A surface-coated natural spherical graphite, its preparation method and application

Abstract

This invention relates to the field of graphite anode material technology, specifically disclosing a surface-coated natural graphite, its preparation method, and its applications. The surface-coated natural graphite comprises a negatively charged natural spherical graphite core, which is coated with a positively charged micelle solution. A negatively charged MOF is grown on the positively charged micelles through electrostatic adsorption and crystal orientation induction. After curing and drying, a hard carbon source is coated on top, and high-temperature calcination is performed to prepare a natural spherical graphite core, an onion-like carbon framework and a nanocrystalline intercalation structure, an electrostatically coupled intermediate layer, and a hard carbon shell. The graphite anode material of this invention achieves optimized ion diffusion channels, optimized conductive networks, and expanded lithium intercalation sites. This structure significantly improves cycle life and specific capacitance, making it suitable for long-life lithium batteries.

Description

Technical Field

[0001] This invention relates to the field of graphite anode material technology, specifically to a surface-coated natural spherical graphite, its preparation method, and its application. Background Technology

[0002] Lithium-ion batteries, as the mainstream energy storage technology, rely heavily on the characteristics of their anode materials for performance. Natural graphite is widely used as a commercial lithium-ion battery anode material due to its high theoretical capacity (approximately 372 mAh / g), low cost, and environmental friendliness. However, natural graphite faces the following key challenges in practical applications:

[0003] 1. During the lithium-ion insertion / extraction process, the layered structure of natural graphite is prone to uneven volume expansion, which leads to particle breakage and repeated formation and breakage of the SEI film, resulting in low initial coulombic efficiency and short cycle life.

[0004] 2. The surface of natural graphite contains a large number of disordered carbon atoms and pores, which can easily lead to the growth of lithium dendrites, which not only reduces safety but also causes electrolyte decomposition and capacity decay.

[0005] 3. Existing technologies, such as single carbon coating or metal oxide coating, often have a simple structure and cannot effectively buffer volumetric stress. For example, the bitumen coating technology in patent CN 111081989 A does not seal pores, leading to increased electrolyte penetration and side reactions; the gas-phase oxidation method in patent CN201310154076.5 only repairs the surface and does not build ion channels. In addition, traditional processes are complex and costly, making it difficult to meet the requirements of high capacity, long cycle time, and high rate capability.

[0006] To address the aforementioned issues, existing technologies have proposed various modification schemes: introducing nanopores into the graphite surface through methods such as KOH etching and strong oxidant oxidation to shorten the lithium-ion diffusion path; introducing elements such as boron and nitrogen to enhance lithium-ion binding energy, for example, boron doping can improve reversibility, but the doping process is complex and the effect is limited; combining soft carbon and hard carbon coating, such as soft carbon reducing specific surface area and hard carbon providing buffer space, however, hard carbon coating tends to increase specific surface area, leading to a decrease in initial efficiency.

[0007] Therefore, this invention studies the volume expansion, surface defects, and limitations of traditional coating technology of natural graphite anodes, and aims to provide a technical solution for a lithium battery with high energy density and long cycle life. Summary of the Invention

[0008] To address related issues, this invention provides a surface-coated natural spherical graphite, its preparation method, and its applications. Positively charged micelles are used as a negatively charged MOF growth template. Through controllable micelle size and electrostatic adsorption of surface charges, the MOF is guided to form a uniform film. After calcination, nanocrystals in the micelles are embedded in an onion-like carbon framework derived from the MOF-derived porous carbon. The onion-like carbon framework enhances conductivity, and the micelle-derived oxides enhance ion diffusion, forming a high-rate anode material. To ensure the performance of the composite structure, a hard carbon shell is applied to form electrolyte isolation and SEI film curing, improving cycle stability.

[0009] The specific details of the invention are as follows:

[0010] A surface-coated natural spherical graphite comprising:

[0011] Natural spherical graphite cores with negative surface charge;

[0012] Electrostatic coupling intermediate layer: prepared by calcination of core-shell type metal hydroxide / oxide positively charged micelles and iron, nickel, and cobalt-based negatively charged MOFs electrostatically adsorbed on the positively charged micelles; the electrostatic coupling intermediate layer contains an onion-like carbon framework and intercalated nanocrystals;

[0013] Hard carbon fiber shell.

