JP5556618B2 - Lithium air battery - Google Patents

Description

  The present invention relates to an electrolytic solution for a lithium secondary battery that can suppress the generation of dendrites, and a lithium secondary battery and a lithium-air battery including the electrolytic solution.

  The secondary battery can convert the decrease in chemical energy associated with the chemical reaction into electrical energy and perform discharge. In addition, the secondary battery converts electrical energy into chemical energy by flowing current in the opposite direction to that during discharge. The battery can be stored (charged). Among secondary batteries, a metal secondary battery represented by a lithium secondary battery has a high energy density, and is therefore widely applied as a power source for notebook personal computers, cellular phones, and the like.

In the lithium secondary battery, when graphite (expressed as C) is used as the negative electrode active material, the reaction of the following formula (I) proceeds in the negative electrode during discharge.
Li _x C → C + xLi ⁺ + xe ⁻ (I)
(In the above formula (I), 0 <x <1.)
The electrons generated by the reaction of the above formula (I) reach the positive electrode after working with an external load via an external circuit. Then, lithium ions (Li ⁺ ) generated by the reaction of the above formula (I) move from the negative electrode side to the positive electrode side by electroosmosis in the electrolyte sandwiched between the negative electrode and the positive electrode.

When lithium cobaltate (Li _1-x CoO ₂ ) is used as the positive electrode active material, the reaction of the following formula (II) proceeds at the positive electrode during discharge.
Li _1-x CoO ₂ + xLi ⁺ + xe ⁻ → LiCoO ₂ (II)
(In the above formula (II), 0 <x <1.)
At the time of charging, reverse reactions of the above formulas (I) and (II) proceed in the negative electrode and the positive electrode, respectively, and in the negative electrode, graphite (Li _x C) containing lithium by graphite intercalation is Since lithium cobaltate (Li _1-x CoO ₂ ) is regenerated, re-discharge is possible.

In recent years, ionic liquids (room temperature molten salts) are less volatile and flammable than organic solvents, and the physical properties can be easily adjusted during synthesis by changing the substituent on the cation center. As a candidate for an electrolyte solution for a lithium secondary battery, it is attracting new attention. Among ionic liquids, in particular, ionic liquids containing quaternary ammonium cations are actively studied as having excellent stability in a cathode environment using lithium ions.
Non-Patent Document 1 describes thermal stability, cycle characteristics, and the like when an ionic liquid containing a quaternary ammonium cation is used as an electrolytic solution for a battery.

H. Sakaebe et al. Electrochimica Acta 53 (2007) 1048-1054

As a result of the study by the present inventors, as shown in Examples described later, the electrolyte solution described in Non-Patent Document 1 is likely to generate dendrites due to the lithium salt concentration in the electrolyte solution being too low. Became clear.
The present invention has been accomplished in view of the above circumstances, and an object thereof is to provide an electrolytic solution for a lithium secondary battery that can suppress the generation of dendrites, and a lithium secondary battery and a lithium air battery including the electrolytic solution. And

The lithium-air battery of the present invention is a lithium-air battery comprising at least an air electrode, a negative electrode, and an electrolyte solution interposed between the air electrode and the negative electrode, wherein the electrolyte solution includes at least an ionic liquid and lithium. containing salt, and the concentration of the lithium salt of the electrolytic solution 0.37~0.75mol / kg der is, and the air electrode is characterized that you containing a nickel catalyst.

  In the present invention, the ionic liquid includes a cation and a counter anion thereof, and the cation is an ammonium cation having a heterocyclic structure closed by a nitrogen atom and an alkyl chain having 2 to 10 carbon atoms, and the counter anion is It is preferably inert to lithium metal.

  In the present invention, the cation is N-methyl-N-propylpiperidinium cation, N-methyl-N-propylpyrrolidinium cation, N-butyl-N-methylpiperidinium cation, N-butyl-N. It is preferably a cation selected from the group consisting of -methylpyrrolidinium cation, N-methyl-N-methoxyethylpiperidinium cation and N-methyl-N-methoxyethylpyrrolidinium cation.

In this invention, it is preferable that the content rate of a nickel catalyst when the total mass of the said air electrode is 100 mass% is 5-15 mass%.

  According to the present invention, when the lithium salt concentration in the electrolytic solution is 0.37 mol / kg or higher, which is higher than the lithium salt concentration in the conventional electrolytic solution, lithium ions are sufficiently contained in the vicinity of the negative electrode during charging. As a result, the generation of dendrite, which is one of the main causes of the lack of lithium ions, can be suppressed. Further, according to the present invention, when the concentration of the lithium salt in the electrolytic solution is 0.75 mol / kg or less, the electrolytic solution has an appropriate viscosity, and sufficient lithium ions are provided near the negative electrode during charging. As a result, the generation of dendrites can be suppressed.

It is a cross-sectional schematic diagram which shows an example of the lithium secondary battery of this invention. When the electrolyte solution of Example 1-3 and Comparative Example 1-3 was used, it is the graph which plotted the time and the ionic conductivity of the lithium which precipitated uniformly. When the electrolyte solution of Example 4 and Comparative Example 4 is used, it is the graph which plotted the value of the time when lithium precipitated uniformly, and the ionic conductivity. 6 is a charge / discharge curve of the lithium-air battery of Example 5. FIG. It is the graph which plotted the value of the discharge capacity of the lithium air battery of Example 5, and the comparative example 5 and the comparative example 6, and coulomb efficiency. It is a charging / discharging curve of the lithium air battery of the comparative example 5 and the comparative example 6.

