Secondary battery and electric device

Abstract

The invention discloses a secondary battery and an electric device, and belongs to the technical field of batteries. According to the secondary battery provided by the invention, the first region and the second region are sequentially arranged on the inner wall of the shell along the direction from the bottom surface of the shell to the top cover, the lithium supplementing layer is arranged on at least part of the surface, close to the roll core, of the first region, and meanwhile, the thickness H1 of the first region and the thickness H2 of the second region are limited to meet a specific relationship; the thickness of the lithium supplementing layer and the thickness of the first region meet a certain relationship; the obtained secondary battery has good cycle performance and relatively low volume expansion, and meanwhile, the prepared secondary battery also has good discharge capacity and first coulombic efficiency.

Description

Technical Field

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

[0002] Currently, commonly used lithium replenishment technologies mainly include negative electrode lithium replenishment and positive electrode lithium replenishment. Negative electrode lithium replenishment often uses lithium foil, lithium powder, or lithium silicide powder to pre-store a certain amount of lithium ions at the negative electrode before charging and discharging with the positive electrode. The pre-existing lithium ions at the negative electrode can compensate for film formation losses, thereby improving the first-cycle efficiency. However, pure lithium replenishment is complex and has high environmental requirements, making practical application challenging. Positive electrode lithium replenishment involves mixing and stirring the positive electrode active material with a lithium replenishing agent to prepare a uniform positive electrode slurry, which is then formed into an electrode sheet. During formation, the lithium replenishing agent decomposes and no longer intercalates lithium; the lithium ions are used to form the SEI film, thereby improving the lithium content in later cycles of the battery cell. However, the decomposition products of the lithium replenishing agent remain in the positive electrode sheet, which may affect ion migration and cause some side reactions, resulting in a less significant improvement in the cycle performance of the secondary battery or a reduction in discharge capacity and first-cycle coulombic efficiency. Summary of the Invention

[0003] The purpose of this application is to solve the technical problems of poor lithium replenishment effect in the prior art, which may also damage capacity and initial coulombic efficiency and cause large volume expansion. It proposes a secondary battery and power device with good cycle performance, small volume expansion before and after cycle, and high discharge capacity and initial coulombic efficiency.

[0004] To achieve the above objectives, a first aspect of this application provides a secondary battery, including a core, a casing, and a top cover, wherein a first region and a second region are sequentially provided on the inner wall of the casing along the direction from the bottom surface of the casing to the top cover; The thickness H1 of the first region and the thickness H2 of the second region satisfy H1 / H2≤0.5; A lithium replenishment layer is provided on at least a portion of the surface of the first region near the core; The thickness h1 of the lithium replenishment layer satisfies 0.5H1≤h1≤1.01H1.

[0005] As an embodiment of this application, the height H3 of the shell and the height H4 of the lithium replenishment layer satisfy 0.05H3≤H4≤0.95H3.

[0006] As an implementation of this application, H1 and H2 satisfy 0.2≤H1 / H2≤0.4.

[0007] As an embodiment of this application, H3 and H4 satisfy 0.4H3≤H4≤0.9H3.

[0008] As an embodiment of this application, the lithium replenishing layer includes a lithium replenishing agent, a conductive agent, and a binder.

[0009] As an embodiment of this application, the conductive agent includes a first conductive agent and a second conductive agent, wherein the Dv50 particle size D1 of the first conductive agent and the Dv50 particle size D2 of the second conductive agent satisfy 5≤D1 / D2≤50.

[0010] As an embodiment of this application, the mass ratio of the first conductive agent to the second conductive agent is 1:(0.05~0.2).

[0011] As an embodiment of this application, D1 is 5μm to 15μm.

[0012] As an embodiment of this application, D2 is 0.1μm ~ 1μm.

[0013] As an embodiment of this application, the Dv50 particle size D3 of the lithium replenishing agent is 0.05μm ~ 0.3μm.

[0014] As an embodiment of this application, the mass ratio of the adhesive to the conductive agent is (0.01~0.3):1.

