Secondary batteries and electronic devices

Abstract

This application provides a secondary battery and an electronic device. The secondary battery of this application includes a positive electrode and a negative electrode. The positive electrode includes a positive electrode active material, polyvinylidene fluoride (PVDF), and a polyacrylonitrile polymer, wherein the mass content of the polyacrylonitrile polymer is 0.01% to 1.1% based on the mass of PVDF. The negative electrode includes a negative electrode active material, which includes graphite and hard carbon. By introducing a polyacrylonitrile polymer into the positive electrode and simultaneously combining it with hard carbon material in the negative electrode, the secondary battery of this application significantly improves its cycle performance and thermal performance under high voltage without affecting other performance characteristics.

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] Further improvements in the energy density of lithium-ion batteries are of great significance. Currently, an important means of increasing energy density is to increase the charging voltage of the positive electrode and increase the amount of lithium delithiation, thereby increasing capacity. However, increasing the charging voltage can lead to problems such as irreversible phase transitions in the positive electrode material, oxygen release from the crystal lattice, and cobalt dissolution. At the same time, the electrolyte is also prone to decomposition under high voltage, resulting in a significant reduction in the high-temperature performance of lithium-ion batteries, especially their cycle and thermal performance.

[0003] To address the aforementioned issues, common solutions in existing technologies include coating and doping the cathode material or optimizing the electrolyte composition. However, coating or doping the cathode material can lead to a decrease in its specific capacity, affecting kinetic performance; electrolyte optimization can also easily result in a decrease in kinetic performance. Summary of the Invention

[0004] In view of the aforementioned problems existing in the prior art, this application provides a secondary battery and an electronic device including the secondary battery. The secondary battery of this application, by introducing a polyacrylonitrile polymer into the positive electrode and simultaneously combining it with a hard carbon material in the negative electrode, significantly improves the cycle performance and thermal performance of the secondary battery under high voltage without affecting other performance characteristics.

[0005] The first aspect of this application provides a secondary battery comprising a positive electrode and a negative electrode. The positive electrode comprises a positive electrode active material, polyvinylidene fluoride (PVDF), and a polyacrylonitrile polymer, wherein the mass content of the polyacrylonitrile polymer is 0.01% to 1.1% based on the mass of PVDF. The negative electrode comprises a negative electrode active material, which includes graphite and hard carbon. The secondary battery of this application, by introducing a polyacrylonitrile polymer into the positive electrode and simultaneously combining it with a hard carbon material in the negative electrode, achieves the following: Firstly, the nitrile bonds in the polyacrylonitrile polymer can coordinate with cobalt in the positive electrode active material, reducing the activity of cobalt and improving the stability of the positive electrode active material. This effectively improves the problem of cobalt leaching during high-temperature cycling and storage. Secondly, the introduction of the hard carbon material can enhance the kinetic performance of the secondary battery, compensating for the slight kinetic loss caused by the introduction of the polyacrylonitrile polymer in the positive electrode. Without affecting other performance characteristics, this significantly improves the cycling performance and thermal performance of the secondary battery at high voltage.

[0006] In some embodiments, the mass content of polyacrylonitrile polymer is 0.3% to 0.8%. When the content of polyacrylonitrile polymer is too high, the kinetic performance of the secondary battery, especially its low-temperature performance, will be significantly affected, and it will be impossible to further balance this performance through the hard carbon content in the negative electrode.

[0007] In some embodiments, the hard carbon content is 0.1% to 2.5% by mass, based on the mass of graphite. The charging cutoff potential of hard carbon material is lower than that of the graphite negative electrode. At the same full-cell voltage, it can effectively lower the cutoff potential of the positive electrode, ensuring the high-temperature performance of the material. Furthermore, hard carbon can improve the kinetic performance of the system, compensating for the slight kinetic losses caused by the polyacrylonitrile polymer in the positive electrode. However, excessively high hard carbon content can lead to an excessively low positive electrode potential, affecting the charge / discharge capacity of the positive electrode and the initial efficiency of the secondary battery. In some embodiments, the hard carbon content is 0.5% to 2% by mass.

[0008] In some embodiments, the weight-average molecular weight of the polyacrylonitrile polymer is between 30,000 and 250,000. If the weight-average molecular weight of the polyacrylonitrile polymer is too high, it will lead to a significant loss of low-temperature performance in the secondary battery; if the weight-average molecular weight is too low, its role in stabilizing the positive electrode active material cannot be effectively exerted. In some embodiments, the weight-average molecular weight of the polyacrylonitrile polymer is between 100,000 and 200,000.

