Secondary battery and electronic device

Abstract

The invention provides a secondary battery which comprises a positive pole piece, 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 second material layer is located between the first material layer and the positive current collector in the thickness direction of the positive pole piece. The first material layer comprises a first positive electrode active material, the first positive electrode active material has a single-crystal structure and a polycrystal structure, and based on the mass of the first positive electrode active material, the mass percentage content of the first positive electrode active material of the single-crystal structure is F1; the second material layer comprises a second positive electrode active material, the second positive electrode active material has a single-crystal structure and a polycrystal structure, based on the mass of the second positive electrode active material, the mass percentage content of the second positive electrode active material with the single-crystal structure is F2, F1 is greater than F2, and F1 is greater than 50%. Through the arrangement, the secondary battery has relatively low discharge temperature rise and relatively high cycle performance and dynamic performance.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, and particularly relates to a secondary battery and an electronic device. BACKGROUND

[0002] With the development of society, the update iteration of technical products such as electric vehicles and portable electronic products puts forward higher requirements for the endurance of secondary batteries (for example, lithium ion batteries), and the improvement of the energy density of lithium batteries is a simple and efficient method to develop new positive and negative active materials, but thick coating weight (≥300 mg / 1540.25 mm 2 ) will also cause other problems, such as uneven coating and increased risk of cracking, increased internal resistance, decreased rate performance, and shortened cycle service life. Therefore, it is crucial to solve the problems of kinetics and cycle decay caused by thick coating while ensuring high energy density. SUMMARY

[0003] The purpose of the present application is to provide a secondary battery and an electronic device to reduce the discharge temperature rise of the secondary battery, and the secondary battery has good kinetic performance and cycle performance.

[0004] It should be noted that the lithium ion battery is taken as an example to explain the present application in the summary of the present application, but the secondary battery of the present application is not limited to the lithium ion battery. The specific technical solutions are as follows:

[0005] The first aspect of the present application provides a secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer located on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a first material layer and a second material layer, the second material layer being located between the first material layer and the positive electrode current collector along the thickness direction of the positive electrode sheet. The first material layer comprises a first positive electrode active material, the first positive electrode active material having a single crystal structure and a polycrystal structure, the mass percentage content of the single crystal structure of the first positive electrode active material being F1 based on the mass of the first positive electrode active material; the second material layer comprises a second positive electrode active material, the second positive electrode active material having a single crystal structure and a polycrystal structure, the mass percentage content of the single crystal structure of the second positive electrode active material being F2 based on the mass of the second positive electrode active material, F1>F2, F1>50%. By setting the double-layer material layer, the proportion of the single crystal structure of the positive electrode active material in the upper first material layer is greater than that in the lower second material layer, forming a gradient distribution in the thickness direction of the secondary battery. The single crystal structure of the positive electrode active material with a higher proportion in the upper layer has good kinetic performance and lower temperature rise of the secondary battery during high-rate discharge, while the polycrystal structure of the positive electrode active material with a higher proportion in the lower layer provides higher energy density and structural stability. The first material layer and the second material layer are collocated, which is beneficial to play a synergistic effect, taking into account the higher energy density while reducing the discharge temperature rise of the secondary battery, and the secondary battery has good kinetic performance and cycle performance.

[0006] In one or more embodiments of the present application, 60%≤F1≤90%; and / or, 10%≤F2<50%. By adjusting the values of F1 and / or F2 within the above range, the structural stability and cycle performance of the secondary battery are further improved. Therefore, while taking into account the higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0007] In one or more embodiments of the present application, the Dv50 of the particles of the first positive electrode active material is B1 µm, the Dv50 of the particles of the second positive electrode active material is B2 µm, and the secondary battery satisfies any one of the following characteristics: (1) 0.15≤(F1×B1) / (F2×B2)<9; (2) 0.5≤(F1×B1) / (F2×B2)≤7. By meeting the above settings, the first material layer and the second material layer are collocated, which is beneficial to play a synergistic effect, taking into account the higher energy density while further reducing the discharge temperature rise of the secondary battery, and the secondary battery has better kinetic performance and cycle performance.

[0008] In one or more embodiments of the present application, the Dv50 of the particles of the first positive electrode active material is B1 µm, the Dv50 of the particles of the second positive electrode active material is B2 µm, and B1 < B2. By the above arrangement, the particle size of the upper layer of the first positive electrode active material is small, which is beneficial to shorten the diffusion path of lithium ions, to make lithium ions smoothly intercalate during the charging and discharging process, to accelerate the transmission speed of lithium ions, and thus to improve the kinetic performance of the secondary battery. During high-rate discharge, lithium ions can be more quickly extracted from the positive electrode active material, reducing polarization and thus reducing the discharge temperature rise of the secondary battery. The particle size of the lower layer of the second positive electrode active material is large, which is beneficial to improve the packing density of the positive electrode active material, and thus to improve the energy density of the secondary battery.

[0009] In one or more embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) 2≤B1<5; or, 3≤B1<5; (2) 5≤B2≤18; or, 6≤B2≤10. By adjusting the value of B1 and / or B2 within the above range, the particle size of the upper layer of the first positive electrode active material is small, which is beneficial to shorten the diffusion path of lithium ions, to make lithium ions smoothly intercalate during the charging and discharging process, to accelerate the transmission speed of lithium ions, and thus to improve the kinetic performance of the secondary battery. During high-rate discharge, lithium ions can be more quickly extracted from the positive electrode active material, reducing polarization and thus reducing the discharge temperature rise of the secondary battery. The particle size of the lower layer of the second positive electrode active material is large, which is beneficial to improve the packing density of the positive electrode active material, and thus to improve the energy density of the secondary battery, and to form a more stable and less likely to crack coating, to reduce the risk of performance degradation of the secondary battery due to uneven coating, sheet cracking and other problems during thick coating, and to reduce the side reaction with the electrolyte, thus further improving the structural stability and cycle performance of the secondary battery.

[0010] In one or more embodiments of the present application, the gram capacity of the first positive electrode active material is a mAh / g, 100≤a<180; the second material layer comprises a second positive electrode active material, the gram capacity of the second positive electrode active material is b mAh / g, 180≤b≤300. In one or more embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the first positive electrode active material comprises at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, lithium cobaltate, lithium iron phosphate or lithium manganate, the molar percentage content of Ni is <80% based on the total moles of Ni, Co and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material; (2) the second positive electrode active material comprises at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material or a lithium-rich manganese-based material, the molar percentage content of Ni is ≥80% based on the total moles of Ni, Co and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material. Through the above settings, at this time the gram capacity of the first positive electrode active material is moderate, has good electronic conductivity and lithium ion diffusion coefficient, has good lithium ion transmission performance in the charging and discharging process, and the structural stability of the first positive electrode active material is good, which can delay the increase of the interfacial impedance between the positive electrode plate surface and the electrolyte in high-rate discharge, reduce the electrochemical polarization, thereby reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. The gram capacity of the second positive electrode active material is high, which can store more lithium ions, thereby improving the energy density of the secondary battery. In addition, the particle surface of the positive electrode active material with high gram capacity has higher reactivity, the first material layer is arranged on the upper surface of the second material layer, which can play a protective role on the surface of the second material layer during the rolling process, reduce the risk of cracks of the positive electrode active material with high gram capacity during the rolling process, and the occurrence of side reactions with the electrolyte causes more active lithium in the secondary battery to be consumed, thereby causing the cycle life to decay. Therefore, while giving consideration to high energy density, the secondary battery has low discharge temperature rise, good kinetic performance and cycle performance.

[0011] In one or more embodiments of the present application, the Dv90 / Dv10 of the particles of the first positive electrode active material is C1, 1.1≤C1≤1.8; and / or, the Dv90 / Dv10 of the particles of the second positive electrode active material is C2, 1.1≤C2≤2.5. By adjusting the value of C1 and / or C2 within the above range, the particle size distribution of the first positive electrode active material in the upper first material layer is more concentrated, and the particle size distribution of the second positive electrode active material in the lower second material layer is relatively dispersed. While giving consideration to high energy density, the secondary battery has low discharge temperature rise, good kinetic performance and cycle performance.

[0012] In one or more embodiments of the present application, the coating weight of the first material layer is CW1 mg / 1540.25mm 2, the coating weight of the second material layer is CW2 mg / 1540.25mm 2 , 300≤CW1+CW2≤500, 0.09≤CW1 / CW2≤1.96. In one or more embodiments of the present application, 0.22≤CW1 / CW2≤1.64; and / or, 0.04≤(CW1×C1) / (CW2×C2)≤2.45. Through the above settings, a higher total coating weight is conducive to increasing the content of active materials of the secondary battery, thereby improving the energy density of the secondary battery. The first material layer and the second material layer are designed in combination, while taking into account a higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance and cycle performance.

[0013] In one or more embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the first material layer includes a first conductive agent, the first conductive agent includes at least one of single-walled carbon nanotubes or graphene, and the mass percentage content w1 of the first conductive agent based on the mass of the first material layer is 2% to 10%; (2) the second material layer includes a second conductive agent, the second conductive agent includes at least one of multi-walled carbon nanotubes, conductive carbon black or conductive graphite, and the mass percentage content w2 of the second conductive agent based on the mass of the second material layer is 0.1% to 6%. Through the above settings, at this time, the content of the first conductive agent is higher, which is conducive to optimizing the conductive network of the first material layer, and the content of the second conductive agent is lower, which reduces the proportion of other substances in the non-positive electrode active material in the second material layer, and is conducive to improving the energy density of the secondary battery. Therefore, while taking into account a higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance and cycle performance.

[0014] In one or more embodiments of the present application, the mass percentage content W1 of the first positive electrode active material based on the mass of the first material layer is 87% to 96%; and / or, the mass percentage content W2 of the second positive electrode active material based on the mass of the second material layer is 90% to 98%. By adjusting the value of W1 and / or W2 within the above range, the first material layer has a good conductive network, which reduces the electronic impedance of the positive electrode sheet, reduces the electrochemical polarization and Joule heat caused by the obstruction of electron transmission, thereby further reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. The content of the second positive electrode active material is higher, which is conducive to improving the energy density of the secondary battery. Therefore, while taking into account a higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance and cycle performance.

[0015] The second aspect of the present application provides an electronic device comprising the secondary battery in any of the preceding embodiments. Therefore, the electronic device provided by the present application has good use performance.

[0016] The beneficial effects of the present application are:

[0017] The present application provides a secondary battery and an electronic device, by setting a double-layer material layer, so that the proportion of single-crystal structure positive electrode active material in the upper layer first material layer is greater than the proportion of single-crystal structure positive electrode active material in the lower layer second material layer, a gradient distribution is formed in the thickness direction of the secondary battery, and the single-crystal structure positive electrode active material with a higher proportion in the upper layer has good kinetic performance and lower temperature rise of the secondary battery during high-rate discharge, while the polycrystal structure positive electrode active material with a higher proportion in the lower layer provides higher energy density and structural stability. The first material layer and the second material layer are arranged in combination, which is beneficial to play a synergistic effect, while taking into account the higher energy density, it is beneficial to reduce the discharge temperature rise of the secondary battery, and the secondary battery has good kinetic performance and cycle performance.

