Ph. Touzain

Insertion des ions PF6−, AsF6− et SbF6− dans le graphite par methode electrochimique. caracterisation des produits obtenus

Abstract PF 6 − , AsF 6 − and SbF 6 − ions were intercalated into graphite from a propylene carbonate electrolyte by an electrochemical method. During the intentiostatic charging of the system Li-Al/P.C., LiX (molar)/graphite one can observe a continuous increase of potential up to 5.2 V. This value is reached for a C 24 + charge of the graphite (Fig. 3). The discharge from the intercalated compound of graphite (I.C.G.) presents three plateaus: at 4.7 V until C 48 + , 4 V between C 48 + and C 96 + and 2.5 V between C 96 + and graphite (Fig. 4). X-Ray powder studies (Fig. 5) of C 48 + X − χ P.C.I.C.G. show that such compound is a second stage one. The anion is solvated by four molecules of propylene carbonate between the layers of graphite. This solvation causes the considerable thickness of the inserted layers (10.6 A for PF 6 − 4 P.C., 10.9 A for SbF 6 − 4 P.C.). The detailed structure (Fig. 8) of these layers is clarified by intensity line calculations. The presence of four molecules of solvent was confirmed by thermogravimetric measurements (Fig. 6). The first stage compound C 24 + X − 4 P.C. could not be obtained outside of the cell. The C, P, As and Sb analysis in the I.C.G. provided a composition in good agreement with the formula C 24n + P.C., where n is the stage. Owing to their high potential of 5.2 V against lithium, these compounds are of great interest for their utilisation as cathodes in high energy density batteries.

Lithium-graphitic oxide cells part II: High specific surface area graphitic oxide as cathode material for lithium batteries

Abstract A high surface area, chemically-prepared graphitic oxide, C 8 O 4 H 2 , has been used as cathodic material in lithium batteries. Good performances have been obtained (2000 W h/kg energy density) because of its physico-chemical and electrochemical properties. Industrial applications can be envisaged.

Lithium — graphite oxide cells Part III: Effect of origin and oxidation of graphite on batteries performances

Abstract In this study graphite oxides prepared by the Brodie method are used as positive electrode materials in lithium primary batteries. Battery performances are related to the discharge rate but also to the original graphite quality (natural graphite, high surface area graphite, ex-CO catalytic graphitized carbon) and to the second oxidation treatment. Energy density of more than 1800 Wh/Kg can be reached under 600 μA/cm 2 .

Intercalation of manganese chloride into mesophase pitch-based graphite fibers via gaseous complexes

The intercalation of certain metal chlorides into carbon fibers presents some problems, which depend on both their structure (three-dimensional ordering in particular) and the intercalation techniques. In this work, the intercalation of manganese chloride into pitch-based carbon fibers was investigated. The use of manganese chloride in the presence of iron(III) chloride or copper(II) chloride enhances the vapor pressure of the manganese salt and thus makes easier the intercalation process. The results of this work confirm the existence of FeCl3–MnCl2 complexes in the gaseous phase during intercalation. Stage-I intercalation compounds have been obtained for different graphite fibers. Several experimental techniques were used to characterize both the pristine and the intercalated graphite fibers, including X-ray diffraction, Raman spectroscopy and electrical resistivity measurements.

Study of piezoresistance effect in carbon fibers

Piezoresistance effect in different carbon and graphite fibers was examined. PAN-based carbon fibers laboratory prepared at various temperatures and pitch-based carbon and graphite fibers have been analyzed. Graphite fibers were intercalated with metal chlorides. The resistance changes during tensile loading were measured. The piezoresistance coefficient and coefficient of strain sensitivity were determined. Dependent on the type of fiber and temperature treatment, negative or positive effect of piezoresistance was observed. The resistance behavior of fibers during stretching was considered as geometrical changes and electrical conduction changes in graphite structure. The relationship between the structure parameters and piezoresistance effect for all fibers has been studied.

Kinetic study of electrochemical intercalation of potassium solvated by tetrahydrofuran into graphite

Abstract The formation of ternary graphite intercalation compounds K(THF) ∼2C∼20n has been followed by neutron diffractometry during cathodic intercalation of K+ (THF)y into a HOPG electrode. The pure first stage appears only after the formation of transient phases from about 4th stage towards the second. All the phases are constituted with organic molecules “standing up” between graphene layers. For the first time in the study of ternary compounds, a new phase of stage 4 3 has been observed for cation concentrations ranging from the second stage stoichiometry to the first one. It corresponds to highly ordered stacking domains consisting of 66% stage 1 and 33 % stage 2.