[0015] As an embodiment of this application, the core includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; Based on the capacity of the secondary battery after its first charge, the capacity that lithium can utilize in the lithium replenishment layer accounts for 1% to 9%.

[0016] As an embodiment of this application, the lithium replenishing agent includes at least one of lithium powder, lithium trifluoromethyl sulfinate, Li5FeO4, Li2NiO2, and Li4SiO4.

[0017] As an embodiment of this application, the conductive agent includes at least one of carbon black and carbon nanospheres.

[0018] As an embodiment of this application, the adhesive includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, and polyacrylic acid.

[0019] A second aspect of this application provides an electrical device including the secondary battery.

[0020] Compared with the prior art, the beneficial effects of this application are: The secondary battery provided in this application has a first region and a second region sequentially arranged on the inner wall of the casing along the direction from the bottom surface of the casing to the top cover, and a lithium replenishment layer is provided on at least a portion of the surface of the first region near the core. At the same time, the thickness H1 of the first region and the thickness H2 of the second region are defined to satisfy a specific relationship, and the thickness of the lithium replenishment layer is related to the thickness of the first region. The resulting secondary battery has good cycle performance, high capacity retention before and after cycling, and small volume expansion. In addition, the prepared secondary battery also has good discharge capacity and initial coulombic efficiency. Attached Figure Description

[0021] Figure 1 Diagram of a secondary battery casing: 1-Lithium replenishment layer, 2-Shell; Figure 2 Here is a cross-sectional view of the secondary battery in Example 1: 1-Lithium replenishment layer; Figure 3 This is a top view of the secondary battery in Example 1 without the top cover: 2-Shell, 3-Insulation layer, 4-Core; Figure 4 This is a schematic diagram of a secondary battery. Detailed Implementation

[0022] 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.

[0023] 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.

[0024] 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.

[0025] In one embodiment of this application, a secondary battery is provided, including a core, a housing, and a top cover, wherein a first region and a second region are sequentially provided on the inner wall of the housing along the direction from the bottom surface of the housing to the top cover; The thickness H1 of the first region and the thickness H2 of the second region satisfy H1 / H2≤0.5; A lithium replenishment layer is provided on at least a portion of the surface of the first region near the core; The thickness h1 of the lithium replenishment layer satisfies 0.5H1≤h1≤1.01H1.

[0026] The secondary battery provided in this application has a first region and a second region sequentially arranged on the inner wall of the casing along the direction from the bottom surface of the casing to the top cover, and a lithium replenishment layer is provided on at least a portion of the surface of the first region near the core. At the same time, the thickness H1 of the first region and the thickness H2 of the second region are defined to satisfy a specific relationship, and the thickness of the lithium replenishment layer is related to the thickness of the first region. The resulting secondary battery has good cycle performance, high capacity retention before and after cycling, and small volume expansion. In addition, the prepared secondary battery also has good discharge capacity and initial coulombic efficiency.

[0027] Specifically, in the first aspect, the thickness H1 of the first region and the thickness H2 of the second region satisfy H1 / H2≤0.5. Since a lithium replenishment layer is subsequently provided on at least a portion of the surface of the first region near the core, this thickness ratio can satisfy the requirement that the thickness of the first region is within a certain range, thereby enabling the introduction of a lithium replenishment layer of a certain thickness, providing a basis for sufficient replenishment of active lithium ions in the later stages of the secondary battery cycle, and thus improving the cycle performance of the secondary battery; at the same time, it can also ensure that the casing has a certain mechanical strength, which can effectively alleviate the casing volume expansion caused by the gas generated during the cycle, that is, reduce the volume expansion rate before and after the cycle. Secondly, a lithium replenishment layer is provided on at least a portion of the surface near the core in the first region. This layer provides a relatively independent additional lithium source. During the first charge of the secondary battery, the active material in the positive electrode lithium replenishment layer reaches its decomposition potential and decomposes, releasing lithium ions and electrons. The released lithium ions enter the electrolyte to replenish the lithium lost by the positive electrode during the first charge film formation, or to replenish the lithium storage in the negative electrode, thereby effectively improving the first discharge capacity of the secondary battery. Furthermore, since the lithium in the replenishment layer is not part of the original positive electrode material during the first charge, it is equivalent to adding more lithium to the original system, thus effectively improving the first discharge capacity and the first coulombic efficiency of the secondary battery. Thirdly, the thickness h1 of the lithium replenishment layer is further limited to 0.5H1≤h1≤1.01H1, providing sufficient lithium ions to improve the first discharge capacity, first coulombic efficiency, and cycle performance of the secondary battery. It also provides a buffer space for volume expansion during battery cycling, reducing the volume expansion of the casing.

