JP6272260B2 - Lithium air secondary battery - Google Patents

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 can realize a discharge capacity that is much larger.

  Lithium-air secondary batteries that use oxygen in the air as the positive electrode active material are always supplied with oxygen from the outside of the battery, and the battery can be filled with a large amount of metallic lithium, which is the negative electrode active material. It has been reported to exhibit a very large discharge capacity.

  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.

For example, as an electrode catalyst for a gas diffusion type air electrode, transition metal oxides such as λ-MnO ₂ are used in Non-Patent Document 1, and iron oxide (Fe ₂ O ₃ ) and cobalt oxide (Co Transition metal oxides such as ₃ O ₄ ) have been studied.

  However, in the secondary battery disclosed in Non-Patent Document 1, the charging voltage is about 4.0 V, which is very large as compared with the average discharge voltage of 2.7 V, and there is a problem that the energy efficiency is low. . Further, the capacity is as small as about 200 mAh / g under the condition of a current density of 1.0 mA / cm 2.

  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, in most cases, the average discharge voltage is about 2.5V, while the charging voltage is 4.0-4.5V, the lowest being about 3.9V. For this reason, the lithium-air secondary battery of Non-Patent Document 2 has low charge / discharge energy efficiency.

In many reports including Non-Patent Documents 1 and 2, lithium salt such as LiClO ₄ , LiPF ₆ , LiTFSI (lithium bistrifluoromethanesulfonylimide) is used as the organic electrolyte of the lithium air secondary battery, propylene carbonate, etc. A solution dissolved in a carbonate ester solvent at a concentration of about 1.0 mol / l is used.

J. et al. 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).

  An object of the present invention is to operate a lithium-air secondary battery as a high-capacity secondary battery, and to realize a high output and a large discharge capacity with a low charging voltage and a small voltage difference between charging and discharging.

  The lithium-air secondary battery according to the present invention includes an air electrode, a negative electrode, and an electrolytic solution in contact with the air electrode and the negative electrode, the electrolytic solution includes a Li salt and an organic solvent, and the electrolytic solution is a liquid. It is characterized by containing hemocyanin as a phase catalyst (additive). Hemocyanin is not present in the air electrode like conventional catalysts and does not exhibit its catalytic action, but it is dissolved in organic electrolytes to react with oxygen reduction (discharge) and oxygen generation (charge) reactions. Have catalytic activity. The higher the hemocyanin concentration in the organic electrolyte, the better the battery performance is obtained. In the present invention, it is preferable that hemocyanin is dissolved in the electrolyte at a saturated concentration.

  By adopting the configuration of the lithium-air secondary battery of the present invention, it is possible to provide a lithium-air secondary battery having a low charge voltage and a small charge / discharge voltage difference, excellent cycle characteristics, and high energy density. .

1 is a basic schematic view of a lithium-air secondary battery according to the present invention. It is a schematic sectional drawing for showing the structure of the lithium air secondary battery used for measurement in an example. 3 is a diagram showing a charge / discharge curve of the lithium-air secondary battery of 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. The following embodiment is merely an example of the present invention, and those skilled in the art can change the design as appropriate.

[Configuration of lithium-air secondary battery]
As shown in FIG. 1, the lithium-air secondary battery according to the present invention includes an air electrode, a negative electrode, and an organic electrolyte in contact with the air electrode and the negative electrode, and the air electrode functions as a positive 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, oxygen is used as the positive electrode active material. Therefore, the positive electrode used in the present invention is an air electrode including an electrically conductive material having an oxygen reduction function and having a space through which oxygen and lithium ions can move, and optionally includes an additive such as a binder. Can do. Moreover, you may contain the catalyst which accelerates | stimulates the oxidation-reduction reaction of oxygen. In the air electrode 1, one side of the electrode constituting the air electrode 1 is exposed to the atmosphere, and the other side is in contact with the electrolyte.

(I-1) Conductive material Carbon materials such as carbon blacks such as ketjen black and acetylene black, activated carbons, graphites, and carbon fibers may be used as the conductive material used in the air electrode of the present invention. it can. Preferably, the carbon has a large surface area in order to sufficiently secure reaction sites in the air electrode, and specifically has a BET specific surface area of 300 m ² / g or more. desirable.

