JP7116464B2 - Positive electrode active material for secondary battery, manufacturing method thereof, and secondary battery - Google Patents

Description

TECHNICAL FIELD The present invention relates to a positive electrode active material for secondary batteries, a method for producing the same, and a secondary battery.

Energy storage media are indispensable for solving energy problems, and the development of high-performance secondary batteries is urgently needed. Lithium-ion secondary batteries currently boast the highest performance, but issues remain in terms of capacity, cost, and safety to realize electric vehicles and smart grids, which are expected to be used in the future. For further development, it is necessary to develop an innovative secondary battery that transcends the current system, and one of the directions is a multivalent ion secondary battery that uses multivalent ions as carriers. For example, a secondary battery system using magnesium as a negative electrode has attracted attention because it has a high theoretical capacity density, is an abundant resource, has a high melting point, and is highly safe.

Magnesium has a high theoretical weight and volume capacity, and exhibits a relatively low oxidation-reduction potential, so secondary batteries using magnesium as a negative electrode are expected to have a high energy density. In particular, the theoretical capacity per volume is superior to that of lithium, which is advantageous in that a large capacity can be packed into a limited space (volume) such as a secondary battery for an electric vehicle. Furthermore, the crustal layer is rich in magnesium reserves, and the problems of resource depletion and cost, which are the drawbacks of lithium-ion secondary batteries, can be overcome. Also, magnesium has a melting point of about 650° C., which is much higher than that of Li (180° C.) and Na (98° C.). The melting point is an indicator of metal stability, and secondary batteries using magnesium are expected to improve safety. In anticipation of the practical use of metal anodes in the future, the safety of the anode metal will be a very important factor. Furthermore, lithium, sodium, etc. are difficult to handle because they react violently with moisture in the air, but magnesium is stable in the air and easy to handle. For these reasons, magnesium secondary batteries are regarded as candidates for post-lithium ion secondary batteries, and have been extensively studied in recent years.

There are still some problems in creating a magnesium secondary battery system. In particular, in positive electrode materials, multivalent ions have twice the Coulomb interaction with anions in the solid phase as compared to lithium ions. becomes smaller and their interaction increases. As a result, it is expected that multivalent ions will be greatly affected by the electrostatic interaction within the positive electrode host compound, so the application of intercalation electrodes based on topochemical reactions is more likely for higher valence ions. Have difficulty.

_On the other hand, in recent _years , many positive electrode materials for _{magnesium secondary batteries} have been reported. Nanostructured materials such as MoS2 have been reported to exhibit stable and reversible charge-discharge characteristics as magnesium _secondary batteries (see, for example, Non-Patent Document 1).

Prototype systems for rechargeable magnesium batteries, Nature 407, 724-727 (2000)

However, there are many problems to be overcome in order to realize the multivalent ion secondary battery, and the most important one is the development of positive electrode materials. Compared to lithium ions, multivalent ions have a dramatically increased Coulomb interaction with anions in the solid phase, so the diffusion of multivalent ions in the positive electrode host compound in the solid phase is slow and the reaction is greatly restricted. can be a factor Even in compounds containing bases such as sulfur ions as disclosed in Non-Patent Document 1, the reaction potential tends to decrease (about 1.1 V), and the Chevrell phase has a large molecular weight and a low theoretical capacity (about 116 mAhg ⁻¹ ) was also a problem. From this point of view, in order to solve the problem of diffusion in the solid phase in the positive electrode and to bring out the potential of the secondary battery, it is necessary to develop a high-capacity positive electrode material made of elements that are abundant in resources like the negative electrode. was wanted.

The present invention has been made in view of the above, and is capable of charging and discharging various secondary batteries and capable of improving the characteristics of these secondary batteries. An object of the present invention is to provide a method and a secondary battery having the positive electrode active material as a component.

Means for Solving the Problems As a result of intensive studies aimed at achieving the above objects, the inventors have found that the above objects can be achieved by using a compound having a specific composition, and have completed the present invention.

That is, the present invention includes, for example, the subject matter described in the following sections.
Section 1. General formula (1) below
M _a OXO _b (1)
(Here, in formula (1), M is at least one selected from the group consisting of Ti, V, Nb and Mo, X is at least one selected from the group consisting of P and S, and 0 .5 ≤ a ≤ 1.2, 3.5 ≤ b ≤ 4.5)
including a compound represented by
A positive electrode active material for a secondary battery in which carrier ions are other than lithium ions.
Section 2. A secondary battery comprising the positive electrode active material according to item 1 as a component.
Item 3. Item 1. A method for producing the positive electrode active material for a secondary battery according to Item 1,
A production method comprising a heating step of heating a raw material mixture containing the raw material containing M and the raw material containing X.
Section 4. 4. The production method according to item 3, wherein the heating temperature in the heating step is 500 to 1500°C.

The positive electrode active material for secondary batteries according to the present invention can charge and discharge various secondary batteries such as magnesium ion secondary batteries and potassium ion batteries, and can increase the energy density and capacity of secondary batteries. , and a post-lithium secondary battery can be constructed at low cost.

