JP6061390B2 - Sodium secondary battery - Google Patents

Description

  The present invention relates to a sodium secondary battery having excellent cycle characteristics.

  Sodium secondary batteries using sodium ion insertion and desorption reactions are superior to lithium secondary batteries currently in widespread use because of the superiority of sodium resources, and thus are superior in cost. Sodium secondary batteries are also expected as a storage battery that can be increased in size in the future, and research and development are underway.

Patent Document 1 discloses a secondary battery using NaCoO ₂ as a positive electrode material and an organic electrolyte as an electrolyte. This secondary battery exhibits a discharge capacity of about 60 mAh / g and enables three stable charge / discharge cycles.

In Non-Patent Document 1, Xin Xia et al. Report that a secondary battery using Na _0.65 CoO ₂ as a positive electrode material and an organic electrolyte as an electrolyte exhibits a discharge capacity of about 70 mAh / g. doing.

Furthermore, in Non-Patent Document 2, Jong-Seon Kim et al. Reported a secondary battery using a material obtained by doping Fe into Ni ₃ S ₂ as a positive electrode material and using an organic electrolyte as an electrolyte. This secondary battery exhibits a very high discharge capacity of about 400 mAh / g, and the discharge capacity is maintained at 64% at the 15th cycle.

JP 2007-35283 A

Xin Xia et al. , Journal of The Electrochemical Society, 159 (5) A647-A1650 (2012). Jong-Seon Kim et al. , Current Applied Physics 11 (2011) S215-S218.

  As described above, a sodium secondary battery has been reported to have a discharge capacity comparable to that of a lithium secondary battery, but has a problem that cycle characteristics are low. An object of this invention is to provide the sodium secondary battery excellent in cycling characteristics compared with the conventional sodium battery.

One example of means for solving the problems of the present invention is a positive electrode containing a substance capable of inserting / desorbing sodium ions, metal sodium, a metal that can be alloyed with metal sodium, or sodium ion insertion / desorption. A sodium secondary battery comprising a negative electrode containing a sodium-containing substance and an electrolyte having sodium ion conductivity, wherein the positive electrode has Mo ₆ S ₈ (for example, Na with respect to 1 mol of Mo ₆ S ₈₎ . The sodium secondary battery further includes a theoretical discharge capacity when 1 mol of is inserted.

Alternatively, another example of the means for solving the problems of the present invention is a metal capable of inserting / extracting sodium ions and capable of alloying with a positive electrode containing sodium-containing substance, metallic sodium, metallic sodium. Or a sodium secondary battery including a negative electrode containing a substance capable of inserting / extracting sodium ions and an electrolyte having sodium ion conductivity, wherein the negative electrode is Mo ₆ S ₈ (for example, 1 mol of Mo ₆ theoretical discharge capacity when Na is 1mol inserted into the S _8: 32mAh / g) is a sodium secondary battery, characterized by further comprising a.

  Here, an organic electrolytic solution containing sodium ions may be used as the electrolyte. Alternatively, an aqueous electrolyte solution containing sodium ions may be used as the electrolyte.

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

The operation principle of the sodium secondary battery is shown. The schematic structure of a 2320 coin cell is shown. An initial charge / discharge curve of a 2320 coin cell using Mo ₆ S ₈ as a positive electrode, sodium metal as a negative electrode, and a solution of NaClO ₄ dissolved in PC at a concentration of 1 mol / L as an electrolytic solution is shown.

  Below, the example of embodiment of the sodium secondary battery by this invention is demonstrated.

<When the positive electrode contains Mo ₆ S ₈ >
When Mo ₆ S ₈ is used as the positive electrode material, the sodium secondary battery can be prepared, for example, by the following means.

The positive electrode includes, for example, Mo ₆ S ₈ , carbon powder such as acetylene black as a substance capable of inserting and removing sodium ions, and a bond such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF). It is mixed with an adhesive powder, rolled with a roll press, cut into a predetermined size, formed into pellets, or dispersed in a solvent such as an organic solvent to prepare a slurry, which is then applied onto a metal foil and dried. Or the like.

  The negative electrode can be formed, for example, by forming metal sodium into a sheet shape and pressing the sheet onto a metal foil such as copper or stainless steel. As the negative electrode material, in addition to metallic sodium, an alloy containing metallic sodium or a material such as amorphous carbon capable of inserting / extracting sodium ions can be used.

