Electrochemical Performance Of Co-MOF Derived Anode Materials For Lithium-Ion Batteries

Energy storage devices with excellent performances have been vigorously developed with inordinate consumption of fossil fuels and ever-growing clean energy.Among all kinds of energy storage devices,lithium-ion batteries（LIBs）have attracted much attention by virtue of high energy density and long-cycle life.As the most successful anode material in LIBs,graphite exhibits a high degree of reversible charge and discharge ability.Nevertheless,graphite cannot satisfy the ever-increasing demand for high-performance batteries due to low theoretical capacity(372 m Ah g^-1)and unsatisfactory rate performance.Therefore,it is urgent to look for anode materials that are featured with high energy density for LIBs.As potential next-generation anode materials,porous carbon and transition metal sulfide have attracted much attention due to their low cost and high capacity.However,such two types of anode materials still have the weakness,such as unsatisfactory conductivity and serious volume expansion,which impede the advances towards the large-scale actual application.Metal-organic frameworks（MOFs）consist of two parts:metal ions and organic ligands.MOFs and their derivatives have been widely investogated as anode nanomaterials for LIBs due to high porosity,great specific surface area,and multiplicity of framework and function.In this paper,aiming at the problems of porous carbon and transition metal sulfide,MOFs/graphene composites were used as precursors to fabricate modified porous carbon materials and transition metal sulfide composites as anode nanomaterials for LIBs.The two main contents of the paper are summarized below:（1）A nitrogen-doped porous carbon composite on reduced graphene oxide（Co@NC/r GO）was prepared by calcinating ZIF-67/GO precursor.In this material,cobalt nanoparticles（18.9±0.4 nm）were uniformly embedded on the nitrogen-doped porous carbon and dispersed on the r GO substrate.The material showed excellent electrochemical performances for LIBs,owing to the introduction of cobalt and graphene,which exhibited specific capacity of 728.6m Ah g^-1 after 100 circles at 0.1 A g^-1and 304.8 m Ah g^-1after 800 cycles at 5 A g^-1.Furthermore,characterization by ex situ XRD,EIS,and GITT reveal that the emtallic cobalt is not involved in contributing to the reversible capacity,but instead improving the electronic conductivity of the anode nanomaterial;and also that the introduction of graphene improves the diffusion kinetics of lithium ions.（2）A Co S₂@NC/r GO composite was prepared from the ZIF-67/GO precursor via carbonizaion and sulfurization to solve the problems of severe volume expansion and insufficient electrical conductivity of transition metal sulfide.The as-prepared material had a large specific surface area of 324.5 m²g^-1and appropriate pore size distribution,which provide rapid electrolyte penetration and rich active sites for lithium ions.When applied to LIBs,the composite delivered a reversible specific capacity of 961.8 m Ah g^-1 after 100circles at 0.1A g^-1,which is quite higher than that of Co S₂@NC(523.1 m Ah g^-1).The enhancement is mainly attributed to the unique double-carbon structure:both r GO and amorphous carbon construct double continuous conductive network and inhibit the volume change of Co S₂.