Battery

Abstract

The invention provides a battery. The battery comprises a first negative plate, a first diaphragm and a first positive plate which are sequentially stacked, wherein the first positive plate is positioned on the outermost side of the battery; the first negative plate comprises a negative current collector and a negative coating covering at least one side surface of the negative current collector, and the negative coating comprises a silicon-based material; and after 50 cycles at 20-35 DEG C, the porosity of the negative electrode coating is K, and K is greater than or equal to 30% and less than or equal to 60%. The battery provided by the invention has excellent cycle performance and safety performance.

Description

Technical Field

[0001] The invention belongs to the technical field of batteries and relates to a battery. Background Art

[0002] With the rapid development of lithium-ion battery technology, people have put forward higher requirements on the energy density, cycle life and safety performance of lithium-ion batteries. Among the many battery cell structures, laminated batteries can take into account both energy density and fast charging performance, becoming the focus of people's attention. The laminated structure can also effectively solve the problems of deformation and breakage of wound lithium-ion batteries.

[0003] However, in existing laminated batteries, the outermost positive electrode sheet is coated with active material only on one side of the positive current collector. This results in uneven stress distribution on both sides of the positive electrode sheet, which can easily cause it to curl and become impossible to flatten, leading to production problems or single-sided warping of the battery cell. To combat this curling, the positive current collector of the single-sided positive electrode sheet is typically thickened. However, as the positive current collector becomes thicker, lithium ions are deposited on the negative electrode sheet, causing lithium plating, which seriously affects the battery's cycle life and safety performance. Summary of the Invention

[0004] The present invention provides a battery that controls the lithium ion migration resistance within a controllable range by limiting the porosity of the negative electrode coating after cycling, thereby avoiding the occurrence of lithium plating, and ensuring that the battery has excellent cycle life and safety performance.

[0005] In one aspect, the present invention provides a battery comprising a first negative electrode sheet, a first separator, and a first positive electrode sheet stacked in sequence, wherein the first positive electrode sheet is located at the outermost side of the battery;

[0006] The first negative electrode sheet includes a negative electrode current collector and a negative electrode coating covering at least one side of the negative electrode current collector, wherein the negative electrode coating includes a silicon-based material;

[0007] The first positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering a surface of one side of the positive electrode current collector;

[0008] After the battery is cycled 50 times at 20-35° C., the porosity of the negative electrode coating is K, and K satisfies 30%≤K≤60%.

[0009] In the battery as described above, the average sphericity of the silicon-based material is 0.5 to 1.0.

[0010] In the battery as described above, the first positive electrode sheet includes a positive electrode active material, and the particle size Dv50 of the positive electrode active material and the diameter R of the silicon-based material satisfy: 0.35≤R / Dv50≤1.32.

[0011] In the battery as described above, the particle size Dv50 of the positive electrode active material is 19 μm to 28 μm, and the diameter R of the silicon-based material is 10 μm to 25 μm.

[0012] In the battery as described above, the negative electrode coating includes N recessed structures, where N≥1.

[0013] In the battery as described above, the depth of the recessed structure is D μm, the mass percentage of the silicon-based material in the first negative electrode sheet is W%, and the following conditions are satisfied: 0.2≤D / W≤25.

[0014] In the battery as described above, the thickness of the positive electrode current collector is H μm, satisfying 0.2≤H / W≤50.

[0015] In the battery as described above, the thickness of the negative electrode active layer of the negative electrode sheet close to the first positive electrode sheet is H2 μm, and satisfies the following: 0.11≤H / H2≤1.43;

[0016] And / or, the thickness of the positive electrode active layer is T, which satisfies: 0.5≤T / H≤7.

[0017] In the battery described above, H is 10 μm to 50 μm, W is 1% to 50%, H2 is 35 μm to 90 μm, D is 10 μm to 25 μm, and T is 25 μm to 70 μm.

[0018] In the battery as described above, the width of the recessed structure is 50 μm-120 μm, and the minimum distance between any adjacent recessed structures is 0.7-2 mm.

[0019] The battery as described above further comprises an electrolyte;

[0020] The electrolyte comprises A% of fluoroethylene carbonate in terms of mass percentage, the mass percentage of the silicon-based material in the first negative electrode sheet is W%, and 0.2≤W / A≤1.75 is satisfied;

[0021] And or, based on the mass percentage, the electrolyte comprises B% of ethylene carbonate and C% of linear carboxylic acid ester, satisfying: 0.15≤B / C≤10.

