v1.1.0 / chapter 12 of 15 / 01 sep 10 / greg goebel / public domain
* One of the most widespread household chemical technologies is the electric storage cell or "battery", with a wide range of old and new technologies in common use. This chapter discusses popular battery technologies.
* As discussed earlier, Alessandro Volta had developed the first electrical storage cell, the "Voltaic pile", in 1800, based on a stack of zinc and silver disks separated by cloths soaked in a weak acid. Following Michael Faraday's pioneering research on electrochemistry, in 1836, the English chemist John Daniell (1790:1845) developed the first modern storage cell using Faraday's principles.
Storage cell operation is based on "reduction-oxidation (redox)" reactions. For example, Volta's scheme can be modeled by placing a bar of zinc at one side of a beaker containing a solution of weak sulfuric acid; placing a bar of silver at the other side of the beaker; and then wiring the two "electrodes" through a light bulb outside the beaker. The light bulb then starts glowing.
In solution, sulfuric acid, H2SO4, breaks down into two H+ ions and a single SO4-- ion, forming an electrolyte that can carry electric current. The SO4-- ion easily "oxidizes" zinc to form zinc sulfate (ZnSO4), which is released into the solution, eating away the negative zinc anode. As each zinc sulfate molecule leaves the electrode, it leaves behind two electrons that flow through the external wire as a current to the positive silver cathode. At the silver cathode, the electrons combine with or "reduce" the hydrogen ions in the solution to form diatomic hydrogen gas. The silver is inert and not consumed in the reaction.
All modern storage cells use similar redox schemes, though the specific implementations vary widely. Some classes of storage cells can be "recharged" by running an electric current through them backwards, which reverses the chemical reactions and more or less restores things to their original condition.
* The popular terminology for storage cells is somewhat confusing. Storage cells are almost always referred to as "batteries" in common usage, but this is not technically correct. The storage cell described above is just that, a "cell", not a "battery". It consists of one cathode and one anode in an electrolyte. A storage cell with specific electrode materials and electrolyte has a certain output voltage, and to get higher voltages with that specific technology, they must be electrically connected together in series as a "battery".
Flashlight cells are just that, cells, but the lead-acid battery used in an automobile consists of several cells packaged and chained together, so it is indeed a battery. A single cell of a lead-acid battery has a voltage of 2 volts, and so a 12-volt lead-acid battery has six cells in series. In practice, people, even manufacturers, call cells "batteries" whether they consist of one cell or many -- but the term "cell" has a way of sneaking itself back in even in popular usage. The default here is to use "cell", meaning "electrical storage cell", except when specifically referring to true "batteries" -- it may sound stuffy, but the alternative is hopeless confusion.
* There are two classes of cells: nonrechargeable or "primary" cells, for example typical cheap throwaway flashlight cells, and rechargeable or "secondary" cells, for example as used in an automotive lead-acid battery. Cells can also be classified as "wet cells", which have liquid electrolytes; "dry cells", which have electrolytes in the form of a paste; and "solid electrolyte" cells, which as their name indicates use a completely solid electrolyte. In addition, there are standardized form factors for certain classes of cells, such as AAA and AA penlight cells; C and D flashlight cells; and the standard nine-volt brick-shaped "transistor radio" battery, a package containing six individual cells. Output voltages are also more or less standardized for these products. However, cells are otherwise not highly standardized items, as shopping for a watch button-style cell quickly proves.
Many cells can maintain their output voltage at a reasonably constant level over a fairly wide range of output currents. In electrical engineering terms, they are said to have a low "internal resistance". Those that have cannot supply high currents without losing voltage have a high internal resistance. Cell specs often include the maximum useful current output, and curves that show the drop in output voltage with increasing current drain. By the way, the low internal resistance, or equivalently current capacity, of big automotive batteries makes them potentially dangerous. While their output voltages are so low that getting a shock off them is not a problem, if the output of a large automotive battery is shorted to the chassis ground the large currents flowing through the short can cause an almost explosive flash and severe burns. It is not usually a good idea to wear a watch with a metal band or a ring while servicing a vehicle, since the vehicle's chassis is ground and a short from a "hot" wire could easily lead to a nasty accident.
