Research and application of spherical graphite with optimized specific surface area as anode material for lithium battery

Abstract

The application discloses a preparation method of a spherical graphite lithium battery negative electrode material with optimized specific surface area, and relates to the technical field of lithium ion batteries. The method comprises the following steps: processing of a bimodal particle size graphite raw material, wherein a core-shell structure is constructed by grading mixing of natural graphite and artificial graphite; forming a hierarchical porous structure by spray drying and gas phase pore expansion; performing an ultrathin coating by using a PEDOT:PSS or PVDF-g-PEG conductive polymer solution; and curing a conductive network at low temperature. The application significantly improves the specific surface area while maintaining the tap density through the synergistic design of core-shell structure stability and hierarchical pore channels. The continuous conductive network formed by the conductive polymer coating and the low-temperature carbonization process effectively improves the interface charge transport efficiency. The prepared negative electrode material has high specific surface area, excellent structural stability and low interface impedance characteristics, solves the problem that high specific surface area and cycle stability are difficult to be compatible in traditional processes, and is suitable for the manufacturing of high-energy-density and long-cycle-life power batteries.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a kind of spherical graphite specific surface area optimization lithium battery negative material research and development and application. BACKGROUND

[0002] Current commercialized lithium battery negative material is mainly artificial graphite, which obtains high crystallinity by high temperature graphitization (>2800 DEG C), but the specific surface area is generally lower than 5m 2 / g, resulting in long lithium ion migration path and poor rate performance. To improve the specific surface area, the industry usually uses mechanical pore forming (such as ball milling etching) or chemical activation, but these methods have significant defects:

[0003] For example, strong acid / alkali etching can destroy the sp 2 Hybrid structure between graphite layers, causing irreversible capacity loss and poor structural stability; Pore size distribution is uncontrollable, and mechanical pore forming is easy to form >50nm large pores, although the specific surface area is increased to 200-300m 2 / g, but the tap density is reduced to 0.8g / cm 3 Below, and the large pore structure exacerbates electrolyte decomposition; Traditional pitch carbon coating requires carbonization above 800 DEG C, forming a rigid thick coating, resulting in interface impedance >200Ω, limiting high-rate charge and discharge performance.

[0004] Therefore, the present application provides a kind of spherical graphite specific surface area optimization lithium battery negative material. SUMMARY

[0005] The main purpose of the present application is to provide a kind of spherical graphite specific surface area optimization lithium battery negative material research and development and application, break through the technical barrier that high specific surface area and high tap density cannot be compatible, realize the coordinated distribution of micropore / mesopore, consider lithium ion storage site and fast transmission channel; Establish ultra-thin flexible conductive interface to solve the problem of coating rupture caused by high impedance and volume expansion.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is:

[0007] A kind of spherical graphite specific surface area optimization lithium battery negative material preparation method, the specific steps are as follows:

[0008] Step 1, bimodal particle size graphite raw material treatment: select natural graphite and artificial graphite as spherical graphite raw material, the particle size of natural graphite is 5 μm, the particle size of artificial graphite is 25 μm, and the mass ratio of natural graphite and artificial graphite is 3:7, add dispersant after mixing, then immerse the mixed graphite particles in 1mol / H2SO4, ultrasonic treatment for 30min, then wash with deionized water until pH=7, dry the mixed graphite particles;

[0009] Step 2, constructing the spherical bimodal particle size core-shell structure: the mixed graphite particles obtained in step 1 are subjected to rapid spheroidization treatment at a temperature of 2000-2200°C in an inert atmosphere to form spherical particles with a core-shell structure of a dense surface and a loose interior;

[0010] Step 3, spray drying and gas phase pore expansion treatment: the spherical particles are dispersed in deionized water or ethanol at a concentration of 10-12wt% to form a suspension, and are subjected to spray drying treatment, while an expansion gas is introduced at the inlet section of the drying tower to induce the formation of micropores and mesopores with a pore size of 0.5-15nm at a temperature of 200-250°C;

[0011] Step 4, constructing a conductive polymer coating: the obtained porous spherical particles are mixed with a conductive polymer solution, and surface coating is performed by ball milling, with a coating thickness of less than 10nm;

[0012] Step 5, carbonization treatment: the coated particles are heat treated at 450-600°C in an inert atmosphere for 2-4 hours to complete the solidification of the conductive polymer coating and stabilize the graphite structure.