[0009] In some embodiments, the molecular weight distribution index of the polyacrylonitrile polymer is 1.0 to 2.0.

[0010] In some embodiments, the polyacrylonitrile polymer includes structural unit A.

[0011]

[0012] R1 to R3 are each independently selected from hydrogen or C1-C4 alkyl groups. In some embodiments, the polyacrylonitrile polymer includes polyacrylonitrile or polymethacrylonitrile.

[0013] In some embodiments, the Dv50 of hard carbon is smaller than that of graphite. In some embodiments, the Dv50 of hard carbon is 1 μm to 10 μm. In some embodiments, the Dv50 of graphite is 10 μm to 20 μm. The fact that hard carbon particles are much smaller than graphite particles can further improve the kinetic performance of the secondary battery, compensating for the slight kinetic loss caused by the polyacrylonitrile polymer in the cathode.

[0014] In some embodiments, the ratio of hydrogen mass content to carbon mass content in hard carbon is 0.05 to 0.2.

[0015] In some embodiments, X-ray diffraction testing reveals that the X-ray diffraction pattern of the negative electrode active material exhibits a first diffraction peak and a second diffraction peak within a 2θ range of 20° to 30°. The ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is greater than or equal to 100. When the ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is within the aforementioned range, there is a superior synergistic effect between hard carbon and graphite, which can effectively reduce the charging temperature rise and further improve the rate performance and high-temperature cycling performance of the secondary battery.

[0016] In some embodiments, the full width at half maximum (FWHM) of the first diffraction peak is 3° to 10°. In some embodiments, the FWHM of the second diffraction peak is less than or equal to 1°.

[0017] In some embodiments, the positive electrode active material includes Li 1-x M x The CoO2 material contains at least one of the following elements: M, which includes at least one of Al, Mg, Ti, Sn, V, Cu, Zn, Zr, Cr, Mn, Ni, Co, Fe, Ga, Mo, Sb, W, Y, and Nb, and 0 ≤ x ≤ 0.5.

[0018] In some implementations, the content of element M is ≥3000 ppm, based on the mass of the positive electrode active material. When the content of element M is within the above range, the high-temperature cycle performance and thermal performance of the secondary battery can be improved.

[0019] A second aspect of this application provides an electronic device comprising the secondary battery of the first aspect.

[0020] The secondary battery of this application introduces polyacrylonitrile polymers into the positive electrode and combines them with hard carbon materials in the negative electrode. The polyacrylonitrile polymers can effectively improve the cobalt dissolution problem of the positive electrode active material during high-temperature cycling and storage, while the hard carbon materials compensate for the slight kinetic loss caused by the introduction of polyacrylonitrile polymers into the positive electrode. The two work synergistically to greatly improve the cycling stability of the secondary battery at high voltage without affecting other performance characteristics. Detailed Implementation

[0021] The embodiments of this application will be described in detail below. The embodiments of this application should not be construed as limiting the application.

[0022] Additionally, quantities, ratios, and other numerical values ​​are sometimes presented in range format in this application. It should be understood that such range format is for convenience and brevity, and should be interpreted flexibly to include not only numerical values ​​explicitly specified as range limits, but also all individual numerical values ​​or subranges covered within the range, as if each numerical value and subrange were explicitly specified.

[0023] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may 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 example, 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 element or multiple elements. Item B may contain a single element or multiple elements. Item C may contain a single element or multiple elements.

[0024] Primary and secondary batteries

[0025] The first aspect of this application provides a secondary battery comprising a positive electrode and a negative electrode. The positive electrode comprises a positive electrode active material, polyvinylidene fluoride (PVDF), and a polyacrylonitrile polymer, wherein the mass content of the polyacrylonitrile polymer is 0.01% to 1.1% based on the mass of PVDF. The negative electrode comprises a negative electrode active material, which includes graphite and hard carbon. The secondary battery of this application, by introducing a polyacrylonitrile polymer into the positive electrode and simultaneously combining it with a hard carbon material in the negative electrode, achieves the following: Firstly, the nitrile bonds in the polyacrylonitrile polymer can coordinate with cobalt in the positive electrode active material, reducing the activity of cobalt and improving the stability of the positive electrode active material. This effectively improves the problem of cobalt leaching during high-temperature cycling and storage. Secondly, the introduction of the hard carbon material can enhance the kinetic performance of the secondary battery, compensating for the slight kinetic loss caused by the introduction of the polyacrylonitrile polymer in the positive electrode. Without affecting other performance characteristics, this significantly improves the cycling performance and thermal performance of the secondary battery at high voltage.

