JP5765705B2 - Composite metal oxide, positive electrode active material for sodium secondary battery, positive electrode for sodium secondary battery, and sodium secondary battery - Google Patents

Description

  The present invention relates to a composite metal oxide, a positive electrode active material for a sodium secondary battery, a positive electrode for a sodium secondary battery, and a sodium secondary battery.

  Lithium secondary batteries are high energy density secondary batteries and have already been put into practical use as small power sources for mobile phones, notebook computers and the like. In addition, since lithium secondary batteries can be used as large-scale 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.

  However, since a rare metal element such as lithium is used in the lithium secondary battery, there is a concern about the unstable supply of the rare metal element when the demand for the lithium secondary battery increases.

  In order to solve the above supply concern problem, research on sodium secondary batteries is underway. As the positive electrode active material of the sodium secondary battery, not only expensive lithium but also abundant and inexpensive sodium is used. Therefore, if the sodium secondary battery can be put into practical use, the problem of supply instability can be solved.

  By the way, as a positive electrode active material for a sodium secondary battery, a composite metal oxide of Na, Mn, and Fe is used (Patent Documents 1 to 3). Since the composite metal oxides described in Patent Documents 1 to 3 do not contain rare metal elements such as Li, Co, and Ni, they contribute to a reduction in production costs of sodium secondary batteries and increase demand for sodium secondary batteries. Can also respond.

JP 2009-135092 A JP 2009-209037 A JP 2010-080424 A

  The performance of the sodium secondary battery using the positive electrode active material composed of the composite metal oxide disclosed in Patent Documents 1 to 3 is sufficient as compared with the performance of the lithium secondary battery currently in practical use. I can't say that. Therefore, there is a need for a sodium secondary battery that stably exhibits high discharge capacity without using rare metal elements such as Co and Ni.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to realize a sodium secondary battery that stably exhibits a high discharge capacity without using rare metal elements such as Co and Ni. Provided is a composite metal oxide, a method for producing the composite metal oxide, a positive electrode active material composed of the composite metal oxide, a positive electrode produced using the positive electrode active material, and a sodium secondary battery including the positive electrode There is to do.

The inventors of the present invention have made extensive studies to solve the above problems. As a result, the present inventors have found that the above problems can be solved by using a composite metal oxide represented by the following formula (I) and composed of an oxide having a P2 structure, and has completed the present invention. More specifically, the present invention provides the following .

（式（ＩＩ）式中のＲ１及びＲ２は、同一又は異なり、夫々フルオロ基又はフルオロアルキル基である） (1) A sodium secondary battery comprising a positive electrode having a positive electrode active material composed of a composite metal oxide, a negative electrode capable of inserting and extracting sodium ions, and an electrolyte, wherein the composite metal oxidation The product is a sodium secondary battery represented by the following formula (I) and composed of an oxide having a P2 structure (excluding an electrolyte containing an anion represented by the following formula (II)).
Na _x Fe _y Mn _1-y O ₂ (I)
(X in the formula (I) is less than 1, and y is 1/3 or more and less than 2/3.)

(R1 and R2 in the formula (II) are the same or different and are each a fluoro group or a fluoroalkyl group)

  When the composite metal oxide of the present invention is used as a positive electrode active material for a sodium secondary battery, the battery performance of the sodium secondary battery can be improved as compared with the performance of a conventional sodium secondary battery. Specifically, the positive electrode active material for a sodium secondary battery using the composite metal oxide of the present invention does not contain a rare metal element such as Co and Ni, and also includes the above-described positive electrode active material. Shows a stable high discharge capacity even after repeated charge and discharge.