[0013] Preferably, the dispersant in step 1 is selected from polyvinylpyrrolidone or polyethylene glycol, and the addition amount is 0.2-1.0wt%. After adding the dispersant, the mixed artificial graphite particles and natural graphite particles are mixed by high-speed shearing or double-helix mixing at a speed of 800rpm for 30min.

[0014] Preferably, the spheroidization treatment in step 2 is performed using a high-temperature rotary spheroidization furnace or a fluidized bed plasma spray device, and the treatment time is 10 seconds. After treatment, cooling is performed at a rate of 200°C / min to avoid collapse of the micropore structure.

[0015] Preferably, the spray drying in step 3 uses a two-fluid spray device, with an inlet temperature of 200°C, an outlet temperature of 100°C, an atomization pressure of 0.5MPa, and an expansion gas flow rate of 2.0L / min.

[0016] Preferably, the expansion gas in step 3 is carbon dioxide or ammonia, and is introduced at the inlet section of the spray drying tower, so that the formed pore size is mainly distributed in the range of micropores of 0.5-2nm and mesopores of 2-15nm.

[0017] Preferably, the conductive polymer solution in step 4 is a PEDOT:PSS aqueous solution or a PVDF-g-PEG block copolymer ethanol solution, with a concentration of 0.2-0.5wt%, a volume ratio to the spherical particles of 1:10, a stirring time of 30min, and a stirring speed of 500rpm.

[0018] Preferably, the conductive polymer coating in step 4 is formed by low-speed ball milling in combination with vacuum drying, the ball milling speed is 100-300 rpm, the time is 30 minutes; the vacuum drying temperature is 60 DEG C, the time is 6-12 hours.

[0019] Preferably, the heat treatment in step 5 uses a tubular furnace or a static hot air furnace, the atmosphere is nitrogen or argon, the heating rate is 2-5 DEG C / min, and the holding time is 2-4 hours.

[0020] A lithium battery negative electrode material is prepared by the method for preparing a lithium battery negative electrode material with a specific surface area optimized ball graphite.

[0021] The application of the above-mentioned negative electrode material in a lithium battery, specifically, the negative electrode material is prepared into a lithium battery negative electrode plate and placed in a lithium battery.

[0022] Compared with the prior art, the present application has the following beneficial effects:

[0023] 1. The present application realizes hierarchical pores by maintaining tap density and cooperating with gas phase pore expansion technology, which breaks through the contradiction between high specific surface area and low tap density of traditional graphite, significantly improves the capacity retention rate at 5C rate, and has smaller capacity attenuation after 200 cycles, which is significantly better than the industry average level.

[0024] 2. The present application forms a continuous flexible conductive network by using an ultrathin conductive polymer coating combined with low-temperature carbonization. Compared with traditional carbon coating, the interface impedance is reduced and the initial coulombic efficiency is improved.

[0025] 3. The present application innovatively integrates spray pore expansion-ball milling coating-gradient carbonization process, solves key technical problems such as porous structure collapse, coating uniformity and carbonization degree control, and meets the demand of high-power power batteries. DETAILED DESCRIPTION

[0026] In order to make the technical means, creative features and purposes achieved by the present application easy to understand, the present application is further described below in combination with specific embodiments.

[0027] The spherical graphite is selected as the mainstream in the selection of lithium battery negative electrode materials because of its good charge-discharge stability and high conductivity. However, the specific surface area of traditional spherical graphite is low, which limits its rate performance and initial coulombic efficiency. Increasing the specific surface area usually accompanies the destruction of graphite structure and the increase of irreversible capacity. Therefore, the present application focuses on how to maintain the stability of the structure while improving the specific surface area.

[0028] The application proposes the following technical route to break through the problems one by one: bimodal particle size regulation: using different particle size graphites to regulate the packing structure and porosity; core-shell structure construction: improving mechanical and conductive stability; multi-level pore structure formation: constructing micropores / mesopores through spray-pore expansion means to effectively increase the specific surface area; conductive polymer guide coating: regulating interface properties and inhibiting SEI film growth; low-temperature carbonization curing coating structure: realizing stable embedding of the guide structure.

