JP7085761B2 - Positive electrode active material for sodium secondary battery and its manufacturing method - Google Patents

Description

The present invention relates to a positive electrode active material for a sodium secondary battery and a method for producing the same.

Lithium-ion secondary batteries, which have the highest energy density among secondary batteries, have been put into practical use, and one of the alternative technologies is to replace lithium ions with sodium ions, which are more resource-rich. It is a secondary battery. Sodium-ion secondary batteries are one of the promising secondary batteries that can achieve a high operating voltage (3V or higher) similar to lithium-ion secondary batteries. Even in a sodium ion secondary battery, the importance of selecting a positive electrode active material is the same as that of a lithium ion secondary battery, and the performance of the positive electrode active material determines the theoretical capacity and operating voltage of the battery.

Of the positive electrode active material candidates, the most important one is sodium ferrite (NaFeO ₂ ). Sodium ferrite has an α phase and a β phase. For example, as described in Patent Document 1, only the α phase functions as a positive electrode active material. However, once the β phase, which is a high temperature phase, is formed, the transition from the β phase to the α phase is not completely completed due to the large difference in crystal structure from the α phase, and the β phase is often mixed with the α phase. Phase synthesis is not easy.

On the other hand, when a step of hydrothermally heat-treating an iron source in sodium hydroxide is used, a highly crystalline sample can be obtained, and sodium ferrite having excellent charge / discharge characteristics when the regulated charge capacity is limited to 70 mAh / g. Is known to be obtained (see, for example, Patent Document 2). However, as described in Comparative Example 1 in Patent Document 3, for example, when the regulated charge capacity is increased to 100 mAh / g, the cycle deterioration of this material is severe, and the characteristics cannot be improved unless a specific amount of cobalt is replaced. rice field. However, cobalt is expensive and hinders its practical use. Therefore, if a positive electrode active material for a sodium-ion secondary battery that can be cycled stably even under these charge / discharge conditions without using expensive and resource-rare cobalt, a great step forward toward the practical application of the sodium-ion secondary battery. do.

Japanese Unexamined Patent Publication No. 2005-317511 Japanese Unexamined Patent Publication No. 2014-08627 JP-A-2015-176662

The present invention has been made in view of the current state of the prior art described above, and exhibits high discharge capacity and excellent cycle characteristics even when the regulated charge capacity is set high using a resource-rich substance as a raw material. It is an object of the present invention to provide a positive electrode active material for a sodium ion secondary battery to be obtained.

The present inventors have carried out diligent research to achieve the above-mentioned object. As a result, when the sodium iron oxide having a hexagonal layered rock salt type crystal structure and having a specific lattice constant and lattice volume is set to a high regulated charge capacity without using an expensive and resource-rare material. It was also found that it can exhibit high discharge capacity and excellent cycle characteristics. The present invention has been completed as a result of further research based on these findings. That is, the present invention includes the following configurations.
Item 1. General formula (1):
Na _x FeO ₂ (1)
[In the formula, x is 0.80 to 1.30. ]
Represented by
It has a hexagonal layered rock salt type crystal structure and has a hexagonal layered rock salt type crystal structure.
The lattice constant is 3.0235 Å or less for a and 16.0820 Å or less for c.
Lattice volume is 127.360 Å ³ or less,
Positive electrode active material for sodium ion secondary batteries made of sodium iron oxide.
Item 2. Item 2. The positive electrode active material for a sodium ion secondary battery according to Item 1, wherein the 3b position iron occupancy in the hexagonal layered rock salt type crystal structure of the sodium iron oxide is 0.920 or more.
Item 3. Item 2. The positive electrode active material for a sodium ion secondary battery according to Item 1 or 2, wherein the sodium iron oxide has a 6c-position sodium occupancy in the hexagonal layered rock salt type crystal structure of 0.050 or less.
Item 4. In the Fourier transform spectrum of the FeK end wide area X-ray absorption (EXAFS) spectrum of the sodium iron oxide, the ratio of Fe-Fe height to Fe-O height (Fe-Fe height / Fe-O height) is 1.050. Item 2. The positive electrode active material for a sodium ion secondary battery according to any one of Items 1 to 3. Item 5. The method for producing a positive electrode material for a sodium secondary battery according to any one of claims 1 to 4.
A production method comprising a step of performing a hydrothermal synthesis reaction at a temperature of 180 ° C. or higher for 30 hours or longer using an alkaline aqueous solution containing a sodium-containing material and an iron-containing material and not containing a lithium-containing material.
Item 6. Item 5. The production method according to Item 5, wherein the sodium-containing material is sodium hydroxide.
Item 7. Item 5. The production method according to Item 5 or 6, wherein the alkaline aqueous solution further contains a potassium-containing material.
Item 8. Item 2. The production method according to Item 7, wherein the potassium-containing material is a neutral or alkaline salt containing potassium.
Item 9. Item 5. A positive electrode for a sodium ion secondary battery, which contains the positive electrode active material for the sodium ion secondary battery according to any one of Items 1 to 4.
Item 10. Item 9. A sodium ion secondary battery comprising the positive electrode for the sodium ion secondary battery according to Item 9.

