Graphite negative electrode active material and preparation method therefor, negative electrode sheet, secondary battery, and electric device

Abstract

A graphite negative electrode active material and a preparation method therefor, a negative electrode sheet containing same, a secondary battery, and an electric device. A particle body of the graphite negative electrode active material comprises an internal area and a surface-layer area at least partially surrounding the internal area; the surface-layer area refers to an area formed by extending from the surface of the particle body of the graphite negative electrode active material to the interior of a particle by a distance of 30 nm; the mass percentage of crystalline carbon in the internal area is denoted as η1; the mass percentage of crystalline carbon in the surface-layer area is denoted as η2; and 10%≤η1-η2≤35%. The graphite negative electrode active material can improve the cycle performance of a battery.

Description

Graphite negative electrode active material and preparation method thereof, negative electrode sheet, secondary battery and electrical device

[0001] Cross-references

[0002] This application claims priority to Chinese Patent Application No. 202311679097.9, filed on December 7, 2023, entitled “Graphite Negative Electrode Active Material, Preparation Method Thereof, Negative Electrode Sheet, Secondary Battery and Electrical Device,” which is incorporated herein by reference in its entirety. Technical Field

[0003] The present disclosure belongs to the field of battery technology, and particularly relates to a graphite negative electrode active material and a preparation method thereof, a negative electrode sheet, a secondary battery and an electrical device. Background Art

[0004] In recent years, secondary batteries have been widely used in energy storage power systems such as hydropower, thermal power, wind power and solar power stations, as well as in power tools, electric bicycles, electric motorcycles, electric vehicles, military equipment, aerospace and other fields.

[0005] As the requirements for the endurance of electrical devices continue to increase, more stringent requirements are placed on the cycle life of secondary batteries. How to further improve the cycle life of secondary batteries is a technical problem that those skilled in the art urgently need to solve.

[0006] Summary of the Invention

[0007] The present disclosure aims to provide a graphite negative electrode active material and a preparation method thereof, a negative electrode plate, a secondary battery, and an electrical device. The graphite negative electrode active material improves battery cycle performance while maintaining good capacity.

[0008] In a first aspect, the present disclosure provides a graphite negative electrode active material, wherein the particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, wherein the mass percentage of crystalline carbon in the internal region is denoted as η1, the mass percentage of crystalline carbon in the surface region is denoted as η2, and 10%≤η1-η2≤35%.

[0009] The relatively low crystalline carbon mass percentage in the surface region of the graphite negative electrode active material compared to the internal region means that the carbon atoms on the surface of the graphite negative electrode active material are arranged in a relatively irregular manner, making the interaction forces between its molecules more complex, resulting in higher strength and hardness than the internal region of the graphite negative electrode active material. On the one hand, it allows the graphite negative electrode active material to retain a high degree of particle integrity during the cold pressing process, reducing the occurrence of side reactions, which is beneficial to improving the cycle stability of the battery. At the same time, during the cycle process, active ions can easily be embedded and de-embedded in the surface region of the graphite negative electrode active material with low crystallinity, which can reduce the expansion rate of the secondary battery during long cycles and further improve the cycle stability of the battery. The relatively high crystallinity in the internal region of the graphite negative electrode active material allows the material to achieve both high capacity and high pressure density while reducing cycle expansion and improving cycle performance, so that the energy density of the battery will not be significantly attenuated.

[0010] In any embodiment, 75%≤η1≤99%, optionally 80%≤η1≤95%.

[0011] The crystalline carbon mass percentage in the internal region of the particle body of the graphite negative electrode active material being within the above-mentioned range is conducive to maintaining a high capacity of the graphite negative electrode active material; and the graphite negative electrode active material will not cause lattice interlacing due to excessive internal defects, so that the graphite negative electrode active material always maintains excellent electrochemical performance during long-term cycling, reducing the probability of battery performance "diving".

[0012] In any embodiment, 55%≤η2≤75%, optionally 60%≤η2≤70%.

[0013] The crystalline carbon mass percentage in the surface area of ​​the graphite negative electrode active material particle body within the above range is beneficial to reducing the physical rebound of the graphite negative electrode active material after cold pressing, reducing the expansion rate of the electrode during the cycle, and thus improving the cycle stability of the secondary battery.

[0014] In any embodiment, the graphite negative electrode active material has a graphitization degree of 88%-95%, and optionally 90%-95%.

[0015] It can be understood that the degree of graphitization is an overall measure of the crystallinity of the graphite negative electrode active material. By regulating the different mass contents of crystalline carbon in the internal and surface areas of the graphite negative electrode active material, the graphite negative electrode active material can have a suitable degree of graphitization, taking into account the capacity and cycle stability of the graphite negative electrode active material.

[0016] In any embodiment, the volume distribution particle size Dv50 of the graphite negative electrode active material is 7.5 μm-14.5 μm, and optionally 8.5 μm-12.5 μm.

[0017] The graphite negative electrode active material with a volume distribution particle size Dv50 within the above range can provide a certain ion channel without causing excessive side reactions due to too small a particle size or too large a specific surface area, and can take into account both kinetic performance and cycle stability.

[0018] In any embodiment, the specific surface area of ​​the graphite negative electrode active material is 1.25 m 2 / g-1.95m 2 / g, optional 1.35m 2 / g-1.75m 2 / g.

[0019] The graphite negative electrode active material has a low specific surface area, so that the graphite negative electrode active material can not only take advantage of the relatively low mass content of crystalline carbon in the surface area to reduce battery expansion, but also reduce the probability of side reactions through the relatively low specific surface area, thereby improving the cycle stability of the battery.

[0020] In any embodiment, the particle size distribution of the graphite negative electrode active material is (Dv90-Dv10) / Dv50 is 1.0-1.6, and optionally 1.1-1.5.

[0021] Controlling the particle size distribution of the graphite negative electrode active material within the above range is beneficial to improving the tight packing of the graphite negative electrode active material and improving the compaction density of the negative electrode film layer. In other words, the cold pressing pressure required for the negative electrode plate to achieve the same compaction density is smaller, thereby reducing the probability of cracking of the graphite negative electrode active material during the cold pressing process, and further improving the integrity of the graphite negative electrode active material during the processing process; and the small stress inside the graphite negative electrode active material particles is beneficial to maintaining the long-period pore structure of the plate during the cycle, and can maintain the original pore structure of the plate during the cycle, so that the lithium ion lithium insertion path remains unobstructed, while reducing the re-filming of the graphite negative electrode active material during the charging process, improving the kinetic performance, cycle life and storage stability. Furthermore, the particle size distribution within the above range can also improve the uniformity of lithium insertion between the graphite negative electrode active material particles, reduce polarization, avoid lithium precipitation caused by uneven current density, and help achieve long-term cycle stability. In addition, the particle size distribution within the above range can also help improve the processing performance of the electrode, and will not affect the uniformity of slurry stirring due to excessive small particles in the graphite negative electrode active material, which is beneficial to improving the uniformity and stability of the electrode quality and helping to achieve long-cycle stability.

[0022] In any embodiment, the powder compaction density of the graphite negative electrode active material under a pressure of 49000N is 1.75g / cm 3 -1.88g / cm 3 , optional 1.78g / cm 3 -1.84g / cm3 .

[0023] Graphite negative electrode active materials with a powder compaction density within the above range are easy to maintain high particle integrity during the cold pressing process, which helps to improve the cycle life of the secondary battery.

[0024] In any embodiment, the tap density of the graphite negative electrode active material is 1.05 g / cm 3 -1.30g / cm 3 .

[0025] In any embodiment, the gram capacity of the graphite negative electrode active material is 345 mAh / g-355 mAh / g, and optionally 347 mAh / g-353 mAh / g.

[0026] The graphite negative electrode active material with a gram capacity within the above range will not undergo significant lattice expansion during the charge and discharge process due to the high graphitization of the graphite negative electrode active material; nor will it be difficult to compact due to the low graphitization of the graphite negative electrode active material. In order to achieve the same electrode compaction density, a higher cold pressing pressure is required, which will cause cracks during the cold pressing process and generate new interfaces during the cycle process to consume excessive active lithium, thereby comprehensively improving the cycle life of the secondary battery.

[0027] In any embodiment, the graphite negative electrode active material I D / I G is 0.05-0.10, where I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at .

[0028] I of graphite negative electrode active material D / I GIt can be used to characterize the surface disorder of graphite negative electrode active materials. Different from the graphite negative electrode active materials that are post-coated on the surface of the graphite particle body, the graphite negative electrode active materials provided by the embodiments of the present disclosure have amorphous carbon on the surface and have a low surface disorder. The amorphous carbon in the active material that is post-coated on the surface of the graphite particle body is usually derived from the coated organic resin, which generates amorphous carbon under low-temperature carbonization, so that the amorphous carbon in the coating layer has a high surface disorder; while the surface area and the internal area of ​​the graphite negative electrode active material provided by the embodiments of the present disclosure are both derived from the same precursor and undergo the same heat treatment process. Therefore, the amorphous carbon of the graphite negative electrode active material is evenly distributed in the surface area of ​​the graphite negative electrode active material, so that the surface area of ​​the graphite negative electrode active material has a low crystalline carbon content and a low surface disorder. On the one hand, this enables the graphite negative electrode active material to take advantage of amorphous carbon, improve the battery's kinetic performance and reduce the cycle expansion rate of the electrode, while not causing excessive side reactions and a significant decrease in battery capacity due to the excessive disorder of the surface of the graphite negative electrode active material, thereby comprehensively improving the battery's cycle stability.