[0018] Of course, implementing any product or method of the present application does not necessarily require all the advantages described above to be achieved at the same time. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other embodiments can also be obtained by those skilled in the art based on these drawings.

[0020] Figure 1 For an embodiment of the present application, a part of the cross-sectional view of the positive electrode sheet along the thickness direction of the positive electrode sheet after being unfolded along the width direction of the positive electrode sheet is shown;

[0021] The drawings show: positive electrode sheet 10; positive electrode current collector 11; positive electrode material layer 12; first material layer 121; second material layer 122. DETAILED DESCRIPTION

[0022] The technical solutions in the present application will be described clearly and completely below in combination with the embodiments of the present application and the drawings. Obviously, the described embodiments are only some of the embodiments of the present application, not all. All other embodiments obtained by those skilled in the art based on the present application are within the scope of protection of the present application.

[0023] It should be noted that in the specific embodiments of the present application, lithium ion batteries are used as examples of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium ion batteries.

[0024] For the problem of kinetic deficiency and rapid cycle life attenuation caused by thick coating of the positive electrode sheet, the root cause lies in the increase of ion diffusion impedance and the extension of electronic conduction distance. Based on the above problems, currently, laser punching or scribing can be used to provide channels for lithium ion transmission; or the proportion of conductive agent in the slurry formula of the positive electrode material layer can be increased. However, laser punching or scribing can easily cause damage to the surface of the positive electrode active material, reducing the reaction activity of the positive electrode active material; and increasing the mass proportion of the conductive agent in the positive electrode material layer can reduce the mass proportion of the positive electrode active material, which is contrary to the original intention of improving the energy density of the secondary battery by increasing the coating weight of the positive electrode material layer. Based on this, the application provides a secondary battery and an electronic device, which can reduce the discharge temperature rise of the secondary battery while maintaining a high energy density, and have good kinetic performance and cycle performance. The specific technical solutions are as follows:

[0025] The first aspect of the application provides a secondary battery, which comprises a positive electrode sheet, the positive electrode sheet comprising a positive electrode current collector and a positive electrode material layer located on at least one surface of the positive electrode current collector, the positive electrode material layer comprising a first material layer and a second material layer, the second material layer being located between the first material layer and the positive electrode current collector along the thickness direction of the positive electrode sheet. The first material layer comprises a first positive electrode active material, the first positive electrode active material having a single crystal structure and a polycrystalline structure, and the mass percentage content of the single crystal structure of the first positive electrode active material based on the mass of the first positive electrode active material is F1; the second material layer comprises a second positive electrode active material, the second positive electrode active material having a single crystal structure and a polycrystalline structure, and the mass percentage content of the single crystal structure of the second positive electrode active material based on the mass of the second positive electrode active material is F2, F1>F2, and F1>50%.

[0026] In the present application, the width direction of the positive electrode sheet itself is defined as the X direction, and the thickness direction of the positive electrode sheet itself is defined as the Z direction. For example, as shown in Figure 1 The positive electrode sheet 10 comprises a positive electrode current collector 11 and a positive electrode material layer 12 located on both surfaces of the positive electrode current collector 11, the positive electrode material layer 12 comprising a first material layer 121 and a second material layer 122, the second material layer 122 being located between the first material layer 121 and the positive electrode current collector 11 along the thickness direction Z of the positive electrode sheet 10 itself.

[0027] The inventors discovered that the first material layer contains a higher percentage of single-crystal structure as the first positive electrode active material. The single-crystal structure exhibits better mechanical properties and thermal stability, and its lattice structure is more stable during charge and discharge. Furthermore, the cold pressing process during fabrication has less impact on the single-crystal structure, thus reducing the internal resistance and polarization of the secondary battery and improving its kinetic performance. The single-crystal structure of the first positive electrode active material provides a more direct diffusion path for lithium ions, accelerating lithium ion transport and further increasing the charge and discharge rate of the secondary battery. This is beneficial for reducing the temperature rise of the secondary battery during high-rate (≥2C) discharge. In the second material layer, the percentage of single-crystal structure as the second positive electrode active material is lower, resulting in a larger proportion of polycrystalline structure. Polycrystalline materials typically have higher specific capacity, enabling them to store more lithium ions, thereby increasing the overall energy density of the secondary battery. Furthermore, polycrystalline particles can form a denser structure during sintering, which is beneficial for improving the mechanical strength and compressive strength of the positive electrode active material, and reducing the risk of performance degradation in secondary batteries due to uneven coating and electrode cracking during thick coating. In addition, the porosity between polycrystalline particles provides additional channels for lithium-ion diffusion, reducing the risk of increased lithium-ion diffusion distance caused by thick coating, thereby further improving the structural stability and cycle performance of the secondary battery. By setting a double-layer material structure, the proportion of single-crystal positive electrode active material in the upper first material layer is greater than that in the lower second material layer, forming a gradient distribution in the thickness direction of the secondary battery. The higher proportion of single-crystal positive electrode active material in the upper layer gives the secondary battery good kinetic performance and low temperature rise during high-rate discharge, while the higher proportion of polycrystalline positive electrode active material in the lower layer provides higher energy density and structural stability. The combination of the first and second material layers is beneficial for synergistic effects, which helps to reduce the discharge temperature rise of the secondary battery while maintaining high energy density, resulting in good kinetic and cycle performance.

[0028] In this context, "single crystal" refers to an active material particle containing only one crystal and without grains or grain boundaries within the particle; "polycrystalline" refers to an active material particle containing multiple (two or more) crystals and with grains or grain boundaries within the particle. The aforementioned "positive electrode material layer located on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be located on one surface of the positive electrode current collector along its thickness direction, or on two surfaces of the positive electrode current collector along its thickness direction. It should be noted that "surface" here can be the entire surface area of ​​the positive electrode current collector, or only a portion of the surface area; this application has no particular limitation, as long as the purpose of this application is achieved.

[0029] In one or more embodiments of the present application, 60%≤F1≤90%. For example, the value of F1 can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, or a range between any two of these values, and the value range of F1 can be 60%-90%, 65%-85%, 70%-80%, 72%-78%, 75%-78%, and all ranges and sub-ranges thereof. By setting the value of F1 within the above range, the proportion of single-crystal structured positive electrode active material in the upper first material layer is high, which reduces the internal resistance and polarization of the secondary battery during charging and discharging, further improves the kinetic performance of the secondary battery, and improves the charging and discharging rate of the secondary battery, further reducing the temperature rise of the secondary battery during high-rate discharge. At the same time, the polycrystalline structured positive electrode active material in the upper first material layer further improves the kinetic performance while taking into account the high energy density of the secondary battery and controlling the actual production cost. Therefore, while taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance, and cycle performance.

[0030] In one or more embodiments of the present application, 10%≤F2<50%. For example, the value of F2 can be 10%, 14%, 15%, 18%, 20%, 22%, 25%, 26%, 30%, 34%, 35%, 38%, 40%, 42%, 45%, 46%, 49%, or a range between any two of these values, and the value range of F2 can be 10%-50% (not including the endpoint value 50%), 15%-45%, 20%-40%, 25%-35%, 30%-35%, and all ranges and sub-ranges thereof. By setting the value of F2 within the above range, the proportion of polycrystalline structured positive electrode active material in the lower second material layer is high, the polycrystalline structured second positive electrode active material has a high specific capacity, which is conducive to storing more lithium ions, thereby improving the overall energy density of the secondary battery, and the polycrystalline structured positive electrode active material has a more compact structure, which is conducive to improving the mechanical strength and compression resistance of the positive electrode active material, reducing the risk of performance degradation of the secondary battery due to uneven coating and electrode cracking during thick coating, thereby further improving the structural stability and cycle performance of the secondary battery. Therefore, while taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance, and cycle performance.

[0031] In one or more embodiments of the present application, 60%≤F1≤90%, for example, the value of F1 can be 60%, 62%, 65%, 68%, 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, or a range consisting of any two of them, the value range of F1 can be 60% to 90%, 65% to 85%, 70% to 80%, 72% to 78%, 75% to 78%, and all ranges and sub-ranges thereof; and / or, 10%≤F2<50%, for example, the value of F2 can be 10%, 14%, 15%, 18%, 20%, 22%, 25%, 26%, 30%, 34%, 35%, 38%, 40%, 42%, 45%, 46%, 49%, or a range consisting of any two of them; the value range of F2 can be 10% to 50% (not including the end value 50%), 15% to 45%, 20% to 40%, 25% to 35%, 30% to 35%, and all ranges and sub-ranges thereof. By adjusting the value of F1 and / or F2 within the above range, the mass percentage of the first positive active material with a single crystal structure in the first material layer is higher, thereby reducing the internal resistance and polarization of the secondary battery, improving the kinetic performance and charge-discharge rate of the secondary battery, and reducing the temperature rise of the secondary battery during high-rate discharge. The mass percentage of the second positive active material with a single crystal structure in the second material layer is lower, and at this time, the proportion of the second positive active material with a polycrystal structure is larger, and the polycrystal material usually has a higher specific capacity and can store more lithium ions, thereby improving the overall energy density of the secondary battery, and due to its more compact structure, it is beneficial to improve the mechanical strength and compression resistance of the positive active material, reduce the risk of performance degradation of the secondary battery caused by uneven coating, sheet cracking, and other problems during thick coating process, thereby further improving the structural stability and cycle performance of the secondary battery. Therefore, while taking into account the higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance, and cycle performance.

[0032] In one or more embodiments of the present application, the Dv50 of the particles of the first positive electrode active material is B1 pm, the Dv50 of the particles of the second positive electrode active material is B2 pm, and the secondary battery satisfies: 0.15≤(F1×B1) / (F2×B2)<9; optionally, 0.5≤(F1×B1) / (F2×B2)≤7. For example, the value of (F1×B1) / (F2×B2) can be 0.15, 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 8.8, 8.99, or a range between any two of them; the value range of (F1×B1) / (F2×B2) can be 0.15 to 9 (not including the end value 9), 0.2 to 8.5, 0.3 to 8, 0.4 to 7.5, 0.5 to 7, 1 to 7, 2 to 6.5, 3 to 6, 3.5 to 5.5, 4 to 5, 4.5 to 5, and all ranges and sub-ranges thereof. By satisfying the above settings, the positive electrode active material in the upper first material layer has a smaller particle size and a higher content of single crystal structure, the smaller particle size is beneficial to shorten the diffusion path of lithium ions, thereby accelerating the transmission speed of lithium ions, further improving the kinetic performance of the secondary battery, and is beneficial to improve the specific surface area of the positive electrode active material, and reduce the discharge temperature rise of the secondary battery; at the same time, the positive electrode active material in the lower second material layer has a larger particle size and a higher content of polycrystalline structure, the larger particle size is beneficial to improve the bulk density of the positive electrode active material, thereby improving the energy density of the secondary battery, and the polycrystalline structure particles can form a more dense structure during the sintering process, thereby being beneficial to improve the mechanical strength and compression resistance of the positive electrode active material, reducing the risk of performance degradation of the secondary battery caused by uneven coating, sheet cracking and other problems during the thick coating process, thereby further improving the structural stability and cycle performance of the secondary battery. The first material layer and the second material layer are collocated, which is beneficial to play a synergistic effect, while taking into account the higher energy density, it is beneficial to further reduce the discharge temperature rise of the secondary battery, and the secondary battery has better kinetic performance and cycle performance.

