Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and relates to the technical field of batteries. According to the secondary battery provided by the invention, the mass percent of the soft carbon in the negative electrode active material, the maximum width of the gap and the number of pores with the diameter greater than 1 mu m in every 100 square micron area of the negative electrode plate are limited in a certain range, so that the energy density of the secondary battery can be effectively improved, lithium precipitation is reduced, and the service life of the secondary battery is prolonged. And the quick charge cycle performance of the secondary battery is improved.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to secondary batteries and electrical devices. Background Technology

[0002] Stacked batteries have advantages in energy density and cycle life, but are limited by higher equipment costs and more complex manufacturing processes. Wound batteries, on the other hand, offer higher production efficiency and yield, lower costs, and wider applicability, making them dominant in the current power battery market. Due to differences in processes and materials, wound batteries are prone to corner gaps in the innermost coil, hindering electrolyte wetting, increasing the unevenness of lithium-ion diffusion, leading to purple spots and lithium plating, deteriorating capacity and cycle performance, and also causing safety issues. Summary of the Invention

[0003] The purpose of this application is to solve the technical problem that improving the energy density of secondary batteries in the prior art leads to poor fast-charging cycle performance of secondary batteries, and to propose a secondary battery and power device with high energy density and excellent fast-charging cycle performance.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a winding core, the winding core including a negative electrode sheet and a positive electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer including a negative electrode active material, the negative electrode active material including soft carbon, and the mass percentage of the soft carbon based on the mass of the negative electrode active material being 1% to 30%; The core has a starting end and a gap at the corner of the core. The gap is located between the innermost positive electrode plate of the core and the adjacent negative electrode plate that is far from the starting end. The maximum width of the gap is 0.01mm to 1.5mm. The negative electrode sheet contains 1 to 50 pores with a diameter greater than 1 μm per 100 square micrometers.

[0005] As an embodiment of this application, the secondary battery satisfies 0.03≤Y≤7500; Where Y = (D × N) / a; D mm is the maximum width of the gap; N represents the number of pores with a diameter greater than 1 μm in every 100 square micrometers of negative electrode sheet; 'a' represents the mass percentage of the soft carbon, based on the mass of the negative electrode active material, and is dimensionless.

[0006] As an embodiment of this application, the negative electrode active material satisfies 0.6≤(Dv90-Dv10) / Dv50≤1.4.

[0007] As an embodiment of this application, the Dv50 is 8μm~15μm.

[0008] As an embodiment of this application, the Dv90 is 10μm~30μm.

[0009] As an embodiment of this application, the Dv10 is 5μm~10μm.

[0010] As an embodiment of this application, the negative electrode active material further includes at least one of artificial graphite, natural graphite, and silicon carbon.

[0011] As an embodiment of this application, in the length direction of the negative electrode sheet, the coverage margin of the negative electrode sheet beyond the edge of the positive electrode sheet is 0.5mm~5mm.

[0012] As an embodiment of this application, the areal density of the negative electrode active material layer is 60 g / m². 2 ~130g / m 2 .

[0013] As an embodiment of this application, 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.

[0014] As an embodiment of this application, the compaction density of the negative electrode sheet is 1.0 g / cm³. 3 ~1.8g / cm 3 .

[0015] A second aspect of this application provides an electrical device comprising the secondary battery described in this application.

[0016] Compared with the prior art, the beneficial effects of this application are: The secondary battery provided in this application effectively improves the energy density of the secondary battery, reduces lithium plating, and enhances the fast-charging cycle performance of the secondary battery by limiting the mass percentage of soft carbon in the negative electrode active material, the maximum width of the gap, and the number of pores with a diameter greater than 1 μm per 100 square micrometers of the negative electrode sheet within a certain range. Attached Figure Description

[0017] Figure 1 A cross-sectional CT image of the secondary battery prepared in Example 1; Figure 2 The image shows a cross-sectional SEM-CP image of the negative electrode sheet prepared in Example 1. Detailed Implementation

[0018] 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.

[0019] 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.

[0020] 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.

[0021] In one embodiment of this application, a secondary battery is provided, comprising a core, the core including a negative electrode sheet and a positive electrode sheet, the negative electrode sheet including 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 including a negative active material, the negative active material including soft carbon, and the mass percentage of the soft carbon being 1% to 30% based on the mass of the negative active material; The core has a starting end and a gap at the corner of the core. The gap is located between the innermost positive electrode plate of the core and the adjacent negative electrode plate that is far from the starting end. The maximum width of the gap is 0.01mm to 1.5mm. The negative electrode sheet contains 1 to 50 pores with a diameter greater than 1 μm per 100 square micrometers.

[0022] The secondary battery provided in this application effectively improves the energy density of the secondary battery, reduces lithium plating, and enhances the room temperature and high temperature fast charging cycle performance of the secondary battery by limiting the mass percentage of soft carbon in the negative electrode active material, the maximum width of the gap, and the number of pores with a diameter greater than 1 μm per 100 square micrometers of the negative electrode sheet within a certain range.

