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Battery Tutorial
Battery Reactions
Probably the simplest battery you can create is called a zinc/carbon battery. By
understanding the chemical reaction going on inside this battery, you can understand how
batteries work in general.
Consider a simple chemistry reaction starting with a jar of sulfuric acid (H2SO4). Immerse a
zinc rod in it, and the acid will immediately start to eat away at the zinc. You will see
hydrogen gas bubbles forming on the zinc, and the rod and acid will start to heat up. The
reaction is summarized as follows:
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The acid molecules break up into three ions: two H+ ions and one SO4-- ion.
The zinc atoms on the surface of the zinc rod lose two electrons (2e-) to become Zn++
ions.
The Zn++ ions combine with the SO4-- ion to create ZnSO4, which dissolves in the
acid.
The electrons from the zinc atoms combine with the hydrogen ions in the acid to
create H2 molecules (hydrogen gas). We see the hydrogen gas as bubbles forming
on the zinc rod.
If you now place a carbon rod in the acid, the acid does nothing to it. But if you connect a
wire between the zinc rod and the carbon rod, two things change:
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The electrons flow through the wire and combine with hydrogen on the carbon rod,
so hydrogen gas begins bubbling off the carbon rod.
There is less heat. You can power a light bulb or similar load using the electrons
flowing through the wire, and you can measure a voltage and current in the wire.
Some of the heat energy is turned into electron motion.
The electrons go to the trouble to move to the carbon rod because they find it easier to
combine with hydrogen there. There is a characteristic voltage in the cell of 0.76 volts.
Eventually, the zinc rod dissolves completely or the hydrogen ions in the acid get used up
and the battery "dies."
In any battery, the same sort of electrochemical reaction occurs so that electrons move from
one pole to the other. The actual metals and electrolytes used control the voltage of the
battery -- each different reaction has a characteristic voltage. For example, here's what
happens in one cell of a car's lead-acid battery:
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The cell has one plate made of lead and another plate made of lead dioxide, with a
strong sulfuric acid electrolyte in which the plates are immersed.
Lead combines with SO4 to create PbSO4 plus one electron.
Lead dioxide, hydrogen ions and SO4 ions, plus electrons from the lead plate, create
PbSO4 and water on the lead dioxide plate.
As the battery discharges, both plates build up PbSO4 (lead sulfate), and water
builds up in the acid. The characteristic voltage is about 2 volts per cell, so by
combining six cells you get a 12-volt battery.
A lead-acid battery has a nice feature -- the reaction is completely reversible. If you apply
current to the battery at the right voltage, lead and lead dioxide form again on the plates so
you can reuse the battery over and over. In a zinc-carbon battery, there is no easy way to
reverse the reaction because there is no easy way to get hydrogen gas back into the
electrolyte.
Modern batteries use a variety of chemicals to power their reactions. Typical battery
chemistries include:
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Zinc-carbon battery - Also known as a standard carbon battery, zinc-carbon
chemistry is used in all inexpensive AA, C and D dry-cell batteries. The electrodes
are zinc and carbon, with an acidic paste between them that serves as the
electrolyte.
Alkaline battery - Used in common Duracell and Energizer batteries, the electrodes
are zinc and manganese-oxide, with an alkaline electrolyte.
Lithium photo battery - Lithium, lithium-iodide and lead-iodide are used in cameras
because of their ability to supply power surges.
Lead-acid battery - Used in automobiles, the electrodes are made of lead and leadoxide with a strong acidic electrolyte (rechargeable).
Nickel-cadmium battery - The electrodes are nickel-hydroxide and cadmium, with
potassium-hydroxide as the electrolyte (rechargeable).
Nickel-metal hydride battery - This battery is rapidly replacing nickel-cadmium
because it does not suffer from the memory effect that nickel-cadmiums do
(rechargeable).
Lithium-ion battery - With a very good power-to-weight ratio, this is often found in
high-end laptop computers and cell phones (rechargeable).
Zinc-air battery - This battery is lightweight and rechargeable.
Zinc-mercury oxide battery - This is often used in hearing-aids.
Silver-zinc battery - This is used in aeronautical applications because the power-toweight ratio is good.
Metal-chloride battery - This is used in electric vehicles.
Definitions
1. Anode: The electrode where oxidation (loss of electrons) takes place. While
discharging, it is the negative electrode; while charging it becomes the positive
electrode.
2. Amps: Also known as Amperes. This is the rate at which electrons flow in a wire.
The units are coulombs per second, or since an electron has a charge of 1.602 x
10-19 columbs, an amp is 6.24 x 10+18 electrons per second. Think of marbles
rolling through a tube. If 6.24 x 10+18 pass by in 1 second you wuld have an amp
of marbles.
3. Amp hours: Also known as ampere hours. This a measure of the amount of
charge stored or used. For example if you had an amp of marbles flowing out of
your tube into a bucket for an hour, you would have one amp-hour of marbles in
the bucket ( 6.24 x 10+18 times 3600 seconds = 2.2 x 10+22 marbles. A 1 amp hour
battery contains enough charge to supply 1 amp for 1 hour, if you discahrge at the
same rate. Usually if you discharge faster than the rate at which the the amp hours
were specified you will get fewer amp-hours out. You may notice that amp-hours
and coulombs measure the same quantity-charge. One amp-hour is 3600
coulombs, but amp hours are easier to use in battery design. So remember, amps
are flow ( "this motor requires 2 amps to run at 1800 rpm.") Amp hours measure
capacity, quanity, or amount of charge ("this 100 amp-hour battery will supply 2
amps for 50 hours before recharge." Amp-hours are amps times hours, not amps
divided by hours.
So Amp-Hours, (AH), or milliamp-Hours (mAH) is a measure of the size of the
battery a 10 mAH battery has half the capacity of a 20 mAH battery, even though
they may be in the same physical package.
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Batteries: Two or more electrochemical cells, electrically interconnected, each of
which contains two electrodes and an electrolyte. The redox (oxidation-reduction)
reactions that occur at these electrodes convert electrochemical energy into
electrical energy. In everyday usage, 'battery' is also used to refer to a single cell.
Capacity: The total quantity of electricity or total ampere-hours available from a
fully charged cell or battery.
Cathode: The electrode where reduction (gain of electrons) takes place. When
discharging, it is the positive electrode, when charging, it becomes the negative
electrode.
Charge: The conversion of electrical energy, provided in the form of current
from an external source, into chemical energy stored at the electrodes of a cell or
battery.
Discharge: The conversion of the chemical energy of a cell into electrical energy,
which can then be used to supply power to a system.
Discharge curve: A plot of cell voltage over time into the discharge, at a constant
temperature and constant current discharge rate.
Each curve in this graph represents cell performance at a different
discharge rate. The farther right the curve ends, the lower the discharge
rate (Crompton 31.4).
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Dry cell: A Leclanché cell, so called because of its nonfluid electrolyte (to
prevent spillage). This is achieved by adding an inert metal oxide so that the
electrolyte forms a gel or paste.
Efficiency: For a secondary cell, the ratio of the output on discharge to the input
required to restore it to its initial state of charge under specified conditions. Can
be measured in ampere-hour, voltage, and watt-hour efficiency.
