Method for producing positive electrode material, and positive electrode material

Abstract

The invention relates to a manufacturing method of a positive electrode material and the positive electrode material. A method for producing a positive electrode material, the method comprising: a step for obtaining a mixture containing a positive electrode active material and a lithium alloy; and a step for heat-treating the mixture.

Description

Technical Field

[0001] The present disclosure relates to a method for manufacturing a positive electrode material and the positive electrode material. Background Art

[0002] In a lithium-ion secondary battery, a portion of lithium ions released from the positive electrode active material during initial charge reacts with the electrolyte on the surface of the negative electrode to form a film called SEI (Solid Electrolyte Interphase).

[0003] The consumption of lithium ions associated with the formation of SEI becomes the cause of the reduction in battery capacity, so it is hoped that it is as small as possible. As a strategy for suppressing the consumption of lithium ions associated with the formation of SEI, it is proposed to make the positive electrode contain a positive electrode active material and a lithium alloy, and use the lithium ions released from the lithium alloy during the initial charge to replenish the amount of lithium ions consumed by the formation of SEI (for example, refer to Japanese Patent Table 2021-520614). Summary of the invention

[0004] The method of making the positive electrode contain lithium alloy is an effective means to suppress the reduction of the initial battery capacity. However, the metal ions from the lithium alloy that has lost lithium ions move to the negative electrode side and precipitate, and the precipitate becomes a resistance layer, which may cause a reduction in battery capacity.

[0005] An object of the present disclosure is to provide a method for producing a positive electrode material for a lithium ion secondary battery that can suppress a decrease in initial battery capacity and a decrease in battery capacity due to repeated charge and discharge, and a positive electrode material.

[0006] Means for solving the above-mentioned problems include the following embodiments.

[0007] <1> A method for producing a positive electrode material, comprising: obtaining a mixture containing a positive electrode active material and a lithium alloy; and heat treating the mixture.

[0008] <2> The method for producing a positive electrode material according to <1>, wherein the lithium alloy is selected from bismuth, tin and antimony.

[0009] <3> The method for producing a positive electrode material according to <1> or <2>, wherein a temperature of the heat treatment is 350° C. or higher.

[0010] <4> The method for producing a positive electrode material according to any one of <1> to <3>, wherein a content of the lithium alloy is 1 part by mass or more and 30 parts by mass or less relative to 100 parts by mass of the positive electrode active material.

[0011] <5> A positive electrode material comprising a composite of a positive electrode active material and a lithium alloy.

[0012] According to one embodiment of the present disclosure, a method for producing a positive electrode material for producing a lithium ion secondary battery that suppresses a decrease in initial battery capacity and a decrease in battery capacity due to repeated charge and discharge, and a positive electrode material can be provided. DETAILED DESCRIPTION

[0013] In the present disclosure, a numerical range expressed using "to" means a range including the numerical values ​​described before and after "to" as the minimum value and the maximum value, respectively.

[0014] In the numerical ranges recorded in stages in the disclosure, the upper limit or lower limit recorded in a certain numerical range can be replaced by the upper limit or lower limit of other numerical ranges recorded in stages. In the numerical ranges recorded in the disclosure, the upper limit or lower limit recorded in a certain numerical range can be replaced by the value shown in the embodiments.

[0015] In the present disclosure, the term "step" refers not only to an independent step but also to a step that cannot be clearly distinguished from other steps as long as the intended purpose of the step is achieved.

[0016] In the present disclosure, a combination of two or more preferred aspects is a more preferred aspect.

[0017] In the present disclosure, when there are plural substances corresponding to each component, the amount of each component refers to the total amount of the plural substances unless otherwise specified.

[0018] Method for manufacturing positive electrode material

[0019] The method for producing a positive electrode material disclosed herein comprises: a step of obtaining a mixture containing a positive electrode active material and a lithium alloy; and a step of heat-treating the mixture.

[0020] In a secondary battery manufactured using a positive electrode material manufactured using the method disclosed herein, the positive electrode includes a lithium alloy. When the secondary battery is initially charged, lithium ions are released from the lithium alloy together with the positive electrode active material, thereby replenishing the reduced portion of lithium ions consumed by the formation of SEI. As a result, the reduction in the initial battery capacity is effectively suppressed.

