JP2010244827A - Lithium air battery - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium air battery serving as a high capacity secondary battery and moreover not getting conspicuous deterioration of a discharging capacity even if charge and discharge cycles are repeated. <P>SOLUTION: In the lithium air battery, oxygen in the atmosphere is used as a positive electrode active material, a metal lithium is used as a negative electrode active material and an organic solution containing lithium salt is used as an electrolyte solution. Powder of La<SB>0.56</SB>Li<SB>0.33</SB>TiO<SB>3</SB>in 40 wt. against acetylene black 30 wt. is added as an electrode catalyst to a gas diffusion electrode which is a positive electrode 1 made mainly of acetylene black, a kind of carbon. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a lithium air battery.

  Commercial zinc-air batteries have a large discharge capacity of about 300 mAh / g per battery weight, and are therefore mainly used for hearing aids. However, compared to a lithium ion battery using a non-aqueous electrolyte, only a voltage of 1V can be obtained, so it is considered difficult to use it widely. In recent years, by using electrochemical reduction of oxygen similar to a zinc-air battery as a positive electrode reaction system, by combining metal lithium instead of zinc as a negative electrode and using a non-aqueous solution medium as an electrolyte, Attempts have been made to make it.

  As reported in the following Patent Document 1 and Non-Patent Document 1, attempts have been made to improve battery performance in terms of discharge capacity and cycle characteristics by adding various catalysts to a gas diffusion electrode as a positive electrode. It has been.

Regarding the electrode catalyst, Patent Document 1 discusses phthalocyanine derivatives and naphthocyanine derivatives, and Non-Patent Document 1 mainly examines transition metal oxides such as Fe ₂ O ₃ and Co ₃ O ₄ . As a result, Patent Document 1 reports that particularly large current discharge characteristics are improved. However, charge / discharge cycle characteristics as a secondary battery are not clearly shown. On the other hand, in Non-Patent Document 1, although the discharge voltage is 2.5 to 3.0 V, which is lower than the current lithium ion battery, a very large discharge capacity of 1000 to 3000 mAh / g is obtained per weight of carbon contained in the positive electrode. Yes. However, when charging and discharging are repeated, the discharge capacity is remarkably reduced. For example, in the case of Co ₃ O ₄ , the capacity retention rate is 65% in 10 cycles, and the capacity is significantly reduced. It is not done.

JP 2004-063262 A

A. Debart et al., Journal of Power Sources, Vol. 174, pp. 1177-1182 (2007).

  The problem to be solved by the present invention is to provide a lithium-air battery that operates as a high-capacity secondary battery and does not show a significant decrease in discharge capacity even after repeated charge / discharge cycles.

In order to solve the above problems, the present invention provides a method as described in claim 1.
A positive electrode mainly composed of carbon in a lithium air battery using oxygen in the air as a positive electrode active material, using metallic lithium or a lithium-containing alloy as a negative electrode active material, and an organic solution containing a lithium salt as an electrolyte. A lithium-air battery is characterized in that a solid electrolyte compound having lithium ion conductivity is added as an electrode catalyst to the gas diffusion type electrode.

In the present invention, as described in claim 2,
The lithium air battery according to claim 1, wherein La _{2 / 3-x} Li _3x TiO ₃ (where 0 <x <2/3) is used as the electrode catalyst.

In the present invention, as described in claim 3,
3. The lithium air battery according to claim 2, wherein x is in a range of 0.10 ≦ x ≦ 0.12.

  According to the present invention, by using a highly active gas diffusion electrode catalyst for the charge / discharge reaction, it operates as a high-capacity secondary battery, and a significant decrease in discharge capacity is observed even after repeated charge / discharge cycles. Lithium-air batteries that are not possible can be provided.

It is sectional drawing of a lithium air battery. 1 is a diagram illustrating a charge / discharge curve of a lithium air battery according to Example 1. FIG. It is a figure which shows the cycling characteristics of the lithium air battery in Example 1 and Comparative Example 1.