It is a figure which shows the result of the powder X-ray-diffraction measurement of the composite metal oxide used in Examples 1 and 2 and Reference Example 1 . It is a figure which shows the relationship between cycle number and discharge capacity about the sodium secondary battery of Example 1. FIG. It is a figure which shows the relationship between cycle number and discharge capacity about the sodium secondary battery of Example 2. FIG. It is a figure which shows the relationship between cycle number and discharge capacity about the sodium secondary battery of the reference example 1. FIG.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

<Composite metal oxide>
The composite metal oxide of the present invention has a composition represented by the following formula (I) and is composed of an oxide having a P2 structure. By using such a composite metal oxide as a positive electrode active material for a sodium secondary battery, the battery performance of the sodium secondary battery can be improved without containing rare metal elements such as Co and Ni. In particular, when y is ¼ or more and less than 2/3, the battery performance is significantly improved. A more preferable range of y is 1/5 or more and less than 2/3.
Na _x Fe _y Mn _1-y O ₂ (I)
(X in the formula (I) is less than 1, and y is 1/3 or more and less than 2/3.)

  The ranges of x and y at which the composite metal oxide tends to have a P2 structure are 2/3 ≦ x ≦ 9/10 and 0.33 ≦ y ≦ 0.60, respectively. Further, the lamination mode is basically a regular P2 type and is characterized by having no clear lamination defects of 3% or more.

  Here, the values of x and y can be adjusted by adjusting the amount of raw material used, manufacturing conditions, and the like. Details will be described later.

  Whether or not the composite metal oxide is an oxide having a P2 structure can be confirmed by X-ray diffraction. Specifically, it can confirm by the method as described in an Example.

  The composite metal oxide of the present invention is composed of an oxide having a P2 structure, but may contain other components as long as the effects of the present invention are not impaired.

<Method for producing composite metal oxide>
The composite metal oxide of the present invention can be produced by firing a mixture of metal-containing compounds as described below.

  Specifically, the metal-containing compound containing the corresponding metal element can be produced by weighing and mixing so as to have a predetermined composition, and then firing the resulting mixture.

  For example, a composite metal oxide having a metal element ratio represented by Na: Fe: Mn = 2/3: 1/2: 1/2 is obtained by using sodium carbonate, iron oxide, and manganese oxide as raw materials. : Fe: Mn can be produced by weighing them so that the molar ratio is 2/3: 1/2: 1/2, mixing them, and firing the resulting mixture.

Metal-containing compounds that can be used to produce composite metal oxides include oxides and compounds that can become oxides when decomposed and / or oxidized at high temperatures, such as hydroxides, carbonates, nitrates, Sulfates, halides, and oxalates can be used. As the sodium compound, Na ₂ CO ₃ , NaHCO ₃ , and Na ₂ O ₂ are preferable, and Na ₂ CO ₃ is more preferable from the viewpoint of handleability. The manganese compound is preferably MnO ₂ and the iron compound is preferably Fe ₃ O ₄ . These metal-containing compounds may be hydrates.

Controlling the crystallinity of the resulting composite metal oxide and the average particle size of the particles constituting the composite metal oxide by adding an appropriate amount of fluoride, chloride, or other halide to the metal-containing compound that is the raw material Can do. In this case, the halide may play a role as a reaction accelerator (flux). Examples of the flux include NaF, MnF ₃ , FeF ₂ , NiF ₂ , CoF ₂ , NaCl, MnCl ₂ , FeCl ₂ , FeCl ₃ , NiCl ₂ , CoCl ₂ , Na ₂ CO ₃ , NaHCO ₃ , NH ₄ Cl, NH _{4. I, B 2 O 3, H} 3 BO 3 and the like. These fluxes may be hydrates.

  For the mixing of the metal-containing compound, an apparatus usually used industrially, such as a ball mill, a V-type mixer, or a stirrer, can be used. The mixing at this time may be either dry mixing or wet mixing. Further, a mixture of metal-containing compounds having a predetermined composition may be obtained by a crystallization method.

