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WO2018225740A1 - Positive electrode active material for sodium secondary batteries and method for producing same - Google Patents

Positive electrode active material for sodium secondary batteries and method for producing same Download PDF

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Publication number
WO2018225740A1
WO2018225740A1 PCT/JP2018/021571 JP2018021571W WO2018225740A1 WO 2018225740 A1 WO2018225740 A1 WO 2018225740A1 JP 2018021571 W JP2018021571 W JP 2018021571W WO 2018225740 A1 WO2018225740 A1 WO 2018225740A1
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Prior art keywords
sodium
positive electrode
ion secondary
secondary battery
electrode active
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PCT/JP2018/021571
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French (fr)
Japanese (ja)
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田渕　光春
理樹 片岡
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国立研究開発法人産業技術総合研究所
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Priority to JP2019523916A priority Critical patent/JP7085761B2/en
Publication of WO2018225740A1 publication Critical patent/WO2018225740A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/054—Accumulators with insertion or intercalation of metals other than lithium, e.g. with magnesium or aluminium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
    • H01M4/485—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of mixed oxides or hydroxides for inserting or intercalating light metals, e.g. LiTi2O4 or LiTi2OxFy
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

Definitions

  • This invention relates to the positive electrode active material for sodium secondary batteries, and its manufacturing method.
  • Lithium ion secondary batteries with the highest energy density among secondary batteries have been put into practical use, but one of the alternative technologies is to replace the charge carriers from lithium ions to more resource-rich sodium ions. It is a secondary battery.
  • the sodium ion secondary battery is one of the secondary batteries that is expected to be able to realize a high operating voltage (3 V or more), similarly to the lithium ion secondary battery. 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.
  • sodium ferrite (NaFeO 2 ) is particularly important.
  • Sodium ferrite has an ⁇ phase and a ⁇ phase.
  • ⁇ phase which is a high temperature phase
  • the transition from the ⁇ phase to the ⁇ phase is not completely completed due to the large difference in crystal structure from the ⁇ phase, and the ⁇ phase often coexists with the ⁇ phase. Phase synthesis is not easy.
  • 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 abundant materials as raw materials. It aims at providing the positive electrode active material for sodium ion secondary batteries obtained.
  • the inventors of the present invention have intensively studied to achieve the above-described object.
  • sodium iron oxide having a hexagonal layered rock-salt crystal structure and a specific lattice constant and lattice volume when the regulated charging capacity is set high without using expensive and resource-rare materials It was also found that high discharge capacity and excellent cycle characteristics can be exhibited.
  • 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) [Wherein x is 0.80 to 1.30.
  • a positive electrode active material for sodium ion secondary batteries comprising sodium iron oxide.
  • 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 crystal structure of the sodium iron oxide is 0.920 or more.
  • Item 3. Item 3.
  • Item 4 In the Fourier transform spectrum of the FeK edge broad X-ray absorption (EXAFS) spectrum of the sodium iron oxide, the ratio of the Fe-Fe height to the Fe-O height (Fe-Fe height / Fe-O height) is 1.050.
  • a 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 more 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 6. The production method according to Item 5, wherein the sodium-containing material is sodium hydroxide.
  • Item 7. Item 7. The method according to Item 5 or 6, wherein the alkaline aqueous solution further contains a potassium-containing material.
  • a positive electrode for a sodium ion secondary battery comprising the positive electrode active material for a sodium ion secondary battery according to any one of items 1 to 4.
  • Item 12. A sodium ion secondary battery comprising the positive electrode for a sodium ion secondary battery according to Item 9.
  • the positive electrode active material for the 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 expensive and resource-rare materials. it can.
  • a and c correspond to lattice constants.
  • 3a, 3b and 6c correspond to (000), (001/2) and (00z) (0.4 ⁇ z ⁇ 0.5), respectively.
  • 3 is a radial distribution function obtained by Fourier transform of the K-edge EXAFS spectrum of Fe of the sample of Example 1.
  • FIG. The charging / discharging characteristic of the sodium secondary battery which used the sample of Example 1 as a positive electrode active material is shown.
  • the curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d).
  • the number indicates the cycle number. It is the measurement (+) and calculation (solid line) X-ray-diffraction pattern of the sample of the comparative example 1. 4 is a radial distribution function obtained by Fourier transform of the K-edge EXAFS spectrum of Fe of the sample of Comparative Example 1; The charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 1 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number.
  • the charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 2 as a positive electrode active material is shown.
  • the curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d).
  • the number indicates the cycle number. It is the measurement (+) and calculation (solid line) X-ray-diffraction pattern of the sample of the comparative example 3.
  • the charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 3 as a positive electrode active material is shown.
  • the curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d).
  • the number indicates the cycle number.
  • containing is a concept including any of “comprise”, “consistently of”, and “consist of”.
  • the notation “A to B” means “A or more and B or less”.
  • Positive electrode active material for sodium ion secondary battery has the general formula (1): Na x FeO 2 (1) [Wherein x is 0.80 to 1.30. ] Represented by Hexagonal layered rock salt type crystal structure, Lattice constant, a is 3.0235mm or less, c is 16.0820mm or less, Lattice volume is 127.360A 3 or less, consisting of sodium iron oxide.
  • the hexagonal layered rock-salt crystal structure of the positive electrode active material for sodium ion secondary batteries of the present invention is a space group:
  • the crystal structure ( ⁇ phase) belonging to A schematic diagram of the crystal structure of the positive electrode active material for a sodium ion secondary battery of the present invention is shown in FIG.
  • the ⁇ phase that has been reported so far is occupied by Na + ions at the 3a position and Fe 3+ ions at the 3b position.
  • 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 of the 3b position. Therefore, the x value corresponding to the Na / Fe value is 0.80 to 1.30, particularly 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 apparatus.
  • the positive electrode active material for the sodium ion secondary battery of the present invention is in this hexagonal layered rock salt type crystal structure.
  • the 3b position iron occupancy is preferably 0.920 or more, more preferably 0.930 to 1.200.
  • the positive electrode active material for the sodium ion secondary battery of the present invention has a sodium occupancy ratio of 6c at 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 total positive electrode active material for 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. As long as it is not impaired, other crystal structures ( ⁇ -NaFeO 2 type crystal structure, P2 type layered structure, etc.) have 20 mol% or less, particularly 10 mol% or less of the positive electrode active material for sodium ion secondary battery of the present invention. You may do it.
  • the positive electrode active material for a sodium ion secondary battery of the present invention also has a unique lattice constant.
  • the a-axis value corresponding to the distance between Fe and Fe ions is 3.0235 mm or less, preferably from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics), etc. when the regulated charge capacity is set high. 3.0000 ⁇ 3.02303.0.
  • the c-axis value reflecting the layered state of the layered lattice is 16.0820 mm or less, preferably 16.0000 to 16.0810 mm, from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics), etc.
  • the positive electrode active material for a sodium ion secondary battery of the present invention has a first adjacent Fe-Fe distance (2.6 to 2.6) calculated from the Fourier transform data of the FeK edge broad X-ray absorption spectrum (EXAFS) based on the characteristics of the crystal structure.
  • the ratio of the peak top intensity of 2.8 mm to the peak top intensity of the distance between the first adjacent Fe-O (1.4 to 1.6 mm) (Fe-Fe peak height / Fe-O peak height) From the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) and the like when set to a high value, 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 sodium ion secondary batteries of the present invention has a 3b-position Fe ion occupancy of 92% or more.
  • the positive electrode active material for a sodium ion secondary battery of the present invention is made of sodium iron oxide having a composition represented by the general formula (1).
  • the positive electrode active material for a sodium ion secondary battery of the present invention may be composed only of sodium iron oxide having the composition represented by the general formula (1), but may contain inevitable impurities. Good. Such inevitable impurities are considered to be due to raw materials, and include the following sodium-containing materials, iron-containing materials, potassium-containing materials, etc., within a range not impairing the effects of the present invention, 10 mol% or less, particularly 5 You may contain below mol%.
  • the positive electrode 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 for 30 hours or more at a temperature of 180 ° C. or higher using an alkaline aqueous solution containing a sodium-containing material and an iron-containing material.
  • this method will be specifically described.
  • raw materials include sodium-containing materials such as metal sodium (Na) sodium oxide (Na 2 O), sodium peroxide (Na 2 O 2 ); sodium hydroxide (NaOH); sodium carbonate (Na 2 CO 3 ).
  • Sodium carbonate such as sodium hydrogen carbonate (NaHCO 3 ), etc.
  • iron-containing materials include metal 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 hydroxide (II) (Fe (OH) 2 ) and iron hydroxide (III) (Fe (OH) 3 ); ⁇ -FeOOH, ⁇ - Iron oxyhydroxides such as FeOOH; iron carbonates such as iron (II) carbonate (FeCO 3 ) and iron (III) (Fe 2 (CO 3 ) 2 ); iron nitrate (II) (Fe (NO 3 )) 2 ), iron nitrates (I
  • the sodium-containing material is preferably sodium hydroxide, and the iron-containing material is iron oxyhydroxide (especially ⁇ -FeOOH). preferable.
