Alkali Metal Graphite Intercalation Compounds Research Articles

Conditions for Reversible Na Intercalation in Graphite: Theoretical Studies on the Interplay Among Guest Ions, Solvent, and Graphite Host

Graphite is the most widely used anode material for Li‐ion batteries and is also considered a promising anode for K‐ion batteries. However, Na+, a similar alkali ion to Li+ or K+, is incapable of being intercalated into graphite and thus, graphite is not considered a potential electrode for Na‐ion batteries. This atypical behavior of Na has drawn considerable attention; however, a clear explanation of its origin has not yet been provided. Herein, through a systematic investigation of alkali metal graphite intercalation compounds (AM‐GICs, AM = Li, Na, K, Rb, Cs) in various solvent environments, it is demonstrated that the unfavorable local Na‐graphene interaction primarily leads to the instability of Na‐GIC formation but can be effectively modulated by screening Na ions with solvent molecules. Moreover, it is shown that the reversible Na intercalation into graphite is possible only for specific conditions of electrolytes with respect to the Na‐solvent solvation energy and the lowest unoccupied molecular orbital level of the complexes. It is believed that these conditions are applicable to other electrochemical systems involving guest ions and an intercalation host and hint at a general strategy to tailor the electrochemical intercalation between pure guest ion intercalation and cointercalation.

Thermoelectric Properties and Performance of n-Type and p-Type Graphite Intercalation Compounds

n-Type alkali metal-graphite intercalation compounds (GICs) and p-type metal chloride-GICs were prepared from commercially available graphite sheets, namely PGS® and GRAFOIL®. Their thermoelectric properties, electrical conductivity, thermal conductivity, and Seebeck coefficient were measured, and their thermoelectric performance was estimated in terms of the figure of merit and power factor. The electrical conductivity (103 S cm−1 to 104 S cm−1) and thermal conductivity (40 W m−1 K−1 to 200 W m−1 K−1) of these GICs are much higher than those of other, conventional thermoelectric materials, whereas their absolute Seebeck coefficients (±30 μV K−1) are lower. Therefore, although the figures of merit of the GICs are somewhat lower (∼105 K−1) than those of other thermoelectric materials, their power factors (∼103 W m−1 K−2) are sufficiently high. The thermoelectric properties of these GICs mainly depend on the host graphite type and slightly on the intercalated species. However, the Seebeck coefficient is independent of both, and the thermoelectric performance of the GICs is strongly governed by their high electrical conductivity. The power factors of almost all the GICs prepared from PGS were greater than 10−3 W m−1 K−2, which is a critical threshold value for use of thermoelectric materials in practical applications. Furthermore, the dilute K-GIC with stage-7 structure had a large Seebeck coefficient (−58 μV Κ−1), which improved the power factor to more than 10−2 W m−1 K−2. Considering the advantages of GICs, this study confirms their significant potential as thermoelectric materials.

First-principles study of alkali metal-graphite intercalation compounds

First-principles calculations were carried out for the alkali (Li, Na, K) metal-graphite intercalation compounds (AM-GICs). From the calculated formation energies (ΔE) of the AM-GICs, it was determined that each Li-GIC and K-GIC were energetically stable until the high alkali metal density LiC6 and KC8. On the other hand, even the low Na density NaC16 and NaC12 are already not stable. Furthermore, the NaC8 and NaC6 are unstable. It has been experimentally reported that Na hardly intercalates into the graphite contrary to the Li and K. Our calculation results agree with these facts. In addition, based on the calculated energy barriers of the Li, Na and K ion jumps between sites in the graphite, it can be deduced that larger radius ions more smoothly diffuse in the graphite.

Reactivity of alkali metal–graphite intercalation compounds

Reactivity of alkali metal–graphite intercalation compounds

Reactivity of alkali metal-graphite intercalation compounds

Three types of reactions of alkali metal-graphite intercalation compounds (AM-GICs) are described: Extraction of alkali metals, penetration of molecules into alkali metal layers and polymerization of molecules on the surface. Oxygen extracts alkali metal atoms of MC8 (and MC24, M=K, Rb) from the bulk to the edge surface, leaving a mixture of higher stage AM-GICs inside and graphitic area on the edge surface. Ammonia molecules penetrate into alkali metal layers of MC8 (M=K, Rb, Cs) by ion-dipole interaction and are accommodated into the nanospace of MC24 (M=K, Rb, Cs). Water molecules penetrate into MC24 (M=K, Cs) and react with alkali metals. Hydrogen is adsorbed in the interlayer nanospace at low temperature (77 K). Butadiene monomers adsorbed on the surface of AM-GICs polymerize.

