High performance zinc powder and battery anodes containing the same

An electrochemically prepared, high-performance, zinc powder has an apparent density of about 0.2-2 g/cc and a surface area of about 0.5-6 m.sup.2 /gm and further has at least one corrosion inhibitor metal intrinsically alloyed therein.

The present invention relates to high-performance zinc powder. 
More particularly, the present invention relates to high-performance zinc 
powder especially for use in primary and secondary-based cells and 
batteries, especially in zinc-alkaline, zinc-manganese dioxide, 
silver-zinc and zinc-air cells which incorporate a zinc anode. 
As is known, it is of importance in batteries containing zinc electrodes 
that the zinc should not be consumed by a reaction with aqueous 
electrolyte, especially alkaline electrolyte which generates hydrogen gas, 
which reaction merely corrodes the zinc and prevents its availability for 
producing electric current. A number of prior patents relate to this 
problem. 
Thus, e.g., in U.S. Pat. No. 4,112,205, double salts containing both 
mercuric and quaternary ammonium ions are used as inhibitors in galvanic 
cells comprising zinc anodes, notably in Leclanche-type batteries 
containing ammonium chloride/zinc chloride electrolyte; U.S. Pat. No. 
3,945,849 employs quaternary ammonium halides as inhibitor for zinc anodes 
in similar primary cells. U.S. Pat. No. 4,195,120 teaches alkaline cells 
containing a predominantly zinc anode and, as a hydrogen evolution 
inhibitor, a surfactant which is an organic phosphate ester of the 
ethylene oxide adduct type. Metal oxide inhibitors for zinc (in practice 
ZnO) electrodes are described in U.S. Pat. No. 4,084,047, in which the 
inhibitors are mixed thoroughly into the ZnO; the inhibitors taught in 
this patent are binary combinations of oxides which exclude mercuric 
oxide, the latter being regarded as an ecologically unsatisfactory 
additive for the ZnO electrode. According to U.S. Pat. No. 4,084,047, it 
was known to mix or alloy the active zinc in zinc-zinc oxide anodes and 
their supporting grid (e.g., copper or silver structures) with 0.5-5.0 wt. 
% mercury or mercuric oxide. Also in U.S. Pat. No. 4,195,120, mercury is 
used; however, more importantly, said patent starts with a block of zinc 
alloy which is then melted into droplets and solidified into particles 
(the so-called thermally atomized zinc), and the contact area between 
these solid particles is therefore quite limited. 
According to the present invention, there is now provided an 
electrochemically prepared, high-performance, zinc powder having an 
apparent density of about 0.2-2 g/cc and a surface area of about 0.5-6 
m.sup.2 /gm and further having at least one corrosion inhibitor metal 
intrinsically alloyed therein. 
In preferred embodiments of the present invention, said corrosion inhibitor 
metal is selected from the group consisting of antimony, bismuth, cadmium, 
gallium, indium, lead, tin and mixtures thereof. 
Preferably, said inhibitor is present in said alloy in a weight ratio of 
zinc to inhibitor of 1:0.00001-0.04. 
The invention also provides a battery anode comprising an 
electrochemically-prepared, high-performance zinc powder; a battery anode 
comprising an aqueous slurry of KOH and electrochemically-prepared, 
high-performance zinc powder pressed onto an anode collector, and a 
battery anode comprising a slurry of KOH and electrochemically-prepared, 
high-performance zinc powder, extruded onto an anode collector. 
