Recycling method and application of waste negative plate

Abstract

The invention discloses a waste negative electrode plate recycling method and a lithium ion battery, and the recycling method comprises the following steps: soaking a waste negative electrode plate in a microbial solution which comprises a gram negative inorganic autotrophic bioleaching strain, adjusting the concentration of the strain to maintain the pH value of the microbial solution at 0.1-1, and carrying out solid-liquid separation to obtain the waste negative electrode plate. A solid-liquid mixture is obtained; performing primary centrifugal treatment on the solid-liquid mixture, and separating to obtain solid-phase graphite; and carrying out vacuum baking on the solid-phase graphite, and then carrying out high-temperature calcination to obtain regenerated graphite. According to the method, the high corrosivity risk of traditional cleaning is avoided, and the crystallinity and cycling stability of graphite are improved; the process cost is reduced through strain circulation; the performance of the regenerated graphite is close to that of commercial graphite, so that high-value recycling of the waste negative plate is realized, and resource waste and environmental pollution are reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of lithium ion batteries, in particular to a recycling method of waste negative electrode sheet and application thereof. BACKGROUND

[0002] Cost reduction and efficiency improvement have become the core strategic objectives of battery enterprises. In the R&D end, the cost reduction strategy mainly focuses on three dimensions of material cost reduction, design cost reduction and technology cost reduction, among which improving material utilization is the key path of technology cost reduction. As the dominant negative electrode material of lithium ion batteries, graphite has long been overlooked due to its relatively low cost and wide source, and is often used for positive electrode material reduction or direct incineration. However, with the exponential growth of graphite demand, its strategic value is increasingly prominent - several countries have listed it as a strategic mineral resource, and the market price has continued to rise. Under this background, the recycling of waste graphite in negative electrode sheet shows multiple values: not only can it significantly reduce the production cost of battery-grade graphite and reduce environmental pollution, but more importantly, this initiative has far-reaching strategic significance for promoting the sustainable development of the lithium ion battery industry. SUMMARY

[0003] In order to overcome the defects in the prior art, the purpose of the present application is to provide a recycling method of waste negative electrode sheet and application thereof.

[0004] To achieve the above purpose, the technical scheme adopted by the present application is:

[0005] In a first aspect, a recycling method of waste negative electrode sheet, comprising the following steps:

[0006] Soaking the waste negative electrode sheet with a microbial solution, wherein the microbial solution comprises a gram-negative inorganic autotrophic bioleaching strain, and the pH value of the microbial solution is maintained at 0.1-1 by adjusting the strain concentration to obtain a solid-liquid mixture;

[0007] Performing a first centrifugal treatment on the solid-liquid mixture to separate and obtain solid-phase graphite;

[0008] After vacuum baking the solid-phase graphite, performing high-temperature calcination to obtain regenerated graphite.

[0009] By utilizing the metabolic activity of the gram-negative inorganic autotrophic bioleaching strains in the microbial solution, on one hand, the trace metal impurities (such as iron, sulfur, etc.) in the waste negative plate are removed, and on the other hand, a biofilm composed of carboxyl groups is formed on the surface of the graphite, covering the cracks and debris on the surface of the graphite, thereby protecting the crystal structure in advance and repairing the surface defects. The concentration of the strain is adjusted to maintain the pH at 0.1-1 (strong acidic environment), because the metabolic activity of the strain is best under strong acidic conditions, which can not only dissolve the trace metal impurities in the negative plate, but also promote the strain to secrete extracellular polymers on the surface of the graphite to form a biofilm and repair the surface defects.

[0010] It should be noted that the gram-negative inorganic autotrophic bioleaching strains used in the present application include Thiobacillus ferrooxidans, Leptospirillum ferriphilum or Thiobacillus thiooxidans, which are known strains that can be obtained by the public through conventional microbial preservation agencies.

[0011] The biofilm formed by the gram-negative inorganic autotrophic bioleaching strains in the microbial solution on the surface of the graphite is mainly composed of extracellular polymers (EPS), which specifically include: extracellular polysaccharides (EPS-P, which are net matrixes formed by monosaccharides such as glucose, mannose and galactose through glycosidic bonds, providing mechanical strength), extracellular proteins (EPS-Pr, which contain adhesion proteins and metabolic enzymes, mediating initial adhesion and participating in metal impurity dissolution), extracellular DNA (eDNA, which forms hydrogen bonds with the hydroxyl and carboxyl groups on the surface of the graphite through phosphate groups to enhance the binding force) and lipids (EPS-L, which are mainly lipopolysaccharides, stabilizing the structure of the biofilm).

