Lithium-graphite Intercalation Compounds Research Articles

Hydrogenation properties of lithium intercalated graphite

Two kinds of nano-structural graphite were prepared from graphite powder by ball-milling under hydrogen or argon atmosphere, where each product is denoted as HG or AG, respectively. And then, non-treated graphite powder (G), HG or AG was milled with Li to synthesize lithium-graphite intercalation compounds, which are denoted as Li-G, Li-HG and Li-AG, respectively. In XRD profiles of Li-G and Li-HG, it was revealed that the peaks correspond to LiC6 and LiCl2, while no peaks were observed in the case of Li-AG. However, similar thermal decomposition reactions were observed for all compounds at about 450°C under an inert gas, indicating that Li-AG includes the lithium-carbon (Li-C) compound. After heat treatment under hydrogen pressure, the formation of LiH was confirmed by XRD measurements for all compounds, indicating that the intercalated lithium reacted with hydrogen. Furthermore, all hydrogenated compounds desorbed hydrogen with heating up to 500°C. In addition, the hydrogenated Li-HG and Li-AG formed lithium carbide (LiC) after hydrogen desorption. On the other hand, in the case of the hydrogenated Li-G, it is confirmed that not only LiC but also LiC12 were formed after dehydrogenation.

Interfacial reactions between graphite electrodes and propylene carbonate-based solutions: Electrolyte-concentration dependence of electrochemical lithium intercalation reaction

This study examines the electrochemical reactions occurring at graphite negative electrodes of lithium-ion batteries in a propylene carbonate (PC) electrolyte that contains different concentrations of lithium salts such as, LiClO 4, LiPF 6 or LiN(SO 2C 2F 5) 2. The electrode reactions are significantly affected by the electrolyte concentration. In concentrated solutions, lithium ions are reversibly intercalated within the graphite to form stage 1 lithium–graphite intercalation compounds (Li–GICs), regardless of the lithium salt used. On the other hand, electrolyte decomposition and exfoliation of the graphene layers occur continuously in the low-concentration range. In situ analysis with atomic force microscopy reveals that a thin film (thickness of ∼8 nm) forms on the graphite surface in a concentrated solution, e.g., 3.27 mol kg −1 LiN(SO 2C 2F 5) 2/PC, after the first potential cycle between 2.9 and 0 V versus Li +/Li. There is no evidence of the co-intercalation of solvent molecules in the concentrated solution.

In situ 7Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite; 1st cycle

We show a continuous, in situ nuclear magnetic resonance (NMR) experiment on a lithium/graphite electrochemical cell. The objective is to study a commercial graphite currently used as negative electrodes in secondary lithium batteries. A plastic cell is made, with metallic lithium as the counter electrode and 1 mol dm −3 LiPF 6/ethylene carbonate (EC) + diethylcarbonate (DEC) electrolyte. The reversible capacity is 346 mAh/g and the irreversible capacity 55 mAh/g, measured in the galvanostatic mode, at a rate of C/20 (20 h for the theoretical capacity of LiC 6) for the first cycle. We show the first discharge and the first charge of the cell inside the magnet and record simultaneously and regularly (in real time) static 7Li NMR spectra. As expected, we observe the quadrupolar lines characteristic of the lithium graphite intercalation compounds (GICs). During the discharge, the two types of in-plane densities of Li are successively found that correspond to the dilute LiC 9, then to the dense LiC 6 configuration; during the charge, we observe the successive decrease of these states. The galvanostatic curve helps to identify the stages NMR signature and the stages coexistence.

Energetic evaluation of possible stacking structures of Li-intercalation in graphite using a first-principle pseudopotential calculation

Energetic evaluation of possible stacking structures of Lithium–graphite intercalation compounds (Li–GICs) has been performed using the density-functional theory under the local density approximation with or without the generalized gradient correction. The calculated formation energies of Li–GICs well reproduced the stability of the structures and their lattice parameters. That is, the calculated formation energies (Δ E) are negative for the equilibrium phases of LiC 6, LiC 12 and LiC 18 with αA, αAA and αAAA stacking where one-third of the carbon hexagons have corresponding Li atoms. In contrast to that, Δ E of hypothetical phases of LiC 2, LiC 4, LiC 6 and LiC 8 with different in-plane ratios of carbon-hexagon to Li are positive. Hypothetical LiC 4 with the Aα′B stacking has also a positive Δ E, but the equilibrium LiC 18 with αABA stacking has a negative Δ E. AA stacking of carbon hexagonal monolayer and graphite with AB stacking are also discussed for the reference state of these Li–GICs.

