Negative plate and lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a negative plate and a lithium ion secondary battery comprising the same. The negative plate comprises a negative active coating, the negative active coating comprises a silicon-based material, and the silicon-based material comprises silicon-carbon particles and a coating layer located on the outer surfaces of the silicon-carbon particles; the coating layer comprises a first coating layer and a second coating layer; the second coating layer at least comprises lithium metaaluminate; based on the total weight of the silicon-based material, the mass content of Al in the second coating layer is A, and the unit is ppm; a plurality of first concave parts are arranged on the surface of the negative electrode active coating on at least one side surface of the negative electrode current collector, the depth of each first concave part is H, and the unit is mu m; and the mass content A of Al in the silicon-based material and the depth H of the first concave part meet the condition that H / A is more than or equal to 0.002 and less than or equal to 0.669. According to the lithium ion secondary battery, the structural stability of the negative plate is remarkably improved, and the lithium ion secondary battery has excellent rate capability and cycle performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a negative electrode and a lithium-ion secondary battery including the negative electrode. Background Technology

[0002] In recent years, with the popularization of electric vehicles and portable devices, the requirements for the energy density of lithium batteries have become increasingly higher. Graphite, as the anode material for lithium batteries, can no longer meet the market's demand for energy density. Silicon-based materials are typically incorporated into the active coating of the anode. Utilizing the high theoretical specific capacity of silicon (up to 4200 mAh / g at high temperatures and approximately 3580 mAh / g at room temperature), the battery energy density can be increased to 300 Wh / kg. -1 That's all. However, silicon-based materials undergo significant volume expansion during charging and discharging, causing the solid electrolyte interphase (SEI) film on their surface to repeatedly rupture and regenerate, disrupting the stability of the electrode structure. This process irreversibly consumes the active lithium and electrolyte within the battery, leading to rapid capacity decay and poor cycle stability. Furthermore, the inherently poor conductivity of silicon-based materials directly impacts the battery's rate performance. Summary of the Invention

[0003] The purpose of this invention is to overcome the aforementioned problems in the prior art and to provide a negative electrode and a lithium-ion secondary battery including the negative electrode. By improving the negative electrode, this invention effectively buffers the volume expansion caused by silicon-based materials, enhances the structural stability of the negative electrode, and thus enables the lithium-ion secondary battery (hereinafter referred to as the battery) to possess both excellent rate performance and cycle performance.

[0004] This invention provides several first recesses on the surface of the negative electrode active coating, which allows for the release of volume expansion of the negative electrode active material and shortens the contact distance between the electrolyte and the negative electrode active material, increasing electrolyte wetting, reducing interfacial impedance, and improving lithium-ion migration rate, thus improving the battery's rate performance to some extent. However, laser wire bonding damages the integrity of silicon-based material particles. The large amount of silicon-based material exposed at the wire groove locations increases side reactions during battery cycling, leading to continuous electrolyte decomposition, continuous loss of active lithium, and capacity decay.

[0005] To address this issue, the present invention provides a coating layer on the outer surface of silicon-carbon particles. This coating layer includes a first coating layer and a second coating layer, with the first coating layer positioned between the silicon-carbon particles and the second coating layer. The first coating layer effectively buffers the volume expansion of the silicon-carbon particles during lithium intercalation and provides a stable ion-conducting channel, improving electronic conductivity and thus enhancing the structural stability of the anode material and the rate performance of the battery. The second coating layer, containing elemental Al (lithium aluminate), possesses a high thermal decomposition temperature, maintaining structural integrity during laser wire bonding. This effectively blocks laser damage to the silicon-carbon particles, reduces fresh interfaces caused by wire bonding exposure, and alleviates direct contact between the electrolyte and silicon-based materials from the source. This reduces interfacial side reactions and slows down the rupture and regeneration of the SEI film. By ensuring that the battery's rate performance remains unaffected, this invention effectively mitigates the continuous consumption of active lithium and electrolyte decomposition, significantly improving the battery's cycle stability.

[0006] Building upon this foundation, the inventors of this invention further discovered that by adjusting the mass content A (in ppm) of elemental Al in the second coating layer and the depth H (in μm) of the first recess on the surface of the negative electrode active coating, the H / A ratio can be made to meet a specific range, thereby further improving the cycle performance and rate performance of the battery. The reason is that when H / A is within a specific range, the depth of the first recess is moderate, which can fully accommodate the huge volume expansion of the silicon-based material during charging and discharging, preventing the silicon-carbon particles from being squeezed, cracked, or detached due to insufficient space. Simultaneously, an appropriate amount of elemental Al ensures that the second coating layer has a high thermal decomposition temperature, forming a dense and stable barrier during laser wire bonding and long-term battery cycling, effectively blocking the laser from damaging the silicon-carbon particles, reducing direct contact between the electrolyte and the silicon-based material, and suppressing side reactions. If the H / A ratio is too high (e.g., greater than 0.669), meaning H is too high and A is too low, although there is sufficient buffer space for the volume expansion of the silicon-based material, the protection of the second coating layer is insufficient. Laser wire bonding will severely damage the integrity of the silicon-based material particles, and the large amount of exposed silicon-based material will trigger side reactions, leading to continuous loss of active lithium and capacity decay. Conversely, if the H / A ratio is too low (e.g., less than 0.002), meaning H is too low and A is too high, there is insufficient buffer space for the volume expansion of the silicon-based material, and internal stress accumulates, causing the silicon-carbon particles to be squeezed, cracked, and pulverized. The negative electrode active coating will detach from the current collector, severely damaging the negative electrode structure. At the same time, the continuously cracked surface will continue to react with the electrolyte, causing the SEI film to repeatedly crack and regenerate, excessively consuming active lithium and electrolyte, ultimately leading to rapid capacity decay and a significant reduction in cycle life of the battery.

[0007] Therefore, this invention, by controlling the mass content of element Al in the second coating layer and the depth of the first recess, achieves a synergistic effect, enabling the negative electrode active coating to effectively improve the battery's cycle stability and rate performance while introducing silicon-based materials to increase battery energy density. When the mass content of element Al in the second coating layer is high, it means that the protective effect of the second coating layer is stronger. In this case, the depth of the first recess can be appropriately increased to obtain a larger expansion space without significantly increasing side reactions. Conversely, when the mass content of element Al in the second coating layer is low, the protective effect of the second coating layer is relatively weaker. Therefore, the depth of the first recess needs to be appropriately reduced to avoid more silicon-based materials being exposed due to excessive depth, thereby maintaining the overall structural stability of the negative electrode sheet.