[0028] It should be noted that the testing methods for the thickness H1 of the first region, the thickness H2 of the second region, and the thickness of the lithium replenishment layer are as follows: H1 is as follows... Figure 2 As shown, the thickness test method is as follows: Let the length, width and height of the secondary battery be the three-dimensional coordinate axis, and let the thickness direction, that is, the direction perpendicular to the large surface of the aluminum shell, be X. Let the thickness H1 of the test point be the straight distance between point a (top) and point b (bottom). The straight line connecting points a and b is parallel to the X-axis. Then H1 is the distance between points a and b at the selected measurement point. Similarly, measure H2 and the thickness of the lithium replenishment layer.

[0029] For example, H1 / H2 can be any point value between ≤0.5 or a range value between any two points, such as 0.01, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, etc.

[0030] In one embodiment, H1 and H2 satisfy 0.2 ≤ H1 / H2 ≤ 0.4. For example, H1 / H2 can be 0.2, 0.22, 0.24, 0.26, 0.28, 0.3, 0.32, 0.34, 0.36, 0.38, 0.4, etc.

[0031] For example, h1 can be any point value between 0.5H1 and 1.01H1 or a range value between any two points, such as 0.5H1, 0.6H1, 0.7H1, 0.8H1, 0.9H1, 1H1, 1.01H1, etc.

[0032] In one embodiment, h1 satisfies 0.7H1≤h1≤0.9H1. For example, h1 can be 0.7H1, 0.72H1, 0.74H1, 0.76H1, 0.78H1, 0.8H1, 0.82H1, 0.84H1, 0.86H1, 0.88H1, 0.9H1, etc.

[0033] In one embodiment, the height H3 of the housing and the height H4 of the lithium replenishment layer satisfy 0.05H3≤H4≤0.95H3.

[0034] It should be noted that the test method for the height of the casing and the height of the lithium replenishment layer is as follows: Let the secondary battery be set as a three-dimensional coordinate axis along the length, width and height directions, and let the height direction, that is, the direction perpendicular to the bottom surface of the aluminum casing, be Z. Let the height H3 of the test point be the straight distance between point c (top) and point d (bottom). The straight line connecting point cd is parallel to the Z axis. Then H3 is the distance between points cd at the selected measurement point; similarly, the thickness H4 is measured.

[0035] For example, H4 can be any point value between 0.05H3 and 0.95H3 or a range value between any two points, such as 0.05 H3, 0.15 H3, 0.25 H3, 0.35 H3, 0.45 H3, 0.55 H3, 0.65 H3, 0.75 H3, 0.85 H3, 0.95 H3, etc.

[0036] In one embodiment, H3 and H4 satisfy 0.4H3≤H4≤0.9H3. For example, H4 can be 0.4 H3, 0.5 H3, 0.6 H3, 0.7 H3, 0.8 H3, 0.9 H3, etc.

[0037] In one embodiment, the lithium replenishing layer includes a lithium replenishing agent, a conductive agent, and a binder.

[0038] In one embodiment, the conductive agent includes a first conductive agent and a second conductive agent, wherein the Dv50 particle size D1 of the first conductive agent and the Dv50 particle size D2 of the second conductive agent satisfy 5≤D1 / D2≤50.

