Negative electrode sheet, secondary battery and electric device

Abstract

Provided in the present application is a negative electrode sheet. The negative electrode sheet comprises: a negative electrode current collector, and a negative electrode active material layer, which is made of a carbon-containing material and is arranged on at least one side of the negative electrode current collector, wherein 0.3≤(C101+C002) / (P×100)≤6, where P is the mercury intrusion porosity of the negative electrode sheet, C101=1.84×λ / (FWHM101×cosθ101), C002=0.84×λ / (FWHM002×cosθ002), and (C101+C002) is within the range of 30-180 nm. By using the negative electrode sheet, a secondary battery having the advantages of fast charging, good cycle performance and a high energy density can be prepared. Further provided in the present application are a secondary battery and an electric device.

Description

Negative electrode sheet, secondary battery and electrical equipment

[0001] Cross-reference to related applications

[0002] This application claims priority to Chinese Patent Application No. 202410703900.6, filed on May 31, 2024, entitled “Negative Electrode Sheet, Secondary Battery and Electrical Equipment”, the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery technology, specifically to a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0004] The limiting factors determining the charging capacity of rechargeable batteries are mainly concentrated in the negative electrode. Currently, the main methods to improve the fast-charging performance of rechargeable batteries from the negative electrode include: improving the active material of the negative electrode, adjusting the formulation of the negative electrode material layer, or adjusting the processing technology of the negative electrode sheet. However, these methods cannot enable the battery to simultaneously achieve good fast-charging performance, good cycle performance, and high energy density. Summary of the Invention

[0005] In view of this, this application provides a secondary battery that can balance good fast charging capability, cycle performance and high energy density, and the negative electrode sheet used therein.

[0006] The first aspect of this application provides a negative electrode sheet, including a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector, wherein the negative electrode active material layer contains a carbon active material having a crystalline structure, and the negative electrode sheet satisfies: 0.3 ≤ (C 101 +C 002 ) / (P×100)≤6;

[0007] Among them, C 101 =1.84×λ / (FWHM) 101 ×cosθ 101 );C 002 =0.84×λ / (FWHM) 002 ×cosθ 002 P represents the mercury porosity of the negative electrode sheet;

[0008] λ represents the wavelength of the cathode rays used in X-ray diffraction testing of the negative electrode active material layer, in nm; θ 101 θ 002 In the X-ray diffraction patterns representing the negative electrode active material layers, the diffraction peaks of the (101) and (002) crystal planes correspond to half the diffraction angles, all in degrees; FWHM 101FWHM 002 The full width at half maximum (FWHM) of the diffraction peaks of the (101) and (002) crystal planes, respectively, in radians; and the (C 101 +C 002 (In the range of 30-180nm)

[0009] Through a series of experiments, the inventors of this application discovered that the negative electrode active material includes a carbon active material with a crystalline structure, and the crystallographic parameters C of the (101) and (002) crystal planes of this carbon active material are... 101 C 002 The porosity P of the negative electrode sheet satisfies 0.3 ≤ (C 101 +C 002 ) / (P×100)≤6, and (C 101 +C 002 Within the 30-180nm range, this negative electrode sheet can be used to provide secondary batteries that combine fast charging performance, good cycle performance, and high energy density.

[0010] A second aspect of this application provides a secondary battery, including the negative electrode provided in the first aspect of this application. This secondary battery enables fast charging and exhibits long cycle life, good safety performance, and high energy density.

[0011] A third aspect of this application provides an electrical device, including the secondary battery provided in the second aspect of this application. This electrical device has good battery life and excellent fast-charging capability. Attached Figure Description

[0012] Figure 1A is a schematic diagram of a negative electrode sheet provided in this application.

[0013] Figure 1B is a schematic diagram of another structure of the negative electrode sheet provided in this application.

[0014] Figure 2 shows the XRD patterns of the negative electrode active material layers provided in Examples 2-4 of this application. Detailed Implementation

[0015] In view of the fact that existing methods for improving the fast-charging performance of secondary batteries from the negative electrode have failed to achieve good fast-charging performance, good cycle performance and high energy density characteristics, this application provides a secondary battery and its negative electrode sheet that can achieve fast charging, long cycle life and high energy density.

