Li-graphite Intercalation Compounds Research Articles

Temperature and pressure stability of Li graphite intercalation compounds

Temperature and pressure stability of Li graphite intercalation compounds

Phase Diagram of Li-Graphite Intercalation Compound Formed by the Charge/Discharge Reaction in Li-Ion Battery

The graphite negative electrode for Li-ion batteries was subjected to precision charge/discharge measurements at low current loads. The derivatives of the potential vs capacity curve and the potential vs C/Li composition curve were used to analyze elementary charge/discharge reactions. The number of peaks in the potential-capacity derivative curve increased with decreasing current load, and the elementary reactions, based on the formation of an unconfirmed superlattice structure, could be clearly separated and analyzed. By synchronizing these results with operando synchrotron X-ray diffraction data, a phase diagram of the Li-graphite intercalation compound formed by the charge/discharge reaction in Li-ion battery was established.

Changes in the stage structure of Li-intercalated graphite electrode at elevated temperatures

To examine the phase transition upon deintercalation of Li from graphite at elevated temperatures, structural changes in C6Lix (x = 0.284, 0.532, 0.666, 0.739, 0.812) samples during heating were evaluated by in situ X-ray diffraction (XRD) analysis. The diffraction patterns of the sample, in which the stage-1 and the stage-2 structures coexist, drastically converted to the profile of a Li-defect stage-1 structure at temperatures above 200 °C without Li intercalation and/or deintercalation. Meanwhile, the reflection peak corresponding to the stage-1 structure in the XRD patterns of C6Li0.812 continuously shifted to higher angles at temperature above 330 °C, which implied deintercalation of Li through reaction with the binder and the formation of the Li-defect stage-1 structure. Li deintercalated from C6Lix while maintaining the same stage structure, likely via a solid-state reaction at temperatures above 330 °C. The lattice misfit calculated from the difference in interlayer spacing between stage-1 and stage-2 in C6Li0.666 drastically decreased from 4.58% at 50 °C to 1.43% at 350 °C. These kinetics for Li graphite intercalation compounds at elevated temperatures are formulated for the first time and should be helpful in understanding the Li intercalation/deintercalation mechanism of the graphite electrodes used in Li-ion batteries.

(Invited) Operando Analysis for Charge/Discharge Reaction Mechanism of Graphite Anode of Li Ion Battery

The first preparation of Li-graphite intercalation compound (Li-GIC) was attempted by Hérold et al. in 1955. Since then, a lot of reports have been made mainly by the research groups in France, and with regard to the stage I and II compounds, it is clarified that they take the in-plane structures called by P(√3×√3)R30°. Though there have been many reports since 1970s, the detail discussion has not been done so far concerning the intercalation mechanism and in-plane structure of higher stage compounds. The first reason is that the concentration of Li species becomes smaller in the higher stages. The second reason is the strong preferred orientation of graphite. Such reasons make difficult to clarify the intercalation mechanism and in-plane structure. In order to improve the performance of Li-ion battery, it is essential to investigate the detail structure change of graphite electrode during the charge/discharge process. In this study, in order to elucidate the intercalation mechanism and structure change not only along c-axis, but also along the a,b-axes, we traced dynamically the intercalation reaction in Li-ion battery negative electrode by Operando measurement with synchrotron radiation diffraction.

In Situ Measurement of Strain Evolution in the Graphene Electrode during Electrochemical Lithiation and Delithiation

The strain/stress of the electrode materials induced in the electrochemical process is the key factor for lithium-ion batteries. However, in situ experimental techniques to quantitatively measure the strain/stress response of electrode materials at different length and time scales are still lacking. In this paper, in situ measurement of strain evolution in the graphene electrode during lithiation/delithiation was performed by micro-Raman spectroscopy. The Raman G and 2D peaks of the graphene electrode were obtained using a modified coin cell with an optical window. The stages of the Li-graphite intercalation compounds were characterized by the G peak, while the in-plane strain of the graphene electrode microstructure was characterized by the 2D peak. Evolution of the biaxial strain during the electrochemical cycles was obtained based on determination of the relationship between the Raman shift and the in-plane strain of the graphene electrode. The experimental results show that the biaxial tensile strain of the graphene electrode almost linearly increases during lithiation. The maximum biaxial tensile strain induced by lithiation is about 0.4%, corresponding to the Li-graphite intercalation compound at stage 3. This study provides an experimental basis for understanding the deformation mechanism of the graphene electrode and developing high-performance graphene-based batteries.

