Prof. Rob Messinger
Department of Chemical Engineering, The City College of New York
“Measuring and Understanding Local (Dis)order: Crystallization of Zeolite Nanosheets & Defects in Li-ion Battery Electrodes”
Measuring and understanding how order develops in crystallizing materials is a challenging problem, especially during the syntheses of self-assembled materials that exhibit both crystalline and mesoscopic order. Surfactant-directed zeolite MFI nanosheets exhibit improved transport and catalytic properties compared to their bulk zeolite counterparts, particularly with respect to large molecules. During their hydrothermal syntheses, coupled framework crystallization and surfactant self-assembly processes occur that are poorly understood and difficult to control. Local compositions, atomic and mesoscale structures, and surfactant-framework interactions were monitored throughout their syntheses by multi-dimensional solid-state 1H and 29Si nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction (XRD), and electron microscopy measurements. The analyses establish how the atomic and mesoscale framework structures evolve during hydrothermal synthesis, revealing that the zeolite MFI nanosheets form via intermediate nano-layered frameworks with 2D crystal-like structures.
Similarly, identifying and characterizing atomic-scale disorder in crystalline solids can be challenging experimentally, particularly for lithium-ion intercalation electrodes that can exhibit multiple oxidation and spin states. LiVPO4F is one of the most energy-dense polyanionic electrode materials currently known for lithium-ion batteries, but recent solid-state NMR measurements have revealed unusual extents of local disorder. Highly crystalline LiVPO4F samples were synthesized, as determined by XRD and scanning transmission electron microscopy (STEM) measurements. Solid-state 7Li NMR spectra reveal unexpected paramagnetic lithium environments that can account for up to 20% of the total lithium content. Multi-dimensional and site-selective solid-state 7Li-7Li dipolar recoupling NMR experiments establish unambiguously that the unexpected lithium environments are defects within the LiVPO4F crystal structure, further revealing the sub-nanometer-scale proximities between them. The lithium defects are shown to exhibit altered electronic environments that result from changed oxidation states of nearby paramagnetic vanadium atoms. The results provide a general strategy for characterizing sub-nanometer-scale disorder in lithium-containing crystalline solids, including highly paramagnetic materials with short NMR relaxation times on the order of ms.
6:00 pm Dinner
7:00 pm Seminar
A57, Frick Chemistry Laboratory Atrium, Princeton University
$15 employed / $5 students, postdoc, retired, unemployed.
No charge for seminar only.