Lithium-intercalated Graphite Research Articles

Operando structural analysis of phase transition of graphite electrode during Li de-intercalation process using neutron and synchrotron radiation X-ray diffraction

Graphite is widely used as a negative electrode material for lithium-ion batteries. Although it is well known lithium ions are inserted between the graphene layers, the details of the structural transition they undergo are not well understood. In this study, we performed operando structural analyses of graphite electrode during the discharge process using neutron diffraction and synchrotron radiation X-ray diffraction and established a solid foundation to clarify the relationship among the LiCx composition, discharging profile, and phase evolution of lithium-intercalated graphite (LIG). The operando analyses enabled us to assess simultaneously the structural changes of LIG along the ab plane (in-plane structure) and the c-axis direction (stage structure). In the de-lithiation process, the intralayer transition was observed on the ab plane from the LiC6-type to the LiC9-type arrangement near the LiC18 composition. The LiC9-type arrangement was retained in the Li-poorer phases such as LiC27, LiC36, LiC54, and LiC72, where the Li de-lithiation proceeded with the interlayer transition to higher-order stages. The phase transition of LIG involves a rapid switchover between the LiC6-type and the LiC9-type intralayer arrangement along with the changes in the interlayer stage structure. A reaction mechanism for the configurational transition on the intralayer and interlayer structure was updated.

SIMS and HR-XPS characterization of lithiated graphite from the magnetic fusion device RFX-mod

Lithium wall conditioning has improved the performance of many magnetic fusion devices. Li conditioning in the RFX-mod device was performed by: (1) a single Li pellet injector and (2) a multi-Li pellet injector during He plasma discharges; (3) a Li evaporator after He glow discharge cleaning and He plasma discharge exposure. This report compares the spatial and depth distributions of Li deposited on polycrystalline graphite witness samples at different locations in RFX-mod and the elemental and chemical compositions of the resulting surfaces. The sample surfaces were analyzed ex situ using secondary ion mass spectrometry (SIMS) and high-resolution X-ray photoelectron spectroscopy (HR-XPS). The results showed that Li pellet injection provided a relatively uniform toroidal coverage while Li evaporation produced highly localized Li deposition. A Li 1s HR-XPS peak at 56.5 eV binding energy (BE) characteristic of lithium-intercalated graphite was only observed with the sample exposed to Li evaporation. All of the samples exhibited a HR-XPS C 1s peak at 285.1–285.2 eV BE that is largely attributed to hydrogenated graphite. This finding suggests that hydrogenation of fresh graphite occurs during He plasma discharge exposures. Our results have implications for density control and the selection of Li conditioning techniques in magnetic fusion devices.

DFTB Modeling of Lithium-Intercalated Graphite with Machine-Learned Repulsive Potential

Lithium ion batteries have been a central part of consumer electronics for decades. More recently, they have also become critical components in the quickly arising technological fields of electric mobility and intermittent renewable energy storage. However, many fundamental principles and mechanisms are not yet understood to a sufficient extent to fully realize the potential of the incorporated materials. The vast majority of concurrent lithium ion batteries make use of graphite anodes. Their working principle is based on intercalation, the embedding and ordering of (lithium-) ions in two-dimensional spaces between the graphene sheets. This important process, it yields the upper bound to a battery's charging speed and plays a decisive role in its longevity, is characterized by multiple phase transitions, ordered and disordered domains, as well as nonequilibrium phenomena, and therefore quite complex. In this work, we provide a simulation framework for the purpose of better understanding lithium-intercalated graphite and its behavior during use in a battery. To address large system sizes and long time scales required to investigate said effects, we identify the highly efficient, but semiempirical density functional tight binding (DFTB) as a suitable approach and combine particle swarm optimization (PSO) with the machine learning (ML) procedure Gaussian process regression (GPR) as implemented in the recently developed GPrep package for DFTB repulsion fitting to obtain the necessary parameters. Using the resulting parametrization, we are able to reproduce experimental reference structures at a level of accuracy which is in no way inferior to much more costly ab initio methods. We finally present structural properties and diffusion barriers for some exemplary system states.

