JP2012252962A - Sodium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sodium secondary battery requiring a smaller amount of Li which is a rare metal than that required for conventional lithium secondary batteries, offering an improved discharge capacity when reaching low voltages during operation, and offering handleability during manufacture and a high discharge capacity superior to conventional sodium secondary batteries.SOLUTION: The sodium secondary battery comprises a positive electrode, a negative electrode, and a nonaqueous electrolyte mainly containing sodium ions, and the positive electrode contains an active material containing a lithium-containing complex metallic oxide represented by the formula (1): LiAMO, where A represents one or more kinds of elements selected from the group consisting of Na and K, M represents one or more kinds of transition metal elements, 0<a≤1.5, 0≤b<1.5, 0<c≤3, 0<d≤6, and 0<a+b≤1.5.

Description

  The present invention relates to a sodium secondary battery.

As secondary batteries, lithium secondary batteries have already been put into practical use as small power sources for mobile phones and laptop computers, and are also used as large power sources such as power sources for automobiles such as electric vehicles and hybrid vehicles, and power sources for distributed power storage. The demand is increasing because it is possible.
However, in the production of materials constituting lithium secondary batteries, a large amount of raw materials containing expensive rare metal elements such as lithium are used, and the supply of the raw materials to cope with an increase in demand for large-scale power supplies. There are concerns. In addition, when a lithium secondary battery is used up to a low voltage for high capacity, there is a drawback that the discharge capacity when charging and discharging are repeated becomes small.

  On the other hand, a sodium secondary battery has been studied as a secondary battery that can solve the above problems. In sodium secondary batteries, sodium, which is a constituent material, is abundant in supply and can be obtained from inexpensive raw materials, so it is expected that a large-scale power supply can be supplied in large quantities by putting this into practical use. Has been.

  As a conventional sodium secondary battery, Patent Document 1 discloses a sodium-containing composite metal oxide obtained by firing a raw material having a composition ratio of Na and Mn (Na: Mn) of 0.7: 1.0. A sodium secondary battery using as a positive electrode active material is specifically described.

JP 2006-216509 A

However, the sodium secondary battery disclosed in Patent Document 1 cannot be said to have sufficient performance as a secondary battery, for example, a discharge capacity.
Further, in the sodium secondary battery, a sodium-containing composite metal oxide is used as the positive electrode active material, but the sodium-containing composite metal oxide tends to be deteriorated and deteriorated by reacting with moisture in the atmosphere. Therefore, in order to maintain the performance as an active material at the time of battery manufacture, handling is difficult, for example, requiring a drying facility.

  Under such circumstances, the object of the present invention is to reduce the amount of Li, which is a rare metal, compared to conventional lithium secondary batteries, and to improve the discharge capacity when used up to a low voltage, An object of the present invention is to provide a sodium secondary battery that is easy to handle at the time of manufacture and has a large discharge capacity as compared with a conventional sodium secondary battery.

  The inventors of the present invention have intensively studied to solve the above problems, and have reached the present invention.

That is, the present invention provides the following inventions.
<1> A sodium secondary battery using a nonaqueous electrolyte mainly containing a positive electrode, a negative electrode, and sodium ions,
A sodium secondary battery, wherein the positive electrode is an electrode having an active material containing a lithium-containing composite metal oxide represented by the following formula (1).
Li _a A _b M _c O _d (1)
(In Formula (1), A represents one or more elements selected from the group consisting of Na and K, M represents one or more transition metal elements, and 0 <a ≦ 1.5, 0 ≦ b <1. 5, 0 <c ≦ 3, 0 <d ≦ 6, and 0 <a + b ≦ 1.5.)
<2> The sodium secondary battery according to <1>, wherein the crystal structure of the lithium-containing composite metal oxide represented by the formula (1) is a layered crystal structure and / or a spinel crystal structure.

  According to the present invention, since the lithium-containing composite metal oxide represented by the above formula (1) is used as an active material, it is easy to handle during production and has a large discharge capacity after repeated charge and discharge. A secondary battery can be provided.

It is a figure which shows the X-ray-diffraction pattern of the composite metal oxide C1 before and behind charging / discharging, (a) is no charging / discharging, (b) is the 1st charging / discharging cycle, (c) is the result of 10th charging / discharging cycle. is there. It is a SEM image of composite metal oxide C1 before and behind charging / discharging, (a) is no charging / discharging, (b) is the result of 1st charging / discharging cycle. It is a figure which shows the X-ray-diffraction pattern of the composite metal oxide C3 before and behind charging / discharging, (a) is no charging / discharging, (b) is the 1st cycle of charging / discharging, (c) is the result of 10th cycle of charging / discharging. is there. It is a SEM image of the composite metal oxide C3 before and after charging / discharging, (a) is no charging / discharging, (b) is the result of charging / discharging 1st cycle. It is a figure which shows the X-ray-diffraction pattern of the composite metal oxide C4 before and behind charging / discharging, and the X-ray diffraction pattern of the composite metal oxide C5, (a) is no charging / discharging (C4), (b) is 1 cycle of charging / discharging. Eyes (C4) and (c) are results of the 10th charge / discharge cycle (C4), and (d) is a result of no charge / discharge (C5). It is a SEM image of composite metal oxide C4 before and behind charging / discharging, (a) is no charging / discharging, (b) is the result of 1st charging / discharging cycle.

