Method for manufacturing electrode active material, method for manufacturing secondary battery, and positive electrode active material

Abstract

The present invention provides a method for manufacturing an electrode active material, a method for manufacturing a secondary battery, and a positive electrode active material, all of which offer improved cycle characteristics while suppressing gas generation compared to conventional methods. [Solution] The method includes a coating step of coating the core portion with precursor particles that serve as precursors for the shell layer, and a heat treatment step of raising the temperature of the core portion coated with precursor particles to a heating temperature of 300°C to 600°C, and then lowering the temperature to 150°C or lower at a cooling rate of 0.70°C / min or less to form the shell layer.

Description

[Technical Field]

[0001] This invention relates to a method for producing an electrode active material, a method for producing a secondary battery, and a positive electrode active material. [Background technology]

[0002] In recent years, core-shell particles have been used as positive electrode active materials for lithium-ion secondary batteries. These particles consist of core particles made of lithium nickel manganese oxide (LNMO) coated with a shell layer made of lithium manganese phosphate (LMP) or lithium iron phosphate (LFP) (for example, Patent Document 1). According to Patent Document 1, by using the above-mentioned core-shell particles as the positive electrode active material, the amount of gas generated can be suppressed and the capacity retention rate can be improved compared to when LNMO alone is used as the positive electrode active material. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2024-149302 [Overview of the project] [Problems that the invention aims to solve]

[0004] However, while the secondary battery described in Patent Document 1 showed significant improvements in gas generation suppression and capacity retention compared to using LNMO alone, it was not sufficient, and there was room for further improvement.

[0005] Therefore, the present invention aims to provide a method for manufacturing an electrode active material, a method for manufacturing a secondary battery, and a positive electrode active material that have improved cycle characteristics while suppressing gas generation compared to conventional methods. [Means for solving the problem]

[0006] One aspect of the present invention for solving the above-mentioned problems is a method for producing an electrode active material including core-shell particles in which the surface of the core portion is coated with a shell layer, comprising: a coating step of coating the core portion with precursor particles that are precursors of the shell layer; and a heat treatment step of raising the temperature of the core portion coated with the precursor particles to a heating temperature of 300°C to 600°C, and then lowering the temperature to 150°C or lower at a cooling rate of 0.70°C / min or less to form the shell layer.

[0007] According to this pattern, cycle characteristics can be improved while suppressing gas generation compared to conventional methods.

[0008] A preferred configuration is that, in the heat treatment step, the heating temperature is maintained for at least one hour before being cooled.

[0009] A preferred configuration is one in which the core portion is mainly composed of lithium nickel manganese oxide having a spinel-type crystal structure.

[0010] In this context, "major component" refers to the component that accounts for more than 50% of the total components. The same applies hereafter.

[0011] A preferred configuration is one in which the shell layer is mainly composed of lithium manganese phosphate having an olivine-type crystal structure.

[0012] One aspect of the present invention is a method for manufacturing a secondary battery having a positive electrode active material, a negative electrode active material, and an electrolyte, the method comprising a positive electrode active material formation step of forming the positive electrode active material using the above-described method for manufacturing electrode active materials.

[0013] According to this pattern, cycle characteristics can be improved while suppressing gas generation compared to conventional methods.

[0014] One aspect of the present invention includes core-shell particles in which the surface of a core portion is coated with a shell layer. The core portion mainly contains lithium nickel manganese oxide having a spinel crystal structure, the shell layer mainly contains lithium manganese phosphate and has an average thickness of less than 5 nm. The core portion includes a nickel-poor region in which the elemental concentration of nickel is lower than that of the central portion of the core portion at least in a range of at least 5 nm from the surface of the core-shell particles. The nickel-poor region is a positive electrode active material in which the ratio of manganese element to nickel element is 10 or more.

[0015] The "average thickness" referred to here means the arithmetic mean value of the film thickness measured at 10 arbitrary points, which is the shortest distance (film thickness) from one main surface to the other main surface when observing a cross section using a microscope such as a scanning electron microscope or a transmission electron microscope.

[0016] According to this aspect, compared with the prior art, the amount of gas generation can be suppressed while the cycle characteristics can be improved.

[0017] A preferred aspect is that the elemental concentration of manganese in the core portion is higher than that of the central portion of the core portion at least in a range of at least 5 nm from the surface of the core-shell particles.

[0018] As long as the above-described aspects are included in the technical scope of the present invention, they can be subordinated to each other, some configurations can be cited, or some configurations can be replaced among the aspects.

Effect of the Invention

[0019] According to the present invention, compared with the prior art, the amount of gas generation is suppressed while the cycle characteristics are improved.

Brief Description of the Drawings

[0020] [Figure 1] It is an explanatory diagram conceptually showing a secondary battery according to a first embodiment of the present invention. (a) is a perspective view of the secondary battery, and (b) is a cross-sectional view taken along line A-A of (a). [Figure 2]It is an explanatory diagram of the electrode part in FIG. 1, (a) is a cross-sectional view of the positive electrode part, and (b) is a cross-sectional view of the negative electrode part.

