Wimperis Stephen, Rudman George E, Johnston Karen E
Department of Chemistry, Faraday Building, Lancaster University, Lancaster LA1 4YB, U.K.
Department of Chemistry, Durham University, Durham DH1 3LE, U.K.
J Phys Chem C Nanomater Interfaces. 2024 Mar 26;128(13):5453-5460. doi: 10.1021/acs.jpcc.4c00262. eCollection 2024 Apr 4.
Spin-lattice relaxation measurements are used in Li NMR studies of materials of potential use in solid-state Li-ion batteries as a probe of ion mobility on a fast (nanosecond to picosecond) time scale. The relaxation behavior is often analyzed by assuming exponential behavior or, equivalently, a single time constant. However, the spin-lattice relaxation of spin = 3/2 nuclei, such as Li, is in general biexponential; this is a fundamental property of = 3/2 nuclei and unrelated to any compartmentalization within the solid. Although the possibility of biexponential Li (and other = 3/2 nuclei) spin-lattice relaxation in the solid state has been noted by a number of authors, it can be difficult to observe unambiguously using conventional experimental NMR techniques, such as inversion or saturation recovery. In this work, we show that triple-quantum-filtered NMR experiments, as previously exploited in = 3/2 NMR of liquids, can be used in favorable circumstances to observe and readily quantify biexponential Li spin-lattice relaxation in solids with high ion mobility. We demonstrate a triple-quantum-filtered inversion-recovery experiment on the candidate solid electrolyte material LiOHCl at 325 K, which has previously been shown to exhibit fast ion mobility, and we also introduce a novel triple-quantum-filtered saturation-recovery experiment. The results of these solid-state NMR experiments are less straightforward than those in liquids as a consequence of the unwanted direct excitation of triple-quantum coherences by the weak (compared with the unaveraged Li quadrupolar interaction) pulses used, but we show that this unwanted excitation can be accounted for and, in the example shown here, does not impede the extraction of the two Li spin-lattice relaxation times.
自旋 - 晶格弛豫测量用于对固态锂离子电池中潜在使用材料的锂核磁共振（Li NMR）研究，作为快速（纳秒到皮秒）时间尺度上离子迁移率的探针。弛豫行为通常通过假设指数行为或等效地假设单个时间常数来分析。然而，自旋为3/2的核（如锂）的自旋 - 晶格弛豫通常是双指数的；这是自旋为3/2的核的基本性质，与固体中的任何分隔无关。尽管许多作者已经指出了固态中双指数锂（以及其他自旋为3/2的核）自旋 - 晶格弛豫的可能性，但使用传统的实验核磁共振技术（如反转或饱和恢复）可能难以明确观察到。在这项工作中，我们表明，如先前在液体的自旋为3/2的核磁共振中所采用的三量子滤波核磁共振实验，在有利的情况下可用于观察并轻松量化具有高离子迁移率的固体中双指数锂自旋 - 晶格弛豫。我们在325 K下对候选固体电解质材料LiOHCl进行了三量子滤波反转恢复实验，该材料先前已被证明具有快速离子迁移率，并且我们还引入了一种新型的三量子滤波饱和恢复实验。由于所使用的弱脉冲（与未平均的锂四极相互作用相比）对三量子相干的不必要直接激发，这些固态核磁共振实验的结果比液体中的结果更不直观，但我们表明这种不必要的激发可以得到解释，并且在这里所示的例子中，并不妨碍提取两个锂自旋 - 晶格弛豫时间。
