Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. The lithium ion secondary battery comprises a positive electrode plate and a negative electrode plate, the negative electrode plate comprises a negative electrode current collector and a first negative electrode active layer, the first negative electrode active layer comprises a first negative electrode active material containing a silicon-carbon material, and a plurality of holes with the size D of 0.1-5 microns are formed in the cross section of the silicon-carbon material; the positive plate comprises a positive current collector and a positive active layer, the positive active layer comprises a positive active material containing a ternary material, the ternary material comprises polycrystalline particles, the chemical formula of the ternary material is Liv1Nix1Coy1Mnz1M1a1M2b1O2, M1 comprises elements Ti and Sr, the content c (Sr) of Sr in the positive active layer is 70ppm-1500ppm, and D / c (Sr) is more than or equal to 0.0003 and less than or equal to 0.02. The lithium ion secondary battery provided by the invention has good cycling stability and rate capability.

Description

Technical Field

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

[0002] With the development of the battery industry, the market demand for lithium-ion batteries that combine high energy density and high-rate discharge performance is increasing. This places higher demands on battery materials and design. Higher energy density requires the selection of positive and negative electrode active materials with higher specific capacity. For example, the positive electrode active material can be a ternary material with a higher nickel content (high nickel refers to the total molar number of transition metals in the ternary material, with Ni molar percentage ≥80%). However, as the nickel content increases, the structural stability of the ternary material decreases, which in turn affects the cycle performance and rate performance of the lithium-ion battery. The negative electrode active material can be achieved by mixing silicon-carbon materials with higher specific capacity. However, silicon-carbon materials face the problems of significant volume expansion and reduced conductivity, which are also detrimental to the cycle stability and rate performance of the battery. Summary of the Invention

[0003] The purpose of this invention is to overcome the above-mentioned problems existing in the prior art and to propose a lithium-ion secondary battery (hereinafter referred to as the battery). The positive electrode of the battery uses polycrystalline particles of ternary material containing Sr element, and the negative electrode contains silicon-carbon material with several pores. By adjusting the size of the pores in the silicon-carbon material and the Sr content in the positive electrode active layer, the battery can have a high energy density while maintaining good cycle stability and rate performance.

[0004] In related technologies, ternary cathode materials and silicon-carbon anode materials are often used to improve battery energy density. However, both ternary materials and silicon-carbon materials suffer from insufficient conductivity, which is detrimental to battery operation at high rates or low temperatures. Increased internal resistance leads to accelerated capacity decay, and increased temperature rise during high-rate charging reduces the structural stability of the positive and negative electrode active materials, making the battery prone to gas generation and heat generation in the later stages of cycling. In addition, silicon-carbon materials also have a significant volume expansion problem when used in batteries, further accelerating the decline in the stability of the anode sheet and leading to battery cycle degradation. The inventors of this invention have discovered that further improvements to ternary materials and silicon-carbon materials can alleviate the above problems. First, constructing several pores inside the silicon-carbon material can increase its specific surface area and increase active sites, thereby improving the diffusion rate of lithium ions inside the silicon-carbon material and thus improving the rate performance of the battery. At the same time, the internal pores can also reserve buffer space for the volume expansion of the silicon-carbon material during battery cycling, mitigating volume changes, reducing the expansion of the anode sheet, and thus improving the cycle stability of the battery. Secondly, polycrystalline particles of ternary materials are used for the cathode, and element M is introduced into the ternary material. 1(For example, Ti and Sr, such as making the Sr mass content in the positive electrode active layer between 70ppm and 1500ppm), can significantly improve the rate performance of the positive electrode and achieve a match between the positive and negative electrode dynamic performance. On the one hand, the diffusion path of lithium ions inside the polycrystalline particles is greatly shortened, the lithium ion insertion-extraction rate is faster, and the specific surface area of ​​the polycrystalline particles is larger, which can provide more active sites for lithium ion insertion and extraction, thereby improving the lithium ion transport rate inside the polycrystalline particles; on the other hand, the introduction of element M into ternary materials 1 This approach can further enhance the structural stability of the positive electrode active material, suppress lattice distortion and structural collapse, thereby compensating to some extent for the shortcomings in cycle performance when polycrystalline particles are used in batteries. Simultaneously, starting from both the positive and negative electrodes, this strategy further adjusts the relationship between c(Sr) and the pore size D in the silicon-carbon material (i.e., D / c(Sr) in the range of 0.0003-0.02), thereby improving the matching degree between the positive and negative electrodes. This not only leverages the effective buffering effect of the internal pores of the silicon-carbon material on the volume expansion of the silicon-carbon negative electrode, but also enhances the lithium-ion insertion / extraction rate, improves the structural stability of the ternary material, and alleviates the volume expansion of the positive electrode during cycling, resulting in a battery with high energy density, good rate performance, and good cycle capacity retention.

[0005] Based on this, the present invention proposes the following technical solution: This invention provides a lithium-ion secondary battery, comprising a positive electrode and a negative electrode. The negative electrode includes a negative current collector and a first negative active layer located on at least one side of the negative current collector. The first negative active layer includes a first negative active material, which is a silicon-carbon material. In a cross-sectional scanning electron microscope image of the negative electrode, the silicon-carbon material has a plurality of pores in the cross-section. The positive electrode includes a positive current collector and a positive active layer located on at least one side of the positive current collector. The positive active layer includes a positive active material, which is a ternary material, comprising polycrystalline particles. The chemical formula of the polycrystalline particles is Li. v1 Ni x1 Co y1 Mn z1 M 1 a1 M 2 b1 O2, where 0.85≤v1≤1.1, 0.8≤x1≤0.98, 0<y1≤0.12, 0<z1≤0.1, 0.001≤a1≤0.006, 0.006≤b1≤0.016, where M 1 Including elements Zr and Sr, M 2The material includes at least one of Al, Mg, Ti, Y, La, P, B, Nb, W, Sb, and V; the size of the pores is D, which is 0.1 μm-5 μm; based on the total mass of the positive electrode active layer, the mass content of the element Sr is c(Sr), which is 70 ppm-1500 ppm; D and c(Sr) satisfy: 0.0003 ≤ D / c(Sr) ≤ 0.02; where D is in μm and c(Sr) is in ppm.

[0006] By employing the above technical solution, the present invention has at least the following advantages compared with the prior art: (1) The lithium-ion secondary battery of the present invention has high energy density.

[0007] (2) The lithium-ion secondary battery of the present invention has good rate performance and cycle capacity retention.

[0008] The endpoints and any values ​​of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0009] Figure 1 The image shown is a cross-sectional scanning electron microscope image of silicon-carbon material in one embodiment of the present invention.

[0010] Figure 2 The figure shows the volume particle size distribution curve of the positive electrode active material in one embodiment of the present invention.

[0011] Figure 3 The diagram shown is a structural schematic of the arc region and the straight region in one embodiment of the present invention.

[0012] Figure 4 The figure shown is a schematic diagram of the planar structure of the positive electrode sheet in one embodiment of the present invention.

[0013] Figure 5 The diagram shown is a structural schematic of the core in one embodiment of the present invention.

[0014] Figure 6 The image shown is a scanning electron microscope image of the negative electrode sheet along the thickness direction in one embodiment of the present invention.

[0015] Reference numerals in the attached figures: 1 is the positive electrode, 2 is the negative electrode, 3 is the separator, 13 is the arc region, 14 is the straight region, 111 is the first convex part, 112 is the first concave part, 121 is the second convex part, and 122 is the second concave part. Detailed Implementation

[0016] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0017] This invention provides a lithium-ion secondary battery, comprising a positive electrode and a negative electrode. The negative electrode includes a negative current collector and a first negative active layer located on at least one side of the surface of the negative current collector. The first negative active layer includes a first negative active material, which is a silicon-carbon material. In a cross-sectional scanning electron microscope (SEM) image of the negative electrode, the silicon-carbon material has a plurality of pores in the cross-section. The term "a plurality of pores" refers to the presence of at least two pores in the cross-section of the silicon-carbon material. Figure 1 The image shown is a cross-sectional scanning electron microscope image of a silicon-carbon material in one embodiment of the present invention. It can be seen that the cross-section of the silicon-carbon material has several holes (dark gray).

[0018] In this invention, the positive electrode sheet includes a positive current collector and a positive active layer located on at least one side surface of the positive current collector. The positive active layer includes a positive active material, which is a ternary material, and the ternary material includes polycrystalline particles. The chemical formula of the polycrystalline particles is Li. v1 Ni x1 Co y1 Mn z1 M 1 a1 M 2 b1 O2, where 0.85≤v1≤1.1 (e.g., 0.85, 0.86, 0.88, 0.9, 0.95, 1, 1.05, or 1.1), 0.8≤x1≤0.98 (e.g., 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, or 0.98), and 0<y1≤0.12 (e.g., 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, 0.1, or 0.12). , 0 < z1 ≤ 0.1 (e.g., 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, or 0.1), 0.001 ≤ a1 ≤ 0.006 (e.g., 0.001, 0.002, 0.003, 0.004, 0.005, or 0.006), 0.006 ≤ b1 ≤ 0.016 (e.g., 0.006, 0.007, 0.008, 0.009, 0.01, 0.012, 0.014, or 0.016), where M 1 Including elements Zr and Sr, M 2It includes at least one of Al, Mg, Ti, Y, La, P, B, Nb, W, Sb, and V. The polycrystalline particle is formed from a plurality of primary particles. The term "a plurality of" refers to a number of primary particles forming the polycrystalline particle that is ≥2.

[0019] In this invention, the size of the hole is D, which is 0.1μm-5μm, for example, 0.1μm, 0.2μm, 0.4μm, 0.6μm, 0.8μm, 1μm, 1.5μm, 2μm, 3μm, 4μm, or 5μm. It is understood that the size of the hole refers to the line connecting the two furthest points on the cross-section of the silicon-carbon material in the cross-sectional SEM image of the negative electrode sheet.

[0020] In one embodiment, D is 0.3 μm-1 μm.

[0021] In this invention, based on the total mass of the positive electrode active material, the mass content of element Sr is c(Sr), c 1 (Sr) ranges from 70ppm to 1500ppm, for example, 70ppm, 80ppm, 100ppm, 200ppm, 300ppm, 400ppm, 600ppm, 800ppm, 1000ppm, 1200ppm, 1400ppm or 1500ppm.

[0022] In this invention, D and c(Sr) satisfy: 0.0003≤D / c(Sr)≤0.02, for example, 0.0003, 0.0005, 0.001, 0.002, 0.004, 0.006, 0.008, 0.01, 0.012, 0.014, 0.016, 0.018 or 0.02.

[0023] In one embodiment, 0.001 ≤ D / c(Sr) ≤ 0.0045. It is understood that D / c(Sr) is calculated by substituting numerical values, where D is in μm and c(Sr) is in ppm. For example, if D is 15 μm and c(Sr) is 500 ppm, then D / c(Sr) = 15 / 500 = 0.03.

