Positive electrode active material, positive electrode, lithium ion polymer solid state secondary battery, lithium ion inorganic all-solid state secondary battery

Abstract

To provide a positive electrode active material that makes it possible to construct a lithium-ion solid state secondary battery equipped with a negative electrode containing lithium without safety concerns due to the formation of dendrites, and can manufacture a lithium-ion polymer solid secondary battery and a lithium-ion inorganic all-solid secondary battery that do not require the addition of a large amount of conductive aid and a binder using iron (III) oxide particles with anisotropic shapes that are free from toxicity and resources, a lithium-ion polymer solid secondary battery and a high lithium-ion inorganic all-solid secondary battery including the positive electrode active material.SOLUTION: A positive electrode active material used in a lithium-ion solid state secondary battery including a lithium-containing negative electrode includes iron (III) oxide or heteroelement-substituted iron (III) oxide, and consists of anisotropic iron oxide particles in which the ratio (b / a) of the half-value width b of the diffraction line derived from the plane index 104 plane obtained by X-ray diffraction measurement to the half-value width a of the diffraction line derived from the plane index 110 plane obtained by X-ray diffraction measurement is 2.0 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a positive electrode active material, a positive electrode, a lithium ion polymer solid state secondary battery, and a lithium ion inorganic all-solid state secondary battery. Note that a lithium ion polymer solid state secondary battery using a polymer electrolyte as an electrolyte and a lithium ion inorganic all-solid state secondary battery using an inorganic solid electrolyte are sometimes simply referred to as lithium ion solid state secondary batteries. [Background technology]

[0002] In recent years, non-aqueous electrolyte secondary batteries such as lithium ion secondary batteries have been proposed and put into practical use as batteries that are expected to be smaller, lighter, and have higher capacities. Lithium ion secondary batteries are composed of a positive electrode and a negative electrode that can reversibly insert and remove lithium ions, and a non-aqueous electrolyte. The negative electrode active material of a lithium-ion secondary battery is generally a carbon-based material, a metal material such as Si or Sn that can reversibly insert and remove lithium ions, or a Li-containing metal oxide. Examples of such Li-containing metal oxides include lithium titanate (Li4Ti5O 12 ) are listed.

[0003] On the other hand, the positive electrode of a lithium-ion secondary battery uses a positive electrode material mixture containing a positive electrode material and a binder. Positive electrode active materials include layered oxides such as lithium cobalt oxide (LCO), ternary layered oxides (NCMs) in which part of the cobalt is replaced with manganese and nickel, and lithium manganese spinel (LMO), lithium iron phosphate (LFP), and lithium iron manganese phosphate (LFMP), which are lithium-containing metal oxides capable of reversibly inserting and extracting lithium ions. The positive electrode of a lithium-ion secondary battery is then formed by applying this positive electrode material mixture to the surface of a metal foil called an electrode current collector.

[0004] A non-aqueous solvent is used as the electrolyte for a lithium-ion secondary battery. A positive electrode active material that undergoes oxidation-reduction at a high potential or a negative electrode active material that undergoes oxidation-reduction at a low potential can be used as the non-aqueous solvent. This allows for the realization of a high-voltage lithium-ion secondary battery.

[0005] Such lithium-ion secondary batteries are lighter and smaller in size and have higher energy than conventional secondary batteries such as lead batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc. Therefore, lithium-ion secondary batteries are used not only as small power sources for portable electronic devices such as mobile phones and notebook personal computers, but also as large stationary emergency power sources.

[0006] In recent years, there has been a demand for improved performance in lithium-ion secondary batteries, and various approaches to this end have been investigated. For example, to further improve the safety of lithium-ion secondary batteries, all-solid-state batteries using nonvolatile polymer electrolyte membranes or inorganic solid electrolytes, rather than flammable organic solvents, and batteries using ionic liquids have been investigated. Among these, lithium-ion polymer secondary batteries using polymer electrolyte membranes have been actively investigated because of their adaptability to the coating manufacturing process used for conventional liquid electrolyte batteries, their low cost, and the ease of thinning due to the high conductivity of the polymer electrolyte membrane. Furthermore, the dense solid state of polymer electrolyte membranes suppresses the formation of needle-shaped metal crystals called dendrites. Therefore, lithium-ion polymer secondary batteries can be used with lithium metal anodes without compromising safety, which is expected to significantly improve capacity.

