Secondary battery and electronic device

Abstract

The application provides a secondary battery and an electronic device. The secondary battery of the application comprises a negative electrode, the negative electrode comprises a current collector and a negative electrode active material layer, the negative electrode active material layer comprises a first active material layer and a second active material layer, the first active material layer is arranged between the current collector and the second active material layer, the first active material layer comprises a first active material, the average particle sphericity SDa of the first active material satisfies 0.4≤SDa≤0.7, the Dv50a of the first active material satisfies 14 mu m≤Dv50a≤18 mu m, the second active material layer comprises a second active material, the average particle sphericity SDb of the second active material satisfies 0.7≤SDb≤1, and the Dv50b of the second active material satisfies 8 mu m≤Dv50b≤16 mu m. The secondary battery of the application has high energy density and excellent fast charging capability.

Description

Technical Field

[0001] This application relates to the field of energy storage. Specifically, this application relates to a secondary battery and an electronic device. Background Technology

[0002] Rechargeable batteries, with their advantages of high operating voltage, wide operating temperature range, and low self-discharge rate, are widely used in mobile electronic devices, home appliances, and electric vehicles. With the development of the new energy industry, higher demands are being placed on rechargeable batteries. How to achieve high energy density while simultaneously maintaining other electrochemical properties, especially fast-charging performance, remains a key challenge in the rechargeable battery field. Currently, conventional electrode designs in rechargeable batteries are typically single-layered, and the active materials used often lack both high energy density and fast-charging capability. This often requires either reducing coating weight to improve fast-charging capability or increasing coating weight to improve the energy density of lithium-ion rechargeable batteries. However, while reducing coating weight to improve fast-charging capability, the limited amount of active material leads to significant energy density loss; similarly, while increasing coating weight to improve energy density, the larger electrode thickness results in longer active ion transport paths, leading to poorer fast-charging capability. Summary of the Invention

[0003] In view of the above-mentioned problems of the prior art, this application provides a secondary battery that improves fast charging performance while minimizing the impact on its volumetric energy density, thus possessing both high energy density and excellent fast charging capability.

[0004] A first aspect of this application provides a secondary battery comprising a negative electrode, which includes a current collector and a negative electrode active material layer. The negative electrode active material layer includes a first active material layer and a second active material layer, wherein the first active material layer is disposed between the current collector and the second active material layer. The first active material layer includes a first active material, wherein the average particle sphericity SDa of the first active material satisfies 0.4 ≤ SDa ≤ 0.7, and the Dv50a of the first active material satisfies 14 μm ≤ Dv50a ≤ 18 μm. The second active material layer includes a second active material, wherein the average particle sphericity SDb of the second active material satisfies 0.7 ≤ SDb ≤ 1, and the Dv50b of the second active material satisfies 8 μm ≤ Dv50b ≤ 16 μm. The particle sphericity of the active material measures the degree to which the particles tend to be spherical. Smaller particle sphericity indicates a more layered structure in the active material, with a relatively larger particle size (Dv50). Under pressure, the particles primarily make surface contact, resulting in denser particle packing. This helps maintain a high compaction density in the electrode, thereby improving the volumetric energy density of the secondary battery. However, excessively dense particle packing leads to fewer pores in the electrode, making it difficult for the electrolyte to wet and penetrate, hindering lithium-ion transport and affecting the battery's kinetic performance. Conversely, larger particle sphericity indicates a more spherical structure in the active material. Smaller particle sizes are conducive to forming a highly spherical particle structure. Under pressure, the particles primarily make point or point-line contact, which helps maintain a richer pore structure in the electrode, promoting electrolyte wetting and penetration, accelerating lithium-ion transport, and improving the battery's kinetic performance. However, sparser particle packing results in a lower electrode compaction density, thus reducing the battery's volumetric energy density. The secondary battery of this application adopts a double-layer active material design. The upper layer (second active material layer) is selected with active material with high sphericity and slightly small particle size, while the lower layer (first active material layer) is selected with active material with low sphericity and large particle size. At the same time, the sphericity and particle size of the upper and lower active materials are controlled within the above range, so that the electrode has both high compaction density and allows lithium ions to diffuse and transfer rapidly on the surface of the electrode. Thus, the secondary battery can achieve both high energy density and excellent kinetic performance.

[0005] In some implementations, 0.4 ≤ SDa ≤ 0.7. When the sphericity of the first active material is too low, the particles are too densely packed, resulting in fewer pores on the electrode, making it difficult for the electrolyte to wet and penetrate, causing difficulties in lithium-ion transport, and thus affecting the kinetic performance of the secondary battery.

[0006] In some embodiments, 0.4 ≤ SDa ≤ 0.5, and 15 μm ≤ Dv50a ≤ 18 μm. When the sphericity and particle size of the first active material are both within this range, the secondary battery has a high volumetric energy density while ensuring certain kinetic performance.

[0007] In some implementations, 0.7 ≤ SDb ≤ 1. When the sphericity of the second active material layer is high, the particles are more sparsely packed, which leads to a lower electrode compaction density, thereby reducing the volumetric energy density of the secondary battery.

