Lithium Storage Performance And Mechanism Of Two-dimensional Layered WX₂ （X=S,Se） Nanosheets

Lithium-ion batteries （LIBs） have been regarded as a priority candidate of energy storage devices for next-generation electric vehicles （EVs） and hybrid electric vehicles （HEVs）, due to their outstanding properties, such as high energy density, long cycle life, no memory effect, and environmental benignity. However, graphite as the commercial anode material for LIBs cannot meet the ever-growing demands in EVs and HEVs owing to their limited specific capacity and unsatisfactory rate capability. Therefore, developing advanced alternative anode materials with increasing energy density and power capability for future LIBs is an urgent task.Transition metal dichalcogenides （TMDs） with the formula MX2 （where M= transition metal and X= chalcogen）, exhibit an immense potential as lithium ion anode materials due to their higher theoretical capacity and layered structure with larger interlayer spacing. In order to optimize their electrochemical performance as anodes for advanced LIBs, two issues should be addressed:fundamental understanding of the chemistry taking place in TMDs anodes and attempts of new-type TMDs. In this thesis, we explored the lithium storage performance and mechanism of WS₂ and WSe₂ nanosheets, which are two typical TMDs with less scientific attention as LIB anodes in comparison to MoS2. The main contents of this paper are as follows:（1） We designed and fabricated 2D WS₂ nanosheets with perfect single crystalline structures. Being used as an anode for LIBs, the WS₂-nanosheet electrode exhibits a high specific capacity of 539.1 mA h g"1 after 60 cycles at 200 mA g"1 and an ultrolong cycle life up to 1000 cycles at 1000 mA g-1 as well as an excellent rate capability. Considering the controversy in the lithium storage mechanism of WS₂, the first discharge-charge process of the WS₂-nanosheet electrode was studied using ex-situ X-ray diffraction （XRD）, Raman and X-ray photoelectron spectroscopy （XPS） measurements. These analyses clearly verified that the recharge product （3.0 V vs. Li+/Li） of the WS₂ electrode after fully discharging to 0.01 V （vs. Li+/Li） tends to reverse to WS₂, confirming the conversion reaction mechanism.（2） 2D layered WSe₂ hexagonal nanosheets were synthesized successfully with perfect single crystalline nature. As lithium ion anode materials, the WSe₂-nanosheet electrodes can achieve a stable reversible capacity and a high rate capability as well as an ultralong cycle life up to 1500 cycles at 1000 mA g"1. Most importantly, in-situ Raman, ex-situ XRD and TEM as well as electrochemical impedance spectroscopy （EIS） measurements during the initial discharge-charge process clearly verified the reversible conversion mechanism which can be summarized as: WSe₂+4 Li++4 e-^ W+2 Li2SeIt should be noted that the deeper understanding of the discharge-charge reaction of WS₂ and WSe₂ electrodes could form the basis of design rules for tailor-made anode materials with enhanced electrochemical performance.