Alkaline electrolyte storage battery systems are suitable in applications where high currents are required, because of the high conductivity of the electrolyte. Additionally, the electrolyte, usually an aqueous solution containing 25â40% potassium hydroxide, KOH, does not enter into the chemical reaction. Thus concentration and cell resistance are invariant with state of discharge, and these battery systems give high performance and have long cycle life. Positive electrode active materials have been made from the oxides or hydroxides of nickel, silver, manganese, copper, mercury, and from oxygen. Negative electrode active materials have been fabricated from various geometric forms of cadmium, Cd, iron, Fe, and zinc, Zn, and from hydrogen. Two different types of hydrogen electrode designs are common: those used in space, which employ hydrogen as a gas, and those used in consumer batteries, where the hydrogen is used as a metallic hydride. Battery system designers are switching to nickelâmetal hydride (MH) cells for some applications, typically in âAAââsize cells. Many of the most recent applications for alkaline storage batteries require higher energy density and lower cost designs than were previously available. Materials such as foam or fiber nickel, Ni, mats as substrates, and new processing techniques including plastic bounded, pasted, or electroplated electrodes, have enabled the alkaline storage battery to meet these new requirements, while reducing environmental problems in the manufacturing plants. The most recent innovations in materials relate to the development of metalâhydride alloys for the storage and electrochemical utilization of hydrogen. Improvements in materials science and electrical circuits have led to better separators, seals, welding techniques, feedthroughs, and charging equipment. A number of different types of nickel oxide electrodes have been used in nickelâcadmium cells. The many varieties of practical nickel electrodes can be divided into two main categories. In the first, the active nickelous hydroxide is blended, admixed, or layered with an electronically conductive material. This active material mixture is afterwards contained in a confining porous metallic structure or pasted onto a metallic mat or grid. The other type of nickel electrode involves constructions in which the active material is deposited
in situ
. This includes the sinteredâtype electrode in which nickel hydroxide is chemically or electrochemically deposited in the pores of a 80â90% porous sintered nickel substrate that may also contain a reinforcing grid. Almost all the methods described for the nickel electrode have been used to fabricate cadmium electrodes. The solidâstate chemistry of the nickel electrode is complex. There are various hydrated and nonhydrated nickel hydroxides that have slightly different crystal habitats and electrochemical potentials. The most common form of charged material observed in batteries is NiOOH,
\documentclass{article}\pagestyle{empty}\begin{document}${{\rm{density}}=4.6{\hskip0.167em}{\hskip0.167em}{\rm{g}}/{\rm{mL}}}$\end{document}
. In comparison, Ni(OH)
2
has a density of 4.15 g/mL. The chemistry, electrochemistry, and crystal structure of the cadmium electrode is much simpler than that of the nickel electrode. Most sealed cells are based on the principles appearing in patents of the early 1950s. The preponderant majority of cells in commercial production use sintered positive (nickel) electrodes, and either sintered or pasted negative (cadmium) electrodes. Pocket cells and tubular cells are also used. The fabrication of sintered electrode batteries can be divided into five principal operations: preparation of sinteringâgrade nickel powder; preparation of the sintered nickel plaque; impregnation of the plaque with active material; assembly of the impregnated plaques (often called plates) into electrode groups and into cells; and assembly of cells into batteries. Other methods to fabricate nickelâcadmium cell electrodes include those for the button cell, used for calculators and other electronic devices. Nickelâcadmium cells represent almost 20% of the market for all storage batteries, including leadâacid, manufactured in the world. Uses are divided into three categories: pocket cells are used in emergency lighting, diesel starting, and stationary and traction applications where the reliability, long life, and low temperature performance characteristics warrant the extra cost over leadâacid storage batteries; sintered, vented cells are used in extremely high rate applications, such as jet engine and large diesel engine starting; and sealed cells, both the sintered and button types, are used in computers, phones, cameras, portable tools, electronic devices, calculators, and in space applications, where nickelâcadmium is optimum because it can be recharged a great number of cycles. There has been renewed interest in the system for electric vehicles (EV). The EV design is based on a high rate, usually sintered, iron electrode as well as high rate nickel electrodes. The sintered nickelâsintered iron design battery has outstanding power characteristics at all states of discharge, making