Hard carbon material, negative pole piece, secondary battery and electric device

Abstract

The invention provides a hard carbon material, a negative pole piece, a secondary battery and an electric device, and relates to the field of electrochemical energy storage. The hard carbon material comprises particles with columnar morphology, and the columnar body comprises at least one of a cylinder, a waist cylinder and a polygon; the polygon comprises at least one of a triangular prism-like prism, a quadrangular prism-like prism and a pentagonal prism-like prism; when the hard carbon material is applied to the secondary battery, the hard carbon material has different small-angle X-ray diffraction characteristics in different states of charge, and scattering intensity convex peaks are observed under 0% SOC and when a scattering vector is in a range of 0.1 nm <-1 > to 7 nm <-1 >; under 100% SOC, when a scattering vector is in a range of 0.1 nm <-1 > to 7 nm <-1 >, no scattering intensity convex peak is observed; according to the hard carbon material disclosed by the invention, the compaction density of the negative electrode active material layer can be improved by optimizing the morphology, so that the energy density, the cycle capacity retention rate and the rate capability of the secondary battery are improved.

Description

Technical Field

[0001] This application relates to the field of electrochemical energy storage, specifically to a hard carbon material, a negative electrode sheet, a secondary battery, and an electrical device. Background Technology

[0002] As a clean energy source, the application of secondary batteries has gradually expanded from portable electronic products to large electrical devices such as electric vehicles. Based on this trend, there is an urgent need to further improve the energy density of secondary batteries.

[0003] Graphite is currently the most commercially available and widely used anode material. However, the specific capacity of graphite materials currently widely used as anodes in secondary batteries is approaching its theoretical limit, making it difficult to achieve further breakthroughs in energy density for secondary batteries using graphite as the anode. Summary of the Invention

[0004] In view of the shortcomings of the above-mentioned related technologies, the purpose of this application is to provide a hard carbon material, a negative electrode sheet, a secondary battery and an electrical device to overcome the technical problem of the difficulty in improving the energy density of secondary batteries in the above-mentioned related technologies, thereby obtaining a secondary battery with high energy density and excellent cycle capacity retention and rate performance.

[0005] To achieve the above and other related objectives, a first aspect of this application provides a hard carbon material comprising particles having a columnar morphology, wherein the columnar morphology includes at least one of a cylinder, an oval cylinder, and a polygon; and the polygon includes at least one of a triangular prism-like structure, a square prism-like structure, and a pentagonal prism-like structure.

[0006] When the hard carbon material is applied to a secondary battery, it exhibits different small-angle X-ray diffraction characteristics under different states of charge, wherein: At 0% SOC, the scattering vector is at 0.1 nm. -1 ~7 nm -1 Within this range, a peak in scattering intensity was observed; At 100% SOC, the scattering vector is 0.1 nm. -1 ~7 nm -1 No scattering intensity peak was observed within the specified range.

[0007] In some embodiments of the first aspect of this application, the hard carbon material, by employing particles with a columnar morphology, can improve its morphological regularity, making the particle-to-particle contact mainly surface-to-surface, effectively increasing the true density, thereby increasing the compaction density of the negative electrode active material layer, while reducing the porosity of the negative electrode active material layer, reducing the amount of electrolyte adsorption, and the hard carbon material exhibits a disordered carbon structure and also has a high specific capacity; small-angle X-ray diffraction characteristics (scattering peak at 0% SOC, no peak at 100% SOC) confirm that the pores are filled with lithium during lithium storage, which is beneficial to improving the lithium storage capacity; therefore, when the above-mentioned hard carbon material is applied to a secondary battery, it is beneficial to improve the energy density, cycle capacity retention rate and rate performance of the secondary battery.

[0008] Based on the first aspect, in some possible implementations, the edges of the polygon are rounded.

[0009] Based on the first aspect, in some possible implementations, the edges of the polygon are rounded.

[0010] Based on the first aspect, in some possible implementations, the hard carbon material satisfies at least one of the following conditions: (1) The edges of the polygon are rounded. (2) The edges of the polygon are rounded.

[0011] Based on the first aspect, in some possible embodiments, the X-ray diffraction pattern of the hard carbon material has broad diffraction peaks between 18° and 30°, and the full width at half maximum (FWHM) of the broad diffraction peaks is between 4° and 12°.

[0012] Based on the first aspect, in some possible embodiments, the pore volume of the hard carbon material is 0.1 cm³. 3 / g to 0.9cm 3 / g.

[0013] Based on the first aspect, in some possible embodiments, the pore volume of the hard carbon material is 0.31 cm³. 3 / g to 0.45cm 3 / g.

[0014] Based on the first aspect, in some possible implementations, the Dv50 of the hard carbon material is 3 μm to 10 μm.

[0015] Based on the first aspect, in some possible implementations, the Dv50 of the hard carbon material is 5 μm.

[0016] Based on the first aspect, in some possible implementations, the true density of the hard carbon material is from 1.01 g / cc to 1.67 g / cc.

[0017] Based on the first aspect, in some possible implementations, the true density of the hard carbon material is from 1.20 g / cc to 1.34 g / cc.

[0018] Based on the first aspect, in some possible embodiments, the specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g.

[0019] Based on the first aspect, in some possible embodiments, the specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 5m 2 / g.

[0020] Based on the first aspect, in some possible implementations, the hard carbon material satisfies at least one of the following characteristics: (1) The pore volume of the hard carbon material is 0.1 cm³. 3 / g to 0.9cm 3 / g; Preferably, the pore volume of the hard carbon material is 0.31 cm³. 3 / g to 0.45cm 3 / g.

[0021] (2) The Dv50 of the hard carbon material is 3 μm to 10 μm; Preferably, the Dv50 of the hard carbon material is 5 μm; (3) The true density of the hard carbon material is 1.01 g / cc to 1.67 g / cc; Preferably, the true density of the hard carbon material is between 1.20 g / cc and 1.34 g / cc. (4) The specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g; Preferably, the specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 5m 2 / g.

[0022] A second aspect of this application also provides a negative electrode sheet, comprising a negative current collector and a negative active material layer located on at least one surface of the negative current collector, the negative active material layer comprising a negative active material; the negative active material comprising the hard carbon material described in the first aspect; and the mass content of the hard carbon material is from 1.0% to 100% based on the mass of the negative active material.

[0023] In some embodiments of the second aspect of this application, the negative electrode sheet can balance high specific capacity and high true density, which is beneficial to improving the energy density, cycle capacity retention rate and rate performance of the secondary battery.

[0024] Based on the second aspect, in some possible implementations, the compaction density of the negative electrode active material layer is 0.9 g / cm³. 3 Up to 1.72 g / cm 3 .

[0025] Based on the second aspect, in some possible embodiments, the compaction density of the negative electrode active material layer is 1.13 g / cm³. 3 Up to 1.40 g / cm 3 .

[0026] Based on the second aspect, in some possible implementations, the porosity of the negative electrode active material layer is 10% to 25%.

[0027] Based on the second aspect, in some possible implementations, the negative electrode active material layer satisfies at least one of the following characteristics: (1) The compaction density of the negative electrode active material layer is 0.9 g / cm³. 3 Up to 1.72 g / cm 3 ; Preferably, the compaction density of the negative electrode active material layer is 1.13 g / cm³. 3 Up to 1.40 g / cm 3 ; (2) The porosity of the negative electrode active material layer is between 10% and 25%.

[0028] A third aspect of this application provides a secondary battery, including the negative electrode sheet described in the second aspect.

[0029] In some embodiments of the third aspect of this application, the secondary battery has high energy density, excellent cycle performance, and good rate performance.

[0030] Based on the third aspect, in some possible implementations, the secondary battery further includes a positive electrode sheet, the positive electrode sheet including a positive current collector and a positive active material layer located on at least one surface of the positive current collector, the positive active material layer including a positive active material; The positive electrode active material includes lithium transition metal oxides, which include at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, and lithium titanate.

