Li Ion Dynamics in Al-Doped Garnet-Type Li7La3Zr2O12 Crystallizing with Cubic Symmetry

Lithium-ion dynamics in the garnet-type solid electrolyte “Li7La3Zr2O12” (LLZ) crystallizing with cubic symmetry was probed by means of variable-temperature 7Li NMR spectroscopy and ac impedance measurements. Li jump rates of an Al-containing sample follow Arrhenius behaviour being characterized by a relatively high activation energy of 0.54(3) eV and a pre-exponential factor of 2.2(5) × 10 13 s-1. The results resemble those which were quite recently obtained for an Al-free LLZ sample crystallizing, however, with tetragonal symmetry. Hence, most likely, the significantly higher Li conductivity previously reported for a cubic LLZ sample cannot be ascribed solely to the slight structural distortions accompanying the change of the crystal symmetry. Here, even Al impurities, acting as stabilizer for the cubic polymorph at room temperature, do not lead to the high ion conductivity reported previously.

Studying Li Dynamics in a Gas-Phase Synthesized Amorphous Oxide by NMR and Impedance Spectroscopy

Li diffusion parameters of a structurally disordered Li-Al-Si-oxide prepared by gas-phase synthesis were complementarily investigated by both time-domain NMR techniques and impedance spectroscopy. The first include 7Li NMR spin-lattice relaxation (SLR) measurements in the laboratory as well as in the rotating frame of reference. An analysis of variable-temperature NMR line widths point to an activation energy Ea of approximately 0.6 eV. The value is confirmed by rotating-frame SLR NMR data recorded at approximately 11 kHz. Above room temperature the low-temperature flank of a diffusion-induced rate peak shows up which can be approximated by an Arrhenius law yielding Ea=0.56(1) eV. This is in very good agreement with the result obtained from 7Li spin-alignment echo (SAE) NMR being sensitive to even slower Li dynamics. For comparison, dc-conductivity measurements, probing long-range motions, yield Ea=0.8 eV. Interestingly, low-temperature SAE NMR decay rates point to localized Li motions being characterized with a very small activation energy of only 0.09 eV.

NMR relaxometry as a versatile tool to study Li ion dynamics in potential battery materials

NMR spin relaxometry is known to be a powerful tool for the investigation of Li(+) dynamics in (non-paramagnetic) crystalline and amorphous solids. As long as significant structural changes are absent in a relatively wide temperature range, with NMR spin-lattice (as well as spin-spin) relaxation measurements information on Li self-diffusion parameters such as jump rates and activation energies are accessible. Diffusion-induced NMR relaxation rates are governed by a motional correlation function describing the ion dynamics present. Besides the mean correlation rate of the dynamic process, the motional correlation function (i) reflects deviations from random motion (so-called correlation effects) and (ii) gives insights into the dimensionality of the hopping process. In favorable cases, i.e., when temperature- and frequency-dependent NMR relaxation rates are available over a large dynamic range, NMR spin relaxometry is able to provide a comprehensive picture of the relevant Li dynamic processes. In the present contribution, we exemplarily present two recent variable-temperature (7)Li NMR spin-lattice relaxation studies focussing on Li(+) dynamics in crystalline ion conductors which are of relevance for battery applications, viz. Li(7) La(3)Zr(2)O(12) and Li(12)Si(7).

Spin-alignment echo NMR: probing Li+ hopping motion in the solid electrolyte Li7La3Zr2O12 with garnet-type tetragonal structure

(7)Li spin-alignment echo (SAE) nuclear magnetic resonance (NMR) spectroscopy has been used to measure single-spin hopping correlation functions of polycrystalline Li(7)La(3)Zr(2)O(12). Damping of the echo amplitude S(2)(t(m),t(p)), recorded at variable mixing time t(m) but fixed preparation time t(p), turns out to be solely controlled by slow Li jump processes taking place in the garnet-like structure. The decay rates τ(SAE)(-1) directly obtained by parametrizing the curves S(2)(t(m),t(p)) with stretched exponential functions show Arrhenius behaviour pointing to an activation energy of approximately 0.5 eV. This value, probed by employing an atomic-scale NMR method, is in very good agreement with that deduced from impedance spectroscopy used to measure macroscopic Li transport parameters. Most likely, the two methods are sensitive to the same hopping correlation function although Li dynamics are probed in a quite different manner.

