A bimetallic sulfide Co₉S₈/MoS₂/C heterojunction in a three-dimensional carbon structure for increasing sodium ion storage

The synthesis of high-rate and long-life anode materials for sodium ion batteries (SIBs) has attracted much attention. However, the slow kinetics and large increase in volume of the batteries remain major problems. Both metal-organic frameworks and MoS₂ have shown properties suitable for SIBs, making research on their composite systems an attractive area of research. We report the formation of flower-like Co₉S₈/MoS₂/C composites by a simultaneous vulcanization-carbonization process using MoCl₅ as the Mo source and a 2-methylimidazole cobalt salt as the Co and C precursor at different temperatures（600, 700 and 800 °C）in sublimed sulfur. The effect of the heterojunction on the diffusion kinetics was analyzed using density functional theory. The results indicate that the electronic structure is different at the interface in the heterogeneous structure, exhibiting typical metallic properties and better electronic conductivity. In addition, the anode material Co₉S₈/MoS₂/C synthesized at 700 °C had the most stable structure and best electrochemical performance of the three samples. Notably, the discharge capacity of Co₉S₈/MoS₂/C-700 fully recovered from 368 to 571 mAh g⁻¹ and then stabilized at 543 mAh g⁻¹ when the current density was restored from 4 000 to 40 mA g⁻¹. This work demonstrates the preparation of heterojunction materials for composite anode materials as a step to producing high-performance metal SIBs.