Method for recycling positive electrode active material

Abstract

A method for recovering and reusing active materials from positive electrode waste is provided. The positive electrode active material recycling method of the present invention comprises the following steps: (a-1) dry grinding positive electrode waste containing a positive electrode active material layer of a lithium composite transition metal oxide on a current collector, thereby desorbing the active material layer in a powder form and thus separating it from the current collector; (a-2) heat-treating the desorbed active material layer in a powder form in air, thereby thermally decomposing a binder and a conductive material contained in the active material layer, and thus recovering the active material; (b) washing the recovered active material with a lithium compound solution that is alkaline in an aqueous solution state and drying it; and (c) adding a lithium precursor to the washed active material and annealing it to obtain a reusable active material.

Description

Technical Field

[0001] The present disclosure relates to a method for recycling resources in the manufacture of lithium secondary batteries. More specifically, the present disclosure relates to a method for collecting and recycling positive electrode waste generated in the lithium secondary battery manufacturing process or positive electrode active materials of lithium secondary batteries discarded after use. This application claims the benefit of Korean Patent Application No. 10-2020-0134325 filed with the Korean Intellectual Property Office on October 16, 2020, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0002] Rechargeable lithium secondary batteries are gaining attention as a substitute for fossil energy. They are mainly used in traditional handheld devices such as mobile phones, cameras and power tools. Recently, the scope of application tends to gradually expand to vehicles powered by electricity (EV, HEV, PHEV), large-capacity energy storage systems (ESS) and uninterruptible power supply systems (UPS).

[0003] A lithium secondary battery comprises: an electrode assembly, the electrode assembly comprises a unit cell, each unit cell comprises a positive electrode plate and a negative electrode plate, the positive electrode plate and the negative electrode plate comprise a current collector and an active material coated on the current collector, a separator is interposed between the positive electrode plate and the negative electrode plate; and a package or a battery case, the electrode assembly is sealed and contained in the package or battery case together with an electrolyte solution. The lithium secondary battery mainly comprises a lithium-based oxide as a positive electrode active material and a carbon-based material as a negative electrode active material. The lithium-based oxide contains metals such as cobalt, nickel or manganese. In particular, cobalt, nickel and manganese are very expensive and valuable metals. Among them, cobalt is a strategic metal, and its supply is the focus of attention around the world. Due to the limited number of cobalt producing countries, the global cobalt supply is unstable. When an imbalance in the supply and demand of strategic metals occurs, the possibility that the cost of raw materials will rise is very high.

[0004] The collection and recovery of valuable metals from lithium secondary batteries (scrap batteries) discarded after expiration has been studied. In addition to scrap batteries, resources can be more preferably collected from scrap discarded after punching positive plates or positive electrodes that have defects or failures during the process.

[0005] Currently, if Figure 1 As shown, a lithium secondary battery is manufactured by coating a positive electrode slurry containing a positive electrode active material, a conductive material, a binder and a solvent on a long sheet type positive electrode collector 10 such as an aluminum (Al) foil to form a positive electrode active material layer 20, manufacturing a positive electrode sheet 30, and punching out a positive electrode sheet 40 having a predetermined size. The remainder after punching is discarded as a positive electrode waste 50. If the positive electrode active material is collected and reused from the positive electrode waste 50, it is very desirable in terms of industrial economy and environment.

[0006] Most existing methods for collecting positive active materials include dissolving the positive electrode with hydrochloric acid, sulfuric acid, nitric acid, etc., extracting active material elements such as cobalt, nickel and manganese, and using them as raw materials for the synthesis of positive active materials. However, the extraction of active material elements using acid uses a non-environmentally friendly method to collect pure raw materials, and requires a neutralization process and a wastewater treatment process, resulting in increased process costs. In addition, lithium, which is one of the key positive active material elements, cannot be collected. In order to overcome these shortcomings, a direct recycling method that does not dissolve the positive active material and does not extract the active material in elemental form is required. Summary of the invention

[0007] Technical issues

[0008] The present disclosure is directed to providing a method for collecting and reusing active materials from cathode waste.

[0009] Technical Solution

[0010] In order to solve the above problems, the method for recycling positive electrode active materials according to the present invention includes the following steps: (a-1): dry grinding positive electrode waste containing a lithium composite transition metal oxide positive electrode active material layer on a current collector to make the active material layer into a powder state and separate it from the current collector; (a-2) heat treating the active material layer in powder form in air to thermally decompose the binder and conductive material in the active material layer to collect the active material; (b) washing the collected active material with an alkaline lithium compound solution in an aqueous solution and drying it; and (c) annealing the washed active material with the addition of a lithium precursor to obtain a reusable active material.

[0011] In the present disclosure, the method of reusing a positive electrode active material may include (d) surface coating the annealed active material.

[0012] The dry grinding may be performed using any one of a pin mill, a disc mill, a cutting mill and a hammer mill.

[0013] The method of recycling a positive electrode active material may further include shredding or cutting the positive electrode scrap before the dry grinding.

[0014] The heat treatment may be performed at 300°C to 1000°C.

[0015] The heat treatment may be performed at 550° C. for 30 minutes at a temperature rise rate of 5° C. / min.

[0016] The lithium compound solution may have a lithium precursor (preferably LiOH) content of 0% or more and 15% or less, and the washing may be performed within 1 hour.

[0017] The washing may be performed by stirring the collected active material while being immersed in the lithium compound solution.

[0018] The lithium precursor may be at least one of LiOH, Li2CO3, LiNO3 and Li2O.

[0019] The amount of addition of the lithium precursor may be used to add lithium to the ratio between lithium and other metals in the raw active material used in the active material layer at a ratio of lost lithium.

[0020] For example, the lithium precursor may be added in an amount to add lithium at a molar ratio of 0.001 to 0.4.

[0021] Furthermore, the addition amount of the lithium precursor is preferably for adding more lithium in a molar ratio of 0.0001 to 0.1 based on a 1:1 molar ratio of lithium: other metals.

[0022] The annealing may be performed in air at 400° C. to 1000° C. The temperature of the annealing step may exceed the melting point of the lithium precursor.

[0023] The active material in the active material layer may be collected in a powder form, and carbon generated by carbonization of the binder or the conductive material does not remain on the surface.

[0024] The surface coating step may include coating at least one of a metal, an organic metal, and a carbon material on the surface by a solid phase method or a liquid phase method, and performing a heat treatment at 100° C. to 1200° C.