  The composite metal oxide can be obtained by firing the mixture of metal-containing compounds obtained as described above. Although it does not specifically limit about baking conditions, It is preferable to set a baking temperature in the range of 800-1000 degreeC, and baking time in the range of 2-24 hours. If the firing temperature is 800 ° C. or higher, it is preferable for suppressing excessive stacking faults, and if the firing temperature is 1000 ° C. or lower, it is preferable for reducing the primary particle size. In addition, if the firing time is 2 hours or more, it is preferable for obtaining a uniform chemical composition with a single particle, and if the firing time is less than 24 hours, crystal growth is performed while maintaining stacking faults at a low temperature. Is also preferable because it becomes possible.

  Depending on the firing conditions, an oxide having a structure other than the P2 structure may be included in the composite metal oxide. When an oxide having a structure other than the P2 structure is included in the composite metal oxide, the battery performance of the sodium secondary battery manufactured using the composite metal oxide of the present invention may be deteriorated. For this reason, the lower the content of the oxide having a structure other than the P2 structure, the better. In order to suppress the content of the oxide having a structure other than the P2 structure, it is preferable to adjust the firing temperature in the range of 850 to 1000 ° C. and the firing time in the range of 6 to 24 hours.

  Examples of the atmosphere during firing include an inert atmosphere such as nitrogen and argon; an oxidizing atmosphere such as air, oxygen, oxygen-containing nitrogen, and oxygen-containing argon; and hydrogen-containing nitrogen containing 0.1 to 10% by volume of hydrogen Any reducing atmosphere such as hydrogen-containing argon containing 0.1 to 10% by volume of hydrogen may be used. In order to fire in a strong reducing atmosphere, an appropriate amount of carbon may be contained in a mixture of metal-containing compounds and fired. As atmosphere at the time of baking, oxidizing atmospheres, such as air, are preferable.

  When a compound that can be decomposed and / or oxidized at a high temperature, such as a hydroxide, carbonate, nitrate, sulfate, halide, or oxalate, is used as a raw material metal-containing compound, before performing the above firing, The metal-containing compound may be calcined in a temperature range of 400 to 1600 ° C. to form an oxide or crystal water may be removed. The atmosphere in which the calcination is performed may be an inert gas atmosphere, an oxidizing atmosphere, or a reducing atmosphere. Moreover, you may grind | pulverize and use the calcined material after calcining.

  In addition, it may be preferable to adjust the particle size by optionally subjecting the composite metal oxide obtained as described above to pulverization, washing, classification and the like using a ball mill, a jet mill or the like. Moreover, you may perform baking twice or more. Further, a surface treatment such as coating the particle surface of the composite metal oxide with an inorganic substance containing Si, Al, Ti, Y or the like may be performed. The composite metal oxide preferably has a crystal structure that is not a tunnel structure.

<Sodium secondary battery>
The sodium secondary battery of this invention is equipped with the positive electrode which has the positive electrode active material comprised from the said composite metal oxide, the negative electrode which can occlude and desorb a sodium ion, and an electrolyte.

[Positive electrode]
The positive electrode has a current collector and a positive electrode active material layer formed on the surface of the current collector, and the positive electrode active material layer has a positive electrode active material, a conductive material, and a binder.

  Although content of the positive electrode active material in a positive electrode active material layer is not specifically limited, It is preferable that it is 80-95 mass%.

  Examples of the conductive material that can be used in the present invention include carbon materials such as natural graphite, artificial graphite, cokes, and carbon black. Although content of the electrically conductive material in a positive electrode active material layer is not specifically limited, It is preferable that it is 5-10 mass%.

  Examples of the binder usable in the present invention include polyvinylidene fluoride (hereinafter sometimes referred to as PVDF), polytetrafluoroethylene (hereinafter sometimes referred to as PTFE), tetrafluoroethylene and hexafluoropropylene. -Vinylidene fluoride copolymers, propylene hexafluoride / vinylidene fluoride copolymers, ethylene tetrafluoride / perfluorovinyl ether copolymers and the like. These may be used alone or in combination of two or more. Other examples of the binder include, for example, polysaccharides such as starch, methylcellulose, carboxymethylcellulose, hydroxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, carboxymethylhydroxyethylcellulose, nitrocellulose, and derivatives thereof. Examples of usable binders include inorganic fine particles such as colloidal silica. Although content of the binder in a positive electrode active material is not specifically limited, It is preferable that it is 5-10 mass%.

