Recycling method of negative electrode graphite in waste lithium battery, regenerated graphite and application

Abstract

The invention discloses a recycling method of negative electrode graphite in a waste lithium battery, regenerated graphite and application, and the recycling method comprises the following steps: carrying out acid leaching reaction on the negative electrode graphite in the waste lithium battery to obtain purified graphite; carrying out doping reaction on the purified graphite and a nitrogen-rich organic matter to obtain intermediate graphite; and grinding and calcining the intermediate graphite and elemental sulfur, and then grinding and calcining the intermediate graphite and red phosphorus to obtain regenerated graphite. According to the recycling method of the negative electrode graphite in the waste lithium battery, the regenerated graphite and the application, the surface of the regenerated graphite obtained through the method is provided with a Li3P-based SEI layer with high ionic conductivity, the electrical performance of the graphite can be repaired, the first coulombic efficiency, the cycling stability and the rate capability of the graphite are further improved, and the service life of the graphite is prolonged. Therefore, reutilization of the graphite as the lithium battery negative electrode material is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium battery recycling and reuse, and particularly relates to a method for recycling anode graphite in waste lithium batteries, regenerated graphite and applications thereof. Background Art

[0002] With the wide popularization and use of new energy vehicles, the demand for lithium batteries is increasing year by year, and the demand for graphite, which is also used as the anode of lithium batteries, is also increasing. As lithium batteries reach the end of their service life and are phased out, a large amount of waste graphite (SG) will appear on the market. Graphite has the advantages of good electrical conductivity and stable structure. Although its electrical performance decreases due to the influence of the SEI film and the repeated insertion and extraction of lithium ions during the operation of the battery, SG still has great reuse value.

[0003] It is reported that the market for graphite anodes will grow from 14.3 billion in 2019 to 21.6 billion in 2027. Therefore, more and more people believe that the crude recycling method of SG is unworkable, and it is necessary to develop a low-cost and high-efficiency technology to recycle the anode graphite in waste lithium batteries, which can not only create certain economic benefits, but also achieve the sustainable development of the lithium-ion battery industry.

[0004] The existing recycling methods first separate the graphite from the copper foil surface through heat treatment or other methods, which not only results in high energy consumption and waste gas generation, but also the recycled graphite will introduce binder, conductive carbon and metal impurities at the same time, directly affecting the performance of secondary utilization. Therefore, more in-depth research on the reuse of SG is needed. Fortunately, the recycled SG still maintains an ordered structure and does not require high-temperature treatment again, thus reducing the cost.

[0005] So far, a few research groups are engaged in the reuse of SG in various energy storage applications, indicating that it has good prospects on an industrial scale. Therefore, how to modify SG to restore or even exceed the performance of commercial graphite, so as to achieve reuse, is of great significance for reducing the cost of graphite anode materials. Summary of the Invention

[0006] Based on the above technical problems, the present invention provides a method for recycling anode graphite in waste lithium batteries, regenerated graphite and applications thereof. The regenerated graphite obtained by the method has an SEI layer based on Li 3 P with high ionic conductivity on its surface, which can not only repair the electrical performance of graphite, but also further improve the first Coulomb efficiency, cycle stability and rate performance of graphite, thereby realizing the reuse of graphite as a lithium battery anode material.

[0007] A method for recycling anode graphite in waste lithium batteries proposed by the present invention includes the following steps:

[0008] S1, subjecting the negative electrode graphite in the waste lithium battery to an acid leaching reaction to obtain purified graphite;

[0009] S2, subjecting the purified graphite to a doping reaction with a nitrogen-rich organic matter to obtain an intermediate graphite;

[0010] S3, grinding and calcining the intermediate graphite with elemental sulfur, and then grinding and calcining it with red phosphorus to obtain regenerated graphite.

