Secondary battery and electronic device

Abstract

The invention provides a secondary battery and an electronic device, the secondary battery comprises a positive pole piece and a diaphragm, the positive pole piece comprises a positive current collector and a positive material layer, the positive material layer comprises a first material layer and a second material layer, and the first material layer is located between the positive current collector and the second material layer; a first region, a boundary region and a second region are respectively arranged from the first material layer to the second material layer. The average particle size of the active material particles in the first region is D1, 3 [mu] m < = D1 < = 5 [mu] m, the average particle size of the active material particles in the second region is D2, 9 [mu] m < = D2 < = 40 [mu] m, and the average particle size of the active material particles in the boundary region is D3, 6 [mu] m < = D3 < = 22 [mu] m; the diaphragm comprises a base film and bonding layers, and at least one bonding layer is in contact with the second material layer. The secondary battery provided by the invention has good safety performance and cycle performance.

Description

Technical Field

[0001] This application relates to the field of electrochemistry technology, and particularly to a secondary battery and an electronic device. Background Art

[0002] With the rapid development of renewable energy and electric transportation, secondary batteries are widely used in modern life and industrial fields. From smart phones, laptops to electric vehicles, energy storage systems, secondary batteries provide efficient and stable energy support for portable electronic devices and new energy technologies.

[0003] However, in the positive electrode active material of the secondary battery obtained by the current preparation process, there are usually residual alkaline substances, which are likely to react with the adhesive layer in the separator, affecting the bonding effect between the separator and the positive electrode plate, resulting in insufficient adhesion between the positive electrode material layer and the separator, and the positive electrode plate and the separator are prone to separation. This will not only cause the secondary battery to deform and affect the cycle performance, but also cause lithium deposition black spots, posing a risk of piercing the separator and then short - circuiting, affecting the safety performance of the secondary battery. Therefore, how to enhance the peel strength between the positive electrode plate and the separator and improve the cycle performance of the secondary battery is an urgent problem to be solved. Summary of the Invention

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

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

[0006] A first aspect of the present application provides a secondary battery, which includes a positive electrode plate and a separator, the positive electrode plate includes a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector, the positive electrode material layer includes a first material layer and a second material layer, the first material layer is located between the positive electrode current collector and the second material layer; the contact surface between the first material layer and the second material layer is an interface, and the interface area includes an area extending 5 μm from the interface along the direction of the first material layer to the second material layer, and an area extending 5 μm from the interface along the direction of the second material layer to the first material layer; The area except the junction area in the first material layer is the first area, and the area except the junction area in the second material layer is the second area; the average particle size of the active material particles in the first area is D1, 3μm≤D1≤5μm, the average particle size of the active material particles in the second area is D2, 9μm≤D2≤40μm, and the average particle size of the active material particles in the junction area is D3, 6μm≤D3≤22μm; the diaphragm includes a base film and an adhesive layer, the adhesive layer is arranged on at least one surface in the thickness direction of the base film, and at least one adhesive layer is in contact with the second material layer. Through the above arrangement, the bonding effect between the positive electrode plate and the diaphragm is good, and more paths are reserved for the transmission of active ions, which is conducive to reducing the deformation of the secondary battery and the generation of lithium plating black spots, and is conducive to better balancing the safety performance, dynamic performance and cycle performance of the secondary battery.

[0007] In some embodiments of the present application, the peel strength between the second material layer and the separator is F, 1N / m≤F≤20N / m. In some embodiments of the present application, 3N / m≤F≤15N / m. When the peel strength F between the second material layer and the separator is within the above range, the bonding effect between the positive electrode sheet and the separator is good, and more paths are reserved for the transmission of active ions, which is beneficial to reduce the deformation of the secondary battery and the generation of lithium plaques, improve the cycle performance and safety performance of the secondary battery, and take into account the dynamic performance.

[0008] In some embodiments of the present application, the adhesive layer includes at least one of polyacrylate, oily polyvinylidene fluoride, polyvinylidene fluoride or polyvinylidene fluoride-hexafluoropropylene copolymer. By selecting the above adhesive with good viscosity, it is beneficial to further enhance the peel strength between the second material layer and the separator, thereby reducing the deformation of the secondary battery and the generation of lithium plaques, and improving the cycle performance and safety performance of the secondary battery.

[0009] In some embodiments of the present application, based on the area of the separator, the area proportion of the adhesive layer is b, and 10% ≤ b×100% ≤ 70%. In some embodiments of the present application, 30% ≤ b×100% ≤ 50%. When the area proportion of the adhesive layer is within the above range, it is beneficial to balance the internal resistance of the positive electrode sheet while reducing the possibility of separation between the positive electrode sheet and the separator, thereby being beneficial to improving the safety performance and cycle performance of the secondary battery while taking into account the kinetic performance.

[0010] In some embodiments of the present application, the second material layer includes a second positive electrode active material, and the specific surface area of the second positive electrode active material is S, 0.07 m 2 / g ≤ S ≤ 0.3 m 2 / g. In some embodiments of the present application, 0.1 m 2 / g ≤ S ≤ 0.25 m 2 / g. In some embodiments of the present application, 0.12 m 2 / g ≤ S ≤ 0.2 m 2 / g. By regulating S within the above range, it is beneficial to reduce the deformation of the secondary battery and the appearance of lithium deposition black spots, and improve the cycle performance and safety performance of the secondary battery. At the same time, the specific surface area within the above range is also beneficial to shortening the transmission path of active ions such as lithium ions, thereby being beneficial to reducing the internal resistance of the positive electrode sheet and improving the kinetic performance of the secondary battery.

[0011] In some embodiments of the present application, the thermal shrinkage rate of the separator is h, and 1% ≤ h ≤ 15%. When the thermal shrinkage rate h of the separator is within the above range, the separator has good high-temperature resistance and is not prone to excessive shrinkage in a high-temperature environment. Furthermore, it is beneficial to reduce the possibility of short circuit of the secondary battery caused by the shrinkage of the separator, thereby being beneficial to improving the safety performance of the secondary battery.

[0012] In some embodiments of the present application, the separator further includes a ceramic layer, the ceramic layer is located between the adhesive layer and the base film, and the total coating amount of the adhesive layer and the ceramic layer is a, 7.7 mg / 5000 mm 2 ≤ a ≤ 26 mg / 5000 mm 2 . By regulating the total coating amount a of the adhesive layer and the ceramic layer within the above range, it is beneficial to further improve the thermal stability and mechanical strength of the separator, and further beneficial to improving the safety performance of the secondary battery.

[0013] In some embodiments of the present application, the thickness of the ceramic layer is d1, 0.5 μm ≤ d1 ≤ 4 μm, and the thickness of the adhesive layer is d2, 0.2 μm ≤ d2 ≤ 3 μm. By regulating d1 and d2 within the above range, it is not only beneficial to improve the thermal stability and mechanical strength of the separator, but also beneficial to regulating the thickness of the secondary battery within a suitable range, thereby being beneficial to improving the safety performance of the secondary battery while taking into account the energy density.

[0014] In some embodiments of the present application, the adhesive layer further includes a ceramic material, and the coating amount of the adhesive layer is c, 3 mg / 5000 mm 2 ≤c≤12 mg / 5000 mm 2 . By adjusting c within the above range, while the separator has appropriate viscosity, its thermal stability and mechanical strength can both be improved, which is beneficial to enhancing the peel strength between the positive electrode sheet and the separator, and further improving the cycle performance and safety performance of the secondary battery.

[0015] In some embodiments of the present application, based on the mass of the adhesive layer, the mass percentage content w of the ceramic material is 40% to 70%. By adjusting the mass percentage content w of the ceramic material within the above range, it is beneficial to further balance the adhesiveness, thermal stability and mechanical strength of the secondary battery, and further beneficial to improving the cycle performance and safety performance of the secondary battery.

