Modified spherical graphite negative electrode material and preparation method and application thereof

Abstract

The invention discloses a modified spherical graphite negative electrode material and a preparation method and application thereof.The modified spherical graphite negative electrode material comprises a graphite core, an embedded structure and a coating layer, the graphite core is spherical or spheroidic graphite, the embedded structure is achieved by embedding boron nitride nanosheets into the surface of the graphite core, and the coating layer is coated with the boron nitride nanosheets. The number of layers of the boron nitride nanosheets is 1-5, the proportion of the embedded structure is 5-15 wt%, the coating layer is one of sulfonated polypyrrole, sulfonated polyaniline or sulfonated polythiophene, the thickness of the coating layer is 5-50 nm, the surface sulfonic acid group content of the coating layer is 5-10 wt%, and the coating layer coats the embedded structure and the graphite core. The graphite negative electrode material is subjected to BNNS embedding through high-pressure homogenization, and then is coated by a sulfonated polymer surface in-situ polymerization process, so that the material is endowed with high tap density, high thermal conductivity and excellent ion transmission capability, and the graphite negative electrode material is suitable for high-energy-density power batteries and fast charging scenes.

Description

Technical Field

[0001] The present invention relates to the technical field of graphite negative electrode materials, and in particular to a modified spherical graphite negative electrode material and a preparation method and application thereof. Background Art

[0002] Currently, graphite is the most important negative electrode material for lithium-ion batteries, with characteristics such as wide availability, high tap density, and electrochemical stability. However, graphite negative electrode materials still have the following disadvantages: 1. Graphite particles have a large specific surface area, which has a significant impact on the initial coulombic efficiency of lithium-ion batteries; 2. Graphite has a layered structure, and the energy storage mechanism is the embedding of Li+, which determines that Li+ can only be embedded from the cross-section of the graphite material and gradually diffuse into the interior of the particles. Due to the anisotropy of graphite particles, the Li+ diffusion path is long and uneven, resulting in poor rate performance and low capacity of the material; 3. The small distance between graphite layers increases the diffusion resistance of Li+, resulting in poor rate performance. During rapid charging, Li+ easily precipitates on the graphite surface, forming lithium dendrites, which poses a safety hazard.

[0003] Therefore, in order to solve the above inherent defects, many researchers have proposed to modify graphite. The main modification methods are spheroidization, surface modification and doping modification. The existing technologies involved are:

[0004] (1) CN110600715A discloses a graphite negative electrode composite material for lithium-ion batteries and its preparation method. By refining the graphite particle size, the lithium ion migration path is shortened, and the sphericity of the material is improved through shaping, which is beneficial to improving the material's isotropy. Furthermore, the amorphous carbon is modified and coated to form a uniform carbon coating, which reduces electrochemical polarization and enhances current performance. The modified structure is more stable, achieving improved cycle performance of the graphite negative electrode material.

[0005] (2) CN110203921A discloses a method for preparing modified natural graphite for lithium-ion batteries. This invention enhances the structural stability of the spherical graphite after rolling by liquefying the modifier and filling it into the voids of the natural spherical graphite, thereby enhancing the liquid phase conduction of the electrode, the rate performance and the cycle performance of the electrode. The internal soft carbon and hard carbon materials have a large diffusion coefficient, which is conducive to the internal diffusion of lithium ions, thereby enhancing the fast charging performance. This invention uses asphalt as a modifier, but the specific capacity and first coulombic efficiency of the prepared material need to be further improved.

[0006] (3) CN111463416B discloses a low-cost, low-expansion, long-cycle natural graphite-based composite material and its preparation method and application. In this inventive method, a modifier is added and pressed into the voids inside the spherical graphite by cold isostatic pressing or warm isostatic pressing, thereby achieving densification of the voids inside the curled carbon layer of the spherical graphite and forming the spherical graphite particles into blocks. During the graphitization process, the blocks can be directly placed in the graphitization furnace, increasing the furnace loading capacity without the need for a crucible, significantly reducing the cost of the product, and further improving the circulation performance of the spherical graphite.

[0007] (4) CN107482203B discloses a coating modification method for graphite negative electrode materials, wherein the method comprises placing graphite in a phenolic resin ethanol solution, stirring, separating, and drying to obtain a primary coated graphite; adding the primary coated graphite to a phenolic resin mixed solution, stirring, separating, and drying to obtain a secondary coated graphite; and calcining the secondary coated graphite, grinding, and screening to obtain a graphite negative electrode material. The coating modification method of this method is simple in process and easy to operate, and improves the defect of the traditional primary coating process in which the phenolic resin easily forms an adhesive system that is easy to settle during stirring or volatilization, resulting in uneven coating. However, the compaction density and expansion rate of the graphite negative electrode material obtained by this modification method are not improved.

[0008] (5) CN111584856A discloses a high-performance silicon-carbon negative electrode material and its preparation method. Polyvinyl pyrrolidone is dissolved in water and stirred to obtain a polyvinyl pyrrolidone dispersion; nano-silicon powder and graphite are added to the polyvinyl pyrrolidone dispersion to obtain a mixed slurry, which is spray-dried to obtain a Si / Graphite / C precursor; the Si / Graphite / C precursor is mixed with spodumene and then solid-phase sintered under an inert atmosphere to obtain a high-performance silicon-carbon negative electrode material. This invention adds polyvinyl pyrrolidone, which has both emulsifying and film-forming effects, to achieve the dual functions of dispersing and coating the carbon source; introduces spodumene to construct a 3D core-shell structure, which plays the quadruple role of buffering, stabilizing the structure, pre-lithiation, and assisting sintering. The prepared silicon-carbon negative electrode material has high reversible specific capacity, first efficiency, cycle life, and high safety, and has high preparation efficiency and low cost. However, the specific capacity, first coulombic efficiency, and expansion rate after 800 cycles need to be further improved.

