Showing papers on "Ion published in 2014"

A sulphide lithium super ion conductor is superior to liquid ion conductors for use in rechargeable batteries

Abstract: We report that a heat-treated Li2S–P2S5 glass-ceramic conductor has an extremely high ionic conductivity of 1.7 × 10−2 S cm−1 and the lowest conduction activation energy of 17 kJ mol−1 at room temperature among lithium-ion conductors reported to date. The optimum conditions of the heat treatment reduce the grain boundary resistance, and the influence of voids, to increase the Li+ ionic conductivity of the solid electrolyte so that it is greater than the conductivities of liquid electrolytes, when the transport number of lithium ions in the inorganic electrolyte is unity.

Identifying the Critical Role of Li Substitution in P2–Nax[LiyNizMn1–y–z]O2 (0 < x, y, z < 1) Intercalation Cathode Materials for High-Energy Na-Ion Batteries

Abstract: Li-substituted layered P2–Na0.80[Li0.12Ni0.22Mn0.66]O2 is investigated as an advanced cathode material for Na-ion batteries. Both neutron diffraction and nuclear magnetic resonance (NMR) spectroscopy are used to elucidate the local structure, and they reveal that most of the Li ions are located in transition metal (TM) sites, preferably surrounded by Mn ions. To characterize structural changes occurring upon electrochemical cycling, in situ synchrotron X-ray diffraction is conducted. It is clearly demonstrated that no significant phase transformation is observed up to 4.4 V charge for this material, unlike Li-free P2-type Na cathodes. The presence of monovalent Li ions in the TM layers allows more Na ions to reside in the prismatic sites, stabilizing the overall charge balance of the compound. Consequently, more Na ions remain in the compound upon charge, the P2 structure is retained in the high voltage region, and the phase transformation is delayed. Ex situ NMR is conducted on samples at different state...

Ion Conducting Membranes for Fuel Cells and other Electrochemical Devices

Abstract: Transport and stability issues of proton and hydroxide ion conducting separator membranes for fuel cells are critically discussed from a fundamental point of view. Considerations of structure and dynamics on the molecular scale to the device level equally imply polymer-chemical and electrochemical aspects which are closely related for this class of materials. The importance of ion/solvent, residual ion/ion, and solvent/polymer interactions for the formation and mobility of ionic charge carriers and selective ionic transport and even as driving forces for nanoscale ordering is emphasized, and it is shown that, apart from simple electrostatics, specific chemical interactions must be considered. On the basis of this understanding, suggestions are being made for the modification of existing and the development of new membrane types, not only for fuel cells but also for other electrochemical energy conversion and storage devices such as redox-flow and alkaline ion batteries.

Conformational ordering of biomolecules in the gas phase: nitrogen collision cross sections measured on a prototype high resolution drift tube ion mobility-mass spectrometer

Abstract: Ion mobility-mass spectrometry measurements which describe the gas-phase scaling of molecular size and mass are of both fundamental and pragmatic utility. Fundamentally, such measurements expand our understanding of intrinsic intramolecular folding forces in the absence of solvent. Practically, reproducible transport properties, such as gas-phase collision cross-section (CCS), are analytically useful metrics for identification and characterization purposes. Here, we report 594 CCS values obtained in nitrogen drift gas on an electrostatic drift tube ion mobility-mass spectrometry (IM-MS) instrument. The instrument platform is a newly developed prototype incorporating a uniform-field drift tube bracketed by electrodynamic ion funnels and coupled to a high resolution quadrupole time-of-flight mass spectrometer. The CCS values reported here are of high experimental precision (±0.5% or better) and represent four chemically distinct classes of molecules (quaternary ammonium salts, lipids, peptides, and carbohyd...

