Negative electrode recovery method and negative electrode graphite powder

Abstract

The invention provides a negative electrode recovery method and negative electrode graphite powder, and relates to the field of resource regeneration. Waste negative electrode powder is obtained from the waste negative electrode piece, the waste negative electrode powder and ozone are subjected to first mixing and first reaction, and pretreated negative electrode powder is obtained; carrying out second mixing on the pretreated negative electrode powder and an alkaline solution, and carrying out second reaction to obtain negative electrode powder subjected to alkali treatment; and carrying out third mixing on the negative electrode powder subjected to alkali treatment and the pitch containing the nitrogen and carbon precursor, and roasting to obtain the negative electrode graphite powder. The strong oxidizing property of ozone molecules attacks a pi electron system of carbon atoms to form epoxy groups and carbonyl groups, and in an oxygen-enriched environment, part of epoxy groups and carbonyl groups are further oxidized into carboxyl groups or hydroxyl groups, so that the surface oxygen content is remarkably increased; performing alkaline solution treatment to selectively etch surface impurities and form a charge difference at the same time; asphalt containing nitrogen and carbon precursors is used for coating, so that the interface resistance is reduced, and the lithium storage kinetics is remarkably improved.

Description

Technical Field

[0001] This application relates to the field of resource recycling, and more particularly to a negative electrode recycling method and negative electrode graphite powder. Background Technology

[0002] With the continuous increase in the global ownership of new energy vehicles, the retirement of power batteries is accelerating. As a core component of lithium-ion batteries, graphite anode materials account for 15%-20% of battery costs, and their recycling is of great significance for resource recycling and cost reduction and emission reduction. Graphite is the most widely used anode material in lithium battery production. With the increase in the number of charge and discharge cycles, the crystal structure of the battery material collapses, its capacity gradually decreases, and it eventually fails. The increasing number of discarded batteries is causing serious environmental pollution and wasting carbon resources.

[0003] Recycling spent lithium-ion batteries has become a crucial link in resource recycling. However, the recycling of anode materials has long faced technical bottlenecks: First, the lithium content in waste graphite reaches 31 mg / g (higher than spodumene), and it also contains high-valence metal impurities (Co, Mn, etc.) and elemental copper and aluminum, requiring the development of efficient impurity removal and structural repair technologies; Second, traditional recycling methods are unable to effectively solve the problems of surface binder residues, metal impurity interference, and crystal structure defects, resulting in low initial efficiency and excessively large specific surface area of ​​recycled graphite, leading to uneven performance recovery. Although conventional high-temperature repair can improve crystallinity, high-temperature treatment consumes a lot of energy and may damage the interlayer structure of graphite, while also aggravating particle breakage, which seriously restricts its electrochemical performance.

[0004] Among them, binder residue refers to the organic binder remaining in the graphite negative electrode, which is difficult to completely remove. However, traditional methods (such as pyrolysis and acid-base leaching) are prone to producing harmful byproducts (such as HF) and are difficult to completely and effectively remove.

[0005] Interference from metallic impurities refers to the presence of metals such as copper, iron, and nickel mixed in graphite, which need to be removed by acid leaching or pyrometallurgical methods. ICP test values ​​are generally excessive (>50ppm), but acid leaching will generate a large amount of heavy metal wastewater.

[0006] Crystal structure defects refer to cracks and peeling between graphite layers caused by repeated lithium ion insertion and extraction during cycling, leading to an increase in specific surface area (typically >5 μm²). 2 / g) and reduced first-efficiency (<90%); in addition, the SEI film on the graphite surface of the waste battery thickens and the particles break, requiring high-temperature graphitization or chemical coating for repair.

[0007] Therefore, there is an urgent need to develop a green resource recycling method to solve the above problems. Summary of the Invention

[0008] The purpose of this application is to provide a negative electrode recovery method and negative electrode graphite powder to solve at least one of the above-mentioned technical problems.

[0009] To achieve the above objectives, the first aspect of this application provides a negative electrode recovery method, comprising: Waste negative electrode powder is obtained from waste negative electrode sheets. The waste negative electrode powder and ozone are mixed and reacted in a first stage to obtain pretreated negative electrode powder. The pretreated negative electrode powder and the alkaline solution are mixed and reacted in a second way to obtain alkaline-treated negative electrode powder. The alkali-treated negative electrode powder and the nitrogen- and carbon-containing precursor pitch are mixed and calcined to obtain negative electrode graphite powder.

[0010] Optionally, the ozone includes an ozone solution with a mass concentration of 100-300 mg / L.

