JP5391934B2 - Secondary battery and method for manufacturing positive electrode - Google Patents

Description

  The present invention relates to a secondary battery and a method for manufacturing a positive electrode, and more particularly to a secondary battery using a polyvalent metal and a method for manufacturing a positive electrode.

  Conventionally, as a secondary battery of this type, it has been proposed to use a vanadium pentoxide / carbon composite material as a positive electrode active material as a material for occluding and releasing magnesium ions (for example, Non-Patent Documents 1 and 2). reference). In this Non-Patent Document 1, there are two internal sites of the vanadium oxide pyramid and an interlayer site as magnesium insertion sites, and the vanadium pentoxide / carbon composite material is a simple mixture of vanadium pentoxide and a carbon material. On the other hand, it has been proposed that the layer spacing is larger and the diffusion length is smaller, the interlayer site is more easily used, and it is useful as a positive electrode active material that occludes and releases magnesium. Further, Non-Patent Document 2 proposes that rapid charging / discharging can be handled by using a vanadium pentoxide / carbon composite material.

Solid State Ionics, 161, 173 (2003) Journal of The Electrochemical Society, 150, A753-A758 (2003)

  In the vanadium pentoxide / carbon composite materials described in Non-Patent Documents 1 and 2 above, the heat treatment temperature at the time of production is relatively low at 120 ° C., and as shown in FIG. Has a laminated structure. In other words, magnesium ions are smoothly occluded / released by a laminated structure containing two types of water. However, in the configuration of the secondary battery, since a non-aqueous electrolyte solution is used, when moisture contained in the positive electrode active material described in Non-Patent Documents 1 and 2 is eluted into the electrolyte solution, for example, magnesium metal or magnesium alloy In order to form an oxide layer (hydroxide layer) by reacting with a negative electrode such as the above, there has been a problem that battery performance deteriorates during repeated charging and discharging.

  The present invention has been made in view of such a problem, and provides a method for manufacturing a secondary battery and a positive electrode that can further improve battery characteristics in the case of containing and releasing polyvalent metals including vanadium. The main purpose.

  As a result of diligent research to achieve the above-described object, the present inventors heat-treated a mixed material containing vanadium and a carbon material in a predetermined temperature range to remove moisture from the vanadium oxide and to control the carbon material. It has been found that the formation of pores by partial oxidation can further improve battery characteristics in those that absorb and release polyvalent metals, and the present invention has been completed.

That is, the secondary battery of the present invention is
A secondary battery using a polyvalent metal,
A negative electrode capable of inserting and extracting the polyvalent metal;
A positive electrode having a positive electrode active material layer containing a positive electrode active material containing vanadium and a carbon material;
A non-aqueous ion conducting medium that is interposed between the negative electrode and the positive electrode and conducts ions of the polyvalent metal,
The positive electrode active material layer has diffraction peaks at 2θ of 12.2 °, 15.4 °, 20.3 °, and 29.2 ° as measured by X-ray diffraction,
When the positive electrode active material layer is subjected to nitrogen adsorption measurement and analyzed by the BJH method, the average pore volume Vd, which is the average value of the Log differential pore volume in a predetermined pore radius range including a pore radius of 2 nm, is 1.50. × 10 ⁻² cm ³ / g or more

Moreover, the manufacturing method of the positive electrode of the present invention includes:
A method for producing a positive electrode of a secondary battery using a polyvalent metal,
A mixing step of preparing a mixed material in which vanadium and a carbon material are mixed;
A heat treatment step of heat-treating the mixed material obtained in the mixing step at 275 ° C. or higher in an oxidizing atmosphere;
Is included.

In the present invention, the battery characteristics can be further improved in the case of containing and releasing polyvalent metals including vanadium. The reason why such an effect is obtained is not clear, but is presumed as follows. For example, for occluding and releasing polyvalent metals (Mg, Al, etc.), it is preferable that crystallization water is present between vanadium oxide layers, because the valence metal is easy to move, so it is possible to remove crystallization water from vanadium oxide. It is common to use it. However, in this case, the negative electrode may be deactivated by elution of water into the ion conductive medium and formation of an oxide or hydroxide on the surface of the negative electrode active material. On the other hand, when crystal water is simply removed from vanadium oxide, the structural change such as crystallization and densification of the oxide progresses, so that the occlusion / release property of polyvalent metals is lowered and the battery performance may be lowered. . In the present invention, when the positive electrode active material layer is measured by X-ray diffraction using a Cu tube by removing moisture present between vanadium oxide layers by heat treatment, 2θ is 12.2 °, 15.4 °, 20 It has a diffraction peak at 3 ° and 29.2 °, and suppresses inactivation of the negative electrode caused by reaction with water. In addition, vanadium and a carbon material are mixed and heat-treated in an oxidizing atmosphere to burn the carbon material and have an average pore volume Vd of 1.50 × 10 ⁻² cm ³ / g or more. That is, by increasing the pore volume, the contact between the active material layer and the ion conductive medium is improved, and the decrease in the occlusion / release property of the polyvalent metal is suppressed. Therefore, it is presumed that the battery characteristics can be further improved in the case of containing and releasing polyvalent metals including vanadium.

