Negative electrode active material and preparation method thereof, secondary battery and electric device

Abstract

The invention relates to a negative electrode active material, the negative electrode active material comprises a core structure and a shell structure arranged on the surface of the core structure, pores are formed in the core structure and the shell structure, and the average pore size of the pores in the shell structure is larger than that of the pores in the core structure. The negative electrode active material provided by the invention can give consideration to excellent dynamic performance under the condition of high compaction density. When the negative electrode active material is used in a battery, the energy density of the battery can be improved, and the purpose of quick charging can be achieved. The invention also relates to a preparation method of the negative electrode active material, a secondary battery and an electric device.

Description

Negative electrode active material and preparation method thereof, secondary battery and electric device

[0001] The present application belongs to the technical field of secondary batteries, and specifically relates to a negative electrode active material and a preparation method thereof, a secondary battery and an electrical device.

[0002] Secondary batteries are now widely used in pure electric vehicles, hybrid electric vehicles, smart grids, etc. With the continuous development of the new energy industry, customers have put forward higher requirements for the use of secondary batteries.

[0003] At present, the research on negative electrode active materials is more focused on how to improve their energy density. Compacted density is one of the reference indicators of energy density. Existing negative electrode active materials cannot take into account excellent kinetic performance under high compacted density conditions.

[0004]

[0005] In view of the problems existing in the background technology, the present application provides a negative electrode active material, which can have excellent kinetic performance under the condition of high compaction density.

[0006] The negative electrode active material provided in the first aspect of the present application includes a core structure and a shell structure arranged on the surface of the core structure. Both the core structure and the shell structure have pores, and the average pore size of the pores in the shell structure is larger than the average pore size of the pores in the core structure.

[0007] In the technical solution of the embodiment of the present application, the negative electrode active material has a gradient pore structure by wrapping a shell structure with a larger average pore size outside a core structure with a smaller average pore size. Such a pore structure design can shorten the transmission path of lithium ions, reduce the time required for lithium ions to reach the graphite surface, and make the negative electrode active material have excellent kinetic properties, and improve the wettability of the electrolyte to the negative electrode active material. In addition, the negative electrode active material with a gradient pore structure also has a high compaction density. Therefore, the negative electrode active material can take into account excellent kinetic properties under high compaction density conditions. When the negative electrode active material is used in a battery, the energy density of the battery can be improved and the purpose of fast charging can be achieved.

[0008] In some embodiments, according to the first aspect, a first example of the first aspect is proposed, the average pore size of the pores in the shell structure is recorded as D1, and the average pore size of the pores in the core structure is recorded as D2, then D1 / D2≥4; optionally, 20≤D1 / D2≤50.

[0009] When D1 / D2 satisfies the above relationship, it is beneficial to shorten the transmission path of lithium ions, reduce the time required for lithium ions to reach the graphite surface, make the negative electrode active material have excellent kinetic properties, improve the wettability of the electrolyte to the negative electrode active material, and at the same time ensure a higher compaction density.

[0010] In some embodiments, according to the first aspect, a second example of the first aspect is proposed, the average pore size of the pores in the shell structure is 100nm~1000nm, optionally 200nm~500nm; and / or, the average pore size of the pores in the core structure is 10nm~100nm, optionally 20nm~50nm.

[0011] By optimizing the average pore size of the shell structure and the core structure, it is beneficial to improve the kinetic performance of the negative electrode active material and ensure a high compaction density at the same time. An average pore size that is too large will lead to a loss of energy density of the negative electrode active material; an average pore size that is too small will lead to too little electrolyte contained in the negative electrode active material, and the electrolyte will be more easily squeezed out during the charge and discharge process, resulting in poor wettability, which in turn deteriorates the kinetic performance.

[0012] In some embodiments, according to the first aspect, a third example of the first aspect is proposed, the thickness of the shell structure is denoted as d, and the volume average particle size Dv50 of the negative electrode active material is denoted as D, then the negative electrode active material satisfies: d≥0.1D, that is, d≥1um; optionally, the negative electrode active material satisfies: 0.1D≤d≤0.3D.

[0013] By optimizing the relationship between the shell structure thickness and the particle size of the negative electrode active material, it is beneficial to further improve the kinetic performance of the negative electrode active material, shorten the transmission path of lithium ions, and reduce the time required for lithium ions to reach the graphite surface. If the shell structure thickness is too large, the negative electrode active material will accommodate more electrolyte, the lithium ion transmission path will be reduced, but the energy density will be lost; if the shell structure is too small, the lithium ion transmission path will be too long and the kinetics will deteriorate.

[0014] In some embodiments, according to the first aspect, a fourth example of the first aspect is proposed, the interlayer spacing d002 of the shell structure is greater than the interlayer spacing d002 of the core structure. Optionally, the interlayer spacing d002 of the core structure is 0.3355nm to 0.3365nm, optionally 0.3355nm to 0.3360nm. Optionally, the interlayer spacing d002 of the shell structure is 0.3500nm to 0.3800nm, optionally 0.3600nm to 0.3700nm.

[0015] By optimizing the interlayer spacing d002 of the shell structure and the core structure, it is beneficial to improve the kinetics of the negative electrode material and increase the compaction density. If the interlayer spacing d002 is too large, the negative electrode active material is more difficult to compact; if the interlayer spacing d002 is too small, the lithium ion transmission is slower and the kinetics deteriorates.

[0016] In some embodiments, according to the first aspect, a fifth example of the first aspect is proposed, and the pore size distribution P is defined as (Pn90-Pn10) / Pn50, wherein Pn10 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 10%, Pn50 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 50%, and Pn90 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 90%, wherein the pore size distribution of the shell structure is recorded as P1, satisfying P1≤0.7, optionally, P1 is 0.4~0.6; and / or, the pore size distribution of the core structure is recorded as P2, satisfying P2≤0.6, optionally, P2 is 0.3~0.5.

[0017] By optimizing the pore size distribution of the shell structure and the core structure, it is beneficial to further improve the kinetic performance of the negative electrode active material, shorten the transmission path of lithium ions, and reduce the time required for lithium ions to reach the graphite surface. The smaller the pore size distribution coefficient, the more uniform the pores, the more uniform the lithium insertion and extraction state of the negative electrode active material, and the better the kinetic performance.

[0018] In some embodiments, according to the first aspect, a sixth example of the first aspect is provided, the porosity of the negative electrode active material is 60% to 80%, and can be optionally 60% to 70%.

[0019] If the porosity is too high, the compaction density of the negative electrode active material will be lower; if the porosity is too small, the kinetic performance of the negative electrode active material will be worse.

[0020] In some embodiments, according to the first aspect, a seventh example of the first aspect is provided, wherein the compaction density of the negative electrode active material after being pressed at a pressure of 2000 kgf is 1.6 to 2.0 g / cm 3 , can be selected as 1.7~1.9g / cm 3 .

[0021] The compaction density of the negative electrode active material within the above range can take into account both fast charging performance.

[0022] In some embodiments, according to the first aspect, an eighth example of the first aspect is provided, wherein the specific surface area of ​​the negative electrode active material is 20 to 100 m 2 / g, optional range is 20~50m 2 / g.

[0023] Keeping the specific surface area of ​​the negative electrode active material within the above range is beneficial to increasing the battery capacity, reducing impedance, and increasing the battery charge and discharge rate. If the specific surface area is too large, there will be more side reactions and a shorter lifespan; if the specific surface area is too small, there will be fewer lithium ion transmission paths and it will be difficult to ensure kinetic performance.

[0024] In some embodiments, according to the first aspect, a ninth example of the first aspect is provided, the reversible gram capacity of the negative electrode active material is ≥355 mAh / g, optionally, the reversible gram capacity is ≥360 mAh / g.

[0025] By optimizing the compaction density and pore structure of the negative electrode active material, we ensure that the electrolyte can still quickly migrate to the graphite surface under high compaction density conditions, the electrolyte polarization is small, and the reversible capacity is correspondingly improved, thereby improving the battery's cycle performance.

[0026] In some embodiments, according to the first aspect, a tenth example of the first aspect is provided, in which the volume average particle size Dv50 of the negative electrode active material is 10 to 30 μm, and can be optionally 15 to 25 μm.

[0027] By optimizing the average particle size of the negative electrode active material, it is beneficial to improve the compaction density and the wettability of the electrolyte to the negative electrode active material. The smaller the average particle size of the negative electrode active material, the more exposed the surface is, the faster the lithium ions reach the material surface, the kinetic performance is improved, but the compaction density is correspondingly deteriorated. If the average particle size is too large, bridging is likely to occur during the slurry production process, making filtration difficult, and the lithium ion transmission path becomes longer, resulting in poor kinetic performance.

[0028] In some embodiments, according to the first aspect, an eleventh example of the first aspect is provided, wherein the particle size distribution of the negative electrode active material is 0.8≤(Dv90-Dv10) / Dv50≤1.8, and may be 1.2≤(Dv90-Dv10) / Dv50≤1.5.

[0029] Optimizing the particle size distribution of the negative electrode active material is beneficial to improving the compaction density and the wettability of the electrolyte to the negative electrode active material. If the particle size distribution is too narrow, the size of the particles will not be suitable for the grading, and the tap density and compaction density will deteriorate; if the particle size distribution is too wide, the tortuosity of the electrode layer will increase, the porosity will decrease, and the dynamic performance will deteriorate.

[0030] In some embodiments, according to the first aspect, a twelfth example of the first aspect is provided, wherein the negative electrode active material includes magnetic impurities. The magnetic impurities include at least one of Fe, Ni, Cr, and Zn. Optionally, the content of the magnetic impurities is less than or equal to 1000 ppm.

