Endogenous Dynamic Nuclear Polarization via Metal Ions in Lithium Titanate

The structure and spatial arrangement in materials is directly linked to their function. Doping is one technique of altering chemical composition and local structure in order to gain new properties (e.g. conductivity). Solid state Nuclear Magnetic Resonance (ssNMR) spectroscopy presents itself as an informative method for probing structural changes in the short range related to the function of materials. Dynamic nuclear polarization (DNP), a technique in which the high spin polarization of unpaired electrons is transferred to surrounding coupled nuclei via microwave radiation, enables to achieve unprecedented sensitivity and selectivity in NMR experiments. The most commonly used polarizing agents for DNP-NMR are nitroxide radicals, and while they reach enhancement factors of ca. 200-fold increase in sensitivity, they are limited to surface probing and additionally might be reactive. Paramagnetic metal ions have recently been introduced as alternatives for nitroxides. Doping functional materials with paramagnetic metal ions can alter their properties and additionally introduces endogenous polarizing agents that enable to correlate properties to structural changes in the bulk as well as the surface. Doping of L₄Ti₅O₁₂ (LTO), used as an anode material for lithium ion batteries, with paramagnetic metal ions has been shown to improve ionic mobility. Herein, we demonstrate the incorporation of metal ions into LTO, as a sensitive probe for structural characterization using DNP-NMR, and show the dependence of the local environments and DNP processes occurring on the concentration of metal ions .