  Examples of the current collector that can be used in the present invention include foil, mesh, expanded grid (expanded metal), punched metal, and the like using a conductive material such as nickel, aluminum, and stainless steel (SUS). The mesh opening, wire diameter, number of meshes and the like are not particularly limited, and conventionally known ones can be used. The general thickness of the current collector is 5 to 30 μm. However, a current collector having a thickness outside this range may be used.

  The size of the current collector is determined according to the intended use of the battery. If a large electrode used for a large battery is manufactured, a current collector having a large area is used. If a small electrode is produced, a current collector with a small area is used.

  As a method for producing the positive electrode, first, a positive electrode active material, a conductive material, a binder, and an organic solvent are mixed to prepare a positive electrode active material slurry. Here, usable organic solvents include amines such as N, N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; dimethyl Examples include aprotic polar solvents such as acetamide and N-methyl-2-pyrrolidone.

  Next, the positive electrode active material slurry is applied onto the positive electrode current collector, and dried and pressed to fix it. Here, examples of the method for coating the positive electrode active material slurry on 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. be able to.

  As a method for forming the positive electrode active material layer on the positive electrode current collector, in addition to the above method, a mixture of the positive electrode active material, the conductive material, and the binder is placed on the positive electrode current collector and pressed. A molding method may be used.

[Negative electrode]
The negative electrode has a current collector and a negative electrode active material layer formed on the surface of the current collector, and the negative electrode active material layer has a negative electrode active material and a binder. Further, as the negative electrode, a negative electrode mixture containing a negative electrode active material supported on a negative electrode current collector, or an electrode capable of occluding and desorbing sodium ions such as sodium metal or sodium alloy can be used.

  Carbon materials such as natural graphite, artificial graphite, coke, hard carbon, carbon black, pyrolytic carbon, carbon fiber, and organic polymer compound fired body capable of absorbing and desorbing sodium ions as the negative electrode active material Is mentioned. The shape of the carbon material may be any of a flake shape such as natural graphite, a spherical shape such as mesocarbon microbeads, a fibrous shape such as graphitized carbon fiber, or an aggregate of fine powder. Here, the carbon material may play a role as a conductive material.

  As described above, the negative electrode active material in the present invention is not limited to a specific material, but it is preferable to use hard carbon. By using hard carbon as the negative electrode active material, it is possible to suppress a decrease in battery performance caused by the negative electrode active material.

  Hard carbon is a carbon material whose stacking order hardly changes even when heat-treated at a high temperature of 2000 ° C. or higher, and is also called non-graphitizable carbon. As hard carbon, carbon fiber obtained by carbonizing an infusible yarn, which is an intermediate product in the production process of carbon fiber, at about 1000 to 1400 ° C., an organic compound is air oxidized at about 150 to 300 ° C., and then about 1000 to 1400 ° C. Examples of the carbon material carbonized in FIG. The method for producing hard carbon is not particularly limited, and hard carbon produced by a conventionally known method can be used.

  There are no particular restrictions on the average particle size, true density, and (002) plane spacing of the hard carbon, and it can be carried out by selecting preferred ones as appropriate.

  Although content of the negative electrode active material in a negative electrode active material layer is not specifically limited, It is preferable that it is 80-95 mass%.

  As the binder, the same binders that can be used for the positive electrode can be used, and thus the description thereof will be omitted. As the current collector, a conductive material such as nickel, copper, and stainless steel (SUS) is used. The current collector is composed of a foil, a mesh, an expanded grid (expanded metal), a punched metal, and the like, like the current collector for the positive electrode.

