JP2015084294A - Lithium air secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium air secondary battery which allows a lithium air secondary battery to be operated as a high-capacity secondary battery, and which is small in voltage difference between charge and discharge, and small in the decrease in discharge capacity even when charge and discharge cycles are repeated.SOLUTION: A lithium air secondary battery comprises: an air electrode including a conductive material, a catalyst and a binding agent; a negative electrode including metallic lithium or a lithium-containing substance; and an electrolyte in contact with the air electrode and the negative electrode. The catalyst of the air electrode includes a praseodymium (Pr) oxide. It is preferable that the praseodymium oxide is hexapraseodymium undecaoxide (PrO-4 PrO). It is particularly preferable that the praseodymium (Pr) oxide includes crystalline water.

Description

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

  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, by adding various catalysts to the gas diffusion type air electrode as the positive electrode, battery performance such as discharge capacity and cycle characteristics can be improved. Has been tried.

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 in Non-Patent Document 2, mainly iron oxide (Fe ₂ O ₃ ), cobalt oxide (Co ₃ O ₄ ), and the like are used. Transition metal oxides have been investigated. These documents 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, the secondary battery disclosed in Non-Patent Document 1 has a problem that the charging voltage is about 4.0 V, which is very large compared to the average discharge voltage of 2.7 V, and the energy efficiency is low. .

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 charge and discharge 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% in 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 lithium-air secondary battery of Non-Patent Document 2 has low charge / discharge energy efficiency.

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 (2007).

  The present invention operates a lithium-air secondary battery as a high-capacity secondary battery and uses a highly active air electrode electrode catalyst for charge and discharge reactions, so that the charge / discharge voltage difference is small. An object of the present invention is to provide a lithium-air secondary battery and a method for producing an air electrode thereof, in which the decrease in discharge capacity is small even if the above is repeated.

A lithium air secondary battery according to the present invention includes an air electrode including a conductive material, a catalyst, and a binder, a negative electrode including metal lithium or a lithium-containing substance, and an electrolyte in contact with the air electrode and the negative electrode. The air electrode catalyst includes praseodymium (Pr) oxide. In one embodiment, the praseodymium oxide is ten monoxide six praseodymium _{(Pr 2 O 3 -4PrO 2)} . In another embodiment, the praseodymium (Pr) oxide preferably contains crystal water.

  Since the lithium-air secondary battery of the present invention includes an electrode catalyst and a sit praseodymium oxide in the air electrode, it can exhibit cycle characteristics, energy efficiency, and the like superior to those of the prior art. Specifically, the voltage difference of charge / discharge is small, and even when the charge / discharge cycle is repeated, a decrease in discharge capacity can be suppressed.

  Moreover, the effect of the catalyst which consists of a praseodymium oxide can be heightened by producing the air electrode of the lithium air secondary battery of this invention.

It is the schematic which shows the basic composition of the lithium air secondary battery which concerns on this invention. It is sectional drawing which shows the structure of the lithium air secondary battery cell used by the present Example. 2 is a diagram illustrating a charge / discharge curve of a lithium-air secondary battery cell according to Example 1. FIG. It is a figure which shows the cycle dependence of the discharge capacity of the lithium air secondary battery which concerns on Example 1, 2 and the comparative example 1. FIG.

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

[Configuration of lithium-air secondary battery]
A basic structure of a lithium air secondary battery according to the present invention is shown in FIG. As shown in FIG. 1, the lithium air secondary battery 100 of the present invention includes at least an air electrode (positive electrode) 102, a negative electrode 104, and an electrolyte 106.

  More specifically, the air electrode 102 may include a catalyst, a conductive material, and a binder as components. The negative electrode 104 can be composed of a material such as a lithium-containing alloy capable of releasing and absorbing metallic lithium or lithium ions. An electrolyte 106 can be disposed between the air electrode and the negative electrode.

  Each of the above components will be described below. In the present specification, the electrolytic solution refers to a case where the electrolyte is in a liquid form.

(I) Air electrode (positive electrode)
In the present invention, the air electrode may include a catalyst, a conductive material, a binder, and the like.