[0031] A fourth aspect of this application provides an electrical device comprising the secondary battery described in the third aspect.

[0032] In some embodiments of the fourth aspect of this application, the electrical device has high endurance and long service life.

[0033] Other features and advantages of this application will be set forth in the following description and will be apparent in part from the description or may be learned by practicing the application. The purposes and other advantages of this application may be realized and obtained by means of the structures particularly pointed out in the written description, claims, and drawings. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not intended to limit this disclosure. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 These are small-angle X-ray scattering characteristic images of the hard carbon material prepared in Example 4 of this application under different charge states; Figure 2 A schematic diagram illustrating the preparation of the irregular fiber precursor provided in this application; Figure 3 This is a schematic diagram of the structure of the quadrilateral-shaped spinneret described in this application; Figure 4 This is a schematic diagram of the structure of the quadrilateral-shaped spinneret described in this application. Detailed Implementation

[0036] The following specific embodiments illustrate the implementation of this application. Those skilled in the art can easily understand other advantages and effects of this application from the content disclosed in this specification.

[0037] The present application will now be described in further detail with reference to specific embodiments. The technical solutions in the embodiments of the present application are clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, and not all of them. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present application.

[0038] The terms "first" and "second" in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, rather than to describe a specific order.

[0039] For the sake of brevity, this article only discloses a few specific numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0040] In this description, unless otherwise stated, "above" and "below" include the stated number.

[0041] Unless otherwise stated, the terms used in this application have their common meanings as commonly understood by those skilled in the art. Unless otherwise stated, the values ​​of the parameters mentioned in this application can be measured using various measurement methods commonly used in the art (e.g., they can be tested according to the methods given in the embodiments of this application).

[0042] The list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms can mean any combination of the listed items. For example, if items A and B are listed, then the phrase "at least one of A and B" means only A; only B; or A and B. In another instance, if items A, B, and C are listed, then the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may contain a single component or multiple components. Item B may contain a single component or multiple components. Item C may contain a single component or multiple components.

[0043] As described in the background section, secondary batteries using graphite as the negative electrode have limited potential for further breakthroughs in energy density. Furthermore, disordered carbon materials, such as hard carbon particles, often exhibit amorphous or near-spherical morphologies, and their uneven surfaces result in a true density far lower than that of graphite. Although hard carbon negative electrode materials have a higher specific capacity, their volumetric capacity is often lower than that of graphite. For the negative electrode active material layer prepared from hard carbon negative electrode materials, the high rigidity of the particles makes it difficult to compact the layer, resulting in point-to-point contact between most particles and a high contact resistance. Additionally, the low true density leads to high porosity between hard carbon particles, causing more electrolyte to be adsorbed between the pores, thus increasing costs.

[0044] In view of this, this application provides a hard carbon material, a secondary battery, and an electrical device. By using hard carbon material comprising particles with columnar morphology and by making hard carbon material a disordered carbon material, the technical problem of difficulty in improving the energy density of secondary batteries in the above-mentioned related technologies is overcome. The secondary battery of this application has high energy density and excellent cycle capacity retention and rate performance.

[0045] In a first aspect, this application provides a hard carbon material, which is a disordered carbon material. The hard carbon material includes particles with a columnar morphology, wherein the columnar particles include at least one of cylinders, oval cylinders, and polygons; and the polygons include at least one of triangular prism-like, quadrangular prism-like, and pentagonal prism-like. When the hard carbon material is applied to a secondary battery, it exhibits different small-angle X-ray diffraction characteristics under different states of charge, wherein: At 0% SOC, the scattering vector is at 0.1 nm. -1 ~7 nm -1 Within this range, a peak in scattering intensity was observed; At 100% SOC, the scattering vector is 0.1 nm. -1 ~7 nm -1 No scattering intensity peak was observed within the specified range.

[0046] When hard carbon materials have columnar morphology particles, their regular shape and strong structural consistency result in high regularity and uniformity. This columnar morphology facilitates ordered arrangement of hard carbon materials during electrode fabrication, with surface contact rather than point contact between particles, significantly enhancing the physical contact area and improving the continuity of the conductive network. Hard carbon materials also possess high true density, thereby increasing the compaction density of the negative electrode active material layer. Simultaneously, the columnar morphology reduces the porosity of the negative electrode, decreasing electrolyte adsorption and side reactions, thus improving ion transport efficiency and reducing internal resistance. Small-angle X-ray diffraction (scattering peaks at 0% SOC, no peaks at 100% SOC) confirms that the pores are filled with lithium during lithium storage, which is beneficial for increasing lithium storage capacity.

[0047] Therefore, when the aforementioned hard carbon materials are applied to secondary batteries, they can significantly improve the energy density, cycle capacity retention, and rate performance of the secondary batteries.

[0048] Furthermore, columnar morphology, such as cylinders, has continuous curved surface symmetry; while cylindrical and polygonal morphologies form regular shapes through the orderly combination of edges and planes. Therefore, columnar morphology particles avoid the disordered arrangement caused by the uneven surface of irregular particles, and can achieve close arrangement through surface contact (rather than point-to-point contact for irregular particles) when stacked, significantly reducing ineffective gaps between particles.

[0049] Understandably, a cylinder refers to a columnar geometric solid with two parallel and congruent circular bases, smooth curved sides, and an overall regular rotationally symmetrical structure.

[0050] Understandably, the oval cylinder is a special type of column with a shape similar to a racetrack on its upper and lower bases. That is, the middle part is a square structure and the two ends are semi-circular structures. It can be prepared by a spinneret with specific orifice holes (such as multiple orifices distributed in a straight line).

[0051] Understandably, a polygon refers to a prism-like structure whose top and bottom faces are polygonal in shape, including triangular prisms, quadrangular prisms, and pentagonal prisms. For example, a spinneret with triangular orifices can be used to produce a triangular prism, and a spinneret with quadrangular orifices can be used to produce a quadrangular prism. For instance, using rounded edges or corners of a polygon can effectively disperse stress concentration points under structural stress, preventing fractures, deformations, or wear caused by excessive local stress, and significantly improving the mechanical stability and durability of the polygon. Furthermore, polygonal structures with rounded transitions are more densely and uniformly packed, which is beneficial for increasing the compaction density of the negative electrode active material.

[0052] Furthermore, the hard carbon material contains pores, which possess lithium storage activity, enhancing the lithium storage capacity of the hard carbon material and thus improving the energy density of the secondary battery. These pores can be distinguished by small-angle X-ray diffraction (SAXD) tests due to differences in electron cloud distribution. In one embodiment, the hard carbon material, when applied to a secondary battery, exhibits different SAXD characteristics at different states of charge; for example, at 0% SOC, the scattering vector is at 0.1 nm. -1 ~7 nm -1 Within this range, a scattering intensity peak was observed, with the scattering vector at 0.1 nm observed at 100% SOC of the secondary battery. -1 ~7 nm -1 Within the specified range, there is no scattering intensity peak. At 0% SOC, the negative electrode active material of the secondary battery is in a lithium-deficient state. The hard carbon active material contains pores, and the local electron cloud concentration in the porous region is low, manifested as a scattering intensity peak in small-angle X-ray diffraction (SAXD). The higher the peak height, the higher the porosity. At 100% SOC, the negative electrode active material is in a lithium-rich state, with lithium stored in lithium clusters within the pores of the hard carbon active material. This leads to an increase in electron cloud concentration in this region, manifested as the disappearance of the scattering intensity peak in SAXDD.

[0053] In some embodiments, the hard carbon material exhibits a disordered carbon structure with high specific capacity. Its disordered microcrystalline structure and abundant nanopores provide more lithium storage sites, thereby helping to improve the energy density, cycle capacity retention and rate performance of the secondary battery.