As the electrolyte, for example, an organic electrolytic solution or an aqueous solution in which a metal salt containing sodium ions is dissolved can be used. For example, a metal salt containing sodium ions such as trifluoromethanesulfonylimide (NaTFSI), sodium perchlorate (NaClO ₄ ), sodium hexafluorophosphate (NaPF ₆ ), a solvent such as polycarbonate (PC), carbonic acid, An organic electrolyte dissolved in a mixed solvent of ethylene (EC) and dimethyl carbonate (DMC) (volume ratio 1: 1), a mixed solvent such as EC and diethyl carbonate (DEC), or a single solvent such as propylene carbonate, or, it may be mentioned aqueous NaOH, _Na 2 SO ₄ aqueous solution, NaCl solution, aqueous solution of metal salt containing sodium ions, such as NaClO ₄ aqueous solution was dissolved in water (aqueous electrolyte).

<When the negative electrode contains Mo ₆ S ₈ >
When Mo ₆ S ₈ is used as the negative electrode material, the sodium secondary battery can be prepared, for example, by the following means.

As the positive electrode, for example, a material such as NaMn _0.5 Fe _0.5 O ₂ containing a sodium-containing substance can be used, in which sodium ions can be inserted and removed, and Mo ₆ S ₈ is used as the positive electrode material. It can be prepared by the same means as used. For example, the positive electrode includes NaMn _0.5 Fe _0.5 O ₂ , carbon powder such as acetylene black as a substance capable of inserting and removing sodium ions, and polytetrafluoroethylene (PTFE) or polyvinylidene fluoride ( PVDF) is mixed with a binder powder, rolled by a roll press, cut into a predetermined size, formed into pellets, or dispersed in a solvent such as an organic solvent to prepare a slurry. It can be produced by means such as coating on a foil and drying.

The negative electrode can be prepared, for example, by the same means as the above positive electrode. For example, the negative electrode includes Mo ₆ S ₈ , carbon powder such as acetylene black as a substance that can insert and desorb sodium ions, and a bond such as polytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF). It is mixed with an adhesive powder, rolled with a roll press, cut into a predetermined size, formed into pellets, or dispersed in a solvent such as an organic solvent to prepare a slurry, which is then applied onto a metal foil and dried. Or the like.

Since the electrolyte can be the same as the electrolyte in the case where the above Mo ₆ S ₈ is used as the positive electrode material, description thereof is omitted.

  A battery using the positive electrode, the negative electrode, and the electrolytic solution as described above can be manufactured in a conventional shape such as a coin shape, a cylindrical shape, or a laminate shape. In addition, as for other elements such as a structural material such as a separator and a battery case, various materials used in conventionally known secondary batteries can be used, and there is no particular limitation.

  Embodiments of a sodium secondary battery according to the present invention will be described below in detail with reference to the accompanying drawings. In addition, this invention is not limited to the content shown to the following Example, In the range which does not change the summary, it can change suitably and can implement.

[Example 1]
The sodium secondary battery was produced by the following procedure.

First, the production of the positive electrode material Mo ₆ S ₈ will be described. _Cu 2 S commercial reagents which is a raw material (manufactured by ALDRICH) 0.3282g, MoS ₂ (Kanto Chemical Co., Ltd.) 1.1788G, and Mo and (Wako Pure Chemical Industries) 0.4930G were combined and vacuum-sealed, It heated at 1000 degreeC for 48 hours using the electric furnace. The sample was pulverized and mixed, and vacuum sealed again. Furthermore, after heating at 1000 ° C. for 48 hours, the mixture was mixed in the air. The obtained Cu ₂ Mo ₆ S ₈ was stirred in 200 ml of HCl (Wako Pure Chemicals, content 35%), the supernatant was removed every 24 h for 7 days, and 100 mL of HCl was added. The solution was subjected to suction filtration, and the solid content was washed 10 times with 25 mL of distilled water at a time, followed by vacuum drying at 120 ° C. to obtain Mo ₆ S ₈ .

When the sample thus obtained was identified using a powder X-ray diffraction measurement method, it was well known as # 82-1709 of PDF (Powder Diffraction File: database of compound diffraction patterns by powder X-ray diffraction). In agreement, it was confirmed that Mo ₆ S ₈ was obtained as the main phase.

Next, each electrode and electrolyte will be described. Mo ₆ S ₈ synthesized as described above, commercially available carbon powder (for example, carbon blacks such as ketjen black powder) and polytetrafluoroethylene (PTFE: manufactured by Daikin Industries, Ltd.) powder in a weight of 70: 25: 5 In the ratio, the mixture was sufficiently pulverized and mixed using a rough machine, roll-formed, and a sheet pellet electrode (thickness: 0.5 mm) was produced, which was used as a positive electrode. This sheet-like electrode was cut out into a circle having a diameter of 15 mm. The negative electrode was formed by pressing a sodium mass of a commercially available reagent to a thickness of 0.5 mm and forming a circular sheet having a diameter of 15 mm. As the electrolyte, a solution (manufactured by Toyama Pharmaceutical Co., Ltd.) in which sodium perchlorate (NaClO ₄ ) was dissolved in propylene carbonate (PC) at a concentration of 1 mol / L was used. The separator used was a polypropylene product (manufactured by Celgard Co., Ltd.) for lithium secondary batteries.