The conceptual diagram of the process which mixes a carbon material after making a vanadium complex sol. The conceptual diagram of the process which makes a sol after mixing a vanadium complex and a carbon material. It shows the relationship between the heat treatment condition and the change in weight of V ₂ O _5. The figure showing the relationship between the heat processing conditions of a carbon material, and a weight change. The measurement result of the X-ray diffraction of the mixed material after heat processing. Pore distribution curve for each sample. The figure showing the relationship between heat processing temperature and vanadium utilization factor. Explanatory drawing of the layered structure of vanadium pentoxide.

  The secondary battery of the present invention includes a negative electrode capable of occluding and releasing a polyvalent metal, a positive electrode having a positive electrode active material layer containing a positive electrode active material containing vanadium and a carbon material, and a negative electrode and a positive electrode. And a non-aqueous ion conducting medium that conducts ions of polyvalent metals. The polyvalent metal is not particularly limited as long as it is a divalent or higher metal that causes a battery reaction, but at least one of magnesium and aluminum is preferable, and magnesium is more preferable.

  The negative electrode of the secondary battery of the present invention may have a negative electrode active material containing a polyvalent metal formed on a current collector. The negative electrode active material is not particularly limited as long as it is a material that can occlude and release polyvalent metals. Carbon materials, metal oxides, etc.) can be used. When magnesium is used as the polyvalent metal, for example, metal magnesium, an alloy of metal magnesium and an alkali metal, or the like can be used. The current collector of the negative electrode includes copper, nickel, stainless steel, titanium, aluminum, noble metal, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., as well as adhesiveness, conductivity and resistance to resistance. For the purpose of improvement, for example, a surface of copper or the like treated with carbon, nickel, titanium, silver or the like can be used. For these, the surface can be oxidized. Examples of the shape of the current collector include foil, film, sheet, net, punched or expanded, lath, porous, foam, and formed fiber group. The thickness of the current collector is, for example, 1 to 500 μm.

The positive electrode of the secondary battery of the present invention is, for example, a mixture of a positive electrode active material, a conductive material, and a binder, and an appropriate solvent is added to form a paste-like positive electrode material, which is applied to the surface of the current collector and dried. Thus, a positive electrode active material layer may be formed and compressed as necessary to increase the electrode density. As the positive electrode active material, an oxide containing vanadium is used. Examples of the oxide containing vanadium include vanadium pentoxide (V ₂ O ₅ ). This positive electrode active material layer has diffraction peaks in the vicinity of 12.2 °, 15.4 °, 20.3 ° and 29.2 ° as measured by X-ray diffraction. The diffraction peaks near 2θ = 12.2 ° and 29.2 ° are derived from orthorhombic V ₂ O ₅ (anhydrous), and the diffraction peaks near 2θ = 15.4 ° and 20.3 ° are square. Derived from crystalline V ₂ O ₅ (anhydrous). That is, the vanadium oxide contained in the positive electrode active material layer does not contain moisture (for example, crystal water). If it carries out like this, inactivation of the negative electrode which arises when water moves and reacts on a negative electrode can be suppressed more, and a battery characteristic can be improved more by extension. Here, the 2θ value in which a diffraction peak exists is intended to have V ₂ O ₅ in which the presence of water is reduced, and a peak shift of about 1 ° above and below is allowed so as to be expressed as “near”. It shall be. Further, when the positive electrode active material layer is subjected to nitrogen adsorption measurement and analyzed by the BJH method, the average pore volume Vd, which is the average value of the Log differential pore volume in a predetermined pore radius range including a pore radius of 2 nm, is 1. It shall be 50 × 10 ⁻² cm ³ / g or more. In this way, it is possible to improve the contact between the positive electrode active material and the electrolytic solution, and occlusion / release of polyvalent metals caused by structural changes (densification, crystallization, etc.) that accompany removal of water. Can be further suppressed. The average pore volume Vd near 2 nm is measured by measuring the nitrogen adsorption isotherm of the positive electrode active material layer, analyzing the measurement result by the BJH method, and using the obtained analysis result, the point closest to the pore radius of 2 nm And the average value of Log differential pore volume (cm ³ / g) at two points (5 points in total) including this center. A range of two points (5 points in total) including the center at the point closest to the pore radius of 2 nm corresponds to the “predetermined pore radius range including the pore radius of 2 nm”. The average pore volume Vd near 2 nm is more preferably 4.0 × 10 ⁻² cm ³ / g or more, and still more preferably 6.0 × 10 ⁻² cm ³ / g or more. The average pore volume Vd in the vicinity of 2 nm is preferably 1.0 × 10 ⁻¹ cm ³ / g or less from the viewpoint of easy preparation of the positive electrode active material layer.

  In the positive electrode active material layer, the weight ratio Wc / Wv, which is the ratio of the weight Wc of the carbon material to the weight Wv of the vanadium oxide, is preferably 0.30 or more and 1.30 or less, and 0.80 or more and 1. More preferably, it is 20 or less. When the weight ratio Wc / Wv is 1.30 or less, the amount of vanadium oxide involved in the battery reaction is relatively large, and the ratio of vanadium oxide to the carbon material is suitable and preferable. This carbon material may function as a conductive material. In this way, the conductivity can be further ensured when the weight ratio Wc / Wv is 0.30 or more. Examples of the carbon material include graphite such as natural graphite (scale-like graphite, scale-like graphite) and artificial graphite, carbon black such as acetylene black and ketjen black, carbon whisker, needle coke, carbon fiber, activated carbon and the like, or What mixed 2 or more types can be used. Of these, acetylene black and ketjen black are more preferable.

