Low-dimensional lithium ion conductors

该项目致力于研究维度效应对锂扩散的影响。研究将在选定的单晶和多晶材料上进行，这些材料具有结构受限的扩散路径。为此，将使用对锂离子的迁移率和扩散过程的维度都敏感的核磁共振技术，通过温度和频率相关的6，7Li核磁共振自旋晶格弛豫(SLR)测量可以区分一维和二维扩散。一维和二维锂离子导体都表现出扩散诱导的SLR核磁共振速率峰的特征不对称性。此外，在所谓的高温极限中，速率表现出与拉莫尔频率ω0的特征依赖关系，其中平均跳跃速率1/τ远大于ω0。通常，在相同的磁场强度下，频率ω0/2π(7Li)的范围从10到300兆赫兹。在一维扩散的情况下，适当的松弛模型预测了速率与ω0的平方根相关性，在二维扩散的情况下预测了速率的对数相关性。类似地，这同样适用于在旋转参照系中执行的单反测量，其中拉莫尔频率被数量级为几千赫兹的锁定频率ω1所取代。在这种情况下，跳跃过程被覆盖，比通常在实验室参照系中探测的过程慢1000倍。甚至更低的跳跃率也可以通过核磁共振自旋排列回波(SAE)测量直接获得，这不需要一个模型来将衰减率转换为Li跳跃率。SAE核磁共振技术本身对基本跳跃过程的几何形状敏感，因此对其维度也很敏感。除了多晶材料就足够的核磁共振技术外，宏观方法也可以用于单晶样品的情况。这里，传输过程的维度由质量或电荷传输的各向异性来反映。宏观技术包括阻抗光谱、质量示踪测量和场梯度核磁共振波谱。

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.