[0011] Optionally, the negative electrode recovery method satisfies at least one of the following conditions: (1) The oxygen content (mass concentration) of the pretreated negative electrode powder is 0.5-1.5%; (2) The Fe content in the pretreated negative electrode powder is ≤20 ppm; (3) The mass content of Cu in the pretreated negative electrode powder is ≤20 ppm; (4) The mass content of Co in the pretreated negative electrode powder is ≤10 ppm; (5) The mass content of Ni in the pretreated negative electrode powder is ≤10 ppm; (6) The mass content of Al in the pretreated negative electrode powder is ≤10 ppm; (7) The mass content of Cr in the pretreated negative electrode powder is ≤10 ppm; (8) The Zn content in the pretreated negative electrode powder is ≤10 ppm; (9) The mass content of Mg in the pretreated negative electrode powder is ≤10 ppm; (10) The mass content of Mn in the pretreated negative electrode powder is ≤10 ppm; (11) The mass content of Na in the pretreated negative electrode powder is ≤10 ppm; (12) The mass content of K in the pretreated negative electrode powder is ≤10 ppm; (13) The mass content of Ca in the pretreated negative electrode powder is ≤10 ppm; (14) The mass content of Si in the pretreated negative electrode powder is ≤10 ppm; (15) The mass content of the binder in the pretreated negative electrode powder is 3%-8%, and the binder includes fluorinated binder.

[0012] Optionally, the temperature of the first reaction is 40-50℃, the humidity is 65-75%, and the time is 10-60 min.

[0013] Optionally, the negative electrode recovery method satisfies at least one of the following conditions: (1) The alkaline solution includes one or more of NaOH, KOH and CaOH; (2) The nitrogen-containing carbon precursor pitch includes one or more of polydopamine, poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline; (3) The mass ratio of the alkali-treated negative electrode powder to the nitrogen-containing carbon precursor asphalt is 100:1-6.

[0014] Optionally, the mass concentration of the polydopamine is 0.8-1.2 g / L.

[0015] Optionally, the negative electrode recovery method satisfies at least one of the following conditions: (1) The pH of the alkaline solution is 8-10; (2) The second reaction takes 5-8 minutes.

[0016] Optionally, the roasting includes a first roasting and a second roasting performed sequentially; The first calcination temperature is 450-550℃, and the holding time is 20-40 minutes; The second roasting temperature is 700-900℃, and the holding time is 2-4 hours.

[0017] The second aspect of this application provides a negative electrode graphite powder, which is prepared by the aforementioned negative electrode recovery method.

[0018] Optionally, the negative electrode graphite powder satisfies at least one of the following conditions: (1) Includes a graphite core and a carbon-containing coating layer disposed on the surface of the graphite core; the thickness of the carbon-containing coating layer is 3-5 nm; (2) The specific surface area of ​​the negative electrode graphite powder is 1.5-1.8 m². 2 / g; (3) The initial coulombic efficiency of the negative electrode graphite powder is 93-94%; (4) The 1C capacity retention rate of the negative electrode graphite powder is greater than 94%.

[0019] Compared with the prior art, the beneficial effects of this application include: The negative electrode recovery method provided in this application utilizes ozone molecules (O3) adsorbed onto the surface of waste negative electrode powder via van der Waals forces. O3's strong oxidizing properties (redox potential +2.07V) can attack the π-electron system of carbon atoms, forming epoxy groups (COC) and carbonyl groups (C=O). In an oxygen-rich environment, some epoxy groups (COC) and carbonyl groups (C=O) are further oxidized to carboxyl groups (-COOH) or hydroxyl groups (-OH), significantly increasing the surface oxygen content. Subsequent alkaline solution treatment selectively etches surface impurities while removing carboxyl atoms from the material surface, lowering the surface potential and providing electrostatic adsorption sites for subsequent coating. This also allows the epoxy groups on the graphite surface to form more carboxyl groups, enhancing the chemical bonding ability of the carbon source precursor. Finally, coating with nitrogen-containing carbon precursor pitch is performed. During pyrolysis, the carbon layer forms COC covalent bonds with the substrate, reducing interfacial resistance and significantly improving lithium storage kinetics.

[0020] The negative electrode graphite powder provided in this application has a simple repair process, is green and energy-saving, and the repaired negative electrode material has high initial efficiency and excellent cycle performance. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly described below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation on the scope of this application.