[0022] The battery provided by the present invention has a certain porosity in the negative electrode coating after 50 cycles at 20-35°C, which reduces the circuitous rate of lithium ions during battery use and reduces the migration resistance of lithium ions, thereby facilitating the embedding of more lithium ions, avoiding the occurrence of lithium plating, and improving the cycle performance and safety performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A partial schematic diagram of a battery provided in a specific embodiment of the present invention;

[0024] Figure 2 A partial schematic diagram of a battery provided in another specific embodiment of the present invention.

[0025] Description of reference numerals:

[0026] 1-first positive electrode sheet; 11-positive electrode current collector; 12-positive electrode active layer; 2-first separator; 3-first negative electrode sheet; 31-negative electrode current collector; 32-negative electrode coating; 4-positive electrode sheet; 5-separator; 6-second negative electrode sheet. DETAILED DESCRIPTION

[0027] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0028] The outermost layer of the existing laminated battery is the first positive electrode sheet, that is, only one side of the positive electrode current collector is coated with positive active material, which leads to uneven stress distribution on both sides of the positive electrode sheet, easily causing it to curl up and unable to flatten, resulting in the inability to produce or the battery cell to have one-sided warping and single-sided zoning problems.

[0029] In order to resist such curling, the positive electrode current collector in the first positive electrode sheet is usually thickened, for example, the thickness of the positive electrode current collector is set to 10-50 μm, preferably 20 μm.

[0030] However, when the thickness of the positive electrode current collector is thicker, the impedance of the positive electrode current collector is lower, and thus the impedance of the first positive electrode sheet is lower, the separation rate of lithium ions increases, and the number of lithium ions migrating to the negative electrode per unit time increases. Therefore, the negative electrode sheet needs to provide more space to accommodate the lithium ions separated from the positive electrode. If the accommodation space is insufficient, the lithium ions will be deposited on the negative electrode sheet, resulting in lithium plating, which will affect the cycle performance and safety performance of the battery.

[0031] To this end, the present invention provides a battery, such as Figure 1 As shown, it includes a first negative electrode sheet 3, a first separator 2, and a first positive electrode sheet 1 stacked in sequence, and the first positive electrode sheet 1 is located at the outermost side of the battery;

[0032] The first negative electrode sheet 3 includes a negative electrode current collector 31 and a negative electrode coating 32 covering at least one side of the negative electrode current collector 31, wherein the negative electrode coating 32 includes a silicon-based material;

[0033] The first positive electrode sheet 1 includes a positive electrode current collector 11 and a positive electrode active layer 12 covering one side surface of the positive electrode current collector 11;

[0034] After the battery is cycled for 50 cycles at 20-35° C., the porosity of the negative electrode coating 32 is K, and K satisfies 30%≤K≤60%.

[0035] The battery in the present invention refers to a laminated battery, and the laminated battery is composed of alternating positive electrodes, negative electrodes and separators. The first positive electrode is located on the outermost side of the battery, which means that the first positive electrode is located on the outermost layer of the entire stacking structure, directly in contact with the battery casing or external circuit, and is responsible for current collection and transmission.

[0036] In detail, the battery provided by the present invention may include, in addition to the first negative electrode sheet 3, the first separator 2, and the first positive electrode sheet 1 stacked in sequence, other stacked negative electrode sheets, separators, and positive electrode sheets.

[0037] For example, in one embodiment, the structural diagram of the battery is as follows Figure 2 As shown, the outermost sides of the battery are the first positive electrode sheet 1, the first negative electrode sheet 3, the first diaphragm 2 and the first positive electrode sheet 1 stacked in sequence along a position away from the center of the stack, and the position close to the center of the stack also includes a positive electrode sheet 4, a diaphragm 5, and a second negative electrode sheet 6 stacked in a circular manner.

[0038] The present invention does not limit the specific structure of the positive electrode sheet 4 and the second negative electrode sheet 6. In one specific embodiment, the positive electrode sheet 4 includes a second positive electrode collector and a positive electrode active layer arranged on at least one side of the second positive electrode collector, and the second negative electrode sheet 6 includes a second negative electrode collector and a negative electrode active layer arranged on at least one side of the second negative electrode collector.

[0039] The first positive electrode sheet in the present invention refers to a positive electrode sheet having a positive electrode active layer disposed only on one side of the positive electrode current collector.

[0040] The present invention does not make any special limitation on the cycling rules of the 50 cycles, as long as the cycle is performed within the allowable range of the battery (the battery is within the allowable range without lithium deposition, fire, bloating, explosion, etc.). In some embodiments, the cycling rules of the 50 cycles are: at 20-35°C, the battery is charged to 5V at 0.7C-3.7C, discharged to 0.05C, and then discharged to 3.0V at 0.5C. This is the first cycle, and this cycle is repeated until the 50th cycle.

