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- x}O₂ 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- x}O₂ 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.