Molybdenum carbide nano-sheet as a high capacity anode material for monovalent alkali metal-ion batteries—Theoretical investigation

This contribution presents a theoretical investigation of monovalent metal-ion adsorption and diffusion on two-dimensional (2D) buckled nanostructure of molybdenum carbide (MoC) by using the first principle method. We find that buckled MoC nanostructure exhibits great stability, semiconducting electronic property, and high performance as electrode material. Interestingly, Crystal Orbital Hamilton population (COHP) method results show that buckled MoC is chemically stable in a wide range of temperatures, and various Li, Na, ions adsorbed configurations, which is beneficial for anode materials. Especially, single-layer MoC exhibits a superior theoretical capacity of 993.16 mA h g⁻¹ for Li-ions and 496.58 mA h g⁻¹ for Na/K-ions. The storage capacity of 1200 mA h g⁻¹ is found for the adsorption of ions on multilayer bulk MoC. Moreover, migration energy barriers are predicted as 0.38 eV for Li, 0.32 eV for Na, and 0.24 eV for K; these remarkable results determine the applicability of buckled MoC as ideal anode material for metal-ion battery applications.     

Study of the temperature dependent transport properties in nanocrystalline lithium lanthanum titanate for lithium ion batteries

The nano-crystalline Li_0.5La_0.5TiO₃ (LLTO) was prepared as an electrolyte material for lithium-ion batteries by the sol–gel method. The prepared LLTO material is characterized by structural, morphological and electrical characterizations. The LLTO shows the cubic perovskite structure with superlattice formation. The uniform distribution of LLTO particles has been analyzed by the SEM and TEM analysis of the sample. Impedance measurements at various temperatures were carried out and the temperature dependent conductivity of as prepared LLTO nanopowders at different temperatures from room temperature to 448 K has been analyzed. The transport mechanism has been analyzed using the dielectric and modulus analysis of the sample. Maximum grain conductivity of the order of 10⁻³ S cm⁻¹ has been obtained for the sample at higher temperatures.     

Silver-coated LiVPO4F composite with improved electrochemical performance as cathode material for lithium-ion batteries

Nano-structured LiVPO₄F/Ag composite cathode material has been successfully synthesized via a sol–gel route. The structural and physical properties, as well as the electrochemical performance of the material are compared with those of the pristine LiVPO₄F. X-ray diffraction (XRD) and scanning electron microscopy (SEM) reveal that Ag particles are uniformly dispersed on the surface of LiVPO₄F without destroying the crystal structure of the bulk material. An analysis of the electrochemical measurements show that the Ag-modified LiVPO₄F material exhibits high discharge capacity, good cycle performance (108.5 mAh g⁻¹ after 50th cycles at 0.1 C, 93% of initial discharge capacity) and excellent rate behavior (81.8 mAh g⁻¹ for initial discharge capacity at 5 C). The electrochemical impedance spectroscopy (EIS) results reveal that the adding of Ag decreases the charge-transfer resistance (R_ct) of LiVPO₄F cathode. This study demonstrates that Ag-coating is a promising way to improve the electrochemical performance of the pristine LiVPO₄F for lithium-ion batteries cathode material.     

Bulk and surface properties of ionic salt CsH5(PO4)2

We report a comparative study of transport and thermodynamic properties of single-crystal and polycrystalline samples of the ionic salt CsH₅(PO₄)₂ possessing a peculiar three-dimensional hydrogen-bond network. The observed potential of electrolyte decomposition ≈ 1.3 V indicates that the main charge carriers in this salt are protons. However, in spite of the high proton concentration, the conductivity appears to be rather low with a high apparent activation energy E_a ≈ 2 eV, implying that protons are strongly bound. The transport anisotropy though is not large, correlates with the crystal structure: the highest conductivity is found in the [001] direction (σ_130 °C ∼ 5.6 × 10^− 6 S cm^− 1) while the minimal conductivity is in the [100] direction (σ_130 °C ∼ 10 ⁻⁶ S cm^− 1). The conductivity of polycrystalline samples appears to exceed the bulk one by 1–3 orders of magnitude with a concomitant decrease of the activation energy (E_a ≈ 1.05 eV), which indicates that a pseudo-liquid layer with a high proton mobility is formed at the surface of grains. Infrared and Raman spectroscopy used to study the dynamics of the hydrogen-bond system in single-crystal and polycrystalline samples have confirmed the formation of such a modified surface layer in the latter. However, no bulk phase transition into the superionic disordered phase is observed in CsH₅(PO₄)₂ up to the melting point T_melt ∼ 151.6 °C, in contrast to its closest relative compound CsH₂PO₄.     

