A secondary battery and an electric device

Abstract

The application discloses a secondary battery and a power utilization device, and belongs to the technical field of batteries. The secondary battery provided by the application can effectively improve the energy density and fast charging cycle performance of the secondary battery by controlling the graphitization degree of a negative active material and the powder resistivity under 12 kN within a certain range, and simultaneously controlling the maximum width of a second area in the direction of the lug close to the negative electrode tab after full charging of the secondary battery within a certain range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a secondary battery and a power utilization device. BACKGROUND

[0002] With the increasing demand of consumers for the endurance and fast charging capability of new energy vehicles, the next generation of power batteries have higher demands for energy density and fast charging performance. Increasing the surface density and compaction density of the negative electrode can significantly improve the energy density of the secondary battery, but deteriorate the kinetic performance, leading to lithium extraction during fast charging and causing safety problems of the secondary battery. In addition, fast charging leads to high temperature rise of the secondary battery and aggravation of side reactions, which deteriorate the cycle life. Therefore, it is a great challenge to develop a negative electrode material with high energy density and excellent fast charging performance and a power secondary battery. SUMMARY

[0003] The present application aims to solve the technical problem that the energy density and fast charging cycle performance of the secondary battery in the prior art are difficult to be considered simultaneously, and proposes a secondary battery with high energy density and excellent fast charging cycle performance and a power utilization device.

[0004] To achieve the above-mentioned purpose, the first aspect of the present application provides a secondary battery, comprising a negative electrode sheet and a tab connected with the negative electrode sheet, the tab being located on one side of the negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer arranged on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, The graphitization degree of the negative electrode active material is 80% to 96%; The powder resistivity of the negative electrode active material under 12kN is 0.0001Ω·cm to 0.01Ω·cm; After the secondary battery is fully charged, the negative electrode sheet comprises a first region and a second region in turn along the direction of the tab close to the negative electrode sheet; the maximum width of the second region is 0.1mm to 24mm.

[0005] As an embodiment of the present application, the secondary battery satisfies 0.03≤A≤10.05, wherein A=H×G×R; H mm is the maximum width of the second region; G % is the graphitization degree of the negative electrode active material; R Ω·cm is the powder resistivity of the negative electrode active material under 12kN.

[0006] As an embodiment of the present application, the maximum width of the first region is 0.5mm to 6mm.

[0007] As an embodiment of the present application, 5 μm≤Dv50≤25 μm.

[0008] As an embodiment of the present application, 1 μm≤Dv10≤10 μm.

[0009] As an embodiment of the present application, Dv99≤50 μm.

[0010] As an embodiment of the present application, the specific surface area B of the negative active material is 0.5 m 2 / g~5 m 2 / g.

[0011] As an embodiment of the present application, at least part of the surface of the negative active material has a first carbon coating layer; the thickness h1 of the first carbon coating layer is 1 nm~200 nm.

[0012] As an embodiment of the present application, at least part of the surface of the first carbon coating layer has a second carbon coating layer; the thickness h2 of the second carbon coating layer is 1 nm~200 nm.

[0013] As an embodiment of the present application, the ratio of the thickness h1 of the first carbon coating layer to the thickness h2 of the second carbon coating layer is 0.1≤h1 / h2≤10.

[0014] As an embodiment of the present application, the area density of the negative active material layer is 50 g / m 2 ~150 g / m 2 .

[0015] As an embodiment of the present application, the compacted density of the negative electrode sheet is 1 g / cm 3 ~2 g / cm 3 .

[0016] As an embodiment of the present application, the secondary battery further comprises a positive electrode sheet, and the ratio of the capacity per unit area of the negative electrode sheet to the capacity per unit area of the positive electrode sheet is (1.01~1.3):1.

[0017] As an embodiment of the present application, the thickness of the negative electrode sheet is 67 μm~194.5 μm when the secondary battery is fully discharged.

[0018] In a second aspect of the present application, a power consuming device is provided, comprising the secondary battery of the present application.

[0019] Compared with the prior art, the present application has the following beneficial effects: The secondary battery provided in this application effectively improves the energy density and fast-charging cycle performance of the secondary battery by controlling the graphitization degree of the negative electrode active material and the powder resistivity at 12kN within a certain range, and by controlling the maximum width of the second region along the direction of the tab close to the negative electrode sheet after the secondary battery is fully charged within a certain range. Attached Figure Description

[0020] Figure 1 This is a photograph of the negative electrode sheet obtained by disassembling the secondary battery prepared in Example 1 after it has been fully charged. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0023] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0024] In one embodiment of this application, a secondary battery is provided, comprising a negative electrode sheet and a tab connected to the negative electrode sheet, the tab being located on one side of the negative electrode sheet. The negative electrode sheet includes a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer comprising a negative active material. The degree of graphitization of the negative electrode active material is 80%~96%; The powder resistivity of the negative electrode active material at 12 kN is 0.0001 Ω·cm to 0.01 Ω·cm; After the secondary battery is fully charged, along the direction of the tab near the negative electrode, the negative electrode includes a first region and a second region in sequence; the maximum width of the second region is 0.1mm~24mm.

[0025] The secondary battery provided in this application effectively improves the energy density and fast-charging cycle performance of the secondary battery by controlling the graphitization degree of the negative electrode active material and the powder resistivity at 12kN within a certain range, and by controlling the maximum width of the second region along the direction of the tab close to the negative electrode sheet after the secondary battery is fully charged within a certain range.