[0033] In one or more embodiments of the present application, the Dv50 of the particles of the first positive electrode active material is B1 µm, the Dv50 of the particles of the second positive electrode active material is B2 µm, and B1 < B2. By the above arrangement, the particle size of the upper layer of the first positive electrode active material is small, which is conducive to shortening the diffusion path of lithium ions, allowing lithium ions to smoothly intercalate during the charging and discharging process, and accelerating the transmission speed of lithium ions, thereby improving the kinetic performance of the secondary battery. During high-rate discharge, lithium ions can be more quickly extracted from the positive electrode active material, reducing polarization, and thereby reducing the discharge temperature rise of the secondary battery. The particle size of the lower layer of the second positive electrode active material is large, which is conducive to improving the packing density of the positive electrode active material, thereby improving the energy density of the secondary battery. Therefore, while taking into account a relatively high energy density, the secondary battery has a relatively low discharge temperature rise, good kinetic performance, and cycle performance.

[0034] In one or more embodiments of the present application, the secondary battery satisfies: 2 ≤ B1 < 5; optionally, 3 ≤ B1 < 5. For example, the value of B1 can be 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 4.9, 4.995, or a range consisting of any two of the values; the value range of B1 can be 2 to 5 (not including the end value 5), 2.5 to 5 (not including the end value 5), 3 to 5 (not including the end value 5), 3.5 to 4.5, 3.5 to 4, and all ranges and sub-ranges thereof. By adjusting the value of B1 within the above range, the particle size of the upper layer of the first positive electrode active material is small, which is conducive to shortening the diffusion path of lithium ions, allowing lithium ions to smoothly intercalate during the charging and discharging process, and accelerating the transmission speed of lithium ions, thereby further improving the kinetic performance of the secondary battery. During high-rate discharge, lithium ions can be more quickly extracted from the positive electrode active material, reducing polarization, and thereby further reducing the discharge temperature rise of the secondary battery. Therefore, while taking into account a relatively high energy density, the secondary battery has a relatively low discharge temperature rise, good kinetic performance, and cycle performance.

[0035] In one or more embodiments of the present application, the secondary battery satisfies: 5≤B2≤18; optionally, 6≤B2≤10. For example, the value of B2may be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 or a range between any two of them; the value range of B2may be 5 to 18, 5 to 15, 6 to 12, 6 to 10, 7 to 9 and all ranges and sub-ranges thereof. By regulating the value of B2within the above range, the particle size of the second positive active material in the lower layer is larger, which is conducive to improving the packing density of the positive active material, thereby improving the energy density of the secondary battery, and when the particle size is smaller, the specific surface area is larger, more conductive agent and binder are needed during slurry preparation, which can affect the uniformity of the slurry coating. By regulating the value of B2within the above range, it is conducive to forming a more stable and less prone to cracking coating, reducing the risk of performance degradation of the secondary battery due to uneven coating, sheet cracking and other problems during thick coating, and reducing the side reaction with the electrolyte, thereby further improving the structural stability and cycle performance of the secondary battery. Therefore, while taking into account the higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0036] In one or more embodiments of this application, the secondary battery satisfies at least one of the following characteristics: (1) 2 ≤ B1 < 5; or, 3 ≤ B1 < 5. For example, the value of B1 can be 2, 2.2, 2.5, 2.8, 3, 3.2, 3.5, 3.8, 4, 4.2, 4.5, 4.8, 4.9, 4.995 or a range consisting of any two of these values; the value range of B1 can be 2 to 5 (excluding the endpoint value 5), 2.5 to 5 (excluding the endpoint value 5), 3 to 5 (excluding the endpoint value 5), 3.5 to 4.5, 3.5 to 4 and all of these ranges, as well as sub-ranges; (2) 5 ≤ B2 ≤ 18; or, 6 ≤ B2 ≤ 10. For example, the value of B2 can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, or any range of two of these values; the value range of B2 can be 5 to 18, 5 to 15, 6 to 12, 6 to 10, 7 to 9, and all of these ranges, as well as sub-ranges. By adjusting the values ​​of B1 and / or B2 within the above ranges, the particle size of the upper first positive electrode active material is smaller, which is beneficial for shortening the diffusion path of lithium ions, allowing lithium ions to be smoothly inserted during charging and discharging, accelerating the lithium ion transport speed, and thus improving the kinetic performance of the secondary battery. During high-rate discharge, lithium ions can be extracted from the positive electrode active material more quickly, reducing polarization and thus lowering the discharge temperature rise of the secondary battery. The larger particle size of the lower-layer second positive electrode active material is beneficial for increasing the packing density of the positive electrode active material, thereby improving the energy density of the secondary battery. It also facilitates the formation of a more stable and crack-resistant coating, reducing the risk of performance degradation caused by uneven coating and electrode cracking during thick coating processes. Furthermore, it reduces side reactions with the electrolyte, further improving the structural stability and cycle performance of the secondary battery. Therefore, while achieving high energy density, the secondary battery exhibits low discharge temperature rise, good kinetic performance, and excellent cycle performance.

[0037] Dv50 refers to the diameter corresponding to when the cumulative volume fraction of particles in the volume reference distribution reaches 50%.

[0038] In one or more embodiments of the present application, the first positive electrode active material has a gravimetric capacity of a mAh / g, 100≤a<180, for example, a can have a value of 100, 105, 110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, 175, 178, 179.9, or a range defined by any two of them; a can have a value in the range of 100 to 180 (not including the end value 180), 105 to 175, 110 to 170, 115 to 165, 120 to 160, 125 to 155, 130 to 150, 135 to 145, 140 to 145, and all ranges and sub-ranges thereof. The second material layer comprises a second positive electrode active material, and the second positive electrode active material has a gravimetric capacity of b mAh / g, 180≤b≤300, for example, b can have a value of 180, 185, 190, 195, 200, 205, 210, 215, 220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285, 290, 295, 300, or a range defined by any two of them, b can have a value in the range of 180 to 300, 190 to 290, 200 to 280, 210 to 270, 220 to 260, 230 to 250, 240 to 250, and all ranges and sub-ranges thereof. By adjusting the values of a and b within the above ranges, the gravimetric capacity of the first positive electrode active material is moderate, and the lithium ion transmission performance is good during the charging and discharging process, and the structural stability of the first positive electrode active material is good, which can delay the increase of the interfacial impedance between the positive electrode plate surface and the electrolyte during high-rate discharging, reduce the electrochemical polarization, and thus reduce the discharge temperature rise of the secondary battery. The second positive electrode active material has a higher gravimetric capacity and can store more lithium ions, thereby improving the energy density of the secondary battery. In addition, the positive electrode active material with a higher gravimetric capacity has a higher reactive particle surface, and the first material layer with a lower gravimetric capacity is arranged on the surface of the second material layer with a higher gravimetric capacity, which can protect the surface of the second material layer during the rolling process, reduce the risk of cracks in the positive electrode active material with a higher gravimetric capacity during the rolling process, and reduce the risk of side reactions with the electrolyte to consume more active lithium in the secondary battery and thus cause the cycle life to decay. Therefore, while taking into account the higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance, and cycle performance.

[0039] In one or more embodiments of the present application, the first positive electrode active material includes at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, lithium cobaltate, lithium iron phosphate, or lithium manganate, and the molar percentage content of Ni is < 80% based on the total moles of Ni, Co, and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material. By selecting the first positive electrode active material of the above type, the specific capacity of the first positive electrode active material is moderate, the electronic conductivity and lithium ion diffusion coefficient are good, the lithium ion transmission performance is good during the charging and discharging process, and the structural stability of the first positive electrode active material is good, which can delay the increase of the interfacial impedance between the positive electrode tab surface and the electrolyte during high-rate discharging, reduce the electrochemical polarization, thereby reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. At the same time, the first positive electrode active material with relatively high mechanical strength is arranged on the surface layer of the positive electrode tab, which can play a protective role on the surface of the second material layer during the rolling process, reduce the risk of cracks in the second positive electrode active material during the rolling process, and reduce the risk of more active lithium being consumed in the secondary battery due to side reactions with the electrolyte, thereby causing the cycle life to decay. Therefore, while taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance, and good cycle performance.

[0040] In one or more embodiments of the present application, the first positive electrode active material includes LiNi 0.33 Co 0.33 Mn 0.33 O2(NCM111), LiNi 0.4 Co 0.2 Mn 0.4 O2(NCM424), LiNi 0.4 Co 0.4 Mn 0.2 O2(NCM442), LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622), LiNi 0.7 Co 0.1 Mn 0.2 O2(NCM712), LiNi 0.7 Co 0.2 Mn 0.1 O2(NCM721), LiNi 0.4 Co 0.3 Al 0.3 O2(NCA433), LiNi 0.5 Co 0.2 Al 0.3 O2(NCA523), LiNi0.7 Co 0.2 Al 0.1 O2(NCA721), lithium cobaltate, lithium iron phosphate or lithium manganate. By selecting the first positive electrode active material of the above type, the specific capacity of the first positive electrode active material is moderate, the electronic conductivity and lithium ion diffusion coefficient are good, the lithium ion transmission performance is good during the charging and discharging process, and the structural stability of the first positive electrode active material is good. The interface impedance between the positive electrode plate surface and the electrolyte can be delayed during high-rate discharging, the electrochemical polarization is reduced, the discharge temperature rise of the secondary battery is reduced, and the kinetic performance of the secondary battery is improved. At the same time, the first positive electrode active material with relatively high mechanical strength is arranged on the surface layer of the positive electrode plate, which can protect the surface of the second material layer during the rolling process, reduce the risk of cracks of the second positive electrode active material during the rolling process, and reduce the risk of more active lithium being consumed in the secondary battery due to the side reaction with the electrolyte, thereby causing the cycle life to decay. Therefore, while taking into account the high energy density, the secondary battery has low discharge temperature rise, good kinetic performance and cycle performance.

[0041] In one or more embodiments of the present application, the second positive electrode active material includes at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, or a lithium-rich manganese-based material, and the mole percentage content of Ni is ≥80% based on the total moles of Ni, Co and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material. By selecting the second positive electrode active material of the above type, the specific capacity of the second positive electrode active material is high, which can store more lithium ions, thereby improving the energy density of the secondary battery. At the same time, the high specific capacity positive electrode active material has a higher reactive particle surface, and the first material layer is arranged on the upper surface of the second material layer with high specific capacity, which can protect the surface of the second material layer during the rolling process, reduce the risk of cracks of the high specific capacity positive electrode active material during the rolling process, and reduce the risk of more active lithium being consumed in the secondary battery due to the side reaction with the electrolyte, thereby causing the cycle life to decay. Therefore, while taking into account the high energy density, the secondary battery has low discharge temperature rise, good kinetic performance and cycle performance.

