Ex-situ X-ray Diffraction Measurements Research Articles

Promoting threshold voltage of P2-Na0.67Ni0.33Mn0.67O2 with Cu2+ cation doping toward high-stability cathode for sodium-ion battery

P2-type Na0.67Ni0.33Mn0.67O2 has attracted considerable attraction as a cathode material for sodium-ion batteries owing to its high operating voltage and theoretical specific capacity. However, when the charging voltage is higher than 4.2 V, the Na0.67Ni0.33Mn0.67O2 cathode undergoes a detrimental irreversible phase transition of P2-O2, leading to a drastic decrease in specific capacity. To address this challenge, we implemented a Cu-doping strategy (Na0.67Ni0.23Cu0.1Mn0.67O2) in this work to stabilize the structure of the transition metal layer. The stabilization strategy involved reinforcing the transition metal–oxygen (TMO) bonds, particularly the MnO bond and inhibiting interlayer slip during deep desodiation. As a result, the irreversible phase transition voltage is delayed, with the threshold voltage increasing from 4.2 to 4.4 V. Ex-situ X-ray diffraction measurements revealed that the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode maintains the P2 phase within the voltage window of 2.5–4.3 V, whereas the P2-Na0.67Ni0.33Mn0.67O2 cathode transforms entirely into O2-type Na0.67Ni0.33Mn0.67O2 when the voltage exceeds 4.3 V. Furthermore, absolute P2-O2 phase transition of the Na0.67Ni0.23Cu0.1Mn0.67O2 cathode occurred at 4.6 V, indicating that Cu2+ doping enhances the stability of the layer structure and increases the threshold voltage. The resulting Na0.67Ni0.23Cu0.1Mn0.67O2 cathode exhibited superior electrochemical properties, demonstrating an initial reversible specific capacity of 89.1 mAh/g at a rate of 2C (360 mA g−1) and retaining more than 78 % of its capacity after 500 cycles.

Revealing Phase Transition in Ni-Rich Cathodes Via Nondestructive Entropymetry Method

With the expanding requirements of recent energy regulations and economic interest in high-performance batteries, the need to improve battery energy density and safety has gained prominence. High-energy-density lithium batteries, employed in next-generation energy storage devices, rely on nickel-rich cathode materials. Since these have extremely high charge/discharge capacity, high operating voltage, prolonged cycle life, and low cost, nickel-rich cathode materials such as Ni-rich NCM (LiNix>0.8CoyMnzO2) and Ni-rich NCA (LiNix>0.8CoyAlzO2) are of particular interest to researchers. Several in situ characterization methodologies are currently used to understand lithium-ion battery electrode response and deterioration better. Nevertheless, in many contexts, these measurement methodologies must be combined with specially designed cells and electrode materials with distinct forms, which is sometimes inconvenient. As an alternative, thermothermo-voltametric dynamic characterization may be utilized to describe the thermal internal characteristics of various electrode materials, such as the structural changes and electrode reactions that occur during charging and discharging. In this paper, a non-destructive entropy measurement method demonstrates that phase change occur for NCM (LiNi0.83Co0.12Mn0.05O2) and NCA (LiNi0.88Co0.09Al0.03O2) at 40-30% of SOC and 90-80% of SOC, respectively. This is confirmed by ex-situ X-ray diffraction measurements for these highly popular cathodes.

Potassium-based dual-carbon battery with pure ionic liquid electrolyte

Dual-carbon batteries (DCBs) have attracted considerable attention as they are environmentally friendly, economical, and exhibit a high cost-to-performance ratio. However, severe electrolyte decomposition and Al current collector corrosion lead to moderate reversible capacity and low average coulombic efficiency in DCBs, limiting their practical application. This paper reports a potassium-based DCB using a pure ionic liquid (IL) electrolyte with asymmetric FTA− anions (FTA = (fluorosulfonyl)(trifluoromethylsulfonyl)amide) and graphite electrodes, with a high charge–discharge capacity and long cycle-life. ILs are intrinsically highly concentrated electrolytes composed of cations and anions that enable the construction of high energy density DCBs. Here, symmetric dual-carbon full cells with the FTA-based IL electrolyte and graphite positive and negative electrodes delivered a high discharge capacity of 81 mAh g−1 with an average discharge voltage of ∼4.35 V up to 300 cycles. Further, ex-situ X-ray diffraction measurements conducted at 298 K indicated the formation of stage 1 FTA-graphite intercalation compounds at the full-charged state. This study facilitates the construction of cost-effective DCBs with a high operating voltage, fast charge–discharge, and long cycling, without the use of toxic metals.

