JP6002110B2 - Sodium secondary battery - Google Patents

Description

The present invention relates to a sodium secondary battery. In particular, the present invention relates to a sodium secondary battery having excellent cycle characteristics using CuFeO ₂ as a negative electrode.

  Sodium secondary batteries using sodium ion insertion and desorption reactions are expected to be cost effective secondary batteries because of the superiority of sodium resources over the widely used lithium secondary batteries. Research and development on electrode materials and electrolyte materials is underway.

  Matsuura et al. In Non-Patent Document 1, in a sodium secondary battery using hard carbon as a negative electrode in an organic electrolyte, when charging / discharging at a current density of 25 mA / g, stable at about 250 mAh / g over 30 cycles. Reported to show reversible capacity. Further, the same document describes that when tin is used as the negative electrode, the initial reversible capacity is as high as 550 mAh / g, but the capacity decreases to about 20% after 5 cycles.

Matsuura et al., 13th Annual Meeting of Chemical Battery Materials Meeting, 2-23 (2011) 97

  As described above, the sodium secondary battery has a problem that the discharge capacity is lower than that of the lithium secondary battery, or a problem that sufficient cycle characteristics cannot be obtained.

  Accordingly, an object of the present invention is to provide a sodium secondary battery having excellent cycle characteristics.

The present invention relates to a sodium secondary battery, which is a sodium secondary battery including a positive electrode, a negative electrode, and an electrolyte having sodium ion conductivity, including a substance capable of inserting and removing sodium ions. The negative electrode contains CuFeO ₂ .

In one embodiment of the present invention, the negative electrode preferably further includes carbon particles. In the present invention, when carbon particles are included in the negative electrode, the negative electrode material is preferably a material that has been subjected to a ball mill treatment in which the CuFeO ₂ and the carbon particles are pulverized and mixed.

  In one embodiment of the present invention, the electrolyte is an organic electrolytic solution or aqueous electrolytic solution containing sodium ions.

  In one embodiment of the present invention, the electrolyte is a solid electrolyte or a polymer electrolyte that allows sodium ions to pass therethrough.

  ADVANTAGE OF THE INVENTION According to this invention, the sodium secondary battery excellent in cycling characteristics can be provided.

It is the schematic which shows the structure of the sodium secondary battery of this invention. Is a graph showing measurement results of X-ray diffraction of CuFeO ₂ powder prepared in Experimental Example. It is the schematic which shows the structure of the sodium secondary battery of one Embodiment of this invention. It is a figure which shows the charging / discharging curve of the sodium secondary battery of embodiment of this invention shown in FIG.

The present invention relates to a sodium secondary battery, particularly one in which the negative electrode is CuFeO ₂ .

  Below, the embodiment of the sodium secondary battery of the present invention is described.

The sodium secondary battery of the present invention includes at least a positive electrode, a negative electrode, and an electrolyte. The positive electrode contains a substance capable of inserting and removing sodium ions, the negative electrode contains CuFeO ₂ , and the electrolyte has sodium ion conductivity.

  The components of the sodium secondary battery of the present invention will be described below.

(1) Negative in the present invention, the negative electrode, as described above, is intended to include CuFeO _2. In this specification, CuFeO ₂ is also referred to as a negative electrode active material.

First, a method for synthesizing CuFeO ₂ used in the sodium secondary battery of the present invention will be described. CuFeO ₂ which is a negative electrode active material can be prepared by both a wet method and a solid phase method. In the present invention, a wet method capable of synthesis with a particle size of 10 to 1000 nm is more preferable.

Hereinafter, a method for synthesizing CuFeO ₂ by a wet method will be specifically described.

Copper salts such as copper nitrate and iron nitrate and iron salts are weighed in an equivalent molar amount, and dissolved in a solvent such as malic acid aqueous solution or ethyl glycol. Then, pH adjustment is performed with ammonia water or the like. The obtained solution is heated to about 100 to 300 ° C. in an evaporating dish to volatilize the solvent to obtain a CuFeO ₂ precursor powder. This precursor powder is pulverized in an agate mortar and fired at 500 to 900 ° C. for 3 to 90 hours in a reducing atmosphere such as an argon atmosphere or a nitrogen atmosphere to obtain a CuFeO ₂ powder.