[0016] In some embodiments of the present application, the thickness of the adhesive layer is d3, 1 μm ≤ d3 ≤ 4 μm. By adjusting d3 within the above range, it is beneficial to improve the thermal stability and mechanical strength of the separator while controlling the thickness of the secondary battery within an appropriate range, thus being beneficial to improving the safety performance of the secondary battery while taking into account the energy density.

[0017] In some embodiments of the present application, the first material layer includes a first positive electrode active material, the second material layer includes a second positive electrode active material, and the first positive electrode active material and the second positive electrode active material each independently include at least one of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate or lithium nickel cobalt aluminate. The average particle size matching degree of the above positive electrode active materials is high, and it can be well adjusted to meet the range of the above D1, D2, D3, which is beneficial to further improving the cycle performance, kinetic performance and safety performance of the secondary battery.

[0018] In some embodiments of the present application, the first positive electrode active material includes at least one of lithium nickel cobalt manganate and lithium nickel cobalt aluminate. In some embodiments of the present application, the first positive electrode active material includes lithium nickel cobalt manganate. In some embodiments of the present application, the second positive electrode active material includes at least one of lithium cobaltate and lithium manganate. In some embodiments of the present application, the second positive electrode active material includes lithium cobaltate. Thus, the above first positive electrode active material cooperates with the above second positive electrode active material and is matched with the average particle sizes of the above D1, D2, D3, which is beneficial to further balancing and improving the kinetic performance, cycle performance, energy density and cost 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] In the first aspect of the present application, a secondary battery and an electronic device are provided. The secondary battery includes a positive electrode plate and a separator. The positive electrode plate 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 positive electrode material layer includes a first material layer and a second material layer. The first material layer is located between the positive electrode current collector and the second material layer; the contact surface between the first material layer and the second material layer is an interface, and the interface region includes a region extending 5 μm in the direction from the interface along the first material layer to the second material layer, and a region extending 5 μm in the direction from the interface along the second material layer to the first material layer; the region other than the interface region in the first material layer is the first region, and the region other than the interface region in the second material layer is the second region; the average particle size of the active material particles in the first region is D1, 3 μm ≤ D1 ≤ 5 μm, the average particle size of the active material particles in the second region is D2, 9 μm ≤ D2 ≤ 40 μm, and the average particle size of the active material particles in the interface region is D3, 6 μm ≤ D3 ≤ 22 μm; the separator includes a base film and an adhesive layer, and the adhesive layer is provided on at least one surface in the thickness direction of the base film, and at least one adhesive layer is in contact with the second material layer. By differentially setting the average particle sizes of the active materials in the first region, the second region, and the interface region, it is beneficial to enhance the peeling strength between the first material layer and the positive electrode current collector, and the second material layer also has appropriate voids for the binder particles of the adjacent adhesive layer provided on the base film to swell and enter during the subsequent formation and hot pressing process of the secondary battery, thereby being beneficial to enhancing the peeling strength between the second material layer and the separator. Thus, through the above settings, the bonding effect between the positive electrode plate and the separator in the secondary battery is enhanced, the lithium plating phenomenon is improved, which is beneficial to improving the cycle performance and safety performance of the secondary battery. At the same time, the first material layer with the average particle size within the above range can also take into account the kinetic performance.

[0022] Of course, it is not necessary for any product or method implementing the present application to achieve all the above-mentioned advantages simultaneously. BRIEF 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 following drawings are only some embodiments of the present application, and those of ordinary skill in the art can also obtain other embodiments based on these drawings.

[0024] Figure 1 Schematic diagram of the positive electrode plate along its own thickness in an embodiment of the present application;

[0025] Figure 2 Schematic diagram of the positive electrode plate along its own thickness in another embodiment of the present application;

[0026] Figure 3 Scanning electron microscope photograph of the first material layer in Example 1-1;

[0027] Figure 4 Scanning electron microscope photograph of the second material layer in Example 1-1;

[0028] Figure 5 Scanning electron microscope photograph of the adhesive layer provided on the surface of the ceramic layer in Example 2-11. Detailed implementation manners

[0029] The technical solutions in the present application will be clearly and completely described below in conjunction with the embodiments and the accompanying drawings of the present application. 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 lithium-ion batteries. The specific technical solutions are as follows:

[0031] In order to enhance the bonding effect between the separator and the positive electrode plate, reduce the deformation of the secondary battery caused by the separation of the positive electrode plate and the separator, and improve the cycle performance, the currently mainly adopted solution is to increase the hot pressing pressure or temperature during the formation stage. However, increasing the hot pressing pressure easily makes the voids between the active material particles in the positive electrode material layer too small, making it difficult to store enough electrolyte. During the cycling process of the secondary battery, problems such as lithium deposition and black spots are likely to occur due to electrolyte shortage, and the cycle performance is affected; increasing the hot pressing temperature also easily causes an increase in side reactions, consumes the electrolyte, and affects the cycle capacity retention rate of the secondary battery.

[0032] Based on the above problems, the present application provides a secondary battery and an electronic device, which are beneficial to the adhesion between the positive electrode plate and the separator, thereby reducing the generation of lithium deposition black spots and the deformation of the secondary battery, and thus being beneficial to improving the cycle performance.

[0033] The first aspect of the present application provides a secondary battery, which includes a positive electrode plate and a separator. The positive electrode plate 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 positive electrode material layer includes a first material layer and a second material layer, and the first material layer is located between the positive electrode current collector and the second material layer. In some embodiments of the present application, such as Figure 1, the positive electrode material layer 02 is disposed on one surface of the positive electrode current collector 10. The contact surface between the first material layer 21 and the second material layer 22 is the interface 20. The junction region 13 includes a second sub-region 132 extending 5 μm from the interface along the direction of the first material layer 21 to the second material layer 22, and a first sub-region 131 extending 5 μm from the interface 20 along the direction of the second material layer 22 to the first material layer 21. The region in the first material layer 21 other than the first sub-region 131 is the first region 11, and the region in the second material layer 22 other than the second sub-region 132 is the second region 12. In some embodiments of the present application, for example Figure 2 , the positive electrode material layer 02 is disposed on two surfaces of the positive electrode current collector 10. The contact surface between the first material layer 21 and the second material layer 22 is the interface 20. The junction region 13 includes a second sub-region 132 extending 5 μm from the interface along the direction of the first material layer 21 to the second material layer 22, and a first sub-region 131 extending 5 μm from the interface 20 along the direction of the second material layer 22 to the first material layer 21. The region in the first material layer 21 other than the first sub-region 131 is the first region 11, and the region in the second material layer 22 other than the second sub-region 132 is the second region 12. The first region 11 includes first positive electrode active material particles 011, the second region 12 includes second positive electrode active material particles 012, and the junction region 13 includes first positive electrode active material particles 011 and second positive electrode active material particles 012. The average particle size of the active material particles in the first region is D1, 3 μm ≤ D1 ≤ 5 μm, the average particle size of the active material particles in the second region is D2, 9 μm ≤ D2 ≤ 40 μm, and the average particle size of the active material particles in the junction region is D3, 6 μm ≤ D3 ≤ 22 μm. In the present application, the separator includes a base film and a bonding layer. The bonding layer includes a separator binder. The bonding layer is disposed on at least one surface in the thickness direction of the base film, and at least one bonding layer is in contact with the second material layer. By differentially setting the average particle size of the active materials in the first region, the second region, and the junction region, the average particle size range of the active material particles in the third region is beneficial for the binder particles of the bonding layer disposed on the base film to swell and enter during the subsequent formation hot pressing process of the secondary battery, thereby enhancing the peel strength between the positive electrode sheet and the separator. The positive electrode sheet and the separator are not easily separated, reducing the generation of lithium deposition and black spots, and reducing the deformation of the secondary battery, thereby improving the cycle performance.

