Linden D., Reddy T.B. (eds.) Handbook of batteries
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PROPULSION AND INDUSTRIAL NICKEL-METAL HYDRIDE BATTERIES 30.37
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31.1
CHAPTER 31
NICKEL-ZINC BATTERIES
Dwaine Coates and Allen Charkey
31.1 GENERAL CHARACTERISTICS
The nickel-zinc (zinc/nickel-oxide) battery is an alkaline rechargeable system. It is a com-
bination of the nickel electrode, as used in other batteries such as nickel-cadmium, nickel-
iron and nickel-metal hydride, and the zinc electrode, which is similar to that used in the
silver-zinc battery system. Currently the nickel-zinc system is capable of delivering about
50 to 60 Watt-hours per kilogram (Wh /kg) and 80 to 120 Watt-hours per liter (Wh/ L)
depending on the specific design. Nickel-zinc batteries are capable of delivering more than
500 cycles at 100% depth-of-discharge (DOD) and up to several thousand cycles at low
DOD. The advantages of the nickel-zinc battery include good specific energy, good cycle
capability, abundant low-cost materials and an environmentally acceptable chemistry. The
nickel-zinc battery is appropriate for a number of commercial applications including electric
bicycles, electric scooters, electric lawn and garden equipment and deep cycle marine ap-
plications.
The generalized advantages and disadvantages of nickel-zinc batteries are shown in Table
31.1.
TABLE 31.1 Advantages and Disadvantages of Nickel-Zinc Batteries
Advantages Disadvantages
Fast recharge capability Relatively low volumetric energy density
Sealed maintenance-free design Higher cost than lead-acid
Good specific energy
Good cycle life
Low environmental impact
Abundant raw materials
Relatively low cost per Watt-hour
31.2 CHAPTER THIRTY-ONE
31.1.1 Background
The goal of the initial development of the nickel-zinc battery was to combine the long cycle
life associated with nickel-cadmium batteries with the high specific energy of the zinc elec-
trode. Historically, the nickel-zinc battery dates back at least to 1901 in a Russian patent by
Michaelowski. Further work was performed by Drumm in Ireland in the 1930s. There was
a serious effort in the 1960s to develop nickel-zinc as a longer-life replacement for the silver-
zinc battery in military applications. Considerable effort was again focused on nickel-zinc
in the 1970s in response to the increased interest in electric vehicles that resulted from an
energy crisis and increasing gasoline prices. System development was hampered for many
years by the limited cycle life associated with the zinc electrode. This cycle life limitation
was primarily due to the solubility of zinc in the alkaline electrolyte. This has been overcome
by recent developments in stabilizing the zinc electrode in alkaline electrolyte and thereby
reducing its solubility and increasing the cycle life of the battery. The improved cycle life
now available has again increased interest in the nickel-zinc battery for a wide variety of
consumer applications. There is currently an extensive patent and scientific literature base
available concerning the nickel-zinc battery system.
