Electronic properties of C60 single crystals doped with lithium by electrodiffusion

Diffusion of lithium cations in C₆₀ single crystals driven by electric field has been detected and studied. A novel technique for fullerene crystal doping based on injection of ions through a “superionic crystal/C₆₀ single crystal” heterojunction has been suggested. It has been found that lithium doping of C₆₀ single crystals brings about an ESR signal, and this signal as a function of time has been investigated. The electronic conductivity in Li_xC₆₀ crystals has a nonmetallic nature. Reflection spectra measured in the IR band have shown that the reflectivity due to free electrons gradually decreases with time, which correlates with the evolution of signals due to ESR and microwave conductivity. Lithium doping of crystals increases the oscillator strength of the T _1u (4) vibrational mode and shifts it to lower frequencies (from 1429 cm⁻¹ to 1413 cm⁻¹), which indicates that one electron is present at the C₆₀ molecule, and this fact may be treated as evidence that the LiC₆₀ phase is generated in a C₆₀ crystal. Zh. éksp. Teor. Fiz. 116, 1706–1722 (November 1999)     

Oxidative decomposition mechanisms of lithium peroxide clusters: an Ab Initio study

Oxidative decomposition of solid lithium peroxide is an important part of the charging process in a Li-O₂ battery. In this paper, we investigate oxidative decomposition mechanisms of lithium peroxide clusters as molecular models for solid lithium peroxide using density functional methods to understand charging processes in advanced energy storage systems. Most calculations are done using a (Li₂O₂)₄ cluster with similar results obtained from a larger (Li₂O₂)₁₆ cluster. Reaction pathways of the clusters involving different sequences of oxidation, oxygen evolution, lithium cation removal, and spin excitation are investigated. The computations suggest that certain oxidative decomposition routes may not have dependence on the oxygen evolution or Li-ion removal kinetics due to the exothermicity of oxygen removal and Li⁺ removal (by solvent) upon oxidation. The computed charge potentials evaluated using a tetramer model indicates that it is possible to have low overcharge potential provided there exists a good electronic conductivity to facilitate the oxidative decomposition. Finally, oxidation potentials of a series of Li_xO_y clusters are investigated to assess their dependence on stoichiometry and how the local site from which the electrons are being removed affects the charge potentials.     

Lithium ion diffusion in LixNiPS3 single crystals

Electrochemical studies have been made of the kinetics and thermodynamics of lithium intercalation into single crystals of NiPS₃. Effective diffusion coefficients for lithium in this layered compound were found to be higher than previously reported for powdered NiPS₃ cathodes, and very much higher than expected from previous nmr studies. Despite considerable electron transfer in Li⁺_xNiPS^x-₃, the electronic conductivity does not approach metallic values until x 1.4. The transition to a previously described state of high ionic conductivity is discussed.     

Phase transitions and the structure of the C60 crystal doped with lithium by electrodiffusion

The structure and phase transitions of C₆₀ crystals doped with lithium by injecting metal ions from the superionic crystal-C₆₀ single crystal heterojunction under electrodiffusion conditions are reported. The sample experienced irreversible transitions resulting in the virtually complete disappearance of EPR signals and MW conduction in the temperature range 320–370 K. In this temperature interval, a new C₆₀ phase was formed; the phase contained polymeric chains of C₆₀ molecules along the crystallographic c axis and lithium clusters. The structure of this phase was determined. Annealing at 620 K restored the EPR signal and, according to the X-ray data, the initial cubic structure of pure C₆₀. The X-ray pattern, however, contained additional diffraction peaks, which was evidence of the presence of one more phase with a structure yet unknown.     