[0034] In this application, the average particle size of the active material particles in the first region is D1, where 3 μm ≤ D1 ≤ 5 μm. For example, D1 can be 3 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4 μm, 4.1 μm, 4.2 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, 5 μm, or any range formed by any two of these values. When the active material particles are too small, such as when D1 is less than 3 μm, during the preparation process of the positive electrode sheet, the active material particles are prone to agglomeration, increasing the internal resistance of the positive electrode sheet, and thus affecting the rate performance and cycle performance of the secondary battery. At the same time, the too-small average particle size will also increase side reactions and consume the electrolyte, thereby affecting the energy density and cycle performance of the secondary battery. When the active material particles are too large, such as when D1 is greater than 5 μm, the amount of active material on the unit positive electrode sheet decreases, and the energy density of the obtained secondary battery is relatively low. Therefore, by controlling D1 within the above range, it is beneficial to balance the energy density, kinetic performance, and cycle performance of the secondary battery. The average particle size of the active material particles in the second region is D2, where 9 μm ≤ D2 ≤ 40 μm. For example, D2 can be 9 μm, 10 μm, 12 μm, 13 μm, 13.2 μm, 13.5 μm, 13.8 μm, 14 μm, 14.2 μm, 14.5 μm, 14.8 μm, 15 μm, 15.2 μm, 15.5 μm, 15.8 μm, 16 μm, 16.2 μm, 16.5 μm, 16.8 μm, 17 μm, 17.2 μm, 17.5 μm, 17.8 μm, 18 μm, 20 μm, 22 μm, 24 μm, 26 μm, 28 μm, 30 μm, 32 μm, 34 μm, 36 μm, 38 μm, 40 μm, or any range formed by any two of these values. When D2 is too small, such as when D2 is less than 13 μm, there are fewer voids between the active material particles, which is not conducive to the swelling of the separator adhesive into the voids during the formation and hot pressing stage, thereby affecting the bonding effect between the positive electrode sheet and the separator. The secondary battery is prone to deformation, resulting in the appearance of lithium deposition black spots, affecting the cycle performance and safety performance of the secondary battery. When D2 is too large, such as when D2 is greater than 18 μm, the amount of active material on the unit positive electrode sheet decreases, and the utilization rate of the active material decreases. The specific capacity of the positive electrode active material decreases. At the same time, the active ion transport path is too long, the impedance of the positive electrode sheet increases, and the energy density of the obtained secondary battery is relatively low and the kinetic performance is poor.The average particle size of the active material particles in the junction region is D3, where 6 μm ≤ D3 ≤ 22 μm; for example, D3 can be 6 μm, 7 μm, 8.2 μm, 8.4 μm, 8.6 μm, 8.8 μm, 9 μm, 9.2 μm, 9.4 μm, 9.6 μm, 9.8 μm, 10 μm, 10.2 μm, 10.4 μm, 10.6 μm, 10.8 μm, 11 μm, 11.2 μm, 11.4 μm, 11.6 μm, 11.8 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm or a range composed of any two of these values. Thus, by controlling D1, D2, and D3 within the above ranges, it is beneficial to better balance the safety performance, kinetic performance, and cycle performance of the secondary battery.

[0035] In the present application, the first region includes a first positive electrode active material, the second region includes a second positive electrode active material, and the particles of the first positive electrode active material and the second positive electrode active material generally do not change significantly during the preparation process of the secondary battery. Therefore, in the present application, the average particle size of the active material in the first region is also the average particle size of the first positive electrode active material, and similarly, the average particle size of the active material in the second region is also the average particle size of the second positive electrode active material.

[0036] During the preparation process of the positive electrode sheet, first, a first material layer is provided on the surface of the positive electrode current collector, and then a second material layer is provided on the surface of the first material layer. Since the first material layer and the second material layer generally use a similar solvent system, after the slurry of the second material layer is coated, the region of the first material layer far from the surface of the positive electrode current collector will partially dissolve and fuse with the slurry of the second material layer, thereby forming a junction region after drying. Therefore, the average particle size of the active material particles in the junction region is between the average particle size of the active material particles in the first region and the average particle size of the active material particles in the second region.

[0037] In some embodiments of the present application, the second material layer includes a second positive electrode active material, and the specific surface area of the second positive electrode active material is S, where 0.07 m 2 / g ≤ S ≤ 0.3 m 2 / g. In some embodiments of the present application, 0.1 m 2 / g ≤ S ≤ 0.25 m 2 / g. In some embodiments of the present application, 0.12 m 2 / g ≤ S ≤ 0.2 m 2 / g. For example, S can be 0.07 m 2 / g, 0.08 m 2 / g, 0.09 m 2 / g, 0.1 m 2 / g, 0.11 m2 / g, 0.12 m 2 / g, 0.14 m 2 / g, 0.16 m 2 / g, 0.18 m 2 / g, 0.2 m 2 / g, 0.22 m 2 / g, 0.24 m 2 / g, 0.26 m 2 / g, 0.28 m 2 / g, 0.3 m 2 / g or a range composed of any two of these values. By controlling S within the above range, during the formation hot pressing stage of the subsequent secondary battery preparation, when the separator binder swells and enters the gaps between the cathode active material particles in the second material layer, the second material layer can come into contact with the binder, enhancing the bonding effect between the cathode electrode sheet and the separator, and thus being beneficial for reducing the deformation and the appearance of lithium deposition black spots in the secondary battery, and improving the cycle performance and safety performance of the secondary battery. At the same time, the specific surface area within the above range is also beneficial for the contact between the second material layer and the electrolyte, and thus is beneficial for shortening the transmission path of active ions such as lithium ions, thereby being beneficial for reducing the internal resistance of the cathode electrode sheet and improving the kinetic performance of the secondary battery. The second cathode active materials with different specific surface areas can be obtained by purchase and tested and calculated by the gas adsorption method (BET method), and those skilled in the art can select the second cathode active materials with corresponding specific surface areas according to needs.

[0038] In some embodiments of the present application, the first material layer includes a first cathode active material, the second material layer includes a second cathode active material, and the first cathode active material and the second cathode active material each independently include at least one of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate, or lithium nickel cobalt aluminate. The average particle size matching degree of the above cathode active materials is high, and it can be well controlled to meet the ranges of D1, D2, and D3 above, thus being beneficial for further improving the cycle performance, kinetic performance, and safety performance of the secondary battery.

[0039] In some embodiments of the present application, the first positive electrode active material includes at least one of lithium nickel cobalt manganese oxide and lithium nickel cobalt aluminate. In some embodiments of the present application, the first positive electrode active material includes lithium nickel cobalt manganese oxide. The second positive electrode active material includes at least one of lithium cobalt oxide and lithium manganese oxide. In some embodiments of the present application, the second positive electrode active material includes lithium cobalt oxide. The first positive electrode active material has high energy density, safety performance, and cycle performance, but has a high cost. Combining it with lithium cobalt oxide is beneficial to further improve the energy density of the secondary battery; when the above first positive electrode active material is combined with lithium manganese oxide, it is beneficial to reduce the cost of the secondary battery. Thus, the above first positive electrode active material is combined with the above second positive electrode active material and matched with the average particle sizes of the above D1, D2, and D3, which is beneficial to further balance and improve the kinetic performance, cycle performance, energy density, and cost of the secondary battery.

[0040] In some embodiments of the present application, the peel strength between the second material layer and the separator is F, and 1 N / m ≤ F ≤ 20 N / m. In some embodiments of the present application, 3 N / m ≤ F ≤ 15 N / m. For example, F can be 1 N / m, 2 N / m, 3 N / m, 4 N / m, 5 N / m, 6 N / m, 7 N / m, 8 N / m, 9 N / m, 10 N / m, 11 N / m, 12 N / m, 13 N / m, 14 N / m, 15 N / m, 16 N / m, 17 N / m, 18 N / m, 19 N / m, 20 N / m, or a range composed of any two of these values. When the peel strength F between the second material layer and the separator is within the above range, the bonding effect between the positive electrode plate and the separator is good, and more paths are reserved for the transmission of active ions, which is beneficial to reducing the deformation of the secondary battery and the generation of lithium deposition black spots, and improving the cycle performance, safety performance, and kinetic performance of the secondary battery.

