Activated carbon with ultrahigh specific surface area synthesized from natural plant material for lithium–sulfur batteries

Abstract

Porous activated carbon with a ultrahigh specific surface area (3164 m² g⁻¹) and large pore volume (1.88 cm³ g⁻¹) was prepared from waste litchi shells with channel-like macropores via a KOH activation method. The macroporous structure of litchi shells is believed to be conducive to distribute the activation agent, which enables sufficient activation. The as-prepared activated carbon was developed as a conducting framework for lithium–sulfur battery cathode materials. The resulting activated carbon/sulfur composite cathode possesses a high specific capacity, good rate capability, and long-term cycling performance. At 200 mA g⁻¹ current density, the initial discharge capacity of the activated carbon/sulfur composite cathode with 60 wt% sulfur content is 1105 mA h g⁻¹. At a current density of 800 mA g⁻¹, the activated carbon/sulfur composite cathode shows 51% capacity retention over 800 cycles with a fade rate of 0.06% per cycle. The coulombic efficiency of the cell remains at approximately 95%. By adding LiNO₃ in the electrolyte, the activated carbon/sulfur composite electrode tested at 800 mA g⁻¹ shows a high coulombic efficiency (>99%). The activated carbon/sulfur composites exhibited similar capacity value and cycling trends with an increase in sulfur content from 60% to 68%. The good electrochemical performance can be attributed to the excellent structural parameters of the activated carbon. The ultrahigh specific surface area and large pore volume not only enhances the sulfur content but also ensures dispersion of elemental sulfur in the conducting framework, thereby improving sulfur utilization. The small nanopores of the activated carbon can effectively inhibit the diffusion of polysulfides during the charge/discharge process.