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 WS₂ micro-flower is synthesized by thermal sulfurization of WO₃. Comprising interconnected thin nanosheets, this structure offers increased surface area, facilitating extensive internal surfaces for electrochemical redox reactions. The WS₂ micro-flower demonstrates a specific capacity of ≈334 mAh g^-1 at 15 mA g^-1, 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^-1 reversible capacity at 100 mA g^-1 current rate. Post-cycling analysis confirms unchanged morphology, highlighting the structural stability and robustness of WS₂. DFT calculations show that the electronic bandgap in both WS₂ and WO₃ increases when going from the bulk to monolayers. Na adsorption calculations show that Na atoms bind strongly in WO₃ with a higher energy diffusion barrier when compared to WS₂, corroborating the experimental findings. This study presents a significant insight into electrode material selection for sodium-ion storage applications.