solvent, a lithium salt, and additives. Based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, and the mass percentage of lithium difluorosulfonylimide is B, with 0.8 ≤ A / B ≤ 1.2 and 10% ≤ A + B ≤ 16%. The additives include a first additive selected from lithium 2,2,2-trifluoroethyl sulfate. Based on the mass of the electrolyte, the mass percentage of lithium 2,2,2-trifluoroethyl sulfate is C, with 0.1% ≤ C ≤ 3%. This configuration helps to improve the corrosion resistance of aluminum foil, enhance the oxidation resistance of the electrolyte, and improve the cycle performance and rate performance of the lithium-ion battery.

[0016] Of course, implementing any product or method of this application does not necessarily require achieving all of the advantages described above at the same time. Detailed Implementation

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

[0018] The first aspect of this application provides an electrolyte comprising an organic solvent, a lithium salt, and an additive, wherein the lithium salt comprises lithium hexafluorophosphate and lithium difluorosulfonylimide, and based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, the mass percentage of lithium difluorosulfonylimide is B, 0.8 ≤ A / B ≤ 1.2, and 10% ≤ A + B ≤ 16%; preferably, A / B = 1; the additive comprises a first additive selected from lithium 2,2,2-trifluoroethyl sulfate, and based on the mass of the electrolyte, the mass percentage of lithium 2,2,2-trifluoroethyl 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 difluorosulfonyl imide can be 0.8, 0.9, 1, 1.1, 1.2, or any range of two of these values; the total mass percentage A+B of lithium hexafluorophosphate and lithium difluorosulfonyl imide can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, or any range of two of these values; and 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 any range of two of these values.

[0019] LiFSI exhibits good thermal stability and its electrolyte possesses high ionic conductivity, which is beneficial for improving 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 a lithium-ion battery, the Al2O3 protective layer naturally formed on the surface of the positive electrode current collector aluminum foil becomes unstable under positive electrode polarization and decomposes, releasing Al. 3+ The aluminum metal exposed by the positive electrode current collector foil is highly reactive 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, namely FSI. -Coordination can lead to the formation of unstable Al(FSI)3, or solvation with the solvent to form complex and easily soluble complexes. These complexes dissolve and diffuse into the electrolyte, causing continuous dissolution and diffusion, which in turn corrodes the aluminum foil, thus affecting the cycle performance of the lithium-ion battery. Furthermore, aluminum-containing compounds formed by aluminum foil corrosion may dissolve and deposit on the negative electrode surface, potentially causing severe electrolyte decomposition and lithium dendrite growth, leading to serious battery failures and safety issues. Therefore, this application uses LiPF6 as another main lithium salt in the electrolyte and adjusts the mass ratio of LiPF6 to LiFSI within the scope of this application to facilitate the formation of a stable AlF3 component on the positive electrode current collector aluminum foil, thereby suppressing aluminum foil corrosion.

[0020] Simultaneously, lithium 2,2,2-trifluoroethyl sulfate is introduced into the electrolyte, and the mass percentage of lithium 2,2,2-trifluoroethyl sulfate is controlled within the scope of this application. Lithium 2,2,2-trifluoroethyl sulfate has fluorinated alkyl groups and strongly coordinated sulfate groups, which can effectively increase the oxidation potential of the electrolyte system. Furthermore, under positive electrode polarization, lithium 2,2,2-trifluoroethyl sulfate can preferentially decompose on the aluminum foil surface before Al2O3 / AlF3 and capture dissolved Al. 3+ A stable organic-inorganic composite passivation film containing aluminum fluoride and sulfur-oxygen bonds is formed on the surface of the aluminum foil, thereby inhibiting 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 2,2,2-trifluoroethyl lithium sulfate as the main lithium salt based on LiPF6 and LiFSI, and adjusts the values ​​of A / B, A+B, and C within the range 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 cycle 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 of lithium hexafluorophosphate is A, the mass percentage of lithium difluorosulfonylimide is B, and 4.5% ≤ A ≤ 8.7%, 4.6% ≤ B ≤ 8.8%. For example, the mass percentage of lithium hexafluorophosphate A can be 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 8.7%, or a range of any two of these values; the mass percentage of lithium difluorosulfonylimide B can be 4.6%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 8.8%, or a range of any two of these values.