[0008] Based on this, the inventors of this invention propose the following solution: The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative electrode current collector and a negative electrode active coating located on at least one side surface of the negative electrode current collector, the negative electrode active coating comprising a silicon-based material, the silicon-based material comprising silicon-carbon particles and a coating layer located on the outer surface of the silicon-carbon particles; the coating layer comprising a first coating layer and a second coating layer; the first coating layer being located between the silicon-carbon particles and the second coating layer; the second coating layer comprising at least lithium aluminate; the second coating layer containing element Al; the mass content of Al is A, in ppm, based on the total weight of the silicon-based material; a plurality of first recesses are provided on the surface of the negative electrode active coating on at least one side surface of the negative electrode current collector, the depth of the first recesses being H, in μm; the mass content of Al A in the silicon-based material and the depth H μm of the first recesses satisfy: 0.002 ≤ H / A ≤ 0.669.

[0009] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the negative electrode sheet described in the first aspect of the present invention.

[0010] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) By improving the negative electrode sheet, the present invention can effectively buffer the huge volume expansion generated by silicon-based materials and improve the structural stability of the negative electrode sheet; (2) The lithium-ion secondary battery of the present invention has excellent cycle stability and rate performance while ensuring high energy density.

[0011] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0012] Figure 1 The diagram shown is a schematic diagram of the silicon-based material in an example of the present invention.

[0013] Figure 2 The diagram shown is a schematic diagram of the first recessed portion of the negative electrode sheet in an embodiment of the present invention.

[0014] Figure 3 The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention. Figure 3 (a) The grooves are continuously arranged; Figure 3 (b) The groove is segmented.

[0015] Figure 4 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 4 (a)- Figure 4 In (c), the two long sides of the groove are straight lines. Figure 4 (d) has two long sides of the groove that are curved.

[0016] Figure 5 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 5 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 5 (b) represents the case where two adjacent long sides are straight lines and not parallel. Figure 5 (c) is the case where two adjacent long sides are curves. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] The first aspect of the present invention provides a negative electrode sheet, the negative electrode sheet comprising a negative current collector and a negative active coating located on at least one side surface of the negative current collector, the negative active coating comprising a silicon-based material; the silicon-based material comprising silicon-carbon particles and a coating layer located on the outer surface of the silicon-carbon particles; the coating layer comprising a first coating layer and a second coating layer; the first coating layer being located between the silicon-carbon particles and the second coating layer.

[0019] like Figure 1The diagram shows a schematic representation of a silicon-based material in an embodiment of the present invention. As can be seen from the diagram, the silicon-based material includes silicon-carbon particles 1 and a coating layer 2 located on the outer surface of the silicon-carbon particles. The silicon-carbon particles include a porous carbon matrix 11 and silicon material 12 located within the pores of the porous carbon matrix. The coating layer includes a first coating layer 21 and a second coating layer 22; the first coating layer is located between the silicon-carbon particles and the second coating layer.

[0020] In this invention, the second coating layer includes at least lithium aluminate. It possesses a high thermal decomposition temperature, effectively blocking laser damage to silicon-carbon particles, inhibiting electrolyte erosion of silicon-carbon particles, and reducing side reactions. Simultaneously, as a fast ion conductor, lithium aluminate exhibits excellent structural stability and high lithium-ion conductivity, significantly improving ion migration efficiency, thereby further enhancing the battery's cycle stability and rate performance.

[0021] In this invention, the second coating layer contains element Al; based on the total weight of the silicon-based material, the mass content of Al is A, and the unit is ppm.

[0022] In one instance, 50ppm ≤ A ≤ 1000ppm (e.g., 50ppm, 100ppm, 200ppm, 300ppm, 400ppm, 500ppm, 600ppm, 700ppm, 800ppm, 900ppm or 1000ppm).

[0023] In one instance, 70ppm < A ≤ 600ppm.

[0024] In this invention, the mass content A of element Al in the second coating layer can be obtained by conventional methods in the art, based on the total weight of the silicon-based material, such as by inductively coupled plasma (ICP) testing.

[0025] In this invention, the surface of the negative electrode active coating on at least one side of the negative electrode current collector is provided with a plurality of first recesses. For example... Figure 2 The diagram shows a structural schematic of the first recessed portion of the negative electrode sheet in an embodiment of the present invention, where h is the depth of the first recessed portion; d is the width of the first recessed portion; and ΔL is the spacing between the first recessed portions. In the present invention, the first recessed portion may include a recessed hole or a groove. When the first recessed portion is a groove, the groove may be continuously arranged or segmented. Figure 3 The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention. Figure 3 (a) The grooves are continuously arranged; Figure 3 (b) The grooves are segmented. As can be seen from the figure, the surface of the negative electrode (i.e., the surface of the negative electrode active coating) has several grooves. Figure 3 In (a), the groove is continuously arranged in the width direction of the negative electrode sheet. Figure 3 In (b), the groove is segmented in the width direction of the negative electrode sheet. Figure 3 The example only shows the case where the groove is set along the width direction of the negative electrode sheet; the groove can also be set along the length direction of the negative electrode sheet.

[0026] In this invention, the depth H of the first recess can be 2μm-35μm (e.g., 2μm, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm). The depth of the first recess has a conventional meaning in the art, referring to the vertical distance from the lowest point within the first recess to the surface of the negative electrode sheet. The depth of the first recess can be measured using conventional methods in the art, such as using a 3D profilometer to measure the depth of all or at least 20 first recesses on the surface of the negative electrode active coating and taking the average value.

[0027] In one instance, H is 5μm-25μm.

[0028] In this invention, the mass content A of Al in the silicon-based material and the depth H μm of the first recess satisfy the following condition: 0.002 ≤ H / A ≤ 0.669 (e.g., 0.002, 0.01, 0.1, 0.2, 0.3, 0.4, 0.5, or 0.669). This can be understood as 0.002 ≤ H / A ≤ 0.669, calculated using the corresponding values ​​when A is in ppm and H is in μm, without considering dimensions. For example, when A is 412 ppm and d1 is 15.3 μm, H / A is 0.037.

[0029] In one instance, 0.01 ≤ H / A ≤ 0.5.

[0030] In one instance, 0.02 ≤ H / A ≤ 0.1.

[0031] In this invention, the width of the first recess can be 20μm-150μm (e.g., 20μm, 40μm, 60μm, 80μm, 100μm, 120μm or 150μm).

[0032] In one example, the width of the first recess is 60μm-100μm.

