JPH04301364A - Hydrogen storage alloy electrode - Google Patents

Abstract

PURPOSE:To improve initial activity and charge/discharge efficiency by sticking the second phase, mainly composed of tungsten carbide, to a surface of hydrogen storage alloy powder by specific weight relating to the hydrogen storage alloy. CONSTITUTION:To a surface of hydrogen storage alloy powder in which a general formula of hydrogen storage alloy is expressed by ABalpha(alpha=1.5 to 2.5) with an alloy phase belonging substantially to Laves phase of intermetal compound and a crystal structure of the alloy in at least one of hexagonal symmetry C14 type and cubic symmetry C15 type, the second phase, mainly composed of tungsten carbide, is attached by a mechanical granulation method or the like. Here, a proportion of the tungsten carbide is set to 0.5 to 20wt.% relating to the hydrogen storage alloy. In this way, electrochemical hydrogen storage reaction at the time of charge is accelerated to improve charge/discharge efficiency.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydrogen storage alloy electrode used in alkaline storage batteries such as nickel-hydrogen storage batteries.

0002

BACKGROUND OF THE INVENTION Lead storage batteries and alkaline storage batteries are widely used as various power sources. Among these, alkaline storage batteries are expected to be highly reliable and can be made smaller and lighter, so small batteries have been used for various portable devices, and large ones for industrial use.

[0003] In this alkaline storage battery, although some air electrodes and silver oxide electrodes are used as the positive electrode, in most cases, the positive electrode is a nickel electrode. The pocket type was replaced by the sintered type, which improved its properties, made it possible to seal it more tightly, and expanded its uses.

On the other hand, in addition to cadmium, zinc,
Targets include iron and hydrogen. Recently, nickel-hydrogen storage batteries using metal hydrides, i.e., hydrogen-absorbing alloy electrodes, have attracted attention in order to achieve even higher energy density.
Many proposals have been made regarding manufacturing methods.

[0005] There are two methods for producing hydrogen storage alloy electrodes: a method in which alloy powder is sintered, and a paste method in which it is filled or applied to a porous support such as foamed, fibrous, or punched metal. Among these, the paste method is the easiest to manufacture. Hydrogen storage alloys, like cadmium electrodes and zinc electrodes, have relatively good electronic conductivity, so there is great potential for non-sintered electrodes. In other words, it is made into a paste with a binder, and this is
It is filled or coated on a porous conductive plate with a dimensional or two-dimensional structure.

Among these, improvements to hydrogen storage alloy electrodes include, for example, forming porous particles by plating the particle surface with nickel or copper in order to improve the oxidation resistance of the hydrogen storage alloy powder, as well as the utilization rate and formability of the electrode. Techniques for forming a transparent metal layer are known. Furthermore, in order to improve the properties, processes such as heat treatment in a vacuum or immersion in an alkaline solution after producing the alloy have been proposed.

Furthermore, when the battery is used in a closed type, attempts have been made to add a fluororesin or a catalyst to improve the absorption of oxygen gas generated from the positive electrode during overcharging.

[0008]

[Problems to be Solved by the Invention] However, it is necessary to improve the charge-discharge characteristics, especially at the beginning of the charge-discharge cycle, to further improve the utilization rate, and to improve the high-rate discharge characteristics of batteries using the conventional hydrogen-absorbing alloy electrodes. There is a need.

Among these, for example, La0.8Nd0.2Ni2 is used to improve the performance of rare earth/nickel alloys.
．． It is known that by producing an alloy with a non-stoichiometric composition such as 9Co2.4Mo0.1Si0.1, MoCo3 can be precipitated at the grain boundaries of the LaNi5-based alloy, which exhibits a good electrochemical reaction ( P.H.L.
Notten and P. Hokkeling,
ECS Fall Meeting Extend
d Abstracts, 120, 1990).

However, this method simultaneously forms a hydrogen storage alloy that absorbs and releases hydrogen electrochemically and MoCo3 as a second phase, and the composition range of intermetallic compounds such as LaNi5-based alloys is relatively narrow. Although it is thought to be effective for narrow alloy systems, it is not necessarily effective for intermetallic compounds that are stable over a wide composition range, and it has been difficult to apply this technique to various alloys. In the past, attempts have been made to add Pd black or the like to improve these properties, but although this is quite effective, a cheaper method has been sought due to cost constraints.