[0039] For example, D1 / D2 can be any point value between 5 and 50 or a range value between any two points, such as 55, 10, 15, 20, 25, 30, 35, 40, 45, 50, etc.

[0040] In one embodiment, the Dv50 particle size D1 of the first conductive agent and the Dv50 particle size D2 of the second conductive agent satisfy 10 ≤ D1 / D2 ≤ 12.5. For example, D1 / D2 can be 10, 10.2, 10.5, 10.8, 11, 11.2, 11.5, 11.8, 12, 12.2, 12.5, etc.

[0041] This application research found that the ratio of the Dv50 particle size D1 of the first conductive agent to the Dv50 particle size D2 of the second conductive agent affects the gap between conductive material particles. When 5≤D1 / D2≤50 is selected, especially 10≤D1 / D2≤12.5, the gap between conductive material particles is smaller. This allows the gas generated by the subsequent lithium replenishment agent to be discharged from the gap, and also allows the lithium replenishment agent to better penetrate the internal pore structure of the conductive agent. At the same time, it can also improve the structural stability of the lithium replenishment layer, enhance the lithium ion transport capacity, and make the overall performance of the resulting secondary battery better.

[0042] In one embodiment, the mass ratio of the first conductive agent to the second conductive agent is 1:(0.05~0.2).

[0043] It should be noted that the test method for the mass ratio of the first and second conductive agents is as follows: The lithium-filled layer is scraped off from the shell surface and then immersed in water or an organic solvent to dissolve the binder. For example, polyvinylidene fluoride (PVDF) is used as the binder, and N-methylpyrrolidone (NMP) is used as the solvent. The conductive agent is then separated and washed several times with water or an organic solvent to remove the PVDF completely, leaving only the conductive agent. The first and second conductive agents can be separated using an air classifier. The powder containing both conductive agents is placed in an air classifier. During operation, the centrifugal force on large particles exceeds air resistance, causing them to be thrown against the wall and collected. Smaller particles are carried to the fine powder collector in the central area. After the first and second conductive agents are collected separately, they are weighed to obtain the mass ratio.

[0044] For example, the mass ratio of the first conductive agent to the second conductive agent can be any point value between 1: (0.05~0.2) or a range value between any two points, such as 1:0.05, 1:0.08, 1:0.1, 1:0.12, 1:0.14, 1:0.16, 1:0.18, 1:0.2, etc.

[0045] In one embodiment, the mass ratio of the first conductive agent to the second conductive agent is 1:(0.12~0.16). For example, it can be 1:0.12, 1:0.13, 1:0.14, 1:0.15, 1:0.16, etc.

[0046] This application research found that when the mass ratio of the first conductive agent and the second conductive agent is within the above-mentioned range, the second conductive agent with a smaller Dv50 particle size can have a good filling ability between the gaps between the first conductive agent with a larger Dv50 particle size, reducing the gaps between conductive agent particles and improving the overall structural stability of the lithium replenishment layer; at the same time, it can also facilitate the wetting and storage of the electrolyte, providing sufficient transport channels for active lithium ions.

[0047] In one embodiment, D1 is 5 μm to 15 μm.

[0048] For example, D1 can be any point value between 5μm and 15μm or a range value between any two points, such as 5μm, 8μm, 10μm, 12μm, 15μm, etc.

[0049] In one embodiment, D1 is 6μm to 10μm. For example, it can be 6μm, 7μm, 8μm, 9μm, 10μm, etc.

[0050] In one embodiment, D2 is 0.1 μm to 1 μm.

[0051] For example, D2 can be any point value between 0.1μm and 1μm or a range value between any two points, such as 0.1μm, 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1μm, etc.

[0052] In one embodiment, D2 is 0.6 μm to 0.8 μm. For example, it can be 0.6 μm, 0.62 μm, 0.65 μm, 0.68 μm, 0.7 μm, 0.72 μm, 0.75 μm, 0.78 μm, 0.8 μm, etc.

