Method for stripping surface positive electrode material of current collector by low-temperature frequency conversion ultrasonic cooperation

Abstract

The application discloses a method for stripping positive electrode material from current collector by low-temperature frequency conversion ultrasonic waves, and belongs to the technical field of resource regeneration of waste lithium batteries. The method comprises the steps of cutting positive electrode sheets, lowering temperature and performing vitrification treatment, and performing low-temperature frequency conversion ultrasonic wave treatment. The method takes advantage of the characteristics that PVDF adhesive vitrifies at-40 DEG C, the bonding strength is reduced and the brittleness is enhanced, rapidly lowers the temperature of the positive electrode sheets to-40 DEG C to make microcracks occur in the positive electrode sheets, and then uses frequency conversion ultrasonic waves to expand the microcracks, so that the positive electrode material is efficiently and cleanly separated from the current collector. The method is suitable for stripping positive electrode material from lithium batteries, such as lithium iron phosphate batteries, ternary lithium batteries, lithium cobalt oxide batteries and lithium manganese oxide batteries, which use PVDF as the adhesive and use aluminum foil as the current collector.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of waste lithium battery resources, and particularly relates to a method for stripping positive electrode material on the surface of a current collector at low temperature through frequency conversion ultrasonic waves. BACKGROUND

[0002] Lithium batteries have a rapidly increasing number of applications in mobile electronic devices, new energy vehicles and the like due to high energy density, good cycle performance, no memory effect, low self-discharge rate, small size and light weight. The average service life of a lithium battery is 3-6 years. Therefore, the popularity of lithium batteries not only brings great convenience to life, but also continuously and rapidly increases the amount of scrap. A large amount of waste lithium batteries, if not properly treated, will not only harm the environment, but also cause resource waste. Therefore, treating waste lithium batteries in an environmentally friendly and economical manner and recycling valuable materials therefrom have far-reaching significance for relieving social resource and environmental pressure and eliminating potential harm to humans.

[0003] Valuable elements such as Li and Co in waste lithium batteries are concentrated in the positive electrode of the battery. The positive electrode material and the current collector are bonded together by an organic binder PVDF. Therefore, the key step for recycling the positive electrode material is to strip the positive electrode material from the aluminum foil current collector while avoiding the introduction of impurities to complicate the subsequent treatment process. At present, the main methods for stripping the positive electrode material from the current collector include mechanical crushing, heat treatment, organic solvent dissolution and alkali dissolution.

[0004] The mechanical crushing method uses a machine to crush and grind the positive electrode sheet of the waste lithium battery, and separates the positive electrode material from the current collector by mechanical forces such as grinding, stripping and shearing. This method has low separation efficiency and serious over-pulverization of the current collector, resulting in excessive copper and aluminum content in the positive electrode material. The heat treatment method separates the current collector from the positive electrode material by high-temperature decomposition of the PVDF binder therebetween. The high-temperature calcination is time-consuming and energy-consuming. Not only does the toxic and harmful gas generated by the decomposition of PVDF need to be specially treated, but also the composition and structure of the positive electrode material are changed. The organic solvent dissolution method uses an organic solvent to dissolve PVDF based on the principle of "like dissolves like". Although this method does not change the composition and structure of the material, the high cost and toxicity of the organic solvent limit the large-scale application of this method. The alkali dissolution method separates the current collector from the positive electrode material by taking advantage of the property that the aluminum foil is dissolved in an alkali solution while the positive electrode material is not dissolved in the alkali solution. This method consumes a large amount of high-concentration acid and alkali reagents and generates a large amount of waste liquid. Not only is the cost high, but also the equipment is severely corroded, and the economic benefit is low.

[0005] Ultrasonic cavitation can be used to remove stubborn stains on the surface of the object, so researchers try to use ultrasonic waves to peel the positive electrode material from the current collector. The positive electrode material and PVDF mixed slurry are dried after rolling, and the combination of the current collector is very firm. Ultrasonic peeling usually needs to be assisted by oscillation, pyrolysis, acid etching, dissolution and other means to enhance the peeling effect. Although the use of ultrasonic waves realizes the peeling of the current collector and the positive electrode material, the introduction of auxiliary means makes the above method still have the defects of long process, high energy consumption, strong toxicity and large pollution. Moreover, when the positive electrode material and the current collector are peeled by ultrasonic waves in the conventional method, the peeling effect is usually improved by increasing the input power, but the current collector is made of aluminum foil and has a thickness of only 8-10 μm. Under the strong cavitation effect of ultrasonic waves, it is easy to be eroded and broken, resulting in the mixing of aluminum scraps in the peeled positive electrode material, which reduces the purity of the peeled product. Therefore, how to use an environmentally friendly method to safely and efficiently realize the peeling of the positive electrode material and the current collector while ensuring the high purity of the peeled product is a technical problem faced by ultrasonic peeling technology. SUMMARY