  As a method for forming the negative electrode active material layer on the current collector, a method similar to the method for forming the positive electrode active material layer on the current collector can be employed.

[Electrolytes]
The electrolyte is not particularly limited, and either a general electrolytic solution or a solid electrolyte can be used. Examples of the electrolyte include propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, isopropyl methyl carbonate, vinylene carbonate, 4-trifluoromethyl-1,3-dioxolan-2-one, 1,2-di ( Carbonates such as methoxycarbonyloxy) ethane; 1,2-dimethoxyethane, 1,3-dimethoxypropane, pentafluoropropyl methyl ether, 2,2,3,3-tetrafluoropropyl difluoromethyl ether, tetrahydrofuran, 2-methyl Ethers such as tetrahydrofuran; esters such as methyl formate, methyl acetate and γ-butyrolactone; nitriles such as acetonitrile and butyronitrile; N, N-dimethylformamide, N, N Amides such as dimethylacetamide; carbamates such as 3-methyl-2-oxazolidone; sulfur-containing compounds such as sulfolane, dimethyl sulfoxide and 1,3-propane sultone; or a fluorine substituent further introduced into the above organic solvent Things can be used. Usually, two or more of these are mixed and used as the organic solvent.

  Among the above electrolytic solutions, it is preferable to employ a non-aqueous solvent consisting essentially of a saturated cyclic carbonate (excluding the sole use of ethylene carbonate) or a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In particular, when any one of these non-aqueous solvents is employed and hard carbon is employed as the negative electrode active material, the sodium ion secondary battery has excellent charge / discharge efficiency and charge / discharge characteristics.

  Here, “substantially” means a non-aqueous solvent composed only of a saturated cyclic carbonate (excluding the single use of ethylene carbonate), a non-aqueous solvent composed of a mixed solvent of a saturated cyclic carbonate and a chain carbonate. In the range which does not affect the performance of the sodium ion secondary battery such as the charge / discharge characteristics, it means that the solvent includes the solvent contained in the non-aqueous solvent used in the present invention.

  Among the saturated cyclic carbonates, use of propylene carbonate is preferable. Of the mixed solvents, it is preferable to use a mixed solvent of ethylene carbonate and diethyl carbonate or a mixed solvent of ethylene carbonate and propylene carbonate.

  As an electrolyte, an electrolyte salt that can be used when an electrolytic solution is employed is not particularly limited, and an electrolyte salt that is generally used for sodium secondary batteries can be used.

Examples of the electrolyte salt generally used for sodium secondary batteries include NaClO ₄ , NaPF ₆ , NaBF ₄ , CF ₃ SO ₃ Na, NaAsF ₆ , NaB (C ₆ H ₅ ) ₄ , CH ₃ SO ₃ Na, _{CF 3 SO 3 Na, NaN (} SO 2 CF 3) 2, NaN (SO 2 C 2 F 5) 2, NaC (SO 2 CF 3) 3, NaN (SO 3 CF 3) may be mentioned _2. In addition, you may use 1 type in the said electrolyte salt, and may use it in combination of 2 or more type.

  The concentration of the electrolyte salt in the electrolytic solution is not particularly limited, but the concentration of the electrolyte salt is preferably 3 to 0.1 mol / l, more preferably 1.5 to 0.5 mol / l. preferable.

As the solid electrolyte, for example, an organic solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain or a polyoxyalkylene chain can be used. Moreover, what is called a gel type which hold | maintained the nonaqueous electrolyte solution in the high molecular compound can also be used. _{Further, Na 2 S-SiS 2,} Na 2 S-GeS 2, NaTi 2 (PO 4) 3, NaFe 2 (PO 4) 3, Na 2 (SO 4) 3, Fe 2 (SO 4) 2 (PO 4 ), Fe ₂ (MoO ₄ ) ₃ or other inorganic solid electrolytes may be used.

