Endogenous Dynamic Nuclear Polarization for Natural Abundance ¹⁷O and Lithium NMR in the Bulk of Inorganic Solids

Abstract

In recent years magic angle spinning-dynamic nuclear polarization (MAS-DNP) has developed as an excellent approach for boosting the sensitivity of solid-state NMR (ssNMR) spectroscopy, thereby enabling the characterization of challenging systems in biology and chemistry. Most commonly, MAS-DNP is based on the use of nitroxide biradicals as polarizing agents. In materials science, since the use of nitroxides often limits the signal enhancement to the materials' surface and subsurface layers, there is need for hyperpolarization approaches which will provide sensitivity in the bulk of micron sized particles. Recently, an alternative in the form of paramagnetic metal ions has emerged. Here we demonstrate the remarkable efficacy of Mn(II) dopants, used as endogenous polarization agents for MAS-DNP, in enabling the detection of ¹⁷O at a natural abundance of only 0.038%. Distinct oxygen sites are identified in the bulk of micron-sized crystals, including battery anode materials Li₄Ti₅O₁₂ (LTO) and Li₂ZnTi₃O₈, as well as the phosphor materials NaCaPO₄ and MgAl₂O₄, all doped with Mn(II) ions. Density functional theory calculations are used to assign the resonances to specific oxygen environments in these phases. Depending on the Mn(II) dopant concentration, we obtain significant signal enhancement factors, 142 and 24, for ⁶Li and ⁷Li nuclei in LTO, respectively. We furthermore follow the changes in the ^6,7Li LTO resonances and determine their enhancement factors as a function of Mn(II) concentration. The results presented show that MAS-DNP from paramagnetic metal ion dopants provides an efficient approach for probing informative nuclei such as ¹⁷O, despite their low gyromagnetic ratio and negligible abundance, without isotope enrichment.