|
As a trade-off, spinel offers a slightly lower energy density,
suffers capacity loss at temperatures above 40°C and ages
quicker than cobalt. Figure 2-6 compares the advantages
and disadvantages of the two chemistries.
|
|
| |
Cobalt |
Manganese
(Spinel) |
|
|
| Energy
density (Wh/kg) |
140 1 |
120 1 |
| Safety |
On overcharge,
the cobalt electrode provides extra lithium, which can
form into metallic lithium, causing a potential safety
risk if not protected by a safety circuit. |
On overcharge,
the manganese electrode runs out of lithium causing the
cell only to get warm. Safety circuits can be eliminated
for small 1 and 2 cell packs. |
| Temperature |
Wide temperature range. Best
suited for operation at elevated temperature. |
Capacity loss above +40°C.
Not as durable at higher temperatures. |
| Aging |
Short-term
storage possible. Impedance increases with age. Newer
versions offer longer storage. |
Slightly
less than cobalt. Impedance changes little over the life
of the cell. Due to continuous improvements, storage time
is difficult to predict. |
| Life
Expectancy |
300 cycles, 50% capacity at
500 cycles. |
May be shorter than cobalt. |
| Cost |
Raw material
relatively high; protection circuit adds to costs. |
Raw material
30% lower than cobalt. Cost advantage on simplified protection
circuit. |
|
|
Figure 2-6: Comparison of cobalt
and manganese as positive electrodes.
Manganese is inherently safer and
more forgiving if abused but offers a slightly lower energy
density. Manganese suffers capacity loss at temperature above
40°C and ages quicker than cobalt.
Based on present generation 18650 cells. The energy density
tends to be lower for prismatic cells.
The choice of metals, chemicals and additives help balance
the critical trade-off between high energy density, long storage
time, extended cycle life and safety. High energy densities
can be achieved with relative ease. For example, adding more
nickel in lieu of cobalt increases the ampere/hours rating
and lowers the manufacturing cost but makes the cell less
safe. While a start-up company may focus on high energy density
to gain quick market acceptance, safety, cycle life and storage
capabilities may be compromised. Reputable manufacturers,
such as Sony, Panasonic, Sanyo, Moli Energy and Polystor place
high importance on safety. Regulatory authorities assure that
only safe batteries are sold to the public.
Li-ion cells cause less harm when disposed of than
lead or cadmium-based batteries. Among the Li-ion family,
the spinel is the friendliest in terms of disposal.
Despite its overall advantages, Li-ion also 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 maximum charge and discharge current is limited
and the cell temperature is monitored to prevent temperature
extremes. With these precautions in place, the possibility
of metallic lithium plating occurring due to overcharge is
virtually eliminated.
Aging is a concern with most Li-ion batteries. For
unknown reasons, battery manufacturers are silent about this
issue. Some capacity deterioration is noticeable after one
year, whether the battery is in use or not. Over two or perhaps
three years, the battery frequently fails. It should be mentioned
that other chemistries also have age-related degenerative
effects. This is especially true for the NiMH if exposed to
high ambient temperatures.
Storing the battery in a cool place slows down the aging
process of the Li-ion (and other chemistries). Manufacturers
recommend storage temperatures of 15°C (59°F). In addition,
the battery should only be partially charged when in storage.
Extended storage is not recommended for Li-ion batteries.
Instead, packs should be rotated. The buyer should be aware
of the manufacturing date when purchasing a replacement Li-ion
battery. Unfortunately, this information is often encoded
in an encrypted serial number and is only available to the
manufacturer.
Manufacturers are constantly improving the chemistry of the
Li-ion battery. Every six months, a new and enhanced
chemical combination is tried. With such rapid progress, it
becomes difficult to assess how well the revised battery ages
and how it performs after long-term storage.
Cost analysis — The most economical lithium-based
battery in terms of cost-to-energy ratio is a pack using the
cylindrical 18650 cell. This battery is somewhat bulky
but suitable for portable applications such as mobile computing.
If a slimmer pack is required (thinner than 18 mm), the
prismatic Li-ion cell is the best choice. There is little
or no gain in energy density per weight and size over the
18650, however the cost is more than double.
If an ultra-slim geometry is needed (less than 4 mm),
the best choice is Li-ion polymer. This is the most
expensive option in terms of energy cost. The Li-ion
polymer does not offer appreciable energy gains over conventional
Li-ion systems, nor does it match the durability of
the 18560 cell.
|
|
|
Advantages
and Limitations of Li-ion Batteries
|
|
|
|
Advantages
|
High energy density — potential for yet higher capacities.
Relatively low self-discharge — self-discharge is less
than half that of NiCd and NiMH.
Low Maintenance — no periodic discharge is needed;
no memory.
|
|
Limitations
|
Requires protection circuit — protection circuit limits
voltage and current. Battery is safe if not provoked.
Subject to aging, even if not in use — storing the
battery in a cool place and at 40 percent state-of-charge
reduces the aging effect.
Moderate discharge current.
Subject to transportation regulations — shipment of
larger quantities of Li-ion batteries 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 NiCd. Better manufacturing techniques and
replacement of rare metals with lower cost alternatives
will likely reduce the price.
Not fully mature — changes in metal and chemical combinations
affect battery test results, especially with some quick
test methods.
|
|
|
Figure 2-7:
Advantages and limitations of Li-ion batteries.
Caution: Li-ion batteries have
a high energy density. Exercise precaution when handling and
testing. Do not short circuit, overcharge, crush, drop, mutilate,
penetrate, apply reverse polarity, expose to high temperature
or disassemble. Only use the Li-ion battery with the designated
protection circuit. High case temperature resulting from abuse
of the cell could cause physical injury. The electrolyte is
highly flammable. Rupture may cause venting with flame.
|