The present invention is a sodium secondary battery having a positive electrode, a negative electrode, and a non-aqueous electrolyte mainly containing sodium ions, wherein the positive electrode includes an active material containing a lithium-containing composite metal oxide represented by the following formula (1). The present invention relates to a sodium secondary battery having an electrode having a substance.
Li _a A _b M _c O _d (1)
(In Formula (1), A represents one or more elements selected from the group consisting of Na and K, M represents one or more transition metal elements, and 0 <a ≦ 1.5, 0 ≦ b <1. 5, 0 <c ≦ 3, 0 <d ≦ 6, and 0 <a + b ≦ 1.5.)
In addition, the sodium secondary battery of this invention has a separator normally further.

The sodium secondary battery of the present invention is characterized in that an electrode having an active material containing a lithium-containing composite metal oxide represented by the above formula (1) is used as a positive electrode.
Here, the lithium-containing composite metal oxide represented by the formula (1) can not only be doped / undoped with sodium ions in a non-aqueous electrolyte mainly containing sodium ions, but the reason is clear at this stage. However, the discharge capacity tends to increase after repeated charging and discharging. Therefore, a high-capacity sodium secondary battery can be obtained by using an electrode having an active material containing the lithium-containing composite metal oxide as the positive electrode.

  Hereinafter, the components of the sodium secondary battery of the present invention will be described in more detail.

<Nonaqueous electrolyte>
In the sodium secondary battery of the present invention, the non-aqueous electrolyte is a liquid or solid composed of a substance containing alkali ions, and mainly contains sodium ions as alkali ions.
The non-aqueous electrolyte may contain alkali ions in addition to sodium ions, and lithium ions and / or potassium ions are preferable as alkali ions other than sodium ions.

  The content ratio of sodium ions contained in the non-aqueous electrolyte is 50% by weight or more of the total alkali ions, preferably 75% by weight or more, more preferably 80% by weight or more (including 100% by weight).

The nonaqueous electrolyte in the sodium secondary battery of the present invention is usually used as a nonaqueous electrolytic solution containing an electrolyte and an organic solvent.
Examples of the electrolyte in the electrolytic solution include NaClO ₄ , NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ , NaN (SO ₂ CF ₃ ) ₂ , lower aliphatic carboxylic acid sodium salt, and NaAlCl _4. Can be mentioned. A mixture of two or more of these may be used. The electrolyte preferably contains at least one fluorine-containing sodium salt selected from the group consisting of NaPF ₆ , NaAsF ₆ , NaSbF ₆ , NaBF ₄ , NaCF ₃ SO ₃ and NaN (SO ₂ CF ₃ ) ₂ .

  Examples of the organic solvent in the electrolytic solution include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1 Carbonates such as 1,2-di (methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, Ethers such as tetrahydrofuran and 2-methyltetrahydrofuran; Esters such as methyl formate, methyl acetate and γ-butyrolactone; Nitriles such as acetonitrile and butyronitrile; N Amides such as N-dimethylformamide and N, N-dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; or the above organic solvents Further, those having a fluorine substituent further introduced can be used.

As the non-aqueous electrolyte in the sodium secondary battery of the present invention, a solid electrolyte may be used instead of the electrolytic solution.
As the solid electrolyte, for example, a so-called gel in which an electrolytic solution is held in a polymer solid electrolyte such as a polymer containing at least one selected from a polyethylene oxide polymer, a polyorganosiloxane chain, and a polyoxyalkylene chain. Types of electrolytes,
_{Na 2 S-SiS 2, Na} 2 S-GeS 2, Na 2 S-P 2 S 5, Na 2 S-B 2 S 3, Na 2 S-SiS 2 -Na 3 PO 4, Na 2 S-SiS 2 An inorganic solid electrolyte such as a sulfide-containing electrolyte such as Na ₂ SO ₄ ; a NASICON-type electrolyte such as NaZr ₂ (PO ₄ ) ₃ ;
When such a solid electrolyte is used, the safety of the secondary battery may be further improved. In the secondary battery, when a solid electrolyte is used, the solid electrolyte may serve as a separator, and in that case, the separator may not be required.

<Positive electrode>
The positive electrode in the sodium secondary battery of the present invention is an electrode having an active material containing the lithium-containing composite metal oxide represented by the above formula (1).

  The transition metal M contained in the lithium-containing composite metal oxide represented by the formula (1) is preferably one or more elements selected from the group consisting of Co, Ni, Mn and Fe. A part of M may be replaced with a metal element other than the above four elements. The battery characteristics of the nonaqueous electrolyte secondary battery may be improved by the replacement. As metals for substituting M, Li, K, Ag, Mg, Ca, Sr, Ba, Al, Ga, In, Ti, V, Cr, Cu, Zn, Sc, Y, Nb, Mo, La, Ce, Pr , Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb and Lu.

Specific examples of the lithium-containing composite metal oxide represented by the formula (1) include the following compounds.
_{That, LiFeO 2, LiMnO 2, LiNiO} 2 and the oxide having a layered-type crystal structure represented by LiMO ₂ such as LiCoO ₂ (where, M represents one or more transition metal elements.);
An oxide having a layered crystal structure represented by Li _0.7 Mn _1-c1 M ¹ _c1 O _d1 (where M ¹ represents one or more transition metal elements excluding Mn, and 0 <c ₁ <1, 1 .9 ≦ d ₁ ≦ 2.1);
An oxide having a spinel crystal structure represented by Li _a1 (Mn _1-c1 M ¹ _c1 ) ₂ O ₄ (where M ¹ represents one or more transition metal elements excluding Mn, and 0 <a ₁ ≦ 1.5, 0 ≦ c ₁ ≦ 1);
Li _0.44 Mn _1-c1 M ¹ _c1 O ₂ oxide having tunnel type crystal structure (where M ¹ represents one or more transition metal elements excluding Mn, and 0 <c ₁ <1 .);
Etc.
Among these, lithium-containing composite metal oxides having a layered crystal structure and / or a spinel crystal structure are preferable.