[0031] Ensuring that the magnetic impurity content of the negative electrode active material is within the above range is beneficial to reducing self-discharge and safety risks.

[0032] In some embodiments, according to the first aspect, a thirteenth example of the first aspect is provided, wherein the negative electrode active material satisfies any one of the following conditions (1)-(4):

[0033] (1) The shell structure is hard carbon and the core structure is soft carbon;

[0034] (2) The shell structure is soft carbon, and the core structure is hard carbon;

[0035] (3) The shell structure is graphite, and the core structure is hard carbon;

[0036] (4) The shell structure is hard carbon, and the core structure is graphite.

[0037] By optimizing the raw materials for forming the shell structure and the core structure, it is beneficial to form a gradient pore structure, thereby improving the electrolyte wettability of the negative electrode active material and increasing its compaction density.

[0038] The second aspect of the present application provides a method for preparing a negative electrode active material, comprising the following steps:

[0039] The first carbon precursor, the catalyst 1 and the dispersant 1 are mixed and calcined to obtain a substance A;

[0040] After mixing substance A, catalyst 2, dispersant 2 and a second carbon precursor, calcining to obtain substance B;

[0041] heat treating the substance B to obtain a negative electrode active material;

[0042] The negative electrode active material includes a core structure and a shell structure arranged on the surface of the core structure, the core structure and the shell structure both have pores, and the average pore size of the pores in the shell structure is larger than the average pore size of the pores in the core structure.

[0043] In the technical solution of the embodiment of the present application, the first carbon precursor and the second carbon precursor are used as raw materials, and after calcination and heat treatment, the negative electrode active material can be obtained, which can take into account excellent kinetic performance under high compaction density conditions. The method is simple to operate, highly repeatable, and is conducive to large-scale industrial production.

[0044] In some embodiments, according to the second aspect, a first example of the second aspect is provided, wherein the first carbon precursor includes at least one of coal tar, coal pitch, petroleum residue, and petroleum pitch.

[0045] In some embodiments, according to the second aspect, a second example of the second aspect is provided, wherein the second carbon precursor includes at least one of a water-soluble phenolic resin, glucose, and sucrose.

[0046] By optimizing the types of raw materials that form the shell structure and the core structure, it is beneficial to further improve the electrolyte wettability of the negative electrode active material and increase its compaction density.

[0047] In some embodiments, according to the second aspect, a third example of the second aspect is proposed, and the preparation method satisfies at least one of the following conditions (1)-(4): (1) Catalyst 1 includes at least one of ferrocene and ferric acetylacetonate; (2) Catalyst 2 includes at least one of ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, and ferric citrate; (3) Dispersant 1 includes at least one of isooctane, xylene, ethyl acetate, cyclohexanone, tetrachloroethylene, tetrahydrofuran, and benzyl alcohol; (4) Dispersant 2 includes at least one of water, ethanol, ethylene glycol, propylene glycol, and oxalic acid.

[0048] By optimizing the types of catalyst 1, catalyst 2, dispersant 1 and dispersant 2, it is helpful to promote the generation of negative electrode active materials with gradient structure. Among them, catalyst 1 and catalyst 2 can promote the complete development of the unit cell of the negative electrode active material under relatively low temperature conditions, ensuring that the battery has a high reversible gram capacity and good cycle performance. In addition, by optimizing the types of catalyst 1, catalyst 2, dispersant 1 and dispersant 2, it is helpful to reduce the requirements for reaction conditions. Compared with the conventional 3000°C synthesis conditions, the energy consumption cost of the preparation method of this application is reduced. In addition, the dispersant helps to evenly disperse the catalyst and the carbon precursor to form a uniform system.

[0049] In some embodiments, according to the second aspect, a fourth example of the second aspect is proposed, the particle size of catalyst 1 is 20nm~200nm, optionally 40nm~100nm; and / or, the particle size of catalyst 2 is 0.2μm~2μm, optionally 0.4μm~1μm.

[0050] By optimizing the particle size of catalyst 1 and catalyst 2, it is beneficial to control the average pore size of the core structure and the shell structure. The larger the particle size of catalyst 1 and catalyst 2, the larger the average pore size of the core structure and the shell structure in the obtained negative electrode active material, and vice versa.

[0051] In some embodiments, according to the second aspect, a fifth example of the second aspect is proposed, wherein the mass ratio of the first carbon precursor to the second carbon precursor is 9:1 to 1:1, which can be optionally 9:1 to 7:3; and / or, the mass ratio of catalyst 1 to the first carbon precursor is (0.1 to 1):1, which can be optionally (0.2 to 0.5):1; and / or, the mass ratio of catalyst 2 to the second carbon precursor is (0.3 to 1):1, which can be optionally (0.4 to 0.7):1.

[0052] By optimizing the mass ratio of the first carbon body and the second carbon body, it is beneficial to ensure that the active material has a high energy density and kinetics. If the mass ratio is too high, the core structure of the negative electrode active material will account for too high a proportion, the porosity will deteriorate, the electrolyte wettability will deteriorate, and the kinetics will deteriorate; if the mass ratio is too low, the core structure of the negative electrode active material will account for too low a proportion, and the energy density will deteriorate.

[0053] By optimizing the amount of catalyst 1 and catalyst 2, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the addition ratio of catalyst 1 and catalyst 2, the larger the average pore size of the core structure and the shell structure in the obtained negative electrode active material, and vice versa.

[0054] In some embodiments, according to the second aspect, a sixth example of the second aspect is proposed, the mass ratio of dispersant 1 to the first carbon precursor is (0.5~5):1, which can be optionally (1~3):1; and / or, the mass ratio of dispersant 2 to the second carbon precursor is (1~5):1, which can be optionally (2~4):1.

[0055] By optimizing the amount of dispersant 1 and dispersant 2, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the addition ratio of dispersant 1 and dispersant 2, the more uniform the average pore size of the core structure and the shell structure in the obtained negative electrode active material.

[0056] In some embodiments, according to the second aspect, a seventh example of the second aspect is proposed, in the step of preparing substance A, the calcination temperature is 600-800°C, and can be optionally 700-800°C; and / or, in the step of preparing substance A, the calcination time is 0.5-5h, and can be optionally 1h-2h.

[0057] By optimizing the calcination temperature of this step, it is beneficial to control the average pore size of the core structure. The higher the calcination temperature, the smaller the average pore size of the core structure in the obtained negative electrode active material.

[0058] The longer the calcination time of the first carbon precursor is, the more completely the dispersant 1 is volatilized, the catalyst 1 is partially decomposed, and the average pore size of the core structure in the obtained negative electrode active material becomes smaller.

[0059] In some embodiments, according to the second aspect, an eighth example of the second aspect is proposed, in the step of preparing substance B, the calcination temperature is 800-1000°C, and can be optionally 800-900°C; and / or, in the step of preparing substance B, the calcination time is 2-5h, and can be optionally 2-3h.

[0060] By optimizing the calcination temperature of this step, it is beneficial to control the average pore size of the shell structure. The higher the calcination temperature, the smaller the average pore size of the shell structure in the obtained negative electrode active material.

[0061] The longer the calcination time of the second carbon precursor is, the more completely the dispersant 2 is volatilized, the catalyst 2 is partially decomposed, and the average pore size of the shell structure in the obtained negative electrode active material becomes smaller.

[0062] In some embodiments, according to the second aspect, a ninth example of the second aspect is proposed, wherein the heat treatment temperature is 1800-2800°C, optionally 2000-2500°C; and / or the heat treatment time is 2-7 days, optionally 3-5 days.

[0063] By optimizing the heat treatment temperature, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the heat treatment temperature, the easier it is for the catalyst to volatilize, and the smaller the average pore size of the core structure and the shell structure in the obtained negative electrode active material.

[0064] As the heat treatment time increases, the average pore size of the core structure and the shell structure gradually decreases. When the heat treatment time increases to a certain extent, the carbon structure is basically stable and the pore size structure no longer changes.

[0065] In some embodiments, according to the second aspect, a tenth example of the second aspect is proposed, after obtaining substance A, substance A is crushed; and / or, after obtaining substance B, substance B is crushed; and / or, after obtaining the negative electrode active material, screening is performed.

[0066] By performing crushing and / or screening, the degree of particle adhesion caused by the heat treatment is reduced, and the particle size of the target size is obtained.

[0067] The third aspect of the present application provides a secondary battery, comprising a negative electrode plate. The negative electrode plate comprises the negative electrode active material described in the first aspect of the present application or the negative electrode active material obtained according to the preparation method described in the second aspect of the present application.

[0068] In the technical solution of the embodiment of the present application, since the negative electrode active material of the first aspect of the present application or the negative electrode active material obtained according to the preparation method of the second aspect of the present application is used, the secondary battery of the present application has improved energy density and can achieve fast charging.

[0069] A fourth aspect of the present application provides an electrical device, comprising the secondary battery described in the third aspect of the present application.

[0070] In the technical solution of the embodiment of the present application, since the negative electrode active material of the first aspect of the present application or the negative electrode active material obtained according to the preparation method of the second aspect of the present application is used, the electrical device of the present application has an improved energy density and can achieve fast charging.

[0071] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, it can be implemented in accordance with the contents of the specification. In order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below.

[0072] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present invention. Moreover, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0073] FIG. 1 is a schematic diagram of one embodiment of a secondary battery.

[0074] FIG. 2 is an exploded view of FIG. 1 .

[0075] FIG. 3 is a schematic diagram of an embodiment of a battery module.

[0076] FIG. 4 is a schematic diagram of an embodiment of a battery pack.