[0012] The formation of the biofilm needs to go through four stages, including an initial adhesion stage, a microcolony formation stage, a biofilm maturation stage and a functional enhancement stage. In the initial adhesion stage, the strain is irreversibly adsorbed to the hydrophobic region of the graphite through the hydrophobic effect of the fimbriae, flagella and adhesion proteins. In the microcolony formation stage, the strain secretes extracellular polysaccharides to construct a net matrix, fix cells and capture solution ions to form microcolonies. In the biofilm maturation stage, the microcolonies fuse to form a stable structure, and the internal pore network maintains metabolic activity. In the functional enhancement stage, the quorum sensing regulates gene expression, up-regulates metabolic enzyme secretion, and at the same time, the carboxyl groups of the EPS coordinate with the graphite cracks / debris, fill the defects and form a continuous protective layer.

[0013] The cell wall structure of the gram-negative inorganic autotrophic bioleaching strain contains a lipopolysaccharide layer, is more suitable for a strong acidic environment (pH 0.1-1), and can efficiently oxidize inorganic sulfides to produce sulfuric acid to maintain the acidity of the solution. The strain uses ferrous ions or sulfur compounds as energy sources through chemoautotrophic metabolism, does not require additional carbon sources, is suitable for processing waste negative plates containing metal impurities, and can reduce process costs. If a gram-positive strain is used, the peptidoglycan layer of the cell wall is easily damaged in a strong acid environment, resulting in inactivation of the strain; if an organic heterotrophic bacterium is used, a carbon source needs to be added, which may introduce organic impurities into the graphite.

[0014] The purpose of centrifugal treatment is to separate the microbial solution from the graphite to ensure that the subsequent heat treatment is only for the graphite. Vacuum baking can remove residual moisture and soluble impurities on the surface of the graphite, avoiding damage to the graphite structure due to water evaporation during high-temperature calcination; high-temperature calcination promotes the migration of carbon atoms from disorder to order through thermal decomposition of the biofilm, forming a dense amorphous carbon layer and a carbon core-shell structure, and significantly improving the crystallinity and graphitization degree of the graphite.

[0015] Optionally, the strain includes at least one of Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum, or Acidithiobacillus thiooxidans. In actual use, the strain can be flexibly adjusted according to the situation. For Acidithiobacillus ferrooxidans, it can efficiently oxidize Fe 2+ to Fe 3+ , promoting the dissolution of metal impurities such as iron. For Leptospirillum ferriphilum, it has strong affinity for iron ions, which can further enrich and remove residual iron impurities. For Acidithiobacillus thiooxidans, it can oxidize elemental sulfur to sulfate, removing sulfur impurities on the surface of the graphite.

[0016] Optionally, the concentration of the strain is 4.2×10 8 -9×10 8 cells per milliliter. The selection of the concentration of the strain directly affects the pH regulation accuracy and the quality of the biofilm formation, such as the thickness of the biofilm formation and the repair effect on the surface of the graphite. When the concentration of the strain is 4.2×10 8 -9×10 8 cells per milliliter, the thickness of the biofilm is moderate, which removes impurities and protects the graphite crystal structure, and does not cause incomplete decomposition of the biofilm during calcination due to excessive thickness, and the graphitization degree is optimal.

[0017] If the concentration of the strain is less than 4.2×10 8 cells per milliliter, the biofilm coverage is incomplete, and cracks and debris are left on the surface of the graphite, and the crystallinity is limited after calcination. If the concentration is higher than 9×10 8 cells per milliliter, the biofilm is too thick, and part of the biofilm reacts with the graphite during calcination, causing ablation and forming pores, reducing lithium ion insertion sites, and reducing capacity and cycle performance.

[0018] Optionally, the centrifugal force of the first centrifugation is 30000-32000g, the temperature is 4-5℃, and the time is 5-10 minutes.

[0019] A high centrifugal force of 30000-32000g is used to quickly precipitate the solid-phase graphite with a large density and separate the metal sulfide impurities with a small density from the supernatant. If the centrifugal force is lower than 30000g, the solid-phase graphite is not completely precipitated, the recovery rate is reduced, the separation of the graphite and the microbial solution is not complete, and the metabolic substances of the residual strains during the subsequent heat treatment are carbonized to form impurities, thereby reducing the purity of the graphite. If the centrifugal force is higher than 32000g, the solid-phase graphite may be broken, thereby increasing the surface defects.

[0020] The centrifugation temperature is set to a low temperature of 4-5℃ to inhibit the metabolic activity of the strains and avoid the decomposition of the graphite during the centrifugation process, thereby affecting the subsequent heat treatment. If the temperature is too high (e.g., >5℃), the activity of the strains is enhanced, the biological membrane on the surface of the graphite may be continuously decomposed, and the pretreatment effect is damaged.

[0021] The centrifugation time is preferably set to 5-10 minutes. If the time is too short, the solid-liquid separation is not complete, and if the time is too long, the energy consumption is increased, or part of the strains are broken, and the intracellular substances (e.g., proteins) released are attached to the surface of the graphite, thereby forming a residual carbon layer after calcination, thereby affecting the graphitization degree.

