KR102321639B1 - Method for manufacturing electrolyte for metal air cell - Google Patents

Abstract

The present invention discloses an electrolyte for a metal-air battery capable of autonomously charging by including graphene nanoflakes and exhibiting excellent stability of the metal-air battery in a long-term charge/discharge cycle.
The electrolyte for a metal-air battery according to the present invention includes an alkali solution; and graphene nanoflakes dispersed in the alkaline solution, wherein the graphene nanoflakes may be surface-modified with a hydrophilic group.

Description

Method for manufacturing an electrolyte for a metal-air battery TECHNICAL FIELD

The present invention relates to an electrolyte for a metal-air battery containing graphene nanoflakes and a method for manufacturing the same.

A metal-air battery uses a metal such as iron, zinc, magnesium, and aluminum as an anode and oxygen in the air as a cathode, and consists of a cathode, a cathode and an electrolyte.

During discharge, the metal is oxidized at the cathode to generate metal ions, and the generated metal ions move across the electrolyte to the oxygen cathode, which is the anode. At the anode, external oxygen is dissolved in the electrolyte through the anode and reduced. For example, a zinc-air battery enables generation of electricity by the following reaction at the negative electrode and the positive electrode during discharge.

Cathode: 2Zn + 4OH ^- → 2ZnO + 2H ₂ O + 4e ^-

Anode: O ₂ + 2H ₂ O + 4e ^- → 4OH ^-

As such, the metal-air battery generates electricity by collecting electrons generated by the reaction between metal and oxygen in the air in the electrolyte. In a metal-air battery, electrons generated at the negative electrode move to the positive electrode along an external conductor to generate an electric current.

Since the metal-air battery uses oxygen, which can be supplied unrestrictedly from the air, as the positive electrode active material, it has a feature that the energy density is significantly higher than that of the lithium ion battery, which is limited by the theoretical capacity of the positive and negative electrode materials.

In addition, the cathode material of the metal-air battery is safe and economical because it uses a relatively safe and cheap metal. Therefore, the metal-air battery has the advantages of being economical and environmentally friendly as well as having characteristics suitable for high capacity required for electric energy storage and transportation equipment such as automobiles.

However, although the metal-air battery has a high energy density, it is difficult to substantially represent all of the theoretical energy density, has a short lifespan, and has disadvantages in that overvoltage occurs due to high polarization. As a representative example, a metal-air battery generates a metal oxide when discharging. The metal oxide has low ionic conductivity, and when the metal oxide covers the anode, the polarization is high, thereby reducing the energy efficiency of the battery. In addition, in the anode of a metal-air battery, an oxygen reduction reaction (ORR) occurs during discharge and an oxygen evolution reaction (OER) occurs during charging. In particular, the oxidation reduction reaction of oxygen gas is very slow Since the reaction rate appears, charging and discharging do not occur smoothly, and there is a problem that the charge/discharge cycle life is reduced.

Therefore, metal-air batteries use catalysts to promote oxygen reduction reaction (ORR) and oxygen production reaction (OER). Catalysts generally use noble metal catalysts such as platinum (Pt) or _{metal oxides such as IrO 2} , RuO ₂ , but these are very expensive and often have low catalytic activity or low efficiency because they are solid catalysts. In recent years, research to include soluble catalysts in the electrolyte has been conducted, which has the advantage of increasing catalyst efficiency because it can freely move in the electrolyte. When the oxidized liquid catalyst moves to the negative electrode and reacts to generate side reactants, there is a problem in that the charge/discharge capacity and lifespan of the battery are also reduced. Therefore, in order to improve the capacity and lifespan of the metal-air battery, there is still a need to develop a novel electrolyte having high catalytic reaction efficiency and high stability.

Registered Patent Publication No. 10-1851564 (2018.04.25.)

It is an object of the present invention to provide an electrolyte including surface functionalized graphene nanoflakes, which has excellent charge/discharge efficiency and exhibits excellent stability of a metal-air battery in a long-term charge/discharge cycle.