[0033] When the first recess is a concave hole, the width of the first recess refers to the diameter of the concave hole. The diameter of the concave hole has a conventional meaning in the art. When the shape of the orthographic projection of the concave hole on the surface of the negative electrode sheet is a "regular circle", the diameter of the concave hole is the diameter of the regular circle; when the shape of the orthographic projection of the concave hole on the surface of the negative electrode sheet is a non-"regular circle" (e.g., ellipse or irregular curved polygon), the diameter of the concave hole is the diameter of an equivalent circle with the same area as the non-"regular circle". The diameter of the concave hole can be tested by conventional means in the art, for example, by using a 3D profilometer to test the diameter of at least 20 concave holes on the surface of the negative electrode active coating and taking the average value.

[0034] When the first recess is a groove, the width of the first recess refers to the width of the groove. The width of the groove has a conventional meaning in the art. The orthographic projection of the groove onto the surface of the negative electrode includes two long sides, and the width of the groove refers to the average distance from one long side to the other along the length or width direction of the negative electrode. For example... Figure 4 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 4 (a)- Figure 4 In (c), the two long sides of the groove are straight lines. Figure 4 In (d), the two long sides of the groove are curved. Figure 4 (a) and Figure 4 In (b), the two long sides are arranged parallel to each other. Therefore, in the width direction of the negative electrode sheet, the perpendicular distance from any point on one long side to the other long side is equal. In this case, the width of the groove is the perpendicular distance d from any point on one long side to the other long side in the length or width direction of the negative electrode sheet. Figure 4 In (c), the two long sides of the groove are straight lines, but not parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can be taken as the average value. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, so that the calculation result is more accurate), and the width d corresponding to each point is measured. The average value is then taken to obtain the width of the groove. Figure 4 In (d), the two long sides are curves. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can also be taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 4 In (d), the two longer sides are curves, and there is no... Figure 4(c) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width d corresponding to each point is measured, and the average value is taken to obtain the width of the groove. The width of the groove can be tested using conventional methods in the art, such as using a 3D profilometer to test the width of all grooves or at least 5 grooves on the surface of the negative electrode active coating and take the average value.

[0035] In this invention, the spacing of the first recess can be 0.5mm-2mm (e.g., 0.5mm, 0.8mm, 1mm, 1.2mm, 1.4mm, 1.6mm, 1.8mm or 2mm).

[0036] In one example, the spacing of the first recess is 0.8mm-1.5mm.

[0037] When the first recess is a hole, the spacing of the first recess refers to the spacing of the holes. The spacing of the holes has a conventional meaning in the art. It refers to the shortest distance between the edges of two adjacent holes on the surface of the negative electrode. The spacing of the holes can be tested by conventional means in the art, such as using a 3D profilometer to select all or at least 10 groups of adjacent holes, measure the spacing, and take the average value.

[0038] When the first recess is a groove, the spacing between the first recesses refers to the spacing between the grooves. It is understood that when there is only one groove on the surface of the negative electrode, there is no spacing between the grooves. The spacing between the grooves has a conventional meaning in the art, referring to the average distance between the two adjacent long sides of two adjacent grooves in the length or width direction of the negative electrode. Figure 5 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 5 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 5 (b) represents the case where two adjacent long sides are straight lines and not parallel. Figure 5 (c) represents the case where two adjacent long sides are curves. Figure 5 In (a), the two adjacent long sides are straight lines and parallel to each other. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. At this time, the spacing of the grooves is the distance ΔL from any point on one long side to the other long side in the width direction. Figure 5 In (b), the two adjacent long sides are straight lines, but not parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can be averaged. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, so that the calculation results are more accurate), and the width ΔL corresponding to each point is measured. The average value is then taken to obtain the spacing. Figure 5 In (c), two adjacent long sides are curves. Therefore, in the width direction, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can also be taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 5 (c) has two long sides that are curves and do not exist. Figure 5 (b) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width ΔL corresponding to each point is measured, and the average value is taken to obtain the spacing. The spacing of the grooves can be tested by conventional means in the art, such as using a 3D profilometer to test the spacing of all grooves or at least 5 grooves on the surface of the negative electrode active coating, and taking the average value.

[0039] In this invention, the second coating layer further includes aluminum oxide and / or aluminum fluoride.

[0040] In this invention, the first coating layer includes at least one of carbon nanotubes, graphene, and amorphous carbon.

[0041] In one instance, the first coating layer comprises amorphous carbon.

[0042] In this invention, the thickness h1 of the first coating layer satisfies: 1nm≤h1≤20nm (for example, 1nm, 5nm, 10nm, 15nm or 20nm).

[0043] In this invention, the thickness h2 of the second coating layer satisfies: 2nm≤h2≤50nm (for example, 2nm, 5nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 45nm or 50nm).

[0044] In one instance, 5nm ≤ h2 ≤ 20nm.

[0045] This invention, by controlling the thickness of the second coating layer to a suitable range, effectively blocks direct contact between the electrolyte and silicon-carbon particles, minimizing side reactions and protecting the internal structure. Simultaneously, it does not increase electron and lithium-ion migration resistance, ensuring that the battery's rate performance is not weakened, achieving synergistic optimization of battery cycle stability and rate performance. When the thickness h2 of the second coating layer is too small (e.g., less than 2 nm), a complete and continuous barrier cannot be formed, and silicon-carbon particles are easily exposed to the electrolyte, leading to intensified side reactions, accelerated loss of active lithium, rapid capacity decay, and affecting the battery's cycle performance. When the thickness h2 of the second coating layer is too large (e.g., greater than 50 nm), although the excessively thick coating layer can provide better protection, it prolongs the lithium-ion diffusion path and increases migration resistance, resulting in a sharp increase in interface resistance and severely impairing the battery's fast-charging capability and rate performance.

[0046] In this invention, the thickness h1 of the first coating layer and the thickness h2 of the second coating layer can be obtained by conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode, dried, and then the silicon-based material is cut using an argon ion milling machine (CP). The positions of the first and second coating layers are determined by TEM observation. Ten test sites are randomly selected on the surface of the first and second coating layers respectively, and the thickness of each site is measured and the average value is taken. At least 10 silicon-based material particles are selected for measurement, and the final average value is taken.

[0047] In this invention, the coverage area of ​​the second coating layer on the surface of the silicon-based material is 60%-100% (e.g., 60%, 70%, 80%, 90% or 100%).

[0048] In one example, the second coating layer covers 80%-100% of the surface area of ​​the silicon-based material.