Realizing rapid electrochemical kinetics of Mg2+ in Ti-Nb oxides through a Li+ intercalation activated strategy toward extremely fast charge/discharge dual-ion batteries

The development of practical magnesium batteries is severely hindered by the sluggish solid diffusion processes resulting from the relatively high charge density of Mg2+ with two unit charges. Therefore, exploiting novel charge storage mechanisms to overcome the inherent kinetic obstruction is urgently needed. Herein, the rapid diffusion kinetics of Mg2+ in Wadsley-Roth phase TiNb2O7 (TNO) is realized through the electrochemical Li+ intercalation boosted approach, contributing to the successful fabrication of high-rate and ultra-long lifespan dual-ion batteries. Through adding lithium salt into the conventional all phenyl complex electrolyte, not only Li+ but also Mg2+ can be intercalated into the TNO, implying the intercalation of Mg2+ can be activated by the intercalation of Li+. Meanwhile, theoretical calculations reveal that the insertion of Li+ can significantly improve the electronic conductivity of TNO and reduce the diffusion energy barrier of Mg2+ in TNO. Moreover, in-situ Raman and ex-situ X-ray diffraction measurements also indicate the improved structural integrity of TNO with Li+ pre-intercalation during cycling. Consequently, such a dual-ion battery system with the TNO cathode and Mg metal anode exhibits a high reversible capacity (243.6 mAh g−1 at 0.1 C) and excellent high-rate cycling stability (capacity retention of 85.1% at 5 C after 2000 cycles)

Enhancement of Lithium-Mediated Ammonia Synthesis By Addition of Oxygen

Ammonia is one of the most produced chemicals worldwide and is currently synthesized by the Haber-Bosch process, which is a thermally catalyzed method that requires high pressures and temperatures. These harsh conditions, in addition to the prerequisite steam reforming process, leads to about 1 % of the annual energy consumption and 1.4 % of the global CO2 emission. One way to mitigate some of the Haber-Bosch process is to produce ammonia electrochemically, utilizing renewable energy sources.The electrochemical synthesis of ammonia faces several big issues. One being the selectivity, since the more facile hydrogen evolution reaction (HER) will always dominate over the nitrogen reduction reaction (NRR), and another issue being the activity, given that the nitrogen triple bond is very stable and therefore hard to split. Hence, most often the reported ammonia contents are in the low ppm regime, which makes it very susceptible to contaminations both from the gas stream (NH3 and NOx impurities) and the system itself (catalyst, cell, chemicals, nitrile gloves, etc.). To avoid misleading results, several protocols have been published on how to correctly perform NRR experiments [1, 2]. One of these confirm that only the Li-mediated ammonia synthesis (LiMeAS) is currently able to produce ammonia electrochemically [3]. The actual mechanism is not fully understood, but it is generally believed that the first step is Li plating from a Li salt containing non-aqueous electrolyte. The very reactive Li will then react with N2 solvated in the electrolyte to form Li3N, which is believed to hydrolyze to ammonia when in contact with a proton source. The currently highest archived faradaic efficiency (FE) is at 69 % at 20 bar N2 pressure when applying an ionic liquid as a proton shuttle [4].In this work, we achieved up to 79 % FE at 20 bar N2 by the addition of 0.8 mol. % O2 in the reaction atmosphere. The positive effect of O2 is a very counterintuitive observation, since the original work by Tsuneto et al. [5] showed that the use of synthetic air significantly hindered the reaction, as it was postulated that O2 inhibits the reaction due to LiO2 formation and/or leading primarily to the oxygen reduction reaction (ORR). We will present experimental results obtained at 10 and 20 bar with varying O2 contents, which were measured accurately by a mass spectrometer probing the atmosphere just above the electrolyte inside the pressure vessel. By combining experimental observations with theoretical modelling, we conclude that the unexpectedly beneficial role of small O2 concentrations has a positive influences the solid electrolyte interface (SEI), which is of great importance in our system. Additional ex-situ X-Ray diffraction (XRD) and X-Ray photoelectron spectroscopy (XPS) measurements were conducted without exposure to air and moisture, to analyze the SEI layer and deposition after electrochemistry. We believe that this study will not only be beneficial for industrializing the LiMeAS, but will also bring us a step further in understanding the complex mechanism behind this process.[1] S. Z. Andersen et al., "A rigorous electrochemical ammonia synthesis protocol with quantitative isotope measurements," Nature, vol. 570, pp. 504-508, 2019.[2] H. Iriawan et al., "Methods for nitrogen activation by reduction and oxidation," Nature Reviews Methods Primers, vol. 1, no. 1, pp. 1-26, 2021.[3] J. Choi et al., "Identification and elimination of false positives in electrochemical nitrogen reduction studies," Nature communications, vol. 11, no. 1, pp. 1-10, 2020.[4] B. H. Suryanto et al., "Nitrogen reduction to ammonia at high efficiency and rates based on a phosphonium proton shuttle," Science, vol. 372, no. 6547, pp. 1187-1191, 2021.[5] A. Tsuneto, A. Kudo, and T. Sakata, "Lithium-mediated electrochemical reduction of high pressure N2 to NH3," Journal of Electroanalytical Chemistry, vol. 367, no. 1-2, pp. 183-188, 1994.

