Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises an electrode assembly of a winding structure, the electrode assembly comprises a negative electrode piece and a diaphragm, the negative electrode piece comprises a negative electrode current collector, a negative electrode material layer and a bonding layer, and the negative electrode material layer is located between the negative electrode current collector and the bonding layer; the bonding layer comprises a first coating area and a second coating area, the first coating area is located in a straight area of the electrode assembly, the second coating area is located in a corner area of the electrode assembly, the first coating area comprises a first coating area material, and the second coating area comprises a second coating area material; in the cross section of the bonding layer in the thickness direction of the bonding layer, the average short-axis size of particles of the material of the first coating area is D1 [mu] m, the average short-axis size of particles of the material of the second coating area is D2 [mu] m, and D2 / D1 is larger than or equal to 2 and smaller than or equal to 6. The secondary battery provided by the invention has good cycle performance.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technologies, and particularly to a secondary battery and an electronic device. Background Art

[0002] In recent years, with the increasing demand for sustainable energy, the market demand for secondary batteries has been continuously growing. Among them, wound-structure secondary batteries are widely used due to their high volumetric energy density, long cycle life, and ease of miniaturization. However, due to the structural characteristics of wound-structure secondary batteries, there are still some challenges in the application process.

[0003] For example, wound-structure secondary batteries are mainly divided into a flat region and a corner region. Among them, the electrolyte storage in the corner region is limited. Along with the cycling process of the secondary battery, the electrolyte in the corner region is insufficient, the internal resistance increases, and problems such as lithium deposition and black spots are likely to occur on the negative electrode sheet, affecting the cycling performance of the secondary battery. Summary of the Invention

[0004] The purpose of the present application is to provide a secondary battery and an electronic device to improve the cycling performance of the secondary battery.

[0005] It should be noted that in the summary of the invention of the present application, lithium-ion batteries are used as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a secondary battery, which includes a wound-structure electrode assembly. The electrode assembly includes a negative electrode sheet and a separator. The negative electrode sheet includes a negative current collector, a negative electrode material layer, and a binder layer. Along the thickness direction of the negative electrode sheet, the negative electrode material layer is located between the negative current collector and the binder layer. The binder layer includes a first coating region and a second coating region. The first coating region is located in the flat region of the electrode assembly, and the second coating region is located in the corner region of the electrode assembly. The first coating region includes a first coating region material, and the second coating region includes a second coating region material. In the cross-section of the binder layer along its own thickness direction, the average short-axis size of the particles of the first coating region material is D1 μm, and the average short-axis size of the particles of the second coating region material is D2 μm, and 2 ≤ D2 / D1 ≤ 6. Thus, through the above arrangement, it is beneficial to balance the cycling performance and energy density of the flat region and the corner region of the secondary battery. The "average short-axis size" in the present application refers to the length of the short axis of an elliptical particle. When the particle presents an irregular shape, it is the diameter of the inscribed circle.

[0007] In some embodiments of the present application, 0.14 ≤ D1 ≤ 0.7. In some embodiments of the present application, 0.2 ≤ D1 ≤ 0.5. By regulating D1 within the above range, it is beneficial to improve the cycling performance of the secondary battery and balance the energy density.

[0008] In some embodiments of the present application, 0.8 ≤ D2 ≤ 1.50. In some embodiments of the present application, 1 ≤ D2 ≤ 1.2. By adjusting D2 within the above range, it is beneficial to improve the liquid storage capacity in the corner area while taking into account the kinetic performance, reduce the generation of lithium deposition and black spots, thereby improving the cycle performance of the secondary battery.

[0009] In some embodiments of the present application, the coating amount of the first coating area is a mg / 5000mm 2 , and the coating amount of the second coating area is b mg / 5000mm 2 , (a + 0.1) ≤ b ≤ (a + 0.6). In some embodiments of the present application, (a + 0.2) ≤ b ≤ (a + 0.4). By adjusting a and b within the scope of the present application, it is beneficial to further improve the kinetic performance and cycle performance of the secondary battery and take into account the energy density.

[0010] In some embodiments of the present application, 0.7 ≤ a ≤ 1.2. By adjusting a within the above range, it is beneficial to better balance the production process yield, kinetic performance and cycle performance of the secondary battery.

[0011] In some embodiments of the present application, the material of the first coating area includes a first binder material and a second binder material, and the material of the second coating area includes a third binder material and a fourth binder material; the first binder material and the third binder material each independently include at least one of methyl acrylate, octyl acrylate, polymethacrylate or isooctyl acrylate; the second binder material and the fourth binder material each independently include at least one of styrene, acrylonitrile or polyvinylidene fluoride; the mass ratio m1 of the first binder material to the second binder material is 1∶(1.2 to 2.0); the mass ratio m2 of the third binder material to the fourth binder material is 1∶(1.5 to 4). By selecting the above materials for the first coating area and the second coating area, and m1 and m2 within the above range, it is beneficial to reduce the internal resistance in the corner area, reduce the generation of lithium deposition black spots, and further improve the cycle stability of the secondary battery.

[0012] In some embodiments of the present application, the separator includes a base film and a composite layer provided on one surface of the base film, the composite layer faces the negative electrode sheet, and the composite layer includes a ceramic material; based on the mass of the composite layer, the mass percentage content of the ceramic material is W2, 78% ≤ W2 ≤ 92%. By introducing a composite layer between the base film and the adhesive layer and adjusting W2 within the above range, it is beneficial to reduce the occurrence of separation between the head separator and the electrode sheet, reduce the internal resistance, reduce the generation of lithium deposition and black spots, and further improve the production process yield and the kinetic performance of the secondary battery.

[0013] In some embodiments of the present application, the composite layer further includes a solid electrolyte; based on the mass of the composite layer, the mass percentage of the solid electrolyte is W1, and 1% ≤ W1 ≤ 8%. In some embodiments of the present application, 3% ≤ W1 ≤ 6%. By adjusting W1 within the above range, it is beneficial to further reduce the internal resistance and improve the phenomena such as lithium deposition and black spots in the corner area, thereby improving the kinetic performance and cycling performance of the secondary battery.

[0014] In some embodiments of the present application, the composite layer includes a third coating area and a fourth coating area. The third coating area is located in the straight area of the electrode assembly, and the fourth coating area is located in the corner area of the electrode assembly. The average particle size of the ceramic material in the third coating area is D3 nm, and the average particle size of the ceramic material in the fourth coating area is D4 nm. In some embodiments of the present application, 200 ≤ D3 ≤ 500. In some embodiments of the present application, 300 ≤ D3 ≤ 400. By adjusting D3 within the above range, it is beneficial to balance the transport rate of active ions and the thickness of the electrode assembly in the straight area, thereby taking into account the cycling performance and energy density of the secondary battery.

[0015] In some embodiments of the present application, 600 ≤ D4 ≤ 800. In some embodiments of the present application, 650 ≤ D4 ≤ 750. By adjusting D4 within the above range, it is beneficial to further improve the electrolyte storage capacity in the corner area, alleviate the generation of lithium deposition and black spots on the negative electrode plate during cycling, and improve the cycling performance of the secondary battery.

[0016] In some embodiments of the present application, the coating amount of the third coating area is c mg / 5000mm 2 , and the coating amount of the fourth coating area is d mg / 5000mm 2 ; (c + 0.1) ≤ d ≤ (c + 2). In some embodiments of the present application, (c + 0.4) ≤ d ≤ (c + 1.2). By adjusting c and d within the above range, it is beneficial to reduce the generation of lithium deposition and black spots, balance the internal resistance of the straight area and the corner area of the secondary battery, and further improve the overall kinetic performance and cycling performance of the secondary battery.

[0017] In some embodiments of the present application, 7 ≤ c ≤ 12. By adjusting c within the above range, it is beneficial to improve the kinetic performance of the secondary battery. At the same time, it is also beneficial to keep the overall thickness of the secondary battery within a suitable range and take into account the energy density of the secondary battery.