them attractive to the design of electric vehicles (EV) which must accelerate with traffic even when almost completely discharged. The silverâzinc battery had the highest attainable energy density of any rechargeable system in use in the early 1990s, and its use was limited almost exclusively to the military for various aerospace applications such as satellites and missiles, submarine and torpedo propulsion applications, and some limited portable communications applications. There are three methods of silver electrode fabrication: the slurry pasting of monovalent or divalent silver oxide to the grid and sintering to agglomerate the fine particles into an integral structure; the dry processing of fine silver powders onto a silver grid followed by sintering; and the use of plasticâbonded active material formed by embedding the active material (fine silver powder) in a plastic vehicle which can then be milled into flexible sheets. These sheets are cut to size, pressed in a mold on both sides of a conductive grid, and the pressed electrode subjected to sintering, leaving the metallic silver. Zinc electrodes for secondary silverâzinc batteries are made by one of three general methods: the dryâpowder process, the slurryâpasted process, or the electroformed process. The active material used in any of the processes for the manufacture of electrodes is a finely divided zinc oxide powder, USP grade 12. The electrolyte in silverâzinc cells is 30â45% KOH. The lower concentrations in this range have higher conductivities and are preferred for high rate cells. Silverâzinc cells have one of the flattest voltage curves of any practical battery system known. Silverâzinc cells are usually manufactured as either low or high rate cells. Approximately 10â30 cycles can be expected for high rate cells, depending on the temperature of use, the rate of discharge, and methods of charging. Low rate cells have been satisfactorily used for 100â300 cycles under the proper conditions. In general, the overall life of the silverâzinc cells with the separator systems normally in use is approximately 1â2 yr. Other silver positive electrode systems include
silverâcadmium cells
, used in satellite applications where the nonmagnetic property of the silverâcadmium battery is of utmost importance, and
silverâiron cells
, which combine the advantages of the high rate capability of the silver electrode and the cycling characteristics of the iron electrode; commercial development has been undertaken to solve problems associated with deep cycling of high power batteries for ocean systems operations. Nickelâzinc cells offer potential advantages over other rechargeable alkaline systems. The singleâlevel discharge voltage, 1.60â1.65 V/cell is approximately 0.35â0.45 V/cell higher than nickelâcadmium or nickelâiron and approximately equal to that of silverâzinc. In addition, the use of zinc as the negative electrode should result in a higher energy density battery than either nickelâcadmium or nickelâiron and a lower cost than silverâzinc. A commercial nickelâzinc battery is considered to be the most likely candidate for electric vehicle development. Work is developmental; there is no commercial production of nickelâzinc batteries. The limited life of nickelâzinc batteries is the principal drawback to widespread use. With the proposed development of the nickelâhydrogen system for electric vehicles, limited attention was directed to the development of a silverâhydrogen cell. Other cell systems include zincâoxygen cells, ironâair cells, hydrogenâoxygen cells, and mechanically rechargeable batteries. Potassium hydroxide is the principal electrolyte of choice for the above batteries because of its compatibility with the various electrodes, good conductivity, and low freezing point temperature. The potassium hydroxide electrolyte used in alkaline batteries is a corrosive, hazardous chemical. It is a poison and if ingested attacks the throat and stomach linings. Immediate medical attention is required. It slowly attacks skin if not rapidly washed away. Extreme care should be taken to avoid eye contact that can result in severe burns and blindness. Protective clothing and face shields or goggles should be worn when filling cells with water or electrolyte and performing other maintenance on vented batteries. Alkaline batteries generate hydrogen and oxygen gases under various operating conditions. In vented batteries free ventilation should be provided to avoid hydrogen accumulations surrounding the battery. Alkaline batteries are capable of high current discharges and accidental short circuits should be avoided. Because of increasing environmental concerns, the disposal of all batteries is being reviewed. Traditionally silver batteries were reclaimed for the silver metal and all other alkaline batteries were disposed of in landfills or incinerators. Some aircraft and industrial nickelâcadmium batteries are rebuilt to utilize the valuable components. To reduce or eliminate the scattering of cadmium in the environment, the disposal of nickelâcadmium batteries is under study. Already a large share of industrial batteries are being reclaimed for the value of their materials.