[0009] (6) CN 117352697 A discloses a polypyrrole-coated spherical graphite / SnO2 composite material for lithium-ion battery negative electrode, its preparation method, and application. The preparation method of the polypyrrole-coated spherical graphite / SnO2 composite material comprises: S1, forming highly dispersed tin dioxide nanoparticles on the surface of oxidized spherical graphite by an in-situ redox reaction to obtain a spherical graphite / SnO2 composite material; S2, forming a polypyrrole coating layer on the surface of the spherical graphite / SnO2 composite material, wherein the polypyrrole coating layer is doped with hydrochloric acid to obtain a spherical graphite / SnO2@PPy composite material; S3, heating the spherical graphite / SnO2@PPy composite material at a rate of 2-7°C / min to 550-650°C and calcining it for 1-2 hours under inert gas protection to obtain a polypyrrole-coated spherical graphite / SnO2 composite material. The polypyrrole-coated spherical graphite / SnO2 composite material obtained by the method provided by the present invention has very excellent rate performance and cycle performance.

[0010] The above technologies are basically modifications to the cycle characteristics, charge rate, charge capacity, etc. However, any factor that affects the migration speed of lithium ions inside the battery will affect the charge and discharge rate performance of the lithium-ion battery. The heat dissipation rate inside the battery is also an important factor affecting the rate performance. If the heat dissipation rate is slow, the heat accumulated during high-rate charge and discharge cannot be transferred out, which will seriously affect the safety and life of the lithium-ion battery. In order to improve the thermal conductivity, the existing technologies involved are:

[0011] (1) CN 110783554 B, specifically relates to a high-rate, low-temperature-resistant, and long-life lithium-ion battery negative electrode material, that is, a graphite-based lithium-ion battery negative electrode material to which nitride is added. The above scheme is adopted to add nitride to the graphite-based lithium-ion battery negative electrode material, and the high electrical conductivity and high thermal conductivity as well as the morphological characteristics of the nitride are utilized to form a conductive and thermally conductive dendritic network and nitrogen-doping sites on the battery negative electrode, thereby increasing the ion migration and movement ability inside and on the surface of the lithium-ion battery negative electrode, reducing internal resistance, promoting heat conduction and heat dissipation during charging and discharging, reducing the polarization phenomenon of the lithium-ion battery and the growth of metal lithium dendrites and maintaining the orderly arrangement of the negative electrode material, and reducing the thickness of the SEI film, thereby achieving the purpose of improving the high-rate charge and discharge performance of the lithium-ion battery, the charge and discharge performance in low-temperature environments, and increasing the service life in various environments.

[0012] (2) CN 112151783 A relates to a graphite electrode composite material doped with carbon nanomaterials, the composite material comprising graphite, carbon nanohorns, carbon nanotubes and graphene nanosheets, wherein graphite constitutes the main body of the composite material, and carbon nanohorns, carbon nanotubes and graphene nanosheets are filled into the material gaps of the main body constituted by graphite to form a three-dimensional bridged structure; the graphene, carbon nanotubes and carbon nanohorns of the present invention are jointly filled into the gaps between the graphite particles, and at the same time play a role in improving the electrical conductivity, ionic conductivity and thermal conductivity of the composite material, thereby improving the safety of the battery due to the increase in thermal conductivity, achieving fast charging due to the increase in electrical conductivity, and improving the energy density due to the increase in ionic conductivity, which can reduce the structural collapse of the negative electrode material caused by the charge and discharge cycle, thereby improving the cycle life of the battery and increasing the service life of the battery.

[0013] Although the above technology solves the problem of heat conduction, further exploration is needed to explore how spherical graphite can be compatible with the conductivity of lithium batteries, reduce internal resistance, improve heat conduction and change the internal temperature, thereby improving the high-rate charge and discharge performance of lithium-ion batteries, the charge and discharge performance in low-temperature environments, and the service life in various environments. Summary of the Invention

[0014] In response to related problems, the present invention provides a modified spherical graphite negative electrode material, a preparation method and application thereof. The modified spherical graphite negative electrode material is realized by sulfonated polymer-coated boron nitride nanosheets (BNNS) embedded in the graphite core, and solves the problem of coexistence of charge and discharge and thermal conductivity through structural design and interface engineering.

[0015] The specific content of the invention is as follows:

[0016] A modified spherical graphite negative electrode material comprises a graphite core, an embedding structure and a coating layer.

[0017] Preferably, the graphite core is spherical or quasi-spherical, and the graphite core is one of natural graphite, artificial graphite, and modified graphite.

[0018] More preferably, the graphite core is natural graphite.

[0019] Preferably, the particle size of the graphite core is 10 to 30 μm.

[0020] More preferably, the particle size of the graphite core is 15 to 25 μm.

[0021] Preferably, the tap density of the graphite core is greater than 1.2 g / cm 3 .

[0022] More preferably, the tap density of the graphite core is 1.3 to 1.6 g / cm 3 .

[0023] Preferably, the embedded structure is realized by embedding boron nitride nanosheets (BNNS) into the surface of the graphite core.

[0024] More preferably, the embedded structure is embedded between the graphite core layers by high-pressure homogenization.

