JP2018170233A - Lithium air secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium air secondary battery which improves charge/discharge energy efficiency and has good charge/discharge cycle performance.SOLUTION: A lithium air secondary battery 100 includes: a positive electrode 101 using oxygen in the air as a positive electrode active material; a negative electrode 102 using metal lithium or a lithium-containing material as a negative electrode active material; and an organic electrolyte 103 containing a lithium salt. The organic electrolyte 103 contains ferric pyrophosphate, and the ferric pyrophosphate is contained in the organic electrolyte at a concentration of 0.1 wt.% or more and 4 wt.% or less.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a lithium air secondary battery using oxygen as a positive electrode active material.

  Lithium-air 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 a large amount of lithium metal, which is a negative electrode active material, can be filled in the battery. For this reason, it is known that the value of the discharge capacity per unit volume of the battery can be increased.

Incidentally, Non-Patent Document 1 reports an example in which a transition metal oxide such as λ-MnO ₂ is used as an electrode catalyst for an air electrode. Non-Patent Document 2 reports an example in which a transition metal oxide such as iron oxide (Fe ₂ O ₃ ) or cobalt oxide (Co ₃ O ₄ ) is mainly used.

J. Read, “Characterization of the Lithium / Oxygen Organic Electrolyte Battery”, Journal of The Electrochemical Society, Vol. 149, pp. A1190-A1195 (2002). Aurelie Debart et al. “An O2 cathode for rechargeable lithium batteries: The effect of a catalyst”, Journal of Power Sources, Vol. 174, pp. 1177 (2007).

  In the secondary battery disclosed in Non-Patent Document 1, the discharge capacity after 4 cycles of charge and discharge is reduced to about 1/4, and the performance as a secondary battery is low. In addition, the charging voltage is about 4.0V, which is higher than the average discharge voltage of 2.7V, and there is a problem that the charge / discharge energy efficiency is low.

  In addition, the secondary battery disclosed in Non-Patent Document 2 can obtain a large discharge capacity of 10003000 mAh / g per weight of carbon contained in the air electrode. However, the capacity retention rate drops to about 65% after 10 cycles of charge / discharge. Thus, the conventional lithium air secondary battery has the subject that charging / discharging energy efficiency is low and charging / discharging cycling performance is bad.

  The present invention has been made in view of this problem, and an object of the present invention is to provide a lithium-air secondary battery with improved charge / discharge energy efficiency and good charge / discharge cycle performance.

  The lithium-air secondary battery according to one aspect of the present embodiment includes a positive electrode that uses oxygen in the air as a positive electrode active material, a negative electrode that uses metallic lithium or a lithium-containing material as a negative electrode active material, and an organic electrolyte that includes a lithium salt. And the organic electrolyte contains ferric pyrophosphate.

  ADVANTAGE OF THE INVENTION According to this invention, charging / discharging energy efficiency can be improved and a lithium air secondary battery with favorable charging / discharging cycling performance can be provided.

It is a figure which shows typically the structure of the lithium air secondary battery which concerns on embodiment of this invention. It is sectional drawing which shows the more detailed structural example of the lithium air secondary battery which concerns on embodiment of this invention. It is a figure which shows the discharge characteristic of the lithium air secondary battery which concerns on embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Outline of lithium-air secondary battery]
FIG. 1 is a basic conceptual diagram of the lithium-air secondary battery of the present embodiment. As shown in the figure, a lithium-air secondary battery 100 is disposed between a positive electrode gas diffusion type air electrode 101, a negative electrode 102 containing lithium, and the air electrode 101 and the negative electrode 102. The organic electrolyte solution 103 is provided.

  The air electrode 101 can include a catalyst and a conductive material as components. 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. The organic electrolyte solution 103 of the present embodiment contains ferric pyrophosphate as an electrolyte additive.

  Hereinafter, each component of the lithium air secondary battery 100 of this embodiment is demonstrated. In addition, electrolyte solution means the case where electrolyte is a liquid form here.

(I) Electrolyte (electrolyte)
The electrolyte of the lithium air secondary battery 100 includes at least ferric pyrophosphate as an additive. More specifically, Li salt and an organic solvent are included, and ferric pyrophosphate is included as an additive.