  • the iron-containing material it is preferable to use a material containing trivalent iron, Fe 2 O 3 , 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 added in an amount of 1 to 3 mol per mol of the iron-containing material, for example.
  • iron-containing material 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 (bubbled) in advance, and then washed with water to remove excess salt. It can also be used as a raw material later.
  • acidic water-soluble compound nitrate, chloride, sulfate, etc.
  • these sodium-containing material and iron-containing material 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-described raw material is not particularly limited, and the sodium-containing material is preferably excessive 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 the alkaline aqueous solution is preferably a high concentration from the viewpoint of easily obtaining the positive electrode active material for a sodium ion secondary battery of the present invention by hydrothermal synthesis, specifically, 20M or more, particularly 50M or more. It is preferable.
  • this alkaline aqueous solution may contain a potassium-containing material in order to further improve charge / discharge characteristics (particularly cycle characteristics).
  • a potassium-containing material Since it processes in alkaline aqueous solution, neutral or alkaline salts, such as potassium hydroxide and potassium chloride, are preferable.
  • the amount of potassium-containing material used is not particularly limited, and from the viewpoint of charge / discharge characteristics (especially cycle characteristics), 0.2 to 10.0 parts by weight, particularly 0.5 to 5.0 parts by weight is used with respect to 100 parts by weight of sodium-containing material. Is preferred. When a lithium-containing material is included, the lattice constants a and c, the lattice volume, etc. are increased, and the cycle characteristics are deteriorated.
  • the hydrothermal synthesis reaction can be advanced by heating the alkaline aqueous solution.
  • the hydrothermal synthesis reaction can be performed using a normal hydrothermal reaction apparatus (commercially available autoclave or the like).
  • the lattice constants a and c, the lattice volume, and the like increase at low temperatures. As a result, not only the discharge capacity is lowered, but 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, etc. are increased, and not only the discharge capacity is lowered, but also the cycle characteristics are dramatically deteriorated. For this reason, it is necessary to increase the temperature in the hydrothermal synthesis reaction, and it is preferably 180 ° C. or higher and 200 to 400 ° C. from the viewpoint of the pressure applied in the hydrothermal treatment furnace. On the other hand, the hydrothermal synthesis reaction time is 30 hours or more, preferably 35 to 100 hours.
  • the reaction product may be washed in order to remove residues such as raw materials and excess alkali components.
  • a nonaqueous polar solvent such as alcohol or acetone can be used for washing.
  • the product after hydrothermal treatment can also be heat-treated in various atmospheres, such as air
  • the upper limit temperature at which the ⁇ phase does not change is preferably 730 ° C. or lower as disclosed in Patent Document 2.
  • the product is filtered and dried at, for example, 80 ° C. or higher (particularly 90 to 200 ° C.), whereby the positive electrode active material for sodium ion secondary batteries of the present invention can be obtained.
  • the positive electrode active material for sodium ion secondary battery of the present invention utilizes the above-described excellent characteristics (discharge capacity and cycle characteristics) to provide a sodium ion secondary battery. It can be effectively used as a positive electrode active material.
  • the “sodium ion secondary battery” is a concept including a metal sodium secondary battery using sodium metal as a negative electrode.
  • the positive electrode active material for a sodium ion secondary battery of the present invention is a material that contains sodium in the structure, and thus can be used not only for a negative electrode that does not contain sodium but also for charging and discharging from charging.
  • the sodium ion secondary battery using the positive electrode active material for sodium ion secondary battery of the present invention may be a non-aqueous electrolyte sodium ion secondary battery using a non-aqueous solvent electrolyte as an electrolyte, and sodium ion conductive
  • the all-solid-state sodium ion secondary battery using the solid electrolyte may be used.
  • 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 a known sodium ion secondary battery except that the positive electrode active material for sodium ion secondary battery of the present invention is used. be able to.
  • the basic structure of the non-aqueous electrolyte sodium ion secondary battery is the same as that of a known non-aqueous electrolyte sodium ion secondary battery except that the positive electrode active material for sodium ion secondary battery is used. be able to.
  • the positive electrode active material for the sodium ion secondary battery described above is used, and the positive electrode mixture prepared by mixing with a conductive agent and a binder as necessary is a positive electrode current collector such as aluminum, nickel, stainless steel, carbon cloth, etc. It can be manufactured by supporting it on the body.
  • a conductive agent for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used.
  • both a material containing sodium and a material not containing sodium can be used.
  • any substance that reacts with sodium such as hardly sinterable 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, using a conductive agent, a binder, or the like as necessary to produce a negative electrode.
  • a polyolefin resin such as polyethylene or polypropylene; a fluororesin; a nylon; an aromatic aramid; an inorganic glass or the like, and a material in the form of a porous film, a nonwoven fabric, a woven fabric, or the like can be used.
  • a known solvent can be used as a solvent for a non-aqueous solvent secondary battery such as carbonate, ether, nitrile, and sulfur-containing compound.
  • the all-solid-state sodium ion secondary battery can have the same structure as a known all-solid-state sodium ion secondary battery except that the positive electrode active material for sodium ion secondary batteries of the present invention is used.
  • a polymer solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain, a sulfide solid electrolyte, an oxidation
  • a physical solid electrolyte or the like can be used as the electrolyte.
  • the positive electrode of the all-solid-type sodium ion secondary battery for example, the positive electrode active material for the sodium ion secondary battery of the present invention is used. It can be produced by supporting it on a positive electrode current collector such as aluminum, nickel or stainless steel.
  • a positive electrode current collector such as aluminum, nickel or stainless steel.
  • the conductive agent for example, a carbon material such as graphite, cokes, carbon black, and acicular carbon can be used as in the case of the non-aqueous solvent 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 shape, a square shape, and the like.
  • Example 1 In a polytetrafluoroethylene (PTFE) beaker, 265 g of sodium hydroxide and 5 g of potassium hydroxide (KOH) were weighed, and 150 mL of distilled water was added and stirred well. 0.2 mol (17.77 g) of ⁇ -FeOOH was added to the resulting mixed alkaline aqueous solution and stirred well. This was left in a hydrothermal reactor, and after sealing, hydrothermal treatment was performed at 220 ° C. for 48 hours.
  • PTFE polytetrafluoroethylene
  • Fig. 2 shows the X-ray diffraction pattern of the obtained sample.
  • the crystal obtained by the Rietveld analysis program RIETAN-FP F. Izumi and K. Momma, Solid State Phenom., 130, 15-20 (2007).
  • Table 1 shows the academic parameters. It is clear that each parameter is within the defined value of the substance of the present invention.
  • FIG. 3 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe
  • Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios.
  • the measurement was performed with a radiation source at the Ritsumeikan University SR Center. It is apparent that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is within the defined value of the present invention.
  • the charge / discharge characteristic evaluation of the sample of Example 1 was performed as follows. In 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 in a mass ratio of 84: 8: 8 and pressed onto an aluminum mesh to form a positive electrode A composite was prepared. A coin battery was prepared using metal sodium as the negative electrode and a support salt NaPF 6 dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) as the electrolyte.
  • EC ethylene carbonate
  • DEC diethyl carbonate
  • Comparative Example 1 A sample was prepared in the same manner as in Example 1 except that the alkali source 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 X-ray fluorescence 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. It was.
  • FIG. 5 shows the X-ray diffraction pattern of the obtained sample, and Table 1 shows the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP. It is clear that each parameter is outside the defined value of the substance of the present invention.
  • FIG. 6 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe
  • Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is clear that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is outside the defined value of the present invention.
  • 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 X-ray fluorescence spectroscopy is 0.915 (14), the composition formula is Na 0.915 (14) FeO 2 and 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.
  • FIG. 9 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe
  • Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is apparent that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is within the defined value of the present invention.
  • the charge / discharge characteristics of the sample of Example 2 were evaluated. The results are shown in FIG. As shown in FIG. 10 and Table 3, the sample of Example 2 shows a high capacity in 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) is 76% after 1 cycle. It is clear that the cycle characteristics are excellent. From the above, it is clear that a sodium iron oxide excellent in charge / discharge characteristics that could not be achieved in Patent Documents 2 and 3 can be obtained.
  • Comparative Example 2 A sample was prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were 150 ° C. and 48 hours. Since the Na / Fe ratio (corresponding to the x value in the composition formula) by X-ray fluorescence 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. It was. 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 defined value of the substance of the present invention.
  • FIG. 12 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe
  • Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is clear that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is outside the defined value of the present invention.
  • Comparative Example 3 A sample was prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were set to 220 ° C. for 20 hours as in Patent Document 2. Since the Na / Fe ratio (corresponding to the x value in the composition formula) by X-ray fluorescence 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. It was.
  • 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 defined value of the substance of the present invention.
  • the charge / discharge characteristics of the sample of Comparative Example 3 were evaluated. The results are shown in FIG. As shown in FIG. 15 and Table 3, the sample of Comparative Example 3 shows a low capacity at 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) is only 28% 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 condition to 220 ° C. for 20 hours.