[0054] Understandably, hard carbon materials exhibit a disordered carbon structure, specifically manifested as amorphous, randomly layered graphite: sp 2 Hybrid carbon networks exhibit disordered stacking, uneven interlayer spacing, and numerous micropores, vacancies, and edge defects. In some embodiments, the disordered carbon structure of hard carbon materials originates from the incomplete graphitization of carbon chains during the high-temperature carbonization of precursors (such as spinning resins), thus preserving the disordered arrangement. This disordered carbon structure provides abundant lithium intercalation sites (interlayers and defects), micropores accommodate more lithium storage, and defects accelerate ion transport; therefore, hard carbon materials possess high specific capacity.

[0055] In this application, the hard carbon material uses particles with a columnar morphology. There are no particular limitations on the preparation method of these columnar particles, as long as the purpose of this application can be achieved. In some embodiments, the hard carbon material is prepared by the following method: like Figure 2 As shown, a spinning process is employed, using spinnerets with different orifice shapes to produce fibers of different cross-sectional shapes. Further carbonization, granulation, and coating of these fibers yields columnar hard carbon materials. For example, a cylindrical fiber material can be obtained using a single circular orifice spinneret; a cylindrical hard carbon material can be obtained using two, three, or four or more linearly distributed orifices; a triangular prism-shaped hard carbon material can be obtained using multiple orifices with a triangular orifice distribution; and a quadrilateral orifice distribution, or similar, can also be obtained using multiple orifices with a quadrilateral orifice distribution. Figure 3 The spinneret with a square-shaped orifice shown, or as... Figure 4 The spinnerets with quadrilateral-shaped orifices shown can all produce hard carbon materials with a quadrangular prism morphology; by using multiple spinnerets with a pentagonal orifice distribution, hard carbon materials with a pentagonal prism morphology can be obtained. The triangular orifice shape refers to the rounded transitions at the angles and sides of the triangle, and the same applies to other polygonal orifices. The orifice shape or distribution of the aforementioned spinnerets can be quasi-triangular, quadrilateral, pentagonal, or hexagonal, or it can be triangular, quadrilateral, rectangular, or pentagonal, resulting in columnar materials with prism morphologies such as regular triangular prisms, regular quadrilateral prisms, regular pentagonal prisms, triangular prisms, quadrilateral prisms, and pentagonal prisms.

[0056] Furthermore, the polygonal fiber material prepared by the above-mentioned spinneret orifice has rounded edges and corners. In the subsequent preparation of hard carbon material, the rounded edges and corners help to alleviate the pulverization caused by stress concentration during continuous charging and discharging, thereby improving the cycle capacity retention rate of the secondary battery.

[0057] Furthermore, the spinning resin is selected from asphalt, phenolic resin, polyacrylonitrile, melamine resin, resorcinol-formaldehyde resin, benzoxazine resin, polyarylacetylene resin, furan resin, polyimide, polyoxydiazole, etc. Of course, the spinning process needs to be selected according to different precursors, and subsequent processing also varies depending on the precursor.

[0058] Furthermore, by controlling the pump feed rate, drawing speed, spinneret orifice size and distribution, and melt viscosity, the diameter of the nascent fiber is adjusted, thereby adjusting the cross-sectional dimensions of the columnar hard carbon material. Specifically, the pump feed rate is the amount of polymer melt extruded from the metering pump per unit time; a larger feed rate results in coarser fibers. The drawing speed is the speed at which the winding roller pulls the fiber; a faster speed results in a larger draw ratio, causing the fiber to be stretched into a finer shape with a smaller diameter, while also increasing molecular orientation and strength. The initial size of the spinneret orifice determines the initial diameter of the melt stream and is a fundamental factor. Melt viscosity is affected by the polymer type and temperature; higher viscosity makes it less likely to become finer under the same traction force.

[0059] In some embodiments, the diameter of the nascent fiber is adjusted by the aforementioned influencing factors to further control the particle size in subsequent granulation processes. In some embodiments, the Dv50 of the hard carbon material is 3 μm to 10 μm, preferably 5 μm. For example, the Dv50 of the hard carbon material can be 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, or a range consisting of any two of these values. By controlling the Dv50 of the hard carbon material within the above range, the lithium-ion diffusion path can be effectively shortened, which helps to improve the true density of the hard carbon material and improve its processing performance, thereby enhancing the energy density, cycle capacity retention, and rate performance of the secondary battery.

[0060] In some embodiments, the X-ray diffraction pattern of the hard carbon material exhibits a broad diffraction peak between 18° and 30°, with a full width at half maximum (FWHM) of 4° to 12°. This indicates that the 002 interplanar spacing of the hard carbon material is between 0.37 nm and 0.41 nm, which is much larger than that of graphite. The larger 002 interplanar spacing facilitates the rapid insertion and extraction of lithium ions within the hard carbon material, effectively improving the reversible capacity and kinetic performance of the negative electrode active material, and helping to optimize battery charge-discharge efficiency and cycle stability.

[0061] In some embodiments, the pore volume of the hard carbon material is 0.1 cm³. 3 / g to 0.9cm 3 / g, preferably, the pore volume is 0.31 cm³. 3 / g to 0.45cm 3 / g, for example, the pore volume of hard carbon material can be 0.1cm³. 3 / g, 0.2cm 3 / g, 0.30cm3 / g, 0.31cm 3 / g, 0.32cm 3 / g, 0.33cm 3 / g, 0.34cm 3 / g, 0.35cm 3 / g, 0.4cm 3 / g, 0.45cm 3 / g, 0.5cm 3 / g, 0.6cm 3 / g, 0.7cm 3 / g, 0.8cm 3 / g, 0.9cm 3 / g or a range consisting of any two of these values; the internal pores of hard carbon materials possess lithium storage activity, providing more lithium storage sites and increasing lithium storage capacity. When the pore volume of hard carbon materials is within the above range, the utilization rate of pore lithium storage can be improved. Combined with the improved true density due to regular morphology, the energy density of secondary batteries can be effectively increased.

[0062] Understandably, hard carbon materials have a dense coating layer on their surface, making it difficult to directly measure the internal pore characteristics of the material using conventional adsorption-desorption tests. For example, this application can use a grinding mill to break up the hard carbon material, thereby destroying the coating layer, and then obtain the internal pore characteristics of the material through carbon dioxide adsorption-desorption tests.

[0063] In one embodiment, the true density of the hard carbon material is from 1.01 g / cc to 1.67 g / cc. For example, the true density of the hard carbon material can be 1.01 g / cc, 1.1 g / cc, 1.2 g / cc, 1.30 g / cc, 1.32 g / cc, 1.34 g / cc, 1.4 g / cc, 1.5 g / cc, 1.67 g / cc, or a range of any two of these values. In some embodiments, the true density of the hard carbon material is from 1.20 g / cc to 1.34 g / cc. When the true density is controlled within the above range, the surface of the hard carbon material contains a large number of closed pores with lithium storage activity, which can improve the lithium storage capacity and thus improve the energy density of the secondary battery.

[0064] In one embodiment, the specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g, preferably, the specific surface area of ​​the hard carbon material is 0.5m². 2 / g to 5m 2 / g; for example, the specific surface area of ​​hard carbon materials can be 0.5m². 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g, 0.9m 2 / g, 1.0m 2 / g、1m 2 / g、2m 2 / g、3m 2 / g、4m 2 / g、5m 2 / g, 10m 2 / g, 15m 2 / g、20m 2 / g、25m 2 / g、30m 2 / g、35m 2 / g、40m 2 / g、45m 2 / g, 50m 2 / g or a range consisting of any two of these values. When the specific surface area is controlled within the above range, hard carbon materials can balance the reactivity and ion loss of the material surface, reduce the area of ​​SEI film formation during the first charge, and thus reduce the loss of active ions. At the same time, it can avoid the increase of side reactions caused by excessive specific surface area, and improve the first efficiency, energy density, cycle capacity retention rate and rate performance of the secondary battery.

[0065] Negative electrode sheet In a second aspect, this application provides a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, wherein the negative electrode active material layer comprises the negative electrode material described in the first aspect; the negative electrode active material layer may be disposed on one surface in the thickness direction of the negative electrode current collector, or it may be disposed on two surfaces in the thickness direction.