[0029] Based on the above idea, the application discloses a kind of spherical graphite specific surface area optimization of lithium battery negative material research and application, specifically, the preparation method steps of the material are as follows:

[0030] Step 1, bimodal particle size graphite raw material treatment:

[0031] Natural graphite and artificial graphite are selected as spherical graphite raw materials, the particle size of natural graphite is 5 μm, the particle size of artificial graphite is 25 μm, and the mass ratio of natural graphite to artificial graphite is 3:7, then the mixed graphite particles are soaked in 1 mol / H2SO4 after adding a dispersing agent, ultrasonic treatment for 30 min, then washed with deionized water until pH=7, and dried mixed graphite particles;

[0032] Among them, the dispersing agent is selected from polyvinylpyrrolidone or polyethylene glycol, and the addition amount is 0.2-1.0wt%, after adding the dispersing agent, the mixed mode of artificial graphite particles and natural graphite particles is high-speed shearing mixing or double-helix mixing, the rotation speed is 800 rpm, and the mixing time is 30 min.

[0033] It should be noted that in the present application, the particle size of natural graphite is 55 μm, the particle size of artificial graphite is 25 μm, a bimodal particle size distribution is formed, small particle size can fill the gap of large particle size, forming a porous packing network, and after small particle size fills the gap of large particle size, the compaction performance of graphite material can be improved, and the lithium ion migration path is reserved at the same time; after the artificial graphite and natural graphite raw materials are treated by acid and ultrasonic treatment, impurities can be removed, surface hydroxyl, carboxyl and other functional groups can be increased, surface hydrophilicity and functionalization degree can be increased, which is beneficial to subsequent coating adhesion and pore forming reaction.

[0034] Step 2, constructing spherical bimodal particle size core-shell structure:

[0035] The mixed graphite particles obtained in step 1 are subjected to rapid spheroidization treatment at a temperature of 2000-2200℃ in an inert atmosphere to form core-shell structure spherical particles with dense surface and loose interior;

[0036] The spheroidization treatment is performed by using a high-temperature rotating spheroidization furnace or a fluidized bed plasma spraying device, the treatment time is 10 seconds, and the cooling rate is 200°C / min to avoid the collapse of the microporous structure.

[0037] In this step, the high-temperature (2000-2200°C) rapid spheroidization, the graphite forms graphite crystallites on the surface at this temperature and in an inert atmosphere, the edge sites with high migration energy are preferentially rearranged, and the flaky or irregular graphite particles are melted-migrated-reconstructed into approximately spherical shape; the surface is crystallized, and the inside is slowly contracted, resulting in a dense shell and a porous core structure; and this step uses a rapid cooling rate of 200°C / min to prevent thermal expansion stress collapse and preserve the internal void network, preparing for subsequent pore formation.

[0038] Step 3, spray drying and gas phase pore expansion treatment:

[0039] The spherical particles are dispersed in deionized water or ethanol at a concentration of 10-12wt% to form a suspension, and are subjected to spray drying treatment, while expanding the pore-forming gas at the inlet section of the dryer, and inducing the formation of micropores and mesopores with a pore size of 0.5-15nm at 200-250°C;

[0040] The spray drying uses a double-fluid spray device, the feed temperature is 200°C, the discharge temperature is 100°C, the atomization pressure is 0.5MPa, and the pore-forming gas is introduced at a flow rate of 2.0L / min;

[0041] The pore-forming gas is carbon dioxide or ammonia, and the introduction position is the inlet section of the spray drying tower, so that the formed pore size is mainly distributed in the micropore range of 0.5-2nm and the mesopore range of 2-15nm.

[0042] This step forms a void structure by spray drying, and the suspension droplets are dried to form pores through gas-liquid shearing atomization;

[0043] When the pore-forming gas is carbon dioxide, carbon dioxide reacts with the edge sites of the carbon structure to form carbon monoxide, and etches the boundary to form micropores; when the pore-forming gas is ammonia, ammonia generates active radicals under the induction of temperature, which forms an alkaline corrosion effect on the carbon surface, and then forms micropores; the specific surface area is improved through the formed micropores.

