JP2014053190A - Sodium secondary battery - Google Patents

Abstract

To provide a sodium secondary battery excellent in cost and cycle characteristics.
The present invention relates to a positive electrode including a substance capable of inserting and desorbing sodium ions, a metallic sodium, a sodium-containing material, or a negative electrode including a substance capable of inserting and desorbing sodium ions, and sodium ions. A sodium secondary battery including an electrolyte having conductivity, wherein the substance capable of inserting and removing sodium ions from the positive electrode includes Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ). To do. The positive electrode material may be a Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (where 0 <x ≦ 4.0)). The electrolyte of the present invention is an organic electrolyte or aqueous electrolyte containing sodium ions.
[Selection] Figure 1

Description

The present invention relates to a sodium secondary battery. In particular, the present invention provides Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ ) (where 0 <X ≦ 4.0)), and relates to a sodium secondary battery excellent in cost and cycle characteristics.

  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.

Park et al. In Non-Patent Document 1 that NaTi ₂ (PO ₄ ) ₃ can be used as a positive electrode in an organic electrolyte and as a negative electrode in an aqueous electrolyte, and a sodium secondary battery using the material is disclosed. In the case of a discharge with a current density of 2.0 mA / cm ² , it is reported that both electrolytes show a relatively large discharge capacity of about 120 mAh / g.

Patent Document 1 describes that NaCoO ₂ can be charged and discharged in a non-aqueous electrolyte and has a high discharge capacity maintenance rate of 94% although the number of cycles is as small as five.

JP 2007-35283 A

Sun Il Park et al., Journal of The Electrochemical Society, 158 (10) A1067-A1070 (2011).

  As described above, sodium secondary batteries having a discharge capacity comparable to that of lithium secondary batteries have been reported so far. There are disadvantages and sufficient cycle characteristics cannot be obtained.

  Therefore, an object of the present invention is to provide a sodium secondary battery excellent in cost performance and cycle characteristics.

The present invention has a positive electrode including a substance capable of inserting and desorbing sodium ions, a negative electrode including a metal sodium, a sodium-containing material, or a substance capable of inserting and desorbing sodium ions, and sodium ion conductivity. It is a sodium secondary battery containing an electrolyte, and the substance capable of inserting and removing sodium ions of the positive electrode contains Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ).

In the present invention, the positive electrode material of the positive electrode may include a Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (where 0 <x ≦ 4)).

In the sodium secondary battery of the present invention, the positive electrode is the Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ ). It is preferable that a material obtained by mixing (wherein 0 <x ≦ 4.0) with carbon particles and performing a treatment with a ball mill is included as a substance capable of inserting and desorbing sodium ions.

  In the sodium secondary battery of the present invention, the electrolyte is preferably an organic electrolytic solution containing sodium ions or an aqueous electrolytic solution containing sodium ions.

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

It is the schematic which shows the structure of the sodium secondary battery of this invention. 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, in particular, Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 < x ≦ 4.0)).

  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 metallic sodium, a sodium-containing substance, or a substance capable of inserting and removing sodium ions, and the electrolyte is sodium ion. It has conductivity.

In the present invention, the positive electrode is made of Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0)). Include as material.

The Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) that can be used in the present invention can be obtained, for example, by the following method.

Commercially available potassium ferricyanide K ₃ [Fe (CN) ₆ ] powder, iron chloride FeCl ₂ , and sodium chloride NaCl are separately dissolved in a solvent such as water, and each solution has a composition molar ratio of a target sample. And stirred overnight to obtain a precipitate. For example, when synthesizing a Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) of x = 4, the respective solutions are mixed at a molar ratio of 3: 4: 4, and precipitation is performed. What is necessary is just to prepare a thing. The precipitate is recovered using an appropriate recovery means such as a centrifugal separator or a suction filter and dried.

The positive electrode of the sodium secondary battery of the present invention has Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ ) (0 <x ≦ 4. It is preferable to include a mixture of 0)) and a carbon material such as carbon powder.

  The above-mentioned positive electrode can be prepared, for example, by the following means, but the present invention is not limited to these.

First, carbon powder (for example, carbon blacks such as acetylene black powder), binder powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and Prussian blue (Fe ₄ [Fe (CN)) ₆ ] ₃ ) or Prussian blue analogue (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0)), then rolled with a roll press, cut into a predetermined size and pelletized The positive electrode can be prepared by molding into a shape.

