The desire to protect the environment is driving
consumer demands for electric vehicles that use
non-polluting propulsion systems. Nickel-metal
hydride and several versions of lithium-ion (Li-ion)
batteries have been used in the power management of electric
vehicles with mixed results.
Some power-management systems also have used ultracapacitors
to augment the performance of electric vehicles.
And though it has been around for about 10 years, the
lithium-iron-phosphate (LiFePO4) battery is getting the attention
of electric vehicle manufacturers (Fig. 1).
Lithium-based batteries offer one of the best energy-toweight
ratios and a short life cycle. Also, they don’t produce
any memory effect. Yet some Li-ion batteries can be dangerous
if they’re mistreated, and a lack of proper care can
reduce their lifespan.
Most lithium batteries previously used in electric vehicles
employed a lithium-cobalt-oxide (LiCoO2) cathode material.
Other lithium battery cathodes used either lithiummanganese
oxide (LiMn2O4) or lithium-nickel oxide
(LiNiO2). Anodes were always made of carbon. But these
lithium batteries have two key disadvantages—slow charge
and discharge rates.
The Pros and Cons of LiFePO4
The LiFePO4 cathode
battery improves the charge/discharge rate as well as the
ability to store energy. And because it is derived from Li-ion
technology, the LiFePO4 chemistry shares many of the advantages
and disadvantages of Li-ion chemistry. For example,
LiFePO4 cells can supply a higher discharge current.
Also, LiFePO4 is an intrinsically safer cathode material
than LiCoO2. The Fe-P-O bond is stronger than the Co-O
bond, so when the battery is abused (short-circuited, overheated,
etc.), the oxygen atoms are much harder to remove.
Breakdown can occur under extreme heating, generally
over 800°C. Yet LiFePO4 batteries don’t have the thermal
runaway that LiCoO2 batteries may exhibit. LiFePO4 batteries
also have the best safety characteristics, accommodating
up to 2000 charge/discharge cycles. But LiFePO4 technology
has a negative side, compared to other Li-ion technologies.
The minimum cell discharge voltage is 2.8 V; its working
voltage is 3.0 to 3.3 V; and the maximum charge voltage is
3.6 V. A conventional Li-ion battery charge voltage is 4.2 V.
Also, the LiFePO4 capacity/size ratio is lower than the Li-
CoO2 battery, so it requires further development to improve
this characteristic.
Still, LiFePO4 batteries have a potentially lower cost than
their lithium-based counterparts, particularly when they
are more widely used. They suit electric bikes, scooters, and
cars, as well as power tools, UPS, and solar energy systems.
Battery manufacturers around the world are currently working
to find a way to get the maximum storage performance
out of smaller and lighter LiFePO4 batteries.
Ultracaps
Electric vehicle batteries have a limited ability
to capture and regenerate energy, or provide bursts of high
power during short duration events, such as acceleration
and braking. This high-power limitation reduces the efficiency
of the electric drive system. Because most vehicles are
in a constant brake/acceleration state, the ability to capture
and regenerate braking energy can be important (Fig. 2).
Vehicle manufacturers require an electric power and
storage system that overcomes the limitations of the batteries
as well as those of the vehicles themselves. Therefore,
ultracapacitors provide a solution by using the regenerative
braking to store energy that could be applied for further
acceleration or for the basic energy needs of supplementary
electrical systems. The associated power-management system
controls ultracap operation.
Ultracaps offer performance usable down to
–40°C, while most batteries don’t operate reliably
below 0°C without heating. Also, ultracaps
have a long life cycle and usually run for the
duration of the lifetime of the machine where
they are installed, resulting in cost savings.
They have an efficiency of 85% to 95%,
compared with an average of 70% or lower for
most batteries. Environmentally friendly, they
are 70% recyclable and do not include heavy
metals. And, they can provide more than 10
times the power of batteries.
Ultracaps are electrochemical capacitors
with an unusually high energy density compared
with common capacitors—on the order of thousands of times
greater than a highcapacity
electrolytic
capacitor. A typical D-cell
sized electrolytic capacitor may
have a value measured in microfarads,
whereas the same size ultracap could
store several farads for an improvement
of about 10,000 times. Larger
commercial ultracaps have capacities
as high as 3000 farads.
A conventional capacitor stores
energy by removing charge carriers,
typically electrons, from one metal
plate and depositing them on another.
The total energy that’s stored in this
fashion is a combination of the number
of charges stored and the potential
between the plates.
The number of charges is essentially
a function of size and the material
properties of the plates, whereas the
potential between the plates is limited
by the dielectric breakdown between
the plates. Various materials can be
inserted between the plates to allow
higher voltages to be stored, leading to
higher energy densities for a given size.
In contrast with traditional capacitors,
ultracaps do not have a conventional
dielectric. Their structure
contains an electrical double layer. In a
double layer, the effective thickness of
the dielectric is exceedingly thin—on
the order of nanometers. Combined
with the very large surface area, that
thinness is responsible for their very
high capacitances in practical sizes.
In an electrical double layer, each
layer by itself is quite conductive. But
the physics at the interface where the
layers are effectively in contact means
that no significant current can flow
between the layers.
However, the double layer can withstand
only a low voltage. That means
that ultracaps that are rated for higher
voltages must be made of matched
series-connected individual ultracaps,
much like series-connected cells in
higher-voltage batteries.
Ultracaps improve storage density
through the use of a nanoporous
material in place of the conventional
insulating barrier, typically activated
charcoal. Activated charcoal is a powder
made up of extremely small and
rough particles. In bulk, they form a
low-density volume of particles with
holes between
them
that resembles a sponge.
The overall surface area of even a
thin layer of such a material is many
times greater than a traditional
material like aluminum, allowing many
more electrons to be stored in any
given volume. The downside is that the
charcoal replaces improved insulators
used in conventional devices. So in
general, ultracaps operate at potentials
of about 2 to 3 V.