[0066] Based on the mass of the negative electrode active material layer, the mass content of the hard carbon material is from 1.0% to 100%. For example, the mass content of the hard carbon material can be 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 100%, or a range of any two of these values. Controlling the mass content of the hard carbon material within the above range helps to balance the relationship between high specific capacity and high true density, optimizes the compaction density and porosity of the negative electrode active material layer, thereby improving the energy density, cycle capacity retention, and rate performance of the secondary battery.

[0067] In some embodiments, the compaction density of the negative electrode active material layer is 0.9 g / cm³. 3 Up to 1.72 g / cm 3 Preferably, the compaction density of the negative electrode active material layer is 1.13 g / cm³. 3Up to 1.40 g / cm 3 For example, the compaction density of the negative electrode active material layer can be 0.9 g / cm³. 3 1.0g / cm 3 1.11 g / cm 3 1.13 g / cm 3 1.2g / cm 3 1.3g / cm 3 1.4g / cm 3 1.5g / cm 3 1.6g / cm 3 1.65g / cm 3 1.7g / cm 3 1.72g / cm 3 Or it can be a range consisting of any two of these values. When the compaction density is controlled within the above range, the negative electrode active material can obtain a higher compaction density, which helps to reduce porosity and electrolyte adsorption, thereby increasing the energy density of the secondary battery, while maintaining an effective ion transport channel, and thus improving cycle capacity retention and rate performance.

[0068] In some embodiments, the porosity of the negative electrode active material layer is 10% to 25%, for example, the porosity of the negative electrode active material layer can be 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or a range consisting of any two of these values. Adjusting the porosity to the above range can, on the one hand, maintain a low porosity of the negative electrode active material layer, thereby reducing the amount of electrolyte adsorbed and helping to improve the energy density of the secondary battery; on the other hand, this porosity range can also ensure sufficient electrolyte penetration and effective construction of ion transport channels, thereby optimizing the cycle capacity retention and rate performance of the secondary battery.

[0069] In one embodiment, the negative electrode active material layer may further include a thickener, which may include, but is not limited to, sodium carboxymethyl cellulose.

[0070] In addition to hard carbon material, the surface of the negative electrode current collector may also include other additional functional layers. In one embodiment, a conductive coating is also included.

[0071] The conductive coating can be located between the surface of the negative electrode active material layer and the surface of the negative electrode current collector.

[0072] The conductive coating includes a conductive agent and a negative electrode binder.

[0073] For example, conductive agents may include, but are not limited to, carbon materials, metals, or conductive polymers. Carbon materials may include at least one of natural graphite, artificial graphite, superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers. Metals may include metal powders or fibers of copper, iron, aluminum, etc. Conductive polymers may include polyphenylene derivatives. Negative electrode binders may include, but are not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0074] In one embodiment, the negative electrode may further include a protective layer located on the air-facing side surface of the negative electrode active material layer.

[0075] The embodiments of this application do not impose any particular limitations on the negative electrode current collector, as long as it can achieve the purpose of this application. For example, the negative electrode current collector may include metal foil, porous metal plate, or composite current collector.

[0076] The metal foil can be copper foil, copper alloy foil, nickel foil, stainless steel foil, or titanium foil. The porous metal plate can be nickel foam or copper foam, etc. The composite current collector can include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector can be obtained by forming a metal material (such as copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0077] The thickness of the negative electrode current collector can be, for example, 4 μm to 12 μm, and the thickness of the single-sided negative electrode active material layer can be 30 μm to 200 μm.

[0078] It should be noted that the aforementioned "surface" can be the entire area of ​​the negative electrode current collector or only a part of it. This application does not have any particular restrictions, as long as the purpose of this application can be achieved.

[0079] Secondary batteries Thirdly, this application provides a secondary battery. The secondary battery includes a negative electrode and a positive electrode, as described in the second aspect. The positive electrode includes a positive current collector and a positive active material layer located on at least one surface of the positive current collector, the positive active material layer comprising a positive active material; The positive current collector can be, for example, a metal foil or a composite current collector. The metal foil can be, for example, an aluminum foil.

[0080] The composite current collector may include a polymeric material base layer and a metallic material layer located on at least one surface of the polymeric material base layer. For example, the material of the metallic material layer may include at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy. The polymeric material base layer may include at least one of polypropylene, polyethylene terephthalate, polybutylene terephthalate, polystyrene, or polyethylene.

[0081] In some embodiments, the positive electrode active material comprises a lithium transition metal oxide selected from at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, and lithium titanate. This positive electrode active material (e.g., lithium nickel cobalt manganese oxide, lithium iron phosphate, etc.) can provide high-capacity lithium ions and synergizes with the excellent lithium storage capacity of hard carbon materials, thereby improving the energy density of the battery. The positive electrode has stable structural characteristics, which, combined with the low expansion characteristics of the negative electrode, can effectively suppress structural damage during charging and discharging, extending its cycle life. Furthermore, the high ion conductivity of the positive electrode combined with the excellent electronic conductivity of the negative electrode helps to accelerate the migration rate of lithium ions and improve rate performance. Therefore, the positive electrode and the negative electrode in the second aspect synergistically improve the energy density, cycle performance, and rate performance of the secondary battery.

[0082] In one embodiment, the positive electrode active material layer may further comprise a conductive agent and a positive electrode binder. The conductive agent may be the same type as that in the negative electrode active material layer described above. The positive electrode binder may be the same type as that in the negative electrode active material layer described above.

[0083] In one embodiment, the positive electrode sheet may further include a conductive coating disposed between the surface of the positive current collector and the positive active material layer. This conductive coating may, for example, consist of a conductive agent and a binder.

[0084] In one embodiment, the positive electrode sheet may further include a protective layer covering the surface of the positive electrode active material layer.

[0085] In one embodiment, the secondary battery is a lithium-ion battery, and the positive electrode active material may include lithium transition metal oxide, which may include, but is not limited to, lithium nickel cobalt manganese oxide (LiNi). 0.90 Co 0.05 Mn 0.05 O2, LiNi 0.80 Co 0.10 Mn 0.10 O2, LiNi 0.60 Co 0.20 Mn 0.20 O2, LiNi 0.50 Co0.20 Mn 0.30 O2, LiNi 0.33 Co 0.33 Mn 0.33 At least one of the following: O2), lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, or lithium titanate. This is beneficial for improving the energy density and cycle stability of secondary batteries, broadening the voltage plateau, enhancing ion conductivity, and improving rate performance.

[0086] Accordingly, the electrolyte salt in the electrolyte may include lithium salts. Lithium salts may include, but are not limited to, at least one of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalate borate (LiODFB), lithium dioxalate borate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorodioxalate phosphate (LiDFOP), and lithium tetrafluorooxalate phosphate (LiTFOP).