The total energy capacity of a cell is measured in the number of hours it can supply a given level of current, or "ampere-hours". This is a straightforward figure of merit for cells based on the same technology, since they will all have the same voltage. However, ampere-hours can be misleading for comparing different cell technologies, since the voltages may differ and the power output of a cell with a lower voltage is lower for the same level of output current. For this reason, the unit of "watt-hours" is used to compare energy storage capacity between different cell technologies.
A related rating is the "specific energy" of a cell, which gives the its storage capacity relative to its mass. For example, a cell could be said to have a given number of watt-hours per kilogram. There is also "energy density", which gives its storage capacity relative to its volume, for example in watt-hours per liter. Specific energy and energy density are used in comparisons between different classes of cells, particularly for automotive propulsion applications. Electric-powered automobiles have long suffered from the limited energy capacity of batteries cells compared to gasoline and other chemical fuels, and so obtaining storage cells with greater specific energy has been one of the most important goals of electric-automobile designers.
* As mentioned, rechargeable cells can be run backwards and more or less restored to their original, charged state. The "more or less" is important. The restored state is not a perfect replica of the original state, and so rechargeable cells degrade slightly every time until their storage capability fades out. For this reason, rechargeable cells are also described by the number of "charging cycles" they will tolerate. The number of cycles tends to fall with greater average depth of discharge; manufacturers may provide curves showing how the cell's capacity slowly falls as the number of cycles increases. Other important specs include shelf life, temperature limits, and physical dimensions.
* The carbon-zinc cell was invented in the 1860s by a French chemist named Georges Leclanche (1839:1882), and is sometimes called the "Leclanche cell". It is also sometimes called the "dry cell", but this term is somewhat misleading since it can be just as legitimately applied to similar storage cell technologies. The carbon-zinc cell consists of zinc cylindrical "cup" that makes up the anode -- which is separated from the external casing of the battery by an insulating spacer -- and a central carbon rod. The electrolyte is powdered ammonium chloride (NH4Cl), an acid, in water, mixed with powdered manganese dioxide (MnO2) and graphite to form a paste. Zinc chloride (ZnCl2) can also be used in place of ammonium chloride, providing longer service life at higher cost, and in fact carbon-zinc cells often use a small amount of zinc chloride along with the ammonium chloride. However, for simplicity this discussion assumes the use of ammonium chloride.
The anode reaction of the carbon-zinc cell involves the double oxidation of a
zinc atom, releasing two electrons into the external circuit:
Zn --> Zn++ + 2e-
The cathode reaction is supported by the manganese dioxide. The graphite
powder is mixed with the manganese dioxide powder to improve the conductivity
of the paste. While some sources call the central carbon rod the cathode, it
is more properly the "cathode collector", since it is inert and simply
provides a conductive path to the positive contact. In fact, this type of
cell should more properly be known as a "manganese-zinc acidic" cell, but
that's not the usage that was adopted. The cathode reaction involves
reduction of MnO2 to Mn2O3 on the inert carbon cathode:
2NH4+ + 2MnO2 + 2e- --> Mn2O3 + 2OH-
Carbon-zinc batteries will go dead prematurely if discharged too quickly, due
to the buildup of reaction products around the carbon cathode, but they will
"rejuvenate" if allowed to rest for a while, allowing the reaction products
to disperse.
* The alkaline cell operates on similar principles, with a zinc anode and
manganese dioxide mixed with graphite for the cathode. However, the
electrolyte is potassium hydroxide (KOH), which is alkaline rather than
acidic, again mixed with manganese dioxide and graphite. It should be
properly known as the "manganese zinc alkaline" cell, but once more that's
not the usage that was adopted. The anode reaction is:
Zn + 2OH- --> ZnO + H2O + 2e-
The cathode reaction is;
2MnO2 + H2O + 2e- --> Mn2O3 + 2OH-
Sodium hydroxide (NaOH) can also be used as the electrolyte. This is the
case for almost all cells that use potassium hydroxide as an electrolyte.
