Batteries, Primary Cells

Abstract

Abstract Primary cells are galvanic cells designed to be discharged only once, and attempts to recharge them can present possible safety hazards. The cells are designed to have the maximum possible energy in each cell size because of the single discharge. The main categories of primary cells are carbon–zinc, known as heavy‐duty and general purpose; alkaline, cylindrical, and miniature; lithium; and reserve or specialty cells. Carbon–zinc batteries are the most commonly found primary cells worldwide and are produced in almost every country. Traditionally there are a carbon rod, for cylindrical cells, or a carbon‐coated plate, for flat cells, to collect the current at the cathode and a zinc anode. There are two basic versions of carbon–zinc cells: the Leclanché cell and the zinc chloride, ZnCl 2 , or heavy‐duty cell. Both have zinc anodes, manganese dioxide, MnO 2 , cathodes, and include zinc chloride in the electrolyte. The Leclanché cell also has an electrolyte saturated with ammonium chloride, NH 4 Cl. Both types are dry cells, in the sense that there is no excess liquid electrolyte in the system. The zinc chloride cell is often made using synthetic manganese dioxide and gives higher capacity than the Leclanché cell, which uses inexpensive natural manganese dioxide for the active cathode material. The MnO 2 is only a modest conductor. Thus the cathodes in both types of cell contain 10–30% carbon black in order to distribute the current. This battery system can be found in many sizes and shapes. Carbon–zinc cells perform best under conditions of intermittent use. The most frequently used tests are American National Standards Institute (ANSI) tests. The tests are carried out at constant resistance and the results reported in minutes or hours of service. To compare one battery with another, it is useful to compute the energy density from these data. The average voltage during the discharge is used to compute an average current, which is then multiplied by the service in hours to give the ampere‐hours of capacity. Primary alkaline cells use sodium hydroxide or potassium hydroxide as the electrolyte. The alkaline cells of the 1990s are mostly of the limited electrolyte, dry cell type. Most primary alkaline cells are made using zinc as the anode material; a variety of cathode materials can be used. Primary alkaline cells are commonly divided into two classes, based on type of construction: the larger, cylindrically shaped batteries, and the miniature, button‐type cells. Cylindrical alkaline cells are zinc–manganese dioxide cells having an alkaline electrolyte, which are constructed in the standard cylindrical sizes, R20 “D”, R14 “C”, R6 “AA”, RO3 “AAA”, as well as a few other less common sizes. They can be used in the same types of devices as ordinary Leclanché and zinc chloride cells. Moreover, the high level of performance makes them ideally suited for applications such as toys, audio devices, and cameras. Alkaline manganese dioxide batteries have relatively high energy density. Moreover, the cells are able to function well with a rather small amount of electrolyte. The result is a cell having relatively high capacity at a fairly reasonable cost. Miniature alkaline cells are small, button‐shaped cells which use alkaline NaOH or KOH electrolyte and generally have zinc anodes, but may have a variety of cathode materials. They are used in watches, calculators, cameras, hearing aids, and other miniature devices. Miniature alkaline cells are made in a large number of different sizes, using many different chemical systems. Whereas the cylindrical alkaline batteries are multipurpose batteries, used for a wide variety of devices under a variety of discharge conditions, miniature alkaline batteries are highly specialized, with the cathode material, separator type, and electrolyte all chosen to match the particular application. Miniature zinc–mercuric oxide batteries have a zinc anode and a cathode containing mercuric oxide, HgO. Although the zinc–mercuric oxide battery has many excellent qualities, increasing environmental concerns in the disposal of the cell have led to a de‐emphasis in the use of this system. Miniature zinc–silver oxide batteries have a zinc anode and a cathode containing silver oxide, Ag 2 O, and are commonly used in electronic watches and in other applications where high energy density, a flat discharge profile, and a higher operating voltage than that of a mercury cell are needed. It is possible to produce a silver oxide in which the silver has a higher oxidation state, approaching a composition of AgO. This material can provide both higher capacity and higher energy density than Ag 2 O. The combination of a zinc anode and manganese dioxide cathode, which is the dominant chemistry in large cylindrical alkaline cells, is used in some miniature alkaline cells as well. Zinc–air batteries offer the possibility of obtaining extremely high energy densities. Instead of having a cathode material placed in the battery when manufactured, oxygen from the atmosphere is used as cathode material, allowing for a much more efficient design. On the inside, the anode occupies much more of the internal volume of the cell. Rather than the thick cathode pellet, there is a thin layer containing the cathode catalyst and air distribution passages. Air enters the cell through the holes in the can and the oxygen reacts at the surface of the cathode catalyst. The performance level of air cells is exceptional, but these are not general‐purpose cells. They must be used in applications where the usage is largely continuous, and where the discharge level is relatively constant and well‐defined. Miniature air cells are mainly used in hearing aids, where they are required to produce a relatively high current for a relatively short time period such as a few weeks. In this application they provide exceptional performance compared to other batteries. Cells having lithium anodes are generally called lithium cells regardless of the cathode. They are of two types: cells having solid cathodes and cells having liquid cathodes. Cells having liquid cathodes also have liquid electrolytes. Cells having solid cathodes may have liquid or solid electrolytes but, except for the lithium–iodine system, those having solid electrolytes are not yet commercial. All of the cells take advantage of the inherently high energy of lithium metal and its unusual film‐forming property. Much analytical study has been required to establish the materials for use as solvents and solutes in lithium batteries. Among the best organic solvents are cyclic esters, such as propylene carbonate (PC), C 4 H 6 O 3 , ethylene carbonate (EC), C 3 H 4 O 3 , and butyrolactone, C 4 H 6 O 2 , and ethers, such as dimethoxyethane (DME), C 4 H 10 O 2 . Among the most useful electrolyte salts are lithium perchlorate, LiClO 4 , lithium trifluoromethanesulfonate, LiCF 3 SO 3 , lithium tetrafluoroborate, LiBF 4 , and lithium hexafluoroarsenate, LiAsF 6 . A limitation of these organic electrolytes is the relatively low conductivity, compared to aqueous electrolytes. This limitation has forced the use of designs such as thin electrodes and very thin separators in all lithium batteries. This usage led to the development of coin cells rather than button cells for miniature batteries and jelly or Swiss roll designs rather than bobbin designs for cylindrical cells. Solid cathode cells include lithium–manganese dioxide cells, lithium–carbon monofluoride cells, lithium–iron disulfide cells, and lithium–iodine cells. Liquid cathode cells include lithium–sulfur dioxide cells and lithium–thionyl chloride cells. Reserve batteries have been developed for applications that require a long inactive shelf period followed by intense discharge during which high energy and power, and sometimes operation at low ambient temperature, are required. These batteries are usually classified by the mechanism of activation which is employed. There are water‐activated batteries that utilize fresh or seawater; electrolyte‐activated batteries, some using the complete electrolyte, some only the solvent; gas‐activated batteries where the gas is used as either an active cathode material or part of the electrolyte; and heat‐activated or thermal batteries which use a solid salt electrolyte activated by melting on application of heat. Activation of these batteries involves adding the missing component which can be done in a simple way, such as pouring water into an opening in the cell, for water‐activated cells, or in a more complicated way by using pistons, valves, or heat pellets activated by gravitational or electric signals for the case of the electrolyte‐ or thermal‐activation types. Such batteries may be stored for 10–20 yr while awaiting use. Reserve batteries are usually manufactured under contract for various government agencies such as the U.S. Department of Defense, although occasional industrial or safety uses have been found. Many millions of these batteries have been manufactured for military ordnance and employed in rockets, bombs, missiles, etc.