Structural form of vibronic intensities in vibronic induced transitions

A general method for calculating the overall shape of the spectrum of vibronic induced transitions in transition-metal ion systems is developed. The vibronic structure is expressed in terms of a weighted sum of medium induced lorentzian lines located at the frequencies of the fundamental odd vibrations of the complex. Each of these lines is followed by a series of vibronic lines assigned to the combination frequencies with one or more even vibrations. The relative intensities of these combination lines are determined by the values of an intramolecular distribution which includes both the effects of geometry and frequency changes of the modes involved in the transition. Values of the linear and the quadratic parameter for the molecule can be estimated from a fit to the experimental spectra.     

Characterization of the surface of positive electrodes for Li-ion batteries using 7Li MAS NMR

The growth and evolution of the interphase, due to contact with the ambient atmosphere or electrolyte, are followed using ⁷Li magic-angle spinning nuclear magnetic resonance (MAS NMR) in the case of two materials amongst the most promising candidates for positive electrodes for lithium batteries: LiFePO₄ and LiMn_0.5Ni_0.5O₂. The use of appropriate experimental conditions to acquire the NMR signal allows observing only the «diamagnetic» lithium species at the surface of the grains of active material. The reaction of LiMn_0.5Ni_0.5O₂ with the ambient atmosphere or LiPF₆ (1 M in Ethylene Carbonated/DiMéthyl Carbonate (EC/DMC)) electrolyte is extremely fast and leads to an important amount of lithium-containing diamagnetic species compared to what can be observed in the case of LiFePO₄. The two studied materials display a completely different surface chemistry in terms of reactivity and/or kinetics of the surface towards electrolyte. Moreover, these results show that MAS NMR is a very promising tool to monitor phenomena taking place at the interface between electrode and electrolyte.     

Analysis on diffusion-induced stress for multi-layer spherical core–shell electrodes in Li-ion batteries

Silicon-based carbon composites are believed as promising anodes in the near future due to their outstanding specific capacity and relatively lower volume effect compared to pure silicon anodes. Herein, a multilayer spherical core–shell(MSCS) electrode with a graphite framework prepared with Si@O-MCMB/C nanoparticles is developed, which aims to realize chemically/mechanically stability during the lithiation/delithiation process with high specific capacity. An electrochemical-/mechanical-coupling model for the M-SCS structure is established with various chemical/mechanical boundary conditions.The simulation of finite difference method(FDM) has been conducted based on the proposed coupling model, by which the diffusion-induced stress along both the radial and the circumferential directions is determined. Moreover, factors that influence the diffusion-induced stress of the M-SCS structure have been discussed and analyzed in detail.     

Review of 5-V electrodes for Li-ion batteries: status and trends

Lithium-ion batteries have dominated the battery industry for the past several years in portable electronic devices due to their high volumetric and gravimetric energy densities. The success of these batteries in small-scale applications translates to large-scale applications, with an important impact in the future of the environment by improving energy efficiency and reduction of pollution. We present the progress that allows several lithium-intercalation compounds to become the active cathode element of a new generation of Li-ion batteries, namely the 5-V cathodes, which are promising to improve the technology of energy storage and electric transportation, and address the replacement of gasoline engine by meeting the increasing demand for green energy power sources. The compounds considered here include spinel LiNi_0.5Mn_1.5O₄ and its related doped-structures, olivine LiCoPO₄, inverse spinel LiNiVO₄ and fluorophosphate Li₂CoPO₄F. LiNi_0.5Mn_1.5O₄ thin films, nanoscale prepared materials and surface-modified cathode particles are also considered. Emphasis is placed on the quality control that is needed to guarantee the reliability and the optimum electrochemical performance of these materials as the active cathode element of Li-ion batteries. The route to increase the performance of Li-ion batteries with the other members of the family is also discussed.     

The study of electron transfer in advanced materials (electrodes for Li-ion batteries or catalysts)

Mössbauer Spectroscopy (MS) is really suitable to study local electronic structures. Its hyperfine parameters, isomer shift (IS) and quadrupole splitting (QS) allow to characterize the oxidation state and coordination of the probed element. So, the capabilities of this powerful technique have been highlighted for the study of electron transfer occurring during electrochemical or catalytic processes. Several examples illustrate how MS can be used for the determination of reaction mechanisms in new electrode materials of Li-ion batteries (Sb or Sn-based, Ti oxides) or reforming catalysts (supported bimetallic PtSn catalysts). Deeper insight into the mechanisms determining the electrochemical or catalytic performances can be expected.     