1–3
31.2 CHEMISTRY
The nickel-zinc battery system uses the nickel/nickel oxide electrode (also known as the
nickel-hydroxide/nickel oxyhydroxide electrode) as the positive and the zinc/ zinc oxide
electrode as the negative. When the battery is discharged, nickel(III) oxyhydroxide is reduced
to nickel(II) hydroxide and metallic zinc (0) is oxidized to zinc(II) oxide /hydroxide. The
electrochemistry of zinc in alkaline solution is actually quite complex so the reactions pre-
sented here are for the purpose of illustration only. The theoretical open-circuit voltage of
this electrochemical couple is 1.73 V. When the battery is overcharged, oxygen is produced
at the nickel electrode and hydrogen is produced at the zinc electrode. These gases may then
recombine to form water. In addition, oxygen produced at the nickel electrode during over-
charge may recombine with metallic zinc directly at the zinc electrode. If the battery is
overdischarged, hydrogen is formed at the nickel electrode and oxygen may be produced at
the zinc electrode. In practical batteries, these reactions are influenced by the balance of
active materials present and the active material utilization of the two electrodes. The sim-
plified, representative electrochemical reactions are as follows:
Discharge E
⬚
Positive electrode 2 NiOOH ⫹ 2H
2
O ⫹ 2e
⫺
→ 2 Ni(OH)
2
⫹ 2OH
⫺
0.49 V
Negative electrode Zn
⫹ 2OH
⫺
→ Zn(OH)
2
⫹ 2e
⫺
1.24 V
Overall reaction 2 NiOOH
⫹ 2H
2
O ⫹ Zn → 2 Ni(OH)
2
⫹ Zn(OH)
2
1.73 V
Charge
Overall Reaction 2 Ni(OH)
2
⫹ Zn(OH)
2
→ 2 NiOOH ⫹ 2H
2
O ⫹ Zn
Overcharge
Positive electrode 2 OH
⫺
→
1
⁄
2
O
2
⫹ H
2
O ⫹ 2e
⫺
Negative electrode 2 H
2
O ⫹ 2e
⫺
→ H
2
⫹ 2OH
⫺
Zn ⫹
1
⁄
2
O
2
→ ZnO (Recombination of oxygen from the
positive electrode)
Overall reaction H
2
O → H
2
⫹
1
⁄
2
O
2
Nickel hydroxide active material has a theoretical specific energy of 289 milliAmpere-hours
per gram (mAh /g) and zinc oxide has a theoretical specific energy of 659 mAh/g. Overall,
the nickel-zinc battery has a theoretical specific energy of 334 Wh /kg, which makes it a
NICKEL-ZINC BATTERIES 31.3
very attractive system for a number of applications. Practical batteries operate at a loaded
discharge voltage of 1.55 to 1.65 V and may deliver up to about 70 Wh/kg, depending on
the specific design. This is only about 20% of the theoretical specific energy, which indicates
that there is room for system improvement.
31.3 CELL COMPONENTS
Nickel-zinc batteries can be constructed in a variety of configurations such as prismatic or
cylindrical cell designs. The specific cell or battery design will dictate the cell component
configurations used. However, all cell designs have basic similarities in terms of the active
materials used, the use of cell containers and covers, the use of metallic conductors and in
the basic manufacturing techniques and application of these materials.
31.3.1 Nickel Electrode
The electrochemically active material of the nickel electrode is nickel hydroxide. This ma-
terial is an amorphous colloid and is only semiconductive at best. It must be supported and
contained by a structural component which provides mechanical support, conductivity and
current collection for the electrode. Standard types of nickel electrodes can be used in the
nickel-zinc system. They can be classified by the type of electrode substrate used and by
the method of preparation. These electrodes consist of two basic types, sintered and nonsin-
tered. Each type has different advantages and disadvantages and may be selected based on
the application. Other types of nickel electrodes, such as pocket plate, are generally not in
common use.
A particular situation with the nickel-zinc battery, which is not present in other nickel
batteries, is the effect of the zinc electrode on the nickel electrode.
4
The observed effect is
that there is essentially an irreversible initial capacity loss due to decreased capacity in the
nickel electrode. There are several possible causes for this effect but the most likely seems
to be pore plugging of the nickel electrode by zinc which is deposited from the electrolyte.
The effect appears in the first few cycles of the battery and then does not seem to further
influence the performance or cycle life of the battery. This must be taken into account in
the design of the battery. The effect is apparently independent of the calcium hydroxide level
in the calcium containing zinc electrode but is influenced by the electrolyte formulation and
concentration.
A similar study showed that the effect of the zinc electrode on the nickel electrode was
primarily mechanical.
5
Previous speculation of the effect of nickel-hydroxyzincates or nickel
zinc double hydroxides on capacity degradation in the nickel electrode were not found to be
significant. However, significant increases in the resistivity of cycled nickel electrodes were
observed which corresponded with mechanical damage observed by optical microscopy.
Further work showed that the performance of the nickel electrode is degraded in the
nickel-zinc battery because of zinc occlusion in the pores of the nickel electrode and the
reduced electrolyte concentration typically used.
6
Deformations in the electric field were also
investigated as a possible mechanism for the observed degradation in performance.
