Cells sold today as<unk> <unk> es have a different design from the older<unk> -ion cells. Unlike<unk> -ion cylindrical, or prismatic cells, which have a rigid metal case,<unk>  cells have a flexible, foil-type<unk>  laminate) case, but they still contain organic solvent.  The main difference between commercial<unk>  and<unk> -ion cells is that in the latter the rigid case presses the electrodes and the separator onto each other, whereas in<unk>  cells this external pressure is not required because the electrode sheets and the separator sheets are laminated onto each other.
Since no metal<unk>  cell casing is needed, the<unk>  can be lighter and it can be specifically shaped to fit the device it will power.  Because of the denser packaging without intercell spacing between cylindrical cells and the lack of metal casing, the energy density of Li-poly<unk> es is over 20% higher than that of a classical Li-ion<unk>  and they store more energy than  nickel-cadmium (NiCd) and nickel metal hydride (NiMH)<unk> es of the same volume.
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.  Overcharging a Li-poly<unk>  will likely result in explosion and/or fire.  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<unk>  will subsequently no longer accept a full charge and may experience problems holding voltage under load.
Early in its development,<unk><unk>  technology had problems with internal resistance.  Other challenges include longer charge times and slower maximum discharge rates compared to more mature technologies.  Li-poly<unk> es typically require more than an hour for a full charge.  Recent design improvements have increased maximum discharge currents from two times to 15 or even 30 times the cell capacity (discharge rate in amps, cell capacity in amp-hours).  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 are expected to have a dramatic effect on the power tool and electric vehicle industries, and a major effect on consumer electronics.[citation needed]
When compared to the<unk> -ion<unk> , Li-poly has a greater life cycle degradation rate.  However, in recent years, manufacturers have been 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<unk><unk> ", has been shown to provide more than 10,000 cycles.[citation needed]
Lithium-ion polymer battery
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