Balancing oxygen evolution reaction and oxygen reduction reaction processes in Li-O-2 batteries through tuning the bond distances of RuO2

Lithium-oxygen (Li-O-2) batteries are promising next-generation energy storage devices due to their ultrahigh theoretical specific energy. Oxide electrocatalysts such as RuO2 are often used to improve the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) processes in Li-O-2 batteries, but the relation between the structural parameters and their catalytic properties is unclear so far. In this work, we synthesize RuO2 nano-particles at different annealing temperatures and investigate the influence of the Ru-O bond distances of RuO2 on the OER and ORR processes in Li-O-2 batteries. The results of X-ray absorption fine structure show that the bond distances decrease with the increase of the annealing temperature. The RuO2 with longer Ru-O bond distances renders a lower charge plateau to Li-O-2 batteries, showing a better OER performance, while that with shorter bond distances gives a larger discharge capacity and is more friendly to the ORR process. The cycle life of Li-O-2 batteries with a carbon nanotube-RuO2 composite cathode is improved to 287 cycles (1148 h) under an extreme testing condition (the voltage range is 2.3-4.2 V), when the Ru-O bond distances are optimized. In addition, the influence of the Ru-O bond distances on the catalytic properties of RuO2 is analyzed by density functional theory calculations. The findings in this work provide a guidance to enhance the ORR and OER efficiency of RuO2 and other oxide catalysts.