Electrolyte: The chemistry of a battery requires a medium that provides the ion
transport mechanism between the positive and negative electrodes of a cell.
Energy density (specific energy): These two terms are often used
interchangeably. Energy density refers mainly to the ratio of a battery's available
energy to its volume (watt hour/liter). Specific energy refers to the ratio of energy
to mass (watt hour/kg). The energy is determined by the charge that can be stored
and the cell voltage (E=qV).
Fuel cell: A cell in which one or both of the reactants are not permanently
contained in the cell, but are continuously supplied from a source external to the
cell and the reaction products continuously removed. Unlike the metal anodes
typically used in batteries, the fuels in a fuel cell are usually gas or liquid, with
oxygen as the oxidant. The hydrogen/oxygen fuel cell is the most common. In this
fuel cell, hydrogen is oxidized at the anode:
half-reaction
V vs SHE
2H2 > 4H+ + 4e+
0
-
4H + O2 + 4e > 2H2O 1.2
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Hydrogen/oxygen fuel cell systems work well in space travel applications because
of their high efficiency, high power-to-weight and volume ratios, and usable
reaction product (water). They can function for many months as long as fuel is
supplied and therefore the energy density cannot be measured.
Half-reaction: Refers to the chemical processes occurring at each electrode. The
potential of the two half-reactions add to give us the overall cell potential. We can
see this in the zinc mercury cell, for example:
Location
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Reaction
-
Potential
-
Anode
Zn + 2OH > Zn(OH)2 + 2e
1.25 V
Cathode
HgO +H2O + 2e- > Hg + 2OH-
0.098 V
Overall
Zn + HgO + H2O > Zn(OH)2 + Hg
1.35 V
Polarization: The voltage drop in a cell during discharge due to the flow of an
electrical current. The cell's internal resistance increases with the buildup of a
product of oxidation or a reduction of an electrode, preventing further reaction.
Power: Defined by voltage (V) and current (I), P=VI.
Since V=IR, P=I2R and P=V2/R
Power also can be described by energy emitted per unit of time: P=E/t.
Thus E=VIt=qV.
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Power density (specific power): Power density is the ratio of the power available
from a battery to its volume (watt/liter). Specific power generally refers to the
ratio of power to mass (watt/kg). Comparison of power to cell mass is more
common.
Primary cells: A cell that is not designed for recharging and is discarded once it
has produced all its electrical energy.
Prismatic: Just a word to say that the cells are not cylindrical, as nature intended
battery cells to be, but fit nicely into a parallelepiped or any other such flattened
shape.
Reserve cell: A cell that may be kept inactive and which is activated by adding an
electrolyte or electrode, or melting an electrolyte in a solid state.
Secondary cells: A cell capable of repeated use. Its charge may be fully restored
by passing an electric current through the cell in the opposite direction to that of
discharge, thus reversing the redox reactions.
Parameters for Battery Performance
No one battery design is perfect for every application. Choosing one requires
compromise. That's why it's important to prioritize your list of requirements.
Decide which ones you absolutely must have and which you can compromise on.
Here are some of the parameters to consider:
1. Voltage: Normal voltage during discharge, maximum and minimum permissible
voltages, discharge curve profile
2. Duty cycle: Conditions the battery experiences during use. Type of discharge and
current drain, e.g., continuous, intermittent, continuous with pulses, etc.
3. Temperature: In storage and in use. Temperatures that are too high or too low
can greatly reduce battery capacity.
4. Shelf life: How rapidly the cell loses potential while unused.
5. Service life: Defined either in calendar time or, for secondary cells, possible
number of discharge/charge cycles, depending on the battery application. Service
life depends on battery design and operational conditions, i.e., the stress put on a
battery. For stationary and motive power application, the end of service life is
defined as the point at which a battery's capacity drops to 80% of its original
capacity. Exceptions would include car batteries where the service life ends when
the capacity falls below 60%.
6. Physical restrictions: These include dimensions, weight, terminals, etc.
7. Maintenance and resupply: Ease of battery acquisition, replacement, charging
facilities, disposal.
8. Safety and reliability: Failure rates, freedom from outgassing or leakage; use of
toxic components; operation under hazardous conditions; environmentally safe
9. Cost: Initial cost, operating cost, use of expensive materials
10. Internal resistance: Batteries capable of a high-rate discharge must have a low
internal resistance.
11. Specific energy: As discussed in the definition section, this is a measurement of
possible stored energy per kilogram of mass. This number is purely theoretical as
it does not take into account the mass of inactive materials, nor the variation in
chemical reactions.
12. Specific power: Also defined in the definitions section, a P=E/t, so the specific
power is discussed at a specific discharge rate. It is possible for batteries with a
high specific energy to have a low power density if they experience large voltage
drops at high discharge rates
13. Unusual requirements: Very long-term or extreme-temperature storage; very
low failure rate; no voltage delay, etc.
Of course the ideal battery would perform well in all these areas with a long shelf
and service life, high specific energy and specific power, low initial and
maintenance costs, low environmental impact, and good performance in a variety
of conditions (temperatures, duty cycles, etc.). When you find one that meets all
these requirements, let us know! In the meantime, we have to make do with
batteries that work very well in specific applications.
Primary Batteries
Leclanché Cells(zinc carbon or dry cell)
Anode: Zinc
Cathode: Manganese Dioxide (MnO2)
Electrolyte: Ammonium chloride or zinc chloride dissolved in water
Applications: Flashlights, toys, moderate drain use
The basic design of the Leclanché cell has been around since the 1860s, and
until World War II, was the only one in wide use. It is still the most commonly
used of all primary battery designs because of its low cost, availability, and
applicability in various situations. However, because the Leclanché cell must be
discharged intermittently for best capacity, much of battery research in the last
three decades has focused on zinc-chloride cell systems, which have been found
to perform better than the Leclanché under heavier drain.
This figure shows typical discharge curves for general-purpose Leclanché zinc
chloride D-size cells discharge 2 h/day at 20º C. Solid line—zinc chloride; broken
line—Leclanché (Linden 8.18). The zinc-chloride cell has a higher service life and
voltage than the Leclanché (at both higher and lower discharge rates).
In an ordinary Leclanché cell the electrolyte consists (in percent of atomic weight)
of 26% NH4Cl (ammonium chloride), 8.8% ZnCl2 (zinc chloride), and 65.2%
water. The overall cell reaction can be expressed:
Zn + 2MnO2 +2NH4Cl —> 2MnOOH + Zn(NH3)2Cl2 E=1.26
The electrolyte in a typical zinc chloride cell consists of 15-40% ZnCl2 and 6085% water, sometimes with a small amount of NH4Cl for optimal performance.
The overall cell reaction of the zinc chloride as the electrolyte can be expressed:
Zn + 2MnO2 + 2H2O + ZnCl2 —> 2MnOOH + 2Zn(OH)Cl
MnO2, is only slightly conductive, so graphite is added to improve conductivity.
The cell voltage increases by using synthetically produced manganese dioxide
instead of that found naturally (called pyrolusite). This does drive the cost up a
bit, but it is still inexpensive and environmentally friendly, making it a popular
cathode.
