A negative electrode active layer, a preparation method therefor, and an application thereof

Abstract

This invention discloses a negative electrode active layer, its preparation method, and its application. The negative electrode active layer comprises a negative electrode active material containing graphite crystals, wherein the full width at half maximum (FWHM) of the 002 and 100 characteristic peaks of the graphite crystals are β0.2 and β100, respectively. 002 β 100 The diffraction angles of the characteristic peaks 002 and 100 are θ, respectively. 002 θ 100 The average width L of the graphite crystal along the a-axis was calculated. a and the average height L along the c-axis c :L a =1.77λ / (β) 100 *cosθ 100 K = 0.089, L c =(K*λ) / (β) 002 *cosθ 002 Wherein, La and Lc satisfy the condition: 1 ≤ La / Lc ≤ 2.5. In this invention, the condition 1 ≤ Lc is satisfied. a / L c ≤2.5, can not only be used for Li + The embedding / extraction provides more edge locations, which can shorten the Li + The diffusion path is beneficial for improving rate performance and also for the reversible capacity of graphite at high current densities.

Description

Technical Field

[0001] This invention belongs to the field of battery materials technology, specifically relating to a negative electrode active layer, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries possess outstanding advantages such as high energy density, no memory effect, long cycle life, rapid charge and discharge, and low self-discharge, making them widely used in consumer electronics, electric vehicles, and energy storage. To enhance product competitiveness, increasing battery energy density through the use of high-capacity anode materials is a common method. Graphite, due to its high specific capacity, is widely used in battery anodes; however, while conventional graphite provides high energy density, it typically exhibits significant volume changes during charge and discharge, which to some extent affects battery performance. Therefore, how to more effectively mitigate the volume expansion of anode materials and ensure further improvement in battery performance remains a pressing challenge in the lithium battery industry. Summary of the Invention

[0003] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a negative electrode active layer, which, when used in a secondary battery, results in a low battery volume expansion rate and good battery safety performance.

[0004] This invention also proposes a method for preparing a negative electrode active layer.

[0005] The present invention also proposes a negative electrode sheet.

[0006] The present invention also proposes a secondary battery.

[0007] In a first aspect, the present invention provides a negative electrode active layer containing a negative electrode active material, wherein the thickness of the negative electrode active layer is H μm, the tortuosity of the negative electrode active layer is τ, and the negative electrode active material is D 50 Particle size D μm, compaction density P of negative electrode active layer 3 The following condition must be met: 1.6≤(H*τ) / (D*P)≤4.7.

[0008] The negative electrode active layer according to embodiments of the present invention has at least the following beneficial effects:

[0009] Tortivity is the ratio of the actual length of the permeation channel to the apparent length / macroscopic distance (which can be considered as the electrode thickness or a related value) through the permeation medium. In this invention, tortivity represents the degree of curvature of the porous electrode transport path, which is related to the ratio of the actual transport path of lithium ions in the negative electrode active layer to the thickness of the negative electrode active layer. Therefore, tortivity is correlated with electrode thickness. The actual length of the permeation channel and the apparent length / macroscopic distance through the permeation medium are significantly affected by P and D factors. Therefore, considering factors such as battery energy density and electrochemical performance, a battery that satisfies 1.6 ≤ (H*τ) / (D*P) ≤ 4.7 can achieve superior overall performance.

[0010] Among these factors, the particle size of the negative electrode active material is closely related to its compaction density. Smaller particle size is more conducive to the compaction density of the electrode, thereby improving the battery's energy density. Furthermore, the determinants of ion diffusion tortuosity include the particle size, compaction density, and electrode thickness. Larger particle size, higher compaction density, and thicker electrodes increase tortuosity and reduce ion transport efficiency. In this invention, under the condition of 1.6 ≤ (H*τ) / (D*P) ≤ 4.7, the ion diffusion path is effectively shortened, reducing ion accumulation on the negative electrode surface and facilitating electrolyte penetration into the negative electrode active layer. This further increases the battery's energy density by increasing the compaction density and electrode thickness.

