KR20190099678A - Carbon materials as the cathode for Zn-air secondary battery - Google Patents

Abstract

The present invention relates to a carbon material for a positive electrode of a zinc-air secondary battery. The technical point of the crystalline graphite is to have a main peak appearing at 2θ = 26.40 to 26.50° in the X-ray diffraction pattern (XRD), and have a full width at half maximum of 0.2 to 0.4° in the main peak. As a result, the present invention can obtain a carbon material for a positive electrode of a zinc-air secondary battery having high crystallinity, constant particle size and shape, less corrosion, and excellent durability.

Description

Carbon materials as the cathode for Zn-air secondary battery}

The present invention relates to a carbon material for the positive electrode of a zinc air secondary battery, and more particularly, the carbon for the positive electrode of a zinc air secondary battery having a high degree of crystallinity, constant particle size and shape, less corrosion, and excellent durability. It is about material.

Zinc-air batteries are attracting attention as fossil fuel exhaustion and demand for eco-friendly, high-efficiency alternative energy increase. In particular, since most of the materials used are environmentally friendly and inexpensive, and there is a lot of potential for development, it is possible to design ultra-high capacity batteries in the future. Such zinc air batteries can convert chemical energy directly into electrical energy by oxidizing zinc using oxygen in the air as a positive electrode. Recently, efforts to develop a rechargeable zinc air secondary battery (zinc-air secondary battery) for these zinc air battery has been continued.

The zinc air secondary battery includes a positive electrode, a separator, an electrolyte, and a negative electrode. Here, the positive electrode of the secondary battery is also referred to as an air electrode, and has a double layer structure of a catalyst layer and a gas diffusion layer. The catalyst layer serves to support the anode catalyst that activates the reduction and production of oxygen, and the gas diffusion layer provides a flow path of gas so that oxygen contained in the outside air can be delivered to the anode catalyst.

Since zinc air secondary batteries use oxygen in the atmosphere as a cathode active material, there is a merit that high energy density can be realized in comparison with other secondary batteries by reducing the weight of the battery as well as using a cathode of light carbon material. However, since the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the anode are very slow compared to the oxidation rate of the cathode, the reaction of the zinc air secondary battery is a reaction of the oxygen reduction reaction of the cathode. Depends on speed Therefore, the research on the increase of the performance of the cathode catalyst in which the oxygen reduction reaction and oxygen generation reaction occurs, but the research on the carbon material contained in the anode has not reached a high level to date.

Korean Patent Office Publication No. 10-2011-0054954 Korean Patent Office Publication No. 10-2014-0065546

Accordingly, an object of the present invention is to provide a carbon material for the positive electrode of a zinc-air secondary battery having high crystallinity, constant particle size and shape, less corrosion, and excellent durability.

The above object is that the crystalline graphite having a main peak at 2θ = 26.40 to 26.50 ° in the X-ray diffraction pattern XRD and a full width at half maximum of 0.2 to 0.4 ° at the main peak. It is achieved by the carbon material for the positive electrode of the zinc-air secondary battery, characterized in that (graphite).

The crystalline graphite may have a peak intensity of 300,000 to 500,000 of the main peak, and the crystalline graphite may be sublimated to 10 wt% or less at 700 ° C. of heat treatment.

In addition, the crystalline graphite has an average particle size of 5 to 50㎛, the total particle size is within the range of ± 20㎛ relative to the average particle size, the crystalline graphite is preferably made of a layered or spherical.

According to the present invention, it is possible to obtain a carbon material for the positive electrode of a zinc-air secondary battery having high crystallinity, constant particle size and shape, less corrosion, and excellent durability.

1 is X-ray diffraction patterns (XRD) of the carbon material according to an embodiment of the present invention,
Figure 2 is a graph showing the thermogravimetric analysis of the carbon material,
3 is a scanning electron microscope (FE-SEM) photograph of the carbon material,
Figure 4 is a graph showing the particle size distribution of the carbon material and the catalyst,
5 is a graph (LSV curves @ 2mV / s) confirming the charge and discharge performance of the air cathode produced through a carbon material,
6 is a cyclic voltammograms of the air anode including the carbon material,
7 is a full cell test graph according to charge and discharge of a zinc air secondary battery composed of an air cathode including a carbon material and an excess of zinc anode,
8 is a graph showing the electrolytic solution photograph and the carbon content quantitative analysis result after the full cell test.

Hereinafter, the carbon material for the positive electrode of the zinc-air secondary battery according to the embodiment of the present invention will be described in detail with reference to the drawings.

Corrosion of the air cathode made of carbon material can be confirmed that the electrolyte of the zinc-air secondary battery turns brown. The color of the electrolyte changes as the carbon of the anode decays into carbonate ions and melts into the electrolyte during repeated charge and discharge cycles. Corrosion reaction of such a carbon material can be represented by the following formula (1).

