WS2@C/rGO复合结构的限域调控及对其赝电容储钠行为的影响机制研究

As one kind of potential anodes in Na-ion battery, WS2 material is in urgent need to improve the charge/discharge mechanism for the promotion of its rate performances. Inspired by this need, we aim to build carbon-coated WS2 composite structures on the surface of graphene. The morphology of the WS nanocrystalline is controlled within the inner spaces of carbon coating by a confined-synthesis technique. This control could further adjust the Na+ pseudocapacity for enhanced rate performances. In this project, a carbon-coated WO3 nanostructure will be firstly prepared on the graphene oxide via a solvothermal process. Secondly, the carbon-coated WO3 will react via a confined sulfuration process with careful adjustment of the reaction condition, resulting, resulting controlled structural features of WS2 nanocrystalline. The influences of these structural features on the pseudocapacitive behavior of the composites will be further explored to define the adjustment mechanism of the pseudocapacity ratio in total Na+ storage of WS2 composites. Thirdly, this ratio will be further researched to clarify its impact on the rate capacity of the material for higher performance in Na-ion storage. A relationship model among the morphology, pseudocapacity and rate performances of WS2 composites will be constructed to explain how the pseudocapacitance can promote the Na-ion storage in this material. The structural features will be summarized to find the effective approach for enhanced electrochemical Na-ion storage, providing foundations for improved Na-ion battery performance and the expanded application fields of WS2-based material.

WS2材料是一类极具潜力的钠离子电池负极材料，但如何改善充放电机制以进一步提升其倍率性能是亟需解决的问题。本项目拟在石墨烯表面构筑碳层包覆的WS2复合结构，利用限域合成技术在碳层空间内调控WS2纳米晶的形貌，以此控制其赝电容储钠行为并提升材料的倍率性能。项目拟首先通过溶剂热法在氧化石墨烯表面制备碳层包覆的WO3纳米结构；以气相反应实现碳层内WO3的限域硫化，控制反应工艺调控WS2纳米晶的结构；研究材料的结构特征对其赝电容储钠行为的作用机制，探索材料结构对赝电容储钠比例的调控机理；明确该比例对材料钠离子电池倍率性能的影响机制，提升WS2材料的储钠性能；构建WS2复合材料的形貌-赝电容储钠行为-倍率性能的关系模型，搞清赝电容机制促进材料储钠性能的原理。总结通过调控WS2复合材料结构特征提升其电化学储钠活性的有效途径，为改善WS2材料的钠离子电池性能并拓展其应用领域奠定基础。

WS2作为一种层状过渡金属硫化物材料，其晶体结构包含有许多三重原子堆叠的二维层状结构（S-M-S），层内的原子之间靠共价键相连，平行层间的结构则依靠范德华力维系。这种结构特征为大尺寸的钠离子嵌入与脱出提供了便利，有助于钠离子实现更快的迁移扩散。但其充放电中间产物Na2S的弱导电性使其电荷迁移能力受到限制，材料较难在高速电流密度下（≥5A/g）展现出良好的大倍率储钠性能（>300 mAh/g）；此外高容量储钠还会引起材料粉化失去有效电接触，进一步制约其倍率性能。这些问题都显著限制了WS2材料在高性能钠离子电池领域的发展潜力。本项目首先采用溶剂热处理法和固相硫化法合成了具有不同结晶度的WS2纳米片。采用固相法制备了高结晶度的WS2纳米片，具有良好的结构稳定性和优异的电化学性能。在200 mA g−1时，300次循环后可实现471 mAh g−1的高可逆容量。即使在5.0 A g−1的高电流密度下进行250次循环后，可逆容量仍可达到240 mAh g−1。进一步研究发现，高结晶度样品的层间有序度高，层间作用力强，具有良好的电化学性能。另外成功地构建了浇筑碳支撑的WS2/CFC复合材料。浇筑碳结构填充在WS2纳米片之间。同时，侧面WS2（002）晶面边缘外露。其可逆容量在100 mA g-1下进行200次循环后保持672 mAh g-1。电极在2 A g-1下循环500次后，维持 297 mAh g-1。浇注碳支撑结构抑制了WS2纳米片的膨胀。另一方面，暴露的（002）侧边实现了Na+的快速迁移。这些优点使快速反应动力学和良好的结构稳定性。这为层状金属硫化物的高效储钠提供了新的思路。

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