KR102215525B1 - Sodium/metal-carbon secondary battery - Google Patents

Abstract

An object of the present invention is to provide a sodium secondary battery capable of preventing a decrease in electronic conductivity due to charging. The sodium/metal-carbon secondary battery according to an aspect of the present invention includes a cathode including sodium chloride (NaCl) and a metal-carbon powder composite. According to an embodiment of the present invention, the metal-carbon powder composite may include a metal powder and a carbon powder, and a binder for bonding the metal powder and the carbon powder to each other.

Description

Sodium/metal-carbon secondary battery

The present invention relates to a sodium/metal-carbon secondary battery, and more particularly, to a sodium secondary battery using molten sodium as an anode active material and a metal chloride as a cathode active material.

In general, batteries can be classified into disposable primary batteries and secondary batteries that can be charged several times. Among them, secondary batteries have been popularized as an essential energy source for portable electronic devices such as notebook computers, camcorders, and mobile phones in that they can be used several times. Recently, secondary batteries have changed the shape and size of the battery according to the purpose of use, ranging from large-capacity batteries for power storage, medium-sized batteries applied to transportation means, and small batteries used as power sources for portable devices, and the range of use has expanded. It is a trend.

This secondary battery is composed of a negative electrode, a positive electrode, an electrolyte, and a current collector. In the positive electrode, a reduction reaction occurs due to electrons generated from the negative electrode, and the current collector serves to supply electrons generated from the negative electrode to the positive electrode active material when the battery is discharged, or to supply electrons supplied from the positive electrode to the negative electrode active material during charging. do. Among secondary batteries, sodium secondary batteries have excellent competitiveness in terms of material supply and demand and manufacturing cost as they use abundant sodium on the earth, and have the advantage of making large-capacity batteries in a simpler structure compared to lithium-ion batteries. Accordingly, the sodium secondary battery has an energy density similar to that of the existing secondary battery, is inexpensive, and can be manufactured with a long power retention time, so that when used as a secondary battery for storing renewable energy such as solar or wind power, large-capacity power is consumed. It is emerging as a next-generation storage medium that can be stored efficiently.

An object of the present invention is to provide a sodium secondary battery capable of preventing a decrease in electronic conductivity due to charging. However, these problems are exemplary, and the scope of the present invention is not limited thereby.

The sodium/metal-carbon secondary battery according to an aspect of the present invention includes a cathode including sodium chloride (NaCl) and a metal-carbon powder composite.

According to an embodiment of the present invention, the metal-carbon powder composite may include a metal powder and a carbon powder, and a binder for bonding the metal powder and the carbon powder to each other.

According to an embodiment of the present invention, in the metal-carbon powder composite, the range of a value obtained by dividing the weight of the carbon powder by the weight of the binder may be 1.1 to 2.45.

According to an embodiment of the present invention, in the metal-carbon powder composite, the metal powder and the carbon powder may be ball-milled to have a combined form.

According to an embodiment of the present invention, the metal powder as the active material may include at least one powder selected from nickel (Ni), iron (Fe), copper (Cu), and zinc (Zn).

According to an embodiment of the present invention, the carbon powder may include at least one selected from carbon black, carbon fiber, CNT, and graphene.

According to an embodiment of the present invention, the binder may include a polymer material having a melting point of 250°C or higher.

According to an embodiment of the present invention, the binder may include at least one of aramid, tetrafluoroethylene, and polyacrylonitrile.

According to an embodiment of the present invention, a sodium/metal-carbon secondary battery includes an anode portion including molten sodium; And a base portion made of a solid electrolyte interposed between the cathode portion and the anode portion so that sodium ions can pass therethrough and move.

According to the exemplary embodiment of the present invention made as described above, even if charging is in progress, there is no phenomenon in which the electronic conductivity decreases, so that the charging characteristics of the sodium secondary battery can be greatly improved. However, these effects are exemplary, and the scope of the present invention is not limited thereby.