[0007] As a positive electrode active material for a lithium ion battery using a lithium metal negative electrode, for example, one made of ultrafine iron oxide particles having an average particle size in the range of 1 to 10 nm and a particle size distribution width in the range of 1 to 10 nm is known (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-204777 Summary of the Invention [Problem to be solved by the invention]

[0009] It is known that iron oxide can function as a positive electrode active material when microparticulated. However, when fabricating a positive electrode containing iron oxide microparticles as a positive electrode active material, a large amount of binder and conductive additive is required, which limits the battery capacity. [Means for solving the problem]

[0010] The present inventors have discovered that in secondary batteries using a dense solid electrolyte, such as lithium ion polymer secondary batteries or inorganic all-solid-state batteries, there are no safety concerns due to dendrite formation, and it is possible to construct a lithium secondary battery equipped with a lithium-containing negative electrode, and that by using iron oxide particles with a special shape, the battery can operate satisfactorily without adding large amounts of a binder or conductive additive, thereby completing the present invention.

[0011] The positive electrode active material of the present invention is a positive electrode active material used in a lithium-ion solid state secondary battery equipped with a lithium-containing negative electrode, and is made of anisotropic iron oxide particles containing iron (III) oxide or iron (III) oxide substituted with a different element, in which the ratio (b / a) of the half-width b of the diffraction line derived from a plane with a plane index of 104 obtained by X-ray diffraction measurement to the half-width a of the diffraction line derived from a plane with a plane index of 110 obtained by X-ray diffraction measurement is 2.0 or more.

[0012] The positive electrode of the present invention contains the positive electrode active material of the present invention.

[0013] The lithium ion polymer solid secondary battery of the present invention comprises a positive electrode containing the positive electrode active material of the present invention, a negative electrode containing lithium, and an ion-conductive polymer electrolyte.

[0014] The lithium ion inorganic all-solid-state secondary battery of the present invention comprises a positive electrode containing the positive electrode active material of the present invention, a negative electrode containing lithium, and an inorganic solid electrolyte. [Effects of the Invention]

[0015] The positive electrode active material of the present invention enables the construction of an all-solid-state lithium ion secondary battery having a lithium-containing negative electrode without the safety concerns due to dendrite formation, and makes it possible to fabricate an all-solid-state lithium ion secondary battery that does not require the addition of large amounts of a binder or conductive additive, is inexpensive, is free from toxicity concerns, and can operate with non-ultrafine particle iron oxide.

[0016] The positive electrode of the present invention makes it possible to construct an all-solid-state lithium ion secondary battery including a lithium-containing negative electrode without any safety concerns due to the formation of dendrites, and by operating at high temperatures, it is possible to provide an all-solid-state lithium ion secondary battery that can operate with non-ultrafine alkali metal-free active material and does not require the addition of large amounts of binders or conductive additives.

[0017] According to the lithium ion polymer solid secondary battery of the present invention, it is possible to construct a lithium ion polymer secondary battery having a lithium-containing negative electrode without any safety concerns due to the formation of dendrites, and it is possible to provide a lithium ion polymer secondary battery that can operate at high temperatures using non-ultrafine alkali metal-free active materials that do not require the addition of large amounts of binders or conductive additives.

[0018] According to the lithium-ion inorganic all-solid-state secondary battery of the present invention, it is possible to construct a lithium-ion inorganic all-solid-state secondary battery having a lithium-containing negative electrode without any safety concerns due to the generation of dendrites, and it is possible to provide a lithium-ion inorganic all-solid-state secondary battery that can operate at high temperatures using an alkali-metal-free active material that is not an ultrafine particle and does not require the addition of large amounts of a binder or conductive additive. [Brief explanation of the drawings]

[0019] [Figure 1]FIG. 1 is a diagram showing X-ray diffraction patterns of Examples 1 to 3 and Comparative Examples 1 and 2. [Figure 2] FIG. 1 is a diagram showing second cycle discharge curves of Examples 1 to 3 and Comparative Examples 1 and 2. DETAILED DESCRIPTION OF THE INVENTION

[0020] Embodiments of the positive electrode active material, positive electrode, lithium ion polymer solid state secondary battery, and lithium ion inorganic all-solid state secondary battery of the present invention will be described. It should be noted that the present embodiment is specifically described to allow a better understanding of the gist of the invention, and does not limit the present invention unless otherwise specified.

[0021] [Cathode active material] The positive electrode active material of this embodiment is used in a lithium-ion solid state secondary battery equipped with a lithium-containing negative electrode. The positive electrode active material of this embodiment is composed of anisotropic iron oxide particles containing iron(III) oxide or heteroelement-substituted iron(III) oxide, and having a ratio (b / a) of 2 or more of the half-width b of the diffraction line derived from the plane with plane index 104 obtained by X-ray diffraction measurement to the half-width a of the diffraction line derived from the plane with plane index 110 obtained by X-ray diffraction measurement. The lithium ion solid state secondary battery includes a lithium ion polymer solid state secondary battery and a lithium ion inorganic all-solid state secondary battery.