Mesopore size dependence of the ionic diffusivity in alumina based composite lithium ionic conductors

Ordered-mesoporous Al₂O₃ was synthesized by a sol–gel method using neutral copolymer surfactants as structure-directing agents. The pore size was controlled over the 3–15 nm range by the use of various surfactants. Composites composed of the synthesized mesoporous Al₂O₃ and a lithium ion conductor (LiI) were prepared. The maximum dc electrical conductivity, 2.6 × 10^− 4 S cm^− 1 at 298 K, was observed for 50 LiI·50 Al₂O₃ composite with 4.2 nm average mesopore size, which was considerably higher than the previously reported LiI-alumina composites. A systematic dependence of conductivity upon pore size was observed, in which conductivity increased with decreasing pore size, except for samples with a pore size of 2.8 nm. The lithium ion diffusion coefficient determined by the ⁷Li pulsed field gradient nuclear magnetic resonance (PFG-NMR) showed excellent agreement with the measured conductivity calculated by the Nernst-Einstein equation. On the other hand, lithium migration activation energies obtained by quasielastic neutron scattering (QENS) and ⁷Li NMR spin-lattice relaxation time (T₁) were considerably smaller than those obtained from electrical conductivity and PFG-NMR. This could be explained by the ion migration mechanism in heterogeneous composites and a possible enhancement of conductivity in mesoscopically confined spaces.     

High lithium ion conductivity glass-ceramics in Li2O–Al2O3–TiO2–P2O5 from nanoscaled glassy powders by mechanical milling

Mechanical milling (MM) has been used to prepare the nanosized Li_1.4Al_0.4Ti_1.6(PO₄)₃ (denoted LATP) glassy powders, which was converted into glass-ceramics through thermal treating at 700–1000 °C. The XRD, TEM, FESEM and ac impedance techniques were used to characterize the products. The results showed that completely amorphous products were prepared by MM for 40 h, and single-phase LiTi₂(PO₄)₃-type structured glass-ceramics were obtained by further heat treatment. The lithium ion conductivity of the glass-ceramics increased with the growth of the crystalline phase and decrease of the grain size. The highest bulk conductivity (σ_b) of 1.09 × 10^− 3 S cm^− 1 with an energy of activation as low as 0.28 eV was obtained at room temperature for the specimen treated at 900 °C for 6 h. The high conductivity, easy fabrication and low cost make the LATP glass-ceramics promising to be used as inorganic solid electrolyte for all-solid-state Li-ion rechargeable batteries.     

Conductivity and thermal studies of blend polymer electrolytes based on PVAc–PMMA

The polymer electrolytes comprising blend of poly(vinyl acetate) (PVAc) and poly(methylmethacrylate) (PMMA) as a host polymer and LiClO₄ as a dopant are prepared by solution casting technique. The amorphous nature of the polymer–salt complex has been confirmed by XRD analysis. The DSC thermograms show two T_g's for PVAc–PMMA blend. A decrease in T_g with the LiClO₄ content reveals the increase of segmental motion. Conductance spectra results are found to obey the Jonscher's power law and the maximum dc conductivity value is found to be 1.76 × 10^− 3 S cm^− 1 at 303 K for the blend polymer complex with 20 wt.% LiClO₄, which is suitable for the Li rechargeable batteries. The conductivity–temperature plots are found to follow an Arrhenius nature. The dc conductivity is found to increase with increase of salt concentration in the blend polymer complexes.     

