Graphite-based negative electrode active material and preparation method thereof, negative electrode plate and secondary battery

Abstract

The invention relates to a graphite-based negative electrode active material and a preparation method thereof, a negative electrode plate and a secondary battery. The graphite-based negative electrode active material comprises an inner core, a first coating layer coating the surface of the inner core and a second coating layer coating the surface of the first coating layer, the inner core comprises hard carbon, the first coating layer comprises graphite, and the second coating layer comprises soft carbon. According to the graphite-based negative electrode active material, a secondary battery can keep relatively high capacity, and relatively good rate capability can be realized at the same time.

Description

Technical Field

[0001] This application relates to batteries, and more particularly to a graphite-based negative electrode active material and its preparation method, a negative electrode sheet, and a secondary battery. Background Technology

[0002] The development of rechargeable batteries has reduced energy waste and environmental pollution, and provided crucial power support for portable electronic devices, electric vehicles, and renewable energy storage. The negative electrode active material in a rechargeable battery is one of its key components, significantly influencing its capacity and rate performance.

[0003] Traditional negative electrode active materials are mainly graphite, including natural and artificial graphite. However, with the accelerating pace of life, people have increasingly higher requirements for the rate performance of secondary batteries, which traditional graphite materials can no longer meet. One method involves coating the surface of graphite with hard carbon and / or soft carbon, utilizing the differences between different carbon materials to improve the rate performance of secondary batteries; however, this has led to a significant decrease in the capacity of the secondary batteries. Summary of the Invention

[0004] Based on this, this application provides a graphite-based negative electrode active material that can effectively improve rate performance while maintaining high capacity, as well as its preparation method, negative electrode sheet, and secondary battery.

[0005] A first aspect of this application provides a graphite-based negative electrode active material, comprising an inner core, a first coating layer covering the surface of the inner core, and a second coating layer covering the surface of the first coating layer;

[0006] The inner core comprises hard carbon, the first coating layer comprises graphite, and the second coating layer comprises soft carbon.

[0007] In some embodiments, the precursor material of the hard carbon includes one or more of sugars, starch, cellulose, acrylonitrile-butadiene-styrene copolymer (ABS resin), acrylic resin, epoxy resin, phenolic resin, urea-formaldehyde resin, polyurethane resin, and silicone resin.

[0008] Optionally, the sugars include one or more of monosaccharides and disaccharides;

[0009] Further, optionally, the monosaccharide includes one or both of glucose and fructose;

[0010] Further, optionally, the disaccharide includes one or more of trehalose, sucrose, maltose, and lactose;

[0011] Optionally, the precursor material for the hard carbon includes disaccharides;

[0012] Further, optionally, the precursor material for the hard carbon includes one or both of maltose and lactose.

[0013] In some embodiments, the precursor material for the soft carbon includes bitumen;

[0014] Optionally, the softening point of the asphalt is ≥180℃, further optionally, the softening point of the asphalt is ≥205℃, and even more preferably, it is 205℃~250℃;

[0015] Optionally, the asphalt includes one or more of coal tar pitch, petroleum asphalt, and synthetic asphalt.

[0016] In some embodiments, the sum of the amounts of the hard carbon precursor material and the soft carbon precursor material is such that the actual residual carbon content relative to the graphite is 1% to 5%;

[0017] Optionally, the mass ratio of the hard carbon, the graphite and the soft carbon is (0.5~4.5):100:(0.5~4.5).

[0018] In some embodiments, the graphite-based anode active material has one or more of the following characteristics:

[0019] (1) Dv50≥10µm, can be selected as 10µm~30µm;

[0020] (2) Tap density TD ≥ 1 g / cm³ 3 1g / cm can be selected. 3 ~1.3g / cm 3 ;

[0021] (3) Specific surface area BET≤1.9m 2 / g, optional 1m 2 / g ~1.9m 2 / g.

[0022] A second aspect of this application provides a method for preparing a graphite-based negative electrode active material, comprising the following steps:

[0023] The first premix is ​​prepared by mixing the first binder, the precursor material of hard carbon, and graphite.

[0024] The first premix is ​​subjected to a first carbonization treatment, pulverized, and a first carbide is prepared.

[0025] The second binder, the first carbide, and the soft carbon precursor material are mixed to prepare the second premix.