[0087] In one embodiment, the secondary battery is a sodium-ion battery, and the positive electrode active material may include at least one of sodium transition metal oxides, polyanionic compounds, and Prussian blue compounds. 1-x Cu h Fe k Mn l M 1 m O 2-y Na 0.67 Mn 0.7 Ni z M 2 0.3-z O2, Na a Li b Ni c Mn d Fe e O2, where M 1 It is at least one of Li, Be, B, Mg, Al, K, Ca, Ti, Co, Ni, Zn, Ga, Sr, Y, Nb, Mo, In, Sn, or Ba, 0 < x ≤ 0.33, 0 < h ≤ 0.24, 0 ≤ k ≤ 0.32, 0 < l ≤ 0.68, 0 ≤ m < 0.1, h + k + l + m = 1, 0 ≤ y < 0.2; M 2It is at least one of Li, Mg, Al, Ca, Ti, Fe, Cu, Zn, or Ba, where 0 < z ≤ 0.1; 0.67 < a ≤ 1, 0 < b < 0.2, 0 < c < 0.3, 0.67 < d + e < 0.8, and b + c + d + e = 1. Polyanionic compounds may include, but are not limited to: A 1 f M 3 g (PO4) i O j X 1 3-j Na n M 4 PO4X 2 Na p M 5 q (SO4)3, Na s Mn t Fe 3-t (PO4)2(P2O7), where A 1 M is at least one of H, Li, Na, K or NH4. 3 X is at least one of Ti, Cr, Mn, Fe, Co, Ni, V, Cu, or Zn. 1 It is at least one of F, Cl or Br, 0 < f ≤ 4, 0 < g ≤ 2, 1 ≤ i ≤ 3, 0 ≤ j ≤ 2; M 4 X is at least one of Mn, Fe, Co, Ni, Cu, or Zn. 2 It is at least one of F, Cl, or Br, 0 < n ≤ 2; M 5 It is at least one of Mn, Fe, Co, Ni, Cu, or Zn, 0 < p ≤ 2, 0 < q ≤ 2; 0 < s ≤ 4, 0 ≤ t ≤ 3. Prussian blue compounds may include, but are not limited to, A. 2 μm 6 v [M 7 (CN)6] w ·xH2O, where A 2 For H + NH4 + M is one or more of alkali metal cations and alkaline earth metal cations. 6 and M 7 Each is independently at least one of the transition metal cations, 0 < u ≤ 2, 0 < v ≤ 1, 0 < w ≤ 1, 0 < x < 6. For example, A 2 For H + Li + Na + K + NH4 + 、Rb+ Cs + 、Fr + Be 2+ Mg 2+ Ca 2+ 、Sr 2+ Ba 2+ and Ra 2 + At least one of them, M 6 and M 7 Each is an independently selected cation of at least one transition metal element chosen from Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sn, and W. Preferably, A 2 For Li + Na + and K + At least one of them, M 6 It is a cation of at least one transition metal element selected from Mn, Fe, Co, Ni, and Cu, M 7 It is a cation of at least one transition metal element selected from Mn, Fe, Co, Ni and Cu.

[0088] Accordingly, the electrolyte salt may include sodium salt, which may include, but is not limited to, at least one of NaPF6, NaClO4, NaBCl4, NaSO3CF3 and Na(CH3)C6H4SO3.

[0089] In one embodiment, the thickness of the positive electrode current collector is 5 μm to 20 μm; preferably 6 μm to 18 μm. The thickness of the single-sided positive electrode active material layer is 30 μm to 400 μm.

[0090] In one embodiment, the positive electrode active material layer can be disposed on one surface in the thickness direction of the positive electrode current collector, or it can be disposed on two surfaces in the thickness direction of the positive electrode current collector.

[0091] It should be noted that the aforementioned "surface" can be the entire area of ​​the positive electrode current collector or only a part of the positive electrode current collector; this application does not impose any particular limitation.

[0092] The secondary battery of this application also includes an electrolyte; the electrolyte may include an organic solvent and an electrolyte salt. The electrolyte salt is determined based on the active ions in the secondary battery.

[0093] The organic solvent may include, for example, at least one of carbonate compounds, carboxylic acid ester compounds, ether compounds, or other non-aqueous organic solvents. The carbonate compound may include, but is not limited to, at least one of chain carbonate compounds, cyclic carbonate compounds, or fluorocarbonate compounds. The aforementioned chain carbonate compounds may include, but are not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), or methyl ethyl carbonate (MEC). The aforementioned cyclic carbonates may include, but are not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butyl carbonate (BC), or vinyl ethylene carbonate (VEC). Fluorocarbonate compounds may include, but are not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The aforementioned carboxylic acid ester compounds may include, but are not limited to, at least one of methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), n-propyl acetate, tert-butyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), ethyl butyrate (EB), 1,4-butyrolactone (GBL), decanolactone, valproic acid lactone, or caprolactone. The aforementioned ether compounds may include, but are not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. Other non-aqueous organic solvents may include, but are not limited to, at least one of dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), diethyl sulfone (ESE), sulfolane (SF), 1,2-dioxolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0094] Furthermore, the electrolyte may also include additives. These additives may include at least one of the following: negative electrode film-forming additives, positive electrode film-forming additives, or additives that can improve certain battery performance characteristics. For example, additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0095] Additives may include, but are not limited to, at least one of the following: fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), vinyl sulfate (DTD), propylene sulfate, vinyl sulfite (ES), 1,3-propanesulfonate lactone (PS), 1,3-propenesulfonate lactone (PST), sulfonate cyclic quaternary ammonium salts, succinic anhydride, succinic anhydride (SN), adiponitrile (AND), tris(trimethylsilane) phosphate (TMSP), and tris(trimethylsilane) borate (TMSB).

[0096] The aforementioned secondary battery also includes a separator to separate the positive and negative electrode plates, prevent internal short circuits, allow electrolyte ions to pass freely, and not affect electrochemical charge-discharge. The separator material may include, but is not limited to, at least one of polyethylene (PE), polyolefins (PO) primarily composed of polypropylene (PP), polyesters (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid.

[0097] The type of diaphragm may include at least one of woven membrane, nonwoven membrane, microporous membrane, composite membrane, rolled membrane or spun membrane.

[0098] The diaphragm may have a porous structure, and the pore size of the porous structure is not limited in the embodiments of this application. For example, the pore size may be from 0.01 μm to 1 μm.

[0099] The thickness of the diaphragm is not limited in the embodiments of this application. For example, the thickness of the diaphragm can be from 3 μm to 20 μm.

[0100] The secondary battery in this application embodiment may also include an outer packaging for encapsulating the positive electrode, separator, negative electrode and electrolyte, as well as other components known in the art in the secondary battery. This application does not limit the above-mentioned other components.

[0101] The outer packaging can be a hard shell. For example, a hard plastic shell, an aluminum shell, a steel shell, etc.

[0102] The outer packaging can also be flexible packaging. For example, a pouch-type flexible packaging. The material of the flexible packaging can be plastic. For example, the plastic can be at least one of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0103] Electrical appliances In a fourth aspect, this application provides an electrical device comprising the aforementioned secondary battery.

[0104] The electrical devices may include, but are not limited to, laptops, pen-based computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, stereo headphones, video recorders, LCD TVs, portable cleaners, portable CD players, mini CDs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, electric bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and lithium-ion capacitors.

[0105] Example The following uses lithium-ion batteries as an example to illustrate the implementation of the secondary battery of this application in more detail through embodiments and comparative examples. Those skilled in the art will understand that the preparation methods described in this application are merely examples, and any other suitable preparation methods are within the scope of this application. Unless otherwise specified, the raw materials used in the following embodiments are all from commercially available products, and the apparatus or equipment used are all purchased from conventional commercial sales channels. Various tests and evaluations were conducted according to the methods described below. Furthermore, unless otherwise specified, "parts" and "%" refer to mass measurements.

[0106] Test methods and equipment (1) Sampling method for negative electrode sheet and negative electrode active material Lithium-ion batteries discharged at 0.2C to 2.0V and charged at 0.2C to the design voltage limit were placed in a glove box. After disassembly, the negative electrode was removed, and powdered negative electrode active material was scraped and filled into a quartz capillary tube. These samples were labeled as 0% SOC negative electrode active material samples and 100% SOC negative electrode active material samples, respectively. Air was isolated during sample preparation, transfer, and testing. Small-angle X-ray scattering test samples were obtained using the above method.

[0107] (2) True density test of hard carbon materials According to the standard GB / T24586-2009 "Determination of apparent density, true density and porosity of iron ore", the true density of the hard carbon material under test is tested using a true density meter.

[0108] (3) Dv50 test of hard carbon materials: The hard carbon material to be tested was taken and its particle size distribution was determined using a laser particle size analyzer (Malvern, UK, model: Mastersizer 2000E) according to GB / T 19077-2016 Particle Size Distribution Laser Diffraction Method.

[0109] (4) Specific surface area test of hard carbon materials: After drying the hard carbon material to be tested in a vacuum drying oven, it was placed into a sample tube. The specific surface area S of the negative electrode active material was measured using a specific surface area analyzer (TristarⅡ3020M) via nitrogen adsorption / desorption. The specific testing was conducted according to GB / T 19587-2017, "Determination of Specific Surface Area of ​​Solid Substances by Gas Adsorption BET Method".