[0008] In some embodiments, 0.9 ≤ SDb ≤ 1, and 8 μm ≤ Dv50b ≤ 10 μm. When the sphericity and particle size of the second active material are both within this range, the secondary battery exhibits superior kinetic performance while ensuring a certain volumetric energy density.

[0009] In some implementations, SDa < SDb, Dv50b < Dv50a.

[0010] In some embodiments, the porosity of the first active material layer is Pa, and the porosity of the second active material layer is Pb, wherein 10% ≤ Pa ≤ 20% and 20% ≤ Pb ≤ 40%. When the porosities of the first and second active material layers are within the above ranges, the secondary battery can have high volumetric energy density and excellent kinetic performance.

[0011] In some implementations, 13% ≤ Pa ≤ 20%, and 25% ≤ Pb ≤ 40%.

[0012] In some embodiments, the first active material is selected from artificial graphite and / or natural graphite, and the second active material is selected from artificial graphite.

[0013] In some embodiments, the first active material is selected from artificial graphite, and the second active material is selected from artificial graphite.

[0014] In some implementations, XRD testing is used to determine the diffraction peak area C004 (004 crystal plane) and the diffraction peak area C110 (110 crystal plane) of the negative electrode, satisfying the following condition: 12 ≤ C004 / C110 ≤ 18. C004 / C110 represents the crystal orientation of the active material particles, where the C004 crystal plane is parallel to the current collector direction, and the C110 crystal plane is perpendicular to the current collector direction. A smaller ratio is more favorable for lithium-ion intercalation, but the electrode is more prone to expansion in multiple directions, leading to a higher risk of deformation. A larger ratio hinders lithium-ion intercalation, affecting kinetic performance.

[0015] In some embodiments, the negative electrode includes a negative electrode active material layer located on the surface of the current collector. The negative electrode active material layer includes a first material layer and a second material layer, wherein the negative electrode active material layer includes a negative electrode active material with a specific surface area of ​​0.8 m². 2 / g to 3.0m 2 / g. If the specific surface area of ​​the negative electrode active material is too small, its exposed surface is too small, which will hinder the intercalation of lithium ions and affect the kinetic performance. If the specific surface area of ​​the negative electrode active material is too large, its exposed surface is too large, the contact area with the electrolyte is increased, resulting in too many side reactions, causing too much irreversible capacity loss, and thus affecting the energy density.

[0016] In some embodiments, the tap density of the first active material is 0.95 g / cm³. 3 Up to 1.2 g / cm 3 The tap density of the second active material is 0.8 g / cm³. 3 Up to 1.05 g / cm 3 Tap density reflects the slurry processing performance of active materials. For active materials with low sphericity, the tap density is relatively high, resulting in better processing performance; conversely, for active materials with high sphericity, the tap density is relatively low, resulting in poorer processing performance. When the tap densities of the first and second active materials are within the aforementioned range, they can adequately meet processing requirements.

[0017] In some embodiments, the compaction density of the negative electrode is 1.70 g / cm³. 3 Up to 1.80 g / cm 3 When the compaction density of the negative electrode is too high, it can easily lead to over-pressure on the electrode, affecting electrode processing and electrical performance. When the compaction density of the negative electrode is too low, it cannot effectively improve the energy density of the secondary battery.

[0018] A second aspect of this application provides an electronic device comprising the secondary battery of the first aspect.

[0019] The secondary battery of this application adopts a double-layer electrode design. The lower layer, which is closer to the current collector, uses an active material with low sphericity and large particle size, so that the negative electrode has a high compaction density, thereby improving the energy density of the secondary battery. The upper layer uses an active material with higher sphericity and slightly smaller particle size, so that the surface of the negative electrode has a better pore structure, which promotes the penetration of electrolyte and the diffusion of active ions, thereby improving the kinetic performance of the secondary battery. In this way, the secondary battery can achieve both high energy density and excellent fast charging capability. Attached Figure Description

[0020] Figure 1 These are SEM images of the first active material and the second active material in the secondary battery of Embodiment 5 of this application, wherein 5a is the SEM image of the first active material and 5b is the SEM image of the second active material.

[0021] Figure 2The particle sphericity distribution of the first active material and the second active material in the secondary battery of Embodiment 5 of this application is shown, wherein 1 is the particle sphericity distribution of the first active material and 2 is the particle sphericity distribution of the second active material.

[0022] Figure 3 This is a comparison diagram of the kinetic performance of the secondary batteries in Example 5 and Comparative Example 1 of this application. Detailed Implementation

[0023] In the description of this application, unless otherwise stated, "above" and "below" include the stated number.