Metal ion intercalation on graphite cathode in MgCl2DMSO and MnCl2DMSO solutions

Cathodic reduction behavior of pyrolytic graphite in MgCl2DMSO and MnCl2DMSO solutions was studied. The cathodic reduction in MgCl2DMSO solution caused the intercalation of DMSO solvated Mg2+ ion to form the graphite intercalation compound. The insertion of a little amount of DMSO solvated Mn2+ ion between layers near the edge part of the graphite was suggested on the cathodic reduction in MnCl2DMSO solution. These results led to an importance of relation between redox potential of metal/metal ion and threshold potential of beginning of cation intercalation on graphite.

Intercalated pyrolytic graphite for neutron monochromatisation

Abstract Highly oriented pyrolytic graphite (HOPG) is actually the most efficient neutron monochromator for wavelengths, λ, from 2 A up to about 6 A. By means of intercalation its use can be extended to λ ∼ 10 A where, up to now, no monochromator crystals of high reflectivity have been available. Both theoretical and experimental results show that even though intercalation strongly increases the mosaic spread, the peak reflectivity of the intercalated HOPG compounds KC8, RbC8 and CsC8 is still about 70% for neutron wavelengths in the range from 7 to 9 A. The variation in d-spacing of these compounds allows enhancement of the neutron flux by increasing the bandwidth, Δλ/λ, up to about 10%. Typical applications are considered in three-axis spectrometry and in diffractometry.

Essais d'insertion dans le graphite d'halogenures de titane(IV): Structure et proprietes du compose lamellaire graphite-tetrafluorure de titane

Abstract Syntheses of lamellar compounds were attempted by heating mixtures of graphite and halides of titanium(lV), TiCl4 and TiF4 in chlorine atmosphere confined in sealed tubes. The whole liquid range of TiCl4 was explored without observing any reaction. Thus Crofts result [4] about the non-intercalation of this compound was confirmed. In case of TiF4, a synthesis was obtained at 300°C. Differential thermal analysis (DTA, see Fig. 1) and X-ray powder diffraction show clearly that TiF4 is intercalated into the lattice of graphite. DTA: Fig. 1(c, d and e) are the curves for the intercalated compound: 1(c and d) are obtained under dry argon, 1(e) under air. No indications of physical transformation of pure TiF4 (Fig. 1a) are observed. An X-ray pattern of a product obtained at 25 atm of chlorine is given in Table 1: the strong band for pure graphite is split in two lines around 3 and 3.7 A. Elemental analysis gives a certain range of non stoichiometry from C19 to C24,TiF4 for the product made under chlorine at a pressure larger than 5 atm. In all cases, no chlorine has been found in the compound. Assuming a 3rd stage layer structure and atomic configuration of TiF4 as shown in Fig. 2. the relative intensities of X-rays diffraction lines were calculated for C15 to C27TiF4. The results are shown in Fig. 3 for the 00l diffraction lines: 001 reflexion has a minimum for C21 TiF4 and as we have never seen this reflexion, we assume that this is the ideal stoichiometry. Table 2 compares the observed and calculated intensities for C21TiF4. Agreement is quite good. Synthesis carried out under pressure of chlorine lower than 5 atm gave disordered products. Table 3 compares the obtained X-ray pattern for a product made at 1 atm of chlorine and the pattern calculated on the following assumptions: 4th stage, stoichiometry of C28TiF4. The pattern of TiOF2 is also present in the table. Agreement is not very good, and moreover the pattern of products depends on the pressure of chlorine as shown on Fig. 4 for the 004 and 005 reflexions. We have had then to introduce a “pseudo-stage” n + x (formula (2) in appendix). On Fig. 5 the inverse of this pseudo-stage is plotted vs the Cl2 pressure. A stable domain of 3rd stage is observed between 5 and 19 atm. Below 5 atm higher stages and disordered products are obtained. Figures 6 and 7 relate to a thermal stability study of the C21 TiF4 product. Figure 6 is a thermolysis curve made under dry argon (a) and under vacuum (10 2torr) (b). Figure 7 is a thermolysis curve made in a diffraction high temperature chamber under vacuum (10−5 torr): 1 (n + x) is plotted vs temperature. In all cases the product gives higher stages and disordered products.