[0011] In particular, in a nickel-metal hydride storage battery using a hydrogen storage alloy whose alloy belongs to the AB2 type Laves phase and whose crystal structure is a C14 type with hexagonal symmetry or a C15 type with cubic symmetry, the initial discharge capacity is small. This is a problem, and there is also the problem that polarization is relatively large with rapid charging and discharging current, resulting in a decrease in potential characteristics during charging and discharging.

[0012] The present invention solves the above problems,
The purpose of the present invention is to provide a hydrogen storage alloy electrode having high charge/discharge efficiency by improving initial activity.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention has a general formula of the main hydrogen storage alloy: ABα (α=
1.5 to 2.5), the alloy phase substantially belongs to the Laves phase of an intermetallic compound, and the crystal structure is at least one of the C14 type with hexagonal symmetry and the C15 type with cubic symmetry. A second phase containing tungsten carbide as a main component is attached to the surface of the alloy powder by mechanical granulation or the like.

[0014]

[Operation] Therefore, according to the present invention, by depositing a second phase mainly composed of tungsten carbide on the hydrogen storage alloy, the electrochemical hydrogen storage reaction during charging is accelerated.
Furthermore, charging and discharging efficiency can be significantly improved.

[0015]

[Embodiment] An embodiment of the present invention will be described below.

[0016] As a hydrogen storage alloy, the main alloy phase is C1.
ZrMn0.3Cr, one of the type 5 Laves phase alloys
A 0.3V0.15Ni1.25 alloy was used. Tungsten carbide passed through 400 mesh was used.

[0017] Tungsten carbide is added to the hydrogen storage alloy that has passed through 100 mesh in an amount of 15% by weight based on the hydrogen storage alloy.
and mixed using a ball mill. When this state was observed using an electron microscope, it was found that tungsten carbide powder was partially scattered on the surface of the hydrogen storage alloy. Furthermore, this mixed powder was crushed to 350
The material was passed through a mesh and used as a material for battery negative electrodes.

Next, the electrochemical properties of this material as a negative electrode material were evaluated. The evaluation electrode was prepared by mixing 1% by weight of a negative electrode material powder with a thermoplastic resin fine powder as a binder based on the electrode material powder. In this example, polyethylene fine powder was used. After filling a foamed nickel plate with a porosity of 95% and a thickness of 0.8 mm with the powder mixed with this binder and pressurizing it,
An electrode was obtained by processing at 120° C. for 1 hour under reduced pressure. This electrode is referred to as electrode A of the example. In order to compare the characteristics of this electrode A, an electrode as a comparative example was also produced using a conventional method. As a conventional method, ZrMn0.3Cr0.
A hydrogen storage alloy having a composition of 3V0.15Ni1.25 was pulverized and the alloy powder obtained by passing it through 350 mesh was made into an electrode in the same manner as Electrode A without adhering tungsten carbide powder. This electrode is referred to as electrode B of the comparative example.

These electrodes are used as negative electrodes, and a nickel oxide electrode having an excessive capacitance is arranged as a counter electrode, and the electrolyte has a specific gravity of 1.
．． Charging and discharging were performed using an aqueous potassium hydroxide solution of No. 30 in an open system in which the capacity was regulated by a hydrogen storage alloy negative electrode under conditions in which the electrolyte was abundant. Charge is 10 per gram of hydrogen storage alloy
0 mA x 5.5 hours, the discharge was 60 mA per 1 g of alloy, and the terminal voltage was up to 0.8 V.

As a result, electrode B of the comparative example had a low discharge capacity at the beginning of the charge/discharge cycle, with a discharge capacity of 45 mAh in the first cycle.
/g, and the saturated capacity was 285mAh/g. Moreover, it took more than 7 cycles to reach saturation. On the other hand, in the case of electrode A in the example, 74% of the saturated discharge capacity in the first cycle and 88% of the saturated discharge capacity in the second cycle.
, reaching saturation in the 5th cycle, and its discharge capacity is 313 m
Ah/g was shown. From these results, it was confirmed that the addition of tungsten carbide as a negative electrode material for alkaline storage batteries is very effective in improving initial activity.

