JP2016157602A - Positive electrode active material for sodium secondary battery and production method therefor, and sodium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high capacity sodium ion secondary battery capable of repeating charge and discharge stably.SOLUTION: A positive electrode active material for sodium secondary battery contains a sodium-containing multiple oxide having P2 type crystal structure, and represented by Na(LiM)O, where 0<x<1.0 and 0.12<a<0.5. The element M contains at least Mn and Ni, and in the powder X-ray diffraction of the sodium-containing multiple oxide using CuKα ray, no peak is observed in a range of 18°-25°.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a positive electrode active material for a sodium secondary battery including a sodium-containing composite oxide having a P2 type crystal structure, a method for producing the same, and a sodium ion secondary battery.

  In place of lithium ion secondary batteries, development of sodium ion secondary batteries using abundant and inexpensive sodium as a main raw material is in progress. As a positive electrode active material for a sodium ion secondary battery, a sodium-containing composite oxide having a P2 type crystal structure is promising because of its high electrochemical stability.

For example, Patent Document 1 proposes that a part of T is replaced with Li in a P2 type sodium-containing composite oxide represented by Na _x TO ₂ (T is a transition metal). Li incorporated in the transition metal site is believed to stabilize the crystal structure and improve cycle characteristics.

JP 2014-160653 A

From the viewpoint of stabilizing the crystal structure of the P2-type sodium-containing composite oxide represented by Na _x TO ₂ , it is desirable to substitute a considerable amount of transition metal with Li. It has also been found that by replacing part of T with Li, the sodium content in the composite oxide can be increased and the capacity can be improved. However, when the amount of Li increases, it becomes difficult to synthesize a P2-type sodium-containing transition metal oxide. For example, the purity of the positive electrode active material decreases, and the stability of the positive electrode active material with respect to moisture decreases. As a result, the production of the positive electrode is hindered.

One aspect of the present invention has a P2-type crystal structure and is represented by Na _x [Li _a M _1-a ] O ₂ , where 0 <x <1.0 and 0.12 <a <0.5. The element M contains at least Mn and Ni, and ranges from 2θ = 18 ° to 25 ° in the powder X-ray diffraction image of the sodium-containing composite oxide using CuKα rays. The present invention relates to a positive electrode active material for a sodium secondary battery in which no peak is observed.

  Another aspect of the present invention relates to a sodium ion secondary battery comprising a positive electrode including the positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte having sodium ion conductivity.

  Still another aspect of the present invention includes a step of preparing a raw material containing a sodium salt, a lithium salt, and a salt containing the element M, and the raw material in an oxidizing atmosphere or an inert gas atmosphere at 700 ° C. And a step of firing at a temperature lower than 800 ° C. to form a sodium-containing composite oxide, and with respect to a total of lithium contained in the lithium salt and element M contained in the salt containing element M The molar ratio x of sodium contained in the sodium salt satisfies 0 <x <1.0, the element M contains at least Mn and Ni, and contains lithium and the element M contained in the lithium salt. The present invention relates to a method for producing a positive electrode active material for a sodium secondary battery, wherein a molar ratio a of lithium contained in the lithium salt with respect to a total of the element M contained in the salt satisfies 0.12 <a <0.5. .

  The positive electrode active material according to the present invention can provide a high-capacity sodium ion secondary battery that can be stably charged and discharged.

It is a longitudinal cross-sectional view of the sodium ion secondary battery which concerns on one Embodiment of this invention. It is a figure which shows the relationship between the calcination temperature of a raw material, and the XRD pattern of the composite oxide to produce | generate. It is a SEM image of complex oxide concerning an example. It is a figure which shows the charging / discharging curve of the sodium ion secondary battery which concerns on an Example. It is a figure which shows the relationship between the charging / discharging cycle number of the sodium ion secondary battery which concerns on an Example, and discharge capacity.

[Description of Embodiment of the Invention]
First, the contents of the embodiment of the present invention will be listed and described.
One embodiment of the present invention has (1) a crystal structure of P2 type, and has a general formula: Na _x [Li _a M _1-a ] O ₂ (where 0 <x <1.0, 0.12 < a <0.5), the element M contains at least Mn and Ni, and 2θ = in the powder X-ray diffraction image of the sodium-containing composite oxide using CuKα rays. The present invention relates to a positive electrode active material for a sodium secondary battery in which no peak is observed in the range of 18 ° to 25 °. The sodium ion secondary battery including the positive electrode active material has a high capacity, and the deterioration of the capacity is small even after repeated charge and discharge.

