A type of secondary battery

Abstract

This application belongs to the field of new energy battery technology, specifically relating to a secondary battery. This application provides a secondary battery including a negative electrode, an electrolyte, and a positive electrode. The positive electrode includes a first positive electrode active layer and a second positive electrode active layer. The first positive electrode active layer includes a monocrystalline ternary material; the second positive electrode active layer includes a hollow polycrystalline ternary material. The average ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle is denoted as m, satisfying 0.04 ≤ m ≤ 0.49. The hollow polycrystalline ternary material is formed by the agglomeration of primary particles. The major axis dimension a and the minor axis dimension b of the primary particles satisfy 1 < a / b ≤ 60. The negative electrode includes a negative electrode active material, and the median volumetric particle size D1 of the negative electrode active material satisfies 1 ≤ D1 ≤ 7. The secondary battery provided by this application has good charge-discharge performance at low temperatures and storage performance at high temperatures.

Description

Technical Field

[0001] This application belongs to the field of new energy battery technology, specifically relating to a secondary battery. Background Technology

[0002] Automobiles are a widely used and essential means of transportation worldwide, and automotive batteries, as a core component of modern vehicles, play multiple crucial roles: not only as the power source for the vehicle's operation, but also providing the starter motor with faster starting power, reducing fuel consumption in road conditions requiring frequent start-stop operations; and providing power to the vehicle's electrical systems when the engine is off. However, existing automotive batteries are significantly affected by ambient temperature. At low temperatures, their charge-discharge performance is poor, and at high temperatures, their cycle life is significantly shortened. They cannot simultaneously achieve good low-temperature charge-discharge performance and high-temperature storage performance, greatly impacting the vehicle's user experience. Summary of the Invention

[0003] Therefore, the technical problem to be solved by this application is to overcome the shortcomings of existing secondary batteries that cannot simultaneously achieve good low-temperature charge-discharge performance and high-temperature storage performance, thereby providing a secondary battery.

[0004] Therefore, this application provides the following technical solution.

[0005] According to an embodiment of this application, in a first aspect, a secondary battery is provided, including a negative electrode, an electrolyte, and a positive electrode. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes a first positive active layer and a second positive active layer stacked thereon, wherein the first positive active layer is located between the positive current collector and the second positive active layer. The first positive electrode active layer comprises a single-crystal ternary material; The second positive electrode active layer comprises a hollow polycrystalline ternary material. On a cross-section of the second positive electrode active layer along its thickness direction, the average ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle is denoted as m, satisfying 0.04≤m≤0.49. The hollow polycrystalline ternary material is formed by the agglomeration of primary particles. The major axis dimension of the primary particles is denoted as a μm, and the minor axis dimension of the primary particles is denoted as b μm, satisfying 1<a / b≤60. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, which includes a carbon-based material. The median particle size of the negative electrode active material is denoted as D1 μm, which satisfies 1≤D1≤7.

[0006] In some implementations, 0.09 ≤ m ≤ 0.25.

[0007] In some implementations, 0.5 ≤ a ≤ 3.

[0008] In some implementations, 0.05 ≤ b ≤ 0.5.

[0009] In some implementations, 2 < a / b ≤ 20.

[0010] In some implementations, 2≤D1≤5.

[0011] In some embodiments, the compressive strength of the hollow polycrystalline ternary material is q1 MPa, and the compressive strength of the single-crystal ternary material is q2 MPa, satisfying: 3≤q2 / q1≤12; and / or, 40≤q1≤100; and / or, 300≤q2≤600.

[0012] In some embodiments, the single-crystal ternary material includes at least one of single-crystal lithium nickel cobalt manganese oxide and single-crystal lithium nickel cobalt aluminum oxide.

[0013] In some embodiments, the hollow polycrystalline ternary material includes at least one of hollow polycrystalline lithium nickel cobalt manganese oxide and hollow polycrystalline lithium nickel cobalt aluminum oxide.

[0014] In some embodiments, the thickness of the first positive electrode active layer is denoted as h1 μm, and the thickness of the second positive electrode active layer is denoted as h2 μm, satisfying 1≤h2 / h1≤4.

[0015] In some embodiments, the sphericity of the hollow polycrystalline ternary material is denoted as F, which satisfies F > 0.8.

[0016] In some embodiments, the median volumetric particle size of the hollow polycrystalline ternary material is 2μm-8μm, and more specifically, 3μm-5μm.

[0017] In some embodiments, the median particle size of the single-crystal ternary material is 1 μm-5 μm, and more specifically, 2 μm-3 μm.

[0018] In some embodiments, at least a portion of the surface of the hollow polycrystalline ternary material is provided with a first coating layer, the first coating layer comprising a first solid electrolyte, the first solid electrolyte comprising at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium aluminum titanium phosphate.

[0019] Furthermore, in some embodiments, the thickness of the first coating layer is 5nm-50nm.

[0020] Furthermore, in some embodiments, the first solid electrolyte further includes silicon, and the mass content of the silicon is 0.02%-0.2% based on the mass of the hollow polycrystalline ternary material.

[0021] In some embodiments, the first positive electrode active layer further includes a second solid electrolyte, the second solid electrolyte including at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium titanium aluminum phosphate.

[0022] Furthermore, in some embodiments, the mass content of the second solid electrolyte is 0.5%-5% based on the mass of the first positive electrode active layer.

[0023] In some embodiments, the carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, soft carbon, and hard carbon.

[0024] In some embodiments, at least a portion of the surface of the carbon-based material is provided with a second coating layer, the second coating layer comprising amorphous carbon, and the thickness of the second coating layer being 5 nm-100 nm.

[0025] In some embodiments, the carbon-based material includes hard carbon, and also includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, and soft carbon.

[0026] Furthermore, in some embodiments, the mass content of the hard carbon is 10%-50% based on the mass of the negative electrode active material, and even more specifically, 15%-30%.

[0027] Furthermore, in some embodiments, the median volumetric particle size of the hard carbon is 1 μm-5 μm, and even more specifically, 2 μm-4 μm.

[0028] Furthermore, in some embodiments, in the Raman spectrum of the negative electrode, 0.2 ≤ I D / I G ≤0.9; wherein, the I D This indicates that the Raman displacement is at 1300 cm. -1 -1400cm -1 The peak intensity of the Raman peak at the location, I G This indicates that the Raman shift is at 1540 cm. -1 -1620cm -1 Peak intensity of the Raman peak at the location.

[0029] In some embodiments, the carbon-based material includes artificial graphite and hard carbon.

[0030] Furthermore, the median particle size of the artificial graphite is 1μm-7μm, and even more specifically, 2μm-5μm.

[0031] In some embodiments, the electrolyte includes a solvent, which includes at least one of a cyclic solvent and a chain solvent, wherein the mass ratio of the chain solvent to the cyclic solvent is (1.5-20):1; the cyclic solvent includes at least one of ethylene carbonate and propylene carbonate; the chain solvent includes at least one of methyl ethyl carbonate, dimethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate, diethyl carbonate, ethyl formate, ethyl acetate, ethyl propionate, propyl propionate, and ethyl butyrate.

