JP2018060625A - Electrode material including oxide ion functioning as movable ion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery in which an entirely new electrode reaction mechanism that is not based on a sodium ion insertion/elimination reaction is used, in order to achieve a power storage device low in cost, high in energy and high in output.SOLUTION: An electrode material for a secondary battery contains an oxide having a composition represented by formula (I). The oxide preferably has an α-PbOtype structure, and M and X preferably represent Fe and Nb, respectively. MXO(I) (where, M represents V, Cr, Mn, Fe, or Co; and X represents Nb, Mo, Ru, Rh, or Pd).SELECTED DRAWING: None

Description

  The present invention relates to an electrode material used for a secondary battery using oxide ions as movable ions. The present invention also relates to a secondary battery electrode including the electrode material and a secondary battery including the electrode.

  Against the backdrop of strong demand for large power storage devices such as electric vehicles and renewable energy output leveling, the development of low-cost large-scale secondary batteries that do not use expensive rare elements is expected. However, the currently developed lithium ion battery requires the use of rare lithium, and thus has a problem that it is difficult to reduce the cost and size.

  On the other hand, a sodium ion battery has been developed as an alternative technology to such a lithium ion battery (for example, Patent Document 1). Although sodium ion batteries are expected to reduce costs because sodium is used instead of lithium, on the other hand, the structure is large and heavy sodium ions are inserted and removed from the electrode material during charge and discharge. At present, there is a problem that the electrode characteristics are easily deteriorated due to the change, and it is difficult to improve the energy density per weight.

  Therefore, there is a demand for the construction of a completely new electrode reaction mechanism that can be used in sodium ion batteries without being based on sodium ion insertion and desorption reactions.

JP 2009-266821 A

  Therefore, the present invention has an object to provide a secondary battery using a completely new electrode reaction mechanism that is not based on the insertion / desorption reaction of sodium ions in order to realize a low-cost, high-energy, high-power storage device. It is what. In particular, an object of the present invention is to develop a low-cost electrode active material that does not use a rare metal and to provide an electrode material used for such a secondary battery.

As a result of diligent studies to solve the above-mentioned problems, the present inventors have newly found that a metal oxide having an α-PbO ₂ type structure functions as an electrode active material by an oxide ion desorption insertion reaction. The present invention has been completed.

That is, the present invention in one aspect,
<1> The following formula (I):
MXO ₄ (I)
(Wherein M is V, Cr, Mn, Fe, or Co; X is Nb, Mo, Ru, Rh, or Pd), and an electrode including an oxide having a composition represented by: material;
<2> The electrode material according to <1>, wherein the oxide has an α-PbO ₂ type structure;
<3> The electrode material according to <1> or <2> above, wherein M is Fe; and <4> X is Nb, according to any one of the above <1> to <3>. Electrode material;
Is to provide.

In another aspect, the invention provides:
<5> An electrode for a secondary battery comprising the electrode material according to any one of <1> to <4>, wherein the oxide is an electrode active material;
<6> A secondary battery comprising the electrode according to <5> as a positive electrode or a negative electrode; and <7> a secondary battery according to <6>, wherein the secondary battery is a sodium ion battery.

  ADVANTAGE OF THE INVENTION According to this invention, the electrode using the completely new electrode reaction mechanism of insertion and desorption of an oxide ion which is not based on the insertion and desorption reaction of sodium ion can be provided.

As a result, it is possible to achieve a low-cost, high-energy, high-output power storage device without using rare metals such as lithium and solving the problems of low energy density and cycle deterioration in conventional sodium ion batteries. .

FIG. 1 is a graph showing a powder X-ray diffraction pattern of FeNbO ₄ which is an active material in the electrode material of the present invention. FIG. 2 is a scanning electron microscope image (SEM image) of FeNbO ₄ which is an active material in the electrode material of the present invention. FIG. 3 is a graph showing (a) a charge / discharge curve obtained with an electrode using FeNbO ₄ of the present invention as an active material, and (b) a graph showing cycle characteristics. FIG. 4 is a graph showing a Mossbauer spectrum of FeNbO ₄ during charge and discharge. FIG. 5 is a graph showing an X-ray diffraction pattern of FeNbO ₄ during charge and discharge. FIG. 6 is a graph showing the O1sXPS spectrum of FeNbO ₄ .