The positive electrode active material for a sodium ion secondary battery of the present invention can exhibit high discharge capacity and excellent cycle characteristics even when the regulated charge capacity is set high, without using an expensive and resource-rare material. can.

It is a schematic diagram of the crystal structure of the positive electrode active material for a sodium ion secondary battery of this invention. a and c correspond to the lattice constant. As for the coordinates of each grid position, 3a, 3b and 6c correspond to (000), (001/2) and (00z) (0.4 ≦ z ≦ 0.5), respectively. It is an actual measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Example 1. FIG. It is a radial distribution function by Fourier transform of the K-end EXAFS spectrum of Fe of the sample of Example 1. The charge / discharge characteristics of the sodium secondary battery using the sample of Example 1 as the positive electrode active material are shown. The upward-sloping curve corresponds to charging (c), and the downward-sloping curve corresponds to discharging (d). The numbers indicate the number of cycles. It is an actual measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Comparative Example 1. It is a radial distribution function by Fourier transform of the K-end EXAFS spectrum of Fe of the sample of Comparative Example 1. The charge / discharge characteristics of the sodium secondary battery using the sample of Comparative Example 1 as the positive electrode active material are shown. The upward-sloping curve corresponds to charging (c), and the downward-sloping curve corresponds to discharging (d). The numbers indicate the number of cycles. It is an actual measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Example 2. FIG. It is a radial distribution function by Fourier transform of the K-end EXAFS spectrum of Fe of the sample of Example 2. The charge / discharge characteristics of the sodium secondary battery using the sample of Example 2 as the positive electrode active material are shown. The upward-sloping curve corresponds to charging (c), and the downward-sloping curve corresponds to discharging (d). The numbers indicate the number of cycles. It is an actual measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Comparative Example 2. It is a radial distribution function by Fourier transform of the K-end EXAFS spectrum of Fe of the sample of Comparative Example 2. The charge / discharge characteristics of the sodium secondary battery using the sample of Comparative Example 2 as the positive electrode active material are shown. The upward-sloping curve corresponds to charging (c), and the downward-sloping curve corresponds to discharging (d). The numbers indicate the number of cycles. It is an actual measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of the comparative example 3. FIG. The charge / discharge characteristics of the sodium secondary battery using the sample of Comparative Example 3 as the positive electrode active material are shown. The upward-sloping curve corresponds to charging (c), and the downward-sloping curve corresponds to discharging (d). The numbers indicate the number of cycles.

As used herein, "contains" is a concept that includes any of "comprise," "consist essentially of," and "consist of." Further, in the present specification, the notation "A to B" means "A or more and B or less".

1. 1. Positive Active Material for Sodium Ion Secondary Battery The positive positive active material for sodium ion secondary battery of the present invention has the general formula (1) :.
Na _x FeO ₂ (1)
[In the formula, x is 0.80 to 1.30. ]
Represented by
It has a hexagonal layered rock salt type crystal structure and has a hexagonal layered rock salt type crystal structure.
The lattice constant is 3.0235 Å or less for a and 16.0820 Å or less for c.
It consists of sodium iron oxide with a lattice volume of 127.360 Å ³ or less.

