Method for regenerating positive electrode active material and the positive electrode active material regenerated therefrom

Abstract

The present invention relates to a method for regenerating positive electrode active material and a positive electrode active material regenerated therefrom. More specifically, it provides a method for regenerating positive electrode active material and a positive electrode active material regenerated therefrom, in which, during the regeneration process, trivalent iron is synthesized in a single crystal structure and trivalent iron is converted to divalent iron, so that no trivalent iron remains inside the regenerated positive electrode active material. This prevents a decrease in battery performance, provides excellent lifespan characteristics in high-voltage environments, has high thermal stability, and generates little gas during charging and discharging.

Description

[Technical Field]

[0001] [Cross-reference with related applications] This application is an application claiming priority based on Korean Patent Application No. 10-2024-0029457 dated February 29, 2024, Korean Patent Application No. 10-2024-0029458 dated February 29, 2024, and Korean Patent Application No. 10-2025-0001945, which was refiled on January 7, 2025, and all the contents disclosed in the documents of said Korean Patent Applications are incorporated herein by reference.

[0002] The present invention relates to a method for regenerating positive electrode active material and positive electrode active material regenerated therefrom. More specifically, the present invention relates to a method for regenerating positive electrode active material and positive electrode active material regenerated therefrom, in which the atmosphere and temperature conditions of the steps of desorption and recovery of positive electrode active material from a waste positive electrode and application of a coating agent to the recovered positive electrode active material and firing are controlled, thereby synthesizing a single-crystal structure during the regeneration process and simultaneously converting trivalent iron to divalent iron, so that no trivalent iron remains inside the regenerated positive electrode active material. As a result, there is no degradation in battery performance, excellent lifespan characteristics in high-voltage environments, high thermal stability, and low gas generation during charging and discharging. [Background technology]

[0003] Demand for lithium-ion batteries has steadily increased since the 1990s, alongside the portable electronics market, and has surged globally recently with the rapid growth of the electric vehicle market. This could lead to instability in the supply and demand of lithium resources in the near future, and the sustained accumulation of end-of-life batteries could also cause significant environmental problems. To address these issues, the reuse of end-of-life lithium-ion batteries is a crucial technological challenge.

[0004] Lithium secondary batteries are broadly classified into a positive electrode, in which a positive electrode active material layer is coated on a metal foil such as aluminum; a negative electrode, in which a negative electrode active material layer is coated on a metal foil such as copper; a separation membrane to prevent the positive and negative electrodes from mixing; and an electrolyte that allows lithium ions to move between the positive and negative electrodes. The positive electrode is manufactured by applying a positive electrode composition containing a positive electrode active material, a binder, a conductive material, and a solvent to a current collector made of a metal foil such as aluminum, drying it, and then press-molding it.

[0005] The positive electrode accounts for more than 60% of the cost of a lithium secondary battery, and the active materials for such positive electrodes include lithium cobalt oxide (LiCoO2), lithium nickel cobalt manganese oxide (LiNiMnCoO2), lithium manganese oxide (LiMnO2), and lithium iron phosphate (LiFePO4). Among these, lithium iron phosphate is increasingly being used in high-capacity lithium secondary batteries for electric vehicles and other applications due to its low cost and stable supply. Therefore, much research is being conducted on recycling technologies to selectively recover valuable metals from the positive electrodes of lithium secondary batteries that are discarded after use, or from positive electrode scrap generated during the manufacturing process of lithium secondary batteries (hereinafter referred to as "waste positive electrodes"), or to directly recover the positive electrode active material.

[0006] There are roughly four main methods described for directly recycling positive electrode active material from waste positive electrodes without decomposing it (direct recycling method): calcination, solvent dissolution, aluminum foil dissolution, and crushing and screening.

[0007] However, although the aforementioned firing method is simple in its process, it has the disadvantages of generating foreign matter on the surface of the recycled positive electrode active material that reduces the output performance of the battery, generating waste gas, and consuming a large amount of energy.

[0008] In addition, although the solvent dissolution method can obtain a relatively clean-surfaced recycled cathode active material, solvents such as N-methyl-2-pyrrolidone (NMP) used to dissolve the binder are toxic gases and pose an explosion risk. Therefore, it has the disadvantages of poor stability and the need for an expensive solvent recovery process.

[0009] In addition, the aluminum foil dissolution method has good process stability, low process cost, and easy removal of the binder. However, it has the disadvantages that foreign substances that are difficult to remove are generated on the surface of the recycled cathode active material, and hydrogen gas is generated during the removal process of the aluminum foil, posing an explosion risk.

[0010] Finally, the crushing and screening method has the advantage of being the simplest process. However, it is difficult to completely separate the current collector and the cathode active material. The particle size distribution of the cathode active material changes during the crushing process, and the binder remains, resulting in deterioration of the battery characteristics of the recycled cathode active material.

[0011] In particular, among the waste cathodes, the waste cathode containing lithium iron phosphate as the cathode active material has the disadvantage that particles with a polycrystalline structure are synthesized during the recycling process, trivalent iron particles are encapsulated in the lithium iron phosphate, and the performance of the battery is deteriorated.

Prior Art Documents

Patent Documents

[0012]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0013] To solve the problems of the conventional technology described above, the present invention aims to provide a method for regenerating positive electrode active material and a regenerated positive electrode active material that, by controlling the atmosphere and temperature conditions in the steps of desorbing and recovering positive electrode active material from a waste positive electrode, and applying a coating agent to the recovered positive electrode active material and firing it, synthesizes a single-crystal structure during the regeneration process, converts trivalent iron to divalent iron, and prevents trivalent iron from remaining inside the regenerated positive electrode active material, thus preventing a decrease in battery performance, exhibiting excellent lifespan characteristics in high-voltage environments, high thermal stability, and low gas generation during charging and discharging.

[0014] The above-mentioned and other objectives of the present invention can all be achieved by the present invention as described below. [Means for solving the problem]

[0015] To achieve the above objective, the present invention provides I)(a) a step of recovering the positive electrode active material by heat-treating the waste positive electrode, which includes a current collector and a positive electrode active material layer formed on its surface, in an oxidizing atmosphere, (b) Adding a coating agent to the recovered positive electrode active material and firing it in a reducing atmosphere to form a coating layer on the surface of the positive electrode active material, while converting the trivalent iron compounds in the positive electrode active material to divalent iron compounds and converting the polycrystalline particles into single-crystal positive electrode active material, (c) A method for regenerating a positive electrode active material is provided, characterized by comprising the step of milling the positive electrode active material, which has been converted into a divalent iron compound and to adjust the particle size of the positive electrode active material.

[0016] II) In I) above, step (a) may include a step of primary heat treatment at 300 to 500°C in an oxidizing atmosphere, and a step of secondary heat treatment at 500 to 700°C in an oxidizing atmosphere after the primary heat treatment.

[0017] III) In I) to II) above, the primary heat treatment step can be carried out for 30 minutes to 10 hours, and the secondary heat treatment step can be carried out for 30 minutes to 10 hours.

[0018] IV) In I) to III) above, step (b) may include the steps of adding a coating agent to the recovered positive electrode active material and pre-milling it, spray-drying the pre-milled positive electrode active material, and firing the spray-dried positive electrode active material at 710 to 900°C in a reducing atmosphere.

[0019] V) In I) to IV) above, the coating agent may contain one or more of the following: metals, organometallics, and carbon components, preferably containing carbon components, and more preferably containing one or more selected from sucrose, glucose, graphite, PVDF (polyvinylidene fluoride), PEG (polyethylene glycol), citric acid, PVA (polyvinyl alcohol), graphene, and fructose.

[0020] VI) In I) to V) above, the firing can be carried out for 1 to 24 hours.

[0021] VII) In I) to VI) above, the preliminary milling can be performed using a ball mill, a high-energy ball mill, a vibratory mill, or a roll mill.

[0022] VIII) In I) to VII) above, the preliminary milling can be carried out for 2 to 24 hours at a stirring speed of 50 to 500 rpm.

[0023] IX) In steps I) to VIII) above, in step (c), milling can be performed using a jet mill.

[0024] X) In steps I) to IX) above, in step (c), the milling may be carried out in air or an inactive atmosphere.

[0025] XI) In I) to X) above, the positive electrode active material may be represented by the following chemical formula 1.

