Negative active material, method for preparing the same, secondary battery, and electric device

Abstract

The application relates to a negative electrode active material and a preparation method thereof, a secondary battery and an electric device. The negative electrode active material comprises: a core material; and a first coating layer arranged on at least part of the surface of the core material, wherein the first coating layer comprises porous carbon material and nano-silicon-based particles, and the nano-silicon-based particles are embedded in the pore structure of the porous carbon material.

Description

TECHNICAL FIELD

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

[0002] Secondary batteries are widely used in various consumer electronic products and electric vehicles due to their light weight, no pollution and no memory effect. With the continuous development of the new energy industry, customers have higher use requirements for secondary batteries.

[0003] Silicon-based negative electrode materials have been widely concerned due to their high capacity. However, silicon-based materials have a serious volume effect and will produce a huge volume expansion during the charging process. Therefore, the silicon-based material is prone to crushing and pulverization during the charging and discharging process, and it is difficult to form a stable SEI film on the surface, thereby causing the capacity of the battery to decay too fast and the cycle performance to be poor. SUMMARY

[0004] In view of the technical problems in the background art, the application provides a negative electrode active material and a preparation method thereof, a secondary battery and an electric device, which aims to make the secondary battery containing the same have good cycle performance and rate performance.

[0005] To achieve the above-mentioned purpose, the first aspect of the application provides a negative electrode active material, comprising:

[0006] a core material; and

[0007] a first coating layer arranged on at least a part of the surface of the core material, the first coating layer comprising a porous carbon material and nano-silicon-based particles, the nano-silicon-based particles being embedded in the pore structure of the porous carbon material.

[0008] Compared with the prior art, the application at least includes the following beneficial effects:

[0009] The negative electrode active material of the present application can improve the compression resistance of the negative electrode active material by coating the first coating layer on the core material, thereby improving the structural stability of the negative electrode active material, avoiding particle rupture during cold pressing of the electrode sheet, and improving the cycle performance. The nano-silicon-based particles are embedded in the pore structure of the porous carbon material, and the utilization rate of the nano-silicon-based particles is improved and the electron and ion conduction rate of the material is improved by coating the surface of the core material. The nano-silicon-based particles are embedded in the pore structure of the porous carbon material, which can disperse the nano-silicon-based particles and provide a certain expansion space in the pore structure of the porous carbon material, effectively relieving the expansion of the nano-silicon-based particles. The above negative electrode active material can effectively improve the cycle performance and rate performance of the secondary battery.

[0010] In any embodiment of the present application, the volume average particle size Dv50 of the core material is denoted as D0, the volume average particle size Dv50 of the core material after 300MPa pressure test is denoted as D1, and the core material satisfies: D0 / (D0-D1)≥3; optionally, 5≤D0 / (D0-D1)≤100.

[0011] In any embodiment of the present application, the core material includes at least one of graphite, hard carbon, mesocarbon microbeads, metal elements, metal alloys, metal compounds and non-metallic compounds;

[0012] Optionally, the graphite includes at least one of natural graphite and artificial graphite;

[0013] Optionally, the metal elements include at least one of Fe, Cu, Mo, Ni or W;

[0014] Optionally, the metal alloy includes an alloy formed by at least two of Fe, Cu, Mo, Ni and W;

[0015] Optionally, the metal compound includes at least one of Al2O3, ZrO2, SiO2, TiO2 and WC;

[0016] Optionally, the non-metallic compound includes at least one of SiC and Si3N4;

[0017] Optionally, the core material includes at least one of natural graphite and artificial graphite.

[0018] In any embodiment of the present application, the number of core materials coated in the same first coating layer is ≤3, and optionally 1-2.

[0019] In any embodiment of the present application, the core material satisfies at least one of the following (1)-(3):

[0020] (1) the volume average particle size Dv50 of the core material is ≤10 μm, and is optionally 2.0-7.0 μm;

[0021] (2) the span of the core material is 0.7-1.2, and is optionally 0.8-1.1;

[0022] (3) the porosity of the core material is ≤5%, and is optionally 1%-5%.

[0023] In any embodiment of the present application, the porosity of the porous carbon material is ≥20%, and is optionally 20%-60%;

[0024] The average pore size of the porous carbon material is ≥1 nm, and is optionally 5-200 nm.

[0025] In any embodiment of the present application, the average pore size of the porous carbon material is greater than the volume average particle size Dv50 of the nanosilicon-based particles;

[0026] Optionally, the ratio of the average pore size of the porous carbon material to the volume average particle size Dv50 of the nanosilicon-based particles is 1.1-4.0.

[0027] In any embodiment of the present application, the volume average particle size Dv50 of the nanosilicon-based particles is 1-100 nm; and is optionally 5-50 nm.

[0028] In any embodiment of the present application, the porous carbon material further has an oxygen-containing group connected to the carbon element of the porous carbon material;

[0029] Optionally, the oxygen-containing group includes at least one of an alcohol group, a phenol group, an ether group, an aldehyde group, a ketone group, a carboxyl group, an ester group, and an epoxy group.

[0030] In any embodiment of the present application, the mass percentage of silicon element in the first coating layer is ≥20%, and is optionally 35%-55%; and / or,

[0031] the mass percentage of carbon element in the first coating layer is ≥30%, and is optionally 42%-60%; and / or,

[0032] the mass percentage of oxygen element in the first coating layer is ≤15%, and is optionally 3%-15%.

[0033] In any embodiment of the present application, the thickness of the first coating layer is ≥0.5 μm, and is optionally 1-5 μm.

[0034] In any embodiment of the present application, the negative active material further comprises a second coating layer, the second coating layer is arranged on at least part of the surface of the first coating layer, and the second coating layer comprises a conductive material;

[0035] Optionally, the conductive material comprises at least one of a conductive carbon material and a conductive polymer.

[0036] In any embodiment of the present application, the thickness of the second coating layer is ≥20 nm, and optionally 30-200 nm.

[0037] In any embodiment of the present application, the powder resistivity of the negative active material under a pressure of 4 MPa is denoted as R4, and the powder resistivity of the negative active material under a pressure of 16 MPa is denoted as R 16 ; and the negative active material satisfies: R4 / R 16 ≤4; and optionally, 1.5≤R4 / R 16 ≤3.

[0038] In any embodiment of the present application, R4≤2 Ω·cm; and optionally, 0.2≤R4≤1.5 Ω·cm.

[0039] In any embodiment of the present application, the negative active material satisfies at least one of the following conditions I-III:

[0040] I, the porosity of the negative active material is ≤30%, and optionally 5%-20%;

[0041] II, the volume average particle size Dv50 of the negative active material is 3-15 μm, and optionally 5-12 μm;

[0042] III, the specific surface area SSA of the negative active material is 0.5-10 m 2 / g, and optionally 1.0-5.0 m 2 / g.

[0043] The second aspect of the present application provides a preparation method of a negative active material, comprising the following steps:

[0044] S1, providing a core material;

[0045] S2, coating a porous carbon material on at least part of the surface of the core material;

[0046] S3, depositing nano-silicon-based particles into the pore structure of the porous carbon material by chemical vapor deposition to form the first coating layer.

[0047] In any embodiment of the present application, step S2 comprises the following steps:

[0048] The carbon material precursor is coated on at least part of the surface of the core material by a spray drying method, and then carbonization treatment is performed to form the porous carbon material.

[0049] In any embodiment of the present application, in step S2,

[0050] The spray-dried liquid flow rate is controlled to be 1-5 L / h, the airflow flow rate is controlled to be 50-120 L / min, the drying temperature is controlled to be 150-200℃, and the air outlet temperature is controlled to be 80-130℃; and / or,

[0051] The carbonization treatment is performed under nitrogen protection at 800-2000℃ for 2-4h.

