KR20010073280A - Activation Treatment Method of Ni/MH Secondary Battery - Google Patents

Abstract

PURPOSE: A method for activating a hydrogen storage alloy for a Ni/MH secondary cell is provided, to lower the activation treatment temperature and to reduce the number of cycles without affecting a Ni anode. CONSTITUTION: The method comprises the steps of dipping a hydrogen storage alloy with the composition of Zr0.7Ti0.3Cr0.3Mn0.3V0.4Ni1.0 into KOH solution at high temperature; and charging and discharging at low speed. Preferably the dipping process is carried out at 50-80 deg.C for 8-20 hours; and the charging and discharging process is carried out by charging the cell by the current density of 5-50 mA/g for 10-100 hours and discharging it by the same current density.

Description

Activation Treatment Method of Ni / MH Secondary Battery Hydrogen Storage Alloy {Activation Treatment Method of Ni / MH Secondary Battery}

The present invention relates to a method for improving the activation characteristics of a hydrogen storage alloy used as a negative electrode material for Ni / MH secondary batteries. More specifically, the Ni / MH secondary battery is generally composed of an MH cathode, a 6M KOH electrolyte, and a Ni (OH) ₂ positive electrode. The activation characteristics of the Ni / MH secondary battery are higher than those of a positive electrode which is relatively easy to activate. It is greatly affected by the difficult MH cathode. This is exposed to the air when the hydrogen storage alloy is manufactured as a negative electrode of the Ni / MH secondary battery, an oxide film is formed on the surface of the hydrogen storage alloy, so that the absorption of hydrogen does not occur during the hydrogenation reaction of the hydrogen storage alloy Because it will result in.

The AB _2- based alloys (A: high hydrogen affinity element, B: transition element) have higher hydrogen storage capacity and longer lifespan compared to the commercially available AB _5- based alloys, so they are the negative electrode of the next-generation Ni / MH secondary battery. It is a promising material. However, since it is difficult to fully activate the electrode configuration, there are many limitations in actual use.

Kim, S. Ret., J. Alloys Comp., 185: L1 (1992), et al., Proposed to improve the activation characteristics of AB ₂ based alloys. It has been reported that by adding Mm, Nd) and the like, the charge / discharge cycles until activation were reduced from 20 to 5 cycles. In addition, Sawa (H. et al., Zeit.fur Phys. Chem., 164: 1527 (1989)) reported that Zr-based AB ₂ hydrogen storage alloys improved the activation characteristics by substituting Ti for Zr. . The above results, however, result in a decrease in theoretical discharge capacity by replacing new elements in the alloy. In contrast to the above results, many studies have been conducted to improve the activation characteristics through surface pretreatment after electrode fabrication. Waka H. et al., "Int. Symp. On Metal HYdrogen Systems" , September, 1990) and the like, by immersing a Zr-V-Ni electrode prepared as a cathode in a KOH solution to remove an oxide film formed on the surface of a Zr-based alloy and forming a new surface of Ni-rich within 5 cycles. Reports that activation has been accomplished. The higher the impregnation temperature and the longer the impregnation time, the more effective activation treatment is reported. In addition, JH Lee, Ph.D. Thesis, KAIST (1993), etc., achieved activation within 2-3 cycles by impregnating ZrCr _0.8 Ni _1.2 alloy in NaBH ₄ solution and in HF + HNO ₃ solution. It is reported that the impregnation achieved activation within 10 cycles. However, in the case of high temperature impregnation (120 ° C.), when the battery is constructed, the Ni (OH) ₂ electrode, which is a positive electrode, may be damaged at a high temperature and thus cannot be used in the actual battery manufacturing process. In addition, the activation treatment through acid (acid) is not only inferior to the activation treatment in alkaline solution in terms of effect, but also requires a complicated process, this also can not be used in the actual battery manufacturing process. In addition, the high temperature impregnation method of US Pat. No. 5,874,824 has a problem that cannot be used in actual battery construction because it affects the Ni anode.

In general, the oxide film formed on the electrode surface of the hydrogen storage alloy acts as a barrier for reducing the active material and hydrogen absorption / emission, thereby increasing contact resistance and charge transfer resistance, thereby maximizing the alloy in the first cycle. It will not show capacity. At this time, the oxide film formed on the surface of the alloy is broken little by little due to the adsorption / release of hydrogen as the cycle progresses, showing the maximum capacity of the alloy after a sufficient cycle. As such, the process of activating the cycle until the alloy exhibits the maximum capacity is called an activation process of the alloy. When an actual battery is configured, such an activation process of the negative electrode causes a large current to be applied to the battery, thus overcharging at the positive electrode. Cause. In general, overcharging at the negative electrode is not a problem, but if overcharging occurs at the positive electrode, irreversible transformation into a new phase in which hydrogen is not absorbed / released is caused, which greatly reduces the performance of the battery. Bring. Therefore, it is essential to shorten the activation process of such a negative electrode when industrially constructing a Ni / MH secondary battery.

In order to solve the above problems, a number of the above-described methods have been proposed, but there are many problems in actual use, and thus a new activation process is required. In the above results, the impregnation method in the KOH solution at high temperature is a promising method except that it is a high temperature process because the process is simpler and the effect is higher than that in other acids.

Therefore, in this invention, the activation process was performed by performing low charge after impregnation at the comparatively low temperature of about 50-80 degreeC as a method for solving the said subject. When charging at low speed, the absorption of hydrogen into the alloy is smoother than the fast charging, and thus the cycle required for activation may be reduced.

1 is a graph showing the activation behavior of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy with different impregnation temperatures.

