JP5596722B2 - 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 excellent in cost performance using a porphyrin-based cyclic organic compound as a positive electrode material of a sodium secondary battery.

  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. ing.

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 a negative electrode in an aqueous electrolyte, and a discharge with a current density of 2.0 mA / cm ² . In this case, both electrolytes have reported a relatively large discharge capacity of about 120 mAh / g.

Yabuuchi et al. Show a discharge capacity of about 150 mAh / g, which is larger than that of Non-Patent Document 1, when Na _2/3 Fe _1/3 Mn _2/3 O ₂ is used as a positive electrode material in Non-Patent Document 2. It is reported that 20 very stable charge / discharge cycles are possible.

  As described above, a discharge capacity at a level comparable to that of a lithium secondary battery has been reported so far. However, as in Non-Patent Document 1 and Non-Patent Document 2, an electrode material containing a rare metal may be used. Many are considered disadvantageous in terms of cost.

Sun Il Park et al. , Journal of The Electrochemical Society, 158 (10) A1067-A1070 (2011). N. Yabuuchi et al. , Meeting Abstracts of 220th ECS Meeting (The Electrochemical Society), Abstract # 649 (2011. October).

  An object of this invention is to provide the sodium secondary battery excellent in cost performance.

The present invention relates to a sodium secondary battery including a positive electrode capable of inserting and removing sodium ions, a negative electrode capable of inserting and removing metallic sodium or sodium ions, and an electrolyte having sodium ion conductivity. In the secondary battery, the positive electrode includes a porphyrin-based cyclic organic compound having a substituent containing sodium in a porphyrin ring structure, that is, protoporphyrin disodium represented by the following formula (I) , carbon, and a binder .

In the sodium secondary battery of the present invention, it is preferable that the positive electrode includes a material obtained by mixing the protoporphyrin disodium powder with carbon particles and performing a treatment with a ball mill , and a binder .

  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 can be provided.

It is the schematic which shows the structure of the sodium secondary battery of this invention. It is a figure which shows the structure of the protoporphyrin disodium which can be used with 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, a battery containing a porphyrin-based cyclic organic compound as a positive electrode material.

  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 can insert and desorb sodium ions, the negative electrode can insert and desorb metallic sodium or sodium ions, and the electrolyte has sodium ion conductivity.

In the present invention, the positive electrode contains a porphyrin cyclic organic compound having a substituent containing sodium as a material in the porphyrin cyclic structure. In the present invention, the sodium-containing porphyrin-based cyclic organic compound that is a positive electrode material is protoporphyrin disodium represented by the following formula (I) (chemical formula: C ₃₄ H ₃₂ N ₄ Na ₂ O ₄ , theoretical discharge capacity: 88 mAh). / G (also shown as FIG. 2)).

  The positive electrode of the sodium secondary battery of the present invention is preferably a mixture of a sodium-containing porphyrin-based cyclic organic compound and a carbon material such as carbon powder.

  Such a positive electrode can be prepared, for example, by the following means, but the present invention is not limited thereto.

  First, carbon powder (for example, carbon black such as acetylene black powder), binder powder such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and sodium-containing porphyrin-based cyclic organic compound are mixed. Then, the positive electrode can be prepared by rolling with a roll press machine, cutting into a predetermined size, and forming into a pellet. In the present invention, when producing the positive electrode, carbon powder and a sodium-containing porphyrin-based cyclic organic compound are mixed, pulverized and mixed by a pulverizer such as a ball mill, and the resulting ball mill (BM) treated mixture is further added to a binder. After mixing the powder, the positive electrode may be formed by rolling as described above.

  Alternatively, after a mixture of the above-described carbon powder, binder powder and sodium-containing porphyrin-based cyclic organic compound is dispersed in a solvent such as an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) to form a slurry. For example, a positive electrode can be prepared by applying on a metal foil such as a copper foil and drying.

  In the present invention, in order to improve the conductivity of sodium-containing porphyrin (positive electrode material), 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 performed for less than 24 hours, preferably 2 to 20 hours, more preferably 2 to 12 hours, and further preferably 6 to 12 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.

  The negative electrode is not particularly limited as long as it can insert and remove metallic sodium or sodium ions. For example, as an example of the negative electrode, a metal sodium sheet or a metal sodium sheet bonded to a metal foil such as nickel or stainless steel can be cited. 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. Moreover, what crimped | bonded the metal sodium sheet | seat to metal foil can be prepared by crimping | bonding the metal sodium sheet | seat prepared as mentioned above to metal foils, such as nickel and stainless steel.

  In addition, 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), or amorphous carbon capable of inserting and removing sodium ions, etc. Materials can be mentioned. 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.

  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 comprises, for example, a positive electrode and a negative electrode, and an electrolyte solution in contact with both electrodes as shown in FIG. In the present invention, a separator may be included between the positive electrode and the negative electrode. Moreover, the electrolyte may be impregnated in the separator. 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, the porphyrin cyclic organic compound having a substituent containing sodium in the porphyrin cyclic structure, which does not contain a metal other than sodium, specifically, a protoporphyrin having a structure as shown in FIG. It is particularly preferable to use disodium (chemical formula: C ₃₄ H ₃₂ N ₄ Na ₂ O ₄ , theoretical discharge capacity: 88 mAh / g) (same as the above formula (I)) as the positive electrode material.

