KR100269515B1 - The surface modification of mg-based alloy of high discharge capacity for improvement in cycle life - Google Patents

Abstract

PURPOSE: A method for producing cathode material for rechargeable nickel/hydrogen storage alloy battery is provided for improving cycle life of the magnesium-based alloy by a sintering and mechanical grinding method. CONSTITUTION: The method for producing cathode material for rechargeable nickel/hydrogen storage alloy (Ni/MH) battery comprises mechanically grinding surface of MgNi alloy to improve its property; mixing Ni or Cu powders to the improved alloy to cool and compress; and producing the final material by a sintering process to result in the improvement of cycle life of high capacity magnesium-based alloy electrode. The cathode material has frequently higher hydrogen storage capacity of about 510mAh/g simultaneously with the improvement of cycle life at recharging/discharging the battery.

Description

Electrode Life Improvement Method of High Capacity Magnesium Alloy

The present invention relates to a method for improving electrode life of high-capacity magnesium-based alloy using surface modification. More specifically, it is one of the anode materials for nickel / hydrogen storage alloy (Ni / MH) secondary batteries, which has a hydrogen storage capacity of 510 mAh / g, which is larger than that of existing alloys, but has a cycle life during charging / discharging of electrodes in alkaline aqueous solution. The present invention relates to a method for improving the service life of a magnesium-containing alloy having poor properties.

The development of anode materials for Ni / MH batteries has been focused on AB ₅ hydrogen storage alloys and AB ₂ Laves phase hydrogen storage alloys. As a result, they have electrochemical discharge capacities of 200-300mAh / g, respectively. Zirconium-based (Zr-based) and titanium-based (Ti-based) alloys have been developed that have a micrometal-nickel (MmNi ₅ ) based and an electrochemical discharge capacity of 300-400 mAh / g. Recently, however, researches on third generation Ni / MH batteries having higher discharge capacities than conventional alloys have been actively conducted to develop high capacity and high performance batteries for small electronic devices and electric vehicles. To date, the performance of Ni / MH batteries is known to be largely dependent on the metal hydride (MH) constituting the negative electrode. Therefore, in order to use the hydrogen storage alloy as an electrode material for a secondary battery, excellent hydrogenation characteristics such as high hydrogen storage capacity, excellent high rate dischargeability, and long cycle life and low life cycle are required. Manufacturing cost is required.

Mg-Ni-based hydrogen storage alloys have a very high hydrogen storage capacity (about 999mAh / g) in theory, and studies have been actively conducted to use them as electrode materials. Mg-Ni-based hydrogen storage alloy has a very low equilibrium pressure and a very slow reaction rate at room temperature, so it is hardly actually discharged at the time of electrode fabrication, but shows a large discharge capacity when it has a disordered structure. According to Sapru et al., A thin film having a composition of Mg ₄₈ Ni ₅₂ was fabricated by sputtering and then electrode test was conducted to obtain a very high discharge capacity of 566 mAh / g. Tatsuoki et al. Also reported that when the alloy having a disordered structure is manufactured by using mechanical grinding, the discharge capacity is about 450-550mAh / g. Indeed, YQLei, Zeitschrift Fur Physika 1ische. Although the maximum capacity was decreased and the life characteristics did not tend to be improved, Iwakura (C. Iwakura Chem. Commun., 1996 p. 1831-1832) showed that graphite was added to the alloy produced by mechanical grinding. In addition, it was reported that the discharge capacity was increased during mechanical grinding, but the effect of improving the service life was insignificant. The electrode life of Mg-Ni-based hydrogen storage alloys reported so far shows a capacity reduction of more than 70% within 10 cycles.

In the present invention, Mg-Ni-based hydrogen storage alloy has many advantages, such as a large hydrogen storage capacity and a cheap alloy manufacturing price, compared to the conventional hydrogen storage alloy for electrodes, but Mg-based hydrogen storage alloy within tens of cycles when charged and discharged in electrolyte An oxide film such as Mg (OH) ₂ is formed on the surface of the electrode, which reduces the active material and increases the contact resistance and charge transfer resistance. This is an unsuitable reality to use as a material. Therefore, in order to use Mg-Ni-based hydrogen storage alloy as an electrode material, the surface of the alloy is mixed with elements such as Ni and Cu in order to have a long life without deterioration during charge / discharge in the electrolyte. After the sintering and mechanical grinding method, the surface improvement method that can improve the life for the MgNi alloy rapidly degenerate in the electrolyte is a problem of the present invention.