[0025] The recyclable active material is represented by the following formula 1:

[0026] [Formula 1]

[0027] Li a Ni x Mn y Co z M w O 2+δ

[0028] wherein M comprises at least one selected from the group consisting of B, W, Al, Ti and Mg, <a≤1.1,0≤x<0.95,0≤y<0.8,0≤z<1.0,0≤w≤0.1,-0.02≤δ≤0.02,x+y+z+w=1。

[0029] The reusable active material may have a fluorine (F) content of 100 ppm or less.

[0030] Beneficial effects

[0031] According to the present disclosure, waste positive electrode active materials, such as positive electrode waste generated in the lithium secondary battery manufacturing process, can be reused without using acid, thereby achieving eco-friendliness. The method according to the present disclosure does not require a neutralization process or a wastewater treatment process, thereby achieving environmental mitigation and process cost reduction.

[0032] According to the present disclosure, all metal elements of the positive electrode active material can be collected. The current collector can be collected because it does not dissolve the current collector. The method does not use the extracted active material elements as raw materials for the synthesis of the positive electrode active material, but directly uses the active material collected in powder form, thereby achieving economic efficiency.

[0033] According to the present disclosure, toxic and explosive solvents such as NMP, DMC, acetone, and methanol are not used, thereby achieving safety, and since simple processes such as heat treatment, washing, and annealing are used, the process is easy to manage and is suitable for large-scale production.

[0034] According to the present disclosure, it is possible to ensure excellent resistance characteristics and capacitance characteristics without deteriorating the electrochemical performance of the collected active materials.

[0035] Specifically, according to the present disclosure, first, dry milling is used to separate the active material from the current collector. By completely separating the active material from the current collector through dry milling, more than 95% of the positive electrode active material can be collected. Since the dry milling has a high hourly production volume, the recycling process of the positive electrode active material can be achieved with high productivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The accompanying drawings illustrate embodiments of the present disclosure and, together with the following detailed description, are used to provide a further understanding of the technical aspects of the present disclosure, and thus the present disclosure should not be construed as being limited to the accompanying drawings.

[0037] Figure 1 : is a figure which shows the positive electrode scrap which is discarded after punching out the positive electrode plate in the positive electrode sheet.

[0038] Figure 2 is a flow chart of a method of recycling active materials according to the present disclosure.

[0039] Figure 3 are photographic images of the results of each step according to the experimental example of the present disclosure.

[0040] Figure 4 is a scanning electron microscope (SEM) image of an active material layer after dry grinding in the method for reusing an active material according to the present disclosure.

[0041] Figure 5 After heat treatment in air Figure 4 SEM image of the active material layer.

[0042] Figure 6 is a SEM image of an active material that has been washed to remove LiF and dried in a method of recycling an active material according to the present disclosure.

[0043] Figure 7 and Figure 8 The results of battery evaluation using the active materials of Examples and Comparative Examples are shown. DETAILED DESCRIPTION

[0044] Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings. Before the description, it should be understood that the terms or words used in the specification and the appended claims should not be interpreted as limited to the general meaning and dictionary meaning, but are interpreted based on the principle of allowing the inventor to appropriately define the terms for the best interpretation, based on the meaning and concept corresponding to the technical aspects of the present disclosure. Therefore, the embodiments described herein and the illustrations in the accompanying drawings are merely embodiments of the present disclosure and do not fully describe the technical aspects of the present disclosure, so it should be understood that various other equivalents and modifications may be made thereto when filing a patent application.

[0045] In the following description, reference is made to the accompanying drawings that constitute some of the present disclosure. The embodiments described in the detailed description, the drawings, and the appended claims are not intended to be limiting. Other embodiments may be used, and modifications and changes may be made thereto, without departing from the technical aspects and scope of the subject matter disclosed herein. As collectively described herein and shown in the accompanying drawings, these aspects of the present disclosure may include arrangements, substitutions, combinations, separations, and designs of a variety of different elements, and it will be immediately understood that all of them are clearly contemplated.

[0046] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present disclosure belongs (hereinafter referred to as “the skilled person”).

[0047] The present disclosure is not limited to the specific embodiments described herein. It is obvious to those skilled in the art that many modifications and changes may be made thereto without departing from the technical aspects and scope of the present disclosure. In addition to those listed herein, functionally equivalent methods within the scope of the present disclosure will be apparent to those skilled in the art based on the previous description. These modifications and changes fall within the scope of the appended claims. The present disclosure will be limited by the appended claims and the full scope of equivalents given by the appended claims. It should be understood that the present disclosure is not limited to specific variant methods. It should also be understood that the terms used herein are for the purpose of describing specific embodiments and are not intended to limit the present disclosure.

[0048] Conventional processes for recycling active materials are mainly aimed at extracting valuable metal elements (nickel, cobalt, manganese) in the active materials of lithium secondary batteries whose performance deteriorates after use and resynthesizing the active materials. In contrast to conventional processes, the present invention is characterized in that active materials are collected from positive electrode waste generated in the lithium secondary battery manufacturing process.

[0049] In addition, known processes for recycling active materials include using reducing agents / additives to produce metals (direct reduction methods) or resynthesized active materials from expensive metals extracted by acid / alkali dissolution or melting, which requires additional chemical methods, making the process complicated and resulting in additional economic costs. However, the present disclosure relates to a method for directly recycling positive electrode active materials without dissolving the positive electrode active materials.

[0050] In order to directly reuse the positive electrode active material, the current collector must be removed from the positive electrode. The current collector can be removed from the positive electrode by removing the binder through high temperature heat treatment, dissolving the binder using a solvent to dissolve the current collector, and screening the active material through dry grinding and screening.

[0051] When using a solvent to dissolve the binder, the stability of the solvent is important. NMP is the most effective solvent, but its disadvantages are toxicity and high cost. Another disadvantage is the need for waste solvent reprocessing or solvent collection processes. The dissolution of the current collector requires a lower process cost than using a solvent. However, it is difficult to remove impurities from the surface of the reusable active material, and hydrogen is generated in the current collector removal process, leading to an explosion risk. Dry grinding and screening cannot perfectly separate the current collector and the active material. The particle size distribution of the active material changes during the grinding process, and it is difficult to remove the binder, resulting in degradation of the characteristics of the battery including the reusable active material.

