JP6302424B2 - Lithium air secondary battery - Google Patents

Description

  The present invention relates to a lithium air secondary battery. The present invention relates to a lithium-air secondary battery that is smaller and lighter than a conventional secondary battery such as a lead storage battery or a lithium ion battery and that can realize a discharge capacity that is much larger.

  A lithium-air secondary battery that uses oxygen in the air as a positive electrode active material is always supplied with oxygen from the outside of the battery, and a large amount of metallic lithium, which is a negative electrode active material, can be filled in the battery. For this reason, it has been reported that the value of the discharge capacity per unit volume of the battery can be greatly increased.

  As reported in Non-Patent Document 1 and Non-Patent Document 2 so far, battery performance such as discharge capacity and cycle characteristics is improved by adding various catalysts to the gas diffusion air electrode as the positive electrode. Attempts have been made.

Transition metal oxides have been studied as electrode catalysts for gas diffusion air electrodes. For example, in the above document, transition metal oxides such as λ-MnO ₂ are used in Non-Patent Document 1, and iron oxide (Fe ₂ O ₃ ), cobalt oxide (Co ₃ O ₄ ) and the like are mainly used in Non-Patent Document 2. Transition metal oxides have been investigated.

J. Read, Journal of The Electrochemical Society, Vol.149, pp.A1190-A1195 (2002). Aurelie Debart et al., Journal of Power Sources, Vol.174, pp.1177-1182 (2007).

  Non-Patent Document 1 and Non-Patent Document 2 show the results of the battery characteristic test of the lithium air secondary battery as follows.

  In the secondary battery disclosed in Non-Patent Document 1, a charge / discharge cycle was possible, but after 4 cycles, the discharge capacity was reduced to about 1/4, and the performance as a secondary battery was low. . Further, in the secondary battery disclosed in Non-Patent Document 1, the charging voltage is about 4.0V, which is very large compared to the average discharge voltage of 2.7V, and the charge / discharge energy efficiency is low. There is.

On the other hand, in Non-Patent Document 2, nine types of catalysts are examined, and a very large discharge capacity of 1000 to 3000 mAh / g is obtained per weight of carbon contained in the air electrode. 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 becomes about 65% after 10 cycles. As described above, the lithium-air secondary battery of Non-Patent Document 2 also shows a significant decrease in capacity, and sufficient characteristics as a secondary battery are not obtained. In most cases, the average discharge voltage is about 2.5 V, while the charging voltage is 4.0 to 4.5 V, and the lowest is about 3.9 V. For this reason, the charge / discharge energy efficiency of the lithium-air secondary battery of Non-Patent Document 2 is low.

  An object of the present invention is to provide a lithium-air secondary battery that operates as a high-capacity secondary battery, has a small charge / discharge voltage difference, and has a small decrease in discharge capacity even after repeated charge / discharge cycles.

  An example of means for solving the problems of the present invention is a lithium-air secondary battery including an air electrode, a negative electrode including lithium, and an electrolyte, and the electrolyte is in contact with the air electrode and the negative electrode, In the lithium-air secondary battery, the air electrode includes a catalyst, and the catalyst includes molybdenum phosphide (MoP).

Here, it is preferable that the molybdenum phosphide contains sulfur (S). Moreover, it is preferable that content of the said sulfur is 1-50% or 20-40% of the whole said molybdenum phosphide containing the said sulfur by atomic ratio. Further, the BET specific surface area of the molybdenum phosphide is preferably 30 m ² / g or more, 40 m ² / g or more, or 50 m ² / g or more.

  Another example of means for solving the problems of the present invention is a lithium air secondary battery cell including the above-described lithium air secondary battery.

  According to the lithium air secondary battery of the present invention described above, the battery performance can be improved. According to the lithium air secondary battery of the present invention, it is possible to realize cycle characteristics superior to those of the prior art, and further improve energy efficiency and the like. Specifically, it is possible to provide a lithium-air secondary battery that has a small charge / discharge voltage difference and that can suppress a decrease in discharge capacity even after repeated charge / discharge cycles.

1 is a basic conceptual diagram of a lithium air secondary battery according to the present invention. It is a schematic longitudinal cross-sectional view for showing the structure of the lithium air secondary battery cell used for the measurement in an Example. It is a graph which shows the first charging / discharging curve of the lithium air secondary battery of Example 1 and Example 2. FIG. It is a figure which shows the cycle dependence of the discharge capacity of the lithium air secondary battery of Example 1, Example 2, and Comparative Example 1. FIG.

