A graphite composite negative electrode material, a preparation method thereof, and a lithium-ion battery

Abstract

The present invention provides a graphite composite negative electrode material, a preparation method thereof, and a lithium-ion battery. The graphite composite negative electrode material has a core-shell structure, including a graphite inner core, a reticular conductive intermediate layer, and a surface soft carbon coating layer; the raw materials of the reticular conductive intermediate layer include polyhydroxybenzene and metal salts, the raw materials of the graphite inner core include natural graphite, and the raw materials of the surface soft carbon coating layer include high-temperature coating pitch; the mass ratio of the natural graphite: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000: 20-80: 20-80: 20-60. The present invention can prepare a natural graphite negative electrode material with high first-cycle Coulomb efficiency, high rate performance, and excellent long-cycle performance.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion battery electrode materials, and particularly to a graphite composite negative electrode material, a preparation method thereof, and a lithium-ion battery. Background Art

[0002] Lithium-ion secondary batteries are widely used in many fields due to their high energy density, long life, low self-discharge, and environmental protection characteristics. As one of the core materials, the negative electrode material directly affects the performance of the battery. Natural graphite is the main raw material for the negative electrode material, but it has defects such as swelling effect and anisotropy, which affect the cycle stability of the battery. To overcome these problems, the industry uses asphalt and resin-based coating materials mixed with spherical graphite and carbonizes them under a protective atmosphere to form an amorphous carbonized layer covering the surface of the graphite. This process effectively repairs the surface defects of the graphite, reduces the co-insertion phenomenon of the electrolyte, and significantly improves its electrochemical performance. The performance of the coating material and the carbonized coating layer after carbonization determines the overall performance of the natural graphite negative electrode material. By adding or plating metal elements inside or on the surface of the coating layer, the conductivity of the negative electrode material can be improved, the internal resistance can be reduced, and the charge and discharge performance of the lithium-ion battery can be enhanced.

[0003] For example, CN113394402A coats a uniform single-layer sieve-like metal copper on the outer layer of spherical graphite. The pores of the copper layer sieve-like structure can allow lithium ions to pass through, and copper can improve the rate performance of the material; CN113991076B forms a layer of nano-copper particles on the surface of graphite by chemical plating to improve the conductivity of electrons on the material surface and the migration rate of lithium ions.

[0004] It can be seen that the metal conductive particles generated by the current surface modification in the prior art only cover the surface of the graphite material and do not extend to the carbonized layer. The copper-containing conductive layer can only improve the conductivity and does not endow more functions through structural design.

[0005] Therefore, providing a new type of graphite composite negative electrode material, a preparation method thereof, and a lithium-ion battery has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] To solve the above technical problems, the purpose of the present invention is to provide a graphite composite negative electrode material, a preparation method thereof, and a lithium-ion battery.

[0007] To achieve the above object, the present invention provides a graphite composite anode material having a core-shell structure. The graphite composite anode material includes a graphite core, a network conductive intermediate layer, and a surface soft carbon coating layer. Among them, the raw materials of the network conductive intermediate layer include polyhydroxybenzene and metal salts, the raw materials of the graphite core include natural graphite, and the raw materials of the surface soft carbon coating layer include high-temperature coating pitch. The mass ratio of natural graphite: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000: 20-80: 20-80: 20-60.

[0008] The present invention creates active sites on the surface of natural graphite, in-situ grows a metal-organic framework network structure under hydrothermal conditions, and after calcination, part of the organic components volatilize to form a network coating layer with certain structural strength and containing uniformly distributed conductive metal particles. A dense soft carbon coating layer is covered on the outer surface to improve the conductivity of the natural graphite anode material, reduce the impedance, utilize the good voids of the network structure to buffer the expansion effect of natural graphite during cycling, and improve the cycling performance of the anode material.

[0009] In the above graphite composite anode material, preferably, the mass ratio of natural graphite: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000: 40-60: 50-70: 45-60. In the above graphite composite anode material, preferably, the monomer carbon number of the polyhydroxybenzene is 6-15, and the carbon-hydrogen ratio is 1-1.2.

[0010] In the above graphite composite anode material, preferably, the polyhydroxybenzene includes hexahydroxybenzene and / or 3,3',4,4',5,5'-hexahydroxybiphenyl.

[0011] In the present invention, the network-shaped conductive layer in-situ grown on the surface of natural graphite is based on polyhydroxybenzene. The polyhydroxybenzene selected in the present invention as the bridging agent has rich hydroxyl groups and high unsaturation. The hydroxyl groups can bridge and grow on the surface of natural graphite with active sites to form a porous network structure. The hydroxyl groups react with metal acetate salts, and copper particles are captured by the network structure and uniformly dispersed in the network structure. Different from other organic ligands, the highly unsaturated π bonds and appropriate carbon number of polyhydroxybenzene ensure that the network structure is not easily collapsed during calcination. The residual carbon and metal ions after high-temperature treatment retain the network structure, have a certain strength to absorb stress, and provide good conductivity.

[0012] In the above graphite composite anode material, preferably, the metal salt includes one or more combinations of copper acetate, cobalt acetate, and zinc acetate.