[0041] In some embodiments of the present application, the separator binder in the bonding layer includes at least one of polyacrylate, oily polyvinylidene fluoride, polyvinylidene fluoride, or polyvinylidene fluoride - hexafluoropropylene copolymer. By selecting the above binders with better viscosity, it is beneficial to further enhance the peel strength between the second material layer and the separator, thereby reducing the possibility of separation between the positive electrode plate and the separator, which is beneficial to reducing the deformation of the secondary battery and the generation of lithium deposition black spots, and improving the cycle performance and safety performance of the secondary battery.

[0042] In some embodiments of the present application, based on the area of the separator, the area proportion of the adhesive layer is b, and 10% ≤ b×100% ≤ 70%. In some embodiments of the present application, 30% ≤ b×100% ≤ 50%. For example, b×100% can be 10%, 12%, 15%, 17%, 20%, 22%, 25%, 28%, 30%, 32%, 35%, 38%, 40%, 42%, 45%, 48%, 50%, 52%, 55%, 58%, 60%, 62%, 65%, 68%, 70% or a range composed of any two of these values. When the area proportion of the adhesive layer is within the above range, it can not only reserve channels for the transport of ions, but also facilitate the adhesion between the second material layer and the separator, thereby facilitating the balance between reducing the possibility of separation between the positive electrode sheet and the separator and the internal resistance of the positive electrode sheet, and thus facilitating the balance between improving the safety performance and cycle performance of the secondary battery and the kinetic performance.

[0043] In some embodiments of the present application, the thermal shrinkage rate of the separator is h, and 1% ≤ h ≤ 15%. For example, h can be 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or a range composed of any two of these values. When the thermal shrinkage rate h of the separator is within the above range, the separator has good high-temperature resistance and is not prone to excessive shrinkage in a high-temperature environment, thereby facilitating the reduction of the possibility of short circuit of the secondary battery caused by the shrinkage of the separator, and thus facilitating the improvement of the safety performance of the secondary battery.

[0044] In some embodiments of the present application, the separator further includes a ceramic layer, and the ceramic layer is located between the adhesive layer and the base film. By providing a ceramic layer between the separator and the adhesive layer, it is beneficial to enhance the thermal stability of the separator, and it can still isolate the positive electrode sheet and the negative electrode sheet well when the temperature of the secondary battery rises, thereby reducing the risk of thermal runaway; at the same time, the introduction of the ceramic layer is also beneficial to improving the mechanical strength of the separator and reducing the possibility of damage to the separator when the secondary battery is subjected to mechanical impact. Therefore, the above setting of the ceramic layer is beneficial to improving the safety performance of the secondary battery.

[0045] In some embodiments of the present application, the total coating amount of the adhesive layer and the ceramic layer is a, and 7.7mg / 5000mm 2 ≤ a ≤ 26mg / 5000mm 2 . For example, a can be 7.7mg / 5000mm 2 , 8mg / 5000mm 2 , 9mg / 5000mm 2 , 10mg / 5000mm 2, 11 mg / 5000 mm 2 , 12 mg / 5000 mm 2 , 13 mg / 5000 mm 2 , 14 mg / 5000 mm 2 , 15 mg / 5000 mm 2 , 16 mg / 5000 mm 2 , 17 mg / 5000 mm 2 , 18 mg / 5000 mm 2 , 19 mg / 5000 mm 2 , 20 mg / 5000 mm 2 , 21 mg / 5000 mm 2 , 22 mg / 5000 mm 2 , 23 mg / 5000 mm 2 , 24 mg / 5000 mm 2 , 25 mg / 5000 mm 2 , 26 mg / 5000 mm 2 Or a range composed of any two of these values. By controlling the total coating amount a of the adhesive layer and the ceramic layer within the above range, it is beneficial to further improve the thermal stability and mechanical strength of the separator, and thus beneficial to further improve the safety performance of the secondary battery.

[0046] In some embodiments of the present application, the thickness of the ceramic layer is d1, 0.5 μm ≤ d1 ≤ 4 μm, and the thickness of the adhesive layer is d2, 0.2 μm ≤ d2 ≤ 3 μm. For example, d1 can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm or a range composed of any two of these values. For example, d2 can be 0.2 μm, 0.4 μm, 0.6 μm, 0.8 μm, 1 μm, 1.2 μm, 1.4 μm, 1.6 μm, 1.8 μm, 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm or a range composed of any two of these values. By controlling d1 and d2 within the above range, it is not only beneficial to improve the thermal stability and mechanical strength of the separator, but also beneficial to control the thickness of the secondary battery within a suitable range, so as to be beneficial to improving the safety performance of the secondary battery while taking into account the energy density. In some embodiments of the present application, the adhesive layer further includes a ceramic material. By adding a ceramic material to the adhesive layer, it is beneficial to improve the thermal stability and mechanical strength of the separator, and thus beneficial to improving the safety performance of the secondary battery when the temperature rises or it is subjected to mechanical shock. At the same time, the ceramic material in the adhesive layer can also create voids in the adhesive layer as channels for active ion transport, thus being beneficial to improving the kinetic performance of the secondary battery.

[0047] In some embodiments of the present application, the coating amount of the adhesive layer is c, 3 mg / 5000 mm 2 ≤c≤12 mg / 5000 mm 2 . For example, c can be 3 mg / 5000 mm 2 , 3.5 mg / 5000 mm 2 , 4 mg / 5000 mm 2 , 4.5 mg / 5000 mm 2 , 5 mg / 5000 mm 2 , 5.5 mg / 5000 mm 2 , 6 mg / 5000 mm 2 , 6.5 mg / 5000 mm 2 , 7 mg / 5000 mm 2 , 7.5 mg / 5000 mm 2 , 8 mg / 5000 mm 2 , 8.5 mg / 5000 mm 2 , 9 mg / 5000 mm 2 , 9.5 mg / 5000 mm 2 , 10 mg / 5000 mm 2 , 10.5 mg / 5000 mm 2 , 11 mg / 5000 mm 2 , 11.5 mg / 5000 mm 2 , 12 mg / 5000 mm 2 or a range composed of any two of these values. By adjusting c within the above range, while the separator has appropriate adhesiveness, its thermal stability and mechanical strength can both be improved, which is beneficial to enhancing the peel strength between the positive electrode sheet and the separator, and further improving the cycling performance and safety performance of the secondary battery.

[0048] In some embodiments of the present application, based on the mass of the adhesive layer, the mass percentage content w of the ceramic material is 40% to 70%. For example, w can be 40%, 42%, 44%, 46%, 48%, 50%, 52%, 54%, 56%, 58%, 60%, 62%, 64%, 66%, 68%, 70% or a range composed of any two of these values. By adjusting the mass percentage content w of the ceramic material within the above range, it is beneficial to further balance the adhesiveness, thermal stability and mechanical strength of the secondary battery, and further beneficial to improving the cycling performance and safety performance of the secondary battery.

[0049] In some embodiments of the present application, the thickness of the adhesive layer is d3, where 1 μm ≤ d3 ≤ 4 μm. For example, d3 can be 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm or a range composed of any two of these values. By controlling d3 within the above range, it is beneficial to improve the thermal stability and mechanical strength of the separator while controlling the thickness of the secondary battery within a suitable range, thereby facilitating the improvement of the safety performance of the secondary battery while taking into account the energy density.

[0050] In the present application, the thickness of the separator is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the separator can be 5 μm to 32 μm.

[0051] In the present application, the thickness of the base film is not particularly limited as long as the object of the present application can be achieved. For example, the thickness of the base film can be 4 μm to 25 μm.

[0052] The present application places no particular restrictions on the positive current collector as long as the object of the present application can be achieved. For example, it can include aluminum foil, aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc.

[0053] The positive electrode material layer may further include a conductive agent and a positive electrode binder. The present application places no particular restrictions on the type of the conductive agent as long as the object of the present application can be achieved. For example, the conductive agent may 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 may include, but is not limited to, at least one of acetylene black or Ketjen black. The above carbon nanotubes may include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above carbon fibers may include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanofibers. The above metal materials may include, but are not limited to, metal powders and / or metal fibers. Specifically, the metal may include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above conductive polymers may include, but are not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole.