[0014] Preferably, the natural spherical graphite core is spherical or near-spherical.

[0015] More preferably, the natural spherical graphite cores have a particle size of 5–40 μm and a specific surface area of ​​2.5–6 m². 2 / g.

[0016] More preferably, the particle size of the natural spherical graphite core is 10–30 μm.

[0017] Preferably, the surface of the natural spherical graphite core has been oxidized to have one or more carboxyl or hydroxyl groups, forming a negative potential surface.

[0018] More preferably, the Zeta potential of the negative potential surface is ≤-30mV.

[0019] Preferably, the positively charged micelles of the metal hydroxide / oxide are one or more of ZnO@Co(OH)2, TiO2@AlO(OH), AlO(OH)@MnO2, H2SnO3@Fe(OH)3, Fe(OH)3@Co(OH)2, AlO(OH)@Fe(OH)3, and SnO2@Ni(OH)2.

[0020] More preferably, the Zeta potential of the outer layer material of the positively charged micelles is adjusted by a pH value ≥15mV.

[0021] More preferably, the thickness of the positively charged micelles is 20–100 nm.

[0022] The negatively charged groups on the graphite surface are electrostatically attracted to the positively charged micelles, providing ordered nucleation sites for negatively charged MOFs.

[0023] Preferably, the negatively charged MOF is electrostatically adsorbed on the positively charged micelle layer, with a thickness of 50–100 nm.

[0024] More preferably, the metal node of the negatively charged MOF is at least one of iron, nickel, and cobalt ions, and the organic ligand of the negatively charged MOF is one of carboxylic acids and imidazoles.

[0025] More preferably, the negatively charged MOF is at least one of Fe-MIL-101, Fe-BTC, Co-ZIF-67, Co-MOF-74, Ni-MOF-74, and Ni-BDC.

[0026] MOF-derived carbon rearranges into an onion-like carbon framework network during calcination, providing support for the nanocrystals formed by the metal hydroxides and oxides of the positively charged micelles to buffer the expansion stress of natural graphite. The nanocrystals converted from metal oxides in the positively charged micelles can increase the number of lithium intercalation sites.

[0027] Carboxylic acid and imidazole organic ligands form stable stimulating building blocks with MOF metal node ions, which are then induced by positively charged micelles to grow along specific crystal orientations, forming 3D mesoporous ion channels.

[0028] Preferably, the negatively charged MOF is a hierarchical porous MOF, wherein the hierarchical pores are micropores and mesopores, and the volume ratio of micropores to mesopores is 1:2 to 4.

[0029] By combining micropores and mesopores, a more diverse range of ion channel configurations can be achieved.

[0030] Preferably, the carbon source for the hard carbon shell can be a common hard carbon source or a nitrogen-doped hard carbon source.

[0031] More preferably, the nitrogen-doped hard carbon source is one or more of polyacrylonitrile, chitosan-based carbon source, melamine phenolic resin, dicyandiamide epoxy resin, and polypyrrole.

[0032] More preferably, the nitrogen-doped hard carbon source contains 5-8 wt% nitrogen and ≥35% pyrrole nitrogen after calcination.

[0033] Nitrogen-doped hard carbon is used as a carbon source. Nitrogen atoms are introduced through in-situ pyrolysis, forming dopants such as pyridine nitrogen and pyrrole nitrogen, significantly improving the material's conductivity and electrochemical activity. When the nitrogen content of the hard carbon source is <5 wt%, a sufficiently lithium-affinity SEI film cannot be formed, resulting in low initial efficiency. When the nitrogen content is >8 wt%, excessive pyrrole nitrogen leads to an overly thick SEI film, reducing rate performance. A nitrogen content of 5-8 wt%, especially pyrrole nitrogen ≥35%, can optimize the SEI film composition. Simultaneously, pyrrole nitrogen ≥35% in the hard carbon shell forms sp... 2 Conjugated domains, in collaboration with onion-like carbon skeletons, construct electron channels.

[0034] More preferably, the thickness of the hard carbon shell is 50-100 nm.

