Regenerated graphite negative electrode and preparation method thereof

Abstract

The invention belongs to the technical field of materials, and particularly relates to a regenerated graphite negative electrode and a preparation method thereof. The method comprises the following steps: in an air atmosphere, heating a cleaned waste lithium ion battery negative electrode to completely remove impurities so as to obtain a pretreated graphite sample; mixing the pretreated graphite sample with oxalic acid, adding water, and stirring until the oxalic acid is completely dissolved to obtain a graphite and oxalic acid mixture; and heating the graphite oxalic acid mixture until the liquid is completely volatilized to obtain the regenerated graphite negative electrode. Due to the adoption of low-temperature weak-acid one-step treatment, the traditional high-temperature treatment repair of graphite is abandoned, and the conditions of low temperature, high pressure and the like are not needed, so that the preparation method is simple in process, relatively mild in condition, convenient in equipment and short in preparation period. The obtained new graphite particles are good in morphology and low in impurity content, lithium oxalate is formed on the surfaces of the graphite particles through acid leaching and evaporation concentration to complete pre-physicochemical treatment, the graphite particles can be directly used as new graphite negative electrodes, and the electrochemical performance is further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of materials, and in particular relates to a regenerated graphite negative electrode and a preparation method thereof. Background Art

[0002] Lithium-ion batteries (LIBs) have become the most promising secondary batteries due to their advantages such as high energy density, low self-discharge and no memory effect, and have been widely used as energy storage units in portable electronics, electric vehicles, aerospace applications and large-scale power energy storage systems. It is predicted that the global demand for LIBs will approach 3600GWh by 2030. However, due to their limited service life (3-10 years), a large amount of spent LIBs have been generated. In 2020 alone, the global production of spent LIBs has exceeded 150,000 tons and will reach 3.7 million tons by 2030. Recycling spent LIBs is of great significance from both economic and environmental perspectives. In particular, recycling valuable components (i.e., cathode, anode and current collector) from spent LIBs is an effective strategy to address the challenge of resource scarcity. At the same time, it can effectively reduce the damage to the environment due to the overexploitation of resources. At present, mature hydrometallurgical, pyrometallurgical and bioleaching technologies have been successfully applied to industrial production to recover valuable metals from spent lithium cathode materials. In contrast, efficient recovery of graphite anode (GA) from waste lithium is also important in economic and environmental terms, but progress lags behind that of cathode recovery. In addition, a little-known fact is that to achieve a battery capacity of 1 kWh of commercial lithium batteries, 1 kg of graphite is required, which means that the demand for GA in commercial lithium batteries is about 10-20 times that of lithium. Considering that the content of GA in LIBs is 12-21 wt% and the number of LIBs consumed is increasing, the treatment of GA in waste LIBs has attracted increasing attention. On the one hand, the disposal of waste graphite slag will cause serious environmental pollution due to the presence of unwanted metal impurities (including Li, Al, Co, Cu, Ni, Fe and Mn) and toxic organic electrolytes, and both disposal and treatment require a lot of time, energy and economy. On the other hand, all grades of natural and synthetic graphite cannot be directly applied to lithium-ion batteries, and the production of battery-grade graphite is a complicated process. Natural graphite mainly exists in associated graphite deposits, and its production mainly includes four steps: mining, beneficiation, purification and processing. The mining of natural graphite not only has a huge impact on the environment (vegetation, air, water pollution, etc.), but also poses a huge health risk to the workers involved (pneumoconiosis). To meet the requirements of battery-grade graphite, acid leaching, alkali roasting or inert atmosphere heat treatment is also required, which also violates the requirements of clean production. Synthetic graphite is made by calcining petroleum coke, needle coke and asphalt at a certain temperature, followed by crushing, grading and high-temperature graphitization treatment (usually 2500°C). High-temperature treatment requires the support of fossil fuels and electricity, which leads to a large amount of carbon emissions. Therefore, the in-situ regeneration of waste lithium-ion battery negative electrodes under high environmental protection and low energy consumption remains a challenge.