[0024] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0025] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0026] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0027] Primary and secondary batteries

[0028] The secondary battery of this application includes a negative electrode, which comprises a current collector and a negative electrode active material layer. The negative electrode active material layer includes a first active material layer and a second active material layer, with the first active material layer disposed between the current collector and the second active material layer. The first active material layer comprises a first active material, the average particle sphericity SDa of which satisfies 0.4 ≤ SDa ≤ 0.7, and the Dv50a of which satisfies 14 μm ≤ Dv50a ≤ 18 μm. The second active material layer comprises a second active material, the average particle sphericity SDb of which satisfies 0.7 ≤ SDb ≤ 1, and the Dv50b of which satisfies 8 μm ≤ Dv50b ≤ 16 μm. The first active material layer uses a first active material with sphericity and particle size within the above-mentioned ranges, resulting in a higher compaction density of the negative electrode, thereby improving the energy density of the secondary battery. The second active material layer uses a second active material with sphericity and particle size within the above-mentioned ranges, resulting in a better porous structure on the surface of the negative electrode, promoting electrolyte penetration and active ion diffusion, thereby improving the kinetic performance of the secondary battery. The synergistic effect of the first and second active materials enables the secondary battery to achieve both high energy density and excellent fast charging capability.

[0029] In this application, Dv50 of the active material indicates that 50% of the particles in the volumetric particle size distribution of the active material are smaller than this value.

[0030] In some embodiments, 0.4 ≤ SDa ≤ 0.7. In some embodiments, SDa is a range of 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, 0.53, 0.55, 0.57, 0.6, 0.63, 0.65, 0.67, 0.7, or any combination of these values. When the sphericity of the first active material is too low, the particles are too densely packed, resulting in fewer pores in the electrode, making it difficult for the electrolyte to wet and penetrate, causing difficulties in lithium-ion transport, and thus affecting the kinetic performance of the secondary battery.

[0031] In some implementations, Dv50a is a range of 14 μm, 14.3 μm, 14.5 μm, 14.7 μm, 15 μm, 15.3 μm, 15.5 μm, 15.7 μm, 16 μm, 16.3 μm, 16.5 μm, 16.7 μm, 17 μm, 17.3 μm, 17.5 μm, 17.7 μm, 18.0 μm, or any combination of these values.

[0032] In some embodiments, 0.4 ≤ SDa ≤ 0.5, and 15 μm ≤ Dv50a ≤ 18 μm. When the sphericity and particle size of the first active material are both within this range, the secondary battery has a high volumetric energy density while ensuring certain kinetic performance.

[0033] In some embodiments, 0.7 ≤ SDb ≤ 1. In some embodiments, SDb is 0.70, 0.75, 0.77, 0.8, 0.83, 0.85, 0.87, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, 1, or a range of any two of these values. When the sphericity of the second active material layer is high, the particles are more sparsely packed, resulting in a lower electrode compaction density, thereby reducing the volumetric energy density of the secondary battery.

[0034] In some embodiments, 0.9 ≤ SDb ≤ 1, and 8 μm ≤ Dv50b ≤ 10 μm. When the sphericity and particle size of the second active material are both within this range, the secondary battery exhibits superior kinetic performance while ensuring a certain volumetric energy density.

[0035] In some implementations, SDa < SDb, Dv50b < Dv50a.

[0036] In some embodiments, the porosity of the first active material layer is Pa, and the porosity of the second active material layer is Pb, wherein 10% ≤ Pa ≤ 20% and 20% ≤ Pb ≤ 40%. When the porosities of the first and second active material layers are within the above ranges, the secondary battery can have high volumetric energy density and excellent kinetic performance.

[0037] In some embodiments, Pa is 10%, 12%, 13%, 13.5%, 14%, 14.5%, 15%, 15.5%, 16%, 16.5%, 17%, 17.5%, 18%, 18.5%, 19%, 19.5%, 20%, or any combination of these values. When Pa is below 10%, the inner layer of the electrode has too few pores, preventing further electrolyte penetration, and even locally preventing electrolyte retention. This leads to lithium ion intercalation and deintercalation, resulting in reduced capacity, affecting energy density, and increased impedance, which reduces kinetic performance. When Pa is above 20%, the inner layer of the electrode has sparse particle packing, making it difficult to maintain a high compaction density, thus affecting energy density.

[0038] In some implementations, Pb is 20%, 22%, 23%, 24%, 25%, 25.5%, 26%, 26.5%, 27%, 27.5%, 28%, 28.5%, 29%, 29.5%, 30%, 30.5%, 31%, 32.5%, 33%, 33.4%, 34%, 34.5%, 35%, 35.5%, 36%, 36.5%, 37%, 37.5%, 38%, 38.5%, 39%, 40%, or a range of any two of these values. When Pb is below 20%, the electrode surface does not have enough pores for the diffusion of electrolyte and lithium ions, thus failing to improve kinetic performance. When Pb is above 40%, the surface particles are too loosely packed, hindering the transport of lithium ions between particles, which also fails to improve kinetic performance. Furthermore, excessive particle exposure leads to increased side reactions, resulting in excessive irreversible capacity loss and affecting energy density.

[0039] In some implementations, 13% ≤ Pa ≤ 20%, and 25% ≤ Pb ≤ 40%.