  In the present invention, in order to solve the above-described problems and achieve the object, oxygen in the air is used as the positive electrode active material, metal lithium or a lithium-containing alloy is used as the negative electrode active material, and an organic solution containing a lithium salt as the electrolytic solution. In a lithium-air battery using a solution, a solid electrolyte compound having lithium ion conductivity is added as an electrode catalyst to a gas diffusion electrode that is a positive electrode mainly composed of carbon. Here, the lithium ion conductivity is conductivity generated by movement of lithium ion ions. By adding a solid electrolyte compound having lithium ion conductivity to the positive electrode, the added solid electrolyte acts as a catalyst, thereby realizing a lithium-air battery having high capacity and battery performance with excellent charge / discharge characteristics.

  A compound having Li ion conductivity as a solid electrolyte has the ability to incorporate Li ions into a crystal structure or an amorphous structure. By using it as a catalyst, oxygen-electrolyte-electrode (catalyst) It has a high ability to trap (occlude or adsorb) Li ions, which are one of the reactive species, at an active site, which is a three-phase interface composed of: and the electrode reaction proceeds smoothly.

  The lithium-air battery according to the present invention comprises a positive electrode (gas diffusion electrode) having carbon and a binder as constituent elements, and a negative electrode having a lithium-containing alloy capable of releasing and absorbing metallic lithium or lithium ions as constituent elements. And an organic electrolyte is disposed between the positive electrode and the negative electrode.

  In the present invention, in order to achieve high performance of the lithium-air battery, as described above, the positive electrode is provided with a highly active electrode catalyst for both oxygen reduction (discharge) and oxygen generation (charge) reactions. Added. The electrode catalyst forms a three-phase interface site where the electrode catalyst-electrolyte solution-gas (oxygen) coexists when the electrolyte solution penetrates into the positive electrode and oxygen gas in the atmosphere is supplied at the same time. If the electrode catalyst is highly active, oxygen reduction (discharge) and oxygen generation (charge) proceed smoothly, and battery performance is greatly improved.

The reaction (discharge reaction) on the positive electrode can be expressed as follows.

2Li ⁺ + (1/2) O ₂ + 2e ⁻ → Li ₂ O (1)

2Li ⁺ + O ₂ + 2e ⁻ → Li ₂ O ₂ (2)

Lithium ions (Li ⁺ ) in the above formula are dissolved in the electrolytic solution from the negative electrode by electrochemical oxidation, and move in the electrolytic solution to the surface of the positive electrode. Furthermore, oxygen (O ₂₎ are those taken into the gas diffusion electrode from the atmosphere.

When using a lithium ion conductive solid electrolyte as an electrode catalyst, a lithium ion is trapped on the catalyst surface or in the lattice, represented by Li ⁺ -O ^2- -Li ⁺ and Li ⁺ -O ₂ -Li ⁺ An intermediate reactant of formula (1) and (2) is formed. Since there are many active sites due to such reaction intermediates in the electrode, the electrode reaction can proceed efficiently. In addition, regarding the charging reaction that is the reverse reaction of the formulas (1) and (2), the electrode catalyst has activity, and the charging of the battery, that is, the oxygen generation reaction on the positive electrode proceeds efficiently.

The solid electrolyte compound to be added as a catalyst is not particularly limited as long as it has lithium ion conductivity. For example, a nitride such as Li ₃ N, Li _x La _{(1-x) / 3} NbO ₃ (0 <x < 1), metal oxides such as La _{2 / 3-x} Li _3x TiO ₃ (0 <x <2/3), thio-LISICON (Li _4−x Ge _1−x P _x S ₄ , 0.6 ≦ x ≦ 0.8) or a lithium-containing glass compound containing phosphorus or sulfur can be used.

In particular, as the electrode catalyst, La _2-3-x Li _3x TiO ₃ (0 <x <2/3) is preferable from the viewpoint of conductivity and stability, and the entire composition range (0 <x <2 / Among the 3), high catalytic activity is obtained in the composition range of 0.10 ≦ x ≦ 0.12 which shows the highest conductivity as a solid electrolyte and has a high ability to trap lithium ions on the catalyst surface.

As a method for synthesizing these catalysts, a known process such as a solid phase method or a liquid phase method can be used. However, it is important to generate a large amount of three-phase interface sites on the surface of the electrode catalyst. A high surface area is desirable and the specific surface area after firing is preferably 10 m ² / g or more. Therefore, an amorphous precursor is obtained by evaporating and drying a mixed aqueous solution of metal acetate or metal nitrate or by hydrolysis of metal alkoxide. It is desirable to use a wet method typified by a technique for obtaining a body.