The invention further provides a high-performance, 
electrochemically-generated, zinc-inhibitor metal alloy powder, prepared 
by a process comprising electrolyzing an admixture of (a) zinc which has 
been at least partly oxidized to an oxidation product selected from the 
group consisting of zinc oxide, zinc hydroxide and zincates; (b) an 
aqueous solution of at least one Group Ia metal comprising anions selected 
from the group consisting of hydroxide and zincate; and (c) an inhibitor 
metal compound, e.g., an inhibitor metal oxide effective to inhibit the 
interaction of zinc and at least one Group Ia metal hydroxide in aqueous 
solution, which would otherwise result in the evolution of hydrogen gas, 
said inhibitor metal compound being capable of forming an alloy with zinc 
and being present in said admixture as a cation species selected from the 
group consisting of antimony, bismuth, cadmium, gallium, indium, lead, tin 
or a mixture thereof in a cell with a corrosion-resistant anode and a 
non-zinc-adherent cathode such that the zinc and inhibitor metal from said 
compound which codeposit on said cathode self-detach or are removable by a 
method selected from brushing, scraping, vibrating, the use of liquid 
jets, either fixed or moving, and the use of electrical pulsing, until no 
more than a preselected amount of zinc remains in the solution, provided 
that the current density at the cathode is preselected so that in 
conjunction with the non-zinc-adherent characteristic of the cathode, the 
electrowon zinc inhibitor metal alloy will have, after homogenizing into 
particles, a density within the range 0.2-2.0 g/cc and a surface area 
within the range 0.5-6.0 m.sup.2 /g; removing zinc inhibitor metal alloy 
from the cathode and homogenizing it into particles by a method selected 
from brushing, stirring or blending; and recovering and drying the 
resulting zinc-inhibitor metal alloy into powder form. 
The cation species is provided by dissolving the oxide, hydroxide, 
carbonate or sulfate of the inhibitor metal(s) in aqueous Group Ia metal 
hydroxide so as to maintain a concentration of 5-1000 ppm. At least one 
inhibitor metal is taken up in the product zinc, and preferably 
constitutes 0.001-4.0 (e.g., 0.04-4.0) percent by weight, based on the 
weight of the zinc. 
In our U.S. Pat. No. 5,206,096 there is described and claimed a slurry for 
use in rechargeable metal-air batteries, comprising particulate, porous, 
zinc-containing material of a particle size within the range of 100-500 
microns and having density within the range of 0.3-1.4 g/cc and a surface 
area within the range of 0.5-6.0 m.sup.2 /g, an aqueous solution of at 
least one Group Ia metal hydroxide, and an inorganic inhibitor ingredient 
effective to inhibit interaction of said particulate, porous, 
zinc-containing material and said aqueous solution to prevent evolution of 
hydrogen gas, wherein the weight ratio in said slurry between said porous 
zinc containing material, said aqueous solution and said inorganic 
inhibitor is 1:0.5-2.0:0.00005-0.04. 
Said patent, however, does not teach or suggest the preparation of a 
high-performance, electrochemically-generated, zinc-inhibitor metal alloy 
powder as described and taught for the first time herein. 
Similarly, in our U.S. Pat. No. 5,228,958 and in our published European 
Application 91312077 there is described and claimed a process for the 
regeneration of an at least partially-spent slurry having a dissolved 
phase and an undissolved phase for use in metal-air batteries, wherein 
zinc is combined with an inorganic corrosion inhibitor; however, a careful 
reading of both of these documents clearly shows that they teach and 
suggest the optional addition of an inorganic and/or organic inhibitor to 
the zinc after it has been removed from the cathode, and do not teach or 
specifically suggest the incorporation of inhibitor metals or ions in the 
zinc plating bath, whereby the zinc powder product is intrinsically 
alloyed with trace inhibitor metals. 
As regards improved properties of the high-performance zinc over the 
previous "pure" zinc/inhibitor oxide mixture, it is obviously more 
convenient to have a one component inhibitor zinc alloy (especially when 
the raw material commodity purchased for manufacturing zinc alkaline or 
zinc-manganese dioxide batteries is a dry powder rather than a slurry, 
which is in fact the case). Superior performance is also to be expected in 
view of the fact that the alloyed single-phase zinc is more homogeneous in 
terms of inhibitor distribution (as a result of the co-plating step in the 
electrolytic process) as compared to physically mixed zinc/inhibitor oxide 
compositions. 
The proposed zinc-inhibitor metal alloy will, of course, be superior to 
zinc from thermal atomization. Not only is the surface area of the 
proposed zinc higher, which allows greater power generation, but the 
proposed zinc is much more porous. In cell fabrication procedures in which 
a zinc/alkaline slurry is extruded (zinc-alkaline application) or pressed 
(zinc-air application) onto the anode current collector, the porous 
dendritic zinc particles can interlock together and densify, giving a 
highly-conductive zinc/KOH matrix much more effective for discharge than 
the equivalent non-compressible zinc particles/KOH mixture from thermal 
zinc. 