Microwave-assisted hydrothermal synthesis of three-dimensional NbOPO4-reduced graphene oxide-carbon nanotube composite for high performance sodium-ion battery anode

Sodium-ion batteries (SIBs) have aroused more and more academic and industrial interest due to their decent electrochemical performance and the cost-effectiveness of sodium (Na) resource. However, fabricating an appropriate anode material with high specific capacity, long-term cycle stability and superior rate capability remains a great challenge. Here, we adopt an efficient microwave-assisted hydrothermal method to prepare layered niobium oxyphosphate (NbOPO4) nanosheets, which grows on the single-walled carbon nanotubes (SWCNT) and reduced graphene oxide (rGO) (NbOPO4-rGO-SWCNT). Benefiting from three-dimensional hierarchical architecture combining NbOPO4 nanosheets with rGO-SWCNT framework with excellent electrical conductivity, NbOPO4-rGO-SWCNT electrode delivers a higher reversible specific capacity (323.2 mAh g−1 at 0.1 A g−1) and superior rate capability (147.9 mAh g−1 at 5 A g−1) than that of NbOPO4 (143.1/43.5 mAh g−1). Ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements further reveal the electrochemical mechanism that reversible reaction between niobium phosphate (NbPO4) and niobium (Nb) occurs after the initial intercalation-conversion reaction, which contributes to the stable reversibility and high-rate capability in the following cycling. This work not only indicates that NbOPO4-rGO-SWCNT can act as a desirable anode material for SIBs, but also provide new insights to material design for niobium oxyphosphate composite.

Influence of Alumina Addition on the Optical Properties and the Thermal Stability of Titania Thin Films and Inverse Opals Produced by Atomic Layer Deposition

TiO2 thin films deposited by atomic layer deposition (ALD) at low temperatures (<100 °C) are, in general, amorphous and exhibit a smaller refractive index in comparison to their crystalline counterparts. Nonetheless, low-temperature ALD is needed when the substrates or templates are based on polymeric materials, as the deposition has to be performed below their glass transition or melting temperatures. This is the case for photonic crystals generated via ALD infiltration of self-assembled polystyrene templates. When heated up, crystal phase transformations take place in the thin films or photonic structures, and the accompanying volume reduction as well as the burn-out of residual impurities can lead to mechanical instability. The introduction of cation doping (e.g., Al or Nb) in bulk TiO2 parts is known to alter phase transitions and to stabilize crystalline phases. In this work, we have developed low-temperature ALD super-cycles to introduce Al2O3 into TiO2 thin films and photonic crystals. The aluminum oxide content was adjusted by varying the TiO2:Al2O3 internal loop ratio within the ALD super-cycle. Both thin films and inverse opal photonic crystal structures were subjected to thermal treatments ranging from 200 to 1200 °C and were characterized by in- and ex-situ X-ray diffraction, spectroscopic ellipsometry, and spectroscopic reflectance measurements. The results show that the introduction of alumina affects the crystallization and phase transition temperatures of titania as well as the optical properties of the inverse opal photonic crystals (iPhC). The thermal stability of the titania iPhCs was increased by the alumina introduction, maintaining their photonic bandgap even after heat treatment at 900 °C and outperforming the pure titania, with the best results being achieved with the super-cycles corresponding to an estimated alumina content of 26 wt.%.