  The above general formula preferably satisfies 0.79 <x. Thereby, since sodium content in sodium content complex oxide becomes large, it becomes easy to obtain high capacity.

  The above general formula preferably satisfies 0.13 <a <0.5. Thereby, it becomes easier to increase the sodium content in the sodium-containing composite oxide.

The element M is preferably represented by Ni _y A _z Mn _1-yz (where 0.1 ≦ y <0.33, 0 ≦ z <0.1). Element A is an element other than Ni and Mn. Thereby, the crystal structure of the sodium-containing composite oxide can be more easily stabilized, and a high-capacity and inexpensive positive electrode active material can be obtained.

  The sodium ion secondary battery which concerns on one Embodiment of this invention comprises the positive electrode containing the said positive electrode active material, the negative electrode containing a negative electrode active material, and the nonaqueous electrolyte which has sodium ion conductivity. By using a positive electrode including the positive electrode active material, a high-capacity sodium ion secondary battery that can stably charge and discharge can be provided.

  A method for producing a positive electrode active material for a sodium secondary battery according to an embodiment of the present invention includes: (i) a step of preparing a raw material containing a sodium salt, a lithium salt, and a salt containing the element M; ) Firing the raw material at a temperature higher than 700 ° C. and lower than 800 ° C. in an oxidizing atmosphere or an inert gas atmosphere to form a sodium-containing composite oxide. However, the molar ratio x of sodium contained in the sodium salt with respect to the sum of the lithium contained in the lithium salt and the element M contained in the salt containing the element M satisfies 0 <x <1.0. The element M includes at least Mn and Ni. Furthermore, the molar ratio “a” of lithium contained in the lithium salt with respect to the sum of lithium contained in the lithium salt and element M contained in the salt containing element M satisfies 0.12 <a <0.5. According to the above production method, it is possible to obtain a sodium-containing composite oxide in which no peak is observed in the range of 2θ = 18 ° to 25 ° in a powder X-ray diffraction image using CuKα rays.

  When the molar ratio of nickel and manganese contained in the salt containing the element M is represented by y: 1-yz, it is preferable that 0.1 ≦ y <0.33 and 0 ≦ z <0.1 are satisfied. Thereby, the crystal structure of the sodium-containing composite oxide can be more easily stabilized, and a high-capacity and inexpensive positive electrode active material can be obtained.

[Details of the embodiment of the invention]
Next, the embodiment of the present invention will be described more specifically. In addition, this invention is not limited to these illustrations, is shown by the attached claim, and is intended that all the changes within the meaning and range equivalent to the claim are included. .

<Positive electrode active material>
The positive electrode active material has a P2 type crystal structure, and has a general formula: Na _x [Li _a M _1-a ] O ₂ (where 0 <x <1.0, 0.12 <a <0.5) A sodium-containing composite oxide represented by That is, the sodium-containing composite oxide has a structure in which a part of the element M of Na _x MO ₂ having a P2 type crystal structure is substituted with Li. The element M has a higher valence than Li and contains at least Mn and Ni. For example, when divalent Ni is substituted with monovalent Li, the positive charge is insufficient. Such deficiencies are compensated by increasing the sodium content.

  As the amount of the element M substituted with Li increases, it becomes increasingly difficult to synthesize a composite oxide having a single-phase P2-type crystal structure. Therefore, the third component having no desired crystal structure is likely to be mixed in the sodium-containing composite oxide. When the above general formula satisfies 0.12 <a <0.5, the amount of the element M substituted with Li exceeds 12 mol%, and thus the third component is usually mixed. In that case, a peak is observed in the range of 2θ = 18 ° to 25 ° in the powder X-ray diffraction image of the sodium-containing composite oxide using CuKα rays.

  On the other hand, by strictly controlling the production conditions of the sodium-containing composite oxide, even when the above general formula satisfies 0.12 <a <0.5, the mixing of the third component can be avoided. This makes it possible to obtain a sodium-containing composite oxide in which no peak is observed in the range of 2θ = 18 ° to 25 ° in a powder X-ray diffraction image using CuKα rays.

The sodium-containing composite oxide does not easily react with water and does not easily change in the air or an aqueous solution. For example, 1 g of the sodium-containing composite oxide is mixed with 20 cm ³ of water, and the pH of water in the dispersion obtained by stirring at 25 ° C. for 5 minutes or more is 12 or less. The fact that no peak is observed in the range of 2θ = 18 ° to 25 ° in the powder X-ray diffraction image using CuKα ray and that the pH of water in the dispersion of the composite oxide is 12 or less is a technique. It means the same thing. That is, it is not always necessary to measure the powder X-ray diffraction image of the sodium-containing composite oxide using CuKα rays, and the conditions of the present invention are satisfied by measuring the pH of water in the dispersion of the composite oxide. It can also be determined.

