JP6865601B2 - Positive electrode active material for sodium ion secondary battery and its manufacturing method, and sodium ion secondary battery - Google Patents

Description

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

Instead of lithium-ion secondary batteries, the development of sodium-ion secondary batteries using sodium, which has abundant resources and is inexpensive, as the main raw material is underway. 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. Among them, the P2-type sodium-containing composite oxide containing Ni and Mn as transition metals has a high operating voltage and can obtain a high capacity (Patent Document 1). However, in terms of stability in the charge / discharge cycle, it cannot be said that it is sufficient as compared with the lithium ion secondary battery currently in practical use, and improvement in cycle characteristics is desired.

On the other hand, a P2-type sodium-containing composite oxide containing Fe has been proposed as a transition metal (Patent Document 2 and Non-Patent Document 1). For example, Na _0.667 Fe _0.5 Mn _0.5 O ₂ has been reported to have excellent cycle characteristics, but the operating voltage is low and it is easy to polarize in the high potential region.

Further, it has been proposed to replace a part of the transition metal with Li in the P2-type sodium-containing composite oxide (Patent Documents 3 and 4). Li incorporated into the transition metal site is thought to stabilize the crystal structure and improve the cycle characteristics. In addition, it has been reported that a high capacity can be obtained by involving oxygen in the redox reaction, but the operating voltage is low.

Japanese Unexamined Patent Publication No. 2016-103463 Japanese Unexamined Patent Publication No. 2015-62161 Japanese Unexamined Patent Publication No. 2016-157602 Japanese Unexamined Patent Publication No. 2014-160653

D. Yuan, Y. Cao et al., Electrochem. Acta., 116, 300, 2014

As described above, various compositions have been studied as P2-type sodium-containing composite oxides, but P2-type sodium-containing composite oxides that exhibit high capacity and high voltage and exhibit good cycle characteristics have been found. Not.

One aspect of the present invention has a P2-type crystal structure, is represented by Na _x [M1 _1-a M2 _a ] O ₂ , and has 0 <x <1.0 and 0.06 ≦ a ≦ 0.19 . when the sodium-containing comprises a composite oxide, the element M1 satisfying include Mn and Ni, the element M2 is seen containing a Li and Fe, which represents the content of Fe in the element M2 in _{a Fe,} 0.04 ≦ a Regarding a positive electrode active material for _{a sodium ion secondary battery satisfying Fe} ≤ 0.13.

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

Yet another aspect of the present invention is a step of preparing a raw material containing a sodium salt, a salt of the element M1 and a salt of the element M2, and the raw material at 700 ° C. or higher in the air or in an oxidizing atmosphere. The step of calcining at a temperature of 1100 ° C. or lower to form a sodium-containing composite oxide, and the above-mentioned with respect to the total of the element M1 contained in the salt of the element M1 and the element M2 contained in the salt of the element M2. The molar ratio x of sodium contained in the sodium salt satisfies 0 <x <1.0, the element M1 contains Mn and Ni, the element M2 contains Li and Fe, and the salt of the element M1 molar ratio a of the element M2 to the total of the element M2 contained the element M1 included in salts of the element M2 is, meets the 0.06 ≦ a ≦ 0.19, the content of Fe in the element M2 a _{The present} invention relates to a method for producing a positive electrode active material for a sodium ion secondary battery, which satisfies 0.04 ≦ a _{Fe ≦ 0.13 when expressed in Fe.}

According to the positive electrode active material according to the present invention, it is possible to provide a sodium ion secondary battery capable of stably repeating charging and discharging with a high capacity and a high voltage.

It is a vertical sectional view of the sodium ion secondary battery which concerns on one Embodiment of this invention. It is a triangular phase diagram of Na _x [FeLiNimn] O _{2 solid solution.} It is a figure which shows the relationship between the composition of Na _x [M1 _1-a M2 _a ] O _{2 and the XRD pattern.} It is a figure which shows the charge / discharge curve of the cell which comprises the positive electrode containing Na _x [M1 _1-a M2 _a ] O _2. It is a figure which shows the relationship between the number of charge / discharge cycles of a cell including a positive electrode containing Na _x [M1 _1-a M2 _a ] O _{2 and a discharge capacity.} The figure which compares the charge / discharge curve at 2.5V-4.5V and the charge / discharge curve at 2.0V-4.5V of the cell including the positive electrode containing _{Na x} [M1 _1-a M2 _a ] O _2. Is. It is a figure which shows the rate characteristic of the cell which comprises the positive electrode containing Na _x [M1 _1-a M2 _a ] O _2. It is a figure which shows the result of the water vapor exposure test of Na _x [M1 _1-a M2 _a ] O _2. It is a figure which shows the result of the inundation test of Na _x [M1 _1-a M2 _a ] O _2.