[0052] In any embodiment of the present application, in step S3, the deposition gas of the chemical vapor deposition is a mixed gas of silane gas and H2, wherein the volume ratio of the silane gas is 2%-20%, the mixed gas is introduced at a flow rate of 100-400 mL / min, the reaction temperature is 400-1000℃, and the deposition reaction time is 2-10h.

[0053] Optionally, the silane gas includes at least one of monosilane, disilane, trisilane, monochlorosilane, dichlorosilane and trichlorosilane.

[0054] In any embodiment of the present application, the following step is further included:

[0055] S4, coating a second coating layer on the surface of the first coating layer, the second coating layer including a conductive material.

[0056] In any embodiment of the present application, in step S4, the second coating layer is formed by chemical vapor deposition. The deposition gas of the chemical vapor deposition is a mixed gas of C2H2 and N2, wherein the volume ratio of C2H2 is 2%-20%, the gas is introduced at a flow rate of 100-300 mL / min, the reaction temperature is 800-1000℃, and the deposition reaction time is 0.5-2h.

[0057] A third aspect of the present application provides a secondary battery including a negative electrode tab including the negative electrode active material according to the first aspect of the present application or including the negative electrode active material prepared according to the preparation method of the second aspect of the present application.

[0058] A fourth aspect of the present application provides an electric device including the secondary battery according to the third aspect of the present application. BRIEF DESCRIPTION OF DRAWINGS

[0059] In order to more clearly illustrate the technical solutions of the present application, the drawings used in the present application will be briefly introduced as follows. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of the drawings.

[0060] Figure 1 is a schematic diagram of the cross-sectional structure of an embodiment of the negative active material of the present application.

[0061] Figure 2 is a scanning electron microscope (SEM) picture of an embodiment of the negative active material of the present application.

[0062] Figure 3 is a schematic diagram of an embodiment of a secondary battery.

[0063] Figure 4 is an exploded view of Figure 3

[0064] Figure 5 is a schematic diagram of an embodiment of a battery module.

[0065] Figure 6 is a schematic diagram of an embodiment of a battery pack.

[0066] Figure 7 is an exploded view of Figure 6

[0067] Figure 8 is a schematic diagram of an embodiment of a device using a secondary battery as a power source.

[0068] Explanation of reference signs:

[0069] 1, battery pack; 2, upper box body; 3, lower box body; 4, battery module; 5, secondary battery; 51, shell; 52, electrode assembly; 53, cover plate; 6, negative active material; 61, core material; 62, first coating layer; 621, porous carbon material; 6221, pore structure; 622, nano-silicon-based particle; 63, second coating layer. DETAILED DESCRIPTION

[0070] The present application will be further described below in conjunction with specific embodiments. It should be understood that these specific embodiments are only used to illustrate the present application and are not used to limit the scope of the present application.

[0071] ​​For the sake of brevity, only some numerical ranges are specifically disclosed herein. However, any upper limit can be combined with any lower limit to make a range not expressly disclosed; and any lower limit can be combined with any other lower limit to make a range not expressly disclosed, and any upper limit can be combined with any other upper limit to make a range not expressly disclosed. Further, each individual disclosed point or single numerical value can be combined with any other point or single numerical value, or with other lower or upper limits, to make a range not expressly disclosed.

[0072] In the description of the present application, it is to be understood that the terms "above", "below", "upper", "lower", "up", "down", "top", "bottom", "over", "under" and the like, are intended to convey a relative position, as opposed to an absolute position, unless specifically stated otherwise.

[0073] In the description of the present application, unless otherwise stated, the term "or" is inclusive. That is, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present). Unless otherwise stated, the terms used in this application have the commonly understood meanings as understood by those skilled in the art. Unless otherwise stated, the values of the parameters mentioned in this application can be measured by various measurement methods commonly used in the art (for example, tests can be performed according to the methods given in the examples of the present application).

[0074] Silicon-based materials are commonly used negative active materials, however, the problem of large volume expansion after multiple cycles is more significant, resulting in poor cycle performance. Silicon-carbon materials can improve the problem of volume expansion of silicon materials to some extent, but it is found in actual production that the current silicon-carbon materials still do not improve the cycle performance obviously enough. The present inventors have found that this is mainly because the current silicon-carbon materials have the problem of poor pressure resistance, which further leads to unstable structure of the silicon-carbon materials in the cold pressing process of the negative electrode sheet and easy damage, especially for porous silicon-carbon materials. On the other hand, the silicon-carbon materials also have the problems of large powder resistance and large DCR (Direct Current Resistance, referred to as DCR) of the prepared battery cell, which the present inventors have analyzed and found that it is mainly caused by the fact that the silicon-carbon materials generally have large particles and the conductivity deteriorates obviously after incorporating a large amount of silicon, which is particularly obvious in porous silicon-carbon materials.

[0075] The negative active material provided by the present application comprises a core material and a first coating layer. The first coating layer is arranged on at least a part of the surface of the core material, and the first coating layer comprises a porous carbon material and nano silicon-based particles, and the nano silicon-based particles are embedded in the pore structure of the porous carbon material.

[0076] Without wishing to be limited to any theory, the negative electrode active material of the present application coats a specific first coating layer on the surface of the core material. Through the synergistic effect of the two, the core material can be used to improve the compressive strength of the negative electrode active material, thereby improving the structural stability of the negative electrode active material and avoiding the deterioration of the cycle performance caused by particle rupture during the cold pressing of the pole piece. At the same time, the nano-silicon-based particles are embedded in the pore structure of the porous carbon material as the active material, and are formed on the surface of the core material by coating, which can improve the utilization rate of the nano-silicon-based particles and improve the electronic and ion conduction rate of the material; at the same time, the above-mentioned first coating layer is used, and the pore structure of the porous carbon material is embedded with nano-silicon-based particles. In this way, the nano-silicon-based particles are arranged in the pores of the porous carbon material, which can, on the one hand, disperse the nano-silicon-based particles. On the other hand, a certain amount of expansion space is reserved in the pores of the porous carbon material, which can effectively alleviate the expansion of the nano-silicon-based particles. In this way, the above-mentioned negative electrode active material is applied to secondary batteries, which can effectively improve the cycle performance and rate performance of the secondary batteries.

[0077] In some embodiments, the aforementioned nano-silicon-based particles include but are not limited to nano-silicon particles, and may also include silicon-based particles such as nano-silicon carbon particles.

[0078] It can be understood that the first coating layer can be coated on the entire surface of the core material, or can be coated on only a portion of the surface of the core material.

[0079] like Figure 1 As shown, in Figure 1 In the specific example shown, the negative electrode active material 6 includes a core material 61 and a first coating layer 62, which coats the entire surface of the core material. The first coating layer 62 includes a porous carbon material 621 and nano-silicon-based particles 622. The porous carbon material 621 has a pore structure 6221, and the nano-silicon-based particles 622 are embedded in the pore structure 6221 of the porous carbon material 621.

[0080] It can be understood that the pore structures 6221 of the porous carbon material 621 may all be embedded with the aforementioned nano-silicon-based particles 622 ; or the aforementioned nano-silicon-based particles 622 may only be embedded in a portion of the pore structures 6221 .

[0081] Furthermore, if Figure 1 As shown, the negative electrode active material 6 further includes a second coating layer 63 (which will be described in detail below) disposed on at least a portion of the surface of the first coating layer 62. It is understood that the second coating layer may be coated on the entire surface of the first coating layer or may be coated on only a portion of the surface of the first coating layer. Figure 1 In the specific example shown, the second coating layer 63 coats the entire surface of the first coating layer 62 .

[0082] like Figure 2Fig. 1 shows a scanning electron microscope (SEM) image of the negative active material according to an embodiment of the present application, and Fig. 2 shows a scanning electron microscope (SEM) image of the negative active material according to another embodiment of the present application. Figure 2 It can be seen that the negative active material according to the embodiments of the present application is spherical and granular.