  The pH of the dispersion is preferably 11.5 or less, more preferably 11.0 or less, and still more preferably 10.5 or less. The pH is usually 7.0 or higher. As the water, pure water having a specific resistance value at 25 ° C. of 1 to 10 MΩ · cm may be used.

Here, the P2-type crystal structure will be further described. A typical P2-type crystal structure belongs to the space group P63 / mmc because of its symmetry. The basic P2 type crystal has a layered structure, but the P2 type mother structure may include P3 type and / or O3 type stacking faults. Whether or not the composite oxide has a P2-type crystal structure can be confirmed by X-ray diffraction. For example, Na _2/3 [Ni _1/3 Mn _2/3 ] O ₂ has a typical P2 type crystal structure.

  The sodium-containing composite oxide according to the present invention has only the above composition and has a P2 type crystal structure, and details of the peak position in X-ray diffraction are not particularly limited. However, in the powder X-ray diffraction measurement using CuKα ray as the X-ray source, no peak is observed in the range of 2θ = 18 ° to 25 °. When no peak is observed in the range of 18 ° to 25 °, it can be determined that the third component other than the P2 type crystal structure is hardly contained in the sodium-containing composite oxide. However, the peak is a substantial peak whose presence can be confirmed objectively, and a peak that can be determined as noise is not included.

  The above general formula only needs to satisfy 0 <x <1.0, but the x value is selected so that the P2-type crystal structure is maintained. For example, it preferably satisfies 0.79 <x, more preferably satisfies 0.80 <x <0.95, and particularly preferably satisfies 0.84 <x <0.90. When the x value is 1 or more, the O3 type crystal structure becomes stable, and it becomes difficult to form the P2 type crystal structure.

  In addition, the said range is a range of x value in the composition of the sodium containing complex oxide produced | generated by baking of a raw material. The x value increases or decreases due to charging / discharging of the sodium ion secondary battery. The x value decreases during charging and increases during discharging. The lower limit of the x value during charging is preferably 0.1 ≦ x, and more preferably 0.2 ≦ x. The upper limit of the x value during discharge is preferably x <1.0, and more preferably x ≦ 0.9.

  The above general formula only needs to satisfy 0.12 <a <0.5, but the value a is selected so that the P2-type crystal structure is maintained and the generation of the third component is suppressed. For example, 0.13 <a <0.5 is preferably satisfied, 0.14 <a <0.3 is more preferable, and 0.15 <a <0.2 is particularly preferable. When the a value is 0.12 or less, it is not necessary to strictly control the production conditions of the sodium-containing composite oxide. For example, even when the raw material is fired at a relatively high temperature, it is possible to produce a sodium-containing composite oxide having a P2 crystal structure substantially free of the third component.

The element M includes at least nickel and manganese. More specifically, the element M is Ni _y A _z Mn _1-yz (where 0.1 ≦ y <0.33, 0 ≦ z <0.1). It is preferable to be represented by However, the element A is an element other than Ni and Mn. Examples of the element A include Mg, Al, Co, Ti, and the like. The y value may satisfy 0.1 ≦ y <0.33, but from the viewpoint of securing the amount of the element M to be substituted with Li, 0.1 <y <0.30 is preferable, and 0.15 <y <0.25 is more preferable, and 0.16 ≦ y ≦ 0.23 is still more preferable.

(Average primary particle size)
The average primary particle size of the sodium-containing composite oxide is not particularly limited, but is, for example, 0.1 μm or more, preferably 0.2 μm or more, and more preferably 0.3 μm or more. If it is this range, it can be said that the P2 type crystal has fully grown. Moreover, an average primary particle diameter is 5 micrometers or less, for example, 3 micrometers or less are preferable and 2 micrometers or less are more preferable. If it is this range, the reaction area of complex oxide can be ensured enough and the positive electrode active material excellent in the output characteristic can be obtained. The average primary particle diameter may be obtained as an average value of the maximum diameters of the selected primary particles by arbitrarily selecting 10 to 30 primary particles in the scanning electron microscope (SEM) image of the composite oxide.