Next, a specific method for synthesizing CuFeO ₂ by a solid phase method will be described.

Copper oxides such as copper oxide Cu ₂ O and iron oxide Fe ₂ O ₃ and iron oxides are weighed in equivalent molar amounts and mixed in an agate mortar. The mixed powder is formed into a pellet shape having a predetermined shape (for example, 13φ × 1 cm) by a press machine, and is fired at 500 to 1000 ° C. for 50 to 180 hours in a reducing atmosphere such as an argon atmosphere or a nitrogen atmosphere, and CuFeO ₂ A powder can be obtained.

In the present invention, CuFeO ₂ is a negative electrode active material, preferably has a particle size of 10 to 1000 nm. This is because, if the particle size is small, the specific surface area of the active material is increased and a high capacity is realized.

In the present specification, the particle size of CuFeO ₂ refers to a measure as determined by scanning electron microscopy (SEM). Specifically, by SEM, 10 points of view of 19 × 25 μm in which at least 100 CuFeO ₂ particles exist are arbitrarily observed, and 90% or more of the observed particle size is 10 to 1000 nm. It belongs.

The negative electrode of the sodium secondary battery of the present invention preferably contains a mixture of the above CuFeO ₂ and a carbon material such as carbon powder.

  The negative electrode containing such a carbon material can be prepared by, for example, the following means, but the present invention is not limited to these.

First, carbon powder (for example, carbon such as acetylene black, ketjen black powder), binder (binder) powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyacrylic acid (PAA), Further, the negative electrode can be prepared by mixing CuFeO ₂ , then rolling with a rolling machine such as a roll press machine, cutting out to a predetermined size and forming into a pellet. In addition, carbon powder, a binder, etc. are available as a commercial reagent, for example.

In the present invention, in order to improve the uniformity and dispersibility of the carbon powder in the negative electrode material and CuFeO ₂ and improve the performance of the secondary battery, the carbon powder and CuFeO ₂ are mixed when the negative electrode is manufactured. Alternatively, the mixture may be pulverized and mixed with a pulverizer such as a ball mill, and the resulting ball mill (BM) treatment mixture may be further mixed with a binder powder and then rolled to form a negative electrode as described above.

Alternatively, a mixture of the above-described carbon powder, binder powder and CuFeO ₂ is dispersed in a solvent such as an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) to form a slurry. The negative electrode can also be prepared by applying on a metal foil such as an aluminum foil and drying.

In the present invention, in order to improve the conductivity of the negative electrode containing CuFeO ₂ , it is preferable to perform a ball mill treatment in which CuFeO ₂ as a negative electrode material is mixed with carbon particles as a conductive material, and pulverized and mixed with a ball mill. By such a ball mill treatment, more excellent battery characteristics can be obtained. The time for the ball mill treatment is less than 9 hours, preferably 1 to 7 hours, and more preferably 1 to 5 hours.

In the present invention, the content of CuFeO _{2 in} the cathode is preferably 60 to 90% by weight based on the total amount of CuFeO ₂ , the conductive material and the binder.

(2) Positive electrode The positive electrode is not particularly limited as long as it contains a substance that can be used in a sodium secondary battery and can insert and desorb sodium ions. For example, compounds capable of inserting and removing sodium ions include compounds such as sodium complex oxides such as metallic sodium, NaCrO ₂ , NaFeO ₂ and NaNi _1/2 Mn _1/2 O ₂ . The positive electrode is preferably made by mixing such a substance capable of inserting and desorbing sodium ions with a conductive material such as carbon powder.

  The above-described positive electrode can be prepared, for example, by the following means when using a substance capable of inserting and desorbing sodium ions, but the present invention is not limited thereto.

  First, binder powder such as carbon powder (for example, carbon blacks such as acetylene black powder and ketjen black powder), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polyacrylic acid (PAA). And a substance capable of inserting and desorbing sodium ions such as sodium complex oxide (also referred to as a positive electrode active material in the present specification) is mixed, and then rolled by a rolling machine such as a roll press machine to obtain a predetermined size. The positive electrode can be produced by cutting out into a pellet shape.

