Friday, March 10, 2017
The safety, or lack of safety, in commercial lithium-ion batteries is well known as news stories continue to appear on sensational fires from technologies ranging from aircraft and cars to e-cigarettes and cell phones, particularly when the batteries are being charged. Lithium-ion batteries can present a challenging safety issue because the cathode (oxidizer) and anode (fuel) are spaced only microns apart in batteries having from several cm2 to m2 of area, and fire prevention requires that they remain separated. However as consumers demand higher energy and higher power products, the “energy in the can” only gets more compacted. At the Naval Research Laboratory, we have been exploring the chemical and materials science behind the safety of lithium-ion batteries toward the objective of having highly reliable, high energy/power batteries. One project was to explore the inherent stability of the oxides and phosphates in the battery cathodes. Numerous theories exist why ~3.3-V LiFePO4 cathodes are less fire prone than standard 3.7-V LiCoO2-ones. We used density functional theory and experimentation to probe these materials and other compounds such as LiCuO2 and Li2RuO3, and found that a key feature for cathode materials is oxygen stability during oxidation of their metal centers. For anode safety research, we used optical microscopy to study dendrite formation during battery charging at temperatures from -10 to 25 °C. Lithium dendrites form when the mass transport and/or kinetics of lithium reduction are limited. We found that for simple cells with Li-metal anodes, the most deleterious temperature for charging was near 2 to 5 °C, at which many “spiky” dendrites form and can short the cathode and anode together. Lastly, we have developed a simple single-point impedance method to determine when a cell has compromised electrodes and is no longer safe. These studies together show a path forward to making safe battery cells for the future.
Dr. Karen Swider-Lyons is the head of the Alternative Energy Section in the Chemistry Division at the U.S. Naval Research Laboratory where she leads research programs in energy materials and systems. Her present work focuses on the science and technology of hydrogen fuel cells, lithium ion battery safety, and electric microgrids. In 2010, she received the Dr. Delores M. Etter Top Scientist Award from the US Navy for developing the long-endurance Ion Tiger fuel cell powered unmanned air vehicle. Dr. Swider-Lyons has authored more than 80 papers in refereed journals and holds 14 patents. She earned her Ph.D. in 1992 in Materials Science and Engineering at the University of Pennsylvania for her work on solid oxide fuel cells and holds a B.S. in Chemistry from Haverford College (1987).