  When a positive electrode including the sodium-containing composite oxide is prepared and a battery is assembled using metallic sodium for the negative electrode (counter electrode), the open circuit voltage (OCV) is, for example, 1.5 V or more. The positive electrode can be charged / discharged in the range of 1.5 V to 4.6 V with respect to the redox potential of metallic sodium. However, from the viewpoint of stably repeating the charge / discharge cycle, the upper limit voltage during charging is preferably less than 4.6V, more preferably 4.5V or less, and even more preferably 4.4V or less. Moreover, 1.7V or more is preferable, as for the lower limit voltage at the time of discharge, 1.8V or more is more preferable, and 1.9V or more is still more preferable.

(Method for producing sodium-containing composite oxide)
Next, as a method for producing the sodium-containing composite oxide, a method having a step (i) of preparing a raw material and a step (ii) of firing the raw material will be described. However, the manufacturing method of a sodium containing complex oxide is not limited to the following.

Process (i)
First, a raw material containing a sodium salt, a lithium salt, and a salt containing the element M is prepared. Examples of the salt containing sodium salt, lithium salt and element M include oxides, hydroxides, oxyhydroxides, bicarbonates, carbonates, nitrates, sulfates, halides, organic acid salts (for example, oxalates) ) Etc. can be used.

Specifically, Na ₂ CO ₃ , NaHCO ₃ , Na ₂ O and the like can be used as the sodium salt. Of these, Na ₂ CO ₃ is the easiest to handle.

As the lithium salt, LiOH, Li ₂ CO ₃ , Li ₂ O, or the like can be used. Of these, Li ₂ CO ₃ is the easiest to handle.

As the salt containing the element M, a double salt containing nickel and manganese may be used, but a nickel salt and a manganese salt as shown below may be used in combination. As the nickel salt, NiCO ₃ , Ni (OH) ₂ , NiOOH, NiO, NiNO ₃ , Ni ₂ SO ₄ or the like can be used. Of these, Ni (OH) ₂ , NiOOH and NiO are easy to handle. As the manganese salt, MnO ₂ , Mn ₂ O ₃ , Mn ₃ O ₄ or the like can be used. The double salt containing nickel and manganese can be prepared as an oxide or hydroxide by, for example, a crystallization method or a coprecipitation method.

  The raw material is a sodium salt such that the molar ratio x of sodium contained in the sodium salt to the total of the lithium contained in the lithium salt and the element M contained in the salt containing the element M is 0 <x <1. And a salt containing element M. The x value is selected so as to satisfy, for example, 0.84 <x <0.90 depending on the composition of the desired sodium-containing composite oxide. Similarly, the molar ratio a of lithium contained in the lithium salt to the sum of the lithium contained in the lithium salt and the element M contained in the salt containing the element M satisfies 0.12 <a <0.5. A salt containing a lithium salt and an element M is included in the raw material. The a value is selected so as to satisfy, for example, 0.15 <a <0.2, depending on the composition of the desired sodium-containing composite oxide.

In the desired sodium-containing composite oxide, when the element M is represented by Ni _y A _z Mn _1-yz (where 0.1 ≦ y <0.33, 0 ≦ z <0.1), the element What is necessary is just to include the salt containing the element A in a predetermined ratio in the salt containing M. Specifically, a salt containing the element M may be used so that the molar ratio of nickel to manganese satisfies y: 1-yz.

  A mixture containing raw materials, that is, a salt containing a sodium salt, a lithium salt, and an element M may be mixed dry or wet using a mixing device such as a ball mill, a V-type mixer, or a stirrer.

Step (ii)
Next, the raw material is fired at a temperature higher than 700 ° C. and lower than 800 ° C. in an oxidizing atmosphere or an inert gas atmosphere to generate a sodium-containing composite oxide. When the firing temperature is 700 ° C. or lower, the P2-type crystal structure does not grow sufficiently, and a sodium-containing composite oxide containing raw material components is generated. When the firing temperature is 800 ° C. or higher, a P2-type crystal structure is obtained, but a sodium-containing composite oxide containing a third component different from the desired crystal structure is generated. In view of the fact that the firing temperature of the composite oxide used as the positive electrode active material is generally 800 ° C. or higher and is often fired near 900 ° C., the firing temperature for obtaining the sodium-containing composite oxide is described above. Can be said to be relatively cold.

  The firing temperature is most desirably 750 ° C. and the vicinity thereof, but it may be 720 ° C. to 780 ° C., and preferably 730 ° C. to 770 ° C. The firing temperature is an average temperature during a period excluding the temperature raising process and the temperature lowering process (hereinafter, the main firing period). Therefore, for example, it is allowed to fall below 700 ° C. or exceed 800 ° C. instantaneously or for a short time during the main baking period. However, about 80% of the main firing period, the temperature of the firing atmosphere is preferably maintained at 720 to 780 ° C. (preferably 730 to 770 ° C.).

