Batteries
Nickel
Cadmium Battery (1)
Since their development and practical application in 1961,
Nickel Cadmium batteries have been widely used due to their
reliability proven through years of use and their ease of
use in a variety of applications such as for rough use. NiCad
rechargeable batteries can be recycled, and it is important
to do so because of the toxic metal cadmium contained in the
batteries. There is growing environmental concern over the
presence of the toxic metal Cd in the battery. Although they
cost 2 to 3 times more than equivalent lead acid batteries,
nickel-cadmium batteries are widely used in portable power
applications, e.g. electric tools and TV cameras: they are
the major battery type in the IRIS system e.g. BB521, BB590.
The main advantages are a higher energy density, especially
at high discharge rates, and a longer cycle life, typically
500 to 2000 cycles, as opposed to the 200 to 700 cycles for
automobile and sealed lead acid batteries. Unlike lead-acid
cells, individual NiCad cells can be deeply discharged and
left in this condition without harm. It is, however, bad practice
to deeply discharge a battery of series-connected cells due
to possible damage through voltage reversal of weaker cells.
Unfortunately, NiCad cells tend to lose their charge fairly
rapidly, especially above 35°C. Furthermore, if partially
discharged to a given level and then recharged on a regular
basis, some types of cell suffer a temporary loss of capacity
called the memory effect. This effect is reversible and
full capacity can be restored by several deep discharge/charge
cycles at a low rate and some modern chargers include a
facility to do this at intervals (called charger-analyzers).
Another operational problem of NiCad batteries is that the
voltage varies very little during discharge making it difficult
to estimate the state of charge.
Nickel
Metal Hydride Battery (2)
The nickel-metal hydride cell chemistry is a hybrid of the
proven positive electrode chemistry of the sealed nickel-cadmium
cell with the energy storage features of metal alloys developed
for advanced hydrogen energy storage concepts. This heritage
in a positive-limited cell design results in batteries providing
enhanced capacities while retaining the well-characterized
electrical and physical design features of the sealed nickel-cadmium
cell design.
Nickel-metal hydride cells, with the exception of the negative
electrode, use the same general types of components as the
sealed nickel-cadmium cell. The basic concept of the nickel-metal
hydride cell negative electrode emanated from research on
the storage of hydrogen for use as an alternative energy
source in the 1970s. Certain metallic alloys were observed
to form hydrides that could capture (and release) hydrogen
in volumes up to nearly a thousand times their own volume.
By careful selection of the alloy constituents and proportions,
the thermodynamics could be balanced to permit the absorption
and release process to proceed at room temperatures and
pressures.
Two general classes of metallic alloys have been identified
as possessing characteristics desirable for battery cell
use. These are rare earth/nickel alloys generally based
around LaNi5 (the so-called AB5 class
of alloys) and alloys consisting primarily of titanium and
zirconium (designated as AB2 alloys). In both
cases, some fraction of the base metals is often replaced
with other metallic elements. The AB5 formulation
appears to offer the best set of features for commercial
nickel-metal hydride cell applications.
The metal hydride electrode has a theoretical capacity
approximately 40 percent higher than the cadmium electrode
in a nickel-cadmium couple. As a result, nickel-metal hydride
cells provide energy densities that are 20-40 percent higher
than the equivalent nickel-cadmium cell.
The nickel-metal hydride positive electrode design draws
heavily on experience with nickel-cadmium electrodes. Electrodes
that are economical and rugged exhibiting excellent high-rate
performance, long cycle life, and good capacity include
pasted and sintered-type positive electrodes.
The balance between the positive and negative electrodes
is adjusted so that the cell is always positive-limited.
This means that the negative electrode possesses a greater
capacity than the positive. The positive will reach full
capacity first as the cell is charged. It then will generate
oxygen gas that diffuses to the negative electrode where
it is recombined. This oxygen cycle is a highly efficient
way of handling moderate overcharge currents.
The electrolyte used in the nickel-metal hydride cell is
alkaline, a dilute solution of potassium hydroxide containing
other minor constituents to enhance cell performance.
The baseline material for the separator, which provides
electrical isolation between the electrodes while still
allowing efficient ionic diffusion between them, is a nylon
blend similar to that currently used in many nickel-cadmium
cells.