[0006] The present application provides a method for low-temperature frequency conversion ultrasonic cooperative peeling of positive electrode material on the surface of a current collector, which aims to overcome the defects of long process, high energy consumption, strong toxicity, large pollution and low product purity in the current ultrasonic peeling method of positive electrode material and current collector, and to provide an environmentally friendly, safe and efficient method for peeling positive electrode material and current collector while ensuring the high purity of the peeled product.

[0007] In a first aspect, a method for low-temperature frequency conversion ultrasonic cooperative peeling of positive electrode material on the surface of a current collector is provided, comprising:

[0008] immersing the positive electrode sheet in a liquid medium at-40℃ to rapidly cool the positive electrode sheet from room temperature to-40℃;

[0009] performing frequency conversion ultrasonic treatment on the positive electrode sheet in the liquid medium at-40℃, so that the positive electrode material and the current collector are completely separated.

[0010] In combination with the first aspect, in some implementations of the first aspect, the liquid medium is an alcohol aqueous solution with a freezing point lower than-40℃.

[0011] In combination with the first aspect, in some implementations of the first aspect, the liquid medium includes any of the following: ethanol, ethylene glycol and propylene glycol.

[0012] In combination with the first aspect, in some implementations of the first aspect, the liquid medium is an ethanol aqueous solution, and the volume fraction of ethanol in the ethanol aqueous solution is greater than or equal to 63%.

[0013] In combination with the first aspect, in some implementations of the first aspect, the liquid medium is an ethylene glycol aqueous solution, and the volume fraction of ethylene glycol in the ethylene glycol aqueous solution is 54% to 68%.

[0014] With reference to the first aspect, in some implementations of the first aspect, the liquid medium is a propylene glycol aqueous solution, and a volume fraction of ethylene glycol in the propylene glycol aqueous solution is 52% to 85%.

[0015] With reference to the first aspect, in some implementations of the first aspect, the cooling rate of the positive electrode sheet in the liquid medium at -40℃ is greater than or equal to 75℃ / s.

[0016] With reference to the first aspect, in some implementations of the first aspect, the frequency variation law of the frequency conversion ultrasonic treatment is that the ultrasonic frequency is 20KHz at the start of the ultrasonic treatment, the ultrasonic frequency is increased at a rate of 2KHz after the start of the ultrasonic treatment, and the ultrasonic treatment is ended when the ultrasonic frequency is increased to 80KHz after 30s.

[0017] With reference to the first aspect, in some implementations of the first aspect, the frequency variation law of the frequency conversion ultrasonic treatment is that the ultrasonic frequency is 20KHz at the start of the ultrasonic treatment, the ultrasonic frequency is increased at a rate of 2KHz after the start of the ultrasonic treatment, and the ultrasonic treatment is ended when the ultrasonic frequency is increased to 80KHz after 30s.

[0018] With reference to the first aspect, in some implementations of the first aspect, before the cooling, the positive electrode sheet obtained by disassembling the battery is first cut into a rectangular sheet shape at 25℃±10℃, and a length of a side of the rectangle is greater than or equal to 10mm and less than or equal to 50mm.

[0019] Compared with the prior art, the scheme provided by the present application has at least the following beneficial technical effects:

[0020] (1) In the process of rapidly immersing the positive electrode sheet at room temperature into the ultrasonic liquid medium at -40℃ to cool the positive electrode sheet at a cooling rate greater than or equal to 75℃ / s, on the one hand, different components of the positive electrode sheet will produce uneven deformation in the rapid cooling process, and when the cohesion between the materials is not enough to resist the anisotropy generated by these uneven changes, micro-cracks will be generated in the electrode material; on the other hand, the thermal expansion coefficient of PVDF is 127.8×10 -6 / K, and the thermal expansion coefficient of the aluminum current collector is 23.6×10 -6 / K, and the difference between the two is large, and the rapid drop in temperature causes a large stress mismatch between the interface of PVDF and the aluminum current collector, which promotes the cracking of the bonding interface, and both effects are conducive to the peeling of the positive electrode material and the current collector.