[0013] In the above graphite composite anode material, preferably, the softening point of the high-temperature coating pitch is 250-300 °C, more preferably 275-300 °C, and the coking value ≥ 70%, more preferably ≥ 75%.

[0014] In the above graphite composite anode material, preferably, the quinoline insoluble content in the high-temperature coated pitch is ≤ 0.3 wt%, and the ash content is ≤ 0.1 wt%; wherein both the quinoline insoluble content and the ash content are calculated based on the total weight of the coated pitch being 100%.

[0015] In the above graphite composite anode material, preferably, the median particle size D50 of the high-temperature coated pitch is 1 - 6 μm, more preferably 2 - 4 μm. In the present invention, the soft carbon coating formed by carbonizing the high-temperature coated pitch can further reduce the specific surface area of the anode material, avoiding the decrease in the first-cycle Coulombic efficiency caused by the large number of holes and large specific surface area in the network structure layer, resulting in the formation of excessive SEI films (solid electrolyte interface films). The combination of the network conductive layer and the soft carbon coating layer not only improves the conductivity of the anode material, provides good buffering performance, reduces the internal resistance, but also forms a dense carbonized layer on the surface, isolating the electrolyte and the copper particles that promote conductivity, and preventing them from escaping into the electrolyte during charge and discharge.

[0016] In the above graphite composite anode material, preferably, the natural graphite is spherical graphite, the median particle size D50 is 16 - 18 μm, and the fixed carbon content is ≥ 99.95 wt%; wherein the fixed carbon content is calculated based on the total weight of the natural graphite being 100%.

[0017] In the above graphite composite anode material, preferably, the tapped density of the natural graphite is ≥ 0.96 g / cm 3 , more preferably ≥ 0.98 g / cm 3 .

[0018] In the above graphite composite anode material, preferably, the specific surface area of the natural graphite is ≤ 6.5 cm 2 / g, more preferably ≤ 6.0 cm 2 / g.

[0019] The present invention also provides a preparation method of the above graphite composite anode material, which includes:

[0020] (1) Surface-modifying natural graphite using a surfactant to obtain surface-modified graphite;

[0021] (2) Mixing the surface-modified graphite with polyhydroxybenzene in water, then adding a metal salt for hydrothermal reaction, and carbonizing the product after the reaction is completed to obtain an intermediate product;

[0022] (3) Mixing the intermediate product with high-temperature coated pitch evenly, and then carbonizing again to obtain the graphite composite anode material.

[0023] The natural graphite, surfactant, polyhydroxybenzene, metal salt, and high-temperature coating pitch used in the present invention can all be obtained by conventional means in the art, such as commercial purchase.

[0024] In the preparation method of the above graphite composite negative electrode material, preferably, in step (1), the method for surface modification of natural graphite using a surfactant is as follows: The surfactant is mixed uniformly with water, and graphite is slowly added under stirring, followed by heating and stirring, filtration, and drying to obtain the surface-modified graphite.

[0025] In the preparation method of the above graphite composite negative electrode material, preferably, during the modification of natural graphite, the temperature for heating and stirring is 40 - 70 °C, and the time is 20 - 120 min; more preferably, the drying temperature does not exceed 80 °C.

[0026] In the preparation method of the above graphite composite negative electrode material, preferably, during the modification of natural graphite, the surfactant includes sodium dodecylbenzenesulfonate and / or cetyltrimethylammonium bromide. In the present invention, the use of the surfactant can improve the wetting contact between graphite and water, help graphite disperse better in the solution medium, and at the same time, the amphiphilic groups of the surfactant can form bonds with the graphite surface, providing active sites for the uniform growth of the metal-organic network structure.

[0027] In the preparation method of the above graphite composite negative electrode material, preferably, during the modification of natural graphite, the mass ratio of the surfactant to natural graphite is 1 - 10:1000.

[0028] In the preparation method of the above graphite composite negative electrode material, preferably, during the modification of natural graphite, the mass ratio of water to natural graphite is 5 - 15:1.

[0029] In the preparation method of the above graphite composite negative electrode material, preferably, in step (2), the temperature of the hydrothermal reaction is 70 - 95 °C, and the time is 20 - 120 min.

[0030] In the preparation method of the above graphite composite negative electrode material, preferably, in step (2), the mass ratio of water to the surface-modified graphite is 15 - 30:1.

[0031] In the preparation method of the above graphite composite negative electrode material, preferably, in step (2), the method for carbonization is as follows: The temperature is raised to 750 - 950 °C at a heating rate of 1 - 10 °C / min and held for 1 - 4 h.

[0032] In the preparation method of the above graphite composite negative electrode material, preferably, in step (3), the method for re-carbonization is as follows: The temperature is raised to 1100 - 1350 °C at a heating rate of 1 - 10 °C / min and held for 1 - 6 h.

[0033] In the preparation method of the above-mentioned graphite composite negative electrode material, preferably, the two carbonization processes are carried out under nitrogen protection.