[0029] In any embodiment, the interlayer spacing of the surface area of ​​the graphite negative electrode active material is recorded as d1, the interlayer spacing of the internal area of ​​the graphite negative electrode active material is recorded as d2, and the graphite negative electrode active material satisfies d1>d2; optionally, 0.3365nm≤d1≤0.3378nm; optionally, 0.3358nm≤d2≤0.3364nm.

[0030] In any embodiment, the graphite negative electrode active material includes both primary particles and secondary particles; optionally, based on the total number of primary particles and secondary particles in the graphite negative electrode active material, the number of secondary particles accounts for less than or equal to 50%.

[0031] Graphite anode active materials with a low proportion of secondary particles are beneficial for maintaining the particle integrity of the graphite anode active material during battery preparation, reducing the formation of new interfaces, reducing the consumption of active lithium during the cycle, and further improving the cycle stability of the secondary battery. At the same time, a certain number of secondary particles can reduce the expansion of the negative electrode while taking into account the kinetic performance of the secondary battery, thus ensuring that the battery cell has a kinetic window for the entire life cycle, and will not cause lithium plating due to uneven current distribution, which will lead to a sharp decline in battery capacity and a sharp deterioration in battery life, thereby comprehensively improving the battery's cycle stability.

[0032] The second aspect of the present disclosure provides a method for preparing a graphite negative electrode active material, comprising the following steps: providing raw materials; processing the raw materials to obtain an intermediate product; graphitizing the intermediate product to obtain a graphitized product; screening the graphitized product to obtain a graphite negative electrode active material; the particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, the mass percentage of crystalline carbon in the internal region is recorded as η1, the mass percentage of crystalline carbon in the surface region is recorded as η2, and 10%≤η1-η2≤35%.

[0033] In any embodiment, the maximum power of the graphitization process is 70% to 90% of the rated power of the graphitization process equipment.

[0034] In any embodiment, the temperature of the graphitization treatment is 2600°C to 3000°C.

[0035] In any embodiment, the graphitization treatment time is 10 hours to 50 hours.

[0036] In any embodiment, the raw material includes one or more of petroleum coke, needle coke, and pitch coke, and needle coke can be selected.

[0037] In any embodiment, based on the total volume of the raw material, the volume proportion of the fibrous structure in the raw material is greater than or equal to 55%, and can be optionally 58%-70%.

[0038] Raw materials with a high volume fraction of fiber-type structures are beneficial for increasing the compaction density and specific capacity of graphite negative electrode active materials, allowing the graphite negative electrode active materials to retain a high degree of integrity during the compaction process, resulting in batteries with both a long cycle life and good energy density. However, an excessively high proportion of fiber-type structures will increase the cost and expansion rate of the graphite negative electrode active materials, and deteriorate the kinetic performance. Raw materials with a volume fraction of fiber-type structures within the above range have both lower costs and good specific capacity of the graphite, allowing the battery cell to have a full life cycle kinetic window, thereby comprehensively improving the long-term cycle life and electrochemical performance of the battery.

[0039] In any embodiment, processing the raw materials specifically includes the following steps: crushing, shaping and classifying the raw materials to obtain a first precursor; granulating the first precursor to obtain a second precursor; and low-temperature carbonizing the first precursor and the second precursor to obtain an intermediate product.

[0040] A third aspect of the present disclosure provides a negative electrode sheet, comprising the graphite negative electrode active material in any embodiment or the graphite negative electrode active material prepared by the preparation method in any embodiment.

[0041] In any embodiment, the compaction density of the negative electrode film layer is 1.5 g / cm 3 ~1.65g / cm 3 .

[0042] A fourth aspect of the present disclosure provides a secondary battery comprising the negative electrode sheet according to the third aspect of the present disclosure.

[0043] A fifth aspect of the present disclosure provides an electric device including the secondary battery according to the fourth aspect of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] To more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments of the present disclosure. Obviously, the drawings described below are only some embodiments of the present disclosure, and those skilled in the art can derive other drawings based on the drawings without inventive effort.

[0045] FIG1 is a schematic diagram of a cross-sectional image of a particle of a graphite negative electrode active material disclosed herein;

[0046] FIG2 is a schematic diagram of an embodiment of a secondary battery disclosed herein;

[0047] FIG3 is an exploded schematic diagram of an embodiment of a secondary battery disclosed herein;

[0048] FIG4 is a schematic diagram of an embodiment of a battery module of the present disclosure;

[0049] FIG5 is a schematic diagram of an embodiment of a battery pack of the present disclosure;

[0050] FIG6 is an exploded schematic diagram of the embodiment of the battery pack shown in FIG5 ;

[0051] FIG. 7 is a schematic diagram of an embodiment of an electric device including the secondary battery of the present disclosure as a power source.

[0052] In the accompanying drawings, which are not necessarily drawn to scale, reference numerals are as follows: 1 battery pack, 2 upper housing, 3 lower housing, 4 battery module, 5 secondary battery, 51 housing, 52 electrode assembly, 53 cover plate, 100 graphite negative electrode active material, 101 surface region, 102 internal region. DETAILED DESCRIPTION

[0053] Below, with appropriate reference to the accompanying drawings, the embodiments of the graphite negative electrode active material and its preparation method, the negative electrode sheet, and the secondary battery and electrical device containing the same are specifically disclosed. However, there may be cases where unnecessary detailed descriptions are omitted. For example, there are cases where detailed descriptions of well-known matters and repeated descriptions of actually the same structures are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate the understanding of those skilled in the art. In addition, the drawings and the following description are provided for those skilled in the art to fully understand the present disclosure and are not intended to limit the subject matter described in the claims.

[0054] " scope " disclosed in the present disclosure is limited in the form of lower limit and upper limit, and given range is limited by selecting a lower limit and an upper limit, and selected lower limit and upper limit define the boundary of special scope.The scope that this mode limits can be to include end value or not include end value, and can be combined arbitrarily, and promptly any lower limit can form a scope with any upper limit combination.For example, if the scope of 60-120 and 80-110 is listed for specific parameter, it is understood that the scope of 60-110 and 80-120 is also expected.In addition, if the minimum range value 1 and 2 listed, and if the maximum range value 3,4 and 5 listed, then the following scope can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.In the present disclosure, unless otherwise specified, numerical range " ab " represents the abbreviation of any real number combination between a and b, and wherein a and b are all real numbers. For example, a numerical range of "0-5" indicates that all real numbers between "0-5" are listed herein, and "0-5" is simply an abbreviation for these numerical combinations. Furthermore, when a parameter is expressed as an integer ≥ 2, this is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0055] Unless otherwise specified, all embodiments and optional embodiments of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0056] Unless otherwise specified, all technical features and optional technical features of the present disclosure can be combined with each other to form new technical solutions, and such technical solutions should be considered to be included in the disclosure content of the present disclosure.

[0057] Unless otherwise specified, all steps of the present disclosure may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0058] Unless otherwise specified, the terms "include" and "comprising" mentioned in this disclosure may be open-ended or closed-ended. For example, the terms "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0059] Unless otherwise specified, the term "or" is inclusive in this disclosure. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, the condition "A or B" is satisfied if any of the following conditions are met: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0060] Unless otherwise specified, the terms used in the present disclosure have the common meanings that are generally understood by those skilled in the art.

[0061] Unless otherwise specified, the numerical values ​​of the parameters mentioned in the present disclosure can be measured using various test methods commonly used in the art, for example, they can be measured according to the test methods given in the present disclosure.

[0062] Unless otherwise specified, in the present disclosure, the term "active ions" refers to ions that can be intercalated and extracted back and forth between the positive electrode and the negative electrode of a secondary battery, including but not limited to lithium ions.

[0063] In the present disclosure, the terms "plurality" and "multiplicity" refer to two or more.

[0064] The electrochemical performance of carbon materials is often related to the mass content of their crystalline carbon. Carbon materials with high crystalline carbon mass content usually have higher capacity, but the crystalline carbon layer spacing is low. During long cycles, the repeated embedding and de-embedding of active ions will cause the lattice to expand greatly, resulting in a high cycle expansion rate of the electrode and poor cycle stability.

[0065] Based on this, the present disclosure provides a graphite negative electrode active material, wherein the particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, the mass percentage of crystalline carbon in the internal region is denoted as η1, the mass percentage of crystalline carbon in the surface region is denoted as η2, and 10%≤η1-η2≤35%.

[0066] Figure 1 is a schematic diagram of a cross-sectional image of a particle of the graphite negative electrode active material 100 disclosed herein. As shown in Figure 1 , the region extending 30 nm inward from the surface of the particle of graphite negative electrode active material 100 is the surface region 101, and the region within surface region 101 is the interior region 102.