(a) shows the XRD pattern of the product (V _a OPO _b ) obtained in Example 1, and (b) shows the XRD pattern refined by the Rietveld method. 1 shows an SEM image of V _a OPO _b obtained in Example 1. FIG. 2 shows the results of the potential-time characteristics of a Mg full battery using V _a OPO _b obtained in Example 1 as a positive electrode active material. 1 shows charge-discharge characteristics when V _a OPO _b obtained in Example 1 is used as a positive electrode active material, and the relationship between each cycle and discharge capacity. 2 shows cycle characteristics when V _a OPO _b obtained in Example 1 is used as a positive electrode active material. 2 shows the XRD pattern of the product (Nb _a OPO _b ) calcined in Example 2. FIG. 2 shows the results (charge/discharge characteristics and relationship between each cycle and discharge capacity) of Test Example 2 when a positive electrode active material for a secondary battery containing Nb _a OPO _b is used as a positive electrode material (C/20rate). 2 shows the results (charge/discharge characteristics and relationship between each cycle and discharge capacity) of Test Example 2 when a positive electrode active material for a secondary battery containing Nb _a OPO _b was used as a positive electrode material (C/50rate). 2 shows the results (charge/discharge characteristics and relationship between each cycle and discharge capacity) of Test Example 2 when a positive electrode active material for a secondary battery containing Nb _a OPO _b is used as a positive electrode material. 1 shows the results of charge-discharge curves of KVOPO ₄ (KVPO ₅ ).

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

1. Positive electrode active material for secondary battery The positive electrode active material for secondary battery of the present embodiment is represented by the following general formula (1)
M _a OXO _b (1)
(Here, in formula (1), M is at least one selected from the group consisting of Ti, V, Nb and Mo, X is at least one selected from the group consisting of P and S, and 0 .5 ≤ a ≤ 1.2, 3.5 ≤ b ≤ 4.5)
Including the compound represented by. The positive electrode active material for a secondary battery of the present embodiment is a positive electrode active material for constituting a secondary battery in which carrier ions are other than lithium ions, that is, a secondary battery other than a lithium ion secondary battery.

Since the positive electrode active material for secondary batteries of the present embodiment can insert and desorb various metal ions, it is useful as a positive electrode active material for various metal ion secondary batteries. For example, the positive electrode active material for a secondary battery of the present embodiment can insert and extract magnesium ions when a compound containing Mg is used as an electrolyte in a secondary battery. It is useful as a positive electrode active material for Further, for example, when a compound containing potassium (K) is used as an electrolyte in a secondary battery, the positive electrode active material for a secondary battery of the present embodiment can insert and desorb potassium ions. It is useful as a positive electrode active material for potassium ion secondary batteries.

In general formula (1), M is at least one selected from the group consisting of Ti, V, Nb and Mo. Among them, M is preferably V. In this case, the positive electrode active material for secondary batteries can be produced at a lower cost, and the theoretical capacity of the positive electrode active material for secondary batteries is higher. It is easy to improve the potential of the secondary battery.

In general formula (1), X is at least one selected from the group consisting of P and S. As a result, the positive electrode active material for secondary batteries can be produced at low cost, and the potential of the secondary battery can be easily improved.

Examples of compounds represented by general formula (1) include V _a OPO _b (eg, VPO _{5 ) and Nba OPO b (} eg, NbPO ₅ ). In this case, the positive electrode active material for secondary batteries is advantageous from the viewpoint of excellent theoretical capacity and high potential, in addition to being less costly.

The crystal structure of the compound represented by general formula (1) is not particularly limited. For example, the crystal structure of the compound represented by general formula (1) (especially VOPO ₄ ) can form various structures such as α-phase, β-phase, γ-phase, δ-phase, ε-phase, and ω-phase. Among them, α-phase and β-phase are preferred, and β-phase is particularly preferred. With such a crystal structure, it is easy to manufacture, and when used as a positive electrode active material for a secondary battery, it is possible to increase the capacity of the secondary battery.

In the above compound, the abundance of the crystal structure, which is the main phase, is not particularly limited, and is preferably 80 mol % or more, more preferably 90 mol % or more, based on the entire compound represented by formula (1). Therefore, the compound represented by general formula (1) can be formed as a material having a single-phase crystal structure. However, the compound represented by the general formula (1) may be formed of a material having multiple crystal structures as long as it does not impair the effects of the present invention. The crystal structure of the compound represented by general formula (1) can be confirmed by X-ray diffraction measurement.

In the X-ray diffraction measurement of the compound represented by the general formula (1), the diffraction angle represented by 2θ is in the range of 16.67 to 17.56 °, the range of 18.95 to 20.03 °, and 21.97. ~23.43° range, 25.04-25.83° range, 26.00-26.84° range, 27.83-28.52° range, 28.79-29.47° range Range 27.79-30.58° Range 31.19-32.37° Range 33.53-35.10° Range 36.61-38.34° Range 38.89- 42.14° range, 42.87-43.93° range, 45.04-51.02° range, 51.81-58.40° range and 59.46-64.04° range preferably has a peak at . In this case, when the compound represented by the general formula (1) is used as the positive electrode active material for a secondary battery, the capacity of the secondary battery can be easily improved.

The compound represented by general formula (1) having the crystal structure and composition as described above can be, for example, in the form of particulate powder. When the compound is particulate, its average particle diameter is preferably 0.2 to 50 μm, more preferably 0.5 to 30 μm, from the viewpoints of ease of insertion and desorption of various metal ions, capacity, and high potential. more preferred. The average particle size of the compound represented by general formula (1) is measured by electron microscope (SEM) observation. The average particle size referred to here is, for example, the arithmetic mean value of equivalent circle diameters measured by direct observation with an electron microscope.