Operando Structural Analysis of Phase Transition of Graphite Electrode during Li De-Intercalation Process Using Synchrotron Radiation X-Ray Diffraction

Introduction : In order to promote development of electric vehicle, it is required to improve the capacity density, the low-temperature performance, and the high-rate property of the batteries. Understanding the lithium intercalation and de-intercalation mechanism at graphite electrode of the storage battery (LIB) is very important for this purpose. Crystal structure changes of lithium-intercalated graphite during charge and discharge processes were investigated by operando X-ray diffraction (XRD) measurements using a synchrotron radiation facility. Low-temperature operando XRD measurements were now available by the development of a new temperature-controlled cell jacket.1-2 This study confirmed in-plane structure change along a- and b-axis as well as stage structure change along c-axis.3 As a result, it was elucidated that crystallinity of graphitic carbon caused differences in phase transition during discharge and this phenomenon affected low-temperature and high-rate discharge performances. Experimental : Coin cells composed of carbon and NMC electrodes were firstly prepared and subjected to the evaluation of the battery performances from the low-temperature (0℃) to the room temperature. Coin cells of several graphitic carbon were evaluated.Crystal structures of lithium-intercalated graphite during discharge process were investigated by operando analysis using synchrotron radiation X-ray diffraction (SR-XRD) at BL28XU beam line of SPring-8. Al-laminated half-cells composed of graphitic carbon and Li electrodes were prepared and subjected to the analysis of operando measurement. The graphitic carbons of different crystallite sizes were analysed. Al-laminated half-cells conditioned by several cycles were used for the measurement. Operando XRD measurements were carried out during charge and discharge processes. X-ray of 25keV (its wavelength of 0.0496 nm) with a beam size of 0.2 mm ×0.5 mm was used to obtain diffraction patterns. The exposure time was set to 10 s. The diffraction patterns were obtained by two-dimensional detector, PILATUS 100K (DECTRIS). Two-dimensional image data were converted to one-dimensional 2q profiles, and the diffraction peaks were separated using the Lorentz functions.The test cell underwent an aging treatment was charged (Li insertion) at room temperature. Thereafter, structural transition were analyzed during the discharging process (Li de-intercalation) in 0.2C at room temperature and in 0.1C at 0℃. Results and Discussion : Figure 1 shows the SR-XRD patterns of lithium-intercalated graphite at the discharge process with current density of 0.2C at room temperature. The result showed the in-plane structure change along a- and b-axis as well as stage structure change along c-axis.The detailed analysis of in-plane structure change and stage structure change for each graphitic carbon was performed using the operando measurements. It was identified that the crystallinity of graphitic carbon caused differences in phase transition during discharge. These results showed that crystallite sizes influence the phase transition and give major effect on LIB performances of graphite negative electrode at low-temperature or high-rate discharge. The Li diffusivity in the graphitic carbon is likely dependent on the crystallite size. The battery reaction mechanism influenced by crystallite size was elucidated by operando XRD measurements. The present results confirm that the crystallinity of the graphitic carbon affects the low-temperature and high-rate discharge characteristics. The detail results will be shown in the conference. Acknowledgment: This work was supported by the Research and Development Initiative for Scientific Innovation of New Generation Battery 2 (RISING2) project administrated by the New Energy and Industrial Technology Development Organization (NEDO).

Characterization of Carbon Properties Influencing on Lib Performance at Low Temperature