Despite the similarity in operation, the alkaline cell's structure is very different from that of the carbon-zinc cell. The alkaline cell is enclosed in a nickel-plated steel can that forms the positive cathode contact, which is separated from the bottom cap, which is the negative anode contact, by a cardboard spacer. The can contains the potassium hydroxide / manganese dioxide / graphite paste for the cathode reaction, separated from a core of powdered zinc by a fabric separator. A tin-plated brass "nail" connected to the bottom cap is inserted up into the powdered zinc to conduct current to the cap. A plastic plug seals the bottom of the can and supports the fabric separator.
* Of course, all cells are covered with a plastic sheath to provide protection, insulation, and labeling. Alkaline cells have about twice the power density of carbon-zinc cells, but are several times more costly. The carbon-zinc cell's virtue, probably its only virtue, is that it is dirt cheap. Both carbon-zinc and alkaline cells have a cell voltage of about 1.5 volts, and are both regarded as environmentally benign, at least by the standards of storage cells. Carbon-zinc and alkaline storage cells are not in general rechargeable, though rechargeable alkaline cells have been produced.
* An Israeli company came up with an interesting variation on this technology in the form of a storage cell that can be literally printed onto cardboard boxes or similar substrates using silkscreen technology. The cell consists of a five layers of silkscreened materials:
The whole assembly is sealed under a protective layer of plastic. The cell provides 1.5 volts, but multiple layers could be used to construct a battery with higher voltage if necessary. Storage capacity is 2.5 milliampere-hours per square centimeter. It has a shelf life of two years. The technology is intended for promotional gimmicks, singing greeting cards, toys or novelties, sensor systems built into packages of perishable foods, and so on.
* Calculators, hearing aids, and other small electronics devices use tiny
nonrechargeable "button" cells. The original technology for button cells was
the mercury cell, which had a mercuric oxide (HgO) cathode, an anode made of
an amalgam of mercury and zinc, and an electrolyte consisting of potassium
hydroxide mixed with zinc hydroxide (or Zn(OH)2). The anode reaction is:
Zn + 2OH- --> ZnO + H2O + 2e-
The cathode reaction is:
HgO + H2O + 2e- --> Hg + 2OH-
Mercury cells had a highly constant cell voltage of 1.35 volts. A similar
cell could be made with cadmium instead of zinc, providing a cell voltage of
0.91 volts. Since mercury is toxic, mercury cells are now banned in the US
and some other countries; they are now only a historical curiosity.
* Modern zinc-air button cells are similar to alkaline cells. The anode is powdered zinc mixed in a gel, the electrolyte is a layer of potassium hydroxide, and the cathode is a carbon disk, designed to support cathode reactions through the oxygen in the air. A porous teflon membrane allows air into the cell while preventing electrolyte from leaking out.
The anode reaction is:
Zn + 2OH- --> Zn(OH)2 + 2e-
The cathode reaction is:
O2 + 2H2O + 4e- --> 4OH-
Zinc-air batteries have a cell voltage of about 1.65 volts. They have a very
high energy density, but also have a high internal resistance and are not
well suited to high-current applications. They have to be sealed while on
the shelf to keep the air out, but as long as they are kept sealed they have
a long shelf life. Large zinc-air cells have been used in consumer
equipment, at least on a limited basis, and very large zinc-air batteries
have experimentally used in vehicular applications.
* The silver oxide cell is similar in construction to the zinc-air type, with an anode of powdered zinc in gel with a potassium hydroxide electrolyte, except that instead of having a cathode made of carbon and exposed to the air, it is a silver screen pasted with silver oxide (Ag2O). They have a cell voltage of 1.55 volts, a flat discharge curve, and long shelf life. They can be recharged a limited number of times, but they are not generally recharged in practice.