The nickel-metal hydride couple lends itself to the wound
construction which is similar to that used by present-day
cylindrical nickel-cadmium cells. The basic components consist
of the positive and negative electrodes insulated by separators.
The sandwiched electrodes are wound together and inserted
into a metallic can that is sealed after injection of a
small amount of electrolyte.
In variation of this design, nickel-metal hydride cells
are also being produced in prismatic versions. The prismatic
cells may fit more easily into volume-critical applications.
The general internal construction of the prismatic cell
is similar to the cylindrical cell except the single positive
and negative electrodes are now replaced by multiple electrode
sets. Thus the trade-off for improved packaging in select
applications is increased complexity in cell assembly with
the corresponding increases in production cost.
Both cylindrical and prismatic nickel-metal hydride cells
are typically two-piece sealed designs with metallic cases
and tops that are electrically insulated from each other.
The case serves as the negative terminal for the cell while
the top is the positive terminal. Some finished cell designs
may use a plastic insulating wrapper shrunk over the case
to provide electrical isolation between cells in typical
battery applications.
Nickel-metal hydride cells contain a resealable safety
vent built into the top. The nickel-metal hydride cell is
designed so the oxygen recombination cycle described earlier
is capable of recombining gases formed during overcharge
under normal operating conditions, thus maintaining pressure
equilibrium within the cell. However, in cases of charger
failure or improper cell/charger design for the operating
environment, it is possible that oxygen, or even hydrogen,
will be generated faster than it can be recombined. In such
cases the safety vent will open to reduce the pressure and
prevent cell rupture. The vent reseals once the pressure
is relieved.
Lead
Acid Battery (3)
Lead acid batteries are usually more economical and have a
high tolerance for abuse.At the positive electrode, lead dioxide
(PbO2) is converted to lead sulfate (PbSO4)
and at the negative electrode, sponge metallic lead (Pb) is
also converted to lead sulfate (PbSO4). The electrolyte
is a dilute mixture of sulfuric acid that provides the sulfate
ion for the discharge reactions. Lead acid has become an accepted
high-performance power source for clean applications. The
sealed lead cell consists of positive and negative electrodes
and their accompanying separators that are wound in a spiral
pattern. The electrodes consist of pure lead grids pasted
with mixtures of lead oxides. These oxides are converted to
the proper active materials when the cell receives its first
charge in a process called formation. The pure lead supporting
grids allow the flexibility needed for winding the plate and
also give excellent corrosion resistance to prolong cell life.
The separator consists of a fibrous glass mat. The cell works
as a starved electrolyte system where the quantity of electrolyte
is limited to the amount that is either absorbed in the plated
or wets the fibers in the separator. The result if open gas
paths between the plates that allow gases evolved during overcharge
to diffuse from one plate to the other where they are recombined.
This recombination provides a closed system reducing venting
of gases under normal overcharge conditions. A resealing safety
vent is provided to handle pressure buildup during abusive
overcharges. Since the electrolyte is recycled, the water
loss that requires routine maintenance or limits life is minimized.
The sealed lead system has proven to provide high performance
and long life in a clean, compact package.
Lithium
Ion Battery (4)
Current lithium battery use is primarily limited to small
electronic devices, like laptop computers and camcorders,
because of their high cost and safety concerns. Several materials
contribute to the high cost, but the most frequently used
cathode material -- lithium cobalt oxide -- is extremely expensive.
Lithium battery research is currently underway at the US Department
of Energy's Sandia National Laboratories in New Mexico to
improve lithium ion battery materials and may result in smaller,
longer-lasting batteries for applications as diverse as portable
computers and electric vehicles. “The research combines
a new mixture of metals to create the cathode portion of the
lithium ion battery -- a high-tech, environmentally friendly
electrical energy storage device. For inorganic chemist Tim
Boyle and chemical engineer Jim Voigt, both in Sandia's Materials
Processing Department, building a better lithium ion battery
is much like baking a cake -- a matter of putting together
the right ingredients in the cathode. ‘We've tried various
combinations of lithium [a lightweight metal] with manganese,
cobalt, nickel, chromium, and aluminum and are making some
breakthroughs,’ Boyle says. If the right combination
of materials can be found -- and the researchers think they
are close -- lithium ion rechargeable batteries may become
economical enough and have a long enough run time to be practical
to power electric cars or replace existing traditional lead-acid
batteries. A battery consists of three basic parts -- two
electrodes (a cathode and anode) separated by an electrolyte.
