Alkaline metal oxide/metal hydride battery

As an active material for its negative electrode, a hydrogen storage alloy for a Ni/H battery has the composition EQU MmNi.sub.v Al.sub.w Mn.sub.x Co.sub.y M.sub.z, where Mm is a misch metal, M is Fe, Cu, or a mixture of Fe and Cu, and where PA1 0.1.ltoreq.z.ltoreq.0.4, PA1 0.2.ltoreq.y.ltoreq.0.4, PA1 0.3.ltoreq.w.ltoreq.0.5, PA1 0.2.ltoreq.x.ltoreq.0.4, and PA1 4.9.ltoreq.v+w+x+y+z.ltoreq.5.1. The partial substitution of Co by M, in conjunction with a special production method including the steps of atomizing the molten alloy, followed by heat-treatment and pulverization, leads to an alloy having a particularly high cycle lifetime and discharge capability.

BACKGROUND OF THE INVENTION 
The present invention generally relates to an alkaline metal oxide/metal 
hydride battery having a positive electrode which contains a metal oxide 
and a negative electrode which is formed of a hydrogen storage alloy 
material. More specifically, the present invention relates to a hydrogen 
storage alloy material for a metal oxide/metal hydride battery which, in 
addition to a misch metal, includes the elements nickel and cobalt, and 
which has a CaCu.sub.5 -type crystalline structure. 
Batteries incorporating a rechargeable metal oxide/metal hydride system 
generally prove superior to conventional storage batteries incorporating 
lead/acid or nickel/cadmium systems. This superiority is primarily due to 
the significantly better charge acceptance of a negative hydrogen storage 
electrode in comparison to that of a negative lead or cadmium electrode. 
Hydrogen storage by the active material (M) of a negative metal hydride 
electrode takes place reversibly, according to the following reactions: 
EQU M+H.sub.2 O+e.sup.- =MH+OH.sup.- for charging, and 
EQU MH+OH.sup.- =M+H.sub.2 O+e.sup.- for discharging. 
During charging, a hydride (MH) is formed by the charging current, with a 
decomposition of water. During discharging, hydrogen is liberated and 
binds to OH.sup.- ions to form water. The simultaneously released 
electron(s) causes a current to flow in an external circuit associated 
with the cell. 
The corresponding positive electrode for use with a negative hydrogen 
storage or metal hydride electrode is generally a nickel hydroxide 
electrode, where the following reversible reactions take place: 
EQU Ni(OH).sub.2 +OH.sup.- =NiOOH+e.sup.- +H.sub.2 O for charging, and 
EQU NiOOH+H.sub.2 O+e.sup.- =Ni(OH).sub.2 +OH.sup.- for discharging. 
The positive and negative electrodes are separated (by a separator 
material), and operate in an alkaline electrolyte. 
In the many metal oxide/metal hydride batteries which have been developed, 
particularly those including the generic formulation previously mentioned, 
the electrochemically active material of the negative electrode is derived 
from an intermetallic compound, LaNi.sub.5, in which both part of the 
lanthanum and part of the nickel is replaced by other metals which do not 
reduce the ability to form a metal hydride. For example, a part of the 
lanthanum can be replaced by other rare-earth metals, and a part of the 
nickel can be replaced by metals such as cobalt, aluminum, manganese, iron 
or chromium. In the literature, all of these alloys are assigned 
(according to the representative, LaNi.sub.5) to a so-called "AB.sub.5 " 
type, having a CaCu.sub.5 structure. 
In contrast, other hydrogen storage alloys have titanium and/or zirconium 
and nickel as essential components, and belong to the so-called "AB" or 
"AB.sub.2 " types (e.g., TiNi and ZrNi.sub.2). 
In the case of alloys derived from LaNi.sub.5, the lanthanum is customarily 
replaced by a so-called "misch metal" (Mm) which contains La, Ce and other 
rare-earth metals. The substitution of nickel by other metals is usually 
carried out for the purpose of reducing the equilibrium pressure of 
hydrogen in the cell. 
Several such alloys are known from the patent literature. For example, U.S. 
Pat. No. 5,008,164 discloses an alloy of a general composition MmNi.sub.a 
Co.sub.b Mn.sub.c, where 2.5&lt;a&lt;3.5. A partial replacement of one of the 
substituents to form an alloy MmNi.sub.a Co.sub.b Mn.sub.c X.sub.d is also 
possible, where X is selected from the group Fe, Cu, Mo, W, B, Al, Si and 
Sn, forming a 5-component B-part of the AB.sub.5 alloy from the original 
4-component B-part. A number of examples of such alloy compositions are 
also found in EP-A-206,776 (e.g., MmNi.sub.3.7 Co.sub.0.5 Mn.sub.0.6 
Al.sub.0.2) and in EP-B-271,043 (e.g., MmNi.sub.3.95 Al.sub.0.3 
Co.sub.0.75). Another known alloy which belongs to this grouping, and 
which is used in actual practice, has the composition MmNi.sub.4.3-y 
Co.sub.y Al.sub.0.4 Mn.sub.0.3 (0.3.ltoreq.y.ltoreq.0.7). 
