Li Hopping Research Articles

Enhancing phase stability and kinetics of lithium-rich layered oxide for an ultra-high performing cathode in Li-ion batteries

To achieve a higher capacity and rate capability, the electrochemical performance of doped Li-rich layered oxide (LLO), in which Co and O ions are substituted with various dopants (Ti, Zr, Ce, Mo, W, and F), is investigated using first-principles calculations. W and Mo are candidate dopants to enhance the phase stability but are excluded due to the decreased average cell voltage of 2.5–5.4 %, lowering the energy density of a battery. Instead, F is selected as a promising dopant because F-doped LLO can achieve high structural stability without a reduction in the average cell voltage compared with un-doped LLO. The Li slab distance in F-doped LLO expands approximately 3–8% depending on the Li concentration, and the activation energy for Li hopping is reduced about 30%, suggesting faster Li ion diffusion. The enthalpy of formation of F-doped LLO is reduced to 5.3–12.4 kJ mol−1 during de-lithiation, implying an increase in phase stability. Based on the DFT prediction, we experimentally demonstrate F-doped LLO (Li1.17Ni0.17Co0.17Mn0.50O1.96F0.04) exhibits a high capacity of 252.2 mAh g−1 at 0.33C rate in the cut-off voltage range of 3.0–4.6 V. The rate capability is enhanced, and the capacity is retained up to 83% at 3C compared with the 0.33C rate.

Evaluation of fundamental transport properties of Li-excess garnet-type Li(5+2x)La(3)Ta(2-x)Y(x)O(12) (x = 0.25, 0.5 and 0.75) electrolytes using AC impedance and dielectric spectroscopy.

The fundamental electrical transport properties including ionic conductivity, dielectric constants, loss tangent, and relaxation time constants of Li-excess garnet-type cubic (space group Ia3̄d) Li5+2xLa3Ta2-xYxO12 (x = 0.25, 0.5 and 0.75) have been studied in the temperature range of -50 to 50 °C using electrochemical AC impedance spectroscopy. A correlation has been established between the excess Li content and the Li(+) ion migration pathways. The loss tangent (tan δ) for all samples exhibits a relaxation peak corresponding to the dielectric loss because of dipolar rotations due to Li(+) migration. Comparing the modulus analysis of Li-excess garnets with fluorite-type oxygen ion conductors, we propose the local migration of Li(+) ions between octahedral sites around the "immobile" Li(+) ions in tetrahedral (24d) sites. In the samples with x = 0.25 and 0.5, Li(+) ions seem to jump from one octahedral (96h) site to another bypassing the tetrahedral (24d) site between them (path A), both in local and long-range order migration processes, with activation energies of ∼0.69 and 0.54 eV, respectively. For the x = 0.75 member, Li(+) ions exhibit mainly long-range order migration, with an activation energy of 0.34 eV, where the Li hopping between two octahedral sites occurs through the edge which is shared between the two LiO6 octahedra and a LiO4 tetrahedron (path B). The present AC impedance analysis is consistent with the ab initio theoretical analysis of Li-excess garnets that showed two conduction paths (A and B) for Li ion conduction with different activation energies.

Motion of Li+ in Nanoengineered LiBH4 and LiBH4:Al2O3 Comparison with the Microcrystalline Form

The introduction of structural disorder and large volume fractions of different kinds of interfaces enables the manipulation of ion dynamics in solids. Variable-temperature solid-state NMR relaxometry is highly useful to study Li(+) jump processes. If carried out as a function of frequency, the resulting NMR relaxation rates also contain information on the dimensionality (1D, 2D, or 3D) of the diffusion process. Recently, NMR relaxometry has revealed the 2D nature of Li hopping in LiBH4 , and thus this hydride is an interesting ion conductor for further diffusion studies on the spatially confined motion of Li spins. Here, nanocrystalline LiBH4 and the two-phase analogue LiBH4 :Al2 O3 , which are prepared by ball milling, serve as interesting model systems to track the changes in NMR relaxation rates with respect to coarse-grained, thermodynamically stable LiBH4 . This reveals that interface (nano)engineering influences the hexagonal-to-orthorhombic phase transition and thus alters the ion-transport properties of Li in one- and two-phase LiBH4 towards higher diffusivities at lower temperatures.