(I-1) Catalyst In the lithium air secondary battery of the present invention, praseodymium oxide is included as a catalyst for the air electrode. In particular, the air electrode preferably contains praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) that is highly active for both oxygen reduction (discharge) and oxygen generation (charge) reactions as an electrode catalyst. By including praseodymium oxide, the lithium air secondary battery of the present invention can enhance the performance as a secondary battery.

  In the air electrode of the lithium-air secondary battery of the present invention, the electrode reaction proceeds in the three-phase portion of electrolyte / electrode catalyst / gas (oxygen). That is, for example, when an organic electrolytic solution (including a solid electrolyte impregnated with an organic electrolytic solution) is used as the electrolyte, the organic electrolytic solution 3 penetrates into the air electrode and simultaneously oxygen gas in the atmosphere is supplied. A three-phase site in which electrolyte, electrode catalyst, and gas (oxygen) coexist is formed. 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 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 from the negative electrode by electrochemical oxidation, and move through the electrolyte to the surface of the air electrode. Oxygen (O ₂ ) is taken into the air electrode from the atmosphere (air). In FIG. 1, a material (Li ⁺ ) that dissolves from the negative electrode, a material (LiO ₂ ) that precipitates at the air electrode, and air (O ₂ ) are shown together with constituent elements.

Praseodymium in praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ), which is preferable as an electrode catalyst for the air electrode (positive electrode), can exist as ions having a valence of +4, +3, +2, and the like. Depending on the conditions for synthesizing praseodymium oxide, there may be a vacancy (also referred to as an oxygen vacancy in this specification) capable of taking oxygen into the praseodymium oxide.

  Since such praseodymium oxide has a strong interaction with oxygen as the positive electrode active material, many oxygen species can be adsorbed on the surface of the praseodymium oxide, or oxygen species can be occluded in oxygen vacancies.

  As described above, the oxygen species adsorbed on the surface of praseodymium oxide or occluded in the oxygen vacancies is subjected to an oxygen reduction reaction as an oxygen source (active intermediate reactant) of the above formulas (1) and (2). The above reaction can easily proceed. Moreover, said praseodymium oxide has activity also with respect to the charge reaction which is a reverse reaction of Formula (1) and Formula (2). Therefore, charging of the battery, that is, the oxygen generation reaction on the air electrode proceeds efficiently. Thus, praseodymium oxide functions effectively as an electrode catalyst.

In the lithium-air secondary battery of the present invention, 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 this point of view, in the present invention, it is important that the above-mentioned three-phase sites are present in a large amount on the surface of the electrode catalyst, and it is desirable that the catalyst used has a high specific surface area. In the present invention, for example, praseodymium oxide has a specific surface area of 30 m ² / g or more, preferably 50 m ² / g or more.

The praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) preferably used in the present invention can be obtained by various methods. For example, praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) can be obtained by various synthesis methods using a known process such as a solid phase method or a liquid phase method.

For example, as an example of a liquid phase method, an praseodymium oxide precursor is prepared using an aqueous solution of praseodymium metal nitrate [praseodymium nitrate hexahydrate (Pr (NO ₃ ) ₃ .6H ₂ O)], and then the precursor The method of heat-treating can be mentioned. In the present invention, it is desirable to prepare praseodymium oxide by a method including this liquid phase method. Specific procedures for obtaining a praseodymium oxide precursor by a liquid phase method include a method of evaporating and drying an aqueous solution, a precipitation method of dropping an alkaline aqueous solution into the aqueous solution, and a method of hydrolyzing an alkoxide of praseodymium metal.

  Next, the praseodymium oxide precursor obtained by the liquid phase method is heat-treated. In many cases, the praseodymium oxide precursor obtained by the liquid phase method is in an amorphous state because crystallization has not progressed. Crystalline praseodymium oxide can be obtained by heat-treating this amorphous precursor at a temperature of about 500 ° C. for 1 to 4 hours, more preferably for 1 hour. The praseodymium oxide thus obtained does not contain crystallization water, and exhibits high performance even when used as an electrode catalyst for the air electrode of the lithium-air secondary battery of the present invention.