[0034] In the preparation method of the above-mentioned graphite composite negative electrode material, preferably, the heating and stirring in step (1) and the hydrothermal reaction process in step (2) are both carried out in a reaction kettle with heating and stirring functions, including but not limited to a glass reaction kettle, a polytetrafluoroethylene reaction kettle, and a stainless steel reaction kettle.

[0035] In the preparation method of the above-mentioned graphite composite negative electrode material, preferably, the two carbonization processes in steps (2) and (3) are carried out in an atmosphere furnace; preferably, step (2) further includes: naturally cooling the obtained carbonized product after carbonization, and then dispersing, sieving; step (3) further includes: naturally cooling the obtained carbonized product after carbonization, and then carrying out operations such as dispersing, sieving, and demagnetization to obtain the graphite composite negative electrode material. The mixing process is carried out in a mixer.

[0036] According to the specific embodiments of the present invention, preferably, the preparation method of the above-mentioned graphite composite negative electrode material includes the following steps:

[0037] S1: Mix the surfactant and water evenly, slowly add natural graphite under stirring, heat and stir for a period of time, then filter and dry to obtain intermediate material 1;

[0038] S2: Mix polyhydroxybenzene and water, slowly add intermediate material 1 under stirring, and then add a metal salt, heat and stir for a period of time, then filter and dry to obtain intermediate material 2;

[0039] S3: Under the protection of an inert gas, carbonize the intermediate material 2 to obtain intermediate material 3;

[0040] S4: Under the protection of an inert gas, mix intermediate material 3 and high-temperature coating pitch evenly to obtain intermediate material 4;

[0041] S5: Under the protection of an inert gas, carbonize the intermediate material 4 to obtain the graphite composite negative electrode material.

[0042] In the graphite composite negative electrode material provided by the present invention, the functions of the network conductive intermediate layer and the surface soft carbon coating layer are to repair the surface phase defects of natural graphite, reduce the specific surface area, improve the conductivity of the graphite material, isolate the electrolyte from the natural graphite body, reduce the co-insertion phenomenon of lithium ions and electrolyte molecules, and at the same time, the network conductive coating layer with a certain structural strength provides a buffer for the expansion effect during the lithium deintercalation process of natural graphite, thereby improving the electrochemical properties such as the first-cycle Coulomb efficiency, rate performance, and long-cycle performance of the natural graphite negative electrode material.

[0043] The present invention also provides a lithium-ion battery, and the negative electrode of the lithium-ion battery is prepared from the above-mentioned graphite composite negative electrode material or the graphite composite negative electrode material prepared by the preparation method of the above-mentioned graphite composite negative electrode material.

[0044] Compared with the prior art, the technical solution provided by the present invention has the following beneficial effects:

[0045] The present invention adopts the in-situ growth technology of metal-organic frameworks to grow a uniform organic framework containing conductive particles on the surface of graphite. Through the appropriate selection of organic ligands, with the appropriate carbon number, high unsaturation and rich hydroxyl structure of polyhydroxybenzene, the calcined network-like conductive layer can obtain a uniform distribution of conductive particles and a high stress absorption intensity. Then, through secondary coating, a dense carbon layer is used to cover the conductive layer with many pores and a large specific surface area, ensuring a lower specific surface area and a higher first-cycle Coulomb efficiency of the graphite composite negative electrode material. The combination of the double-layer structure also avoids the exposure of metal conductive particles in the conductive network buffer layer, resulting in a decrease in battery safety caused by the diffusion of conductive particles into the electrolyte. At the same time, the presence of the conductive layer also compensates for the disadvantages of poor conductivity of high-temperature pitch and poor adhesion to the graphite matrix. Through the technical solution provided by the present invention, a natural graphite negative electrode material with high first-cycle Coulomb efficiency, high rate performance and excellent long-cycle performance can be prepared. Description of the Drawings

[0046] Figure 1 It is a scanning electron microscope image of the graphite composite negative electrode material provided in Example 1 of the present invention;

[0047] Figure 2 It is a scanning electron microscope image of the graphite composite negative electrode material provided in Comparative Example 1 of the present invention;

[0048] Figure 3 It is a scanning electron microscope image of the graphite composite negative electrode material provided in Comparative Example 2 of the present invention;

[0049] Figure 4 It is a scanning electron microscope image of the graphite composite negative electrode material provided in Comparative Example 3 of the present invention;

[0050] Figure 5 It is a transmission electron microscope image of the graphite composite negative electrode material provided in Example 1 of the present invention;

[0051] Figure 6 It is a transmission electron microscope image of the graphite composite negative electrode material provided in Comparative Example 1 of the present invention;

[0052] Figure 7 It is a transmission electron microscope image of the graphite composite negative electrode material provided in Comparative Example 2 of the present invention;

[0053] Figure 8 It is a transmission electron microscope image of the graphite composite negative electrode material provided in Comparative Example 3 of the present invention;

[0054] Figure 9 AC impedance spectra of the anode materials provided in Embodiments 1-6 and Comparative Examples 1-5 of the present invention;

[0055] Figure 10 Cycling curves of the anode materials provided in Embodiments 1-6 and Comparative Examples 1-5 of the present invention at different rates;

[0056] Figure 11 1C / 1C cycling diagrams of the anode materials provided in Embodiments 4-6 and Comparative Examples 1-5 of the present invention. Detailed implementation manners

[0057] It should be noted that the term "including" and any variations thereof in the description, claims and above-mentioned drawings of the present invention are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or devices.