  Alternatively, a mixture of the above-described carbon powder, binder powder and positive electrode active material is dispersed in a solvent such as an organic solvent (for example, NMP) to form a slurry, and then applied onto a metal foil such as an aluminum foil. The positive electrode can also be produced by drying.

The material contained in the above-described positive electrode is available as a commercial product or can be appropriately prepared by synthesis. For example, a sodium composite oxide such as NaCrO ₂ can be obtained by mixing commercially available reagents sodium carbonate Na ₂ CO ₃ and chromium oxide Cr ₂ O ₃ in a predetermined ratio and baking them in an inert atmosphere. it can. Moreover, carbon powder, a binder, etc. are available as a commercial reagent, for example.

(3) Electrolyte The sodium secondary battery of the present invention including the above negative electrode can use an organic electrolytic solution or aqueous electrolytic solution containing sodium ions as an electrolyte. Furthermore, the sodium secondary battery including the negative electrode can use a solid electrolyte or a polymer electrolyte that allows sodium ions to pass therethrough as an electrolyte.

For the electrolyte, metal salts containing sodium ions such as sodium bistrifluoromethanesulfonylimide (NaTFSI), sodium perchlorate (NaClO ₄ ), sodium hexafluorophosphate (NaPF ₆ ), such as ethylene carbonate (EC) and Mention organic solvent dissolved in mixed solvent of dimethyl carbonate (DMC) (volume ratio 1: 1), mixed solvent such as EC and diethyl carbonate (DEC), or single solvent such as propylene carbonate (PC) (But not limited to). Further, examples of the electrolyte include an aqueous solution (aqueous electrolytic solution) in which a metal salt containing sodium ions such as an aqueous NaOH solution, an aqueous Na ₂ SO ₄ solution, and an aqueous NaClO ₄ solution is dissolved in water (but is not limited thereto). .

As another electrolyte of the sodium secondary battery of the present invention, a solid electrolyte having sodium ion conductivity [for example, a glassy substance such as 75Na ₂ S · 25P ₂ S ₅ , NASICON such as Na ₃ Zr ₂ Si ₂ PO ₁₂ ( Na ⁺ Super Ionic Conductor)], a polymer electrolyte having sodium ion conductivity (for example, a material obtained by compositing the above-mentioned organic electrolyte and polyethylene oxide (PEO)), and the like, but is not limited thereto. In the present invention, any known solid electrolyte passing through sodium ions or polymer electrolyte passing through sodium ions used in sodium secondary batteries can be used.

  The sodium secondary battery of the present invention can also include other elements such as a structural material such as a separator and a battery case. Also for these elements, various materials used in conventionally known secondary batteries can be used, and there is no particular limitation.

(4) Configuration of sodium secondary battery A battery using the positive electrode, the negative electrode, the electrolytic solution and the like as described above can be manufactured in a conventional shape such as a coin shape, a cylindrical shape, or a laminate shape. And the manufacturing method of these secondary batteries can also use the method similar to the past.

  For example, the sodium secondary battery of the present invention includes a positive electrode and a negative electrode as shown in FIG. 1 and an electrolyte in contact with both electrodes. In the present invention, a separator may be included between the positive electrode and the negative electrode. When an organic electrolyte is used as the electrolyte, for example, the separator can be used by impregnating the electrolyte. Further, the organic electrolyte may be impregnated in a polymer electrolyte or the like. Moreover, what is necessary is just to arrange | position so that both poles may contact | connect these, when using a solid electrolyte, a polymer electrolyte, etc.

Furthermore, although not clearly shown in FIG. 1, the sodium secondary battery of the present invention may include a battery case that covers a positive electrode, a negative electrode, an electrolyte, a separator, and the like. In the present invention, it is particularly preferable to use CuFeO ₂ as a negative electrode material.