In the present invention, the carbon powder in the positive electrode material and Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analogue (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4 0.0)) in order to improve the uniformity and dispersibility of the secondary battery and improve the performance of the secondary battery, when manufacturing the positive electrode, carbon powder and Prussian blue or Prussian blue analogue are mixed, The positive electrode may be formed by pulverizing and mixing with a pulverizer and further mixing the binder powder with the obtained ball mill (BM) treatment mixture, followed by rolling and forming as described above.

Alternatively, the aforementioned carbon powder, binder powder and Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ ) (0 <x ≦ 4 0.0)) is dispersed in a solvent such as an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) to form a slurry, which is then applied onto a metal foil such as a copper foil and dried. By doing so, a positive electrode can be prepared.

In the present invention, Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0)) (positive electrode material) In order to improve the electroconductivity of the positive electrode containing, it is preferable to mix the positive electrode material with carbon particles as a conductive material and perform ball mill treatment. By such a ball mill treatment, more excellent battery characteristics can be obtained. The ball mill treatment is less than 12 hours, preferably 3 to 10 hours, more preferably 3 to 6 hours.

  In the positive electrode of the present invention, both an electrolyte solution containing an organic electrolyte solution containing sodium ions and an aqueous electrolyte solution containing sodium ions can be used as the electrolyte solution. Furthermore, the sodium secondary battery including the positive electrode of the present invention can also use a solid electrolyte or polymer electrolyte that allows sodium ions to pass therethrough as an electrolyte.

  The negative electrode is not particularly limited as long as it includes a sodium-containing material such as metallic sodium, a sodium-containing alloy, or a material capable of inserting and removing sodium ions. For example, examples of the negative electrode include a metal sodium sheet, or a metal sodium sheet bonded to a metal foil such as aluminum. The negative electrode of such a metal sodium sheet can be prepared by rolling metal sodium into a sheet shape with a press or the like and forming it into a desired shape. Further, a metal sodium sheet that has been pressure-bonded to a metal foil can be prepared by pressure-bonding the metal sodium sheet prepared as described above to a metal foil such as aluminum.

  Moreover, as a negative electrode material other than metallic sodium as described above, an alloy containing sodium as a main component as a negative electrode active material (for example, a sodium-tin alloy) (sodium-containing material), or insertion and removal of sodium ions is possible. Examples thereof include amorphous carbon and other materials (substances capable of inserting and removing sodium ions). The negative electrode containing these negative electrode active materials is, for example, a metal foil such as copper foil and a negative electrode active material and a binder such as polyvinylidene fluoride (PVDF), such as N-methyl-2-pyrrolidone (NMP). A slurry dispersed in a simple organic solvent can be applied and dried.

As the electrolyte, a metal salt containing sodium ions such as sodium bistrifluoromethanesulfonylimide (NaTFSI), sodium perchlorate (NaClO ₄ ), sodium hexafluorophosphate (NaPF ₆ ), for example, ethylene carbonate (EC) And a mixed solvent of dimethyl carbonate (DMC) (volume ratio 1: 1), a mixed solvent such as EC and diethyl carbonate (DEC), or an organic electrolyte dissolved in a single solvent such as propylene carbonate, or an aqueous NaOH solution An aqueous solution (aqueous electrolyte solution) in which a metal salt containing sodium ions, such as an aqueous Na ₂ SO ₄ solution, an aqueous NaCl solution, and an aqueous NaClO ₄ solution, is dissolved.

As another electrolyte of the sodium secondary battery of the present invention, a solid electrolyte that passes sodium ions (for example, 75Na ₂ S.25P ₂ S ₅ , NASICON (Na ⁺ Super Ionic Conductor, Na ₃ Zr ₂ Si ₂ PO ₁₂ etc.)) , Polymer electrolytes that allow sodium ions to pass therethrough (for example, polyethylene oxide (PEO) type), and the like, but are 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.

  A battery using the positive electrode, the negative electrode, the electrolytic solution, or 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 using an organic electrolyte or an aqueous electrolyte as the electrolyte solution, for example, a separator can be used by impregnating the electrolyte solution. Moreover, the organic electrolyte or the aqueous 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.