[0032] Furthermore, in some embodiments, the ethylene carbonate content is less than or equal to 7% based on the mass of the electrolyte.

[0033] In some embodiments, the electrolyte includes additive N, which includes fluorine and phosphorus elements. The mass content of additive N is denoted as n% based on the mass of the electrolyte, satisfying 0.5 ≤ n ≤ 3.

[0034] Furthermore, in some embodiments, the additive N includes at least one of lithium difluorophosphate and lithium difluorooxalate phosphate.

[0035] The technical solution of this application has the following advantages: The secondary battery provided in this application includes a negative electrode, an electrolyte, and a positive electrode. The positive electrode includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes a first positive active layer and a second positive active layer stacked together. The first positive active layer is located between the positive current collector and the second positive active layer. The first positive electrode active layer comprises a single-crystal ternary material; the second positive electrode active layer comprises a hollow polycrystalline ternary material. On a cross-section of the second positive electrode active layer along its thickness direction, the average ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle is denoted as m, satisfying 0.04 ≤ m ≤ 0.49; the hollow polycrystalline ternary material is formed by the agglomeration of primary particles, the major axis dimension of the primary particles is denoted as a μm, and the minor axis dimension of the primary particles is denoted as b μm, satisfying 1 < a / b ≤ 60; The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, which includes a carbon-based material. The median particle size of the negative electrode active material is denoted as D1 μm, which satisfies 1≤D1≤7.

[0036] Hollow polycrystalline ternary materials are composed of many small grains with numerous grain boundaries, providing diffusion paths for ions in different directions. The hollow interior effectively shortens the diffusion path of lithium ions within the polycrystalline particles. These characteristics all contribute to improving the ion diffusion rate. Placing hollow polycrystalline ternary materials in the second positive electrode active layer, far from the positive electrode current collector, allows for better electrolyte wetting. The hollow structure can also store some electrolyte, further enhancing the ion transport rate and enabling the second active layer to respond quickly to charge transfer. In contrast, single-crystal ternary materials have a single crystal structure, making them less prone to cracking or breakage during cycling. This design maintains the stability of the positive electrode structure and places it in the first positive electrode active layer near the positive electrode current collector. While ensuring low-temperature charge-discharge performance, it fully utilizes the performance of the single-crystal ternary material and improves the high-temperature storage performance of the battery. At the same time, this application uses a specific negative electrode active material (the median volume particle size D1 satisfies 1≤D1≤7), which can balance good structural stability and a short ion transport path, which is beneficial to kinetic performance. When this smaller particle size negative electrode active material is used in combination with the positive electrode sheet containing hollow polycrystalline ternary material, the positive and negative electrodes have well matched kinetic performance, thereby giving the battery better low-temperature charge-discharge performance.

[0037] Furthermore, hollow polycrystalline ternary materials formed by the agglomeration of primary particles are used. In particular, the major axis dimension a and minor axis dimension b of the primary particles satisfy 1 < a / b ≤ 60. The primary particles with strong structural stability will intertwine and interweave with each other, which improves the mechanical strength of the hollow polycrystalline ternary material and thus improves the high-temperature storage performance of the battery. At the same time, the gaps between the agglomerated primary particles are large, which can further improve the wettability of the electrolyte to the hollow polycrystalline ternary material, better help ion transport, and benefit the low-temperature charge and discharge performance of the battery.

[0038] Additional aspects and advantages of the embodiments of this application will be described and shown in part in the following description, or illustrated by practice of the embodiments of this application. Attached Figure Description

[0039] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0040] Figure 1 This is an electron microscope image of the hollow polycrystalline ternary material formed from primary particles in this application; Figure 2 This is an electron microscope image of a cross-section of the hollow polycrystalline ternary material of this application; Figure 3 This is a schematic diagram of the structure of the first positive electrode active layer and the second positive electrode active layer of this application; Figure 4 This is another schematic diagram of the structure of the first positive electrode active layer and the second positive electrode active layer of this application; Figure label: 1-Positive electrode sheet, 101-Second positive electrode active layer, 102-First positive electrode active layer, 103-Positive electrode current collector, 111-Hollow polycrystalline ternary material, 112-Single crystal ternary material, 113-Second solid electrolyte, 1111-First coating layer. Detailed Implementation

[0041] The following embodiments are provided to better understand this application and are not limited to the preferred embodiments described herein. They do not constitute a limitation on the content and scope of protection of this application. Any product that is the same as or similar to this application, derived by anyone under the guidance of this application or by combining features of this application with other prior art, falls within the scope of protection of this application.

[0042] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0043] To address the problem that existing batteries cannot simultaneously achieve good charge / discharge performance at low temperatures and good storage performance at high temperatures, according to an embodiment of this application, in a first aspect, a secondary battery is provided, comprising a negative electrode, an electrolyte, and a positive electrode. The positive electrode includes a positive current collector and a positive active layer disposed on at least one surface of the positive current collector. The positive active layer includes a first positive active layer and a second positive active layer stacked together, wherein the first positive active layer is located between the positive current collector and the second positive active layer. The first positive electrode active layer comprises a single-crystal ternary material; The second positive electrode active layer comprises a hollow polycrystalline ternary material. On a cross-section of the second positive electrode active layer along its thickness direction, the average ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle is denoted as m, satisfying 0.04≤m≤0.49. The hollow polycrystalline ternary material is formed by the agglomeration of primary particles. The major axis dimension of the primary particles is denoted as a μm, and the minor axis dimension of the primary particles is denoted as b μm, satisfying 1<a / b≤60. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, which includes a carbon-based material. The median particle size of the negative electrode active material is denoted as D1 μm, which satisfies 1≤D1≤7.

[0044] This study found that hollow polycrystalline ternary materials are composed of many small grains with numerous grain boundaries, providing diffusion paths for ions in different directions. The hollow interior effectively shortens the diffusion path of lithium ions within the polycrystalline particles. These characteristics all contribute to improving the ion diffusion rate. The second active layer, located far from the positive electrode current collector, is in direct contact with the electrolyte and has a shorter distance from the negative electrode compared to the first active layer. Therefore, it needs to have a rapid charge transfer response capability. Thus, a hollow polycrystalline ternary material is placed in the second positive electrode active layer, far from the positive electrode current collector. Specifically, when the area ratio of the hollow region of the hollow polycrystalline ternary material satisfies 0.04 ≤ m ≤ 0.49, the hollow polycrystalline ternary material can be better wetted by the electrolyte. The hollow structure can also store some electrolyte, greatly increasing the contact area between the electrolyte and the hollow polycrystalline ternary material. All of these factors contribute to improving the ion transport rate, thereby enhancing the low-temperature charge-discharge performance of the battery. Single-crystal ternary materials have a single crystal structure, making them less prone to cracking or breakage during cycling, thus maintaining the stability of the positive electrode structure. Placing them in the first positive electrode active layer near the positive electrode current collector helps achieve a higher electrode compaction density, thereby compensating for the energy density loss caused by the second active layer and improving the battery's energy density. At the same time, the first positive electrode active layer near the positive electrode current collector requires a weaker ability to respond quickly to charge transfer, which can match the long diffusion path of single-crystal ternary materials. This allows the performance of single-crystal ternary materials to be fully utilized while ensuring low-temperature charge-discharge performance, thereby improving the battery's high-temperature storage performance. Meanwhile, this application uses a specific negative electrode active material (with a median volume particle size D1 satisfying 1≤D1≤7), which can balance good structural stability and a short ion transport path, which is beneficial to kinetic performance. When this smaller particle size negative electrode active material is used in combination with a positive electrode containing hollow polycrystalline ternary materials, the positive and negative electrodes have well-matched kinetic performance, thus giving the battery better low-temperature charge-discharge performance.