[0083] The inventors have found through in-depth research that when the negative active material according to the present application meets the above design conditions, and optionally meets one or several of the following conditions, the performance of the secondary battery can be further improved.

[0084] In some embodiments, the volume average particle size Dv50 of the core material is denoted as D0, the volume average particle size Dv50 of the core material after 300 MPa pressure testing is denoted as D1, and the core material satisfies: D0 / (D0-D1)≥3; optionally, 5≤D0 / (D0-D1)≤100. For example, the core material satisfies: D0 / (D0-D1) is 3, 5, 8, 10, 15, 20, 30, 50, 70, 80, 90, 100.

[0085] The D0 and D1 of the core material mentioned above are tested by the following method: using a UTM7305 electronic pressure testing machine, 10g of the powder of the core material is placed in a cylindrical mold with an inner cavity diameter of 20mm, a pressure of 300MPa is applied to the powder in the mold, and the pressure is maintained for 30s, and after continuous pressing for 3 times. The powder is taken out, and the Dv50 of the powder after pressing is denoted as D1. At the same time, the Dv50 of the core material before pressing is denoted as D0. The Dv50 of the core material can be tested by a method known in the art. For example, it can be tested by referring to the standard GB / T 19077.1-2016, and using a laser particle size analyzer (such as Malvern Master Size3000). The volume average particle size Dv50 change of the core material before and after pressing is represented by D0 / (D0-D1), denoted as P; that is, P=D0 / (D0-D1). The P value can represent the pressure resistance of the core material. The larger the P value, that is, the less the powder is broken after pressing, the better the pressure resistance, and vice versa, the worse the pressure resistance.

[0086] By adjusting the D0 / (D0-D1) of the core material to meet the above specific conditions, the core material has good pressure resistance, thereby improving the pressure resistance of the negative active material and reducing the risk of structural damage of the material under pressure and reducing its performance; at the same time, the first coating layer is used, which can effectively alleviate the expansion of the nano-silicon-based particles. Therefore, the negative active material described above can effectively improve the cycle performance and rate performance of the secondary battery when applied to the secondary battery.

[0087] In some embodiments, the core material includes at least one of graphite, hard carbon, mesocarbon microbeads, a metal element, a metal alloy, a metal compound. Optionally, the core material includes graphite, hard carbon, mesocarbon microbeads, which have good compression resistance and can be used as negative active materials, so that the energy density of the secondary battery can be further improved.

[0088] Optionally, the graphite includes at least one of natural graphite and artificial graphite.

[0089] Optionally, the metal element includes at least one of Fe, Cu, Mo, Ni, and W.

[0090] Optionally, the metal alloy includes an alloy formed by at least two of Fe, Cu, Mo, Ni, Cr, and W.

[0091] Optionally, the metal compound includes at least one of Al2O3, ZrO2, SiO2, TiO2, and WC.

[0092] Optionally, the non-metal compound includes at least one of SiC and Si3N4.

[0093] In some embodiments, the number of core materials coated in the same first coating layer is ≤3, and is optionally 1-2. Limiting the number of core materials in the negative active material particles can ensure the consistency and compression resistance of the negative active material.

[0094] The number of core material particles can be tested by cross-section SEM. The negative active material is ion polished. Specifically, the negative active material powder is placed in an IB-19500CP ion polisher. Under vacuum conditions, argon ions are generated by ionizing argon gas, and the high-speed argon ions will impact the atoms or molecules on the surface of the sample after acceleration and focusing, so as to realize ion polishing and cross-section detection. Then, the obtained cross-section is placed in a Sigma 300 ZEISS SEM for testing, and the data of the number of core material particles can be obtained.

[0095] In some embodiments, the volume average particle size Dv50 (i.e., D0) of the core material is ≤10 μm, and is optionally 2.0-7.0 μm. By setting the volume average particle size Dv50 of the core material in a given small range, a larger thickness space is reserved for the first coating layer containing silicon, so as to ensure the silicon content in the negative active material and the high capacity of the negative active material. Meanwhile, the small particle size of the core material can improve the compression resistance of the negative active material.

[0096] The volume average particle size Dv50 of the core material can be tested by cross-section SEM. The negative electrode active material is ion polished. Specifically, the powder is placed in an IB-19500CP ion polisher. In a vacuum condition, argon ions are generated by ionization of argon gas. After acceleration and focusing, the high-speed argon ions impact the atoms or molecules on the surface of the sample to achieve ion polishing, so that cross-section detection can be achieved. Then, the obtained cross-section is placed in a Sigma 300 ZEISS SEM for testing, and a large number of particle sizes of the core material can be obtained. After sorting, the particle size distribution of the core material can be obtained.

[0097] In some embodiments, the span of the core material is 0.7-1.2, which can be 0.8-1.1. The span (Span) refers to (Dv90-Dv10) / Dv50. Limiting the span of the core material in a given range can ensure the consistency of the particle size of the negative electrode active material.

[0098] In some embodiments, the porosity of the core material is ≤5%, which can be 1%-5%. By limiting the porosity of the core material in a given range, the pressure resistance of the core material can be further ensured.

[0099] In some embodiments, the porosity of the porous carbon material is ≥20%, which can be 20%-60%, 40%-60%. The porous carbon material with the given porosity can provide space for the subsequent embedding of silicon nanoparticles and the expansion of silicon nanoparticles, and improve the cycle performance of the prepared negative electrode active material.

[0100] In some embodiments, the average pore size of the porous carbon material is ≥1 nm, which can be 5-200 nm, such as 5 nm, 10 nm, 50 nm, 100 nm, 150 nm, 180 nm, or 200 nm. Limiting the average pore size of the porous carbon material in the given range can provide space for the subsequent embedding of silicon nanoparticles and the expansion of silicon nanoparticles.

[0101] The pore size evaluation method refers to GB / T 19587-2017 and GB / T 21650.2-2008. A TriStarII3020 pore size distribution instrument is used for testing. By adsorbing gas on the material to be tested under a series of gradually increasing pressures at a constant temperature, the pore size distribution of the porous material can be characterized by the curve of the volume of each pore size and the corresponding partial pressure. Meanwhile, the average pore size can be calculated.

[0102] Further, the average pore size of the porous carbon material is greater than the volume average particle size Dy50 of the nano-silicon-based particles. Further, the ratio of the average pore size of the porous carbon material to the volume average particle size Dv50 of the nano-silicon-based particles is 1.1-4.0. In this way, the pores of the porous carbon material are not filled with the nano-silicon-based particles, and the un-filled pores provide a certain expansion space for the expansion of the nano-silicon-based particles, which can effectively alleviate the expansion of the nano-silicon-based particles.

[0103] The volume average particle size Dv50 of the nano-silicon-based particles can be tested by scanning electron microscopy (TEM test) to test the particle sizes of 100 nano-silicon-based particles, and the average value is obtained by measurement. For irregularly shaped particles, the particle size is taken as the particle size at the maximum particle size.

[0104] In some embodiments, the volume average particle size Dv50 of the nano-silicon-based particles is 1-100 nm; optionally 1-50 nm, 5-50 nm, 5-20 nm.

[0105] In some embodiments, the porous carbon material further has oxygen-containing groups connected to the carbon elements of the porous carbon material; optionally, the oxygen-containing groups include at least one of an alcohol group, a phenol group, an ether group, an aldehyde group, a ketone group, a carboxyl group, an ester group, and an epoxy group. The porous carbon material has a certain oxygen content due to the presence of the oxygen-containing groups, which can improve the Si-O-C bonding and the elasticity of the porous carbon material, thereby improving the pressure resistance of the first coating layer.

