Electrochemical insertion of lithium ions into disordered carbons derived from reduced graphite fluoride

Both covalent (obtained by direct fluorination at high temperature) and semi-ionic carbon fluorides (synthesized at room temperature) were reduced in order to obtain disordered carbons containing very small content of fluorine and different physical properties according to the reduction treatment (chemical, thermal or electrochemical). After a physical characterization (X-ray diffraction, electron spin resonance and FT-IR spectroscopies), the electrochemical behaviours of the pristine carbon fluorides and of the treated samples were investigated during the insertion of lithium using liquid carbonate-based electrolytes (LiClO₄-EC/PC, 50:50%, v/v). Both galvanostatic and voltammetric modes were performed and revealed that the voltage profiles and the capacities differed according to the starting material and the reduction treatment. Semi-ionic carbon fluoride treated in F₂ atmosphere for 2 h at 150 °C and then chemically reduced in KOH exhibits high reversible capacities (the reversible capacity is 530 mAh g⁻¹ in the second cycle); in this case, the voltage profiles show a large flat portion at potentials lower than 0.3 V which is attributed to the insertion/deinsertion of lithium ions between the small graphene sheets and/or the absorption of pseudo metallic lithium into the microporosity of the sample. Nevertheless, a part of the lithium ions are removed at potentials higher than 0.5 V versus Li⁺/Li limiting the useful capacity.     

Deposition of a thin film of carbon nanotubes onto a glassy carbon electrode by electropolymerization

A novel and easy route for the deposition of a thin film of carbon nanotubes onto an electrode surface by electropolymerization is described. Multi-wall carbon nanotubes (MWNTs) were “dissolved” in aqueous alizarin red S (ARS, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid, sodium salt) solution, and a very stable and well-distributed aqueous MWNTs–ARS solution was obtained. A thin film of MWNTs–ARS was successfully deposited onto the electrode surface by an in situ electropolymerization in aqueous MWNTs–ARS solution. The MWNTs–ARS thin film was characterized by scanning electron microscopy, Raman spectroscopy, UV–Vis spectroscopy and electrochemical techniques.     

Electrochemical, textural and microstructural effects of mechanical grinding on graphitized petroleum coke for lithium and sodium batteries

A graphitized coke material obtained from petroleum residua was mechanically ground at different milling times between 0 and 100 h. Electrochemical reactions with both lithium and sodium are significantly altered as a function of grinding time. Short-time ball milling of graphite (1 and 5 h) induces a limited decrease in particle size and an increase in microstrain content. Simultaneously, alkali metal intercalation and electrolyte decomposition are hindered, and thus the irreversible and reversible capacities decrease. For longer milling time (up to 100 h), average crystallite size decreases and particles adopt a lamellar shape. Simultaneously, the irreversible capacity increases and correlates with an increase of the resistance, as obtained by impedance spectroscopy. Ex-situ XRD shows that extensively ground graphite samples need a higher discharge specific capacity to reach the formation of n-stages as compared to non-ground graphite, this being indicative of lithium incorporation in energetically different sites to the interlayer space. Sodium storage capacity increases with prolonged grinding time. This effect is shown here for the first time for graphitized cokes.     

Study on the microstructural evolution of high temperature adhesives for graphite bonding

Development of high-temperature adhesives (HTAs) can provide a way to produce carbon materials with a large size or a complex shape. Some HTAs were developed for joining graphite materials. These HTAs were prepared using phenol formaldehyde (PF) resin as matrix and B₄C powders as fillers. The results showed that these HTAs had satisfactory adhesive properties for graphite bonding even though the bonded graphite components were heat-treated at temperatures up to 1500 °C. In order to clarify the effects of the microstructure of HTAs on adhesive strength, the bonded graphite parts were heat-treated at high temperatures ranging from 400 to 1500 °C. The microstructural evolution of HTAs was investigated using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Analyses of the HTAs at the interface revealed that during microstructural evolution, carbonization of PF resin and complex interactions between PE and B₄C powders occurred. A high chemical bonding force was introduced at the interface through interactions between the glassy carbon from the PF resin and B₄C fillers, and the B₄C fillers restrained the volume shrinkage of PF resin during carbonization, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

EPR study on petroleum cokes annealed at different temperatures and used in lithium and sodium batteries

