Tungsten diselenide (WSe2) as a high capacity, low overpotential conversion electrode for sodium ion batteries

Abstract

Tungsten diselenide (WSe₂) is demonstrated as an efficient electrode for sodium ion batteries for the first time. A high reversible capacity above 200 mA h g⁻¹ is observed at 20 mA g⁻¹ rate, with over 250 mA h g⁻¹ capacity measured in the first sodium extraction. Assessment of electrolyte and binder materials on performance was examined and an EC/DEC electrolyte with CMC binder emerges to yield the highest capacity and cycling retention. Comparison between WS₂ and WSe₂ distinguishes WSe₂ to exhibit superior performance due to more efficient energetics bearing a small overpotential <0.30 V. Ex situ analysis and imaging after cycling confirms a sodium-mediated conversion reaction that yields isolated domains of W metal or Na_xSe and reformation of WSe₂ upon sodium extraction, enabling insight into the chemical storage pathway. This work highlights the promise of WSe₂ compared to other conversion-based transition metal dichalcogenides as a practical material for sodium ion batteries.