[0016] Please refer to Figures 1A and 1B together. This embodiment of the application provides a negative electrode sheet 10, which includes a current collector 11 and a negative electrode active material layer 12 sequentially disposed on at least one side of the current collector 11. The negative electrode active material layer 12 contains a negative electrode active material, which includes a carbon active material with a crystalline structure. The negative electrode sheet 10 satisfies: 0.3 ≤ (C 101 +C 002 ) / (P×100)≤6;

[0017] C 101 =1.84×λ / (FWHM) 101 ×cosθ 101 ), C 002 =0.84×λ / (FWHM) 002 ×cosθ 002 P represents the mercury intrusion porosity of the negative electrode 10;

[0018] λ represents the wavelength of the cathode rays used in X-ray diffraction (XRD) testing of the negative electrode active material layer 12, in nm; θ 101 θ 002 In the X-ray diffraction patterns representing the negative electrode active material layer 12, the diffraction angles corresponding to the diffraction peaks of the (101) and (002) crystal planes are half of each other, with units of °; FWHM 101 FWHM 002 These represent the full width at half maximum (FWHM) of the diffraction peaks of the (101) and (002) crystal planes, respectively, in radians; and the (C) 101 +C 002 (In the range of 30-180nm)

[0019] The applicant considered the influence of the intrinsic material properties of the negative electrode active material and the electrode property parameters of the negative electrode sheet on the fast-charging performance of the secondary battery. The crystallographic parameters C101 and (002) crystal planes of the carbon active material were extracted using the evolved Scherrer formula. 101 C 002 and control C 101 C 002 The porosity P of the negative electrode sheet satisfies 0.3 ≤ (C 101 +C 002 ) / (P×100)≤6, and (C 101 +C 002Within the 30-180nm range, it can ensure that carbon active materials with crystalline structures can fully exert their kinetic performance. As a result, secondary batteries using this negative electrode can have good fast charging performance (which can be reflected in a high lithium plating rate), good cycle performance (which can be reflected in a long cycle life), good safety performance (which can be reflected in a high lithium plating rate), and high energy density.

[0020] Furthermore, this application provides a negative electrode for secondary batteries that exhibits excellent kinetic performance while also possessing long lifespan and high energy density by establishing a link between the aforementioned crystal structure characteristics of carbon active materials with crystalline structures and the pore structure parameter P of the negative electrode sheet. Based on this, secondary batteries with excellent overall performance can be rapidly constructed. For ease of description, (C) will be referred to as such in this application. 101 +C 002 Let K be denoted as (P×100), then 0.3≤K≤6. K can reflect the overall capacity, kinetics, and cycle performance of the negative electrode.

[0021] In this application, the above C 101 It can reflect the height of the unit cell of carbon-active materials, C 002 It can reflect the length or width of the unit cell of carbon-active materials, and control (C 101 +C 002 Within the 30-180 nm range, the ordered cell size of carbon-active materials can be kept at an appropriately low level, ensuring the energy storage active ions (such as Li) are within acceptable limits. + The solid-phase migration rate in this carbon active material is high, and the migration distance and resistance of active ions in the solid-phase active material do not increase significantly. Therefore, the intrinsic kinetic performance of this carbon active material is good, which is beneficial to improving the rate performance of the battery. At the same time, the ordered cell size C of this carbon active material is high. 101 +C 002 It will not be too small, causing carbon-active materials to even tend towards a disordered structure, thereby reducing their energy density, etc.

[0022] For example, (C 101 +C 002 Specifically, it can be 35nm, 40nm, 50nm, 60nm, 70nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 145nm, etc. In some embodiments, the (C) 101 +C 002 Within the range of 50-150 nm, the carbon active material in the negative electrode is more likely to exhibit excellent kinetic performance and has a sufficiently large energy storage space to achieve a higher capacity.

[0023] In this application, since the (101) and (002) crystal plane diffraction peaks originate from carbon active materials with crystal structures, and the binder of the negative electrode active material layer 12 usually does not produce peaks, the XRD spectrum of the negative electrode active material layer 12 reflects the XRD characteristics of the carbon active material with crystal structures in the negative electrode sheet 10.

[0024] In some embodiments of this application, the cathode rays used for XRD testing of the negative electrode active material layer are copper Kα rays with a wavelength λ of 0.15418 nm.

