Unveiling the Nature of Superior Sodium Storage in the CoSe₂/rGO Nanocomposite

Sodium-ion batteries (SIBs) are considered the most promising alternatives to lithium-ion batteries (LIBs) due to the abundant availability of sodium and their cost-effectiveness. Transition metal selenides (TMSes) are considered promising anodes for SIBs due to their economic efficiency and high theoretical capacity. Nevertheless, overcoming the challenges of sluggish reaction kinetics and severe structural damage is crucial to improving cycle life and rate capability. Herein, a simple microwave hydrothermal process was used to synthesize a nanocomposite of CoSe₂ nanoparticles uniformly anchored on reduced graphene oxide nanosheets (CoSe₂/rGO). The influences of rGO on the structure and electrochemical performance and Na+ diffusion kinetics are investigated through a series of characterization and electrochemical tests. The resulting CoSe₂/rGO nanocomposite exhibits a remarkable initial specific capacity of 544 mAh g^-1 at 0.5 A g^-1, impressive rate capability (368 mAh g^-1 at 20 A g^-1), and excellent cycle life and maintains 348 mAh g^-1 at 5 A g^-1 over 1200 cycles. In addition, the in situ electrochemical impedance spectroscopy (EIS), ex situ X-ray diffraction (XRD), and transmission electron microscopy (TEM) tests are selected to further investigate the sodium storage mechanism.