[0022] In some embodiments of this application, the additive further includes a second additive 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, where 0.5% ≤ D ≤ 10%, preferably 0.8% ≤ D ≤ 5%. For example, the mass percentage D of the second additive can be 0.5%, 0.8%, 1%, 3%, 5%, 8%, 10%, or a range of any two of these values. By further introducing the second additive into the electrolyte, the second additive alters the solvation structure of the positive electrode and the aluminum foil surface. The second additive decomposes and releases fluoride ions, which can rapidly combine with aluminum ions dissolved from the aluminum foil, generating an inorganic layer structure rich in aluminum fluoride with higher mechanical strength on the aluminum foil surface. When used simultaneously with the first additive, 2,2,2-trifluoroethyl lithium sulfate, it synergistically generates an organic-inorganic composite passivation layer that combines mechanical strength and flexibility, effectively inhibiting aluminum foil corrosion and improving electron conductivity. The first additive and the second additive in this application work synergistically to further improve the cycle performance and rate performance of lithium-ion batteries.

[0023] In some embodiments of this 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. This application does not impose any particular limitation on the content of the organic solvent in the electrolyte, as long as it achieves the purpose of this application. For example, based on the mass of the electrolyte, the mass percentage of the organic solvent is 71% to 89%. Organic solvents within the above range have a lower dielectric constant, which is beneficial for reducing Al. 3+ The solvation energy of the solvents is beneficial for inhibiting the dissolution of aluminum-containing substances, and the organic solvents within the above-mentioned range have good compatibility with aluminum foil, which is beneficial for keeping the aluminum foil in a more stable passivation state. The electrolyte of this application includes organic solvents within the above-mentioned range, which is beneficial for further improving the corrosion phenomenon of aluminum foil, thereby improving the cycle performance and rate performance of lithium-ion batteries.

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

[0025] In some embodiments of this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive 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 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 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 consisting of any two of these values. Studies have found that by adjusting the values ​​of t, Ra, A / B, and C to satisfy the above relationship, the positive electrode current collector aluminum foil and the electrolyte can have a good compatibility, which is beneficial to further improve the corrosion phenomenon of aluminum foil, thereby improving the cycle performance and rate performance of lithium-ion batteries.

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

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

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

[0029] In this application, the positive electrode includes a positive current collector and a positive electrode material layer disposed on at least one surface of the aluminum foil of the positive current collector. The phrase "positive electrode material layer disposed on at least one surface of the aluminum foil of the positive current collector" means that the positive electrode material layer can be disposed on one surface of the aluminum foil of the positive current collector along its thickness direction, or on two surfaces of the aluminum foil of the positive current collector along its thickness direction. It should be noted that "surface" here can be the entire surface of the positive current collector or a portion of the surface of the positive current collector; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the thickness of the positive electrode material layer, as long as the purpose of this application is achieved; for example, the thickness of a single-sided positive electrode material layer is 50 μm to 250 μm.

[0030] The positive electrode material layer includes a positive electrode active material. This application does not impose any particular restrictions on the positive electrode active material, as long as it can achieve the purpose of this application. For example, the positive electrode active material may include, but is not limited to, lithium nickel cobalt manganese oxide (e.g., NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0031] The positive electrode material layer also includes a positive electrode conductive agent and a positive electrode binder. This application does not impose any particular limitation on the types of positive electrode conductive agents and positive electrode binders, as long as they achieve the purpose of this application. For example, the positive 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. For example, the positive electrode binder may include, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin. This application does not impose any particular limitation on the mass ratio of the positive electrode active material, conductive agent, and binder in the positive electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0032] In this application, there are no particular limitations on the preparation method of the positive electrode sheet, as long as it achieves the purpose of this application. For example, it can be prepared by the following method: mixing positive electrode active material, positive electrode conductive agent, and positive electrode binder, adding N-methylpyrrolidone (NMP) and stirring evenly to obtain a positive electrode slurry with a solid content of 50wt% to 85wt%. The positive electrode slurry is uniformly coated on one surface of the positive electrode current collector, and after drying, a positive electrode sheet with a single-sided coating of positive electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the positive electrode current collector, and after drying, a positive electrode sheet with a double-sided coating of positive electrode material layer is obtained. After coating, the positive electrode sheet is obtained by cold pressing and cutting.