[0018] In some embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the solid electrolyte includes at least one of lithium titanium aluminum phosphate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium titanium oxide, or lithium lanthanum titanium oxide; (2) the ceramic material includes at least one of boehmite, alumina, magnesium hydroxide, or barium sulfate; (3) based on the mass of the composite layer, the mass percentage content of the ceramic material is W2, and 78% ≤ W2 ≤ 92%. The secondary battery satisfying at least one of the above characteristics is beneficial to further improving the transport rate of active ions in the secondary battery, thereby reducing the internal resistance, reducing the generation of lithium deposition and black spots, and improving the kinetic performance and cycling performance of the secondary battery.

[0019] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the foregoing embodiments.

[0020] Advantages of the present application:

[0021] The present application provides a secondary battery and an electronic device. By differentially setting bonding layers with different average sizes in the flat area and the corner area of the wound structure electrode assembly, and 2 ≤ D2 / D1 ≤ 6, it is beneficial to specifically improve the electrolyte storage capacity in the corner area, alleviate the generation of lithium deposition and black spots in the negative electrode plate during cycling, and improve the cycling performance of the secondary battery. At the same time, it is also beneficial to reduce the impact on the overall thickness of the flat area of the wound structure electrode assembly, thereby reducing the impact on the thickness of the secondary battery and taking into account the energy density.

[0022] Of course, implementing any product or method of the present application does not necessarily require achieving all of the above advantages simultaneously. Description of the Drawings

[0023] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other embodiments can also be obtained based on these drawings.

[0024] Figure 1 Schematic diagram of a wound structure electrode assembly in an embodiment of the present application;

[0025] Figure 2 Schematic diagram of the structure of the negative electrode plate along its own thickness direction in an embodiment of the present application;

[0026] Figure 3 Schematic diagram of a bonding layer in an embodiment of the present application;

[0027] Figure 4 Schematic diagram of the structure of the separator along its own thickness direction in an embodiment of the present application;

[0028] Figure 5 Schematic diagram of a composite layer in an embodiment of the present application. Detailed implementation manners

[0029] Next, the technical solutions in the present application will be clearly and completely described in conjunction with the embodiments of the present application and the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on the present application belong to the scope of protection of the present application.

[0030] It should be noted that in the specific implementation manners of the present application, a lithium-ion battery is taken as an example of a secondary battery to explain the present application, but the secondary battery of the present application is not limited to the lithium-ion battery. The specific technical solutions are as follows:

[0031] In the corner area of a wound structure secondary battery, there is usually a problem of insufficient storage capacity of the electrolyte, that is, insufficient "liquid storage capacity", which is prone to lithium deposition and black spots, thereby affecting the cycle performance of the secondary battery. To improve the liquid storage capacity of the corner area and thus reduce the occurrence of lithium deposition and black spots in the corner area, a bonding layer with a larger particle size can be coated on the surface of the separator to provide space for the storage of the electrolyte, thereby improving the cycle performance of the secondary battery. However, as the particle size of the bonding layer particles on the overall surface of the separator increases, the thickness of the main body of the wound structure secondary battery increases significantly, which will reduce the overall energy density of the secondary battery.

[0032] Based on the above problems, the present application provides a secondary battery and an electronic device, which can improve the liquid storage capacity of the corner area, reduce the occurrence of lithium deposition and black spots, thereby improving the cycle performance of the secondary battery. At the same time, it can also reduce the impact on the thickness of the main body of the wound structure secondary battery and take into account the energy density.

[0033] In the first aspect of the present application, a secondary battery is provided. If it includes a wound structure electrode assembly, the electrode assembly includes a negative electrode tab and a separator. The negative electrode tab includes a negative electrode current collector, a negative electrode material layer, and a bonding layer. Along the thickness direction of the negative electrode tab, the negative electrode material layer is located between the negative electrode current collector and the bonding layer. Specifically, such as Figures 1 to 3As shown, the electrode assembly 03 of the winding structure includes a negative electrode tab 02, a positive electrode tab 04, and a separator 01. The negative electrode tab 02 includes a negative current collector 021, a negative electrode material layer 022, and an adhesive layer 023, and the negative electrode material layer 022 and the adhesive layer 023 are disposed on one surface of the negative current collector 021. The adhesive layer 023 includes a first coating region 231 and a second coating region 232. The first coating region 231 is located in the flat region 032 of the electrode assembly 03, and the second coating region 232 is located in the corner region 031 of the electrode assembly 03. Compared with the current setting of the adhesive layer on the separator, setting the adhesive layer on the surface of the negative electrode material layer in this application is beneficial to reducing the blockage of the separator, improving the transport of active ions such as lithium ions, and further improving the kinetic performance of the secondary battery. It can be understood that the flat region of the secondary battery corresponds to the flat region of the electrode assembly, and the corner region of the secondary battery corresponds to the corner region of the electrode assembly. In some embodiments of this application, the negative electrode material layer and the adhesive layer can also be disposed on two surfaces of the negative current collector.

[0034] The first coating area includes the first coating area material, and the second coating area includes the second coating area material. In the cross-section of the bonding layer along its thickness direction, the average short-axis size of the particles of the first coating area material is D1 μm, and the average short-axis size of the particles of the second coating area material is D2 μm, where 2 ≤ D2 / D1 ≤ 6. In some embodiments of the present application, 3 ≤ D2 / D1 ≤ 5. For example, D2 / D1 can be 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3.8, 4, 4.2, 4.4, 4.6, 4.8, 5, 5.2, 5.4, 5.6, 5.8, 6, or a range composed of any two of these ratios. When D2 / D1 is too small, for example, less than 2, that is, D2 is too small or D1 is too large. When D2 is too small, the second coating area cannot store enough electrolyte, and lithium deposition and black spots are still likely to occur in the corner area. When D1 is too large, the thickness of the flat area will be too large, affecting the energy density of the secondary battery. When D2 / D1 is too large, for example, greater than 6, that is, D2 is too large or D1 is too small. When D2 is too large, the active ion transport distance is too long, the internal resistance increases, and thus the kinetic performance of the secondary battery is affected. When D1 is too small, the pores between the first coating area materials are reduced, affecting the transport of active ions, increasing the internal resistance, and lithium deposition and black spots are likely to occur in the flat area. Therefore, by differentially setting the bonding layers with different average size on the negative electrode tab in the flat area and the corner area of the wound structure electrode assembly, and 2 ≤ D2 / D1 ≤ 6, the first coating area material and the second coating area material are matched, which is beneficial to specifically improving the electrolyte storage capacity of the corner area, alleviating the generation of lithium deposition and black spots on the negative electrode tab during the cycling process, and improving the cycling performance of the secondary battery. At the same time, it is also beneficial to reduce the impact on the overall thickness of the flat area of the wound structure electrode assembly, and thus reduce the impact on the thickness of the secondary battery, taking into account the energy density. Thus, through the above settings, it is beneficial to balance the cycling performance and energy density of the flat area and the corner area of the secondary battery. It should be noted that during the formation hot pressing stage of the lithium-ion battery, the particles of the first coating area material raw material and the second coating area material raw material usually deform under hot pressing. For example, the shape of a single particle of the first coating area material changes after hot pressing, and the dimensions of its outer contour in two mutually perpendicular directions are different, usually elliptical or quasi-elliptical, and the average short-axis size is usually smaller than the average particle size of its raw material particles. The average short-axis size is also D1, and the same applies to D2. For another example, there are particles formed by swelling or adhesion between two or more adjacent particles, and the average short-axis size of these particles is usually larger than the average particle size of their raw material particles. It can be understood that the average short-axis size D1 of the particles of the first coating area material and the average short-axis size D2 of the particles of the second coating area material can be adjusted by changing the pressure during the formation hot pressing in the lithium-ion battery manufacturing process. Exemplarily, when the pressure in the formation hot pressing stage increases, D1 and D2 usually decrease; when the pressure in the formation hot pressing stage decreases, D1 and D2 usually increase.The present application places no particular restrictions on the temperature and pressure during the formation hot pressing stage, as long as the objectives of the present application can be achieved. Exemplarily, the temperature during the formation hot pressing stage is 70°C to 80°C, and the pressure is 1.0 MPa to 1.8 MPa.

