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

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.


X-ray powder diffraction 2. Room temperature 6 Li NMR measurements
Room temperature 6 Li NMR measurements were performed in order to determine the Fermi contact shift of Li sites around the Fe dopants. Rotor synchronized Hahn echo sequence was used on samples spinning at 60 kHz. A short recycle delay of 50 ms was chosen in order to give the Li environments in the vicinity of the dopant higher weight in the spectrum. We note that the we expect here that Li sites bonded to Fe (III) through a bridging oxygen are not quenched at room temperature and can therefore be detected even though at 100 K they may be quenched. This expectation is based on two assumptions: (i) The main cause for quenching is fast transverse nuclear relaxation which results in severe broadening. In this case transverse relaxation will be mostly driven by the electron longitudinal relaxation through its modulation of the electron-nuclear couplings. Treating the electron relaxation time, T1e, as the correlation time of the motion in the system, when 1/T1e is of the order of the nuclear Larmor frequency (n=58 MHz), it will drive Figure S1 X ray powder diffraction patterns of the undoped and doped Fe LTO samples compared with the simulated pattern taken from the ICSD. both longitudinal and transverse nuclear relaxation since the spectral density, J(), will have nonnegligible contribution at both J(0) and J(n). As the electron relaxation shortens, due to temperature and electron-electron interactions with increasing Fe doping, its efficiency in driving nuclear relaxation will decrease with decreasing T1e, since the spectral density will be broader with lower contribution at J(0) and J(n). (ii) A second contribution to quenching is spectral broadening due to electron-nuclear through space dipolar interactions. These are averaged with increasing efficiency as the electron relaxation becomes faster. Thus, here again we expect that with increasing dopant concentration and temperature, quenching effects will be significantly reduced and finally eliminated. These assumptions will be investigated thoroughly and quantitatively in a future study. The room temperature 6 Li spectra of Fe00125-10 are plotted in Figure S2. The low Fe content sample displayed only a single sharp resonance corresponding to the majority of Li in the sample which are not in close proximity with the dopant. Due to the low concentration of dopant only a small fraction of Li will be broadened and/or shifted by the Fe(III) electrons and are most likely below the detection limit of this room temperature measurement. As the Fe content increased, a broad spectral component appeared, growing in contribution with Fe content. Taking into account the two assumptions discussed above, we assign this resonance to the Li in close proximity of the dopant, which in the Fe10 sample are the major component in the spectrum. This resonance is centered at 1 ppm and has a width of 36 ppm, suggesting the Fermi contact shift is negligible in this compound.

ENDOR Simulations
7 Li Mims ENDOR spectra were simulated for the two possible doping sites in the spinel structure (based on the undoped structure).
In each site the distances to the first three nearest Li ions was simulated.

Quenching, absolute and components enhancement factors
The absolute enhancement factors were obtained by taking into account the increasing loss of signal due to quenching by the Fe dopant at 100 K ( Figure S4), the shortening of nuclear relaxation of doped samples compared to undoped sample at room temperature and the gain in sensitivity due to performing the experiments at low temperature 1,2 . For 6 Li, due to the long relaxation of the undoped sample at 100 K we were not able to acquire a quantitative measurement. Since for 7 Li there is no significant quenching effect comparing the undoped and Fe00125 samples, we use the 6 Li Fe00125 signal as a reference point to estimate quenching with increasing Fe content. Absolute enhancement factors obtained for 6,7 Li are plotted in Figure S5, in comparison to the enhancement determined from the ratio of signal intensity with and without waves. As mentioned in the main text, with increasing Fe(III) content a second broad component was observed in the spectrum ( Figure S6a). The spectra of the various samples were deconvoluted in DMFIT 3 and the spectral contribution of the broad and narrow components is plotted in Figures  S6b. This analysis shows that the ratio between these two components was maintained in spectra acquired with and without waves. This is also reflected by the Figure S4 Quantitative measurement of Li signal in the doped samples in comparison with undoped LTO measured at 100 K without waves. The values were obtained by acquiring the Li spectra with a single pulse excitation with a recovery delay set at 5*T1. For 6 Li the lowest dopant concentration was used as reference since the undoped sample had a relaxation time exceeding 20 h at 100 K.

Figure S5
The enhancement and absolute enhancement factors obtained for 7,6 Li as a function of Fe content. similar enhancement factors obtained for the two components as a function of Fe content ( Figure  S6c).

Quantification of the Fe(III) reduction process
To support the assignment of the redox process observed at about 1.8 V on the cathodic scan in the CV experiments performed on Fe doped LTO we calculate the mole of electrons transferred during that process. The integrated area under this redox peak vs. time is about 50mAs. Thus the total amount of charge passed in the cell during that process: 0.05 C / 96485 C/mole= 5.2x10 -7 mole of electrons. The active mass of the electrode was 2.75 mg. Neglecting the change in the molecular weight of the LTO with doping (459.083 g/mol) we obtain that for x=0.1 Fe in LTO formula there are 6x10 -7 mole of Fe(III) in the electrode. Since there is a transfer of approximately one electron per Fe(III) site we assign this process to the reduction of Fe(III) to Fe(II).

Sweep profile of cycled samples
The electrochemistry data suggest that not all Fe is oxidized back to Fe(III) even with the extended potential window since the reduction peak is larger than the oxidation one. In that case, either the concentration of Fe(III) within the particles will be lower compared to the concentration prior to cycling or that the sample will become heterogenous, varying in the Fe(III) content. The change in the shape of the DNP sweep profile may be interpreted as a situation where a fraction of the LTO particles is not active in DNP and contribute a constant value across the sweep profile, independent of the magnetic field. This scenario is described schematically in Figure S8. To examine this point, we used the experimental result of the DNP sweep profile obtained for the pristine powder (pink curve in Figure S8) and calculated the needed contribution of non-active LTO (with a constant signal intensity across the sweep) in order to reproduce the shape obtained in the sweep profile of the cycler Fe-LTO. We found that with about 25% non-active LTO (blue curve in Figure S8) we can reproduce the experimental result for the sweep fairly well (comparing the dark purple calculated curve with the experimental curve in light purple, Figure S8).