[0023] Specifically, firstly, the mass percentage of soft carbon in the negative electrode active material affects the ionic and electronic conductivity of the negative electrode active material, as well as its structural stability. Furthermore, it also affects the kinetic performance of the secondary battery, thus influencing the degree of lithium plating after cycling. Further, the mass percentage of soft carbon in the negative electrode active material also affects the gravimetric and volumetric energy density of the secondary battery. When the mass percentage of soft carbon in the negative electrode active material is limited to between 1% and 30%, the fast-charging cycle performance of the secondary battery can be effectively improved while ensuring a relatively high energy density. Secondly, the maximum width of the gap affects the space utilization rate inside the core, thus affecting the energy density of the secondary battery. Simultaneously, it also affects the wettability of the electrolyte and its buffering capacity against the volume expansion of the electrode sheets after cycling, thereby affecting the fast-charging cycle performance of the secondary battery. When the maximum width of the gap is limited to between 0.01 mm and 1.5 mm, the secondary battery can maintain a high energy density while exhibiting excellent fast-charging cycle performance. Thirdly, the number of pores with a diameter greater than 1 μm per 100 square micrometers of the negative electrode sheet affects the wettability of the electrolyte and the degree of side reactions in the electrolyte. It also affects the resistance to ion transport, thereby affecting the fast-charging cycle performance of the secondary battery. In addition, the number of pores also affects the lithium storage capacity, thus affecting the energy density of the secondary battery. When the number of pores with a diameter greater than 1 μm per 100 square micrometers of the negative electrode sheet is further limited to 1 to 50, the energy density of the secondary battery can be higher and the fast-charging cycle performance can be better.

[0024] For example, based on the mass of the negative electrode active material, the mass percentage of the soft carbon can be any point value between 1% and 30% or a range between any two points, such as 1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28%, 30%, etc.

[0025] In one embodiment, the mass percentage of the soft carbon, based on the mass of the negative electrode active material, is 10% to 20%. For example, it can be 10%, 11%, 13%, 15%, 17%, 19%, 20%, etc.

[0026] This study found that by further selecting the mass percentage of soft carbon in the negative electrode active material within the above-mentioned range, the ionic and electronic conductivity of the negative electrode active material can be better improved while ensuring energy density, thereby enhancing the structural stability of the negative electrode active material, improving the kinetic performance of the secondary battery, reducing lithium plating, and optimizing the fast-charging cycle performance of the secondary battery.

[0027] It should be noted that the mass percentage of soft carbon in the negative electrode active material was tested by thermogravimetric analysis (TGA). The specific method was as follows: a secondary battery was taken, the negative electrode sheet was separated, and then the negative electrode sheet was soaked in dimethyl carbonate solvent to remove residual electrolyte. After drying, the active material layer was carefully scraped off and sieved to remove current collector fragments, yielding the negative electrode active material. The negative electrode active material powder was then subjected to TGA in air, and the TGA curves from room temperature to 900℃ were recorded. The mass percentage of soft carbon was calculated by analyzing the weight loss ratio in the 400-600℃ temperature range of the TGA curve.

[0028] For example, the maximum width of the gap can be any point value between 0.01mm and 1.5mm or a range value between any two points, such as 0.01mm, 0.05mm, 0.1mm, 0.2mm, 0.4mm, 0.6mm, 0.8mm, 1mm, 1.2mm, 1.4mm, 1.5mm, etc.

[0029] In one embodiment, the maximum width of the gap is 0.3mm to 1.2mm. For example, it can be 0.3mm, 0.35mm, 0.4mm, 0.45mm, 0.5mm, 0.55mm, 0.6mm, 0.65mm, 0.7mm, 0.75mm, 0.8mm, 0.85mm, 0.9mm, 0.95mm, 1mm, 1.05mm, 1.1mm, 1.15mm, 1.2mm, etc.

[0030] This study found that when the maximum width of the further gap is within the above range, it can better provide buffer space for the electrode, and is conducive to the wetting of the electrolyte and avoids a significant decrease in the utilization rate of the internal space; thus, it can effectively ensure that the energy density of the secondary battery is high and the fast charging cycle performance is excellent.

[0031] It should be noted that the innermost circle of the core refers to the first turn of the core at the beginning of winding; the starting end of the core is the starting point of winding; the test method for the maximum width of the gap is as follows: take a secondary battery, obtain a core sample, and then perform CT scanning and three-dimensional reconstruction on the core sample. On the central cross-section parallel to the core axis, measure the maximum distance in the radial direction between the innermost positive electrode and the adjacent negative electrode that is far away from the starting end at the first corner of the core. This distance is the maximum gap width.

[0032] For example, in every 100 square micrometers of the negative electrode sheet, the number of pores with a diameter greater than 1 μm can be any point value between 1 and 50 or a range between any two points, such as 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, etc.

[0033] In one embodiment, the negative electrode sheet has 20 to 40 pores with a diameter greater than 1 μm per 100 square micrometers. For example, it can have 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, etc.