[0022] The positive electrode active material of this embodiment is mainly composed of iron (III) oxide or iron (III) oxide substituted with a different element.

[0023] When the positive electrode active material of this embodiment is mainly composed of hetero-element-substituted iron (III) oxide, it may contain various metal elements or non-metal elements as appropriate dopants for the purpose of improving electronic conductivity, stabilizing the structure, etc. That is, when the positive electrode active material of this embodiment is mainly composed of hetero-element-substituted iron (III) oxide, at least a portion of the iron element constituting the iron (III) oxide may be substituted with various metal elements other than iron or non-metal elements. Examples of metal elements include titanium (Ti), chromium (Cr), manganese (Mn), cobalt (Co), nickel (Ni), and aluminum (Al). Examples of non-metallic elements include boron (B), phosphorus (P), arsenic (As), and silicon (Si).

[0024] When at least a portion of the iron element constituting the iron(III) oxide is substituted with various metal elements other than iron or nonmetal elements, i.e., when the hetero-element-substituted iron(III) oxide contains a metal element or a nonmetal element as a dopant, the content of the dopant is preferably 0.001% by mass to 10% by mass, more preferably 0.01% by mass to 5% by mass, and even more preferably 0.1% by mass to 1% by mass, based on the total mass of the hetero-element-substituted iron(III) oxide being 100% by mass. When the content of the dopant is equal to or greater than the lower limit, improved electronic conductivity and structural stabilization effects can be expected. When the content of the dopant is equal to or less than the upper limit, problems such as reduced capacity and increased cost due to an increase in inert elements can be suppressed.

[0025] The lower limit of the average particle size of the anisotropic iron oxide particles is preferably 30 nm or more, more preferably 200 nm or more, and even more preferably 0.5 μm or more, and the upper limit of the average particle size of the iron oxide particles is preferably 50 μm or less, more preferably 30 μm or less, and even more preferably 25 μm or less. If the average particle size of the anisotropic iron oxide particles is less than the lower limit, an excessive amount of additives such as a conductive additive and a binder may be required, which may result in a decrease in the electrode capacity.If the average particle size of the anisotropic iron oxide particles is equal to or less than the upper limit, the particles can function well as an active material.

[0026] Here, the average particle size of the anisotropic iron oxide particles can be measured using a laser diffraction scattering particle size distribution measuring device or the like.

[0027] The BET specific surface area of ​​the above anisotropic iron oxide particles is 10 m 2 / g or more is preferable, and 20m 2 / g or more is more preferable, and 60m 2 / g or more. The BET specific surface area of ​​the iron oxide particles is preferably 500 m 2 / g or less, and 2 / g or less is more preferable, and 200m 2 It is more preferable that the saturation coefficient is 1 / g or less. When the BET specific surface area of ​​the anisotropic iron oxide particles is equal to or greater than the lower limit, improved electronic conductivity and structural stabilization can be expected.When the BET specific surface area of ​​the anisotropic iron oxide particles is equal to or less than the upper limit, the amount of binder and conductive material added can be reduced, thereby preventing problems such as reduced capacity and increased cost. Here, the BET specific surface area of ​​the anisotropic iron oxide particles can be measured by a gas adsorption method using a gas such as nitrogen.

[0028] The anisotropic iron oxide particles have a ratio (b / a) of 2.0 or more, preferably 2.2 or more. The upper limit of the ratio (b / a) is preferably 4.0 or less, more preferably 3.3 or less. If the ratio (b / a) is less than the lower limit, the non-ultrafine particle iron(III) oxide will not exhibit sufficient capacity as a positive electrode active material.

[0029] [Method of manufacturing positive electrode active material] The method for producing the positive electrode active material of this embodiment is not particularly limited. The positive electrode active material of this embodiment can be produced by appropriately selecting a method that can obtain desired iron oxide particles, such as a wet method or a dry method. If necessary, wet carbon coating by thermal decomposition of organic matter, or dry or semi-dry carbon compounding using an attritor or planetary ball mill and a carbonaceous material may be performed. Similarly, an ion-conductive polymer coating may be applied for lithium ion polymer secondary batteries, and an inorganic solid electrolyte coating may be formed for lithium ion inorganic all-solid-state batteries.

