A porous graphite material for sodium-ion battery negative electrode and a preparation method and application thereof

Abstract

The application discloses a kind of porous graphite materials for sodium-ion battery negative electrode and its preparation method and application, belong to waste lithium-ion battery recycling and battery electrode material preparation technical field.The method includes: waste graphite slag roasting pretreatment;With the composite activator of potassium hydroxide and potassium carbonate is mixed;Heating to 700~900 ℃ under inert protective atmosphere and constant temperature incubation, utilize the residual transition metal in waste graphite slag in-situ catalysis pore-making;After cooling, water washing, acid washing purification and drying, obtain the porous graphite material for sodium-ion battery negative electrode.The application utilizes harmful transition metal impurities in-situ as pore-making catalyst, realizes purification, repair and pore-making integration in the same heat treatment process, low in cost, product has connected micropore-mesoporous pore channel structure and excellent sodium storage performance, suitable for sodium-ion battery negative electrode.

Description

Technical Field

[0001] This invention belongs to the technical field of waste lithium-ion battery recycling and battery electrode material preparation, specifically relating to a porous graphite material for sodium-ion battery anodes, its preparation method, and its application. Background Technology

[0002] With the explosive growth of the global electric vehicle and energy storage industries, the retirement of lithium-ion batteries has arrived. Currently, the recycling of waste lithium-ion batteries mainly focuses on the high-value cathode materials (such as lithium, cobalt, nickel, and manganese), while the anode graphite materials, which account for about 15% to 20% of the battery mass, are often landfilled or incinerated as waste, causing great waste of resources and environmental pressure.

[0003] Waste graphite slag obtained from the recycling of waste lithium-ion batteries often has the following defects, which limit its direct reuse: First, the impurity content is high. The surface of the waste graphite slag is covered with broken conductive carbon black, residual polyvinylidene fluoride binder, electrolyte decomposition products, and residual trace amounts of transition metals and aluminum, etc. Second, the structure is damaged. Long-term charge and discharge cycles cause graphite sheets to peel off and collapse, increasing defects. When the waste graphite slag is used directly as the negative electrode material of lithium-ion batteries or sodium-ion batteries, the initial efficiency and cycle life are extremely poor.

[0004] Existing technologies include methods such as refining and purifying recycled waste graphite, using it as a seed for induced growth, mixing it with biomass carbon precursors, calcining and carbonizing it, and then removing the graphite to obtain hard carbon anode materials. However, this method uses waste graphite as a sacrificial template, failing to preserve the graphite skeleton and achieve high-value utilization. Another method involves using a sodium-potassium alloy to etch and create pores in failed graphite at room temperature. This method requires the use of reactive metal alloys, posing safety risks and having a narrow process window. A third method uses steam annealing combined with water immersion and ball milling to treat waste graphite, but its pore-forming efficiency and pore structure controllability are limited. Directly repairing waste graphite slag using acid washing followed by high-temperature graphitization can even exceed the cost of purchasing new graphite.

[0005] Therefore, developing a low-cost, high-value-added technology for reusing waste graphite slag, and converting waste graphite slag into high-performance porous graphite materials for sodium-ion battery anodes, is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] The technical problem this invention aims to solve is to address the shortcomings of existing technologies, such as high impurity content, structural damage, poor direct reuse performance, and high repair costs of waste graphite slag. This invention provides a method for preparing porous graphite materials for sodium-ion battery anodes obtained from recycled waste lithium-ion batteries. This method synergistically achieves purification, structural repair, and directional pore formation of waste graphite slag within the same heat treatment process. Furthermore, this invention also provides the porous graphite materials for sodium-ion battery anodes prepared by this method and their application in the preparation of sodium-ion battery anodes.

