Research on new cathodes for lithium-ion batteries has long been directed towards crystalline metal oxide-based materials, with charge stored by lithium insertion into the material matrix. Research in the Energy Frontier Research Center is pursuing an alternate approach to battery cathodes based on the reaction of lithium with naturally abundant, light-weight, and amorphous organic materials. Chemical conversion reactions can in principle be limited by the rate of solution-phase lithium diffusion, which is 1000-10000 times faster than the solid-state diffusion required for traditional lithium insertion compounds. As a result, organic materials have promise for high-rate battery applications.
Achieving the full possibilities of organic materials requires the diligent application of chemical understanding and electrochemical experimentation. For example, the well-known organosulfur compound DMcT exhibits very slow reaction rates at a carbon electrode but , as seen in figure 1, is dramatically accelerated by introducing a conducting polymer (PEDOT). The rates shown in figure 1 correspond to complete battery charging and discharging in ~3 minutes! Additional compounds have been identified with quantum chemical computations and prepared by the techniques of organic chemistry synthesis, demonstrating the extraordinary versatility of these materials and capability to systematically tune battery voltage.
This Energy Frontiers Research Center also brings together an exceptional suite of characterization resources to better understand the chemical processes occurring in operational batteries, including in-situ x-ray techniques at the Cornell High Energy Synchrotron Source (CHESS). X-ray radiation can be tuned to the right energy to probe structural changes, through x-ray diffraction (XRD), and/or to detect chemical changes in specific elements, through x-ray absorption spectroscopy (XAS). Hard (energetic) x-rays have enough energy to probe these changes while the battery is operating, allowing direct correlation between electrochemical, chemical, and structural responses. For example, the diffraction data in figure 2 shows clear changes during charging and discharging that can be understood by changes in the material crystal structure.