Self-Assembling Hollow Carbon Nanobeads into Double-Shell Microspheres as a Hierarchical Sulfur Host for Sustainable Room-Temperature Sodium-Sulfur Batteries

We report the use of passion fruit-like double-carbon-shell porous carbon microspheres (PCMs) as the sulfur substrate in room-temperature sodium-sulfur batteries. The PCMs are covered by microsized carbon shells on the outside and consisted of carbon nanobeads with hollow structure inside, leading to a unique multidimensional scaling double-carbon-shell structure with high electronic conductivity and strengthened mechanical properties. Sulfur is filled inside the PCMs (PCMs-S) and protected by the unique double-carbon-shell, which means the subsequently generated intermediate sodium polysulfide species cannot be exposed to the electrolyte directly and well protected inside. In addition, the inner interconnected porous structure provides room for the volume expansion of sulfur during discharge processes. It is found that the PCMs-S with a 63.6% initial Coulombic efficiency contributed to the 290 mA h g^-1 at the current density of 100 mA g^-1 after 350 cycles. More importantly, PCMs-S exhibited good rate performance with a capacity of 113 and 56 mA h g^-1 at the current densities of 1000 and 2000 mA g^-1, respectively.