[0011] The present invention essentially proposes a new strategy for recycling and modifying graphite of the negative electrode of waste lithium batteries. Specifically, purified graphite is obtained by acid leaching, and then the surface is modified by nitrogen-rich organic matter to obtain an intermediate graphite with N-doped C surface coating. Then, the intermediate graphite is mixed and ground with elemental sulfur and red phosphorus in sequence, and then calcined at high temperature to obtain CSP bonded regenerated graphite (RG).

[0012] In the present invention, graphite purification can remove the presence of residual impurities (such as SEI layer, binder, transition metal, etc.); surface modification of nitrogen-rich organic matter can repair the graphite structure and form an N-doped C layer on its surface, thereby increasing the surface activity of graphite and improving its electrochemical performance; when graphite is mixed and ground with elemental sulfur and red phosphorus in sequence, since S decomposes into S2-S4 molecules at the melting point, S acts as an intermediate bridge to promote the gasification of P into P 4 The CSP bonds formed by the post-deposition of molecules can generate continuous crystal lines in situ during battery cycling. 3 P-based SEI layer (Li 3 P has high ionic conductivity), which helps Li + Effective desolvation and Li + Migration across the SEI layer; ultimately, the present invention obtains a graphite negative electrode that has both high initial coulombic efficiency and large current charge and discharge capability, which can be reapplied to lithium batteries to effectively improve the cycle stability and rate performance of the battery.

[0013] Preferably, in step S1, the acid leaching reaction is achieved by soaking the negative electrode graphite in a concentrated sulfuric acid solution and then solidifying it;

[0014] Preferably, the concentration of concentrated sulfuric acid is 90-99wt%;

[0015] Preferably, the soaking temperature is 20-30°C and the soaking time is 0.5-1h;

[0016] Preferably, the curing temperature is 180-220° C. and the curing time is 16-24 hours.

[0017] Preferably, step S1 further comprises subjecting the purified graphite to a secondary acid leaching reaction;

[0018] Preferably, the secondary acid leaching reaction is achieved by soaking the purified graphite in dilute sulfuric acid.

[0019] Preferably, the concentration of the dilute sulfuric acid is 15-25 wt%;

[0020] Preferably, the soaking temperature is 70-90 °C and the time is 0.5-2 h.

[0021] Preferably, in step S2, the doping reaction is achieved by ultrasonically dispersing the purified graphite and the nitrogen-rich organic matter in a solvent uniformly and then drying;

[0022] Preferably, the mass ratio of the purified graphite to the nitrogen-rich organic matter is 1:1-3;

[0023] Preferably, the ultrasonic dispersion power is 500-1000 W and the time is 1-3 h;

[0024] Preferably, the drying temperature is 70-90 °C and the time is 8-16 h.

[0025] Preferably, the nitrogen-rich organic matter is at least one of urea, melamine or dicyandiamide.

[0026] Preferably, in step S2, it further includes performing a covalent bond functionalization reaction on the intermediate graphite and the polyamine;

[0027] Preferably, the polyamine is at least one of ethylenediamine, butanediamine or hexanediamine;

[0028] Preferably, the mass ratio of the intermediate graphite to the polyamine is 1:0.05-0.1;

[0029] Preferably, the temperature of the covalent bond functionalization reaction is 80-100 °C and the time is 3-6 h.

[0030] In the present invention, there are certain oxygen-containing groups on the surface of the intermediate graphite, which can undergo a condensation reaction with the polyamine, enabling the organic amine group to be grafted onto the surface of the intermediate graphite. This organic amine group can activate the basic unit P 4 molecule of red phosphorus, further promoting the deposition of red phosphorus on the graphite surface, and thus being more conducive to obtaining the regenerated graphite (RG) with C-S-P bond connection.

[0031] Preferably, in step S3, the mass ratio of the intermediate graphite to red phosphorus and elemental sulfur is 1:0.05-0.06:0.0005-0.01;

[0032] Preferably, the grinding is achieved in a mortar.

