Stage Graphite Intercalation Compounds Research Articles

Electrochemical Formation of a Covalent–Ionic Stage-1 Graphite Intercalation Compound with Trifluoroacetic Acid

A pure stage-1 graphite intercalation compound (GIC) has been prepared by the electrochemical oxidation of graphite in trifluoroacetic acid (CF3COOH). During the CF3COOH-GIC formation, covalent as well as ionic bonds between the trifluoroacetic acid and graphite layers are formed. Electrochemical reduction of the stage-1 GIC leads to the deintercalation of ionically bonded intercalate, yielding the stage-2 covalent GIC. The interlayer spacing is 8.10 Å for stage-1 and 11.62 Å for stage-2 GIC. Further electrochemical oxidation of stage-1 CF3COOH-GIC does not lead to the exfoliation of the GIC product, and the GIC has relatively high stability in air.

Electrochemical Intercalation of Magnesium into Graphite Via an Ethylenediamine-Based Electrolyte

The formation of dimeric or larger cluster metal intercalates within layered hosts may reveal new chemistries for charge storage electrodes. In previous work, we reported a series of novel ternary stage-1 graphite intercalation compounds (GICs) containing alkaline earth metal cations (M = Mg, Ca, Sr, or Ba) and ethylenediamine (en). Pair distribution function (PDF) analyses indicated the presence M-M bonding within the intercalates in the monolayer galleries.In this work, we present the results of electrochemical intercalation reactions for Mg-en complexes within graphite. Using Mg(SO3CF3)2 in en as the electrolyte, galvanostatic charge and discharge profiles show capacities of 810 and 455 mAh g-1 during the first intercalation and deintercalation steps, respectively. After this formation cycle, a capacity of ~120 mAh g-1 can be retained for over 100 cycles with a columbic efficiency of >93%. Ex-situ X-ray diffraction confirms that stage-1 GICs are produced in the fully reduced state.Cyclic voltammetry (CV) and first-cycle differential capacity measurements (dQ/dV) both show that four redox couples arise in the first charge/discharge process. Very large polarizations (2.2 V) are observed, suggesting that there may be changes in Mg-N bonding in this redox chemistry. Galvanostatic intermittent titration technique (GITT) studies as well as X-ray Raman spectroscopy are currently underway to further examine reaction kinetics and confirm the presence of Mg-Mg bonding. This work may provide a new paradigm toward charge storage in intercalation electrodes.

Intrinsic phonon anharmonicity in heavily doped graphene probed by Raman spectroscopy

The temperature-dependent (T-dependent) linewidth (ΓG) and frequency shift (ΔωG) of the G mode provide valuable information on the phonon anharmonicity of graphene-based materials. In contrast to the negligible contribution from electron-phonon coupling (EPC) to the linewidth of a Raman mode in semiconductors, ΓG in pristine graphene is dominated by EPC contribution at room temperature due to its semimetallic characteristics. This leads to difficulty in resolving intrinsic contribution from phonon anharmonicity to ΓG. Here, we probed the intrinsic phonon anharmonicity of heavily-doped graphene by T-dependent Raman spectra based on FeCl3-based stage-1 graphite intercalation compound (GIC), in which the EPC contribution is negligible due to the large Fermi level (EF) shift. The ΔωG and ΓG exhibit a nonlinear decrease and noticeable broadening with increasing temperature, respectively, which are both dominated by phonon anharmonicity processes. The contribution of phonon anharmonicity to ΓG of heavily-doped graphene decreases as the EF approaches to the Dirac point. However, the T dependence of ΔωG is almost independent on EF and qualitatively agrees with the theoretical result of pristine graphene. These results provide a deeper understanding of the role of phonon anharmonicity on the Raman spectra of heavily doped graphene.

Peculiarities of phase transformations in graphite intercalation compounds with bromine

The X-Ray (00L) and (HK0) patterns of third stage graphite intercalation compound with bromine have been experimentally investigated in the range of (150–290) K. The dependence of the intercalate identity parameter on temperature is determined. In the temperature range of (150–290) K there are three temperature intervals with different values of the linear thermal expansion coefficient (α): К−1 in range I (238–300)K, К−1 in range II (174–238) K and К−1 in range III (150–174) K. The abrupt change of the linear thermal expansion coefficient has been detected at 238 K and 174 K. The redistribution of the intensities of the (HK0) pattern main components on cooling from the 290 K to 150 K, in particular, a decrease of some maxima intensities indicates the active migration processes of bromine molecules in the two-dimensional lattice of the intercalate.