[0052] The present disclosure uses dry grinding to separate the active material from the current collector. By completely separating the active material from the current collector through dry grinding, more than 95% of the positive electrode active material can be collected. Subsequently, the binder and the conductive material are removed by heat treatment. Since the heat treatment is carried out in the air, the process only requires heating without any special device construction, so it is relatively simple, and it is suitable for large-scale production and commercialization. However, impurities should not remain on the surface of the reusable active material. The present disclosure proposes to remove impurities from the surface of the reusable active material.

[0053] In the following, reference will be made to Figure 2 A method of recycling active materials according to an embodiment of the present disclosure is described. Figure 2 is a flow chart of a method of recycling active materials according to the present disclosure.

[0054] Reference Figure 2 First, waste positive electrode waste is prepared (S10).

[0055] As mentioned above Figure 1 As described above, after manufacturing a positive electrode sheet including a positive electrode active material layer on a current collector and punching the positive electrode sheet, positive electrode waste may remain. In addition, positive electrode waste can be prepared by collecting positive electrodes that have defects or failures during the process. In addition, positive electrode waste can be prepared by separating positive electrodes from lithium secondary batteries discarded after use.

[0056] For example, the positive electrode waste may be the residue after coating a slurry on a sheet-like current collector of aluminum foil and then drying it in a vacuum oven at about 120° C. to manufacture a positive electrode sheet and punching out a positive electrode plate having a predetermined size, wherein the slurry is prepared by mixing a lithium cobalt oxide active material such as LiCoO2 (LCO) or an NCM active material containing nickel, cobalt and manganese, a carbon-based material such as carbon black as a conductive material and polyvinylidene fluoride (PVdF) as a binder in N-methylpyrrolidone (NMP).

[0057] The positive electrode active material of the lithium secondary battery includes a lithium composite transition metal oxide, in particular, a lithium cobalt oxide such as LiCoO2, a lithium manganese oxide (LiMnO2 or LiMn2O4), a lithium iron phosphate compound (LiFePO4) or a lithium nickel oxide (LiNiO2). In addition, in order to improve the low thermal stability while maintaining the excellent reversible capacity of LiNiO2, a nickel-manganese-based lithium composite metal oxide partially substituted with manganese (Mn) having excellent thermal stability to nickel (Ni) and an NCM lithium composite transition metal oxide substituted with manganese (Mn) and cobalt (Co) are used. In particular, the present disclosure is applicable to the reuse of LCO or NCM active materials.

[0058] The positive electrode waste has an active material layer on a current collector of a metal foil such as an aluminum foil. The active material layer is formed by coating a slurry including a mixture of an active material, a conductive material, a binder and a solvent, and after the solvent is volatilized, the active material and the conductive material are bonded by the binder. Therefore, when the binder is removed, the active material can be separated from the current collector.

[0059] Subsequently, the positive electrode waste is crushed into pieces of appropriate size (S20). Crushing into pieces means cutting the positive electrode waste into pieces of appropriate size for processing. After crushing, the size of the positive electrode waste is about 10 cm×10 cm. In addition, cutting can be performed to cut into smaller sizes. When cutting, the size of the positive electrode waste is reduced to, for example, 1 cm×1 cm.

[0060] Considering the characteristics required for the treatment of the positive electrode waste and the equipment used in the subsequent process, the crushing into pieces including shredding and cutting can be performed. For example, when using equipment that requires continuous processing in the loading and unloading of the positive electrode waste, the oversized positive electrode waste must be crushed into pieces to facilitate the movement of the positive electrode waste.

[0061] Subsequently, the positive electrode waste material broken into pieces is dry-milled to make the active material layer into a powder state and separate it from the current collector (S25). Dry milling can use any one of a pin mill, a disc mill, a cutting mill and a hammer mill, and a pin mill is preferred. More than 95% of the positive electrode active material in the form of a powder of 5 mm or less can be collected by a pin mill. The pin mill equipment has an hourly production of more than 500 kg. Therefore, when the process is carried out for 10 hours a day, there is a total of 5 tons of electrode separation effect.

[0062] The pin mill is a device having a structure in which a rotor pin and a stator pin are engaged with each other, and when a raw material is fed into the mill, the raw material is dispersed by an air flow through rotation, and is crushed by the impact of the rotor and the stator using the brittleness of the raw material. The ground product is allowed to pass through an opening in a screen formed in a ring shape, and only a portion of the ground product that has passed is taken out.

[0063] Using a pin mill, grinding can be performed at the desired level and desired particle size by adjusting the type and number of pins and the size range of the screen used for screening. When a pin mill is used to grind the positive electrode waste that is broken into pieces, the current collector sheet is cut into smaller sizes and the highly brittle active material layer is separated from the current collector sheet. The highly flexible current collector sheet is rolled into a circle and retained in the screen, and only the active material layer in the form of a fine powder that has passed through the screen can be obtained separately. The active material layer loses the continuity that makes it called a layer and is broken into pieces, but the active material, binder and conductive material are still gathered together in powder form. The active material layer and the current collector can be separated by dry grinding with the help of the difference in brittleness and flexibility of the active material layer and the current collector.

[0064] Subsequently, the active material layer in powder form is heat-treated in air ( S30 ).

[0065] In the present disclosure, heat treatment is performed to thermally decompose the binder and the conductive material in the active material layer. The heat treatment can be performed at 300°C to 1000°C and can be referred to as high-temperature heat treatment. At a temperature below 300°C, it is difficult to remove the binder. When the positive electrode waste is heat treated in the presence of a current collector without dry grinding, it is necessary to perform heat treatment at a temperature below the melting point of the current collector, but the present disclosure performs heat treatment on the active material layer that has been separated from the current collector, so there is no limitation on the heat treatment temperature by the current collector.

[0066] The heat treatment time is long enough to thermally decompose the binder. For example, the heat treatment time is about 30 minutes. Preferably, the heat treatment time is more than 30 minutes. As the heat treatment time increases, the thermal decomposition of the binder is prolonged, but when the heat treatment time is equal to or longer than the predetermined time, there is no difference in the thermal decomposition effect. Preferably, the heat treatment time is more than 30 minutes and less than 5 hours.

[0067] The heat treatment equipment may include various types of furnaces. For example, the heat treatment equipment may be a box-type furnace, and when productivity is considered, it may be a rotary kiln capable of continuous processing.