1 is a diagram conceptually illustrating a unit cell of a sodium/metal-carbon secondary battery according to an embodiment of the present invention.
FIG. 2 is a diagram schematically illustrating a reaction performed in a charging and discharging process in a unit cell of a sodium/metal-carbon secondary battery according to an embodiment of the present invention.
3A is a view illustrating a good connection state of nickel powder particles before charging is performed in a unit cell of a sodium secondary battery according to a comparative example of the present invention.
3B is a diagram illustrating a poor connection state of nickel powder particles as charging proceeds in a unit cell of a sodium secondary battery according to a comparative example of the present invention.
4 is a graph showing the change in the maximum dose according to the experimental example of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and the following embodiments make the disclosure of the present invention complete, and the scope of the invention to those of ordinary skill in the art. It is provided to fully inform you. In addition, in the drawings for convenience of description, at least some of the constituent elements may be exaggerated or reduced in size. In the drawings, the same reference numerals refer to the same elements.

Throughout the specification, when mentioning that one component such as a layer or region is located “on” another component, the one component directly contacts “on” the other component, or between them. It can be interpreted that there may be other intervening elements. On the other hand, when it is mentioned that one component is positioned "directly on" another component, it is interpreted that there are no other components interposed therebetween.

FIG. 1 is a diagram conceptually illustrating a unit cell of a sodium/metal-carbon secondary battery according to an embodiment of the present invention, and FIG. 2 is a diagram schematically illustrating a reaction performed during charging and discharging in area E of FIG. 1 to be. The sodium/metal-carbon secondary battery according to an exemplary embodiment of the present invention may be understood as a battery of an improved form from a Na/NiCl ₂ battery or a ZEBRA battery.

Referring to FIGS. 1 and 2, a unit cell 10 of a sodium/metal-carbon secondary battery according to an embodiment of the present invention includes a cathode portion as a positive electrode and an anode portion as a negative electrode.

The cathode portion includes a mixture 20 including an active material and a cathode current collector 50. The mixture 20 may optionally further include a secondary electrolyte 30 such as NaAlCl ₄ (melting point 150 ~ 160°C) in a molten state in order to allow sodium ions to move quickly. The anode portion includes molten sodium 40 and an anode current collector 70. The cathode current collector 50 may be made of nickel, and the anode current collector 70 may be provided on the outer portion of the battery.

Between the cathode portion and the anode portion, a base (beta-alumina solid electrolyte) portion 60 made of a solid electrolyte may be interposed. The solid electrolyte constituting the base portion 60 may include, for example, beta alumina. The crystal structure of beta alumina may have a structure consisting of alternating densely packed layers and loosely filled layers. In the conventional Na/S (sodium/sulfur) battery, when the base part made of beta-alumina solid electrolyte is destroyed, the S of the positive electrode and the Na of the negative electrode generate an accumulation reaction at high temperature, and there is a risk of explosion. . In addition, the materials constituting the cell were highly corrosive, making it very difficult to select the material to be used. However, in the sodium/metal-carbon secondary battery according to the embodiment of the present invention, this problem can be solved by using NaCl instead of S in the positive electrode. Since the sodium ion conductivity of the base portion 60 has a temperature dependence (eg, ≥ 0.2 Scm ^-1 at 260°C), the operating temperature of the sodium/metal-carbon secondary battery may have a predetermined range.

The mixture 20 constituting the cathode portion may include sodium chloride (NaCl), a metal powder as an active material, and a carbon powder as an electron conductor. According to the technical specification of the present invention, the metal powder and the carbon powder are combined with each other by a binder, and hereinafter, this is referred to as a'metal-carbon powder composite', which will be described in detail later.

The metal powder as the active material may include at least one powder selected from nickel (Ni), iron (Fe), copper (Cu), and zinc (Zn). In addition, the carbon powder may include at least one selected from carbon black, carbon fiber, CNT, and graphene.