[0030] [Lithium-ion polymer solid secondary battery] The lithium ion polymer solid secondary battery of this embodiment includes a positive electrode for a lithium ion polymer solid secondary battery containing the positive electrode active material of this embodiment, a negative electrode containing lithium, and an ion-conductive polymer electrolyte present between the positive electrode for the lithium ion polymer solid secondary battery and the negative electrode containing lithium.

[0031] "Positive electrode for lithium-ion polymer solid secondary battery" The positive electrode for a lithium ion polymer solid secondary battery includes an electrode current collector made of a metal foil and a positive electrode mixture layer formed on the electrode current collector. The positive electrode mixture layer includes the positive electrode active material of the present embodiment and an ion-conductive polymer electrolyte. The positive electrode mixture layer may also include a conductive additive such as carbon black and a binder, as needed.

[0032] The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited, but is preferably 50% by mass to 95% by mass, more preferably 60% by mass to 90% by mass, and even more preferably 60% by mass to 70% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. If the content of the positive electrode active material is below the lower limit, the capacity of a lithium ion polymer secondary battery equipped with a positive electrode for a lithium ion polymer solid secondary battery containing the positive electrode active material will be low. If the content of the positive electrode active material exceeds the upper limit, ions and electrons will not be able to sufficiently reach the surface of the active material, thereby reducing the capacity of a lithium ion polymer secondary battery equipped with a positive electrode for a lithium ion polymer solid secondary battery containing the positive electrode active material.

[0033] "Ion-conductive polymer electrolyte" As the ion-conductive polymer electrolyte, various types are widely known, such as polyethylene oxide containing Li electrolytes such as lithium perchlorate, lithium hexafluorophosphate, and Li triflimide (LiTFSI), modified polyethylene oxide, and polyvinylidene fluoride, and can be used as appropriate.

[0034] The positive electrode mixture layer must contain an ion-conductive polymer electrolyte, which is an electrolyte. The content of the ion-conductive polymer electrolyte in the positive electrode mixture layer is not particularly limited, but is preferably 5% by mass to 50% by mass, more preferably 10% by mass to 40% by mass, and even more preferably 30% by mass to 40% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. When the content of the ion-conductive polymer electrolyte is equal to or greater than the lower limit, sufficient ion-conductive paths are formed on the surface of the positive electrode active material, preventing the generation of active material that does not contribute to the reaction and suppressing a decrease in battery capacity. When the content of the ion-conductive polymer electrolyte is equal to or less than the upper limit, the ion-conductive polymer electrolyte is not wasted and the proportion of active material in the electrode is not too low, preventing a decrease in battery capacity.

[0035] "Binder" If the ion-conductive polymer electrolyte has adhesiveness, a binder is not necessarily required. Suitable binders, i.e., binder resins, include polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, and fluororubber.

[0036] The content of the binder in the positive electrode mixture layer is not particularly limited, but is preferably 5% by mass or less, and more preferably 3% by mass or less, when the total mass of the positive electrode mixture layer is 100% by mass. When the content of the binder is equal to or less than the above upper limit, the binding strength between the positive electrode mixture layer and the electrode current collector can be sufficiently increased. This can prevent the positive electrode mixture layer from cracking or falling off during compaction formation, etc. During the charge / discharge process of a lithium ion polymer secondary battery, it can also prevent the positive electrode mixture layer from peeling off from the electrode current collector, resulting in a decrease in battery capacity and charge / discharge rate characteristics. When the content of the binder is equal to or less than the above upper limit, the internal resistance of the positive electrode material for a lithium ion polymer solid secondary battery is reduced, and a decrease in battery capacity at high-speed charge / discharge rates can be prevented.

[0037] "Conductive additive" The conductive additive is not particularly limited, but at least one selected from the group consisting of particulate carbon such as acetylene black (AB), ketjen black, and furnace black, fibrous carbon such as vapor grown carbon fiber (VGCF) and carbon nanotubes, and graphene can be used.

[0038] The content of the conductive additive in the positive electrode mixture layer is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 10% by mass, and even more preferably 2% by mass to 5% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. When the content of the conductive additive is equal to or greater than the lower limit, the lithium ion polymer secondary battery can operate satisfactorily. When the content of the conductive additive exceeds the upper limit, not only is it wasted, but it may also result in the positive electrode active material falling off due to insufficient binding strength and a decrease in capacity.

[0039] Since the positive electrode of this embodiment contains the positive electrode active material of this embodiment, a lithium ion polymer secondary battery using the positive electrode of this embodiment has excellent battery capacity.