Stage-number dependence of intercalated species for fluorosilicate graphite intercalation compounds: pentafluorosilicate vs. hexafluorosilicate

Most intercalated ions ever reported for graphite intercalation compounds (GICs) are singly charged, and the number of reports on GICs with doubly charged ions is limited. For fluorosilicate complex anions, only the intercalation of SiF5− was reported as stage-2 GICs, although SiF62− is more commonly known in inorganic compounds. In the present study, chemical states of fluorosilicate GICs (SiFy-GICs) are investigated along with the change in stage numbers. Syntheses of SiFy-GICs at various stage numbers (the mixtures of stage-5 and -4, stage-4 and -3, stage-3 and -2, and stage-3 and -2) clarify that SiFy-GICs at a low stage number have a larger gallery height than those at higher stage numbers. In addition, reactions of SiFy-GICs with PF5 formed PF6-GICs with large weight increase and stage-number decrease, which cannot be explained by the substitution of SiF4 with PF5 to intercalate PF6−. The model that SiF62− and SiF5− are present in GICs at high and low stage numbers, respectively (SiF62− for stage-n (n ≥ 3) and SiF5− for stage-2), can explain this phenomenon, suggesting intercalation of SiF62− into graphite for the first time and stage-number dependency of intercalated species for fluorosilicate GICs.

Intercalation of sodium and heavy alkali metals into graphenic foams

Abstract Most of alkali metals are well-known to easily intercalate into graphite to form stage-1 graphite intercalation compounds (GIC) whose structure and physical properties have been extensively studied. This work is focused on the intercalation of potassium, sodium and cesium into graphenic foams prepared by a solvothermal-based process. The as-obtained samples are analyzed by means of X-ray diffraction and Raman spectroscopy, and reveal a double phenomenon: the intercalation of alkali metals between the interplanar spaces of the multi-layer graphene stackings, and their sorption into the porosity of the host sample. This kind of behavior has already been reported when studying the reactivity of alkali metals with disordered carbon. Interestingly, our study shows a low stage intercalation in the case of sodium. Moreover, the dispersion of the potassium-intercalated samples into a polar solvent leads to a few-layer graphene dispersion stable for months.

Formation of a N2O5–graphite intercalation compound by ozone treatment of natural graphite

In this work, for the first time a stage-2 graphite intercalation compound with dinitrogen pentoxide (N2O5-GIC) by only ozonation of natural graphite is prepared.

Boosting Aqueous Batteries by Conversion-Intercalation Graphite Cathode Chemistry

In their recent publication in Nature, Kang Xu, Chunsheng Wang, and co-authors reported for the first time on a novel halogen conversion-intercalation graphite cathode chemistry for the development of high-energy aqueous batteries. As evidenced by experimental works and modeling, a densely packed stage-1 graphite intercalation compound (GIC) with a stoichiometry of C3.5[Br0.5Cl0.5] is reversibly formed in a water-in-bisalt (WiBS) electrolyte, enabling an outstandingly high specific cathode capacity of 243 mAh g−1 at an average potential of 4.2 V versus Li/Li+.

A New and Facile Route Using Electride Solutions To Intercalate Alkaline Earth Ions into Graphite

A series of new ternary graphite intercalation compounds (GICs) containing alkaline earth metal cations (M = Mg, Ca, Sr, Ba) and ethylenediamine (en) are reported. These GICs are deep blue to green in color and can be prepared as phase-pure compounds by the direct reaction of graphite powder with the metal in liquid en at mild temperatures (25–100 °C) under an inert atmosphere. X-ray diffraction and thermal analyses were employed to determine the structural and compositional details. [Mg2(en)2.0]C26 and [Ba2(en)2.0]C34 can be obtained as stage-1 GICs with gallery expansions of 0.55 and 0.46 nm, respectively, indicating the presence of intercalate monolayers with en cointercalates oriented perpendicular to the encasing graphene layers. Reactions with Ca and Sr metals form [Ca(en)2.0]C26 and [Sr(en)2.0]C22, which are stage-1 GICs with intercalate bilayers and gallery expansions of 0.76 nm. Titration indicates that each metal intercalate is associated with a 2-electron reduction process. Details on the effec...