  In addition, the lithium-containing composite metal oxide is more stable against moisture in the atmosphere than the sodium-containing composite metal oxide (for example, Na-Mn composite oxide) conventionally used as a positive electrode active material for sodium secondary batteries. It is easy to handle at the time of production because of its high tendency.

The lithium-containing composite metal oxide preferably has a BET specific surface area of 0.1 to ⁵ m ² / g, which tends to increase the energy density. The BET specific surface area is more preferably 0.3 m ² / g or more, and even more preferably 0.5 m ² / g or more. Further, the BET specific surface area is more preferably 4.5 m ² / g or less, and even more preferably 4 m ² / g or less.

<Method for producing lithium-containing composite metal oxide>
The lithium-containing composite metal oxide represented by the above formula (1) can be produced by firing a raw material having a composition that becomes the target lithium-containing composite metal oxide. Examples of the raw material include a mixture of metal-containing compounds. Specifically, the metal-containing compounds containing the corresponding metal elements are weighed so as to have a predetermined composition, and they are mixed to obtain a mixture. A composite metal oxide can be manufactured by baking the obtained mixture.
A lithium-containing composite metal oxide having a metal element ratio of Li: Mn = 1: 2 as one of the preferred metal element ratios is obtained by using Li ₂ CO ₃ and MnO ₂ raw materials with a molar ratio of Li: Mn of 1: It can be manufactured by weighing to 2 and mixing them and firing the resulting mixture.

Examples of the metal-containing compound include compounds that can change to oxides at high temperatures, such as oxides, hydroxides, carbonates, nitrates, halides, and oxalates containing the corresponding metal elements. The cobalt compound is preferably Co ₃ O ₄ , the manganese compound is preferably Mn ₂ O ₃ , the nickel compound is preferably NiO, and the iron compound is preferably Fe ₃ O ₄ .
Examples of the lithium compound include one or more compounds selected from the group consisting of lithium hydroxide, lithium chloride, lithium nitrate, lithium peroxide, lithium sulfate, lithium hydrogen carbonate, lithium oxalate, and lithium carbonate. The compound may be a hydrate. Lithium carbonate is preferable because of its low hygroscopicity from the viewpoint of handleability, and lithium hydroxide is preferable because of its high reactivity at low temperatures. If lithium hydroxide is used, it can be fired at a relatively low firing temperature. These compounds may be hydrates.

  The mixture of metal-containing compounds can also be obtained by mixing a metal-containing compound obtained by the following coprecipitation method and a lithium compound.

  Specifically, as compounds of Co, Mn, Ni and Fe, compounds such as chloride, nitrate, acetate, formate, oxalate or sulfate are dissolved in water, and a mixed aqueous solution is prepared. obtain. By bringing the aqueous solution into contact with a precipitant, a precipitate containing a metal-containing compound can be obtained. Among these compounds, chloride or sulfate is preferable. When using raw materials that are difficult to dissolve in water, for example, when using oxides, hydroxides, or metal materials, these are dissolved in acids such as hydrochloric acid, sulfuric acid, nitric acid, or aqueous solutions thereof to obtain aqueous solutions. be able to.

Examples of the precipitant include LiOH (lithium hydroxide), NaOH (sodium hydroxide), KOH (potassium hydroxide), Li ₂ CO ₃ (lithium carbonate), Na ₂ CO ₃ (sodium carbonate), K ₂ CO Examples include compounds selected from the group consisting of ₃ (potassium carbonate), (NH ₄ ) ₂ CO ₃ (ammonium carbonate) and (NH ₂ ) ₂ CO (urea). The precipitating agent may be one or more of the compounds described above, may be one or more hydrates of the compounds described above, and a compound and a hydrate may be used in combination. These precipitating agents are preferably aqueous precipitant solutions. The aqueous precipitant solution is obtained by dissolving the precipitant in water. The concentration of the precipitant in the aqueous precipitant solution is about 0.5 to 10 mol / liter, preferably about 1 to 8 mol / liter. The precipitating agent is preferably KOH or NaOH. The aqueous precipitant solution is preferably an aqueous KOH solution or an aqueous NaOH solution. Aqueous ammonia can also be mentioned as the precipitant aqueous solution. Aqueous ammonia and other aqueous precipitant solutions may be used in combination.

  Examples of the method of bringing the mixed aqueous solution into contact with the precipitant include a method of adding a precipitant (including a precipitant aqueous solution) to the mixed aqueous solution, a method of adding the mixed aqueous solution to the precipitant aqueous solution, and a mixed aqueous solution of water. And a method of adding a precipitant (including a precipitant aqueous solution). These additions are preferably accompanied by stirring. Among the above methods, a method of adding a mixed aqueous solution to a precipitant aqueous solution is preferable. According to this method, it is easy to maintain pH and control the particle size of the resulting precipitate. The pH tends to decrease as the mixed aqueous solution is added to the precipitant aqueous solution. It is preferable to add the mixed aqueous solution while adjusting the pH to be 9 or more, preferably 10 or more. This adjustment can be performed by adding a precipitant aqueous solution. The atmosphere at the time of contact is preferably nitrogen or argon in order to suppress impurity generation.

  A precipitate can be obtained by the above contact. This precipitate contains a metal-containing compound.