The electrochemically-generated, zinc-inhibitor metal alloy powder of the 
present invention may additionally be used in other applications of zinc 
powder, for example, as a reducing agent for organic and inorganic 
synthesis reactions, and as an anti-corrosive pigment in paints. The zinc 
powder is characterized by a high porosity (apparent density 0.2-2 g/cc) 
and high surface area (0.5-6 m.sup.2 /g), while the particle size can be 
chosen over a range from 5-1000 microns. The zinc-inhibitor metal alloy 
powder is produced by electrowinning means as a slurry in alkaline 
solution having a zinc:alkaline solution ratio of 1:0.05-12. For battery 
applications particularly, the zinc slurry may be used directly to form 
the anode; for other applications, it may be preferable to separate the 
zinc from the alkali (for example, by water washing and drying) and to use 
the zinc as a dry powder. The parameters of the electrowinning process 
(e.g., current density of the electroplating stage, concentration of 
corrosion inhibitors, etc.) may be varied to give a variety of zinc powder 
types and zinc compositions, depending on application. 
As far as we are aware, the prior art (Linden, Handbook of Batteries and 
Fuel Cells, p. 7-5; also, Kirk Othmer Encyclopedia of Chemical Technology, 
Vol. 24, p. 205) is based on zinc powders from thermal atomization of 
molten zinc in an airjet, which produces lower porosity and lower surface 
area zinc powders (e.g., density 2.5-3.5 g/cc, surface area 0.1-0.4 
m.sup.2 /g) compared with the electrochemically-generated zinc, with the 
thermal zinc giving substantially lower performance ratings. Performance 
improvements in the case of batteries are evident, especially in the areas 
of high discharge rate, zinc utilization and capability of sustained high 
power discharge. 
Furthermore, the electrochemical process route is associated with fewer 
environmental problems as compared with the thermal one, in view of the 
relative difficulty of preventing toxic traces (e.g., lead) entering the 
atmosphere from a thermal process, versus the relative ease of process 
effluent control in a wet electrochemical process.

While the invention will now be described in connection with certain 
preferred embodiments in the following examples so that aspects thereof 
may be more fully understood and appreciated, it is not intended to limit 
the invention to these particular embodiments. On the contrary, it is 
intended to cover all alternatives, modifications and equivalents as may 
be included within the scope of the invention as defined by the appended 
claims. Thus, the following examples which include preferred embodiments 
will serve to illustrate the practice of this invention, it being 
understood that the particulars shown are by way of example and for 
purposes of illustrative discussion of preferred embodiments of the 
present invention only and are presented in the cause of providing what is 
believed to be the most useful and readily understood description of 
formulation procedures as well as of the principles and conceptual aspects 
of the invention. 
EXAMPLE 1 
A clear solution (5 liters) containing 30 wt. % aqueous potassium 
hydroxide, 200 g dissolved zinc oxide and 0.5 g inhibitor lead (II) oxide, 
was transferred to an electrolytic bath which contained two immersed 
nickel anodes flanking a central vitreous carbon cathode. Each plate had 
the dimensions 50.times.50.times.3 mm, and was fitted with 
current-carrying leads; there was a 20 mm space on each side between the 
cathode and the anodes. 
An electrowinning current of 15 A (300 milliamp/cm.sup.2 at the cathode) 
was applied to the electrowinning cell at a voltage of 2.4 V. The bath 
temperature stabilized at 70.degree. C. without the need for external 
cooling. From time to time, deionized water or alkali was added to the 
bath to maintain the alkali concentration. 
The cathode was scraped every 2 minutes for 10 seconds with a plastic 
blade, and every half hour the zinc that fell to the bottom of the bath 
was transferred to a separate container. This zinc was then blended with a 
blender into a particulate structure. The blending step afforded 
alkali-moist zinc particles below about 30 mesh particle size, and having 
a bulk density of 0.6 g/cc. 
After about 115 minutes of electrolysis, there was obtained a quantity of 
alkali-moist zinc, containing about. 33 g dry zinc, thus indicating a 
current efficiency of about 95% at the specified current density. The zinc 
contained about 1000 ppm lead. By gasometric methods, the zinc was found 
to have a low gassing rate for hydrogen, 0.04 ml/hr/g zinc (compared to 
0.2 ml/hr/g zinc for undoped zinc), on attempted reaction with 30 wt. % 
KOH at 30.degree. C. Similar results were obtained using inhibitor 
cation(s) other than those from lead (II) oxide. 