[0026] (chemical formula 1) Li 1-a M a Fe 1-b N b P 1-c X c O 4-d Y d (In Chemical Formula 1, M contains one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N contains one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X contains Si; Y contains one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are each 0 ≦ a < 1, 0 ≦ b < 1, 0 ≦ c < 1, and 0 ≦ d < 4.)

[0027] XII) In the above I) to XI), the positive electrode active material can consist of 99 mol% or more or 100 mol% of single crystals.

[0028] XIII) In the above I) to XII), based on a total of 一百 mol% of the divalent iron compound and the trivalent iron compound, the positive electrode active material may contain less than 1 mol% or 0 mol% of the trivalent iron compound based on XRD (X-ray diffraction) analysis or EPR (Electron Paramagnetic Resonance) analysis.

[0029] Further, the present invention provides a regenerated positive electrode active material characterized by being represented by the following Chemical Formula 1 and consisting of 99 mol% or more or 100 mol% of single crystals.

[0030] (Chemical Formula 1) [[ID=三十六]]Li 1-a M a Fe 1-b N b P 1-c X c O 4-d Y d It should be noted that in the translation of "一百 mol%", it seems there is an incorrect expression in the original text. It might be "100 mol%". I translated it according to the literal content provided. If this is an error, please correct the original text for a more accurate translation.(In the above chemical formula 1, M comprises one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N comprises one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X comprises Si; Y comprises one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are 0≦a<1, 0≦b<1, 0≦c<1, and 0≦d<4, respectively.)

[0031] Furthermore, the present invention provides a secondary battery characterized by including the regenerated positive electrode active material (XV). [Effects of the Invention]

[0032] According to the present invention, during the regeneration process, trivalent iron is synthesized in a single-crystal structure while simultaneously converting it to divalent iron. As a result, no trivalent iron remains inside the regenerated positive electrode active material, preventing a decrease in battery performance. This method provides a positive electrode active material regeneration method that exhibits excellent lifespan characteristics in high-voltage environments, high thermal stability, and low gas generation during charging and discharging.

[0033] Furthermore, because it directly regenerates the positive electrode active material without decomposing it using a simple and environmentally friendly method, it offers a method for regenerating positive electrode active material that significantly improves both economic efficiency and productivity. [Brief explanation of the drawing]

[0034] [Figure 1] This is a scheme of the positive electrode active material regenerated in Example 2 and Comparative Examples 3 and 4 according to the present invention. [Figure 2] This figure shows TEM images of the positive electrode active material regenerated in Example 1 and Comparative Examples 1 and 2. [Figure 3] This figure compares TEM images and EDP (Electron Diffraction Pattern) images of the positive electrode active material regenerated in Example 2 and Comparative Example 3. [Figure 4] This figure shows the TEM image and TEM-EDS mapping image of the cathode active material regenerated in Comparative Example 4. [Figure 5]This figure compares the XRD graphs of the regenerated positive electrode active materials in Example 2 and Comparative Examples 3 and 4. [Figure 6] This figure compares the XPS graphs of the regenerated positive electrode active materials in Example 2 and Comparative Examples 3 and 4. [Figure 7] This figure compares the EPR graphs of the regenerated positive electrode active materials in Example 2 and Comparative Examples 3 and 4. [Figure 8] This figure compares the electrochemical performance of the positive electrode active materials regenerated in Example 1 and Comparative Examples 1 and 2, including rated voltage, energy capacity, CHC capacity retention rate, and 45°C lifetime capacity. [Figure 9] This figure compares the electrochemical performance, such as energy capacity and CHC capacity retention rate, of the regenerated or newly created cathode active materials in Example 2 and Comparative Examples 3 and 4. In the figure, Ref refers to Comparative Example 1, 700C refers to Comparative Example 2, 800C refers to Example 1, a) refers to Example 2, b) refers to Comparative Example 3, and c) refers to Comparative Example 4. [Modes for carrying out the invention]

[0035] The inventors of this invention were researching a direct recycled method for directly regenerating waste cathode material into a cathode material with superior battery performance without decomposing it. They found that by controlling the atmosphere and temperature conditions in the steps of desorbing and recovering the cathode active material from the waste cathode, and applying a coating agent to the recovered cathode active material and firing it, they were able to synthesize the cathode active material in a single-crystal structure during the regeneration process, while simultaneously converting trivalent iron to divalent iron. As a result, no trivalent iron remains inside the regenerated cathode active material, preventing a decrease in battery performance, resulting in excellent lifespan characteristics in high-voltage environments, high thermal stability, and low gas generation during charging and discharging. Based on this, they continued their research and completed the present invention.

[0036] In this description, the active material layer of the discarded positive electrode may include a positive electrode active material, a binder, and a conductive material.

[0037] In this description, "oxidizing atmosphere" specifically refers to an atmosphere containing air or oxygen with a purity of 30% or higher.

[0038] In this document, unless otherwise specified, "trivalent iron compound" may refer to Li3Fe(PO4)3 and Fe2O3, but is not limited to these.

[0039] In this description, "fresh" positive electrode active material refers to positive electrode active material that is newly synthesized and manufactured, rather than material obtained by recovering and recycling from waste batteries.

[0040] The positive electrode active material, the method for regenerating the positive electrode active material, and the secondary battery described herein will be explained in detail below.

[0041] However, the terms and words used in this specification and in the claims are not to be interpreted in their ordinary or dictionary sense, but rather in a sense and concept consistent with the technical idea of ​​the present invention, based on the principle that inventors may appropriately define the concepts of terms in order to best describe their invention. Therefore, it should be understood that the examples and configurations shown in the drawings described herein are merely examples of the present invention and do not represent the entire technical idea of ​​the present invention, and that there may be various equivalents and modifications that can substitute for them, and that they may be arranged, substituted, combined, separated or designed in various other configurations.

[0042] All technical and scientific terms used herein have the same meaning as those commonly understood by those with ordinary skill in the art to which this invention pertains, unless otherwise defined.

[0043] Method for regenerating positive electrode active material The present invention provides a method for regenerating positive electrode active material, comprising: (a) heat-treating a waste positive electrode including a current collector and a positive electrode active material layer formed on its surface in an oxidizing atmosphere to recover the positive electrode active material; (b) adding a coating agent to the recovered positive electrode active material and firing it in a reducing atmosphere to form a coating layer on the surface of the positive electrode active material, while converting the trivalent iron compounds in the positive electrode active material to divalent iron compounds and converting polycrystalline particles to single-crystal positive electrode active material; and (c) milling the positive electrode active material to which the coating layer has been formed and which has been converted to divalent iron compounds to adjust the particle size of the positive electrode active material. In this case, the crystal structure of the single crystal is restored, and at the same time, the trivalent iron generated during the regeneration process is completely converted to divalent iron, which has the effect of realizing excellent charging capacity, resistance characteristics and capacitance characteristics.

[0044] The following describes in detail, step by step, the method for regenerating the positive electrode active material.

[0045] (a) Step to recover positive electrode active material In the present invention, the method for regenerating positive electrode active material includes the step of heat-treating a waste positive electrode, including a current collector and a positive electrode active material layer formed on its surface, in an oxidizing atmosphere. This method has the advantages of being easy to recover and improving the purity of the recovered positive electrode active material.

[0046] The aforementioned waste positive electrode may preferably be a positive electrode separated from a secondary battery that has been discarded after use, or a positive electrode sheet or positive electrode scrap that is discarded after defects or cutting during the manufacturing process of the secondary battery. For example, in the case of positive electrode scrap generated during the manufacturing process, there is no loss of lithium ions in the positive electrode active material, which has the advantage of enabling better battery characteristics.

[0047] The secondary battery may preferably be a lithium secondary battery.

[0048] In this description, the olivine structure is a type of crystalline structure for positive electrode active materials. Its lattice structure is a three-dimensional (3D) hexahedron, and because PO (phosphorus-oxygen) atoms are strongly bonded, it can maintain its structure even after all lithium ions have escaped. Therefore, it exhibits less performance degradation due to charging and discharging, and also has excellent thermal stability. Although it has the disadvantage of having a lower energy density, electrical conductivity, and lithium ion diffusivity compared to other positive electrode active materials, it offers significant economic advantages by using inexpensive iron instead of expensive cobalt.

[0049] The olivine structure can be confirmed by measurement methods commonly used in the art to which the present invention belongs, and as a specific example, it can be confirmed by X-ray diffraction analysis (XRD).