When a secondary battery such as a magnesium ion secondary battery or a potassium ion secondary battery is formed using the positive electrode active material for a secondary battery of the present embodiment, the secondary battery can have a high capacity. Moreover, in the compound represented by the general formula (1), when M is at least one selected from the group consisting of Ti, V, Nb and Mo, a two-stage redox reaction occurs, which results in secondary Battery capacity tends to increase. For example, if M is V, a redox reaction occurs in two steps, V ⁵⁺ /V ⁴⁺ and V ⁴⁺ /V ³⁺ .

In addition, the compound represented by the above general formula (1) can be produced by a cheap and easy process. Therefore, according to the positive electrode active material for secondary batteries of the present embodiment, various secondary batteries can be provided at low cost. It is suitable as a positive electrode material for secondary batteries (for example, magnesium ion secondary batteries and potassium ion secondary batteries).

The positive electrode active material for secondary batteries may contain other materials as long as the performance of the positive electrode active material for secondary batteries of the present embodiment is not hindered. In addition, the positive electrode active material for secondary batteries may contain inevitable impurities in addition to the compound represented by the general formula (1). Examples of such unavoidable impurities include raw material mixtures to be described later. can contain

In the positive electrode active material for secondary batteries, the compound represented by the general formula (1) and a carbon material (for example, carbon black such as acetylene black) may form a composite. As a result, the carbon material suppresses grain growth of the compound represented by the general formula (1) during sintering, so that a particulate positive electrode active material for secondary batteries having excellent electrode properties can be obtained. In this case, the content of the carbon material is preferably adjusted to 3 to 20% by mass, particularly 5 to 15% by mass in the positive electrode active material for secondary batteries.

2. Method for Producing Positive Electrode Active Material for Secondary Battery The method for producing the positive electrode active material for secondary battery is not particularly limited. For example, it is possible to obtain a positive electrode active material for a secondary battery by producing the compound by a production method comprising a heating step of heating a raw material mixture containing the raw material containing M and the raw material containing X. can. This method will be specifically described below.

(1) Raw material mixture The raw material mixture in the above production method includes a raw material containing M and a raw material containing X above. The raw material mixture may be a compound having both M and X, and may contain one or more such compounds. Moreover, the raw material mixture may contain two or more kinds of raw materials containing M, and may contain two or more kinds of raw materials containing X.

The raw material containing M may be, for example, metal M (M is one selected from the group consisting of Ti, V, Nb and Mo), or may be a compound containing M. Also, the raw material containing M may be an alloy of two or more selected from the group consisting of Ti, V, Nb and Mo, or may contain the metal M alone in addition to this alloy. Examples of compounds containing M include vanadium oxide (V ₂ O ₅ ), titanium oxide, niobium oxide, molybdenum oxide, and the like. In addition, examples of compounds containing M include metal M hydroxides, chlorides, carbonates, nitrates, oxalates, and the like. A compound containing M may be a hydrate.

The raw material containing X may be a single substance (that is, P or S) or a compound containing X. Examples of compounds containing X include NH ₄ H ₂ PO ₄ (ammonium dihydrogen phosphate). In addition, examples of compounds containing X include oxides, hydroxides, chlorides, carbonates, nitrates, and oxalates of X. A compound containing X may be a hydrate.

The raw material mixture preferably does not contain metal elements other than M and X (especially rare metal elements). Moreover, when other metal elements are contained, it is preferable that the metal elements are detached and volatilized by heat treatment in a non-oxidizing atmosphere.

In addition, each raw material constituting the raw material mixture may be used as a commercially available product, or may be synthesized separately and used.

The shape of the raw material mixture is not particularly limited, and powdery form is preferable from the viewpoint of handleability. From the viewpoint of reactivity, fine particles are preferred, and powdery particles having an average particle size of 1 μm or less (particularly about 60 to 80 nm) are preferred. The average particle size of each raw material is measured by electron microscopy (SEM).

The raw material mixture can be prepared by mixing a raw material containing M and a raw material containing X in a predetermined mixing ratio. The mixing method is not particularly limited, and for example, a method capable of uniformly mixing each raw material can be adopted. Specifically, mortar mixing, mechanical milling, coprecipitation, a method of dispersing each raw material in a solvent and then mixing, a method of dispersing and mixing each raw material in a solvent at once, and the like can be adopted. can. Among these methods, mortar mixing is a simpler method to obtain the compound represented by the general formula (1). A coprecipitation method can be employed to obtain a more homogeneous mixture of raw materials.

The mixing ratio of each raw material in each raw material mixture is not particularly limited. For example, it is preferable to blend the raw materials so as to obtain the compound represented by the general formula (1) having the desired composition as the final product. Specifically, it is preferable to adjust the mixing ratio of each raw material so that the ratio of each element contained in each raw material is the same as the ratio of each element in the target compound.

(2) Heating step In the heating step, a raw material mixture containing a raw material containing M and a raw material containing X is heated.

The heating step can be performed, for example, in an atmosphere of an inert gas such as argon or nitrogen. Alternatively, the heating step may be performed under reduced pressure such as vacuum.