[0026] In some embodiments, the mass content of the polyacrylonitrile polymer is 0.01%, 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.33%, 0.35%, 0.37%, 0.39%, 0.4%, 0.43%, 0.45%, 0.47%, 0.49%, 0.5%, 0.53%, 0.55%, 0.57%, 0.59%, 0.6%, 0.63%, 0.65%, 0.67%, 0.69%, 0.7%, 0.73%, 0.75%, 0.77%, 0.79%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05%, or a range of any two of these values. In some embodiments, the mass content of polyacrylonitrile polymer is 0.3% to 0.8%. When the content of polyacrylonitrile polymer is too high, the kinetic performance of the secondary battery, especially its low-temperature performance, will be significantly affected, and it will be impossible to further balance this performance through the hard carbon content in the negative electrode.

[0027] In some implementations, the mass content of hard carbon is 0.1% to 2.5% based on the mass of graphite. In some embodiments, the mass content of hard carbon is 0.2%, 0.3%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.8%, 0.85%, 0.9%, 0.95%, 1.0%, 1.05%, 1.1%, 1.15%, 1.2%, 1.25%, 1.3%, 1.35%, 1.4%, 1.45%, 1.5%, 1.55%, 1.6%, 1.65%, 1.7%, 1.75%, 1.8%, 1.85%, 1.9%, 1.95%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, or a range of any two of these values. Hard carbon materials have a lower charging cutoff potential compared to graphite anodes. Under the same full-cell voltage, this effectively lowers the cathode cutoff potential, ensuring the material's high-temperature performance. Furthermore, hard carbon can improve the system's kinetic performance, compensating for the slight kinetic losses caused by the polyacrylonitrile polymer in the cathode. However, excessively high hard carbon content can lead to an excessively low cathode potential, affecting the charge / discharge capacity of the cathode and the initial efficiency of the secondary battery. In some embodiments, the hard carbon content is 0.5% to 2% by mass.

[0028] In some embodiments, the weight-average molecular weight of the polyacrylonitrile polymer is between 30,000 and 250,000. In other embodiments, the weight-average molecular weight of the polyacrylonitrile polymer is within the range of 35,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, 105,000, 110,000, 115,000, 120,000, 125,000, 130,000, 135,000, 140,000, 145,000, 150,000, 155,000, 160,000, 165,000, 170,000, 175,000, 180,000, 185,000, 190,000, 195,000, 200,000, 210,000, 220,000, 230,000, 240,000, or any combination of these values. When the weight-average molecular weight of the polyacrylonitrile polymer is too high, it leads to a significant loss of low-temperature performance in the secondary battery; when the weight-average molecular weight is too low, its role in stabilizing the positive electrode active material cannot be effectively exerted. In some embodiments, the weight-average molecular weight of the polyacrylonitrile polymer is between 100,000 and 200,000.

[0029] In some embodiments, the molecular weight distribution index of the polyacrylonitrile polymer is 1.0 to 2.0, for example, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8 or 1.9.

[0030] In some embodiments, the polyacrylonitrile polymer includes structural unit A.

[0031]

[0032] R1 to R3 are each independently selected from hydrogen or C1-C4 alkyl groups.

[0033] In some embodiments, R1 to R3 are each independently selected from hydrogen, methyl, ethyl, n-propyl, or isopropyl. In some embodiments, R1 and R2 may be the same or different, each independently selected from hydrogen, methyl, ethyl, n-propyl, or isopropyl, and R3 is hydrogen. In some embodiments, the polyacrylonitrile polymer includes polyacrylonitrile or polymethacrylonitrile.

[0034] In some embodiments, the purity of the polyacrylonitrile polymer is greater than or equal to 99%.