[0035] A method for preparing surface-coated natural spherical graphite, characterized by comprising the following steps:

[0036] S1, Graphite Oxidation: The natural spherical graphite core is oxidized to introduce a negatively charged surface;

[0037] S2, Micellar Coating: Metal salt precursors and polyacrylic acid self-assemble in water to form a positively charged micelle solution, and then oxidized natural spherical graphite cores are added for homogeneous coating;

[0038] S3, MOF adsorption growth: Add negatively charged MOF precursor and organic ligand solution dropwise to the product of step S2, and allow electrostatic adsorption reaction for 6-12 h, followed by centrifugation and drying.

[0039] S4, Hard carbon coating: Impregnate the product of step S3 in a carbon source, then cure and dry;

[0040] S5. One-step calcination: High-temperature calcination in an inert atmosphere simultaneously achieves carbon source carbonization and electrostatic coupling intermediate layer structure.

[0041] Preferably, the oxidation treatment in step S1 is performed using a dilute nitric acid / hydrogen peroxide solution, with the oxygen content controlled at 1-3 wt%.

[0042] More preferably, the ratio of dilute nitric acid to hydrogen peroxide solution is 3:1, the treatment temperature is 50-70℃, and the treatment time is 5-10 min.

[0043] More preferably, the surface Zeta potential of the natural spherical graphite core is controlled to be ≤-30mV.

[0044] By introducing appropriate amounts of negatively charged hydroxyl and carboxyl groups, the electrostatic adsorption capacity of natural graphite for positively charged micelles is enhanced, and the subsequent electrostatic adsorption efficiency exceeds 90%.

[0045] Preferably, in step S2, the positively charged micelles are self-assembled from a metal salt precursor and PAA, and the molar ratio of the metal salt precursor to PAA is 0.1-0.25:0.08-0.2.

[0046] More preferably, the positively charged micelles achieve a Zeta potential ≥15 mV by adjusting the pH value, with the pH value adjustment range being 7–10.

[0047] More preferably, the self-assembly temperature is 55-65°C and the self-assembly time is 30-45 min.

[0048] Preferably, the homogenization conditions in step S2 are 1300-1500 rpm for 10 min.

[0049] Preferably, the positively charged micelles have a coverage rate of greater than 90% and a coverage thickness of 20–100 nm.

[0050] Preferably, in step S3, the growth time of the negatively charged MOF is 6–24 h, and the growth pH is controlled at 7–9.

[0051] More preferably, when the pH value is controlled at 7-8, a transition metal salt precursor and an organic ligand solution are added dropwise to form a uniform negatively charged MOF adsorption layer.

[0052] More preferably, the organic ligand is at least one of carboxylic acids and imidazoles.

[0053] More preferably, the zeta potential of the negatively charged MOF is ≤-10mV.

[0054] Positively charged micelles can act as "microreactors" for MOF nucleation at the formed hydrophilic-hydrophobic interface. Transition metal ions Fe³⁺ / Ni²⁺ / Co²⁺ are adsorbed on the micelle surface, and the organic ligands are guided to connect in an oriented manner through electrostatic interactions or coordination bonds, thereby controlling the crystal growth rate.

[0055] Preferably, in step S4, the ratio of carbon source to product of step S3 is 1:2 to 5.

[0056] More preferably, when the carbon source is PAN, its concentration in the NMP solution is 10-15 wt%, and it is cured at 80°C for 2 hours to form a PAN coating layer.

[0057] More preferably, the viscosity of the PAN-NMP solution is controlled at 800-1200 cP to improve the uniformity of the coating thickness.

[0058] Preferably, in step S5, the nanocrystals formed by the metal hydroxide and metal oxide after the positive micelles are calcined are embedded in the onion-like carbon framework formed by the negatively charged MOF.

[0059] More preferably, the nanocrystals D50 formed by the positively charged micelles after calcination have a particle size of 40-60 nm.

[0060] The onion-like carbon skeleton is cross-linked with the matrix through M1-OC chemical bonds. M1 is a metal element in positively charged micelles, which significantly enhances the interfacial bonding force and prevents interlayer delamination during cycling.

[0061] Preferably, the calcination atmosphere is Ar-5wt%H2, 1000-1300℃, and held for 2 hours.

[0062] More preferably, the calcination includes a pre-oxidation stage, in which NH3 is introduced, the pre-oxidation temperature is 200-300℃, and the holding time is 30-45 min, thereby increasing the pyrrole nitrogen content.

[0063] More preferably, the calcination temperature rise rate is 2-4°C / min.

