Method for stripping positive electrode material on surface of current collector through low-temperature variable-frequency ultrasonic cooperation

Abstract

The invention discloses a method for stripping a positive electrode material on the surface of a current collector through low-temperature variable-frequency ultrasonic cooperation, and belongs to the technical field of waste lithium battery resource regeneration. The method comprises the steps of positive plate cutting, cooling vitrification treatment, low-temperature variable-frequency ultrasonic treatment and the like. According to the invention, the characteristics that the PVDF binder is vitrified at-40 DEG C, the bonding strength is reduced and the brittleness is enhanced are utilized, the battery positive plate is rapidly cooled to-40 DEG C, so that microcracks are generated in the battery positive plate, and then the microcracks are expanded by utilizing variable-frequency ultrasonic waves, so that the efficient cleaning separation between the positive material and the current collector is realized. The method is suitable for stripping the positive electrode material of the lithium battery, such as a lithium iron phosphate battery, a ternary lithium battery, a lithium cobalt oxide battery, a lithium manganate battery and the like, which takes PVDF (Polyvinylidene Fluoride) as a binder and an aluminum foil as a current collector.

Description

Technical Field

[0001] The invention relates to the technical field of waste lithium battery resources, in particular to a method for low-temperature variable-frequency ultrasound-assisted stripping of positive electrode materials on the surface of a current collector. Background Art

[0002] Lithium batteries are rapidly increasing in the fields of mobile electronic devices, new energy vehicles, etc. due to their high energy density, good cycle performance, no memory effect, low self-discharge rate, small size and light weight. The average service life of lithium batteries is 3-6 years. Therefore, while the popularity of lithium batteries has brought great convenience to life, the amount of scrapped lithium batteries has also continued to grow rapidly. If a large number of waste lithium batteries are not properly handled, they will not only harm the environment, but also cause waste of resources. Therefore, using environmentally friendly and economical methods to handle waste lithium batteries and recover valuable materials in them has far-reaching significance for alleviating social resource and environmental pressures and eliminating potential hazards 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 the organic binder PVDF. Therefore, the key step in recycling the positive electrode material is to peel the positive electrode material from the current collector aluminum foil while avoiding the introduction of impurities that complicate the subsequent processing process. At present, the main methods for peeling the positive electrode material from the current collector are mechanical crushing, heat treatment, organic solvent dissolution, and alkaline dissolution.

[0004] The mechanical crushing method is to use machinery to crush and grind the positive electrode sheets of waste lithium batteries, and separate the positive electrode material from the current collector by mechanical forces such as grinding, peeling and shearing. This method has low separation efficiency and serious over-crushing of the current collector, resulting in excessive copper and aluminum content in the positive electrode material; the heat treatment method is to separate the current collector and the positive electrode material by high-temperature decomposition of the PVDF binder between the current collector and the positive electrode material. High-temperature calcination takes a long time and consumes high energy. Not only does the toxic and harmful gases produced by the decomposition of PVDF require special treatment, but it will also change the composition and structure of the positive electrode material; the organic solvent dissolution method is to use 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 alkaline dissolution method is to separate the two by using the property that aluminum foil dissolves in an alkaline solution and the positive electrode material is insoluble in alkaline solution. This method consumes a large amount of high-concentration acid and alkali reagents and produces a large amount of waste liquid. It is not only costly, but also seriously corrodes the equipment and has low economic benefits.

[0005] Ultrasonic waves can produce cavitation in liquids and can be used to remove stubborn stains on the surface of objects. Therefore, researchers have tried to use ultrasound to strip the positive electrode material from the current collector. After roller pressing and drying, the mixed slurry of positive electrode material and PVDF is very firmly bonded to the current collector. Ultrasonic stripping usually requires oscillation, pyrolysis, acid etching, dissolution and other means to enhance the stripping effect. Although the stripping of the current collector and the positive electrode material is achieved by ultrasound, the introduction of auxiliary means still makes the above method have defects such as long process, high energy consumption, strong toxicity and high pollution. Not only that, when the positive electrode material and the current collector are ultrasonically stripped by conventional methods, the method of increasing the input power is usually used to improve the stripping effect. However, since the current collector is made of aluminum foil and the thickness is only 8 to 10 μm, it is very easy to be eroded and broken under the strong cavitation effect of ultrasound, resulting in aluminum chips mixed in the stripped positive electrode material, reducing the purity of the stripped product. Therefore, how to use an environmentally friendly way to safely and efficiently achieve the stripping of the positive electrode material and the current collector while ensuring the high purity of the stripped product is a technical problem faced by ultrasonic stripping technology. Summary of the invention

[0006] The present invention provides a method for low-temperature variable-frequency ultrasonic synergistic stripping of positive electrode materials on the surface of a current collector, with the aim of overcoming the defects of the current ultrasonic stripping of positive electrode materials and current collectors, such as long process, high energy consumption, strong toxicity, high pollution, and low product purity, and providing an environmentally friendly, safe and efficient method for stripping positive electrode materials and current collectors, while ensuring the high purity of the stripping products.

