Carboxyl grafted sulfur-expanded graphite composites as cathodes for lithium-sulfur batteries

Abstract

Main challenging issues of sulfur as cathodes for lithium-sulfur (Li-S) batteries are poor conductivity of sulfur and dissolution of long-chain polysulfides (Li2Sx, x = 4 to 8) formed during the redox processes. Herein, we demonstrate an advantageous method for embedding sulfur into the voids of the expanded graphite (EG) layers, forming S-EG composite. A further modification of the surface of S-EG with 3-cyanpropyl-trichlorosilane (CPS) was attempted to functionalize EG with carboxyl groups to form the carboxyl-grafted sulfur/expanded graphite composites, S-(EG-COOH). Thus, in addition to improvement in conductivity of the composites via the conductive framework of EG, the negatively charged carboxyl groups on EG could generate an environment to confine the polysulfide anions formed during charge/discharge cycling. It is demonstrated that modification of the carboxy groups on EG leads to an alteration of the surface properties of the material, including hydrophilicity and electrolyte wettability. Importantly, notable improvement in the initial discharge capacity (1058 mA h g−1) and Coulombic efficiency (87.47%), and cyclability (capacity retention of 70% after 100 cycles) of sulfur indicates that carboxyl groups could relieve the dissolution of polysulfides and promote the charge transfer at the sulfur/electrolyte interface. High dispersion of sulfur in EG, good conductivity with the aid of EG, and fast Li diffusion are ascribed to be responsible for the superior electrochemical performance of S-(EG-COOH).