[0024] The positive electrode uses a high-nickel ternary material (e.g., x1≥0.8), but under deep charge-discharge and high-temperature conditions, the Ni content of the high-nickel ternary material... 2+Li⁺ is more likely to migrate into the lithium layer (3a site) and occupy a position, while Li⁺ may also occupy the transition metal layer (3b site), resulting in Li / Ni mixing. This mixing hinders lithium-ion diffusion, increases lithium-ion insertion / extraction resistance, leads to increased internal resistance and reduced reversible capacity, destroys the layered structure stability of the ternary material, and exacerbates structural degradation during cycling. Furthermore, during high-voltage charging and discharging, the cell parameters (especially the c-axis) of high-nickel ternary materials change more significantly, leading to huge internal stress within the polycrystalline grains. This causes microcracks within the polycrystalline particles, resulting in poor electrical contact between particles. The newly exposed surfaces continuously undergo side reactions with the electrolyte, consuming active lithium and electrolyte, and forming a thicker CEI film at the positive electrode, leading to deterioration in the cycling stability and rate performance of the polycrystalline particles. To address these issues, Zr and Sr are introduced into the polycrystalline particles. During battery charging, the positive electrode material transforms from the H₂ phase to the H₃ phase with volume shrinkage, generating mechanical stress that leads to microcracks. The introduction of Zr can buffer and delay this phase transition, and even guide a milder monoclinic (M) phase transition, thus protecting the granular structure. Strong Zr-O bonds act as a support, slowing the collapse of the layered structure and promoting the formation of a more stable local superlattice, suppressing anisotropic lattice distortion. Zr can fundamentally improve the structural stability of ternary materials by strengthening the crystal framework and suppressing destructive phase transitions. In addition, Zr can also suppress transition metal ions (such as Ni) in ternary cathodes. 4+ The dissolution of Sr can alleviate electrolyte side reactions and prevent the deposition of leached transition metals on the negative electrode, which can damage the electrode. The introduction of Sr can suppress lattice distortion and microcracks in polycrystalline particles. Sr has a much larger ionic radius than Ni, and its introduction can partially replace Ni ions, reducing lattice stress. During battery charging and discharging, Sr... 2+ It can buffer the volume change of the positive electrode active material, reduce anisotropic strain, and suppress the generation and propagation of microcracks within polycrystalline particles and between primary particles; additionally, some Sr 2+Sr combines with Ni on the surface of polycrystalline particles to form a highly stable SrNiO3 coating layer, which reduces electrolyte corrosion, suppresses side reactions between Ni ions and the electrolyte, and thus lowers interfacial impedance. Sr also contributes to oxygen vacancy formation energy, reduces oxygen release at high temperatures, and improves battery stability. When c(Sr) is too low (e.g., <70ppm), the Sr content in the positive electrode active layer is too low, resulting in insufficient suppression of expansion of the ternary material. The positive electrode sheet is susceptible to expansion stress, leading to poor structural stability. When c(Sr) is too high (e.g., >1500ppm), the Sr content in the positive electrode active layer is too high, and Sr segregation on the surface of the ternary positive electrode is prone to occur, leading to blockage of lithium-ion transport paths and hindering the improvement of battery cycle stability. Since Zr can stabilize the structure of the positive electrode material, increase the cutoff voltage of the positive electrode material, and suppress transition metal dissolution, thereby improving the cycle performance of the battery, the corresponding negative electrode material also needs to be a material with a longer lifespan. Using porous silicon-carbon negative electrode materials can alleviate volume expansion during battery cycling and improve cycle stability. Furthermore, when the Zr content in ternary materials is high, it can block lithium-ion transport channels, potentially affecting the lithium-ion insertion / extraction rate. Porous silicon-carbon anode materials can provide more active sites for lithium insertion / extraction, thereby improving the battery's rate performance. Sr can reduce the volume change of polycrystalline particles during battery cycling. For silicon-based anodes, which exhibit more significant volume expansion, further mitigation of this expansion is needed to improve battery cycle stability. Therefore, combining the anode active material with porous silicon-carbon materials offers advantages such as high specific surface area and numerous active sites. This provides more insertion / extraction sites and diffusion channels for lithium-ion diffusion within the silicon-carbon material, thereby improving the ion / electronic conductivity of the silicon-carbon anode material and the battery's rate performance. Simultaneously, the internal pores can provide a buffer space for the volume expansion of silicon materials during battery cycling, mitigating volume changes, reducing anode sheet expansion, and thus improving battery cycle stability. At this point, when a positive electrode sheet with polycrystalline particles of ternary materials containing Zr and Sr is matched with a negative electrode sheet with silicon-carbon anode material containing several pores, it can not only ensure high battery energy density, but also improve the battery's rate performance, cycle stability and safety performance under high voltage and high temperature conditions.Adjusting the D / c(Sr) ratio within a suitable range can improve the matching degree between the positive and negative electrodes. When the D / c(Sr) ratio is too low (e.g., <0.0003), the size of the internal pores in the silicon-carbon material is too small, while the Sr content in the positive electrode active material is too high. In this case, Sr is prone to segregation from the surface of the ternary positive electrode, leading to blockage of the lithium-ion transport path. At the same time, the volume expansion buffer of the silicon-carbon negative electrode is insufficient, the number of active sites is reduced, and the ion transport rate of the positive and negative electrodes decreases, which is not conducive to improving the rate performance of the battery. When the D / c(Sr) ratio is too high (e.g., >0.02), the size of the internal pores in the silicon-carbon material is too large, while the Sr content in the positive electrode active material is too low. The positive electrode has poor suppression of the volume expansion of the ternary material, and the structural stability of the ternary material decreases. Meanwhile, the diameter of the internal pores in the silicon-carbon negative electrode is too large, which leads to a decrease in the strength of the silicon-carbon material and an aggravation of the side reactions of the electrolyte. Similarly, it is not conducive to improving the cycle stability of the battery under high temperature and high voltage.

[0025] In this invention, the values ​​of a1, b1, x1, y1, z1, and v1 can be determined by conventional testing methods in the art, for example, by the following method: Take about 0.1g of polycrystalline particles, add 10mL of hydrochloric acid, digest on a hot plate at 350℃ for 10min, cool, and bring the volume to 100ml. Then dilute it 10 times, take a portion of the solution, and analyze it using an inductively coupled plasma spectrometer (ICP). After obtaining the corresponding test parameters, a1, b2, x1, y1, z1, and v2 can be obtained by formula conversion. c(Sr) can be obtained by conventional testing methods in the field, such as by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), as follows: The lithium-ion secondary battery is discharged to 0% SOC, the positive electrode is disassembled and removed, soaked in DMC solvent for 12 hours, rinsed with DMC solvent to remove the lithium salt attached to the positive electrode, and then washed with deionized water to remove the residue on the surface of the positive electrode. After drying, the positive electrode active layer is scraped as a sample, 10 mL of hydrochloric acid is added and digested on a hot plate at 350 °C for 10 min. After cooling, the volume is adjusted to 100 mL, and then diluted 10 times. A portion of the solution is taken for analysis by inductively coupled plasma spectrometry (ICP).

[0026] In this invention, the mass content of element Zr is c based on the total mass of the polycrystalline particles. 1 (Zr), c 1 (Zr) is 300ppm-4500ppm, for example, 300ppm, 400ppm, 6000ppm, 1000ppm, 150ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, or 4500ppm. Based on the total mass of the polycrystalline particles, the mass content of element Sr is c. 1 (Sr), c 1(Sr) is 300ppm-1500ppm, for example, 300ppm, 500ppm, 800ppm, 1000ppm or 1500ppm.

[0027] In one embodiment, c 1 (Zr) is 800ppm-2500ppm.

[0028] In this invention, the average particle size of the polycrystalline particles is R1, which is 3μm-20μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 10μm, 12μm, 14μm, 16μm, 18μm or 20μm.

[0029] In this invention, the polycrystalline particles have grain boundaries, and the grain boundaries contain the elements Zr and / or Sr.

[0030] In one embodiment, the grain boundary contains the element Zr.

[0031] In one embodiment, the grain boundary contains the element Sr.

[0032] In one embodiment, the grain boundary contains the elements Zr and Sr.

[0033] Adjust c 1 (Zr) and c 1 (Sr) within a suitable range allows the introduction of Zr and Sr to achieve the best effect, when c 1 When the Zr content is too low (e.g., <300 ppm), the improvement on the thermal stability, structural stability, and ion transport performance of the polycrystalline particles is not significant. 1 Excessive Zr content (e.g., >4500ppm) can clog lithium-ion transport channels, hindering the improvement of polycrystalline particle capacity and stability. When c 1 When the Sr content is too low (e.g., <300ppm), the Sr content is insufficient to suppress the expansion of the ternary material, resulting in poor crystal structure stability and a tendency for irreversible phase transitions, which is detrimental to improving battery cycle stability; when c 1If the Sr content is too high (e.g., >1500ppm), the Sr content is too high, the Sr is unevenly distributed in the crystal, and it is easy to segregate on the surface, which hinders lithium-ion transport and is not conducive to improving the cycle stability and rate performance of polycrystalline particles. When R1 is too low (e.g., <3μm), the specific surface area of ​​the polycrystalline particles increases dramatically, leading to a larger contact area with the electrolyte. This exacerbates the decomposition of the electrolyte at the positive terminal, and the generated gaseous byproducts increase the cell thickness. Additionally, other byproducts increase the interfacial impedance. Furthermore, excessively small particle size increases the risk of polycrystalline particles pulverizing and detaching during battery cycling due to volume changes, which is detrimental to further improving battery cycle stability. When R1 is too high (e.g., >20μm), the diffusion path of lithium ions within the polycrystalline particles is longer, resulting in a decrease in lithium ion insertion / extraction efficiency at high voltages. This is detrimental to improving battery rate performance. Moreover, excessively large particle size can lead to uneven distribution of expansion stress, exacerbating the generation and propagation of microcracks within the polycrystalline particles and at grain boundaries, which is also detrimental to improving battery cycle stability and safety performance.

[0034] In this invention, c 1 (Zr) and c 1 (Sr) can be obtained by conventional testing methods in the art, such as by ICP spectrometry, as follows: Take about 0.1g of polycrystalline particles, add 10mL of hydrochloric acid, digest on a hot plate at 350℃ for 10min, cool, and bring the volume to 100ml. Then dilute it 10 times, take a portion of the solution and analyze it using inductively coupled plasma spectrometry (ICP) to measure c. 1 (Zr) and c 1 (Sr). Whether the grain boundaries of polycrystalline particles contain elements Zr and / or Sr can be determined by conventional testing methods in the art, such as by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), as follows: The lithium-ion secondary battery is discharged to 0% SOC, the positive electrode is disassembled and removed, and then soaked in dimethyl carbonate (DMC) solvent for 12 hours. After that, it is rinsed with DMC to remove the lithium salt attached to the positive electrode. The positive electrode is then kept at 450°C for 180 minutes. After cooling, the positive electrode is brushed off with a brush to obtain positive electrode active material powder. After sample preparation, it is observed under a scanning electron microscope at a certain magnification to find the location of polycrystalline particles and grain boundaries. The elemental distribution at the grain boundaries is determined by EDS spot scanning. R1 can be obtained through conventional testing methods in the field, such as SEM testing: after discharging the lithium-ion secondary battery to 0% SOC, disassemble and remove the positive electrode sheet, polish the cross-section of the positive electrode sheet with an argon ion mill, image it under a scanning electron microscope, magnify it to a certain magnification, find 30 different polycrystalline particles in the SEM image, and measure their diameter using image analysis software (such as Image Pro Plus) and take the average value, which is R1.

[0035] In this invention, the average particle size of the silicon-carbon material is R2, which is 3μm-17μm, for example, 3μm, 4μm, 6μm, 8μm, 10μm, 12μm, 14μm, 16μm, or 17μm. "The average particle size of the silicon-carbon material" has a conventional meaning in the art, that is, the maximum distance between two opposite sides of the outer contour of the cross-section of the silicon-carbon material is recorded as the particle size of the silicon-carbon material. The average particle size of the silicon-carbon material is obtained by averaging several particle sizes; "several" refers to a number greater than or equal to 2, and the specific value can be determined based on actual testing, for example, 50. R2 can also be obtained by SEM or focused ion beam (FIB) testing.

[0036] In one embodiment, R2 is 6μm-13μm.

[0037] In this invention, D and R2 satisfy: 0.02≤D / R2≤0.3, for example, 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14, 0.46, 0.18, 0.2, 0.25 or 0.3.