[0026] Specifically, firstly, the graphitization degree of the negative electrode active material reflects the regularity of its carbon layer to a certain extent, which not only affects the reversible insertion / extraction ratio of lithium ions, thus affecting the specific capacity, but also affects the lithium ion diffusion path and diffusion rate. When the graphitization degree of the negative electrode active material selected in this application is within the above range, the energy density and fast-charge cycle performance of the secondary battery can be better improved. Secondly, the powder resistivity of the negative electrode active material at 12kN affects the amount of conductive agent to be added to the subsequent negative electrode sheet, thus affecting the mass ratio of the negative electrode active material, and consequently affecting the energy density of the secondary battery. At the same time, the powder resistivity of the negative electrode active material at 12kN also affects the electron transport resistance between the negative electrode active material particles and the reaction uniformity, thus affecting the fast-charge cycle performance of the secondary battery. Thirdly, after the secondary battery is fully charged, the maximum width of the second region along the direction of the tab close to the negative electrode sheet will affect the dynamic performance of the secondary battery and also affect the energy density of the secondary battery. When the maximum width of the second region of the negative electrode sheet from the tab side to the center of the negative electrode sheet after the secondary battery is fully charged is within the above range, it is possible to balance the high energy density and excellent fast charging cycle performance of the secondary battery.

[0027] It should be noted that the testing method for the graphitization degree and powder resistivity at 12kN of the negative electrode active material is as follows: The secondary battery is discharged at a 1C current to the lower voltage limit (exemplarily, if the positive electrode active material is lithium iron phosphate, the lower cutoff voltage limit is 2.5V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the lower cutoff voltage limit is 2.8V), and the negative electrode sheet is obtained after disassembly. The negative electrode sheet is cut into 3cm*3cm pieces and placed in a beaker in an ultrasonic cleaner. Water is used as the solvent, the ultrasonic power is 50W, the ultrasonic wave length is 1000nm, the temperature is 45℃, and the ultrasonic time is 30min, to peel the negative electrode active material off the copper foil. The material is then sieved using a 2mm sieve, and the sieved material is dried at 120℃ for 20min to obtain the negative electrode active material F (the negative electrode active material F used for the following tests of graphitization degree, powder resistivity, powder compaction, particle size, specific surface area, and carbon coating thickness are all peeled using the above method).

[0028] After grinding the above-mentioned negative electrode active material F uniformly, its XRD pattern was measured, and the degree of graphitization was calculated using the following formula: G=(0.344-d002 ) / (0.344-0.3354), where G is the degree of graphitization, in % d 002 d represents the (002) interplanar spacing, in nm. 002 =λ / (2sinθ), where λ is the X-ray wavelength in nm and θ is the diffraction angle of the (002) crystal plane in °.

[0029] After the above-mentioned negative electrode active material F is pressed into a dense sheet, the powder resistivity at 12kN is measured using a four-probe resistance tester.

[0030] It should be noted that, after the secondary battery is fully charged, the test method for the maximum width of the second region along the direction of the tab near the negative electrode is as follows: fully charge the secondary battery with a 1C current to the upper limit voltage V. max After constant voltage charging until the current is less than 0.05C, the secondary battery is disassembled to obtain the fully charged negative electrode sheet. The maximum width of the second region (which appears purple in color) is measured using a micrometer. For example, if the positive electrode active material is lithium iron phosphate, the upper limit of the cutoff voltage is 3.65V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the upper limit of the cutoff voltage is 4.4V.

[0031] For example, the degree of graphitization of the negative electrode active material can be any point value or a range between any two points between 80% and 96%, such as 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, etc.

[0032] In one embodiment, the degree of graphitization of the negative electrode active material is 90% to 96%. For example, it can be 90%, 90.5%, 91%, 91.5%, 92%, 92.5%, 93%, 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, etc.

[0033] This study found that when the graphitization degree of the negative electrode active material is further selected to be 90%~96%, the resulting secondary battery has a higher energy density and better fast-charging cycle performance.

[0034] For example, the powder resistivity of the negative electrode active material at 12kN is any point value or a range between any two points between 0.0001Ω·cm and 0.01Ω·cm, such as 0.0001Ω·cm, 0.0002Ω·cm, 0.0003Ω·cm, 0.0004Ω·cm, 0.0005Ω·cm, 0.0006Ω·cm, 0.0007Ω·cm, 0.0008Ω·cm, 0.0009Ω·cm, 0.001Ω·cm, 0.002Ω·cm, 0.003Ω·cm, 0.004Ω·cm, 0.005Ω·cm, 0.006Ω·cm, 0.007Ω·cm, 0.008Ω·cm, 0.009Ω·cm, 0.01Ω·cm, etc.

[0035] For example, after the secondary battery is fully charged, the maximum width of the second region along the direction of the tab near the negative electrode can be any point value or any range between two points between 0.1mm and 24mm, such as 0.1mm, 0.5mm, 1mm, 2mm, 4mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm, 20mm, 22mm, 24mm, etc.

[0036] In one embodiment, after the secondary battery is fully charged, the maximum width of the second region along the direction of the tab near the negative electrode is 2mm to 5mm. For example, it can be 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, etc.

[0037] This application research found that after the secondary battery is fully charged, the maximum width of the second region along the direction of the tab close to the negative electrode sheet will affect the lithium plating phenomenon during fast charging of the secondary battery, and will also affect the energy density of the secondary battery; when the maximum width of the second region is further selected to be 2mm~5mm, the overall performance of the secondary battery is better.

[0038] In one embodiment, the secondary battery satisfies: 0.03 ≤ A ≤ 10.05. Where A = H × G × R; H mm is the maximum width of the second region; G% represents the degree of graphitization of the negative electrode active material; R Ω·cm is the powder resistivity of the negative electrode active material at 12kN.

[0039] This study found that by controlling the maximum width H of the second region, the graphitization degree G of the negative electrode active material, and the powder resistivity R of the negative electrode active material at 12kN to satisfy 0.03≤H×G×R≤10.05, the energy density of the secondary battery and the fast-charging cycle performance of the secondary battery can be improved.

[0040] For example, A can be any point value between 0.03 and 10.05 or a range value between any two points, such as 0.03, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 10.05, etc.