[0025] Preferably, the number of layers of the boron nitride nanosheets is 1 to 5.

[0026] More preferably, the number of layers of the boron nitride nanosheets is 2 to 3.

[0027] Preferably, the embedding ratio is 5 to 15 wt%.

[0028] More preferably, the embedding ratio is 8 to 12 wt%.

[0029] The calculation formula of the embedding ratio is: BNNS mass / (BNNS mass+graphite core mass)×100%.

[0030] Preferably, the lateral size of the boron nitride nanosheets is 100 to 500 nm.

[0031] The embedded structure acts as a relay structure to improve the thermal conductivity of the material, inhibit excessive expansion of the graphite core surface and passivation of the SEI film, and enhance the structural stability of the electrode material.

[0032] Preferably, the coating layer is one of sulfonated polypyrrole, sulfonated polyaniline or sulfonated polythiophene.

[0033] More preferably, the coating layer is sulfonated polypyrrole.

[0034] More preferably, the electrical conductivity of the sulfonated polypyrrole is 400-450 S / cm, and the ionic conductivity is 0.9×10 -8 ~1.2×10 -7 cm 2 / s.

[0035] Preferably, the coating layer has a thickness of 5 to 30 nm.

[0036] More preferably, the coating layer has a thickness of 10 to 20 nm.

[0037] Preferably, the surface sulfonic acid group content of the coating layer is 5 to 10 wt%.

[0038] More preferably, the surface sulfonic acid group content is 7-9 wt%.

[0039] More preferably, the coating layer is in situ polymerized on the surface of the graphite core and the embedded structure to form a continuous or discontinuous film.

[0040] More preferably, the coverage of the continuous film is greater than 60%, and the coverage of the discontinuous film is 60-90%.

[0041] The coating layer can improve the wettability of the interface and promote the penetration of the electrolyte; it can also improve the conductivity of electrons and ions, balance the conductivity reduced by the embedding of BNNS, inhibit the abnormal growth of the SEI film, and improve the cycle life.

[0042] A method for preparing a modified spherical graphite negative electrode material comprises the following steps:

[0043] S1: h-BN pretreatment: h-BN and urea were ball-milled in a ratio of 1:4-5 wt%, and ball-milled at 200-300 rpm for 2-3 h. The mixture was then heat-treated at 150°C in a nitrogen atmosphere for 2 h, and finally washed with dilute hydrochloric acid and deionized water and dried.

[0044] S2: BNNS embedded in graphite cores: h-BN powder is ultrasonically exfoliated in NMP to obtain a BNNS dispersion; spherical graphite is mixed with the BNNS dispersion in a mass ratio of 6-8:1; after high-pressure homogenization, the mixture is dried at 120°C for 6 h to obtain a BNNS embedded in graphite core composite;

[0045] S3: Sulfonated polymer coating: the polymer monomer is subjected to a sulfonation reaction with fuming sulfuric acid at 0±5℃ for 5-8h. Ammonium persulfate is used as an oxidant to in situ polymerize the graphite core and the embedded structure surface to form a polymer coating layer. After the polymerization is completed, the polymer is washed and dried.

[0046] Preferably, the ultrasonic stripping power is 550W to 650W.

[0047] More preferably, the ultrasonic stripping power is 600W.

[0048] Preferably, the ultrasonic peeling time is 3 to 4 hours.

[0049] More preferably, the ultrasonic stripping time is 3 hours.

[0050] Preferably, the high-pressure homogenization treatment is cycled 2 to 5 times.

[0051] More preferably, the high-pressure homogenization treatment is repeated three times.

[0052] Preferably, the coating method may also be solution coating or vapor deposition.

[0053] Preferably, the pressure of the high-pressure homogenization treatment is 80 to 120 MPa, and the number of cycles is 2 to 5 times.

[0054] More preferably, the number of cycles is 3 times.

[0055] Preferably, the molar ratio of the polymer monomer to fuming sulfuric acid is 1:1.2-1.5.

[0056] Preferably, the in-situ polymerization temperature is 35-45° C. and the reaction time is 6-10 h.

[0057] A method for applying a modified spherical graphite negative electrode material is as follows:

[0058] S1: Preparation of negative electrode slurry, SPPy-coated spherical graphite / BNNS composite 90wt%, conductive carbon black 2wt%, PVDF binder 3wt%, sodium carboxymethyl cellulose (CMC) 1wt%, deionized water 4wt%, CMC is slowly added to deionized water, magnetic stirring at 500rpm until completely dissolved, conductive carbon black is added, high-speed shear stirring is performed at 2000rpm for 30min, SPPy-coated spherical graphite / BNNS composite is added, planetary stirring is performed at 2000rpm for 2h, pre-diluted emulsion of PVDF binder (solid content 5wt%) and 0.5wt% polydopamine are slowly added dropwise, stirring is continued at 1000rpm for 30min, and vacuum degassing is performed to form a homogeneous slurry with a viscosity of 3000-5000mPa·s;

[0059] S2: Pre-shear coating, the slurry temperature is controlled to 25 ° C, and the linear shear rate is scanned to 5000s using a rotational rheometer -1 , keep for 5 minutes, then use micro gravure coating, gravure roller line number 200 lines / inch, transfer speed 15 / min, shear rate 5000s -1 , substrate tension 20N / m, automatic closed-loop control gap;

[0060] S3: Gradient drying and roller pressing, the first stage: infrared rapid drying (wind speed 5m / s) drying for 1 minute (quickly fix the BNNS orientation structure); the second stage: vacuum drying at 160℃ for 1 hour (completely remove the solvent); use a bidirectional roller press at 60℃ and a pressure of 10MPa to further strengthen the BNNS orientation arrangement and ensure the in-plane heat conduction path is connected.

