Endogenous Dynamic Nuclear Polarization for Sensitivity Enhancement in Solid-State NMR of Electrode Materials

Abstract

Rational design of materials for energy storage systems relies on our ability to probe these materials at various length scales. Solid-state NMR spectroscopy is a powerful approach for gaining chemical and structural insights at the atomic/molecular level, but its low detection sensitivity often limits applicability. This limitation can be overcome by transferring the high polarization of electron spins to the sample of interest in a process called dynamic nuclear polarization (DNP). Here, we employ for the first time metal ion-based DNP to probe pristine and cycled composite battery electrodes. A new and efficient DNP agent, Fe­(III), is introduced, yielding lithium signal enhancement up to 180 when substituted in the anode material Li4Ti5O12. In addition for being DNP active, Fe­(III) improves the anode performance. Reduction of Fe­(III) to Fe­(II) upon cycling can be monitored in the loss of DNP activity. We show that the dopant can be reactivated (return to Fe­(III)) for DNP by increasing the cycling potential window. Furthermore, we demonstrate that the deleterious effect of carbon additives on the DNP process can be eliminated by using carbon free electrodes, doped with Fe­(III) and Mn­(II), which provide good electrochemical performance as well as sensitivity in DNP-NMR. We expect that the approach presented here will expand the applicability of DNP for studying materials for frontier challenges in materials chemistry associated with energy and sustainability.

Enabling Natural Abundance 17O Solid-State NMR by Direct Polarization from Paramagnetic Metal Ions

“…An increment of dopant concentration between 0.0025 and 0.01 merely reduces the average distance between nuclei and unpaired electrons, shortening the buildup times. A further increment in concentration will ultimately shorten the electronic relaxation times, as we previously reported for this system, 29 dampening the polarization efficiency, vide infra. In Fe00125LTO, the quadrupolar relaxation mechanism has a significant contribution.…”

“… 24 − 26 In particular, introducing small amounts of metal ions 27 as polarization agents in the MIDNP 28 approach has proven efficient for obtaining signal enhancements in the bulk of inorganic materials. 18 , 29 Still, it has been assumed that also in these approaches, efficient polarization buildup requires spin-diffusion to spread the hyperpolarization over the sample volume.…”

“…Not requiring efficient spin diffusion mechanisms will mostly benefit nuclear spins with limited sensitivity due to low gyromagnetic ratios or low isotope abundance. We examine this concept by measuring 17 O MAS DNP NMR spectra in a series of Fe(III)-doped Li 4 Ti 5 O 12 (FeLTO) powders prepared via solid-state synthesis, as described in ref ( 29 ), with a varying Fe(III) mole fraction per LTO unit of x = 0.00125, 0.0025, 0.005, 0.01, and 0.02 (hereon labeled Fe00125-02LTO). These mole fractions are equivalent to a paramagnetic agent molarity ranging from 9.5 to 152 mM.…”

“…This is in line with the experimentally observed smaller enhancement in the sample with the highest Fe(III) content, for which we reported a decrease in the electronic relaxation times. 29 …”

Non-oxidative dehydrogenation of isobutane over supported vanadium oxide: nature of the active sites and coke formation

“…[39][40][41][42][43][46][47][48][49][50][51][52] Ruling out the possibility of using highly expensive 13 C-enriched reactants to address low sensitivity, we have recently demonstrated that the utilization of dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP SENS) is an ideal alternative. 53 Prior to this work, DNP SENS was conventionally applied for the characterization of inorganic materials, [54][55][56][57][58][59][60][61][62] including surface organometallic fragments/catalysts, metal-organic frameworks (MOFs), and amorphous aluminosilicates. [63][64][65][66][67] Here, we further demonstrate the potential application of this advanced technique to elucidate the nature of coke deposits during the dehydrogenation of isobutane.…”