1. Electrolyte for lithium secondary battery The electrolyte for lithium secondary battery of the present invention is an electrolyte for lithium secondary battery containing at least an ionic liquid and a lithium salt, and the concentration of the lithium salt in the electrolyte is It is 0.37 to 0.75 mol / kg.

As described above, the electrolytic solution described in Non-Patent Document 1 has a low lithium salt concentration of 0.32 mol / kg in the electrolytic solution. As shown in Examples described later, the inventors of the electrolytic solution of Comparative Example 2 in which the lithium salt concentration in the electrolytic solution is 0.32 mol / kg has a time for which lithium is uniformly deposited on the negative electrode for 60 seconds. They found that lithium dendrite was likely to be generated in the negative electrode.
As a result of intensive studies, the inventors of the present invention have a remarkably long time during which lithium is uniformly deposited on the negative electrode by setting the concentration of the lithium salt in the electrolytic solution within the range of 0.37 to 0.75 mol / kg. Thus, the inventors have found that generation of lithium dendrite in the negative electrode can be suppressed, and completed the present invention.

  The cation in the ionic liquid used in the present invention is not particularly limited as long as it is usually used as a cation species of the ionic liquid, but has a heterocyclic structure closed by a nitrogen atom and an alkyl chain having 2 to 10 carbon atoms. It is preferable that it is the ammonium cation which has.

  Specific examples of the cation used in the present invention include N-methyl-N-propylpiperidinium cation (PP13), N-methyl-N-propylpyrrolidinium cation (P13), and N-butyl-N. -Methylpiperidinium cation (PP14), N-butyl-N-methylpyrrolidinium cation (P14), N-methyl-N-methoxyethylpiperidinium cation (PP1.1o2) or N-methyl-N-methoxy It is preferable to use an ethylpyrrolidinium cation (P1.1o2).

The counter anion used in the present invention is not particularly limited as long as it is normally used as an anionic species of an ionic liquid, but an anion which is inactive with respect to lithium metal is preferable.
The anion inactive to the lithium metal herein refers to a stable anion that does not change its chemical structure even when the lithium metal is immersed in an electrolytic solution containing the anion for 100 minutes. On the other hand, an anion active against lithium metal refers to an anion that decomposes by immersing lithium metal in an electrolyte containing the anion for 100 minutes.
The counter anions used in the present invention are specifically [N (CF ₃ ) ₂ ] ⁻ , [N (SO ₂ CF ₃ ) ₂ ] ⁻ , [N (SO ₂ C ₂ F ₅ ) ₂ ] ^−. Imide anions such as RSO ₃ ⁻ (hereinafter R represents an aliphatic hydrocarbon group or aromatic hydrocarbon group), RSO ₄ ⁻ , R ^f SO ₃ ⁻ (hereinafter R ^f is a fluorine-containing halogenated hydrocarbon group) Sulfite anion or sulfate anion such as R ^f SO ₄ ^— ; phosphate anions such as R ^f ₂ P (O) O ⁻ , R ^f ₃ PF ₃ ^— ; and others such as lactate anion, trifluoroacetate anion, etc. Can be mentioned.
Among these anions, the counter anion used in the present invention is preferably a bis (trifluoromethanesulfonyl) imide anion ([N (SO ₂ CF ₃ ) ₂ ] ⁻ ).

The electrolytic solution used in the present invention further contains a lithium salt. Examples of lithium salts include inorganic lithium salts such as LiPF ₆ , LiBF ₄ , LiClO ₄ and LiAsF ₆ ; LiCF ₃ SO ₃ , LiN (SO ₂ CF ₃ ) ₂ (Li-TFSI), LiN (SO ₂ C ₂ F ₅ ) ₂ and organic lithium salts such as LiC (SO ₂ CF ₃ ) ₃ . Two or more such lithium salts may be used in combination.

In the present invention, one of the main features is that the concentration of the lithium salt in the electrolytic solution is 0.37 to 0.75 mol / kg. As shown in the examples described later, when the concentration of the lithium salt in the electrolytic solution is less than 0.37 mol / kg, lithium is uniformly deposited only for an extremely short time of about 1 minute at the time of charging. Is extremely likely to generate lithium dendrite on the negative electrode. This is considered to be due to the fact that the lithium ion concentration in the electrolyte is too low, so that lithium ions cannot be sufficiently supplied to the vicinity of the negative electrode during charging. On the other hand, when the concentration of the lithium salt in the electrolytic solution exceeds 0.75 mol / kg, lithium is uniformly deposited only for an extremely short time of about 1 minute during charging, and then lithium dendrite is generated on the negative electrode. The fear is very high and the ionic conductivity is also very low. This is considered to be due to the fact that the concentration of the lithium salt in the electrolytic solution is too high, the viscosity of the electrolytic solution becomes too high, and lithium ions cannot be sufficiently supplied to the vicinity of the negative electrode during charging.
In the present invention, the concentration of the lithium salt in the electrolytic solution is preferably 0.4 to 0.7 mol / kg, and more preferably 0.45 to 0.6 mol / kg.