[0067] In the present disclosure, the mass percentage of crystalline carbon can be tested by methods known in the art. As an example, a slice with a thickness of about 20nm to 50nm is cut from the middle of the graphite negative electrode active material particle body by a focused ion beam (FIB), and then the slice is tested by transmission electron microscopy (TEM). The content of different bond carbon elements in graphite is tested by transmission electron microscopy-energy loss spectroscopy (TEM-EELS) using energy loss spectroscopy (EELS), and the sp 2 Carbon is taken as crystalline carbon, and the mass percentage of crystalline carbon in different regions of the graphite negative electrode active material is determined by integrating the area ratio. At least 10 points and at least 5 samples are selected in each region, and the average mass percentage of crystalline carbon in different regions is calculated.

[0068] In some embodiments, η1-η2 is 10%, 13%, 15%, 16%, 20%, 22%, 24%, 25%, 26%, 28%, 30%, 31%, 35%, or any range therebetween. For example, η1-η2 can be 10%-31%, 10%-28%, 13%-35%, 13%-31%, 13%-28%, 15%-35%, 15%-31%, 15%-28%, 16%-35%, 16%-31%, 16%-28%, 20%-35%, 20%-31%, 20%-28%, 22%-35%, 22%-31%, 24%-35%, 24%-31%, 25%-35%, 25%-31%.

[0069] The relatively low crystalline carbon mass percentage in the surface region of the graphite negative electrode active material compared to the internal region means that the carbon atoms on the surface of the graphite negative electrode active material are arranged in a relatively irregular manner, making the interaction forces between its molecules more complex, resulting in higher strength and hardness than the internal region of the graphite negative electrode active material. On the one hand, it allows the graphite negative electrode active material to retain a high degree of particle integrity during the cold pressing process, reducing the occurrence of side reactions, which is beneficial to improving the cycle stability of the battery. At the same time, during the cycle process, active ions can easily be embedded and de-embedded in the surface region of the graphite negative electrode active material with low crystallinity, which can reduce the expansion rate of the secondary battery during long cycles and further improve the cycle stability of the battery. The relatively high crystallinity in the internal region of the graphite negative electrode active material allows the material to reduce cycle expansion and improve cycle performance while taking into account the high pressure density of the battery, thereby achieving high capacity of the battery.

[0070] In some embodiments, 75%≤η1<99%. In some embodiments, 80%≤η1≤95%.

[0071] In some embodiments, η1 is 75%, 80%, 85%, 90%, 95%, 99%, or any range therebetween.

[0072] The crystalline carbon mass percentage in the internal region of the particle body of the graphite negative electrode active material being within the above-mentioned range is conducive to maintaining a high capacity of the graphite negative electrode active material; and the graphite negative electrode active material will not cause lattice interlacing due to excessive internal defects, so that the graphite negative electrode active material always maintains excellent electrochemical performance during long-term cycling, reducing the probability of battery performance "diving".

[0073] In some embodiments, 55%≤η2≤75%. In some embodiments, 60%≤η2≤70%.

[0074] In some embodiments, η2 is 55%, 57%, 60%, 63%, 65%, 67%, 70%, 73%, 75%, or any range therebetween.

[0075] The crystalline carbon mass percentage in the surface area of ​​the graphite negative electrode active material particle body within the above range is beneficial to reducing the physical rebound of the graphite negative electrode active material after cold pressing, reducing the expansion rate of the electrode during the cycle, and thus improving the cycle stability of the secondary battery.

[0076] In some embodiments, the graphite negative electrode active material has a degree of graphitization of 88% to 95%. In some embodiments, the graphite negative electrode active material has a degree of graphitization of 90% to 95%.

[0077] As used herein, the term "degree of graphitization" refers to an indicator measuring the degree to which carbon atoms form a close-packed hexagonal graphite crystal structure.

[0078] In the present disclosure, the degree of graphitization of the graphite negative electrode active material can be tested using methods known in the art. As an example, high-purity silicon powder (purity ≥ 99.99%) is used as an internal standard for calibration. The graphite negative electrode active material and silicon are mixed in a weight ratio of 5:1, ground uniformly, and pressed into a pellet. Testing is performed using an X-ray diffractometer (e.g., a Bruker D8 Discover), and with reference to JIS K 0131-1996 and JB / T 4220-2011, the average interlayer spacing d002 of the (002) plane in the crystal structure of the graphite negative electrode active material is obtained. The degree of graphitization is then calculated using the formula g = (0.344 - d002) / (0.344 - 0.3354) × 100%. In the above formula, d002 is the average interlayer spacing of the (002) plane in the crystal structure of the graphite negative electrode active material expressed in nanometers (nm).

[0079] In some embodiments, the graphite negative electrode active material has a degree of graphitization of 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, or any range therebetween.

[0080] It can be understood that the degree of graphitization is an overall measure of the crystallinity of the graphite negative electrode active material. By regulating the different mass contents of crystalline carbon in the internal and surface areas of the graphite negative electrode active material, the graphite negative electrode active material can have a suitable degree of graphitization, taking into account the capacity and cycle stability of the graphite negative electrode active material.

[0081] In some embodiments, the volume distribution particle size Dv50 of the graphite negative electrode active material is 7.5 μm to 14.5 μm. In some embodiments, the volume distribution particle size Dv50 of the graphite negative electrode active material is 8.5 μm to 12.5 μm.

[0082] As used herein, the term "volume distribution particle size Dv50" refers to the particle size corresponding to when the cumulative volume distribution number of particles reaches 50% in the particle size distribution curve.

[0083] In the present disclosure, the volume distribution particle size Dv50 of the active material can be measured using methods known in the art. For example, referring to GB / T 19077-2016, it can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0084] In some embodiments, the volume distribution particle size Dv50 of the graphite negative electrode active material is 7.5 μm, 8.5 μm, 9.5 μm, 10.5 μm, 11.5 μm, 12.5 μm, 13.5 μm, 14.5 μm, or any range therebetween.

[0085] The graphite negative electrode active material with a volume distribution particle size Dv50 within the above range can provide a certain ion channel without causing excessive side reactions due to too small a particle size or too large a specific surface area, and can take into account both kinetic performance and cycle stability.

[0086] In some embodiments, the specific surface area of ​​the graphite negative electrode active material is 1.25 m 2 / g-1.95m 2 In some embodiments, the specific surface area of ​​the graphite negative electrode active material is 1.35 m 2 / g-1.75m 2 / g.

[0087] In the present disclosure, the specific surface area of ​​the graphite negative electrode active material can be measured using methods known in the art. As an example, the specific surface area of ​​the graphite negative electrode active material can be measured using the nitrogen adsorption specific surface area analysis method according to GB / T 19587-2017 and calculated using the BET (Brunauer Emmett Teller) method. The testing instrument can be a Tri-Star 3020 specific surface area pore size analyzer available from Micromeritics, Inc., USA.

[0088] In some embodiments, the specific surface area of ​​the graphite negative electrode active material can be 1.25 m 2 / g, 1.35m 2 / g, 1.45m 2 / g, 1.55m 2 / g, 1.65m 2 / g, 1.75m 2 / g, 1.85m 2 / g, 1.95m 2 / g or any range of values ​​between them.

[0089] The graphite negative electrode active material has a low specific surface area, so that the graphite negative electrode active material can not only take advantage of the relatively low mass content of crystalline carbon in the surface area to reduce battery expansion, but also reduce the probability of side reactions through the relatively low specific surface area, thereby improving the cycle stability of the battery.

[0090] In some embodiments, the particle size distribution of the graphite negative electrode active material (Dv90-Dv10) / Dv50 is 1.0-1.6. In some embodiments, the particle size distribution of the graphite negative electrode active material (Dv90-Dv10) / Dv50 is 1.1-1.45.

[0091] In this article, the terms "Dv90" and "Dv10" refer to the particle sizes corresponding to when the cumulative volume distribution number of particles reaches 90% and 10% in the particle size distribution curve, respectively.

[0092] In the present disclosure, the volume distribution particle size Dv90 and Dv10 of the graphite negative electrode active material can be measured using methods known in the art. As an example, referring to GB / T 19077-2016, they can be measured using a laser particle size analyzer. The testing instrument can be a Mastersizer 3000 laser particle size analyzer manufactured by Malvern Instruments Ltd., UK.

[0093] In some embodiments, the particle size distribution of the graphite negative active material (Dv90-Dv10) / Dv50 is 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3, 1.35, 1.4, 1.45, 1.5, 1.55, 1.6, or any range therebetween.

[0094] Controlling the particle size distribution of the graphite negative electrode active material within the above range is beneficial to improving the tight packing of the graphite negative electrode active material and improving the compaction density of the negative electrode film layer. In other words, the cold pressing pressure required for the negative electrode plate to achieve the same compaction density is smaller, thereby reducing the probability of cracking of the graphite negative electrode active material during the cold pressing process, and further improving the integrity of the graphite negative electrode active material during the processing process; and the small stress inside the graphite negative electrode active material particles is beneficial to maintaining the long-period pore structure of the plate during the cycle, and can maintain the original pore structure of the plate during the cycle, so that the lithium ion lithium insertion path remains unobstructed, while reducing the re-filming of the graphite negative electrode active material during the charging process, improving the kinetic performance, cycle life and storage stability. Furthermore, the particle size distribution within the above range can also improve the uniformity of lithium insertion between the graphite negative electrode active material particles, reduce polarization, avoid lithium precipitation caused by uneven current density, and help achieve long-term cycle stability. In addition, the particle size distribution within the above range can also help improve the processing performance of the electrode, and will not affect the uniformity of slurry stirring due to excessive small particles in the graphite negative electrode active material, which is beneficial to improving the uniformity and stability of the electrode quality and helping to achieve long-cycle stability.