[0038] Adjusting the D / R2 ratio can balance the gains from the internal pores of silicon-carbon materials in improving kinetics and mitigating volume expansion, as well as the overall structural stability of the silicon-carbon material. When D / R2 is too low (e.g., <0.02), D is too small and / or R2 is too large, resulting in a large overall particle size of the silicon-carbon material. This increases the diffusion distance of lithium ions within the material, reducing diffusion efficiency. Meanwhile, the small size of the internal pores is detrimental to improving the kinetic performance of the silicon-carbon material, and the limited internal buffer space prevents further improvement in battery cycle stability and rate performance. When D / R2 is too high (e.g., >0.3), D is too large and / or R2 is too small, resulting in large internal pore sizes of the silicon-carbon material. This reduces the mechanical strength of the porous carbon matrix, making the silicon-carbon material prone to breakage during battery cycling and affecting the structural stability of the particles.

[0039] In this invention, D and R2 can be obtained using conventional testing methods in the art, for example, by the following method: discharging a lithium-ion secondary battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly removing the negative electrode sheet, polishing its cross-section with an argon-ion polisher, and then imaging the obtained cross-section using backscatter imaging mode on a scanning electron microscope (SEM); in the image, silicon-carbon material particles appear grayish-white, and pores appear grayish-black. Twenty different silicon-carbon materials are randomly selected, and particle size and distribution statistics are performed using electron microscopy image analysis software such as ImageJ to obtain the average particle size of the silicon-carbon material, which is R2. Then, cross-sections with pores are selected, and the size of the pores is measured using electron microscopy image analysis software such as ImageJ. The sizes of the 20 different pores are determined, and the average value is taken as D. It should be noted that since the cross-section cutting is random during the test, only cross-sections with pores are included in the statistics when measuring D; silicon-carbon material particles without pores are not included in the statistics.

[0040] In this invention, the projected area of ​​a single pore on the cross-section of the silicon-carbon material is S1, where S1 is 0.01 μm. 2 -20μm 2 For example, 0.01μm 2 0.05μm 2 0.1μm 2 0.2μm 2 0.5μm 2 1μm 2 2μm 2 4μm 2 6μm 2 8μm 2 10μm 2 12μm 2 14μm 2 16μm 2 or 20μm 2 The sum of the projected areas of the pores on the cross-section of the silicon-carbon material is S2, where S2 is 0.1 μm. 2 -25μm 2 For example, 0.1μm 2 0.4μm 2 0.6μm 2 1μm 2 2μm 2 4μm 2 6μm 2 8μm 2 10μm 2 12μm 2 14μm 2 16μm 2 18μm2 20μm 2 or 25μm 2 The cross-sectional area of ​​the silicon-carbon material is S3, where S3 is 5 μm. 2 -250μm 2 For example, 5μm 2 10μm 2 20μm 2 30μm 2 40μm 2 60μm 2 80μm 2 100μm 2 120μm 2 140μm 2 160μm 2 180μm 2 200μm 2 or 250μm 2 .

[0041] In this invention, based on the cross-sectional area of ​​the silicon-carbon material, the sum of the areas of the pores is S2 / S3, where S2 / S3 is 0.04%-11%, for example, 0.04%, 0.05%, 0.1%, 0.5%, 1%, 1.5%, 2%, 3%, 4%, 5%, 6%, 8%, 10% or 11%.

[0042] In one embodiment, S2 / S3 is 0.5%-3%.

[0043] Adjusting the S2 / S3 ratio can balance the gains from the internal pores of silicon-carbon materials in improving kinetics and mitigating volume expansion, as well as the overall structural stability of the silicon-carbon material. When S2 / S3 is too low (e.g., <0.04%), S2 is too small and / or S3 is too large, resulting in a low proportion of pores in the silicon-carbon material, which is detrimental to improving the kinetic performance of the silicon-carbon material and leaves little internal buffer space, which is not conducive to further improving the cycle stability and rate performance of the battery. When S2 / S3 is too high (e.g., >11%), S2 is too large and / or S3 is too small, resulting in a high proportion of pores in the silicon-carbon material. On the one hand, this reduces the mechanical strength of the porous carbon matrix, making the silicon-carbon material prone to breakage during battery cycling, affecting the structural stability of the particles. On the other hand, it also reduces the specific capacity of the silicon-carbon material, which is not conducive to improving the energy density of the battery.

[0044] In this invention, the number of pores on the cross-section of the silicon-carbon material is N, where N ≤ 20, and the unit is individual pores, for example, 1, 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18, or 20. The number of pores refers to the number of pores on the cross-section of a single silicon-carbon material particle.

[0045] In one embodiment, N is 1 to 10.

[0046] Adjusting the combination of N and D with S1 and S2 can ensure that the specific surface area and porosity of silicon-carbon materials are within a suitable range, enabling the formation of a dense and stable SEI film at the negative electrode. The pores inside the silicon-carbon material extend into the particle interior, which helps to improve the lithium-ion insertion / extraction capability and improve the rate performance of the battery.

[0047] In this invention, S1, S2, S3, and N can be obtained through conventional testing methods in the art, for example, by the following method: discharging a lithium-ion secondary battery to 0% SOC, disassembling and removing the negative electrode sheet, or directly removing the negative electrode sheet, polishing its cross-section with an argon-ion polisher, and then imaging the obtained cross-section using a scanning electron microscope (SEM) in backscatter imaging mode; in the image, silicon-carbon material particles appear grayish-white, and pores appear grayish-black. Ten different silicon-carbon materials are randomly selected, and the sum of the areas of all pores on each cross-section is measured using electron microscope image analysis software such as ImageJ, which is S2. The average of the sum of the areas of all pores is S1. The area of ​​each of the ten silicon-carbon material cross-sections is measured and the average is taken as S3. The number of pores in the cross-sections of different silicon-carbon material particles is counted as N. It should be noted that since the cutting of the cross-section is random during the test, when measuring S1 and S2, only those cross-sections with pores are counted, and silicon-carbon material particles without pores are not included in the count.

[0048] In this invention, the silicon-carbon material comprises a porous carbon matrix and silicon particles located in the pores within the porous carbon matrix. The porous carbon matrix comprises biochar.

[0049] In this invention, based on the total mass of the silicon-carbon material, the mass content of silicon element is w1, which is 30%-70%, for example, 30%, 32%, 36%, 40%, 45%, 50%, 60%, or 70%. Based on the total mass of the first negative electrode active layer, the mass content of silicon element is w2, which is 5%-40%, for example, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 20%, 25%, 30%, 35%, or 40%.

[0050] In this invention, w2 and w1 satisfy: 0.03≤w1×w2≤0.3, for example, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.1, 0.12, 0.14, 0.16, 0.2, 0.25 or 0.3.

[0051] Adjusting w1×w2 within a suitable range can balance the contribution of silicon-carbon anode material to battery capacity and structural stability. When w1 is low, the mass content of silicon in silicon-carbon material is low. To ensure high energy density, the amount of silicon-carbon material mixed in the anode active layer needs to be increased. When w1 is high, the mass content of silicon in silicon-carbon material is high, resulting in more significant volume expansion and a decrease in the overall conductivity of silicon-carbon material. Therefore, it is necessary to reduce the amount of silicon-carbon material mixed in the anode active layer, increase the proportion of graphite to further buffer the volume expansion of silicon-carbon anode material and maintain the overall conductivity of the anode sheet, thereby enabling the battery to have good rate performance and cycle stability.

[0052] In this invention, w1 can be obtained by conventional testing methods in the art, such as the following method: using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), specifically as follows: the lithium-ion secondary battery is discharged to empty charge, the battery is disassembled, the negative electrode is taken out, and after processing by argon ion polishing technology, a cross-sectional sample of the negative electrode can be obtained. Then, the backscatter mode of the scanning electron microscope is used for testing, with a magnification of 10K. The middle region of a single silicon-carbon material particle is selected for EDS analysis, and only the content of carbon and silicon elements is tested. The mass content of silicon element is w1. A total of 20 different particles are measured, and the average value is taken.

[0053] In this invention, the mass content w2 of silicon in the negative electrode active layer can be obtained by conventional methods in the art. For example, after discharging a lithium-ion secondary battery to 0% SOC, the negative electrode sheet is disassembled and soaked in dimethyl carbonate (DMC) solvent for 12 hours, then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active layer can then be peeled off from the negative electrode current collector, and the negative electrode active layer is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a rate of 10°C / min, and held at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active layer volatilize while the silicon is fully oxidized to silicon dioxide. The remaining substance is the ash of the negative electrode active layer. The mass content of silicon in the negative electrode active layer can be calculated based on the mass of ash. The calculation formula is as follows: Mass content of silicon in the negative electrode active layer = 7 × mass of ash / (15 × mass of test sample).

[0054] In this invention, the ternary material further includes single-crystal particles, the single-crystal particles having the chemical formula Li. v1 Ni x2 Co y2 Mnz2 M 3 a2 O2, where 0.85 ≤ v1 ≤ 1.1 (e.g., 0.85, 0.86, 0.88, 0.9, 0.95, 1, 1.05, or 1.1), 0.8 ≤ x2 ≤ 0.98 (e.g., 0.8, 0.82, 0.84, 0.86, 0.88, 0.9, 0.92, 0.94, 0.96, or 0.98), and 0 < y2 ≤ 0.12 (e.g., 0.01, 0.0...). 2. 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.1 or 0.12), 0 < z2 ≤ 0.1 (e.g., 0.01, 0.02, 0.03, 0.04, 0.06, 0.08 or 1), 0.008 ≤ a2 ≤ 0.03 (e.g., 0.008, 0.01, 0.015, 0.02, 0.025 or 0.03), where M 3 It includes at least one of the elements Al, Ti, Sr, Mg, Zr, Y, La, P, B, Nb, Sb, W, and V.

[0055] In this invention, the values ​​of a2, b2, x2, y2, z2 and v2 can be determined with reference to the test methods of a1, b1, x1, y1, z1 and v1.

[0056] In this invention, the volumetric particle size distribution curve of the ternary material has a first peak and a second peak. The first peak is located between 1 μm and 5 μm, for example, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm or 5 μm. The second peak is located between 3 μm and 20 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm. Figure 2 The figure shows the volume particle size distribution curve of the positive electrode active material in one embodiment of the present invention, which has a first peak and a second peak. The first peak represents single crystal particles, and the second peak represents polycrystalline particles.

[0057] In one embodiment, the first peak is located at 1.2 μm-3 μm, and the second peak is located at 5.5 μm-10 μm.

[0058] In this invention, based on the total mass of the ternary material, the mass content of the polycrystalline particles is w3, and the mass content of the single crystal particles is w4. w3 and w4 satisfy: 0.6≤w4 / w3≤7.5, for example, 0.6, 0.8, 1, 2, 3, 4, 5, 6, 7 or 7.5.

[0059] In one embodiment, 0.8 ≤ w4 / w3 ≤ 3.

[0060] In this invention, the mass content of element Zr is c based on the total mass of the single crystal particles. 2 (Zr), c 2 (Zr) is 0ppm-3000ppm, for example, 0ppm, 10ppm, 50ppm, 100ppm, 200ppm, 400ppm, 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, or 3000ppm. Based on the total mass of the single crystal particles, the mass content of element Sr is c. 2 (Sr), c 2 (Sr) ranges from 0ppm to 2000ppm, for example, 0ppm, 10ppm, 50ppm, 100ppm, 200ppm, 400ppm, 500ppm, 1000ppm, 1500ppm or 2000ppm.

[0061] In this invention, the mass content of element Ni is c based on the total mass of the polycrystalline particles. 1 (Ni), based on the total mass of the single crystal particles, the mass content of elemental Ni is c. 2 (Ni), c 1 (Ni)≤c 2 (Ni).