The electrolytic solution used in the present invention may contain a non-aqueous electrolyte in addition to the ionic liquid and the lithium salt.
As the non-aqueous electrolyte, a non-aqueous electrolyte solution and a non-aqueous gel electrolyte can be used.
The nonaqueous electrolytic solution used in the present invention usually contains the above-described lithium salt and nonaqueous solvent. Examples of the non-aqueous solvent include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), ethyl carbonate, butylene carbonate, γ-butyrolactone, and sulfolane. , Acetonitrile, dimethoxymethane, 1,2-dimethoxyethane, 1,3-dimethoxypropane, diethyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, and mixtures thereof. Further, from the viewpoint that dissolved oxygen can be efficiently used for the reaction, the non-aqueous solvent is preferably a solvent having high oxygen solubility.

  Further, the non-aqueous gel electrolyte used in the present invention is usually a gel obtained by adding a polymer to a non-aqueous electrolyte solution. For example, it can be obtained by adding a polymer such as polyethylene oxide (PEO), polyacrylonitrile (PAN), or polymethyl methacrylate (PMMA) to the non-aqueous electrolyte solution described above and gelling.

2. Lithium secondary battery and lithium air battery The lithium secondary battery of the present invention is a lithium secondary battery comprising at least a positive electrode, a negative electrode, and an electrolyte solution interposed between the positive electrode and the negative electrode. The liquid is the above-mentioned electrolytic solution for a lithium secondary battery.
The lithium-air battery of the present invention is a lithium-air battery comprising at least an air electrode, a negative electrode, and an electrolyte solution interposed between the air electrode and the negative electrode, wherein the electrolyte solution is for the lithium secondary battery. It is an electrolytic solution, and the air electrode contains a nickel catalyst.

FIG. 1 is a diagram showing an example of a layer configuration of a lithium secondary battery according to the present invention, and is a diagram schematically showing a cross section cut in a stacking direction. The lithium secondary battery according to the present invention is not necessarily limited to this example.
The lithium secondary battery 100 is sandwiched between the positive electrode 6 including the positive electrode active material layer 2 and the positive electrode current collector 4, the negative electrode 7 including the negative electrode active material layer 3 and the negative electrode current collector 5, and the positive electrode 6 and the negative electrode 7. An electrolytic solution 1 is provided.
Among the lithium secondary batteries according to the present invention, the electrolytic solution is as described above. Hereinafter, the positive electrode, the negative electrode, the separator, and the battery case, which are components of the lithium secondary battery according to the present invention, will be described in detail.

(Positive electrode)
The positive electrode of the lithium secondary battery according to the present invention preferably comprises a positive electrode active material layer having a positive electrode active material, and is usually connected to the positive electrode current collector and the positive electrode current collector. The positive electrode lead is provided. When the lithium secondary battery according to the present invention is a lithium-air battery, an air electrode including an air electrode layer is provided instead of the positive electrode.

(Positive electrode active material layer)
Hereinafter, the case where the positive electrode provided with a positive electrode active material layer is employ | adopted as a positive electrode is demonstrated.
Specific examples of the positive electrode active material used in the present invention include LiCoO ₂ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ , LiNiPO ₄ , LiMnPO ₄ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoMnO _4. , Li ₂ NiMn ₃ O ₈ , Li ₃ Fe ₂ (PO ₄ ) _3, Li ₃ V ₂ (PO ₄ ) _{3 and the} like. Among these, in the present invention, LiCoO ₂ is preferably used as the positive electrode active material.

  The thickness of the positive electrode active material layer used in the present invention varies depending on the intended use of the lithium secondary battery, but is preferably in the range of 10 μm to 250 μm, and in the range of 20 μm to 200 μm. It is particularly preferred that it is most preferably in the range of 30 μm to 150 μm.

  The average particle diameter of the positive electrode active material is, for example, preferably in the range of 1 μm to 50 μm, more preferably in the range of 1 μm to 20 μm, and particularly preferably in the range of 3 μm to 5 μm. If the average particle size of the positive electrode active material is too small, the handleability may be deteriorated. If the average particle size of the positive electrode active material is too large, it may be difficult to obtain a flat positive electrode active material layer. Because. The average particle diameter of the positive electrode active material can be determined by measuring and averaging the particle diameter of the active material carrier observed with, for example, a scanning electron microscope (SEM).

The positive electrode active material layer may contain a conductive material, a binder, and the like as necessary.
The conductive material included in the positive electrode active material layer used in the present invention is not particularly limited as long as the conductivity of the positive electrode active material layer can be improved. For example, carbon black such as acetylene black and ketjen black Etc. Moreover, although content of the electrically conductive material in a positive electrode active material layer changes with kinds of electrically conductive material, it is in the range of 1 mass%-10 mass% normally.

  Examples of the binder included in the positive electrode active material layer used in the present invention include polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE). In addition, the content of the binder in the positive electrode active material layer may be an amount that can fix the positive electrode active material or the like, and is preferably smaller. The content of the binder is usually in the range of 1% by mass to 10% by mass.

