Three-dimensional CNT/graphene–sulfur hybrid sponges with high sulfur loading as superior-capacity cathodes for lithium–sulfur batteries

Abstract

A facile method is presented to synthesize three-dimensional carbon nanotube/graphene–sulfur (3DCGS) sponge with a high sulfur loading of 80.1%. In the well-designed 3D architecture, the two-dimensional graphene nanosheets function as the 3D porous backbone and the one-dimensional (1D) highly conductive carbon nanotubes (CNTs) can not only significantly enhance the conductivity, but also effectively tune the mesoporous structure. Compared to the three-dimensional graphene–sulfur (3DGS) sponge without CNTs, the conductivity of 3DCGS is enhanced by 324.7%; most importantly, compared to the monomodal mesopores (with a size of 3.5 nm) formed in the 3DG, bimodal mesopores (with sizes of 3.5 and 32.1 nm) were formed in 3DCG; the bimodal mesopores, especially the newly formed 32.1 nm mesopores, provide abundant electrochemical nanoreactors, accommodate plenty of sulfur and polysulfides, and facilitate charge transportation and electrolyte penetration. The significantly enhanced conductivity and the unique bimodal-mesopore structure in 3DCGS result in its superior electrochemical performance. The reversible discharge capacity for sulfur is 1217 mA h g⁻¹; the corresponding capacity for the whole electrode (including the 3DCGS, the conductive additive and the binder) is 877.4 mA h g_e⁻¹, which is the highest ever reported. In addition, the capacity decay is as low as 0.08% per cycle, and the high-rate capacity up to 4C is as large as 653.4 mA h g⁻¹. The 3DCGS sponge with high sulfur loading is promising as a superior-capacity cathode for lithium–sulfur batteries.