These cells are the cheapest ones in wide use, but they also have the lowest
energy density and perform poorly under high-current applications. Still, the zinc
carbon design is reliable and more than adequate for many everyday
applications.
Alkaline Cells
Anode: Zinc powder
Cathode: Manganese dioxide (MnO2) powder
Electrolyte: Potassium hydroxide (KOH)
Applications: Radios, toys, photo-flash applications, watches, high-drain
applications
This cell design gets its name from its use of alkaline aqueous solutions as
electrolytes. Alkaline battery chemistry was first introduced in the early ’60s. The
alkaline cell has grown in popularity, becoming the zinc-carbon cell's greatest
competitor. Alkaline cells have many acknowledged advantages over zinc-
carbon, including a higher energy density, longer shelf life, superior leakage
resistance, better performance in both continuous and intermittent duty cycles,
and lower internal resistance, which allows it to operate at high discharge rates
over a wider temperature range.
Zinc in a powdered form increases the surface area of the anode, allowing more
particle interaction. This lowers the internal resistance and increases the power
density. The cathode, MnO2, is synthetically produced because of its superiority
to naturally occurring MnO2. This increases the energy density. Just as in the
zinc carbon cell, graphite is added to the cathode to increase conductivity. The
electrolyte, KOH, allows high ionic conductivity. Zinc oxide is often added to slow
down corrosion of the zinc anode. A cellulose derivative is thrown in as well as a
gelling agent. These materials make the alkaline cell more expensive than the
zinc-carbon, but its improved performance makes it more cost effective,
especially in high drain situations where the alkaline cell's energy density is much
higher.
The half-reactions are:
Zn + 2 OH- —> ZnO + H2O + 2 e2 MnO2 + H2O + 2 e- —>Mn2O3 + 2 OHThe overall reaction is:
Zn + 2MnO2 —> ZnO + Mn2O3 E=1.5 V
There are other cell designs that fit into the alkaline cell category, including the
mercury oxide, silver oxide, and zinc air cells. Mercury and silver give even
higher energy densities, but cost a lot more and are being phased out through
government regulations because of their high toxicity as heavy metals. The
mercury oxide, silver oxide, and zinc air (which is being developed for electronic
vehicles) are all discussed below.
Mercury Oxide Cells
Anode: Zinc (or cadmium)
Cathode: Mercuric Oxide (HgO)
Electrolyte: Potassium hydroxide
Applications: Small electronic equipment, hearing aids, photography, alarm
systems, emergency beacons, detonators, radio microphones
This is an obsolete technology. Most if not all of the manufacture of these cells
has been stopped by government regulators. Mercury batteries come in two main
varieties: zinc/mercuric oxide and cadmium/mercuric oxide. The zinc/mercuric
oxide system has high volumetric specific energy (400 Wh/L), long storage life,
and stable voltage. The cadmium/mercuric oxide system has good high
temperature and good low temperature (-55 C to +80 C, some designs to +180
C) and has very low gas evolution.
Basic Cell Reaction
Voltage Electrochemical Efficiency
Zn + HgO = ZnO + Hg
1.35 V
Cd + HgO + H2O = Cd(OH2) + Hg 0.91 V
820 mAH/g(Zn), 250 mAH/g(Hg)
480 mAH/g(Cd)
The electrolytes used in mercury cells are sodium and/or potassium hydroxide solutions,
making these alkaline cells. These cells are not rechargeable.
Zinc/Air Cells
Anode: Amalgamated zinc powder and electrolyte
Cathode: Oxygen (O2)
Electrolyte: Potassium hydroxide (KOH)
Applications: Hearing aids, pagers, electric vehicles
The zinc air cell fits into the alkaline cell category because of its electrolyte. It
also acts as a partial fuel cell because it uses the O2 from air as the cathode.
This cell is interesting technology, even aside from the question "how do you use
air for an electrode?" Actually, oxygen is let in to the cathode through a hole in
the battery and is reduced on a carbon surface.
A number of battery chemistries involve a metal oxide and zinc. The metal oxide
reduces, the zinc becomes oxidized, and electric current results. A familiar
example is the old mercury oxide/zinc batteries used for hearing aids. If you
leave out the metal oxide you could double the capacity per unit volume
(roughly), but where would you get the oxygen? Right!
First let's look at the electrochemical reactions and find that the open cell voltage
should be 1.65 volts:
Location Half Cell reactions
Anode
Voltage
Zn + 2OH —> Zn(OH)2
2+
-
1.25
Cathode 1/2 O2 + H2O + 2e —> 2 OH
-
0.4
Overall 2Zn +O2 +2H2O —> 2Zn(OH)2 1.65
The electrolyte is an alkali hydroxide in 20-40% weight solution with water. One
disadvantage is that since these hydroxides are hygroscopic, they will pick up or
lose water from the air depending on the humidity. Both too little and too much
humidity reduces the life of the cell. Selective membranes can help. Oxygen from
the air dissolves in the electrolyte through a porous, hydrophobic electrode—a
carbon-polymer or metal-polymer composite.
Since there is no need to carry around the cathode, the energy density of these
batteries can be quite high, between 220–300 Wh/kg (compared to 99–123
Wh/kg with a HgO cathode), although the power density remains low. However,
the use of potassium or sodium hydroxides as the electrolyte is a problem, since
these can react with carbon dioxide in the air to form alkali carbonates. For this
reason large zinc air batteries usually contain a higher volume of CO2 absorbing
material (calcium oxide flake) than battery components. This can cancel out the
huge increase in energy density gained by using the air electrode.
This cell has the additional benefits of being environmentally friendly at a
relatively low cost.
These batteries can last indefinitely before they are activated by exposing them
to air, after which they have a short shelf life. For this reason (as well as the high
energy density) most zinc-air batteries are used in hearing aids. There is a
company promoting them for use in electric vehicles also because they are
environmentally friendly and cost relatively little. The idea is to have refueling
stations where the zinc oxide waste can be replaced by fresh zinc pellets.
Aluminum / Air Cells
Although, to our way of thinking, the metal/air batteries are strictly primary, cells
have been designed to have the metal replaceable. These are called
mechanically rechargeable batteries. Aluminum/air is an example of such a cell.
Aluminum is attractive for such cells because it is highly reactive, the aluminum
oxide protective layer is dissolved by hydroxide electrolytes, and it has a nice,
high voltage. The overall chemical reaction is:
Location Half Cell reactions
Anode
Voltage
Al + 4 OH —> Al(OH)4 + 3e
-
-
Cathode 3/4 O2 + 3/2 H2O + 3e—> 3OH-
-2.35
0.40
Overall Al + 3/2 HO + 3/4 O2 —> Al(OH)3 2.75 V
As I mentioned above, alkali (chiefly potassium hydroxide) electrolytes are used,
but so also are neutral salt solutions. The alkali cell has some problem with the
air electrode, because the hydroxide ion makes a gell in the porous electrode,
polarizing it. The typical aluminum hydroxide gell is a problem on either electrode
because it sucks up a lot of water. Using a concentrated caustic solution
prevents this, but is very reactive with the aluminum electrode, producing
hydrogen gas. Another way to prevent the gel formation is to seed the electroylte
with aluminum trihydroxide crystals. These act to convert the aluminum
hydroxide to aluminum trihydroxide as the crystals grow. To prevent hydrogen
gas evolution tin and zinc have been used as corrosion inhibitors. A number of
additives are used to control the reactions. A disadvantae of the alkaline
electrolyte is that it reacts with atmospheric carbon dioxide.