[0061] Compared with the existing technology, the present invention significantly improves the comprehensive performance of modified spherical graphite negative electrode materials through structural design and interface engineering:

[0062] 1. High tap density: Through the synergistic effect of spherical structure and high-pressure homogenization process, the tap density of the material reaches 1.78~1.88g / cm 3 , while traditional graphite is about 1.5g / cm 3 ;

[0063] 2. Excellent cycle stability: capacity retention rate ≥90% after 500 charge and discharge cycles;

[0064] 3. High thermal conductivity and low volume expansion: In-plane thermal conductivity is 110-130W / (m·K); volume expansion rate is ≤5%;

[0065] 4. Adapt to fast charging and high energy density scenarios: Suitable for power batteries and fast charging scenarios, it solves the problems of poor rate performance and difficult thermal management of lithium-ion batteries. BRIEF DESCRIPTION OF THE DRAWINGS

[0066] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0067] Figure 1 TEM image of BNNS embedded in graphite core of Example 1 of the present invention.

[0068] Figure 2 2 are AC impedance diagrams of Example 1 and Comparative Example 2 of the present invention.

[0069] Figure 3 500 cycle capacity retention graph of the embodiments of the present invention and the comparative example. DETAILED DESCRIPTION

[0070] The endpoints of the ranges and any values ​​disclosed herein 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 endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to form one or more new numerical ranges, and these numerical ranges should be considered to be specifically disclosed in this invention.

[0071] In the following examples, the chemical reagents and main equipment used are as follows:

[0072] Natural spherical graphite: Qingdao Haida Graphite Co., Ltd., model SG15, carbon content ≥99.5%, D50 15μm;

[0073] Artificial spherical graphite: Zhongke Xingcheng, model XC-SG15, D50 is 15μm, tap density is 1.3g / cm 3 ;

[0074] h-BN powder: Zhejiang Yamei Nanotechnology, purity ≥99.5%, D50 100nm;

[0075] N-Methylpyrrolidone (NMP): Aladdin.com, product number M103246;

[0076] Pyrrole monomer: Aladdin.com, product number P104878, purity 99%;

[0077] Oleum: Shuangshi Zhangjiagang Fine Chemical Co., Ltd., SO3 content 20-30%;

[0078] Ammonium persulfate (APS): Sinopharm Chemical Reagent, catalog number 10002616;

[0079] Conductive carbon black: Aladdin Network, product number C742510, D50 is 30nm;

[0080] PVDF binder: Funolin, Model FL2052;

[0081] Ultrasound equipment: Ningbo Xinzhi, SCIENTZ-950E;

[0082] High-pressure homogenizer: Microfluidics, M-110P;

[0083] Ball mill: Nanjing Sunchi, QM-3SP4 planetary ball mill;

[0084] Infrared dryer: Wuhan Youer, IR-100. DETAILED DESCRIPTION

[0085] Example 1: Preparation and testing of artificial spherical graphite composite materials and composite negative electrode slurry

[0086] S1: h-BN pretreatment: 1 g of h-BN and 5 g of urea were ball-milled at 200 rpm for 2 h, followed by heat treatment at 150 °C in nitrogen atmosphere for 2 h, and finally washed with dilute hydrochloric acid and deionized water and dried;

[0087] S2: BNNS embedded in spherical graphite composite: 1 g of h-BN was dispersed in 200 mL of NMP and ultrasonicated at 600 W for 4 h. Then, 8 g of artificial spherical graphite was added, and the mixture was homogenized in a high-pressure homogenizer at 100 MPa for 3 cycles and dried in a vacuum at 120 °C for 6 h.

[0088] S3: SPPy coating: 10 mmol of pyrrole and 12 mmol of fuming sulfuric acid were reacted at 0°C with slow stirring for 5 h; the BNNS embedded in the spherical graphite composite prepared in step S1 was added, 10 mmol of ammonium persulfate was added dropwise, and the mixture was polymerized at 40°C with slow stirring for 8 h; the mixture was washed with ethanol three times and dried at 80°C for 12 h to obtain an SPPy-coated BNNS embedded in the spherical graphite composite;

[0089] S4: Preparation of negative electrode slurry, SPPy-coated BNNS embedded spherical graphite composite 90wt%, conductive carbon black 2wt%, PVDF binder 3wt%, sodium carboxymethyl cellulose (CMC) 1wt%, deionized water 4wt%, CMC is slowly added to deionized water, magnetic stirring at 500rpm until completely dissolved, conductive carbon black is added, high-speed shear stirring is performed at 2000rpm for 30min, SPPy-coated spherical graphite / BNNS composite is added, planetary stirring is performed at 2000rpm for 2h, pre-diluted emulsion of PVDF binder (solid content 5wt%) and 0.5wt% polydopamine are slowly added dropwise, stirring is continued at 1000rpm for 30min, vacuum degassing is performed to form a homogeneous slurry with a viscosity of 3000-5000mPa·s;

[0090] S4: Pre-shear coating, slurry temperature controlled to 25 ° C, using a rotational rheometer to perform a linear shear rate scan to 5000s -1 , keep for 5 minutes, then use micro gravure coating, gravure roller line number 200 lines / inch, transfer speed 15 / min, shear rate 5000s -1 , substrate tension 20N / m, automatic closed-loop control gap;

[0091] S5: Gradient drying and roller pressing, the first stage: infrared rapid drying (wind speed 5m / s) drying for 1 minute (quickly fix the BNNS orientation structure); the second stage: vacuum drying at 160℃ for 1 hour (completely remove the solvent); use a bidirectional roller press at 60℃ and a pressure of 10MPa to further strengthen the BNNS orientation arrangement and ensure the in-plane heat conduction path is connected.