[Structure of sodium secondary battery]
The structure of the sodium secondary battery of the present invention is not particularly limited, and when distinguished by the form and structure, any conventionally known structure such as a stacked (flat) battery or a wound (cylindrical) battery can be used. It can also be applied to form and structure. Moreover, when viewed in terms of electrical connection form (electrode structure) in the sodium ion secondary battery, it can be applied to both (internal parallel connection type) batteries and bipolar (internal series connection type) batteries. .

  Hereinafter, the present invention will be described in detail with reference to examples. In addition, this invention is not limited to the Example shown below at all.

<Example 1>
Sodium peroxide (Na ₂ O ₂ : manufactured by Wako Pure Chemical Industries, Ltd .: purity 99.8%), manganese (III) oxide (Mn ₂ O ₃ : manufactured by Kojundo Chemical Laboratory Co., Ltd .: purity 99.9%) , And iron (III) oxide (Fe ₂ O ₃ : manufactured by Kojundo Chemical Laboratory Co., Ltd .: purity 99%), the molar ratio of Na: Mn: Fe is 5/6: 1/2: 1/2. And weighed for 30 minutes in an agate mortar to obtain a mixture of metal-containing compounds. The obtained mixture was filled in an alumina boat and fired at 900 ° C. for 12 hours in an air atmosphere using an electric furnace, whereby the composite metal oxide of Example 1 (Na _5/6 Fe _1/2 Mn _1/2 O ₂ ) was obtained.

  A positive electrode active material composed of a composite metal oxide, acetylene black as a conductive material, and polybilinidene fluoride as a binder, positive electrode active material: conductive material: binder = 80: 10: 10 (mass ratio) ) To obtain the composition of Then, first, the positive electrode active material and the conductive material were sufficiently mixed in an agate mortar, and N-methylpyrrolidone was added to the mixture, and the mixture was then mixed uniformly to make a slurry. Next, the obtained positive electrode active material slurry was applied to an aluminum foil having a thickness of 20 μm, which is a current collector, with a thickness of 80 μm using an applicator, and this was put into a dryer, and N-methylpyrrolidone was added. While being removed, the electrode sheet was obtained by sufficiently drying. This electrode sheet was punched out to a diameter of 1.0 cm with an electrode punching machine, and then sufficiently pressed with a hand press to obtain a positive electrode.

  NMP, which is a solvent, with respect to a solid content of 90% by mass of hard carbon (“Carbotron P”, manufactured by Kureha Co., Ltd.) having an average particle size of about 10 μm as a negative electrode active material and 10% by mass of polyvinylidene fluoride as a binder An appropriate amount of (N-methylpyrrolidone) was added to prepare a negative electrode active material slurry. On the other hand, a nickel mesh was prepared as a current collector for the negative electrode. The negative electrode active material slurry prepared above was applied to one surface of the prepared current collector by the doctor blade method to form a coating film. Next, this coating film was vacuum dried at 80 ° C. to obtain a negative electrode.

A coin-type sodium secondary battery was produced using a negative electrode produced using the above hard carbon as the working electrode and a positive electrode produced using the composite metal oxide as the counter electrode. As the electrolytic solution, a 1M electrolyte salt (NaClO ₄ ) dissolved in a non-aqueous solvent (propylene carbonate) was used. The sodium secondary battery was produced in a glove box filled with argon.

<Example 2>
In the production of the composite metal oxide, the composite metal oxide of Example 2 (Na _{0.86 was} used) in the same manner as in Example 1 except that the molar ratio of Na: Mn: Fe was changed to 86:45:55. Fe _0.55 Mn _0.45 O ₂ ) was produced. And the coin type | mold sodium secondary battery of Example 2 was produced by the method similar to Example 1 except having used the positive electrode produced using the composite metal oxide of Example 2 for a counter electrode.