[0034] This application research found that the number of pores with a diameter greater than 1 μm per 100 square micrometers of negative electrode sheet is further optimized within the above range. This can not only ensure the lithium storage capacity to improve energy density, but also effectively improve the wetting ability of the electrolyte, reduce the lithium ion transport resistance and electrolyte side reactions, thereby improving the fast charging cycle performance of the secondary battery.

[0035] It should be noted that the testing method for the number of pores with a diameter greater than 1 μm per 100 square micrometers of the negative electrode sheet is as follows: The secondary battery is disassembled to obtain the negative electrode sheet. Then, a cross-sectional sample of the negative electrode sheet is prepared, and SEM analysis is performed on the cross-section to obtain an SEM image of the electrode sheet at 10K~20K magnification. The number of pores per unit area in the negative electrode sheet is then calculated using the following formula: N = n / S*100. Where n represents the number of pores with a diameter greater than 1 μm in the cross-sectional SEM image of the negative electrode sheet. S represents the actual sample area corresponding to the SEM image, in square micrometers (μm). 2 ).

[0036] In one embodiment, the secondary battery satisfies 0.03≤Y≤7500; Where Y = (D × N) / a; D mm is the maximum width of the gap; N represents the number of pores with a diameter greater than 1 μm in every 100 square micrometers of negative electrode sheet; 'a' represents the mass percentage of the soft carbon, based on the mass of the negative electrode active material, and is dimensionless.

[0037] For example, Y can be any point value between 0.03 and 7500 or a range value between any two points, such as 0.03, 1, 5, 10, 20, 40, 60, 80, 100, 200, 400, 600, 800, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 7500, etc.

[0038] In one embodiment, the secondary battery satisfies 30≤Y≤240. For example, it can be 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, etc.

[0039] This study found that by further controlling 0.03 ≤ (D×N) / a ≤ 7500, the secondary battery can be linked from three dimensions: macroscopic mechanical structure (D), microscopic electrode structure (N), and intrinsic material properties (a). These three dimensions can synergistically and effectively improve the energy density and fast-charge cycle performance of the secondary battery. Specifically, D×N, to a certain extent, reflects the "total efficiency of macroscopic and microscopic transport channels" of ions from the electrolyte bulk to the surface of the negative electrode active material particles. D, to a certain extent, ensures the overall wettability of the electrolyte and the number of channels for ion migration. N, to a certain extent, ensures the short path for ions to jump in the pores inside the electrode. a, to a certain extent, represents the contribution and cost of high ionic conductivity materials introduced to improve fast-charging capability. Limiting 0.03 ≤ (D×N) / a ≤ 7500 can achieve better ion transport effect at the cost of better energy density, thereby effectively improving the energy density and fast-charge cycle performance of the secondary battery simultaneously. Increasing (D×N) / a to a certain extent can effectively improve energy density and rate performance, while decreasing (D×N) / a to a certain extent can effectively improve cycle performance; especially when further selecting 30≤(D×N) / a≤240, the overall performance of the secondary battery is better.

[0040] In one embodiment, the negative electrode active material satisfies 0.6≤(Dv90-Dv10) / Dv50≤1.4.

[0041] For example, the negative electrode active material can satisfy (Dv90-Dv10) / Dv50 as a point value between 0.6 and 1.4 or a range value between any two points, such as 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, etc.

[0042] In one embodiment, the negative electrode active material satisfies 0.8 ≤ (Dv90 - Dv10) / Dv50 ≤ 1.0. For example, it can be 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, 0.98, 1, etc.

[0043] This study found that (Dv90-Dv10) / Dv50 reflects the uniformity of particle size distribution of the negative electrode active material. When the (Dv90-Dv10) / Dv50 of the negative electrode active material is further selected within the above range, especially within the further preferred range, the small particles can better fill the gaps between large particles, reduce the overall porosity of the material, effectively improve the compaction density of the electrode, and thus improve the energy density of the secondary battery. In addition, the particles have good uniformity, the volume change is more synchronous during cycling, the stress distribution is uniform, and the electrode structure is more stable, thereby effectively improving the cycle performance of the secondary battery.

[0044] It should be noted that the test methods for Dv90, Dv10 and Dv50 of the negative electrode active material are as follows: the negative electrode active material is separated by the steps in the test of the mass percentage of soft carbon in the negative electrode active material, and then tested by a laser particle size analyzer. For details, please refer to GB / T 19077-2016 "Particle size distribution by laser diffraction".

[0045] In one embodiment, the Dv50 is 8μm to 15μm.

[0046] For example, Dv50 can be a point value between 8μm and 15μm or a range value between any two points, such as 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, etc.

[0047] In one embodiment, the Dv50 is 9μm to 11μm. For example, it can be 9μm, 9.2μm, 9.5μm, 9.8μm, 10μm, 10.2μm, 10.5μm, 10.8μm, 11μm, etc.