[0035] In some embodiments, the Dv50 of hard carbon is smaller than that of graphite. In some embodiments, the Dv50 of hard carbon is 1 μm to 10 μm. In some embodiments, the Dv50 of graphite is 10 μm to 20 μm. The fact that hard carbon particles are much smaller than graphite particles can further improve the kinetic performance of the secondary battery, compensating for the slight kinetic loss caused by the polyacrylonitrile polymer in the cathode.

[0036] In some embodiments, the Dv50 of hard carbon is a range of 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or any combination of these values. In some embodiments, the Dv50 of graphite is a range of 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or any combination of these values.

[0037] In some implementations, the specific surface area of ​​graphite is 1.0 m². 2 / g to 1.8m 2 / g, for example, 1.1m 2 / g, 1.2m 2 / g, 1.3m 2 / g, 1.4m 2 / g, 1.5m 2 / g, 1.6m 2 / g or 1.7m 2 / g.

[0038] In some embodiments, the ratio of hydrogen mass content to carbon mass content in hard carbon is from 0.05 to 0.2, for example, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18 or 0.19.

[0039] In some embodiments, X-ray diffraction testing reveals that the X-ray diffraction pattern of the negative electrode active material exhibits a first diffraction peak and a second diffraction peak within a 2θ range of 20° to 30°. The ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is greater than or equal to 100. When the ratio of the peak intensity of the second diffraction peak to that of the first diffraction peak is within the aforementioned range, there is a superior synergistic effect between hard carbon and graphite, which can effectively reduce the charging temperature rise and further improve the rate performance and high-temperature cycling performance of the secondary battery.

[0040] In this application, the first diffraction peak corresponds to the characteristic peak of hard carbon materials. The second diffraction peak corresponds to the characteristic peak of graphite materials.

[0041] In some implementations, the ratio of the peak intensity of the second diffraction peak to the peak intensity of the first diffraction peak is 100 to 10000.

[0042] In some embodiments, the full width at half maximum (FWHM) of the first diffraction peak is 3° to 10°. In some embodiments, the FWHM of the second diffraction peak is less than or equal to 1°.

[0043] In some embodiments, the positive electrode active material includes Li 1-x M xThe CoO2 material contains at least one of the following elements: M, which includes at least one of Al, Mg, Ti, Sn, V, Cu, Zn, Zr, Cr, Mn, Ni, Co, Fe, Ga, Mo, Sb, W, Y, and Nb, and 0 ≤ x ≤ 0.5.

[0044] In some embodiments, the content of element M is ≥3000 ppm based on the mass of the positive electrode active material. When the content of element M is within the above range, the high-temperature cycle performance and thermal performance of the secondary battery can be improved. In some embodiments, the content of element M is from 3000 ppm to 10000 ppm. In some embodiments, the content of element M is 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm, 5500 ppm, 6000 ppm, 6500 ppm, 7000 ppm, 7500 ppm, 8000 ppm, 8500 ppm, 9000 ppm, or any combination of these values.

[0045] In some implementations, the full charge voltage of the secondary battery is greater than or equal to 4.5V, for example, 4.53V, 4.55V or 4.6V.

[0046] In some embodiments, the negative electrode further includes a conductive agent. In some embodiments, the conductive agent includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0047] In some embodiments, the positive electrode further includes a conductive agent and a binder. The conductive agent includes carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, or carbon fiber; metal-based materials such as metal powders or fibers of copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof. The binder includes at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamide-imide, or styrene-butadiene rubber.

[0048] In some embodiments, the positive electrode also includes a positive current collector, which can be a metal foil or a composite current collector. For example, aluminum foil can be used. The composite current collector 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.

[0049] In some embodiments, the negative electrode further includes a binder and a conductive agent. The binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride (PVDF), polyethylene, polypropylene, styrene-butadiene rubber (SBR), acrylated SBR, epoxy resin, or nylon. The conductive agent includes at least one of carbon-based materials, metal-based materials, and conductive polymers. In some embodiments, the carbon-based material is selected from natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0050] In some embodiments, the negative electrode further includes a negative electrode current collector, which includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or any combination thereof.

[0051] In some embodiments, the electrolyte includes an organic solvent, a lithium salt, and optional additives. There are no limitations on the electrolyte used in the electrolyte according to this application; it may be any electrolyte known in the prior art. The additives in the electrolyte according to this application may be any additives known in the prior art that can be used as electrolyte additives.