It is also an object of the present invention to provide an electrolyte capable of self-charging by increasing the efficiency of an oxygen production reaction in a positive electrode of a metal-air battery.

Another object of the present invention is to provide a metal-air battery using the electrolyte.

The objects of the present invention are not limited to the above-mentioned objects, and other objects and advantages of the present invention not mentioned may be understood by the following description, and will be more clearly understood by the examples of the present invention. Moreover, it will be readily apparent that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

The electrolyte for a metal-air battery according to the present invention includes an alkali solution; and graphene nanoflakes dispersed in the alkaline solution, wherein the graphene nanoflakes are surface-modified with a hydrophilic group.

A method for preparing an electrolyte for a metal-air battery according to the present invention comprises the steps of: (a) hydrothermal treatment of a carbon-containing precursor; (b) carbonizing the hydrothermal-treated carbon-containing precursor to prepare graphene nanoflakes; and (c) treating the graphene nanoflakes with an acid solution to modify the surface thereof with a hydrophilic group; and (d) dispersing and mixing the surface-modified graphene nanoflakes in an alkaline solution.

A metal-air battery according to the present invention includes a negative electrode comprising a metal; a positive electrode using oxygen as an active material; and an electrolyte interposed between the negative electrode and the positive electrode, wherein the electrolyte includes an alkaline solution and graphene nanoflakes dispersed in the alkaline solution, wherein the graphene nanoflakes are surface-modified with a hydrophilic group.

The electrolyte according to the present invention is applied to a metal-air battery, and by including graphene nanoflakes, it can be charged autonomously, has high charge and discharge efficiency, and can exhibit excellent charge and discharge cycle life.

In addition, the electrolyte according to the present invention has a simple manufacturing process, and the metal-air battery using the same is characterized in that it exhibits significantly higher charge-discharge efficiency and charge-discharge cycle life compared to the conventional metal-air battery.

In addition to the above-described effects, the specific effects of the present invention will be described together while describing specific details for carrying out the invention below.

1 is a process diagram showing a method of manufacturing an electrolyte for a metal-air battery according to the present invention.
2 and 3 show the properties of the surface-modified graphene nanoflakes included in the electrolyte for a metal-air battery according to the present invention.
4 is a diagram illustrating a configuration of a metal-air battery according to the present invention and a state in which the metal-air battery is connected to an external circuit.
5 and 6 show the characteristics of the metal-air battery according to the present invention.
7 shows the self-charging characteristics of the metal-air battery according to the present invention.
8 shows the self-charging mechanism of the metal-air battery according to the present invention.

The above-described objects, features and advantages will be described below in detail with reference to the accompanying drawings, and accordingly, those of ordinary skill in the art to which the present invention pertains will be able to easily implement the technical idea of the present invention. In describing the present invention, if it is determined that a detailed description of a known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description will be omitted. Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. In the drawings, the same reference numerals are used to indicate the same or similar components.

In the following, that an arbitrary component is disposed on the "upper (or lower)" of a component or "upper (or below)" of a component means that any component is disposed in contact with the upper surface (or lower surface) of the component. Furthermore, it may mean that other components may be interposed between the component and any component disposed on (or under) the component.

Also, when it is described that a component is "connected", "coupled" or "connected" to another component, the components may be directly connected or connected to each other, but other components are "interposed" between each component. It is to be understood that “or, each component may be “connected,” “coupled,” or “connected” through another component.

Hereinafter, an electrolyte for a metal-air battery according to an embodiment of the present invention, a method for manufacturing the same, and a metal-air battery using the same will be described.

In the present invention, a metal-air battery refers to a secondary battery that generates electricity by combining a metal such as zinc, lithium, magnesium, aluminum, etc. with oxygen in the air. Such a metal-air battery includes a negative electrode including a metal, a positive electrode using oxygen as an active material, and an electrolyte interposed between the negative electrode and the positive electrode.

First, an electrolyte applicable to the metal-air battery and a method for manufacturing the same will be described.