[0053] This study found that the Dv50 particle size of the second conductive agent affects its filling ability in the gaps between the first conductive agent, increases the contact points between conductive agent particles, and reduces the contact resistance; at the same time, the excellent filling effect can better improve the stability of the lithium replenishment layer. In one embodiment, the particle size D3 of the lithium replenishing agent is 0.05 μm to 0.3 μm.

[0054] It should be noted that the particle size D3 of lithium supplements can be tested using a Malvern MasterSizer2000 particle size analyzer. Water or an organic solvent is used as the dispersant. The sample is added, and the test is repeated three times until the result is obtained. The average Dv50 value is then calculated as D3. If a particle size analyzer cannot be used to test the lithium supplement, a scanning electron microscope (SEM) can be used to measure the particle size.

[0055] For example, the particle size D3 of the lithium supplement can be any point value between 0.05μm and 0.3μm or a range value between any two points, such as 0.05μm, 0.1μm, 0.15μm, 0.2μm, 0.25μm, 0.3μm, etc.

[0056] In one embodiment, the particle size D3 of the lithium supplement is 0.1 μm to 0.2 μm. For example, it can be 0.1 μm, 0.12 μm, 0.14 μm, 0.16 μm, 0.18 μm, 0.2 μm, etc.

[0057] In one embodiment, the mass ratio of the adhesive to the conductive agent is (0.01~0.3):1.

[0058] For example, the mass ratio of the adhesive to the conductive agent can be any point value between (0.01~0.3):1 or a range value between any two points, such as 0.01:1, 0.05:1, 0.1:1, 0.15:1, 0.2:1, 0.25:1, 0.3:1, etc.

[0059] In one embodiment, the mass ratio of the adhesive to the conductive agent is (0.08~0.2):1. For example, it can be 0.08:1, 0.1:1, 0.12:1, 0.14:1, 0.16:1, 0.18:1, 0.2:1, etc.

[0060] This study found that the quality of the binder and conductive agent affects the mechanical strength of the lithium replenishment layer, thereby affecting its stability; it also affects the electron transport pathway. When the mass ratio of the binder and conductive agent is selected within the above range, it is beneficial to ensure the structural stability of the lithium replenishment layer, while also ensuring a good lithium-ion transport pathway.

[0061] In one embodiment, the core includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector; Based on the capacity of the secondary battery after its first charge, the capacity that lithium can utilize in the lithium replenishment layer accounts for 1% to 9%.

[0062] It should be noted that, based on the capacity after the first charge of the secondary battery, the test method for the capacity ratio of lithium in the lithium replenishment layer is as follows: At 25°C, for a fresh secondary battery, the charging and discharging equipment is first connected to the aluminum shell and the negative electrode, and charged for a certain period of time with a constant current (0.01C-0.1C) to allow all the lithium replenishment agent to decompose and lithium to enter the negative electrode. The capacity C1 is recorded. Then, the charging and discharging equipment is connected to the positive and negative electrodes, and charged at 0.05C for 1 hour. Then, it is charged at a constant current of 0.1C until the cutoff voltage is reached and then constant at 0.05C. Charging is stopped, and the capacity is recorded as C2. The capacity ratio of the lithium replenishment agent is then C1 / (C1+C2).

[0063] For example, based on the capacity after the first charge of the secondary battery, the capacity ratio that lithium can exert in the lithium replenishment layer can be any point value between 1% and 9% or a range between any two points, such as 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, etc.

[0064] In one embodiment, based on the capacity of the secondary battery after its first charge, the capacity that lithium can utilize in the lithium replenishment layer is 3% to 8%. For example, it can be 3%, 3.5%, 4.5%, 5.5%, 6.5%, 7.5%, 8%, etc.