[0025] In this embodiment, the mercury porosity P of the negative electrode 10 can be in the range of 20%-50%. The abundant pore structure and appropriately high porosity of the negative electrode 10 facilitate the wetting of the electrolyte within the electrode, increase the electrolyte retention capacity of the electrode, and reduce the concentration of active ions (such as Li). + The liquid phase diffusion resistance in the negative electrode 10 can reduce battery polarization, improve kinetic performance, and reduce lithium plating under high-rate charging or low-temperature conditions. Furthermore, excessively high negative electrode porosity reduces battery energy density, and because the contact between the negative electrode active materials is a partial contact, their contact resistance increases, leading to decreased electrode conductivity and excessive polarization, thus resulting in decreased kinetic performance. Therefore, this application controls the mercury porosity P of the negative electrode 10 within the range of 25%-50% (i.e., 25% ≤ P ≤ 50%), which can better balance the electrode's liquid retention capacity, battery cycle performance, and kinetic performance.

[0026] For example, P can specifically be 25%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, or 48%, etc. In some embodiments, P is in the range of 30%-45%. Controlling P within this range, while satisfying the aforementioned 0.3≤K≤6, is more conducive to balancing the battery's cycle performance and kinetic performance. The mercury intrusion porosity P of the negative electrode sheet can be controlled by adjusting the rolling pressure during the negative electrode preparation process. This P can be obtained by mercury intrusion testing (also known as the "mercury injection method").

[0027] In this application, the parameter K can be specifically 0.4, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 1.9, 2.0, 2.2, 2.5, 2.8, 3.0, 3.2, 3.5, 3.8, 4.0, 4.2, 4.5, 4.8, 5.0, 5.5, 5.8, etc.

[0028] In some embodiments of this application, 1.4 ≤ K ≤ 5. In this case, the secondary battery using the above-mentioned negative electrode sheet exhibits good fast-charging performance, cycle performance, and energy density. Further, in some embodiments, 1.4 ≤ K < 1.9. In this case, the secondary battery shows the best rate performance improvement and further improvement in cycle performance, while maintaining a good energy density, making it more suitable for high-power applications (such as drones). In other embodiments, 1.9 ≤ K ≤ 3. In this case, the secondary battery using the above-mentioned negative electrode sheet 10 can better balance fast-charging performance, long cycle performance, and high energy density, with improvements in all three aspects. In still other embodiments, 3 < K ≤ 5. In this case, the secondary battery shows the best energy density improvement and some improvement in cycle performance, while maintaining a good rate performance, making it more suitable for scenarios requiring high energy density and long lifespan.

[0029] In this embodiment, the areal density M of the negative electrode active material layer 12 can be 10-200 g / m². 2 Within a certain range. The areal density of the negative electrode sheet is related to its thickness. A suitable areal density M on one side of the negative electrode active material layer 12 ensures high electrode energy density without excessively increasing the ion liquid phase diffusion distance, thus affecting battery kinetic performance (such as fast charging performance). For example, M can specifically be 15 g / m³. 2 20g / m 2 30g / m 2 40g / m 2 50g / m 2 60g / m 2 70g / m 2 80g / m 2 90g / m 2 100g / m 2 110g / m 2 120g / m 2 130g / m 2 140g / m 2 150g / m 2 160g / m 2 170g / m 2 180g / m 2 190g / m 2 195g / m 2 Etc. In some embodiments, M is 50-150 g / m³. 2 Within the range of 80-150g / m 2 Within the range.

[0030] It should be noted that in this application, the negative electrode current collector 11 may have a negative electrode active material layer 12 on one side surface (as shown in Figure 1A), or it may have a negative electrode active material layer 12 on both opposite sides surface of the negative electrode current collector. When both opposite sides surface of the negative electrode current collector have a negative electrode active material layer 12, it is sufficient for one negative electrode active material layer 12 to satisfy the above parameter K being in the range of [0.3, 6]. Of course, both negative electrode active material layers 12 may also satisfy K being in the range of [0.3, 6]. In addition, it should be noted that when both opposite sides surface of the negative electrode current collector 11 have a negative electrode active material layer 12, the above (C 101 +C 002 Both M and M refer to the negative electrode active material layer located on the same side as the negative electrode current collector 11.