[0022] Figure 1 A cross-sectional SEM image of the negative electrode graphite powder provided in Example 1; Figure 2 This is a high-resolution SEM image of the negative electrode graphite powder provided in Example 1. Detailed Implementation

[0023] First, the solution provided in this application will be explained in more detail as follows: The first aspect of this application provides a negative electrode recovery method, comprising: Waste negative electrode powder is obtained from waste negative electrode sheets. The waste negative electrode powder and ozone are mixed and reacted in a first stage to obtain pretreated negative electrode powder. It is important to note that ozone molecules (O3) are adsorbed onto the surface of waste negative electrode powder via van der Waals forces. The adsorbed O3 chemically decomposes into adsorbed oxygen and ground-state oxygen molecules at active sites (such as defect or heteroatom doping sites). 3 O2) enables primary and deep oxidation. Primary oxidation specifically involves adsorbing oxygen to attack the sps of graphite in waste negative electrode powder. 2Hybridized carbon forms epoxy groups (COC) and carbonyl groups (C=O). Deep oxidation specifically involves the further oxidation of some epoxy groups (COC) and carbonyl groups (C=O) into carboxyl groups (-COOH) or hydroxyl groups (-OH) under oxygen-rich conditions, significantly increasing the surface oxygen content. -OH radicals (oxidation potential 2.8V) preferentially attack the CF bonds (bond energy 485kJ / mol) of the PVDF binder, achieving a dual effect of binder degradation and defect site activation. In some embodiments, TG curves confirmed a residual organic matter removal rate >95%, while retaining graphite sp. 2 The structural integrity of the hybrid carbon skeleton (bond energy 607 kJ / mol) is maintained; furthermore, ozone oxidation converts metallic impurities (such as copper and lithium) into soluble oxides (such as CuO and Li2O), which can then be separated by acid leaching or water washing. The pretreated negative electrode powder and the alkaline solution are mixed and reacted in a second way to obtain alkaline-treated negative electrode powder. It is important to note that an alkaline solution treatment is performed before coating. Firstly, this reduces the surface potential of the material (the surface potential is lower than the internal potential, resulting in a charge difference distribution). This increases the potential difference between the material and the coating agent, providing electrostatic adsorption sites for subsequent coating, guiding the directional deposition of the coating agent, and improving coating uniformity and strength. Secondly, the epoxy groups generated during ozone treatment are further converted into more reactive carboxyl or hydroxyl groups, preparing for bonding during subsequent nitrogen-containing asphalt coating. The alkali-treated negative electrode powder and the nitrogen- and carbon-containing precursor pitch are mixed and calcined to obtain negative electrode graphite powder.

[0024] It should be noted that ozone treatment introduces carboxyl / epoxy groups onto the surface of the negative electrode powder after pretreatment, thereby enhancing its chemical bonding ability with the carbon source precursor and increasing the bond energy.

[0025] In some embodiments, the ozone comprises an ozone solution having a mass concentration of 100-300 mg / L.

[0026] Optionally, the mass concentration of the ozone solution can be any value between 100 mg / L, 150 mg / L, 200 mg / L, 250 mg / L, 300 mg / L, or 100-300 mg / L.

[0027] It is important to note that when the ozone solution concentration is too low or the reaction time is too short, the number of oxygen-containing functional groups formed is insufficient, failing to provide a adequate bonding base for subsequent nitrogen-containing carbon coating, and also hindering the deep decomposition of the binder and the removal of impurities. When the ozone solution concentration is too high or the reaction time is too long, the effect will not be further improved, but energy consumption and costs will increase.

[0028] In some embodiments, the negative electrode recovery method satisfies at least one of the following conditions: (1) The oxygen content (mass concentration) of the pretreated negative electrode powder is 0.5-1.5%; Optionally, the oxygen content mass concentration of the pretreated negative electrode powder can be any value between 0.5%, 1.0%, 1.5%, or 0.5-1.5%. It is important to note that the oxygen concentration on the surface of the negative electrode powder will affect the bonding form with the nitrogen-containing precursor asphalt during the subsequent coating process. The higher the oxygen concentration, the better in this process, but it generally should not exceed 1.5%. (2) The Fe content in the pretreated negative electrode powder is ≤20 ppm; Optionally, the Fe content in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm, 15 ppm, 20 ppm or ≤20 ppm; It should be noted that the metal element content in the pretreated negative electrode powder is basically the same as that in the negative electrode graphite powder obtained after the third mixing and calcination. In some embodiments, the metal element content of the negative electrode graphite powder is usually tested to obtain the metal element content in the pretreated negative electrode powder. (3) The mass content of Cu in the pretreated negative electrode powder is ≤20 ppm; Optionally, the Cu content in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm, 15 ppm, 20 ppm or ≤20 ppm; (4) The mass content of Co in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Co in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (5) The mass content of Ni in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Ni in the pretreated negative electrode powder can be any value between 1ppm, 5ppm, 10ppm or ≤10ppm; (6) The mass content of Al in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Al in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (7) The mass content of Cr in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Cr in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (8) The Zn content in the pretreated negative electrode powder is ≤10 ppm; Optionally, the Zn content in the pretreated negative electrode powder can be any value between 1ppm, 5ppm, 10ppm or ≤10ppm; (9) The mass content of Mg in the pretreated negative electrode powder is ≤10 ppm; Optionally, the Mg content in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm. (10) The mass content of Mn in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Mn in the pretreated negative electrode powder can be any value between 1ppm, 5ppm, 10ppm or ≤10ppm; (11) The mass content of Na in the pretreated negative electrode powder is ≤10 ppm; Optionally, the Na content in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (12) The mass content of K in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of K in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (13) The mass content of Ca in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Ca in the pretreated negative electrode powder can be any value between 1 ppm, 5 ppm, 10 ppm or ≤10 ppm; (14) The mass content of Si in the pretreated negative electrode powder is ≤10 ppm; Optionally, the mass content of Si in the pretreated negative electrode powder can be any value between 1ppm, 5ppm, 10ppm or ≤10ppm; (15) The mass content of the binder in the pretreated negative electrode powder is 3%-8%, and the binder includes fluorinated binder.

