Charge distribution and mobility of lithium ions in Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript> from <Superscript>6,7</Superscript>Li NMR data

A comparative analysis of ^6,7Li NMR spectra is performed for the samples of monoclinic lithium titanate obtained at different synthesis temperatures. In the ⁷Li NMR spectra three lines are found, which differ in quadrupole splitting frequencies v _Q and according to ab initio EFG calculations are assigned to three crystallographic sites of lithium: Li1 (v _Q ～ 27 kHz); Li2 (v _Q ～ 59 kHz); Li3 (v _Q ～ 6 kHz). The dynamics of lithium ions is studied in a wide temperature range from 300 K to 900 K. It is found that the narrowing of ⁷Li NMR spectra as a result of thermally activated diffusion of lithium ions in the low-temperature Li₂TiO₃ sample is observed at a higher temperature in comparison with a sample of high-temperature lithium titanate. Based on the analysis of ⁶Li NMR spectra it is assumed that there is mixed occupancy of lithium and titanium sites in the corresponding layers of the crystal structure of low-temperature lithium titanate, which hinders lithium ion transfer over regular crystallographic sites.     

Cycling Performance at Li<Subscript>7</Subscript>La<Subscript>3</Subscript>Zr<Subscript>2</Subscript>O<Subscript>12</Subscript>|Li Interface

Cyclability of the Li|Li₇La₃Zr₂O₁₂ interface was tested by voltammetry under externally applied potential difference. It was found that the solid electrolyte synthesized in the study contains a minor amount of an impurity in the form of lithium carbonate. This impurity forms, when brought in contact with metallic lithium, carbon that pierces the whole volume of the ceramic separator and produces a channel for a flow of electrons through the material, which leads to a poor cyclability of the solid electrolyte. A possible way to solve the given problem is via a purposeful replacement of the carbonate in the intergrain space of Li₇La₃Zr₂O₁₂ with another crystalline or glassy plasticizer that possesses an acceptable unipolar lithium conductivity (no less than 10^–6 S cm^–1) and forms, when brought in contact with metallic lithium, no electrically conducting compound or a compound capable of reversibly intercalating/deintercalating lithium.     

Ionic mobility and conductivity in PbSnF<Subscript>4</Subscript> doped with CaF<Subscript>2</Subscript> from the NMR and impedance spectroscopy data

The ionic mobility and conductivity in the crystalline phases of PbSnF_4–xCaF₂ systems (x = 2.5 mol.%, 5 mol.%, 7.5 mol.%, and 10 mol.%) in the temperature range of 150-500 K are studied by NMR and impedance spectroscopy. The parameters of ¹⁹F NMR spectra, types of ion motions, and ionic conductivity in the PbSnF₄ compound doped with calcium fluoride are found to be determined by the temperature and concentration of calcium fluoride. The specific conductivity of the crystalline phases in the PbSnF₄–CaF₂ systems is rather high at room temperature, and hence, one cannot exclude the possibility to use them for the creation of functional materials with a high ionic (superionic) conductivity.     

Correlation between the structural,electrical and electrochemical performance of layered Li(Ni<Subscript>0.33</Subscript>Co<Subscript>0.33</Subscript>Mn<Subscript>0.33</Subscript>)O<Subscript>2</Subscript> for lithium ion battery

The Li(Ni_0.33Co_0.33Mn_0.33)O₂ (LNCMO) cathode material is prepared by poly(vinyl pyrrolidone) (PVP)-assisted sol-gel/hydrothermal and poly(ethylene glycol)-block-poly(propylene glycol)-block-poly (ethylene glycol) (Pluronic-P123)-assisted hydrothermal methods. The compound prepared by PVP-assisted hydrothermal method shows a comparatively higher electrical conductivity of ~2?×?10^?5 S cm^?1 and exhibits a discharge capacity of 152 mAh g^?1 in the voltage range of 2.5 to 4.4 V, for a C-rate of 0.2 C, whereas the compounds prepared by P123-assisted hydrothermal method and PVP-assisted sol-gel method show a total electrical conductivity in the order of 10^?6 S cm^?1 and result in poor electrochemical performance. The structural and electrical properties of LNCMO (active material) and its electrochemical performance are correlated. The difference in percentage of ionic and electronic conductivity contribution to the total electrical conductivity is compared by transference number studies. The cation disorder is found to be the limiting factor for the lithium ion diffusion as determined from ionic conductivity values.     