(I-2) Air Electrode Catalyst In the lithium-air secondary battery of the present invention, the air electrode catalyst is oxygen reduction (discharge) such as manganese oxide (MnO ₂ ), ruthenium oxide (RuO ₂ ), and oxygen generation (charge). The catalyst is not particularly limited as long as it is a conventionally known oxide catalyst that is highly active in both reactions. Specifically, single oxides such as MnO ₂ , Mn ₃ O ₄ , MnO, FeO ₂ , Fe ₃ O ₄ , FeO, CoO, Co ₃ O ₄ , NiO, NiO ₂ , V ₂ O ₅ , WO _3, etc. La _0.6 Sr _0.4 MnO ₃ , La _0.6 Sr _0.4 FeO ₃ , La _0.6 Sr _0.4 CoO, La _0.6 Sr _0.4 CoO ₃ , Pr _0.6 Ca _0.4 MnO ₃ , LaNiO ₃ , La _0.6 Sr _0.4 Mn _0.4 Fe _0.6 O ₃ A composite oxide having a perovskite structure can be used. These catalysts can be synthesized using a known process such as a solid phase method or a liquid phase method.

  Further, as the catalyst added to the air electrode, a macrocyclic metal complex such as porphyrin or phthalocyanine containing at least one transition metal such as Mn, Fe, Co, Ni, V, or W as the central metal can be used. These metal complexes may be activated by heat treatment in an inert gas atmosphere after mixing with carbon.

  As the catalyst added to the air electrode of the present invention, not only the above compound system, but also a noble metal such as Pt, Au, Pd, and a transition metal such as Co, Ni, Mn may be used. For example, high activity can be expressed by carrying these metals in high dispersion on carbon.

  In the air electrode of the lithium secondary battery of the present invention, the electrode reaction proceeds in the three-phase portion of electrolyte / electrode catalyst / gas (oxygen). Specifically, the organic electrolyte 3 penetrates into the air electrode 1 and oxygen gas in the atmosphere is supplied at the same time, thereby forming a three-phase site where the electrolyte-electrode catalyst-gas (oxygen) coexists (FIG. 1). reference). If the electrode catalyst is highly active, oxygen reduction (discharge) and oxygen generation (charge) proceed smoothly and battery performance is greatly improved.

The discharge reaction at the air electrode 1 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 organic electrolyte 3 from the negative electrode 2 by electrochemical oxidation, and have moved through the organic electrolyte 3 to the surface of the air electrode 1. Oxygen (O ₂ ) is taken into the air electrode 1 from the atmosphere (air). The material that dissolves from the negative electrode 2 (Li ^+), the material to be deposited at the cathode 1 (Li ₂ O or Li ₂ O _2), and showed air (O ₂₎ together with the components of FIG.

In an oxide that can be used as an electrode catalyst for the air electrode 1 (positive electrode), particularly manganese oxide (MnO ₂ ), ruthenium oxide (RuO ₂ ), etc., manganese and ruthenium have positive valences such as +4 and +3. It exists in the air electrode as ions. Depending on the conditions for synthesizing these oxides, there are vacancies (also referred to as oxygen vacancies in this specification) that can incorporate oxygen into oxides such as manganese oxide and ruthenium oxide. It is considered to function as a site. Therefore, such an oxide catalyst has a strong interaction with oxygen, which is a positive electrode active material, and can adsorb many oxygen species on the surface of the oxide or occlude oxygen species in oxygen vacancies. .

  Thus, the oxygen species adsorbed on the oxide surface or occluded in the oxygen vacancies is used as an oxygen source (active intermediate reactant) of the above formulas (1) and (2) for the oxygen reduction reaction. The above reaction can easily proceed. Moreover, said oxide has activity also with respect to the charging 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.

In the lithium air secondary battery of the present invention, in order to increase the efficiency of the battery reaction, there may be more reaction sites (the above three-phase portion of the electrolyte / electrode catalyst / air (oxygen)) that cause the electrode reaction. desirable. From such a viewpoint, 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 the catalyst used preferably has a high specific surface area. For example, the specific surface area after firing is preferably 10 m ² / g or more.

(I-3) Binder (binder)
The air electrode of the present invention can further contain a binder (binder). Although this binder is not specifically limited, Polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polybutadiene 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 1 can be prepared as follows. For example, an oxide powder as a catalyst, a carbon powder as a conductive material, and a polyvinylidene fluoride (PVDF) powder as a binder are mixed, and this mixture is pressure-bonded onto a support such as a titanium mesh, whereby the air electrode 1 is formed. Can be molded. The compression molding method may be a method well known in the art, for example, by applying not only a cold press but also a hot press to increase the strength of the electrode and prevent leakage of the electrolyte. However, it is possible to produce an air electrode with better stability. Alternatively, the air electrode 1 may be formed by dispersing the above-described mixture in a solvent such as an organic solvent to form a slurry, applying the mixture onto 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 1 is preferably, for example, more than 0 and less than 100% by weight based on the weight of the air electrode 1. The ratio of other components is the same as that of the conventional lithium air secondary battery.