  As a more specific embodiment, the present invention can be applied using a coin cell type secondary battery. As shown in FIG. 3, the coin cell type secondary battery includes a positive electrode 1 and a negative electrode 3, and further includes a separator 2 containing an electrolytic solution between these electrodes. Further, the secondary battery structure may include a positive electrode case 4, a gasket 5, and a negative electrode case 6. In this secondary battery, for example, the positive electrode 1, the negative electrode 3, and the separator 2 containing the electrolytic solution are arranged as desired in the positive electrode case 4 and the negative electrode case 6, and both cases where the respective components are arranged are arranged. It can be prepared by fixing.

  In the present invention, a solid electrolyte, a polymer electrolyte, or the like as described above can be used instead of or in addition to the separator.

[Experimental example]
Hereinafter, experimental examples of the sodium secondary battery of the present invention will be described in detail with reference to the drawings. In addition, this invention is not limited to what was shown to the following experiment example, In the range which does not change the meaning and scope of this invention, it can change suitably and can implement.

(Experimental example 1)
A metal sodium sheet was used for the counter electrode, and the performance of the negative electrode alone was evaluated.

(I) Production of test cell for performance evaluation A test cell for performance evaluation was produced by the following procedure.

  Since the performance evaluation cell is different from a normal battery, the positive electrode and the negative electrode are not indicated, but the working electrode and the counter electrode are indicated.

(A) CuFeO shows ₂ Synthesis of synthesis CuFeO ₂ are shown below.

_{Cu (NO 3) 2 · 3H} 2 O ( Kanto Chemical, FW 241.60, Purity min. 99.9 %) and _{8.0614g, Fe (NO 3) 3} · 9H 2 O ( Kanto Chemical, FW 404.00 , Purity min. 99.0%), weighed 13.6027 g, malic acid aqueous solution (distilled water: about 300 ml, malic acid (Kanto Chemical, FW 134.09, Purity min. 99.0%): 13.35444 g) And dissolved with stirring at room temperature. Thereafter, aqueous ammonia (Kanto Chemical) was added and the pH was adjusted to pH = 2.5. The solution was heated to about 250 to 300 ° C. while stirring using a hot stirrer in a fume hood in an evaporating dish to volatilize the solvent, thereby obtaining an amorphous precursor powder of CuFeO ₂ . The obtained precursor powder was pulverized in an agate mortar for 15 minutes and fired at 750 ° C. for 10 hours in an argon atmosphere to obtain CuFeO ₂ powder. FIG. 2 shows the result of X-ray diffraction measurement of the obtained powder. As a result of the X-ray diffraction measurement, it was found that the X-ray diffraction measurement was in good agreement with PDF (Powder Diffraction File: a database of compound diffraction patterns by powder X-ray diffraction) 01-075-2146. Moreover, the CuFeO ₂ produced in this way was confirmed by SEM observation to have a particle size of about 500 nm, which was consistent with the range of 10 to 1000 nm of the present invention.

CuFeO ₂ powder, acetylene black powder (manufactured by Denki Kagaku Kogyo Co., Ltd.) and polytetrafluoroethylene (PTFE) powder (manufactured by Daikin Co., Ltd.) in a weight ratio of 70: 25: 5 are sufficiently pulverized and mixed using a rough machine. Then, roll forming was performed to produce a sheet pellet-shaped electrode (thickness: 0.2 mm). This sheet-like electrode was cut out into a circle having a diameter of 15 mm. The counter electrode was prepared by pressing a sodium reagent (manufactured by Kanto Chemical Co., Ltd.), which is a commercially available reagent, to a thickness of 0.8 mm and molding it into a circular sheet having a diameter of 15 mm. The electrolyte was sodium bistrifluoromethane at a concentration of 1 mol / L in a mixed solvent prepared by mixing ethylene carbonate (EC) (manufactured by Kishida Chemical) and dimethyl carbonate (DMC) (manufactured by Kishida Chemical) at a volume ratio of 1: 1. It was prepared by dissolving methanesulfonylimide (NaTFSI) (manufactured by Kishida Chemical). The separator was made of polypropylene (made by Celgard) for lithium secondary batteries.