Structural and optical properties of La-doped BaSnO3 thin films grown by PLD

In this study the structural and optical properties of lanthanum-doped BaSnO₃ powder samples and thin films deposited on fused silica were investigaed using laser ablation. Under an oxygen pressure of 5×10⁻⁴ mbar, phase pure BaSnO₃ films with a lattice constant of 0.417 nm and grain size of 21 nm were prepared at 630 °C. The band gap of BaSnO3 powder sample and thin films was calculated to be 3.36 eV and 3.67 eV, respectively. There was a progressive increase in conductivity for thin films of BaSnO₃ doped with 0~7 at% of La. The highest conductivity, 9 Scm⁻¹, was obtained for 7 at% La-doped BaSnO₃. Carrier concentration, obtained from Burstein-Moss (B-M) shift, nearly matches the measured values except for 3 at% and 10 at% La-doped BaSnO₃ thin films.     

Strongly anisotropic thermal conductivity in planar hexagonal borophene oxide sheet

The flat hexagonal borophene oxide (B₂O) has the highest Li storage capacity among existing two-dimensional materials. Thermal conductivity is an important parameter for the safety of Li-ion batteries. We investigate the lattice thermal conductivity of B₂O by solving phonon Boltzmann transport equation combined with the first-principles calculations. We found that the relaxation time approximation remarkably underestimate the thermal conductivity (κ) of monolayer B₂O, revealing phonon hydrodynamics characteristic. The κ of B₂O from the exact solution of Boltzmann transport equation is 53 W m⁻¹ K⁻¹ and 130 W m⁻¹ K⁻¹ along armchair-direction and zigzag-direction at 300 K, respectively. B₂O exhibits strong thermal transport anisotropy due to anisotropic phonon group velocity, obviously larger than that of other borophene allotropes. At room temperature, the phonon mean free path of B₂O is about 231 nm and 49 nm along armchair-direction and zigzag-direction, respectively. The highly anisotropic thermal conductivity of B₂O offers new possibilities for its applications in thermal management.     

Electronic conductivity and chemical diffusion in n-conducting barium titanate ceramics at high temperatures

The electronic conductivity as well as the chemical diffusion coefficient of barium titanate ceramics doped with Y and Mn (donor-doped and acceptor co-doped) have been determined by application of conductivity relaxation experiments. The equilibrium values of the electronic conductivity of n-conducting BaTiO₃ have been analyzed by application of a defect chemical model involving electrons and cation vacancies as the predominant defect species at oxidizing conditions (fairly high oxygen partial pressures). The relaxation curves of the electronic conductivity yield the chemical diffusion coefficient of the bulk by employing a spherical grain model where the appropriate diffusion length is the radius of grains (average grain size). The conductivity relaxation experiments have been performed as a function of temperature ranging from 1100 to 1250 °C at oxygen partial pressures between 0.01 and 1 bar. The kinetics of the oxygen exchange process can be interpreted in terms of extremely fast diffusion of oxygen via oxygen vacancies along the grain boundaries and slow diffusion of Ti (cation)-vacancies from the grain boundaries into the grains. The Ti-vacancy diffusion coefficients were extracted from the chemical diffusion coefficients as a function of temperature. Typical values for the Ti-vacancy diffusivity are around 10^− 15 cm² s^− 1 with an activation energy of 3.9 ± 0.7 eV.     

Inorganic polymer phosphazene disulfide as cathode material for rechargeable lithium batteries

Novel inorganic network polymer phosphazene disulfide [(NPS₂)₃]_n was synthesized by a solution cross-link method. IR and element content analysis confirmed the polymer's molecular structure. The polymer has an average particle size of d_0.5 = 7.7 μm and the specific surface area is 57.4 m² g^− 1. TG/DTA analysis showed that [(NPS₂)₃]_n underwent a decomposition reaction from 200 to 300 °C. When used as cathode material in lithium batteries, its initial discharge capacity was 459.1 mAh g^− 1, which is almost 93.5% of theoretical specific capacity (490.9 mAh g^− 1). After 30 charge–discharge cycles, the discharge capacity of [(NPS₂)₃]_n stabilized at approximately 400.1 mAh g^− 1 which revealed an excellent cyclic ability. Therefore [(NPS₂)₃]_n is of great potential as cathode material for secondary lithium batteries.     