Unexpected Relation between Degradation Rate of Lithium Ion Battery and Stage Structure of Graphite Anode

It is very important to understand the degradation behavior of lithium ion battery under various conditions (ex.; state of charge (SOC), temperature) in order to predict the lifetime of the battery. We performed long-term storage tests for lithium ion batteries which consist of layered rock-salt structure cathode and graphite anode on various conditions. During analyzing these test results, we found that the relation between the degradation rate and the SOC is not monotonous. In order to reveal the reason of this peculiar result, we paid attention to the stage structure of graphite. The electrochemical potential of graphite anode is determined by two-phase coexistence relation of stages of Li-graphite intercalation compound. Therefore we can specify the fractions of stages by using differential voltage analysis (dV/dQ). We adopted five stages to characterize the state of anode; stage1, stage2a, stage2b, stage3, and higher stage, and non-lithiated graphite [1]. We estimated the degradation rate of individual stage by solving simultaneous equations of the fraction of each stage and degradation rate. The degradation rate are expected to have negative correlation with the electrochemical potential of stage, however we revealed that the degradation rate of stage2a was much lower than expected rate, as shown in Figure 1. This result suggests that stage2a has high stability against the irreversible reaction accompanied by formation of solid electrolyte interface. Figure 1

Electrochemical Stiffness Measurements of Graphite Anodes for Li-Ion Batterie

We report on measurement of the electrochemical stiffness of the graphite anodes during Li exchange in Li-ion batteries. Graphite is the most common anode material used in commercial Li-ion batteries. During lithiation and delithiation of the graphite anodes, Li-graphite intercalation compounds with varying Li content (Li x C6, 0 ≤ x ≤ 1), and graphene layer spacing are formed. Hence, Li intercalation and de-intercalation causes considerable mechanical changes in the whole electrode. We use curvature bending measurements to monitor stress changes in a thin graphite electrode constrained on an inert, rigid substrate. In a separate experiment, we employ digital image correlation to track strain changes in a free-standing, unconstrained graphite electrode. Combining in-situstress and strain analyses enables us to extract the potential-dependent stiffness of the graphite anodes during lithiation-delithiation cycles. As expected, graphite anodes develop compressive stress during lithiation (ca. 10 MPa). The compressive stress is removed during delithiation. Strain response also corresponds with the Li deposition and removal, as graphite anodes expand during lithiation (ca. 1.5 % linear strain) and contract over delithiation. The electrode exhibits an initially stiff response at high potentials and low lithium contents, and then gradually softens during early formation of dilute stage I of the Li-graphite intercalation compound. As the potential is lowered and lithium content increases, several oscillations in the stiffness are observed, which correspond with the phase transitions during different stages of graphite lithiation. The evolution of stress, strain and stiffness data provides quantitative information on the coupled electro-chemo-mechanical response of the battery electrodes and insight on material strategies to increase battery reliability.

Intercalation Compounds from LiH and Graphite: Relative Stability of Metastable Stages and Thermodynamic Stability of Dilute Stage Id

The intercalation of lithium into graphite was studied at temperatures between 400 and 550 °C by heating mixtures of LiH and graphite powders with molar ratios 4:1, 1:1, and 1:6 under dynamic vacuum for periods between 1 and 72 h. These conditions probe the formation and thermal stability of metastable staged Li–graphite intercalation compounds (Li-GICs) close to the competing formation of the thermodynamically stable carbide Li2C2. Li-GICs of stages I (LiC6, Aα), IIa (Li0.5C6, AαA), IIb (Li∼0.33C6, AαABβB), III (Li∼0.22C6, AαAB), IV (Li∼0.167C6), and dilute stage lithium Id have been identified and characterized by powder X-ray diffraction and Raman spectroscopy. The rate and extent of intercalation (i.e., the achieved stage of Li-GIC) depends on LiH activity and temperature. Stage I was only observed for temperatures above 500 °C. At 400 °C, the highest intercalation corresponded to stage IIb, which was obtained after 2 and 24 h for 4:1 and 1:1 reaction mixtures, respectively. Lower-staged Li-GICs attained at temperatures below 500 °C deintercalate upon prolonged dwelling with the exception of stage IIa, which can be maintained for very long periods (several days) in the presence of LiH. At temperatures above 500 °C, the kinetically controlled formation of Li-GICs is followed by Li2C2 carbide formation. It is shown that the Li-GIC Id coexists with Li2C2 at temperatures up to 800 °C and that the Li content of Id (solubility of Li in graphite) increases between 550 and 800 °C. Consequently, Id with a temperature-dependent homogeneity range should be added as a stable phase in the Li–C phase diagram. A sketch of a revised Li–C phase diagram is provided.