[0064] During the high-temperature calcination of transition metal MOFs, organic ligands decompose to generate a porous carbon framework, while metal ions are reduced to nanoparticles or single atoms, which are uniformly dispersed in the carbon substrate. Nitrogen-doped hard carbon, as a carbon source, undergoes carbonization at high temperatures, and its amorphous structure combines with MOF-derived carbon to form continuous electron channels.

[0065] The use of PAN coating can produce multiple effects: (1) The cyano group in the PAN molecular chain forms M2–N≡C coordinate bond with the metal node of the negatively charged MOF. M2 is the transition metal element in the negatively charged MOF, which makes the MOF tightly anchored to the graphite surface; (2) The benzene ring structure of PAN and the organic ligand of MOF produce a π-π stacked conjugated structure, forming a continuous electron channel; (3) Pyrrole nitrogen catalyzes the decomposition of electrolyte to form a stable SEI film; (4) The shell acts as a "protective barrier", physically isolating the electrolyte from the core and reducing side reactions. At the same time, its microporous structure optimizes the lithium ion diffusion path.

[0066] An application of surface-coated natural spherical graphite, wherein the surface-coated natural spherical graphite is used as a negative electrode in a lithium battery.

[0067] Preferably, when the surface-coated natural spherical graphite is used as the negative electrode material, the initial discharge capacity is ≥500mAh / g, the capacity retention rate after 500 cycles is ≥92%, and the 2C rate capacity retention rate is ≥80%.

[0068] Compared with existing technologies, this invention significantly improves the overall performance of natural spherical graphite through structural design and interface engineering:

[0069] 1. Ultra-high capacity: The metal oxide nanocrystals in the electrostatic coupling intermediate layer enhance the conversion reaction capacity, resulting in an initial discharge capacity ≥500 mAh / g, which is greater than that of traditional graphite;

[0070] 2. First-time efficiency: The nitrogen-doped stable SEI film with a hard carbon shell improves the first-time coulombic efficiency to 90-93%;

[0071] 3. Cyclic stability: The onion-like carbon skeleton structure effectively disperses volumetric stress and buffers graphitization expansion, with a capacity retention of ≥92% after 500 cycles at 0.5C.

[0072] 4. Excellent rate performance: The onion-like carbon framework and nitrogen-doped hard carbon construct a low-impedance ion / electron dual channel, with a 2C capacity retention of ≥80%, and in some cases, 10C capacity retention of 65%.

[0073] 5. Process economy and environmental protection: One-step calcination simplifies the process and increases the utilization rate of raw materials; aqueous phase self-assembly avoids organic solvent pollution, which is in line with the trend of green manufacturing. Attached Figure Description

[0074] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0075] Figure 1 This is a schematic diagram of the coating process of natural spherical graphite for surface coating treatment according to the present invention.

[0076] Figure reference numerals: 1-Natural spherical graphite core, 2-Positively charged micelle layer, 3-Negatively charged MOF layer, 4-Hard carbon source coating layer, 5-Electrostatic coupling intermediate layer, 6-Hard carbon shell. Detailed Implementation

[0077] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this invention.

[0078] Both the examples and comparative examples were measured using CR2032 coin cells. The electrolyte was 1M LiPF6 in EC / DMC, the positive electrode material was Li metal sheet, and the negative electrode material was natural spherical graphite coated with copper foil.

[0079] To illustrate the rationality of the technical solution of the present invention, specific embodiments are shown below:

[0080] Example 1: Preparation of a reference coated graphite anode material using iron-based MOFs. The specific steps are as follows:

[0081] S1. Graphite oxidation: Take 10g of natural spherical graphite with a D50 particle size of 15μm, oxidize it in 200ml of HNO3:H2O2=3:1 mixed solution at 60℃ for 10min, centrifuge, wash with deionized water until neutral, dry at 80℃ for 12h and filter.

[0082] S2, Micelle Coating: Add the metal salt SnCl4:FeCl3:PAA in a molar ratio of 0.02:0.03:0.015 to 100 ml of deionized water, stir in a 55 °C water bath for 40 min to self-assemble, adjust the pH to 9 with ammonium acetate to prepare H2SnO3@Fe(OH)3 micelle solution, then add the product prepared in step S1, and homogenize at 1500 rpm for 10 min;

[0083] S3, MOF growth: Add 0.03 mol-FeCl3 and 0.04 mol-terephthalic acid DMF solution to the product of step S2, adjust the pH to 7.5 with sodium acetate and react for 8 h to grow Fe-MIL-101, centrifuge, filter, wash with water and dry.

