Thursday, 5 October 2017: 10:10
National Harbor 8 (Gaylord National Resort and Convention Center)
Energy density of current Li-ion cells still need improvement for wide-spreading sales of electric vehicles. One can increase energy density of cells based on state-of-the-art active materials, such as graphite, by using thicker and/or lower-porosity electrodes, which contain a higher active-material loading. A downside of this approach is that in addition to enhancing the cell energy density, these thick/dense porous electrodes suffer lower cell power capability because of higher electrolyte transport limitations across the electrode. Large concentration and potential gradients develop across the electrode, leading to significant polarization increase and the possible occurrence of side reactions, such as Li plating. Active particle shape may also contribute to electrolyte transport limitations; Anisotropic particles may stack in a preferred direction that likely impede ionic transport. When studying high-energy-density porous electrodes, it is then essential to correlate the electrochemical performance with design parameters. Physics-based models, such as the Newman porous electrode model , prove effective to attempt such a correlation.
In this work, industry-grade graphite porous electrodes with different thicknesses and porosities are studied by a variety of electrochemical techniques such as rate-capability tests, pulse tests and electrochemical impedance spectroscopy. Electrode tortuosities are estimated with a model analysis of impedance diagrams of symmetric graphite/graphite cells. Tortuosity values lie well above what is predicted by the Bruggeman relation possibly because the graphite particles have a platelet-like shape that promotes a stacking perpendicular to ionic pathways. Cycling and rate performance as well as electrochemical impedance of several high-loading-electrode designs are compared.
 T.F. Fuller, M. Doyle, J. Newman, Simulation and Optimization of the Dual Lithium Ion Insertion Cell, J. Electrochem. Soc. 141 (1994).