Here, as described above, in the present invention, the specific surface area of praseodymium oxide is 30 m ² / g or more, preferably 50 m ² / g or more. This is because when praseodymium oxide is prepared by a liquid phase method, The value after heat treatment. As will be described later, in the present invention, praseodymium oxide containing water of crystallization can be suitably used as a catalyst, but also in such praseodymium oxide, the specific surface area has the above range, particularly 50 m ² / g or more. Preferably there is.

In addition, the praseodymium oxide produced by heat treatment may have a specific surface area that is remarkably reduced, and only about 30 m ² / g may be obtained. In particular, the specific surface area may decrease when baked at a high temperature. In order to prevent such a decrease in specific surface area, the above-mentioned amorphous praseodymium precursor can be heat-treated at a low temperature. For example, the above-mentioned amorphous praseodymium precursor can be heat-treated at a relatively low temperature of preferably 50 ° C. or higher, more preferably about 50 to 200 ° C. Moreover, the time of this heat processing is 6 to 24 hours, Preferably it is 12 to 18 hours.

By such heat treatment in a relatively low temperature range and treatment time, praseodymium oxide having a high specific surface area can be obtained. That is, the praseodymium oxide powder obtained by heat-treating the praseodymium oxide precursor in an amorphous state at a relatively low temperature can have a very large specific surface area of about 150 m ² / g because sintering is hardly progressed. . Therefore, when used as an electrode catalyst of the present invention, it is suitable as a catalyst and can realize excellent battery performance. When the above-mentioned praseodymium oxide precursor is dried at a relatively low temperature of about 50 to 200 ° C., the precursor powder is not completely dehydrated from the praseodymium oxide precursor, and is in an amorphous state. While maintaining the above, water of crystallization is present. This praseodymium oxide is formally expressed as Pr ₂ O ₃ -4PrO ₂ .nH ₂ O (n is the number of moles of H ₂ O contained in the crystal with respect to 1 mol of Pr ₂ O ₃ -4PrO ₂ ). In the present invention, praseodymium oxide containing such crystal water can also be suitably used as an electrode catalyst.

In the present invention, in addition to the above-mentioned praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ), other praseodymium oxides such as praseodymium nickel oxide (Pr ₂ NiO ₄ ) can be used as the air electrode catalyst. . Note that in this specification, metal oxides containing praseodymium such as praseodymium oxide and praseodymium nickel are collectively referred to as “praseodymium oxide”. Furthermore, with respect to praseodymium itself, its isotopes can also be used. In the lithium-air secondary battery of the present invention, praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) or praseodymium oxide [Pr ₂ O ₃ -4PrO ₂ .nH ₂ O (n is 1 mol) containing crystal water as a catalyst for the air electrode. The number of moles of H ₂ O contained in the crystal of Pr ₂ O ₃ -4PrO ₂ is preferred.

(I-2) Conductive material In this invention, a conductive material can be included in an air electrode. 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, carbon having a large specific surface area is suitable. Specifically, a material having a BET specific surface area of 300 m ² / g or more is desirable. These carbons can be obtained, for example, as commercial products or by known synthesis.

  In the lithium air secondary battery of the present invention, as described above, it is desirable that the specific surface area of the catalyst and carbon used for the air electrode have a predetermined value. In the present invention, the specific surface area can be measured by the BET method.

(I-3) Binder (binder)
The air electrode can contain a binder (binder). The binder is not particularly limited, and examples thereof include polytetrafluoroethylene (PTFE), polyethylene, polypropylene, polyvinylidene fluoride (PVDF), polybutadiene rubber, and styrene butadiene rubber. These binders can be used as a powder or as a dispersion.

(I-4) Preparation of air electrode An air electrode can be prepared as follows. The air electrode can be formed by mixing a catalyst powder such as praseodymium oxide powder, carbon powder and polytetrafluoroethylene (PTFE), and then pressing the mixture onto a support such as a titanium mesh. it can. Moreover, an air electrode can be formed by disperse | distributing the above-mentioned mixture in solvents, such as an organic solvent, and making it into a slurry form, apply | coating on a metal mesh or a carbon cloth, or a carbon sheet, and drying.

  In the lithium air secondary battery of the present invention, the catalyst content in the air electrode is preferably 1 to 50% by weight, more preferably 5 to 50% by weight.