[0029] Optionally, the mass content of the binder in the pretreated negative electrode powder can be any value between 3%, 4%, 5%, 6%, 7%, 8%, or 3-8%.

[0030] In some embodiments, the temperature of the first reaction is 40-50°C, the humidity is 65-75%, and the time is 10-60 min.

[0031] Optionally, the temperature of the first reaction can be any value between 40℃, 45℃, 50℃ or 40-50℃, the humidity can be any value between 65%, 70%, 75% or 65-75%, and the time can be any value between 10min, 20min, 30min, 40min, 50min, 60min or 10-60min.

[0032] It is important to note that due to the relatively high ozone concentration, the temperature during ozone treatment does not need to be too high, ideally controlled at 40-50℃ and humidity at approximately 65%-75%. At these temperature and humidity levels, the reaction between ozone and graphite is rapid, while maintaining a certain ozone concentration. Excessive temperature will cause ozone solution loss. The entire reaction time is approximately 10-60 minutes; too short a time will result in an incomplete reaction, while too long a time will diminish the subsequent effects.

[0033] In some embodiments, the negative electrode recovery method satisfies at least one of the following conditions: (1) The alkaline solution includes one or more of NaOH, KOH and CaOH; (2) The nitrogen-containing carbon precursor pitch includes one or more of polydopamine, poly(3,4-ethylenedioxythiophene) (PEDOT) and polyaniline; It is important to note that the amino and oxygen-containing groups in the nitrogen-containing carbon precursor form strong bonds, resulting in a stable and tight coating, which improves coating uniformity and strength, and enhances first-efficiency. During the coating process, the carboxyl group (-COOH) and the amino group (-NH2) first form a peptide bond (-CO-NH-), i.e., an amino-carboxyl condensation reaction to form a uniform pre-coating layer. Then, during further high-temperature pyrolysis, the more stable COC or CN bonds are finally formed. Preferably, the nitrogen-containing carbon precursor bitumen includes polydopamine; It is worth noting that polydopamine monomers contain amino groups (-NH2), which can be well coated on the material surface after carbonization, maintaining the material structure and enhancing the cycling stability of the sample during electrochemical processes. At the same time, after high-temperature annealing, nitrogen elements in polydopamine will remain in the carbon layer on the sample surface, which is beneficial to improving the charge interface transfer ability and electronic conductivity of the carbon layer, thereby improving the electrochemical performance of the sample. (3) The mass ratio of the alkali-treated negative electrode powder to the nitrogen-containing carbon precursor asphalt is 100:1-6.

[0034] Optionally, the mass ratio of the alkali-treated negative electrode powder to the nitrogen-carbon precursor pitch can be any value between 100:1, 100:2, 100:3, 100:4, 100:5, 100:6 or 100:1-6.

[0035] In some embodiments, the mass concentration of the polydopamine is 0.8-1.2 g / L.

[0036] Optionally, the mass concentration of polydopamine can be any value between 0.8 g / L, 0.9 g / L, 1 g / L, 1.1 g / L, 1.2 g / L, or 0.8-1.2 g / L.

[0037] In some embodiments, the negative electrode recovery method satisfies at least one of the following conditions: (1) The pH of the alkaline solution is 8-10; Optionally, the pH of the alkaline solution can be any value between 8, 8.5, 9, 9.5, 10, or 8-10; It should be noted that polydopamine can adhere very tightly to the surface of materials in an alkaline environment, enhancing the adhesion of active substances. Therefore, after carbonization, it can be well coated on the surface of materials, maintaining the structure of materials and enhancing the cycling stability of samples in electrochemical processes. (2) The second reaction takes 5-8 minutes.

[0038] Optionally, the time for the second reaction can be any value between 5 min, 6 min, 7 min, 8 min, or 5-8 min.