[0041] In the present invention, the direction perpendicular to the thickness of the first negative electrode sheet is defined as the X direction, the direction perpendicular to the thickness of the first negative electrode sheet is defined as the Y direction, and the direction perpendicular to the plane formed by the X and Y directions is defined as the Z direction. The first negative electrode sheet or a partial region thereof is defined as a rectangular parallelepiped region, the length of the rectangular parallelepiped region in the X direction is denoted as a, the length of the rectangular parallelepiped region in the Y direction is denoted as H, and the length of the rectangular parallelepiped region in the Z direction is denoted as b; the volume of the rectangular parallelepiped region Vtotal = a×b×H; and the horizontal area S of the rectangular parallelepiped region in the X and Z directions is denoted as a×b.

[0042] Referring to the volume calculation method of the rectangular area, the porosity of the negative electrode coating is measured in the following way: using SEM to obtain any cross-section of the negative electrode sheet in the thickness direction, taking a three-dimensional electron microscope image containing the cross-section, and using Image J software to obtain the total volume of pores (Vpore) and the total volume of active materials (Vactive) in the three-dimensional electron microscope image, and using Vpore / Vactive to calculate the porosity of the negative electrode coating.

[0043] In one specific embodiment, a is 50 to 200 μm, and b is 50 to 200 μm.

[0044] In detail, the porosity of the negative electrode coating 32 includes but is not limited to 30%, 32%, 35%, 37%, 40%, 42%, 45%, 47%, 50%, 52%, 55%, 57%, 59%, 60% or a range between any two thereof.

[0045] During the cycle, due to the expansion and contraction characteristics of the negative electrode material itself, that is, the negative electrode material embeds lithium ions during charging and releases lithium ions during discharging, the volume of the negative electrode material will change. When multiple particles in the negative electrode sheet expand and shrink in volume at the same time, the adhesive on the surface of the particles will be pulled, causing fatigue changes and failure of the adhesive, resulting in an increase in the porosity of the negative electrode sheet, and ultimately a decrease in the structural integrity of the negative electrode material, affecting the embedding and de-embedding efficiency of lithium ions, which may lead to battery capacity decay and even lithium plating.

[0046] The battery provided by the present invention can effectively improve the problem of lithium plating and improve the cycle life and safety performance of lithium batteries. On the one hand, by controlling the porosity in the negative electrode coating after the cycle within a certain range, space can be reserved for the volume expansion of the silicon-based material during the cycle, reducing mechanical stress. At the same time, the negative electrode material undergoes a smaller volume change during the cycle, and less expansion, pulling, and extrusion and slippage occur between the particles. At this time, the adhesive on the surface of the negative electrode material can firmly grasp the active material so that it does not squeeze and slip between the particles, allowing it to reduce the volume expansion outside the particles, thereby preventing the electrode structure from collapsing and maintaining the integrity of the battery structure. On the other hand, the appropriate porosity range can reduce the migration resistance of lithium ions, reduce the circuitous rate of lithium ions, facilitate the embedding of more lithium ions, and avoid lithium plating. Moreover, the porous structure enhances the penetration of the electrolyte, ensures the uniform transmission of lithium ions, avoids lithium plating caused by excessive local concentration gradients, and thus improves the cycle stability and safety performance of the battery.

[0047] In some embodiments, in order to ensure that the battery meets the following requirements: after 50 cycles at 20-35°C, the porosity of the negative electrode coating is 27% to 52%, the porosity range of the negative electrode coating can be adjusted by regulating at least one of the sphericity of the silicon-based material, the particle size of the positive electrode active material and the diameter of the silicon-based material, and the relationship between the two, the structure of the negative electrode coating, the thickness of the negative electrode active layer, the thickness of the positive electrode current collector, the thickness of the positive electrode active layer, and the composition of the electrolyte. In detail, as a negative electrode active material, the sphericity of the silicon-based material also has a certain impact on the cycle performance and safety performance of the battery. In one specific embodiment, the average sphericity of the silicon-based material is 0.5 to 1.0.

[0048] In the present invention, sphericity characterizes the morphological characteristics of particles. The closer the particle is to a sphere in morphology, the closer its sphericity is to 1. Calculation of average sphericity typically involves measuring the sphericity of a group of particles or objects and then averaging these measured values.