[0007] In a first aspect, a method for low-temperature variable frequency ultrasound synergistic stripping of positive electrode materials on the surface of a current collector is provided, comprising:

[0008] Immerse the positive electrode in a liquid medium at -40°C to rapidly cool the positive electrode from room temperature to -40°C;

[0009] The positive electrode sheet was subjected to variable frequency ultrasonic treatment in a liquid medium at -40°C, and the positive electrode material was completely separated from the current collector.

[0010] In combination with the first aspect, in certain implementations of the first aspect, the liquid medium is an alcohol aqueous solution with a freezing point below -40°C.

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

[0012] In combination with the first aspect, in certain 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 ≥ 63%.

[0013] In combination with the first aspect, in certain 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] In combination with the first aspect, in certain implementations of the first aspect, 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%.

[0015] In combination with the first aspect, in certain implementations of the first aspect, the cooling rate of the positive electrode sheet in a liquid medium at -40°C is ≥75°C / s.

[0016] In combination with the first aspect, in certain implementations of the first aspect, the variable frequency ultrasonic treatment meets the following conditions: the ultrasonic frequency is 20 to 80 kHz, the power density is 20 W / L, and the variable frequency ultrasonic treatment duration is 30 s.

[0017] In combination with the first aspect, in certain implementations of the first aspect, the frequency change rule of the variable frequency ultrasonic processing is: the frequency is 20KHz when the ultrasonic wave is started, and the ultrasonic frequency increases at a rate of 2KHz after starting. When the ultrasonic frequency increases to 80KHz after 30s, the ultrasonic processing ends.

[0018] In combination with the first aspect, in certain implementations of the first aspect, before cooling the positive electrode sheet, the positive electrode sheet obtained by disassembling the battery is cut into rectangular sheets at 25°C±10°C, and the side length of the rectangle is greater than or equal to 10 mm and less than or equal to 50 mm.

[0019] Compared with the prior art, the solution provided by the present invention includes at least the following beneficial technical effects:

[0020] (1) When the positive electrode sheet at room temperature is rapidly immersed in a -40℃ ultrasonic liquid medium and cooled at a cooling rate of ≥75℃ / s, on the one hand, different components of the positive electrode sheet will produce uneven deformation during the rapid cooling process. When the cohesive force between the materials is not enough to resist the anisotropy caused by these uneven changes, microcracks will appear in the electrode material; on the other hand, the thermal expansion coefficient of PVDF is 127.8×10 -6 K, the thermal expansion coefficient of the aluminum current collector is 23.6×10 -6 / K, the difference between the two is large. The rapid drop in temperature causes a huge stress mismatch between the interface of PVDF and the aluminum current collector, which causes the bonding interface to crack. 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%. It undergoes a glass transition at -40°C, transforming from a highly elastic state to a brittle glass state, which greatly weakens the adhesion of the polymer molecules and is beneficial to the peeling of the positive electrode material. The current collector aluminum foil is a face-centered cubic crystal. Its strength does not decrease at a low temperature of -40°C, but is increased by about 10%, which is beneficial to ensure the integrity of the current collector during ultrasonic treatment.

[0022] (3) Variable frequency ultrasonic treatment method: In the initial stage, low-frequency ultrasonic waves are used to strip the positive electrode material. Under low-frequency conditions, the size of the cavitation nuclei in the ultrasonic medium is large, and the impact effect on the PVDF adhesive on the surface of the positive electrode sheet is stronger, which can quickly strip the surface positive electrode material. As the positive electrode material on the surface of the current collector gradually falls off, the integrity of the positive electrode material membrane structure decreases and the aluminum foil is exposed. At this time, the ultrasonic frequency is increased to reduce the size of the cavitation nuclei while increasing the density of the cavitation nuclei. The impact force on the stripping surface is weakened but the effect is more uniform, ensuring the positive electrode material stripping effect while ensuring the integrity of the current collector structure and avoiding the introduction of metal impurities. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 A flow chart of a method for low-temperature variable-frequency ultrasound coordinated stripping of positive electrode materials on the surface of a current collector provided by the present invention. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below 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 ultrasound synergistic stripping of positive electrode materials on the surface of a current collector, comprising the following steps.

[0026] Step (1), cutting the positive electrode sheet obtained by disassembling the battery into sheets at room temperature.

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

[0028] Step (2), immersing the cut positive electrode sheet in a liquid medium at -40°C, so that the positive electrode sheet is quickly cooled from room temperature to -40°C.

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

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

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

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

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

[0034] Step (3), subjecting the positive electrode sheet to variable frequency ultrasonic treatment in a liquid medium at -40°C, so that the positive electrode material and the current collector are completely separated.