[0022] Optionally, the temperature of the vacuum baking is 65℃, and the time is 6 hours. The purpose of the vacuum baking is to remove the residual water and soluble inorganic salts (e.g., sulfate salts produced by the metabolism of the microorganisms) in the solid-phase graphite. The reason for selecting 65℃ is that this temperature can quickly evaporate the water (the boiling point of water is reduced in a vacuum environment) and avoid the decomposition of the biological membrane on the surface of the graphite due to the high temperature (the main component of the biological membrane is an organic polymer, which is easily decomposed at a high temperature). The baking time of 6 hours can ensure that the water content of the graphite is <0.5%, thereby creating conditions for the subsequent high-temperature calcination. If the temperature is too low (e.g., <65℃), the water evaporation is slow, the baking time needs to be extended, and the process efficiency is reduced. If the temperature is too high (e.g., >65℃), the biological membrane may be partially decomposed, the complete carbon core-shell structure cannot be formed during the calcination, and the cycle stability is affected. If the time is insufficient (e.g., <6 hours), the residual water in the graphite is evaporated to generate bubbles during the calcination, thereby damaging the crystal structure of the graphite and reducing the capacity.

[0023] Optionally, the temperature of the high-temperature calcination is 1800-3000℃, such as 1800℃, 2000℃, 2200℃, 2400℃, 2600℃, 2800℃, 3000℃, or any value between 1800-3000℃. Preferably, the temperature of the high-temperature calcination is 2500℃.

[0024] High-temperature calcination is a key step in the regeneration of graphite, which includes the following functions: decomposing the biofilm on the surface of graphite (forming amorphous carbon layer), promoting the rearrangement of carbon atoms (increasing crystallinity), and removing residual impurities (such as sulfur and nitrogen).

[0025] The calcination temperature is controlled within the range of 1800-3000℃. At this temperature, the biofilm (mainly composed of proteins and polysaccharides) is gradually decomposed into amorphous carbon layer, and the carbon atoms in the graphite lattice obtain enough energy for rearrangement, repairing defects. When the temperature is lower than 1800℃, the biofilm is not completely decomposed, and organic impurities remain on the surface of graphite, which leads to insufficient rearrangement of carbon atoms and low crystallinity. When the temperature exceeds 3000℃, the graphite may be over-carbonized, causing irreversible structural damage.

[0026] Optionally, the calcination equipment is a muffle furnace, a tube furnace, or a box furnace. All of these are general high-temperature equipment, which can provide a uniform heating environment, are suitable for different scales of regeneration processes, and can provide inert gas protection to prevent graphite from being oxidized at high temperatures. Among them, the tube furnace is more suitable for batch production due to its good airflow uniformity.

[0027] Optionally, the method further comprises the following steps:

[0028] The supernatant after the first centrifugation is subjected to at least one centrifugal purification to recover the strain and extracellular polymeric substances;

[0029] The recovered strain and extracellular polymeric substances are resuspended in a solution, and the pH value of the solution is adjusted to 0.1-1 for recycling for soaking the waste negative plate.

[0030] The supernatant contains unreacted strains and extracellular polymeric substances (EPS). After recovery by centrifugal purification, they can be recycled, which can significantly reduce the cost of the microbial solution. The extracellular polymeric substances refer to the high-molecular polymers actively secreted by the Gram-negative inorganic autotrophic bioleaching strain outside the cell, mainly composed of polysaccharides (such as β-glucan), proteins (such as adhesion proteins), and a small amount of nucleic acids (such as extracellular DNA). The EPS can tightly adhere to the surface of the strain to form a "strain cell-EPS complex", or part of it can be dispersed in the solution in a free state.

[0031] Adjusting the pH to 0.1-1 is to maintain the active environment of the strain and ensure the stability of the metabolic capacity of the microorganism during recycling. If the strain is not recovered, it needs to be inoculated every time, which increases the cost. If the pH is not adjusted properly, the activity of the strain will decrease, the soaking effect will decrease, and additional strains need to be added to maintain the concentration, which increases the cost.

[0032] Optionally, the centrifugal purification comprises the following steps:

[0033] The supernatant after the first centrifugation is subjected to a second centrifugation, and the precipitate is collected and washed.

[0034] The precipitate after washing is resuspended in ultrapure water, and a third centrifugation is performed to obtain the mixed strains.

[0035] Optionally, the second centrifugation is performed at a centrifugal force of 8000-9000 g, at a temperature of 4-5°C, and for a time of 5-10 minutes.

[0036] The third centrifugation is performed at a centrifugal force of 12000-13000 g, at a temperature of 4-5°C, and for a time of 20-25 minutes.