[0007] To address the aforementioned technical problems, this invention provides a method for preparing porous graphite material for sodium-ion battery anodes, comprising the following steps: (1) Pretreatment: The waste graphite slag is roasted to remove the residual organic impurities in the waste graphite slag, and pretreated graphite slag is obtained. (2) Mixing: The pretreated graphite slag is mixed evenly with the composite activator and dried to obtain a mixture; the composite activator is composed of potassium hydroxide and potassium carbonate; (3) In-situ autocatalytic pore formation: Under an inert protective atmosphere, the mixture is heated to 700~900℃ and kept at a constant temperature. The residual transition metal in the waste graphite slag is used to catalyze the gasification reaction between the carbon matrix of the pretreated graphite slag and the composite activator in situ, forming pores inside the pretreated graphite slag to obtain the activated product. (4) Acid washing and purification: After cooling the activated product, wash it with water and then soak it in acid solution to remove the transition metal and other metal impurities in the activated product. Then wash it with water until neutral and dry it to obtain the porous graphite material for sodium-ion battery negative electrode.

[0008] The inventive concept and reaction principle of the above technical solution are as follows: This invention turns waste into treasure, cleverly utilizing the trace amounts of transition metals and coated amorphous carbon impurities remaining in waste graphite slag to introduce an environmentally friendly potassium hydroxide-potassium carbonate composite pore-forming system. At a high temperature of 700~900℃, the residual transition metal nanoparticles come into contact with the carbon matrix, catalyzing the redox gasification reaction between carbon and potassium salt. The gasification reaction preferentially occurs around the transition metal nanoparticles. As the reaction proceeds, the transition metal nanoparticles migrate inside the graphite, leaving interconnected channels along their migration path, thus generating a catalytic tunneling effect and achieving in-situ self-catalytic pore-forming. Meanwhile, the composite activator composed of potassium hydroxide and potassium carbonate provides a stepped gasification reaction rate: potassium hydroxide first reacts with carbon in the low-temperature region of 400~600℃ to form micropores, while potassium carbonate catalytically etches and expands the pores in the high-temperature region of 700~900℃, forming a connected micropore-mesopore structure. Furthermore, compared with using potassium hydroxide alone, the amount of strong alkali required is reduced by approximately 50%, greatly protecting the macroscopic conductive framework of graphite. The typical gasification reaction occurring at high temperatures is as follows: 6KOH + 2C → 2K + 3H2↑ + 2K2CO3 (Potassium hydroxide first creates micropores in the low-temperature region); K2CO3 + 2C → 2K + 3CO↑ (Potassium carbonate catalyzes etching and expands pores in the high-temperature zone).

[0009] In the above reaction formula, KOH represents potassium hydroxide, K2CO3 represents potassium carbonate, C represents carbon in the pretreated graphite slag, K represents metallic potassium, H2 represents hydrogen, CO represents carbon monoxide, ↑ indicates that the product escapes in gaseous form, the number in the lower right corner of the element symbol indicates the number of atoms of that element in the corresponding chemical formula, and the number in front of the chemical formula indicates the stoichiometric coefficient of the reaction. Potassium hydroxide reacts with carbon in the low temperature zone to produce metallic potassium vapor, hydrogen and potassium carbonate, and the escape of the gaseous products leaves micropores in the carbon matrix; the potassium carbonate produced in the reaction combines with the added potassium carbonate and continues to react with carbon in the high temperature zone to produce metallic potassium vapor and carbon monoxide, and the escape of the gaseous products achieves pore expansion; metallic potassium vapor is discharged from the system with an inert protective atmosphere, and the residual potassium salt is dissolved and recovered in the water washing in step (4).

[0010] While creating pores at high temperatures, vaporization removes residual polyvinylidene fluoride binder and electrolyte byproducts, and the layered lattice of graphite is partially repaired at high temperatures; acid washing converts harmful transition metals into easily washable metal salts for removal, thus achieving integrated synergy of purification, repair, and pore creation.

[0011] Preferably, the waste graphite slag is the graphite slag obtained from the recycling and sorting of the negative electrode of the waste lithium-ion battery, and the transition metal remaining in the waste graphite slag is one or more of iron, nickel, and copper. The aforementioned transition metals naturally remain in the graphite slag recovered from the negative electrode of the waste lithium-ion battery. This invention directly utilizes these metals as an in-situ catalyst, eliminating the need for the addition of expensive catalytic metals, which is one of the key factors in reducing the process cost of this invention.

