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Transitions to Alternative Transportation Technologies—Plug-In Hybrid Electric Vehicles
FIGURE 2.1 Plug-in hybrid electric vehicle concepts. SOURCE: Toyota.
Thus, the PHEV-10 requires 2.0 kWh of battery energy (actually used) to drive its 10-mile AER. The PHEV-40 draws 8 kWh of battery-stored energy to meet its 40-mile AER in charge-depletion mode before the engine starts and begins supplying power to operate the vehicle in charge-sustaining mode.4 For the 50 percent SOC assumed in this report for the first generation of vehicles, the nameplate capacities are 4 kWh for the PHEV-10 and 16 kWh for the PHEV-40.
LITHIUM-ION BATTERY CELL CHEMISTRIES
For PHEVs to be widely accepted by consumers, batteries must be significantly cheaper than they are now, durable enough to have a long life, and safe. In addition, they will have to meet performance goals, which will require
High power density to deliver the current needed for demanding driving conditions;
High energy density for storing the needed energy for an extended all-electric range; and
Wide range of SOC while maintaining a long cycle life.
Li-ion batteries currently are the only serious option for PHEVs. They are smaller and lighter than other batteries, and they promise to withstand multiple large SOC swings while maintaining their performance. They have more than twice the energy density and about three times the power density of the nickel-metal-hydride (NiMH) batteries used in current HEVs, and four times the energy density of the lead-acid batteries used in most vehicles today.
What no Li-ion battery can do—yet—is simultaneously deliver both high power density and high energy density at a reasonable cost. To meet this challenge, several promising Li-ion chemistries are being vigorously pursued by companies, research institutions, and governments. The technology is advancing rapidly, but there is no guarantee that any Li-ion battery will be developed that meets all goals for vehicle use. Table 2.1 compares the attributes of four of the more promising Li-ion battery chemistries.
Li-ion battery manufacturing technology is essentially the same for all battery chemistries. Typically the electrodes of Li-ion batteries are coated on metal foils, usually copper foil for the negative electrode and aluminum foil for the positive electrode, separated by an electrolyte (Nelson et al., 2009). Typical electrolytes are derived from solutions of LiPF6 salt in a solvent blend of ethylene carbonate and various linear carbonates, such as dimethyl carbonate (Tikhonov and Koch, 2009; Zhang et al., 2002).
The batteries for these two vehicles are not identical because they are optimized for different conditions. For example, the PHEV-10 is likely to operate more in a charge, sustaining mode at minimum SOC than the PHEV-40.