[0092] Example 1 achieves the following through the collaborative design of BNNS embedding and SPPy coating:

[0093] (1)1.85g / cm 3 High tap density improves electrode compaction density;

[0094] (2) Low expansion rate of 4.5%, extending cycle life to 500 times, and capacity retention rate greater than 90%;

[0095] (3) High thermal conductivity of the interface structure of 122W / (m·K), supporting 6C fast charging.

[0096] The performance improvement of this embodiment is explained as follows: (1) BNNS are uniformly embedded between the graphite core layers, expanding the lithium ion transmission channel, improving the reversible capacity, and constructing an in-plane thermal conductive network; (2) The SPPy coating layer improves the interface conductivity, reduces polarization loss, buffers volume changes, and inhibits SEI film rupture.

[0097] according to Figure 1It can be seen that the degree of embedding of BNNS in the graphite core is partially inserted between the graphite core layers.

[0098] according to Figure 2 As can be seen in the EIS spectrum, the diameter of the semicircle represents the charge transfer impedance. As can be seen from the figure, the semicircle diameter of the graphite negative electrode material of Example 1 is small, indicating that the charge transfer rate within the electrode is fast, the material has good conductivity, and the overall conductivity of the material is improved.

[0099] Example 2, preparation and testing of natural spherical graphite composite material and composite negative electrode slurry, based on the material dosage and process adjustment of Example 1:

[0100] (1) Natural spherical graphite: 10g, particle size 15μm, tap density 1.3g / cm 3 ;

[0101] (2) BNNS embedding amount: 1.5 g (13% by mass), high-pressure homogenization pressure 120 MPa, 4 cycles;

[0102] (3) SPPy coating: sulfonic acid group content 8 wt %, pyrrole to fuming sulfuric acid molar ratio 1:1.5, in situ polymerization temperature 45 °C, reaction time 10 h;

[0103] (4) BNNS embedding: h-BN was ultrasonically exfoliated and mixed with spherical graphite at a mass ratio of 7:1, homogenized under high pressure, and then dried;

[0104] (5) SPPy coating: The molar ratio of pyrrole to fuming sulfuric acid is 1:1.5, and a uniform coating layer is formed after polymerization reaction.

[0105] (6) Electrode preparation: Same as in Example 1, the tap density after roller pressing reached 1.88 g / cm 3 .

[0106] Performance changes:

[0107] (1) Tap density: 1.88g / cm 3 , higher than 1.85g / cm in Example 1 3 ;

[0108] (2) In-plane thermal conductivity: 130 W / (m·K), the amount of BNNS increases, and the thermal network becomes denser;

[0109] (3) Cycling performance: The capacity retention rate after 500 cycles was 93.2%, indicating that the SPPy coating layer was denser and suppressed volume expansion.

[0110] The performance results of this example show that higher tap density can be achieved thanks to the characteristics of natural graphite and the high-pressure homogenization process.

[0111] Example 3, preparation and testing of artificial spherical graphite composite material and composite negative electrode slurry, material dosage and process adjustment:

[0112] (1) Artificial spherical graphite: 8g, particle size 25μm, tap density 1.2g / cm 3 ;

[0113] (2) BNNS embedding amount: 0.8 g, mass percentage 9%, high-pressure homogenization pressure 80 MPa, 2 cycles;

[0114] (3) SPPy coating: sulfonic acid group content 10 wt%, in-situ polymerization temperature 35 °C, reaction time 6 h;

[0115] (4) BNNS embedding: Ultrasonic stripping time was shortened to 3 h, and high-pressure homogenization pressure was reduced to reduce structural damage;

[0116] (5) SPPy coating: the content of sulfonic acid groups is increased, which enhances the ionic conductivity;

[0117] (6) Electrode preparation: Tap density after roller pressing: 1.78 g / cm 3 .

[0118] Performance Effect:

[0119] (1) Ionic conductivity: 1.2×10 -7 cm 2 / s, indicating that the content of sulfonic acid groups is increased, which promotes lithium ion transmission;

[0120] (2) Electronic conductivity: 450 S / cm, SPPy coating optimizes conductivity.

[0121] (3) Cyclic stability: After 500 cycles, the capacity retention rate is 91%, the amount of BNNS embedded is reduced, and the thermal conductive network is partially missing.

[0122] The technical effect of this embodiment shows that the high content of sulfonic acid groups provides more lithium ion transmission channels, promoting electrolyte penetration and ion diffusion; the small amount of BNNS embedded in the thermal conductive network is partially missing, resulting in local heat accumulation, but the SPPy coating inhibits the abnormal growth of the SEI film and still maintains high cycle stability.

[0123] Example 4, preparation and testing of modified spherical graphite (natural graphite oxidized) composite material and composite negative electrode slurry, material dosage and process adjustment:

[0124] (1) Modified spherical graphite is used as the core material, natural graphite with a particle size of 20 μm and a tap density of 1.45 g / cm 3 , the surface was slightly oxidized by argon plasma treatment (50W, 2min), which increased the active sites on the graphite surface;

[0125] (2) BNNS embedding amount: 1.2 g (10.7% by mass), high-pressure homogenization pressure 90 MPa, 3 cycles;

[0126] (3) SPPy coating: sulfonic acid group content 9 wt %, pyrrole to fuming sulfuric acid molar ratio 1:1.3, in situ polymerization temperature 38 °C, reaction time 9 h;

[0127] (4) A two-step coating process was used: first, a 50 nm polydopamine transition layer was vapor deposited, and then SPPy was in situ polymerized.

