In situ Li nuclear magnetic resonance observation of the electrochemical intercalation of lithium in graphite; 1st cycle

We show a continuous, in situ nuclear magnetic resonance (NMR) experiment on a lithium/graphite electrochemical cell. The objective is to study a commercial graphite currently used as negative electrodes in secondary lithium batteries. A plastic cell is made, with metallic lithium as the counter electrode and 1 mol dm⁻³ LiPF₆/ethylene carbonate (EC) + diethylcarbonate (DEC) electrolyte. The reversible capacity is 346 mAh/g and the irreversible capacity 55 mAh/g, measured in the galvanostatic mode, at a rate of C/20 (20 h for the theoretical capacity of LiC₆) for the first cycle. We show the first discharge and the first charge of the cell inside the magnet and record simultaneously and regularly (in real time) static ⁷Li NMR spectra. As expected, we observe the quadrupolar lines characteristic of the lithium graphite intercalation compounds (GICs). During the discharge, the two types of in-plane densities of Li are successively found that correspond to the dilute LiC₉, then to the dense LiC₆ configuration; during the charge, we observe the successive decrease of these states. The galvanostatic curve helps to identify the stages NMR signature and the stages coexistence.     

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF₆ in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.     

Isotope-specific analysis of neutron-irradiated lithium aluminate ceramics by nuclear magnetic resonance spectroscopy

The effects of neutron irradiation on lithium aluminate ceramics and the transport and incorporation of tritium into surrounding components are primary concerns in the development of materials for use in tritium production. The high specific activity and volatility of the tritium in activated samples greatly increase the challenges of experimental analysis. This paper reports the determination of structural forms and concentrations of ³H and ⁶Li in irradiated and nonirradiated LiAlO₂ by nuclear magnetic resonance spectroscopy. The experiments find tritium uniformly distributed in hydroxyl groups bridging aluminum centers and lithium in four- and six-coordinated sites. These results demonstrate that measurements can be performed in a nondestructive, safe manner to obtain isotope-selective, quantitative information even on highly hazardous materials.     

Discharge characteristics of graphite fluorides prepared via graphite intercalation compounds in nonaqueous lithium cells

Two types of graphite fluorides (C₂F)_n and (CF)_n were prepared by refluorination of thermally decomposed ionic graphite intercalation compounds of fluorine. Discharge behavior and effect of the heat treatment of graphite fluoride were investigated. This method drastically decreased the reaction time compared with the direct fluorination of graphite. OCV of (C₂F)_n was higher by 0.3 V than that of the conventionally prepared one, however, overpotential was the same at the constant current density of 0.5 mA cm⁻². On the other hand, the same OCV and less overpotential by 0.3 V were observed for (CF)_n. The heat treatment of these samples in fluorine atmosphere at higher temperatures increased the discharge capacity and provided a more flat discharge potential.     

Analysis of bis(trifluoromethylsulfonyl)imide-doped paramagnetic graphite intercalation compound using F very fast magic angle spinning nuclear magnetic resonance

F atoms bonding to paramagnetic/conductive graphene layers in accepter-type graphite intercalation compounds (GICs) are analyzed using very fast magic angle spinning nuclear magnetic resonance, which is applied for the first time on ¹⁹F nuclei to investigate paramagnetic materials. In the bis(trifluoromethylsulfonyl)imide(TFSI)-doped GIC, C–F bonds between fluorine atoms and graphene layers conform to a weak bonding of F to the graphene sheets. TFSI anions intercalated in the GIC do not show overall molecular motion; even at room temperature only the CF₃ groups rotate.     

In situ TEM observation of lithium nanoparticle growth and morphological cycling

Lithium fluoride crystals were subjected to electron beam irradiation at 200 and 300 keV using transmission electron microscopy in order to study in situ fabrication of Li nanostructures. We observed that LiF crystals decompose in a unique way different to all other metal halides: Fluorine ablation and salt-to-metal conversion is non-local and due to a rapid lateral diffusion of Li, the life cycle from nucleation to annihilation of fresh Li nano-crystals can be observed at a distance from the Li-source, the irradiated salt. Growth, shape transition and annihilation of Li nanostructures follow at slow enough speed for live video recording with resolution of 25 frames per second. The equilibrium shapes of pure Li nano-crystals range from cubic to rod-shaped and ball-shaped and up to 300 nm size. By varying the e-beam flux of irradiation, transitions from cube to spherical shape can be induced cyclically.     