  The main firing period is desirably 5 hours or longer, more preferably 8 hours to 15 hours, and still more preferably 10 hours to 12 hours. Thereby, a P2 type crystal structure can be grown more stably.

When the raw material is fired in an oxidizing atmosphere, it may be fired in air at atmospheric pressure. Alternatively, the raw material may be fired in an atmosphere having a higher oxygen concentration than air. When the raw material is fired in an inert gas atmosphere, it is desirable to use a raw material containing a sufficient oxygen source. Examples of the inert gas atmosphere include an inert atmosphere containing nitrogen, argon, helium and the like. The pressure of the oxidizing atmosphere and inert gas atmosphere is not particularly restricted as long as it is for example ^{0.9 × 10 5 Pa~1.1 × 10 5} Pa.

  Before performing the firing, the raw material may be temporarily fired in a temperature range of 200 to 500 ° C. in the same atmosphere as described above. By calcination, volatile components (for example, moisture) contained in the raw material can be removed gently. This facilitates crystal growth in the subsequent main firing period.

<Sodium ion secondary battery>
Next, a sodium ion secondary battery will be described. The sodium ion secondary battery includes a positive electrode including the positive electrode active material, a negative electrode including a negative electrode active material, and a nonaqueous electrolyte having sodium ion conductivity. FIG. 1 is a longitudinal sectional view schematically showing a sodium ion secondary battery 100 according to an embodiment. The sodium ion secondary battery includes a stacked electrode group, an electrolyte (not shown), and a rectangular battery case 10 that houses them. The battery case 10 is made of, for example, aluminum, and includes a bottomed container main body 12 having an upper opening and a lid 13 that closes the upper opening.

  In the center of the lid 13, a safety valve 16 is provided for releasing gas generated inside when the internal pressure of the battery case 10 rises. An external negative electrode terminal 14 that penetrates the lid body 13 is provided near one side of the lid body 13 with the safety valve 16 at the center, and an external side that penetrates the lid body 13 is located at the other side of the lid body 13. A positive terminal is provided.

  Each of the stacked electrode groups is composed of a plurality of sheet-like positive electrodes 2, a plurality of negative electrodes 3, and a plurality of separators 1 interposed therebetween. A positive electrode lead piece 2 a is formed at one end of each positive electrode 2. The positive electrode lead pieces 2 a of the plurality of positive electrodes 2 are bundled and connected to an external positive electrode terminal provided on the lid 13 of the battery case 10. Similarly, a negative electrode lead piece 3 a is formed at one end of each negative electrode 3. The negative electrode lead pieces 3 a of the plurality of negative electrodes 3 are bundled and connected to an external negative electrode terminal 14 provided on the lid 13 of the battery case 10.

  Both the external positive terminal and the external negative terminal 14 are columnar, and at least a portion exposed to the outside has a screw groove. A nut 7 is fitted in the screw groove of each terminal, and the nut 7 is fixed to the lid 13 by rotating the nut 7. A flange 8 is provided in a portion of each terminal accommodated in the battery case 10, and by rotation of the nut 7, the flange 8 attaches an O-ring-shaped gasket 9 to the inner surface of the lid 13. Fixed through.

(Positive electrode)
The positive electrode includes, for example, a positive electrode current collector and the positive electrode active material (or positive electrode mixture) supported on the positive electrode current collector. The positive electrode current collector may be a metal foil or a metal porous body. As the material of the positive electrode current collector, aluminum, an aluminum alloy, or the like is preferable from the viewpoint of stability at the positive electrode potential.

As the positive electrode active material, the above-mentioned sodium-containing composite oxide may be used alone, or may be used in combination with a material that occludes and releases (or inserts and desorbs) other sodium ions. The material used in combination is not particularly limited. For example, sodium chromite (NaCrO ₂ ), sodium nickel manganate (NaNi _0.5 Mn _0.5 O ₂ , Na _2/3 Ti _1/6 Ni _1/3 Mn _1/2 O ₂ ), sodium iron cobaltate (NaFe _0.5 Co _0.5 O ₂ etc.), sodium manganate (Na _2/3 Fe _1/3 Mn _2/3 O ₂ etc.) and the like.

  In addition to the positive electrode active material, the positive electrode mixture can further contain a conductive additive and / or a binder. The positive electrode is obtained by applying or filling a positive electrode mixture to a positive electrode current collector, drying, and rolling the dried product in the thickness direction as necessary. The positive electrode mixture is usually used in the form of a slurry containing a dispersion medium.