[0077] FIG. 5 is an exploded view of FIG. 4 .

[0078] FIG. 6 is a schematic diagram of an embodiment of a device in which a secondary battery is used as a power source.

[0079] FIG. 7 is a schematic diagram of the structure of the negative electrode active material prepared in Example 1 of the present application.

[0080] FIG8 is a cross-sectional TEM image of the negative electrode active material prepared in Example 1 of the present application.

[0081] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments described herein. On the contrary, these embodiments are provided in order to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0082] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit this application. The terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned figure descriptions are intended to cover non-exclusive inclusions.

[0083] For simplicity, only some numerical ranges are explicitly disclosed herein. However, any lower limit can be combined with any upper limit to form an unambiguous range; and any lower limit can be combined with other lower limits to form an unambiguous range, and any upper limit can be combined with any other upper limit to form an unambiguous range. In addition, although not explicitly stated, each point or single value between the range endpoints is included in the range. Thus, each point or single value can be combined with any other point or single value as its own lower limit or upper limit or with other lower limits or upper limits to form an unambiguous range.

[0084] In the description of this article, it should be noted that, unless otherwise specified, "above" and "below" are inclusive of the number itself, and the "multiple" in "one or more" means more than two (including two).

[0085] In the description of the embodiments of the present application, the term "and / or" is merely a term used to describe the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists, A and B exist at the same time, and B exists.

[0086] In the description of the embodiments of the present application, unless otherwise clearly specified and limited, technical terms such as "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, and it can be the internal connection of two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the embodiments of the present application can be understood according to the specific circumstances.

[0087] The above-mentioned summary of the invention of the present application is not intended to describe each disclosed embodiment or each implementation in the present application. The following description more specifically illustrates exemplary embodiments. In many places throughout the application, guidance is provided by a series of examples, which can be used in various combinations. In each example, enumeration is only used as a representative group and should not be interpreted as exhaustive.

[0088] Secondary batteries are now widely used in pure electric vehicles, hybrid electric vehicles, smart grids and other fields. With the continuous development of the new energy industry, customers have put forward higher requirements for the use of secondary batteries. At present, the research on negative electrode active materials is more focused on how to improve their energy density. Compacted density is one of the reference indicators of energy density. Existing negative electrode active materials cannot take into account excellent kinetic performance under high compacted density conditions.

[0089] In order to solve the above problems, the inventors have designed a negative electrode active material after in-depth research. By wrapping a shell structure with a larger average pore size outside a core structure with a smaller average pore size, the negative electrode active material has a gradient pore structure. The negative electrode active material is first embedded with lithium ions in the part close to the electrolyte, and then embedded with lithium ions in the part away from the electrolyte. This gradient pore structure matches the concentration distribution of lithium ions in the negative electrode sheet, shortens the transmission path of lithium ions, and reduces the time required for lithium ions to reach the graphite surface, so that the negative electrode active material has excellent kinetic properties and improves the wettability of the electrolyte to the negative electrode active material. In addition, the negative electrode active material with a gradient pore structure also has a high compaction density. Therefore, the negative electrode active material can take into account excellent kinetic properties under high compaction density conditions. When the negative electrode active material is used in a battery, the energy density of the battery can be improved and the purpose of fast charging can be achieved.

[0090] The technical solutions described in the embodiments of the present application are applicable to negative electrode active materials, and are also applicable to the preparation process of negative electrode active materials, secondary batteries using the negative electrode active materials, and electrical devices using the secondary batteries.

[0091] In a first aspect, according to some embodiments of the present application, the present application provides a negative electrode active material. The negative electrode active material includes a core structure and a shell structure disposed on the surface of the core structure. The core structure and the shell structure both have pores, and the average pore size of the pores in the shell structure is greater than the average pore size of the pores in the core structure.

[0092] In some embodiments, the average pore size of the pores in the shell structure is denoted as D1, and the average pore size of the pores in the core structure is denoted as D2, then D1 / D2≥4; optionally, 20≤D1 / D2≤50.

[0093] When D1 / D2 satisfies the above relationship, it is beneficial to shorten the transmission path of lithium ions, reduce the time required for lithium ions to reach the graphite surface, make the negative electrode active material have excellent kinetic properties, improve the wettability of the electrolyte to the negative electrode active material, and at the same time ensure a higher compaction density.

[0094] In some specific embodiments, D1 / D2 may be, for example, 4, 10, 15, 20, 25, 30, 35, 40, 45 or 50. D1 / D2 may be within a numerical range formed by any two of the above-listed numerical values ​​as end values.

[0095] In some embodiments, the average pore size of the pores in the shell structure is 100 nm to 1000 nm, optionally 200 nm to 500 nm; and / or the average pore size of the pores in the core structure is 10 nm to 100 nm, optionally 20 nm to 50 nm.

[0096] By optimizing the average pore size of the shell structure and the core structure, it is beneficial to improve the kinetic performance of the negative electrode active material and increase the compaction density. If the average pore size is too large, the energy density of the negative electrode active material will be lost; if the average pore size is too small, the negative electrode active material will contain too little electrolyte, and the electrolyte will be more easily squeezed out during the charge and discharge process, resulting in poor wettability, which in turn deteriorates the kinetic performance.

[0097] In some specific embodiments, the average pore size of the pores in the shell structure may be, for example, 110 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm, 400 nm, 450 nm, 500 nm, 550 nm, 600 nm, 650 nm, 700 nm, 750 nm, 800 nm, 850 nm, 900 nm, 950 nm, or 1 μm. The average pore size of the pores in the shell structure may be within a numerical range consisting of any two of the above listed values ​​as end values. For example, the average pore size of the pores in the shell structure may be 110 nm to 500 nm, 200 to 500 nm, or 500 nm to 1 μm.

[0098] In some specific embodiments, the average pore size of the pores in the core structure may be, for example, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm, 50 nm, 55 nm, 60 nm, 65 nm, 70 nm, 75 nm, 80 nm, 85 nm, 90 nm, 95 nm or 100 nm. Alternatively, the average pore size of the pores in the core structure may be 10 to 50 nm, 20 to 50 nm or 50 nm to 100 nm. The average pore size of the pores in the core structure may be within a numerical range consisting of any two of the above listed values ​​as end values.

[0099] In some embodiments, the thickness of the shell structure is denoted as d, and the volume average particle size Dv50 of the negative electrode active material is denoted as D, then the negative electrode active material satisfies: d≥0.1D, that is, d≥1um; optionally, the negative electrode active material satisfies: 0.1D≤d≤0.3D.

[0100] By optimizing the relationship between the shell structure thickness and the particle size of the negative electrode active material, it is beneficial to further improve the kinetic performance of the negative electrode active material, shorten the transmission path of lithium ions, and reduce the time required for lithium ions to reach the graphite surface. If the shell structure thickness is too large, the negative electrode active material will accommodate more electrolyte, the lithium ion transmission path will be reduced, but the energy density will be lost; if the shell structure is too small, the lithium ion transmission path will be too long and the kinetics will deteriorate.

[0101] In some specific embodiments, d may be equal to 0.1D, 0.2D, 0.3D, 0.4D or 0.5D. Optionally, 0.1D≤d≤0.3D. d may be within a numerical range formed by any two of the above listed values ​​as end values.

[0102] In some embodiments, according to the first aspect, a fourth example of the first aspect is proposed, the interlayer spacing d002 of the shell structure is greater than the interlayer spacing d002 of the core structure. Optionally, the interlayer spacing d002 of the core structure is 0.3355nm to 0.3365nm, optionally 0.3355nm to 0.3360nm. Optionally, the interlayer spacing d002 of the shell structure is 0.3500nm to 0.3800nm, optionally 0.3600nm to 0.3700nm.

[0103] By optimizing the interlayer spacing d002 of the shell structure and the core structure, it is beneficial to improve the kinetics of the negative electrode material and increase the compaction density. If the interlayer spacing d002 is too large, the negative electrode active material is more difficult to compact; if the interlayer spacing d002 is too small, the lithium ion transmission is slower and the kinetics deteriorates.

[0104] In some specific embodiments, the interlayer spacing d002 of the shell structure may be, for example, 0.3355 nm, 0.3356 nm, 0.3357 nm, 0.3358 nm, 0.3359 nm, 0.3360 nm, 0.3361 nm, 0.3362 nm, 0.3363 nm, 0.3364 nm or 0.3365 nm. Optionally, the interlayer spacing d002 of the shell structure may be 0.3355 to 0.3360 nm or 0.3360 to 0.3365 nm. The interlayer spacing d002 of the shell structure may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0105] In some specific embodiments, the interlayer spacing d002 of the core structure may be, for example, 0.3500 nm, 0.3550 nm, 0.3600 nm, 0.3650 nm, 0.3700 nm, 0.3750 nm or 0.3800 nm. Alternatively, the interlayer spacing d002 of the core structure may be 0.3600 to 0.3700 nm or 0.3700 to 0.3800 nm. The interlayer spacing d002 of the core structure may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0106] In some embodiments, the pore size distribution P is defined as (Pn90-Pn10) / Pn50, wherein Pn10 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 10%, Pn50 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 50%, and Pn90 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 90%, wherein the pore size distribution of the shell structure is recorded as P1, satisfying P1≤0.7, optionally, P1 is 0.4~0.6; and / or, the pore size distribution of the core structure is recorded as P2, satisfying P2≤0.6, optionally, P2 is 0.3~0.5.

[0107] By optimizing the pore size distribution of the shell structure and the core structure, it is beneficial to further improve the kinetic performance of the negative electrode active material, shorten the transmission path of lithium ions, and reduce the time required for lithium ions to reach the graphite surface. The smaller the pore size distribution coefficient, the more uniform the pores, the more uniform the lithium insertion and extraction state of the negative electrode active material, and the better the kinetic performance.

