Composite electrode materials for lithium-ion batteries obtained by metal oxide addition to petroleum vacuum residua

New composites of carbon with different metal compounds (tin, iron, nickel and vanadium oxides and sulfides) have been obtained at low temperature from petroleum residua, and their electrochemical behaviour has been evaluated in lithium test cells. X-ray diffraction evidenced a partial metal reduction and, in the case of tin, nickel and iron composites, their conversion to sulfides. Optical and SEM microscopy observations confirmed the presence of the metal compounds embedded in the carbonaceous matrix. Electrochemical techniques including impedance spectroscopy were used to evaluate the electrode performance, which was correlated to the microstructural and morphological properties. We have demonstrated the beneficial contribution of the metal sulfides formed in-situ to the electrochemical performance of the electrode material as compared to the carbon material without metal addition. V₂O₅ was not converted to a sulfide. Instead the reduction to V₂O₃ was observed, and the resulting composite material exhibited a good capacity retention, ascribable to a modification of the carbon surface properties.     

Lowering sensitivity of anode materials for lithium ion batteries towards humidity

Sensitivity of anode materials towards humidity is an important factor for the performance of lithium ion batteries. Here it is demonstrated for the first time that the sensitivity of composite anode materials prepared of metals such as copper and silver with natural graphite can be strikingly lowered. The composites are prepared by adsorbing metal ions from solutions onto the surface of natural graphite followed by heat-treatment at high temperature. Results from X-ray photoelectron spectroscopy, high resolution electron microscopy, thermogravimmetry, differential thermal analysis, and capacity measurements indicate that the deposited metals exist in two forms, viz. metallic and carbidic M_xC (M=Cu and Ag), and remove/cover (i.e. deactivated) the active hydrophilic sites at the surface of graphite. As a result, in the presence of high humidity the composites absorb less water, and the obtained electrochemical performance including reversible capacity, coulombic efficiency in the first cycle and cycling behavior is markedly improved. This approach provides a potentially powerful method to manufacture lithium ion batteries under less critical conditions.     

Carbon molecular sieves as model active electrode materials in supercapacitors

Various microporous carbon molecular sieves are studied as active electrode material for supercapacitors in order to clarify the controversy about the accessibility of the electrolyte to the micropores. Cyclic voltammetry experiments were performed in electrolytes with different ion size. The results showed a clear ion sieving effect when the porosity of the carbon was similar to that of the ions of the electrolyte. Impedance spectroscopy was also useful to evidence diffusion restrictions of the ions into the pores. The results obtained in this study clearly demonstrate that in aqueous media very narrow micropores (0.5 nm) are still capable of forming the electrical double layer. Therefore, the majority of microporous carbons, with wider porosity, are perfectly suitable as active electrode materials for supercapacitors when aqueous electrolytes are used.     

锂硫电池正极复合材料研究现状

锂硫电池由于其高理论能量密度(2600W·h/kg)而受到了广泛的关注,是极具应用前景的电池体系.硫基正极材料作为锂硫电池的重要组成部分,是提高电池性能的关键.然而锂硫电池还存在一些问题,如硫的利用率低及正极结构的稳定性差等.本文综述了近几年锂硫电池硫正极复合材料的研究现状,分别从硫/碳复合、硫/导电聚合物复合、硫/氧化物复合3个方面进行介绍,指出了未来锂硫电池正极材料要注意结合硫/导电聚合物及硫/氧化物的优势并注重材料结构的设计,向核壳或类核壳结构方向发展的趋势,同时还要提高载硫量,提高循环稳定性,以获得高性能的锂硫电池.     

Ultrafine SnO2 dispersed carbon matrix composites derived by a sol-gel method as anode materials for lithium ion batteries

Ultrafine crystalline SnO₂ particles (2-3 nm) dispersed carbon matrix composites are prepared by a sol-gel method. Citric acid and hydrous SnCl₄ are used as the starting constituents. The effect of the calcination temperatures on the structure and electrochemical properties of the composites has been studied. Structure analyses show that ultrafine SnO₂ particles form and disperse in the disordered carbon matrix in the calcination temperature range of 500-800 °C, forming SnO₂/C composites, and the carbon content shows only a slight increase from 35.8 wt.% to 39.1 wt.% with the temperature. Nano-Sn particles form when the calcination temperature is increased to 900 °C, forming a SnO₂/Sn/C composite, and the carbon content is increased to 49.3 wt.%. Electrochemical testing shows that the composite anodes provide high reversible cycle stability after several initial cycles, maintaining capacities of 380-400 mAh g⁻¹ beyond 70 cycles for the calcination temperature of 600-800 °C. The effect of the structure feature of the ultrafine size of SnO₂ and the disordered carbon matrix on the lithium insertion and extraction process, especially on the reversible behavior of the lithium ion reaction during cycling, is discussed.     