A process for preparing a hydrogen storage alloy powder, known as gas 
atomization, is disclosed in EP 420,669. In this process, argon gas jets 
discharged from nozzles are directed perpendicular to a pressurized liquid 
jet of the alloy (that flows out of a melting vessel). This results in 
atomization of the molten material to form spherical particles. The 
surfaces of the resulting particles are allowed to cool in a free 
environment, and are collected at the bottom of a cooling chamber. 
While known hydrogen storage alloys with a low cobalt content tend to have 
a good discharge capability, even at low temperatures, it has been found 
that a high cycle lifetime can only be achieved with a higher cobalt 
content. The scarcity and high price of this raw material constitutes a 
significant disadvantage. 
SUMMARY OF THE INVENTION 
It is therefore the object of the present invention to provide a modified 
hydrogen storage alloy material which is derived from the previously 
described alloy composition, but which is capable of lengthening the cycle 
lifetime of the cell while containing the lowest possible proportion of 
cobalt. 
This and other objects are achieved in accordance with the present 
invention by providing a metal oxide/metal hydride battery having a 
hydrogen storage alloy as an active material of its negative electrode 
which, in addition to a misch metal, includes the elements nickel and 
cobalt, and which has a CaCu.sub.5 -type crystal structure, wherein a part 
of the cobalt in the alloy is replaced by iron, copper, or a mixture of 
iron and copper, according to the composition 
EQU MmNi.sub.v Al.sub.w Mn.sub.x Co.sub.y M.sub.z, 
where Mm is the misch metal, M is Fe, Cu, or a mixture of Fe and Cu, and 
where 
0.2.ltoreq.x.ltoreq.0.4, 
0.1.ltoreq.z.ltoreq.0.4, 
0.2.ltoreq.y.ltoreq.0.4, 
0.3.ltoreq.w.ltoreq.0.5, and 
4.9.ltoreq.v+w+x+y+z.ltoreq.5.1. 
For further detail regarding the hydrogen storage alloy of the present 
invention, reference is made to the detailed description which follows, 
and the single accompanying figure which illustrates a comparative testing 
of cells in terms of their discharge capacity.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
An alloy produced in accordance with the present invention has the general 
composition: 
EQU MmNi.sub.v Al.sub.w Mn.sub.x Co.sub.y M.sub.z. 
Mm is a misch metal having a lanthanum content of from 25 to 60% by weight, 
and preferably from 40 to 60% by weight. The remainder of the composition, 
to 100% by weight, is predominantly Ce. M is one of the metals Cu or Fe, 
or mixtures thereof. The proportions of the individual components can vary 
within the following limits: 
0.1.ltoreq.z.ltoreq.0.4, 
0.2.ltoreq.y.ltoreq.0.4, 
0.3.ltoreq.w.ltoreq.0.5, 
0.2.ltoreq.x.ltoreq.0.4, and 
4.9.ltoreq.v+w+x+y+z.ltoreq.5.1. 
In addition to La, the misch metal contains Ce (more than 25% by weight) as 
well as Pr and Nd. When Cu and Fe are used, the preferred Cu/Fe ratio is 
in the range of 0.5.ltoreq.Cu/Fe.ltoreq.2. 
In tests with the alloy MmNi.sub.3.8 Al.sub.0.4 Mn.sub.0.3 Co.sub.0.3 
M.sub.0.2, with M=Fe, Cu, it has been ascertained that the cycle lifetime 
of the cell can be considerably increased if an alloy material with the 
substituents Cu and/or Fe is substituted for part of the cobalt used for 
the negative electrode, as compared to a conventional hydrogen storage 
alloy MmNi.sub.4.3-y Co.sub.y Al.sub.0.4 Mn.sub.0.3 
(0.3.ltoreq.y.ltoreq.0.7) having the same low cobalt content (i.e., 
y=0.3). The use of Si, V, Sn or Cr instead of Fe or Cu results in lower 
capacities or shorter cycle lifetimes. 
It is particularly advantageous to produce the alloys of the present 
invention by the atomization of molten alloys, followed by heat-treatment 
and pulverization. The heat-treatment is preferably carried out at 
temperatures of from 700.degree. C. to 900.degree. C., for a period of 
several hours (e.g., 2 to 4 hours) and under vacuum. 
For purposes of electrical testing, size AANi/H cells were used having 
alloys with compositions according to the present invention as the 
negative electrodes. Cells for comparison contained negative electrodes 
made of a conventional alloy. The alloy samples were produced either in 
the conventional way (i.e., by subjecting a melted alloy to a casting 
heat-treatment for 12 hours at 1000.degree. C. in a vacuum furnace, 
pulverizing the heat-treated material, and screening the pulverized 
material to a particle size of less than 75 .mu.m) or according to the 
present invention (i.e., by the atomization of a molten alloy, followed by 
heat-treatment and pulverization). According to their X-ray diffraction 
patterns, all samples were found to be monophase and to exhibit, 
exclusively, the typical peaks of a CaCu.sub.5 structure. 
