JP2015053136A - Lithium air battery and positive complex - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium air battery that enables continuous heavy-current discharge.SOLUTION: A lithium air battery comprises at least a positive complex that uses oxygen as a cathode active substance, an anode that uses lithium as an anode active substance, and an aqueous electrolyte that is interposed between a cathode and the anode. The aqueous electrolyte includes lithium chloride and a boric acid, a phosphoric acid or a combination of them. The positive complex comprises at least the cathode, the boric acid and a metal catalyst.

Description

  The present invention relates to a lithium-air battery and a positive electrode composite, and more particularly to a lithium-air battery that enables continuous high-current discharge.

  A lithium air battery that uses oxygen in the air as the positive electrode active material and lithium as the negative electrode active material has a theoretically high energy density. Therefore, a lithium air battery is expected as a battery capable of obtaining an energy density several times higher than that of a lithium ion battery required for full-scale spread of electric vehicles.

  Lithium-air batteries are broadly classified into batteries using aqueous electrolytes and batteries using non-aqueous electrolytes, depending on the type of electrolyte. As the aqueous electrolyte, a lithium hydroxide aqueous solution, a potassium hydroxide aqueous solution, or the like is used. Moreover, ethylene carbonate etc. are used as a non-aqueous electrolyte.

  The mainstream of lithium-air battery research and development is a battery using a non-aqueous electrolyte. This is because the structure of the battery is simple and the technology of the lithium ion battery can be used except for the positive electrode.

  On the other hand, a lithium-air battery using a water-based electrolyte has been studied although the number is still small (for example, Patent Document 1). Lithium air batteries using aqueous electrolytes have the advantage that the aqueous electrolyte used is cheaper and nonflammable compared to lithium air batteries using non-aqueous electrolytes.

  Generally, in a battery, if the current is the same, the electric power (W) expressed as current (A) × voltage (V) increases by increasing the discharge voltage. Therefore, the amount of electric power (Wh) expressed by electric power (W) × time (h) is also increased, so that the discharge electric energy of the battery can be increased by increasing the discharge voltage. That is, in order to improve the electric energy of the battery, it is necessary to increase the initial voltage.

  Here, the reaction of the positive electrode and the negative electrode during discharge of the lithium-air battery is as shown in the following formulas (1) and (2).

At the time of discharging, Li ⁺ generated by the formula (2) and OH ⁻ generated by the formula (1) react to generate lithium hydroxide (LiOH). When lithium hydroxide is dissolved in the aqueous electrolyte, the aqueous electrolyte becomes alkaline. Here, when the pH of the aqueous electrolyte rises and becomes alkaline, the activity of the positive electrode is lowered, and the amount of OH ^{− in} the formula (1) is reduced. That is, when the aqueous electrolyte becomes alkaline, the reaction of the formula (1) does not easily occur, and the amount of electrons e ⁻ decreases, so the current value decreases. When the current value is large, the pH rises momentarily and partially, making it difficult to continuously discharge.

Japanese Patent No. 4298234

  In view of the above problems, an object of the present invention is to provide a lithium-air battery capable of continuously discharging a large current.

  In order to solve the above problems, the present inventor has intensively studied a lithium water-based air battery capable of discharging a large current. As a result, it has been found that by making the pH of the aqueous electrolyte weakly acidic, the activity of the metal catalyst of the positive electrode composite is increased and the reaction at the positive electrode is promoted, so that a large current can be discharged. Furthermore, if a predetermined inorganic acid is contained in the positive electrode composite, the pH of the aqueous electrolyte can be kept weakly acidic, and a large current discharge can be continued. The present inventor has come up with the present invention from these findings.

  That is, the lithium-air battery according to the present invention includes at least a positive electrode composite having oxygen as a positive electrode active material, a negative electrode having lithium as a negative electrode active material, and an aqueous electrolyte interposed between the positive electrode and the negative electrode. And the aqueous electrolyte includes lithium chloride and boric acid or phosphoric acid, or a combination thereof, and the positive electrode composite is a lithium-air battery including at least a positive electrode, boric acid, and a metal catalyst. is there.

  In another aspect, the present invention is a positive electrode composite used for a lithium-air battery, and the positive electrode composite includes at least a positive electrode, boric acid, and a metal catalyst.

  According to the lithium air battery of the present invention, it is possible to continuously discharge a large current.