[0033] Preferably, in step S3, the calcination with elemental sulfur includes: first heating to 200-300°C at a rate of 1-10°C / min, holding for 16-24 h, and then naturally cooling to room temperature;

[0034] The calcination with red phosphorus includes: first heating to 400-500°C at a rate of 1-10°C / min, holding for 2-4 h, then cooling to 200-300°C at a rate of 1-10°C / min, holding for 16-24 h, and then naturally cooling to room temperature.

[0035] The present invention also provides a regenerated graphite obtained by the above recycling method.

[0036] The present invention also provides an application of the above regenerated graphite in a lithium battery.

[0037] Compared with the prior art, the present invention has the following technical effects:

[0038] By combining heat treatment and acid leaching, the present invention can effectively remove impurity elements in the negative electrode graphite of waste lithium batteries, including metal oxides such as copper, nickel, cobalt, and manganese, and other surface pollutants. This process helps to reduce the negative impact of these impurities on the performance of the negative electrode graphite; thereafter, through doping with nitrogen-rich organic matter and grinding reaction with elemental sulfur and red phosphorus, C-S-P bonds are formed, which can in-situ generate a continuous crystal line Li 3 P-based SEI layer on the surface of graphite, thereby repairing the surface defects of graphite, increasing the channels for lithium ion migration, and thus improving the capacity performance and cycling performance of the obtained regenerated graphite.

[0039] Compared with the prior art which often involves flash Joule heat and has high energy consumption, and generally uses coating materials such as carbon materials, inorganic oxides, and lithium carbonate salts, these coating materials cannot achieve the charge and discharge stability of the graphite negative electrode at high currents due to their low ionic conductivity; by mixing and treating waste graphite with nitrogen-rich organic matter, sulfur, red phosphorus, etc. to form C-S-P bonds, the present invention can in-situ generate a continuous crystal line Li 3 P-based SEI layer during the cycling process, and at the same time solve the problems that the waste graphite negative electrode is difficult to recycle and causes resource waste, and the cycling performance of the graphite negative electrode is poor at high currents, so that the negative electrode has high initial efficiency, excellent cycling performance, and high rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 is a process schematic diagram of the recycling method described in Embodiment 1 of the present invention;

[0041] Figure 2 is an XRD pattern of the regenerated graphite obtained by the recycling method described in Embodiment 1 of the present invention.

[0042] Figure 3 High-resolution transmission electron microscope image and EDS energy spectrum of the regenerated graphite obtained by the reuse method described in Embodiment 1 of the present invention;

[0043] Figure 4 XPS spectrum analysis of the regenerated graphite obtained by the reuse method described in Embodiment 1 of the present invention;

[0044] Figure 5 Discharge specific capacity curves of the regenerated graphite obtained by the reuse method described in Embodiment 1 of the present invention and the original waste graphite at a 3C rate. Detailed implementation mode

[0045] Next, the present invention will be described in detail with reference to specific embodiments. However, it should be clearly stated that these embodiments are for illustrative purposes only and are not to be construed as limiting the scope of the present invention.

[0046] Embodiment 1

[0047] Refer to Figure 1 , this embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries, including the following steps:

[0048] (1) Add the original waste graphite (SG) to concentrated sulfuric acid with a concentration of 98.3% at a mass ratio of 1:1, then add deionized water with a total mass of 20%. After stirring and mixing at room temperature for 0.5 h, filter to obtain a mixture of SG and sulfuric acid. Put this mixture into a muffle furnace for curing, cure at 200 °C for 20 h to obtain primary purified graphite; add the primary purified graphite to dilute sulfuric acid with a concentration of 20 wt%, stir and mix at 80 °C for 1 h, then filter, wash with deionized water until neutral, and dry at 80 °C for 12 h to obtain secondary purified graphite (PG);

[0049] (2) Add 5 g of PG to 50 mL of water dissolving 10 g of urea, stir evenly at room temperature, then transfer to an ultrasonic cell disruptor, ultrasonicate at 800 W for 2 h. After completion, centrifuge and separate, wash with deionized water to remove residual urea on the surface, filter by suction, and dry at 80 °C for 12 h to obtain intermediate graphite (N-PG);