[0045] Further research revealed that if hollow polycrystalline ternary materials formed by the agglomeration of primary particles are used, especially when the major axis dimension *a* and minor axis dimension *b* of the primary particles satisfy 1 < *a / b* ≤ 60, the primary particles with strong structural stability will intertwine and interlock, improving the mechanical strength of the hollow polycrystalline ternary material and thus enhancing the high-temperature storage performance of the battery. Simultaneously, the increased gaps between the agglomerated primary particles further improve the wettability of the electrolyte to the hollow polycrystalline ternary material, better facilitating ion transport and benefiting the low-temperature charge-discharge performance of the battery. Figure 1As shown, polycrystalline ternary materials are formed from primary particles.

[0046] If m is greater than 0.49, the area ratio of the hollow region is too large, which affects the intrinsic volumetric energy density and mechanical strength of the hollow polycrystalline ternary material. At the same time, the contact area between the material and the electrolyte is too large, resulting in more side reactions and affecting the low-temperature charge-discharge and high-temperature storage performance of the battery. If m is less than 0.04, the area ratio of the hollow region is too small, and the improvement effect of the hollow polycrystalline ternary material is limited. If a / b is greater than 60, the structural stability of the primary particles is poor, which is not conducive to the structural stability of the hollow polycrystalline ternary material and affects the low-temperature charge-discharge and high-temperature storage performance of the battery. If a / b equals 1, it is a polycrystalline material formed by conventional spherical primary particles, which is difficult to effectively improve the low-temperature charge-discharge and high-temperature storage performance of the battery.

[0047] The average value m of the ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle can be tested using conventional methods in the art. For example, the cross-section of the first positive electrode active layer can be placed under a scanning electron microscope (SEM) in a 15μm × 15μm magnification area, and the total area m1 of all hollow polycrystalline ternary materials and the total area m2 of the hollow regions in all hollow polycrystalline ternary materials can be recorded, where m = m1 / m2; Figure 2 As shown, in the cross-section of the first positive electrode active layer, the hollow region of the hollow polycrystalline ternary material is represented by the black frame. For example, m can be 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, or 0. 24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, etc., or values ​​within the range of any two of the above values.

[0048] The test method for the major axis dimension a and minor axis dimension b of the primary particle can be any conventional method in the art, such as using a scanning electron microscope (SEM). For example, a / b can be 1.1, 2, 2.1, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, etc., or values ​​within the range of any two of the above values.

[0049] The volume median particle size D1 of the negative electrode active material can be tested using conventional methods in the art, such as using a Malvern Mastersizer 3000 laser particle size analyzer. For example, D1 (unit μm) can be 1, 1.4, 1.9, 2.1, 2.5, 3.1, 3.4, 3.9, 4.6, 4.9, 5.4, 5.9, 6.5, 6.9, 7, or a value within any two of the above values.

[0050] like Figure 3 As shown, the positive electrode 1 includes a positive current collector 103, a first positive active layer 102, and a second positive active layer 101. The second positive active layer 101 includes a hollow polycrystalline ternary material 111, and the first positive active layer 102 includes a single-crystal ternary material 112. In the hollow polycrystalline ternary material 111, the white lines represent the pores between primary particles, and the white circles represent the hollow regions. It can be understood that... Figure 3 This is merely an exemplary schematic diagram of the structure of the positive electrode sheet. The dimensions of the positive current collector 103, the first positive active layer 102, the second positive active layer 101, the single crystal ternary material 112, and the hollow polycrystalline ternary material 111 are for reference only and do not constitute a limitation on the scope of protection of this application.

[0051] In some embodiments, 0.5 ≤ a ≤ 3. In some embodiments, 0.05 ≤ b ≤ 0.5. Thus, while facilitating processing, the primary particles can be better wound and interwoven, further improving the structural stability of the hollow polycrystalline ternary material, which is beneficial for the low-temperature charge-discharge and high-temperature storage performance of the battery. For example, a can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, etc., or a value within the range of any two of the above values. For example, b can be 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, 0.40, 0.41, 0.42, 0.43, 0.44, 0.45, 0.46, 0.47, 0.48, 0.49, 0.5, etc., or a value within the range of any two of the above values.

[0052] In some implementations, 0.09 ≤ m ≤ 0.25. This can further improve the structural stability of hollow polycrystalline ternary materials, leverage the improving effect of hollow polycrystalline ternary materials, and thus further enhance the low-temperature charge-discharge performance of batteries.

[0053] In some implementations, 2 < a / b ≤ 20. This can further improve the ion transport performance and mechanical strength of hollow polycrystalline ternary materials, thereby enhancing the high-temperature storage and low-temperature charge-discharge performance of the battery.

[0054] In some implementations, 2 ≤ D1 ≤ 5. This can further improve the structural stability and kinetic performance of the negative electrode active material, which is beneficial to the low-temperature charge-discharge performance of the battery.

[0055] In some embodiments, the compressive strength of the hollow polycrystalline ternary material is q1 MPa, and the compressive strength of the single-crystal ternary material is q2 MPa.

[0056] In some implementations, 3 ≤ q2 / q1 ≤ 12. In some implementations, 40 ≤ q1 ≤ 100, and in some implementations, 300 ≤ q2 ≤ 600.

[0057] Further research in this application revealed that when 40≤q1≤100 or 300≤q2≤600 is controlled, hollow polycrystalline ternary materials or single-crystal ternary materials have good mechanical strength, are not easily broken during the rolling process, and have good structural stability, thus ensuring the energy density, high-temperature storage performance and low-temperature charge-discharge performance of the battery. Similarly, when the ternary material undergoes volume expansion or contraction during cycling, its good compressive strength can effectively resist the generation of expansion stress and reduce stress accumulation. When the rolling force is applied to the positive electrode sheet, the second positive electrode active layer on the surface undergoes micro-plastic deformation, while the first positive electrode active layer remains rigid due to the high compressive strength of the monocrystalline ternary material. If the compressive strength difference between the hollow polycrystalline ternary material and the monocrystalline ternary material is too large, stress will accumulate at the interface between the first and second positive electrode active layers. When the accumulated stress exceeds the interfacial bonding force, the second positive electrode active layer will peel off. Therefore, this application controls 3≤q2 / q1≤12, which enables the hollow polycrystalline ternary material and the monocrystalline ternary material to have better compatibility, thereby giving the positive electrode sheet better structural stability and further improving the compaction density of the active layer, thereby further improving the energy density, high-temperature storage performance and low-temperature charge and discharge performance of the battery.