[0106] In some embodiments, the mass percentage of silicon elements in the first coating layer is ≥20%, optionally 35%-55%. In some embodiments, the mass percentage of carbon elements in the first coating layer is ≥30%, optionally 42%-60%. In some embodiments, the mass percentage of oxygen elements in the first coating layer is ≤15%, optionally 3%-15%; optionally 3%-10%. However, too high oxygen content will affect the capacity and cycle performance of the negative electrode active material, so the mass content of oxygen elements is preferably controlled within the above given range.

[0107] The oxygen content in the negative electrode active material is evaluated based on the "Microbeam Analysis Energy Dispersive Spectroscopy Quantitative Analysis" in GB / T 17359-2012, using cross-section EDS spectroscopy. The specific operation is as follows: place the powder to be tested in an IB-19500CP ion polisher, ionize argon under vacuum conditions using an ion source to generate argon ions, accelerate and focus the high-speed argon ions to knock off the atoms or molecules on the surface of the sample, and realize ion polishing, thereby realizing cross-section detection. Place the obtained powder cross-section in an OXFORD EDS instrument to test the oxygen content of each point on the cross-section of the material, and the average value of 10 points can be obtained to obtain the oxygen content of the material.

[0108] The silicon content in the negative active material is evaluated by ICP (inductively coupled plasma emission spectrometer) according to the standard EPA 6010D-2018 of the United States Environmental Protection Agency. The specific operation is as follows: the powder to be measured is placed in a CEM-Mars6 type MARS microwave digestion instrument for digestion. After digestion, the solution is introduced into an ICAP7400 type ICP tester. By comparing the silicon concentration in the standard solution, the silicon content in the material is calculated.

[0109] The carbon content in the negative active material is tested by a carbon-sulfur analyzer according to GB / T 20123-2006 / ISO 15350:200. The principle is as follows: the sample is heated and combusted by a high-frequency furnace under oxygen-rich conditions, so that the carbon is oxidized to carbon dioxide. The gas enters the absorption cell, absorbs the corresponding infrared radiation, and is then converted into a corresponding signal by the detector. The carbon content in the material is calculated by calculation.

[0110] In some embodiments, the thickness of the first coating layer is ≥0.5 μm, optionally 1-5 μm, and further optionally 1-3 μm. Controlling the thickness of the first coating layer is conducive to improving the electrical conductivity of the silicon-carbon material. Meanwhile, the thickness is preferably 1-5 μm to avoid the inner nanosilicon-based particles in the first coating layer from failing to provide activity due to excessive thickness.

[0111] In some embodiments, the negative active material further comprises a second coating layer, which is arranged on at least a part of the surface of the first coating layer and comprises a conductive material. The second coating layer can improve the electrical conductivity of the prepared negative active material, so that the compression resistance and electrical conductivity of the negative active material are simultaneously improved. In addition, the second coating layer can also reduce the contact between the first coating layer and the electrolyte, thereby improving the cycle performance of the negative active material.

[0112] Optionally, the conductive material comprises at least one of a conductive carbon material and a conductive polymer.

[0113] The conductive carbon material can be at least one of amorphous carbon, conductive carbon black, carbon nanotube, carbon fiber, graphene, etc. In a specific example, the second coating layer is an amorphous carbon coating layer. The amorphous carbon comprises at least one of soft carbon and hard carbon.

[0114] The conductive polymer can be at least one of polyacrylonitrile, polyaniline, polypyrrole, polythiophene, polyacetylene, polyphenylacetylene, and polyphenylene, etc.

[0115] Further, the thickness of the second coating layer is ≥20 nm, optionally 20-200 nm, 30 nm-200 nm, 50-200 nm, 30 nm-100 nm.

[0116] In some embodiments, the powder resistivity of the negative active material under a pressure of 4 MPa is denoted as R4, and the powder resistivity of the negative active material under a pressure of 16 MPa is denoted as R 16 ; and the negative active material satisfies: R4 / R 16 ≤ 4; and optionally, 1.5 ≤ R4 / R 16 ≤ 3. The applicant has found that the ratio of R4 / R 16 can be used to characterize the pressure resistance of the negative active material, and the smaller the value is, the better the pressure resistance of the material is, because the powder resistivity of the material under high pressure will not decrease due to the increase of powder contact caused by powder deformation.

[0117] In some embodiments, R4≤ 2 Ω·cm; and optionally, 0.2 ≤ R4≤ 1.5 Ω·cm.

[0118] In some embodiments, the porosity of the negative active material is ≤ 30%, and is optionally 5%-20%.

[0119] In some embodiments, the volume average particle size Dv50 of the negative active material is 3-15 μm, and is optionally 5-12 μm.

[0120] In some embodiments, the specific surface area SSA of the negative active material is 0.5-10 m2 / g, and is optionally 1.0-5.0 m 2 / g.

[0121] The present application also provides a preparation method of the negative active material described above, comprising the following steps S1-S3.

[0122] S1, providing a core material.

[0123] S2, coating a porous carbon material on at least part of the surface of the core material.

[0124] S3, depositing nano-silicon-based particles into the pore structure of the porous carbon material by chemical vapor deposition (CVD) to form a first coating layer.

[0125] In some embodiments, the step S2 coats a carbon material precursor by spray drying, and then performs carbonization treatment to form the porous carbon material. Further, the carbon material precursor can be at least one of an organic carbon source, such as phenolic resin, furan resin, urea-formaldehyde resin, melamine resin, epoxy resin, polyformaldehyde methyl acrylate resin, and polyacrylonitrile.

[0126] Further, in the step S2, the coating step specifically comprises the following steps S21-S23:

[0127] Step S21, stirring the core material in a carbon material precursor solution to obtain a mixed solution A.

[0128] The ratio of the core material to the carbon material precursor solution is adjusted according to the material design requirements, for example, the mass ratio of the core material to the carbon material precursor in the carbon material precursor solution is 1:1-1:100, and optionally 1:5-1:30.

[0129] In step S22, the mixed solution A is coated on the core material in a spray drying form to form a carbon material precursor, and a powder B is obtained.

[0130] Optionally, the liquid flow rate of the spray drying is controlled to be 1-5 L / h, the airflow flow rate is controlled to be 50-120 L / min, the drying temperature is controlled to be 150-200°C, and the air outlet temperature is controlled to be 80-130°C.

[0131] Further, the spray drying is performed in a spray drying device. Taking a 5L spray drying device as an example, by controlling the liquid flow rate of the spray drying device to be 3 L / h, the airflow flow rate to be 80 L / min, the drying temperature to be 180°C, and the air outlet temperature to be 110°C, the particle size of the powder obtained by the spray drying is controlled to be the target particle size, and an intermediate B is obtained.

[0132] In step S23, the intermediate B is carbonized to obtain an intermediate C, that is, a core material coated with a porous carbon material.

[0133] Optionally, the carbonization treatment is performed under the following conditions: carbonization at 800-2000°C for 2-4h under nitrogen protection. Further, the carbonization can be performed in a high-temperature furnace, so that by controlling the carbonization conditions of the organic carbon source, a certain amount of oxygen-containing groups can be retained in the porous carbon material.

[0134] In step S3, nano-silicon-based particles are embedded in the pore structure of the intermediate C by chemical vapor deposition, thereby obtaining the negative electrode active material of the present application.

[0135] Further, the deposition gas of the chemical vapor deposition is a mixed gas of silane gas and H2, wherein the volume ratio of the silane gas is 2%-20%, the inlet gas flow rate of the mixed gas is 100-400 mL / min, the reaction temperature is 400-1000°C, and the deposition reaction time is 2-10h.

[0136] Optionally, the silane gas includes at least one of monosilane (SiH4), disilane, trisilane, monochlorosilane, dichlorosilane, and trichlorosilane.

[0137] In some embodiments, the above preparation method further includes the following step S4 after step S3:

[0138] S4, a second coating layer is coated on the surface of the first coating layer, and the second coating layer includes a conductive material.