[0068] The heat treatment may be followed by rapid or slow cooling in air.

[0069] For example, the heat treatment may be performed at 550° C. for 30 minutes at a temperature rise rate of 5° C. / minute. For example, the temperature rise rate is within the allowable range of a box furnace and is sufficient to heat the powdered active material without thermal shock. 550° C. is conducive to thermal decomposition of the binder. When the heat treatment is performed at the above temperature for less than 10 minutes, thermal decomposition is insufficient, so the heat treatment needs to be performed for more than 10 minutes, preferably more than 30 minutes.

[0070] The binder and the conductive material in the active material layer undergo thermal decomposition into CO 2 and H 2 O which are removed by heat treatment in air. Since the binder is removed, the active material to be collected can be deagglomerated and sieved in the form of powder.

[0071] It is important to perform the heat treatment of S30 in air. When the heat treatment is performed in a reducing or inert gas atmosphere, the binder and the conductive material undergo carbonization rather than thermal decomposition. When carbonized, carbon remains on the surface of the active material and reduces the performance of the reusable active material. When the heat treatment is performed in air, the carbon in the binder or the conductive material is removed by a combustion reaction with oxygen, thereby generating CO, CO2 gas, so the binder and the conductive material do not remain and are almost completely removed.

[0072] Therefore, according to the present disclosure, the active material is collected in a powder form, and carbon generated by carbonization of the binder or the conductive material does not remain on the surface.

[0073] Subsequently, the collected active material is washed and dried (S40). It is important to wash with a lithium compound solution that is alkaline in an aqueous solution. The lithium compound (preferably LiOH) content of the lithium compound solution is above 0% and below 15%. Preferably, the amount of LiOH is below 15%. When an excess of LiOH is included, excess LiOH may remain on the surface of the active material after washing, which may affect the subsequent annealing process. In order to make the surface of the active material before annealing as clean as possible, adding excess LiOH is not good for the process, and the amount of LiOH is limited to below 15%.

[0074] Washing can be carried out by soaking the collected active material in a lithium compound solution. Washing can be carried out within a week after soaking, preferably one day, more preferably 1 hour. When washing is carried out for a week or longer, the capacity may be reduced due to excessive lithium dissolution. Therefore, it is desirable to wash within 1 hour. Washing includes soaking the active material in a lithium compound solution that is alkaline in an aqueous solution and stirring under soaking. It is desirable to soak and stir together. When the active material is soaked in a lithium compound solution without stirring, the washing process is slow, resulting in lithium dissolution. When stirring together, the process time is minimized, so it is desirable to stir while soaking in a lithium compound solution. Drying can be carried out in a (convection) oven in air after filtering.

[0075] The lithium compound solution that is alkaline in aqueous solution can be used for washing to remove LiF and metal fluorides present on the surface of the collected active material and perform surface modification. During the heat treatment of S30, the binder and conductive material in the active material layer are removed by evaporating CO2 and H2O, and in this process, CO2 and H2O react with lithium on the surface of the active material to produce Li2CO3, LiOH, and F such as PVdF present in the binder reacts with lithium or any other metal in the positive active material and lithium in the added lithium precursor to produce LiH or metal fluorides. When LiF or metal fluorides remain, the characteristics of the battery including the reusable active material are reduced. The present disclosure adds a washing step of S40 to remove the reactants produced on the surface of the reusable active material during the heat treatment step (S30), thereby preventing impurities from remaining on the surface of the reusable active material.

[0076] In S40, it is important to wash with a lithium compound solution that is alkaline in aqueous solution. When sulfuric acid or hydrochloric acid aqueous solution is used instead of a lithium compound solution that is alkaline in aqueous solution, F on the surface of the active material is washed away, but the performance of the reusable positive active material may be degraded due to the dissolution of transition metals (Co, Mg) present in the active material. The alkaline lithium compound solution in the aqueous solution used in the present disclosure plays a role in removing trace binders that may remain after thermal decomposition in S30, does not dissolve transition metals present in the active material, and compensates for the amount of lithium that may be dissolved during the washing process.

[0077] LiF acts as a resistance layer when it remains. Through S40, the present disclosure can adjust the LiF content on the surface of the collected active material to less than 500 ppm, thereby improving the capacity. Preferably, the F content can be less than 100 ppm. More preferably, the F content can be less than 30 ppm.

[0078] Subsequently, the washed active material is annealed with the addition of a lithium precursor ( S50 ).

[0079] The lithium in the active material may be lost through the previous steps S30 and S40. S50 compensates for the lithium loss.

[0080] Furthermore, in S50 , the crystal structure of the active material is restored by annealing, and thus the characteristics of the reusable active material are restored or improved to the level of unused fresh active material.

[0081] After the previous steps S30 and S40, a transformation structure may be found in the surface of the active material. For example, the NCM-based lithium composite transition metal oxide as an active material may have a spinel structure due to the moisture in S40 converting Ni into rock salt [NiCO3·2Ni(OH)2)H20]. When the battery is manufactured in this state, the battery characteristics may deteriorate, such as reduced capacity. The present disclosure restores the crystal structure through S50. For example, the NCM-based lithium composite transition metal oxide as an active material is restored to a hexagonal structure. Therefore, the initial characteristics can be restored or improved to a level similar to that of the fresh active material.

[0082] The lithium precursor in S50 may be at least one of LiOH, Li2CO3, LiNO3 and Li2O.

[0083] The amount of lithium precursor added is used to add lithium to the ratio between lithium and other metals in the raw active material (i.e., fresh active material) used in the active material layer at the ratio of lithium loss. For example, when the ratio between lithium and other metals in the fresh active material is 1, the amount of lithium precursor added can be used to add lithium at a molar ratio of 0.001 to 0.4. It is appropriate to add lithium at a molar ratio of 0.01 to 0.2. Adding lithium precursor that exceeds the lithium lost by washing causes unreacted lithium precursor to remain on the reusable active material, resulting in increased resistance during the reuse of the active material, and therefore it is necessary to feed the lithium precursor in an optimal amount.

[0084] In addition, based on a molar ratio of lithium: other metals of 1: 1, the amount of the lithium precursor added is preferably used to add more lithium at a molar ratio of 0.0001 to 0.1. The addition of excess lithium is to form a surface protective layer on the active material by surface coating, which will be described in more detail below. When a secondary battery is manufactured using an active material, the side reaction of the electrolyte solution can be suppressed and the life characteristics can be maintained.