<Formula 1>

_{^{C + 6OH - → CO 3 2-}} + 3H 2 O + 4e -

Corrosion of carbon materials not only increases the oxygen diffusion resistance by changing the surface water repellency and porosity, but also increases the electrical resistance. As a result, deterioration of the anode and carbonate ions dissolved in the electrolyte negatively affect the performance of the zinc air secondary battery. Get mad. Therefore, in order to solve the problem of the carbon material, the present invention invented a carbon material for the positive electrode that can increase the performance of the zinc-air secondary battery.

The carbon material according to the present invention has a main peak at 2θ = 26.40 to 26.50 ° in the X-ray diffraction pattern (XRD), and has a full width at half maximum of 0.2 to 0.4 °. It is crystalline graphite. In addition, the crystalline graphite preferably has a peak intensity of 300,000 to 500,000 of the main peak.

As such, when the half width at the main peak is 0.2 to 0.4 ° and the peak intensity is 300,000 to 500,000, it means that the crystallinity of graphite is high. In the case of graphite having low crystallinity rather than crystalline graphite, the half width is 5 ° or more, and the peak intensity is 30,000 or less, thereby showing peaks outside the scope of the present invention. In the case of graphite having low crystallinity, the degree of corrosion is high and thereby the durability is not excellent, thereby degrading the performance of the zinc air secondary battery. However, in the case of the crystalline graphite of the present invention, since the crystallinity is high, the corrosion degree is small and the durability is excellent, so that the performance of the zinc-air secondary battery may be excellent and the life may be extended.

Particularly, crystalline graphite is sublimated to 10 wt% or less at 700 ° C. heat treatment. This is because amorphous graphite is sublimated to 20 wt% or more at 700 ° C. heat treatment, which is not suitable for zinc air secondary batteries. Corresponding.

In addition, the crystalline graphite has an average particle size of 5 to 50 μm, and the total particle size preferably falls within a range of ± 20 μm relative to the average particle size. That is, the crystalline graphite is preferably a uniform particle size, the more uniform the particle size is excellent durability and easy to apply to the product. Therefore, the average particle size of crystalline graphite is one of the very important points in the present invention. Such crystalline graphite may consist of layers or spheres.

The carbon material of the present invention was compared with the carbon material applied to a conventional zinc air secondary battery. In order to investigate the characteristics of carbon materials affecting anode performance and durability of zinc air secondary batteries, air cathodes were manufactured and tested with carbon materials having different specific surface area, particle size, particle shape, and crystallinity.

The properties of the carbon materials used in the test can be seen in Table 1. Gr1 and Gr2 are crystalline graphite according to the present invention, Gr3 is graphite having low crystallinity, and CB is carbon black used in a zinc air secondary battery.

Carbon material type Peak position (°) Peak intensity Half width (°) graphite1 (Gr1) 26.49 374,093 0.3 graphite2 (Gr2) 26.46 459,421 0.23 graphite3 (Gr3) 25.88 14,715 5.7 carbon black (CB) 24.62 5,734 6.2

Figure 1 shows the X-ray diffraction patterns (XRD) according to the carbon material type, and means (a) Gr1, (b) Gr2, (c) Gr3, (d) CB peaks. Through XRD, the crystallinity of the carbon material can be seen that Gr1 and Gr2 are composed of highly crystalline particles, whereas Gr3 and CB are amorphous with low crystallinity. That is, Gr1 and Gr2 correspond to crystalline graphite, and Gr3 and CB correspond to graphite and amorphous carbon black having low crystallinity. At this time, it can be seen that Gr2 has more excellent crystallinity than Gr1.

Figure 2 is a graph showing the results of thermogravimetric analysis according to the type of carbon material, it is possible to confirm the degree of sublimation by checking the weight after heat treatment to the carbon material. As can be seen from the graph, low crystalline graphite and amorphous carbon black sublimate at a low temperature, whereas crystalline graphite has a low degree of sublimation even at a high temperature. This can be seen that shows the same tendency with the crystallinity confirmed through XRD of FIG.

Figure 3 shows a scanning electron microscope (FE-SEM) of the carbon material, and corresponds to (a) Gr1, (b) Gr2, (c) Gr3, (d) CB. In the case of Gr1, a lamellar structure of 8 to 10 mu m size is shown, and Gr2 shows a uniform spherical structure of about 15 mu m size. In addition, although Gr3 is spherical, it can be seen that particles of large size and small size coexist with each other, and CB is made of very small particles of 500 nm or less. Gr1 and Gr2 have a uniform size, whereas Gr3 and CB have a problem in that the particle size is not uniform and the deterioration progresses quickly during charge and discharge for a long time, thereby decreasing the lifespan. Therefore, it is important to have an appropriate size with a uniform particle size, such as Gr1 and Gr2.