* The modern lead-acid battery is by far the most familiar rechargeable
storage cell technology. The lead-acid cell was invented in 1859 by a French
physicist, Gaston Plante (1834:1889). It uses dilute sulfuric acid for an
electrolyte, lead for the anode, and lead oxide for the cathode. The
sulfuric acid dissociates into two hydrogen ions (protons) and a sulfate
group. The sulfate group reacts with the lead anode to form lead sulfate and
releases two electrons through the external circuit. This is the oxidation
reaction, which can be summarized as:
Pb + HSO4- --> PbSO4 + H+ + 2e-
At the cathode, the two electrons cause a reaction to create lead sulfate and
water. This is the reduction reaction, which can be summarized as:
PbO2 + HSO4- + 3H+ + 2e- --> PbSO4 + 2H2O
At full discharge, both anode and cathode are covered with lead sulfate, and
the electrolyte is mostly water. As the sulfuric acid solution is denser
than water, a "densitometer", consisting of no more than a dropper with
pellets of varying densities and different colors, can be used to examine the
cell's charge level. Reversing the current flow reverses the reactions,
recharging the cell.
A standard automotive battery consists of a box-shaped casing with internal divider walls to separate its series-connected cells. The electrodes in each cell are built as sets of interleaved plates to provide the maximum surface area for the electrochemical reaction. Each cell in a lead-acid battery provides about two volts. Lead-acid batteries usually have large capacities, though they tend to run down quickly. They can be recharged hundreds of times until their electrodes are too eroded to allow the battery to hold a charge. They have indefinite shelf lives if stored without electrolyte. Lead-acid batteries are cheap and effective, and at present are the only practical choice where high power capacities are required at low cost.
Ruggedized and sealed lead-acid storage batteries are in common use in portable equipment with large power requirements. However, lead-acid batteries are bulky, and their active materials are environmentally hazardous, demanding recycling as an environmental safety measure.
* A new type of lead-acid battery was introduced in the late 1990s that operates on the same chemical principles, but has a radically different construction. The electrodes are formed as thin plates, with the electrolyte stored in a separator sheet between the plates, and stored in a sealed can in a "wound" or "jelly-roll" configuration. The improved battery configuration provides higher energy density, though the environmental issues remain much the same. This is about the only significant innovation in lead-acid battery design in over a century of the technology's existence. To be sure, there have been improvements in packaging materials for lighter weight and greater reliability, but Gaston Plante would see little in a modern lead-acid battery that he didn't find familiar.
Trying to come up with a high-capacity rechargeable cell with a higher energy density at a reasonable cost has proven extremely difficult. This was frustrating even a century ago, and the well-known American inventor Thomas Alva Edison (1847:1939) spent a fortune trying to build a rechargeable cell that could improve on Plante's invention. The result was the "nickel-iron" cell, or "Edison cell", and though it still lives on in industrial uses, it never came close to displacing the lead-acid battery. The Edison cell uses an iron anode, a nickel oxide cathode, and a potassium hydroxide electrolyte. The Edison cell provides a voltage of about 1.15 volts per cell. Its main virtue is that it is extremely rugged, tolerating aggressive discharges that would ruin other types of storage cells, and has a very long service life.
* The "nickel-cadmium" or "nicad" cell is similar to the Edison cell, but
uses a cadmium instead of an iron anode. A nicad cell is generally a
cylinder with layers of cadmium and nickel oxide separated by absorbent
layers containing KOH electrolyte. The anode reaction is:
Cd + 2OH- --> Cd(OH)2 + 2e-
The cathode reaction is:
NiO2 + 2H2O + 2e- --> Ni(OH)2 + 2OH-
These are reversible reactions. The nicad produces about 1.2 volts per cell.
It has a low internal resistance and its cell voltage remains remarkably
constant until the cell is almost discharged.
While Edison batteries are generally built as large industrial units that physically resemble lead-acid batteries, nicads are built mostly for rechargeable consumer equipment and so have smaller form factors. Nicads were once the predominant rechargeable cell technology in consumer gear, but they tended to be ruined by complete discharge, and the heavy-metal cadmium anode made them an environmental nuisance. Nicads are still in widespread use, particularly for portable power tools where their ability to provide large amounts of current on demand makes them particularly useful, but are now increasingly being replaced by improved rechargeable cell technologies.
