Balance between reversible and irreversible processes during lithium intercalation in graphite

The effect of the measurements’ speed on reversible and irreversible processes occurring during intercalation and deintercalation of lithium in graphite out of a 1 M LiClO₄ solution in a propylene carbonate-dimethoxyethane mixture is studied by the chronopotentiometry and cyclic voltammetry methods. Dependence of reversible and irreversible capacity on the potential scan rate during potentiodynamic measurements is shown to be quite involved. Lithium diffusion coefficients in graphite are calculated by different methods.     

Decreasing Irreversible Capacity of Graphite Electrodes in Lithium-Ion Batteries by Direct Contact of Graphite with Metallic Lithium

The feasibility of reducing the irreversible capacity of negative graphite electrodes in lithium-ion batteries by a direct contact of such electrodes with lithium in the electrolyte is studied. It is shown that the dynamics of the formation of the passive film on graphite and the degree of the decrease in the irreversible capacity depend on the ratio between weights of graphite and lithium in contact. This method of reducing the irreversible capacity does not diminish the reversible capacity of graphite during the cycling. The irreversible capacity of the initial graphite cycled in 1 M LiPF₆ in a mixture of propylene carbonate and diethyl carbonate at a current density of 20 mA g^–1 is 550–1150 mA h g^–1. The reversible capacity of electrodes cycled in the same conditions reaches 290 mA h g^–1.     

Aqueous electrochemistry of binuclear copper complex with Robson-type ligand: dissolved versus surface-immobilized reactant

The electrochemical behaviour of binuclear copper complex with Robson-type ligand [Cu₂L]Cl₂ in aqueous medium is studied by cyclic voltammetry at highly oriented pyrolytic graphite, glassy carbon and gold electrodes. The overall reduction from solution of this reactant is found to be irreversible resulting in metallic copper formation. It is also complicated by chemical transformations of Cu(I) containing species. When attached to carbon support, [Cu₂L]Cl₂ is redox active in aqueous medium in the same potential range. The reduction is more reversible if reactant is immobilized at HOPG surface, and is in general agreement with reversible copper demetallation scheme. For dissolved reactant, the contribution of surface-attached species is screened by predominating voltammetric response of irreversible reduction. These conclusions are supported by data on the reduction of free protonated ligand and its hydrolysis products. Ex situ STM is applied to characterize electrode surfaces modified by [Cu₂L]Cl₂. Adsorbate monolayer of periodic structure is observed at highly oriented pyrolytic graphite (HOPG). Adsorption is more disordered at GC and less strong at polycrystalline gold support.     

Mechanistic studies on the trapping and desorption of volatile hydrides and mercury for their determination by electrothermal vaporization-inductively-coupled plasma mass spectrometry

Metallic coatings in the pyrolytic graphite furnace have been used for the pre-concentration of Hg (using electrochemically reduced Au) and AsH₃ (using both thermally and electrochemically reduced Ir and Pd/Ir) prior to electrothermal volatilization. The analyte trapping efficiency during accumulation and the analyte and modifier release processes during volatilization were monitored in real time. This was achieved by using the fast response capability of inductively-coupled plasma-mass spectrometry to determine simultaneously both the analyte and modifier elements as a function of time. The temperature dependence of the analyte trapping process points to contrasting mechanisms for Hg adsorption on Au (reversible, physical adsorption/amalgamation) and for AsH₃ adsorption on Pd, Ir or Pd/Ir (physical adsorption followed by an irreversible hydrogen abstraction reaction at active sites). Results also indicate that only minor volatilization of modifier occurs at the temperature required for volatilization of analyte from the furnace.     

Synthesis and Redox Behavior of Novel 9,10‐Diphenylphenanthrenes

The synthesis and electrochemical investigations of 9,10‐diphenylphenanthrene 2a and its derivatives 2b – 2e are reported. The cyclic voltammetry of derivatives 2a – 2c and 2e in different solvent/Bu₄NPF₆ electrolyte systems reveals that the redox properties are dependent on solvent, temperature, and sweep rate. The oxidation of 9,10‐diphenylphenanthrene 2a occurred as an irreversible process, while two fully reversible oxidation waves were observed for dimethoxy derivative 2c . The room‐temperature oxidation of brominated compound 2b is reversible, whereas AcO‐substituted phenanthrene 2e displayed a reversible oxidation peak only at low temperature. Furthermore, the electronic nature of the substituent affects the oxidation potentials. In the CH₂Cl₂‐based electrolyte system, the first oxidation potentials increase in the order 2c < 2e < 2b .     

