Unveiling the Double-Edged Behavior of Controlled Selenium Substitution in Cobalt Sulfide for Balanced Na-Storage Capacity and Rate Capability

Transition metal sulfide-based anodes usually suffer from huge volumetric change and sluggish reaction kinetics, hindering their application for long-term and high-power/energy sodium-ion batteries. Herein, a new design of CoS2-xSex (0 <= x <= 2) nanocrystals with highly controllable selenium substitution and S, Se-codoped graphene immobilization (CoS2-xSex@SG) is proposed to tune the reaction kinetics and structural stability. The nanocrystal-on-graphene structure and robust C & horbar;S & C & horbar;Se bonding rivets between CoS2-xSex and SG greatly improve the structural stability of the CoS2-xSex@SG. Electrochemical performance, kinetic analysis, and theoretical calculation reveal that Se substitution plays a double-edged role in sodium storage: the increase of Se substitution content enhances the Na+ diffusion kinetics but decreases the Na-storage capacity. When the Se substitution content is 0.4, the CoS1.6Se0.4@SG electrode demonstrates the best performance: high initial Coulombic efficiency (95.5%), ultrahigh rate capability (412.8 mAh g(-1) at 30 A g(-1)), and ultra-stable cycling performance (97.6% capacity retention after 1000 cycles). In situ/ex situ measurements further unveil that the conversion reaction between Co-0 and Na2S/Na2Se generates the micro-scaled CoSe2-CoS2 heterostructure, synergistically improving the Na-storage active sites and reaction kinetics. This work provides a controllable anion substitution strategy to balance the Na+ storage active sites and kinetics with potential applications for high-power/energy sodium-ion batteries.