* One such improved rechargeable technology is the "nickel-metal hydride
(NiMH)" cell. Most NiMH designs are similar to nicads, but replace the
cadmium anode with a "metal hydride", based on complex metallic alloys that
can store large quantities of hydrogen, The cathode is nickel oxide, the
electrolyte is a solution of potassium hydroxide, stored in a polymer
separator sheet. The anode reaction, with "(M)" representing the metal
hydride, is:
(M)H + OH- --> (M) + H2O + e-
The cathode reaction is:
NiOOH + H2O + e- --> Ni(OH)2 + OH-
NiMH cells have a typical cell voltage of 1.2 volts, which tends to remain
flat through the cell discharge cycle. They tend to have a high
self-discharge rate, but are environmentally benign, at least by the
standards of storage cells.
* Lithium is an excellent material for making storage cell anodes, since it gives up electrons very easily and is very light. Lithium cells can provide an order of magnitude better energy density than lead-acid cells. One of the big problems with lithium is that reacts violently with moisture, and manufacturing lithium cells requires a moisture-free environment. Lithium cells also require venting and other safety systems to keep them from exploding if moisture does infiltrate the case, or if such cells are heated. The safety issues delayed their use for a very long time. There are a bewildering range of lithium cell technologies. They can be basically divided into non-rechargeable lithium cells, and rechargeable "lithium-ion" cells.
* The conceptually simplest and most common nonrechargeable lithium cell is
the "lithium-manganese" cell. This has a lithium anode, a manganese dioxide
cathode, and a carbonate electrolyte. The anode reaction is:
Li --> Li+ + e-
The cathode reaction is:
MnO2 + Li+ + e- --> MnO2(Li)
The cell voltage is about 3 volts. Such cells are constructed in a jelly
roll configuration, with a sheet of lithium foil, a separator sheet
containing electrolytic salts, and a sheet of manganese dioxide rolled up
together. They have an indefinite shelf life.
There are many other nonrechargeable lithium cell configurations, such as "lithium sulfur dioxide", "lithium thionyl chloride", and "lithium polycarbonate monofluoride", with complicated constructions and chemistries that are substantially more capable than lithium-manganese but not as cheap, and so not in as widespread use. The latest generation of nonrechargeable lithium cells uses a polymeric electrolyte. Such "lithium polymer" cells have electrical characteristics similar to those of the predecessors, but they can be more easily built in flat or rectangular configurations that are very useful for lightweight portable equipment.
* The high cell voltage of the lithium ion cell means that it is not interchangeable with standard zinc-carbon or alkaline cells. A nonrechargeable "lithium disulfide" cell or "voltage compatible lithium cell" has been introduced that does provide a cell voltage of 1.5 volts. The lithium disulfide cell is also built in a jelly roll configuration, with a lithium anode, an electrolytic separator sheet, an iron disulfide (FeS2) cathode, and an aluminum cathode collector. It is lighter than an alkaline cell, has high capacity, and has a very long shelf life.
* Lithium is easier to handle in its ionized form, and so rechargeable lithium cells, which have to deal with the hazards of being recharged, have been traditionally based on lithium compounds. Again, there are many variations, but a typical "lithium ion" cell has a carbon anode, a lithium cobalt dioxide or manganese dioxide cathode, and an electrolyte consisting of a lithium salt in solution. Lithium-ion cells have a cell voltage of about 3.6 volts. They have high internal resistance and are not suited to high current applications.
They have very long cycle lives, up to a thousand cycles for single cells, and their storage capacity does not degrade significantly with cycling. They are increasingly becoming the rechargeable battery of choice for portable consumer electronics equipment. They did acquire a reputation for being temperamental, with recalls of laptop computers with lithium battery packs that had an unpleasant tendency to catch fire, and despite the long cycle life the overall lifetime of lithium cells was only a few years.
The latest generation of rechargeable lithium-ion cells feature electrodes made of phosphides of manganese and iron, made of "nanosized" particles. These new cells are much more robust, have longer lifetimes, and can take a full charge in a matter of minutes.