Change of the stoichiometry and electrocoloration due to the injection of Li and O ions into lithium niobate crystals

Congruently grown LiNbO₃ single crystals show both high oxygen and lithium ion conductivity at temperatures above 500 °C. The high oxygen ion conductivity can be understood in terms of a certain amount of oxygen vacancies already present in congruently grown LiNbO₃ single crystals. Thermal treatment of LiNbO₃ produces additional oxygen vacancies. The absorption bands introduced by this procedure are investigated. It is found that the electrons which are generated during the reduction process are homogeneously distributed among all oxygen vacancies in the LiNbO₃ single crystals. The electrocoloration phenomenon in LiNbO₃ single crystals is due to the process of injection of lithium ions and electrons into LiNbO₃ by a double charge mechanism. Investigations of the optical and electrical properties of electrocolored LiNbO₃ crystals are reported. It is shown that the absorption spectra of thermally and electrochemically reduced samples are identical and that the origin of the absorption processes has to be therefore the same in both cases. That means, additional electrons produced by the double charge injection of lithium ions and electrons are also homogeneously distributed among the oxygen vacancies. This supports our hypothesis that a certain amount of oxygen vacancies has to be present already in as-grown LiNbO₃ single crystals.     

Clustering and polymerization of Li15C60

The structural and electronic properties of lithium intercalated fullerides (of which Li₁₅C₆₀ is the most representative) are still puzzling and unclear. Above 520 K, x-ray/neutron diffraction shows an fcc phase in which the 15 Li atoms clusterize in the octahedral interstices. However, at lower temperatures, a change in the crystalline symmetry and also in the electronic properties takes place as observed from ¹³C, ⁷Li/⁶Li NMR and x-ray diffraction measurements. X-ray diffraction data suggest the presence of two different stable structures: a tetragonal monomeric and an orthorhombic polymerised phase. Detailed ¹³C magic angle spinning NMR experiments in the latter phase indicate sp ³ bondings among the carbon atoms, whereas the relative (sp ²/sp ³) intensities, together with x-ray data, suggest the C₆₀ polymerization to be a [2+2] cycloaddition. Multiple quantum NMR experiments on ⁷Li confirm the presence of lithium clusters, as observed by x-ray diffraction in the high temperature phase, also at lower temperatures. However, the inferred cluster size is significantly smaller than that suggested by the stoichiometry. The distortion in the low-T structure of L₁₅C₆₀ is supposed to induce the migration of Li atoms from octahedral to tetrahedral voids, thus accounting for the lower number of Li atoms in the clusters. Further evidence of this scenario is obtained also from preliminary measurements of line shapes and T ₁ relaxation times, which exhibit a multiexponential recovery with very different constants that are hardly compatible with a single family of Li atom sites.     

Electronic to ionic conductivity of glasses in the Li2S–Sb2S3–P2S5 system

There is great interest in sulfide glasses because of their high lithium ion conductivity. New sulfide glasses in the Li₂S–Sb₂S₃–P₂S₅ system have been synthesized by classical quenching technique. The glass domain relays on the medium-lithium content (up to 50% molar).Electrical conductivities of the samples have been determined by Impedance Spectroscopy. The isothermal conductivity curves exhibit two regions on dependence of lithium content implying that the conductivity mechanisms in these two regions are different. The compositions of low lithium content (below 20% mol.) have presented low electronic conductivities close to 10^− 8 S/cm at room temperature. The compositions of medium lithium content (30–50% mol.) could be mixed ionic–electronic conductors with predominant ionic conductivities with a maximum close to 10^− 6 S/cm for sample with 50% Li₂S at room temperature. Arrhenius exponential behavior is verified between 25 °C and T_g for all glasses. The activation energies, determined from temperature dependence, are 0.55–0.64 eV. A comparative study with glasses belonging to the other chalcogenide systems has been undertaken on base of the weak electrolyte model and the values of decoupling index, R_τ, are reported. The impedance of the 0.5Li₂S–0.3Sb₂S₃–0.2P₂S₅ ionic conductor can be described by an equivalent circuit R(RQ)(RQ).     