[0139] Further, the second coating layer in step S4 can also be formed by chemical vapor deposition.

[0140] Further, the deposition process conditions for forming the second coating layer on the core material coated with the first coating layer are as follows: the deposition gas is a mixed gas of C2H2 and N2, wherein the volume ratio of C2H2 is 2%-20%, the gas flow rate is 100-300 mL / min, the reaction temperature is 800-1000°C, and the deposition reaction time is 0.5-2h.

[0141] The above raw materials not specifically described can be obtained by purchase in the market.

[0142] Secondary battery

[0143] The secondary battery refers to a battery that can continue to be used by activating the active material through charging after the battery is discharged.

[0144] Generally, the secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte. During the charging and discharging process of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet and plays a role of isolation. The electrolyte plays a role of ion conduction between the positive electrode sheet and the negative electrode sheet.

[0145] Negative electrode sheet

[0146] In the secondary battery, the negative electrode sheet generally includes a negative electrode current collector and a negative electrode film layer arranged on the negative electrode current collector, and the negative electrode film layer includes the negative electrode active material provided in the present application.

[0147] The negative electrode current collector can adopt a conventional metal foil or a composite current collector (for example, a metal material can be arranged on a polymer substrate to form a composite current collector). As an example, the negative electrode current collector can adopt a copper foil.

[0148] The negative electrode film layer generally can also optionally include a binder, a conductive agent, and other optional additives.

[0149] As an example, the conductive agent can be one or several of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0150] As an example, the binder can be one or several 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).

[0151] As an example, other optional additives can be thickening and dispersing agents (e.g., sodium carboxymethylcellulose, CMC-Na), PTC thermistor materials, etc.

[0152] [Positive electrode sheet]

[0153] In the secondary battery, the positive electrode sheet generally includes a positive current collector and a positive film layer disposed on the positive current collector, the positive film layer including a positive active material.

[0154] The positive current collector can employ a conventional metal foil or a composite current collector (a metal material can be disposed on a polymer substrate to form a composite current collector). As an example, the positive current collector can employ an aluminum foil.

[0155] The specific type of the positive active material is not limited, and a known active material capable of being used for a positive electrode of a secondary battery can be employed, and a person skilled in the art can select it according to the actual needs.

[0156] As an example, the positive active material can include, but is not limited to, one or more of lithium transition metal oxides, olivine-structured lithium-containing phosphates, and their respective modified compounds. Examples of lithium transition metal oxides can 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 olivine-structured lithium-containing phosphates can 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 manganese iron phosphate, a composite material of lithium manganese iron phosphate and carbon, and their modified compounds. These materials can all be obtained through commercial channels.

[0157] In some embodiments, the modified compounds of the above-mentioned materials can be doping modification and / or surface coating modification of the materials.

[0158] The positive film layer can also generally optionally include a binder, a conductive agent, and other optional additives.

[0159] As an example, the conductive agent can be one or more of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, Super P (SP), graphene, and carbon nanofibers.

[0160] As an example, the binder can be one or several 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).

[0161] [Separator]

[0162] In some embodiments, the secondary battery further includes a separator. The type of the separator is not particularly limited in the present application, and any publicly known porous structure separator having good chemical stability and mechanical stability can be used.

[0163] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film, and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different, and are not particularly limited.

[0164] [Electrolyte]

[0165] The secondary battery can include an electrolyte, which functions to conduct ions between the positive electrode and the negative electrode. The electrolyte can include an electrolyte salt and a solvent.

[0166] As an example, the electrolyte salt can be selected from one or several of lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium perchlorate (LiClO4), lithium hexafluoroarsenate (LiAsF6), lithium bisfluorosulfonylimide (LiFSI), lithium bis-trifluoromethanesulfonylimide (LiTFSI), lithium trifluoromethanesulfonate (LiTFS), lithium difluoro-oxalato-borate (LiDFOB), lithium bis-oxalato-borate (LiBOB), lithium difluoro-phosphate (LiPO2F2), lithium difluoro-di-oxalato-phosphate (LiDFOP), and lithium tetrafluoro-oxalato-phosphate (LiTFOP).

[0167] As an example, the solvent can be selected from one or more of ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), and diethyl sulfone (ESE).

[0168] In some embodiments, the electrolyte further comprises an additive. For example, the additive can comprise a negative electrode film-forming additive, a positive electrode film-forming additive, or an additive capable of improving certain properties of the battery, such as an additive capable of improving overcharge performance, an additive capable of improving high-temperature performance, an additive capable of improving low-temperature performance, and the like.

[0169] In some embodiments, the secondary battery of the present application is a lithium ion secondary battery.

[0170] The secondary battery can be prepared according to conventional methods in the art, for example, by winding (or stacking) the positive electrode sheet, the separator, and the negative electrode sheet in order, with the separator between the positive electrode sheet and the negative electrode sheet to serve as a separator, to obtain an electrode core, placing the electrode core in an outer package, injecting the electrolyte, and sealing to obtain the secondary battery.

[0171] The shape of the secondary battery of the embodiments of the present application is not particularly limited, and the secondary battery can be cylindrical, square, or any other shape. For example, Figure 3 is a square structure as an example of a secondary battery 5.

[0172] In some embodiments, the secondary battery can comprise an outer package. The outer package is used to package the positive electrode sheet, the negative electrode sheet, and the electrolyte.

[0173] In some embodiments, with reference to Figure 4 , the outer package can comprise a housing 51 and a cover plate 53. The housing 51 can comprise 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 housing 51 has an opening communicating with the receiving cavity, and the cover plate 53 can be provided on the opening to close the receiving cavity.

[0174] The positive electrode sheet, the negative electrode sheet, and the separator film can be formed into the electrode assembly 52 through a roll-pressing process or a stacking process. The electrode assembly 52 is enclosed in the accommodation cavity. The electrolyte solution is impregnated in the electrode assembly 52. The number of the electrode assemblies 52 included in the secondary battery 5 can be one or more, which can be adjusted according to the needs.

[0175] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch-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.

[0176] In some embodiments, the secondary battery can be assembled into a battery module, and the number of the secondary batteries included in the battery module can be multiple, which can be adjusted according to the application and capacity of the battery module.

[0177] Figure 5 The battery module 4 is an example. In the battery module 4, the plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangements can also be used. The plurality of secondary batteries 5 can be further fixed by fasteners.

[0178] Optionally, the battery module 4 can further include a housing having an accommodation space, and the plurality of secondary batteries 5 are accommodated in the accommodation space.

[0179] In some embodiments, the above-described battery module can be further assembled into a battery pack, and the number of the battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0180] Figure 6 and Figure 7 The battery pack 1 is an example. The battery pack 1 can include a battery box and a plurality of battery modules 4 arranged 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 arranged 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 an arbitrary manner.

[0181] [Electric device]

[0182] The application also provides a device using electricity, which comprises at least one of the secondary battery, the battery module, or the battery pack. The secondary battery, the battery module, or the battery pack can be used as a power supply of the device, or as an energy storage unit of the device. The device can be, but is not limited to, a mobile device (such as a mobile phone, a notebook 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 and a satellite, an energy storage system, etc.

[0183] The device can select a secondary battery, a battery module, or a battery pack according to its use requirements.

[0184] Figure 8 The device is 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 requirements of the device for high power and high energy density of the secondary battery, a battery pack or a battery module can be used.

[0185] The device as another example can be a mobile phone, a tablet computer, a notebook computer, etc. The device usually requires thinning, and a secondary battery can be used as a power supply.

[0186] The beneficial effects of the application will be further illustrated below in combination with examples.