[0128] Performance Effect:

[0129] (1) Tap density: 1.82g / cm 3 ;

[0130] (2) Cycling performance: Capacity retention rate after 500 cycles is 94.6%;

[0131] (3) Volume expansion rate: 4.3%.

[0132] In this embodiment, the polydopamine transition layer enhances the interfacial bonding between BNNS and the graphite core, and the optimized two-step coating process makes the coating layer more uniform and dense, demonstrating the scalability of the present invention in the composite coating process.

[0133] Example 5, preparation and testing of artificial spherical graphite (KOH activated) composite material and composite negative electrode slurry, material dosage and process adjustment:

[0134] (1) Artificial spherical graphite (activated by KOH) was used as the core material, with a particle size of 18 μm and a tap density of 1.35 g / cm 3 ;

[0135] (2) BNNS embedding amount: 1.8 g (15% by mass), high-pressure homogenization pressure 110 MPa, 4 cycles;

[0136] (3) using sulfonated polyaniline (SPAN) as the coating layer with a thickness of 15 nm and a sulfonic acid group content of 8.5 wt%;

[0137] (4) In-situ polymerization temperature was 42°C and reaction time was 7 h.

[0138] Performance Effect:

[0139] (1) Tap density: 1.83g / cm 3 ;

[0140] (2) Cycling performance: Capacity retention rate after 500 cycles is 93%;

[0141] (3) In-plane thermal conductivity: 126 W / (m·K).

[0142] This example verifies the feasibility of using sulfonated polyaniline as a coating layer, demonstrates the flexibility of the present invention in selecting coating materials, and confirms that different sulfonated conductive polymers can achieve good results.

[0143] Comparative Example 1, based on Example 1, with differences in materials and process:

[0144] (1) No BNNS embedded, only SPPy coated spherical graphite;

[0145] (2) Other steps are the same as in Example 1.

[0146] Performance drawbacks:

[0147] (1) Tap density: 1.5g / cm 3 ,without BNNS support, the structure is loose;

[0148] (2) In-plane thermal conductivity: 72W / (m·K);

[0149] (3) Cyclic performance: After 500 cycles, the capacity retention rate dropped to 77%, and the volume expansion rate was as high as 15%.

[0150] Comparative Example 2, based on Example 1, with differences in materials and process:

[0151] (1) Unsulfonated polypyrrole coating, using ordinary polypyrrole without sulfonic acid groups;

[0152] (2) The BNNS embedding amount is the same as in Example 1.

[0153] Performance drawbacks:

[0154] (1) Ionic conductivity: 5×10 -9 cm 2 / s, missing sulfonic acid group, lithium ion transport is hindered;

[0155] (2) Electronic conductivity: 200S / cm, the coating has poor conductivity;

[0156] (3) Cyclic stability: After 500 cycles, the capacity retention rate is 82%, and the interface polarization is aggravated.

[0157] according to Figure 1 It can be seen that in the EIS spectrum, the diameter of the semicircle represents the charge transfer impedance. As can be seen from the figure, the semicircle diameter of the graphite negative electrode material in Comparative Example 2 is larger, indicating that the charge transfer rate inside the electrode is slow and the conductivity of the material is poor. Ordinary polypyrrole cannot achieve good conductivity.

[0158] Comparative Example 3, based on Example 1, with differences in materials and processes:

[0159] (1) High-pressure homogenization pressure 60MPa;

[0160] (2) The BNNS embedding amount is the same as in Example 1.

[0161] Performance drawbacks:

[0162] (1) The BNNSs are not uniformly embedded, the number of layers is greater than 10, and insufficient high pressure leads to inadequate peeling;

[0163] (2) Tap density: 1.6g / cm 3 , the structural density decreases;

[0164] (3) In-plane thermal conductivity: 95 W / (m·K), BNNS stacking results in discontinuous heat conduction paths. Comparative Example 4, based on Example 1, material and process differences:

[0165] (1) The amount of BNNS embedded is too high: 2.5 g, accounting for 20% of the mass;

[0166] (2) Other parameters are the same as in Example 1

[0167] Performance drawbacks:

[0168] (1) Tap density: 1.65g / cm 3 , excessive BNNS leads to loose structure;

[0169] (2) Electronic conductivity: 180 S / cm, excess BNNS destroys the conductive network;

[0170] (3) Cyclic performance: The capacity retention rate after 500 cycles is 78%.

[0171] This comparative example shows that the performance will be degraded when the BNNS embedding ratio exceeds 15 wt %, which verifies the rationality of the embedding ratio range of 5 to 15 wt % in the claims.

[0172] Comparative Example 5, based on Example 1, with differences in materials and process:

[0173] (1) The coating thickness exceeds the range: 70nm;

[0174] (2) Other parameters are the same as in Example 1

[0175] Performance drawbacks:

[0176] (1) Ionic conductivity: 3×10 -8 cm 2 / s, too thick coating hinders ion transmission;

[0177] (2) Volume expansion rate: 12%, the coating layer is too thick, resulting in increased structural stress;

[0178] (3) Tap density: 1.52g / cm 3 .

[0179] The results of this comparative example show that the thickness of the coating layer needs to be controlled within the range of 5 to 50 nm. Too thick a coating layer will deteriorate the overall performance of the material.