The hexagonal layered rock salt type crystal structure composed of the positive electrode active material for the sodium ion secondary battery of the present invention is a space group:

It is a crystal structure (α phase) attributed to. FIG. 1 shows a schematic diagram of the crystal structure of the positive electrode active material for a sodium ion secondary battery of the present invention. In this structure, the α phase reported so far is occupied by Na ⁺ ions at the 3a position and Fe ³⁺ ions at the 3b position. In addition, the positive electrode active material for a sodium ion secondary battery of the present invention has a unique structure in which excess Na ⁺ ions occupy the 6c position corresponding to the upper and lower tetrahedral positions at the 3b position. Therefore, the x value corresponding to the Na / Fe value is 0.80 to 1.30, especially 0.90 to 1.20. The cation distribution is calculated by powder X-ray Rietveld analysis, and the Na / Fe ratio is calculated by elemental analysis using a fluorescent X-ray device. In particular, when the charge / discharge characteristics (discharge capacity, cycle characteristics, etc.) are excellent when the regulated charge capacity is set high, the positive electrode active material for a sodium ion secondary battery of the present invention is contained in this hexagonal layered rock salt type crystal structure. The 3b position iron occupancy rate is preferably 0.920 or more, more preferably 0.930 to 1.200. Further, when the charge / discharge characteristics (discharge capacity, cycle characteristics, etc.) are particularly excellent when the regulated charge capacity is set high, the positive electrode active material for a sodium ion secondary battery of the present invention has a 6c position Na occupancy rate of 0.050. The following is preferable, and 0.010 to 0.040 is more preferable. The hexagonal layered rock salt type crystal structure is not particularly limited, but is preferably 80 mol% or more, more preferably 90 mol% or more, based on the entire positive electrode active material for the sodium ion secondary battery of the present invention.

The positive electrode active material for a sodium ion secondary battery of the present invention may be a material having only a single-phase hexagonal layered rock salt type crystal structure, that is, a hexagonal layered rock salt type crystal structure. Other crystal structures (β-NaFeO type ₂ crystal structure, P2 type layered structure, etc.) are contained in 20 mol% or less, particularly 10 mol% or less of the positive electrode active material for the sodium ion secondary battery of the present invention, as long as they are not impaired. You may be doing it.

Further, in addition to the above-mentioned characteristics of the cation distribution, the positive electrode active material for a sodium ion secondary battery of the present invention also has a peculiar lattice constant. The a-axis value corresponding to the distance between Fe-Fe ions (see the center of Fig. 1) is 3.0235 Å or less, preferably 3.0235 Å or less from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) when the regulated charge capacity is set high. It is 3.0000 to 3.0230 Å. On the other hand, the c-axis value that reflects the laminated state of the layered lattice is 16.8020 Å or less, preferably 16.0000 to 16.0810 Å, from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) when the regulated charge capacity is set high. be. Further, the lattice volume is 127.360 Å ³ or less, preferably 127.000 to 127.300 Å ³ from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) when the regulated charge capacity is set high.

Further, in the positive electrode active material for a sodium ion secondary battery of the present invention, the first proximity Fe-Fe distance (2.6 to Fe) calculated from the Fourier conversion data of the FeK end wide area X-ray absorption spectrum (EXAFS) from the characteristics of the crystal structure described above. The ratio of the peak top intensity of 2.8 Å) to the peak top intensity of the first proximity Fe-O distance (1.4 to 1.6 Å) (Fe-Fe peak height / Fe-O peak height) is the regulated charge capacity. From the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) when set high, 1.05 or more is preferable, and 1.10 to 1.50 is more preferable. This seems to correspond to the fact that the positive electrode active material for a sodium ion secondary battery of the present invention has a Fe ion occupancy rate of 92% or more at the 3b position.