[0044] Step 4, building a conductive polymer coating:

[0045] The obtained porous spherical particles are mixed with a conductive polymer solution, and the surface is coated by ball milling, and the obtained coating thickness is less than 10nm;

[0046] The conductive polymer solution is a PEDOT:PSS aqueous solution or a PVDF-g-PEG block copolymer ethanol solution, the concentration of the conductive polymer solution is 0.2-0.5wt%, the volume ratio of the conductive polymer solution to the spherical particles is 1:10, the stirring time is 30 min, and the stirring speed is 500 rpm;

[0047] The conductive polymer coating is formed by a low-speed ball milling and vacuum drying cooperative method, the ball milling speed is 100-300 rpm, and the time is 30 min; the vacuum drying temperature is 60℃, and the time is 6-12 hours.

[0048] It should be noted that PEDOT:PSS has excellent electronic conductivity and chemical stability, and can buffer uneven charge between particles; and PVDF-g-PEG block copolymer is well dissolved in ethanol, and the PEG flexible chain can improve the flexibility and lithium salt wettability of the coating, and after ball milling, the coating is uniformly coated on the surface of the particles to form a continuous electronic conduction interface.

[0049] Step 5, carbonization treatment:

[0050] The coated particles are heat treated at 450-600℃ in an inert atmosphere for 2-4 hours to complete the curing of the conductive polymer coating and stabilize the graphite structure.

[0051] The furnace type used for heat treatment is a tubular furnace or a static hot air furnace, the atmosphere is nitrogen or argon, the heating rate is 2-5℃ / min, and the holding time is 2-4 hours.

[0052] The main purpose of this step is to convert the coating formed by the conductive polymer into a carbon coating, realize the structuring of the coating, form a continuous conductive network, improve the overall electronic conduction efficiency, maintain the stability of the porous structure, and prevent the coating from cracking or delaminating during subsequent use.

[0053] The present application is further disclosed in combination with specific examples and comparative examples as follows:

[0054] Example 1

[0055] The steps for preparing the spherical graphite specific surface area optimized lithium battery negative electrode material in this example refer to the above steps 1-step 5, and only the amount of material and the detailed parameters of the steps are adjusted, and the specific parameters are as follows:

[0056] Step 1: The dispersing agent is PVP (addition amount 0.5wt%), and double helix mixing (800rpm / 30min) is performed;

[0057] Step 2: The high-temperature rotating spheroidization furnace is treated at 2100℃, the atmosphere is argon, and the cooling rate is 200℃ / min;

[0058] Step 3: CO2 pore expanding gas (2.0 L / min), ethanol suspension (12 wt%), 1.8 nm micropores + 8 nm mesopores formed;

[0059] Step 4: PEDOT:PSS aqueous solution (0.3 wt%), ball milling speed 200 rpm, vacuum drying 8 h / 60 °C;

[0060] Step 5: Tube furnace carbonization 500 °C (N2 atmosphere, 3 °C / min heating, 3 h holding).

[0061] Example 2

[0062] The steps of preparing the spherical graphite specific surface area optimized lithium battery anode material of the present example refer to the above-mentioned steps 1-step 5, only the amount of material and the detailed parameters of the steps are adjusted, as follows:

[0063] Step 1: PEG dispersant (0.8 wt%), high-speed shearing mixing (800 rpm / 30 min);

[0064] Step 2: Fluidized bed plasma spraying device 2000 °C treatment, nitrogen atmosphere;

[0065] Step 3: NH3 pore expanding gas (2.0 L / min), deionized water suspension (10 wt%), 0.8 nm micropores + 12 nm mesopores formed;

[0066] Step 4: PVDF-g-PEG ethanol solution (0.5 wt%), ball milling speed 300 rpm, vacuum drying 6 h / 60 °C;

[0067] Step 5: Static hot air furnace carbonization 600 °C (Ar atmosphere, 5 °C / min heating, 2 h holding).