In situ observation of damage nucleation in graphite and carbon/carbon composites

The flexure strength and the fracture toughness at 300 K and 77 K were measured in two isotropic polycrystalline graphites with very different microstructure and in one carbon/carbon composite. In addition, the micromechanisms of damage initiation at the notch tip were examined in situ during the fracture tests through a long focal distance microscope. It was found that the mechanical response of carbon-based materials was insensitive to the effect of cryogenic temperatures. In graphite with coarse microstructure, cracks appeared at very low stresses in various points of an ample region surrounding the notch tip, and damage progressed by their stable crack growth and link up. On the contrary, damage was localized at the notch root in graphite with a fine microstructure. High stresses were necessary to nucleate a single crack, which grew unstably, leading to immediate specimen failure. Damage in carbon/carbon composites was nucleated in the form of matrix cracks around the notch tip, but fiber yarns impeded the crack propagation until the load had increased significantly. This process was repeated several times, leading to a serrated load-deflection curve and to a marked increase in the overall fracture resistance.     

Competition between Li+ and Mg2+ for red blood cell membrane phospholipids: A 31P, 7Li, and 6Li nuclear magnetic resonance study

The mode of action of the lithium ion (Li⁺) in the treatment of manic depression or bipolar illness is still under investigation, although this inorganic drug has been in clinical use for 50 yr. Several research reports have provided evidence for Li⁺/Mg²⁺ competition in biomolecules. We carried out this study to characterize the interactions of Li⁺ and Mg²⁺ with red blood cell (RBC) membrane components to see whether Li⁺/Mg²⁺ competition occurs. ³¹P nuclear magnetic resonance chemical shift measurements of the phospholipids extracted from the RBC membranes indicated that the anionic phospholipids, phosphatidylserine and phosphatidylinositol, bind Li⁺ and Mg²⁺ most strongly. From ⁶Li relaxation measurements, the Li⁺ binding constant to the phospholipid extract was found to be 45±5M⁻¹. Thus, these studies showed that the phospholipids play a major role in metal ion binding. ⁷Li spin-lattice relaxation measurements conducted on unsealed and cytoskeleton-depleted RBC membrane in the presence of magnesium indicated that the removal of the cytoskeleton increases lithium binding to the more exposed anionic phospholipids (357±24 M⁻¹) when compared to lithium binding in the unsealed RBC membrane (221±21 M⁻¹). Therefore, it can be seen that the cytoskeleton does not play a major role in Li⁺ binding or in Li⁺/Mg²⁺ competition.     

Carbon-13 nuclear magnetic resonance analysis of intact oilseeds

High resolution natural abundance¹³C nuclear magnetic resonance (NMR) spectra of intact oilseeds have been obtained by Fourier transform techniques. The spectra can be interpreted in terms of the relative concentrations of the major fatty acids in the oilseed. The¹³C NMR spectra are well resolved despite the fact that¹H NMR spectra of the same seeds are poorly resolved. The difference in resolution can be attributed to the simplicity of the¹³C NMR spectra in which all spin-spin coupling can be removed, in which the separation of lines is increased by a factor of 30, and in which line broadening due to intermolecular dipolar interactions is not important. The¹³C NMR Fourier transform technique is sufficiently sensitive that a high quality spectrum can be obtained from a single soybean, for example, in about 10 minutes. Similar spectra of single seeds can be obtained in comparable times for corn, castor bean, peanut, sunflower seed, and rapeseed. Because the NMR technique is nondestructive, it can be used to select individual oilseeds for use in breeding programs designed to improve oil quality. By employing some special experimental NMR line narrowing techniques, it also appears feasible to obtain moderately well resolved, natural abundance¹³C NMR spectra of the immobile, rigid protein, and carbohydrate components of an intact oilseed, as well as the more mobile oil components.     

Enzyme-catalyzed polymerization of 8-hydroxyquinoline-5-sulfonate by in situ nuclear magnetic resonance spectroscopy

In this report, we describe the use of in situ NMR spectroscopy to elucidate the mechanism of horseradish peroxidase-catalyzed oxidative free-radical coupling of phenols. We demonstrate the potential of the technique for the polymerization of 8-hydroxyquinoline-5-sulfonate (HQS). Based on the structural changes, we establish the involvement of ortho- and para-position protons (to the hydroxyl group) in the oxidative free-radical coupling polymerization with their relative preferences. For example, in HQS, we establish that the positions 2, 4, and 7 are involved in the chemical bonding with the order of preference being 7 ≥ 2 > 4. Analyses of ¹³C-NMR data suggest the formation of C—C- and C—O—C-type coupling bonds during enzymatic polymerization. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 1257–1264, 1998     

Effect of the electrochemical oxidation/reduction cycle on the electrochemical capacitance of graphite oxide