  Examples of the conductive assistant include carbon black, graphite, and / or carbon fiber. Examples of the binder include a fluorine resin, a polyolefin resin, a rubber-like polymer, a polyamide resin, a polyimide resin (such as polyamideimide), and / or a cellulose ether. As the dispersion medium, for example, water is used in addition to an organic solvent such as N-methyl-2-pyrrolidone (NMP).

(Negative electrode)
The negative electrode includes, for example, a negative electrode current collector and a negative electrode active material (or a negative electrode mixture) supported on the negative electrode current collector. Similar to the positive electrode current collector, the negative electrode current collector may be a metal foil or a metal porous body. The material of the negative electrode current collector is preferably copper, copper alloy, nickel, nickel alloy, stainless steel, or the like because it is not alloyed with sodium and is stable at the negative electrode potential.

  Examples of the negative electrode active material include a material that reversibly occludes and releases (or inserts and desorbs) sodium ions, and a material that is alloyed with sodium. Any of these materials is a material that develops capacity by a Faraday reaction. Examples of such a negative electrode active material include metals such as sodium, titanium, zinc, indium, tin, and silicon, or alloys thereof, or compounds thereof; and carbonaceous materials.

Examples of the metal compound include lithium-containing titanium oxides such as lithium titanate (such as Li ₂ Ti ₃ O ₇ and / or Li ₄ Ti ₅ O ₁₂ ), and sodium titanate (Na ₂ Ti ₃ O ₇ and / or Na _4). Examples thereof include sodium-containing titanium oxides such as Ti ₅ O ₁₂ . Examples of the carbonaceous material include graphitizable carbon (soft carbon) and / or non-graphitizable carbon (hard carbon). A negative electrode active material can be used individually by 1 type or in combination of 2 or more types.

  The negative electrode can be formed by, for example, applying or filling a negative electrode mixture containing a negative electrode active material to a negative electrode current collector, drying it, and rolling the dried product in the thickness direction in accordance with the case of the positive electrode. The negative electrode active material may be pre-doped with sodium ions as necessary.

  The negative electrode mixture can further contain a conductive additive and / or a binder in addition to the negative electrode active material. The negative electrode mixture is usually used in the form of a slurry containing a dispersion medium. As a conductive support agent, a binder, and a dispersion medium, it can respectively select from what was illustrated about the positive electrode suitably.

(Separator)
As the separator, for example, a resin microporous film, a nonwoven fabric, or the like can be used. Examples of the material of the separator include polyolefin resin, polyphenylene sulfide resin, polyamide resin, and polyimide resin.

[Nonaqueous electrolyte]
The nonaqueous electrolyte is not particularly limited as long as it has sodium ion conductivity. For example, in addition to an organic electrolyte in which a salt (sodium salt) of sodium ions and anions is dissolved in an organic solvent, an ionic liquid containing sodium ions and anions is used. The concentration of the sodium salt in the nonaqueous electrolyte may be, for example, 0.3 to 3 mol / liter.

The kind of the anion (first anion) constituting the sodium salt is not particularly limited. For example, hexafluorophosphate ion (PF ₆ ⁻ ), tetrafluoroborate ion (BF ₄ ⁻ ), perchlorate ion (ClO _4). ^-), lithium bis (oxalato) borate ion _{(B (C 2 O 4)} 2 -), trifluoromethanesulfonate ion (CF ₃ SO ₃ ^-), etc. bissulfonylamide anions. A sodium salt may be used individually by 1 type, and may use it in combination of 2 or more types of sodium salts from which the kind of 1st anion differs.

  When an ionic liquid is used for the nonaqueous electrolyte, the content of the ionic liquid in the nonaqueous electrolyte is preferably 80% by mass or more, and more preferably 90% by mass or more. The non-aqueous electrolyte may contain a small amount of an organic solvent or an additive in addition to the ionic liquid. The “ionic liquid” is a molten salt (molten salt).

  When an organic electrolyte is used for the nonaqueous electrolyte, the total amount of the organic solvent and the sodium salt in the nonaqueous electrolyte is preferably 80% by mass or more of the nonaqueous electrolyte, and more preferably 90% by mass or more. . The non-aqueous electrolyte may contain a small amount of an ionic liquid or an additive in addition to the organic electrolyte.

  As the organic solvent, for example, cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate; cyclic carbonates such as γ-butyrolactone can be preferably used. . An organic solvent may be used individually by 1 type, and may be used in combination of 2 or more type.

