Rapid cycling of lithium-ion batteries (LIBs), being developed for electric vehicles, is highly desirable. Rapid charge is needed to meet customer demands of time-parity with today’s gasoline-powered cars; rapid discharge is needed to deliver the high performance expected of electric vehicles. The U.S. DOE has set a target charging time of 15 minutes (4C rate) as a near-term goal, further indicating that a 5-minute charge (12C rate) could be a long-term goal. During discharge, the batteries should sustain rates ≥4C and ≥10C for full discharge and pulse discharge, respectively.

At higher cycling rates, however, the capacity, cycle life, and thermal stability of these LIBs is known to degrade. The mechanisms responsible for this performance decline are being examined at Argonne National Laboratory as part of projects funded by the U.S. DOE. These mechanisms are believed to include the following: heterogeneous Li intercalation/extraction reactions in the active particles, non-uniform reactions across the porous electrode cross-sections, fracture of anode and cathode particles, and lithium metal plating on the anode. A fundamental understanding of these processes is being gained by the application of various diagnostic techniques, including electrochemistry, diffraction, microscopy, spectroscopy and tomography.

During the presentation, results from the study of cells, containing layered oxide cathodes and graphite anodes, subjected to rapid cycling will be highlighted. In particular, the use of a microprobe reference electrode to monitor the onset of Li plating conditions will be examined and methodologies to investigate the lithium concentration gradients that develop in the electrodes during rapid cycling will be discussed.