[0033] In this application, the negative electrode sheet 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 phrase "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 on two surfaces of the negative electrode current collector along its thickness direction. It should be noted that the "surface" here can be the entire surface area of ​​the negative electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved. This application has no particular limitation on the negative electrode current collector, as long as the purpose of this application is achieved; for example, the negative electrode current collector can be copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foamed nickel or foamed copper, aluminum foil, or a composite negative electrode current collector. The aforementioned 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 base layer. The material of the polymer material base layer can be, but is not limited to, at least one of polypropylene (PP), polyethylene terephthalate (PET), or polybutylene terephthalate (PBT). The material of the metal layer can be, but is not limited to, at least one of copper, copper alloy, nickel, or nickel alloy. This application does not impose any particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector, as long as the purpose of this application can be achieved. For example, the thickness of the single-sided negative electrode material layer can be from 50 μm to 180 μm, and the thickness of the negative electrode current collector can be from 3 μm to 10 μm.

[0034] The negative electrode material layer includes a negative electrode active material. This application does not have a particular limitation on the type of negative electrode active material, as long as it can achieve the purpose of this application. For example, the negative electrode active material may include, but is not limited to, graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. Graphite may include, but is not limited to, at least one of natural graphite or artificial graphite; the aforementioned silicon-based materials may include, but are not limited to, at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, or silicon alloys; the aforementioned tin-based materials may include, at least one of elemental tin, tin oxide compounds, or tin alloys.

[0035] The negative electrode material layer also includes a negative electrode conductive agent and a negative electrode binder. This application does not impose any particular limitation on the types of negative electrode conductive agents and negative electrode binders, as long as they achieve the purpose of this application. 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 fibers. The aforementioned carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The aforementioned carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or carbon nanofibers. 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), polymethyl methacrylate (PMAA), or carboxymethyl chitosan (CMCS). This application does not impose any particular limitation on the mass ratio of the negative electrode active material, conductive agent, and binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of this application is achieved.

[0036] In this application, there are no particular limitations on the preparation method of the negative electrode sheet, as long as it can achieve the purpose of this application. For example, it can be prepared by the following method: adding negative electrode active material, negative electrode conductive agent, and negative electrode binder to deionized water and stirring evenly to obtain a negative electrode slurry with a solid content of 45wt% to 70wt%. The negative electrode slurry is uniformly coated on one surface of the negative electrode current collector, and after drying, a negative electrode sheet with a single-sided negative electrode material layer is obtained. Then, the above coating steps are repeated on the other surface of the negative electrode current collector, and after drying, a negative electrode sheet with a double-sided negative electrode material layer is obtained. After coating, the negative electrode sheet is obtained by cold pressing and cutting.

[0037] In this application, the lithium-ion battery may further include a separator. This application does not impose any particular limitation on the separator, as long as it achieves the purpose of this application. For example, the separator material 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), and polyamide (PA). The type of separator may include at least one of woven membrane, nonwoven fabric, microporous membrane, composite membrane, rolled membrane, or spun membrane. In this application, the thickness of the separator is not particularly limited, as long as it achieves the purpose of this application; for example, the separator thickness may be from 4 μm to 20 μm.

[0038] In this application, the lithium-ion battery also includes a casing for housing the positive electrode, separator, negative electrode, and gel electrolyte, as well as other components known in the field of lithium-ion batteries. This application does not limit the scope of these other components. This application does not impose any particular limitation on the casing; it can be a casing known in the art, as long as it achieves the purpose of this application. For example, the casing can be a rigid casing or a flexible casing. The material of the rigid casing can be metal; this application does not limit the type of metal and can use known metal rigid casings, as long as they achieve the purpose of this application. The flexible casing can be a metal plastic film, such as aluminum-plastic film, steel-plastic film, etc.

[0039] The manufacturing process of the lithium-ion battery described in this application is well known to those skilled in the art, and this application does not impose any particular limitations. For example, the manufacturing process of a lithium-ion battery may include, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and performing operations such as winding and folding as needed to obtain a wound electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a lithium-ion battery. Alternatively, stacking the positive electrode, separator, and negative electrode in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain a stacked electrode assembly; placing the electrode assembly into a housing; injecting electrolyte into the housing and sealing it to obtain a lithium-ion battery. In addition, overcurrent protection elements, conductive plates, etc., may be placed in the housing as needed to prevent the internal pressure of the lithium-ion battery from rising and causing overcharging and over-discharging.

