Is lithium-ion the ideal battery?
For many years,
nickel-cadmium was the only suitable battery for portable applications
from wireless communications to mobile computing. In 1990, the
nickel-metal-hydride and lithium-ion emerged, offering higher
capacities. Both chemistries fought nose to nose, each claiming better
performance and smaller sizes. Today, lithium-ion has won the limelight
and has become the most talked-about battery. It's the fastest growing
and most promising battery chemistry of today. The lithium-ion battery
Pioneer work with the
lithium battery began in 1912 under G.N. Lewis but it was not until the
early 1970s when the first non-rechargeable lithium batteries became
commercially available. lithium is the lightest of all metals, has the
greatest electrochemical potential and provides the largest energy
density for weight.
Attempts to develop
rechargeable lithium batteries failed due to safety problems. Because of
the inherent instability of lithium metal, especially during charging,
research shifted to a non-metallic lithium battery using lithium ions.
Although slightly lower in energy density than lithium metal,
lithium-ion is safe, provided certain precautions are met when charging
and discharging. In 1991, the Sony Corporation commercialized the first
lithium-ion battery. Other manufacturers followed suit.
The energy density of
lithium-ion is typically twice that of the standard nickel-cadmium.
There is potential for higher energy densities. The load characteristics
are reasonably good and behave similarly to nickel-cadmium in terms of
discharge. The high cell voltage of 3.6 volts allows battery pack
designs with only one cell. Most of today's mobile phones run on a
single cell. A nickel-based pack would require three 1.2-volt cells
connected in series.
Lithium-ion is a low
maintenance battery, an advantage that most other chemistries cannot
claim. There is no memory and no scheduled cycling is required to
prolong the battery's life. In addition, the self-discharge is less than
half compared to nickel-cadmium, making lithium-ion well suited for
modern fuel gauge applications. lithium-ion cells cause little harm when
Despite its overall
advantages, lithium-ion has its drawbacks. It is fragile and requires a
protection circuit to maintain safe operation. Built into each pack, the
protection circuit limits the peak voltage of each cell during charge
and prevents the cell voltage from dropping too low on discharge. In
addition, the cell temperature is monitored to prevent temperature
extremes. The maximum charge and discharge current is limited to between
1C and 2C. With these precautions in place, the possibility of metallic
lithium plating occurring due to overcharge is virtually eliminated.
Aging is a concern with
most lithium-ion batteries and many manufacturers remain silent about
this issue. Some capacity deterioration is noticeable after one year,
whether the battery is in use or not. The battery frequently fails after
two or three years. It should be noted that other chemistries also have
age-related degenerative effects. This is especially true for
nickel-metal-hydride if exposed to high ambient temperatures.
constantly improving lithium-ion. New and enhanced chemical combinations
are introduced every six months or so. With such rapid progress, it is
difficult to assess how well the revised battery will age.
Storage in a cool place
slows the aging process of lithium-ion (and other chemistries).
Manufacturers recommend storage temperatures of 15ºC. In
addition, the battery should be partially charged during storage. The
manufacturer recommends a 40% charge.
The most economical
lithium-ion battery in terms of cost-to-energy ratio is the cylindrical
18650 (18 is the diameter and 650 the length in mm). This cell is used
for mobile computing and other applications that do not demand
ultra-thin geometry. If a slim pack is required, the prismatic
lithium-ion cell is the best choice. These cells come at a higher cost
in terms of stored energy.
High energy density - potential for yet higher capacities.
Does not need prolonged priming when new. One regular charge is all
Relatively low self-discharge - self-discharge is less than half that of
Low Maintenance - no periodic discharge is needed; there is no memory
Requires protection circuit to maintain voltage and current within safe
Subject to aging, even if not in use - storing the battery in a cool
place and at 40% charge reduces the aging effect.
Moderate discharge current - not suitable for heavy loads.
Transportation restrictions - shipment of larger quantities may be
subject to regulatory control. This restriction does not apply to
personal carry-on batteries.
Expensive to manufacture - about 40 percent higher in cost than
Not fully mature - metals and chemicals are changing on a continuing
basis. The lithium Polymer battery
differentiates itself from conventional battery systems in the type of
electrolyte used. The original design, dating back to the 1970s, uses a
dry solid polymer electrolyte. This electrolyte resembles a plastic-like
film that does not conduct electricity but allows ions exchange
(electrically charged atoms or groups of atoms). The polymer electrolyte
replaces the traditional porous separator, which is soaked with
The dry polymer design
offers simplifications with respect to fabrication, ruggedness, safety
and thin-profile geometry. With a cell thickness measuring as little as
one millimeter (0.039 inches), equipment designers are left to their own
imagination in terms of form, shape and size.
Unfortunately, the dry
lithium-polymer suffers from poor conductivity. The internal resistance
is too high and cannot deliver the current bursts needed to power modern
communication devices and spin up the hard drives of mobile computing
equipment. Heating the cell to 60ºC and higher increases the
conductivity, a requirement that is unsuitable for portable
To compromise, some gelled
electrolyte has been added. Most of the commercial lithium-polymer
batteries used today for mobile phones are a hybrid cells and contain
gelled electrolyte. The correct term for this system is
lithium-ion-polymer. This is the only functioning polymer battery for
portable use today.
With gelled electrolyte
added, what then is the difference between classic lithium-ion and
lithium-ion-polymer? Although the characteristics and performance of the
two systems are similar, the lithium-ion-polymer is unique in that solid
electrolyte replaces the porous separator. The gelled electrolyte is
simply added to enhance ion conductivity.
Lithium-ion-polymer has not
caught on as quickly as some analysts had expected. Its superiority to
other systems and low manufacturing costs has not been realized. No
improvements in capacity gains are achieved - in fact, the capacity is
slightly less than that of the standard lithium-ion battery.
lithium-ion-polymer finds its market niche in wafer-thin geometries,
such as batteries for credit cards and other such applications.
Very low profile - batteries resembling the profile of a credit card are
Flexible form factor - manufacturers are not bound by standard cell
formats. With high volume, any reasonable size can be produced
Lightweight - gelled electrolytes enable simplified packaging by
eliminating the metal shell.
Improved safety - more resistant to overcharge; less chance for
electrolyte leakage. Limitations
Lower energy density and decreased cycle count compared to lithium-ion.
Expensive to manufacture.
No standard sizes. Most cells are produced for high volume consumer
Higher cost-to-energy ratio than lithium-ion