Pocket Plate Electrode. This is the same type of electrode used in pocket plate nickel-
cadmium and nickel-iron batteries.
2
Electrodes are prepared by loading nickel hydroxide
hydrate active material and a conductive additive (graphite and /or nickel flake) into tubular
flat pockets which are then assembled into electrodes. Little interest currently exists in using
this type of electrode in nickel-zinc cells since modern cells attempt to utilize lightweight
electrode construction.
31.4 CHAPTER THIRTY-ONE
Sintered Nickel Electrodes. Until recently, the sintered electrode was the most common
type of nickel electrode used in commercial nickel-cadmium batteries. It has been widely
replaced in cylindrical cells by the pasted type electrode, discussed below. The electrode
substrate is prepared by sintering high surface area carbonyl nickel powder into a porous
structure. Sintering involves heating the nickel powder to just below its melting point so that
the individual particles are fused together. This is done under a reducing atmosphere to avoid
oxidation of the metallic nickel. This electrode structure is typically 80% to 84% porous
(known as the nickel plaque electrode) before the introduction of the nickel hydroxide active
material. The electrode substrate also typically contains a current collector support such as
nickel wire mesh, perforated nickel or nickel-plated steel. The nickel hydroxide active ma-
terial can be introduced into this porous structure in a number of ways. The most common
is chemical impregnation. The porous substrate is alternately dipped in a bath of nickel
nitrate and a separate bath of potassium hydroxide, with the resultant chemical precipitation
of nickel hydroxide into the structure. This method is cumbersome in that as many as 8 to
10 cycles may be required to achieve adequate active material loading. A refinement of this
process involved dipping the substrate into the nickel nitrate bath under a vacuum. This was
typically done for high-performance aerospace cells but still required multiple cycles. Sin-
tered electrodes are capable of high energy density and long cycle life but are relatively
expensive to produce because of the high nickel content and the expensive sintering process.
A further refinement involved the electrochemical deposition of nickel hydroxide into the
porous electrode substrate directly from the nitrate bath. This is accomplished by cathodically
polarizing the nickel electrode substrate such that a local change in pH inside the substrate
pores causes the deposition of the nickel-hydroxide active material. This improvement allows
the impregnation process to be completed in a single step. An important advantage of this
process is that the electrodes are impregnated with active material from the inside out,
resulting in a much more uniform cross-sectional loading. The resultant electrodes display
better performance and active material utilization than other impregnation methods. This
process is typically used only in aerospace nickel-cadmium and nickel-hydrogen cells and
has not generally been adopted in commercial batteries. Other types of impregnation are
possible, such as the thermal decomposition of nickel salts, but these have also not found
extensive commercial application.
The primary disadvantage of sintered nickel electrodes is that they contain a relatively
low ratio of active materials to inactive materials compared to some types of non-sintered
electrodes. Sintered electrodes typically contain as much as 60% inactive material, in the
form of the porous electrode substrate and/or current collector. Practical sintered nickel
electrodes yield about 100 to 120 mAh/g, depending on the method of preparation and the
active material utilization. This is only about 40% of the theoretical specific energy of the
active material. Also, the sintering process itself is relatively expensive and must be done at
high temperatures under a reducing atmosphere.
Non-sintered Nickel Electrodes. Two types of nonsintered nickel electrodes have been
developed for commercial applications. The most common type is the pasted nickel electrode.
In this method, the nickel-hydroxide active material formulation is mechanically introduced
into a porous nickel substrate. The substrate can be nonwoven nickel fiber or a variety of
other materials, but nickel foam is the most commonly used. Nickel foam is produced by
depositing metallic nickel on a porous plastic foam and then burning away the polymer in
an oven, leaving only the nickel metal. Electrode substrates can be produced in this manner
with a porosity as high as 95%. Many small form-factor cylindrical nickel-cadmium and
nickel-metal hydride cells use a pasted nickel foam electrode. This type of electrode is
generally not used in prismatic flooded cells due to the tendency for the electrode to extrude
the active material out of the structure during cycling. The pasted type of electrode suffers
from relatively poor active material utilization, a tendency towards significant swelling during
cycling and consequently relatively poor cycle life. The advantage is that it is relatively
cheap and efficient to produce in large quantities.