[0040] "Method for manufacturing positive electrodes for lithium ion polymer solid secondary batteries" The method for producing a positive electrode for a lithium ion polymer solid secondary battery is not particularly limited as long as it is a method that can form a positive electrode mixture layer on at least one main surface of an electrode current collector using the positive electrode active material of this embodiment. Examples of the method for producing a positive electrode include the following methods. First, the positive electrode active material of this embodiment, an ion-conductive polymer electrolyte, and a solvent are mixed to prepare a positive electrode material paste. At this time, a conductive additive such as carbon black and a binder may be added to the positive electrode material paste of this embodiment as needed.

[0041] "solvent" The solvent used in the positive electrode material paste containing the positive electrode active material of this embodiment is appropriately selected depending on the properties of the binder. By appropriately selecting the solvent, the positive electrode material paste can be easily applied to a coating target such as an electrode current collector. Examples of the solvent include water, alcohols, esters, ethers, ketones, amides, and glycols. These solvents may be used alone or in combination of two or more.

[0042] The content of the solvent in the positive electrode material paste is preferably small from the viewpoint of cost, but may be determined appropriately taking into consideration the positive electrode formability and coating properties.

[0043] The method for mixing the positive electrode active material of this embodiment, the ion-conductive polymer electrolyte, the solvent, and optionally the conductive additive and binder is not particularly limited as long as it can uniformly mix these components. For example, mixing methods using a kneader such as a ball mill, a sand mill, a planetary mixer, a paint shaker, or a homogenizer can be used.

[0044] The positive electrode material paste is applied to at least one main surface of an electrode current collector to form a coating film, and then this coating film is dried to obtain an electrode current collector having a coating film made of the mixture of the positive electrode material and the binder formed on at least one main surface thereof. Thereafter, the coating film may be press-bonded as necessary.

[0045] "Lithium-containing negative electrode" Examples of lithium-containing negative electrodes include metallic Li, Li alloys, and Li4Ti5O 12 , Si-based materials (Li 4.4 Examples include those containing a negative electrode material such as silicon.

[0046] The lithium ion polymer solid secondary battery of this embodiment has a positive electrode containing the positive electrode active material of this embodiment, and therefore has excellent battery capacity.

[0047] [Lithium-ion inorganic all-solid-state secondary battery] The lithium ion inorganic all-solid-state secondary battery of this embodiment includes a positive electrode for a lithium ion inorganic all-solid-state secondary battery containing the positive electrode active material of this embodiment, a negative electrode containing lithium, and an inorganic solid electrolyte present between the positive electrode for the lithium ion inorganic all-solid-state secondary battery and the negative electrode containing lithium.

[0048] "Positive electrode for lithium-ion inorganic all-solid-state secondary batteries" The positive electrode for a lithium-ion inorganic all-solid-state secondary battery includes an electrode current collector made of a metal foil and a positive electrode mixture layer formed on the electrode current collector. The positive electrode mixture layer includes the positive electrode active material of the present embodiment and an inorganic solid electrolyte. The positive electrode mixture layer may also include a conductive additive such as carbon black and a binder, as needed.

[0049] The content of the positive electrode active material in the positive electrode mixture layer is not particularly limited, but is preferably 50% by mass to 95% by mass, more preferably 60% by mass to 90% by mass, and even more preferably 60% by mass to 70% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. If the content of the positive electrode active material is less than the lower limit, the capacity of a lithium-ion inorganic all-solid-state secondary battery equipped with a positive electrode for a lithium-ion inorganic all-solid-state secondary battery containing the positive electrode active material will be low. If the content of the positive electrode active material exceeds the upper limit, ions and electrons will not be able to sufficiently reach the surface of the active material, resulting in a decrease in the capacity of a lithium-ion inorganic all-solid-state secondary battery equipped with a positive electrode for a lithium-ion inorganic all-solid-state secondary battery containing the positive electrode active material.

[0050] "Inorganic solid electrolyte" Examples of inorganic solid electrolytes include Li7La3Zr2O 12 Various oxides such as those mentioned above and Li-Sn-Si-PS sulfides are widely known and can be used as appropriate.

[0051] The positive electrode mixture layer must contain an inorganic solid electrolyte as an electrolyte. The content of the inorganic solid electrolyte in the positive electrode mixture layer is not particularly limited, but is preferably 5% by mass to 50% by mass, more preferably 10% by mass to 40% by mass, and even more preferably 30% by mass to 40% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. When the content of the inorganic solid electrolyte is equal to or greater than the lower limit, sufficient ion-conductive paths are formed on the surface of the positive electrode active material, preventing the generation of active material that does not contribute to the reaction and suppressing a decrease in battery capacity. When the content of the inorganic solid electrolyte is equal to or less than the upper limit, the inorganic solid electrolyte is not wasted and the proportion of active material in the electrode is not too low, preventing a decrease in battery capacity.

