ok thanks i found the answers
so means the polymer is better
lithium polymer cell
Lithium polymer batteries (Li-Poly or LiPo) are rechargeable
batteries which have technologically evolved from lithium ion
batteries. Ultimatively the lithium salt electrolyte is not held in
an organic solvent, like in the proven lithium ion design, but in a
solid polymer composite such as polyacrylonitrile. There are many
advantages of this design over the classic lithium ion design. The
solid polymer electrolyte is not flammable like the organic solvent
that the Li-Ion cell uses. Thus these batteries are less hazardous
if mistreated.
Cells sold today as polymer batteries are different from this
ultimate design. Different from lithium ion cylindrical or
prismatic cells which have a rigid metal case, polymer cells have a
flexible, foil-type (polymer laminate) case, but they still contain
organic solvent. The main difference between commercial polymer and
to lithium ion cells is that in the latter cells, the rigid case
presses the electrodes and the separator onto each other, whereas
in polymer cells this external pressure is not required because the
electrode sheets and the separator sheets are laminated onto each
other.
Since no metal battery cell casing is needed, the battery can be
lighter and it can be specifically shaped to fit the device it will
power. Because of the denser packaging without the holes between
cylindrical cells and the lack of metal casing, the energy density
of Li-Poly batteries is over 20% higher than that of a classical
Li-Ion battery and approximately three times better than NiCd and
NiMH batteries.
The voltage of a Li-Poly cell varies from about 2.7 V (discharged)
to about 4.23 V (fully charged), and Li-Poly cells have to be
protected from overcharge by limiting the applied voltage to no
more than 4.235 V per cell used in a series combination. During
discharge on load, the load has to be removed as soon as the
voltage drops below approximately 3.0 V per cell (used in a series
combination), or else the battery will subsequently no longer
accept a charge.
Early in its development, lithium polymer technology had problems
with internal resistance. Other challenges include longer charge
times and slower maximum discharge rates compared to more mature
technologies. Li-Po batteries typically require more than an hour
for a full charge. Recent design improvements have increased
maximum discharge currents from two times the cell capacity to 15
or even 20 times it. In March 2005 Toshiba announced a new design
offering a much faster (about 1-3 minutes) rate of charge. These
cells have yet to reach the market but should have a dramatic
effect on the power tool and electric vehicle industries, and a
major effect on consumer electronics; especially
electrically-powered model aeroplanes.
When compared to the lithium ion battery, Li-Poly had a greater
life cycle degradation rate. However, in recent years,
manufacturers are declaring upwards of 500 charge-discharge cycles
before the capacity drops to 80% (see Sanyo). Another variant of
Li-Poly cells, the "thin film rechargeable lithium battery" has
been shown to provide more than 10,000 cycles.
The lithium in a Li-Poly cell is hazardous and will react violently
with water. Cell disposal instructions usually suggest puncturing
the cell and immersing it in salt water for several hours.