ChemInform Abstract: Hydrogen in Alkali Metal-Graphite Intercalation Compounds.

Abstract The sorption and desorption of hydrogen by alkali-metal-graphite intercalation compounds (AGICs), MC 8 and MC 24 (M = K, Rb and Cs), were studied by the constant-volume closed method and mass-analyzed thermal desorption spectroscopy. Three forms of hydrogen were detected in KC 8 H x and KC 24 H x , while two forms in RbC 8 H x and one form in RbC 24 H x were detected. The energy diagrams for the systems composed of AGICs and hydrogen demonstrate that the type of alkali metal intercalated in graphite and the stage structure of AGICs have greater influence on the absorption process than on the adsorption process of hydrogen.

Fermi-surface-dependent superconducting gap in C6Ca

High-resolution angle-resolved photoemission measurements of the Fermi-surface and superconducting gap of high-quality C6Ca crystals should help resolve the nature of the high-temperature superconducting behaviour of this and related intercalated graphite materials. The discovery of superconductivity in C6Yb and C6Ca (ref. 1) has activated fierce debates on whether it is described within the conventional Bardeen–Cooper–Schrieffer scheme or some other exotic mechanisms are involved, because the superconducting transition temperature (Tc) is significantly higher than that of the alkali-metal graphite intercalation compounds intensively studied in the 1980s (refs 2, 3, 4). The key to understand the mechanism of superconductivity lies in the superconducting energy gap associated with the formation of superconducting pairs. Here, we report the first direct observation of a superconducting gap in C6Ca by high-resolution angle-resolved photoemission spectroscopy. We found that the superconducting gap of 1.8–2.0 meV opens on the intercalant Fermi surface, whereas the gap is very small or absent on the Fermi surface derived from the π* band of graphene layers. These experimental results unambiguously establish that the interlayer band has an essential role for the high-Tc superconductivity in C6Ca.

A density functional theory study of the alkali metal graphite intercalation compounds using model complexes

To analyze the electronic state of alkaline metal-graphite intercalation compounds (AM-GIC), model complexes where two aromatic molecules coordinate to an alkaline metal center (Li, K, Rb, and Cs) were employed, and their geometry optimization, population analysis, and donor-accepter interaction analysis were performed using the density functional theory (DFT) with B3LYP level and Natural Bond Orbital (NBO). The distance of two aromatic ligands of each model complexes showed good agreement with the corresponding identity period of stage 1 AM-GIC. The electronic charges on alkali metal of model complexes and stabilization energies by electron donation from aromatic ligand to alkali metal were calculated by the population analysis.The theoretical study of the mechanism of ethylene origomerization that proceeds inside the graphite layers was also carried out at DFT with B3LYP level. The Cs-C bond formation through the combination of two cationic Cs+ (η2-H2C=CH2) while accepting electrons from graphite layers is proposed as the initiation reaction of the origomerization. Additionally, we found that the carbon chain elongation reaction holds promise to proceed smoothly with a free energy of activation (ΔG‡) 16.29 [kcal/mol] under the presence of the chemical species having Cs-C bond.

Chemical Reduction of SiCl4 for the Preparation of Silicon–Graphite Composites used as Negative Electrodes in Lithium-Ion Batteries

Silicon–graphite composites were prepared by direct reduction of liquid by various alkali metal graphite-intercalation compounds (GICs). The reaction occurred in all cases and led to intimate mixtures of nanosized silicon and graphite. A cleaning procedure has been used to solubilize the alkali chlorides formed as by-products. Even if silicon is hardly detected when the synthesis is performed from , the experiments conducted with lead to nanocrystallized silicon particles well dispersed in the graphite matrix. The Si–graphite composite (9:91 by weight) issued from displays a first-charge capacity of , which is in fairly good agreement with the theoretical expected value , assuming the formation of the phase. Interestingly, this composite exhibits a better capacity retention than similar composites prepared by ballmilling; the charge capacity is still above after 10 cycles. This suggests the existence of a strong interaction between carbon and Si nanoparticles, which may be explained by the formation and growth of silicon when in contact with the graphene sheets. These preliminary electrochemical results point out the use of such an approach of chemical reaction between GICs and for the preparation of composites.

