Development of High-performance Cathodic Catalysts for Non-Aqueous Lithium-Oxygen Batteries

Abstract

Lithium-oxygen (Li-O2) batteries have attracted intensive attention in last decade, due to its high theoretical energy densities and environmental benignity that satisfy the need for large energy storage systems including electric vehicles. However, they are still in their infancy and several challenges remain to be addressed immediately. In addition to the degradation of anode and electrolyte, one of the biggest challenges is the structure and catalytic design for oxygen electrode to achieve high capacity and long cycle life. In this thesis, size-controlled polystyrene (PS) spheres were introduced to a polydopamine derived N-doped reduced graphene (N-rGO) to explore the impact of the pore size of carbon oxygen electrodes to the performance of Li-O2 batteries. The battery containing N-rGO with 170 nm pores revealed a high specific capacity of 16777 mA h g-1, which is one of the highest among the reported carbon-based Li-O2 batteries. Field emission scanning electron microscope (FESEM) of cathode morphologies before and after discharge/charge showed that the N-rGO with 170 nm pores could hold most discharge products at a cut-off capacity of 1000 mA h g-1 without deformation to achieve a long stable cycle life. Furthermore, cobalt sulfides with controlled phases being synthesized via thermal decomposition of Co(TU)4(NO3)2 were studied as the bi-functional catalysts towards both ORR and OER of Li-O2 batteries in order to improve specific capacity and cycling life. A dual-phase cobalt sulfide prepared at 900 °C (CoS-900) contains both Co9S8 and CoS exhibited excellent ORR and OER catalytic activities with a low overvoltage (1.25 V) for Li-O2 batteries. The designated CoS-900@NG cathode achieved large discharge capacity at 7410 mA h g-1 with 100% charge capacity recovery as well as a super long cycle life at 108 cycles for Li-O2 battery. The excellent Li-O2 batteries performance can be attributed to the generation of both crystalline and amorphous film-like Li2O2 that effectively improve ORR/OER kinetics of Li-O2 batteries. Finally, Co9S8 nanoparticles were anchored to N, S co-doped graphene to form leave-like Co9S8/N, S-GO composites through hydrothermal treatment. The composite was further optimized by adjusting cobalt sulfide precursor amount to achieve an improvement of battery performance. As a result, the Li-O2 battery with Co9S8/N, S-GO composite can achieve a 100% recoverable high discharge capacity at 4884 mA h g-1 and a stable cycle life. The thesis systematically explored the relationship between the structures of oxygen electrode and electrochemical performance of Li-O2 batteries, including surface structure, heteroatom doping and cobalt sulphide hybridization. The outcomes provide new perspectives for the future development of high-performance Li-O2 batteries by strategically designing ORR/OER catalysts.