[0031] In this embodiment, the aforementioned carbon-active material with a crystalline structure may include one or more of artificial graphite, natural graphite, and mesophase carbon microspheres. These carbon-active materials may possess distinctive (101) and (002) crystal plane diffraction peaks to obtain the aforementioned parameters (C). 101 +C 002 In some embodiments, the carbon-active material comprises artificial graphite and / or natural graphite. The artificial graphite may be produced from raw materials such as petroleum coke, needle coke, isotropic coke, pitch coke, or anthracite.

[0032] In some embodiments of this application, the negative electrode active material layer 12 may contain one or more of the following active materials in addition to the aforementioned carbon active materials with crystalline structures: hard carbon, soft carbon, silicon-based materials, tin-based materials, lithium titanate, etc., but is not limited thereto. Further, in this case, the mass percentage of the aforementioned carbon active materials with crystalline structures in the negative electrode active material may be 50%-100%. This helps to improve the overall rate performance of the negative electrode active material, thereby ensuring good fast-charging performance. Soft carbon and hard carbon are highly irregular, amorphous carbon active materials. Silicon-based materials may include one or more of elemental silicon (such as monocrystalline silicon), silicon alloys, silicon oxides, silicon-carbon composites, etc. Tin-based materials may include one or more of elemental tin, tin oxides, tin alloys, etc.

[0033] The other active materials mentioned above can be mixed and dispersed with the carbon active material with a crystalline structure, or they can be coated. For example, the surface of the carbon active material with a crystalline structure can have a hard carbon or soft carbon coating layer. Of course, in other embodiments of this application, the surface of the carbon active material with a crystalline structure can also have other coating layers, such as an oxide coating layer.

[0034] In this application, the negative electrode active material layer 12 may include a binder in addition to the negative electrode active material. In some cases, it may also include a conductive agent. Binders and conductive agents are conventional choices in the battery field and can be selected according to actual needs. For example, binders may include, but are not limited to, one or more of styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyimide (PI), polyacrylic acid (PAA), polyacrylates (such as polymethyl methacrylate, polymethyl acrylate, polyethyl acrylate, etc.), polyolefins (such as polypropylene, polyethylene, etc.), carboxymethyl cellulose (CMC), sodium alginate, etc. For example, conductive agents include, but are not limited to, one or more of carbon nanotubes, graphene, carbon fibers, carbon black (such as acetylene black, Ketjen black), etc. It should be noted that conductive agents such as carbon black are amorphous carbon and will not show crystal diffraction peaks in XRD. Therefore, they will not affect the reading of the peak positions and full width at half maximum (FWHM) data of the (101) and (002) crystal diffraction peaks of the above-mentioned carbon active materials with crystal structure.

[0035] In addition, the negative electrode current collector 11 that carries the negative electrode active material layer 12 can be selected according to the battery system, and can be copper foil, aluminum foil, etc. For lithium secondary batteries, the negative electrode current collector 11 can be, but is not limited to, copper foil, copper alloy foil, carbon-coated copper foil, or copper-plated film.

[0036] This application also provides a secondary battery, including the negative electrode sheet described above. Because this secondary battery uses the aforementioned negative electrode sheet, it exhibits good fast-charging performance, long cycle life, good safety performance, and high energy density.

[0037] In this application, the secondary battery includes a negative electrode and a positive electrode. The positive electrode may include a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector. The positive active material layer includes a positive active material, and optionally a binder and a conductive agent.

[0038] In this embodiment of the application, the secondary battery can specifically be a lithium secondary battery. For a lithium secondary battery, the positive electrode active material can include one or more of the following: lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), and lithium phosphates with an olivine structure (such as lithium iron phosphate (LFP) and lithium manganese iron phosphate (LFMP)).

[0039] In this application, the aforementioned secondary battery may be a liquid battery using a liquid electrolyte, a semi-solid battery using a semi-solid electrolyte, or a solid battery using a solid electrolyte.