[Explanation of Embodiments of the Invention]
First, the contents of the embodiments of the present invention will be listed and described.
(1) The positive electrode active material according to the embodiment of the present invention has a P2-type crystal structure, is represented by the general formula: Na _x [M1 _1-a M2 _a ] O ₂ , and 0 <x <1. It contains a sodium-containing composite oxide (hereinafter, also referred to as M1M2 composite oxide) that satisfies 0 and 0.06 ≦ a ≦ 0.2. The element M1 contains Mn and Ni, and the element M2 contains Li and Fe. By using the positive electrode active material, a sodium ion secondary battery that exhibits high capacity and high voltage and exhibits good cycle characteristics can be obtained.

(2) The above general formula preferably satisfies 0.666 <x. As a result, the sodium content in the M1M2 composite oxide is increased, so that a high capacity can be easily obtained.

(3) The above general formula preferably satisfies 0.06 ≦ a ≦ 0.19. As a result, an excellent sodium-ion secondary battery can be obtained with a good balance between capacity and cycle characteristics.

(4) When the Li content in the element M2 is represented by a _Li and the Fe content is _{represented by a Fe} , the above general formulas are 0.02 ≤ a _Li ≤ 0.07 and 0.04 ≤ a _Fe ≤ 0. It is preferable to satisfy 13 and a = a _Li + a _Fe. Further, it is more preferable to satisfy (5) 0.02 ≦ a _Li ≦ 0.06 and 0.04 ≦ a _{Fe ≦ 0.12.} Further, it is more preferable to satisfy (6) 0.042 ≦ a _Li ≦ 0.06 and 0.085 ≦ a _{Fe ≦ 0.11.} As a result, the balance between the capacity of the sodium ion secondary battery and the cycle characteristics is further improved.

(7) the element M1 _{is, Ni y A z Mn 1-} yz ( although, 0.2 ≦ y ≦ 0.3,0 ≦ z <0.1) is preferably represented by. Element A is an element other than Ni and Mn. This makes it easier for the M1M2 composite oxide to develop a high capacity and a high voltage.

(8) The sodium ion secondary battery according to the embodiment of the present invention includes a positive electrode containing the positive electrode active material, a negative electrode containing the negative electrode active material, and a non-aqueous electrolyte having sodium ion conductivity. By using the positive electrode provided with the positive electrode active material, it is possible to provide a sodium ion secondary battery capable of stably repeating charging and discharging with a high capacity and a high voltage.

(9) The method for producing a positive electrode active material for a sodium ion secondary battery according to an embodiment of the present invention prepares a raw material containing (i) a sodium salt, a salt of element M1, and a salt of element M2. It comprises (ii) a step of calcining the raw material in air or an oxidizing atmosphere at a temperature of 700 ° C. or higher and 1100 ° C. or lower to produce a sodium-containing composite oxide. However, the molar ratio x of sodium contained in the sodium salt to the total of the element M1 contained in the salt of the element M1 and the element M2 contained in the salt of the element M2 satisfies 0 <x <1.0. The element M1 contains Mn and Ni. The element M2 contains Li and Fe. The molar ratio a of the element M2 to the total of the element M1 contained in the salt of the element M1 and the element M2 contained in the salt of the element M2 satisfies 0.06 ≦ a ≦ 0.2. According to the above method, an M1M2 composite oxide having few impurities can be efficiently produced.

[Details of Embodiments of the Invention]
Next, an embodiment of the present invention will be described in more detail. It should be noted that the present invention is not limited to these examples, and is indicated by the appended claims, and is intended to include all modifications within the meaning and scope equivalent to the claims. ..