[0026] The second premix is ​​subjected to a second carbonization treatment to prepare a graphite-based anode active material;

[0027] Wherein, the Dv50 of the hard carbon precursor material is greater than the Dv50 of the graphite;

[0028] The Dv50 of the first carbide is greater than the Dv50 of the soft carbon precursor material.

[0029] In some embodiments, the method for preparing the graphite-based negative electrode active material has one or more of the following features:

[0030] (1) The Dv50 of the precursor material of the hard carbon is ≥30µm, and can be ≥1cm;

[0031] (2) The graphite has a Dv50 ≤ 10µm;

[0032] (3) The Dv50 of the first carbide is 10µm~20µm;

[0033] (4) The Dv50 of the soft carbon precursor material is ≤5µm.

[0034] In some embodiments, the carbonization temperature of the first carbonization treatment and the carbonization temperature of the second carbonization treatment are each independently 600°C to 1200°C, and optionally 1100°C to 1200°C.

[0035] Optionally, during the first carbonization treatment and / or the second carbonization treatment, the temperature is first raised to 150℃~250℃ and held for 60min~100min, and then the temperature is further raised to the carbonization temperature for further treatment.

[0036] A third aspect of this application provides a negative electrode sheet, comprising the graphite-based negative electrode active material described in the first aspect or the graphite-based negative electrode active material prepared by the preparation method described in the second aspect.

[0037] A fourth aspect of this application provides a secondary battery, including the negative electrode sheet described in the third aspect.

[0038] The aforementioned graphite-based anode active material has a hard carbon core coated with graphite and soft carbon in sequence. This structure facilitates the construction of efficient ion transport channels, accelerates lithium-ion insertion / extraction, and thus improves the rate performance of the secondary battery. At the same time, the structure has good stability and can maintain a high battery capacity.

[0039] In addition, the graphite-based anode active material mentioned above also has a high initial efficiency. Attached Figure Description

[0040] Figure 1 This is a SEM image of the graphite-based anode active material prepared in Example 1. Detailed Implementation

[0041] The graphite-based negative electrode active material, its preparation method, negative electrode sheet, and secondary battery of this application are further described in detail below with reference to specific embodiments. This application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0042] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein in the specification of this application is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0043] In this article, "one or more" refers to any one, two or more of the listed items.

[0044] In this application, terms such as "first aspect," "second aspect," "third aspect," and "fourth aspect" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or quantity, nor should they be construed as implicitly indicating the importance or quantity of the indicated technical features. Moreover, terms such as "first," "second," "third," and "fourth" serve only as a non-exhaustive enumeration and should be understood not to constitute a closed limitation on quantity.

[0045] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0046] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0047] Unless otherwise specified, the percentage content mentioned in this application refers to mass percentage for solid-liquid mixtures and solid-phase-solid mixtures, and volume percentage for liquid-phase-liquid mixtures.

[0048] Unless otherwise specified, all percentage concentrations mentioned in this application refer to the final concentration. The final concentration refers to the proportion of the added component in the system after the addition of that component.

[0049] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows for temperature fluctuations within the precision range controlled by the instrument.

[0050] In this application, room temperature generally refers to 4℃~30℃, and preferably 20±5℃.

[0051] Some embodiments of this application provide a graphite-based negative electrode active material, including an inner core, a first coating layer covering the surface of the inner core, and a second coating layer covering the surface of the first coating layer;

[0052] The inner core comprises hard carbon, the first coating layer comprises graphite, and the second coating layer comprises soft carbon.

[0053] Understandably, "covering" can refer to partial coverage or complete coverage.

[0054] In some embodiments, the precursor material of the hard carbon includes one or more selected from sugars, starch, cellulose, acrylonitrile-butadiene-styrene copolymer (ABS resin), acrylic resin, epoxy resin, phenolic resin, urea-formaldehyde resin, polyurethane resin, and silicone resin. The sugars include one or more monosaccharides and disaccharides. Further, the monosaccharides include one or both of glucose and fructose; the disaccharides include one or more of trehalose, sucrose, maltose, and lactose.

[0055] Furthermore, the precursor material for the hard carbon includes disaccharides. Using disaccharides as precursor materials for hard carbon enables better rate performance. Even further, the precursor material for the hard carbon includes one or both of maltose and lactose. Compared to other disaccharides such as trehalose and sucrose, maltose and lactose contain hemiacetal hydroxyl groups, exhibiting mutarotation. They also have more planar structures compared to similar disaccharides, allowing for better binding with the coating layer, resulting in structural stability and maintaining higher battery capacity.