Insight into the mechanical integrity of few-layers graphene upon lithiation/delithiation via in situ monitoring of stress development

By monitoring the stress developments in situ during lithiation/delithiation of few layers graphene films, we report here for the first time an interesting observation of stress release/reversal just within the potential ranges corresponding to the co-existences of pristine graphene, dilute stage I and stage IV Li-graphite intercalation compounds (Li-GICs). This, along with supporting observations related to the presence of cracks in the graphene films upon electrochemical cycling and enhanced ID/IG ratio in Raman spectra recorded upon cycling primarily within the potential regimes corresponding to the stress release/reversal, indicate possible occurrences of mechanical/structural degradation mainly during the initial stages of lithiation and later stages of delithiation. This observation is being supported by a geometric model which estimates the strains induced in the individual graphene layers at the interfaces between the different Li-GICs at the various stages of lithiation; and as a function of distance from the current collector. On a slightly different note, the experimentally recorded magnitude of net lithiation induced reversible in-plane stress development was ∼11GPa. This is in fair agreement with that expected based on the theoretical dilation of ∼1% along the graphene layers upon lithiation as per the classical Li-intercalation mechanism (i.e., up to formation of LiC6).

In situ7Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite: second cycle analysis

Graphite is used in the lithium-ion batteries as a negative electrode. We use continuous, in situ, operando 7Li nuclear magnetic resonance (NMR) to show, in real time, the progressive intercalation and de-intercalation of lithium in graphite when a battery is charged and discharged. We obtain all the Li–graphite intercalation compound stages through an electrochemical path. We explain the overvoltages by transient entropic and Peltier effects. The sample is a plastic cell, NMR compatible, made of commercial graphite, commercial electrolyte and lithium metal foil. We analyze the NMR characteristics of the Li–GIC stages: line shift, quadrupolar frequencies, line width (Li diffusion), line intensity and area as a function of x=Li/C6. This allows us to estimate the lithium quantities in each stage at each step. Two facts differ from the theoretical stage n formation: for the C/20 cycling rate, we find an hysteresis in the filling/emptying of the dilute (LiC9n) stages, and we find another NMR line synchronous with LiC6. The lithium metal line also provides quantitative information on the lithium deposited as dendrites when x diminishes, in de-intercalation. This paper presents experimental NMR data over two cycles, and is an extension of the first cycle analysis published earlier.

Enhanced Thermal Diffusion of Li in Graphite by Alternating Vertical Electric Field: A Hybrid Quantum-Classical Simulation Study

Enhancing the diffusivity of the Li ion in a Li-graphite intercalation compound that has been used as a negative electrode in the Li-ion rechargeable battery, is important in improving both the recharging speed and power of the battery. In the compound, the Li ion creates a long-range stress field around itself by expanding the interlayer spacing of graphite. We advance the hybrid quantum-classical simulation code to include the external electric field in addition to the long-range stress field by first-principles simulation. In the hybrid code, the quantum region selected adaptively around the Li ion is treated using the real-space density-functional theory for electrons. The rest of the system is described with an empirical interatomic potential that includes the term relating to the dispersion force between the C atoms in different layers. Hybrid simulation runs for Li dynamics in graphite are performed at 423 K under various settings of the amplitude and frequency of alternating electric fields perpen...

Electrodeposition of Zr on graphite in molten fluorides

The electrochemical behavior of Zr 4+ using graphite working electrode has been studied in fused LiF–NaF–ZrF 4 eutectic mixture (temperature range 923–1123 K). In addition to the electrochemical profile related to the deposition/dissolution of Zr metal, the cyclic voltammograms exhibit well-defined peaks attributed to the formation of zirconium carbide. A third contribution relative to the formation of Li–graphite intercalation compound and/or lithium carbide was also proposed for a better interpretation of the voltammogram. Depending on the experimental deposition conditions, the analysis of the deposits by SEM/EDX has revealed the presence of a Zr metal thin film with various porosities and compositions. Moreover, the electrochemical reduction of Zr(IV) in molten fluorides using a graphite electrode allows the extraction of pure Zr metal.