XRD evidence for the electrochemical formation of [formula omitted] in PC-based electrolytes

The reduction of graphite in propylene carbonate (PC) based electrolytes is a complex process involving solvated lithium–graphite intercalation compounds (GICs), Li + ( PC ) y C n - . XRD performed in situ to cyclic voltammetry allows to monitor the intercalation/de-intercalation of PC-solvated lithium into/from a crystalline graphite electrode, which is associated with up to 374% expansion of the basal plane spacing in c-direction of the graphite structure. The XRD data further allows to estimate the structure and stoichiometry of the ternary GIC: Li + ( PC ) y C n - , with y ⩾ 4, indicating a 3-dimensional, tetrahedral solvate shell. The stability of ternary GICs vs. reduction is addressed in view of graphite exfoliation and electrode failure.

Analysis of layered structures of lithium–graphite intercalation compounds by one-dimensional Rietveld method

Lithium–graphite intercalation compounds were synthesized by intercalating various amount of lithium into graphite electrochemically. X-ray diffraction was measured and the layered structures of the lithium–graphite intercalation compounds (Li-GICs) were precisely analyzed by one-dimensional Rietveld method. The layered structures for stage 1, stage 2, stage 2L, stage 3, and stage 4 were verified precisely. The spacings both between graphene layers with a lithium layer intercalated and between graphene layers with no lithium layer intercalated were obtained. The changes of these values were discussed based on the structure of stage compounds.

Lithium Isotope Effect Accompanying Electrochemical Intercalation of Lithium into Graphite

Lithium has been electrochemically intercalated from a 1:2 (v/v) mixed solution of ethylene carbonate (EC) and methylethyl carbonate (MEC) containing 1 M LiClO4 into graphite, and the lithium isotope fractionation accompanying the intercalation was observed. The lighter isotope was preferentially fractionated into graphite. The single-stage lithium isotope separation factor ranged from 1.007 to 1.025 at 25 °C and depended little on the mole ratio of lithium to carbon of the lithium-graphite intercalation compounds (Li-GIC) formed. The separation factor inceased with the relative content of lithium. This dependence seems consistent with the existence of an equilibrium isotope effect between the solvated lithium ion in the EC/MEC electrolyte solution and the lithium in graphite, and with the formation of a solid electrolyte interfaces on graphite at the early stage of intercalation.

Lithium Isotope Effect Accompanying Chemical Insertion of Lithium into Graphite

Lithiumwas chemically intercalated from 1-methoxybutane solution of lithiumand naphthalene into graphite and vice versa, and lithium isotope fractionation accompanying those intercalation and deintercalation processes was observed. 6Li was always preferentially fractionated into the graphite phase. The single-stage lithium isotope separation factor upon intercalation was about 1.023 at 25 ºC, nearly independent of the structure of the lithium-graphite intercalation compounds formed. A much smaller separation factor was observed for the deintercalation process, suggesting the existence of lithium sites (surface areas) other than the sites between graphene layers of the host graphite. Separation factor data were consistent with the following decreasing order of the 6Li-to-7Li reduced partition function ratio (RPFR): RPFR of 1-methoxybutane solution > RPFR of surface areas > RPFR of metal lithium > RPFR of graphene interlayer sites.

Nucleation and phase-boundary movement upon stage transformation in lithium–graphite intercalation compounds

Nucleation and phase-boundary movement upon stage transformation in lithium–graphite intercalation compounds were investigated by potential-step chronoamperometry and alternating current impedance spectroscopy. Highly oriented pyrolytic graphite was used as a host graphite material. The current response obtained by potential step was strongly dependent on the kind and direction of stage transformation. A current hump due to the nucleation and nuclear growth of a new stage was observed during the stage transformations from dilute stage 1 to stage 4 and from stage 2 to stage 1 upon lithium intercalation, and from stage 4 to dilute stage 1 upon deintercalation. The rate-determining steps for nucleation and for phase-boundary movement were discussed in the stage transformation between dilute stage 1 and stage 4. It was shown that the nucleation rate was controlled by the reaction at the graphite/electrolyte interface in both directions. The rate of the phase-boundary movement was affected greatly by the diffusivity of lithium ions in a growing phase; that is, it was controlled mainly by the diffusion in stage 4 in the direction from dilute stage 1 to stage 4, whereas by the interfacial reaction in the reverse direction.

Influence of defects on the phase-boundary movement in a stage transformation of lithium-graphite intercalation compounds

The influence of defects on the phase-boundary movement in the stage transformation from random stage (dilute stage 1) to stage 4 of lithium-graphite intercalation compounds was investigated using highly oriented pyrolytic graphite (HOPG). Potential-step chronoamperometry and in situ optical microscopy coupled with micro Raman spectroscopy were used to clarify macroscopic and microscopic influence of defects. Potential-step chronoamperometry confirmed that the diffusion-limited rate of the phase-boundary movement in the bulk of HOPG was independent of the crystallinity of the samples. This result indicates that defects have macroscopically no appreciable influence on the phase-boundary movement. In situ optical microscopy on the basal plane of HOPG revealed that the nucleation and growth of the stage-4 phase took place quite inhomogeneously in the initial stage. The rate of the phase-boundary movement in the surface region of HOPG was roughly evaluated to be in the range of 5 to 10 μm min−1. The phase-boundary movement was seriously retarded by cracks formed on the basal plane during the stage transformation.