Introduction : In order to promote development of electric vehicle, it is required to improve the capacity density of a battery, the battery performance at a low temperature, and the high rate property of a battery. Characterization of the size distribution and the average size of carbon in the negative electrode significantly influence on the battery performance. We have studied clarification of graphitic carbon’s properties influencing on performance of LIB especially at low temperature. Firstly, the X-ray diffraction profiles of several graphitic carbons were analyzed by the fundamental parameter (FP) method. Further, the structure changes of lithium-intercalated graphite during the charge and discharge processes were investigated by operando measurement using synchrotron radiation at BL28XU of SPring-8 in Japan. The time resolution of the apparatus is the order of milliseconds for analyzing one diffraction pattern. Experimental : Crystallite size and its distribution of several graphitic carbons were analyzed by the fundamental parameter (FP) method using X-ray diffraction profiles. X-ray wavelength of 0.154 nm was used to obtain X-ray diffraction profiles. In order to analyze crystallite size and its distribution, X-ray diffraction profiles were calculated by using software called PDXL2 (Rigaku Corporation).Coin cells composed of carbon and NMC electrodes were prepared and subjected to the evaluation of the battery performances from the low temperature to the room temperature. Coin cells conditioned by several cycles were evaluated.Further, crystal structures of lithium-intercalated graphite during charge and discharge processes were investigated by operando analysis using synchrotron radiation diffraction at BL28XU beam line of SPring-8. Al-laminated half-cells composed of graphitic carbon and Li electrodes were prepared and subjected to the evaluation of operando measurement. The graphitic carbons of different crystallite sizes and their distributions were analysed. Al-laminated half-cells conditioned by several cycles were used for the measurement. Operando measurements were carried out during charge and discharge processes. X-ray of 25keV (its wavelength of 0.0496 nm) with a beam size of 0.2 mm ×0.5 mm was used to obtain diffraction patterns. The exposure time was set from 0.5 s to 10 s. The diffraction patterns were obtained by two-dimensional detector (Rigaku Corporation). Results and Discussion : The experimental X-ray diffraction profile of graphitic carbon and that calculated profile by the fundamental parameter (FP) method were obtained. The calculated profile showed a good fit with the measured one. The each plane of graphitic carbon was analyzed properly.The analysis of FP method revealed that crystallite sizes and their distributions of graphitic carbons influences on LIB performance especially at low temperature. The results indicate that the low temperature characteristics improve as the crystallite size and distribution of the graphite (102) plane decreases. The same results were also obtained for the relationship between the crystallite size distribution of the graphite (102) plane and the high-rate discharge characteristics of LIB. The crystallite size distribution of the graphite (102) plane is thought to express the Li diffusivity.Figure shows the result of synchrotron radiation diffraction pattern of lithium-intercalated graphite at the discharge process with current density of 0.2C. The stage change was analysed clearly as shown in Figure.The difference of stage change of each carbon was revealed in considerable detail by Operando measurement.The crystallite size and its distribution influencing on the battery performance at low temperature was examined in detail. The result showed that crystallite sizes and their distributions influence on the phase structure changes and give major effect on charge and discharge performances of graphite negative electrode at low temperature. Their battery reaction mechanism influenced by crystallite size distribution was elucidated by the Operando measurement. The detail results will be shown in the conference. Acknowledgment: This work was supported by the Research and Development Innovative for Scientific Innovation of New Generation Battery 2 (RISING2) project of the New Energy and Industrial Technology Development Organization (NEDO). Figure 1

Electrochemical properties of p-terphenyl based lithium solvated electron solutions

Abstract Concentrated lithium solvated electron solutions (LiSESs) consisting of p-terphenyl (TER) in tetrahydrofuran (THF) prepared using suspensions of TER in THF, exhibit a more than twelve times increase in conductivity compared to more dilute solutions. The temperature dependent conductivity of LiSES has metallic-like characteristics, i.e. linear decrease of conductivity with increasing temperature, except for the solution of higher concentration (less THF) with the ratio of lithium to TER being 2 which conductivity increases as raising temperature due to the higher contribution of ionic conductivity. From temperature dependent open circuit voltage temperature measurement, the values of changes in entropy (ΔS) and enthalpy (ΔH) were determined. Unlike lithium-intercalated graphite, the ΔS decreases linearly as the ratio of lithium to TER increases, which demonstrates fundamental differences in the lithium charging/discharging processes between solution and solid state. The TER based LiSES with higher concentrations (less THF) show deviations from the ΔS and ΔH values of solutions with lower concentrations. The ΔS values for the TER based LiSES are higher than for naphthalene and biphenyl based Li SES having the same composition, but lower than those for solid lithium ion battery anode materials. LiSES with the maximum ratio of lithium to TER is 2.4 to 1 exhibits minimum conductivity which indicates the formation of a dimeric structure under these conditions. The LiSES provides higher energy density comparing to vanadium redox batteries, which demonstrates that LiSES holds good promise as anolytes in liquid-based refuelable batteries.