Electrical conductivity studies in the system Li<Subscript>2</Subscript>TiO<Subscript>3</Subscript>-Li<Subscript>1.33</Subscript>Ti<Subscript>1.67</Subscript>O<Subscript>4</Subscript>

Electrical conductivity in the monoclinic Li₂TiO₃, cubic Li_1.33Ti_1.67O₄, and in their mixture has been studied by impedance spectroscopy in the temperature range 20–730 °C. Li₂TiO₃ shows low lithium ion conductivity, σ₃₀₀≈10^–6 S/cm at 300 °C, whereas Li_1.33Ti_1.67O₄ has 3×10^–8 at 20 °C and 3×10^–4 S/cm at 300 °C. Structural properties are used to discuss the observed conductivity features. The conductivity dependences on temperature in the coordinates of 1000/T versus log_e(σT) are not linear, as the conductivity mechanism changes. Extrinsic and intrinsic conductivity regions are observed. The change in the conductivity mechanism in Li₂TiO₃ at around 500–600 °C is observed and considered as an effect of the first-order phase transition, not reported before. Formation of solid solutions of Li_{2– x} Ti_{1+ x} O₃ above 900 °C significantly increases the conductivity. Irradiation by high-energy (5 MeV) electrons causes defects and the conductivity in Li₂TiO₃ increases exponentially. A dose of 144 MGy yields an increase in conductivity of about 100 times at room temperature. Electronic Publication     

The impedance of the Ag-CeF<Subscript>3</Subscript>/CeF<Subscript>3</Subscript> bicrystal-Ag system

The impedance spectra of CeF₃/CeF₃ bicrystal (two single crystals separated by a single intercrystalline boundary) between Ag-electrodes are studied over a 135 to 410 K temperature interval (including temperatures below room temperature). The bicrystal was prepared by thermal-diffusion welding under a pressure of 1.5 × 10⁷ Pa at 1473 K in vacuum (∼10⁻² Pa). It is shown that the intercrystalline boundary affects but insignificantly the bicrystal bulk impedance. The CeF₃/CeF₃ ionic conductivity is 3 × 10⁻⁶ S/cm at 293 K; it is mainly determined by transfer processes in the single crystal bulk.     

<Superscript>1</Superscript>H and <Superscript>7</Superscript>Li NMR in defect cobaltite Li<Subscript>0.6</Subscript>CoO<Subscript>2</Subscript>

Within a temperature range of 120–330 K, ⁷Li NMR spectra in Li_0.6CoO₂ are obtained. It is shown that as the temperature increases, both smooth and stepwise variation of ⁷Li NMR contact shifts occurs. The observed effects are explained by the occupation of the excited levels of cobalt ions. The stepwise change of the resonance line width depending on the temperature is revealed. It is driven by the features of the diffusive motion of lithium ions. The calculation of the ¹H NMR line shape provides the determination of the ratio of one-, two-, and three-spin proton clusters in Li_0.6CoO₂·xH₂O.     

Crystal structure of Li(H<Subscript>2</Subscript>O)<Subscript>3</Subscript>[GaEdta]

The ethylenediaminetetraacetate complex Li(H₂O)₃[Ga(Edta)] was synthesized and its crystal structure composed of octahedral (Ga(Edta)⁻ anions connected to the Li(H₂O)₃⁺ ion through the oxygen atom was studied. Five of the six hydrogen atoms of water molecules are involved in weak hydrogen bonds with the oxygen atoms of four Ga(Edta)⁻ complexes, the complex anion is hydrogen-bonded to five water molecules. In addition, shortened contacts C(221)–H(22A)…O(112) between the Ga(Edta)⁻ anions were found. As a result, the molecular packing in the crystal is determined by the three-dimensional lace of hydrogen bonds. The results are compared with published data for the lithium salts of Bi(III), Sb(III), Fe(III), Ni(II), and Hg(II) ethylenediaminetetraacetates.     

Ion mobility,phase transitions,and conductivity of crystal phases in KF-CsF-SbF<Subscript>3</Subscript>-H<Subscript>2</Subscript>O system according to data of NMR and impedance spectroscopy

The methods of NMR, thermogravimetric analysis, and impedance spectroscopy were used to study ion mobility, phase transitions, and ion conductivity in crystal phases in the KF-CsF-SbF₃-H₂O system. Analysis of ¹⁹F NMR spectra allowed tracing the dynamics of ion movement in the fluoride sublattice under temperature variations, determining their types and temperature ranges, in which they are implemented. The observed phase transitions in potassium-cesium fluoroantimonates(III) are phase transitions to the superionic state. It is found that the predominant form of ion movement in the high-temperature modifications formed as a result of phase transitions becomes diffusion of fluoride ions. According to the results of electrophysical studies the K_{1 − x} Cs _x SbF₄ (x ≤ 0.2) high-temperature phases are superionic. Their conductivity reaches the values of ∼10⁻² to 10⁻³ S/cm at 463–483 K. The high-temperature phases are stabilized under cooling, which results in a significant increase in conductivity at the room temperature.     