[0052] In some embodiments, the organic solvent includes, but is not limited to, ethylene carbonate (EC), 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-dioxapentane (DOL) and dimethyl glycol ether (DME).

[0053] In some embodiments, the lithium salt includes at least one of organic 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.

[0054] 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.

[0055] 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.

[0056] 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).

[0057] In some embodiments, the secondary battery of this application includes, but is not limited to, a lithium-ion battery or a sodium-ion battery. In some embodiments, the secondary battery includes a lithium-ion battery.

[0058] II. Electronic Devices

[0059] This application further provides an electronic device that includes the secondary battery of the first aspect of this application.

[0060] 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.

[0061] Unless otherwise specified, all reagents, materials and instruments used in the following examples and comparative examples are commercially available.

[0062] Examples and Comparative Examples

[0063] Example 1

[0064] Positive Electrode: A mixture of lithium cobalt oxide (LiCoO2) and polyacrylonitrile (PAN) was prepared. This mixture was then thoroughly stirred with conductive agent Super P, carbon nanotubes (CNTs), and polyvinylidene fluoride (PVDF) in an appropriate amount of N-methylpyrrolidone (NMP) solvent at a weight ratio of lithium cobalt oxide:Super P:CNT:(PVDF+PAN) = 97.3:0.8:0.5:1.4 to form a uniform positive electrode slurry with a solid content of 75 wt%. The purity of PAN was 99%, its weight-average molecular weight was 50,000, and its molecular weight distribution width (Mw / Mn) was 1.5. Based on the mass of PVDF, the PAN content was 0.1%. This slurry was coated onto aluminum foil used as a positive electrode current collector, dried at 85°C, and then cold-pressed, cut, and slit. Finally, it was dried under vacuum at 85°C for 4 hours to obtain the positive electrode sheet.

[0065] Negative electrode: Artificial graphite and hard carbon are mixed together as the negative electrode active material. The content of hard carbon is 2% based on the mass of the artificial graphite; the graphite particles have a Dv50 of 14 μm and a BET of 1.4 μm. 2 / g, the mass ratio of hydrogen to carbon in hard carbon is 0.1. The above-mentioned negative electrode active material is thoroughly mixed with styrene-butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) in an appropriate amount of deionized water at a weight ratio of 98:1:1 to form a uniform negative electrode slurry, wherein the solid content of the negative electrode slurry is 50wt%. This slurry is coated onto the negative electrode current collector copper foil, dried at 85℃, then cold-pressed, cut, and slit, and finally dried under vacuum at 120℃ for 12 hours to obtain the negative electrode sheet.

[0066] Electrolyte: Ethyl carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC:PC:DEC = 50:20:30. Then, 1.5% of 1,3-propanesulfonyl lactone is added, dissolved, and stirred thoroughly. Lithium salt LiPF6 is then added and mixed evenly to obtain the electrolyte, wherein the molar concentration of LiPF6 is 1.15 mol / L.

[0067] Separating membrane: a 5μm thick porous polyethylene (PE) polymer film with a porosity of 35%.

[0068] Lithium-ion batteries are produced by stacking the positive electrode, separator, and negative electrode in sequence, with the separator acting as a barrier between the positive and negative electrodes. After winding and welding the tabs, the batteries are placed in an outer packaging foil-aluminum-plastic film, and electrolyte is injected. The batteries then undergo vacuum sealing, settling, formation, shaping, and capacity testing to obtain a soft-pack lithium-ion battery.

[0069] Examples 2 to 16, Comparative Examples 1 to 2

[0070] The positive and negative electrodes of Examples 2 to 16, as well as Comparative Examples 1 and 2, were achieved by adjusting the content of polyacrylonitrile and hard carbon based on Example 1. Specific adjustment measures are detailed in Table a, and detailed data are shown in Table 1.

[0071] Table a

[0072] Examples and Comparative Examples Polyacrylonitrile mass content Hard carbon mass content Example 2 0.01％ 1％ Example 3 0.01％ 1.5％ Example 4 0.01％ 2.0％ Example 5 0.01％ 0.1％ Example 6 0.01％ 2.5％ Example 7 0.02％ 1％ Example 8 0.05％ 1％ Example 9 0.1％ 1％ Example 10 0.3％ 1％ Example 11 0.5％ 1％ Example 12 0.7％ 1％ Example 13 1％ 1％ Example 14 0.5％ 1.5％ Example 15 1％ 1.5％ Example 16 1％ 2％ Comparative Example 1 0 0 Comparative Example 2 0 1％ Comparative Example 3 0.5％ 0

[0073] Examples 17 to 27

[0074] The positive electrodes of Examples 17 to 27 were achieved by adjusting the molecular weight of polyacrylonitrile and the content of element M in the positive electrode active material based on Example 1. For details of the adjustment measures, please refer to Table b, and for detailed data, please refer to Table 2.

