Na、Li互掺杂Na2Li2Ti6O14材料相图的构建及储锂过程的原位研究

Advanced energy materials not only are the key components for power batteries, but also are the core materials for the development of new energy industry. However, material technologies have hindered the development of power batteries, hybrid and electric vehicles in recent years. Furthermore, the development of in-situ techniques is very important to know the working situation of batteries. Based on the problems found in batteries, the researchers can make an improvement with the electrode materials, and then enhance the electrochemical properties. Na2Li2Ti6O14 is a kind of novel anode material with low operating potential (1.25 V) and stable framework. To be suitable lithium storage hosts for power lithium-ion batteries, a series of Ti-based anode materials are proposed by forming a phase diagram for Na and Li doped Na2Li2Ti6O14. This phase diagram is built by preparing a series of NaxLiyTi6O14 samples at different Na/Li contents (x+y=4) and different calcination temperatures. This phase diagram can provide useful information for the preparation of various Ti-based anode materials. To understand the differences in structure and electrochemical behavior after Na/Li-site substitution, several Na2Li2Ti6O14-based compounds with the same Na and Li doping contents are selected as research objectives according to their superior electrochemical results. The electrochemical reaction processes of selected compounds with lithium ions are thoroughly studied by in-situ X-ray diffraction, in-situ Raman and in-situ synchrotron techniques. Based on various in-situ analytical results, the Wyckoff positions and orders for lithium ions uptake/removal at different insertion/extraction stages are displayed and compared. As a result, the characteristics and differences of lithium storage mechanisms for different Na2Li2Ti6O14-based compounds can be described. Besides, the effect of Na/Li-site substitution with the same or different contents on the storage positions and transportation channels for lithium ions can also be presented in detail. Finally, according to the ion diffusion pathways, the morphologies and structures of samples can be controlled via electrospinning methods to form advanced Ti-based anode materials for power lithium-ion batteries.

高性能电池材料是动力电池的核心部分，也是加速新能源产业发展的关键材料，然而，近年来材料技术成了阻碍动力电池进一步转型升级的瓶颈；再者，开发高精尖的原位技术也是实时掌握电池材料工作状态的重要手段。Na2Li2Ti6O14是一种新型的电极材料，较低的工作电位和稳定的晶体结构使得它有可能成为动力锂离子电池的负极材料。本项目首先通过构建Na、Li互掺杂Na2Li2Ti6O14材料相图来掌握合成新型钛基负极材料的技术特点；接着，优选几种电化学性能优异的化合物进行原位X射线衍射、原位拉曼光谱和原位同步辐射的多重研究，分析不同充放电阶段锂的存储位置、顺序及输运步骤，通过对比不同掺杂状态化合物的储锂机制的异同点，分析Na位和Li位等量及不等量掺杂对锂离子在结构中存储及输运的影响。最后，基于锂离子的传输路径和特点，通过调控样品的形貌和结构来获得具有高倍率和长寿命特性的负极材料。

高性能电池材料是动力电池的核心，也是加速新能源产业发展的关键材料。在Na、Li互掺杂思想的指导下，通过对Na2±xLi2∓xTi6O14结构中Na、Li的等量变化（-2.0≤x≤2.0），获得了Na2Ti3O7、Na3LiTi6O14、Na2Li2Ti6O14、NaLi3Ti6O14和Li4Ti6O14等目标样品，分析并鉴定了所合成样品的物相种类，依此数据构建了Na、Li互掺杂Na2Li2Ti6O14物相XRD组图。针对所合成的一系列Na2±xLi2∓xTi6O14，分析测量了其可逆容量、库伦效率、倍率性能和离子扩散系数，基于所得的数据构建了Na2±xLi2∓xTi6O14的电化学性能对比图，从而掌握了Na/Li位掺杂对Na2±xLi2∓xTi6O14电化学性能的影响，确定了Na2Li2Ti6O14作为本项目的主要研究目标。再利用原位联用非原位技术对Na2Li2Ti6O14实现了原位电化学观察，解析了Na2Li2Ti6O14结构中的脱嵌锂机制，可知其结构中的四面体4a、4b位置和正八面体8c位置能够容纳锂离子嵌入和扩散，在嵌锂过程中，锂离子能够依次占据Na2Li2Ti6O14晶体结构中的4a、4b和8c空位。项目组还进一步研究了其他金属元素掺杂对Na2Li2Ti6O14晶体结构、嵌脱锂机制及电化学性能的影响，发现高价元素取代低价元素后所产生的五面体8f空位是额外锂存储容量的来源。通过原位联用非原位技术解析了Na位异价掺杂Na2Li2Ti6O14的嵌脱锂过程，发现每个单元结构中8f空位最大能够填充2个锂离子。在放电过程中，锂离子先进入8f位置，接着嵌入到4a位置，最后，再进入到4b和8c位置。同时，研究还发现掺杂对Na2Li2Ti6O14的晶胞参数、结构变化和锂离子输运通道均没有明显影响，但是部分异价掺杂元素在一定程度上可以改善Na2Li2Ti6O14的电子结构，从而提升其本征电子电导率，改善Na2Li2Ti6O14的倍率性能。为了进一步改善Na2Li2Ti6O14的储锂能力，项目组还利用静电纺丝技术合成了Na2Li2Ti6O14纳米线，并对其进行表面碳包覆，得到了Na2Li2Ti6O14@C纳米线。对比块状和颗粒状材料，Na2Li2Ti6O14@C纳米线具有超高的倍率性能和极其稳定的长循环性能，有望成为高倍率、长寿命动力电池的负极材料。