  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. 3, 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.

  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.

  Examples 1 to 6 below are examples in which a sodium secondary battery was produced using an organic electrolyte using an organic solvent, and Example 7 was an example in which a sodium secondary battery was produced using an aqueous electrolyte solution. It is.

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

  As materials for the positive electrode, commercially available reagents such as protoporphyrin disodium powder (manufactured by Tokyo Chemical Industry Co., Ltd.), acetylene black powder (manufactured by Denki Kagaku Kogyo Co., Ltd.) and polytetrafluoroethylene (PTFE) powder (manufactured by Daikin) are 70. : 25: 5 weight ratio. These materials were sufficiently pulverized and mixed using a rough machine, and then roll-formed 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 to prepare a positive electrode. The negative electrode was prepared by pressing a sodium reagent (manufactured by Kanto Chemical), which is a commercially available reagent, to a thickness of 0.5 mm and molding it into a circular sheet shape 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.

  As the sodium secondary battery, a 2320 coin cell type as shown in FIG. 3 was manufactured. 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 spot welding a titanium mesh (manufactured by Niraco) (not shown) to the negative electrode case 6 and fixing it by crimping a sodium sheet thereon. 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 a glove box in an argon atmosphere with a dew point of −80 ° C. or lower.

(Ii) Discharge test The discharge test of the sodium secondary battery was conducted by supplying 0.2 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). A charge / discharge test was performed in a voltage range of a final voltage of 3.0V and a final discharge voltage of 2.0V. 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, this sodium secondary battery can be charged and discharged, and showed an initial discharge capacity of 57 mAh / g (normalized per weight of protoporphyrin disodium powder) and an average discharge voltage of 2.5V. Table 1 shows the discharge capacity retention rates at the 20th and 50th cycles.

  As described above, it was found that the sodium secondary battery according to this example was chargeable / dischargeable and had some cycle stability.

(Examples 2 to 6)
Protoporphyrin disodium powder and acetylene black powder were pulverized and mixed with a ball mill (BM) (ball mill treatment) to improve battery performance.

  Protoporphyrin disodium 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, 2 hours (Example 2), 6 hours (Example 3), 12 hours (Example 4), 20 hours (Example 5), 24 hours (Example). The ball mill process (BM process) of 6) was performed. In any of the BM treatments, the mixing ratio of the protoporphyrin disodium-acetylene black mixture and the ball was 1:10 by weight, and the rotation speed during mixing was 300 rpm.

  A PTFE binder was further added to the resulting BM-treated protoporphyrin disodium-acetylene black mixture, and the mixture was mixed with a cracker 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 battery characteristics improved until the BM treatment time was 12 hours. In the BM treatment for 12 hours, the discharge capacity showed 98% of the theoretical value, and the discharge capacity maintenance rate after 50 cycles achieved a high value of 92%. This is presumably because the contact property between protoporphyrin disodium powder and acetylene black powder was improved by BM treatment, and the interfacial resistance between the two powders was reduced. On the other hand, in the BM treatment for 24 hours, battery performance such as discharge capacity was lowered. This is considered to be due to the denaturation of protoporphyrin disodium caused by 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, for BM processing, it is considered necessary to appropriately select parameters such as processing time and optimize processing conditions.

(Example 7)
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 protoporphyrin disodium-acetylene black mixture prepared under the conditions of Example 3 was used.

The battery discharge test was conducted in the same manner as in Example 1, using a charge / discharge measurement system, with a current density per effective area of the positive electrode of 0.2 mA / cm ² , a charge end voltage of 1.4 V, and a discharge end. The charge / discharge test was conducted in the voltage range of 0.5V.

The results of the charge / discharge test are shown in Table 1. Since an aqueous electrolyte was used, the discharge voltage was 1 V class, but the discharge capacity showed 95% of the theoretical value, and the discharge capacity retention rate after 50 cycles achieved a high value of 90%. Also in acidic 1mol / L Na ₂ SO ₄ aqueous solution, was confirmed to show similar results. These results indicate that protoporphyrin disodium 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 4.

  Compared with Example 4, the battery according to this comparative example showed excellent characteristics in terms of voltage and discharge capacity. 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 4, although the initial performance is inferior to that of the comparative example (however, the characteristics as a secondary battery are sufficient), the discharge capacity of about 85% is maintained even after 100 cycles. It was found that the stability is 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 using disodium protoporphyrin according to the present invention is a high-performance battery having excellent cost performance and further excellent charge / discharge cycle characteristics.

  According to the present invention, it is possible to manufacture a sodium secondary battery with excellent cost performance, and it 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 (4)

A sodium secondary battery comprising a positive electrode capable of inserting and removing sodium ions, a negative electrode capable of inserting and removing metallic sodium or sodium ions, and an electrolyte having sodium ion conductivity,
The said positive electrode contains the protoporphyrin disodium represented by following formula (I) as a positive electrode substance , carbon, and a binder, The secondary battery characterized by the above-mentioned.

The positive electrode, the protoporphyrin disodium powder of the positive electrode material and a mixture of powder of the carbon material was subjected to ball mill treatment, and sodium of claim 1, characterized in that it comprises a binder two Next battery.   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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