  After bringing the mixed aqueous solution into contact with the precipitating agent, a slurry containing a precipitate is usually obtained, and the precipitate may be recovered by solid-liquid separation. Solid-liquid separation may be performed by any method. From the viewpoint of operability, solid-liquid separation such as filtration is preferable. A method of volatilizing the liquid by heating such as spray drying may be used. The collected precipitate may be washed and dried. The precipitate obtained after the solid-liquid separation may have an excessive component of the precipitant attached thereto, and the component can be reduced by washing. The cleaning liquid used for cleaning is preferably water, and may be a water-soluble organic solvent such as alcohol or acetone. Examples of drying include heat drying, air drying, and vacuum drying. Heat drying is normally performed at 50-300 degreeC, Preferably it is performed at about 100-200 degreeC. Washing and drying may be performed twice or more.

  Examples of the mixing method include dry mixing and wet mixing. From the viewpoint of simplicity, dry mixing is preferred. Examples of the mixing device include a stirring and mixing device, a V-type mixer, a W-type mixer, a ribbon mixer, a drum mixer, and a ball mill. The firing temperature depends on the type of lithium compound used, and is usually about 400 to 1200 ° C., preferably about 500 to 1000 ° C. Moreover, the time hold | maintained at the said calcination temperature is 0.1 to 20 hours normally, Preferably it is 0.5 to 10 hours. The rate of temperature rise to the firing temperature is usually 50 to 400 ° C./hour, and the rate of temperature fall from the firing temperature to room temperature is usually 10 to 400 ° C./hour. Examples of the firing atmosphere include air, oxygen, nitrogen, argon, and a mixed gas thereof. Air is preferable from the viewpoint of easy atmosphere control, and oxygen, nitrogen, argon, or a mixed gas thereof is preferable from the viewpoint of stability of the sample after firing.

Controlling the crystallinity of the resulting lithium-containing composite metal oxide and the average particle size of the particles comprising the lithium-containing composite metal oxide by using an appropriate amount of a halide such as fluoride or chloride as the metal-containing compound Can do. The halide may play a role as a reaction accelerator (flux). Examples of the flux include LiF, NaF, MnF ₃ , FeF ₂ , NiF ₂ , CoF ₂ , LiCl, NaCl, MnCl ₂ , FeCl ₂ , FeCl ₃ , NiCl ₂ , CoCl ₂ , Li ₂ CO ₃ , Na ₂ CO ₃ , LiHCO ₃ , NaHCO ₃ , NH ₄ Cl, NH ₄ I, B ₂ O ₃ and H ₃ BO ₃ can be mentioned, and these are used as a raw material (metal-containing compound) of the mixture or by adding an appropriate amount to the mixture be able to. These fluxes may be hydrates.

  When using a lithium-containing composite metal oxide as a positive electrode active material, the obtained lithium-containing composite metal oxide is optionally pulverized using an industrially commonly used apparatus such as a ball mill, a jet mill, or a vibration mill. Alternatively, it may be washed and classified. By these operations, the particle size of the lithium-containing composite metal oxide may be adjusted. Firing may be performed twice or more. Surface treatment such as coating the particle surface of the lithium-containing composite metal oxide with an inorganic substance containing Si, Al, Ti, Y or the like may be performed.

  In addition, you may heat-process after said surface treatment. Depending on the temperature of the heat treatment, the BET specific surface area of the powder after the heat treatment may vary from the BET specific surface area of the lithium-containing composite metal oxide.

<Method for producing positive electrode>
The positive electrode contains the lithium-containing composite metal oxide described above as a positive electrode active material. The positive electrode can be produced by supporting a positive electrode mixture containing a positive electrode active material, a conductive material and a binder on a positive electrode current collector.

  Examples of the conductive material include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black.

  A thermoplastic resin is mentioned as a binder. Specific examples of thermoplastic resins include polyvinylidene fluoride (hereinafter sometimes referred to as “PVDF”), polytetrafluoroethylene (hereinafter sometimes referred to as “PTFE”), tetrafluoroethylene · Fluororesin such as hexafluoropropylene / vinylidene fluoride copolymer, hexafluoropropylene / vinylidene fluoride copolymer, tetrafluoroethylene / perfluorovinyl ether copolymer; and polyolefins such as polyethylene and polypropylene Resin. Two or more of these may be used.

  Examples of the positive electrode current collector include Al, Ni, and stainless steel. Al is preferable from the viewpoint of being easily processed into a thin film and inexpensive. Examples of the method of supporting the positive electrode mixture on the positive electrode current collector include a pressure molding method; a positive electrode mixture paste is obtained by further using an organic solvent, and the paste is applied to the positive electrode current collector and dried. Then, a method of fixing the positive electrode mixture to the current collector by pressing the obtained sheet is obtained. The positive electrode mixture paste contains a positive electrode active material, a conductive material, a binder, and an organic solvent. Examples of organic solvents include amine solvents such as N, N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; dimethylacetamide, N- Examples thereof include amide solvents such as methyl-2-pyrrolidone (hereinafter sometimes referred to as NMP).

  Examples of the method of applying the positive electrode mixture to the positive electrode current collector include a slit die coating method, a screen coating method, a curtain coating method, a knife coating method, a gravure coating method, and an electrostatic spray method. As described above, the positive electrode can be manufactured.