EPR spectroscopy was used for characterisation of petroleum cokes heat-treated in the temperature range of 480-2800 °C and after their electrochemical reactions with lithium and sodium. Unpaired spins with localised character were detected for cokes treated below 1000 °C, while delocalised electrons were found to contribute to the EPR profile of cokes treated between 1400 and 2800 °C. After electrochemical interaction of cokes with lithium, the EPR signal due to delocalised electrons undergoes a strong line narrowing combined with an increase in signal intensity. Localised paramagnetic centres interact with Li and Na, as a result of which there is a line narrowing and decrease in the signal intensity. A new narrow signal is detected for low-temperature cokes after reaction with lithium. The origin of this signal could be associated with ‘near-metallic’ lithium, which is accommodated in non-ordinary carbon sites such as microcavities. The intensity of this signal is higher for samples treated at 600 °C, where a higher hysteresis in the voltage versus composition curve is observed.     

Influence of the soluble organic fraction on the thermal behaviour, texture and surface chemistry of diesel exhaust soot

Diesel soot samples collected on a SiC filter while using or not an oxidation catalyst, were solvent extracted or heat treated under inert atmosphere. All studied soot were characterised by XPS, DRIFTS, TGA-MS, Py-GC-MS and gas sorption. In the first part, the obtained results are discussed in order to show the influence of an oxidation catalyst on some properties of the raw soot materials, such as their soluble organic fraction (SOF) content and composition, their surface chemistry and textural characteristics. Then, it is seen that the thermal decomposition of SOF adsorbed on the particulate matter leads under inert atmosphere to the formation of a microporous carbonaceous layer at 600 °C. On the other hand, textural analyses have revealed that the structure of the raw soot material is non-porous in nature, as subsequent extractions in DCM and toluene do not seem to create any porosity within the samples. Finally, we have described and discussed the composition of the particulate matter as a function of its respective components, namely SOF, volatile organic fraction (VOF), carbonaceous matrix and ashes.     

Mechanism of lithium storage in low temperature carbon

Through measurement of the intensity of the EPR signal of carbon anodes at different discharge and charge potentials, a micropore mechanism is suggested for the storage of lithium in low temperature carbons (LTCs), and it is further confirmed by results from the addition of pore-genic agent and introduction of crosslinker DVB into addition polymers PAN and P(4-VP). The size of micropores acting effectively as ‘reservoirs’ for lithium storage is suggested to be below 100 nm. The phenomena, which are characteristic in LTCs such as voltage hysteresis and capacity fading, are explained through the suggested mechanism.     

Surface compatibility in a carbon-alloy composite and its influence on the electrochemical performance of Li/ion batteries

Carbon-alloy composites were prepared by coating carbon materials with different surfaces with Sn, Sb or SnSb. The SnSb-hard carbon spherule (HCS) composite electrode shows the best cycling performance. The matching of crystalline parameters between SnSb and the carbon ensures a good dispersion of SnSb alloy on the surface. The high density of the nucleation centers on the HCS surface leads to a small SnSb crystallite size. Open pores on the surface of HCS, into which the alloy crystals are allowed to grow, act as pinning centers, which further stabilize the composite. These three factors are supposed to be responsible for the best cyclic performance of SnSb-HCS composite. The latter two factors also result in a large BET surface area, which leads to a large initial irreversible capacity loss, because more solid electrolyte interface film is formed. It seems that the best cyclic performance and the highest initial efficiency are paradoxical in the SnSb-carbon composite. Further surface modification should be conducted to obtain better electrode materials.     

Thin-film particles of graphite oxide. 2: Preliminary studies for internal micro fabrication of single particle and carbonaceous electronic circuits

Some preliminary studies to realize a carbonaceous electronic circuit were carried out using the liquid-dispersible thin-film particles of graphite oxide and their conductive reduction product. (1) The affinity of insulator substrates (diamond, silicon carbide, silicon, sapphire and three kinds of glass) for the particles was improved by heating and immersion in water. (2) The electric conductivity was measured for a wide wiring pattern formed by a large number of the reduced thin-film particles mounted on the substrate, and the value was 1600 S/m after heating at 500 °C. (3) To make the prototype of a narrow wiring or a micro device, the internal micro fabrication (position-selective removal) of a single particle was attempted using focused ion beam. Many kinds of patterns which contain narrow wiring having a width of 200 nm etc. were formed in the reduced thin-film particle having about 10 nm thickness. (4) The simple model calculation of anisotropic conductivity was executed for thin graphite which has a small number of layers and finite size. In the case of the fine graphitic carbonaceous devices and circuits, much attention must be paid to their sizes in all directions.     