[0042] In one or more embodiments of the present application, the second positive electrode active material includes LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.075 Mn 0.075 O2(NCM85), LiNi 0.88 Co 0.06 Mn 0.06 O2(NCM88), LiNi 0.9Co 0.05 Mn 0.05 O2(NCM90), LiNi 0.92 Co 0.05 Mn 0.03 O2(NCM92), LiNi 0.95 Co 0.025 Mn 0.025 O2(NCM95), LiNi 0.8 Co 0.1 Al 0.1 O2(NCA80), LiNi 0.85 Co 0.09 Al 0.06 O2(NCA85), LiNi 0.88 Co 0.06 Al 0.06 O2(NCA88), LiNi 0.9 Co 0.05 Al 0.05 O2(NCA90), LiNi 0.92 Co 0.05 Al 0.03 O2(NCA92), LiNi 0.95 Co 0.03 Al 0.02 O2(NCA95), Li 1.17 Mn 0.54 Ni 0.22 Co 0.07 O2or Li 1.2 Mn 0.54 Co 0.13 Ni 0.13 O2. By selecting the second positive electrode active material of the above-mentioned kind, the specific capacity of the second positive electrode active material is higher, which can store more lithium ions, thereby improving the energy density of the secondary battery. At the same time, the positive electrode active material with higher specific capacity has a higher reactive particle surface. By arranging the first material layer on the upper surface of the second material layer with higher specific capacity, the surface of the second material layer can be protected during the rolling process, thereby reducing the risk of cracks in the positive electrode active material with higher specific capacity during the rolling process, and the risk of more active lithium being consumed in the secondary battery due to the side reaction with the electrolyte, thereby causing the cycle life to deteriorate. Therefore, while taking into account the higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0043] In one or more embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the first positive electrode active material comprises at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, lithium cobaltate, lithium iron phosphate, or lithium manganate, and the molar percentage content of Ni is < 80% based on the total moles of Ni, Co, and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material; (2) the second positive electrode active material comprises at least one of a nickel-cobalt-manganese ternary material, a nickel-cobalt-aluminum ternary material, or a lithium-rich manganese-based material, and the molar percentage content of Ni is > 80% based on the total moles of Ni, Co, and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material. Through the above settings, the gram capacity of the first positive electrode active material is moderate, has good electronic conductivity and lithium ion diffusion coefficient, has good lithium ion transmission performance in the charging and discharging process, and the structural stability of the first positive electrode active material is good, which can delay the increase of the interfacial impedance between the positive electrode plate surface and the electrolyte during high-rate discharging, reduce the electrochemical polarization, thereby reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. The gram capacity of the second positive electrode active material is high, which can store more lithium ions, thereby improving the energy density of the secondary battery. In addition, the positive electrode active material with high gram capacity has a particle surface with higher reactivity, and the first material layer is arranged on the upper surface of the second material layer, which can play a protective role on the surface of the second material layer during the rolling process, reduce the risk of cracks in the positive electrode active material with high gram capacity during the rolling process, and the occurrence of side reactions with the electrolyte causes more active lithium in the secondary battery to be consumed, thereby causing the cycle life to decay. Therefore, while taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance, and cycle performance.

[0044] In one or more embodiments of the present application, the Dv90 / Dv10 of the particles of the first positive electrode active material is C1, 1.1≤C1≤1.8. For example, the value of C1may be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or a range between any two of them, and the value range of C1may be 1.1 to 1.8, 1.2 to 1.7, 1.3 to 1.6, 1.4 to 1.5, and all ranges and sub-ranges thereof. The particle size distribution width reflects the uniformity of the particle size, wherein the particle size distribution width is characterized by Dv90 / Dv10, the smaller the value, the more concentrated the particle size distribution, and the more uniform the particle size. By adjusting the value of C1within the above range, the particle size distribution of the first positive electrode active material in the upper first material layer is more concentrated, and the particle size is more uniform at this time, which is beneficial to shorten the diffusion path of lithium ions, thereby accelerating the transmission speed of lithium ions, reducing the concentration polarization, thereby reducing the discharge temperature rise of the secondary battery, and improving the kinetic performance of the secondary battery. And the uniform particle size changes in volume synchronously during charging and discharging, and the internal stress distribution is uniform, thereby reducing the risk of particle pulverization and damage to the surface structure of the positive electrode sheet caused by stress concentration, and the structure of the first material layer is relatively stable at this time. Therefore, while giving priority to higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0045] In one or more embodiments of the present application, the Dv90 / Dv10 of the particles of the second positive electrode active material is C2, 1.1≤C2≤2.5. For example, the value of C2may be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, or a range between any two of them, and the value range of C2may be 1.1 to 2.5, 1.2 to 2.4, 1.3 to 2.3, 1.4 to 2.2, 1.5 to 2.1, 1.6 to 2, 1.7 to 1.9, 1.8 to 1.9, and all ranges and sub-ranges thereof. The particle size distribution width reflects the uniformity of the particle size, wherein the particle size distribution width is characterized by Dv90 / Dv10, the smaller the value, the more concentrated the particle size distribution, and the more uniform the particle size. By adjusting the value of C2within the above range, the particle size distribution of the second positive electrode active material in the lower second material layer is relatively dispersed, and the particle size has certain differences at this time, and when the particle size distribution is wide, small particles are easy to fill into the pores between large particles, so that more positive electrode active material particles can be accommodated in a unit volume, thereby improving the packing density of the positive electrode active material, and thereby improving the energy density of the secondary battery. Coupled with the first material layer in the upper layer, while giving priority to higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0046] In one or more embodiments of the present application, the Dv90 / Dv10 of the particles of the first positive electrode active material is C1, 1.1≤C1≤1.8, for example, the value of C1 can be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, or a range composed of any two of them, the value range of C1 can be 1.1 to 1.8, 1.2 to 1.7, 1.3 to 1.6, 1.4 to 1.5, and all ranges and sub-ranges thereof; and / or, the Dv90 / Dv10 of the particles of the second positive electrode active material is C2, 1.1≤C2≤2.5, for example, the value of C2 can be 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1, 2.2, 2.3, 2.4, 2.5, or a range composed of any two of them, the value range of C2 can be 1.1 to 2.5, 1.2 to 2.4, 1.3 to 2.3, 1.4 to 2.2, 1.5 to 2.1, 1.6 to 2, 1.7 to 1.9, 1.8 to 1.9, and all ranges and sub-ranges thereof. By adjusting the value of C1 and / or C2 within the above range, the particle size distribution of the first positive electrode active material in the upper first material layer is more concentrated, at this time the particle size is uniform, which is beneficial to shorten the diffusion path of lithium ions, thereby accelerating the transmission speed of lithium ions, reducing the concentration polarization, thereby reducing the discharge temperature rise of the secondary battery, and improving the kinetic performance of the secondary battery. And the particles with uniform particle size change synchronously in volume during charging and discharging, the internal stress distribution is uniform, thereby reducing the risk of particle pulverization and damage to the surface structure of the positive electrode sheet caused by stress concentration, at this time the structure of the first material layer is relatively stable. The particle size distribution of the second positive electrode active material in the lower second material layer is relatively dispersed, at this time there is a certain difference in particle size, when the particle size distribution is wide, small particles are easy to fill into the pores between large particles, so that more positive electrode active material particles can be accommodated in a unit volume, thereby improving the packing density of the positive electrode active material, and thereby improving the energy density of the secondary battery. While taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance and cycle performance.

[0047] wherein Dv90 refers to the diameter corresponding to the cumulative volume fraction of 90% in the volume-based distribution; and Dv10 refers to the diameter corresponding to the cumulative volume fraction of 90% in the volume-based distribution.

[0048] In one or more embodiments of the present application, the Dv90 of the first positive electrode active material is 2.4 pm to 6 pm; and / or, the Dv10 of the first positive electrode active material is 1.2 pm to 4.2 pm. In one or more embodiments of the present application, the Dv90 of the second positive electrode active material is 6.5 pm to 25 pm; and / or, the Dv10 of the second positive electrode active material is 2.5 pm to 15 pm. By the above settings, while the energy density is relatively high, the secondary battery has a low discharge temperature rise, good kinetic performance and cycle performance.

[0049] In one or more embodiments of the present application, the coating weight of the first material layer is CW1 mg / 1540.25 mm 2 , the coating weight of the second material layer is CW2 mg / 1540.25 mm 2 , 300≤CW1+CW2≤500, for example, the value of CW1+CW2 can be 300, 320, 350, 380, 400, 420, 450, 480, 500 or a range composed of any two of them, the value range of CW1+CW2 can be 300 to 500, 350 to 480, 400 to 460, 420 to 450 and all ranges and sub-ranges thereof. 0.09≤CW1 / CW2≤1.96, optionally, 0.22≤CW1 / CW2≤1.64, for example, the value of CW1 / CW2 can be 0.09, 0.1, 0.2, 0.22, 0.25, 0.3, 0.5, 0.8, 1, 1.2, 1.5, 1.6, 1.64, 1.7, 1.8, 1.9, 1.95, 1.96 or a range composed of any two of them, the value range of CW1 / CW2 can be 0.09 to 1.96, 0.1 to 1.9, 0.2 to 1.7, 0.22 to 1.64, 0.5 to 1.5, 0.8 to 1.2, 1 to 1.2 and all ranges and sub-ranges thereof. By adjusting the value of CW1+CW2 and CW1 / CW2 within the above range, a higher total coating weight is conducive to increasing the content of active material of the secondary battery, thereby improving the energy density of the secondary battery. The coating weight of the upper first material layer is relatively low, at this time the lithium ion diffusion path is short, which is conducive to speeding up the transmission speed of lithium ions and improving the kinetic performance of the secondary battery. The coating weight of the lower second material layer is relatively high, a higher coating weight can increase the content of active material, which is conducive to improving the energy density of the secondary battery. The first material layer and the second material layer are designed in combination, while the energy density is relatively high, the secondary battery has a low discharge temperature rise, good kinetic performance and cycle performance.

[0050] In one or more embodiments of the present application, 24≤ CW1≤ 331. For example, the value of CW1 can be 24, 25, 28, 29, 30, 50, 80, 100, 120, 150, 180, 200, 220, 250, 280, 300, 320, 330, 331, or a range between any two of them, the value range of CW1 can be 24 to 331, 29 to 331, 30 to 330, 50 to 300, 80 to 280, 100 to 250, 120 to 220, 150 to 200, 150 to 180, and all ranges and sub-ranges thereof. By regulating the value of CW1 within the above range, the coating weight of the upper first material layer is relatively low, and the lithium ion diffusion path is relatively short, which is conducive to accelerating the transmission speed of lithium ions and improving the kinetic performance of the secondary battery. In combination with the second material layer, the secondary battery has a relatively high energy density, a low discharge temperature rise, good kinetic performance, and cycle performance.