* Another relatively recent innovation in battery technology is the "sodium-sulfur" cell, which uses a core of molten sodium as the anode, separated by an inert solid electrolyte from an outer layer of carbon containing molten sulfur. The cell voltage is about 2 volts. They have several times the energy density and cycle lives of lead-acid cells and demand much less maintenance.
Sodium-sulfur batteries date back in concept back to the 1960s, but they didn't come into commercial use until about 2000 or so. Thanks to their requirement for molten metals, they are generally only suited to large-scale power storage at fixed sites, with installations providing up to tens of megawatt-hours of capacity.
* The following table summarizes electrical storage cell technologies:
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CARBON-ZINC (LECLANCHE) CELL:
anode: zinc cup
cathode: manganese dioxide in graphite powder
electrolyte: ammonium chloride & zinc chloride in water
cell voltage: 1.5 volts
Non-rechargeable, poor storage density, but very cheap.
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ALKALINE CELL:
anode: nickel-plated steel cup
cathode: manganese dioxide in graphite powder
electrolyte: potassium hydroxide in water
cell voltage: 1.5 volts
(Generally) non-rechargeable, storage density about twice that of the
carbon-zinc cell, but several times more expensive.
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MERCURY BUTTON CELL:
anode: zinc
cathode: mercuric oxide
electrolyte: potassium hydroxide in paste
cell voltage: 1.35 volts
Non-rechargeable. A variation on this technology used cadmium instead of
zinc and provided a cell voltage of 0.91 volts. The first button cell
technology, now obsolete due to environmental concerns.
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ZINC-AIR BUTTON CELL:
anode: powdered zinc in gel
cathode: carbon disk exposed to air
electrolyte: potassium hydroxide layer
cell voltage: 1.65 volts
Non-rechargeable. Most popular current button cell technology, also some
applications in larger cell formats.
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SILVER OXIDE BUTTON CELL:
anode: powdered zinc in gel
cathode: silver grid pasted with silver oxide
electrolyte: potassium hydroxide layer
cell voltage: 1.55 volts
Nonrechargeable.
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LEAD-ACID CELL:
anode: lead
cathode: lead oxide
electrolyte: sulfuric acid
cell voltage: 2 volts
The standard large capacity battery technology. Can be recharged
hundreds of times and very cheap, but bulky and environmentally
noxious.
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NICKEL-IRON (EDISON) CELL:
anode: iron
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.15 volts
Heavy-duty rechargeable unit, used in some industrial applications.
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NICKEL-CADMIUM (NICAD) CELL:
anode: cadmium
cathode: nickel oxide
electrolyte: potassium hydroxide
cell voltage: 1.2 volts
The original rechargeable cell for portable gear, now used mostly in
gear that needs high power levels on demand.
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NICKEL-METAL HYDRIDE (NIMH) CELL:
anode: metal hydride
cathode: nickel oxide
electrolyte: potassium hydroxide solution in separator sheet
cell voltage: 1.2 volts
Greater capacity than nicads but more expensive.
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LITHIUM-MANGANESE DIOXIDE CELL:
anode: lithium foil
cathode: manganese dioxide
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 3 volts
The most common non-rechargeable lithium cell.
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LITHIUM DISULFIDE CELL:
anode: lithium foil
cathode: iron disulfide with aluminum cathode contact
electrolyte: separator sheet impregnated with electrolytic salts
cell voltage: 1.5 volts
"Voltage compatible" lithium cell as direct replacement for carbon-zinc
or alkaline cells.
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LITHIUM-ION CELL:
anode: inert carbon sheet
cathode: manganese dioxide
electrolyte: electrolyte separator sheet with lithium ions
cell voltage: 3.6 volts
Rechargeable lithium cell.
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SODIUM-SULFUR CELL
anode: molten sodium
cathode: molten sulfur
electrolyte: solid carbon with molten sulfur
cell voltage: 2 volts
Rechargeable, useful for fixed sites, requires molten electrodes.
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