[0039] In some embodiments, the roasting includes a first roasting and a second roasting performed sequentially; The first calcination temperature is 450-550℃, and the holding time is 20-40 minutes; Optionally, the temperature of the first firing can be any value between 450℃, 500℃, 550℃ or 450-550℃, and the holding time can be any value between 20min, 30min, 40min or 20-40min. The second roasting temperature is 700-900℃, and the holding time is 2-4 hours.

[0040] Optionally, the temperature of the second roasting can be any value between 700℃, 800℃, 900℃ or 700-900℃, and the holding time can be any value between 2h, 3h, 4h or 2-4h.

[0041] It should be noted that the two-step calcination process achieves a graphitization degree of over 85% in the carbon layer, which improves conductivity and enhances the first-efficiency performance of the product. In some embodiments, the carbon layer forms COC covalent bonds with the substrate during the calcination pyrolysis process, which reduces the interfacial resistance and increases the capacity retention rate to 98.6% after 500 cycles.

[0042] The second aspect of this application provides a negative electrode graphite powder, which is prepared by the aforementioned negative electrode recovery method.

[0043] In some embodiments, the negative electrode graphite powder satisfies at least one of the following conditions: (1) Includes a graphite core and a carbon-containing coating layer disposed on the surface of the graphite core; the thickness of the carbon-containing coating layer is 3-5 nm; Optionally, the thickness of the carbon coating can be any value between 3nm, 4nm, 5nm, or 3-5nm; (2) The specific surface area of ​​the negative electrode graphite powder is 1.5-1.8 m². 2 / g; Optionally, the specific surface area of ​​the negative electrode graphite powder can be 1.5 m². 2 / g, 1.6m 2 / g, 1.7m 2 / g, 1.8m 2 / g or 1.5-1.8m 2 Any value between / g; (3) The initial coulombic efficiency of the negative electrode graphite powder is 93-94%; Optionally, the initial coulombic efficiency of the negative electrode graphite powder can be 93%, 93.5%, 94%, or any value between 93% and 94%. (4) The 1C capacity retention rate of the negative electrode graphite powder is greater than 94%.

[0044] Optionally, the 1C capacity retention rate of the negative electrode graphite powder can be any value of 94.5%, 95%, 96%, 97%, 98%, 99%, or greater than 94%.

[0045] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0046] Example 1 This embodiment provides a negative electrode recovery method, the specific steps of which are as follows: S1: Waste negative electrode powder obtained by crushing and sieving waste negative electrode sheets, and TG test the raw material to confirm the binder content and carbon content in the waste negative electrode powder; S2: An acidic oxidation environment with an ozone concentration of 100 mg / L and pH=5 was constructed using an ozone generator (SMC-3T-300). The waste negative electrode powder was then soaked in the ozone water for 4 hours. After that, the negative electrode powder was filtered and dried to obtain ozone-treated graphite negative electrode powder. The obtained sample was then subjected to ICP and TG tests. S3: Treat with NaOH solution at pH=8 for 5 min to form a surface charge gradient and obtain alkali-treated graphite anode powder; S4: Modified asphalt containing nitrogen-carbon precursor (polydopamine, mass concentration 0.8g / L) is mixed with alkali-treated graphite anode powder (mass ratio of nitrogen-carbon precursor asphalt to alkali-treated graphite anode powder is 100:2), heated to 60℃ and stirred for 30 minutes to coat the surface of graphite anode powder with modified asphalt. S5: Then, the temperature was raised to 550℃ and held for 30 minutes in an argon atmosphere, and then raised to 700℃ and held for 2 hours to obtain the coated pyrolyzed graphite anode powder.

[0047] The second aspect of this embodiment provides a negative electrode graphite powder, which is prepared by the above method.

[0048] The negative electrode graphite powder includes a graphite core and a carbon-containing coating layer disposed on the surface of the graphite core; the thickness of the carbon-containing coating layer is 3-5 nm.

[0049] SEM image of the cross-section of the negative electrode graphite powder is as follows: Figure 1 As shown, high-magnification SEM Figure 2 As shown.

[0050] Example 2 The difference from Example 1 is that the concentration of the ozone aqueous solution is 200 ppm.

[0051] Example 3 The difference from Example 1 is that the concentration of the ozone aqueous solution is 300 ppm.

[0052] Example 4 The difference from Example 1 is that the ozone aqueous solution soaking time is 6 hours.

[0053] Example 5 The difference from Example 1 is that the ozone aqueous solution soaking time is 8 hours.

[0054] Example 6 The difference from Example 1 is that the pH of the weak alkaline solution is 9.

[0055] Example 7 The difference from Example 1 is that the pH of the weak alkaline solution is 10.

[0056] Example 8 The difference from Example 1 is that in step S5, the temperature is raised to 550°C and held for 30 minutes, and then raised to 800°C and held for 2 hours.