Effect of crystallographic structure on electrical and mechanical characteristics of Sm<Subscript>2</Subscript>O<Subscript>3</Subscript>-Doped CeO<Subscript>2</Subscript> films

Electrical conductivity, dielectric permittivity and mechanical hardness of the polycrystalline CeO₂ + xSm₂O₃ (x = 0, 10.9–15.9 mol %) films prepared by Electron Beam Physical Vapour Deposition (EB-PVD) and Ionic Beam Assisted Deposition, (IBAD), techniques were investigated in dependence on their structure and microstructure influenced by the deposition conditions, namely composition, deposition temperature and Ar⁺ ion bombardment. The electrical conductivity of doped ceria prepared without Ar⁺ ion bombardment and investigated by the impedance spectroscopy, IS, was found to be predominantly ionic one under the oxidizing atmosphere/low-temperature conditions and the higher amounts of Sm₂O₃ (>10 mol %) used. The bulk conductivity as a part of total measured conductivity was a subject of interest because the grain boundary conductivity was found to be ∼3 orders of magnitude lower than the corresponding bulk conductivity. Ar⁺ ion bombardment acted as a reducer (Ce⁴⁺ → Ce³⁺) resulting in the development of electronic conductivity. Dielectric permittivity determined from the bulk parallel capacitance measured at room temperature and the frequency of 1 MHz, similarly as the mechanical hardness measured by indentation (classical Vickers and Depth Sensing Indentation-DSI) techniques were also found to be dependent on the deposition conditions. The approximative value of hardness for the investigated films deposited on the substrate was estimated using a simple phenomenological model described by the power function HV = HV ₀ + aP ^b and compared with the so-called apparent hardness (substrate + investigated film) determined by the classical Vickers formula. Results obtained are analyzed and discussed.     

The influence of Li sources on physical and electrochemical properties of LiNi<Subscript>0.5</Subscript>Mn<Subscript>1.5</Subscript>O<Subscript>4</Subscript> cathode materials for lithium-ion batteries

LiNi_0.5Mn_1.5O₄ cathode materials were successfully prepared by sol–gel method with two different Li sources. The effect of both lithium acetate and lithium hydroxide on physical and electrochemical performances of LiNi_0.5Mn_1.5O₄ was investigated by scanning electron microscopy, Fourier transform infrared, X-ray diffraction, and electrochemical method. The structure of both samples is confirmed as typical cubic spinel with Fd3m space group, whichever lithium salt is adopted. The grain size of LiNi_0.5Mn_1.5O₄ powder and its electrochemical behaviors are strongly affected by Li sources. For the samples prepared with lithium acetate, more spinel nucleation should form during the precalcination process, which was stimulated by the heat released from the combustion of extra organic acetate group. Therefore, the particle size of the obtained powder presents smaller average and wider distribution, which facilitates the initial discharge capacity and deteriorates the cycling performance. More seriously, there exists cation replacement of Li sites by transition metal elements, which causes channel block for Li ion transference and deteriorates the rate capability. The compound obtained with lithium hydroxide exhibits better electrochemical responses in terms of both cycling and rate properties due to higher crystallinity, moderate particle size, narrow size distribution and lower transition cation substitute content.     

<Superscript>27</Superscript>Al NMR Study of the Metal Cluster Compound Al<Subscript>50</Subscript>C<Subscript>120</Subscript>H<Subscript>180</Subscript>

We present an ²⁷Al NMR study of the metal cluster compound Al₅₀Cp*₁₂ which is composed of (identical) Al₅₀ clusters, each surrounded by a Cp* ligand shell, and arranged in a crystalline 3D array (here Cp* = pentamethylcyclopentadienyl = C₅(CH₃)₅). The compound is found to be non-conducting, the nuclear spin-lattice relaxation in the temperature range 100–300 K being predominantly due to reorientational motions of the Cp* rings. These lead to a pronounced maximum in the relaxation rate at T ∼ 170 K, corresponding to an activation energy of about 850 K. Data for the related compound Al₄Cp*₄, containing very much smaller Al₄ clusters are also presented. A comparison is drawn with the quadrupolar relaxation recently observed for the non-conducting fraction of Ga₈₄ molecules in the metal cluster compound Ga₈₄[N(SiMe₃)₂]₂₀-Li₆Br₂(thf)₂₀·2toluene. It is our pleasure to dedicate this paper to our colleague professor Günter Schmid at the occasion of his 70th birthday.     

