Secondary battery and electric device

Abstract

The embodiment of the invention provides a secondary battery and a power utilization device, a negative pole piece of the secondary battery is provided with a convex surface and a concave surface, the negative pole piece comprises a current collector and active substance layers, the active substance layers comprise a first active substance layer arranged on the convex surface and a second active substance layer arranged on the concave surface, each active substance layer comprises graphite and a silicon-based material, and on the convex surface, the second active substance layer comprises graphite and a silicon-based material; the first active material layer at least comprises a first part far away from the current collector and a second part close to the current collector at the concave surface, and the second active material layer at least comprises a third part close to the current collector and a fourth part far away from the current collector at the concave surface. The mass percents of the silicon-based materials in the first part, the second part, the third part and the fourth part are A1, A2, A3 and A4 in sequence, A1: A2 and A3: A4 are in the range of 10: 1-6: 1, the mass percents of the silicon-based materials in the first active material layer and the second active material layer are B1 and B2 in sequence based on the total mass of the graphite and the silicon-based materials of each active material layer, B1 is larger than 0% and smaller than or equal to 6%, and B2 is larger than 0% and smaller than or equal to 6%.

Description

Technical Field

[0001] Embodiments of this application relate to secondary batteries and electrical devices. Background Technology

[0002] Cylindrical batteries are widely used in the new energy field. However, after cycling, cylindrical batteries often exhibit differences in the interface between the concave and convex surfaces of the negative electrode. This phenomenon has two main consequences: firstly, the current density is higher on the convex surface of the negative electrode, leading to severe lithium plating when the battery kinetic window margin is insufficient; secondly, the concave surface of the negative electrode is subjected to compressive stress, causing severe bending of the innermost electrode ring during winding. Due to this compressive stress, the inner concave electrode ring is highly susceptible to electrode delamination during the later stages of cycling due to particle expansion and significant bending, posing a significant safety risk. Summary of the Invention

[0003] The secondary battery provided in this application improves the interface defects of the uneven surface of the negative electrode sheet by using different formulation designs for the corresponding uneven surface of the negative electrode sheet.

[0004] Some embodiments of this application provide a secondary battery comprising a negative electrode, a positive electrode, and a separator wound around a winding center. The negative electrode has a convex surface away from the winding center and a concave surface near the winding center. The negative electrode includes a negative current collector and a negative active material layer, comprising a first negative active material layer disposed on the convex surface of the negative current collector and a second negative active material layer disposed on the concave surface. Both the first and second negative active material layers comprise graphite and silicon-based materials. On the convex surface, the first negative active material layer includes at least a first portion with a first thickness away from the negative current collector and a second portion with a second thickness near the negative current collector along the thickness direction. Both the first and second thicknesses are less than or equal to half the thickness of the first negative active material layer. On the concave surface, the second negative active material layer includes at least a portion near the negative current collector along the thickness direction. The material comprises a third portion having a third thickness and a fourth portion having a fourth thickness, both of which are less than or equal to half the thickness of the second negative electrode active material layer. Based on the total mass of the graphite and silicon-based material in each of the first to fourth portions, the mass percentages of the silicon-based material in the first, second, third, and fourth portions are A1, A2, A3, and A4, respectively, satisfying that A1:A2 and A3:A4 are both within the range of 10:1 to 6:1, and further within the range of 10:1 to 8:1. Furthermore, based on the total mass of the graphite and silicon-based material in each of the first and second negative electrode active material layers, the mass percentages of the silicon-based material in the first and second negative electrode active material layers are B1 and B2, respectively, satisfying that 0% < B1 ≤ 6% and 0% < B2 ≤ 6%. The secondary battery provided in this application uses a larger amount of silicon-based material in the upper layer (the first part of the first negative electrode active material layer that is farther away from the negative electrode current collector) and a smaller amount of silicon-based material in the lower layer (the second part of the first negative electrode active material layer that is closer to the negative electrode current collector) on the convex side (i.e., the side away from the winding center) of the negative electrode sheet, and a smaller amount of silicon-based material in the upper layer (the fourth part of the second negative electrode active material layer that is farther away from the negative electrode current collector) and a larger amount of silicon-based material in the lower layer (the third part of the second negative electrode active material layer that is closer to the negative electrode current collector) on the concave side (the side closer to the winding center). This design can significantly improve the cycle performance of the battery.

[0005] In some embodiments, A1 and A3 are the same, and A2 and A4 are the same. In some embodiments, A1:A2 and A3:A4 are different, and A1:A2 is greater than A3:A4.

[0006] In some embodiments, 0.1% ≤ A1 ≤ 6%, and 0.1% ≤ A3 ≤ 6%. In further embodiments, 3% ≤ A1 ≤ 6%, and 3% ≤ A3 ≤ 6%. In some other embodiments, 0.1% ≤ A1 ≤ 3%, and 0.1% ≤ A3 ≤ 3%.

[0007] In some embodiments, B1 ≤ 3.5% and B2 ≤ 3.5%. In this application, when the silicon-based material accounts for more than 4% of the total mass of silicon-based material and graphite, the cycle performance retention rate may decay, resulting in limited performance improvement of the secondary battery.

[0008] In some embodiments, the first to fourth portions further include an adhesive, wherein the mass percentage of the adhesive in the first portion is X1 based on the total mass of the first portion, and the mass percentage of the adhesive in the second portion is X2 based on the total mass of the second portion, satisfying: X1 > X2; and the mass percentage of the adhesive in the third portion is X3 based on the total mass of the third portion, and the mass percentage of the adhesive in the fourth portion is X4 based on the total mass of the fourth portion, satisfying: X3 > X4. In the first and third portions, the silicon-based material content is relatively high (larger mass percentage). Increasing the adhesive content in the corresponding first and third portions can improve the electrode peeling force without affecting fast charging. Specifically, for the convex first portion, the higher silicon-based material content can improve porosity; the second portion has a relatively high graphite content, resulting in less expansion and requiring less electrode bonding force; for the concave portion, the compressive stress is relatively high, but the fast charging requirement is low, leading to more severe negative electrode extrusion and easier demolding; the third portion has a relatively high silicon-based material content, therefore the adhesive content in the third portion will be increased accordingly, thereby improving the electrode peeling force.