  Hereinafter, embodiments of the present invention will be described. The scope of the present invention is not limited to these descriptions, and other than the following examples, the scope of the present invention can be appropriately changed and implemented without departing from the spirit of the present invention.

1. Electrode Material (1) Active Material The electrode material according to the present invention is a transition metal oxide having an α-PbO ₂ type structure as an active material. Specifically, the following formula (I):
MXO ₄ (I)
It is characterized by including the oxide which has the composition represented by these.

In the formula, M is an element selected from transition metals in the fourth period, specifically, V, Cr, Mn, Fe, or Co. M is preferably Fe (iron) because it is inexpensive, easily available, and has a low environmental burden. X is an element selected from transition metals in the fifth period, specifically Nb, Mo, Ru, Rh, or Pd, and preferably Nb. In the most typical example, M is Fe and X is Nb, in which case the oxide of formula (I) is FeNbO ₄ .

The oxide having the composition represented by the above formula (I) has a three-dimensional structure called “α-PbO ₂ type structure”. In such an α-PbO ₂ type structure, transition metals M and X are 6-coordinated to oxide ions. Meanwhile, coordinating oxide ions simultaneously is subjected to edge-sharing between MO ₆ octahedra or XO ₆ octahedra, is subjected to another MO ₆ octahedra or XO ₆ octahedra and vertices share 3 A dimensional structure is formed.

In the present invention, it has been found that the oxide of the above formula (I) having the α-PbO ₂ type structure functions as an electrode active material not by insertion / release reaction of sodium ions but by insertion / release reaction of oxide ions. It is characterized by this.

The specific electrode reaction of the oxide ion insertion / desorption reaction can be expressed as follows.
_{2 ×} Na ⁺ + 2xe ⁻ + MXO ₄ ⇔ (Na ₂ O) _x (MXO _4−x )
In the formula, x is in the range of 0 <x <4.

  The oxide of formula (I) contained in the electrode material of the present invention is prepared by dissolving M and X in methanol containing citric acid and stirring at room temperature for 30 minutes, adding ethylene glycol and stirring at 80 ° C. for 5 hours, Then, it can obtain by baking at 350 degreeC for 3 hours and 500 degreeC for 5 hours. Such a firing step can be performed using means known in the art.

(2) Other electrode materials The electrode material of the present invention contains an oxide of the above formula (I) as an active material, and may contain only the oxide. In addition, in order to improve the rate characteristics of the electrode, etc., it may contain at least one of a known conductive material and binder. These can be carried on an electrode current collector to produce an electrode. For example, it can be produced by preparing a slurry containing an active material, a binder, etc., applying the slurry on a current collector, and drying it. The electrode active material used for an electrode can be used individually or in mixture of 2 or more types.

Examples of the conductive material that can be used include carbon materials, conductive fibers such as metal fibers, metal powders such as copper, silver, nickel, and aluminum, and organic conductive materials. As the carbon material, graphite, soft carbon, hard carbon, carbon black, ketjen black, acetylene black, graphite, activated carbon, carbon nanotube, carbon fiber and the like can be used. In addition, mesoporous carbon obtained by firing a synthetic resin containing an aromatic ring, petroleum pitch, or the like can also be used. Examples of organic conductive materials include polyphenylene derivatives,
Examples thereof include conductive polymers such as polypyrrole and polyacrylonitrile. Acetylene black is preferred from the viewpoint of high conductivity when used as an electrode.

  When the conductive material is contained, the electrode can be prepared by pulverizing and mixing it together with the active material. Such pulverization / mixing is not particularly limited, but can be performed using a pulverizer such as a vibration mill, a jet mill, or a ball mill. As a method for forming an electrode using a powdery active material, a doctor blade method, a molding method using a pressure press, or the like can be used.

  Moreover, in a preferable aspect, when an electroconductive material is contained, an active material can also be coat | covered with the said electroconductive material. As a method for performing such coating, the active material powder is immersed in a liquid containing a conductive material or a precursor of the conductive material, and then subjected to heat treatment to deposit the conductive material on the surface of the active material powder. A method is mentioned. Alternatively, it is also possible to use a technique in which a powder of an active material is flowed into a gas phase containing a conductive material or a precursor of a conductive material, and then heat treatment is performed as necessary. By covering the active material with a conductive material, a decrease in capacity when a large current is applied is suppressed, which is beneficial in high load characteristics.