[0187] Embodiments

[0188] In order to make the technical problems, technical solutions and beneficial effects solved by the application clearer, the following will be further described in combination with examples and drawings. Obviously, the described embodiments are only a part of the embodiments of the application, rather than all the embodiments. The following description of at least one exemplary embodiment is actually only illustrative, but not as any limitation on the application and its application. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0189] The materials used in the embodiments of the application can be obtained by commercial purchase.

[0190] I. Preparation of the negative electrode material

[0191] Embodiment 1:

[0192] The negative electrode material comprises an artificial graphite inner core and a first coating layer coated on the surface of the artificial graphite inner core. The volume average particle size Dv50 of the artificial graphite inner core is 3.0 μm, the span is 0.9, and the porosity is 3%. The first coating layer comprises a porous carbon material and nano-silicon particles, and the nano-silicon particles are embedded in the pore structure of the porous carbon material. Specifically, the preparation method is as follows:

[0193] 1) The core material artificial graphite is put into the phenolic resin precursor solution and stirred uniformly to obtain a mixed solution A;

[0194] 2) The mixed solution A is put into a 5L spray drying device, the liquid flow rate of the spray drying device is controlled to be 3L / h, the airflow flow rate is 80L / min, the drying temperature is 180℃, the air outlet temperature is 110℃, and the particle size of the powder sprayed and dried is controlled to be the target particle size to obtain intermediate B;

[0195] 3) The intermediate B is placed in a high-temperature furnace, nitrogen protection is performed, carbonization is performed at 1000℃ for 2h, and intermediate C is obtained. That is, the core material coated with a porous carbon material.

[0196] 4) The intermediate C obtained in step 3) is put into a CVD furnace, a mixed gas of SiH4:H2 with a volume ratio of 5:95 is introduced, the gas flow rate is 300mL / min, the reaction temperature is 600℃, and the deposition reaction time is 7h. After the reaction is completed, the negative electrode active material is obtained.

[0197] Example 2:

[0198] The negative electrode material is basically the same as that of Example 1, and the only difference is that a second coating layer is further coated on the surface of the first coating layer, and the second coating layer is a conductive carbon layer. Specifically, after step 4), it further includes the following step:

[0199] 5) The powder D obtained in step 4) is put into a CVD furnace, a mixed gas of C2H2:N2 with a volume ratio of 20:80 is introduced, the gas flow rate is 200mL / min, the reaction temperature is 800℃, and the deposition reaction time is 2h. After the reaction is completed, the negative electrode active material of Example 2 is obtained.

[0200] Examples 3-20:

[0201] The negative electrode material is basically the same as that of Example 2, and the only difference is that at least one of the following parameters is not the same: the type of the core material, the compressive resistance D0 / (D0-D1) of the core material, the mass ratio of Si, C and O in the first coating layer, the porosity of the porous carbon material, the average pore size of the porous carbon material, the volume average particle size Dv50 of the nanosilicon particles, the thickness of the first coating layer, the thickness of the second coating layer, R4, R 16 , the porosity of the negative electrode material, and the volume average particle size Dv50 of the negative electrode material. The specific values are shown in Table 1.

[0202] Among them, the mass ratio of Si, C and O in the first coating layer can be adjusted by adjusting the carbonization temperature in step 3) and / or the deposition reaction time in step 4) in Examples 16-18;

[0203] Specifically, the carbonization temperature in step 3) of Example 16 is 1500℃; the deposition reaction time in step 4) is 5h; specifically, the carbonization temperature in step 3) of Example 17 is 1800℃; the deposition reaction time in step 4) is 11.5h; specifically, the carbonization temperature in step 3) of Example 18 is 800℃; the deposition reaction time in step 4) is 7.5h.

[0204] In Examples 10-12, the porosity of the porous carbon material can be adjusted by adjusting the type of resin in step 1). Specifically, the type of resin in step 1) of Examples 10-12 is furan resin, urea-formaldehyde resin and epoxy resin, respectively.

[0205] Comparative Example 1

[0206] Comparative Example 1 does not put the core material, and is synthesized by the same synthesis method as Example 1 to obtain the negative electrode active material of Comparative Example 1.

[0207] The phenolic resin precursor solution is stirred and uniformly dispersed to obtain a mixed solution A.

[0208] The mixed solution A is put into a 5L spray drying device, and the liquid flow rate of the spray drying device is controlled to be 3L / h, the gas flow rate is 80L / min, the drying temperature is 180℃, the outlet air temperature is 110℃, and the particle size of the powder sprayed out is controlled to be the target particle size of 9μm to obtain intermediate B.

[0209] The intermediate B is placed in a high-temperature furnace and carbonized at 1000℃ for 2h under nitrogen protection to obtain the porous carbon material, i.e. powder C. The intermediate B is put into a CVD furnace, and a mixed gas of SiH4:H2 with a volume ratio of 5:95 is introduced at a flow rate of 300mL / min, the reaction temperature is 600℃, and the deposition reaction time is 7h. After the reaction, the negative electrode active material is obtained.

[0210] Comparative Example 2

[0211] Comparative Example 2 is basically the same as Comparative Example 1, except that the negative electrode active material prepared in Comparative Example 1 is further subjected to the following steps:

[0212] The negative electrode active material prepared in Comparative Example 1 is put into a CVD furnace, and a mixed gas of C2H2:N2 with a volume ratio of 20:80 is introduced at a flow rate of 200mL / min, the reaction temperature is 800℃, and the deposition reaction time is 2h. After the reaction, the negative electrode active material is obtained.

[0213] The partial parameters of the negative electrode materials prepared in each example and comparative example are shown in Table 1.

[0214] Table 1

[0215]

[0216]

[0217] II. Performance test of negative electrode material

[0218] 1) Test of powder resistivity of negative electrode material

[0219] A resistivity tester (Suzhou Jingge Electronics ST2722) was used to take 1 g of powder sample, and the sample was placed between the electrodes of the tester. The electronic press was used to maintain a constant pressure of different pressures (for example, 4 MPa and 16 MPa) for 15-25 s, and the sample height h (cm), the voltage U between the two ends, the current I, and the resistance R (KΩ) were read. The area S of the powder after pressing was 1 cm 2 . The powder resistivity was calculated according to the formula δ = S * R * 1000 / h, and the unit was Ω·cm. The powder resistivity under the pressure of 4 MPa was R4, and the powder resistivity under the pressure of 16 MPa was R 16 .

[0220] 2) Test of porosity of negative electrode material

[0221] Porosity test method: The true density of the material was tested by AccuPyc II1340 true density tester. The specific operation was as follows: a certain mass of sample was weighed and placed in the true density tester, and the closed test system was filled with helium according to the program. The gas pressure in the sample chamber and the expansion chamber was detected, and then the real volume Vr was calculated according to the Boyle's law (PV = nRT), and then the real density ρ r = m / Vr was calculated. The apparent density of the material could be obtained by filling a certain mass of powder into a cylindrical mold with an inner diameter of 10 mm, and applying a pressure of 200 MPa to obtain the apparent volume V0 of the corresponding powder, and then the apparent density of the material was ρ0 = m / V0.

[0222] The porosity P of the material was 1 - ρ0 / ρ r × 100%.

[0223] The porosity in this paper was tested by the same method.

[0224] 3) Test of particle size of negative electrode material

[0225] The Dv10, Dv50 and Dv90 of the negative electrode active material were determined by referring to the standard GB / T 19077.1-2016 and using a laser particle size analyzer (such as Malvern Master Size 3000). The physical definitions of Dv10, Dv50 and Dv90 are as follows:

[0226] Dv10: the particle size corresponding to the cumulative volume distribution percentage of the negative electrode active material reaching 10%;

[0227] Dv50: the particle size corresponding to the cumulative volume distribution percentage of the negative active material reaching 50%;

[0228] Dv90: the particle size corresponding to the cumulative volume distribution percentage of the negative active material reaching 90%.

