SECONDARY BATTERIES – ZINC SYSTEMS | Zinc–Silver

Zinc–Silver AJ Salkind, Rutgers University, Piscataway, NJ, USA AP Karpinski and JR Serenyi, Yardney Technical Products, Inc., Pawcatuck, CT, USA & 2009 Elsevier B.V. All rights reserved.

Background

At the negative electrode:

The zinc–silver-oxide system, commonly designated as silver–zinc or Ag–Zn, has been known at least since the early nineteenth century when Alessandro Volta experimented with rudimentary batteries of this electrochemistry. In the later part of that century, extensive work on the system and its electrodes was undertaken by Jungner in Sweden and Edison in the United States, among others. Nevertheless, it was not until the 1940s that the meticulous work of the French Professor Henri Andre´ resulted in a commercially viable, truly rechargeable battery. His contributions to the development of the Ag–Zn system are too numerous to be listed in a short article, but the most important was the introduction of cellophane as a semipermeable membrane to prevent or retard the migration of colloidal silver to the zinc electrode, which caused all prior cells to short out quickly. Without detracting from the notable achievements of Andre´, his typical cell had some shortcomings, namely: Because of the emphasis on long cycle life, the cell • could only sustain low-to-moderate discharge rates,

•

and thereby did not take advantage of the system’s inherent high specific power and power density. The cell required maintenance by the user, viz electrolyte-level adjustments, and very careful charge and discharge controls; the latter were cumbersome with the electronic equipment available at the time.

In the late 1940s, Prof. Andre´ became associated with Michel Yardney, who introduced the system to the United States and, through his company, Yardney Electric Corporation, established in New York City, further developed it in the 1950s, and thereby attracted the immediate attention of the US Navy for use in submersibles and electrical torpedoes. Later on, a large variety of applications arose, as described elsewhere in this article.

Electrochemistry The electrochemistry of the silver–zinc system is simple. It is represented by the following reactions: At the positive electrode: Ag þ 2OH

Charge

$ AgO þ H2 O þ 2e

Discharge

½I

ZnO þ H2 O þ 2e

Charge

$ Zn þ 2OH

Discharge

½II

Overall: Ag þ ZnO

Charge

$ AgO þ Zn

Discharge

½III

The only peculiarity is that silver, unlike other divalent elements, experiences a double valence change (from 0 to 2 and vice versa) in two distinct steps, as both monovalent (Ag2O) and divalent (AgO) silver oxides, commonly known as silver monoxide and silver peroxide, respectively, are stable, as expressed by 2Ag þ 2OH 2Ag2 O þ H2 O þ 2e

½IV

Ag2 O þ 2OH 22AgO þ H2 O þ 2e

½V

The two levels of oxidation of the silver electrode are responsible for the peculiar shape of the charge and discharge curves of the silver–zinc cells. An example is shown in Figures 1(a) and 1(b). Charging is normally performed at a low rate (as recommended by the manufacturer, see disadvantages in the following section) for maximum efficiency, which usually exceeds 99%. The discharge rate, dictated by the application, can be very high, and the cells are designed to accommodate such rates. If a relatively new cell is overcharged (i.e., charged beyond the exhaustion of the active materials) the positive electrodes will be fully charged first, generating oxygen; eventually, if the overcharge is continued, the negative electrodes will be fully charged as well, generating hydrogen. After many cycles, near the end of the cell’s useful life, the balance of the active materials changes, the negative electrodes reach a full state of charge first, and the gases are generated in reverse order. Overcharging should be avoided, because the gases generated electrolyze the water from the electrolyte, thus drying out the cell. Furthermore, overcharging the negative electrodes generates zinc dendrites, needle-like formations that tend to perforate the separators and thus cause internal short circuits. Over-discharging or reversing the cells generates hydrogen at the positive electrodes and oxygen at the negatives. Cell reversal is even more harmful than

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Secondary Batteries – Zinc Systems | Zinc–Silver 2.05 2.00 1.95

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Figure 1 (a) Typical charge of a 200-Ah cell at 8 A. (b) Typical discharge of a 70 Ah cell at 10 A.

overcharging, because in addition to the gas generation (which usually is much more vigorous, depending on the rate of discharge), zinc may plate on the silver electrodes, which is an irreversible reaction that severely degrades cell performance.

Practically no size limitations (refer to the section • entitled ‘Availability’). High charge efficiency, typically >99%. • Safer to operate other systems. • Can be designedthanformost use the rechargeable and • primary modes. The latter ininclude the so-called reserve types, where the electrolyte is stored outside the cells, and which can be activated by a pyrotechnic devise (usually in less than 1 s).