In situ X-ray Raman scattering spectroscopy of a graphite electrode for lithium-ion batteries

Lithium-intercalated graphite (LIG) is the most commonly used anode material for Li-ion batteries; however, the change in the electronic structure of LIG during battery operation is not fully understood. We have developed an in situ cell and a measurement method to elucidate this change using X-ray Raman scattering (XRS) spectroscopy. A confocal-like method enables the C K-edge spectrum of LIG alone to be extracted while avoiding the spectral overlap of carbon-containing components in the cell. In situ XRS and X-ray diffraction measurements are conducted simultaneously for LiC6, LiC12, and fully delithiated graphite during the delithiation process. The results indicate that the intensity of the π* peak of the XRS spectrum increases upon delithiation, and the σ* onset shifts to higher energy, which are consistent with prior spectroscopic studies on LIG. These results demonstrate that the in situ XRS method established in this study is useful to characterize the electronic properties of LIG during the operation of batteries.

Comprehensive elucidation of crystal structures of lithium-intercalated graphite

Graphite is widely used as an anode material in almost all Lithium (Li)-ion batteries. Li is easily absorbed into the layer structure of graphite, forming lithium-carbon intercalation compounds LixC between LiC6 and C. However, although they are simple binary compounds, their structures have been left unelucidated for a long time, especially for the compounds between LiC12 and C. This is probably because the Li concentrations are too low to make a big difference in diffraction pattern between these Li-intercalated graphites and pure graphite. It has been known, however, that they have stacked layer structures, which strongly suggests that they are very likely to form modulated structures. Structural analyses of these compounds based on the modulated structure model were successfully performed, comprehensively elucidating all the LixC structures lying between LiC6 and C.

Piezoelectrochemical Energy Harvesting in Commercial Lithium Ion Batteries

As technology becomes smaller and smaller, the need for micro-energy sources becomes increasingly imperative. One promising technology to address this need is piezoelectrochemical harvesting, a recently identified mechanism to directly convert mechanical energy to electrochemical potential[1-4]. In piezoelectrochemical (PEC) materials, the chemical potential of ions is affected by an applied stress, and under such circumstances these materials can be used in a thermodynamic cycle to harvest energy, at a relatively slow rate commensurate with the kinetic transport in electrochemical systems. Previous work[1] has demonstrated that commercial lithium cobalt oxide (LCO) batteries exhibit the PEC effect, as both the lithium cobalt oxide cathode and lithium-intercalated graphite anode are PEC materials. The coupling factor between the change in equilibrium potential and applied mechanical stress has been found to be linear. In this presentation, we will discuss our research to use piezoelectrochemical energy harvesting to increase the voltage generated from commercial lithium ion batteries. We measured the differential expansion and differential voltage of a lithium ion battery, and used this data to estimate the coupling factor as a function of state-of-charge (SOC). We analyzed the coupling factor for commercial LCO batteries, and found the SOC where the coupling factor was maximized. At this SOC, batteries were placed under a mechanical load to harvest energy. The voltage generated was quantified by measuring the voltage drop across a resistor. To understand how the PEC effect operates in multiple batteries, we wired cells in series and parallel, and performed similar mechanical load experiments. As expected, the PEC voltage can be increased by compressing batteries in series. Increasing the PEC voltage generated would allow the effect to be used in practical applications such as micro-energy devices.

Influence of the Graphite Morphology on Anion Intercalation

The redox-amphoteric character of graphitic carbon makes it possible to intercalate a broad range of anions and cations. These graphite intercalation compounds (GICs) are divided into two types: donor-type GICs, which host cations and acceptor-type GICs, which are formed by anions. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC6) as anode material in lithium-ion batteries. Besides this example, also acceptor-type GICs are known as active material in batteries. In this context, Placke et al.1 presented the “dual-ion cell”, that also deals with anion intercalation into the graphite structure. During charge anions and cations are inserted into the respective electrode and are released back into the electrolyte during discharge. In the charged state the cathode reaches a potential of 5 V (vs. Li/Li+). In order to avoid electrolyte degradation, an ionic liquid mixture is chosen as electrolyte. It is composed of N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide (Pyr14TFSI) and lithium bis(trifluoromethanesulfonyl) imide (LiTFSI). The focus of this work is to investigate the influence of different graphite characteristics on the intercalation of the bis(trifluoromethanesulfonyl) imide anion into the graphite structure. For that purpose, nitrogen adsorption measurements were performed in order to determine the porosity of the graphite samples and X-ray diffraction was carried out to identify the crystallite size. Both structural parameters are correlated to the electrochemical performances. 1. Placke, T.; Bieker, P.; Lux, S. F.; Fromm, O.; Meyer, H. W.; Passerini, S.; Winter, M., Dual-ion cells based on anion intercalation into graphite from ionic liquid-based electrolytes. Zeitschrift für Physikalische Chemie 2012, 226, 391-407.