Reduction largely affects the properties of graphene oxide (GO), because the content of each functional group changes by the oxidation degree. So far no study about re-oxidation and re-reduction of GO has been carried out. Herein, we report the effects of oxidation/reduction cycle of graphite oxide (GtO, multilayered GO) on its composition and electrochemical capacitance property. Electrochemical oxidation and reduction of glassy carbon (GC) were conducted at potentials of +2.0 and −1.1 V (vs. Ag/AgCl), respectively, and then the electrochemical capacitances were measured. According to X-ray photoelectron spectroscopy (XPS) analysis, CC bonds were produced from CH defects of the reduced graphite oxide (rGtO) by the electrochemical re-oxidation. The conductivities of the reduced samples, with CH defects, and re-oxidized samples, with newly produced CC bonds, were high and low, respectively. The high electrochemical capacitance observed for the rGtO electrodes is caused by the activity of the CH defects and good conduction.     

Preparation of high-impact polystyrene nanocomposites with organoclay by melt intercalation and characterization by low-field nuclear magnetic resonance

Recycled high-impact polystyrene nanocomposites with organoclay were prepared. Clay of the smectite group (montmorillonite) with two types of intercalated compounds was used (Viscogel S4 and Viscogel S7). The polymer nanocomposites were prepared by melt intercalation, applying two shear intensities in a twin screw extruder. The nanostructured materials obtained were characterized by NMR relaxometry, X-ray diffraction, thermogravimetric analysis, melt flow index and mechanical analyses. The results showed that the nanostructured materials presented a mixed intercalated/exfoliated morphology. The organophilic clay, Viscogel S7, generated polymer nanocomposites with better dispersion and distribution (at low concentrations) than those produced with the Viscogel S4. The shear rate was effective for dispersion of the nanoparticles. The materials processed at 600 rpm showed better dispersion than those processed at 450 rpm. The characterization techniques chosen were effective. They were complementary and permitted comparison among the polymer nanocomposites. The use of low-field NMR relaxometry allowed measurement of the spin–lattice relaxation time of hydrogen (T₁H), which provided more precise information on the mobility of the materials, thus complementing and explaining the results obtained by X-ray diffraction.     

Electrochemical intercalation of lithium into a natural graphite anode in quaternary ammonium-based ionic liquid electrolytes

Electrochemical intercalation of lithium into a natural graphite anode was investigated in electrolytes based on a room temperature ionic liquid consisting of trimethyl-n-hexylammonium (TMHA) cation and bis(trifluoromethanesulfone) imide (TFSI) anion. Graphite electrode was less prone to forming effective passivation film in 1 M LiTFSI/TMHA-TFSI ionic electrolyte. Reversible intercalation/de-intercalation of TMHA cations into/from the graphene interlayer was confirmed by using cyclic voltammetry, galvanostatic measurements, and ex situ X-ray diffraction technique. Addition of 20 vol% chloroethylenene carbonate (Cl-EC), ethylene carbonate (EC), vinyl carbonate (VC), or ethylene sulfite (ES) into the ionic electrolyte resulted in the formation of solid electrolyte interface (SEI) film prior to TMHA intercalation and allowed the formation of Li-C₆ graphite interlayer compound. In the ionic electrolyte containing 20 vol% Cl-EC, the natural graphite anode exhibited excellent electrochemical behavior with 352.9 mAh/g discharge capacity and 87.1% coulombic efficiency at the first cycle. A stable reversible capacity of around 360 mAh/g was obtained in the initial 20 cycles without any noticeable capacity loss. Mechanisms concerning the significant electrochemical improvement of the graphite anode were discussed. Ac impedance and SEM studies demonstrated the formation of a thin, homogenous, compact and more conductive SEI layer on the graphite electrode surface.     

C nuclear magnetic resonance spectral analysis of stereosequences in ethylene-propylene copolymer

A number of split peaks dependent on both comonomer sequences and stereosequences were observed in the ¹³C nuclear magnetic resonance (n.m.r.) spectrum of ethylene-propylene (E-P) copolymer. The ¹³C chemical shifts of methylene carbon in stereoisomers of the respective hexad comonomer sequences were predicted by a chemical-shift calculation using the gamma effect on ¹³C chemical shifts and Mark's rotational isomeric state model for E-P copolymer. Assignments of the split peaks that arise from different hexad stereosequences were given by comparison between the observed and calculated chemical shifts. Reference was made to the hexad assignments of comonomer-sequence-dependent peak splittings determined in our previous calculation of ¹³C n.m.r. chemical shifts of stereoregular E-P copolymers. The tacticities were estimated for successive (not separated by ethylene units) propylene units in the hexad sequences.     