[0057] Example 9 The difference from Example 1 is that in step S5, the temperature is raised to 550°C and held for 30 minutes, and then raised to 900°C and held for 2 hours.

[0058] Comparative Example 1 The difference from Example 1 is that ozone treatment in step S3 was not performed; instead, 1wt% HCl purification treatment was used.

[0059] Comparative Example 2 The difference from Example 1 is that the concentration of the ozone aqueous solution is 50 ppm.

[0060] Comparative Example 3 The difference from Example 1 is that step S3 was not performed, that is, the ozone-treated sample was not treated with an alkaline solution.

[0061] Comparative Example 4 The difference from Example 1 is that conventional asphalt is used for coating in step S5.

[0062] Comparative Example 5 The difference from Example 1 is that in step S5, the temperature is directly raised to 700°C and held for 2 hours.

[0063] The pretreated negative electrode powder or negative electrode powder prepared in the above embodiments and comparative examples were tested, specifically including specific surface area testing, metal element testing, particle size testing, carbon content testing, TG testing, and electrochemical performance testing after the battery was prepared.

[0064] Metal element testing: A certain amount of sample was weighed and added to a certain amount of concentrated nitric acid for microwave digestion to obtain a solution. The obtained solution and filter residue were washed multiple times and the volume was adjusted to a certain level. The plasma intensity of the metal elements was tested by ICP-OES. The metal content in the solution was calculated based on the standard curve of the measured metal, thereby calculating the amount of metal elements contained in the material. The specific element content of the pretreated negative electrode powder prepared in the examples and comparative examples is shown in Table 1. Except for oxygen content and binder content, which are expressed in % (%), the unit of other element content is ppm.

[0065] Table 1 Elemental Content Test

[0066] Specific surface area test: Under constant temperature and low temperature, the amount of gas adsorbed on the solid surface at different relative pressures is measured. Based on the Brownauer-Etter-Taylor adsorption theory and its formula (BET formula), the amount of monolayer adsorption of the sample is calculated, thereby calculating the specific surface area of ​​the solid. Approximately 1.5-3.5 g of powder sample is weighed and placed into the test sample tube of TriStar II 3030, degassed at approximately 200°C for 120 min, and then tested.

[0067] Particle size test: Add about 0.02g of powder sample to a 50mL clean beaker, add about 20mL of deionized water, and then add a few drops of 1% surfactant to completely disperse the powder in the water. Sonicate in a 120W ultrasonic cleaner for 5 minutes and test the particle size distribution using MasterSizer 2000.

[0068] Carbon content testing: The sample is heated at high temperature in a high-frequency furnace under oxygen-enriched conditions, causing carbon and sulfur to be oxidized into carbon dioxide and sulfur dioxide, respectively. This gas is then processed and enters the corresponding absorption cell, where it absorbs the corresponding infrared radiation, which is then converted into corresponding signals by a detector. This signal is sampled by a computer, linearly corrected, and converted into a value proportional to the concentrations of carbon dioxide and sulfur dioxide. The values ​​from the entire analysis process are then accumulated. After analysis, this accumulated value is divided by the weight value in the computer, multiplied by the correction factor, and the blank is subtracted to obtain the percentage carbon and sulfur content in the sample. The sample was tested using a high-frequency infrared carbon-sulfur analyzer (Shanghai Dekai HCS-140).

[0069] TG test: Refer to GB / T 27761-2011 Test method for weight loss and residual amount in thermogravimetric analyzer.

[0070] The negative electrode powders prepared in the above embodiments and comparative examples were then assembled to obtain coin cells. The specific preparation method is as follows: In a dry argon atmosphere, LiPF6 was added to a solvent consisting of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a weight ratio of 1:1:1, and the mixture was stirred until homogeneous. The concentration of LiPF6 was approximately 1.15 mol / L. Then, approximately 7.5 wt% of fluoroethylene carbonate (FEC) was added and the mixture was stirred until homogeneous to obtain the electrolyte.

[0071] The negative electrode powder, conductive carbon black, and binder PAA (modified polyacrylic acid) obtained in the above examples and comparative examples were added to deionized water at a weight ratio of approximately 80:10:10, stirred to form a slurry, and coated with a doctor blade to form a coating with a thickness of approximately 100 μm. The coating was then dried in a vacuum drying oven at approximately 85°C for approximately 12 hours. The coating was then cut into circular pieces with a diameter of approximately 1 cm using a punch press in a drying environment. In a glove box, a lithium metal sheet was used as the counter electrode, and a Ceglard composite membrane was selected as the separator. Electrolyte was added to assemble a coin cell.