As used herein, the term “air” is not limited to atmospheric air, and may include a combination of gases including oxygen, or pure oxygen gas.

Electrolyte for Metal Air Cells

The electrolyte for a metal-air battery of the present invention includes surface functionalized graphene nanoflakes.

The electrolyte for a metal-air battery is a space in which an electrochemical reaction of an electrode occurs and ions move. The electrolyte for a metal-air battery may be an aqueous electrolyte obtained by dissolving a metal salt in water. For example, in the case of a lithium-air battery, it may be an aqueous solution of a lithium salt. In addition, in the case of a zinc-air battery, it may be an aqueous alkali solution. As the aqueous alkali solution, an aqueous potassium hydroxide (KOH) solution is used, but is not particularly limited thereto. In some cases, the aqueous alkali solution may include an aqueous sodium hydroxide (NaOH) solution, an ammonium hydroxide (NH ₄ OH) aqueous solution, and the like. The aqueous alkali solution can ^{prevent a deficiency of hydroxide ions (OH −} ) in the negative electrode, and thus serves to improve the output characteristics and discharge efficiency of the metal-air battery. The alkali solution used as the electrolyte of the present invention ^{can serve as a buffer capable of maintaining the concentration of hydroxide ions (OH −} ) in the negative electrode, and improves the performance of the metal-air battery by reducing electrical resistance.

To this end, the electrolyte for a metal-air battery preferably includes an alkali solution, for example, potassium hydroxide (KOH), sodium hydroxide (NaOH), may include one or more of _{ammonium hydroxide (NH 4 OH).} . The concentration of the alkali solution may be approximately 1 ~ 10M, preferably may be 2 ~ 6M. When the concentration of the alkali solution is 1M or less, it may not be sufficient to prevent the hydroxide ion deficiency in the negative electrode. Conversely, when the concentration of the alkali solution is 10 M or more, the effect of increasing the electrolyte ion concentration may be insignificant.

More specifically, the alkali solution has a metal salt dissolved therein, and the metal salt may include the same metal as the metal used for the negative electrode. For example, zinc acetate or the like may be used as the metal salt included in the electrolyte in the zinc-air battery. The concentration of the metal salt may be about 0.01 ~ 1M, preferably 0.01 ~ 0.5M. When the concentration of the metal salt is out of 0.01 ~ 1M, the balance of the concentration of metal ions is broken, and a problem in which charge/discharge cycle characteristics are deteriorated may occur.

The surface-functionalized nanoflakes are carbon having a two-dimensional structure including a graphene crystal structure, and include functional groups including oxygen, nitrogen, sulfur, phosphorus, etc. on the surface. The surface-functionalized graphene nanoflakes act as catalysts that allow the oxygen reduction reaction (ORR) or oxygen production reaction (OER) reaction in the anode of the metal-air battery to occur well while being oxidized or reduced during charging and discharging. .

Surface-functionalized graphene nanoflakes are included in the electrolyte and serve as redox mediators to facilitate ORR or OER reactions at the positive electrode, thereby smoothly charging and discharging, and improving the capacity efficiency of metal-air batteries. ) can be improved.

In addition, the graphene nanoflakes included in the electrolyte have high catalytic reaction efficiency due to their high specific surface area, leading to improved battery performance.

The surface functionalized graphene nanoflakes used in the present invention are carbon having a two-dimensional structure including a graphene crystal structure, and include a functional group including oxygen on the surface.

A plurality of graphene nanoflakes are dispersed in the alkaline solution. When the size of the graphene nanoflakes of the present invention is more than several tens of μm, the graphene may be precipitated in the electrolyte solution, which may cause a partial short circuit.

Accordingly, the size of the graphene nanoflakes is preferably about 1 nm to 10 μm. The size of the graphene nanoflakes may be a length in a longitudinal direction of a major axis, a length in a transverse direction, or a length in a longitudinal direction. The graphene nanoflakes may be reduced or oxidized graphene, and may be single-layered or multi-layered.

