Highly Conductive Two-Dimensional FeTHBQ/Graphene Nanocomposite as the Cathode Material for Lithium-Ion Batteries

Constructing high-performance coordination polymer (CP) cathodes for lithium-ion batteries based on the redox reactions of both high-potential transition metal ions and high-capacity organic ligands has attracted extensive attention. However, CP cathodes suffer from structural degradation, low electrical conductivity, and sluggish diffusion kinetics, resulting in poor cycling stability and inferior rate capability. Herein, the ultrafine FeTHBQ (THBQ = tetrahydroxy-1,4-benzoquinone) CP nanoparticles in situ grew on both sides of graphene nanosheets to form the uniform two-dimensional (2D) FeTHBQ/Graphene nanocomposite with a sandwich structure via a one-pot solvothermal method. The highly conductive graphene skeleton promotes the electronic conduction and structural stability for the 2D FeTHBQ/Graphene nanocomposite. Besides, compared with bulk FeTHBQ, the primary FeTHBQ nanoparticles in the FeTHBQ/Graphene nanocomposite have smaller particle sizes with larger specific surface areas. This not only shortens the Li⁺ diffusion distance in the FeTHBQ crystal but also benefits Li⁺ transfer between the electrolyte and the electrode. In the FeTHBQ/Graphene nanocomposite, the active material of FeTHBQ manifested multiple redox centers of transition metal ions (Fe³⁺/Fe²⁺) and carbonyls (C═O/C-O^-) in THBQ ligands. Owing to the enhancements of structural stability, electronic conduction, and Li⁺ diffusion kinetics, the 2D FeTHBQ/Graphene nanocomposite presented a high lithium-ion storage capacity of 217.2 mA h g^-1 at 50 mA g^-1, a fast rate capability of 79.1 mA h g^-1 at 5000 mA g^-1, and a stable cycling performance of 87.2 mA h g^-1 at 500 mA g^-1 after 100 cycles. This work sheds light on the great opportunity for optimizing the electrochemical performances of CP-based functional electrode materials by combining with conductive substrates.