[0041] In one embodiment, 0.03 ≤ A ≤ 3.96. For example, it can be 0.03, 0.05, 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 3.96, etc.

[0042] This study found that when A is further selected to satisfy 0.03≤A≤3.96, the overall performance of the secondary battery is better.

[0043] In one embodiment, after the secondary battery is fully charged, the maximum width of the first region of the negative electrode sheet from the tab side to the center of the negative electrode sheet is 0.5mm to 6mm.

[0044] It should be noted that the test method for the maximum width of the first region of the negative electrode plate from the tab side to the center of the negative electrode plate after the secondary battery is fully charged is as follows: fully charge the secondary battery with a 1C current to the upper limit voltage V. max After constant voltage charging until the current is less than 0.05C, the secondary battery is disassembled to obtain the fully charged negative electrode sheet. The maximum width of the first region (which appears black in color) is measured using a micrometer. For example, if the positive electrode active material is lithium iron phosphate, the upper limit of the cutoff voltage is 3.65V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the upper limit of the cutoff voltage is 4.4V.

[0045] For example, the maximum width of the first region can be any point value between 0.5mm and 6mm or a range value between any two points, such as 0.5 mm, 1 mm, 1.5 mm, 2 mm, 2.5 mm, 3 mm, 3.5 mm, 4 mm, 4.5 mm, 5 mm, 5.5 mm, 6 mm, etc.

[0046] This application research found that after a secondary battery is fully charged, the maximum width of the first region along the direction of the tab close to the negative electrode sheet affects the edge polarization phenomenon of the negative electrode sheet, thereby affecting the degree of edge lithium deposition. When the maximum width of the first region of the negative electrode sheet from the tab side to the center of the negative electrode sheet after the secondary battery is fully charged is selected to be 0.5mm~6mm, the edge polarization can be effectively reduced, thereby reducing edge lithium deposition and improving the fast charging cycle performance of the secondary battery.

[0047] In one embodiment, the Dv50 of the negative electrode active material satisfies: 5μm≤Dv50≤25μm.

[0048] For example, the Dv50 of the negative electrode active material can be any point value between 5μm and 25μm or a range value between any two points, such as 5μm, 8μm, 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, etc.

[0049] This study found that the Dv50 of the negative electrode active material affects the length of the lithium ion diffusion path and the size of the diffusion surface, thus affecting the diffusion time; at the same time, it also affects the compaction density of the negative electrode active material; when the Dv50 is selected to be 5μm~25μm, it is possible to achieve both good fast charging performance and high energy density of the secondary battery.

[0050] In one embodiment, the Dv10 of the negative electrode active material satisfies: 1μm≤Dv10≤10μm.

[0051] For example, the Dv10 of the negative electrode active material can be any point value between 1μm and 10μm or a range value between any two points, such as 1μm, 2μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc.

[0052] This study found that the Dv10 of the negative electrode active material affects the diffusion path length and diffusion surface size of lithium ions; it also affects the dispersion ability of the negative electrode active material during the preparation process, thereby affecting the structural stability of the negative electrode sheet; when the Dv10 is selected to be 1μm~10μm, the overall performance of the secondary battery is better.

[0053] In one embodiment, the Dv99 of the negative electrode active material satisfies: Dv99≤50μm.

[0054] In one embodiment, the Dv99 of the negative electrode active material satisfies: 37μm≤Dv99≤50μm.

[0055] For example, the Dv99 of the negative electrode active material can be any point value between 37μm and 50μm or a range value between any two points, such as 37μm, 38μm, 39μm, 40μm, 41μm, 42μm, 43μm, 44μm, 45μm, 46μm, 47μm, 48μm, 49μm, 50μm, etc.

[0056] This study found that the Dv99 of the negative electrode active material affects the kinetic performance of the secondary battery and also affects the stability of the separator. When the Dv99 is selected to be 37μm~50μm, the secondary battery has excellent kinetic performance and good safety performance.

[0057] It should be noted that the testing method for Dv50, Dv10 and Dv99 of the negative electrode active material is as follows: the Dv50, Dv10 and Dv99 of the above-mentioned stripped negative electrode active material F are measured using a laser particle size analyzer, where Dv50 is the particle size corresponding to the cumulative volume percentage of the negative electrode active material reaching 50%, and Dv10 and Dv99 are the particle sizes corresponding to the cumulative volume percentage of the negative electrode active material reaching 10% and 99%, respectively.

[0058] In one embodiment, the specific surface area B of the negative electrode active material is 0.5 m². 2 / g~5 m 2 / g.

[0059] It should be noted that the test method for the specific surface area of ​​the negative electrode active material is as follows: the specific surface area B of the stripped negative electrode active material F is measured using a BET analyzer.

[0060] For example, the specific surface area B of the negative electrode active material can be 0.5 m². 2 / g~5m 2 Any point value between / g or a range between any two points, for example, 0.5m. 2 / g、1m 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g、4m 2 / g, 4.5m 2 / g、5m 2 / g etc.

[0061] This study found that the specific surface area of ​​the negative electrode active material affects the degree of side reactions occurring in the negative electrode active material, thereby affecting the thickness of the SEI film and consequently the capacity of the secondary battery. Simultaneously, it also affects the number of lithium-ion insertion / extraction pathways, thus influencing the degree of lithium plating. Further optimization of the specific surface area B of the negative electrode active material to 0.5 m²... 2 / g~5m 2 When the ratio is / g, the resulting secondary battery exhibits superior fast-charging and cycle performance.

[0062] In one embodiment, at least a portion of the surface of the negative electrode active material has a first carbon coating layer; the thickness h1 of the first carbon coating layer is 1 nm to 200 nm.

[0063] For example, h1 can be any point value between 1nm and 200nm or a range value between any two points, such as 1 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, 200 nm, etc.