  As the battery, a 2320 coin cell as shown in FIG. 2 was used. For the positive electrode, the above-described pellet electrode 1 was set in the positive electrode case 4, covered with a titanium mesh (manufactured by Nilaco Co., Ltd.) not shown, and the peripheral edge thereof was fixed by spot welding. The negative electrode was fixed to the negative electrode case 6 by fixing a titanium mesh (not shown) (manufactured by Niraco Co., Ltd.) by spot welding and pressing the sodium sheet 3 thereon. Next, the separator 2 is set in the positive electrode case 4 to which the pellet electrode 1 is fixed, the electrolytic solution is further injected, the negative electrode case 6 to which the sodium sheet 3 is fixed is covered, and caulking with a coin cell caulking machine. A coin cell including the gasket 5 was produced.

The battery discharge test was conducted using a charge / discharge measurement system (Hokuto Denko Corporation SD8 charge / discharge system) with a current density of 0.5 mA / cm ² per effective area of the positive electrode, a charge end voltage of 3.0 V, A charge / discharge test was performed in a voltage range of a final discharge voltage of 0.9V. The battery was produced in a glove box having an argon atmosphere with a dew point of −80 ° C. or less, and the charge / discharge test of the battery was carried out in a thermostatic bath at 25 ° C. (the atmosphere was normal air). The charge / discharge capacity (mAh / g) was standardized per weight of Mo ₆ S ₈ .

  The charge / discharge curve of the battery produced in Example 1 is shown in FIG. From the figure, it was found that the battery showed an initial discharge capacity of 140 mAh / g and an average discharge voltage of 1.4 V, and could be charged and discharged. Table 1 shows the discharge capacity retention rates at the 20th and 50th cycles.

As described above, the sodium secondary battery according to Example 1 is chargeable / dischargeable, and the discharge capacity is 4 of the theoretical discharge capacity (32 mAh / g) when 1 mol of Na is inserted into 1 mol of Mo ₆ S ₈ . It was found that about 4.4 mol of sodium was inserted. Moreover, it turned out that it has favorable cycle stability.

[Example 2]
NaFe _0.4 Ni _0.3 Mn _0.3 O ₂ which is a known material was used as the positive electrode material. NaFe _0.4 Ni _0.3 Mn _0.3 O ₂ was synthesized by firing a transition metal-containing precursor and sodium carbonate in air at 800 degrees for 24 hours. Mo ₆ S ₈ produced under the conditions of Example 1 was used as the negative electrode material. The coin cell is composed of synthesized NaFe _0.4 Ni _0.3 Mn _0.3 O ₂ , carbon powders of commercially available reagents (for example, carbon blacks such as ketjen black powder) and polytetrafluoroethylene (PTFE: manufactured by Daikin Industries, Ltd.). ) The powder was sufficiently pulverized and mixed using a roughing machine at a weight ratio of 70: 25: 5 and roll-formed to produce a sheet-pellet electrode (thickness: 0.5 mm), which was used as the positive electrode. This sheet-like electrode was cut out into a circle having a diameter of 15 mm. Mo ₆ S ₈ synthesized under the same conditions as in Example 1, carbon powder of commercially available reagents (for example, carbon blacks such as ketjen black powder) and polytetrafluoroethylene (PTFE: manufactured by Daikin Industries, Ltd.) powder, A weight ratio of 70: 25: 5 was sufficiently pulverized and mixed using a roughing machine, and roll-formed to produce a sheet pellet electrode (thickness: 0.5 mm), which was used as a negative electrode. This sheet-like electrode was cut out into a circle having a diameter of 15 mm. As the electrolyte, a solution (manufactured by Toyama Pharmaceutical Co., Ltd.) in which sodium perchlorate (NaClO ₄ ) was dissolved in propylene carbonate (PC) at a concentration of 1 mol / L was used. The separator used was a polypropylene product (manufactured by Celgard Co., Ltd.) for lithium secondary batteries.

In the same manner as in Example 1, the battery discharge test was conducted by applying 0.5 mA / cm ² at a current density per effective area of the positive electrode, using a charge / discharge measurement system, and having a charge end voltage of 4.0 V and a discharge end voltage. The charge / discharge test was conducted in a voltage range of 0.9V. The electrolytic solution is a solution (manufactured by Toyama Pharmaceutical Co., Ltd.) in which trifluoromethanesulfonylimide (NaTFSI) is dissolved in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1: 1) at a concentration of 1 mol / L. Using.