[0049] The sphericity of the present invention is determined by discharging a lithium-ion secondary battery to 0% SOC, disassembling and removing the first negative electrode sheet, soaking it in a dimethyl carbonate (DMC) solvent for 12 hours, then rinsing it with DMC to remove lithium salts attached to the sheet. The negative electrode active material layer is then rinsed from the sheet with deionized water. After ultrasonic treatment, the filtrate is removed by centrifugation and then air-dried. The resulting powder is observed using a scanning electron microscope in backscattered mode. In this mode, the silicon-based material has a brighter contrast and can be distinguished from graphite and conductive carbon. Using image processing software (such as Image Pro Plus), the image of each brighter particle in the SEM photograph at a certain magnification (e.g., 2500x) is analyzed to obtain the perimeter and area of ​​each particle. The perimeter equivalent radius r1 and area equivalent radius r2 of each particle are calculated, respectively. The sphericity of each particle is then calculated as S = r2 / r1. The sphericity of each particle is then weighted averaged to obtain the average sphericity of the silicon-based material in the first negative electrode sheet.

[0050] In detail, the average sphericity of the silicon-based material includes but is not limited to 0.5, 0.6, 0.8, 1.0 or a range between any two thereof.

[0051] The appropriate sphericity not only ensures the uniform distribution of particles and low contact resistance, but also provides sufficient specific surface area and pore structure, optimizing ion transport and electrolyte infiltration.

[0052] In a specific embodiment, the first positive electrode sheet includes a positive electrode active material, and the particle size Dv50 of the positive electrode active material and the diameter R of the silicon-based material satisfy: 0.35≤R / Dv50≤1.32.

[0053] The particle size Dv50 of the positive electrode active material refers to the particle size value when the cumulative volume fraction reaches 50% in the particle size distribution of the positive electrode active material, reflecting the average particle size of the positive electrode active material. The particle size Dv50 of the positive electrode active material in the present invention is expressed in μm.

[0054] The diameter R of the silicon-based material in the present invention refers to the average equivalent diameter of the silicon-based material, which is expressed in μm and can be measured by common measurement methods in the art, such as laser particle size analysis.

[0055] In detail, R / Dv50 includes but is not limited to 0.35, 0.4, 0.6, 0.8, 1.0, 1.2, 1.32 or a range between any two thereof.

[0056] When the particle size Dv50 of the positive electrode active material and the diameter R of the silicon-based material satisfy the above relationship, it can not only ensure that the positive electrode active material and the negative electrode active material maintain good interface contact during the charge and discharge process, reduce the interface impedance, and improve the rate performance of the battery, but also by controlling R / Dv50, it can better adapt to the volume change of the silicon-based material and reduce the stress concentration and damage of the electrode structure.

[0057] The present invention does not limit the specific type of positive electrode active material, and common positive electrode active materials in the field can be selected. In a specific embodiment, the positive electrode active material is at least one of lithium cobalt oxide and a ternary material, and the positive electrode active material is doped with at least one of Al, Zr, Mg, Y, Ti, B, Sr, W, Si, La, and Nb.

[0058] In another embodiment, the positive electrode active material is doped with Al, Mg, and Zr, with the Al+Mg+Zr content ranging from 1000 to 7000 ppm. Doping with Al, Mg, and Zr improves ionic conductivity, reduces material migration resistance, prevents lithium plating, and increases the cycle life of lithium-ion batteries.

[0059] In another specific embodiment, the positive electrode active material is doped with Ni. After 10 T of cycling, the battery is discharged to 3.0 V and disassembled. The negative electrode powder is then tested for ICP, where Ni can be detected. The Ni content is 2-7%. This Ni content results from the migration of trace transition metal elements from the positive electrode material during charge and discharge, which participate in the formation of the SEI on the surface of the negative electrode active material. Ni improves the stability of the SEI, reduces SEI damage and new SEI formation during cycling, reduces electrolyte consumption, and thus increases the cycle life of the lithium battery.

[0060] In a specific embodiment, the particle size Dv50 of the positive electrode active material is 19 μm to 28 μm, and the diameter R of the silicon-based material is 10 μm to 25 μm.

[0061] Specifically, the particle size Dv50 of the positive electrode active material includes, but is not limited to, 19 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, or any range therebetween. The diameter R of the silicon-based material includes, but is not limited to, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, 22 μm, 24 μm, 25 μm, or any range therebetween.

[0062] Furthermore, in a specific embodiment of the present invention, the negative electrode coating includes N recessed structures, where N≥1.

[0063] The present invention does not limit the specific shape of the recessed structure, and the shape of the recessed structure can be adjusted according to actual needs. For example, along the stacking direction of the negative electrode current collector and the negative electrode coating, the cross-section of the recessed structure can be circular, triangular, sector-shaped, etc. The negative electrode coating can be provided with the recessed structure by conventional methods in the art. For example, mechanical drilling, laser drilling, electric or thermal beam melting, radiation melting, chemical etching, friction drilling, etc. can be used.

[0064] In a specific embodiment, along the stacking direction of the negative electrode current collector and the negative electrode coating, the cross-section of the recessed structure is V-shaped.