It is a cross-sectional schematic diagram of the lithium air battery 1 of the present invention. 2 is a schematic diagram of a positive electrode composite 2. FIG. It is a figure which shows the discharge characteristic of a lithium air battery.

  Hereinafter, the general form of the present invention will be described in detail. However, this invention is not limited by the form demonstrated below.

  The lithium-air battery of the present invention includes at least a positive electrode composite, a negative electrode, and an aqueous electrolyte. The positive electrode composite uses oxygen as a positive electrode active material, and the negative electrode uses lithium as a negative electrode active material.

  The aqueous electrolyte is interposed between the positive electrode and the negative electrode, and includes lithium chloride, boric acid, phosphoric acid, or a combination thereof. The electrolyte is a battery that generates direct-current power by being interposed between the positive electrode and the negative electrode. And lithium chloride precipitates LiOH produced at the time of discharge, and prevents the increase in pH due to excessive dissolution.

Boric acid and phosphoric acid are inorganic acids that make the pH of the aqueous electrolyte weakly acidic. Since the aqueous electrolyte becomes weakly acidic, the reaction at the positive electrode is promoted, so that a large current can be discharged. Boric acid and phosphoric acid can contain only one or in combination. The pH of the aqueous electrolyte gradually increases as Li ⁺ generated by the formula (2) and OH ⁻ generated by the formula (1) react to generate lithium hydroxide during discharge. In order to continue discharging with a large current, it is necessary to maintain the pH of the aqueous electrolyte in a weakly acidic region for a long time. For that purpose, it is preferable that the concentration of the inorganic acid is high, and it is more preferable that a saturated amount of the inorganic acid is dissolved in the aqueous electrolyte.

The lithium air battery of the present invention can include an organic electrolyte and a solid electrolyte in addition to the positive electrode composite, the negative electrode composite, and the aqueous electrolyte. The organic electrolyte is interposed between the negative electrode and the solid electrolyte and serves as a path for lithium ions (Li ⁺ ) dissolved from lithium of the negative electrode, and examples thereof include a dry polymer electrolyte composed of a polymer. In addition, the solid electrolyte is interposed between the organic electrolyte and the aqueous electrolyte and selectively passes only Li ⁺ from the negative electrode to the aqueous electrolyte. For example, LTAP (Li _{1 + x + y} Ti _2-x Al _{x of} glass ceramics). _{P 3-y Si y O 12} ) can be mentioned.

  In the present invention, the positive electrode composite includes at least a positive electrode, boric acid, and a metal catalyst. Boric acid gradually dissolves in the aqueous electrolyte, so that the pH of the aqueous electrolyte can be kept weakly acidic, and a large current discharge can be continued. Moreover, a metal catalyst is a catalyst which can accelerate | stimulate reaction of Formula (1) in a positive electrode. Examples of the metal catalyst include platinum-supported carbon powder and transition metal oxide. Boric acid can increase the activity of the metal catalyst by arranging it in the vicinity of the metal catalyst.

  In the present invention, the positive electrode is preferably a carbon cloth, carbon non-woven fabric, or carbon paper. These materials are suitable for use as a positive electrode of a lithium air battery because they are porous for taking in oxygen in the air, have conductivity as a current collector, and have corrosion resistance to withstand alkaline electrolyte.

  The positive electrode composite can be formed, for example, by binding boric acid to carbon powder having carbon cloth as a positive electrode and carrying platinum on the surface of the carbon cloth. As a specific example of the manufacturing method of the positive electrode composite, first, carbon powder carrying platinum, boric acid powder, and polyvinylidene fluoride (PVDF) as a binder are mixed to form a mixed powder, and the mixed powder is used as a solvent. N-methylpyrrolidone (NMP) is added to form a paste. Next, the paste is evenly applied to the carbon cloth with a thickness of 200 μm to 400 μm, heated to about 90 ° C. and vacuum dried. Through these steps, the positive electrode composite can be produced.

  As the binder, polytetrafluoroethylene (PTFE), carboxymethylcellulose (CMC), or the like can be used in addition to the PVDF. As the solvent, in addition to NMP, an organic solvent that can dissolve the binder, such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), tetrahydrofuran (THF), or the like can be used.