[0075] Table b

[0076]

[0077] Test methods

[0078] 1. Polyacrylonitrile molecular weight and content test

[0079] The molecular weight of polyacrylonitrile was determined using a GPC (gel permeation chromatography) instrument. First, the positive electrode active material mixture sample was dissolved in a suitable solvent and then injected into a GPC column. The elution time and relative peak height were determined and used to calculate the molecular weight distribution.

[0080] 2. Hard carbon content test

[0081] A fully discharged lithium-ion battery was disassembled, and the negative electrode was soaked in DMC (ethylene carbonate) for 20 minutes. It was then rinsed with DMC and acetone sequentially to remove the electrolyte and the surface SEI film. The battery was then placed in an oven and baked at 80°C for 12 hours to obtain the treated negative electrode sheet. Powder was scraped off the negative electrode sheet with a scraper, and the scraped powder was heat-treated in a tube furnace at 400°C for 4 hours under argon protection to obtain the negative electrode active material.

[0082] The content of hard carbon in the negative electrode active material can be determined by mass spectrometry. The specific steps are as follows:

[0083] Step 1: Measure the weight of the sample to be tested.

[0084] Step 2: Place the sample in an inert atmosphere (such as argon or nitrogen) and decompose it at high temperature.

[0085] Step 3: Collect the decomposition products, such as gases and vapors.

[0086] Step 4: Use a mass spectrometer to separate the components of the sample.

[0087] Step 5: Analyze the composition using a mass spectrometry library and determine if hard carbon is present in the sample.

[0088] Step 6: Calculate the hard carbon content in the sample based on the peak area of ​​hard carbon in the mass spectrometer.

[0089] 3. Test of the mass ratio of hydrogen to carbon in hard carbon

[0090] Remove the negative electrode sheet, clean it with anhydrous ethanol and dry it. Scrape off the negative electrode active material layer from the negative electrode current collector and use an elemental analyzer to test the content of elements such as C / H / O to obtain the content ratio of hydrogen to carbon.

[0091] 4. Testing of metal element content in positive electrode active materials

[0092] The content of metal elements was quantitatively tested using ICP.

[0093] 5. Lithium-ion battery hot box test

[0094] In an environment of 25℃, the lithium-ion battery is first charged at a constant current of 1.0C to a voltage of 4.5V, and then charged at a constant voltage to a cutoff current of 0.025C. The battery is then placed in a hot box, the corresponding temperature is set, and the battery is left to stand for the corresponding time. The passing criteria are no smoke, no fire, and no explosion.

[0095] 6. Low-temperature discharge performance test of lithium-ion batteries

[0096] In an environment of 25℃, the lithium-ion battery was first charged at a constant current of 1.0C to a voltage of 4.5V, and then charged at a constant voltage to a cutoff current of 0.025C. After that, the battery was left to stand at 25℃ for 1 hour, and then discharged at 0.2C to 3.0V. The discharge capacity at this point was recorded as the reference discharge capacity C0.

[0097] In an environment of 25℃, the lithium-ion battery was first charged at a constant current of 1.0C to a voltage of 4.5V, and then charged at a constant voltage to a cutoff current of 0.025C. After that, the battery was placed in a low-temperature chamber with a set low temperature and left to stand for 1 hour. Then, it was discharged at 0.2C to 3.0V, and the discharge capacity at this time was recorded as C1.

[0098] The low-temperature discharge capacity retention rate is: C0 / C1×100%.

[0099] 7. Lithium-ion battery cycle performance test

[0100] In an environment of 45℃, the lithium-ion battery was first charged at a constant current of 1.0C to a voltage of 4.5V, then charged at a constant voltage to a cutoff current of 0.025C; and then discharged at a constant current of 0.5C to a voltage of 3V. This is recorded as one cycle, and the discharge capacity of the first cycle is recorded. This process is repeated n times, and the discharge capacity of each cycle is recorded. The discharge capacity retention rate is calculated as (discharge capacity of the nth cycle / discharge capacity of the first cycle) × 100%.