[0049] The second coating layer of this invention has a high coverage area on the surface of the silicon-based material, forming a uniform film distribution. This minimizes the direct contact area between the silicon-based material and the electrolyte, suppresses side reactions, significantly reduces active lithium loss and electrolyte decomposition, and effectively stabilizes the SEI film. Simultaneously, it ensures a uniform and continuous lithium-ion migration path, avoiding increased impedance due to uncoated areas, further improving the battery's cycle life and rate performance. If the coverage area of ​​the second coating layer on the silicon-based material is too small (e.g., less than 60%), a large amount of silicon-based material will be directly exposed to the electrolyte, exacerbating side reactions, causing continuous electrolyte decomposition, accelerating the active lithium loss rate, and causing the unstable SEI film to repeatedly rupture and regenerate in exposed areas, further consuming the negative electrode active material, significantly shortening the battery capacity decay cycle, and severely affecting the battery's cycle stability.

[0050] In this invention, the tap density of the silicon-based material is 0.6 g / cm³. 3 -2g / cm 3 (For example, 0.6 g / cm³) 3 0.8g / cm 3 1g / cm 3 1.2g / cm 3 1.4g / cm 3 1.6g / cm 3 1.8g / cm 3 or 2g / cm 3 ).

[0051] In this invention, the specific surface area of ​​the silicon-based material is 1m².2 / g-3m 2 / g (e.g., 1m) 2 / g, 1.5m 2 / g、2m 2 / g, 2.5m 2 / g or 3m 2 / g).

[0052] In this invention, the specific surface area of ​​the silicon-based material can be obtained by conventional methods in the art, such as using a McMurray TriStar II 3020 Plus high-throughput specific surface area and pore size analyzer.

[0053] In this invention, the particle size Dv50 of the silicon-based material is 3μm-15μm (e.g., 3μm, 5μm, 7μm, 9μm, 11μm, 13μm, or 15μm); and Dv99 is 16μm-30μm (e.g., 16μm, 18μm, 20μm, 22μm, 24μm, 26μm, 28μm, or 30μm). When the particle size Dv50 and Dv99 of the silicon-based material are within the above ranges, it is beneficial to improve the ion transport rate of the material, reduce interfacial side reactions, and reduce SEI film consumption, thereby enabling the battery to have excellent cycle performance.

[0054] In this invention, the particle sizes Dv50 and Dv99 of the silicon-based material can be obtained by conventional methods in the art, such as by laser particle size analyzer.

[0055] In this invention, the average sphericity s of the silicon-based material is 0.5-1 (e.g., 0.5, 0.6, 0.7, 0.8, 0.9 or 1).

[0056] In one example, the average sphericity s of the silicon-based material is 0.7-1.

[0057] Spherical silicon-based materials typically exhibit better packing density and higher specific surface area, which can improve battery energy density, effectively alleviate the volume expansion problem of silicon-based materials during charge and discharge, reduce internal stress during battery cycling, minimize electrode structure damage caused by volume changes, and improve battery cycle stability. Secondly, the more regular surface of spherical silicon-based materials facilitates the formation of a stable solid electrolyte interphase (SEI) film, reduces the occurrence of side reactions, and further enhances battery cycle performance. Simultaneously, it can shorten the lithium-ion diffusion path, facilitating electron conduction and ion transport, and improving the conductivity and rate performance of the anode material.

[0058] In this invention, the average sphericity s of the silicon-based material can be tested using conventional methods in the art. For example, the battery is discharged to 0% SOC, the negative electrode is disassembled and removed, the negative electrode is cut using an argon ion milling machine (CP), and observed using backscatter imaging mode on a scanning electron microscope (SEM) or transmission electron microscope (TEM). At least 10 silicon-based material particles are selected from the obtained SEM / TEM images, and the perimeter and area of ​​each particle are measured using Image Pro Plus image processing software. The perimeter equivalent radius r1 and area equivalent radius r2 of each particle are calculated respectively, and the sphericity s = r2 / r1 is taken as the average value to obtain the sphericity s of the silicon-carbon composite material.

[0059] In this invention, the mass content of silicon in the negative electrode active coating is 2%-50% (for example, 2%, 5%, 10%, 20%, 30%, 40% or 50%).

[0060] In one example, the silicon content in the negative electrode active coating is 5%-25% by mass.

[0061] In this invention, the silicon-based material contains 30%-70% silicon by mass (e.g., 30%, 30%, 40%, 50%, 60% or 70%).

[0062] In this invention, the negative electrode active coating further includes graphite.

[0063] In one example, the average particle size of the graphite is 5 μm to 25 μm (e.g., 5 μm, 10 μm, 15 μm, 20 μm or 25 μm).

[0064] In this invention, the thickness of the negative electrode active coating on the surface of the negative electrode current collector is 15μm-50μm (e.g., 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm or 50μm).

[0065] In this invention, the mass content of silicon in the silicon-based material can be obtained by conventional methods in the art, such as using an energy dispersive spectroscopy (EDS) instrument. Specifically, the battery is discharged to 0% SOC, the negative electrode is removed from the battery, soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC to remove the lithium salt adhering to the negative electrode. After drying, the sample is processed using an argon ion cutter (e.g., a NEC IB-19530CP argon ion section polisher). The cut sample is then quickly transferred to the scanning electron microscope sample chamber for observation. Using the EDS spot scan mode, at least 5 points on the surface of the silicon-based material are measured to obtain the mass content of silicon at each point, and the average value is calculated.

[0066] In this invention, the mass content of silicon in the negative electrode active coating can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active coating volatilize while the silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of silicon in the negative electrode active coating can be calculated based on the mass of ash. The calculation formula is as follows: Mass content of silicon in negative electrode active coating = 7 × mass of ash / (15 × mass of test sample).

[0067] In this invention, the average particle size of the graphite can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet can be disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove lithium salts adhering to the negative electrode sheet. Alternatively, the negative electrode sheet can be directly removed, and the negative electrode sheet can be cut using an argon-ion milling machine with a CP laser. Then, SEM can be used for observation, and at least 20 graphite particles can be randomly selected to measure the particle size of each particle and take the average value. When the particles in the SEM image are regular circles, the particle size is the diameter of the regular circle; when the particles in the SEM image are not "regular circles", the particle size is the diameter of an equivalent circle with the same area as the "irregular circle".

[0068] A second aspect of the present invention provides a lithium-ion secondary battery, the lithium-ion secondary battery comprising the negative electrode sheet described in the first aspect of the present invention.

[0069] In this invention, the compaction density of the negative electrode sheet is 0.8 g / cm³. 3 -1.8g / cm 3 (e.g., 0.8g / cm) 3 1g / cm 3 1.2g / cm 3 1.4g / cm 3 1.6g / cm 3 Or 1.8g / cm 3 ).