  The ionic liquid may further contain a second cation in addition to the sodium ion (first cation). Such a second cation is preferably an organic cation. A 2nd cation can be used individually by 1 type or in combination of 2 or more types. As the organic cation, a nitrogen-containing onium cation is preferable.

  Hereinafter, the present invention will be described in more detail based on examples. However, the following examples do not limit the present invention.

Example 1
<Synthesis of sodium-containing composite oxide>
Na ₂ CO ₃ , Li ₂ CO ₃ , Ni (OH) ₂ and Mn ₂ O ₃ have a Na: Li: Ni: Mn molar ratio of 0.85: 0.15: 0.2: 0.65. And weighed for 12 hours in a 600 rpm ball mill to obtain a raw material. The obtained raw material was molded into pellets and then calcined at 750 ° C. for 12 hours in air at atmospheric pressure to obtain a composite oxide of Example 1.

[Evaluation 1]
Powder X-ray diffraction measurement of the composite oxide of Example 1 was performed under the following conditions to identify the crystal structure. As the measuring device, a powder X-ray diffraction measuring device (MultiFlex) manufactured by Rigaku Corporation was used.
X-ray: CuKα
Voltage-current: 40kV-20mA
Measurement angle range: 2θ = 10 to 70 °
Step: 0.02 °
Scanning speed: 6 ° / min

As a result of the powder X-ray diffraction image (XRD pattern) and Rietveld analysis, the composite oxide of Example 1 has a P2-type crystal structure and is represented by Na _0.85 [Li _0.15 Ni _0.2 Mn _0.65 ] O _2. found.

(Comparative Examples 1-4)
Example 1 except that the firing temperature of the raw material was changed from 750 ° C. to 700 ° C. (Comparative Example 1), 800 ° C. (Comparative Example 2), 850 ° C. (Comparative Example 3) or 900 ° C. (Comparative Example 4). Similarly, a composite oxide was synthesized.

  The XRD patterns of Example 1 and Comparative Examples 1 to 4 are shown in FIG. From FIG. 2, it can be understood that the XRD pattern of the composite oxide to be generated varies greatly depending on the firing temperature of the raw material. Specifically, when the firing temperature is 700 ° C. (Comparative Example 1), the growth of the P2 type crystal structure is insufficient, and the peak of the P3 type crystal is also observed. When the firing temperature is 800 ° C. or higher (Comparative Examples 2 to 4), one or two or more peaks are observed in the range of 2θ = 18 ° to 25 °. On the other hand, when the firing temperature is 750 ° C. (Example 1), it can be seen that a single-phase P2-type crystal structure is obtained.

  Next, FIG. 3 shows an SEM image of the composite oxide of Example 1. From FIG. 3, it can be seen that the crystals have grown until the average primary particle size becomes 1-2 μm.

[Evaluation 2]
The pH was measured for the composite oxides of Example 1 and Comparative Examples 1 to 4. A dispersion was prepared by mixing 1 g of the composite oxide with 20 cm ³ of pure water, and the dispersion was stirred at 25 ° C. for 5 minutes, and then the pH of water in the dispersion was measured. As a result, the pH of water in the dispersion containing the composite oxide of Example 1 was 10.0. On the other hand, the pH of the water in the dispersion liquid containing the composite oxides of Comparative Examples 1 to 4 exceeded 12.

[Evaluation 3]
An appropriate amount of N-methyl-2-pyrrolidone (NMP) containing the composite oxide of Example 1, acetylene black as a conductive material, and polyvinylidene fluoride as a binder in a mass ratio of 80:10:10. Was prepared as a dispersion medium. The obtained slurry was applied to one side of a 20 μm thick aluminum foil serving as a current collector. The thickness of the coating film was 40 μm. After sufficiently drying the coating film, it was punched out together with an aluminum foil to obtain a coin-shaped positive electrode having a diameter of 1.0 cm.

A coin-shaped sodium ion secondary battery (hereinafter referred to as a test battery) was prepared using the positive electrode and a negative electrode made of metallic sodium. As the non-aqueous electrolyte, a mixed solvent of propylene carbonate and fluoroethylene carbonate containing 1 mol / L NaClO ₄ in a volume ratio of 98: 2 was used. A glass filter was used as the separator.

