xO2 Materials Research Articles

Substituting Na for Excess Li in Li1+x(Ni0.6Mn0.4)1−xO2 Materials

NMC640, a series of Li1+x(Ni0.6Mn0.4)1−xO2 materials, are important Co-free mid-Ni cathode materials for Li-ion batteries, offering high energy density and better cost-efficiency than Ni-rich counterparts. These materials require excess Li compared to stoichiometric composition to improve the electrochemical performance in terms of rate capability and cycling stability. Although lithium-to-transition metal ratios up to 1.15 can be used to optimize the performance, less than 80% of this lithium is electrochemically active during cycling up to a 4.4 V upper cut off. This study explores whether some percentage of the inactive Li can be replaced by sodium to make these materials more cost-effective and bring potential improvements in electrochemical performance. Various amounts of excess Li were substituted by sodium in the structure. The results show that sodium can be integrated into the layered oxide structure without forming any impurity phases and effectively decreases the cation mixing observed in these layered structures. However, this does compromise cycling stability and rate capability. Na tends to occupy Li sites rather than transition metal sites, resulting in electrochemical instability and capacity loss. Even though excess Li is not electrochemically active, it cannot be effectively replaced by sodium without compromising battery performance of Li1+x(Ni0.6Mn0.4)1−xO2 materials.

Impact of Cobalt Addition on Single-Crystal Li1+x(Ni0.6Mn0.4)1−xO2 Cathode Material Performance

Nickel and manganese-based layered oxides with a nickel content ranging from 50% to 80% are promising cathode materials for high-energy density lithium-ion batteries. However, these materials face challenges such as poor rate capability and limited cycling stability. The addition of excess lithium can mitigate these issues to some extent. This study examines the impact of incorporating small amounts of cobalt (5% or 10%) into these materials through an “all-dry” synthesis approach in stoichiometric and excess lithium-containing compositions. Results indicate that adding even these small amounts of cobalt decreases the cation mixing, improves crystallinity, reduces electronic resistance, and influences the morphology depending on whether nickel or manganese is replaced. The materials can accommodate up to 15% excess lithium without significant surface impurities. The addition of cobalt further enhances the rate capability of the material in excess lithium materials, but increasing cobalt content tends to compromise cycling stability when the materials are cycled up to 4.4 V. Materials in which 5% cobalt replaces nickel still exhibit superior rate capability and cycling performance compared to materials without cobalt. Therefore, incorporating small amounts of cobalt can positively impact the performance of Li1+x(Ni0.6Mn0.4)1−xO2 materials, offering a balance between improved rate capability and cycling stability.

Mg-doped Na[Ni1/3Fe1/3Mn1/3]O2 with enhanced cycle stability as a cathode material for sodium-ion batteries

O3-type Na[Ni1/3Fe1/3Mn1/3]O2 (NaNFM) is considered as a promising cathode material for sodium-ion batteries; however, its poor cycling stability is still a concern. In this study, we discuss the structural, surface and electrochemical properties of Mg-doped NaMgx[Ni1/3Fe1/3Mn1/3]1–xO2 materials and their enhanced cycling performance. The variations of the lattice parameters by substitution of Mg ion and its uniform distribution on the particles are confirmed using X-ray diffraction and transmission electron microscopy. The optimized NaMg0.05[Ni1/3Fe1/3Mn1/3]0.95O2 delivers a discharge capacity of ~120 mAh g−1 and has a diffusion coefficient of Na ranging from 6.5 × 10−13 to 2.7 × 10−10 cm2 s−1. In particular, it shows a relatively high discharge capacity of 42 mAh g−1 even at a high current density of 1200 mA g−1 and exhibits considerably enhanced cycling stability (77% capacity retention after 50 cycles), compared with that of the undoped NaNFM (40%). Based on structural and electrochemical analyses, it is suggested that Mg doping can effectively suppress the irreversible structural degradation and induce more reversible phase transitions; this results in a more stable cycling performance of the Mg-doped NaNFM than that of undoped NaNFM.