[0085] Annealing can be performed at 400°C to 1000°C in air. The annealing temperature can be 600°C to 900°C. Depending on the type of lithium precursor, the temperature varies within a limited range. The annealing time can be at least 1 hour. Preferably, the annealing time is about 5 hours. When the annealing time is long, the crystal structure can be fully restored, but the extended annealing does not greatly affect the performance. The annealing time is, for example, less than about 15 hours. The annealing equipment can be similar or identical to the annealing equipment of the heat treatment step S30.

[0086] For example, when Li2CO3 is used as a lithium precursor, the annealing temperature is preferably 700°C to 900°C, more preferably 710°C to 780°C. This is because the melting point of Li2CO3 is 723°C. Most preferably, annealing is performed at 750°C. When LiOH is used as a lithium precursor, the annealing temperature is preferably 400°C to 600°C, more preferably 450°C to 480°C. This is because the melting point of LiOH is 462°C.

[0087] Preferably, the annealing temperature is higher than the melting point of the lithium precursor. However, at a temperature higher than 1000°C, the positive electrode active material undergoes thermal decomposition and reduces the performance of the active material, so the annealing temperature does not exceed 1000°C.

[0088] Through S50, recyclable active materials can be obtained.

[0089] The present disclosure is characterized in that the active material in powder form is separated from the current collector in the positive electrode waste by dry grinding in S25. Due to the mechanical separation, the chemical properties of the current collector or the active material are not changed.

[0090] The positive electrode active material layer can be separated by heat treatment in the presence of the current collector without dry grinding. However, when the current collector is separated by heat treatment alone without dry grinding, the volume of the current collector and the electrode is large, and the mixing process through oxygen contact is important, so the production per unit volume is lower than the production of the proposed dry grinding. The active material powder separated from the current collector by the proposed dry grinding method can be processed in large quantities by a heat treatment process. The use of a pin mill device achieves more than 95% positive electrode active material collection and an electrode separation production of at least 500 kg per hour, and allows a continuous process, thereby maximizing productivity.

[0091] In addition, in the case of heat-treating the positive electrode waste containing the current collector without dry grinding, there is a limitation that the heat treatment is performed at a temperature below 550° C. due to the oxidation problem of the aluminum current collector. However, according to the present disclosure, since the current collector is separated before the heat treatment, the heat treatment temperature can exceed 550° C., which is higher than the melting point of the current collector. That is, by first separating the current collector, there is no limitation on the heat treatment temperature in the heat treatment for removing the binder and the conductive material, and high temperature heating is feasible.

[0092] Optionally, S60 may be performed. In S60, the active material annealed in S50 is surface-coated.

[0093] The surface coating step may include coating at least one of a metal, an organic metal, and a carbon material on the surface by a solid phase method or a liquid phase method and performing a heat treatment at 100° C. to 1200° C. When the heat treatment is performed at a temperature higher than 1200° C., performance degradation may occur due to thermal decomposition of the positive electrode active material. In the surface coating, coating on the surface by a solid phase method or a liquid phase method may use mixing, grinding, spray drying, and milling.

[0094] In the case of NCM active materials, it can be coated with B, BW, W, and in the case of LCO active materials, it can be coated with Al, Mg, Ti. In the case where the coating requires a high heat treatment temperature, S60 can be performed before S50. That is, annealing is performed after the surface coating with the addition of a lithium precursor.

[0095] By surface coating, a surface protective layer of different metals is formed. In the case where the molar ratio of lithium in the positive active material: other metals is 1: 1, when the molar ratio of lithium in the positive active material: other metals is reduced to less than 1: 1 due to the reaction of lithium in the active material with the surface coating material, 100% capacity cannot be exhibited. Therefore, lithium is added by adding a lithium precursor in the previous step S30 so that the molar ratio of lithium in the positive active material: other metals is 1: 1, and more lithium is added in excess in the positive active material at a molar ratio of 0.0001 to 0.1 with other metals. Therefore, a surface protective layer can be formed so that the molar ratio of lithium in the positive active material: other metals in the surface coating is 1: 1.

[0096] In detail, when the active material is coated with a metal oxide such as B, W, BW and heat-treated, a lithium boron oxide layer may be formed on the surface of the active material, and this serves as a surface protective layer. The lithium additionally contained in S50 at a molar ratio of 0.0001 to 0.1 reacts with the metal oxide such as B, W, BW in S60, and the molar ratio of lithium: other metals in the positive electrode active material does not decrease to less than 1:1, so there is no capacity reduction.

[0097] The recyclable active material obtained by the above method can be represented by Formula 1.

[0098] [Formula 1]

[0099] Li a Ni x Mn y Co z M w O2+δ

[0100] wherein M comprises at least one selected from the group consisting of B, W, Al, Ti and Mg, <a≤1.1,0≤x<0.95,0≤y<0.8,0≤z<1.0,0≤w≤0.1,-0.02≤δ≤0.02,x+y+z+w=1。

[0101] The recyclable active material may have an F content of 100 ppm or less. According to the present disclosure, an active material having a reduced F content may be collected, and when reused as an active material, may provide excellent resistance characteristics and capacity characteristics.

[0102] According to the present disclosure, LiF or metal fluoride is removed in the washing step of S40. The step of washing and drying using a lithium compound solution that is alkaline in an aqueous solution is safe and inexpensive, and can remove lithium or metal fluoride without losing other elements and prevent transition metal dissolution, thereby compensating for lithium loss that occurs during the process. The annealing step is also safe and inexpensive, and can restore the crystal structure, that is, improve crystallinity, thereby enhancing the battery characteristics of the reusable active material.

[0103] The reusable active material obtained according to the present disclosure may have a particle size distribution similar to that of fresh active material, thereby eliminating the need for any treatment to control the particle size distribution. Since the carbon produced by carbonization of the binder or conductive material does not remain on the surface, no carbon removal step is required. Figure 2 The active material obtained by the method can be used to manufacture the positive electrode without any treatment.

[0104] The recyclable active material can be used 100% without adjusting the composition or mixing with fresh active material, and can be used to prepare a slurry in combination with a conductive material, a binder, and a solvent.

[0105] Hereinafter, experimental examples of the present disclosure will be described in detail.

