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

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 Na_2‐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 Na_2‐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.