Stanje B, Epp V, Nakhal S, Lerch M, Wilkening M
Christian Doppler Laboratory for Lithium Batteries, and Institute for Chemistry and Technology of Materials (Member of NAWI Graz), Graz University of Technology , Stremayrgasse 9, A-8010 Graz, Austria.
ACS Appl Mater Interfaces. 2015 Feb 25;7(7):4089-99. doi: 10.1021/am5078655. Epub 2015 Feb 11.
Layer-structured materials, such as graphite (LiCy) or Lix(Co,Ni,Mn)O2, are important electrode materials in current battery research that still relies on insertion materials. This is due to their excellent ability to reversibly accommodate small alkali ions such as Li(+) and Na(+). Despite of these applications, microscopic information on Li ion self-diffusion in transition metal sulfides are relatively rare. Here, we used (7)Li nuclear magnetic resonance (NMR) spectroscopy to study translational Li ion diffusion in hexagonal (2H) LixNbS2 (x = 0.3, 0.7, and 1) by means of variable-temperature NMR relaxometry. (7)Li spin-lattice relaxation rates and (7)Li NMR spectra were used to determine Li jump rates and activation barriers as a function of Li content. Hereby, NMR spin-lattice relaxation rates recorded with the spin-lock technique offered the possibility to study Li ion dynamics on both the short-range and long-range length scale. Information was extracted from complete diffusion-induced rate peaks that are obtained when the relaxation rate is plotted vs inverse temperature. The peak maximum of the three samples studied shifts toward higher temperatures with increasing Li content x in 2H-LixNbS2. Information on the dimensionality of the diffusion process was experimentally obtained by frequency dependent Rρ measurements carried out at T = 444 K, that is in the high-temperature regime of the rate peaks. A slight, but measurable frequency-dependence within this limit is found for all samples; it is in good agreement with predictions from relaxation models developed to approximate low-dimensional (2D) jump diffusion.
层状结构材料，如石墨（LiCy）或Lix(Co,Ni,Mn)O2，是当前仍依赖嵌入材料的电池研究中的重要电极材料。这是因为它们具有出色的可逆容纳Li(+)和Na(+)等小碱金属离子的能力。尽管有这些应用，但关于过渡金属硫化物中锂离子自扩散的微观信息相对较少。在这里，我们使用（7）Li核磁共振（NMR）光谱，通过变温NMR弛豫测量法研究六方（2H）LixNbS2（x = 0.3、0.7和1）中Li离子的平移扩散。（7）Li自旋晶格弛豫率和（7）Li NMR光谱用于确定Li跳跃率和作为Li含量函数的活化能垒。在此，用自旋锁定技术记录的NMR自旋晶格弛豫率提供了在短程和长程长度尺度上研究Li离子动力学的可能性。信息是从当弛豫率与绝对温度的倒数作图时获得的完整扩散诱导率峰中提取的。在所研究的三个样品中，随着2H-LixNbS2中Li含量x的增加，峰最大值向更高温度移动。通过在T = 444 K（即率峰的高温区域）进行的频率相关Rρ测量，实验获得了关于扩散过程维度的信息。在这个极限范围内，所有样品都发现了轻微但可测量的频率依赖性；这与为近似低维（2D）跳跃扩散而开发的弛豫模型的预测非常吻合。