Wet synthesis route of Li1+xV1−xO2 for lithium-ion batteries

Li1+xV1−xO2 samples with different ratios of Li/V (1.12 (x = 0.056), 1.17 (x = 0.078), 1.22 (x = 0.1), 1.27 (x = 0.118), and 1.32 (x = 0.138) are synthesized via wet synthesis route followed by a high temperature treatment. As anode materials for lithium-ion batteries, the electrochemical performances of as-synthesized Li1+xV1−xO2 materials are investigated by galvanostatic charge–discharge test. The Li1+xV1−xO2 compound synthesized with the ratio of Li/V = 1.32 possesses the optimal performance, delivering an initial discharge capacity of 183.8 mAh g−1 between 0.005 V and 3 V at a current density of 0.2C, and exhibiting a good cycling stability over 20 cycles.

Influences of W Content on the Phase Transformation Properties and the Associated Stress Change in Thin Film Substrate Combinations Studied by Fabrication and Characterization of Thin Film V1- xW xO2 Materials Libraries.

The mechanical stress change of VO2 film substrate combinations during their reversible phase transformation makes them promising for applications in micro/nanoactuators. V1- xW xO2 thin film libraries were fabricated by reactive combinatorial cosputtering to investigate the effects of the addition of W on mechanical and other transformation properties. High-throughput characterization methods were used to systematically determine the composition spread, crystalline structure, surface topography, as well as the temperature-dependent phase transformation properties, that is, the hysteresis curves of the resistance and stress change. The study indicates that as x in V1- xW xO2 increases from 0.007 to 0.044 the crystalline structure gradually shifts from the VO2 (M) phase to the VO2 (R) phase. The transformation temperature decreases by 15 K/at. % and the resistance change is reduced to 1 order of magnitude, accompanied by a wider transition range and a narrower hysteresis with a minimal value of 1.8 K. A V1- xW xO2 library deposited on a Si3N4/SiO2-coated Si cantilever array wafer was used to study simultaneously the temperature-dependent stress change σ( T) of films with different W content through the phase transformation. Compared with σ( T) of ∼700 MPa of a VO2 film, σ( T) in V1- xW xO2 films decreases to ∼250 MPa. Meanwhile, σ( T) becomes less abrupt and occurs over a wider temperature range with decreased transformation temperatures.

Microstructure-performance study of cerium-doped TiO2 prepared by using pressurized fluids in photocatalytic mitigation of N2O

Parent TiO2 and cerium-doped TiO2 photocatalysts with various Ce molar loadings (Ce0.05Ti0.95O2, Ce0.30Ti0.70O2) were prepared unconventionally using the sol–gel process controlled within the reverse micelles and the processing by pressurized hot fluids as an alternative to standard calcination. Purity, textural, micro/structural and optical properties of prepared photocatalysts were characterized by organic elementary analysis, nitrogen physisorption, powder X-ray diffraction, and DRS UV–Vis spectroscopy. The activity of developed cerium-doped TiO2 catalysts was investigated in the photocatalytic decomposition of N2O for the first time. It was revealed that photocatalysts processed by pressurized hot fluids show significantly improved textural properties and different crystallinity compared to their calcined analogues. Ce loading as well as the processing procedure had a key effect on the crystallization of CexTi1−xO2 materials. The maximum N2O conversion (77 % after 20 h of illumination) in inert gas was reached over the Ce0.05Ti0.95O2 photocatalyst and it can be attributed to the simultaneous N2O photocatalytic decomposition and N2O photolysis. Reaction kinetics of N2O photocatalytic decomposition was described well by the pseudo-first-rate law. The lower photocatalytic activity of the Ce0.30Ti0.70O2 photocatalyst compared to the Ce0.05Ti0.95O2 photocatalyst can be correlated with two factors; the lower disorder within anatase crystal structure and possible presence of some cerium-containing amorphous phase (e.g., CeO2 or CeTi2O6), which can partially block the surface active sites within the anatase crystal structure.