[0003] On the other hand, during the cycle of lithium-ion batteries, lithium ions are constantly embedded and migrated out of the positive and negative electrodes. However, due to the formation of the SEI layer and lithium dendrites and the reaction of electrolyte byproducts, the lithium content at the positive electrode becomes less and less, while the lithium content at the negative electrode gradually increases. Generally, there will be residual intercalation compounds such as lithium carbide and dead lithium in the negative electrode of waste lithium-ion batteries. At present, most scholars only extract valuable metals without maximizing their utilization. In some large enterprises, graphite is generally discarded during recycling, which also means that this part of lithium will be wasted in battery recycling and processing, resulting in a huge waste of resources. Therefore, in addition to the problem of environmental protection treatment, the efficient use of graphite and the valuable metals it contains is also a challenge.

[0004] The information disclosed in this background technology section is only intended to enhance the understanding of the overall background of the invention and should not be regarded as an acknowledgment or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art. Summary of the invention

[0005] The purpose of the present invention is to provide a regenerated graphite negative electrode and a preparation method thereof in order to solve the problems of high energy consumption in repairing graphite negative electrodes and incomplete utilization of lithium elements in graphite. The greatest feature of this method is that only one green and environmentally friendly reagent is used to achieve two purposes: using oxalic acid to leach the remaining lithium elements in waste graphite without using additional strong acids and alkalis; at the same time, by forming lithium oxalate on the surface of graphite particles, the effect of pre-physicalization is achieved, which has the characteristics of simple and green process and lower energy consumption.

[0006] The present invention provides a method for preparing a regenerated graphite negative electrode, which has the following characteristics and comprises the following steps:

[0007] Step S1, heating the cleaned waste lithium-ion battery negative electrode to 500° C. to 550° C. in an air atmosphere to completely remove impurities and obtain a pretreated graphite sample;

[0008] Step S2, mixing the pretreated graphite sample with oxalic acid at a ratio of 2.5 g / L to 5 g / L, adding water and stirring until the oxalic acid is completely dissolved, to obtain a graphite-oxalic acid mixture;

[0009] Step S3, heating the graphite-oxalic acid mixture until the liquid is completely volatilized to obtain a regenerated graphite negative electrode;

[0010] Wherein, in step S2, the concentration of oxalic acid in the graphite-oxalic acid mixture is 0.8M.

[0011] The method for preparing the regenerated graphite negative electrode provided by the present invention may also have the following characteristics: wherein, in step S1, the cleaned waste lithium-ion battery negative electrode is heated to 500°C to 550°C in a tube furnace in an air atmosphere, and kept at this temperature for 2 hours to completely remove impurities, thereby obtaining a pretreated graphite sample. The waste lithium-ion battery negative electrode is also called a negative electrode material.

[0012] The preparation method of the regenerated graphite negative electrode provided by the present invention may also have the following characteristics: wherein, in step S2, the pretreated graphite sample is mixed with oxalic acid in a ratio of 2.5 g / L to 5 g / L, deionized water is added, and stirred at room temperature at a speed of 200 r / min until the oxalic acid is completely dissolved to obtain a graphite oxalic acid mixture.

[0013] The preparation method of the regenerated graphite negative electrode provided by the present invention may also have the following characteristics: wherein, in step S3, the graphite oxalic acid mixture is placed in an oil bath, heated to 130° C. under stirring conditions at a speed of 200 r / min, and heated at 130° C. until the liquid is completely volatilized to obtain a regenerated graphite negative electrode.