[0058] The compressive strength q1 of the hollow polycrystalline ternary material and the compressive strength q2 of the single-crystal ternary material can be tested using conventional methods in the art. For example, a mechanical strength tester can be used to gradually apply pressure; the compressive strength at which the particle breaks is the compressive strength of the particle. For example, q1 (unit MPa) can be 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 5... Values ​​such as 6, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, or values ​​within the range of any two of the above values. q2 (unit MPa) can be 300, 305, 310, 315, 320, 325, 330, 335, 340, 345, 350, 355, 360, 365, 370, 375, 380, 385, 390, 395, 400, 410, 420, 430, 440, 450, 460, 470, 480, 490, 500, 510, 520, 530, 540, 550, 560, 570, 580, 590, 600, etc., or a value within the range of any two of the above values. q2 / q1 can be 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1, 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1, 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9, 6, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9, 7, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, Values ​​of 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 9.7, 9.8, 9.9, 10.0, 10.1, 10.2, 10.3, 10.4, 10.5, 10.6, 10.7, 10.8, 10.9, 11.0, 11.1, 11.2, 11.3, 11.4, 11.5, 11.6, 11.7, 11.8, 11.9, 12.0, etc., or values ​​within the range of any two of the above values. It should be noted that q1 may be affected by m and a / b.q2 can be adjusted by regulating the preparation process of single-crystal ternary materials, including but not limited to adjusting at least one of the following in the sintering process: sintering temperature, sintering atmosphere, sintering time, heating rate, and cooling rate.

[0059] In some embodiments, the single-crystal ternary material includes at least one of single-crystal lithium nickel cobalt manganese oxide and single-crystal lithium nickel cobalt aluminum oxide.

[0060] In some embodiments, the hollow polycrystalline ternary material includes at least one of hollow polycrystalline lithium nickel cobalt manganese oxide and hollow polycrystalline lithium nickel cobalt aluminum oxide.

[0061] In some embodiments, the thickness of the first positive electrode active layer is denoted as h1 μm, and the thickness of the second positive electrode active layer is denoted as h2 μm, satisfying 1≤h2 / h1≤4. This fully leverages the kinetic improvement effect of hollow polycrystalline ternary materials, enhancing the battery's low-temperature charge-discharge performance, while also utilizing the structural stability enhancement effect of single-crystal ternary materials, ensuring the stability of the positive electrode sheet and benefiting the battery's high-temperature storage performance.

[0062] In some implementations, 6 ≤ h1 ≤ 20. In some implementations, 12 ≤ h2 ≤ 32.

[0063] The thickness h1 of the first positive electrode active layer and the thickness h2 of the second positive electrode active layer can be tested using conventional methods in the art, such as scanning electron microscopy (SEM). For example, h2 / h1 can be 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, or values ​​within the range of any two of the above values. h1 (in μm) can be 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or values ​​within the range of any two of the above values. h2 (unit μm) can be 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, etc., or a value within the range of any two of the above values.

[0064] In some embodiments, the sphericity of the hollow polycrystalline ternary material is denoted as F, satisfying F > 0.8. Thus, the hollow polycrystalline ternary material possesses good compressive strength and packing density, helping to improve the compaction density of the cathode sheet, which is beneficial to the energy density of the battery while also ensuring good structural stability, improving the battery's high-temperature storage and low-temperature charge-discharge performance. The sphericity of the hollow polycrystalline ternary material can be tested using conventional methods in the art. For example, using image processing software (ImageProPlus) to analyze the SEM image of the first cathode active layer at 2500x magnification, the perimeter and area of ​​each hollow polycrystalline ternary material particle in the image are obtained. The equivalent radius r1 of the perimeter and the equivalent radius r2 of the area of ​​each hollow polycrystalline ternary material particle are calculated, and the sphericity S = r2 / r1 is obtained. Then, the sphericity of each hollow polycrystalline ternary material particle is... The sphericity of the material particles is weighted by quantity to obtain the sphericity of the hollow polycrystalline ternary material; for example, F can be 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99, etc., or a value within the range of any two of the above values.

[0065] In some embodiments, the median volumetric particle size of the hollow polycrystalline ternary material is 2μm-8μm, particularly 3μm-5μm, which balances good structural stability and a short ion transport path, further improving the kinetic performance of the cathode and the low-temperature charge-discharge performance of the battery. The median volumetric particle size of the hollow polycrystalline ternary material can be tested using conventional methods in the art, such as a Malvern Mastersizer 3000 laser particle size analyzer. Exemplarily, the median volumetric particle size (in μm) of the hollow polycrystalline ternary material can be 2, 2.1, 2.5, 3, 3.1, 3.4, 3.9, 4.6, 4.9, 5, 5.4, 5.9, 6.5, 6.9, 7, 7.4, 7.9, 8, or a value within any two of the above ranges.

[0066] In some embodiments, the median volumetric particle size of the single-crystal ternary material is 1μm-5μm, particularly 2μm-3μm, which balances good structural stability and a short ion transport path, further improving the kinetic performance of the cathode and the low-temperature charge-discharge performance of the battery. The median volumetric particle size of the single-crystal ternary material can be tested using conventional methods in the art, such as a Malvern Mastersizer 3000 laser particle size analyzer. Exemplarily, the median volumetric particle size (in μm) of the single-crystal ternary material can be 1, 1.4, 1.9, 2, 2.1, 2.5, 3, 3.1, 3.4, 3.9, 4.6, 4.9, 5, or a value within any two of the above ranges.

[0067] In some embodiments, at least a portion of the surface of the hollow polycrystalline ternary material is provided with a first coating layer. The first coating layer includes a first solid electrolyte, which includes at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium aluminum titanium phosphate. In particular, when the thickness of the first coating layer is 5nm-50nm, the coated first solid electrolyte can effectively reduce the active sites on the surface of the hollow polycrystalline ternary material, thereby reducing the side reactions of the electrolyte on the positive electrode surface, improving the interface stability of the positive electrode, and benefiting the high-temperature storage performance of the battery. At the same time, the first solid electrolyte can improve the ion transport rate at the interface, thereby improving the low-temperature charge and discharge performance of the battery. The thickness of the first coating layer can be tested using conventional methods in the art, such as by using a transmission electron microscope (TEM). For example, the thickness (in nm) of the first coating layer can be 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, or a value within the range of any two of the above values.