[0048] This study found that Dv50 affects the specific surface area of ​​the negative electrode active material, thereby affecting the degree of side reactions; Dv50 also affects the stacking effect of the negative electrode active material, thereby affecting the volumetric energy density; at the same time, Dv50 also affects the solid-phase diffusion path of lithium ions inside the negative electrode active material, thereby affecting the degree of lithium plating during fast charging; when the Dv50 value is further selected within the above range, the resulting secondary battery has a better energy density and better fast charging cycle performance.

[0049] In one embodiment, the Dv90 is 10μm~30μm.

[0050] For example, Dv90 can be a point value between 10μm and 30μm or a range value between any two points, such as 10μm, 12μm, 15μm, 18μm, 20μm, 22μm, 25μm, 28μm, 30μm, etc.

[0051] In one embodiment, the Dv90 is 14μm to 16μm. For example, it can be 14μm, 14.2μm, 14.5μm, 14.8μm, 15μm, 15.2μm, 15.5μm, 15.8μm, 16μm, etc.

[0052] This study found that Dv90 affects the diffusion kinetics of lithium ions inside the negative electrode active material, thereby affecting the degree of lithium plating caused by local over-lithiation on the surface of the negative electrode active material. When Dv90 is further selected within the above range, the fast charging cycle performance of the secondary battery can be effectively improved.

[0053] In one embodiment, the Dv10 is 5μm to 10μm.

[0054] For example, Dv10 can be a point value between 5μm and 10μm or a range value between any two points, such as 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, etc.

[0055] In one embodiment, the Dv10 is 6μm to 7μm. For example, it can be 6μm, 6.2μm, 6.4μm, 6.6μm, 6.8μm, 7μm, etc.

[0056] This study found that Dv10 affects the degree of side reactions with the electrolyte, thus affecting cycle performance; it also affects the amount of binder and conductive agent to be added subsequently, thus affecting energy density; when Dv10 is further selected within the above range, the overall performance of the secondary battery is better.

[0057] In one embodiment, the negative electrode active material further includes at least one of artificial graphite, natural graphite, and silicon carbon.

[0058] This study found that different negative electrode active materials can affect the interaction with soft carbon to some extent. When the negative electrode active materials are further selected to include the above-mentioned types of substances, the overall performance of the secondary battery is better.

[0059] It should be noted that the silicon-carbon is a material formed by combining silicon and carbon materials through physical or chemical methods; for example, the silicon-carbon may be a material in which silicon is set as the core and carbon material is coated on at least part of its surface; or the silicon-carbon may be a material in which silicon is dispersed in the pores of carbon material.

[0060] In one embodiment, the negative electrode sheet extends beyond the edge of the positive electrode sheet by 0.5mm to 5mm along its length.

[0061] For example, in the length direction of the negative electrode sheet, the coverage margin of the negative electrode sheet beyond the edge of the positive electrode sheet can be any point value or any range between two points between 0.5mm and 5mm, such as 0.5mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm, 5mm, etc.

[0062] In one embodiment, the negative electrode sheet extends beyond the edge of the positive electrode sheet by 2.5mm to 3.5mm along its length. For example, it can be 2.5mm, 2.7mm, 2.9mm, 3.1mm, 3.3mm, 3.5mm, etc.

[0063] This application research found that, in the length direction of the negative electrode sheet, the amount of coverage of the negative electrode sheet beyond the edge of the positive electrode sheet affects the risk of lithium plating at the inner corner, and also indirectly affects the ion transport capability due to the voids formed by material accumulation at the inner corner. When the coverage amount is further selected within the above range, the risk of lithium plating can be effectively reduced, the ion transport efficiency can be improved, and thus the fast charging cycle performance of the secondary battery can be improved.

[0064] It should be noted that the test method for the coverage allowance of the negative electrode sheet extending beyond the edge of the positive electrode sheet along its length is as follows: After disassembling the secondary battery, carefully unfold the core and lay it flat and fixed. Using an image measuring instrument, measure the distance along the length between the edge of the negative electrode sheet and the edge of the positive electrode sheet at the starting end of the core. This distance is the coverage allowance H, in millimeters (mm).

[0065] In one embodiment, the areal density of the negative electrode active material layer is 60 g / m². 2 ~130g / m 2 .

[0066] For example, the areal density of the negative electrode active material layer can be 60 g / m³. 2 ~130g / m 2 The value at any point between or between any two points, for example, could be 60g / 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 wait.

[0067] In one embodiment, the areal density of the negative electrode active material layer is 90 g / m². 2 ~110g / m 2 For example, it could be 90 g / m³. 2 92 g / m 2 95 g / m 2 98 g / m 2 100 g / m 2 102 g / m 2 105 g / m 2 108 g / m 2 110 g / m 2 wait.

[0068] This study found that the areal density of the negative electrode active material layer affects the ion transport resistance and the amount of negative electrode active material. When the areal density of the negative electrode active material layer is further selected within the above range, good fast-charging cycle performance of the secondary battery can be achieved while ensuring energy density.