[0110] (5) X-ray diffraction (XRD) test of hard carbon materials: The hard carbon material under test was tested using an X-ray powder diffractometer (XRD, instrument model: Bruker D8 ADVANCE), with Cu Kα as the target material; the voltage and current were 40 kV / 40 mA, the scanning angle range was 5° to 80°, the scanning step size was 0.00836°, and the time of each step was 0.3 s.

[0111] (6) Small-angle X-ray diffraction test of hard carbon materials Powder samples were subjected to small-angle X-ray scattering (SAXS) measurements using a Xeuss 3.0 HR and a Sinop Xenocs detector. The target material was CuKα, and the detector was an Eiger 2 R 1M DECTRIS. After slit collimation, the scattering vector of the sample was measured to be at 0.01 nm. -1 Up to 100 nm -1 Changes in internal scattering intensity.

[0112] (7) Pore volume test of hard carbon materials The hard carbon material was crushed by external force using an IKA grinder (model MultiDrive control MT Package). Specific parameters: 20g of hard carbon material powder was taken, the grinder speed was 10000rpm, the crushing time was 30s, and the crushing was repeated 5 times. After that, the crushed powder was taken out for testing.

[0113] Hard carbon materials without external force fracture and those with damaged surface coatings were tested for pore structure characteristics using an ASAP2460 physical adsorption analyzer. Specifically, after drying and degassing pretreatment, the adsorption capacity of the material for nitrogen gas at different pressures was measured using the ASAP2460 physical adsorption analyzer, and adsorption and desorption isotherms were plotted. The pore shape was determined based on the shape of the hysteresis loop, and the pore size distribution curve of the micropores was fitted using a DFT model to obtain the pore volume information of the hard carbon material.

[0114] The internal pore volume of hard carbon material = pore volume after destruction - pore volume before destruction.

[0115] (8) Specific capacity and initial efficiency tests of the negative electrode active material: The negative electrode active material, styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) were mixed at a mass ratio of 97:1.5:1.5. Deionized water was then added as a solvent and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was uniformly coated onto one surface of a 10 μm thick copper foil current collector, with a single-sided coating thickness of 60 μm. The coating was then dried at 85°C to obtain the negative electrode sheet. The negative electrode sheet was cut into 14 mm diameter discs to serve as the working electrode. A lithium metal sheet was used as the counter electrode, and a 7 μm thick polyethylene (PE) membrane was used as the separator. After injecting test electrolyte, the cells were assembled into a coin cell.

[0116] The coin cell was subjected to charge-discharge cycles. The coin cell was first discharged at 0.05C to 0mV, then discharged at a constant voltage of 0mV to 10μA, and the initial discharge specific capacity was recorded. Then, it was charged at a constant current of 0.05C to 2.5V, and the initial charge specific capacity was recorded. Based on the coating quality and area of ​​the negative electrode slurry during the electrode preparation process, the mass of the negative electrode active material in the negative electrode sheet was calculated. The initial efficiency and specific capacity were then calculated using the following expressions: Initial efficiency = (First charge specific capacity / First discharge specific capacity) × 100%; The specific capacity of the negative electrode active material, Q, is calculated as: initial charge specific capacity / mass of the negative electrode active material, expressed in mAh / g.

[0117] (9) Test of compaction density of negative electrode active material layer In this application, the compaction density of the negative electrode active material layer has a meaning known in the art and can be determined by methods known in the art. For example, the treated negative electrode sheet (with the negative electrode current collector coated on both sides) of area S is weighed using an electronic balance, and the mass is recorded as W1. The thickness T1 of the negative electrode sheet is measured using a micrometer. The negative electrode active material layer is washed off with a solvent, dried, and the mass of the negative electrode current collector is measured and recorded as W2. The thickness T2 of the negative electrode current collector is measured using a micrometer. The compaction density PD of the negative electrode active material layer on one side of the negative electrode current collector is PD = (W1-W2) / [(T1-T2)·S].

[0118] (10) Test method for porosity of negative electrode active material layer A test area of ​​1000μm×100μm was selected on the surface of the negative electrode active material layer. The surface of the negative electrode active material layer refers to the surface of the active material layer on the side away from the current collector. All the pores between particles within this area were counted. The brightness of particles and pores in the image was distinguished, with brighter particles being particles and darker pores being pores. Then, ImageJ software was used to process the image, adjusting the particles to a black background and the pores to white. The area occupied by pores in the entire image was counted to obtain the porosity.

[0119] (11) Energy density test Energy density was tested using lithium-ion batteries prepared according to the method described in the test paper. Five batteries were used in each test group, and charging and discharging were performed at 25°C. A constant current was applied at a charging current of 0.2C until the upper limit of the charging cutoff voltage. Then, the batteries were charged at a constant voltage to 0.02C at the charging cutoff voltage, and then discharged at a constant current of 0.2C until the lower limit of the discharge cutoff voltage. The average discharge capacity D and average discharge voltage of the lithium-ion batteries at 0.2C were obtained. The lithium-ion batteries were then charged at a charging current of 0.2C to the average discharge voltage, and the length, width, and thickness of the cells were measured to calculate the cell volume V (in mm). 3 Volumetric energy density calculation: VED = (D × 3.89 × 1000) / V, unit is Wh / L, and the energy density ratio of each embodiment to Comparative Example 1 is calculated.

[0120] (12) 45℃ Cyclic Capacity Retention Rate Test Cyclic performance testing was conducted using lithium-ion batteries prepared according to the method described in the test paper. Five lithium-ion batteries were taken from each test group, and the following steps were performed to repeatedly charge and discharge the batteries, calculating the discharge capacity retention rate. The first charge and discharge cycle was conducted at 45℃. Constant current charging was performed at 1C until the charging cutoff voltage was reached, then constant voltage charging was switched to 0.05C at the charging cutoff voltage. Then, constant current discharging was performed at 1C until the cutoff voltage was reached, and the discharge capacity of the first cycle was recorded. Subsequently, 1000 charge and discharge cycles were performed, and the discharge capacity of the 1000th cycle was recorded.

[0121] The loop performance can then be calculated using the following expression: Cycle capacity retention (%) = (Discharge capacity of the 1000th cycle / Discharge capacity of the first cycle) × 100%.

[0122] (13) Rate discharge temperature rise test: Five lithium-ion batteries were taken from each group of tested batteries and subjected to the first charge and discharge at 25°C. A multi-channel temperature sensor probe was attached to the center of the lithium-ion battery surface to measure the temperature change during discharge. A constant current was maintained at 0.2C until the charging cutoff voltage, followed by constant voltage charging to 0.02C at the charging cutoff voltage. Then, a constant current discharge was maintained at 0.2C until the discharge cutoff voltage, obtaining the average 0.2C discharge capacity of the lithium-ion battery. The above charging process was repeated, followed by constant current discharge at 5C until the discharge cutoff voltage, obtaining the highest temperature value at the center of the lithium-ion battery surface at a 5C discharge rate. Therefore, the temperature rise at a 5C discharge rate = the highest temperature value - 25°C, and the improvement rate of temperature rise at 5C discharge rate = (temperature rise of Comparative Example 1 - temperature rise of each embodiment / temperature rise of Comparative Example 1) × 100%.

[0123] The charging cutoff voltage and discharging cutoff voltage of the lithium-ion battery using the negative electrode active material in this application are determined according to different positive electrode materials. The embodiments provided in this application take lithium cobalt oxide as an example, with a lithium-ion battery charging cutoff voltage of 4.53V and a discharging cutoff voltage of 2.0V. Specifically, the lithium-ion batteries in Comparative Example 1, Example 8, and Example 9 have a charging cutoff voltage of 4.48V and a discharging cutoff voltage of 3.0V.