Na3V2(PO4)3@C core–shell nanocomposites for rechargeable sodium-ion batteries

Abstract: Na3V2(PO4)3 (NVP) is an attractive cathode material for sodium ion batteries due to its high theoretical energy density and stable three-dimensional (3D) NASICON structure. In this paper, a NVP@C core–shell nanocomposite has been synthesized through a hydrothermal assisted sol–gel method. Ascorbic acid and polyethylene glycol 400 (PEG-400) were synergistically used to control the particle growth and provide the surface coating of conductive carbon. The as-prepared nanocomposite was composed of a nanosized Na3V2(PO4)3 core with a typical size of ∼40 nm and a uniformly amorphous carbon shell with the thickness of a few nanometers. The electrode performance of the NVP@C core–shell nanocomposite as cathode for sodium ion batteries is investigated and compared with that of bare NVP and NVP/C. Among the samples examined, the NVP@C nanocomposite showed the best cycle life and rate capability. It rendered an initial capacity of 104.3 mA h g−1 at 0.5 C and 94.9 mA h g−1 at 5 C with a remarkable capacity retention of 96.1% after 700 cycles. Moreover, a full cell using the as-prepared nanocomposite as both the cathode and the anode active material has been successfully built, showing a reversible capacity of 90.9 mA h g−1 at 2 C with an output voltage of about 1.7 V and a specific energy density of about 154.5 W h kg−1. The enhanced electrode performance is attributed to the combination of particle downsizing and carbon coating, which can favor the migration of both electrons and ions.

A new ultrafast superionic Li-conductor: ion dynamics in Li11Si2PS12 and comparison with other tetragonal LGPS-type electrolytes

Abstract: We report on a new ultrafast solid electrolyte of the composition Li11Si2PS12, which exhibits a higher room-temperature Li ion diffusivity than the present record holder Li10GeP2S12. We discuss the high-pressure synthesis and ion dynamics of tetragonal Li11Si2PS12, and comparison is made with our investigations of related members of the LMePS family, i.e. electrolytes of the general formula Li11−xMe2−xP1+xS12 with Me = Ge, Sn : Li10GeP2S12, Li7GePS8, Li10SnP2S12. The structure and dynamics were studied with multiple complementary techniques and the macroscopic diffusion could be traced back to fast Li ion hopping in the crystalline lattice. A clear correlation between the diffusivity and the unit cell volume of the LGPS-type electrolytes was observed.

Electrochemical Quartz Crystal Microbalance (EQCM) Study of Ion Dynamics in Nanoporous Carbons

Abstract: Electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry (CV) measurements were used to characterize ion adsorption in carbide-derived carbon (CDC) with two different average pore sizes (1 and 0.65 nm), from neat and solvated 1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMI-TFSI) electrolytes. From the electrode mass change in neat EMI-TFSI, it was shown that one net charge stored corresponds almost to one single ion at high polarization; in that case, no ion-pairing or charge screening by co-ions were observed. In 2 M EMI-TFSI in acetonitrile electrolyte, experimental solvation numbers were estimated for EMI+ cation, showing a partial desolvation when cations were adsorbed in confined carbon pores. The extent of desolvation increased when decreasing the carbon pore size (from 1 down to 0.65 nm). The results also suggest that EMI+ cation owns higher mobility than TFSI– anion in these electrolytes.

Ion permeation in K + channels occurs by direct Coulomb knock-on

Abstract: Potassium channels selectively conduct K(+) ions across cellular membranes with extraordinary efficiency. Their selectivity filter exhibits four binding sites with approximately equal electron density in crystal structures with high K(+) concentrations, previously thought to reflect a superposition of alternating ion- and water-occupied states. Consequently, cotranslocation of ions with water has become a widely accepted ion conduction mechanism for potassium channels. By analyzing more than 1300 permeation events from molecular dynamics simulations at physiological voltages, we observed instead that permeation occurs via ion-ion contacts between neighboring K(+) ions. Coulomb repulsion between adjacent ions is found to be the key to high-efficiency K(+) conduction. Crystallographic data are consistent with directly neighboring K(+) ions in the selectivity filter, and our model offers an intuitive explanation for the high throughput rates of K(+) channels.