[0108] In some specific embodiments, P1 may be, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6 or 0.7. Alternatively, P1 may be 0.4 to 0.6, 0.1 to 0.4 or 0.2 to 0.5. P1 may be within a numerical range formed by any two of the above listed values ​​as end values.

[0109] In some specific embodiments, P2 may be, for example, 0.1, 0.2, 0.3, 0.4, 0.5 or 0.6, and optionally, P2 may be 0.3 to 0.5, 0.1 to 0.3 or 0.2 to 0.4. P2 may be within a numerical range formed by any two of the above listed values ​​as end values.

[0110] In some embodiments, the porosity of the negative electrode active material is 60% to 80%, and optionally 60% to 70%.

[0111] By optimizing the porosity of the negative electrode active material, it is beneficial to improve the kinetic performance of the negative electrode active material and increase the compaction density. If the porosity is too high, the compaction density of the negative electrode active material will be lower; if the porosity is too small, the kinetic performance of the negative electrode active material will be worse.

[0112] In some specific embodiments, the porosity of the negative electrode active material may be, for example, 60%, 61%, 62%, 63%, 64%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79% or 80%. Optionally, the porosity of the negative electrode active material may be 60% to 70% or 70% to 80%. The porosity of the negative electrode active material may be within a numerical range consisting of any two of the above listed values ​​as end values.

[0113] In some embodiments, the compaction density of the negative electrode active material after being pressed at a pressure of 2000 kgf is 1.6 to 2.0 g / cm 3 , can be selected as 1.7~1.9g / cm 3 .

[0114] The compaction density of the negative electrode active material is within the above range, while taking into account fast charging performance.

[0115] In some specific embodiments, the compaction density of the negative electrode active material after being pressed at a pressure of 2000 kgf may be, for example, 1.6 g / cm 3 , 1.7g / cm 3 , 1.8g / cm 3 , 1.9g / cm 3 or 2.0g / cm 3 The compacted density may be within a numerical range consisting of any two of the above-listed numerical values ​​as end values.

[0116] In some embodiments, the specific surface area of ​​the negative electrode active material is 20 to 100 m 2 / g, optional range is 20~50m 2 / g.

[0117] Keeping the specific surface area of ​​the negative electrode active material within the above range is beneficial to increasing the battery capacity, reducing impedance, and increasing the battery charge and discharge rate. If the specific surface area is too large, there will be more side reactions and a shorter lifespan; if the specific surface area is too small, there will be fewer lithium ion transmission paths and it will be difficult to ensure kinetic performance.

[0118] In some specific embodiments, the specific surface area of ​​the negative electrode active material may be, for example, 20 m 2 / g, 25m 2 / g, 30m 2 / g, 35m 2 / g, 40m 2 / g, 45m 2 / g, 50m 2 / g, 55m 2 / g, 60m 2 / g, 65m 2 / g, 70m 2 / g, 75m 2 / g, 80m 2 / g, 85m 2 / g, 90m 2 / g, 95m 2 / g or 100m 2 Optionally, the specific surface area of ​​the negative electrode active material may be 20 to 50 m 2 / g, 30~70m 2 / g or 50m 2 / g~100m 2 The specific surface area of ​​the negative electrode active material may be within a numerical range consisting of any two of the above-listed numerical values ​​as end values.

[0119] In some embodiments, the reversible gram capacity of the negative electrode active material is ≥355 mAh / g, and optionally, the reversible gram capacity is ≥360 mAh / g.

[0120] By optimizing the compaction density and pore structure of the negative electrode active material, we ensure that the electrolyte can still quickly migrate to the graphite surface under high compaction density conditions, the electrolyte polarization is small, and the reversible capacity is correspondingly improved, thereby improving the battery's cycle performance.

[0121] In some specific embodiments, the reversible gram capacity of the negative electrode active material can be, for example, 355 mAh / g, 356 mAh / g, 357 mAh / g, 358 mAh / g, 359 mAh / g, 360 mAh / g, 365 mAh / g, 370 mAh / g, 375 mAh / g. The reversible gram capacity of the negative electrode active material can be within a numerical range formed by any two of the above-listed values ​​as end values.

[0122] In some embodiments, the volume average particle size Dv50 of the negative electrode active material is 10 to 30 μm, and may be 15 to 25 μm.

[0123] By optimizing the average particle size of the negative electrode active material, it is beneficial to improve the compaction density and the wettability of the electrolyte to the negative electrode active material. The smaller the average particle size of the negative electrode active material, the more exposed the surface is, the faster the lithium ions reach the material surface, the kinetic performance is improved, but the compaction density is correspondingly deteriorated. If the average particle size is too large, bridging is likely to occur during the slurry production process, making filtration difficult, and the lithium ion transmission path becomes longer, resulting in poor kinetic performance.

[0124] In some specific embodiments, the volume average particle size Dv50 of the negative electrode active material may be, for example, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm or 30 μm. Optionally, the volume average particle size Dv50 of the negative electrode active material may be 15 to 25 μm, 10 to 20 μm or 20 to 30 μm. The volume average particle size Dv50 of the negative electrode active material may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0125] In some embodiments, the particle size distribution of the negative electrode active material is 0.8≤(Dv90-Dv10) / Dv50≤1.8, and optionally 1.2≤(Dv90-Dv10) / Dv50≤1.5.

[0126] Optimizing the particle size distribution of the negative electrode active material is beneficial to improving the compaction density and the wettability of the electrolyte to the negative electrode active material. Too narrow particle size distribution leads to mismatch of large and small particle grading, and poor tap density and compaction density; too wide particle size distribution leads to increased tortuosity of the electrode layer, reduced porosity, and thus deterioration of dynamic performance.

[0127] In some specific embodiments, the particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material may be equal to 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7 or 1.8. The particle size distribution (Dv90-Dv10) / Dv50 of the negative electrode active material may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0128] In some embodiments, the negative electrode active material includes magnetic impurities. The magnetic impurities mainly come from raw materials, raw material calcination, catalyst 1 and catalyst 2. The magnetic impurities include at least one of Fe, Ni, Cr and Zn. The content of the magnetic impurities is less than or equal to 1000 ppm, for example, less than or equal to 500 ppm.

[0129] Keeping the magnetic impurities in the negative electrode active material within the above range is beneficial to reducing self-discharge and safety risks.

[0130] In some embodiments, the negative electrode active material satisfies the requirement that the shell structure is hard carbon and the core structure is soft carbon.

[0131] In some embodiments, the negative electrode active material satisfies a shell structure of soft carbon and a core structure of hard carbon.

[0132] In some embodiments, the negative electrode active material satisfies a shell structure of graphite and a core structure of hard carbon.

[0133] In some embodiments, the negative electrode active material satisfies a shell structure of hard carbon and a core structure of graphite.

[0134] By optimizing the raw materials for forming the shell structure and the core structure, it is beneficial to form a gradient pore structure, thereby improving the electrolyte wettability of the negative electrode active material and increasing its compaction density. Preferably, the negative electrode active material satisfies the requirements that the shell structure is soft carbon and the core structure is hard carbon.

[0135] The second aspect of the present application provides a method for preparing a negative electrode active material, comprising the following steps:

[0136] The first carbon precursor, the catalyst 1 and the dispersant 1 are mixed and calcined to obtain a substance A;

[0137] After mixing substance A, catalyst 2, dispersant 2 and a second carbon precursor, calcining to obtain substance B;

[0138] The substance B is subjected to heat treatment to obtain the negative electrode active material provided in the first aspect of the present application.

[0139] In the technical solution of the embodiment of the present application, the first carbon precursor, the second carbon precursor and a specific catalyst are used as raw materials, and after calcination and heat treatment, a negative electrode active material can be obtained, which can take into account excellent kinetic performance under high compaction density conditions. The method is simple to operate, highly repeatable, and is conducive to large-scale industrial production.

[0140] In some embodiments, the first carbon precursor includes at least one of coal tar, coal pitch, petroleum residue, and petroleum pitch.

[0141] In some embodiments, the second carbon precursor includes at least one of a water-soluble phenolic resin, glucose, and sucrose.

[0142] By optimizing the types of raw materials that form the shell structure and the core structure, it is beneficial to further improve the electrolyte wettability of the negative electrode active material and increase its compaction density.

[0143] In some embodiments, the catalyst 1 includes at least one of ferrocene and iron acetylacetonate.

[0144] In some embodiments, the catalyst 2 includes at least one of ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, and ferric citrate.

[0145] In some embodiments, the dispersant 1 includes at least one of isooctane, xylene, ethyl acetate, cyclohexanone, tetrachloroethylene, tetrahydrofuran, and benzyl alcohol.

[0146] In some embodiments, the dispersant 2 includes at least one of water, ethanol, ethylene glycol, glycerol, and oxalic acid.

[0147] By optimizing the types of catalyst 1, catalyst 2, dispersant 1 and dispersant 2, it is helpful to promote the generation of negative electrode active materials with gradient structure. Among them, catalyst 1 and catalyst 2 can promote the complete development of the unit cell of the negative electrode active material under relatively low temperature conditions, ensuring that the battery has a high reversible gram capacity and good cycle performance. In addition, by optimizing the types of catalyst 1, catalyst 2, dispersant 1 and dispersant 2, it is helpful to reduce the requirements for reaction conditions. Compared with the conventional 3000°C synthesis conditions, the energy consumption cost of the preparation method of this application is reduced. In addition, the dispersant helps to evenly disperse the catalyst and the carbon precursor to form a uniform system.