[0011] In some embodiments of the present invention, the τ satisfies: 1.2≤τ≤2.5.

[0012] In some embodiments of the present invention, the tortuosity τ is calculated using the following formula:

[0013] τ=γε 1-ɑ

[0014] Where ε is the porosity of the negative electrode active layer, γ is 1, and α is 1.5. α is the Bruggeman exponent, and γ is the pore shape factor.

[0015] In some embodiments of the present invention, H satisfies: 5≤H≤100, such as 25≤H≤60.

[0016] In some embodiments of the present invention, D satisfies: 5≤D≤25, such as 11≤D≤17.

[0017] In some embodiments of the present invention, P satisfies: 1.55≤P≤1.85, such as 1.55≤P≤1.80.

[0018] In some embodiments of the present invention, the negative electrode active layer comprises a negative electrode active material containing graphite, and the XRD pattern of the graphite contains characteristic peaks (002) and (100); the full width at half maximum (FWHM) of the (002) peak and the (100) peak are β and β, respectively. 002 β 100 The diffraction angles of the (002) peak and the (100) peak are θ, respectively. 002 θ 100 ;

[0019] The average width L of graphite along the a-axis direction a and the average height L along the c-axis c for:

[0020] L a =1.77λ / (β) 100 *cosθ 100 ), L c =(K*λ) / (β) 002 *cosθ 002 ), where K = 0.089,

[0021] And L a and L c The condition is satisfied between L and L: 1.4 ≤ L a / L c ≤2.4.

[0022] a-axis: parallel to the surface within the graphite layer, usually called the prism (1010); c-axis: perpendicular to the graphite layer, representing the stacking direction between layers, usually called the basal plane (0001). Based on the XRD pattern of graphite crystals, L a The value L represents the base plane parallel to the graphite layer. c The value represents the edge position of the graphite crystal end face. (This is related to the basal plane (L)). a Compared to the edge location (L), c It has higher Li + Insertion / extraction activity, thus achieving higher rate performance. And high L... a Graphite exhibits higher capacity. In this invention, the condition 1.4 ≤ L is satisfied. a / L c ≤2.4, not only can it be used for Li + The embedding / extraction provides more edge locations, which can shorten the Li +The diffusion path is more conducive to improving rate performance and also to the reversible capacity of graphite at high current density. At the same time, satisfying this relationship has a certain effect on improving the volume expansion rate of battery materials: the length of graphite material on the horizontal base plane is relatively short, the anisotropy of the material in the negative electrode slurry coating process of electrode preparation will be more obvious, and the lithium intercalation expansion of the material is decomposed from the Z direction to the X and Y directions, which can alleviate the thickness expansion of the battery.

[0023] In some embodiments of the present invention, the θ 100 The angle is 42.8° to 44.3°, θ 002 The range is 23.0° to 26.5°.

[0024] In some embodiments of the present invention, 20nm≤L a ≤100nm, 10nm≤L c ≤70nm, or 30nm≤L can be selected. a ≤80nm, 20nm≤L c ≤60nm.

[0025] In some embodiments of the present invention, the graphite includes at least one of natural graphite or synthetic graphite. Optionally, the graphite contains graphite crystals.

[0026] In some embodiments of the present invention, the negative electrode active material further includes at least one of silicon or carbon materials. Optionally, the negative electrode active material includes at least one of hard carbon or silicon-carbon materials. In some embodiments of the present invention, the negative electrode active material may include a graphite and silicon negative electrode material, or a mixed material containing graphite and hard carbon, or a mixed material containing graphite, silicon negative electrode material, and hard carbon, etc. Optionally, the silicon negative electrode material may be silicon.