[0009] In some embodiments, the silicon-based material is selected from one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon nitrides, and silicon alloys, and the graphite is selected from artificial graphite or natural graphite.

[0010] In some embodiments, the sum of the first thickness and the second thickness is less than or equal to the thickness of the first negative electrode active material layer, and the sum of the third thickness and the fourth thickness is less than or equal to the thickness of the second negative electrode active material layer, or the compaction density of the negative electrode sheet is 1.5-1.7 g / cm³. 3 .

[0011] Other embodiments of this application provide an electrical device including the aforementioned secondary battery. Attached Figure Description

[0012] Various aspects of the invention will be best understood from the following detailed description when read in conjunction with the accompanying drawings.

[0013] Figure 1 A schematic diagram of a negative electrode sheet according to some embodiments of this application is shown.

[0014] Figure 2 A schematic diagram of a negative electrode sheet according to some embodiments of this application is shown.

[0015] Figure 3 The graph shows the capacity retention of Example 1, Comparative Example 1, and Comparative Example 2 under 35°C 3N3F cycling. Detailed Implementation

[0016] In the field of new energy, for cylindrical batteries, the convex surface of the negative electrode is more prone to lithium plating in wound cells, especially in corner areas and high-energy-density designs. Specifically: ① The arc length of the convex surface of the negative electrode (i.e., the side away from the winding center) is less than the arc length of the concave surface of the positive electrode (the side closer to the winding center). Under the same unit area capacity, the convex surface of the negative electrode can provide relatively insufficient lithium intercalation capacity; ② The current density and curvature are relatively smaller in the wound core near the center hole, and the concave surface is subjected to greater compression. Although the migration of lithium ions is not as concentrated, in the initial ring directly opposite the positive electrode, due to excessive compression, the presence of the dynamic window between the positive and negative electrodes, and uneven electrolyte distribution, ion transport is hindered, making lithium plating more likely. In addition, the convex surface is subjected to tensile stress, which is prone to microcracks, which will damage the SEI film (solid electrolyte interface film) and further aggravate lithium plating; and ③ for the middle part of the winding core, the current density is relatively large (the convex surface itself has a larger current density than the concave surface), and the degree of compression in the middle part of the winding core is also higher than that of the inner and outer rings of the core, and the electric field of the convex surface is more likely to concentrate at this location, so the problem is relatively more serious.

[0017] To address the aforementioned issues, the porosity of the convex surface relative to the concave surface is typically increased. However, since both surfaces have the same porosity, this doesn't specifically improve the porosity of either the concave or convex surface. Adjusting the overall porosity of both surfaces would be costly. Alternatively, conventional compaction treatment can be applied to the negative electrode sheet, but this results in a higher compaction density on the upper (surface) layer. Since lithium ions must first pass through the upper (surface) layer for immersion, the compaction density of the surface layer has a greater impact on immersion performance than the lower layer. Furthermore, the binder in the active material layer (single-sided double-layer) formulation of the negative electrode sheet can be adjusted (by mass percentage). Specifically, the mass percentage of the main components in the upper layer could be: silicon 3.92%, graphite 94%, binder 1.2%, 0.3% (binder for different layers within the upper layer); the mass percentage of the components in the lower layer could be: silicon 3.88%, graphite 93.2%, binder 1.3%, 1.5% (binder for different layers within the lower layer). Other components, such as conductive agents, can also be included. Furthermore, two layers of negative electrode active material can be coated onto the negative electrode current collector at once, but this does not allow for overall adjustment of the porosity of the active material layer.

[0018] To address the aforementioned issues, the secondary battery provided in this application utilizes a higher amount of silicon-based material in the upper layer (the first part of the first negative electrode active material layer farther from the negative electrode current collector) and a lower amount of silicon-based material in the lower layer (the second part of the first negative electrode active material layer closer to the negative electrode current collector) on the convex side (i.e., the side away from the winding center) of the negative electrode sheet, while using a lower amount of silicon-based material in the upper layer (the fourth part of the second negative electrode active material layer farther from the negative electrode current collector) and a higher amount of silicon-based material in the lower layer (the third part of the second negative electrode active material layer closer to the negative electrode current collector) on the concave side (the side closer to the winding center). This design significantly improves the battery's cycle performance.

[0019] Specifically, some embodiments of this application provide a secondary battery, which is formed by winding a negative electrode sheet, a positive electrode sheet, and a separator around a winding center. The negative electrode sheet has a convex surface away from the winding center and a concave surface near the winding center. The negative electrode sheet includes: a negative current collector; and a negative active material layer, including a first negative active material layer disposed on the convex surface of the negative current collector and a second negative active material layer disposed on the concave surface. Both the first and second negative active material layers comprise graphite and silicon-based materials. On the convex surface, the first negative active material layer includes at least a first portion with a first thickness away from the negative current collector and a second portion with a second thickness near the negative current collector along the thickness direction. Both the first and second thicknesses are less than or equal to half the thickness of the first negative active material layer. On the concave surface, the second negative active material layer includes at least a portion near the negative current collector along the thickness direction. The current collector has a third portion with a third thickness and a fourth portion has a fourth thickness away from the negative electrode current collector, the third thickness and the fourth thickness are both less than or equal to half the thickness of the second negative electrode active material layer, wherein, based on the total mass of graphite and silicon-based materials in each of the first to fourth portions, the mass percentage of silicon-based materials in the first, second, third and fourth portions is A1, A2, A3 and A4 respectively, satisfying that: A1:A2 and A3:A4 are both in the range of 10:1-6:1, further, in the range of 10:1-8:1, and wherein, based on the total mass of graphite and silicon-based materials in each of the first and second negative electrode active material layers, the mass percentage of silicon-based materials in the first and second negative electrode active material layers is B1 and B2 respectively, satisfying that: 0% < B1 ≤ 6% and 0% < B2 ≤ 6%. In summary, the secondary battery provided in this application significantly improves the cycle performance of the battery by using a larger amount of silicon-based material in the first part (upper layer) of the convex surface of the negative electrode and a smaller amount of silicon-based material in the second part (lower layer); and a smaller amount of silicon-based material in the fourth part (upper layer) of the concave surface and a larger amount of silicon-based material in the third part (lower layer).