[0068] Furthermore, in some embodiments, the first solid electrolyte further includes silicon, with a silicon content of 0.02%-0.2% based on the mass of the hollow polycrystalline ternary material. Silicon can complex free fluoride ions in the electrolyte, reducing the corrosive effect of fluoride ions on the positive electrode interface, further improving the stability of the positive electrode interface, and benefiting the high-temperature storage performance of the battery. The mass content of silicon can be tested using conventional methods in the art, such as inductively coupled plasma (ICP). Exemplarily, the mass content of silicon can be 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.10%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.20%, or values ​​within any two of the above ranges. The mass content of silicon can be controlled by adjusting the silicon content in the first solid electrolyte, the mass content of the first solid electrolyte, the thickness of the first coating layer, the mass content of the hollow polycrystalline ternary material, and the particle size of the hollow polycrystalline ternary material.

[0069] In some embodiments, the first positive electrode active layer further includes a second solid electrolyte, which includes at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium aluminum titanium phosphate. In particular, when the second solid electrolyte accounts for 0.5%-5% of the mass content of the first positive electrode active layer, it can effectively improve the lithium-ion transport rate in the first positive electrode active layer, helping to improve the low-temperature charge-discharge performance of the battery. The mass content of the second solid electrolyte can be tested using conventional methods in the art, such as inductively coupled plasma (ICP). Exemplarily, the mass content of the second solid electrolyte can be 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, or a value within any two of the above ranges.

[0070] like Figure 4 As shown, the positive electrode 1 includes a positive current collector 103, a first positive active layer 102, and a second positive active layer 101. The second positive active layer 101 includes a hollow polycrystalline ternary material 111, which includes a first coating layer 1111. The first positive active layer 102 includes a single-crystal ternary material 112 and a second solid electrolyte 113. It is understood that... Figure 4This is merely an exemplary schematic diagram of the structure of the positive electrode sheet. The dimensions of the positive current collector 103, the first positive active layer 102, the second positive active layer 101, the single crystal ternary material 112, the hollow polycrystalline ternary material 111, the second solid electrolyte 113, and the first coating layer 1111 are for reference only and do not constitute a limitation on the scope of protection of this application.

[0071] In some embodiments, the carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, soft carbon, and hard carbon.

[0072] In some embodiments, at least a portion of the surface of the carbon-based material is provided with a second coating layer, the second coating layer comprising amorphous carbon, and the thickness of the second coating layer being 5 nm-100 nm. Transmission electron microscopy (TEM) characterization results show that the amorphous carbon has a particle size distribution between 10 nm and 100 nm, exhibiting a short-range ordered structure and lacking a continuous, regularly extending long-range lattice arrangement. It displays amorphous characteristics of short-range order and long-range disorder. This structure can provide more ion transport channels, and its larger interlayer spacing (0.34 nm-0.36 nm) can reduce ion migration resistance and improve the low-temperature charge-discharge performance of the battery. When the thickness of the second coating layer is 5 nm-100 nm, while ensuring the coating effect, it can reduce side reactions between the second coating layer and lithium ions, ensuring the battery's first-efficiency performance and low-temperature charge-discharge performance.

[0073] The thickness of the second coating layer can be tested using conventional methods in the art, such as by transmission electron microscopy (TEM). For example, the thickness (in nm) of the second coating layer can be 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, or a value within any two of the above values.

[0074] In some embodiments, the carbon-based material includes hard carbon, and also includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, and soft carbon.

[0075] When hard carbon is selected as the negative electrode active material, it has high structural stability and less expansion during lithium-ion insertion / extraction, which is beneficial to the high-temperature storage performance of the battery. At the same time, the large interlayer spacing and disordered structure of hard carbon can reduce the resistance to ion insertion / extraction and provide more ion transport channels, thereby improving the ion transport rate and the low-temperature charge / discharge performance of the battery. However, hard carbon has a high surface defect degree and is prone to react with active lithium to form an SEI film. Furthermore, in some embodiments, based on the mass of the negative electrode active material, the mass content of hard carbon is 10%-50%, especially 15%-30%. This can fully utilize the improvement effect of hard carbon while avoiding an excessively thick SEI film, reducing the loss of active lithium, and thus improving the energy density, low-temperature charge / discharge performance, and high-temperature storage performance of the battery.

[0076] The test method for the mass content of hard carbon can be any conventional method in the art. For example, take the negative electrode sheet, scrape off the powder, clean the powder, separate the hard carbon using density gradient centrifugation (e.g., using the density difference between artificial graphite and hard carbon), wash it clean, weigh it, and calculate the mass content. For example, the mass content of hard carbon can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, etc., or values ​​within any two of the above ranges.

[0077] Furthermore, in some embodiments, the median volumetric particle size of the hard carbon is 1μm-5μm, and more particularly 2μm-4μm, which can balance good structural stability and a shorter ion transport path, further improving the kinetic performance of the negative electrode and benefiting the low-temperature charge-discharge performance of the battery. The median volumetric particle size of the hard carbon can be tested using conventional methods in the art, such as using a Malvern Mastersizer 3000 laser particle size analyzer; exemplaryly, the median volumetric particle size (in μm) of the hard carbon can be 1, 1.4, 1.9, 2, 2.1, 2.5, 3, 3.1, 3.4, 3.9, 4, 4.6, 4.9, 5, etc., or values ​​within any two of the above ranges.

[0078] Furthermore, in some embodiments, in the Raman spectrum of the negative electrode, 0.2 ≤ I D / I G ≤0.9; wherein, the I D This indicates that the Raman displacement is at 1300 cm. -1-1400cm -1 The peak intensity of the Raman peak at the location, I G This indicates that the Raman shift is at 1540 cm. -1 -1620cm -1 The peak intensity of the Raman peak at the specified location indicates that the proportion of hard carbon in the negative electrode active material is moderate. This ensures that the structural stability of the negative electrode sheet is maintained while fully utilizing the improving effect of hard carbon, allowing the battery to achieve both good low-temperature charge / discharge and high-temperature storage performance. The Raman spectrum of the negative electrode sheet shows the I... D / I G The peak intensity ratio can be obtained by taking the arithmetic mean of the surface scan of the negative electrode. This application does not specify the specific scanning sites or number.

[0079] In some embodiments, the carbon-based material includes artificial graphite and hard carbon.

[0080] Furthermore, the median volumetric particle size of the artificial graphite is 1μm-7μm, particularly 2μm-5μm, which balances good structural stability and a short ion transport path, further improving the kinetic performance of the negative electrode and benefiting the low-temperature charge-discharge performance of the battery. The median volumetric particle size of the artificial graphite can be tested using conventional methods in the art, such as a Malvern Mastersizer 3000 laser particle size analyzer. For example, the median volumetric particle size (in μm) of the artificial graphite can be 1, 1.4, 1.9, 2, 2.1, 2.5, 3, 3.1, 3.4, 3.9, 4, 4.6, 4.9, 5, 5.4, 5.8, 6, 6.3, 6.6, 7, or a value within any two of the above ranges.