[0040] In some embodiments, the first active material is selected from artificial graphite and / or natural graphite, and the second active material is selected from artificial graphite.

[0041] In some embodiments, the first active material is selected from artificial graphite. In some embodiments, the second active material is selected from artificial graphite.

[0042] In some embodiments, artificial graphite is obtained by high-temperature graphitization of carbonaceous raw materials such as needle coke, petroleum coke, and pitch coke.

[0043] In some embodiments, XRD testing is used to determine that the diffraction peak area C004 of the 004 crystal plane and the diffraction peak area C110 of the 110 crystal plane of the negative electrode satisfy the following condition: 12 ≤ C004 / C110 ≤ 18. In some embodiments, C004 / C110 is a range of 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, or any combination of these values. C004 / C110 represents the crystal orientation of the active material particles, where the C004 crystal plane is parallel to the current collector direction and the C110 crystal plane is perpendicular to the current collector direction. A smaller ratio is more conducive to lithium-ion intercalation, but the electrode is more prone to expansion in multiple directions, leading to a higher risk of deformation; a larger ratio hinders lithium-ion intercalation and affects kinetic performance.

[0044] In some embodiments, the negative electrode includes a negative electrode active material layer located on the surface of the current collector. The negative electrode active material layer includes a first material layer and a second material layer, wherein the negative electrode active material layer includes a negative electrode active material with a specific surface area of ​​0.8 m². 2 / g to 3.0m 2 / g. In some embodiments, the specific surface area of ​​the active material is 0.8m². 2 / g, 1.0m 2 / g, 1.2m 2 / g, 1.4m 2 / g, 1.6m 2 / g, 1.8m 2 / g, 2.0m 2 / g, 2.2m 2 / g, 2.4m 2 / g, 2.6m 2 / g, 2.8m 2 / g, 3.0m 2 / g or a range of any two of these values. If the specific surface area of ​​the negative electrode active material is too small, its exposed surface is too small, which will hinder lithium-ion intercalation and affect kinetic performance. If the specific surface area of ​​the negative electrode active material is too large, its exposed surface is too large, increasing the contact area with the electrolyte, leading to excessive side reactions, excessive irreversible capacity loss, and consequently affecting energy density.

[0045] In some embodiments, the tap density of the first active material is 0.95 g / cm³. 3 Up to 1.2 g / cm 3 For example, 0.95 g / cm³ 3 1.0g / cm 3 1.05g / cm 3 1.1g / cm 3 Or 1.15g / cm 3 1.2g / cm 3 In some embodiments, the tap density of the second active material is 0.8 g / cm³. 3 Up to 1.05 g / cm 3 For example, 0.8 g / cm³ 3 0.85g / cm 3 0.9g / cm 3 0.95g / cm 3 or 1.0g / cm 3 1.05g / cm 3 Tap density reflects the slurry processing performance of active materials. For active materials with low sphericity, the tap density is relatively high, resulting in better processing performance; conversely, for active materials with high sphericity, the tap density is relatively low, resulting in poorer processing performance. When the tap densities of the first and second active materials are within the aforementioned range, they can adequately meet processing requirements.

[0046] In some embodiments, the compaction density of the negative electrode is 1.70 g / cm³. 3Up to 1.80 g / cm 3 In some embodiments, the compaction density of the negative electrode is 1.73 g / cm³. 3 1.75g / cm 3 1.77g / cm 3 1.79g / cm 3 Or a range of any two of these values. If the compaction density of the negative electrode is too high, it can easily lead to over-pressure of the electrode, which will negatively affect the electrode processing and electrical performance. If the compaction density of the negative electrode is too low, it will not be able to effectively improve the energy density of the secondary battery.

[0047] In some embodiments, the negative electrode active material layer further includes a binder and a conductive agent. In some embodiments, the binder includes at least one selected from styrene-butadiene rubber, polyacrylic acid, polyacrylate, polyimide, polyamide-imide, polyvinylidene fluoride, polydifluoroethylene, polytetrafluoroethylene, waterborne acrylic resin, polyvinyl alcohol formal, or styrene-acrylic acid copolymer resin. In some embodiments, any conductive material can be used as the conductive material, as long as it does not cause a chemical change. In some embodiments, the conductive material includes at least one selected from conductive carbon black, acetylene black, carbon nanotubes, Ketjen black, conductive graphite, or graphene.

[0048] In some embodiments, the current collector of the negative electrode may be copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

[0049] In some embodiments, the secondary battery also includes a positive electrode, which comprises a positive electrode current collector and a positive electrode active material layer.

[0050] In some embodiments, the positive electrode active material layer includes a positive electrode active material, a binder, and a conductive agent. In some embodiments, the positive electrode active material may include at least one of lithium cobalt oxide, lithium nickel manganese cobalt oxide, lithium nickel manganese aluminum oxide, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganese oxide, spinel-type lithium nickel manganese oxide, and lithium titanate. In some embodiments, the binder may include at least one of various adhesive polymers, such as polyvinylidene fluoride, polytetrafluoroethylene, polyolefins, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, modified polyvinylidene fluoride, modified SBR rubber, or polyurethane. In some embodiments, any conductive material may be used as the conductive agent, as long as it does not cause a chemical change. Examples of conductive agents include: carbon-based materials, such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.; conductive polymers, such as polyphenylene derivatives, etc.; or mixtures thereof.