<Positive electrode active material>
The positive electrode active material according to one embodiment of the present invention has a P2-type crystal structure, is represented by the general formula: Na _x [M1 _1-a M2 _a ] O ₂ , and has 0 <x <1.0 and 0. .. Contains a sodium-containing composite oxide (M1M2 composite oxide) satisfying 06 ≦ a ≦ 0.2. The element M1 contains Mn and Ni, and the element M2 contains Li and Fe. The M1M2 composite oxide has a structure in which a part of the element M1 of _{Na x} M1O _{2 having a P2-type crystal structure is replaced with the element M2.} The M1M2 composite oxide exhibits high working voltage and high capacity derived from _{Na x} M1O ₂ (typically Na _2/3 [Ni _1/3 Mn _2/3 ] O _2).

Among the elements M1, Ni mainly contributes to the redox reaction. The Ni valence varies between divalent and tetravalent. Among the elements M2, Fe contributes to the redox reaction. The Fe valence varies between trivalent and tetravalent.

Among the elements M2, Li stabilizes the crystal structure and suppresses deterioration of the M1M2 composite oxide due to charging and discharging, so that improvement in cycle characteristics can be expected. Further, when divalent Ni is replaced with monovalent Li, the positive charge is insufficient, but the insufficient charge is compensated by increasing the sodium content. By increasing the sodium content, the capacity is improved.

Of the elements M2, Fe is abundant in resources and is also involved in the redox reaction. Since the element M2 contains Fe together with Li, deterioration of the M1M2 composite oxide due to charging and discharging is suppressed. Further, by substituting a part of Li with Fe, it becomes possible to design a sodium ion secondary battery having a high capacity, low cost, and good cycle characteristics while reducing the amount of rare Li used.

As the amount of the element M1 substituted with the element M2 increases, it becomes increasingly difficult to synthesize an M1M2 composite oxide having a single-phase P2-type crystal structure. Therefore, a third component that does not have a desired crystal structure is likely to be mixed in the M1M2 composite oxide. When the above general formula satisfies 0.06 ≦ a ≦ 0.2, mixing of the third component can be avoided.

The M1M2 composite oxide is less likely to react with water and is less likely to be altered in air or in aqueous solution. For example, even if 0.1 g of M1M2 composite oxide is ^{mixed with 2 cm 3} of water and stirred at 25 ° C. for 30 minutes, almost no change is observed in the powder X-ray diffraction image using CuKα rays.

Here, the P2-type crystal structure will be further described. A typical P2-type crystal structure is assigned to the space group P63 / mmc because of its symmetry. The basic P2-type crystal has a layered structure, but the P2-type matrix structure may contain P3-type and / or O3-type stacking defects. 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.

When no peak is observed in the range of 18 ° to 25 ° in the powder X-ray diffraction measurement using CuKα ray as the X-ray source, most of the third component other than the P2 type crystal structure is contained in the M1M2 composite oxide. It can be determined that it is not included. However, the peak is a substantial peak whose existence can be objectively confirmed, and does not include a peak judged to be noise.

The above general formula may satisfy 0 <x <1.0, and the x value is selected so that the P2-type crystal structure is maintained. For example, 0.666 <x is preferable, 0.68 ≦ x <0.95 is more preferable, and 0.69 ≦ x <0.90 is particularly preferable. When the x value is 1 or more, the O3 type crystal structure becomes stable, so that the P2 type crystal structure is difficult to form.

The range of the x value is the range in the composition of the M1M2 composite oxide produced by firing the raw material. The x value increases or decreases depending on the charge and discharge of the sodium ion secondary battery. The x value decreases during charging and increases during discharging. The lower limit of the x value at the time of charging is preferably 0.1 ≦ x, and more preferably 0.2 ≦ x. The upper limit of the x value at the time of discharge is preferably x <1.0, and more preferably x ≦ 0.9.

The above general formula may satisfy 0.06 ≦ a ≦ 0.2, preferably 0.06 ≦ a ≦ 0.19, and more preferably 0.06 ≦ a ≦ 0.15. When the Li content in the element M2 is represented by a _Li and the Fe content is _{represented by a Fe} , 0.02 ≤ a _Li ≤ 0.07 and 0.04 ≤ a _Fe ≤ 0.13 and a = a _Li. It is preferable to satisfy + a _Fe. Further, it is more preferable to satisfy 0.02 ≤ a _Li ≤ 0.06 and 0.04 ≤ a _Fe ≤ 0.12, and 0.042 ≤ a _Li ≤ 0.06 and 0.085 ≤ a _Fe ≤ 0. It is more preferable to satisfy 11.