[0035] In some embodiments of the present application, the adhesive layer is an aqueous adhesive layer. The aqueous adhesive layer in the present application is prepared by using water as a solvent to prepare an aqueous adhesive layer slurry. Compared with other types of adhesive layers, both the aqueous adhesive layer and the negative electrode sheet are usually prepared by using water as a solvent during the preparation process. Therefore, there is a better bonding effect between the obtained negative electrode sheet and the aqueous adhesive layer, which is conducive to alleviating the swelling phenomenon of the secondary battery after long-term cycling tests and improving the cycling performance of the secondary battery.

[0036] In some embodiments of the present application, 0.14 ≤ D1 ≤ 0.7. In some embodiments of the present application, 0.2 ≤ D1 ≤ 0.5. For example, D1 can be 0.14, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, 0.65, 0.7 or a range composed of any two of these values. By adjusting D1 within the above range, the particle size of the material in the first coating area is moderate, and thus the flat area of the obtained electrode assembly has an appropriate thickness. At the same time, a suitable D1 is also conducive to the transmission of active ions in the flat area of the secondary battery. Therefore, D1 within the above range is conducive to improving the cycling performance of the secondary battery and taking into account the energy density.

[0037] In some embodiments of the present application, 0.8 ≤ D2 ≤ 1.50. In some embodiments of the present application, 1 ≤ D2 ≤ 1.2. For example, D2 can be 0.8, 0.85, 0.9, 0.95, 1, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5 or a range composed of any two of these values. By adjusting D2 within the above range, more electrode liquid can be stored while promoting the better transmission of active ions, thereby improving the liquid storage capacity of the corner area and taking into account the kinetic performance, reducing the generation of lithium deposition and black spots, and thus improving the cycling performance of the secondary battery.

[0038] In some embodiments of the present application, the coating amount of the first coating area is a mg / 5000mm 2 , and the coating amount of the second coating area is b mg / 5000mm 2, (a + 0.1) ≤ b ≤ (a + 0.6). In some embodiments of the present application, (a + 0.2) ≤ b ≤ (a + 0.4). By adjusting a and b within the scope of the present application and matching with the average sizes that are different from those of the flat region and the corner region, it is beneficial to improve the liquid storage capacity of the second coating region while enhancing the adhesion between the electrode sheet corresponding to the corner region and the separator, reducing the internal resistance, and further improving the cycling performance. At the same time, the first coating region also has a high active ion transport rate, and the thickness of the electrode assembly in the flat region is appropriate, thereby improving the overall kinetic performance and cycling performance of the secondary battery and taking into account the energy density.

[0039] In some embodiments of the present application, 0.7 ≤ a ≤ 1.2. For example, a can be 0.7, 0.72, 0.75, 0.78, 0.8, 0.82, 0.85, 0.88, 0.9, 0.95, 1.0, 1.02, 1.05, 1.08, 1.1, 1.12, 1.15, 1.18, 1.2 or a range composed of any two of these values. By adjusting a within the above range, the coating amounts of the adhesive layers in the first coating region and the second coating region are appropriate, which is beneficial to the adhesion between the negative electrode sheet and the separator, and at the same time can provide a channel for the transport of active ions in the secondary battery, thus better taking into account the production process yield, kinetic performance and cycling performance of the secondary battery.

[0040] In some embodiments of the present application, 1.1 ≤ b ≤ 1.6. For example, b can be 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6 or a range composed of any two of these values.

[0041] In some embodiments of the present application, the material of the first coating region includes a first adhesive material and a second adhesive material, and the material of the second coating region includes a third adhesive material and a fourth adhesive material.

[0042] In some embodiments of the present application, the first adhesive material and the third adhesive material each independently include at least one of methyl acrylate, octyl acrylate, polymethacrylate or isooctyl acrylate. By selecting the above first binder and / or third adhesive material, it is beneficial to improve the viscosity of the material in the first coating region, and thus beneficial to reducing the internal resistance of the flat region of the secondary battery and improving the kinetic performance and cycling stability of the secondary battery.

[0043] In some embodiments of the present application, the second adhesive material and the fourth adhesive material each independently comprise at least one of styrene, acrylonitrile or polyvinylidene fluoride. By selecting the above-mentioned second binder and / or fourth adhesive material, a supporting effect can be achieved in the adhesive layer, and the obtained adhesive layer is not easily melted and softened during the hot pressing process, thereby reserving channels for the transport of active ions, reducing the generation of lithium deposition and black spots, and thus being beneficial to improving the cycling performance of the secondary battery.

[0044] The mass ratio m1 of the first adhesive material to the second adhesive material is 1:(1.2 to 2.0). For example, m1 can be 1:2, 1:1.25, 1:1.3, 1:1.35, 1:4, 1:1.45, 1:1.5, 1:1.55, 1:1.6, 1:1.65, 1:1.7, 1:1.75, 1:1.8, 1:1.85, 1:1.9, 1:1.95, 1:2 or a range composed of any two of these ratios. By selecting the first adhesive material and the second adhesive material with the above mass ratio, the obtained adhesive layer has both good viscosity and appropriate support, which is beneficial to improving the adhesion between the flat area negative electrode sheet and the separator, maintaining channels for ion transport, reducing internal resistance, and thus being beneficial to improving the kinetic performance and cycling performance of the secondary battery.

[0045] The mass ratio m2 of the third adhesive material to the fourth adhesive material is 1:(1.5 to 4). For example, m2 can be 1:1.5, 1:1.7, 1:2.0, 1:2.2, 1:2.6, 1:2.8, 1:3.0, 1:3.2, 1:3.4, 1:3.6, 1:3.8, 1:4 or a range composed of any two of these ratios. By selecting the third adhesive material and the fourth adhesive material with the above mass ratio, the obtained adhesive layer has both good viscosity and good support, which is beneficial to reducing the internal resistance in the corner area, reducing the generation of lithium deposition black spots, and thus being beneficial to further improving the cycling stability of the secondary battery.

[0046] In some embodiments of the present application, as Figure 1 and Figure 4 shown, the separator 01 includes a base film 011 and a composite layer 012 provided on one surface of the base film 011, and the composite layer 012 faces the negative electrode sheet 02. The composite layer includes a solid electrolyte and a ceramic material. Providing a composite layer on the surface of the base film can provide channels for the transport of active ions in the secondary battery. Even in the case of insufficient electrolyte in the corner area, the active ions can be transported through the solid electrolyte, reducing the internal resistance, reducing the generation of lithium deposition and black spots, and improving the kinetic performance and cycling performance of the secondary battery.

[0047] In some embodiments of the present application, based on the mass of the composite layer, the mass percentage content of the ceramic material is W2, and 78% ≤ W2 ≤ 92%. For example, W2 can be 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, or a range composed of any two of these values. By controlling W2 within the above range, it is beneficial to maintain a good active ion transport rate in the composite layer while improving the liquid storage capacity in the corner area, thereby reducing the generation of lithium deposition and black spots, reducing the internal resistance, and further improving the kinetic performance and cycling performance of the secondary battery.

[0048] In some embodiments of the present application, based on the mass of the composite layer, the mass percentage content of the solid electrolyte is W1, and 1% ≤ W1 ≤ 8%. In some embodiments of the present application, 3% ≤ W1 ≤ 6%. For example, W1 can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, or a range composed of any two of these values. By controlling W1 within the above range, it is beneficial to further reduce the internal resistance and improve the phenomena such as lithium deposition and black spots in the corner area, thereby further improving the kinetic performance and cycling performance of the secondary battery.