[0020] In some embodiments, A1 and A3 are the same, and A2 and A4 are the same. This reduces the engineering difficulty during fabrication, and the error rate can reach ±0.05%. In some embodiments, A1:A2 and A3:A4 are different, and A1:A2 is greater than A3:A4. When A1:A2 is greater than A3:A4, the porosity of the convex surface of the negative electrode sheet can be effectively enhanced, thereby further improving the performance of the secondary battery. In this application, the sameness of A1 and A3, and the sameness of A2 and A4, indicates that the design values ​​are the same. However, there may be deviations in actual situations. These deviations are within the scope of common knowledge among those skilled in the art, such as within ±10% of the design values.

[0021] In some embodiments, 0.1% ≤ A1 ≤ 6% and 0.1% ≤ A3 ≤ 6%, that is, based on the total mass of graphite and silicon-based materials in the first part, the mass percentage A1 of the silicon-based material in the first part ranges from 0.1% to 6%, and based on the total mass of graphite and silicon-based materials in the third part, the mass percentage A3 of the silicon-based material in the third part ranges from 0.1% to 6%. This ensures that the mass percentage of silicon-based material in the first part (upper layer) of the convex surface of the negative electrode is ≤ 6%, and the mass percentage of silicon-based material in the third part (lower layer) of the concave surface of the negative electrode is ≤ 6%. Further, in some embodiments, 3% ≤ A1 ≤ 6% and 3% ≤ A3 ≤ 6%, that is, the mass percentage of silicon-based material in the first and third parts is 3%-6%. In some other embodiments, 0.1% ≤ A1 ≤ 3% and 0.1% ≤ A3 ≤ 3%.

[0022] In some embodiments, 0% < B1 ≤ 3.5% and 0% < B2 ≤ 3.5%, that is, based on the total mass of graphite and silicon-based materials in the first negative electrode active material layer, the mass percentage B1 of silicon-based materials in the first negative electrode active material layer satisfies 0% < B1 ≤ 3.5%, and based on the total mass of graphite and silicon-based materials in the second negative electrode active material layer, the mass percentage B2 of silicon-based materials in the second negative electrode active material layer satisfies 0% < B1 ≤ 3.5%. In this application, when the silicon-based material accounts for more than 4% of the total mass of silicon-based materials and graphite, the cycle performance retention rate may decay, thus limiting the performance improvement of the secondary battery. In some embodiments, the first to fourth portions further include an adhesive. The mass percentage of the adhesive in the first portion is X1 based on the total mass of the first portion, and the mass percentage of the adhesive in the second portion is X2 based on the total mass of the second portion, satisfying X1 > X2; and the mass percentage of the adhesive in the third portion is X3 based on the total mass of the third portion, and the mass percentage of the adhesive in the fourth portion is X4 based on the total mass of the fourth portion, satisfying X3 > X4. In the first and third portions, the higher content (larger mass percentage) of silicon-based material increases the adhesive content of the corresponding first and third layers, improving the electrode peel strength without affecting fast charging. Specifically, for the convex first portion, the higher silicon-based material content improves porosity; the second portion has a higher graphite content, resulting in less expansion and requiring less electrode adhesion. For the concave portion, the compressive stress is higher, but the fast charging requirement is lower, leading to more severe negative electrode extrusion and easier demolding. The third portion has a higher silicon-based material content, thus increasing the adhesive content and improving the electrode peel strength.

[0023] In summary, for the convex side of the negative electrode (i.e., the side furthest from the winding center), due to the high current density and higher kinetic requirements, a double-layer coating design is adopted, with a higher content of silicon-based material in the first part (upper layer) and a higher content of graphite in the second part (lower layer) (mass percentage of silicon-based material A1 ≤ 6%). During charging, the volume of the silicon-based material expands, and after discharging, the volume of the silicon-based material shrinks to a certain extent. During cycling, the expansion and contraction of the silicon-based material improves the porosity of the surface electrode, and the increased porosity can significantly optimize the fast-charging performance of the battery; and

[0024] For the concave side of the negative electrode (the side near the winding center), the negative electrode is prone to expansion during cycling due to compressive stress, especially in the inner ring. The inner ring electrode also experiences severe bending, leading to greater compression and extrusion, resulting in large-scale lithium plating in the inner ring of the battery and easy delamination, posing a safety risk. Therefore, a double-layer coating design is adopted, with a higher graphite content in the fourth part (upper layer) and a higher silicon-based material content in the third part (lower layer) (the mass percentage of silicon-based material in the lower layer, A3, is ≤ 6%). The advantage of this design is that the silicon-based material is distributed near the negative electrode current collector, which can significantly increase the binder content on the negative electrode current collector side (when the mass content of silicon-based material is higher than that of graphite and silicon-based materials, the binder content also increases accordingly), significantly improving the electrode peeling force. This does not affect the battery's fast-charging performance and optimizes the electrode adhesion of the concave electrode, preventing delamination or even lithium plating of the inner concave electrode due to compressive stress or high degree of electrode bending, thus significantly improving battery safety and reliability.

[0025] In some embodiments, the mass percentage of the total mass of silicon-based material and graphite in each of the first to fourth portions is in the range of 96% to 98%, based on the total mass of each of the first to fourth portions respectively.

[0026] In some embodiments of this application, the graphite is selected from artificial graphite and / or natural graphite, and the silicon-based material is selected from one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon nitrides, and silicon alloys.

[0027] In some embodiments, the compaction density of the negative electrode sheet is 1.5-1.7 g / cm³, and more specifically, it can be 1.62 g / cm³. In these embodiments, the compaction density can affect the porosity, thus promoting an increase in porosity.