Mode for Carrying Out the Invention

[0021] Hereinafter, embodiments of the present invention will be described in detail.

[0022] The secondary battery 1 of the first embodiment of the present invention is a lithium-ion secondary battery, and as shown in FIG. 1, it includes a battery laminate 2, electrode extraction members 3a and 3b, an electrolyte 5, and an exterior body 6.

[0023] <Battery laminate 2> As shown in FIG. 1(b), the battery laminate 2 has a plurality of positive electrode parts 10, a plurality of negative electrode parts 11, and a plurality of separators 12. In the battery laminate 2 of the present embodiment, the electrode parts 10 and 11 and the separators 12 are arranged alternately, such as... / separator 12 / positive electrode part 10 / separator 12 / negative electrode part 11 / separator 12 / ..., and the separators 12 are respectively arranged on the outermost side in the stacking direction.

[0024] (Positive electrode part 10) As shown in FIG. 2(a), the positive electrode part 10 is a structure in which a positive electrode active material layer 21 is laminated on at least one main surface of a positive electrode current collector 20, and it is an intercalation electrode in which lithium ions can be inserted and desorbed. The positive electrode active material layer 21 is a layered body having a positive electrode active material 22 (electrode active material), a conductive assistant, and a binder.

[0025] As shown in the enlarged view of FIG. 2(a), the positive electrode active material 22 is composed of a plurality of first core-shell particles 23. The first core-shell particle 23 is one in which a first shell layer 26 is coated on the surface of a first core part 25.

[0026] The first core part 25 is composed of a lithium ion conductive oxide, and lithium ions can be inserted and desorbed. The average potential of lithium deintercalation and insertion in the first core part 25 is preferably 4.5 V or more and 5.0 V or less with respect to the deposition potential of Li (vs. Li / Li + ; also shown). That is, the first core part 25 preferably has an operating potential of 4.5 V or more and 5.0 V or less based on lithium metal alone. The potential of the lithium ion insertion / desorption reaction (hereinafter also referred to as voltage) (vs. Li / Li + ) can be obtained, for example, by measuring the charge / discharge characteristics of a half-cell with the working electrode using the first core part 25 and lithium metal as the counter electrode, and reading the voltage values at the start and end of the plateau. When there are two or more plateaus, it is sufficient that the plateau with the lowest voltage value is 4.5 V (vs. Li / Li + ) or more, and the plateau with the highest voltage value is 5.0 V (vs. Li / Li + ) or less.

[0027] The first core part 25 is not particularly limited as long as it has ion conductivity, but it is preferably mainly composed of a spinel-type lithium manganese-based oxide represented by the following formula (1), and more preferably contains 90% of the spinel-type lithium manganese-based oxide represented by the following formula (1) in all components. Li 1+x M y Mn 2-x-y O4 ···(1) In the above formula (1), x and y satisfy 0 ≦ x ≦ 0.2 and 0 < y ≦ 0.8 respectively, and M is at least one selected from the group consisting of Al, Mg, Zn, Ni, Co, Fe, Ti, Cu, and Cr.

[0028] Among the above formula (1), lithium nickel manganese oxide (hereinafter also referred to as LNMO) in which M is Ni is preferable.

[0029] The first core part 25 of the present embodiment is lithium nickel manganese oxide, and as shown in FIG. 2(a), it includes a nickel poor region 27 and a nickel rich region 28. The nickel-poor region 27 is a region located at least 5 nm from the surface of the first shell layer 26, and has a lower nickel element concentration and a higher manganese element concentration compared to the central nickel-rich region 28. The nickel-poor region 27 preferably contains manganese elements migrated from the nickel-rich region 28, and also contains manganese elements originating from the first shell layer 26. Nickel-rich region 28 is a region where the concentration of nickel is higher and the concentration of manganese is lower than in nickel-poor region 27.

[0030] The first shell layer 26 is a coating mainly composed of a lithium-ion conductive oxide having an olivine-type crystal structure, and is composed of an intercalation material that functions as a positive electrode active material on its own. Specifically, the first shell layer 26 is preferably mainly composed of lithium manganese phosphate (LiMnPO4, hereinafter also referred to as LMP) having an olivine-type crystal structure, and it is more preferable that LMP accounts for 90% or more of the total components. The average thickness of the first shell layer 26 is preferably greater than 0 nm and less than 5 nm, and thinner than the particle size of the first core portion 25. The first shell layer 26 covers at least a portion of the surface of the first core portion 25, more preferably 95% or more, and more preferably completely.