The positive electrode active material for a sodium ion secondary battery of the present invention is made of a sodium iron oxide having a composition represented by the above general formula (1). The positive electrode active material for a sodium ion secondary battery of the present invention may consist only of a sodium iron oxide having a composition represented by the above general formula (1), but may contain unavoidable impurities. good. Such unavoidable impurities may be due to raw materials, and include sodium-containing materials, iron-containing materials, potassium-containing materials, etc., which will be described later, and are 10 mol% or less, particularly 5) as long as the effects of the present invention are not impaired. It may be contained in a molar% or less.

2. 2. Method for Producing Positive Active Material for Sodium Ion Secondary Battery The positive positive active material for sodium ion secondary battery of the present invention is, for example,
It can be obtained by a production method comprising a step of performing a hydrothermal synthesis reaction at a temperature of 180 ° C. or higher for 30 hours or longer using an alkaline aqueous solution containing a sodium-containing material and an iron-containing material. Hereinafter, this method will be specifically described.

Specific examples of raw materials include metallic sodium (Na) sodium oxide (Na ₂ O), sodium peroxide (Na ₂ O ₂ ); sodium hydroxide (NaOH); sodium carbonate (Na ₂ CO ₃ ) as sodium-containing materials. , Sodium carbonate such as sodium hydrogen carbonate (NaHCO ₃ ) can be exemplified, and as iron-containing materials, metallic iron (Fe); iron (II) oxide (FeO), iron (III) oxide (Fe ₂ O ₃ ), Iron oxides such as Fe ₃ O ₄ ; Iron hydroxides such as iron (II) hydroxide (Fe (OH) ₂ ), iron (III) hydroxide (Fe (OH) ₃ ); α-FeOOH, β- Iron oxyhydroxides such as FeOOH; iron carbonates such as iron (II) carbonate (FeCO ₃ ) and iron (III) carbonate (Fe ₂ (CO ₃ ) ₂ ); iron nitrate (II) (Fe (NO ₃ )) ₂ ), iron nitrates such as iron (III) nitrate (Fe (NO ₃ ) ₃ ); iron chlorides such as iron (II) chloride (FeCl ₂ ), iron (III) chloride (FeCl ₃ ); iron sulfate (II) ) (FeSO ₄ ), iron (III) sulfate (Fe ₂ (SO ₄ ) ₃ ) and other iron sulfates can be exemplified. From the viewpoint that the positive electrode active material for a sodium ion secondary battery of the present invention can be easily obtained by hydrothermal synthesis, sodium hydroxide is preferable as the sodium-containing material, and iron oxyhydroxide (particularly α-FeOOH) is used as the iron-containing material. preferable. Further, from the viewpoint that the positive electrode active material for a sodium ion secondary battery of the present invention can be easily obtained by hydrothermal synthesis, it is preferable to use a material containing trivalent iron as the iron-containing material, but Fe ₂ O ₃ and Fe. When a material containing divalent iron such as ₃ O ₄ is used, as described in Patent Document 2, an oxidizing agent such as sodium chlorate is used, for example, 1 to 3 mol per 1 mol of iron-containing material. It can also be added. Furthermore, when an acidic water-soluble compound (nitrate, chloride, sulfate, etc.) is used as the iron-containing material, it is neutralized with an alkali and air-oxidized (bubbling) in advance, and then washed with water to remove excess salt. It can also be used later as a raw material. These sodium-containing materials and iron-containing materials can be used alone or in combination of two or more.

The content ratio of the sodium-containing material and the iron-containing material in the above-mentioned raw materials is not particularly limited, and it is preferable to use an excess amount of the sodium-containing material with respect to the iron-containing material. Specifically, it is preferable to use 5 to 50 parts by mass, particularly 10 to 30 parts by mass, of the sodium-containing material with respect to 100 parts by mass of the iron-containing material.

The alkaline aqueous solution is not particularly limited, but sodium hydroxide is used as the sodium-containing material, and a sodium hydroxide aqueous solution is preferable. The concentration of this alkaline aqueous solution is preferably high, preferably 20 M or more, particularly 50 M or more, from the viewpoint that the positive electrode active material for a sodium ion secondary battery of the present invention can be easily obtained by hydrothermal synthesis. Is preferable.