Element M1 may include Ni and Mn, specifically, element M1 is represented by _{Ni y A z Mn 1-yz} ( although, 0.2 ≦ y ≦ 0.3,0 ≦ z <0.1) Is preferable. 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 0.2 ≦ y ≦ 0.3, but 0.2 ≦ y ≦ 0.26 is preferable from the viewpoint of ensuring better cycle characteristics.

(Average primary particle size)
The average primary particle size of the M1M2 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. Within this range, it can be said that the P2-type crystal is sufficiently grown. The average primary particle size is, for example, 5 μm or less, preferably 3 μm or less, and more preferably 2 μm or less. The average primary particle size may be obtained by arbitrarily selecting 10 to 30 primary particles in a scanning electron microscope (SEM) image of the composite oxide and determining the average value as the average value of the maximum diameters of the selected primary particles.

When a positive electrode containing the sodium-containing composite oxide is produced and a battery is assembled using metallic sodium for the negative electrode (counter electrode), the open circuit voltage (OCV) thereof is, for example, 1.5 V or more. The positive electrode can be charged and 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 4.6 V, more preferably 4.5 V or less, and further preferably 4.4 V or less. Further, the lower limit voltage at the time of discharging is preferably 1.7 V or more, more preferably 2.0 V or more, and further preferably 2.5 V or more.

(Method for producing M1M2 composite oxide)
Next, a method for producing an M1M2 composite oxide having a step of preparing a raw material (i) and a step of firing the raw material (ii) will be described. However, the manufacturing method is not limited to the following.

Process (i)
First, a raw material containing a Na salt, a salt of element M1, and a salt of element M2 is prepared. As the salt of the element M1, a double salt containing Li and Fe may be used, but it is preferable to use the Li salt and the Fe salt as shown below in combination. Further, as the salt of the element M2, a double salt containing Ni and Mn may be used, but it is preferable to use a Ni salt and an Mn salt as shown below in combination. Each double salt can be prepared as an oxide or hydroxide by, for example, a crystallization method or a coprecipitation method.

Examples of Na salt, Li salt, Fe salt source, Ni salt and Mn salt include oxides, hydroxides, oxyhydroxides, hydrogen carbonates, carbonates, nitrates, sulfates, halides and organic acid salts. (For example, oxalate) and the like can be used.

Specifically, as the Na salt, or the like can be used _{Na 2 CO 3, NaHCO 3,} Na 2 O. Of these, Na ₂ CO ₃ is the easiest to handle.

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

As the Fe salt, Fe ₂ O ₃ , FeO, Fe (OH) ₃ , Fe (OH) _{2 and the} like can be used. Of these, Fe ₂ O ₃ is the easiest to handle.

As the Ni salt, NiCO ₃ , Ni (OH) ₂ , NiOOH, NiO, NiNO ₃ , Ni ₂ SO _{4 and the} like can be used. Of these, Ni (OH) ₂ , NiOOH and NiO are easy to handle.

As the Mn salt, MnO ₂ , Mn ₂ O ₃ , Mn ₃ O _{4 and the} like can be used.

The raw materials are blended so that the molar ratio x of sodium to the total of the element M1 contained in the salt of the element M1 and the element M2 contained in the salt of the element M2 is 0 <x <1.0. Similarly, the molar ratio a of element M2 to the total of the element M2 contained in the salt of the element M1 and the element M2 contained in the salt of the element M1 is blended raw material to satisfy 0.06 ≦ a ≦ 0.2 To do. The x-value and a-value are selected according to the composition of the desired M1M2 composite oxide.

In M1M2 composite oxide, if the element M1 is _{Ni y A z Mn 1-yz} ( although, 0.1 ≦ y <0.33,0 ≦ z <0.1) represented by the salt of the element M1, A salt containing the element A may be contained in a predetermined ratio. Specifically, the salt of the element M2 may be prepared so that the molar ratio of nickel, manganese and the element A satisfies y: (1-yz): z.