[0069] It should be noted that the method for testing the areal density of the negative electrode active material layer is as follows: disassemble the secondary battery to obtain the negative electrode sheet, and then cut the negative electrode sheet into multiple (the number can be 3 to 100) small circular pieces (the area can be 1540.25 mm²). 2 The mass of the small disc is weighed using a high-precision balance. After deducting the mass of the negative electrode current collector, the mass is divided by the disc area and then by 2 to calculate the surface density of the negative electrode sheet.

[0070] In one embodiment, 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.

[0071] 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, 1.05:1, 1.1:1, 1.15:1, 1.2:1, 1.25:1, 1.3:1, etc.

[0072] In one embodiment, the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area is (1.1~1.2):1. For example, it can be 1.1:1, 1.12:1, 1.14:1, 1.16:1, 1.18:1, 1.2:1, etc.

[0073] This application research found that the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area affects the energy density and the risk of lithium plating on the negative electrode. When the ratio of the capacity of the positive electrode per unit area to the capacity of the negative electrode per unit area is further selected within the above range, it is possible to balance the high energy density and excellent fast-charging cycle performance of the secondary battery.

[0074] It should be noted that the test method for the ratio of the capacity of the negative electrode per unit area to the capacity of the positive electrode per unit area is as follows: CB = (C n / A n ) / (C p / A p ). Wherein, CB represents the ratio. C n The reversible capacity of the negative electrode is indicated by a charge-discharge test performed on the negative electrode at a rate of 0.1C, and is expressed in milliampere-hours (mAh). An This indicates the single-sided coating area of ​​the negative electrode sheet, expressed in square centimeters (cm²). 2 C p The reversible capacity of the positive electrode is indicated by charging and discharging the positive electrode at a rate of 0.1C, and the unit is milliampere-hours (mAh). A p This indicates the single-sided coating area of ​​the positive electrode sheet, expressed in square centimeters (cm²). 2 ).

[0075] In one embodiment, the compaction density of the negative electrode sheet is 1.0 g / cm³. 3 ~1.8g / cm 3 .

[0076] For example, the compaction density of the negative electrode sheet can be 1.0 g / cm³. 3 ~1.8g / cm 3 The value at any point between or between any two points, for example, 1 g / cm³. 3 1.1 g / cm 3 1.2 g / cm 3 1.3 g / cm 3 1.4 g / cm 3 1.5g / cm 3 1.6 g / cm 3 1.7 g / cm 3 1.8 g / cm 3 wait.

[0077] In one embodiment, the compaction density of the negative electrode sheet is 1.6 g / cm³. 3 ~1.7g / cm 3 For example, it could be 1.6 g / cm³. 3 1.62 g / cm 3 1.64 g / cm 3 1.66 g / cm 3 1.68 g / cm 3 1.7 g / cm 3 wait.

[0078] This study found that the compaction density of the negative electrode sheet affects the wettability of the electrolyte and also affects the ion transport path at the corner gap. When the compaction density of the negative electrode sheet is further selected within the above range, the resulting secondary battery has better energy density and fast charge cycle performance.

[0079] It should be noted that the compaction density of the negative electrode sheet = the surface density of the negative electrode sheet / (the thickness of the negative electrode sheet after rolling - the thickness of the negative current collector).

[0080] In one embodiment, the negative electrode active material layer further includes a negative electrode binder, a negative electrode conductive agent, and a negative electrode thickener.

[0081] This application does not have any special requirements for the selection of the negative electrode binder, and any negative electrode binder conventionally available in the art can be used. For example, the negative electrode binder can be any one of styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC-Na), gum arabic (GA), polypyrrole (PPy), polyaniline (PANI), polyethylene dioxythiophene (PEDT), polythiophene (PTh), aramid (PPTA), etc.

[0082] This application does not have any special requirements for the selection of the negative electrode thickener; any negative electrode thickener conventionally available in the art can be used. For example, the negative electrode thickener may be any one of sodium carboxymethyl cellulose (CMC-Na), polyacrylic acid (PAA), polyurethane (PU), or guar gum (GG).

[0083] This application does not have any special requirements for the selection of the negative electrode conductive agent; any negative electrode conductive agent conventionally available in the art can be used. For example, the negative electrode conductive agent can be any one of carbon black (Super P), graphite, carbon nanotubes (CNTs), and graphene.

[0084] In one embodiment, the core further includes a positive electrode sheet, which 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 conductive agent, and a positive binder.

[0085] This application does not impose any particular limitation on the selection of the positive electrode active material, and any positive electrode active material conventionally used in the art can be used; for example, the positive electrode active material can be any one of lithium iron phosphate and lithium manganese iron phosphate.

[0086] This application does not impose any particular limitation on the selection of the positive electrode binder, and binders conventionally used in the art can be used; for example, the positive electrode binder can be any one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), styrene-butadiene rubber (SBR), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), and polymethyl methacrylate (PAMA).

[0087] This application does not have any special requirements for the selection of the positive electrode conductive agent; any positive electrode conductive agent conventionally available in the art can be used. For example, the positive electrode conductive agent can be any one of carbon black (Super P), graphite, carbon nanotubes (CNTs), and graphene.