[0037] After the first centrifugation, the solid-phase graphite is separated by precipitation, and the supernatant still contains microbial components that have not been separated. The microbial components include strains, extracellular polymeric substances, and free metabolites. The free metabolites refer to soluble small-molecule substances produced by gram-negative inorganic autotrophic bioleaching strains (such as Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum, or Acidithiobacillus thiooxidans) during the metabolic process, including but not limited to organic acids (such as sulfuric acid), biological enzymes (such as iron-oxidizing enzymes), metal ion complexes (such as Fe 3+ -EPS complexes), and the like. The metabolites are usually dissolved in the microbial solution in a free state and have a low density, and are difficult to precipitate under a low centrifugal force.

[0038] At this time, the supernatant is subjected to a second separation, and the initial separation is achieved by using the density difference between different components. The “strain cell-EPS complex” with a high density is precipitated due to gravity, and the free metabolites and loosely bound EPS with a low density are retained in the supernatant. By collecting the precipitate, the target microbial components can be initially concentrated.

[0039] A small amount of free metabolites and loose EPS may be retained in the precipitate obtained by the second centrifugation (if such substances enter the subsequent circulating solution, they may cause pH fluctuations or inhibit the activity of the strains). Therefore, the precipitate needs to be washed with ultrapure water, and the number of washing times can be 2-3 times.

[0040] The precipitate after washing is resuspended in ultrapure water, and a third centrifugation is performed. This step further separates by using a higher centrifugal force: the “strain cell-EPS complex” that is tightly bound is precipitated due to a high density, and a small amount of loose EPS or free metabolites (with a lower density) that are not washed off are still retained in the supernatant. The final precipitate obtained is the “mixed strains”. The mixed strains refer to a population of strain cells obtained after centrifugal purification, which mainly contains intact gram-negative inorganic autotrophic bioleaching strain cells, and also contains a small amount of extracellular polymeric substances (EPS) that are tightly bound to the cell surface.

[0041] In a second aspect, a lithium ion battery includes a negative electrode material comprising the regenerated graphite prepared by the method described above.

[0042] In a third aspect, a power utilization device comprises the lithium ion battery described above.

[0043] Due to the technical scheme described above, the present application has the following advantages compared with the prior art:

[0044] 1. Through the metabolic activity of gram-negative inorganic autotrophic bioleaching strains (such as Thiobacillus ferroxidans, etc.) in a strong acidic environment (pH 0.1-1), iron, sulfur and other metal impurities in the negative plate can be simultaneously dissolved, and extracellular polymeric substances (EPS) are secreted to form a biofilm on the surface of graphite, covering cracks and debris, thereby protecting the crystal structure and repairing surface defects in advance; compared with the traditional physical / chemical cleaning method, the high corrosive risk of strong acid and strong base is avoided, and the biofilm is converted into a carbon core-shell structure in subsequent calcination, further improving the crystallinity and cycle stability of graphite.

[0045] 2. The supernatant after centrifugation is subjected to at least one centrifugal purification, and the strain cells and extracellular polymeric substances in the supernatant are recovered and recycled for soaking in a new batch of waste negative plates, so as to reduce the cost of microbial solution, reduce the consumption of strain culture, avoid resource waste and environmental pollution caused by one-time use, realize cost reduction and efficiency increase, and achieve sustainable development. Moreover, the strain is chemotrophic autotrophic, and no additional carbon source is needed, further reducing the process energy consumption and raw material cost.

[0046] 3. By controlling the centrifugal conditions, efficient solid-liquid separation is realized, and graphite breakage and impurity residue are avoided; vacuum baking removes water and soluble impurities, preventing structure damage during high-temperature calcination; high-temperature calcination promotes the decomposition of the biofilm and the rearrangement of carbon atoms, forming a dense amorphous carbon layer and a carbon core-shell structure, significantly improving the crystallinity and graphitization degree of graphite, and the surface carbon core-shell structure can reduce the penetration of electrolyte and reduce the interfacial impedance, so that the performance of the regenerated graphite is close to that of commercial graphite.

[0047] 4. High-value recycling of waste negative plates is realized, and resource waste and environmental pollution caused by direct incineration or low-value reduction treatment of graphite in traditional processes are avoided.

[0048] In order to make the above and other objects, features and advantages of the present application more apparent, the following will describe a preferred embodiment, and the accompanying drawings will be described in detail as follows. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description. Obviously, the drawings in the following description are only some embodiments of the present application, and those skilled in the art can obtain other drawings according to these drawings without creative labor.

[0050] Figure 1 are results of X-ray diffraction (XRD), Raman spectroscopy of the embodiments of the present application. DETAILED DESCRIPTION

[0051] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. If specific conditions are not specified in the embodiments, conventional conditions or manufacturer recommended conditions are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0052] The embodiments of the present application provide a recycling method of waste negative electrode sheet, comprising the following steps:

[0053] S1, microbial solution soaking pretreatment;

[0054] Take the waste lithium ion battery negative electrode sheet (the graphite layer obtained after disassembling and removing the copper foil), and place it in a reaction kettle. A microbial solution with a pH of 0.1-1 is injected into the reaction kettle to soak the waste negative electrode sheet, and a solid-liquid mixture is obtained.