Further, the alkaline aqueous solution may contain a potassium-containing material in order to further improve the charge / discharge characteristics (particularly the cycle characteristics). The potassium-containing material is not particularly limited, but a neutral or alkaline salt such as potassium hydroxide or potassium chloride is preferable because it is treated in an alkaline aqueous solution. The amount of the potassium-containing material used is not particularly limited, and from the viewpoint of charge / discharge characteristics (particularly cycle characteristics), 0.2 to 10.0 parts by mass, particularly 0.5 to 5.0 parts by mass, should be used with respect to 100 parts by mass of the sodium-containing material. Is preferable. When the lithium-containing material is contained, it is preferable not to include it because the lattice constants a and c, the lattice volume, and the like become large and the cycle characteristics deteriorate.

Next, the hydrothermal synthesis reaction can be allowed to proceed by heating this alkaline aqueous solution. The hydrothermal synthesis reaction can be carried out using a normal hydrothermal reaction device (commercially available autoclave or the like).

Regarding the conditions of the hydrothermal synthesis reaction, the lattice constants a and c, the lattice volume, and the like become large at low temperatures. As a result, not only the discharge capacity is reduced, but also the cycle characteristics are dramatically deteriorated. On the other hand, even in the case of a short time, the lattice constants a and c, the lattice volume, and the like increase, and not only the discharge capacity decreases, but also the cycle characteristics deteriorate dramatically. Therefore, it is necessary to raise the temperature in the hydrothermal synthesis reaction, which is 180 ° C. or higher, preferably 200 to 400 ° C. from the viewpoint of the pressure applied in the hydrothermal heat treatment furnace. On the other hand, the hydrothermal synthesis reaction takes 30 hours or more, preferably 35 to 100 hours.

After the hydrothermal synthesis reaction is carried out by the above method, the reaction product may be washed in order to remove residues such as raw materials and excess alkaline components. For washing, a non-aqueous polar solvent such as alcohol or acetone can be used in order to further suppress the change to spinel ferrite due to the liberation of the Na component. Further, if necessary, the product after the hydrothermal treatment can be heat-treated in various atmospheres such as in the atmosphere. As for the heat treatment temperature, the upper limit temperature that does not change to the β phase is preferably 730 ° C. or lower as described in Patent Document 2. Then, the product is filtered and dried at, for example, 80 ° C. or higher (particularly 90 to 200 ° C.) to obtain the positive electrode active material for a sodium ion secondary battery of the present invention.

3. 3. Positive electrode for sodium ion secondary battery and sodium ion secondary battery The positive electrode active material for sodium ion secondary battery of the present invention utilizes the above-mentioned excellent characteristics (discharge capacity and cycle characteristics) to form a sodium ion secondary battery. It can be effectively used as a positive electrode active material. In the present specification, the "sodium ion secondary battery" is a concept including a metallic sodium secondary battery using a sodium metal as a negative electrode. In particular, since the positive electrode active material for a sodium ion secondary battery of the present invention is a material containing sodium in its structure, it is a material that can be charged and discharged as well as a negative electrode containing no sodium. Moreover, since the discharge capacity and the average voltage are high and the cycle characteristics are excellent, it is useful as a positive electrode active material for a sodium ion secondary battery. The sodium ion secondary battery using the positive electrode active material for the sodium ion secondary battery of the present invention may be a non-aqueous electrolyte sodium ion secondary battery using a non-aqueous solvent-based electrolytic solution as the electrolyte, and may be sodium ion conductivity. It may be an all-solid-state sodium-ion secondary battery using the solid electrolyte of.

The structures of the non-aqueous electrolyte sodium ion secondary battery and the all-solid-state sodium ion secondary battery are the same as those of the known sodium ion secondary battery except that the positive electrode active material for the sodium ion secondary battery of the present invention is used. be able to.

For example, the basic structure of the non-aqueous electrolyte sodium ion secondary battery is the same as that of the known non-aqueous electrolyte sodium ion secondary battery, except that the above-mentioned positive electrode active material for the sodium ion secondary battery is used. be able to.