Synthesis of Spherical Graphite Particles Possessing a Definable Size and Their Application in Dual-Graphite Cells

Graphitic carbon provides a redox-amphoteric character, which leads to the capability to intercalate a broad range of different cations and anions between its planar graphene sheets, resulting in so-called graphite intercalation compounds (GICs). Graphite intercalation compounds, formed by cations are called donor-type GICs and those intercalated by anions are called acceptor type GICs [1-4]. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC6) as the anode material in lithium-ion batteries. In recent publications, we reported an electrochemical energy storage system using graphite as both anion and cation intercalation host. During charge, cations and anions, stemming from the electrolyte, are intercalated into the graphitic lattice structure of negative and positive electrode, respectively. During discharge, they are released back into the electrolyte. Hence, the electrolyte can be considered as active material, as it does not only act as charge carrier for lithium ions [4, 5]. Since a high oxidative stability of the electrolyte is mandatory, it is most likely composed of an ionic liquid and a suitable conductive salt, possessing the same anion as the ionic liquid. This system has been proposed as dual-graphite cell [6]. The electrolyte of the dual-graphite system used in this work is composed of the ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesul-fonyl) imide (Pyr14TFSI) using lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as conductive and electro-active salt. This work deals with the synthesis of spherical graphite particles, their physical characterization and the application as active material in dual-graphite cells. First, carbon spheres, possessing different sizes, are synthesized by a hydrothermal bottom-up synthesis from glucose as precursor. Afterwards, a heat treatment at 1200 °C under argon atmosphere was performed in order to decompose the glucose structure to a soft carbon structure. Finally, the resulting soft carbon spheres were heat treated at different temperatures above 2000 °C in order to investigate the graphitization degree as well as the influence of the carbonaceous material structure to the electrochemical performance for application in a dual-graphite cell. The structure of the novel materials was characterized by X-ray diffraction, Raman spectroscopy, specific surface area investigations, scanning electron microscopy and electrochemical measurements.

Predicting the voltage dependence of interfacial electrochemical processes at lithium-intercalated graphite edge planes

The applied potential governs lithium-intercalation and electrode passivation reactions in lithium ion batteries, but are challenging to calibrate in condensed phase DFT calculations. In this work, the "anode potential" of charge-neutral lithium-intercalated graphite (LiC6) with oxidized edge planes is computed as a function of Li-content (nLi) at edge planes, using ab initio molecular dynamics (AIMD), a previously introduced Li(+) transfer free energy method, and the experimental Li(+)/Li(s) value as reference. The voltage assignments are corroborated using explicit electron transfer from fluoroethylene carbonate radical anion markers. PF6(-) is shown to decompose electrochemically (i.e., not just thermally) at low potentials imposed by our voltage calibration technique. We demonstrate that excess electrons reside in localized states-in-the-gap in the organic carbonate liquid region, which is not semiconductor-like (band-state-like) as widely assumed in the literature.

Preparation, characterization and electrochemical properties of a graphene-like carbon nano-fragment material

A graphene-like nanomaterial, carbon nano-fragments (CNFs), is obtained using the graphite anodes of spent lithium-ion batteries (LIBs) as carbon source, and its morphology, structure, functional groups, and reactivity are characterized to evaluate the properties and potential applications. The interlayer space increase, layer distortion, and remnant lithium of the waste lithium-intercalated graphite are utilized to prepare the oxidized CNFs (ox-CNFs) through a chemical oxidation and ultrasonic crushing process. These ox-CNFs exhibit a size distribution of 15nm to 2μm and excellent hydrophilicity, and disperse well in an aqueous suspension. Under the hydrothermal condition at 180°C for 12h, the ox-CNFs are converted into a suspension of reduced CNFs (re-CNFs), or a cylindrical aggregate when the concentration exceeds 2 mg·mL−1. The spectroscopic results demonstrate that there are abundant edges, defects, and functional groups existing on the CNFs, which affect their reactive, electronic, and electrochemical properties. Thereinto, the vacuum-dried ox-CNFs film can be converted from an insulator to a conductor after a chemical reduction by hydroiodic acid. And the re-CNFs modified glass carbon electrode (re-CNFs/GCE) exhibits enhanced electrocatalytic activity of about 8 times than the GCE to the oxidation reaction of dopamine. Furthermore, with the addition of the carboxylic ox-CNFs in aniline, the CNFs/polyaniline composite discharges a capacitance of 356.4 F·g−1 at 2 mV·s−1, an increase of 80.5% compared to the polyaniline. This preparation entails not only novel carbon nanomaterials but also an excellent disposal method of spent graphite, showing special significance in materials innovation and environmental science.