  Moreover, in order to increase the strength of the electrode and prevent leakage of the electrolytic solution, it is possible to produce a more stable air electrode by applying not only a cold press but also a hot press.

  In the air electrode, one surface of the electrode constituting the air electrode is exposed to the atmosphere, and the other surface is in contact with the electrolytic solution.

(II) Negative Electrode The lithium air secondary battery of the present invention contains a negative electrode active material in the negative electrode. 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 for a lithium secondary battery. For example, metallic lithium can be mentioned. Alternatively, examples of the lithium-containing substance include lithium and silicon or tin alloy, or lithium nitride such as Li _2.6 Co _0.4 N, which is a substance that can release and occlude lithium ions. be able to.

  In addition, when using said silicon or tin alloy as a negative electrode, the silicon | silicone or tin etc. which do not contain lithium can also be used when synthesize | combining a negative electrode. However, in this case, prior to the production of the air battery, 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) is used to form silicon or silicon. It is necessary to treat so that 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 of the lithium air secondary battery of the present invention can be formed by a known method. For example, when lithium metal is used as the negative electrode, the negative electrode may be produced 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 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 contains an electrolyte. This electrolyte should just be what can move a lithium ion between an air electrode (positive electrode) and a negative electrode. In the present invention, an organic electrolytic solution in which a metal salt containing lithium ions is dissolved in an appropriate solvent can be used. Specific examples of the solute metal salt include lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), and LiTFSI [(CF ₃ SO ₂ ) ₂ NLi]. Examples of the solvent include dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), methyl propyl carbonate (MPC), methyl isopropyl carbonate (MIPC), methyl butyl carbonate (MBC), diethyl carbonate (DEC), 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 1,2 Carbonate ester solvents such as butylene (1,2-BC), ether solvents such as 1,2-dimethoxyethane (DME) and tetraethylene glycol dimethyl ether (TEGDME), and lactone solvents such as γ-butyrotactone (GBL) Or dimethyl sulfoxide DMSO) can be mentioned sulfoxide type solvents such as. Furthermore, the solvent which mixed 2 or more types from these can also be used.

As another electrolyte of the lithium air secondary battery of the present invention, a solid electrolyte having lithium ion conductivity [for example, a glassy substance such as 75Li ₂ S · 25P ₂ S ₅ , a LISICON (Li ₁₄ ZnGe ₄ O ₁₆ or the like) ⁺ Super Ionic Conductor)], a polymer electrolyte having lithium ion conductivity (for example, a material in which the above-mentioned organic electrolyte and polyethylene oxide (PEO) are composited), and the like are not limited thereto. In the present invention, any known solid electrolyte that passes lithium ions or polymer electrolyte that passes lithium ions used in lithium-air secondary batteries can be used.

(IV) Other Elements The lithium air secondary battery of the present invention is required for structural members such as separators, battery cases, metal meshes (for example, titanium mesh), and other lithium air secondary batteries in addition to the above components. Can contain elements. Conventionally known ones can be used.

  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
In this example, praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) powder is used as an electrode catalyst for a gas diffusion type air electrode.

(Preparation of praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) powder)
The praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) powder used as an electrode catalyst in the air electrode of the lithium air secondary battery of the present invention was synthesized as follows.

Commercial praseodymium nitrate hexahydrate (manufactured by Sigma-Aldrich _{Corp.) (Pr (NO 3) 3} · 6H 2 O) was dissolved in distilled water, with stirring, until an aqueous solution of sodium hydroxide to pH7.0 Slowly dropped. Sodium nitrate (NaNO ₃ ) was removed by centrifuging the resulting solution, and all the solution was evaporated at 60 ° C. to obtain praseodymium hydroxide powder. The obtained powder was dried at 60 ° C. overnight and then heat-treated at 500 ° C. for 1 hour in the air using an electric furnace. The powder after firing was evaluated by X-ray diffraction (XRD) measurement, TG-DTA analysis, and BET specific surface area measurement.

The powder after the heat treatment was confirmed to be a praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ , PDF file No. 42-1121) single phase by XRD measurement. Moreover, it was confirmed by the TG-DTA measurement that the crystal water was not contained in the powder. Moreover, it was 30 m < ² > / g when the specific surface area of the powder was measured by BET method.