Urchin-like nano/micro hybrid anode materials for lithium ion battery

Inspired by a special biological structure in nature, a kind of urchin-like nano/micro hybrid design was proposed to modify conventional micrometer-sized electroactive materials for lithium ion battery (LIB). By catalytic chemical vapor deposition to in situ grow carbon nanofibers on the surface of natural graphite spheres, we fabricated the nano/micro hybrid composite with an urchin-like structure. Scanning electron microscopy (SEM), focused ion beam (FIB) workstation, transmission electron microscopy (TEM), and Raman spectroscopy were employed to characterize the composite. Electrochemical measurements indicated that the cyclability and rate capability of the composite as anode material for LIB were significantly improved. Furthermore, the design also demonstrated its effectiveness in other kinds of anode materials such as transition metal oxides.     

Novel carbon nanofibers of high graphitization as anodic materials for lithium ion secondary batteries

Carbon nanofibers (CNFs) of high graphitization degree were prepared by a CVD process at 550-700 °C. They showed different structures according to catalyst and preparation temperatures. The structure of CNF prepared from CO/H₂ over an iron catalyst was controlled from platelet (P) to tubular (T) by raising the decomposition temperature from 550 to 700 °C. The CNFs prepared over a copper-nickel catalyst from C₂H₄/H₂ showed the typical herringbone (HB) structure regardless of the reaction temperatures. The CNFs prepared over Fe showed d₀₀₂ of 0.3363-0.3381 nm, similar to that of graphite, indicating very high graphitization degree in spite of the low preparation temperature. Such CNFs of high graphitization degree showed high capacity of 297-431 mA h/g, especially in the low potential region. However, low first cycle coulombic efficiency of ≈60% is a problem to be solved. The graphitization of the CNF preserved the platelet texture, however, and formed the loops to connect the edges of the graphene sheets. Higher graphitization temperatures made the loop more definite. The graphitized CNF showed high capacity (367 mA h/g); however, its coulombic efficiency was not so large despite its modified edges by graphitization, indicating that the graphene edges were not so influential for the irreversible reaction of Li ion battery.     

Reduction of irreversible capacities of amorphous carbon materials for lithium ion battery anodes by Li2CO3 addition

Amorphous carbon materials for lithium ion battery anodes which contain a small amount of Li₂CO₃ were prepared by three methods. The obtained materials were characterized using X-ray diffraction (XRD) analysis, Raman spectroscopy and CO₂ adsorption experiments. Although the XRD profiles and Raman spectra of these materials were similar to those of carbon materials synthesized with no addition, the amount of CO₂ adsorbed was largely decreased by Li₂CO₃ addition. These results suggest that the micropores in these materials were plugged and/or filled with Li₂ CO₃. Galvanostatic lithium charging and discharging experiments showed that the irreversible capacity of the material can be significantly decreased by Li₂CO₃ addition, which is thought to be due to the plugging of the pore inlets by Li₂CO₃. Moreover, it was also found that the reversible capacities of the materials can be increased by adjusting both the amount of Li₂CO₃ addition and carbonization temperature.     

In situ Mössbauer spectroscopy of carbon-supported iron catalysts at cryogenic temperatures and in external magnetic fields

A highly dispersed carbon-supported iron catalyst has been studied within situ Mössbauer spectroscopy at temperatures down to 5 K and with external magnetic fields. It is shown that measurements of spectra in the presence of large magnetic fields considerably improves the information obtained from Mössbauer spectra.     

Microstructure of carbon derived from mangrove charcoal and its application in Li-ion batteries

In this study, the microstructure of mangrove-charcoal-derived carbon (MC) was studied using XRD, STM and TEM. MC was found to consist of aligned quasi-spherical structural units with diameters of around 5-20 nm. It shows typical hard carbon characteristics, including a strongly disoriented single graphene layer and BSU, formed by two or three graphene layers stacked nearly parallel. Some curved and faceted graphene layers, especially closed carbon nanoparticles with fullerene-like, were observed in the as-prepared samples. MC was also evaluated as an anodic material for Li-ion batteries. MC carbonized at 1000 °C possessed the highest available discharge capacity (below 0.5 V) of 335 mAh g⁻¹, the high first-cycle coulombic efficiency of 73.7%, good rate and cyclic capability and PC-based electrolyte compatibility. ⁷Li nuclear magnetic resonance (NMR) spectra of fully lithiated mangrove charcoal-derived carbons indicated the co-existence of three Li species.     