(Positive electrode current collector)
The positive electrode current collector used in the present invention has a function of collecting the positive electrode active material layer. Examples of the material for the positive electrode current collector include aluminum, SUS, nickel, iron, and titanium. Among these, aluminum and SUS are preferable. Moreover, as a shape of a positive electrode electrical power collector, foil shape, plate shape, mesh shape etc. can be mentioned, for example, Foil shape is preferable.

  The electrode active material layer of at least one of the positive electrode and the negative electrode can also be configured to contain at least an electrode active material and an electrode electrolyte. In this case, as the electrode electrolyte, a solid electrolyte such as a solid oxide electrolyte or a solid sulfide electrolyte, the above-described gel electrolyte, or the like can be used.

  The method for producing the positive electrode used in the present invention is not particularly limited as long as it is a method capable of obtaining the positive electrode. In addition, after forming a positive electrode active material layer, in order to improve an electrode density, you may press a positive electrode active material layer.

(Air electrode layer)
Hereinafter, a case where an air electrode including an air electrode layer is employed as the positive electrode will be described. The air electrode layer used in the present invention preferably contains at least a catalyst and a conductive material. The air electrode layer used in the present invention may further contain a binder as necessary.

The air electrode used in the present invention contains a nickel catalyst, and the content ratio of the nickel catalyst is preferably 5 to 15% by mass when the total mass of the air electrode is 100% by mass.
As shown in the examples described later, when the content ratio of the nickel catalyst is less than 5% by mass, the deposition site of the discharge product in the air electrode can be effectively used because the nickel content ratio is too low. In addition, since the discharge products are preferentially deposited, the discharge capacity may be lowered, and the lithium oxide (Li ₂ O ₂ and / or Li ₂ O) generated at the time of discharge is all electrically charged at the time of charge. The coulomb efficiency is likely to be low.
On the other hand, when the content ratio of the nickel catalyst exceeds 15% by mass, the content ratio of the conductive material such as the carbon material in the air electrode is relatively reduced due to the nickel content being too high, Since the active sites on the surface of the carbon material and the like are reduced, the discharge capacity may be lowered, and further, the size of the precipitate of lithium oxide (Li ₂ O ₂ and / or Li ₂ O) generated during discharge is small. Since it becomes large and it becomes difficult to decompose | disassemble all the said deposits at the time of charge, there exists a possibility that coulomb efficiency may become low.
In this invention, it is more preferable that the content rate of a nickel catalyst when the total mass of an air electrode is 100 mass% is 5-15 mass%, and it is further more preferable that it is 7-12 mass%.

  The nickel catalyst that can be used in the present invention is nickel metal (Ni); an alloy of nickel and at least one metal selected from the group consisting of Fe, Cu, Ti, Au, Pt, Zu, Cr, Mo, W, and the like; A nickel compound comprising at least one nonmetallic element selected from the group consisting of B, C, Si and the like and nickel; and the like can be used.

In addition to the nickel catalyst described above, examples of a catalyst that can be used for the air electrode layer include an oxygen active catalyst. Examples of oxygen active catalysts include, for example, platinum groups such as palladium and platinum; perovskite oxides containing transition metals such as cobalt, manganese or iron; inorganic compounds containing noble metal oxides such as ruthenium, iridium or palladium; porphyrins Metal coordination organic compounds having a skeleton or a phthalocyanine skeleton; manganese oxide and the like.
From the viewpoint that the electrode reaction is performed more smoothly, it is preferable that the catalyst is supported on a conductive material described later.

The conductive material used for the air electrode layer is not particularly limited as long as it has conductivity. For example, a carbon material, a perovskite-type conductive material, a porous conductive polymer, a metal porous body, etc. Can be mentioned. In particular, the carbon material may have a porous structure or may not have a porous structure, but in the present invention, the carbon material preferably has a porous structure. This is because the specific surface area is large, so that many reaction fields can be provided, and it also serves as a gas diffusion layer. Specific examples of the carbon material having a porous structure include mesoporous carbon. On the other hand, specific examples of the carbon material having no porous structure include graphite, acetylene black, carbon nanotube, and carbon fiber.
As a content rate of the electroconductive material in an air electrode layer, it is preferable to exist in the range of 65 mass%-99 mass%, for example in the range of 75 mass%-95 mass% especially. When the content ratio of the conductive material is too small, the reaction field is decreased, and the battery capacity may be reduced. When the content ratio of the conductive material is too large, the content of the catalyst is relatively reduced, and there is a possibility that a sufficient catalytic function cannot be exhibited.

  The air electrode layer only needs to contain at least a catalyst and a conductive material, but preferably further contains a binder. Examples of the binder include rubber resins such as polyvinylidene fluoride (PVdF), polytetrafluoroethylene (PTFE), and styrene / butadiene rubber (SBR rubber). Although it does not specifically limit as a content rate of the binder in an air electrode layer, For example, it is preferable that it is in the range of 30 mass% or less, especially 1 mass%-10 mass%.

  The thickness of the air electrode layer varies depending on the application of the air battery and the like, but is preferably in the range of 2 μm to 500 μm, and more preferably in the range of 5 μm to 300 μm.

