Mesoporous Carbon-Sulfur Composite Microspheres with Multiscale Morphology Control to Achieve High Areal Capacity for Lithium-Sulfur Battery Cathodes

Lithium-sulfur (Li-S) batteries are widely regarded as the next-generation energy storage technology, due to their high potential in energy density and a low projected cost. Practical applications of such batteries are plagued with radical challenges that mainly stem from two factors. First, both sulfur and its final lithiation product, Li2S, are insulating. Thus, the reaction kinetics is slow and often leads to a low attainable capacity. Second, active material dissolve in the electrolyte in the form of lithium polysulfides. This dissolution causes a series of problems, including loss of cathode materials by diffusion away from the cathode, collapse of the cathode structure, and the polysulfide shuttle effect. As a result, Li-S batteries commonly show fast capacity decay and low coulombic efficiency.

Herein, we report the exceptional battery performance of porous carbon-sulfur composite microspheres, PSC-S, that benefited from simultaneous control of nanostructure and microscale particle morphology. The porous carbon microspheres (PSC) was synthesized as the sulfur host,by combining emulsion polymerization and evaporation-induced self-assembly (EISA) process, with a good control of both pore structure and microscale particle morphology. Such material not only demonstrated superioty in attainable capacity, but also in several other critical parameters for the development of high-energy Li-S batteries. PSC-S composite boasts a high tap density of 1.08 g/ml and contains 75 wt.% sulfur embedded into the mesopores of its carbon framework. At a high sulfur loading of 5 mg-S/cm² and 60wt.% sulfur content in the electrode, this material delivers both high gravimetric and high volumetric capacity with excellent retention. Carbon nanotubes (CNTs) can also be readily incorporated into the microspherical structure in the synthesis process to form a porous spherical carbon-carbon nanotube-sulfur composite (PSC-CNT-S). CNT incorporation effectively raises the conductivity of the composite, and thus significantly improves its high-rate performance. At a high current density of 2.8 mA/cm², an initial capacity of 1100 mAh/g and a reversible capacity of 700 mAh/g after 200 cycles were achieved in PSC-CNT-S. The resultant areal capacity of over 3.5 mAh/cm² is much higher than in previous reports of Li-S cathodes fabricated using coating techniques.