MXene Ti2CTx表界面调控及其钠离子混合电容器的储能机理研究

Hybrid sodium-ion capacitors (HNICs) offer advantages such as low cost, high energy density and high power density, thus they are very suitable for large scale energy storage. Two-dimensional layered MXene Ti2CTx are attractive and promising for HNICs. However, the techniques including exfoliation, delamination and intercalation are dangerous, laborious and low-efficiency. In addition, surface covered F and OH terminals severely degrades the conductivity and hampers the electrochemical performance of MXene. In this project, we design a safer and faster mechanochemical route to achieve the efficient exfoliation, spontaneous delamination and straightforward intercalation in one step, resulting in high quality MXene with high yield and non-restacking. Surface engineering of MXene via microwave plasma induces a clean or re-functionalized surface with improved conductivity. Comparative investigation will be carried out for MXene based hybrid Na-ion energy storage devices, revealing the electrode kinetics in aqueous and nonaqueous electrolytes and demonstrating the effects of layer distance and surface terminals. This work will provide new insight for optimizing 2D materials via more efficient mechanochemical route and microwave plasma surface engineering. The surface engineered MXene will be promising for hybrid energy storage with high energy density and power density.

MXene二维材料剥离后的氟氧等表面基团使其导电性急剧下降，导致其构筑的储能器件性能无法达到最佳；同时，与表界面环境相关的离子输送和电子传导机制还不清晰，MXene基钠离子混合电容器的电化学储能机理亟需探讨。针对上述问题，本项目设计安全、便捷、高效的机械化学法剥离策略，发挥化学刻蚀和机械球磨的协同剥离优势，借助大尺寸钠离子的层间自发嵌入特性，同步实现MXene快速剥离分层和离子嵌入插层，获得高质量、大产率、可自发抑制团聚的MXene；项目还借助微波等离子体进行MXene表界面处理工程，实现界面清洁或基团再修饰，通过调整费米能级提升其电子导电性；最后研究该MXene基钠离子混合储能器件的电化学性能，探讨二维MXene层片间距、离子嵌入、表面基团等对电极材料电子传导、离子输传的影响，对比其在水系和有机系电解液中的电极动力学差异，为设计高功率密度和高能量密度的混合型储能器件提供指导。

MXene二维材料在电化学储能中具有独特优势，特别是大尺寸离子的存储如钠离子。二维材料所具有的层间距既可实现钠离子的嵌入，也可进行表界面的吸附存储。这意味着MXene二维材料可实现高能量密度和高功率密度的储能效果。针对MXene二维材料的化学剥离制备缓慢耗时，且材料表面富集F/O等化学基团降低其导电性，不利于快速储能。本项目首先设计了高效便捷的化学机械方法进行二维材料的剥离，发现采用化学刻蚀和机械球磨结合的协同剥离技术，实现快速剥离和离子嵌入。该方法采用盐粒为球磨粒剂，结合NaF/HCl化学剥离，加速推动材料剥离的同时实现钠离子的预嵌入，经济实惠且快速高效。其次，发现采用微波等离子体技术对MXenes进行表面处理，Ar惰性气氛气体等离子体可降低表面F/O基团；而氮气等离子体气氛可在表面实现含氮基团的修饰；含氧等离子体处理MXenes时，低浓度可实现二维MXenes表面F基团被氧取代，但高强度的氧会导致部分表层MXenes转变为TiO2。最后，研究上述剥离制备和表面修饰的MXene的电化学行为，揭示了层间距、离子嵌入、表界面基团对电子传输和离子传导的影响，归纳了提升电池能量密度和功率密度的关键因素，梳理了MXene二维材料层间结构和表面基团与其电化学特性之间的依赖关系，为高能量密度和高功率密度储能材料提供新的思路。