[0180] Comparative Example 6, based on Example 1, with differences in materials and process:

[0181] (1) High-pressure homogenization pressure exceeds the upper limit: 150MPa;

[0182] (2) The BNNS embedding amount is the same as in Example 1.

[0183] Performance drawbacks:

[0184] (1) The graphite core structure is damaged and obvious cracks appear;

[0185] (2) Cycling performance: after 500 cycles, the capacity retention rate dropped to 60%;

[0186] (3) Volume expansion rate: 18%.

[0187] The results of this comparative example show that the high-pressure homogenization pressure needs to be controlled within the range of 80-120 MPa, as excessively high pressure will destroy the structural integrity of the graphite core.

[0188] Performance testing:

[0189] 1. Tap density test

[0190] Test standard: Refer to GB / T 5162-2021 "Determination of tap density of metal powders".

[0191] Test method:

[0192] (1) Sample preparation: Take about 50 g of dried negative electrode material powder and pass it through a 200-mesh sieve to ensure uniform particles.

[0193] (2) Test steps: Powder was placed in a graduated cylinder and vibrated 1000 times using a tap density meter at a fixed amplitude of 3 mm and a frequency of 250 times / min. The volume after tapping was recorded and the density was calculated.

[0194] (3) Repeatability: The average value of the test is 3 times, and the deviation is less than ±0.05g / cm 3 .

[0195] 2. Ionic conductivity test

[0196] Test standard: Reference EIS (Electrochemical Impedance Spectroscopy), according to IEC 62620-2014.

[0197] Test method:

[0198] An electrochemical workstation was used to apply a 10 mV AC signal in the frequency range of 0.1 Hz–1 MHz. The bulk resistance (Rb) was obtained by Nyquist plot fitting, and the ionic conductivity was calculated according to the formula σ = L / (Rb × A) (L is the thickness and A is the electrode area).

[0199] 3. Electronic conductivity test

[0200] Test standard: Refer to the four-probe method (GB / T 30835-2014).

[0201] Test method:

[0202] (1) Sample preparation: The negative electrode material powder was pressed into a 10 mm × 10 mm × 1 mm block with a force of 20 MPa and the surface was polished to Ra less than 0.1 μm.

[0203] (2) Test steps:

[0204] A four-probe tester was used with a probe spacing of 1 mm. A constant current (10 mA) was applied and the voltage difference was recorded.

[0205] The conductivity was calculated according to the formula σ = (I × L) / (V × W × T) (I is the current, L is the probe distance, V is the voltage, W and T are the sample width / thickness).

[0206] 4. Cycle performance test (capacity retention rate)

[0207] Test standard: Refer to GB / T 37201-2018 "Test method for electrochemical performance of lithium-ion battery negative electrode materials".

[0208] Test method:

[0209] (1) Sample preparation

[0210] Negative electrode: Prepare the sample according to the invention and coat it on copper foil with a required surface density of 3 mg / cm 2 , rolling to a porosity of 20±2%;

[0211] Assemble the half-cell: use metallic lithium as the counter electrode, 1M LiPF6 / EC:DMC=1:1 as the electrolyte, and Celgard 2400 as the separator.

[0212] (2) Test steps:

[0213] Charge and discharge conditions: 0.5C constant current charge and discharge, voltage range 0.005–2V, 500 cycles.

[0214] 5. In-plane thermal conductivity test

[0215] Test standard: Reference laser flash method (ASTM E1461-2022).

[0216] Test method:

[0217] (1) Sample preparation: The negative electrode material was pressed into a disc with a diameter of 12.7 mm and a thickness of 1 mm at a pressure of 15 MPa, and a graphite layer was sprayed on the surface to reduce the infrared emission error.

[0218] (2) Test steps:

[0219] Using a laser thermal conductivity meter, the laser pulse heats the lower surface of the sample, and the infrared detector records the temperature rise curve of the upper surface.

[0220] By the formula α=0.1388×d 2 / t 0.5 Calculate the thermal diffusion coefficient (d is the thickness, t 0.5 is the half heating time), and the thermal conductivity λ = α × ρ × Cp is calculated based on the specific heat capacity and density.

[0221] 6. Volume expansion rate test

[0222] Test standard: Refer to GB / T 24533-2019 "Graphite Anode Materials for Lithium-ion Batteries".

[0223] Test method:

[0224] (1) Sample preparation: Assemble 10 mm × 10 mm electrodes into a half-cell.

[0225] (2) Test steps:

[0226] After the first charge and discharge, disassemble the battery and measure the change in electrode thickness.

[0227] Volume expansion ratio = [(thickness after cycle - initial thickness) / initial thickness] x 100%.

[0228] (3) Repeatability: Three parallel samples were tested, and the deviation was less than ±0.5%.

[0229] 7. Energy density test

[0230] Test standard: GB / T 37201-2018 "Test method for electrochemical properties of lithium-ion battery negative electrode materials".

[0231] Test method:

[0232] (1) Activation cycle: 0.1C constant current charge and discharge 3 times, 0.005-2V;

[0233] (2) Energy density calculation: E = ∫V·dQ / m, where V is the real-time voltage (accuracy ±0.1 mV), Q is the capacity (0.5 C constant current charge and discharge), and m is the mass of active material (accuracy ±0.01 mg).

[0234] (3) Data collection: Sampling interval 10s, temperature control 25±0.5℃.

