Low-dimensional lithium ion conductors

本项目主要研究Li扩散受维数效应的影响。该研究将在具有结构限制扩散路径的单晶和多晶材料上进行。为此，将采用核磁共振技术，它对锂离子的迁移率和扩散过程的维度都很敏感。一维和二维扩散可以通过温度相关和频率相关的6,7 li核磁共振自旋晶格弛豫（SLR）测量来区分。一维和二维锂离子导体均表现出扩散诱导单反核磁共振率峰的不对称性特征。此外，在所谓的高温极限中，速率显示出对拉莫尔频率ω0的特征依赖，其中平均跳变率1/τ远大于ω0。通常，在普通磁场强度下，频率ω0/2π(7Li)的范围为10到300 MHz。在一维扩散情况下，适当的松弛模型预测速率与ω0的平方根依赖关系，而在二维扩散情况下则是对数依赖关系。类似地，在旋转参照系中进行的单反测量也是如此，其中拉莫尔频率被锁定频率ω1所取代，其数量级为几kHz。在这种情况下，所涉及的跳跃过程比通常在实验室参考系中探测的过程慢1000倍。通过核磁共振自旋对准回波（SAE）测量可以直接获得更低的跳变率，而不需要模型将衰减率转换为Li跳变率。SAE核磁共振技术本身对基本跳变过程的几何形状很敏感，因此对其维数也很敏感。除了核磁共振技术，多晶材料就足够了，宏观方法可以用于单晶样品。在这里，输运过程的维度是由质量或电荷输运的各向异性来反映的。宏观技术包括阻抗谱、质量示踪测量和场梯度核磁共振谱。

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.