NICKEL-ZINC BATTERIES 31.5
A second type of nonsintered nickel electrode which has been developed for commercial
applications is the plastic-bonded electrode. An example of this process is shown in Fig.
31.1. This electrode has been developed primarily for the nickel-zinc battery and optimized
for lightweight and low cost. In this process, the active materials are blended together with
a polytetrafluoroethylene (PTFE) binder and then roll-bonded into a porous three-dimensional
structure by fibrillating the binder. PTFE has the unique property of forming nanometer scale
fibers through a combination of heat, compression, shear and mechanically working the
material. By properly controlling the process, the resultant orientation of the nanofibers and
thus the resultant electrode structure can be controlled. The fibrillated PTFE forms an elastic
three-dimensional network which locks the active materials into the structure. This unique
electrode structure is shown in Figure 31.2. This electrode structure overcomes the major
disadvantage of the pasted foam electrode, which is shedding the active material from the
electrode structure during cycling and a resultant reduction in cycle life. The active material
is essentially locked into place by the nano-fibrillated structure. The composite electrode
active material matrix is supported in the electrode by a very thin (typically less than 0.05
millimeter) metallic substrate. This provides mechanical support and current collection and
also provides a point of electrical contact.
FIGURE 31.1 Roll-bonding process. (Courtesy of Evercel Corp.)
The plastic-bonded nickel positive uses a graphite composite structure which replaces the
metallic nickel, used in standard sintered and foam electrodes, with high surface area carbon.
This results in an electrode with a much lower metallic content, with a savings in cost and
weight. Graphite corrosion in the highly oxidizing cell environment is controlled by treating
the carbon with a cobalt spinel.
7
The increased impedance of the graphite composite electrode
is partially offset by the increased surface area of carbon as compared to nickel powder or
foam. The conductivity of the carbon is also enhanced by the cobalt treatment. The increased
surface area provides greater interfacial contact between the active material and the carbon
support and between the active material composite structure and the electrolyte. The electrode
is able to support current densities suitable for high-rate battery applications. Since graphite
has a much lower density than nickel metal, the plastic bonded electrode is much lighter in
weight than standard nickel electrodes for an equivalent energy storage capacity. Plastic-
bonded graphite composite nickel electrodes yield about 140 to 150 mAh/ g, depending on
the formulation, use of additives, the method of preparation and the active material utiliza-
tion. By the same token, since graphite has a lower density it also occupies a larger volume
for the same weight as metallic nickel. This results in a somewhat lower energy density for
the electrode (milliAmpere-hours per cubic centimeter).
31.6 CHAPTER THIRTY-ONE
FIGURE 31.2 Scanning electron microphotograph of roll-bonded nickel electrode material. (Courtesy of
Evercel Corp.)
31.3.2 Zinc Electrode
Traditionally, zinc electrodes have been prepared by a variety of means including pressed
powders, electrodeposition and pasting methods. A thorough review of the state of zinc
electrode technology has been previously published.
2
Zinc electrodes can be prepared in
either the charged or discharged state depending on whether the starting material is metallic
zinc (charged) or zinc oxide (discharged). Typically charged zinc electrodes are used only
in primary batteries while zinc electrodes for secondary batteries are usually manufactured
in the discharged state. Like the nickel electrode, the zinc electrode contains a mechanical
support/ current collector. This is usually copper or plated copper and can be perforated foil,
expanded metal, wire mesh or other similar materials.
Zinc oxide electrodes, which are manufactured in the discharged state, have a relatively
high initial impedance due to the semiconductive nature of the oxide. During cell formation,
a significant quantity of the oxide is reduced to metallic zinc. This greatly improves electrode
performance because of the increased conductivity imparted by zinc metal. Zinc electrodes
may take as many as 50 cycles to become fully formed, but performance is adequate after
normal battery formation of 3 or 4 cycles. The zinc electrode will continue to improve in
performance over the initial usage cycle as the metallic zinc content of the electrode is
gradually increased. Alternatively, some metallic zinc can be added to the electrode during
manufacture to offset the initial high impedance.
