Recent progress in sulfur cathodes for application to lithium–sulfur batteries

Owing to the extensive use of fossil fuels for energy, environmental problems are becoming increasingly severe. Therefore, renewable clean energy sources must be urgently developed. As an environmentally friendly electrochemical energy-storage system, lithium-ion batteries (LIBs) are widely used in portable devices, electric vehicles, and medical equipment. However, owing to their high cost and low theoretical energy density, LIBs are far from meeting the current energy demand. Lithium–sulfur batteries (LSBs) (wherein lithium metal and sulfur are the anode and cathode, respectively) are one of the most valuable secondary batteries because of their high theoretical energy density (~2600 Wh·kg–1). However, the intrinsic conductivity of sulfur cathode materials is poor, and the lithium polysulfide formed during lithiation dissolves easily. Moreover, the volumetric expansion during charging and discharging adversely affects the LSB electrochemical performance, including the rate performance, cycle life, and coulombic efficiency. Therefore, to improve the LSB electrochemical performance, various sulfur composites have been prepared using carbon materials, metallic oxides, and conductive polymers, and various composite cathode materials recently developed for application to LSBs were reviewed. Finally, research directions were proposed for modifying LSB cathode materials.