[0081] In some embodiments, the electrolyte includes a solvent, which comprises a cyclic solvent and a chain solvent, with a mass ratio of the chain solvent to the cyclic solvent of (1.5-20):1. The cyclic solvent includes at least one of ethylene carbonate and propylene carbonate; the chain solvent includes at least one of methyl ethyl carbonate, dimethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate, ethyl formate, ethyl acetate, ethyl propionate, propyl propionate, and ethyl butyrate. Cyclic solvents exhibit good solubility for lithium salts, while chain solvents have lower viscosity, better electrochemical stability, and better kinetic performance at low temperatures. Combining cyclic and chain solvents can improve the stability and kinetic performance of the electrolyte while maintaining the lithium-ion concentration, which is beneficial for the low-temperature charge-discharge performance of the battery.

[0082] Furthermore, this application research found that propylene carbonate has a low melting point and viscosity, exhibiting good kinetic performance at low temperatures. It also has a uniform electron distribution, strong oxidation resistance, and good stability at high temperatures. Furthermore, when propylene carbonate is used in combination with hard carbon, the larger interlayer spacing of hard carbon ensures good compatibility between the two. This is because the solid electrolyte interphase (SEI) film formed by propylene carbonate on the negative electrode side has low stability. During cycling, propylene carbonate molecules easily embed with lithium ions into the negative electrode active material. The larger interlayer spacing of hard carbon prevents significant damage to the hard carbon structure even if propylene carbonate embeds, thus ensuring the stability of the negative electrode and benefiting the battery's high-temperature storage performance. In contrast, the SEI film formed by ethylene carbonate has high density and stability, reducing solvent molecule embedding and improving the stability of the negative electrode interface. However, its high melting point and viscosity make it prone to solidification and viscosity increase at low temperatures, which is detrimental to the battery's low-temperature charge-discharge performance. Furthermore, in some embodiments, the mass content of ethylene carbonate is less than or equal to 7% based on the mass of the electrolyte. It can ensure the low-temperature charge and discharge performance of the battery while giving the SEI film good stability and improving the high-temperature storage performance of the battery.

[0083] The test methods for the mass content of the chain solvent and the cyclic solvent include: measuring by GC (gas chromatography) or GC-MS (gas chromatography-mass spectrometry); for example, the mass ratio of the chain solvent to the cyclic solvent can be 1.5:1, 2:1, 3:1, 4:1, 5:1, 6:1, 7:1, 8:1, 9:1, 10:1, 11:1, 12:1, 13:1, 14:1, 15:1, 16:1, 17:1, 18:1, 19:1, 20:1, etc., or values ​​within the range of any two of the above values.

[0084] In some embodiments, the electrolyte includes additive N, which comprises fluorine and phosphorus elements. The mass content of additive N, denoted as n%, is based on the mass of the electrolyte and satisfies 0.5 ≤ n ≤ 3. Additive N can participate in the formation of a solid electrolyte interphase (SEI) film on the surfaces of the positive and negative electrodes. Particularly at the negative electrode, the SEI film it forms can prevent solvent molecules from embedding into the negative electrode active material, maintaining the structural stability of the negative electrode active material and benefiting the low-temperature charge-discharge performance of the battery. Specifically, satisfying 0.5 ≤ n ≤ 3 ensures the effectiveness of additive N while preventing the SEI film from becoming too thick, which could affect ion transport.

[0085] Furthermore, in some embodiments, the additive N includes at least one of lithium difluorophosphate and lithium difluorooxalate phosphate.

[0086] The test method for the mass content n% of additive N includes: determination by ion chromatography; for example, the mass content n% of additive N can be 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, etc., or values ​​within the range of any two of the above values.

[0087] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium difluorooxalate borate (LiDFOB), lithium bis(oxalate borate) (LiBOB), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium 4,5-dicyano-2-trifluoromethylimidazolium (LiDTI).

[0088] In some embodiments, the secondary battery further includes a separator, which includes a base membrane. This application does not have any particular limitation on the type of base membrane, and any known base membrane with good chemical and mechanical stability can be selected.

[0089] In some embodiments, the base film is made of at least one of polyethylene, polypropylene, polyethylene, polypropylene, polyamide, polyimide, polyethylene terephthalate, polybutylene terephthalate, polystyrene, and aramid.

[0090] The base film can be a single-layer film or a multi-layer composite film, without particular restrictions. When the base film is a multi-layer composite film, the materials of each layer can be the same or different, without particular restrictions.

[0091] In some embodiments, a heat-resistant layer is provided on at least one surface of the base film.

[0092] In some embodiments, the heat-resistant layer includes heat-resistant particles and a heat-resistant layer binder, wherein the heat-resistant particles account for 90%-99% of the mass of the heat-resistant layer, and the heat-resistant layer binder accounts for 1%-10% of the mass of the heat-resistant layer.

[0093] In some embodiments, the heat-resistant layer adhesive includes at least one of polyvinyl alcohol, styrene-butadiene rubber, ethylene-vinyl acetate copolymer, sodium carboxymethyl cellulose, polyvinylpyrrolidone, polymethyl methacrylate, polybutyl methacrylate, styrene-acrylic latex, polyacrylonitrile, polyethyl acrylate, polyvinyl acetate, polyacrylic acid, polyacrylate, polyvinylidene fluoride polyurethane, polyvinylidene fluoride-hexafluoropropylene, or copolymer systems derived from the above polymers.

[0094] In some embodiments, the heat-resistant particles include ceramic particles and / or nitrogen-containing organic particles.

[0095] In some embodiments, the ceramic particles are selected from at least one of aluminum oxide, boehmite, silicon dioxide, zirconium dioxide, barium sulfate, magnesium hydroxide, fluorapatite, fluorophlogopite, mullite, aluminum titanate, copper oxide, titanium dioxide, and zinc oxide.

[0096] In some embodiments, the nitrogen-containing organic particles include at least one of melamine, melamine polyphosphate, melamine thiocyanate, melamine cyanurate, formaldehyde polymelamine hydrochloride, melamine polyphosphate, piperazine, piperazine pyrophosphate, urea-formaldehyde resin, melamine formaldehyde resin, polyacrylonitrile, and 1,3,5-triazine-2,4,6-triamine, piperazine hydrochloride, piperazine citrate, and piperazine sulfate.

[0097] The heat-resistant layer can be coated using conventional processes in the field, such as gravure coating, transfer coating, dip coating, and spray coating, for single-sided or double-sided coating.

[0098] The present application is further described in detail below with reference to specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in this application. Where specific experimental steps or conditions are not specified in the embodiments and comparative examples, they can be performed according to the conventional experimental steps or conditions described in the literature in the art. Reagents or instruments used, unless otherwise specified, are all commercially available conventional reagent products. In all embodiments and comparative examples of this application, the unit % represents mass percentage.