[0050] (3) Add 4 g of N-PG and 1.2 mg of sulfur (S) powder into a mortar, grind to mix them evenly, seal the obtained mixture in a glove box with water and oxygen content both less than 0.1 ppm, take it out and pump it to vacuum, then place it in a muffle furnace for sintering. First, heat it from room temperature to 280 °C at a rate of 5 °C / min, keep the temperature for firing for 20 h, then cool it naturally to room temperature. Add the obtained mixed powder and 118.8 mg of red phosphorus (P) into the mortar, grind to mix them evenly. At this time, the mass ratio of each substance satisfies S / (S + P) = 1 / 100 and (S + P) / N-PG = 3 / 100. Seal the obtained mixture in a glove box with water and oxygen content both less than 0.1 ppm, take it out and pump it to vacuum, then place it in a muffle furnace for sintering. First, heat it from room temperature to 480 °C at a rate of 5 °C / min, keep the temperature for firing for 3 h, then cool it to 280 °C at a rate of 5 °C / min, keep the temperature for 20 h, and then cool it naturally to room temperature. The obtained sintered powder is successively washed with CS 2 and alcohol, centrifuged three times, and dried to obtain regenerated graphite (RG).

[0051] Figure 2 is the XRD pattern of the regenerated graphite obtained by the reuse method described in Example 1 of the present invention. Referring to Figure 2 it can be seen that compared with the waste graphite, the overall lattice structure of the regenerated graphite described in Example 1 is not damaged and maintains the original lattice structure. At the same time, the peak represented by the (002) crystal plane of the regenerated graphite shifts to the left. Thus, it can be seen that the interlayer spacing of the regenerated graphite expands, which is more conducive to the + extraction and insertion of Li

[0052] Figure 3 are the high-resolution transmission electron microscope image and EDS energy spectrum diagram of the regenerated graphite obtained by the reuse method described in Example 1 of the present invention. Referring to Figure 3 it can be seen that the regenerated graphite described in Example 1 uses S as an intermediate bridge, so that P is evenly attached to the graphite surface to form a coating layer. Figure 4 is the XPS spectrum analysis of the regenerated graphite obtained by the reuse method described in Example 1 of the present invention. Referring to Figure 4 it can be seen that the main component of this coating layer is the C-S-P bond. The formation of the C-S-P bond can generate a continuous crystal line Li 3 P-based SEI layer (Li 3 P has high ionic conductivity) during the battery cycling process, which helps the effective + desolvation process and the migration of Li + across the SEI layer.

[0053] Example 2

[0054] This embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (2), 5 g of PG is added to 50 mL of water dissolving 5 g of urea.

[0055] Embodiment 3

[0056] This embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (2), 5 g of PG is added to 50 mL of water dissolving 15 g of urea.

[0057] Embodiment 4

[0058] This embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (3), the temperature is raised from room temperature to 420 °C at a rate of 5 °C / min instead of 480 °C.

[0059] Embodiment 5

[0060] This embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (3), 12 mg of sulfur (S) powder is used instead of 1.2 mg of sulfur (S) powder, and 108 mg of red phosphorus (P) is used instead of 118.8 mg of red phosphorus (P).

[0061] Embodiment 6

[0062] This embodiment proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (2), 4 g of N-PG is further added to N-methyl-2-pyrrolidone. After ultrasonic dispersion until uniform, 0.3 g of ethylenediamine is added, and the mixture is stirred and reacted at 90 °C for 4 h. After filtration, washing, and drying, an amino-functionalized intermediate graphite is obtained; in step (3), 4 g of this amino-functionalized intermediate graphite is used instead of 4 g of N-PG.

[0063] Comparative Example 1

[0064] This comparative example proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (2), adding 10 g of urea is omitted, and 5 g of PG is directly added to 50 mL of water.