[0062] Polycrystalline particles offer better rate performance, but their higher grain boundaries make them prone to side reactions with the electrolyte, leading to battery gas production. Adding monocrystalline particles to the positive electrode active material addresses this issue. Monocrystalline particles have larger grains and smaller specific surface areas, resulting in a lower lithium-ion insertion / extraction rate compared to polycrystalline particles. This slows down the charge / discharge rate and reduces electrolyte consumption, contributing to better battery cycle stability. Therefore, a blend of monocrystalline and polycrystalline particles provides better structural stability and rate performance. Furthermore, the strong side reactions between silicon and the electrolyte at high temperatures lead to significant volume expansion. When the positive electrode active material includes both monocrystalline and polycrystalline particles, it exhibits a lower charging temperature rise, effectively suppressing silicon volume expansion and resulting in higher cycle performance and energy density. The content of monocrystalline and polycrystalline particles needs to be carefully controlled. Monocrystalline particles have poor rate performance; introducing too many can degrade the battery's rate performance. Furthermore, M can be further introduced into the monocrystalline particles. 3 When c 2 (Ti) and c 2 When (Sr) is 0 ppm, single-crystal particles have a high specific capacity, which is beneficial for further improving the capacity of the cathode and the energy density of the battery; when c 2 (Ti) and c 2When the Sr content is not 0 ppm, single-crystal particles exhibit better stability and kinetic performance. Furthermore, single-crystal particles demonstrate better stability than polycrystalline particles. Therefore, single-crystal particles without Ti or Sr doping are suitable for use in battery systems requiring high energy density but low rate capability. Additionally, since single-crystal particles offer better structural stability than polycrystalline particles, setting a higher Ni content in single-crystal particles and blending them with porous particles can not only improve the overall capacity of the ternary material but also maintain its overall structural stability.

[0063] In this invention, the volumetric particle size distribution curve of the ternary material can be obtained using conventional testing methods in the art, for example, by: discharging a lithium-ion secondary battery to 0% SOC, disassembling and removing the positive electrode sheet, immersing it in dimethyl carbonate (DMC) solvent for 12 hours, then rinsing it with DMC to remove the lithium salt adhering to the positive electrode sheet, then keeping the positive electrode sheet at 450°C for 180 minutes, and brushing off the positive electrode active material from the cooled positive electrode sheet to obtain positive electrode active material powder; taking the collected positive electrode active material powder as a sample, and measuring the volume distribution curve using a laser diffraction particle size analyzer, wherein the volume distribution curve has a bimodal structure. w4 / w3 can be obtained by the ratio of the peak areas of the second peak to the first peak. 2 (Zr) and c 2 For the determination of (Sr), please refer to c. 1 (Zr) and c 1 (Sr) proceeds.

[0064] In this invention, the lithium-ion secondary battery further includes a separator. The positive electrode, the separator, and the negative electrode are stacked and wound to form a core. The lithium-ion secondary battery has an arc-shaped region and a flat region located between the arc-shaped regions. The positive electrode located in the arc-shaped region has a first convex portion and a first concave portion. The positive electrode located in the flat region has a second convex portion and a second concave portion. The positive electrode includes a first surface and a second surface. The first surface is close to the winding center, and the second surface is away from the winding center. The first convex portion, the first concave portion, the second convex portion, and the second concave portion can be obtained by conventional methods in the art, for example, by using an embossing roller (with raised dots) through an embossing process.

[0065] In one embodiment, the first surface located in the arc region has a plurality of first recesses, and the second surface located in the arc region has a plurality of first protrusions. The positions of the first recesses and the first protrusions correspond one-to-one. It is understood that "a plurality of first protrusions" means that the number of first protrusions is ≥2, and "a plurality of first recesses" means that the number of first recesses is ≥2.

[0066] In one embodiment, the first surface located in the flat region has a plurality of second protrusions, and the second surface located in the flat region has a plurality of second recesses. The positions of the second recesses and the second protrusions correspond one-to-one. It is understood that "a plurality of second protrusions" means that the number of second protrusions is ≥2, and "a plurality of second recesses" means that the number of second recesses is ≥2.

[0067] like Figure 3 The diagram shown is a structural schematic of the arc region and the straight region in one embodiment of the present invention. It can be seen that the concave and convex parts of the positive electrode sheet located in the arc region and the positive electrode sheet located in the straight region are arranged in opposite directions. Among them, 13 is the arc region, 14 is the straight region, 111 is the first convex part, 112 is the first concave part, 121 is the second convex part, and 122 is the second concave part.

[0068] In this invention, the depth of the first recess is 10μm-35μm, for example, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm; the depth of the second recess is 10μm-35μm, for example, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm. The height of the first protrusion is 10μm-35μm, for example, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm; the height of the second protrusion is 10μm-35μm, for example, 10μm, 15μm, 20μm, 25μm, 30μm, or 35μm.

[0069] In this invention, the spacing between the first recesses is 1mm-6mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, or 6mm; the spacing between the second recesses is 2mm-6mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, or 6mm. The spacing between the first protrusions is 1mm-6mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, or 6mm; the spacing between the second protrusions is 1mm-6mm, for example, 1mm, 2mm, 3mm, 4mm, 5mm, or 6mm.

[0070] In this invention, the first recess and the second recess constitute a recessed region, and the projected area of ​​the recessed region on the surface of the positive electrode sheet accounts for 40%-90% of the area of ​​one side of the positive electrode sheet, for example, 40%, 50%, 60%, 70%, or 90%. The first convex portion and the second convex portion constitute a convex region, and the projected area of ​​the convex region on the surface of the positive electrode sheet accounts for 40%-90% of the area of ​​one side of the positive electrode sheet, for example, 40%, 50%, 60%, 70%, or 90%.

[0071] In this invention, the shape of the first and second recesses projected onto the surface of the positive electrode is not limited; they can be circular or rectangular. The spacing between the first and second recesses refers to the distance between the lowest points of two adjacent first recesses (or two adjacent second recesses); the depth of the first and second recesses refers to the vertical distance from the lowest point of the first recess (or second recess) to the surface of the positive electrode active layer on one side away from the positive electrode current collector (for example, when the first surface has the first recess, the depth of the first recess refers to the distance from the lowest point of the first recess to the first surface; when the second surface has the second recess, the depth of the second recess refers to the distance from the lowest point of the second recess to the second surface).

[0072] In this invention, the shape of the orthographic projection of the first and second protrusions onto the surface of the positive electrode is not limited; it can be circular or rectangular. The spacing between the first and second protrusions refers to the distance between the highest points of two adjacent first protrusions (or two adjacent second protrusions); the height of the first and second protrusions refers to the vertical distance from the highest point of the first protrusion (or second protrusion) to the surface of the positive electrode active layer on one side (for example, when the second surface has the first protrusion, the height of the first protrusion refers to the distance from the highest point of the first protrusion to the second surface; when the first surface has the second protrusion, the height of the second protrusion refers to the distance from the highest point of the second protrusion to the first surface).

[0073] In this invention, the distance between the first and second recesses, and their depths, can be obtained using conventional testing methods in the art. For example, after discharging a lithium-ion secondary battery to 0% SOC, the positive electrode is disassembled and removed. A 3D profilometer is used to photograph the surface of the electrode recesses, and the recess distance and depth are measured. Ten different first and second recesses are selected from the positive electrode in the arc region and the flat region, respectively, and their depths are measured and averaged to obtain the depths of the first and second recesses. Ten groups are randomly selected, with two adjacent first (or second) recesses as a group. The shortest distance between the edges of the contours of two adjacent first (or second) recesses in each group is measured, and the average value is taken as the distance between the first and second recesses. The distance between the first and second convex parts, and their heights, can be determined using the same method.

[0074] By adding a first concave and a first convex portion, as well as a second concave and a second convex portion, to the two sides of the positive electrode sheet located in the arc region and the straight region, respectively, the gap between the positive and negative electrode sheets can be increased, increasing the contact area between the electrolyte and the positive electrode active material. This provides buffer space for the volume expansion of the negative electrode active material. Simultaneously, for wound batteries, the adhesive layer of the separator may experience uneven melting and softening during hot pressing in the arc region, creating some spaces. This is especially true for spray-coated adhesive layers, which have poor flatness. Similarly, this provides gaps between the positive and negative electrode sheets, helping to release the stress generated by expansion, reducing the expansion stress on the positive and negative electrode sheets, and improving the battery's cycle life and charge / discharge performance. When the concave and convex portions in the arc region and the straight region are arranged in the above manner, the first concave and first convex portions in the arc region and the second concave and second convex portions in the straight region are arranged in opposite directions. The first convex portion in the arc region expands away from the winding center, increasing the gap between the negative and positive electrode sheets in the arc region, alleviating stress concentration in the arc region, and improving the dynamic performance in the arc region.

[0075] In this invention, the concave region refers to the combination of the shortest-circumference closed shape formed by connecting the lowest points of the first and second concave portions on the outer periphery of the positive electrode surface in three-dimensional space. The orthographic projection area of ​​the closed shape on the surface of the positive electrode is the area of ​​the concave region. The area of ​​the concave region can be measured and calculated by the following method: along the width direction of the positive electrode, connect the lowest points of the outermost first concave portion of each arc region and the outermost second concave portion of each straight region in three-dimensional space, and denot their dimensions as Y1 and Y1'. Along the length direction of the positive electrode, connect the outermost first concave portion of each arc region and the outermost second concave portion of each straight region in three-dimensional space respectively. The lowest point of the outermost second concave portion in three-dimensional space is denoted as Y2 and Y2'. The width of the positive electrode is Y3, and the length of the positive electrode is Y4. The sum of the areas defined by line segments Y1 and Y2 in all the arc regions and the areas defined by line segments Y1' and Y2' in all the straight regions together constitute the concave region. The number of arc regions is denoted as k1, and the number of straight regions is denoted as k2. The area of ​​the concave region is k1(Y1×Y2)+k2(Y1'×Y2'), and the proportion of the area of ​​the concave region in the positive electrode is [k1(Y1×Y2)+k2(Y1'×Y2')] / (Y3×Y4). Figure 3 The figure shown is a schematic diagram of the planar structure of the positive electrode sheet in one embodiment of the present invention.

[0076] In this invention, the area of ​​the concave region on the positive electrode sheet can be obtained by conventional testing methods in the art, such as measuring Y1, Y2, Y1', Y2', Y3, and Y4 with a tape measure and then calculating the area. Similarly, the area of ​​the convex region on the positive electrode sheet can be determined.

[0077] In this invention, the positive electrode, the separator, and the negative electrode are stacked and wound to form a core. Along the winding direction of the core, at the winding tail of the core, the separator extends beyond the negative electrode, and the negative electrode extends beyond the positive electrode.

[0078] In this invention, the ratio of the radius of the arc region to the thickness of the core is 0.45-0.6, for example, 0.45, 0.46, 0.48, 0.5, 0.52, 0.54, 0.56, 0.58, or 0.6. The radius of the arc region is as follows... Figure 4 As shown in L1, the thickness of the core is as follows: Figure 5 As shown in L2. The radius of the arc region and the thickness of the core can be obtained by conventional testing methods in the art, for example, by discharging the lithium-ion secondary battery to 0% SOC and then taking a computed tomography (CT) scan and measuring the cross-section.

[0079] In this invention, the winding tail of the core does not include an empty foil area, which refers to an empty current collector without an active layer. Figure 5 The diagram shown is a schematic diagram of the core structure in one embodiment of the present invention, wherein the diaphragm 1 extends into the negative electrode 2, the negative electrode 2 extends into the positive electrode 1, 13 is the arc region, and 14 is the straight region.

[0080] In this invention, the lithium-ion secondary battery further includes an aluminum-plastic film located outside the core. The aluminum-plastic film includes an inner layer, an outer layer, and an intermediate layer located between the inner layer and the outer layer. The inner layer is close to the core, and the outer layer is away from the core. The inner layer includes a polypropylene layer (PP layer) and / or a modified PP layer. The intermediate layer includes an aluminum layer, and the outer layer includes nylon.

[0081] In this invention, the ratio of the thickness of the aluminum layer to the thickness of the aluminum-plastic film is 33%-40%, for example, 33%, 33%, 34%, 35%, 36%, 37%, 38%, 39% or 40%.