  The conductive material included in the positive electrode is not particularly limited as long as it is an electron conductive material that does not adversely affect the battery performance of the positive electrode. For example, the above-described carbon materials and metals (copper, nickel, aluminum, silver, gold, etc.) What mixed 1 type, or 2 or more types etc. can be used. Among these, as the conductive material, carbon black and acetylene black are preferable from the viewpoints of electron conductivity and coatability. The binder serves to bind the active material particles and the conductive material particles. For example, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluorine-containing resin such as fluorine rubber, or polypropylene, Thermoplastic resins such as polyethylene, ethylene-propylene-dienemer (EPDM), sulfonated EPDM, natural butyl rubber (NBR) and the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of cellulose or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. Examples of the solvent for dispersing the positive electrode active material, the conductive material, and the binder include N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, and N, N-dimethylaminopropyl. Organic solvents such as amine, ethylene oxide, and tetrahydrofuran can be used. Moreover, a dispersing agent, a thickener, etc. may be added to water, and an active material may be slurried with latex, such as SBR. As the thickener, for example, polysaccharides such as carboxymethyl cellulose and methyl cellulose can be used alone or as a mixture of two or more. Examples of the application method include roller coating such as applicator roll, screen coating, doctor blade method, spin coating, bar coater, and the like, and any of these can be used to obtain an arbitrary thickness and shape. Current collectors include aluminum, titanium, stainless steel, nickel, iron, noble metals, calcined carbon, conductive polymers, conductive glass, etc., as well as aluminum and titanium for the purpose of improving adhesion, conductivity and oxidation resistance. What processed the surface of copper etc. with carbon, nickel, titanium, silver, etc. can be used. For these, the surface can be oxidized. The shape of the current collector can be the same as that of the negative electrode.

  As the ion conductive medium of the secondary battery of the present invention, a non-aqueous electrolyte solution containing a supporting salt, a non-aqueous gel electrolyte solution, or the like can be used. Examples of the solvent for the nonaqueous electrolytic solution include carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Chain carbonates such as butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyllactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate, ethers such as dimethoxyethane, ethoxymethoxyethane, and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as lan, methyltetrahydrofuran, sulfolanes such as sulfolane and tetramethylsulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Of these, acetonitrile is more preferred.

As the supporting salt included in the secondary battery of the present invention, for example, a salt containing a polyvalent metal can be used, and for example, a perchlorate of a polyvalent metal can be used. In addition, for example, M (SO ₂ CF ₃ ) _n (M is a polyvalent metal, n is a value associated with the valence of M), borofluoride (M (BF ₄ ) _n ), trifluoromethylsulfonic acid Examples thereof include salts such as a salt (M (CF ₃ SO ₃ ) _n ) and hexafluorophosphate (M (PF ₆ ) _n ), and one or more salts may be used in combination. When the polyvalent metal is magnesium, for example, Mg (ClO ₄ ) ₂ , Mg (SO ₂ CH ₃ ) ₂ , Mg (BF ₄ ) ₂ , Mg (CF ₃ SO ₃ ) ₂ , Mg (PF ₆ ) _2, etc. Of these, it is preferable to use Mg (ClO ₄ ) ₂ . The supporting salt preferably has a concentration in the non-aqueous electrolyte of 0.1 mol / L or more and 5 mol / L or less, and more preferably 0.5 mol / L or more and 2 mol / L or less. If the concentration of the supporting salt is 0.1 mol / L or more, a sufficient current density can be obtained, and if it is 5 mol / L or less, the electrolytic solution can be made more stable. Moreover, you may add flame retardants, such as a phosphorus type and a halogen type, to this non-aqueous electrolyte.

  Further, instead of the liquid ion conducting medium, a solid ion conducting polymer may be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, and polyvinylidene fluoride and a supporting salt can be used. Further, an ion conductive polymer and a non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, an inorganic solid powder bound by an organic binder, or the like can be used as the ion conductive medium.

  The secondary battery of the present invention may include a separator between the negative electrode and the positive electrode. The separator is not particularly limited as long as it has a composition that can withstand the usage range of the secondary battery. For example, a polymer nonwoven fabric such as a polypropylene nonwoven fabric or a polyphenylene sulfide nonwoven fabric, or an olefin resin such as polyethylene or polypropylene. The microporous film of this is mentioned. These may be used alone or in combination.

  The shape of the secondary battery of the present invention is not particularly limited, and examples thereof include a coin type, a button type, a sheet type, a laminated type, a cylindrical type, a flat type, and a square type. Moreover, you may apply to the large sized thing etc. which are used for an electric vehicle etc.

