N. Brambilla, J. Cooley
Keywords: nanocomposite electrode
Summary:Energy storage electrodes are often limited in their electrochemical stability and electrical performance by polymer binders used in the active material. These binders also adhere the active material to the current collector. Binders are usually polymeric resins used in electrodes to mechanically hold active material powders together. Active materials may be activated carbons, in the case of EDLCs, or graphite and lithium oxides, in the case of li-ion batteries. The binders are a passive component in energy storage electrodes and they carry limitations in terms of thermal stability and chemical stability. Capacitance and ESR suffer because inactive binder material takes up space within the electrode material. Meanwhile lifetime at temperature and voltage can suffer as the binder material interacts with the internal electrochemical system of the energy storage device. Additionally, the electrode’s thermal stability can be limited by both the electrochemical activity and phase transitions of the binder material. For instance, commonly used polymer binders may melt at application temperatures in harsh environment end-use applications or harsh manufacturing processes like solder reflow. Performance envelopes of energy storage systems using conventional electrodes in particularly high power applications may be limited by the thermal stability of the binder as well. Internal heat dissipation in those applications can be a serious concern when combined with ambient temperatures, especially in space-constrained environments or in applications where thermal management is at a premium. Nanoramic has developed an alternate solution - a binderless electrode platform technology that effectively replaces polymer binders and primers with various forms and methods exploiting mechanical and electrical properties of carbon nanotubes(1). Carbon nanotubes provide an electrically conductive network that can host active material powders used in energy storage devices. Results have been demonstrated for both EDLC electrodes and Li-ion Cathodes. Nanoramic electrodes have significantly lower ESR, thanks to the highly conductive carbon nanotube network, while also retaining or improving specific capacitance. Lifetime at high temperature is also significantly improved, because of the increased thermal and chemical stability of the binderless electrodes. Eliminating thermal stability limitations in the electrode is at least one key thermal concern in the overall energy storage system design. With a significantly lower ohmic resistance and also an adaptable morphology, there is a secondary opportunity to co-design the binderless electrode with more auspicious electrolyte systems, e.g. those with an advantageous combination of thermal and electrochemical stability, and conductivity. These considerations can ultimately broaden available design spaces for energy storage systems with far-reaching implications for safety, cost and performance. Common wound cell format EDLC products have been demonstrated with a cell-level ESR reduction of about 50%. Direct mechanical tests of the electrode itself have demonstrated thermal stability to at least 200°C. Cell-level tests using the electrode in an 8 x 11 x 2.3mm SMD format have demonstrated suitable performance under repeated JDEC standard reflow profiles to 260°C. References: 1. Brambilla, N. et al., "Composite Electrode." International Pat. Pub. No. WO/2018/102652 published June 7, 2018.