Deciphering Sodium-Ion Storage: 2D-Sulfide versus Oxide Through Experimental and Computational Analyses.

Transition metal derivatives exhibit high theoretical capacity, making them promising anode materials for sodium-ion batteries. Sulfides, known for their superior electrical conductivity compared to oxides, enhance charge transfer, leading to improved electrochemical performance. Here, a hierarchical WS micro-flower is synthesized by thermal sulfurization of WO. Comprising interconnected thin nanosheets, this structure offers increased surface area, facilitating extensive internal surfaces for electrochemical redox reactions. The WS micro-flower demonstrates a specific capacity of ≈334 mAh g at 15 mA g, nearly three times higher than its oxide counterpart. Further, it shows very stable performance as a high-temperature (65 °C) anode with ≈180 mAh g reversible capacity at 100 mA g current rate. Post-cycling analysis confirms unchanged morphology, highlighting the structural stability and robustness of WS. DFT calculations show that the electronic bandgap in both WS and WO increases when going from the bulk to monolayers. Na adsorption calculations show that Na atoms bind strongly in WO with a higher energy diffusion barrier when compared to WS, corroborating the experimental findings. This study presents a significant insight into electrode material selection for sodium-ion storage applications.