[0035] In step (3), the ultrasonic frequency is 20 to 80 kHz, the power density is 20 W / L, and the variable frequency ultrasonic treatment lasts for 30 seconds.

[0036] In step (3), the frequency change rule of the variable frequency ultrasonic wave is that the frequency is 20KHz when the ultrasonic wave is started, and the ultrasonic wave frequency increases at a rate of 2KHz after starting. When the ultrasonic wave frequency increases to 80KHz after 30s, the ultrasonic treatment is completed.

[0037] The method proposed in the present invention is suitable for the 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 the 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 the positive electrode sheet of the lithium iron phosphate battery are peeled off. The steps are as follows:

[0040] (1) At 25°C, the positive electrode sheet obtained by disassembling the lithium iron phosphate battery was cut into 10 mm × 10 mm sheets.

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

[0042] (3) The positive electrode sheet was subjected to variable frequency ultrasonic treatment in a liquid medium at -40°C, with an ultrasonic power density of 20W / L and an ultrasonic start frequency of 20KHz. After starting, the ultrasonic frequency increased at a rate of 2KHz. After 30s, the ultrasonic frequency increased to 80KHz, and the ultrasonic treatment was completed. The positive electrode material and the current collector were completely separated.

[0043] Example 2

[0044] The positive electrode material and the current collector on the surface of the positive electrode sheet of the ternary lithium battery are peeled off in the following steps:

[0045] (1) At 30°C, the positive electrode sheet obtained by disassembling the ternary lithium battery was cut into 15 mm × 15 mm sheets.

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

[0047] (3) The positive electrode sheet was subjected to variable frequency ultrasonic treatment in a liquid medium at -40°C, with an ultrasonic power density of 20W / L and an ultrasonic start frequency of 20KHz. After starting, the ultrasonic frequency increased at a rate of 2KHz. After 30s, the ultrasonic frequency increased to 80KHz, and the ultrasonic treatment was completed. The positive electrode material and the current collector were completely separated.

[0048] Example 3

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

[0050] (1) At 20°C, the positive electrode sheet obtained by disassembling the lithium cobalt oxide battery was cut into 20 mm × 20 mm sheets.

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

[0052] (3) The positive electrode sheet was subjected to variable frequency ultrasonic treatment in a liquid medium at -40°C, with an ultrasonic power density of 20W / L and an ultrasonic start frequency of 20KHz. After starting, the ultrasonic frequency increased at a rate of 2KHz. After 30s, the ultrasonic frequency increased to 80KHz, and the ultrasonic treatment was completed. The positive electrode material and the current collector were completely separated.

[0053] Although the present invention is disclosed as above in the form of a preferred embodiment, it is not intended to limit the present invention. Any person skilled in the art may make possible changes and modifications without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be based on the scope defined by the claims of the present invention.

Claims

1. A method for stripping positive electrode materials on the surface of a current collector by low-temperature variable frequency ultrasound, characterized in that: include: Immerse the positive electrode in a liquid medium at -40°C to rapidly cool the positive electrode from room temperature to -40°C; The positive electrode sheet was subjected to variable frequency ultrasonic treatment in a liquid medium at -40°C, and the positive electrode material was completely separated from the current collector.

2. The method according to claim 1, characterized in that The liquid medium is an alcohol aqueous solution with a freezing point below -40°C.

3. The method according to claim 2, characterized in that The liquid medium includes any of the following: ethanol, ethylene glycol and propylene glycol.

4. The method according to claim 3, characterized in that The liquid phase medium is an ethanol aqueous solution, and the volume fraction of ethanol in the ethanol aqueous solution is ≥63%.

5. The method according to claim 3, characterized in that: 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 according to claim 3, characterized in that The liquid phase medium is a propylene glycol aqueous solution, in which the volume fraction of ethylene glycol is 52% to 85%.

7. The method according to claim 3, characterized in that The cooling rate of the positive electrode in a liquid medium at -40°C is ≥75°C / s.

8. The method according to claim 1, characterized in that The variable frequency ultrasonic treatment meets the following conditions: the ultrasonic frequency is 20-80KHz, the power density is 20W / L, and the variable frequency ultrasonic treatment duration is 30s.

9. The method according to claim 8, characterized in that The frequency variation rule of variable frequency ultrasonic treatment is as follows: the frequency is 20KHz when the ultrasonic wave is started, and the ultrasonic wave frequency increases at a rate of 2KHz after starting. When the ultrasonic wave frequency increases to 80KHz after 30s, the ultrasonic treatment ends.

10. The method according to claim 1, characterized in that Before cooling the positive electrode sheet, the positive electrode sheet obtained by disassembling the battery is cut into rectangular sheets at 25°C±10°C, and the side length of the rectangle is greater than or equal to 10 mm and less than or equal to 50 mm.

Images