[0031] (Negative electrode part 11) As shown in Figure 2(b), the negative electrode portion 11 is an intercalation electrode in which a negative electrode active material layer 31 is laminated on at least one main surface of the negative electrode current collector 30, and lithium ions can be inserted into and removed from it. The negative electrode active material layer 31 comprises a negative electrode active material 32 (electrode active material), a conductive additive, and a binder.

[0032] The negative electrode active material 32 is composed of multiple second core-shell particles 33, as shown in the enlarged view in Figure 2(b). The second core-shell particle 33 has a second shell layer 36 covering the surface of the second core portion 35.

[0033] The second core portion 35 is preferably composed mainly of lithium titanate (hereinafter also referred to as LTO) from the viewpoint of reducing the likelihood of lithium deposition and improving safety, and it is more preferable that LTO accounts for 90% or more of the total components. Among lithium titanates, lithium titanate with a spinel structure is particularly preferred for the second core portion 35 because it exhibits less expansion and contraction of the negative electrode active material 32 during lithium ion insertion and removal reactions.

[0034] The second shell layer 36 is a coating made of a lithium-ion conductive oxide containing phosphorus as an element, and is composed of an intercalation material. Specifically, the second shell layer 36 is preferably mainly composed of lithium iron phosphate (LiFePO4, hereinafter also referred to as LFP) having an olivine-type crystal structure, and it is more preferable that LFP accounts for 90% or more of the total components. The thickness of the second shell layer 36 is preferably thinner than the particle size of the second core portion 35. The second shell layer 36 covers at least a portion of the surface of the second core portion 35, more preferably 95% or more, and more preferably completely.

[0035] The current collectors 20 and 30 are not particularly limited, but are preferably made of aluminum or an aluminum alloy because they are stable in both positive and negative electrode reaction atmospheres. The current collectors 20 and 30 can also be made of a metal other than aluminum (copper, stainless steel, nickel, titanium, and their alloys) coated with a metal that does not react to the potential of the positive electrode portion 10 and the negative electrode portion 11.

[0036] The conductive additive used in the active material layers 21 and 31 is not particularly limited, but carbon materials are preferred. The carbon material is preferably at least one selected from natural graphite, artificial graphite, vapor-grown carbon fibers, carbon nanotubes, acetylene black, Ketjen black, and furnace black. The amount of conductive additive contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active material 22 and 32. Within the above range, the conductivity of the active material layers 21 and 31 can be ensured while maintaining adhesion to the binder and achieving sufficient adhesion to the current collectors 20 and 30.

[0037] The binder used in the active material layers 21 and 31 is not particularly limited, but for either of the active material layers 21 or 31, at least one selected from the group consisting of polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), styrene-butadiene rubber, polyimide, and derivatives thereof can be used. The amount of binder contained in the active material layers 21 and 31 is preferably 1 part by weight or more and 30 parts by weight or less per 100 parts by weight of the active material 22 and 32. Within the above range, the adhesion between the active materials 22 and 32 and the conductive additive is maintained, and sufficient adhesion to the current collectors 20 and 30 can be obtained.

[0038] <Electrode extraction member 3a, 3b> As shown in Figure 1(b), the positive electrode extraction member 3a is a positive electrode terminal that is at least electrically connected to one of the positive electrode portions 10 constituting the battery stack 2 within the casing 6 and extends both inside and outside the casing 6. The negative electrode extraction member 3b is a negative electrode terminal that is at least electrically connected to one of the negative electrode portions 11 constituting the battery stack 2 within the casing 6 and extends both inside and outside the casing 6. The electrode extraction members 3a and 3b are conductive plate-like bodies, and are not particularly limited as long as they are conductive; for example, metals such as aluminum or their alloys can be used.

[0039] <Electrolyte 5> The electrolyte 5 is not particularly limited as long as it has lithium ion conductivity, but a non-aqueous electrolyte obtained by dissolving a solute in a non-aqueous solvent, or a gel electrolyte obtained by impregnating a polymer with a non-aqueous electrolyte obtained by dissolving a solute in a non-aqueous solvent, can be used. The electrolyte 5 may be in solid form, or it may be a solid electrolyte. In this embodiment, the electrolyte 5 is a non-aqueous electrolyte.

[0040] <Separator 12> The separator 12 is installed between the positive electrode portion 10 and the negative electrode portion 11, and should have an insulating structure that can also contain the electrolyte 5. Examples of separators 12 include woven fabrics, nonwoven fabrics, and microporous membranes made from nylon, cellulose, polysulfone, polyethylene, polypropylene, polybutene, polyacrylonitrile, polyimide, polyamide, polyethylene terephthalate, and composites of two or more of these materials. The separator 12 may contain various plasticizers, antioxidants, and flame retardants, or it may be coated with metal oxides or the like.