[0040] In some embodiments, a liquid secondary battery may include a positive electrode, a negative electrode, and a separator and an electrolyte disposed between the positive and negative electrodes. The separator isolates the positive and negative electrodes, maintaining electrical insulation between them. This application does not limit the type of separator; any separator material from existing batteries can be used. Exemplary separators include, but are not limited to, polypropylene (PP) films, polyethylene (PE) films, glass fiber films, and multilayer composite films composed of these (such as bilayer PP / PE films, triplelayer PP / PE / PP films, etc.). The electrolyte includes an electrolyte salt and an organic solvent; the specific types and compositions of the electrolyte salt and organic solvent are conventional choices in the battery field and can be selected according to actual needs.

[0041] In some other embodiments of this application, the semi-solid secondary battery may include a positive electrode, a negative electrode, and a semi-solid electrolyte disposed between the positive and negative electrode. In still other embodiments, the solid secondary battery may include a positive electrode, a negative electrode, and a solid electrolyte disposed between the positive and negative electrode.

[0042] This application also provides an electrical device including the aforementioned secondary battery. Because the electrical device uses the aforementioned secondary battery, the battery has a long battery life, good cycle performance, and excellent fast charging capability.

[0043] This application does not impose any particular restrictions on the electrical devices that use the aforementioned secondary batteries. Exemplarily, the electrical devices include, but are not limited to, mobile phones, laptops, tablets, cameras, televisions, radios, wearable devices (such as smartwatches, smart bracelets, stereo headphones, and Bluetooth headsets), electric vehicles (such as new energy vehicles and electric bicycles), electric toys, backup power supplies, and large household batteries.

[0044] The technical solution of this application will be further described below with reference to several specific embodiments.

[0045] Before introducing the specific embodiments of this application, the method for testing the parameters mentioned above from the perspective of a secondary battery will be described first.

[0046] The testing methods for the porosity of the negative electrode sheet include: 1) drying the coated and rolled negative electrode sheet of the secondary battery; 2) testing the porosity of the dried negative electrode sheet according to GB / T 21650.1-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Method - Part 1: Mercury Intrusion Porosimetry". Alternatively, the porosity of the negative electrode material layer can be obtained by disassembling the secondary battery, as follows: 1) Disassemble the secondary battery after complete discharge to obtain the negative electrode sheet; 2) Immerse the electrode sheet in dimethyl carbonate (DMC) for a period of time to clean the residual electrolyte, and then dry the negative electrode sheet; 3) Finally, test the porosity of the dried negative electrode sheet according to GB / T 21650.1-2008 "Determination of Pore Size Distribution and Porosity of Solid Materials by Mercury Intrusion Porosimetry and Gas Adsorption Method - Part 1: Mercury Intrusion Porosimetry". The porosity results obtained by using both methods are basically consistent, and the data differences are within the error range.

[0047] The method for testing the areal density of the negative electrode active material layer includes: 1) Drying the negative electrode sheet after single-sided coating, cutting out small circular pieces of a certain diameter, weighing and recording the mass m1 of the small circular pieces; 2) Taking the current collector without coating layer, cutting out small circular pieces of the same size as mentioned above, weighing and recording the mass m2 of the current collector; 3) The difference between the mass m1 of the small circular pieces and the mass m2 of the current collector is the total weight m3 of the negative electrode active material layer with single-sided coating; 4) The ratio of the total weight m3 of the single-sided coating to the area of ​​the small circular pieces is the single-sided areal density of the negative electrode active material layer. In addition, the surface density of the negative electrode material can also be obtained by disassembling the secondary battery. The method is as follows: 1) Disassemble the secondary battery after it is fully discharged to obtain the negative electrode sheet; soak the negative electrode sheet in the solvent DMC for a period of time to clean the residual electrolyte, and then dry the negative electrode sheet; 2) Take the above negative electrode sheet and cut small circular pieces of a certain diameter in a certain direction. Scrape off the coating layer on one side of the small circular piece with a ceramic scraper (or polish it off with sandpaper), measure the total weight of the coating on one side of the small circular piece, and then calculate the weight of the coating on one side of the small circular piece per unit area, which is the surface density of the negative electrode active material layer. The porosity results obtained by the two methods are basically consistent, and the data difference is within the error range.

