The use of 6Li{7Li}-REDOR NMR spectroscopy to compare the ionic conductivities of solid-state lithium ion electrolytes.

Garnet-like solid-state electrolyte materials for lithium ion batteries are promising replacements for the currently-used liquid electrolytes. This work compares the temperature dependent Li(+) ion hopping rate in Li6BaLa2M2O12 (M = Ta, Nb) using solid-state (6)Li{(7)Li}-REDOR NMR. The slope of the (6)Li{(7)Li}-REDOR curve is highly temperature dependent in these two phases, and a comparison of the changes of the REDOR slopes as a function of temperature has been used to evaluate the Li(+) ion dynamics. Our results indicate that the Nb phase has a higher overall ionic conductivity in the range of 247 K to 350 K, as well as a higher activation energy for lithium ion hopping than the Ta counterpart. For appropriate relative timescales of the dipolar couplings and ion transport processes, this is shown to be a facile method to compare ion dynamics among similar structures.

Studies of lithium ion dynamics in paramagnetic cathode materials using (6)Li 1D selective inversion methods.

The effectiveness of two different selective inversion methods is investigated to determine timescales of Li ion mobility in paramagnetic Li intercalation materials. The first method is 1D exchange spectroscopy, which employs a 90°-τ(1)-90° sequence for selective inversion of a Li resonance undergoing site exchange. The experiment is most easily applied when the first delay period, τ(1), is set to the frequency difference between two resonances undergoing ion exchange. This enables the determination of ion hopping timescales for single exchange pair systems only. To measure ion dynamics in systems having more than one exchange process, a second selective inversion method was tested on two paramagnetic Li intercalation materials. This second technique, replaces the 90°-τ(1)-90° portion of 1D EXSY with a long, selective shaped pulse (SP). Two paramagnetic solid-state materials, which are both cathode materials for lithion ion batteries, were chosen as model compounds to test the effectiveness of both the selective inversion methods. The first compound, Li(2)VPO(4)F, was chosen as it hosts two Li sites with 1-exchange process. The second model compound is a 3-site, 3-exchange process system, Li(2)VOPO(4). For the 2-site material, Li(2)VPO(4)F, the timescales of the single A-B exchange process were found to be within error of one another regardless of the inversion method. For the 3 Li-site material Li(2)VOPO(4), the three exchange processes, AB, BC, and AC, were found to be on the millisecond timescale as revealed using the SP method. These timescales were determined over a variable temperature range where activation energies extended from 0.6 ± 0.1 eV up to 0.9 ± 0.2 eV.

Influence of particle size on solid solution formation and phase interfaces in Li0.5FePO4 revealed by 31P and 7Li solid state NMR spectroscopy.

Here we report the observation of electron delocalization in nano-dimension xLiFePO(4):(1 - x)FePO(4) (x = 0.5) using high temperature, static, (31)P solid state NMR. The (31)P paramagnetic shift in this material shows extreme sensitivity to the oxidation state of the Fe center. At room temperature two distinct (31)P resonances arising from FePO(4) and LiFePO(4) are observed at 5800 ppm and 3800 ppm, respectively. At temperatures near 400 °C these resonances coalesce into a single narrowed peak centered around 3200 ppm caused by the averaging of the electronic environments at the phosphate centers, resulting from the delocalization of the electrons among the iron centers. (7)Li MAS NMR spectra of nanometre sized xLiFePO(4):(1 - x)FePO(4) (x = 0.5) particles at ambient temperature reveal evidence of Li residing at the phase interface between the LiFePO(4) and FePO(4) domains. Moreover, a new broad resonance is resolved at 65 ppm, and is attributed to Li adjacent to the anti-site Fe defect. This information is considered in light of the (7)Li MAS spectrum of LiMnPO(4), which despite being iso-structural with LiFePO(4) yields a remarkably different (7)Li MAS spectrum due to the different electronic states of the paramagnetic centers. For LiMnPO(4) the higher (7)Li MAS paramagnetic shift (65 ppm) and narrowed isotropic resonance (FWHM ≈ 500 Hz) is attributed to an additional unpaired electron in the t(2g) orbital as compared to LiFePO(4) which has δ(iso) = -11 ppm and a FWHM = 9500 Hz. Only the delithiated phase FePO(4) is iso-electronic and iso-structural with LiMnPO(4). This similarity is readily observed in the (7)Li MAS spectrum of xLiFePO(4):(1 - x)FePO(4) (x = 0.5) where Li sitting near Fe in the 3+ oxidation state takes on spectral features reminiscent of LiMnPO(4). Overall, these spectral features allow for better understanding of the chemical and electrochemical (de)lithiation mechanisms of LiFePO(4) and the Li-environments generated upon cycling.

(7)Li NMR and two-dimensional exchange study of lithium dynamics in monoclinic Li(3)V(2)(PO(4))(3).

High-resolution solid-state (7)Li NMR was used to characterize the structure and dynamics of lithium ion transport in monoclinic Li(3)V(2)(PO(4))(3). Under fast magic-angle spinning (MAS) conditions (25 kHz), three resonances are clearly resolved and assigned to the three unique crystallographic sites. This assignment is based on the Fermi-contact delocalization interaction between the unpaired d-electrons at the vanadium centers and the lithium ions. One-dimensional variable-temperature NMR and two-dimensional exchange spectroscopy (EXSY) are used to probe Li mobility between the three sites. Very fast exchange, on the microsecond time scale, was observed for the Li hopping processes. Activation energies are determined and correlated to structural properties including interatomic Li distances and Li-O bottleneck sizes.

Solid-state 2H NMR study of methyl-d3-cobalamin.

A solid-state 2H NMR study of methyl-d3-cobalamin has been performed as a function of temperature to provide information concerning the character and energetics of the motion performed by this unique bioorganometallic methyl group. Analysis of the 2H NMR line shape indicates that the methyl group undergoes rapid three-fold rotation, and that the Co-C-2H angle lies between 105.9 and 109.5 degrees. Determination of the spin-lattice relaxation times T1 shows that the relaxation is anisotropic, consistent with a "jumping" motion of the methyl group rather than rotational diffusion. This also provides the activation energy to methyl jumps as 8.3 +/- 1.3 kJ/mol. It is proposed that this energetic barrier may be a useful probe of changes in the electronic character of the Co-C bond that accompany the biological role of this molecule in such enzymes as methionine synthase.