[0095] In some embodiments, the powder compaction density of the graphite negative electrode active material under a pressure of 49000N is 1.75g / cm 3 -1.88g / cm3 In some embodiments, the powder compaction density of the graphite negative electrode active material under a pressure of 49000N is 1.78g / cm 3 -1.84g / cm 3 .

[0096] In this article, the term "powder compaction density" refers to the mass of powder particles per unit volume under a certain pressure.

[0097] In the present disclosure, the powder compaction density of the graphite negative electrode active material under a pressure of 49000N can be tested by methods known in the art. As an example, referring to GB / T 24533-2009, 1g of graphite negative electrode active material powder is weighed and added to a bottom area of ​​1.327cm 2 The mold is pressurized to 5000 kg (equivalent to 49000 N), the pressure is maintained for 30 seconds, and then the pressure is released and maintained for 10 seconds. The powder compaction density of the graphite negative electrode active material under a pressure of 49000 N is measured by an electronic pressure testing machine (such as UTM7305 electronic pressure testing machine).

[0098] In some embodiments, the powder compaction density of the graphite negative electrode active material under a pressure of 49000N can be 1.75g / cm 3 , 1.78g / cm 3 , 1.82g / cm 3 , 1.84g / cm 3 , 1.85g / cm 3 , 1.88g / cm 3 or any range of values ​​between them.

[0099] Graphite negative electrode active materials with a powder compaction density within the above range are easy to maintain high particle integrity during the cold pressing process, which helps to improve the cycle life of the secondary battery.

[0100] In some embodiments, the tap density of the graphite negative electrode active material is 1.05 g / cm 3 -1.30g / cm 3 .

[0101] As used herein, the term "tap density" refers to the mass per unit volume of powder in a container measured after being tapped under specified conditions.

[0102] In the present disclosure, the tap density of the graphite negative electrode active material can be measured using methods known in the art. For example, GB / T 5162-2006 can be used for the tap density test using a powder tap density tester. The test instrument can be a Dandong Better BT-301, with the following test parameters: vibration frequency of 250 ± 15 times / minute, amplitude of 3 ± 0.2 mm, vibration count of 5000 times, and a 25 mL graduated cylinder.

[0103] In some embodiments, the tap density of the graphite negative electrode active material is 1.05 g / cm 3 , 1.10g / cm 3 , 1.15g / cm 3 , 1.20g / cm 3 , 1.25g / cm 3 , 1.30g / cm 3 or any range of values ​​between them.

[0104] In some embodiments, the gram capacity of the graphite negative electrode active material is 345 mAh / g to 355 mAh / g. In some embodiments, the gram capacity of the graphite negative electrode active material is 347 mAh / g to 353 mAh / g.

[0105] As used herein, the term "gram capacity" refers to the ratio of the amount of electricity that an active material can release to the mass of the active material.

[0106] In the present disclosure, the gram capacity of the graphite negative electrode active material can be tested using methods known in the art. As an example, a sample of the graphite negative electrode active material is thoroughly stirred and mixed with the conductive agent carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 91.6:1.8:6.6 in an appropriate amount of solvent NMP to form a uniform negative electrode slurry; the negative electrode slurry is evenly coated on the surface of the negative electrode current collector copper foil and dried and cold pressed; then a metal lithium sheet is used as the counter electrode and a polypropylene (PP) film is used as the separator, and an electrolyte is injected. The electrolyte formula used is as follows: dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) are mixed in a weight ratio of 1:1:1 to obtain an organic solvent, and then LiPF6 is dissolved in the above organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. CR2430 button cells are assembled in an argon-protected glove box. At 25°C, the prepared button cell was first discharged at a constant current of 0.05C to 0.005V, then discharged at a constant current of 10μA to 0.005V. The cell was allowed to rest for 5 minutes, and the first-cycle discharge capacity was recorded. The cell was then charged at a constant current of 0.1C to 2.0V, and the charge capacity was recorded. The ratio of the charge capacity of the button cell to the mass of the graphite anode active material sample is the gram capacity of the graphite anode active material.

[0107] In some embodiments, the gram capacity of the graphite negative electrode active material is 345 mAh / g, 346 mAh / g, 347 mAh / g, 348 mAh / g, 349 mAh / g, 350 mAh / g, 351 mAh / g, 352 mAh / g, 353 mAh / g, 354 mAh / g, 355 mAh / g, or any range therebetween.

[0108] The graphite negative electrode active material with a gram capacity within the above range will not undergo significant lattice expansion during the charge and discharge process due to the high graphitization of the graphite negative electrode active material; nor will it be difficult to compact due to the low graphitization of the graphite negative electrode active material. In order to achieve the same electrode compaction density, a higher cold pressing pressure is required, which will cause cracks during the cold pressing process and generate new interfaces during the cycle process to consume excessive active lithium, thereby comprehensively improving the cycle life of the secondary battery.

[0109] In some embodiments, the graphite negative electrode active material is D / I G is 0.05-0.10, where I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at .

[0110] In the present disclosure, the graphite negative electrode active material I D / I G Any known mode of Raman spectroscopy can be used for testing. As an example, referring to GB / T 40219-2021, an InVia Qontor (Reflex) Raman spectrometer is used for testing; a solid-state laser with a wavelength of 523 nm is used as the light source, 100 points are sampled in an area of ​​100 μm × 100 μm, and I D / I G The median is the median of the collected I D / I G The middle number in the data.

[0111] In some embodiments, the graphite negative electrode active material is D / I G 0.05, 0.06, 0.07, 0.08, 0.09, 0.10 or any numerical range therebetween.

[0112] Unlike active materials in which amorphous carbon is coated on the surface of the graphite particle body, the graphite negative electrode active material provided by the embodiment of the present disclosure has amorphous carbon on the surface and has a low degree of surface disorder. The amorphous carbon in the active material in which amorphous carbon is coated on the surface of the graphite particle body is usually derived from a coated organic resin, which generates amorphous carbon under low-temperature carbonization, so that the amorphous carbon in the coating layer has a high degree of surface disorder; while the surface region and the internal region of the graphite negative electrode active material provided by the embodiment of the present disclosure are both derived from the same precursor and undergo the same heat treatment process. Therefore, the amorphous carbon of the graphite negative electrode active material is evenly distributed in the surface region of the graphite negative electrode active material, so that the surface region of the graphite negative electrode active material has a low crystalline carbon content and a low degree of surface disorder. On the one hand, this enables the graphite negative electrode active material to take advantage of amorphous carbon, improve the battery's kinetic performance and reduce the cycle expansion rate of the electrode, while not causing excessive side reactions and a significant decrease in battery capacity due to the excessive disorder of the surface of the graphite negative electrode active material, thereby comprehensively improving the battery's cycle stability.

[0113] In some embodiments, the interlayer spacing of the surface region of the graphite negative electrode active material is denoted as d1, and the interlayer spacing of the inner region of the graphite negative electrode active material is denoted as d2, and the graphite negative electrode active material satisfies d1>d2. In some embodiments, 0.3365nm≤d1≤0.3378nm. In some embodiments, 0.3358nm≤d2≤0.3364nm.

[0114] As used herein, the term "interlayer spacing" refers to the shortest distance between two adjacent carbon atoms in the same carbon layer in graphite.

[0115] In the present disclosure, the interlayer spacing between different regions of the graphite negative electrode active material can be measured using instruments and methods known in the art. For example, a high-resolution transmission electron microscope (HRTEM) can be used for measurement. The measurement instrument can be a Thermo Fisher Scientific Spectra S / TEM scanning transmission electron microscope.

[0116] In some embodiments, the interlayer spacing d1 of the surface region of the graphite negative electrode active material can be selected from 0.3365 nm, 0.3367 nm, 0.3369 nm, 0.3371 nm, 0.3372 nm, 0.3373 nm, 0.3375 nm, 0.3378 nm, or any range therebetween. In some embodiments, the interlayer spacing d2 of the inner region of the graphite negative electrode active material can be selected from 0.3358 nm, 0.3359 nm, 0.3361 nm, 0.3363 nm, 0.3364 nm, or any range therebetween.

[0117] The interlayer spacing in the surface area is higher than that in the internal area of ​​the graphite negative electrode active material, which can play the role of a lithium insertion buffer layer, improve the wettability of the electrolyte to the graphite negative electrode active material, and improve the fast charging performance of the battery.

[0118] In some embodiments, the graphite negative electrode active material includes both primary particles and secondary particles. In some embodiments, the secondary particles account for less than or equal to 50% of the total number of primary particles and secondary particles in the graphite negative electrode active material.

[0119] As used herein, the term "primary particles" refers to particles in a non-agglomerated state.

[0120] As used herein, the term "secondary particles" refers to particles in an agglomerated state formed by the aggregation of two or more primary particles.