[0012] Preferably, in step (1), the calcination is carried out in an air atmosphere, the calcination temperature is 200~400℃, and the calcination time is 1~4h; more preferably, the calcination temperature is 300℃, and the calcination time is 2h. Light calcination can remove residual organic electrolyte and organic additives without oxidizing the graphite matrix, preventing them from carbonizing, coking, and clogging the pores during subsequent high-temperature heat treatment.

[0013] Preferably, in the composite activator, the mass ratio of potassium hydroxide to potassium carbonate is 1:1 to 1:3; more preferably, the mass ratio of potassium hydroxide to potassium carbonate is 1:2. At this ratio, the reaction rates for micropore formation in the low-temperature zone and pore expansion in the high-temperature zone are optimally matched, resulting in the most developed micropore-mesopore structure. Furthermore, the proportion of the strong alkali potassium hydroxide is minimized, significantly reducing corrosion to the equipment.

[0014] Preferably, in step (2), the mass ratio of the pretreated graphite slag to the composite activator is 1:0.5 to 1:1; more preferably, the mass ratio of the pretreated graphite slag to the composite activator is 1:0.8. The mixing method is wet ball milling with anhydrous ethanol as the dispersion medium. At this material-to-agent ratio, the composite activator can fully penetrate the graphite flakes without excessively etching the graphite skeleton; wet ball milling with anhydrous ethanol as the dispersion medium ensures that the composite activator is uniformly dispersed on the surface of the pretreated graphite slag, avoiding dust and agglomeration caused by dry mixing, and the anhydrous ethanol completely evaporates during the drying process, without introducing new impurities.

[0015] Preferably, in step (3), the heating rate is 5~10℃ / min, more preferably 8℃ / min; the isothermal holding time is 1~5h, more preferably 3h; and the inert protective atmosphere is argon. The slow heating rate allows potassium hydroxide and potassium carbonate to be activated sequentially within their respective temperature windows, achieving a hierarchical construction of micropores and mesopores; the chemical properties of argon are stable, which can prevent the carbon matrix from being oxidized and ablated at high temperatures.

[0016] Preferably, in step (4), the acid solution is a hydrochloric acid solution with a concentration of 1-3 mol / L, more preferably 2 mol / L; the soaking and washing temperature is 50-70℃, more preferably 60℃; and the soaking and washing time is 2-6 h, more preferably 4 h. The hot hydrochloric acid solution can completely dissolve and remove transition metal salts and aluminum and copper impurities, and the potassium salt recovered from the water washing can be recycled, further reducing process costs.

[0017] This invention also provides a porous graphite material for sodium-ion battery anodes, which is prepared by the above method and has a connected micropore-mesopore channel structure. Testing shows that the specific surface area of ​​the porous graphite material for sodium-ion battery anodes can reach 210 m² / g, the purity can reach 99.8%, and the conductivity can reach 60~90 S / cm (Siemens per centimeter).

[0018] This invention also provides the application of the aforementioned porous graphite material for sodium-ion battery anodes in the preparation of sodium-ion battery anodes. The unique catalytic tunnel pores in the porous graphite material for sodium-ion battery anodes facilitate the rapid insertion and extraction of sodium ions. When this porous graphite material is used as a sodium-ion battery anode, the sodium storage specific capacity reaches over 265 mAh / g at a current density of 0.1 A / g, and the capacity retention rate reaches 92.4% after 2000 cycles at a high current density of 5 A / g.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: (1) Turning waste into treasure and low cost: This invention uses inexpensive waste graphite slag as raw material and cleverly utilizes the harmful transition metal impurities in it as an in-situ pore-forming catalyst. No additional expensive catalytic metals are needed, thus achieving deep recycling of resources and low cost.

