Material engineering for Li-ion capacitors and Li-ion batteries
Ajayan, Pulickel M
Doctor of Philosophy
Electrochemical energy storage devices are fundamental driving force behind personal and industrial electronics. Li-ion batteries became the most prevalent rechargeable energy storage technology in market because of a high energy density. However, a power density (especially charging) of Li-ion batteries is not satisfactory for certain applications. In this regard, supercapacitors serve as a complementary role. To combine the advantages of Li-ion batteries and supercapacitors and bridge the technological gap, Li-ion capacitors (LICs) are invented. A typical Li-ion capacitor consists of a battery-type anode and supercapacitor-type cathode in Li-ion containing carbonate-based electrolytes. The major challenge of LICs arises from such disparity in charge-storage mechanism and kinetic. The present work addresses the issue by engineering electrode and electrolyte materials. 1) Two-dimensional material (vanadium disulfide anode and nitrogen-doped reduced graphene oxide cathode) are developed to combat the low power density of battery-type electrodes and the low energy density of supercapacitor-type electrodes. 2) We demonstrated that the energy and power densities achievable by LICs are largely influenced (and perhaps determined) by the anion adsorption at the positive electrodes, and by the ion transport within the electrolytes. Another challenge of the current Li-ion battery technology is an environmental and sustainability aspects because of a use of toxic and scarce transitional metals. Electroactive organic molecule-based cathodes which can reversibly store Li-ions are environmentally benign alternatives. Here, we assessed electrochemical performance of a plant-based organic molecule (lawsone) and showed that its oligomer structure stabilizes the molecules, which led to an improvement in a capacity retention over repeated cyclings. Next, we exploited the light-harvesting and Li-storing capabilities of the organic molecules to demonstrate light charging capability of the molecule. This work sheds light on the unique capability of organic cathode materials and paves the way for the future development of environmentally friendly and light rechargeable Li-ion batteries.