[0056] In some embodiments, the precursor material for the soft carbon includes bitumen.

[0057] Furthermore, the softening point of the asphalt is ≥180℃. Asphalt with a higher softening point has a higher residual carbon content, which is beneficial for coating, making the structure stable, and maintaining a higher battery capacity. Specifically, the softening point of the asphalt includes, but is not limited to: 205℃, 210℃, 215℃, 220℃, 225℃, 230℃, 235℃, 240℃, 245℃, 250℃, or any range between the foregoing. Further, the softening point of the asphalt is ≥205℃. Even further, the softening point of the asphalt is 205℃~250℃.

[0058] Without limitation, the bitumen includes one or more of coal tar pitch, petroleum bitumen, and synthetic bitumen.

[0059] In some embodiments, the sum of the amounts of the hard carbon precursor material and the soft carbon precursor material is such that the actual residual carbon content relative to the graphite is 1% to 5%. Specifically, the actual residual carbon content includes, but is not limited to: 1%, 1.5%, 2%, 2.2%, 2.3%, 2.5%, 2.7%, 3%, 3.1%, 3.5%, 4%, 4.5%, 5%, or any range between the foregoing.

[0060] Understandably, the actual residual carbon content = the percentage of precursor material added relative to graphite × the residual carbon rate of the precursor material.

[0061] The residual carbon content of some precursor materials is shown in Table 1 below:

[0062] Table 1

[0063]

[0064] In some embodiments, the mass ratio of hard carbon, graphite, and soft carbon is (0.5~4.5):100:(0.5~4.5). Specifically, this mass ratio includes, but is not limited to: 0.5:100:4.5, 1:100:4, 4.5:100:0.5, 1:100:1, 1.5:100:1.5, 2:100:2, 2.5:100:2.5, 3:100:3, 3.5:100:3.5, 4:100:4, 4:100:1, 4.5:100:4.5, or a range between any two of the foregoing. Further, the mass ratio of hard carbon, graphite, and soft carbon is (1~3):100:(3~4).

[0065] In some embodiments, the Dv50 of the graphite-based anode active material is ≥10µm. Further, the Dv50 of the graphite-based anode active material is 10µm to 30µm. Specifically, the Dv50 includes, but is not limited to, 10µm, 15µm, 20µm, 25µm, 30µm, or any range between the foregoing.

[0066] In some embodiments, the tap density (TD) of the graphite-based anode active material is ≥0.95 g / cm³. 3 Furthermore, the tap density (TD) of the graphite-based anode active material is 0.95 g / cm³. 3 ~1.3g / cm 3 Specifically, the tap density TD includes, but is not limited to, 0.95 g / cm³. 3 1g / cm 3 1.02g / cm 3 1.06 g / cm 3 1.07 g / cm 31.11 g / cm 3 1.12 g / cm 3 1.15 g / cm 3 1.17 g / cm 3 1.2g / cm 3 1.22 g / cm 3 1.25 g / cm 3 1.27g / cm 3 1.3g / cm 3 Or the range between any two of the aforementioned.

[0067] In some embodiments, the specific surface area (BET) of the graphite-based anode active material is ≤3.5 m². 2 / g. Furthermore, the specific surface area (BET) of the graphite-based anode active material is 1m². 2 / g~3.5m 2 / g. Specifically, the specific surface area BET includes, but is not limited to: 1m² 2 / g, 1.04 m 2 / g、1.1 m 2 / g, 1.15 m 2 / g, 1.20 m 2 / g, 1.23 m 2 / g, 1.3 m 2 / g, 1.32 m 2 / g, 1.34 m 2 / g, 1.4 m 2 / g, 1.45 m 2 / g, 1.51 m 2 / g, 1.6 m 2 / g, 1.65 m 2 / g, 1.7 m 2 / g, 1.8m 2 / g, 1.9 m 2 / g、2 m 2 / g、2.2 m 2 / g, 2.36 m 2 / g, 2.5 m 2 / g, 2.7 m 2 / g, 3.00 m 2 / g, 3.46 m 2 / g, 3.5m 2 / g or any of the two mentioned above.