[0051] In some embodiments, the positive electrode current collector can be a metal foil or a composite current collector. For example, aluminum foil can be used. Composite current collectors can be formed by forming a metallic material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy, etc.) on a polymer substrate.

[0052] The secondary battery of this application also includes a separator. The material and shape of the separator used in the secondary battery of this application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator comprises a polymer or inorganic material formed from a material stable to the electrolyte of this application.

[0053] For example, the separator may include a substrate layer and a surface treatment layer. The substrate layer is a nonwoven fabric, membrane, or composite membrane with a porous structure, and the material of the substrate layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous membrane, a polyethylene porous membrane, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite membrane may be selected.

[0054] A surface treatment layer is disposed on at least one surface of the substrate layer. The surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixture of polymer and inorganic material. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of polyvinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the polymer material is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl alkoxy, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0055] The secondary battery of this application also includes an electrolyte. The electrolyte that can be used in this application can be any electrolyte known in the prior art.

[0056] According to some embodiments of this application, the electrolyte includes an organic solvent, a lithium salt, and optional additives. The organic solvent in the electrolyte of this application can be any organic solvent known in the prior art that can be used as an electrolyte solvent. There are no limitations on the electrolyte used in the electrolyte of this application; it can be any electrolyte known in the prior art. The additives in the electrolyte of this application can be any additives known in the prior art that can be used as electrolyte additives. In some embodiments, the organic solvent includes, but is not limited to: ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes ether solvents, such as at least one selected from 1,3-dioxane (DOL) and dimethyl glycol ether (DME). In some embodiments, the lithium salt includes at least one selected from organic lithium salts or inorganic lithium salts. In some embodiments, the lithium salt includes, but is not limited to: lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis(trifluoromethanesulfonyl)imide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalatoborate)borate LiB(C2O4)2 (LiBOB), or lithium difluorooxalatoborate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0057] In some implementations, the secondary battery is a wound secondary battery or a stacked secondary battery.

[0058] According to some embodiments of this application, the secondary battery of this application includes, but is not limited to, lithium-ion batteries or sodium-ion batteries. In some embodiments, the secondary battery includes a lithium-ion battery.

[0059] II. Electronic Devices

[0060] This application further provides an electronic device that includes the secondary battery of the first aspect of this application.

[0061] The electronic devices or apparatus described in this application are not particularly limited. In some embodiments, the electronic devices described in this application include, but are not limited to, laptops, pen input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors, etc.

[0062] Unless otherwise specified, all reagents, materials and instruments used in the following examples and comparative examples are commercially available.

[0063] Examples and Comparative Examples

[0064] 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.

[0065] Examples 1 to 8, Comparative Examples 1 to 4

[0066] 1. Preparation of negative electrode active materials

[0067] The graphite material used in this case is artificial graphite, specifically made from at least one of the carbonaceous raw materials such as needle coke, petroleum coke, and pitch coke as a precursor. The graphitized product is obtained through crushing, grading, granulation, and high-temperature graphitization. The aggregate particle size of the graphitized product can be adjusted by controlling the degree of crushing of the precursor. The obtained graphitized product is fed into a shaping zone for grinding. The sphericity of the sample can be adjusted by regulating the working frequency of the shaping equipment and the grinding time. After adjusting the degree of crushing of the active material precursor (referring to the incoming material) and the aforementioned sphericity shaping, the sample is graded in a self-diverting grading zone to obtain a sample with the desired particle size distribution.

[0068] 2. Preparation of the negative electrode

[0069] The first negative electrode active material (the aforementioned artificial graphite), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) are uniformly dispersed in an appropriate amount of deionized water at a mass ratio of 97.5:1.2:1.3 to obtain slurry 1. The second negative electrode active material (the aforementioned artificial graphite), sodium carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) are uniformly dispersed in an appropriate amount of deionized water at a mass ratio of 97.5:1.2:1.3 to obtain slurry 2. Copper foil is used as a current collector. Slurry 1 is used as the first active material layer, and slurry 2 is used as the second active material layer, and both are uniformly coated on the current collector. After coating, the material is dried and cold-pressed to obtain the negative electrode sheet, also known as the negative electrode. The porosity of the first active material layer is 9%, and the porosity of the second active material layer is 20%.

[0070] 3. Preparation of the positive electrode

[0071] The positive electrode uses lithium cobalt oxide (chemical formula: LiCoO2) as the active material. It is mixed with conductive agent acetylene black and binder polyvinylidene fluoride (PVDF) in a weight ratio of 96.3:2.2:1.5 in an appropriate amount of N-methylpyrrolidone (NMP) solvent to form a uniform positive electrode slurry. The slurry is coated onto the current collector Al foil, dried and cold-pressed to obtain the positive electrode sheet, also known as the positive electrode.