According to the technical idea of the present invention, the metal powder, which is the active material, plays a role as an electron conductor in the cathode, and the carbon powder also plays a role of a secondary electron conductor responsible for electron conduction like the metal powder Is done. Therefore, according to an embodiment of the present invention, as the added carbon plays the role of the secondary electron conductor, the charge/discharge capacity of the sodium/metal secondary battery is increased by increasing the proportion of the role as the positive electrode active material among the roles of the metal powder. It can be significantly improved.

Hereinafter, a case in which a mixed powder containing Ni powder and an auxiliary small amount of Fe powder is used as a metal powder for the cathode part will be described in detail with respect to the technical idea of the present invention.

In the case of using a mixed powder in which Ni powder and Fe powder are mixed as a metal powder in the unit cell 10 of the sodium/metal-carbon secondary battery according to an embodiment of the present invention, when charging the sodium/metal-carbon secondary battery Since the cut-off voltage is greater than the open circuit voltage (OCV) of Ni and Fe, both Ni and Fe participate in the charging reaction as follows during charging. Specifically, the cut-off voltage is 2.67 to 2.7 V, and the open circuit voltage (OCV) of Ni and Fe is 2.58 V and 2.35 V, respectively.

Reaction Scheme (1): Ni + 2NaCl → NiCl 2 + 2Na + + 2e -

Reaction Scheme (2): Fe + 2NaCl → FeCl 2 + 2Na + + 2e -

Meanwhile, Reaction Formula (1) corresponds to the anode reaction in the charging process, and the cathode reaction in the charging process corresponding thereto is shown in Reaction Formula (3).

Reaction Scheme ^{(3): 2Na + + 2e} - → 2Na

At this time, Ni is an active material and acts as an electron conductor in the positive electrode. That is, during charging, sodium ions move from the cathode portion to the base portion 60 made of a solid electrolyte, and electrons move toward the current collector 50 of the cathode portion through nickel particles connected to each other. If the connection between the nickel particles is poor, electron conduction is not good and battery performance is degraded.

3A is a diagram illustrating a good connection state of nickel powder particles before charging in a unit cell of a sodium secondary battery according to a comparative example of the present invention, and FIG. 3B is a unit of a sodium secondary battery according to a comparative example of the present invention. A diagram illustrating a poor connection state of nickel powder particles as charging proceeds in the cell.

Referring to FIG. 3A, the nickel particles 22a are connected to each other before charging is performed or during the initial charging process, so that electronic conductivity may be secured.

Referring to FIG. 3B, as charging proceeds, a reaction product 24 including NiCl ₂ (24), which is an electrical nonconductor, appears on the surface of the nickel particles 22a. As nickel is consumed by the reaction, the nickel particles The connection between (22a) becomes thinner (A) or breaks (B). Since NiCl _{2, which} is the reaction product 24, is a non-conductor, electron conductivity through the nickel particles 22a and the reaction product 24 deteriorates as charging proceeds. That is, in the sodium secondary battery according to the comparative example of the present invention, since nickel powder is used as both the positive electrode active material and the electron conductor, the difference in performance is remarkable depending on the connection state between the nickel powder particles. There is a problem that the conductivity inevitably decreases.

Therefore, in the prior art, in order to solve this problem, excess Ni was added in excess of the theoretically necessary Ni content. That is, in theory, in order for all of the added Ni to participate in the battery reaction, the ratio of Ni and NaCl should be 1:2 by mass. However, as described above, due to the problem that NiCl ₂ is formed on the surface of Ni during charging, Ni has been added in excess of the theoretical ratio, and Ni:NaCl = 1.6 to 1.9:1 by mass ratio is usually added. .

In the sodium/metal-carbon secondary battery according to an embodiment of the present invention, it is a technical feature that carbon is used as a material that plays a role of an electron conductor without participating in a battery reaction among positive electrode constituent materials. Carbon powder does not participate in the battery reaction even if charging proceeds, so electron conductivity does not deteriorate.