[0065] Silicon-based materials will undergo significant volume expansion (up to 300%) during the charge and discharge process, leading to damage to the electrode structure and capacity attenuation. The concave structure provides a buffer space for the volume expansion of silicon-based materials, reduces stress concentration inside the coating, and thus improves the cycle stability of the electrode. The concave structure increases the specific surface area of ​​the negative electrode coating, making it easier for the electrolyte to penetrate into the coating, improving the diffusion efficiency of lithium ions, and promoting electrolyte infiltration and ion transmission. Moreover, the concave structure can disperse the stress inside the coating, reduce cracking and shedding of the coating during charge and discharge, and thus ensure the cycle stability and safety performance of the battery.

[0066] In a specific embodiment, the depth of the concave structure is D μm, the mass percentage of the silicon-based material in the first negative electrode sheet is W%, and the following ratio is satisfied: 0.2≤D / W≤25.

[0067] The depth of the recessed structure refers to the average vertical distance from the outermost surface of the recessed portion to its deepest point along the thickness direction of the first negative electrode sheet. The depth of the recessed structure can be measured by microscopic observation, scanning electron microscope (SEM) imaging or other surface profile measurement techniques.

[0068] The mass percentage of the silicon-based material in the first negative electrode sheet refers to the mass proportion of the silicon-based material based on the total mass of the first negative electrode sheet.

[0069] In detail, D / W includes but is not limited to 0.2, 0.5, 1, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 25 or a range between any two thereof.

[0070] The depth of the recessed structure needs to match the silicon content of the base material to ensure the structural integrity and functionality of the electrode. Deeper recesses may be suitable for higher silicon content to provide sufficient buffer space, while shallower recesses may be suitable for lower silicon content.

[0071] Furthermore, in a specific embodiment of the present invention, the first positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering a surface of one side of the positive electrode current collector;

[0072] The thickness of the positive electrode current collector is H μm, satisfying 0.2≤H / W≤50.

[0073] The present invention does not limit the specific type of the positive electrode current collector, and common types in the art, such as copper foil and aluminum foil, can be selected.

[0074] In detail, the thickness H of the positive electrode current collector and the mass percentage W of the silicon-based material in the first negative electrode sheet satisfy 0.2≤H / W≤50. Specifically, H / W includes but is not limited to 0.2, 0.5, 1, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50 or a range between any two of them.

[0075] When H / W is lower than 0.2, the cathode current collector is too thin, which may lead to insufficient mechanical strength and affect the cycle life and safety of the battery. When H / W is higher than 50, the cathode current collector is too thick, which may increase the weight and volume of the battery and reduce the energy density.

[0076] Furthermore, in a specific embodiment of the present invention, the thickness of the negative electrode active layer of the first negative electrode sheet close to the first positive electrode sheet is H2 μm, satisfying: 0.11≤H / H2≤1.43.

[0077] In the present invention, the thickness of the negative electrode active layer of the first negative electrode sheet close to the first positive electrode sheet refers to the average thickness of the negative electrode active layer.

[0078] Specifically, the thickness H of the positive current collector in the first positive electrode sheet and the thickness H2 of the negative active layer in the first negative electrode sheet adjacent to the first positive electrode sheet satisfy the following: 0.11 ≤ H / H2 ≤ 1.43. Specifically, H / H2 includes, but is not limited to, 0.11, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.43, or any range therebetween.

[0079] The silicon-based material in the first negative electrode sheet undergoes significant volume changes during charge and discharge. An appropriate thickness ratio can mitigate damage to the electrode structure caused by volume expansion. Furthermore, a reasonable ratio between the current collector thickness and the active layer thickness helps reduce the battery's internal resistance, ensuring a good capacity match between the positive and negative electrodes, avoiding performance losses caused by excessive thickness or thinness of one electrode, and improving the battery's power performance.

[0080] In another specific embodiment, the thickness of the positive electrode active layer is T, which satisfies: 0.5≤T / H≤7.

[0081] Specifically, T / H includes but is not limited to 0.5, 1, 2, 3, 4, 5, 6, 7 or a range between any two thereof.

[0082] When the thickness of the positive electrode active layer and the thickness of the positive electrode current collector meet the above ratio, the thickness of the positive electrode active layer and the current collector can be matched, avoiding the difficulty of lithium ion diffusion caused by an overly thick active layer or the loss of energy density caused by an overly thick current collector. In addition, the appropriate ratio ensures that the thickness of the positive electrode current collector provides sufficient support for the positive electrode active layer, preventing deformation or fracture of the first positive electrode sheet during charge and discharge.

[0083] Furthermore, in a specific embodiment, H is 10 μm to 50 μm, W is 1% to 50%, H2 is 35 μm to 90 μm, D is 10 μm to 25 μm, and T is 25 μm to 70 μm.