(Air current collector)
The air electrode current collector used in the present invention collects current in the air electrode layer. The material for the air electrode current collector is not particularly limited as long as it has conductivity, and examples thereof include stainless steel, nickel, aluminum, iron, titanium, and carbon. Examples of the shape of the air electrode current collector include a foil shape, a plate shape, and a mesh (grid) shape. Especially, in this invention, it is preferable that the shape of an air electrode electrical power collector is a mesh form. This is because the current collection efficiency is excellent. In this case, usually, a mesh-shaped air electrode current collector is disposed inside the air electrode layer. Furthermore, the lithium secondary battery of the present invention may include another air electrode current collector (for example, a foil-shaped current collector) that collects charges collected by the mesh-shaped air electrode current collector. good. In the present invention, a battery case to be described later may also have the function of an air electrode current collector.
The thickness of the air electrode current collector is, for example, preferably in the range of 10 μm to 1000 μm, and more preferably in the range of 20 μm to 400 μm.

(Negative electrode)
The negative electrode in the lithium secondary battery according to the present invention preferably comprises a negative electrode active material layer containing a negative electrode active material, and is usually connected to the negative electrode current collector and the negative electrode current collector in addition to this. The negative electrode lead is provided.

(Negative electrode active material layer)
The negative electrode layer in the lithium secondary battery according to the present invention contains a negative electrode active material including a metal and an alloy material. The negative electrode active material used for the negative electrode active material layer is not particularly limited as long as it can occlude / release lithium ions. For example, metal lithium, lithium alloy, metal oxide containing lithium element, and lithium element Examples thereof include metal sulfides, metal nitrides containing lithium element, and carbon materials such as graphite. The negative electrode active material may be in the form of a powder or a thin film.
Examples of the lithium alloy include a lithium aluminum alloy, a lithium tin alloy, a lithium lead alloy, and a lithium silicon alloy. Moreover, as a metal oxide containing a lithium element, lithium titanium oxide etc. can be mentioned, for example. Examples of the metal nitride containing a lithium element include lithium cobalt nitride, lithium iron nitride, and lithium manganese nitride. Further, lithium coated with a solid electrolyte can also be used for the negative electrode layer.

The negative electrode layer may contain only a negative electrode active material, or may contain at least one of a conductive material and a binder in addition to the negative electrode active material. For example, when the negative electrode active material is in the form of a foil, a negative electrode layer containing only the negative electrode active material can be obtained. On the other hand, when the negative electrode active material is in a powder form, a negative electrode layer having a negative electrode active material and a binder can be obtained. The conductive material and the binder are the same as the contents described in the above-mentioned “positive electrode active material layer” or “air electrode layer”, and thus the description thereof is omitted here.
Although it does not specifically limit as a film thickness of a negative electrode active material layer, For example, it is preferable to exist in the range of 10 micrometers-100 micrometers, especially in the range of 10 micrometers-50 micrometers.

(Negative electrode current collector)
As the material and shape of the negative electrode current collector, the same materials and shapes as those of the positive electrode current collector described above can be employed.

(Separator)
The lithium secondary battery according to the present invention may include a separator impregnated with an electrolytic solution between the positive electrode and the negative electrode. Examples of the separator include porous films such as polyethylene and polypropylene; and nonwoven fabrics such as a resin nonwoven fabric and a glass fiber nonwoven fabric.

(Battery case)
The lithium secondary battery according to the present invention usually has a battery case that houses a positive electrode, an electrolyte, a negative electrode, and the like. Specific examples of the shape of the battery case include a coin type, a flat plate type, a cylindrical type, and a laminate type.
When the battery according to the present invention is a lithium-air battery, the battery case may be an open-air battery case or a sealed battery case. An open-air battery case is a battery case having a structure in which at least the air electrode layer can sufficiently come into contact with the atmosphere. On the other hand, when the battery case is a sealed battery case, it is preferable to provide a gas (air) introduction pipe and an exhaust pipe in the sealed battery case. In this case, the gas to be introduced / exhausted preferably has a high oxygen concentration, and more preferably pure oxygen. In addition, it is preferable to increase the oxygen concentration during discharging and decrease the oxygen concentration during charging.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. However, the present invention is not limited to these examples.

1. Preparation of electrolyte [Example 1]
N-methyl-N-propylpiperidinium bis (trifluoromethanesulfonyl) imide (manufactured by Kanto Chemical Co., Inc .; hereinafter referred to as PP13TFSI) and lithium bis (trifluoromethanesulfonyl) imide (hereinafter referred to as LiTFSI) Was dissolved to a concentration of 0.4 mol / kg to prepare an electrolytic solution of Example 1.

[Example 2]
LiTFSI was dissolved in PP13TFSI so as to have a concentration of 0.5 mol / kg to prepare an electrolytic solution of Example 2.

[Example 3]
LiTFSI was dissolved in PP13TFSI so as to have a concentration of 0.75 mol / kg to prepare an electrolytic solution of Example 3.

[Example 4]
LiTFSI is dissolved in N-butyl-N-methylpyrrolidinium bis (trifluoromethanesulfonyl) imide (manufactured by Kanto Chemical Co., Inc .: hereinafter sometimes referred to as P14TFSI) to a concentration of 0.5 mol / kg. The electrolyte solution of Example 4 was prepared.

[Comparative Example 1]
LiTFSI was dissolved in PP13TFSI so as to have a concentration of 0.1 mol / kg to prepare an electrolytic solution of Comparative Example 1.

