Lithium Ion Hopping Research Articles

Study of Lithium Dynamics in Monoclinic Li3Fe2(PO4)3 using 6Li VT and 2D Exchange MAS NMR Spectroscopy

Details of Li-mobility in Li3Fe2(PO4)3 are elucidated using solid-state 6Li NMR. Three crystallographically unique Li sites were resolved under magic angle spinning (25 kHz) with paramagnetic shifts arising at 45 ppm, 102 ppm, and 216 ppm. These resonances were assigned to the crystallographic positions based on the degree of the Fermi-contact interaction with the paramagnetic iron center. 6Li 2D exchange NMR experiments were performed under variable temperature conditions in order to determine the activation energies for hopping between lithium sites. Activation energies ranged from 0.59 (± 0.05) eV to 0.81 (± 0.04) eV, where shorter Li internuclear distances and larger Li−O bottlenecks yielded lower activation energies. These results were compared to a previous study on the isostructural Li3V2(PO4)3, which showed similar trends of increased internuclear distance (and constricted bottlenecks) yielding larger energy barriers for Li−Li exchange. Overall, the average activation energy for lithium ion hopping in the iron-based structure is lower than the vanadium analogue, which is attributed to the more open framework of the former.

6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the 7Li NMR chemical shifts. Our research includes 6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times ( T 1) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter. Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. 7Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH 3SO 3–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.

Lithium dynamics in the fast ionic conductorLi0.18La0.61TiO3probed byLi7NMR spectroscopy

Lithium dynamics in the ${\mathrm{Li}}_{0.18}{\mathrm{La}}_{0.61}{\mathrm{TiO}}_{3}$ perovskite quenched from $1623\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ has been analyzed by means of $^{7}\mathrm{Li}$ Nuclear magnetic resonance (NMR) spectroscopy and neutron powder diffraction. The Rietveld analysis of the diffraction pattern shows rhombohedral symmetry with lithium ions occupying square windows that connect contiguous $A$ sites of the perovskite. The hopping of lithium ions through these windows produces the line narrowing detected above $170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in $^{7}\mathrm{Li}$ NMR spectra. Deconvolution of NMR spectra shows the existence of two lithium species that exchange their positions along the temperature range $250--350\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this temperature range, a plateau is detected in ${T}_{2}^{\ensuremath{-}1}$ plots, which has been ascribed to the existence of two-dimensional motions of lithium in ordered domains of the perovskite. Evidence of this limited motion comes from the frequency dependence of the spin-lattice relaxation rates measured at the high temperature side of the asymmetric $1∕{T}_{1}$ maximum ($\ensuremath{\omega}\ensuremath{\tau}⪡1$ regime). ${T}_{1\ensuremath{\rho}}$ measurements indicate that there is a slower motion of lithium with a characteristic time of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\phantom{\rule{0.3em}{0ex}}\mathrm{s}$ at room temperature, assigned to isotropic three-dimensional diffusion.

Investigation of Imidazole-Based Lithium Conducting Materials

ABSTRACTThis study aims to develop novel polyelectrolytes including lithiated imidazole heterocycles for use in lithium ion rechargeable batteries. Lithium ion local mobility in these materials is characterized by 6, 7Li solid-state NMR. By comparing these results with macroscopic ionic conductivity, measured by impedance spectroscopy, we will be able to develop a picture of the ionic conductivity at the microscopic level. Multinuclear solid state NMR provides information on microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Our research includes 6, 7Li variable MAS NMR studies at intermediate spinning speeds, relaxation investigations to determine spin-lattice relaxation times (T1) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. A very long T1 (135 s at ambient temperature) and an activation energy Ea = 17.2 kJ/mol suggests rigid molecule structure and the absence of the ring reorientation of the model compound, lithium imidazolium (LiIm). We compare this to the behavior of LiIm doped with lithium methanesulfonate, which we show to form a new ionic complex with lower T1 and corresponding lower activation energy. With the goal of creating new polyelectrolytes, we have synthesized electrolytes incorporating lithiated imidazole rings, where lithium transport may be independent of polymer-backbone flexibility, and thus polymers with high Tg may be viable. Such materials are highly desirable for secondary lithium polymer battery applications.

A Molecular Dynamics Simulation Study on Ion‐Conducting Polymer sPBI‐PS(Li+)

AbstractTo understand the mechanism of ionic migration in the amorphous matrixes of polymer electrolytes is crucial for their applications in modern technologies. Here, molecular dynamics (MD) simulation was carried out to investigate the ionic conduction mechanism of a particular conjugated rigid‐rod polymer, sPBI‐PS(Li+). The backbone of this polymer is poly[(1,7‐dihydrobenzo[1,2‐d:4,5‐d']diimidazole‐2,6‐diyl)‐2‐(2‐sulfo)‐p‐phenylene]. The polymer has pendants of propane sulfonate Li+ ionomer. The MD simulations showed that the main chains of sPBI‐PS(Li+) are in a layer‐like structure. The further detailed structure analysis suggested that the π‐electron of this polymer is not delocalized among aromatic rings. This agrees with the experimental result that sPBI‐PS(Li+) shows no electronic conductivity, and the conductivity of this polymer is mainly ionic. The calculated migration channels of lithium ions and electrostatic potential distributions indicated clearly that the polymer matrix is anisotropic for the migrations of ions. The migration of lithium ions along the longitudinal direction is more preferable than that along the transverse direction. The relaxations of the polymer host were found to play important roles in the transfer process of lithium ions. The hopping of lithium ions from one ‐SO3−1 group to another correlated strongly with characteristic motions of ‐SO3−1 group on a time scale of about 10−13 s.

NMR STUDIES ON SELF DIFFUSION COEFFICIENTS OF LITHIUM IONS IN PAN-BASED GEL POLYMER ELECTROLYTES

The self diffusion coefficients D of lithium ions in PAN-EC/PC-LiClO4 gel polymer electrolytes with different compositions and at different temperatures have been directly measured by field gradient NMR spin echo technique. The results indicated that self diffusion coefficients D of lithium ions were dependent on the mass ratio x% of lithium salt, and had a maxiumum at x=10 in the range of x=5 to 20. It was suggested that this was probably involved with transport mechanisms of the hopping of lithium ions, and with two kinds of interactions: the interaction between Li+ ions and plasticizer EC, and the interaction between Li+ ions and network polymer PAN.

A computer simulation of LiKSO4

Molecular-dynamics computer-simulation of an ionic molecular solid LiKSO4 has been carried out at 300 and 1000 K using the atom-atom potentials obtained from lattice dynamical studies. We observe hopping of lithium ions to interstitial positions which is related to reorientations of sulphate tetrahedra.

“Glassy” properties of crystalline Li 3N at low temperatures

We have studied the acoustic and dielectric behaviour of the superionic conductor Li 3N at temperatures between 100 K and 20 mK. At low temperatures its properties are determined by low-energy excitations similar to those observed in glasses. In Li 3N these excitations are ascribed to the tunneling of hydrogen impurities, whereas thermally activated hopping of lithium ions accounts for the behaviour at higher temperatures.