Effects of sulfuric acid treatment on the microstructure and electrochemical performance of a polyacrylonitrile (PAN)-based carbon anode

To examine whether acid treatment of a non-graphitizing hard carbon influences positively or negatively its electrochemical anodic performance, this study reports the effects of sulfuric acid treatment on the microstructural changes and electrochemical performance of PAN-based hard carbons prepared at various temperatures. It was found that PAN-based hard carbons heat treated at 900 °C (TAN9) exhibit an increased reversible capacity by up to ∼20% and a decreased irreversible capacity by up to ∼8% following the sulfuric acid treatment (TAN9S series). Since small changes in microstructure, except for slight reductions in surface area and crystallite size (L_a) value, were observed after sulfuric acid treatment, it was speculated that the capacity responses of samples in series TAN9S were due to the introduction of new functional groups such as SO₃H and SO₄H. As the newly introduced functional groups are strong acids but their conjugates ( and ) are weak bases, those conjugates were thus considered to be able to react with Li⁺ ion relatively weakly and reversibly. For PAN-based hard carbons heat treated at 1100 °C (TAN11), there were much smaller changes, as compared with series TAN9S samples, in surface chemistry and microstructure by sulfuric acid treatment. Consequently, we observed an analogous but smaller influence of sulfuric acid oxidation on electrochemical performance. The samples from series TAN11S exhibited very stable cycling behavior. It was suggested that the acidity/basicity of surface functional groups may be an influential factor for improving the electrochemical performance of hard carbon-based anodic materials for lithium ion batteries.     

Magnetoelectronic properties of a graphite sheet

Magnetoelectronic structures of a two-dimensional (2D) graphite sheet are calculated by the tight-binding model. They are very sensitive to the magnitude of perpendicular magnetic field (B). B imposes the periodical boundary condition on the Bloch functions in the real and momentum spaces. Thus, B changes energy dispersions, energy spacing, bandwidth, and oscillation period of Landau levels. B could reduce the dimensionality of a graphite sheet. Energy dispersions mainly exhibit zero-dimensional (or 1D) characteristics. A lot of delta-function-like peaks (or square-root peaks) in the density of states can be clearly found. The magnetic field dramatically changes the joint density of states and the magnetoabsorption spectra. So, many peaks with different structures are produced.     

Influence of oxygen treatment on electric double-layer capacitance of activated carbon fabrics

Oxygen treatment at 250°C on polyacrylonitrile-based activated carbon fabric was conducted to explore the influence of carbon-oxygen complexes on the performance of capacitors fabricated with the carbon fabric. Surface analysis showed that most of the oxygen functional groups created from the oxygen treatment were the carbonyl or quinone type. The performance of the capacitors was tested in 1 M H₂SO₄, using potential sweep cyclic voltammetry and constant current charge-discharge cycling. It was found that the Faradaic current, the contributor of pseudocapacitance, increased significantly with the extent of oxygen treatment, while the increase in the double-layer capacitance was minor. Due to the treatment the overall specific capacitance showed an increase up to 25% (e.g., from 120 to 150 F g⁻¹ at a current density of 0.5 mA cm⁻²). However, the distributed capacitance effect, the inner resistance and the leakage current were found to increase with the extent of oxidation. It is suggested that due to the local changes of charge density and the increase in redox activity the presence of the carbonyl- or quinone-type functional groups may induce double-layer formation, Faradaic current, surface polarity, and electrolyte decomposition.     

Chemical modification of carbon fiber surfaces

Surface properties of a high modulus unsized carbon fiber modified by aqueous ammonia were studied. Basic groups were detected by potentiometric titration in addition to the pristine acid groups in untreated fiber. Surface composition of fiber modified with maleic anhydride or tetracyanoethylene was determined by XPS analysis. The modified fiber interaction with epoxide resin was studied by the Single Fiber Composite test. Ammonia treated fiber showed an improvement of the fiber/matrix stress transfer.     

Development of a dispersion process for carbon nanotubes in ceramic matrix by heterocoagulation

The surface properties of carbon nanotubes have been changed by heat treatment with ammonia to yield a basic surface or with N₂ to yield an acidic surface without destroying the integrity of the nanotubes. The dispersion state of CNTs could be much improved by the adsorption of a cationic dispersant of poly(ethylenimine) (PEI) or an ionic dispersant of sodium dodecyl sulfate (SDS). Through the electrostatic interaction between components, CNTs could be successfully coated with particles of titania or alumina by colloidal heterocoagulation. A titania coating on 1.5wt% CNTs, shows the same photocatalytic properties in phenol degradation and has advantages in separation and reuse.