[0068] Some embodiments of this application provide a method for preparing a graphite-based negative electrode active material, comprising the following steps:

[0069] The first premix is ​​prepared by mixing the first binder, the precursor material of hard carbon, and graphite.

[0070] The first premix is ​​subjected to a first carbonization treatment, crushed, and a first carbide is prepared (the first carbide has a hard carbon material core and graphite coated on the core surface).

[0071] The second binder, the first carbide, and the soft carbon precursor material are mixed to prepare the second premix.

[0072] The second premix is ​​subjected to a second carbonization treatment to prepare a graphite-based anode active material;

[0073] Wherein, the Dv50 of the hard carbon precursor material is greater than the Dv50 of the graphite;

[0074] The Dv50 of the first carbide is greater than the Dv50 of the soft carbon precursor material.

[0075] Understandably, the technical solutions for the hard carbon precursor material, the soft carbon precursor material, and the graphite in the above preparation method are the same as those for the graphite-based anode active material, and will not be repeated here.

[0076] In the above preparation method, controlling the particle size of the materials during mixing is beneficial for coating. For example, if the Dv50 of the hard carbon precursor material is greater than that of graphite, the graphite tends to aggregate around the hard carbon precursor material during the bonding process, forming a core-shell structure. After carbonization, the structural relationship between the two will be fixed as a hard carbon core-graphite shell structure (first carbide). Similarly, if the Dv50 of the first carbide is greater than that of the soft carbon precursor material, the soft carbon precursor material tends to aggregate around the first carbide during the bonding process, forming a hard carbon-graphite-soft carbon structure after carbonization.

[0077] In some embodiments, the Dv50 of the hard carbon precursor material is ≥15µm. Further, the Dv50 of the hard carbon precursor material is 15µm to 50µm. Specifically, the Dv50 of the hard carbon precursor material includes, but is not limited to: 15µm, 17.5µm, 20µm, 25µm, 30µm, 35µm, 40µm, 45µm, 50µm, or any range between the foregoing.

[0078] In some embodiments, the graphite has a Dv50 ≤ 10µm. Further, the graphite has a Dv50 of 5µm to 10µm. Specifically, the Dv50 of the graphite includes, but is not limited to, 5µm, 6µm, 7µm, 8µm, 9µm, 10µm, or any range between the foregoing.

[0079] In some embodiments, the Dv50 of the first carbide is 10µm to 20µm. Specifically, the Dv50 of the first carbide includes, but is not limited to: 10µm, 11µm, 12µm, 13µm, 14µm, 15µm, 16µm, 17µm, 18µm, 19µm, 20µm or any range between the two for the foregoing.

[0080] In some embodiments, the Dv50 of the soft carbon precursor material is ≤ 5µm. Further, the Dv50 of the soft carbon precursor material is 1µm to 5µm. Specifically, the Dv50 of the soft carbon precursor material includes, but is not limited to, 1µm, 2µm, 3µm, 4µm, 5µm, or any range between the foregoing.

[0081] In some embodiments, the carbonization temperature of the first carbonization treatment and / or the second carbonization treatment is 600°C to 1200°C. Specifically, the carbonization temperature includes, but is not limited to: 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, 900°C, 950°C, 1000°C, 1050°C, 1100°C, 1150°C, 1200°C, or a range between any two of the foregoing. Further, the carbonization temperature is 1100°C to 1200°C.

[0082] Furthermore, during the first carbonization treatment and / or the second carbonization treatment, the temperature is first raised to 150℃~250℃ and held for 60min~100min, and then the temperature is further raised to the carbonization temperature for further treatment. Without limitation, the time for heating to the carbonization temperature for treatment is 1h~3h.

[0083] Without limitation, the first binder and the second binder can be binders commonly used in the art, and may be one or more of polyvinyl alcohol and polyethylene glycol, which leave little residue after carbonization. In some embodiments, the mass percentage of the first binder and / or the second binder is 1% to 3% based on the mass of the graphite.

[0084] Furthermore, without limitation, depending on the original state of the materials, mixing can be either dry or wet mixing, and can be carried out in equipment such as a VC mixer. There are no particular restrictions on the mixing time; the goal is to ensure the materials are thoroughly and evenly mixed. For example, the mixing time for dry mixing should be ≥5 minutes, and the mixing time for wet mixing should be ≥1 minute. A drying step is also included after mixing to ensure the moisture content of the materials is <1%.