[0055] The microbial solution includes a gram-negative inorganic autotrophic bioleaching strain. By utilizing the metabolic activity of the gram-negative inorganic autotrophic bioleaching strain in the microbial solution, on the one hand, trace metal impurities (such as iron, sulfur, etc.) in the waste negative electrode sheet are removed, and on the other hand, a biofilm composed of carboxyl groups is formed on the surface of the graphite, covering the cracks and debris on the surface of the graphite, thereby protecting the crystal structure in advance and repairing the surface defects.

[0056] In an optional embodiment, the strain includes at least one of Acidithiobacillus ferrooxidans, Leptospirillum ferriphilum or Acidithiobacillus thiooxidans. For Acidithiobacillus ferrooxidans, it can efficiently oxidize Fe 2+ to Fe 3+ , promoting the dissolution of metal impurities (such as iron). For Leptospirillum ferriphilum, it has strong affinity for iron ions, which can further enrich and remove residual iron impurities. For Acidithiobacillus thiooxidans, it can oxidize elemental sulfur to sulfate, removing sulfur impurities on the surface of the graphite.

[0057] In an optional embodiment, the pH value of the microbial solution is achieved by controlling the strain concentration. The strain concentration is 4.2×10 8 -9×10 8 cells per milliliter, such as any value between 4.2×10 8 , 6.5×10 8 , 9×10 8 or 4.2×10 8 -9×10 8 .

[0058] In an alternative embodiment, the soaking time of the waste negative electrode sheet is 24-48 hours, during which the solid-liquid is ensured to be in sufficient contact through stirring.

[0059] S2, first centrifugal separation of the solid-phase graphite;

[0060] The solid-liquid mixture after soaking is transferred to a high-speed centrifuge, and the solid-liquid mixture is subjected to first centrifugal treatment. After centrifugation, the solid-phase graphite with a larger density is precipitated at the bottom of the centrifuge tube, and the supernatant with a smaller density remains in the upper layer.

[0061] In an alternative embodiment, in the centrifugal conditions of the first centrifugation, the centrifugal force is 30000-32000g, for example, 30000g, 31000g, 32000g, or any value between 30000g and 32000g. The centrifugal temperature is 4-5℃, for example, 4℃, 5℃, or any value between 4℃ and 5℃. The centrifugal time is 5-10 minutes, for example, 5 minutes, 6 minutes, 7 minutes, 8 minutes, 9 minutes, 10 minutes, or any value between 5 minutes and 10 minutes.

[0062] S3, vacuum baking to remove impurities;

[0063] The separated solid-phase graphite is transferred to a vacuum oven, and the temperature is set to 65℃ for baking for 6 hours. During the baking process, the water and soluble sulfate salts remaining on the surface of the graphite are fully removed.

[0064] S4, high-temperature calcination to prepare regenerated graphite;

[0065] The baked graphite is placed in a calcination device for high-temperature calcination to obtain regenerated graphite.

[0066] In an alternative embodiment, the calcination device is a muffle furnace, a tube furnace, or a box furnace. The muffle furnace, the tube furnace, or the box furnace are general high-temperature devices that can provide a uniform heating environment, are suitable for different scales of regeneration processes, and can provide inert atmosphere protection to prevent graphite from being oxidized at high temperatures. Among them, the tube furnace is more suitable for batch production due to its good airflow uniformity.

[0067] In an alternative embodiment, the temperature of the high-temperature calcination is 1800-3000℃, for example, 1800℃, 2000℃, 2200℃, 2400℃, 2600℃, 2800℃, 3000℃, or any value between 1800℃ and 3000℃. Preferably, the temperature of the high-temperature calcination is 2500℃.

[0068] During the calcination process, the biological membrane is thermally decomposed into an amorphous carbon layer, and the carbon atoms in the graphite lattice obtain energy to rearrange, repair defects, and form a dense carbon core-shell structure.

[0069] S5, centrifugal purification and recycling of the strain.

[0070] The supernatant after the first centrifugation is subjected to at least one centrifugal purification to recover the strain and extracellular polymer substances;

[0071] The recovered strain and extracellular polymer substances are resuspended in a solution, the pH value of the solution is adjusted to 0.1-1, and the solution is recycled for soaking the waste negative plate.

[0072] In an optional embodiment, the supernatant after the first centrifugation is subjected to two centrifugal purifications.

[0073] Specifically, the centrifugal purification comprises the following steps:

[0074] The supernatant after the first centrifugation is subjected to a second centrifugation, and the precipitate is collected and washed;

[0075] The washed precipitate is resuspended in ultrapure water, subjected to a third centrifugation, and a mixed strain is obtained.