[0121] Primary particles and secondary particles can be distinguished by observing the particle cross-section of the graphite negative electrode active material using a scanning electron microscope (SEM). In the present disclosure, the proportion of the number of secondary particles in the graphite negative electrode active material can be tested by methods known in the art. As an example, a cross-section polisher (such as the IB-09010CP argon ion cross-section polisher of Japan's JEOL company) can be used to prepare the cross-section of the negative electrode sheet; then, referring to JY / T010-1996, a scanning electron microscope (such as the Sigma 300 scanning electron microscope of Germany's ZEISS company) is used to scan the cross-section of the negative electrode sheet, and multiple test areas are randomly selected in the test sample. Images of the multiple test areas are obtained using a scanning electron microscope, and the number of graphite negative electrode active materials with secondary particle morphology in each image is counted as the ratio of the total number of graphite negative electrode active material particles. The average value of the multiple statistical results is the number proportion of secondary particles in the graphite negative electrode active material.

[0122] In some embodiments, based on the total number of primary particles and secondary particles in the graphite negative active material, the proportion of secondary particles can be selected as 50%, 40%, 30%, 20%, 10% or any numerical range therebetween.

[0123] Graphite anode active materials with a low proportion of secondary particles are beneficial for maintaining the particle integrity of the graphite anode active material during battery preparation, reducing the formation of new interfaces, reducing the consumption of active lithium during the cycle, and further improving the cycle stability of the secondary battery. At the same time, a certain number of secondary particles can reduce the expansion of the negative electrode while taking into account the kinetic performance of the secondary battery, thus ensuring that the battery cell has a kinetic window for the entire life cycle, and will not cause lithium plating due to uneven current distribution, which will lead to a sharp decline in battery capacity and a sharp deterioration in battery life, thereby comprehensively improving the battery's cycle stability.

[0124] The second aspect of the present disclosure provides a method for preparing a graphite negative electrode active material, comprising the following steps: providing raw materials; processing the raw materials to obtain an intermediate product; graphitizing the intermediate product to obtain a graphitized product; screening the graphitized product to obtain a graphite negative electrode active material; the particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, the mass percentage of crystalline carbon in the internal region is recorded as η1, the mass percentage of crystalline carbon in the surface region is recorded as η2, and 10%≤η1-η2≤35%.

[0125] In this article, the term "graphitization treatment" refers to the heat treatment process of carbon materials. Under the action of high temperature, the carbon material transforms from a two-dimensional carbon network structure to a three-dimensional ordered structure through "microcrystal" growth.

[0126] In some embodiments, the maximum power of the graphitization process is 70% to 90% of the rated power of the graphitization process equipment.

[0127] In some embodiments, the power of the graphitization process may be selected to be 70%, 75%, 80%, 85%, 90% of the rated power of the equipment, or any range of values ​​therebetween.

[0128] It is understood that graphitization equipment refers to any device capable of graphitization, including but not limited to Acheson furnaces, box furnaces, internal furnaces, continuous graphitization furnaces, electric calcining furnaces, medium frequency furnaces, and tubular furnaces. The rated power of graphitization equipment produced by different manufacturers may vary, so you can select the right one based on your actual needs.

[0129] The graphitization treatment power used in the present disclosure needs to be lower than the rated power of the graphitization treatment equipment to achieve uniformity of the temperature field during the graphitization process, ensure the consistency of the material's gram capacity, and help improve the cycle life of the battery.

[0130] In some embodiments, the graphitization treatment equipment is an inner string furnace, and the rated power of the inner string furnace is 25000W-32000W.

[0131] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the rated power of the Acheson furnace is 28,000W-30,000W.

[0132] In some embodiments, the temperature of the graphitization treatment is 2600°C to 3000°C.

[0133] In some embodiments, the temperature of the graphitization treatment is 2600° C., 2700° C., 2800° C., 2900° C., 3000° C., or any range therebetween.

[0134] In some embodiments, the graphitization treatment time is 10 hours to 50 hours.

[0135] In some embodiments, the graphitization treatment time is 10 h, 15 h, 20 h, 25 h, 30 h, 35 h, 40 h, 45 h, 50 h, or any range therebetween.

[0136] In some embodiments, the graphitization treatment equipment is an internal series furnace, and the graphitization treatment time at maximum power is 10 hours to 30 hours.

[0137] In some embodiments, the graphitization treatment equipment is an Acheson furnace, and the graphitization treatment is performed at maximum power for 30 hours to 50 hours.

[0138] In some embodiments, the raw material comprises at least one of petroleum coke, needle coke, and pitch coke. In some embodiments, the raw material is needle coke.

[0139] In this article, the term "petroleum coke" refers to the coke formed by high-temperature carbonization of petroleum residue or petroleum asphalt.

[0140] In this article, the term "needle coke" refers to coal tar pitch or petroleum pitch, which, after undergoing liquid phase carbonization to generate an anisotropic mesophase, can produce coke with a needle-like texture through processes such as high-temperature carbonization.

[0141] As used herein, the term "pitch coke" refers to the solid material produced by carbonizing coal tar pitch at high temperatures.

[0142] In some embodiments, the volume proportion of the fiber-type structure in the raw material is greater than or equal to 55%. In some embodiments, the volume proportion of the fiber-type structure in the raw material is 58%-70%.

[0143] In this article, "fibrous structure" is also called streamlined structure, which refers to the structure of the raw material with obvious fibrous texture observed under a microscope.

[0144] The microstructure of char materials can be generally classified into mosaic, regional, and fibrous types based on their morphological characteristics and the size of their isochromatic zones under a polarizing microscope. Generally, isochromatic zones with a size less than 30 μm are classified as mosaic, those with a size greater than 30 μm as regional, and anisotropic banded isochromatic zones as fibrous.

[0145] In the present disclosure, the volume percentage of the fiber structure in the raw material can be tested using methods known in the art. As an example, according to the provisions of GB 1997-89, the raw material is crushed to 1mm and mixed, and 40g to 50g is separated. A square hole sieve is used to take 4g to 5g of a 0.07mm to 1.0mm grade sample for film making; according to the provisions of MT 116.1-86, powder coke and block coke optical films are prepared. The diameter of the powder coke optical film shall not be less than 22mm, and the volume occupied by the cement shall be less than 1 / 3; the sample is placed on a slide with clay, flattened, and then placed on the stage for focusing. After calibrating the microscope, the polarizer and analyzer are adjusted to make them orthogonal. Insert the azurite inspection plate (1λ) so that the field of view shows the interference color of the first-order red; determine the step length of the moving ruler to ensure that more than 400 effective measuring points are evenly distributed, with a point spacing of 0.3 to 0.5mm and a line spacing of 0.5 to 0.8mm. Starting from one end of the sample, determine the microstructure category under the intersection of the crosshairs, and divide the number of effective measuring points of the fiber-type structure optical organization by the total number of statistical testing points as the volume content of the fiber-type structure in the raw material.

[0146] In some embodiments, based on the total volume of the raw material structure, the volume proportion of the raw material fiber structure can be selected as 55%, 58%, 60%, 63%, 65%, 68%, 70% or any numerical range therebetween.

[0147] Raw materials with a high volume fraction of fiber-type structures are beneficial for increasing the compaction density and specific capacity of graphite negative electrode active materials, allowing the graphite negative electrode active materials to retain a high degree of integrity during the compaction process, resulting in batteries with both a long cycle life and good energy density. However, an excessively high proportion of fiber-type structures will increase the cost and expansion rate of the graphite negative electrode active materials, and deteriorate the kinetic performance. Raw materials with a volume fraction of fiber-type structures within the above range have both lower costs and good specific capacity of the graphite, allowing the battery cell to have a full life cycle kinetic window, thereby comprehensively improving the long-term cycle life and electrochemical performance of the battery.

[0148] In some embodiments, the maximum gram capacity that can be achieved by the raw material is greater than the gram capacity of the graphite negative electrode active material.

[0149] High-grade raw materials are used and the degree of graphitization is controlled so that the maximum gram capacity that the raw materials can achieve is not fully utilized, so that the mass percentage of crystalline carbon in the surface area of ​​the graphite negative electrode active material is lower than the mass percentage of crystalline carbon in the internal area of ​​the graphite negative electrode active material, so as to reduce the cycle expansion of the graphite negative electrode active material and improve the cycle stability of the graphite negative electrode active material.

[0150] In some embodiments, the processing of raw materials specifically includes: crushing, shaping and grading the raw materials to obtain a first precursor; granulating the first precursor to obtain a second precursor; and low-temperature carbonizing a mixture of the first precursor and the second precursor to obtain the intermediate product.

[0151] Crushing is the process of reducing the particle size of raw materials. The raw materials can be crushed by any mechanical device such as crusher, mechanical mill, etc.

[0152] Shaping and grading is the process of adjusting the particle size distribution of the raw materials to obtain a first precursor that meets the particle size requirements. The particle size and particle size distribution of the first precursor can be controlled by adjusting the grading frequency and air intake volume. In some embodiments, the grading frequency is 40 Hz to 50 Hz, and the air damper opening is 20% to 70%.

[0153] It can be understood that low-temperature carbonization of the first precursor and the second precursor to obtain the intermediate product includes low-temperature carbonization of a mixture of the first precursor and the second precursor to obtain the intermediate product; it also includes low-temperature carbonization of the first precursor and the second precursor separately to obtain the first intermediate product and the second intermediate product respectively.

[0154] In some embodiments, the Dv50 particle size of the first precursor is 6.0 μm to 10.0 μm.