Synthesis and electrochemical studies on LiV<Subscript>3</Subscript>O<Subscript>8</Subscript>

The compound, lithium trivanadate (LiV₃O₈), was synthesized by the polymer precursor method, using the polymer polyvinylpyrrolidone. The electrochemical performance of LiV₃O₈ was compared with LiV₃O₈ synthesized by the solid state reaction method. The prepared compounds were characterized by X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy techniques. The electrochemical performances were studied by cyclic voltammetry and galvanostatic cycling in the voltage range of 2.0 to 4.0 V at room temperature (25 °C). The compound prepared by the polymer precursor method was found to have a good cycling stability. A reversible capacity value of 203 mAh/g (2.18 mol of Li) and 170 mAh/g (1.83 mol of Li) was obtained at the end of the 70th cycle, at a current density of 30 and 120 mA/g, respectively.     

Synthesis and charge-discharge performance of Li<Subscript>5</Subscript>SiN<Subscript>3</Subscript> as a cathode material of lithium secondary batteries

Li₅SiN₃ crystals are synthesized by direct reaction between Li₃N and Si₃N₄ with the molar ratio Li₃N/Si₃N₄ of 10:1. Reaction is performed at 1073 K for 1 h under a nitrogen atmosphere of 700 Torr. The lattice constant determined by the X-ray powder diffraction method is 4.718 Å. Four broad Raman peaks are observed at 196, 286, 580, and 750 cm^?1. By analogy with LiMgN, the broad peak at 580 cm^?1 with a half width of 140 cm^?1 is attributed to homogenously random distribution of Li and Si atoms. The band gap of Li₅SiN₃ is found to be a direct gap of about 2.5 eV by optical absorption and photoacoustic spectroscopy methods. Comparison with the conventional cathode materials for lithium ion batteries, this gap value is close to d-d transition energy of Mn in LiMn₂O₄ (1.63 eV or 2.00 eV) and that of Co in LiCoO₂ (2.1 eV), suggesting that Li₅SiN₃ is a possible cathode material. The 5 × 5 mm²-sized lithium secondary battery of Li₅SiN₃ cathode/propylene carbonate + LiClO₄ electrolyte/Li anode structure shows a discharge capacity of 2.4 μAh cm^?2 for a discharge current of 1.0 μA.     

Structure and lithium mobility of Li<Subscript>4</Subscript>Pt<Subscript>3</Subscript>Si

The lithium-rich silicide Li₄Pt₃Si was synthesised from the elements by high-temperature synthesis in a sealed niobium ampoule. Its structure was refined on the basis of single-crystal X-ray diffraction data: R32, a = 693.7(2), c = 1627.1(4) pm, wR2 = 0.0762, 525 F² values and 21 variables. The striking structural motifs of the Li₄Pt₃Si structure are silicon atoms with a slightly distorted trigonal prismatic platinum coordination with short Si–Pt distances (238–246 pm). Always two trigonal prisms are condensed via a common Pt₃ triangle, and these double units built up a three-dimensional network by condensation via common corners. The channels left by this prismatic network are filled by two crystallographically independent lithium sites in a 3:1 ratio. The single crystal X-ray data were fully confirmed by neutron powder diffraction and ⁷Li magic-angle spinning (MAS)–nuclear magnetic resonance (NMR) results. The two distinct lithium sites are well differentiated by their ⁷Li isotropic chemical shift and nuclear electric quadrupolar interaction parameters. MAS-NMR spectra reveal signal coalescence effects above 300 K, indicating chemical exchange between the lithium sites on the millisecond timescale. The spectra can be simulated with a simple two-site exchange model. From the resulting temperature-dependent correlation times, an activation energy of 50 kJ/mol is extracted.     