Bilayer-rich graphene suspension from electrochemical exfoliation of graphite

The superior electronic properties of graphene change with number of layers, making different layered flakes suitable for different applications. Bilayer has a tunable band gap and is promising material for electronic applications. Liquid phase synthesis methods are scalable, can provide flakes with size suitable for microelectronic fabrication technologies and allow easy, defect-free transfer of flakes to the application substrate. However, these methods yield flakes with different number of layers. Here, we show a modified electrochemical exfoliation of graphite that gives a bilayer-rich graphene suspension. The method first obtains particles of stage-2 graphite intercalation compound. These particles have alternating weak and strong attraction between the two nearest neighboring layers which allowed the tuning of ultrasonic agitation intensity, such that cleavages occurred at weak attraction sites yielding primarily bilayer graphene flakes. Graphene flakes were produced only on ultrasonic cleaving and were done in DMF, a medium favorable for graphene dispersion thus avoiding aggregation of the freshly generated bilayer flakes. In principle, our method can be tuned for the liquid phase synthesis of layer-specific graphene flakes for any given number of layers. The method could therefore provide a general synthesis process for graphene required in different end-use.

Intercalation of imidazolium cations into graphite via ion exchange

New graphite intercalation compounds (GICs) containing 1-alkyl-2,3-dimethyl imidazolium cations (Im1-1-a, a = alkyl length) are obtained from [Na(ethylenediamine)1.0]C15 by cation exchange. Exchange reactions occur rapidly, but require higher temperature than for substituted alkylammoniums. Powder X-ray diffraction, thermogravimetry and structural modeling indicate that [Im1-1-4]C47 is a stage-2 GIC with 0.36 nm monolayer galleries and cations oriented parallel to graphene sheets. [Im1-1-12]C44 is a stage-1 GIC with a gallery expansion of 0.40 nm. The lower sheet charge density for [Im1-1-12]C44 is commensurate with its larger intercalate. Imidazolium cations without alkyl substitution at the imidazolium ring C2 do not form stable GICs by this route.

Preparation, Characterization, and Structure Trends for Graphite Intercalation Compounds Containing Pyrrolidinium Cations

New graphite intercalation compounds (GICs) containing N,N-n-alkyl substituted pyrrolidinium cation intercalates (Pyn.m, n, m = alkyl chain lengths) are obtained via cationic exchange from stage-1 donor-type GIC [Na(ethylenediamine)1.0]C15. Powder X-ray diffraction and thermogravimetric analyses are used to determine the GIC structures and compositions. [Py4.8]C47·0.71DMSO and [Py8.8]C48 with intercalate monolayers are obtained as stage-1 GICs with gallery expansions of 0.48 nm, whereas [Py1.18]C47 and [Py12.12]C80·0.25DMSO form stage-1 GICs with intercalate bilayers and gallery expansions of 0.81 nm. The gallery dimensions require that alkyl chain substituents orient parallel to the encasing graphene sheets. Smaller intercalate cations such as Py1.4, Py4.4, and Py1.8 either form high-stage GICs or do not form stable intercalation compounds. These results, along with those reported for graphite intercalation of other quaternary ammonium cations, indicate trends in graphite chemistry where larger intercala...

Superconductivity in the Graphite Intercalation Compound BaC(6).

Among many two-dimensional (2D) high T(C) superconductors, graphite intercalation compounds (GICs) are the most famous intercalation family, which are classified as typical electron-phonon mediated superconductors. We show unambiguous experimental facts that BaC(6), the superconductivity of which has been missing for many years so far among various alkaline earth metal (Ca, Sr, and Ba) intercalted GICs, exhibits superconductivity at T(C)=65 mK. By adding this finding as the additional experimental point, a complete figure displaying the relationship between T(C) and interlayer distance (d) for GICs is now provided, and their possible superconducting mechanisms raised so far are revisited. The present study settles a long-running debate between theories and experiments on the superconductivity in the first stage GICs.

Doping inhomogeneity and staging of ultra-thin graphite intercalation compound flakes probed by visible and near-infrared Raman spectroscopy**Project supported by the National Natural Science Foundation of China (Grant Nos. 11225421, 11474277, and 11434010).

When ultra-thin graphite intercalation compounds (GICs) are deposited on the SiO2/Si substrate, it is found that their colors are dependent on the thickness of GIC flakes. The sample colors of ultrathin GIC flakes can no longer provide qualitative information on the stage index. Here, multi-wavelength Raman spectroscopy is thus applied to study the doping inhomogeneity and staging of ultra-thin GICs by FeCl3 intercalation. The G band intensity of stage-1 GIC flakes is strongly enhanced by 532-nm laser excitation, while that of stage-2 and stage-3 flakes exhibits strong intensity enhancement for 785-nm laser excitation. The near-infrared lasers are suggested to probe the doping inhomogeneity and staging of ultra-thin GIC flakes.

Diffusion of alkali metals in the first stage graphite intercalation compounds by vdW-DFT calculations

The diffusion constant of alkali metals in graphite intercalation compounds is proportional to the graphene interlayer distance.