[0027] In some embodiments of the present invention, the negative electrode active layer further includes a conductive agent and a binder. The types of conductive agent and binder are not limited; for example, in some embodiments of the present invention, the conductive agent may be one or more of SP, CNT, etc.; and in some embodiments of the present invention, the binder may be one or more of SBR, PAALi, etc.

[0028] In a second aspect, the present invention provides a method for preparing a negative electrode active layer, comprising the following steps: coating a negative electrode slurry containing graphite onto a substrate surface to form a wet film, magnetizing it, and then drying it to obtain the negative electrode active layer; the negative electrode active layer contains a negative electrode active material, and the thickness of the negative electrode active layer is H μm, the tortuosity of the negative electrode active layer is τ, and the negative electrode active material is D 50 Particle size D μm, compaction density P of negative electrode active layer 3The following condition must be met: 1.6 ≤ (H*τ) / (D*P) ≤ 4.7. Optionally, the graphite includes, but is not limited to, one or more of natural graphite, artificial graphite, etc.

[0029] The method for preparing the negative electrode active layer according to embodiments of the present invention has at least the following beneficial effects:

[0030] Graphite has a layered structure. Typically, lithium ions need to insert into the gaps between the graphite layers from the edges, diffuse through the SEI film, and reach the reaction site to combine with electrons. The tortuous path of lithium insertion / extraction easily leads to lithium deposition. In this invention, under the influence of an external magnetic field, the electron spin in the graphite changes, causing electron rearrangement and affecting the electronic structure and properties of the graphite, thereby altering its magnetization. After magnetization, the alignment direction of the graphite is changed, satisfying the relationship: 1.6 ≤ (H*τ) / (D*P) ≤ 4.7. Using this in lithium-ion batteries is more conducive to lithium ion insertion and extraction, reduces the tortuosity of the electrode, improves rate performance and electrolyte wetting, reduces thickness expansion during battery cycling, alleviates the problem of expansion in the thickness direction, and extends battery cycle life.

[0031] In some embodiments of the present invention, the magnetic field strength of the magnetization is 3000-8000 Gs, such as 4000-6000 Gs.

[0032] In some embodiments of the present invention, the magnetization time is 3 to 30 seconds, such as 5 to 15 seconds.

[0033] In some embodiments of the present invention, the negative electrode slurry includes a negative electrode active material, and the negative electrode active material includes the graphite.

[0034] In some embodiments of the present invention, the negative electrode active material further includes at least one of silicon material or carbon material. Optionally, the negative electrode active material includes at least one of hard carbon or silicon-carbon material. In some embodiments of the present invention, the negative electrode slurry can be a slurry containing graphite and silicon negative electrode material, or a slurry containing a mixture of graphite and hard carbon, or a slurry containing a mixture of graphite, silicon negative electrode material and hard carbon, etc.

[0035] In some embodiments of the present invention, the negative electrode slurry further includes a conductive agent.

[0036] In some embodiments of the present invention, the conductive agent includes at least one of conductive carbon black (SP) or carbon nanotubes (CNTs). Optionally, the mass ratio of SP to CNTs is (3-20):1.

[0037] In some embodiments of the present invention, the negative electrode slurry further includes a binder.

[0038] In some embodiments of the present invention, the adhesive comprises SBR and PAALi. Optionally, the mass ratio of SBR to PAALi is 1:(0.5 to 10).

[0039] In some embodiments of the present invention, the mass ratio of the negative electrode active material, the conductive agent and the binder in the negative electrode slurry is (80-99.5):(0.1-10):(0.1-10), or optionally (90-99.5):(0.2-5):(0.3-5).

[0040] In some embodiments of the present invention, the magnetic field used in the magnetization step is not parallel to the wet film; alternatively, the magnetic field direction is perpendicular to the wet film.

[0041] In some embodiments of the present invention, a magnetic rod is used to magnetize the wet film.

[0042] In some embodiments of the present invention, in the preparation method, the wet film is magnetized, dried, and compacted to obtain the negative electrode active layer.