Calix[4]pyrrole-Based Ion Pair Receptors

Abstract: ConspectusIon pair receptors, which are able to bind concurrently both a cation and an anion, often display higher selectivity and affinity for specific ion pairs than simple ion receptors capable of recognizing primarily either a cation or an anion. This enhancement in recognition function is attributable to direct or indirect cooperative interactions between cobound ions via electrostatic attractions between oppositely charged ions, as well as to positive allosteric effects. In addition, by virtue of binding the counterions of the targeted ion, ion pair receptors can minimize the solvation of the counterions, which can otherwise have a negative effect on the interactions between the receptors and the targeted ions. As a result of their more favorable interactions, ion pair receptors are attractive for use in applications, such as extraction and sensing, where control of the binding interactions is advantageous. In this Account, we illustrate this potential in the context of ion pair receptors based on t...

Amorphous iron phosphate: potential host for various charge carrier ions

Abstract: Unlike crystalline electrodes wherein ion insertion is crucially dependent on the presence of energetically equivalent sites, nanostructured amorphous iron(III) phosphate hosts prepared by room temperature strategies and possessing porous properties facilitate the insertion of alkali ions with different sizes and also higher charge carriers including divalent cations (Mg2+−0.72A, Zn2+-0.74 A) or trivalent cations (Al3+−0.53 A). This versatile cathode stores electrical energy by a reversible amorphous to crystalline reconstitutive reaction that occurs during electrochemical reaction with monovalent sodium, potassium and lithium. The study presents opportunities to develop amorphous electrodes with similar phase behavior for energy storage applications.

Surface Structure Evolution of LiMn2O4 Cathode Material upon Charge/Discharge

Abstract: Surface dissolution of manganese is a long-standing issue hindering the practical application of spinel LiMn2O4 cathode material, while few studies concerning the crystal structure evolution at the surface area have been reported. Combining X-ray photoelectron spectroscopy, electron energy loss spectroscopy, scanning transmission electron microscopy, and density functional theory calculations, we investigate the chemical and structural evolutions on the surface of a LiMn2O4 electrode upon cycling. We found that an unexpected Mn3O4 phase was present on the surface of LiMn2O4 via the application of an advanced electron microscopy. Since the Mn3O4 phase contains 1/3 soluble Mn2+ ions, formation of this phase contributes significantly to the Mn2+ dissolution in a LiMn2O4 electrode upon cycling. It is further found that the Mn3O4 appears upon charge and disappears upon discharge, coincident with the valence change of Mn. Our results shed light on the importance of stabilizing the surface structure of cathode m...

Probing molecular interaction in ionic liquids by low frequency spectroscopy: Coulomb energy, hydrogen bonding and dispersion forces

Abstract: Ionic liquids are defined as salts composed solely of ions with melting points below 100 °C. These remarkable liquids have unique and fascinating properties and offer new opportunities for science and technology. New combinations of ions provide changing physical properties and thus novel potential applications for this class of liquid materials. To a large extent, the structure and properties of ionic liquids are determined by the intermolecular interaction between anions and cations. In this perspective we show that far infrared and terahertz spectroscopy are suitable methods for studying the cation–anion interaction in these Coulomb fluids. The interpretation of the measured low frequency spectra is supported by density functional theory calculations and molecular dynamics simulations. We present results for selected aprotic and protic ionic liquids and their mixtures with molecular solvents. In particular, we focus on the strength and type of intermolecular interaction and how both parameters are influenced by the character of the ions and their combinations. We show that the total interaction between cations and anions is a result of a subtle balance between Coulomb forces, hydrogen bonds and dispersion forces. For protic ionic liquids we could measure distinct vibrational modes in the low frequency spectra indicating clearly the cation–anion interaction characterized by linear and medium to strong hydrogen bonds. Using isotopic substitution we have been able to dissect frequency shifts related to pure interaction strength between cations and anions and to different reduced masses only. In this context we also show how these different types of interaction may influence the physical properties of ionic liquids such as the melting point, viscosity or enthalpy of vaporization. Furthermore we demonstrate that low frequency spectroscopy can also be used for studying ion speciation. Low vibrational features can be assigned to contact ion pairs and solvent separated ion pairs. In conclusion we showed how detailed knowledge of the low frequency spectra can be used to understand the change in interaction strength and structure by variation of temperature, solvent polarity and solvent concentration in ionic liquids and their mixtures with molecular solvents. In principle the used combination of methods is suitable for studying intermolecular interaction in pure molecular liquids and their solutions including additive materials such as nanoparticles.