[0065] Comparative Example 2

[0066] This comparative example proposes a method for reusing the negative electrode graphite in waste lithium batteries. The specific steps refer to Embodiment 1, except that in step (3), adding 1.2 mg of sulfur (S) powder is omitted, and sintering in a muffle furnace is omitted. 4 g of N-PG and 120 mg of red phosphorus (P) are added to a mortar.

[0067] Comparative Example 3

[0068] This comparative example proposes a method for recycling negative electrode graphite in waste lithium batteries. The specific steps are as described in Example 1, except that in step (3), 4 g of N-PG, 1.2 mg of sulfur (S) powder, and 118.8 mg of red phosphorus (P) are added to a mortar and ground to make them uniformly mixed. The resulting mixture is sealed in a glove box with a water and oxygen content of less than 0.1 ppm, taken out and evacuated to a vacuum, and placed in a muffle furnace for sintering. The temperature is first raised from room temperature to 480°C at a rate of 5°C / min, and the temperature is kept at this temperature for 3 hours. The temperature is then lowered to 280°C at a rate of 5°C / min, and the temperature is kept at this temperature for 20 hours. Then, the sintered powder is naturally cooled to room temperature. The sintered powder is successively sintered with CS 2 , washed with alcohol, centrifuged three times, and dried to obtain regenerated graphite (RG).

[0069] Comparative Example 4

[0070] This comparative example proposes a method for recycling negative electrode graphite in waste lithium batteries. The specific steps are referred to Example 1, except that in step (3), the temperature is first increased from room temperature to 480°C at a rate of 5°C / min, kept at this temperature for 3 hours, and then directly cooled naturally to room temperature.

[0071] The present invention also provides a lithium battery, which is prepared by the following method:

[0072] In a glove box with water and oxygen content less than 0.1 ppm, a lithium sheet is used as a counter electrode, and the regenerated graphite (RG) described in the above embodiment or comparative example is used as a negative electrode, and is assembled into a half-cell with an electrolyte and a separator;

[0073] The electrolyte is prepared by mixing ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 1:1 in a glove box with a water and oxygen content of less than 0.1 ppm at room temperature, and adding 1 mol / L LiPF 6 , mixed thoroughly and allowed to stand for 24 hours to obtain; the diaphragm was punched into a 16 mm diameter disc through a polypropylene material film and used as a diaphragm, and transferred to a vacuum drying oven at 55°C and vacuum dried for 24 hours to obtain.

[0074] The above lithium battery was tested by Arbin BT2000 test system, with a charge and discharge voltage range of 0.01-3V, and 100 cycles (25°C) at 0.2C to obtain the first coulombic efficiency. In addition, the lithium battery was tested for high-rate performance by Arbin BT2000 test system, with a voltage range of 0.01-0.8V and a current density of 3C. The test results are shown in Table 1 below.

[0075] Table 1 Test results of lithium batteries corresponding to the embodiments or comparative examples

[0076]

[0077]

[0078] Figure 5 The discharge specific capacity curves of the regenerated graphite obtained by the reuse method described in Embodiment 1 of the present invention and commercial graphite at a 3C rate. From the above table and Figure 5 the results, it can be seen that the formation of C-S-P bonds in the regenerated graphite of the present invention can generate continuous crystal lines of Li in situ during the first charge and discharge process of the lithium battery 3 P-based SEI layer (Li 3 P has high ionic conductivity), which helps the effective Li + desolvation process and the migration of Li + across the SEI layer, enabling the regenerated graphite to still have a specific capacity of more than 90 mAh / g at a 3C current density (in contrast, commercial graphite only has a specific capacity of about 40 mAh / g at a 3C current density), and the regenerated graphite has excellent cycle stability. It can be seen that the regenerated graphite with a high ionic conductivity coating on the outside and N atoms inside can simultaneously have the advantages of high initial efficiency, excellent cycle stability, and rate performance. This is due to the ingenious utilization of the transformation of C-S-P bonds during the cycling process in this method, which enables the in-situ reaction to generate a coating layer with excellent physical and chemical properties, thereby improving the performance of secondary batteries.