  Next, the manufacturing method of the positive electrode of the secondary battery of this invention using a polyvalent metal is demonstrated. The positive electrode manufacturing method of the present invention includes a mixing step of preparing a mixed material in which vanadium and a carbon material are mixed, and a heat treatment step of heat-treating the mixed material obtained in the mixing step at 275 ° C. or higher in an oxidizing atmosphere. It is a waste.

(Mixing process)
In this step, vanadium and a carbon material are mixed to prepare a mixed material. As vanadium, metal powder, salt, oxide, or the like can be used. Examples of the carbon material include graphite such as natural graphite (scale-like graphite, scale-like graphite) and artificial graphite, carbon black such as acetylene black and ketjen black, carbon whisker, needle coke, carbon fiber, activated carbon and the like. A seed or a mixture of two or more kinds can be used. Among these, acetylene black, ketjen black, etc. are more preferable from a viewpoint of improving electroconductivity. When mixing acetylene black and ketjen black, empirically determine the mixing ratio of acetylene black, which is solid particles and not easily burned down by heat treatment, and ketjen black, which is hollow particles and easily burned off by heat treatment. Thus, characteristics such as the pore volume of the positive electrode active material layer can be easily controlled. In this mixing step, a mixed material may be prepared by generating a sol containing vanadium from a vanadium complex solution in which vanadium is dissolved and then mixing a carbon material with the sol. As the vanadium complex solution, for example, an aqueous hydrogen peroxide solution is preferable because it is easily solated thereafter. Alternatively, in the mixing step, the mixed material may be prepared by mixing a vanadium complex solution in which vanadium is dissolved and a carbon material, and then generating a sol containing vanadium. By doing so, vanadium and the carbon material can be mixed more uniformly.

(Heat treatment process)
In this step, the mixed material is heat-treated at 275 ° C. or higher in an oxidizing atmosphere. The heat treatment temperature is 275 ° C. or higher, but is preferably 300 ° C. or higher. When the treatment is performed at 275 ° C. or higher, moisture contained in vanadium oxide can be further removed, which is preferable. Moreover, it is considered that a part of the carbon material is combusted and the positive electrode active material layer can be made porous (particularly, the average pore volume Vd near 2 nm can be further increased), which is preferable. FIG. 1 is a conceptual diagram of a process of mixing a carbon material after solating a vanadium complex. As shown in FIG. 1, it is presumed that the carbon material is burned out by the heat treatment, whereby the pore volume is increased and the contact area with the ion conductive medium is increased. Generally, vanadium oxide has a layered structure containing crystal water, and can absorb and release polyvalent metals smoothly. Moreover, when water elutes and moves to the negative electrode, the negative electrode may be deactivated by forming an oxide or hydroxide on the negative electrode surface. Here, the moisture contained in the vanadium oxide is removed to suppress such inactivation of the negative electrode, and the pore volume is increased to improve the contact between the active material and the ion conductive medium. This suppresses a decrease in the mobility of the valent metal. As a result, the battery characteristics can be further improved. In this heat treatment step, when the carbon material also serves as a conductive material, it is preferably treated at a temperature that does not completely burn out, for example, preferably 400 ° C. or lower, more preferably 350 ° C. or lower, and more preferably 325 ° C. or lower. preferable. Further, in the heat treatment step, after the mixed material is formed on the current collector, the current collector may be heat treated. In this case, the fixing process of the active material layer on the current collector and the heat treatment of the mixed material can be performed at the same time, and the positive electrode active material layer can be manufactured by a simpler process. When the mixed material is formed on the current collector, the above-described conductive material or binder may be mixed in advance with the mixed material. Alternatively, when heat treatment is performed without forming a mixed material on the current collector, for example, the mixed material is placed in a container and heat treatment is performed, and the obtained mixed material is mixed with a conductive material or a binder. The positive electrode active material layer may be formed on the current collector.

  In the mixing step, after mixing the vanadium complex solution and the carbon material, and preparing the mixed material by generating a sol containing vanadium, in this heat treatment step, the mixed material obtained in the mixing step is used. Heat treatment may be performed at 250 ° C. or higher in an inert atmosphere. Even in this case, it is possible to suppress the inactivation of the negative electrode by removing water, and by increasing the pore volume, the contact between the active material layer and the ion conductive medium is increased, and the mobility of the polyvalent metal is increased. Can be suppressed. This is because, for example, as shown in FIG. 2, vanadium and the carbon material are uniformly dispersed, the vanadium complex is adsorbed on the surface of the carbon material, and the vanadium oxide is formed on the surface of the carbon material when it is solated. By coating, it is presumed that the pore volume can be increased even under an inert atmosphere. FIG. 2 is a conceptual diagram of a process for forming a sol after mixing a vanadium complex and a carbon material. Here, the heat treatment is described as being performed in an inert atmosphere, but heat treatment may be performed in an oxidizing atmosphere. Even in this case, the pore volume can be increased. Moreover, when heat-processing, it is preferable to process at 250 degreeC for 4 hours or more from a viewpoint of removing a water | moisture content. As described above, the positive electrode of the present invention can be produced.

  In the secondary battery and the method for manufacturing the positive electrode thereof according to the present embodiment described in detail above, the battery characteristics can be further improved in the case where the multivalent metal is occluded and released including vanadium. This is because the inactivation of the negative electrode can be suppressed by removing water, and the active material layer and the ion conducting medium are increased by increasing the pore volume (especially the average pore volume Vd near 2 nm). It is presumed that this is because the lowering of the occlusion / release property of the polyvalent metal can be suppressed by improving the contact property with the metal.