  In order to form a gas diffusion electrode (positive electrode), a mixture of a catalyst powder, carbon powder and a binder powder such as polytetrafluoroethylene (PTFE) is formed on a support such as a titanium mesh, or the above-mentioned The mixture is dispersed in a solvent such as an organic solvent to form a slurry, which is then applied onto a metal mesh or carbon cloth and dried, and one side of the electrode is exposed to the atmosphere, and the other side Is in contact with the electrolyte. Further, in order to increase the strength of the electrode and prevent leakage of the electrolytic solution, it is possible to produce an electrode having higher stability by performing not only cold pressing but also hot pressing. As the binder, not only the above PTFE but also a powder or dispersion such as polyvinylidene fluoride and polybutadiene rubber can be used.

As an active material of the negative electrode, metallic lithium or lithium nitride such as lithium-containing silicon or tin alloy or Li _2.6 Co _0.4 N, which is a material capable of releasing and absorbing lithium ions Any material that can be used as a negative electrode material for a lithium secondary battery can be used. However, when silicon or tin that does not contain lithium is used as a starting material, it is necessary to make these materials contain lithium in advance by chemical treatment or electrochemical treatment.

The reaction of the negative electrode (metallic lithium) during discharge can be expressed as follows.

Li → Li ⁺ + e ⁻ (3)

The electrolyte solution may be any substance that can move lithium ions between the positive and negative electrodes. A non-aqueous solvent in which a metal salt containing lithium ions is dissolved can be used. A molecular electrolyte or an ionic liquid in which a lithium metal salt is dissolved can also be used.

  For the elements such as the separator and the battery case, conventionally known various materials can be used and there is no particular limitation.

  Embodiments of a lithium-air 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 what was shown to the following Example, In the range which does not change the summary, it can change suitably and can implement.

To synthesize _{La 0.56 Li 0.33 TiO 3 (x} = 0.11) having high lithium ion conductivity was added to the positive electrode as an electrode catalyst. The titanate was synthesized by mixing lithium carbonate, lanthanum oxide, and titanium dioxide at a molar ratio in the synthesis target compound, and further mixing and pulverizing using an agate mortar. The resulting mixture had a diameter of 10 mm. The coin mold was uniaxially pressed to form a pellet. The pellets were heat treated at 1150 ° C. for 12 hours. The pellets after the heat treatment were pulverized and mixed in the same manner as described above, and further subjected to heat treatment. These steps were repeated so that the heat treatment was performed 4 times. The finally obtained pellet was sufficiently pulverized and used as an electrode catalyst. The obtained powder was confirmed to be La _0.56 Li _0.33 TiO ₃ single phase by X-ray diffraction measurement.

  Next, a method for producing a gas diffusion type carbon positive electrode and a lithium air battery cell will be described.

La _0.56 Li _0.33 TiO ₃ powder, acetylene black powder, which is a kind of carbon, and polytetrafluoroethylene (PTFE) powder are sufficiently pulverized in a weight ratio of 40:30:30 using a coarse machine. It mixed, roll-formed, and produced the sheet-like electrode (thickness 0.5mm). The sheet electrode was cut into a circle with a diameter of 23 mm and pressed onto a titanium mesh to obtain a gas diffusion electrode.

FIG. 1 shows a cross-sectional view of a cylindrical lithium-air battery cell. The positive electrode 1 was disposed in a concave portion of the positive electrode support 2 coated with PTFE, and fixed with a positive electrode fixing PTFE ring 3. Note that the portion where the positive electrode 1 and the positive electrode support 2 are in contact with each other is not covered with PTFE in order to make electrical contact. Moreover, the effective area of the electrode which the positive electrode 1 and air contact is 2 cm < ² >.

Next, a separator 5 for a lithium secondary battery was disposed on the bottom surface of the recess on the side opposite to the surface of the positive electrode 1 that contacts the atmosphere. The negative electrode fixing washer 7 was pressure-bonded onto four metallic lithium foils (effective area 2 cm ² ) having a thickness of 150 μm as the negative electrode 8 in a concentric manner. The negative electrode fixing PTFE ring 6 was disposed in a concave portion opposite to the concave portion in which the positive electrode 1 is disposed, and a negative electrode fixing washer 7 having metallic lithium bonded thereto was further disposed in the center. The O-ring 9 was set as shown in the figure. Inside the cell, the organic electrolyte is 10 1 mol / l (mole / liter) of hexafluorophosphate lithium / propylene carbonate (LiPF ₆ / PC) solution was filled with, covered with a negative electrode support 11, cells fixed The entire cell was fixed with the screw 12. The positive electrode terminal 4 and the negative electrode terminal 13 were used for the battery performance measurement test.