[0229] III. Preparation of the battery

[0230] 1. Preparation of the positive electrode sheet: the positive active material LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), the binder polyvinylidene fluoride (PVDF), and the conductive agent acetylene black are dissolved in the solvent N-methyl pyrrolidone (NMP) in a mass ratio of 97%: 1.5%: 1.5%, and after being fully stirred and mixed uniformly, a positive electrode slurry is prepared; the positive electrode slurry is uniformly coated on the positive current collector aluminum foil, and then after drying, cold pressing, and slitting, a positive electrode sheet is obtained.

[0231] 2. Preparation of the negative electrode sheet:

[0232] The negative active material, artificial graphite, binder styrene-butadiene rubber (SBR), binder polyacrylic acid (PAA), dispersant (CMC-Na), and conductive carbon black (Super-P, SP) and carbon nanotubes (CNT) prepared in the above examples and comparative examples are fully stirred and mixed in a weight ratio of 10%: 85%: 2%: 1%: 1%: 0.7%: 0.3% in an appropriate amount of deionized water to prepare a negative electrode slurry; the negative electrode slurry is applied to the current collector copper foil by a coating device, and then after drying, cold pressing, and slitting, a negative electrode sheet is obtained.

[0233] 3. Separating film: a polyethylene film (PE) with a thickness of 12 μm is used as the separating film.

[0234] 4. Preparation of the electrolyte: ethylene carbonate (EC), methyl ethyl 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.

[0235] 5. Preparation of the secondary battery: the above positive electrode sheet, separating film, and negative electrode sheet are stacked and wound in order to obtain an electrode assembly; the electrode assembly is placed in an outer package, and the above prepared electrolyte is added, and after processes such as packaging, standing, formation, and aging, a secondary battery is obtained.

[0236] IV. Performance test of the battery

[0237] 1. The above prepared secondary batteries are subjected to a 45°C cycle performance test method:

[0238] The prepared secondary battery was charged at 45°C in a constant temperature environment at a rate of 1C to a voltage of 4.25V, and then charged at a constant voltage of 4.25V until the current was less than or equal to 0.05mA, and then left to stand for 5min, and then discharged at a rate of 1C to a voltage of 2.5V, and left to stand for 5min, which was one cycle of charging and discharging. The discharge capacity of this cycle was recorded as the discharge capacity of the first cycle of the secondary battery. The secondary battery was subjected to 300 cycles of charging and discharging according to the above method, and the discharge capacity of the 300th cycle was recorded.

[0239] The capacity retention rate CR45°C(%) of the secondary battery after 300 cycles at 45°C was calculated according to the following formula: CR45°C(%) = discharge capacity of the 300th cycle / discharge capacity of the first cycle x 100%.

[0240] 2. Battery rate performance test

[0241] Rate performance test method:

[0242] The prepared secondary battery was charged at 25°C at a rate of 0.33C (i.e. the current value at which the theoretical capacity is completely discharged within 3h) to a charge cut-off voltage of 4.25V, and then charged at a constant voltage until the current was 0.05C, and then left to stand for 5min, and then discharged at a rate of 0.33C to a discharge cut-off voltage of 2.5V, and the actual capacity was recorded as C0.

[0243] The battery was then charged at a rate of 0.33C to a charge cut-off voltage of 4.25V, and then charged at a constant voltage until the current was 0.05C, and then left to stand for 5min, and then discharged at a rate of 3C to a discharge cut-off voltage of 2.5V, and the actual capacity was recorded as C3.

[0244] The rate performance of the secondary battery can be characterized by DR = C3 / C0 x 100%. The higher the DR value, the better the rate performance of the material.

[0245] The performance parameters of the negative electrode materials prepared in each example and comparative example are shown in Table 2 below.

[0246] Table 2

[0247]

[0248]

[0249] As can be seen from each of the examples and Comparative Example 1 in Table 1 and Table 2, the negative electrode material of the present application can improve the rate performance and cycle performance of the secondary battery at the same time. In particular, by further optimizing the type of the core material, the pressure resistance of the core material, the volume average particle size Dv50 of the core material, the mass ratio of Si, C and O in the first coating layer, the porosity of the porous carbon material, the volume average particle size Dv50 of the nano-silicon particles, the thickness of the first coating layer, the thickness of the second coating layer, the type of the conductive material in the second coating layer, R4, R 16 , the porosity of the negative electrode material, the volume average particle size Dv50 of the negative electrode material, and further improve the rate performance and cycle performance of the secondary battery.

[0250] Further, the conductive carbon in the second coating layer in Example 2 is mainly amorphous carbon, and may also contain a small amount of microcrystalline carbon.

[0251] Further, the main difference between Examples 2-5 is that the D0 / (D0-D1) of the negative electrode active material is different, wherein the D0 / (D0-D1) in Example 5 is the smallest, which is 3, and the pressure resistance of the core material used in Example 5 is relatively poor, so the cycle performance of Example 5 is lower than that of Examples 2-4, but the technicians find that the rate performance of Example 5 is slightly improved compared with Examples 2-4. The technicians analyze the reason, which may be because the relatively poor pressure resistance of the negative electrode active material in Example 5 leads to easy fragmentation of the particles, and the fragmentation of the particles has a slight improvement effect on the rate performance.

[0252] Comparing Example 6 with Example 2, the main difference is that the number of particles of the core material of the negative electrode active material in Example 6 is different, and the number of particles of the core material in the negative electrode active material in Example 6 is 3, which has a relatively lower pressure resistance, and further leads to a lower cycle performance.

[0253] Comparing Examples 7-9 with Example 2, the main difference is that the type of the core material of the negative electrode active material in Examples 7-9 is different, and accordingly the D0 / (D0-D1) is different. Compared with the hard carbon core material in Example 7, the cycle performance of Examples 2 and 8-9 is better. More preferably, Examples 2 and 8 can simultaneously obtain better cycle performance and battery rate performance.

[0254] Comparing Examples 10-12 with Example 2, the main difference is that the porosity of the porous carbon material in the first coating layer of the negative electrode active material in Examples 10-12 is different, which is 20%-60%, preferably 40%-60%.

[0255] Examples 13-15 and Example 2 are compared, the main difference is that the average pore size of the porous carbon material of the negative active material in Examples 13-15 is different, the average pore size of the porous carbon material ranges from 5 nm to 200 nm, and both good cycle performance and battery rate performance can be obtained.

[0256] Examples 16-18 and Example 2 are compared, the main difference is that the mass ratio of Si:C:O in the first coating layer of the negative active material in Examples 16-18 is different, the mass ratio of Si:C:O in the first coating layer ranges from (35%-55%) :(42%-60%) :(3%-15%), and both good cycle performance and battery rate performance can be obtained.

[0257] Examples 19-20 and Example 2 are compared, the main difference is that the thickness of the first coating layer and the second coating layer of the negative active material in Examples 19-20 is different, and Examples 2 and 20 have better cycle performance and battery rate performance compared with Example 19.

[0258] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited to this, any person skilled in the art can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present application, and these modifications or replacements should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A negative electrode active material comprising: A core material, wherein the volume average particle size Dv50 of the core material is ≤10 μm, the porosity of the core material is ≤5%, the core material comprises at least one of graphite, hard carbon, mesocarbon microbeads, a metal element, a metal alloy, a metal compound, and a non-metallic compound, the metal compound comprises at least one of Al2O3, ZrO2, TiO2, and WC, and the non-metallic compound comprises at least one of SiC and Si3N4, the volume average particle size Dv50 of the core material is denoted as D0, and the volume average particle size Dv50 of the core material after a 300 MPa pressure test is denoted as D1, then the core material satisfies the following: 3≤D0 / (D0-D1)≤100; and The first coating layer is provided on at least a portion of the surface of the core material. The first coating layer comprises a porous carbon material and nano-silicon-based particles. The nano-silicon-based particles are embedded in the pore structure of the porous carbon material.