[0070] In this invention, the compaction density of the negative electrode sheet can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. Then, the surface residue of the negative electrode sheet is washed off with deionized water and dried. At least 20 sites are selected on the negative electrode sheet, and the thickness of the negative electrode sheet at each site is measured using a micrometer. The average value h (in μm) is taken. The negative electrode sheet is punched into a disc with a diameter of 44.3 mm using a punching die. Ten discs are taken, and the mass of each disc is weighed. The average value m (in mg) is taken. The areal density M1 is calculated as M1 = (m - m1) × 100 / 1540.25, where m1 is the mass of the negative electrode current collector in the disc (in mg), and the areal density is in mg / cm³. 2 The compaction density is calculated using the areal density M1 and the average thickness h, as follows: Compaction density = M1 × 20 / (h - thickness of the negative electrode current collector), where the unit of the thickness of the negative electrode current collector is μm.

[0071] In this invention, the negative electrode active coating further includes a negative electrode conductive agent and a negative electrode binder. The negative electrode conductive agent includes at least one selected from superconducting carbon black, acetylene black, carbon black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers; the negative electrode binder includes at least one selected from polyvinylidene fluoride (PVDF), carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, styrene-butadiene rubber, polyacrylonitrile (PAN), polyacrylic acid, and polytetrafluoroethylene. Based on the total weight of the negative electrode active coating, the content of the conductive agent is 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the binder is 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0072] In this invention, the lithium-ion secondary battery further includes a positive electrode sheet, which comprises a positive current collector and a positive active coating located on at least one side of the surface of the positive current collector. The positive active coating comprises a positive electrode material, a positive conductive agent, and a positive electrode binder. The positive electrode material comprises at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium nickel cobalt manganese aluminum oxide, lithium manganese oxide, lithium nickel manganese oxide, lithium nickel oxide, lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, and lithium-rich manganese-based materials. The positive conductive agent comprises at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes (including at least one of single-walled carbon nanotubes and multi-walled carbon nanotubes), and carbon fibers. The positive electrode binder may comprise at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose, styrene-butadiene rubber, polytetrafluoroethylene, polyethylene oxide, polyacrylic acid, polyacrylonitrile (PAN), and derivatives of the above substances. Based on the total weight of the positive electrode active coating, the content of the positive electrode material can be 80%-98% (e.g., 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, or 98%), the content of the positive electrode conductive agent can be 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%), and the content of the positive electrode binder can be 1%-10% (e.g., 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, or 1%).

[0073] In this invention, the lithium-ion secondary battery further includes an electrolyte, which comprises an organic solvent, an electrolyte salt, and additives. The organic solvent includes carbonate solvents and / or carboxylic acid ester solvents. The carbonate solvents include cyclic carbonate solvents and chain carbonate solvents. The cyclic carbonate solvents include at least one of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), and butylene carbonate (BC). The chain carbonate solvents include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl ethyl carbonate (EMC), methyl propyl carbonate (MPC), and ethyl propyl carbonate (EPC). The carboxylic acid ester solvents include at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), and ethyl butyrate (EB). The electrolyte salt comprises at least one of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide, lithium hexafluoroantimonyate, lithium hexafluoroarsenate, lithium bis(trifluoromethanesulfonyl)imide, lithium bis(pentafluoroethylsulfonyl)imide, lithium tri(trifluoromethanesulfonyl)methyl, or lithium bis(trifluoromethanesulfonyl)imide. The additive comprises at least one of 2,2-difluoroethyl acetate (DFEA), vinylene carbonate, 1,3-propenesulfonate lactone, vinyl ethylene carbonate, vinyl sulfate, succinic acid nitrile, glutaronitrile, adiponitrile, heptaonitrile, octanoic acid nitrile, sebaconitrile, 1,3,6-hexanetrionitrile (HTCN), glycerol trionitrile, and 1,2-bis(2-cyanoethoxy)ethane.

[0074] In one example, the electrolyte comprises fluoroethylene carbonate (FEC) and / or 2,2-difluoroethyl acetate (DFEA).

[0075] In one instance, the FEC content in the electrolyte is 5%-25% by mass (e.g., 5%, 10%, 15%, 20% or 25%).

[0076] In one instance, the FEC content in the electrolyte is 8%-20% by mass.

[0077] In one example, the mass content of DFEA in the electrolyte is 5%-60% (e.g., 5%, 10%, 20%, 30%, 40%, 50% or 60%).

[0078] In one example, the mass content of DFEA in the electrolyte is 10%-55%.

[0079] The second coating layer of this invention possesses a high thermal decomposition temperature, enabling it to form a dense and stable barrier during laser wire bonding and long-term battery cycling, essentially acting as an artificial SEI film. This effectively protects the silicon-based material and reduces side reactions in the electrolyte. Therefore, based on the effective protection of the negative electrode interface by the second coating layer, the battery system can correspondingly reduce its dependence on FEC to maintain the SEI film, avoiding the effects of decreased ionic conductivity and deterioration in battery rate performance caused by excessive FEC addition. This enhances the structural stability of the negative electrode material and effectively improves the problem of negative electrode cycling expansion.

[0080] When the electrolyte contains DFEA, it can synergistically interact with the second coating layer. The fluorine atoms in its molecules are absorbed by Lewis acid sites (such as Al) abundant on the surface of the second coating layer. 3+ Selective adsorption and catalytic decomposition promote the preferential reduction of fluorinated solvents on the surface of silicon-based materials, generating a LiF-rich interfacial protective film. This film further blocks direct contact between the silicon-based material and the electrolyte, reduces side reactions, and lowers the loss of electrolyte and active lithium, thereby further improving the cycle stability of the battery.

[0081] In this invention, the mass content of FEC or DFEA in the electrolyte can be obtained by methods conventional in the art, such as gas chromatography (GC) or gas chromatography-mass spectrometry (GCMS).

[0082] In this invention, the battery further includes a separator, which includes a substrate layer, a ceramic layer located on at least one side of the substrate layer, and adhesive layers located on both outer surfaces of the separator.

[0083] In one example, the substrate layer includes a matrix that may include at least one of polyethylene, polyvinyl chloride, polyoxyethylene, polypropylene, nylon, glass fiber, polyethylene phthalate (PET), polyimide (PI), aramid, cellulose, and nonwoven fabric.