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Abstract

A positive electrode active material for sodium ion secondary batteries, which is composed of a sodium iron oxide that is represented by general formula (1) NaxFeO2 (wherein x is from 0.80 to 1.30), and which has a hexagonal layered rock salt-type crystal structure, while having a lattice constant a of 3.0235 Å or less, a lattice constant c of 16.0820 Å or less and a lattice volume of 127.360 Å3 or less. This positive electrode active material for sodium ion secondary batteries is able to be obtained using abundant resources as the starting materials, and is capable of exhibiting excellent cycle characteristics and high discharge capacity even in cases where the limited charge capacity is set to high values.

Description

ナトリウム二次電池用正極活物質及びその製造方法Positive electrode active material for sodium secondary battery and method for producing the same
　本発明は、ナトリウム二次電池用正極活物質及びその製造方法に関する。 This invention relates to the positive electrode active material for sodium secondary batteries, and its manufacturing method.
　二次電池のなかで最も高いエネルギー密度を有するリチウムイオン二次電池が実用化されているが、その代替技術の一つが電荷担体をリチウムイオンからより資源的に豊富なナトリウムイオンに代えたナトリウムイオン二次電池である。ナトリウムイオン二次電池は、リチウムイオン二次電池と同様に高い作動電圧（3V以上）を実現できることが有望視される二次電池の一つである。ナトリウムイオン二次電池においても、正極活物質の選択の重要性はリチウムイオン二次電池と変わりなく、正極活物質の性能が電池の理論容量と作動電圧を決定づける。 Lithium ion secondary batteries with the highest energy density among secondary batteries have been put into practical use, but one of the alternative technologies is to replace the charge carriers from lithium ions to more resource-rich sodium ions. It is a secondary battery. The sodium ion secondary battery is one of the secondary batteries that is expected to be able to realize a high operating voltage (3 V or more), similarly to the lithium ion secondary battery. 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.
　正極活物質候補のなかで特に重要なものがナトリウムフェライト（NaFeO2）である。ナトリウムフェライトにはα相とβ相とがあり、例えば、特許文献１に記載されているように、α相のみが正極活物質として機能する。しかしながら、高温相であるβ相がいったんできると、α相との結晶構造の大きな違いから完全にβ相からα相への転移が完了せず、β相がしばしばα相と混在するため、α相の合成は容易ではない。 Among the positive electrode active material candidates, sodium ferrite (NaFeO 2 ) is particularly important. 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 often coexists with the α phase. Phase synthesis is not easy.
　一方、水酸化ナトリウム中で鉄源を水熱処理する工程を用いた場合に、高結晶性の試料が得られ、規制充電容量を70mAh/gに制限したときに優れた充放電特性を有するナトリウムフェライトが得られることが知られている（例えば、特許文献２参照）。しかしながらこの材料は、例えば、特許文献３に比較例１として記載されているように、規制充電容量を100mAh/gに上昇したときにはサイクル劣化が激しく、特定量のコバルトを置換しないと特性改善できなかった。しかしながら、コバルトは高価であるため実用化の妨げとなっている。このため、高価で資源的に希少なコバルトを使うことなくこの充放電条件下でも安定にサイクル可能なナトリウムイオン二次電池用正極活物質ができればナトリウムイオン二次電池の実用化に向けて大きく前進する。 On the other hand, when using the process of hydrothermal treatment of iron source in sodium hydroxide, a highly crystalline sample is obtained, and sodium ferrite with 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 Patent Document 3, for example, as Comparative Example 1, this material has severe cycle deterioration when the regulated charge capacity is increased to 100 mAh / g, and the characteristics cannot be improved unless a specific amount of cobalt is substituted. It was. However, since cobalt is expensive, it has hindered practical use. Therefore, if a positive electrode active material for a sodium ion secondary battery that can be stably cycled under this charge / discharge condition without using expensive and resource-rare cobalt, a significant advance will be made toward the practical application of sodium ion secondary batteries. To do.
特開２００５－３１７５１１号公報JP 2005-317511 A 特開２０１４－０８６２７９号公報JP 2014-086279 A 特開２０１５－１７６６６２号公報Japanese Patent Laying-Open No. 2015-17662
　本発明は、上記した従来技術の現状に鑑みてなされたものであり、資源的に豊富な物質を原料として、規制充電容量を高く設定した場合にも高い放電容量と優れたサイクル特性を発揮し得るナトリウムイオン二次電池用正極活物質を提供することを目的とする。 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 abundant materials as raw materials. It aims at providing the positive electrode active material for sodium ion secondary batteries obtained.
　本発明者らは、上記した目的を達成すべく鋭意研究を重ねてきた。その結果、六方晶層状岩塩型結晶構造を有し、特定の格子定数及び格子体積を有するナトリウム鉄酸化物が、高価で資源的に希少な材料を使わずとも、規制充電容量を高く設定した場合にも高い放電容量と優れたサイクル特性を発揮できることを見出した。本発明は、これらの知見に基づいてさらに研究を重ねた結果、完成されたものである。即ち、本発明は、以下の構成を包含する。
項１．一般式（1）：
NaxFeO2　　　（1）
［式中、xは0.80～1.30である。］
で表され、
六方晶層状岩塩型結晶構造を有し、
格子定数が、aは3.0235Å以下、cは16.0820Å以下であり、
格子体積が127.360Å3以下である、
ナトリウム鉄酸化物からなるナトリウムイオン二次電池用正極活物質。
項２．前記ナトリウム鉄酸化物が有する六方晶層状岩塩型結晶構造中の3b位置鉄占有率が0.920以上である、項１に記載のナトリウムイオン二次電池用正極活物質。
項３．前記ナトリウム鉄酸化物が有する六方晶層状岩塩型結晶構造中の6c位置ナトリウム占有率が0.050以下である、項１又は２に記載のナトリウムイオン二次電池用正極活物質。
項４．前記ナトリウム鉄酸化物のFeK端広域X線吸収（EXAFS）スペクトルのフーリエ変換スペクトルにおいて、Fe-O高さに対するFe-Fe高さの比（Fe-Fe高さ／Fe-O高さ）が1.050以上である、項１～３のいずれか1項に記載のナトリウムイオン二次電池用正極活物質。項５．請求項１～４のいずれか1項に記載のナトリウム二次電池用正極材料の製造方法であって、
ナトリウム含有材料及び鉄含有材料を含み、リチウム含有材料を含まないアルカリ水溶液を用いて、180℃以上の温度で30時間以上水熱合成反応を行う工程
を備える、製造方法。
項６．前記ナトリウム含有材料が水酸化ナトリウムである、項５に記載の製造方法。
項７．前記アルカリ水溶液が、さらに、カリウム含有材料を含む、項５又は６に記載の製造方法。
項８．前記カリウム含有材料が、カリウムを含む中性又はアルカリ性塩である、項７に記載の製造方法。
項９．項１～４のいずれか1項に記載のナトリウムイオン二次電池用正極活物質を含有する、ナトリウムイオン二次電池用正極。
項１０．項９に記載のナトリウムイオン二次電池用正極を備えるナトリウムイオン二次電池。 The inventors of the present invention have intensively studied to achieve the above-described object. As a result, sodium iron oxide having a hexagonal layered rock-salt crystal structure and a specific lattice constant and lattice volume, when the regulated charging capacity is set high without using expensive and resource-rare materials It was also found that high discharge capacity and excellent cycle characteristics can be exhibited. 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)
[Wherein x is 0.80 to 1.30. ]
Represented by
Hexagonal layered rock salt type crystal structure,
Lattice constant, a is 3.0235mm or less, c is 16.0820mm or less,
Lattice volume is 127.360A 3 or less,
A positive electrode active material for sodium ion secondary batteries comprising 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 crystal structure of the sodium iron oxide is 0.920 or more.
Item 3. Item 3. The positive electrode active material for a sodium ion secondary battery according to Item 1 or 2, wherein a sodium occupancy at a 6c position in the hexagonal layered rock salt crystal structure of the sodium iron oxide is 0.050 or less.
Item 4. In the Fourier transform spectrum of the FeK edge broad X-ray absorption (EXAFS) spectrum of the sodium iron oxide, the ratio of the Fe-Fe height to the 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 Items 1 to 3, which is as described above. Item 5. A 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 more 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 6. The production method according to Item 5, wherein the sodium-containing material is sodium hydroxide.
Item 7. Item 7. The method according to Item 5 or 6, wherein the alkaline aqueous solution further contains a potassium-containing material.
Item 8. Item 8. 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, comprising the positive electrode active material for a sodium ion secondary battery according to any one of items 1 to 4.
Item 10. Item 12. A sodium ion secondary battery comprising the positive electrode for a sodium ion secondary battery according to Item 9.
　本発明のナトリウムイオン二次電池用正極活物質は、高価で資源的に希少な材料を使わずとも、規制充電容量を高く設定した場合にも高い放電容量と優れたサイクル特性を発揮することができる。 The positive electrode active material for the 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 expensive and resource-rare materials. it can.