[0099] Example 1 This embodiment provides a method for preparing a battery, including the following steps: (1) Preparation of positive electrode Coated with LATSP (Li 2.05 Al 0.3 Ti 1.7 Si 0.75 P 2.25 O 12 Hollow polycrystalline ternary material (parameters detailed in Table 1-2), polyvinylidene fluoride, and acetylene black were added to a vacuum mixer in a mass ratio of 93:2:5, along with an appropriate amount of N-methylpyrrolidone (NMP). The mixture was thoroughly stirred under vacuum until a homogeneous, highly fluid second cathode slurry was formed, with a solid content of 50 wt%. The chemical formula of the hollow polycrystalline ternary material is LiNi. 0.6 Co 0.2 Mn 0.2 O2; The single-crystal ternary material (parameters are detailed in Table 1-2), the second solid electrolyte (LATP(Li)) 1.3 Al 0.3 Ti 1.7 (PO4)3), polyvinylidene fluoride, and acetylene black were added to a vacuum mixer in a mass ratio of 92:3:2:3, along with an appropriate amount of N-methylpyrrolidone (NMP). The mixture was thoroughly stirred under vacuum until a homogeneous, highly fluid first cathode slurry was formed, with a solid content of 50 wt%. The chemical formula of the single-crystal ternary material was LiNi. 0.6 Co 0.2 Mn 0.2 O2; The first positive electrode slurry and the second positive electrode slurry are uniformly coated on both sides of an aluminum foil with a thickness of 12μm through a double-layer coating nozzle to obtain the first positive electrode active layer and the second positive electrode active layer. The first positive electrode active layer is located on the surface of the aluminum foil, and the second positive electrode active layer is located on the surface of the first positive electrode active layer. After drying, rolling and cutting, the positive electrode sheet is obtained.

[0100] (2) Preparation of negative electrode The negative electrode active material (hard carbon and artificial graphite coated with soft carbon material in a mass ratio of 15:85, parameters of the negative electrode active material are detailed in Table 1-2), sodium carboxymethyl cellulose, polyacrylic acid, and acetylene black were added to a vacuum mixer in a mass ratio of 95:0.7:2.5:1.8, along with an appropriate amount of deionized water. The mixture was thoroughly mixed under vacuum until a uniform, free-flowing negative electrode slurry was formed, with a solid content of 48 wt%. The negative electrode slurry was then uniformly coated onto a 6 μm thick copper foil, dried, rolled, and slit to obtain the negative electrode sheet.

[0101] (3) Electrolyte preparation In an argon-filled glove box (moisture <1ppm, oxygen <1ppm), ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed evenly. Thoroughly dried lithium hexafluorophosphate (LiPF6) was added and stirred to dissolve. Then, lithium difluorophosphate (LiPO2F2) was added to prepare the electrolyte. The electrolyte consisted of 12% lithium hexafluorophosphate, 5% EC, 10% PC, 1.5% LiPO2F2, 35.75% EMC, and 35.75% DEC. The mass ratio of the chain solvent to the cyclic solvent was 4.77:1.

[0102] (4) Preparation of the diaphragm 8μm thick polyethylene diaphragm.

[0103] (5) Preparation of lithium-ion batteries The prepared positive electrode, separator, and negative electrode are stacked to form a bare cell, and tabs are welded on. Then, the bare cell is placed in an aluminum-plastic film, and the prepared electrolyte is injected into the dried bare cell. After vacuum sealing, room temperature standing, and high temperature formation, the desired lithium-ion battery is obtained.

[0104] The preparation methods of Examples 2-27 and Comparative Examples 1-5 are basically the same as those of Example 1, with differences shown in Tables 1-4. In the table, " / " indicates that the item does not exist. The symbol indicates that it is the same as in Example 1, but there may be reasonable deviations due to the testing process. In Tables 1-4, m is the average ratio of the area of ​​the hollow region of a single particle of hollow polycrystalline ternary material to the area of ​​the particle, a μm is the major axis dimension of the primary particle, b μm is the minor axis dimension of the primary particle, D1 μm is the volume median particle size of the negative electrode active material, q1 MPa is the compressive strength of the hollow polycrystalline ternary material, q2 MPa is the compressive strength of the single crystal ternary material, and the hard carbon mass content refers to the mass content of hard carbon in the mass of the negative electrode active material, with the balance of the negative electrode active material being artificial graphite; I D / I G This refers to the ratio of peak intensities in the Raman spectrum of the negative electrode. The cyclic solvent includes ethylene carbonate and propylene carbonate, with the balance of the cyclic solvent being propylene carbonate. The mass ratio of EMC to DEC in the chain solvent is always 1:1. The mass content of the second solid electrolyte refers to the mass content of the second solid electrolyte in the mass of the first positive electrode active layer. In Example 24, artificial graphite coated with hard carbon material is used instead of the artificial graphite coated with soft carbon material in Example 1.

[0105] Table 1. Variables in the Examples and Comparative Examples

[0106] Table 2 Variable 2 of Example

[0107] Table 3 Variable 3 of the Example

[0108] Table 4 Variables in Example 4

[0109] Comparative Example 6 This comparative example provides a method for preparing a battery. The difference from Example 1 is that the positions of the first positive electrode active layer and the second positive electrode active layer are different. Specifically, the second positive electrode active layer is located on the surface of the aluminum foil, and the first positive electrode active layer is located on the surface of the second positive electrode active layer.

[0110] Test case (1) Low temperature discharge time test: Under (25+2)℃ environment, standard constant current 1C discharge to 3V, rest for 30min; then 1C standard constant current constant voltage charge to 4.2V, cut-off current 0.05C, rest for 30min; then standard constant current 1C discharge to 3V, obtain the actual capacity C0 of the cell; rest for 30min; standard constant current constant voltage charge to 4.2V, cut-off current 0.05C, use 1C0 discharge to 50% SOC; -30℃ stand for 4h; constant power (25*C0)W, constant power discharge to 2.8V cut-off (protection voltage), sample every 50ms, record discharge time, the longer the time, the better the low temperature discharge performance.

[0111] (2) Low temperature charging time test: Under (25+2)℃ environment, discharge to 3V with standard constant current 1C, and rest for 30min; then charge to 4.2V with standard constant current constant voltage 1C, cut-off current 0.05C, and rest for 30min; then discharge to 3V with standard constant current 1C, and obtain the actual capacity C0 of the cell; rest for 30min; charge to 4.2V with standard constant current constant voltage, cut-off current 0.05C, and discharge to 50% SOC with 1C0; stand at -30℃ for 4h; charge to 4.3V cut-off (protection voltage) with constant power (24*C0)W, sample every 50ms and record the charging time. The longer the time, the better the low temperature charging performance.

[0112] (3) High temperature storage test: Under (25±2)℃ environment, 1C constant current discharge to 3V, rest for 30min; then 1C constant current and constant voltage charge to 4.2V, 0.05C cutoff, rest for 30min; 1C constant current discharge to 3V, obtain the initial capacity C0 of the cell; rest for 30min, 1C0 constant current and constant voltage charge to 4.2V, 0.05C0 cutoff, then open circuit rest for 30 days under 60℃ environment; after 30 days, take it out, stand at (25±2)℃ for 1h, 1C0 constant current discharge to 3V, rest for 30min, 1C0 constant current and constant voltage charge to 4.2V, cutoff current 0.05C0, rest for 30min, 1C0 constant current discharge to 3V to obtain the recovered capacity, high temperature storage performance (%) = recovered capacity / C0 × 100%.

[0113] The test results are shown in Table 5.