Preparation of graphite supported Li–N–H hydrogen storage material from Li-graphite intercalation compound and their unique hydrogen release property

A highly dispersed Li–N–H hydrogen storage material supported on graphite was prepared from a Li-graphite intercalation compound with ammonia, and their hydrogen release property was investigated. In addition, the changes of the chemical state of the lithium species responsible for hydrogen storage/release were investigated using 7Li NMR and XRD. Those spectral analyses revealed that the graphite supported Li–N–H hydrogen storage material released hydrogen at lower temperature than in the absence of graphite support and Li 3N was directly formed in one single step without going through the formation of Li 2NH. Also we found that durability to hydrogen storage/release cycles was improved in the presence of graphite support.

Study of the electronic properties of Li-intercalated nitrogen doped graphite

First-principles calculations are used to investigate the electronic properties of N-doped graphenes, including graphitic, pyridinic and pyrrolic nitrogens. The pristine, graphitic and pyridinic N-doped graphenes are also employed in stage-1 Li–graphite intercalation compounds and their electronic properties were investigated. The results indicate that the pristine graphene shows a zero-gap semiconducting behaviour, while the conductive behaviour in graphitic graphene changes to n-type. The pyridinic and pyrrolic defects make a p-type semiconductor. In the Li intercalation compounds, the greatest contribution of Li atoms to the total density of states is from the hybridization of 2s and 2p states in the conduction band.

1506 ハイブリッド量子古典シミュレーション : オーダーN型実空間DFT法の開発とLiグラファイト層間化合物への応用(OS15.固体物理/流体物理のマルチフィジックス/マルチスケール解析(2),OS・一般セッション講演)

1506 ハイブリッド量子古典シミュレーション : オーダーN型実空間DFT法の開発とLiグラファイト層間化合物への応用(OS15.固体物理/流体物理のマルチフィジックス/マルチスケール解析(2),OS・一般セッション講演)

Li-intercalation into hexagonal boron nitride

A Li hexagonal boron nitride (hBN) intercalation compound (Li-hBNIC) was successfully synthesized by the annealing of powder or bulk hBN and Li at 1523 K. By an XRD analysis, a strong peak indicating the expansion of BN interlayer distance due to Li-intercalation was observed at an angle lower than that of hBN (0 0 2). In the sample, the interlayer distance and its expansion ratio were 3.76 Å and 12.6%, respectively, and these values were similar to those of a first stage Li-graphite intercalation compound (Li-GIC), LiC 6. The electrical conductivity of the sample was increased by several orders of magnitude, from 10 −15 to 10 −7 Ω −1 cm −1 at room temperature. Li de-intercalation was confirmed by the dispersion of the sample in purified water.

High-pressure synthesis of a novel form of endohedral Li diamond from Li graphite intercalation compound

A novel form of hexagonal diamond containing Li atoms in the open rooms surrounded by sp 3-bonded carbon atoms was successfully synthesized from a Li graphite intercalation compound under high pressure, as had been predicted by theoretical studies. High-pressure experiments with LiC 6 were performed in the pressure range from 0.1 MPa to 43 GPa using a diamond-anvil cell. In situ X-ray diffractometry and optical microscopy revealed that LiC 6 was transformed to a hexagonal-diamond form without losing Li atoms. The c-axis of the hexagonal-diamond form was considerably longer than that of the hexagonal diamond transformed from pure graphite, which was consistent with the predicted structure of the endohedral Li diamond. The observed high-pressure form exhibited a golden metallic gloss, which was also consistent with the calculated metallic band structure.

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.

7Li ‐ NMR of Well‐Graphitized Vapor‐Grown Carbon Fibers and Natural Graphite Negative Electrodes of Rechargeable Lithium‐Ion Batteries

Lithium intercalation of natural graphite and well‐graphitized vapor‐grown carbon fibers has been investigated by solid‐state and by cyclic voltammetry. Chemical shift of in Li‐graphite intercalation compounds (Li‐GICs) of natural graphite occurs in two regions, >40 and <10 ppm. The former shifts were assigned to GICs with the in‐plane density of and the latter shifts were assigned to those with the in‐plane density of . Phase diagram of Li‐GICs during electrochemical intercalation is discussed. In comparison to natural graphite, in the vapor‐grown carbon fibers showed higher chemical shifts. The passivating film was also examined by and structural change of the film during the first charge‐discharge cycle is indicated. Scanning electron microscopy imaging of the passivation film supports the NMR data during charge‐discharge. The swelling and shrinking of passivation is evident by scanning elecron microscope image and nuclear magnetic resonance data. The thickness of the solid electrolyte interface layer depends on the nature of salt, solvent, carbon origin, and current density. © 1999 The Electrochemical Society. All rights reserved.

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.