A technical guideline for preparing alkali metal-graphite intercalation compounds – Vapor phase and solid phase reactions

A technical guideline for preparing alkali metal-graphite intercalation compounds – Vapor phase and solid phase reactions

A density functional theory study of the alkali metal graphite intercalation compounds using model complexes

To analyze the electronic state of alkaline metal-graphite intercalation compounds (AM-GIC), model complexes where two aromatic molecules coordinate to an alkaline metal center (Li, K, Rb, and Cs) were employed, and their geometry optimization, population analysis, and donor-accepter interaction analysis were performed using the density functional theory (DFT) with B3LYP level and Natural Bond Orbital (NBO). The distance of two aromatic ligands of each model complexes showed good agreement with the corresponding identity period of stage 1 AM-GIC. The electronic charges on alkali metal of model complexes and stabilization energies by electron donation from aromatic ligand to alkali metal were calculated by the population analysis.The theoretical study of the mechanism of ethylene origomerization that proceeds inside the graphite layers was also carried out at DFT with B3LYP level. The Cs-C bond formation through the combination of two cationic Cs+ (η2-H2C=CH2) while accepting electrons from graphite layers is proposed as the initiation reaction of the origomerization. Additionally, we found that the carbon chain elongation reaction holds promise to proceed smoothly with a free energy of activation (ΔG‡) 16.29 [kcal/mol] under the presence of the chemical species having Cs-C bond.

Sorption of molecules by alkali metal–graphite intercalation compounds

Sorption of molecules by alkali metal–graphite intercalation compounds

Sorption of molecules by alkali metal-graphite intercalation compounds

Several aspects of sorption behavior of alkali metal-graphite intercalation compounds were described. Dependence of hydrogen-sorption capacity of potassium-graphite intercalation compounds (K-GICs) on the potassium concentration of KGICs was found to be analogous to those of resistivity and conduction electron spin resonance (CESR) g-value of the K-GICs. Host effect on the hydrogen-sorption behavior of CsC24 was also described. The saturated amount of sorption increased with heat-treatment temperature (HTT) of the host carbons. It was confirmed that H2 was able to be sorbed even for CsC24 prepared from less graphitized carbons such as HTT-1300. The change of electrical resistivity (conductivity) and CESR spectra of alkali metal-graphite intercalation compounds (AM-GICs) during sorption of ammonia was also described.

Effect of sorbed molecules on the resistivity of alkali metal–graphite intercalation compounds

Alkali metal–graphite intercalation compounds with the composition of MC 24 ( M=K, Rb, Cs) were prepared by heating a mixture of MC 8 (saturated compound) and graphite sheet (Grafoil) at 350–450 °C. The resistivity perpendicular to the layer planes ( ρ c) of the resulting compounds was determined by the two-terminal method. The anisotropy factor of the resistivity, ( ρ c/ ρ a), of KC 24 prepared from Grafoil was ∼130, being about 1/6–1/10 in magnitude compared with that of KC 24 prepared from highly oriented pyrolytic graphite. The resistivity change during sorption of hydrogen (at 90 K), ethylene (at 194 K) and acetylene (at 194 K) was determined. The resistivity of MC 24 increased with increase of the sorbed amount of H 2. The magnitude of the increase was in the order KC 24>RbC 24>CsC 24. This resistivity increase was considered to be due to the expansion along c-direction which reduces the charge-transfer interaction between the carbon layers and potassium ions, resulting in the decrease of the density of the conduction electron. The resistivity of MC 24 increased extensively during sorption of C 2H 4 and C 2H 2. It was discussed in connection with the in-plane structural transition and chemical interaction between alkali metal ions and sorbed molecules.

Occlusion of Hydrogen in Alkali Metal-Intercalated Cup-Stacked Carbon Nanofibers

Hydrogen storage in alkali metal-graphite intercalation compounds (AGICs) prepared from cup-stacked carbon nanofibers (A-CNFs, A = K, Rb and Cs) was studied by means of thermal desorption of hydrogen. The hydrogen sorbed to stage-1 K-CNFs amounts to 0.067 dm3 (STP) per gram of carbon, most of which is fixed to KGIC as a chemical component.