Effects of heteroatoms on electrochemical performance of electrode materials for lithium ion batteries

Recent studies of lithium ion batteries focus on improving electrochemical performance of electrode materials and/or lowering cost. Doping of active materials with heteroatoms is one promising method. This paper reviews the effects of heteroatoms on anode materials such as carbon- and tin-based materials, and cathode materials such as LiCoO₂, LiNiO₂, LiMn₂O₄ and V₂O₅. There are favorable and unfavorable effects, which depend on the species and physicochemical states of heteroatoms and the parent electrode materials. In the application of lithium ion batteries advantageous factors should be exploited, unwelcome side effects should be avoided as far as possible. Considerable gains towards improved electrochemical performance of the electrode materials have been achieved. Nevertheless, there are still problems needing further investigation including theoretical aspects, which will in the meanwhile stimulate the investigation for better electrode materials.     

Characterization of carbon nanofiber composites synthesized by shaping process

Catalytically grown carbon nanofibers (CNFs) are shaped into pellets in desired size and configuration by a conventional molding process so as to extend the potential applications of CNFs in industrial heterogeneous catalysis. After shaping, a novel carbon nanofiber composite with sufficient mechanical strength is produced, in which isolated CNFs are connected by a carbon network formed through polymer binder carbonization. Characterization of the synthesized CNF composite is performed by using HRTEM, XRD, Raman, N₂ physisorption, TPD and TGA. A comparison of the textural and structural properties, as well as the surface chemistry is made amongst the CNFs, the CNF composite, and a commercial coal-based activated carbon, in order to attain a comprehensive understanding of the CNF composite. The results show that the CNF composite preserves the mesoporous texture of the CNFs which will be beneficial to those reactions of mass transfer control. The modification effect of oxidative treatments on physico-chemical properties of the CNF composite is also investigated. More surface oxygen-containing groups are introduced to the composite by treating the material either in boiling HNO₃ solution or in static air at 400 °C.     

Numerical modeling of the carbonization process in the manufacture of carbon/carbon composites

A method for the numerical simulation of the carbonization process is introduced. A general model for the transient analyses of heat and mass transfer together with stress and displacement predictions is constructed using two-dimensional FEM (finite element method) for arbitrary geometry. The established model is applied to the carbonization of a single-phase, homogeneous, isotropic phenolic foam, and an anisotropic, two-phase composite material. A damage model is introduced to account for the development of shrinkage cracks, and a CDM (continuum damage mechanics) model is implemented for the calculation of mechanical property degradation due to crack evolution. The established model is verified by comparison with experimental results, and is applied to various numerical examples.     

Characterization of electrodeposited thin layers of magnetic alloys by Mössbauer spectroscopy

Rare earth and transition metal alloys are interesting magnetic materials for applications in the field of magnetooptical memories. We describe the preparation of Dy/Fe alloys by electrodeposition using a pulsed galvanostatic process. Samples were characterized by Mössbauer spectroscopy. Thin layers obtained with short electrolysis pulses (0.1 s) give a Mössbauer spectrum analysed as the sum of two subspectra: the first corresponds to a spectrum originating from very small sized particles of Fe and the second is attributed to a Dy/Fe phase. Longer electrolysis pulses (1 s) give samples whose spectrum is also the sum of two subspectra: the first is attributed to metallic Fe with an extended particle-size distribution while the second is attributed to a Fe/Dy alloy. Mössbauer spectroscopy was found useful in determining the best electrolysis parameters leading to domains of uniform magnetization with high coercive fields.     

Synthesis and characterization of chemically and electrochemically prepared conducting polymer/iron oxalate composites

Poly(3-octyl-thiophene) (POT) and polypyrrole (PPy) iron oxalate composites were synthesized through a post-polymerization oxidative treatment. The composite of the latter has been prepared also by electrochemical polymerization. The samples have been characterized by X-ray diffraction (XRD), impedance spectroscopy, scanning electron microscope (SEM) combined with energy dispersive X-ray (EDX) spectroscopy, Mössbauer spectroscopy, cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). In case of PPy, two peaks in the XRD spectra show the presence of iron containing composite, while with POT only the layered structure originating from the octyl side-chain interactions was modified by the composite formation. The assumption of the weakening of short- and long-range interactions was proven by the decrease in conductivity of the composite. The successful electrochemical synthesis resulted a composite of ∼5% iron content, determined by EDX. Mössbauer spectroscopy measurements evidenced a composite containing mixed valence iron oxalate doping ions, which supports the indirect EQCM data.     