Aluminum / air cells have also been made for marine applications. These are
"rechargeable" by replacing the seawater electrolyte until the aluminum is
exhausted, then replacing the aluminum. Some cells that are open to seawater
have also been researched. Since salt water solutions tend to passivate the
aluminum, pumping the electrolyte back and forth along the cell surface has been
successful. For those cells that don't need to use ocean water, an electrolyte of
KCL and KF solutions is used.
Air electrodes of Teflon-bonded carbon are used without a catalyst.
Lithium Cells
Applications: Pacemakers, defibrillators, watches, meters, cameras, calculators,
portable, low-power use
Lithium battery chemistry comprise a number of cell designs that use lithium as
the anode. Lithium is gaining a lot of popularity as an anode for a number of
reasons. In this comparison of anode materials, we can see some reasons why:
Anode
Atomic Standard Density Melting Electrochemical
mass (g) potential (V) g/cm3 point ºC Equivalence (Ah/g)
Li
6.94
3.05
0.54
180
3.86
Na
23.0
2.7
0.97
97.8
1.16
Mg
24.3
2.4
1.74
650
2.20
Al
26.9
1.7
2.7
659
2.98
Ca
40.1
2.87
1.54
851
1.34
Fe
55.8
0.44
7.85
1528
0.96
Zn
65.4
0.76
7.1
419
0.82
Cd
112
0.40
8.65
321
0.48
Pb
207
0.13
11.3
327
0.26
Note that lithium, the lightest of the metals, also has the highest standard
potential of all the metals, at over 3 V. Some of the lithium cell designs have a
voltage of nearly 4 V. This means that lithium has the highest energy density.
Many different lithium cells exist because of its stability and low reactivity with a
number of cathodes and nonaqueous electrolytes. The most common
electrolytes are organic liquids with the notable exceptions of SOCl2 (thionyl
chloride) and SO2Cl2 (sulfuryl chloride). Solutes are added to the electrolytes to
increase conductivity.
Lithium cells have only recently become commercially viable because lithium
reacts violently with water, as well as nitrogen in air. This requires sealed cells.
High-rate lithium cells can build up pressure if they short circuit and cause the
temperature and pressure to rise. Thus, the cell design needs to include weak
points, or safety vents, which rupture at a certain pressure to prevent explosion.
Lithium cells can be grouped into three general categories: liquid cathode, solid
cathode, and solid electrolyte. Let's look at some specific lithium cell designs
within the context of these three categories.
Liquid cathode lithium cells:
These cells tend to offer higher discharge rates because the reactions occur at
the cathode surface. In a solid cathode, the reactions take longer because the
lithium ions must enter into the cathode for discharge to occur. The direct contact
between the liquid cathode and the lithium forms a film over the lithium, called
the solid electrolyte interface (SEI). This prevents further chemical reaction when
not in use, thus preserving the cell's shelf life. One drawback, though, is that if
the film is too thick, it causes an initial voltage delay. Usually, water
contamination is the reason for the thicker film, so quality control is important.
LiSO2 Lithium–Sulfur Dioxide
This cell performs very well in high current applications as well as in low
temperatures. It has an open voltage of almost 3 V and a typical energy density
of 240–280 Wh/kg. It uses a cathode of porous carbon with sulfur dioxide taking
part in the reaction at the cathode. The electrolyte consists of an acetonitrile
solvent and a lithium bromide solute. Polypropylene acts as a separator. Lithium
and sulfur dioxide combine to form lithium dithionite:
2Li + 2SO2 —> Li2S2O4
These cells are mainly used in military applications for communication because
of high cost and safety concerns in high-discharge situations, i.e., pressure
buildup and overheating.
LiSOCl2 Lithium Thionyl Chloride
This cell consists of a high-surface area carbon cathode, a non-woven glass
separator, and thionyl chloride, which doubles as the electrolyte solvent and the
active cathode material. Lithium aluminum chloride (LiAlCl4) acts as the
electrolyte salt.
The materials react as follows:
Location
Reaction
+
Anode Li —> Li + e
Cathode 4Li+ + 4e- + 2SOCl2 —> 4LiCl + SO2 + S
Overall
4Li + 2SOCl2 —> 4LiCl + SO2 + S
During discharge the anode gives off lithium ions. On the carbon surface, the
thionyl chloride reduces to chloride ions, sulfur dioxide, and sulfur. The lithium
and chloride ions then form lithium chloride. Once the lithium chloride has
deposited at a site on the carbon surface, that site is rendered inactive. The
sulfur and sulfur dioxide dissolve in the electrolyte, but at higher-rate discharges
SO2 will increase the cell pressure.
This system has a very high energy density (about 500 Wh/kg) and an operating
voltage of 3.3–3.5 V. The cell is generally a low-pressure system
In high-rate discharge, the voltage delay is more pronounced and the pressure
increases as mentioned before. Low-rate cells are used commercially for small
electronics and memory backup. High-rate cells are used mainly for military
applications.
Solid cathode lithium cells:
These cells cannot be used in high-drain applications and don't perform as well
as the liquid cathode cells in low temperatures. However, they don't have the
same voltage delay and the cells don't require pressurization. They are used
generally for memory backup, watches, portable electronic devices, etc.
LiMnO2
These account for about 80% of all primary lithium cells, one reason being their
low cost. The cathode used is a heat-treated MnO2 and the electrolyte a mixture
of propylene carbonate and 1,2-dimethoyethane. The half reactions are
Anode
Li —> Li+ + e
Cathode MnIVO2 + Li+ + e —> MnIIIO2(Li+)
Overall Li + MnIVO2 —> MnIIIO2(Li+)
At lower temperatures and in high-rate discharge, the LiSO2 cell performs much
better than the LiMnO2 cell. At low-rate discharge and higher temperatures, the
two cells perform equally well, but LiMnO2 cell has the advantage because it
doesn't require pressurization.
Li(CF)n Lithium polycarbon monofluoride
The cathode in this cell is carbon monofluoride, a compound formed through
high-temperature intercalation. This is the process where foreign atoms (in this
case fluorine gas) incorporate themselves into some crystal lattice (graphite
powder), with the crystal lattice atoms retaining their positions relative to one
another.
A typical electrolyte is lithium tetrafluorobate (LiBF4) salt in a solution of
propylene carbonate (PC) and dimethoxyethane (DME).
Anode
Li —> Li+ + e
Cathode MnIVO2 + Li+ + e —> MnIIIO2(Li+)
Overall Li + MnIVO2 —> MnIIIO2(Li+)
These cells also have a high voltage (about 3.0 V open voltage) and a high
energy density (around 250 Wh/kg). All this and a 7-year shelf life makes them
very suitable for low- to moderate-drain use, e.g., watches, calculators, and
memory applications.