[0072] 4. Preparation of electrolyte

[0073] In a dry argon-atmospheric glove box, ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a mass ratio of EC:PC:EMC:DEC = 1:3:3:3. Then, fluoroethylene carbonate and 1,3-propanesulfonyl lactone were added, dissolved, and thoroughly stirred. Lithium salt LiPF6 was then added and mixed evenly to obtain the electrolyte. The mass percentages of LiPF6, fluoroethylene carbonate, and 1,3-propanesulfonyl lactone were all 12.5% ​​and 2% respectively, calculated based on the mass of the electrolyte.

[0074] 5. Preparation of the separating membrane

[0075] Polyethylene porous polymer film is used as the separator.

[0076] 6. Preparation of lithium-ion batteries

[0077] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, the electrode assembly is wound up. After welding the tabs, the electrode assembly is placed in the outer packaging foil aluminum-plastic film. The prepared electrolyte is injected into the dried electrode assembly. After vacuum sealing, settling, formation, shaping, and capacity testing, a soft-pack lithium-ion battery is obtained.

[0078] Examples 9 to 18

[0079] Preparation of negative electrode active materials

[0080] The preparation process of the negative electrode active material is the same as in Example 5.

[0081] The negative electrode preparation process is similar to that in Example 5, except that the porosity of the first active material layer and the second active material layer is adjusted by adjusting the sphericity and particle size distribution. The porosity of the first active material layer and the second active material layer after the adjustment is shown in Table 2.

[0082] The preparation of the positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 5.

[0083] Examples 19 to 26

[0084] Preparation of negative electrode active materials

[0085] The preparation process of the negative electrode active material is the same as in Example 11.

[0086] The negative electrode preparation process is similar to that in Example 11, except that the C004 / C110 ratio of the negative electrode sheet is adjusted by adjusting the granulation degree of the second active material. The specific adjusted ratio is shown in Table 3.

[0087] The preparation of the positive electrode, separator, electrolyte, and lithium-ion battery is the same as in Example 11.

[0088] Examples 27 to 36

[0089] Preparation of negative electrode active materials

[0090] The preparation process of the negative electrode active material is the same as in Example 21.

[0091] The preparation process of the negative electrode is similar to that in Example 21, except that the specific surface area of ​​the obtained negative electrode sheet is adjusted by adjusting the sphericity and particle size distribution of the first active material and the second active material respectively. The specific surface area of ​​the negative electrode sheet after adjustment is shown in Table 4.

[0092] Test methods

[0093] Testing of relevant parameters of negative electrode active materials

[0094] Take a fully discharged lithium-ion battery, disassemble it, remove the negative electrode and soak it in DMC (dimethyl carbonate) for 20 minutes, then rinse it with DMC and acetone in turn to remove the electrolyte and the surface SEI film. Then place it in an oven and bake it at 80°C for 12 hours to obtain the treated negative electrode sheet.

[0095] The powder within a 10μm thickness of the negative electrode sheet was scraped off with a scraper, and the scraped powder was calcined in air at 500℃ for 3 hours to obtain the second active material.

[0096] The powder within a 10 μm thickness range between the negative electrode sheet and the current collector was scraped off with a scraper, and the scraped powder was calcined in air at 500°C for 3 hours to obtain the first active material.

[0097] All M grams of powder above the current collector of the negative electrode sheet were scraped off with a scraper, and the scraped powder was calcined in air at 500°C for 3 hours to obtain the negative electrode active material.

[0098] The obtained first active material, second active material, and negative electrode active material were subjected to the following tests:

[0099] 1. Average particle sphericity test

[0100] Images of the first and second active materials were captured using a scanning electron microscope (SEM). The sphericity of each particle was calculated by determining the ratio of its major to minor axis in the SEM image. In this application, the sphericity of 100 randomly selected particles was statistically analyzed, and the average sphericity of the active material was obtained through a distribution curve.

[0101] 2. Particle size test

[0102] Using a laser diffraction particle size distribution measuring instrument (Malvem Mastersizer 3000), and in accordance with the particle size distribution laser diffraction method GB / T19077 2016, the particle size distribution can be measured to obtain the Dv50 of the corresponding active material.

[0103] 3. Porosity test of active material layer

[0104] Take the negative electrode sheet after the above treatment, and take an electron microscope image of the cross-section of the electrode sheet by SEM backscattering. Since the conductivity of the active material, CMC and SBR are different, the obtained cross-sectional SEM image has different contrast. Different colors can be used to distinguish the pores, active material, CMC and SBR. By statistically analyzing the area ratio of pores at different positions in the thickness direction, the porosity distribution of the electrode sheet can be obtained, and then the porosity of the first active material layer and the second active material layer can be calculated.