本発明のナトリウムイオン二次電池用正極活物質の結晶構造の概略図である。a及びcは格子定数に相当する。各格子位置の座標は、3a、3b及び6cがそれぞれ(000)、(001/2)及び(00z)（0.4≦z≦0.5）に相当する。It is the schematic of the crystal structure of the positive electrode active material for sodium ion secondary batteries of this invention. a and c correspond to lattice constants. As for the coordinates of each lattice position, 3a, 3b and 6c correspond to (000), (001/2) and (00z) (0.4 ≦ z ≦ 0.5), respectively. 実施例１の試料の実測(+)及び計算（実線）X線回折パターンである。It is the measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Example 1. 実施例１の試料のFeのK端EXAFSスペクトルのフーリエ変換による動径分布関数である。3 is a radial distribution function obtained by Fourier transform of the K-edge EXAFS spectrum of Fe of the sample of Example 1. FIG. 実施例１の試料を正極活物質として用いたナトリウム二次電池の充放電特性を示す。右上がりの曲線が充電（c）、右下がりの曲線が放電（d）に対応している。数字はサイクル数を示す。The charging / discharging characteristic of the sodium secondary battery which used the sample of Example 1 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number. 比較例１の試料の実測(+)及び計算（実線）X線回折パターンである。It is the measurement (+) and calculation (solid line) X-ray-diffraction pattern of the sample of the comparative example 1. 比較例１の試料のFeのK端EXAFSスペクトルのフーリエ変換による動径分布関数である。4 is a radial distribution function obtained by Fourier transform of the K-edge EXAFS spectrum of Fe of the sample of Comparative Example 1; 比較例１の試料を正極活物質として用いたナトリウム二次電池の充放電特性を示す。右上がりの曲線が充電（c）、右下がりの曲線が放電（d）に対応している。数字はサイクル数を示す。The charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 1 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number. 実施例２の試料の実測(+)及び計算（実線）X線回折パターンである。It is the measurement (+) and calculation (solid line) X-ray diffraction pattern of the sample of Example 2. 実施例２の試料のFeのK端EXAFSスペクトルのフーリエ変換による動径分布関数である。7 is a radial distribution function obtained by Fourier transform of the K-edge EXAFS spectrum of Fe of the sample of Example 2. FIG. 実施例２の試料を正極活物質として用いたナトリウム二次電池の充放電特性を示す。右上がりの曲線が充電（c）、右下がりの曲線が放電（d）に対応している。数字はサイクル数を示す。The charging / discharging characteristic of the sodium secondary battery which used the sample of Example 2 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number. 比較例２の試料の実測(+)及び計算（実線）X線回折パターンである。It is the measurement (+) and calculation (solid line) X-ray-diffraction pattern of the sample of the comparative example 2. 比較例２の試料のFeのK端EXAFSスペクトルのフーリエ変換による動径分布関数である。It is a radial distribution function by the Fourier transform of the K edge EXAFS spectrum of Fe of the sample of Comparative Example 2. 比較例２の試料を正極活物質として用いたナトリウム二次電池の充放電特性を示す。右上がりの曲線が充電（c）、右下がりの曲線が放電（d）に対応している。数字はサイクル数を示す。The charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 2 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number. 比較例３の試料の実測(+)及び計算（実線）X線回折パターンである。It is the measurement (+) and calculation (solid line) X-ray-diffraction pattern of the sample of the comparative example 3. 比較例３の試料を正極活物質として用いたナトリウム二次電池の充放電特性を示す。右上がりの曲線が充電（c）、右下がりの曲線が放電（d）に対応している。数字はサイクル数を示す。The charge / discharge characteristic of the sodium secondary battery which used the sample of the comparative example 3 as a positive electrode active material is shown. The curve that rises to the right corresponds to charge (c), and the curve that falls to the right corresponds to discharge (d). The number indicates the cycle number.
　本明細書において、「含有」は、「含む（comprise）」、「実質的にのみからなる（consist essentially of）」、及び「のみからなる（consist of）」のいずれも包含する概念である。また、本明細書において、「A～B」との表記は、「A以上且つB以下」を意味する。 In the present specification, “containing” is a concept including any of “comprise”, “consistently of”, and “consist of”. In the present specification, the notation “A to B” means “A or more and B or less”.
　１．ナトリウムイオン二次電池用正極活物質
　本発明のナトリウムイオン二次電池用正極活物質は、一般式（1）：
NaxFeO2　　　（1）
［式中、xは0.80～1.30である。］
で表され、
六方晶層状岩塩型結晶構造を有し、
格子定数が、aは3.0235Å以下、cは16.0820Å以下であり、
格子体積が127.360Å3以下である、ナトリウム鉄酸化物からなる。 1. Positive electrode active material for sodium ion secondary battery The positive electrode active material for sodium ion secondary battery of the present invention has the general formula (1):
Na x FeO 2 (1)
[Wherein x is 0.80 to 1.30. ]
Represented by
Hexagonal layered rock salt type crystal structure,
Lattice constant, a is 3.0235mm or less, c is 16.0820mm or less,
Lattice volume is 127.360A 3 or less, consisting of sodium iron oxide.
　本発明のナトリウムイオン二次電池用正極活物質が構成する六方晶層状岩塩型結晶構造は、空間群： The hexagonal layered rock-salt crystal structure of the positive electrode active material for sodium ion secondary batteries of the present invention is a space group:
Figure JPOXMLDOC01-appb-M000001
Figure JPOXMLDOC01-appb-M000001
に帰属される結晶構造（α相）である。本発明のナトリウムイオン二次電池用正極活物質の結晶構造の概略図を図１に示す。この構造において、今まで報告されてきたα相は、3a位置にNa+イオンが占有し、3b位置にFe3+イオンが占有している。本発明のナトリウムイオン二次電池用正極活物質においては、それに加えて、3b位置の上下の四面体位置に相当する6c位置に過剰のNa+イオンが占有する特異な構造を有する。したがって、Na/Fe値に相当するx値は0.80～1.30、特に0.90～1.20である。上記陽イオン分布は粉末X線リートベルト解析により、Na/Fe比は蛍光X線装置による元素分析により算出する。特に規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性等）に優れる場合には、本発明のナトリウムイオン二次電池用正極活物質は、この六方晶層状岩塩型結晶構造中の3b位置鉄占有率は0.920以上が好ましく、0.930～1.200がより好ましい。また、特に規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性等）に優れる場合には、本発明のナトリウムイオン二次電池用正極活物質は、6c位置Na占有率は0.050以下が好ましく、0.010～0.040がより好ましい。この六方晶層状岩塩型結晶構造は、特に限定的ではないが、本発明のナトリウムイオン二次電池用正極活物質全体を基準として80モル%以上が好ましく、90モル%以上がより好ましい。 The crystal structure (α phase) belonging to A schematic diagram of the crystal structure of the positive electrode active material for a sodium ion secondary battery of the present invention is shown in FIG. In this structure, the α phase that has been 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 of the 3b position. Therefore, the x value corresponding to the Na / Fe value is 0.80 to 1.30, particularly 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 apparatus. In particular, in the case where 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 the sodium ion secondary battery of the present invention is in this hexagonal layered rock salt type crystal structure. The 3b position iron occupancy is preferably 0.920 or more, more preferably 0.930 to 1.200. In addition, in the case where 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 the sodium ion secondary battery of the present invention has a sodium occupancy ratio of 6c at 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 total positive electrode active material for sodium ion secondary battery of the present invention.
　本発明のナトリウムイオン二次電池用正極活物質は、単相の六方晶層状岩塩型結晶構造、つまり、六方晶層状岩塩型結晶構造のみからなる材料であってもよいが、本発明の効果を損なわない範囲で、他の結晶構造（β-NaFeO2型結晶構造、P2型層状構造等）を、本発明のナトリウムイオン二次電池用正極活物質の20モル%以下、特に10モル%以下有していてもよい。 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. As long as it is not impaired, other crystal structures (β-NaFeO 2 type crystal structure, P2 type layered structure, etc.) have 20 mol% or less, particularly 10 mol% or less of the positive electrode active material for sodium ion secondary battery of the present invention. You may do it.
　また上記陽イオン分布の特徴に加えて、本発明のナトリウムイオン二次電池用正極活物質は、格子定数も特異な値を有している。Fe-Feイオン間距離（図１中央参照）に相当するa軸値は、規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性）等の観点から、3.0235Å以下、好ましくは3.0000～3.0230Åである。一方、層状格子の積層状態を反映するc軸値は、規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性）等の観点から、16.0820Å以下、好ましくは16.0000～16.0810Åである。さらに、格子体積は、規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性）等の観点から、127.360Å3以下、好ましくは127.000～127.300Å3である。 In addition to the characteristics of the cation distribution, the positive electrode active material for a sodium ion secondary battery of the present invention also has a unique lattice constant. The a-axis value corresponding to the distance between Fe and Fe ions (see the center of Fig. 1) is 3.0235 mm or less, preferably from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics), etc. when the regulated charge capacity is set high. 3.0000 ～ 3.02303.0. On the other hand, the c-axis value reflecting the layered state of the layered lattice is 16.0820 mm or less, preferably 16.0000 to 16.0810 mm, from the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics), etc. when the regulated charge capacity is set high. is there. Furthermore, cell volume, charge and discharge characteristics (discharge capacity, the cycle characteristics) in the case of setting a high regulatory charge capacity from the viewpoint of, 127.360A 3 or less, preferably 127.000 ~ 127.300Å 3.