[0028] In some embodiments, the sum of the first thickness and the second thickness is less than or equal to the thickness of the first negative electrode active material layer, and the sum of the third thickness and the fourth thickness is less than or equal to the thickness of the second negative electrode active material layer. In some embodiments, the ratio of the first thickness of the first portion to the second thickness of the second portion can be about 1, where about represents an error range of ±10% of the described range. In some embodiments, the ratio of the first thickness to the second thickness can also be in the range of 0.9-1.1. Similarly, the ratio of the third thickness of the third portion to the fourth thickness of the fourth portion can also be about 1, where about represents an error range of ±10% of the described range. In some embodiments, the ratio of the third thickness to the fourth thickness can also be in the range of 0.9-1.1.

[0029] Other embodiments of this application provide an electrical device including the secondary battery, which is a cylindrical battery and may be a lithium-ion battery, sodium-ion battery, sodium-lithium-ion battery, etc., and this application is not limited thereto.

[0030] In some embodiments, the battery is a lithium-ion battery, which includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0031] Negative electrode sheet

[0032] The negative electrode sheet can be made using the aforementioned negative electrode sheet, and the formulation of this negative electrode sheet mainly consists of negative electrode active material, binder, and conductive agent. Furthermore, the negative electrode sheet can be prepared using methods conventional in the art, such as coating and drying with a negative electrode slurry. The negative electrode slurry is typically formed by dispersing the negative electrode active material, conductive agent, and binder in a solvent and stirring uniformly. For example, the following method can be used: mixing the negative electrode active material, binder, conductive agent, and optionally other additives in a certain weight ratio, adding a solvent and mixing uniformly to obtain a negative electrode slurry; then uniformly coating the negative electrode slurry onto a negative electrode current collector; air-drying at room temperature, transferring to an oven for further drying, and then cold-pressing, slitting, and cutting to obtain the negative electrode sheet. The negative electrode active material accounts for 96%-98% of the mass of the negative electrode slurry, and the compacted density of the negative electrode sheet is 1.5 g / cm³-1.8 g / cm³. 3 .

[0033] In the aforementioned negative electrode sheet, the negative electrode active material includes silicon-based materials and graphite. Graphite includes artificial graphite and / or natural graphite, and the silicon-based materials include one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon nitrides, and silicon alloys. The negative electrode active material may include optional binders, optional conductive agents, and other optional additives. The conductive agent may include one or more of superconducting carbon, acetylene black, carbon black (SP), Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. The binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resin, polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS). Other optional additives include thickeners (such as sodium carboxymethyl cellulose CMC-Na, CMCLi), PTC thermistor materials, etc.

[0034] For the negative electrode current collector, the negative electrode current collector can be a current collector conventionally used for negative electrodes in the art, and can be a common current collector or a composite current collector. The negative electrode current collector can be made of a non-chemically reactive and conductive material without limitation. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum-cadmium alloys can be used, or copper, stainless steel, or aluminum-cadmium alloys surface-treated with carbon, nickel, titanium, or silver. Furthermore, to enhance the adhesion of the negative electrode active material, micro-embossing can be formed on the surface of the negative electrode current collector. The negative electrode current collector can be used in various forms, such as a membrane, sheet, foil, mesh, or porous body.

[0035] Positive electrode sheet

[0036] In some implementations, the positive electrode active material, conductive agent and binder are mixed in a mass ratio of 96-98:1-3:0.5-1.5, a solvent (such as N-methylpyrrolidone) is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry. The positive electrode slurry is then uniformly coated on a positive electrode current collector, air-dried at room temperature, transferred to an oven for further drying, and then cold-pressed, slit and cut to obtain a positive electrode sheet.

[0037] The positive electrode active material can be a conventional positive electrode active material used in the field for the positive electrode of pouch cells. The positive electrode active material may include, but is not limited to, one or more of lithium iron phosphate, lithium manganese iron phosphate, ternary positive electrode active material (NCM), and lithium-rich manganese-based material.

[0038] The binder can be a component that facilitates the bonding between the active material and the conductive agent, and also facilitates the bonding between the active material and the current collector. It can typically be selected from polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, and various copolymers. In some embodiments, polyvinylidene fluoride (PVDF) can be used as the binder.

[0039] Conductive agents are reagents used to ensure that the electrodes have good charge and discharge performance. They can be selected from graphite materials such as natural graphite, carbon nanotubes (CNTs), and artificial graphite; carbon black materials such as carbon black (SP), acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermally cracked carbon black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorinated carbon powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; and conductive metal oxides or polyphenylene derivatives such as titanium dioxide. In some embodiments, carbon nanotubes (CNTs) and carbon black (SP) can be used as conductive agents.

[0040] For the positive electrode current collector, materials that do not cause chemical changes and have high conductivity can be used without restriction. Commonly used materials include stainless steel, aluminum, nickel, titanium, or calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. To enhance the adhesion of the positive electrode active material, micro-embossing can be formed on the surface of the positive electrode current collector. Positive electrode current collectors can be used in various forms, such as films, sheets, foils, meshes, or porous bodies.

[0041] diaphragm

[0042] The separator placed between the positive and negative electrode plates uses an insulating film with high ion permeability and high mechanical strength. The separator typically has a thickness of 9 μm-18 μm; a pore size of 5 μm-300 μm; an air permeability of 180 s / 100 mL-380 s / 100 mL; and a porosity of 30% to 50%. As a separator, it is chemically resistant and hydrophobic, and is usually made of sheets or nonwoven fabrics made of olefin polymers such as polypropylene or polyethylene membranes; glass fibers; or, further, a coating is applied to the aforementioned sheets or nonwoven fabrics as a base film.

[0043] electrolyte

[0044] In this application, the electrolyte can be a conventional electrolyte used in lithium-ion batteries, generally including non-aqueous solvents, lithium salts, and additives.

[0045] In some embodiments, the non-aqueous solvent may be a conventional non-aqueous solvent in the art, preferably an ester solvent, and more preferably a carbonate solvent. The carbonate solvent may be one or more of ethylene carbonate, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), methyl ethyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), and butyl carbonate (BC).

[0046] In some embodiments, the additive may be one or more selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene ethylene carbonate (VEC), vinyl sulfate (DTD), vinylene sulfate, 1,3-propane sulfonyl lactone (PS), propylene sulfonate lactone, and 1,4-butane sulfonate lactone. The conventional amount of the additive in the electrolyte is 1%-4% of the electrolyte, for example, 2%.