The heating temperature (that is, the firing temperature) in the heating step is preferably 500 to 1500°C. In this case, the operation of the heating step can be performed more easily, and the obtained compound can easily form the desired crystal structure. As a result, the crystallinity and electrode properties (especially capacity and potential) of the resulting compound are likely to be improved.

The lower limit of the heating temperature in the heating step is preferably 700°C, more preferably 800°C. The upper limit of the calcination temperature can be appropriately determined within a range in which the operation in the production of the compound represented by the general formula (1) can be easily performed (for example, 1500°C).

The heating time in the heating step is not particularly limited, and is preferably 10 minutes to 48 hours, more preferably 30 minutes to 24 hours.

After heating for a predetermined time, the desired compound is obtained by cooling. A cooling rate is not particularly limited. Moreover, after cooling once, you may heat-process again at the said heating temperature, and may bake.

In addition, the compound represented by the general formula (1) can be produced by, for example, a coprecipitation method, a sol-gel method, a hydrothermal synthesis method, or the like, in addition to the above production methods.

3. Secondary Battery The secondary battery of the present embodiment has the positive electrode active material for a secondary battery as a constituent element. In particular, the secondary battery of the present embodiment refers to secondary batteries other than lithium ion secondary batteries, such as magnesium ion secondary batteries and potassium ion secondary batteries.

The basic structure of the secondary battery can be constructed with reference to known non-aqueous electrolyte secondary batteries, except that the positive electrode active material for secondary batteries is used as the positive electrode active material. For example, a positive electrode, a negative electrode and a separator can be placed in a battery container such that the positive electrode and negative electrode are separated from each other by the separator. After that, after filling the battery container with the non-aqueous electrolyte, the battery container is sealed, and the like, whereby the secondary battery can be manufactured. The material, size and shape of the battery container can be appropriately determined according to the application of the secondary battery.

The positive electrode can employ a structure in which a positive electrode current collector carries a positive electrode material containing the positive electrode active material for a secondary battery of the above embodiment. For example, it can be produced by coating a positive electrode current collector with a positive electrode mixture containing the positive electrode active material for a secondary battery, a conductive aid, and, if necessary, a binder.

Examples of conductive aids that can be used include carbon materials such as acetylene black, ketjen black, carbon nanotubes, vapor-grown carbon fibers, carbon nanofibers, graphite, and cokes. The shape of the conductive aid is not particularly limited, and for example, a powdery shape can be used.

Examples of the binder include polyvinylidene fluoride resin, fluororesin such as polytetrafluoroethylene, and the like, but are not limited to these.

The content of various components in the positive electrode material is not particularly limited, and can be appropriately determined according to the type of material. % by volume), 2.5 to 25% by volume (especially 5 to 15% by volume) of a conductive aid, and 2.5 to 25% by volume (especially 5 to 15% by volume) of a binder.

Materials constituting the positive electrode current collector include, for example, aluminum, titanium, platinum, molybdenum, stainless steel, and copper. Examples of the shape of the positive electrode current collector include porous bodies, foils, plates, meshes made of fibers, and the like.

The amount of the positive electrode material applied to the positive electrode current collector is preferably determined appropriately according to the intended use of the secondary battery.

Examples of the negative electrode active material constituting the negative electrode of the magnesium ion ^secondary battery include magnesium metal; silicon; silicon ^- containing _{Clathrate compound} ; magnesium ^alloy ; , M ² : Mn, Fe, Zn, etc.); M ³ ₃ O ₄ (M ³ : Fe, Co, Ni, Mn, etc.), M ⁴ ₂ O ₃ (M ⁴ : Fe, Co, Ni, Mn, etc.), MnV ₂ O ₆ , M ⁵ O ₂ (M ⁵ : Sn, Ti, etc.), M ⁶ O (M ⁶ : Fe, Co, Ni, Mn, Sn, Cu, etc.) Graphite, hard carbon, soft carbon, _graphene ; _Carbon materials mentioned above; _Organic compounds such _{as MgC8H4O4} , _{MgC8H4O4.2H2O} , _etc. mentioned. Examples of magnesium alloys include alloys containing magnesium and aluminum as constituent elements, alloys containing magnesium and zinc as constituent elements, alloys containing magnesium and manganese as constituent elements, alloys containing magnesium and bismuth as constituent elements, and magnesium and nickel. as constituent elements, alloys containing magnesium and antimony as constituent elements, alloys containing magnesium and tin as constituent elements, alloys containing magnesium and indium as constituent elements; metals (scandium, titanium, vanadium, chromium, zirconium, niobium , molybdenum, hafnium, tantalum, etc.) and carbon as constituent elements, ^M7xBC3 _- based alloys ₍ M7: Sc, Ti, ^V , Cr, Zr, Nb, Mo, Hf, Ta, etc.) quaternary layered carburized or nitrided compounds; and alloys containing magnesium and lead as constituent elements. Such a negative electrode may be an electrode in which metallic magnesium, graphite, or a magnesium alloy is supported on a current collector, and metallic magnesium, graphite, carbon, or a magnesium alloy is formed into a shape suitable for the electrode (for example, a plate shape). It may be an electrode obtained by