[0054] The present application places no particular limitation on the cathode binder, as long as the object of the present application can be achieved. For example, the cathode binder may include, but is not limited to, at least one of polyacrylic acid, sodium polyacrylate, potassium polyacrylate, lithium polyacrylate, polyimide, polyvinyl alcohol, carboxymethyl cellulose, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, polyimide, polyamideimide, styrene-butadiene rubber, or polyvinylidene fluoride.

[0055] The present application places no particular limitation on the thickness of the cathode current collector and the cathode material layer, as long as the object of the present application can be achieved. For example, the thickness of the cathode current collector is 8 μm to 20 μm, and the thickness of the cathode material layer is 60 μm to 140 μm.

[0056] Optionally, the cathode electrode sheet may further include a conductive layer, which is located between the cathode current collector and the cathode material layer. The composition of the conductive layer is not particularly limited and may be a commonly used conductive layer in the art. The conductive layer includes a conductive agent and a cathode binder. The present application places no particular limitation on the conductive agent and the cathode binder in the conductive layer. For example, it may be at least one of the above-mentioned conductive agents and the above-mentioned cathode binders.

[0057] In the present application, the secondary battery further includes a negative electrode sheet, which includes a negative current collector and a negative material layer provided on at least one surface of the negative current collector. The above "the negative material layer is provided on at least one surface of the negative current collector" means that the negative material layer may be provided on one surface of the negative current collector along its thickness direction, or may be provided on both surfaces of the negative current collector along its thickness direction. It should be noted that the "surface" here may be the entire area of the surface of the negative current collector or a partial area of the surface of the negative current collector. The present application places no particular limitation as long as the object of the present application can be achieved.

[0058] The present application places no particular limitation on the negative current collector, as long as the object of the present application can be achieved. For example, it may include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a composite current collector. Exemplarily, the composite current collector may 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.

[0059] The negative material layer includes a negative active material. The present application places no particular limitation on the negative active material, as long as the object of the present application can be achieved. For example, the negative active material may include, but is not limited to, at least one of natural graphite, artificial graphite, mesophase microcarbon microspheres, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured lithiated TiO2-Li4Ti5O 12 or Li-Al alloy.

[0060] 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 does not particularly limit the types of the conductive agent and the binder, as long as the object of the present application can be achieved. For example, it may be at least one of the above-mentioned conductive agent and the above-mentioned positive electrode binder. The present application does not particularly limit the mass ratio of the negative electrode active material, the conductive agent, and the negative electrode binder in the negative electrode material layer, and those skilled in the art can select according to actual needs as long as the object of the present application can be achieved.

[0061] The present application does not particularly limit the thickness of the negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode material layer is 80 μm to 140 μm.

[0062] The present application does not particularly limit the thickness of the negative electrode current collector, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm.

[0063] Optionally, the negative electrode sheet may further include a conductive layer, and the conductive layer is located between the negative electrode current collector and the negative electrode material layer. The present application does not particularly limit the composition of the conductive layer, 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. The present application does not particularly limit the conductive agent and the negative electrode binder in the conductive layer. For example, it may be at least one of the above-mentioned conductive agent and the above-mentioned negative electrode binder.

[0064] In the present application, the secondary battery further includes an electrolyte, and the electrolyte includes a lithium salt and a non-aqueous solvent.

[0065] The present application does not particularly limit the lithium salt, as long as the object of the present 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. The present application does not particularly limit the content of the lithium salt in the electrolyte, as long as the object of the present application can be achieved.

[0066] The present application does not particularly limit the non-aqueous solvent, as long as the object of the present 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.

[0067] 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 carboxylate 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.

[0068] The secondary battery further includes a housing for accommodating the positive electrode sheet, the separator, the negative electrode sheet, 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 can be a housing well-known in the art, as long as the object of the present application can be achieved. For example, the housing can be a hard shell housing or a flexible housing. The material of the hard shell housing can be metal. The present application does not limit the type of metal, and a metal hard shell housing known in the art can be used, as long as the object of the present application can be achieved. The flexible housing can be a metal plastic film, such as an aluminum plastic film, a steel plastic film, etc.

[0069] The preparation process of the secondary battery of the present application is well-known to those skilled in the art, and the present application has no special restrictions. 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, and the negative electrode sheet in sequence, and winding, folding, etc. as needed to obtain a wound electrode assembly, placing the electrode assembly into a housing, injecting electrolyte into the housing and sealing it to obtain a secondary battery. Or, stacking the positive electrode sheet, the separator, and the negative electrode sheet in sequence, and then fixing the four corners of the entire laminated structure with tape to obtain a laminated electrode assembly, placing the electrode assembly into the housing, injecting electrolyte into the housing and sealing it to obtain a secondary battery. 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 / discharging.

[0070] 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.

[0071] 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.

[0072] Examples

[0073] Hereinafter, examples and comparative examples are given to illustrate the embodiments of the present application more specifically. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0074] Testing methods and equipment:

[0075] Sampling of positive electrode sheet and separator:

[0076] First, disassemble the lithium-ion battery, take out the positive electrode sheet and the separator, wash them with dimethyl carbonate (DMC), and then dry the positive electrode sheet at 80 °C to obtain a negative electrode sheet sample; dry the separator at room temperature to obtain a separator sample.

[0077] Unless otherwise specified, the following test methods are all carried out using the positive electrode sheet sample and the separator sample obtained above.

[0078] Average particle size test:

[0079] Take a positive electrode sample. Polish the cross-section of the positive electrode sample along the thickness direction by ion polishing, and measure it under a scanning electron microscope (SEM). First, find the interface between the first material layer and the second material layer. The junction region is the region extending 5 μm from the interface along the direction of the first material layer to the second material layer, and the region extending 5 μm from the interface along the direction of the second material layer to the first material layer. Combine the energy dispersive spectrometer (EDS) function of the SEM, select 10 particles containing metal elements from the junction region, measure the circumscribed circle diameter of each of them respectively, and calculate the average value, which is D3; then, select 10 particles in the region outside the junction region of the first material layer (the first region) and 10 particles in the region outside the junction region of the second material layer (the second region) according to the above method, measure the circumscribed circle diameter of each of them respectively, and calculate the average values as D1 and D2 respectively.

[0080] Peeling strength test:

[0081] Disassemble the lithium-ion battery, take out the positive electrode, separator and negative electrode bonded together, and separate the negative electrode to obtain a peeling strength test sample. Cut the above sample into a spline with a size of 100 mm × 30 mm. Clamp one end of the positive electrode on the fixture of the tensile machine, stretch it at 180°, turn on the tensile machine for testing, and pull the positive electrode at a constant speed of 50 mm / min until the positive electrode is peeled off from the separator. After the test is completed, obtain the peeling strength F between the positive electrode and the separator.

[0082] Area ratio test of the bonding layer b×100%:

[0083] Take a separator sample. Use a scanning electron microscope (SEM) to select the area with angular particle distribution as the area where the bonding layer is set in the separator, and record the total area of the remaining areas with non-angular particle distribution as S1. Measure the total area S of the separator with a micrometer. Combine the SEM image processing software, perform threshold segmentation processing on the SEM image according to different contrasts, and calculate to obtain b×100% = S1 / S×100%.

[0084] Specific surface area test:

[0085] Scrape the powder of the second material layer from the second region, calcine it in an air atmosphere at 500 °C for 2 h to remove the conductive agent, binder, moisture and impurities in the positive electrode material layer, and then use a specific surface area analyzer (TriStarⅡ3020) to test its specific surface area by the N2 adsorption method. The determination of the second region refers to the average particle size test method.

[0086] Thermal shrinkage rate test:

[0087] Take the diaphragm sample X i mm×Y i mm, and place it in an oven at 130 °C for 1 h. After heating, let the diaphragm cool down, and measure the minimum dimensions in the longitudinal direction (TD) and the transverse direction (MD), which are X a mm and Y a mm respectively.