31P nuclear magnetic resonance phospholipid analysis of anionic-enriched lecithins

Phosphorus-31 nuclear magnetic resonance provides well-dispersed, quantifiable phospholipid profiles of commercial and laboratory anionic lecithin preparations. Seventeen phospholipids were determined in commercial soybean lecithins. Lecithin refinement to produce anionic-enriched lecithins may be monitored precisely.     

Quantitative analysis of oil removal from cotton fabrics through thein situ formation of microemulsions by solid-state nuclear magnetic resonance

Solubilization and subsequent removal of soybean oil from cotton fabrics through thein situ formation of microemulsions were evaluated by solid-state nuclear magnetic resonance (NMR) spectrometry. Regions of water-in-oil and oil-in-water microemulsions were identified for systems that contained polyoxyethylene (60) sorbitol hexaoleate, soybean oil, and an aqueous phase composed of water/ethanol or isopropanol (80:20 wt%) at 25°C. The amount of oil removed from the cotton fabrics was determined by solid-state NMR after constructing a calibration curve relating the intensity of camphor/oil NMR signals (I_c/I_o) to their molar ratio (M_c/M_o). A precision Crockmeter (Mul-Tech Industries, New York, NY) was used to reproducibly remove soybean oil stain from cotton fabric, which was subsequently analyzed by NMR. Typically, more than 90% of the oil stain was removed after 200 revolutions of the Crockmeter finger with 2 wt% surfactant at 25°C. Increasing the amount of surfactant to 6 wt% improved soybean oil removal from the fabric to 99 wt%.     

In situ neutron radiography analysis of graphite/NCA lithium-ion battery during overcharge

Overcharge of lithium-ion batteries can lead to the deposition of lithium ions on the surface of graphite electrodes. The phenomenon of lithium deposition causes reduced electrochemical performance and presents safety concerns for lithium-ion batteries in high-power applications. This study presents a technique using neutron radiography (NR) for in situ visualization of the effects of overcharge in a graphite/NCA (LiNi_0.8Co_0.15Al_0.05O₂) lithium-ion cell. Patterns of deposition of solid material on the surface of the graphite electrode observed in the radiographs were confirmed by direct observation of the electrode. Inductively coupled plasma mass spectrometry was used to verify the elemental contents of the deposited material. NR is shown to be a promising tool for the study of lithium-ion batteries in high-power applications.     

On the great difficulty of intercalating lithium with a second element into graphite

Lithium is able to intercalate into graphite leading to various binary graphite intercalation compounds, that are well defined by their stage. Concerning the ternaries, there is little literature on the subject. Thermodynamical and structural data, that differ largely from those of the other alkali metals, lead one to foresee some serious difficulties in synthesising such ternary compounds. Many experiments have attempted to synthesise ternary graphite intercalation compounds with lithium, using successively very electronegative elements, then fairly electronegative species and lastly electropositive metals. Numerous results, that are wholly negative, are described in this paper. The calcium-lithium system only allows one to prepare a novel intercalation compound, that is a first stage ternary phase exhibiting a large interplanar distance. This latter suggests that the intercalated sheets consist of several superimposed atomic layers. The synthesis of this ternary is not easy, because it needs reagents of very high purity. It possesses the brightness of metals and its strong hardness is very unusual among graphite intercalation compounds. On the other hand, the charge transfer between the graphene planes and the intercalated sheets, that just allows the intercalation, is especially high, and much higher than the LiC₆ compound.     

The nature of the chromium trioxide intercalation in graphite

X-ray diffraction, thermal, and chemical techniques have been used to demonstrate that chromium trioxide will intercalate graphite in the presence of a glacial acetic acid solvent. The product is a third stage ionic compound of graphite with an identity period I_c of 14·92 A, which may be hydrolyzed to give a molecular intercalate. Contrary to previous reports, it appears that chromium trioxide and graphite do not react directly to form an analogous intercalation product, yielding instead a mixture of lower oxides of chromium and unreacted graphite.     

Effect of polypyrrole film on the stability and electrochemical activity of fluoride based graphite intercalation compounds in HF media

Fluoride intercalation/deintercalation cycles on commercially available high purity graphite electrodes leads to powder formation and electrode damage. Formation of polypyrrole films of optimum thickness by potential cycling on the graphite surface before fluoride intercalation leads to good mechanical stability to the electrode during intercalation/deintercalation cycles. The intercalation potential shifts by 200 mV in the positive direction. The intercalation and deintercalation charges (Q _a, Q _c) also decrease slightly. However the charge recovery ratio (Q _c/Q _a) improves significantly. Since the polypyrrole layer is compact on the graphite surface, the present study indicates that the film offers mechanical stability to the graphite film without affecting the electronic conductivity of the surface. F^– ion transport through the film also occurs with a small overvoltage.