The organic electrolyte 103 may be any substance that can move lithium ions between the positive and negative electrodes. For example, a nonaqueous solvent in which a metal salt containing lithium ions is dissolved can be used. Examples of the solute include lithium hexafluorophosphate (LiPF ₆ ), lithium perchlorate (LiClO ₄ ), lithium bistrifluoromensulfonylimide [ (CF ₃ SO ₂ ) 2NLi] (LiTFSI) or the like can be used.

  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), 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), 1,2-carbonate Carbonate ester solvents such as butylene (1,2-BC), ether solvents such as 1,2-dimethoxyethane (DME), lactone solvents such as γ-butyrolactone (GBL), tetraethylene glycol dimethyl ether (TEGDME, etc.) Glyme solvents and dimethyl The mixing ratio of the case of using Rusuruhokishido (DMSO) sulfoxide solvent or can be given solvent obtained by mixing two or more from these. Mixed solvent, such as is not particularly limited.

(II) Air electrode (positive electrode)
The air electrode 101 of the lithium air secondary battery 100 includes at least a conductive material, and includes a catalyst and / or a binder as necessary.

(II-1) Conductive Material Carbon is suitable for the conductive material included in the air electrode 101 of the present embodiment. Examples of the conductive material include carbon blacks such as ketjen black and acetylene black, activated carbons, graphites, carbon fibers, carbon sheets, and carbon cloth.

(II-2) Catalyst The catalyst of the air electrode 101 of the present embodiment is for both oxygen reduction (discharge) and oxygen generation (charge) reactions such as manganese oxide (MnO ₂ ) and ruthenium oxide (RuO ₂ ). There is no particular limitation as long as it is a highly active oxide catalyst. 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 , and WO ₃ 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 A composite oxide having a perovskite structure such as _0.4 MnO ₃ , LaNiO ₃ , La _0.6 Sr _0.4 Mn _0.4 Fe _0.6 O ₃ can be used. These catalysts can be synthesized using conventional processes such as solid phase methods and liquid phase methods.

  Moreover, as a catalyst added to the air electrode 101, a macrocyclic metal complex such as porphyrin or phthalocyanine containing at least one transition metal such as Mn, Fe, Co, Ni, V, and W as a central metal can also 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 101, 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 101, an electrode reaction proceeds in a three-phase portion of electrolyte / electrode catalyst / gas (oxygen). That is, the organic electrolyte solution 103 penetrates into the air electrode 101, and oxygen gas in the atmosphere is supplied at the same time, thereby forming a three-phase site where electrolyte solution-electrode catalyst-gas (oxygen) coexists. 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 101 can be expressed as follows.

2Li ⁺ + O ₂ + 2e ⁻ → Li ₂ O ₂ (1)
Lithium ions (Li ⁺ ) in the formula (1) are dissolved in the organic electrolytic solution 103 from the negative electrode 102 by electrochemical oxidation, and move through the organic electrolytic solution 103 to the surface of the air electrode 101. Further, 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 to be deposited at the cathode 101 _{(Li 2 O 2),} and showed air _{(O 2)} together with the components of FIG.

An oxide that can be used as an electrode catalyst for the air electrode 101 (positive electrode), particularly manganese oxide (MnO ₂ ) or a perovskite oxide containing manganese (such as La _0.6 Sr _0.4 MnO ₃ ) is manganese. It can exist in ions having a valence of +4, +3, etc. In addition, depending on the conditions for synthesizing these oxides, oxygen vacancies capable of taking oxygen into the oxide exist and are considered to function as active sites. 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. .

  As described above, the oxygen species adsorbed on the oxide surface or occluded in the oxygen vacancies is used as an oxygen source (active intermediate reactant) in the oxygen reduction reaction so that the reaction proceeds easily. (Equation (1)). Moreover, said oxide has activity also with respect to the charging reaction which is a reverse reaction of Formula (1). Therefore, battery charging, that is, oxygen generation reaction on the air electrode 101 also proceeds efficiently. Thus, the oxide containing manganese effectively functions as an electrode catalyst.

In the lithium air secondary battery 100 of the present embodiment, in order to increase the efficiency of the battery, there are more reaction sites (the above-described three-phase portion of electrolyte / electrode catalyst / air (oxygen)) that cause an electrode reaction. Is desirable. From such a viewpoint, 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.

(II-3) Binder (binder)
The air electrode 101 can 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.