[0020] (2) Purification, pore formation and repair combined: While forming pores, high-temperature gasification and acid washing thoroughly remove impurities from waste graphite slag, and the layered lattice of graphite is partially repaired at high temperature, restoring high conductivity with a conductivity of 60~90S / cm.

[0021] (3) Excellent sodium storage electrochemical performance: The unique catalytic tunnel pores formed by composite pore formation are conducive to the rapid insertion and extraction of sodium ions. The sodium storage specific capacity reaches more than 265 mAh / g at 0.1 A / g, and the capacity retention rate reaches 92.4% after 2000 cycles.

[0022] (4) The amount of alkali used is greatly reduced: the composite activator reduces the proportion of strong alkali potassium hydroxide, and the amount of alkali used is reduced by about 50% compared with potassium hydroxide alone, which greatly reduces the corrosion of equipment and is in line with the development direction of green chemical industry. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort, wherein: Figure 1 This is a schematic diagram of the process flow for preparing porous graphite material for sodium-ion battery anodes according to the present invention. Figure 2 This is a schematic diagram of the in-situ autocatalytic pore-forming mechanism of the present invention.

[0024] Explanation of reference numerals in the attached figures: 1. Graphite sheets; 2. Transition metal nanoparticles; 3. Catalytic tunnel pores; 4. Micropores; 5. Mesopores. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; the scope of protection of this invention is defined by the claims.

[0026] like Figure 1 As shown, the preparation method of porous graphite material for sodium-ion battery anode of the present invention includes, in sequence, pretreatment S1, mixing S2, in-situ autocatalytic pore formation S3, and acid washing and purification S4: waste lithium-ion batteries are recycled and sorted to obtain waste graphite slag; the waste graphite slag is calcined by pretreatment S1 to obtain pretreated graphite slag; the pretreated graphite slag is mixed with a composite activator composed of potassium hydroxide and potassium carbonate by wet ball milling in mixing S2 and dried to obtain a mixture; the mixture is subjected to in-situ autocatalytic pore formation S3 and heat-treated at 700~900℃ under an inert protective atmosphere to obtain an activated product; the activated product is cooled by acid washing and purification S4, washed with water to recover potassium salt, soaked and washed with acid solution, washed with water until neutral and dried to obtain the porous graphite material for sodium-ion battery anode.

[0027] Regarding pretreatment S1: The waste graphite slag originates from the graphite slag obtained by recycling and sorting the negative electrode of the waste lithium-ion battery. Its surface is coated with broken conductive carbon black, residual polyvinylidene fluoride binder, and electrolyte decomposition products, and contains trace amounts of transition metals (one or more of iron, nickel, and copper) and aluminum impurities. The calcination is carried out in an air atmosphere at a temperature of 200-400℃ for 1-4 hours, preferably at 300℃ for 2 hours. This light calcination removes residual organic electrolyte and organic additives without oxidizing the graphite matrix, preventing organic residues from carbonizing and coking during the subsequent high-temperature heat treatment at 700-900℃, thus blocking the formed pores and ensuring the smooth progress of the subsequent pore-forming reaction.

[0028] Regarding mixing S2: First, the content of metallic impurities in the waste graphite slag is determined (e.g., by inductively coupled plasma atomic emission spectrometry). Then, a composite activator is prepared and mixed with the pretreated graphite slag. The composite activator consists of potassium hydroxide and potassium carbonate, with a mass ratio of potassium hydroxide to potassium carbonate of 1:1 to 1:3, preferably 1:2; the mass ratio of the pretreated graphite slag to the composite activator is 1:0.5 to 1:1, preferably 1:0.8; the mixing method is wet ball milling with anhydrous ethanol as the dispersion medium, for example, 2 hours, followed by drying (e.g., drying at 80°C) to obtain the mixture. In this way, the combination of potassium hydroxide and potassium carbonate provides a stepped gasification reaction rate, reducing the amount of strong alkali by about 50% while constructing a well-developed pore structure, thus protecting the macroscopic conductive framework of graphite; wet ball milling ensures that the composite activator is uniformly dispersed on the surface of the pretreated graphite slag, avoiding dust and agglomeration as in dry mixing; and the anhydrous ethanol completely evaporates during the drying process, without introducing new impurities.