[0051] In one or more embodiments of the present application, 101≤ CW2≤ 459. For example, the value of CW2 can be 101, 118, 120, 150, 180, 200, 220, 250, 280, 300, 320, 350, 380, 400, 420, 450, 459, or a range between any two of them, the value range of CW2 can be 101 to 459, 118 to 459, 120 to 450, 150 to 420, 180 to 400, 200 to 380, 220 to 350, 250 to 320, 280 to 300, and all ranges and sub-ranges thereof. By regulating the value of CW2 within the above range, the coating weight of the lower second material layer is relatively high, and the higher coating weight can increase the content of the active material, which is conducive to improving the energy density of the secondary battery. In combination with the first material layer, the secondary battery has a relatively high energy density, a low discharge temperature rise, good kinetic performance, and cycle performance.

[0052] In one or more embodiments of the present application, 0.04≤(CW1×C1) / (CW2×C2)≤2.45. For example, the value of (CW1×C1) / (CW2×C2) can be 0.04, 0.1, 0.3, 0.5, 0.8, 1, 1.2, 1.5, 1.8, 2, 2.2, 2.4, 2.45, or a range between any two of them, the value range of (CW1×C1) / (CW2×C2) can be 0.04 to 2.45, 0.1 to 2.2, 0.3 to 2, 0.5 to 1.8, 0.8 to 1.5, 1 to 1.2, and all ranges and sub-ranges thereof. By adjusting the value of (CW1×C1) / (CW2×C2) within the above range, the transmission impedance of lithium ions inside the thickly coated positive electrode material layer is adjusted, and the overall electrochemical polarization is reduced. At this time, the first material layer and the second material layer work together to achieve a higher energy density while the secondary battery has a lower discharge temperature rise, good kinetic performance, and cycle performance.

[0053] The present application does not have a particular limitation on the adjustment method of the parameters of the positive electrode active material, as long as the purpose of the present application can be achieved. For example, different commercially available positive electrode active materials can be purchased according to the required parameters of the positive electrode active material (such as type, single crystal structure, polycrystalline structure, Dv50, Dv90, Dv10, specific capacity, etc.). Alternatively, the Dv50, Dv90, and Dv10 of the positive electrode active particles can also be adjusted by grinding and other methods.

[0054] In one or more embodiments of the present application, the secondary battery satisfies: the first material layer comprises a first conductive agent, the first conductive agent comprises at least one of single-walled carbon nanotubes or graphene, and the mass percentage content w1 of the first conductive agent is 2% to 10% based on the mass of the first material layer. For example, the value of the mass percentage content w1 of the first conductive agent can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range between any two of them, the value range of the mass percentage content w1 of the first conductive agent can be 2% to 10%, 3% to 9%, 4% to 8%, 5% to 7%, and all ranges and sub-ranges thereof. By selecting the above type of first conductive agent and adjusting the mass percentage content w1 of the first conductive agent within the above range, the conductive network of the first material layer is optimized, the electronic impedance of the positive electrode sheet is reduced, the electrochemical polarization and Joule heat caused by the obstruction of electronic transmission are reduced, thereby further reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. Therefore, while taking into account a higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance, and cycle performance.

[0055] In one or more embodiments of the present application, the secondary battery satisfies: the second material layer comprises a second conductive agent, the second conductive agent comprises at least one of multi-walled carbon nanotubes, conductive carbon black or conductive graphite, and a mass percentage content w2 of the second conductive agent is 0.1% to 6% based on a mass of the second material layer. For example, the value of the mass percentage content w2 of the second conductive agent can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6% or a range composed of any two of the numerical values, the value range of the mass percentage content w2 of the second conductive agent can be 0.1% to 6%, 0.5% to 5.5%, 1% to 5%, 2% to 4%, 3% to 4% and all ranges and sub-ranges thereof. By selecting the second conductive agent of the above type and adjusting the mass percentage content w2 of the second conductive agent within the above range, the content of the second conductive agent is low, which reduces the proportion of other substances in the second material layer other than the positive active material, is conducive to improving the energy density of the secondary battery, and the second conductive agent can form a supporting network to enhance the mechanical strength of the second material layer, effectively reducing the risk of uneven coating or cracking of the pole piece, thereby improving the cycle stability of the secondary battery. Therefore, while giving consideration to a higher energy density, the secondary battery has a lower discharge temperature rise, good kinetic performance and cycle performance.

[0056] In one or more embodiments of the present application, the secondary battery satisfies at least one of the following characteristics: (1) the first material layer comprises a first conductive agent, the first conductive agent comprises at least one of single-walled carbon nanotubes or graphene, and the mass percentage content w1 of the first conductive agent is 2% to 10% based on the mass of the first material layer, for example, the value of the mass percentage content w1 of the first conductive agent can be 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or a range formed by any two of them, the value range of the mass percentage content w1 of the first conductive agent can be 2% to 10%, 3% to 9%, 4% to 8%, 5% to 7%, and all ranges and sub-ranges thereof; (2) the second material layer comprises a second conductive agent, the second conductive agent comprises at least one of multi-walled carbon nanotubes, conductive carbon black or conductive graphite, and the mass percentage content w2 of the second conductive agent is 0.1% to 6% based on the mass of the second material layer, for example, the value of the mass percentage content w2 of the second conductive agent can be 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, or a range formed by any two of them, the value range of the mass percentage content w2 of the second conductive agent can be 0.1% to 6%, 0.5% to 5.5%, 1% to 5%, 2% to 4%, 3% to 4%, and all ranges and sub-ranges thereof. Through the above setting, at this time, the content of the first conductive agent is high, which is conducive to optimizing the conductive network of the first material layer, reducing the electronic impedance of the positive electrode sheet, reducing the electrochemical polarization and Joule heat caused by the obstruction of electron transmission, thereby further reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. The content of the second conductive agent is low, which reduces the proportion of other substances in the second material layer that are not positive active materials, which is conducive to improving the energy density of the secondary battery, and the second conductive agent can form a supporting network to enhance the mechanical strength of the second material layer, effectively reducing the risk of uneven coating or sheet cracking, thereby improving the cycle stability of the secondary battery. Therefore, while taking into account the high energy density, the secondary battery has low discharge temperature rise, good kinetic performance and cycle performance.

[0057] In one or more embodiments of the present application, the mass percentage content W1 of the first positive electrode active material is 87% to 96% based on the mass of the first material layer. For example, the mass percentage content W1 of the first positive electrode active material can be 87%, 88%, 90%, 92%, 94%, 95%, 96%, or a range consisting of any two of the above values, and the mass percentage content W1 of the first positive electrode active material can be in the range of 87% to 96%, 88% to 95%, 89% to 94%, 90% to 93%, 91% to 92%, and all ranges and sub-ranges thereof. By adjusting the mass percentage content W1 of the first positive electrode active material within the above range, the first material layer has a good conductive network while taking into account high energy density, reducing the electronic impedance of the positive electrode sheet, reducing the electrochemical polarization and Joule heat caused by blocked electron transport, thereby further reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. Therefore, while taking into account higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0058] In one or more embodiments of the present application, the mass percentage content W2 of the second positive electrode active material is 90% to 98% based on the mass of the second material layer. For example, the mass percentage content W2 of the second positive electrode active material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or a range consisting of any two of the above values, and the mass percentage content W2 of the second positive electrode active material can be in the range of 90% to 98%, 91% to 97%, 92% to 96%, 93% to 95%, 94% to 95%, and all ranges and sub-ranges thereof. By adjusting the mass percentage content W2 of the second positive electrode active material within the above range, the content of the second positive electrode active material is higher, which is beneficial to improve the energy density of the secondary battery. In combination with the first material layer, while taking into account higher energy density, the secondary battery has lower discharge temperature rise, good kinetic performance and cycle performance.

[0059] In one or more embodiments of the present application, the mass percentage content W1 of the first positive electrode active material is 87% to 96% based on the mass of the first material layer, for example, the mass percentage content W1 of the first positive electrode active material can be 87%, 88%, 90%, 92%, 94%, 95%, 96%, or a range composed of any two of the above values, the mass percentage content W1 of the first positive electrode active material can be in the range of 87% to 96%, 88% to 95%, 89% to 94%, 90% to 93%, 91% to 92%, and all ranges and sub-ranges thereof; and / or, the mass percentage content W2 of the second positive electrode active material is 90% to 98% based on the mass of the second material layer, for example, the mass percentage content W2 of the second positive electrode active material can be 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or a range composed of any two of the above values, the mass percentage content W2 of the second positive electrode active material can be in the range of 90% to 98%, 91% to 97%, 92% to 96%, 93% to 95%, 94% to 95%, and all ranges and sub-ranges thereof. By adjusting the value of W1 and / or W2 within the above range, the first material layer has a good conductive network, reducing the electronic impedance of the positive electrode sheet, reducing the electrochemical polarization and Joule heat caused by the obstruction of electron transmission, thereby further reducing the discharge temperature rise of the secondary battery and improving the kinetic performance of the secondary battery. The high content of the second positive electrode active material is beneficial to improve the energy density of the secondary battery. Therefore, while taking into account the high energy density, the secondary battery has a low discharge temperature rise, good kinetic performance and cycle performance.

[0060] The positive electrode current collector of the present application is not particularly limited as long as it can achieve the purpose of the present application, for example, it can include an aluminum foil, an aluminum alloy foil, or a composite current collector (such as an aluminum-carbon composite current collector), etc. The thickness of the positive electrode current collector of the present application is not particularly limited as long as it can achieve the purpose of the present application. The positive electrode material layer can also include a binder, and the type of the binder of the present application is not particularly limited as long as it can achieve the purpose of the present application, for example, the binder can 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, polyamide-imide, butadiene-styrene rubber, or polyvinylidene fluoride.

[0061] The preparation method of the positive electrode sheet is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode sheet includes but is not limited to the following steps: (1) preparing a first material layer slurry; (2) preparing a second material layer slurry; (3) using a coating machine to simultaneously and uniformly coat the prepared first material layer slurry and the second material layer slurry on one surface of the positive current collector, and drying to obtain a positive electrode sheet coated with the first material layer and the second material layer on one side; (4) repeating the above operation on the other surface of the positive current collector to obtain a positive electrode sheet coated with the first material layer and the second material layer on both sides; (5) after cold pressing, cutting and stripping, etc., a positive electrode sheet is obtained.

[0062] It can be understood that when coating the first material layer slurry and the second material layer slurry, direct double-layer coating can be performed according to the above preparation method; or secondary coating can be performed, i.e., the second material layer slurry is first coated on one surface of the positive current collector, and after drying, a positive electrode sheet coated with the second material layer on one side is obtained, the first material layer slurry is coated on the surface of the second material layer away from the positive current collector, and after drying, a positive electrode sheet coated with the first material layer and the second material layer on one side is obtained, and the above operation is repeated on the other surface of the positive current collector to obtain a positive electrode sheet coated with the first material layer and the second material layer on both sides.