[0049] In some embodiments of the present application, as Figure 5 shown, the composite layer 012 includes a third coating area 121 and a fourth coating area 122. The third coating area 121 is located in the straight area 032 of the electrode assembly 03, and the fourth coating area 122 is located in the corner area 031 of the electrode assembly 03. The average particle size of the ceramic material in the third coating area is D3 nm, and the average particle size of the ceramic material in the fourth coating area is D4 nm.

[0050] In some embodiments of the present application, 200 ≤ D3 ≤ 500. In some embodiments of the present application, 300 ≤ D3 ≤ 400. For example, D3 can be 200, 220, 240, 260, 280, 300, 320, 340, 360, 380, 400, 420, 440, 460, 480, 500, or a range composed of any two of these values. By controlling D3 within the above range, the average particle size of the ceramic material particles is appropriate, which is beneficial to balance the active ion transport rate and the thickness of the straight area electrode assembly, thereby taking into account the cycling performance and energy density of the secondary battery.

[0051] In some embodiments of the present application, 600 ≤ D4 ≤ 800. In some embodiments of the present application, 650 ≤ D4 ≤ 750. For example, D4 can be 600, 610, 620, 630, 640, 650, 660, 670, 680, 690, 700, 710, 720, 730, 740, 750, 760, 770, 780, 790, 800 or a range composed of any two of these values. By controlling D4 within the above range, the particle size of the ceramic material in the corner region is relatively large, which is beneficial to further improving the electrolyte storage capacity in the corner region, alleviating the generation of lithium deposition and black spots on the negative electrode sheet during cycling, and improving the cycling performance of the secondary battery.

[0052] In some embodiments of the present application, the coating amount in the third coating region is c mg / 5000mm 2 , and the coating amount in the fourth coating region is d mg / 5000mm 2 ; (c + 0.1) ≤ d ≤ (c + 2). In some embodiments of the present application, (c + 0.4) ≤ d ≤ (c + 1.2). By controlling c and d within the above range, differentially setting the composite layer in the straight region and the corner region is beneficial to further providing more channels for electron transport in the corner region, reducing the generation of lithium deposition and black spots in the corner region, balancing the internal resistance of the straight region and the corner region of the secondary battery, and thus improving the overall kinetic performance and cycling performance of the secondary battery.

[0053] In some embodiments of the present application, 7 ≤ c ≤ 12. For example, c can be 7, 7.3, 7.6, 8, 8.2, 8.6, 9, 9.3, 9.5, 9.8, 10, 10.2, 10.4, 10.6, 10.8, 11, 11.2, 11.6, 11.8, 12 or a range composed of any two of these values. By controlling c within the above range, it is beneficial to improve the transport rate of active ions, and thus improve the kinetic performance of the secondary battery. At the same time, it is also beneficial to keep the overall thickness of the secondary battery within a suitable range, taking into account the energy density of the secondary battery.

[0054] In some embodiments of the present application, 7.1 ≤ d ≤ 14. In some embodiments of the present application, 10.1 ≤ d1 ≤ 12. For example, d1 can be 7.1, 7.5, 8, 8.5, 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14 or a range composed of any two of these values.

[0055] In some embodiments of the present application, the above solid electrolyte includes lithium aluminum titanium phosphate (Li 1.5 Al 0.5 Ti 1.5(PO4)3), lithium lanthanum zirconium oxide, lithium lanthanum zirconium titanium oxide, or lithium lanthanum titanium oxide. By selecting the above solid electrolyte, it is beneficial to further improve the transport rate of active ions in the secondary battery, thereby reducing the internal resistance, reducing the generation of lithium deposition and black spots, and improving the kinetic performance of the secondary battery.

[0056] In some embodiments of the present application, the above ceramic material includes at least one of boehmite, alumina, magnesium hydroxide, or barium sulfate. The above ceramic material has good heat resistance and electrochemical stability in the secondary battery, can further improve the liquid storage capacity in the corner area, reduce the generation of lithium deposition and black spots, and basically does not produce side reactions, thereby further improving the cycle performance of the secondary battery.

[0057] In the present application, the technical features in the above embodiments can be combined arbitrarily as long as the purpose of the present application can be achieved.

[0058] In some embodiments of the present application, the composite layer further includes a dispersant, a separator binder, and a wetting agent. The present application does not particularly limit the types of the dispersant, the separator binder, and the wetting agent as long as the purpose of the present application can be achieved. For example, the dispersant may include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. For example, the separator binder may include, but is not limited to, at least one of methyl acrylate, ethyl acrylate, butyl acrylate, octyl acrylate, isooctyl acrylate, acrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, polyimide, polyamideimide, styrene-butadiene rubber, or polyvinylidene fluoride. For example, the wetting agent may include, but is not limited to, at least one of sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, or sodium stearate.

[0059] The present application does not particularly limit the content of the dispersant in the composite layer as long as the purpose of the present application can be achieved. For example, based on the mass of the composite layer, the mass percentage content of the dispersant is 0.5% to 1%. The present application does not particularly limit the content of the separator binder in the composite layer as long as the purpose of the present application can be achieved. For example, based on the mass of the composite layer, the mass percentage content of the separator binder is 5% to 10%. The present application does not particularly limit the content of the wetting agent in the composite layer as long as the purpose of the present application can be achieved. For example, based on the mass of the composite layer, the mass percentage content of the wetting agent is 1% to 3%.

[0060] The present application does not particularly limit the preparation of the negative electrode sheet as long as the purpose of the present application can be achieved. For example, when the adhesive layer in the negative electrode sheet includes a first coating area and a second coating area, its preparation method includes, but is not limited to, the following steps:

[0061] First, mix the first adhesive material, the second adhesive material, and water to obtain the first coating area material slurry, and then mix the third adhesive material, the fourth adhesive material, and a solvent to obtain the second coating area material slurry; then, on one surface of the negative electrode plate, respectively set the first coating area material slurry and the second coating area material slurry with corresponding coating amounts in the areas corresponding to the first coating area and the second coating area, and dry them to obtain a negative electrode plate with a bonding layer set on one side. Then, repeat the above steps on the other surface of the negative electrode plate to obtain a negative electrode plate with bonding layers set on both sides.

[0062] This application has no special limitation on the solid content of the bonding layer slurry, as long as the purpose of this application can be achieved.

[0063] This application has no special limitation on the method of setting the bonding layer slurry. For example, the bonding layer slurry can be set on a coating roller, and the pattern of the coating roller can be set according to the requirements of the coating area. For example, the size of the coating area can be adjusted by adjusting the line width and line spacing in the wire groove. When the coating roller rotates and fits with the base film, the bonding layer slurry is set on the base film. The coating amount can be adjusted by adjusting the load amount of the bonding layer on the coating roller. The larger the load amount, the higher the coating amount, and vice versa.

[0064] This application has no special limitation on the preparation of the separator, as long as the purpose of this application can be achieved. For example, when the composite layer in the separator includes a third coating area and a fourth coating area, its preparation method includes but is not limited to the following steps:

[0065] First, mix the solid electrolyte, the ceramic material, the dispersant, the separator binder, and the solvent to obtain the composite layer slurry, and then, on one surface of the base film, respectively set the composite layer slurry with corresponding coating amounts in the areas corresponding to the third coating area and the fourth coating area, and dry them to obtain a separator with a composite layer set on one side. This application has no special limitation on the type of the solvent, as long as the purpose of this application can be achieved. For example, the solvent can include at least one of water, acetone, or N-methylpyrrolidone (NMP). This application has no special limitation on the solid content of the composite layer slurry, as long as the purpose of this application can be achieved. It can be understood that the ceramic material has a high hardness, and during the preparation of the secondary battery, its average particle size will not change significantly.

[0066] This application has no special limitation on the method of setting the composite layer. For example, it can be the method of setting the bonding layer as described above.