Ionic and electronic conductivity of nitrogen-doped YSZ single crystals

The ionic and electronic charge transport was studied for single crystals of 9.5 mol% yttria-stabilized zirconia with additional nitrogen doping (YSZ:N) of up to 7.5 at.% (referred to the anion sublattice and formula unit Zr_0.83Y_0.17O_1.91) as a function of temperature and nitrogen content. The total conductivity being almost equivalent to the oxygen ion conductivity has been measured by AC impedance spectroscopy under vacuum conditions in order to prevent re-oxidation and loss of nitrogen. The electronic conductivity has been determined by Hebb–Wagner polarization using ion-blocking Pt microelectrodes in N₂ atmosphere. The ionic conductivity of YSZ:N decreases in the presence of nitrogen at intermediate temperatures up to 1000 °C. The mean activation energy of ionic conduction strongly increases with increasing nitrogen content, from 1.0 eV for nitrogen-free YSZ up to 1.9 eV for YSZ containing 7.3 at.% N. Compared to nitrogen-free YSZ, the electronic conductivity first decreases at nitrogen contents of 2.17 and 5.80 at.%, but then increases again for a sample with 7.53 at.%. At temperatures of 850 °C and above, the presence of the N^3? dopant fixes the electrode potential and thus the oxygen partial pressure at the Pt electrode to very low values. This corresponds to a pinning of the Fermi level at a relatively high energy in the upper half of the band gap. At 7.53 at.% N and 950 °C, the oxygen partial pressure in YSZ:N corresponds to p_O2 = 3 × 10^? 18 bar. At temperatures above 850 °C, even in the presence of a very small oxygen concentration in the surrounding gas phase, the nitrogen ion dopant becomes highly mobile and thus diffuses to the surface where it is oxidized to gaseous N₂. The results are discussed in terms of the ionic and electronic defect structures and the defect mobilities in YSZ:N.     

Super-protonic phase transition and fast ionic conductivity of Li+ in [Li0.2(NH4)0.8]2TeCl6

The differential scanning calorimetry diagram of [Li_0.2(NH₄)_0.8]₂TeCl₆ showed one anomaly at 526 K accompanied with a shoulder at 505 K.The conductivity plot exhibits two anomalies at 496 and 526 K, which characterize the beginning and the end of the crossing to superionic conductor state. The low temperature conduction is ensured essentially by Li⁺. A sudden jump confirms the presence of a superionic protonic transition related to the fast motion of Li⁺ and H⁺ ions. Above 526 K, the high temperature phase is characterized by high electrical conductivity (10^− 3 Ω^− 1 m^− 1) and low activation energy (E_a < 0.3 eV).The dielectric constant evolution as a function of frequency and temperature revealed the same anomaly.Transport properties in this material appear to be due to Li⁺ and H⁺ ions' hopping mechanism.     

Kinetic studies of electronically conducting Yb3Al5O12 garnet

Upon reduction, originally fully transparent and insulating ytterbium alumina garnet single crystals, Yb₃Al₅O₁₂, become deeply colored and electrically conducting with a conductivity of the order of 10^− 3 Ω^− 1 cm^− 1 in the temperature range of 550 °C to 1000 °C. The redox kinetics of the material is studied by means of conductivity relaxation experiments performed at oxidising and reducing conditions. Good agreement is obtained with an optical study into the redox kinetics of Yb₃Al₅O₁₂.     