[0041] <Exterior 6> As shown in Figure 1(b), the outer casing 6 has an internal space 40 and is a sealing member that houses and seals the battery stack 2 and electrolyte 5 in the internal space 40. It is chemically stable with respect to the electrolyte 5 and has water vapor barrier properties. As shown in Figure 1, the outer casing 6 is composed of a first outer casing film 41 and a second outer casing film 42, and can be sealed by sandwiching the battery stack 2 and electrolyte 5 between the outer casing films 41 and 42. The outer films 41 and 42 are made of laminate films containing a laminating resin.

[0042] Next, the manufacturing method of the secondary battery 1 of this embodiment will be described.

[0043] The manufacturing method of the secondary battery 1 in this embodiment mainly includes a positive electrode formation step, a negative electrode formation step, and a secondary battery assembly step.

[0044] The positive electrode formation process consists of a first core-shell formation process (positive electrode active material formation process) for forming first core-shell particles 23, and a positive electrode active material layer formation process for coating a positive electrode active material 22 containing a plurality of first core-shell particles 23 onto a positive electrode current collector 20 to form a positive electrode active material layer 21.

[0045] The first core-shell formation step is a step of forming a first shell layer 26 on the surface of the first core portion 25 to form first core-shell particles 23.

[0046] Specifically, the first core-shell formation process consists of a crushing process, a fluid formation process, a coating process, and a heat treatment process.

[0047] Specifically, first, lithium manganese phosphate (LMP) is placed in a solvent and stirred, then pulverized and crushed using a wet bead mill, and the beads are removed to form the first precursor particles of LMP (pulverization step).

[0048] In this case, it is preferable that the beads used in the bead mill have a diameter of 0.015 mm or more and 2.0 mm or less. In this case, the processing time of the bead mill can be appropriately set depending on the bead diameter and the particle size of the desired LMP, but it is preferably 30 minutes to 24 hours, and more preferably 1 hour to 24 hours. Furthermore, the bead milling process may be terminated when the cumulative load power (Wh / g) reaches a predetermined amount. In this case, the solvent is not particularly limited as long as it can disperse without reacting with lithium manganese phosphate, but it is preferably one or more alcohol solutions, and ethanol is more preferable from the viewpoint of volatility and safety. The bead mill may also be a dry-type bead mill.

[0049] Next, the first precursor particles, which are pulverized in the pulverization process and dispersed in the solvent, are concentrated as needed to form a first precursor fluid (fluid formation process).

[0050] Next, a shell layer 26 composed of LMP is formed on the surface of the core portion 25 by a mechanical coating method, in which at least one type of energy—shear force, compressive force, impact force, and centrifugal force—is applied to the lithium-ion conductive oxide constituting the core portion 25 and / or the first precursor particles in the first precursor fluid constituting the shell layer 26, while the core portion 25 and the first precursor particles in the first precursor fluid are mechanically brought into contact. In this embodiment, the surface of the core portion 25 is ground using a grinding device such as a grinding mill, and the ground material is bonded to a fine particle fluid to form a grinding product (coating step).

[0051] The amount of first precursor particles (LMP) in the fine particle fluid introduced into the grinding device at this time is preferably 0.5 wt% or more. The amount of first precursor particles (LMP) in the fine particle fluid introduced into the grinding device at this time is preferably 2.5 wt% or less, and more preferably 1.6 wt% or less. The processing temperature in the grinding apparatus at this time is preferably 5°C to 100°C, more preferably 10°C to 90°C, and even more preferably 20°C to 80°C. The processing time in the grinding device at this time is preferably 5 minutes or more and 90 minutes or less, and more preferably 10 minutes or more and 60 minutes or less. Furthermore, the grinding process may be terminated when the cumulative load power (Wh / g) reaches a predetermined amount. The atmosphere in the grinding apparatus at this time is preferably an inert gas atmosphere or an air atmosphere.

[0052] Next, the ground material is heat-treated at a heating temperature T1, the heat-treated state is maintained for a heating time t1 to remove the dispersion solvent from the ground material, and the temperature is lowered to 150°C or below at a cooling rate v1 to form the first shell layer 26 and the first core-shell particles 23 (heat treatment step).

[0053] In this case, the heating temperature T1 is 300°C or higher and 600°C or lower, and preferably 400°C or lower. The heating time t1 is 1 hour or more, preferably 12 hours or less, and more preferably 6 hours or less. The cooling rate v1 is preferably greater than 0°C / min and less than or equal to 0.70°C / min.

[0054] The positive electrode active material layer formation process consists of a paste application process and a firing process. Specifically, the first core-shell particles 23 constituting the positive electrode active material 22 are mixed with a conductive material and a binder, dispersed in a solvent to form a paste-like slurry, and the slurry is applied to the positive electrode current collector 20 to form a slurry-coated body (paste application step).

[0055] At this time, the solvent used in the paste coating process is not particularly limited as long as it can dissolve or disperse the first core-shell particles 23, the conductive material and the binder, but for example, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, methyl ethyl ketone, methyl acetate, ethyl acetate and tetrahydrofuran can be used. Dispersants and thickeners may be added to these.