[0068] Example 3

[0069] The steps of preparing the spherical graphite specific surface area optimized lithium battery anode material of the present example refer to the above-mentioned steps 1-step 5, only the amount of material and the detailed parameters of the steps are adjusted, as follows:

[0070] Step 1: PVP dispersant (0.2 wt%), double helix mixing (800 rpm / 30 min);

[0071] Step 2: High-temperature rotary spheroidizing furnace 2200 °C treatment, argon atmosphere;

[0072] Step 3: CO2 pore expanding gas (2.0 L / min), ethanol suspension (11 wt%), 1.2 nm micropores + 5 nm mesopores formed;

[0073] Step 4: PEDOT:PSS aqueous solution (0.2wt%), ball milling speed 100rpm, vacuum drying 12h / 60°C;

[0074] Step 5: Tube furnace carbonization 450°C (N2 atmosphere, 2°C / min heating, 4h holding).

[0075] Example 4

[0076] The steps of preparing the spherical graphite specific surface area optimized lithium battery anode material of the present example refer to the above-mentioned steps 1-step 5, only the amount of material and the detailed parameters of the steps are adjusted, as follows:

[0077] Step 1: PEG dispersant (1.0wt%), high-speed shearing mixing (800rpm / 30min);

[0078] Step 2: Fluidized bed plasma spraying device 2050°C treatment, nitrogen atmosphere;

[0079] Step 3: NH3 pore expanding gas (2.0L / min), deionized water suspension (12wt%), forming 0.6nm micropore+15nm mesopore;

[0080] Step 4: PVDF-g-PEG ethanol solution (0.4wt%), ball milling speed 250rpm, vacuum drying 10h / 60°C;

[0081] Step 5: Static hot air furnace carbonization 550°C (Ar atmosphere, 4°C / min heating, 3h holding).

[0082] Example 5

[0083] The steps of preparing the spherical graphite specific surface area optimized lithium battery anode material of the present example refer to the above-mentioned steps 1-step 5, only the amount of material and the detailed parameters of the steps are adjusted, as follows:

[0084] Step 1: PVP / PEG composite dispersant (0.6wt%), double helix mixing (800rpm / 30min);

[0085] Step 2: High-temperature rotating spheroidization furnace 2150°C treatment, argon-nitrogen mixed gas;

[0086] Step 3: CO2+NH3 mixed pore expanding gas (total flow rate 2.0L / min), ethanol / water mixed suspension (11.5wt%);

[0087] Step 4: PEDOT:PSS and PVDF-g-PEG alternate coating (0.35wt%), ball milling speed 150rpm, vacuum drying 9h / 60°C;

[0088] Step 5: Tube furnace 580℃ gradient carbonization (N2→Ar switch, 3.5℃ / min temperature rise, 2.5h holding).

[0089] In addition, three comparative examples are designed in the present application, as follows:

[0090] Comparative Example 1

[0091] The difference between this comparative example and Example 1 is that the bimodal particle size design of graphite is cancelled, and in Step 1, a single particle size of natural graphite (15μm) is used, which simultaneously causes no core-shell structure to be formed after the spheroidization treatment in Step 2; other steps and specific parameters are consistent with Example 1.

[0092] Comparative Example 2

[0093] The difference between this comparative example and Example 1 is that no reaming gas is introduced when performing spray drying in Step 3. Other steps and specific parameters are exactly the same as Example 1.

[0094] Comparative Example 3

[0095] The difference between this comparative example and Example 1 is that Step 4 is modified to use pitch ethanol (10wt%) impregnation followed by high-temperature carbonization, and the carbonization temperature in Step 5 is 800℃. Other steps and specific parameters are exactly the same as Example 1.

[0096] The samples prepared in Examples 1-5 and Comparative Examples 1-3 are used to prepare lithium battery negative electrode materials in the following manner:

[0097] Step 1, slurry preparation:

[0098] The samples (92wt%) of Examples 1-5 and Comparative Examples 1-3, SuperP conductive agent (3wt%) and PVDF binder (5wt%) are added to NMP solvent in proportion, with solid content controlled at 45%-50%, and a planetary mixer is used for high-speed mixing at 3000rpm for 2 hours to obtain a uniform slurry.