　また、本発明のナトリウムイオン二次電池用正極活物質は、上記結晶構造の特徴からFeK端広域X線吸収スペクトル（EXAFS）のフーリエ変換データから算出した第一近接Fe-Fe間距離（2.6～2.8Å）のピークトップ強度が、第一近接Fe-O間距離（1.4～1.6Å）のピークトップ強度に対する比率（Fe-Feピーク高さ／Fe-Oピーク高さ）は、規制充電容量を高く設定した場合の充放電特性（放電容量、サイクル特性）等の観点から、1.05以上が好ましく、1.10～1.50がより好ましい。これは、本発明のナトリウムイオン二次電池用正極活物質が、3b位置Feイオン占有率が92%以上であることと対応しているものと思われる。 Further, the positive electrode active material for a sodium ion secondary battery of the present invention has a first adjacent Fe-Fe distance (2.6 to 2.6) calculated from the Fourier transform data of the FeK edge broad X-ray absorption spectrum (EXAFS) based on the characteristics of the crystal structure. The ratio of the peak top intensity of 2.8 mm to the peak top intensity of the distance between the first adjacent Fe-O (1.4 to 1.6 mm) (Fe-Fe peak height / Fe-O peak height) From the viewpoint of charge / discharge characteristics (discharge capacity, cycle characteristics) and the like when set to a high value, 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 sodium ion secondary batteries of the present invention has a 3b-position Fe ion occupancy of 92% or more.
　本発明のナトリウムイオン二次電池用正極活物質は、上記一般式（1）で表される組成を有するナトリウム鉄酸化物からなるものである。本発明のナトリウムイオン二次電池用正極活物質は、上記一般式（1）で表される組成を有するナトリウム鉄酸化物のみからなるものであってもよいが、不可避不純物が含まれていてもよい。このような不可避不純物としては、原料によるものが考えられ、後述のナトリウム含有材料、鉄含有材料、カリウム含有材料等が挙げられ、本発明の効果を損なわない範囲で、10モル%以下、特に5モル%以下含有していてもよい。 The positive electrode active material for a sodium ion secondary battery of the present invention is made of sodium iron oxide having a composition represented by the general formula (1). The positive electrode active material for a sodium ion secondary battery of the present invention may be composed only of sodium iron oxide having the composition represented by the general formula (1), but may contain inevitable impurities. Good. Such inevitable impurities are considered to be due to raw materials, and include the following sodium-containing materials, iron-containing materials, potassium-containing materials, etc., within a range not impairing the effects of the present invention, 10 mol% or less, particularly 5 You may contain below mol%.
　２．ナトリウムイオン二次電池用正極活物質の製造方法
　本発明のナトリウムイオン二次電池用正極活物質は、例えば、
ナトリウム含有材料及び鉄含有材料を含むアルカリ水溶液を用いて、180℃以上の温度で30時間以上水熱合成反応を行う工程
を備える製造方法により得ることができる。以下、この方法について具体的に説明する。 2. Method for producing positive electrode active material for sodium ion secondary battery The positive electrode 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 for 30 hours or more at a temperature of 180 ° C. or higher using an alkaline aqueous solution containing a sodium-containing material and an iron-containing material. Hereinafter, this method will be specifically described.
　原料の具体例としては、ナトリウム含有材料として、金属ナトリウム（Na）酸化ナトリウム（Na2O）、過酸化ナトリウム（Na2O2）；水酸化ナトリウム（NaOH）；炭酸ナトリウム（Na2CO3）、炭酸水素ナトリウム（NaHCO3）等のナトリウム炭酸塩等が例示でき、鉄含有材料として、金属鉄（Fe）；酸化鉄(II)（FeO）、酸化鉄(III)（Fe2O3）、Fe3O4等の鉄酸化物；水酸化鉄(II)（Fe(OH)2）、水酸化鉄(III)（Fe(OH)3）等の鉄水酸化物；α-FeOOH、β-FeOOH等の鉄オキシ水酸化物；炭酸鉄(II)（FeCO3）、炭酸鉄(III)（Fe2(CO3)2）等の鉄炭酸塩；硝酸鉄(II)（Fe(NO3)2）、硝酸鉄(III)（Fe(NO3)3）等の鉄硝酸塩；塩化鉄(II)（FeCl2）、塩化鉄(III)（FeCl3）等の鉄塩化物；硫酸鉄(II)（FeSO4）、硫酸鉄(III)（Fe2(SO4)3）等の鉄硫酸塩等が例示できる。本発明のナトリウムイオン二次電池用正極活物質を水熱合成により得やすい観点から、ナトリウム含有材料としては水酸化ナトリウムが好ましく、鉄含有材料としては鉄オキシ水酸化物（特にα-FeOOH）が好ましい。また、本発明のナトリウムイオン二次電池用正極活物質を水熱合成により得やすい観点から、鉄含有材料としては、3価の鉄を含む材料を用いることが好ましいが、Fe2O3、Fe3O4等の2価の鉄を含む材料を用いる場合は、特許文献２に記載されているように、塩素酸ナトリウム等の酸化剤を、例えば鉄含有材料1モルに対して1～3モル添加することもできる。さらに、鉄含有材料として、酸性の水溶性化合物（硝酸塩、塩化物、硫酸塩等）を用いる場合は、あらかじめアルカリで中和及び空気酸化（バブリング）した後、水洗して過剰の塩を取り除いた後に原料として用いることもできる。これらのナトリウム含有材料及び鉄含有材料は、それぞれ、単独で用いることもでき、2種以上を組合せて用いることもできる。 Specific examples of raw materials include sodium-containing materials such as metal sodium (Na) sodium oxide (Na 2 O), sodium peroxide (Na 2 O 2 ); sodium hydroxide (NaOH); sodium carbonate (Na 2 CO 3 ). , Sodium carbonate such as sodium hydrogen carbonate (NaHCO 3 ), etc., and iron-containing materials include metal 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 hydroxide (II) (Fe (OH) 2 ) and iron hydroxide (III) (Fe (OH) 3 ); α-FeOOH, β- Iron oxyhydroxides such as FeOOH; iron carbonates such as iron (II) carbonate (FeCO 3 ) and iron (III) (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 sulfate (III) (Fe 2 (SO 4 ) 3 ), and the like. From the viewpoint of easily obtaining the positive electrode active material for sodium ion secondary battery of the present invention by hydrothermal synthesis, the sodium-containing material is preferably sodium hydroxide, and the iron-containing material is iron oxyhydroxide (especially α-FeOOH). preferable. Further, from the viewpoint of easily obtaining the positive electrode active material for the sodium ion secondary battery of the present invention by hydrothermal synthesis, as the iron-containing material, it is preferable to use a material containing trivalent iron, Fe 2 O 3 , 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 added in an amount of 1 to 3 mol per mol of the iron-containing material, for example. 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 (bubbled) in advance, and then washed with water to remove excess salt. It can also be used as a raw material later. These sodium-containing material and iron-containing material can be used alone or in combination of two or more.
　上記した原料におけるナトリウム含有材料と鉄含有材料との含有割合は、特に制限されず、鉄含有材料に対してナトリウム含有材料を過剰量とすることが好ましい。具体的には、鉄含有材料100質量部に対して、ナトリウム含有材料を5～50質量部、特に10～30質量部使用することが好ましい。 The content ratio of the sodium-containing material and the iron-containing material in the above-described raw material is not particularly limited, and the sodium-containing material is preferably excessive 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.
　アルカリ水溶液としては、特に制限はないが、ナトリウム含有材料として水酸化ナトリウムを使用し、水酸化ナトリウム水溶液が好ましい。このアルカリ水溶液の濃度は、本発明のナトリウムイオン二次電池用正極活物質を水熱合成により得やすい観点から、高濃度とすることが好ましく、具体的には、20M以上、特に50M以上とすることが好ましい。 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 the alkaline aqueous solution is preferably a high concentration from the viewpoint of easily obtaining the positive electrode active material for a sodium ion secondary battery of the present invention by hydrothermal synthesis, specifically, 20M or more, particularly 50M or more. It is preferable.
　また、このアルカリ水溶液には、充放電特性（特にサイクル特性）をより向上させるため、カリウム含有材料を含ませることもできる。カリウム含有材料としては、特に制限はないが、アルカリ水溶液中で処理するため、水酸化カリウム、塩化カリウム等の中性又はアルカリ性塩が好ましい。カリウム含有材料の使用量は、特に制限されず、充放電特性（特にサイクル特性）の観点から、ナトリウム含有材料100質量部に対して、0.2～10.0質量部、特に0.5～5.0質量部使用することが好ましい。なお、リチウム含有材料を含む場合は、格子定数a及びc、格子体積等が大きくなり、かえってサイクル特性が悪化するため含まないことが好ましい。 Further, this alkaline aqueous solution may contain a potassium-containing material in order to further improve charge / discharge characteristics (particularly cycle characteristics). Although there is no restriction | limiting in particular as a potassium containing material, Since it processes in alkaline aqueous solution, neutral or alkaline salts, such as potassium hydroxide and potassium chloride, are preferable. The amount of potassium-containing material used is not particularly limited, and from the viewpoint of charge / discharge characteristics (especially cycle characteristics), 0.2 to 10.0 parts by weight, particularly 0.5 to 5.0 parts by weight is used with respect to 100 parts by weight of sodium-containing material. Is preferred. When a lithium-containing material is included, the lattice constants a and c, the lattice volume, etc. are increased, and the cycle characteristics are deteriorated.