The raw material, that is, the mixture containing the sodium salt, the salt of the element M1 and the salt of the element M2 may be mixed dry or wet using a mixing device such as a ball mill, a V-type mixer, or a stirrer.

Process (ii)
Next, the raw material is calcined in air or in an oxidizing atmosphere at a temperature of 700 ° C. or higher (preferably 800 ° C. or higher) and 1100 ° C. or lower (preferably 1000 ° C. or lower) to produce an M1M2 composite oxide. If the calcination temperature is less than 700 ° C., the P2 type crystal structure does not grow sufficiently, and an M1M2 composite oxide containing a raw material component as an impurity is likely to be produced. When the firing temperature exceeds 1100 ° C., a P2-type crystal structure can be obtained, but a third component having a crystal structure different from the desired crystal structure is likely to be produced.

The firing temperature is an average temperature during a period excluding the temperature raising process and the temperature lowering process (hereinafter referred to as the main firing period). Therefore, for example, it is permissible that the temperature drops below 700 ° C. or exceeds 1100 ° C. momentarily or for a short time during the main firing period. However, it is preferable that the temperature of the firing atmosphere is maintained at 700 ° C. to 1100 ° C. (preferably 800 ° C. to 1000 ° C.) for about 80% of the main firing period.

The main firing period may be, for example, 5 hours or more, more preferably 8 hours to 15 hours, and even more preferably 10 hours to 12 hours. As a result, the 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 under atmospheric pressure. Further, the raw material may be calcined in an atmosphere having a higher oxygen concentration than air. The pressure of air or an oxidizing atmosphere is not particularly restricted as long as it is for example ^{0.9 × 10 5 Pa~1.1 × 10 5} Pa.

Before the above firing, the raw material may be provisionally fired in a temperature range of 200 to 500 ° C. in the same atmosphere as above. By the calcination, the volatile components (for example, water) contained in the raw material are slowly removed, and the crystal growth in the main calcination period is facilitated.

<Sodium ion secondary battery>
Next, the sodium ion secondary battery will be described. The sodium ion secondary battery includes a positive electrode containing the positive electrode active material, a negative electrode containing the negative electrode active material, and a non-aqueous electrolyte having sodium ion conductivity. FIG. 1 is a vertical cross-sectional view schematically showing a sodium ion secondary battery 100 according to an embodiment. The sodium-ion secondary battery includes a laminated electrode group, an electrolyte (not shown), and a square battery case 10 for accommodating them. The battery case 10 is made of, for example, aluminum, and is composed of a bottomed container body 12 having an open top and a lid 13 that closes the top opening.

At the center of the lid 13, a safety valve 16 is provided to release the gas generated inside when the internal pressure of the battery case 10 rises. An external positive electrode terminal 14 penetrating the lid 13 is provided on one side of the lid 13 with the safety valve 16 in the center, and an external penetrating the lid 13 is provided on the other side of the lid 13. A negative electrode terminal is provided.

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

Both the external positive electrode terminal 14 and the external negative electrode terminal 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 collar 8 is provided in a portion of each terminal housed inside the battery case 10, and the collar 8 attaches an O-ring-shaped gasket 9 to the inner surface of the lid 13 by rotation of the nut 7. It is fixed through.

(Positive electrode)
The positive electrode includes a positive electrode current collector and the positive electrode active material (or a 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 M1M2 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 materials used in combination are not particularly limited, but are, for example, sodium chromate (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 etc.), sodium iron cobaltate (NaFe _0.5 Co _0.5 O ₂ etc.), sodium iron manganate (Na _2/3 Fe _1/3 Mn _2/3 O ₂ etc.), etc. Can be mentioned.

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

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

(Negative electrode)
The negative electrode includes a negative electrode current collector and a negative electrode active material (or a negative electrode mixture) supported on the negative electrode current collector. The negative electrode current collector may be a metal foil or a metal porous body in the same manner as the positive electrode current collector. As the material of the negative electrode current collector, copper, copper alloy, nickel, nickel alloy, stainless steel and the like are preferable because they are not alloyed with sodium and are 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, a material that alloys with sodium, and the like. Both materials are materials that develop their capacity by the 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.

Metallic compounds 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 easily graphitizable carbon (soft carbon) and / or non-graphitizable carbon (hard carbon). The negative electrode active material may be used alone or in combination of two or more.