[0148] In some embodiments, the particle size of the catalyst 1 is 20 nm to 200 nm, and can be optionally 40 nm to 100 nm.

[0149] In some embodiments, the particle size of the catalyst 2 is 0.2 μm to 2 μm, and can be optionally 0.4 μm to 1 μm.

[0150] By optimizing the particle size of catalyst 1 and catalyst 2, it is beneficial to control the average pore size of the core structure and the shell structure. The larger the particle size of catalyst 1 and catalyst 2, the larger the average pore size of the core structure and the shell structure in the obtained negative electrode active material, and vice versa.

[0151] In some specific embodiments, the particle size of the catalyst 1 may be, for example, 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm or 200 nm. Alternatively, the particle size of the catalyst 1 may be 20 to 100 nm, 40 to 100 nm or 100 to 200 nm. The particle size of the catalyst 1 may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0152] In some specific embodiments, the particle size of the catalyst 2 may be, for example, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm or 2 μm. Optionally, the particle size of the catalyst 2 may be 0.2-1 μm, 0.4-1 μm or 1 μm-2 μm. The particle size of the catalyst 2 may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0153] In some embodiments, the mass ratio of the first carbon precursor to the second carbon precursor is 9:1 to 1:1, and can be optionally 9:1 to 7:3.

[0154] In some embodiments, the mass ratio of the catalyst 1 to the first carbon precursor is (0.1-1):1, and can be optionally (0.2-0.5):1.

[0155] In some embodiments, the mass ratio of the catalyst 2 to the second carbon precursor is (0.3-1):1, and can be optionally (0.4-0.7):1.

[0156] By optimizing the mass ratio of the first carbon body and the second carbon body, it is beneficial to ensure that the active material has a high energy density and kinetics. If the mass ratio is too high, the core structure of the negative electrode active material will account for too high a proportion, the porosity will deteriorate, the electrolyte wettability will deteriorate, and the kinetics will deteriorate; if the mass ratio is too low, the core structure of the negative electrode active material will account for too low a proportion, and the energy density will deteriorate.

[0157] By optimizing the amount of catalyst 1 and catalyst 2, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the addition ratio of catalyst 1 and catalyst 2, the larger the average pore size of the core structure and the shell structure in the obtained negative electrode active material, and vice versa.

[0158] In some specific embodiments, the mass ratio of the first carbon precursor to the second carbon precursor may be, for example, 9: 1, 8: 1, 7: 1, 6: 1, 5: 1, 9: 2, 4: 1, 7: 2, 3: 1, 7: 3, 2: 1, 7: 5, or 1: 1. The mass ratio of the first carbon precursor to the second carbon precursor may be within a numerical range consisting of any two of the above listed values ​​as end values.

[0159] In some specific embodiments, the mass ratio of the catalyst 1 to the first carbon precursor may be, for example, 0.1:1, 0.2:1, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1 or 1:1, and may be (0.1-1):1 or (0.2-0.5):1. The mass ratio of the catalyst 1 to the first carbon precursor may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0160] In some specific embodiments, the mass ratio of the catalyst 2 to the second carbon precursor may be, for example, 0.3:1, 0.4:1, 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1 or 1:1, and may be (0.3-1.0):1, (0.4-0.7):1. The mass ratio of the catalyst 2 to the second carbon precursor may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0161] In some embodiments, the mass ratio of the dispersant 1 to the first carbon precursor is (0.5-5):1, and can be optionally (1-3):1.

[0162] In some embodiments, the mass ratio of the dispersant 2 to the second carbon precursor is (1-5):1, and can be optionally (2-4):1.

[0163] By optimizing the amount of dispersant 1 and dispersant 2, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the addition ratio of dispersant 1 and dispersant 2, the more uniform the average pore size of the core structure and the shell structure in the obtained negative electrode active material.

[0164] In some specific embodiments, the mass ratio of the dispersant 1 to the first carbon precursor may be, for example, 0.5: 1, 1: 1, 1.5: 1, 2: 1, 2.5: 1, 3: 1, 3.5: 1, 4: 1, 4.5: 1 or 5: 1, and may be (0.5-2): 1, (2-5): 1 or (1-3): 1. The mass ratio of the dispersant 1 to the first carbon precursor may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0165] In some specific embodiments, the mass ratio of the dispersant 2 to the second carbon precursor may be, for example, 1: 1, 1.5: 1, 2: 1, 2.5: 1, 3: 1, 3.5: 1, 4: 1, 4.5: 1 or 5: 1, and may be (1-2): 1, (2-4): 1 or (1-3): 1. The mass ratio of the dispersant 2 to the second carbon precursor may be within a numerical range formed by any two of the above-listed values ​​as end values.

[0166] In some embodiments, in the step of preparing substance A, the calcination temperature is 600-800°C, and optionally 700-800°C.

[0167] In some embodiments, in the step of preparing substance A, the calcination time is 0.5 to 5 hours, and can be 1 hour to 2 hours.

[0168] By optimizing the calcination temperature of this step, it is beneficial to control the average pore size of the core structure. The higher the calcination temperature, the smaller the average pore size of the core structure in the obtained negative electrode active material.

[0169] The longer the calcination time of the first carbon precursor is, the more completely the dispersant 1 is volatilized, the catalyst 1 is partially decomposed, and the average pore size of the core structure in the obtained negative electrode active material becomes smaller.

[0170] In some specific embodiments, in the step of preparing substance A, the calcination temperature may be, for example, 600° C., 650° C., 700° C., 750° C. or 800° C. The calcination temperature may be within a numerical range consisting of any two of the above-listed numerical values ​​as end values.

[0171] In some specific embodiments, in the step of preparing substance A, the calcination time can be, for example, 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h or 5 h. The calcination time can be within a numerical range consisting of any two of the above listed values ​​as end values.

[0172] In some embodiments, in the step of preparing substance B, the calcination temperature is 800-1000°C, and optionally 800-900°C.

[0173] In some embodiments, in the step of preparing substance B, the calcination time is 2 to 5 hours, and optionally 2 to 3 hours.

[0174] By optimizing the calcination temperature of this step, it is beneficial to control the average pore size of the shell structure. The higher the calcination temperature, the smaller the average pore size of the shell structure in the obtained negative electrode active material.

[0175] The longer the calcination time of the second carbon precursor is, the more completely the dispersant 2 is volatilized, the catalyst 2 is partially decomposed, and the average pore size of the shell structure in the obtained negative electrode active material becomes smaller.

[0176] In some specific embodiments, in the step of preparing substance B, the calcination temperature may be, for example, 800° C., 850° C., 900° C., 950° C. or 1000° C. The calcination temperature may be within a numerical range consisting of any two of the above-listed numerical values ​​as end values.

[0177] In some specific embodiments, in the step of preparing substance B, the calcination time can be, for example, 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, 4.5 h, or 5 h. The calcination time can be within a numerical range consisting of any two of the above listed values ​​as end values.

[0178] In some embodiments, the temperature of the heat treatment is 1800-2800°C, and optionally 2000-2500°C.

[0179] In some embodiments, the heat treatment time is 2 to 7 days, and optionally 3 to 5 days.

[0180] By optimizing the heat treatment temperature, it is beneficial to control the average pore size of the core structure and the shell structure. The higher the heat treatment temperature, the easier it is for the catalyst to volatilize, and the smaller the average pore size of the core structure and the shell structure in the obtained negative electrode active material.

[0181] As the heat treatment time increases, the average pore size of the core structure and the shell structure gradually decreases. When the heat treatment time increases to a certain extent, the carbon structure is basically stable and the pore size structure no longer changes.

[0182] In some specific embodiments, the temperature of the heat treatment may be, for example, 1800° C., 1900° C., 2000° C., 2100° C., 2200° C., 2300° C., 2400° C., 2500° C., 2600° C., 2700° C. or 2800° C. The temperature of the heat treatment may be within a numerical range consisting of any two of the above-listed numerical values ​​as end values.

[0183] In some specific embodiments, the heat treatment time can be, for example, 2 days, 2.5 days, 3 days, 3.5 days, 4 days, 4.5 days, 5 days, 5.5 days, 6 days, 6.5 days or 7 days. The heat treatment time can be within a numerical range consisting of any two of the above listed values ​​as end values.

[0184] In some embodiments, according to the second aspect, a tenth example of the second aspect is proposed, after obtaining substance A, substance A is crushed; and / or, after obtaining substance B, substance B is crushed; and / or, after obtaining the negative electrode active material, screening is performed.

[0185] By performing crushing and / or screening, the degree of particle adhesion caused by the heat treatment is reduced, and the particle size of the target size is obtained.

[0186] In some specific embodiments, after obtaining substance A, substance A is crushed to an average particle size Dv50 of 8um to 27um, optionally 12um to 22um. For example, substance A is crushed to an average particle size Dv50 of 8um, 9um, 10um, 11um, 12um, 13um, 14um, 15um, 16um, 17um, 18um, 19um, 20um, 21um, 22um, 23um, 24um, 25um, 26um or 27um. The average particle size Dv50 of substance A after crushing can be within the numerical range formed by any two of the above-listed values ​​as end values.

[0187] In some specific embodiments, after obtaining substance B, substance B is crushed to an average particle size Dv50 of 12um to 32um, optionally 17um to 27um. For example, substance B is crushed to an average particle size Dv50 of 12um, 13um, 14um, 15um, 16um, 17um, 18um, 19um, 20um, 21um, 22um, 23um, 24um, 25um, 26um, 27um, 28um, 29um, 30um, 31um or 32um. The average particle size Dv50 of substance B after crushing can be within the numerical range formed by any two of the above-listed values ​​as end values.