[0067] In the present application, the negative electrode sheet further includes a negative electrode current collector, and the negative electrode material layer is disposed on at least one surface of the negative electrode current collector. The above-mentioned "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along its own thickness direction, or can be disposed on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the surface of the negative electrode current collector, or can be a partial area of the surface of the negative electrode current collector. There is no special limitation in the present application, as long as the purpose of the present application can be achieved.

[0068] The present application has no special limitation on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam or composite current collector. Exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc.

[0069] The negative electrode material layer includes a negative electrode active material. The present application has no special limitation on the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material can include, but is not limited to, natural graphite, artificial graphite, mesophase microcarbon beads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, lithiated TiO2-Li4Ti5O with a spinel structure 12 or at least one of Li-Al alloy.

[0070] In some embodiments of the present application, the negative electrode material layer may further include a conductive agent and a negative electrode binder. The present application has no special limitation on the types of the conductive agent and the negative electrode binder, as long as the purpose of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The conductive carbon black can include, but is not limited to, at least one of acetylene black or Ketjen black. The above-mentioned carbon nanotubes can include, but is not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers can include, but is not limited to, vapor-grown carbon fibers (VGCF) and / or nanofibers. The above-mentioned metal materials can include, but is not limited to, metal powders and / or metal fibers. Specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymers can include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. For example, the negative electrode binder can be at least one of the above-mentioned separator binders. The present application has no special limitation on the mass ratio of the negative electrode active material, the conductive agent and the negative electrode binder in the negative electrode material layer. Those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0071] There is no particular limitation on the thickness of the negative electrode material layer in this application, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode material layer is 30 μm to 120 μm.

[0072] There is no particular limitation on the thickness of the negative electrode current collector in this application, as long as the purpose of this application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 15 μm.

[0073] Optionally, the negative electrode sheet may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. There is no particular limitation on the composition of the conductive layer in this application, and it may be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a negative electrode binder. There is no particular limitation on the conductive agent and the negative electrode binder in the conductive layer in this application. For example, it may be at least one of the above-mentioned conductive agents and the above-mentioned separator binder.

[0074] There is no particular limitation on the material of the base film in this application, as long as the purpose of this application can be achieved. For example, the material of the base film may include, but is not limited to, at least one of polyolefins (PO) mainly composed of polyethylene (PE) and polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of the separator may include at least one of a woven film, a non-woven film, a microporous film, a composite film, a rolled film or a spun film. There is no particular limitation on the thickness of the base film in this application, as long as the purpose of this application can be achieved. For example, the thickness of the base film may be 3 μm to 20 μm.

[0075] In this application, there is no particular limitation on the thickness of the separator, as long as the purpose of this application can be achieved. For example, the thickness of the separator may be 5 μm to 30 μm.

[0076] In this application, the secondary battery further includes a positive electrode sheet, which includes a positive electrode current collector and a positive electrode material layer provided on at least one surface of the positive electrode current collector. The above-mentioned "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer may be provided on one surface of the positive electrode current collector along its own thickness direction, or may be provided on two surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here may be the entire area of the positive electrode current collector surface, or may be a partial area of the positive electrode current collector surface. There is no particular limitation in this application, as long as the purpose of this application can be achieved.

[0077] There is no particular limitation on the positive electrode current collector in this application, as long as the purpose of this application can be achieved. For example, it may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector), etc.

[0078] The positive electrode material layer includes a positive electrode active material. There are no particular limitations on the positive electrode active material in this application, as long as the objectives of this application can be achieved. For example, the positive electrode active material may include, but is not limited to, at least one of lithium nickel cobalt manganese oxide (such as NCM811, NCM622, NCM523, NCM111), lithium nickel cobalt aluminate, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt oxide (LiCoO2), lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate.

[0079] The positive electrode material layer may further include a conductive agent and a positive electrode binder. There are no particular limitations on the types of the conductive agent and the positive electrode binder in this application, as long as the objectives of this application can be achieved. For example, it may be at least one of the above-mentioned conductive agents and the above-mentioned separator binder. There are no particular limitations on the mass ratio of the positive electrode active material, the conductive agent, and the positive electrode binder in the positive electrode material layer. Those skilled in the art can select according to actual needs as long as the objectives of this application can be achieved.

[0080] There are no particular limitations on the thickness of the positive electrode current collector and the positive electrode material layer in this application, as long as the objectives of this application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, and the thickness of the positive electrode material layer is 30 μm to 120 μm.

[0081] Optionally, the positive electrode sheet may further include a conductive layer, which is located between the positive electrode current collector and the positive electrode material layer. There are no particular limitations on the composition of the conductive layer, and it may be a commonly used conductive layer in the art. The conductive layer includes a conductive agent and a positive electrode binder. There are no particular limitations on the conductive agent and the positive electrode binder in the conductive layer in this application. For example, it may be at least one of the above-mentioned conductive agents and the above-mentioned separator binder.

[0082] In this application, the secondary battery further includes an electrolyte, which includes a lithium salt and a non-aqueous solvent.

[0083] There are no particular limitations on the lithium salt in this application, as long as the objectives of this application can be achieved. For example, the lithium salt may include, but is not limited to, at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalato)borate (LiBOB), or lithium difluoroborate. There are no particular limitations on the content of the lithium salt in the electrolyte as long as the objectives of this application can be achieved.

[0084] There are no particular limitations on the non-aqueous solvent in this application, as long as the objectives of this application can be achieved. For example, the non-aqueous solvent may include, but is not limited to, at least one of carbonate compounds, carboxylate compounds, ether compounds, or other organic solvents.

[0085] The above-mentioned carbonate compounds may include, but are not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds or fluorinated carbonate compounds. The above-mentioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or methyl ethyl carbonate (MEC). The above-mentioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinylene ethylene carbonate (VEC). The fluorinated carbonate compounds may include, but are not limited to, at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethyl ethylene carbonate. The above-mentioned carboxylic ester compounds may include, but are not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone or caprolactone. The above-mentioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate or trioctyl phosphate. The present application does not particularly limit the content of the non-aqueous solvent in the electrolyte, as long as the object of the present application can be achieved.

[0086] The secondary battery further includes a housing for accommodating the positive electrode plate, the separator, the negative electrode plate and the electrolyte, as well as other components known in the field of secondary batteries. The present application does not limit the above-mentioned other components. The present application does not particularly limit the housing, and it may be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing may be a hard shell housing or a flexible housing. The material of the hard shell housing may be metal. The present application does not limit the type of metal, and a metal hard shell housing known in the art may be used, as long as the object of the present application can be achieved. The flexible housing may be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0087] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and there is no special limitation in the present application. For example, the preparation process of the secondary battery may include but is not limited to the following steps: stacking the positive electrode sheet, the separator, the negative electrode sheet, and the separator in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into a housing, injecting an electrolyte into the housing and sealing it to obtain a secondary battery. Among them, when the separator includes a composite layer, one side of the composite layer is arranged facing the negative electrode sheet. In addition, an overcurrent protection element, a guide plate, etc. can also be placed in the housing as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging.

[0088] The second aspect of the present application provides an electronic device, which includes the secondary battery in any of the foregoing embodiments. Thus, the electronic device provided by the present application has good performance in use.

[0089] The present application does not particularly limit the type of the electronic device, and it can be any electronic device known in the prior art. In some embodiments of the present application, the electronic device may include but is not limited to a laptop computer, a pen input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal TV, a portable cleaner, a portable CD player, a minidisc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a moped, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flash, a camera, a large household battery, and a lithium-ion capacitor, etc.

[0090] Examples

[0091] Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0092] Testing method and device:

[0093] Sampling of the negative electrode sheet and the separator:

[0094] First, disassemble the lithium-ion battery, take out the negative electrode sheet and the separator, wash them with dimethyl carbonate (DMC), and then dry the negative electrode sheet at 80 °C to obtain a negative electrode sheet sample; dry the separator at room temperature to obtain a separator sample.