(II) Negative electrode The lithium-air secondary battery of the present invention contains a negative electrode active material capable of occluding and releasing lithium ions 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 material include lithium and silicon or tin alloy, or lithium nitride such as Li _2.6 Co _0.4 N, which is a material that can release and occlude lithium ions.

  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.

  Note that when the alloy of lithium and silicon or tin is used as the negative electrode, the electrode may be initially formed using silicon or tin that does not contain lithium. In this case, prior to the production of the air battery, the chemical method or the electrochemical method (for example, a method of alloying lithium and silicon or tin with an electrochemical cell) is used to convert silicon or tin into lithium. It is necessary to process so as to include a state. 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. 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) Organic Electrolyte Solution As the organic electrolyte solution 3, it is only necessary that lithium ions can move between the positive electrode and the negative electrode. For example, a nonaqueous solvent in which a metal salt containing lithium ions is dissolved can be used. As a metal salt containing lithium ions, lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium bistrifluoromethanesulfonylimide [(CF ₃ SO ₂ ) 2NLi] (LiTFSI), or the like is used. Can do. Examples of the non-aqueous solvent 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 esters such as 2-butylene (1,2-BC), ethers such as 1,2-dimethoxyethane (DME), triglyme and tetraglyme, lactones such as γ-butyrotactone (GBL), Also use a solvent that is a mixture of two or more It can be.

  The organic electrolytic solution 3 of the present invention further contains hemocyanin. Although not limited to theory, hemocyanin is used as a liquid phase catalyst that has a strong interaction with oxygen as a positive electrode active material and can store oxygen species in the structure, as shown in the following chemical formula. be able to. Specifically, the oxygen species moves to the electrolyte / air electrode interface while being occluded within the structure of hemocyanin, and at this interface, the oxygen source (active intermediate reactant) of the above formulas (1) and (2). ) Is used for oxygen reduction reaction, and the above reaction proceeds easily. Moreover, said hemocyanin has activity also with respect to the charging reaction which is a reverse reaction of Formula (1) and Formula (2). Therefore, charging of the battery, that is, oxygen generation reaction on the electrolyte / air electrode interface proceeds efficiently. As described above, since oxygen storage / release of hemocyanin occurs reversibly, the speed of the electrode reaction is improved, and large current discharge / charge is possible.

  The hemocyanin in the organic electrolyte is added at a concentration of 0.05% by weight or more, preferably at a concentration of 0.1% by weight or more, based on the weight of the organic electrolyte. The higher the hemocyanin concentration in the organic electrolytic solution, the better the battery performance is obtained. Therefore, in the present invention, it is desirable that hemocyanin is added at the saturated concentration of the organic electrolytic solution to be added.

(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.

[Example]
Hereinafter, embodiments of the lithium-air secondary battery according to the present invention will be described 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
An organic electrolyte stock solution prepared by dissolving LiTFSI in an organic solvent tetraglyme (TEGDME) at a concentration of 1 mol / L was prepared. Commercially available hemocyanin powder (manufactured by Sigma-Aldrich) was added to the organic electrolyte stock solution so as to be 0.5% by weight with respect to the weight of the organic electrolyte stock solution, and sufficiently stirred. After stirring, dehydration treatment was performed by adding a lump of metallic lithium to the solution. The charged lithium mass was recovered after several days to prepare the organic electrolyte solution 3 of the present invention.

Using manganese oxide (MnO ₂ ) as a catalyst for the air electrode, a lithium air secondary battery cell was produced by the following procedure. A commercially available reagent (manufactured by Kanto Chemical Co., Inc.) was used as the manganese oxide (MnO ₂ ).

Manganese oxide (MnO ₂ ) powder, ketjen black powder and polyvinylidene fluoride (PVDF) powder are mixed at a weight ratio of 10:72:18, and sufficient for N-methyl-2-pyrrolidone (NMP) using a mixer Dispersed to prepare a slurry. The slurry was applied to a carbon sheet having a diameter of 16 mm, put in a 90 ° C. vacuum dryer, and dried overnight to obtain a gas diffusion type air electrode 1.

  Next, a cylindrical lithium-air secondary battery cell having a cross-sectional structure shown in FIG. 3 was produced in the following procedure in dry air having a dew point of −60 ° C. or less. The slurry application amount was adjusted so that the thickness after drying was 10 μm.

The air electrode (positive electrode) 1 prepared by the above method was placed in the concave portion of the air electrode support 10 covered with PTFE, and fixed with a PTFE ring 8 for fixing the air electrode. In addition, the effective area of the electrode where the air 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 surface opposite to the surface where the air electrode 1 and the atmosphere contacted. Subsequently, four metallic lithium foils (effective area: 2 cm ² ) having a thickness of 150 μm were stacked on the negative electrode fixing washer 7 as the negative electrode 2 and pressed on the concentric circles. 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 was disposed, and a negative electrode fixing washer 7 having metal lithium bonded thereto was further disposed in the center. Subsequently, the O-ring 9 was disposed at the bottom of the air electrode support 10 as shown in FIG.