[0021] (2) PVDF is a crystalline polymer with a crystallinity of about 50%, which undergoes glass transition at -40℃, thereby converting from a high-elastic state to a glass state with brittleness, greatly weakening the adhesion properties of the polymer molecules, which is conducive to the peeling of the positive electrode material; and the aluminum foil current collector is a face-centered cubic crystal, and the strength is not only not reduced at -40℃, but also increased by about 10%, which is conducive to ensuring the integrity of the current collector during ultrasonic treatment.

[0022] (3) The variable frequency ultrasonic treatment method uses low frequency ultrasonic waves to peel off the positive electrode material in the initial stage. Under low frequency conditions, the cavitation nuclei in the ultrasonic medium are large, and the impact effect on the PVDF adhesive on the surface of the positive electrode sheet is stronger, which can quickly peel off the surface positive electrode material. As the positive electrode material on the current collector surface gradually falls off, the integrity of the positive electrode material film structure decreases and the aluminum foil is exposed. At this time, the ultrasonic frequency is increased, the cavitation nuclei size is reduced and the cavitation nuclei density is increased. The impact force on the peeling surface is weakened but the effect is more uniform, ensuring the peeling effect of the positive electrode material while ensuring the integrity of the current collector structure and avoiding the introduction of metal impurities. Attached Figure Description

[0023] Figure 1 The present invention provides a flowchart of a method for low-temperature variable frequency ultrasonic synergistic stripping of cathode material on the surface of current collector. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0025] like Figure 1 As shown, the present invention provides a method for low-temperature variable frequency ultrasonic synergistic stripping of positive electrode material on the surface of current collector, comprising the following steps.

[0026] Step (1): Cut the positive electrode obtained from disassembling the battery into sheets at room temperature.

[0027] In step (1), the ambient temperature is 25℃±10℃, and the positive electrode is cut into a rectangle with a side length greater than or equal to 10mm and less than or equal to 50mm.

[0028] Step (2): Immerse the cut positive electrode sheet in a liquid medium at -40°C to rapidly cool the positive electrode sheet from room temperature to -40°C.

[0029] In step (2), the liquid medium is an aqueous solution of an alcohol with a freezing point below -40°C, preferably an aqueous solution of ethanol, ethylene glycol, or propylene glycol.

[0030] Furthermore, the volume fraction of ethanol in the aqueous ethanol solution is ≥63%.

[0031] Furthermore, the volume fraction of ethylene glycol in the aqueous ethylene glycol solution is 54%–68%.

[0032] Furthermore, the volume fraction of ethylene glycol in the propylene glycol aqueous solution is 52%–85%.

[0033] In step (2), the cooling rate of the positive electrode in the -40℃ liquid medium is ≥75℃ / s.

[0034] Step (3), the positive electrode sheet is subjected to frequency conversion ultrasonic treatment in a liquid medium at -40℃, and the positive electrode material is completely separated from the current collector.

[0035] In step (3), the ultrasonic frequency is 20-80 KHz, the power density is 20 W / L, and the duration of the frequency conversion ultrasonic treatment is 30 s.

[0036] In step (3), the frequency conversion ultrasonic frequency varies as follows: the ultrasonic frequency is 20 KHz at the start of the ultrasonic treatment, the ultrasonic frequency increases at a rate of 2 KHz after the start of the ultrasonic treatment, the ultrasonic treatment ends when the ultrasonic frequency increases to 80 KHz after 30 s.

[0037] The method disclosed in the present application is suitable for stripping of positive electrode materials of lithium batteries using PVDF as a binder and aluminum foil as a current collector, including but not limited to stripping of positive electrode materials and current collectors of lithium iron phosphate batteries, ternary lithium batteries, lithium cobalt oxide batteries and lithium manganese oxide batteries.

[0038] Example 1

[0039] The positive electrode material and the current collector on the surface of a positive electrode sheet of a lithium iron phosphate battery are stripped, and the steps are as follows:

[0040] (1) The positive electrode sheet obtained by disassembling a lithium iron phosphate battery is cut into a 10 mm x 10 mm sheet shape at 25℃.

[0041] (2) The cut positive electrode sheet is immersed in a -40℃, 75% by volume ethanol aqueous solution, and the positive electrode sheet is cooled from 25℃ to -40℃ at a rate of 100℃ / s.

[0042] (3) The positive electrode sheet is subjected to frequency conversion ultrasonic treatment in a liquid medium at -40℃, and the positive electrode material is completely separated from the current collector.

[0043] Example 2

[0044] The positive electrode material and the current collector on the surface of a positive electrode sheet of a ternary lithium battery are stripped, and the steps are as follows:

[0045] (1) The positive electrode sheet obtained by disassembling a ternary lithium battery is cut into a 15 mm x 15 mm sheet shape at 30℃.

