The term “lithium-ion” refers not to a single electrochemical couple but to a wide array of different chemistries, all of which are characterized by the transfer of lithium ions between the electrodes during the charge and discharge reactions. Li-ion cells do not contain metallic lithium; rather, the ions are inserted into the structure of other materials, such as lithiated metal oxides or phosphates in the positive electrode (cathode) and carbon (typically graphite) or lithium titanate in the negative (anode).

Technologies with lithiated metal oxide positives and carbon negatives have high cell voltages (typically 3.6 V to 3.7 V) and correspondingly high energy density. These technologies have widely differing life and safety characteristics. Cells with positive materials based on lithium iron phosphate are inherently safer than their metal oxide/carbon counterparts but the voltage is lower (around 3.2 V), as is the energy density. Designs with lithiated metal oxide positives and lithium titanate negatives have the lowest voltage (around 2.5 V) and low energy density but have much higher power capability and safety advantages.

                                                                                                                                                                                                                                                    Above is from energystorage.org.