A negative electrode, a method for preparing the same, a secondary battery, and an electrical device

Abstract

To overcome the problem of large contact impedance existing between existing anode materials, the present invention provides an anode material, which includes an anode active material, a carbon layer, and carbon nanotubes. The carbon layer coats at least part of the surface of the anode active material. One end of at least part of the carbon nanotubes is located on the outer surface of the carbon layer away from the anode active material, and the other end of the at least part of the carbon nanotubes is a free end extending out of the carbon layer. At the same time, the present invention also discloses a preparation method of the above anode material, a secondary battery including the above anode material, and an electrical device including the above secondary battery. The anode material provided by the present invention can effectively improve the electron conductivity between adjacent anode materials.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrode materials, and in particular relates to a negative electrode and a preparation method thereof, a secondary battery and an electrical device. Background Art

[0002] Lithium-ion batteries have been widely used as power sources in our daily lives due to their irreplaceable advantages such as low self-discharge rate, long cycle life, high operating voltage, and low pollution. They have broad application prospects in new energy vehicles and other fields.

[0003] The negative electrode is an electrode with a lower potential that provides sites for lithium ion insertion or removal in lithium-ion batteries. The selection of negative electrode materials is the key to the capacity of the negative electrode materials. However, existing negative electrode materials generally have poor electronic conductivity. It is usually necessary to add conductive carbon black, carbon nanotubes and other conductive agents to the negative electrode materials for blending in order to improve the electronic conductivity between the negative electrode materials.

[0004] However, although the conductivity of the negative electrode can be improved to a certain extent by direct blending, the degree of improvement in conductivity is greatly limited because the conductive agent is not in sufficient contact with the negative electrode material. In order to improve the conductivity disadvantage of the negative electrode material, it is an effective technical means to modify it, such as spheroidization, carbon coating, surface oxidation, surface reduction, etc. However, in the existing methods for reducing the contact impedance between negative electrode materials, the obtained negative electrode materials are still in point-to-point contact or point-to-face contact, which also has great limitations and is difficult to highly improve the electronic conductivity. Summary of the invention

[0005] In view of the problem of large contact impedance among existing negative electrode materials, the present invention provides a negative electrode and a preparation method thereof, a secondary battery and an electrical device.

[0006] The technical solution adopted by the present invention to solve the above technical problems is as follows:

[0007] On the one hand, the present invention provides a negative electrode material, including a negative electrode active material, a carbon layer and carbon nanotubes, wherein the carbon layer is coated on at least a portion of the surface of the negative electrode active material, one end of at least a portion of the carbon nanotubes is located on the outer surface of the carbon layer away from the negative electrode active material, and the other end of at least a portion of the carbon nanotubes is a free end extending out of the carbon layer.

[0008] Optionally, the length of the carbon nanotube is 0.01-10 um.

[0009] Optionally, the carbon layer has a thickness of 0.01 to 5 um.

[0010] Optionally, the median particle size of the negative electrode active material is 4 to 30 um.

[0011] Optionally, the negative electrode active material includes one or more of a carbon-based negative electrode active material, a silicon-based negative electrode active material, a germanium-based negative electrode active material, a tin-based negative electrode active material, and a transition metal oxide negative electrode active material.

[0012] Optionally, the carbon-based negative electrode active material includes one or more of natural graphite, artificial graphite, hard carbon materials, soft carbon materials, graphene, and mesophase carbon microspheres;

[0013] The silicon-based negative electrode active material includes one or more of silicon materials, silicon oxides, silicon-carbon composite materials, and silicon alloy materials;

[0014] The germanium-based negative electrode active material includes one or more of germanium, germanium-carbon, germanium-oxygen, and germanium metal compounds;

[0015] The tin-based negative electrode active material includes one or more of tin, tin-carbon, tin-oxygen, and tin metal compounds;

[0016] In the transition metal oxide negative electrode active material, the transition metal includes, but is not limited to, one or more of Fe, Co, Mn, Cu, Zn, Ni, Ti, V, and Cr.

[0017] On the other hand, the present invention provides a method for preparing the negative electrode material as described above, including the following operations:

[0018] Mix the negative electrode active material and carbon source A, so that carbon source A coats at least part of the surface of the negative electrode active material to obtain a negative electrode precursor;

[0019] Carburize carbon source A to obtain a carbon layer;

[0020] Introduce carbon source gas B, and under the catalytic action of a metal catalyst, in-situ growth of carbon nanotubes is carried out on the surface of the carbon layer to obtain the negative electrode material.

[0021] Optionally, carbon source A includes one or more of asphalt, heavy oil, organic resin, starches, and sugars.

[0022] Optionally, carbon source gas B includes one or more of alkanes, alkenes, alkynes, ethanol, and acetone.

[0023] Optionally, the carburization temperature of carbon source A is 300-2000 °C.

