Materials Design toward High Performance Electrodes for Advanced Energy Storage Applications

Abstract

Rechargeable batteries, especially lithium ion batteries, have greatly transformed mobile electronic devices nowadays. Due to the ever-depletion of fossil fuel and the need to reduce CO2 emissions, the development of batteries needs to extend the success in small electronic devices to other fields such as electric vehicles and large-scale renewable energy storage. Li-ion batteries, however, even when fully developed, may not meet the requirements for future electric vehicles and grid-scale energy storage due to the inherent limitations related with intercalation chemistry. As such, alternative battery systems should be developed in order to meet these important future applications. This dissertation presents our successes in improving Li-O2 battery performance for electric vehicle application and integrating a redox flow battery into a photoelectrochemical cell for direct solar energy storage application. Li-O2 batteries have attracted much attention in recent years for electric vehicle application since it offers much higher gravimetric energy density than Li-ion ones. However, the development of this technology has been greatly hindered by the poor cycling performance. The key reason is the instability of carbon cathode under operation conditions. Our strategy is to protect the carbon cathode from reactive intermediates by a thin uniform layer grown by atomic layer depostion. The protected electrode significantly minimized parasitic reactions and enhanced cycling performance. Furthermore, the well-defined pore structures in our carbon electrode also enabled the fundamental studies of cathode reactions. Redox flow batteries (RFB), on the other hand, are well-suited for large-scale stationary energy storage in general, and for intermittent, renewable energy storage in particular. The efficient capture, storage and dispatch of renewable solar energy are major challenges to expand solar energy utilization. Solar rechargeable redox flow batteries (SRFBs) offer a highly promising solution by directly converting and storing solar energy in a RFB with the integration of a photoelectrochemical cell. One major challenge in this field is the low cell open-circuit potential, mainly due to the insufficient photovoltages of the photoelectrode systems. By combining two highly efficient photoelectrodes, Ta3N5 and Si (coated with GaN), we show that a high-voltage SRFB could be unassistedly photocharged and discharged with a high solar-to-chemical efficiency.