Synthesis and Characterization of a Composite Cathode Material (Li2S@rGO) for Li-S Batteries Made By in Situ Electrochemical Conversion of MoS2@rGO

Abstract

Lithium disulfide (Li2S) could be used as cathode material for high-energy-density lithium–sulfur (Li–S) batteries, having a large theoretical specific capacity (1166 mAh g–1). However, the material has low conductivity. Additionally, soluble lithium polysulfide species can be formed during cell operation in carbonate-based electrolytes resulting in a poor cycling stability. In this work, Li2S particles are wrapped in reduced graphene oxide (Li2S@rGO) to produce cathode materials for Li–S batteries in carbonate-based electrolytes. First GO is produced by a modified Hummers method. Then, rGO is reduced hydrothermally on MoS2 particles. The composite is then fully lithiated and irreversible decomposed at 0.01 V vs. Li/Li+ to produce a Li2S@rGO with a high Li2S (loading of ≈5 mg cm–2). The material is characterized by SEM, TEM, EDXS, N2 isotherm and XRD (before and after cell cycling). The material is then used as cathode with Li foil as counter electrode. The system is mounted as coin cells (CR2025) in an Ar filled glove box, filled with 1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC; 1:1 v/v). The cells are sealed and then cycled in air (constant temperature and humidity). The electrochemical properties are investigated using galvanostatoc charge/discharge cycles, cyclic voltammetry and electrochemical impedance spectroscopy measurements. The cathode material exhibits a high initial capacity: 1401 mAh g–1 (based on S weight) at 0.1 C. Besides, it shows a good capacity retention (411 mAh g–1 at 2 C after 150 cycles) and excellent Coulombic efficiency (≈99.5%). The material could be used in high-loading Li2S cathodes for high-performance Li–S batteries without the disadvantage of polysulfide formation.