Structural and transport properties of Li1+xV1−xO2 anode materials for Li-ion batteries

Recently, layered Li1 + xV1 − xO2 with x ≥ 0 has attracted significant attention as an anode material for Li-ion batteries. Its high volumetric and gravimetric capacities (1360 mAh·cm− 3 and 300 mAh·g− 1 respectively) make it particularly interesting. During lithium intercalation Li1 + xV1 − xO2 with x > 0 exhibits wide potential plateau below 0.1 V vs. Li/Li+, while for stoichiometric LiVO2 lithium uptake hardly occurs. In this work evaluation of crystal structure, transport and electrochemical properties is given for Li1 + xV1 − xO2 materials with x = 0, 0.03 and 0.07. Li1 + xV1 − xO2 showed activated character of conductivity with activation energy about 0.2–0.3 eV. Thermoelectric power values exceeding 80 μV·K− 1 point to electron holes as the main charge carriers. Thermogravimetric measurements carried out in air indicated only minor variations of mass of the materials up to 215 °C, suggesting high stability along with low level of oxygen nonstoichiometry.

Core level spectroscopy of oxidized and reduced CexU1−xO2 materials

A series of CexU1−xO2 has been studied by core level spectroscopy, where x=0, 0.25, 0.5, 0.75, 0.95, and 1. These materials have shown potential for thermal generation of hydrogen from water with better activity than CeO2 alone. In addition, Ce0.5U0.5O2 was also studied by in situ XRD as a function of temperature (from 300 to 973K). The fresh material had the fluorite structure with crystallite size of about 15nm. The lattice spacing is typical of complete miscibility of Ce and U cations in the fluorite structure. Phase segregation, of the uranium oxide, when the temperature was raised to 500K and above was seen for the fresh material. The phase segregated uranium oxide is tentatively attributed to α-U3O8 (although it can be attributed to the hexagonalα−phase or the orthorhombic phase of U2O5). The used Ce0.5U0.5O2, composed of larger crystallite size (about 60nm), was more stable with negligible phase segregation.Study of the XPS U4f regions indicated the presence of three types of U cations (U6+, U5+, and U4+). U6+ cations are mainly present in the fresh material with high % independent of U concentration (XPS U4f7/2 binding energy at 381.1eV and associated pairs of satellites at 4.5 and 10.0eV above each split). Upon sputtering with Ar ions, a large fraction of these U6+ cations are reduced to U5+(XPS U4f7/2 binding energy at 380.4eV and satellite at 8.2eV above each split line) then to U4+ cations (XPS U4f7/2 at 379.9eV and its typical satellite at 6.9eV above). After five minutes Ar ion sputtering the CexU1−xO2 materials were mainly composed of U4+ cations. On the other hand, XPS Ce3d lines indicated the presence of some contribution of Ce3+ cation in addition to lines characteristic of Ce4+ cations in the as prepared materials. Upon reduction considerable amount of Ce4+ is reduced to Ce3+ (up to 75%) in the mixed oxides while CeO2 alone showed very mild reduction. This is interpreted as due to charge transfer from the U cations further reducing Ce4+ cations. Valence band analyses corroborated the core level results with the reduced U containing mixed oxide showing large contribution of U5f/Ce4f lines while CeO2 alone has small Ce4f contribution even upon Ar ion sputtering.

Composition-Dependent Performance of CexZr1–xO2Mixed-Oxide-Supported WO3Catalysts for the NOxStorage Reduction–Selective Catalytic Reduction Coupled Process

WO3/CexZr1–xO2 materials were evaluated as a possible NH3-selective catalytic reduction (SCR) active catalyst in a NOx storage reduction (NSR) + SCR combined system. The effect of the support composition was investigated at a constant WO3 loading (9.1 wt % of WO3). The impact of WO3 promotion over textural, structural, acid–base, and redox properties of SCR samples was characterized by means of nitrogen adsorption–desorption isotherms, XRD, NOx storage capacity, NH3 temperature programmed desporption, pyridine adsorption followed by FTIR, and H2-TPR. Catalytic activities in NH3-SCR and NH3-SCO reactions as well as corresponding kinetics parameters are also discussed. All WO3/Ce–Zr materials are active and fully selective in N2 for NOx reduction by NH3 and ammonia oxidation by O2. For the SCR reaction, the rate of NO conversion is found approximately half-order with respect to NO, and negative to nearly zero-order with respect to NH3. Tungstated ceria–zirconia materials were then associated downstream with...