[0064] This application research found that the first carbon coating layer has an ordered layered structure, which can effectively improve fast charging performance. When the thickness of the first carbon coating layer is selected within the above range, the overall performance of the secondary battery is better.

[0065] In one embodiment, the first carbon coating layer has at least a portion of its surface covered by a second carbon coating layer; the thickness h2 of the second carbon coating layer is 1 nm to 200 nm.

[0066] For example, h2 can be any point value between 1nm and 200nm or a range value between any two points, such as 1 nm, 10 nm, 20 nm, 50 nm, 100 nm, 150 nm, 200 nm, etc.

[0067] This study found that the second carbon coating layer has a loose and porous amorphous structure, which can improve the specific capacity and cycle performance. When the thickness of the second carbon coating layer is selected within the above range, the resulting secondary battery has excellent overall performance.

[0068] It should be noted that the test methods for the thickness h1 of the first carbon coating layer and the thickness h2 of the second carbon coating layer are as follows: a high-resolution transmission electron microscope is used to measure the TEM image of the surface of the stripped negative electrode active material F, and the thickness h1 of the first carbon coating layer (ordered layered structure) and the thickness h2 of the second carbon coating layer (amorphous structure) can be measured using TEM analysis software.

[0069] In one embodiment, the ratio of the thickness h1 of the first carbon coating layer to the thickness h2 of the second carbon coating layer is 0.1 ≤ h1 / h2 ≤ 10.

[0070] For example, the ratio h1 / h2 of the thickness h1 of the first carbon coating layer and the thickness h2 of the second carbon coating layer can be any point value between 0.1 and 10 or a range value between any two points, such as 0.1, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, etc.

[0071] This study found that controlling the thickness ratio of the first carbon coating layer and the second carbon coating layer within the above-mentioned range can effectively improve the specific capacity and energy density, while also reducing the degree of interfacial side reactions and improving the fast-charging cycle performance of the secondary battery.

[0072] In one embodiment, the areal density of the negative electrode active material layer is 50 g / m². 2 ~150g / m 2 .

[0073] It should be noted that the method for testing the areal density of the negative electrode active material layer is as follows: The secondary battery is discharged at a 1C current to the lower voltage limit (for example, if the positive electrode active material is lithium iron phosphate, the lower cutoff voltage limit is 2.5V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the lower cutoff voltage limit is 2.8V), and the negative electrode sheet is obtained after disassembly. A 0.5mm × 0.5mm piece of copper foil above the negative electrode tab is taken, weighed, and the areal density C1 (g / m³) of the copper foil is calculated. 2 Take a 3cm × 3cm negative electrode sheet, weigh it, and calculate the negative electrode surface density C2 (g / m³). 2 The areal density of the negative electrode active material layer is CW = (C2 - C1) / 2.

[0074] For example, the areal density of the negative electrode active material layer can be 50 g / m². 2 ~150g / m 2 The value at any point between or between any two points, for example, could be 50 g / m². 2 60 g / m 2 70 g / m 2 80 g / m 2 90 g / m 2 100 g / m 2 110 g / m 2 120 g / m 2 130 g / m 2 140 g / m 2 150 g / m 2 wait.

[0075] This study found that the areal density of the negative electrode active material affects the length of the lithium-ion diffusion path, thus affecting the fast-charging performance of the secondary battery and its energy density. When the areal density of the negative electrode active material layer is selected to be 50 g / m², the effect is significant. 2 ~150g / m 2 When the time is right, the overall performance of the secondary battery obtained is better.

[0076] In one embodiment, the compaction density of the negative electrode sheet is 1 g / cm³. 3 ~2g / cm 3 .

[0077] It should be noted that the test method for the compaction density of the negative electrode sheet is as follows: the secondary battery is discharged at a 1C current to the lower voltage limit (for example, if the positive electrode active material is lithium iron phosphate, the lower cutoff voltage limit is 2.5V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the lower cutoff voltage limit is 2.8V), and the negative electrode sheet is obtained after disassembly. The thickness T of the copper foil above the negative electrode tab is measured using a micrometer. c， The thickness T of the negative electrode sheet was measured at 10 random points, and the average value was taken as the final thickness measurement result. The compaction density PD of the negative electrode sheet is calculated as 2 × CW / (TT). c ).

[0078] For example, the compaction density of the negative electrode sheet can be 1 g / cm³. 3 ~2g / cm 3 The value at any point between or between any two points, for example, could be 1 g / cm³. 3 1.1 g / cm 3 1.2 g / cm 3 1.3 g / cm 3 1.4 g / cm 3 1.5 g / cm 3 1.6 g / cm 3 1.7 g / cm 3 1.8 g / cm 3 1.9 g / cm 3 2 g / cm 3 wait.

[0079] This application research found that the compaction density of the negative electrode sheet affects its electrolyte wettability, thereby affecting the degree of lithium plating during fast charging of the secondary battery. Simultaneously, the compaction density of the negative electrode sheet also affects the energy density of the secondary battery. Furthermore, when the compaction density of the negative electrode sheet is further selected to be 1 g / cm³... 3 ~2g / cm 3 When the time is right, the overall performance of the secondary battery obtained is better.

[0080] In one embodiment, the secondary battery further includes a positive electrode, and the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area is (1.01~1.3):1:.

[0081] It should be noted that the test method for the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area is as follows: The secondary battery is discharged at a 1C current to the lower voltage limit (exemplarily, if the positive electrode active material is lithium iron phosphate, the lower cutoff voltage limit is 2.5V; if the positive electrode active material is lithium manganese iron phosphate or nickel-cobalt-manganese ternary material, the lower cutoff voltage limit is 2.8V), and the positive and negative electrode sheets are obtained after disassembly. The positive electrode sheet is coated with NMP solvent to form a single-sided coating, and the negative electrode sheet is coated with a water-based solvent to form a single-sided coating. These are then dried at 120℃ for 20 minutes to obtain single-sided coated positive and negative electrode sheets. The above positive electrode sheets are assembled into a coin cell, and the discharge capacity Q1 per unit area at 0.1C is measured in mAh / mm². 2 Assemble the above negative electrode sheets into a coin cell and measure the charging capacity Q2 per unit area at 0.1C, in mAh / mm². 2 The ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area is Q2 / Q1. If the positive electrode active material is lithium manganese iron phosphate or nickel cobalt manganese ternary material, the coin cell test voltage range is 2.8V~4.4V; if the positive electrode active material is lithium iron phosphate, the coin cell test voltage range is 2.5V~3.7V; the negative electrode test voltage range is 0V~2V.