As the positive electrode, the above-mentioned positive electrode active material for a sodium ion secondary battery is used, and a positive electrode mixture prepared by mixing with a conductive agent and a binder as necessary is used to collect positive electrodes such as aluminum, nickel, stainless steel, and carbon cloth. It can be manufactured by supporting it on the body. As the conductive agent, for example, a carbon material such as graphite, coke, carbon black, or needle-shaped carbon can be used.

As the negative electrode, both a sodium-containing material and a sodium-free material can be used. For example, any substance that reacts with sodium, such as difficult-to-sinter carbon, sodium metal, tin, and alloys containing these, can be used. These negative electrode active materials can also be supported on a negative electrode current collector made of aluminum, copper, nickel, stainless steel, carbon or the like by using a conductive agent, a binder or the like, if necessary, to manufacture a negative electrode.

As the separator, for example, a polyolefin resin such as polyethylene or polypropylene; a fluororesin; nylon; an aromatic aramid; a material such as inorganic glass can be used, and a material in the form of a porous film, a non-woven fabric, a woven fabric or the like can be used.

As the solvent for the non-aqueous electrolyte, for example, a solvent known as a solvent for a non-aqueous solvent-based secondary battery such as carbonate, ether, nitrile, and sulfur-containing compound can be used.

Further, the all-solid-state sodium-ion secondary battery can have the same structure as the known all-solid-state sodium-ion secondary battery except that the positive electrode active material for the sodium-ion secondary battery of the present invention is used.

In this case, the electrolyte includes, for example, a polymer-based solid electrolyte such as a polyethylene oxide-based polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain, a sulfide-based solid electrolyte, and oxidation. A physical solid electrolyte or the like can be used.

As the positive electrode of the all-solid-state sodium-ion secondary battery, for example, the positive electrode active material for the sodium-ion secondary battery of the present invention is used, and if necessary, a positive electrode mixture containing a conductive agent, a binder, a solid electrolyte, etc. is used in titanium. It can be manufactured by supporting it on a positive electrode current collector such as aluminum, nickel, or stainless steel. As the conductive agent, carbon materials such as graphite, coke, carbon black, and acicular carbon can be used in the same manner as in the non-aqueous solvent type secondary battery.

The shape of the non-aqueous electrolyte sodium ion secondary battery and the all-solid-state sodium ion secondary battery is not particularly limited, and may be any of a cylindrical type, a square type, and the like.

Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples, but it goes without saying that the present invention is not limited to these.

Example 1
265 g of sodium hydroxide and 5 g of potassium hydroxide (KOH) were weighed in a polytetrafluoroethylene (PTFE) beaker, 150 mL of distilled water was added, and the mixture was stirred well. 0.2 mol (17.77 g) of α-FeOOH was added to the obtained mixed alkaline aqueous solution, and the mixture was stirred well. This was allowed to stand in a hydrothermal reaction furnace, and after sealing, hydrothermal treatment was performed at 220 ° C. for 48 hours. After hydrothermal treatment, cool the reactor to around room temperature, take out the PTFE beaker, mix with 1 L of ethanol in a mixer, remove excess sodium hydroxide by filtering, and dry at 100 ° C to obtain the desired sodium. Iron oxide was obtained. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by fluorescent X-ray spectroscopy is 1.18 (7), the composition formula is Na _{1.18 (7)} FeO ₂ , which is within the range of the composition formula of the present invention. there were.

The X-ray diffraction pattern of the obtained sample is shown in Fig. 2, and the crystals obtained by the Rietveld analysis program RIETAN-FP (F. Izumi and K. Momma, Solid State Phenom., 130, 15-20 (2007).) The academic parameters are shown in Table 1. It is clear that each parameter is within the defined value of the substance of the present invention.

Next, the Fourier transform diagram of the K-end EXAFS spectrum of Fe is shown in FIG. 3, and the first proximity Fe-O and Fe-Fe peak heights and their intensity ratios are shown in Table 2. The measurement was carried out with a radiant light source at the SR Center of Ritsumeikan University. It is clear that the ratio (B / A) of the Fe-Fe peak height to the obtained Fe-O peak height is within the definition value of the present invention.