[0084] Specifically, H includes but is not limited to 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, 40 μm, 45 μm, 50 μm or a range between any two of them, W includes but is not limited to 1%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or a range between any two of them, H2 includes but is not limited to 35 μm, 40 μm, 45 μm, 50 μm, 55 μm, 60 μm, 65 μm, 70μm, 75μm, 80μm, 85μm, 90μm or a range between any two of them, D includes but is not limited to 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, 22μm, 24μm, 25μm or a range between any two of them, and T includes but is not limited to 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, 65μm, 70μm or a range between any two of them.

[0085] Furthermore, in a specific embodiment, the width of the concave structure is 50 μm-120 μm, and the minimum distance between any adjacent concave structures is 0.7-2 mm.

[0086] The width of the recessed structure in the present invention refers to the maximum distance along the extension direction of the negative electrode sheet when the recessed structure is cross-sectioned along the thickness direction of the negative electrode sheet. For example, when the recessed structure is V-shaped, the distance between the opening of the V-shape and the extension direction of the negative electrode sheet is the width of the recessed structure.

[0087] In detail, the width of the concave structure includes but is not limited to 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, or a range between any two thereof.

[0088] When the width of the recessed structure is within the above range, it is sufficient to accommodate the volume expansion of the silicon-based material without causing the negative electrode coating structure to be too loose. Moreover, the width within this range can provide sufficient buffer space without weakening the overall mechanical strength of the negative electrode coating. At the same time, the width within the above range can increase the surface area of ​​the electrode, provide more infiltration channels for the electrolyte, improve the ion transmission efficiency, and improve the rate performance and charge and discharge performance of the battery.

[0089] The minimum distance between any adjacent recessed structures refers to the shortest distance between adjacent recessed structures along the extension direction of the negative electrode sheet. Specifically, the minimum distance between any adjacent recessed structures includes but is not limited to 0.7 mm, 1 mm, 1.2 mm, 1.4 mm, 1.6 mm, 1.8 mm, 2.0 mm or a range between any two of them.

[0090] When the minimum distance between any two adjacent recesses is within the above range, the appropriate spacing ensures uniform distribution of the recesses, avoiding local stress concentration and thus reducing the risk of coating cracking or shedding. Furthermore, the appropriate spacing ensures that the electrolyte is evenly distributed across the entire electrode surface, avoiding localized insufficient wetting, thereby ensuring the battery's cycling stability and safety.

[0091] Furthermore, in a specific embodiment of the present invention, it further includes an electrolyte;

[0092] The electrolyte comprises A% of fluoroethylene carbonate, and the mass percentage of the silicon-based material in the first negative electrode sheet is W%, satisfying 0.2≤W / A≤1.75.

[0093] In detail, W / A includes but is not limited to 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.75 or a range between any two thereof.

[0094] When W / A is between 0.2 and 1.75, it can balance the stability of the electrolyte and the performance of the silicon-based material, avoiding the decrease in ion transmission efficiency caused by excessive FEC content or the volume expansion problem caused by excessive silicon-based material content.

[0095] In one embodiment, A is 5 to 20, that is, the mass percentage of fluoroethylene carbonate in the electrolyte includes but is not limited to 5%, 10%, 15%, 20% or any range between two of them.

[0096] In another specific embodiment of the present invention, the electrolyte comprises B% of ethylene carbonate (EC) and C% of linear carboxylic acid ester, in terms of mass percentage, satisfying the following relationship: 0.15≤B / C≤10.

[0097] The linear carboxylic acid esters in the present invention include, but are not limited to, one or more of ethyl methyl carbonate (EMC), diethyl carbonate (DEC), and dimethyl carbonate (DMC).

[0098] Adding EC to the electrolyte will increase the viscosity. It is necessary to add some low-viscosity linear carbonates such as EMC, DEC, and DMC to adjust the viscosity of the electrolyte, enable the lithium ions to be quickly embedded, and reduce the difference in lithium ion concentration inside and outside the particles.

[0099] In detail, B / C includes but is not limited to 0.15, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or a range between any two thereof.

[0100] The ratio of ethylene carbonate (EC) to linear carboxylate affects the ionic conductivity of the electrolyte. A proper ratio optimizes ion transport and improves overall battery performance. It also prevents electrolyte decomposition or side reactions at high temperatures or during prolonged use, helping to reduce electrolyte flammability and explosion risks, thereby ensuring battery cycle stability and safety.

[0101] In one embodiment, B is 10 to 50, and C is 5 to 65. Specifically, the mass percentage of ethylene carbonate is 10% to 50%, and the mass percentage of linear carboxylic acid ester is 5% to 65%.