Preparation and characterization of Ce1−x(Gd0.5Pr0.5)xO2 electrolyte for IT-SOFCs

The Ce1−x(Gd0.5Pr0.5)xO2 (x = 0–0.24) compositions were synthesized through the sol–gel process followed by low temperature combustion. X-ray diffraction data analysis showed that all the samples exhibit a cubic structure with single phase. The lattice parameter was calculated by rietveld refinement of XRD patterns. Dense ceramics were prepared by sintering the pellets at 1300 °C. The relative density of the samples was over 98%. The surface morphology was studied by Scanning electron microscopy (SEM). Chemical composition was analyzed by Energy dispersive spectroscopy (EDX). A.C. impedance spectroscopy measurements were carried out to study the grain, grain boundary and total ionic conductivity of co-doped ceria samples in the temperature range 150–700 °C. The Ce0.84(Gd0.5Pr0.5)0.16O2 composition showed highest grain ionic conductivity i.e., 1.059 × 10−2 S/cm at 500 °C which is 11.5% higher than the Ce0.9Gd0.1O2 (with an activation energy 0.62 eV). At intermediate temperatures, the Ce1−x(Gd0.5Pr0.5)xO2 materials were found to be ionic in nature.

Impact of Excess Li on the Structural and Electrochemical Properties of Li1+x(NizCo1-2zMnz)1-xO2 Materials for Lithium-Ion Batteries

LiNizCo1−2zMnzO2 materials have been investigated as promising positive electrode materials for lithium-ion secondary batteries, in which cobalt ions are simultaneously substituted with nickel and manganese ions while preserving the α-NaFeO2 structure. The materials can be regarded as solid solutions of LiNiO2, LiCoO2, and LiMnO2, in which the transition metals are trivalent in all three materials, whereas the transition metals found in LiNizCo1−2zMnzO2 materials are Ni2+, Co3+, and Mn4+. The electrochemical lithium deintercalation-intercalation processes of these materials are mainly accompanied by the Ni2+/4+ and Co3+/4+ redox reactions, whereas Mn4+ ions do not participate in the electrochemical redox reaction. Recently, Li+ substitution for transition metals in Li1+x(NizCo1−2zMnz)1−xO2, so called overlithiation, has been introduced to improve their structural integrity and electrochemical performances.In this paper, we introduce the overlithiation effects on the structural and electrochemical properties of the Li1+x(NizCo1−2zMnz)1−xO2 materials having diffrrent x- and z-values in the ranges from x=0.00 to x=0.17 and z=0.00 to z=0.50.

Structure and Electrochemical Performances of the Li1+x(Ni0.425Mn0.425Co0.15)1−xO2 Materials

ABSTRACTLayered Li1+x(Ni0.425Mn0.425Co0.15)1−xO2 materials (x = 0 and 0.12) have been prepared by a coprecipitation method. The substitution of × Li+ ions for × metal ions in the transition metal layers leads to an increase in the average transition metal oxidation state for charge compensation, which was found to be due to the oxidation of Ni2+ ions into Ni3+ ions. The refinement in the R-3m space group of the X-ray diffraction data has shown an increasing lamellar character for the structure of these materials, with increasing overlithiation. Electron diffraction has revealed the presence of a √3.ahex. × √3.ahex. superstructure due to a cationic ordering in the transition metal layers, those being packed with faults along the chex.-axis. From an electrochemical point of view, the reversible capacity in the 2–4.3V decreased with overlithiation, in agreement with a decreasing amount of exchangeable electrons.