In the lithium-air battery of the present invention, the negative electrode is made of metallic lithium, Li ₄ SiO ₄ , Li ₇ Sn ₃ , LiSn, Li ₂ Sn ₅ , Li ₂ SO ₄ H ₂ O, Mg-9% Li, LiAlH ₄ , LiBH _4. Or LiC ₆ . If these lithium compounds are used, the reaction of the formula (2) can be caused and a negative electrode can be obtained.

  Hereinafter, embodiments of the lithium-air battery of the present invention will be described with reference to the drawings. In this case, the present invention is not limited to the embodiments of the drawings.

  FIG. 1 is a schematic cross-sectional view of a lithium air battery 1 of the present invention. A positive electrode composite 2 using oxygen as a positive electrode active material, a negative electrode 3 using lithium as a negative electrode active material, and an aqueous electrolyte 4 interposed between the positive electrode composite 2 and the negative electrode 3 are provided. The organic electrolyte 5 is interposed between the negative electrode 3 and the solid electrolyte 6, and the solid electrolyte 6 is interposed between the organic electrolyte 5 and the aqueous electrolyte 4.

Like the reaction shown in the following formula (2), lithium in the negative electrode 3 is dissolved in the organic electrolyte 5 to become Li ⁺ , and the electron e ⁻ is supplied to the positive electrode composite 2 through the conductive wire 7. The dissolved Li ⁺ passes through the solid electrolyte 6 and moves to the aqueous electrolyte 4.

Like the reaction shown in the following formula (1), oxygen in the air, water in the aqueous electrolyte 4 and the electron e ⁻ supplied from the negative electrode 3 react to generate hydroxide ions (OH ⁻ ). . This OH ⁻ and Li ⁺ moved from the negative electrode react to form lithium hydroxide (LiOH).

When lithium hydroxide is dissolved in the aqueous electrolyte and the pH of the aqueous electrolyte is increased to become alkaline, the activity of the positive electrode is lowered, and the amount of OH ⁻ of the formula (1) is reduced. Therefore, an inorganic acid is dissolved in the aqueous electrolyte 4 in order to keep the aqueous electrolyte 4 weakly acidic.

  FIG. 2 is a schematic diagram of the positive electrode composite 2. A metal catalyst 9 and boric acid 10 are bound to the surface of the positive electrode 8 by a binder 11. The metal catalyst 9 promotes the reaction of the above formula (1). On the other hand, the boric acid 10 is gradually dissolved in the aqueous electrolyte 4 to suppress an increase in pH of the aqueous electrolyte 4 due to the dissolution of lithium hydroxide and to maintain weak acidity.

  As described above, according to the lithium-air battery of the present invention, a large current can be discharged by making the pH of the aqueous electrolyte weakly acidic. Furthermore, by containing an inorganic acid in the positive electrode composite, the pH of the aqueous electrolyte can be kept weakly acidic, and a large current discharge can be continued.

  EXAMPLES Hereinafter, although an Example etc. are shown and this invention is demonstrated concretely, this invention is not limited to these.

[Production of aqueous electrolyte]
A boric acid powder (4.328 g) was dissolved in ion-exchanged water (100 ml) to prepare a 0.7M saturated boric acid solution, and then lithium chloride (8.4 g) was added to prepare an aqueous electrolyte A.

  Further, 8.4 g of lithium chloride was added to 100 ml of ion-exchanged water without adding boric acid to prepare an aqueous electrolyte B.

[Preparation of positive electrode composite]
60 mg of a metal catalyst (TEC10E50E manufactured by Tanaka Kikinzoku Co., Ltd.) carrying platinum powder (average particle diameter 3 nm) on carbon powder (average particle diameter 50 μm) 45.8 mass%, boric acid powder (average particle diameter 100 μm) 60 mg, and 25 mg of polyvinylidene fluoride (Wako Pure Chemical Industries, Ltd.) was weighed into a beaker and mixed to obtain a mixed powder. To this mixed powder, 1.5 ml of N-methylpyrrolidone was added, and 35 KHz ultrasonic vibration was added at room temperature for 30 minutes with an ultrasonic cleaner (manufactured by ASONE) to obtain a catalyst paste. The paste was applied to the carbon cloth so that the thickness was equal to 300 μm by placing a square formwork on the carbon cloth and pouring the paste onto the carbon cloth. Thereafter, the carbon cloth to which the paste was applied was vacuum-dried at 90 ° C. for 1 hour to produce a positive electrode composite A.