[0101] 8. High-Temperature Storage Performance Test of Lithium-ion Batteries

[0102] In an environment of 25℃, the lithium-ion battery was first charged at a constant current of 1.0C to a voltage of 4.5V, and then charged at a constant voltage to a cutoff current of 0.025C. The initial thickness L1 was recorded. The battery was then placed in an 85℃ oven and left to stand for 6 hours. The battery thickness L2 was then measured. The expansion rate was calculated as (L2 / L1-1)×100%.

[0103] Test Results

[0104] Table 1

[0105]

[0106]

[0107] The data in Table 1 show that when the content of polyacrylonitrile polymer is in the range of 0.01% to 1% and the content of hard carbon is in the range of 0.1% to 2.0%, the secondary battery exhibits excellent high-temperature cycling performance, thermal performance, and kinetic performance.

[0108] Table 2

[0109]

[0110] As can be seen from the data in Table 2, when the molecular weight of the polyacrylonitrile polymer in the positive electrode is in the range of 50,000 to 200,000 and the Al content in the positive electrode active material is in the range of 3,000 ppm to 6,000 ppm, the high-temperature performance and cycle performance of the secondary battery are further improved.

[0111] 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: The positive electrode comprises a positive electrode active material, polyvinylidene fluoride (PVDF), and a polyacrylonitrile polymer, wherein the mass content of the polyacrylonitrile polymer is from 0.5% to 1.05% based on the mass of PVDF. A negative electrode, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprising graphite and hard carbon; Based on the mass of graphite, the mass content of the hard carbon is 1.0% to 2.2%; The polyacrylonitrile polymer includes structural unit A. Structural Unit A R1 to R3 are each independently selected from hydrogen or C1-C4 alkyl; The weight-average molecular weight of the polyacrylonitrile polymer is between 60,000 and 200,000. The positive electrode active material includes lithium cobalt oxide material doped with Al, and the content of Al is 3000ppm to 6000ppm based on the mass of the positive electrode active material.

2. The secondary battery according to claim 1, wherein The mass content of the polyacrylonitrile polymer is 0.55% to 0.8%; and / or The hard carbon content is 1.1% to 2.1% by mass.

3. The secondary battery according to claim 1, wherein The polyacrylonitrile polymer satisfies at least one of the following conditions (ii) to (iii): (ii) The weight-average molecular weight of the polyacrylonitrile polymer is between 70,000 and 190,000; (iii) The molecular weight distribution index of the polyacrylonitrile polymer is 1.0 to 2.

0.

4. The secondary battery according to claim 3, wherein The polyacrylonitrile polymers include polyacrylonitrile or polymethacrylonitrile; and / or The weight-average molecular weight of the polyacrylonitrile polymer is between 80,000 and 150,000.

5. The secondary battery according to claim 3, wherein The weight-average molecular weight of the polyacrylonitrile polymer is between 90,000 and 120,000.

6. The secondary battery according to claim 1 or 2, wherein Based on the mass of the positive electrode active material, the content of the Al element is between 4000 ppm and 6000 ppm.

7. The secondary battery according to claim 1 or 2, wherein The negative electrode satisfies at least one of the following conditions (iv) to (vi): (iv) The Dv50 of the hard carbon is less than that of the graphite; (v) In the hard carbon, the ratio of the mass content of hydrogen to the mass content of carbon is 0.05 to 0.2; (vi) The X-ray diffraction pattern of the negative electrode active material is tested by X-ray diffraction method. It has a first diffraction peak and a second diffraction peak in the range of 2θ from 20° to 30°. The ratio of the peak intensity of the second diffraction peak to the peak intensity of the first diffraction peak is greater than or equal to 100.

8. The secondary battery according to claim 7, wherein The negative electrode satisfies at least one of the following conditions (vii) to (x): (vii) The Dv50 of the hard carbon is 1 μm to 10 μm; (viii) The Dv50 of the graphite is 10 μm to 20 μm; (ix) The full width at half maximum (FWHM) of the first diffraction peak is 3° to 10°; (x) The full width at half maximum (FWHM) of the second diffraction peak is less than or equal to 1°.

9. An electronic device comprising a secondary battery as claimed in any one of claims 1 to 8.

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