[0043] In a third aspect, the present invention provides a negative electrode sheet comprising the aforementioned negative electrode active layer. In this invention, magnetization alters the graphite arrangement orientation in the negative electrode sheet, which is more conducive to the insertion and extraction of lithium ions, reduces the tortuosity of the electrode sheet, and satisfies the relationship: 1.6 ≤ (H*τ) / (D*P) ≤ 4.7. This improves the rate performance and electrolyte wetting of the battery using the negative electrode sheet, and effectively alleviates the problem of expansion rate in the thickness direction of the electrode sheet.

[0044] In some embodiments of the present invention, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode active layer is disposed on the surface of the negative electrode current collector.

[0045] In some embodiments of the present invention, the negative electrode active layer is disposed on at least one side surface of the negative electrode current collector.

[0046] In a third aspect, the present invention provides a secondary battery comprising the aforementioned negative electrode sheet.

[0047] In some embodiments of the present invention, the secondary battery includes at least one of a lithium-ion battery or a sodium-ion battery.

[0048] In some embodiments of the present invention, the secondary battery further includes a positive electrode.

[0049] In some embodiments of the present invention, the positive electrode sheet includes a positive electrode active material. The type of material of the positive electrode active material is not limited. In some embodiments of the present invention, the positive electrode active material includes, but is not limited to, one or more of lithium cobalt oxide, ternary materials, lithium-rich materials, or other positive electrode materials.

[0050] In some embodiments of the present invention, the secondary battery further includes an electrolyte. The composition of the electrolyte is not limited. In some embodiments of the present invention, the electrolyte includes an electrolyte salt and an organic solvent, wherein the specific types and compositions of the electrolyte salt and the organic solvent are not specifically limited, and may also include positive electrode film-forming additives, negative electrode film-forming additives, cycle-improving additives, and low-temperature additives, etc.

[0051] In some embodiments of the present invention, the secondary battery further includes a separator. The material of the separator is not limited. In some embodiments of the present invention, the separator includes, but is not limited to, one or more of polyethylene, polypropylene, polyvinylidene fluoride, and their multilayer composite films.

[0052] In some embodiments of the present invention, the secondary battery further includes a packaging film. Detailed Implementation

[0053] The following will describe the concept and technical effects of the present invention clearly and completely with reference to embodiments, so as to fully understand the purpose, features and effects of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the scope of protection of the present invention.

[0054] Unless otherwise specified, the experimental methods used in the following examples and comparative examples are generally performed under conventional conditions in the art or as recommended by the manufacturer; the raw materials and reagents used are all commercially available from the conventional market unless otherwise specified.

[0055] Graphite: Purchased from Jiangxi Zichen Technology Co., Ltd., part number: YP-APG-0058-0000; graphite can also be purchased from other manufacturers.

[0056] SBR, PAALi, and PVDF can be purchased from Yitong New Energy Materials Co., Ltd. (or from other manufacturers producing the corresponding materials); among them, SBR part number: DM-GAS-0002-0077; PAALi part number: YP-GAA-0007-0000; PVDF part number: DM-GPK-0001-0007.

[0057] Conductive agent: Purchased from Yitong New Energy Materials Co., Ltd., DM-GCN-0002-0138, a slurry containing CNTs and dispersants, aqueous, black liquid or jelly-like slurry, slurry solid content 1.00±0.20, CNT content 0.40±0.15, dispersant content 0.60±0.15, viscosity 500~10000mPa·s;

[0058] Example 1

[0059] This embodiment discloses a negative electrode active layer, the preparation process of which includes the following steps:

[0060] A negative electrode active material (graphite), a conductive agent (a mixture of SP and CNT in a mass ratio of 0.45:0.05), and a binder (a mixture of SBR and PAALi in a mass ratio of 0.5:1.8) are mixed with water in a weight ratio of 97.7:1.1:1.2 to prepare a negative electrode active material slurry (the solid content of the slurry can be 65%–80%). The negative electrode active material slurry is uniformly coated onto both sides of a negative electrode current collector (a copper foil with a thickness of 5 μm) to form a wet film. During the wet film coating stage, the graphite on the wet film is magnetized using a magnetic field (the magnetic rod is positioned directly above the wet film, with a magnetic field strength of 5000 gs and a magnetization time of 8 s), with the magnetic field direction perpendicular to the wet film. The film is then dried, cold-pressed, and slit to form a negative electrode active layer on the surface of the negative electrode current collector.