[Comparative Example 2]
LiTFSI was dissolved in PP13TFSI so as to have a concentration of 0.32 mol / kg to prepare an electrolytic solution of Comparative Example 2.

[Comparative Example 3]
LiTFSI was dissolved in PP13TFSI so as to have a concentration of 1.0 mol / kg to prepare an electrolytic solution of Comparative Example 3.

[Comparative Example 4]
LiTFSI was dissolved in P14TFSI so as to have a concentration of 0.32 mol / kg to prepare an electrolytic solution of Comparative Example 4.

2. Production of lithium-air battery [Example 5]
First, ketjen black (ECP600JD; hereinafter may be referred to as KB) as a conductive material, PTFE (trade name: F-104, manufactured by Daikin Industries, Ltd.) as a binder, and nickel powder as an air electrode catalyst Were prepared. The conductive material, the binder and the air electrode catalyst were mixed at a ratio of KB: PTFE: Ni = 80 mass%: 10 mass%: 10 mass% to prepare an air electrode paste.

As an air electrode current collector, SUS100 mesh (manufactured by Niraco Co., Ltd.) was prepared, and the air electrode paste was applied to one surface side of the SUS mesh to produce an air electrode. Moreover, a SUS plate was prepared as a negative electrode current collector, and lithium metal (Far East Metal, thickness 200 μm, Φ15 mm) was bonded to one surface side of the SUS plate to produce a negative electrode.
An electrolyte layer was prepared by immersing 0.75 mL of the electrolyte solution (lithium salt concentration: 0.5 mol / kg) of Example 2 described above into a polypropylene nonwoven fabric (JH1004N). In order to prevent bubbles from entering the electrolyte layer, the negative electrode current collector-lithium metal-electrolyte layer-air electrode paste-air electrode current collector are arranged in this order from the lower side in the direction of gravity by the air electrode and the negative electrode. Thus, a lithium air battery of Example 5 was produced.
All the above steps were performed in a glove box under a nitrogen atmosphere.
The metal-air battery was placed in an F-type cell (manufactured by Hokuto Denko Co., Ltd.), and the cell was placed in a glass desiccator with a gas replacement cock. About 50 mL of molecular sieve was placed in the desiccator, and the inside of the container was replaced with oxygen.

[Comparative Example 5]
The conductive material, the binder, and the air electrode catalyst were mixed in a ratio of KB: PTFE: Ni = 70% by mass: 10% by mass: 20% by mass to prepare an air electrode paste as in Example 5. Similarly, a lithium air battery of Comparative Example 5 was produced.

[Comparative Example 6]
Example 5 except that the air electrode paste was prepared by mixing the conductive material and the binder at a ratio of KB: PTFE = 90 mass%: 10 mass% without mixing the nickel powder with the air electrode paste. Similarly, a lithium air battery of Comparative Example 6 was produced.

3. Comparison of time during which lithium was deposited uniformly and ionic conductivity The electrolytes of Examples 1-3 and 1-3 were introduced into lithium / lithium symmetrical cells, respectively, and the current density was 0.2 mA / cm ² . The constant current was maintained for a certain time under the conditions. After a certain period of time, the negative electrode in the cell was taken out, and the surface of the lithium solid deposited on the negative electrode surface was observed with an optical microscope. The observation of the negative electrode surface was repeated several times under the same conditions at regular intervals, and the time during which lithium was uniformly deposited on the negative electrode surface was defined as the time during which lithium was uniformly deposited in the electrolyte. All of these operations were performed under an argon atmosphere.

  For the electrolyte solutions of Examples 2 and 3 and Comparative Examples 2 and 3, ionic conductivity was measured at an evaluation temperature of 25 ° C. using a conductivity meter (manufactured by TOA Electronics Ltd., product number: CM-40S).

  FIG. 2 is a graph plotting the time during which lithium was deposited uniformly and the value of ionic conductivity in the case where the electrolytic solutions of Example 1-3 and Comparative Example 1-3 were used. In the graph of FIG. 2, the left vertical axis represents the time (seconds) in which lithium was uniformly deposited, the right vertical axis represents the ionic conductivity (mS / cm), and the horizontal axis represents the lithium salt concentration (mol / kg). ). The black rhombus plot shows the time during which lithium was deposited uniformly, and the black square plot shows the ionic conductivity.

  First, Comparative Example 1 (lithium salt concentration: 0.1 mol / kg) and Comparative Example 2 (lithium salt concentration: 0.32 mol / kg) will be examined. The ionic conductivity of the electrolytic solution of Comparative Example 2 is 0.78 mS / cm. However, the time during which lithium was uniformly deposited in the electrolytic solution of Comparative Example 1 was 55 seconds, and the time during which lithium was uniformly deposited in the electrolytic solution of Comparative Example 2 was 60 seconds. From these results, it can be seen that when the lithium salt concentration is 0.32 mol / kg or less, the time during which lithium is uniformly deposited is extremely short. This is because the lithium salt concentration in the electrolyte solution is too low, 0.32 mol / kg or less, similar to the lithium salt concentration in the conventional electrolyte solution, so that sufficient lithium ions are supplied near the negative electrode during charging. As a result, it is considered that dendrite, which is one of the main causes of lithium ion deficiency, is likely to occur.