  Hereinafter, an embodiment of the lithium-air secondary battery of the present invention will be described in detail with reference to the drawings.

[Configuration of lithium-air secondary battery]
As shown in FIG. 1, the lithium-air secondary battery 100 of this embodiment includes at least an air electrode (positive electrode) 101, a negative electrode 102, and an electrolyte (for example, an organic electrolyte) 103. The air electrode 101 functions as a positive electrode. The electrolyte 103 is in contact with the air electrode 101 and the negative electrode 102. The electrolyte 103 can be disposed between the air electrode 101 and the negative electrode 102.

Each of the above components will be described below.
(I) Air electrode (positive electrode)
The air electrode 101 includes at least a catalyst.

(I-1) Catalyst The air electrode 101 contains molybdenum phosphide (MoP) as a catalyst (electrode catalyst). In particular, by including molybdenum phosphide, the air electrode 101 becomes highly active with respect to both reactions of oxygen reduction (discharge) and oxygen generation (charge), and the performance as a secondary battery can be enhanced.

  In the air electrode 101, an electrode reaction proceeds at a three-phase interface site of electrolyte / electrode catalyst / air (oxygen). That is, the electrolyte 103 (typically an organic electrolyte solution) penetrates into the air electrode 101, and at the same time, oxygen gas in the atmosphere is supplied to form a three-phase interface site in which electrolyte-electrode catalyst-air (oxygen) coexists. Is done. If the electrode catalyst is highly active, oxygen reduction (discharge) and oxygen generation (charge) proceed smoothly, and battery performance is greatly improved.

  The reaction at the air electrode 101 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 electrolyte 103 from the negative electrode 102 by electrochemical oxidation, and move through the electrolyte 103 to the surface of the air electrode 101. Oxygen (O ₂ ) is taken into the air electrode 101 from the atmosphere (air). The material (Li ⁺⁾ to dissolve from the anode 102, the material (Li ₂ O ₂₎ to be deposited in the air electrode 101, and shows air (O ₂₎ together with the components of FIG.

  When sulfur (S) is contained in molybdenum phosphide used as the electrode catalyst of the air electrode 101, sulfur contained in the molybdenum phosphide serves as an active center of oxygen, so that the interaction of the electrode catalyst with oxygen is reduced. It becomes stronger and can adsorb many oxygen species on the surface of molybdenum phosphide. As described above, the oxygen species adsorbed on the molybdenum phosphide surface is used in the oxygen reduction reaction as an oxygen source (active intermediate reactant) of the above formulas (1) and (2), and the above reaction can be easily performed. Come on. Moreover, said phosphorus (P) compound has activity also with respect to the charging reaction which is a reverse reaction of Formula (1) and Formula (2). Therefore, battery charging, that is, oxygen generation reaction on the air electrode 101 also proceeds efficiently. Thus, molybdenum phosphide containing sulfur functions effectively as an electrode catalyst.

In a lithium-air secondary battery, in order to increase the efficiency of the battery, it is desirable that there are more reaction sites (the above-described electrolyte / electrode catalyst / air (oxygen) three-phase portion) that cause an electrode reaction. From such a viewpoint, it is important that the above-mentioned three-phase sites are present in a large amount on the electrode catalyst surface, and it is desirable that the catalyst used has a high specific surface area. For example, molybdenum phosphide has a specific surface area of 30 m ² / g or more, preferably 40 m ² / g or more, more preferably 50 m ² / g or more.

  Molybdenum phosphide can be obtained by various methods. For example, molybdenum phosphide can be obtained by various synthesis methods using known processes such as a solid phase method, a liquid phase method, and a gas phase method.