Lithium motion in the anode material LiC6as seen via time-domain7Li NMR

Since the commercialization of rechargeable lithium-ion energy storage systems in the early 1990s, graphite intercalation compounds (GICs) have served as the number one negative electrode material in most of today's batteries. During charging the performance of a battery is closely tied with facile Li insertion into the graphite host structure. So far, only occasionally time-domain nuclear magnetic resonance (NMR) measurements have been reported to study Li self-diffusion parameters in GICs. Here, we used several NMR techniques to enlighten Li hopping motions from an atomic-scale point of view. Li self-diffusion in the stage-1 GIC LiC${}_{6}$ has been studied comparatively by temperature-variable spin-spin relaxation NMR as well as (rotating frame) spin-lattice relaxation NMR. The data collected yield information on both the relevant activation energies and jump rates, which can directly be transformed into Li self-diffusion coefficients. At room temperature the Li self-diffusion coefficient turns out to be ${10}^{\ensuremath{-}15}\phantom{\rule{0.28em}{0ex}}{\mathrm{m}}^{2}\phantom{\rule{0.28em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, thus, slightly lower than that for layer-structured cathode materials such as Li${}_{x\ensuremath{\approx}0.7}$TiS${}_{2}$.

Fast Li diffusion in crystallineLiBH4due to reduced dimensionality: Frequency-dependent NMR spectroscopy

The hexagonal and orthorhombic form of lithium borohydride, ${\text{LiBH}}_{4}$, are investigated by temperature and frequency-dependent nuclear magnetic resonance (NMR) spectroscopy. The local electronic structure and microscopic diffusion parameters are determined by recording both $^{6,7}\text{L}\text{i}$ NMR spectra and spin-lattice relaxation (SLR) rates. The rates of the high-temperature flank of the SLR-NMR peaks of hexagonal ${\text{LiBH}}_{4}$ clearly depend on resonance frequency which unequivocally reveals a low-dimensional diffusion process. Due to the very limited number of suitable model substances this makes lithium borohydride an extremely attractive material to study the effect of reduced dimensionality on Li dynamics. Most likely, the spatial confinement of Li hopping is also responsible for the very high ionic conductivity found for the hexagonal polymorph, recently.

Polar clusters in impurity-doped quantum paraelectricK1−xLixTaO3

From density-functional calculations, we show that large off-center motions (approx =1.0 A) of Li impurities in the KTaO{sub 3} matrix (studied at 3.7% concentration) create very anisotropic polar clusters oriented along the Li off-center dipole. The polarization induced by Li in the matrix decreases very sharply in the lateral directions so that polar clusters are only approx = two lattice constants thick (one-dimensional or needlelike clusters). The polarization in such polar regions is mainly constituted by the displacements in the (highly polarizable) matrix rather than by the impurity itself. These results suggest that Li-doped potassium tantalate (3.7% concentration) is not ferroelectric at low temperature and rather behaves as a relaxor. These small polar zones around Li correlate at T{sub B} to form larger polar nanoregions, in which the matrix remains however nonpolar. This is confirmed by a low temperature neutron-diffraction analysis showing that the KTaO{sub 3} matrix remains paraelectric. Li-doped KTaO{sub 3} is an order-disorder system with a very deep local potential felt by the Li impurities (approx =-200 meV). The energy barrier for Li hopping is estimated at 80-90 meV. An analytic expression for this local potential is provided, as well as a simple model describing the energetics ofmore » K{sub 1-x}Li{sub x}TaO{sub 3}.« less

Lithium dynamics in the zeolite-like lithosilicate RUB-29 and its high-temperature structure after dehydration (Cs14Li42Si72O172)

Abstract The thermally induced modifications of the zeolite-like lithosilicate RUB-29 (Cs 14 Li 24 [Li 18 Si 72 O 172 ] · 14H 2 O, space group I 222, a = 11.208(1), b = 17.286(1), c = 23.536(1) A, and V = 4660(1) A 3 [S.-H. Park, J.B. Parise, H. Gies, H. Liu, C.P. Grey, B.H. Toby, J. Am. Chem. Soc. 122 (44) (2000) 11023–11024]) have been studied focusing on static and dynamical disorder of Li. After the water loss, dehydrated RUB-29 (HT-d-RUB-29) maintains the basic space group I 222 at high temperatures from 473 up to 1073 K. However, the distribution of extra-framework Li and Cs cations in HT-d-RUB-29 became distinctly different from those in the original structure. In particular, extra-framework Li cations located within porous 8- and 10-membered ring-channels, where these Li cations were partially coordinated with zeolitic water in the original structure, migrate into densely packed Li 2 O-layers after dehydration. As a result of the migration, the number of empty sites relevant for Li hopping in Li 2 O-layers decreases, which give a negative effect on the long-range charge transfer in dehydrated RUB-29. Its DC conductivity values estimated from AC impedance spectra lie between 2 × 10 −5 and 6 × 10 −5 S cm −1 at 873 K, and a low activation energy of 51 kJ/mol (≅ 0.53 eV) could be determined for the bulk charge transfer process in dehydrated RUB-29.