[0040] Example

[0041] The embodiments and comparative examples provided below illustrate the implementation of this application in more detail. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" are quality standards.

[0042] Test methods and equipment:

[0043] Aluminum foil thickness testing

[0044] The thickness of the aluminum foil is calculated by weighing a sample with a known area S and density ρ. Specifically, an analytical balance with an accuracy of 0.1 mg is used, and a sample with a punching area of ​​100 cm² is used. 2 The quantitative standard sampler removes the wrinkled outer layer of the aluminum foil roll, cuts a sample from the roll, and then uses a sampler larger than 100cm. 2 The paper sheets are sandwiched together, ensuring that the paper does not cause defects in the sample. Using a sampler, one sample is punched from the sandwiched aluminum foil at a time; the area deviation of the sample should not exceed ±0.35 cm². 2Before testing, wipe the sample with anhydrous ethanol to remove oil or dirt. Weigh the clean, dry sample on an analytical balance to four decimal places and record its mass m. Calculate the thickness t of the aluminum foil using the formula t=m / (ρ×S), in μm.

[0045] Testing the roughness of aluminum foil

[0046] Three aluminum foil samples with a side length of 50±0.2mm and consistent thickness were taken from the center and two sides of the aluminum foil in the width direction. A roughness standard sample was used for calibration. A surface roughness measuring instrument with a probe was used to measure one data point in the longitudinal and transverse directions of each sample. A total of 6 data points were obtained from the 3 samples. The arithmetic mean of them was taken as the result of this test, and recorded as Ra.

[0047] Electrolyte linear sweep voltammetry (LSV) test

[0048] A dual-electrode electrolytic cell was constructed for LSV testing. A platinum electrode was selected as the working electrode, and a lithium sheet was used as the reference electrode. The electrolytes of each embodiment or comparative example were added to the electrolytic cell, with the electrolyte level covering the electrodes. The LSV test was performed using an electrochemical workstation (model: Shanghai Chenhua Electrochemical Workstation CHI604e) with a scan frequency of 1 mV / s and a scan voltage range of OCP~5.5V (OCP refers to the open-circuit voltage value obtained by the open-circuit potential test of the electrolytic cell). The initial oxidation voltage of the electrolyte was recorded 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℃ Cyclic Performance Test

[0050] The lithium-ion battery was placed in a 25°C constant temperature test chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 1C to 4.35V, followed by constant voltage charging at 4.35V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C to 3V, and the initial discharge capacity was recorded as C0. This constitutes one charge-discharge cycle. The above charge-discharge cycle was repeated, and the number of cycles when the discharge capacity decreased to 80% of the initial discharge capacity was recorded. A higher number of cycles indicates better cycle performance of the lithium-ion battery.

[0051] 45℃ Cyclic Performance Test

[0052] The lithium-ion battery was placed in a 45°C constant temperature test chamber and left to stand for 30 minutes to allow it to reach a constant temperature. It was then charged at a constant current of 1C to 4.35V, followed by constant voltage charging at 4.35V until the cutoff current reached 0.05C. Finally, it was discharged at a constant current of 1C to 3V, and the initial discharge capacity was recorded as Q0. This constitutes one charge-discharge cycle. The above charge-discharge cycle was repeated, and the number of cycles when the discharge capacity decreased to 80% of the initial discharge capacity was recorded. A higher number of cycles indicates better cycle performance of the lithium-ion battery.

[0053] 25℃ Rate Performance Test

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

[0055] Capacity retention rate R (%) = D4 / D1 × 100%. The higher the value of 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 atmosphere with a water content of less than 0.1 ppm and an oxygen content of less than 1 ppm, ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a mass ratio of 3:7 to obtain a basic organic solvent. LiPF6 and LiFSI were then added to the basic organic solvent and mixed thoroughly. Finally, the first additive, 2,2,2-trifluoroethyl lithium sulfate, was added and mixed thoroughly to obtain the electrolyte. Based on the mass of the electrolyte, the mass percentages of LiPF6, LiFSI, and 2,2,2-trifluoroethyl lithium sulfate were 6.25% and 1%, respectively, with the remainder being the basic organic solvent.