[0079] As described above, the above are only the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present invention, according to the technical solution and inventive concept of the present invention, makes equivalent substitutions or changes, and all should be covered by the protection scope of the present invention.

Claims

1. A method for recycling negative electrode graphite in waste lithium batteries, characterized in that: The steps include: S1, subjecting the negative electrode graphite in the waste lithium battery to an acid leaching reaction to obtain purified graphite; S2, performing a doping reaction on the purified graphite and nitrogen-rich organic matter to obtain an intermediate graphite; S3, grinding and calcining the intermediate graphite with elemental sulfur, and then grinding and calcining it with red phosphorus to obtain regenerated graphite.

2. The method for recycling negative electrode graphite in waste lithium batteries according to claim 1, characterized in that: In step S1, the acid leaching reaction is achieved by soaking the negative electrode graphite in a concentrated sulfuric acid solution and then solidifying it; Preferably, the concentration of concentrated sulfuric acid is 90-99wt%; Preferably, the soaking temperature is 20-30°C and the soaking time is 0.5-1h; Preferably, the curing temperature is 180-220° C. and the curing time is 16-24 hours.

3. The method for recycling negative electrode graphite in waste lithium batteries according to claim 1 or 2, characterized in that: Step S1 also includes subjecting the purified graphite to a secondary acid leaching reaction; Preferably, the secondary acid leaching reaction is achieved by immersing the purified graphite in dilute sulfuric acid. Preferably, the concentration of the dilute sulfuric acid is 15-25wt%; Preferably, the soaking temperature is 70-90° C. and the soaking time is 0.5-2 h.

4. The method for recycling negative electrode graphite in waste lithium batteries according to any one of claims 1 to 3, characterized in that: In step S2, the doping reaction is achieved by adding the purified graphite and the nitrogen-rich organic matter into a solvent, ultrasonically dispersing them uniformly, and then drying them; Preferably, the mass ratio of the purified graphite to the nitrogen-rich organic matter is 1:1-3; Preferably, the ultrasonic dispersion power is 500-1000W, and the time is 1-3h; Preferably, the drying temperature is 70-90° C. and the drying time is 8-16 hours.

5. The method for recycling negative electrode graphite in waste lithium batteries according to claim 4, characterized in that: The nitrogen-rich organic matter is at least one of urea, melamine or dicyandiamide.

6. The method for recycling negative electrode graphite in waste lithium batteries according to any one of claims 1 to 5, characterized in that: Step S2 also includes subjecting the intermediate graphite to a covalent functionalization reaction with a polyamine; Preferably, the polyamine is at least one of ethylenediamine, butanediamine or hexamethylenediamine; Preferably, the mass ratio of the intermediate graphite to the polyamine is 1:0.05-0.1; Preferably, the covalent bond functionalization reaction temperature is 80-100° C. and the reaction time is 3-6 h.

7. The method for recycling negative electrode graphite in waste lithium batteries according to any one of claims 1 to 6, characterized in that: In step S3, the mass ratio of the intermediate graphite to red phosphorus and elemental sulfur is 1:0.05-0.06:0.0005-0.01; Preferably, the grinding is achieved in a mortar.

8. The method for recycling negative electrode graphite in waste lithium batteries according to any one of claims 1 to 7, characterized in that: In step S3, the calcination with elemental sulfur includes: firstly heating to 200-300°C at a rate of 1-10°C / min, keeping the temperature for 16-24h, and then naturally cooling to room temperature; The calcination with red phosphorus comprises: firstly heating to 400-500°C at a rate of 1-10°C / min, keeping the temperature for 2-4h, then cooling to 200-300°C at a rate of 1-10°C / min, keeping the temperature for 16-24h, and then naturally cooling to room temperature.

9. A regenerated graphite, characterized in that: It is obtained by the recycling method described in any one of claims 1 to 8.

10. Use of the regenerated graphite according to claim 9 in lithium batteries.

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