Solid electrolyte lithium cells:
All commercially manufactured cells that use a solid electrolyte have a lithium
anode. They perform best in low-current applications and have a very long
service life. For this reason, they are used in pacemakers
LiI2—Lithium iodine cells use solid LiI as their electrolyte and also produce LiI
as the cell discharges. The cathode is poly-2-vinylpyridine (P2VP) with the
following reactions:
Anode
2Li —> 2Li+ + 2e
Cathode 2Li+ + 2e + P2VP· nI2 —> P2VP· (n–1)I2 + 2LiI
Overall 2Li + P2VP· nI2 —> P2VP· (n–1)I2 +2LiI
LiI is formed in situ by direct reaction of the electrodes.
Lithium-Iron Cells
The Lithium-Iron chemistry deserves a separate section because it is one of a handful of
lithium metal systems that have a 1.5 volt output (others are lithium/lead bismuthate,
lithium/bismuth trioxide, lithiumr/copper oxdide, and lithium/copper sulfide). Recently
consumer cells that use the Li/Fe have reached the market, including the Energizer. These
have advantage of having the same voltage as alkaline batteries with much more energy
storage capacity, so they are called "voltage compatible" lithiums. They are not
rechargeable. They have about 2.5 times the capacity of an alkaline battery of the same
size, but only under high current discharge conditions (digital cameras, flashlights, motor
driven toiys, etc.). For small currents they don't have any advantage. Another advantage
is the low self-discharge rate–10 years storage is quoted by the manufacturer. The
discharge reactions are:
Type
Reaction
Nominal Voltage Range
FeS2 Version 2 FeS2 + 4 Li —> Fe + 2Li2S 1.6 Volts
1.6-1.4 V
FeS Version FeS + 2Li —> Fe + Li2S
1.5 Volts
1.5-1.2 V
Both Iron sulfide and Iron disulfide are used, the FeS2 is used in the Energizer.
Electrolytes are organic materials such as propylene carbonate, dioxolane and
dimethoxyelthane
Secondary batteries
Lead–acid Cells
Anode: Sponge metallic lead
Cathode: Lead dioxide (PbO2)
Electrolyte: Dilute mixture of aqueous sulfuric acid
Applications: Motive power in cars, trucks, forklifts, construction equipment,
recreational water craft, standby/backup systems
Used mainly for engine batteries, these cells represent over half of all battery
sales. Some advantages are their low cost, long life cycle, and ability to
withstand mistreatment. They also perform well in high and low temperatures and
in high-drain applications. The chemistry lead acid battery half-cell reactions are:
Pb + SO4
2-
half-reaction
—> PbSO4 + 2e-
V vs SHE
.356
PbO2 + SO42- + 4H+ + 2e- —> PbSO4 + 2H2O 1.685
There are a few problems with this design. If the cell voltages exceed 2.39 V, the
water breaks down into hydrogen and oxygen (this so-called gassing voltage is
temperature dependent, for a chart of the temperature dependence click here
).
This requires replacing the cell's water. Also, as the hydrogen and oxygen vent
from the cell, too high a concentration of this mixture will cause an explosion.
Another problem arising from this system is that fumes from the acid or hydroxide
solution may have a corrosive effect on the area surrounding the battery.
These problems are mostly solved by sealed cells, made commercially available
in the 1970s. In the case of lead acid cells, the term "valve-regulated cells" is
more accurate, because they cannot be sealed completely. If they were, the
hydrogen gas would cause the pressure to build up beyond safe limits. Catalytic
gas recombiners do a great deal to alleviate this problem. They convert the
hydrogen and oxygen back into water, achieving about 85% efficiency at best.
Although this doesn't entirely eliminate the hydrogen and oxygen gas, the water
lost becomes so insignificant that no refill is needed for the life of the battery. For
this reason , these cells are often referred to as maintenance-free batteries. Also,
this cell design prevents corrosive fumes from escaping.
These cells have a low cycle life, a quick self discharge, and low energy densities
(normally between 30 and 40 Wh/kg). However, with a nominal voltage of 2 V
and power densities of up to 600 W/kg, the lead-acid cell is an adequate, if not
perfect, design for car batteries.
Nickel/Cadmium Cells
Anode: Cadmium
Cathode: Nickel oxyhydroxide Ni(OH)2
Electrolyte: Aqueous potassium hydroxide (KOH)
Applications: Calculators, digital cameras, pagers, lap tops, tape recorders,
flashlights, medical devices (e.g., defibrillators), electric vehicles, space
applications
The cathode is nickel-plated, woven mesh, and the anode is a cadmium-plated
net. Since the cadmium is just a coating, this cell's negative environmental
impact is often exaggerated. (Incidentally, cadmium is also used in TV tubes,
some semiconductors, and as an orange-yellow dye for plastics.) The electrolyte,
KOH, acts only as an ion conductor and does not contribute significantly to the
cell's reaction. That's why not much electrolyte is needed, so this keeps the
weight down. (NaOH is sometimes used as an electrolyte, which doesn't conduct
as well, but also doesn't tend to leak out of the seal as much). Here are the cell
reactions:
Reaction
Cd + 2OH- —> Cd(OH)2 + 2e-
V vs SHE
0.81
NiO2 + 2H2O + 2e- —> Ni(OH)2 + 2OH-
0.49
Cd +NiO2 + 2H2O —> Cd(OH)2 + Ni(OH)2 1.30
Advantages include good performance in high-discharge and low-temperature
applications. They also have long shelf and use life. Disadvantages are that they
cost more than the lead-acid battery and have lower power densities. Possibly its
most well-known limitation is a memory effect, where the cell retains the
characteristics of the previous cycle.
This term refers to a temporary loss of cell capacity, which occurs when a cell is
recharged without being fully discharged. This can cause cadmium hydroxide to
passivate the electrode, or the battery to wear out. In the former case, a few
cycles of discharging and charging the cell will help correct the problem, but may
shorten the lifetime of the battery. The true memory effect comes from
experience with a certain style of NiCad in space use, which were cycled within a
few percent of discharge each time.
An important thing to know about "conditioning " a NiCd battery is that the deep
discharge spoken of is not a discharge to zero volts, but to about 1 volt per cell.
Nickel/Metal Hydride (NiMH) Cells
Anode: Rare-earth or nickel alloys with many metals
Cathode: Nickel oxyhydroxide
Electrolyte: Potassium hydroxide
Applications: Cellular phones, camcorders, emergency backup lighting, power
tools, laptops, portable, electric vehicles
This sealed cell is a hybrid of the NiCd and NiH2 cells. Previously, this battery
was not available for commercial use because, although hydrogen has wonderful
anodic qualities, it requires cell pressurization. Fortunately, in the late 1960s
scientists discovered that some metal alloys (hydrides such as LiNi5 or ZrNi2)
could store hydrogen atoms, which then could participate in reversible chemical
reactions. In modern NiMH batteries, the anode consists of many metals alloys,
including V, Ti, Zr, Ni, Cr, Co, and Fe.