[0024] Optionally, the mass ratio of the negative electrode active material to carbon source A is 1:(0.005-0.2).

[0025] Optionally, before the high-temperature carbonization of carbon source A, a metal catalyst precursor is dispersed on the surface of carbon source A of the anode precursor. The metal catalyst precursor is selected from metal oxides, hydroxides, metal salts, and / or metal organic compounds including one or more of Fe, Co, Ni, Mn, Al, Mg, and Mo; during the high-temperature carbonization process, the metal catalyst precursor decomposes to obtain a metal catalyst.

[0026] Optionally, pickling is performed after the operation of "in-situ growth of carbon nanotubes on the surface of the carbon layer" to remove the metal catalyst.

[0027] Optionally, the metal catalyst is dispersed on the surface of the carbon layer, and the metal catalyst is a metal single substance and / or alloy including one or more of Fe, Co, Ni, Mn, Al, Mg, and Mo.

[0028] Optionally, the mass ratio of the anode precursor to the metal catalyst precursor is 1:(0.001 - 0.5).

[0029] On the other hand, the present invention provides a secondary battery, including a positive electrode, an electrolyte, and a negative electrode, and the negative electrode includes the negative electrode material as described above.

[0030] On yet another aspect, the present invention provides an electrical device including the secondary battery as described above.

[0031] According to the negative electrode material provided by the present invention, a carbon layer is coated on at least part of the surface of the negative electrode material. The carbon layer can preferably reduce the surface contact impedance and interfacial reaction impedance of the negative electrode material. At the same time, the carbon layer serves as the in-situ growth action point of carbon nanotubes, ensuring the uniform distribution of carbon nanotubes, inhibiting the agglomeration of carbon nanotubes, and effectively improving the electron conductivity and binding strength between the carbon nanotubes and the negative electrode material. Multiple carbon nanotubes form a hair-like structure on the outer surface of the carbon layer, playing a bridging role, enabling a large contact area between adjacent negative electrode materials, and further enhancing the electron conductivity between adjacent negative electrode materials. Description of the Drawings

[0032] Figure 1 is a schematic structural diagram of the negative electrode material provided by the present invention;

[0033] Figure 2 is the lithium intercalation kinetics test result of Example 1 and Comparative Example 1 provided by the present invention;

[0034] Figure 3 is the charging capacity test result provided by the present invention;

[0035] Figure 4 is the pole piece resistivity test result of Example 1 and Comparative Example 1 provided by the present invention;

[0036] Figure 5 It is the front scanning electron microscope photograph of the negative electrode material provided in Embodiment 1 of the present invention;

[0037] Figure 6 It is the side scanning electron microscope photograph of the negative electrode material provided in Embodiment 1 of the present invention;

[0038] Figure 7 It is the scanning electron microscope photograph of the negative electrode material provided in Comparative Example 1 of the present invention.

[0039] 1. Negative electrode active material; 2. Carbon layer; 3. Carbon nanotube. Detailed implementation manners

[0040] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0041] As Figure 1 shown, an embodiment of the present invention provides a negative electrode material, including a negative electrode active material 1, a carbon layer 2 and carbon nanotubes 3. The carbon layer 2 covers at least part of the surface of the negative electrode active material 1. One end of at least part of the carbon nanotubes 3 is located on the outer surface of the carbon layer 2 facing away from the negative electrode active material 1, and the other end of the at least part of the carbon nanotubes 3 is a free end extending out of the carbon layer 2.

[0042] The carbon layer 2 is coated on at least part of the surface of the negative electrode material. The carbon layer 2 can preferably reduce the surface contact impedance and interfacial reaction impedance of the negative electrode material. At the same time, the carbon layer 2 serves as an attachment point for the carbon nanotubes 3, ensuring the uniform distribution of the carbon nanotubes 3 and inhibiting the agglomeration of the carbon nanotubes 3. It can effectively improve the electron conductivity and bonding strength between the carbon nanotubes 3 and the negative electrode material. A plurality of the carbon nanotubes 3 form a hair-like structure on the outer surface of the carbon layer 2, playing a bridging role, enabling a large contact area between adjacent negative electrode materials, and thus enhancing the electron conductivity between adjacent negative electrode materials.

[0043] In the description of the present invention, "the carbon layer coats at least a part of the surface of the negative electrode active material" includes that the carbon layer coats a part of the surface of the negative electrode active material and that the carbon layer coats the entire surface of the negative electrode active material. When the carbon layer coats a part of the surface of the negative electrode active material, the coating area can be 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or 99% of the outer surface of the negative electrode active material.

[0044] In some embodiments, one end of at least a part of the carbon nanotubes grows in situ on the outer surface of the carbon layer facing away from the negative electrode active material. Using the carbon layer as the in-situ growth action point of the carbon nanotubes can improve the distribution uniformity of the carbon nanotubes, inhibit the agglomeration of the carbon nanotubes, and effectively improve the electron conductivity and the bonding strength between the carbon nanotubes and the negative electrode material.