The electric double layer has a life of its own

Abstract: Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit.

On the origin of ionicity in ionic liquids. Ion pairing versus charge transfer

Abstract: In this paper we show by using static DFT calculations and classical molecular dynamics simulations that the charge transfer between ionic liquid ions plays a major role in the observed discrepancies between the overall mobility of the ions and the observed conductivities of the corresponding ionic liquids, while it also directly suppresses the association of oppositely charged ions, thus the ion pairing. Accordingly, in electrochemical applications of these materials it is important to consider this reduction of the total charges on the ions, which can greatly affect the performance of the given process or device in which the ionic liquid is used. By slightly shifting from the salt-like to a molecular liquid-like system via the decreased charges, the charge transfer also fluidizes the ionic liquid. We believe that this vital information on the molecular level structure of ionic liquids offers a better understanding of these materials, and allows us to improve the a priori design of ionic liquids for any given purpose.

Ion dynamics in a trapped ion mobility spectrometer

Abstract: In the present paper, theoretical simulations and experimental observations are used to describe the ion dynamics in a trapped ion mobility spectrometer. In particular, the ion motion, ion transmission and mobility separation are discussed as a function of the bath gas velocity, radial confinement, analysis time and speed. Mobility analysis and calibration procedure are reported for the case of sphere-like molecules for positive and negative ion modes. Results showed that a maximal mobility resolution can be achieved by optimizing the gas velocity, radial confinement (RF amplitude) and ramp speed (voltage range and ramp time). The mobility resolution scales with the electric field and gas velocity and R = 100–250 can be routinely obtained at room temperature.

Study of the lithium/nickel ions exchange in the layered LiNi0.42Mn0.42Co0.16O2 cathode material for lithium ion batteries: experimental and first-principles calculations

Abstract: Mix transition metal layered oxide materials are much attractive for cathode materials in lithium ion batteries. However, the disordered arrangement between lithium and transition metal ions in local regions of these materials always occurs, and seriously affects their electrochemical performance. Here we report experimental and first-principles calculations of Li+/Ni2+ ion exchange in the LiNi0.42Mn0.42Co0.16O2 materials prepared by solid state reaction and co-precipitation methods. The impact of Li+/Ni2+ ions exchange on the crystal/electronic structure, electrochemical performance and stress are investigated in detail. The results show that there are obvious anisotropic stress and smaller inter-slab space of the unit cell associated with greater Li+/Ni2+ ion exchange. During the delithiation process, the distortion force in the unit cell of the material with large Li+/Ni2+ ion exchange increases sharply and presents strong spin-flip transition of Ni ions (magnetic moment direction change). These issues are closely associated with the electrochemical performance of these layered oxide materials and definitely affect their reversible capacity, cycle stability and rate performance.

Tuning charge–discharge induced unit cell breathing in layer-structured cathode materials for lithium-ion batteries

Abstract: For LiMO2 (M=Co, Ni, Mn) cathode materials, lattice parameters, a(b), contract during charge. Here we report such changes in opposite directions for lithium molybdenum trioxide (Li2MoO3). A 'unit cell breathing' mechanism is proposed based on crystal and electronic structural changes of transition metal oxides during charge-discharge. Metal-metal bonding is used to explain such 'abnormal' behaviour and a generalized hypothesis is developed. The expansion of the metal-metal bond becomes the controlling factor for a(b) evolution during charge, in contrast to the shrinking metal-oxygen bond as controlling factor in 'normal' materials. The cation mixing caused by migration of molybdenum ions at higher oxidation state provides the benefits of reducing the c expansion range in the early stage of charging and suppressing the structure collapse at high voltage charge. These results may open a new strategy for designing layered cathode materials for high energy density lithium-ion batteries.