For example, as an example of a synthesis method, a molybdenum (Mo) salt contained in molybdenum phosphide (eg, molybdenum nitrate, molybdenum sulfate, molybdenum ammonium salt) and an ammonium salt of phosphorus (eg, diammonium hydrogen phosphate) A phosphide precursor is prepared by a liquid phase method using, and then this precursor is heat-treated. It is desirable to prepare molybdenum phosphide by methods including this liquid phase method. Examples of the liquid phase method include a method in which the aqueous solution is evaporated to dryness, a precipitation method in which an alkaline aqueous solution is dropped into the aqueous solution, and a method in which a metal alkoxide is hydrolyzed. As a more specific procedure, an aqueous solution of molybdenum salt and phosphorus ammonium salt was mixed, and the resulting product was filtered, dried, and then helium (He), argon (Ar), nitrogen (N ₂ ). For example, there is a method of heat treatment under an inert atmosphere. The heat treatment procedure is 100 ° C. or higher, preferably 100 to 800 ° C., more preferably 300 to 700 ° C., 0.5 to 24 hours, preferably 1 to 5 hours, hydrogen (H ₂ ), etc. Temperature reduction in an inert atmosphere such as helium (He), argon (Ar), nitrogen (N ₂ ), etc., mixed with a reducing gas. Molybdenum phosphide having a high specific surface area can be obtained by a synthesis method including heat treatment in such a temperature range and processing time.

  Molybdenum phosphide obtained by the liquid phase method may spontaneously oxidize in air. Therefore, stable molybdenum phosphide can be obtained by subjecting the surface of molybdenum phosphide obtained by the liquid phase method to passivation treatment. The passivation treatment is performed at a temperature of about room temperature under an inert atmosphere (for example, He, Ar, nitrogen, etc.) containing, for example, low concentration oxygen (about 0.1 to 1%, preferably about 0.5%). And standing for 1 to 10 hours, preferably 1 to 5 hours.

  The molybdenum phosphide thus obtained exhibits high performance when used as an electrode catalyst for the air electrode of a lithium air secondary battery.

Here, as described above, the specific surface area of molybdenum phosphide is, for example, 30 m ² / g or more, preferably 40 m ² / g or more, more preferably 50 m ² / g or more. When the phosphide is prepared by the above liquid phase method, the value after the heat treatment is 41 m ² / g, which satisfies this condition.

Further, the molybdenum phosphide obtained by the above synthesis method is at a temperature of 100 ° C. or higher, preferably 100 to 800 ° C., more preferably 300 to 500 ° C. in a hydrogen (H ₂ ) and hydrogen sulfide (H ₂ S) atmosphere. The molybdenum phosphide can contain sulfur by heat treatment. In the present invention, molybdenum phosphide containing such sulfur can also be suitably used as an electrode catalyst. The sulfur content of the molybdenum phosphide containing sulfur is preferably 1 to 50%, more preferably 20 to 40% in terms of atomic ratio.

(I-2) Conductive Material The air electrode 101 can include a conductive material. An example of the conductive material is carbon. Specific examples include carbon blacks such as ketjen black and acetylene black, activated carbons, graphites, and carbon fibers. In order to secure sufficient reaction sites in the air electrode 101, carbon having a large specific surface area is suitable. Specifically, those having a BET specific surface area of 300 m ² / g or more are desirable. These carbons can be obtained, for example, as commercial products or by a known synthesis method.

  The specific surface area of the catalyst used for the air electrode and preferable carbon desirably has a predetermined value. The specific surface area can be measured using a commercially available apparatus. For example, the specific surface area can be measured by a BET method using liquid nitrogen as a cooling medium using a commercially available measuring apparatus.

(I-3) Binder (binder)
The air electrode 101 preferably contains a binder or the like for integrating the materials as necessary. Although this binder is not specifically limited, Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), styrene butadiene rubber, etc. can be mentioned as an example. These binders can be used as a powder or as a dispersion.

(I-4) Preparation of air electrode The air electrode 101 can be prepared as follows. For example, by mixing a catalyst such as molybdenum phosphide, carbon powder, and, if necessary, a binder powder such as polytetrafluoroethylene (PTFE), the mixture is pressure-bonded onto a support such as a titanium mesh, whereby air The pole 101 can be formed. Moreover, the air electrode 101 can be formed by dispersing the above-described mixture in a solvent such as an organic solvent to form a slurry, and applying the mixture onto a metal mesh or carbon cloth or carbon sheet and drying.

  Moreover, in order to increase the strength as an electrode and prevent leakage of the electrolytic solution, the air electrode 101 with more stability can be produced by applying not only a cold press but also a hot press. .