  The test cell for performance evaluation was manufactured as a 2032 coin type as shown in FIG. For the working electrode, the above-described pellet electrode was set in the working electrode case 4, covered with a titanium mesh (manufactured by Niraco) (not shown), and the peripheral edge thereof was fixed by spot welding. The counter electrode was fixed by fixing a titanium mesh (manufactured by Niraco) (not shown) to the counter electrode case 6 by spot welding and crimping a sodium sheet thereon. Next, the separator 2 is set in the working electrode case to which the pellet is fixed, and the electrolytic solution is injected into the separator 2, and the counter electrode case to which the sodium sheet is fixed is covered, and the working electrode case 4 and the counter electrode case are covered with a coin cell caulking machine. By crimping 6, a coin cell including a polypropylene gasket 5 was produced. The performance evaluation test cell was produced in a glove box in an argon atmosphere with a dew point of −85 ° C. or less.

(Ii) Charge / Discharge Test The charge / discharge test of the test cell for performance evaluation is performed by using a commercially available charge / discharge measurement system (manufactured by Hokuto Denko Co., Ltd.) at a current density per effective area of the working electrode of 0.5 mA / cm ² . It energized and performed in the voltage range of charge final voltage 2.7V and discharge final voltage 0.0V. The measurement of the charge / discharge test of the sodium secondary battery of this experimental example was performed in a thermostatic chamber at 25 ° C. (atmosphere is in a normal atmospheric environment).

A charge / discharge curve of the test cell for performance evaluation produced in this experimental example is shown in FIG. From the figure, CuFeO ₂ can be charged and discharged, and the initial reversible capacity is 322 mAh / g (standardized per CuFeO ₂ powder weight), when Na is desorbed (corresponding to discharge when used as the negative electrode of Na secondary battery) The average potential of was 1.3V. Thus, CuFeO ₂ is applicable as a negative electrode because of its low potential with respect to the metal Na counter electrode. Table 1 shows capacity retention rates at the 30th and 50th cycles. From the table, it can be seen that only about 0.5% capacity reduction per cycle is observed, and that stable cycle characteristics are obtained.

As described above, it was found from this experimental example that CuFeO ₂ has a low potential and can be applied as a negative electrode, and has excellent cycle characteristics.

(Experimental Examples 2-6)
In order to verify the effect of the ball mill (BM) using a metallic sodium sheet as the counter electrode, the performance of the negative electrode alone was evaluated.

An attempt was made to improve battery performance by pulverizing and mixing CuFeO ₂ powder and acetylene black powder with BM (ball mill treatment).

CuFeO ₂ powder and acetylene black powder (weight ratio 70:25) were mixed in a mixer for about several minutes. To this mixture, a zirconia ball having a diameter of 7 mm was added, 1 hour (Experimental Example 2), 3 hours (Experimental Example 3), 5 hours (Experimental Example 4), 7 hours (Experimental Example 5), and 9 hours (Experimental Example). The BM process of 6) was performed. In any of the BM treatments, the mixing ratio of the CuFeO ₂ acetylene black mixture and the balls was 1:10 by weight, and the rotation speed during the BM treatment was 300 rpm.

The resulting BM-treated CuFeO ₂ acetylene black mixture was further mixed with a PTFE binder (binder) and mixed with a coarse machine to produce working electrode pellets in the same manner as in Experimental Example 1. Using this pellet, a coin cell was produced in the same manner as in Experimental Example 1. The charge / discharge test was also performed in the same manner as in Experimental Example 1.

  Table 2 shows the results of the charge / discharge test. In Table 2, the results of Experimental Example 1 are also shown for reference.

The characteristics improved remarkably up to BM treatment time up to 5 h. In addition, in the BM treatment for 5 hours, the capacity retention rate after 50 cycles also achieved a high value of 98%. This is presumably because the contact property between the CuFeO ₂ powder and the acetylene black powder was improved by the BM treatment, and the interfacial resistance between the powders was reduced. On the other hand, in the BM treatment of 9h (Experimental Example 6), the battery performance such as capacity decreased as compared with Experimental Example 4 (however, as a sodium secondary battery, it had characteristics similar to Experimental Example 1). And has sufficient characteristics). This is considered to be due to the modification of CuFeO ₂ due to local heat generation during the BM treatment. Thus, in the present invention, it was revealed that the battery performance is improved by BM treatment of CuFeO ₂ .