[0050] The positive electrode active material having the olivine structure may, for example, include a compound represented by the following chemical formula 1, which has the advantages of excellent high-temperature stability and lifespan characteristics, as well as being economically viable.

[0051] (chemical formula 1) Li 1-a M a Fe 1-b N b P 1-c X c O 4-d Y d (In the above chemical formula 1, M comprises one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N comprises one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X comprises Si; Y comprises one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are 0≦a<1, 0≦b<1, 0≦c<1, and 0≦d<4, respectively.)

[0052] In the above chemical formula 1, a, b, c, and d may each be 0≦a≦0.5, 0≦b≦0.5, 0≦c≦0.5, and 0≦d≦0.1, respectively.

[0053] The compound represented by the chemical formula 1 may preferably be lithium iron phosphate, which has the advantages of excellent high-temperature stability and lifespan characteristics, as well as being economically viable.

[0054] The lithium iron phosphate may preferably contain LiFePO4 with an olivine structure, which has the advantages of excellent high-temperature stability and lifespan characteristics, as well as being economically advantageous.

[0055] The conductive material may, for example, be a carbon-based conductive material, and preferably, carbon black, carbon nanotubes (CNTs), or a mixture thereof.

[0056] The binder may be, for example, a polymer binder, preferably polyvinylidene fluoride (PVdF), acrylonitrile-butadiene rubber (NBR), or a mixture thereof, and more preferably polyvinylidene fluoride.

[0057] The active material layer of the waste positive electrode may, for example, contain a solvent, which is used to mix the positive electrode active material, binder, and / or conductive material, and may be a solvent commonly used in the art to which the present invention belongs. For example, the solvent may be one or more selected from the group consisting of dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methyl-2-pyrrolidone (NMP), acetone, and water.

[0058] The active material layer of the aforementioned discarded positive electrode may, as an example, further contain a dispersant.

[0059] The dispersant may, as an example, be one or more selected from the group consisting of cellulosic compounds, polyalkylene oxides, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl acetal, polyvinyl ether, polyvinyl sulfonic acid, polyvinyl chloride (PVC), polyvinylidene fluoride, chitosans, starch, amylose, polyacrylamide, poly-N-isopropylacrylamide, poly-N,N-dimethylacrylamide, polyethyleneimine, polyoxyethylene, poly(2-methoxyethoxyethylene), poly(acrylamide-co-diallyldimethylammonium chloride), acrylonitrile-butadiene-styrene (ABS) copolymer, acrylate-styrene-acrylonitrile (ASA) copolymer, a mixture of acrylate-styrene-acrylonitrile (ASA) copolymer and propylene carbonate, styrene-acrylonitrile (SAN) copolymer, and methyl methacrylate-acrylonitrile-butadiene-styrene (MABS) copolymer.

[0060] The (a) positive electrode active material recovery step may, for example, include (a1) a step of primary heat treatment at 300 to 440°C in an oxidizing atmosphere; and (a2) a step of secondary heat treatment at 500 to 650°C in an oxidizing atmosphere after the primary heat treatment. In this case, foreign substances such as binders and conductive materials contained in the active material of the waste positive electrode can be easily removed, and the positive electrode active material precursor can be recovered in high yield, and this can simply be called a "desorption step".

[0061] In this description, the term "positive electrode active material precursor" is used to distinguish it from the positive electrode active material that is ultimately regenerated after the calcination and milling processes following desorption, and refers to a material that can be subjected to calcination and milling to provide the final regenerated positive electrode active material.

[0062] The heat treatment is not particularly limited as long as it is a method commonly used in the art to which the present invention belongs, and the heating rate and heating time can be adjusted as needed.

[0063] The (a) positive electrode active material recovery step may, as a preferred example, include (a1) a step of primary heat treatment at 350 to 440°C in an oxidizing atmosphere, and (a2) a step of secondary heat treatment at 520 to 630°C in an oxidizing atmosphere after the primary heat treatment. More preferably, the primary heat treatment temperature can be lower than the secondary heat treatment temperature. In this case, there is an advantage that, in the process of recovering the positive electrode active material from the waste positive electrode, not only the binder and conductive material but also foreign matter such as metals that have flowed in from the current collector, such as aluminum, can be smoothly removed, thereby enabling the recovery of high-purity positive electrode active material.

[0064] As a preferred example, the present invention allows for the pre-desorption of the waste positive electrode by oxidizing it at a low temperature range, and then completing the desorption by oxidizing the positive electrode active material powder containing the pre-desorbed positive electrode active material again at a high temperature. In this case, even without a separate pretreatment step for separating or removing the current collector, the amount of residual metal flowing in from the current collector is greatly reduced, which improves the purity of the positive electrode active material and simplifies the process.

[0065] More specifically, in the recovery process for desorbing and recovering positive electrode active material from a waste positive electrode, as described above, the heat treatment can be performed in two steps: under predetermined temperature conditions and under higher temperature conditions. In this case, in the primary heat treatment process (preliminary desorption process), metallic foreign matter that may flow in from the current collector is removed, and in the secondary oxidation heat treatment process (desorption process) at a higher temperature, carbon-based foreign matter such as binders, conductive materials, and surface coating agents of the positive electrode active material is removed. This significantly reduces the amount of foreign matter remaining other than the components that make up the positive electrode active material, and through this, high-purity positive electrode active material can be recovered in a simple process.

[0066] Furthermore, the positive electrode active material in the waste positive electrode may, for example, be coated with a coating agent containing metal and / or carbon. Thus, sometimes the positive electrode active material may be coated with various metal and / or carbon-containing coating agents for the purpose of improving battery performance. However, if the structure of such a coating layer is destroyed during the process of recovering the positive electrode active material from the waste positive electrode, and it is reused in the battery without removing it, it may cause a decrease in battery performance. For this reason, if the positive electrode active material in the waste positive electrode is coated, it is advantageous to remove the coating. In the secondary heat treatment step of (a2), the carbon coating on the surface of the positive electrode active material may be removed. Therefore, in this case, the purity of the recovered positive electrode active material is improved, which has the advantage of preventing a decrease in battery performance.

[0067] The primary and secondary heat treatments can, for example, be carried out in an oxidizing atmosphere including air or oxygen. In this case, foreign matter such as metal that has flowed in from the binder, conductive material, and current collector can be smoothly removed, and the target positive electrode active material can be recovered in high purity and high yield.

[0068] The oxygen atmosphere may, for example, have an oxygen purity of 30% or more, or 50% or more, preferably 70% or more, more preferably 80% or more, even more preferably 90% or more, and even more preferably 90-99%. Within this range, there is the advantage that the target positive electrode active material can be recovered with high purity and high yield.

[0069] The purity percentage of the oxygen may be in volume percentage or mol%.

[0070] The purity of oxygen described herein is not particularly limited when measured by a measurement method commonly used in the art to which this invention pertains.

[0071] The primary heat treatment can preferably be performed at 330 to 440°C, more preferably at 350 to 440°C, even more preferably at 360 to 440°C, and even more preferably at 360 to 420°C. Within this range, there is the advantage that the purity and recovery yield of the positive electrode active material precursor are greatly improved without excessive energy consumption.

[0072] The heating rate to reach the primary heat treatment temperature may be, for example, 1 to 10°C / min, preferably 2 to 9°C / min, and more preferably 3 to 7°C / min. Within this range, there is the advantage that the target positive electrode active material can be recovered with high purity and high yield.

[0073] The primary heat treatment time may be, for example, 30 minutes to 10 hours, preferably 40 minutes to 8.5 hours, more preferably 1 to 8 hours, and even more preferably 3 to 6 hours. Within this range, there is the advantage that the target positive electrode active material can be recovered with high purity and high yield.

[0074] In the primary heat treatment step (a1) described above, for example, the active material layer of the waste positive electrode can be separated from the current collector. In this process, some of the binder and conductive material contained in the positive electrode active material layer can be removed, but the positive electrode material that has been pre-detached in the primary heat treatment step (a1) may still contain the binder and conductive material that have not been removed. As a result, the positive electrode material recovered in the primary heat treatment step (a1) may contain the positive electrode active material, residual binder, and residual conductive material.

[0075] The cathode material pre-desorbed in the primary heat treatment step (a1) above can be recovered in powder form.