  In the lithium air secondary battery 100 of the present embodiment, the catalyst content of the air electrode 101 is preferably, for example, more than 0 and 100% by weight or less based on the weight of the air electrode 101. The ratio of other components is the same as that of the conventional lithium air secondary battery.

(III) Negative Electrode The lithium-air secondary battery 100 of this embodiment includes a negative electrode active material in the negative electrode 102. 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.

  Note that when the above-described silicon or tin alloy is used as the negative electrode, 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 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.

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

(Discharge reaction)
Li → Li ⁺ + e ⁻ (2)
In addition, in the negative electrode 102 at the time of charge, the lithium precipitation reaction which is the reverse reaction of Formula (2) occurs.

(IV) Other Elements In addition to the above-described components, the lithium-air secondary battery 100 of this embodiment is required for structural members such as separators, battery cases, metal meshes (for example, titanium mesh), and other lithium-air secondary batteries. Can be included. These can use conventional ones.

[Configuration of lithium-air secondary battery]
Next, the structure of the lithium air secondary battery 100 of this embodiment is demonstrated.

  FIG. 2 is a cross-sectional view showing a more detailed configuration example of the lithium air secondary battery 100 of the present embodiment.

  2 is a cylindrical lithium-air battery, and includes an air electrode 101, a negative electrode 102, an organic electrolyte 103, a separator 105, an air electrode support 115, an air electrode fixing ring 104, a negative electrode. A fixing ring 107, a negative electrode fixing washer 108, a negative electrode support 109, a fixing screw 110, an O-ring 111, an air electrode terminal 121, and a negative electrode terminal 122 are provided.

  The air electrode 101, the negative electrode 102, the organic electrolyte solution 103, and the separator 105 are accommodated in a cylindrical air electrode support 115. The air electrode support 115 is made of metal and makes electrical contact with the air electrode 101. However, the portion where the air electrode support 115 is in contact with the organic electrolyte solution 103 and the separator 105 is covered with polytetrafluoroethylene (PTFE) and insulated and separated.

  The air electrode support 115 has a partition 151 at the center in the cylinder. By the partition 151, the inside of the cylinder of the air electrode support 115 is divided into a region where the air electrode 101 is disposed and a region where the negative electrode 102 and the separator 105 are disposed. The partition 151 is open at the center, and both regions communicate with each other.

  The organic electrolyte solution 103 is disposed in the opening of the partition 151 and is sandwiched between the air electrode 101 and the separator 105 serving as a salt bridge. The organic electrolyte solution 103 is impregnated in the separator 105 and is also present around the separator 105.

  The air electrode 101 is sandwiched and fixed between a cylindrical air electrode fixing ring 104 and a partition 151 made of PTFE.

  The separator 105 is sandwiched and fixed between a cylindrical negative electrode fixing ring 107 made of PTFE and a partition 151.

  The negative electrode 102 is laminated on the negative electrode fixing washer 108 and is in contact with the separator 105 inside the cylinder of the negative electrode fixing ring 107.

  The negative electrode fixing washer 108 is covered with a negative electrode support 109 made of metal. The negative electrode support 109 is fixed to the air electrode support 115 by a fixing screw 110. An O-ring 111 is disposed between the air electrode support 115 and the negative electrode support 109. The fixing screw 110 is covered with PTFE so that the air electrode support 115 and the negative electrode support 109 are electrically separated.

  The negative electrode support 109 is pressed against the air electrode support 115 by a fixing screw 110. The negative electrode 102 is pressed through the negative electrode fixing washer 108 and pressed against the separator 105.

  The air electrode 101 is electrically connected to the air electrode support 115 and the negative electrode terminal 122 is electrically connected to the negative electrode support 109.

[Procedure for Lithium-Air Secondary Battery]
Next, a manufacturing procedure of the lithium air secondary battery 100 of FIG. 2 will be described.

  Lithium-air battery cells are produced in the following procedure in dry air with a dew point of -60 ° C or lower.

First, La _0.6 Sr _0.4 MnO ₃ powder, ketjen black powder and polyvinylidene fluoride (PVDF) powder in a weight ratio of 10:72:18 were mixed with N-methyl-2-pyrrolidone (NMP) using a mixer. ) To prepare a slurry. This slurry is applied to a carbon sheet having a diameter of 17 mm, placed in a 90 ° C. vacuum dryer, and dried overnight to produce a gas diffusion type air electrode 101. La _0.6 Sr _0.4 MnO ₃ was synthesized by a method using citric acid.