Further, although not explicitly shown in FIG. 1, a battery case covering a positive electrode, a negative electrode, an electrolyte, a separator, and the like can be included. In the present invention, Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) or Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0)) is used as the positive electrode material. It is particularly preferable to use as

  As a more specific embodiment, the present invention can be applied as a coin cell type secondary battery as shown in FIG. As shown in FIG. 2, the coin cell type secondary battery includes a positive electrode 1 and a negative electrode 3, and further includes a separator 2 impregnated with an electrolyte 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 impregnated with the electrolytic solution are arranged in the positive electrode case 4 and the negative electrode case 6 as desired, and both cases in which 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.

  Hereinafter, embodiments 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 Example, In the range which does not change the meaning and scope of this invention, it can change suitably and can implement.

In the following, preparation of the cathode material _{(Na x Fe 4 [Fe (} CN) 6] 3 (0 <x ≦ 4.0)), and show an embodiment 1-14. Examples 1 to 11 are examples in which a sodium secondary battery was produced using an organic electrolyte using an organic solvent, and Example 12 was an example in which a sodium secondary battery was produced using an aqueous electrolyte solution. is there. Examples 13 and 14 are examples in which a sodium secondary battery was produced using a solid electrolyte and a polymer electrolyte, respectively.

(Preparation of positive electrode material)
Preparation of Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0)

(Preparation Example 1)
Preparation of Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) This compound was synthesized by the following procedure.

First, commercially available potassium ferricyanide K ₃ [Fe (CN) ₆ ] powder (manufactured by Kanto Chemical Co., Inc.), iron chloride FeCl ₂ (manufactured by Kanto Chemical Co., Ltd.), sodium chloride NaCl (manufactured by Kanto Chemical Co., Ltd.) Dissolved in distilled water. Each solution was mixed so that it might become the composition molar ratio of the target sample, and it stirred all day and night, and obtained the deposit. In this preparation example, a sample with x = 4 was synthesized. In Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ , the respective solutions were mixed at a molar ratio of 3: 4: 4 to obtain a precipitate. The precipitate was collected using a centrifuge and a suction filter and dried at 60 ° C. for 1 week. The dried precipitate was sufficiently pulverized with a mixer and used as a positive electrode material. When X-ray diffraction measurement was performed on the obtained powder, no impurity was produced, and a diffraction pattern similar to that of Prussian blue having a host crystal structure was obtained. Further, ICP analysis of the powder revealed that the composition ratio (molar ratio) of the powder was Na: Fe: C: N = 4: 7: 18: 18. From the above results, it was confirmed that the obtained powder was Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ .

(Preparation Example 2)
_{Na x Fe 4 [Fe (CN} ) 6] as the compound of Preparation other compositions of the compounds of the ₃ (x = 1.0 and 3.0), in _{Na x Fe 4 [Fe (CN} ) 6] 3 x = 1 0.0 and 3.0 compounds were prepared in the same procedure as in Preparation Example 1 above.

(Reference example)
As a reference example, a compound of Na _x Fe ₄ [Fe (CN) ₆ ] ₃ and x = 4.1 was prepared in the same procedure as in Preparation Example 1 described above. The compound of Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (x = 4.1) is a sodium secondary solution using Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0). It used as a positive electrode material of a sodium secondary battery for comparison with the battery.

Example 1
(I) Production of sodium secondary battery A sodium secondary battery was produced by the following procedure.

  The positive electrode comprises commercially available Prussian blue powder (manufactured by ACROS), acetylene black powder (manufactured by Denki Kagaku Kogyo) and polytetrafluoroethylene (PTFE) powder (manufactured by Daikin) in a weight ratio of 70: 25: 5. The mixture was sufficiently pulverized and mixed using a rough machine, and then roll-formed 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 negative electrode was produced 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 sodium secondary battery was manufactured as a 2320 coin type as shown in FIG. 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 prepared by fixing a titanium mesh (manufactured by Niraco) (not shown) to the negative electrode case 6 by spot welding and fixing a sodium sheet thereon by pressure bonding. Next, the separator 2 is set in the positive electrode case to which the pellet is fixed, the electrolytic solution is injected into the separator 2, and the negative electrode case to which the sodium sheet is fixed is covered, and the positive electrode case 4 and the negative electrode case 6 are covered with a coin cell caulking machine. By crimping, 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 Using a commercially available charge / discharge measurement system (manufactured by Hokuto Denko Co., Ltd.), a sodium secondary battery discharge test was conducted with a current density of 0.5 mA / cm ² per effective area of the positive electrode. A charge / discharge test was performed in a voltage range of a charge end voltage of 3.5 V and a discharge end voltage of 2.0 V. The battery charge / discharge test was performed in a thermostatic chamber at 25 ° C. (atmosphere was in a normal living environment).