[0045] In some embodiments, the length of the carbon nanotubes is 0.01 - 10 μm. For example, the length of the carbon nanotubes can be 0.02 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm.

[0046] The carbon nanotubes serve as a bridging structure between adjacent negative electrode materials, and their length is related to the contact impedance between the negative electrode materials. When the length of the carbon nanotubes is within the above range, it can effectively ensure the mutual contact between the negative electrode materials, reduce the contact impedance, and at the same time avoid the influence of too long a length of the carbon nanotubes on the negative electrode energy density.

[0047] In some embodiments, the thickness of the carbon layer is 0.01 - 5 μm.

[0048] As the thickness of the carbon layer increases, the surface impedance of the negative electrode active material can be reduced. When the thickness of the carbon layer is within the above range, it can avoid the influence of too large a thickness of the carbon layer on the insertion or extraction of lithium ions while maintaining the low surface impedance of the negative electrode active material, and ensure the ionic conductivity. For example, the thickness of the carbon layer can be 0.02 μm, 0.05 μm, 0.1 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm.

[0049] In some embodiments, the median particle size of the negative electrode active material is 4 - 30 μm.

[0050] The median particle size of the negative electrode active material is related to the porosity between the negative electrode materials and the contact area of adjacent negative electrode active materials, thereby affecting the penetration efficiency of the electrolyte and the internal resistance of the negative electrode. When the median particle size of the negative electrode active material is within the above range, it is beneficial to maintain the porosity for electrolyte penetration and at the same time reduce the internal resistance of the negative electrode.

[0051] In some embodiments, the negative electrode active material includes one or more of a carbon-based negative electrode active material, a silicon-based negative electrode active material, a germanium-based negative electrode active material, a tin-based negative electrode active material, and a transition metal oxide negative electrode active material.

[0052] In some embodiments, the carbon-based negative electrode active material includes one or more of natural graphite, artificial graphite, hard carbon materials, soft carbon materials, graphene, and mesophase carbon microspheres;

[0053] The silicon-based negative electrode active material includes one or more of silicon materials, silicon oxides, silicon-carbon composite materials, and silicon alloy materials;

[0054] The germanium-based negative electrode active material includes one or more of germanium, germanium-carbon, germanium-oxygen, and germanium metal compounds;

[0055] The tin-based negative electrode active material includes one or more of tin, tin-carbon, tin-oxygen, and tin metal compounds;

[0056] In the transition metal oxide negative electrode active material, the transition metal includes, but is not limited to, one or more of Fe, Co, Mn, Cu, Zn, Ni, Ti, V, and Cr.

[0057] In some embodiments, the negative electrode active material is selected from natural graphite or artificial graphite.

[0058] The natural graphite or the artificial graphite has a relatively low and flat lithium ion intercalation potential, can provide a relatively high and stable working voltage for the lithium ion battery, has a relatively high theoretical capacity, and has good charge and discharge performance.

[0059] Another embodiment of the present invention provides a method for preparing the negative electrode material as described above, including the following operations:

[0060] Mix the negative electrode active material and carbon source A so that carbon source A coats at least part of the surface of the negative electrode active material to obtain a negative electrode precursor;

[0061] Carburize carbon source A to obtain a carbon layer;

[0062] Introduce carbon source gas B, and under the catalytic action of a metal catalyst, in-situ growth of carbon nanotubes is carried out on the surface of the carbon layer to obtain the negative electrode material.

[0063] This preparation method performs a double-layer coating on the negative electrode active material. Among them, carbon source A serves as the first-layer coating precursor, carbonizes at high temperature to form a carbon layer, and carbon nanotubes are catalytically grown on the surface of the carbon layer, which can effectively improve the connection strength between the carbon nanotubes and the negative electrode active material. The obtained negative electrode material has the advantage of low contact impedance and meets the application requirements of secondary batteries for high-rate charge and discharge.

[0064] In some embodiments, the carbon source A includes one or more of asphalt, heavy oil, organic resin, starches, and sugars.

[0065] In some embodiments, the carbon source A is selected from asphalt.

[0066] Compared with other carbon sources, asphalt has a good coating effect on the negative electrode active material, and the formed carbon layer has a good bonding effect with the negative electrode active material. At the same time, asphalt has a low cost and a mature coating process.

[0067] In some embodiments, the carbonization temperature of the carbon source A is 300 - 2000 °C.

[0068] In some embodiments, the mass ratio of the negative electrode active material to the carbon source A is 1:(0.005 - 0.2).