[0052] "Binder" If the inorganic solid electrolyte has adhesiveness, a binder is not necessarily required. Suitable binders, i.e., binder resins, include polytetrafluoroethylene (PTFE) resin, polyvinylidene fluoride (PVdF) resin, and fluororubber.

[0053] The content of the binder in the positive electrode mixture layer is not particularly limited, but is preferably 5% by mass or less, and more preferably 3% by mass or less, when the total mass of the positive electrode mixture layer is 100% by mass. When the content of the binder is equal to or less than the upper limit, the binding strength between the positive electrode mixture layer and the electrode current collector can be sufficiently increased. This can prevent the positive electrode mixture layer from cracking or falling off during compaction formation, etc. In addition, during the charge / discharge process of a lithium-ion inorganic all-solid-state battery, peeling of the positive electrode mixture layer from the electrode current collector, which can prevent a decrease in battery capacity and charge / discharge rate, can be prevented. When the content of the binder is equal to or less than the upper limit, the internal resistance of the positive electrode material for a lithium-ion inorganic all-solid-state secondary battery is reduced, and a decrease in battery capacity at high-speed charge / discharge rates can be prevented.

[0054] "Conductive additive" The conductive additive is not particularly limited, but at least one selected from the group consisting of particulate carbon such as acetylene black (AB), ketjen black, and furnace black, fibrous carbon such as vapor grown carbon fiber (VGCF) and carbon nanotubes, and graphene can be used.

[0055] The content of the conductive additive in the positive electrode mixture layer is not particularly limited, but is preferably 1% by mass to 20% by mass, more preferably 1% by mass to 10% by mass, and even more preferably 2% by mass to 5% by mass, assuming the total mass of the positive electrode mixture layer to be 100% by mass. When the content of the conductive additive is equal to or greater than the lower limit, the lithium ion inorganic all-solid-state secondary battery can be operated satisfactorily. When the content of the conductive additive exceeds the upper limit, not only is it wasted, but it may also lead to detachment of the positive electrode active material due to insufficient binding strength and a decrease in capacity.

[0056] "Method for manufacturing positive electrodes for lithium-ion inorganic all-solid-state secondary batteries" The method for producing a positive electrode for a lithium-ion inorganic all-solid-state secondary battery is not particularly limited as long as it is a method that can form a positive electrode mixture layer on at least one main surface of an electrode current collector using the positive electrode active material of this embodiment. Examples of the method for producing a positive electrode include the following methods. First, the cathode active material of this embodiment, an inorganic solid electrolyte, and a solvent are mixed to prepare a cathode material paste. At this time, a conductive additive such as carbon black and a binder may be added to the cathode material paste of this embodiment as needed.

[0057] "solvent" The solvent used in the positive electrode material paste containing the positive electrode active material of this embodiment is appropriately selected depending on the properties of the binder. By appropriately selecting the solvent, the positive electrode material paste can be easily applied to a coating target such as an electrode current collector. Examples of the solvent include water, alcohols, esters, ethers, ketones, amides, and glycols. These solvents may be used alone or in combination of two or more.

[0058] The content of the solvent in the positive electrode material paste is preferably small from the viewpoint of cost, but may be determined appropriately taking into consideration the positive electrode formability and coating properties.

[0059] The method for mixing the positive electrode active material of this embodiment, the inorganic solid electrolyte, the solvent, and optionally the conductive additive and binder is not particularly limited as long as it can uniformly mix these components. For example, mixing methods using a kneader such as a ball mill, a sand mill, a planetary mixer, a paint shaker, or a homogenizer can be used.

[0060] The positive electrode material paste is applied to at least one main surface of an electrode current collector to form a coating film, and then this coating film is dried to obtain an electrode current collector having a coating film made of the mixture of the positive electrode material and the binder formed on at least one main surface thereof. Thereafter, the coating film may be press-bonded as necessary.

[0061] "Lithium-containing negative electrode" Examples of lithium-containing negative electrodes include metallic Li, Li alloys, and Li4Ti5O 12 , Si-based materials (Li 4.4 Examples include those containing a negative electrode material such as silicon.