[0047] In some embodiments, the lithium salt may be a conventional lithium salt in the art, such as at least one selected from LiPF6, LiBF4, LiN(SO2F)2 (abbreviated as LiFSI), LiClO4, LiAsF6, LiB(C2O4)2 (abbreviated as LiBOB), LiBF2(C2O4) (abbreviated as LiDFOB), LiN(SO2RF)2, and LiN(SO2F)(SO2RF). The concentration of the lithium salt may be conventional in the art, and the lithium salt is preferably present in the electrolyte at a concentration of 5%-20%, typically 1 mol / L-2 mol / L.

[0048] In this application, the preparation method of the electrolyte can be conventional in the art, generally involving mixing a non-aqueous solvent, lithium salt, and additives.

[0049] Secondary batteries (taking lithium-ion batteries as an example)

[0050] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound around the center to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a secondary battery, such as a lithium-ion battery, is obtained.

[0051] Electrical appliances

[0052] In this application, the electrical device includes the aforementioned secondary battery, and the electrical device can be a vehicle, mobile phone, portable device, laptop, ship, spacecraft, electric toy, and power tool, etc. Vehicles can be gasoline-powered cars, natural gas-powered cars, or new energy vehicles; new energy vehicles can be pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles, etc.; spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc.; electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc.; power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. It can also be an energy storage electronic device that stores energy and then discharges it externally. This application does not impose any special limitations on the aforementioned electronic devices.

[0053] The present application is further illustrated below by way of embodiments, but these embodiments are not intended to limit the scope of the present application. Experimental methods not specifically described in the following embodiments are performed according to conventional methods and conditions in the art, or as selected according to the product specification.

[0054] The positive and negative electrode plates and electrolytes of the batteries in Examples 1-13 and Comparative Examples 1-3 were prepared using the following methods.

[0055] Example 1

[0056] Preparation of negative electrode sheet

[0057] A mixture of graphite (artificial graphite) and silicon-based material (silicon oxide, SiO) as the negative electrode active material was prepared according to a first mass percentage. This mixture was then combined with conductive agent SP, thickener CMC-Na, and binder SBR at a mass ratio of 96:1:1.2:1.8, along with deionized water. The mixture was stirred under vacuum until homogeneous, yielding a first slurry. Using the same method, a mixture of the above graphite and silicon-based material as the negative electrode active material was prepared according to a second mass percentage. This mixture was then combined with conductive agent SP, thickener CMC-Na, and binder SBR. A second slurry is obtained by mixing the above-mentioned graphite and silicon-based material as negative electrode active materials at a third mass percentage, and then mixing them with conductive agent SP, thickener CMC-Na, and binder SBR at a mass ratio of 96:1:1.2:1.8 to obtain a third slurry. Furthermore, a fourth slurry is obtained by mixing the above-mentioned graphite and silicon-based material as negative electrode active materials at a fourth mass percentage, and then mixing them with conductive agent SP, thickener CMC-Na, and binder SBR at a mass ratio of 97:1:1.2:0.8. The first to fourth mass ratios are shown in Table 1 below.

[0058] For the convex side of the negative electrode sheet, the first and second slurries are simultaneously coated on the corresponding sides of the negative electrode current collector copper foil, and the coating is uniform to form a first part and a second part of the first negative electrode active material layer, wherein the first part is away from the negative electrode current collector and has a first thickness, the second part is close to the negative electrode current collector and has a second thickness, and the first thickness is approximately equal to the second thickness; and for the concave side of the negative electrode sheet, the third and fourth slurries are simultaneously coated on the other corresponding side of the negative electrode current collector copper foil, and the coating is uniform to form a third part and a fourth part of the second negative electrode active material layer, wherein the third part is close to the negative electrode current collector and has a third thickness, the fourth part is away from the negative electrode current collector and has a fourth thickness, and the third thickness is approximately equal to the fourth thickness.

[0059] After air-drying at room temperature, the material is transferred to an oven for further drying, followed by cold pressing, slitting, and cutting to obtain the negative electrode sheet. In this embodiment, the compacted density of the negative electrode sheet is 1.62 g / cm³. It should be noted that after the first and second slurries, or the third and fourth slurries, are coated, it is difficult to clearly distinguish the boundaries of the corresponding layers through observation and testing. Therefore, independent tests are conducted using 50% or 25% of the coating thickness. Specifically, when using 50% of the coating thickness, the overall thickness of the first negative electrode active material layer is D. The third thickness of the third part and the fourth thickness of the fourth part are similarly related to the overall thickness of the second negative electrode active material layer; when the coating thickness is 25%, the overall thickness of the first negative electrode active material layer is D, then... The third thickness of the third part and the fourth thickness of the fourth part are similar to the overall thickness of the second negative electrode active material layer. It should be noted that when 25% of the coating thickness is used, the corresponding first and second thicknesses are measured from the corresponding edges of the first negative electrode active material layer, and the third and fourth thicknesses are measured from the corresponding edges of the second negative electrode active material layer.

[0060] The first to fourth slurries were used to prepare the first to fourth parts, respectively, and the specific mass percentages of graphite and silicon-based materials are shown in Table 1 below:

[0061] Table 1 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials of Parts 1 to 4.

[0062]

[0063] Preparation of positive electrode sheet

[0064] LiNi, the positive electrode active material 0.8 Co 0.1 Mn 0.1O2, conductive agents CNT and SP (mass ratio 1:1), and binder PVDF are mixed at a mass ratio of 97:2:1. NMP solvent is added, and the mixture is stirred under vacuum until the system is homogeneous to obtain a positive electrode slurry. The positive electrode slurry is then coated onto a positive electrode current collector aluminum foil, dried at room temperature, and transferred to an oven for further drying. After cold pressing, slitting, and cutting, the positive electrode sheet is obtained.

[0065] Preparation of electrolyte

[0066] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1 to obtain an organic solvent. Then, fully dried lithium salt LiPF6 was dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L. Finally, 2% VC was added as a film-forming additive.

