Micrometer‐Sized RuO2 Catalysts Contributing to Formation of Amorphous Na‐Deficient Sodium Peroxide in Na–O2 Batteries

Abstract

The results obtained herein demonstrate that the oxygen electrode plays a critical role in determining the morphology and chemical composition of discharge products in Na–O2 batteries. Micrometer‐sized cubic NaO2, film‐like NaO2, and nano‐sized amorphous spherical Na2‐xO2 are characterized as the main discharge products on the surface of reduced graphite oxide (rGO), boron‐doped rGO (B‐rGO), and micrometer‐sized RuO2 catalyst‐coated B‐rGO (m‐RuO2‐B‐rGO) cathodes, respectively. The Na–O2 battery with m‐RuO2‐B‐rGO as the cathode exhibits a much longer cycle life than those with the other cathodes, maintaining an unchanged capacity (0.5 mAh cm‐2) after 100 cycles at a current density of 0.05 mA cm‐2. A good rate capability and deep discharge–charge energy efficiency are also obtained. The excellent electrochemical performance of the battery is attributed to the effect of the micrometer‐sized RuO2 catalyst. Owing to the high affinity of RuO2 for oxygen, the amorphous phase Na2‐xO2 discharge product, which has good electrical contact with the RuO2 particles, can decompose completely under 3.1 V without a sudden voltage jump. Meanwhile, the micrometer‐sized RuO2 catalysts also provide enough active sites and space for the reactions, and effectively minimize the occurrence of side reactions between discharge products and carbon defects.