[0063] In the present application, the mass percentage content of the first conductive agent and the mass percentage content of the first positive electrode active material in the first material layer can be controlled by adjusting the mass ratio of each substance in the first material layer slurry; the mass percentage content of the second conductive agent and the mass percentage content of the first positive electrode active material in the second material layer can be controlled by adjusting the mass ratio of each substance in the second material layer slurry; the mass percentage content of the first positive electrode active material with a single crystal structure in the first positive electrode active material can be controlled by adjusting the mass ratio of the first positive electrode active material with a single crystal structure and the first positive electrode active material with a polycrystal structure in the preparation of the first material layer slurry; and the mass percentage content of the second positive electrode active material with a single crystal structure in the second positive electrode active material can be controlled by adjusting the mass ratio of the second positive electrode active material with a single crystal structure and the second positive electrode active material with a polycrystal structure in the preparation of the second material layer slurry.

[0064] In the present application, the secondary battery further comprises a negative electrode tab, the negative electrode tab comprising a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The above-mentioned "the negative electrode material layer is disposed on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be disposed on one surface of the negative electrode current collector along the thickness direction of the negative electrode current collector, or can be disposed on two surfaces of the negative electrode current collector along the thickness direction of the negative electrode current collector. It should be noted that the "surface" herein can be the entire area of the surface of the negative electrode current collector, or can be part of the area of the surface of the negative electrode current collector, and the present application does not have a particular limitation as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the negative electrode current collector as long as the purpose of the present application can be achieved, for example, it can comprise a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam or a composite current collector, and exemplarily, the composite current collector can be a lithium-copper composite current collector, a carbon-copper composite current collector, a nickel-copper composite current collector, a titanium-copper composite current collector, etc. The present application does not have a particular limitation on the thickness of the negative electrode material layer and the negative electrode current collector as long as the purpose of the present application can be achieved. The negative electrode material layer comprises a negative electrode active material, and the present application does not have a particular limitation on the negative electrode active material as long as the purpose of the present application can be achieved, for example, the negative electrode active material can comprise but is not limited to natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon, soft carbon, silicon, silicon-carbon composite, Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel-structured Li4Ti5O12, etc. 12The negative active material layer can further include at least one of a lithium metal, a lithium alloy, or a lithium-aluminum alloy. The negative active material layer can further include a conductive agent and a binder, and the type of the conductive agent and the binder is not particularly limited as long as the purpose of the present application is achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black (Super P), carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, a metal material, or a conductive polymer, and the conductive carbon black can include, but is not limited to, at least one of acetylene black or Ketjen black. The carbon nanotubes can include, but are not limited to, single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The carbon fibers can include, but are not limited to, vapor-grown carbon fibers (VGCF) and / or nanocarbon fibers. The metal material can include, but is not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The conductive polymer can include, but is not limited to, at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene, or polypyrrole. For example, the binder can 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, polyamide-imide, styrene butadiene rubber, or polyvinylidene fluoride. The mass ratio of the negative active material, the conductive agent, and the binder in the negative active material layer is not particularly limited, and a person skilled in the art can select them according to the actual needs as long as the purpose of the present application is achieved.

[0065] In the present application, the secondary battery includes a separator. The separator is not particularly limited in the present application as long as the object of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include at least one of a woven film, a nonwoven film, a microporous film, a composite film, a calendered film, or a spunlaced film. In one or more embodiments of the present application, the separator can include a base layer and a surface treatment layer. The base layer can be a nonwoven fabric film or a composite film having a porous structure, and the material of the base layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used. Optionally, a surface treatment layer is provided on at least one surface of the base layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. In one or more embodiments of the present application, the inorganic layer includes inorganic particles and a binder. The inorganic particles are not particularly limited in the present application, and for example, the inorganic particles can include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The binder is not particularly limited in the present application, and for example, the binder can be at least one of the above-described binders. In some embodiments of the present application, the polymer layer includes a polymer, and the material of the polymer includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or poly(vinylidene fluoride-hexafluoropropylene). 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, and for example, the thickness of the separator can be 3 μm to 30 μm.

[0066] In the present application, the secondary battery includes an electrolyte including a lithium salt and a non-aqueous solvent. The lithium salt is not particularly limited in the present application as long as the object of the present application is achieved. For example, the lithium salt can 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 content of the lithium salt in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved. The non-aqueous solvent is not particularly limited in the present application as long as the object of the present application is achieved, for example, the non-aqueous solvent can include, but is not limited to, at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound, or other organic solvents. The carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound, or a fluorinated carbonate compound. The chain carbonate compound can include, but is 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 cyclic carbonate can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound can include, but is not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate, or trifluoromethyl ethylene carbonate. The carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone, or caprolactone. The ether compound can include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The other organic solvents can include, but is not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidinone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The content of the non-aqueous solvent in the electrolyte is not particularly limited in the present application as long as the object of the present application is achieved.

[0067] In the present application, the secondary battery further includes a case for accommodating the positive electrode sheet, the separator, the negative electrode sheet, and the electrolyte solution, and other components known in the art of secondary batteries, and the present application does not limit the above-mentioned other components. The case is not particularly limited in the present application, and can be a case known in the art as long as the purpose of the present application is achieved. For example, the case can be a hard case or a flexible case. The material of the hard case can be a metal or a hard plastic, and the type of the metal is not limited in the present application, and a metal case known in the art can be used as long as the purpose of the present application is achieved. The flexible case can be a metal-plastic film such as an aluminum-plastic film, a steel-plastic film, or the like.

[0068] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and the present application is not particularly limited, for example, the preparation process of the secondary battery can include but is not limited to the following steps: stacking the separator, the positive electrode sheet, the separator, and the negative electrode sheet in order or stacking the positive electrode sheet, the separator, the negative electrode sheet, and the separator in order, and winding them as needed to obtain an electrode assembly with a wound structure, then placing the electrode assembly into the case, injecting the electrolyte solution into the case and sealing it to obtain the secondary battery. Alternatively, the positive electrode sheet, the separator, and the negative electrode sheet, the separator are stacked in order to obtain an electrode assembly with a laminated structure, and the four corners of the electrode assembly are fixed with adhesive tape, then the electrode assembly is placed into the case, the electrolyte solution is injected into the case and sealed to obtain the secondary battery.

[0069] The second aspect of the present application provides an electronic device comprising the secondary battery of any one of the preceding embodiments. Thus, the electronic device provided by the present application has good use performance.

[0070] The type of the electronic device is not particularly limited in the present application, and it can be any electronic device known in the art. In some embodiments of the present application, the electronic device can include but is not limited to a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, an illuminating appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery, and a lithium-ion capacitor, etc.

[0071] Examples

[0072] Hereinafter, examples and comparative examples are given to more specifically explain the embodiments of the present application. Various tests and evaluations were carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are on a mass basis.

[0073] Test methods and equipment:

[0074] Positive electrode tab sampling:

[0075] At 25°C, the lithium ion battery was discharged to the discharge cut-off voltage by constant current discharge at 0.5C, and then the discharged lithium ion battery was disassembled under an argon atmosphere, the positive electrode tab was taken out, and the positive electrode tab was soaked with dimethyl carbonate solvent for 2h, and dried at 60°C for 1h to obtain the positive electrode tab. In the present application and the comparative examples, the discharge cut-off voltage of the lithium ion battery was 2.5V. It can be understood that when the voltage range marked on the factory battery packaging is 2.5V to 4.3V, the charge cut-off voltage is 4.3V, and the discharge cut-off voltage is 2.5V.

[0076] The following test methods were tested using the positive electrode tab obtained in the above manner unless otherwise specified. Among them, the material layer on the surface of the positive electrode tab is the first material layer, and the material layer between the first material layer and the positive electrode current collector is the second material layer.

[0077] Test of F1, F2:

[0078] The scanning electron microscope (SEM) combined with argon ion polishing (CP-SEM) was used to take SEM photos of the positive electrode cross section obtained by disassembling the lithium ion battery, and the upper first active material layer area was selected, and the active material area S 总 in the CP field of view was randomly cut off. 单晶 The single crystal structure particle area S 多晶 in the field of view was obtained by morphology difference. 总 S 单晶+ S 多晶 Since the different morphologies of the same material do not affect the density value of the material, the mass percentage content of the first positive electrode active material with single crystal structure in the first material layer F1=S 单晶 / (S 单晶 +S 多晶 ); The mass percentage content of the second positive electrode active material with single crystal structure in the second material layer F2 can be measured in the same way.

[0079] Test of Dv50, Dv10 and Dv90 of the first positive electrode active material:

[0080] Take the positive electrode tab to be tested, determine the boundary line of the first material layer and the second material layer by SEM-CP cross section, use a blade to scrape the first material layer, place the scraped first material layer powder in a crucible, place it in a muffle furnace at 800°C for 30 min in a nitrogen atmosphere, remove the binder in the first material layer, then add 0.02 g of powder sample in a 50 mL clean beaker, add 20 mL of deionized water, and then add a surface dispersing agent CTAB (cetyltrimethylammonium bromide) dropwise to make the powder fully dispersed in water, ultrasonic cleaning machine for 5 min at 120 W, and then test the particle size distribution by laser particle size tester (MasterSizer 2000). In the volume-based particle size distribution of the material, from the small particle size, the particle size reaching 10% of the volume accumulation is Dv10, the particle size reaching 50% of the volume accumulation is Dv50, and the particle size reaching 90% of the volume accumulation is Dv90.

[0081] It can be understood that, due to the low content of the first conductive agent in the first material layer and the large difference between the particle size of the active material and the particle size of the conductive agent, the influence of the first conductive agent can be ignored when testing the particle size of the first positive electrode active material.

[0082] Test of Dv50, Dv10 and Dv90 of the second positive electrode active material:

[0083] Scrape the positive electrode tab to remove the first material layer, use the same method to scrape the second material layer, and use the same particle size test method as the first positive electrode active material to measure the Dv50, Dv10 and Dv90 of the second positive electrode active material. It can be understood that, due to the low content of the second conductive agent in the second material layer and the large difference between the particle size of the active material and the particle size of the conductive agent, the influence of the second conductive agent can be ignored when testing the particle size of the second positive electrode active material.