[0085] Without limitation, the Dv50 of the first carbide can be controlled by pulverization, for example, by using equipment such as a wall-breaking machine, depolymerizer, or mechanical mill, and the yield of the pulverized material after sieving must be >70%.

[0086] Without restriction, the particle size of graphite-based anode active materials can be pulverized manually or by using equipment such as rod mills. The yield of the pulverized material after sieving must be >80%.

[0087] In other embodiments of this application, a negative electrode sheet is provided, comprising the graphite-based negative electrode active material as described above or the graphite-based negative electrode active material prepared by the preparation method described above.

[0088] In other embodiments of this application, a secondary battery is provided, including the negative electrode sheet as described above.

[0089] For experimental parameters not specified in the following specific embodiments, please refer to the guidelines given in this application document first, or refer to experimental manuals or other experimental methods known in the art, or refer to the experimental conditions recommended by the manufacturer.

[0090] The raw materials and reagents involved in the following specific embodiments can be obtained commercially or prepared by those skilled in the art using known methods.

[0091] Example 1

[0092] This embodiment describes a graphite-based anode active material, and its preparation method is as follows:

[0093] (1) Mix 20g polyvinyl alcohol, 30g maltose (Dv50=17.5µm) and 1kg artificial graphite material (Dv50=10.0µm) in a VC mixer for 5min and dry for 4h.

[0094] (2) Carbonize the material after mixing and drying in step (1). First, heat it to 200℃ and keep it at that temperature for 80 minutes. Then, continue to heat it to the carbonization temperature of 1100℃ and keep it at that temperature for 2 hours.

[0095] (3) The carbonized material from step (2) is crushed using a depolymerizer at a frequency of 20 Hz, and the crushed material has a Dv50 of 13.0 µm.

[0096] (4) The crushed material from step (3) is mixed with 30g of petroleum asphalt (softening point is 250℃, Dv50=3.0µm), 20g of polyvinyl alcohol, and dry-mixed for 5min using a VC mixer;

[0097] (5) The material mixed in step (4) is carbonized a second time. First, the temperature is raised to 200℃ and held for 80 minutes, then the temperature is raised to 1100℃ and held for 2 hours.

[0098] (6) The carbonized material was crushed using a pin mill at a frequency of 30 Hz. After crushing, Dv50 = 15.0 µm, resulting in a graphite-based negative electrode active material with an actual residual carbon rate of 2.3%. The mass ratio of hard carbon, graphite and soft carbon was 3:100:3.

[0099] The SEM and TEM test results of the graphite-based anode active material prepared in Example 1 are as follows: Figure 1 As shown, the graphite-based anode active material has a core of large-particle-size hard carbon material, and its surface is coated with layers of graphite and soft carbon material of different small particle sizes.

[0100] Example 2

[0101] This embodiment is a graphite-based anode active material, and its preparation method is the same as that in Example 1. The main difference is that the maltose in step (1) is replaced with lactose, and the actual residual carbon rate is 2.5%.

[0102] Example 3

[0103] This embodiment is a graphite-based anode active material, and its preparation method is the same as that in Example 1. The main difference is that the maltose in step (1) is replaced with trehalose, and the actual residual carbon rate is 2.2%.

[0104] Example 4

[0105] This embodiment is a graphite-based negative electrode active material. Its preparation method is the same as that in Example 1. The main difference is that the maltose in step (1) is replaced by phenolic resin powder, and the actual residual carbon rate is 3.1%.

[0106] Example 5

[0107] This embodiment is a graphite-based negative electrode active material. Its preparation method is the same as that in embodiment 1. The main difference is that the petroleum asphalt (softening point of 250°C) in step (4) is replaced with artificial asphalt (softening point of 205°C).

[0108] Example 6

[0109] This embodiment is a graphite-based negative electrode active material. Its preparation method is the same as that in embodiment 1. The main difference is that the petroleum pitch (softening point of 250°C) in step (4) is replaced by coal pitch (softening point of 180°C).

[0110] Example 7

[0111] This embodiment is a graphite-based negative electrode active material, and its preparation method is the same as that in Example 1. The main difference is that different amounts of maltose and pitch are used to make the mass ratio of hard carbon, graphite and soft carbon 1:100:4.