[0059] <Preparation of the positive electrode>

[0060] LiNi, the positive electrode active material 0.6 Co 0.1 Mn 0.3 O2 (NCM613), positive electrode conductive agent Super P, conductive agent CNT, and positive electrode binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 95:3:0.5:1.5. N-methylpyrrolidone (NMP) was added as a solvent to prepare a slurry with a solid content of 65wt%. After vacuum stirring, a positive electrode slurry was obtained. The positive electrode slurry was uniformly coated on both surfaces of a positive electrode current collector aluminum foil with a thickness t of 12μm and a roughness Ra of 0.35μm. After drying at 85℃, it was cold-pressed to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. The thickness of the single-sided positive electrode material layer was 125μm. The sheet was then trimmed, cut, and slit. After slitting, it was dried at 85℃ under vacuum for 4 hours, and electrode tabs were welded to obtain a positive electrode sheet with a specification of 540mm×50mm for later use.

[0061] <Preparation of Negative Electrode Sheets>

[0062] A mixture of graphite (model: Zichen QC-6, manufacturer: Jiangxi Zichen Technology Co., Ltd.), conductive agent Super P, binder styrene-butadiene rubber (SBR), and sodium carboxymethyl cellulose (CMC) was prepared at a mass ratio of 95:1.5:2:1.5. Deionized water was added as a solvent to form a slurry with a solid content of 49wt%. The slurry was stirred evenly in a vacuum mixer to obtain the negative electrode slurry. The negative electrode slurry was uniformly coated on both surfaces of a 9μm thick copper foil current collector. After drying at 85℃, it was cold-pressed to obtain a negative electrode sheet with a double-sided coated negative electrode material layer. The thickness of the single-sided negative electrode material layer was 73μm. The sheet was then trimmed, cut, and slit. After slitting, it was dried at 85℃ under vacuum for 4 hours. The tabs were then welded to obtain a negative electrode sheet with a size of 660mm×59mm for later use.

[0063] <Preparation of the diaphragm>

[0064] The diaphragm is a PE-coated ceramic diaphragm, which was purchased from Shenzhen Xingyuan Material Technology Co., Ltd., model PE ceramic 10+2, with a total thickness of 12μm.

[0065] <Preparation of Lithium-ion Batteries>

[0066] The prepared positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrode assembly is then wound to obtain the electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag and vacuum-baked at 85°C for 48 hours. The electrolyte prepared above is then injected, and the lithium-ion battery is obtained through vacuum sealing, settling, formation, and shaping processes. The settling time is 12 hours, the formation current is 0.1C, the formation time is 6.5 hours, and the formation temperature is 25°C.

[0067] Examples 2 to 14

[0068] Except for adjusting the relevant parameters according to Table 1 in the <Preparation of Electrolyte> section, the rest is the same as in Example 1. Specifically, when the mass percentage of lithium hexafluorophosphate, lithium difluorosulfonylimide, the first additive, or the second additive in the electrolyte changes, the mass percentage of the basic organic solvent also changes accordingly.

[0069] Examples 15 to 22

[0070] Except for adjusting the relevant parameters of the positive current collector aluminum foil according to Table 1 in the <Preparation of Positive Electrode Sheet>, the rest is the same as in Example 1.

[0071] Example 23, Example 24

[0072] Except for adjusting the relevant parameters according to Table 1 in <Preparation of Electrolyte> and <Preparation of Positive Electrode>, the rest is the same as in Example 1.

[0073] Example 25

[0074] Except for the preparation of the electrolyte, in which 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 is the same as in Example 1.

[0075] Comparative Examples 1 to 6

[0076] Except for adjusting the relevant parameters according to Table 1 in the <Preparation of Electrolyte> section, the rest is the same as in Example 1. Specifically, when the mass percentage of lithium hexafluorophosphate, lithium difluorosulfonylimide, the first additive, or the second additive in the electrolyte changes, the mass percentage of the basic organic solvent also changes accordingly.

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

[0078] Table 1

[0079]

[0080] Note: In Table 1, " / " indicates that the corresponding preparation parameters or substances do not exist.