[0021] Furthermore, the reduction in battery capacity is suppressed in a secondary battery manufactured using a positive electrode material manufactured using the method disclosed herein compared to a secondary battery manufactured using a positive electrode material not manufactured using the method disclosed herein (specifically, without heat treatment of the mixture of the positive electrode active material and the lithium alloy). As the main reason for this, it is believed that by heat treatment of the mixture of the positive electrode active material and the lithium alloy, solid solution diffusion of the positive electrode active material and the lithium alloy occurs to form a complex composed of the positive electrode active material and the lithium alloy. It is believed that by making the lithium alloy be incorporated into the complex, the movement of metal ions from the lithium alloy to the negative electrode side is suppressed, and the reduction in battery capacity is suppressed.

[0022] Hereinafter, the step of obtaining a mixture containing a positive electrode active material and a lithium alloy is referred to as a first step, and the step of heat-treating the mixture is referred to as a second step.

[0023] First process

[0024] In the first step, a mixture containing a positive electrode active material and a lithium alloy is obtained.

[0025] The lithium alloy used in the first step preferably contains a lithium alloy having an alloying potential with lithium of 0.5 V (vs. Li / Li + ) or more metal elements.

[0026] Since the metal elements contained in the lithium alloy have the above alloying potential, the reduced lithium ions released from the positive electrode active material and consumed by the formation of SEI during the initial charge of the battery are effectively replenished. As a result, the initial reduction in battery capacity is effectively suppressed.

[0027] The alloying potential with lithium is 0.5 V (vs. Li / Li + ) and above, examples of the metal elements include bismuth (Bi, alloying potential: 0.81 to 0.83 V), tin (Sn, alloying potential: 0.57 to 0.66 V) and antimony (Sb, alloying potential: 0.94 to 0.96 V).

[0028] That is, as lithium alloys contained in the positive electrode material, there can be listed lithium alloys containing bismuth (hereinafter also referred to as Li—Bi alloys), lithium alloys containing tin (hereinafter also referred to as Li—Sn alloys), and lithium alloys containing antimony (hereinafter also referred to as Li—Sb alloys).

[0029] Specific examples of the Li-Bi alloy include Li3Bi.

[0030] Specific examples of the Li—Sb alloy include Li 3 Sb.

[0031] Specific examples of the Li—Sn alloy include LiSn.

[0032] From the viewpoint of suppressing a decrease in battery capacity due to repeated charge and discharge, the lithium alloy contained in the positive electrode material is preferably a Li—Bi alloy.

[0033] The positive electrode material contains one type of lithium alloy. In the present disclosure, the positive electrode material “contains one type of lithium alloy” means that among the metal elements other than lithium constituting the lithium alloy contained in the positive electrode material, there is only one type of metal element with the highest content rate.

[0034] From the viewpoint of fully replenishing the consumption of lithium ions accompanying the formation of SEI, the content of lithium alloy contained in the positive electrode material is preferably 1 mass part or more, more preferably 2 mass parts or more, and further preferably 5 mass parts or more relative to 100 mass parts of the positive electrode active material.

[0035] From the viewpoint of ensuring sufficient energy density, the content of the lithium alloy contained in the positive electrode material is preferably 30 parts by mass or less, more preferably 20 parts by mass or less, and further preferably 15 parts by mass or less, based on 100 parts by mass of the positive electrode active material.

[0036] The lithium alloy may be in the form of particles. The volume average particle size of the lithium alloy in the form of particles is not particularly limited, and can be selected from a range of, for example, 1 μm to 50 μm.

[0037] The volume average particle size of particles in the present disclosure is a value (D50) when the cumulative volume is 50% in a volume-based particle size distribution measured by a laser diffraction / scattering method.

[0038] Examples of the positive electrode active material used in the first step include composite oxides of lithium and transition metals (hereinafter also referred to as lithium transition metal composite oxides).

[0039] Examples of the lithium transition metal composite oxide include layered lithium transition metal composite oxides, spinel lithium transition metal composite oxides, and olivine lithium transition metal composite oxides.