[0069] When the mass ratio of the negative electrode active material to the carbon source A is within the above range, it can ensure the surface coating of the carbon source A on the negative electrode active material, and at the same time avoid problems such as too thick a carbon layer or agglomeration of the negative electrode active material caused by excessive carbon source A. For example, the mass ratio of the negative electrode active material to the carbon source A can be 1:0.005, 1:0.01, 1:0.05, 1:0.1, 1:0.2, etc.

[0070] In some embodiments, the carbon source gas B includes one or more of alkanes, alkenes, alkynes, ethanol, and acetone.

[0071] In some embodiments, after the operation of "in-situ growth of carbon nanotubes on the surface of the carbon layer", the obtained material is pickled to remove the metal catalyst.

[0072] The metal elements contained in the metal catalyst will occupy the lithium ion insertion and extraction sites in the negative electrode material during battery cycling, resulting in a decrease in the capacity of the negative electrode material. By pickling, the metal catalyst in the negative electrode material can be effectively removed, thereby avoiding the adverse effects of the residual metal catalyst on the performance of the negative electrode material.

[0073] In some embodiments, in the "pickling" operation, the acid used is one or more of hydrochloric acid, nitric acid, sulfuric acid, and hydrofluoric acid, with a concentration of 0.01 - 2 M, a pickling temperature of 25 °C - 90 °C, and a pickling time of 0.5 - 12 h.

[0074] In some embodiments, after the "pickling" operation, water washing is carried out, and then drying is performed to remove moisture.

[0075] In some embodiments, the metal catalyst is dispersed on the surface of the carbon layer, and the metal catalyst includes one or more metal elements and / or alloys selected from Fe, Co, Ni, Mn, Al, Mg, and Mo.

[0076] When the catalytic growth of the carbon nanotubes is carried out, the carbon nanotubes start to grow with the metal catalyst as the catalytic site. Therefore, by dispersing the metal catalyst on the surface of the carbon layer, the uniformity of the growth of the carbon nanotubes on the carbon layer surface can be effectively ensured, agglomeration can be inhibited, and thus the bridging probability between the negative electrode materials is increased and the contact impedance is reduced.

[0077] In some embodiments, before the high-temperature carbonization of carbon source A, a metal catalyst precursor is dispersed on the surface of carbon source A of the negative electrode precursor, and the metal catalyst precursor is selected from one or more metal oxides, hydroxides, metal salts, and / or metal organic compounds including Fe, Co, Ni, Mn, Al, Mg, and Mo;

[0078] During the carbonization process, the carbonization temperature of carbon source A is 300 - 2000 °C.

[0079] During the carbonization process, the metal catalyst precursor decomposes to obtain the metal catalyst.

[0080] When the catalytic growth of the carbon nanotubes is carried out, the actual catalysts are one or more of the simple substances and / or alloys of Fe, Co, Ni, Mn, Al, Mg, and Mo. When the metal catalyst precursor is selected from the metal oxides of the above metal elements, the metal oxide can be reduced by a reducing substance to obtain a simple substance or alloy to play a catalytic role; when the metal catalyst precursor is selected from the hydroxides, metal salts, and / or metal organic compounds of the above metal elements, it can be converted into a metal oxide by high temperature, and then the metal oxide can be reduced by a reducing substance to obtain a simple substance or alloy to play a catalytic role. For example, when the catalyst precursor is selected from metal oxides, during the process of carbonizing carbon source A to form a carbon layer under high temperature conditions, the formed carbon layer has a certain reducing property and can reduce the metal from the metal oxide at high temperature, so as to obtain a simple substance or alloy to play a catalytic role in the growth of carbon nanotubes; when the catalyst precursor is selected from hydroxides, metal salts, and / or metal organic compounds, the hydroxides, metal salts, and / or metal organic compounds can be converted into metal oxides by high temperature, and then the same conversion process as that of the metal oxide is carried out; in some embodiments, the metal oxide can also be reduced by introducing a reducing gas, such as hydrogen, into the reaction system to obtain a simple substance or alloy with catalytic activity.

[0081] By first preparing a dispersion of the metal catalyst precursor on the surface of carbon source A and then decomposing it to form the metal catalyst, since the metal catalyst precursor generates gas during thermal decomposition and is cracked into finer particles, it is beneficial to improve the dispersion uniformity of the metal catalyst on the surface of carbon source A.

[0082] In some embodiments, the dispersion of the metal catalyst precursor is mixed and stirred with the anode precursor, the stirring speed is 10 - 500 rpm, the dispersion time is 0.5 - 12 h, and the obtained mixed dispersion liquid is coated on the surface of carbon source A, and then vacuum dried at 60 - 120 °C for 0.5 - 12 h to disperse the metal catalyst precursor on the surface of carbon source A.

[0083] In some embodiments, the dispersion of the metal catalyst precursor includes the metal catalyst precursor and glycerol, and the solid content of the dispersion of the metal catalyst precursor is 0.1% - 10%.