[0065] This study found that the capacity ratio of lithium in the replenishment layer after the first charge of a secondary battery not only affects the actual replenishment effect, i.e., the ability to provide active lithium ions, but also affects the degree of subsequent lithium plating and the degree of side reactions. When the capacity ratio of lithium in the replenishment layer after the first charge of a secondary battery is controlled at 1% to 9%, especially 3% to 8%, the overall performance of the secondary battery is better.

[0066] In one embodiment, the lithium replenishing agent includes at least one of lithium powder, lithium trifluoromethyl sulfinate, Li5FeO4, Li2NiO2, and Li4SiO4.

[0067] In one embodiment, the conductive agent includes at least one of carbon black and carbon nanospheres.

[0068] In one embodiment, the adhesive includes at least one of polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA).

[0069] In one embodiment, the secondary battery further includes an insulating layer located between the core and the casing.

[0070] In one embodiment, the thickness of the insulating layer is 0.1~0.3 mm. In one embodiment, the area of ​​the insulating layer can be any area that completely covers the lithium replenishment layer to wrap the core.

[0071] This application does not have any special requirements for the material of the insulating layer, and any insulating material that can be used in the art can be used. For example, the material of the insulating layer can be polyethylene, polypropylene, paraffin wool, insulating rubber, etc.

[0072] In one embodiment, the core includes a positive electrode sheet, a negative electrode sheet, and a separator.

[0073] 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 including a positive active material, a positive conductive agent, and a positive binder.

[0074] 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.

[0075] 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), etc.

[0076] 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.

[0077] In one embodiment, 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 including a negative active material, a negative conductive agent and a negative binder.

[0078] This application does not impose any particular restrictions on the selection of the negative electrode active material; conventional negative electrode active materials in the art can be used. For example, the negative electrode active material may be artificial graphite, natural graphite, silicon carbide, silicon oxide, etc.

[0079] 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.

[0080] 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 carboxymethyl cellulose (CMC), styrene-butadiene rubber, etc.

[0081] In one embodiment, the secondary battery further includes an electrolyte comprising an organic solvent and a lithium salt.

[0082] 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.

[0083] 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.

[0084] In one embodiment of this application, an electrical device is provided, which includes the secondary battery described in this application.

[0085] 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.

[0086] 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) are mixed in a weight ratio of 97:0.7:2.3. Then, N-methylpyrrolidone (NMP) is added and mixed thoroughly to obtain a positive electrode slurry. The positive electrode slurry is coated on both sides of a carbon-coated aluminum foil (13μm empty foil + 1μm carbon layer on each side). The electrode is then dried, rolled, slit, and cut to obtain the positive electrode sheet. (2) Preparation of negative electrode sheet The negative electrode active material (artificial graphite), negative electrode conductive agent (conductive carbon black SP), negative electrode binder (CMC), and negative electrode binder (SBR) are 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. The negative electrode slurry is coated on both surfaces of a 4.5μm copper foil, and then the electrode is dried, rolled, slit, and cut to obtain a negative electrode sheet. (3) 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 additive was added. Based on the mass of the electrolyte, the content of lithium hexafluorophosphate was 12%, and the content of vinylene carbonate additive was 0.5%. (4) Preparation of the shell A square aluminum shell is selected as the shell. The distance from the bottom surface of the shell to the top cover, i.e., the height H3 of the shell, is 100mm. The inner wall of the shell is sequentially set with a first region and a second region along the direction from the bottom surface to the top cover. The second region is the thickness H2 (1mm) of the shell itself. The first region is a groove opened by die casting, with a thickness H1 (0.3mm) and a height H4 (40mm). Then, lithium supplement (lithium trifluoromethyl sulfinate, Dv50 particle size of 0.1μm), conductive agent (a mixture of the first conductive agent and the second conductive agent in a mass ratio of 1:0.12, wherein the Dv50 of the first conductive agent is 6μm and the Dv50 of the second conductive agent is 0.6μm, and the conductive agent is carbon black) and binder (PVDF) are mixed, wherein the mass ratio of binder to conductive agent is 0.08:1. After mixing, the mixture is sprayed onto the first region and then dried to obtain the shell. (5) Preparation of secondary batteries The prepared positive and negative electrode sheets are rolled and die-cut, then wound with a separator (polyethylene film) in a separator-negative electrode-separator-positive electrode stacking manner to obtain a core. After shaping, the core tabs are connected to the terminals, and an insulating layer (0.2 mm thick, made of polyethylene) is coated on the surface. The core is then placed in a casing, and laser welding sealing, baking, electrolyte injection, formation, and capacity testing are performed to obtain a secondary battery. A schematic diagram of the resulting secondary battery is shown below. Figures 1-4 As shown.