[0076] Optionally, the second centrifugation is performed under the following conditions: centrifugal force 8000-9000g, temperature 4-5℃, and time 5-10 minutes.

[0077] The third centrifugation is performed under the following conditions: centrifugal force 12000-13000g, temperature 4-5℃, and time 20-25 minutes.

[0078] The application soaks the waste negative plate in a microbial solution of a Gram-negative inorganic autotrophic bioleaching strain, synchronously dissolves iron, sulfur and other metal impurities through metabolic activity of the strain, and secretes extracellular polymer substances (EPS) to form a biological membrane to repair defects on the graphite surface; after centrifugal separation of the solid-phase graphite, water and soluble impurities are removed through vacuum baking, and then high-temperature calcination is performed to promote decomposition of the biological membrane and rearrangement of carbon atoms, thereby forming a dense carbon core-shell structure; meanwhile, the centrifugal supernatant is subjected to multiple centrifugal purifications to recycle the strain and EPS for cyclic utilization. The above method avoids the high corrosion risk of traditional cleaning, improves the crystallinity and cyclic stability of graphite, reduces the process cost through cyclic utilization of the strain, and realizes high-value recycling of the waste negative plate, thereby reducing resource waste and environmental pollution.

[0079] In a second aspect, a lithium ion battery, wherein the negative material of the lithium ion battery comprises the regenerated graphite prepared by the above method.

[0080] In a third aspect, a power-using device, wherein the power-using device comprises the lithium ion battery.

[0081] The application is implemented to test the related performance by using the following detection method:

[0082] X-ray diffraction (XRD) analysis (D8 Advance, Burker)

[0083] Specific steps: Take the regenerated graphite sample, grind it into a uniform powder, and then lay it in the sample groove. Set the scanning range, rate, voltage, current, and other parameters. By analyzing the intensity and sharpness of the (002) crystal face diffraction peak at about 26.5°, the crystal structure integrity and crystallinity changes of the graphite can be evaluated.

[0084] Raman spectroscopy analysis (Raman, inVia, Renishaw)

[0085] Specific steps: Select different areas on the surface of the graphite sample, use laser excitation, and set the scanning range, time, and number of times. By calculating the intensity ratio of D peak to G peak (ID / IG), the graphitization degree of the graphite can be quantified (the smaller the ID / IG value, the higher the graphitization degree).

[0086] Gravimetric capacity test

[0087] Battery formula:

[0088] Positive electrode sheet (16.0 mg / cm 2 ): composed of lithium iron phosphate (LFP), conductive carbon black (Super P), polyvinylidene fluoride (PVDF 5130), carbon nanotubes (CNT) in a weight ratio of 96:1.8:1.7:0.5;

[0089] Negative electrode sheet (10.1 mg / cm 2 ): composed of regenerated graphite, water-based binder (LA136D), Super P, CNT, sodium carboxymethyl cellulose (CMC) in a weight ratio of 96:2.3:0.9:0.4:0.4;

[0090] Electrolyte: 1M lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (EMC) (volume ratio 3:7);

[0091] Separator: ceramic-coated polyethylene film;

[0092] Negative current collector: copper foil with a thickness of 6.0 μm;

[0093] N / P value (ratio of negative capacity to positive capacity): about 1.13.

[0094] Test steps: After assembling the button cell, use the Bluecell battery test system to perform cycle testing at a voltage range of 2.0-4.5V with a charge-discharge rate of 0.5C / 0.5C. Record the first discharge capacity as the gravimetric capacity (unit: mAh / g).

[0095] Normal temperature cycle performance test (blue light workstation)

[0096] Test procedure: using the same battery formula as the specific capacity test, at 25℃ normal temperature environment, 0.5C / 0.5C rate for 500 times of charge-discharge cycle (voltage range 2.0-4.5V), record the ratio of discharge capacity after 500 cycles to the first discharge capacity (capacity retention rate), evaluate the cycle stability of the regenerated graphite.

[0097] The features and performance of the application are further described in detail below in conjunction with the examples.

[0098] Examples 1-2 respectively provide a method for recycling waste negative electrode sheet, which is described below by taking the method of Example 1 as an example.

[0099] The method for recycling waste negative electrode sheet of Example 1 comprises the following steps:

[0100] S1, microbial solution soaking pretreatment;

[0101] The ferrous iron sulfur bacillus was selected, and the ph was adjusted to 1 by controlling the strain concentration. The strain solution with a concentration of 4.2×10 8 cells / ml was used to soak the waste negative electrode sheet to obtain a solid-liquid mixture.

[0102] S2, first centrifugal separation of solid phase graphite;

[0103] The 3000g solid-liquid mixture was centrifuged at 4℃ for 5 minutes. After centrifugation, the solid phase graphite with a higher density was precipitated at the bottom of the centrifuge tube, and the supernatant with a lower density remained in the upper layer.