[0056] Next, the slurry coating formed in the paste coating step is fired for a predetermined firing time and at a predetermined firing temperature to form the positive electrode active material layer 21 and the positive electrode portion 10 (firing step).

[0057] In this case, the firing time is not particularly limited as long as the slurry-coated body is fired to a temperature at which the slurry solidifies, but it is preferably 1 minute or more and 1 hour or less, and more preferably 20 minutes or more. The firing temperature is not particularly limited as long as it is the temperature at which the slurry-coated body is fired and the slurry solidifies, but it is preferably, for example, 80°C to 200°C.

[0058] The above is a description of the positive electrode formation process.

[0059] The negative electrode formation process is the same as the positive electrode formation process described above, except that lithium iron phosphate (LFP) is pulverized using a bead mill or ball mill in the pulverization process (second pulverization process) of the first core-shell formation process to form second precursor particles, and the second precursor particles are used instead of the first precursor particles in each process, and lithium titanate (LTO) is used as the core 25 in the coating process of the first core-shell formation process, so the explanation is omitted. In other words, the negative electrode formation process consists of a second core-shell formation process and a negative electrode active material layer formation process. The second core-shell formation process consists of a second grinding process, a fluid formation process, a coating process, and a heat treatment process, while the negative electrode active material layer formation process consists of a paste application process and a firing process.

[0060] The secondary battery assembly process comprises a battery stack formation process, an interface electrolyte injection process, a voltage application process, and an electrolyte injection process. Specifically, a separator 12 is sandwiched between the positive electrode portion 10 and the negative electrode portion 11 to form a battery stack 2 (battery stack formation process).

[0061] Next, electrode extraction members 3a and 3b are attached to the battery stack 2, the battery stack 2 is placed inside the outer casing 6 which has been partially opened beforehand, and the interface electrolyte is injected into the outer casing 6 (interface electrolyte injection step).

[0062] The interface electrolyte used at this time is preferably the same as that of electrolyte 5, and may also be electrolyte 5 with an electrolyte additive containing a silicon-containing compound added. As electrolyte additives, for example, compounds containing siloxanes such as 2,4,6,8-tetravinyl-2,4,6,8-tetramethylcyclotetrasiloxane (hereinafter also referred to as 4VC4S), octamethylcyclotetrasiloxane, octadecamethylcyclooctadecanenonasiloxane, 1,3,5-tris(trifluoropropylmethyl)cyclotrisiloxane, and tetrakis(trimethylsilyl) orthosilicate can be used.

[0063] Next, a charge / discharge device is connected between the electrode extraction members 3a and 3b, and charging and discharging are performed (work-in-progress charge / discharge process).

[0064] The positive electrode potential at this time is between 4.5V and 5V (vs. Li / Li + ) is preferable.

[0065] Next, if necessary, the interface electrolyte is discharged from inside the outer casing 6, the electrolyte 5 is injected into the outer casing 6 (electrolyte injection step), the outer casing 6 is sealed to enclose the battery stack 2, and the secondary battery 1 is completed.

[0066] The manufacturing method for the secondary battery 1 of this embodiment includes a coating step of coating the core portions 25 and 35 with precursor particles that are precursors to the shell layers 26 and 36, and a heat treatment step of raising the temperature of the core portions 25 and 35 with the precursor particles coated to a heating temperature of 300°C to 600°C, and then lowering the temperature to 150°C or lower at a cooling rate of 0.70°C / min or less to form the shell layers 26 and 36. A secondary battery 1 formed by this method of electrode active material can have improved capacity retention and suppress the generation of gas due to the decomposition of the electrolyte 5 compared to conventional batteries.

[0067] The positive electrode active material 22 of the secondary battery 1 of this embodiment includes first core-shell particles 23 in which the surface of a first core portion 25 is coated with a first shell layer 26. The first core portion 25 is mainly composed of lithium nickel manganese oxide having a spinel-type crystal structure, and the first shell layer 26 is mainly composed of lithium manganese phosphate with an average thickness of less than 5 nm. The first core portion 25 includes a nickel-poor region 27 in which the elemental concentration of nickel is lower than that of the central portion of the first core portion 25, at least in a range of at least 5 nm from the surface of the first core-shell particles 23. The nickel-poor region 27 has a ratio of manganese element to nickel element (Mn(Atomic%) / Ni(Atomic%)) of 10 or more. With a secondary battery 1 equipped with such a positive electrode active material 22, the capacity retention rate can be improved and the generation of gas due to the decomposition of the electrolyte 5 can be suppressed compared to conventional batteries.

[0068] According to the secondary battery 1 of this embodiment, the capacity retention rate can be improved compared to conventional batteries, and the generation of gas due to the decomposition of the electrolyte 5 can be suppressed.