  It should be noted that the present invention is not limited to the above-described embodiment, and it goes without saying that the present invention can be implemented in various modes as long as it belongs to the technical scope of the present invention.

  Below, the example which produced the secondary battery of this invention concretely is demonstrated as an experiment example.

(Preparation of positive electrode active material)
0.1 g of vanadium (manufactured by High Purity Chemical Laboratory, 99.9%) is added to ice-cooled 10 ml of hydrogen peroxide water (manufactured by Wako Pure Chemical Industries, Ltd., 30% for precision analysis) and dissolved to obtain a vanadium complex solution. Was made. The resulting vanadium complex solution to decompose excess hydrogen peroxide was held at room temperature to produce a V ₂ O ₅ sol to obtain a vanadium sol solution containing V ₂ O ₅ sol. Next, 1 ml of the obtained vanadium sol solution, 1 ml of pure water, 1 ml of acetone, and 23.2 mg of acetylene black (Denka Black manufactured by Denki Kagaku Kogyo Co., Ltd., hereinafter also referred to as “ab”) are used with a homogenizer (T-25 manufactured by Ika). Mixed materials were prepared by mixing. The mixing conditions were 24000 rpm and dispersion for 5 minutes. The abbreviation of the obtained mixed material is referred to as V ₂ O ₅ / ab-1. This V ₂ O ₅ / ab-1 was a weight ratio V ₂ O ₅ : ab = 1: 1.3.

4. 1 ml of the obtained vanadium sol solution, 1 ml of pure water, 1 ml of acetone, 23.2 mg of acetylene black, Ketjen Black (Ketjen Black EC manufactured by Ketjen Black International, hereinafter also referred to as kb) 3 mg was mixed with a homogenizer under the same conditions as described above to prepare a mixed material. The abbreviation of the obtained mixed material is referred to as V ₂ O ₅ / ab-2. This V ₂ O ₅ / ab-2 was a weight ratio V ₂ O ₅ : ab: kb = 1: 1.3: 0.3.

In addition, 1 ml of the prepared vanadium complex solution was ice-cooled, and 1 ml of pure water, 1 ml of acetone, and 23.2 mg of acetylene black were added thereto, and the liquid temperature was gradually raised to room temperature while stirring with a magnetic stirrer to make excess. An amount of hydrogen peroxide was decomposed to prepare a mixed material of V ₂ O ₅ sol and acetylene black. The abbreviation of the obtained mixed material is referred to as V ₂ O ₅ / ab-3. This V ₂ O ₅ / ab-3 was a weight ratio V ₂ O ₅ : ab = 1: 1.3.

(Preparation of positive electrode)
Using a Pt plate as a current collector, the mixed materials V ₂ O ₅ / ab-1, V ₂ O ₅ / ab-2 and V ₂ O ₅ prepared as described above so that the V ₂ O ₅ sol was about 100 μg. Any one of / ab-3 is supported on a Pt plate, heat-treated under atmospheric conditions and temperature conditions described later, and pressure-molded at 100 MPa after the heat treatment, and the resultant was used as a positive electrode.

(Production of battery)
Using the positive electrode described above and two magnesium ribbons (99.9%, high purity chemical research laboratory, size 3.5 cm × 0.32 cm × 0.24 mm) as the counter electrode, Pt foil is wound around the end and lead A line. As the separator, a PE single layer (thickness 25 μm, porosity 37%) manufactured by Tonen Chemical was used. In order to prevent the supported positive electrode active material layer from falling off, a PTFE plate was placed outside these components and sandwiched between them to constitute an electrode system. Ag wire was used as a reference electrode. This electrode system was put into a beaker, and 1M (mol / L) magnesium perchlorate (manufactured by Wako Pure Chemical Industries, Ltd., for elemental analysis) dissolved in acetonitrile (manufactured by Wako Pure Chemical Industries, Ltd., dehydrated) was injected as an electrolyte. A battery was constructed. Made of acrylic to supply high-purity nitrogen (> 99.999%, dew point <-70 ° C., oxygen <0.5 ppm) to prevent moisture contamination during measurement and to prevent evaporation of the electrolyte. A battery was put in a sealed glass container in a glove box (manufactured by ASONE, positive pressure type) and used.

(Examination of weight change due to heat treatment of V ₂ O ₅ )
1 ml of the obtained vanadium sol solution, 1 ml of pure water and 1 ml of acetone are added and mixed by stirring with a magnetic stirrer, and coated on a Pt plate so that V ₂ O ₅ is about 600 μg, and at 60 ° C. Dried for 1 hour. Prepare a plurality of these samples, and heat treatment conditions are 1 hour at 250 ° C., 2 hours, 3.5 hours, 6 hours, 8 hours, 1 hour at 300 ° C., 2 hours, 3 hours, 4 hours, 1 hour at 350 ° C. Each was heat-treated in air for 2 hours, 4 hours. Then, the weight after drying at 60 ° C. was defined as the initial weight W ₀ , the weight after the heat treatment was defined as the post-treatment weight W, and the weight reduction rate W / W ₀ was determined. FIG. 3 is a diagram showing the relationship between the heat treatment condition of V ₂ O ₅ and the weight change. As shown in FIG. 3, it was found that a constant weight was exhibited when a predetermined time or more passed under any heat treatment condition. For this reason, it was guessed that the weight loss by this heat treatment represents the amount of water removed from V ₂ O ₅ . From this result, it was found that the moisture contained in V ₂ O ₅ was almost completely removed by heat treatment at 250 ° C. for 3.5 hours or more and at 300 ° C. or more for 1 hour or more. Further, from this weight reduction, it was possible to estimate that the initial composition was V ₂ O ₅ · 1.9H ₂ O.

