Environmental Engineering Reference
In-Depth Information
The performance of Li-ion batteries can be improved by nanostructuring the
electrodes, to enlarge the effective surface area. For example, it is stated that both
higher power and higher storage capacity can be obtained by applying the active
anode and cathode materials in a very thin
film to copper nanorods anchored to
sheets of copper foil. Enlarged electrode surface area by nanostructuring can increase
the short-circuit current of a battery, assuming good electrical conduction to the
nanostructured areas.
10.6.3
NiMH
A nickel metal hydride battery, abbreviated NiMH, is a type of rechargeable battery,
with 1.2 V nominal voltage, similar to a nickel
-
cadmium battery, but has an anode of
hydrogen-absorbing alloy, instead of cadmium. The anode reaction occurring in a
NiMH battery is
e
$
OH
þ
H
2
O
þ
M
þ
MH
ð
10
:
4
Þ
The battery is charged in the forward direction of this equation and discharged in
the reverse direction. Nickel (II) hydroxide forms the cathode. The metal M in the
anode of a NiMH battery is typically an intermetallic compound. Several different
intermetallic compounds have been developed, which mainly fall into two classes.
The most common form of anode metal M is AB
5
, where A is a rare earth mixture of
lanthanum, cerium, neodymium, and praseodymium and B is nickel, cobalt,
manganese, and/or aluminum. Other batteries use higher capacity negative electro-
des based on AB
2
compounds, where A is titanium and/or vanadium and B is
zirconium or nickel, modi
ed with chromium, cobalt, iron, and/or manganese. Any
of these compounds M serves the same purpose, reversibly forming a mixture of
metal hydride compounds. When hydrogen ions are forced out of the potassium
hydroxide electrolyte solution by the charging voltage, it is essential that hydride
formation is more favorable than forming a gas, allowing a low pressure and volume
to be maintained. As the battery is discharged, these same ions are released to
participate in the reverse reaction.
NiMHbatteries have an alkaline electrolyte, usually potassiumhydroxide. ANiMH
battery can have two
-
three times the capacity of an equivalent size NiCd battery.
However, compared to the lithium ion battery, the volumetric energy density is lower.
The speci
c energy density for the NiMH battery is approximately 70Wh/kg. It is
reported that a NiMHPrius car battery contains 12 kg of lanthanum. An advantage of
the NiCd battery over the NiMH is typically higher short-circuit current.
Batteries used in fully electric vehicles EV (Nissan Leaf) and plug-in electric
vehicles PHEV (Chevy Volt) are in the capacity range 20
50 kWh. The batteries in
hybrid electric vehicles, HEV, like the Toyota Prius are generally in the range of
2 kWh.
The broader question of energy storage in the power grid can be discussed in terms
of many small batteries. The total sales of the Toyota Prius are about 1 million, half in
the United States. The storage capacity of 0.5million Prius cars, assuming 2 kWh per
-