[0188] In some specific embodiments, after obtaining the negative electrode active material, screening is performed until Dv50 is 10um to 30um, and can be 15um to 25um. For example, screening is performed until Dv50 is 10um, 11um, 12um, 13um, 14um, 15um, 16um, 17um, 18um, 19um, 20um, 21um, 22um, 23um, 24um, 25um, 26um, 27um, 28um, 29um or 30um. The Dv50 after screening can be within the numerical range formed by any two of the above listed values ​​as end values.

[0189] In some embodiments, the heat treatment may be performed in a graphitization furnace.

[0190] In some embodiments, after the heat treatment, the obtained negative electrode active material is cooled naturally.

[0191] The third aspect of the present application provides a secondary battery, comprising a negative electrode plate. The negative electrode plate comprises the negative electrode active material described in the first aspect of the present application or the negative electrode active material obtained according to the preparation method described in the second aspect of the present application.

[0192] In the technical solution of the embodiment of the present application, since the negative electrode active material of the first aspect of the present application or the negative electrode active material obtained according to the preparation method of the second aspect of the present application is used, the secondary battery of the present application has improved energy density and can achieve fast charging.

[0193] [Secondary battery]

[0194] A secondary battery is a battery that can be recharged to activate the active materials after being discharged and continue to be used.

[0195] Generally, a secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte. During the battery charging and discharging process, active ions are embedded and released back and forth between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet to play a role of isolation. The electrolyte plays a role of conducting ions between the positive electrode sheet and the negative electrode sheet.

[0196] [Negative electrode]

[0197] In a secondary battery, the negative electrode plate generally includes a negative electrode current collector and a negative electrode film layer disposed on the negative electrode current collector, and the negative electrode film layer includes the negative electrode active material provided above in the present application.

[0198] The negative electrode current collector may be a conventional metal foil or a composite current collector (for example, a metal material may be disposed on a polymer substrate to form a composite current collector). As an example, the negative electrode current collector may be a copper foil.

[0199] The negative electrode film layer may also optionally include a binder, a conductive agent and other optional additives.

[0200] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene and carbon nanofibers.

[0201] As an example, the binder may be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[0202] As an example, other optional auxiliary agents may be thickeners and dispersants (such as sodium carboxymethyl cellulose CMC-Na), PTC thermistor materials, etc.

[0203] [Positive electrode]

[0204] In a secondary battery, the positive electrode plate generally includes a positive electrode current collector and a positive electrode film layer disposed on the positive electrode current collector, wherein the positive electrode film layer includes a positive electrode active material.

[0205] The positive electrode current collector may be a conventional metal foil or a composite current collector (a metal material may be disposed on a polymer substrate to form a composite current collector). As an example, the positive electrode current collector may be an aluminum foil.

[0206] The specific type of the positive electrode active material is not limited, and any active material known in the art that can be used for the positive electrode of a secondary battery can be used, and those skilled in the art can select it according to actual needs.

[0207] As an example, the positive electrode active material may include, but is not limited to, one or more of lithium transition metal oxides, lithium-containing phosphates of olivine structure and their respective modified compounds. Examples of 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 modified compounds. Examples of lithium-containing phosphates of olivine structure may include, but are not limited to, one or more of 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 iron manganese phosphate, a composite material of lithium iron manganese phosphate and carbon and their modified compounds. These materials can all be obtained commercially.

[0208] In some embodiments, the modified compounds of the above materials may be doping-modified and / or surface-coated modified materials.

[0209] The positive electrode film layer may also optionally include a binder, a conductive agent and other optional additives.

[0210] As an example, the conductive agent may be one or more of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, SuperP (SP), graphene, and carbon nanofibers.

[0211] As an example, the binder can be one or more of styrene-butadiene rubber (SBR), water-based acrylic resin, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-vinyl acetate copolymer (EVA), polyacrylic acid (PAA), carboxymethyl cellulose (CMC), polyvinyl alcohol (PVA) and polyvinyl butyral (PVB).

[0212] [Isolation film]

[0213] In some embodiments, the secondary battery further includes a separator. The present application has no particular limitation on the type of separator, and any known porous separator with good chemical stability and mechanical stability can be selected.

[0214] In some embodiments, the material of the isolation membrane can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene and polyvinylidene fluoride. The isolation membrane can be a single-layer film or a multi-layer composite film, without particular limitation. When the isolation membrane is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0215] [Electrolyte]

[0216] The secondary battery may include an electrolyte that conducts ions between the positive electrode and the negative electrode. The electrolyte may include an electrolyte salt and a solvent.

[0217] As an example, the electrolyte salt can be selected from one or more of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluorooxalatoborate (LiDFOB), lithium dioxalatoborate (LiBOB), lithium difluorophosphate (LiPO2F2), lithium difluorobis(oxalatophosphate) (LiDFOP) and lithium tetrafluorooxalatophosphate (LiTFOP).

[0218] As an example, the solvent can be selected from one or more of 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), cyclopentane (SF), dimethyl sulfone (MSM), ethyl methyl sulfone (EMS) and diethyl sulfone (ESE).

[0219] In some embodiments, the electrolyte further includes additives, such as additives that may include negative electrode film-forming additives, positive electrode film-forming additives, and additives that can improve certain battery properties, such as additives that improve battery overcharge performance, additives that improve battery high temperature performance, and additives that improve battery low temperature performance.

[0220] In some embodiments, the secondary battery of the present application is a lithium ion secondary battery.

[0221] Secondary batteries can be prepared according to conventional methods in the art, for example, the positive electrode sheet, the separator, and the negative electrode sheet are wound (or stacked) in sequence, so that the separator is placed between the positive electrode sheet and the negative electrode sheet to play an isolating role, to obtain a battery cell, the battery cell is placed in an outer package, the electrolyte is injected and the package is sealed to obtain a secondary battery.

[0222] The embodiment of the present application has no particular limitation on the shape of the secondary battery, which can be cylindrical, square or any other shape. FIG1 is a secondary battery 5 of a square structure as an example.

[0223] In some embodiments, the secondary battery may include an outer package for packaging a positive electrode sheet, a negative electrode sheet, and an electrolyte.

[0224] In some embodiments, referring to FIG. 2 , the outer package 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 can be covered on the opening to close the receiving cavity.

[0225] The positive electrode sheet, the negative electrode sheet and the separator can be wound or laminated to form an electrode assembly 52. ​​The electrode assembly 52 is encapsulated in the housing cavity. The electrolyte is infiltrated 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 needs.

[0226] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer packaging of the secondary battery 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), polybutylene succinate (PBS), etc.

[0227] In some embodiments, secondary batteries 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.

[0228] FIG3 is a battery module 4 as an example. In the battery module 4, a plurality of secondary batteries 5 may be arranged in sequence along the length direction of the battery module 4. Of course, they may also be arranged in any other manner. Further, the plurality of secondary batteries 5 may be fixed by fasteners.

[0229] 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.

[0230] 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.

[0231] FIG4 and FIG5 are battery packs 1 as an example. The battery pack 1 may include a battery box and a plurality of battery modules 4 disposed in the battery box. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be covered on the lower box body 3 to form a closed space for accommodating the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0232] [Electrical devices]

[0233] The present application also provides an electrical device, which includes at least one of the secondary battery, battery module, and battery pack. The secondary battery, battery module, or battery pack can be used as a power source for the electrical device, or as an energy storage unit for the electrical device. The electrical device can be, but is not limited to, a mobile device (such as a mobile phone, 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.

[0234] The electric device can select a secondary battery, a battery module or a battery pack according to its usage requirements.

[0235] FIG6 is a device as an example. The device is a pure electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle, etc. In order to meet the device's requirements for high power and high energy density of secondary batteries, a battery pack or a battery module may be used.

[0236] Another example of a device may be a mobile phone, a tablet computer, a notebook computer, etc. Such a device is usually required to be thin and light, and a secondary battery may be used as a power source.

[0237] Since the negative electrode active material of the first aspect of the present application or the negative electrode active material obtained by the preparation method of the second aspect of the present application is used, the electric device of the present application has an improved energy density and can achieve fast charging.

[0238] The beneficial effects of the present application are further illustrated below in conjunction with embodiments.

[0239] Example

[0240] In order to make the technical problems, technical solutions and beneficial effects solved by the present application clearer, the following will be further described in detail with reference to the embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and is by no means intended to limit the present application and its application. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field without creative work belong to the scope of protection of the present application.

[0241] The materials used in the examples of this application can all be obtained commercially.

[0242] Preparation of negative electrode active materials

[0243] Example 1

[0244] Coal tar, catalyst 1 ferric acetylacetonate (particle size 40nm), and ethyl acetate were mixed uniformly in a mass ratio of 1:0.2:1, calcined at 700℃ for 1h, and crushed to an average particle size Dv50 of 20um to obtain substance A. The prepared substance A was then soaked in an aqueous solution of phenolic resin (phenolic resin: coal tar mass ratio of 7:2) mixed with 300nm ferric chloride, wherein the mass ratio of phenolic resin: ferric chloride: water was 1:0.3:2, calcined at 800℃ for 2h, and crushed to an average particle size Dv50 of 27um to obtain substance B. Substance B was heat treated at 2000℃ in a graphitization furnace for 5 days, and after natural cooling, it was sieved to an average particle size Dv50 of 25um to obtain a negative electrode active material with a gradient pore structure. The structural schematic diagram of the negative electrode active material is shown in Figure 7. The cross-sectional TEM image of the negative electrode active material is shown in Figure 8.