  A charge / discharge curve of the sodium secondary battery produced in this example is shown in FIG. From the figure, the irreversible capacity (the difference between the charge capacity and the discharge capacity) is also small, and the sodium secondary battery according to the present invention can be charged and discharged, and the initial discharge capacity is 81 mAh / g (standardized per weight of Prussian blue powder). The average discharge voltage was 2.5V. Table 1 shows the discharge capacity retention rates at the 20th and 50th cycles. From the table, it can be seen that only a capacity decrease of about 0.4 to 0.5% per cycle is observed, and the cycle characteristics are stable.

(Example 2)
Using the Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) prepared according to Preparation Example 1 above, in the same procedure as in Example 1, the production of the sodium secondary battery and the charge / discharge characteristics Measurements were made.

Table 1 shows the average discharge voltage and discharge capacity of the first cycle, and the discharge capacity maintenance rates of the 20th and 50th cycles. _{Na 4 Fe 4 [Fe (CN} ) 6] 3 shown in this embodiment, ensure that they have substantially the same discharge voltage and cycle characteristics as _{Fe 4 [Fe (CN) 6} ] 3 of Example 1 did. Moreover, when samples of x = 1.0 and 3.0 were synthesized with a compound of Na _x Fe ₄ [Fe (CN) ₆ ] ₃ and the same evaluation was performed, the same results as in Example 2 were obtained. It was.

Further, when a compound of x = 4.1 is synthesized from a compound of Na _x Fe ₄ [Fe (CN) ₆ ] _3, a secondary battery is manufactured in the same procedure as in Example 1, and charge / discharge characteristics are measured. The discharge voltage and discharge capacity decreased. This is considered due to the generation of an impurity phase. This result shows that the range of the Na content defined in the present invention is appropriate.

From the above results, Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (0 <x ≦ 4.0) contains Na ions in the crystal structure in the initial state, and thus contains Na ions. A rocking chair type secondary battery can be formed in combination with a negative electrode material (for example, amorphous carbon).

(Examples 3 to 6)
Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) powder and acetylene black powder were pulverized and mixed with a ball mill (BM) (ball mill treatment) to improve battery performance.

  Prussian blue powder and acetylene black powder (weight ratio 70:25) were mixed in a mixer for several minutes. A zirconia ball having a diameter of 7 mm was added to this mixture, and BM treatment was performed for 3 hours (Example 3), 6 hours (Example 4), 10 hours (Example 5), and 12 hours (Example 6). . In any of the BM treatments, the mixing ratio of the Prussian blue-acetylene black mixture and the ball was 1:10 by weight, and the rotation speed during the BM treatment was 300 rpm.

  The obtained Prussian blue-acetylene black mixture after the BM treatment was further added with a PTFE binder and mixed with a rake machine to produce a positive electrode pellet in the same manner as in Example 1. Using this pellet, a coin cell was produced in the same manner as in Example 1. The charge / discharge test was also performed in the same manner as in Example 1.

  The results of the charge / discharge test are shown in Table 1. The characteristics improved until the BM treatment time was 6 hours (6 hours). In addition, in the BM treatment for 6 hours, the discharge capacity retention rate after 50 cycles achieved a high value of 90%. This is presumably because the contact property between the Prussian blue 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 12 h BM treatment, battery performance such as discharge capacity decreased. This is considered to be due to the modification of Prussian blue due to local heat generation during BM treatment. Thus, in this invention, it became clear that battery performance improves by carrying out BM process of the active material of positive electrode material. However, the optimum conditions for BM treatment differ depending on the particle size of the sample and the material and size of the ball to be used.