[0084] In one example, the ceramic layer comprises a nitrogen-containing material and / or an inorganic material; the nitrogen-containing material includes melamine, melamine polyphosphate, melamine thiocyanate, melamine cyanurate, formaldehyde polymelamine hydrochloride, melamine polyphosphate, piperazine pyrophosphate, 1,3,5-triazine-2,4,6-triamine, symmetrical triaminotriazine, 2-(4-bromophenyl)-4,6-dimethyl-1,3,5-triazine, 1-(4,6-diamino-1,3,5-triazine-2-yl)guanidine, 2,4-diamino- The inorganic material comprises at least one of 6-dimethylamino-1,3,5-triazine, cyanuric chloride, 2,4,6-tris(2-pyridyl)triazine, 2,4,6-triphenyl-1,3,5-triazine, tris(tribromophenoxy)triazine, and 2-amino-4,6-methoxy-1,3,5-triazine; the inorganic material comprises at least one of boehmite, alumina, barium sulfate, magnesium oxide, magnesium hydroxide, silicon dioxide, tin dioxide, titanium dioxide, calcium oxide, zinc oxide, zirconium oxide, yttrium oxide, nickel oxide, cerium oxide, zirconium titanate, barium titanate, and magnesium fluoride.

[0085] In one example, the adhesive layer comprises at least one of polyvinylidene fluoride (PVDF), sodium carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose (CMC-Na), lithium carboxymethyl cellulose (CMC-Li), polyvinylpyrrolidone (PVP), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-HFP), and acrylate polymers such as polymethyl methacrylate (PMMA).

[0086] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0087] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0088] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0089] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0090] Example 1 (1) Preparation of positive electrode sheet Lithium cobalt oxide, conductive agent (conductive carbon black and carbon nanotubes mixed at a mass ratio of 1:1) and PVDF are mixed at a mass ratio of 98.2:1:0.8. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil, and after baking, rolling and cutting, the positive electrode sheet is obtained. (2) Preparation of negative electrode sheet Graphite (average particle size of 15.3 μm), silicon-based materials, conductive carbon black and PVDF were mixed in a mass ratio of 72.1:24:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of a copper foil and dried in an oven at 80°C for 10 hours. After cold pressing and slitting, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer. The silicon-carbon particles consist of a porous carbon matrix and silicon material located within the pores of the porous carbon matrix. The coating layer comprises a first coating layer and a second coating layer. A is 412 ppm, H is 15.3 μm, H / A is 0.037, the width of the first recess is 90.7 μm, and the spacing between the first recesses is 1.2 mm. The first coating layer comprises amorphous carbon with h1 of 10.2 nm, and the second coating layer comprises lithium aluminate with h2 of 12.5 nm. The specific surface area of ​​the silicon-based material is 2.2 m². 2 The silicon-based material has a particle size Dv50 of 9.4 μm and a Dv99 of 23.5 μm, an average sphericity s of 0.85, and a silicon content of 50.45% by mass. The negative electrode active coating has a silicon content of 12% by mass and a thickness of 35.5 μm. The compaction density of the negative electrode sheet is 1.2 g / cm³. 3 ; (3) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a 9 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side and a 1 μm thick polyvinylidene fluoride adhesive layer on both sides) are stacked in the order of positive electrode sheet, separator, and negative electrode sheet, ensuring that the separator is between the positive and negative electrodes to provide isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DEC / EMC with a volume ratio of 1:1:1, with the addition of FEC and DFEA) is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion battery is obtained. The electrolyte contains 15% FEC by mass, 35% DFEA by mass, and 1 mol / L of lithium hexafluorophosphate.

[0091] Example 2 (1) Preparation of positive electrode sheet Lithium cobalt oxide, conductive agent (conductive carbon black and carbon nanotubes mixed at a mass ratio of 1:1) and PVDF are mixed at a mass ratio of 98.2:1:0.8. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil, and after baking, rolling and cutting, the positive electrode sheet is obtained. (2) Preparation of negative electrode sheet Graphite (average particle size of 5.8 μm), silicon-based materials, conductive carbon black and PVDF were mixed in a mass ratio of 72.1:24:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of a copper foil and dried in an oven at 80°C for 10 hours. After cold pressing and slitting, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer. The silicon-carbon particles consist of a porous carbon matrix and silicon material located within the pores of the porous carbon matrix. The coating layer comprises a first coating layer and a second coating layer. A is 75 ppm, H is 7.4 μm, H / A is 0.099, the width of the first recess is 61.2 μm, and the spacing between the first recesses is 0.8 mm. The first coating layer comprises amorphous carbon with h1 of 1.6 nm, and the second coating layer comprises lithium aluminate with h2 of 5.4 nm. The specific surface area of ​​the silicon-based material is 3 m². 2 The silicon-based material has a particle size Dv50 of 3.1 μm and a Dv99 of 16.3 μm, an average sphericity s of 0.75, and a silicon content of 50.61% by mass. The negative electrode active coating has a silicon content of 12% by mass and a thickness of 15.2 μm. The compaction density of the negative electrode sheet is 0.8 g / cm³. 3 ; (3) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a 9 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side and a 1 μm thick polyvinylidene fluoride adhesive layer on both sides) are stacked in the order of positive electrode sheet, separator, and negative electrode sheet, ensuring that the separator is between the positive and negative electrodes to provide isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DEC / EMC with a volume ratio of 1:1:1, with the addition of FEC and DFEA) is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion battery is obtained. The electrolyte contains 8% FEC, 10% DFEA, and 1 mol / L of lithium hexafluorophosphate.

[0092] Example 3 (1) Preparation of positive electrode sheet Lithium cobalt oxide, conductive agent (conductive carbon black and carbon nanotubes mixed at a mass ratio of 1:1) and PVDF are mixed at a mass ratio of 98.2:1:0.8. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil, and after baking, rolling and cutting, the positive electrode sheet is obtained. (2) Preparation of negative electrode sheet Graphite (average particle size of 9.9 μm), silicon-based materials, conductive carbon black and PVDF were mixed in a mass ratio of 72.1:24:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of a copper foil and dried in an oven at 80°C for 10 hours. After cold pressing and slitting, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer. The silicon-carbon particles consist of a porous carbon matrix and silicon material located within the pores of the porous carbon matrix. The coating layer comprises a first coating layer and a second coating layer. A is 315 ppm, H is 8 μm, H / A is 0.025, the width of the first recess is 79.6 μm, and the spacing between the first recesses is 1.1 mm. The first coating layer comprises amorphous carbon with h1 of 8.3 nm, and the second coating layer comprises lithium aluminate with h2 of 10.2 nm. The specific surface area of ​​the silicon-based material is 1.9 m². 2The silicon-based material has a particle size Dv50 of 10.3 μm and a Dv99 of 25.1 μm, an average sphericity s of 0.95, and a silicon content of 50.32% by mass. The negative electrode active coating has a silicon content of 12% by mass and a thickness of 28.8 μm. The compaction density of the negative electrode sheet is 1.5 g / cm³. 3 ; (3) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a 9 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side and a 1 μm thick polyvinylidene fluoride adhesive layer on both sides) are stacked in the order of positive electrode sheet, separator, and negative electrode sheet, ensuring that the separator is positioned between the positive and negative electrodes to provide isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DEC / EMC with a volume ratio of 1:1:1, with the addition of FEC and DFEA) is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion battery is obtained. The electrolyte contains 5% FEC by mass, 55% DFEA by mass, and 1 mol / L of lithium hexafluorophosphate.