[0069] In the embodiment described above, a first precursor fluid in which first precursor particles are dispersed in a solvent is introduced into the grinding apparatus during the coating process, but the present invention is not limited thereto. Before the coating process, the first precursor fluid formed in the fluid formation process may be mixed with a lithium-ion conductive oxide, heated under reduced pressure to volatilize and remove the solvent, forming a mixture in which the first precursor fluid adheres to the surface of the lithium-ion conductive oxide (pre-mixing process). This mixture may then be introduced into the grinding apparatus in place of the first precursor fluid during the coating process. The mixing time in this pre-mixing step is preferably 5 minutes or more and 1 hour or less, and more preferably 15 minutes or less.

[0070] In the embodiments described above, the interface electrolyte injection step and the voltage application step were performed before the electrolyte injection step, but the present invention is not limited thereto. The interface electrolyte injection step and the voltage application step may be omitted.

[0071] In the embodiment described above, the negative electrode active material 32 was composed of second core-shell particles 33 in which a second shell layer 36 was coated on the surface of a second core portion 35, but the present invention is not limited thereto. The negative electrode active material 32 may consist only of the second core portion 35. That is, the second core portion 35 does not have to be coated with the second shell layer 36.

[0072] In the embodiments described above, the components can be freely substituted or added between each embodiment, as long as they fall within the technical scope of the present invention. [Examples]

[0073] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples and can be implemented with appropriate modifications without altering its essence.

[0074] (Example 1) (a) Preparation of the positive electrode active material First, a predetermined amount of ethanol as a solvent was mixed with lithium manganese phosphate (LMP) powder having an olivine-type crystal structure, and bead milling treatment was performed for 7 hours using beads with a bead diameter of 0.1 mm. After removing the beads from the treated mixture, a part of the ethanol was removed to obtain a slurry containing 16.4 wt% of the first precursor particles (LMP fine powder).

[0075] As the first core part of the positive electrode, lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4, hereinafter also referred to as LNMO) was used. 60 g of LNMO was put into a attrition mill and rotated at a clearance of 0.8 mm, a rotor load power of 1.5 kW, and 2600 rpm. The ethanol dispersion slurry of the first precursor particles was added in two portions so that the addition amount of the first precursor particles was 1.2 wt%. Thereafter, the rotor rotation speed was maintained in the range of 2600 rpm to 3000 rpm, and the treatment was performed at room temperature for 10 minutes in an air atmosphere. Thereafter, heat treatment was performed at 370 °C for 3 hours, and then the temperature was lowered to room temperature so that the temperature lowering rate was 0.45 ± 0.25 °C / min, thereby obtaining a positive electrode active material containing first core-shell particles in which the surface of LMNO (first core part) was coated with LMP (first shell layer).

[0076] (b) Preparation of the positive electrode part A slurry was prepared by dispersing a mixture containing the obtained positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in N-methyl-2-pyrrolidone (NMP) at solid content concentrations of 90 parts by weight, 6 parts by weight, and 4 parts by weight, respectively. The binder was used after being adjusted to a solution of N-methyl-2-pyrrolidone (NMP) with a solid content concentration of 5 wt%, and NMP was further added to adjust the viscosity for easy coating.

[0077] The slurry was coated onto a 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the positive electrode portion was fabricated by further vacuum drying at 170°C.

[0078] (c) Preparation of negative electrode active material First, lithium iron phosphate (LFP) powder, which has an olivine-type crystalline structure, was mixed with a predetermined amount of ethanol as a solvent, and the mixture was ball-milled for 3 hours using zirconia balls with a ball diameter of 0.5 mm. After removing the zirconia balls from the mixture, a portion of the ethanol was removed to obtain a slurry containing 16.4 wt% of second precursor particles (LFP fine powder).

[0079] As the second core of the negative electrode, a spinel-type lithium titanate (Li4Ti5O) with a median diameter of 8.5 μm is used. 12 (hereinafter also referred to as LTO) was used. 80g of LTO was placed in a grinding mill, and while rotating at a clearance of 0.8mm, rotor load power of 1.5kW, and 2600rpm, an ethanol-dispersed slurry of second precursor particles was added in two portions so that the amount of second precursor particles added was 3.6wt%. Then, the process was carried out at room temperature in an air atmosphere while maintaining the rotor rotation speed in the range of 2600rpm to 3000rpm, and then heat-treated at 350°C for 1 hour to obtain a negative electrode active material containing second core-shell particles in which the surface of LTO (second core portion) was coated with LFP (second shell layer).

[0080] (d) Fabrication of the negative electrode A slurry was prepared by dispersing a mixture containing 92 parts by weight, 3 parts by weight, and 5 parts by weight of the obtained negative electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, respectively, in N-methyl-2-pyrrolidone (NMP) at solid content concentrations. The binder used was an N-methyl-2-pyrrolidone (NMP) solution with a solid content concentration of 5% by weight, and further NMP was added to adjust the viscosity for easier coating.