Next, using the obtained praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) powder, an air electrode and a lithium-air secondary battery cell using the air electrode were produced as follows.

A praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) powder, a ketjen black powder having a BET specific surface area of 500 m ² / g and a polytetrafluoroethylene (PTFE) powder in a weight ratio of 50:30:20 It was sufficiently pulverized and mixed, and roll-formed to produce a sheet-like electrode (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.

(Preparation of lithium-air secondary battery cell)
A cylindrical lithium-air secondary battery cell having the cross-sectional structure shown in FIG. 2 was produced. FIG. 2 is a cross-sectional view of a lithium air secondary battery cell. The lithium air battery cell was produced according to the following procedure in dry air having a dew point of −60 ° C. or lower.

The air electrode (positive electrode) 1 prepared by the above method was placed in the concave portion of the PTFE-coated air electrode support 2 and fixed with a PTFE ring 3 for fixing the air electrode. The portion where the air electrode 1 and the air electrode support 2 are in contact with each other was not subjected to PTFE coating in order to make electrical contact. The effective area of the electrode in contact between the air electrode 1 and air is 2 cm ² .

Next, a separator 5 for a lithium-air secondary battery was disposed on the bottom surface of the recess on the surface opposite to the surface of the air electrode 1 that contacts the atmosphere. Subsequently, four metal lithium foils (effective area: 2 cm ² ) having a thickness of 150 μm as the negative electrode 8 were stacked on the concentric circle and pressure-bonded to the negative electrode fixing washer 7 as shown in FIG. Subsequently, the negative electrode fixing PTFE ring 6 was disposed in a concave portion opposite to the concave portion in which the air electrode 1 is disposed, 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 positive electrode support 2 as shown in FIG. Next, the inside of the cell (between the positive electrode 1 and the negative electrode 8) was filled with an electrolyte (organic electrolyte solution) 10, covered with a negative electrode support 11, and the entire cell was fixed with a cell fixing screw 12. As the electrolyte, a 1 mol / l lithium hexafluorophosphate / propylene carbonate (LiPF ₆ / PC) solution was used.

  Subsequently, the positive electrode terminal 4 was installed on the positive electrode support 2, 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. Note that the positive electrode terminal 4 and the negative electrode terminal 13 shown in FIG. 2 were used for a battery performance measurement test.

In the battery cycle test, a charge / discharge measurement system (manufactured by Bio Logic) was used, and a current density of 0.1 mA / cm ² per effective area of the air electrode 1 was applied. Measurements were taken until the voltage dropped to 0.0V. The battery charge test was performed until the battery voltage increased to 4.5 V at the same current density as during discharge. The charge / discharge test of the battery 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 + praseodymium oxide + PTFE).

  The initial discharge and charge curves are shown in FIG.

As shown in FIG. 3, the average discharge voltage when praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) is used as an air electrode catalyst is 2.75 V, and the discharge capacity is 400 mAh / g (per carbon weight, 700 mAh / g).

  Further, the initial charge capacity is 393 mAh / g, which is substantially the same as the discharge capacity, and it can be seen that the lithium-air secondary battery of the present invention is excellent in reversibility.

  FIG. 4 shows the cycle dependency of the discharge capacity. As shown in FIG. 4, in this example (Example 1), even when the charge / discharge cycle was repeated 100 times, a decrease in the discharge capacity (mAh / g) was hardly observed.

  Moreover, about the voltage at the time of this charge, the flat part was seen by about 3.7V from FIG. 3, and it turned out that it shows a low value compared with the conventional report example.

The charge / discharge cycle was repeated to measure the transition of the charge / discharge voltage. The results are 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 was found that praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ) has a very excellent activity as a catalyst for the air electrode of a lithium air secondary battery.

(Example 2)
In this example, praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ · nH ₂ O) containing crystal water was used as an electrode catalyst for a gas diffusion type air electrode.