[0087] 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 thickness of the first region and the lithium replenishment layer and the weight of the added lithium replenishment agent are adjusted to achieve the parameters in Tables 1-2.

[0088] 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 thickness of the lithium replenishment layer and the weight of the lithium replenishment agent are adjusted to achieve the parameters in Tables 1-2.

[0089] 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 height of the first region is adjusted to achieve the parameters in Tables 1-2.

[0090] Examples 11-14 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the Dv50 particle size of the first conductive agent and the second conductive agent is adjusted to achieve the parameters in Tables 1-2.

[0091] Examples 15-17 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the weight of the first conductive agent and the second conductive agent are adjusted to achieve the parameters in Tables 1-2.

[0092] Examples 18-20 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the Dv50 particle size of the lithium replenishing agent is adjusted to achieve the parameters in Tables 1-2.

[0093] 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 weight of the conductive agent and binder added is adjusted to achieve the parameters in Tables 1-2.

[0094] Examples 24-26 This application provides a secondary battery. The difference between the preparation method of the secondary battery and that of Example 1 is that the weight of the lithium replenishing agent is adjusted to achieve the parameters in Tables 1-2.

[0095] Example 27 This application provides a secondary battery. The preparation method of the secondary battery differs from that of Example 1 in that the types of lithium replenishing agent, conductive agent and binder are adjusted to achieve the parameters in Tables 1-2.

[0096] Comparative Example 1 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 thickness of the first region and the lithium replenishment layer and the weight of the added lithium replenishing agent are adjusted to achieve the parameters in Tables 1-2.

[0097] Comparative Examples 2-3 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 thickness of the lithium replenishment layer and the weight of the added lithium replenishment agent are adjusted to achieve the parameters in Tables 1-2.

[0098] The ratios of H1 / H2, h1 / H1, H4 / H3, particle sizes of conductive agents D1, D2, D1 / D2, mass ratios of the first and second conductive agents M1 and D3, mass ratios of binder and conductive agent M2, the percentage of lithium capacity that can be utilized in the lithium replenishment layer based on the capacity after the first charge of the secondary battery M3, the types of lithium replenishment agents, conductive agents, and binders in the examples and comparative examples are shown in Tables 1 and 2. Table 1. Parameters of Secondary Batteries Table 2 Secondary Battery Parameters The performance of the secondary batteries prepared in the examples and comparative examples was tested, including the following: 1. Discharge capacity retention rate: The secondary battery was charged and discharged at 0.5P at an ambient temperature of 25℃, with a voltage range of 2.5V~3.65V. When switching between charge and discharge, it was allowed to stand for 30 minutes and cycled for 4000cls. The discharge capacity of the first cycle and the discharge capacity of the first 4000cls cycle were recorded. Then the capacity retention rate was calculated. Capacity retention rate = discharge capacity at 4000cls cycle / discharge capacity of the first cycle. 2. Initial Coulomb Efficiency: Initial Coulomb Efficiency = Initial Discharge Capacity / Initial Charge Capacity; The secondary battery is charged and discharged at an ambient temperature of 25℃, with a voltage range of 2.5V~3.65V. Specifically, the initial charging process is as follows: charge at 0.05C for 1 hour, then charge at 0.2C for 2 hours, and finally charge at 0.5P to the upper limit voltage; let stand for 30 minutes; the initial discharging process is as follows: discharge at 0.5P to 2.5V; 3. Secondary battery volume expansion rate: Initial (early cycle) secondary battery thickness test: The secondary battery was fully charged to the upper limit voltage at 0.33C constant current and constant voltage, and then cut off at 0.05C current; after standing for 30 minutes, it was discharged to 50% SOC at 0.33C, and then the thickness of the center position of the large surface of the secondary battery was measured 3 times with vernier calipers, and the average value was its thickness value h1. Later (after a certain number of cycles, such as 4000cls) secondary battery thickness test: The secondary battery is fully charged to the upper limit voltage at 0.33C constant current and constant voltage, and then cut off at 0.05C current; after standing for 30 minutes, it is discharged to 50% SOC at 0.33C, and then the thickness at the center of the large surface of the secondary battery is measured 3 times with vernier calipers, and the average value is its thickness value h2. Secondary battery volume expansion rate = (h2-h1) / h1×100%; The results are shown in Table 3. 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 first-cycle discharge capacity and first-cycle coulombic efficiency, and a high capacity retention rate before and after cycling; specifically, the obtained secondary battery has a first-cycle discharge capacity of more than 101.6 Ah, a first-cycle coulombic efficiency of more than 91.8%, a 4000cls capacity retention rate of more than 80.6%, and a volume expansion rate of less than 15.6%. As can be seen from Examples 1-27 and Comparative Examples 1-3, volume expansion can be effectively reduced and the cycle performance and first coulombic efficiency of the secondary battery can be improved only when within the range given by the present invention.