4 is a graph showing particle size distribution of a carbon material and a catalyst. In the case of Gr1 and Gr2, it can be seen that Gr3 and CB exhibit bimodal distributions, whereas the average particle sizes are constant at 5 µm and 15 µm, respectively. That is, crystalline graphite shows one distribution, whereas graphite and amorphous carbon black having low crystallinity mean that the average particle size is represented by two distributions.

Figure 5 is a graph (LSV curves @ 2mV / s) to confirm the charge and discharge performance of the air cathode produced through a carbon material, (a) is a discharge graph and (b) corresponds to a charge graph. Through the discharge graph, the Gr1 anode has a current density of 20.78mAcm ^-2 , the Gr2 anode has a 8.17mAcm ^-2 , the Gr3 anode has 0.21mAcm ^-2 , and the CB anode has a 13.01mAcm ^You can see that it is ^-2 . The charge graph shows that the Gr1 anode is 154.11 mAcm ^-2 , the Gr2 anode is 65.50 mAcm ^-2 , the Gr3 anode is 4.93 mAcm ^-2 , and the CB anode is 119.51 mAcm ^-2 . You can see that.

6 is a cyclic voltammograms of the air anode including the carbon material, which corresponds to (a) Gr1, (b) Gr2, (c) Gr3, and (d) CB. In this case, the smaller the current density change at both terminal voltages, the more excellent the durability. In the case of Gr1, it can be seen that durability is excellent due to the small difference between one cycle and 40 cycles at both ends, and in the case of Gr2, the current density increases as the cycle progresses, because the specific surface area is small and the activation time is long. . In the case of Gr3, the electrochemical reaction hardly occurs, and in the case of CB, the change of one cycle and 40 cycles is large, indicating that the durability is not excellent.

7 shows a full cell test graph according to charge and discharge of a zinc air secondary battery composed of an air cathode including a carbon material and an excess of zinc anode. The voltage difference between charge and discharge is small, and the charge and discharge end voltage intervals are increased as the cycle progresses. Maintaining the constant means stable charging and discharging, and the smaller the voltage difference is, the better the durability is. At this time, the graph corresponds to (a) Gr1, (b) Gr2, (c) Gr3, and (d) CB. Gr1 shows a stable voltage due to a small and constant difference in charge and discharge voltage, which corresponds to excellent zinc air secondary battery performance. Gr2 and Gr3 have a larger charge / discharge voltage difference than Gr1, and in the case of CB, the initial charge / discharge voltage difference is small, but the difference rapidly increases as the cycle progresses, which means that durability is not good.

FIG. 8 shows the results of quantitative analysis of carbon content in the (a) electrolyte photograph and (b) electrolyte after the full cell test according to FIG. 7. As shown in FIG. 8 (a), the browning degree of the electrolyte is CB> Gr3> Gr1, Gr2, and the graphite and the amorphous carbon black having low crystallinity have a high degree of carbon corrosion and thus, a large amount of the electrolyte is dissolved in the electrolyte. In addition, it can be seen from FIG. 8B that the carbon content of the electrolyte is Gr 3> CB> Gr 1, Gr 2, and the corrosion degree of graphite and amorphous carbon black with low crystallinity is also large. The carbon content of the electrolyte affects the degree of deterioration of the anode, and these results indicate that Gr1 and Gr2 have uniform carbon crystal grains and high crystallinity.

Claims (5)

In the carbon material for positive electrode of zinc air secondary battery,
In the X-ray diffraction pattern XRD, the main peak appears at 2θ = 26.40 to 26.50 °, and the crystalline graphite having a full width at half maximum of 0.2 to 0.4 ° Carbon material for positive electrode of zinc-air secondary battery characterized by. The method of claim 1,
The crystalline graphite is a carbon material for the positive electrode of the zinc-air secondary battery, characterized in that the peak intensity (peak intensity) of the main peak is 300,000 to 500,000. The method of claim 1,
The crystalline graphite is a carbon material for the positive electrode of the zinc-air secondary battery, characterized in that sublimation (sublimation) to 10wt% or less in the heat treatment at 700 ℃. The method of claim 1,
The crystalline graphite has an average particle size of 5 to 50㎛, the total particle size of the carbon material for the positive electrode of the zinc-air secondary battery, characterized in that within the range of ± 20㎛ relative to the average particle size. The method of claim 1,
The crystalline graphite is a carbon material for the positive electrode of the zinc-air secondary battery, characterized in that consisting of a layer or sphere.

Images