Combining and Charging multiple Lithium Ion Battery Cells (3.6 V or 3.7 Volts)
Well, actually I was also interested here.


Rated voltage might be rated different to make it serve more in expense of capacity.
LiIon batteries have lots of variants of chimestry - all are LiIon, but different voltages, prices, reliability.



LiCoO2    3.7 V   140 mA·h/g  0.518 kW·h/kg
LiMn2O4   4.0 V   100 mA·h/g  0.400 kW·h/kg
LiNiO2    3.5 V   180 mA·h/g  0.630 kW·h/kg
LiFePO4   3.3 V   150 mA·h/g  0.495 kW·h/kg
Li2FePO4F     3.6 V   115 mA·h/g  0.414 kW·h/kg
LiCo1/3Ni1/3Mn1/3O2   3.6 V   160 mA·h/g  0.576 kW·h/kg
Li(LiaNixMnyCoz)O2    4.2 V   220 mA·h/g  0.920 kW·h/kg



http://en.wikipedia.org/wiki/Lithium-ion_battery#Positive_electrodes