Density Functional Theory Study on Structural and Energetic Characteristics of Graphite Intercalation Compounds

The structures and energetics of a number of graphite intercalation compounds (GICs) having a relatively wide range of chemistry have been investigated by density functional theory calculations with the van der Waals correction using the dispersion correction method within the framework of generalized gradient approximation. The GICs studied included potassium-intercalated graphite (KCn), lithium-intercalated graphite (LiCn), lithium solvated by dimethyl sulfone (DMSO)-intercalated graphite (Li(DMSO)4Cn), lithium solvated by dibuthoxy ethane (DBE)-intercalated graphite (Li(DBE)2Cn), perchlorate (ClO4)-intercalated graphite (ClO4Cn), and hexafluorophosphate (PF6)-intercalated graphite (PF6Cn). Our calculations show reasonable agreement with experimental data for the interlayer distances of the GICs. A correlation between the size of the intercalate and the interlayer distance of the GIC has been observed. Our study has also predicted that all the GICs studied here are energetically stable except for Li(DBE)2Cn, consistent with experimental observations. Our results have suggested that there is a strong correlation between the intercalation energy and the electron transfer between the intercalate and graphite. On the basis of our results, we propose that the ionization potential or the electron affinity of the intercalate, along with the size of the intercalate, is a good measure for the stability of the resulting GIC in general.

Carbon Nano-Fragments Derived from the Lithium-Intercalated Graphite

Along with the discoveries of carbon nanotube, fullerenes, and graphene, 1,2 the insight on the properties of carbon nanomaterials has been greatly promoted, and carbon nanomaterials have been applied to various fields such as energy, environment, and materials based on their unique nanostructure and properties. 3‐7 As the results, the applications of these known materials not only enrich the knowledge of carbon nanomaterials but also encourage the expectation of other novel carbon nanomaterials. Graphite, a basic carbon material and the raw material of carbon nanomaterials, has been largely used as the active material in the anodes of lithium ion batteries (LIBs) recently. Although the annual output of LIBs has increased quickly and their cathode materials are generally recycled, 8,9 the waste graphite anode materials are usually abandoned because of the knowledge lack on their recovery value. In practice, the status quo on the incessant accumulation of spent anode active materials can’t be neglected from the environmental viewpoint, and the remnant lithium in the active materials must also be considered, showing the essentiality to reclaim the graphite in spent LIBs. Because the perfect layered structure of spent graphite has been changed and an uncertain amount of lithium ions remain in the graphite lattice voids, the reusing of spent graphite, even after a regeneration, as the anode active material of LIBs is impossible. In addition, the uncertainty for the lifetime and states of the spent LIBs collected from different resources has to be faced, which also puzzles the reclaiming of the graphite anodes in spent LIBs. Recently, we prepared a novel carbon nanomaterial that is denoted as carbon nano-fragments (CNFs) by using the graphite in spent LIBs as raw material. And this CNFs material exhibits some similar properties to carbon nanotube and graphene 10,11 and excellent electrocatalytic oxidation activity to organic molecules such as dopamine, β-cyclodextrin, and phenol, showing potential application in the electrochemical fields. In this paper, the preparation and the characterization on the morphology, structure, functional groups, and electrocatalytic activity of the as-prepared CNFs are presented.