The electrochemistry of the zinc electrode in alkaline solution is actually quite complex.
Zinc commonly has a coordination number of 4, such as in zinc oxide, or of 6, as in some
crystals. The electronic configuration of zinc makes the
⫹2 oxidation state the most stable,
although references are occasionally made to monovalent zinc. Zinc has a tendency towards
NICKEL-ZINC BATTERIES 31.7
FIGURE 31.3 Scanning electron microphotograph of fresh roll-bonded zinc electrode. (Courtesy
of Evercel Corp.)
dissolution in the electrolyte and the subsequent formation of a variety of solvated complex
ions. The species involved in zinc electrochemistry are typically oxides and hydroxides, and
the products of their interactions with the aqueous alkaline electrolyte.
Zinc is partially soluble in the alkaline electrolyte. Herein lies the basic problem with the
zinc electrode, which is manifested as electrode shape change and the formation of dendrites.
Redistribution of zinc on the electrode is commonly referred to as ‘‘shape change.’’ During
discharge, zinc is oxidized from metallic zinc to zinc (II) and dissolves in the electrolyte in
the form of zincate anions (e.g. Zn(OH)
4
2
⫺
). During charging, zinc ions in the electrolyte
are reduced back to metallic zinc and are replated back onto the electrode. The zinc does
not necessarily replate onto the electrode in the same place from which it came. Therefore
the ‘‘shape’’ of the electrode changes. Typically, the zinc tends to migrate to the lower one-
third of the electrode which becomes much more dense. This densification reduces the elec-
trochemical activity of the zinc and may lead to cell failure through electrical shorting. An
associated phenomenon is zinc dendrite formation. If the surface of the zinc electrode is not
uniform, zinc may preferentially replate onto the ‘‘high’’ spots. The zinc may gradually build
up until the separator is penetrated and a short-circuit occurs. Dendrites will form under
diffusion controlled deposition conditions.
Zinc electrodes can also be manufactured by the plastic-bonded method, similar to that
of the nickel electrode described above. The zinc oxide dry powder, PTFE binder and other
additives are blended with an organic solvent and then the mixture is passed through a
calendaring process similar to that in Fig. 31.1. The PTFE fibrillates into a nano-structured
three-dimensional fiber matrix. This electrode structure for the calcium zincate electrode is
shown in Fig. 31.3 for a freshly prepared electrode. The active materials are locked into the
electrode structural matrix which helps to reduce the tendency towards shape change and
dendritic growth.
31.8 CHAPTER THIRTY-ONE
Zinc Electrode Additives. In addition to its partial solubility in the alkaline electrolyte,
metallic zinc has a tendency to oxidize (corrode), evolving hydrogen in the process. This is
undesirable in a sealed battery for a number of reasons. Many additives have been investi-
gated to reduce hydrogen evolution and to improve zinc electrode cycle life performance.
8
The traditional mercury amalgamation zinc electrode process has been largely abandoned
because of the environmental problems associated with mercury. A wide variety of substitutes
has been investigated. These include various mixed oxides and hydroxides of lead, bismuth,
indium, thallium, tin and antimony in addition to fluorides, carbonates, borates and others.
Currently, lead provides the best observed performance in sealed batteries. Even though lead
is present in a relatively small amount in the nickel-zinc battery, it is still an environmentally
objectionable material and efforts are underway to reduce or completely eliminate lead from
the battery.
The average zinc electrode active material utilization was found to be about 60% of the
theoretical capacity.
9
The reason for this relatively low electrochemical utilization has not
been reported in the literature but may be related to the solubility and complex chemistry
of zinc in alkaline solution. The utilization obtained will be a function of the electrode design
and composition and will be affected by the use of any additives either to the electrode or
the electrolyte. The utilization of the zinc electrode may also vary for a specific cell design
and must be taken into account in the electrochemical design and active material balance of
the cell.
Calcium-Zincate Electrode. The use of calcium as an additive to the zinc electrode dates
to at least 1962.
