High-Performance Rechargeable Batteries for Electric Vehicles
Developing an advanced battery for automotive applications is a difficult undertaking. Identifying the limitations and understanding the performances and lifetimes of batteries are required to guide scaling up and other activities. High cell potentials and demanding cycling requirements lead to chemical and mechanical instabilities—important issues that must be addressed. The recently reorganized Batteries for Advanced Transportation Technologies (BATT) Program addresses fundamental issues of chemistries and materials that confront all lithium battery candidates for Department of Energy electric vehicle and hybrid electric vehicle applications. The Program emphasizes the synthesis of components into battery cells with determination of failure modes, coupled with strong efforts in materials synthesis and evaluation, advanced diagnostics, and improved electrochemical models. The selected battery chemistries are monitored continuously with periodic substitution of more promising components.
Battery research with these lofty goals has been underway at Berkeley Lab for more than 20 years. The electrochemical programs in the Environmental Energy Technologies and Materials Sciences Divisions have traditionally focused on interfacial studies of ideal systems and materials invention. The research also characterizes very small cells, using measurements pertaining to one electrode, often in a thin film form. However, the two electrodes in a cell frequently affect the performance of each other, and materials and systems that look good in the ideal lab experiment sometimes do not translate into a practical battery.
For these reasons, EETD researchers have begun a new cell fabrication and testing task. The goal is to take new materials, developed at Berkeley Lab and elsewhere in the DOE-supported Electric Vehicle (EV) Battery program, and build them into a full cell that resembles something that could be built commercially for an electric vehicle. These efforts are not targeting the production of 40 kWh batteries required to run a standard sedan automobile. Rather the work focuses on single cells with two electrodes, and energies in the realm of 40 mWh.The first year of effort has been devoted to gathering the equipment required for lithium-ion cell production, developing electrode preparation and cell-assembly techniques, and testing protocols. The tools used include commercially produced lithium-ion cells and a battery cycler capable of cycling 64 cells simultaneously. Rechargeable lithium-ion batteries are available commercially, in sizes suitable for cell phones and portable computers. However, this technology costs more than 30 times that for a practical vehicular power source and has severe safety issues on the large scale. Materials in the cathode such as LixCoO2 need to be replaced with oxides or phosphates containing manganese or iron, and the designer graphites in the anode with materials derived from naturally occurring graphites. In addition, the liquid electrolyte should be replaced with a gel polymer electrolyte for improved safety. Cells with all these different components can be assembled and tested with consistent protocols in EETD laboratories.
A nearer term battery chemistry, being developed for the hybrid car (the Partnership for a New Generation of Vehicles (PNGV) program) has been already been assembled at LBNL. Figure 1 shows the cycling performance of this cell at 25 and 60 °C. These cells were disassembled and the electrodes were tested individually in a separate diagnostic cell. The voltage profiles for the cathode samples, tested against fresh lithium electrodes (Figure 2), show clearly that the source of the full cell capacity loss noted in Figure 1 for the cell cycled at 60 °C is due to the cathode.
Together with the diagnostic capabilities within EETD and the materials development efforts in the MSD arm of the Electrochemical Program, this cell fabrication and testing task should provide a real boost to the larger DOE effort to produce an all-electric car.
For more information, contact:
- Kathryn Striebel
- (510) 486-4385; fax (510) 486-7303
This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Transportation Technologies, Office of Advanced Automotive Technologies.