Two-Dimensional Mesoporous Cobalt Sulfide Nanosheets as a Superior Anode for a Li-Ion Battery and a Bifunctional Electrocatalyst for the Li–O<sub>2</sub> System

We report the synthesis of two-dimensional (2D) Co<sub>3</sub>S<sub>4</sub> in a nanothickness sheetlike morphology via simple hydrothermal process and its application to electrochemical energy-storage devices. The presence of unique mesopores with a combination of core/shell nanoparticles in the nanosheets showed superior electrochemical performances as a negative electrode for a Li-ion battery (LIB) and an electrocatalyst in Li–O<sub>2</sub> battery applications. A high discharge capacity of ∼968 mAh g<sup>–1</sup> is noted after 60 cycles with excellent cycling stability when evaluated as an anode for a LIB. On the other hand, the first discharge capacity of ∼5917 mAh g<sup>–1</sup> is observed with a high reversibility of 95.72% for the Li–O<sub>2</sub> battery point of view. This exceptional electrochemical performance in both applications is mainly attributed to the presence of mesoporous with core/shell 2D nanostructure, which translates more catalytic bifunctional (oxygen reduction reaction/oxygen evolution reaction) active sites for Li–O<sub>2</sub> and sustains the volume variations that occur in a three-dimensional manner upon the charge–discharge process for LIB applications. <i>Ex situ</i> studies, such as transmission electron microscopy, X-ray photoelectron spectroscopy, and impedance spectroscopy studies, are also conducted to validate the reaction mechanisms.