[0088] The thermal shrinkage rate h in the TD direction TD =(X i -X a ) / X i ×100%);

[0089] The thermal shrinkage rate h in the MD direction MD (=(Y i- -Y a ) / Y i ×100%).

[0090] Finally, take the larger value between h TD and h TD as the thermal shrinkage rate h of the diaphragm.

[0091] Coating amount test:

[0092] a: Take a diaphragm sample with an area of 5000 mm 2 , weigh it to get the diaphragm mass as n1; put the above sample into a container, first put the sample into water and perform ultrasonic treatment in an ultrasonic instrument with a heating function, control the temperature at 45 °C, and ultrasonic for 3 h. When the diaphragm becomes completely transparent, take it out to obtain the base film, dry it and weigh the mass of the base film as m2; the coating amount a = (m1 - m2) mg / 5000 mm 2 .

[0093] If the ceramic layer and the adhesive layer on the diaphragm do not fall off after ultrasonic treatment in water, then put the diaphragm sample into N-methylpyrrolidone (NMP) solvent and perform ultrasonic treatment in an ultrasonic instrument with a heating function, control the temperature at 45 °C, and ultrasonic for 3 h. When the diaphragm becomes completely transparent, take it out to obtain the base film, weigh the mass of the base film as m2; the coating amount a = (m1 - m2) mg / 5000 mm 2 .

[0094] c. The mass percentage content w of the ceramic material:

[0095] The test method of the coating amount c is the same as that of a above. Record the mass difference of the diaphragm before and after ultrasonic treatment as m3, which is the coating amount c.

[0096] Centrifuge the solution after ultrasonic treatment, dry and weigh the solid material obtained by centrifugation, and record the mass as m4.

[0097] w = m4 / m3 × 100%.

[0098] Thickness test:

[0099] Take the positive electrode sample. The cross-section of the positive electrode sample along the thickness direction is ion-polished, and the cross-section of the positive electrode sample is measured under a scanning electron microscope (SEM). According to the position of the base film and the different morphologies of the adhesive layer and the ceramic layer, their thicknesses are measured respectively to obtain d1 and d2.

[0100] The test method of d3 is the same as the above test method.

[0101] Cycle capacity retention rate:

[0102] At 25°C, the lithium-ion battery is charged at a constant current of 1C to a voltage of 4.45V, then charged at a constant voltage of 4.45V to 0.05C, and left standing for 10 min; then discharged at a constant current of 1C to a voltage of 3.0V and left standing for 5 min. The above is one cycle. Follow the above cycle to 1000 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 1000th cycle as Q1. Calculate the cycle capacity retention rate of the lithium-ion battery in the 1000th cycle through the following formula: Cycle capacity retention rate (%) = Q1 / Q0 × 100%.

[0103] Lithium plating test:

[0104] Take the lithium-ion battery after the above cycle capacity test, disassemble it, and mark the surface of the negative electrode material layer close to the winding center in the outermost winding of the negative electrode of the lithium-ion battery as the observation interface.

[0105] Judgment criteria for the degree of lithium plating: When the lithium plating area is 0, it is judged as no lithium plating; when the proportion of the lithium plating area in the observation interface area is less than 5%, it is judged as mild lithium plating; when the proportion of the lithium plating area in the observation interface area is 5% to 10%, it is judged as moderate lithium plating; when the proportion of the lithium plating area in the observation interface area is greater than 10%, it is judged as severe lithium plating.

[0106] In this application, the degree of lithium plating is used to characterize the safety performance of the lithium-ion battery.

[0107] Critical lithium plating rate test:

[0108] At 25°C, the lithium-ion battery is charged at a constant current of 0.7C to 4.45V, charged at a constant voltage of 4.45V to 0.05C, and the lithium-ion battery is disassembled to observe whether metallic lithium is precipitated on the interfaces of all negative electrodes of the lithium-ion battery. If no metallic lithium is precipitated, then increase the charging rate by 0.05C in turn until metallic lithium is precipitated on the surface of the disassembled lithium-ion battery. Record the charging rate of the last non-lithium-plating as the critical lithium plating rate of the lithium-ion battery.

[0109] The critical lithium plating rate is used in this application to characterize the kinetic performance of lithium-ion batteries. The higher the critical lithium plating rate, the better the kinetic performance of the lithium-ion battery.

[0110] Pinprick test:

[0111] Charge the lithium-ion battery at a rate of 0.5C to 4.45V, then charge it at a constant voltage until 0.05C. Use a 3mm steel needle to completely penetrate the lithium-ion battery from the direction perpendicular to the largest surface of the lithium-ion battery at a speed of 10 cm / s. If the lithium-ion battery does not catch fire, explode, or emit smoke, it is considered to pass. Test 10 lithium-ion batteries for each example or comparative example, record the number of passes, denoted as: number of passes / number of tests. For example, 9 / 10 means that 9 out of 10 lithium-ion batteries pass the test.

[0112] Energy density test:

[0113] Use a 3D scanner to measure the length L, width W, and thickness H of the battery.

[0114] At an ambient temperature of 25°C, charge the lithium-ion battery at a constant current of 0.2C to a voltage of 4.5V on a charge-discharge tester, then charge it at a constant voltage of 4.5V until the cut-off current of 0.05C. Let it stand for 10 min, then discharge it 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 = (C × V) / (L × W × H), with the unit of Wh / L.

[0115] Example 1-1

[0116] <Preparation of the positive electrode sheet>

[0117] Mix the first positive electrode active material NCM811, conductive agent Super P, and binder polyvinylidene fluoride in a mass ratio of 96:2:2, add N-methylpyrrolidone (NMP) as a solvent, and formulate a slurry with a solid content of 75 wt%. After vacuum stirring evenly, obtain the first material layer slurry. Among them, the average particle size of the first positive electrode active material is 4 μm.

[0118] Mix the second positive electrode active material lithium cobaltate, conductive agent Super P, and binder polyvinylidene fluoride in a mass ratio of 96:2:2, add N-methylpyrrolidone (NMP) as a solvent, and formulate a slurry with a solid content of 75 wt%. After vacuum stirring evenly, obtain the second layer positive electrode slurry. Among them, the average particle size of the second positive electrode active material is 16 μm.

[0119] The first material layer is evenly coated on one surface of the aluminum foil positive current collector with a thickness of 10 μm, dried at 120 °C, and then the second material layer is evenly coated on the surface of the first material layer and dried at 120 °C to obtain a positive electrode sheet with the first material layer and the second material layer coated on one side. The coating mass per unit area of the positive electrode material layer is 327 mg / 1540 mm 2 , where the coating mass per unit area of the first material layer is equal to that of the second material layer. Then, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with the first material layer and the second material layer coated on both sides, dried at 120 °C, and cold-pressed to obtain the positive electrode sheet. After cutting and welding the tab, a positive electrode sheet with a specification of 74 mm × 867 mm is obtained for use. Among them, the thickness of the single-sided positive electrode material layer is 60 μm. The specific parameters are shown in Tables 1 to 2.

[0120] <Preparation of negative electrode sheet>

[0121] The negative electrode active material artificial graphite, the binder styrene-butadiene rubber, and the conductive agent acetylene black are mixed according to a mass ratio of 97.4:1.4:1.2, and deionized water is added as a solvent to prepare a slurry with a solid content of 45 wt%. After being stirred evenly by a vacuum mixer, a negative electrode slurry is obtained. The negative electrode slurry is evenly coated on one surface of the negative current collector copper foil with a thickness of 6 μm and dried at 120 °C to obtain a negative electrode sheet with the negative electrode material layer coated on one side. The coating mass per unit area of the negative electrode material layer is 173 mg / 1540 mm 2 . Then, the above steps are repeated on the other surface of the copper foil to obtain a negative electrode sheet with the negative electrode material layer coated on both sides. After drying at 120 °C, it is cold-pressed, and then after cutting and welding the tab, a negative electrode sheet with a specification of 78 mm × 875 mm is obtained for use. Among them, the thickness of the single-sided negative electrode material layer is 66.5 μm.