1 (a) and (b) are SEM results of the MgNi alloy,

(a) shows the image of the alloy,

(b) shows the result of EDX analysis.

2 is an XRD analysis result of MgNi alloy.

Figure 3 shows the degeneration behavior of the MgNi alloy according to the charge / discharge.

Figure 4 shows the degradation behavior according to the charge / discharge after sintering the MgNi alloy with Ni.

The alloy used in the present invention was prepared by mechanical grinding under argon atmosphere. The structure of the prepared alloy was observed by XRD analysis, and the particle size and composition were observed by SEM and EDS analysis. The prepared alloy was mixed with Cu powder and cold pressed to produce a pellet-type electrode. For the surface improvement, the alloy prepared by mechanical grinding method was mixed with Ni powder and cold pressed, and then the electrode was manufactured by sintering method. The prepared electrode was subjected to a half cell experiment, and the charge / discharge current density was 300 mA / g and charged for 3 hours.

1 is a SEM analysis result of MgNi alloy. As shown in (a) and (b), after the mechanical grinding, the particle size was pulverized very finely about several micrometers (um), and the EDX analysis showed that the uniform alloy had a Mg / Ni ratio of about 1.

2 is an XRD analysis result of MgNi alloy. Crystal peaks appearing at the initial loading of Ni (a) and Mg ₂ Ni (b) disappeared and wide peaks typically seen in disordered structures can be observed.

Figure 3 shows the capacity change according to the charge / discharge cycle of the MgNi alloy. The maximum discharge capacity is about 500 mAh / g, which shows a very large discharge capacity and the maximum discharge capacity in the first cycle, indicating that the activation characteristics are very good. However, the discharge capacity is greatly reduced from the second cycle, and the large discharge capacity is reduced by 60% in 25 cycles.

Figure 4 shows the degradation behavior according to the charge / discharge after mixing the MgNi alloy with Ni and surface improvement by the sintering method. In the case of the sintered electrode with Ni, the lifespan characteristics were greatly improved without a significant decrease in the maximum capacity.

<Example>

In order to prepare the alloy of the present invention, Mg ₂ Ni and Ni are precisely weighed at 1: 1 (mol fraction), charged with stainless steel balls in a chamber of a mechanical grinding device, and then vacuumed at a vacuum of 10 ^-3 torr. Vacuum for 60 minutes. At this time, a stir type system (manufactured by Zoz GmbH, Germany) was used, the diameter of the ball was 4.762 mm, the ratio of ball and alloy was 40, and all work was performed in a glove box. Was performed in. After vacuum treatment, Ar was injected into the chamber, and then, 16 cycles (80 hours total) were repeated for 4 hours at 500 rpm and 1 hour at 200 rpm. The size of the final alloy was found to be very finely crushed to 1-10um as a result of SEM analysis, and it was confirmed that the alloy had a disordered structure through XRD analysis.

The electrode manufacturing method for the charge / discharge characteristics test is as follows. When the surface was not improved, Cu was used as a conductive collector, and the Cu / alloy ratio was 3, mixed, and then pressed into a pressure of 10 to prepare a pellet. In the case of surface improvement by the sintering method, the MgNi alloy prepared by mechanical grinding is mixed with Ni powder and pressed into a pellet at 10t. The prepared pellets were vacuum treated at a vacuum heat treatment furnace at a vacuum of 10 ^-3 torr for 30 minutes and then quenched after 5-20 minutes of heat treatment while maintaining a temperature of 200-300 ^o C.

The charge / discharge characteristics test is as follows. Hg / HgO was used as the reference electrode and platinum wire was used as the counter electrode, and the electrolyte was composed of a half cell using 30 wt% KOH solution. Charge current is 300mAh / g, discharge current is 100mAh / g, charge time is 3 hours, rest time after charge and discharge is 1 minute, and cutoff voltage at discharge is -0.6V Hg / HgO was used as the electrode).

Mechanical Grinding Method of the Invention After the alloy is mixed with other elements such as Ni and Cu, the surface is improved by the sintering method. It is effective.

Claims (1)

In the electrode life improvement method of the negative electrode material of the Ni / MH secondary battery, A method of improving electrode life of a high-capacity magnesium-based alloy, which comprises manufacturing an electrode by sintering after mixing Ni or Cu powder with a surface-improved MgNi alloy by mechanical grinding.