[0076] The positive electrode active material recovery step may, for example, include a step of cooling the positive electrode active material powder recovered in the primary heat treatment step between the primary heat treatment step (a1) and the secondary heat treatment step (a2). In this case, the inclusion rate of current collector constituent metals can be minimized, and a high-purity positive electrode active material precursor can be recovered in high yield. At the same time, the crystallinity of the regenerated positive electrode active material obtained after the regeneration process is increased, which has the advantage of improving battery characteristics when used in a secondary battery.

[0077] The aforementioned cooling may, as a specific example, be natural cooling, which involves interrupting the heat supply to the heating furnace where the primary heat treatment is performed and leaving it at room temperature. In this case, there is the advantage that subsequent processes can proceed easily without additional energy consumption.

[0078] In this document, "room temperature" refers to a single point within a range of 20±5℃.

[0079] The aforementioned secondary heat treatment can be carried out at 500 to 650°C, preferably at 520 to 630°C, more preferably at 530 to 620°C, and even more preferably at 520 to 610°C. Within this range, there is the advantage that the purity and recovery yield of the positive electrode active material precursor are greatly improved without excessive energy consumption.

[0080] The heating rate to reach the secondary heat treatment temperature may be, for example, 1 to 10°C / min, preferably 5 to 10°C / min, and more preferably 7 to 9°C / min. Within this range, there is the advantage that the target positive electrode active material can be recovered with high purity and high yield.

[0081] The aforementioned secondary heat treatment time may be, for example, 30 minutes to 10 hours, preferably 40 minutes to 8.5 hours, more preferably 1 to 8 hours, and even more preferably 3 to 6 hours. Within this range, there is the advantage that the target positive electrode active material can be recovered with high purity and high yield.

[0082] The positive electrode active material recovered after the completion of the secondary heat treatment can, for example, be naturally cooled, which has the advantage of being easily incorporated into subsequent processes.

[0083] The positive electrode active material (positive electrode active material precursor) recovered after the completion of step (a) positive electrode active material recovery may, for example, include a compound represented by chemical formula 1, preferably an LFP-based positive electrode active material, and specifically, may include Fe2O3 and Li3Fe2(PO4)3. In this case, a high-purity regenerated positive electrode active material can be provided through subsequent regeneration processing, with a significantly reduced amount of residual metallic foreign matter such as aluminum and carbon-based foreign matter. Because the regenerated positive electrode active material has high purity, it can provide good battery characteristics when applied as a positive electrode active material for a secondary battery.

[0084] In this description, the LFP-based cathode active material is not particularly limited as long as it is an LFP-based cathode active material that is generally defined or used in the art to which the present invention pertains.

[0085] After the completion of step (a) positive electrode active material recovery, the recovered positive electrode active material precursor may, for example, have a metal content of 390 ppm or less that has flowed in from the current collector, preferably 250 ppm or less, more preferably 240 ppm or less, even more preferably 230 ppm or less, and even more preferably 225 ppm or less. The lower limit is not particularly limited, but from the viewpoint of balancing the purity of the positive electrode active material with the recovery rate and process efficiency, it may be 10 ppm or more, or 50 ppm or more. In this case, there is an advantage in that high-purity positive electrode active material can be recovered.

[0086] The metal flowing in from the current collector is not particularly limited as long as it is a metal commonly used in current collectors in the art to which the present invention belongs, and a specific example of this may be aluminum.

[0087] In this description, the measurement of the content of metal elements is not particularly limited, as long as it is carried out by methods commonly used in the art to which the present invention pertains. As a specific example, it can be measured by ICP (Inductively Coupled Plasma) analysis.

[0088] After the completion of step (a) positive electrode active material recovery, the recovered positive electrode active material precursor may, for example, have a carbon (C) content of 1.0% by weight or less, preferably 0.5% by weight or less, more preferably 0.1% by weight or less, even more preferably 0.08% by weight or less, and even more preferably 0.06% by weight or less. The lower limit is not particularly limited, but from the viewpoint of balancing the purity of the positive electrode active material with the recovery rate and process efficiency, it may be 0.001% by weight or more, or 0.01% by weight or more. In this case, there is the advantage that a high-purity positive electrode active material can be recovered.

[0089] In this description, the measurement of the carbon content is not particularly limited as long as it is carried out by methods commonly used in the art to which the present invention pertains. As a specific example, it can be measured by quantitative analysis using a CS analyzer (Carbon / Sulfur Determinator).

[0090] (b) Step to restore the divalent structure of the trivalent iron compound-free in the positive electrode active material. In the present invention, the method for regenerating positive electrode active material includes the steps of (b) adding a coating agent to the recovered positive electrode active material and firing it in a reducing atmosphere to form a coating layer on the surface of the positive electrode active material, thereby converting the trivalent iron compounds in the positive electrode active material to divalent iron compounds and converting the polycrystalline particles into single-crystal positive electrode active material. In this case, a coating layer is formed on the surface of the particles of the regenerated positive electrode active material, and when this is applied to a secondary battery, it has the advantage of providing good battery characteristics at a level equivalent to that when fresh positive electrode active material is applied, such as improving the output characteristics, charge / discharge performance and life performance of the battery. Furthermore, before forming the coating layer, the trivalent iron compound-free divalent structure is restored to be similar to the crystal structure of fresh positive electrode active material, allowing the coating layer to be formed uniformly on the surface of the particles, preventing aggregation of the positive electrode active material particles, which has the advantage of improving battery performance.

[0091] Step (b) above may include the steps of: adding a coating agent to the recovered positive electrode active material (precursor) and pre-milling it; spray-drying the pre-milled positive electrode active material; and firing the spray-dried positive electrode active material at 710-900°C in a reducing atmosphere. In this case, the particle size is made uniform, a coating layer of uniform thickness is produced, and the positive electrode active material has a divalent iron compound without trivalent iron compounds, thereby restoring it to a structure similar to the crystal structure of fresh positive electrode active material.

[0092] The coating agent may, for example, be a coating agent containing one or more of metals, organometallics, and carbon components, and preferably a coating agent containing a carbon component configured to form a carbon coating, in which case there is the advantage of even better battery characteristics.

[0093] The carbon component is not particularly limited as long as it is a carbon component commonly used in the art to which the present invention belongs. For example, it may be one or more selected from the group consisting of sugars such as sucrose, glucose, and fructose, graphite, and polyvinylidene fluoride. Preferably, it may be a sugar, and more preferably sucrose. In this case, coating is easy, there are economic advantages, and when applied to a battery, there is an advantage in improving battery characteristics.

[0094] The aforementioned metal-containing coating agent is preferably a coating agent containing one or more selected from the group consisting of B, W, Al, Ti, Mg, Ni, Co, Mn, Si, Zr, Ge, Sn, Cr, Fe, V, Nb, and Y; more preferably a coating agent containing one or more selected from the group consisting of B, W, Al, Ti, and Mg; even more preferably a coating agent containing boron (B), tungsten (W), or a mixture thereof; and even more preferably a coating agent containing tungsten (W) and / or boron (B). A specific example is a coating agent containing tungsten boride (WB), in which case there is an effect of improving resistance characteristics and life characteristics.

[0095] The aforementioned metal may include, for example, oxides or acids that contain a metallic element as an element in their molecule.

[0096] The aforementioned organometallic coating agent is not particularly limited as long as it is an organometallic commonly used as a coating agent in the art to which the present invention belongs, and specific examples include metal alkoxides.

[0097] As an example, the coating agent can be a coating agent solution obtained by mixing a metal, organometallic, or carbon component with a suitable solvent. In this case, the solvent is not particularly limited as long as it is a commonly used solvent, and as a specific example, an aqueous solvent, or more specifically deionized water, can be used. The ratio of solids in the coating agent solution may be 20% by weight or less, preferably 1 to 15% by weight, and more preferably 2 to 10% by weight, relative to the total weight of the coating agent solution. In this case, there is an advantage that the coating efficiency is excellent and the subsequent milling process is carried out smoothly, resulting in the uniform formation of the coating layer that is ultimately formed on the surface of the positive electrode active material particles.

[0098] The method for applying the coating agent to the recovered positive electrode active material (precursor) is not particularly limited as long as it is a coating method commonly used in the art to which the present invention belongs. Specific examples include a liquid-phase method in which a liquid coating agent is manufactured and mixed with the positive electrode active material; a mechanochemical method using the high mechanical energy of ball milling; a fluidized bed coating method; a spray drying method; a precipitation method in which the coating agent is precipitated on the surface of the positive electrode active material in an aqueous solution; a method utilizing the reaction between the gas-phase coating agent and the positive electrode active material; and a sputtering method, which can be appropriately selected from this group.