Experimental and theoretical characterization of SmAlO3 perovskite synthesized by mechanical alloying

The synthesis of a nanometric and monophasic SmAlO3 perovskite by utilizing the process of mechanical alloying at room temperature is reported for the first time. The structural evolutions are examined by the ex-situ X-Ray diffraction measurements, which demonstrate that the perovskite nucleation occurs on 2 ​h of milling, and its phase fraction increases proportionally with the processing time. Thermal stability analysis is performed in order to accomplish the annealing at 1050 ​°C. The obtained sample is found to be orthorhombic with nanometric crystallites of 41 ​nm. The optical and vibrational properties of the prepared SmAlO3 sample have been investigated by Raman and UV–Vis measurements along with DFT calculations. The SmAlO3 Raman peaks are appropriately assigned to the fundamental vibrational modes and an excellent agreement between the experimental and theoretical Raman modes is observed. The vibrational modes, band gap, density of states and the energy bands are also demonstrated.

Microstructure of α + β dual phase formed from isothermal α″phase via novel decomposition pathway in metastable β-Ti alloy

Abstract Recently, the formation of the isothermal α″ phase (α″iso, orthorhombic) via aging heat treatment has been reported in several β-Ti alloys. The microstructure of the α + β dual phase that was obtained from the α″iso phase via a novel decomposition pathway (β → α″iso (single phase) → α + β) in metastable β-Ti alloys was investigated in the present study. Ex-situ X-ray diffraction measurements and scanning electron microscopy were performed to elucidate the phase constitution and the microstructural evolution, respectively, during the decomposition. The detailed microstructure of the α + β phases was observed using transmission electron microscopy. The microstructure of the α + β dual phase inherited the characteristics of the microstructure of the α″iso phase. The microstructural features included irregular-shaped variants, dense and fine twinning (width ~ 50–100 nm). The Burgers orientation relationship was satisfied between the initial β, α″iso, decomposed β, and decomposed α phases. Furthermore, only a specific variant combination of the decomposed α and β phases was observed. The crystallographic orientation (i.e., variant) of the decomposed β phase was similar to that of the initial β phase. These results indicated the existence of a strict variant selection rule between the α and β phases. The origin of this rule was the α″iso variant that was formed before the decomposition. When the Burgers orientation relationship was established, the α″iso variant determined the parallel plane and parallel direction between the α and β phases.

Interface mixing and hydrogen absorption in Pd/Mg and Pd/Al/Mg thin films

Pd/Mg bilayers and Pd/Al/Mg trilayers were prepared onto glass substrates at room temperature (RT) by UHV magnetron sputtering. Mixing effects at the Pd–Mg and Al–Mg interfaces were studied in-situ, immediately after deposition, by X-ray Photoelectron Spectroscopy (XPS). Additionally, the interfaces of the Pd/Al/Mg trilayer for the Al thickness equal to 1 nm were examined. Hydrogen absorption was monitored in-situ at RT by simultaneous resistivity and optical transmittance measurements. Formation of MgH2 phase was confirmed by ex-situ X-ray diffraction measurements. The XPS studies revealed rather sharp interface between Al and Mg layers. On the other hand, a significant interface mixing for the Pd/Mg bilayers and Pd/1 nm – Al/Mg trilayers was observed. Further studies showed that an additional layer of Al, deposited between magnesium and palladium layers, can significantly improve the hydrogen absorption kinetics at RT. The optimal thickness of the Al layer was found to be 0.5 nm.

In-situ curvature measurements of AlInN/GaN distributed Bragg reflectors during growths containing substrate temperature ramping steps

• A model for in-situ wafer curvature values of AlInN/GaN distributed Bragg reflectors was developed. • Contributions of substrate temperature ramping and GaN growth to the in-situ curvature values during the AlInN/GaN DBR growth was experimentally investigated. • InN mole fractions in the AlInN/GaN distributed Bragg reflectors with the model was determined. We developed a model for in-situ wafer curvature values of AlInN/GaN distributed Bragg reflectors (DBRs) and determined InN mole fractions in the DBRs with the model. In order to develop the model, we experimentally investigated contributions of substrate temperature ramping and a GaN growth to changes in the in-situ curvature values during the AlInN/GaN DBR growth. We found that an increase of curvature changes at the substrate temperature ramping steps was explained by an increase of the total epitaxial layer thicknesses. Another finding was that strain in the GaN layers at the GaN layer growth steps was almost zero. Finally, we determined the InN mole fractions in the AlInN layers by using the model, showing excellent agreements with the values estimated from ex-situ X-ray diffraction measurements. The model reveals not only the entire in-situ curvature change profile but also the InN mole fraction under the precisely lattice-matched condition of AlInN/GaN DBRs.