[0067] Preparation of diaphragm

[0068] A polyethylene film with a thickness of 12 μm was selected as the diaphragm.

[0069] Preparation of lithium-ion batteries

[0070] The positive electrode, separator, and negative electrode are stacked in sequence, with the separator acting as a separator between the positive and negative electrodes. Then, they are wound to obtain a bare cell. The bare cell is placed in an outer packaging shell, dried, and then injected with electrolyte. After vacuum sealing, settling, formation, and shaping, a lithium-ion battery is obtained.

[0071] Example 2

[0072] Similar to Example 1, except that in Example 2, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 2 below:

[0073] Table 2 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials (parts 1-4).

[0074]

[0075] Example 3

[0076] Similar to Example 1, except that in Example 3, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 3 below:

[0077] Table 3 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials in Parts 1 to 4.

[0078]

[0079] Example 4

[0080] Similar to Example 1, except that in Example 4, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts of the negative electrode active material are shown in Table 4 below:

[0081] Table 4 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials (Parts 1-4).

[0082]

[0083] Example 5

[0084] Similar to Example 1, except that in Example 5, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 5 below:

[0085] Table 5 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials in Parts 1 to 4.

[0086]

[0087] Example 6

[0088] Similar to Example 1, except that in Example 6, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 6 below:

[0089] Table 6 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials of Parts 1 to 4.

[0090]

[0091] Example 7

[0092] Similar to Example 1, except that in Example 7, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 7 below:

[0093] Table 7 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials in Parts 1 to 4.

[0094]

[0095] Example 8

[0096] Similar to Example 1, except that in Example 8, the negative electrode active material is artificial graphite and silicon-based material (silicon carbide, SiC).

[0097] Example 9

[0098] Similar to Example 1, except that in Example 9, the negative electrode active material is made of artificial graphite and silicon-based material (elemental silicon, Si).

[0099] Example 10

[0100] Similar to Example 1, except that in Example 10, the negative electrode active material is selected from natural graphite and silicon-based materials (silicon nitride).

[0101] Example 11

[0102] Similar to Example 1, except that in Example 11, the negative electrode active material is a mixture of artificial graphite and silicon-based materials (silicon oxide and silicon carbide in a mass ratio of 1:1).

[0103] Example 12

[0104] Similar to Example 1, except that in Example 12, the compaction density of the negative electrode sheet is 1.5 g / cm³. 3 .

[0105] Example 13

[0106] Similar to Example 1, except that in Example 13, the compaction density of the negative electrode sheet is 1.7 g / cm³. 3 .

[0107] Comparative Example 1

[0108] Similar to Example 1, except that in Comparative Example 1, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 8 below:

[0109] Table 8 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials of Parts 1 to 4.

[0110]

[0111] Comparative Example 2

[0112] Similar to Example 1, except that in Comparative Example 2, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 9 below:

[0113] Table 9 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials of Parts 1 to 4.

[0114]

[0115]

[0116] Comparative Example 3

[0117] Similar to Example 1, except that in Comparative Example 3, the specific mass percentages of graphite and silicon-based materials in the first to fourth parts are shown in Table 10 below:

[0118] Table 10 shows the mass percentage of graphite and silicon-based materials in the negative electrode active materials of Parts 1 to 4.

[0119]

[0120] Test the first through fourth parts.

[0121] (1) Sampling and testing before coating: test the mass percentage of the corresponding silicon-based material;

[0122] (2) At 23±2℃ and a dew point ≤-30℃ (preferably below -40℃), the coating of the negative electrode sheet of the battery cell (the first and second parts of the first negative electrode active material layer and the third and fourth parts of the second negative electrode active material layer) is tested to avoid bulging of the negative electrode sheet due to low moisture content and to control the moisture content, for example, by using an infrared moisture meter to detect that the moisture content is not greater than 300ppm. The specific test method is as follows:

[0123] ① Perform EDS (energy dispersive spectroscopy) line scan and area scan on the cross-section of the negative electrode, and then use coin cell to test the specific capacity;

[0124] ② The boundary between the first and second parts can be defined manually (in reality, after coating, it is difficult to clearly distinguish the boundaries between the two parts through observation and testing). Therefore, for the above embodiments and comparative examples, 50% of the coating thickness was used for independent testing, and the same applies to the third and fourth parts. Thickness was measured using a 0.1μm resolution thickness gauge. Specifically, when using 50% of the coating thickness for independent testing: see [link to relevant documentation]. Figure 1 , Figure 1 A schematic diagram of a negative electrode sheet 100 according to some embodiments of this application is shown. A first negative electrode active material layer 101 and a second negative electrode active material layer 102 are respectively disposed on the convex and concave surfaces of the negative electrode current collector 103. Here, 50% of the coating thickness represents the overall thickness of the first negative electrode active material layer 101 as D1. Then, the first thickness D11 of the first portion and the second thickness D12 of the second portion are... Furthermore, if the overall thickness of the second negative electrode active material layer 102 is D2, then the third thickness D21 of the third part and the fourth thickness D22 of the fourth part are... However, it should be noted that when conducting independent tests using 25% of the coating thickness: see [link to relevant documentation]. Figure 2 , Figure 2 A schematic diagram of a negative electrode sheet 100 according to some embodiments of this application is shown. In this diagram, 25% of the coating thickness of the negative electrode sheet 100 represents the overall thickness of the first negative electrode active material layer 101 as D1. Then, the first thickness D11 of the first portion and the second thickness D12 of the second portion are both... Furthermore, if the overall thickness of the second negative electrode active material layer 102 is D2, then the third thickness D21 of the third part and the fourth thickness D22 of the fourth part are both... The first and second thicknesses are measured from the corresponding edges of the first negative electrode active material layer 101, and the third and fourth thicknesses are measured from the corresponding edges of the second negative electrode active material layer 102; and

[0125] ③ Based on the test method in ②, the overall silicon-based material content of the coating per unit area can be measured as the obtained value. The unit area is, for example, an area of ​​1 cm2, and in some cases, in order to avoid the coating edge effect area, there is a certain error, which can be an area of ​​(1±0.2) cm2.

