High-power lithium-ion, a new area in portable power.
recently, applications for high current rate capabilities
were reserved for nickel-cadmium and nickel-metal-hydride
batteries. These applications include power tools and medical
equipment. Much has changed in the fields of lithium-based
batteries during the last few years and high current lithium-ion
now claim similar or higher load capabilities to nickel-based
system. These new-technology batteries are expected to have
a similar impact on high-power portable products as the introduction
of lithium-ion had on the consumer electronics market in the
||Most lithium-ion batteries
used for portable applications are cobalt-based. High
production volume enables the battery to be manufactured
at a relatively low cost. They consist of a cobalt oxide
positive electrode (cathode) and a graphic carbon in the
negative electrode (anode). One of the main advantages
of the cobalt-based battery is its high energy density.
This makes this chemistry attractive for cell phones,
laptops and cameras.
In spite of its great popularity, the cobalt-based lithium-ion
has some limitations. It is not very robust and cannot take
a high charge and discharge currents. Trying to force a rapid
charge or loading the battery with excess discharge current
would overheat the pack and its safety would be jeopardized.
The safety circuit of the cobalt-based battery is typically
limited to a charge and discharge rate of about 1C. This means
that a 2400mAh 18650 cell can only be charged and discharged
with a maximum current of 2.4A. Another drawback of the cobalt
system is the increase of the internal resistance that occurs
with cycling and aging. After 2-3 years of use, the pack often
becomes unserviceable due to a high voltage drop under load
that is caused by elevated internal resistance. This condition
cannot be reversed.
New cathode material opened the door for higher rate capability.
Moving away form cobalt also helped in lowering manufacturing
costs. In 1996, scientists succeeded in using lithium manganese
oxide as a cathode material. This substance forms a three-dimensional
spinel structure that provides improved ion flow on the electrode.
High ion flow reflects in a lower internal resistance and
hence higher loading capability. Unlike the cobalt-based lithium-ion,
the resistance stays low with cycling and aging. The battery
does age, however, and the overall service life is similar
to that of the cobalt system. A further advantage of spinel
is its inherent high stability. Spinel needs less in terms
of safety circuit compared to the cobalt system.
Low internal cell resistance is the key to high rate capability.
This characteristic benefits both in fast-charging and high-current
discharging. A spinel-based lithium-ion in an 18650 package
can, for example, be discharged at currents of 20-30A with
marginal heat buildup. One-second load pulses of twice the
specified current are permissible. At continuous high load
requirements, a heat build-up will occur and a cell temperature
cannot exceed 80°C. Beside power tools and medical instruments,
the spinel-based lithium-ion is a candidate for hybrid cars.
Manufacturing cost will need to be lowered and the service
life prolonged before this battery system can be used for
automotive propulsion applications.
The spinel battery has some disadvantages, however. One of
the largest drawbacks is the lower capacity compared to the
cobalt-based system. Spinel provides roughly 1200mAh in an
18650 package, about half that of the cobalt equivalent. In
spite of this, spinel provides an energy density that is about
50% higher than that of a nickel-based equivalent.
Types of lithium-ion batteries
Lithium-ion has not yet reached full maturity and the technology
is continually improving. The anode in today's cells is made
up of a graphite mixture and the cathode is a combination
of lithium and other choice metals. It should be noted that
all materials in a battery have a theoretical energy density.
With lithium-ion, the anode is well optimized and little improvements
can be gained in terms of design changes. The cathode, however,
shows promise for further enhancements. Battery research is
therefore focusing on the cathode material. Another part that
has potential is the electrolyte. The electrolyte serves as
a reaction medium between the anode and the cathode.
The battery industry is making incremental capacity gains
of 8-10% per year. This trend is expected to continue. This,
however, is a far cry from Moore's Law that specifies a doubling
of transistors on a chip every 18 to 24 months. Translating
this increase to a battery would mean a doubling of capacity
every two years. Instead of two years, lithium-ion has doubled
its energy capacity in 10 years.
Today's lithium-ion comes in many "flavours" and
the differences in the composition are mostly related to the
cathode material. Table 1 below summarizes the most commonly
used lithium-ion on the market today. For simplicity, we summarize
the chemistries into four groupings, which are Cobalt, Manganese,
NCM and Phosphate.
|Table 1: Most common types of lithium-ion batteries.
The cobalt-based lithium-ion appeared first in 1991, introduced
by Sony. This battery chemistry gained quick acceptance because
of its high energy density. Possibly due to lower energy density,
spinel-based lithium-ion had a slower start. When introduced
in 1996, the world demanded longer runtime above anything
else. With the need for high current rate on many portable
devices, spinel has now moved to the frontline and is in hot
demand. The requirements are so great that manufacturers producing
these batteries are unable to meet the demand. This is one
of the reasons why so little advertising is done to promote
this product. E-One Moli Energy (Canada) is a leading manufacturer
of the spinel lithium-ion in cylindrical form. They are specializing
in the 18650 and 26700 cell formats. Other major players of
spinel-based lithium-ion are Sanyo, Panasonic and Sony.
Sony is focusing on the nickel-cobalt manganese (NCM) version.