Tailoring the structural and magnetic properties of epitaxially growth Ni–Mn–Ga magnetic films by Fe addition

ABSTRACTNi–Mn–Ga is one of the major magnetic materials in Heusler family due to its interesting magnetic and structural properties. In this study, the epitaxial 200 nm thin films are prepared by magnetron co-sputtering from a NiMnGa alloy target and an elemental Fe target on MgO substrates. The epitaxial growth mechanism is pursued and controlled by ex-situ X-ray refractometry (XRR) and X-ray diffraction (XRD) measurements. On account of analysing the physical parameters, temperature-dependent X-ray diffraction (XRD), transmission electron (TEM), scanning electron microscopy (SEM), temperature dependence of resistivity (ρ(T)), atomic force microscopy (AFM) and magnetisation (MOKE) measurements are performed systematically for different Fe contents. Structural and magnetic measurements show that the NiMnGa parent thin film transitions could be adjusted from martensitic to austenitic phases by adding Fe element from high temperature to the vicinity of room temperature. Measurements are carried out to get a further structural characterisation of 14M martensite which was found to consist of nano-twinned structures. The epitaxial growth and the stable coexisting austenite and martensitic phases for the thin film of the NiMnGa chemical composition are shown near room temperature. Due to this feature, magnetic thin films could increase the likelihood of possible room temperature using thin film applications.

Diffraction Signal from Sodium Cluster in Hard Carbon: A Combined Experimental and Computational Study

Na-ion batteries have attracted increasing attention as large-scale energy storage devices. Hard carbon is a standard anode material owing to its large capacity and good cycling stability1, 2. However, sodium storage mechanism of hard carbon has not been fully understood due to its low crystallinity and diversity of microstructure. This study aimed to systematic understanding of the relationship between microstructure and sodium storage behavior with special attention to the diffraction signal from sodium cluster confined in nanopores. Hard carbons were synthesized by hydrothermal treatment of sucrose and subsequent high-temperature carbonizations at T = 1000-1900 °C (denoted as HC-T). The charge-discharge tests were carried out using half-cells. As the heat-treatment temperature increased, the capacity of the high potential region (> 0.2 V) decreased while the plateau region (< 0.2 V) become dominant. HC-1400 showed the highest overall capacity (347 mAh g-1) and initial coulombic efficiency (95.2%). Reaction mechanism analyses were performed on three representative samples, HC-1000, HC-1400, and HC-1900. Ex-situ small angle X-ray scattering (SAXS) measurements revealed (i) the nanopores become larger with decreasing their number for higher annealing temperatures, (ii) insertion of sodium into the nanopores proceeded under ca. 0.07 V. When electrodes were overcharged exceeding the sodium deposition voltage (ca. –0.01 V), the sodium density in the nanopores became comparable to that of bulk bcc sodium. Besides, apparent smaller capacity of HC-1900 was analyzed to be a kinetic observation with larger cathodic polarization. Careful ex-situ X-ray diffraction measurements have detected smaller mean interlayer distances in order of HC-1900 < HC-1400 < HC-1000. Expansion of interlayer distance upon sodium insertion was observed for HC-1000 and HC-1400, but not at all for HC-1900, suggesting the existence of a threshold interlayer distance to allow sodium insertion. Importantly, for the first time, we have succeeded to detect a broad peak appeared around 29° as a signature of sodium insertion into the nanopores, and its origin was analyzed to be diffraction from sodium clusters in the nanopores using DFT-MD simulations. Based on these analyses, criteria for better hard carbon will be discussed in the poster. Figure 1

Synthesis of porous MoV2O8 nanosheets as anode material for superior lithium storage