[0102] Hereinafter, the battery provided by the present invention will be described in detail through specific embodiments.

[0103] Example 1

[0104] The method for preparing the battery provided in this embodiment comprises the following steps:

[0105] 1. Preparation of positive electrode sheet: The positive electrode material (lithium cobalt oxide), conductive agent (conductive carbon black), and binder (PVDF) are mixed in a mass ratio of 97:1:2, and then NMP solvent is added to prepare a positive electrode slurry; the positive electrode slurry is then coated on a specific aluminum foil current collector using a sprayer to obtain a positive electrode sheet; the above-mentioned positive electrode sheet is baked at high temperature and then cooled to room temperature in an environment with a humidity of less than 5%; then rolled and then cut according to the requirements of the battery cell model to obtain the finished positive electrode sheet.

[0106] 2. Preparation of negative electrode sheet: The negative electrode material (graphite and silicon-carbon negative electrode material with a mass ratio of 70:30), the conductive agent (conductive carbon black + conductive carbon tube with a mass ratio of 1:1), the binder (PAA), and the thickener (CMC-Li) are mixed in a mass ratio of 95:1.5:2.8:0.7, and then a deionized solvent is added to prepare a negative electrode slurry; the negative electrode slurry is then coated on a specific copper foil current collector using a sprayer to obtain a negative electrode sheet; the above-mentioned negative electrode sheet is baked at a high temperature and then cooled to room temperature in an environment with a humidity of <5%; then rolled and cut according to the requirements of the battery cell model to obtain a finished negative electrode sheet; then a concave portion is etched on the negative electrode sheet using a laser to obtain the final negative electrode sheet.

[0107] 2. Diaphragm: Use commercially available diaphragms, with a base material of 5μm thick polyethylene (PE) porous film, and coat both surfaces with a 2um thick glue solution mixed with PVDF and ceramics as the diaphragm.

[0108] 3. Electrolyte: Ethyl methyl carbonate and diethyl carbonate were mixed in a mass ratio of 60:30 to obtain an organic solution. A certain amount of ethylene carbonate, fluoroethylene carbonate, and linear carboxylate were then added as shown in Table 1. Lithium hexafluorophosphate was then added to the organic solvent to dissolve and mix thoroughly to obtain an electrolyte with a lithium salt concentration of 1.0 mol / L.

[0109] The prepared positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order to form a core stack. The core stack is placed in an aluminum-plastic film bag, dried, and then filled with electrolyte. The battery is then allowed to stand and undergoes a formation process to produce a lithium-ion battery. The formation process is as follows: 15 minutes of constant current charging at 0.2C, followed by 45 minutes of constant current charging at 1.0C, to obtain the final battery.

[0110] The preparation methods of the batteries provided in Examples 2 to 24 and Comparative Examples 1 and 2 are basically the same as the preparation method provided in Example 1, with some parameters adjusted, as shown in Table 1 for details.

[0111] Test Case

[0112] Performance tests were performed on the batteries provided in all the embodiments and comparative examples.

[0113] 1. Porosity of negative electrode coating

[0114] At 25°C, the lithium-ion battery was charged at 1.5C to 5V and then discharged at 0.05C to 3.0V. This cycle was repeated until the 50th cycle. The battery was disassembled, and any cross-section of the negative electrode sheet in the thickness direction was obtained using SEM. A three-dimensional electron microscope image containing the cross-section was taken. Image J software was used to obtain the total volume of pores (Vpore) and the total volume of active material (Vactive) of the negative electrode sheet or a part of it in the three-dimensional electron microscope image. The porosity of the negative electrode coating was calculated using Vpore / Vactive. This was repeated three times and the average value was calculated. See Table 1 for details.

[0115] 2. Cycle performance test

[0116] Test method: At 25°C, charge and discharge the lithium-ion battery at a rate of 1.5C to 4.5V and then 0.05C / 0.5C to 3.0V. Record the discharge capacity Q2 of the 800th charge and discharge cycle and the discharge capacity Q1 of the first charge and discharge cycle. Capacity retention = Q2 / Q1×100%. Specific results are shown in Table 2.

[0117] 3. Cyclic expansion

[0118] Test method: At 25°C, a lithium-ion battery was charged at 1.5C to 3.9V and then discharged at 0.05C using a 600g ppg tester to measure the cell thickness T1. The lithium-ion battery was then charged and discharged 800 times at a rate of 1.5C to 4.5V and then discharged at 0.05C / 0.5C to 3.0V. Finally, the lithium-ion battery was charged at 1.5C to 4.5V and then discharged at 0.05C using a 600g ppg tester to measure the cell thickness T2. The expansion rate of the lithium battery was calculated as follows: (T2 / T1-1)×100%. Specific results are shown in Table 2.