[0088] In one embodiment, the secondary battery further includes an electrolyte; the electrolyte includes an organic solvent, a lithium salt, and additives.

[0089] This application does not impose any particular limitation on the selection of organic solvents, and organic solvents conventionally used in the art can be used. For example, the organic solvent may be ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, ethyl acetate, propyl propionate, ethyl propionate, dimethyl carbonate, etc.

[0090] This application does not impose any particular limitation on the selection of lithium salt, and lithium salts conventionally available in the art can be used. For example, the lithium salt may be lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium tetrafluoroborate (LiBF4), etc.

[0091] This application does not impose any particular limitation on the selection of additives, and additives conventionally available in the art can be used. For example, the additives may be vinyl sulfate (DTD), vinylene carbonate (VC), fluoroethylene carbonate (FEC), etc.

[0092] In one embodiment, the method for preparing the secondary battery includes the following steps: (1) 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. (2) Preparation of negative electrode sheet: The pretreated 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. (3) Preparation of electrolyte: Lithium salt and additives are added to an organic solvent to obtain an electrolyte; (4) Preparation of secondary battery: The positive electrode, negative electrode, separator and other components are assembled and then subjected to processes such as winding, hot pressing, super welding, core assembly, casing, baking, liquid injection, high temperature immersion, formation, aging, sealing and capacity testing to obtain secondary battery.

[0093] It should be noted that the maximum width of the gap can be adjusted by controlling the winding tension. For example, increasing the winding tension during the winding process can make the electrode and the separator adhere more tightly to the inner ring of the core, thereby reducing the maximum width of the gap.

[0094] It should be noted that the number of pores with a diameter greater than 1 μm per 100 square micrometers of negative electrode sheet can be controlled by adjusting the rolling pressure. For example, increasing the rolling pressure during the electrode rolling process will compress the gaps between the active material particles, thereby reducing the number of pores with a diameter greater than 1 μm per unit area.

[0095] Furthermore, the number of pores can also be controlled by adjusting the type and amount of binder or the solid content of the slurry. For example, reducing the amount of binder or lowering the solid content of the slurry will usually increase the number of pores after the electrode is formed.

[0096] It should be noted that the Dv90, Dv50, and Dv10 of the negative electrode active material can be controlled by adjusting the pretreatment process, specifically the sintering process. For example, the pretreatment can be sintering at a higher temperature, which usually leads to particle growth or fusion, thereby increasing Dv90, Dv50, and Dv10.

[0097] Furthermore, precise particle size control can also be achieved by controlling the pretreatment process of the negative electrode active material, specifically the crushing and classification process of the raw materials. For example, extending the mechanical grinding time will reduce the particle size; while using air classification technology can precisely control the distribution of Dv90 and Dv10.

[0098] It should be noted that by adjusting the cutting width of the electrode slitting process, the amount of coverage allowance of the negative electrode extending beyond the edge of the positive electrode in the width direction can be changed. For example, increasing the cutting width of the negative electrode during slitting will directly increase its coverage allowance beyond the positive electrode.

[0099] It should be noted that the areal density of the negative electrode active material layer can be controlled by adjusting the solid content and flow rate of the negative electrode slurry during the preparation of the negative electrode sheet; for example, increasing the solid content and the flow rate of the slurry can increase the areal density of the negative electrode active material layer.

[0100] It should be noted that by controlling the coating surface density of the positive and negative electrode sheets, the ratio of the capacity of the negative electrode sheet per unit area to the capacity of the positive electrode sheet per unit area can be adjusted; for example, if the surface density of the positive electrode remains unchanged, increasing the coating surface density of the negative electrode can improve the capacity ratio.

[0101] It should be noted that the compaction density of the negative electrode sheet can be controlled by adjusting the rolling process parameters (such as rolling pressure and roll gap width) or by selecting active materials with different physical properties (such as artificial graphite with different tap densities). For example, under the premise of fixed pore structure, using active materials with higher tap densities can increase the compaction density of the negative electrode sheet.

[0102] In one embodiment of this application, an electrical device is provided, including the secondary battery described in this application.

[0103] 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.

[0104] Example 1 This application provides a secondary battery, the preparation method of which includes the following steps: (1) Preparation of positive electrode sheet The positive electrode active material (lithium iron phosphate), positive electrode conductive agent (conductive carbon black SP), and positive electrode binder (PVDF) were mixed in a weight ratio of 97.6:0.6:1.8. Then, N-methylpyrrolidone (NMP) was added and thoroughly mixed to obtain a positive electrode slurry with a solid content of 67%. The positive electrode slurry was double-coated onto a (13+1+1) μm carbon-coated aluminum foil at a flow rate of 15 m / min, with the single-sided coating weight controlled at 217.5 g / m². 2 The electrode sheets are then dried, rolled, slit, and cut to obtain the positive electrode sheet. During the rolling process, the pressure is 80t, the roll gap thickness is 95μm, and the compaction density of the positive electrode sheet is controlled to be 2.64g / cm³. 3 .

