TransForum Vol. 5, No. 2

Argonne's "Composite-Structure" Electrodes Promise to Enable the Switch to Lithium-Ion Batteries in Hybrids

High-resolution transmission electron micrograph (TEM) (in background) and schematic illustration of a composite “layered-spinel” lithium-manganese-oxide electrode structure.

Skyrocketing fuel prices have prompted automakers to begin adding fuel-saving hybrid electric vehicles (HEVs) to their product lines, thereby creating a burgeoning demand for the nickel-metal-hydride (NiMH) batteries these vehicles require. This, in turn, has prompted battery makers to look into ways of replacing the NiMH batteries with lithium-ion batteries. The reason is that "lithium-ion batteries would allow them to put more energy into the same size package for less money," says Gary Henriksen, manager of the Chemical Engineering Division's Battery Department at Argonne.

Despite their promise, current lithium-ion batteries are plagued by several problems of their own, including safety issues, rising cost, and limited calendar life. As it happens, all three of these problems stem largely from the current reliance on lithium cobalt oxide as an electrode material. The effort to address these problems has brought lithium battery makers from around the globe to Argonne's door, to learn about Argonne's newly patented manganese-based "composite-structure" family of electrode materials. The new materials, which can be tailored to various applications, would eliminate the need for lithium cobalt oxide electrodes, thus enabling rechargeable lithium-ion batteries that are much cheaper, safer, and more stable. Also, some versions of these new materials could significantly increase the energy capacity of current lithium batteries. This new family of cathode materials is already starting to replace conventional materials in lithium batteries for consumer electronics applications.

"The composite structures of the high-capacity, manganese-rich electrodes have a Li2MnO3 layered component that is structurally integrated with either a layered LiMO2 component or a spinel LiM2O4 component — the 'M' in either case is typically manganese or nickel," explains Michael Thackeray, a senior scientist and the group leader responsible for materials development in the Battery Department. The strategy being adopted is to use the layered component predominantly to provide increased energy capacity and the spinel component to provide high power. "We are trying to integrate these two types of structures to achieve the best of both worlds," Thackeray adds.

What constitutes the best of both worlds will depend on the type of hybrid being targeted. The HEVs that have already been commercialized vary in their degree of hybridization; that is, the Toyota Prius and Honda Insight require more onboard energy storage than the so-called "mild" hybrids, such as the Honda Civic. However, they all use rechargeable batteries primarily for leveling the load on the internal combustion engine (ICE) and for capturing energy associated with regenerative braking.

If fuel prices continue to rise, automakers may be led to consider introducing "plug-in" hybrids, which possess sufficient battery power and energy to provide a limited range capability while operating only on battery power, without the ICE. For longer trips, the ICE would be used in combination with the battery, much like the conventional HEV. Batteries in this type of HEV could be recharged from the electric power grid while the vehicles aren't in use, particularly during off-peak periods at night when energy demand is low. This would allow various types of electrical power generation, rather than gasoline, to be used as the "fuel" for the vehicles during local commutes. This type of HEV requires a battery not only with more power but also with considerably more energy. This is where Argonne's high-capacity cathode materials come in; they would provide maximum benefit in terms of allowing a greater range capability on battery power.

The design and selection of electrode and electrolyte materials that will continue to improve the performance of state-of-the-art HEVs will challenge Argonne's lithium battery researchers in the years to come.

Argonne's battery research is funded by the U.S. Department of Energy's Office of Basic Energy Sciences and FreedomCAR and Vehicle Technologies Program.

For more information, contact Michael Thackeray, phone: 630/252-9184.

November 3, 2005