The results of the charge / discharge test are shown in Table 1. From Table 1, this battery showed an initial discharge capacity of 135 mAh / g and an average discharge voltage of 1.9 V, and was found to be chargeable / dischargeable. Table 1 shows the discharge capacity retention rates at the 20th and 50th cycles. From this result, it was found that Mo ₆ S ₈ can also be used as a negative electrode material.

[Example 3]
Mo ₆ S ₈ produced under the conditions of Example 1 was used as the positive electrode material. Amorphous carbon was used as the negative electrode material. An 8 mol / L NaOH aqueous solution was used as the aqueous electrolyte. A coin cell was manufactured under the same conditions as in Example 1 except for the above.

In the same manner as in Example 1, the battery discharge test was conducted by applying 0.5 mA / cm ² at a current density per effective area of the positive electrode, using a charge / discharge measurement system, and having a charge end voltage of 2.5 V and a discharge end voltage. 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 together with Example 1. Since the aqueous electrolyte solution is used, the discharge voltage is 1 V class, but the discharge capacity retention rate after 50 cycles also achieved a high value of 73%. Also in acidic 1mol / L _Na 2 SO ₄ aqueous solution, was confirmed to show similar results. These results indicate that Mo ₆ S ₈ according to the present invention can function as a positive electrode material even in an aqueous electrolyte. 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.

[Example 4]
Mo ₆ S ₈ produced under the conditions of Example 1 was used as the negative electrode material. NaFe _0.4 Ni _0.3 Mn _0.3 O ₂ which is a known material was used as the positive electrode material. An 8 mol / L NaOH aqueous solution was used as the aqueous electrolyte. A coin cell was manufactured under the same conditions as in Example 2 except for the above.

In the same manner as in Example 1, the discharge test of the battery was conducted by applying 0.5 mA / cm ² at the current density per effective area of the positive electrode using the charge / discharge measurement system, the charge end voltage 3.0V, and the discharge end voltage. 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 the aqueous electrolyte was used, the discharge voltage was 1 V class, but the discharge capacity retention rate after 50 cycles was 61%. Also in acidic 1mol / L _Na 2 SO ₄ aqueous solution, was confirmed to show similar results. These results indicate that Mo ₆ S ₈ 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 1]
In Comparative Example 1, NaCoO ₂ which is a known material was used as the 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. Except for the above, a coin cell was prepared and evaluated under the same conditions as in Example 1. The results are shown in Table 2 together with the results of Example 1.

  In the battery according to Comparative Example 1, the capacity decrease due to the cycle was remarkable, and only 100% of the initial discharge capacity was obtained after 100 cycles.

On the other hand, in the case of Example 1, it was found that the discharge capacity of about 68% was maintained even after 100 cycles, and the stability was high. In the case of NaCoO ₂ , Co, which is a transition metal, has been eluted, and this is considered to be due to inducing a decrease in capacity.

From the above, it was confirmed that the sodium secondary battery using Mo ₆ S ₈ according to the present invention as the positive electrode is a high-performance battery having excellent charge / discharge cycle characteristics.

[Comparative Example 2]
In Comparative Example 2, NaFe _0.4 Ni _0.3 Mn _0.3 O ₂ which is a known material was used as the positive electrode material, and NaCoO ₂ which was a known material was used as the negative electrode material. Except for the above, a coin cell was prepared and evaluated under the same conditions as in Example 2. The results are shown in Table 2 together with the results of Example 2. Compared with the battery of Example 2, the battery according to Comparative Example 2 showed superior characteristics in Example 2 both in terms of voltage and discharge capacity. Further, the capacity decrease due to the cycle was remarkable, and after 100 cycles, only the initial 12% discharge capacity was obtained.

  On the other hand, in the case of Example 2, it was found that the discharge capacity of about 41% was maintained even after 100 cycles, and the stability was relatively high.

From the above, it was confirmed that the sodium secondary battery using Mo ₆ S ₈ according to the present invention as a negative electrode is a battery having excellent 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.

Claims (3)

A positive electrode containing a sodium-containing substance capable of inserting / extracting sodium ions, a metal sodium, a metal capable of alloying with sodium metal, or a negative electrode containing a substance capable of inserting / extracting sodium ions, and sodium ions A sodium secondary battery including a conductive electrolyte,
The sodium secondary battery, wherein the negative electrode further contains Mo ₆ S ₈ . 2. The sodium secondary battery according to claim 1, wherein an organic electrolytic solution containing sodium ions is used as the electrolyte. The sodium secondary battery according to claim 1, wherein an aqueous electrolyte containing sodium ions is used as the electrolyte.

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