Except for the anode, the NiMH cell very closely resembles the NiCd cell in
construction. Even the voltage is virtually identical, at 1.2 volts, making the cells
interchangeable in many applications. Here are the cell reactions:
Location
Reactions
Anode MH + OH- —> M + H2O + e-
Voltage
0.83
Cathode NiOOH + H2O + e- —> Ni(OH)2 + OH- 0.52
Overall
NiOOH + MH —> Ni(OH)2 + M
1.35
The anodes used in these cells are complex alloys containing many metals, such
as an alloy of V, Ti, Zr, Ni, Cr, Co, and (!) Fe. The underlying chemistry of these
alloys and reasons for superior performance are not clearly understood, and the
compositions are determined by empirical testing methods.
A very interesting fact about these alloys is that some metals absorb heat when
absorbiong hydrogen, and some give off heat when absorbing hydrogen. Both of
these are bad for a battery, since we would like the hydregen to move easily in
and out without any energy transfer. The sucessful alloys are all mixtures of
exothermic and endothermic metals to achieve this.
Hydrogen Storage Metals Comparison:
Material
Density H2 Storage Capacity
LaNi5
8.3
0.11 g/cc
FeTi
6.2
0.11
Mg2Ni
4.1
0.15
Mg
1.74
0.13
MgNi Eutectic 2.54
0.16
liquid H2
0.07
0.07
Please notice that the density of hydrogen stored in a metal hydride is higher
than that of pure liquid hydrogen! Commercial NiMH batteries are mostly of the
rare earth-nickel type, of which LaNi5 is a representative. These alloys can store
six hydrogen atoms per unit cell such as LaNi5H6. Even misch metal nickel alloys
are used to save the cost of separation.
The electrolyte of commercial NiMH batteries is typically 6 M KOH
The NiMH cell does cost more and has half the service life of the NiCd cell, but it
also has 30% more capacity, increased power density (theoretically 50% more,
practically 25% more). The memory effect, which was at one time thought to be
absent from NiMH cells, is present if the cells are treated just right. To avoid the
memory effect fully discharge once every 30 or so cycles. There is no clear
winner between the two. The better battery depends on what characteristics are
more crucial for a specific application.
Sodium/Sulfur Cells
Anode: Molten sodium
Cathode: Molten sulfur
Electrolyte: Solid ceramic beta alumina (ß"-Al2O3)
Applications: Electric vehicles, aerospace (satellites)
This cell have been studied extensively for electric vehicles because of its
inexpensive materials, high cycle life, and high specific energy and power.
Specific energies have reached levels of 150 W-h/kg and specific powers of 200
W/kg. The half-reactions are:
half-reaction
V vs SHE
2Na —> 2Na+ + 2e3S + 2e- —> S322Na + 3S —> Na2S3 2.076 V
Despite these advantages there are couple of disadvantages serious enough that
other alternatives, such as lithium-ion, nickel-metal hydride, and lithium polymer,
have emerged as the most promising solutions to electric vehicle power. One is
that the power output is very small at room temperature. The temperature must
be kept at around 350 ºC to keep the sulfur in liquid form and to be effective in
motive power applications. This is achieved through insulation or heating through
the cells own power. This lowers the energy density.
The second problem has to do with electrolyte breakdown, which is one of the
principal causes of sodium sulfur cell failure. The electrolyte, ceramic beta"alumina, has several attractive characteristics. It has all the benefits of a solid
electrolyte with the added qualities of a high ionic conductivity with a small
electronic transfer, all with the added benefit of being a solid. However, ceramic
beta"-alumina also is brittle and develops microfissures. Thus the liquid sodium
and sulfur come in contact—with explosively violent results.
Recently, some research efforts have focussed on replacing the molten sulfur
cathode with a poly(disulfide) such as poly(ethylenedisulfide), (SSCH2CH2)n.
These cells can be discharged just above the melting temperature of Na (90 °C).
The net cell reaction becomes:
2 Na + (SSR)n=Na2SSR
where the discharge reaction involves scission of the S-S disulfide linkage in the
polymer backbone, and charge involves repolymerization of the resulting
dithiolate salt.
One of these is the sodium/metal chloride, which in addition to beta"-alumina has
a secondary electrolyte (NaAlCl4) to conduct ions from the first electrolyte to the
cathode. This is necessary because the metal chloride is a solid.
Nickel/Sodium Cells
These are specialty cells made by one manufacturer in England, Beta Research. They
have advantages for electric vehicles. The cell runs hot, about 300 degrees C, but this
isn't a worry, since they heat themselves up during discharge. The discharge reaction is:
Location
Charge
Half Reaction
Voltage
2 NaCl + Ni Z —> 2Na +NiCl2
Discharge NiCl2 + 2 Na —> Ni + 2NaCl 2.58 V
The electrolyte on the nickel side of the alumina separator is sodium
tetracloroaluminate.NaAlCl4, which melts at 151 degrees C.. Energy density is 100 to 150
Wh/kg. These use an aluminum oxide ceramic as a separator, similar to that of the
sodium-sulfur cell. They have the same danger of rupture of the separator, but have a
unique solution to the problem. The cells is incased in a two-wall steel thermally
insulated package. If the separator breaks the energy is confined within this package. A
cell that is broken in this way has a low resistance, so it can continue to reside in the
battery pack without causing a vehicle break-down. This double-insulated case also
prevents the cell from spilling in car crashes.
There are no higher-voltage reactions or other side reactions, so the inventors claim that
up to the point of full charge the cell is 100% coulomb efficient–meaning that the amphours you put in is exactly the same as the amp-hours you get out. Overcharging does not
damage the cell, so the battery packs are easy to keep in balance–just overcharge the
whole pack.
It seems that the cell has no self-discharge if the batteries are cold, (solid blocks of
sodium don't migrate at room temperature) and that a pack requires about 24 hours to get
to temperature with a 230 VAC input to the pack heater.
Lithium Ion Cells
Anode: Carbon compound, graphite
Cathode: Lithium oxide
Electrolyte:
Applications: Laptops, cellular phones, electric vehicles
Lithium batteries that use lithium metal have safety disadvantages when used as
secondary (rechargeable) energy sources. For this reason a series of cell
chemistries have been developed using lithium compounds instead of lithium
metal. These are called generically Lithium ion Batteries.
Cathodes consist of a a layered crystal (graphite) into which the lithium is
intercalated. Experimental cells have also used lithiated metal oxide such as
LiCoO2, NiNi0.3Co0.7O2, LiNiO2, LiV2O5, LiV6O13, LiMn4O9, LiMn2O4, LiNiO0.2CoO2.
Electrolytes are usually LiPF6, although this has a problem with aluminum
corrosion, and so alternatives are being sought. One such is LiBF4. The
electrolyte in current production batteries is liquid, and uses an organic solvent.
Membranes are necessary to separate the electrons from the ions. Currently the
batteries in wide use have microporous polyethylene membranes.