[0112] Example 8

[0113] This embodiment is a graphite-based negative electrode active material, and its preparation method is the same as that in Example 1. The main difference is that different amounts of maltose and pitch are used to make the mass ratio of hard carbon, graphite and soft carbon 4:100:1.

[0114] Example 9

[0115] This embodiment is a graphite-based negative electrode active material, and its preparation method is the same as that in Example 1. The main difference is that the carbonization temperature in step (2) and the carbonization temperature in step (5) are different. Specifically, the temperature is first raised to 200°C and held for 80 minutes, and then the temperature is raised to the carbonization temperature of 600°C and held for 2 hours.

[0116] Comparative Example 1

[0117] This comparative example is a graphite-based anode active material, which is prepared in the same way as in Example 1. The main difference is that the prepared graphite-based anode active material has artificial graphite as the core and is coated with a hard carbon layer and a soft carbon layer in sequence on the surface.

[0118] Specifically, step (1) is as follows:

[0119] Mix 30g of maltose (Dv50=10.0µm) with 1kg of artificial graphite material (Dv50=11.0µm) using a VC mixer for 5 minutes and then dry for 4 hours.

[0120] The remaining steps are the same as in Example 1.

[0121] Test case

[0122] 1. Material property testing methods:

[0123] (1) Tap density of the material: Tested in accordance with GB / T 24533-2019, using Dandong Baite BT313 instrument;

[0124] (2) Specific surface area of ​​the material: The test was conducted in accordance with GB / T 19587-2017, and the test instrument was Best 3H-2000A.

[0125] 2. Battery performance testing methods:

[0126] 2.1 Experimental Battery Construction:

[0127] Conductive agent: Conductive carbon material Super P Li;

[0128] Polyvinylidene fluoride (PVDF): Battery grade, 5130, moisture content not exceeding 0.1%;

[0129] N-Methylpyrrolidone (NMP): Battery grade, purity not less than 99.9%, moisture content not greater than 0.02%;

[0130] Copper foil: 10μm thick;

[0131] Lithium-ion battery separator: 0.5mm thick;

[0132] Lithium metal sheet: 18 mm in diameter and 0.5 mm in thickness;

[0133] Battery standard structural components: LIR2430, including positive and negative electrode battery casings; the supporting structure is nickel foam, 2.3 mm thick;

[0134] Weigh and mix PVDF (5130) and dispersant (NMP) at a mass ratio of 1:20 to prepare PVDF solution; homogenize the slurry at a mass ratio of 91.6 wt% (anode material): 1.8 wt% (Super P): 6.6 wt% (PVDF solution) until the slurry is uniform, free of particles and has a certain fluidity.

[0135] The slurry is coated onto copper foil and dried under vacuum (-0.1MPa) at 80℃ for at least 3 hours. The roller gap of the roller press is adjusted to 90μm. The copper foil end of the electrode is steadily inserted into the roller press by hand, with the electrode entering from the front end and exiting from the back end. The electrode is pressed once until the compacted density is 1.0±0.10 g / cm³. 3 scope;

[0136] The rolled electrode sheets are cut using a 14 mm die to obtain graphite material or graphite-coated negative electrode sheets.

[0137] Ethyl carbonate (EC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1:1, and then LiPF6 was uniformly dissolved in the above solution to obtain an electrolyte with a concentration of 1 mol / L.

[0138] Take out the electrode sheets from the glove box and assemble the battery. The assembly sequence is as follows: negative electrode shell - φ20 mm nickel foam (convex side up) - φ18 mm lithium sheet (brushed side up) - 3 drops of electrolyte - φ23 mm separator - 3 drops of electrolyte - φ14 mm negative electrode sheet (material side facing lithium sheet) - positive electrode shell.

[0139] 2.2 Test Method:

[0140] The experimental battery was placed in a constant temperature chamber at 25±0.5℃.

[0141] (1) Battery capacity test method

[0142] The experimental battery was left to stand for 12 hours, then discharged at a constant current of 0.05C to 0.005V. After standing for 10 minutes, it was discharged at a constant current of 10μA to 0.005V, and the discharge capacity Q was recorded. D Then, it is charged at a constant current of 0.1C to 2V, and the charging capacity Q is recorded. C The ratio of charging capacity to the mass of the negative electrode material is the initial charging specific capacity Q of the prepared negative electrode material. C1 The ratio of discharge capacity to the mass of the negative electrode material is the initial discharge specific capacity Q of the prepared negative electrode material. D1 .