[0155] In some embodiments, the Dv50 particle size of the first precursor is 6.0 μm, 7.0 μm, 8.0 μm, 9.0 μm, 10.0 μm, or any range therebetween.

[0156] In some embodiments, the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05-1.75.

[0157] In some embodiments, the particle size distribution of the first precursor (Dv90-Dv10) / Dv50 is 1.05, 1.15, 1.25, 1.35, 1.45, 1.55, 1.65, 1.75, or any range therebetween.

[0158] In some embodiments, the tap density of the first precursor is 0.5 g / cm 3 ~0.7g / cm 3 .

[0159] In some embodiments, the tap density of the first precursor is 0.5 g / cm 3 , 0.55g / cm 3 , 0.6g / cm 3 , 0.65g / cm 3 , 0.7g / cm 3or any range of values ​​between them.

[0160] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm to 15.0 μm.

[0161] In some embodiments, the Dv50 particle size of the second precursor is 11.0 μm, 12.0 μm, 13.0 μm, 14.0 μm, 15.0 μm, or any range therebetween.

[0162] The second precursor is obtained by granulating the first precursor, and therefore, the second precursor mainly forms secondary particles in the graphite negative electrode active material.

[0163] Controlling the particle sizes of the first precursor and the second precursor helps to regulate the particle size and particle size distribution of the graphite negative electrode active material and improve the cycle stability of the battery.

[0164] In some embodiments, the temperature of low-temperature carbonization is 900° C.-1300° C., and the time of low-temperature carbonization is 24 hours-240 hours.

[0165] In some embodiments, the temperature of the low-temperature carbonization may be selected to be 900° C., 1000° C., 1100° C., 1200° C., 1300° C., or any range therebetween.

[0166] In some embodiments, the low-temperature carbonization time may be 24 h, 50 h, 75 h, 100 h, 150 h, 200 h, 240 h, or any range therebetween.

[0167] [Negative electrode]

[0168] In some embodiments, the negative electrode plate includes a negative electrode current collector and a negative electrode film layer disposed on at least one surface of the negative electrode current collector. For example, the negative electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the negative electrode film layer is disposed on either or both of the two opposing surfaces of the negative electrode current collector.

[0169] In some embodiments, the negative electrode film layer includes the graphite negative electrode active material of the first aspect of the embodiment of the present disclosure or the graphite negative electrode active material prepared by the method of the second aspect of the embodiment of the present disclosure, thereby enabling the secondary battery to have high first coulombic efficiency, high energy density and good cycle performance.

[0170] In some embodiments, the compaction density of the negative electrode film layer is 1.5 g / cm 3 ~1.65g / cm 3 .

[0171] In the present disclosure, the compaction density of the negative electrode film layer can be tested using methods known in the art. As an example, an electronic balance is used to weigh a negative electrode sheet test sample with an area of ​​S, and the weight is recorded as W1. The thickness of the negative electrode sheet T1 is measured using a caliper. The weighed electrode sheet film layer is then wiped off, the weight of the negative electrode current collector is weighed, recorded as W2, and the thickness of the negative electrode current collector T2 is measured using a caliper. The compaction density of the negative electrode film layer PD = (W1-W2) / [(T1-T2)×S].

[0172] In some embodiments, the compaction density of the negative electrode film layer is 1.5 g / cm 3 , 1.55g / cm 3 , 1.6g / cm 3 , 1.65g / cm 3 or any range of values ​​between them.

[0173] In some embodiments, the negative electrode film layer may further include other negative electrode active materials in addition to the above-mentioned graphite negative electrode active material. In some embodiments, the other negative electrode active materials include, but are not limited to, one or more of conventional natural graphite, other artificial graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate. The silicon-based material may include one or more of elemental silicon, silicon oxide, silicon-carbon composite, silicon-nitrogen composite, and silicon alloy material. The tin-based material may include one or more of elemental tin, tin oxide, and tin alloy material.

[0174] In some embodiments, the negative electrode film layer may further include a negative electrode conductive agent. The present disclosure does not particularly limit the type of the negative electrode conductive agent. For example, the negative electrode conductive agent may include one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0175] In some embodiments, the negative electrode film layer may further optionally include a negative electrode binder. The present disclosure does not particularly limit the type of the negative electrode binder. As examples, the negative electrode binder may include one or more of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, water-based acrylic resin (e.g., polyacrylic acid PAA, polymethacrylic acid PMAA, sodium polyacrylate PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), and carboxymethyl chitosan (CMCS).

[0176] In some embodiments, the negative electrode film layer may further include other additives. For example, the other additives may include a thickener, such as sodium carboxymethyl cellulose (CMC), a PTC thermistor material, and the like.

[0177] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, copper foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), and polyethylene (PE).

[0178] The negative electrode film layer is typically formed by coating a negative electrode slurry onto a negative electrode current collector, drying, and cold pressing. The negative electrode slurry is typically formed by dispersing the negative electrode active material, an optional conductive agent, an optional binder, and other optional additives in a solvent and stirring until uniformly mixed. The solvent may be, but is not limited to, N-methylpyrrolidone (NMP) or deionized water.

[0179] The negative electrode plate does not exclude other additional functional layers in addition to the negative electrode film layer. For example, in some embodiments, the negative electrode plate described in the present disclosure further includes a conductive primer layer (e.g., composed of a conductive agent and a binder) sandwiched between the negative electrode current collector and the negative electrode film layer and disposed on the surface of the negative electrode current collector; in some embodiments, the negative electrode plate described in the present disclosure further includes a protective layer covering the surface of the negative electrode film layer.

[0180] [Positive electrode]

[0181] In some embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode film layer disposed on at least one surface of the positive electrode current collector. For example, the positive electrode current collector has two surfaces that are opposite to each other in its thickness direction, and the positive electrode film layer is disposed on either or both of the two opposing surfaces of the positive electrode current collector.

[0182] The positive electrode current collector may be a metal foil or a composite current collector. As an example of a metal foil, aluminum foil may be used. The composite current collector may include a polymer material base layer and a metal material layer formed on at least one surface of the polymer material base layer. As an example, the metal material may include one or more of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy. As an example, the polymer material base layer may include one or more of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS) and polyethylene (PE).

[0183] The positive electrode film layer generally includes a positive electrode active material, an optional binder, and an optional conductive agent. The positive electrode film layer is usually formed by coating a positive electrode slurry on the positive electrode current collector and then drying and cold pressing. The positive electrode slurry is usually formed by dispersing a positive electrode active material, an optional conductive agent, an optional binder, and any other components in a solvent and stirring evenly. The solvent can be N-methylpyrrolidone (NMP), but is not limited thereto. As an example, the binder used for the positive electrode film layer may include one or more of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin. As an example, the conductive agent used for the positive electrode film layer includes one or more of superconducting carbon, conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0184] The positive electrode active material can be a positive electrode active material for secondary batteries known in the art.

[0185] When the secondary battery of the present disclosure is a lithium-ion battery, the positive electrode active material may include, but is not limited to, one or more of lithium-containing transition metal oxides, lithium-containing phosphates, and their respective modified compounds. Examples of the lithium transition metal oxides may include, but are not limited to, one or more of lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide, lithium nickel cobalt oxide, lithium manganese cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, and their respective modified compounds. Examples of the lithium-containing phosphates may include, but are not limited to, lithium iron phosphate, a composite material of lithium iron phosphate and carbon, lithium manganese phosphate, a composite material of lithium manganese phosphate and carbon, lithium manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their respective modified compounds.

[0186] In some embodiments, in order to further improve the energy density of the secondary battery, the positive electrode active material for the lithium-ion battery may include a lithium transition metal oxide represented by the general formula Li a Ni b Co c M d O e A f and its modified compounds. 0.8 ≤ a ≤ 1.2, 0.5 ≤ b < 1, 0 < c < 1, 0 < d < 1, 1 ≤ e ≤ 2, 0 ≤ f ≤ 1, M is selected from one or more of Mn, Al, Zr, Zn, Cu, Cr, Mg, Fe, V, Ti, and B, and A is selected from one or more of N, F, S, and Cl.

[0187] In some embodiments, as examples, the positive electrode active material for lithium ion batteries may include LiCoO2, LiNiO2, LiMnO2, LiMn2O4, LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2(NCM333),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.85 Co 0.15 Al 0.05 One or more of O2, LiFePO4 and LiMnPO4.

[0188] In the present disclosure, the modified compounds of the above-mentioned positive electrode active materials may be the ones subjected to doping modification and / or surface coating modification on the positive electrode active materials.

[0189] [Electrolytes]

[0190] In some embodiments, the electrolyte is an electrolyte solution comprising an electrolyte salt and a solvent.

[0191] The type of the electrolyte salt is not particularly limited and can be selected according to actual needs.

[0192] When the secondary battery of the present disclosure is a lithium ion battery, as an example, the electrolyte salt may include one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonyl imide (LiFSI), lithium bistrifluoromethanesulfonyl imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobisoxalatophosphate (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0193] The type of the solvent is not specifically limited and can be selected according to actual needs. In some embodiments, for example, the solvent may include ethylene carbonate (EC), propylene carbonate (PC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE). One or more.