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

The negative electrode mixture may further contain a conductive auxiliary agent 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. The conductive auxiliary agent, the binder, and the dispersion medium can be appropriately selected from those exemplified for the positive electrode, respectively.

(Separator)
As the separator, for example, a resin microporous membrane, a non-woven 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.

[Non-aqueous electrolyte]
The non-aqueous 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 of sodium ions and anions (sodium salt) is dissolved in an organic solvent, an ionic liquid containing sodium ions and anions is used. The concentration of the sodium salt in the non-aqueous electrolyte may be, for example, 0.3 to 3 mol / liter.

Kind of 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 (oxalate) borate ion (B (C ₂ O ₄ ) ₂ ⁻ ), trifluoromethanesulfonate ion (CF ₃ SO ₃ ⁻ ), bissulfonylamide anion and the like. As the sodium salt, one kind may be used alone, or two or more kinds of sodium salts having different types of the first anion may be used in combination.

When an ionic liquid is used as the non-aqueous electrolyte, the content of the ionic liquid in the non-aqueous electrolyte is preferably 80% by mass or more, 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 salt in a molten state (molten salt).

When an organic electrolyte is used as the non-aqueous electrolyte, the total amount of the organic solvent and the alkali metal salt in the non-aqueous electrolyte is preferably 80% by mass or more, more preferably 90% by mass or more of the non-aqueous electrolyte. preferable. 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 ethylmethyl carbonate; and cyclic carbonates such as γ-butyrolactone can be preferably used. .. As the organic solvent, one type may be used alone, or two or more types may be used in combination.

The ionic liquid may further contain a second cation in addition to the sodium ion (first cation). As such a second cation, an organic cation is preferable. The second cation may be used alone or in combination of two or more. 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 in any way.

(Examples 1 to 4)
<Synthesis of sodium-containing composite oxide>
Na ₂ CO ₃ , Li ₂ CO ₃ , Fe ₂ O ₃ , Ni (OH) ₂ and Mn ₂ O ₃ were mixed at a predetermined ratio and mixed in a ball mill at 600 rpm for 12 hours to obtain a raw material. The obtained raw material was molded into pellets and then calcined in air at 900 ° C. for 12 hours to synthesize the composite oxides A1 to A4 of Examples 1 to 4 below.

(A1) Na _0.680 [Li _0.021 Fe _0.042 Ni _0.280 Mn _0.660 ] O ₂
(A2) Na _0.694 [Li _0.0417 Fe _0.0833 Ni _0.222 Mn _0.653 ] O ₂
(A3) Na _0.699 [Li _0.0486 Fe _0.0972 Ni _0.204 Mn _0.650 ] O ₂
(A4) Na _0.708 [Li _0.0625 Fe _0.125 Ni _0.167 Mn _0.646 ] O ₂

(Comparative Example 1)
The following composite oxide B1 was synthesized by the same procedure as in Examples 1 to 4 using a raw material in which _{Na 2} CO ₃ , Ni (OH) ₂ and Mn ₂ O _{3 were mixed at a predetermined ratio.}

(B1) Na _0.667 Ni _0.333 Mn _0.667 O ₂

FIG. 2 shows _{a triangular phase diagram of the Na x} [FeLiNiMn] O ₂ solid solution and the coordinates of the M1M2 composite oxides A1 to A4. The composite oxides A1 to A4 are Na _0.667 Ni _0.333 Mn _0.667 O ₂ (abbreviated as NiMn), Na _0.667 Fe _0.5 Mn _0.5 O ₂ (abbreviated as FeMn), and Na _5/6 Li _1/4 Mn _3/4. It can be seen as _{a solid solution (Na x} [FeLiNimn] O ₂ ) containing O ₂ (abbreviated as LiMn) in the following proportions.