Etude cinetique et structurale par diffraction neutronique en temps reel de l'insertion du tétrahydrofuranne dans KC24 et RbC24

Abstract The reaction of tetrahydrofuran (THF) with KC24 and RbC24 graphite intercalation compounds has been studied in situ by neutron diffraction measurements. The patterns obtained during intercalation and deintercalation of the organic molecules were recorded on a time scale of two minutes/spectrum using the DIB multidetector at I.L.L. (Figs. 1 and 2). We have successively observed the formation of two first stage ternary phases: MC24 (THF)1 (M = K, Rb) where the THF molecules lie parallel to the graphitic planes (//), then the richer ternary phase MC24 (THF)2 in which the organic molecules stay perpendicular to the planes (⊥). By cryopumping, MC24 (THF)2 (⊥) can be reversibly transformed into MC24 (THF)1 (//). The analysis of the (100) graphite diffraction peak shows a shift to the lower theta during the THF intercalation into the binary compounds indicating an increase of dc-c with the ternary compound formation. This increase, related to a charge transfer from the intercalate to the graphite layers, is more important with the potassium ( Δd = 0.007 A ) than with the rubidium ( Δd = 0.004 A ) like it has been previously observed during benzene intercalation. dc-c remains quite constant in the two ternary phases (// and ⊥). Owing to the scattering cross-section of the deuterium comparable to those of carbon and oxygen, the structural analysis of the (00f) neutron lines provides more informations than X-ray diffraction measurements can do. So, we have been able to fit all the (00f) intensities observed and calculated with different ratios m = THF/MC24. The better agreement was found with m1 = 1.6 for the (//) phase and m2 = 2.45 for the (⊥) one. This result confirms that the intercalation number m is limited by the free space between two adjacent graphitic planes. On the other hand, m2 was calculated with two hypotheses: all the molecules standing parallel to the c axis with the oxygen atom leveling at the same graphite plane (A model) or half leveling at a plane and half at the adjacent one (B model). The cristallographic results can only be fitted to the A model (Fig. 6). The in situ structural observation during intercalation revealed that the THF action on KC24 and RbC24 second stage binary compounds leads to first stage ternary compounds via intermediary second stages ternary phases. Nevertheless, the second stage phases diffract with a weak intensity and during a relatively short time range. Furthermore, and for the first time, it has been observed the appearance of the first stage binary compound MC−8 during the first stage ternary MC24 (THF)1.6 (//) formation, then its decrease with the transformation MC24 (THF)1.6 (//) → MC24 (THF)2.45 (⊥). During cyropumping, the formation of the (//) phase from the (⊥) one goes again with the MC−8 growth (Fig. 3). Such a mechanism can be explained by the alkali atoms reorganisation inside the graphite layers: the bulky organic molecules penetration induces a back flow for some alkali atoms which involves the growth of the binary domains MC−8 in the same time that the ternary domains formation. This necessarily implies a ratio M C weaker in the ternary phase Mx1 C24 (THF)1.6 (//) than in the initial binary compound Mx0 C24 (x1 Furthermore, the intensity disappearance for the (00f) diffraction peaks related to the RbC−8 phase while the (//) phase is transformed into the (⊥) one: Rbx2 C24 (THF)2.45 indicates that X2 is close to X0. So, in the ternary compounds as in the binary ones, the ratio C/M = 24 is an ideal integer value which may vary from one phase to another. Our results tend to a ratio THF/M = 2 in agreement with EXAFS measurements repons where it was found only one distance M-0 in the two phases (// and ⊥), and the same oxygen number around the alkali atoms[20]. This observation is consistent with the two solvating THF molecules lying parallel to the graphitic sheets before straightening and becoming perpendicular when the THF pressure increases. In this hypothesis, the phases would be represented by MC30 (THF)2 (//) and MC20 (THF)2 (⊥) where x2/C − 1 20 close to the ratio x0/C in the initial binary, agrees with the existence of several binary phases with different stoichiometries[24].