Vibrational spectra of superdense lithium graphite intercalation compounds

IR investigations of lithium graphite intercalation compounds with different lithium in-plane density (LiC 6, LiC 4, LiC 2) were carried out. The spectra interpretation was done on the basis of normal coordinate analysis for model structures. LiC 2 and LiC 4 spectra exhibit the bands corresponding to a complex skeleton stretching in Li 7 or LiC 6 clusters including equilateral triangles of metal atoms at short distances which shows that covalence exists between Li atoms in the intercalated layer and that LiC 2 decomposition proceeds keeping these clusters in LiC x structure for x < 4 at least.

On the possibility of hydrogen intercalation of graphite-like carbon materials—electrochemical and molecular orbital studies

Graphite-like carbon materials were investigated to learn whether they form hydrogen-graphite intercalation compounds (HGICs), which would be useful as hydrogen storage materials in rechargeable batteries. The ability of graphite-like materials to absorb hydrogen was examined with cyclic voltammetry (CV) and constant-current charge and discharge tests in 6 mol dm −3 KOH. It was concluded that H-GICs did not form under our experimental conditions. In order to understand the difference between hydrogen and lithium ions in ability to form the intercalation compounds with graphite-like carbon materials, molecular orbital analyses were carried out. The results demonstrate that lithium-graphite intercalation compounds (Li-GICs) are stable because of the presence of lithium 2 p-orbitals, whereas such stable graphite intercalation compounds do not form with hydrogen ions because no three-dimensional p- rmorbitals are available.

In situ X-ray diffraction studies of a graphite-based Li-ion battery negative electrode

Lithium-graphite intercalation compounds were synthesised in an electrochemical cell by the application of a series of current pulses, during which non-equilibrium distributions of intercalation phases were created in the graphite. The phase formation and subsequent interconversion were studied by a combination of chronopotentiometry and in situ X-ray diffraction.

Revisited structures of dense and dilute stage II lithium-graphite intercalation compounds

Electrochemical intercalation of unsolvated lithium into pyrographite has been carried out in LiClO4-ethylene carbonate electrolyte. Pure phases have been isolated and characterized by X-ray diffraction studies. The orientated texture of pyrographite has allowed the study of the different families of Bragg reflections (001, hk0, hkl) especially for stage I LiC6, 3D stage II LiC12 and dilute stage II ‘LiC18’ compounds. Experimental data and some theoretical models have been compared.

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.

ChemInform Abstract: Synthesis and Compressibility of First Order Lithium Graphite Intercalation Compounds.

ChemInform Abstract: Synthesis and Compressibility of First Order Lithium Graphite Intercalation Compounds.

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.

Synthesis of Lithium-Graphite Intercalation Compounds and the Reactivity Against Oxygen

A simple preparation technique of Li-GICs was described. Graphite sample and lithium metal was placed separately in a steel tube, which was then encapsulated in a pyrex glass tube. Several days were needed to obtain Li-GICs under the condition of 400-450°C. The X-ray diffraction measurement and the chemical analysis were performed for the products. The compositions of the stage 1 and 2 Li-GICs were determined to be LiC6 and LiC18. It must be noted that Li-GICs had a remarkable stability against oxygen. The stage 1 LiC6 was found to be almost insnsitive again oxygen at 0°C, and at 35°C, LiC7.5 was also proved to be stable as compared with KC8 or KC24.

Structure and transitions in coordinated lithium graphite intercalation compounds

The LiC n (THF) m ternaries obtained by the direct action of THF on the parent LiC n binaries seem to be quite similar whether the initial binary be 1st stage LiC 6 or 2nd stage LiC 12 or LiC 18. The basic structure is a slightly flattened [Li(THF) 4] + tetrahedral complex with THF molecules probably rapidly exchanging between adjacent complexes. Some excess non complexed Li could induce a coupling of the conduction electrons with the intercalated layer.

Physical properties of first and second stage lithium graphite intercalation compounds

The second stage lamellar compound of graphite with lithium (LiC 12) has been investigated by specific heat and magnetic susceptibility experiments at low temperatures. The heat capacity thermal variation in the He 4 temperature range is fitted, as usual, with linear and cubic T-dependent terms. This fit allows us to calculate the density of levels at Fermi energy: N( E F) = 2.2 ± 0.2 eV −1 molecule −1 and the Debye temperature θ D = 142 K for a mole of product. The magnetic measurements between 295 and 5 K, indicate a temperature independent term χ exp = (0.85 ± 0.10)10 −6 emu CGS g −1 . After corrections for the usual core diamagnetism we obtain a mean Pauli component equal to (1.85 ± 0.2)10 −4 emu CGS mole −1 which is about twice as large as expected from the above calculated N( E F). Comparison of these experimental data with those already obtained for the first stage compound has been made. Referring to other physical properties, such as e.p.r., and plasma resonance, it is shown that these materials behave as weakly anisotropic 3D electronic systems.