[0095] Unless otherwise specified, the following test methods are all carried out using the obtained negative electrode sheet sample and separator sample.

[0096] The average short-axis size D1 of the particles of the material in the first coating area and the average short-axis size D2 of the particles of the material in the second coating area:

[0097] Take the negative electrode sheet sample corresponding to the flat area of the electrode assembly. Ion-polish the cross-section of the negative electrode sheet along the thickness direction, measure it under a scanning electron microscope (SEM), and combine with the element analysis function of the SEM. Select an area without metal elements, and then select 10 particles without metal elements from this area. Measure their short-axis sizes respectively, and calculate the average value, which is D1.

[0098] Then take the negative electrode sheet sample corresponding to the corner area of the electrode assembly, and measure D2 by the same method as above.

[0099] The average particle size D3 of the ceramic particles in the third coating area and the average particle size D4 of the ceramic particles in the fourth coating area:

[0100] Take the separator sample corresponding to the area (overhang area) of the electrode assembly flat area that extends beyond the electrode sheet. Ion-polish the cross-section of the separator sample along the thickness direction, measure it under a scanning electron microscope (SEM), and combine with the element analysis function of the SEM. Select 10 particles containing metal elements but no transition metal elements, measure their inscribed circle diameters respectively, and calculate the average value, which is D3.

[0101] Then take the separator sample corresponding to the corner area of the electrode assembly, and measure D4 by the same method as above.

[0102] The coating amount c in the third coating area and the coating amount d in the fourth coating area:

[0103] Take the separator sample corresponding to the area (overhang area) of the electrode assembly flat area that extends beyond the electrode sheet, cut it into samples with an area of s mm 2 Test it with a thermogravimetry-mass spectrometry (TGA-MS) instrument. Set the starting temperature at 25 °C, the ending temperature at 450 °C, and the heating rate at 5 °C / min. Test it in an air atmosphere, record the mass change curve with temperature, the sum of the masses lost at 180 °C, u3, is the mass of the dispersant, binder, and wetting agent in the composite layer, and record the final remaining sample weight as the mass u4 mg of the composite layer.

[0104] The coating amount of the composite layer = (u3 + u4) / s × 5000.

[0105] Energy density:

[0106] Use a 3D scanner to measure the length L, width W, and thickness H of the battery.

[0107] At an ambient temperature of 25°C, the lithium-ion battery is charged at a constant current of 0.2C to a voltage of 4.5V by a charge and discharge tester, then charged at a constant voltage of 4.5V until the cut-off current is 0.05C, left standing for 10 min, and then discharged at a constant current of 0.2C to 3V. Record the discharge capacity C and the discharge platform V. Then the volume energy density VED of the lithium-ion battery is VED=(C×V) / (L×W×H), with the unit of Wh / L.

[0108] Lithium plating test at 0°C and 1.5C:

[0109] Place the lithium-ion battery in an environment of 10°C, charge it at a constant current of 1.5C to a voltage of 4.5V, then charge it at a constant voltage of 4.5V until the cut-off current is 0.05C, leave it standing for 5 min, discharge it at a constant current of 0.5C to a voltage of 3.0V, and leave it standing for 5 min. This is one charge and discharge cycle. Then perform 10 cycles of charging and discharging in the same steps.

[0110] Then charge it at a constant current of 1.5C to 4.5V, then charge it at a constant voltage of 4.5V until the cut-off current is 0.05C, and leave it standing for 5 min; then disassemble the lithium-ion battery and confirm the surface of the negative electrode sheet adjacent to the separator as the observation interface.

[0111] Use the lithium plating test at 0°C and 1.5C to characterize the kinetic performance of the lithium-ion battery.

[0112] Interface observation:

[0113] Observe whether the interface on the surface of the negative electrode sheet is "golden" and uniform. If there are grayish-white bright spots, it is recorded as "lithium plating". If there is no grayish-white lithium metal precipitation, it is recorded as "no lithium plating". If there is grayish-white lithium metal precipitation and the proportion of the lithium plating area is less than 5%, it is recorded as slight lithium plating; if there is grayish-white lithium metal precipitation and the proportion of the lithium plating area is greater than or equal to 5%, it is recorded as severe lithium plating. Among them, the proportion of the lithium plating area is the percentage of the lithium plating area in the area of the observation interface of the negative electrode sheet. Among them, the observation interface is the observation interface in the "lithium plating test at 0°C and 1.5C" test method.

[0114] Cycle capacity retention rate:

[0115] At 25°C, charge the lithium-ion battery at a constant current of 2.0C to a voltage of 4.5V, then charge it at a constant voltage of 4.5V to 0.05C, and leave it standing for 10 min; then discharge it at a constant current of 1C to a voltage of 3.0V and leave it standing for 5 min. The above is one cycle. Follow the above cycle to 800 cycles, record the discharge capacity of the first cycle as the initial capacity Q0, and the discharge capacity of the lithium-ion battery in the 800th cycle as Q1. Calculate the cycle capacity retention rate of the lithium-ion battery in the 800th cycle through the following formula: Cycle capacity retention rate (%) = Q1 / Q0×100%.

[0116] Cycle thickness expansion rate:

[0117] At 25 °C, the lithium-ion battery is charged at a constant current of 0.2C until the voltage reaches 4.5V, and then charged at a constant voltage of 4.5V until 0.05C, reaching the fully charged state. Record the thickness H1 of the lithium-ion battery at this time. Then discharge it at a constant current of 1C until the voltage reaches 3.0V and let it stand for 5 minutes. The above is one cycle. Follow the above cycle for 800 cycles, and then charge the lithium-ion battery to the fully charged state according to the above method and test its thickness H2.

[0118] Cyclic thickness expansion rate = (H2 - H1) / H1 × 100%.

[0119] Example 1-1

[0120] <Preparation of the positive electrode sheet>

[0121] Mix the positive active material lithium cobaltate, the conductive agent Super P, and the positive binder polyvinylidene fluoride in a mass ratio of 97.9:0.9:1.2, add N-methylpyrrolidone (NMP) as a solvent, and formulate a slurry with a solid content of 75 wt%. After vacuum stirring evenly, the positive electrode slurry is obtained. The positive electrode slurry is evenly coated on one surface of a positive current collector aluminum foil with a thickness of 10 μm and dried at 120 °C to obtain a positive electrode sheet with a single-sided coated positive electrode material layer. The coating weight of the positive electrode material layer is 267.8 mg / 1540 mm 2 . Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. After drying at 120 °C, it is cold-pressed, and then cut into pieces and the tabs are welded to obtain a positive electrode sheet with a specification of 74 mm × 867 mm for use. Among them, the thickness of the single-sided positive electrode material layer is 42 μm.

[0122] <Preparation of the negative electrode sheet>

[0123] Binder layer slurry: Mix the first binder material methyl acrylate and the second binder material styrene in a mass ratio m1 of 1:1.5 in water as a solvent to obtain a first coating area material slurry with a solid content of 20 wt%. Then mix the third binder material methyl acrylate and the fourth binder material styrene in a mass ratio m2 of 1:2.7 in water as a solvent to obtain a second coating area material slurry with a solid content of 25 wt%.

[0124] Mix the negative active material artificial graphite, the negative binder styrene-butadiene rubber, and the conductive agent acetylene black in a mass ratio of 97.4:1.4:1.2, add deionized water as a solvent, and formulate a slurry with a solid content of 45 wt%. After vacuum stirring evenly with a vacuum mixer, the negative electrode slurry is obtained.