  The obtained coin battery was charged and discharged at a current density of 13.4 mA / g per weight of the positive electrode active material in a range of 2.0 V to 4.4 V. FIG. 4 shows a charge / discharge curve of the sodium ion secondary battery up to 20 cycles. FIG. 5 shows the relationship between the number of charge / discharge cycles and the discharge capacity. 4 and 5, it can be understood that although a special plateau is observed at the time of charging in the first cycle, good cycle characteristics are obtained as a whole.

[Evaluation 4]
Four test batteries were prepared as described above. Of these, 3 cells were charged to 3.4V, 3.9V and 4.4V. One cell was charged to 4.2V and then discharged to 2.0V. The composite oxide was taken out from the positive electrodes of the cells in these four states and subjected to powder X-ray diffraction measurement. As a result, it was found that the P2-type crystal structure was maintained in any state, that is, the crystal structure was stable.

(Example 2)
Na ₂ CO ₃ , Li ₂ CO ₃ , Ni (OH) ₂ and Mn ₂ O ₃ have a Na: Li: Ni: Mn molar ratio of 0.90: 0.15: 0.23: 0.63. Thus, the composite oxide of Example 2 was obtained in the same manner as in Example 1 except that the raw material was obtained by weighing.

(Example 3)
Na ₂ CO ₃ , Li ₂ CO ₃ , Ni (OH) ₂ , Mn ₂ O ₃ and Co ₂ O ₃ are mixed at a molar ratio of Na: Li: Ni: Mn: Co of 0.85: 0.15: 0. The composite oxide of Example 3 was obtained in the same manner as in Example 1 except that the raw materials were obtained by weighing and mixing to 16: 0.61: 0.08.

[Evaluation 5]
Each test battery was produced in the same manner as described above except that the composite oxides of Examples 2 and 3 were used. The obtained coin battery was charged and discharged at a current density of 13.4 mA / g per weight of the positive electrode active material in a range of 2.0 V to 4.4 V. The discharge capacity obtained at this time is shown in Table 1 together with the results of Example 1.

  From Table 1, it was found that a high capacity can be obtained when any composite oxide is used.

  The sodium ion secondary battery according to the present invention is useful as a power source for various applications because of its high capacity and excellent cycle characteristics.

  1: separator, 2: positive electrode, 2a: positive electrode lead piece, 3: negative electrode, 3a: negative electrode lead piece, 7: nut, 8: flange, 9: gasket, 10: battery case, 12: container body, 13: lid Body, 14: external negative terminal, 16: safety valve

Claims (8)

Sodium-containing composite oxide having a P2 type crystal structure and represented by Na _x [Li _a M _1-a ] O ₂ , where 0 <x <1.0 and 0.12 <a <0.5 Including
The element M includes at least Mn and Ni,
A positive electrode active material for a sodium secondary battery, wherein no peak is observed in the range of 2θ = 18 ° to 25 ° in a powder X-ray diffraction image of the sodium-containing composite oxide using CuKα rays.   The positive electrode active material for sodium secondary batteries according to claim 1, wherein 0.79 <x.   The positive electrode active material for sodium secondary batteries according to claim 1 or 2, wherein 0.13 <a <0.5. The element M is Ni _y A _z Mn _1-yz , provided that 0.1 ≦ y <0.33 and 0 ≦ z <0.1, and the element A is an element other than Ni and Mn. The positive electrode active material for sodium secondary batteries of any one of Claims 1-3. A positive electrode comprising the positive electrode active material according to claim 1;
A negative electrode containing a negative electrode active material;
A sodium ion secondary battery comprising: a nonaqueous electrolyte having sodium ion conductivity. Preparing a raw material containing a sodium salt, a lithium salt, and a salt containing element M;
Firing the raw material in an oxidizing atmosphere or an inert gas atmosphere at a temperature higher than 700 ° C. and lower than 800 ° C. to produce a sodium-containing composite oxide,
The molar ratio x of sodium contained in the sodium salt to the sum of the lithium contained in the lithium salt and the element M contained in the salt containing the element M satisfies 0 <x <1.0,
The element M includes at least Mn and Ni,
Sodium in which a molar ratio a of lithium contained in the lithium salt with respect to the sum of lithium contained in the lithium salt and element M contained in the salt containing the element M satisfies 0.12 <a <0.5 A method for producing a positive electrode active material for a secondary battery.   The molar ratio between nickel and manganese contained in the salt containing the element M is y: 1-yz, where 0.1 ≦ y <0.33 and 0 ≦ z <0.1. The manufacturing method of the positive electrode active material for sodium secondary batteries as described in 2 ..   The manufacturing method of the positive electrode active material for sodium secondary batteries of Claim 6 or 7 which is 0.79 <x.