[0082] For example, the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area can be any point value between (1.01~1.3):1 or a range value between any two points, such as 1.01:1, 1.02:1, 1.05:1, 1.08:1, 1.1:1, 1.12:1, 1.15:1, 1.18:1, 1.2:1, 1.22:1, 1.25:1, 1.28:1, 1.3:1, etc.

[0083] This study found that increasing the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area within a certain range can effectively improve the lithium plating phenomenon of the negative electrode and enhance the fast-charging cycle performance of the secondary battery; decreasing the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area within a certain range can effectively improve the energy density of the secondary battery; when the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area is selected as (1.01~1.3):1, the overall performance of the obtained secondary battery is better.

[0084] In one embodiment, when the secondary battery is fully discharged, the thickness of the negative electrode sheet is 67μm~194.5μm.

[0085] It should be noted that the method for testing the thickness of the negative electrode sheet when the secondary battery is fully discharged is as follows: The secondary battery is discharged at a 1C current to the lower voltage limit (for example, if the positive electrode active material is lithium iron phosphate, the lower cutoff voltage limit is 2.5V; if the positive electrode active material is lithium manganese iron phosphate or nickel cobalt manganese ternary material, the lower cutoff voltage limit is 2.8V), and the negative electrode sheet is obtained after disassembly. The thickness T of the negative electrode sheet is measured using a micrometer, and 10 points are randomly tested. The average value is taken as the final thickness test result.

[0086] For example, when the secondary battery is fully discharged, the thickness of the negative electrode sheet can be any value between 67μm and 194.5μm or a range between any two points, such as 67μm, 70μm, 80μm, 90μm, 100μm, 110μm, 120μm, 130μm, 140μm, 150μm, 160μm, 170μm, 180μm, 190μm, 194.5μm, etc.

[0087] This study found that increasing the thickness of the negative electrode sheet when the secondary battery is fully discharged can effectively improve the energy density of the secondary battery to a certain extent, while reducing the thickness of the negative electrode sheet when the secondary battery is fully discharged can reduce the lithium-ion diffusion path and improve the kinetic performance of the secondary battery. When the thickness of the negative electrode sheet when the secondary battery is fully discharged is selected to be 67μm~194.5μm, the overall performance of the secondary battery is better.

[0088] In one embodiment, the negative electrode active material includes at least one of artificial graphite, natural graphite, silicon carbide, and silicon oxide.

[0089] In one embodiment, at least a portion of the surface of the negative electrode active material includes a first carbon coating layer and a second carbon coating layer disposed on at least a portion of the surface of the first carbon coating layer; wherein the carbon source of the first carbon coating layer includes at least one of pitch, petroleum coke, and coal tar, and the carbon source of the second carbon coating layer includes at least one of propanol, acetone, tartaric acid, and citric acid.

[0090] In one embodiment, the negative electrode active material layer further includes a negative electrode conductive agent and a negative electrode binder.

[0091] This application does not impose any particular restrictions on the selection of the negative electrode conductive agent; conventional negative electrode conductive agents in the art can be used. For example, the negative electrode conductive agent may be acetylene black, graphene, carbon nanotubes (CNTs), etc.

[0092] This application does not impose any particular restrictions on the selection of the negative electrode binder; conventional negative electrode binders in the art can be used. For example, the negative electrode binder may be polyvinylidene fluoride, styrene-butadiene rubber, etc.

[0093] In one embodiment, the positive electrode sheet includes 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, a positive binder, and a positive conductive agent.

[0094] This application does not impose any particular restrictions on the selection of the positive electrode active material; conventional positive electrode active materials in the art can be used. For example, the positive electrode active material may be lithium iron phosphate, lithium manganese iron phosphate, lithium cobalt oxide, lithium nickel cobalt manganese oxide, etc.

[0095] This application does not impose any particular restrictions on the selection of the positive electrode binder; conventional positive electrode binders in the art can be used. For example, the positive electrode binder may be polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), sodium polyacrylate (PAANa), carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), etc.

[0096] This application does not impose any particular restrictions on the selection of the positive electrode conductive agent; conventional positive electrode conductive agents in the art can be used. For example, the positive electrode conductive agent may be acetylene black, SuperP, carbon nanotubes, graphene, etc.

[0097] In one embodiment, the secondary battery further includes an electrolyte comprising an organic solvent and a lithium salt.

[0098] This application does not impose any particular restrictions on the selection of organic solvents; conventional organic solvents in the art can be used. For example, the organic solvent may be ethylene carbonate, ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate, etc.

[0099] This application does not impose any particular restrictions on the selection of lithium salts, and conventional lithium salts in the art can be used. For example, the lithium salt may be lithium hexafluorophosphate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethylsulfonyl)imide, etc.