Vanadium-based mixed oxides are promising for high-energy-density lithium-ion batteries (LIBs) due to their abundant oxidation states, complex chemical compositions and synergistic effects, but the large volume change upon lithiation/delithiation limits the electrochemical properties. Two-dimensional nanostructures have great potential in the development of electrode materials by promoting the ion transport and alleviating the volume change. Herein, porous MoV2O8 nanosheets are synthesized via a solvothermal process followed by thermal treatment without using any surfactants. Benefiting from the nanosheet structure with large surface area and abundant pores, the MoV2O8 electrode exhibits impressive lithium storage properties with superior rate performance and prolonged cyclability. Superior reversible capacities of 1396 and 570 mA h g–1 can be maintained after 120 cycles at 200 mA g–1 and after 1000 cycles at 10 A g–1, respectively. Furthermore, the lithium storage mechanism of the MoV2O8 nanosheets was investigated by in-situ X-ray diffraction, ex-situ X-ray diffraction and X-ray photoelectron spectroscopy measurements, which confirms the synergistic effects of Mo and V upon lithiation/delithiation. In addition, a full cell consisting of the MoV2O8 nanosheets and commercial LiFePO4 exhibited good electrochemical performance, demonstrating the great potential of the MoV2O8 nanosheets as an anode material for LIBs.

Epitaxial growth of Pd1−xFex films on MgO single-crystal substrate

In the paper we report on growth conditions, morphology, crystallographic structure and magnetic anisotropy of 20 nm thick, palladium-rich Pd1−xFex (0.01 < x < 0.1) alloy films grown on (001)-oriented MgO single-crystal substrate. Molecular beam deposition at ultra-high vacuum conditions has been utilized along with the three-step procedure to achieve the epitaxy conditions for the synthesized films. The scanning electron microscopy and atomic force microscopy have shown flat and smooth morphology of the films studied in a wide range of lateral scales. Insitu low energy electron diffraction and exsitu X-ray diffraction measurements confirmed that our Pd1−xFex alloy films are epitaxial. Ferromagnetic resonance investigations have shown the in-plane four-fold magnetocrystalline anisotropy characteristic for single-crystalline films with the cubic structure. The whole set of our measurements testifies the epitaxial cube-on-cube growth and excellent magnetic homogeneity of Pd1−xFex films on MgO substrate obtained within the three-step deposition process.

Influence of Li substitution on the structure and electrochemical performance of P2-type Na0.67Ni0.2Fe0.15Mn0.65O2 cathode materials for sodium ion batteries

A series of layered Na0.67Lix(Ni0.2Fe0.15Mn0.65)1-xO2+δ (x = 0, 0.1, 0.2 and 0.3) compounds is prepared via a facile oxalate coprecipitation method and a solid-state reaction process, and investigated as the promising positive electrode materials for Na-ion batteries. As the Li content increases, O3 phase and Li2MnO3 gradually grow at the expense of the P2 phase and the particles become smaller and more agglomerated based on X-ray diffraction, scanning electron microscopy and transmission electron microscope results. The optimal Na0.67Li0.2(Ni0.2Fe0.15Mn0.65)0.8O2+δ shows the improved electrochemical performance with a high specific capacity of 151 mAh g−1 and 78% capacity retention over 50 cycles at 0.1 C (15 mA g−1). When tested at 5 C (750 mA g−1) rate, the electrode exhibits a discharge capacity of 68 mAh g−1. The results of electrochemical impedance spectroscopic and ex-situ X-ray diffraction measurements demonstrate that the improved cyclability and rate capability of the Li-substituted cathode can be ascribed to the decreased resistance and the enhanced structure stability in the high voltage of 4.3 V.

Structure and magnetic properties of an epitaxial Fe(110)/MgO(111)/GaN(0001) heterostructure

We present the structural and magnetic properties of fully epitaxial Fe(110)/MgO(111)/GaN(0001) tunnel barrier structures grown by molecular beam epitaxy. In-situ reflection high-energy electron diffraction and ex-situ X-ray diffraction measurements indicate epitaxial Fe(110) films on top of an epitaxial 2 nm MgO(111) tunnel barrier on GaN(0001). X-ray reflectivity measurements confirm a roughness of approximately 0.3 nm and 0.7 nm for the MgO/GaN and the Fe/MgO interfaces, respectively. Results of in-situ magneto-optical Kerr effect measurements indicate that 1 nm thick Fe film shows signs of in-plane ferromagnetism at room temperature. Vibrating sample magnetometer measurements determine the saturation magnetisation of the 5 nm thick film to be 1660 ± 100 emu/cm3 and show that this system has a predominant uniaxial anisotropy contribution despite the presence of cyclic twinned crystals. We estimate the values of effective uniaxial (KUeff) and cubic (K1eff) anisotropy constants to be 11700 ± 170 erg cm−3 and −3300 ± 700 erg cm−3 by fitting the angular dependence of the magnetising energy.