[0084] In some embodiments, the metal catalyst precursor is prepared by the sol-gel method, including the following operations:

[0085] Dissolve one or more acetates or nitrates or sulfates or hydrochlorides of Fe, Co, Ni, Mn, Al, Mg, and Mo in water, add citric acid and ammonium nitrate, and stir at 60°C to 120°C at 10 to 500 rpm for 0.5 to 12 h to complete gelation. Then bake at 100°C to 200°C for 0.5 to 12 h, and grind the obtained solid into fine powder to obtain the catalyst precursor, with the grinding time being 10 to 60 min.

[0086] In some embodiments, the mass ratio of the negative electrode precursor to the metal catalyst precursor is 1:(0.001 to 0.5).

[0087] Another embodiment of the present invention provides a secondary battery, including a positive electrode, an electrolyte, and a negative electrode, wherein the negative electrode includes the negative electrode material as described above.

[0088] Due to the adoption of the negative electrode material as described above, the secondary battery has a lower battery internal resistance and meets the requirements of high-power charge and discharge.

[0089] In some embodiments, the negative electrode includes a current collector and a negative electrode material layer, the negative electrode material layer includes the negative electrode material and a binder, and the binder includes one or more of polyvinylidene fluoride, copolymers of vinylidene fluoride, polytetrafluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene, copolymers of tetrafluoroethylene - hexafluoropropylene, copolymers of tetrafluoroethylene - perfluoroalkyl vinyl ether, copolymers of ethylene - tetrafluoroethylene, copolymers of vinylidene fluoride - tetrafluoroethylene, copolymers of vinylidene fluoride - trifluoroethylene, copolymers of vinylidene fluoride - trichloroethylene, copolymers of vinylidene fluoride - fluoroethylene, copolymers of vinylidene fluoride - hexafluoropropylene - tetrafluoroethylene, thermoplastic polyimides, thermoplastic resins such as polyethylene and polypropylene; acrylic resins; sodium carboxymethyl cellulose; polyvinyl butyral; ethylene - vinyl acetate copolymer; polyvinyl alcohol; and styrene - butadiene rubber.

[0090] In some embodiments, the negative electrode material layer further includes a conductive agent, and the conductive agent includes one or more of conductive carbon black, conductive carbon spheres, conductive graphite, conductive carbon fibers, carbon nanotubes, graphene, or reduced graphene oxide.

[0091] The current collector is selected from metal materials capable of conducting electrons. In some embodiments, the current collector includes one or more of Al, Ni, tin, copper, and stainless steel. In some embodiments, the current collector is selected from copper foils.

[0092] In some embodiments, the electrolyte is selected from solid electrolytes or electrolytic solutions.

[0093] In some embodiments, the solid electrolyte includes a polymer and an electrolyte salt dispersed in the polymer.

[0094] The electrolyte includes a non-aqueous organic solvent and an electrolyte salt.

[0095] Another embodiment of the present invention provides an electrical device, including the secondary battery as described above.

[0096] The electrical device can be a vehicle, a ship or other transportation means, or can be computer, communication and consumer electronic products (3C products), etc.

[0097] The present invention will be further described by way of examples below.

[0098] Example 1

[0099] This example is used to illustrate the negative electrode material and its preparation method disclosed by the present invention, including the following operations:

[0100] (1) Mix and disperse spherical natural graphite and pitch evenly at a mass ratio of 1:0.1 to obtain a spherical natural graphite / pitch mixture. The median particle size of the spherical natural graphite is 10 μm, the softening point of the pitch is 200 °C, the stirring speed is 300 rpm; the dispersion time is 5 h.

[0101] (2) Stir the spherical natural graphite / pitch mixture at 200 °C at a speed of 300 rpm for 5 h, cool down, and disperse to obtain pitch-coated natural graphite.

[0102] (3) Prepare a catalyst precursor by the sol-gel method. Dissolve ferric nitrate, molybdenum nitrate and aluminum nitrate at a molar ratio of 1:1:1 in water to obtain a nitrate solution with a total molar concentration of 1 mol / L, and at the same time add citric acid and ammonium nitrate to obtain a mixed solution; in the mixed solution, the mass percentages of citric acid and ammonium nitrate are 3% and 3% respectively. Stir the mixed solution at 90 °C at a rotation speed of 300 rpm for 5 h to complete the gelation, then bake at 150 °C for 5 h, and then grind for 30 min to obtain the catalyst precursor.

[0103] (4) Uniformly ultrasonically disperse the above catalyst precursor in glycerol, control the solid content at 5%, and the ultrasonic dispersion time is 10 h to obtain a catalyst precursor dispersion.