Therefore, according to an embodiment of the present invention, the added carbon is used as an auxiliary electron conductor to perform electron conduction, thereby further increasing the specific gravity of the Ni particles as a positive electrode active material, thereby increasing the charge/discharge capacity of the sodium/metal secondary battery. It can be significantly improved. In addition, even if the mass ratio of Ni: NaCl is maintained at the theoretical value of 1:2 without adding excessive Ni as in the prior art, the phenomenon of deteriorating battery characteristics as in the prior art does not occur. Due to this, it is possible to obtain an economical effect that can reduce the cost required for using Ni compared to the conventional one.

The metal powder and the carbon powder must be physically bonded to each other in order for electrons made from the metal powder, which is an active material, to be well conducted through the carbon powder to the current collector by the battery reaction. To implement this. A binder for bonding the metal powder and the carbon powder may be used. Therefore, according to an embodiment of the present invention, the cathode portion may be composed of a metal-carbon powder composite in a form in which metal powder and carbon powder are bonded to each other by a binder.

Such a binder includes a polymer material, and may have a melting point of 250° C. or higher, preferably 300° C. or higher in consideration of the charging/discharging temperature of the sodium/metal secondary battery. When the melting point is less than the above numerical range, problems such as separation of the metal-carbon powder composite constituting the cathode portion may occur due to melting of the binder during the charging/discharging process of the sodium/metal secondary battery. As such a binder, for example, polyimide may be included. In addition to polyimide, aramid, tetrafluoroethylene, polyacrylonitrile, and the like may be used.

The bonding strength between the metal powder and carbon can be improved by combining the metal powder and carbon with a binder and then further milling by ball-milling. In this case, the microstructure of the metal-carbon powder composite may have a ball-milling structure in a form mechanically alloyed by balls in the ball-milling process.

In the metal-carbon powder composite, the ratio of carbon to be added can be appropriately determined in terms of optimizing the performance of the metal powder, which is a positive electrode active material. If the proportion of carbon added is too high, the proportion of the metal powder, which is a positive electrode active material, is relatively decreased, resulting in a decrease in battery characteristics. On the other hand, when the value is too small, when the value is less than the above numerical range, the role of carbon as a secondary conductor may not be sufficiently performed.

In the metal-carbon powder composite, the ratio of the binder to be added may be appropriately determined in consideration of both the bonding strength of the metal powder and carbon and the electrical conductivity aspects. When the addition ratio of the binder is too small, the function of bonding the metal powder and carbon to each other is not smoothly performed, so that improvement in battery characteristics may not appear. On the other hand, if too much is added, the ratio of the non-conductor binder interposed between the metal powder and carbon increases, and electrical conduction may not be smoothly achieved due to the insulating effect between the metal powder and carbon, which deteriorates battery characteristics. It leads to. In this aspect, in the metal-carbon powder composite, the value obtained by dividing the weight of carbon by the weight of the binder may have a range of 1.1 to 2.45, preferably 1.3 to 2.0.

Hereinafter, an experimental example for aiding understanding of the present invention is presented. The experimental examples presented below are intended to aid understanding of the present invention, and the present invention is not limited to the following experimental examples.

As an experimental example, in order to prepare a metal powder-carbon powder composite (hereinafter referred to as a'composite'), Ni powder, carbon black and polyimide were prepared as metal powder, carbon, and binder, respectively. Table 1 shows the weight of each prepared material.

The prepared polyimide was dissolved in methanol as a solvent. After adding the carbon black powder to the solution in which the polyimide was dissolved, the mixture was sufficiently stirred with a stirrer so that the polyimide and the carbon black powder were uniformly mixed as a whole. After the stirring was completed, the mixed solution and Ni powder were added to a ball-mill equipment, and a composite was manufactured by ball-milling treatment.

In Table 1, C/P is a value obtained by dividing the weight of carbon black in the composite by the weight of polyimide.

On the other hand, as a comparative example, the result of using only Ni powder as a cathode material (sample 1 in Table 1) and the result of using a composite manufactured by ball-milling only Ni powder and carbon black powder without a binder (sample 2 in Table 1) are also shown. It is shown in Table 1.