[0014] In the preparation method of the regenerated graphite negative electrode provided by the present invention, the following characteristics may also be provided: wherein, the cleaning method of the negative electrode of the waste lithium ion battery is:

[0015] The disassembled waste lithium-ion battery negative electrode was placed in anhydrous ethanol and soaked for 1 hour and then placed in a beaker; since the binder on the negative electrode side is mostly an aqueous binder, the copper foil and graphite can be separated after adding deionized water and stirring. The liquid after taking out the copper foil was filtered and rinsed with ethanol several times to remove the residual electrolyte. After the filter residue was dried, the cleaned waste negative electrode sample was obtained.

[0016] The invention also provides a regenerated graphite negative electrode, which is prepared according to the preparation method of the regenerated graphite negative electrode.

[0017] Functions and Effects of the Invention

[0018] The preparation method of the regenerated graphite negative electrode provided by the present invention adopts a one-step treatment with low-temperature weak acid, abandons the traditional high-temperature treatment and repair of graphite, and does not require low temperature, high pressure and other conditions. Therefore, the preparation method is simple in process, mild in conditions, convenient in equipment, and short in preparation cycle;

[0019] Furthermore, the present invention promotes the ionization of oxalic acid by increasing the temperature, thereby leaching out the remaining lithium element in the waste graphite, and combined with the reaction effect of oxalic acid itself on lithium element, the efficient leaching of lithium element in the graphite is achieved.

[0020] Furthermore, the new graphite particles obtained by the regeneration method of the present invention have good morphology and low impurity content. The graphite particles are pre-chemically prepared by forming lithium oxalate on the surface of the graphite particles through acid leaching and evaporation concentration, and can be directly used as new graphite negative electrodes, further improving the electrochemical performance.

[0021] Therefore, the preparation method of the present invention does not involve the pollution of strong acids and alkalis, uses only a green and environmentally friendly solvent, and has low energy consumption. The electrochemical performance of the regenerated graphite is greatly improved compared to the original graphite. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the X-ray diffraction pattern of the negative electrode material before and after treatment in Example 1 of the present invention;

[0023] Figure 2 is a comparison chart of X-ray diffraction of the recycled material and the commercial material in Example 1 of the present invention;

[0024] Figure 3 are scanning electron microscope images and transmission electron microscope images of the negative electrode material before and after treatment in Example 1 of the present invention;

[0025] Figure 4 is an infrared spectrum of the negative electrode material before and after treatment in Example 1 of the present invention;

[0026] Figure 5 is the Raman image of the negative electrode material before and after treatment in Example 1 of the present invention;

[0027] Figure 6 is a Raman comparison diagram of the recycled material and the commercial material in Example 1 of the present invention;

[0028] Figure 7 is a half-cell cycle diagram made of recycled materials, pretreated materials and original materials of the present invention;

[0029] Figure 8 It is a half-cell cycle diagram made of commercial materials of the present invention. DETAILED DESCRIPTION

[0030] In order to make the technical means, creative features, objectives and effects achieved by the present invention easy to understand, the following is a detailed description of a regenerated graphite negative electrode and a preparation method thereof in conjunction with embodiments and drawings.

[0031] Unless otherwise specified, the raw materials used in the present invention are purchased through general commercial channels. All test standards not mentioned are national standards.

[0032] The source information of some materials and instruments involved in the examples are as follows:

[0033] Anhydrous oxalic acid, Aladdin Company, ≥99%; the negative electrode of the used lithium-ion battery comes from the used lithium cobalt oxide battery of Apple mobile phones.

[0034] The negative electrode of the waste lithium-ion battery was pretreated using an OTF-1200X high-temperature tube furnace. The phase characterization and crystal orientation of the powder sample were analyzed using a Bruker D8ADVANCE x-ray diffractometer. The morphology image was taken at 5.0 kV using a field emission scanning electron microscope (JEM-7800F). The lattice structure of the sample was photographed using a field emission transmission electron microscope (JEM-2100F). The material composition and changes were analyzed using an infrared spectrometer (IRTracer-100) and a Raman spectrometer (JYH R800).