[0105] 4. XRD test

[0106] The negative electrode sheet after the above treatment was subjected to XRD testing. Specifically, CuKα radiation was used for X-rays, and the CuKα radiation was removed by a filter or monochromator. The operating voltage of the X-ray tube was (30-35) kV, and the operating current was (15-20) mA. The scanning speed of the counter was 1 / 4 (°) / min. When recording the 004 diffraction pattern, the scanning range of the diffraction angle 2θ was 53°-57°. When recording the 110 diffraction pattern, the scanning range of the diffraction angle 2θ was 75°-79°. The peak area obtained from the (004) plane diffraction pattern was denoted as C004. The peak area obtained from the (110) plane diffraction pattern was denoted as C110.

[0107] 5. Specific surface area test

[0108] Take m grams of the above active material and pretreat it at 200°C in a nitrogen atmosphere for 2 hours to remove impurities adsorbed on the material surface. Then, adsorb the sample in a liquid nitrogen environment using nitrogen as the adsorbate. After adsorption saturation, calculate the specific surface area of ​​the corresponding material using the BET method.

[0109] Lithium-ion battery performance testing

[0110] 6. Volumetric energy density

[0111] The capacity of lithium-ion batteries is tested according to the following procedure:

[0112] 1) Let it stand at 25℃ for 30 minutes;

[0113] 2) Charge at 0.5C to 4.48V, then maintain constant voltage at 0.05C;

[0114] 3) Let it sit for 5 minutes;

[0115] 4) Discharge at 0.2C to 3.0V;

[0116] 5) Let it sit for 5 minutes, then the test is over.

[0117] Record the discharge capacity as C, the discharge plateau voltage as P, the thickness of the lithium-ion battery as G, the length of the lithium-ion battery as L, and the width of the lithium-ion battery as W. Then, the volumetric energy density E of the lithium-ion battery can be calculated according to the following formula:

[0118] E = (C × P) / (G × L × W).

[0119] 7. Electrochemical impedance spectroscopy

[0120] 1) The test temperature is 25℃;

[0121] 2) Let stand for 60 minutes;

[0122] 3) Constant current (CC) up to 4.48V, constant voltage (CV) up to 0.025C;

[0123] 4) Let stand for 10 minutes;

[0124] 5) 0.1C DC to 3V;

[0125] 6) Let stand for 10 minutes;

[0126] 7) 0.5C CC to 4.48V, CV to 0.025C;

[0127] 8) Let stand for 1 hour;

[0128] 9) 0.5C DC to 3V;

[0129] 10) Let stand for 30 minutes;

[0130] 11) 0.1C DC to 3V;

[0131] 12) Let stand for 30 minutes;

[0132] 13) 12mA DC to 3V;

[0133] 14) Let stand for 30 minutes;

[0134] 15) 0.1C CC to 10s;

[0135] 16) 1C CC to 360s;

[0136] 17) Let stand for 30 minutes;

[0137] Repeat steps 15)-17) until the voltage is ≥4.48V. The difference between the 360s DC resistance and the 10s DC resistance of the battery at 70% SOC is denoted as Rcp. The smaller Rcp is, the more favorable it is for lithium-ion diffusion, indicating better kinetic performance of the lithium-ion battery.

[0138] 8. Electrode XY direction expansion rate test

[0139] In the lithium-ion battery manufacturing process, the width of the negative electrode sheet during winding is measured using a charge-coupled device (CCD) and recorded as W1. The width of the negative electrode sheet from a fully charged lithium-ion battery is measured using a CCD and recorded as W2. The expansion rate of the electrode sheet in the XY direction (where XY is the plane perpendicular to the thickness direction of the electrode sheet) can be calculated using the following formula:

[0140] Expansion rate in the XY direction = (W2-W1) / W1×100%.

[0141] Test Results

[0142] Table 1 shows the effect of the average particle sphericity SDa and Dv50a of the first active material and the average particle sphericity SDb and Dv50b of the second active material on the performance of lithium-ion batteries.

[0143] Table 1

[0144]

[0145] As can be seen from Examples 1 to 8 in Table 1, when SDa satisfies 0.4≤SDa≤0.7, Dv50a is 14μm to 18μm, and SDb satisfies 0.7≤SDb≤1.0, Dv50b is 8μm to 16μm, the lithium-ion battery has a high volumetric energy density and maintains a low Rcp level (≤25mΩ). The lithium-ion battery also has good kinetic performance.

[0146] Comparative Example 1 shows that when SDa is less than 0.4 and Dv50a is greater than 18μm, the lithium-ion battery exhibits high energy density but a large Rcp, resulting in poor kinetic performance. Comparative Examples 2 and 4 show that when SDa is greater than 0.7 and Dv50a is less than 14μm, the lithium-ion battery has a small Rcp and excellent kinetic performance, but its volumetric energy density is low. Furthermore, when 0.4 ≤ SDa ≤ 0.5 and 15μm ≤ Dv50a ≤ 18μm, a high volumetric energy density can be guaranteed for the lithium-ion battery.