Electrical conductivity change of AM-GICs by ternarization with ammonia

Electrical conductivity change of alkali metal–graphite intercalation compounds such as MC 8 and MC 24 (M=K, Rb, Cs) was determined during ternarization with ammonia. The conductivity of MC 8 decreased by the ammoniation. A remarkable decrease of the conductivity during initial stage of ammoniation was observed for KC 8. The conductivity of RbC 8 decreased rather moderately and that of CsC 8 was most insensitive to the ammoniation. The energy of interaction of alkali metal ion with ammonia molecule was evaluated by theoretical calculation using density functional theory and obtained the following result: K +–NH 3, −28.9 kcal/mol; Rb +–NH 3, −24.4 kcal/mol; Cs +–NH 3, −19.3 kcal/mol. This result clearly explained the observed difference in the degree of the conductivity decrease of the MC 8 compounds against the ammoniation. Thus, the conductivity decrease can be considered to be originated from the interaction of ammonia molecules with alkali metal ions, resulting in the backtransfer of conduction electrons from carbon layers to alkali metal ions. The conductivity change for KC 24–NH 3 and KC 48–NH 3 systems was relatively small compared to the system KC 8–NH 3. This was explained by taking into account the fact that the nanospace of KC 24 and KC 48 is larger than that of KC 8.

Preparation and characterization of sodium-graphite intercalation compounds

Preparation and characterization of sodium-graphite intercalation compounds were investigated. It was confirmed that the resulting compound (NaC x ) had the stage 8 structure with identity period of 2.8 nm. The Raman spectroscopy showed that the G-band signal of NaC x was observed at 1582 cm −1 and at 1608 cm −1 , which is a typical indication of the formation of intercalation compounds. The g-value of NaC x was determined to be 2.0018 by the ESR measurement, which was very close to the reported values for the other alkali metal-graphite intercalation compounds. The electrical resistivity of NaC x was about 1/3 of that of host graphite at room temperature and showed metallic temperature dependence.

Conduction electron spin resonance of ternary am-nh 3 -gics

Conduction electron spin resonance (CESR) of alkali metal-graphite intercalation compounds (AM-GICs) such as KC 8 , KC 24 , KC 36 , RbC 8 , RbC 24 etc. and their ternary compounds with NH 3 was investigated. It was shown that ternary compound of RbC 8 with ammonia gave a clear ESR signal, in contrast to no signal for binary RbC 8 and that the line width of ESR spectra of KC 8 (NH 3 ) x decreased monotonically with increasing ammonia content. These observations suggested that ammonia molecules reduce the spin-orbit interaction.

Electrical Conductivity of Ternary Alkali Metal-Ammonia-Graphite Intercalation Compounds

The stage-1 alkali metal-graphite intercalation compounds, MC8 (M=K, Rb, Cs), were prepared from Grafoil, and then ternary compounds were obtained by additional intercalation of ammonia. The electrical conductivity of those compounds were determined. The conductivity of KC8 decreased considerably along with ammoniation in the region of ammonia content, nNH3/nK, less than 0.2. Similar behavior of conductivity change was also observed for RbC8 for nNH3 / n K≤ 0.4, except for slight decrease in the early stage up to nNH3/nK=0.1. In contrast, the conductivity of CsC8 decreased slightly by ammoniation in the region of nNH3/nK≤0.6.

Discrete–variational Xα calculations of graphite and alkali–metal graphite intercalation compounds

First-principles molecular orbital calculations using discrete–variational (DV)- Xα method are carried out on the model clusters of graphite and alkali–metal graphite intercalation compounds MC x (M=Li, Na, K, Rb, and Cs). Results of the calculations are used so as to simulate the experimentally observed near-edge X-ray-absorption fine-structures (XANES) and ultraviolet photoelectron spectra. For the clusters of graphite and KC x , the calculated partial electronic densities of states (PDOSs) are in good agreement with the experimental C K-edge and K K-edge XANES spectra, respectively. Furthermore, the accordance between the calculated DOSs and the observed UPS spectra of graphite and RbC x is also satisfactory. It is shown that the Fermi level is pushed up into the conduction band of graphite by doping alkali metals.