The low-cost room-temperature sodium-sulfur battery system is arousing extensive interest owing to its promise for large-scale applications. Although significant efforts have been made, resolving low sulfur reaction activity and severe polysulfide dissolution remains challenging. Here, a sulfur host comprised of atomic cobalt-decorated hollow carbon nanospheres is synthesized to enhance sulfur reactivity and to electrocatalytically reduce polysulfide into the final product, sodium sulfide. The constructed sulfur cathode delivers an initial reversible capacity of 1081 mA h g⁻¹ with 64.7% sulfur utilization rate; significantly, the cell retained a high reversible capacity of 508 mA h g⁻¹ at 100 mA g⁻¹ after 600 cycles. An excellent rate capability is achieved with an average capacity of 220.3 mA h g⁻¹ at the high current density of 5 A g⁻¹. Moreover, the electrocatalytic effects of atomic cobalt are clearly evidenced by operando Raman spectroscopy, synchrotron X-ray diffraction, and density functional theory.

Controversial results still exist about the activities of tetrahedral (Co²⁺) and octahedral (Co³⁺) sites in Co₃O₄ toward the OER. Theoretical and experimental data confirm that octahedral sites are responsible for the OER, using model catalysts.