Advantages/Disadvantages The major advantages of silver–zinc as compared to the systems prevalent at the time of its introduction (mainly lead–acid, nickel–cadmium, nickel–iron) and some of the more recent ones (lithium-ion, nickel–metal hydride, nickel–zinc, nickel–hydrogen, etc.) are given as follows:

The system has some disadvantages as well, namely: High cost, which can be linked to the use of a precious • metal, silver, as an active material and to the fact that

Highest specific power (up to 600 W kg continuous • and • 2500 W kg for short-duration pulses). Very high specific energy (up to 250 W h kg ), sur• passed only by Li-ion. 1

1

1

many cells and batteries are custom-made for a given application. Limited cycle life and wet life, typically no more than 100 cycles and 2 years; ways to overcome these limitations are listed under the section entitled ‘Recent Improvements’.

Secondary Batteries – Zinc Systems | Zinc–Silver

Relatively slow recharge; while high-rate cells can • usually be recharged (from a fully discharged condition) in 12 h or less, low-rate models can take anywhere from 24 to 96 h, depending on design. Higher rates reduce the charge efficiency and, if used repeatedly, accelerate the rate of capacity fade of the cells.

Components and Processes The main components of a silver–zinc cell are: (silver) electrodes; • positive negative • separators;(zinc) electrodes; • electrolyte; • cell case, cover, and hardware; and • cements, solder, vent valves. • Some cells have additional components, for particular purposes. For example, cells that must withstand high levels of shock, acceleration, and/or vibration may be equipped with hold-down blocks and/or epoxy cement at the top and bottom of the cell packs to prevent or minimize the movement of the electrodes (and possible damage to them) during such events. A discussion of the components, and the methods employed for their manufacture, with emphasis on the positive and negative electrodes follows. Positive (Silver) Electrode Positive electrodes are made from high-purity (>99.9%), small-particle-size (generally 1–10 mm) silver powder, and a current collector or grid, also made of silver, in the form of a solid sheet, or more commonly as expanded metal (Exmet), which has a particular pattern of diamond-shaped perforations. Silver electrodes can be made by the following methods: 1. Mold pressing: This is an old method, used to make single plates or larger master plates. Silver powder is pre-weighed and uniformly distributed into the cavity of a suitable mold, with grid and pre-attached leads. A plunger is located on top, and the assembly is pressed to the required thickness. Finally, the plates are sintered and removed from the mold. The method requires little capital investment, but it is labor intensive. Furthermore, the quality and uniformity of the resulting electrodes may be questionable. 2. Continuous rolling: This method, used by most (if not all) battery manufacturers, requires a sizeable initial capital investment, but involves considerably less labor. It results in excellent weight and thickness uniformity. Silver powder is fed onto a conveyor belt lined with special paper and passed through adjustable

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doctor blades, which spread it uniformly across the width of the belt. Next, a ribbon of expanded silver foil from a supply roll is embedded into the powder, and the composite sheet, called strip, is pressed to the desired thickness by means of an adjustable roller. The paper is then removed to a take-up spool and the strip is passed through a sintering furnace, set at 600– 800 1C. After sintering and cooling, the strip is cut into slabs of suitable length or travels through a cutting die that produces electrodes of the desired size. All of the above operations are performed by a rolling mill, which may be more or less automated. A fully automated mill controls the speed of the conveyor belt, the setting of the doctor blades, the tension on the roller, and the sintering temperature. It also samples the strip for weight and thickness and makes corrections, as needed. After the strips are cut to electrode size, the leads are welded directly to the surface of the plate. The silver electrodes are used as manufactured by one of the above methods for ‘dry unformed’ (i.e., Ag– ZnO) cells, but require further processing for ‘dry charged’ (i.e., AgO–Zn) cells, which presently constitute the vast majority. Dry charged cells, by definition, require silver oxide electrodes, which are usually by electroforming the silver electrodes as described above. The electrodes are set in frames against inert nickel or steel counterelectrodes and placed in large tanks. After filling with electrolyte, a potassium hydroxide solution, current is passed through the cells at a low rate (2–3 mA cm2) until the electrodes are fully charged or converted into divalent silver oxide (in practice this is not possible: a charge level of at least 85% of that needed to convert all of the silver into the divalent oxide is considered satisfactory; the rest of the silver remains uncharged or converts only into monoxide). The electrodes are then sampled to verify that they meet capacity requirements, by force discharging against inert nickel counter-electrodes, a procedure called dummy discharge, and, if satisfactory, they are removed from the tanks, thoroughly washed and dried, and the oxide is removed from the leads by dipping them into a nitric acid solution. If one or more sample plates fail to meet requirements, the tank is reprocessed by continuing the charge at a lower rate or, more effectively, by a partial discharge of the electrodes, followed by a recharge. Should the electrodes still fail, an unlikely event, the tank is rejected and the plates scrapped. 3. As an alternative procedure, chemically prepared divalent silver oxide (AgO) may be extruded, allowing for the direct use of the electrodes without having to electrochemically oxidize or form them. Although this one-step process appears quite simple, working with divalent silver oxide is notoriously difficult, requiring

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very high pressures, and results in excessive thickness and weight variability, thus increasing the proportion of rejects. Some cells are required not to exceed maximum voltage limits on open circuit or at low-rate discharge: to accomplish this objective the positive electrodes may require further treatment to remove part or all of the silver peroxide (AgO) by converting it into monoxide (Ag2O). This may be accomplished by preloading (back-discharging) the plates or by hot water or hot air treatments. While preloading removes a significant part of the electrode capacity (up to 50%), and for that reason is used only as a last resort, the hot water or hot air treatments have only a relatively minor impact on capacity (o10%), but are not as effective in depressing the initial discharge voltage, although quite sufficient for most applications. Negative (Zinc) Electrode The active material of the negative electrode is zinc for dry charged cells and zinc oxide for dry unformed cells together with additives and binders, as follows: An anti-gassing and depolarizing agent, which was • mercury or mercuric oxide until the mid-1970s.

