Kinetics of Li intercalation and de-intercalation in tantalum chalcogenide single crystals

锂在层状材料中的嵌入动力学还不是很清楚，将以硫族化合物为例在TP中进行系统的研究。插层过程涉及几个步骤，其中哪一个步骤控制着插层速度尚不清楚。在项目的第一阶段，我们研究了锂在钛硫化物单晶中的化学嵌入。主要结果是锂在这些层状晶体中的插入是不均匀的，局部应力起着重要的作用。在第二阶段，我们想要研究锂在硫化钽中的插入。与钛硫化物相反，这种化合物在加入锂之前就是一种很好的导电体。因此，可以在同一材料上同时研究化学插层和电化学插层，从而对锂电池充放电过程的机理有了新的认识。化学插层将通过用正丁基锂在己烷或正庚烷中的溶液包围单晶来实现。对于电化学插层实验，将使用金属锂作为第二电极，无水锂化合物作为电解液对所建立的装置进行修饰。根据施加的电压，锂将被插入晶体或从晶体中移除。实验结束后，将使用激光烧蚀和质谱学相结合的方法测量锂的分布。垂直于各层的扩散将使用SIMS(二次离子质谱仪)进行研究。利用飞行时间-二次离子质谱仪(TOF-SIMS)可以观察到锂在层内的分布。我们的初步研究结果表明，压力对锂离子的插层动力学有重要影响。在密封金胶囊中的样品组合后，将在气体压力容器中研究静水压力下的化学插层。在TP6第一阶段开发的新电池中，化学和电化学插层都将在几个千巴的轴向压力下进行研究，最高可达千巴的十分之几。要改变的其他参数包括温度(20-80°C)、溶液中的锂浓度，以及在电化学插层的情况下，电压。由于TaS_2可以通过气体传输反应和随后的退火或淬火来合成不同晶型的TaS_2，因此也可以研究层的顺序对TaS_2的影响。

The kinetics of lithium intercalation in layered materials is poorly understood and will be systematically investigated in TP using chalcogenides as examples. Several steps are involved in the intercalation process and it is not known a priori which of them controls the rate of intercalation. In the first period of the project we have studied chemical intercalation of lithium in titanium chalcogenide single crystals. Major findings were that the insertion of lithium in these layered crystals is inhomogeneous, and local stress plays an important role. In the second period we want to study insertion of lithium in tantalum sulfides. In contrary to titanium chalcogenides, this compound is a good electrical conductor already before incorporation of lithium. Thus, both chemical and electrochemical intercalation can be studied on the same material, giving new insights to the mechanisms of processes relevant for charge and discharge of Li-batteries. Chemical intercalation will be performed by surrounding single crystals with a solution of n-buthyl lithium in hexane or heptane. For electrochemical intercalation experiments the set up will be modified using lithium metal as a second electrode and anhydrous lithium compounds as electrolytes. Depending on applied voltage lithium will be either inserted or removed from the crystals. After the experiments lithium profiles will be measured using laser ablation combined with mass spectrometry. Diffusion perpendicular to the layers will be studied using SIMS (secondary ion mass spectrometry). Distribution of lithium within layers can be observed using ToF-SIMS (time of flight - secondary ion mass spectrometry). Findings of our first studies have demonstrated that pressure has major influence on the kinetics of Li-intercalation. Chemical intercalation under hydrostatic pressure will be studied in gas pressure vessels after sealing of the sample assemblage in gold capsules. Both chemical and electrochemical intercalation will be studied under axial pressures of several kbars up to several tenths of kbar in a new cell developed during the first period of TP6. Additional parameters to be varied are temperature (20 - 80 °C), lithium concentration in the solutions and, in the case of electrochemical intercalation, voltage. Since different polymorphs of TaS2 can be synthesized by gas transport reactions and subsequent annealing or quenching, the influence of order of layers can be investigated as well.