[0040] As the layered lithium transition metal composite oxide, specifically, a compound represented by LiMO2 (M is at least one transition metal selected from Ni, Co and Mn) and a compound to which a heterogeneous element is added can be listed. As the heterogeneous element, Al, Mg, La, Ti, Zn, B, W, Fe, Cr, V, Ru, Cu, Cd, Ag, Y, Sc, Ga, In, As, Sb, Pt, Au, Si, etc. can be listed.

[0041] Specific examples of spinel-type lithium transition metal composite oxides include LiMn 2 O 4 .

[0042] Specific examples of the olivine-type lithium transition metal composite oxide include LiMPO 4 (M is Fe, Co, Ni, or Mn).

[0043] The positive electrode active material contained in the positive electrode material may be a single type or two or more types.

[0044] Among the lithium transition metal composite oxides, a layered lithium transition metal composite oxide containing at least one selected from Ni, Co and Mn as a transition metal is more preferred. A layered lithium transition metal composite oxide containing Ni and at least one selected from Co and Mn as a transition metal is further preferred. A layered lithium transition metal composite oxide (NCM, nickel cobalt manganese oxide) containing Ni, Co and Mn as transition metals is further preferred.

[0045] Regarding the molar ratio of Ni, Co and Mn contained in NCM, for example, the molar ratio of Ni to Co (Ni:Co) can be selected from the range of 1:0.1 to 1:1, and the molar ratio of Ni to Mn (Ni:Mn) can be selected from the range of 1:0.1 to 1:1.

[0046] The molar ratio of Ni to Co (Ni:Co) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2.

[0047] The molar ratio of Ni to Mn (Ni:Mn) may be selected from the range of 1:0.1 to 1:0.5, 1:0.1 to 1:0.3, or 1:0.1 to 1:0.2.

[0048] The positive electrode active material may be in the form of particles.

[0049] The volume average particle size of the positive electrode active material in particulate form is not particularly limited and can be selected from a range of 5 μm to 30 μm, for example. When the positive electrode active material is a secondary particle that is an aggregate of a plurality of primary particles, the above volume average particle size is the volume average particle size of the secondary particles.

[0050] The method for obtaining the mixture containing the positive electrode active material and a lithium alloy in the first step is not particularly limited, and a common method can be used.

[0051] Second process

[0052] In the second step, the mixture containing the positive electrode active material and the lithium alloy obtained in the first step is heat treated.

[0053] As shown in the embodiments described later, the battery manufactured by using the positive electrode material obtained by heat treatment of the mixture of the positive electrode active material and the lithium alloy maintains the battery capacity well compared with the battery manufactured by using the positive electrode material obtained by heat treatment of the mixture. As the main reason, it is believed that the solid solution diffusion of the positive electrode active material and the lithium alloy is produced by the heat treatment of the mixture, forming a complex composed of the positive electrode active material and the lithium alloy. By becoming a state in which the lithium alloy is collected in the complex, the movement of metal ions from the lithium alloy to the negative electrode side is effectively suppressed.

[0054] The temperature of the heat treatment is preferably 350°C or higher, and may be 450°C or higher, 550°C or higher, or 650°C or higher.

[0055] The temperature of the heat treatment may be 1000° C. or lower, 900° C. or lower, or 800° C. or lower.

[0056] The heat treatment time is not particularly limited and can be selected from a period of 30 minutes to 5 hours.

[0057] The heat treatment is preferably performed in an inert atmosphere such as nitrogen or argon.

[0058] The mixture after the heat treatment is preferably in a state where at least a part of the positive electrode active material and at least a part of the lithium alloy form a complex (solid solution, etc.).

[0059] The heat-treated mixture may be crushed to form particles.

[0060] The volume average particle size of the particles obtained by crushing the heat-treated mixture can be selected from the range of 5 μm to 30 μm, for example.

[0061] Cathode Materials

[0062] The positive electrode material disclosed herein includes a composite body composed of a positive electrode active material and a lithium alloy.