　次に、このアルカリ水溶液を加熱することによって、水熱合成反応を進行させることができる。水熱合成反応は、通常の水熱反応装置（市販のオートクレーブ等）を用いて行うことができる。 Next, the hydrothermal synthesis reaction can be advanced by heating the alkaline aqueous solution. The hydrothermal synthesis reaction can be performed using a normal hydrothermal reaction apparatus (commercially available autoclave or the like).
　水熱合成反応の条件については、低温では格子定数a及びc、格子体積等が大きくなる。この結果、放電容量が低下するのみならず、サイクル特性が劇的に悪化する。一方、短時間の場合も、格子定数a及びc、格子体積等が大きくなり、放電容量が低下するのみならず、サイクル特性が劇的に悪化する。このため、水熱合成反応における温度は高くすることが必要であり、180℃以上、水熱処理炉内にかかる圧力の観点から好ましくは200～400℃である。一方、水熱合成反応の時間は30時間以上、好ましくは35～100時間である。 Regarding the conditions for the hydrothermal synthesis reaction, the lattice constants a and c, the lattice volume, and the like increase at low temperatures. As a result, not only the discharge capacity is lowered, but 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, etc. are increased, and not only the discharge capacity is lowered, but also the cycle characteristics are dramatically deteriorated. For this reason, it is necessary to increase the temperature in the hydrothermal synthesis reaction, and it is preferably 180 ° C. or higher and 200 to 400 ° C. from the viewpoint of the pressure applied in the hydrothermal treatment furnace. On the other hand, the hydrothermal synthesis reaction time is 30 hours or more, preferably 35 to 100 hours.
  上記した方法で水熱合成反応を行った後、原料等の残存物や過剰のアルカリ成分を除去するために、反応生成物を洗浄してもよい。洗浄には、Na成分の遊離によりスピネルフェライトに変化することをより抑制するため、例えば、アルコール、アセトン等の非水極性溶媒を用いることができる。また、必要に応じて、水熱処理後の生成物を大気中等各種雰囲気下で熱処理することもできる。熱処理温度に関してはβ相に変化しない上限温度として、特許文献２にあるように730℃以下が好ましい。次いで、生成物を濾過し、例えば、80℃以上（特に90～200℃）で乾燥することにより、本発明のナトリウムイオン二次電池用正極活物質を得ることができる。 後 After performing the hydrothermal synthesis reaction by the method described above, the reaction product may be washed in order to remove residues such as raw materials and excess alkali components. In order to further suppress the change to spinel ferrite due to the liberation of the Na component, for example, a nonaqueous polar solvent such as alcohol or acetone can be used for washing. Moreover, the product after hydrothermal treatment can also be heat-treated in various atmospheres, such as air | atmosphere, as needed. Regarding the heat treatment temperature, the upper limit temperature at which the β phase does not change is preferably 730 ° C. or lower as disclosed in Patent Document 2. Next, the product is filtered and dried at, for example, 80 ° C. or higher (particularly 90 to 200 ° C.), whereby the positive electrode active material for sodium ion secondary batteries of the present invention can be obtained.
　３．ナトリウムイオン二次電池用正極及びナトリウムイオン二次電池
　本発明のナトリウムイオン二次電池用正極活物質は、上記した優れた特性（放電容量及びサイクル特性）を利用して、ナトリウムイオン二次電池の正極活物質として有効に利用できる。なお、本明細書において、「ナトリウムイオン二次電池」とは、ナトリウム金属を負極に用いた金属ナトリウム二次電池も包含する概念である。特に、本発明のナトリウムイオン二次電池用正極活物質は、構造中にナトリウムを含有する材料であるため、ナトリウムを含まない負極が使えるだけでなく充電から充放電を行うことができる材料であり、しかも、放電容量及び平均電圧が高く、サイクル特性に優れることから、ナトリウムイオン二次電池用の正極活物質として有用である。本発明のナトリウムイオン二次電池用正極活物質を使用するナトリウムイオン二次電池は、電解質として非水溶媒系電解液を用いる非水電解質ナトリウムイオン二次電池であってもよく、ナトリウムイオン伝導性の固体電解質を用いる全固体型ナトリウムイオン二次電池であってもよい。 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-described excellent characteristics (discharge capacity and cycle characteristics) to provide 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 metal sodium secondary battery using sodium metal as a negative electrode. In particular, the positive electrode active material for a sodium ion secondary battery of the present invention is a material that contains sodium in the structure, and thus can be used not only for a negative electrode that does not contain sodium but also for charging and discharging from charging. Moreover, since the discharge capacity and average voltage are high and the cycle characteristics are excellent, it is useful as a positive electrode active material for sodium ion secondary batteries. The sodium ion secondary battery using the positive electrode active material for sodium ion secondary battery of the present invention may be a non-aqueous electrolyte sodium ion secondary battery using a non-aqueous solvent electrolyte as an electrolyte, and sodium ion conductive The all-solid-state sodium ion secondary battery using the solid electrolyte may be used.
　非水電解質ナトリウムイオン二次電池及び全固体型ナトリウムイオン二次電池の構造は、本発明のナトリウムイオン二次電池用正極活物質を用いること以外は、公知のナトリウムイオン二次電池と同様とすることができる。 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 a known sodium ion secondary battery except that the positive electrode active material for sodium ion secondary battery of the present invention is used. be able to.
　例えば、非水電解質ナトリウムイオン二次電池については、上記したナトリウムイオン二次電池用正極活物質を使用する他は、基本的な構造は、公知の非水電解質ナトリウムイオン二次電池と同様とすることができる。 For example, the basic structure of the non-aqueous electrolyte sodium ion secondary battery is the same as that of a known non-aqueous electrolyte sodium ion secondary battery except that the positive electrode active material for sodium ion secondary battery is used. be able to.
　正極としては、上記したナトリウムイオン二次電池用正極活物質を用い、必要に応じて導電剤及びバインダーと混合することで作製した正極合剤をアルミニウム、ニッケル、ステンレス、カーボンクロス等の正極集電体に担持させることで製造することができる。導電剤としては、例えば、黒鉛、コークス類、カーボンブラック、針状カーボン等の炭素材料を用いることができる。 As the positive electrode, the positive electrode active material for the sodium ion secondary battery described above is used, and the positive electrode mixture prepared by mixing with a conductive agent and a binder as necessary is a positive electrode current collector such as aluminum, nickel, stainless steel, carbon cloth, etc. It can be manufactured by supporting it on the body. As the conductive agent, for example, carbon materials such as graphite, cokes, carbon black, and acicular carbon can be used.
　負極としては、ナトリウムを含有する材料とナトリウムを含有しない材料共に用いることが可能である。例えば、難焼結性炭素、ナトリウム金属、スズ及びこれらを含む合金等、ナトリウムと反応する物質であれば用いることができる。これらの負極活物質についても、必要に応じて導電剤、バインダー等を用いて、アルミニウム、銅、ニッケル、ステンレス、カーボン等からなる負極集電体に担持させて、負極を製造することができる。 As the negative electrode, both a material containing sodium and a material not containing sodium can be used. For example, any substance that reacts with sodium, such as hardly sinterable 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, using a conductive agent, a binder, or the like as necessary to produce a negative electrode.
　セパレータとしては、例えば、ポリエチレン、ポリプロピレン等のポリオレフィン樹脂；フッ素樹脂；ナイロン；芳香族アラミド；無機ガラス等の材質からなり、多孔質膜、不織布、織布等の形態の材料を用いることができる。 As the separator, for example, a polyolefin resin such as polyethylene or polypropylene; a fluororesin; a nylon; an aromatic aramid; an inorganic glass or the like, and a material in the form of a porous film, a nonwoven fabric, a woven fabric, or the like can be used.
　非水電解質の溶媒としては、例えば、カーボネート、エーテル、ニトリル、含硫黄化合物等の非水溶媒系二次電池の溶媒として公知の溶媒を用いることができる。 As the solvent for the non-aqueous electrolyte, for example, a known solvent can be used as a solvent for a non-aqueous solvent secondary battery such as carbonate, ether, nitrile, and sulfur-containing compound.
　また、全固体型ナトリウムイオン二次電池についても、本発明のナトリウムイオン二次電池用正極活物質を用いる以外は、公知の全固体型ナトリウムイオン二次電池と同様の構造とすることができる。 The all-solid-state sodium ion secondary battery can have the same structure as a known all-solid-state sodium ion secondary battery except that the positive electrode active material for sodium ion secondary batteries of the present invention is used.