[0061] This embodiment discloses a negative electrode sheet, which is formed by the negative current collector and the negative active layer formed on its two sides.

[0062] This embodiment discloses a lithium-ion battery, including a negative electrode, a positive electrode, an electrolyte, and a separator.

[0063] The preparation method of the positive electrode sheet includes: fully dispersing the positive electrode active material LiCoO2, acetylene black, conductive carbon nanotubes, and binder polyvinylidene fluoride (PVDF) in an N-methylpyrrolidone solvent system at a weight ratio of 97.6:0.5:0.6:1.3 (the solid content of the slurry can be 50% to 80%), and coating it onto both sides of the positive electrode current collector (aluminum foil with a thickness of 8μm).

[0064] Separating membrane: A ceramic layer is formed by coating the surface of a PE base film with a ceramic mixture to obtain a composite separating membrane; the thickness of the PE base film is 4μm, the ceramic layer is a ceramic layer formed by a mixed slurry of PVDF and Al2O3, and the total thickness of the composite separating membrane is 6μm.

[0065] Electrolyte: Ethyl carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and propyl propionate (PP) are mixed in a volume ratio of 1.2:1:4:4 to obtain a mixed organic solvent. Then, fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent at a ratio of 1 mol / L to prepare the electrolyte.

[0066] The preparation of a lithium-ion battery includes: winding the above-mentioned negative electrode sheet, separator, and positive electrode sheet (or stacking them in other embodiments) to make a bare cell, encapsulating and injecting electrolyte to obtain a lithium-ion battery.

[0067] Examples 1-15 and Comparative Examples 1-2 disclose a series of negative electrode active layers, negative electrode sheets, and lithium-ion batteries. Among them:

[0068] The only difference between the active layers of Examples 1-15 and Comparative Examples 1-2 and Example 1 is that the negative active layer adopts the design parameters in Table 1. The magnetization intensity (4000-6000 Gs) and magnetization time (5-15 s) of each example are adjusted to obtain the negative active layers of each example and each comparative example (strengthening the magnetic field strength and extending the magnetization time will reduce the OI value of the electrode graphite, which is beneficial to the diffusion of lithium ions). Other conditions are the same as in Example 1.

[0069] The negative electrode sheets in Examples 1-15 and Comparative Examples 1-2 differ from those in Example 1 only in that the negative electrode active layer in Examples 1-15 and Comparative Examples 1-2 is used instead of the negative electrode active layer in Example 1, while the rest is the same as in Example 1.

[0070] The lithium-ion batteries in Examples 1-15 and Comparative Examples 1-2 differ from those in Example 1 only in that the negative electrode sheet in Examples 1-15 and Comparative Examples 1-2 is used instead of the negative electrode sheet in Example 1, while the rest is the same as in Example 1.

[0071] Table 1

[0072]

[0073] The formula for calculating tortuosity τ is as follows:

[0074] τ=γε 1-α

[0075] Where ε is the electrode porosity, γ is 1, and α is 1.5.

[0076] Electrode porosity ε test: A mercury porosimeter (PoreMaster 60) was used to apply pressures of approximately 0.6–50 PSI at a low-pressure station and 20–60,000 PSI at a high-pressure station. A glass cone probe was used as the sample container, and pressure was applied to the sample. The sample container had a volume of 0.5 cm³ and a length of 3.8 cm. The volume of mercury applied to the sample is the pore volume V. If we assume the sample volume is V... 总 Then the porosity ε = V / V 总 *100%.

