Lithium Graphite Intercalation Compound Research Articles

Relaxation Analysis of Silicon Monoxide-Graphite Composite Anode

Relaxation analysis based on X-ray diffraction has been carried out on SiO-graphite composite anode of the lithium-ion battery, focusing on the lithium migration during the relaxation accompanied by the stage change of lithium graphite intercalation compound (Li-GIC). After the termination of electrochemical lithium insertion, XRD peaks of Li-GIC change from stage I into stage II during the relaxation. Due to the slower reaction for lithiation of SiO in comparison with the intercalation of Li-GIC, lithium migration from Li-GIC toward SiO to change the Li-GIC from stages I into II toward the equilibrium lithium distribution. The stage change is more apparent for higher current density at the charging, presumably due to the core–shell model of lithium concentration of SiO after charging.

Relaxation stage analysis of lithium inserted graphite

Relaxation analysis has been carried out on lithium graphite intercalation compound (Li-GIC) focusing on the variation of stage structure and graphene interlayer distances. X-ray diffraction coupled with the one-dimensional Rietveld analysis revealed that, after the termination of electrochemical lithium insertion, excessively formed stage I partly transforms into stage II during the relaxation process. In addition, the periodic stacking of defective lithium layers forming stage II orders the lithium occupation at the relaxation, which is more apparently observed for faster charged sample. Even after the lithium extraction, gradual stage changes from I to II accompanied by the ordering of lithium occupation in stage II have been observed at the relaxation process. Lithium ion is rather uniformly intercalated or deintercalated in the graphene interlayers, while Li-GIC turned into ordered stage structure to achieve the stable state at the relaxation time.

Criticality of the phase transition on stage two in a lattice-gas model of a graphite anode in a lithium-ion battery.

Herein, a Monte Carlo study within the canonical assembly has been applied to elucidate the lithium-ion phase transition order of a stage II lithium-graphite intercalation compound (LiC12) around the critical point. The results reveal a weakly first-order phase transition at 354.6 ± 0.5 K via measurements that follows the power laws with effective exponents. The graphite-lithium system was emulated within a lattice-gas model, comprising specific insertion sites arranged in four parallel planes with a triangular geometry. Moreover, two different types of energetic interactions were used: a Lennard-Jones potential, for particle interactions in the same plane, and a power law potential that decreased with distance, for particles in different planes. The energy per site and order parameter distribution were used to classify the order of the transition. Furthermore, the order parameters, susceptibility, and heat capacity were computed and analyzed.

Investigating the Phase Transitions of Graphite By In Situ Neutron Diffraction

Graphite is one of the most common negative electrode materials in today’s lithium-ion batteries [1]. Intensive research has revealed most of its thermodynamic properties [2-5] and modeling of its free energy was able to determine some of these properties like the phase diagram and the staging phenomena to some extent [6, 7]. However, these tools were partially insufficient to describe the open-circuit potential and phase transitions in the low state-of-charge regime of the Li-graphite system.The phase transitions during the intercalation of lithium in graphite were therefore investigated using in situ neutron diffraction. In situ neutron diffraction data have been collected to determine the transitions of the lithium-graphite intercalation compound in the range of 7-30 % of the state-of-charge (100 % = LiC6). Only this limited SOC range has been investigated in detail since previous findings by Billaud and Dahn already describe the range from 0-7 % and 30-100 % very precisely [3,5]. This new set of data suggests a continuous lithium intercalation in a certain solid-solution regime for each pure phase (stage) at room temperature. This result is found to be consistent with the phase diagram of Woo et al. [4] and is able to extract the in-plane density of lithium in each stage.

Lithium Diffusion in Lithium-Graphite Intercalation Compound from First Principles; (I) Calculation Methodology

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Ball-milling with a fluid: A powerful means for new syntheses

By reactive ball-milling in the presence of a fluid (either liquid as dodecane or gaseous as hydrogen), it is possible to synthesize nanomaterials as diverse as: alkali metal hydrides; well-crystallized highly anisometric graphite (HAG) whose typical particle size is a thickness of 40 nm for a mean diameter of several micrometers; a superdense lithium–graphite intercalation compound: LiC 3; maghemite (γFe 2O 3) and its composites with graphite. Most of those compounds present interesting electrochemical properties and can be used as anodes in lithium-ion batteries. Depending on the system, the role of the fluid can be either a shock moderator, avoiding the amorphization of the material (e.g. graphite), or a reagent (e.g. hydrogen forms alkali metal hydride, water oxidizes transition metal powders). Some reaction mechanisms are described in this paper with a particular attention to the oxidation by water of chromium, nickel and copper.