粒度对石墨负极材料嵌锂性能的影响

研究了不同粒径（13～80 μm）石墨材料作为锂离子电池负极材料的嵌锂性能.结果表明，石墨粒度大小对嵌锂性能有明显影响，石墨的不可逆容量随着粒径的减小而逐渐增大，当粒径从80 μm减小到13 μm时，其不可逆容量增大了10％.而对可逆容量来说，随着粒径的减小，可逆容量逐渐增大;当粒径减小到20 μm时，可逆容量达到最大;再进一步减小石墨颗粒的粒径，可逆容量则随之减小.这表明石墨颗粒过大或过小都不利于锂离子的可逆脱嵌，只有合适的粒度才能最大限度地可逆脱嵌锂离子.根据不同粒度石墨的比表面的变化趋势，阐述了嵌锂性能随粒度变化的原因.     

Specific reversible melting of polyethylene

The specific reversibility of the crystallization and melting of linear and branched polyethylene has been determined as function of temperature by temperature‐modulated differential scanning calorimetry. The specific reversibility of crystallization and melting is defined as the ratio of the reversible enthalpy to the total enthalpy of the transition, both measured at the same temperature. This definition emphasizes a close connection between the reversible and irreversible parts of the transition. As one would expect, the crystal‐to‐melt transition of a given portion of a sample can only be reversible at a temperature close to its own temperature of irreversible melting. Reversible melting is absent at temperatures far from irreversible melting, and this is usually seen by experimentation as its zero‐entropy production melting temperature. The reversible change in the fold length, in contrast, is observed far from the melting temperature of the crystal involved. The specific reversibility of the crystallization and melting of polyethylene crystals may exceed 50% outside the temperature range of the main crystallization and melting. The specific reversibility seems rather independent of the branch concentration, and this points to similar mechanisms of the reversible transition in linear polyethylene of high crystallinity and in branched polyethylene of low crystallinity. The reversible transition is due to a local equilibrium at the crystal surface and is, therefore, largely independent of the overall morphology of the sample. In this study, a model is developed that is based on partial molecular melting, which avoids the need of molecular nucleation and permits, therefore, reversible melting as seen for small molecules in the presence of crystal nuclei. It provides an explanation of the rather large number of the crystals that may participate in reversible melting and allows a connection to the fully reversible crystallization of paraffins and the fully irreversible crystallization of extended‐chain crystals of high crystallinity. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2157–2173, 2003     

Enhanced high temperature cycling performance of LiMn2O4/graphite cells with methylene methanedisulfonate (MMDS) as electrolyte additive and its acting mechanism

The effects of methylene methanedisulfonate(MMDS) on the high-temperature(~50℃) cycle performance of LiMn_2O_4/graphite cells are investigated.By addition of 2 wt%MMDS into a routine electrolyte,the high-temperature cycling performance of LiMn204/graphite cells can be significantly improved.The analysis of differential capacity curves and energy-dispersive X-ray spectrometry(EDX) indicates that MMDS decomposed on both cathode and anode.The three-electrode system of pouch cell is used to reveal the capacity loss mechanism in the cells.It is shown that the capacity fading of cells without MMDS in the electrolytes is due to irreversible lithium consumption during cycling and irreversible damage of LiMn_2O_4 material,while the capacity fading of cell with 2 wt%MMDS in electrolytes mainly originated from irreversible lithium consumption during cycling.     

Irreversible capacity elimination via immediate contact of carbon with lithium metal

A new method for elimination of irreversible capacity during lithium intercalation into graphite is described. The method consists of bringing the graphite electrode into tight contact with lithium metal in an electrolyte. As a result of such shorting, a passive film is formed at the graphite surface. The dynamics of the film formation and its properties depend on the correlation between the masses of lithium and graphite. The method does not result in a decrease of the reversible capacity.Presented at the 3rd International Conference on Advanced Batteries and Accumulators, 16–20 June 2002, Brno, Czech Republic     

Cycling parameters of silicon anode materials for lithium-ion batteries

Integrated analysis of the cycling parameters (reversible specific capacity, Coulomb efficiency, irreversible loss of cycle capacity, accumulated irreversible capacity, and retention of reversible capacity) of synthetic graphite of MAG brand as an active material for the negative electrode of lithium-ion batteries was made.     