[0104] (5) Stir and mix the pitch-coated natural graphite and the above catalyst precursor dispersion at a mass ratio of pitch-coated natural graphite to catalyst precursor of 1:0.05, stirring speed: 300 rpm, dispersion time: 5 h. Subsequently, vacuum dry at 90 °C for 5 h to obtain pitch-coated natural graphite with the catalyst precursor dispersed on the surface.

[0105] (6) Place the asphalt-coated natural graphite with the catalyst precursor dispersed on its surface obtained above in a large CVD furnace. First, heat it to 380 °C and maintain for 5 h, then raise the temperature to 1200 °C and carry out high-temperature carbonization for 2 h to convert the asphalt into a carbon layer with a thickness of 0.05 - 0.2 μm. Subsequently, introduce methane and hydrogen with flow rates of 100 sccm and 20 sccm respectively, and the introduction time of methane and hydrogen is 20 min to complete the catalytic growth of carbon nanotubes and obtain a composite coating material; the length of the carbon nanotubes is 0.05 - 0.3 μm.

[0106] (7) Wash the composite coating material obtained in step (6) with an acidic solution at 80 °C for 5 h to remove metal impurities. The acidic solution is a 1M dilute hydrochloric acid aqueous solution. After the pickling is completed, wash it 5 times with deionized water and dry it in vacuum at 100 °C for 5 h to obtain the negative electrode material.

[0107] (8) Mix the negative electrode material obtained in step (7) and the carbon black conductive agent in proportion and add them to an aqueous solution of carboxymethyl cellulose (CMC) with a solid content of 1.5%, disperse, and then add an aqueous solution of styrene-butadiene rubber (SBR) with a solid content of 40% and disperse; after forming a slurry, coat it on a copper foil, then place it in an oven at 80 °C and dry for 24 h, and then make a negative electrode sheet after pressing and cutting; the proportion of each solid component in the negative electrode sheet is negative electrode material: carbon black conductive agent: styrene-butadiene rubber (SBR) emulsion: CMC thickener = 100:1.2:1.6:1.5.

[0108] Use metallic lithium as the counter electrode, the above negative electrode sheet as the working electrode, a 12-μm-thick polyethylene (PE) membrane as the separator, and a mixture containing 0.5% by mass of vinylene carbonate (VC), a concentration of 1M LiPF6, and a volume ratio of ethylene carbonate (EC): ethyl methyl carbonate (EMC): dimethyl carbonate (DMC) = 1:1:1 as the electrolyte to assemble a coin-type half-cell.

[0109] Use ternary material NCM622 as the positive electrode active material to prepare the positive electrode, the above negative electrode sheet as the negative electrode, a 12-μm-thick PE membrane coated with a 2-μm ceramic coating as the separator, and a mixture containing 0.5% by mass of VC, a concentration of 1M LiPF6, and a volume ratio of EC: EMC: DMC = 1:1:1 as the electrolyte to assemble a full cell.

[0110] Example 2

[0111] This example is used to illustrate the negative electrode material and its preparation method disclosed in the present invention, including most of the operations in Example 1, and the differences are as follows:

[0112] In step (1), the mass ratio of spherical natural graphite to asphalt is 1:0.15.

[0113] In step (6), the obtained carbon layer thickness is 0.09 - 0.32 um.

[0114] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0115] Example 3

[0116] This example is used to illustrate the negative electrode material and its preparation method disclosed in the present invention, including most of the operations in Example 1, and the differences are as follows:

[0117] In step (1), the mass ratio of spherical natural graphite to pitch is 1:0.05.

[0118] In step (6), the obtained carbon layer thickness is 0.02 - 0.11 um.

[0119] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0120] Example 4

[0121] This example is used to illustrate the negative electrode material and its preparation method disclosed in the present invention, including most of the operations in Example 1, and the differences are as follows:

[0122] In step (6), methane and hydrogen are introduced with flow rates of 100 sccm and 20 sccm respectively, and maintained for 40 min to complete the catalytic growth of carbon nanotubes. The length of the carbon nanotubes is 0.08 - 0.7 um.

[0123] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0124] Example 5

[0125] This example is used to illustrate the negative electrode material and its preparation method disclosed in the present invention, including most of the operations in Example 1, and the differences are as follows:

[0126] In step (6), methane and hydrogen are introduced with flow rates of 100 sccm and 20 sccm respectively, and maintained for 10 min to complete the catalytic growth of carbon nanotubes. The length of the carbon nanotubes is 0.03 - 0.16 um.

[0127] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0128] Example 6

[0129] This example is used to illustrate the negative electrode material and its preparation method disclosed by the present invention, including most of the operations in Example 1, and the differences are as follows:

[0130] In step (1), the mass ratio of spherical natural graphite to asphalt is 1:1.

[0131] In step (6), the obtained carbon layer thickness is 3 - 9 μm.