Oxygen nonstoichiometry and transport properties of strontium substituted lanthanum cobaltite

Oxygen nonstoichiometry, structure and transport properties of the two compositions (La_0.6Sr_0.4)_0.99CoO_3−δ (LSC40) and La_0.85Sr_0.15CoO_3−δ (LSC15) were measured. It was found that the oxygen nonstoichiometry as a function of the temperature and oxygen partial pressure could be described using the itinerant electron model. The electrical conductivity, σ, of the materials is high (σ > 500 S cm^− 1) in the measured temperature range (650–1000 °C) and oxygen partial pressure range (0.209–10^− 4 atm). At 900 °C the electrical conductivity is 1365 and 1491 S cm^− 1 in air for LSC40 and LSC15, respectively. A linear correlation between the electrical conductivity and the oxygen vacancy concentration was found for both samples. The mobility of the electron-holes was inversely proportional with the absolute temperature indicating a metallic type conductivity for LSC40. Using electrical conductivity relaxation the chemical diffusion coefficient of oxygen was determined. It was found that accurate values of the chemical diffusion coefficient could only be obtained using a sample with a porous surface coating. The porous surface coating increased the surface exchange reaction thereby unmasking the chemical diffusion coefficient. The ionic conductivity deduced from electrical conductivity relaxation was determined to be 0.45 S cm^− 1 and 0.01 S cm^− 1 at 1000 and 650 °C, respectively. The activation energy for the ionic conductivity at a constant vacancy concentration (δ = 0.125) was found to be 0.90 eV.     

Synthesis and characterization of (Pr0.75Sr0.25)1 − xCr0.5Mn0.5O3 − δ as anode for SOFCs

The complex perovskite (Pr_0.75Sr_0.25)_1 − xCr_0.5Mn_0.5O_3 − δ (PSCM) has been prepared and studied as possible anode material for high-temperature solid oxide fuel cells (SOFCs). PSCM exhibits GdFeO₃-type structure and is both physically and chemically compatible with the conventional YSZ electrolyte. The reduction of PSCM resulted in structural change from orthorhombic Pbnm to cubic Pm-3m. Selected area electron diffraction (SAED) analysis on the reduced phases indicated the presence of a √2 × √2 × 2 superlattice. The total conductivity values of ∼ 75% dense Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ at 900 °C in air and 5% H₂/Ar are 9.6 and 0.14 S cm^− 1 respectively. The conductivity of PSCM drops with decreasing Po₂ and is a p-type conductor at all studied Po₂. The average TEC of Pr_0.75Sr_0.25Cr_0.5Mn_0.5O_3 − δ is 9.3 × 10^− 6 K^− 1, in the temperature range of 100–900 °C and is close to that of YSZ electrolyte. The anode polarization resistance of PSCM in wet 5%H₂ is 1.31 Ω cm² at 910 °C and in wet CH₄ at 930 °C; the polarization resistance is 1.29 Ω cm². PSCM was unstable at 900 °C in unhumidified hydrogen. Cell performance measurements carried out using graded PSCM and La_0.8Sr_0.2MnO₃ as anode and cathode respectively yielded a maximum power density of 0.18 W cm^− 2 in wet 5%H₂/Ar at 910 °C and the corresponding current density was 0.44 A cm^− 2 at 0.4 V. The activation energy for the electrochemical cell operating in wet (3% H₂O) 5%H₂/Ar fuel is 85 kJ mol^− 1.     

Electrical and dielectric properties of β-BaB2O4 (BBO) and CsLiB6O10 (CLBO) crystals

In barium borate (BBO) crystals, sodium and potassium ions, inherited due to the preparation technique, are dominant charge carriers. The conductivity between layers is higher; the conductivity activation energy and the conductivity at 350 °C being equal to 1.01±0.05 eV and (1.3±0.2)×10⁻⁸ S/cm, respectively. The conductivity activation energy and the conductivity at 350 °C along the channels are equal to 1.13±0.05 eV and to (4±0.2)×10⁻⁹ S/cm, respectively. Relative static permittivity is almost isotropic, and equal to 7.65±0.05. Upon storing of cesium–lithium borate (CLBO) crystals, pre-heating to 600 °C eliminates the influence of surface humidity. At 500 K, the ionic conductivity ranges from 4×10⁻¹² to 2×10⁻¹⁰ S/cm; the conductivity activation energy ranges from 1.01 to 1.17 eV. Relative static permittivity is equal to 7.4±0.3.     