FIG. 2 is a graph showing the results of AES analysis on the surface of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy before (a) and (b) impregnation for 12 hours in KOH electrolyte at 80 ° C.

3 is a graph showing the activation behavior of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy when the current density during charging was varied (10-100 mA / g).

4 (a) is a SEM photograph of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy after 10 cycles of charging at a current density of 10 mA / g.

4 (b) is a SEM photograph of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy after 10 cycles of charging at a current density of 100 mA / g.

5 is a graph showing the activation behavior of Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy, where (-□-) is the case without any pretreatment, and (-▲-) is 80 ° C. KOH solution (-▽-) is the case where charging / discharging was performed at the current cycle of 10mA / g in the first cycle after 12 hours of impregnation in 80 ℃ KOH solution, and (-●-) is 10mA Sample cycle with current density of / g.

Figure 6 is a photograph schematically showing the effect of the activation treatment by the low-speed charging after the high temperature impregnation to be described in the present invention.

The present invention is to improve the activation characteristics of the AB type ₂ alloy, which has a larger capacity than the commercially available type AB ₅ but has poor activation characteristics and thus cannot constitute an actual battery.

The alloy used in the present invention was prepared by Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 through a known arc-melting under argon atmosphere.

The activation treatment method used in the present invention is largely divided into two processes. Firstly, Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy was impregnated for 8-20 hours in high temperature (50-80 ° C.) KOH electrolyte, and second, Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 in KOH electrolyte. Ni _1.0 alloy is activated by slow charging for 10 to 100 hours at a current density of 5 to 50 mA / g in the first cycle after impregnation. Since each of these processes has different effects on the activation process in the AB ₂ alloy, more effective activation characteristics can be achieved by using both processes simultaneously.

Hereinafter, the above-described activation process of the hydrogen storage alloy will be described in more detail by examples. However, these examples are merely for illustrating the present invention, it is obvious to those skilled in the art that the scope of the present invention to the equivalent range including the contents of the above embodiments.

<Example 1>: Effect on activation treatment and alloy by high temperature impregnation

In the present invention, the effects of the two processes used as the activation treatment method and the effect on the alloy during high temperature impregnation of KOH electrolyte were examined. Figure 1 shows the activation behavior of the alloy according to the impregnation temperature at different impregnation temperatures. It can be seen from FIG. 1 that the activation characteristic is improved as the impregnation temperature increases. To find out the cause of this, AES (Auger Electron Spectroscopy) analysis was performed. 2 shows the results of AES analysis on the alloy surface after high temperature impregnation. It can be seen that after impregnation, the concentration of Zr on the surface is greatly reduced and the concentration of Ni is increasing instead. The results show that the impregnation causes the Zr-oxide present on the surface to elute into the electrolyte, and instead Ni is enriched on the surface. This result greatly affects the activation characteristics. However, it can be seen that the degree of oxygen penetration into the alloy is greatly increased after impregnation. In addition, the experimental results show that the higher the impregnation temperature, the deeper the impregnation time becomes. Therefore, when the activation treatment is performed only by high temperature impregnation, such oxygen penetration may be detrimental to the improvement of activation characteristics.

Example 2 Effect on Activation Treatment and Alloy by Low-Speed Charge / Discharge

Figure 3 shows the activation behavior when changing the current density during charging (10 ~ 100mA / g). As the charging current density decreases, it can be seen that the activation characteristics are improved. Figure 4 shows the SEM photograph of the alloy after 10 cycles when charged with different current density. As a result of the SEM photograph, it can be seen that hydrogen is more easily absorbed due to the presence of many cracks when charged with a low current density than when charged with a high current density. In other words, the low-speed filling formed cracks on the surface of the alloy, thereby improving the activation characteristics.

<Example 3>: Activation treatment method by low-speed charging and discharging after high temperature impregnation (new activation treatment method)

Figure 5 shows the activation behavior for each of the methods described above and the alloy treated with the two processes together. In the present invention, after impregnating Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 alloy for 12 hours in a KOH electrolyte with a new activation treatment method and charging 50 hours at a current density of 10 mA / g in the first cycle, Discharge was performed to -0.7 V compared to Hg / HgO at a current density of 20 mA / g. In the drawings, it can be seen that the newly proposed activation treatment method by low speed charge and discharge after the high temperature impregnation shows better activation characteristics than the activation treatment method using only the high temperature impregnation method or the low speed charging method. Fig. 6 shows how this activation processing method exerts a synergistic effect. In other words, by removing the zirconium oxide (Zr-oxide) layer formed on the surface through high temperature impregnation, and then cracks formed on the surface through low-speed charging it was able to facilitate the absorption / release of hydrogen.

The present invention is a method for improving the activation characteristics of the Zr-based hydrogen storage alloy, in particular, the conventional method affects the Ni anode by treatment in a boiling KOH electrolyte, which is not actually used industrially. The activation process can be effectively performed at a lower temperature, so that actual industrial use is possible. In addition, the activation process also showed the highest discharge capacity in the first cycle, showing the best results among the known activation methods.

Claims (3)

In the Zr-based alloy, Zr _0.7 Ti _0.3 Cr _0.3 Mn _0.3 V _0.4 Ni _1.0 hydrogen storage alloy is impregnated in KOH electrolyte at high temperature, and then charged / discharged at low speed, hydrogen storage for Ni / MH secondary battery Method of activation treatment of alloys. The method of claim 1, wherein the high temperature impregnation impregnates the alloy for 8 to 20 hours using a KOH electrolyte between 50 and 80 ° C. 2. The hydrogen storage alloy of claim 1, wherein the low-speed charge / discharge is charged at a current density of 5 to 50 mA / g for 10 to 100 hours and then discharged at a current density in the same range. How to deal with activation.