(Experimental example 7)
Used CuFeO ₂ to prepare a negative electrode of (i) a sodium secondary battery using the NaCrO ₂ in the positive electrode, to produce a sodium secondary battery.

As the negative electrode, a CuFeO ₂ / acetylene black mixture prepared under the conditions of Experimental Example 4 was used.

For the positive electrode, sodium chromate NaCrO ₂ was synthesized and used as the positive electrode active material. Sodium chromate NaCrO ₂ was synthesized by the following procedure. By mixing 10.6 g of commercially available reagent sodium carbonate Na ₂ CO ₃ (manufactured by Kanto Chemical Co., Ltd.) and 15.2 g of chromium oxide Cr ₂ O ₃ (manufactured by Kanto Chemical Co., Ltd.), and firing it at 1000 ° C. for 6 hours in an argon atmosphere. Sodium chromate NaCrO ₂ was obtained.

NaCrO ₂ , acetylene black powder (manufactured by Denki Kagaku Kogyo Co., Ltd.) and polytetrafluoroethylene (PTFE) powder (manufactured by Daikin Co., Ltd.) in a weight ratio of 70: 25: 5 are sufficiently pulverized and mixed using a rough machine. Then, roll forming was performed to produce a sheet pellet-shaped electrode (thickness: 0.5 mm). This sheet-like electrode was cut out into a circle having a diameter of 15 mm. The separator was made of polypropylene (made by Celgard) for lithium secondary batteries.

  As the sodium secondary battery, a 2032 coin type battery as shown in FIG. 3 was manufactured in the same manner as the performance evaluation test cell. For the positive electrode, the above-mentioned pellet electrode was set in the positive electrode case 4, covered with a titanium mesh (manufactured by Niraco) (not shown), and the peripheral edge thereof was fixed by spot welding. The negative electrode was fixed by fixing a titanium mesh (manufactured by Niraco) (not shown) to the negative electrode case 6 by spot welding and crimping a sodium sheet thereon. Next, the separator 2 is set in the positive electrode case to which the pellet is fixed, and an electrolyte solution (the same as in Experimental Example 1) is injected into the separator 2, and the negative electrode case to which the sodium sheet is fixed is covered, and a coin cell caulking machine is used. By crimping the positive electrode case 4 and the negative electrode case 6, a coin cell including a polypropylene gasket 5 was produced. The sodium secondary battery was produced in an argon atmosphere glove box having a dew point of −85 ° C. or lower.

(Ii) Charge / Discharge Test The charge / discharge test of the sodium secondary battery was conducted by applying 0.5 mA / cm ² at a current density per effective area of the positive electrode using a commercially available charge / discharge measurement system (made by Hokuto Denko). The charge / discharge test was conducted in a voltage range of a charge end voltage of 3.0 V and a discharge end voltage of 1.2 V. The battery charge / discharge test was performed in a thermostatic chamber at 25 ° C. (atmosphere was in a normal atmospheric environment).

  Table 3 shows the average discharge voltage and reversible capacity of the first cycle, and the discharge capacity retention ratio of the 30th and 50th cycles. The sodium secondary battery according to the present invention can be charged and discharged, and showed a reversible capacity of 390 mAh / g (normalized per negative electrode active material weight) and an average discharge voltage of 2.1 V in the first cycle. Table 3 also shows the discharge capacity retention rates at the 30th and 50th cycles. From the table, it can be seen that only about 0.2% capacity reduction per cycle is seen, and that stable cycle characteristics are obtained.

  As described above, it was found that the sodium secondary battery according to this experimental example has excellent cycle characteristics and can be charged and discharged.

(Experimental example 8)
Experimental Example 8 is an example in which a solid electrolyte is used in the sodium secondary battery of the present invention.

NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) as the solid electrolyte, NaCrO ₂ used in Experimental Example 1 as the positive electrode material, and CuFeO ₂ acetylene black mixture prepared under the conditions of Experimental Example 4 as the negative electrode, Each was used.

The NASICON disk was prepared by the following procedure. First, ZrO (NO ₃ ) ₂ · 8H ₂ O (Kanto Chemical Co., Inc.), NH ₄ H ₂ PO ₄ (Kanto Chemical Co., Ltd.), and Na ₂ SiO ₃ · 9H ₂ O (Kanto Chemical Co., Ltd.) : Zr: Si: P = 3: 2: 2: 1 were mixed and pre-baked at 850 ° C. The obtained powder was formed into a disk shape by a pellet molding machine and subjected to main firing at 1100 ° C. for 24 hours.

  A coin cell was fabricated in the same manner as in Experimental Example 1. The solid electrolyte was manufactured in a disk (thickness: about 1 mm) shape so as to be accommodated in the coin cell, and set in the illustrated separator portion. Also, a disc-shaped spacer made of the same material as the coin cell of an arbitrary thickness was welded to the coin cell case so that there was no gap at each interface of the positive electrode / solid electrolyte / negative electrode.

In the charge / discharge test of the battery, in the same manner as in Experimental Example 1, a charge / discharge measurement system was used, and a current density per effective area of the positive electrode of 0.3 mA / cm ² was applied, and the end-of-charge voltage was 2.9 V. The charge / discharge test was conducted in a voltage range of a final voltage of 1.1V.

  Table 4 shows the results of the charge / discharge test. Table 4 also shows the results of Experimental Example 1 for reference. Compared with Experimental Example 1, since the contact resistance at the positive electrode / electrolyte and negative electrode / electrolyte interfaces increases, the voltage decreases by 0.2 V, and the discharge capacity also decreases slightly. However, no significant change was observed in the cycle stability indicated by the discharge capacity. From this experimental example, it was confirmed that the sodium secondary battery can operate according to the present invention including the solid electrolyte.

(Experimental example 9)
Experimental Example 9 is an example in which a polymer electrolyte is used in the sodium secondary battery of the present invention.

A PEO polymer electrolyte membrane was used as the electrolyte, NaCrO ₂ used in Experimental Example 1 was used as the positive electrode material, and a CuFeO ₂ acetylene black mixture prepared under the conditions of Experimental Example 4 was used as the negative electrode.

The PEO polymer electrolyte membrane (thickness: about 2 mm) was prepared as follows. PEO (Aldrich, Mw = 6 × 10 ⁵ ) and NaTFSI (Kishida Chemical) as a solute were added to acetonitrile solvent (Kishida Chemical) together with 10 wt% BaTiO ₃ filler so that Na / O = 1/18. As BaTiO ₃ , a commercially available reagent (particle size: <0.2 μm) manufactured by Aldrich was used. After stirring overnight, the resulting mixture was applied onto a PTFE plate to completely volatilize acetonitrile. Then, the PEO polymer electrolyte membrane was obtained by drying at 90 ° C. for 12 hours under vacuum.

  A coin cell was fabricated in the same manner as in Experimental Example 1. The polymer electrolyte membrane was cut out in a disk shape so as to fit in the coin cell, and set in the illustrated separator portion. Also, a disc-shaped spacer made of the same material as the coin cell of an arbitrary thickness was welded to the coin cell case so that there was no gap at each interface of the positive electrode / solid electrolyte / negative electrode.

The charge / discharge test of the battery was conducted in the same manner as in Experimental Example 7, using a charge / discharge measurement system, with a current density per effective area of the positive electrode of 0.3 mA / cm ² , an end-of-charge voltage of 2.9 V, and a discharge. The charge / discharge test was conducted in a voltage range of a final voltage of 1.1V.

  Table 4 shows the results of the charge / discharge test. Table 4 also shows the results of Experimental Example 1 for reference. Compared with Experimental Example 7, the contact resistance at the positive electrode / electrolyte and negative electrode / electrolyte interfaces increases, so that the voltage decreases by 0.2 V, and the discharge capacity also decreases slightly. However, no significant change was observed in the cycle stability indicated by the discharge capacity. From this experimental example, it was confirmed that the sodium secondary battery according to the present invention including the polymer electrolyte was operable.