[0058] The "ranges" disclosed in the present invention are given in the form of lower limits and upper limits. There may be one or more lower limits, and one or more upper limits. A given range is defined by selecting a lower limit and an upper limit. The selected lower limit and upper limit define the boundary of a particular range. All ranges defined in this way are combinable, that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, ranges of 60-110 and 80-120 are also contemplated. In addition, if the minimum range values listed are 1 and 2, and the maximum range values listed are 3, 4 and 5, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5.

[0059] In the present invention, unless otherwise stated, the numerical range "a-b" represents an abbreviated representation of any real number combination between a and b, where a and b are both real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" have been fully listed in the present invention, and "0-5" is only an abbreviated representation of these numerical combinations.

[0060] In the present invention, if there is no special description, all the embodiments and preferred embodiments mentioned in the present invention can be combined with each other to form a new technical solution.

[0061] In the present invention, if there is no special description, all the technical features and preferred features mentioned in the present invention can be combined with each other to form a new technical solution.

[0062] In the present invention, unless otherwise specified, all steps mentioned herein can be carried out sequentially or randomly, but preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) carried out sequentially, or may include steps (b) and (a) carried out sequentially. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0063] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the attached tables, drawings and embodiments. The following described embodiments are some embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be construed as limiting the scope of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention. For those conditions not specified in the embodiments, they are carried out according to conventional conditions or conditions recommended by the manufacturer. For reagents or instruments whose manufacturers are not specified, they are all conventional products that can be obtained through commercial purchase.

[0064] The raw materials used in the embodiments of the present invention are as follows:

[0065] The natural graphite is spherical graphite, which is a commercially available conventional product, with a median particle size D50 = 16.39 μm, a fixed carbon content of 99.97 wt%, a tapped density of 1.00 g / cm 3 , and a specific surface area of 5.80 cm 2 / g;

[0066] The high-temperature coated pitch is a commercially available conventional product, with a measured softening point of 279 °C, a coking value of 65.7%, a quinoline insoluble content of 0.16 wt%, an ash content of 0.05 wt%, and a median particle size D50 = 3.32 μm.

[0067] Example 1

[0068] This embodiment provides a graphite composite negative electrode material, and its preparation method includes the following specific steps:

[0069] S1: Add cetyltrimethylammonium bromide, a surfactant, into a reaction kettle filled with deionized water at 60 °C, and start stirring. After the cetyltrimethylammonium bromide is completely dissolved, add natural graphite in small amounts and slowly, where the mass ratio of natural graphite: surfactant: water is 1000:1:10000. After the natural graphite is evenly dispersed in the aqueous solution and there are no floating or agglomerated powders on the surface, keep stirring at 60 °C for 1 h, filter to obtain a filter cake and dry it in an oven at 80 °C to obtain Intermediate Material 1, i.e., surface-modified graphite.

[0070] S2: Add hexahydroxybenzene (polyhydroxybenzene) into a reaction kettle filled with deionized water at 60 °C, and start stirring. After hexahydroxybenzene is completely dissolved, add Intermediate Material 1 (surface-modified graphite) in small amounts and slowly, where the mass ratio of water to surface-modified graphite is 20:1. After Intermediate Material 1 is evenly dispersed in the aqueous solution and there are no floating or agglomerated powders on the surface, slowly drop an aqueous solution of copper acetate (metal salt), then raise the temperature to 80 °C, stir and keep warm for 90 min, filter to obtain a filter cake and dry it in an oven at 80 °C to obtain Intermediate Material 2.

[0071] S3: Place the above Intermediate Material 2 into an atmosphere furnace. Under the protection of high-purity nitrogen, heat it to 850 °C at a heating rate of 5 °C / min, and keep warm for 2 h for carbonization. After the carbonization is completed, let the obtained product cool naturally, and then break it up to obtain Intermediate Material 3.

[0072] S4: Mix Intermediate Material 3 and high-temperature coating pitch evenly in a high-speed cyclone mixer. During the mixing process, introduce high-purity nitrogen for protection to obtain Intermediate Material 4.

[0073] S5: Place the above Intermediate Material 4 into an atmosphere furnace. Under the protection of high-purity nitrogen, heat it to 1150 °C at a heating rate of 5 °C / min, and keep warm for 2 h for carbonization. After the carbonization is completed, let the obtained product cool naturally, and then break it up, screen it, and demagnetize it to obtain the graphite composite anode material.

[0074] Among them, the feeding mass ratio of natural graphite: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000:20:20:20.

[0075] Example 2

[0076] This embodiment provides a graphite composite anode material, and its preparation method is the same as that of Embodiment 1, except that: the surfactant used in S1 is sodium dodecylbenzenesulfonate, the polyhydroxybenzene in S2 is 3,3',4,4',5,5'-hexahydroxybiphenyl, and the metal salt is a composition of cobalt acetate and zinc acetate; the feeding mass ratio of natural graphite: polyhydroxybenzene: cobalt acetate (metal salt): zinc acetate (metal salt): high-temperature coating pitch is 1000:80:40:40:60, and the mass ratio of natural graphite: surfactant: water in S1 is 1000:10:10000.