(Examination of weight change due to heat treatment of carbon materials)
2 ml of pure water, 1 ml of acetone, and 23.2 mg of a carbon material (either ab or kb) were added and mixed by stirring with a magnetic stirrer, coated on a Pt plate, and dried at 60 ° C. for 1 hour. . Several samples were prepared. The ab sample was heat-treated in nitrogen or air, and the kb sample was heat-treated in air. The ab and kb samples were heat-treated at 250 ° C., 275 ° C., 300 ° C., 325 ° C. and 350 ° C. for 4 hours, respectively. Then, the weight after drying at 60 ° C. was defined as the initial weight W ₀ , the weight after the heat treatment was defined as the post-treatment weight W, and the weight reduction rate W / W ₀ was determined. FIG. 4 is a diagram illustrating the relationship between the heat treatment condition of the carbon material and the weight change. As shown in FIG. 4, weight reduction hardly occurred in nitrogen, but weight reduction was observed at 250 ° C. or higher in the atmosphere that is an oxidizing atmosphere. For this reason, it was guessed that the weight reduction by this heat processing represents the amount of carbon burnt down. From this result, the susceptibility to burning differs depending on the type of carbon material. For example, ab shows a weight reduction ratio W / W ₀ of 0.85 (15% burned out) at 300 ° C. for 4 hours and kb shows 300 ° C. It was found that the weight reduction ratio W / W ₀ showed 0.00 (100% burned out) in 4 hours. Note that kb is hollow spherical carbon, which is easily burned and burned, whereas ab is solid spherical carbon, which is difficult to burn. Therefore, it was found that the pore diameter distribution of the positive electrode active material layer can be controlled by mixing kb, which has a high degree of oxidation, and ab, which has a low degree of oxidation, in an empirical amount.

(X-ray diffraction measurement)
Samples obtained by preparing a plurality of mixed materials V ₂ O ₅ / ab-1 coated on a glass plate and heat-treating them at 60 ° C., 120 ° C., 200 ° C. and 250 ° C. for 8 hours in a nitrogen atmosphere, respectively. Were each measured by X-ray diffraction. The X-ray diffraction measurement was performed using CuKα rays with an X-ray diffractometer (RINT-TTR manufactured by Rigaku) in a range of 2θ of 5 ° to 75 °. FIG. 5 is a measurement result of X-ray diffraction of the mixed material after the heat treatment. As shown in FIG. 5, in the heat treatment up to 200 ° C., a broad diffraction peak derived from acetylene black or the like near 2θ = 26 °, 30 °, and 2θ = 6.5 °, around 24.2 °. A diffraction peak derived from V ₂ O ₅ .1.6H ₂ O was obtained. That is, V ₂ O ₅ was in a state having crystal water. On the other hand, in the heat treatment at 250 ° C. for 8 hours, the peak derived from V ₂ O ₅ .1.6H ₂ O is greatly reduced, and 2θ = from orthorhombic V ₂ O ₅ (anhydrous). The diffraction peaks of 12.2 ° and 29.2 ° and the diffraction peaks of 2θ = 15.4 ° and 20.3 ° derived from tetragonal V ₂ O ₅ (anhydrous) have been obtained. . That is, it was found that most of the crystal water can be removed from the structure of V ₂ O ₅ . Considering the characteristic changes due to these basic heat treatments, the heat treatment conditions shown in the following experimental examples were set, and the battery characteristics were examined.

[Experimental Example 1]
The mixed material V ₂ O ₅ / ab-1 supported on a Pt plate and heat-treated at 250 ° C. for 4 hours in a nitrogen atmosphere was used as the positive electrode of Experimental Example 1. Using this positive electrode, the battery obtained through the above-described production process was used as the secondary battery of Experimental Example 1.

[Experimental Examples 2 to 5]
The mixed materials V ₂ O ₅ / ab-1 were supported on a Pt plate and heat-treated at 275 ° C., 300 ° C., 325 ° C., and 350 ° C. for 4 hours in the atmosphere, respectively. No. 5 positive electrode. Using these positive electrodes, the batteries obtained through the above-described production steps were used as secondary batteries of Experimental Examples 2 to 5, respectively.

[Experimental Example 6]
The mixed material V ₂ O ₅ / ab-2 supported on a Pt plate and heat-treated at 300 ° C. for 4 hours in an air atmosphere was used as the positive electrode of Experimental Example 6. Using this positive electrode, a battery obtained through the above-described production process was used as a secondary battery of Experimental Example 6.