  Moreover, the positive electrode composite B was produced by the method similar to the positive electrode composite A except not adding a boric acid powder.

[Manufacture of lithium-air batteries]
Glass ceramic LTAP was used as a solid electrolyte, an SBR rubber adhesive was adhered to the end of the solid electrolyte, and an aluminum laminate (PP resin / Al / PET resin) packaging material was adhered. Then, the metallic lithium attached to one side of the copper foil as the negative electrode in an Ar gas atmosphere of an inert gas is placed between the aluminum laminate wrapping materials, and between the glass ceramics and the metallic lithium, the cellulose separator is organic. A protective layer soaked with the electrolyte was disposed. In order to obtain a sealed structure, the four sides of the aluminum laminate packaging material were thermally welded and sealed. In addition, as for metallic lithium, a composite negative electrode having a thickness of 200 μm and an area of 0.25 cm ² and corresponding to 21 mh was prepared. Comparative Example 1 using aqueous electrolyte A and positive electrode composite B, 500 μl of aqueous electrolyte A in a cellulose separator on LTAP of the composite negative electrode, placing positive electrode composite B thereon, and using aqueous electrolyte A containing boric acid A lithium air battery was manufactured.

  Using the aqueous electrolyte A and the positive electrode composite A, the lithium-air battery of Example 1 using the aqueous electrolyte containing boric acid and the positive electrode composite containing boric acid was manufactured by the same process as in Example 1.

  Using the aqueous electrolyte B and the positive electrode composite B, a lithium air battery of Comparative Example 2 containing no boric acid was produced by the same process as in Example 1.

[Evaluation of discharge characteristics]
Lithium-air batteries of Example 1 and Comparative Examples 1 and 2 were connected by connecting the tab of the produced air battery to a charge / discharge device (ALS electrochemical analyzer) and performing constant current discharge at the current density shown in Table 1. The discharge characteristics of were evaluated. The results are shown in Table 1 and FIG.

The current density increases as the discharge rate increases. Table 1 shows the results of measuring the initial voltage and the voltage after 300 seconds of discharge, and FIG. 3 shows the results of the initial voltage. Further, in Table 1, “x” indicates that the voltage could not be measured. From the results of Table 1 and FIG. 3, when the discharge rate was 12 mA / cm ² where the voltage could not be measured in the lithium air battery of Comparative Example 2, the voltage was measured in Comparative Example 1 and the air battery of Example 1. We were able to. Moreover, only the lithium air battery of Example 1 was able to measure the voltage when the discharge rate was 16 mA / cm ² .

  Further, if the open circuit voltage is high, the voltage at the time of discharge is high even with the same resistance, which is advantageous. From the result of the open circuit voltage, the open circuit voltage of Example 1 was higher than those of Comparative Examples 1 and 2.

  From the above results, by adding boric acid to the aqueous electrolyte, continuous discharge in a state where the current density is large is possible, and by further mixing boric acid into the positive electrode composite, in a state where the current density is higher It was found that continuous discharge becomes possible.

  According to the present invention, a lithium-air battery capable of continuously discharging a large current can be provided, which is industrially useful.

DESCRIPTION OF SYMBOLS 1 Lithium air battery 2 Positive electrode composite body 3 Negative electrode 4 Water-system electrolyte 5 Organic electrolyte 6 Solid electrolyte 7 Conductor 8 Positive electrode 9 Metal catalyst 10 Boric acid 11 Binder

Claims (4)

A positive electrode composite having oxygen as a positive electrode active material, a negative electrode having lithium as a negative electrode active material, and an aqueous electrolyte interposed between the positive electrode and the negative electrode,
The aqueous electrolyte includes lithium chloride, boric acid, phosphoric acid, or a combination thereof,
The positive electrode composite is a lithium air battery including at least a positive electrode, boric acid, and a metal catalyst.   The lithium-air battery according to claim 1, wherein the positive electrode is one of carbon cloth, carbon non-woven fabric, or carbon paper. The negative electrode is any one of metallic lithium, Li ₄ SiO ₄ , Li ₇ Sn ₃ , LiSn, Li ₂ Sn ₅ , Li ₂ SO ₄ H ₂ O, Mg-9% Li, LiAlH ₄ , LiBH ₄ or LiC _6. The lithium air battery according to claim 1 or 2.   A positive electrode composite used for a lithium air battery including at least a positive electrode, boric acid, and a metal catalyst.

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