[0099] The method for drying the positive electrode active material coated with the aforementioned coating agent may preferably be a spray drying method. In this case, the coating is formed uniformly, aggregation of positive electrode active material particles is prevented, and the coating process is carried out smoothly, which has the advantage of excellent productivity.

[0100] The aforementioned spray drying is not particularly limited when using spray drying equipment commonly used in the art to which the present invention pertains. For example, an ultrasonic spray dryer, an air nozzle spray dryer, an ultrasonic nozzle spray dryer, a filter expansion droplet generator, or an electrostatic spray dryer may be used. Specifically, it can be carried out using PSD-05 equipment (manufacturer: Eugene Tech Co., Ltd.), but is not limited thereto.

[0101] Other parameters, such as spray pressure and the supply rate of the coating solution, can be appropriately selected considering the amount of coating agent that will ultimately be applied to the surface of the regenerated cathode active material.

[0102] After the spray drying, the temperature at which the coated positive electrode active material is fired in a reducing atmosphere may be 710 to 900°C, preferably 710 to 850°C, more preferably 710 to 800°C, and even more preferably 710 to 750°C. In this case, there is an advantage that the coating agent is stably coated onto the surface of the positive electrode active material while maintaining the properties inherent to the positive electrode active material. Examples 1 and 2, described later, confirm that the atmosphere and temperature conditions of the firing process are one of the variables that allow the active material to recover in a single-crystal state with a trivalent iron compound-free divalent structure.

[0103] The interior of the regenerated cathode active material according to the present invention and the interior of a regenerated cathode active material not according to the present invention are schematically shown in Figure 1 below.

[0104] Figure 1 below is a schematic diagram illustrating the positive electrode active material regenerated in Example 2 and Comparative Examples 3 and 4, which do not conform to the present invention. As can be seen from Comparative Example 4 on the right side of Figure 1, when heat treatment is performed in an oxidizing atmosphere and then calcination treatment is not applied in a reducing atmosphere, trivalent iron compounds remain inside. Even when calcined in a reducing atmosphere, in Comparative Example 3 in the middle of Figure 1, where the calcination temperature conditions are inappropriate, some trivalent iron compounds that were not converted to divalent iron compounds remain. However, in Example 2 on the left side of Figure 1, where all heat treatment conditions and calcination temperature conditions conform to the present invention, it is confirmed that no trivalent iron compounds are present and all are converted to divalent iron compounds.

[0105] For example, the firing process may involve a heating rate of 1 to 20°C / min, preferably 1 to 10°C / min, and more preferably 2 to 7°C / min to reach the firing temperature. This has the advantage that the desired firing effect can be fully achieved within this range.

[0106] The firing process can, for example, be carried out at the firing temperature for 1 to 24 hours, preferably 1 to 16 hours, and more preferably 3 to 16 hours. Within this range, the desired firing effect can be fully achieved.

[0107] The reducing atmosphere may, for example, be an argon (Ar) or nitrogen (N2) atmosphere. In a preferred example, the reducing atmosphere may have a nitrogen purity of 80% or more, preferably 90% or more, more preferably 90-99.8%, and even more preferably 95-99.8%. In this case, there is the advantage that oxidation of the coating agent is prevented during the firing process, and a coating layer is stably formed on the surface of the positive electrode active material.

[0108] The purity percentage of the nitrogen may be expressed as volume percentage or mol%.

[0109] The purity of nitrogen described herein is not particularly limited when measured by a measurement method commonly used in the art to which this invention pertains.

[0110] The amount of the coating layer formed on the surface of the regenerated positive electrode active material particles may, for example, be 0.1 to 15% by weight, preferably 0.2 to 10% by weight, more preferably 0.5 to 5% by weight, even more preferably 0.7 to 3% by weight, and even more preferably 0.8 to 2% by weight, based on the total weight of the regenerated positive electrode active material including the weight of the coating layer. Within this range, there is the advantage that the desired coating effect can be sufficiently expressed.

[0111] The amount of the coating layer formed on the surface of the regenerated positive electrode active material particles can be measured by methods commonly used in the art to which the present invention belongs. Specifically, it can be measured by quantitative analysis using thermogravimetric analysis (TGA) or a carbon / sulfur analyzer (CS analyzer).

[0112] The thickness of the coating layer applied to the surface of the regenerated positive electrode active material particles can be appropriately controlled according to the desired coating amount. In this description, the thickness of the coating layer can be measured by methods commonly used in the art to which the present invention belongs. For example, it can be determined by measuring the major axes of 5 to 100 positive electrode active material particles observed using a transmission electron microscope (TEM) or scanning electron microscope (SEM), and then taking the arithmetic mean of these measurements.

[0113] Step (b) may include a pre-milling step, in which case the particle size of the recovered cathode active material is adjusted to a predetermined range before being fed into the subsequent regeneration step, thereby uniformly controlling the particle size and particle size distribution of the regenerated cathode active material that is ultimately obtained. This has the advantage of controlling the particles to a state that is favorable for restoring the crystal structure of the cathode active material in subsequent steps, and significantly improving the battery characteristics.

[0114] The preliminary milling can be performed using, for example, a ball mill, a high-energy ball mill, a vibrating mill, or a roll mill. Preferably, a ball mill can be used, which has the advantage of allowing for easy control of the particle size distribution of the regenerated cathode active material and the average particle size of the regenerated cathode active material obtained, and is advantageous for restoring the crystal structure of the cathode active material in subsequent steps.

[0115] The aforementioned preliminary milling can be performed for, for example, 2 to 24 hours, preferably 2 to 20 hours, and more preferably 5 to 16 hours. Within this range, there is the advantage of suppressing the generation of fine particles and smoothly controlling the particle size distribution of the regenerated positive electrode active material within a narrow range.

[0116] The aforementioned preliminary milling can be carried out under conditions such as 50-500 rpm, preferably 100-450 rpm, more preferably 150-420 rpm, even more preferably 180-410 rpm, even more preferably 200-400 rpm, and especially more preferably 250-320 rpm. In this case, there is the advantage that the generation of fine powder is suppressed and the crystalline structure of the positive electrode active material is maintained while the desired effect can be fully expressed.

[0117] The average particle size (D) of the positive electrode active material powder obtained after the preliminary milling. 50 The particle size may be 0.3 to 0.7 μm, preferably 0.3 to 0.65 μm, more preferably 0.35 to 0.65 μm, even more preferably 0.4 to 0.6 μm, and even more preferably 0.45 to 0.55 μm. In this case, there is an advantage in suppressing the generation of fine particles and maintaining the crystalline structure of the positive electrode active material while controlling the particle size of the regenerated positive electrode active material obtained to within the desired range.

[0118] (c) Single crystal particle synthesis step The method for regenerating positive electrode active material of the present invention may, as an example, include the step of milling a positive electrode active material in which the trivalent iron compound-free divalent phase structure has been restored. In this case, aggregation, particle cracking, and the generation of fine powder are prevented from occurring in the regenerated positive electrode active material obtained, and the particle size distribution is controlled to a narrow range, which has the advantage of enabling the positive electrode active material to be manufactured as single crystal particles (single particles). This has the advantage of preventing a decrease in battery performance due to fine powder and further improving the thermal stability and life characteristics of the battery. Furthermore, when the regenerated positive electrode active material is ultimately applied to the positive electrode of a secondary battery, it has the advantage of providing battery characteristics at a level equivalent to or better than that of fresh positive electrode active material.

[0119] The single particle may, for example, be a particle consisting of 30 or fewer nodules, preferably 1 to 20, more preferably 1 to 10, even more preferably 1 to 5, and most preferably a particle consisting of 1 nodule. In this case, the performance degradation of the battery is prevented during the electrode manufacturing process, and a positive electrode active material with excellent thermal stability and life characteristics of the battery is provided.

[0120] In this description, a nodule refers to a particle unit body that constitutes a single particle, and can mean either a single crystal lacking crystalline grain boundaries, or a polycrystalline material in which grain boundaries are not visible when observed at a magnification of 5,000 to 20,000 times using a scanning electron microscope (SEM) or electron backscatter diffraction pattern analyzer (EBSD).