An Electrochemical Sensor Based on Carbon Nano-Fragments and β-cyclodextrin Composite-Modified Glassy Carbon Electrode for the Determination of Rutin

An electrochemical sensor for the determination of rutin using differential pulse voltammetry (DPV) is developed based on a novel β-cyclodextrin (β-CD) and reduced carbon nano-fragments (re-CNFs) hybrid-modified glassy carbon electrode (β-CD/re-CNFs/GCE). The re-CNFs are synthesized by hydrothermally reducing the oxidized CNF suspension derived from the lithium-intercalated graphite, and then mixed with β-CD and pasted on the GCE. The electrochemical reaction of rutin at the β-CD/re-CNFs/GCE is shown to be a two-electron and two-proton reversible process. Compared to the re-CNFs/GCE and the GCE, the β-CD/re-CNFs/GCE indicates an enhanced electrocatalytic activity toward the oxidation of rutin with about 1.69 and 4.03 times higher current response, respectively. Under the optimized conditions, the β-CD/re-CNFs/GCE displays a linear peak current response toward the electrochemical oxidation of rutin in the concentration range of 3 × 10−7 to 1.5 × 10−5 mol L−1 with a detection limit of 9.0 × 10−8 mol L−1 (S/N = 3). In addition, this β-CD/re-CNFs/GCE exhibits other beneficial features including high selectivity, reproducibility and stability, and can be successfully applied to determine rutin in real sample with satisfied recovery.

Density-functional theory calculations of the electron energy-loss near-edge structure of Li-intercalated graphite

We have studied the structural and electronic properties of lithium-intercalated graphite (LIG) for various Li content. Atomic relaxation shows that Li above the center of the carbon hexagon in a AAAA stacked graphite is the only stable Li configuration in stage 1 intercalated graphite. Lithium and Carbon 1 s energy-loss near-edge structure (ELNES) calculations are performed on the Li-intercalated graphite using the core-excited density-functional theory formulation. Several features of the Li 1 s ELNES are correlated with reported experimental features. The ELNES spectra of Li is found to be electron beam orientation sensitive and this property is used to assign the origin of the various Li 1 s ELNES features. Information about core-hole screening by the valence electrons and charge transfer in the LIG systems is obtained from the C 1 s ELNES and valence charge density difference calculations, respectively.

Thermodynamics and crystal structure anomalies in lithium-intercalated graphite

We found anomalous behaviors during lithium electrochemical intercalation into graphite in the entropy curve and the crystal structure, especially, during the stages from 2 to 1 transition at x = 0.5 in Li x C 6. The entropy first decreases monotonously, then re-increased sharply at the transition. The average interlayer spacing displays a hysteresis during intercalation and de-intercalation and departs from linearity. The results are discussed in relation with a contribution of the vibrational entropy and possible occurrence of intermediary phase(s) between the two lithium-rich intercalation stages.

The Structure of Lithium Intercalated Graphite Using an Effective Atomic Charge of Lithium

Using the experimental graphene layer spacing of the stage 1 model as a constraint, the effective atomic charge of lithium, in lithium-intercalated graphite (LIG) was determined. In order to confirm that lithium in LIG exists in a partially ionic state, quantum mechanical calculations were also carried out for several lithium-carbon systems. Using a fixed the graphene layer spacing and structures for hexagonal graphite, stage 3, stage 2, and stage 1 models, were obtained. The more lithium is intercalated into the graphite, the wider the layer spacing becomes. The distortion of structures due to lithium intercalation was not observed until the stage 1 model was formed. In stage 1 and stage 2 models, the graphene layers shifted from ABAB to AAAA stacking as lithium was intercalated to the hexagonal graphite. However, the stage 3 model showed a shift of layers from ABABAB to AB′AAB″A stacking, where B′ and B″ represent the graphene layers which have shifted slightly from B. Only the graphene layers that have the intercalated lithium layers between them shifted to AA stacking. © 2001 The Electrochemical Society. All rights reserved.

Relationship between the charge capacity of a turbostratic carbon anode for a Li secondary battery and its structure

The relation between the charge capacity of a turbostratic carbon anode for a lithium secondary battery and carbon structure was investigated using mesocarbon microbeads, and a new theoretical equation for estimating the charge capacity was proposed. There was a linearity between the observed charge capacity and Warren's P value, which denotes the 1 probability for finding the AB-stacking area in a pair of adjacent carbon layer planes. Fourier analysis of X-ray diffraction patterns proposed by Houska and Warren revealed that an island structure composed of AA- and AB-stacking area by the rotational misorientation existed. Such an island structure reduced the formation area of in-plane ordering of stage I lithium-intercalated graphite LiC , because the intercalation reaction of Li into graphite requires the slipping of AB-stacking 6 )) ˛˛ to AA for the formation of the super lattice, p (3 3 3)R308. © 2000 Elsevier Science Ltd. All rights reserved.