Structural studies of the stage III lithium–graphite intercalation compound

Electrochemical intercalation of unsolvated lithium ions into pyrographite has been carried out in LiClO 4–carbonate electrolyte solution. The pure stage III lithium–graphite intercalation compound has been isolated and characterized by X-ray diffraction studies. The oriented texture of pyrographite has allowed the separated study of the different families of Bragg reflections. The identity period along the c-axis, I c , is equal to 1040.80 pm corresponding to an intercalation distance d I equal to 374.80 pm. There is no evidence for a 2D structural organization of the intercalated lithium species. The a and b parameters of the graphite sublattice are equal to 246.84 pm corresponding to an in-plane C–C distance of 142.50±0.05 pm. Comparison between experimental data and some theoretical models suggests a ABA/ACA stacking of the graphene layers along the c-axis with a parameter c equal to 2 I c . However, a contribution of a ABA/ABA stacking with c= I c is also probable.

Structural observation of energy storage materials prepared by MA

Mechanical alloying (MA) was employed to synthesize hydrogen storage materials and cell materials as anodes of secondary lithium-ion batteries. Amorphous MgNi alloy was prepared by MA. The structures of dilutely and fully deuterated MgNi alloys (MgNiD 0.1, MgNiD 1.6) were observed using a combination of X-ray and neutron diffractions. The radial distribution functions RDF(r) of the amorphous MgNiD x alloys provided us information that deuterium atom occupies the tetrahedral site composed of nearly 2Mg 2Ni. Hexagonal graphite was found out to absorb deuterium during the MA process. It changed into the nano-crystalline one after milling. Two types of the location of deuterium atoms for CD 0.661 powder were found in RDF(r) observed by neutron diffraction. One is the C–D covalent bond and the other is due to deuterium located between layers of the graphite. Lithium–graphite intercalation compound was obtained from graphite and lithium mixed powders by MA. The short-range structure around a Li atom was observed by using the advantage of neutron diffraction. The coordination number of the C–Li correlation indicates the formation of LiC 3.1 intercalation compound.

Potential and Thermodynamics of Graphite Anodes in Li‐Ion Cells

Lithium graphite intercalation compound (Li‐GIC) has potential plateaus depending on its staging in Li/Li‐GIC cells. It is believed that the entropy of each stage is different depending on the total number of states created by the inserted and electrons in Li‐GIC. On this basis, the entropy change with Li insertion has been calculated and the potential, using the reaction Gibbs function, has been estimated. The entropy term in the usual potential equation diverges for Li vacant and Li full GICs. The equation was improved and had finite values for these GICs depending on the mean size of the crystallites in the GIC. By the introduction of the intercalation pressure to our model, we confirmed that Li‐GIC is unstable and decomposes into Li‐poor and Li‐rich phases. The potential was calculated using the entropy term for Li full GIC. The result agreed well with the experimental value. The entropy term contribution was very large for the potential difference of each plateau. © 2000 The Electrochemical Society. All rights reserved.

Structure of functional graphite as hydrogen storage and cell materials.

Mechanical alloying was employed to synthesize hydrogen storage graphite materials and cell materials as anodes of secondary lithium-ion batteries. First of all, the mechanical alloying of graphite was carried out under Ar gas atmosphere. Hexagonal graphite was used as a starting material. The transformation from the hexagonal graphite composed of layers into amorphous-like carbon was observed by using a highly resolved neutron diffraction technique. In case of the mechanical alloying under D2 gas atmosphere, deuterium was absorbed by the solid-gas reaction. Two types of the location of deuterium atoms were found in RDF(r). One is the C-D covalent bond and the other is due to deuterium located between layers of the graphite. Lithium-graphite intercalation compound was obtained by mechanical alloying of graphite and lithium mixed powders. The short range structure around a Li atom was observed by using the advantage of neutron diffraction. The coordination number of the C-Li correlation indicates the formation of LiC3.1 intercalation compound.

A.c. impedance analysis of electrochemical lithium intercalation into highly oriented pyrolytic graphite

Electrochemical lithium intercalation into graphite was studied by cyclic voltammetry and a.c. impedance spectroscopy. Highly oriented pyrolytic graphite was used as a model graphite material to distinguish the difference in electrochemical behavior between the basal and the edge planes at graphite. A comparison between cyclic voltammograms of the basal plane and the whole surface of highly oriented pyrolytic graphite revealed that electrochemical lithium intercalation proceeds predominantly at the edge plane/electrolyte interface. The charge-transfer resistance changed continuously with electrode potential, and no significant change was observed at stage transition potentials (210, 120, and 90 mV versus Li/Li +). From the variations of the Warburg impedance of samples of different sizes, it was concluded that lithium diffuses from the edge plane to the interior in the direction parralel to the basal plane and that its diffusivity changes with the stage structure of the bulk lithium—graphite intercalation compound.

Synthesis and XPS investigation of superdense lithium-graphite intercalation compound, LiC 2

The superdense lithium-graphite intercalation compound (GIC), LiC 2, first synthesized by the author was prepared by the original technique in a piston-cylinder high-pressure chamber with internal heater. Cleaved surfaces of this compound, LiC 3, LiC 6 and pristine graphite were studied by the X-ray photoelectron spectroscopy (XPS) technique. It was shown that chemical shifts of carbon do not vary in the range LiC 6 to LiC 2 which suggests that the additional lithium intercalation into LiC 6 gives no significant change in the charge transfer rate per carbon atom. Carbon/lithium atom ratios were obtained for freshly cleaved surfaces and for the surfaces after argon ion bombardment. It was shown that the bombardment results in the surface decomposition of LiC 2-GIC.