[0082] Eliminating empty foil areas in the core reduces its weight and increases the battery's energy density. When the separator at the winding tail extends into a negative electrode, and this extension leads to a positive electrode, overall battery safety is improved. This prevents the negative electrode from contacting and compressing the aluminum-plastic film, which could cause the film to crack, exacerbate electrolyte corrosion, and reduce battery safety. When the aluminum layer thickness accounts for 33%-40% of the aluminum-plastic film thickness, the aluminum layer is thicker than in traditional aluminum layers. This prevents the negative electrode from stretching and compressing the aluminum-plastic film under expansion stress in high-silicon negative electrode systems, thus preventing cracking. The ratio of the aluminum layer thickness to the aluminum-plastic film thickness can be obtained using conventional testing methods, such as SEM cross-sectioning to observe and measure the total thickness of the aluminum-plastic film and the aluminum layer thickness, and then calculating the ratio.

[0083] In this invention, the puncture strength of the diaphragm is 280 gf-400 gf, for example, 280 gf, 300 gf, 320 gf, 340 gf, 360 gf, 380 gf, or 400 gf. The tensile strength of the diaphragm is 1200 kgf / cm². 2 -4000 kgf / cm 2 For example, 1200 kgf / cm 2 1400kgf / cm 2 1600kgf / cm 2 2000 kgf / cm 2 2400kgf / cm 2 2800kgf / cm 2 3200kgf / cm 2 3400kgf / cm 2 3800kgf / cm 2 Or 4000 kgf / cm 2 The membrane has an elongation of 40%-100% in the TD and MD directions, for example, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%. The membrane has an air permeability of 600s / cc-500s / cc, for example, 60s / cc, 80s / cc, 100s / cc, 150s / cc, 200s / cc, 250s / cc, 300s / cc, 400s / cc, or 500s / cc.

[0084] In this invention, the “TD direction” and the “MD direction” have conventional meanings in the art. The “TD direction” refers to the direction perpendicular to the diaphragm winding direction, and the “MD direction” refers to the direction along the diaphragm winding direction.

[0085] In this invention, the diaphragm may include a polyethylene diaphragm and / or a polypropylene diaphragm.

[0086] In this invention, the diaphragm may further include a substrate layer, a coating layer, and an adhesive layer, wherein the adhesive layer is located on at least one outer surface of the diaphragm, and the coating layer is located on at least one surface of the substrate layer.

[0087] In one embodiment, the coating is located on one side surface of the substrate layer, and the adhesive layer is located on both outer surfaces of the diaphragm.

[0088] In one embodiment, the coating is located on both sides of the substrate layer, and the adhesive layer is located on both sides of the outer surface of the diaphragm, that is, the coating is located between the substrate layer and the adhesive layer.

[0089] In one embodiment, the coating comprises functional particles, the functional particles being at least one selected from alumina, boehmite, magnesium oxide, magnesium hydroxide, barium titanate, piperazine pyrophosphate, polyamide, polycarbonate, phenolic resin, aluminum nitride, melamine polyphosphate, melamine thiocyanate, melamine cyanurate, formaldehyde polymelamine hydrochloride, and melamine polyphosphate. The substrate comprises polyethylene (PE) and / or polypropylene (PP). The adhesive layer comprises at least one selected from polyvinylidene fluoride (PVDF), polymethyl methacrylate (PMMA), polyacrylonitrile (PAN), and polyacrylic acid (PAA).

[0090] In silicon-carbon anode systems (especially for double-coated anode sheets with silicon-carbon material on the surface), the volume of silicon material expands significantly during battery cycling. The increased size of silicon material can compress the separator, thus requiring a separator with high puncture strength to prevent short circuits during charging. At the same time, the stretching and shrinking of the anode sheet may cause lithium plating at the edges of the anode sheet, requiring a separator with high tensile strength and elongation to counteract the stretching of the anode sheet, and high puncture strength to prevent lithium dendrites from piercing the separator, thereby improving battery safety performance. The permeability of the separator affects the electrolyte transport rate and lithium-ion migration efficiency. When the permeability is within the above range, the rate performance of the battery can be further improved.

[0091] In this invention, the puncture strength of the separator can be obtained by conventional testing methods in the art, such as using a WD-D3 universal testing machine. After discharging the lithium-ion secondary battery to 0% SOC, the separator is disassembled and removed, for example, by removing the separator from the cell. The separator is selected from the head, tail, or superanode area of ​​the cell, and cleaned with anhydrous ethanol to remove electrolyte and residual lithium salt. Then, it is placed in a 40°C vacuum oven to dry and remove the solvent, thus obtaining the test sample. Using a WD-D3 universal testing machine, the separator is fixed on the base so that there is no relative displacement between the separator and the base. The puncture force test head is installed for testing, referring to GB / T 36363-2018. The puncture strength of the separator is tested in a flattened state, where "flattened" means that the separator is neither bent nor stretched.

[0092] In this invention, the tensile strength and elongation of the separator can be obtained by conventional testing methods in the art. For example, after discharging the lithium-ion secondary battery to 0% SOC, the separator is disassembled and removed, for example, the separator is removed from the cell. The separator is selected from the head, tail, or superanode area of ​​the cell, and cleaned with anhydrous ethanol to remove electrolyte and residual lithium salt. Then, it is placed in a 40°C vacuum oven to dry and remove the solvent, thus obtaining a test sample. The WD-D3 universal testing machine is used to install different test fixtures for testing (there is no relative displacement between the separator and the fixtures). The separator is stretched between two fixtures along the length and width directions until it breaks. The maximum tensile strength after breakage is the tensile strength of the separator in the length and width directions. At the same time, the elongation is obtained (elongation = (length at break - original length) / original length). The stretching speed is 100 mm / min, and the gauge length is 30 mm.

[0093] In this invention, the air permeability of the diaphragm can be obtained by testing, for example, using an air permeability testing device.

[0094] In this invention, the negative electrode sheet may further include a second negative electrode active layer located between the first negative electrode active layer and the negative electrode current collector. The first negative electrode active layer and the second negative electrode active layer form a negative electrode active layer, where w2 is the mass content of silicon in the negative electrode active layer, and w2 is 5%-40%.

[0095] In this invention, the first negative electrode active layer comprises a first negative electrode active material, which includes the silicon-carbon material; the second negative electrode active layer comprises a second negative electrode active material, which includes a second graphite. The average particle size of the second graphite is T2, where 5 μm ≤ T2 ≤ 20 μm, for example, 5 μm, 6 μm, 8 μm, 10 μm, 12 μm, 14 μm, 18 μm, or 20 μm. The first negative electrode active material further comprises a first graphite, where the average particle size of the first graphite is T1, and T1 and T2 satisfy: 5 μm ≤ T2 ≤ T1 ≤ 20 μm, for example, 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, or 20 μm.

[0096] In this invention, the thickness of the first negative electrode active layer on one side of the negative electrode current collector is H1, and the thickness of the first negative electrode active layer on the opposite side of the negative electrode current collector is H2, where 10μm ≤ H1 < H2 ≤ 65μm, for example, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, 60μm, or 65μm. The thickness of the second negative electrode active layer is 20μm-60μm, for example, 20μm, 22μm, 24μm, 26μm, 28μm, 30μm, 35μm, 40μm, 45μm, 50μm, 55μm, or 60μm. Figure 6 The image shown is a scanning electron microscope image of the negative electrode sheet along the thickness direction in one embodiment of the present invention. Silicon carbon material and first graphite are disposed in the first negative electrode active layer, and second graphite is disposed in the second negative electrode active layer. The thickness of the first negative electrode active layer on both sides of the negative electrode current collector is inconsistent.

[0097] In one embodiment, the thickness of the first negative electrode active layer near the winding center of the negative electrode sheet is H1, and the thickness of the first negative electrode active layer away from the winding center of the negative electrode sheet is H2.

[0098] In another embodiment, the thickness of the first negative electrode active layer near the winding center of the negative electrode sheet is H2, and the thickness of the first negative electrode active layer away from the winding center of the negative electrode sheet is H1.

[0099] During charging, silicon preferentially undergoes lithium intercalation. Therefore, placing silicon-carbon material on the surface layer (i.e., the first negative electrode active layer) helps shorten the migration distance of lithium ions, enabling rapid lithium intercalation on the negative electrode surface and improving the battery charging rate. Conversely, during discharging, graphite preferentially discharges. Therefore, using smaller-sized second graphite in the bottom layer (i.e., the second negative electrode active layer) shortens the diffusion path of lithium ions within the second graphite, improving its kinetic performance and facilitating high-rate discharge. Using larger-sized first graphite in the surface layer increases the inter-particle spacing, which is beneficial for silicon-carbon material filling. Since silicon expands more than first graphite, it maintains good contact with the first graphite particles during expansion and contraction, further improving cycle performance. The bottom layer has the same thickness on both sides of the negative electrode current collector, while the surface layer thickness can be the same or different. Different surface layer thicknesses can alter the NP ratio on both sides of the negative electrode, balancing high-temperature cycle capacity retention and negative electrode lithium plating during cycle. The side of the arc area of ​​the core that is closer to the winding center is usually subjected to greater compressive stress, which can easily cause compression of the separator and lead to local micro-short circuits. Reducing the thickness of the first negative electrode active layer on this side can reduce the compression on the side of the arc area that is closer to the winding center, reduce the risk of micro-short circuits, and improve the safety performance of the battery.

[0100] In this invention, the thicknesses of the first and second negative electrode active layers can be obtained using conventional testing methods in the art. For example, a lithium-ion secondary battery is discharged to 0% SOC, the negative electrode sheet is disassembled and removed, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salts attached to the negative electrode sheet. After drying the negative electrode sheet, its cross-section is polished with an argon ion mill. The obtained cross-section is imaged using a backscattering imaging mode on a scanning electron microscope (SEM). In the obtained cross-sectional image, the shortest distance from the outline of the silicon-carbon material to the surface of the negative electrode active layer away from the negative electrode current collector is measured using image processing software such as Image Pro Plus. This is the thickness of the first negative electrode active layer. The shortest distance from the same point to the surface of the negative electrode current collector is measured at the same location. This is the thickness of the second negative electrode active layer.

[0101] The present invention also provides a method for preparing silicon-carbon materials, comprising at least the following steps: Step 1: Pre-treat the biomass carbon source to obtain uniform particles; Step 2: The pretreated biomass carbon source is subjected to high-temperature pyrolysis under an inert atmosphere to obtain biochar, which is then pore-formed to obtain a porous carbon matrix. Step 3: Place the porous carbon matrix in a chemical vapor deposition (CVD) device, introduce an inert protective gas to purge the air, heat to the deposition temperature, then introduce a silicon source gas, and deposit for a certain period of time to obtain the silicon-carbon material.

[0102] In this invention, the biomass carbon source includes at least one selected from rice husks, wheat husks, bamboo, walnut shells, straw, coconut shells, and wood. The pretreatment includes at least washing, drying, and pulverizing. The high-temperature pyrolysis temperature is 600℃-1000℃ (e.g., 600℃, 700℃, 800℃, 900℃, or 1000℃). The inert atmosphere includes an argon atmosphere and / or a nitrogen atmosphere.

[0103] In this invention, the pore formation can be achieved through physical or chemical methods, such as heat treatment in a mixed atmosphere of CO2 or water vapor and nitrogen, at a temperature of 600℃-1000℃ (e.g., 600℃, 700℃, 800℃, or 1000℃) for 1h-10h (e.g., 1h, 5h, or 10h). Based on the total volume of the mixed atmosphere, the volume percentage of CO2 or water vapor is 10%-50% (e.g., 10%, 20%, 30%, 40%, or 50%).