Metal oxide anodes for Li-ion batteries

Recently metal oxides, especially tin oxides, have been investigated as negative electrodes in Li-ion batteries. Different compounds such as amorphous SnO₂, SnO and SnSiO₃ have been electrochemically cycled versus a metallic lithium electrode. In this study, the reversible capacities as well as the cycling behavior of crystalline SnO₂ thin films and powders have been investigated. SnO₂ powder exhibits a reversible capacity as high as 600 mAh/g over more than 50 cycles versus a metallic lithium electrode. Based on these results, we give clues for the future investigations of metal oxides as anodes in lithium ion batteries and discuss what can be the expected capacities of such negative electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland, Sept. 13–19, 1997     

Electrical and electrochemical properties of carbon nanotube-based free standing LTO electrodes for current collector-free Li-ion batteries

The spinel lithium titanate oxide (Li₄Ti₅O₁₂, LTO) has been extensively studied as one of the most promising alternatives to carbon materials in energy conversion and storage devices, due to its high structural and thermal stability, rate capability, and excellent cycling stability. In this study, Li₄Ti₅O₁₂/multi-walled carbon nanotubes (LTO-MWCNTs) free-standing and flexible composite electrodes/buckypapers were prepared via tape casting technique and well compared with commercially available LTO. The structural, morphological, electrical and electrochemical properties of LTO-MWCNTs buckypaper as well as LTO were studied. The electrical conductivity of LTO-based buckypaper was found to be very high i. e, 4.4 × 10² Scm⁻¹ at room temperature. Further, the buckypaper prepared by synthesized LTO showed higher specific capacity (166 mAhg⁻¹) compared to commercially available LTO (137 mAhg⁻¹) at 0.2 C rate. The enhanced electrochemical performance of as-synthesized LTO-based buckypaper is mainly attributed to the higher electronic conductivity and homogeneous distribution of particles with its small size which facilitates large amount of active sites for lithium insertion and also short diffusion paths.     

Optimized LiVOPO4 for cathodes in Li-ion rechargeable batteries

Improvement of the rate properties of orthorhombic LiVOPO₄ by using a milling approach and acetylene black additives as electronic binder is investigated. The average particle size of orthorhombic LiVOPO₄ was reduced from 12.0 μm to 6.1 μm by milling process by which the Li intercalation capacity into LiVOPO₄ increased to 40 mAhg⁻¹ at 0.4 mAcm⁻² (C/5). At an optimized acetylene black amount of 15 wt.%, a better uniformity in particle size distribution and dispersion of the current distribution was obtained. Thus, enhancing the kinetic performance a fairly large reversible intercalation capacity of Li was achieved with values of 100 and 60 mAhg⁻¹ at high rate conditions of C/5 (0.4 mAcm⁻²) and 1C (2 mAcm⁻²), respectively. Paper presented at the International Conference on Functional Materials and Devices 2005, Kuala Lumpur, Malaysia, June 6 – 8, 2005.     

Nanopowders of spinel-type electrode materials for Li-ion batteries

Nanomaterials are becoming important for use in Li-ion battery electrodes as these can deliver increased capacity and improved power performance. Our work is focused on Mg-doped high-voltage spinel materials, such as LiNi_0.5Mn_1.5O₄, in order to improve its stability. LiMg_δNi_0.5−δMn_1.5O₄ with δ = 0.05, having the cubic spinel structure (P4₃32) were made via four different synthesis routes – a solid-state route, a sol–gel method, a xerogel route and an auto ignition method.The powders were investigated with SEM and TEM analysis. XRD was used to determine the crystallographic structure. Electrochemical tests were performed in CR2320 coin cells built with 1 M LiPF₆ in EC/EMC/DMC 1:2:2 as electrolyte and metallic Li as negative electrode – cells were measured with a MACCOR cycler.LiMg_0.05Ni_0.45Mn_1.5O₄ made via the sol–gel and xerogel routes revealed agglomerated nanoparticles with sizes ranging from 10 to 200 nm, whereas the auto ignition method gives particle sizes between 10 and 50 nm. Although agglomerated, often residual LiMn₂O₄ is observed, with increasing concentration going from solid-state, sol–gel, xerogel to auto ignition.Hence, thanks to these different synthesis routes, we are able to obtain particle sizes reaching from 10 to 200 nm, with a narrow particle size distribution. The electrochemical tests of the xerogel particles showed promising results. The auto ignition method show also promising results, however, the impurity phase needs to be suppressed significantly. The sol–gel method, the xerogel route and the auto ignition method show increased capacity retention at high power rates compared to the solid state method.     