  Next, Comparative Example 3 (lithium salt concentration: 1.0 mol / kg) will be examined. The ionic conductivity of the electrolytic solution of Comparative Example 3 is 0.015 mS / cm, and the time during which lithium is uniformly deposited is 80 seconds. From these results, it is understood that when the lithium salt concentration is 1.0 mol / kg or more, the ionic conductivity is extremely low as less than 0.1 mS / cm, and the time during which lithium is uniformly deposited is extremely short. . This is because when the concentration of the lithium salt in the electrolytic solution is too high, 1 mol / kg, the viscosity of the electrolytic solution becomes too high and lithium ions cannot be sufficiently supplied to the vicinity of the negative electrode during charging. It is thought that dendrite, which is one of the main causes due to the lack of ions, is likely to occur. The high viscosity of the electrolyte solution of Comparative Example 3 is apparent from the fact that the ionic conductivity of Comparative Example 3 is extremely low, less than 0.1 mS / cm.

On the other hand, in the electrolytic solution of Example 1 (lithium salt concentration: 0.4 mol / kg), the time during which lithium is uniformly deposited is 350 seconds. This result shows that lithium was deposited uniformly for a time longer than the result of Comparative Example 2 by 5 times or more.
Moreover, the ionic conductivity of the electrolyte solution of Example 2 (lithium salt concentration: 0.5 mol / kg) is 0.60 mS / cm, and the time during which lithium is uniformly deposited is 600 seconds. The result of Example 2 shows that lithium was uniformly deposited for 10 times longer than the result of Comparative Example 2 while keeping the ionic conductivity sufficiently high.
Furthermore, the ionic conductivity of the electrolyte solution of Example 3 (lithium salt concentration: 0.75 mol / kg) is 0.28 mS / cm, and the time during which lithium is deposited uniformly is 240 seconds. The results of Example 3 indicate that lithium was uniformly deposited for a time four times longer than the results of Comparative Example 2, although the ionic conductivity was about 1/3 that of Comparative Example 2.

  In addition, the ionic conductivity of the electrolytic solution of Comparative Example 2 is slightly higher than the ionic conductivity of the electrolytic solution of Example 1. However, considering the viewpoint of improving cycle characteristics and the case where the electrolytic solution of the present invention is used in a high capacity battery such as a lithium-air battery, the time during which lithium is deposited more uniformly than the electrolytic solution of Comparative Example 2 is longer. The electrolytic solution of Example 1 that is five times longer is more practical and preferred.

  From the above results, in the electrolytic solution of Example 1-3, the concentration of the lithium salt in the electrolytic solution is in the range of 0.37 to 0.75 mol / kg, so that the electrolytic solution has an appropriate viscosity. At the time of charging, it can be seen that lithium ions can be sufficiently supplied in the vicinity of the negative electrode, and the generation of dendrites can be suppressed.

  FIG. 3 is a graph plotting the time during which lithium was uniformly deposited and the value of ionic conductivity when the electrolytic solutions of Example 4 and Comparative Example 4 were used. The left vertical axis, right vertical axis, horizontal axis, black rhombus plot, and black square plot of the graph of FIG. 3 are the same as the graph of FIG.

First, Comparative Example 4 (lithium salt concentration: 0.32 mol / kg) will be examined. The ionic conductivity of the electrolytic solution of Comparative Example 4 exceeds 1.4 mS / cm. However, the time during which lithium is uniformly deposited in the electrolytic solution of Comparative Example 4 is less than 100 seconds. From these results, it can be seen that when the lithium salt concentration is 0.32 mol / kg or less, the time during which lithium is uniformly deposited is extremely short.
On the other hand, the ionic conductivity of the electrolyte solution of Example 4 (lithium salt concentration: 0.5 mol / kg) exceeds 0.9 mS / cm, and the time during which lithium is uniformly deposited is 400 seconds. The results of Example 4 indicate that lithium was uniformly deposited for a time that is at least 4 times longer than the results of Comparative Example 4 while keeping the ionic conductivity sufficiently high.
From the above results, not only when the piperidinium salt was included as in the electrolyte solution of Example 1-3 above, but also when the pyrrolidinium salt was included as in the electrolyte solution of Example 4, the lithium salt in the electrolyte solution When the concentration is in the range of 0.37 to 0.75 mol / kg, the electrolyte solution has an appropriate viscosity, can sufficiently supply lithium ions near the negative electrode during charging, and can suppress the generation of dendrites. I understand.

4). Charge / Discharge Experiment of Lithium Air Battery Charge / discharge was performed on the lithium air batteries of Example 5 and Comparative Examples 5 and 6, and the discharge capacity and Coulomb efficiency were calculated from the obtained charge / discharge curves. Detailed charge / discharge conditions are as follows.
Charge / discharge device: BTS2004H, manufactured by Nagano Discharge cut-off voltage: 2V
Discharge rate: 0.02 mA / cm ²
Charge cut-off voltage: 3.85V
Charging rate: 0.02 mA / cm ²
Charge / discharge temperature: 60 ° C
Atmosphere: Pure oxygen 1 atm

  4 is a charge / discharge curve of the lithium-air battery of Example 5. FIG. FIG. 6 is a charge / discharge curve of the lithium air battery of Comparative Example 5 (FIG. 6A) and Comparative Example 6 (FIG. 6B). FIG. 5 is a graph plotting discharge capacity and coulomb efficiency values of the lithium-air batteries of Example 5 and Comparative Examples 5 and 6 obtained from the charge / discharge curves of FIGS. 4 and 6. The graph of FIG. 5 is a graph in which the discharge capacity (mAh / g) is taken on the left vertical axis, the Coulomb efficiency (%) is taken on the right vertical axis, and the nickel addition amount (mass%) is taken on the horizontal axis. . The black diamond plot shows the discharge capacity, and the black square plot shows the Coulomb efficiency.