[0084] S4, Hard carbon coating: Impregnate with 12wt% PAN in NMP solution, viscosity 1000cP, carbon source: product = 1:3, cure at 80℃ for 2h;

[0085] S5. High-temperature calcination: NH3 atmosphere, 250℃, pre-oxidation for 40 min, then Ar-5%H2 atmosphere, 3℃ / min to 1200℃, heat preservation for 2 h, and dispersed into spheres by air jet mill.

[0086] The properties of the products prepared in each step of this embodiment are as follows:

[0087] 1. The surface-oxidized natural spherical graphite prepared in step S1 has a Zeta potential of -35mV and an oxygen content of 2.1wt%.

[0088] 2. The H2SnO3@Fe(OH)3 micelles prepared in step S2 have a Zeta potential of +25mV, a micelle coating thickness of 35nm, and a coating efficiency >95%.

[0089] 3. The negatively charged MOF layer prepared in step S3 has a Zeta potential of -28mV and a thickness of 70nm;

[0090] 4. After calcination in step S5, an onion-like carbon skeleton with a thickness of 40 nm is formed, cross-linked with 45 nm Fe3O4 nanocrystals via CO-Fe bonds. The hard carbon shell is 55 nm thick, contains 6.8 wt% nitrogen, and pyrrole nitrogen accounts for 48% of the total nitrogen.

[0091] The properties of surface-coated natural spherical graphite are as follows:

[0092] 1. Electrochemical performance: initial discharge capacity 532 mAh / g, initial coulombic efficiency 92.5%, capacity retention after 500 cycles 96%, 2C rate capacity 82.5%;

[0093] 2. Physical properties: Tapped density 1.18 g / cm³ 3 Specific surface area 17.3m² 2 / g.

[0094] The product prepared in this embodiment is suitable for use in energy storage, backup power and other systems with high requirements for ultra-long cycle stability.

[0095] Example 2: Preparation of highly conductive graphite anode material using cobalt-based MOFs. The specific steps are as follows:

[0096] S1. Graphite oxidation: Take 10g of natural spherical graphite with a D50 particle size of 20μm, oxidize it in 200ml of HNO3:H2O2=3.5:1 mixed solution at 60℃ for 10min, centrifuge and wash with deionized water until neutral, and vacuum dry at 80℃ for 12h.

[0097] S2, Micelle Coating: Add ZnO@Co(OH)2 micelle solution to 100ml of deionized water at a molar ratio of ZnCl2:Co(NO3)2:PAA of 0.025:0.03:0.018. Stir in a 60℃ water bath for 45min to self-assemble. Adjust the pH to 8.0 with TMAH to prepare ZnO@Co(OH)2 micelle solution. Then add the product prepared in step S1 and homogenize at 1400rpm for 10min.

[0098] S3, MOF growth: A methanol solution of 0.04 mol-Co(NO3)2 and 0.16 mol-2-methylimidazole was added dropwise to the product of step S2. The pH was adjusted to 7 with sodium acetate and the reaction was carried out for 12 h to grow and prepare a Co-ZIF-67 layer. The layer was then centrifuged, filtered, washed with water and dried.

[0099] S4, Hard carbon coating: Impregnate with 10wt% polypyrrole ethanol solution, viscosity 8000cP, carbon source: product = 1:4, cure at 80℃ for 2h;

[0100] S5. High-temperature calcination: NH3 atmosphere, 270℃, pre-oxidation for 40 min, Ar-5%H2 atmosphere, increase to 1300℃ at 4℃ / min, hold for 2 h, disperse into spheres by air jet mill.

[0101] The properties of the products prepared in each step of this embodiment are as follows:

[0102] 1. The surface-oxidized natural spherical graphite prepared in step S1 has a Zeta potential of -29 mV and an oxygen content of 2.4 wt%.

[0103] 2. The ZnO@Co(OH)2 micelles prepared in step S2 have a Zeta potential of +22mV, a micelle coating thickness of 50nm, and a coating efficiency >95%.