On the theory of lasing at vibronic transitions in impurity crystals

Some aspects of lasing at vibronic transitions in impurity crystals are theoretically studied. The threshold conditions for a vibronic laser are shown to be dependent on the strength of the interaction of optical centers with a local vibration, which forms the vibronic spectrum, and the crystal lattice temperature. The theory can easily be generalized to the spectrum containing a structureless phonon sideband and well agrees with the experimental temperature dependence of the output power of a Mg₂SiO₄:Cr⁴⁺ forsterite laser.     

The Dushinsky effect and sum rules for vibronic transitions in polyatomic molecules

A method is presented for analyzing, in terms of sum rules, the intensity distribution among the vibronic bands in electronic spectra of polyatomic molecules, taking into account the rotation of the excited-state normal coordinates relative to those of the ground state (the Dushinsky effect). In the harmonic oscillator approximation, a quantitative criterion for the occurrence of the Dushinsky effect is obtained. The existence of this effect leads to non-product formulas for probabilities of the joint excitation of different vibrational modes. Expressions are obtained for the mean number of quanta excited in a given mode, and for its standard deviation, as well as for the correlation coefficients. The proposed method does not require a complete vibrational analysis of the spectrum of interest. As an example, the calculation of the correlation coefficient for the 3700-Å band system of the absorption spectrum of SO₂ is given.     

Surface structure evolution of cathode materials for Li-ion batteries

Lithium ion batteries are important electrochemical energy storage devices for consumer electronics and the most promising candidates for electrical/hybrid vehicles. The surface chemistry influences the performance of the batteries significantly. In this short review, the evolution of the surface structure of the cathode materials at different states of the pristine, storage and electrochemical reactions are summarized. The main methods for the surface modification are also introduced.     

Preparation of V-LiFePO4 cathode material for Li-ion batteries

The preparation of vanadium-modified olivine LiFePO₄ was attempted using vanadium-modified FePO₄ precursor which was synthesized by controlled crystallization. The structure and electrochemical behavior of V-LiFePO₄ with different vanadium contents were investigated. The electrochemical behavior of V-LiFePO₄ materials at high rate and low temperature was compared with that of the LiFePO₄ material. Incorporation of vanadium improved the electrochemical performance of LiFePO₄. The investigation showed that the 3%V-modified LiFePO₄ presented the best electrochemical performance.     

Comparative study of NiSb2 and FeSb2 as negative electrodes for Li-ion batteries

Crystalline FeSb₂ powder prepared by ceramic route is examined as negative electrodes for lithium-ion batteries. The complete reaction mechanism of FeSb₂ is investigated by ¹²¹Sb and ⁵⁷Fe Mössbauer spectroscopy as well as magnetic measurements and the results are correlated with a previous in situ XRD characterization. On the first discharge the reaction with Li proceeds through a biphasic process transforming FeSb₂ into a new Li_xFe_ySb₂ phase, and this ternary phase is then converted into fcc Li₃Sb and metallic Fe nanoparticles. The combination of Mössbauer spectroscopy and magnetic analyses leads i) to a better understanding of the FeSb₂ → ternary phase reaction and concomitantly allowed ii) to specify the stoichiometry of the new ternary phase. On charge, the extrusion of lithium includes the back conversion of the Li₃Sb/Fe mixture into both Li₄Fe_0.5Sb₂ and metallic Sb, which are the main active species for the following cycles, responsible for the poor cycling life of the FeSb₂ electrode. The nature of these resulting products is quite different from that previously observed for the isotype NiSb₂ electrode which is characterized by a highly reversible mechanism.     

Two-dimensional MnN utilized as high-capacity anode for Li-ion batteries

When developing high performance lithium-ion batteries,high capacity is one of the key indicators.In the last decade,the progress of two-dimensional(2 D) materials has provided new opportunities for boosting the storage capacity.Here,based on first-principles calculation method,we predict that MnN monolayer,a recently proposed 2 D nodal-loop halfmetal containing the metallic element Mn,can be used as a super high-capacity lithium-ion batteries anode.Its theoretical capacity is above 1554 mA-h/g,more than four times that of graphite.Meanwhile,it also satisfies other requirements for a good anode material.Specifically,we demonstrate that MnN is mechanically,dynamically,and thermodynamically stable.The configurations before and after lithium adsorption exhibit good electrical conductivity.The study of Li diffusion on its surface reveals a very low diffusion barrier(～ 0.12 eV),indicating excellent rate performance.The calculated average open-circuit voltage of the corresponding half-cell at full charge is also very low(～0.22 V),which facilitates higher operating voltage.In addition,the lattice changes of the material during lithium intercalation are very small(～ 1.2%-～4.8%),which implies good cycling performance.These results suggest that 2 D MnN can be a very promising anode material for lithium-ion batteries.     