[0121] In this description, the number of nodules refers to the average number of nodules in the positive electrode active material particles. This can be determined by cutting the positive electrode containing the positive electrode active material using ion milling, obtaining a cross-sectional image of the thickness direction of the cut positive electrode using a scanning electron microscope (SEM), selecting at least 30 particles each for the largest and smallest positive electrode active material particles in the cross-sectional image, and then measuring the number of nodules in the cross-section of each positive electrode active material particle through SEM image analysis, and then taking the arithmetic mean of these values.

[0122] In this description, secondary particles refer to aggregates formed by the aggregation of multiple single particles, and which contain more than 30 nodules.

[0123] In step (c) above, the milling may be performed by a jet mill, for example. In this case, it is possible to precisely control the particle size and particle distribution of the regenerated cathode active material obtained within a narrow range while preventing damage to the crystal structure of the cathode active material. Furthermore, it has the advantage of preventing the inflow of foreign matter that may be generated during the milling process, thereby improving the purity of the regenerated cathode active material.

[0124] The jet mill can, for example, be operated using air or an inactive gas under conditions of a temperature of -30°C to 30°C, preferably -20°C to 20°C, and a pressure of 0.8 to 10 bar. In this case, it is possible to precisely control the particle size and particle distribution of the regenerated cathode active material within a narrow range while preventing damage to the crystal structure of the cathode active material. Furthermore, it has the advantage of preventing the inflow of foreign matter that may be generated during the milling process, thereby improving the purity of the regenerated cathode active material.

[0125] In particular, when jet milling is performed using an inert gas that does not react with the regenerated positive electrode active material, it is preferable because the regeneration of trivalent iron compounds can be prevented by the energy of jet milling. The inert gas may be, for example, argon (Ar) or nitrogen (N2).

[0126] More specifically, the jet milling can be performed in 5 minutes or less under conditions where the pressure of the feeding line is 2 to 8 bar, preferably 2.5 to 6 bar, more preferably 3 to 5 bar, and the pressure of the grinding line is 0.8 to 2 bar, preferably 0.9 to 1.5 bar, more preferably 1 to 1.3 bar. Within this range, it is advantageous to prevent damage to the crystal structure of the positive electrode active material, precisely control the particle size and particle distribution of the regenerated positive electrode active material obtained within a narrow range, and realize a single-particle structure.

[0127] The average particle size (D) of the regenerated cathode active material finally recovered in step (c) above. 50The particle size may be, for example, 0.6 to 3.0 μm, preferably 0.7 to 2.0 μm, more preferably 0.8 to 1.5 μm, even more preferably 0.9 to 1.2 μm, and even more preferably 0.955 to 1.255 μm, in which case there is the advantage that excellent battery characteristics are exhibited.

[0128] In this description, the average particle size (D) of the positive electrode active material 50 The measurement method is not particularly limited as long as it is a measurement method commonly used in the art to which the present invention belongs, and as an example, it may be the average particle size on a 50% cumulative basis of the particle size distribution measured using laser diffraction.

[0129] The regenerated cathode active material recovered in step (c) above may, for example, have a crystal size of 120 to 180 nm as measured by XRD (X-Ray Diffraction), preferably 125 to 170 nm, more preferably 130 to 167 nm, even more preferably 140 to 166 nm, and even more preferably 150 to 165 nm. In this case, the structure of the regenerated cathode active material is restored to that of the fresh cathode active material, which has the advantage of providing good battery characteristics.

[0130] Furthermore, the regenerated cathode active material finally recovered in step (c) above, for example, has a cell volume of 291.00~291.12 Å measured by XRD. 3 It may be, preferably 291.02~291.12 Å 3 More preferably 291.15~291.11Å 3 This is also possible, and in this case, the structure of the regenerated positive electrode active material is restored to the structure of the fresh positive electrode active material, which has the advantage of providing good battery characteristics.

[0131] Regenerated cathode active material The regenerated positive electrode active material of the present invention is characterized by being regenerated by the method for regenerating positive electrode active material of the present invention. In this case, in addition to the advantage of being environmentally friendly, it is synthesized in a single-crystal structure and simultaneously converts trivalent iron to divalent iron, so no trivalent iron compounds remain inside the regenerated positive electrode active material. When applied as a positive electrode active material for a secondary battery, it has the advantage of excellent charge / discharge characteristics, cycle characteristics, and life characteristics of the battery, as well as excellent thermal stability, which improves high-temperature performance. Furthermore, the regenerated positive electrode active material of the present invention can ultimately provide battery characteristics at a level equivalent to or better than a secondary battery using fresh positive electrode active material, thus having the advantage of being able to replace newly generated (fresh) positive electrode active material.

[0132] The regenerated positive electrode active material may, for example, be a compound represented by chemical formula 1, preferably a lithium iron phosphate (LFP)-based positive electrode active material, and more preferably a LiFePO4 with an olivine structure. In this case, there are advantages such as excellent high-temperature stability and lifespan characteristics, as well as economic advantages.

[0133] (chemical formula 1) Li 1-a M a Fe 1-b N b P 1-c X c O 4-d Y d (In the above chemical formula 1, M comprises one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N comprises one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X comprises Si; Y comprises one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are 0≦a<1, 0≦b<1, 0≦c<1, and 0≦d<4, respectively.)

[0134] In the above chemical formula 1, it is preferable that a, b, c, and d are 0 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.5, and 0 ≤ d ≤ 0.1, respectively.

[0135] As an example, the regenerated cathode active material has a crystal size of 120-180 nm as measured by XRD, and a cell volume of 291.00-291.12 Å as measured by XRD. 3 It may also be the case that the crystal size is preferably 125-170 nm, more preferably 130-167 nm, even more preferably 140-166 nm, and even more preferably 150-165 nm, and the cell volume is 291.02-291.12 Å. 3 More preferably 291.15~291.11Å 3 This is also acceptable, and in this case, there is the advantage of being able to provide good battery characteristics.

[0136] The regenerated positive electrode active material may preferably be a single particle in which no boundary layer is observed within the particle, and more preferably a single particle consisting only of a single crystal, without secondary particles. In this case, during the electrode manufacturing process, it is converted into a positive electrode active material containing only a divalent iron compound, which has the effect of providing a positive electrode active material that does not cause a decrease in battery performance, has excellent life characteristics in high voltage environments, has high thermal stability, and generates little gas during charging and discharging.

[0137] The regenerated positive electrode active material may, for example, consist of a positive electrode active material comprising 99 mol% or more or 100 mol% single crystals, preferably a compound represented by the chemical formula 1, and more preferably a lithium iron phosphate compound. In this case, synthesizing single crystal particles during the regeneration process of the positive electrode active material provides the effect of providing a regenerated positive electrode active material that does not cause a decrease in battery performance, has excellent lifespan characteristics in high-voltage environments, has high thermal stability, and generates little gas during charging and discharging.

[0138] The aforementioned regenerated positive electrode active material, based on XRD analysis or EPR (Electron Paramagnetic Resonance) analysis, contains a total of 100 mol% of divalent iron compounds and trivalent iron compounds, with the trivalent iron compound being less than 1 mol% or 0 mol%, more preferably 0 mol%. In this case, by synthesizing positive electrode active material particles containing only divalent iron compounds during the electrode manufacturing process, it is possible to provide a regenerated positive electrode active material that does not experience a decrease in battery performance, has excellent lifespan characteristics in high-voltage environments, high thermal stability, and generates little gas during charging and discharging.

[0139] In this invention, mole percent may be referred to as volume percent as needed.

[0140] The regenerated positive electrode active material may, for example, have a fluorine (F) content of 250 mg / kg or less, preferably 200 mg / kg or less, and more preferably 10 to 200 mg / kg. Within this range, improving the particle strength offers the advantage of excellent charging capacity, resistance characteristics, and capacitance characteristics.

[0141] In this description, the fluorine (F) content can be measured using an ICP analyzer, and while this can be done with a general ICP analyzer commonly used in laboratories, there is no deviation due to the measuring device or method.

[0142] The regenerated positive electrode active material may, for example, have an average crystal size of 50 to 500 nm, preferably 50 to 300 nm, and more preferably 50 to 200 nm. Within this range, there is an advantage in improving the conductivity of the regenerated positive electrode active material and extending the battery life.

[0143] In this description, the average crystal size can be measured by XRD crystallography, and there is no deviation due to the measurement device or method. Specifically, it can be determined by placing 5g of positive electrode active material particles in a holder and analyzing the diffraction grating produced when the particles are irradiated with X-rays. In this case, the calculation method can be determined from the main peak or the full width at half maximum of three or more peaks, which can be considered to correspond to the average crystal size of the primary particles of the positive electrode active material.