[0084] Test of a and b:

[0085] Referring to "Test of Dv50, Dv10 and Dv90 of the first positive electrode active material" and "Test of Dv50, Dv10 and Dv90 of the second positive electrode active material", respectively, the first positive electrode active material and the second positive electrode active material are obtained, and the positive electrode slurry is reconfigured for coating and tab assembly for discharge test, the specific process is as follows: take 100 mg of first positive electrode active material powder, 10 mg of carbon black (Super P), and 10 mg of polyvinylidene fluoride (PVDF) and put them into a mortar for thorough mixing, add the mixed material into NMP, stir until a uniform slurry with a solid content of 70 wt% is formed, evenly coat the prepared slurry on an aluminum foil current collector with a thickness of 8 μm, and the coating weight is 150 mg / 1540.25 mm 2The coated electrode sheet is placed in a vacuum drying oven, and the temperature is set to 120°C and the time is set to 12 hours to remove the solvent. The dried electrode sheet is placed in a tablet press, and a pressure of 10 MPa is applied. The pressed electrode sheet is punched into a circular sheet with a diameter of 14 mm using a punch machine. Lithium sheets are used as negative electrodes. The positive electrode sheet, the separator, and the negative electrode sheet are sequentially placed in a battery case, and an electrolyte is added. The battery is sealed using a battery sealing machine. The assembled button cell is connected to a charge-discharge device, and the charge-discharge parameters are set. The charge voltage is 4.2 V, the discharge voltage is 2.5 V, and the charge-discharge current is 0.1 C. The charge-discharge cycle test is performed, and the charge-discharge capacity is recorded. The specific capacities a and b are calculated based on the discharge capacity and the mass of the positive active material in the button cell.

[0086] The composition of the electrolyte used in the test is the same as that of the electrolyte in Example 1-1.

[0087] Measurement of CW1 and CW2:

[0088] A sample of the electrode sheet to be tested with an area of 1540.25 mm 2 is cut from the positive electrode sheet and placed on a balance to weigh the weight, which is recorded as m1, in mg. Then, the first material layer on the sample of the electrode sheet to be tested is scraped off to obtain a sample of the electrode sheet coated with a second material layer, which is placed on a balance to weigh the weight, which is recorded as m2, in mg.

[0089] If the positive electrode sheet is single-sided coated, the value of CW1 is m1-m2, in mg / 1540.25 mm 2 .

[0090] If the positive electrode sheet is double-sided coated, the value of CW1 is (m1-m2) / 2, in mg / 1540.25 mm 2 .

[0091] The second material layer on the sample of the electrode sheet to be tested is scraped off, and the positive current collector is placed on a balance to weigh the weight, which is recorded as m3, in mg.

[0092] If the positive electrode sheet is single-sided coated, the value of CW2 is m2-m3, in mg / 1540.25 mm 2 .

[0093] If the positive electrode sheet is double-sided coated, the value of CW2 is (m2-m3) / 2, in mg / 1540.25 mm 2 .

[0094] Discharge temperature rise test:

[0095] The temperature sensing line is attached to the center position corresponding to the intersection of the diagonals on the surface of the lithium ion battery at 25°C to monitor the process temperature, and then the lithium ion battery with the temperature sensing line is wrapped in foam for testing. The lithium ion battery is charged at 1.5C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, and then discharged at 4C constant current to 2.5V, and then discharged at 4C constant current to 2.5V. 30min, the highest temperature of 4C discharge is obtained.

[0096] Cycle performance test:

[0097] (1) At 25°C, the lithium ion battery is charged at 1.5C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, and then discharged at 4C constant current to 2.5V, and then discharged at 4C constant current to 2.5V. 30min, the highest temperature of 4C discharge is obtained. 400 , the discharge capacity retention rate of the lithium ion battery at the 400th cycle is calculated.

[0098] The discharge capacity retention rate at the 400th cycle (%) = (Q 400 / Q0) x 100%.

[0099] (2) The PPG thickness of the fresh lithium ion battery at 30% SOC (3.6V) is T0; at 25°C, the lithium ion battery is charged at 1.5C constant current to 4.3V, charged at 4.3V constant voltage to 0.05C, and then discharged at 4C constant current to 2.5V, and then discharged at 4C constant current to 2.5V. 30min, the PPG thickness of the lithium ion battery at full charge at the 400th cycle is T 400 , then the expansion rate of the lithium ion battery at the 400th cycle (%) = (T 400 -T0) / T0 x 100%.

[0100] Lithium analysis:

[0101] At 5°C, the lithium ion battery is charged at 1C constant current to 4.3V, then charged at 4.3V constant voltage to 0.05C (full charge state), and then discharged at 1C constant current to 2.5V, and then discharged at 1C constant current to 2.5V. 5min, 10 cycles. The full charge state lithium ion battery is disassembled to obtain the negative electrode sheet. If any area of the negative electrode sheet is found to have an area of ≥2mm 2 , it is determined that the negative electrode sheet is lithium precipitation, and the unit is C. If it is determined that the negative electrode sheet does not precipitate lithium, the charge rate is increased by 0.1C in turn until the negative electrode sheet of the disassembled lithium ion battery precipitates lithium. The last charge rate without lithium precipitation is recorded as the critical lithium precipitation rate of the lithium ion battery.

[0102] The critical lithium precipitation rate of the lithium ion battery is used to characterize the kinetic performance of the lithium ion battery, and the higher the critical lithium precipitation rate is, the better the kinetic performance of the lithium ion battery is.

[0103] Example 1-1

[0104] Preparation of the positive electrode sheet

[0105] LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622) as the first positive electrode active material, vacuum drying at 120°C for 12h to make its moisture ≤500ppm; polyvinylidene fluoride PVDF with a weight average molecular weight Mw=1 million as the binder, single-walled carbon nanotube (SWCNT) as the first conductive agent, and N-methyl pyrrolidone (NMP) as the solvent, to prepare a first material layer slurry with a solid content of 70% and a weight ratio of the first positive electrode active material, the binder and the first conductive agent of 92:4:4. The slurry preparation process is as follows: (1) dry powder pre-dispersion: add the first conductive agent into a high-speed mixer, mix at 2000 rpm for 10 min, add the first positive electrode active material in three times with an interval of 2 min, and mix at 3000 rpm for 15 min until a dry powder mixture is formed; (2) initial mixing stage: add the solvent NMP into a stirring tank, pour in 80% of the PVDF glue, start low-speed stirring at 300 rpm, add the dry powder mixture, and stir for 30 min until there is no dry powder lump; (3) high-speed dispersion: switch to double planetary stirring paddle, and vacuum degree is extracted to -0.08 MPa, and the gradient speed is increased: first 800 rpm (10 min), then 1500 rpm (40 min), and then 2000 rpm (10 min).

[0106] LiNi 0.9 Co 0.05 Mn 0.05 O2(NCM90) as the second positive electrode active material, vacuum drying at 120°C for 12h to make its moisture ≤500ppm; polyvinylidene fluoride PVDF with a weight average molecular weight Mw=1 million as the binder, multi-walled carbon nanotube (MWCNT) as the second conductive agent, and N-methyl pyrrolidone (NMP) as the solvent, to prepare a second material layer slurry with a solid content of 70% and a weight ratio of the second positive electrode active material, the binder and the second conductive agent of 95:2.5:2.5.

[0107] The first material layer slurry and the second material layer slurry are simultaneously and uniformly coated on one surface of the positive current collector aluminum foil with a thickness of 8 μm using a coating machine, dried at 120°C, to obtain a positive electrode sheet coated with the first material layer and the second material layer on one side; the above operation is repeated on the other surface of the positive current collector to obtain a positive electrode sheet coated with the first material layer and the second material layer on both sides. After drying under vacuum at 120°C for 1 h, cold pressing, sheet cutting, and slitting, a positive electrode sheet with a specification of 75 mm x 867 mm is obtained.

[0108] wherein the Dv10 of the first positive electrode active material is 3.3 μm, and the Dv90 is 4.8 μm; the Dv10 of the second positive electrode material is 5.2 μm, and the Dv90 is 9.3 μm; the coating weight CW1 of the first material layer is 131 mg / 1540.25 mm 2 , the coating weight CW2 of the second material layer is 219 mg / 1540.25 mm 2 ; and the compaction density of the positive electrode material layer is 3.4 g / cm 3 .

[0109] <Preparation of a negative electrode sheet>

[0110] The silicon-based material SiC and artificial graphite are used as the negative electrode active material, multi-walled carbon nanotubes are used as the conductive agent, carboxymethyl cellulose sodium (CMC) is used as the dispersant, and polyacrylic acid (PAA) is used as the binder. The solvent (deionized water: ethanol = 9:1) is weighed into a stirring tank, the binder is added first under stirring at 300 rpm, the dispersant (of which the CMC is a 1.5%wt% solid content aqueous solution) is added after stirring for 5 min, the conductive agent is added after stirring for another 5 min, the stirring speed is increased to 800 rpm, and the homogeneous colloid is formed after stirring for 30 min. The negative electrode active material is weighed and added in 4 portions with an interval of 10 min, and the stirring speed is increased in steps, i.e., pre-mixing: 500 rpm x 15 min, high-shear dispersion: 1500 rpm x 45 min, and stabilization: 800 rpm x 30 min, to prepare a negative electrode material layer slurry with a solid content of 48 wt%. The solid weight ratio of the silicon-based material, artificial graphite, PAA, CMC, and multi-walled carbon nanotubes is 35:61:3:0.5:0.5.

[0111] The negative electrode material layer slurry is uniformly coated on one surface of the negative current collector copper foil with a thickness of 6 μm, dried at 120°C, to obtain a negative electrode sheet coated with the negative electrode material layer on one side; the above operation is repeated on the other surface of the negative current collector to obtain a negative electrode sheet coated with the negative electrode material layer on both sides. After drying under vacuum at 120°C for 1 h, cold pressing, sheet cutting, and slitting, a negative electrode sheet with a specification of 78 mm x 875 mm is obtained. The coating weight of the negative electrode material layer is 150 mg / 1540.25 mm 2, the compaction density of the negative material layer is 1.50 g / cm 3 .

[0112] <Preparation of electrolyte>

[0113] Ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) are mixed in a mass ratio of 20:30:40:10 to obtain an organic solvent, and then lithium salt lithium hexafluorophosphate (LiPF6) is added to the organic solvent to obtain an electrolyte. The concentration of the lithium salt is 1 mol / L, and the rest is the organic solvent.

[0114] <Separator>

[0115] A porous polyethylene film with a thickness of 7 μm is used as the separator.

[0116] <Preparation of lithium ion battery>

[0117] The separator, the negative electrode sheet, the separator, and the positive electrode sheet prepared above are stacked in order, with the separator between the positive electrode sheet and the negative electrode sheet to play a separating role, and the electrode assembly is obtained by winding. The electrode assembly is placed in an aluminum plastic film packaging bag, and the water is removed at 80℃, and the electrolyte prepared above is injected, and the lithium ion battery is obtained after vacuum packaging, standing, formation, degassing, and edge cutting processes. The upper limit voltage of the formation is 4.15V, the formation temperature is 70℃, and the standing time of the formation is 2h.

[0118] Examples 1-2 to 1-30

[0119] Except for adjusting the relevant preparation parameters according to Tables 1 and 2, the rest is the same as Example 1-1. Among them, when the value of W1 and / or w1 changes, the content of the binder in the first material layer changes accordingly; when the value of W2 and / or w2 changes, the content of the binder in the second material layer changes accordingly.

[0120] Examples 2-1 to 2-15

[0121] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1. Among them, when the value of W1 and / or w1 changes, the content of the binder in the first material layer changes accordingly; when the value of W2 and / or w2 changes, the content of the binder in the second material layer changes accordingly.