[0104] S3, vacuum baking to remove impurities;

[0105] The separated solid phase graphite was transferred to a vacuum oven, and the temperature was set to 65℃ for 6 hours. During the baking process, the water and soluble sulfate on the surface of the graphite were fully removed.

[0106] S4, high temperature calcination to prepare regenerated graphite;

[0107] The baked graphite was placed in a box furnace for high temperature calcination at a temperature of 2500℃ to obtain regenerated graphite.

[0108] The method for recycling waste negative electrode sheet of Example 2 refers to the recycling method of Example 1, with only some operations different, as follows:

[0109] Example 2: compared with Example 1, the strain solution in S1 step is 9×10 8 cells / ml.

[0110] Comparative Example 1:

[0111] The untreated waste graphite from disassembled retired lithium-ion batteries was selected.

[0112] Comparative Example 2:

[0113] The commercially available graphite (BETTERRAY (Jiangsu) New Energy Materials Co., Ltd.) was selected.

[0114] Test Example:

[0115] The performance tests of the graphite in Examples 1-2 and Comparative Examples 1-2 were carried out, and the results were as follows:

[0116] Referring to FIG. 1, Figure 1 The results of X-ray diffraction (XRD) and Raman spectrum analysis of the graphite in Examples 1-2 and Comparative Example 1 were shown in FIG. 1. It can be seen from the figure that:

[0117] Each sample showed a strong characteristic diffraction peak at about 26.5°, corresponding to the (002) crystal plane of the graphite material. This indicates that the graphite crystal of the graphite material after microbial solution soaking treatment and heat treatment still maintains a 2H-type hexagonal crystal structure with a space group of P63 / MMC. It should be noted that after microbial solution soaking treatment and heat treatment, the intensity of the (002) crystal plane diffraction peak decreases slightly, which may be due to the successful coating of the biofilm on the graphite, introducing amorphous carbon or organic impurities, and reducing the overall crystallinity of the graphite material. However, as the heat treatment temperature increases, the intensity of the (002) crystal plane diffraction peak increases continuously. In particular, when the temperature exceeds 1800℃, the graphite diffraction peak becomes sharp and strong, indicating that the crystallinity of the graphite after heat treatment is improved.

[0118] In addition, it can be found from the ratio of Raman D peak and G peak that when the strain solution concentration is 4.2×10 8 The degree of graphitization of the graphite is best at 0.59 when the strain solution concentration is 4.2×10

[0119] The surface of Comparative Example 1 is relatively rough due to the presence of cracks and debris. However, after soaking in the microbial solution, a clear coating layer appears on the surface of the graphite, covering the cracks and debris. Although there are many impurities attached to the coating layer, they gradually disappear as the heat treatment temperature increases, and the disappearance of the debris and amorphous aggregates in Examples 1-2 also indicates that the heat treatment process promotes the transformation of carbon from disorder to order. However, when the strain concentration is too high, the smoothness of some graphite surfaces is reduced due to the presence of pores, which may be due to the reaction between the biofilm coated on the graphite and the graphite at this temperature and the resulting ablation, which consumes the normal reversible lithium ion intercalation sites and affects its electrochemical performance. Therefore, when the strain concentration is 4.2×10 8The strain solution of 4.2 x 105cells / mL showed the best morphology and structure, which was beneficial to the insertion and extraction of lithium ions.

[0120] Therefore, the structure repair mechanism of the strain immersion after the impurity removal and the graphite heat treatment is as follows: after the biological film coating, the graphite can effectively fill the edge carbon layer defects and repair the surface broken carbon layer, so that a smoother morphology is formed, which is beneficial to the formation of a uniform and complete solid electrolyte interface film. The heat further decomposes the biological film into a dense amorphous carbon layer to form a carbon core-shell structure, thereby reducing the direct contact of the graphite active surface with the electrolyte.

[0121] II. Gram capacity test and room temperature cycle performance test

[0122] Referring to Table 1, the results of the gram capacity and room temperature cycle performance test of the graphite in Examples 1-2 and Comparative Examples 1-2 are shown.