Characterization of LSGM films obtained by electrophoretic deposition (EPD)

The application of the electrophoretic deposition (EPD) technique to the preparation of high quality electrolyte films for intermediate temperature solid oxide fuel cells (IT-SOFCs) was investigated. Films of La_0.83Sr_0.17Ga_0.83Mg_0.17O_2.83 (LSGM) were deposited on Pt and La_0.8Sr_0.2MnO₃ (LSM) substrates from suspensions in acetone/ethanol (3:1 by volume) mixture solvent and sintered at 1300 °C. Pt supported LSGM films, 10–20 μm thick, exhibited good adhesion to the Pt substrate, well-distributed microporosity and some surface roughness. LSM supported films exhibited cracking after sintering at 1300 °C for 3 h. Up to 900 °C the bulk conductivity of the Pt supported LSGM film showed the same behaviour of LSGM pellet (E_a = 0.93 eV and 0.99 eV, respectively). The LSGM film exhibited lower bulk electrical conductivity than the latter (4.1 × 10^− 3 and 4.4 × 10^− 2 Ω^− 1 cm^− 1, respectively, at 700 °C). This difference should be ascribed to the slight Ga depletion in the LSGM film. An important issue remains the selection of adequate electrode for LSGM electrolyte films.     

Transport and phase dynamics of poly(vinyl pyrrolidone) doped plastically crystalline N-methyl-N-propylpyrrolidinium tetrafluoroborate

The plastic crystal phase forming N-methyl-N-propylpyrrolidinium tetrafluoroborate organic salt (P₁₃BF₄) was combined with 2, 5 and 10 wt.% poly(vinyl pyrrolidone) (PVP). The ternary 2 wt.% PVP/2 wt.% LiBF₄/P₁₃BF₄ was also investigated. Thermal analysis, conductivity, optical thermomicroscopy, and Nuclear Magnetic Resonance (¹¹B, ¹⁹F, ¹H, ⁷Li) were used to probe the fundamental transport processes. Both the onset of phase I and the final melting temperature were reduced with increasing additions of PVP. Conductivity in phase I was 2.6 × 10^− 4 S cm^− 1 5.2 × 10^− 4 S cm^− 1 1.1 × 10^− 4 S cm^− 1 and 3.9 × 10^− 5 S cm^− 1 for 0, 2, 5 and 10 wt.%PVP/P₁₃BF₄, respectively. Doping with 2 wt.% LiBF₄ increased the conductivity by up to an order of magnitude in phase II. Further additions of 2 wt.% PVP slightly reduced the conductivity, although it remained higher than for pure P₁₃BF₄.     

Synthesis,crystal structure,spectroscopic characterization and optical properties of bis(4-acetylanilinium) tetrachlorocobalt (II)

The chemical preparation, crystal structure, spectroscopic investigations and optical features are given for a novel organic–inorganic hybrid material [C₈H₁₀NO]₂CoCl₄.The compound is crystallized in the orthorhombic space group Cmca, with the following unit cell parameters: a=19.461(2) Å, b=15.523(2) Å, c=13.7436(15) Å, and Z=8. The atomic arrangement shows an alternation of organic and inorganic layers along the b-axis. The cohesion between these entities is performed by N–H…Cl and N–H…O hydrogen bonds and π–π stacking interactions.Infrared and Raman spectra at room temperature are recorded in the 4000−400 and 4000−0 cm⁻¹ frequency regions, respectively and analyzed on the basis of literature data. This study confirms the presence of the organic cation [C₈H₁₀NO]⁺ and of the [CoCl₄]²⁻ anion. UV–vis spectroscopy results showed the indirect transition with band gap energy 2.98 eV.