(Examples 7 to 11)
Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) powder and acetylene black powder were pulverized and mixed with a ball mill (BM) (ball mill treatment) to improve battery performance.

  Prussian blue analog powder and acetylene black powder (weight ratio 70:25) were mixed in a mixer for about several minutes. A zirconia ball having a diameter of 7 mm was added to this mixture, and 3 hours (Example 7), 4 hours (Example 8), 6 hours (Example 9), 10 hours (Example 10), and 12 hours (Example). The BM treatment of 11) was performed. In any of the BM treatments, the mixing ratio of the Prussian blue analog-acetylene black mixture and the ball was 1:10 by weight, and the rotation speed during the BM treatment was 300 rpm.

  The obtained Prussian blue analog-acetylene black mixture after the BM treatment was added with a PTFE binder and mixed with a milling machine to prepare a positive electrode pellet in the same manner as in Example 1. Using this pellet, a coin cell was produced in the same manner as in Example 1, and a charge / discharge test was performed.

  The results of the charge / discharge test are shown in Table 1. The characteristics improved until the BM treatment time was up to 3 h, and the discharge capacity retention rate after 50 cycles achieved a high value of 91%. This is presumably because the contact property between the Prussian blue analog 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 for 4 hours or more, battery performance such as discharge capacity was lowered. This is considered to be due to the modification of Prussian blue due to local heat generation during BM treatment. Thus, in the present invention, it was revealed that the battery performance is improved by subjecting the active material of the positive electrode material to BM treatment within a range not exceeding 3 hours. However, the optimum conditions for BM treatment differ depending on the particle size of the sample and the material and size of the ball to be used.

(Example 12)
A coin cell was manufactured in the same manner as in Example 1 using an 8 mol / L NaOH aqueous solution as the aqueous electrolyte and amorphous carbon as the negative electrode material.

For the positive electrode, a Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) / acetylene black mixture prepared under the conditions of Example 7 was used.

In the battery discharge test, in the same manner as in Example 1, a charge / discharge measurement system was used, and a current density per effective area of the positive electrode was applied at ² mA / cm ² , and the charge end voltage was 1.4 V and the discharge end voltage was 0. The charge / discharge test was conducted in the voltage range of 5V.

The results of the charge / discharge test are shown in Table 1. Since the aqueous electrolyte was used, the discharge voltage was 1 V class, but the discharge capacity retention rate after 50 cycles achieved a high value of 91%. Also in acidic 1mol / L Na ₂ SO ₄ aqueous solution, was confirmed to show similar results. These results indicate that the Prussian blue material according to the present invention can function as a positive 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 a positive electrode material containing a rare metal was produced. NaCoO ₂ was evaluated as a positive electrode material. NaCoO ₂ was synthesized by mixing Na ₂ CO ₃ and Co ₃ O ₄ at a predetermined molar ratio (3: 2) and firing at 1000 ° C.

A coin cell using NaCoO ₂ was produced and evaluated in the same manner as in Example 1. The results are shown in Table 2 in comparison with Example 7.

  The battery according to this comparative example showed superior characteristics in terms of voltage and discharge capacity as compared with Example 7. However, the capacity decrease due to the charge / discharge cycle was significant, and only about 30% of the initial discharge capacity was obtained after 100 cycles.

On the other hand, in the case of Example 7, although the initial performance is inferior to that of the comparative example (however, the characteristics as a secondary battery are sufficient), the discharge capacity is maintained at about 82% even after 100 cycles. It was found that the stability was high. In the case of NaCoO ₂ , the elution of Co, which is a transition metal, has occurred, which is considered to have caused a decrease in capacity.

  As described above, it was found that the sodium secondary battery according to the present invention is a high-performance battery having excellent cost performance and excellent charge / discharge cycle characteristics.

(Example 13)
NASICON (Na ₃ Zr ₂ Si ₂ PO ₁₂ ) as the solid electrolyte, amorphous carbon as the negative electrode material, and Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) produced under the conditions of Example 7 as the positive electrode. ) / Acetylene black mixtures were used respectively.

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 at a molar ratio, and calcined at 850 ° C. The obtained powder was molded 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 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 separator 2 portion of FIG. 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 no gap was formed at each interface of the positive electrode / solid electrolyte / negative electrode.