The graphene nanoflakes are preferably surface-modified with a hydrophilic group in order to improve the properties of graphene. The surface-modified graphene nanoflakes can prevent precipitation, agglomeration or suspension in an electrolyte and provide improved dispersibility. The hydrophilic group may include at least one of a hydroxyl group, a carboxyl group, a carbonyl group, an amino group, and a phosphoric acid group.

The surface-modified graphene nanoflakes may be present in an amount of about 5 to 50% by weight based on 100% by weight of the alkali solution. When the surface-modified graphene nanoflakes are present in an amount less than 5% by weight, the effect of improving the charge/discharge cycle life is reduced.

Conversely, when present in an amount greater than 50% by weight, graphene may be precipitated in an alkaline aqueous solution.

Method for manufacturing electrolyte for metal-air battery

1 is a process diagram showing a method of manufacturing an electrolyte for a metal-air battery according to the present invention.

First, the carbon-containing precursor is subjected to hydrothermal treatment.

Using the carbon-containing precursor as a carbon source, hydrothermal treatment is performed at 180-250° C. for 12-48 hours.

The hydrothermal-treated carbon-containing precursor may be purified with an organic solvent such as acetone or methanol using a soxhlet extractor.

As the carbon-containing precursor, plant-derived organic substances, high molecular compounds, pitches, fruits, starch, and the like may be used. The plant-derived organic material may include an organic material derived from coffee beans. The polymer compound may include a thermosetting resin such as a phenol resin. The pitch is petroleum-based or coal-based. The fruit may include a pear, an apple, and the like.

Then, the hydrothermal-treated carbon-containing precursor is carbonized to prepare graphene nanoflakes.

The carbonization is preferably made at 300 ~ 1000 ℃. If the carbonization temperature is less than 300 °C, carbonization may not be sufficiently performed, and thus the production of carbon crystals including graphene may not be performed well. Conversely, when the temperature is higher than 1000°C, the yield of finally obtained graphene nanoflakes may be reduced.

The carbonization time may be performed for 1 to 10 hours, but is not limited thereto.

Then, in order to functionalize the prepared graphene nanoflakes, the surface of the graphene nanoflakes is modified with a hydrophilic group by treating the graphene nanoflakes with an acid solution.

The mixing ratio of the graphene nanoflakes: the acid solution may be 1:1 to 1:10 by weight, but is not limited thereto. At this stage, it is important to agitate the graphene nanoflakes sufficiently to mix them with the acid solution. The acid solution may be nitric acid, sulfuric acid, hydrochloric acid, or the like.

After the surface modification is made by the acid solution, the acid solution is dried and removed to obtain graphene nanoflakes surface-modified with a hydrophilic group. Here, the hydrophilic group may include one or more of a hydroxyl group, a carboxyl group, a carbonyl group, an amino group, and a phosphoric acid group.

Then, the surface-modified graphene nanoflakes are dispersed and mixed in an alkaline solution to prepare an electrolyte.

In this step, stirring is performed at 25±10° C., and surface-modified graphene nanoflakes are uniformly dispersed in an alkaline solution.

metal air battery

The electrolyte prepared according to the manufacturing method of the present invention can be applied to a metal-air battery. The metal-air battery includes a negative electrode including a metal, a positive electrode using oxygen as an active material, and an electrolyte interposed between the negative electrode and the positive electrode. One surface of the negative electrode and one surface of the positive electrode are in contact with the electrolyte.

The electrolyte includes an alkali solution and graphene nanoflakes dispersed in the alkali solution, and the graphene nanoflakes are surface-modified with a hydrophilic group.

The negative electrode may act as a metal plate or an alloy plate itself, and may be replaced when the metal is consumed. The metal included in the negative electrode may include at least one of zinc, lithium, magnesium, and aluminum.

The positive electrode is a plate-shaped electrode having a relatively thin thickness compared to the negative electrode, and may be prepared by adding a positive electrode material to a binder solution and stirring to prepare a slurry, and then applying it to a current collector.