7Li ‐Nuclear Magnetic Resonance Observation of Lithium Insertion into Mesocarbon Microbeads

The stacking order of graphite layers in mesocarbon microbeads (MCMBs) heat‐treated between 700 and 3000°C was examined by analyses of x‐ray diffraction measurements, and lithium insertion into the MCMBs has been observed using solid‐state ‐nuclear magnetic resonance (‐NMR) spectroscopy. In MCMBs heat‐treated above 2000°C, the fully lithiated MCMBs showed two bands at ca. 45 ppm (vs. LiCl) and ca. 27 ppm in their ‐NMR spectra. The profile of the band at 45 ppm was very close to that for the first‐stage lithium graphite intercalation compound (Li‐GIC), though the other band at 27 ppm could not be assigned to any phases of Li‐GICs. From these results, it is suggested that the structures of the MCMBs heat‐treated above 2000°C for lithium insertion are classified as graphitic structure, which has the AB stacking order of graphite layers, and turbostratic structure with a random stacking sequence of graphite layers; the fully lithiated compositions of both structures were estimated as and ca. , respectively. Although MCMB heat‐treated at 700°C gave a higher capacity than , the line shift in the ‐NMR spectra indicated that lithium stored in the MCMB displayed an ionic character. Capacity change of the MCMBs during charge‐discharge cycling up to 20 cycles and capacity loss at higher current densities were also examined.

Tritium behavior in neutron irradiated lithium graphite intercalation compound (LiC 6)

Tritium release behavior from neutron irradiated LiC 6 was investigated by heating up to 1000 °C. Released chemical species were found to be mainly molecular tritiated hydrogen (HT or T 2), accompanied by a small portion of CH 3T and HTO. Regarding tritium release kinetics, two peaks in tritium release were observed at 200–300 °C and 500–600 °C. The latter could be analyzed by diffusion-controlling kinetics, while the former by both diffusion-controlling and 1st order reaction kinetics. Judging from the values of diffusion coefficients and of activation energy, it is concluded that the tritium species behave with some interaction to lithium ions in LiC 6.

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.

Phonons in graphite and LiC 6

A two-body potential has been proposed to study the phonon dispersion in graphite. The two-body potential contains, in addition to the Born-Mayer constants A and ϱ, two parameters for the stability of the graphite crystal along the a and c axes. The phonon dispersion of graphite is well explained by this potential, especially the A 2 u out-of-plane mode. The analysis is further extended for the evaluation of phonon dispersion in the first-stage lithium-graphite intercalation compound LiC 6. Good agreement was obtained between the theory and the available experimental results.

Some Basic Studies on Tritium Processing in CTR Solid Blanket

AbstractSome basic studies related to tritium processes in the solid blanket of CTR have been made.These are:(1) Evaluation of somewhat new blanket materials such as lithium-graphite intercalation compound. In this study, fractional tritium release was measured when lithium compound was heated after irradiated in a fast reactor.(2) Isotope effect, isotopic swamping effect and exchange reaction in tritiated water vapor adsorption on molecular sieve were studied. These are important items in the design of tritiated water vapor recovery in the gas cooled blanket.

Pressure effects on the quadrupole coupling constant of 7Li in first stage lithium graphite

The pressure dependence of the experimental 7Li NMR spectra is reported for first stage lithium graphite (LiC 6) intercalation compound at temperatures T = 232 and 293 K. This experiment together with the presented point charge model calculation of the 7Li quadrupole coupling constant ( e 2qQ h ) allows an unambiguous determination of the sign of e 2 qQ/h which is negative: e 2qQ h=-52 kHz at p = 1 bar and T = 232 K. The averaged location of the electrons transferred from the Li intercalant to the graphite layers, as estimated in this study, is in excellent agreement with earlier theoretical energy-band calculations. The compressibility of LiC 6 in the c-direction is predicted to be k c = 1.7 × 10 -12cm 2dyn -1, it agrees with estimates derived from the available phonon dispersion relations.

Energy band structure of lithium—graphite intercalation compound

The energy band structure for C 6Li has been calculated using a two-step procedure. In the first step the ab initio KKR method has been applied to a pseudo-muffin tin potential, where the potential within the atomic spheres is not spherically averaged. The resulting wave functions are then used to obtain the corrections to these bands resulting from the deviation of the actual interstitial potential from its volume averaged value. The results indicate a band structure composed of the energy bands of two-dimensional graphite, folded into the Brillouin zone of C 6Li, plus a metal-like band originating from the lithium s level. The minimum of the metal-like band is about 2 eV above the Fermi level, indicating sizable transfer of charge between the Li and the graphite layers. The interaction of the lithium s level with occupied carbon π and σ bands leads to a covalent contribution to interlayer bonds. However, the interaction of the Li s level with the C π levels near the Fermi level is negligible. The Fermi surface contains two electron sheets originating from the folded, two-dimensional, weakly antibonding graphite π bands.