The discharge test of the battery was conducted in the same manner as in the example by supplying a current density per effective area of the positive electrode of 0.5 mA / cm ² using a commercially available discharge / charge measurement system, with a charge end voltage of 3.5 V and a discharge. A charge / discharge test was performed in a voltage range of a final voltage of 2.0V.

  The results of the charge / discharge test are shown in Table 1. Compared with Example 7, the contact resistance at the positive electrode / electrolyte and negative electrode / electrolyte interface increased, so that the voltage decreased by 0.2 V and the discharge capacity decreased slightly. However, there was no change in the cycle stability indicated by the discharge capacity. According to this example, it was confirmed that the sodium secondary battery of the present invention including the solid electrolyte was operable.

(Example 14)
Prussian blue analog (Na ₄ Fe ₄ [Fe (CN) ₆ ] ₃ ) / acetylene black produced under the conditions of Example 7 using PEO polymer electrolyte membrane as the polymer electrolyte, amorphous carbon as the negative electrode material, and positive electrode as the positive electrode Each mixture was used.

  A PEO-based electrolyte membrane (thickness: about 2 mm) was prepared by the following procedure.

First, PEO (Aldrich, Mw = 6 × 10 ⁵ ) and LiTFSI (Kishida Chemical), which is a solute, are 10 wt% BaTiO ₃ filler (BaTiO ₃ manufactured by Aldrich) so that Li / O = 1/18. Reagent (with particle size: <2 μm)) was added to the acetonitrile solvent (Kishida Chemical). After stirring overnight, the resulting solution was applied on a PTFE plate to completely volatilize acetonitrile. Then, it dried at 90 degreeC under vacuum for 12 hours.

  A coin cell was fabricated in the same manner as in Example 1. The polymer electrolyte was cut out in a disk shape so as to fit in the coin cell, and set in the separator 2 portion of FIG. 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 no gap was formed at each interface of the positive electrode / solid electrolyte / negative electrode.

The discharge test of the battery was conducted in the same manner as in the example by supplying a current density per effective area of the positive electrode of 0.5 mA / cm ² using a commercially available discharge / charge measurement system, with a charge end voltage of 3.5 V and a discharge. A charge / discharge test was performed in a voltage range of a final voltage of 2.0V.

  The results of the charge / discharge test are shown in Table 1. Compared with Example 7, the contact resistance at the positive electrode / electrolyte and negative electrode / electrolyte interfaces increased, so that the voltage decreased by 0.1 V, and the discharge capacity also decreased slightly. However, there was no change in the cycle stability indicated by the discharge capacity. This example confirmed that the sodium secondary battery of the present invention containing a polymer electrolyte was operable.

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

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

Claims (6)

A positive electrode including a substance capable of inserting and desorbing sodium ions, a sodium including a sodium metal, a sodium-containing substance, or a negative electrode including a substance capable of inserting and desorbing sodium ions, and an electrolyte having sodium ion conductivity A sodium secondary battery, which is a secondary battery, wherein the substance capable of inserting and removing sodium ions from the positive electrode contains Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ). A positive electrode including a substance capable of inserting and desorbing sodium ions, a sodium including a sodium metal, a sodium-containing material, or a negative electrode including a substance capable of inserting and desorbing sodium ions, and an electrolyte having sodium ion conductivity In the secondary battery, a substance capable of inserting and desorbing sodium ions from the positive electrode is a Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (where 0 <x ≦ 4.0). )). The sodium secondary battery characterized by including. 2. The sodium secondary according to claim 1, wherein the positive electrode includes a material obtained by mixing the Prussian blue (Fe ₄ [Fe (CN) ₆ ] ₃ ) with carbon particles and performing a ball mill treatment. battery. The positive electrode is a material in which the Prussian blue analog (Na _x Fe ₄ [Fe (CN) ₆ ] ₃ (where 0 <x ≦ 4.0)) is mixed with carbon particles and ball milled. The sodium secondary battery according to claim 2, comprising:   The sodium secondary battery according to claim 1, wherein the electrolyte is an organic electrolytic solution containing sodium ions.   The sodium secondary battery according to claim 1 or 2, wherein the electrolyte is an aqueous electrolyte containing sodium ions.

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