<Electrochemical treatment of positive electrode>
The lithium-containing composite metal oxide represented by the above formula (1) can be changed to a sodium-containing composite metal oxide having a layered crystal structure by performing an electrochemical treatment under specific conditions.
Specifically, in the configuration of the sodium secondary battery of the present invention, that is, in a non-aqueous electrolyte mainly containing sodium ions, a positive electrode having a lithium-containing composite metal oxide as a positive electrode active material is subjected to the following two procedures. The method of repeating charging / discharging is mentioned.
(1) Charge to 3.8 V or higher, preferably 4.0 V or higher (withdrawing alkali metal elements such as Li) based on Na metal.
(2) Discharge (insert Na) to 3.5 V or less, preferably 3.0 V or less based on Na metal.
Although the detailed reason is not clear at this stage, the discharge capacity tends to increase after repeated charging and discharging under such conditions.
The ratio of the lithium-containing composite metal oxide to the sodium-containing composite metal oxide in the active material after the electrochemical treatment is appropriately determined in consideration of the type of target lithium-containing composite metal oxide and the required electrode performance. do it. Therefore, most of the lithium-containing composite metal oxide may be changed to a sodium-containing composite metal oxide having a layered crystal structure.

<Negative electrode>
As the negative electrode in the sodium secondary battery of the present invention, an electrode that can be doped with sodium ions at a potential lower than that of the positive electrode and that can be dedoped is used.
Specific examples of the negative electrode include an electrode in which a negative electrode mixture containing a negative electrode material is supported on a negative electrode current collector, or an electrode made of a negative electrode material alone.
The negative electrode material may be a carbon material, a chalcogen compound (oxide, sulfide, etc.), a nitride, a metal or an alloy, which can be doped with sodium ions at a lower potential than that of the positive electrode and can be undoped. Possible materials are listed. These negative electrode materials may be mixed.

As an example of the carbon material, specifically, in graphite such as natural graphite and artificial graphite, coke, carbon black, pyrolytic carbon, carbon fiber, polymer fired body, etc., at a lower potential than the positive electrode, Examples include materials that can be doped with sodium ions and can be dedope. Examples of the oxide include Li ₄ Ti ₅ O ₁₂ . Examples of sulfides include TiS ₂ , NiS ₂ , FeS ₂ , Fe ₃ S _{4 and the} like. Examples of nitrides include Li _{3 -X} M _x N such as Li ₃ N and Li _2.6 Co _0.4 N (where M is a transition metal element, 0 ≦ X ≦ 3), Na ₃ N, Na _2.6 Co _0.4 N Na _3−X M _X N (where M is a transition metal element, 0 ≦ X ≦ 3) and the like.
These carbon materials, oxides, sulfides, and nitrides may be used in combination, and may be crystalline or amorphous. These carbon materials, oxides, sulfides and nitrides are mainly carried on the negative electrode current collector and used as the negative electrode.
Specific examples of the metal as the negative electrode material include sodium metal, silicon metal, tin metal, bismuth metal, and germanium metal. Examples of the alloy include alloys made of the metals exemplified as the negative electrode material, sodium alloys such as Na—Al, Na—Ni, Na—Si; silicon alloys such as Si—Zn; Sn—Mn, Sn—Co. , Sn—Ni, Sn—Cu, Sn—La and other tin alloys; Cu ₂ Sb, La ₃ Ni ₂ Sn ₇ and other alloys. These metals and alloys are mainly used alone as electrodes (for example, used in a foil shape).

  In addition, the said negative mix may contain a binder as needed. A thermoplastic resin is mentioned as a binder. Specific examples of the thermoplastic resin include PVDF, thermoplastic polyimide, carboxymethyl cellulose, polyethylene, polypropylene, polyacrylic acid, sodium polyacrylate, and the like. In addition, when electrolyte solution does not contain the below-mentioned ethylene carbonate and a negative electrode mixture contains polyethylene carbonate, the cycling characteristics and large current discharge characteristic of the battery obtained may improve.

  Examples of the negative electrode current collector include Cu, Ni, stainless steel, and Al. From the viewpoint that it is difficult to make an alloy with sodium and it is easy to process into a thin film, Cu or Al is preferable. The method of supporting the negative electrode mixture on the negative electrode current collector is the same as the method of forming the positive electrode; pressure molding; obtaining a negative electrode mixture paste by further using an organic solvent and the like, and applying the paste to the negative electrode current collector And drying to obtain a sheet, and pressing the obtained sheet to fix the negative electrode mixture to the current collector.

<Separator>
A sodium secondary battery usually has a separator. Examples of the separator that can be used in the sodium secondary battery of the present invention include porous films, nonwoven fabrics, woven fabrics, and the like made of materials such as polyolefin resins such as polyethylene and polypropylene, fluororesins, and nitrogen-containing aromatic polymers. A material having a form can be used. Moreover, it is good also as a single layer or laminated separator which used 2 or more types of these materials. Examples of the separator include separators described in JP 2000-30686 A, JP 10-324758 A, and the like. The thickness of the separator is preferably as thin as possible as long as the mechanical strength is maintained in that the volume energy density of the battery is increased and the internal resistance is reduced. In general, the thickness of the separator is preferably about 5 to 200 μm, more preferably about 5 to 40 μm.

The separator preferably has a porous film containing a thermoplastic resin. In a sodium secondary battery, normally, when an abnormal current flows in the battery due to a short circuit between the positive electrode and the negative electrode, the current is interrupted to prevent an excessive current from flowing (shut down). is important. Therefore, the separator shuts down at the lowest possible temperature when the normal use temperature is exceeded (if the separator has a porous film containing a thermoplastic resin, the pores of the porous film are blocked). In addition, even if the temperature in the battery rises to a certain high temperature after the shutdown, it is required to maintain the shutdown state without breaking the film due to the temperature, in other words, to have high heat resistance. .
By using a separator made of a laminated porous film in which a heat-resistant porous layer containing a heat-resistant resin and a porous film containing a thermoplastic resin are laminated as the separator, thermal breakage can be further prevented. Here, the heat-resistant porous layer may be laminated on both surfaces of the porous film.