[0106] <Experimental Example 1>

[0107] Figure 3 are photographic images of the results of each step according to the experimental example of the present disclosure.

[0108] Figure 3 (a) is a photographic image of pulverized cathode waste. The discarded cathode waste was collected and chopped into a size of about 10 cm×10 cm.

[0109] Figure 3 (b) is a photographic image of the cut positive electrode scrap. The cut positive electrode scrap was cut into a size of 1 cm×1 cm.

[0110] Figure 3 (c) is a photographic image of the current collector sheet after needle grinding of the cut cathode waste, Figure 3 (d) is a photographic image of the active material layer in powder form separated from the current collector sheet after pin milling. It has been found that the active material layer is almost completely separated from the current collector by dry milling using a pin mill.

[0111] Figure 4 yes Figure 3 (d) is a scanning electron microscope (SEM) image of the active material layer shown in FIG. Figure 5 yes Figure 4 SEM image of the active material layer after heat treatment in air. SEM imaging is performed using SEM equipment commonly used in laboratories. For example, imaging can be performed using a HITACHI s-4200. However, there is no difference depending on the measuring device or method.

[0112] Figure 4 The agglomeration of particles in the active material layer due to the presence of a binder in the active material layer prior to heat treatment is shown. Figure 5 It is shown that the particles are deagglomerated by removing the binder through heat treatment.Thus, the binder and the conductive material are removed by the proposed heat treatment in air, so that the binder or the conductive material hardly remains on the surface of the active material.

[0113] Figure 6 is a SEM image of an active material that has been washed to remove LiF and dried in a method of recycling an active material according to the present disclosure. When the active material is annealed with the addition of a lithium precursor, a recyclable active material having almost the same shape as a fresh active material can be obtained.

[0114] <Experimental Example 2>

[0115] Positive electrode waste using NCM lithium composite transition metal oxide positive electrode active material and positive electrode waste using LCO active material were prepared. The active material in powder form was separated by a dry grinding step in the method for recycling active materials according to the present disclosure, and heat-treated at 550° C. in air for 30 minutes. Subsequently, some were immersed in a LiOH aqueous solution for 10 minutes for washing in the method for recycling active materials according to the present disclosure, while others were not washed. In order to determine the Li / metal molar ratio and the amount of residual LiF in the active material, residual F content analysis was performed, and the results are summarized in Table 1.

[0116] Sample 1 is LCO fresh active material. Sample 2 is LCO cathode waste that has been heat treated but not washed. Sample 3 is LCO cathode waste that has been heat treated and washed.

[0117] Sample 4 is NCM fresh active material. Sample 5 is NCM cathode waste that has been heat treated but not washed. Sample 6 is NCM cathode waste that has been heat treated and washed.

[0118] [Table 1]

[0119]

[0120] ND means the measurement is below 30 ppm. Since samples 1 and 4 are fresh active materials, F is hardly detected.

[0121] In the unwashed samples 2 and 5, the amount of residual F was measured to be 1900 mg / kg and 1450 mg / kg. However, it can be seen that in the washed samples 3 and 6, LiF was completely dissolved in the washing solution and thus completely removed to be undetectable. Therefore, LCO and NCM cathode wastes produce LiF during thermal treatment, but LiF is completely removed by the proposed washing step.

[0122] At the same time, it was found that the Li / metal molar ratio in Samples 2 and 5 was reduced by about 0.2 to 0.5 compared to the fresh active materials of Samples 1 and 4, and the Li / metal molar ratio in Samples 3 and 6 was reduced by about 0.2 to 0.5 compared to Samples 2 and 5. In particular, it seems that the NCM active material has a significant reduction in the Li / metal molar ratio due to a larger particle specific surface area than LCO and a transformation into a spinel structure. That is, it can be seen that lithium loss occurs in the heat treatment step for removing the binder and the conductive material, and lithium loss also occurs in the surface modification step by washing. Therefore, the method for reusing active materials according to the present disclosure proposes to include annealing to compensate for lithium loss while adding a lithium precursor.

[0123] <Experimental Example 3>

[0124] A cathode waste is prepared, which contains NCM lithium composite transition metal oxide: carbon black conductive material: PVdF binder = 96.25:1.5:2.25.

[0125] Comparative Example 1 is fresh active material.

[0126] Comparative Examples 2 and 3 are positive electrode wastes heat-treated in air in the presence of a positive electrode active material layer on the current collector without dry grinding of the positive electrode waste. The heat treatment was performed at 550°C for 30 minutes. The current collector was separated by heat treatment, and the active material was collected in the form of powder. In Comparative Example 2, the collected active material was not washed. In Comparative Example 3, washing was performed.

[0127] Comparative Example 4 and Example 1 are positive electrode wastes in which the active material layer is separated from the current collector by dry grinding using a pin mill, and only the active material layer in the form of a powder separated from the current collector is heat-treated in air. Heat treatment was performed at 550°C for 30 minutes in the same manner as Comparative Examples 2 and 3. In Comparative Example 4, the collected active material was not washed. In Example 1, washing was performed in the same manner as Comparative Example 3.

[0128] In order to determine the Li / metal molar ratio and the amount of residual LiF in the active material, residual F content analysis was performed and the results are summarized in Table 2.

[0129] [Table 2]

[0130]

[0131] The experimental results show that the amount of F remaining after heat treatment is slightly different, but there is no significant difference in the amount of F remaining after washing. In addition, the Li / metal molar ratio measurement results of Comparative Example 3 and Example 1 are within a similar level (error range ± 0.02), so it can be seen that even if the initial electrode separation process is different, the analytical values ​​after washing are at a similar level.

[0132] <Experimental Example 4>

[0133] Each positive electrode active material was prepared by the method of the embodiment and comparative example described below, and the electrochemical performance evaluation was performed. 96.25 wt % of the positive electrode active material, 1.5 wt % of carbon black as a conductive material, and 2.25 wt % of PVdF as a binder collected or prepared in the embodiment and comparative example were measured and mixed in NMP to prepare a slurry for making a positive electrode, and a coin half cell (CHC) was made and the electrochemical performance was evaluated. The voltage was 3V to 4.3V, and the initial formation charge / discharge was performed at 0.1C / 0.1C. The electrolyte solution for the battery was a carbonate-based electrolyte solution, including ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 3:7, with additives added. The discharge conditions used 0.5C / 1C and 0.5C / 2C.