The resulting alloys were further processed to form negative electrodes by 
admixing the alloys with carbon and a polytetrafluoroethylene (PTFE) 
binder, and by rolling the admixed alloys onto a perforated nickel plate. 
Nickel foam electrodes obtained by pasting nickel hydroxide into a nickel 
foam frame were used as the corresponding positive electrodes. The paste 
was composed of 90% spherical nickel hydroxide, with the remainder being 
CoO, a binder (PTFE) and water. 
The separators used were commercially available types made, for example, of 
a polyamide nonwoven material. The electrolyte was a 6.5 molar KOH and 0.5 
molar LiOH solution, in a proportion of 2.1 ml/cell. 
At the start of their actual cycling, all of the cells were first 
conditioned ("run in"). To this end, the cells were initially subjected to 
a single cycle including charging for 15 hours with 0.1 C (i.e., a current 
in amperes which is 0.1 times the value of the rated capacity of the 
cell), followed by storage for 24 hours at 60.degree. C. and discharging 
with 0.2 C to a final voltage of 1 V. The cells were then subjected to 
three cycles including charging for 7 hours with 0.2 C, followed by 
waiting for 0.25 hours and discharging with 0.2 C to a final voltage of 
0.9 V. 
The results of such cycle testing are given in an accompanying figure which 
shows discharging capacity C (Ah) as a function of cycle number (n). Curve 
1 relates to a conventional alloy with a cobalt content, Co.sub.0.3. Curve 
2 relates to another conventional alloy, with a cobalt content, 
Co.sub.0.7, having a cycle life which is clearly very good. However, this 
is obtained at the expense of using a correspondingly large amount of 
cobalt. 
Alloys a (with M=Cu) and b (with M=Fe) were produced according to the 
present invention, and are clearly superior to the known alloy (Curve 1) 
with regard to cycle life. It is further possible for the alloys of the 
present invention to have cycle lifetimes which approach even the 
conventional, cobalt-rich alloy of Curve 2, despite a low cobalt content. 
This was achieved by using a gas atomization process to produce such 
alloys. To this end, the alloys of the present invention were subjected to 
processing steps including melting of the starting materials, followed by 
atomization, screening of the resulting spherical particles (&lt;125 .mu.m 
screen), heat-treating the screened spherical particles for 3 hours at 
800.degree. C. in a vacuum furnace, and pulverizing the heat-treated 
spherical particles. Curves A (M=Cu) and B (M=Fe) relate to the alloys 
produced according to this particularly advantageous production method. 
Both the atomization and the subsequent heat-treatment and pulverization 
steps contribute considerably to the capacity and cycle lifetime of the 
cells. As a result of such treatment, the cycle lifetime is considerably 
higher than the cycle lifetime of the more conventionally produced samples 
(Curves a and b). 
The special nature of alloys produced in accordance with the present 
invention (by atomization, followed by heat-treatment and pulverization) 
resides in the fact that the powder particles are spherically shaped and, 
under scanning electron microscopy (SEM), exhibit a cell-type 
substructure. The substructures are separated from one another by boundary 
regions. These boundary regions, which constitute up to approximately 20% 
by volume of the particle, differ significantly in chemical composition 
(and probably also in crystallographic ordering) from the substructures. 
It is suspected that the boundary regions have a low hydrogen storage 
capacity. The suspected effect of the heat-treatment is to decompose these 
boundary regions to some extent, by diffusion processes. This is suspected 
to be the reason for the increase in capacity which results. The suspected 
effect of pulverization is that the spherical particles are broken up. As 
a result, it is suspected that the electrode particles exhibit better 
electrical contact with one another, in turn yielding a more efficient use 
of material, and therefore, a further increase in capacity. 
The primary advantage of the present invention is the ability to replace 
cobalt, which is relatively expensive, with copper and/or iron. Ni/H cells 
equipped with such negative electrode materials can achieve virtually the 
same cycle lifetime (approximately 1000 cycles) as cells with conventional 
alloys, having a cobalt content which is approximately twice as great. 
With conventional alloys which contain approximately the same (low) 
quantity of cobalt as the alloys of the present invention, it is only 
possible to achieve cycle lifetimes of approximately 400 cycles. This is 
insufficient for commercial applications. 
The practical requirements for loading capacity (of the cells) are fully 
satisfied by the conventional alloys, even with a low cobalt content. The 
alloys of the present invention are no less satisfactory in this regard 
(i.e., a partial substitution of Co by Cu or Fe does not lead to a loss in 
capacity at higher loading). The measurement of discharge capacities under 
different loading levels from 1 C to 5 C has shown that the resulting 
capacities are actually approximately 10% higher for the higher loading 
levels (i.e., around 3 C) than the capacities which are achieved with 
conventional alloys. The alloys of the present invention exhibit a 
capacity behavior which has a less sensitive reaction to loading changes. 
Other properties such as self-discharge and pressure behavior are also not 
detrimentally affected as compared to the conventional alloys. 
It will be understood that various changes in the details, materials and 
arrangement of parts which have been herein described and illustrated in 
order to explain the nature of this invention may be made by those skilled 
in the art within the principle and scope of the invention as expressed in 
the following claims.