•

Without such an agent, the active material would gas vigorously upon coming in contact with the electrolyte, and would dissolve into it, thereby reducing the life of the cell to no more than 2 or 3 cycles. Concerns about the toxicity of mercury and its ability to contaminate valuable electronic equipment resulted in it being replaced in some applications by other additives, in particular a combination of lead and cadmium oxides. Although these are no less toxic than mercury, their high vapor pressure makes them safer in case of an internal short circuit or if the cells are accidentally exposed to external heat. Mercury is still used in most applications that do not expressly forbid it. Other, less toxic additives that show promise are bismuth oxide and indium hydroxide. The function of the binders is to increase the mechanical strength of the strips and electrodes; otherwise these components would fall apart. The binder concentration must be kept as low as possible, because these materials tend to be electrical insulators, and therefore increase the internal resistance of the electrodes. Two types of binders may be used: (1) mechanical binders such as nylon or rayon fibers and (2) chemical binders, such as TeflonTM, neoprene, carboxymethyl cellulose, or polyvinyl alcohol (PVA).

Zinc oxide electrodes were originally made by mold pressing, in a way similar to, but more complex than, that described above for positive electrodes. The method is very labor intensive, and results in electrodes that are difficult to handle because of poor mechanical strength

and vary in thickness from spot to spot and from plate to plate. At present, pasting is the preferred method of manufacture of zinc oxide electrodes. For zinc electrodes, the additional step of electroforming is required. Pasting and electroforming are multiple-step wet processes that are complex, and energy and labor intensive, as briefly described below. Pasting consists of the following operations: 1. Preparing a wet paste that includes water, zinc oxide, and all of the additives and binders described above. All these components are added to a blender in a predetermined order and quantity, and thoroughly mixed over a period of several hours. Uniformity of the mix is indispensable for the good quality and performance of the electrodes. 2. Feeding the paste to a pasting machine, which spreads it uniformly between two layers of a special tissue paper by means of adjustable oscillating doctor blades, forming strips B30 cm wide. The amount of material per unit area, excluding the water, is controlled to half of the area density of the plates to be built. 3. Conveying the strips to a drying section (hot air at about 90 1C) at a pre-determined speed so that they are free of water when they emerge from the machine. 4. At this point, one layer of the tissue paper is removed, and samples of the strips are weighed to verify that the area density is correct. 5. Finally, the strips are cut to the proper size and the electrodes are made by pressing two strips, with the current collector (usually a lightly perforated silver sheet) and the leads pre-attached, in between, to the specified thickness. Very thick electrodes may require four strips (two at each side of the collector). Electroforming requires the operations 1 to 4 above, and the following additional steps: 6. Two strips (four for thick electrodes) are put together as described in operation 5 above, but without cutting to electrode size or attaching the leads to the collector. These laminates are wrapped in several layers of cellophane, stacked with alternating nickel or stainless steel counter-electrodes, wrapped in nylon or polypropylene and placed in a charging tank, which may contain up to 30 of these stacks, depending on the thickness of the laminates. Leads are attached to both the zinc oxide laminates and the counter-electrodes and connected to the negative and positive terminals of the tank, respectively. 7. The tank is filled with electrolyte, a concentrated solution of potassium hydroxide, and the zinc oxide is electroformed (reduced or converted to zinc) by passing a predetermined current for a period of 18– 24 h, including a significant excess of charge to insure full conversion to zinc.