< Reference Example 1 >
In the production of the composite metal oxide, the composite metal oxide of Reference Example 1 (Na _{1) was} prepared in the same manner as in Example 1 except that the molar ratio of Na: Mn: Fe was changed to 1: 1/3: 2/3. Fe _1/3 Mn _2/3 O ₂ ) was produced. And the coin type | mold sodium secondary battery of the reference example 1 was produced by the method similar to Example 1 except having used the positive electrode produced using the composite metal oxide of the reference example 1 for a counter electrode.

<Evaluation 1>
Powder X-ray diffraction measurement was performed on the composite metal oxides of Examples and Reference Examples . The measurement was performed under the following conditions using a Rigaku powder X-ray diffraction measurement device MultiFlex. The measurement result of Example 1 is shown in FIG. 1 (a), the measurement result of Example 2 is shown in FIG. 1 (b), and the measurement result of Reference Example 1 is shown in FIG. 1 (c).
X-ray: CuKα
Voltage-current: 30kV-20mA
Measurement angle range: 2θ = 10 to 70 °
Step: 0.01 °
Scan speed: 1 ° / min

It can be confirmed from FIGS. 1A to 1C that the width of the peak near 48 is about 0.3 degrees. Thereby, it was confirmed that the composite metal oxide of an Example and a reference example is an oxide which has P2 structure.

<Evaluation 2>
The charge / discharge evaluation of the sodium secondary battery of Example 1 was performed. The current density was set to 12 mA / g for each electrode, and constant current charging was performed up to 4.3 V (charging voltage). After charging, the current density was set to 12 mA / g for each electrode, and constant current discharge was performed up to 1.5 V (discharge voltage). This charge / discharge cycle was performed 30 times, and the relationship between the number of cycles from the first cycle to the 12th cycle and the discharge capacity is shown in FIG. The charge / discharge of evaluation 1 was performed under the condition of a temperature of 25 ° C.

  As is clear from the results of FIG. 2, it was confirmed that the sodium secondary battery of Example 1 exhibited a high discharge capacity (capacity) exceeding about 200 mAh / g even after several cycles of charge / discharge depending on conditions. It was. In addition, it was confirmed that when the charge and discharge are repeated, the discharge capacity may be slightly lower than 200 mAh / g, but a high discharge capacity is stably exhibited.

<Evaluation 2>
The charge / discharge evaluation of the sodium secondary battery of Example 2 was performed in the same manner as the charge / discharge evaluation of the sodium secondary battery of Example 1. FIG. 3 shows the relationship between the number of cycles from the first cycle to the 10th cycle and the discharge capacity.

  As shown in FIG. 3, it was confirmed that the sodium secondary battery of Example 2 exhibited a high discharge capacity (capacity) exceeding about 150 mAh / g even after several cycles of charge / discharge depending on the conditions.

<Evaluation 3>
Charging / discharging evaluation of the sodium secondary battery of Example 1 was conducted except that the charging / discharging evaluation of the sodium secondary battery of Reference Example 1 was performed with 20 cycles of charging / discharging with a charging voltage of 4 V and a discharging voltage of 1.5 V. The same method was used. The relationship between the number of cycles and the discharge capacity is shown in FIG.

It was confirmed that the sodium secondary battery of Reference Example 1 stably exhibited a high discharge capacity exceeding 100 mAh / g even after 20 cycles of charge and discharge.

Claims (1)

A sodium secondary battery comprising a positive electrode having a positive electrode active material composed of a composite metal oxide, a negative electrode capable of inserting and extracting sodium ions, and an electrolyte,
The composite metal oxide is represented by the following formula (I) and is composed of an oxide having a P2 structure, and the electrolyte containing an anion represented by the following formula (II): except.).
Na _x Fe _y Mn _1-y O ₂ ... (I)
(X in the formula (I) is less than 1, and y is 1/3 or more and less than 2/3.)

(R1 and R2 in the formula (II) are the same or different and are each a fluoro group or a fluoroalkyl group)

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