[0077] Embodiment 3

[0078] This embodiment provides a graphite composite anode material, and its preparation method is the same as that of Embodiment 1, except that: the polyhydroxybenzene in S2 is a composition of hexahydroxybenzene and 3,3',4,4',5,5'-hexahydroxybiphenyl; the feeding mass ratio of natural graphite: hexahydroxybenzene (polyhydroxybenzene): 3,3',4,4',5,5'-hexahydroxybiphenyl (polyhydroxybenzene): metal salt: high-temperature coating pitch is 1000: 25:25:50:40; the mass ratio of natural graphite: surfactant: water in S1 is 1000:5:10000.

[0079] Embodiment 4

[0080] This embodiment provides a graphite composite anode material, and its preparation method is the same as that of Embodiment 1, except that: the polyhydroxybenzene in S2 is a composition of hexahydroxybenzene and 3,3',4,4',5,5'-hexahydroxybiphenyl; the metal salt is a composition of cobalt acetate and zinc acetate; the feeding mass ratio of natural graphite: hexahydroxybenzene (polyhydroxybenzene): 3,3',4,4',5,5'-hexahydroxybiphenyl (polyhydroxybenzene): cobalt acetate (metal salt): zinc acetate (metal salt): high-temperature coating pitch is 1000: 20:20:25:25:45; the mass ratio of natural graphite: surfactant: water in S1 is 1000:5:10000.

[0081] Embodiment 5

[0082] This embodiment provides a graphite composite anode material, and its preparation method is the same as that of Embodiment 1, except that: the feeding mass ratio of natural graphite: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000:60:70:60; the mass ratio of natural graphite: surfactant: water in S1 is 1000:5:10000.

[0083] Embodiment 6

[0084] This embodiment provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: in S2, the polyhydroxybenzene is a composition of hexahydroxybenzene and 3,3',4,4',5,5'-hexahydroxybiphenyl; the feeding mass ratio of natural graphite: hexahydroxybenzene (polyhydroxybenzene): 3,3',4,4',5,5'-hexahydroxybiphenyl (polyhydroxybenzene): metal salt: high-temperature coating pitch is 1000: 25:25:60:52.5; in S1, the mass ratio of natural graphite: surfactant: water is 1000:5:10000; in S5, the carbonization temperature is 1350 °C and the heat preservation time is 4 h.

[0085] Comparative Example 1

[0086] This comparative example provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: after obtaining the intermediate material 3, demagnetization is carried out, and the surface soft carbon coating steps of S4 and S5 are not carried out.

[0087] Comparative Example 2

[0088] This comparative example provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: only the surface soft carbon coating steps of S4 and S5 are carried out on the natural graphite, and the reticulated conductive layer coating steps of S1, S2, and S3 are not carried out.

[0089] Comparative Example 3

[0090] This comparative example provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: polyhydroxytoluene is replaced with 10-hydroxystearic acid.

[0091] Comparative Example 4

[0092] This comparative example provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: the mass ratio of natural graphite: surfactant: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000:5:75:95:45;

[0093] Comparative Example 5

[0094] This comparative example provides a graphite composite negative electrode material, and its preparation method is the same as that of Embodiment 1, except that: the mass ratio of natural graphite: surfactant: polyhydroxybenzene: metal salt: high-temperature coating pitch is 1000:5:18:22:45.

[0095] Test Example 1:

[0096] This test example uses a ZEISS SUPRA55 scanning electron microscope and under conventional test conditions, field emission scanning electron microscope analysis is performed on the graphite composite anode materials provided in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention. The obtained scanning electron microscope images are as shown in Figures 1-4 shown. Among them, Figure 1 is the scanning electron microscope image of the graphite composite anode material provided in Example 1 of the present invention. Figure 2 is the scanning electron microscope image of the graphite composite anode material provided in Comparative Example 1 of the present invention. Figure 3 is the scanning electron microscope image of the graphite composite anode material provided in Comparative Example 2 of the present invention. Figure 4 is the scanning electron microscope image of the graphite composite anode material provided in Comparative Example 3 of the present invention. The graphite composite anode materials obtained in Example 1 and Comparative Examples 1-3 are all potato-like, and it can be observed that Figure 1 , Figure 3 the surfaces of the particles are relatively smooth and flat, Figure 2 , Figure 4 the surfaces of the particles are relatively rough and sharp.