  The air electrode 101 is disposed in a recess of the air electrode support 115 covered with PTFE, and is fixed by the air electrode fixing ring 104. It should be noted that the portion where the air electrode 101 and the air electrode support 115 are in contact with each other is not covered with PTFE for electrical contact.

  For example, in the case of using lithium metal, the negative electrode 102 is manufactured by stacking a plurality of metal lithium foils into a predetermined shape. The negative electrode 102 is fixed by pressing four metallic lithium foils having a thickness of 150 μm on a negative electrode fixing washer 108.

  The separator 105 is disposed inside the cylinder of the air electrode support 115 so as to be in contact with the partition 151 from the side opposite to the air electrode 101, and the negative electrode fixing ring 107 is disposed on the cylinder of the air electrode support 115 from the same side as the separator 105. A negative electrode fixing washer 108 to which the negative electrode 102 is crimped is disposed inside the cylinder of the negative electrode fixing ring 107.

  The inside of the cell (between the air electrode 101 and the negative electrode 102) is filled with an electrolyte containing ferric pyrophosphate as described above, and the O-ring 111 is disposed at the bottom of the air electrode support 115, thereby supporting the negative electrode. The body 109 is covered, and the air electrode support 115 and the negative electrode support 109 are fixed with the fixing screw 110. As the organic electrolyte, ferric pyrophosphate-containing organic electrolyte (1 mol / l: LiTFSI / TEGDME solution) was used.

  Finally, the air electrode terminal 121 is connected and fixed to the air electrode support 115, and the negative electrode terminal 122 is connected and fixed to the negative electrode support 109.

[Battery cycle test]
Next, a battery cycle test will be described. In the battery cycle test, a charge / discharge measurement system (manufactured by Bio Logic) was used to conduct 0.1 mA / cm ² at a current density per area of the air electrode 101, and the battery voltage was changed from 2.0 V to 2.0 V. The discharge voltage was measured until it decreased. The battery charge test was performed until the battery voltage reached 4.2 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 air electrode (carbon + oxide + PVDF).

  An experimental example in which battery performance was evaluated by changing the concentration of the organic electrolyte solution 103 of the lithium-air secondary battery 100 according to the present embodiment and the concentration of ferric pyrophosphate contained in the electrolyte solution will be described.

  In Experimental Examples 1 to 5, the organic electrolyte solution 103 was prepared by dissolving LiTFSI at a concentration of 1 mol / L in the organic solvent TEGDME. In Experimental Example 1, ferric pyrophosphate (Tonda Pharmaceutical Co., Ltd.) was mixed with the solution in a weight of 0.09 wt%. When mixing, dispersion was performed for about 2 hours using an ultrasonic cleaner.

  In Experimental Example 2, ferric pyrophosphate was mixed with the same solution as in Example 1 at a weight of 0.1 wt%. In Experimental Example 3, the weight of ferric pyrophosphate was set to 2.0 wt%. In Experimental Example 4, the weight of ferric pyrophosphate was 4.0 wt%. In Experimental Example 5, the weight of ferric pyrophosphate was 4.1 wt%.

  In Experimental Examples 6 to 10, the organic electrolyte solution 103 was prepared by dissolving LiTFSI at a concentration of 1 mol / L in the organic solvent DMSO. The amount of ferric pyrophosphate added is the same as in Experimental Examples 1-5. That is, the weight of ferric pyrophosphate in Experimental Example 6 is 0.09 wt%, the same weight in Experimental Example 7 is 0.1 wt%, the same weight in Experimental Example 8 is 2.0 wt%, and the same weight in Experimental Example 9 is 4.0 wt%. And the same weight of Experimental Example 10 is 4.1 wt%.