[0081] The slurry was coated onto a 15 μm aluminum foil and then dried in an oven at 120°C. After performing this operation on both sides of the aluminum foil, the negative electrode was fabricated by further vacuum drying at 170°C.

[0082] (e) Fabrication of lithium-ion secondary batteries Using the positive and negative electrode sections prepared in (b) and (d) above, and a 20 μm polypropylene separator, a battery was fabricated according to the following procedure. First, the positive and negative electrode sections were dried under reduced pressure at 80°C for 12 hours. Next, 15 positive electrodes and 16 negative electrodes were stacked in the order of negative electrode / separator / positive electrode to form a battery stack. The outermost layer of both battery stacks was made up of separators. Next, aluminum tabs were vibration-welded to the positive and negative electrode sections at both ends.

[0083] Two aluminum laminate films were prepared to serve as the exterior material. After forming recesses for the battery section and the gas collection section using a press, the battery stack was inserted. The outer periphery, leaving space for electrolyte injection, was heat-sealed at 180°C for 7 seconds. A non-aqueous electrolyte was then added to an unsealed area, which was a solvent prepared by mixing ethylene carbonate, propylene carbonate, and ethyl methyl carbonate in a volume-based ratio of ethylene carbonate / propylene carbonate / ethyl methyl carbonate = 15 / 15 / 70, to which LiPF6 was dissolved at a concentration of 1 mol / L. 3 wt% of 4VC4S was added to the non-aqueous electrolyte as an electrolyte additive to create an electrolyte with additives. The unsealed area was then heat-sealed at 180°C for 7 seconds under reduced pressure. The obtained battery was charged with a constant current at a current equivalent to 0.2C until the battery voltage reached the cutoff voltage of 3.4V, and then the charging was stopped. After that, it was left to stand in a 60°C environment for 24 hours, and then discharged with a constant current at a current equivalent to 0.2C, stopping the discharge when the battery voltage reached 2.5V. After stopping the discharge, the gas accumulated in the gas collection section was removed and resealed. Through the above procedure, a lithium-ion secondary battery for evaluation was fabricated.

[0084] (Comparative Example 1) Comparative Example 1 was prepared in the same manner as in Example 1 (a), except that the positive electrode active material was obtained by lowering the temperature to room temperature at a rate of 1.0 ± 0.2 °C / min.

[0085] (Gas generation amount measurement) The amount of gas generated by the lithium-ion secondary batteries before and after cycle characteristic evaluation in Example 1 and Comparative Example 1 was evaluated using the Archimedes method, i.e., the buoyancy of the lithium-ion secondary batteries. The evaluation was performed as follows.

[0086] First, the weight of the lithium-ion secondary battery was measured using an electronic balance. Next, the weight in water was measured using a hydrometer (Alpha Mirage Co., Ltd., model number: MDS-3000), and the buoyancy was calculated by taking the difference between these weights. This buoyancy was calculated using the density of water (1.0 g / cm³). 3 The volume of the lithium-ion secondary battery was calculated by dividing by ( ). The amount of gas generated was calculated by comparing the volume after aging with the volume after the cycle characteristic evaluation described below.

[0087] (Evaluation of cycle characteristics of lithium-ion secondary batteries) The lithium-ion secondary batteries prepared in Example 1 and Comparative Example 1 were connected to a charge / discharge device (HJ1005SD8, manufactured by Hokuto Denko Co., Ltd.) and cycled operation was performed. Under conditions of 45°C, constant current charging was performed at a current value equivalent to 1.0C until the battery voltage reached the cutoff voltage of 3.4V, and then charging was stopped. Subsequently, constant current discharge was performed at a current value equivalent to 1.0C, and discharge was stopped when the battery voltage reached 2.5V. This constituted one cycle, and the charge / discharge cycle was repeated. Furthermore, the stability of the cycle characteristics was evaluated by using the discharge capacity at the 400th discharge cycle, with the discharge capacity at the first discharge cycle set to 100, as the capacity retention rate (%).

[0088] First, the evaluation results for Example 1 and Comparative Example 1 are shown in Table 1.

[0089] [Table 1]

[0090] In Example 1, where the cooling rate after heat treatment was 0.7°C / min or less and the positive electrode active material was formed by slow cooling, the amount of gas generated was suppressed and the capacity retention rate was improved compared to Comparative Example 1, where the cooling rate was 0.8°C / min or more and the cooling rate was faster than that of Example 1. This suggests that the crystal structure changes when the cooling rate after heat treatment is reduced.

[0091] (XPS measurement) The positive electrode active materials of Example 1 and Comparative Example 1 were subjected to compositional analysis using X-ray photoelectron spectroscopy (XPS) under the following analytical conditions. In addition, to analyze the compositional changes before and after heat treatment, the positive electrode active material of Example 1 before heat treatment was designated as Reference Example 1, and the positive electrode active material of Reference Example 1 was also subjected to compositional analysis.