  One side of the air electrode 101 is exposed to the atmosphere, and the other side is in contact with the electrolyte 103. Note that the phrase “the electrolyte 103 is in contact with the air electrode 101 and the negative electrode 102” means that the electrolyte 103 is in direct or indirect contact with at least one of the air electrode 101 and the negative electrode 102. The meaning of being in contact with each other is included.

  As described above, by producing the air electrode 101 to which molybdenum phosphide is added, it is possible to obtain an air electrode electrode that is highly active against charging and discharging reactions. Furthermore, the effect of the catalyst containing molybdenum phosphide can be enhanced by producing the air electrode of the lithium-air secondary battery having the above configuration.

(II) Negative electrode The negative electrode 102 contains lithium. For example, the negative electrode 102 can be made of a material such as lithium-containing alloy that can release and absorb metallic lithium or lithium ions.

Typically, the negative electrode 102 includes a negative electrode active material containing lithium, and the negative electrode active material is not particularly limited as long as it is a material that can be used as a negative electrode material of a lithium secondary battery. Examples of the negative electrode active material include metallic lithium. Alternatively, examples of the lithium-containing material include lithium and silicon or tin alloy, or lithium nitride such as Li _2.6 Co _0.4 N, which is a material capable of releasing and occluding lithium ions.

  Note that when the above-described silicon or tin alloy is used as the negative electrode 102, silicon or tin that does not contain lithium may be used when the negative electrode 102 is synthesized. However, in this case, prior to the production of the air secondary battery, by a chemical method or an electrochemical method (for example, a method in which an electrochemical cell is assembled and lithium and silicon or tin are alloyed), It is necessary to process so that silicon or tin is in a state containing lithium. Specifically, it is preferable that the working electrode contains silicon or tin, the counter electrode is lithium, and a treatment such as alloying is performed by flowing a reducing current in the organic electrolyte.

  The negative electrode 102 can be formed by a known method. For example, in the case where lithium metal is used as the negative electrode 102, the negative electrode 102 may be manufactured by stacking a plurality of metal lithium foils into a predetermined shape.

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

(Discharge reaction)
Li → Li ⁺ + e ⁻ (3)
In addition, in the negative electrode 102 at the time of charge, the lithium precipitation reaction which is the reverse reaction of Formula (3) occurs.

(III) Electrolyte The lithium-air secondary battery of the present invention includes an electrolyte 103. Note that in this specification, the electrolytic solution refers to a case where the electrolyte 103 is in a liquid form. The electrolyte 103 may be any substance that can move lithium ions between the air electrode (positive electrode) 101 and the negative electrode 102. For example, a nonaqueous solvent in which a metal salt containing lithium ions is dissolved can be used. Specifically, as a metal salt containing lithium ions, a metal salt containing lithium ions such as lithium bistrifluoromethanesulfonylimide (LiTFSI), lithium perchlorate (LiClO ₄ ), lithium hexafluorophosphate (LiPF ₆ ), etc. Can be mentioned. Nonaqueous solvents include dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate (MBC), diethyl carbonate (DEC), and ethyl propyl carbonate. (EPC), ethyl isopropyl carbonate (EIPC), ethyl butyl carbonate (EBC), dipropyl carbonate (DPC), diisopropyl carbonate (DIPC), dibutyl carbonate (DBC), ethylene carbonate (EC), propylene carbonate (PC), carbonic acid 1, Carbonate ester solvents such as 2-butylene (1,2-BC), ether solvents such as 1,2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME), and lactone systems such as γ-butyrotactone (GBL) Solvent or dimethylsulfoxy A sulfoxide solvent such as (DMSO), and a solvent in which two or more of these are mixed [for example, a mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) (volume ratio 1: 1), EC and carbonic acid Mixed solvent such as diethyl (DEC) and the like.

Other electrolytes 103 include solid electrolytes that pass lithium ions (for example, sulfide-based solid electrolytes including Li ₂ S and P ₂ S ₅ ), polymer electrolytes that pass lithium ions (for example, polyethylene oxides, specifically, For example, a material obtained by compositing the organic electrolyte and polyethylene oxide may be used. However, this invention is not limited to these, It can use suitably if it is a solid electrolyte which lets the known lithium ion used for a lithium air secondary battery, or a polymer electrolyte which lets lithium ion pass.

(IV) Other elements The lithium air secondary battery includes structural elements such as a separator, a battery case, a metal mesh (for example, titanium mesh), and other elements required for the lithium air secondary battery, in addition to the above components. be able to. Conventionally known ones can be used.