　この場合、電解質としては、例えば、ポリエチレンオキサイド系の高分子化合物、ポリオルガノシロキサン鎖及びポリオキシアルキレン鎖の少なくとも一種を含む高分子化合物等のポリマー系固体電解質の他、硫化物系固体電解質、酸化物系固体電解質等を用いることができる。 In this case, as the electrolyte, for example, a polymer solid electrolyte such as a polyethylene oxide polymer compound, a polymer compound containing at least one of a polyorganosiloxane chain and a polyoxyalkylene chain, a sulfide solid electrolyte, an oxidation A physical solid electrolyte or the like can be used.
　全固体型ナトリウムイオン二次電池の正極としては、例えば、本発明のナトリウムイオン二次電池用正極活物質を用い、必要に応じて導電剤、バインダー、固体電解質等を含む正極合剤をチタン、アルミニウム、ニッケル、ステンレス等の正極集電体に担持させることで製造することができる。導電剤については、非水溶媒系二次電池と同様に、例えば、黒鉛、コークス類、カーボンブラック、針状カーボン等の炭素材料を用いることができる。 As the positive electrode of the all-solid-type sodium ion secondary battery, for example, the positive electrode active material for the sodium ion secondary battery of the present invention is used. It can be produced by supporting it on a positive electrode current collector such as aluminum, nickel or stainless steel. As for the conductive agent, for example, a carbon material such as graphite, cokes, carbon black, and acicular carbon can be used as in the case of the non-aqueous solvent secondary battery.
　非水電解質ナトリウムイオン二次電池及び全固体型ナトリウムイオン二次電池の形状についても特に限定はなく、円筒型、角型等のいずれであってもよい。 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 shape, a square shape, and the like.
　以下、実施例及び比較例を示して本発明を具体的に説明するが、本発明は、これらのみに限定されないことは言うまでもない。 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 thereto.
　実施例１
　ポリテトラフルオロエチレン（PTFE）製ビーカーに水酸化ナトリウム265gと水酸化カリウム（KOH）5gを秤量し、蒸留水150mLを加えてよく攪拌した。得られた混合アルカリ水溶液に0.2mol（17.77g）のα-FeOOHを加えてよく攪拌した。これを水熱反応炉内に静置し、密閉後220℃で48時間水熱処理を行った。水熱処理後、室温付近まで反応炉を冷却後、PTFEビーカーを取り出し、1Lのエタノールでミキサー混合後、濾過することによって過剰の水酸化ナトリウムを除去し、100℃で乾燥させることによって目的とするナトリウム鉄酸化物を得た。蛍光X線分光法によるNa/Fe比（組成式のx値相当）は1.18(7)であることから、組成式はNa1.18(7)FeO2であり、本発明の組成式の範囲内であった。 Example 1
In a polytetrafluoroethylene (PTFE) beaker, 265 g of sodium hydroxide and 5 g of potassium hydroxide (KOH) were weighed, and 150 mL of distilled water was added and stirred well. 0.2 mol (17.77 g) of α-FeOOH was added to the resulting mixed alkaline aqueous solution and stirred well. This was left in a hydrothermal reactor, and after sealing, hydrothermal treatment was performed at 220 ° C. for 48 hours. After hydrothermal treatment, after cooling the reactor to near room temperature, take out the PTFE beaker, mix with 1 L ethanol in a mixer, filter to remove excess sodium hydroxide, and dry at 100 ° C to achieve the desired sodium An iron oxide was obtained. Since the Na / Fe ratio (corresponding to the x value of the composition formula) by X-ray fluorescence spectroscopy is 1.18 (7), the composition formula is Na 1.18 (7) FeO 2 and is within the range of the composition formula of the present invention. there were.
　得られた試料のX線回折パターンを図２に、リートベルト解析プログラムRIETAN-FP（F. Izumi and K. Momma, Solid State Phenom., 130, 15-20 (2007).）で得られた結晶学パラメータを表１に示す。各パラメータは本発明物質の定義値内であることが明らかである。 Fig. 2 shows the X-ray diffraction pattern of the obtained sample. The crystal obtained by the Rietveld analysis program RIETAN-FP (F. Izumi and K. Momma, Solid State Phenom., 130, 15-20 (2007).) Table 1 shows the academic parameters. It is clear that each parameter is within the defined value of the substance of the present invention.
　次に、FeのK端EXAFSスペクトルのフーリエ変換図を図３に、第一近接Fe-O及びFe-Feピーク高さとその強度比を表２に示す。測定は立命館大学SRセンターの放射光源にて実施した。得られたFe-Oピーク高さに対するFe-Feピーク高さの比（B/A）は本発明の定義値内であることが明らかである。 Next, FIG. 3 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe, and Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. The measurement was performed with a radiation source at the Ritsumeikan University SR Center. It is apparent that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is within the defined value of the present invention.
　実施例１の試料の充放電特性評価は以下のように行った。露点-50℃以下の超低湿度環境下で、得られたナトリウム鉄酸化物粉末、ケッチェンブラック、及びPTFEを質量比84: 8: 8の割合で混合してアルミニウムメッシュ上に圧着して正極合材を作製した。負極として金属ナトリウム、電解液として支持塩NaPF6を炭酸エチレン（EC）及び炭酸ジエチル（DEC）混合溶媒に溶解させたものを用いてコイン電池を作製した。作製した電池を充放電試験機にて、+30℃において、正極活物質あたりの電流密度10mA/gで充電開始且つ充電容量を100mAh/gに規制し、電位範囲1.5-4.0Vに固定して充放電試験を行った。その結果を図４及び表３に示す。図４及び表３から理解できるように、本発明のナトリウムイオン二次電池用正極活物質は、20サイクルまで高い容量を維持し、1サイクル後に対する20サイクル後の容量維持率（Q20d/Q1d）が99%以上であり、高容量のみならず高サイクル特性を示すことが明らかである。 The charge / discharge characteristic evaluation of the sample of Example 1 was performed as follows. In 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 in a mass ratio of 84: 8: 8 and pressed onto an aluminum mesh to form a positive electrode A composite was prepared. A coin battery was prepared using metal sodium as the negative electrode and a support salt NaPF 6 dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) as the electrolyte. Charge the prepared battery at + 30 ° C with a current density of 10 mA / g per positive electrode active material at + 30 ° C and regulate the charge capacity to 100 mAh / g and fix it to a potential range of 1.5-4.0 V. A charge / discharge test was conducted. The results are shown in FIG. As can be understood from FIG. 4 and Table 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 ratio after 20 cycles (Q 20d / Q It is clear that 1d ) is 99% or more and shows not only high capacity but also high cycle characteristics.
　比較例１
　水酸化ナトリウムに添加するアルカリ源をKOHからLiOH.H2Oを5gに変更した以外は実施例１と同様に試料作製を行った。蛍光X線分光法によるNa/Fe比（組成式のx値相当）は1.14(16)であることから、組成式はNa1.14(16)FeO2であり、本発明組成式の範囲内であった。得られた試料のX線回折パターンを図５に、リートベルト解析プログラムRIETAN-FPで得られた結晶学パラメータを表１に示す。各パラメータは本発明物質の定義値外であることが明らかである。 Comparative Example 1
A sample was prepared in the same manner as in Example 1 except that the alkali source 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 X-ray fluorescence 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. It was. FIG. 5 shows the X-ray diffraction pattern of the obtained sample, and Table 1 shows the crystallographic parameters obtained by the Rietveld analysis program RIETAN-FP. It is clear that each parameter is outside the defined value of the substance of the present invention.
　次に、FeのK端EXAFSスペクトルのフーリエ変換図を図６に、第一近接Fe-O及びFe-Feピーク高さとその強度比を表２に示す。得られたFe-Oピーク高さに対するFe-Feピーク高さの比（B/A）は本発明の定義値外であることが明らかである。 Next, FIG. 6 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe, and Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is clear that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is outside the defined value of the present invention.
　実施例１の試料と同様に、比較例１の試料の充放電特性評価を行った。その結果を図７及び表３に示す。図７及び表３にあるように比較例１の試料は20サイクルにおいて低い容量を示し、1サイクル後に対する20サイクル後の容量維持率（Q20d/Q1d）が57%しかなくサイクル特性に劣ることが明らかである。以上のことから、水酸化ナトリウムに添加するアルカリ源としてリチウム源を使用した場合は、かえってサイクル特性が悪化することがわかる。 In the same manner as the sample of Example 1, the charge / discharge characteristics of the sample of Comparative Example 1 were evaluated. The results are shown in FIG. As shown in FIG. 7 and Table 3, the sample of Comparative Example 1 shows a low capacity at 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) is only 57%, and the cycle characteristics are inferior. It is clear. From the above, it can be seen that when a lithium source is used as an alkali source added to sodium hydroxide, the cycle characteristics deteriorate.
　実施例２
　水酸化ナトリウム量を270gとし、KOHを加えない以外は実施例１と同様に試料作製を実施した。蛍光X線分光法によるNa/Fe比（組成式のx値相当）は0.915(14)であることから、組成式はNa0.915(14)FeO2であり、本発明の組成式の範囲内であった。得られた試料のX線回折パターンを図８に、リートベルト解析プログラムRIETAN-FPで得られた結晶学パラメータを表１に示す。各パラメータは本発明物質の定義値内であることが明らかである。 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 X-ray fluorescence spectroscopy is 0.915 (14), the composition formula is Na 0.915 (14) FeO 2 and 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.