[0063] The detailed information and preferred embodiments of the positive electrode active material and the lithium alloy contained in the positive electrode material are the same as those of the positive electrode active material and the lithium alloy used in the above-mentioned method for producing the positive electrode material.

[0064] The composite of the positive electrode active material and a lithium alloy contained in the positive electrode material may be in a state where solid solution diffusion of the positive electrode active material and the lithium alloy occurs by heat treatment of the mixture of the positive electrode active material and the lithium alloy.

[0065] The composite material composed of the positive electrode active material and a lithium alloy contained in the positive electrode material may be in the form of particles.

[0066] The volume average particle size of the particles obtained by crushing the heat-treated mixture can be selected from the range of 5 μm to 30 μm, for example.

[0067] The positive electrode material of the present disclosure may further include other components used as a material for a positive electrode of a lithium ion secondary battery.

[0068] For example, the positive electrode material may be in the form of a mixture containing a positive electrode active material such as a conductive additive and a binder and components other than the lithium alloy. If necessary, a solvent may be added to the mixture to adjust the viscosity of the mixture.

[0069] Specific examples of the conductive auxiliary agent include carbon materials such as carbon black (acetylene black, thermal black, furnace black, etc.), carbon nanotubes, and graphite.

[0070] The conductive material contained in the positive electrode material may be a single type or two or more types.

[0071] Specific examples of the binder include polyvinylidene fluoride (PVDF), polyethylene, polypropylene, polyethylene terephthalate, cellulose, nitrocellulose, carboxymethyl cellulose, polyethylene oxide, polyepichlorohydrin, polyacrylonitrile, styrene-butadiene rubber (SBR), acrylonitrile-butadiene rubber (NBR), polyacrylate, and polymethacrylate.

[0072] The binder contained in the positive electrode material may be a single type or two or more types.

[0073] The positive electrode material of the present disclosure can be used as a material for a positive electrode of a lithium ion secondary battery. The positive electrode includes, for example, a current collector and a positive electrode layer disposed on the current collector, and the positive electrode layer contains the positive electrode material of the present disclosure.

[0074] The positive electrode layer may be disposed on one surface of the current collector or on both surfaces of the current collector.

[0075] Examples of the material of the current collector constituting the positive electrode include aluminum, aluminum alloys, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector include foil, mesh, and the like.

[0076] The positive electrode layer is disposed on the current collector by, for example, applying a slurry of positive electrode material to one or both sides of the current collector. If necessary, a pressurization treatment for adjusting the density of the positive electrode layer may be performed. There is no particular limitation on the thickness of the positive electrode layer, and it can be selected, for example, from a range of 10 μm to 100 μm.

[0077] A lithium ion secondary battery obtained by using the positive electrode material of the present disclosure includes, for example, a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and an electrolyte.

[0078] The negative electrode includes, for example, a current collector and a negative electrode layer disposed on the current collector and containing a negative electrode active material.

[0079] Examples of the negative electrode active material include carbon materials such as graphite, hard carbon, soft carbon, and activated carbon, silicon, metallic lithium, lithium alloys, and lithium titanate (LTO).

[0080] Examples of the material of the current collector constituting the negative electrode include copper, copper alloys, nickel, titanium, stainless steel, etc. Examples of the shape of the current collector of the negative electrode include foil, mesh, and the like.

[0081] Examples of the separator include nonwoven fabrics, cloths, and microporous films containing polyolefins such as polyethylene and polypropylene as a main component.

[0082] As for the electrolyte, liquid or solid is OK. As liquid electrolyte (electrolyte), the product which makes known electrolyte such as LiPF6 dissolve in organic solvent can be used without particular restriction. As solid electrolyte, known solid electrolytes such as sulfide solid electrolyte, oxide solid electrolyte, halide solid electrolyte can be used without particular restriction.

[0083] The present disclosure will be described in more detail below using examples, but the present invention is not limited to these examples.

[0084] Preparation of evaluation batteries

[0085] In the production of the batteries of the respective examples, NCM (LiNi 0.8 Co 0.1 Mn 0.1 O2) and LiB3, Li3Sb and LiSn as lithium alloys.