[0105] (2) Preparation of negative electrode sheet The tap density of soft carbon and artificial graphite (tested according to GB / T 24533-2019, is 1.04 g / cm³) was determined. 3 The mixture was mixed at a mass ratio of 10:90 and then crushed using an air jet mill at a grinding pressure of 0.7 MPa. Subsequently, the crushed material was finely classified by an air classifier at a classifying wheel speed of 2500 rpm. Fine powder meeting the requirements was collected to obtain negative electrode active materials with Dv90, Dv50 and Dv10 of 15μm, 10μm and 6μm, respectively. The negative electrode active material, negative electrode conductive agent (conductive carbon black SP), negative electrode thickener (CMC), and negative electrode binder (SBR) were mixed in a mass ratio of 96.5:0.7:1:1.8, and then thoroughly mixed in water to obtain a negative electrode slurry with a solid content of 49%. The negative electrode slurry was coated on both surfaces of a 0.45 μm copper foil at a flow rate of 10 m / min, with the single-sided coating weight controlled at 97 g / m. 2 The electrode sheets are then dried, rolled, slit, and cut to obtain the negative electrode sheets. During the rolling process, the pressure is 60t, the roll gap thickness is 70μm, and the compaction density of the negative electrode sheets is controlled to be 1.65 g / cm³. 3 .

[0106] (3) Preparation of electrolyte Ethylene carbonate, ethyl methyl carbonate, and dimethyl carbonate were mixed in a mass ratio of 1:1:1, and then lithium hexafluorophosphate was added. After mixing evenly, the additive vinylene carbonate was added. The mass content of lithium hexafluorophosphate was 12% and the mass content of the additive vinylene carbonate was 0.5% based on the mass of the electrolyte.

[0107] (4) Preparation of secondary batteries The positive electrode sheet, negative electrode sheet, and separator (polyethylene film) prepared above are wound to obtain a core. The core undergoes hot pressing, super welding, core assembly, casing, baking, liquid injection, high-temperature wetting, formation, aging, sealing, and capacity testing to obtain a secondary battery. The tension during winding is 35N.

[0108] The interface CT image of the secondary battery prepared in Example 1 is shown below. Figure 1 As shown, the interface SEM-CP image of the negative electrode sheet is as follows. Figure 2 As shown.

[0109] Specifically, from Figure 1 It can be observed that the core structure is regular, and the electrode sheets and diaphragm interface are tightly bonded. In particular, at the inner corner of the starting end of the core, no obvious electrode sheet springback or wrinkles are observed. The maximum gap width between the innermost positive electrode sheet and the next innermost negative electrode sheet at the first corner at the starting end of the core is effectively controlled and remains within a small range. Figure 2 It can be observed that the negative electrode sheet has a uniform and abundant porous structure. A good three-dimensional network channel is formed between the active material particles, and there are a large number of pores with a diameter greater than 1 μm per unit area.

[0110] Examples 2-4 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of embodiment 1 is that the maximum width D of the gap is changed by adjusting the tension during winding, so as to achieve the parameters in Table 1.

[0111] Examples 5-7 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 process parameters (specifically the rolling pressure) in the preparation process of the negative electrode sheet are adjusted to change the number N of pores with a diameter greater than 1 μm in every 100 square micrometers of the negative electrode sheet, so as to achieve the parameters in Table 1.

[0112] Examples 8-10 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the amount of soft carbon added is adjusted to change the mass percentage 'a' of soft carbon based on the mass of the negative electrode active material, so as to achieve the parameters in Table 1.

[0113] Examples 11-12 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the winding tension, negative electrode rolling pressure and the amount of soft carbon added are adjusted to change D, N and a, so as to achieve the parameters in Table 1.

[0114] Examples 13-14 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the grinding pressure of the negative electrode active material and the airflow classification process parameters are adjusted to change the negative electrode active materials Dv90, Dv50 and Dv10, so as to achieve the parameters in Table 1.

[0115] Examples 15-16 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the cutting width of the negative electrode sheet in the slitting process is adjusted to change the coverage allowance of the negative electrode sheet beyond the edge of the positive electrode sheet in the width direction, so as to achieve the parameters in Table 1.

[0116] Examples 17-18 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 parameters of the negative electrode slurry are adjusted to change the areal density of the negative electrode active material layer, so as to achieve the parameters in Table 1.

[0117] Examples 19-20 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the areal density of the positive electrode active material layer 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 Table 1.

[0118] Examples 21-22 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that artificial graphite with different tap densities is used, and the rolling pressure and roll gap are adjusted during the rolling process. Under the premise of achieving the same pore structure (i.e., the number of pores N per unit area is equivalent to that in Example 1), the compaction density of the negative electrode sheet is changed to achieve the parameters in Table 1.

[0119] Example 23 This application provides a secondary battery, the preparation method of which differs from that of Example 1 in that silicon carbon is used instead of artificial graphite to achieve the parameters in Table 1.