(V) Production of Lithium-Air Secondary Battery As described above, the lithium-air secondary battery includes at least an air electrode (positive electrode), a negative electrode, and an electrolyte. For example, as shown in FIG. It is configured to sandwich the electrolyte in between. The lithium air secondary battery having such a configuration can be prepared in the same manner as a conventional secondary battery.

  In a further embodiment, for example, a cylindrical lithium-air secondary battery cell 200 as shown in FIG. 2 can be prepared. Specifically, first, the air electrode 1 is disposed and fixed on the air electrode support 10 that is coated with insulation. The negative electrode 2 is fixed to the negative electrode support 11. The inside of the secondary air battery cell 200 (the portion between the air electrode 1 and the negative electrode 2) is filled with an electrolyte (typically an organic electrolyte) 3, and the surface of the negative electrode 2 in contact with the atmosphere of the air electrode 1; The whole of the air secondary battery cell 200 is fixed by covering the negative electrode support 11 so as to be disposed on the opposite surface.

  In addition to the above components, a member such as a separator 5 can be disposed in a portion between the air electrode 1 and the negative electrode 2. Other fixtures such as the air electrode terminal 4, the negative electrode terminal 13, and the negative electrode fixing PTFE ring 6, the negative electrode fixing washer 7, the air electrode fixing PTFE ring 8, the O ring 9, and the cell fixing screw (PTFE coating) 12. Insulating members and the like can be appropriately disposed.

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

Example 1
Molybdenum phosphide (MoP) powder used as the electrode catalyst for the air electrode 1 was synthesized by the following procedure.

Commercially available diammonium hydrogen phosphate ((NH ₄ ) ₂ HPO ₄ ) (manufactured by Sigma-Aldrich) and commercially available hexaammonium heptamolybdate tetrahydrate ((NH ₄ ) ₆ Mo ₇ O ₂₄ · 4H ₂ O) (Manufactured by Wako Pure Chemical Industries, Ltd.) was dissolved in distilled water and adjusted so that phosphorus and molybdenum were in an atomic weight ratio of 1: 1. The obtained solution was heat-treated at 650 ° C. for 2 hours in a mixed gas atmosphere of 95% nitrogen (N ₂ ) and 5% hydrogen (H ₂ ) to produce molybdenum phosphide. The powder was passivated for 4 hours in a He atmosphere containing 0.5% oxygen (O ₂ ) so that it was not oxidized in the air. This powder was evaluated by X-ray diffraction (XRD) measurement and BET specific surface area measurement.

The powder after the heat treatment was confirmed to contain no impurities in molybdenum phosphide (JCPDS No. 24-0771) by XRD measurement. Moreover, it was 41 m < ² > / g when the specific surface area of the powder was measured by BET method.

(Preparation of air electrode)
Next, an air electrode 1 and a lithium-air secondary battery cell 200 using the air electrode 1 using such a molybdenum phosphide powder were produced as follows.

  Molybdenum phosphide powder, ketjen black powder and polytetrafluoroethylene (PTFE) powder are thoroughly pulverized and mixed using a rough machine at a weight ratio of 50:30:20, roll-molded, and a sheet electrode (thickness) Thickness: 0.5 mm). The sheet electrode was cut into a circle having a diameter of 23 mm and pressed onto a titanium mesh to obtain a gas diffusion type air electrode.

(Production of lithium secondary battery cells)
A cylindrical lithium-air secondary battery cell 200 having the cross-sectional structure shown in FIG. 2 was produced. FIG. 2 is a schematic longitudinal sectional view of the lithium air secondary battery cell 200. The lithium air secondary battery cell 200 was produced in the following procedure in dry air having a dew point of −60 ° C. or less.

The air electrode (positive electrode) 1 prepared by the above method was placed in the recess of the PTFE-coated air electrode support 10 and fixed with the air electrode fixing PTFE ring 8. The portion where the air electrode 1 and the air electrode support 10 are in contact with each other was not covered with PTFE in order to make electrical contact. The effective area of the electrode in contact with the air electrode 1 and air was 2 cm ² .