The cathode incorporates cobalt, nickel and manganese in the
crystal structure that forms a multi-metal oxide material
to which lithium is added. The manufacturer offers a range
of different products within this battery family, catering
to users that either needs high energy density or high load
capability. It should be noted that these two attributes could
not be combined in one and the same package; there is a compromise
between the two. Note that the NCM charges to 4.10V/cell,
100mV lower than cobalt and spinel. Charging this battery
chemistry to 4.20V/cell would provide higher capacities but
the cycle life would be cut short. Instead of the customary
800 cycles achieved in a laboratory environment, the cycle
count would be reduced to about 300.
The newest addition to the lithium-ion family is the A123
System in which nano-phosphate materials are added in the
cathode. Although the manufacturer has not officially announced
what metal is being used, it is widely believed to be iron.
They claim to have the highest energy density of a commercially
available lithium-ion battery. The cell can be continuously
discharged to 100% depth-of-discharge at 35C and endures discharge
pulses as high as 100C. The phosphate-based system has a nominal
voltage of about 3.25V/cell. The charge limit is 3.60V. This
is far lower than the customary 4.20V/cell of the cobalt-based
lithium-ion. Because of these lower voltages, the A123 System
will need be charged with a special charger. Due to the anticipated
strong demand, this cell is expected to be in short supply.
A123 Systems was founded in 2001, the company is privately
held and the major shareholders include Motorola, Qualcomm
Confusion with voltages
For the last 10 years or so, the nominal voltage of lithium-ion
was known to be 3.60V/cell. This was a rather handy figure
because it made up for three nickel-based batteries (1.2V/cell)
connected in series. Using the higher cell voltages for lithium-ion
reflects in better watt/hours readings on paper and poses
a marketing advantage, however, the equipment manufacturer
will continue assuming the cell to be 3.60V.
The nominal voltage of a lithium-ion battery is calculated
by taking a fully charged battery of about 4.20V, fully discharging
it to about 3.00V at a rate of 0.5C while measuring the average
voltage. Because of the lower internal resistance, the average
voltage of a spinel system will be higher than that of the
cobalt-based equivalent. Pure spinel has the lowest internal
resistance and the nominal cell voltage is 3.80V. The exception
again is the phosphate-based lithium-ion. This system deviates
the furthest from the conventional lithium-ion system.
Prolonged battery life through moderation
Batteries live longer if treated in a gentle manner. High
charge voltages, excessive charge rate and extreme load conditions
will have a negative effect and shorten the battery life.
This also applies to high current rate lithium-ion batteries.
The longevity is often a direct result of the environmental
stresses applied. The following guidelines suggest how to
prolong battery life.
§ The time at which the battery stays at 4.20/cell should
be as short as possible. Prolonged high voltage promotes corrosion,
especially at elevated temperatures. Spinel is less sensitive
to high voltage.
§ 3.92V/cell is the best upper voltage threshold for
cobalt-based lithium-ion. Charging batteries to this voltage
level has been shown to double cycle life. Lithium-ion systems
for defense applications make use of the lower voltage threshold.
The negative is reduced capacity.
§ The charge current of Li-ion should be moderate (0.5C
for cobalt-based lithium-ion). The lower charge current reduces
the time in which the cell resides at 4.20V. It should be
noted that a 0.5C charge only adds marginally to the charge
time over 1C because the topping charge will be shorter. A
high current charge tends to push the voltage up and forces
it into the voltage limit prematurely.
§ A depth of discharge to 80% or less poses less strain
on the battery than a full 100% discharge. It's better to
charge lithium-ion more often than letting it down too deeply.
One does not need to worry about memory as was the case with
Note: In respect to fast-charging and topping charge, the
charge behavior of lithium-ion is similar to lead acid. Here,
the voltage threshold of 2.35V/cell during regular charge
needs to be lowered to 2.27V/cell when the VRLA is on standby.
Keeping the voltage at the high threshold would contribute
to corrosion. A similar effect occurs with lithium-ion.
Not only does a lithium-ion battery live longer with a slower
charge rate, high discharge rates also contribute the extra
wear and tear. Figure 1 shows the cycle life as a function
of charge and discharge rates. Observe the good laboratory
performance if the battery is charged and discharged at 1C.
(A 0.5C charge and discharge would further improve this rating.)
||Figure 1: Longevity of lithium-ion as a
function of charge and discharge rates. A moderate charge
and discharge puts less stress on the battery, resulting
in a longer cycle life.
Battery experts agree that the longevity of lithium-ion is
shortened by other factors than charge and discharge rates
alone. Elevated temperature while the battery is fully charged
is one such element that shortens service life. Even though
incremental improvements can be achieved with careful use
of the battery, our environment and the services required
are not always conducive to achieve optimal battery life.
In this respect, the battery behaves much like us humans -
we cannot always live a life that is favorable of achieving
a maximum life span.
About the Author
Isidor Buchmann is the founder and CEO of Cadex Electronics
Inc., in Vancouver BC. He has studied the behavior of rechargeable
batteries in practical, everyday applications for two decades.
Award winning author of many articles and books on batteries,
Mr. Buchmann has delivered technical papers around the world.
Isidor Buchmann is the founder and CEO of Cadex Electronics
Inc., in Vancouver Canada. Founded in 1980, Cadex specializes
in the design and manufacturing of advanced battery charging
and testing instruments. For product information please visit