State of hydrogen electrochemically stored using nanoporous carbons as negative electrode materials in an aqueous medium

Nanoporous carbons were used as negative electrode material in aqueous KOH medium to store hydrogen by electrodecomposition of water at atmospheric pressure. The storage capacity by this process is approximately one order of magnitude higher than in the gas phase at ambient conditions. By considering the particularities of the electrochemical characteristics, this paper gives information on the mechanism and on the kind of bond between hydrogen and the carbon host. For most experiments, a self-standing porous carbon cloth electrode has been used in order to avoid any side effect which could be due to additives. After galvanostatic hydrogen charging, the carbon material was analyzed by galvanostatic discharge and temperature-programmed desorption in order to determine the nature of the carbon-hydrogen bond and the amount of hydrogen sorbed. The activation energy for hydrogen desorption was estimated to be 110 kJ/mol, that confirms a weak chemical character of the hydrogen-carbon bond. Although the bond is stronger than in the case of physisorption, the fraction of hydrogen irreversibly trapped is low compared to the reversible fraction. Finally, we show that the reversible capacity can be significantly enhanced by increasing the temperature to 60 °C during the electrochemical reduction of water. The well-defined plateau during the oxidation step demonstrates high potentialities of this technique for electrochemical energy storage in nanoporous carbons using an aqueous medium.     

High density carbon fiber/boron nitride matrix composites: Fabrication of composites with exceptional wear resistance

This paper describes the fabrication of high density (ρ ∼ 1.75 g/cc) composites containing a hybrid (carbon and boron nitride), or complete boron nitride matrix. The composites were reinforced with either chopped or 3D needled carbon fibers. The boron nitride was introduced via liquid infiltration of a borazine oligomer that can exhibit liquid crystallinity. The processing scheme was developed for the chopped carbon fiber/boron nitride matrix composites (C/BN) and later applied to the 3D carbon fiber reinforced/boron nitride matrix composites (3D C/BN). The hybrid matrix composites were produced by infiltrating the borazine oligomer into a low density 3D needled C/C composite to yield 3D C/C-BN. In addition to achieving high densities, the processing scheme yielded d₀₀₂ spacings of 3.35 Å, which afforded boron nitride with excellent hydrolytic stability. The friction and wear properties of the composites were explored over the entire energy spectrum for aircraft braking using an inertial brake dynamometer. The C/BN composites outperformed both the previously reported C/C-BN and chopped fiber reinforced C/C. The high density 3D C/BN performed as well as both the 3D C/C and the C/BN. The 3D C/C-BN provided outstanding wear resistance, incurring nearly zero wear across the entire testing spectrum. The coefficient of friction was relatively stable with respect to energy level, varying from 0.2 to 0.3.     

Novel tin-graphite composites as negative electrodes of Li-ion batteries

For the purpose of obtaining an improved performance of the graphite negative electrode of Li-ion batteries, a novel graphite-tin composite has been synthesized by reduction of tin chloride (SnCl₂) with KC₈ in THF medium. This composite contains nano-sized tin particles dispersed on the graphite surface and free tin aggregates. Lithium electrochemical insertion occurs both in graphite and in tin. An experimental reversible specific charge of 489 mA h g⁻¹ is found stable upon cycling. Such a value is lower than the maximum theoretical one of 609 mA h g⁻¹ suggesting that only a part of tin is involved in the lithium insertion/extraction process. This part of active tin responsible for the stable capacity could be that bound to graphite. To the contrary, free tin aggregates could contribute to an extra capacity that decreases upon cycling in relation with the volume changes that occurs during alloying/dealloying.     

Friction behaviors of carbon/carbon composites with different pyrolytic carbon textures

Friction and wear properties of carbon/carbon (C/C) composites with a smooth laminar (SL), a medium textured rough laminar (RL) and a high textured RL pyrolytic carbon texture were investigated with a home-made laboratory scale dynamometer to simulate airplane normal landing (NL), over landing (OL) and rejected take-off (RTO) conditions. The morphology of worn surfaces at different braking levels was observed with scanning electron microscopy. The results show that C/C composites with RL have nearly constant friction coefficients, stable friction curves and proper wear loss at different braking levels, while friction coefficients of C/C composites with SL pyrolytic carbon decrease intensely and their oxidation losses increase greatly under OL and RTO conditions. Therefore, C/C composites with a high and medium textured RL pyrolytic carbon may satisfy the requirements of aircraft brakes. The good friction and wear properties of C/C composites with RL are due to the properties of RL, which leads to a uniform friction film forming on the friction surface.