Secondary Batteries – Zinc Systems | Zinc–Silver

8. The electroformed zinc laminates are removed from the tanks and the cellophane is peeled off. Next, they are pressed to the required thickness, rinsed in water to eliminate the potassium hydroxide, and dried. Because of the high reactivity of wet zinc, especially in the presence of potassium hydroxide, these operations must be done rapidly to prevent reoxidation. 9. The laminates are cut to electrode size and the leads are welded to the collector after removing the active material from one corner of the plate. The early method for making zinc electrodes was by electrodeposition. This process consists of passing a current through a tank containing large sheets of copper or silver, a concentrated potassium hydroxide solution as the electrolyte, and zinc in the form of 5–7.5-cm diameter zinc balls. Alternatively, the zinc may be dissolved in the electrolyte in the form of potassium zincate. Either way, zinc sponge is deposited on the copper or silver sheet that will serve as the current collector of the finished electrode. The amount of zinc deposited per unit area depends on the magnitude and duration of the current, and the thickness is controlled by means of pressing the deposited zinc between rollers. Mercury is added to the strips by amalgamating in a separate tank, and after washing and drying, the strip is cut into electrodes of the required size. This process is, in theory, simpler than pasting and electroforming. In practice, however, the density of the deposits tends to vary from spot to spot, and especially from the center to the upper and lower edges of the strips. The end result is a large weight variability with potential for exceeding the specification limits. In addition, there is an upper limit to thickness of the electrodes of about 0.125 cm, beyond which the active material delaminates from the collector. For these reasons, electrodepositing is used only for electrodes that are too thin for pasting (below B0.04 cm), and where mercury is allowed, most of them for primary reserve batteries. Note that it may be possible to make electrodeposited negatives with additives other than mercury, but a method for their manufacture has, to our knowledge, not been developed. New procedures are being evaluated that include a metallic zinc electrode process that eliminates the waste associated with the wet processing of strips and electrodes. An advantage of this approach is increased cycle life (i.e., two to three times) due to reduced capacity fade during cycling (refer to the section entitled ‘Recent Improvements’).

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To a lesser extent, fibre-reinforced sausage casing, also a cellulosic material, is used in some applications. Batterygrade cellophane has been the mainstay for the rechargeable Ag–Zn cell since its introduction by Professor Andre´ in the 1940s. In the late 1950s Yardney developed a process for silver treatment of cellophane that improves the cycle life and wet life of the cells built with it. However, cellulosic materials have some disadvantages that include limited resistance to attack by the aqueous alkaline electrolyte and by silver oxides that diffuse from the silver electrode to the separator. These attacks are exacerbated at higher temperatures, thus reducing the calendar (wet) life of cells exposed to those temperatures. Other main separators for rechargeable applications include grafted and cross-linked polyethylene (i.e., radiated or chemically treated) and microporous polypropylene, which must be coated with other materials (e.g., cellulose acetate). In addition to the main separator, Ag–Zn cells have a so-called positive inter-separator, which may consist of a layer of woven or nonwoven nylon, or polypropylene. The function of the inter-separator is to protect the main separator from direct attack by silver oxides and to act as an electrolyte reservoir to keep the electrodes wet at all times. A small percentage of Ag–Zn cells, mostly those designed for long cycle life and/or wet life, have also a negative inter-separator, usually made of nonwoven nylon or polypropylene. Its function is to act as an electrolyte reservoir and to contain the ‘shape change’ of the negative electrodes (refer to the section entitled ‘Negative (Zinc) Electrodes’). The merits of using a negative inter-separator are debatable, especially in the prevention of ‘shape change’ where the improvement appears to be purely cosmetic, as the shedding of the zinc is hidden from view. The negative inter-separator also takes up some of the room that otherwise could be occupied by active materials, reducing the energy density and specific energy of the cells. For short-life applications (i.e., minutes of operations vs days, months, or years) where primary reserve batteries are used, the separator requires unique characteristics, including (1) rapid electrolyte absorption, (2) low resistivity, and (3) minimum thickness. The most common materials used for these applications are polypropylene, PVA, rayon, alpha cellulose, nylon, and asbestos. Asbestos possesses excellent characteristics for both primary and secondary Ag–Zn cells, but has been phased out due to environmental and health-related issues.

Separators The main separator for rechargeable Ag–Zn cells is primarily regenerated cellulose, known as battery-grade cellophane. This material is either plain or silver-treated.

Electrolyte The electrolyte for rechargeable Ag–Zn cells is an aqueous solution of potassium hydroxide, with a

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Secondary Batteries – Zinc Systems | Zinc–Silver

concentration from 40 to 45 wt% (10–11.7 mol L1). Lower concentrations, around 30–35%, would be preferable in terms of higher conductivity, lower zinc oxide solubility, and better wettability, but as long as cellophane or other cellulosic membranes are used as the separators, they are out of the question, because the lower the concentration, the more vigorous the attack on those separators. In fact, for long-life cells (>12 months), 45% potassium hydroxide is exclusively used. Also, lowerconcentration potassium hydroxide has a higher silver solubility, which adversely affects cycle life and wet life. For reserve primary cells, the adverse effects of low concentration are irrelevant, and the benefits are important; therefore, most of them use 31–35% potassium hydroxide. The reason for not exceeding 45% for long-life, lowrate cells, which appears logical from the preceding discussion, is that there is a change of phase of the H2O/ potassium hydroxide solution at 45.5%, and the freezing temperature increases rapidly, from –30 1C for 45% to 0 1C for 48%.

covers, and usually there are two kinds: a plastic washer under the cell cover, and a metal one on top of it. Vent valves have a double function, that is, they prevent the electrolyte from leaking out of the cell, while allowing the gases to escape to the outside at a relatively high pressure, usually about 70 000 Pa. If the relief pressure is too high, the gas pressure may cause deformation and even cracking of the cell cases; if too low (as was the case in the early days of development), the cells lose capacity on charged stand at a relatively high rate (up to 15% per month at room temperature) because the self-discharge reactions that are responsible, namely, the oxidation of the zinc and the reduction of the silver peroxide (eqns [VI] and [VII]), proceed rapidly from left to right. If the gas pressure produced by the reactions is allowed to rise, however, the increased concentration of the products of reaction (hydrogen and oxygen) slows down the process and the cells lose capacity at an acceptable rate, typically B5% per month at room temperature (25 1C) and slower at low temperatures.

