Tungsten diselenide nanoplates as advanced lithium/sodium ion electrode materials with different storage mechanisms

Transition-metal dichalcogenides (TMDs) exhibit immense potential as lithium/ sodium-ion electrode materials owing to their sandwich-like layered structures. To optimize their lithium/sodium-storage performance, two issues should be addressed: fundamentally understanding the chemical reaction occurring in TMD electrodes and developing novel TMDs. In this study, WSe₂ hexagonal nanoplates were synthesized as lithium/sodium-ion battery (LIB/SIB) electrode materials. For LIBs, the WSe₂-nanoplate electrodes achieved a stable reversible capacity and a high rate capability, as well as an ultralong cycle life of up to 1,500 cycles at 1,000 mA·g^–1. Most importantly, in situ Raman spectroscopy, ex situ X-ray diffraction (XRD), transmission electron microscopy, and electrochemical impedance spectroscopy measurements performed during the discharge–charge process clearly verified the reversible conversion mechanism, which can be summarized as follows: WSe₂ + 4Li⁺ + 4e^– ↔ W + 2Li₂Se. The WSe₂ nanoplates also exhibited excellent cycling performance and a high rate capability as SIB electrodes. Ex situ XRD and Raman spectroscopy results demonstrate that WSe₂ reacted with Na⁺ more easily and thoroughly than with Li⁺ and converted to Na₂Se and tungsten in the 1^st sodiated state. The subsequent charging reaction can be expressed as Na₂Se → Se + 2Na⁺ + 2e^–, which differs from the traditional conversion mechanism for LIBs. To our knowledge, this is the first systematic exploration of the lithium/sodium-storage performance of WSe₂ and the mechanism involved.