[0097] Test Example 2:

[0098] This test example uses a JEM-F200 field emission transmission electron microscope and under conventional test conditions, transmission scanning electron microscope analysis is performed on the graphite composite anode materials provided in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention. The obtained scanning electron microscope images are as shown in Figures 5-8 shown. Among them, Figure 5 is the transmission scanning electron microscope analysis of the graphite composite anode material provided in Example 1 of the present invention. Figure 6 is the transmission scanning electron microscope analysis of the graphite composite anode material provided in Comparative Example 1 of the present invention. Figure 7 is the transmission scanning electron microscope analysis of the graphite composite anode material provided in Comparative Example 2 of the present invention. Figure 8 is the transmission scanning electron microscope analysis of the graphite composite anode material provided in Comparative Example 3 of the present invention. Through the transmission electron microscope, the uniformity of the coating layer thickness and the compactness of the coating layer can be observed. As shown in Figure 5 , on the surface of the anode particles in Example 1, an obvious double-layer coating structure is presented. The inner coating layer has no obvious crystal structure, and the outer coating layer has a typical soft carbon-like structure. A short-range irregular layered structure can be observed. The inner and outer coating layers are uniform, and the inner coating layer is slightly thicker than the outer coating layer. Figure 6 , Figure 7 shown, on the surfaces of the particles of Comparative Example 1 and Comparative Example 2, only a single-layer coating layer can be observed respectively. Among them, the coating layer on the surface of the particles in Comparative Example 1 has no obvious crystal structure, while the layered structure of the coating layer on the surface of the particles in Comparative Example 2 is obvious, which is a typical soft carbon coating layer. Figure 8As shown, the surface of the negative electrode material particles in Comparative Example 3 also exhibits an obvious double-layer coating layer. Compared with Example 1, the inner coating layer is thicker and has poor color uniformity.

[0099] Test Example 3:

[0100] In this test example, a Bettersize 2600 laser particle size analyzer and a Micromeritics TriStar II 3020 full-automatic specific surface area and pore size distribution analyzer were used to measure the particle size and specific surface area of the negative electrode materials provided in each example and comparative example under conventional test conditions. A CHI660E was also used to measure the AC impedance spectra of the negative electrode materials provided in each example and comparative example under conventional test conditions. The obtained AC impedance spectra are as Figure 9 shown, and the obtained particle size and specific surface area data are shown in Table 1 below;

[0101] This test example also measured the electrochemical performance of the negative electrode materials provided in each example and comparative example, including:

[0102] The natural graphite negative electrode materials provided in each example and comparative example were used as the negative electrode active materials. The three were mixed evenly according to the mass ratio of negative electrode active material: LA133: Super P = 94: 3: 3, and then coated on a copper foil current collector. After drying, cutting, and rolling, a negative electrode sheet was obtained and reserved.

[0103] The obtained negative electrode sheets were assembled into 2032 coin cells and tested for the first charge capacity and the first-week Coulombic efficiency. Among them, the counter electrode was a lithium sheet, the solute of the electrolyte was LiPF 6 , the solvent was a mixed solution formed by DEC, DMC, and EC in a volume ratio of 1: 1: 1, the concentration of the solute was 1 mol / L, the separator was a commercially available Celgard 2320, and the test was carried out on a battery test instrument CT300A1U. The charge-discharge voltage was 0.005 - 1.5 V. Deep discharge (0.1C - 0.01C) and 0.1C charge were carried out in the first week. The obtained first charge capacity and the first-week Coulombic efficiency are also listed in Table 1 below.

[0104] For the rate performance and cycle performance of the negative electrode materials, they were carried out according to the mass ratio of active material: LA133: Super P = 90: 5: 5. Charge and discharge were carried out at 0.3C, 0.5C, 1C, 3C, and 5C respectively to test the rate performance of the negative electrode materials; charge and discharge were carried out under the condition of 1C / 1C to test the cycle performance of the negative electrode materials. The other test conditions were the same as those for the first charge capacity and the first-week Coulombic efficiency test. The obtained cycle curves at different rates are as Figure 10 shown, the capacity retention rates at different rates are listed in Table 1 below, and the obtained 500-week 1C / 1C cycle diagram is as Figure 11As shown, the 1C / 1C 500-cycle capacity retention rate results are also listed in Table 1 below.

[0105] Table 1 Battery performance tests of graphite composite anode materials of examples and comparative examples

[0106]

[0107] Note (in Table 1): In this invention, the anode material is tested by making a coin-type half-cell. Generally, an increase in the 1C capacity retention rate of 15% - 20%, an increase in the 3C capacity retention rate of 10% - 20%, and an increase in the 5C capacity retention rate of 5% - 10% are considered effective. An increase of more than 25%, 20%, and 10% respectively is considered a significant improvement effect; an increase in the 1C / 1C 500-cycle capacity retention rate of 3% - 5% is considered an improvement effect, and an increase in the 1C / 1C 500-cycle capacity retention rate of more than 5% is considered a significant improvement effect.

[0108] From Table 1 and Figures 9-11 it can be seen that compared with the double-coated graphite composite anode material provided in Example 1, Comparative Example 1 only coated a network conductive layer. From Figure 6 it can be seen that the coating layer is in a dispersed state without an obvious boundary, and the specific surface area increases from 2.25 to 3.48 m 2 / g compared with Example 1, and the initial efficiency drops significantly to 92.74%. The 1C / 1C 500-cycle capacity retention rate also decreases to 86.3%. The rate performance and internal impedance have no obvious difference compared with Example 1, indicating that a single network copper particle conductive layer can improve the conductivity of the anode material. However, due to the large number of voids and low density in the conductive coating layer, the specific surface area is much higher than that of the conductive coating layer formed by high-temperature pitch. The excessive specific surface area consumes more Li to form the SEI film during the first charge and discharge process, and at the same time, the diffusion of copper particles into the electrolyte during the charge and discharge process also causes a decline in the cycle performance.