It was surprisingly found that the zinc from electroplating baths with a 
low or insignificant amount of inhibitor cation(s) could be given 
additional corrosion protection by simply reacting for some hours with a 
solution (e.g., alkaline) containing inhibitor cations. For example, 
lead-free zinc (33 g) stirred overnight with a liter of 30 wt. % KOH 
containing dissolved PbO (0.1 g) provided acceptable inhibition of 
corrosion on repeated recycling, with minimal makeup inhibitor. The 
lead-doped, alkaline-moist zinc was mixed with 25 g of 30 wt. % aqueous 
potassium hydroxide, and gave a slurry having a density of about 1.5 g/ml. 
Enough slurry to provide 33 g of zinc which exhibited no generation of 
hydrogen bubbles was introduced into the anode frame compartment of a 
zinc-air cell. The cell provided 2A for 10 hours at an average discharge 
rate of 1.2 V, until a cut-off voltage of 1.1 V. Since there were about 33 
g zinc in the cell, the zinc utilization was about 75%. When the discharge 
was run with untreated zinc, the cell passivated after 15 minutes, due to 
excessive hydrogen gassing which blocked the electrolyte path to the air 
electrodes of the cell. 
EXAMPLE 2 
Following the details of Example 1, but substituting 0.05 g Bi.sub.2 
O.sub.3 and 0.05 g In.sub.2 O.sub.3 in place of PbO gave similar results, 
but the corrosion rate was somewhat higher, 0.06 ml/hr/g zinc. 
EXAMPLE 3 
1 kg of electrochemically-generated, mercury-free zinc powder alloyed with 
bismuth and indium was prepared according to the procedure of Examples 1 
and 2. The powder, after washing and drying, had a surface area of 0.5 
m.sup.2 /g, a density of 1.5 g/cm.sup.2, and trace alloying constituents 
of 200 ppm bismuth and 200 ppm indium, which gave a corrosion rate at 
30.degree. C. in 30 wt. % KOH of less than 0.1% per week. A gelled anode 
mixture was made, containing 70 wt. % zinc., 6 wt. % carboxymethyl 
cellulose binder and balance aqueous 30 wt. % KOH. The viscous mixture was 
extruded into the anode compartment of a small number of AA size cells of 
the zinc-manganese dioxide type. Upon running controlled discharges with 
the electrochemical zinc bearing cells at the 10 hour discharge rate, a 
consistent 10% higher capacity was obtained with these cells, compared 
with equivalent cells containing the same zinc content and anode 
formulation based on thermally-generated zinc. 
EXAMPLE 4 
The cell performance in Example 3 may have been limited by the invariable 
manganese dioxide cathode. In order to show the high power capabilities of 
electrochemically-generated zinc, two zinc-air cells were constructed with 
high-performance air cathodes. The cells each comprised two AE-20 type air 
electrodes (Electromedia Ltd.) of active area 10.times.10 cm, flanking a 
central zinc anode. The zinc anode comprised zinc powder (100 g) mixed 
with 30 wt. % KOH (50 g) pressed onto a copper screen, and once the anode 
was inserted between the cell electrodes, the cell was topped off with 
electrolyte (30 wt. % KOH). One cell was assembled using 
electrochemically-generated zinc (see Example 3) and a second cell with 
commercially-available, thermally-generated zinc. For both cells, under a 
five times stoichiometric air flow, the open current voltage was 1.45 V. 
The electrochemically-generated zinc cell sustained 80A at 0.9 V for 30 
seconds, and could be continuously discharged at 30A for 2 hours to a 0.85 
V cutoff, delivering 60 Ahr capacity. By comparison, the 
thermally-generated zinc cell could sustain a peak of only 50A at 0.6 V 
for 10 seconds, while the maximum achievable continuous discharge to the 
0.85 V cutoff was 10A for 5 hours, giving a capacity of 50 Ahr. 
It will be evident to those skilled in the art that the invention is not 
limited to the details of the foregoing illustrative examples and that the 
present invention may be embodied in other specific forms without 
departing from the essential attributes thereof, and it is therefore 
desired that the present embodiments and examples be considered in all 
respects as illustrative and not restrictive, reference being made to the 
appended claims, rather than to the foregoing description, and all changes 
which come within the meaning and range of equivalency of the claims are 
therefore intended to be embraced therein.