[0072] The LAND series battery was used to conduct charge-discharge tests on the battery to measure its charge-discharge capacity. First, after resting for 120 minutes, the battery was discharged at 10 μA to 0.001V to obtain the initial lithium insertion capacity. Then, it was charged at 0.1C to 1.5V to obtain the initial lithium extraction capacity. Finally, the initial efficiency of the material was obtained by dividing the initial lithium extraction capacity by the initial lithium insertion capacity.

[0073] The negative electrode powders prepared in the above embodiments and comparative examples were then assembled to obtain a lithium-ion battery. The specific preparation method is as follows: Preparation of the positive electrode: LiCoO2, conductive carbon black, and polyvinylidene fluoride (PVDF) were thoroughly mixed in an N-methylpyrrolidone solvent system at a weight ratio of approximately 95%:2.5%:2.5% to obtain a positive electrode slurry. The obtained positive electrode slurry was coated onto a positive electrode current collector aluminum foil, dried, and cold-pressed to obtain the positive electrode.

[0074] Preparation of the negative electrode: Graphite, silicon-based negative electrode active material prepared according to the examples and comparative examples, conductive agent (conductive carbon black, Super), and binder PAA were mixed in a weight ratio of approximately 95%:1.2%:5%:3.8%, and an appropriate amount of water was added. The mixture was kneaded at a solid content of approximately 40 wt%. An appropriate amount of water was added to adjust the viscosity of the slurry to approximately 2500 Pa·s to prepare the negative electrode slurry. The prepared negative electrode slurry was coated onto the negative electrode current collector copper foil, dried, and cold-pressed to obtain the negative electrode.

[0075] Preparation of electrolyte: Under a dry argon atmosphere, LiPF6 was added to a solvent composed of propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a weight ratio of approximately 1:1:1 and mixed thoroughly. The concentration of LiPF6 was approximately 1.15 mol / L. Then, approximately 7.5 wt% of fluoroethylene carbonate (FEC) was added and mixed thoroughly to obtain the electrolyte.

[0076] Preparation of the separator: PE porous polymer film was used as the separator.

[0077] Lithium-ion battery fabrication: The positive electrode, separator, and negative electrode are stacked in sequence, with the separator positioned between the positive and negative electrodes for isolation. The cells are then wound to obtain bare cells. These bare cells are placed in outer packaging, injected with electrolyte, and sealed. After formation, degassing, and edge trimming processes, the lithium-ion battery is obtained.

[0078] Cyclic performance testing: The test temperature was 25℃ / 45℃. The battery was charged at a constant current of 0.7C to 4.4V, then charged at a constant voltage of 0.025C, and after a 5-minute rest period, discharged at 0.5C to 3.0V. The capacity obtained from this process was used as the initial capacity. Cyclic tests were then conducted using 0.7C charging / 0.5C discharging. The capacity decay curve was obtained by comparing the capacity at each step with the initial capacity. The number of cycles at 25℃ until 90% capacity retention was recorded as the room temperature cycle performance, and the number of cycles at 45℃ until 80% capacity retention was recorded as the high-temperature cycle performance. The cycle performance of the material was compared by comparing the number of cycles under these two conditions. Specific test results are shown in Table 2.

[0079] Table 2 Test Results

[0080] analyze: The results above show that Comparative Example 1 has poorer capacity and first-efficiency performance compared to Examples 1-9. Example 1 has better capacity and first-efficiency performance. This is because ozone treatment of waste negative electrode powder can remove the binder more thoroughly, reducing the negative impact of the binder on product performance. On the other hand, ozone treatment can form active oxygen-containing groups on the graphite surface. After coating and pyrolysis, a dense and stable coating layer can be formed with lower internal resistance, resulting in better first-efficiency and capacity performance.

[0081] Compared with Comparative Example 2, Examples 1-9 showed better capacity and first-efficiency performance. This is because the high-concentration ozone water treatment of the negative electrode powder resulted in the formation of more active groups on the surface. After subsequent coating and pyrolysis, a more stable and dense coating layer could be formed, thus the first-efficiency and capacity performance were significantly improved compared to low-concentration ozone.

[0082] Compared with Comparative Example 3, Examples 1-9 showed better cycle and first-effect performance. This is because after treatment with alkaline solution and then coating pyrolysis, more active groups can be formed on the graphite surface, which can better react with the active groups in the modified asphalt, thereby forming a more stable coating layer structure. Therefore, the conductivity is better, and the first-effect and cycle performance is superior.

[0083] Compared with Comparative Example 4, Examples 1-9 showed better cycle performance because the modified asphalt contains specific active groups that can interact with the active groups on the surface of the graphite anode after alkaline solution treatment, thereby forming a directional and stable coating layer on the surface of the graphite anode. This coating layer is denser and more stable than that of conventional asphalt, thus exhibiting better cycle performance.