[0122] <Preparation of electrolyte>

[0123] In an environment with a water content of less than 10 ppm, dimethyl carbonate, diethyl carbonate, and ethylene carbonate are mixed according to a mass ratio of 1:1:1 to obtain an organic solvent, and then the 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.

[0124] <Separator>

[0125] Preparation of the bonding layer slurry:

[0126] Bonding layer slurry: The separator binder polyvinylidene fluoride is dissolved in the solvent NMP to obtain a bonding layer slurry with a solid content of 5 wt%.

[0127] Use a 5-μm thick polyethylene film as the base film of the separator. Apply the adhesive layer slurry on one surface of the base film and dry it. Then repeat the above steps on the other surface of the base film to obtain a separator with a double-sided coated adhesive layer. Among them, the coating amount of the single-sided adhesive layer is 2 mg / 5000 mm 2 , the single-sided thickness is 0.5 μm, and the porosity of the separator is 39%. The specific parameters are shown in Tables 1 to 4.

[0128] <Preparation of Lithium-Ion Batteries>

[0129] Stack the above-prepared positive electrode sheet, separator, and negative electrode sheet in sequence, with the separator in the middle of the positive electrode sheet and the negative electrode sheet to play an isolation role, and wind to obtain an electrode assembly. Put the electrode assembly into an aluminum-plastic film packaging bag, remove the 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 4.15 V, the formation temperature is 70 °C, the formation pressure is 1.8 Mpa, the time is 50 min, and the formation standing time is 2 h.

[0130] Examples 1-2 to Examples 1-16

[0131] Except for adjusting the relevant preparation parameters according to Table 1, the rest are the same as in Example 1-1.

[0132] Examples 1-17 to Examples 1-22

[0133] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as in Example 1-1.

[0134] Example 1-23

[0135] Except for preparing the adhesive layer slurry according to the following method, the rest are the same as in Example 1-1.

[0136] Adhesive layer slurry: Dissolve the separator binder polyacrylate in the solvent water to obtain an adhesive layer slurry with a solid content of 5 wt%.

[0137] Example 2-1

[0138] Except for preparing the separator and lithium-ion battery according to the following method, the rest are the same as in Example 1-1.

[0139] The preparation method of the adhesive layer slurry is the same as that in Example 1-1.

[0140] Ceramic layer slurry: Dissolve the ceramic material alumina and the binder polyvinylidene fluoride in the solvent NMP to obtain a ceramic layer slurry with a solid content of 30 wt%.

[0141] Use a 5-μm thick polyethylene film as the base film of the separator. Apply a ceramic layer slurry on one surface of the base film, dry it, and then apply a bonding layer slurry on the surface of the ceramic layer slurry and dry it. Then, apply a bonding layer slurry on the other surface of the base film to obtain the separator. Among them, the coating amount of the ceramic layer is 13.5 mg / 5000 mm 2 , and the thickness is 3 μm; the coating amount of the bonding layer is 3.5 mg / 5000 mm 2 , and the thickness is 1.6 μm. The porosity of the separator is 39%. The specific parameters are shown in Table 3.

[0142] <Preparation of Lithium-Ion Batteries>

[0143] Stack the above-prepared positive electrode plate, separator, and negative electrode plate in sequence, with the separator in the middle between the positive electrode plate and the negative electrode plate to play an isolation role, and wind it to obtain an electrode assembly. Among them, the side of the separator with the ceramic layer faces the positive electrode plate. Put the electrode assembly into an aluminum-plastic film packaging bag, remove the 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 4.15 V, the formation temperature is 70 °C, the formation pressure is 1.8 Mpa, the time is 50 min, and the formation standing time is 2 h.

[0144] Examples 2-2 to 2-10

[0145] Except for adjusting the relevant preparation parameters according to Table 2, the rest are the same as Example 2-1.

[0146] Example 2-11

[0147] Except for preparing the bonding layer slurry according to the method of Example 1-23, the rest are the same as Example 2-1.

[0148] Example 3-1

[0149] Except for preparing the separator according to the following method, the rest are the same as Example 1-1.

[0150] Bonding layer slurry: Dissolve the separator binder polyvinylidene fluoride and the ceramic material alumina in the solvent NMP according to a mass ratio of 35:65 to obtain a bonding layer slurry with a solid content of 15 wt%.

[0151] Use a 5-μm thick polyethylene film as the base film of the separator. Apply a bonding layer slurry on one surface of the base film and dry it. Among them, the coating amount of the bonding layer is 5 mg / 5000 mm 2 , and the thickness is 1.5 μm; the porosity of the separator is 39%. The specific parameters are shown in Table 4.

[0152] Examples 3-2 to 3-7

[0153] Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as in Example 1-1.

[0154] Comparative Examples 1 to 7

[0155] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as in Example 1-1.

[0156] Table 1

[0157]

[0158]

[0159] It can be seen from Example 1-1 to Example 1-16 and Comparative Examples 1 to 7 that the average particle size D1 of the active material particles in the first region in the examples satisfies 3 μm ≤ D1 ≤ 5 μm, the average particle size D2 of the active material particles in the second region satisfies 9 μm ≤ D2 ≤ 40 μm, and the average particle size D3 of the active material particles in the junction region satisfies 6 μm ≤ D3 ≤ 22 μm; and the separator includes a base film and a bonding layer, the peel strength between the second material layer and the separator is large, the cycle capacity retention rate of the obtained lithium-ion battery is high, there is no severe lithium plating phenomenon, and the critical lithium plating rate is high. Specifically, Figure 3 is the scanning electron microscope photograph of the first material layer in Example 1-1, Figure 4 is the scanning electron microscope photograph of the second material layer in Example 1-1. From Figure 3 and Figure 4It can be seen that the average particle size of the active material in the second material layer is larger than that in the first material layer. In Comparative Example 1, D2 and D3 are too large. Although the peel strength between the second material layer and the separator is relatively high, the cycle capacity retention rate of the obtained lithium-ion battery is low, and the critical lithium deposition rate is low. In Comparative Example 2, the average particle size of the cathode material in different regions is not differentially set, and D1 is too large. Although the peel strength between the second material layer and the separator is relatively high, the cycle capacity retention rate of the obtained lithium-ion battery is low, and the critical lithium deposition rate is low. In Comparative Example 3, D1 is too large. Although the peel strength between the second material layer and the separator is relatively high, the critical lithium deposition rate of the obtained lithium ions is relatively low. In Comparative Example 4, the average particle size of the cathode material in different regions is not differentially set, and D2 and D3 are too small. The peel strength between the second material layer and the separator is too low, and severe lithium deposition occurs in the obtained lithium-ion battery, and the cycle capacity retention rate is low. In Comparative Example 5, D1 is too large and D3 is too small. The peel strength between the second material layer and the separator is too low, and severe lithium deposition occurs in the obtained lithium-ion battery, and the cycle capacity retention rate is low. In Comparative Examples 6 and 7, D1 is too small. Although the peel strength between the second material layer and the separator is relatively high, the cycle capacity retention rate of the obtained lithium-ion battery is low. Therefore, it shows that the lithium-ion battery satisfying this application has good cycle performance, safety performance, and kinetic performance.

[0160] The specific surface area S of the second cathode active material usually affects the cycle performance, safety performance, and kinetic performance of the lithium-ion battery. It can be seen from Examples 1-1, 1-4 to 1-14 that when S is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, no severe lithium deposition occurs, and the critical lithium deposition rate is relatively high. Therefore, it shows that when S is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and kinetic performance.

[0161] The types of the first cathode active material and the second cathode active material usually affect the cycle performance, safety performance, and kinetic performance of the lithium-ion battery. It can be seen from Examples 1-1, 1-15 to 1-16 that when the types of the first cathode active material and the second cathode active material are within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, no severe lithium deposition occurs, and the critical lithium deposition rate is relatively high. Among them, due to the differences in morphology and particle distribution of different cathode active materials, there are significant differences in their specific surface areas. For example, the NCM811 used in the second material layer in Example 1-16 has a relatively high residual alkali content, which affects the peel strength between the second material layer and the separator. Therefore, it shows that when S is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and kinetic performance.