_In addition, as the negative electrode active material constituting the negative electrode of the potassium ion ^secondary battery, for example, potassium metal; silicon; silicon ^- containing _Clathrate ^compound ; potassium alloy; Sn, etc., M ² : Mn, Fe, Zn, etc.); M ³ ₃ O ₄ (M ³ : Fe, Co, Ni, Mn, etc.), M ⁴ ₂ O ₃ ( M4: Fe, Co, Ni, Mn, etc.), ^MnV2O6 , _M5O2 ^{( M5} : ^Sn , Ti, etc.), _{M6O (} ^M6 : Fe, Co, Ni, Mn, Sn, Cu etc.), etc _{.; graphite} , hard carbon, soft carbon _{, graphene ;} the above _- described carbon materials _{; K2Ti6O13} ; _{K2TiO2n + 1 (} n= _3,4,6,8 ); _K2SiP2 ; _{KSi2P3 ; MnSnO3 ; K1.4Ti8O16 ; K1.5 Ti6.5V1.5O16} ; _{K1.4Ti6.6Mn1.4O16 ; Zn3 ( HCOO} ) ₆ ; _{Co3 ( HCOO} ) ₆ ; _Zn1.5Co1.5 ( _HCOO ) ₆ ; _KVMoO6 ; _AV2O6 (A ₌ Mn, Co, Ni, Cu); _Mn2GeO4 ; _{Ti2 ( SO4} ) ₃ ; _{KTi2 ( PO4} ) ₃ ; _SnO2 ; _Nb2O5 ; TiO ₂ ; Te; VOMoO ₄ ; , poly(2,5-dimercapto-1,3,4-thiadiazole) (PDMcT), poly(2,2'-dithiodianiline) (PDTDA), poly(5,8-dihydro-1H,4H-2,3, 6,7-tetrathia-anthracene) (PDTTA), poly(2,2,6,6 _{-tetramethylpiperidine - 1 - oxyl - 4 -} yl methacrylate) _{( PTMA} ), _K2C6H4O4 , _K2C5O5.2H2O , _K4C8H2O6 , _{K2C6H 4O4 , K2C14H6O4 , K4C6O6 , K4C24H8O8 , K4C6O6 , K2C6O6 , K2C6H2O _ _ _ _ _ _ _ _ _ 4 , K2C14H6O4 , K2C8H4O4 , K2C14H4N2O4 , K2C6H4O4 , K2C18H12O8 , K _ _ _ _ _ _ _ _ _ Organic compounds such as 2C16H8O4 , K2C10H2N2O4} and the _{like are} included. Potassium alloys include, for example, alloys containing potassium and aluminum as constituent elements, alloys containing potassium and zinc as constituent elements, alloys containing potassium and manganese as constituent elements, alloys containing potassium and bismuth as constituent elements, and potassium and nickel. as constituent elements, alloys containing potassium and antimony as constituent elements, alloys containing potassium and tin as constituent elements, alloys containing potassium and indium as constituent elements; metals (scandium, titanium, vanadium, chromium, zirconium, niobium , molybdenum, hafnium, tantalum, etc.) and carbon as constituent elements, ^M7xBC3 _- based alloys ₍ M7: Sc, Ti, ^V , Cr, Zr, Nb, Mo, Hf, Ta, etc.) quaternary layered carbide or nitride compounds; alloys containing potassium and lead as constituent elements; Such a negative electrode may be an electrode in which metallic potassium, graphite, or a potassium alloy is supported on a current collector. It may be an electrode obtained by

The negative electrode can also be composed of a negative electrode active material, and adopts a configuration in which a negative electrode material containing a negative electrode active material, a conductive aid, and, if necessary, a binder is supported on a negative electrode current collector. can also When adopting a configuration in which the negative electrode material is supported on the negative electrode current collector, a negative electrode mixture containing a negative electrode active material, a conductive aid, and optionally a binder is applied to the negative electrode current collector. can be manufactured.

When the negative electrode is composed of a negative electrode active material, it can be obtained by forming the above negative electrode active material into a shape (such as a plate shape) suitable for the electrode.

In addition, when adopting a configuration in which the negative electrode material is supported on the negative electrode current collector, the types of the conductive aid and the binder, and the contents of the negative electrode active material, the conductive aid, and the binder are those of the positive electrode described above. can be applied. Examples of materials that constitute the negative electrode current collector include aluminum, copper, nickel, and stainless steel. Examples of the shape of the negative electrode current collector include a porous body, a foil, a plate, and a mesh made of fibers. The amount of the negative electrode material applied to the negative electrode current collector is preferably determined appropriately according to the usage of the secondary battery.

The separator is not particularly limited as long as it is made of a material that separates the positive electrode and the negative electrode in the battery, holds an electrolytic solution, and ensures ion conductivity between the positive electrode and the negative electrode. For example, polyolefin resin such as polyethylene and polypropylene; polyimide; polyvinyl alcohol; fluorine resin such as terminally aminated polyethylene oxide polytetrafluoroethylene; acrylic resin; nylon; aromatic aramid; Materials in the form of membranes, nonwovens, wovens, etc. can be used.

The non-aqueous electrolyte can be appropriately selected according to the type of secondary battery. For example, if the secondary battery is a magnesium ion secondary battery, an electrolytic solution containing magnesium ions is preferable. Examples of such an electrolytic solution include a magnesium salt solution, an ionic liquid composed of an inorganic material containing magnesium, and the like. Examples of such an electrolytic solution include a solution in which a magnesium salt is dissolved in a solvent, an ionic liquid composed of an inorganic material containing magnesium, and the like, but are not limited to these examples.