[0126] Determining the lithium plating status after disassembling the negative electrode: Disassemble the fully charged cell and confirm the lithium plating status at the corresponding position of the negative electrode corresponding to the third region of the positive electrode. Define the lithium plating level based on the lithium plating area (lithium plating area / total area of ​​the negative electrode active material layer in the target area): 1. No lithium plating; 2. Lithium plating area less than 10%, light lithium plating; 3. Lithium plating area 10%-30%, moderate lithium plating; 4. Lithium plating area > 30%, heavy lithium plating. The total area of ​​the negative electrode active material layer in the target area can be selected according to actual testing needs. In this embodiment, the total area of ​​the negative electrode active material layer in the target area is the total area of ​​the first negative electrode active material layer or the second negative electrode active material layer.

[0127] Performance testing of lithium-ion batteries.

[0128] ① Test conditions: 35℃

[0129] ②Testing process:

[0130] 1) A regular charging cycle consisting of 3 cycles + a fast charging cycle consisting of 3 cycles;

[0131] 2) Normal cycle SOC (State of Charge) range: 10%-100% SOC; Fast charging cycle: 10%-80% SOC;

[0132] 3) Standard charging cycle: 0.5C rate; Fast charging cycle: 3C to 1.1C stepped charging, charging from 10% SOC to 80% SOC within 22 minutes; and

[0133] 4) The process consists of 3 regular cycles plus 3 fast charging cycles, which are repeated repeatedly. This is known as the 3N3F cycle.

[0134] Battery fast charging cycle process

[0135] At 35°C, the batteries of the above embodiments and comparative examples were charged and discharged for the first time at a current of 0.5C (i.e., the current value at which the theoretical capacity is completely discharged within 1 hour). Specifically, the batteries were charged at a constant current rate of 0.5C to a voltage of 4.25V, then charged at a constant voltage rate to a current ≤0.05C, left to stand for 5 minutes, and then discharged at a constant current rate of 0.33C to a voltage of 2.8V. The actual capacity was recorded as C0.

[0136] Then, the battery is continuously charged at constant current from 3.0C0 to 1.0C0, selecting several charging rate values ​​until the full battery charging cutoff voltage of 4.25V or the negative electrode cutoff potential of 0V (whichever comes first). After each charging, the battery is discharged at 0.5C0 until the full battery discharge cutoff voltage of 2.8V. The negative electrode potential corresponding to different percentages of SOC is recorded at different charging rates. The SOC percentage ranges from 10% to 80%. There is a fixed charging rate difference between adjacent charging rates. For example, 3.0C0, 2.8C0, 2.6C0, etc. are selected, with a charging rate value of 0.2C0 between them, until the 1.0C0 charging rate is selected.

[0137] Battery's typical cycle performance process

[0138] At 35°C, the secondary batteries prepared in each embodiment and comparative example were charged at a constant current rate of 0.5C to the charging cutoff voltage of 4.25V, then charged at a constant voltage until the current ≤0.05C, allowed to stand for 5 minutes, and then discharged at a constant current rate of 0.5C to the discharge cutoff voltage of 2.8V, allowed to stand for 5 minutes. This constitutes one charge-discharge cycle. The batteries were subjected to cyclic charge-discharge tests according to this method until the battery capacity decreased to 80%. The number of cycles at this point is the cycle life of the battery at 35°C.

[0139] The test results for Examples 1-13 and Comparative Examples 1-3 are shown in Table 11 below.

[0140]

[0141]

[0142]

[0143] Furthermore, Figure 3 The graphs showing the capacity retention of Example 1, Comparative Example 1, and Comparative Example 2 under 35°C 3N3F cycling are illustrated. Figure 3As can be seen from the comparison of the test results of Example 1 and Comparative Examples 1-2 in Table 11 above, the fast charging cycle of the improved (Example 1) 3N3F battery is significantly improved. The cycle life of Example 1 is extended by more than 335cls. When the retention rate decays to 80% in the conventional cycle process, the number of cycles is greater than 800cls, which can meet the customer's needs.

[0144] Furthermore, a comparison of Examples 1-6 and Comparative Examples 1-2 shows that for the convex side of the negative electrode (i.e., the side away from the winding center), due to the high current density and higher dynamic requirements, a double-layer coating design is adopted, with a higher content of silicon-based material in the first part (upper layer) and a higher content of graphite in the second part (lower layer) (mass percentage of silicon-based material A1 ≤ 6%). During charging, the volume of the silicon-based material expands, and after discharging, the volume of the silicon-based material shrinks to a certain extent. During cycling, the expansion and contraction of the silicon-based material improves the porosity of the surface electrode, and the increased porosity can significantly optimize the fast-charging performance of the battery. For the concave side of the negative electrode (the side closer to the winding center), the negative electrode is prone to expansion during cycling due to compressive stress, especially in the inner ring. Furthermore, the severe bending and compression of the inner electrode sheets lead to lithium plating in the inner circle of the battery, and also cause easy delamination and material loss, posing a safety risk. Therefore, a double-layer coating design is adopted, with a higher content of graphite in the fourth part (upper layer) and a higher content of silicon-based material in the third part (lower layer) (mass percentage of silicon-based material A3 ≤ 6%). The advantage of this design is that the silicon-based material is distributed near the negative electrode current collector, which can significantly increase the binder content on the negative electrode current collector side (when the mass content of silicon-based material is higher than that of graphite and silicon-based materials, the binder content also increases accordingly), significantly improving the peeling force of the electrode sheets. This does not affect the fast charging performance of the battery, and can also optimize the electrode adhesion of the concave electrode sheets, avoiding delamination or even lithium plating of the inner concave electrode sheets due to compressive stress or high degree of electrode bending, which can significantly improve the safety and reliability of the battery.

[0145] Furthermore, as can be seen from Examples 1 to 4, based on the total mass of graphite and silicon-based materials in the first negative electrode active material layer, the mass percentage of the silicon-based material in the first negative electrode active material layer is B1, and based on the total mass of graphite and silicon-based materials in the second negative electrode active material layer, the mass percentage of the silicon-based material in the second negative electrode active material layer is B2, satisfying: 0% < B1 ≤ 6%, and 0% < B2 ≤ 6%, preferably 0%-3.5%. Within this range, the cycle performance of the battery is significantly improved.