[0109] Compared with the double-coated graphite composite anode material provided in Example 1, Comparative Example 2 only coated a soft carbon coating layer formed by high-temperature pitch, which is a conventional process for natural graphite anode materials. From Figure 7It can be seen that the coating layer has a short-range layered structure with obvious boundaries, which is a typical soft carbon structure. Since the outer coating structure is the same as that in Example 1, the specific surface area and the first efficiency are also similar to those in Example 1. The difference is that after adding the inner conductive coating layer in Example 1, the internal impedance drops significantly, and the capacity retention rates at 1C / 3C / 5C are increased by 19.4%, 18.8% and 9.7% respectively. The capacity retention rate at 1C / 1C for 500 cycles also increases significantly from 89.1% to 94.5%. The surface network conductive layer structure provides better electrical conductivity through copper particles, thereby reducing the internal resistance and improving the rate performance. At the same time, the buffer of the network structure also provides a buffer for the expansion effect during the cycling process of the natural graphite negative electrode material, improving the cycling performance.

[0110] Compared with the graphite composite negative electrode material provided in Example 1, in Comparative Example 3, the polyhydroxybenzene in the network conductive layer coating is replaced with long-chain 10-hydroxy stearic acid, and its structure is different from that of the polyhydroxybenzene of the present invention. The number of monomer carbon atoms is 18, and the carbon-hydrogen ratio is 1:2, which does not belong to the scope of the present invention. Since the outer coating structure is the same as that in Example 1, compared with Comparative Example 2, the specific surface area only increases slightly, and the first efficiency also decreases slightly. The internal resistance and the rate performance hardly increase, indicating that the electrical conductivity of the negative electrode material has not changed. It is speculated that because the number of hydroxyl groups in the 10-hydroxy stearic acid molecule is small, there are few active sites that can provide links to the graphite surface and metal ions, and not enough metal ions are bound in the coating layer. At the same time, from Figure 8 It can be seen that the thickness of the inner network coating layer structure is 2-3 times that of the surface soft carbon coating layer, the color is uneven, the internal pore distribution is uneven, and a good network structure is not formed. The capacity retention rate at 1C / 1C for 500 cycles of the negative electrode material decreases significantly by 8.2% compared with Comparative Example 2. From the cycling curve, it can be seen that the cycling capacity is relatively high in the first 250 cycles and then drops rapidly. It is speculated that although the unsaturated long-chain hydrocarbon 10-hydroxy stearic acid is coated on the surface of natural graphite, the fragile carbon-carbon single bond and inappropriate carbon-hydrogen ratio cause the network conductive layer to boil and escape rapidly during the carbonization process, forming a coating layer with larger pores, uneven structure and lower strength. At the beginning of the cycle, the lithium ions provide additional capacity for the negative electrode material due to the adsorption of the high-void conductive layer. However, as the cycle progresses, obvious volume expansion of the graphite occurs, and the internal stress generated causes the network conductive layer to collapse rapidly, and the structure can no longer be maintained, resulting in a rapid drop in the cycling capacity.

[0111] Compared with the graphite composite negative electrode material provided in Example 1, in Comparative Example 4, the addition amounts of polyhydroxybenzene and metal salt in the network conductive layer coating increased, exceeding the scope of the present invention. Since the outer layer coating structure is the same as that in Example 1, the specific surface area slightly increased, the initial efficiency slightly decreased, and the internal resistance also slightly increased. The 1C capacity retention rate decreased by 2% compared with Example 1, and the 3C / 5C capacity retention rates were similar to those in Example 1. The difference is that the 1C / 1C 500-cycle capacity retention rate of the negative electrode material decreased by 0.7% compared with Comparative Example 2, far from reaching the level of Example 1, indicating that when the network conductive layer is too thick, it is easy to cause the structure to be loose and fragile, and cannot help improve the cycling performance of the graphite composite negative electrode material.

[0112] Compared with the graphite composite negative electrode material provided in Example 1, in Comparative Example 5, the addition amounts of polyhydroxybenzene and metal salt in the conductive layer coating decreased, falling below the scope of the present invention. Similarly, since the outer layer coating structure is the same as that in Example 1, the initial efficiency and specific surface area are also similar. The difference is that due to the thin and sparse coating layer, the conductive particles are also sparsely distributed in the coating layer, and a good conductive path cannot be formed, especially unable to carry a large number of electrons passing quickly. As a result, the 1C / 3C / 5C capacity retention rates of the negative electrode material did not increase significantly, and at the same time, a small amount of polyhydroxybenzene did not form a good network structure, and the 1C / 1C 500-cycle capacity retention rate did not increase significantly, and the cycling performance hardly improved.