[0092] (XPS analysis conditions) Equipment: ULVAC-PHI "PHI5000 VersaProbe II" X-ray source: Monochromatic AlKα rays X-ray intensity: 15kV (25W) X-ray measurement range: 700 μm square (X-ray spot diameter 200 μmΦ)

[0093] Table 2 shows the evaluation results for Example 1, Comparative Example 1, and Reference Example 1.

[0094] [Table 2]

[0095] In Example 1, where slow cooling was performed after heat treatment, the ratio of manganese element concentration to nickel element concentration was 10 or more, which was a larger value than in Comparative Example 1 and also larger than the value before heat treatment.

[0096] Here, since the core is largely covered by the shell layer, it is thought that when heat treatment is performed, only the manganese elements on the surface of the shell layer will scatter and disappear, and there will be almost no loss of nickel elements in the core. In other words, in the range of 5 nm from the shell layer, which is the resolution of the XPS used for evaluation, it was thought that the amount of nickel elements in the core did not change before and after heat treatment, while the amount of manganese elements decreased. However, the results in Table 2 show that in the 5nm range from the shell layer, which is the resolution of the XPS used for evaluation, the amount of nickel decreased and the amount of manganese increased in both Example 1 and Comparative Example 1. From these results, it is considered that in Example 1 and Comparative Example 1, during the cooling process after heat treatment, migration of manganese elements to the shell layer occurred in the core, resulting in the localization of manganese elements within the core and replenishment of the shell layer, while nickel elements migrated towards the center within the core. Furthermore, in Example 1, where the heat treatment was followed by slow cooling, the migration of manganese and nickel elements was significantly greater compared to Comparative Example 1, which had a higher cooling rate than Example 1. From this, it can be concluded that in Example 1, since the material was slowly cooled at a cooling rate of 0.70°C / min after heat treatment, the migration of manganese elements to the shell layer side and the migration of nickel elements to the center side of the core became rate-limiting processes compared to Comparative Example 1, and the material progressed until it became structurally stable. As a result, Example 1 exhibited less strain and greater structural stability compared to Comparative Example 1, and it is believed that the cycle characteristics were improved while suppressing gas generation compared to Comparative Example 1.

[0097] From these results, it was found that by performing heat treatment at over 300°C followed by cooling at a rate of 0.70°C / min or less, the core undergoes a structural exchange of nickel and manganese elements, at least within a 5nm range from the surface of the shell layer, until it becomes structurally stable. As a result, it was found that this leads to a reduction in gas generation and an improvement in cycle characteristics. [Explanation of symbols]

[0098] 1. Rechargeable battery (lithium-ion rechargeable battery) 5 Electrolytes 22 Positive electrode active material (electrode active material) 23. First core-shell particle 25. First Core Section 26. First Shell Layer 27 Nickel Poor Region 28 Nickel-rich regions 32 Negative electrode active material (electrode active material) 33. Second core-shell particle 35. Second Core Section 36. Second Shell Layer

Claims

1. A method for producing an electrode active material containing core-shell particles in which the surface of the core portion is coated with a shell layer, A coating step of coating the core portion with precursor particles that are precursors to the shell layer, A method for producing an electrode active material, comprising a heat treatment step of heating the core portion with the precursor particles coated on it to a heating temperature of 300°C to 600°C, and then cooling it down to 150°C or below at a cooling rate of 0.70°C / min or less to form the shell layer.

2. The method for producing an electrode active material according to claim 1, wherein in the heat treatment step, the heating temperature is maintained for at least one hour and then cooled.

3. The method for producing an electrode active material according to claim 1, wherein the core portion mainly consists of lithium nickel manganese oxide having a spinel-type crystal structure.

4. The method for producing an electrode active material according to claim 1, wherein the shell layer mainly consists of lithium manganese phosphate having an olivine-type crystal structure.

5. A method for manufacturing a secondary battery having a positive electrode active material, a negative electrode active material, and an electrolyte, A method for manufacturing a secondary battery, comprising a positive electrode active material formation step of forming a positive electrode active material using the method for manufacturing an electrode active material described in any one of claims 1 to 4.

6. It contains core-shell particles in which the surface of the core is coated with a shell layer, The core portion mainly consists of lithium nickel manganese oxide having a spinel-type crystal structure. The aforementioned shell layer is mainly composed of lithium manganese phosphate and has an average thickness of less than 5 nm. The core portion includes a nickel-poor region in which the elemental concentration of nickel is lower than that of the central portion of the core portion, at least in a range of at least 5 nm from the surface of the core-shell particles. The nickel-poor region is a positive electrode active material in which the ratio of manganese elements to nickel elements is 10 or more.

7. The positive electrode active material according to claim 6, wherein the core portion has a higher manganese elemental concentration than the central portion of the core portion, at least in a range of at least 5 nm from the surface of the core shell particles.

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