Batteries for vehicle propulsion - The next generation

May 1, 2007
Hybrid-electric vehicles represent the best solution that we must pursue with a sense of urgency. Even though they are not the silver bullet, hybrids have significant advantages.

Hybrid-electric vehicles (HEVs) represent the best solution that we must pursue with a sense of urgency to address these is-sues: a) the crossing of curves for cumulative oil supply and demand that may occur in the 2040-2050 timeframe driven by the global growth in the number of vehicles; b) the political turmoil that is having an impact on fuel prices and begs for a solution that provides flexibility in propulsion energy sources; and c) the burning of petroleum fuels that contribute about one-half of the total carbon dioxide (CO2) resulting from human activities and whose contribution is about equal to the net rate of the increase in atmospheric CO2.

Even though they are not the silver bullet, hybrids have significant advantages. For example, a full hybrid such as the Ford Escape, the Toyota Prius or the GM AHS2 can offer a fuel economy improvement of 100% in city driving without compromises in power or package, while qualifying to be the cleanest production vehicles. In addition, HEV technology is forward compatible, meaning, there is synergy between HEV technology and homogeneous charge compression ignition and fuel cell electric vehicle technology.

A critical component affecting the value equation for the end customer and the vehicle OEM is the battery. It may represent up to one-third of the incremental cost of hybridization or approximately 5% to 10% of the cost of the full vehicle for a strong hybrid and significantly higher proportions for a plug-in depending on pure electric range requirements. And, ensuring that the battery lasts for the vehicle's life is a critical element of the value equation.

Fortunately, lithium-ion battery technology can deliver the performance requirements established by the United States Advanced Battery Consortium, including increased life, and it is on a cost curve. With cell level energy density of 200 Wh/kg and power density of 3000 W/kg, it deliver improvement over any other battery, including nickel-metal hydride (Ni-MH), which is its nearest competitor, and its capability is at the theoretical limit achievable by Ni-MH, which is the battery used in all of the hybrids on the road today. On a $/kWh basis, lithium-ion is cost competitive with Ni-MH, has achieved a cost reduction of about 12x in the last 15 years and will continue to get better. Hybrids are compatible with many forms of fuel, including diesel, E85 and bio-diesel. This leads to the conclusion that lithium-ion batteries are poised to dominate the HEV market.

This does not mean that all lithium-ion batteries are equal. It does mean there are trade-offs and lithium-ion battery chemistry can and should be optimized. For example, Cobalt-based chemistries used in consumer electronics have good energy density but poor abuse tolerance, whereas manganese (spinel) or phosphate-based chemistries provide superior abuse tolerance but with a loss in energy density. A “mixed” cathode can be designed to maintain the strengths of the spinel chemistry (low/stable cost, high-power density and superior abuse tolerance) while overcoming calendar life limitations associated with spinel chemistries.

The good news with regard to mechanical and electrical abuse tolerance is that these issues have been successfully addressed, in some cases using external protection devices such as a current interrupt device. Certain chemistries, such as spinel or phosphate, that do not release oxygen as they get hot, have an inherent advantage as does the safety-reinforced separator (SRS) developed by LG Chem, CPI's parent company, that eliminates the need for external protection devices. Furthermore, it protects against internal shorts where external devices are ineffective.

Finally, for certain lithium-ion chemistries, a laminated package is the right future direction for thermal efficiency, manufacturability and cost resulting from their geometry, simplicity and reduced part count.


Prabhakar Patil is chief executive officer of Compact Power Inc. (CPI), the North American subsidiary of LG Chem (LGC), Korea. In this position, he has responsibility for the strategic direction, engineering and business development activities. Prior to joining CPI in 2005, Patil spent his professional career of 27 years at Ford Motor Company.

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