[0143] (2) Battery first-efficiency test method

[0144] The first-efficiency of graphite anode materials is calculated according to formula (1):

[0145] = Q C1 / Q D1 ×100% (1)

[0146] In the formula:

[0147] —Initial charge / discharge efficiency;

[0148] Q C1 —Specific capacity of the first charge, in milliampere-hours per gram (mAh / g).

[0149] Q D1 —First discharge specific capacity, in milliampere-hours per gram (mAh / g).

[0150] The calculation result is rounded to one decimal place.

[0151] (3) Battery rate test method

[0152] The experimental battery was processed as follows:

[0153] 1. Let it sit for 6 hours;

[0154] 2. Constant current discharge at 0.1 C, discharging to 0.01 V;

[0155] Constant voltage discharge at 0.01 V for 5 hours;

[0156] 3. Let it sit for 3 minutes;

[0157] 4. Constant current charging at 0.1C, charging to 1.5V;

[0158] 5. Let it sit for 3 minutes;

[0159] 6. Constant current discharge at 0.1 C, discharging to 0.01 V;

[0160] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0161] 7. Let it sit for 3 minutes;

[0162] 8. Constant current charging at 0.2 C, charging to 1.5 V;

[0163] 9. Let it sit for 3 minutes;

[0164] 10. Constant current discharge at 0.2 C, discharge to 0.01 V;

[0165] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0166] 11. Let it sit for 3 minutes;

[0167] 12. Constant current charging at 0.2 C, charging to 1.5 V;

[0168] 13. Let it sit for 3 minutes;

[0169] 14. Constant current discharge at 0.5 C, discharge to 0.01 V;

[0170] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0171] 15. Let it sit for 3 minutes;

[0172] 16. Charge at a constant current of 0.2 C until it reaches 1.5 V;

[0173] 17. Let it sit for 3 minutes;

[0174] 18. Constant current discharge at 1.0 C, discharge to 0.01 V;

[0175] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0176] 19. Let it sit for 3 minutes;

[0177] 20. Constant current charging at 0.2 C, charging to 1.5 V;

[0178] 21. Let it sit for 3 minutes;

[0179] 22. Constant current discharge at 2.0 C, discharging to 0.01 V;

[0180] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0181] 23. Let it sit for 3 minutes;

[0182] 24. Constant current charging at 0.2 C, charging to 1.5 V;

[0183] 25. Let it sit for 3 minutes;

[0184] 26. Constant current discharge at 3.0 C, discharging to 0.01 V;

[0185] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0186] 27. Let it sit for 3 minutes;

[0187] 28. Constant current charging at 0.2 C, charging to 1.5 V;

[0188] 29. Let it sit for 3 minutes;

[0189] 30. Constant current discharge at 4.0 C, discharging to 0.01 V;

[0190] Constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C;

[0191] 31. Let it sit for 3 minutes;

[0192] 32. Constant current charging at 0.2 C, charging to 1.5 V;

[0193] 33. Let it sit for 3 minutes;

[0194] 34. Constant current discharge at 5.0 C, discharging to 0.01 V;

[0195] The constant voltage discharge is 0.01 V, and the cutoff current is 0.01 C.

[0196] The rate performance of the material is calculated based on the rate at the lithium plating window (the point where the voltage spikes) at 80% SOC during constant current discharge.

[0197] The test results are shown in Table 2 below:

[0198] Table 2

[0199]

[0200] Note: The base group is the artificial graphite in Example 1.

[0201] A comparison between Examples 1-9 and the base group shows that this application achieves improved rate performance while maintaining high capacity and first-efficiency. Furthermore, a comparison between Examples 1-9 and Comparative Example 1 shows that by rationally designing the coating structure between hard carbon, graphite, and soft carbon, high capacity, first-efficiency, and rate performance can be achieved simultaneously.

[0202] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0203] The embodiments described above are merely illustrative of several implementation methods of this application, intended to facilitate a detailed understanding of the technical solutions of this application, but should not be construed as limiting the scope of protection of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the scope of protection of this application. It should be understood that technical solutions obtained by those skilled in the art based on the technical solutions provided in this application through logical analysis, reasoning, or limited experimentation are all within the scope of protection of the appended claims. Therefore, the scope of protection of this patent application should be determined by the content of the appended claims, and the specification can be used to interpret the content of the claims.