[0048] The method for obtaining the XRD-related parameters of the negative electrode active material layer is as follows: 1) Dry the negative electrode sheet of the secondary battery after coating and rolling; 2) Scrape the negative electrode active material layer on the negative electrode current collector side with a ceramic scraper (or polish with sandpaper) to obtain the negative electrode active material layer powder, and then perform XRD testing on the powder in an X-ray diffractometer to obtain the XRD spectrum, and record the wavelength λ (in nm) of the cathode ray used in the XRD test. 3) From the obtained XRD spectrum of the negative electrode active material layer, the diffraction angle 2θ (in °) of the diffraction peaks corresponding to the (101) and (002) crystal planes of the carbon material and the half-width of these two diffraction peaks (i.e., the diffraction angle corresponding to 1 / 2 peak height, in radians) can be read, and then C can be calculated according to the aforementioned formula. 101 With C 002 It should be noted that in XRD patterns, the diffraction angle and half-width at half-maximum (FWHM) of a diffraction peak on a specific crystal plane are directly read in degrees. When calculating the value of parameter K, the FWHM value in degrees needs to be converted to a value in radians.

[0049] In addition, the XRD parameters of the negative electrode material layer can also be obtained by disassembling the secondary battery. The method is as follows: 1) Disassemble the secondary battery after it is fully discharged to obtain the negative electrode sheet; immerse the negative electrode sheet in dimethyl carbonate (DMC) for 10-20 minutes to clean the residual electrolyte, and then dry the negative electrode sheet. 2) Scrape the negative electrode active material layer on the negative electrode current collector side with a ceramic scraper (or polish with sandpaper) to obtain the negative electrode active material layer powder, and then perform XRD testing on the powder in an X-ray diffractometer to obtain the XRD spectrum, and record the wavelength λ (in nm) of the cathode ray used in the XRD test. 3) From the obtained XRD spectrum of the negative electrode active material layer, the diffraction angle 2θ (in °) of the diffraction peaks corresponding to the (101) and (002) crystal planes of the carbon material and the half-peak width of these two diffraction peaks (i.e., the diffraction angle corresponding to 1 / 2 peak height, in radians) can be read, and then C can be calculated according to the aforementioned formula. 101 With C 002 It should be noted that in XRD patterns, the diffraction angle and half-width at half-maximum (FWHM) of a diffraction peak on a specific crystal plane are directly read in degrees. When calculating the value of parameter K, the FWHM value in degrees needs to be converted to a value in radians.

[0050] The XRD test conditions were standard, for example, using copper Kα rays as cathode rays, using 40kV and 20mA to excite the cathode ray source to generate X-rays, using a test scan speed of 5° / min, and using a Kβ filter to remove the influence of Kβ rays on the peak shape and position of the spectral lines.

[0051] Example 1

[0052] The preparation of a negative electrode sheet includes:

[0053] The negative electrode active material (materials shown in Table 1) was mixed with conductive agent carbon black and binder (CMC and SBR, mass ratio 1.5:2.2) at a mass ratio of 95.4:0.9:3.7. The mixed powder was placed in a vacuum mixer, and deionized water was added and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of the negative electrode current collector copper foil and baked in an oven at 100°C until dry to form a negative electrode active material layer. The copper foil with the negative electrode active material layer was pressed to the target compaction by a hot roller at 105°C and then cut to obtain a negative electrode sheet. The porosity P, the single-sided areal density M and C of the negative electrode active material layer of the negative electrode sheet were determined. 101 +C 002 The test results of the value and the calculation results of the aforementioned defined parameter K are summarized in Table 1.

[0054] A method for preparing a lithium secondary battery, comprising:

[0055] 1) Preparation of the positive electrode sheet:

[0056] The positive electrode active material (materials shown in Table 1), conductive agent carbon black, and binder PVDF are mixed in a mass ratio of 96:2:2. The mixed powder is placed in a vacuum mixer, and solvent N-methylpyrrolidone (NMP) is added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is sieved (through a 200-mesh sieve) and coated onto the positive electrode current collector aluminum foil. After drying in an oven at 120°C, it is then rolled and slit to obtain the positive electrode sheet.

[0057] 2) Electrolyte preparation: Ethyl carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1 to obtain a mixed solvent. Then, dry lithium salt LiPF6 is added to it to prepare an electrolyte with a LiPF6 concentration of 1 mol / L.