Phase change effect on the structural and electrochemical behaviour of pure and doped vanadium pentoxide as positive electrodes for lithium ion batteries

Abstract Electrospun ceramic oxide fibers find myriad uses as energy materials such as in battery electrodes and vanadium oxides are one such family of materials. In this study, the structural and energy storage properties of electrospun vanadium pentoxide are compared to approximately 10 at% barium and titanium-doped equivalents. The vanadium pentoxide was doped in order to improve its electrochemical performance. The materials are characterised using powder X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller measurements, transmission electron microscopy and potentiostatic and galvanostatic analysis. X-ray diffraction analysis showed that each dopant has a critical effect on lattice distortions whilst showing no influence over the overall crystal structure, which is unusual for such large dopant amounts. The doped materials show better cyclability and higher efficiencies than the pure equivalent. Ex-situ X-ray diffraction measurements show detrimental phase changes within undoped V 2 O 5 whereas the titanium-doped V 2 O 5 predominantly remains as α-V 2 O 5 after the first cycle.

Layered P2-Na2/3Co1/2Ti1/2O2 as a high-performance cathode material for sodium-ion batteries

Abstract Layered oxides are regarded as promising cathode materials for sodium-ion batteries. We present Na 2/3 Co 1/2 Ti 1/2 O 2 as a potential new cathode material for sodium-ion batteries. The crystal features and morphology of the pristine powder were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The cathode material is evaluated in galvanostatic charge-discharge and galvanostatic intermittent titration tests, as well as ex-situ X-ray diffraction analysis. Synthesized by a high-temperature solid state reaction, Na 2/3 Co 1/2 Ti 1/2 O 2 crystallizes in P2-type structure with P 6 3 / mmc space group. The material presents reversible electrochemical behavior and delivers a specific discharge capacity of 100 mAh g −1 when tested in Na half cells between 2.0 and 4.2 V (vs. Na + /Na), with capacity retention of 98% after 50 cycles. Furthermore, the electrochemical cycling of this titanium-containing material evidenced a reduction of the potential jumps recorded in the Na x CoO 2 parent phase, revealing a positive impact of Ti substitution for Co. The ex-situ XRD measurements confirmed the reversibility and stability of the material. No structural changes were observed in the XRD patterns, and the P2-type structure was stable during the charge/discharge process between 2.0 and 4.2 V vs. Na + /Na. These outcomes will contribute to the progress of developing low cost electrode materials for sodium-ion batteries.

Investigation of the electrochemical features of carbon-coated TiO2 anode for application in lithium-ion battery using high voltage LiNi0.5Mn1.5O4 spinel cathode

In this paper we propose a carbon-coated, nano-sized TiO2 anode for application in lithium-ion batteries. The lithiation-delithiation process characteristic of this mixed anatase/rutile material has been investigated in detail, in order to define the optimal operating voltage range and to further enhance the electrode cycle life. Ex-situ x-ray diffraction measurements demonstrate that the rutile phase becomes electrochemically inactive toward lithium intercalation after the first cycle and remains inactive by cycles. The TiO2 electrochemical behavior is studied by means of various techniques, including galvanostatic cycling and potentiodynamic cycling with galvanostatic acceleration. We show that the combination of the TiO2 anode with a high-voltage, LiNi0.5Mn1.5O4 spinel cathode results in an advanced li-ion battery able to exchange reversibly a capacity higher than 100 mAh/g for over 70 cycles at the high rate of 1C. Considering an average working voltage of about 2.9V, the theoretical energy content of the cell here disclosed is about 300 Wh kg−1. Taking into account the energy content and high safety level of the full cell, due to the use of a TiO2-based electrode, by operating at a voltage value well far from the one associated to the common electrolyte decomposition, i.e. about 1.7V, we may propose the anode here studied as suitable material for advanced energy storage systems.