[0194] In some embodiments, the electrolyte may further optionally include additives. For example, the additives may include negative electrode film-forming additives, positive electrode film-forming additives, or additives capable of improving certain properties of the secondary battery, such as additives that improve the overcharge performance of the secondary battery, additives that improve the high-temperature performance of the secondary battery, and additives that improve the low-temperature power performance of the secondary battery.

[0195] [Isolation film]

[0196] The present disclosure has no particular limitation on the type of the isolation membrane, and any known isolation membrane with a porous structure having good chemical stability and mechanical stability can be selected.

[0197] In some embodiments, the material of the isolation membrane may include one or more of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The isolation membrane may be a single-layer film or a multi-layer composite film. When the isolation membrane is a multi-layer composite film, the materials of each layer may be the same or different.

[0198] In some embodiments, the positive electrode sheet, the separator, and the negative electrode sheet can be formed into an electrode assembly through a winding process or a lamination process.

[0199] secondary batteries

[0200] A fourth aspect of the embodiments of the present disclosure provides a secondary battery.

[0201] The present disclosure has no particular restrictions on the types of secondary batteries. For example, the secondary battery can be a lithium-ion battery, etc. In general, a secondary battery includes a positive electrode plate, a negative electrode plate, and an electrolyte, etc. During the charge and discharge process of the secondary battery, active ions are embedded and extracted back and forth between the positive electrode plate and the negative electrode plate, and the electrolyte plays a role in conducting active ions between the positive electrode plate and the negative electrode plate. The present disclosure has no particular restrictions on the type of the electrolyte, and it can be selected according to actual needs. For example, the electrolyte can be selected from at least one of a solid electrolyte and a liquid electrolyte (i.e., an electrolyte). Secondary batteries using electrolytes and some secondary batteries using solid electrolytes can also include an isolation membrane, which is arranged between the positive electrode plate and the negative electrode plate to play an isolation role.

[0202] In some embodiments, the secondary battery may include an outer packaging that can be used to encapsulate the electrode assembly and the electrolyte.

[0203] In some embodiments, the outer packaging can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging can also be a soft package, such as a bag-type soft package. The material of the soft package can be plastic, such as one or more of polypropylene (PP), polybutylene terephthalate (PBT), and polybutylene succinate (PBS).

[0204] The present disclosure has no particular limitation on the shape of the secondary battery, which can be cylindrical, square, or any other shape. FIG2 shows a secondary battery 5 with a square structure as an example.

[0205] In some embodiments, as shown in FIG3 , the outer packaging may include a shell 51 and a cover plate 53. The shell 51 may include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate enclose a receiving cavity. The shell 51 has an opening connected to the receiving cavity, and the cover plate 53 is used to cover the opening to close the receiving cavity. The positive electrode sheet, the negative electrode sheet and the isolation film can be formed into an electrode assembly 52 through a winding process or a lamination process. The electrode assembly 52 is encapsulated in the receiving cavity. The electrolyte is impregnated in the electrode assembly 52. ​​The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, which can be adjusted according to demand.

[0206] The preparation method of the secondary battery disclosed herein is well known. In some embodiments, a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte can be assembled to form a secondary battery. As an example, the positive electrode sheet, separator, and negative electrode sheet can be wound or laminated to form an electrode assembly. The electrode assembly is then placed in an outer packaging, dried, and then injected with electrolyte. The secondary battery is then vacuum packaged, allowed to stand, formed, and shaped to obtain the secondary battery.

[0207] In some embodiments of the present disclosure, the secondary batteries according to the present disclosure may be assembled into a battery module. The battery module may contain multiple secondary batteries, and the specific number may be adjusted according to the application and capacity of the battery module.

[0208] Figure 4 is a schematic diagram of an exemplary battery module 4. As shown in Figure 4 , within the battery module 4, multiple secondary batteries 5 may be arranged sequentially along the length of the battery module 4. Of course, any other arrangement is also possible. Furthermore, the multiple secondary batteries 5 may be secured using fasteners.

[0209] Optionally, the battery module 4 may further include a housing having a receiving space, and the plurality of secondary batteries 5 are received in the receiving space.

[0210] In some embodiments, the battery modules described above may also be assembled into a battery pack, and the number of battery modules contained in the battery pack may be adjusted according to the application and capacity of the battery pack.

[0211] Figures 5 and 6 are schematic diagrams of an exemplary battery pack 1. As shown in Figures 5 and 6, the battery pack 1 may include a battery box and multiple battery modules 4 disposed within the box. The battery box comprises an upper case 2 and a lower case 3. The upper case 2 covers the lower case 3 and forms an enclosed space for accommodating the battery modules 4. The multiple battery modules 4 may be arranged in any manner within the battery box.

[0212] Electrical devices

[0213] The present disclosure also provides an electric device, which includes at least one of the secondary battery, battery module, or battery pack of the present disclosure. The secondary battery, battery module, or battery pack can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be, but is not limited to, a mobile device (such as a mobile phone, a tablet computer, a laptop computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship, a satellite, an energy storage system, etc.

[0214] The electrical device may select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0215] Figure 7 is a schematic diagram of an exemplary electric device. This device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. To meet the high power and high energy density requirements of this device, a battery pack or battery module may be used.

[0216] As another example, the electric device may be a mobile phone, a tablet computer, a laptop computer, etc. Such an electric device is usually required to be lightweight and thin, and may use a secondary battery as a power source.

[0217] Example

[0218] The following examples describe the present disclosure in more detail. These examples are intended to be illustrative only, as various modifications and variations within the scope of the present disclosure will be apparent to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are by mass, and all reagents used in the examples are commercially available or synthesized according to conventional methods and can be used directly without further processing, and all instruments used in the examples are commercially available.

[0219] Example 1

[0220] (1) Preparation of graphite negative electrode active materials

[0221] The needle coke with a fiber structure of 60.8% was crushed; the crushed material was shaped and finely powdered to obtain the first precursor. The Dv50 particle size of the first precursor was 9.1 μm, the particle size distribution (Dv90-Dv10) / Dv50 was 1.35, and the tap density of the first precursor was 0.67 g / cm 3 .

[0222] A portion of the first precursor was granulated and reformed in a reactor to obtain a second precursor with a particle size Dv50 of 14.4 μm;

[0223] The ungranulated first precursor and the second precursor are respectively placed in a kiln for carbonization at a carbonization temperature of 1150° C. and a carbonization time of 24 h to obtain a first intermediate product and a second intermediate product, respectively;

[0224] The first intermediate product and the second intermediate product are respectively placed in an inner string furnace for graphitization treatment at a temperature of 2800°C. The rated power of the inner string furnace is 28000W, and the maximum power of the graphitization treatment is 22400W. The maximum power is 80% of the rated power of the equipment. The maximum power is maintained for 25 hours to obtain primary particles and secondary particles respectively.

[0225] The primary particles and the secondary particles are mixed uniformly in a mass ratio of 1:1, and the mixture is sieved to remove magnetism to obtain the graphite negative electrode active material.

[0226] The crystalline carbon mass percentage η1 in the inner region of the graphite negative electrode active material is 87%, the crystalline carbon mass percentage η2 in the surface region is 65%, η1-η2=22%, the graphitization degree is 92.97%, and the specific surface area is 1.49m 2 / g, the volume distribution particle size Dv50 is 10.5μm, and the powder compaction density under a pressure of 49000N is 1.78g / cm 3 , gram capacity is 350.2mAh / g, I D / I G The particle size distribution (Dv90-Dv10) / Dv50 is 1.36.

[0227] (2) Preparation of negative electrode sheet

[0228] The graphite negative electrode active material prepared above, the conductive agent Super P, the thickener sodium carboxymethyl cellulose CMC-Na, and the binder styrene-butadiene rubber SBR were mixed in a dry material mass ratio of 96:1:1.2:1.8, and deionized water was added as a solvent. The mixture was stirred in a vacuum mixer until the system became uniform to obtain a negative electrode slurry. The negative electrode slurry was evenly coated on the negative electrode current collector copper foil, and the negative electrode sheets were obtained after drying, cold pressing, and slitting. The compacted density of the negative electrode film layer was 1.60g / cm 3 , the surface density is 9.48mg / cm 2 .

[0229] (3) Preparation of positive electrode sheet

[0230] The positive electrode active material lithium iron phosphate (LFP), the conductive agent Super P, and the binder polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 97:1:2, and the solvent N-methylpyrrolidone (NMP) was added. The mixture was stirred in a vacuum mixer until the system became uniform to obtain a positive electrode slurry. The positive electrode slurry was evenly coated on the positive electrode current collector aluminum foil, and the positive electrode sheets were obtained after drying, cold pressing, and slitting. The compacted density of the positive electrode film layer is 2.50g / cm 3 , the surface density is 19.48 mg / cm 2 .

[0231] (4) Preparation of electrolyte

[0232] In an argon atmosphere glove box with a water content of less than 10 ppm, diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC) were mixed in a weight ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to prepare an electrolyte with a concentration of 1.0 mol / L. Vinylene carbonate (VC) was then added, and the VC content was 2% of the total mass of the electrolyte.

[0233] (5) Preparation of isolation membrane

[0234] Polypropylene film is used as the isolation film.