[0058] 3) Battery assembly: In an argon-filled glove box, the positive electrode, separator (specifically a polyethylene film), and negative electrode are stacked in sequence to obtain a battery cell. The separator must completely separate the positive and negative electrode cells. The stacked battery cells are then placed into an aluminum-plastic film soft package and injected with the electrolyte. After vacuum sealing, settling, formation, cutting, and sealing, a lithium-ion battery is obtained.

[0059] Other embodiments

[0060] The negative electrode sheets and lithium-ion batteries of the remaining examples and comparative examples were prepared according to the parameters listed in Table 1.

[0061] To strongly support the beneficial effects of this application, the following electrochemical performance of the batteries in the above embodiments and comparative examples was tested, and the results are summarized in Table 2 above.

[0062] The method for testing the specific capacity of the negative electrode is as follows: At 25°C, the negative electrode of each lithium-ion battery is weighed using an electronic balance. In an argon-filled glove box, lithium sheets larger than the negative electrode, a separator (specifically a polyethylene film), and the negative electrode are stacked in sequence to obtain a battery cell. At 25°C, each lithium-ion battery is discharged at a rate of 0.2C and charged at a rate of 0.2C (voltage range 2V-3.8V), and the actual charge amount is recorded. The ratio of the battery's actual 0.2C charge amount to the weight of the negative electrode is the specific capacity of the negative electrode, expressed in mAh / g.

[0063] The energy density testing method is as follows: At 25℃, the negative electrode, separator, and positive electrode of each battery are weighed using an electronic balance and recorded as the cell weight. Under the same conditions, each lithium-ion battery is first charged at a 1 / 3C rate and then discharged at a 1 / 3C rate (voltage range 2V-3.8V), and the actual discharge amount is recorded. The product of the actual discharge amount at 1 / 3C and the average voltage during discharge is the battery's energy. The ratio of this energy to the cell weight is the actual energy density of the cell using that negative electrode.

[0064] The battery kinetic performance testing method is as follows: At 25℃, each lithium-ion battery is fully charged at nC and fully discharged at 1C for 10 charge-discharge cycles. Then, the battery is fully charged at nC, and the negative electrode is disassembled to observe the lithium deposition on its surface. If the area of ​​the lithium deposition region on the negative electrode surface is less than 5%, it is considered slight lithium deposition; if the area of ​​the lithium deposition region on the negative electrode surface is 5%–40%, it is considered moderate lithium deposition; if the area of ​​the lithium deposition region on the negative electrode surface is greater than 40%, it is considered severe lithium deposition. If no lithium deposition occurs on the negative electrode surface (i.e., no lithium deposition region exists on the negative electrode surface), the charging rate is increased from nC in increments of 0.1C, and the test is repeated until slight lithium deposition occurs on the negative electrode surface. The test is then stopped. The maximum charging rate of the battery under non-lithium deposition conditions is nC minus 0.1C.

[0065] The test method for battery cycle performance is as follows: At 25℃, each lithium-ion battery is charged at a rate of 0.33C and discharged at a rate of 1C to perform a full charge and discharge cycle test until the discharge capacity of the lithium-ion battery after a certain number of cycles is 80% of the discharge capacity of the first cycle. The number of cycles at this time is recorded, which is the cycle life of the battery.

[0066] Table 1

[0067] Table 2

[0068] Figure 2 shows the XRD patterns of the negative electrode active material layers provided in Examples 2-4 of this application. From Figure 2, it can be seen that the 2θ angle corresponding to the (002) crystal plane diffraction peak is 26.5059° (i.e., θ...). 002 (13.253°), FWHM (002) The value in degrees is 0.1382°. Multiplying this degree value by π (3.14) and then dividing by 180° yields the FWHM in radians. (002) The value is 0.002412045; while the 2θ angle corresponding to the diffraction peak of the (101) crystal plane is 44.575° (i.e., θ). 101 (22.2875°), FWHM (101) The value in degrees is 0.506°. Multiplying this degree by π and then dividing by 180° yields the half-width in radians, which is 0.008831466. According to the formula listed earlier in this application, C can be calculated. 101 The calculated value is 34.7, C 002 The calculated value is 58.5, C 101 +C 002 It is 93.2.