[0093] Example 4 (1) Preparation of positive electrode sheet Lithium cobalt oxide, conductive agent (conductive carbon black and carbon nanotubes mixed at a mass ratio of 1:1) and PVDF are mixed at a mass ratio of 98.2:1:0.8. N-methylpyrrolidone is added and the mixture is stirred under vacuum until the mixture becomes a uniform and fluid positive electrode slurry. The positive electrode slurry is uniformly coated on both sides of aluminum foil, and after baking, rolling and cutting, the positive electrode sheet is obtained. (2) Preparation of negative electrode sheet Graphite (average particle size of 24.5 μm), silicon-based materials, conductive carbon black and PVDF were mixed in a mass ratio of 72.1:24:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry. The negative electrode slurry was uniformly coated on both sides of a copper foil and dried in an oven at 80°C for 10 hours. After cold pressing and slitting, grooves were etched on the outer surface of the negative electrode active layer using laser processing technology to obtain the negative electrode sheet. The silicon-based material comprises silicon-carbon particles and a coating layer. The silicon-carbon particles consist of a porous carbon matrix and silicon material located within the pores of the porous carbon matrix. The coating layer comprises a first coating layer and a second coating layer. A is 501 ppm, H is 24.6 μm, H / A is 0.049, the width of the first recess is 148.6 μm, and the spacing between the first recesses is 1.5 mm. The first coating layer comprises amorphous carbon with h1 of 18.9 nm, and the second coating layer comprises lithium aluminate with h2 of 19.8 nm. The specific surface area of ​​the silicon-based material is 1.1 m². 2 The silicon-based material has a particle size Dv50 of 14.6 μm, a Dv99 of 29.8 μm, an average sphericity s of 0.99, and a silicon content of 50.54% by mass. The negative electrode active coating has a silicon content of 12% by mass and a thickness of 45.8 μm. The compaction density of the negative electrode sheet is 1.8 g / cm³. 3 ; (3) Battery preparation The positive electrode sheet prepared in step (1), the negative electrode sheet prepared in step (2), and the separator (a 9 μm thick polyethylene film coated with a 2 μm thick boehmite ceramic layer on one side and a 1 μm thick polyvinylidene fluoride adhesive layer on both sides) are stacked in the order of positive electrode sheet, separator, and negative electrode sheet, ensuring that the separator is positioned between the positive and negative electrodes to provide isolation. Then, the bare cell is obtained by winding. The bare cell is placed in an aluminum-plastic film shell, and the electrolyte (lithium hexafluorophosphate dissolved in a mixed solution of EC / DEC / EMC with a volume ratio of 1:1:1, with the addition of FEC and DFEA) is injected into the dried bare cell. After vacuum sealing, settling, formation, shaping, and sorting, a lithium-ion battery is obtained. The electrolyte contains 20% FEC, 5% DFEA, and 1 mol / L of lithium hexafluorophosphate.

[0094] Example 5 group This set of embodiments is based on Embodiment 1, except that the H / A ratio is adjusted by changing the mass content A of Al in the silicon-based material and the depth H of the first recess, as follows: Example 5a: A is 597 ppm, H is 5.8 μm, H / A is 0.01; the mass content of silicon in the negative electrode active coating is 48.6%, and the thickness of the negative electrode active coating is 20.4 μm; Example 5b: A is 71 ppm, H is 24.9 μm, H / A is 0.351; the mass content of silicon in the negative electrode active coating is 24.9%, and the thickness of the negative electrode active coating is 40.5 μm; Example 5c: A is 50 ppm, H is 24.9 μm, H / A is 0.498; the mass content of silicon in the negative electrode active coating is 30.5%, and the thickness of the negative electrode active coating is 40.5 μm; Example 5d: A was 52 ppm, H was 34.8 μm, and H / A was 0.669; the mass content of silicon in the negative electrode active coating was 2%, and the thickness of the negative electrode active coating was 49.8 μm. Example 5e: A is 986 ppm, H is 2.1 μm, H / A is 0.002; the mass content of silicon in the negative electrode active coating is 4.8%, and the thickness of the negative electrode active coating is 19.9 μm.

[0095] Example 6 group This set of embodiments is based on Embodiment 1, except that the mass content A of Al in the material of the second coating layer and the silicon-based material is changed, as follows: Example 6a, the second coating layer comprises lithium aluminate and aluminum oxide; A is 398 ppm, H / A is 0.038; Example 6b, the second coating layer includes lithium aluminate and aluminum fluoride; A is 612 ppm, H / A is 0.025.

[0096] Example 7 group This set of embodiments is based on Embodiment 1, except that the average sphericity of the silicon-based material is changed, as follows: In Example 7a, the average sphericity s of the silicon-based material is 0.5; In Example 7b, the average sphericity s of the silicon-based material is 0.45.

[0097] Example 8 group This set of embodiments is based on Embodiment 1, except that the mass content of silicon in the negative electrode active coating is changed, as follows: In Example 8a, graphite (average particle size 15.3 μm), silicon-based materials, conductive carbon black, and PVDF were mixed in a mass ratio of 46.6:49.5:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry; the silicon content in the negative electrode active coating was 25% by mass. In Example 8b, silicon-based materials, conductive carbon black, and PVDF were mixed in a mass ratio of 96.1:0.5:3.4, deionized water was added, and the mixture was stirred under vacuum to prepare a negative electrode slurry; the mass content of silicon in the negative electrode active coating was 50%.

[0098] Example 9 group This set of embodiments is based on Embodiment 1, except that the composition of the electrolyte is changed, as follows: Example 9a: The FEC content in the electrolyte is 25% by mass; Example 9b: The FEC content in the electrolyte was 26% by mass. Example 9c: The DFEA content in the electrolyte was 4% by mass. In Example 9d, the mass content of DFEA in the electrolyte was 60%.

[0099] Example 10 group This set of embodiments is based on Embodiment 1, except that the mass content of silicon in the silicon-based material is changed, as follows: Example 10a: The silicon-based material has a silicon content of 30.06% by mass. Example 10b: The silicon-based material has a silicon content of 69.98% by mass.

[0100] Comparative Example 1 This comparative example is based on Example 1, except that the silicon-based material contains only the first coating layer.