(Preparation of praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ · nH ₂ O) powder containing crystal water)
In the procedure shown in Example 1, praseodymium oxide containing crystal water (Pr ₂ O ₃ -4PrO ₂ .nH ₂ O) is subjected to the final heat treatment performed at 500 ° C. for 1 hour at 100 ° C. for 12 hours. Modified and synthesized. The evaluation of the physical properties of the powder, the method for producing the electrode and the battery, and the evaluation of the electrode and the battery were performed in the same manner as in Example 1. From the XRD measurement, it was confirmed that the obtained powder was amorphous. From the TG-DTA measurement, it was found that the crystallization water contained in the obtained powder was n = 0.8. Moreover, the BET specific surface area was 150 m < ² > / g.

The cycle dependency of the charge / discharge capacity and the charge / discharge voltage of a lithium-air secondary battery using praseodymium oxide (Pr ₂ O ₃ -4PrO ₂ .0.8H ₂ O) containing crystal water as an electrode catalyst for the air electrode 1 It shows in FIG.

As shown in FIG. 4, the discharge capacity of this example (Example 2) is 610 mAh / g for the first time, and praseodymium oxide (Pr ₂ O ₃ −) having a specific surface area of 30 m ² / g as in Example 1. 4PrO ₂ ). It was also found that even when the cycle was repeated, stable behavior was exhibited.

  As shown in Table 1, the charge / discharge voltage was also reduced more than in Example 1, and the energy efficiency of charge / discharge was improved. 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 battery characteristics as described above is considered to be due to the fact that praseodymium oxide functions effectively as a catalyst for the air electrode. In other words, by using praseodymium oxide having a very large surface area as an electrode catalyst, praseodymium oxide increases the deposition site of lithium oxide at the time of discharge at the air electrode and improves the adsorption ability of oxygen in the air electrode. It is thought that it led to making it.

(Comparative Example 1)
This comparative example is an example of a lithium air secondary battery using cobalt oxide (Co ₃ O ₄ ).

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 Sigma-Aldrich) was used. The conditions of the battery cycle test are the same as in Example 1.

  FIG. 4 shows the cycle performance of the discharge capacity of the lithium-air secondary battery according to this comparative example together with the results of Examples 1 and 2.

  As shown in FIG. 4, in the first comparative example, the initial discharge capacity was about 500 mAh / g, which was larger than that in the first 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 20% of the initial value.

  The cycle dependency of the charge / discharge voltage is shown in Table 1 together with the results of Examples 1 and 2.

  As can be seen from Table 1, the charge / discharge voltage according to this comparative example 1 is clearly lower than those of Examples 1 and 2, and when the cycle is repeated, the overvoltage clearly increases and the cycle is difficult at the 20th time. became.

  From the above results, praseodymium oxide, as in the present invention, is superior as an electrode catalyst for air electrodes in terms of capacity and voltage as compared with known materials, and is effective as an air electrode catalyst for lithium-air secondary batteries. It was confirmed that.

  By using praseodymium oxide as an electrode catalyst for an air electrode, a lithium-air secondary battery excellent in charge / discharge cycle performance can be produced, and can be effectively used as a drive source for various electronic devices.

1 Air electrode (positive electrode)
2 Positive electrode support (PTFE coating)
3 Positive electrode fixing ring (PTFE ring)
4 Air electrode terminal 5 Separator 6 Negative electrode fixing ring (PTFE ring)
7 Washer for fixing negative electrode 8 Negative electrode 9 O-ring 10 Organic electrolyte 11 Negative electrode support 12 Cell fixing screw (PTFE coating)
13 Negative terminal 100 Lithium air secondary battery 102 Air electrode 104 Negative electrode 106 Organic electrolyte

Claims (3)

A lithium-air secondary battery comprising a conductive material, an air electrode containing a catalyst and a binder, a negative electrode containing metallic lithium or a lithium-containing substance, and an electrolyte in contact with the air electrode and the negative electrode,
The lithium-air secondary battery, wherein the air electrode catalyst includes praseodymium (Pr) oxide. The praseodymium (Pr) oxide is a mixed oxide in which praseodymium is present as trivalent and tetravalent ions, and is hexapraseodymium oxide (Pr ₂ O ₃ -4PrO ₂ ). Item 2. The lithium-air secondary battery according to Item 1.   The lithium air secondary battery according to claim 1, wherein the praseodymium (Pr) oxide contains crystal water.

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