Electrochemical Characteristics of Negative Electrodes Made of Ozone-Treated Graphite for Lithium-Ion Batteries

Electrochemical characteristics of negative electrodes made of graphite treated in ozone–oxygen environment are studied in an operating lithium-ion battery. Successively treating graphite with ozone and then with a sec-butyllithium solution in cyclohexane dramatically reduces the irreversible capacity and stabilizes discharge characteristics of graphite electrodes cycled in a propylene carbonate-based electrolyte. An ozone–oxygen gaseous mixture containing 5 vol % O₃is most effective for the stabilization. According to IR spectroscopy, the treatment gives rise largely to –COLi groups, which ex situform a passivating solid-electrolyte film on the graphite surface. The film hampers the electroreduction of the organic solvent and reduces the irreversible lithium consumption.     

Kinetic investigation of the textile bleaching process

Volumetric H₂-uptake measurements on an Mo₂N (79 m²g^–1) sample reduced at 673 K have been carried out and the uptake isotherms in the temperature range of 308–623 K have been determined. Both the total and reversible hydrogen uptake increased with the uptake temperature. The irreversible hydrogen uptake increased abruptly when the uptake temperature was raised up to 423 K. The maximum of irreversible hydrogen uptake was measured at 473 K. The H_IR/Mo ratio calculated from the uptakes obtained in the temperature range of 308–623 K varies in the range of 0.0010–0.0202. One possible mechanism for hydrogen adsorption is proposed to be heterolytic dissociation on Mo-N paris, in which the molybdenum atoms are in unsaturated coordination.     

Cycling parameters of MAG graphite as anode material for lithium-ion batteries

Integrated analysis of the cycling parameters (reversible specific capacity, Coulomb efficiency, irreversible loss of cycle capacity, accumulated irreversible capacity, and retention of reversible capacity) of synthetic graphite of MAG brand as an active material for the negative electrode of lithium-ion batteries was made.     

Voltammetry of Organic Microparticles

 Solid microparticles of several different insoluble organic compounds were mechanically immobilized on the surface of graphite electrodes and immersed into a liquid electrolyte in order to study their electrochemical reactions. Cyclic staircase voltammetry and square-wave voltammetry were used. Quinhydrone was found to be a stable intermediate in the reversible redox reaction of solid quinone and hydroquinone on the electrode surface. The reaction occurs on the surface of the solid particle which is in contact with water. Indigo can be reduced to leucoindigo and oxidized to dehydroindigo in two separate reversible redox reactions. In strongly basic medium indigo dissolves in water upon electroreduction. A hydroacridine radical was detected as a stable intermediate in the electrochemically irreversible redox reaction of acridine and dihydroacridine. Famotidine can be electrooxidized and the product of this reaction can be electroreduced in two separate chemically irreversible reactions. Probucol is oxidized to a semiquinone radical which can be re-reduced in an electrochemically irreversible redox reaction. Propyl- thiouracil can be also oxidized to an unknown product which can be re-reduced in a chemically reversible, but slow solid state surface redox reaction. Reductions of solid thionicotinoylanilide and nicotinoylanilide are totally irreversible. Received September 22, 1998. Revision March 19, 1999.     

Thin‐Film Voltammetry of a Lutetium Bisphthalocyanine at Ionic Liquid|Water Interface

At room temperature, tetraoctylphosphonium bromide is a viscous ionic liquid, this gel‐like organic phase can be cast over a basal‐plane graphite electrode (BPGE). Cyclic voltammetry at such a modified electrode, in contact with an aqueous solution have revealed one reversible oxidation and five reversible reduction steps for a Lu^III bisphthalocyanine dissolved in the ionic liquid film, a proof that the highly reactive reduced species were protected from interaction with water in this highly lipophilic phase. It has also been shown that the redox properties are influenced by the ions in the aqueous phase, a property which has been attributed to ion‐pairing effects; obviously, the ion transfers at the organic|aqueous interface has been ignored. Electrochemistry of Lu(III)[(_tBu)₄Pc]₂ (cyclic voltammetry and square wave voltammetry) under similar conditions shows that the nature and concentration of the anion in the aqueous solution in contact with the ionic liquid film influences the potential of the electrode reaction. This can be attributed to variations of the interfacial potential and also because the organic phase is an anion exchanger. Moreover, SWV experiments suggest that the rate of the overall reaction varies with the nature and concentration of the anion of the aqueous electrolyte, which implies that the ion transfer through the organic|aqueous interface is slower than the electron exchange rate of the molecule at the surface of graphite.     