A conductive material may be used for the positive electrode using oxygen as an active material. The conductive material may be used without limitation as long as it has porosity and conductivity, for example, a carbon-based material having porosity may be used. As such a carbon-based material, carbon black, graphite, graphene, activated carbon, carbon fibers, and the like may be used alone or in combination.

The operation process of the metal-air battery of the present invention is as follows. Metal-air batteries enable the generation of electricity by the following reactions.

Cathode: 2Zn + 4OH ^- → 2ZnO + 2H ₂ O + 4e ^- (1)

Anode: O ₂ + 2H ₂ O + 4e ^- → 4OH ^- (2)

During discharge ((1) to (2)), the metal is oxidized at the cathode to generate electrons, and at the anode, oxygen meets the electrons moved from the cathode and an oxygen reduction reaction (ORR) occurs.

During discharge, the graphene nanoflakes of the present invention are included in the electrolyte and serve as a catalyst to promote the oxygen reduction reaction at the anode, and the reaction formulas ((3) to (7)) are as follows.

O ₂ + * → O ₂ * (3)

O ₂ * + H ₂ O (l) + e ^- → OOH* + OH ^- (4)

OOH* + e ^- → O* + OH ^- (5)

O* + H ₂ O (l) + e ^- → OH* + OH ^- (6)

OH* + e ^- → * + OH ^- (7)

Here, * represents an active site on the surface of the graphene nanoflakes, and O*, OH*, and OOH* are intermediate products of the graphene surface.

During charging ((8) to (9)), the reverse reaction of the reaction during discharge ((1) to (2)) occurs.

Cathode: 2ZnO + 2H ₂ O + 4 e ^- → 2Zn + 4OH ^- (8)

Anode: 4OH ^- → O ₂ + 2H ₂ O +4e ^- (9)

During charging ((8) to (9)), an oxygen generation reaction (OER) in which oxygen is generated occurs at the positive electrode. During charging, the graphene nanoflakes of the present invention serve as a catalyst to promote the oxygen generation reaction at the positive electrode. will do

The reaction formula for this is as follows. ((10) to (14))

OH ^- + * → OH* + e ^- (10)

OH* + OH ^- → O* + H ₂ O (l) + e ^- (11)

O* + OH ^- → OOH* + e ^- (12)

OOH* + OH ^- → * + O ₂ (g) + H ₂ O (l) + e ^- (13)

Here, * represents an active site on the surface of the graphene nanoflakes, and O*, OH*, and OOH* are intermediate products of the graphene surface.

As such, the metal-air battery according to the present invention can be autonomously charged by including the surface-modified graphene nanoflakes as an electrolyte, and can exhibit excellent stability of the metal-air battery in a long-term charge/discharge cycle.

In addition, the electrolyte according to the present invention has a simple manufacturing process, and a metal-air battery using the same can generate electricity in a short time without using a catalyst.

The metal-air battery according to the present invention may be a rechargeable battery capable of charging and discharging, and may be used as a power source for an electric vehicle, a hybrid electric vehicle, or the like, or as a power source for a power storage device. In addition, the metal-air battery according to the present invention can generate energy at low cost and can be manufactured without limitation in shape and size, so that it can be used without limitation in space.

As described above, a detailed example of an electrolyte for a metal-air battery, a method for manufacturing the same, and a metal-air battery using the same will be described as follows.

2 and 3 show the properties of the surface-modified graphene nanoflakes included in the electrolyte for a metal-air battery according to the present invention. The surface-modified graphene nanoflakes measured in FIGS. 2 and 3 were prepared as follows.

First, using the pear as a carbon source, hydrothermal treatment was carried out at 250° C. for 48 hours. After purification with acetone using a Soxhlet extractor, graphene nanoflakes were prepared by carbonization at 400° C. for 2 hours. Then, the prepared graphene nanoflakes _{were mixed with nitric acid (HNO 3} ), and then the remaining nitric acid solution was removed to prepare surface-modified graphene nanoflakes.