2. The negative electrode active material according to claim 1, wherein The volume average particle size Dv50 of the core material is recorded as D0, and the volume average particle size Dv50 of the core material after 300 MPa pressure test is recorded as D1. Then the core material satisfies: 5≤D0 / (D0-D1)≤90.

3. The negative electrode active material according to claim 2, wherein 5≤D0 / (D0-D1)≤50.

4. The negative electrode active material according to claim 1, wherein The core material includes at least one of graphite, hard carbon and mesocarbon microbeads.

5. The negative electrode active material according to claim 1, wherein At least one of the following conditions is met: (1) The graphite includes at least one of natural graphite and artificial graphite; (2) The metal element includes at least one of Fe, Cu, Mo, Ni, Cr or W; (3) The metal alloy includes an alloy formed by at least two of Fe, Cu, Mo, Ni, Cr and W.

6. The negative electrode active material according to claim 1, wherein The number of the core materials coated in the same first coating layer is ≤3.

7. The negative electrode active material according to claim 6, wherein The number of the core materials coated in the same first coating layer is 1-2.

8. The negative electrode active material according to claim 1, wherein The core material satisfies at least one of the following conditions (1)-(3): (1) The volume average particle size Dv50 of the core material is 2.0-7.0 μm; (2) The diameter pitch of the core material is 0.7-1.2; (3) The porosity of the core material is 1%-5%.

9. The negative electrode active material according to claim 1, wherein The diameter pitch of the core material is 0.8-1.

1.

10. The negative electrode active material according to any one of claims 1 to 9, wherein The porosity of the porous carbon material is ≥20%; The average pore diameter of the porous carbon material is ≥1 nm.

11. The negative electrode active material according to claim 10, wherein The porosity of the porous carbon material is 20%-60%; The average pore diameter of the porous carbon material is 5-200 nm.

12. The negative electrode active material according to any one of claims 1 to 9, wherein The average pore size of the porous carbon material is greater than the volume average particle size Dv50 of the nano-silicon-based particles.

13. The negative electrode active material according to claim 12, wherein The ratio of the average pore size of the porous carbon material to the volume average particle size Dv50 of the nano-silicon-based particles is 1.1-4.

0.

14. The negative electrode active material according to any one of claims 1 to 9, wherein The volume average particle size Dv50 of the nano silicon-based particles is 1-100 nm.

15. The negative electrode active material according to claim 14, wherein The volume average particle size Dv50 of the nano silicon-based particles is 5-50 nm.

16. The negative electrode active material according to any one of claims 1 to 9, wherein The porous carbon material further comprises an oxygen-containing group connected to the carbon element of the porous carbon material.

17. The negative electrode active material according to claim 16, wherein The oxygen-containing group includes at least one of an alcohol group, a phenol group, an ether group, an aldehyde group, a ketone group, a carboxyl group, an ester group and an epoxy group.

18. The negative electrode active material according to any one of claims 1 to 9, wherein The mass proportion of silicon in the first coating layer is ≥20%; and / or, The mass proportion of carbon in the first coating layer is ≥30%; and / or, The mass proportion of oxygen in the first coating layer is ≤15%.

19. The negative electrode active material according to claim 18, wherein The mass proportion of silicon in the first coating layer is 35%-55%; and / or, The mass proportion of carbon in the first coating layer is 42%-60%; and / or, The mass proportion of oxygen element in the first coating layer is 3%-15%.

20. The negative electrode active material according to any one of claims 1 to 9, wherein The thickness of the first coating layer is ≥0.5 μm.

21. The negative electrode active material according to claim 20, wherein The thickness of the first coating layer is 1-5 μm.

22. The negative electrode active material according to any one of claims 1 to 9, wherein The negative electrode active material further includes a second coating layer disposed on at least a portion of a surface of the first coating layer, and the second coating layer includes a conductive material.

23. The negative electrode active material according to claim 22, wherein The conductive material includes at least one of a conductive carbon material and a conductive polymer.

24. The negative electrode active material according to claim 22, wherein The thickness of the second coating layer is ≥20 nm.

25. The negative electrode active material according to claim 24, wherein The thickness of the second coating layer is 30-200 nm.

26. The negative electrode active material according to any one of claims 1 to 9, wherein The powder resistivity of the negative electrode active material under a pressure of 4 MPa is recorded as R4, and the powder resistivity of the negative electrode active material under a pressure of 16 MPa is recorded as R 16 , then the negative electrode active material satisfies: R4 / R 16 ≤4.

27. The negative electrode active material according to claim 26, wherein 1.5≤R4 / R 16 ≤3。 28. The negative electrode active material according to claim 26, wherein R4≤2 Ω•cm.

29. The negative electrode active material according to claim 28, wherein 0.2≤R4≤1.5Ω•cm.

30. The negative electrode active material according to any one of claims 1 to 9, wherein The negative electrode active material satisfies at least one of the following conditions I-III: Ⅰ. The porosity of the negative electrode active material is ≤30%; II. The volume average particle size Dv50 of the negative electrode active material is 3-15 μm; III. The specific surface area SSA of the negative electrode active material is 0.5-10m 2 / g.

31. The negative electrode active material according to claim 30, wherein The negative electrode active material satisfies at least one of the following conditions I-III: Ⅰ. The porosity of the negative electrode active material is 5%-20%; II. The volume average particle size Dv50 of the negative electrode active material is 5-12 μm; III. The specific surface area SSA of the negative electrode active material is 1.0-5.0 m 2 / g.

32. A method for preparing the negative electrode active material according to any one of claims 1 to 31, comprising the following steps: S1. Provide core materials; S2. coating a porous carbon material on at least a portion of the surface of the core material; S3. Depositing nano-silicon-based particles into the pore structure of the porous carbon material using a chemical vapor deposition method to form the first coating layer.

33. The preparation method according to claim 32, wherein Step S2 includes the following steps: A carbon material precursor is coated on at least a portion of the surface of the core material by a spray drying method, and then carbonization treatment is performed to form a porous carbon material.

34. The preparation method according to claim 33, wherein In step S2: The liquid flow rate of the spray drying is controlled at 1-5 L / h, the air flow rate is controlled at 50-120 L / min, the drying temperature is controlled at 150-200° C., and the air outlet temperature is controlled at 80-130° C.; and / or, The carbonization treatment conditions are: carbonization at 800-2000° C. for 2-4 hours under nitrogen protection.

35. The preparation method according to any one of claims 32 to 34, wherein In step S3, the deposition gas for chemical vapor deposition is a mixture of silane gas and H2, wherein the volume ratio of the silane gas is 2%-20%, the inlet flow rate of the mixed gas is 100-400 mL / min, the reaction temperature is 400-1000°C, and the deposition reaction time is 2-10h.

36. The preparation method according to claim 35, wherein The silane gas includes at least one of monosilane, disilane, trisilane, monochlorosilane, dichlorosilane and trichlorosilane.

37. The preparation method according to any one of claims 32 to 34, wherein The following steps are also included: S4. Covering the surface of the first covering layer with a second covering layer, wherein the second covering layer includes a conductive material.

38. The preparation method according to claim 37, wherein In step S4, chemical vapor deposition is used to form a second coating layer. The deposition gas of chemical vapor deposition is a mixture of C2H2 and N2, wherein the volume ratio of C2H2 is 2%-20%, the gas flow rate is 100-300 mL / min, the reaction temperature is 800-1000°C, and the deposition reaction time is 0.5-2h.

39. A secondary battery comprising a negative electrode plate, wherein the negative electrode plate comprises the negative electrode active material according to any one of claims 1 to 31 or comprises the negative electrode active material prepared by the preparation method according to any one of claims 32 to 38.

40. An electric device comprising the secondary battery according to claim 39.

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