Lithium ion batteries use host materials for the electrodes
(for example, carbon as the anode and lithium cobalt oxide
as the cathode) to avoid using metallic lithium, thereby improving
safety. Electrochemical reactions at the electrodes produce
an electric current that powers an external circuit. During
charge and discharge of lithium ion rechargeable batteries,
lithium ions are shuttled between the cathode and anode host
materials in a ‘rocking horse’ fashion. Sandia
has done extensive past work to improve carbons for use as
anodes. The cathode work builds on the previous anode endeavors,
says Dan Doughty, manager of Sandia's Lithium Battery R&D
Department. This is where Boyle and Voigt's research could
make a difference.
Two factors drive the quest for a better lithium ion
rechargeable battery, Boyle says. First, the batteries are
more ‘environmentally friendly.’ ‘Lithium
manganate is like sand. It has almost no environmental impact
-- unlike lead acid batteries that contain poisonous heavy
metal.’ Boyle says. ‘Also, the lithium battery
can be recharged -- meaning that it isn't thrown out, but
used over and over again.’
The second reason is that lithium batteries are lightweight
and provide more electricity than non-lithium batteries
of equal size and weight. As a result, they are ideal to
power portable electronics, a rapidly growing market. Also,
they might be used in electric cars, which require batteries
that are cheap, light, powerful, and long-lasting. The challenge,
then, is to find the right combination of cathode elements.
Boyle and Voigt are in a unique position to do this because
of a process they invented and patented three years ago
to combine elements.
Their system is a simple waterless process in which the
materials being combined are dissolved in methanol. The
solution is then dried in a vacuum, baked at 200 degrees
C in a box furnace for 24 hours, transferred to a tube furnace
where it is heated to 800 degrees C, and held for 24 hours
under a flowing oxygen atmosphere. The result is a homogenous
powder. Deciding which elements to combine is not a "hit
or miss" testing process, Boyle says. Before elements
are combined, computer models are developed showing the
structural integrity of the final material. After determining
via the computer modeling which combinations are best, the
solutions are mixed, powders processed and batteries tested.
The material's performance is tested by measuring the capacity
and useful life of the new cathode materials using electrochemical
methods. Also, X-ray diffraction is used to prove these
materials are phase pure. Boyle's experiments show that
cobalt, nickel, manganese, and other transition metals might
be the most effective combination of materials. The introduction
of the nickel to replace some of the cobalt would reduce
the cost of the final material while maintaining the high
capacity. The manganese allows for more flexibility in the
charge distribution and also would reduce costs because
it is replacing the expensive cobalt. Another advantage
of using manganese is that it is a benign material and therefore
environmentally less damaging than some of the other elements
used in lithium batteries. Sandia has long been a leader
in designing and building batteries for defense applications.
The lithium battery cathode development program is funded
by a Department of Energy Office of Basic Energy Science
initiative to develop novel, high performance battery materials.
‘They want us to find a higher capacity material for
the cathode that will give these batteries a longer life,’
Doughty says. ‘Boyle and Voigt's research fits right
into this goal.’
Work Cited
(1) “Nickel-Cadmium Batteries”.
Retrieved 19 June 2002 [online]
http://www.corrosion-doctors.org/secondaries/nickel.htm
(2) “Nickel-Metal Hydride Application
Manual”. Retrieved 19 June 2002 [online]
http://data.energizer.com/batteryinfo/
application/manuals/nickel_metal_hydride.htm
(3) “Introduction to Batteries: Lead-Acid
Batteries”. Retrieved 19 June 2002 [online]
http://www.hepi.com/basics/pb.htm
(4) “Sandia Research May Bring Smaller,
Longer Life Lithium Batteries Into Our Lives”. Retrieved
20 June 2002 [online]
http://www.sandia.gov/media/lithium.htm
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