[0081] As can be seen from Examples 1 to 25 and Comparative Examples 1 to 6, the lithium-ion batteries of each embodiment of this application have electrolytes including lithium hexafluorophosphate, lithium difluorosulfonylimide, and the first additive 2,2,2-trifluoroethyl lithium sulfate. By adjusting the ranges of A / B, A+B, and C within the scope of this application, the resulting electrolyte has a larger V1 value, and the resulting lithium-ion batteries have a higher number of cycles at 25°C, a higher number of cycles at 45°C, and a higher capacity retention rate R, indicating that the lithium-ion batteries have good cycle performance and rate performance.

[0082] The mass percentage of the second additive affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1, 10 to 14, by adding the second additive to the electrolyte and controlling the mass percentage D of the second additive within the range of 0.5% to 10%, the resulting electrolyte has a larger V1 value, and the resulting lithium-ion battery has a higher number of cycles at 25°C, a higher number of cycles at 45°C, and a higher capacity retention rate R, indicating that the cycle performance and rate performance of the lithium-ion battery are further improved.

[0083] The thickness of the positive electrode current collector aluminum foil affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1, 15 to 18, by adjusting the thickness of the positive electrode current collector aluminum foil to the range of 8 μm to 20 μm, the resulting lithium-ion batteries have a higher number of cycles at 25°C, a higher number of cycles at 45°C, and a higher capacity retention rate R, indicating that the lithium-ion batteries have good cycle performance and rate performance.

[0084] The roughness Ra of the positive electrode current collector aluminum foil affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1, 19 to 22, by adjusting the roughness Ra of the positive electrode current collector aluminum foil to the range of 0.1 μm to 0.5 μm, the resulting lithium-ion batteries have higher 25°C cycle test cycle count, higher 45°C cycle test cycle count, and higher capacity retention R, indicating that the lithium-ion batteries have good cycle performance and rate performance.

[0085] The value of Equation I affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1 to 25, by adjusting the relationship between t, Ra, A / B, and C to satisfy Equation I, the resulting lithium-ion batteries have a higher number of cycles at 25°C, a higher number of cycles at 45°C, and a higher capacity retention rate R, indicating that lithium-ion batteries have good cycle performance and rate performance.

[0086] The type of organic solvent affects the cycle performance and rate performance of lithium-ion batteries. As can be seen from Examples 1 and 25, the lithium-ion batteries obtained by using organic solvents selected from the scope of this application have a higher number of cycles at 25°C, a higher number of cycles at 45°C, and a higher capacity retention rate R, indicating that the lithium-ion batteries have good cycle performance and rate performance.

[0087] The above description is only a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A lithium-ion battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the electrolyte comprises an organic solvent, a lithium salt, and additives, characterized in that, The lithium salt comprises lithium hexafluorophosphate and lithium bis(fluorosulfonyl)imide. Based on the mass of the electrolyte, the mass percentage of lithium hexafluorophosphate is A, the mass percentage of lithium bis(fluorosulfonyl)imide is B, 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, wherein the mass percentage of lithium 2,2,2-trifluoroethyl sulfate is C, based on the mass of the electrolyte, and 0.1% ≤ C ≤ 3%; The additive further includes a second additive, which is selected from at least one of fluoroethylene carbonate and difluoroethylene carbonate; based on the mass of the electrolyte, the mass percentage of the second additive is D, where 0.5% ≤ D ≤ 10%; The positive electrode sheet includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive 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; 8≤t≤20, 0.1≤Ra≤0.5; The lithium-ion battery satisfies Equation II: 9.83≤t×(A / B)-0.06 / (Ra×C×100)≤13.

83.

2. The lithium-ion battery according to claim 1, characterized in that, A / B=1.

3. The lithium-ion battery according to claim 1, characterized in that, 0.5%≤C≤1.5%。 4. The lithium-ion battery according to claim 1, characterized in that, The organic solvent is selected from at least one of carbonate compounds and ether compounds; the carbonate compound is selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, and dimethyl carbonate; the ether compound is 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.

5. The lithium-ion battery according to claim 1, characterized in that, 10≤t≤14。 6. The lithium-ion battery according to claim 1, characterized in that, 0.25≤Ra≤0.4.

Images