Examples of magnesium salts include magnesium halides such as magnesium chloride, magnesium bromide, and magnesium iodide; magnesium inorganic salts such as magnesium perchlorate, magnesium tetrafluoroborate, magnesium hexafluorophosphate, and magnesium hexafluoroarsenate; Compound; Magnesium organic salt compounds such as bis(trifluoromethylsulfonyl)imide magnesium, magnesium benzoate, magnesium salicylate, magnesium phthalate, magnesium acetate, magnesium propionate, magnesium tris(pentafluoroethane) trifluorophosphate, Grignard reagents, etc. However, it is not limited only to such examples.

A solid electrolyte can also be used instead of the non-aqueous electrolyte. Examples of solid electrolytes include (Mg _0.1 Hf _0.9 ) _4/3.8 Nb(PO ₄ ) ₃ , (Mg _0.1 Hf _0.9 ) _4/3.8 (Nb _1-y W _y ) (PO4) ₃ ( _0≤y≤3 ), _MgaZrb (PO4) _c (a, _b = 1, 2, ₄ , 5, 7; c = 2, 4, 6, 8, 10 ), Mg(BH ₄ ) ₂ (NH ₃ ) ₂ and other magnesium ion conductors.

Moreover, if the secondary battery is a potassium ion secondary battery, the electrolytic solution may be an electrolytic solution containing potassium cations. Examples of the electrolytic solution include a solution in which a potassium salt is dissolved in a solvent, an ionic liquid composed of an inorganic material containing potassium, and the like, but are not limited to these examples.

Examples of potassium salts include potassium halides such as potassium chloride, potassium bromide, potassium fluoride, and potassium iodide, potassium perchlorate, potassium tetrafluoroborate, potassium hexafluorophosphate, potassium hexafluoroarsenate, and the like. Potassium inorganic salt compound of; , potassium pentafluoropropionate, potassium ethoxide, potassium tetraphenylborate, potassium tetrafluoroborate, potassium hexafluorophosphate, potassium bis(trifluoromethylsulfonyl)imide, potassium bis(pentafluoroethanesulfonyl)imide, tris Potassium organic salt compounds such as (pentafluoroethane) trifluorophosphate potassium, potassium benzoate, potassium salicylate, potassium phthalate, potassium acetate, potassium propionate, and Grignard reagents, etc., but are limited only to these examples. is not.

A solid electrolyte can also be used instead of the non-aqueous electrolyte. Solid electrolytes for potassium secondary batteries include, for example, KBiO ₃ , KNO ₂ —Gd ₂ O ₃ solid solution system, KSbO ₃ , K ₂ SO ₄ , KH ₂ PO ₄ , KZr ₂ (PO ₄ ) ₃ , K ₉ Fe( Potassium ion conductors such as MoO ₄ ) ₆ , K ₄ Fe ₃ (PO ₄ ) ₂ P ₂₇ and K ₃ MnTi(PO ₄ ) ₃ are enumerated.

Examples of solvents include carbonate compounds such as propylene carbonate, ethylene carbonate, dimethol carbonate, ethylmethyl carbonate and diethyl carbonate; lactone compounds such as γ-butyrolactone and γ-valerolactone; tetrahydrofuran, 2-methyltetrahydrofuran, diethyl ether, Diisopropyl ether, dibutyl ether, methoxymethane, N,N-dimethylformamide, glyme, N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, dimethoxyethane, dimethoxymethane, diethoxymethane, diethoxyethane, propylene glycol dimethyl ether, etc. Ether compounds; acetonitrile and the like.

Since the secondary battery of the present embodiment uses the compound having the composition represented by formula (1), it is possible to ensure higher potential and energy density during the oxidation-reduction reaction (charge-discharge reaction). Moreover, it is excellent in safety (polyanion skeleton) and practicality. Therefore, the secondary battery of the present embodiment can be suitably used, for example, for devices that require miniaturization and high performance.

In this specification, "magnesium ion secondary battery" means a secondary battery with magnesium ions as carrier ions, and "magnesium secondary battery" uses magnesium metal or a magnesium alloy as a negative electrode active material. means a secondary battery. In addition, "potassium ion secondary battery" means a secondary battery with potassium ions as carrier ions, and "potassium secondary battery" means a secondary battery that uses potassium metal or a potassium alloy as a negative electrode active material. do.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the embodiments of these examples.

(Example 1)
NH ₄ H ₂ PO ₄ and V ₂ O ₅ were prepared as raw material powders, and the NH ₄ H ₂ PO ₄ and V ₂ O ₅ were weighed so that the molar ratio of vanadium and phosphorus was 1:2. A raw material mixture was obtained by mixing in a mortar for about 30 minutes. After that, the raw material mixture was placed in a chromium steel container together with zirconia balls (15 mmΦ×10), acetone was added, and the mixture was pulverized and mixed at 400 rpm for 6 hours in a planetary ball mill (Fritsch; P-6). Then, after acetone was distilled off under reduced pressure, the recovered powder was pellet-molded at 40 MPa and fired at a firing temperature of 700° C. for 2 hours in argon. At this time, the temperature increase rate was set to 400° C./h. The cooling rate was 100° C./h up to 300° C., and thereafter, it was allowed to cool to room temperature by natural cooling. Then, it was fired again at 700° C. for 3 hours to obtain V _a OPO _b .