Next, a separator 5 for a lithium secondary battery was disposed on the bottom surface of the recess on the surface opposite to the surface where the air electrode 1 and the atmosphere contacted. Subsequently, four metal lithium foils (effective area: 2 cm ² ) having a thickness of 150 μm as the negative electrode 2 were stacked on the concentric circle and pressure bonded to the negative electrode fixing washer 7 as shown in FIG. Next, the negative electrode fixing PTFE ring 6 was disposed in a concave portion opposite to the concave portion in which the air electrode 1 was installed, and a negative electrode fixing washer 7 having metal lithium bonded thereto was further disposed in the central portion. Subsequently, the O-ring 9 was disposed at the bottom of the air electrode (positive electrode) support 10 as shown in FIG.

Next, the inside of the cell (between the positive electrode 1 and the negative electrode 8) was filled with the organic electrolyte solution 10, covered with the negative electrode support 11, and the entire cell was fixed with the cell fixing screws 12. The organic electrolyte 10 was a 1 mol / l lithium hexafluorophosphate / propylene carbonate (LiPF ₆ / PC) solution.

  Subsequently, the air electrode (positive electrode) terminal 4 was installed on the air electrode (positive electrode) support 10, and the negative electrode terminal 13 was installed on the negative electrode support 11.

(Battery performance)
The battery performance of the lithium air secondary battery cell prepared by the above procedure was measured. In addition, the air electrode (positive electrode) terminal 4 and the negative electrode terminal 13 shown in FIG. 2 were used for the measurement test of battery performance.

In the battery cycle test, a charge / discharge measurement system (manufactured by Bio Logic) was used, and a current density per effective area of the air electrode 1 of 0.1 mA / cm ² was applied. The discharge voltage was measured until the voltage dropped to 0.0V. Further, the battery charge test was performed until the battery voltage increased to 4.4 V at the same current density as during discharge. The battery discharge test was performed in a normal living environment. The charge / discharge capacity was expressed as a value (mAh / g) per weight of the air electrode (carbon + molybdenum phosphide + PTFE).

  The initial discharge and charge curves are shown in FIG.

  FIG. 3 shows that when molybdenum phosphide is used as the air electrode catalyst, the average discharge voltage is 2.74 V, and the discharge capacity is 650 mAh / g (2200 mAh / g per carbon weight). Here, the average charge / discharge voltage is defined as a discharge voltage and a charge voltage at an intermediate value of the total discharge capacity in the figure.

  In addition, the initial charge capacity is 660 mAh / g which is almost the same as the discharge capacity, and it can be seen that the reversibility is excellent.

  FIG. 4 shows the cycle dependency of the discharge capacity. In this example (Example 1), even when the charge / discharge cycle was repeated 100 times, the discharge capacity (mAh / g) was hardly reduced.

  Further, it was found from FIG. 3 that the voltage at the time of charging had a flat portion at about 3.75 V, which was lower than the conventional report.

  The transition of the charge / discharge voltage is shown in Table 1 below. In this example (Example 1), it was found that a slight increase in overvoltage was observed during charging and discharging, but an almost stable voltage was exhibited. Thus, it has been found that molybdenum phosphide has very excellent activity as a catalyst for the air electrode of a lithium-air secondary battery.

(Example 2)
A molybdenum phosphide powder containing sulfur (S) was synthesized by the following procedure.

Molybdenum phosphide was produced in the same manner as in Example 1. The produced molybdenum phosphide was heat-treated at 400 ° C. for 15 minutes in an atmosphere of 10% hydrogen sulfide (H ₂ S) and 90% hydrogen (H ₂ ) to obtain molybdenum phosphide containing sulfur.

  The production of the air electrode, the production of the battery, and the evaluation method were performed in the same manner as in Example 1. Regarding the evaluation method of the obtained powder, ICP emission analysis was performed in addition to the evaluation method of Example 1 in order to quantitatively evaluate the sulfur content.

It was confirmed by XRD measurement that the powder after the heat treatment had a crystal structure similar to that of molybdenum phosphide. ICP emission analysis confirmed that the sulfur content was 30% of the total by atomic ratio. Moreover, it was 52 m < ² > / g when the specific surface area of the powder was measured by BET method.

(Battery performance)
The cycle dependency of the discharge capacity and the charge / discharge voltage of the lithium-air secondary battery using the molybdenum phosphide containing sulfur of this example as the electrode catalyst of the air electrode 1 is shown in FIGS.