[0104] 3. The negatively charged MOF layer prepared in step S3 has a Zeta potential of -26mV and a thickness of 65nm;

[0105] 4. After calcination in step S5, an onion-like carbon skeleton with a thickness of 55 nm is formed, cross-linked with 45 nm CoO nanocrystals via CO-Co bonds. The hard carbon shell is 70 nm thick, contains 7.1 wt% nitrogen, and pyrrole nitrogen accounts for 50% of the total nitrogen.

[0106] The properties of surface-coated natural spherical graphite are as follows:

[0107] 1. Electrochemical performance: Initial discharge capacity 542 mAh / g, initial coulombic efficiency 90.2%, capacity retention after 500 cycles 94%, 2C rate capacity 86.0%, 10C rate capacity 65.2%;

[0108] 2. Physical properties: Tapped density 1.15 g / cm³ 3 True density 2.26 g / cm³ 3 Specific surface area 18.2m² 2 / g.

[0109] The product prepared in this embodiment is suitable for use in fields such as drones and hybrid vehicles with instantaneous high-rate output.

[0110] Example 3: Preparation of highly nitrogen-doped graphite anode material using nickel-based MOFs. The specific steps are as follows:

[0111] S1. Graphite oxidation: Take natural spherical graphite with a D50 particle size of 30μm, oxidize it in a 4:1 mixture of HNO3:H2O2 at 60℃ for 8 minutes, wash it with water until neutral, dry and filter.

[0112] S2, Micelle Coating: Add Al(NO3)3:KMnO4:PAA in a molar ratio of 0.02:0.015:0.018 to 100 ml of deionized water, stir in a 60 °C water bath for 40 min to self-assemble, adjust the pH to 7.5 with ammonia water to prepare AlO(OH)@MnO2 micelle solution, then add the product prepared in step S1, and homogenize at 1450 rpm for 10 min;

[0113] S3, MOF growth: 0.035 mol of NiCl2 and 0.052 mol of 2,5-dihydroxyterephthalic acid dissolved in ethanol solution were added dropwise to the product of step S2. The pH value was adjusted to 7.5 by acetic acid and the reaction was carried out for 9 h to grow and prepare Ni-MOF-74 layer. The layer was then centrifuged, filtered, washed with water and dried.

[0114] S4, Hard carbon coating: Impregnated with an ethanol solution of 14wt% dicyandiamide epoxy resin, viscosity 1100 cP, carbon source: product = 1:3, cured at 80℃ for 2h;

[0115] S5. High-temperature calcination: NH3 atmosphere, 300℃, pre-oxidation for 35 min, Ar-5%H2 atmosphere, increase to 1250℃ at 4℃ / min, hold for 2 h, disperse into spheres by air jet mill.

[0116] The properties of the products prepared in each step of this embodiment are as follows:

[0117] 1. The surface-oxidized natural spherical graphite prepared in step S1 has a Zeta potential of -27mV;

[0118] 2. The AlO(OH)@MnO2 micelles prepared in step S2 have a Zeta potential of +28mV, a micelle coating thickness of 40nm, and a coating efficiency of >93%.

[0119] 3. The negatively charged MOF layer prepared in step S3 has a Zeta potential of -30mV and a thickness of 75nm;

[0120] 4. After calcination in step S5, an onion-like carbon skeleton with a thickness of 60 nm is formed, cross-linked with 50 nm Mn3O4 nanocrystals via CO-Mn bonds. The hard carbon shell is 70 nm thick, contains 7.8 wt% nitrogen, and pyrrole nitrogen accounts for 47% of the total nitrogen.

[0121] The properties of surface-coated natural spherical graphite are as follows:

[0122] 1. Electrochemical performance: initial discharge capacity 518 mAh / g, initial coulombic efficiency 93%, capacity retention after 500 cycles 94%, 2C rate capacity 83%, 5C rate capacity 76%;

[0123] 2. Physical properties: Tap density 1.19 g / cm³ 3 True density 2.26 g / cm³ 3 Specific surface area 21m² 2 / g, volume expansion rate 9.1% (500 cycles).

[0124] The product prepared in this embodiment is suitable for use in wearable devices and medical devices where space is limited, long-cycle operation is required, and high safety is needed.

