Li-intercalated Graphite Research Articles

Electrochemical Li Insertion in B-Doped Multiwall Carbon Nanotubes

Electrochemical Li insertion into boron-doped multiwall carbon nanotubes (B-MWNTs) was investigated in a nonaqueous medium. Transmission electron microscopy observations showed that the walls of the tubes consisted of highly aligned ca. 35-45 graphene layers with good 3D ordering feature. Raman studies revealed that boron doping in multiwall carbon nanotubes (MWNTs) destroyed the local hexagonal symmetry. X-ray photoelectron spectra of B-MWNTs further supported the results of Raman spectra and confirmed the presence of nanodomains. adsorption measurements indicated that the Brunauer-Emmett-Teller (BET) surface areas of undoped and doped nanotubes were 10 and 12 cm2/g, respectively, with almost similar mesopore volumes. Galvanostatic discharge-charge measurements revealed that the reversible capacity was 156 mAh/g for undoped and 180 mAh/g for B-doped nanotubes in the first cycle with almost equal coulomb efficiencies of 55-58%. The coulomb efficiency increased to more than 90% after the second cycle. Cyclic voltammetry (CV) showed that highly reversible intercalation/deintercalation of Li occurred with some undesirable reduction processes in the initial discharge process. The cycle lives of both undoped and doped samples were quite satisfactory. Slow-scan CV confirmed that the intercalation of lithium in these nanotubes occurred through staging transition, usually observed in Li graphite intercalation compounds. © 2001 The Electrochemical Society. All rights reserved.

Stacking fault analysis in layered materials

Ordered and disordered stacking sequences in graphite and Li-intercalated graphite were modelled. The X-ray powder patterns were simulated by the diffracted intensities from faulted xtals (DIFFaX) program. Resulting diffraction patterns show characteristic differences for hexagonal, rhombohedral and statistical intermixed stacking orders. For this reason, simulated patterns can be used for quantitative analysis of stacking faults by profile fitting, using the DIFFaX simulation parameters as fit parameters.

Rapid diffusion of lithium into bulk graphite in lithium conditioning

One of the known lithium conditioning effects is the reduction of carbon impurities released from the graphite walls of a tokamak. However, little is known about the role of lithium in graphite–plasma interactions. Graphite intercalates lithium atoms between the hexagonal layer planes. The Li diffusion constant in the direction perpendicular to the basal planes of highly oriented pyrolytic graphite (HOPG) is measured for the first time using Rutherford backscattering spectrometry and is found to be D ⊥=2.1×10 −10 cm 2 /s with an activation energy of 0.26 eV, in the case of low temperatures (300 K) and low Li concentration (Li/C atomic ratio <3%). This value increases by a factor of two when HOPG is exposed to hydrogen plasma before lithium deposition. The implications resulting from the Li–graphite intercalation in lithium wall conditioning are discussed, focusing on Li-induced suppression of graphite sputtering.

Thermogravimetric Measurement of Hydrogen Absorption in Alkali-Modified Carbon Materials

We have prepared Li-doped multiwall carbon nanotubes and Li- and K-intercalated graphite and measured their hydrogen storage properties using a thermogravimetric analyzer (TGA). In a flowing H2 atmosphere Li-doped nanotubes and Li-intercalated graphite both exhibit a cyclable weight gain between 200 and 400 °C and weight loss between 400 and 500 °C characterized by a distinct and unusual temperature profile. We find, however, that neither H2 nor carbon is required to generate this TGA feature; we observe it even in Li-containing samples measured in flowing Ar without H2 and in LiOH samples measured in either H2 or Ar. Potassium-intercalated graphite shows mass cycling with a different thermal character between 40 and 250 °C, but as with Li, observation of a large cyclable feature does not rely on the presence of H2. In both cases we identify the cycling mass to be absorption/desorption of H2O present as an impurity in the TGA atmosphere. The temperature signatures we observe are strikingly similar to thos...

Electron energy-loss spectrometry on lithiated graphite

Transmission electron energy-loss spectrometry was used to investigate the electronic states of metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges of metallic Li and LiC6 were nearly identical, and the C K edges were only weakly affected by the presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation damage and sample oxidation in the transmission electron microscopy are also reported.

The effect of various electrolyte additives on reversible Li-graphite intercalation

The effect of various electrolyte additives on reversible Li-graphite intercalation

Li-Graphite Intercalation Compounds Synthesized in Various Ether-Type Organic Solvents

Intercalation of Li into graphite was performed by a chemical method in various ether-type solvents oftetrahydrofuran (THF), 2-methyl tetrahydrofuran (2-MeTHF), 2, 5-dimethyl tetrahydrofuran (2, 5-diMeTHF), 1, 2-dimethoxyethane (DME), 1, 2-diethoxyethane (DEE), 1, 2-dibutoxyethane (DBE), 1, 2-dimethoxymethane (DMM), 1, 2-diethoxymethane (DEM), 1-methoxypropane (MP), 1-methoxy-butane (MB), diethyl ether (Et2O) and resultant products were studied by X-ray diffraction. For the solvents of THF, DME, DEE, DBE, DMM, DEM, ternary Li-solvent-graphite intercalation compounds (GIC) were obtained. On the contrary, for the other solvents, only Li intercalated into graphite to form binary Li-GIC. The criteria for the solvent co-intercalation were discussedin terms of the electron donicity of the solvents, the size of the solvents and also the size of Li-solvent complexes.