[0084] Compared with Comparative Example 5, Examples 1-9 showed poor cycling and first-effect performance. The possible reason is that direct heating can easily destroy the chemical bonds of the coating layer, resulting in more micropores in the coating layer, a larger specific surface area, and poorer conductivity, thus reducing the first-effect and cycling performance.

[0085] In summary, by adjusting the ozone concentration and treatment time, the pH value of the weak alkaline solution, and the pyrolysis temperature, the capacity and initial efficiency of Examples 1-9 of this application are improved, and the specific surface area is controlled, thus solving the problems of poor performance of recycled graphite, large specific surface area, and difficulty in removing binders.

[0086] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

[0087] Furthermore, those skilled in the art will understand that although some embodiments herein include certain features included in other embodiments but not others, combinations of features from different embodiments are intended to be within the scope of this application and form different embodiments. For example, any of the claimed embodiments can be used in any combination. The information disclosed in this background section is intended only to enhance the understanding of the general background of this application and should not be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

Claims

1. A method for recovering negative electrodes, characterized in that, include: Waste negative electrode powder is obtained from waste negative electrode sheets. The waste negative electrode powder and ozone are mixed and reacted in a first stage to obtain pretreated negative electrode powder. The pretreated negative electrode powder and the alkaline solution are mixed and reacted in a second way to obtain alkaline-treated negative electrode powder. The alkali-treated negative electrode powder and the nitrogen- and carbon-containing precursor pitch are mixed and calcined to obtain negative electrode graphite powder.

2. The negative electrode recovery method according to claim 1, characterized in that, The ozone includes an ozone solution with a mass concentration of 100-300 mg / L.

3. The negative electrode recovery method according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The oxygen content (mass concentration) of the pretreated negative electrode powder is 0.5-1.5%; (2) The Fe content in the pretreated negative electrode powder is ≤20 ppm; (3) The mass content of Cu in the pretreated negative electrode powder is ≤20 ppm; (4) The mass content of Co in the pretreated negative electrode powder is ≤10 ppm; (5) The mass content of Ni in the pretreated negative electrode powder is ≤10 ppm; (6) The mass content of Al in the pretreated negative electrode powder is ≤10 ppm; (7) The mass content of Cr in the pretreated negative electrode powder is ≤10 ppm; (8) The Zn content in the pretreated negative electrode powder is ≤10 ppm; (9) The mass content of Mg in the pretreated negative electrode powder is ≤10 ppm; (10) The mass content of Mn in the pretreated negative electrode powder is ≤10 ppm; (11) The mass content of Na in the pretreated negative electrode powder is ≤10 ppm; (12) The mass content of K in the pretreated negative electrode powder is ≤10 ppm; (13) The mass content of Ca in the pretreated negative electrode powder is ≤10 ppm; (14) The mass content of Si in the pretreated negative electrode powder is ≤10 ppm; (15) The mass content of the binder in the pretreated negative electrode powder is 3%-8%, and the binder includes fluorinated binder.

4. The negative electrode recovery method according to claim 1, characterized in that, The temperature of the first reaction is 40-50℃, the humidity is 65-75%, and the time is 10-60 min.

5. The negative electrode recovery method according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The alkaline solution includes one or more of NaOH, KOH and CaOH; (2) The nitrogen-containing carbon precursor pitch includes one or more of polydopamine, poly(3,4-ethylenedioxythiophene) and polyaniline; (3) The mass ratio of the alkali-treated negative electrode powder to the nitrogen-containing carbon precursor asphalt is 100:1-6.

6. The negative electrode recovery method according to claim 5, characterized in that, The mass concentration of the polydopamine is 0.8-1.2 g / L.

7. The negative electrode recovery method according to claim 1, characterized in that, At least one of the following conditions must be met: (1) The pH of the alkaline solution is 8-10; (2) The second reaction takes 5-8 minutes.

8. The negative electrode recovery method according to any one of claims 1-7, characterized in that, The roasting includes a first roasting and a second roasting performed sequentially. The first calcination temperature is 450-550℃, and the holding time is 20-40 minutes; The second roasting temperature is 700-900℃, and the holding time is 2-4 hours.

9. A negative electrode graphite powder, characterized in that, It is prepared by the negative electrode recovery method according to any one of claims 1-8.

10. The negative electrode graphite powder according to claim 9, characterized in that, At least one of the following conditions must be met: (1) Includes a graphite core and a carbon-containing coating layer disposed on the surface of the graphite core; the thickness of the carbon-containing coating layer is 3-5 nm; (2) The specific surface area of ​​the negative electrode graphite powder is 1.5-1.8 m². 2 / g; (3) The initial coulombic efficiency of the negative electrode graphite powder is 93-94%; (4) The 1C capacity retention rate of the negative electrode graphite powder is greater than 94%.

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