Intercalation (rhymes with relation—not inter-cal, but in-tercal-ation) is a longstudied process which has finally found a practical use. It has long been known
that small ions (such as lithium, sodium, and the other alkali metals) can fit in the
interstitial spaces in a graphite crystal. Not only that, but these metallic atoms
can go farther and force the graphitic planes apart to fit two, three, or more layers
of metallic atoms between the carbon sheets. You can imagine what a great way
this is to store lithium in a battery—the graphite is conductive, dilutes the lithium
for safety, is reasonably cheap, and does not allow dendrites or other unwanted
crystal structures to form.
Manganese-Titanium (Lithium) Cells
Anode: Lithium-Titanium Oxide
Cathode: Lithium intercalated Mangaese Dioxide
Electrolyte:
Applications: Watches, other ultra-low discharge applicatons
This technology might be called Manganese-Titanium, but it is just another
lithium coin cell. It has "compatible" voltage – 1.5 V to 1.2 Volts, like the LithiumIron cell, which makes it convenient for applications that formerly used primary
coin cells. It is unusual for a lithium based cell because it can withstand a
continuous overcharge at 1.6 to 2.6 volts without damage. Although rated for 500
full discharge cycles, it only has a 10% a year self-discharge rate, and so is used
in solar charged watches with expected life of 15+ years with shallow
discharging. The amp-hour capacity and available current output of these cells is
extremely meager. The range of capacities from Panasonic is 0.9 to 14 mAH
(yes, 0.9 milliamp hours). The maximum continuous drain current is 0.1 to 0.5
mA.
Rechargeable Alkaline Manganese Cells
Anode: Zinc
Cathode:Mangaese Dioxide
Electrolyte: Potassium Hydroxide Solution
Applications: Consumer devices
Yes, this is the familiar alkaline battery, but specially designed to be
rechargeable, and with a hot new acronym—RAM (haven't I seen that acronym
somewhere before?). In the charging process, direct-current electrical power is
used to reform the active chemicals of the battery system to their high-energy
charge state. In the case of the RAM battery, this involves oxidation of
manganese oxyhydroxide (MnOOH) in the discharged positive electrode to
manganese dioxide (MnO2), and of zinc oxide (ZnO) in the negative electrode to
metallic zinc.
Care must be taken not to overcharge to prevent electrolysis of the KOH solution
electrolyte, or to charge at voltages higher than 1.65 V (depending on
temperature) to avoid the formation of higher oxides of manganese.
Nickel Zinc Cells
Anode: Zinc
Cathode: Nickel oxide
Electrolyte: Potassium hydroxide
Applications:Electric vehicles, standby load service
The combination of nickel and zinc is very interesting because of the low cost
and low toxicity of the constituents. There have been many technical obstacles,
but a string of recent patents and a commercial start-up based on a KOH
electrolyte holds great promise for applications where light weight is an issue.
The nickel/zinc battery uses zinc as the negative electrode and nickel hydroxide
as the positive. The discharge reactions are:
Location
Half Reaction
Voltage
Zn + 2OH —> Zn(OH)2+ 2e
1.24 V
Cathode 2NiOOH + 2H2O —> 2Ni(OH)2 + 2OH-
0.49 V
Anode
Overall
-
2NiOOH + Zn + 2H2O —> 2Ni(OH)2 + Zn(OH)2 1.73
These cells run between 1.55 and 1.65 V. Theoretical energy density is 334
Wh/kg, or about 1.3 kg of nickel and 0.7 kg of zinc per kilowatt-hour. The internal
resistance of nickel/zinc batteries is remarkably low, which makes this system
particularly attractive for high charge and discharge rates
Practical specific energy is around 60 Wh/kg. The technical problems that have
plagued these batteries so far are dissolution of the zinc in the electrolyte, and
uneven redepositing of the zinc during charging. Progress in these batteries has
been mostly in the improvement of the zinc electrode. The charging is tricky
because the termination voltage is a strong function of temperature.
Iron Nickel Cells
Anode: Iron
Cathode: Nickel oxyhydroxide
Electrolyte: Potassium hydroxide
Applications:
This battery was introduced by Thomas Edison. It is a very robust battery: it can
withstand overcharge, overdischarge, and remaining discharged for long periods
of time without damage. It is good for high depths of discharge and can have
very long life even if so treated. It has low energy density, a high self-discharge
rate, and evolves hydrogen during both charge and discharge. It is often used in
backup situations where it can be continuously charged and can last for 20
years.
The chemistry involves the movement of oxygen from one electrode to the other:
3Fe + 8NiOOH + 4H2O=8 Ni(OH)2 +Fe3O4.
Half Reaction
Voltage
Fe + 2OH —> Fe(OH)2 +2e
-
-
3Fe(OH)2 + 2OH- —> Fe3O4 + 4H2O + 2eThe open circuit voltage of this system is 1.4 V, and the discharge voltage is
about 1.2 V. The electrolyte is 30% KOH solution, with some additives.
The ability of this system to survive frequent cycling is due to the low solubility of
the reactants in the electrolyte. The formation of metallic iron on charge is slow
because of the low solubility of the Fe3O4, which is good and bad. It is good
because the slow formation of iron crystals preserves the electrode morphology.
It is bad because it limits the high rate performance: these cells take a charge
slowly, and give it up slowly.
Iron Air Cells
The Iron/Air is another of the air-electrode batteries. The electrochemistry is as
follows:
Half Reaction
Voltage
O2 + 2Fe +2H2O=2Fe(OH)2
O2 +2H2O +2e=H2O2 +2(OH)
These batteries require a high degree of support, since the CO 2 must be taken
out of the air in order to prevent potassium carbonates forming in the KOH
electrolyte. They have been built in large backup systems. The air electrode
consists of a catalyst on a support. For example a carbon particle substrate held
together with Teflon, coated with a silver complex catalyst. Support is provided
by a silver-plated nickel screen.
Iron Silver Cells
These have a very high energy density, and a good cycle life. It is an alkaline
battery with a KOH electrolyte, and the working materials are silver oxide and
metallic iron. The high cost of these batteries have long been a problem, but an
ounce of silver in a cell phone battery would probably cost less than an ounce of
the rare earths now used in some NiMH batteries.
Redox (Liquid Electrode) Cells
These consist of a semipermeable membrane having different liquids on either
side. The membrane permits ion flow but prevents mixing of the liquids. Electrical
contact is made through inert conductors in the liquids. As the ions flow across
the membrane an electric current is induced in the conductors. These cells and
batteries have two ways of recharging. The first is the traditional way of running
current backwards. The other is replacing the liquids, which can be recharged in
another cell. A small cell can also be used to charge a great quantity of liquid,
which is stored outside the cells. This is an interesting way to store energy for
alternative energy sources that are unreliable, such as solar, wind, and tide.
These batteries have low volumetric efficiency, but are reliable and very long
lived.
Electrochemical systems that can be used are FeCl3 (cathode) and TiCl3 or CrCl2
(anode).
Vanadium redox cells: A particularly interesting cell uses vanadium oxides of different
oxidation states as the anode and cathode. These solutions will not be spoiled if the
membrane leaks, since the mixture can be charged as either reducing or oxidizing
components.