As can be seen from FIG. 5, the discharge capacity of the lithium air battery of Comparative Example 5 is 728 mAh / g, and the discharge capacity of the lithium air battery of Comparative Example 6 is 794 mAh / g. On the other hand, the discharge capacity of the lithium air battery of Example 5 is 859 mAh / g. From these results, it can be seen that the discharge capacity is highest when the amount of nickel added is 10% by mass.
First, the results of Example 5 and Comparative Example 6 are compared. The lithium air battery of Example 5 to which nickel was added had a higher discharge capacity than the lithium air battery of Comparative Example 6 to which nickel was not added. The reason is that by realizing a uniform discharge reaction, uneven precipitation and cohesive growth of the discharge product can be suppressed, and the precipitation site of the discharge product in the air electrode can be effectively used.
Next, the results of Example 5 and Comparative Example 5 are compared. The lithium air battery of Comparative Example 5 to which 20% by mass of nickel was added has a lower discharge capacity than the lithium air battery of Example 5 to which 10% by mass of nickel was added. The reason is that nickel is not a catalyst for the oxidation reaction of lithium in the air electrode of a metal-air battery, that is, the reaction shown in the following formula (1) and / or the following formula (2). This is because if the amount is too high, the content of the conductive material such as the carbon material in the air electrode is relatively decreased, and the active points on the surface of the carbon material and the like are decreased.
2Li ⁺ + O ₂ + 2e ⁻ → Li ₂ O ₂ (1)
2Li ⁺ + 1 / 2O ₂ + 2e ⁻ → Li ₂ O (2)

Further, as can be seen from FIG. 5, the coulomb efficiency of the lithium air battery of Comparative Example 5 is 55.8%, and the Coulomb efficiency of the lithium air battery of Comparative Example 6 is 32.7%. On the other hand, the coulomb efficiency of the lithium-air battery of Example 5 is 95.6%. From these results, it can be seen that the Coulomb efficiency is highest when the amount of nickel added is 10% by mass.
First, the results of Example 5 and Comparative Example 6 are compared. The lithium air battery of Example 5 to which nickel was added has higher coulomb efficiency than the lithium air battery of Comparative Example 6 to which nickel was not added. The reason is that in the air electrode of the lithium-air battery of Example 5, almost all of the lithium oxide (Li ₂ O ₂ and / or Li ₂ O) generated during discharging is decomposed by the added nickel during charging. Because it is done.
Next, the results of Example 5 and Comparative Example 5 are compared. The lithium air battery of Comparative Example 5 to which 20% by mass of nickel was added has lower Coulomb efficiency than the lithium air battery of Example 5 to which 10% by mass of nickel was added. The reason for this is that, as described above, when the addition ratio of nickel is made too high, the content ratio of conductive materials such as carbon materials in the air electrode relatively decreases, and as a result, lithium oxide generated during discharge This is because the size of the precipitate of (Li ₂ O ₂ and / or Li ₂ O) becomes too large, and it becomes difficult to decompose all the precipitate during charging.

  As can be seen from FIG. 4, the first-time discharge curve (thick line graph) and the second-time discharge curve (thin line graph) of Example 5 almost overlap each other. From this result, it can be seen that the lithium-air battery according to the present invention has high cycle characteristics.

DESCRIPTION OF SYMBOLS 1 Electrolytic solution 2 Positive electrode active material layer 3 Negative electrode active material layer 4 Positive electrode collector 5 Negative electrode collector 6 Positive electrode 7 Negative electrode 100 Lithium secondary battery

Claims (4)

A lithium-air battery comprising at least an air electrode, a negative electrode, and an electrolyte solution interposed between the air electrode and the negative electrode,
The electrolyte contains at least an ionic liquid and a lithium salt, and the concentration of the lithium salt in the electrolyte is 0.37 to 0.75 mol / kg , and
The lithium air battery, wherein the air electrode contains a nickel catalyst. The ionic liquid contains a cation and its counter anion,
The cation is an ammonium cation having a heterocyclic structure closed by a nitrogen atom and an alkyl chain having 2 to 10 carbon atoms,
The lithium air battery according to claim 1, wherein the counter anion is inert to lithium metal. The cations are N-methyl-N-propylpiperidinium cation, N-methyl-N-propylpyrrolidinium cation, N-butyl-N-methylpiperidinium cation, N-butyl-N-methylpyrrolidinium. The lithium air battery according to claim 2, wherein the battery is a cation selected from the group consisting of a cation, an N-methyl-N-methoxyethylpiperidinium cation, and an N-methyl-N-methoxyethylpyrrolidinium cation. The lithium air battery according to any one of claims 1 to 3, wherein a content ratio of the nickel catalyst is 5 to 15% by mass when the total mass of the air electrode is 100% by mass.

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