[0101] Comparative Example 2 This comparative study was conducted with reference to Example 1, except that the ratio H / A was adjusted by changing the mass content A of Al in the silicon-based material and the depth H of the first recess, as detailed below: Comparative Example 2a: A was 48 ppm, H was 34 μm, and H / A was 0.708; Comparative Example 2b: A = 869 ppm, H = 0.8 μm, H / A = 0.001.

[0102] Comparative Example 3 Groups This comparative study was conducted with reference to Example 1, except that the mass content A of Al in the material of the second coating layer and the silicon-based material was changed, as follows: Comparative Example 3a, the second coating layer includes aluminum fluoride; A is 395 ppm, and H / A is 0.039; Comparative Example 3b: The second coating layer consists of alumina; A is 802 ppm, and H / A is 0.019.

[0103] Test case (1) Loop test The lithium-ion batteries prepared in the examples and comparative examples were subjected to cycle tests. The specific test methods are as follows: At 25°C, the battery was discharged at 1C to 3.0V, then allowed to rest for 10 minutes, and then charged at 1C to 4.55V with a cutoff current of 0.025C, followed by a 10-minute rest. The thickness of the battery at this point was recorded as the initial thickness. The battery was then discharged at a 1.2C rate to the cutoff voltage of 3.0V, allowed to rest for 10 minutes, and the discharge capacity was recorded as the initial capacity. The cycle was repeated as follows: charging at 1C to 4.55V, with a cutoff current of 0.025C, allowed to rest for 10 minutes, then discharging at a 1.2C rate to the cutoff voltage of 3.0V, allowed to rest for 10 minutes, and so on, until the 400th cycle. After the 600th charging cycle, the battery was allowed to rest for 10 minutes, and the thickness was measured and recorded as the post-cycle thickness. The battery was then discharged at 1.2C to 3.0V, allowed to rest for 10 minutes, and the discharge capacity was recorded as the post-cycle capacity. The cycle capacity retention rate = capacity after cycling × 100% / initial capacity, and the thickness expansion rate = (thickness after cycling - initial thickness) × 100% / initial thickness. The cycle capacity retention rate and thickness expansion rate are recorded in Table 1.

[0104] (2) Ratio test The batteries prepared in the examples and comparative examples were subjected to discharge rate tests. The specific test methods are as follows: At 25℃±2℃, the battery was left to stand for 10 minutes; then discharged at 0.2C to 3V, and left to stand for 10 minutes; fully charged at 1.5C in a 25℃ constant temperature chamber, with a cutoff voltage of 4.55V and a cutoff current of 0.025C, and left to stand for 10 minutes; then discharged at 2.5C to 3V in a 25℃ constant temperature chamber or incubator environment, and left to stand for 10 minutes; this charging and discharging process was repeated until the 200th cycle of constant voltage charging was completed, followed by a 10-minute stand, and then discharged again at a current density of 0.2C to 3.0V, and left to stand for 10 minutes. The discharge capacity of the battery at this point was recorded as the capacity C1 after the cycle. The cycle capacity retention rate is calculated as C1×100% / C0, and the results are recorded in Table 1.

[0105] Table 1 As can be seen from Table 1, the battery of the present invention has significantly improved cycle performance and rate performance at 25°C compared with the comparative example.

[0106] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A negative electrode sheet, characterized in that, The negative electrode includes a negative current collector and a negative active coating located on at least one side of the negative current collector. The negative active coating includes a silicon-based material, which includes silicon-carbon particles and a coating layer located on the outer surface of the silicon-carbon particles. The coating layer includes a first coating layer and a second coating layer. The first coating layer is located between the silicon-carbon particles and the second coating layer. The second coating layer includes at least lithium aluminate. The second coating layer contains Al; based on the total weight of the silicon-based material, the mass content of Al is A, and the unit is ppm; The surface of the negative electrode active coating on at least one side of the negative electrode current collector is provided with a plurality of first recesses, the depth of the first recesses being H, in μm; The mass content A of Al in the silicon-based material and the depth H of the first recess satisfy the following condition: 0.002 ≤ H / A ≤ 0.

669.

2. The negative electrode sheet according to claim 1, wherein, 0.01≤H / A≤0.5; Preferably, 0.02 ≤ H / A ≤ 0.

1.

3. The negative electrode sheet according to claim 1, wherein, 50ppm≤A≤1000ppm; preferably, 70ppm<A≤600ppm; And / or, H is 2μm-35μm; preferably 5μm-25μm; And / or, the width of the first recess is 20μm-150μm, and the spacing between the first recesses is 0.5mm-2mm.

4. The negative electrode sheet according to claim 1 or 2, wherein, The second coating layer further includes aluminum oxide and / or aluminum fluoride; And / or, the first coating layer comprises at least one of carbon nanotubes, graphene, and amorphous carbon; preferably amorphous carbon.

5. The negative electrode sheet according to claim 1 or 2, wherein, The thickness h1 of the first coating layer satisfies: 1nm ≤ h1 ≤ 20nm; And / or, the thickness h2 of the second coating layer satisfies: 2nm≤h2≤50nm.

6. The negative electrode sheet according to claim 5, wherein, 5nm≤h2≤20nm; And / or, the silicon-carbon particles comprise a porous carbon matrix and silicon material located in the pores of the porous carbon matrix.

7. The negative electrode according to claim 1 or 2, wherein, The specific surface area of ​​the silicon-based material is 1m². 2 / g-3m 2 / g; And / or, the particle size Dv50 of the silicon-based material is 3μm-15μm, and the particle size Dv99 is 16μm-30μm; And / or, the average sphericity s of the silicon-based material is 0.5-1; preferably 0.7-1.

8. The negative electrode according to claim 1 or 2, wherein, The silicon content in the negative electrode active coating is 2%-50% by mass; more preferably 5%-25%. And / or, the silicon-based material contains 30%-70% silicon by mass; And / or, the negative electrode active coating further includes graphite; preferably, the average particle size of the graphite is 5μm-25μm; And / or, the thickness of the negative electrode active coating on the surface of the negative electrode current collector is 15μm-50μm.

9. A lithium-ion secondary battery, characterized in that, The lithium-ion secondary battery includes the negative electrode sheet according to any one of claims 1-8; the compaction density of the negative electrode sheet is 0.8 g / cm³. 3 -1.8g / cm 3 .

10. The lithium-ion secondary battery according to claim 9, wherein, The lithium-ion secondary battery also includes an electrolyte, which comprises fluoroethylene carbonate and / or 2,2-difluoroethyl acetate. Preferably, the fluoroethylene carbonate has a mass content of 5%-25% in the electrolyte; more preferably, it has a mass content of 8%-20%. Preferably, the mass content of 2,2-difluoroethyl acetate in the electrolyte is 5%-60%.

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