[0086] The amounts of the components were adjusted so that the total volume of the positive electrode active material and the lithium alloy was equal in each example.

[0087] In Examples 1 to 3, the mixture of the positive electrode active material and the lithium alloy shown in Table 1 was heat treated at 700° C. for 2 hours in a nitrogen atmosphere and then crushed to obtain composite particles of the positive electrode active material and the lithium alloy.

[0088] In Comparative Examples 1 to 5, the mixtures of the positive electrode active material and the lithium alloy shown in Table 1 were not subjected to heat treatment.

[0089] A mixture (93 g) of a positive electrode active material and a lithium alloy, carbon black (4 g) as a conductive aid, PVDF (3 g) as a binder, and a solvent (NMP) were mixed to obtain a positive electrode material in a slurry state.

[0090] The obtained positive electrode material was applied to aluminum foil and dried to obtain a positive electrode. The obtained positive electrode, separator (polyethylene microporous membrane), and negative electrode containing graphite as active material were stacked in sequence to produce an electrode body. Using the electrode body and an electrolyte (a mixed solvent of ethylene carbonate / dimethyl carbonate / ethyl methyl carbonate containing LiPF6 as an electrolyte), a laminated evaluation battery was produced.

[0091] Evaluation of battery performance

[0092] CCCV charging and CCCV discharging were performed under the following conditions, with the current value of 210 mA / g defined as 1C rate based on the mass of the positive electrode active material contained in the battery. The obtained CCCV capacity was defined as the capacity of the battery (initial discharge capacity).

[0093] Lower voltage limit: 2.5V

[0094] Upper voltage limit: 4.25V

[0095] Charge and discharge current value: 0.2C

[0096] Termination current value: 0.03C

[0097] Then, a cycle test (CC charge and CC discharge) of the battery was carried out. Table 1 shows the number of cycles at which the CC discharge capacity of the battery became 40% or less of the initial discharge capacity.

[0098] It can be seen that as the number of cycles shown in Table 1 increases, the decrease in battery capacity due to repeated charge and discharge is suppressed.

[0099] Table 1

[0100]

[0101] As shown in Table 1, the batteries of Examples 1 to 3 in which the mixture of the positive electrode active material and the lithium alloy was heat-treated showed excellent capacity retention rates compared with the batteries of Comparative Examples 1 to 5 in which the mixture of the positive electrode active material and the lithium alloy was not heat-treated.

[0102] Evaluation of metal ion release from lithium alloys

[0103] After the cycle test, the battery (cell) was disassembled and the positive electrode was taken out. A sample solution was prepared from the positive electrode layer using acid, and elemental analysis was performed using ICP-AES (ICP atomic emission spectroscopy).

[0104] The concentrations of the metal elements in the lithium alloy used for preparing the positive electrode are shown in Table 2 as ratios when the initial concentration is set to 100.

[0105] Table 2

[0106]

[0107] As shown in Table 2, the batteries of Examples 1 to 3 in which the mixture of the positive electrode active material and the lithium alloy was heat treated have a higher residual rate of the metal element from the lithium alloy in the positive electrode than the batteries of Comparative Examples 1 to 5 in which the mixture of the positive electrode active material and the lithium alloy was not heat treated.

[0108] The above results suggest that the migration of metal elements from the lithium alloy to the negative electrode side is effectively suppressed by heat-treating the mixture of the positive electrode active material and the lithium alloy.

Claims

1. A method for manufacturing a positive electrode material, comprising: A step of obtaining a mixture comprising a positive electrode active material and a lithium alloy; and A step of heat treating the mixture is performed.

2. The method for producing a positive electrode material according to claim 1, wherein: The lithium alloy is selected from bismuth, tin and antimony.

3. The method for producing a positive electrode material according to claim 1 or claim 2, wherein: The temperature of the heat treatment is 350° C. or higher.

4. The method for producing a positive electrode material according to claim 1 or claim 2, wherein: The content of the lithium alloy is 1 part by mass or more and 30 parts by mass or less relative to 100 parts by mass of the positive electrode active material.

5. A positive electrode material comprising a composite composed of a positive electrode active material and a lithium alloy.

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