[0122] Comparative Example 1

[0123] Except that the first material layer in <Preparation of positive electrode sheet> is the same as the second material layer in Example 1, and the second material layer is the same as the first material layer in Example 1, the rest is the same as Example 1.

[0124] Comparative Example 2

[0125] The rest is the same as Example 1 except that the positive electrode sheet is prepared according to the following steps.

[0126] Preparation of the positive electrode sheet

[0127] A nickel-cobalt-manganese ternary material NCM90 (LiNi 0.9 Co 0.05 Mn 0.05 O2) with Dv50 (B2) = 7 μm, a ratio of single crystal structure to polycrystal structure of 30:70 is used as the second positive electrode active material, vacuum dried at 120°C for 12 h to make its moisture ≤500 ppm; polyvinylidene fluoride (PVDF) with a weight average molecular weight Mw = 1 million is used as the binder, multi-walled carbon nanotubes (MWCNT) are used as the second conductive agent, and N-methyl pyrrolidone (NMP) is used as the solvent to prepare a second material layer slurry with a solid content of 70% and a weight ratio of the second positive electrode active material, the binder, and the second conductive agent of 95:2.5:2.5.

[0128] The second material layer slurry is uniformly coated on one surface of the positive current collector aluminum foil with a thickness of 8 μm, and dried at 120°C to obtain a positive electrode sheet coated with the second material layer on one side; the above operation is repeated on the other surface of the positive current collector to obtain a positive electrode sheet coated with the second material layer on both sides. After drying under vacuum at 120°C for 1 h, cold pressing, sheet cutting, and slitting, a positive electrode sheet with a specification of 75 mm x 867 mm is obtained.

[0129] wherein the Dv10 of the second positive electrode material is 5.2 μm, and the Dv90 is 9.3 μm; the coating weight of the second material layer is 350 mg / 1540.25 mm 2 ; and the compaction density of the positive electrode material layer is 3.4 g / cm 3 .

[0130] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 and 2.

[0131] Table 1

[0132] Note: “ / ” in Table 1 means no relevant preparation parameter.

[0133] Table 2

[0134] Note: “ / ” in Table 2 means no relevant preparation parameter.

[0135] As can be seen from Examples 1-1 to 1-30, Examples 2-1 to 2-15, and Comparative Examples 1 to 2, by setting the positive electrode active material with a single crystal structure in the upper first material layer to be greater than the positive electrode active material with a single crystal structure in the lower second material layer, while satisfying F1>50%, the total coating weight of the first material layer and the second material layer is greater, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, and has good kinetic performance and cycle performance. In Comparative Example 1, the proportion of the positive electrode active material with a single crystal structure in the upper first material layer is less than the proportion of the positive electrode active material with a single crystal structure in the lower second material layer; in Comparative Example 2, only the second material layer is provided; in Comparative Examples 1 to 2, the 4C discharge temperature rise of the lithium ion battery is higher, the critical lithium precipitation rate is lower, the discharge capacity retention rate at the 400th cycle is lower and the lithium ion battery expansion rate at the 400th cycle is higher, indicating that the kinetic performance and cycle performance of the lithium ion battery in the comparative examples are poorer, and the discharge temperature rise is higher. In Examples 1-1 to 1-30, Examples 2-1 to 2-15, the total coating weight of the first material layer and the second material layer is greater, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, and has good kinetic performance and cycle performance.

[0136] The value of F1 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Examples 1-1 to 1-4, when the value of F1 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, and has good kinetic performance and cycle performance.

[0137] The value of F2 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Examples 1-1, 1-5 to 1-6, when the value of F2 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, and has good kinetic performance and cycle performance.

[0138] The value of B1 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-7 to Example 1-10, when the value of B1 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0139] The value of B2 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-11 to Example 1-14, when the value of B2 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0140] The value of (F1 x B1) / (F2 x B2) affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1 to Example 1-20, when the value of (F1 x B1) / (F2 x B2) is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0141] The type of the first positive electrode active material and / or the second positive electrode active material affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-19 to Example 1-26, when the type of the first positive electrode active material and / or the second positive electrode active material is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0142] The value of a affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-23 to Example 1-24, when the value of a is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0143] The value of b affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-25 to Example 1-26, when the value of b is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0144] The value of C1 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-27 to Example 1-28, when the value of C1 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0145] The value of C2 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-29 to Example 1-30, when the value of C2 is within the range of the present application, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0146] The value of CW1+CW2 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-1 to Example 2-3, when the value of CW1+CW2 is within the range of the present application, the total coating weight of the first material layer and the second material layer is large, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0147] The value of CW1 / CW2 affects the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-4 to Example 2-9, when the value of CW1 / CW2 is within the range of the present application, the total coating weight of the first material layer and the second material layer is large, the 4C discharge temperature rise of the lithium ion battery is small, the critical lithium precipitation rate is high, the cycle capacity retention rate is high and the cycle expansion rate is low, which shows that the lithium ion battery of the present application has low discharge temperature rise while taking into account high energy density, good kinetic performance and cycle performance.

[0148] The type of the first conductive agent and / or the type of the second conductive agent can affect the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 1-19 to Example 1-20, when the type of the first conductive agent and / or the type of the second conductive agent is within the scope of the present application, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, has good kinetic performance and cycle performance.

[0149] The value of w1 can affect the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-10 to Example 2-12, when the value of w1 is within the scope of the present application, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, has good kinetic performance and cycle performance.

[0150] The value of w1 can affect the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-10 to Example 2-12, when the value of w1 is within the scope of the present application, the total coating weight of the first material layer and the second material layer is larger, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, has good kinetic performance and cycle performance.

[0151] The value of w2 can affect the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-13 to Example 2-15, when the value of w2 is within the scope of the present application, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, has good kinetic performance and cycle performance.

[0152] The value of w2 can affect the kinetic performance, cycle performance and discharge temperature rise of the lithium ion battery. As can be seen from Example 1-1, Example 2-13 to Example 2-15, when the value of w2 is within the scope of the present application, the total coating weight of the first material layer and the second material layer is larger, the 4C discharge temperature rise of the lithium ion battery is smaller, the critical lithium precipitation rate is higher, the cycle capacity retention rate is higher and the cycle expansion rate is lower, indicating that the lithium ion battery of the present application has lower discharge temperature rise while taking into account higher energy density, has good kinetic performance and cycle performance.

[0153] It is to be noted that, in the present document, relational terms such as first and second and the like can be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. Moreover, the terms "comprises", "comprising", or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but can include other elements not expressly listed or inherent to such process, method, article, or apparatus.

[0154] Various embodiments are described herein with reference to particular applications with a specific configuration and contents for convenience. It is to be understood that the application is not limited to those embodiments but cover any technical equivalents in the spirit and principles of the application.

[0155] The preferred embodiments of the present application have been disclosed herein and are intended to be used only as examples and illustrations of the present application and not for the purpose of limiting the same. Any modification, equivalent replacement, improvement, etc. made within the spirit and principles of the present application shall be included in the scope of the present application.

Claims

1. A secondary battery, comprising a positive electrode sheet, the positive electrode sheet comprising a positive current collector and a positive electrode material layer located on at least one surface of the positive current collector, the positive electrode material layer comprising a first material layer and a second material layer, wherein the second material layer is located between the first material layer and the positive current collector along the thickness direction of the positive electrode sheet; The first material layer includes a first positive electrode active material, which has a single crystal structure and a polycrystalline structure. Based on the mass of the first positive electrode active material, the mass percentage of the single crystal structure of the first positive electrode active material is F1. The second material layer includes a second positive electrode active material, which has a single crystal structure and a polycrystalline structure. Based on the mass of the second positive electrode active material, the mass percentage of the single crystal structure of the second positive electrode active material is F2, F1 > F2, and F1 > 50%.

2. The secondary battery according to claim 1, wherein, 60%≤F1≤90%; and / or, 10%≤F2<50%.

3. The secondary battery according to claim 1, wherein, The first positive electrode active material particles have a Dv50 of B1 µm, the second positive electrode active material particles have a Dv50 of B2 µm, and the secondary battery satisfies any one of the following characteristics: (1) 0.15 ≤ (F1×B1) / (F2×B2) < 9; (2) 0.5≤(F1×B1) / (F2×B2)≤7.

4. The secondary battery according to claim 1, wherein, The Dv50 of the first positive electrode active material particles is B1 µm, and the Dv50 of the second positive electrode active material particles is B2 µm, where B1 < B2.

5. The secondary battery according to claim 4, wherein it satisfies at least one of the following characteristics: (1) 2≤B1<5; or, 3≤B1<5; (2) 5≤B2≤18; or, 6≤B2≤10.

6. The secondary battery according to claim 1, wherein, The specific capacity of the first positive electrode active material is a mAh / g, 100≤a<180; the second material layer includes a second positive electrode active material, the specific capacity of the second positive electrode active material is b mAh / g, 180≤b≤300.

7. The secondary battery according to claim 1, wherein it satisfies at least one of the following characteristics: (1) The first positive electrode active material includes at least one of nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material, lithium cobalt oxide, lithium iron phosphate or lithium manganese oxide, and the molar percentage of Ni is <80% based on the total molar number of Ni, Co and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material; (2) The second positive electrode active material includes at least one of nickel-cobalt-manganese ternary material, nickel-cobalt-aluminum ternary material or lithium-rich manganese-based material, and the molar percentage of Ni is ≥80% based on the total molar number of Ni, Co and Mn in the nickel-cobalt-manganese ternary material or the nickel-cobalt-aluminum ternary material.

8. The secondary battery according to any one of claims 1 to 7, wherein, The Dv90 / Dv10 ratio of the particles of the first positive electrode active material is C1, 1.1≤C1≤1.8; and / or, the Dv90 / Dv10 ratio of the particles of the second positive electrode active material is C2, 1.1≤C2≤2.

5.

9. The secondary battery according to claim 8, wherein, The coating weight of the first material layer is CW1 mg / 1540.25mm. 2 The coating weight of the second material layer is CW2 mg / 1540.25mm. 2 , 300≤CW1+CW2≤500, 0.09≤CW1 / CW2≤1.

96.

10. The secondary battery according to claim 9, wherein, 0.22≤CW1 / CW2≤1.64; and / or, 0.04≤(CW1×C1) / (CW2×C2)≤2.

45.

11. The secondary battery according to claim 1, wherein it satisfies at least one of the following characteristics: (1) The first material layer includes a first conductive agent, which includes at least one of single-walled carbon nanotubes or graphene, and the mass percentage of the first conductive agent is 2% to 10% based on the mass of the first material layer; (2) The second material layer includes a second conductive agent, which includes at least one of multi-walled carbon nanotubes, conductive carbon black or conductive graphite, and the mass percentage of the second conductive agent is 0.1% to 6% based on the mass of the second material layer.

12. The secondary battery according to claim 1, wherein, Based on the mass of the first material layer, the mass percentage of the first positive electrode active material is 87% to 96%; and / or, based on the mass of the second material layer, the mass percentage of the second positive electrode active material is 90% to 98%.

13. An electronic device comprising a secondary battery according to any one of claims 1 to 12.

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