(Example 2)
NH ₄ H ₂ PO ₄ and Nb ₂ O ₅ were prepared as raw material powders, and the NH ₄ H ₂ PO ₄ and Nb ₂ O ₅ were weighed so that the molar ratio of niobium and phosphorous was 1:2. After mixing for about 30 minutes, a raw material mixture was obtained. After that, the raw material mixture was placed in a chromium steel container together with zirconia balls (15 mmΦ×10), acetone was added, and the mixture was pulverized and mixed at 400 rpm for 6 hours in a planetary ball mill (Fritsch; P-6). Thereafter, after acetone was distilled off under reduced pressure, the recovered powder was pellet-molded at 40 MPa and fired at a firing temperature of 900° C. for 8 hours or more in argon. At this time, the temperature increase rate was set to 400° C./h. The cooling rate was 100° C./h up to 300° C., after which the product was naturally cooled to room temperature to obtain _{Nba OPO b .}

<Evaluation method>
[Powder X-ray diffraction (XRD) measurement]
The synthesized sample was measured using an X-ray diffractometer (RINT-UltimaIII/G manufactured by Rigaku Corporation). CuKα rays were used as the X-ray source, and the applied voltage was 40 kV and the current value was 40 mA. Measurements were made over an angular range of 10° to 80° at a scanning speed of 0.02°/sec.

[ICP-AES measurement]
ICP-AES measurement was performed using an inductively coupled plasma atomic emission spectrometer (“iCAP6500” manufactured by Thermo Fisher Scientific).

[Test Example 1: Examination of magnesium ion desorption and insertion (Mg full battery)]
A CR2032 type coin cell was used as the cell. Using the Mg disc as the negative electrode, the electrolytic solution was prepared by dissolving Mg(TFSI) ₂ in ethylene glycol dimethyl ether dimethoxyethylene glycol (trade name: monoglyme) to a concentration of 0.5M. The constant current charge/discharge measurement was performed by using a voltage switch, setting the current to 5 mAg ⁻¹ , the upper limit voltage to 3.6 V, and the lower limit voltage to 0 V, and starting from charging. In addition, the measurement was performed at room temperature.

[Test Example 2: Examination of cation detachment and insertion (potassium secondary battery)]
A CR2032 type coin cell was used as the cell. Natural graphite was used as the negative electrode, and 1M KTFSI (PC solvent) was used as the electrolyte. The constant current charge/discharge measurement was performed by using a voltage switch, setting the current to 10 mAg ⁻¹ , the upper limit voltage to 4.2 V, and the lower limit voltage to 1.5 V, and started from charging. Charge/discharge measurement was performed with the cell placed in a 55° C. constant temperature bath.

FIG. 1(a) shows the XRD pattern of the calcined product (V _a OPO _b ) in Example 1. FIG. The X-ray wavelength used is 1.5418 Å.

From FIG. 1(a), all the Bragg peaks of the obtained sample can be indexed with the orthorhombic lattice, and the presence of the impurity phase seen in the starting material, its product, etc. was almost not confirmed. rice field. Further, from the XRD results, V _a OPO _b could be attributed to the orthorhombic system and space group Pnma (No. 62). According to the Rietveld method, lattice constants a = 7.7849 (7) Å, b = 6.1329 (4) Å, c = 6.9692 (5) Å, α = β = γ = 90 °, unit cell volume ( V) is 323.74(7) Å ³ , which agrees with the literature value (Gopal et al. Solid State Chem., 5, 432-435 (1972)) (that is, it is a β phase). . The reliability factors were R _wp =6.27%, R _p =4.78%, and χ ² =1.40.

V _a OPO _b is known to exhibit polymorphism, among which the β-V _a OPO _b polymorph is the most stable phase. This polymorphic V _a OPO _b was evaluated as a positive electrode material for magnesium and potassium secondary batteries (Test Examples 1 and 2).

Further, from FIG. 1, the obtained crystals of V _a OPO _b have diffraction angles represented by 2θ in the range of 16.67 to 17.56°, 18.95 to 20.03°, and 21.97°. ~23.43° range, 25.04-25.83° range, 26.00-26.84° range, 27.83-28.52° range, 28.79-29.47° range Range 27.79-30.58° Range 31.19-32.37° Range 33.53-35.10° Range 36.61-38.34° Range 38.89- 42.14° range, 42.87-43.93° range, 45.04-51.02° range, 51.81-58.40° range and 59.46-64.04° range , and the peak position of V _a OPO _b was found to be almost close to that of β-VPO ₅ . From this result, it can be seen that the lattice constant of V _a OPO _b is almost close to that of β-VPO ₅ .

FIG. 1(b) shows the XRD pattern refined by the Rietveld method. As a result, the lattice constants a = 7.7849 (7) Å, b = 6.1329 (4) Å, c = 6.9692 (5) Å, α = β = γ = 90 °, unit cell volume (V ) is 323.74(7) Å ³ , which agrees with the literature value (Gopal et al. Solid State Chem., 5, 432-435 (1972)) (that is, the β phase is the main phase). I found out. The reliability factors were R _wp =6.27%, R _p =4.78%, and χ ² =1.40.