Cell Hardware (Cases, Covers, Vent Valves) These items shall be discussed briefly because they offer little or no interest from the electrochemical point of view, but they are very important in practice. Most noncommercial Ag–Zn cells use plastic cases that are prismatic in configuration. The preferred plastics are acrylonitrile butadiene styrene (ABS) and polysulfone. Other materials that have been used in the past (and that are still in use in some cells) are styrene acrylonitrile (SAN), copolymer nylon, polyaryl ether, and modified polyphenylene oxide. All of these offer advantages and present drawbacks depending on the application. For example, polysulfone has excellent mechanical properties and is heat resistant to about 165 1C (330 1F). Polysulfone is difficult to mold, however, and is therefore expensive. It is used only in applications where cells are likely to get very hot and exceed the softening point of ABS or SAN. Cell covers are made of the same materials, sometimes glass-filled for added strength. It is also possible to make cell cases and covers out of metal, such as stainless steel, which would obviously be stronger and more heat resistant than plastic. On the contrary, the problems involved in keeping the electrode materials and the electrolyte out of electrical contact with the cases, thus shorting out the cells, more than negate the possible benefits of using metallic cases and covers. The cell hardware consists of the terminals, top and bottom nuts, washers, and vent valves. Terminals and nuts are usually threaded and made of steel or copper, the former being stronger and the latter more conductive. Washers improve the sealing of the terminals to the

Zn þ H2 O2ZnO þ H2 Z

½VI

2AgO2Ag2 O þ 12O2 Z

½VII

Availability One of the advantages of the Ag–Zn system is that it is available in practically any size and shape. Although most existing cell models are prismatic, it is possible to build them in cylindrical form and as ‘button cells’, which are used in calculators, hearing aids, watches, etc. Referring to availability of rechargeable cells and batteries, we have to differentiate between those models presently available (off-the-shelf items), those that were available in the past and may require new or refurbished cell cases and covers, and those that may be built specially for a new application. The same could be said about the availability of reserve primary batteries; however, these are almost invariably custom-made for a particular application. Among the rechargeable cells a distinction can be made between those designed for high-rate (10-min to 1-h discharge) and low-rate (>2-h discharge) applications. The former are designed for maximum power density and specific power, at the expense of cycle life, which is generally limited to 5–20 cycles; while the latter offer high energy density and specific energy, and cycle lives in the order of 40–100 cycles. A partial list of rechargeable cells that are within the above categories of availability includes: Present models range in capacity from 1 to 2500 Ah • (Yardney Technical Products models only; other

Secondary Batteries – Zinc Systems | Zinc–Silver

•

•

manufacturers may have somewhat different models). Some of the most frequently used are 1, 1.5, 2, 2.5, 3, 4, 5, 10, 12, 15, 16, 18, 20, 21, 30, 35, 40, 58, 60, 70, 80, 85, 90, 100, 112, 130, 140, 170, 190, 200, 215, 300, 400, 660, 700, 750, 850, and 1000 Ah cells. Cells that were available in the past include most of the above models plus smaller cells (0.1, 0.2, and 0.5 Ah) as well as larger models, which were employed in submarines, e.g., the USS Dolphin (330–4000 Ah cells) and the USS Trieste (5000 Ah cells). The largest battery of all was built in the late 1950s and early 1960s for the USS Albacore, with a complement of 560 20 000 Ah cells, that weighed 232 metric tons when filled with electrolyte. (It is reported that the former Soviet Union built a 300-ton silver–zinc battery for one of their submarines.) For new applications it is always possible to design and build cells and batteries of almost any size and configuration, using existing cell cases and covers, or into newly designed ones.

Recently, a newly established company, ZPower Inc. (formerly Zinc Matrix Power), has developed a line of small Ag–Zn batteries for notebook computers, cell phones, and other consumer electronics, which they expect to be commercially available in the very near future. They claim higher power density and specific power and longer cycle life than similar lithium-ion batteries. Recent Improvements Since the days of Andre´, many improvements have been made to the Ag–Zn system in order to enhance its performance and manufacturability. Without those improvements, the cells would have had limited high-rate capability, and their cost would have been prohibitive. In this review, however, improvements achieved only within the past 10 years are considered. Not all of these have yet been implemented. The two components that are most responsible for the drawbacks of the system are the negative electrodes and the separators. The positive electrodes, to a lesser extent, also bear some responsibility but are not discussed here. Negative (Zinc) Electrodes This component bears the major responsibility for the limitation to the cycle life of Ag–Zn cells. The reasons are ‘shape change’ and formation of zinc dendrites. ‘Shape change’ is a phenomenon whereby zinc oxide formed during discharge is partially dissolved in the electrolyte and redeposited during the subsequent recharge in a location different from where it originated. The result is a gradual depletion of active material from the top and sides of the electrodes, and a densification at and around the center, with a corresponding reduction in cell capacity.