[0029] Regarding in-situ autocatalytic pore formation S3: The mixture is placed in a tube furnace under an inert protective atmosphere (preferably argon), heated to 700-900°C at a heating rate of 5-10°C / min (preferably 8°C / min), and held at that temperature for 1-5 hours (preferably 850°C for 3 hours). Figure 2 As shown, transition metal nanoparticles 2 embedded in graphite layers 1 catalyze the gasification reaction between the carbon matrix of the pretreated graphite slag and the composite activator in situ. The gasification reaction preferentially occurs at the interface between the transition metal nanoparticles 2 and the carbon matrix. As the reaction proceeds, the transition metal nanoparticles 2 migrate along the interlayer of graphite layers 1, leaving catalytic tunnel pores 3 penetrating the graphite layers 1 along their migration path. Simultaneously, potassium hydroxide undergoes a gasification reaction with carbon in a low-temperature region of 400-600℃, forming micropores 4 on the graphite layers 1. Potassium carbonate then catalytically etches and expands the pores in a high-temperature region of 700-900℃, forming mesopores 5 with larger pore sizes. The micropores 4, mesopores 5, and catalytic tunnel pores 3 are interconnected, forming a connected micropore-mesopore channel structure. In this way, directional pore formation can be achieved without the addition of additional catalytic metal, and the gasification removal of organic impurities and partial repair of the graphite layered lattice can be completed simultaneously in the same heat treatment process.

[0030] Regarding acid washing and purification S4: After cooling the activated product, wash it with water and filter it to recover the potassium salt in the filtrate; then soak and wash the solid product with an acid solution, preferably hydrochloric acid solution, with a concentration of 1~3 mol / L (preferably 2 mol / L), at a temperature of 50~70℃ (preferably 60℃), for a time of 2~6 h (preferably 4 h); finally, wash with deionized water until neutral and dry (e.g., dry at 110℃ for 12 h) to obtain the porous graphite material for the sodium-ion battery negative electrode. In this way, the hot hydrochloric acid solution completely removes transition metals and other metal impurities by converting them into soluble metal salts, achieving a product purity of up to 99.8%; the potassium salt recovered from water washing can be recycled, further reducing process costs.

[0031] The present invention will now be described in detail with reference to specific embodiments.

[0032] Example 1 100g of waste graphite slag from the sorting of waste lithium-ion batteries was calcined in air at 300℃ for 2 hours to remove organic residues, yielding pretreated graphite slag. Inductively coupled plasma atomic emission spectrometry (ICP-AES) analysis showed that the residual transition metals (iron, nickel, and copper) in the waste graphite slag comprised 0.5% by mass. 20g of the pretreated graphite slag was mixed with 4g of potassium hydroxide and 12g of potassium carbonate (the mass ratio of potassium hydroxide to potassium carbonate was 1:3, within the range of 1:1 to 1:3). 30mL of anhydrous ethanol was added, and the mixture was ball-milled for 2 hours. The mixture was then dried at 80℃ to obtain a final product. Under argon protection, the final product was heated to 850℃ at a rate of 8℃ / min and held at this temperature for 3 hours to perform in-situ autocatalytic pore formation, yielding an activated product. The activated product was cooled to room temperature, washed with water and filtered to recover the potassium salt; the solid product was transferred to 200 mL of 2 mol / L hydrochloric acid solution and stirred at 60 °C for 4 h to remove transition metal impurities; filtered, washed with deionized water until the pH value was 7, and dried at 110 °C for 12 h to obtain porous graphite material for sodium-ion battery anode.