  As the conductive material that can be used for the coating, carbon, graphite, vapor-grown carbon fiber, carbon nanotube, iron phosphide, conductive oxide, conductive polymer (polypyrrole, polyacrylonitrile, etc.) and the like can be used. However, it is not limited to these.

  Examples of the binder (binder) include fluorine resins such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and ethylenetetrafluoroethylene (ETFE); styrene-butadiene rubber (SBR), acrylonitrile-butadiene. Rubber-based binders such as rubber (NBR); or polyethylene, polypropylene and the like can be preferably used.

  As the electrode current collector, Al, Ni, Cu, stainless steel, or the like can be used. As a method of supporting the electrode mixture on the electrode current collector, it is fixed by press molding or pasting using an organic solvent, coating on the electrode current collector, drying and pressing. A method is mentioned. In the case of forming a paste, a slurry composed of an electrode active material, a conductive material, a binder, and an organic solvent is prepared.

  The electrode obtained from the electrode material of the present invention can be used as either a positive electrode or a negative electrode, but is preferably a positive electrode. In that case, a well-known electrode structure in the said technical field can be used as a negative electrode of a secondary battery.

  For example, when the secondary battery is a sodium ion battery, an electrode containing a negative electrode active material capable of electrochemically occluding and releasing sodium ions can be used as the negative electrode. As such a negative electrode active material, known negative electrode active materials for sodium ion secondary batteries can be used. For example, hard carbon, soft carbon, carbon black, ketjen black, acetylene black, activated carbon, carbon nanotube, carbon Examples thereof include carbonaceous materials such as fibers and amorphous carbon. Alternatively, a sodium ion metal or an alloy containing a sodium ion element, a metal oxide, a metal nitride, or the like can be used. Other components such as the conductive material and the binder are as described above.

  Even when the electrode obtained from the electrode material of the present invention is used as a negative electrode, a positive electrode known in the art can be used.

2. Secondary battery A secondary battery including an electrode obtained by the electrode material of the present invention is provided with an electrode containing the oxide of the above formula (I) as an active material as a positive electrode or a negative electrode, preferably as a positive electrode. The secondary battery includes a positive electrode, a negative electrode, an electrolytic solution, a separator, and other components.

(1) Electrolytic Solution In the secondary battery including the electrode obtained by the electrode material of the present invention, a known electrolytic solution can be used. For example, in the case of a sodium ion battery, the electrolytic solution is not particularly limited as long as it uses a sodium salt or a lithium salt as an electrolyte. For example, examples of the sodium salt used as the electrolyte include NaClO ₄ , NaPF ₆ , NaBF ₄ , NaNO ₃ , NaOH, NaCl, NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , Na ₂ SO ₄ and Na _2. S etc. are mentioned. These sodium salts can be used alone or in combination of two or more.

  Examples of the solvent in the electrolytic solution include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate, 4-trifluoromethyl-1,3-dioxolane-2- ON, carbonates such as 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyldifluoro Ethers such as methyl ether, tetrahydrofuran and 2-methyltetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide Amides such as N, N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide, 1,3-propane sultone, or the above organic solvents, and further fluorine substitution A group into which a group is introduced can be used. Among these, one type may be used alone, or two or more types may be used in combination. However, it is not limited to these.

  Moreover, a solid electrolyte can be used instead of the electrolytic solution. Examples of the solid electrolyte include polymer electrolytes such as polyethylene oxide polymer compounds, polymer compounds containing at least one polyorganosiloxane chain or polyoxyalkylene chain. Moreover, it can also be used with the form of the non-aqueous gel electrolyte which added and polymerized the said electrolyte solution. In the sodium ion secondary battery of the present invention, when a solid electrolyte is used, the solid electrolyte may serve as a separator described later, and in that case, the separator may not be required.

  Further, the electrolytic solution may contain other components as necessary for the purpose of improving the function thereof. Examples of the other components include conventionally known overcharge inhibitors, dehydrating agents, deoxidizing agents, capacity maintenance characteristics after high-temperature storage, and property improvement aids for improving cycle characteristics.