[0100] In one embodiment, the method for preparing the secondary battery includes the following steps: (1) Preparation of negative electrode active material: The negative electrode active material is placed in a vapor deposition reaction chamber, and after heating under vacuum conditions, a first carbon source solution is introduced for a first reaction, followed by a second carbon source solution for a second reaction. After the second reaction is completed, the material is cooled to room temperature to obtain a negative electrode active material having a first carbon coating layer and a second carbon coating layer, wherein the second carbon coating layer is disposed on at least a portion of the surface of the first carbon coating layer, and the first carbon coating layer is disposed on at least a portion of the surface of the negative electrode active material. (2) Preparation of positive electrode sheet: The positive active material, positive conductive agent and positive binder are mixed and then added to N-methylpyrrolidone to obtain positive electrode slurry. The positive electrode slurry is coated on at least one surface of the positive current collector, and then dried, rolled, slit and cut to obtain positive electrode sheet. (3) Preparation of negative electrode sheet: The negative electrode active material, negative electrode conductive agent and negative electrode binder are mixed and added to water to obtain negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector, and then dried, rolled, slit and cut to obtain negative electrode sheet. (4) Preparation of electrolyte: Lithium salt and additives are added to an organic solvent to obtain an electrolyte; (5) Preparation of secondary battery: The positive electrode, negative electrode, separator and other components are assembled and then subjected to winding, hot pressing, super welding, core assembly, casing, baking, liquid injection, high temperature wetting, formation, aging, sealing and capacity testing to obtain secondary battery.

[0101] In some embodiments, the heating rate in step (1) is 4°C / min to 6°C / min.

[0102] In some embodiments, in step (1), the temperature of the first reaction is 750°C to 850°C, and the time of the first reaction is 25 min to 35 min.

[0103] In some embodiments, in step (1), the flow rate of the first carbon source solution is 25 cm⁻¹. 3 / min ~35cm 3 / min.

[0104] In some embodiments, in step (1), the mass percentage of the solute in the first carbon source solution is 1% to 20%.

[0105] In some embodiments, in step (1), the solvent of the first carbon source solution includes toluene.

[0106] In some embodiments, in step (1), the temperature of the second reaction is 750°C to 850°C, and the time of the second reaction is 25 min to 35 min.

[0107] In some embodiments, in step (1), the flow rate of the second carbon source solution is 45 cm⁻¹. 3 / min ~55cm 3 / min.

[0108] It should be noted that the degree of graphitization of the negative electrode active material can be altered by controlling the type and amount of carbon coating source. For example, increasing the mass of the first carbon coating source will increase the degree of graphitization of the negative electrode active material. Similarly, changing the type of negative electrode active material will also change its degree of graphitization. Furthermore, reaction parameters during the introduction of the carbon coating layer, such as reaction temperature and reaction time, will also affect the degree of graphitization. Other factors, such as changing the particle size of the negative electrode active material, will also influence its degree of graphitization, but these will not be listed here.

[0109] It should be noted that the powder resistivity of the negative electrode active material at 12 kN can be altered by controlling the type and amount of carbon coating source. For example, increasing the carbon coating amount will decrease the powder resistivity of the negative electrode active material at 12 kN. Simultaneously, changing the type of negative electrode active material will also change its powder resistivity at 12 kN. Furthermore, reaction parameters during the introduction of the carbon coating layer, such as reaction temperature and reaction time, will also affect the powder resistivity of the negative electrode active material at 12 kN. Other factors, such as changing the particle size of the negative electrode active material, will also influence the powder resistivity at 12 kN, but these will not be listed here.

[0110] It should be noted that before placing the negative electrode active material in the vapor deposition reaction chamber, the process includes ball milling and sieving of the negative electrode active material. By adjusting the ball milling speed and time, the Dv50, Dv10, and Dv99 of the negative electrode active material can be changed. For example, increasing the ball milling speed will reduce the Dv50, Dv10, and Dv99 of the negative electrode active material.

[0111] It should be noted that the specific surface area of ​​the negative electrode active material can be changed by adjusting the particle size, carbon coating source, and carbon coating amount. For example, increasing the particle size will reduce the specific surface area of ​​the negative electrode active material.

[0112] It should be noted that the thickness of the first carbon coating layer and the thickness of the second carbon coating layer can be changed by adjusting the reaction time. For example, increasing the first reaction time will increase the thickness of the first carbon coating layer; increasing the second reaction time will increase the thickness of the second carbon coating layer.

[0113] It should be noted that by adjusting the carbon coating source, carbon coating amount, electrode coating weight, and compaction, the maximum width of the second region along the direction of the tab near the negative electrode can be changed after the secondary battery is fully charged. For example, when the first carbon coating amount is increased, the maximum width of the second region will decrease.

[0114] It should be noted that by adjusting the width of the negative electrode plate, the maximum width of the first region along the direction of the tab close to the negative electrode plate can be changed after the secondary battery is fully charged. For example, when the width of the negative electrode plate is increased, the maximum width of the first region will increase.

[0115] It should be noted that the areal density of the negative electrode active material layer can be changed by adjusting the solid content and flow rate of the negative electrode slurry. For example, increasing the solid content and flow rate will increase the areal density of the negative electrode active material layer.

[0116] It should be noted that the compaction density of the negative electrode sheet can be changed by adjusting the rolling pressure and the roll gap width during the preparation process. For example, increasing the rolling pressure and decreasing the roll gap thickness will increase the compaction density of the negative electrode sheet.

[0117] It should be noted that the thickness of the negative electrode sheet when the secondary battery is fully discharged can be changed by adjusting the coating weight and compaction density of the negative electrode sheet. For example, increasing the coating weight and decreasing the compaction density will increase the thickness of the negative electrode sheet when the secondary battery is fully discharged.

[0118] It should be noted that by adjusting the coating weight of the negative electrode, the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area can be changed. For example, when the coating weight of the negative electrode is increased, the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area will increase.

[0119] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0120] For example, the aforementioned electrical devices may include mobile devices (such as mobile phones, laptops, etc.), electric vehicles (such as pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc., but are not limited thereto.