[0062] The lithium ion inorganic all-solid-state secondary battery of this embodiment has an excellent battery capacity because it includes a positive electrode containing the positive electrode active material of this embodiment as the positive electrode. [Example]

[0063] The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0064] [Example 1] "Preparation of iron oxide particles" An excess amount of sodium hydroxide solution was added to a 1 mol / L aqueous solution of iron(II) sulfate to neutralize it to pH 12. Air was then bubbled through the solution with stirring for 24 hours to fully oxidize the iron. The resulting precipitate was washed with water, dried, and then calcined in air at 450°C for 3 hours to obtain a powder sample. The resulting sample was pulverized using a ball mill to prepare an electrode active material. XRD analysis of the resulting powder confirmed that it was iron(III) oxide (Fe2O3) with a hematite structure. The half-width a of the diffraction line originating from the 110 plane was 0.1893°, and the half-width b of the diffraction line originating from the 104 plane was 0.6030°. The ratio of the half-width b to the half-width a (b / a) was approximately 3.2.

[0065] "Fabrication of Lithium-ion Polymer Solid Secondary Batteries" The above iron oxide (III) (Fe2O3), polyethylene oxide (PEO20000, average molecular weight 20000 g / mol) as an ion-conductive polymer (base material), LiTFSI as a lithium salt, and acetylene black (AB) as a conductive additive were mixed into N-methyl-2-pyrrolidinone (NMP) as a solvent so that the mass ratio in the paste was iron oxide (III) (Fe2O3):PEO20000:LiTFSI:AB = 70:22:6:2, and the total solids content of the paste was 36 mass%. The mixture was then kneaded for 15 minutes using a kneader (product name: Awatori Rentaro, manufactured by Thinky Corporation) at 2000 rpm revolution and 1000 rpm rotation to prepare a positive electrode material paste (for the positive electrode). This positive electrode material paste (for positive electrode) was applied to the surface of a 20 μm thick aluminum foil (electrode current collector) to form a coating film, which was then dried to form a positive electrode mixture layer on the surface of the aluminum foil. Thereafter, the positive electrode mixture layer was pressed at a linear pressure of 4 kN / 100 mm to prepare the positive electrode of Example 1.

[0066] An ion-conductive polymer film was placed on the positive electrode as an electrolyte and lithium metal was placed on the negative electrode. After pressing the positive electrode with a predetermined pressure, a 2cm 2 The resulting material was cut into pieces of this size and used as battery components. Next, the battery member was placed in a CR2032 coin cell to prepare the lithium ion polymer solid secondary battery of Example 1.

[0067] [Example 2] "Preparation of iron oxide particles" Polyethylene glycol 400 was added to a 1 mol / L aqueous solution of iron(III) nitrate so that the ratio of Fe ions to polyethylene glycol -CH2CHO- units was 1:1 and the mixture was left overnight. The resulting solution was neutralized with 28% aqueous ammonia, and the resulting slurry was dried using a spray dryer. The dried material was then calcined at 500°C for 2 hours to obtain a sample. The evolved gas was absorbed and detoxified by passing it through a scrubber containing 10% aqueous sodium hydroxide. XRD analysis of the resulting powder confirmed that it was iron(III) oxide (Fe2O3) with a hematite structure. The half-width a of the diffraction line originating from the 110 plane was 0.1632°, and the half-width b of the diffraction line originating from the 104 plane was 0.5377°. The ratio of the half-width b to the half-width a (b / a) was approximately 3.3.

[0068] "Fabrication of Lithium-ion Polymer Solid Secondary Batteries" A lithium ion polymer solid secondary battery of Example 2 was fabricated in the same manner as in Example 1, except that Fe2O3 of Example 2 was used.

[0069] [Example 3] "Preparation of iron oxide particles" The sample of Example 3 was obtained in the same manner as in Example 2, except that the amount of polyethylene glycol 400 in the aqueous solution was halved (Fe ions: -CH2CHO- unit amount of substance: 2:1). XRD measurement of the obtained powder confirmed that it was iron (III) oxide (Fe2O3) with a hematite structure. The half-width a of the diffraction line originating from the plane with plane index 110 was 0.1667°, and the half-width b of the diffraction line originating from the plane with plane index 104 was 0.3734°. The ratio of half-width b to half-width a (b / a) was approximately 2.2.

[0070] "Fabrication of Lithium-ion Polymer Solid Secondary Batteries" A lithium ion polymer solid secondary battery of Example 3 was fabricated in the same manner as in Example 1, except that Fe2O3 of Example 3 was used.

[0071] [Comparative Example 1] "Preparation of iron oxide particles" A sample for Comparative Example 1 was obtained in the same manner as in Example 2, except that polyethylene glycol 400 was not added. XRD measurement of the obtained powder confirmed that it was iron (III) oxide (Fe2O3) with a hematite structure. The half-width a of the diffraction line derived from the plane with a plane index of 110 was 0.1759°, and the half-width b of the diffraction line derived from the plane with a plane index of 104 was 0.1836°. The ratio of the half-width b to the half-width a (b / a) was approximately 1.0.