[0125] Example 4: Preparation of thin-layer coated graphite anode material using nickel-based MOFs. The specific steps are as follows:

[0126] S1. Graphite oxidation: Take natural spherical graphite with a D50 particle size of 25μm, oxidize it in a 60℃ water bath for 10min in a mixed solution of HNO3:H2O2=3.5:1, wash it with water until neutral, dry and filter.

[0127] S2, Micellar Coating: SnO2@Ni(OH)2 micelle solution was prepared by adding SnCl4:NiSO4:PAA in a molar ratio of 0.18:0.18:0.02 to 100 ml of deionized water and self-assembling in a 60℃ water bath for 40 min. The pH was adjusted to 8.5 with ammonium bicarbonate. Then the product prepared in step S1 was added and homogenized at 1500 rpm for 10 min.

[0128] S3, MOF growth: Add 0.03 mol of Ni(NO3)2 and 0.036 mol of terephthalic acid in ethanol solution to the product of step S2, with a molar ratio of 1:1.2. Adjust the pH value to 8.0 with acetic acid and react for 9 h to grow Ni-BDC. Centrifuge, filter, wash with water and dry.

[0129] S4, Hard carbon coating: Impregnate with 10wt% PAN in NMP solution, viscosity 1000cP, carbon source: product = 1:4.5, cure at 80℃ for 2h;

[0130] S5. High-temperature calcination: NH3 atmosphere, 260℃, pre-oxidation for 40 min, Ar-5%H2 atmosphere, increase to 1100℃ at 2℃ / min, hold for 2.5 h, disperse into spheres by air jet mill.

[0131] The properties of the products prepared in each step of this embodiment are as follows:

[0132] 1. The surface-oxidized natural spherical graphite prepared in step S1 has a Zeta potential of -31mV;

[0133] 2. The micelles prepared in step S2 have a Zeta potential of +21mV, a micelle coating thickness of 30nm, and a coating efficiency >91%;

[0134] 3. The negatively charged MOF layer prepared in step S3 has a Zeta potential of -32mV and a thickness of 50nm;

[0135] 4. After calcination in step S5, an onion-like carbon skeleton with a thickness of 40 nm is formed, which is cross-linked with 40 nm NiO nanocrystals through CO-Ni bonds. The hard carbon shell is 85 nm thick, contains 5.9 wt% nitrogen, and pyrrole nitrogen accounts for 55% of the total nitrogen.

[0136] The properties of surface-coated natural spherical graphite are as follows:

[0137] 1. Electrochemical performance: initial discharge capacity 515 mAh / g, initial coulombic efficiency 91%, capacity retention after 500 cycles 92%, 5C rate capacity 81%;

[0138] 2. Physical properties: Tap density 1.22 g / cm³ 3 True density 2.26 g / cm³ 3 Specific surface area 18m² 2 / g.

[0139] The product prepared in this embodiment is suitable for use in the consumer electronics field with high-speed charging and high energy density.

[0140] To fully illustrate the substantial progress of the technical solution of the present invention, the following comparative examples are provided:

[0141] Comparative Example 1, without a MOF interlayer, was prepared based on Example 1, with step S3 omitted. The prepared product had a capacity retention of 82.4% after 500 cycles, significantly lower than the 96% of Example 1, indicating that the onion-like carbon framework lost its stress buffering capacity.

[0142] Comparative Example 2: Low Zeta potential positively charged micelles. Using Example 1 as the base column, the amount of PAA was reduced to 0.005 in step S2 to prepare a product with a positively charged micelle zeta of 8 mV. The MOF coverage was only 30-35%, and the 2C rate capacity retention was only 72%.

[0143] Comparative Example 3 showed excessive nitrogen content in the hard carbon. Based on Example 1, in step S4, a 21 wt% PAN solution was used for coating, resulting in a nitrogen content of 10.1 wt% in the hard carbon. The excessive pyrrole nitrogen in the prepared graphite anode led to a thicker SEI film, resulting in an initial coulombic efficiency of 85.7% and a 10C rate capacity retention of 58.6%, indicating significant ion diffusion obstruction.

[0144] Comparative Example 4: Micellar-MOF Mismatch. Based on Example 1, a Zn-ZIF-8 MOF material was prepared in step S3. The inability to form CO-Fe bonds led to easy interface delamination, resulting in significant interlayer cracks and high interfacial impedance after cycling.