Cycle test of the battery using the charge and discharge measurement system, energizes the 0.1 mA / cm ² at a current density per effective area of the positive electrode, the battery voltage from the open circuit voltage was measured until reduced to 2.0 V. Charging was performed at the same current density until the battery voltage increased to 4.5V.

  The battery was manufactured in dry air with a dew point of -60 ° C. or lower, and the battery charge / discharge test was performed in a normal living environment.

  The charge / discharge capacity is represented by a value (mAh / g) per weight of carbon (acetylene black), and FIG. 2 shows the first discharge / charge curve and the second discharge curve.

FIG. 2 shows that the lithium air battery using the La _0.56 Li _0.33 TiO ₃ catalyst exhibits a very flat discharge voltage characteristic unique to a 2.7 V air battery. In addition, the initial discharge capacity was as large as about 1700 mAh / g, and it was found that the battery had a very large energy density. Moreover, although discharge after the first charge shows an overshoot in the initial stage, it shows the same voltage as the first time, and it turns out that it is operating as a secondary battery. Further, the second discharge capacity was larger than the first discharge capacity, and showed a large value of about 2600 mAh / g. This is considered to be because the wettability of the carbon surface was improved by performing the charge / discharge cycle, and the electrolyte solution penetrated into the electrode, and stable discharge behavior was shown in the subsequent cycles.

  Next, the cycle characteristics of this battery are shown in FIG. 3 together with the cycle characteristics of Comparative Example 1 below. In the 50 cycles of measurement, a large discharge capacity of about 2500 mAh / g was shown except for the first time. From the second time to the 50th time, the maintenance rate of the discharge capacity was about 96%, and it was confirmed that it had excellent charge / discharge cycle characteristics.

These _results are La _{0.56 Li} 0.33 TiO ₃ shows acting effectively to charge and discharge reactions as a positive electrode catalyst for a lithium-air battery.

[Comparative Example 1]
A lithium air battery was produced in the same manner as in Example 1 using Co ₃ O ₄ which is known as a positive electrode catalyst. Co ₃ O ₄ used a commercially available reagent. The conditions of the battery cycle test are the same as in Example 1.

  The cycle characteristics of the lithium air battery according to this comparative example are shown in FIG.

  FIG. 3 shows that the initial discharge capacity is about 2500 mAh / g, which is almost the same as that of Example 1. However, when the charge / discharge cycle was repeated, unlike in Example 1, a large decrease in discharge capacity was observed, and the capacity retention rate after 20 cycles was about 20%.

  From these results, it was confirmed that the electrode catalyst according to the present invention is more excellent in charge / discharge cycle characteristics than known materials and can be used effectively as a positive electrode catalyst for a lithium air battery.

  By using the electrode catalyst according to the present invention, a lithium-air battery having high energy density and excellent charge / discharge cycle performance can be produced, and this can be used as a drive source for various electronic devices.

  1: positive electrode (gas diffusion type electrode), 2: positive electrode support (PTFE coating), 3: positive electrode fixing PTFE ring, 4: positive electrode terminal, 5: separator, 6: negative electrode fixing PTFE ring, 7: negative electrode fixing Washer, 8: negative electrode, 9: O-ring, 10: organic electrolyte, 11: negative electrode support, 12: screw for fixing cells (PTFE coating), 13: negative electrode terminal.

Claims (3)

In a lithium air battery using oxygen in the air as a positive electrode active material, using metal lithium or a lithium-containing alloy as a negative electrode active material, and using an organic solution containing a lithium salt as an electrolyte,
A lithium-air battery, wherein a solid electrolyte compound having lithium ion conductivity is added as an electrode catalyst to a gas diffusion electrode which is a positive electrode mainly composed of carbon. The lithium air battery according to claim 1, wherein La _{2 / 3-x} Li _3x TiO ₃ (where 0 <x <2/3) is used as the electrode catalyst.   3. The lithium air battery according to claim 2, wherein x is in a range of 0.10 ≦ x ≦ 0.12.

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