Electrical properties of LiBSe ternary system

Amorphous products were obtained in the LiBSe ternary system by quenching melts of Li₂Se, B and Se mixture prepared at 100°C in sealed silica tubes. The vitreous region was slightly lower selenium composition than that of the Li₂SeB₂Se₃ tie-line. The amorphous products were lithium ionic conductors and most of them showed contributions to their total conductivity. The amorphous product of composition Li₂₅B₃₆Se₃₉ has the least electronic contribution to its total conductivity of 6.0 × 10⁻⁶ S/cm at room temperature. A new crystalline compound and crystalline LiBH₄ were also obtained in LiBSe ternary. Both of them were lithium ionic conductors having conductivities of about 1 × 10⁻⁶ S/cm at room temperature.     

Ab initio calculations of endo-and exohedral C60 fullerene complexes with Li+ ion and the endohedral C60 fullerene complex with Li2 dimer

The results of ab initio Hartree-Fock calculations of endo-and exohedral C₆₀ fullerene complexes with the Li⁺ ion and Li₂ dimer are presented. The coordination of the Li⁺ ion and the Li₂ dimer in the endohedral complexes and the coordination of Li⁺ ion in the exohedral complex of C₆₀ fullerene are determined by the geometry optimization using the 3–21G basis set. In the endohedral Li⁺C₆₀ complex, the Li⁺ ion is displaced from the center of the C₆₀ cage to the centers of carbon hexa-and pentagons by 0.12 nm. In the Li₂ dimer encapsulated inside the C₆₀ cage, the distance between the lithium atoms is 0.02 nm longer than that in the free molecule. The calculated total and partial one-electron densities of states of C₆₀ fullerene are in good agreement with the experimental photoelectron and X-ray emission spectra. Analysis of one-electron density of states of the endohedral Li⁺@C₆₀ complex indicates an ionic bonding between the Li atoms and the C₆₀ fullerene. In the Li⁺C₆₀ and Li⁺@C₆₀ complexes, there is a strong electrostatic interaction between the Li⁺ ion and the fullerene.     

First order phase transition of Li3 ThF7 at 281 K: A comparative study between EPR and Raman scattering

Abstract

Electron Paramagnetic Resonance (EPR) and Raman Spectroscopy were used to study the low temperature phase transition of pulled Li₃ThF₇ single crystals, occurring at 281 K. In both cases, the room temperature spectra were very broad, owing to the statistical disorder and high ionic mobility of the lithium ions, which occupy only 3/4 of their structural sites. The results are compatible with a first order ferroelastic transition from the room temperature orthorhombic D_2h ²² phase to a monoclinic C_2h ^(x) one. The symmetry rules are well respected assuming a model with four lithium ions in average per chemical formula. The EPR spectra show also the appearing of additional ferroelastic domains.     

Anisotropy of the electrical conductivity of lithium heptagermanate crystals

The electrical conductivity σ of single crystals of lithium heptagermanate Li₂Ge₇O₁₅ is studied in an electric field in the frequency range 0.5–100 kHz at temperatures ranging from 300 to 700 K. Heating the crystal above 500 K gives rise to a pronounced anisotropy in the electrical conductivity, which differs in magnitude by one to two orders of magnitude for different directions of the measurement field along the crystallographic axes. It is shown that an increase in the electrical conductivity σ with increasing temperature originates from charge transfer with an activation energy U = 1.04 eV. It is assumed that the thermally activated contribution to the electrical conductivity is governed by transport of lithium interstitial ions along channels in the structure of the Li₂Ge₇O₁₅ compound.     

Recent progress of glass and glass-ceramics as solid electrolytes for lithium secondary batteries

In recent years many new glass-based solid electrolytes with high Li⁺ conductivity have been developed. In the present paper, we review the preparation and characterization of Li₂S-based oxysulfide glasses and sulfide glass-ceramics on the basis of two strategies of enhancing Li⁺ conductivity: the utilization of “mixed-anion effect” by combining sulfide and oxide anions, and the precipitation of superionic metastable crystals by careful heat-treatment of glasses. The superior Li⁺ conducting solid electrolytes with the highest conductivity and the lowest activation energy for conduction have been achieved in the Li₂S–P₂S₅ glass-ceramics. The use of these glass-ceramic solid electrolytes leads to the development of a bulk-type all solid-state lithium secondary battery with excellent cycling performance.     