[0104] In this invention, the deposition temperature is 400℃-800℃ (e.g., 400℃, 500℃, 600℃ or 800℃), and the deposition time is 1h-10h (e.g., 1h, 5h or 10h). The silicon source gas includes at least one of silane (SiH4), dichlorosilane, trichlorosilane and tetrachlorosilane.

[0105] In this invention, step 3 further includes cooling to room temperature after deposition. Step 2 further includes crushing and sieving after pore formation.

[0106] In this invention, the term "0% SOC" refers to the battery being discharged to a lower limit voltage (e.g., 3V) at 0.1C.

[0107] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0108] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0109] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0110] The following preparation examples illustrate the silicon-carbon material of the present invention.

[0111] Preparation Example 1: Rice husks were washed, dried, and pulverized to obtain uniform particles. The pretreated rice husks were then subjected to high-temperature pyrolysis under a nitrogen atmosphere at 800℃ to obtain biochar. The biochar was then heat-treated at 800℃ for 2 hours in a mixed atmosphere of 20% water vapor and 80% nitrogen. After sieving and pulverizing, a porous carbon matrix was obtained. The porous carbon matrix was placed in a CVD apparatus, nitrogen was introduced to purge air, the temperature was raised to 550℃, and then silane gas was introduced. After deposition for 4 hours, the material was cooled to room temperature to obtain the silicon-carbon material, denoted as SC-1. At this point, D was 0.75 μm, R2 was 10.6 μm, D / R2 was 0.071, and S1 was 0.44 μm. 2 S2 is 2.64 μm 2 S3 is 88.5 μm 2 S2 / S3 is 2.98%, N is 6, and w1 is 48.52%.

[0112] Preparation Example 2: Rice husks were washed, dried, and pulverized to obtain uniform particles. The pretreated rice husks were then subjected to high-temperature pyrolysis under a nitrogen atmosphere at 800°C to obtain biochar. The biochar was then heat-treated at 800°C for 1 hour in a mixed atmosphere of 20% water vapor and 80% nitrogen. After sieving and pulverizing, a porous carbon matrix was obtained. The porous carbon matrix was placed in a CVD apparatus, nitrogen was introduced to purge air, the temperature was raised to 550°C, and then silane gas was introduced. After deposition for 4 hours, the material was cooled to room temperature to obtain the silicon-carbon material, denoted as SC-2. At this point, D was 0.31 μm, R2 was 12.8 μm, D / R2 was 0.024, and S1 was 0.078 μm. 2 S2 is 0.78 μm 2 S3 is 128.5 μm 2 S2 / S3 is 0.61%, N is 10, and w1 is 48.43%.

[0113] Preparation Example 3: Rice husks were washed, dried, and pulverized to obtain uniform particles. The pretreated rice husks were then subjected to high-temperature pyrolysis under a nitrogen atmosphere at 800℃ to obtain biochar. The biochar was then heat-treated at 800℃ for 3 hours in a mixed atmosphere of 20% steam and 80% nitrogen. After sieving and pulverizing, a porous carbon matrix was obtained. The porous carbon matrix was placed in a CVD apparatus, nitrogen was introduced to purge air, the temperature was raised to 550℃, and then silane gas was introduced. After deposition for 4 hours, the material was cooled to room temperature to obtain the silicon-carbon material, denoted as SC-3. At this point, D was 0.97 μm, R2 was 6.2 μm, D / R2 was 0.156, and S1 was 0.75 μm. 2 S2 is 0.75μm 2 S3 is 30.18 μm 2 S2 / S3 is 2.49%, N is 1, and w1 is 48.64%.

[0114] Preparation Example Group 4: This group of preparation examples is used to verify the effects of changes in "D and R2", which is achieved by changing the temperature and time of the heat treatment during pore formation, as follows: Preparation Example 4a was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 600°C and the heat treatment time was 1 hour. The resulting silicon-carbon material is designated as SC-4a. In this case, D is 0.12 μm, R2 is 3 μm, D / R2 is 0.04, and S1 is 0.01 μm. 2 S2 is 0.06 μm 2 S3 is 7.1 μm 2 The ratio of S2 to S3 is 0.85%, N is 6, and w1 is 48.51%. Preparation Example 4b was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 1000℃ and the heat treatment time was 10h. The resulting silicon-carbon material was designated SC-4b, with a diameter (D) of 5 μm, a relative thickness (R2) of 16.8 μm, a D / R2 ratio of 0.298, and a Si ratio (S1) of 19.6 μm. 2 S2 is 19.6 μm 2 S3 is 221.5 μm 2 S2 / S3 is 8.85%, N is 1, and w1 is 48.55%. Preparation Example 4c was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 500°C and the heat treatment time was 1 hour. The resulting silicon-carbon material was designated SC-4c, with a diameter (D) of 0.08 μm, a relative thickness (R2) of 2.5 μm, a D / R2 ratio of 0.032, and a Si ratio of 0.005 μm. 2 S2 is 0.1 μm 2 S3 is 5μm 2 The ratio of S2 to S3 is 2.01%, N is 20, and w1 is 48.49%. Preparation Example 4d was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 1000℃ and the heat treatment time was 12h. The resulting silicon-carbon material is designated SC-4d, with a diameter (D) of 5.2 μm, a relative thickness (R2) of 17.8 μm, a D / R2 ratio of 0.292, and a Si content (S1) of 21.25 μm. 2 S2 is 21.25 μm 2 S3 is 248.5 μm 2 S2 / S3 is 8.55%, N is 1, and w1 is 48.5%. Preparation Example 4e was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 800°C and the heat treatment time was 1 hour. The resulting silicon-carbon material was designated SC-4e, with a diameter (D) of 0.3 μm, a relative thickness (R²) of 17 μm, a D / R² ratio of 0.0176, and a Si content (S₁) of 0.07 μm. 2 S2 is 0.42 μm 2S3 is 226.7 μm 2 The ratio of S2 to S3 is 0.19%, N is 6, and w1 is 48.46%. Preparation Example 4f was performed in accordance with Preparation Example 1, except that the heat treatment temperature was 1000℃ and the heat treatment time was 9h. The resulting silicon-carbon material was designated SC-4f, with a diameter (D) of 4.5 μm, a relative thickness (R2) of 13.7 μm, a D / R2 ratio of 0.328, and a surface area (S1) of 16 μm. 2 S2 is 16μm 2 S3 is 147.5 μm 2 S2 / S3 is 10.85%, N is 1, and w1 is 48.47%.

[0115] Preparation Example Group 5: This group of preparation examples was used to verify the effect of changes in "S2 / S3". This was achieved by changing the heat treatment time and the type of biomass carbon source during pore formation, as detailed below: Preparation Example 5a was performed in accordance with Preparation Example 1, except that bamboo was used as the biomass carbon source, and the heat treatment time was 1 hour. The resulting silicon-carbon material was designated SC-5a, with a diameter (D) of 0.25 μm, a relative density (R2) of 15.5 μm, a D / R2 ratio of 0.0161, and a surface area (S1) of 0.05 μm. 2 S2 is 0.05μm 2 S3 is 189μm 2 S2 / S3 is 0.03%, N is 1, and w1 is 48.59%; Preparation Example 5b was performed in accordance with Preparation Example 1, except that straw was used as the biomass carbon source, and the heat treatment time was 9 hours. The resulting silicon-carbon material was designated SC-5b, with a diameter (D) of 3.8 μm, a relative density (R²) of 15.5 μm, a D / R² ratio of 0.2452, and a Si content (S₁) of 11.4 μm. 2 S2 is 22.8 μm 2 S3 is 189μm 2 S2 / S3 is 12.06%, N is 2, and w1 is 48.57%.

[0116] Preparation Example 6: This group of preparation examples is used to verify the effect of changes in "w1 and w2", which is achieved by changing the deposition time, as follows: Preparation Example 6a was performed in accordance with Preparation Example 1, except that the deposition time was 2 hours. The resulting silicon-carbon material was designated SC-6a, and the w1 was 55.12%. Preparation Example 6b was carried out in accordance with Preparation Example 1, except that the deposition time was 6 hours. The resulting silicon-carbon material was denoted as SC-6b. In this case, w1 was 69.47%.

[0117] Preparation Example 6c was carried out in accordance with Preparation Example 1, except that the deposition time was 6 hours. The resulting silicon-carbon material was denoted as SC-6c. In this case, w1 was 30.34%.

[0118] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0119] Example 1: (1) Preparation of the positive electrode: Ternary materials, conductive carbon black (SP), carbon nanotubes (CNT), and polyvinylidene fluoride (PVDF) were mixed in a ratio of 95:2:1:2, and N-methylpyrrolidone (NMP) was added and stirred evenly to prepare a positive electrode slurry. The positive electrode slurry was coated onto both sides of an aluminum foil, baked, and rolled. Then, it was processed using a special roller with raised sections to obtain a positive electrode sheet with a concave section on one side and a convex section on the other. The ternary material consisted of polycrystalline and monocrystalline particles; at this point, w3 was 41%, w4 was 59%, w4 / w3 was 1.44, and c... 1 (Zr) was 1848 ppm, c 1 (Sr) was 680 ppm, R1 was 12.6 μm, and c 2 (Zr) is 0 ppm, c 2 (Sr) is 0 ppm, c 1 (Ni)<c 2 (Ni), the grain boundaries of the polycrystalline particles contain Zr and Sr, and the chemical formula of the polycrystalline particles is LiNi. 0.89818 Co 0.05836 Mn 0.03242 Zr 0.00193 Sr 0.00073 Al 0.00837 O2, the chemical formula of single crystal particles is LiNi 0.9049 Co 0.06428 Mn 0.01978 Al 0.011 O2, c(Sr) is 265ppm.

[0120] (2) Preparation of negative electrode: The first negative electrode active material was obtained by mixing silicon carbide material SC-1 and artificial graphite at a mass ratio of 21:79. The negative electrode active material was then mixed with conductive carbon black (SP), carbon nanotubes (CNT), sodium carboxymethyl cellulose (CMC-Li), styrene-butadiene rubber (SBR), and polyacrylic acid (PAA) at a ratio of 96:1:0.5:0.5:1:1, and deionized water was added to prepare the first negative electrode slurry. The first negative electrode slurry was coated on both sides of a carbon-coated copper foil. After baking and rolling, the negative electrode sheet was obtained. At this time, the average particle size of the artificial graphite was 15 μm, w2 was 25%, w1×w2 was 0.049, and D / c(Sr) was 0.00283.

[0121] (3) Diaphragm: The diaphragm uses polypropylene as the substrate (5μm), with a boehmite coating (1μm) on one side of the substrate and PVDF adhesive layers (2μm) on both sides of the diaphragm. The puncture strength is 354gf, and the tensile strength is 3320kgf / cm². 2 The elongation rate in the TD and MD directions is 72%, and the air permeability is 383s / cc.

[0122] (4) Preparation of lithium-ion secondary batteries: After the positive and negative electrode sheets are slit and fabricated, the positive electrode sheet, separator, and negative electrode sheet are stacked sequentially and wound to obtain a core. This core is then encapsulated in an aluminum-plastic film, baked, injected with electrolyte, formed, resealed, sorted, and subjected to OCV (Optical Characteristic Cell) processing to obtain a lithium-ion secondary battery. The aluminum layer in the aluminum-plastic film accounts for 36% of the battery's thickness. The separator extends beyond the negative electrode sheet, and the negative electrode sheet extends beyond the positive electrode sheet. The winding tail of the core has no empty foil area. The arc-shaped area has a first convex portion on the second surface and a first concave portion on the first surface. The flat area... The electrode has a second convex portion located on a first surface and a second concave portion located on a second surface. The depth of the first and second concave portions is 15 μm, the height of the first and second convex portions is 15 μm, the spacing between the first and second concave portions is 4.3 mm, the spacing between the first and second concave portions is 4.3 mm, the spacing between the first and second convex portions is 4.3 mm, and the projection area of ​​the concave portion on the surface of the positive electrode sheet accounts for 63% of the surface area of ​​one side of the positive electrode sheet. The ratio of the radius of the arc area to the thickness of the core is 0.5.