[0132] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0133] Example 7

[0134] This example is used to illustrate the negative electrode material and its preparation method disclosed by the present invention, including most of the operations in Example 1, and the differences are as follows:

[0135] In step (6), methane and hydrogen are introduced with flow rates of 100 sccm and 20 sccm respectively, and maintained for 10 h to complete the catalytic growth of carbon nanotubes. The length of the carbon nanotubes is 5 - 25 μm.

[0136] The obtained negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0137] Comparative Example 1

[0138] This comparative example uses spherical natural graphite as the negative electrode material.

[0139] The negative electrode material is made into a pole piece under the same conditions as in step (8) of Example 1 and assembled into a button-type half-cell and a full cell.

[0140] Comparative Example 2

[0141] This comparative example is used to compare and illustrate the negative electrode material and its preparation method disclosed by the present invention, including steps (1) - (5) in Example 1, and the differences are as follows:

[0142] (6) The carbon nanotubes are uniformly ultrasonically dispersed in glycerol with a solid content controlled at 5% and a dispersion time of 10 h to obtain a carbon nanotube dispersion. The natural graphite coated with asphalt with a catalyst precursor dispersed on its surface and the carbon nanotube dispersion are stirred and mixed at a solid content mass ratio of 1:0.01. Stirring speed: 300 rpm, dispersion time: 5 h. Subsequently, it is vacuum dried at 90°C for 5 h.

[0143] (7) Place the product of step (6) in a CVD furnace. First, heat it to 380 °C and maintain for 5 h, then raise the temperature to 1200 °C and perform high-temperature carbonization for 2 h to obtain a composite coating material including spherical natural graphite, a carbon layer, and carbon nanotubes dispersed on the surface of the carbon layer. The thickness of the carbon layer is 0.05 - 0.2 μm.

[0144] (8) Wash the composite coating material obtained in step (7) with an acidic solution at 80 °C for 5 h to remove metal impurities. The acidic solution is a 1 M dilute hydrochloric acid aqueous solution. After the pickling is completed, wash it 5 times with deionized water and dry it in vacuum at 100 °C for 5 h to obtain the negative electrode material.

[0145] (9) Make the negative electrode material into a pole piece under the same conditions as in step (8) of Example 1 and assemble it into a button-type half-cell and a full-cell.

[0146] Performance Test

[0147] (I) Conduct physical property tests on the negative electrodes prepared in the above Example 1 and Comparative Example 1, and conduct the following electrical property tests based on the button-type half-cell and full-cell:

[0148] 1.1 Conduct a power-type discharge capacity test on the button-type half-cells obtained in Example 1 and Comparative Example 1 under the test conditions: 50% SOC, 3C lithium insertion for 10 s, and the test results are as Figure 2 shown.

[0149] Conduct a charging capacity test on the full-cells obtained in Example 1 and Comparative Example 1 under the test conditions: 50% SOC, 3C charging for 30 s, and the test results are as Figure 3 shown.

[0150] From Figure 2 and 3 of the test results, it can be seen that compared with the natural graphite negative electrode without coating and carbon nanotube growth, the lithium insertion kinetics / charging performance of the negative electrode material prepared by the method provided by the present invention has been significantly improved.

[0151] 1.2 Test the resistivity of the negative electrodes obtained in Example 1 and Comparative Example 1, and the test results are as Figure 4 shown.

[0152] From Figure 4 the results, it can be seen that compared with the natural graphite negative electrode without coating and carbon nanotube growth, the impedance of the negative electrode material prepared by the method provided by the present invention is significantly reduced.

[0153] 1.3 Conduct scanning electron microscope observations on the negative electrode material prepared in Example 1 and the negative electrode material of the comparative example. The electron microscope photos obtained in Example 1 are as Figure 5 and Figure 6As shown, the electron microscope photograph obtained in Comparative Example 1 is as Figure 7 shown.

[0154] From Figures 5 - 7 the results, it can be seen that there are more carbon nanotubes formed on the surface of the negative electrode material prepared by the method provided by the present invention, and there is obvious dispersion between the carbon nanotubes, and no agglomeration problem occurs.

[0155] (2) After the coin-type half-cells prepared in the above Examples 1 to 5 and Comparative Examples 1 to 2 are subjected to charge and discharge activation, the EIS impedance of the battery at 50% SOC is tested, and the test results are filled in Table 1. The activation conditions are as follows: the coin-type half-cells are discharged to 5 mV at 0.1C, 0.09C, 0.08C... 0.02C respectively, and the interval time between each step is 10 min; then the coin-type half-cells are charged to 2V at 0.1C, 0.05C, 0.02C respectively, and the interval time between each step is 10 min; the coin cells are cycled 3 weeks according to the above steps; the 1C capacity input value is calculated by multiplying the mass of graphite (in g) by 360 mAh / g. The charging capacity of the last cycle is marked as the actual capacity of the material, and then the coin-type half-cell is discharged, and the discharge is stopped when it reaches half of the actual capacity, that is, the coin-type half-cell at 50% SOC is obtained.