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Past methods of eliminating or restricting shape change of the zinc electrodes, which started with Andre´ and span the history of the system, had only limited success. Recently, however, the use of bismuth oxide as an additive and a new manufacturing method have brought a significant improvement and are discussed below. A zinc dendrite is a sharp, needle-like crystalline form of the metal, mostly produced during overcharge. It shows a tendency to perforate the separators, making a direct electronic contact with the adjacent positive electrode, thereby causing the cell to short-circuit, sometimes at a very high rate, a so-called hot short. The formation of zinc dendrites can be avoided by careful charge controls to prevent overcharge, which include individual cell voltage monitoring. These controls, which were cumbersome in the past, have been made simple with the advent of modern computer-controlled charging equipment. The use of bismuth oxide, Bi2O3, as an additive to the zinc electrode was reported by scientists at the Brookhaven National Laboratory. The beneficial effects of the bismuth, that is, longer cycle life due to a reduced rate of shape change were attributed to several phenomena, but mostly to the formation of a needle-like matrix of bismuth in the zinc oxide paste, which helped to keep it in place, rather than to dissolve in the electrolyte. Work on the additive started at Yardney Technical Products in 1991, when a group of 12–A h cells was built with Bi2O3, in combination with lead(II) oxide (PbO) and cadmium oxide (CdO). Further work on the additive was continued with a scale-up of the manufacturing process. This resulted in encouraging results in both small (8.5 Ah) and large (200 Ah) cells, with increases in cycle life ranging from 60% to 120%. In view of the above, Yardney applied for and was granted a patent on the use of this additive in Ag–Zn cells. Another recent improvement was the development of a metallic zinc electrode, directly from zinc powder. The procedure fundamentally involves the blending of fine zinc powders, which may be pre-alloyed with the required depolarizers, adding a bonding agent (e.g., latexbased) and extruding and pressing to the desired thickness. This process, which already has been developed to the pilot plant scale, has substantial advantages over the wet process of pasting and electroforming described in the section entitled ‘Negative (Zinc) Electrode’. Some of these include the following: Improved quality: The blending of dry powders re• sults in a more uniform distribution of the additives(s); this is especially true when the powders are prealloyed. Lack of uniformity can create potential gradients, and consequently, undesirable secondary currents within an electrode, superimposed to the

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Secondary Batteries – Zinc Systems | Zinc–Silver 250 240

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normal charge and discharge currents, leading to excessive gassing and accelerated ‘shape change’. Cost savings: These include reduced water and energy consumption. Safety: It eliminates the use of caustic electrolyte for processing, and the washing operations are no longer required. Performance: It was expected that the improvement in the quality of the electrodes would translate into better results; this proved to be true in that testing of small (12 Ah) and large (190 Ah) cells which showed a major reduction in the rate of ‘shape change’ of the new electrodes that resulted in an increase in cycle life in the order of 100–200%!

Refer to Figure 2 for comparative performance of the larger, 190-Ah cells. New Separators Over the years several new separators have been evaluated with the intention of replacing the cellophane (the drawbacks of cellophane are described in the section entitled ‘Separators’) as the main separator of silver–zinc cells, thus providing improvements in wet and cycle life. One promising material, designated as flexible alkaline separator and manufactured by Advanced Membrane Systems (AMS), consists of a microporous polyolefin film with a titanium dioxide filler. Its physical properties include excellent resistance to potassium hydroxide solutions, even at high temperature (70 1C), excellent dimensional stability, and low electrolytic resistance. Cells (12 Ah) were tested along with controls using

standard separator (silver-treated cellophane) mostly for cycle life, but also for high-rate and low-temperature discharges. While the discharge tests indicated little or no difference with the standard separators, the cycle life tests produced truly remarkable results: the cells with polyolefin separators lasted up to 250 cycles, compared to 75 cycles for those with standard separators. Unfortunately, the results of this program, which included both small and large cells, could not be duplicated; therefore, the use of the separator was temporarily discontinued.