[0162] Table 2

[0163]

[0164]

[0165] The area ratio b×100% of the adhesive layer usually affects the cycle performance, safety performance and kinetic performance of lithium-ion batteries. It can be seen from Example 1-1, Example 1-17 to Example 1-22 that when b×100% is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and no serious lithium plating occurs, and the critical lithium plating rate is relatively high. Thus, it shows that when b×100% is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance and kinetic performance.

[0166] The type of binder usually affects the cycle performance, safety performance and kinetic performance of lithium-ion batteries. It can be seen from Example 1-1 and Example 1-23 that when the type of binder is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and no serious lithium plating occurs, and the critical lithium plating rate is relatively high. Thus, it shows that when the type of binder is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance and kinetic performance.

[0167] Table 3

[0168]

[0169] When the separator further includes a ceramic layer, the total coating amount a of the adhesive layer and the ceramic layer usually affects the cycle performance, safety performance and energy density of lithium-ion batteries. It can be seen from Example 1-1, Example 2-1 to Example 2-11 that when a is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and no serious lithium plating occurs, the passing rate of the needle-punching test is relatively high and the energy density is relatively high. Thus, it shows that when a is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance and energy density. Specifically, as Figure 5 shown, it is a scanning electron microscope photograph of the adhesive layer arranged on the surface of the ceramic layer in Example 2-11. The area where the angular particles are distributed in the figure is the area where the ceramic layer is distributed, and the area where the non-angular particles are distributed in the rest is the area where the adhesive layer is distributed.

[0170] The thickness d1 of the ceramic layer usually affects the cycle performance, kinetic performance, safety performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 2-1 to 2-10 that when d1 is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and there is no serious lithium plating. The passing rate of the needle-punching test is relatively high and the energy density is relatively high. Thus, it shows that when d1 is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and energy density.

[0171] The thickness d2 of the adhesive layer usually affects the cycle performance, kinetic performance, safety performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 2-1 to 2-10 that when d2 is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and there is no serious lithium plating. The passing rate of the needle-punching test is relatively high and the energy density is relatively high. Thus, it shows that when d2 is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and energy density.

[0172] Table 4

[0173]

[0174] When the adhesive layer further includes a ceramic material, the coating amount c of the adhesive layer usually affects the cycle performance, kinetic performance, safety performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 3-1 to 3-3 that when c is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and there is no serious lithium plating. The passing rate of the needle-punching test is relatively high and the energy density is relatively high. Thus, it shows that when c is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and energy density.

[0175] The thickness d3 of the adhesive layer usually affects the cycle performance, kinetic performance, safety performance, and energy density of the lithium-ion battery. It can be seen from Examples 1-1, 3-1 to 3-3 that when d3 is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and there is no serious lithium plating. The passing rate of the needle-punching test is relatively high and the energy density is relatively high. Thus, it shows that when d3 is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance, and energy density.

[0176] The mass percentage content w of the ceramic material usually affects the cycle performance, kinetic performance, safety performance and energy density of lithium-ion batteries. It can be seen from Example 1-1, Example 3-1, Example 3-4 to Example 3-7 that when w is within the scope of this application, the peel strength between the second material layer and the separator is relatively high, and the obtained lithium-ion battery has a relatively high cycle capacity retention rate, and there is no serious lithium plating. The passing rate of the needle-punching test is relatively high and the energy density is relatively high. Therefore, it shows that when w is within the scope of this application, the obtained lithium-ion battery has good cycle performance, safety performance and energy density.

[0177] The above are only the preferred embodiments of this application, and are not intended to limit this application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of this application shall be included within the scope of protection of this application.

Claims

1. A secondary battery, which includes a positive electrode sheet and a separator. The positive electrode sheet 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 positive electrode material layer includes a first material layer and a second material layer, and the first material layer is located between the positive electrode current collector and the second material layer; The contact surface between the first material layer and the second material layer is an interface, and the interface region includes a region extending 5 μm from the interface along the direction from the first material layer to the second material layer, and a region extending 5 μm from the interface along the direction from the second material layer to the first material layer; The region in the first material layer other than the interface region is the first region, and the region in the second material layer other than the interface region is the second region; The average particle size of the active material particles in the first region is D1, 3 μm ≤ D1 ≤ 5 μm, the average particle size of the active material particles in the second region is D2, 9 μm ≤ D2 ≤ 40 μm, and the average particle size of the active material particles in the interface region is D3, 6 μm ≤ D3 ≤ 22 μm; The separator includes a base film and an adhesive layer, and the adhesive layer is provided on at least one surface in the thickness direction of the base film, and at least one of the adhesive layers is in contact with the second material layer.

2. The secondary battery according to claim 1, wherein, The peel strength between the second material layer and the separator is F, 1 N / m ≤ F ≤ 20 N / m.

3. The secondary battery according to claim 2, wherein, 3 N / m ≤ F ≤ 15 N / m.

4. The secondary battery according to claim 1, wherein, The adhesive layer includes at least one of polyacrylate, oily polyvinylidene fluoride, polyvinylidene fluoride or polyvinylidene fluoride - hexafluoropropylene copolymer.

5. The secondary battery according to claim 1, wherein, Based on the area of the separator, the area ratio of the adhesive layer is b, 10% ≤ b×100% ≤ 70%.

6. The secondary battery according to claim 5, wherein, 30％≤b×100％≤50％。 7. The secondary battery according to claim 1, wherein, The second material layer includes a second positive electrode active material, and the specific surface area of the second positive electrode active material is S, 0.07 m 2 / g ≤ S ≤ 0.3 m 2 / g.

8. The secondary battery according to claim 7, wherein, 0.1 m 2 / g ≤ S ≤ 0.25 m 2 / g。 9. The secondary battery according to claim 8, wherein, 0.12 m 2 / g ≤ S ≤ 0.2 m 2 / g。 10. The secondary battery according to claim 1, wherein, The thermal shrinkage rate of the separator is h, 1% ≤ h ≤ 15%.

11. The secondary battery according to any one of claims 1 to 10, wherein, The diaphragm further includes a ceramic layer, the ceramic layer is located between the adhesive layer and the base film, and the total coating amount of the adhesive layer and the ceramic layer is a, 7.7 mg / 5000 mm 2 7.7 mg / 5000 mm ≤ a ≤ 26 mg / 5000 mm 2 .

12. The secondary battery according to claim 11, wherein, The thickness of the ceramic layer is d1, 0.5 μm ≤ d1 ≤ 4 μm, and the thickness of the adhesive layer is d2, 0.2 μm ≤ d2 ≤ 3 μm.

13. The secondary battery according to any one of claims 1 to 10, wherein, The bonding layer further includes a ceramic material, and the coating amount of the bonding layer is c, 3 mg / 5000 mm 2 ≤ c ≤ 12 mg / 5000 mm 2 .

14. The secondary battery according to claim 13, wherein, Based on the mass of the adhesive layer, the mass percentage content w of the ceramic material is 40% to 70%.

15. The secondary battery according to claim 13, wherein, The thickness of the adhesive layer is d3, 1 μm ≤ d3 ≤ 4 μm.

16. The secondary battery according to any one of claims 1 to 10, wherein, The first material layer includes a first positive electrode active material, the second material layer includes a second positive electrode active material, and the first positive electrode active material and the second positive electrode active material each independently include at least one of lithium cobaltate, lithium manganate, lithium nickel cobalt manganate or lithium nickel cobalt aluminate.

17. The secondary battery according to claim 16, wherein, The first positive electrode active material includes at least one of lithium nickel cobalt manganate and lithium nickel cobalt aluminate, and the second positive electrode active material includes at least one of lithium cobaltate and lithium manganate.

18. The secondary battery according to claim 17, wherein The first positive electrode active material includes lithium nickel cobalt manganate, and the second positive electrode active material includes lithium cobaltate.

19. An electronic device, which includes the secondary battery according to any one of claims 1 to 18.

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