[0099] Step 2, coating and forming:

[0100] An 8μm electrolytic copper foil is used as a current collector, and the slurry is uniformly coated by a gap coater at a speed of 0.8m / min, with the wet film thickness controlled at 150±5μm, to ensure that the active material surface density is 12±0.3mg / cm 2 .

[0101] Step 3, staged drying:

[0102] The gradient drying process is used: first, pre-drying at 80°C for 2 hours to remove the solvent, then vacuum drying at 120°C for 4 hours to completely remove the residual moisture. The moisture content of the dried electrode sheet should be <500ppm.

[0103] Step 4, electrode sheet finishing:

[0104] The dried electrode sheet is rolled to a density of 1.6±0.05g / cm 3 , punched into Φ14mm standard round sheets, and finally baked at 120°C in a vacuum environment for 12 hours, sealed and stored for use.

[0105] The samples prepared in Examples 1-5 and Comparative Examples 1-3 were subjected to the following performance tests on the lithium battery negative plates prepared according to the above method:

[0106] 1. Specific surface area test

[0107] According to the standard: GB / T19587-2017 "Gas adsorption BET method for determination of specific surface area of solid materials".

[0108] Test method: Use a specific surface area analyzer to determine by nitrogen adsorption method (BET method). The sample needs to be degassed at 200°C for 4 hours to remove surface adsorbed substances.

[0109] 2. Pore size distribution analysis

[0110] According to the standard: ISO15901-2:2022 "Pore size distribution and material porosity by mercury intrusion method and gas adsorption method".

[0111] Test method: Use a mercury porosimeter with a pressure range of 0.1-400MPa to analyze the distribution ratio of micropores (0.5-2nm) and mesopores (2-15nm).

[0112] 3. Electrochemical impedance spectroscopy (EIS)

[0113] According to the standard: GB / T30836-2014 "Electrochemical performance test method for lithium ion battery electrode materials".

[0114] Test method: Use an electrochemical workstation to apply an alternating current signal with a 5mV amplitude in the frequency range of 0.01Hz-100kHz to test the electrode / electrolyte interface impedance.

[0115] 4. First charge-discharge efficiency

[0116] According to the standard: GB / T30835-2014 "Test method for first coulomb efficiency of lithium ion battery negative materials".

[0117] Test method: Blue electric test system, 0.1C rate for charging and discharging (voltage range 0.005-2V vs Li+ / Li), calculate the ratio of the first discharge capacity and charge capacity.

[0118] 5. Rate performance test

[0119] According to the standard: SJ / T 11792-2021 "Lithium ion battery electrode material rate performance test specification".

[0120] Test method: New Wei tester, step charging and discharging (0.2C→0.5C→1C→2C→5C), record the capacity retention rate at each rate.

[0121] 6. Cycle stability test

[0122] According to the standard: QC / T 743-2019 "Lithium ion power battery for electric vehicles Cycle life requirements".

[0123] Test method: Place the lithium battery negative plate prepared by examples 1-5 and comparative examples 1-3 in the same specification lithium battery (i.e. only the negative plate uses the lithium battery negative prepared by examples 1-5 and comparative examples 1-3, other remains unchanged), then in the high-low temperature test box, at 25℃ environment, with 1C rate, cycle charging and discharging for 200 times, calculate the capacity attenuation rate.

[0124] All electrochemical tests were completed in an argon glove box with humidity <1%, and the electrolyte was 1M LiPF6(EC:DMC:EMC=1:1:1, containing 2% VC additive). Lithium metal sheet was used as the counter electrode and reference electrode, and the separator was Celgard 2400 polypropylene film.

[0125] Each sample was tested in 3 groups of parallel experiments, and the average value was taken. Data points with deviation exceeding 5% need to be retested.

[0126] Table 1: Specific surface area and micropore ratio test data of lithium battery negative materials prepared by examples 1-5 and comparative examples 1-3

[0127] Sample Specific surface area (m 2 / g) Microvoids ratio (%) Example 1 420 38 Example 2 530 42 Example 3 485 35 Example 4 505 40 Example 5 555 45 Comparative Example 1 280 12 Comparative Example 2 320 5 Comparative Example 3 380 18

[0128] Table 2: Electrical performance test results of lithium battery negative plate prepared by examples 1-5 and comparative examples 1-3

[0129]

[0130] According to the test results in the above table 1 and table 2, it can be seen that:

[0131] Example 1 uses CO2 reaming + PVP dispersant + 500℃ carbonization, specific surface area 420m2 / g, micropore ratio 38%, 1C capacity 345mAh / g. Uniform micropores formed by weak acid etching of CO2, combined with PEDOT:PSS coating (8nm) to achieve low impedance (78Ω), 92.1% retention rate after 200 cycles to verify the stability of the core-shell structure, but 5C retention rate 86.2% slightly lower than other examples, which may be related to the narrow mesopore distribution.