[0099] 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, a casing, and a top cover, characterized in that, The inner wall of the shell is provided with a first region and a second region in sequence along the direction from the bottom surface of the shell to the top cover; The thickness H1 of the first region and the thickness H2 of the second region satisfy H1 / H2≤0.5; A lithium replenishment layer is provided on at least a portion of the surface of the first region near the core; The thickness h1 of the lithium replenishment layer satisfies 0.5H1≤h1≤1.01H1.

2. The secondary battery according to claim 1, characterized in that, The height H3 of the shell and the height H4 of the lithium replenishment layer satisfy 0.05H3≤H4≤0.95H3.

3. The secondary battery according to claim 2, characterized in that, H1 and H2 satisfy 0.2 ≤ H1 / H2 ≤ 0.4; And / or, H3 and H4 satisfy 0.4H3≤H4≤0.9H3.

4. The secondary battery according to claim 1, characterized in that, The lithium replenishing layer includes a lithium replenishing agent, a conductive agent, and a binder.

5. The secondary battery according to claim 4, characterized in that, The conductive agent includes a first conductive agent and a second conductive agent, wherein the Dv50 particle size D1 of the first conductive agent and the Dv50 particle size D2 of the second conductive agent satisfy 5≤D1 / D2≤50.

6. The secondary battery according to claim 5, characterized in that, The mass ratio of the first conductive agent to the second conductive agent is 1:(0.05~0.2). And / or, the D1 is 5μm ~ 15μm; And / or, the D2 is 0.1 μm ~ 1 μm; And / or, the Dv50 particle size D3 of the lithium supplement is 0.05 μm ~ 0.3 μm.

7. The secondary battery according to claim 4, characterized in that, The mass ratio of the adhesive to the conductive agent is (0.01~0.3):

1.

8. The secondary battery according to claim 4, characterized in that, The core 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; Based on the capacity of the secondary battery after its first charge, the capacity that lithium can utilize in the lithium replenishment layer accounts for 1% to 9%.

9. The secondary battery according to claim 4, characterized in that, The lithium replenishing agent includes at least one of lithium powder, lithium trifluoromethyl sulfinate, Li5FeO4, Li2NiO2, and Li4SiO4; And / or, the conductive agent includes at least one of carbon black and carbon nanospheres; And / or, the adhesive includes at least one of polyvinylidene fluoride, carboxymethyl cellulose, and polyacrylic acid.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in any one of claims 1 to 9.

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