The charge / discharge characteristics of the sample of Example 1 were evaluated as follows. Under an ultra-low humidity environment with a dew point of -50 ° C or less, the obtained sodium iron oxide powder, Ketjen black, and PTFE are mixed at a mass ratio of 84: 8: 8 and pressure-bonded onto an aluminum mesh to form a positive electrode. A mixture was prepared. A coin battery was prepared by dissolving metallic sodium as a negative electrode and supporting salt NaPF ₆ as an electrolytic solution in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC). Charge the manufactured battery with a charge / discharge tester at + 30 ° C with a current density of 10 mA / g per positive electrode active material, regulate the charge capacity to 100 mAh / g, and fix it in the potential range of 1.5-4.0 V. A charge / discharge test was performed. The results are shown in FIGS. 4 and 3. As can be understood from FIGS. 4 and 3, the positive electrode active material for a sodium ion secondary battery of the present invention maintains a high capacity up to 20 cycles, and the capacity retention rate after 20 cycles (Q _20d / Q) with respect to after 1 cycle. It is clear that _1d ) is 99% or more, showing not only high capacity but also high cycle characteristics.

Comparative Example 1
Samples were prepared in the same manner as in Example 1 except that the alkali source to be added to sodium hydroxide was changed from KOH to 5 g of LiOH.H ₂ O. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by fluorescent X-ray spectroscopy is 1.14 (16), the composition formula is Na _{1.14 (16)} FeO ₂ , which is within the range of the composition formula of the present invention. rice field. The X-ray diffraction pattern of the obtained sample is shown in FIG. 5, and the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP are shown in Table 1. It is clear that each parameter is outside the definition of the substance of the present invention.

Next, the Fourier transform diagram of the K-end EXAFS spectrum of Fe is shown in FIG. 6, and the first proximity Fe-O and Fe-Fe peak heights and their intensity ratios are shown in Table 2. It is clear that the ratio (B / A) of the Fe-Fe peak height to the obtained Fe-O peak height is outside the definition value of the present invention.

Similar to the sample of Example 1, the charge / discharge characteristics of the sample of Comparative Example 1 were evaluated. The results are shown in FIG. 7 and Table 3. As shown in FIGS. 7 and 3, the sample of Comparative Example 1 showed a low capacity in 20 cycles, and the capacity retention rate (Q _20d / Q _1d ) after 20 cycles was only 57% with respect to that after 1 cycle, and the cycle characteristics were inferior. It is clear that. From the above, it can be seen that when a lithium source is used as the alkali source to be added to sodium hydroxide, the cycle characteristics are rather deteriorated.

Example 2
Sample preparation was carried out in the same manner as in Example 1 except that the amount of sodium hydroxide was 270 g and KOH was not added. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by fluorescent X-ray spectroscopy is 0.915 (14), the composition formula is Na _{0.915 (14)} FeO ₂ , which is within the range of the composition formula of the present invention. there were. The X-ray diffraction pattern of the obtained sample is shown in FIG. 8, and the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP are shown in Table 1. It is clear that each parameter is within the defined value of the substance of the present invention.

Next, the Fourier transform diagram of the K-end EXAFS spectrum of Fe is shown in FIG. 9, and the first proximity Fe-O and Fe-Fe peak heights and their intensity ratios are shown in Table 2. It is clear that the ratio (B / A) of the Fe-Fe peak height to the obtained Fe-O peak height is within the definition value of the present invention.

Similar to the sample of Example 1, the charge / discharge characteristics of the sample of Example 2 were evaluated. The results are shown in FIG. 10 and Table 3. As shown in FIG. 10 and Table 3, the sample of Example 2 showed a high capacity at 20 cycles, and the capacity retention rate (Q _20d / Q _1d ) after 20 cycles with respect to after 1 cycle was 76%. It is clear that the cycle characteristics are excellent. From the above, it is clear that sodium iron oxide having excellent charge / discharge characteristics, which could not be achieved in Patent Documents 2 and 3, can be obtained.