[0035] In the embodiment of the present invention, the negative electrode sample of the waste lithium-ion battery is used as the original graphite sample after preliminary treatment, and the cleaning treatment method is as follows:

[0036] The waste lithium cobalt oxide batteries from Apple mobile phones were manually disassembled in a fume hood, and the disassembled negative electrode sheets were soaked in an ethanol solution for 30 minutes to remove the electrolyte. Since the binder in the negative electrode is an aqueous binder, the negative electrode material and copper foil can be separated by washing with deionized water. After washing away most of the electrolyte and binder, the powder was collected and dried in a drying oven for 12 hours to obtain the original graphite sample (SG).

[0037] <Example 1>

[0038] A method for preparing a regenerated graphite negative electrode comprises the following steps:

[0039] Step S1, the cleaned waste lithium-ion battery negative electrode is kept at 550° C. for 2 hours in an air atmosphere in a tubular furnace to completely remove impurities such as electrolyte and binder to obtain a pretreated graphite sample;

[0040] Step S2, mixing the pretreated graphite sample obtained in step S1 with oxalic acid, slowly adding deionized water, and stirring at a speed of 200 r / min at room temperature until the oxalic acid is completely dissolved to obtain a graphite oxalic acid mixture.

[0041] Step S3, then put the graphite oxalic acid mixture into an oil bath, raise the temperature to 130° C. at a speed of 200 r / min, and heat at 130° C. until the liquid is completely volatilized to obtain regenerated pre-lithiated graphite, that is, regenerated graphite negative electrode.

[0042] In the above step S2, the amount of oxalic acid and negative electrode graphite is calculated based on the ratio of negative electrode graphite: oxalic acid of 5 g / L; the oxalic acid is anhydrous oxalic acid, and the concentration of oxalic acid in the graphite-oxalic acid mixture is 0.8 M.

[0043] The new graphite negative electrode material (RG) described below is the regenerated graphite negative electrode in step S3; the original sample (SG) is the graphite after disassembly and cleaning; the pretreated sample (PG) is the graphite after impurities are removed in step S1; and the commercial graphite sample (CG) is purchased commercial graphite.

[0044] The new graphite negative electrode material (RG), the original sample (SG), the pretreated sample (PG) and the commercial graphite sample (CG) obtained above were measured by X-ray diffractometer. The X-ray diffraction patterns before and after treatment are shown in FIG. Figure 1 and Figure 2 shown.

[0045] from Figure 1 and Figure 2 It can be seen that the peaks of SG, PG, RG and CG are very similar, and all conform to the standard XRD spectrum of graphite. The difference is that the size of d(002) is different. Generally, we believe that the larger the d(002), the greater the probability that the layered structure of the sample is perpendicular to the current collector, that is, the greater the preferred orientation of the (002) plane perpendicular to the current collector, the more conducive to the ion extraction / embedding movement. Therefore, it can be clearly seen from the figure that the peak of the (002) plane of RG is the highest, which also means that RG's ion extraction and embedding movement ability is better than the other three.