Applications The initial interest created in the 1950s by the newly developed Ag–Zn system in the field of torpedo propulsion and submersibles expanded to a variety of applications. The interest was worldwide, but most of the development work took place in the United States and in the former Soviet Union, where it was shrouded by secrecy. Other countries involved include France, Germany, the UK, and Sweden, and later, Japan. The torpedo applications included primary and secondary batteries for the Mk 32, 37, 43 Mod 1 and 2, 44, and 45 torpedoes and the Mk 30, Mod 1, and Mod 2 torpedo targets, used by the US Navy, some of which are still in use. Even today, silver–zinc batteries are the preferred choice for several non-US Navy torpedoes, such as the SST4 (Peru, Greece), the Mk 24 Tigerfish (Turkey, Brazil), the DM2A3 and DM2A4 (Norway,

Secondary Batteries – Zinc Systems | Zinc–Silver

Turkey, Israel), and the Black Shark secondary battery (Chile, Portugal, Spain, Greece), to name a few. Note: Some of the above batteries may have been used, or considered for use by countries other than those mentioned above. In the ensuing years even larger silver–zinc batteries were integrated into submarines. The first was the Barracuda, which had 165 cells rated at 1600 Ah. As mentioned above, the largest was probably the battery used as the main power source for the USS Albacore (G5) submarine, a two-section battery, each containing 280 cells rated at 20 000 Ah, capable of delivering 100 cycles over a 2-year wet life. Each cell was roughly the size of a standard four-drawer filing cabinet and contained B80 kg of silver or 45 metric tons of silver per battery (i.e., active and structural). For commercial applications, silver–zinc cells and batteries have been and are still used in portable video cameras (electronic news gathering) and high-intensity emergency flashlights, and have recently been suggested as an option for the mobile electronics market for cell phones and notebook computers. Some battery applications require operation up to 6000-m depths, outside of a pressure-resistant enclosure. For this technology, the batteries are built into a leakproof container, the void spaces of which, including the upper portion of the cells, are filled with nonreactive, dielectric compensating oil. Pressure is transmitted through a flexible member connected to the submersible compensating system. The increase in pressure as the vehicle descends causes volume changes within the battery that are due to a decrease in volume of the gas held in and around the porous electrodes, but also to the compressibility of the potassium hydroxide electrolyte (B2%), and that of the mineral oil (5–7%) between 0 and 105 kPa. As hydrostatic pressure increases in the system, additional oil enters the battery box from the compensator and flows into the cells. Pressure is equalized outside and within the battery box by means of a diaphragm, bellows, or bag and can be mounted internally or externally to the box. Batteries built as described above are called pressure-compensated batteries. One of the programs still utilizing pressure-compensated silver– zinc batteries is the Deep Submergence Rescue Vehicle. In the field of electric vehicles, silver–zinc batteries have been used in a variety of applications. In the 1950s, the first was the Dyna-Panhard, which had a range of almost 200 km per charge. Other electric vehicles included the Renault Dauphine and a motorcycle that until recently held the world speed record for an electricpowered motorcycle. Unfortunately, high cost and relatively short cycle-life limited the use of Ag–Zn cells to experimental and demonstration models. Ag–Zn primary reserve batteries are nonrechargeable systems, that is, one-shot devices, where the cells are in

Table 1

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Typical characteristics of primary batteries

Non-rechargeable, one-shot device Rapid activation (typically o500 ms) Short operating life (typically from a few minutes to a few hours) Long storage life (5–25 years) Sealed constructiona Electrolyte contained within the battery, separate from the cellsb Capable of discharge at complex load profiles, with narrow voltage limits Operate in any orientation Require electrical signal for activation High specific power and power densitiesc High reliability (>99.9%) Custom-designed for specific applications Low-temperature operation (up to –40 1C)d a Most batteries are equipped with a high-pressure emergency relief valve. b Typically the electrolyte is contained within a tank or a coil. c However, these figures are less than ideal, because of the added weight and volume of the activation system, battery case, and miscellaneous hardware. d Batteries required to operate at low temperatures are usually equipped with heater blankets, to heat the electrolyte to a preestablished temperature.

the dry charged condition and are activated by injecting electrolyte just prior to power demand. They are used for missiles, rocket igniters, flares, launch or atmosphere reentry vehicles, and torpedo applications. These batteries are pneumatic–hydraulic-electrochemical devices (the electrical activation signal, often originated at a remote location, triggers a controlled explosion, generating gasses that compress the electrolyte, forcing it to rupture a metallic disk, and to flow into a manifold that carries it to each of the battery cells). They supply electric power in less than a second after activation, for short periods of time, typically a few minutes to a few hours. They can be discharged at very high currents with excellent voltage regulation and their power-to-weight and -volume ratios are high, particularly at high discharge rates. In addition, they have demonstrated reliable performance over extended dry storage life (i.e., more than 25 years). The typical characteristics of primary reserve silver–zinc batteries are shown in Table 1. Some of the current programs utilizing primary silver–zinc batteries include missiles such as the Hawk, Harm, Sparrow, Trident II, Harpoon, and Standard Missile, and the Mk 12 reentry vehicle. Silver–zinc batteries are still utilized in a number of critical space programs. Provided below is a partial list of applications for past and current programs. The Portable Life Support System (PLSS), which was • originally developed for the Apollo program supporting lunar exploration and evolved into the Extravehicular Mobility Unit (EMU) used by the astronauts in their extravehicular activities, such as the construction of the Space Station and the refurbishment of the Hubble

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• • • •

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Space Telescope. A larger version of the EMU battery is presently used in the Space Station. Upper stages, such as the Boeing Inertial Upper Stage (IUS), the Centaur, the Payload Assist Module (PAM), and the Spinning Stage of the IRIS system. Unmanned launch vehicles such as the Delta II and IV and the Atlas V. The Mars Pathfinder Lander. The Russian Sputnik spacecraft.