Psalter nickel
(g) Carbon black
(g) Polyimide
(g) C/P Maximum capacity
(mAh/g) One 1.0 0 0 - 217 2 1.0 0.02 0 - - 3 0.5 0.125 0.18 0.69 157 4 0.5 0.125 0.09 1.39 263 5 0.5 0.125 0.0625 2.0 363 6 0.5 0.125 0.05 2.5 196

After configuring the secondary battery shown in FIG. 1 using the Experimental Examples and Comparative Examples of Table 1, a charge/discharge test was performed to investigate the maximum capacity of the secondary battery, and the results are shown in Table 1. 4 shows the maximum dose according to the C/P value.

Referring to Tables 1 and 4, the results of Specimen 4 and Specimen 5 using the composite showed a higher maximum capacity result than that of Specimen 1 using only nickel powder as the anode material.

On the other hand, in the case of Specimen 2, in which only Ni powder and carbon black powder were ball-milled, a normal battery reaction did not appear, and the maximum capacity could not be measured. From this, it can be seen that the improvement of battery characteristics cannot be expected by the method of treating only Ni powder and carbon black powder by ball-milling.

In contrast, specimens 4 and 5 using the binder exhibited superior maximum capacity compared to specimen 1. This is interpreted to be because when polyimide as a binder is used, the probability that the Ni powder and the carbon black powder are physically bonded to each other increases, thereby maintaining a good bonding state between the two powders.

However, in the case of specimen 3, which had the highest polyimide weight in the composite, the maximum capacity was rather lower than that of specimen 1. This is interpreted as because, when an excessive amount of polyimide is included in the composite, the electrical connection between the two powders is poor as polyimide, which is an electrical nonconductor, is excessively interposed between the Ni powder and the carbon black powder.

In addition, specimen 6, which has the smallest polyimide weight in the composite, exhibited a lower maximum capacity than that of specimen 1. This improves the bonding strength of nickel powder and carbon black when the content of polyimide, a binder, is less than the appropriate range. It is interpreted that it does not have a significant effect on the battery and rather degrades the battery characteristics.

From the graph of FIG. 4, it can be seen that the range of C/P (marked by the arrow in FIG. 4) indicating a value greater than 217 mAh/g, which is the maximum capacity of the Comparative Example 1, has a range of 1.1 to 2.45.

The present invention has been described with reference to the embodiments shown in the drawings, but these are merely exemplary, and those of ordinary skill in the art will appreciate that various modifications and equivalent other embodiments are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

Claims (8)

A cathode portion including sodium chloride (NaCl) and a metal-carbon powder complex is provided,
The metal-carbon powder composite is
Metal powder and carbon powder,
It includes a binder for bonding the metal powder and the carbon powder to each other,
The binder is a polymer material having a melting point of 250° C. or higher,
Sodium/metal-carbon secondary battery. The method of claim 1,
In the metal-carbon powder composite,
A sodium/metal-carbon secondary battery having a range of 1.1 to 2.45, obtained by dividing the weight of the carbon powder by the weight of the binder. The method of claim 1,
In the metal-carbon powder composite,
The metal powder and the carbon powder are ball-milled to be bonded to each other,
Sodium/metal-carbon secondary battery. The method according to any one of claims 1 to 3,
The metal powder as an active material includes at least one powder selected from nickel (Ni), iron (Fe), copper (Cu), and zinc (Zn),
Sodium/metal-carbon secondary battery. The method according to any one of claims 1 to 3,
The carbon powder contains at least one selected from carbon black, carbon fiber, CNT and graphene,
Sodium/metal-carbon secondary battery. delete The method of claim 1,
The binder includes at least one of polyimide, aramid, tetrafluoroethylene, and polyacrylonitrile, sodium/metal-carbon secondary battery. The method of claim 1,
An anode portion containing molten sodium; And
A base portion made of a solid electrolyte interposed between the cathode portion and the anode portion to allow sodium ions to pass therethrough;
Having,
Sodium/metal-carbon secondary battery.

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