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 1st 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.