Graphites from the Zavalie deposit as active materials for lithium-ion batteries

We study the utility of standard graphites (GAK-2, GL-1, EUZ-M, and others) produced by the Zavalie Integrated Graphite Plant (ZGP) as active materials in lithium-ion batteries (LIBs). The structure and main electrochemical characteristics of these graphites are studied for choosing the best type of graphite and evaluating its utility for LIB production. The electrochemical characteristics of the best ZGP graphites and graphite for batteries produced by Superior Graphite Co. (USA), a worldwide leader of the graphite industry, are compared. Some tendencies in the effect of the structure and particle-size distribution on the electrochemical characteristics of graphite electrodes are determined. EUZ-M graphite modified by tin with amorphous carbon is prepared. The reversible capacity of this graphite in the cell against LiCoO₂ exceeds 400 mA h/g. The increased reversible capacity is due to the contribution of components having higher specific parameters; the cycling stability is due to the core-shell structure.     

Enhancement of Li-ion battery performance of graphite anode by sodium ion as an electrolyte additive

Electrochemical performance of a graphite electrode for lithium-ion batteries was successfully and easily improved by sodium ion dissolved in an electrolyte solution. Sodium ion was added by dissolving 0.22 mol dm⁻³ NaClO₄ into a 1 mol dm⁻³ LiClO₄ ethylene carbonate–diethyl carbonate (1:1 by volume) electrolyte solution prior to charge–discharge cycle. By sodium-ion addition, an irreversible capacity at the initial cycle was obviously reduced, and reversible discharge capacities increased with better capacity retention. From ac impedance measurements, a graphite electrode in the sodium ion added electrolyte had much smaller interface resistance compared to that obtained in sodium ion free one. Furthermore, the electrode surface morphology observed by electron microscopes after charge–discharge tests got more uniform in the sodium added electrolyte.     

Synthesis of a silicon-graphite composite for the hybrid electrode of lithium-ion batteries

Comprehensive analysis was made of the cycling parameters (reversible specific capacity, Coulomb efficiency of cycles, accumulated irreversible capacity, and retention of reversible capacity) of a hybrid electrode based on a mixture of MAG synthetic graphite and silicon-graphite composite produced by mechanical grinding.     

Reversible Entropy-Driven Defect Migration and Insulator-Metal Transition Suppression in VO2 Nanostructures for Phase-Change Electronic Switching

Oxygen defects are among essential issues and required to be manipulated in correlated electronic oxides with insulator-metal transition (IMT). Besides, surface and interface control are necessary but challenging in field-induced electronic switching towards advanced IMT-triggered transistors and optical modulators. Herein, we demonstrated reversible entropy-driven oxygen defect migrations and reversible IMT suppression in vanadium dioxide (VO₂) phase-change electronic switching. The initial IMT was suppressed with oxygen defects, which is caused by the entropy change during reversed surface oxygen ionosorption on the VO₂ nanostructures. This IMT suppression is reversible and reverts when the adsorbed oxygen extracts electrons from the surface and heals defects again. The reversible IMT suppression observed in the VO₂ nanobeam with M2 phase is accompanied by large variations in the IMT temperature. We also achieved irreversible and stable IMT by exploiting an Al₂O₃ partition layer prepared by atomic layer deposition (ALD) to disrupt the entropy-driven defect migration. We expected that such reversible modulations would be helpful for understanding the origin of surface-driven IMT in correlated vanadium oxides, and constructing functional phase-change electronic and optical devices.     

Triggering the spin-crossover of Fe(phen)2(NCS)2 by a pressure pulse. Pressure and magnetic field induce ‘mirror effects’

We have studied the static and dynamic effects of pressure on the spin-transition in the temperature range of the thermal hysteresis loop for the compound Fe(phen)₂(NCS)₂. The high-spin fraction (n_HS) as a function of pressure and temperature has been determined by optical reflectivity. In this compound, the pressure was found to upward shift the spin-transition temperature by 23 K per kbar. During the dynamic pressure pulse, a decrease in n_HS is observed, with an irreversible (reversible) character in the descending (ascending) branch of the hysteresis loop. In this respect, pressure has a ‘mirror effect’ compared to the application of an intense and pulsed magnetic field, for which – as reported previously – an increase in n_HS is observed, with an irreversible (reversible) character in the ascending (descending) branch of the hysteresis loop. To cite this article: A. Bousseksou et al., C. R. Chimie 6 (2003).