Dynamics of li-carbon and carbon solids

Abstract Neutron scattering methods have been used to measure vibrational densities of states (VDOS) for a variety of carbon solids, including diamond, graphite, glassy carbon, C60 and a sputtered amorphous carbon sample. While glassy carbon has a VDOS very similar to that of graphite, a very different, almost featureless, spectrum was obtained in the case of sputtered a-C. Li-graphite intercalation compounds have also been studied in an experiment to investigate the dependence of shear properties on the concentration and degree of order in the Li substructure.

Diffusive Motion in Stage-1 and Stage-2 Li-Graphite Intercalation Compounds: Results of ß-NMR and Quasielastic Neutron Scattering

Investigations of Li diffusion in stage-1 and stage-2 Li graphite intercalation compounds with neutron time-of-flight and backscattering techniques and β-radiation detected nuclear magnetic resonance/relaxation (β-NMR) with the nucleus 8Li(T1/2 = 0.8 s) are reviewed and compared. Depending on temperature, the spin-lattice relaxation-rate T1 -1of 8Li is governed by different processes. Above 300 K, T1 -1 (T) shows maxima induced by long-range Li+ diffusion. Jump correlation times are estimated. Inspection of the B field dependence of T1 -1 revealed two-dimensional diffusion behaviour. The neutron spectra showed a quasielastic line broadening above 500 K, which was used to obtain diffusion coefficients and to trace jump vectors of the in-plane motion. The diffusion parameters observed with both techniques are compared, and differences that show up are discussed. In addition, the low-temperature spin-lattice relaxation rates, being due to coupling to conduction electrons, are used to explore electronic properties.

X-ray investigation of highly saturated Li-graphite intercalation compound

Highly saturated lithium-graphite intercalation compounds (of a composition LiC2LiC4) synthesized under high-pressure conditions were investigated using X-ray diffraction. It was shown that these compounds present the structure with hexagonal unit cell with a parameter 8.63 Å, c = 3Ic = 3 · 3.7 = 11.1 Å. The most probable stoichiometry for this unit cell is LiC2.67, as deduced from the comparison of the calculated and observed intensities of (hkℓ) reflections. It is supposed that this structure is the most “stable” step in decomposition of LiC2 compound obtained under pressure.

NMR Study of LICXGraphite Intercalation Compounds Prepared Under High Pressure

Abstract A highly saturated Li graphite intercalation compound synthesized under high pressure, was investigated after pressure relaxation by Li NMR spectroscopy. At low temperature the sample exhibits a doublet at 256 ppm downfield from Li+. The lineshape agrees with the simulation of an island of 7 Li atoms with nearest neighbours at 2.46° distances, in good agreement with a local LiC2 structure. This confirms short metal-metal bondlengths and possible Li-Li covalent bonds which were shown by IR Spectroscopy.

Theoretical and Experimental Investigation of Electronic Structure of High Pressure Phases of Li Intercalated Graphite

Abstract Electronic structure of a high pressure phase of lithium intercalated graphite has been investigated by theoretical and experimental means. Total energy calculation has shown the existence of a stable phase with LiC2 composition at c=3.38°. A Compton measurement indicates a possible existence of excess Li in the sample.

In-Plane Phonon Density of States of Li-Graphite Intercalation Compounds

Abstract The in-plane phonon densities of states of Li-graphite intercalation compounds of stage-1 and stage-2 are obtained from neutron time-of-flight spectra. For the stage-2 the ordered compound LiC12 which has the same in-plane structure as the stage-1 compound LiC6 as well as the dilute compound LiC16 which only reveals short range order were studied. The temperature in the measurements was extended above expected phase transitions for both stages and the density of states is analysed in terms of the effective temperatures T ∥ for the in-plane vibrational energies. T ∥ shows characteristic changes at the thermodynamic temperatures of the phase tansitions. Comparison of the T ∥ values shows similar in-plane dynamics of all Li-compounds at the highest temperatures.

Electrochemical characterization of Li-intercalated pyrolytic graphite

A new type of Li-intercalated graphite C 9Li as well as thermodynamic data have been obtained from the study of characteristics of electrochemical deintercalation for the Li-intercalated graphites made by vapor transport method.

Polarization of carbon electron-momentum density in lithium-graphite intercalation compounds.