Structural, magnetic and redox properties of a new cathode material for Li-ion batteries: the iron-based metal organic framework

The iron-based metal organic framework (MOF) presently studied is the first example of MOF showing a reversible electrochemical Li insertion with a very good cycling life. Its potential application as a cathode material in Li-ion battery is nevertheless curbed by a rather poor capacity of 70 mAh/g. To figure out the origin of this limited insertion, first-principles density functional theory (DFT)+U calculations were performed. The results show that Fe^III{OH(BDC)} is a weak anti-ferromagnetic charge transfer insulator at T = 0 K with iron in the high-spin S = 5/2 state. In agreement with the absence of electronic de-localisation along the inorganic chains, lithium insertion leads to the stabilisation of a Fe^II/Fe^III mixed-valence state of class I or II in the Robin–Day classification, whatever the Li sites considered in the calculations. Among these Li sites, the most probable site I (OH-Li) and site II (O=CO-Li) are shown to induce incompatible structural changes on the reduced Li_0.5Fe{OH(BDC)} form that could be at the origin of the small capacity measured for this compound. Paper presented at the 11th EuroConference on the Science and Technology of Ionics, Batz-sur-Mer, Sept. 9–15, 2007.     

X-ray study of metal oxide based anodes for Li-ion batteries

Cells were made from Co₃O₄ and Co₂SnO₄ and lithium metal and tested to determine reversible lithium capacity. Li is reversibly inserted into Co₃O₄, as was observed by electrochemistry, coupled with changes of cobalt oxidation state as observed by Co K-edge EXAFS. On lithium insertion the Co₃O₄ is reduced to yield only metallic cobalt species, and then on lithium removal an oxide of Co is formed. The EXAFS data further showed that the initial reduction was to Co(II) and then to metallic Co, and that both the metallic and oxide phases were disordered, having low co-ordination numbers and large shell spacings. The electrochemical behaviour of the Co₂SnO₄ cells was closer to that of SnO₂ than Co₃O₄, but did exhibit changes obviously caused by the cobalt. EXAFS on Co₂SnO₄ cells revealed that the Co is reduced to metallic cobalt on the initial discharge, but does not convert back to an oxide on cycling. Paper presented at the 7th Euroconference on Ionics, Calcatoggio, Corsica, France, Oct. 1–7, 2000.     

Ultrafast vibronic phase transitions induced in semiconductors by femtosecond laser pulses

The intermode anharmonic interaction in the theory of ultrafast (t∼10⁻¹³ s) vibronic phase transitions induced on semiconductor surfaces (Si, GaAs) by femtosecond laser pulses is calculated. The conditions for plasma-induced transitions either to a state of chaotic disorder in the positions of the atoms (“cold liquid”) or into a state with crystal symmetry different from the initial symmetry (a new crystalline phase) are determined. It is shown that a NaCl-type structure is realized in GaAs for a transition of the second type, the transition being due to the instability of the longitudinal optical phonon branch. The corresponding numerical estimates are made for Si and GaAs. Fiz. Tverd. Tela (St. Petersburg) 41, 1462–1466 (August 1999)     

Nanoflake CoN as a high capacity anode for Li-ion batteries

CoN films with nanoflake morphology are prepared by RF magnetron sputtering on Cu and oxidized Si substrates and characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscopy (HR-TEM) and selected area electron diffraction (SAED) techniques. The thickness and composition of the films are determined by the Rutherford back scattering (RBS) technique confirming the stoichiometric composition of CoN with a thickness, 200 (± 10) nm. Li-storage and cycling behavior of nanoflake CoN have been evaluated by galvanostatic discharge–charge cycling and cyclic voltammetry (CV) in cells with Li–metal as counter electrode in the range of 0.005–3.0 V at ambient temperature. Results show that a first-cycle reversible capacity of 760 (± 10) mAhg^? 1 at a current rate 250 mAg^? 1(0.33 C) increases consistently to yield a capacity of 990 (± 10) mAhg^? 1 after 80 cycles. The latter value corresponds to 2.7 mol of cyclable Li/mol of CoN vs. the theoretical, 3.0 mol of Li. Very good rate capability is shown when cycled at 0.59 C (up to 80 cycles) and at 6.6 C (up to 50 cycles). The coloumbic efficiency is found to be 96–98% in the range of 10–80 cycles. The average charge and discharge potentials are 0.7 and 0.2 V, respectively for the decomposition/formation of Li₃N as determined by CV. However, cycling to an upper cut-off voltage of 3.0 V is essential for the completion of the “conversion reaction”. Based on the ex-situ-XRD, -HR-TEM and -SAED data, the plausible Li-cycling mechanism is discussed. The results show that nanoflake CoN film is a prospective anode material for Li-ion batteries.