The Joint Center for Energy Storage Research (JCESR), Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States.
J Phys Chem B. 2021 Nov 18;125(45):12574-12583. doi: 10.1021/acs.jpcb.1c08729. Epub 2021 Nov 8.
The diffusion behavior of Mg in electrolytes is not as readily accessible as that from Li or Na utilizing PFG NMR, due to the low sensitivity, poor resolution, and rapid relaxation encountered when attempting Mg NMR. In MgTFSI/DME solutions, "bound" DME (coordinating to Mg) and "free" DME (bulk) are distinguishable from H NMR. With the exchange rates between them obtained from 2D H EXSY NMR, we can extract the self-diffusivities of free DME and bound DME (which are equal to that of Mg) before the exchange occurs using PFG diffusion NMR measurements coupled with analytical formulas describing diffusion under two-site exchange. The high activation enthalpy for exhange (65-70 kJ/mol) can be explained by the structural change of bound DME as evidenced by its reduced C-H bond length. Comparison of the diffusion behaviors of Mg, TFSI, DME, and Li reveals a relative restriction to Mg diffusion that is caused by the long-range interaction between Mg and solvent molecules, especially those with suppressed motions at high concentrations and low temperatures.
利用 PFG NMR 研究镁在电解液中的扩散行为不如研究锂离子或钠离子那样容易，因为在尝试进行镁 NMR 时会遇到灵敏度低、分辨率差和弛豫迅速等问题。在 MgTFSI/DME 溶液中，“配位”的 DME（与镁配位）和“游离”的 DME（体相）可以通过 H NMR 进行区分。通过二维 H EXSY NMR 获得它们之间的交换速率，我们可以在交换发生之前使用 PFG 扩散 NMR 测量并结合描述两个位点交换下扩散的分析公式来提取游离 DME 和配位 DME（等于镁）的自扩散系数。高交换活化焓（65-70 kJ/mol）可以通过配位 DME 的结构变化来解释，这可以通过其 C-H 键长的缩短来证明。对镁、TFSI、DME 和 Li 的扩散行为进行比较表明，镁的扩散受到限制，这是由于镁与溶剂分子之间的远程相互作用造成的，尤其是在高浓度和低温下，溶剂分子的运动受到抑制。
