Low-dimensional lithium ion conductors

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The project is concerned with the study how Li diffusion is influenced by dimensionality effects. The investigations are to be performed on selected single-crystalline and polycrystalline materials which feature structurally confined diffusion pathways. To this end NMR techniques will be employed which are sensitive to both the mobility of the Li ions and the dimensionality of the diffusion process.One and two dimensional diffusion can be distinguished by means of temperature dependent and frequency dependent 6,7Li NMR spin-lattice relaxation (SLR) measurements. Both 1D and 2D lithium-ion conductors show a characteristic asymmetry of the diffusion-induced SLR NMR rate peak. Furthermore, the rates reveal a characteristic dependence on the Larmor frequency ω0 in the so-called high temperature limit where the mean jump rate 1/τ is much larger than ω0. Typically, at common magnetic field strengths, the frequency ω0/2π(7Li) ranges from 10 to 300 MHz. In the case of 1D diffusion appropriate relaxation models predict a square root dependence of the rate on ω0 and a logarithmic dependence in the case of 2D diffusion. Analogously, the same holds true for SLR measurements performed in the rotating frame of reference where the Larmor frequency is replaced by the locking frequency ω1 being of the order of several kHz. In this case jump processes are covered which are 1000 times slower than those usually probed in the laboratory frame of reference. Even lower jump rates are directly accessible by NMR spin-alignment echo (SAE) measurements which do not require a model to convert decay rates into Li jump rates. The SAE NMR technique is per se sensitive to the geometry of the elementary jump process, and thus to its dimensionality.Besides NMR techniques, for which polycrystalline materials are sufficient, macroscopic methods can be employed in the case of single crystalline samples. Here, the dimensionality of the transport process is reflected by the anisotropy of the mass or charge transport. Macroscopic techniques include impedance spectroscopy, mass tracer measurements and field gradient NMR spectroscopy.