(Experimental example 10)
Experimental Example 10 is an example in which an aqueous electrolyte is used in the sodium secondary battery of the present invention.

As the aqueous electrolyte, an 8 mol / L NaOH aqueous solution, NaCrO ₂ used in Experimental Example 7 as a positive electrode, and a CuFeO ₂ acetylene black mixture prepared under the conditions of Experimental Example 4 as a negative electrode were used.

  A coin cell was fabricated in the same manner as in Experimental Example 1.

The charge / discharge test of the battery was conducted in the same manner as in Experimental Example 7, using a charge / discharge measurement system, with a current density per effective area of the positive electrode of 0.5 mA / cm ² , an end-of-charge voltage of 1.5 V, and a discharge. The charge / discharge test was conducted in a voltage range of a final voltage of 0.5V.

Table 4 shows the results of the charge / discharge test. Since the aqueous electrolyte is used, the discharge voltage is 1V class, and the charge / discharge capacity is reduced because the voltage range is narrowed. However, the discharge capacity retention rate after 50 cycles achieved a high value of 85%. Also in acidic 1mol / L _Na 2 SO ₄ aqueous solution, was confirmed to show similar results. These results indicate that the CuFeO ₂ acetylene black mixture according to the present invention can function as a negative electrode material even in an aqueous electrolyte solution. The aqueous electrolyte solution is generally less expensive than the organic electrolyte solution, so it is considered advantageous for reducing the cost of the sodium secondary battery.

(Comparative example)
As a comparative example, a sodium secondary battery using tin as a negative electrode material was prepared and evaluated.

  Coin cells were produced and evaluated in the same manner as in Experimental Example 7 above. The results are shown in Table 5 in comparison with Experimental Example 7.

  The battery according to this comparative example showed excellent characteristics in terms of voltage and discharge capacity as compared with Experimental Example 7. However, the reduction of the discharge capacity due to the cycle was remarkable, and only about 5% of the initial discharge capacity was obtained after 50 cycles.

  On the other hand, in the case of Experimental Example 7, although the initial performance such as voltage and discharge capacity is inferior to that of the comparative example (however, it has sufficient performance as a sodium secondary battery), the discharge capacity is about 89% even after 50 cycles. It was maintained and was found to be highly stable. In the case of a negative electrode made of tin alone, the decrease in the discharge capacity of such a comparative example is that the negative electrode active material expands due to alloying of tin with sodium during discharge, and the negative electrode loses due to peeling from the electrode. It is thought that this is due to the fact that it was alive.

  As described above, it has been found that the sodium secondary battery according to the present invention is a high-performance battery having stable charge / discharge cycle characteristics.

  According to the present invention, a sodium secondary battery having excellent cycle characteristics can be manufactured and used as a drive source for various electronic devices.

1 Positive electrode (or working electrode)
2 Separator (impregnated with electrolyte)
3 Negative electrode (or counter electrode)
4 Positive electrode (or working electrode) case 5 Gasket 6 Negative electrode (or counter electrode) case

Claims (5)

A sodium secondary battery including a positive electrode, a negative electrode, and a sodium ion conductive electrolyte containing a substance capable of inserting and removing sodium ions, wherein the negative electrode contains CuFeO _2. Next battery.   The sodium secondary battery according to claim 1, wherein the negative electrode further includes carbon particles. 3. The sodium secondary battery according to claim 2, wherein the negative electrode includes a material that has been subjected to a ball mill process of pulverizing and mixing the CuFeO ₂ and the carbon particles.   The sodium secondary battery according to any one of claims 1 to 3, wherein the electrolyte is an organic electrolytic solution or an aqueous electrolytic solution containing sodium ions.   The sodium secondary battery according to any one of claims 1 to 3, wherein the electrolyte is a solid electrolyte or a polymer electrolyte that allows sodium ions to pass therethrough.

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