[Experimental Example 7]
The mixed material V ₂ O ₅ / ab-3 supported on a Pt plate and heat-treated at 250 ° C. for 4 hours in a nitrogen atmosphere was used as the positive electrode of Experimental Example 7. Using this positive electrode, the battery obtained through the above-described production process was used as the secondary battery of Experimental Example 7.

[Experimental Examples 8 to 10]
The mixed material V ₂ O ₅ / ab-1 was supported on a Pt plate and heat-treated at 60 ° C., 120 ° C., and 200 ° C. for 4 hours in a nitrogen atmosphere, respectively. It was. Using these positive electrodes, the batteries obtained through the above-described preparation steps were used as secondary batteries of Experimental Examples 8 to 10, respectively.

[Experimental Example 11]
2 ml of pure water, 1 ml of acetone, and 23.2 mg of acetylene black were added and mixed by stirring with a magnetic stirrer, coated on a Pt plate, dried at 60 ° C. for 1 hour, and then at 250 ° C. in a nitrogen atmosphere. The product obtained by heat treatment for 4 hours was used as the positive electrode of Experimental Example 11. Using this positive electrode, the battery obtained through the above-described production process was used as the secondary battery of Experimental Example 11.

[Experimental example 12]
1 ml of the obtained vanadium sol solution, 1 ml of pure water and 1 ml of acetone are added and mixed by stirring with a magnetic stirrer, and coated on a Pt plate so that V ₂ O ₅ is about 600 μg, and at 60 ° C. Dried for 1 hour. Thereafter, a positive electrode of Experimental Example 12 was obtained by heat treatment at 250 ° C. for 4 hours in a nitrogen atmosphere. Using this positive electrode, a battery obtained through the above-described production process was used as a secondary battery of Experimental Example 12.

[Experimental Examples 13 to 16]
Except that the atmosphere during the heat treatment was changed to a nitrogen atmosphere, positive electrodes and secondary batteries obtained through the same steps as in Experimental Examples 2 to 5 were set as Experimental Examples 13 to 16, respectively.

(Battery characteristics measurement)
Using the secondary battery configured as described above, current-potential (IV) characteristics were measured. Three cycle potential sweep at a sweep rate of 0.2 mV · s ^{−1 in} the potential range of −0.4 V to 1.5 V (2.0 V to 3.9 V based on Mg / Mg ²⁺ ) at the Ag reference potential. In the third cycle, in the potential range of 2.0 V to 3.5 V on the Mg / Mg ²⁺ basis, when the potential is swept in the noble direction and when the potential was swept in the base direction The amount of electricity Q = (Qa−Qc) / 2 was calculated as the difference from the amount of electricity Qc that flowed. In addition, a battery was constructed in the same manner as described above using a positive electrode carrying only acetylene black, and the current-potential measurement was performed to determine the amount of electricity Qe generated by the electric double layer capacity formed at the acetylene black / electrolyte interface. It was. When the quantity of electricity (Q-Qe) generated by doping (de-doping) with Mg / Mg ²⁺ is calculated, and the valence of all the supported V ₂ O ₅ changes between V ³⁺ and V ⁵⁺ The vanadium utilization rate Urv = (Q−Qe) / Q ₀ was calculated from the ratio of the amount of electricity to Q ₀ .

(Measurement of pore distribution)
The pore distribution measurement of the positive electrode active material layers of Experimental Examples 1 to 7 and Experimental Examples 11 and 12 was performed. The pore distribution was obtained by measuring a nitrogen adsorption isotherm using AUTOSORP-1 manufactured by QUANTACHROME and analyzing the measurement result by the BJH method. Using the obtained analysis results, calculate the average value of the Log differential pore volume (cm ³ / g) at the two points on the both sides including the center (5 points in total) centered on the point closest to the pore radius of 2 nm. The obtained value was defined as the average pore volume Vd (cm ³ / g). FIG. 6 is a pore distribution curve of each sample. As shown in FIG. 6, in the sample of only V ₂ O ₅ or acetylene black, the average pore volume Vd (cm ³ / g) in the region near the pore radius of 2 nm was 0. In contrast, in Experimental Example 1, there is a small amount of average pore volume Vd in the region near the pore radius of 2 nm. In Experimental Examples 2 to 7, this average pore volume Vd is 1.50 × 10 ⁻² (cm ³ / g) or more.

(Calculation of weight ratio Wc / Wv)
The weight ratio Wc / Wv between the weight Wc of carbon after the heat treatment in Experimental Examples 1 to 16 and the weight Wv of V ₂ O ₅ was calculated. The weight ratio is calculated by measuring the weight of the positive and negative electrodes before and after heat treatment, and calculating the weight reduction amount in the heat treatment. The value obtained by subtracting the weight loss from the weight reduction amount is the carbon material weight loss and is included in the mixing. The weight of the carbon material remaining in each experimental example was calculated by subtracting the weight loss of the carbon material from the weight of the carbon material. Then, the weight ratio Wc / Wv was calculated by dividing the weight of the remaining carbon material by the weight of V ₂ O ₅ contained during mixing. In addition, the weight ratio Wc / Wv was calculated on the assumption that there was no weight loss of the carbon material when heat-treated in a nitrogen atmosphere.