[0046] The treated graphite negative electrode material, the original sample and the pretreated sample were scanned by a scanning electron microscope. The scanning electron microscope images and transmission electron microscope images of the three materials were as follows: Figure 3 As shown, Figure 3 ac are scanning electron microscope images of different samples. We found that there are many impurities in SG, especially lithium dendrites are formed on the surface. After pretreatment, the surface impurities of PG samples are significantly reduced, and pores appear on the surface. This may be caused by impurities in waste graphite and a small amount of graphite being decomposed at high temperatures. It is worth noting that the appropriate specific surface area is conducive to ion transport and electrolyte penetration, while the appropriate micropore diameter is conducive to ion storage capacity. Therefore, the formation of this pore not only provides a site for the attachment of lithium oxalate, but also has a positive effect on the migration of lithium ions in subsequent battery work. From Figure 3 As can be seen in Figure c, a relatively uniform lithium oxalate is produced on the graphite surface, and the graphite itself also has a relatively excellent layered structure. Combined with the morphology analysis of the transmission electron microscope image, Figure 3 d It can be seen that there are a lot of agglomerated substances in SG, which are generally composed of impurities. Figure 3 e In the pre-treated PG, there are irregular black granular objects, which may be dead lithium and lithium carbide. Figure 3As shown in f, there are abundant particles on the surface or between the layers. Combining the scanning electron microscope image and the transmission electron microscope image, it is further proved that there is lithium oxalate on the surface of the regenerated graphite. In addition, the lattice fringes in the figure are analyzed. Figure 3 gi are the lattice fringe diagrams of SG, PG and RG, respectively. The lattice distribution is observed at 10nm. The lattice spacing of different samples can be calculated by Fourier transform and inverse Fourier calculation. It can be calculated that the lattice spacing of SG and PG is and The standard lattice spacing of the (002) plane in the graphite standard card is In comparison, the spacing of the two samples is larger. The lattice spacing of SG is larger due to the presence of impurities such as electrolytes and binders in the sample, while PG also contains substances such as dead lithium and lithium carbide, so it is slightly larger than the standard lattice spacing. In RG, it is calculated that the internal lattice spacing is The standard lattice spacing of the (002) plane in the graphite standard card is consistent with that of the surface, while the lattice fringes in the red frame on the surface are obviously inconsistent with the internal fringes, which are the lattice fringes of the new substance lithium oxalate. Combined with the analysis of scanning electron microscopy and transmission electron microscopy, it further proves the successful formation of lithium oxalate and its presence on the sample surface.

[0047] The new graphite negative electrode material, the original sample and the pre-treated sample obtained above were measured by infrared spectrometer. The infrared images of the three materials obtained are as follows: Figure 4 As shown, from Figure 4 It can be seen that the new graphite negative electrode material has characteristic peaks of carbon-oxygen double bonds and lithium oxalate, which can prove the formation of lithium oxalate.

[0048] The new graphite negative electrode material, the original sample, the pretreated sample and the commercial graphite sample obtained above were measured by Raman spectrometer. The Raman images of the four materials are shown in the figure below. Figure 5 and Figure 6 As shown in the figure, it can be seen that there are significant differences in the Raman spectra of SG, PG, RG and CG. For the Raman analysis of graphite, the D peak mainly comes from the stretching vibration of the CC bond and the C=C double bond, which is a typical Raman peak in carbon materials. In addition, the G peak is also a typical Raman peak in graphite materials, mainly from the sp 2 The stretching vibration of hybrid carbon atoms and π electrons, and the ratio of the two peak intensities I D / I G Represents the degree of graphitization of the material. The I of the four samples can be calculated. D / I GThe values ​​are 0.29, 0.423, 0.26 and 0.36 respectively. The overall trend of the experimental samples shows a trend of increasing first and then decreasing. This is because the PG is pretreated to remove impurities in the waste graphite, resulting in an increase in defects, and the residual lithium in it causes the sample to have a low degree of graphitization. The regenerated RG extracts lithium by using oxalic acid to form lithium oxalate, which occupies the defect position, so I D / I G The value decreased from 0.423 to 0.26, the defects decreased and the degree of graphitization increased. D / I G The value is at an intermediate level, which is consistent with the actual situation and shows that the lithium oxalate in the recycled graphite fills some gaps in the graphite.

[0049] <Example 2>

[0050] A method for preparing a regenerated graphite negative electrode comprises the following steps:

[0051] Step S1, heat the cleaned waste lithium-ion battery negative electrode in an air atmosphere at 500° C. for 2 h in a tubular furnace to completely remove impurities such as electrolyte and binder to obtain a pretreated graphite sample;

[0052] Step S2, mixing the pretreated graphite sample obtained in step S1 with oxalic acid, slowly adding deionized water, and stirring at a speed of 200 r / min at room temperature until the oxalic acid is completely dissolved to obtain a graphite oxalic acid mixture.