[0124] Example 1 <Preparation of Hard Carbon Materials> S1. Take asphalt with a softening point of 260℃, heat it to a molten state, and extrude the fibers through a round-hole spinneret. After extrusion, the fibers are solidified by cooling water. Collect the asphalt fibers and carry out oxidative crosslinking at 200℃, 240℃, 280℃, and 300℃ for 3h, 3h, 2h, and 2h respectively in an air atmosphere to obtain pre-oxidized asphalt fibers.

[0125] S2. Pre-oxidized pitch fibers are placed in a rotary kiln and heated to 600°C at a rate of 3°C / min under nitrogen atmosphere protection, and held for 1 hour to obtain pre-carbonized fibers. The pre-carbonized fibers are then coarsely crushed using a mechanical crusher, followed by the addition of sodium hydroxide for further crushing and mixing, with the mass ratio of sodium hydroxide to pre-carbonized carbon fibers being 1. The mixture is then collected and placed in nickel saggers, 2 kg of material per sagger, and placed in a roller kiln. Under nitrogen atmosphere protection, the temperature is increased to 700°C at a rate of 3°C / min and held for 2 hours. After cooling, the material is removed and subjected to acid washing and water washing to obtain activated carbon material.

[0126] S3. The particle size of the activated carbon material is further adjusted to Dv50 of 5μm using an airflow crusher and classifier. Then, the powder is placed in a rotary kiln and heated to 950℃ under a nitrogen atmosphere and held at that temperature. A mixed atmosphere of methane and argon with a methane content of 20wt% is introduced at 950℃ for 6 hours. Finally, the temperature is lowered under a nitrogen atmosphere to obtain hard carbon material with Dv50 of 5μm.

[0127] <Preparation of Negative Electrode Sheets> Hard carbon material (negative electrode active material), styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) were mixed in a mass ratio of 97:1.5:1.5. Deionized water was then added as a solvent and the mixture was stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 85°C to obtain a single-sided coated negative electrode sheet with an 80 μm thick negative electrode active material layer. The above steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. After cold pressing, cutting, and slitting, the negative electrode sheets were obtained.

[0128] <Preparation of the positive electrode> Lithium cobalt oxide (LCO), conductive carbon black (Super P), and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1.4:1.6. N-methylpyrrolidone (NMP) was added as a solvent and the mixture was stirred until homogeneous, resulting in a positive electrode slurry with a solid content of 72 wt%. The positive electrode slurry was uniformly coated onto one surface of a 10 μm thick aluminum foil used as a positive electrode current collector and dried at 85 °C to obtain a positive electrode sheet with a single-sided coating of positive active material. The above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode sheet with a double-sided coating of positive active material. After cold pressing, cutting, and slitting, the positive electrode sheets were obtained, with the N / P ratio set to 1.0, meaning the ratio of the negative electrode capacity per unit area to the positive electrode capacity per unit area was 1.0.

[0129] <Preparation of Electrolyte> In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), and diethyl carbonate (DEC) were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. Lithium salt LiPF6 was dissolved in the organic solvent, and then fluoroethylene carbonate (FEC) was added and mixed thoroughly to obtain the electrolyte. Based on the total mass of the electrolyte, the mass content of LiPF6 was 12.5%, the mass content of FEC was 5%, and the remainder was the organic solvent.

[0130] <Septum> Polyethylene (PE) with a thickness of 7μm is used.

[0131] <Preparation of Lithium-ion Batteries> The positive electrode, separator, and negative electrode are stacked sequentially, with the separator positioned between the positive and negative electrodes to provide insulation. The electrodes are then wound to form the electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag and dried in an 80°C vacuum oven for 12 hours to remove moisture. The prepared electrolyte is then injected, and the battery undergoes vacuum sealing, settling, formation, degassing, and edge trimming to obtain the lithium-ion battery. The designed potential range for lithium-ion batteries is 2.0–4.53V.

[0132] Example 2 In the <Preparation of Hard Carbon Materials>, in step S1, the spinneret holes prepared from pitch fibers are three circular holes arranged in a straight line. The other steps S2 and S3 of the preparation of hard carbon materials are the same as in Example 1. The remaining conditions are the same as in Example 1.

[0133] Example 3 In the <Preparation of Hard Carbon Materials>, in step S1, the spinneret holes prepared from pitch fibers are quasi-equilateral triangles, with rounded corners and rounded edges. The other steps S2 and S3 for the preparation of hard carbon materials are the same as in Example 1; The remaining conditions are the same as in Example 1.

[0134] Example 4 In the <Preparation of Hard Carbon Materials>, in step S1, the spinneret holes prepared from pitch fibers are quasi-square, and the corners and edges of the quasi-square are rounded. The other steps S2 and S3 of the preparation of hard carbon materials are the same as in Example 1. The remaining conditions are the same as in Example 1.

[0135] Example 5 In the <Preparation of Hard Carbon Materials>, in step S1, the spinneret holes prepared from pitch fibers are quasi-regular pentagons, with rounded corners and edges; the other steps S2 and S3 of the preparation of hard carbon materials are the same as in Example 1. The remaining conditions are the same as in Example 1.

[0136] Example 6 <Preparation of Hard Carbon Materials> S1. A spinning solution was prepared by dissolving a methyl phenolic resin with a number-average molecular weight of 1100 g / mol in an ethanol solution. After degassing the spinning solution, it was extruded through a spinneret with near-square orifices into an acidic coagulation bath. The fibers were washed with water to remove residual solvent and acid. The fibers were then cured under a nitrogen atmosphere at 100℃, 120℃, and 150℃ for 6 h, 3 h, 2 h, and 2 h respectively to obtain phenolic fibers. The acidic coagulation bath was a formaldehyde-hydrochloric acid aqueous solution with a formaldehyde mass fraction of 18.5% and a hydrochloric acid mass fraction of 12%. The corners and edges of the near-square orifices were rounded.

[0137] The other steps S2 and S3 in the preparation of hard carbon materials are the same as in Example 4; Other conditions are the same as in Example 1.

[0138] Example 7 <Preparation of Hard Carbon Materials> S1. Acrylonitrile monomer and azobisisobutyronitrile (0.2% by mass of monomer) are used as initiators, dissolved in dimethylformamide and added to a polymerization reactor. The temperature is raised to 60°C for polymerization. When the monomer conversion rate reaches 60%, the temperature is lowered. After passing through a monomer removal tower and vacuum degassing, bubbles and unreacted monomers are removed to obtain a pure, uniform, and viscosity-stable spinning solution.

[0139] The spinning solution was extruded through a spinneret with near-square orifices into an aqueous solution of dimethylformamide. After drawing and washing, polyacrylonitrile fibers were obtained. The fibers were then subjected to oxidative crosslinking at air at 100℃, 150℃, 180℃, and 250℃ for 6 hours, 3 hours, 2 hours, and 2 hours respectively, to obtain pre-oxidized polyacrylonitrile fibers. The corners and edges of the near-square orifices were rounded.

[0140] The other steps S2 and S3 in the preparation of hard carbon materials are the same as in Example 4; Other conditions are the same as in Example 4.

[0141] Example 8 In the <Preparation of Hard Carbon Material>, in step S2, the mass ratio of sodium hydroxide to pre-carbonized carbon filament is 0.5; the other steps S1 and S3 of the preparation of hard carbon material are the same as in Example 4. The remaining conditions are the same as in Example 4.

[0142] Example 9 In the <Preparation of Hard Carbon Material>, in step S2, the mass ratio of sodium hydroxide to pre-carbonized carbon filament is 0.5; the other steps S1 and S3 of the preparation of hard carbon material are the same as in Example 4. The remaining conditions are the same as in Example 4.

[0143] Example 10 In the preparation of hard carbon materials, in step S2, the mass ratio of potassium hydroxide to pre-carbonized carbon filament is 1.0, and the activation condition is 700℃ for 4 hours; the other steps S1 and S3 of the preparation of hard carbon materials are the same as in Example 4. The remaining conditions are the same as in Example 4.

[0144] Example 11 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 90% and the mass content of artificial graphite is 10%. The remaining conditions are the same as in Example 1.