Further, as a result of ICP-AES measurement, it was found that V _a OPO _b obtained in Example 1 was a=0.722 .

FIG. 2 shows an SEM image of V _a OPO _b obtained in Example 1. As shown in FIG. In the figure, the scale bar indicates 23.2 μm. From this result, it can be seen that the particles of V _a OPO _b obtained in Example 1 tend to be agglomerated, but the average particle size is around 30 μm.

FIG. 3 shows the results of Test Example 1 when the positive electrode active material for secondary batteries containing V _a OPO _b obtained in Example 1 was used as the positive electrode material (results of potential-time characteristics of Mg full battery). showing. As a result, it was found that potential responses suggesting the insertion and desorption of magnesium were observed.

FIG. 4 shows the results of Test Example 2 when a positive electrode active material for a secondary battery containing V _a OPO _b was used as a positive electrode material (charge/discharge characteristics and relationship between each cycle and discharge capacity). . From this result, it can be seen that good cycle characteristics are exhibited by taking in and out potassium ions. It was also found that about 70% of the initial capacity was maintained at the 50th cycle. Furthermore, as shown in FIG. 5, a good reversible capacity was obtained in the cycle test, and the capacity retention after 50 cycles was 70%, indicating stable characteristics.

The drawn capacity (charged capacity) was 70 mAhg ⁻¹ and the average voltage was about 3.75 V. It can be seen that the obtained positive electrode material is expected as a high capacity and high potential material.

The theoretical capacity that can be extracted when K ⁺ or Mg ²⁺ intercalates and desorbs is about 331 mAhg ⁻¹ . This is because when a positive electrode active material for a secondary battery containing V _a OPO _b is used as a positive electrode material, V in V _a OPO _b containing a strong polyanion unit (PO ₄ ³⁻ ) undergoes a two-step redox electron reaction (V ⁵⁺ /V ⁴⁺ and V ⁴⁺ /V ³⁺ pairs).

FIG. 6 shows the XRD pattern of the product (Nb _a OPO _b ) calcined in Example 2. The X-ray wavelength used is 1.5418 Å.

From FIG. 6, all the Bragg peaks of the obtained sample could be indexed with an orthorhombic lattice, and the existence of the impurity phase seen in the starting material, its product, etc. was hardly confirmed. Further, from the XRD results, it was found that Nb _a OPO _b exhibited polymorphism in the same manner as V _a OPO _b , and in particular, it was found that the orthorhombic Nb _a OPO _b polymorph was the main phase.

7 and 8 show the results of Test Example 2 when a positive electrode active material for a secondary battery containing Nb _a OPO _b was used as the positive electrode material (charge/discharge characteristics and relationship between each cycle and discharge capacity). (C/20rate in FIG. 7, C/50rate in FIG. 8, both 25 cycles). From this result, it can be seen that good cycle characteristics are exhibited by the inflow and outflow of potassium ions at both C/20rate and C/50rate. It was also found that about 70% of the initial capacity was maintained at the 50th cycle. Furthermore, as shown in FIGS. 7 and 8, good reversible capacity was obtained in the cycle test, and capacity retention after 25 cycles was about 70%, indicating stable characteristics.

FIG. 9 shows a summary of the results of Test Example 2 when a positive electrode active material for a secondary battery containing Nb _a OPO _b was used as the positive electrode material (charge/discharge characteristics and relationship between each cycle and discharge capacity). ing. As shown in FIG. 9, good reversible capacity was obtained in the cycle test (relationship between each cycle and discharge capacity), and a relatively high capacity of about 100 mAhg ⁻¹ was obtained at a rate of C/5. Although the mixture electrode has not been optimized, it can be said that _NbOPO4 exhibits good rate characteristics.

(Comparative example 1)
K ₂ CO ₃ , _{Va OPO b ,} and carbon were prepared as raw material powders, weighed so that the molar ratio of K, V, and C was 2:4:1, and mixed in an agate mortar for about 30 minutes to obtain raw materials. A mixture was obtained. After that, the raw material mixture was formed into pellets at 40 MPa, and fired at a firing temperature of 700° C. for 1 hour or more in argon. At this time, the temperature increase rate was set to 400° C./h. The cooling rate was set to 100°C/h up to 300°C, and _thereafter , _KVaOPOb was obtained by naturally cooling to room temperature.

FIG. 10 shows the results of charge-discharge curves of KV _a OPO _b . From this result, it can be seen that although the operating potential is high (around 4 V), the capacity that can be extracted is small, and it is effective to use a potassium-free V _a OPO _b skeleton to improve the performance of the secondary battery. I understand.

Claims (4)

General formula (1) below
Nb _a OPO _b (1)
(Here, in formula (1) , 0.5≦a≦1.2 and 3.5≦b≦4.5)
including a compound represented by
The compound is a particulate powder having an average particle size of 0.2 to 50 μm,
The crystal structure of the compound is α phase or β phase,
A positive electrode active material for a secondary battery in which carrier ions are other than lithium ions. A secondary battery comprising the positive electrode active material according to claim 1 as a component. A method for producing the positive electrode active material for a secondary battery according to claim 1,
A production method comprising a heating step of heating a raw material mixture containing a Nb -containing raw material and a P -containing raw material. The production method according to claim 3, wherein the heating temperature in the heating step is 500 to 1500°C.

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