[0053] Step S3, then put the graphite oxalic acid mixture into an oil bath, raise the temperature to 130° C. at a speed of 200 r / min, and heat at 130° C. until the liquid is completely volatilized to obtain regenerated pre-lithiated graphite, that is, regenerated graphite negative electrode.

[0054] In the above step S2, the amount of oxalic acid and negative electrode graphite is calculated based on the ratio of negative electrode graphite: oxalic acid of 5 g / L; the oxalic acid is anhydrous oxalic acid, and the concentration of oxalic acid in the graphite-oxalic acid mixture is 0.8 M.

[0055] <Application Examples>

[0056] The SG, PG, RG and CG samples were mixed with conductive carbon and sodium alginate in a mortar at a mass ratio of 8:1:1 and ground evenly. The mixture was then dissolved in deionized water to form a slurry, spread on a copper collector, and dried at 80°C for 12 hours to obtain a half-cell positive electrode material. A lithium sheet was used as the negative electrode material to make a half-cell.

[0057] The above four batteries were charged and discharged at a current density of 0.2C. Figure 7 and Figure 8The results show that after 150 cycles, the samples showed different trends, and RG maintained a high specific capacity of 320.5 mAh g after 200 cycles. -1 , the capacity retention rate is 90.4%.

[0058] The above are only examples of the embodiments of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a regenerated graphite negative electrode, characterized in that: The following steps are involved: Step S1, heating the cleaned waste lithium-ion battery negative electrode to 500° C. to 550° C. in an air atmosphere to completely remove impurities and obtain a pretreated graphite sample; Step S2, mixing the pretreated graphite sample with oxalic acid at a ratio of 2.5 g / L to 5 g / L, adding water and stirring until the oxalic acid is completely dissolved, to obtain a graphite-oxalic acid mixture; Step S3, heating the graphite-oxalic acid mixture until the liquid is completely volatilized to obtain a regenerated graphite negative electrode; Wherein, in step S2, the concentration of oxalic acid in the graphite-oxalic acid mixture is 0.8M.

2. The method for preparing a regenerated graphite negative electrode according to claim 1, characterized in that: in, In step S1, the cleaned waste lithium-ion battery negative electrode is heated to 500° C. to 550° C. in an air atmosphere in a tube furnace, and kept at this temperature for 2 hours to completely remove impurities, thereby obtaining the pretreated graphite sample.

3. The method for preparing a regenerated graphite negative electrode according to claim 1, characterized in that: in, In step S2, the pretreated graphite sample is mixed with oxalic acid in a ratio of 2.5 g / L to 5 g / L, deionized water is added, and the mixture is stirred at a speed of 200 r / min at room temperature until the oxalic acid is completely dissolved to obtain a graphite oxalic acid mixture.

4. The method for preparing a regenerated graphite negative electrode according to claim 1, characterized in that: in, In step S3, the graphite oxalic acid mixture is placed in an oil bath, heated to 130° C. under stirring conditions at a speed of 200 r / min, and heated at 130° C. until the liquid is completely volatilized to obtain a regenerated graphite negative electrode.

5. The method for preparing a regenerated graphite negative electrode according to claim 1, characterized in that: in, The cleaning method of the negative electrode of the waste lithium-ion battery in step S1 is: The disassembled waste lithium-ion battery negative electrode is placed in anhydrous ethanol and soaked for 1 hour, and then placed in a beaker; deionized water is added and stirred to separate the copper foil and graphite; the liquid after the copper foil is taken out is filtered, and the residual electrolyte is rinsed with ethanol for multiple times, and the cleaned waste negative electrode sample can be obtained after the filter residue is dried.

6. A regenerated graphite negative electrode, characterized in that: The regenerated graphite negative electrode is prepared according to the method for preparing the regenerated graphite negative electrode according to any one of claims 1 to 5.

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