Hierarchical CoNi ₂ S ₄ /C Hollow Nanospheres as Anode Materials for High-Performance Sodium-Ion Batteries

Transition metal sulfides have garnered considerable interest as promising anode material candidates for sodium-ion batteries, owing to their superior theoretical capacity and cost efficiency. Nevertheless, the real-world applications of these materials are hindered by their slow reaction kinetics and considerable volumetric expansion during cycling, which results in a less-than-ideal electrochemical performance. Herein, a novel composite of CoNi₂S₄/C hollow nanospheres (CoNi₂S₄/C HSs) is successfully prepared through a one-step solvothermal approach. The unique hollow structure of the CoNi₂S₄/C HSs composite provides it with an increased specific surface area, accelerated Na⁺ diffusion kinetics, and reduced electrical resistance compared to CoNi₂S₄/C nanoparticles (CoNi₂S₄/C NPs). As a result, CoNi₂S₄/C HSs demonstrate an exceptional average capacity of 649.5 mAh g^-1 at a current density of 0.1 A g^-1 and maintain a capacity of 495.5 mAh g^-1 after 1000 cycles at a high current density of 10.0 A g^-1. These values exceed those of the CoNi₂S₄/C NPs, highlighting their superior electrochemical sodium storage performance. It is demonstrated through density functional theory calculations that the interfacial dipole enhances electronic coupling, thereby facilitating rapid interfacial electron transfer. Furthermore, due to the interfacial dipole effect, Na⁺ is found to preferentially adsorb at the interface of C and CoNi₂S₄, where they exhibit a relatively lower diffusion energy barrier. The proposed structural design offers a promising approach for the advancement of the electrochemical sodium storage performance of bimetallic sulfides.