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Transitions to Alternative Transportation Technologies—Plug-In Hybrid Electric Vehicles
TABLE 2.1 Characteristics of Li-Ion Batteries Involving Different Chemistries
Nickel Cobalt Aluminum Oxide/Graphite
Manganese Spinel/Lithium Titanium Oxide
Safety and abuse tolerance
Poor; safety concerns
Some battery developers
LG Chem Ltd.
Associated vehicle manufacturers
Daimler HEV buses
NOTE: Cathode chemistries are frequently referred to as involving a spinel crystal structure. Actually there are no pure spinel structures present in Li-ion batteries; spinel-like would be more accurate.
While the power density (W/kg) of the cell is fixed by the surface area of the electrode foil, the energy density (Wh/kg) can be varied over a limited, but significant, range simply by increasing or decreasing coating thickness. HEV batteries, which require high power more than high energy storage, have thin electrode coatings. By contrast, electric vehicle (EV) batteries require high energy density and have thicker electrode coatings. Research has yielded new concepts for better electrodes and electrolytes. For example, raising the cell voltage to 5 V would increase the battery’s energy density. Could this lead to a better PHEV battery? It is simply too early to tell.
In this report overall properties such as energy density, power density, and total energy available refer to the full range from 100 percent to 0 percent SOC. Energy and power density are intrinsic properties, and total available energy is the nameplate capacity of the cell or battery. For batteries in battery packs for vehicle operation, this report refers to the energy (kWh) actually used—that is, the nameplate capacity of all the cells in the pack multiplied by the allowable SOC.
LITHIUM-ION BATTERY PACKS
For PHEV applications, about 100 Li-ion cells are connected in series to provide the design voltage to operate the electrical propulsion motors. These cell groups are then installed in parallel, as needed, to provide the energy to drive the motor for the distance desired. Battery packs consist of these groups of cells, the supporting frame, electronic controls, and cooling systems to protect the cells. The current focus is on improving battery durability, safety, and cost competitiveness.
Auto manufacturers have indicated that they intend to offer an 8-year warranty in 49 states and a 10-year warranty in California on PHEV battery systems as part of the drive-train warranty. Current commercial Li-ion batteries typically last 3 to 4 years, which is a function of both the number of charge/discharge cycles and calendar life (Howell, 2009). Some degradation is inevitable; for the purposes of this report, about 20 percent over the warranty period is assumed. If the PHEV-40 is expected to still have its required 8 kWh (actually used) of energy needed for an AER of 40 miles with the same 50 percent SOC range in 10 years, it could be sized to provide 10 kWh (actually used) energy initially. The other option is to assume that the SOC range is increased over time to account for battery degradation, which could be adjusted every year when the vehicle is brought in for servicing. This is the approach that the committee chose for the estimations that follow. If degradation is not too large or does not accelerate with larger SOC range, this should be satisfactory, but until demonstrated it remains a concern.5
Figure 2.2 compares the SOC variation for PHEV and HEV batteries. In a PHEV, batteries must undergo multiple large SOC range cycles without significant degradation. A 10-year life would require the batteries to undergo at least 2,000 cycles and still stay within the prescribed performance range. At present, this requires limiting the SOC range. The right-hand figure shows the much narrower (and less demanding) SOC range of an HEV.
This study assumes that SOC varies at most between 30 and 80 percent, or 50 percent of the total charge. The 30 percent lower limit is near the minimum and serves to maintain power and energy during charge-sustaining mode. The upper limit allows charging from regenerative braking while preventing overcharging and the resultant rapid battery degradation. A 50 percent range in SOC does, however, come at a price: The battery must have a nameplate capacity twice as high as the amount of energy actually needed and delivered to meet performance targets. In other words, the PHEV-40 will need a nameplate battery rating of 16 kWh to supply 8 kWh of the energy actually used for its 40 miles of charge-
If after 10 years of operation the battery pack has lost 20 percent of its capacity (16 kWh down to 12.8 kWh for the PHEV-40), the SOC would have to be raised to 62.5 percent to maintain the required 8 kWh usable energy.