[0134] By introducing 0.1 mol of lithium precursor (Li2CO3) into Comparative Example 3 of Experimental Example 3, Comparative Example 5 was annealed at 750°C for 5 hours, and then surface-coated heat-treated at 300°C for 5 hours by adding H3BO3, so that the boron content as a surface protective layer was 500 ppm.

[0135] Example 2 Annealing and surface coating were performed on Comparative Example 1 of Experimental Example 3 in the same manner as Comparative Example 5.

[0136] Table 3 summarizes the experimental results.

[0137] [Table 3]

[0138]

[0139] The electrical performance evaluation results show that Comparative Example 5 and Example 2 show equivalent levels at the initial capacity (0.1C / 0.1C) and also at high C rates. That is, even if the initial current collector separation process is different, the same battery performance results can be obtained by performing the same washing, annealing of adding lithium precursors, and surface coating processes.

[0140] <Experimental Example 5>

[0141] Each positive electrode active material was prepared by the methods of the experimental examples and comparative examples described below, and the electrochemical performance was evaluated.

[0142] Example 3: Reusable active materials are collected by the method for recycling active materials according to the present disclosure as described above. A positive electrode waste discarded after punching out a positive electrode plate having an NCM-based lithium composite transition metal oxide as an active material is prepared, and S20 and S25 are performed to obtain an active material layer in powder form. The heat treatment of S30 is performed at 550°C for 30 minutes in air. The washing of S40 is performed using LiOH for 10 minutes. In S50, annealing is performed at 750°C for 15 hours by introducing a lithium precursor (Li2CO3), and the amount of the lithium precursor is used to add more lithium at a molar ratio of 0.09 during the process based on the molar ratio of lithium and other metals in the raw active material (inductively coupled plasma (ICP) analysis). Theoretically, in the case of fresh active materials, the molar ratio of lithium: other metals is 1: 1, but since the average error of the ICP equipment used to measure the molar ratio is ± 0.05, preferably about ± 0.02, the molar ratio of lithium: other metals in the raw active material can be 1 ± 0.05: 1 by ICP measurement. In this experiment, the lithium precursor was added based on the analytical ratio analyzed by ICP.

[0143] Example 4: In addition to Example 3, the active material surface protection layer recovery process of S60 is performed.

[0144] Comparative Example 6: Fresh NCM-based lithium composite transition metal oxide was used instead of the recycled active material.

[0145] Comparative Example 7: Only the heat treatment of S30 in the method for recycling active materials according to the present disclosure as described above was performed to remove the binder and the conductive material. Since S25 in the method for recycling active materials according to the present disclosure was not performed, the Al current collector was separated during S30. S30 was performed under the same conditions as in Example 3. In the method for recycling active materials according to the present disclosure, the surface modification of S40, the crystal structure recovery of S50, and the surface coating process of S60 were not performed.

[0146] Comparative Example 8: On the basis of Comparative Example 7, active material was collected by surface modification of S40 in the method for recycling active material according to the present disclosure. That is, surface modification was performed, but the crystal structure recovery of S50 and the surface coating process of S60 in the method for recycling active material according to the present disclosure were not performed. S40 was performed under the same conditions as in Example 3.

[0147] Comparative Example 9: Based on Comparative Example 7, the NCM-based lithium composite transition metal oxide active material was collected by crystal structure recovery of S50 in the above method for recycling active materials according to the present disclosure without surface modification of S40. In contrast to Example 3, annealing for crystal structure recovery was performed without adding a lithium precursor.

[0148] Comparative Example 10: S30, S40 and S50 were performed in the same manner as in Example 3. S25 was not performed. In contrast to Example 3, annealing for crystal structure recovery was performed without adding a lithium precursor.

[0149] ICP analysis was performed on each of the positive electrode active materials collected or prepared in Examples and Comparative Examples to analyze the amount of residual LiF, the ratio of lithium and other metals in the active material, and the amount of a specific element such as B or W.

[0150] In addition, 96.25 wt % of each positive electrode active material collected or prepared in the examples and comparative examples, 1.5 wt % of carbon black as a conductive material, and 2.25 wt % of PVdF as a binder were measured and mixed in NMP to prepare a slurry for a positive electrode, and a coin half cell (CHC) was manufactured and the electrochemical performance was evaluated.

[0151] In order to determine the amount of LiF remaining in the active materials collected in Comparative Examples 7 and 8, F detection and analysis were performed by ICP. The results are shown in Table 4 below.

[0152] [Table 4]

[0153] Comparative Example 7 Comparative Example 8 F content (mg / kg) 1450 ND

[0154] Referring to Table 4, it was found that the F content in the collected positive electrode active material was significantly reduced in Comparative Example 8 compared with Comparative Example 7. That is, it can be seen that Lif was completely dissolved in the lithium compound solution by washing and thus completely removed, so that it could not be detected by ICP. Therefore, it can be seen that the removal was performed very well by S40.

[0155] In order to determine whether there is a change in lithium in the positive electrode active material during S30 and S40 of the present disclosure, the ratio of lithium / other metals in the active material was analyzed by ICP. The results are shown in Table 5 below.

[0156] [Table 5]

[0157]

[0158] Referring to Table 5, it can be seen that Comparative Example 7 reduces the ratio of lithium to other metals by about 0.2 to 0.5 by heat treatment S30 compared to Comparative Example 6, and Comparative Example 8 reduces the ratio of lithium to other metals by about 0.2 to 0.5 by washing and drying S40 compared to Comparative Example 7. It seems that the NCM-based lithium composite transition metal oxide significantly reduces the ratio of lithium to other metals due to the relatively large specific surface area of ​​the particles and the transition to the spinel structure. Therefore, it can be seen that insufficient lithium must be compensated.

[0159] Table 5 shows the values ​​measured by ICP analysis, and as described above, the ICP analysis has an error value of about ± 0.02. Therefore, in Comparative Example 6 of the new active material, the ratio of lithium to other metals may be less than 1. Therefore, the amount of lithium precursor added to compensate for lithium loss is the amount of lithium reduced based on the ratio of lithium to other metals in the raw material active material (i.e., fresh active material) used in the active material layer (i.e., the molar ratio by ICP analysis).