[0235] The specific test results are shown in Table 1:

[0236] By comparing the preparation and test results of Examples 1-5 of the present invention and Comparative Examples 1-6, it can be found that:

[0237] (1) The electrode compaction density is significantly improved to 1.78-1.88 g / cm by synergistically integrating BNNS and high-pressure homogenization process. 3 High tap density;

[0238] (2) The SPPy coating inhibits the rupture of the SEI film, and the BNNS embedding buffers the volume expansion, achieving excellent cycling stability with an expansion rate of ≤5% and a capacity retention rate of ≥90% after 500 cycles. The performance comparison of capacity retention is shown in Figure 3 ;

[0239] (3) BNNS constructs an in-plane thermal conductivity network to achieve a high thermal conductivity of 109 to 130 W / (m·K), solving the problem of heat dissipation during charging and discharging;

[0240] (4) 7-9 wt% sulfonic acid groups improve ionic conductivity, up to 1.2×10-8 cm 2 / s, the electronic conductivity reaches 400-450S / cm, and it has strong adaptability to fast charging.

[0241] Table 1

[0242]

[0243] In Comparative Example 1, due to the lack of BNNS embedding, the tap density and thermal conductivity decreased significantly, and the cycle retention rate was only 77%.

[0244] In Comparative Example 2, the absence of sulfonic acid groups resulted in extremely low ion conductivity, demonstrating the key role of sulfonic acid groups in ion transport.

[0245] In Comparative Examples 3-6, insufficient high-pressure homogenization pressure resulted in uneven embedding of BNNS, which reduced thermal conductivity; excessive BNNS destroyed the conductive network, which reduced electrical conductivity; and an overly thick coating layer hindered ion transport, resulting in an increase in the expansion rate.

[0246] Therefore, the "core-embedded-shell" structure of SPPy-coated BNNS embedded in the graphite core disclosed in the present invention has achieved breakthroughs in the tap density, thermal conductivity, cycle life and fast charging performance of graphite negative electrode materials through multi-scale interface engineering. It is suitable for high-energy-density power batteries and extreme working conditions (such as low temperature and fast charging), providing a new solution for lithium-ion battery technology.

[0247] The basic principles, main features, and advantages of the present invention are shown and described above. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The above embodiments and descriptions are merely illustrative of the principles of the present invention. Various changes and modifications may be made to the present invention without departing from the spirit and scope of the present invention. Such changes and modifications are intended to fall within the scope of the present invention. The scope of protection claimed in the present invention is defined by the appended claims and their equivalents.

Claims

1. A modified spherical graphite negative electrode material, characterized in that: Including graphite core, embedded structure and cladding: The graphite core is spherical or quasi-spherical graphite with a particle size of 10 to 30 μm and a tap density greater than 1.2 g / cm 3 ; The embedded structure is achieved by embedding boron nitride nanosheets into the surface of the graphite core, wherein the number of layers of the boron nitride nanosheets is 1 to 5 and the embedding ratio is 5 to 15 wt%; The coating layer is one of sulfonated polypyrrole, sulfonated polyaniline or sulfonated polythiophene, the coating layer thickness is 5 to 50 nm, and the surface sulfonic acid group content is 5 to 10 wt%; The coating layer covers the graphite core and the embedded structure.

2. The modified spherical graphite negative electrode material according to claim 1, characterized in that: The lateral size of the boron nitride nanosheet is 300-500 nm.

3. The modified spherical graphite negative electrode material according to claim 1, characterized in that: The coating layer forms a continuous or discontinuous film on the surface of the graphite core and the embedded structure.

4. The modified spherical graphite negative electrode material according to claim 3, characterized in that: The modified spherical graphite negative electrode material has an electrical conductivity of 400-450 S / cm and an ionic conductivity of 0.9×10 -8 ~1.2×10 -7 cm 2 / s.

5. The modified spherical graphite negative electrode material according to claim 1, characterized in that: The coating layer has a thickness of 5 to 20 nm and a surface sulfonic acid group content of 7 to 9 wt%.

6. A method for preparing the modified spherical graphite negative electrode material according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: Boron nitride nanosheets (BNNS) are embedded in graphite cores. h-BN powder is ultrasonically exfoliated in NMP to obtain a BNNS dispersion. The graphite core and BNNS dispersion are mixed at a mass ratio of 6 to 8:

1. The mixture is then homogenized under high pressure and dried to obtain an embedded structure of BNNS embedded in the graphite core. S2: Sulfonated polypyrrole coating: pyrrole monomer is sulfonated with fuming sulfuric acid at 0±5℃, and ammonium persulfate is used as an oxidant to in situ polymerize the graphite core and embedded structure surface to form an SPPy coating layer. After the polymerization is completed, the SPPy coating layer is washed and dried.

7. The preparation method according to claim 6, characterized in that The ultrasonic stripping conditions are: power 5503-650W, time 3-4h, high-pressure homogenization pressure 80-120MPa, and cycle number 2-5 times.

8. The preparation method according to claim 6, characterized in that: The temperature of the in-situ polymerization reaction is 35-45° C., and the reaction time is 6-10 hours.

9. The preparation method according to claim 6, characterized in that: The molar ratio of pyrrole to fuming sulfuric acid in the sulfonation reaction is 1:1.2-1.5, and the reaction time is 5-8 hours.

10. Use of the modified spherical graphite negative electrode material according to any one of claims 1 to 5, characterized in that: After the modified spherical graphite negative electrode material was prepared into a negative electrode slurry, it was coated on copper foil and further strengthened the BNNS orientation arrangement by a bidirectional roller press at 60°C and 10MPa.

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