Thermally stimulated recombination processes and luminescence in Li<Subscript>6</Subscript>(Y,Gd,Eu)(BO<Subscript>3</Subscript>)<Subscript>3</Subscript> crystals

The thermally stimulated recombination processes and luminescence in crystals of the lithium borate family Li₆(Y,Gd,Eu)(BO₃)₃ have been investigated. The steady-state luminescence spectra under X-ray excitation (X-ray luminescence spectra), the temperature dependences of the X-ray luminescence intensity, and the glow curves for the Li₆Gd(BO₃)₃, Li₆Eu(BO₃)₃, Li₆Y_0.5Gd_0.5(BO₃)₃: Eu, and Li₆Gd(BO₃)₃: Eu compounds have been measured in the temperature range 90–500 K. In the X-ray luminescence spectra, the band at 312 nm corresponding to the ⁶ P _J → ⁸ S _7/2 transitions in the Gd³⁺ ion and the group of lines at 580–700 nm due to the ⁵ D ₀ → ⁷ F _J transitions (J = 0–4) in the Eu³⁺ ion are dominant. For undoped crystals, the X-ray luminescence intensity of these bands increases by a factor of 15 with a change in the temperature from 100 to 400 K. The possible mechanisms providing the observed temperature dependence of the intensity and their relation to the specific features of energy transfer of electronic excitations in these crystals have been discussed. It has been revealed that the glow curves for all the crystals under investigation exhibit the main complex peak with the maximum at a temperature of 110–160 K and a number of weaker peaks with the composition and structure dependent on the crystal type. The nature of shallow trapping centers responsible for the thermally stimulated luminescence in the range below room temperature and their relation to defects in the lithium cation sublattice have been analyzed.     

Defect dependence of the dielectric permeability of Qn(TCNQ)2

The conductivity and 9.1 GHz dielectric constant of neutron irradiated Qn(TCNQ)₂ single crystals are strongly affected by defect concentrations less than 1 %. This indicates long range electronic coherence of the order of 100 lattice constants in the non-irradiated material for which the coherence length may still be impurity limited. It is suggested that a collective excitation of electrons is responsible for the temperature dependence of the large dielectric constant.     

Study of the electronic structure and properties of 13C-isotope-based composites

The morphology and electronic structure of ¹³C-isotope-based graphite composites were studied by transmission electron microscopy (TEM), x-ray diffraction, and x-ray fluorescence spectroscopy. High-resolution TEM images showed that composites contain several forms of carbon particles. According to an x-ray diffraction analysis, the size of graphene stacks of graphite particles is 20 and 40 Å. The CK _α x-ray fluorescence spectra of the initial ¹³C isotope powder and composites based on it detected an increase in the density of high-energy occupied states in comparison with the graphite spectrum. Ab initio quantum chemical calculation of the structure of C₁₅₀ graphene showed that the increase in the density of states stems from the electrons of dangling bonds of boundary carbon atoms of particles ～20 Å in size. Electrical properties of ¹³C-isotope-based samples were studied by analyzing the temperature dependence of the conductivity. It was assumed that the change in the logarithmic dependence of the conductivity observed at liquid-helium temperatures to the linear dependence as the temperature increases is caused by carrier transfers between the disordered graphene layers forming a nanocomposite.     

Morphological investigations on the solid electrolyte Li3.6Ge0.6V0.4O4 and use in a solid state lithium battery