[0125] The negative electrode slurry is evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 6 μm, and dried at 120 °C to obtain a negative electrode sheet with a single-sided coated negative electrode material layer. The coating weight of the negative electrode material layer is 142 mg / 1540 mm 2 Then, in the area corresponding to the first coating area on one surface of the negative electrode material layer, a coating amount of a mg / 5000 mm 2 = 0.9 mg / 5000 mm 2 of the first coating area material slurry is set. In the area corresponding to the second coating area on this surface, a coating amount of b mg / 5000 mm 2 = 1.2 mg / 5000 mm 2 of the second coating area material slurry is set and dried to obtain a negative electrode sheet with a single-sided bonding layer. Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with a double-sided bonding layer. After drying at 120 °C, it is cold-pressed, and then cut into pieces and the tabs are welded to obtain a negative electrode sheet with a specification of 78 mm × 875 mm for use. Among them, the thickness of the single-sided negative electrode material layer is 54.5 μm. Among them, in the raw materials of the first coating area material, the average particle sizes of the first binder and the second binder are the same and are both 0.7 μm. In the raw materials of the second coating area material, the average particle sizes of the third binder and the fourth binder are the same and are both 4 μm.

[0126] <Preparation of electrolyte>

[0127] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate are mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Then, an electrolyte salt LiPF6 is added to the organic solvent and mixed evenly to obtain an electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of the electrolyte salt is 12.5%, and the rest is the organic solvent.

[0128] <Separator>

[0129] Composite layer slurry: Boehmite, a ceramic material with an average particle size of 350 nm, sodium carboxymethyl cellulose as a dispersant, styrene-butadiene rubber as a binder, and sodium dodecyl sulfate as a wetting agent are mixed evenly in solvent water in a mass ratio of 92∶0.8∶6∶1.2 to obtain a third coating area composite layer slurry with a solid content of 30 wt%; then, boehmite, a ceramic material with an average particle size of 700 nm, is mixed evenly according to the above method to obtain a fourth coating area composite layer slurry with a solid content of 30 wt%.

[0130] A 5-μm-thick polyethylene film is used as the base film of the separator. In the area corresponding to the third coating area on one surface of the base film, a coating amount of c mg / 5000 mm 2 = 10 mg / 5000 mm 2The composite layer slurry, and a coating amount d mg / 5000mm is set in the area corresponding to the fourth coating area on the surface 2 = 11mg / 5000mm 2 of the adhesive layer slurry, and it is dried. Among them, the thickness of the composite layer is 1.5μm, and the porosity of the separator is 39%. The specific parameters are shown in Tables 1 to 2.

[0131] <Preparation of Lithium-Ion Batteries>

[0132] Stack the above-prepared positive electrode sheet, separator, negative electrode sheet, and separator in sequence, with the separator in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, wind it up to obtain a wound electrode assembly, put the electrode assembly into an aluminum-plastic film packaging bag, remove moisture at 80°C, inject the above-prepared electrolyte, and obtain a lithium-ion battery through processes such as vacuum packaging, standing, formation, degassing, and trimming. Among them, the upper limit voltage of formation is 4.15V, the formation temperature is 70°C, the formation hot pressing pressure is 1.5MPa, the formation time is 0.5h, and the formation standing time is 2h.

[0133] Examples 1-2 to Examples 1-20

[0134] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Example 1-1. Among them, in Examples 1-1 to 1-12, D1 and D2 are as shown in Table 1 by changing the formation hot pressing pressure during the preparation of the lithium-ion battery.

[0135] Examples 1-21 to Examples 1-30

[0136] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as Example 1-1.

[0137] Examples 2-1 to Examples 2-9

[0138] Except for adjusting the relevant preparation parameters according to Table 3, the rest are the same as Example 1-1.

[0139] Examples 2-10 to Examples 2-28

[0140] Except for adjusting the relevant preparation parameters according to Table 4, the rest are the same as Example 1-1.

[0141] Comparative Examples 1 to Comparative Examples 5

[0142] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as Example 1-1.

[0143] Table 1

[0144]

[0145]

[0146] It can be seen from Examples 1-1 to 1-20 and Comparative Examples 1-1 to 1-5 that the relationship between D1 and D2 in the examples satisfies 2 ≤ D2 / D1 ≤ 6. For the obtained lithium-ion batteries, there is no serious lithium plating or black spots in both the flat area and the corner area, and they have a high cycle capacity retention rate, a small cycle thickness expansion rate, and can balance the kinetic performance and energy density at the same time. In Comparative Example 1, although the lithium plating phenomenon is not serious, due to the too large average short-axis size D1 of the particles of the coating material in the first coating area, the adhesion is worse under the same coating amount, and D2 / D1 < 2. Therefore, the obtained lithium-ion battery has a large cycle capacity retention rate and a large cycle thickness expansion rate. In Comparative Example 2, the too large average D2 of the material in the second coating area along the thickness direction of the adhesive layer itself leads to D2 / D1 > 6, resulting in too long an ion transport path in the corner area, an increase in internal resistance, and then a decrease in the cycle capacity retention rate of the lithium-ion battery, and gas swelling occurs, the cycle thickness expansion rate is large, and the volume of the corner area increases, affecting the energy density. In Comparative Example 3, the too large average short-axis size D1 of the particles of the material in the first coating area leads to D2 / D1 < 2. The obtained lithium-ion battery has a large thickness and low energy density, and the adhesion in the first coating area is poor. Therefore, the obtained lithium-ion battery has a low cycle capacity retention rate and a high cycle thickness expansion rate. In Comparative Example 4, the too small average short-axis size D2 of the particles of the material in the second coating area leads to D2 / D1 < 2. The too small D2 results in insufficient liquid storage in the corner area and serious lithium plating. The obtained lithium-ion battery has poor cycle performance and a high thickness change rate. In Comparative Example 5, the too large average short-axis size D1 of the particles of the material in the first coating area and the too small average short-axis size D2 of the particles of the material in the second coating area lead to D2 / D1 < 2. The too small D2 results in insufficient liquid storage in the corner area and serious lithium plating, while the too large D1 makes the viscosity in the flat area insufficient. Therefore, the obtained lithium-ion battery has a poor cycle capacity retention rate and a high cycle thickness expansion rate. This shows that while the liquid storage capacity in the corner area of the lithium-ion battery that meets the present application is improved, the flat area still has a good ion transport rate, the viscosity between the negative electrode sheet and the separator is moderate, there is no serious lithium plating in both the flat area and the corner area, the lithium-ion battery has good cycle performance and kinetic performance, and can balance the energy density.

[0147] The coating amount b of the second coating area usually affects the kinetic performance, cycle performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 1-13 to 1-18 that when the coating amount b of the second coating area is within the scope of the present application, there is no lithium plating phenomenon in the corner area, and the obtained lithium-ion battery has a high cycle capacity retention rate, a low cycle thickness expansion rate, and a high energy density. This shows that when the coating amount b of the second coating area is within the scope of the present application, the obtained lithium-ion battery has good cycle performance and kinetic performance, and can balance the energy density.

[0148] The coating amount a of the first coating area usually affects the kinetic performance, cycling performance and energy density of the lithium-ion battery. It can be seen from Example 1-1, Example 1-19 to Example 1-20 that when the coating amount a of the first coating area is within the scope of this application, there is no serious lithium plating phenomenon in the flat area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate and a high energy density. Thus, it shows that when the coating amount a of the first coating area is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0149] Table 2

[0150]

[0151]

[0152] The mass ratio m1 of the first binder material to the second binder material usually affects the kinetic performance, cycling performance and energy density of the lithium-ion battery. It can be seen from Example 1-1, Example 1-21 to Example 1-24 that when the mass ratio m1 of the first binder material to the second binder material is within the scope of this application, there is no serious lithium plating phenomenon in both the flat area and the corner area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate and a high energy density. Thus, it shows that when the mass ratio m1 of the first binder material to the second binder material is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0153] The mass ratio m2 of the third binder material to the fourth binder material usually affects the kinetic performance, cycling performance and energy density of the lithium-ion battery. It can be seen from Example 1-1, Example 1-25 to Example 1-28 that when the mass ratio m2 of the third binder material to the fourth binder material is within the scope of this application, there is no serious lithium plating phenomenon in both the flat area and the corner area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate and a high energy density. Thus, it shows that when the mass ratio m2 of the third binder material to the fourth binder material is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0154] The types of the first binder material, the third binder material, the second binder material, and the fourth binder material usually affect the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 1-29 to 1-30 that when the types of the first binder material, the third binder material, the second binder material, and the fourth binder material are within the scope of this application, there is no serious lithium plating phenomenon in the straight area and the corner area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. Thus, it shows that when the types of the first binder material, the third binder material, the second binder material, and the fourth binder material are within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0155] Table 3

[0156]

[0157]

[0158] In Table 3, " / " represents the absence of relevant parameters.