Superionic fluoride ceramics RF<Subscript>3</Subscript> and R<Subscript>0.95</Subscript>Sr<Subscript>0.05</Subscript>F<Subscript>2.95</Subscript> (R = La,Ce, Pr,Nd) prepared by hot pressing

RF₃ and R_0.95Sr_0.05F_2.95 (R = La, Ce, Pr, Nd) ceramic specimens were prepared by hot pressing at 1173 K under pressure of 3 × 10⁸ Pa for 20 min. The ionic conductivity value was determined by means of impedance spectroscopy in vacuum from 293 to 823 K. For LaF₃ at 350 K, the single crystal / ceramics conductivity ratio is about 5. The difference decreases at higher temperature and disappears about 500 K. The ionic conductivity activation energy is 0.30 ± 0.05 eV. For La_0.95Sr_0.05F_2.95, the conductivity of ceramics below 500 K is slightly lower that of single crystals. At T > 500 K, the conductivity values of ceramic and single crystal specimens practically coincide. The ionic conductivity of hot pressed ceramics is about 10^?2 S/cm at 500 K and activation energy is 0.25 ± 0.02 eV.     

Synthesis and electrochemical properties of yttrium-doped LiMn<Subscript>0.98</Subscript>Y<Subscript>0.02</Subscript>O<Subscript>2</Subscript> for lithium secondary batteries

Yttrium-doped lithium manganese oxide (LiMn_0.98Y_0.02O₂) was prepared by ion exchange of lithium for sodium in NaMn_0.98Y_0.02O₂ precursors obtained by using rheological phase reaction method. This material had small particle size, which was composed of grain size of about 100 nm. Especially, LiMn_0.98Y_0.02O₂ delivered the initial discharge capacity of about 191 mA h g⁻¹ at room temperature when cycled between 2.0 and 4.4 V vs Li/Li⁺. Moreover, it showed an excellent cycling behavior, its specific capacity remained above 173 mA h g⁻¹ after 20 cycles, and the material did not transform into spinel structure during the electrochemical cycling according to the cyclic voltammograms and X-ray powder diffraction. The electrochemical results revealed that the doping of Y³⁺ improved the performance of LiMnO₂ considerably.     

Ionic mobility in ammonium-rubidium heptafluorozirconate (NH<Subscript>4</Subscript>)<Subscript>2.4</Subscript>Rb<Subscript>0.6</Subscript>ZrF<Subscript>7</Subscript> by (<Superscript>1</Superscript>H, <Superscript>19</Superscript>F) NMR data

NMR (¹⁹F, ¹H) methods are used to study ionic mobility in heptafluorozirconate (NH₄)_2.4Rb_0.6ZrF₇ in a range of temperatures from 150 K to 430 K. Types of ionic movements are determined, and their activation energy is evaluated. As a result of a phase transition a modification forms in which diffusion in the ammonium sublattice and isotropic reorientations of ZrF ₇ ^3? complex anions are observed. According to preliminary data, due to diffusion of ammonium ions the compound has relatively high ionic conductivity (σ ≈ 8.3 × 10^?5 S/cm at 423 K).     

Structure and electric properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-NaPO<Subscript>3</Subscript> glasses

The influence of the SO₄²⁻ ion on the temperature and concentration dependences of electric conductivity and the structure of sodium phosphate oxide glasses was studied. The increased electric conductivity of sulfate-phosphate glasses was explained by the formation of mixed sulfate-phosphate fragments with terminal SO₄²⁻ ions in the structure of glasses in the Na₂SO₄-NaPO₃ system. The dissociation energies of the sodium sulfate fragments are lower than those of pure oxide sodium phosphate structural units. As a result, the number of dissociated sodium ions increases, the activation energy of electric conductivity falls, and the conductivity (at 25°C) increases approximately 270-fold relative to the conductivity of NaPO₃. The arrangement of SO₄²⁻ ions in the structure was evaluated from the IR spectra of the glasses.     

Ionic conductivity of bismuth oxofluoride BiO<Subscript>0.1</Subscript>F<Subscript>2.8</Subscript> with tysonite-type structure (LaF<Subscript>3</Subscript>)

The complex impedance method in the temperature range of 291–660 K was used to study conductivity of oxofluoride BiO_0.1F_2.8 belonging to the tysonite structural type (LaF₃). Bismuth oxofluoride was synthesized using a solid-phase method at 770–870 K for 1–2 h in an argon atmosphere. Heterovalent substitution of fluoride ions F⁻ by oxygen ions O²⁻ in the anionic BiF₃ matrix sublattice results in high ionic conductivity (∼0.1 S/cm at 660 K) of BiO_0.1F_2.8 ceramic samples.