[0146] A comparison of Examples 1, 5-6, and Comparative Examples 1 and 3 shows that A1:A2 and A3:A4 are both within the range of 10:1-6:1, and further within the range of 10:1-8:1. When the range exceeds this range (10:1-6:1), the cycle performance of the battery will significantly decrease.

[0147] As can be seen from the comparison between Example 1 and Comparative Example 2, 0.1% ≤ A1 ≤ 6% and 0.1% ≤ A3 ≤ 6%, further, 3% ≤ A1 ≤ 6% and 3% ≤ A3 ≤ 6%. When the range is exceeded as in Example 2, the cycle performance of the battery will also degrade.

[0148] As can be seen from Examples 1-6 and Example 7, A1 can be the same as A3, and A2 can be the same as A4. However, as can be seen from Example 7, A1:A2 and A3:A4 are different, and A1:A2 is greater than A3:A4. When A1:A2 is greater than A3:A4, the porosity of the convex surface of the negative electrode sheet can be effectively enhanced, thereby further improving the performance of the secondary battery.

[0149] As shown in Examples 1 and 8-11, the graphite includes artificial graphite and / or natural graphite, and the silicon-based material includes one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon nitrides, and silicon alloys, all of which can improve battery performance. Furthermore, as seen in Examples 12-13, the compaction density of the negative electrode sheet is 1.5-1.7 g / cm³, and can be further expressed as 1.62 g / cm³. In this application, compaction density can affect porosity; a range of 1.5-1.7 g / cm³ is more conducive to increasing porosity.

[0150] The foregoing outlines features of several embodiments to enable those skilled in the art to better understand aspects of the invention. Those skilled in the art should understand that they can readily use this invention as a basis to design or modify other processes and structures for implementing the same purposes and / or achieving the same advantages as the embodiments described herein. Those skilled in the art should also recognize that such equivalent constructions do not depart from the spirit and scope of the invention, and that various changes, substitutions, and modifications can be made to them herein without departing from the spirit and scope of the invention.

Claims

1. A secondary battery, characterized in that, The electrode is formed by winding a negative electrode, a positive electrode, and a separator around a winding center. The negative electrode has a convex surface away from the winding center and a concave surface near the winding center. The negative electrode comprises: Negative current collector; and The negative electrode active material layer includes a first negative electrode active material layer disposed on the convex surface of the negative electrode current collector and a second negative electrode active material layer disposed on the concave surface. Both the first negative electrode active material layer and the second negative electrode active material layer comprise graphite and silicon-based materials. Wherein, at the convex surface, the first negative electrode active material layer includes at least a first portion having a first thickness and being away from the negative electrode current collector, and a second portion having a second thickness and being close to the negative electrode current collector, wherein both the first thickness and the second thickness are less than or equal to half the thickness of the first negative electrode active material layer; and at the concave surface, the second negative electrode active material layer includes at least a third portion having a third thickness and being close to the negative electrode current collector, and a fourth portion having a fourth thickness and being away from the negative electrode current collector, wherein both the third thickness and the fourth thickness are less than or equal to half the thickness of the second negative electrode active material layer. Wherein, based on the total mass of the graphite and the silicon-based material in each of the first to fourth parts, the mass percentages of the silicon-based material in the first, second, third, and fourth parts are A1, A2, A3, and A4 respectively, satisfying that: A1:A2 and A3:A4 are both within the range of 10:1-6:

1. And wherein, based on the total mass of the graphite and the silicon-based material in each of the first and second negative electrode active material layers, the mass percentages of the silicon-based material in the first and second negative electrode active material layers are B1 and B2 respectively, satisfying that: 0% < B1 ≤ 6%, and 0% < B2 ≤ 6%.

2. The secondary battery according to claim 1, characterized in that, A1 and A3 are the same, and A2 and A4 are the same.

3. The secondary battery according to claim 1, characterized in that, A1:A2 and A3:A4 are different, and A1:A2 is greater than A3:A4.

4. The secondary battery according to claim 1, characterized in that, Both A1:A2 and A3:A4 are within the range of 10:1 to 8:

1.

5. The secondary battery according to claim 1, characterized in that, 0.1% ≤ A1 ≤ 6%, and 0.1% ≤ A3 ≤ 6%.

6. The secondary battery according to claim 5, characterized in that, 3% ≤ A1 ≤ 6%, and 3% ≤ A3 ≤ 6%.

7. The secondary battery according to claim 5, characterized in that, 0.1% ≤ A1 ≤ 3%, and 0.1% ≤ A3 ≤ 3%.

8. The secondary battery according to claim 1, characterized in that, B1 ≤ 3.5%, and B2 ≤ 3.5%.

9. The secondary battery according to claim 1, characterized in that, The first to fourth portions also include an adhesive, wherein the mass percentage of the adhesive in the first portion is X1 based on the total mass of the first portion, and the mass percentage of the adhesive in the second portion is X2 based on the total mass of the second portion, satisfying: X1 > X2; and Based on the total mass of the third part, the mass percentage of the adhesive in the third part is X3, and based on the total mass of the fourth part, the mass percentage of the adhesive in the fourth part is X4, satisfying: X3 > X4.

10. The secondary battery according to claim 1, characterized in that, The silicon-based material is selected from one or more of elemental silicon, silicon oxides, silicon-carbon composites, silicon-nitrogen compounds, and silicon alloys, and the graphite is selected from artificial graphite or natural graphite.

11. The secondary battery according to claim 1, characterized in that, The sum of the first thickness and the second thickness is less than or equal to the thickness of the first negative electrode active material layer, and the sum of the third thickness and the fourth thickness is less than or equal to the thickness of the second negative electrode active material layer, or The compacted density of the negative electrode sheet is 1.5-1.7 g / cm³. 3 .

12. An electrical appliance, characterized in that, Includes the secondary battery according to any one of claims 1-11.

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