(Measurement results and discussion)
Table 1 shows the contents of each sample, heat treatment conditions, weight ratio Wc / Wv of carbon / V ₂ O ₅ after heat treatment, average pore volume Vd (cm ³ / g), and vanadium utilization rate for Experimental Examples 1 to 11. The experimental examples 1, 12 to 15 are shown in Table 2. The relationship between the heat treatment temperature and the vanadium utilization rate is shown in FIG. As shown in FIG. 7, in Experimental Examples 8 to 10 having a low heat treatment temperature, since water of crystallization was present in V ₂ O ₅ , the vanadium utilization rate was a relatively high value of about 0.2 or more. However, in Experimental Examples 8 to 10, when repeated charging and discharging are performed, it is predicted that a defect due to the crystal water occurs. When this positive electrode active material layer was heat-treated at 250 ° C., moisture could be completely removed, but the vanadium utilization rate decreased to 0.12 (Experimental Example 1). Moreover, in Experimental Examples 13-16 heat-processed in nitrogen, even if the heat processing temperature was 250 degreeC or more, the vanadium utilization factor did not improve. This was presumed to be due to the fact that magnesium was not able to be fully occluded / released by removing water of crystallization from V ₂ O ₅ . Further, as shown in FIG. 4, it is presumed that this is because, in a nitrogen atmosphere, there is almost no weight loss of carbonaceous material even when the heat treatment temperature is increased, that is, no increase in pore volume occurs. On the other hand, when heat treatment at 275 ° C. or higher was performed in the air atmosphere, it was found that the vanadium utilization rate was improved as compared with Experimental Example 1 where heat treatment was performed at 250 ° C. This improvement in the vanadium utilization rate was presumed to be related to an increase in the average pore volume Vd near 2 nm. The reason why the vanadium utilization rate decreases when heat-treated at 325 ° C. or higher in the air atmosphere is that, under this experimental condition, the amount of acetylene black responsible for conductivity in the battery reaction is increased and the conductivity is decreased. Therefore, it was speculated that the vanadium utilization rate seemed to decrease. For this reason, it was expected that if a conductive material is further added after the heat treatment of the mixed material, the vanadium utilization rate can be suppressed from being lowered even if the heat treatment is performed at 325 ° C. or higher. In Experimental Example 6 using acetylene black and ketjen black, the average pore volume Vd and vanadium utilization rate were highest when the pore radius was around 2 nm, and the contact between the electrolyte and the positive electrode active material was better. It was guessed that. In Experimental Example 7 in which vanadium sol was formed after acetylene black was added to the vanadium complex solution, the uniform dispersibility of vanadium and the carbon material was the best, and despite the heat treatment at 250 ° C. in a nitrogen atmosphere. The average pore volume Vd was 1.50 × 10 ⁻² cm ³ / g or more, and the vanadium utilization rate exceeded 0.3. From these facts, if V ₂ O ₅ has no water and the average pore volume Vd near 2 nm is 1.50 × 10 ⁻² cm ³ / g or more, it contains vanadium and a polyvalent metal. It has been clarified that the battery characteristics, particularly the vanadium utilization rate, can be further improved in those that absorb and release.

  The present invention can be used in the field of manufacturing secondary batteries.

Claims (6)

A method for producing a positive electrode of a secondary battery using a polyvalent metal,
A mixing step of preparing a mixed material in which vanadium and a carbon material are mixed by forming a sol containing vanadium after mixing the vanadium complex solution and the carbon material ;
A heat treatment step of heat-treating the mixed material obtained in the mixing step at 250 ° C. or higher in an inert atmosphere;
The manufacturing method of the positive electrode containing this. The method for producing a positive electrode according to claim 1 , wherein, in the heat treatment step, the current collector is heat-treated after the mixed material is formed on the current collector. The method for producing a positive electrode according to claim 1 or 2 , wherein in the mixing step, the mixed material is prepared using at least one or more of acetylene black and ketjen black as the carbon material. A secondary battery using a polyvalent metal,
A negative electrode capable of inserting and extracting the polyvalent metal;
A positive electrode produced by the method for producing a positive electrode according to any one of claims 1 to 3, and having a positive electrode active material layer containing a positive electrode active material containing vanadium and a carbon material,
A non-aqueous ion conducting medium that is interposed between the negative electrode and the positive electrode and conducts ions of the polyvalent metal,
The positive electrode active material layer has diffraction peaks at 2θ of 12.2 °, 15.4 °, 20.3 °, and 29.2 ° as measured by X-ray diffraction,
When the positive electrode active material layer is subjected to nitrogen adsorption measurement and analyzed by the BJH method, the average pore volume Vd, which is the average value of the Log differential pore volume in a predetermined pore radius range including a pore radius of 2 nm, is 1.50. × 10 ⁻² cm ³ / g or more,
Secondary battery. The secondary battery according to claim 4 , wherein the polyvalent metal is at least one of magnesium and aluminum. The positive active material layer, the weight ratio Wc / Wv is the ratio of the vanadium oxide weight Wv carbon weight Wc is at least 0.30 to 1.30, according to claim 4 or 5 secondary battery according.