The development of a solid state thin film lithium battery system needs a detailed investigation of the electrolyte as well as the electrodes. The realisation of a total solid state battery includes the assumption, that the interfacial contact between the electrodes and the electrolyte is as good as possible. The interfacial resistance should be low for an easily intercalation or deintercalation of lithium at the electrodes and the ionic conductivity of the electrolyte should be high. A further critical aspect of the solid state battery is the porosity of the electrodes and the electrolyte. A homogeneous surface coverage and a high density are useful for a high contact concentration between the grains of electrolyte itself and between the electrolyte and the electrodes. As a possible electrolyte we have investigated the system Li₄GeO₄/Li₃VO₄, which we prepared in the stoichiometry Li_3.6 Ge_0.6 V_0.4 O₄. This lithium conducting system shows a high lithium conductivity of about 4 x 10⁻⁵ S/cm at room temperature and higher values at elevated temperatures. The conductivity is the result of interstitial Li cations in the solid state solution. The deposition of the inorganic thin film was done by a spray pyrolysis technique on different substrates. A variety of different substrates were investigated as a function of adhesion and modified surface conditions. The aim is to find a correlation between the substrate and the morphology of the thin film. The temperature of the deposition was varied between 400 and 600 °C. The temperature dependent cristallinity was also studied. Furthermore the change of the unit cell volume and its constants a, b and c has been investigated as a function of temperature by high temperature diffractometry. The thermal expansion coefficient of the electrolyte could be calculated, to examine stresses with various substrates. Paper presented at the 2nd Euroconference on Solid State Ionics, Funchal, Madeira, Portugal, Sept. 10–16, 1995     

Electronic and geometric structure of acetylide carbon on the surface of lithium

The reaction products formed on clean evaporated lithium films exposed to acetylene gas under UHV conditions were examined with ultraviolet photoelectron and Auger electron spectroscopies. Good agreement was obtained between the observed ultraviolet photoelectron spectrum and a spectrum simulated from ab initio calculations for C₂Li₂ in D_2h symmetry. The components of the C(KW) Auger signal were also consistent with carbon being present as C₂^n? units (acetylide) rather than as carbonaceous aggregates or as monomeric carbon. The observed carbon AES structure was fit in good agreement by a simulation using experimental electronic binding energies and Auger transition intensities calculated for C₂Li₂. Saturation of the ultraviolet and Auger electron signals with increasing exposure to HC₂H was not found, although thin films of lithium were slowly consumed on long exposure to acetylene. A loss of the reaction product from the interfacial region was indicated. Cluster models of acetylide on the (100) face of lithium, C₂/Li₈(6, 2)-(100), were used to investigate geometric arrangement of C₂^n? relative to lithium. The results show that acetylide resident in the interface below the first layer of lithiums of the model was energetically favored compared to C₂^n? resident on top of the first layer of lithium. The stability of this conformation was calculated to be further increased by a small expansion of lithiums near neighbor to C₂Li₂. The restructuring calculated for the model complements the absent saturation and suggests that C₂^n? units diffuse into the actual film via a local expansion.     

Specific features of the dark conductivity in lithium niobate crystals of congruent composition

The temperature dependence of the dark conductivity in a series of doped and nominally undoped LiNbO₃ crystals of congruent composition is studied. The activation energies for the ionic and electronic contributions to the dark conductivity are determined, and the H⁺, ion diffusion coefficient is estimated. The correlation between the ionic and electronic contributions to the dark conductivity is established.     

Ionic-electronic mixed conduction in LixV2O5

The dc conductivity of lithium vanadium bronze, Li_xV₂O₅ was measured on polycrystal prepared by solid-state reaction in the x region 0.25–0.70. Both electronic and ionic conduction was observed. The former increased with increase of lithium content and was nearly equal to the total conductivity 10^-1–10⁰ S/cm. The ionic conductivity (∽10^-4 S/cm) measured by dc four-probe technique decreased as the lithium content increased in the range 400–500°C. The apparent activation energy for ionic conduction varied from 57 kJ/mol for x of 0.25 to 82 kJ/mol for x of 0.50.     

Special temperature features of the birefringence of lithium tetraborate crystals

Using the Senarmont polarimetric method, the optical birefringence of lithium tetraborate crystals Li₂B₄O₇ in the 290–480 K range was measured. The steplike temperature dependence of birefringence, the thermal hysteresis, and the thermooptic memory effect were observed, which are most likely associated with the presence of incommensurate phase modulation in lithium tetraborate.