Lithium-ion battery electrodes are composed of lightweight lithium and carbon. Lithium is a highly reactive element, meaning that a lot of energy can be stored in its atomic bonds. This translates to excellent energy density in a small, lightweight package. If you have a cell phone, iPod or almost any other portable electronic device, you've seen lithium-ion batteries in action.
Lithium-ion batteries are promising for pure electric and plug-in hybrid applications that require higher energy density to meet the higher demands of charge depleting operation (large swings in the battery state-of-charge). It is important to note that although lithium-ion batteries are less expensive in terms of materials, they are more expensive than nickel-metal batteries in terms of production costs. Lithium-ion batteries require higher quality-control, often including clean-room production facilities, which increases the overall cost of production.
Toyota began dedicated lithium-ion battery research and development in the mid 1990s. A lithium-ion stop/start battery was used in the Vitz, which went on sale in Japan in 2003. The battery powering the current Prius Plug-in Hybrid (PHV) demonstration program vehicle is the first lithium-ion drive-battery developed by Toyota and its joint venture battery production company, Panasonic Electric Vehicle Energy (PEVE). In early November 2009, PEVE began producing the first of more than 600 lithium batteries on a dedicated assembly line at its Teiho production facility in Japan.
This first-generation lithium battery has undergone more than three years of coordinated field testing in Japan, North America and Europe in a wide variety of climatic environments and driving conditions. Using approximately 150 conventional hybrids (mostly Prius), the field test vehicles logged well over a million combined miles. In the end, the battery was deemed both reliable and durable, confirming that it could indeed be used in conventional hybrid applications in the future, depending on further developments in cost reduction.
The battery will now be placed into service in the 600 Prius PHVs dedicated to Toyota's global demonstration program, which begins in December 2009. Operating in a more severe charge-depleting mode, the battery's overall performance in a broad range of vehicle use applications will be confirmed.