[0147] As can be seen from Comparative Example 3, when SDb is less than 0.7 and Dv50b is greater than 16 μm, the Rcp is relatively large, and its kinetic performance is poor. Furthermore, when 0.9 ≤ SDb ≤ 1 and 8 μm ≤ Dv50b ≤ 10 μm, the Rcp is relatively small, and its kinetic performance is better.

[0148] Table 2 further investigates the effects of the porosity Pa of the first active material layer and the porosity Pb ​​of the second active material layer on lithium-ion performance, based on Example 5.

[0149] Table 2

[0150]

[0151] Example 9 shows that when Pa is less than 10%, the Rcp of the lithium-ion battery is relatively large, and its kinetic performance is poor. Example 14 shows that when Pa is greater than 20%, the energy density loss of the lithium-ion battery is significant. Example 17 shows that when Pb is less than 20%, the Rcp of the lithium-ion battery is relatively large, and its kinetic performance is poor. Example 18 shows that when Pb is greater than 40%, the energy density of the lithium-ion battery is relatively low.

[0152] Table 3 further investigates the effect of the C004 / C110 value of the negative electrode on lithium-ion performance, based on Example 11.

[0153] Table 3

[0154]

[0155] As can be seen from Example 19, when the C004 / C110 ratio is less than 12, the lithium-ion battery exhibits better kinetic performance, but the expansion rate of the negative electrode in the XY direction is larger, leading to a higher risk of electrode deformation during cycling. As can be seen from Example 26, when the C004 / C110 ratio is greater than 18, the lithium-ion battery has a larger Rcp, resulting in poorer kinetic performance.

[0156] Table 4 further investigates the effect of the BET specific surface area of ​​the negative electrode active material on lithium-ion performance based on Example 21.

[0157] Table 4

[0158]

[0159]

[0160] As can be seen from Example 27, when the BET specific surface area is less than 0.8m² 2 At a specific surface area of ​​ / g, the kinetic performance of lithium-ion batteries is poor. As can be seen from Example 36, when the BET specific surface area is greater than 3m², the kinetic performance of lithium-ion batteries is relatively poor. 2 When the energy density of lithium-ion batteries is reduced to a certain level, excessive side reactions lead to irreversible capacity loss, significantly impacting the energy density of lithium-ion batteries.

[0161] While some exemplary embodiments of this application have been described and illustrated, this application is not limited to the disclosed embodiments. Rather, those skilled in the art will recognize that modifications and changes may be made to the described embodiments without departing from the spirit and scope of this application as described in the appended claims.

Claims

1. A secondary battery comprising a negative electrode, the negative electrode comprising a current collector and a negative electrode active material layer, the negative electrode active material layer comprising a first active material layer and a second active material layer, the first active material layer being disposed between the current collector and the second active material layer, and the second active material layer being disposed on the surface of the first active material layer away from the current collector. The first active material layer includes a first active material, wherein the average particle sphericity SDa of the first active material satisfies 0.4 ≤ SDa ≤ 0.6, and the Dv50a of the first active material satisfies 14 μm ≤ Dv50a ≤ 18 μm. The second active material layer includes a second active material, wherein the average particle sphericity SDb of the second active material satisfies 0.7≤SDb≤1, and the Dv50b of the second active material satisfies 8μm≤Dv50b≤16μm.

2. The secondary battery according to claim 1, wherein, 0.4≤SDa≤0.5, and 15μm≤Dv50a≤18μm.

3. The secondary battery according to claim 1, wherein, 0.9≤SDb≤1, and 8μm≤Dv50b≤10μm.

4. The secondary battery according to claim 1, wherein, SDa < SDb, Dv50b < Dv50a.

5. The secondary battery according to claim 1, wherein, The porosity of the first active material layer is Pa, and the porosity of the second active material layer is Pb, wherein 10%≤Pa≤20% and 20%≤Pb≤40%.

6. The secondary battery according to claim 5, wherein, 13%≤Pa≤20%, 25%≤Pb≤40%.

7. The secondary battery according to claim 1, wherein, The first active material is selected from artificial graphite, and the second active material is selected from artificial graphite.

8. The secondary battery according to claim 1, wherein, The negative electrode active material layer includes a negative electrode active material, and the specific surface area of ​​the negative electrode active material is 0.8 m². 2 / g to 3.0m 2 / g.

9. The secondary battery according to claim 1, wherein, The negative electrode satisfies at least one of the following conditions (i) to (iii): (i) Using XRD testing, the diffraction peak area C004 of the 004 crystal plane and the diffraction peak area C110 of the 110 crystal plane of the negative electrode active material layer satisfy: 12≤C004 / C110≤18; (ii) The tap density of the first active material is 0.95 g / cm³. 3 Up to 1.2 g / cm 3 The tap density of the second active material is 0.8 g / cm³. 3 Up to 1.05 g / cm 3 ; (iii) The compaction density of the negative electrode active material layer is 1.70 g / cm³. 3 Up to 1.80 g / cm 3 .

10. An electronic device comprising a secondary battery according to any one of claims 1 to 9.

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