The electric double layer has a life of its own

Abstract: Using molecular dynamics simulations with recently developed importance sampling methods, we show that the differential capacitance of a model ionic liquid based double-layer capacitor exhibits an anomalous dependence on the applied electrical potential. Such behavior is qualitatively incompatible with standard mean-field theories of the electrical double layer, but is consistent with observations made in experiment. The anomalous response results from structural changes induced in the interfacial region of the ionic liquid as it develops a charge density to screen the charge induced on the electrode surface. These structural changes are strongly influenced by the out-of-plane layering of the electrolyte and are multifaceted, including an abrupt local ordering of the ions adsorbed in the plane of the electrode surface, reorientation of molecular ions, and the spontaneous exchange of ions between different layers of the electrolyte close to the electrode surface. The local ordering exhibits signatures of a first-order phase transition, which would indicate a singular charge-density transition in a macroscopic limit.

Specific ion effects on membrane potential and the permselectivity of ion exchange membranes

Abstract: Membrane potential and permselectivity are critical parameters for a variety of electrochemically-driven separation and energy technologies. An electric potential is developed when a membrane separates electrolyte solutions of different concentrations, and a permselective membrane allows specific species to be transported while restricting the passage of other species. Ion exchange membranes are commonly used in applications that require advanced ionic electrolytes and span technologies such as alkaline batteries to ammonium bicarbonate reverse electrodialysis, but membranes are often only characterized in sodium chloride solutions. Our goal in this work was to better understand membrane behaviour in aqueous ammonium bicarbonate, which is of interest for closed-loop energy generation processes. Here we characterized the permselectivity of four commercial ion exchange membranes in aqueous solutions of sodium chloride, ammonium chloride, sodium bicarbonate, and ammonium bicarbonate. This stepwise approach, using four different ions in aqueous solution, was used to better understand how these specific ions affect ion transport in ion exchange membranes. Characterization of cation and anion exchange membrane permselectivity, using these ions, is discussed from the perspective of the difference in the physical chemistry of the hydrated ions, along with an accompanying re-derivation and examination of the basic equations that describe membrane potential. In general, permselectivity was highest in sodium chloride and lowest in ammonium bicarbonate solutions, and the nature of both the counter- and co-ions appeared to influence measured permselectivity. The counter-ion type influences the binding affinity between counter-ions and polymer fixed charge groups, and higher binding affinity between fixed charge sites and counter-ions within the membrane decreases the effective membrane charge density. As a result permselectivity decreases. The charge density and polarizability of the co-ions also appeared to influence permselectivity leading to ion-specific effects; co-ions that are charge dense and have low polarizability tended to result in high membrane permselectivity.

Profiling the nanoscale gradient in stoichiometric layered cathode particles for lithium-ion batteries

Abstract: Chemical and structural evolution in battery materials influences properties relevant to ionic and electronic transport and ultimately impacts the battery performance. Although chemical and structural gradients have been observed in several cathode materials, the origin(s) of these phenomena are poorly understood. Via high-throughput core-level spectroscopies {i.e., X-ray absorption spectroscopy (XAS), depth-profiled X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS)}, as well as scanning transmission electron microscopy (STEM), the present study seeks to achieve mechanistic understanding for these phenomena in a stoichiometric Rm layered cathode material (e.g., LiNixMnxCo1−2xO2, NMC). We observed that the surfaces of particles in the composite electrode are complicated by the presence of a surface reaction layer resulting from electrolyte decomposition. In large particle ensembles, the global nickel oxidation state switches between Ni2+ and Ni2+x (x = 1–2) during charging/discharging processes, and hole states are also created at the O2p level due to the TM3d–O2p hybridization states. In primary particles, the surface is less oxidized than the bulk counterpart of the same particle whenever the particle has been cycled. This is partially attributed to the reconstruction from an Rm structure to an Fmm structure at the surfaces of NMC particles. This work provides a unique insight into correlating crystal structures with charge compensation mechanisms and performance fading in stoichiometric layered cathode materials.