Next, a sealed battery was constructed using these electrodes A and B, and the sealed battery was evaluated. A method for manufacturing a sealed electrode will be explained. First, the electrodes A and B are each 3.3 cm wide, 21 cm long, and 0 thick.
．． The length was adjusted to 50 mm, and lead plates were attached to two predetermined locations. Then, it was combined with a positive electrode and a separator to form a cylindrical three-layer spiral and housed in an SC size battery case. The positive electrode at this time was a well-known foamed nickel electrode with a width of 3.3 mm.
cm, and the length was 16 cm. In this case as well, lead plates were attached at two locations. Moreover, a polypropylene nonwoven fabric imparted with hydrophilicity was used as the separator. As the electrolytic solution, 30 g/l of lithium hydroxide was dissolved in an aqueous potassium hydroxide solution having a specific gravity of 1.30. This was sealed to form a sealed battery. This battery has a nominal capacity of 2 due to positive electrode capacity regulations.
．． It is 5Ah. In this sealed battery, a battery constructed with electrode A, which is a hydrogen storage alloy electrode, is designated as battery A, and a battery similarly constructed with electrode B is designated as battery B.

[0022] Ten of each of these batteries were prepared and evaluated by a normal charge/discharge cycle test, and the results will be described.

First, the initial discharge voltage and capacity were compared. 1
When battery A was charged at a constant current of 130% of its capacity at a 0-hour rate and similarly discharged at a constant current to 0.9V at a 10-hour rate, the average voltage of battery A was 1.25V, and the discharge capacity was 2 cycles. After that, it was approximately 2.5 Ah. However, in battery B, the average discharge voltage is 1.20V, and the discharge capacity is 2.
The discharge capacity did not reach 2.5Ah during the cycle, and as the number of cycles increased, the discharge capacity increased, and it took five cycles to reach a nearly constant level.

Similarly, charging at 1C (1 hour rate)
%, the discharge is the same at 1C (1 hour rate) and the final voltage is 1.
As a result of repeated charging and discharging cycles at 0V and 20°C, battery A had an average discharge voltage of 1.23V, while battery B had an average discharge voltage of 1.11V, indicating that battery A had even better discharge characteristics with rapid charging and discharging. It was also found that it has

[0025] If the amount of the second phase mainly composed of tungsten carbide is less than 0.5% by weight, the charge/discharge efficiency will decrease, and if it exceeds 20% by weight, the energy density per unit weight will decrease. do.

Although the above is the case of AB2 type Laves phase alloy, similar excellent results were also obtained with LaNi5 base alloy.

Although the hydrogen storage alloy in this example was mixed using a ball mill, excellent results were also obtained when tungsten carbide powder was compositely immobilized on the surface of the hydrogen storage alloy using a high-speed air impact method. I was able to get it.

[0028]

Effects of the Invention As is clear from the above examples, the hydrogen storage alloy electrode of the present invention improves the initial activity and improves charging by attaching a second phase mainly composed of tungsten carbide to the surface of the alloy powder. The discharge efficiency can also be improved by very inexpensive means.

Claims (3)

[Claims] Claim 1: A second phase containing tungsten carbide as a main component is attached to the surface of a hydrogen storage alloy powder that electrochemically absorbs and releases hydrogen in an amount of 0.5 to 20% by weight based on the hydrogen storage alloy. Hydrogen storage alloy electrode. Claim 2: The general formula of the main hydrogen storage alloy is ABα(α
= 1.5 to 2.5), the alloy phase substantially belongs to the intermetallic compound Laves phase, and the crystal structure is at least one of the C14 type with hexagonal symmetry and the C15 type with cubic symmetry. 1. The hydrogen storage alloy electrode according to 1. [Claim 3] The hydrogen storage alloy to which tungsten carbide powder is attached is processed by either mechanical granulation or high-speed air impact method.
2. The hydrogen storage alloy electrode according to claim 1, which is produced by a surface modification method using a method or a combination thereof.