Ultra-slow Li ion dynamics in Li(2)C(2)--on the similarities of results from (7)Li spin-alignment echo NMR and impedance spectroscopy

Li diffusion and transport parameters of binary lithium carbide Li(2)C(2) were complementarily investigated by (7)Li (nuclear magnetic resonance) NMR and impedance spectroscopy. Long-range Li diffusion parameters were measured by using mixing-time-dependent and temperature-variable stimulated echo NMR spectroscopy. The method is sensitive to ultra-slow Li hopping processes which were probed from an atomic-scale point of view. Two-time phase correlation functions S(2) obtained can be parameterized by stretched exponentials only. The corresponding echo decay rates τ(-1), which were recorded at a resonance frequency of e.g. 155.5 MHz, show Arrhenius behaviour revealing an activation energy of 0.80(2) eV. This value is in very good agreement with that deduced from dc conductivity measurements (0.79(2) eV) probing Li transport processes on a macroscopic length scale. The comparison of impedance data with the measured NMR echo decay functions showed that both methods reflect diffusion processes being characterized by very similar motional correlation functions.

Microscopic Access to Long-Range Diffusion Parameters of the Fast Lithium Ion Conductor Li7BiO6 by Solid State 7Li Stimulated Echo NMR

Li self-diffusion in rhombohedral Li7BiO6, being a promising basic material for cathodes of rechargeable ion batteries, is studied by means of 7Li stimulated echo NMR. Using the pulse sequence introduced by Jeener and Broekaert, a spin-alignment echo is created whose amplitude decay is recorded as a function of mixing time. The so-obtained two-time correlation functions follow stretched exponential behavior and lead to decay rates which can be identified directly with microscopic Li motional correlation rates (tau(-1)). Using a jump distance of about 0.2 nm, this results in a diffusion coefficient (D) of about 0.5 x 10(-16) m(2) s(-1) at 294 K. The activation energy turned out to be 0.53(3) eV which is in very good agreement with recently obtained results by means of dc-conductivity measurements probing long-range diffusion parameters. This shows that stimulated echo NMR, due to its inherent time scale, gives microscopic access to long-range transport. The prefactor tau(0)(-1) of the corresponding Arrhenius law lies in the typical range of phonon frequencies, tau(0)(-1) = 3 x 10(12) s(-1).

Ultraslow Li diffusion in spinel-type structured Li4Ti5O12 - a comparison of results from solid state NMR and impedance spectroscopy

The cubic spinel oxides Li(1+x)Ti(2-x)O(4) (0 < or =x< or = 1/3) are promising anode materials for lithium-ion rechargeable batteries. The end member of the Li-Ti-O series, Li(4)Ti(5)O(12), can accommodate Li ions up to the composition Li(7)Ti(5)O(12). Whereas a number of studies focus on the electrochemical behaviour of Li insertion into and Li diffusion in the Li intercalated material, only few investigations about low-temperature Li dynamics in the non-intercalated host material Li(4)Ti(5)O(12) have been reported so far. In the present paper, Li diffusion in pure-phase microcrystalline Li(4)Ti(5)O(12) with an average particle size in the microm range was probed by (7)Li solid state NMR spectroscopy using spin-alignment echo (SAE) and spin-lattice relaxation (SLR) measurements. Between T = 295 K and 400 K extremely slow Li jump rates tau(-1) ranging from 1 s(-1) to about 2200 s(-1) were directly measured by recording the decay of spin-alignment echoes as a function of mixing time and constant evolution time. The results point out the slow Li diffusion in non-intercalated Li(4)Ti(5)O(12) x tau(-1) (1/T) follows Arrhenius behaviour with an activation energy E(ASAE) of about 0.86 eV. Interestingly, E(ASAE) is comparable to activation energies deduced from conductivity measurements (0.94(1) eV) and from SLR measurements in the rotating frame (0.74(2) eV) rather than from those performed in the laboratory frame, E(A)(low-T) = 0.26(1) eV at low T.

NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system

Lithium niobate has been chosen as a model system for spectroscopic studies of the influence of different structural forms and preparation routes of an ionic conductor on its ion transport properties. The Li diffusivity in nanocrystalline LiNbO3, prepared either mechanically by high energy ball milling or chemically by a sol-gel route, was studied by means of impedance and solid state 7Li NMR spectroscopy. The Li diffusivity turned out to be strongly correlated with the different grain boundary microstructures of the two nanocrystalline samples and with the degree of disorder introduced during preparation, as seen especially by HRTEM and EXAFS. Although in both samples nanostructuring yields an enhancement of the Li diffusivity compared to that in coarse grained LiNbO3, the Li diffusivity in ball milled LiNbO3 is much higher than in chemically prepared nanocrystalline LiNbO3. The former LiNbO3 sample has a large volume fraction of highly disordered interfacial regions which seem to be responsible for fast Li diffusion and to have a structure very similar to that of the amorphous form. This is in contrast to the chemically prepared sample where these regions have a smaller volume fraction.