[0077] Test case

[0078] This experimental example tested the performance of the batteries obtained in the embodiment and the comparative example, specifically including:

[0079] (1) Figure 1 The image shows the XRD pattern of the negative electrode sheet in Example 1, where the red peak represents Cu current collector, the blue peak represents graphite, the (002) characteristic peak is around 26.5°, and the (100) characteristic peak is around 43°.

[0080] (2) Rate performance, including:

[0081] Charge to 4.5V with constant current and constant voltage at 0.5C, cut-off current 0.02C; let stand for 0.5 hours; discharge to 3.0V with constant current at 0.2C, and read the capacity C0.

[0082] Charge to 4.5V with constant current and constant voltage at 0.5C, cut-off current 0.02C; let stand for 0.5 hours; discharge to 3.0V with constant current at 1.0C, and read the capacity C1.

[0083] Ratio performance = C1 / C0 * 100%.

[0084] (3) Loop testing, including:

[0085] In a 25℃ environment, perform cyclic testing according to the following methods: charging mode: 2.7C CC to 4.25V, 2C CC to 4.35V, CV to 1.8C, 1.8C CC to 4.4V, CV to 1.5C, 1.5C CC to 4.5V, CV to 1.2C, 1.2C CC to 4.55V, CV to 0.25C; discharging mode: 0.7C DC to 3.0V.

[0086] 500-week Fading = Discharge capacity in week 500 / Discharge capacity in week 1 * 100%;

[0087] 500-week Swelling = Full charge thickness at week 500 / Initial half charge thickness * 100%;

[0088] All battery thicknesses were tested using a 600PPG method.

[0089] (4) Liquid retention test: Liquid retention = Injection volume - Liquid loss.

[0090] The test results are shown in Table 2 below:

[0091] Table 2

[0092]

[0093] As can be seen from the examples and comparative examples, by magnetizing the negative electrode, the following relationships are satisfied: 1.6≤(H*τ) / (D*P)≤4.7, 1.4≤L a / L c Under conditions ≤2.4, the battery's rate performance is significantly improved, and its liquid retention capacity is enhanced, providing a guarantee for long-cycle performance testing. Meeting the condition 1.6≤(H*τ) / (D*P)≤4.7 allows lithium ions to be better embedded within the negative electrode and to travel a shorter distance to the insertion site, preventing lithium ion accumulation and deposition on the negative electrode surface. Meeting the condition 1.4≤L a / L c With a thickness ≤2.4, lithium ions can more quickly insert into the graphite to complete the lithium intercalation process, effectively dispersing free lithium ions within the electrode and promoting the battery's rate performance and charging capability. Furthermore, batteries using the negative electrode of this invention exhibit superior cycle performance, with less sinking in the later stages of cycling, indicating improved expansion in the thickness direction, which better meets market demands. Specifically:

[0094] Compared to (H*τ) / (D*P)<1.6, under the condition that 1.6≤(H*τ) / (D*P)≤4.7, the ion diffusion path is effectively shortened, reducing the accumulation of ions on the surface of the negative electrode sheet. It also helps the electrolyte to penetrate the active material layer, which can further increase the compaction density and electrode thickness, thereby improving the energy density of the battery.

[0095] L a The value L represents the base plane parallel to the graphite layer. c The value represents the edge position of the graphite crystal end face. Compared to the basal plane (La), the edge position (Lc) has a higher Li value. + Insertion / extraction activity results in higher rate performance. High-La graphite exhibits higher capacity. The condition 1.4 ≤ L must be met. a / L c ≤2.4, not only can it be used for Li + The embedding / extraction provides more edge locations, which can shorten the Li + The diffusion path is beneficial for improving rate performance and also for the reversible capacity of graphite at high current densities.