[0121] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of negative electrode active material Graphite raw material was placed in a ball mill, using zirconia balls as the milling medium. The mass ratio of zirconia balls to graphite was 20:1. The milling speed was 300 rpm, and the milling time was 2 hours, yielding artificial graphite with a Dv50 of 14.9 μm. This artificial graphite was then placed on a quartz boat and placed in a vapor deposition reaction chamber, where a vacuum of 10⁻⁶ ppm was applied. -3 Pa, removing air and moisture; then heating to 800℃ at a rate of 5℃ / min, followed by a 30cm increment.3 The first carbon source solution (a toluene solution of asphalt, with an asphalt mass percentage of 5%) was continuously sprayed at a flow rate of / min to initiate the first reaction, which lasted for 30 min. Then, the solution was sprayed at a flow rate of 50 cm⁻¹ to initiate the first reaction. 3 The second carbon source solution (propanol solution) is sprayed into the second carbon source solution (propanol solution) at a flow rate of / min for the second reaction, and the second reaction time is 30min. After the second reaction is completed, the temperature is cooled to room temperature (25℃) to obtain the negative electrode active material. The surface of the negative electrode active material includes a first carbon coating layer, and the first carbon coating surface is coated with a second carbon coating layer. (2) Preparation of negative electrode sheet The negative electrode active material, negative electrode conductive agent (carbon black), negative electrode binder (CMC), and negative electrode binder (SBR) were mixed in a mass ratio of 96.3:0.7:1.1:1.9, and then thoroughly mixed in water to obtain a negative electrode slurry with a solid content of 50%. The negative electrode slurry was then coated onto both surfaces of a 4.5 μm copper foil at a flow rate of 1 m / min, with a single-sided coating weight of 95 g / m². 2 After coating, the electrode sheet is dried, rolled (rolling pressure is 80t, roll gap thickness is 120μm), slitting and cutting to obtain the negative electrode sheet; (3) Preparation of positive electrode sheet The positive electrode active material (lithium iron phosphate), positive electrode conductive agent (carbon black), and positive electrode binder (PVDF) were mixed in a weight ratio of 97:0.7:2.3. Then, N-methylpyrrolidone (NMP) was added and thoroughly mixed to obtain a positive electrode slurry with a solid content of 65%. The positive electrode slurry was then coated on both sides of a (13+1+1) μm carbon-coated aluminum foil at a flow rate of 2 m / min, with a coating weight of 199 g / m². 2 After coating, the electrode is dried, rolled (rolling pressure is 100t, roll gap thickness is 170μm), slitting and cutting to obtain the positive electrode. (4) Preparation of electrolyte Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a mass ratio of 1:1:1, then lithium hexafluorophosphate was added, and after thorough mixing, vinylene carbonate was added as an additive. Based on the mass of the electrolyte, the mass content of lithium hexafluorophosphate was 12%, and the mass content of vinylene carbonate was 0.5%. (5) Preparation of secondary batteries The positive electrode, negative electrode, separator (polyethylene film) and other battery components are assembled and then processed through shaping, baking, packaging, liquid injection, formation and capacity testing to obtain a secondary battery.

[0122] In addition, after the secondary battery in Example 1 was fully charged, it was disassembled to obtain the negative electrode plate. The corresponding photograph of the negative electrode plate is shown below. Figure 1As shown, H represents the maximum width of the second region, and Z represents the maximum width of the first region.

[0123] Examples 2-4 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the ball milling speed is adjusted to change the negative electrode active materials Dv50, Dv10 and Dv99, so as to achieve the parameters in Tables 1 to 2.

[0124] Examples 5-8 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Embodiment 1 is that the width of the negative electrode sheet is adjusted to change the maximum width of the first region from the tab side of the negative electrode sheet to the center of the negative electrode sheet after the secondary battery is fully charged, so as to achieve the parameters in Tables 1-2.

[0125] Examples 9-12 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the solid content and flow rate of the negative electrode slurry are adjusted to change the density of the negative electrode active material layer, so as to achieve the parameters in Tables 1-2.

[0126] Examples 13-16 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the rolling pressure and the roll gap thickness are adjusted to change the compaction density of the negative electrode sheet, so as to achieve the parameters in Tables 1-2.

[0127] Examples 17-20 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the coating weight of the negative electrode sheet is adjusted to change the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area, so as to achieve the parameters in Tables 1-2.

[0128] Examples 21-23 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the type of negative electrode active material is adjusted to achieve the parameters in Tables 1-2.

[0129] Examples 24-25 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the type of the first carbon source is adjusted to achieve the parameters in Tables 1-2.

[0130] Examples 26-28 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the type of the second carbon source is adjusted to achieve the parameters in Tables 1-2.

[0131] Examples 29-31 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the first and second reaction times are adjusted to change the thickness of the first carbon coating layer, the thickness of the second carbon coating layer, and the ratio of their thicknesses, so as to achieve the parameters in Tables 1-2.

[0132] Comparative Examples 1-2 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that, in Comparative Example 1, the maximum width of the second region along the direction of the tab near the negative electrode is too large after the secondary battery is fully charged by adjusting the particle size; in Comparative Example 2, the maximum width of the second region along the direction of the tab near the negative electrode is too small after the secondary battery is fully charged by adjusting the width of the negative electrode, so as to achieve the parameters in Tables 1 to 2.

[0133] Comparative Examples 3-4 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and that of Example 1 is that in Comparative Example 3, the first reaction time is adjusted to achieve an excessively thick first coating layer and an excessively high degree of graphitization of the negative electrode active material; in Comparative Example 4, there is no first coating layer, resulting in an excessively low degree of graphitization of the negative electrode active material, so as to achieve the parameters in Tables 1-2.

[0134] Comparative Examples 5-6 This application provides a secondary battery in a comparative example. The difference between the preparation method of the secondary battery and Example 1 is that in Comparative Example 5, the second reaction time is adjusted to achieve an excessively thick second coating layer of the negative electrode active material and an excessively low powder resistance of the 12kN negative electrode active material; in Comparative Example 6, there is no second coating layer and the powder resistance of the 12kN negative electrode active material is too high, so as to achieve the parameters in Tables 1 to 2.