[0245] Example 2-21

[0246] Examples 2-21 were carried out according to the method described in Example 1, except that the parameters listed in the following Table 1 were different from those in Example 1.

[0247] Comparative Examples 1-4

[0248] The method described in Example 8 was followed, except that the parameters listed in the following Table 1 were different from those in Example 8.

[0249] Comparative Example 5

[0250] Coal tar was calcined at 700°C for 1 hour, crushed to an average particle size Dv50 of 20um, heat treated at 3000°C in a graphitization furnace for 7 days, naturally cooled, and sieved to remove the adhering large particles to an average particle size Dv50 of 20um to obtain substance A. Substance A was mixed with petroleum asphalt at a ratio of 20:1 and calcined at 1100°C for 2 hours to obtain substance B, which is the negative electrode active material.

[0251] Table 1

[0252]

[0253]

[0254]

[0255]

[0256]

[0257]

[0258] Using the negative electrode active materials prepared in the above examples and comparative examples, half cells and secondary batteries were prepared according to the following general preparation method.

[0259] Preparation of half-cell

[0260] The prepared negative electrode active material, conductive agent SuperP, binder (PVDF) and solvent NMP (N-methylpyrrolidone) were mixed evenly in a mass ratio of 91.6:1.8:6.6, coated on a 6um copper foil, and vacuum dried at 120°C for 12h to prepare a negative electrode sheet, which was then rolled on a roller press to a compaction density of 1.5g / cm 3The negative electrode was punched into a small disc, which was the working electrode. The lithium sheet was used as the counter electrode, a 12 μm polypropylene film was used as the isolation membrane, and LiPF6 solute was dissolved in ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) (1:1:1) solvent as the electrolyte. The electrolyte was assembled in a CR2032 button cell to make a half-cell.

[0261] Preparation of secondary batteries

[0262] The positive electrode active material LiNi 0.8 Mn 0.1 Co 0.1 O2 is mixed with conductive carbon black and PVDF in a weight ratio of 96:2.5:1.5, and an appropriate amount of N-methylpyrrolidone is added and stirred evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on aluminum foil and dried after coating to obtain a positive electrode sheet. The loading amount of positive electrode active material on the positive electrode sheet is 0.02g / cm 2 .

[0263] The prepared negative electrode active material was mixed with conductive carbon black, sodium carboxymethyl cellulose, and styrene-butadiene rubber in a weight ratio of 96.5:0.8:1.2:1.5, and an appropriate amount of deionized water was added and stirred evenly to obtain a negative electrode slurry. The negative electrode slurry was coated on a 6μm copper foil and dried after coating to obtain a negative electrode sheet. The loading amount of the negative electrode active material on the negative electrode sheet was 0.012g / cm 2 .

[0264] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a volume ratio of 1:1:1, and then LiPF6 is uniformly dissolved in the above solution to obtain an electrolyte. In the electrolyte, the concentration of LiPF6 is 1 mol / L.

[0265] Using polypropylene film as a separator, the positive electrode sheet, separator, and negative electrode sheet prepared above are placed in order, so that the separator is placed between the positive and negative electrode sheets to play a role of isolation, and then wound into shape and packaged in an aluminum-plastic bag. Electrolyte is injected, and after packaging, the capacity is formed to obtain a secondary battery.

[0266] Characterization test of negative electrode active materials

[0267] 1. Reversible gram capacity test of negative electrode active materials

[0268] After the above half-cell was placed at 25°C for 6 hours, it was discharged to 5.0mV at a constant current rate of 0.05C (1C is the current that theoretically charges the battery in 1 hour) in sequence, discharged to 5.0mV at a constant current rate of 50μA, and discharged to 5.0mV at a constant current rate of 10μA; after being placed for 10 minutes, it was charged to 2.0V at a constant current rate of 0.1C, and the constant current charging capacity was recorded as the reversible gram capacity of the negative electrode active material. The test results are shown in Table 2 below.

[0269] 2. Average pore size test of the pores in the shell structure of the negative electrode active material

[0270] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOELJEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selected areas (size 1um*9um) were selected for the shell structure of each particle. The pore size in the selected area was measured, and the average pore size was calculated by statistical analysis. The test results are shown in Table 2 below.

[0271] 3. Average pore size test of the core structure of the negative electrode active material

[0272] According to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOEL JEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selection areas (size 1um*9um) were made for the core structure of each particle. The pore size in the selection area was measured, and the average pore size was calculated by statistical analysis. The test results are shown in Table 2 below.

[0273] 4. Thickness test of the shell structure of the negative electrode active material

[0274] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (such as JOEL JEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selection areas (size of 1um*9um) were made for the shell structure of each particle, and the thickness in the selection area was measured. The test results are shown in Table 2 below.

[0275] 5. Volume average particle size Dv50 test of negative electrode active materials

[0276] Referring to the particle size analysis laser diffraction method of GB / T 19077-2016, the volume distribution average particle size Dv50 of the negative electrode active material was measured using a Mastersizer 3000 laser particle size analyzer. Dv50 represents the particle size corresponding to when the cumulative volume distribution percentage of the material reaches 50%. The test results are shown in Table 2 below.

[0277] 6. Pore size distribution P1 test of the shell structure of negative electrode active materials

[0278] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOEL JEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selection areas (size of 1um*9um) were made for the shell structure of each particle. The size of the pores in the selection area was measured, and the pore size distribution P1 was obtained through statistical analysis. The pore size distribution P1 is (Pn90-Pn10) / Pn50, where Pn10 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 10%, Pn50 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 50%, and Pn90 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 90%. The test results are shown in Table 2 below.

[0279] 7. P2 test of pore size distribution of core structure of negative electrode active material

[0280] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOEL JEM-2100Plus) was used to select n1≥20 particles, and the core structure of each particle was selected as n2≥20 areas (size 1um*9um), and the size of the pores in the selected areas was measured. The pore size distribution P2 was obtained through statistical analysis. The pore size distribution P2 is (Pn90-Pn10) / nP50, where Pn10 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 10%, Pn50 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 50%, and Pn90 represents the pore size corresponding to when the cumulative distribution percentage of the number of holes reaches 90%. The test results are shown in Table 2 below.

[0281] 8. Porosity test of negative electrode active materials

[0282] The porosity of the negative electrode active material was tested by referring to the true density method of GB / T 24586-2009. The test results are shown in Table 2 below.

[0283] 9. Test of the interlayer spacing d002 of the shell structure of the negative electrode active material

[0284] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOEL JEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selection areas (size 1um*9um) were made for the shell structure of each particle. The size of the lattice fringes in the selection area was measured, and the crystal plane spacing d002 was obtained through statistical analysis. The test results are shown in Table 2 below.

[0285] 10. Test of the interlayer spacing d002 of the core structure of the negative electrode active material

[0286] Referring to the JY / T0581-2020 transmission electron microscope analyzer test method, a transmission electron microscope (JOELJEM-2100Plus) was used to select n1≥20 particles, and n2≥20 selection areas (size 1um*9um) were made for the core structure of each particle. The size of the lattice fringes in the selection area was measured, and the crystal plane spacing d002 was obtained through statistical analysis. The test results are shown in Table 2 below.

[0287] 11. Compaction density test of negative electrode active material after being pressed at 2000kgf pressure

[0288] The compaction density of the powder was tested according to the compaction density test method of GB / T 24533-2009. The test results are shown in Table 2 below.

[0289] 12. Specific surface area test of negative electrode active materials

[0290] The specific surface area of ​​the powder was measured by the gas adsorption BET method according to GB / T 19587-2004. The test results are shown in Table 2 below.

[0291] 13. Particle size distribution test of negative electrode active materials

[0292] Referring to the particle size analysis laser diffraction method of GB / T 19077-2016, the volume distribution particle size Dv10, Dv50 and Dv90 of the negative electrode active material were measured using a Mastersizer 3000 laser particle size analyzer, where Dv10 represents the particle size corresponding to 10% of the cumulative volume distribution percentage of the particles, Dv50 represents the particle size corresponding to 50% of the cumulative volume distribution percentage of the particles, and Dv90 represents the particle size corresponding to 90% of the cumulative volume distribution percentage of the particles. The test results are shown in Table 2 below.

[0293] 14. Test of the types and contents of magnetic impurities in negative electrode active materials

[0294] The Fe, Ni, Cr and Zn elements were tested according to EPA 6010D-2014 elemental analysis - inductively coupled plasma emission spectrometry. The test results are shown in Table 2 below.

[0295] Table 2

[0296]

[0297]

[0298]

[0299]

[0300]

[0301] Electrical performance characterization of secondary batteries

[0302] 1. Fast charging performance test:

[0303] The secondary batteries prepared in each embodiment and comparative example were placed at room temperature of 25°C, charged to 4.25V with a constant current of 0.33C, then charged to a current of 0.05C at a constant voltage, and then left for 5 minutes, and then discharged to 2.5V with a constant current of 0.33C, and the constant current discharge capacity was recorded as the initial capacity C0. Then the battery was charged to a full battery potential of 4.25V or a negative electrode cutoff potential of 0mV with a constant current of 0.5C0, 1C0, 1.5C0, 2C0, 2.5C0, 3C0, and 3.5C0 in sequence (reaching any one of the conditions indicates that charging is completed), and discharged to 2.5V with 0.33C0 after each charging. At every interval of 10% SOC (State of Charge), the corresponding negative electrode potential at different charging rates was recorded, and the rate-negative electrode potential curve at different SOCs was plotted. After linear fitting, the charging rate corresponding to the negative electrode potential of 0mV at different SOCs was obtained, which was recorded as Cx (x=2~8). According to the formula (1 / C2+1 / C3+1 / C4+1 / C5+1 / C6+1 / C7+1 / C8)×0.1×60, the secondary battery is charged from 10% SOC to

[0304] The charging time T (min) for 80% SOC. The shorter the time, the better the fast charging performance of the battery. The test results are shown in Table 3 below.