  As shown in FIG. 4, the discharge capacity of this example (Example 2) was 830 mAh / g for the first time, which was larger than that of molybdenum phosphide not containing sulfur in Example 1. It was also found that even when the cycle was repeated, stable behavior was exhibited.

  Further, as shown in Table 1, the charge / discharge voltage was also reduced more than in Example 1, and the improvement of the energy efficiency of charge / discharge could be achieved. In addition, regarding the charge / discharge voltage, it was confirmed that no significant overvoltage increase was observed even when the cycle was repeated, and the operation was stable. The improvement in these characteristics is considered to be due to the smooth reaction of both oxygen reduction (discharge) and oxygen generation (charge) because sulfur contained in molybdenum phosphide served as the active center in the battery reaction. .

(Comparative Example 1)
A lithium air secondary battery cell was produced in the same manner as in Example 1 using cobalt oxide (Co ₃ O ₄ ), which is known as an electrode catalyst for the air electrode 1. As the cobalt oxide (Co ₃ O ₄ ), a commercially available reagent (manufactured by Wako Pure Chemical Industries, Ltd.) was used. The conditions of the battery cycle test were the same as in Example 1.

  The cycle performance regarding the discharge capacity of the lithium air secondary battery according to this comparative example is shown in FIG. 4 together with the results of Examples 1-2.

  As shown in the figure, in the first comparative example, the initial discharge capacity is about 800 mAh / g, which is larger than that in the first embodiment, for example. However, when the charge / discharge cycle was repeated, an extreme decrease in the discharge capacity was observed unlike Example 1, and the capacity retention rate after 20 cycles was about 13% of the initial value.

  In addition, the cycle dependency of the charge / discharge voltage is shown in Table 1 together with the results of Examples 1-2.

  As can be seen from Table 1, the charge / discharge voltage according to the first comparative example is clearly higher than those of Examples 1 and 2, and the discharge voltage is a low value. However, the cycle became difficult at the 20th time.

  From the above results, the lithium-air secondary battery including the electrode containing molybdenum phosphide as an electrode catalyst as the air electrode as in the present invention is superior in cycle characteristics with respect to capacity and voltage, compared to the case where a known material is used. Thus, it was confirmed that molybdenum phosphide is effective as a catalyst for an air electrode for a lithium-air secondary battery.

  By using molybdenum phosphide as an electrode catalyst for the air electrode of a lithium-air secondary battery, it is possible to produce a lithium-air secondary battery with excellent charge / discharge cycle performance and effectively use it as a drive source for various electronic devices. can do.

1, 101 Air electrode (positive electrode)
2, 102 Negative electrode 3, 103 Electrolyte 4 Air electrode (positive electrode) terminal 5 Separator 6 Negative electrode fixing PTFE ring 7 Negative electrode fixing washer 8 Air electrode (positive electrode) fixing PTFE ring 9 O-ring 10 Air electrode (positive electrode) support ( PTFE coating)
11 Negative electrode support 12 Cell fixing screw (PTFE coating)
13 Negative terminal 100 Lithium air secondary battery 200 Lithium air secondary battery cell

Claims (8)

An air electrode, a negative electrode containing lithium, and an electrolyte, wherein the electrolyte is a lithium-air secondary battery in contact with the air electrode and the negative electrode,
The air electrode includes a catalyst;
The lithium air secondary battery, wherein the catalyst contains molybdenum phosphide (MoP).   The lithium air secondary battery according to claim 1, wherein the molybdenum phosphide contains sulfur (S). The lithium air secondary battery according to claim 2 , wherein the sulfur content is 1 to 50% in terms of atomic ratio of the entire molybdenum phosphide containing sulfur. 3. The lithium air secondary battery according to claim 2 , wherein the sulfur content is 20 to 40% of the entire molybdenum phosphide containing the sulfur in an atomic ratio. The lithium air secondary battery according to claim 1, wherein the molybdenum phosphide has a BET specific surface area of 30 m ² / g or more . The lithium air secondary battery according to claim 1, wherein the molybdenum phosphide has a BET specific surface area of 40 m ² / g or more . The lithium air secondary battery according to claim 1, wherein the molybdenum phosphide has a BET specific surface area of 50 m ² / g or more .   The lithium air secondary battery cell containing the lithium air secondary battery of any one of Claims 1-7.