　次にFeのK端EXAFSスペクトルのフーリエ変換図を図９に、第一近接Fe-O及びFe-Feピーク高さとその強度比を表２に示す。得られたFe-Oピーク高さに対するFe-Feピーク高さの比（B/A）は本発明の定義値内であることが明らかである。 Next, FIG. 9 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe, and Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is apparent that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is within the defined value of the present invention.
　実施例１の試料と同様に、実施例２の試料の充放電特性評価を行った。その結果を図１０及び表３に示す。図１０及び表３に記載されているように、実施例２の試料は20サイクルにおいて高い容量を示し、1サイクル後に対する20サイクル後の容量維持率（Q20d/Q1d）が76%であり、サイクル特性に優れることが明らかである。以上のことから特許文献２及び３で達成できなかった充放電特性に優れたナトリウム鉄酸化物が得られることが明らかである。 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. As shown in FIG. 10 and Table 3, the sample of Example 2 shows a high capacity in 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) is 76% after 1 cycle. It is clear that the cycle characteristics are excellent. From the above, it is clear that a sodium iron oxide excellent in charge / discharge characteristics that could not be achieved in Patent Documents 2 and 3 can be obtained.
　比較例２
　水熱処理条件を150℃、48時間とした以外は実施例２と同様に試料作製を行った。蛍光X線分光法によるNa/Fe比（組成式のx値相当）は1.00(7)であることから、組成式はNa1.00(7)FeO2であり、本発明組成式の範囲内であった。得られた試料のX線回折パターンを図１１に、リートベルト解析プログラムRIETAN-FPで得られた結晶学パラメータを表１に示す。各パラメータは本発明物質の定義値外であることが明らかである。 Comparative Example 2
A sample was prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were 150 ° C. and 48 hours. Since the Na / Fe ratio (corresponding to the x value in the composition formula) by X-ray fluorescence 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. It was. 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 defined value of the substance of the present invention.
　次に、FeのK端EXAFSスペクトルのフーリエ変換図を図１２に、第一近接Fe-O及びFe-Feピーク高さとその強度比を表２に示す。得られたFe-Oピーク高さに対するFe-Feピーク高さの比（B/A）は本発明の定義値外であることが明らかである。 Next, FIG. 12 shows the Fourier transform diagram of the K-edge EXAFS spectrum of Fe, and Table 2 shows the first adjacent Fe—O and Fe—Fe peak heights and their intensity ratios. It is clear that the ratio of the Fe—Fe peak height to the obtained Fe—O peak height (B / A) is outside the defined value of the present invention.
　実施例１の試料と同様に、比較例２の試料の充放電特性評価を行った。その結果を図１３及び表３に示す。図１３及び表３に記載されているように比較例２の試料は20サイクルにおいて低い容量を示し、1サイクル後に対する20サイクル後の容量維持率（Q20d/Q1d）が32%しかなく、サイクル特性に劣ることが明らかである。以上のことから水熱処理条件を150℃、48時間に変更することにより目的のナトリウム鉄酸化物が得られないことが明らかである。 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. As shown in FIG. 13 and Table 3, the sample of Comparative Example 2 showed a low capacity in 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) was only 32% 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.
　比較例３
　水熱処理条件を特許文献２と同様の220℃、20時間とした以外は実施例２と同様に試料作製を行った。蛍光X線分光法によるNa/Fe比（組成式のx値相当）は1.332(7) であることから、組成式はNa1.332(7)FeO2であり、本発明組成式の範囲外であった。得られた試料のX線回折パターンを図１４に、リートベルト解析プログラムRIETAN-FPで得られた結晶学パラメータを表１に示す。各パラメータは本発明物質の定義値外であることが明らかである。 Comparative Example 3
A sample was prepared in the same manner as in Example 2 except that the hydrothermal treatment conditions were set to 220 ° C. for 20 hours as in Patent Document 2. Since the Na / Fe ratio (corresponding to the x value in the composition formula) by X-ray fluorescence 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. It was. 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 defined value of the substance of the present invention.
　実施例１の試料と同様に、比較例３の試料の充放電特性評価を行った。その結果を図１５及び表３に示す。図１５及び表３に記載されているように、比較例３の試料は20サイクルにおいて低い容量を示し、1サイクル後に対する20サイクル後の容量維持率（Q20d/Q1d）が28%しかなく、サイクル特性に劣ることが明らかである。以上のことから水熱処理条件を220℃、20時間に変更することにより目的のナトリウム鉄酸化物が得られないことが明らかである。 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. As shown in FIG. 15 and Table 3, the sample of Comparative Example 3 shows a low capacity at 20 cycles, and the capacity retention rate after 20 cycles (Q 20d / Q 1d ) is only 28% 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 condition to 220 ° C. for 20 hours.
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004

Claims (10)

  1. 一般式（1）：
    NaxFeO2　　　（1）
    ［式中、xは0.80～1.30である。］
    で表され、
    六方晶層状岩塩型結晶構造を有し、
    格子定数が、aは3.0235Å以下、cは16.0820Å以下であり、
    格子体積が127.360Å3以下である、
    ナトリウム鉄酸化物からなるナトリウムイオン二次電池用正極活物質。     General formula (1):
    Na x FeO 2 (1)
    [Wherein x is 0.80 to 1.30. ]
    Represented by
    Hexagonal layered rock salt type crystal structure,
    Lattice constant, a is 3.0235mm or less, c is 16.0820mm or less,
    Lattice volume is 127.360A 3 or less,
    A positive electrode active material for sodium ion secondary batteries comprising sodium iron oxide.
  2. 前記ナトリウム鉄酸化物が有する六方晶層状岩塩型結晶構造中の3b位置鉄占有率が0.920以上である、請求項１に記載のナトリウムイオン二次電池用正極活物質。 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.
  3. 前記ナトリウム鉄酸化物が有する六方晶層状岩塩型結晶構造中の6c位置ナトリウム占有率が0.050以下である、請求項１又は２に記載のナトリウムイオン二次電池用正極活物質。 The positive electrode active material for sodium ion secondary batteries according to claim 1 or 2, wherein a sodium occupancy at the 6c position in the hexagonal layered rock salt crystal structure of the sodium iron oxide is 0.050 or less.
  4. 前記ナトリウム鉄酸化物のFeK端広域X線吸収（EXAFS）スペクトルのフーリエ変換スペクトルにおいて、Fe-O高さに対するFe-Fe高さの比（Fe-Fe高さ／Fe-O高さ）が1.050以上である、請求項１～３のいずれか1項に記載のナトリウムイオン二次電池用正極活物質。 In the Fourier transform spectrum of the FeK edge broad X-ray absorption (EXAFS) spectrum of the sodium iron oxide, the ratio of the Fe-Fe height to the 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, which is as described above.
  5. 請求項１～４のいずれか1項に記載のナトリウム二次電池用正極材料の製造方法であって、
    ナトリウム含有材料及び鉄含有材料を含み、リチウム含有材料を含まないアルカリ水溶液を用いて、180℃以上の温度で30時間以上水熱合成反応を行う工程
    を備える、製造方法。     A 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 more using an alkaline aqueous solution containing a sodium-containing material and an iron-containing material and not containing a lithium-containing material.
  6. 前記ナトリウム含有材料が水酸化ナトリウムである、請求項５に記載の製造方法。 The manufacturing method according to claim 5, wherein the sodium-containing material is sodium hydroxide.
  7. 前記アルカリ水溶液が、さらに、カリウム含有材料を含む、請求項５又は６に記載の製造方法。 The manufacturing method according to claim 5 or 6, wherein the alkaline aqueous solution further contains a potassium-containing material.
  8. 前記カリウム含有材料が、カリウムを含む中性又はアルカリ性塩である、請求項７に記載の製造方法。 The production method according to claim 7, wherein the potassium-containing material is a neutral or alkaline salt containing potassium.
  9. 請求項１～４のいずれか1項に記載のナトリウムイオン二次電池用正極活物質を含有する、ナトリウムイオン二次電池用正極。 A positive electrode for a sodium ion secondary battery, comprising the positive electrode active material for a sodium ion secondary battery according to any one of claims 1 to 4.
  10. 請求項９に記載のナトリウムイオン二次電池用正極を備えるナトリウムイオン二次電池。 A sodium ion secondary battery comprising the positive electrode for a sodium ion secondary battery according to claim 9.
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JP2014086279A (en) * 2012-10-24 2014-05-12 National Institute Of Advanced Industrial & Technology Positive electrode active material for sodium ion secondary battery
CN106684369A (en) * 2017-02-16 2017-05-17 长沙理工大学 Sodium ion battery anode material embedded and coated with sodium fast ion conductor and synthetic method thereof

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JP2014086279A (en) * 2012-10-24 2014-05-12 National Institute Of Advanced Industrial & Technology Positive electrode active material for sodium ion secondary battery
CN106684369A (en) * 2017-02-16 2017-05-17 长沙理工大学 Sodium ion battery anode material embedded and coated with sodium fast ion conductor and synthetic method thereof

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