Bimetallic Sulfides Cr_0.99V_1.8S₄ with Loosely Packed Structure: Exploring the Boundary of Conversion and Intercalation Sodium-Ion Storage Mechanism

Metal sulfide electrodes for sodium-ion batteries face trade-offs among high capacity, fast kinetics, and stability. The challenge lies in breaking and restoring metal-sulfur bonds and allowing rapid ionic transport. Here we explore the boundary of conversion- and intercalation-type metal sulfides to develop ideal sodium-ion storage materials. We focus on sulfides of vanadium and chromium because of their adjacent atomic numbers but different energy storage mechanism. Among various sulfides of vanadium and chromium, a loosely packed bimetallic sulfide, Cr_0.99V_1.8S₄, with cationic vacancies and metallic conductivity (4.28 S m^-1), shows optimal sodium-ion storage performance: an initial Coulombic efficiency of 95.6%, a reversible capacity of 551 mAh g^-1 at 1.6 C, and maintaining 100% capacity after 600 cycles at a high rate of 16-66 C.