[0145] Example 12 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 80% and the mass content of artificial graphite is 20%. The remaining conditions are the same as in Example 1.

[0146] Example 13 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 60% and the mass content of artificial graphite is 40%. The remaining conditions are the same as in Example 1.

[0147] Example 14 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 30% and the mass content of artificial graphite is 70%. The remaining conditions are the same as in Example 1.

[0148] Example 15 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 5% and the mass content of artificial graphite is 95%. In the preparation of lithium-ion batteries, the N / P ratio of the positive and negative electrodes in the lithium-ion battery is set to 1.04, and the potential range is adjusted to 3.0V to 4.48V (that is, the charging cut-off voltage is adjusted to 4.48V and the discharging cut-off voltage is adjusted to 3.0V).

[0149] The rest of the steps and parameters are the same as in Example 1.

[0150] Example 16 The preparation of hard carbon materials is consistent with Example 4; In the preparation of the negative electrode sheet, the negative electrode active material includes hard carbon material and artificial graphite. Based on the mass of the negative electrode active material, the mass content of hard carbon material is 1% and the mass content of artificial graphite is 99%. In the preparation of lithium-ion batteries, the N / P ratio of the positive and negative electrodes in the lithium-ion battery is set to 1.04, and the potential range is adjusted to 3.0V to 4.48V (that is, the charging cut-off voltage is adjusted to 4.48V and the discharging cut-off voltage is adjusted to 3.0V).

[0151] The rest of the steps and parameters are the same as in Example 1.

[0152] Comparative Example 1 <Preparation of Negative Electrode Sheets> Artificial graphite (anode active material), styrene-butadiene rubber (SBR) binder, and sodium carboxymethyl cellulose (CMC-Na) were mixed in a mass ratio of 97:1.5:1.5. Deionized water was then added as a solvent and stirred until homogeneous, resulting in a negative electrode slurry with a solid content of 40 wt%. The negative electrode slurry was uniformly coated onto one surface of a 6 μm thick copper foil current collector and dried at 85°C to obtain a single-sided coated negative electrode sheet with an 80 μm thick negative electrode active material layer. The above steps were repeated on the other surface of the copper foil to obtain a double-sided coated negative electrode sheet. After cold pressing, cutting, and slitting, the negative electrode sheets were obtained.

[0153] In the preparation of lithium-ion batteries, the N / P ratio of the positive and negative electrodes in the lithium-ion battery is set to 1.04, and the potential range is adjusted to 3.0V to 4.48V (that is, the charging cut-off voltage is adjusted to 4.48V and the discharging cut-off voltage is adjusted to 3.0V).

[0154] The rest of the steps and parameters are the same as in Example 1.

[0155] Comparative Example 2 <Preparation of Hard Carbon Materials> S1: Take asphalt with a softening point of 260℃, crush it to a Dv50 of 5~6μm particles, collect the asphalt powder, and perform oxidative crosslinking in a rotary kiln under air atmosphere at 200℃, 240℃, 280℃, and 300℃ for 3h, 3h, 2h, and 2h respectively to obtain pre-oxidized asphalt powder. Subsequent steps S2 and S3 are the same as in Example 1; The lithium-ion battery preparation steps and parameters are the same as in Example 1.

[0156] Small-angle X-ray diffraction, pore volume, true density, and Dt of hard carbon materials obtained from tests in various embodiments and comparative examples are shown. v 50. The compaction density and porosity of the negative electrode active material layer are shown in Table 1 below.

[0157] Table 1

[0158] Note: In Table 1, " / " indicates that the corresponding substance or parameter does not exist.

[0159] As shown in Table 1, the hard carbon materials with regular morphology prepared using Examples 1 to 16 have higher compaction density and lower porosity compared with the negative electrode active material layer using the comparative hard carbon material of Comparative Example 2.

[0160] The specific capacity, initial efficiency, energy density ratio to Comparative Example 1, capacity retention after 1000 cycles at 45°C, and temperature rise improvement rate at 5C discharge rate at room temperature compared to Comparative Example 1 obtained from the tests of each embodiment and comparative example are shown in Table 2 below.

[0161] Table 2

[0162] Table 2 shows that hard carbon materials have high specific capacity and initial efficiency. For example... Figure 2 As shown, the hard carbon material provided in this application has a large number of pores inside, which can serve as active sites for lithium ions. Lithium-ion batteries prepared from hard carbon materials with regular morphology have higher energy density and improved cycle and discharge temperature rise characteristics. For example, the energy density in Example 4 is increased by 10% compared to Comparative Example 1. Furthermore, after the hard carbon material provided in this application is combined with graphite material, the lithium-ion battery exhibits even better cycle capacity retention after 1000 cycles at 45°C.

[0163] It should be noted that, in this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, or article.

[0164] The various embodiments in this specification are described in a related manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0165] The above are merely embodiments of this application and are not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application.

Claims

1. A hard carbon material, characterized in that, The hard carbon material includes particles with a columnar morphology, wherein the columnar morphology includes at least one of cylinders, oval cylinders, and polygons; and the polygons include at least one of triangular prism-like, quadrangular prism-like, and pentagonal prism-like. When the hard carbon material is applied to a secondary battery, it exhibits different small-angle X-ray diffraction characteristics under different states of charge, wherein: At 0% SOC, the scattering vector is at 0.1 nm. -1 ~7 nm -1 Within this range, a peak in scattering intensity was observed; At 100% SOC, the scattering vector is 0.1 nm. -1 ~7 nm -1 No scattering intensity peak was observed within the specified range.

2. The hard carbon material according to claim 1, characterized in that, The hard carbon material satisfies at least one of the following conditions: (1) The edges of the polygon are rounded. (2) The edges of the polygon are rounded.

3. The hard carbon material according to claim 1 or 2, characterized in that, The X-ray diffraction pattern of the hard carbon material has broad diffraction peaks between 18° and 30°, and the full width at half maximum (FWHM) of the broad diffraction peaks is between 4° and 12°.

4. The hard carbon material according to claim 1 or 2, characterized in that, The hard carbon material satisfies at least one of the following characteristics: (1) The pore volume of the hard carbon material is 0.1 cm³. 3 / g to 0.9cm 3 / g; (2) The Dv50 of the hard carbon material is 3 μm to 10 μm; (3) The true density of the hard carbon material is 1.01 g / cc to 1.67 g / cc; (4) The specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 50m 2 / g.

5. The hard carbon material according to claim 1 or 2, characterized in that, The hard carbon material satisfies at least one of the following characteristics: (1) The pore volume of the hard carbon material is 0.31 cm³. 3 / g to 0.45cm 3 / g; (2) The true density of hard carbon materials is 1.20 g / cc to 1.34 g / cc; (3) The specific surface area of ​​the hard carbon material is 0.5 m². 2 / g to 5m 2 / g.

6. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active material layer located on at least one surface of the negative electrode current collector, the negative electrode active material layer comprising a negative electrode active material; Its features are, The negative electrode active material includes the hard carbon material according to any one of claims 1 to 5; Based on the mass of the negative electrode active material, the mass content of the hard carbon material is from 1.0% to 100%.

7. The negative electrode sheet according to claim 6, characterized in that, The negative electrode active material layer satisfies at least one of the following characteristics: (1) The compaction density of the negative electrode active material layer is 0.9 g / cm³. 3 Up to 1.72 g / cm 3 ; (2) The porosity of the negative electrode active material layer is between 10% and 25%.

8. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 6 or 7.

9. The secondary battery according to claim 8, characterized in that, The secondary battery further includes a positive electrode sheet, which includes a positive current collector and a positive active material layer located on at least one surface of the positive current collector, the positive active material layer including a positive active material; The positive electrode active material includes lithium transition metal oxides, which include at least one of lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium iron phosphate, lithium-rich manganese-based materials, lithium cobalt oxide, lithium manganese oxide, lithium manganese iron phosphate, and lithium titanate.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 8 or 9.

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