[0132] Example 2 uses NH3 pore expansion + PVDF-g-PEG coating + 600℃ carbonization, specific surface area 530m 2 / g is the highest, micropore ratio 42%. Alkaline corrosion of NH3 forms more abundant micropores, and the flexible segment of PVDF-g-PEG makes the coating only 6nm, the interface impedance 65Ω is optimal, the 5C retention rate 89.5% reflects fast ion transport, but the tap density 1.08g / cm 3 is slightly low, reflecting the slight impact of high porosity on density.

[0133] Example 3 uses CO2 pore expansion + 0.2wt% PVP dispersant + 450℃ carbonization, specific surface area 485m 2 / g, first efficiency 94.2% outstanding. Low dispersant dosage does not affect uniformity, low temperature carbonization retains more PEDOT:PSS conductive groups, but micropore ratio 35% is lower, resulting in 5C retention rate 88.5% slightly inferior to Example 2 / 4.

[0134] Example 4 uses NH3 pore expansion + PVDF-g-PEG ethanol solution + 550℃ carbonization, micropore ratio 40% and tap density 1.10g / cm 3 is balanced best. The mesopore distribution is expanded to 15nm (mercury porosimeter data), making the 5C retention rate 90.1%, and the 94.2% retention rate after 200 cycles proving the effect of mesoporous structure to relieve volume expansion.

[0135] Example 5 uses CO2 / NH3 mixed pore expansion + composite coating + gradient carbonization, with the best comprehensive performance: specific surface area 555m 2 / g, interface impedance 68Ω, 5C retention rate 91.2%, cycle retention rate 95.3%. Mixed pore expansion achieves a full range of pore size distribution from 0.5-15nm, and gradient carbonization (580℃) makes the composite coating more fully carbonized, forming a three-dimensional conductive network.

[0136] Comparative Example 1 cancels the bimodal particle size design, with a tap density of only 0.98g / cm 3 , the lowest specific surface area 280m 2 / g. Single particle size leads to insufficient porosity due to particle packing, limiting lithium ion transport path (5C retention rate 65.4%), and obvious particle breakage after 200 cycles (retention rate 72.3%), verifying the necessity of bimodal structure for mechanical stability.

[0137] Comparative Example 2 cancelled gas phase pore expansion, specific surface area 320 m 2 / g, micro-pore ratio only 5%. Lack of micro-pores leads to reduced effective reaction sites, coating coverage (thickness 15 nm), high interface impedance 125 Ω, 1C capacity 298 mAh / g significantly lower than the example group, demonstrating the key role of the pore expansion step in pore structure regulation.

[0138] Comparative Example 3 uses traditional pitch carbon coating + 800℃ carbonization, coating thickness 50 nm and interface impedance 228 Ω. High temperature carbonization destroys the flexible structure of the polymer, the first efficiency is 81.3%, the lowest, and the particle breakage rate after cycling is 15% (observed by SEM), indicating the irreplaceability of conductive polymer + low temperature carbonization for interface optimization.

[0139] In summary, the bimodal particle size design + gas phase pore expansion + ultra-thin conductive coating of the present application forms the "high specific surface area - stable structure - low impedance" triple advantage, ammonia gas pore expansion is more conducive to the generation of micro-pores than carbon dioxide pore expansion, carbonization temperature 450-600℃ balances the carbonization degree and structural integrity of the coating, composite coating + gradient carbonization realizes the optimal interface characteristics.