[0119] 4. Furnace temperature test

[0120] The lithium battery was charged at 1.5C to 4.5V and then cut off at 0.05C in a constant temperature box. The temperature was raised to 130℃ at a rate of 2℃ / min and maintained for 30 minutes to confirm the fire status of the lithium battery. If the lithium battery did not catch fire, it was considered to have good safety performance. If it caught fire, it was considered to have bad safety performance. For details, see Table 2.

[0121] Table 1

[0122]

[0123]

[0124] Table 2

[0125]

[0126]

[0127] As can be seen from Table 1-2, compared with the comparative example, the negative electrode sheet of the embodiment can better improve the cycle performance, expansion rate and safety performance of the battery by controlling the porosity of the negative electrode coating after 50 cycles of the battery to be in the range of 30%-60%.

[0128] Furthermore, by comparing the data of Examples 1-4, it can be seen that appropriate sphericity can further ensure the cycle performance, expansion rate and safety performance of the battery.

[0129] Furthermore, by comparing the data of Examples 5-19, it can be seen that regulating any one of the ratio of the diameter of the silicon-based material to the particle size of the positive electrode active material, the ratio of the depth of the concave structure of the negative electrode sheet to the mass percentage of the silicon-based material, the ratio of the mass percentage of the silicon-based material of the negative electrode sheet to the thickness of the positive electrode current collector, the ratio of the thickness of the positive electrode current collector to the thickness of the negative electrode active layer, and the ratio of the thickness of the positive electrode active layer to the thickness of the positive electrode current collector can further regulate the cycle performance, expansion rate and safety performance of the battery.

[0130] Furthermore, by comparing the data of Examples 22-24, it can be seen that the components of the battery electrolyte affect the porosity range of the negative electrode coating after 50 cycles of the battery, and thus regulating the components of the battery electrolyte can further regulate the cycle performance, expansion rate and safety performance of the battery.

[0131] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A battery, characterized in that: The battery comprises a first negative electrode sheet, a first separator, and a first positive electrode sheet stacked in sequence, wherein the first positive electrode sheet is located at the outermost side of the battery; The first negative electrode sheet includes a negative electrode current collector and a negative electrode coating covering at least one side of the negative electrode current collector, wherein the negative electrode coating includes a silicon-based material; The first positive electrode sheet includes a positive electrode current collector and a positive electrode active layer covering a surface of one side of the positive electrode current collector; After the battery is cycled 50 times at 20-35° C., the porosity of the negative electrode coating is K, and K satisfies 30%≤K≤60%.

2. The battery according to claim 1, characterized in that The average sphericity of the silicon-based material is 0.5-1.

0.

3. The battery according to claim 1 or 2, characterized in that The first positive electrode sheet includes a positive electrode active material, and the particle size Dv50 of the positive electrode active material and the diameter R of the silicon-based material satisfy the following: 0.35≤R / Dv50≤1.

32.

4. The battery according to claim 3, characterized in that The particle size Dv50 of the positive electrode active material is 19 μm to 28 μm, and the diameter R of the silicon-based material is 10 μm to 25 μm.

5. The battery according to any one of claims 1 to 4, characterized in that The negative electrode coating comprises N concave structures, where N is greater than or equal to 1; The depth of the concave structure is D μm, the mass percentage of the silicon-based material in the first negative electrode sheet is W%, and the following conditions are satisfied: 0.2≤D / W≤25.

6. The battery according to claim 5, characterized in that The thickness of the positive electrode current collector is H μm, satisfying 0.2≤H / W≤50.

7. The battery according to claim 6, characterized in that The thickness of the negative electrode active layer of the first negative electrode sheet close to the first positive electrode sheet is H2 μm, satisfying the following: 0.11≤H / H2≤1.43; And / or, the thickness of the positive electrode active layer is T, which satisfies: 0.5≤T / H≤7.

8. The battery according to claim 7, characterized in that H is 10μm~50μm, W is 1%~50%, H2 is 35μm~90μm, D is 10μm~25μm, and T is 25μm~70μm.

9. The battery according to any one of claims 5 to 8, characterized in that: The width of the concave structure is 50 μm-120 μm, and the minimum distance between any adjacent concave structures is 0.7-2 mm.

10. The battery according to any one of claims 1 to 9, characterized in that Also includes electrolyte; The electrolyte comprises A% of fluoroethylene carbonate in terms of mass percentage, the mass percentage of the silicon-based material in the first negative electrode sheet is W%, and 0.2≤W / A≤1.75 is satisfied; And or, based on the mass percentage, the electrolyte comprises B% of ethylene carbonate and C% of linear carboxylic acid ester, satisfying: 0.15≤B / C≤10.

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