<Type of sodium secondary battery>
In a sodium secondary battery, an electrode group obtained by laminating or laminating and winding a positive electrode, a separator, a negative electrode, and a separator in this order is housed in a battery case such as a battery can, and the electrolyte and It can manufacture by inject | pouring the electrolyte solution containing an organic solvent.
In the case where the separator is not provided, the sodium secondary battery includes, for example, an electrode group obtained by laminating, or laminating and winding, a positive electrode, a solid electrolyte, a negative electrode, and a solid electrolyte in this order. Can be stored in a battery case.

  As the shape of the electrode group, for example, a cross section when the electrode group is cut perpendicularly to the axis of winding or a cross section when the electrode group is cut parallel to the stacking direction has a circle, an ellipse, a rectangle, and a corner. The shape which becomes such a rectangle etc. is mentioned. Examples of the shape of the battery include a paper shape, a coin shape, a cylindrical shape, and a square shape.

<Uses of sodium secondary batteries>
Since the sodium secondary battery of the present invention has a high energy density, it is a small battery, automobile, motorcycle, electric chair, forklift, train, airplane, ship, which is a power source for small devices such as mobile phones, portable audios, and notebook computers. Power supply for transportation equipment such as spacecraft and submarines; Power supply for machinery such as cultivators; Outdoor power supply for camping applications; Suitable for medium and large-sized batteries that are mobile batteries for outdoor / indoor power supplies such as vending machines is there.

  In addition, since the sodium secondary battery of the present invention uses abundant and inexpensive raw materials, it can be used in outdoor / indoor installation power supplies for factories, houses, etc .; solar battery chargers, wind power generator chargers, etc. Load leveling power source for various power generations; Installation power source for low temperature environments such as in refrigerated / refrigerated warehouses and extremely cold areas; Installation power source for high temperature environments such as deserts; Installation power source for space environments such as space stations It is suitable as a medium / large battery.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited by these. In addition, unless otherwise indicated, the manufacturing method of the electrode for a charging / discharging test and a secondary battery is as follows.

(1) Production of electrode (positive electrode) Composite metal oxide described later as an electrode active material, acetylene black (manufactured by Denki Kagaku Kogyo Co., Ltd.) as a conductive material, and PVDF (manufactured by Polysciences, PolyVinyleneDiFluoride) as a binder Each was weighed so as to have a composition of metal oxide: acetylene black: PVDF = 8: 1: 1 (weight ratio). Thereafter, an appropriate amount of N-methyl-2-pyrrolidone (NMP: manufactured by Tokyo Chemical Industry Co., Ltd.) is first added to the mixture obtained by thoroughly mixing the composite metal oxide and acetylene black in an agate mortar, and then PVDF is added. Then, they were mixed so as to be uniform and slurried. The obtained slurry was applied onto a stainless mesh as a current collector, put in a drier, and sufficiently dried while removing NMP to obtain an electrode (positive electrode).

(2) Production of battery A battery was produced by sandwiching a positive electrode between clips of a lead wire of a beaker cell (a bottle having a height of 80 mm and a diameter of 30 mm) and combining an electrolyte and an alkali metal as a negative electrode. . The battery was assembled in a glove box in an argon atmosphere.

(3) Powder X-ray diffraction measurement The measurement was performed under the following conditions using a powder X-ray diffraction measurement apparatus RINT2500TTR manufactured by Rigaku Corporation.
X-ray: CuKα
Voltage-current: 40 kV-140 mA
Measurement angle range: 2θ = 10-90 °
Step: 0.02 °
Scan speed: 4 ° / min

(4) Scanning electron microscope (SEM) observation
Measurement was carried out using a Carl Zeis FE-SEM (supra40) under an acceleration voltage of 10 kV.

Production Example 1
(1) Production of Composite Metal Oxide As a metal-containing compound, lithium carbonate (Li ₂ CO ₃ : Wako Pure Chemical Industries, Ltd .: purity 99.8%) and cobalt oxide (II, III) (Co ₃ O ₄ : Made by Kojundo Chemical Laboratories Co., Ltd .: purity 99%), weighed so that the molar ratio of Li: Co would be 1.00: 1.00, mixed for 4 hours with a dry ball mill, and contained metal A mixture of compounds was obtained. The obtained mixture of metal-containing compounds was filled in an alumina boat, heated in an air atmosphere using an electric furnace, held at 700 ° C. for 12 hours, and fired. Firing was performed once. In this way, lithium-containing composite metal oxide C1 of Production Example 1 was obtained. The X-ray diffraction measurement result and SEM image of the composite metal oxide C1 are shown in FIG. 1 (a) and FIG. 2 (a), respectively.
As a result of the X-ray diffraction measurement, the composite metal oxide C1 was LiCoO ₂ having a layered crystal structure.

Comparative Example 1
(2) Evaluation of charge / discharge performance of lithium secondary battery An electrode was prepared using the composite metal oxide C1, the electrode was used as the positive electrode of the lithium secondary battery, and LiClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using lithium metal (manufactured by Honjo Metal Co., Ltd.) as a counter electrode was prepared, and a constant current charge / discharge test was performed under the following conditions.

Charging / discharging conditions:
Charging was performed by CC (constant current: constant current) charging at a 0.1 C rate (a rate of complete charging in 10 hours) to 4.0 V. The discharge was performed by performing CC discharge at the same rate as the charge rate and cutting off at a voltage of 2.0V. Charging and discharging after the next cycle were performed at the same rate as the charging rate, and cut off at a charging voltage of 4.0 V and a discharging voltage of 2.0 V as in the first cycle.

