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

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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.