[0113] In Examples 4-6 of the present invention, through a more reasonable proportioning of polyhydroxybenzene, metal salt, and surface soft carbon coating, negative electrode material products with better initial efficiency, rate performance, and cycling performance, and lower impedance and specific surface area were obtained. At the same time, in Example 6, by increasing the carbonization temperature, a product with better properties was obtained. Thus, it can be proved that the present invention can produce natural graphite composite negative electrode materials with different properties, and negative electrode products with different properties and production costs can be selected according to customer customization requirements.

[0114] As described above, the above are only specific embodiments of the present invention, and the scope of the invention implementation cannot be limited by them. Therefore, the replacement of equivalent components, or equivalent changes and modifications made according to the scope of the present invention patent protection, should still fall within the scope covered by this patent. In addition, the technical features in the present invention can be freely combined with each other between technical features, between technical features and technical inventions, and between technical inventions.

Claims

1. A graphite composite negative electrode material, characterized in that: The graphite composite negative electrode material has a core-shell structure, which includes a graphite core, a mesh conductive intermediate layer and a surface soft carbon coating layer; wherein the raw material of the mesh conductive intermediate layer includes polyhydroxybenzene and metal salt, the raw material of the graphite core includes natural graphite, and the raw material of the surface soft carbon coating layer includes high-temperature coated asphalt; the mass ratio of the natural graphite: polyhydroxybenzene: metal salt: high-temperature coated asphalt is 1000:20-80:20-80:20-60; The preparation method of the graphite composite negative electrode material comprises: (1) Surface modification of natural graphite using a surfactant to obtain surface-modified graphite; (2) mixing the surface-modified graphite and polyhydroxybenzene in water, then adding a metal salt to carry out a hydrothermal reaction, and carbonizing the product after the reaction is completed to obtain an intermediate product; (3) The intermediate product is mixed evenly with high-temperature coated asphalt, and then carbonized again to obtain the graphite composite negative electrode material.

2. The graphite composite negative electrode material according to claim 1, characterized in that: The mass ratio of the natural graphite: polyhydroxybenzene: metal salt: high temperature coated asphalt is 1000:40-60:50-70:45-60.

3. The graphite composite negative electrode material according to claim 1, characterized in that: The monomer carbon number of the polyhydroxybenzene is 6-15, and the carbon-hydrogen ratio is 1-1.

2.

4. The graphite composite negative electrode material according to claim 1, characterized in that: The polyhydroxybenzene includes hexahydroxybenzene and / or 3,3',4,4',5,5'-hexahydroxybiphenyl.

5. The graphite composite negative electrode material according to claim 1, characterized in that: The metal salt includes one or a combination of two or more of copper acetate, cobalt acetate and zinc acetate.

6. The graphite composite negative electrode material according to claim 1, characterized in that: The high temperature coated asphalt has a softening point of 250-300° C. and a coking value of ≥70%.

7. The graphite composite negative electrode material according to claim 1, characterized in that: The quinoline insoluble matter content in the high-temperature coated asphalt is ≤0.3wt%, and the ash content is ≤0.1wt%; the median particle size D50 of the high-temperature coated asphalt is 1-6μm.

8. The graphite composite negative electrode material according to claim 1, characterized in that: The natural graphite is spherical graphite, with a median particle size D50 of 16-18 μm and a fixed carbon content of ≥99.95 wt %.

9. The graphite composite negative electrode material according to claim 1, characterized in that: The tap density of the natural graphite is ≥0.96 g / cm 3 ; The specific surface area of ​​the natural graphite is ≤6.5cm 2 / g.

10. The graphite composite negative electrode material according to claim 1, characterized in that: In step (1), the method for surface modifying natural graphite using a surfactant is as follows: uniformly mixing the surfactant with water, slowly adding graphite under stirring, heating and stirring, filtering, and drying to obtain the surface-modified graphite.

11. The graphite composite negative electrode material according to claim 10, characterized in that: The surfactant includes sodium dodecylbenzene sulfonate and / or hexadecyltrimethylammonium bromide.

12. The graphite composite negative electrode material according to claim 10, characterized in that: The mass ratio of the surfactant to the natural graphite is 1-10:1000.

13. The graphite composite negative electrode material according to claim 10, characterized in that: The mass ratio of water to natural graphite is 5-15:

1.

14. The graphite composite negative electrode material according to claim 1, characterized in that: In step (2), the temperature of the hydrothermal reaction is 70-95° C. and the time is 20-120 min.

15. The graphite composite negative electrode material according to claim 1, characterized in that: The mass ratio of water to the surface modified graphite is 15-30:

1.

16. The graphite composite negative electrode material according to claim 1, characterized in that: In step (2), the carbonization method is: heating to 750-950°C at a heating rate of 1-10°C / min and keeping the temperature for 1-4h.

17. The graphite composite negative electrode material according to claim 1, characterized in that: In step (3), the method of re-carbonization is: heating to 1100-1350°C at a heating rate of 1-10°C / min and keeping the temperature for 1-6h.

18. A lithium ion battery, characterized in that: The negative electrode of the lithium-ion battery is prepared from the graphite composite negative electrode material according to any one of claims 1 to 17.

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