Comparative Example 2
Samples were prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were set to 150 ° C. and 48 hours. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by fluorescent X-ray spectroscopy is 1.00 (7), the composition formula is Na _{1.00 (7)} FeO ₂ , which is within the range of the composition formula of the present invention. rice field. The X-ray diffraction pattern of the obtained sample is shown in FIG. 11, and the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP are shown in Table 1. It is clear that each parameter is outside the definition of the substance of the present invention.

Next, the Fourier transform diagram of the K-end EXAFS spectrum of Fe is shown in FIG. 12, and the first proximity Fe-O and Fe-Fe peak heights and their intensity ratios are shown in Table 2. It is clear that the ratio (B / A) of the Fe-Fe peak height to the obtained Fe-O peak height is outside the definition value of the present invention.

Similar to the sample of Example 1, the charge / discharge characteristics of the sample of Comparative Example 2 were evaluated. The results are shown in FIG. 13 and Table 3. As shown in FIG. 13 and Table 3, the sample of Comparative Example 2 showed a low capacity at 20 cycles, and the capacity retention rate after 20 cycles (Q _20d / Q _1d ) was only 32% with respect to after 1 cycle. It is clear that the cycle characteristics are inferior. From the above, it is clear that the desired sodium iron oxide cannot be obtained by changing the hydrothermal treatment conditions to 150 ° C. for 48 hours.

Comparative Example 3
A sample was prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were 220 ° C. and 20 hours, which were the same as in Patent Document 2. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by fluorescent X-ray spectroscopy is 1.332 (7), the composition formula is Na _{1.332 (7)} FeO ₂ , which is outside the range of the composition formula of the present invention. rice field. The X-ray diffraction pattern of the obtained sample is shown in FIG. 14, and the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP are shown in Table 1. It is clear that each parameter is outside the definition of the substance of the present invention.

Similar to the sample of Example 1, the charge / discharge characteristics of the sample of Comparative Example 3 were evaluated. The results are shown in FIG. 15 and Table 3. As shown in FIG. 15 and Table 3, the sample of Comparative Example 3 showed a low capacity at 20 cycles, and the capacity retention rate after 20 cycles (Q _20d / Q _1d ) was only 28% with respect to after 1 cycle. , It is clear that the cycle characteristics are inferior. From the above, it is clear that the desired sodium iron oxide cannot be obtained by changing the hydrothermal treatment conditions to 220 ° C. for 20 hours.

Claims (8)

General formula (1):
Na _x FeO ₂ (1)
[In the formula, x is 0.80 to 1.30. ]
Represented by
It has a hexagonal layered rock salt type crystal structure and has a hexagonal layered rock salt type crystal structure.
The lattice constant is 3.0235 Å or less for a and 16.0820 Å or less for c.
Lattice volume is 127.360 Å ³ or less,
Positive electrode active material for sodium ion secondary batteries made of sodium iron oxide. The positive electrode active material for a sodium ion secondary battery according to claim 1, wherein the 3b position iron occupancy in the hexagonal layered rock salt type crystal structure of the sodium iron oxide is 0.920 or more. The positive electrode active material for a sodium ion secondary battery according to claim 1 or 2, wherein the 6c position sodium occupancy in the hexagonal layered rock salt type crystal structure of the sodium iron oxide is 0.050 or less. In the Fourier transform spectrum of the FeK end wide area X-ray absorption (EXAFS) spectrum of the sodium iron oxide, the ratio of Fe-Fe height to Fe-O height (Fe-Fe height / Fe-O height) is 1.050. The positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 3 as described above. The method for producing a positive electrode material for a sodium secondary battery according to any one of claims 1 to 4.
A step of performing a hydrothermal synthesis reaction at a temperature of 180 ° C. or higher for 30 hours or longer using an alkaline aqueous solution containing a sodium-containing material , an iron-containing material , and a neutral or alkaline salt containing potassium and not containing a lithium-containing material. A manufacturing method. The production method according to claim 5, wherein the sodium-containing material is sodium hydroxide. A positive electrode for a sodium ion secondary battery containing the positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 4. The sodium ion secondary battery comprising the positive electrode for the sodium ion secondary battery according to claim 7 .

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