[0120] 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 the maximum width D of the gap is changed by adjusting the winding tension, so as to achieve the parameters in Table 1.

[0121] Comparative Examples 3-4 This application provides a secondary battery in comparison. The difference between the preparation method of the secondary battery and that of Example 1 is that the rolling pressure of the negative electrode sheet is adjusted to change the number N of pores with a diameter greater than 1 μm in every 100 square micrometers of the negative electrode sheet, so as to achieve the parameters in Table 1.

[0122] 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 that of Example 1 is that the amount of soft carbon added is adjusted to change the mass percentage 'a' of soft carbon based on the mass of the negative electrode active material, so as to achieve the parameters in Table 1.

[0123] Comparative Example 7 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 soft carbon is not added, so as to achieve the parameters in Table 1.

[0124] In the examples and comparative examples, Dmm, N, a, Y, P = (Dv90 - Dv10) / Dv50, Dv90 μm, Dv10 μm, Dv50 μm, the coverage margin Hmm of the negative electrode extending beyond the edge of the positive electrode in the length direction of the negative electrode sheet, and the areal density CW of the negative electrode active material layer are all mentioned. 2 The ratio of the capacity of the positive electrode sheet per unit area to the capacity of the negative electrode sheet per unit area (CB); the compacted density of the negative electrode sheet (PD) (g / cm³). 3 As shown in Table 1; Table 1. Parameters of Secondary Batteries The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following: 1. Energy density test: At room temperature, the battery is charged at a constant current of 1C to 3.65V, then charged at a constant voltage of 3.65V until the current is less than 0.05C. After standing for 5 minutes, it is discharged at a current of 1C to 2.5V to obtain the cell capacity C0. The mass of the secondary battery is denoted as W0, and the energy density of the secondary battery is ED = C0 / W0 (Wh / kg). 2. Fast charging cycle performance test: At 25℃ and 45℃, charge at a constant current of 4C to 3.65V, then charge at a constant voltage of 3.65V until the current is less than 0.05C. After resting for 5 minutes, discharge at a current of 1C to 2.5V. Repeat this charge-discharge cycle 2000 times. The capacity retention rate on the 2000th cycle = (Capacity on the 2000th cycle / Capacity on the first cycle) × 100%. The results are shown in Table 2; Table 2 Performance Data of Secondary Batteries As can be seen from Table 2, 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 160Wh / kg, the capacity retention rate after 2000 cycles at 25℃ is above 80%, and the capacity retention rate after 2000 cycles at 45℃ is above 67%. As can be seen from Examples 1-23 and Comparative Examples 1-6, the effects of this application can be achieved when D, N and a are limited to the ranges given in this application; as can be seen from Examples 1-23 and Comparative Example 7, the effects of this application can be achieved when soft carbon is introduced into the negative electrode active material.

[0125] 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 winding core, the winding core comprising a negative electrode sheet and a positive electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material, characterized in that, The negative electrode active material includes soft carbon, and the mass percentage of the soft carbon is 1% to 30% based on the mass of the negative electrode active material. The core has a starting end and a gap at the corner of the core. The gap is located between the innermost positive electrode plate of the core and the adjacent negative electrode plate that is far from the starting end. The maximum width of the gap is 0.01mm to 1.5mm. The negative electrode sheet contains 1 to 50 pores with a diameter greater than 1 μm per 100 square micrometers.

2. The secondary battery according to claim 1, characterized in that, The secondary battery satisfies 0.03≤Y≤7500; Where Y = (D × N) / a; D mm is the maximum width of the gap; The number of pores with a diameter greater than 1 μm in every 100 square micrometers of negative electrode sheet; 'a' represents the mass percentage of the soft carbon, based on the mass of the negative electrode active material, and is dimensionless.

3. The secondary battery according to claim 1, characterized in that, The negative electrode active material satisfies 0.6≤(Dv90-Dv10) / Dv50≤1.

4.

4. The secondary battery according to claim 3, characterized in that, The negative electrode active material satisfies at least one of the following conditions: (1) The Dv50 is 8μm~15μm; (2) The Dv90 is 10μm~30μm; (3) The Dv10 is 5μm~10μm.

5. The secondary battery according to claim 1, characterized in that, The negative electrode active material also includes at least one of artificial graphite, natural graphite, and silicon carbon.

6. The secondary battery according to claim 1, characterized in that, Along the length of the negative electrode sheet, the overlap of the negative electrode sheet with the edge of the positive electrode sheet is 0.5mm to 5mm.

7. The secondary battery according to claim 1, characterized in that, The areal density of the negative electrode active material layer is 60 g / m³. 2 ~130g / m 2 .

8. The secondary battery according to claim 1, characterized in that, 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.

9. The secondary battery according to claim 1, characterized in that, The compaction density of the negative electrode sheet is 1.0 g / cm³. 3 ~1.8g / cm 3 .

10. An electrical device, characterized in that, The electrical device includes a secondary battery as described in any one of claims 1 to 9.

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