Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
Phys Chem Chem Phys. 2019 Feb 20;21(8):4538-4546. doi: 10.1039/c8cp07776b.
Anisotropic battery electrodes that allow enhanced diffusion through the thickness of the electrode can be engineered to improve the rate performance, but direct measurement of 3D diffusion in this pore structure is extremely challenging. To address this, we used 1H and 7Li pulsed field gradient (PFG) NMR to measure anisotropic diffusion in a model porous silicon substrate. We show that NMR spectroscopy can resolve solvent molecules and ions (here, in H2O, DMSO, and the battery electrolyte LIPF6:DC:EMC) in and outside of the pores of the Si substrate, allowing the diffusion coefficients of the ion/molecules in the two components to be individually determined. Exchange between ions/molecules inside and outside of the pores is observed with 1H 2D exchange spectroscopy (EXSY). The pore dimensions can extracted from the diffusivity of the in-pore component and the results are in reasonable agreement with the pore dimensions measured with electron microscopy. Better agreement is obtained for pore diameters; for pore length measurements, exchange between the in-pore and ex-pore solvents should be accounted for. These results suggest that PFG-NMR can serve as a non-destructive characterisation method for both in situ and ex situ analyses of materials ranging from complex battery and supercapacitor electrodes to catalyst supports and tissue scaffolds.
各向异性的电池电极可以通过工程设计来允许增强通过电极厚度的扩散，从而提高倍率性能，但直接测量这种孔结构中的 3D 扩散极具挑战性。为了解决这个问题，我们使用 1H 和 7Li 脉冲梯度（PFG）NMR 来测量模型多孔硅衬底中的各向异性扩散。我们表明，NMR 光谱可以分辨出溶剂分子和离子（这里是 H2O、DMSO 和电池电解质 LIPF6：DC：EMC）在 Si 衬底的孔内外，从而可以单独确定离子/分子在两个组件中的扩散系数。通过 1H 2D 交换光谱（EXSY）观察到孔内外离子/分子之间的交换。可以从孔内组分的扩散系数中提取出孔径，结果与电子显微镜测量的孔径尺寸基本一致。对于孔径测量，更好的一致性；对于孔长度的测量，应该考虑孔内和孔外溶剂之间的交换。这些结果表明，PFG-NMR 可以作为一种无损的表征方法，用于从复杂的电池和超级电容器电极到催化剂载体和组织支架等材料的原位和非原位分析。