[0159] The mass percentage contents of the ceramic material and the solid electrolyte in the composite layer usually affect the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 2-1 to 2-8 that when the mass percentage contents of the ceramic material and the solid electrolyte in the composite layer are within the scope of this application, there is no serious lithium plating phenomenon in the straight area and the corner area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. Thus, it shows that when the mass percentage contents of the ceramic material and the solid electrolyte in the composite layer are within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0160] The types of the ceramic material and the solid electrolyte in the composite layer usually affect the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 2-1, 2-8, and 2-9 that when the types of the ceramic material and the solid electrolyte in the composite layer are within the scope of this application, there is no serious lithium plating phenomenon in the straight area and the corner area, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. Thus, it shows that when the types of the ceramic material and the solid electrolyte in the composite layer are within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0161] Table 4

[0162]

[0163]

[0164] The average particle size D3 of the ceramic material in the third coating region generally affects the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 2-1, 2-10 to 2-15 that when the average particle size D3 of the ceramic material in the third coating region is within the scope of this application, there is no serious lithium plating phenomenon in both the straight region and the corner region, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. This shows that when the average particle size D3 of the ceramic material in the third coating region is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0165] The average particle size D4 of the ceramic material in the fourth coating region generally affects the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 2-1, 2-16 to 2-21 that when the average particle size D4 of the ceramic material in the fourth coating region is within the scope of this application, there is no serious lithium plating phenomenon in both the straight region and the corner region, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. This shows that when the average particle size D4 of the ceramic material in the fourth coating region is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0166] The coating amount of the third coating region generally affects the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 2-1, 2-27, and 2-28 that when the coating amount of the third coating region is within the scope of this application, there is no serious lithium plating phenomenon in both the straight region and the corner region, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. Among them, when the coating amount of the third coating region is too low, such as in Example 2-27, the mechanical strength and heat shrinkage resistance of the obtained separator are low, which will affect the safety performance of the lithium-ion battery. This shows that when the coating amount of the third coating region is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0167] The coating amount of the fourth coating region generally affects the kinetic performance, cycling performance, and energy density of the lithium-ion battery. It can be seen from Examples 2-1, 2-22 to 2-26 that when the coating amount of the fourth coating region is within the scope of this application, there is no serious lithium plating phenomenon in both the straight region and the corner region, and the obtained lithium-ion battery has a high cycling capacity retention rate, a low cycling thickness expansion rate, and a high energy density. This shows that when the coating amount of the fourth coating region is within the scope of this application, the obtained lithium-ion battery has good cycling performance and kinetic performance, and takes into account the energy density.

[0168] It should be noted that in this text, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the term "comprising", "including" or any other variant thereof is intended to cover non-exclusive inclusion, so that a process, method or article comprising a series of elements not only includes those elements, but also includes other elements not expressly listed, or further includes elements inherent to such process, method or article.

[0169] Each embodiment in this specification is described in a related manner. For the same or similar parts among the embodiments, reference can be made to each other. Each embodiment focuses on the differences from other embodiments.

[0170] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included within the scope of protection of the present application.

Claims

1. A secondary battery, comprising an electrode assembly of a wound structure, the electrode assembly comprising a negative electrode sheet and a separator, the negative electrode sheet comprising a negative electrode current collector, a negative electrode material layer and a bonding layer, and along the thickness direction of the negative electrode sheet, the negative electrode material layer is located between the negative electrode current collector and the bonding layer; The bonding layer includes a first coating area and a second coating area, the first coating area is located in a straight area of ​​the electrode assembly, the second coating area is located in a corner area of ​​the electrode assembly, the first coating area includes a first coating area material, and the second coating area includes a second coating area material; In the cross section of the bonding layer along its own thickness direction, the average minor axis size of the particles of the first coating area material is D1 μm, the average minor axis size of the particles of the second coating area material is D2 μm, and 2≤D2 / D1≤6.

2. The secondary battery according to claim 1, wherein 0.14≤D1≤0.7, 0.8≤D2≤1.

5.

3. The secondary battery according to claim 1, which satisfies at least one of the following characteristics: (1)3≤D2 / D1≤5; (2)0.2≤D1≤0.5； (3)1≤D2≤1.2。 4. The secondary battery according to claim 1, wherein The coating amount of the first coating area is a mg / 5000mm 2 The coating amount of the second coating area is b mg / 5000mm 2 , (a+0.1)≤b≤(a+0.6).

5. The secondary battery according to claim 4, wherein (a+0.2)≤b≤(a+0.4).

6. The secondary battery according to claim 4, wherein 0.7≤a≤1.2。 7. The secondary battery according to claim 1, wherein The first coating area material includes a first bonding material and a second bonding material, and the second coating area material includes a third bonding material and a fourth bonding material; The first bonding material and the third bonding material each independently include at least one of methyl acrylate, octyl acrylate, polymethacrylate or isooctyl acrylate; the second bonding material and the fourth bonding material each independently include at least one of styrene, acrylonitrile or polyvinylidene fluoride; The mass ratio m1 of the first bonding material to the second bonding material is 1:(1.2 to 2.0); A mass ratio m2 of the third bonding material to the fourth bonding material is 1:(1.5 to 4).

8. The secondary battery according to any one of claims 1 to 7, wherein The diaphragm includes a base film and a composite layer arranged on a surface of the base film, the composite layer faces the negative electrode plate, and the composite layer includes a ceramic material. Based on the mass of the composite layer, the mass percentage of the ceramic material is W2, 78%≤W2≤92%.

9. The secondary battery according to claim 8, wherein The composite layer further includes a solid electrolyte. Based on the mass of the composite layer, the mass percentage of the solid electrolyte is W1, 1%≤W1≤8%.

10. The secondary battery according to claim 9, wherein 3％≤W1≤6％。 11. The secondary battery according to claim 8, wherein The composite layer includes a third coating area and a fourth coating area, the third coating area is located in the straight area of ​​the electrode assembly, the fourth coating area is located in the corner area of ​​the electrode assembly, the average particle size of the ceramic material in the third coating area is D3 nm, the average particle size of the ceramic material in the fourth coating area is D4 nm, 200≤D3≤500,600≤D4≤800.

12. The secondary battery according to claim 11, which satisfies at least one of the following characteristics: (1)300≤D3≤400； (2)650≤D4≤750。 13. The secondary battery according to claim 11, wherein The coating amount of the third coating area is c mg / 5000mm 2 The coating amount of the fourth coating zone is d mg / 5000mm 2 ; (c+0.1)≤d≤(c+2).

14. The secondary battery according to claim 13, wherein (c+0.4)≤d≤(c+1.2).

15. The secondary battery according to claim 13, wherein 7≤c≤12。 16. The secondary battery according to claim 9, which satisfies at least one of the following characteristics: (1) The solid electrolyte comprises at least one of lithium aluminum titanium phosphate, lithium lanthanum zirconium oxide, lithium lanthanum zirconium titanium oxide or lithium lanthanum titanium oxide; (2) The ceramic material includes at least one of boehmite, alumina, magnesium hydroxide or barium sulfate. 17 . An electronic device comprising the secondary battery according to claim 1 .

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