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

Abstract 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 CoS_2‐xSe_x (0≤x≤2) nanocrystals with highly controllable selenium substitution and S, Se‐codoped graphene immobilization (CoS_2‐xSe_x@SG) is proposed to tune the reaction kinetics and structural stability. The nanocrystal‐on‐graphene structure and robust C─S &C─Se bonding rivets between CoS_2‐xSe_x and SG greatly improve the structural stability of the CoS_2‐xSe_x@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 CoS_1.6Se_0.4@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 Co⁰ 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.