Factors that affect Li mobility in layered lithium transition metal oxides

The diffusion constant of Li in electrode materials is a key aspect of the rate capability of rechargeable Li batteries. The factors that affect Li mobility in layered lithium transition metal oxides are systematically studied in this paper by means of first-principles calculations. In close packed oxides octahedral ions diffuse by migrating through intermediate tetrahedral sites. Our results indicate that the activation barrier for Li hopping is strongly affected by the size of the tetrahedral site and the electrostatic interaction between ${\mathrm{Li}}^{+}$ in that site and the cation in the octahedron that shares a face with it. The size of the tetrahedral site is determined by the $c$-lattice parameter which has a remarkably strong effect on the activation barrier for Li migration. The effect of other factors such as cation mixing and doping with nontransition metal ions can be interpreted quantitatively in terms of the size and electrostatic effect. A general strategy to design high rate electrode materials is discussed.

Infrared spectroscopy of superionic conductor LiNaSO 4: Vibrational modes and thermodynamics

Infrared active phonon spectra of lithium sodium sulphate, LiNaSO 4, were studied at temperatures between 20 K and 780 K. Dielectric constant [ ε = ε′ + iε″] and energy loss function [− Im(1/ ε)] were obtained from Kramers–Kronig analysis. Our IR data show a more complete set of vibrational modes than previous investigations. The IR data of LiNaSO 4 at 20 K are consistent with the P31c symmetry, indicating that LiNaSO 4 shows no structural phase transitions between 20 K and 300 K, in contrast to LiKSO 4. On heating from 20 K, phonon modes related to Li and Na vibrations show a dramatic line broadening and decrease in intensity. An extra mode is recorded near 380 cm − 1 at 500 K. The absorption shows a systematic increase in intensity on further heating. These changes are attributed to anharmonic effects and Li diffusion or hopping. Dramatic spectral changes in the internal modes occur near 620 K on heating, suggesting the onset of the rotational disorder of SO 4 tetrahedra, but the Li atom spectrum shows weak response to the rotational disorder.

Cross relaxation and atomic motion in LiNbO2.

Nuclear magnetic relaxation rates and linewidths are reported for the $^{7}\mathrm{Li}$ and $^{93}\mathrm{Nb}$ resonances in the layered semiconducting compound ${\mathrm{LiNbO}}_{2}$. $ sup 7---Li line narrowing and ${\mathit{T}}_{1\mathrm{\ensuremath{\rho}}}$ measurements provide unambiguous signatures of Li diffusive motion and the Li long-range hopping rate is determined as a function of temperature. Rotating-sample experiments show that the frequency dependence of the Li spin-lattice relaxation rate at low resonance frequencies results from cross relaxation to the quadrupole-broadened $^{93}\mathrm{Nb}$ spins. A crucial aspect of the cross relaxation is the large frequency variations of the $^{93}\mathrm{Nb}$ transitions resulting from the diffusion of Li vacancies. $^{93}\mathrm{Nb}$ relaxation rates are reported and shown to support both the cross relaxation and the Li hopping interpretations. In high magnetic fields, cross relaxation is suppressed and the $^{7}\mathrm{Li}$ and $^{93}\mathrm{Nb}$ spin-lattice relaxation rates differ by three orders of magnitude. The frequency and temperature dependences of these high field relaxation rates indicate that the relaxation mechanisms are not associated with diffusive Li motion. For both Li and Nb, nearly linear temperature dependences of ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ result from interaction with conduction electrons. The electrons are believed to be due to the acceptorlike behavior of Li vacancies. The much slower $^{7}\mathrm{Li}$ relaxation rate ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ reflects the much smaller electron density (at the Fermi surface) at the Li sites than at the Nb sites. Sample-to-sample variations in the Li ion mobility and in the Nb ${\mathit{T}}_{1}^{\mathrm{\ensuremath{-}}1}$ are believed to reflect variations of the Li deficiency of the material.

Li diffusion in NbSe 2 and Ag 0.25NbSe 2 single crystals

Measurements are reported of the diffusion of Lithium into NbSe 2 and into NbSe 2 preintercalated with silver. Diffusion parallel to the layers is found to be at least 2 orders of magnitude greater than perpendicular to the layers, but not to be greatly affected by the presence of intercalated silver. The activation energy for Li hopping in NbSe 2 is estimated to be 520 meV. The staging structure of Ag xNbSe 2 is also reported. These results have implications for the characterisation of cathode materials in intercalation batteries.