[0123] Example 2: (1) Preparation of the positive electrode: Ternary materials, SP, CNT, and PVDF were mixed in a ratio of 95:2:1:2, and NMP was added. The mixture was stirred until homogeneous to prepare a positive electrode slurry. This slurry was coated onto both sides of an aluminum foil. After baking and rolling, a special roller with raised sections was used for further processing, resulting in a positive electrode sheet with a concave portion on one side and a convex portion on the other. The ternary material consisted of polycrystalline and monocrystalline particles. At this point, w3 was 26%, w4 was 74%, w4 / w3 was 2.85, and c... 1 (Zr) was 310 ppm, c 1 (Sr) was 304 ppm, R1 was 3.2 μm, and c 2 (Zr) is 0 ppm, c 2 (Sr) is 0 ppm, c 1 (Ni)<c 2 (Ni), the grain boundaries of the polycrystalline particles contain Zr and Sr, and the chemical formula of the polycrystalline particles is LiNi. 0.89 Co 0.05848 Mn 0.03249Zr 0.00032 Sr 0.00033 Al 0.00839 O2, the chemical formula of single crystal particles is LiNi 0.90625 Co 0.06438 Mn 0.01981 Al 0.00997 O2c(Sr) was 75 ppm.

[0124] (2) Preparation of negative electrode: The first negative electrode active material was obtained by mixing silicon-carbon material SC-2 and artificial graphite at a mass ratio of 22:78. The first negative electrode active material was then mixed with SP, CNT, CMC-Li, SBR, and PAA at a ratio of 96:1:0.5:0.5:1:1, and deionized water was added to prepare the first negative electrode slurry. The first negative electrode slurry was coated on both sides of a carbon-coated copper foil. After baking and rolling, the negative electrode sheet was obtained. At this time, the average particle size of the artificial graphite was 15μm, w2 was 25%, w1×w2 was 0.048, and D / c(Sr) was 0.00413.

[0125] (3) Diaphragm: The diaphragm uses polypropylene as the substrate (5μm), with a boehmite coating (1μm) on one side of the substrate. PVDF adhesive layers (2μm) are applied to both sides of the diaphragm. The puncture strength is 280gf, and the tensile strength is 1210kgf / cm². 2 The elongation in the TD and MD directions is 53%, and the air permeability is 62s / cc.

[0126] (4) Preparation of lithium-ion secondary batteries: After the positive and negative electrode sheets are slit and fabricated, the positive electrode sheet, separator, and negative electrode sheet are stacked sequentially and wound to obtain a core. This core is then encapsulated in an aluminum-plastic film, baked, injected with electrolyte, formed, resealed, sorted, and subjected to OCV (Optical Characteristic Cell) processing to obtain a lithium-ion secondary battery. The aluminum layer in the aluminum-plastic film accounts for 33% of the battery's thickness. The separator extends beyond the negative electrode sheet, and the negative electrode sheet extends beyond the positive electrode sheet. The winding tail of the core has no empty foil area. The arc-shaped area has a first convex portion on the second surface and a first concave portion on the first surface. The flat area has… There is a second convex portion located on the first surface and a second concave portion located on the second surface. The depth of the first concave portion and the second concave portion is 10 μm, the height of the first convex portion and the second convex portion is 10 μm, the spacing between the first concave portions is 1.1 mm, the spacing between the second concave portions is 1.1 mm, the spacing between the first convex portions is 1.1 mm, and the spacing between the second convex portions is 1.1 mm. The projected area of ​​the concave portion region on the surface of the positive electrode sheet accounts for 88% of the surface area of ​​one side of the positive electrode sheet, and the ratio of the radius of the arc region to the thickness of the core is 0.45.

[0127] Example 3: (1) Preparation of the positive electrode: Ternary materials, SP, CNT, and PVDF were mixed in a ratio of 95:2:1:2, and NMP was added. The mixture was stirred until homogeneous to prepare a positive electrode slurry. This slurry was coated onto both sides of an aluminum foil. After baking and rolling, a special roller with raised sections was used for further processing, resulting in a positive electrode sheet with a concave portion on one side and a convex portion on the other. The ternary material consisted of polycrystalline and monocrystalline particles. At this point, w3 was 55%, w4 was 45%, w4 / w3 was 0.82, and c... 1 (Zr) was 4496 ppm, c 1 (Sr) was 1492 ppm, R1 was 19.5 μm, and c 2 (Zr) is 0 ppm, c 2 (Sr) is 0 ppm, c 1 (Ni)<c 2 (Ni), the grain boundaries of the polycrystalline particles contain Zr and Sr, and the chemical formula of the polycrystalline particles is LiNi. 0.89493 Co 0.05815 Mn 0.0323 Zr 0.00468 Sr 0.0016 Al 0.00834 O2, the chemical formula of single crystal particles is LiNi 0.9035 Co 0.06418 Mn 0.01975 Al 0.01257 O2, c(Sr) is 780ppm.

[0128] (2) Preparation of negative electrode: The first negative electrode active material was obtained by mixing silicon carbide material SC-3 and artificial graphite at a mass ratio of 21:79. The first negative electrode active material was then mixed with SP, CNT, CMC-Li, SBR, and PAA at a ratio of 96:1:0.5:0.5:1:1, and deionized water was added to prepare the first negative electrode slurry. The first negative electrode slurry was coated on both sides of a carbon-coated copper foil. After baking and rolling, the negative electrode sheet was obtained. At this time, the average particle size of the artificial graphite was 15 μm, w2 was 25%, w1×w2 was 0.049, and D / c(Sr) was 0.00124.

[0129] (3) Diaphragm: The diaphragm uses polypropylene as the substrate (5μm), with a boehmite coating (1μm) on one side of the substrate and PVDF adhesive layers (2μm) on both sides of the diaphragm. The puncture strength is 397gf, and the tensile strength is 3970kgf / cm². 2 The elongation rate in the TD and MD directions is 98%, and the air permeability is 487s / cc.

[0130] (4) Preparation of lithium-ion secondary batteries: After the positive and negative electrode sheets are slit and fabricated, the positive electrode sheet, separator, and negative electrode sheet are stacked sequentially and wound to obtain a core. This core is then encapsulated in an aluminum-plastic film, baked, injected with electrolyte, formed, resealed, sorted, and subjected to OCV (Optical Characteristic Cell) processing to obtain a lithium-ion secondary battery. The aluminum layer in the aluminum-plastic film accounts for 40% of the battery's thickness. The separator extends beyond the negative electrode sheet, and the negative electrode sheet extends beyond the positive electrode sheet. The winding tail of the core has no empty foil area. The arc-shaped area has a first convex portion on the second surface and a first concave portion on the first surface. The flat area... The electrode has a second convex portion located on a first surface and a second concave portion located on a second surface. The depth of the first and second concave portions is 35 μm, the height of the first and second convex portions is 35 μm, the spacing between the first and second concave portions is 5.7 mm, the spacing between the first and second concave portions is 5.7 mm, the spacing between the first and second convex portions is 5.7 mm, and the projection area of ​​the concave portion on the surface of the positive electrode sheet accounts for 41% of the surface area of ​​one side of the positive electrode sheet. The ratio of the radius of the arc area to the thickness of the core is 0.6.

[0131] Example 4: This set of examples is used to verify "c 1 The specific impact of the change in "(Sr)" is as follows: Example 4a, based on Example 1, except that c 1 The Sr content is 262 ppm, and the polycrystalline particles have the chemical formula LiNi. 0.89859 Co 0.05838 Mn 0.03244 Zr 0.00193 Sr 0.00028 Al 0.00838 O2, c(Sr) is 102ppm, D / c(Sr) is 0.00735; Example 4b, based on Example 1, except that c 1 (Sr) content is 1588 ppm, polycrystalline particles and chemical formula LiNi 0.8973 Co 0.0583 Mn 0.03239 Zr 0.00193 Sr 0.00171 Al 0.00836 O2, c(Sr) is 619ppm, D / c(Sr) is 0.0012.

[0132] Examples 5-7 were performed in accordance with Example 1, except that the silicon-carbon materials were different. The specific settings are shown in Table 1. In Examples 5a-5e, the D / c(Sr) ratios were 0.00045, 0.0189, 0.0003, 0.0196, 0.00113, and 0.0167, respectively. In Examples 6a and 6b, the D / c(Sr) ratios were 0.00094 and 0.01435, respectively. In Example 7a, the proportion of silicon-carbon material in the first negative electrode active material was 76%, w2 was 40%, and w1×w2 was 0.22. In Example 7b, the proportion of silicon-carbon material in the first negative electrode active material was 7%, w2 was 5%, and w1×w2 was 0.035. In Example 7c, the proportion of silicon-carbon material in the first negative electrode active material was 41%, w2 was 12%, and w1×w2 was 0.036.

[0133] Example 8: This set of examples is used to verify the impact of changes in "w3 and w4", as detailed below: Example 8a is based on Example 1, except that w3 is 12%, w4 is 88%, w4 / w3 is 7.33, c(Sr) is 78ppm, and D / c(Sr) is 0.00967. Example 8b is based on Example 1, except that w3 is 60%, w4 is 40%, w4 / w3 is 0.67, c(Sr) is 388ppm, and D / c(Sr) is 0.00193; Example 8c is based on Example 1, except that w3 is 100%, c(Sr) is 646ppm, and D / c(Sr) is 0.00116.

[0134] Example 9: Refer to Example 1, except that c 2 (Zr) was 2984 ppm, c 2 The Sr content is 1896 ppm, and the chemical formula of the single crystal particles is LiNi. 0.90265 Co 0.06412 Mn 0.01973 Zr 0.00311 Sr 0.00204 Al 0.00835 O2, c(Sr) is 1328ppm, D / c(Sr) is 0.00056.

[0135] Example 10: This set of examples is used to verify the effect of "the setting of concave and convex parts", as follows: Example 10a is based on Example 1, except that the first recess and the second recess are located on the first surface, and the first convex portion and the second convex portion are located on the second surface; Example 10b is based on Example 1, except that the first protrusion is located on the first surface, the first concave portion is located on the second surface, the second protrusion is located on the second surface, and the second concave portion is located on the first surface.

[0136] Example 11: This set of examples is used to verify the impact of changes in the "negative electrode coating method", as detailed below: This set of embodiments refers to Embodiment 1, except that the preparation method of the negative electrode sheet is different, as follows: Silicon carbon material SC-1 and first graphite (artificial graphite, average particle size T1) are mixed at a mass ratio of 54:46 to obtain the first negative electrode active material. The first negative electrode active material is mixed with SP, CMC-Li, and SBR at a ratio of 97:0.5:1.5:1, and deionized water is added to prepare the first negative electrode slurry. Second graphite (artificial graphite, average particle size T2) is mixed with SP, CNT, and CMC-Li... SBR and PAA are mixed in a ratio of 96:1:0.5:0.5:1:1, and deionized water is added to prepare a second negative electrode slurry. The second negative electrode slurry is coated on both sides of the carbon-coated copper foil and dried to obtain a second negative electrode active layer. Then, the first negative electrode slurry is coated on the surface of the first negative electrode active layer to obtain a first negative electrode active layer. After baking and rolling, a negative electrode sheet is obtained. After winding, a battery is obtained. The thickness of the first negative electrode active layer near the winding center is H1, and the thickness of the first negative electrode active layer away from the winding center is H2. Example 11a, wherein T1 is 20 μm, T2 is 5 μm, H1 is 30 μm, and H2 is 30 μm; Example 11b, wherein T1 is 20 μm, T2 is 5 μm, H1 is 15 μm, and H2 is 55 μm; Example 11c, wherein T1 is 20 μm, T2 is 5 μm, H1 is 55 μm, and H2 is 15 μm; Example 11d, wherein T1 is 5 μm, T2 is 20 μm, H1 is 15 μm, and H2 is 55 μm.