[0144] Furthermore, the regenerated positive electrode active material of the present invention contains a lithium iron phosphate compound coated with a carbon-containing coating agent, and is characterized by the absence of trivalent iron compound peaks in XRD analysis and EPR (Electron Paramagnetic Resonance) analysis. In this case, it has the effect of having excellent battery performance such as charging capacity, resistance characteristics, and capacity characteristics.

[0145] secondary battery The secondary battery of the present invention includes the regenerated positive electrode active material, and in this case, the trivalent iron compound inside the regenerated positive electrode active material is completely converted to a divalent iron compound, and at the same time a single crystal structure is realized, which has the advantages of not causing a decrease in battery performance, having excellent life characteristics in high voltage environments, high thermal stability, and low gas generation during charging and discharging.

[0146] The secondary battery of the present invention may include all of the above-described details regarding the regenerated positive electrode active material and its regeneration method. Therefore, redundant descriptions thereof are omitted here.

[0147] The method for manufacturing a secondary battery according to the present invention is not particularly limited if it is a method for manufacturing a lithium secondary battery commonly used in the art to which the present invention belongs.

[0148] The following are preferred embodiments to aid in understanding the present invention. However, these embodiments are merely illustrative of the present invention, and it will be obvious to those skilled in the art that various changes and modifications are possible within the scope of the present invention and the technical concept, and that such changes and modifications fall within the scope of the appended claims.

[0149] [Examples] Example 1 As waste positive electrode material, a positive electrode active material layer containing an olivine-structured LFP positive electrode active material, binder, and conductive material was coated onto an aluminum current collector. Positive electrode scrap remaining after punching out positive electrode plates was prepared and crushed into 2cm x 2cm pieces.

[0150] Next, the temperature was increased at a rate of 5°C / min under an oxygen atmosphere, followed by primary heat treatment at 390°C for 5 hours. During this time, 3 L / min of oxygen gas with a purity of 95% was supplied. In this process, the binder in the waste positive electrode was thermally decomposed, and the positive electrode active material powder separated from the current collector was recovered.

[0151] After the primary heat treatment, the heat supply was stopped, and the recovered positive electrode active material powder was allowed to cool completely at room temperature. Then, under an air atmosphere, it was heated again at a rate of 8°C / min and then heated to 580°C for 5 hours for secondary heat treatment. During this time, oxygen gas was supplied at 3 L / min.

[0152] After the secondary heat treatment, the heat supply was stopped and the material was allowed to cool to room temperature. Then, a coating agent composition (solvent water) was added to the recovered positive electrode active material (precursor) powder. This composition was prepared by mixing sucrose as a coating agent with deionized water so that the carbon content was 4.4 parts by weight per 100 parts by weight of positive electrode active material. The material was then pre-milled using a ball mill at 300 rpm for 12 hours, and the coated positive electrode active material was dried by spray drying. The average particle size (D) of the spray-dried positive electrode active material powder was determined. 50 The thickness was 0.5 μm.

[0153] The spray-dried positive electrode active material was heated in a furnace under a nitrogen atmosphere at a heating rate of 3°C / min, and then calcined at 800°C for 12 hours to form a carbon (C) coating layer on the surface of the positive electrode active material. At this time, the nitrogen supply rate during calcination was 3 L / min, and after the completion of calcination, CS analysis revealed that the coating amount was 1.43% by weight.

[0154] The coated cathode active material was milled using a jet mill under an air atmosphere with a feeding line pressure of 4 bar and a grinding line pressure of 1 bar to obtain regenerated cathode active material with a final particle size of 200 to 1500 nm.

[0155] Example 2 In Example 1, a regenerated cathode active material was produced by repeating the same process as in Example 1, except that milling was performed using a jet mill under an inactive (Ar) atmosphere with a feeding line pressure of 4 bar and a grinding line pressure of 1 bar.

[0156] Comparative Example 1 Instead of recycled cathode active material, a newly generated (fresh) LFP cathode active material was used. Analysis of the newly generated LFP cathode active material by ICP confirmed that it was an LFP (LiFePO4) cathode active material with elemental ratios of Li / Fe:1.06, Li / P:1.00, and P / Fe:1.06.

[0157] Comparative Example 2 In Example 1, the regenerated cathode active material was produced in the same manner as in Example 1, except that the firing was performed at 700°C after the spray drying.

[0158] Comparative Example 3 In Example 2, the regenerated cathode active material was produced in the same manner as in Example 2, except that the firing was performed at 600°C after the spray drying.

[0159] Comparative Example 4 In the above-mentioned Example 2, a regenerated cathode active material was produced by performing a secondary heat treatment step.

[0160] [Test Example I: TEM Analysis] TEM analysis was performed on the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4.

[0161] Specifically, scanning electron microscope images of the positive electrode active material were acquired using a scanning electron microscope (TITAN G2 800-200) and are shown in Figure 2 below.

[0162] Figure 2 below shows TEM images of the regenerated or newly created cathode active material in Example 1 and Comparative Examples 1 and 2 according to the present invention.

[0163] As shown in Figure 2 below, while Example 1 and Comparative Example 2 appear similar when viewing the carbon coating layer and its interface, the EDP (Electron Diffraction Pattern) diagram on the right reveals that Comparative Example 2 clearly exhibits both a ring structure (single-particle crystal structure) and a spot structure (multi-particle crystal structure), confirming the coexistence of single crystals and polycrystals. In contrast, Example 1 only exhibits a spot structure (single-particle crystal structure), confirming that particles consisting solely of single crystals were produced.

[0164] From this, we confirmed that, using the regeneration process of the present invention, a regenerated cathode active material with a single crystal structure and single particles is synthesized.

[0165] [Test Example II: TEM Analysis] TEM analysis was performed on the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4.

[0166] Specifically, TEM images of the cathode active material were acquired using a TEM (TITAN G2 800-20) system.

[0167] Figure 3 below compares TEM images and EDP (Electron Diffraction Pattern) images of the cathode active material regenerated in Example 2 and Comparative Example 3 according to the present invention, and Figure 4 below shows the TEM image and TEM-EDS mapping image of the cathode active material regenerated in Comparative Example 4.

[0168] As shown in Figure 3 below, in Example 2 and Comparative Example 3, Fe 3+ In the case where the reduction reaction proceeds appropriately and no grain boundaries are observed, it can be confirmed that grain boundaries are observed in the cathode active material regenerated in Comparative Example 4 of Figure 4 because trivalent iron compounds (Fe2O3 and Li3Fe2(PO4)3) are present within the particles.

[0169] The similarity between the images of Example 2 and Comparative Example 3 can be inferred to be because Comparative Example 3 contains localized foreign matter (ferric iron compounds) that is difficult to observe on a TEM.

[0170] In Figure 4 below, the TEM-EDS mapping image of Comparative Example 4 shows that there is a high concentration of P (phosphorus) on the inside and a high concentration of Fe (ferrous metal) on the surface of the particles. Here, the presence of P indicates the presence of the trivalent iron compound Li3Fe2(PO4)3, and the Fe on the surface is inferred to be Fe2O3.

[0171] Therefore, since Comparative Example 4 did not undergo the reduction and calcination process, it existed almost entirely as a trivalent iron compound Fe oxide phase compound, exhibiting a core / shell morphology. According to the TEM-EDS mapping in Figure 3, both Example 2 and Comparative Example 3 show a mixed P and Fe structure, which is presumed to be because most of the trivalent iron compounds (Li3Fe2(PO4)3 and Fe2O3) were reduced to the form of divalent iron compounds LFP.

[0172] [Test Example III: XRD Analysis] XRD analysis was performed on the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4, and XRD graphs were obtained.

[0173] Specifically, Figure 5 below is a comparison of the XRD graphs of the regenerated cathode active materials of Example 2 and Comparative Examples 3-4 according to the present invention.

[0174] As shown in Figure 5 below, in Example 2 and Comparative Examples 3-4 according to the present invention, the reduction reaction of the trivalent iron compound to the divalent iron compound proceeded appropriately, and an XRD graph consistent with the fresh state of LFP was obtained.

[0175] On the other hand, in the case of Comparative Example 4, the reduction reaction of the trivalent iron compound to the divalent iron compound did not occur well, and it was confirmed that Fe2O3 and Li3Fe2(PO4)3 were observed.