[0033] The characterization and electrochemical performance test results of the porous graphite material for sodium-ion battery anode obtained in Example 1 are as follows: Based on the nitrogen adsorption-desorption isotherm and measured using the BET method (i.e., the Brunauer-Emmett-Teller multilayer molecular adsorption model), the specific surface area is 210 m² / g, the purity reaches 99.8%, and the conductivity is 60~90 S / cm. Electrochemical tests were conducted using the porous graphite material as the sodium-ion battery anode: at a current density of 0.1 A / g, the initial discharge specific capacity was 272 mAh / g, and the initial coulombic efficiency was 80.2%; at a high current density of 5 A / g, the specific capacity still reached 152 mAh / g, and the capacity retention rate after 2000 cycles was 92.4%.

[0034] The test results of Example 1 show that the present invention, through the synergistic effect of in-situ catalysis of residual transition metals and step-by-step pore formation by potassium hydroxide-potassium carbonate composite activator, constructs a connected microporous-mesoporous channel structure while preserving and repairing the graphite conductive framework. The unique catalytic tunnel pores facilitate the rapid insertion and extraction of sodium ions, enabling the porous graphite material for the sodium-ion battery anode to possess high specific capacity, high initial coulombic efficiency, excellent rate performance, and ultra-long cycle life, thus realizing the low-cost and high-value utilization of waste graphite slag.

[0035] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing porous graphite material for sodium-ion battery negative electrode, characterized in that, Includes the following steps: (1) Pretreatment: The waste graphite slag is roasted to remove the residual organic impurities in the waste graphite slag, and pretreated graphite slag is obtained. (2) Mixing: The pretreated graphite slag is mixed evenly with the composite activator and dried to obtain a mixture; the composite activator is composed of potassium hydroxide and potassium carbonate; (3) In-situ autocatalytic pore formation: Under an inert protective atmosphere, the mixture is heated to 700~900℃ and kept at a constant temperature. The residual transition metal in the waste graphite slag is used to catalyze the gasification reaction between the carbon matrix of the pretreated graphite slag and the composite activator in situ, forming pores inside the pretreated graphite slag to obtain the activated product. (4) Acid washing and purification: After cooling the activated product, wash it with water and then soak it in acid solution to remove the transition metal and other metal impurities in the activated product. Then wash it with water until neutral and dry it to obtain the porous graphite material for sodium-ion battery negative electrode.

2. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1, characterized in that, The waste graphite slag is the graphite slag obtained by recycling and sorting the negative electrode of the waste lithium-ion battery, and the transition metal remaining in the waste graphite slag is one or more of iron, nickel and copper.

3. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1 or 2, characterized in that, In step (1), the roasting is carried out in an air atmosphere, the roasting temperature is 200~400℃, and the roasting time is 1~4h.

4. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1 or 2, characterized in that, In the composite activator, the mass ratio of potassium hydroxide to potassium carbonate is 1:1 to 1:

3.

5. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 4, characterized in that, The mass ratio of potassium hydroxide to potassium carbonate is 1:

2.

6. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1 or 2, characterized in that, In step (2), the mass ratio of the pretreated graphite slag to the composite activator is 1:0.5 to 1:1; the mixing method is wet ball milling with anhydrous ethanol as the dispersion medium.

7. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1 or 2, characterized in that, In step (3), the heating rate is 5~10℃ / min, the constant temperature holding time is 1~5h, and the inert protective atmosphere is argon.

8. The method for preparing porous graphite material for sodium-ion battery negative electrode according to claim 1 or 2, characterized in that, In step (4), the acid solution is a hydrochloric acid solution with a concentration of 1~3 mol / L, the soaking and washing temperature is 50~70℃, and the soaking and washing time is 2~6h.

9. A porous graphite material for the negative electrode of a sodium-ion battery, characterized in that, The porous graphite material for the sodium-ion battery anode is prepared by the method for preparing the porous graphite material for the sodium-ion battery anode according to claim 1, and the porous graphite material for the sodium-ion battery anode has a connected micropore-mesopore channel structure.

10. The application of the porous graphite material for sodium-ion battery anode according to claim 9 in the preparation of sodium-ion battery anode.

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