Figure 2(a) shows the presence of oxygen functional groups in the graphene skeleton as a result of FTIR measurement. Figure 2 (b) shows the graphene properties as a result of Raman measurement. Figure 2(c) shows the phase purity of the synthetic material as a result of XRD measurement. Figure 2(d) determines the composition and surface function as a result of XPS measurement. 2(e) and 2(f) show the formation of functional groups containing oxygen on the surface of graphene nanoflakes as a result of XPS measurement.

3(a) shows that the surface-modified graphene nanoflakes have a sheet-like shape as an SEM image. 3(b) shows a randomly oriented multi-layered graphene nanoflake sheet as a TEM image, and folds can be observed. Fig. 3(c) shows lattice fringes as an HRTEM image. 3(d) to 3(f) show a uniform distribution of carbon and oxygen as a result of element mapping measurement.

4 is a diagram illustrating a configuration of a metal-air battery according to the present invention and a state in which the metal-air battery is connected to an external circuit.

4(a) is an electrolyte in which surface-modified graphene nanoflakes are dispersed in 6M calcium hydroxide Q(KOH) containing 0.2M zinc acetate (Zn(CH ₃ CO ₂ ) _{2 ), zinc foil as an anode, and an anode.} It is an exploded perspective view of a carbon fiber sheet steel (strip, diameter 0.05mm). Figure 4 (b) is a state in which the metal-air battery of Figure 4 (a) is connected to an external circuit.

5 to 7 show the characteristics of the zinc-air battery prepared in FIG.

Fig. 5(a) is a polarization curve and power profile of a zinc-air battery, Fig. 5(b) is a discharge profile of a zinc-air battery at different current densities, Fig. 5(c) is a typical proportional capacity curve, and Fig. 5(d) is Long ^- term stability experiment of self-charging in discharging and self-charging mode (at 15-minute pause) up to 1000 cycles at a current density of 5 mAcm -2 , Fig. 5(e) is the first 4 cycles and Fig. 5(f) is the last 4 cycles Constant current discharge and self-charge curves for

Referring to FIGS. 5 (e) and 5 (f), it can be seen that the zinc-air battery is self-charging for a long period of time.

7 shows a self-charging process in air and a constant current discharging process at ^{5mAcm -2 .} The bright yellow region (left) is self-charged by air respiration of surface-modified graphene nanoflakes (f-GNS).

8 shows the self-charging mechanism of the metal-air battery according to the present invention.

The surface-modified graphene nanoflakes (f-GNS) contained in the electrolyte serve as a catalyst for accelerating the oxygen reduction reaction at the anode during discharge, and as a catalyst for accelerating the oxygen production reaction at the anode during charging. will do

As described above, the present invention has been described with reference to the illustrated drawings, but the present invention is not limited by the embodiments and drawings disclosed in the present specification. It is obvious that variations can be made. In addition, although the effects according to the configuration of the present invention are not explicitly described and described while describing the embodiments of the present invention, it is natural that the effects predictable by the configuration should also be recognized.

Claims (8)

delete delete (a) hydrothermal treatment of the carbon-containing precursor;
(b) carbonizing the hydrothermal-treated carbon-containing precursor to prepare graphene nanoflakes; and
(c) treating the graphene nanoflakes with an acid solution to modify the surface thereof with a hydrophilic group; and
(d) dispersing and mixing the surface-modified graphene nanoflakes and an alkali solution;
The surface-modified graphene nanoflakes are a method of manufacturing an electrolyte for a metal-air battery in the form of a corrugated sheet.
4. The method of claim 3,
The step (a) is a method for producing an electrolyte for a metal-air battery that is hydrothermal treatment at 180 ~ 250 ℃.
4. The method of claim 3,
The step (b) is a method of manufacturing an electrolyte for a metal-air battery that is carbonized at 300 ~ 1000 ℃.
4. The method of claim 3,
In step (c), the hydrophilic group includes at least one of a hydroxyl group, a carboxyl group, a carbonyl group, an amino group, and a phosphoric acid group.
delete delete

Images