Claims

1. A graphite-based anode active material, characterized in that, It includes an inner core, a first covering layer covering the surface of the inner core, and a second covering layer covering the surface of the first covering layer; The inner core comprises hard carbon, the first coating layer comprises graphite, and the second coating layer comprises soft carbon.

2. The graphite-based anode active material according to claim 1, characterized in that, The precursor materials for the hard carbon include one or more of the following: sugars, starch, cellulose, acrylonitrile-butadiene-styrene copolymer, acrylic resin, epoxy resin, phenolic resin, urea-formaldehyde resin, polyurethane resin, and silicone resin. Optionally, the sugars include one or more of monosaccharides and disaccharides; Further, optionally, the monosaccharide includes one or both of glucose and fructose; Further optionally, the disaccharide includes one or more of trehalose, sucrose, maltose, and lactose; Optionally, the precursor material for the hard carbon includes disaccharides; Further, optionally, the precursor material for the hard carbon includes one or both of maltose and lactose.

3. The graphite-based anode active material according to claim 1, characterized in that, The precursor material for the soft carbon includes asphalt; Optionally, the softening point of the asphalt is ≥180℃, further optionally, the softening point of the asphalt is ≥205℃, and even more preferably, it is 205℃~250℃; Optionally, the asphalt includes one or more of coal tar pitch, petroleum asphalt, and synthetic asphalt.

4. The graphite-based anode active material according to claim 1, characterized in that, The sum of the amounts of the hard carbon precursor material and the soft carbon precursor material is such that the actual residual carbon content relative to the graphite is 1% to 5%; Optionally, the mass ratio of the hard carbon, the graphite and the soft carbon is (0.5~4.5):100:(0.5~4.5); further optionally, the mass ratio of the hard carbon, the graphite and the soft carbon is (1~3):100:(3~4).

5. The graphite-based anode active material according to any one of claims 1 to 4, characterized in that, The graphite-based anode active material has one or more of the following characteristics: (1) Dv50≥10µm, can be selected as 10µm~30µm; (2) Tap density TD ≥ 0.95 g / cm³ 3 The option is 0.95g / cm³. 3 ~1.3g / cm 3 ; (3) Specific surface area BET≤3.5m 2 / g, optional 1m 2 / g~3.5m 2 / g.

6. A method for preparing graphite-based negative electrode active materials, characterized in that, Includes the following steps: The first premix is ​​prepared by mixing the first binder, the precursor material of hard carbon, and graphite. The first premix is ​​subjected to a first carbonization treatment, pulverized, and a first carbide is prepared. The second binder, the first carbide, and the soft carbon precursor material are mixed to prepare the second premix. The second premix is ​​subjected to a second carbonization treatment to prepare a graphite-based anode active material; Wherein, the Dv50 of the hard carbon precursor material is greater than the Dv50 of the graphite; The Dv50 of the first carbide is greater than the Dv50 of the soft carbon precursor material.

7. The method for preparing the graphite-based negative electrode active material according to claim 6, characterized in that, It has one or more of the following characteristics: (1) The Dv50 of the precursor material of the hard carbon is ≥30µm, and can be ≥1cm; (2) The graphite has a Dv50 ≤ 10µm, and can be selected as 5µm~10µm; (3) The Dv50 of the first carbide is 10µm~20µm; (4) The Dv50 of the soft carbon precursor material is ≤5µm, and can be selected as 1µm~5µm.

8. The method for preparing the graphite-based negative electrode active material according to claim 6 or 7, characterized in that, The carbonization temperature of the first carbonization treatment and the carbonization temperature of the second carbonization treatment are each independently 600℃~1200℃, and can be selected as 1100℃~1200℃; Optionally, during the first carbonization treatment and / or the second carbonization treatment, the temperature is first raised to 150℃~250℃ and held for 60min~100min, and then the temperature is further raised to the carbonization temperature for further treatment.

9. A negative electrode sheet, characterized in that, The graphite-based anode active material includes the graphite-based anode active material according to any one of claims 1 to 5 or the graphite-based anode active material prepared by the preparation method according to any one of claims 6 to 8.

10. A secondary battery, characterized in that, Includes the negative electrode sheet as described in claim 9.

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