[0160] Figure 7 and Figure 8 Results of battery evaluation using active materials of Examples and Comparative Examples are shown. Capacity as a function of cycle number was evaluated at different currents to determine rate performance. The equipment used for evaluation is a charge / discharge tester commonly used in laboratories. There are no differences depending on the measuring device or method. Figure 7 and Figure 8 In the curve graph, the horizontal axis represents the number of cycles and the vertical axis represents the capacity.

[0161] The voltage was 3 V to 4.3 V, and the initial formation charge / discharge was performed at 0.1 C / 0.1 C. The electrolyte solution used for the battery was a carbonate-based electrolyte solution including EC:EMC=3:7, to which additives were added.

[0162] First, refer to Figure 7, after the primary heat treatment (550°C / 30 minutes) for removing the binder and the conductive material, in the case of Comparative Example 7 before the surface modification and Comparative Example 8 after the surface modification, Comparative Example 8 which has undergone the surface modification shows a rapid decrease in the electrode capacity. This is because, as described above, Ni in the NCM-based lithium composite transition metal oxide is converted into rock salt by moisture, resulting in a decrease in capacity.

[0163] However, when annealing (750°C / 15 hours) was performed without surface modification, this corresponds to Comparative Example 9, and there was a capacity improvement effect when compared with Comparative Example 7. This is because when surface modification is not performed, LiF is present on the surface of the active material. As shown in Table 4 above, LiF was removed at a satisfactory level by washing.

[0164] When surface modification and annealing are performed after the primary heat treatment, the capacity increases as shown in Comparative Example 10. The reason is that, although the capacity decreases after surface modification as shown in Comparative Example 8, after LiF is removed by surface modification, Ni rock salt is reduced by annealing and its structure is restored to a hexagonal shape.

[0165] Then, refer to Figure 8 , it is found that Example 3 has an improved capacity compared to Comparative Example 10. Compared to Comparative Example 10, Example 3 adds a lithium precursor during annealing. It can be seen that the capacity is improved by adding a lithium precursor to compensate for the lithium lost in the previous step. The lithium loss that occurs during heat treatment and washing is described with reference to Table 5.

[0166] Based on the ICP analysis results (Table 5), the lithium compound was added to the lithium content present in the positive electrode active material at a lithium loss ratio, and as a result, as can be seen from additional experiments, addition at a molar ratio of 0.09 to 0.1 showed a capacity improvement effect comparable to that of Comparative Example 6.

[0167] According to the present disclosure, active materials can be collected from cathode waste at a directly usable level. It is safe because no toxic and explosive solvents such as NMP, DMC, acetone and methanol are used, and it is suitable for large-scale production because simple and safe processes such as heat treatment, washing and drying, and annealing are used.

[0168] According to the present disclosure, cathode waste can be reused using a simple, environmentally friendly and economical method, and a lithium secondary battery manufactured by reusing the prepared NCM-based lithium composite transition metal oxide cathode active material does not have battery performance problems.

[0169] Although the present disclosure has been described above with respect to a limited number of embodiments and drawings, the present disclosure is not limited thereto, and it will be apparent to those skilled in the art that various modifications and changes may be made thereto within the technical aspects of the present disclosure and the scope of the appended claims and their equivalents.

Claims

1. A method for recycling a positive electrode active material, the method for recycling a positive electrode active material comprising the following steps: (a-1): dry grinding the positive electrode waste including a lithium composite transition metal oxide positive electrode active material layer on a current collector to make the active material layer into a powder state and separate it from the current collector; (a-2) heat-treating the active material layer in powder form in air to thermally decompose the fluorine-containing binder and the conductive material in the active material layer, thereby collecting the active material; (b) washing the collected active material with a lithium compound solution which is alkaline in aqueous solution and drying, wherein the washing is performed by stirring the collected active material while being immersed in the lithium compound solution; as well as (c) Annealing the washed active material with the addition of a lithium precursor to obtain a reusable active material.

2. The method for recycling positive electrode active materials according to claim 1, further comprising the following steps: (d) Surface coating of active material after annealing.

3. The method for recycling positive electrode active materials according to claim 1, wherein: The dry grinding uses any one of a pin mill, a disc mill, a cutting mill and a hammer mill.

4. The method for recycling positive electrode active materials according to claim 1, further comprising the following steps: The cathode waste is shredded or cut prior to the dry grinding.

5. The method for recycling positive electrode active materials according to claim 1, wherein: The heat treatment is performed at 300°C to 1000°C.

6. The method for recycling positive electrode active materials according to claim 1, wherein: The lithium compound solution has a lithium precursor content of at most 15%, and the washing is performed within 1 hour.

7. The method for recycling positive electrode active materials according to claim 1, wherein: The lithium precursor is at least one of LiOH, Li2CO3, LiNO3 and Li2O.

8. The method for recycling positive electrode active materials according to claim 1, wherein: The amount of the lithium precursor added is for adding lithium to the ratio between lithium and other metals in the raw active material used in the active material layer at a ratio of lost lithium.

9. The method for recycling positive electrode active materials according to claim 8, wherein: The lithium precursor is added in an amount for adding lithium at a molar ratio of 0.001 to 0.

4.

10. The method for recycling positive electrode active materials according to claim 8, wherein: The lithium precursor is added in an amount for adding more lithium at a molar ratio of 0.0001 to 0.1 based on a 1:1 molar ratio of lithium: other metals.

11. The method for recycling positive electrode active materials according to claim 1, wherein: The annealing is performed at 400° C. to 1000° C. in air.

12. The method for recycling positive electrode active materials according to claim 1, wherein: The annealing temperature exceeds the melting point of the lithium precursor.

13. The method for recycling positive electrode active materials according to claim 2, wherein: The surface coating step includes coating at least one of a metal, an organic metal, and a carbon material on the surface by a solid phase method or a liquid phase method, and performing a heat treatment at 100° C. to 1200° C.

14. The method for recycling positive electrode active materials according to claim 1, wherein: The recyclable active material is represented by the following formula 1: [Formula 1] Li a Ni x Mr y Co z M w O 2+δ wherein M comprises at least one selected from the group consisting of B, W, Al, Ti and Mg, <a≤1.1,0≤x<0.95,0≤y<0.8,0≤z<1.0,0≤w≤0.1,-0.02≤δ≤0.02,x+y+z+w=1。 15. The method for recycling positive electrode active materials according to claim 5, wherein: The heat treatment was performed at 550° C. for 30 minutes at a temperature rise rate of 5° C. / min.

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