(B1) (Nimn: Femn: LiMn) = (1: 0: 0)
(A1) (Nimn: Femn: LiMn) = (10: 1: 1)
(A2) (Nimn: Femn: LiMn) = (4: 1: 1)
(A3) (Nimn: Femn: LiMn) = (22: 7: 7)
(A4) (Nimn: Femn: LiMn) = (2: 1: 1)

[Evaluation 1]
Powder X-ray diffraction measurement of the composite oxides B1 and A1 to A4 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 Co., Ltd. was used.
X-ray: CuKα
Voltage-current: 40kV-20mA
Measurement angle range: 2θ = 10 to 70 °
Step: 0.02 °
Scan speed: 6 ° / min

FIG. 3 shows powder X-ray diffraction images (XRD patterns) of the composite oxides B1 and A1 to A4. All of them show a typical pattern of P2-type crystal structure, and no peak of impurities is observed in the range of 2θ = 18 ° to 25 °. Table 1 shows the lattice constants based on the XRD pattern.

[Evaluation 2]
A predetermined composite oxide (B1 or A1 to A4), acetylene black (AB), and polyvinylidene fluoride (PVdF) are blended in a mass ratio of 80:10:10, and an appropriate amount of N-methyl-2-pyrrolidone is blended. A slurry was prepared using (NMP) as a dispersion medium. The obtained slurry was applied to one side of an aluminum foil having a thickness of 20 μm. The thickness of the coating film was 220 μm. After the coating film was sufficiently dried, 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 cell (hereinafter referred to as a test battery) was produced using a positive electrode and a negative electrode composed of metallic Na. As the non-aqueous electrolyte, a solvent obtained by dissolving _{NaPF 6} in a mixed solvent having a volume ratio of propylene carbonate and fluoroethylene carbonate at a volume ratio of 98: 2 at a concentration of 1 mol / L was used. A glass filter was used as the separator.

FIG. 4 shows the charge / discharge curves of the test battery up to 20 cycles. The charging / discharging current density was approximately 13 mA / g per mass of the positive electrode active material, and was carried out in the range of 2.5 V to 4.5 V.

In the test battery using the composite oxide B1, the cycle characteristics are insufficient, and a step that seems to be derived from the localization of electrons by the charge alignment of the transition metal is observed in the charge / discharge curve. On the other hand, the test batteries using the composite oxides A1 to A4 have significantly improved cycle characteristics. Above all, the improvement of the cycle characteristics of the composite oxides A3 and A4 is remarkable. Further, the steps of the charge / discharge curve observed in the test battery using the composite oxide B1 are eliminated in the test battery using the composite oxides A3 and A4.

FIG. 5 shows the relationship between the number of charge / discharge cycles of the test battery and the discharge capacity. Table 2 shows the discharge capacity and average operating voltage in the second cycle and the capacity retention rate in the 30th cycle. It can be understood that the balance between the capacity and the cycle characteristics of the test battery using the composite oxide A3 is the best.

FIG. 6 shows a charge / discharge curve when the lower limit voltage of charge / discharge is changed to 2.0 V for the test battery using the composite oxide A3. Here, charging / discharging is performed at 2.5 to 4.5 V from the 1st cycle to the 5th cycle, and charging / discharging is performed at 2.0 to 4.5 V from the 6th cycle to the 8th cycle. Was compared. Table 3 shows the discharge capacity and the average operating voltage (Vave) in the first cycle and the sixth cycle. From FIG. 6, it can be understood that stable charging / discharging is possible even in a low voltage region. In the low voltage region, the capacitance based on the redox of Mn was observed, and the discharge capacitance increased by that amount. On the other hand, the average working voltage decreased.

FIG. 7 shows the relationship between the discharge rate and the discharge curve of the test battery using the composite oxide A3. The charge rate was fixed at a current value of C / 20 with respect to the design capacity of the test battery, and the discharge rate was changed from C / 20 to 2C. From FIG. 7, it can be understood that a high discharge capacity can be obtained even in the case of high rate discharge. The discharge capacity during 2C discharge maintained 88.2% of the discharge capacity during C / 20 discharge. A large polarization was observed at a high rate discharge of 1C or higher, but this did not reflect the performance of the positive electrode active material itself, but was affected by the manufacturing conditions of the test battery and PVdF that binds the positive electrode active material. Seems to be large.

[Evaluation 3]
A water vapor exposure test was carried out on the composite oxide A3. The composite oxide was exposed to saturated steam for 90 hours at room temperature, and the XRD pattern was confirmed at predetermined time intervals. Then, the composite oxide was dried at 80 ° C., and an XRD pattern was further confirmed.