[0176] [Example Test IV: XPS Analysis] XPS analysis was performed on the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4, and XPS graphs were obtained.

[0177] Specifically, Figure 6 below compares the XPS graphs of the regenerated cathode active materials of Example 2 and Comparative Examples 3-4 according to the present invention. XPS analysis is performed using O 1s We analyzed the differences between them.

[0178] As shown in Figure 6 below, no difference could be found between the graphs of the reference (fresh LFP) and comparative example 3. This is understood to be because the oxygen elements present in the positive electrode active material are located within the same crystal.

[0179] However, a shoulder is observed around 530 eV in Comparative Example 4. This is because, as can be seen from the EDP image in Figure 3, Comparative Example 4 has a core / shell configuration where the outside of the particle is in the form of Fe2O3, and the inside of the particle contains many Li3Fe2(PO4)3 molecules.

[0180] Similar to the XRD in Figure 5, when a trace amount of trivalent iron compound is present in the newly generated positive electrode active material of Comparative Example 1, it is not observed by XPS, and therefore the results of the XPS analysis of Example 2 and Comparative Example 3 become similar.

[0181] [Example Test V: EPR Analysis] EPR analysis was performed on the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4, and EPR graphs were obtained.

[0182] Specifically, Figure 7 below is a comparison of the EPR graphs of the regenerated cathode active materials of Example 2 and Comparative Examples 3-4 according to the present invention.

[0183] As shown in Figure 7 below, in Comparative Example 4, large peaks were observed that were due to the trivalent iron compounds present in Li3Fe2(PO4)3 and Fe2O3.

[0184] However, in Example 2, the trivalent iron compound was completely reduced and converted to a divalent iron compound, so the peak in question was not observed.

[0185] Furthermore, unlike the results of XRD and XPS, EPR analysis revealed that Comparative Example 3 also showed a different slope pattern in its graph compared to Example 2. This is because trace amounts of the trivalent iron compound, which were not observed in XRD and XPS, remained in Comparative Example 3, resulting in the EPR slope pattern shown in the graph.

[0186] Through this, we confirmed that, unlike Comparative Examples 3 and 4, Example 2 produced a regenerated positive electrode active material in which the trivalent iron compound was completely converted and only the divalent iron compound was present.

[0187] [Test Example VI: Evaluation of CHC Cells] The electrochemical performance of the regenerated or newly generated cathode active materials obtained in Examples 1-2 and Comparative Examples 1-4 was measured through evaluation using a CHC cell as described below. *Evaluation of CHC cells: 97.5% by weight of recycled or newly generated positive electrode active material, 1% by weight of conductive carbon black, and 1.5% by weight of PVdF as a binder were weighed and mixed with LFP to produce a slurry. After coating aluminum foil with this slurry to produce a positive electrode, a cell (Coin Half Cell, CHC) was manufactured. Under conditions of a voltage of 3-4.3V, with charging and discharging proceeding at 0.1C / 0.1C, and containing ethylene carbonate (EC):ethyl methyl carbonate (EMC) = 3:7 (by weight ratio) and other additives as the electrolyte, the electrochemical performance (charging capacity, discharging capacity, and efficiency) was evaluated.

[0188] The results of the evaluation are shown in Figures 8 and 9 below, respectively.

[0189] Figure 8 below compares the electrochemical performance of the regenerated or newly generated positive electrode active materials in Example 1 and Comparative Examples 1 and 2, including rated voltage, energy capacity, CHC capacity retention rate, and 45°C lifetime capacity. Figure 9 below compares the electrochemical performance of the regenerated or newly generated positive electrode active materials in Example 2 and Comparative Examples 3 and 4, including energy capacity and CHC capacity retention rate.

[0190] As shown in the upper right figure of Figure 8 below, in the case of Example 1, indicated in blue ink, the charge / discharge capacity was found to be equivalent, similar, or improved compared to the fresh positive electrode active material of Comparative Example 1, indicated in black ink, and the regenerated positive electrode active material of Comparative Example 2, indicated in red ink.

[0191] As shown in Figure 9 below, Example 2, indicated in blue ink, showed generally superior cell characteristics compared to Comparative Example 3, indicated in black ink. In particular, as shown in the lower right of Figure 9, Example 2 showed remarkably superior results in high-temperature (45°C) lifetime characteristics, while Comparative Example 4 was so poor that it was difficult to measure its cell characteristics.

Claims

1. (a) A step of recovering the positive electrode active material by heat-treating the current collector and the positive electrode active material layer formed on its surface in an oxidizing atmosphere, (b) Adding a coating agent to the recovered positive electrode active material and firing it in a reducing atmosphere to form a coating layer on the surface of the positive electrode active material, while converting the trivalent iron compounds in the positive electrode active material to divalent iron compounds and converting the polycrystalline particles into single-crystal positive electrode active material, (c) A method for regenerating a positive electrode active material, comprising the step of milling the positive electrode active material, which has been coated and converted into a divalent iron compound, to adjust the particle size of the positive electrode active material.

2. The method for regenerating a positive electrode active material according to claim 1, wherein step (a) comprises a step of primary heat treatment at 300 to 440°C in an oxidizing atmosphere, and a step of secondary heat treatment at 500 to 650°C in an oxidizing atmosphere after the primary heat treatment.

3. The method for regenerating a positive electrode active material according to claim 2, wherein in step (a) above, the primary heat treatment is performed for 30 minutes to 10 hours, and the secondary heat treatment is performed for 30 minutes to 10 hours.

4. The method for regenerating a positive electrode active material according to claim 1, wherein step (b) comprises adding a coating agent to the recovered positive electrode active material and pre-milling it, spray-drying the pre-milled positive electrode active material, and firing the spray-dried positive electrode active material at 710 to 900°C in a reducing atmosphere.

5. The method for regenerating a positive electrode active material according to claim 1 or 4, wherein the coating agent comprises one or more of the following: metal, organometallic, and carbon components.

6. The method for regenerating a positive electrode active material according to claim 1 or 4, wherein in step (b) above, the firing is performed for 1 to 24 hours.

7. The method for regenerating a positive electrode active material according to claim 4, wherein the preliminary milling is performed using a ball mill, a high-energy ball mill, a vibratory mill, or a roll mill.

8. The method for regenerating a positive electrode active material according to claim 7, wherein the preliminary milling is performed for 2 to 24 hours at an agitation speed of 50 to 500 rpm.

9. The method for regenerating a positive electrode active material according to claim 1, wherein in step (c) above, milling is performed using a jet mill.

10. The positive electrode active material is represented by the following chemical formula 1, (Chemical formula 1) Li １－ａ M ａ Fe １－ｂ N ｂ P １－ｃ X ｃ O ４－ｄ Y ｄ A method for regenerating a positive electrode active material according to claim 1, wherein in the chemical formula 1, M comprises one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N comprises one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X comprises Si; Y comprises one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are, respectively, 0 ≤ a < 1, 0 ≤ b < 1, 0 ≤ c < 1, and 0 ≤ d < 4.

11. The method for regenerating a positive electrode active material according to claim 1, wherein the positive electrode active material consists of 99 mol% or more or 100 mol% single crystals.

12. The method for regenerating a positive electrode active material according to claim 1, wherein the positive electrode active material contains less than 1 mol% or 0 mol% of the trivalent iron compound, based on a total of 100 mol% of the divalent iron compound and trivalent iron compound, as determined by XRD (X-ray diffraction) analysis or EPR (electron paramagnetic resonance) analysis.

13. A regenerated positive electrode active material represented by the following chemical formula 1, consisting of 99 mol% or more or 100 mol% single crystals, (Chemical formula 1) Li １－ａ M ａ Fe １－ｂ N ｂ P １－ｃ X ｃ O ４－ｄ Y ｄ In the above chemical formula 1, M comprises one or more elements selected from the group consisting of Nb, Al, Na, Ti, Zr, and K; N comprises one or more elements selected from the group consisting of Mo, V, Mn, Sm, Eu, Yb, Cu, Mg, Ni, and Co; X comprises Si; Y comprises one or more elements selected from the group consisting of F, S, and N; and a, b, c, and d are, respectively, 0 ≤ a ≤ 0.5, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.5, and 0 ≤ d ≤ 0.1, which constitutes a regenerated positive electrode active material.

14. A secondary battery comprising the regenerated positive electrode active material according to claim 13.

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