  For this battery, the discharge capacity at the first charge / discharge cycle was 154 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 114 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity retention ratio) to the discharge capacity at the first charge / discharge cycle was 74%. The results are shown in Table 1.

Example 1
(3) Evaluation of charge / discharge performance of sodium secondary battery An electrode was prepared using the composite metal oxide C1, the electrode was used as the positive electrode of the sodium secondary battery, and NaClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using sodium metal (manufactured by Aldrich) as a counter electrode was produced, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 1.
For this battery, the discharge capacity at the first charge / discharge cycle was 119 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 120 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity retention ratio) to the discharge capacity at the 1st charge / discharge cycle was 101%. The results are shown in Table 1.
Moreover, about the positive electrode of this battery, after 1 cycle of charging / discharging, the X-ray-diffraction measurement result after 10 cycles is shown in FIG.1 (b) and FIG.1 (c), respectively. From the result of X-ray diffraction measurement, LiCoO ₂ in the composite metal oxide C1 is changed to Na _x CoO ₂ having a layered structure (provided that 0.6 ≦ x ≦ 0.9) by performing a charge / discharge cycle. I understood that.
Moreover, although the SEM image after 1 cycle of charging / discharging is shown in FIG.2 (b), compared with the composite metal oxide C1 (Fig.2 (a)) before charging / discharging cycle, the change which was conspicuous in the particle | grain surface and form Was not confirmed.

Production Example 2
(1) Production of Composite Metal Oxide As a metal-containing compound, sodium carbonate (Na ₂ CO ₃ : Wako Pure Chemical Industries, Ltd .: purity 99.8%) and cobalt oxide (II, III) (Co ₃ O ₄ : Made by Kojundo Chemical Laboratory Co., Ltd .: purity 99%), weighed so that the molar ratio of Na: Co would be 1.00: 1.00, mixed for 4 hours with a dry ball mill, and contained metal A mixture of compounds was obtained. The obtained mixture of metal-containing compounds was filled in an alumina boat, heated in an air atmosphere using an electric furnace, held at 800 ° C. for 24 hours, and fired. Firing was performed once. In this way, sodium-containing composite metal oxide C2 of Production Example 2 was obtained.
As a result of the X-ray diffraction measurement, the composite metal oxide C2 was NaCoO ₂ having a layered crystal structure.

Comparative Example 2
(2) Evaluation of charge / discharge performance of sodium secondary battery An electrode was prepared using the composite metal oxide C2, the electrode was used as the positive electrode of the sodium secondary battery, and NaClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using sodium metal (manufactured by Aldrich) as a counter electrode was prepared, and a constant current charge / discharge test was performed under the following conditions.

Charging / discharging conditions:
Charging was performed by CC (constant current: constant current) charging at a 0.1 C rate (a rate of complete charging in 10 hours) to 4.0 V. The discharge was performed by performing CC discharge at the same rate as the charge rate and cutting off at a voltage of 2.0V. Charging and discharging after the next cycle were performed at the same rate as the charging rate, and cut off at a charging voltage of 4.0 V and a discharging voltage of 2.0 V as in the first cycle.

  With respect to this battery, the discharge capacity at the first charge / discharge cycle was 115 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 104 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity maintenance ratio) to the discharge capacity at the first charge / discharge cycle was 90%. The results are shown in Table 1.

Commercial product 1
(1) Evaluation of composite metal oxide LiCo _0.33 Mn _0.33 Ni _0.33 O ₂ manufactured by Nippon Chemical Industry Co., Ltd. was used as the lithium-containing composite metal oxide C3. The X-ray diffraction measurement result of the composite metal oxide C3 is shown in FIG. As a result of X-ray diffraction measurement, it was confirmed that the composite metal oxide C3 was LiCo _0.33 Mn _0.33 Ni _0.33 O ₂ having a layered crystal structure.

Comparative Example 3
(2) Evaluation of charge / discharge performance of lithium secondary battery An electrode was prepared using the composite metal oxide C3, the electrode was used as the positive electrode of the lithium secondary battery, and LiClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using lithium metal (manufactured by Honjo Metal Co., Ltd.) as a counter electrode was prepared, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 1. For this battery, the discharge capacity at the first charge / discharge cycle was 150 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 95 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity maintenance ratio) to the discharge capacity at the first charge / discharge cycle was 63%. The results are shown in Table 1.

Example 2
(3) Evaluation of charge / discharge performance of sodium secondary battery An electrode was prepared using the composite metal oxide C3, the electrode was used as the positive electrode of the sodium secondary battery, and NaClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using sodium metal (manufactured by Aldrich) as a counter electrode was produced, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 1. With respect to this battery, the discharge capacity at the first charge / discharge cycle was 142 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 155 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity maintenance ratio) to the discharge capacity at the first charge / discharge cycle was 109%. The results are shown in Table 1.
Moreover, about the positive electrode of this battery, the X-ray-diffraction measurement result after 10 cycles after charging / discharging is shown in FIG.3 (b) and FIG.3 (c), respectively. From the result of the X-ray diffraction measurement, LiCo _0.33 Mn _0.33 Ni _0.33 O ₂ in the composite metal oxide C1 changes to Na _x MnO ₂ having a layered structure (however, x≈1) by performing a charge / discharge cycle. I understood that.
Moreover, although the SEM image after 1 cycle of charging / discharging is shown in FIG.4 (b), compared with the composite metal oxide C3 (Fig.4 (a)) before charging / discharging cycle, the change which was conspicuous in the particle | grain surface and form Was not confirmed.