[0096] Therefore, the negative electrode sheet of the present invention can significantly improve the rate performance and cycle performance of lithium-ion batteries, solve the battery failure problem caused by large material expansion, and at the same time, the cell also improves the liquid retention performance and energy density.

[0097] Unless otherwise specified, the term "about" in this invention actually means that the allowable error is within ±2%, for example, about 100 is actually 100 ± 2% × 100. The terms "room temperature" and "room temperature" in this invention, unless otherwise specified, are approximately 20-30°C. The phrase "between..." in this invention includes the number itself; for example, "between 2 and 3" includes the endpoints 2 and 3.

[0098] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments, and various changes can be made within the scope of knowledge possessed by those skilled in the art without departing from the spirit of the present invention. Furthermore, the embodiments of the present invention and the features thereof can be combined with each other unless otherwise specified.

Claims

1. A secondary battery, characterized in that, The device includes a negative electrode sheet and an electrolyte. The negative electrode sheet includes a negative current collector, and a negative active layer is disposed on the surface of the negative current collector. The negative current collector is a composite separator, comprising a PE base film and a ceramic layer formed by coating the surface of the PE base film with a ceramic mixture containing PVDF and Al2O3. The electrolyte is a mixture of ethylene carbonate, propylene carbonate, diethyl carbonate, propyl propionate, and lithium salt. The negative electrode active layer comprises a conductive agent, a binder, and a negative electrode active material containing graphite; the mass ratio of the negative electrode active material, the conductive agent, and the binder is (80~99.5):(0.1~10):(0.1~10), and the negative electrode active material further comprises at least one of silicon or carbon materials; the thickness H μm of the negative electrode active layer, the tortuosity τ of the negative electrode active layer, and the negative electrode active material D 50 The particle size D μm and the compaction density P g / cm³ of the negative electrode active layer satisfy the following relationship: 1.6 ≤ (H τ) / (D P)≤4.7; The tortuosity τ is calculated using the following formula: τ=γɛ 1-ɑ Where ɛ is the porosity of the negative electrode active layer, γ is 1, and ɑ is 1.5; It satisfies: 1.2≤τ≤2.5, 5≤H≤100, 5≤D≤25, 1.55≤P≤1.85; The negative electrode active layer is prepared by a method including the following steps: A graphite-containing negative electrode slurry is coated onto the substrate surface to form a wet film. After magnetization, it is dried to obtain the negative electrode active layer. The magnetic field strength of the magnetization is 4000~6000Gs. The magnetization time is 5~15s. During the magnetization process, the magnetic field direction is perpendicular to the wet film.

2. The secondary battery according to claim 1, characterized in that, The condition H satisfies: 25 ≤ H ≤ 60.

3. The secondary battery according to claim 1, characterized in that, The condition D satisfies: 11≤D≤17.

4. The secondary battery according to claim 1, characterized in that, The condition P satisfies: 1.55 ≤ P ≤ 1.

80.

5. The secondary battery according to claim 1, characterized in that, The negative electrode active material includes graphite, and the XRD pattern of the graphite contains characteristic peaks (002) and (100); the full width at half maximum (FWHM) of the (002) and (100) peaks are respectively... β 002 , β 100 The diffraction angles of peaks (002) and (100) are respectively θ 002 , θ 100 ; The average width L of graphite along the a-axis direction a and the average height L along the c-axis c for: L a =1.77λ / ( β 100 cos θ 100 ),L c =(K λ) / ( β 002 cosθ 002 ), among them,K=0.089,λ=1.54182Å; And L a and L c The condition is satisfied between L and L: 1.4 ≤ L a / L c ≤2.

4.

6. The secondary battery according to claim 5, characterized in that, 20nm≤L a ≤100nm,10nm≤L c ≤70nm。 7. The secondary battery according to claim 1, characterized in that, In the preparation method, the wet film is magnetized, dried, and compacted to obtain the negative electrode active layer.

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