[0114] Table 5 Test results of the examples and comparative examples

[0115] As can be seen from Tables 1-5, the secondary battery provided in this application can achieve both low-temperature charge-discharge performance and high-temperature storage performance, with both improved to varying degrees. However, in Comparative Example 1, the m is too large, resulting in poor structural stability and affecting high-temperature storage performance. Conversely, in Comparative Example 2, the m is too small, failing to leverage the advantages of the hollow structure, and the low-temperature charge-discharge performance is significantly reduced. In Comparative Example 3, the a / b ratio is too large, resulting in poor structural stability of the primary particles and affecting the high-temperature storage performance of the secondary battery. In Comparative Example 4, the a / b ratio is too small, which also affects the low-temperature charge-discharge performance and high-temperature storage performance of the secondary battery.

[0116] In Comparative Example 5, the median particle size D1 of the negative electrode active material was too large, resulting in poor kinetics and severely affecting the low-temperature charge-discharge performance of the secondary battery. In Comparative Example 6, the positions of the monocrystalline ternary material and the hollow polycrystalline ternary material were unreasonable, failing to effectively utilize their improvement effects and thus impacting the low-temperature charge-discharge performance and high-temperature storage performance of the secondary battery.

[0117] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A secondary battery, comprising a negative electrode, an electrolyte, and a positive electrode, characterized in that, The positive electrode sheet includes a positive current collector and a positive active layer disposed on at least one side surface of the positive current collector. The positive active layer includes a first positive active layer and a second positive active layer stacked together, with the first positive active layer located between the positive current collector and the second positive active layer. The first positive electrode active layer comprises a single-crystal ternary material; The second positive electrode active layer comprises a hollow polycrystalline ternary material. On a cross-section of the second positive electrode active layer along its thickness direction, the average ratio of the area of ​​the hollow region of a single particle of the hollow polycrystalline ternary material to the area of ​​the particle is denoted as m, satisfying 0.04≤m≤0.

49. The hollow polycrystalline ternary material is formed by the agglomeration of primary particles. The major axis dimension of the primary particles is denoted as a μm, and the minor axis dimension of the primary particles is denoted as b μm, satisfying 1<a / b≤60. The negative electrode sheet includes a negative electrode current collector and a negative electrode active layer disposed on at least one side surface of the negative electrode current collector. The negative electrode active layer includes a negative electrode active material, which includes a carbon-based material. The median particle size of the negative electrode active material is denoted as D1 μm, which satisfies 1≤D1≤7.

2. The secondary battery according to claim 1, characterized in that, 0.09≤m≤0.25； And / or, 0.5≤a≤3, and / or, 0.05≤b≤0.5, and / or, 2<a / b≤20; And / or, 2≤D1≤5.

3. The secondary battery according to claim 2, characterized in that, The compressive strength of the hollow polycrystalline ternary material is q1 MPa, and the compressive strength of the single-crystal ternary material is q2 MPa, satisfying the following: 3≤q2 / q1≤12; and / or, 40≤q1≤100; and / or, 300≤q2≤600; And / or, the single-crystal ternary material includes at least one of single-crystal lithium nickel cobalt manganese oxide and single-crystal lithium nickel cobalt aluminum oxide; And / or, the hollow polycrystalline ternary material includes at least one of hollow polycrystalline lithium nickel cobalt manganese oxide and hollow polycrystalline lithium nickel cobalt aluminum oxide; And / or, the thickness of the first positive electrode active layer is denoted as h1 μm, and the thickness of the second positive electrode active layer is denoted as h2 μm, satisfying 1≤h2 / h1≤4; And / or, the sphericity of the hollow polycrystalline ternary material is denoted as F, which satisfies F > 0.8; And / or, the median volumetric particle size of the hollow polycrystalline ternary material is 2μm-8μm, preferably 3μm-5μm; And / or, the median particle size of the single-crystal ternary material is 1μm-5μm, preferably 2μm-3μm.

4. The secondary battery according to any one of claims 1-3, characterized in that, At least a portion of the surface of the hollow polycrystalline ternary material is provided with a first coating layer, the first coating layer comprising a first solid electrolyte, the first solid electrolyte comprising at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium titanium aluminum phosphate; Preferably, the thickness of the first coating layer is 5nm-50nm; Preferably, the first solid electrolyte further includes silicon, and the mass content of silicon is 0.02%-0.2% based on the mass of the hollow polycrystalline ternary material.

5. The secondary battery according to any one of claims 1-3, characterized in that, The first positive electrode active layer further includes a second solid electrolyte, which includes at least one of lithium lanthanum zirconate, lithium lanthanum titanate, and lithium titanium aluminum phosphate. Preferably, based on the mass of the first positive electrode active layer, the mass content of the second solid electrolyte is 0.5%-5%.

6. The secondary battery according to any one of claims 1-3, characterized in that, The carbon-based material includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, soft carbon, and hard carbon. And / or, at least a portion of the surface of the carbon-based material is provided with a second coating layer, the second coating layer comprising amorphous carbon, and the thickness of the second coating layer being 5 nm-100 nm.

7. The secondary battery according to claim 6, characterized in that, The carbon-based material includes hard carbon, and also includes at least one of artificial graphite, natural graphite, mesophase carbon microsphere graphite, and soft carbon; Preferably, based on the mass of the negative electrode active material, the mass content of the hard carbon is 10%-50%, more preferably 15%-30%; Preferably, the median particle size of the hard carbon is 1 μm-5 μm, more preferably 2 μm-4 μm; Preferably, in the Raman spectrum of the negative electrode, 0.2 ≤ I D / I G ≤0.9; wherein, the I D This indicates that the Raman displacement is at 1300 cm. -1 -1400cm -1 The peak intensity of the Raman peak at the location, I G This indicates that the Raman displacement is at 1540 cm. -1 -1620cm -1 Peak intensity of the Raman peak at the location.

8. The secondary battery according to claim 7, characterized in that, The carbon-based materials include artificial graphite and hard carbon; Preferably, the median particle size of the artificial graphite is 1μm-7μm, more preferably 2μm-5μm.

9. The secondary battery according to any one of claims 1-3, characterized in that, The electrolyte includes a solvent, which includes at least one of a cyclic solvent and a chain solvent, wherein the mass ratio of the chain solvent to the cyclic solvent is (1.5-20):1; the cyclic solvent includes at least one of ethylene carbonate and propylene carbonate; the chain solvent includes at least one of methyl ethyl carbonate, dimethyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, diethyl carbonate, ethyl formate, ethyl acetate, ethyl propionate, propyl propionate, and ethyl butyrate. Preferably, the ethylene carbonate content is less than or equal to 7% based on the mass of the electrolyte.

10. The secondary battery according to any one of claims 1-3, characterized in that, The electrolyte includes additive N, which includes fluorine and phosphorus elements. The mass content of additive N is denoted as n% based on the mass of the electrolyte, and satisfies 0.5≤n≤3. Preferably, the additive N includes at least one of lithium difluorophosphate and lithium difluorooxalate phosphate.

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