[0235] (6) Preparation of lithium-ion batteries

[0236] The positive electrode sheet, separator, and negative electrode sheet are stacked in order, with the separator placed between the positive and negative electrode sheets to serve as an isolation, and then wound to obtain an electrode assembly; the electrode assembly is placed in an outer package, dried, and then injected with electrolyte, and after vacuum packaging, standing, forming, shaping and other processes, a lithium-ion battery is obtained.

[0237] Examples 2-8

[0238] The preparation method is basically the same as that of Example 1, except that the raw material structure or preparation process is adjusted, thereby adjusting the crystallinity of the inner region and / or surface region of the graphite negative electrode active material. The specific preparation parameters are shown in Table 1.

[0239] Table 1

[0240] Comparative Example 1

[0241] The preparation method of Comparative Example 1 is basically the same as that of Example 1, except that the maximum power of the graphitization treatment is 28000W (rated power of the equipment) and the constant power time at maximum power is 35h, and the surface area of ​​the prepared graphite negative electrode active material body particles has no disordered layer.

[0242] Comparative Example 2

[0243] The preparation method was essentially the same as that of Comparative Example 1, except that the surface of the graphite negative electrode active material prepared in Comparative Example 1 was carbon-coated. Specifically, the material prepared in Comparative Example 1 was mixed with a coating agent (asphalt) at a ratio of 100%:3%. The mixed product was carbonized at a temperature of 1100°C for 2 hours. After cooling to room temperature, the material was sieved and demagnetized to obtain the finished material.

[0244] Performance Testing

[0245] (1) Negative electrode cold pressure rebound rate test

[0246] The thickness of the negative electrode sheet after cold pressing is L0. After the cold pressed negative electrode sheet is placed in an environment of 25°C and 10% humidity for 24 hours, the thickness of the negative electrode sheet is measured to be L1. The cold pressing rebound rate V0 of the negative electrode sheet is calculated by the following formula: V0 = (L1-L0) / L0.

[0247] (2) Negative electrode cycle expansion rate test

[0248] The thickness of the negative electrode sheet after cold pressing in each embodiment and comparative example is recorded as L0. Take the secondary batteries prepared in each embodiment and comparative example, and carry out charge and discharge tests in an environment of 25°C. At a discharge current of 1.0C (i.e., the current value of completely discharging the theoretical capacity within 1 hour), constant current discharge is carried out to 2.5V. Then, constant current charge is carried out to 3.65V at a charging current of 1.0C, and constant voltage charge is continued to be carried out until the current is 0.05C. After the fully charged battery cell is allowed to stand for 5 minutes, constant current discharge is carried out to 2.5V at a discharge current of 1.0C. The discharge capacity at this time is recorded as C0. Charge the secondary battery to 3.65V at 25°C, disassemble the secondary battery and test the thickness of the negative electrode sheet at this time, which is recorded as L2. The cyclic expansion rate of the negative electrode sheet is: (L2-L0) / L0×100%.

[0249] (3) Cyclic stability test

[0250] At 60°C, the batteries of the above examples and comparative examples were charged at a constant current of 1C to a voltage of 3.65V. They were then charged at a constant voltage of 3.65V to a current ≤ 0.05C. The batteries were then discharged at a constant current of 1C to a voltage of 2.5V. This constituted one charge and discharge cycle, and the discharge capacity (C1) of the first cycle was recorded. This charge and discharge cycle was repeated until the battery capacity decayed to 80% of the initial capacity (C1). The test was then stopped, and the number of test cycles was recorded.

[0251] Test results

[0252] As can be seen from Table 2, the graphite negative electrode active material in which the difference in the mass percentage of crystalline carbon between the inner region and the surface region is 10%-35% can achieve improved battery cycle performance.

[0253] Table 2

[0254] As can be seen from Table 3, when 80%≤η1<95%, the battery can achieve both good gram capacity and long cycle life.

[0255] Table 3

[0256] As can be seen from Table 4, when 60%≤η2≤70%, the battery has low cold pressure rebound rate, cycle expansion rate and excellent cycle life.

[0257] Table 4

[0258] It should be noted that the present disclosure is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and within the scope of the technical solution of the present disclosure, embodiments having substantially the same structure as the technical concept and exerting the same effects are all included in the technical scope of the present disclosure. In addition, within the scope of the present disclosure, various modifications that can be imagined by those skilled in the art to the embodiments, and other methods constructed by combining some of the constituent elements of the embodiments are also included in the scope of the present disclosure.

Claims

1. A graphite negative electrode active material, characterized in that: The particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, the mass percentage of crystalline carbon in the internal region is denoted as η1, the mass percentage of crystalline carbon in the surface region is denoted as η2, and 10%≤η1-η2≤35%.

2. The graphite negative electrode active material according to claim 1, characterized in that: 75%≤η1≤99%, optionally 80%≤η1≤95%.

3. The graphite negative electrode active material according to claim 1 or 2, characterized in that: 55%≤η2≤75%, optionally 60%≤η2≤70%.

4. The graphite negative electrode active material according to any one of claims 1 to 3, characterized in that The graphitization degree of the graphite negative electrode active material is 88%-95%, and can be optionally 90%-95%.

5. The graphite negative electrode active material according to any one of claims 1 to 4, characterized in that The volume distribution particle size Dv50 of the graphite negative electrode active material is 7.5 μm-14.5 μm, and can be optionally 8.5 μm-12.5 μm.

6. The graphite negative electrode active material according to any one of claims 1 to 5, characterized in that The graphite negative electrode active material satisfies at least one of the following: (1) The specific surface area of ​​the graphite negative electrode active material is 1.25 m 2 / g-1.95m 2 / g, optional 1.35m 2 / g-1.75m 2 / g; (2) The particle size distribution (Dv90-Dv10) / Dv50 of the graphite negative electrode active material is 1.0-1.6, and can be optionally 1.1-1.5; (3) The powder compaction density of the graphite negative electrode active material under a pressure of 49000N is 1.75g / cm 3 -1.88g / cm 3 , optional 1.78g / cm 3 -1.84g / cm 3 ; (4) The tap density of the graphite negative electrode active material is 1.05 g / cm 3 -1.30g / cm 3 ; (5) The gram capacity of the graphite negative electrode active material is 345 mAh / g-355 mAh / g, and can be optionally 347 mAh / g-353 mAh / g; (6) I of the graphite negative electrode active material D / I G is 0.05-0.10, among which I D / I G is the ratio of the D peak intensity to the G peak intensity obtained from the Raman spectrum, I D Indicates that the Raman spectrum is at 1350±100cm -1 The D peak intensity at I G Indicates that the Raman spectrum is at 1580±100cm -1 The G peak intensity at (7) The interlayer spacing of the surface region of the graphite negative electrode active material is denoted as d1, and the interlayer spacing of the inner region of the graphite negative electrode active material is denoted as d2, and the graphite negative electrode active material satisfies d1>d2; optionally, 0.3365nm≤d1≤0.3378nm; optionally, 0.3358nm≤d2≤0.3364nm; (8) The graphite negative electrode active material includes both primary particles and secondary particles; optionally, based on the total number of primary particles and secondary particles in the graphite negative electrode active material, the number of the secondary particles accounts for less than or equal to 50％。 7. A method for preparing a graphite negative electrode active material, characterized in that: The following steps are involved: Provide raw materials; Processing the raw materials to obtain intermediate products; performing graphitization treatment on the intermediate product to obtain a graphitized product; Screening the graphitized product to obtain a graphite negative electrode active material; The particle body of the graphite negative electrode active material includes an internal region and a surface region that at least partially surrounds the internal region, wherein the surface region refers to a region extending 30 nm from the surface of the particle body of the graphite negative electrode active material to the interior of the particle, the mass percentage of crystalline carbon in the internal region is denoted as η1, the mass percentage of crystalline carbon in the surface region is denoted as η2, and 10%≤η1-η2≤35%.

8. The preparation method according to claim 7, characterized in that: The graphitization treatment satisfies at least one of the following conditions: (1) The maximum power of the graphitization treatment is 70%-90% of the rated power of the graphitization treatment equipment; (2) The temperature of the graphitization treatment is 2600° C. to 3000° C.; (3) The graphitization treatment time is 10 h to 50 h.

9. The preparation method according to claim 7 or 8, characterized in that: The raw material includes one or more of petroleum coke, needle coke, and asphalt coke, and needle coke can be selected.

10. The preparation method according to claim 9, characterized in that: Based on the total volume of the raw material, the volume proportion of the fiber-type structure in the raw material is greater than or equal to 55%, and can be optionally 58%-70%.

11. The preparation method according to any one of claims 7 to 10, characterized in that: The processing of raw materials specifically comprises the following steps: crushing, shaping and classifying the raw materials to obtain a first precursor; Granulating the precursor to obtain a second precursor; The first precursor and the second precursor are carbonized at low temperature to obtain the intermediate product.

12. A negative electrode plate, characterized in that: The negative electrode sheet comprises the graphite negative electrode active material according to any one of claims 1 to 6 or the graphite negative electrode active material prepared by the preparation method according to any one of claims 7 to 11.

13. The negative electrode sheet according to claim 12, characterized in that: The compaction density of the negative electrode film layer is 1.5 g / cm 3 ~1.65g / cm 3 .

14. A secondary battery, characterized in that: Including the negative electrode sheet as described in claim 12 or 13.

15. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 14.

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