  Examples of the overcharge inhibitor include aromatic compounds such as biphenyl, alkylbiphenyl, terphenyl, partially hydrogenated terphenyl, cyclohexylbenzene, t-butylbenzene, t-amylbenzene, diphenyl ether, and dibenzofuran; 2-fluoro Partially fluorinated products of the above aromatic compounds such as biphenyl, o-cyclohexylfluorobenzene, p-cyclohexylfluorobenzene; fluorinated anisole such as 2,4-difluoroanisole, 2,5-difluoroanisole and 2,6-difluoroaniol Compounds. An overcharge inhibitor may be used individually by 1 type, and may use 2 or more types together.

  When electrolyte solution contains an overcharge inhibitor, it is preferable that content of the overcharge inhibitor in electrolyte solution is 0.01-5 mass%. By containing 0.1% by mass or more of the overcharge inhibitor in the electrolytic solution, it becomes easier to suppress the rupture / ignition of the secondary battery due to overcharge, and the secondary battery can be used more stably.

  Examples of the dehydrating agent include molecular sieves, mirabilite, magnesium sulfate, calcium hydride, sodium hydride, potassium hydride, lithium aluminum hydride and the like. As the solvent used in the electrolytic solution of the present invention, a solvent obtained by performing rectification after dehydrating with the above dehydrating agent may be used. Moreover, you may use the solvent which performed only the dehydration by the said dehydrating agent, without performing rectification.

  Examples of the characteristic improvement aid for improving capacity maintenance characteristics and cycle characteristics after high-temperature storage include succinic anhydride, glutaric anhydride, maleic anhydride, citraconic anhydride, glutaconic anhydride, itaconic anhydride, dihydrate Carboxylic anhydride such as glycolic acid, cyclohexanedicarboxylic anhydride, cyclopentanetetracarboxylic dianhydride, phenylsuccinic anhydride; ethylene sulfite, 1,3-propane sultone, 1,4-butane sultone, methanesulfonic acid Methyl, busulfan, sulfolane, sulfolene, dimethylsulfone, diphenylsulfone, methylphenylsulfone, dibutyldisulfide, dicyclohexyldisulfide, tetramethylthiuram monosulfide, N, N-dimethylmethanesulfonamide, N, N-diethylmethanes Sulfur-containing compounds such as honamide; 1-methyl-2-pyrrolidinone, 1-methyl-2-piperidone, 3-methyl-2-oxazolidinone, 1,3-dimethyl-2-imidazolidinone, N-methylsuccinimide, etc. Nitrogen compounds; hydrocarbon compounds such as heptane, octane, cycloheptane; fluorine-containing aromatic compounds such as fluorocarbonic acid ethylene (FEC), fluorobenzene, difluorobenzene, hexafluorobenzene, and benzotrifluoride. These characteristic improvement aids may be used alone or in combination of two or more. When the electrolytic solution contains a property improving aid, the content of the property improving aid in the electrolytic solution is preferably 0.01 to 5% by mass.

(2) Separator The separator used in the secondary battery of the present invention is not particularly limited as long as it has a function of electrically separating the positive electrode layer and the negative electrode layer. For example, polyethylene (PE) Examples thereof include a porous sheet made of a resin such as polypropylene (PP), polyester, cellulose, and polyamide, and a porous insulating material such as a nonwoven fabric such as a nonwoven fabric and a glass fiber nonwoven fabric.

(3) Shape, etc. The shape of the secondary battery of the present invention is not particularly limited as long as it can accommodate a positive electrode, a negative electrode, and an electrolytic solution. For example, a cylindrical type, a coin type, a flat plate type, a laminate type Etc. Further, the case for storing the battery may be an open-air battery case or a sealed battery case.

  In addition, although the secondary battery of this invention is suitable for the use as a secondary battery, using as a primary battery is not excluded.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these.

1. Synthesis of oxides Fe and Nb were dissolved in methanol containing citric acid and stirred at room temperature for 30 minutes. Further, ethylene glycol was added and stirred at 80 ° C for 5 hours, then 350 ° C for 3 hours and 500 ° C for 5 hours. By firing, α-PbO ₂ type FeNbO ₄ was obtained.