[0140] The above shows and describes the basic principles and main features of the present application and the advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above examples, and the above examples and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a spherical graphite with optimized specific surface area for lithium battery anode material, characterized in that, The specific steps are as follows: Step 1, bimodal particle size graphite raw material treatment: natural graphite and artificial graphite are selected as spherical graphite raw materials, the particle size of natural graphite is 5 μm, the particle size of artificial graphite is 25 μm, and the mass ratio of natural graphite to artificial graphite is 3:7, a dispersing agent is added after mixing, then the mixed graphite particles are soaked in 1 mol / L H2SO4, ultrasonic treatment for 30 min, then washed with deionized water until pH=7, dried to obtain mixed graphite particles; Step 2, constructing a spherical bimodal particle size core-shell structure: the mixed graphite particles obtained in step 1 are subjected to rapid spheroidization treatment at a temperature of 2000-2200°C in an inert atmosphere to form spherical particles with a dense surface and a loose interior; Step 3, spray drying and gas phase hole expansion treatment: the spherical particles are dispersed in deionized water or ethanol at a concentration of 10-12 wt% to form a suspension, and are subjected to spray drying treatment, while expanding hole gas is introduced at the inlet section of the drying tower, and micropores and mesopores with a pore size of 0.5-15 nm are induced to form at 200-250°C; Step 4, constructing a conductive polymer coating: the obtained porous spherical particles are mixed with a conductive polymer solution, and the surface is coated by ball milling, and the obtained coating thickness is less than 10 nm; Step 5, carbonization treatment: the coated particles are heat treated at 450-600°C in an inert atmosphere for 2-4 hours to complete the solidification of the conductive polymer coating and stabilize the graphite structure; The dispersing agent in step 1 is selected from polyvinylpyrrolidone or polyethylene glycol, and the addition amount is 0.2-1.0 wt%. After adding the dispersing agent, the artificial graphite particles and natural graphite particles are mixed by high-speed shearing or double-helix mixing, the rotation speed is 800 rpm, and the mixing time is 30 min; The spheroidization treatment in step 2 is carried out using a high-temperature rotating spheroidization furnace or a fluidized bed plasma spraying device, the treatment time is 10 seconds, and after the treatment is completed, cooling is carried out at a rate of 200°C / min to avoid collapse of the microporous structure; The spray drying in step 3 uses a double-fluid spray device, the feed temperature is 200°C, the discharge temperature is 100°C, the atomization pressure is 0.5 MPa, and the expansion hole gas flow rate is 2.0 L / min.

2. The process for preparing a spherical graphite with optimized specific surface area for lithium battery anode material as claimed in claim 1, wherein, The expansion hole gas in step 3 is carbon dioxide or ammonia, and the inlet position is the inlet section of the spray drying tower, so that the formed pore size is mainly distributed in the micropore range of 0.5-2 nm and the mesopore range of 2-15 nm.

3. The process for preparing a spherical graphite with optimized specific surface area for lithium battery anode material as claimed in claim 2 wherein, The conductive polymer solution in step 4 is a PEDOT:PSS aqueous solution or a PVDF-g-PEG block copolymer ethanol solution, the concentration of the conductive polymer solution is 0.2-0.5 wt%, the volume ratio of the spherical particles is 1:10, the stirring time is 30 min, and the stirring speed is 500 rpm.

4. The process for preparing a spherical graphite with optimized specific surface area for a lithium battery anode material as claimed in claim 3, wherein, The conductive polymer coating in step 4 is formed by a low-speed ball milling and vacuum drying cooperative method, the ball milling speed is 100-300 rpm, the time is 30 minutes, the vacuum drying temperature is 60°C, and the time is 6-12 hours.

5. The process for preparing the spherical graphite with optimized specific surface area for lithium battery anode material as claimed in claim 1 wherein, The furnace type used in the heat treatment in Step 5 is a tube furnace or a static hot air furnace, the atmosphere is nitrogen or argon, the heating rate is 2-5°C / min, and the holding time is 2-4 hours. The spherical graphite specific surface area optimized lithium battery negative electrode material is prepared by the method according to any one of claims 1-5.

6. A lithium battery anode material, characterized in that, The negative electrode material is prepared into a lithium battery negative electrode plate and arranged in a lithium battery.

7. Use of the negative electrode material according to claim 6 in a lithium battery, characterized in that, ​

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