THE ROLE OF THE

Developing Technology 179
Positive
LiCoO 2
The general term for the process of moving from anode to cathode by the lithium
ion or insertion is intercalation. The reverse of this process—moving from cathode
to anode—is called extraction or deintercalation. Figure 12.3 shows how lithium
ions are transported to and from the cathode and anode. The cobalt in the positive
electrode is being oxidized from the Co +3 state to the Co +4 state during the charge
phase and reduced from Co +4 to Co +3 during the discharge phase. The open crystal
structure allows extraction of the lithium ions. As stated, the cathode has many possible
chemistries, including manganese dioxide. Table 12.4 shows the average voltages
of some possible cathode materials.
When discharge occurs, lithium ions from the anode migrate across the electrolyte
to the cathode. Migration of electrons across the circuit will balance the reaction.
The migration occurs in an organic solvent such as ether because these cells
contain no aqueous media. Lithium produces a violent reaction with water. The electrolyte
is lithium salts, typically LiPF 6 or LiClO 4. Unfortunately, decomposition of
the solvent occurs on the anode during charging. When decomposition does occur, a
solid electrolyte interphase (SEI) forms [20]. This SEI prevents decomposition of the
electrolyte after the second charge.
The mass of lithium ion batteries allows the automotive chemist and design engineers
to decrease the mass of the overall vehicle as well as increase the performance
of the vehicle. They are much lighter than other rechargeable battery designs.
Lithium ions are also a wise choice for automotive use because self-discharge is very
low (5% per month vs. 30% for nickel–metal hydride [NiMH] batteries).
OEM research around lithium ion technology is ongoing and many issues still
need to be overcome. Lithium ion batteries have a life span that is age dependent,
table 12.4
average voltage for cathode materials
material average voltage (v)
LiCoO 2
LiMnO 2
LiFePO 4
Charge
Li +
Li +
Discharge
FIgure 12.3 Lithium ion flow in lithium ion battery system.
3.7
4.0
3.3
Li 2FePO 4F 3.6
Negative
LiyC 6