  In Experimental Examples 11 to 15, the organic electrolytic solution 103 was prepared by dissolving LiTFSI in the organic solvent PC at a concentration of 1 mol / L. In Experimental Examples 16 to 20, the organic electrolyte solution 103 was prepared by dissolving LiTFSI at a concentration of 1 mol / L in an organic solvent / DMC (volume ratio 1: 1). In Experimental Examples 21 to 25, the organic electrolyte solution 103 was prepared by dissolving LiTFSI at a concentration of 1 mol / L in the organic solvent GBL. In addition, the weight of each ferric pyrophosphate of Experimental Examples 11 to 25 is the same as that of Experimental Examples 1 to 5 and the like. For the relationship between each experimental example and the weight of ferric pyrophosphate, see Table 1 below.

  FIG. 3 shows an example of the initial discharge and charging curve of the lithium-air secondary battery 100 of Experimental Example 2. The horizontal axis in FIG. 3 is the charge / discharge capacity (mAh / g), and the vertical axis is the battery voltage (V). The characteristic indicated by the solid line in the figure is the charging characteristic, and the characteristic indicated by the broken line is the discharge characteristic.

  It was confirmed that the average discharge voltage was 2.65 V and the discharge capacity was 825 mAh / g, which were relatively large values. 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 FIG. In addition, the initial charge voltage is 3.78V, and the charge capacity is 730 mAh / g, which is almost the same as the discharge capacity.

  Table 1 shows the results of the battery performance tests of Experimental Examples 1 to 25.

  As shown in Table 1, when the addition amount of ferric pyrophosphate is 2.0 wt%, the largest discharge capacity, the highest discharge voltage, and the lowest charge voltage are shown.

  When the addition amount of ferric pyrophosphate is 0.09 wt% and 4.1 wt%, the discharge capacity is severely degraded after 50 cycles. This is because the amount of ferric pyrophosphate added is low at low concentrations, and the effect is limited. At high concentrations, the conductivity decreases due to increased viscosity due to the addition of ferric pyrophosphate. This is considered to be because the charge / discharge cycle stability decreased due to the influence.

  From the results in Table 1, it can be seen that the addition amount of ferric pyrophosphate has a large discharge capacity and sufficient charge / discharge cycle performance in the concentration range of 0.1 wt% or more and 4.0 wt% or less.

  Table 2 shows an example compared with an organic electrolyte without adding ferric pyrophosphate. In Comparative Example 1, a 1 mol / l LiTFSI / TEGDME solution was used as the organic electrolyte. Comparative Example 2 used DMSO, Comparative Example 3 used PC, Comparative Example 4 used EC / DMC, and Comparative Example 5 used GBL solution. Conditions other than the addition of ferric pyrophosphate were the same as in the above experimental example.

  Table 2 shows that the discharge capacity of the initial performance is almost the same or a little higher in the comparative example when comparing the result with no addition of ferric pyrophosphate according to this comparative example and the addition amount of 0.09 wt%. The result was obtained. However, when the charge / discharge cycle is repeated, the discharge capacity is remarkably reduced in the case of no addition, and only a value of several percent of the initial capacity can be obtained after 50 cycles. This indicates that the addition of ferric pyrophosphate to the electrolyte is an effective technique for long-term stable operation of the lithium-air secondary battery.

  As described above, according to the present invention, by including ferric pyrophosphate as an additive in an electrolyte, it is possible to provide a lithium air secondary battery having a large discharge capacity and good charge / discharge cycle performance. it can. In addition, this invention is not limited to said embodiment, A deformation | transformation is possible within the range of the summary.

  By using ferric pyrophosphate as an additive for the organic electrolyte, a high-performance lithium-air secondary battery can be produced and can be effectively used as a driving source for various electronic devices and automobiles.

100: Lithium air secondary battery 101: Positive electrode (air electrode)
102: Negative electrode 103: Organic electrolyte 104: Air electrode fixing ring 105: Separator 107: Negative electrode fixing ring 108: Negative electrode fixing washer 109: Negative electrode support 110: Fixing screw 111: O-ring 121: Air electrode terminal 122: Negative terminal 151: Partition

Claims (2)

In a lithium air secondary battery having a positive electrode using oxygen in the air as a positive electrode active material, a negative electrode using metallic lithium or a lithium-containing material as a negative electrode active material, and an organic electrolyte containing a lithium salt,
A lithium-air secondary battery, wherein the organic electrolyte contains ferric pyrophosphate.   2. The lithium air secondary battery according to claim 1, wherein the ferric pyrophosphate is contained in the organic electrolyte solution at a concentration of 0.1 wt% or more and 4 wt% or less.

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