[0072] "Fabrication of Lithium-ion Polymer Solid Secondary Batteries" A lithium ion polymer solid secondary battery of Comparative Example 1 was fabricated in the same manner as in Example 1, except that Fe2O3 of Comparative Example 1 was used.

[0073] Comparative Example 2 "Fabrication of Lithium-ion Polymer Solid Secondary Batteries" A lithium ion polymer solid state secondary battery of Comparative Example 2 was fabricated in the same manner as in Example 1 using commercially available hematite-type iron oxide (Fujifilm Wako Pure Chemical Industries, Ltd., Reagent Grade 1). The half-width a of the diffraction line originating from the plane with a plane index of 110 of the iron oxide used was 0.1684°, and the half-width b of the diffraction line originating from the plane with a plane index of 104 was 0.1703°. The ratio of the half-width b to the half-width a (b / a) was approximately 1.0.

[0074] [X-ray diffraction measurement] X-ray diffraction measurements of the samples were performed using a powder X-ray diffractometer (Aeris) manufactured by Malvern Panalytical, using CuKα radiation (600 W) by the powder method. The half-widths of the obtained diffraction lines were determined by least-squares fitting with a pseudo-Voigt function using the analysis software X'Pert Pro provided with the instrument. X-ray diffraction patterns for the examples and comparative examples are shown in Figure 1, and the half-widths a of the diffraction lines derived from the plane with plane index 110 and b of the diffraction lines derived from the plane with plane index 104 are shown in Table 1, along with the evaluation results of the secondary batteries.

[0075] [Evaluation of secondary batteries] A constant current charge / discharge test was carried out using the lithium ion polymer solid secondary batteries of Examples 1 to 3 and Comparative Examples 1 and 2. The test temperature was 50°C, the measurement current was 100 mA / g (per active material weight), and the cutoff voltage was 1 V-4 V. The discharge curve for the second cycle is shown in Figure 2, and the discharge capacity for the second cycle and the results of the cycle test (capacity and retention rate after 10 cycles) are shown in Table 1.

[0076] [Table 1]

[0077] The results shown in FIG. 2 and Table 1 demonstrate that the lithium ion polymer solid secondary batteries of Examples 1 to 3, which contain as their positive electrode active material iron oxide particles having a ratio (b / a) of 2 or more of the half-width b of the diffraction line derived from a plane with a plane index of 104 obtained by X-ray diffraction measurement to the half-width a of the diffraction line derived from a plane with a plane index of 110 obtained by X-ray diffraction measurement, are excellent in discharge capacity and capacity retention rate. On the other hand, it was found that the lithium ion polymer solid secondary batteries of Comparative Examples 1 and 2, which contain iron oxide particles as positive electrode active materials having a ratio (b / a) of less than 2, where the half-width b of the diffraction line derived from the plane with a plane index of 104 obtained by X-ray diffraction measurement is relative to the half-width a of the diffraction line derived from the plane with a plane index of 110 obtained by X-ray diffraction measurement, were inferior in discharge capacity and capacity retention rate. [Industrial Applicability]

[0078] A lithium ion polymer solid state secondary battery or a lithium ion inorganic all-solid state secondary battery using the positive electrode active material of the present invention has excellent battery capacity, and can therefore make a significant contribution to the advancement of the reliability of lithium ion solid state secondary batteries for use in mobile devices and other applications.

Claims

1. A positive electrode active material for use in a lithium ion solid state secondary battery having a lithium-containing negative electrode, iron(III) oxide or heteroelement-substituted iron(III) oxide, A positive electrode active material comprising anisotropic iron oxide particles having a ratio (b / a) of 2.0 or more of the half-width b of a diffraction line derived from a plane with a plane index of 104 obtained by X-ray diffraction measurement to the half-width a of a diffraction line derived from a plane with a plane index of 110 obtained by X-ray diffraction measurement.

2. A positive electrode comprising the positive electrode active material according to claim 1 .

3. A lithium ion polymer solid secondary battery comprising: a positive electrode containing the positive electrode active material according to claim 1; a negative electrode containing lithium; and an ion-conductive polymer electrolyte.

4. A lithium ion inorganic all-solid-state secondary battery comprising: a positive electrode containing the positive electrode active material according to claim 1; a negative electrode containing lithium; and an inorganic solid electrolyte.

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