10
Studies have been done on the formation of calcium zincate during the
corrosion of zinc in cement. The calcium zincate complex was found to improve the strength
of concrete. Calcium zincate materials are also used in a variety of industrial process and
materials such as dielectrics, catalysis and glass. A thorough discussion of calcium zincate
electrode theory and operation has been published.
4
This work found that zinc electrode
shape change was dramatically reduced by the addition of calcium to the zinc electrode. It
was further determined that the optimum concentration of calcium is about 25% by weight,
which is near the stoichiometric ratio depending on the assumed structure of the zincate
complex. It was also found that the rate of decomposition of the zincate complex is relatively
fast but that the rate of formation can be slow, and perhaps even rate limiting in some cases.
The rate of formation of calcium zincate is favored by lower concentrations of potassium
hydroxide. Therefore the use of lower electrolyte concentrations in the nickel-zinc cell has
a positive effect on calcium zincate formation which helps to offset the reduced conductivity
and ionic strength of the weaker electrolyte.
A study
11
showed that reduced zinc oxide solubility improved the cycle life of Zn/NiOOH
batteries. There have been several studies of the rate of formation and decomposition of
calcium zincate in solutions of Ca(OH)
2
, ZnO and KOH.
11–13
The rate equations developed
in these studies evaluate the effectiveness of adding Ca(OH)
2
with calcium zincate formation
and decomposition reactions. At a discharge rate of C /3, about 26% of the zincate is still
in the solution. At a charge rate of C /6, the liberation of zincate from calcium zincate is
complete. Because the calcium zincate formation is a slower process, the discharge rate could
effect how well the addition of Ca(OH)
2
controls shape change in a battery. Since the calcium
zincate decomposition is a faster process, the addition of Ca(OH)
2
will reduce the growth
of dendrites.
A zinc electrode with reduced solubility has been developed and patented.
12
This tech-
nology has made nickel-zinc batteries practical from a cycle-life point of view. The ‘‘shape
change’’ that occurs in the standard zinc electrode during charge /discharge cycling is due
to the formation of the intermediate zincate ion. The zinc active material passes through this
soluble phase during oxidation and reduction in alkaline solution. The calcium-zincate elec-
trode design will reduce the formation of the soluble zincate intermediate reaction product
by proceeding directly from the metallic oxidation state to an insoluble phase. Calcium
hydroxide is used as an additive to the electrode to promote the formation of calcium zincate
NICKEL-ZINC BATTERIES 31.9
FIGURE 31.4 Roll-bonded calcium zincate electrode after 500 cycles. (Courtesy of Ev-
ercel Corp.)
which is thermodynamically stable and remains substantially insoluble in the electrolyte.
Zinc dissolution is mitigated by formation of a passivating layer at the electrode /electrolyte
interface. A potential concern is electronic conduction through this passivating, insoluble
film on the zinc electrode. This can be addressed by doping the electrode with electronically
conductive metallic oxides, such as PbO or Bi
2
O
3
.
The relative lack of shape change which occurs after cycling in the calcium zincate
electrode is shown in Fig. 31.4. Only a slight redistribution of zinc from around the edges
of the electrode is observed. There is a slight densification of zinc which occurs over the
bottom one-fourth of the electrode, but it is relatively minor compared to the massive changes
seen in zinc electrodes which do not contain calcium hydroxide. The detailed structure of a
calcium zincate electrode which has been cycled 500 times at 100% depth-of-discharge is
shown in the scanning electron microphotograph of Fig. 31.5.
The formation of calcium zincate is given by the reaction:
Ca(OH) ⫹ 2 ZnO ⫹ 4 H O Ca[Zn(OH) ] 䡠 2HO
22322
The stoichiometry of calcium zincate has also been reported in the literature as Ca(OH)
2
䡠 2
Zn(OH)
2
䡠 2H
2
O.
13
This reference also contains a detailed study of the kinetics of calcium
zincate formation in potassium hydroxide solutions. This study concluded that the rate of
formation of calcium zincate was not a function of the dissolution and transport of zinc
oxide or of the transport of water, but is a function of the calcium hydroxide concentration.