[0123] g of active material -1 ]] 143.1 142.6 86.3 142.7 0.5C / 0.5C room temperature cycling - 500 cycles 97.6％ 97.1％ 66.30％ 96.80％

[0124] As can be seen from the table, the initial discharge gram capacity of Example 1 is 143.1 mAh / g, and that of Example 2 is 142.6 mAh / g, which is basically consistent with that of Comparative Example 2, 142.7 mAh / g, which is much higher than that of Comparative Example 1, 86.3 mAh / g. This shows that the lithium storage capacity of the regenerated graphite after microbial repair and heat treatment is close to that of commercial graphite, verifying the effectiveness of the recycling process. The gram capacity of Example 1 is slightly higher than that of Example 2, further indicating that when the strain concentration is controlled at 4.2 x 105cells / mL, the biological film thickness is moderate, which can effectively repair the surface defects of the graphite and avoid excessive reaction during calcination to cause loss of active sites. 8

[0125] At 25°C, 500 charge-discharge cycles were carried out at a rate of 0.5C / 0.5C (voltage range 2.0-4.5V), and the ratio of the discharge capacity after the 500th cycle to the initial discharge capacity (capacity retention rate) was recorded. The capacity retention rate of Example 1 was 97.6%, and that of Example 2 was 97.1%, both of which were better than that of Comparative Example 2, 96.8%, and significantly higher than that of Comparative Example 1, 66.3%. This is due to the carbon core-shell structure on the surface of the regenerated graphite, which can reduce the penetration of the electrolyte, reduce the interface impedance, and improve the cycle stability. The cycle performance of Example 1 is slightly better than that of Example 2, which again verifies that the optimal strain concentration (4.2 x 105cells / mL) can form an ideal biological film protective layer, which is converted into a uniform carbon core-shell structure after calcination, thereby optimizing the lithium ion insertion / extraction kinetics. 8

[0126] ​​The method for recycling waste negative pole pieces provided in the application can effectively repair the graphite crystal structure, improve the crystallinity and electrochemical performance by combining the metabolic activity of gram-negative inorganic autotrophic bioleaching strains (such as ferrous iron oxidizing Thiobacillus) with a heat treatment process. When the strain concentration is controlled at 4.2x10 8 -9x10 8 After vacuum baking at 65 DEG C for 6 hours and high-temperature calcination at 2500 DEG C, the specific capacity of the regenerated graphite is close to the commercial level, and the cycle stability is even better, realizing the high-value recycling of the waste negative pole pieces. The process recycles the strains and extracellular polymers by centrifugal purification and recycling, which not only reduces the production cost, but also reduces the environmental pollution, and provides a feasible path for the sustainable development of the lithium ion battery industry.

[0127] The principles and implementation manners of the application are described by using specific examples in the application, and the above examples are only used to help understand the method and core idea of the application; meanwhile, for the general skilled person in the art, the specific implementation manners and application ranges will be changed according to the idea of the application, and the above description should not be understood as the limitation of the application.

Claims

1. A method for recycling waste negative electrode sheets, characterized in that: The following steps are involved: soaking the waste negative electrode sheet in a microbial solution, wherein the microbial solution includes a Gram-negative inorganic autotrophic bioleaching strain, and maintaining the pH value of the microbial solution at 0.1-1 by adjusting the strain concentration to obtain a solid-liquid mixture; performing a first centrifugal treatment on the solid-liquid mixture to separate and obtain solid-phase graphite; The solid phase graphite is vacuum baked and then calcined at high temperature to obtain regenerated graphite.

2. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The bacterial strains include Thiobacillus ferrooxidans, Leptospira ferrooxidans or Thiobacillus thiooxidans.

3. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The strain concentration was 4.2×10 8 -9×10 8 cells / ml.

4. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The centrifugal conditions of the solid-liquid mixture are: centrifugal force 30000-32000g, temperature 4-5°C, and time 5-10 minutes.

5. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The vacuum baking was carried out at a temperature of 65° C. and for 6 hours.

6. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The temperature of the high-temperature calcination is 1800-3000° C., and the calcination equipment is a muffle furnace, a tube furnace or a box furnace.

7. The method for recycling waste negative electrode sheets according to claim 1, characterized in that: The following steps are also included: The supernatant obtained after the first centrifugation is subjected to at least one centrifugation purification to recover the strain and extracellular polymeric substances; The recovered strains and extracellular polymeric substances are resuspended in a solution, the pH value of the solution is adjusted to 0.1-1, and the solution is recycled for soaking waste negative electrode sheets.

8. The method for recycling waste negative electrode sheets according to claim 7, characterized in that: The centrifugal purification comprises the following steps: The supernatant obtained after the first centrifugation was centrifuged a second time, and the precipitate was collected and washed; The washed precipitate was resuspended in ultrapure water and centrifuged for the third time to obtain a mixed strain.

9. The method for recycling waste negative electrode sheets according to claim 8, characterized in that: The conditions for the second centrifugation are: centrifugal force 8000-9000g, temperature 4-5°C, time 5-10 minutes; The conditions for the third centrifugation are: centrifugal force 12000-13000g, temperature 4-5°C, and time 20-25 minutes.

10. A lithium ion battery, characterized in that: The negative electrode material of the lithium-ion battery comprises regenerated graphite prepared by the method according to any one of claims 1 to 9.

11. An electrical device, characterized in that: The electrical device comprises the lithium-ion battery according to claim 10.

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