[0137] Comparative Example 1: This set of examples is used to verify "c 1 (Sr) and c 1 The specific impacts of the changes in "(Zr)" are as follows: Comparative Example 1a, based on Example 1, except that c 1 (Zr) is 0 ppm, polycrystalline particles and chemical formula LiNi 0.89992 Co 0.05847 Mn 0.03248 Sr 0.00074 Al 0.00839 O2; Comparative Example 1b, based on Example 1, except that c 1(Sr) is 0 ppm, polycrystalline particles and chemical formula LiNi 0.89994 Co 0.0584 Mn 0.03245 Zr 0.00193 Al 0.00838 O2; Comparative Example 1c, based on Example 1, except that c 1 (Zr) is 0 ppm, c 1 (Sr) is 0 ppm, polycrystalline particles and chemical formula LiNi 0.90058 Co 0.05851 Mn 0.03251 Al 0.00839 O2.

[0138] Comparative Example 2: Based on Example 1, except that the silicon carbide material was replaced with a non-porous silicon carbide material (commercially available) prepared from phenolic resin.

[0139] Comparative Example 3: This group of comparative examples is used to verify the effects of changes in "c(Sr) and / or D / c(Sr)", as detailed below: Comparative Example 1a is based on Example 3, except that the silicon-carbon material is replaced with an equal mass fraction of SC-4a, and the D / c(Sr) is 0.00015; Comparative Example 1b is based on Example 2, except that the silicon-carbon material is replaced with an equal mass fraction of SC-5b, and the D / c(Sr) is 0.03725; Comparative Example 1c is based on Example 2, except that w3 is 22%, w4 is 78%, c(Sr) is 64ppm, and D / c(Sr) is 0.00488. Comparative Example 1d is based on Example 4b, except that w3 is 100%, w4 is 0%, c(Sr) is 1509ppm, and D / c(Sr) is 0.0005.

[0140] Test example: (1) Battery gravimetric energy density: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were charged to the upper limit voltage of 4.53V at 0.2C (cut off at 0.02C) and then discharged to the lower limit voltage at 0.2C using a Xinwei battery tester. This process was repeated 3 times. The discharge capacity of the 3rd discharge was taken as the battery capacity Q. The weight of the battery was measured as W using an electronic balance. The gravimetric energy density was Q / W. The test results are recorded in Table 1.

[0141] (2) 45℃ Cyclic Expansion Rate: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were placed at 45℃ for 1 hour. The batteries were fully charged before cycling, and the battery thickness Z1 before cycling was measured. After 50 cycles, the batteries were fully charged, and the battery thickness Z2 was measured. The expansion rate is calculated as (Z2-Z1) / Z1. The cycling conditions are: 1C constant current charging cutoff voltage 4.53V, constant voltage charging cutoff current 0.2C, constant current 0.7C discharge, cutoff voltage 3V. The test results are recorded in Table 1.

[0142] (3) 45℃ cycle capacity retention rate: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were placed at 45℃ for 1 hour. They were first charged at 1C to the cutoff voltage of 4.53V, then charged at constant voltage to the cutoff current of 0.05C, and then discharged at 1C to 3V. The above charge and discharge steps were repeated 500 times. The discharge capacity of the 500th cycle was Q2, and the maximum discharge capacity in the first 3 cycles was Q1. The 45℃ cycle capacity retention rate was Q2 / Q1×100%. The test results are recorded in Table 1.

[0143] (4) Constant current charging ratio at 25℃: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were discharged to 3V at 0.2C in an environment of 25℃, and then charged to the upper limit cutoff voltage at 0.5C. The constant current charging capacity R1 was recorded. The constant voltage charging was continued to the cutoff current of 0.05C and the constant voltage charging capacity R2 was recorded. The constant current charging ratio = R1 / (R1+R2) and the test results are recorded in Table 1.

[0144] (5) 25℃ charging temperature rise: The lithium-ion secondary batteries prepared in the embodiments and comparative examples of the present invention were charged at 25℃ using a Xinwei battery tester at 1C to the upper limit voltage of 4.53V (cutoff at 0.02C). The charging time and charging temperature rise were recorded as batteries. The test results are recorded in Table 1.

[0145] Table 1: Note: " / " indicates that the silicon-carbon material obtained from the preparation example is not used.

[0146] As can be seen from Table 1, the lithium-ion secondary battery prepared in this invention maintains good cycle stability and rate performance while having high energy density compared to the comparative example.

[0147] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises a positive electrode sheet and a negative electrode sheet, the negative electrode sheet comprises a negative electrode current collector and a first negative electrode active layer located on at least one side surface of the negative electrode current collector, the first negative electrode active layer comprises a first negative electrode active material, the first negative electrode active material comprises a silicon-carbon material, and the silicon-carbon material has a plurality of pores in the cross section in the cross-sectional scanning electron microscope image of the negative electrode sheet. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one side surface of the positive electrode current collector, the positive electrode active layer comprises a positive electrode active material, the positive electrode active material comprises a ternary material, and the ternary material comprises polycrystalline particles. The chemical formula of the polycrystal particles is Li v1 Ni x1 Co y1 Mn z1 M 1 a1 M 2 b1 O2, wherein, 0.85≤v1≤1.1, 0.8≤x1≤0.98, 0 1 including elements Zr and Sr, M 2 including at least one of Al, Mg, Ti, Y, La, P, B, Nb, W, Sb and V; The size of the pore is D, D is 0.1 μm-5 μm; The mass content of element Sr is c(Sr) based on the total mass of the positive electrode active layer, c(Sr) is 70 ppm-1500 ppm; The D and c(Sr) satisfy: 0.0003≤D / c(Sr)≤0.02; wherein, the unit of D is μm; the unit of c(Sr) is ppm.

2. The lithium-ion secondary battery according to claim 1, wherein The mass content of the element Zr is c 1 (Zr), c 1 (Zr) is 300 ppm - 4500 ppm; And / or, the average particle size of the polycrystalline particles is R1, R1 is 3 μm-20 μm; And / or, the polycrystalline particles have grain boundaries, and the grain boundaries contain element Zr and / or Sr.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein D is 0.3 μm-1 μm; And / or, the average particle size of the silicon-carbon material is R2, R2 is 3 μm-17 μm; preferably 6 μm-13 μm; And / or, the single hole has a normal projection area S1 on the cross section of the silicon-carbon material, S1 is 0.01 μm 2 - 20 μm 2 ; And / or, the sum of the normal projection areas of the holes on the cross section of the silicon-carbon material is S2, S2 is 0.1 μm 2 - 25 μm 2 ; Preferably, D and R2 satisfy: 0.02≤D / R2≤0.3; Preferably, the cross-sectional area of the silicon-carbon material is S3, and the proportion of the sum of the areas of the pores based on the cross-sectional area of the silicon-carbon material is S2 / S3, S2 / S3 is 0.04%-11%; More preferably, the number of the pores on the cross section of the silicon-carbon material is N, N≤20, unit: pieces; further preferably 1-10 pieces.

4. The lithium-ion secondary battery according to claim 1, wherein The mass content of silicon element is w1 based on the total mass of the silicon-carbon material, w1 is 30%-70%; And / or, the mass content of silicon element is w2 based on the total mass of the first negative electrode active layer, w2 is 5%-40%; Preferably, w2 and w1 satisfy: 0.03≤w1×w2≤0.

3.

5. The lithium-ion secondary battery according to claim 1, wherein The ternary material further includes single-crystal particles having a chemical formula of Li v2 Ni x2 Co y2 Mn z2 M 3 a2 O2, wherein 0.85≤v2≤1.1, 0.8≤x2≤0.98, 0.<y2≤0.12, 0<z2≤0.1, 0.008≤a2≤0.03, and M 3 includes at least one of elements Al, Ti, Sr, Mg, Zr, Y, La, P, B, Nb, Sb, W, and V. And / or, in the volume particle size distribution curve of the ternary material, there are a first peak and a second peak, the first peak is located at 1 μm-5 μm, and the second peak is located at 3 μm-20 μm; Preferably, in the volume particle size distribution curve of the ternary material, the first peak is located at 1.2 μm-3 μm, and the second peak is located at 5.5 μm-10 μm.

6. The lithium-ion secondary battery according to claim 5, wherein The mass content of the element Zr is c 2 (Zr), c 2 (Zr) is 0 ppm to 3000 ppm; and / or the mass content of the element Sr, based on the total mass of the single-crystal particles, is c 2 (Sr), c 2 (Sr) is 0 ppm to 2000 ppm; Preferably, the mass content of the element Ni is c 1 (Ni), the mass content of the element Ni is c 2 (Ni), c 1 (Ni) ≤ c 2 (Ni).

7. The lithium-ion secondary battery according to claim 1, wherein The lithium ion secondary battery further comprises a separator, the positive electrode sheet, the separator and the negative electrode sheet are laminated and wound to form a roll core, the lithium ion secondary battery has a circular arc region and a flat region located between the circular arc regions, the positive electrode sheet located in the circular arc region has a first convex part and a first concave part, the positive electrode sheet located in the flat region has a second convex part and a second concave part, the positive electrode sheet comprises a first surface and a second surface, the first surface is close to the winding center, and the second surface is away from the winding center; Preferably, the first surface located in the circular arc region has a plurality of first concave parts, and the second surface located in the circular arc region has a plurality of first convex parts; Preferably, the first surface located in the flat region has a plurality of second convex parts, and the second surface located in the flat region has a plurality of second concave parts.

8. The lithium-ion secondary battery according to claim 7, wherein The depth of the first concave part is 10 μm-35 μm, and the depth of the second concave part is 10 μm-35 μm; And / or, the interval of the first concave part is 1 mm-6 mm, and the interval of the second concave part is 1 mm-6 mm; And / or, the first concave part and the second concave part form a concave part region, and the area of the orthographic projection of the concave part region on the surface of the positive electrode sheet accounts for 40%-90% of the area of the surface of the positive electrode sheet; And / or, the ratio of the radius of the circular arc region to the thickness of the roll core is 0.45-0.

6.

9. The lithium-ion secondary battery according to claim 1, wherein, The positive electrode sheet, the separator, and the negative electrode sheet are laminated and wound to form a roll core, and along the winding direction of the roll core, at the winding tail of the roll core, the separator extends beyond the negative electrode sheet, and the negative electrode sheet extends beyond the positive electrode sheet; And / or, the puncture strength of the separator is 280 gf-400 gf; and / or the tensile strength of the diaphragm is 1200 kgf / cm 2 - 4000 kgf / cm 2 ; And / or, the elongation of the separator in the TD direction and the MD direction is 40%-100%; And / or, the air permeability of the separator is 60 s / cc-500 s / cc.

10. The lithium-ion secondary battery according to claim 1, wherein, The negative electrode sheet further comprises a second negative electrode active layer between the first negative electrode active layer and the negative electrode current collector; And / or, the first negative electrode active material comprises the silicon-carbon material, the second negative electrode active layer comprises a second negative electrode active material, and the second negative electrode active material comprises a second graphite; the average particle size of the second graphite is T2, and 5 μm≤T2≤20 μm is satisfied; Preferably, the first negative electrode active layer further comprises a first graphite, and the average particle size of the first graphite is T1, and TI and T2 satisfy 5 μm≤T2≤T1≤20 μm; More preferably, the thickness of the first negative electrode active layer located on one side surface of the negative electrode current collector is H1, the thickness of the first negative electrode active layer located on the other side surface opposite to the negative electrode current collector is H2, and 10 μm≤H1<H2≤65 μm is satisfied; Further preferably, the thickness of the first negative electrode active layer of the negative electrode sheet close to the winding center is H1, and the thickness of the first negative electrode active layer of the negative electrode sheet away from the winding center is H2.

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