[0046] (2) The cut positive electrode sheet is immersed in a -40℃, 60% by volume ethylene glycol aqueous solution, and the positive electrode sheet is cooled from 30℃ to -40℃ at a rate of 80℃ / s.

[0047] (3) The positive electrode sheet is subjected to variable frequency ultrasonic treatment in a liquid medium at -40 DEG C, the ultrasonic power density is 20 W / L, the ultrasonic frequency is 20 KHz when starting, the ultrasonic frequency is increased at a rate of 2 KHz after starting, the ultrasonic frequency is increased to 80 KHz after 30 s, the ultrasonic treatment is ended, and the positive electrode material is completely separated from the current collector.

[0048] Example 3

[0049] The positive electrode material and the current collector on the surface of the positive electrode sheet of the lithium cobalt oxide battery are stripped, and the steps are as follows:

[0050] (1) The positive electrode sheet obtained by disassembling the lithium cobalt oxide battery is cut into a 20 mm x 20 mm sheet at 20 DEG C.

[0051] (2) The cut positive electrode sheet is immersed in a 60% volume fraction of the glycol aqueous solution at -40 DEG C, and the positive electrode sheet is cooled from 20 DEG C to -40 DEG C at a rate of 120 DEG C / s.

[0052] (3) The positive electrode sheet is subjected to variable frequency ultrasonic treatment in a liquid medium at -40 DEG C, the ultrasonic power density is 20 W / L, the ultrasonic frequency is 20 KHz when starting, the ultrasonic frequency is increased at a rate of 2 KHz after starting, the ultrasonic frequency is increased to 80 KHz after 30 s, the ultrasonic treatment is ended, and the positive electrode material is completely separated from the current collector.

[0053] Although the present application is disclosed with the preferred embodiments as above, it is not intended to limit the present application, and any person skilled in the art can make possible changes and modifications without departing from the spirit and scope of the present application, therefore, the protection scope of the present application should be defined by the scope of the claims of the present application.

Claims

1. A method for low-temperature frequency conversion ultrasonic synergistic stripping of current collector surface positive electrode material, characterized by, The application relates to a method for separating a positive electrode material from a current collector of a positive electrode sheet. The positive electrode sheet is immersed in a liquid medium at-40 DEG C, so that the positive electrode sheet is rapidly cooled from normal temperature to-40 DEG C. The positive electrode sheet is subjected to frequency-variable ultrasonic treatment in the liquid medium at-40 DEG C, so that the positive electrode material is completely separated from the current collector.

2. The method of claim 1, wherein, The liquid medium is an alcohol aqueous solution with a freezing point lower than-40 DEG C.

3. The method of claim 2, wherein, The liquid medium comprises any one of the following: ethanol, ethylene glycol and propylene glycol.

4. The method of claim 3, wherein, The liquid medium is an ethanol aqueous solution, and the volume fraction of ethanol in the ethanol aqueous solution is greater than or equal to 63%.

5. The method of claim 3, wherein, The liquid medium is an ethylene glycol aqueous solution, and the volume fraction of ethylene glycol in the ethylene glycol aqueous solution is 54% to 68%.

6. The method of claim 3, wherein, The liquid medium is a propylene glycol aqueous solution, and the volume fraction of ethylene glycol in the propylene glycol aqueous solution is 52% to 85%.

7. The method of claim 3, wherein, The cooling rate of the positive electrode sheet in the liquid medium at-40 DEG C is greater than or equal to 75 DEG C / s.

8. The method of claim 1, wherein, The frequency-variable ultrasonic treatment satisfies the following conditions: the ultrasonic frequency is 20 to 80 KHz, the power density is 20 W / L, and the duration of the frequency-variable ultrasonic treatment is 30 s.

9. The method of claim 8, wherein, The frequency variation rule of the frequency-variable ultrasonic treatment is as follows: the ultrasonic frequency is 20 KHz when the ultrasonic treatment is started, the ultrasonic frequency is increased at a rate of 2 KHz after the ultrasonic treatment is started, the ultrasonic treatment is ended when the ultrasonic frequency is increased to 80 KHz after 30 s.

10. The method of claim 1, wherein, Before cooling, the positive electrode sheet obtained by disassembling the battery is cut into a rectangular sheet shape at 25 DEG C plus or minus 10 DEG C, and the length of the rectangle is greater than or equal to 10 mm and less than or equal to 50 mm.

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