[0156] Table 1

[0157] Group Battery EIS Impedance Example 1 40.27 ohm Example 2 38.79 ohm Example 3 45.28 ohm Example 4 37.88 ohm Example 5 42.43 ohm Example 6 48.07 ohm Example 7 43.35 ohm Comparative Example 1 53.29 ohm Comparative Example 2 48.95 ohm

[0158] From the test results in Table 1, it can be seen that the negative electrode material prepared by the method provided by the present invention has significantly lower impedance. At the same time, compared with the method of directly blending carbon nanotubes, the in-situ growth method is more conducive to increasing the contact area between the negative electrode materials and reducing the internal resistance.

[0159] The above are only the preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A negative electrode material, characterized in that, The invention comprises a negative electrode active material, a carbon layer and carbon nanotubes, wherein the negative electrode active material is selected from a carbon-based negative electrode active material, the carbon layer is coated on at least a portion of the surface of the negative electrode active material, one end of at least a portion of the carbon nanotubes is located on the outer surface of the carbon layer away from the negative electrode active material, and the other end of at least a portion of the carbon nanotubes is a free end extending out of the carbon layer; The method for preparing the negative electrode material comprises the following operations: Mixing the negative electrode active material and the carbon source A so that the carbon source A is coated on at least a portion of the surface of the negative electrode active material to obtain a negative electrode precursor; Carbon source A is carbonized to obtain a carbon layer; before carbonizing carbon source A, a metal catalyst precursor is dispersed on the surface of carbon source A of the negative electrode precursor, wherein the metal catalyst precursor is a metal oxide, hydroxide, metal salt and / or metal organic compound including one or more of Fe, Co, Ni, Mn, Al, Mg and Mo, and during the carbonization process, the metal catalyst precursor is decomposed to obtain a metal catalyst, and the metal catalyst is dispersed on the surface of the carbon layer; A carbon source gas B is introduced, and under the catalytic action of a metal catalyst, carbon nanotubes are grown in situ on the surface of the carbon layer to obtain a negative electrode material.

2. The negative electrode material according to claim 1, characterized in that, The length of the carbon nanotube is 0.01-10 um.

3. The negative electrode material according to claim 1, wherein The thickness of the carbon layer is 0.01-5 um.

4. The negative electrode material according to claim 1, characterized in that, The median particle size of the negative electrode active material is 4-30 um.

5. The negative electrode material according to claim 1, wherein The carbon-based negative electrode active material includes one or more of natural graphite, artificial graphite, hard carbon material, soft carbon material, graphene and mesophase carbon microbeads.

6. The preparation method of the negative electrode material according to any one of claims 1 to 5, characterized in that The following operations are included: Mixing the negative electrode active material and the carbon source A so that the carbon source A is coated on at least a portion of the surface of the negative electrode active material to obtain a negative electrode precursor; Carbon source A is carbonized to obtain a carbon layer; before carbonizing carbon source A, a metal catalyst precursor is dispersed on the surface of carbon source A of the negative electrode precursor, wherein the metal catalyst precursor is a metal oxide, hydroxide, metal salt and / or metal organic compound including one or more of Fe, Co, Ni, Mn, Al, Mg and Mo, and during the carbonization process, the metal catalyst precursor is decomposed to obtain a metal catalyst, and the metal catalyst is dispersed on the surface of the carbon layer; A carbon source gas B is introduced, and under the catalytic action of a metal catalyst, carbon nanotubes are grown in situ on the surface of the carbon layer to obtain a negative electrode material.

7. The preparation method of the negative electrode material according to claim 6, characterized in that, The carbon source A includes one or more of asphalt, heavy oil, organic resin, starch and sugar; the carbon source gas B includes one or more of alkane, alkene, alkyne, ethanol and acetone; the carbonization temperature of the carbon source A is 300~2000°C.

8. The method for preparing the negative electrode material according to claim 6 or 7, characterized in that, The mass ratio of the negative electrode active material to the carbon source A is 1:(0.005-0.2).

9. The preparation method of the negative electrode material according to claim 6 or 7, characterized in that, After the in-situ growth of carbon nanotubes is performed on the surface of the carbon layer, acid washing is performed to remove the metal catalyst; The metal catalyst is a metal single substance and / or alloy including one or more of Fe, Co, Ni, Mn, Al, Mg and Mo; The mass ratio of the negative electrode precursor to the metal catalyst precursor is 1:(0.001-0.5).

10. A secondary battery, characterized in that, It includes a positive electrode, an electrolyte, and a negative electrode, and the negative electrode includes the negative electrode material described in any one of claims 1 to 5.

11. An electrical device, characterized in that, It includes the secondary battery described in claim 10.

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