Supercapacitors
Unlike batteries, which store energy chemically, capacitors store energy as an
electrostatic field. Typically, a battery is known for storing a lot of energy and little
power; a capacitor can provide large amounts of power, but low amounts of
energy. A capacitor is made of two conducting plates and an insulator called the
dielectric, which conducts ionically, but not electrically. In a capacitor,
Ecap = qV = ½CV2
where the capacitance, C, is directly proportional to the surface area of the plates
and inversely proportional to the distance between them.
So in other words, as the plate surface area increases and the distance between
the plates decreases, the energy you can store in a capacitor increases. Normal
every-day capacitors have capacity on the orders of millifarads per cubic foot.
Aluminum electrolytics are about a farad per cubic foot. But for useful energy
storage we need farads per cubic inch. That is where supercapacitors come in.
First let's see how clever we can get to obtain a big surface area in a small
volume. Imagine a polymer foam cleaning sponge. It has a tremendous amount
of surface area in a small area because of all the crenulations (OK, nooks and
crannies). Now, put it in a furnace, excluding the oxygen and bake it until only the
carbon is left. You now have a conductive carbon surface with an incredible
surface area in a small volume.
But to get a high capacitance there has to be two plates. You can't just go in
there and create complimentary surface as the other electrode—or can you?
Yes, just fill it with a conductive liquid (e.g., an aqueous acid or salt solution). The
last thing you need is an ultra-thin insulator on the carbon. Ultra thin to get high
capacitance, and insulator so the carbon and the liquid don't short out. This is
also easy, you can electrochemically deposit an insulator on the carbon surface
(or electrochemically deposit something that could be turned into an insulator
upon baking).
Now attach one electrode to the carbon, one to the liquid, and you can have a
capacitor that can have Farads of capacitance per cubic inch. Very nice.
Most practical supercapacitors have low voltage (2 to 5 V—remember that
insulator is ultra-thin and so can break down at low voltages), which is a problem
for energy storage, since the stored energy is proportional to the square of the
voltage. Also, conduction through an ionic liquid is slow, so these capacitors
cannot be discharged quickly compared with standard capacitors, but can be
discharged very quickly compared to batteries!
Typical numbers for capacitors and batteries are given below:
device
batteries
volumetric
power
number of
discharge
energy
density charge/discharge time
density
W/L
cycles
s
Wh/L
50-250
capacitors 0.05 - 5
150
1 - 103
105 - 108 105 - 106
> 1000
<1
Supercapacitors have several advantages over batteries: they can experience
virtually indefinite number of cycles (charging and discharging), they are
maintenance free, they work well in high-rate discharge, they recharge quickly,
and they have no negative environmental impact.
References and Other Helpful Sources
1. Berndt, D. Maintenance-Free Batteries. New York:: John Wiley & Sons, 1997.
2. Crompton, T. R. Battery Reference Book. London: Butterworth–Heinemann,
1990.
3. Linden, D. (Ed), Handbook of Batteries. Maidenhead: McGraw–Hill, 1995.
4. Linford, R. G. (Ed), Electrochemical Science and Technology of Polymers. New
York: Elsevier, 1990.
5. Ovshinsky, S. R., Fetcenko, M. A., and Ross, J. A. "A Nickel Metal Hydride
Battery for Electric Vehicles", Science 260: 1993, 176–81.
6. Rechargeable Batteries Applications Handbook. Stoneham: Butterworth–
Heinemann, 1992.
7. Wells, A. F. Structural Inorganic Chemistry. Oxford: Clarendon Press, 1975.
Standard Cell Sizes for Alkaline, NiCaD, NiMH Batteries:
Cell Size
AAAA
8.4
Diameter
Length
Weight
Alkaline
Weight
NiCad
Weight
NiMH
mm
mm
grams
grams
grams
40.2
6
10
10
4/3 AAAA 8.4
67
12-13
13
1/4 AAA
10.5
14
2.5-3.5
2.5-4
1/3 AAA
10.5
16
5.5
5.5
1/2 AAA
10.5
22
2/3 AAA
10.5
30
AAA36
10.5
36
11
4/5 AAA
10.5
37
11
AAA38
10.5
38
11
3/4 AAA
10.5
39.5
AAA42
10.5
42
AAA
5/4 AAA
10.5
10.5
44.5
50
L-AAA
10.5
4/3 AAA
7
6-8
12
8-9
12
12
12
10
14
13
15
50
13
14
10.5
67
17
18
5/3 AAA
10.5
67
19
19
LL-AAA
10.5
67
17
18
3/2 AAA
10.5
67
19
20
6/4 AAA
10.5
67
20
20
7/5 AAA
10.5
66.5
15
15
7/4 AAA
10.5
76
19
20-21
7/3 AAA
10.5
80
23
SL AAA
10.5
80
23
1/3 N
11.5
10.8
N
11.5
28
4/3 N
11.5
44.5
6.6
6
6
8-10
11
18
18
Cell Type
Diameter
mm
Length Alkaline weight
mm
NiCad
Weight
grams
NiMH
Weight
grams
grams
1/3 AA
14.2
17.5
6.5
7
1/2 AA
14.2
30
12
15
2/3 AA
14.2
28.7
13-15
13-16
4/5 AA
14.2
43
20
22
AA
14.2
AA flat top 14.2
50
48
21
21
27
27
5/4 AA
14.2
64.5
L-AA
14.2
65
29
30
4/3 AA
14.2
65.2
30
30
7/5 AA
14.2
70
29
39
1/3 A
17
21
1/2 A
17
25
17
21
2/3 A
17
28.5
18-20
20-23
4/5 A
17
43
26-31
32-35
A
17
50
32
40
4/3 A
17
67
50
55
L-A
17
67
48
53
7/5 A
17
70
44.8
56
Fat A
18
50
38
42
4/3 Fat A
18
67
56
60
L-Fat A
18
67
55
60
24
24
29
Cell Type
Diameter
mm
Length
Alkaline
Weight
mm
NiCad
Weight
grams
NiMH
Weight
grams
grams
1/2 SC
23
26
30
2/3 SC
23
28
25
28
4/5 SC
23
34
38
42
SC (sub C) 23
43
52
55
RR
23
42.2
50
5/4 Sub C
23
49.5
65-67
70
4/3 SC
23
50
60
66
L-SC
23
50
57
63
1/2 C
26
24
31
34
3/5 C
26
30
40
44
2/3 C
26
31
45
50
C
26
46
72
80
5/4 C
26
58
90
100
1/2 D
33
37
81-84
81
2/3 D
33
43.4
98-105
115
D
33
58
105-145
105-160
4/3 D
33
89
140-190
175 g
3/2 D
33
90.3
195-236
240 g
F
33
91.2
231
255 g
SF (super
F)
41.4
89.1
393
425 g
G
32
105
181
J
32
150
272
6
67
172
998
F3
Prismatic
5.6 x 16.5 x 22 mm
8g
F4
Prismatic
5.6 x 16.5 x 31.5
mm
11 g
F5
Prismatic
5.6 x 16.5 x 35.5
mm
12 g
F6
Prismatic
5.6 x 16.5 x 48 mm
18 g
F8
Prismatic
5.6 x 16.5 x 66 mm
25 g
65
135
Data from Powerstream Inc.