[0145] Comparative Example 5: Soft carbon coating with asphalt, based on Example 1, with impregnation of asphalt solution in step S4, without a micelle-MOF layer. The prepared product had high surface porosity, and electrolyte penetration caused incomplete SEI film. The initial coulombic efficiency was 87.5%, and the retention rate after 500 cycles was 67.9%. Lithium dendrite growth was observed.

[0146] Through performance tests of examples and comparative examples (see Table 1), the surface-coated natural spherical graphite anode material of the composite structure of the present invention has significant advantages. By connecting the inner and outer layers of the electrostatic adsorption intermediate layer of the transition metal MOF that can catalyze the preparation of onion carbon, the ion diffusion, conductivity and cycle strength are improved.

[0147] Table 1. Performance tests of examples and comparative examples

[0148]

[0149] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A surface-coated natural spherical graphite, characterized by, Comprise: A natural spherical graphite core with a negatively charged surface; An electrostatically coupled intermediate layer: prepared by calcining a shell-core type metal hydroxide / oxide positively charged micelle and a negatively charged iron, nickel, cobalt-based MOF electrostatically adsorbed on the positively charged micelle; the electrostatically coupled intermediate layer contains an onion-like carbon skeleton and chimeric nanocrystals; A hard carbon shell; The iron, nickel, cobalt-based negatively charged MOF is at least one of Fe-MIL-101, Fe-BTC, Co-ZIF-67, Co-MOF-74, Ni-MOF-74, and Ni-BDC, and the positively charged micelle is at least one of ZnO@Co(OH)2, TiO2@AlO(OH), AlO(OH)@MnO2, H2SnO3@Fe(OH)3, Fe(OH)3@Co(OH)2, AlO(OH)@Fe(OH)3, and SnO2@Ni(OH)2; The carbon source of the hard carbon shell is a nitrogen-doped hard carbon source, and the nitrogen content of the hard carbon shell is 5-8wt%, of which the pyrrole nitrogen accounts for ≥35%.

2. The surface-coated natural spherical graphite of claim 1, wherein: The nitrogen-doped hard carbon source is one or more of polyacrylonitrile, chitosan-based carbon source, melamine phenolic resin, dicyandiamide epoxy resin, and polypyrrole.

3. The surface-coated natural spherical graphite of claim 1, wherein: The thickness of the electrostatically coupled intermediate layer is 50-100nm, and the thickness of the hard carbon shell layer is 50-100nm.

4. A method for producing the surface-coated natural spherical graphite according to any one of claims 1 to 3, characterized by, Comprise the following steps: S1, graphite oxidation: introducing negative charges to the surface of the natural spherical graphite core through oxidation treatment; S2, micelle coating: self-assembling metal salt precursors and polyacrylic acid in deionized water at a molar ratio of 0.1-0.25:0.08-0.2, forming a positively charged micelle solution under the condition of pH 7-10, adding the oxidized natural spherical graphite core, and uniformly coating; S3, MOF adsorption and growth: adding a solution of a negatively charged MOF precursor and at least one organic ligand of a carboxylic acid or an imidazole to the product of step S2, electrostatically adsorbing under the condition of pH 7-9 for 6-12h, and centrifuging and drying; S4, hard carbon coating: immersing the product of step S3 in a carbon source, the ratio of the carbon source to the product of step S3 being 1:2-5, and solidifying and drying; S5, one-step calcination: calcining at 1000-1300℃ in an inert atmosphere for 2h, simultaneously realizing carbonization of the carbon source and structure of the electrostatically coupled intermediate layer.

5. The method of claim 4, wherein: The oxidation treatment in step S1 makes the surface Zeta potential ≤-30mV, the positively charged micelle Zeta potential in step S2 ≥15mV, and the negatively charged MOF Zeta potential in step S3 ≤-10mV.

6. The method of claim 4, wherein: The step S5 further comprises a pre-oxidation stage, the pre-oxidation conditions being NH3 atmosphere, 200-300℃, and holding for 30-45min.

7. Use of the surface-coated natural spherical graphite according to any one of claims 1-3 for a lithium battery negative electrode, the lithium battery negative electrode having a first discharge capacity ≥500mAh / g, a capacity retention rate after 500 cycles ≥92%, and a 2C rate capacity retention rate ≥80%.

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