[0305] 2. Negative electrode sheet limit compaction density test:

[0306] The negative electrode sheets prepared in each embodiment and comparative example were placed on a roller press for a rolling test. The roller gap and rolling pressure were adjusted until the sheet could not be pressed down or the edge was exposed. The thickness of the sheet was measured with a micrometer perpendicular to the rolling direction. The average of 12 thicknesses was recorded, i.e., h1 (um). The rolled sheet was punched with a punching machine to 1540.25 mm. 2 Take the copper foil without negative electrode active material and measure its thickness with a micrometer. Record the average of 12 thicknesses and get h0 (um). Punch it to 1540.25mm with a punching machine. 2 The weight of the 12 small discs is recorded and the average value is M0 (g). According to the formula (M1-M0) / 1540.25*(h1-h0)*10 6The ultimate compaction density of the negative electrode sheet is calculated. The larger the value, the higher the energy density of the battery. The test results are shown in Table 3 below.

[0307] Table 3

[0308]

[0309]

[0310] It can be seen from Tables 2 and 3 that the ultimate compaction density of the negative electrode sheet containing the negative electrode active material of the present application is comparable to that of the negative electrode sheet of Comparative Examples 1-5, but at such a high compaction density, the charging time of the secondary battery of the present application is significantly lower than the charging time of the secondary battery corresponding to Comparative Examples 1-5. It can be seen that the negative electrode active material of the present application can take into account excellent kinetic performance under high compaction density conditions and can achieve the purpose of fast charging of the battery.

[0311] The above is only a preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

A negative electrode active material, characterized in that The negative electrode active material includes a core structure and a shell structure disposed on the surface of the core structure. Both the core structure and the shell structure have pores, and the average pore size of the pores in the shell structure is larger than the average pore size of the pores in the core structure. The negative electrode active material according to claim 1, characterized in that The average pore size of the pores in the shell structure is recorded as D1, and the average pore size of the pores in the core structure is recorded as D2, then D1 / D2≥4; optionally, 20≤D1 / D2≤50. The negative electrode active material according to claim 1 or 2, characterized in that The average pore size of the pores in the shell structure is 100 nm to 1000 nm, and may be 200 nm to 500 nm; and / or the average pore size of the pores in the core structure is 10 nm to 100 nm, and may be 20 nm to 50 nm. The negative electrode active material according to any one of claims 1 to 3, characterized in that The thickness of the shell structure is denoted as d, and the volume average particle size Dv50 of the negative electrode active material is denoted as D, then the negative electrode active material satisfies: d≥0.1D, that is, d≥1um; optionally, the negative electrode active material satisfies: 0.1D≤d≤0.3D. The negative electrode active material according to any one of claims 1 to 4, characterized in that The crystal plane interlayer spacing d002 of the shell structure is greater than the crystal plane interlayer spacing d002 of the core structure; optionally, the crystal plane interlayer spacing d002 of the core structure is 0.3355nm~0.3365nm, optionally 0.3355nm~0.3360nm; optionally, the crystal plane interlayer spacing d002 of the shell structure is 0.3500nm~0.3800nm, optionally 0.3600nm~0.3700nm. The negative electrode active material according to any one of claims 1 to 5, characterized in that The pore size distribution P is defined as (Pn90-Pn10) / Pn50, wherein Pn10 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 10%, Pn50 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 50%, and Pn90 represents the pore size corresponding to when the cumulative distribution percentage of the number of pores reaches 90%, wherein the pore size distribution of the shell structure is recorded as P1, satisfying P1≤0.7, optionally, P1 is 0.4~0.6; and / or, the pore size distribution of the core structure is recorded as P2, satisfying P2≤0.6, optionally, P2 is 0.3~0.

5. The negative electrode active material according to any one of claims 1 to 6, characterized in that The porosity is 60% to 80%, and can be optionally 60% to 70%. The negative electrode active material according to any one of claims 1 to 7, characterized in that The compaction density of the negative electrode active material after being pressed at a pressure of 2000 kgf is 1.6 to 2.0 g / cm 3 , can be selected as 1.7~1.9g / cm 3 . The negative electrode active material according to any one of claims 1 to 8, characterized in that The specific surface area of ​​the negative electrode active material is 20 to 100 m 2 / g, optional 20~50m 2 / g. The negative electrode active material according to any one of claims 1 to 9, characterized in that The reversible gram capacity of the negative electrode active material is ≥355 mAh / g, and optionally, the reversible gram capacity is ≥360 mAh / g. The negative electrode active material according to any one of claims 1 to 10, characterized in that The volume average particle size Dv50 of the negative electrode active material is 10 μm to 30 μm, and can be optionally 15 μm to 25 μm. The negative electrode active material according to any one of claims 1 to 11, characterized in that The particle size distribution of the negative electrode active material is 0.8≤(Dv90-Dv10) / Dv50≤1.8, and can be optionally 1.2≤(Dv90-Dv10) / Dv50≤1.

5. The negative electrode active material according to any one of claims 1 to 12, characterized in that The negative electrode active material includes magnetic impurities, and the magnetic impurities include at least one of Fe, Ni, Cr, and Zn; optionally, the content of the magnetic impurities is less than or equal to 1000 ppm. The negative electrode active material according to any one of claims 1 to 13, characterized in that The negative electrode active material satisfies any one of the following conditions (1)-(4): (1) the shell structure is hard carbon and the core structure is soft carbon; (2) the shell structure is soft carbon and the core structure is hard carbon; (3) the shell structure is graphite and the core structure is hard carbon; (4) the shell structure is hard carbon and the core structure is graphite. A method for preparing a negative electrode active material, characterized in that: The invention comprises the following steps: mixing a first carbon precursor, a catalyst 1 and a dispersant 1, and calcining the mixture to obtain a substance A; mixing the substance A, a catalyst 2, a dispersant 2 and a second carbon precursor, and calcining the mixture to obtain a substance B; heat-treating the substance B to obtain a negative electrode active material; wherein the negative electrode active material comprises a core structure and a shell structure arranged on the surface of the core structure, the core structure and the shell structure both have pores, and the average pore size of the pores in the shell structure is larger than the average pore size of the pores in the core structure. The preparation method according to claim 15, characterized in that The first carbon precursor includes at least one of coal tar, coal pitch, petroleum residue, and petroleum pitch. The preparation method according to claim 15 or 16, characterized in that The second carbon precursor includes at least one of water-soluble phenolic resin, glucose and sucrose. The preparation method according to any one of claims 15 to 17, characterized in that The preparation method satisfies at least one of the following conditions (1)-(4): (1) the catalyst 1 includes at least one of ferrocene and ferric acetylacetonate; (2) the catalyst 2 includes at least one of ferric chloride, ferrous chloride, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, and ferric citrate; (3) the dispersant 1 includes at least one of isooctane, xylene, ethyl acetate, cyclohexanone, tetrachloroethylene, tetrahydrofuran, and benzyl alcohol; (4) the dispersant 2 includes at least one of water, ethanol, ethylene glycol, propylene glycol, and oxalic acid. The preparation method according to any one of claims 15 to 18, characterized in that The particle size of the catalyst 1 is 20 nm to 200 nm, and can be 40 nm to 100 nm; and / or the particle size of the catalyst 2 is 0.2 μm to 2 μm, and can be 0.4 μm to 1 μm. The preparation method according to any one of claims 15 to 19, characterized in that The mass ratio of the first carbon precursor to the second precursor is 9:1 to 1:1, and can be optionally 9:1 to 7:3; and / or, the mass ratio of the catalyst 1 to the first carbon precursor is (0.1 to 1):1, and can be optionally (0.2 to 0.5):1; and / or, the mass ratio of the catalyst 2 to the second carbon precursor is (0.3 to 1):1, and can be optionally (0.4 to 0.7):

1. The preparation method according to any one of claims 15 to 20, characterized in that The mass ratio of the dispersant 1 to the first carbon precursor is (0.5-5):1, and can be optionally (1-3):1; and / or the mass ratio of the dispersant 2 to the second carbon precursor is (1-5):1, and can be optionally (2-4):

1. The preparation method according to any one of claims 15 to 21, characterized in that In the step of preparing the substance A, the calcination temperature is 600-800° C., and can be 700-800° C.; and / or, in the step of preparing the substance A, the calcination time is 0.5-5 h, and can be 1 h-2 h. The preparation method according to any one of claims 15 to 22, characterized in that In the step of preparing the substance B, the calcination temperature is 800-1000° C., and can be 800-900° C.; and / or, in the step of preparing the substance B, the calcination time is 2-5 hours, and can be 2-3 hours. The preparation method according to any one of claims 15 to 23, characterized in that The heat treatment temperature is 1800-2800° C., and may be 2000-2500° C.; and / or the heat treatment time is 2-7 days, and may be 3-5 days. The preparation method according to any one of claims 15 to 24, characterized in that After obtaining the substance A, the substance A is crushed; and / or, after obtaining the substance B, the substance B is crushed; and / or, after obtaining the negative electrode active material, screening is performed. A secondary battery, comprising a negative electrode plate, characterized in that: The negative electrode sheet comprises the negative electrode active material according to any one of claims 1 to 14 or the negative electrode active material obtained by the preparation method according to any one of claims 15 to 25. An electrical device, characterized in that: Includes the secondary battery as claimed in claim 26.

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