Other current applications of Ag–Zn include aircraft starting, high-intensity portable lighting, and, more recently, mobile electronics.

room temperature). Long-term high-temperature operation of Ag–Zn cells is undesirable for the following reasons: Decreased cycle life and wet life due to accelerated • attack on the cellulosic separators. Accelerated shape change of the negative electrodes, • leading to shorter cycle life. Changes in the crystalline structure of the active sil• ver, increasing the rate of capacity degradation. Ideal storage conditions, both in the dry and activated condition, are low temperature ( þ 10 to –10 1C) and relative humidity below 60%.

Cost The cost of Ag–Zn cells and batteries varies greatly, depending on size, quantities, and whether the model purchased is an off-the-shelf item or is specially designed for a given application. Large cells, of an established design, purchased in large quantities (>200 units) are the least expensive, down to about US$1.60 per Wh. Small cells, designed for a new application, in small quantities (o50 units), are quite costly at US$32 per Wh or higher. Included in that price are not only the cost of labor and materials, but also the engineering effort of developing, designing, and testing the cells. In addition, when a battery is purchased, it will be necessary to add the cost of the battery hardware (case, cover, connectors, valves, etc.) to the above costs.

Operating and Storage Conditions Ag–Zn cells and batteries operate best at the temperature ranges of 10–30 1C. Below 10 1C the initial cell voltage may be too low for a given application, and the capacity is also negatively affected. However, as the cells generate heat during discharge, the temperature will increase, and so does the voltage. Furthermore, if the end-of-discharge temperature is at or above room temperature (B25 1C), the impact on capacity will be minimal. In general, the lower the temperature, the impact on initial voltage and on capacity is more pronounced, until the specification requirements can no longer be met. If such is the case, the batteries are provided with a heater blanket and a thermostat, which will bring the temperature up to the desired level (usually 15–20 1C), just prior to the beginning of discharge. Heater blankets may be powered by the battery cells, or by an external power source. The reasons for not operating at initial temperatures above 30 1C are quite different, as voltage and capacity are not affected (in fact they are slightly better than at

Nomenclature Abbreviations and Acronyms ABS AMS EMU PAM PLSS PVA SAN

acrylonitrile butadiene styrene Advanced Membrane Systems Extravehicular Mobility Unit Payload Assist Module Portable Life Support System polyvinyl alcohol styrene acrylonitrile

See also: Chemistry, Electrochemistry, and Electrochemical Applications: Silver; Zinc; Secondary Batteries – Zinc Systems: Zinc Electrodes: Overview.

Further Reading Carey J, Harma DA, and Karpinski AP (1992) High energy density batteries for undersea applications. Proceedings of Mastering the Oceans through Technology. Newport, RI, USA. Falk SU and Salkind AJ (1969) Alkaline Storage Batteries. New York: John Wiley & Sons. Fleischer A and Lander JJ (1971) Zinc–Silver Oxide Batteries. New York: John Wiley & Sons. Himy A (1986) Silver–Zinc Battery: Phenomena and Design Principles. New York: Vantage Press. Himy A (1995) Silver–Zinc Battery: Best Practices, Facts and Reflections. New York: Vantage Press. Himy A (2003) Silver–Zinc Battery: Phenomena, Design Principles, Manufacturing and Various Topics. College Park, MD: University Press. Karpinski AP (1993) Bipolar silver–zinc technology. Proceedings of the European Space Power Conference. Graz, Austria. Karpinski AP, Makovetski B, Russell SJ, Serenyi JR, and Williams DC (1999) Silver–zinc: Status of technology and applications. Journal of Power Sources 80: 53--60. Karpinski AP and Patten JA (1990) Performance characteristics of silver–zinc cells for orbiting spacecraft. Proceeding of the 25th International Energy Conversion Engineering Conference, vol. 3, pp.105–110, August 12–17. Karpinski AP, Russell SJ, Serenyi JR, and Murphy JP (2000) Silver based batteries for high power applications. Journal of Power Sources 91: 77--82.

Secondary Batteries – Zinc Systems | Zinc–Silver

Linden D and Reddy TB (2002) Handbook of Batteries, 3rd edn. New York: McGraw-Hill. McBreen J and Gannon E (1985) Bismuth oxide as an additive in pasted Zinc electrodes. Journal of Power Sources 15: 169--177. Serenyi JR (1982) Silver electrode technology – an overview, 162nd Meeting of the Electrochemical Society. Detroit, MI.

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Serenyi R (1998) Rechargeable alkaline silver–zinc cell with improved negative electrode. Yardney Technical Products, U.S. patent No. 5,773,176. Serenyi JR, Kuklinski J, Williams DC, and Thompson AF (1995) Development of silver–zinc cells of improved cycle life and energy density. Proceedings of the Symposium on Rechargeable Zinc Batteries, The Electrochemical Society.