Mechanism of Graphene Oxide Formation

Despite intensive research, the mechanism of graphene oxide (GO) formation remains unclear. The role of interfacial interactions between solid graphite and the liquid reaction medium, and transport of the oxidizing agent into the graphite, has not been well-addressed. In this work, we show that formation of GO from graphite constitutes three distinct independent steps. The reaction can be stopped at each step, and the corresponding intermediate products can be isolated, characterized, and stored under appropriate conditions. The first step is conversion of graphite into a stage-1 graphite intercalation compound (GIC). The second step is conversion of the stage-1 GIC into oxidized graphite, which we define as pristine graphite oxide (PGO). This step involves diffusion of the oxidizing agent into the preoccupied graphite galleries. This rate-determining step makes the entire process diffusive-controlled. The third step is conversion of PGO into conventional GO after exposure to water, which involves hydrolysis of covalent sulfates and loss of all interlayer registry.

Effect of pre-lithiation degrees of mesocarbon microbeads anode on the electrochemical performance of lithium-ion capacitors

Lithium ion capacitors are assembled with pre-lithiated mesocarbon microbeads (LMCMB) anode and activated carbon (AC) cathode. The effect of pre-lithiation degrees on the crystal structure of MCMB electrode and the electrochemical capacitance behavior of LIC are investigated by X-ray diffraction (XRD) and the charge-discharge test of three-electrode cell. The structure of graphite still maintained when the pre-lithiation capacity is less than 200 mAh g−1, phase transition takes place with the increase of pre-lithiation capacity from 250 mAh g−1 to 350 mAh g−1. Pre-lithiation degrees of MCMB anode greatly affect the charge-discharge process and behavior, which impact on the electrochemical performance of LIC. The LIC with pre-lithiation capacity of 300 mAh g−1 has the optimal electrochemical performance. The energy density of LIC300 is up to 92.3 Wh kg−1, the power density as high as 5.5kWkg−1 and the capacity retention is 97.0% after 1000 cycles. The excellent electrochemical performance benefits from the appropriate pre-lithiation capacity of negative electrode. The appropriate pre-lithiation ensures the working voltage of negative electrode in low and relative stable charge-discharge platform corresponding to the mutual phase transition from the second stage graphite intercalation compound (LiC12) to the first stage graphite intercalation compound (LiC6). The stable charge-discharge platform of negative electrode is conductive to the sufficient utilization of AC positive electrode.

Manifestation of Charged and Strained Graphene Layers in the Raman Response of Graphite Intercalation Compounds

We present detailed multifrequency resonant Raman measurements of potassium graphite intercalation compounds (GICs). From a well-controlled and consecutive in situ intercalation and high-temperature deintercalation approach the response of each stage up to stage VI is identified. The positions of the G and 2D lines as a function of staging depend on the charge transfer from K to the graphite layers and on the lattice expansion. Ab initio calculations of the density and the electronic band structure demonstrate that most (but not all) of the transferred charge remains on the graphene sheets adjacent to the intercalant layers. This leads to an electronic decoupling of these “outer” layers from the ones sandwiched between carbon layers and consequently to a decoupling of the corresponding Raman spectra. Thus, higher stage GICs offer the possibility to measure the vibrations of single, double, and multilayer graphene under conditions of biaxial strain. This strain can additionally be correlated to the in-plane lattice constants of GICs determined by X-ray diffraction. The outcome of this study demonstrates that Raman spectroscopy is a very powerful tool to identify local internal strain in pristine and weakly charged single and few-layer graphene and their composites, yielding even absolute lattice constants.

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${}_{2}$.

First-principles study of electronic and magnetic properties of FeCl3-based graphite intercalation compounds

The structural, electronic and magnetic properties of stage-1 and -2 FeCl3-based graphite intercalation compounds (GICs) are studied in the framework of the GGA+U implementation of density functional theory. The intercalation process extends the c-axis remarkably and modulates the band structure of graphite to p-type doped. A linearly dispersing band structure is observed for stage-1 GIC. The carrier density shows a weak descending tendency from stage-1 GIC to stage-2 GIC. The dependence of the energy level positions of Fe 3d orbitals on parameter U is strong. With the increase of U, the spin-up states move to the deeper energy levels, while the spin-down states move to the shallower energy levels. Stage-1 GIC has antiferromagnetic (AFM) order and stage-2 GICs has ferromagnetic (FM) orders at the ground states, and two combinative effects are proposed to explain the origin of the magnetic transformation from stage-1 GIC to stage-2 GIC.