[0135] The parameters in the examples and comparative examples are shown in Tables 1-2, including the following parameters: Dv50, Dv10, and Dv99 of the negative electrode active material; B of the specific surface area of ​​the negative electrode active material; H of the maximum width of the second region along the direction of the tab near the negative electrode sheet after the secondary battery is fully charged; G of the graphitization degree of the negative electrode active material; R and A of the powder resistivity of the negative electrode active material at 12 kN; Z of the maximum width of the first region along the direction of the tab near the negative electrode sheet after the secondary battery is fully charged; CW of the areal density of the negative electrode active material layer; PD of the compaction density of the negative electrode sheet; T of the negative electrode sheet when the secondary battery is fully discharged; CB of the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area; type of negative electrode active material; type of the first carbon source and the second carbon source; h1 of the thickness of the first carbon coating layer; h2 of the thickness of the second carbon coating layer; and h1 / h2 of the thickness of the first carbon coating layer and the thickness of the second carbon coating layer.

[0136] Table 1. Parameters of Secondary Battery Table 2 Secondary Battery Parameters The performance tests of the obtained secondary batteries include the following: (1) Energy density test: At room temperature, the battery was charged at a constant current of 1C to 3.65V, and then charged at a constant voltage of 3.65V until the current was less than 0.05C. After standing for 5 minutes, the battery was discharged at a current of 1C to 2.5V to obtain the secondary battery capacity C0. The mass of the secondary battery was denoted as W0, and the energy density of the secondary battery was ED=C0 / W0 (Wh / kg). The results are shown in Table 3.

[0137] (2) Fast charging cycle performance test: At 25℃ and 45℃, the capacitor was charged at a constant current of 4C to 3.65V, and then charged at a constant voltage of 3.65V until the current was less than 0.05C. After resting for 5 minutes, it was discharged at a current of 1C to 2.5V. This charge-discharge cycle was repeated 2000 times. The capacity retention rate of the 2000th cycle = capacity of the 2000th cycle / capacity of the first cycle × 100%. The results are shown in Table 3.

[0138] Table 3 As can be seen from Table 3, when the technical solution provided in this application is adopted, the obtained secondary battery has a high energy density and excellent fast-charging cycle performance; specifically, the energy density of the obtained secondary battery is above 150Wh / kg, and the capacity retention rate after 2000 cycles at 25℃ and 45℃ is above 75% and above 70%, respectively.

[0139] As can be seen from Examples 1-31 and Comparative Examples 1-2, when the second region along the direction of the tab close to the negative electrode sheet after the secondary battery is fully charged, the maximum width of the second region is within the range given in this application, and the resulting secondary battery has both high energy density and excellent fast-charging cycle performance.

[0140] As can be seen from Examples 1-31 and Comparative Examples 3-4, when the degree of graphitization of the negative electrode active material is within the range given in this application, the resulting secondary battery has excellent overall performance.

[0141] As can be seen from Examples 1-31 and Comparative Examples 5-6, when the powder resistivity of the negative electrode active material is within the range given in this application at 12 kN, the resulting secondary battery has excellent overall performance.

[0142] Finally, it should be noted that the above embodiments are used to illustrate the technical solutions of this application and not to limit the scope of protection of this application. Although this application has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this application without departing from the substance and scope of the technical solutions of this application.

Claims

1. A secondary battery, comprising a negative electrode sheet and a tab connected to the negative electrode sheet, the tab being located on one side of the negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative active material layer disposed on at least one surface of the negative current collector, the negative active material layer comprising a negative active material, characterized in that, The degree of graphitization of the negative electrode active material is 80%~96%; The powder resistivity of the negative electrode active material at 12 kN is 0.0001 Ω·cm to 0.01 Ω·cm; After the secondary battery is fully charged, along the direction of the tab near the negative electrode, the negative electrode includes a first region and a second region in sequence; the maximum width of the second region is 0.1mm~24mm.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies: 0.03 ≤ A ≤ 10.

05. Where A = H × G × R; H mm is the maximum width of the second region; G% represents the degree of graphitization of the negative electrode active material; R Ω·cm is the powder resistivity of the negative electrode active material at 12kN.

3. The secondary battery according to claim 1, characterized in that, The maximum width of the first region is 0.5mm to 6mm.

4. The secondary battery according to claim 1, characterized in that, The negative electrode active material satisfies any one of the following: a. 5μm≤Dv50≤25μm; b. 1μm≤Dv10≤10μm; c. Dv99≤50μm; d. The specific surface area B of the negative electrode active material is 0.5 m². 2 / g~5 m 2 / g.

5. The secondary battery according to claim 4, characterized in that, At least a portion of the surface of the negative electrode active material has a first carbon coating layer; the thickness h1 of the first carbon coating layer is 1 nm to 200 nm.

6. The secondary battery according to claim 5, characterized in that, At least a portion of the surface of the first carbon coating layer has a second carbon coating layer; the thickness h2 of the second carbon coating layer is 1 nm to 200 nm.

7. The secondary battery according to claim 6, characterized in that, The ratio of the thickness h1 of the first carbon coating layer to the thickness h2 of the second carbon coating layer is 0.1 ≤ h1 / h2 ≤ 10.

8. The secondary battery according to claim 1, characterized in that, The areal density of the negative electrode active material layer is 50 g / m³. 2 ~150g / m 2 ; And / or, the compaction density of the negative electrode sheet is 1 g / cm³. 3 ~2g / cm 3 .

9. The secondary battery according to claim 1, characterized in that, The secondary battery also includes a positive electrode, and the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area is (1.01~1.3):

1.

10. The secondary battery according to claim 1, characterized in that, When the secondary battery is fully discharged, the thickness of the negative electrode sheet is 67μm~194.5μm.

11. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 10.

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