FIG. 8 shows the XRD pattern measured in the water vapor exposure test. Table 4 shows the lattice constants based on the XRD pattern. No impurities were formed when exposed to water vapor. The change in the lattice constant suggested that Na was slightly desorbed from the layers of the composite oxide, but it was shown that the composite oxide is less likely to react with moisture and is less likely to deteriorate in the atmosphere.

[Evaluation 4]
An inundation test was carried out on the composite oxide A3. 0.1 g of the composite oxide ^{was put into 2 cm 3} of pure water, stirred for 30 minutes, and then allowed to stand for 1 hour. Then, the composite oxide was separated by suction filtration and dried at 80 ° C. overnight to confirm the XRD pattern.

FIG. 9 shows the XRD pattern measured in the inundation test. Table 5 shows some of the Rietveld analysis results based on the XRD pattern. No impurities were formed by flooding, indicating that the composite oxide is not easily altered even in water. The change in the lattice constant suggested that Na was slightly desorbed from the layers of the composite oxide.

The sodium-ion secondary battery according to the present invention is useful as a power source for various purposes because it exhibits high capacity and high voltage and exhibits good cycle characteristics.

1: Separator 2: Positive electrode, 2a: Positive electrode lead piece 3: Negative electrode, 3a: Negative electrode lead piece, 7: Nut, 8: Collar, 9: Gasket, 10: Battery case, 12: Container body, 13: Lid Body, 14: External positive electrode terminal, 16: Safety valve

Claims (8)

It has a P2-type crystal structure and is represented by Na _x [M1 _1-a M2 _a ] O ₂ .
Containing sodium-containing composite oxides satisfying 0 <x <1.0 and 0.06 ≤ a ≤ 0.19,
The element M1 contains Mn and Ni, and contains Mn and Ni.
The element M2 is seen including Li and Fe,
A positive electrode active material for a sodium ion secondary battery that satisfies 0.04 ≤ a _Fe ≤ 0.13 when the content of Fe in the element M2 is _{expressed by a Fe.} The positive electrode active material for a sodium ion secondary battery according to claim 1, wherein 0.666 <x. When referring to the content of Li in the element M2 in a _Li,
0.02 ≦ _a meet _Li ≦ 0.07 and _{a =} a Li ₊ a _Fe, positive electrode active material for a sodium ion secondary battery according to claim 1 or claim 2. The positive electrode active material for a sodium ion secondary battery according to claim 3 , which satisfies 0.02 ≤ a _Li ≤ 0.06 and 0.04 ≤ a _{Fe ≤ 0.12.} The positive electrode active material for a sodium ion secondary battery according to claim 4 , which satisfies 0.042 ≤ a _Li ≤ 0.06 and 0.085 ≤ a _{Fe ≤ 0.11.} The sodium-containing composite _{oxide, Na x [(Ni y A} z Mn 1-yz) 1-a M2 a] O 2 ( however, 0.2 ≦ y ≦ 0.3 and 0 ≦ z <0.1) Represented by
The positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 5 , wherein the element A contains at least one selected from the group consisting of Mg, Al, Co and Ti. A positive electrode containing the positive electrode active material according to any one of claims 1 to 6.
Negative electrode A negative electrode containing an active material and a negative electrode
A sodium ion secondary battery comprising a non-aqueous electrolyte having sodium ion conductivity. A step of preparing a raw material containing a sodium salt, a salt of element M1, and a salt of element M2, and
The raw material is calcined in air or in an oxidizing atmosphere at a temperature of 700 ° C. or higher and 1100 ° C. or lower to produce a sodium-containing composite oxide.
The molar ratio x of sodium contained in the sodium salt to the total of the element M1 contained in the salt of the element M1 and the element M2 contained in the salt of the element M2 satisfies 0 <x <1.0.
The element M1 contains Mn and Ni, and contains Mn and Ni.
The element M2 contains Li and Fe, and contains Li and Fe.
Molar ratio a of the element M2 to the total of the element M2 contained in the salt of the element M1 and the element M2 contained in salt of the element M1 is, meets the 0.06 ≦ a ≦ 0.19,
A method for producing a positive electrode active material for a sodium ion secondary battery, which satisfies 0.04 ≦ a _Fe ≦ 0.13 when the content of Fe in the element M2 is _{expressed by a Fe.}

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