Production Example 3
(1) Production of composite metal oxide As metal-containing compounds, lithium carbonate (Li ₂ CO ₃ : Wako Pure Chemical Industries, Ltd .: purity 99.8%) and manganese oxide (II, III) (Mn ₂ O ₃ : 99% pure purity chemical laboratory Co., Ltd.), weighed so that the molar ratio of Li: Mn is 1.00: 2.00, mixed for 4 hours in a dry ball mill, and contained metal A mixture of compounds was obtained. The obtained mixture of metal-containing compounds was filled in an alumina boat, heated in an air atmosphere using an electric furnace, held at 700 ° C. for 12 hours, and fired. Firing was performed once. Thus, the lithium-containing composite metal oxide C4 of Production Example 3 was obtained. FIG. 5A shows the X-ray diffraction measurement result of the composite metal oxide C4. As a result of the X-ray diffraction measurement, the composite metal oxide C4 was LiMn ₂ O ₄ having a spinel crystal structure.

Comparative Example 4
(2) Evaluation of charge / discharge performance of lithium secondary battery An electrode was prepared using the composite metal oxide C4, the electrode was used as the positive electrode of the lithium secondary battery, and LiClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) ), A battery using lithium metal (manufactured by Honjo Metal Co., Ltd.) as a counter electrode was prepared, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 1. For this battery, the discharge capacity at the first charge / discharge cycle was 250 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 64 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity retention ratio) to the discharge capacity at the first charge / discharge cycle was 25%. The results are shown in Table 1.

Example 3
(3) Evaluation of charge / discharge performance of sodium secondary battery An electrode was prepared using the composite metal oxide C4, the electrode was used as the positive electrode of the sodium secondary battery, and NaClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using sodium metal (manufactured by Aldrich) as a counter electrode was produced, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 1. For this battery, the discharge capacity at the first charge / discharge cycle was 199 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 190 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity retention ratio) to the discharge capacity at the first charge / discharge cycle was 95%. The results are shown in Table 1.
Moreover, about the positive electrode of this battery, the X-ray-diffraction measurement result after 10 cycles after charging / discharging is shown in FIG.5 (b) and FIG.5 (c), respectively. From the result of the X-ray diffraction measurement, it was found that LiMn ₂ O ₄ in the composite metal oxide C4 changed to NaMnO ₂ having a layered structure by performing a charge / discharge cycle.
Moreover, although the SEM image after 1 cycle of charging / discharging is shown in FIG.6 (b), compared with the composite metal oxide C4 (Fig.6 (a)) before charging / discharging cycle, the change which was conspicuous in the particle | grain surface and form Was not confirmed.

Production Example 4
(1) Production of composite metal oxide As metal-containing compounds, sodium carbonate (Na ₂ CO ₃ : Wako Pure Chemical Industries, Ltd .: purity 99.8%), and manganese oxide (II, III) (Mn ₂ O ₃ : Made by Kojundo Chemical Laboratory Co., Ltd .: purity 99%), weighed so that the molar ratio of Na: Mn was 1.00: 1.00, mixed for 4 hours with a dry ball mill, and contained metal A mixture of compounds was obtained. The obtained mixture of metal-containing compounds was filled in an alumina boat, heated in an air atmosphere using an electric furnace, held at 800 ° C. for 24 hours, and fired. Firing was performed once. Thus, sodium-containing composite metal oxide C5 of Production Example 4 was obtained. FIG. 5D shows the X-ray diffraction measurement result of the composite metal oxide C5. As a result of the X-ray diffraction measurement, the composite metal oxide C5 was NaMnO ₂ having a layered crystal structure.

Comparative Example 5
(2) Evaluation of charge / discharge performance of sodium secondary battery An electrode was prepared using the composite metal oxide C5, the electrode was used as the positive electrode of the sodium secondary battery, and NaClO ₄ / PC (manufactured by Kishida Chemical Co., Ltd.) was used as the electrolyte. ), A battery using sodium metal (manufactured by Aldrich) as a counter electrode was produced, and a constant current charge / discharge test was performed under the same conditions as in Comparative Example 2. With respect to this battery, the discharge capacity at the first charge / discharge cycle was 156 mAh / g, and the discharge capacity at the 10th charge / discharge cycle was 92 mAh / g. The ratio of the discharge capacity at the 10th charge / discharge cycle (discharge capacity retention ratio) to the discharge capacity at the first charge / discharge cycle was 59%. The results are shown in Table 1.

The sodium secondary battery of the present invention can provide a large discharge capacity even after repeated charge and discharge, has a large discharge capacity per weight, and can reduce the amount of expensive lithium used.
Furthermore, the present invention is extremely useful industrially because an electrode and a battery can be easily produced in an air atmosphere or the like without requiring a large drying facility.

Claims (2)

A sodium secondary battery using a positive electrode, a negative electrode and a non-aqueous electrolyte mainly containing sodium ions,
The sodium secondary battery, wherein the positive electrode is an electrode having an active material containing a lithium-containing composite metal oxide represented by the following formula (1).
Li _a A _b M _c O _d (1)
(In Formula (1), A represents one or more elements selected from the group consisting of Na and K, M represents one or more transition metal elements, and 0 <a ≦ 1.5, 0 ≦ b <1. 5, 0 <c ≦ 3, 0 <d ≦ 6, and 0 <a + b ≦ 1.5.)   2. The sodium secondary battery according to claim 1, wherein a crystal structure of the lithium-containing composite metal oxide represented by the formula (1) is a layered crystal structure and / or a spinel crystal structure.