  Next, the inside of the cell (between the air electrode (positive electrode) 1 and the negative electrode 2) is filled with the organic electrolyte solution 3, covered with the negative electrode support 11, and the entire cell is fixed with the cell fixing screws 12. . As the organic electrolyte 3, the above-mentioned hemocyanin-containing organic electrolyte [1 mol / L LiTFSI (hemocyanin 0.5 wt%) / TEGDME solution] was used.

  Then, the air electrode terminal 4 was installed in the air electrode support body 10, and the negative electrode terminal 13 was installed in the negative electrode support body 11, and the lithium air secondary battery cell 200 was obtained.

(Example 2)
A lithium air secondary battery cell 200 was produced in the same manner as in Example 1 by using an electrolytic solution in which hemocyanin was added to a 1 mol / L LiTFSI / TEGDME solution at a saturated concentration (about 1 wt%) as the organic electrolytic solution 3.

(Comparative Example 1)
Using the 1 mol / L LiTFSI / TEGDME solution with no hemocyanin added as the organic electrolyte 3, a lithium air secondary battery cell 200 was produced in the same manner as in the example.

(performance test)
In the battery cycle test, a current density per effective area of the air electrode 1 of 0.1 mA / cm ² or 1.0 mA / cm ² was energized using a charge / discharge measurement system (VMP-3, manufactured by Bio Logic). Then, the discharge voltage was measured until the battery voltage dropped from the open circuit voltage to 2.0V. The battery charge test was performed until the battery voltage reached 4.0 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 (the total of the conductive material, the air electrode catalyst and the binder).

FIG. 3 shows the initial discharge and charge curves when a current density of 0.1 mA / cm ² is applied to the lithium-air secondary battery of Example 1. As shown in the figure, the average charge / discharge voltage is defined as a discharge voltage and a charge voltage at an intermediate value of the total discharge capacity.

  From FIG. 3, the average discharge voltage of Example 1 is 2.77 V, the initial discharge capacity is as large as 1200 mAh / g, and the difference (ΔV) between the average discharge voltage and the average charge voltage is 1.02 V. It was.

To lithium-air secondary batteries of Examples 1 and 2 and Comparative Example 1, the charge and discharge current density: shown in Table 1 0.1 mA / cm ² or the charge-discharge cycle test results conducted in 1.0 mA / cm ² .

In Example 1, even after 100 cycles both current density values, were able to hold the 685mAh / g respectively high discharge capacity at 0.1mA / cm ² 950mAh / g, at 1.0 mA / cm ^2.

  Further, in Example 2, an increase in initial capacity was observed for both current density values compared to Example 1, and even when the charge / discharge cycle was repeated, the average discharge voltage decreased, ΔV increased, and discharge capacity decreased. It was even smaller than Example 1.

On the other hand, in Comparative Example 1, the initial capacity is as low as 723 mAh / g at a current density value of 0.1 mA / cm ² and 428 mAh / g at 1.0 mA / cm ² , and the discharge capacity decreases significantly when the cycle is repeated. After 100 cycles, the capacity retention ratios were only 29% and 5.4%, which were very small values, respectively.

  Thus, the usefulness of adding hemocyanin as a liquid phase catalyst could be verified. It was also confirmed that the high current density characteristics and cycle characteristics were further improved by dissolving hemocyanin at a high concentration.

  By using hemocyanin as an additive for the organic electrolyte, a lithium-air secondary battery having excellent 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 Negative Electrode 3 Organic Electrolyte 4 Air Electrode Terminal 5 Separator 6 Negative Electrode Fixing PTFE Ring 7 Negative Electrode Fixing Washer 8 Air Electrode Fixing PTFE Ring 9 O Ring 10 Air Electrode Support 11 Negative Electrode Support 12 Cell Fixing Screw 13 Negative Electrode Terminal 200 Lithium air secondary battery cell

Claims (4)

An air electrode containing a conductive material;
A negative electrode comprising lithium metal or a lithium-containing material;
An organic electrolyte solution in contact with the air electrode and the negative electrode;
A lithium-air secondary battery, wherein the electrolyte contains hemocyanin.   The lithium-air secondary battery according to claim 1, wherein hemocyanin is dissolved in the electrolyte at a concentration of 0.05% by weight or more based on the total weight of the electrolyte.   The lithium-air secondary battery according to claim 1, wherein hemocyanin is dissolved in the electrolyte at a saturation concentration of the electrolyte.   The lithium air secondary battery according to any one of claims 1 to 3, wherein the conductive material is carbon.