[0069] From the comparison between Examples 9-10 and Comparative Examples 1-3 in Table 1, it can be seen that when the positive and negative active materials are the same, the (C) of the negative active material... 101 +C 002 Within the range of 30-180 nm, and when the composition of the negative electrode sheet ensures that the aforementioned custom parameter K is within the range of [0.3, 6], the lithium-ion battery using this negative electrode sheet exhibits a longer cycle life and a higher rate of return during lithium plating at the negative electrode, reflecting better battery safety and kinetic performance, while maintaining a relatively high energy density. A comparison between Examples 11-12 and Comparative Examples 4-5 reveals that if the negative electrode active material (C... 101 +C 002 If the nanometer size is less than 30nm, even if the parameter K is in the range of 0.3-6, the energy density of the battery will still be too low.

[0070] Other embodiments of this application (such as embodiments 1-8, 13-16) satisfy the (C) of the negative electrode active material. 101 +C 002Within the range of 30-180nm, and with the composition of the negative electrode sheet such that the aforementioned custom parameter K is within the range of [0.3, 6], the battery can achieve a good balance of long cycle life, high lithium plating rate, and high energy density. In particular, when K is within the range of 1.9-3 (as in Examples 1-4), the battery's cycle life, rate, and energy density can achieve a good balance.

[0071] Furthermore, a comparison between Examples 1 and Examples 13-15 reveals that when the negative electrode active material is the same and the composition of the negative electrode sheet makes the aforementioned custom parameter K the same, the battery's performance is good in all aspects. However, the lower the single-sided areal density of the negative electrode sheet, the smaller the diffusion resistance of the electrolyte, which improves the specific capacity of the negative electrode active material and the rate performance of the battery, but reduces the battery's energy density. The higher the single-sided areal density of the negative electrode sheet, the higher the energy density of the cell, but the rate performance decreases to some extent.

[0072] The above description is an exemplary embodiment of this application. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principles of this application, and these improvements and modifications are also considered to be within the scope of protection of this application.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector. The negative electrode active material layer contains a carbon active material with a crystalline structure. The negative electrode sheet satisfies: 0.3 ≤ (C 101 +C 002 ) / (P×100)≤6; Among them, C 101 =1.84×λ / (FWHM) 101 ×cosθ 101 );C 002 =0.84×λ / (FWHM) 002 ×cosθ 002 P represents the mercury porosity of the negative electrode sheet; λ represents the wavelength of the cathode rays used in X-ray diffraction testing of the negative electrode active material layer, in nm; θ 101 θ 002 In the X-ray diffraction patterns representing the negative electrode active material layers, the diffraction peaks of the (101) and (002) crystal planes correspond to half the diffraction angles, all in degrees; FWHM 101 FWHM 002 The full width at half maximum (FWHM) of the diffraction peaks of the (101) and (002) crystal planes, respectively, in radians; and the (C 101 +C 002 (In the range of 30-180nm) 2. The negative electrode sheet as described in claim 1, characterized in that, The (C) 101 +C 002 (In the range of 50-150nm) 3. The negative electrode sheet as described in claim 1, characterized in that, The value of P is in the range of 20%-50%.

4. The negative electrode sheet as described in any one of claims 1-3, characterized in that, 1.4≤(C 101 +C 002 ) / (P×100)≤5。 5. The negative electrode sheet as described in claims 1-4, characterized in that, The areal density of the negative electrode active material layer is 10-200 g / m². 2 Within the range.

6. The negative electrode sheet as described in any one of claims 1-5, characterized in that, The carbon-active material with a crystalline structure includes one or more of artificial graphite, natural graphite, and mesophase carbon microspheres.

7. The negative electrode sheet as described in claim 6, characterized in that, The carbon-active materials with crystalline structures include artificial graphite and / or natural graphite.

8. The negative electrode sheet according to any one of claims 1-7, characterized in that, The carbon active material with a crystalline structure accounts for 50%-100% of the mass of the negative electrode active material.

9. The negative electrode sheet according to any one of claims 1-8, characterized in that, The negative electrode active material also includes one or more of silicon-based materials, tin-based materials, lithium titanate, soft carbon, and hard carbon.

10. A secondary battery, characterized in that, The secondary battery includes the negative electrode sheet as described in any one of claims 1-9.

11. An electrical appliance, characterized in that, The electrical equipment includes the secondary battery as described in claim 10.

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