The result of having measured the powder X-ray-diffraction pattern of the obtained oxide is shown in FIG. As a result, alpha-PbO from the crystal structure of type ₂ FeNbO ₄ coincides with the diffraction pattern simulated, alpha-PbO ₂ type FeNbO ₄ that was synthesized was confirmed. Moreover, it was suggested from the broad diffraction peak that it is a nanoparticle with a small crystallite size. Further, when a scanning electron microscope image (SEM image) was measured, it was confirmed to be a fine aggregate, which was consistent with the result of a broad diffraction peak of powder X-ray diffraction (FIG. 2).

2. Evaluation of electrochemical characteristics The obtained FeNbO ₄ was mixed with acetylene black 10 wt% and polyvinylidene fluoride 5 wt% to prepare a sodium ion battery electrode. As the electrolytic solution, an electrolytic solution of 1M NaPF ₆ / ethylene carbonate-diethyl carbonate (volume ratio of 1: 1) was used. For the negative electrode / reference electrode, sodium metal was used to accurately record the reaction voltage.

First, when a charge / discharge test was conducted at a constant current of 10 mA / g (active material), decomposition of the electrolyte solution at a low potential and formation of an interface layer (stable surface coating) were observed when the first reduction current was applied. (Figure 3a). Thereafter, 0.6E respect FeNbO ₄ ^- shows the charge-discharge capacity corresponding to, was confirmed to function as reversible electrodes.

  Moreover, even if it repeated 80 times charging / discharging cycles, charging / discharging capacity | capacitance did not reduce and it was confirmed that it is a stable electrode reaction (FIG. 3). The charge / discharge capacity obtained here is considered to be an electrode reaction due to insertion / desorption of oxide ions.

Next, in order to clarify the electrode reaction mechanism, the change in the electronic state during charge / discharge was measured by ⁵⁷ Fe Mossbauer spectroscopy (FIG. 4). As a result, it was confirmed that 60% of Fe exhibited a redox reaction of Fe ³⁺ / Fe ²⁺ with charge / discharge. That is, it was found that the reversible charge / discharge capacity corresponding to 0.6e ⁻ obtained in FIG. 3a is responsible for the oxidation and reduction of Fe in FeNbO ₄ .

  Furthermore, the X-ray diffraction pattern during charge / discharge was measured to verify the structural change. The obtained results are shown in FIG. As a result, it was confirmed that the diffraction peak shifted with charge / discharge, and the electrode reaction proceeded in a solid solution state. The lattice volume change at that time was about 6%.

In order to confirm the generation and disappearance of Na ₂ O along with charge and discharge, an O1sXPS spectrum was measured. As a result, it was confirmed that the obtained electrode reaction was not due to insertion of sodium ions, but due to insertion / extraction of oxide ions. When a reduction current was applied to 1 V, a peak of 528 eV corresponding to Na ₂ O was clearly confirmed, and when it was oxidized to 4.0 V, the peak intensity of Na ₂ O was lowered. This result shows that oxide ions function as mobile ions,
_{2 ×} Na ⁺ + 2xe ⁻ + FeNbO ₄ ⇔ (Na ₂ O) _x (FeNbO _4−x )
It is shown that the reaction is occurring.

In the above examples, the oxide of formula (I) having an α-PbO ₂ type structure is used as an active material, so that the insertion and desorption of oxide ions can be completely performed without being based on the insertion and desorption of sodium ions. This demonstrates that a new electrode reaction can provide a stable reversible capacity and achieve a sodium ion battery.

Claims (7)

The following formula (I):
MXO ₄ (I)
(Wherein M is V, Cr, Mn, Fe, or Co; X is Nb, Mo, Ru, Rh, or Pd), and an electrode including an oxide having a composition represented by: material. The electrode material according to claim 1, wherein the oxide has an α-PbO ₂ type structure. The electrode material according to claim 1, wherein M is Fe. The electrode material according to claim 1, wherein X is Nb. An electrode for a secondary battery comprising the electrode material according to claim 1, wherein the oxide is an electrode active material. A secondary battery comprising the electrode according to claim 5 as a positive electrode or a negative electrode. The secondary battery according to claim 6 which is a sodium ion battery.

Images