Compton-profile measurements on the second-stage lithium-graphite intercalation compound, ${\mathrm{LiC}}_{12}$, have been performed with the high-resolution spectrometer of Laboratoire pour l'Utilisation du Rayonnement Electromagn\'etique for two directions, parallel to the c axis and average in the basal plane. Simultaneously, a new ab initio self-consistent-field energy-band structure has been calculated for ${\mathrm{LiC}}_{12}$ with the actual stacking of graphite layers. The wave functions have been used to obtain Compton profiles. Both theory and experiment confirm the failure of the rigid-band model for Li-graphite intercalation compounds, as well as the importance of the polarization of graphite valence orbitals due to the presence of lithium ions.

Evidence for interlayer band shifts upon lithium intercalation in graphite from inelastic x-ray scattering.

The dielectric function $\ensuremath{\epsilon}(\mathbf{q},\ensuremath{\omega})$ of Li-intercalated graphite (Li${\mathrm{C}}_{6}$) with $\mathbf{q}\ensuremath{\parallel}c$ axis was deduced from inelastic x-ray scattering data and compared with $\ensuremath{\epsilon}(\mathbf{q},\ensuremath{\omega})$ of pristine graphite. A displacement and broadening of the first peak of $\mathrm{Im}\ensuremath{\epsilon}(\mathbf{q},\ensuremath{\omega})$ upon intercalation was found, which we attribute to a 2-eV shift of the "metal" $\ensuremath{\sigma}$ state of Li${\mathrm{C}}_{6}$ to lower energies relative to the $\ensuremath{\sigma}$-type interlayer state of graphite.

Neutron scattering studies of dilute stage-1 Li-graphite intercalation compounds: Elastic interactions, phase transitions and the phase diagram

Neutron scattering studies of dilute stage-1 Li-graphite intercalation compounds: Elastic interactions, phase transitions and the phase diagram

Raman scattering study of the high-frequency graphitic intralayer modes in Li-graphite and the stage dependence of the mode frequency in donor graphite intercalation compounds.

Raman spectra of the high-frequency (\ensuremath{\sim}1600 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) graphite interlayer modes have been studied in several Li-graphite intercalation compounds (GIC's) ${\mathrm{LiC}}_{\mathrm{p}}$=6,12,17,18. In stage-2 ${\mathrm{LiC}}_{17}$ we have also used these modes to study the T=250 K reversible structural phase transition from high-T disordered to low-T ordered Li intercalate layers and found that a Raman singlet at room temperature evolves into an unresolved Raman doublet (or triplet) below the phase transition temperature. A careful line-shape study of the low-T spectrum could not clearly distinguish between these two possibilities. A triplet is consistent, however, with previous x-ray diffraction studies of the phase transition in the bulk. Raman spectra of ${\mathrm{LiC}}_{6}$ were obtained and found to be in good agreement with previous studies. Spectra of pure stage-2 ${\mathrm{LiC}}_{12}$ were recorded for the first time. The high-frequency graphitic intralayer mode frequencies obtained in this work, together with other previously published data, indicate that a ``universal'' linear relation between mode frequency and reciprocal stage index does not exist in donor-type GIC's. That is, well outside of experimental error, the Li results show clearly that several factors (i.e., intercalate layer order, charge transfer, etc.) all make significant contributions to the small fractional change (\ensuremath{\sim}0.5%) observed in the graphitic mode frequencies. The net effect of all factors in the graphitic intralayer mode frequency is not well parametrized by the stage index alone.

Cohesion and structure in stage-1 graphite intercalation compounds.

We have developed a Thomas-Fermi theory for the structural and elastic properties of the first-stage alkali-metal graphite intercalation compounds. We use a simplified model for the electronic structure of these materials which assumes full charge transfer between the alkali metal and the graphite, no hybridization between metal and carbon states, and a uniform distribution of the donated charge on the graphite planes. We have computed lattice constants, compressibilities, shear moduli, alkali-metal diffusion constants and activation energies, and domain-wall thicknesses; general agreement with available experiments is found. These results indicate that our model of the electronic properties is consistent with known elastic and structural properties. The interplane metal-carbon interaction is mostly determined by a competition between Coulomb attraction and hard-core repulsion. The Li ion is much more compact than that of K, Rb, or Cs, which explains the higher compressibility of the Li graphite intercalation compound and the lower Li ionic mobility. The Na--C bond is found to be very soft, explaining the lack of formation of Na-intercalated graphite. The in-plane alkali-metal--alkali-metal interaction is determined almost entirely by the Coulomb interaction, and is thus relatively independent of the alkali-metal species.

Theoretical phase diagram for Li-intercalated graphite

A mean-field model for the phase diagram of Li-intercalated graphite is presented. Recent observations indicate that different Li graphite compounds occur with the same stage but with different, nearly commensurate in-plane Li densities. This phenomenon, which is a consequence of the corrugation potential of the host graphite lattice, is accounted for approximately in the present model by a small modification of the effective in-plane alkali-alkali potential. The resulting calculated phase diagram agrees well with experiment. The stability of the dilute stage-two phase is identified as a consequence of the high entropy of the compound. The dense stage-two compound has low entropy and decomposes at much lower temperature. A comparison of the present model with previous models is given, and the extension to other alkali-metal compounds is considered.