How Does Nanoscale Crystalline Structure Affect Ion Transport in Solid Polymer Electrolytes

Abstract: Polymer electrolytes have attracted intensive attention due to their potential applications in all-solid-state lithium batteries. Ion conduction in this system is generally considered to be confined in the amorphous polymer/ion phase, where segmental relaxation of the polymer above glass transition temperature facilitates ion transport. In this article, we show quantitatively that the effect of polymer crystallization on ion transport is twofold: structural (tortuosity) and dynamic (tethered chain confinement). We decouple these two effects by designing and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity, and orientation. Ion conduction is confined within the chain fold region and guided by the crystalline lamellae. We show that, at low content, due to the tortuosity effect, the in-plane conductivity is 2000 times greater than through-plane one. Contradictory to the general view, the dynamic effect is negligible at moderate ion contents....

Water-induced correlation between single ions imaged at the solid-liquid interface

Abstract: When immersed into water, most solids develop a surface charge, which is neutralized by an accumulation of dissolved counterions at the interface. Although the density distribution of counterions perpendicular to the interface obeys well-established theories, little is known about counterions' lateral organization at the surface of the solid. Here we show, by using atomic force microscopy and computer simulations, that single hydrated metal ions can spontaneously form ordered structures at the surface of homogeneous solids in aqueous solutions. The structures are laterally stabilized only by water molecules with no need for specific interactions between the surface and the ions. The mechanism, studied here for several systems, is controlled by the hydration landscape of both the surface and the adsorbed ions. The existence of discrete ion domains could play an important role in interfacial phenomena such as charge transfer, crystal growth, nanoscale self-assembly and colloidal stability.

Multi-ion cooperative processes in Yb3+ clusters

Abstract: Cooperative luminescence (CL) occurs in spectral regions in which single ions do not have energy levels. It was first observed more than 40 years ago, and all results reported so far are from a pair of ions. In this work, upconverted CL of three Yb3+ ions was observed in the ultraviolet (UV) region under near-infrared (NIR) excitation. The UV CL intensity showed a cubic dependence on the NIR pump power, whereas the luminescence lifetime was nearly one-third the luminescence lifetime of single Yb3+ ions. The triplet CL (TCL) has a clear spectral structure, in which most emission peaks are consistent with the self-convoluted spectra from single Yb3+ ions. Blue shifts were observed for certain peaks, indicating complex interactions among the excited Yb3+ ions. The probability of the TCL process versus the average distances among three Yb3+ ions was derived via the first- and second-order corrections to the wave functions of lanthanide ions, indicating that the formation of Yb3+ clusters containing closely spaced ions favors the occurrence of the multi-ion interaction processes. Furthermore, the cooperative sensitization of one Gd3+ ion by four excited Yb3+ ions (Yb3+-tetramer) was demonstrated experimentally, which exhibited a novel upconversion mechanism—cluster sensitization. Our results are intriguing for further exploring quantum transitions that simultaneously involve multiple ions. Scientists in China have observed ultraviolet light emission from a cluster of rare-earth Yb3+ ions in a CaF2 matrix. Zhen-Yu Liu and co-workers from Jilin University excited the polycrystalline powders with 978 nm near-infrared laser light. The resulting up-converted emission in the ultraviolet is believed to be due to a phenomenon called co-operative luminescence, whereby multiple Yb3+ ions emit a single shorter-wavelength photon by simultaneous depopulation from their excited states. In this particular case, three Yb3+ ions are thought to be involved — a hypothesis confirmed by low-temperature laser spectroscopy of the sample. This process is interesting because it could lead to light emission at wavelengths that lie outside the absorption and emission bands of single ions.

Enhanced up-conversion and temperature-sensing behaviour of Er3+ and Yb3+ co-doped Y2Ti2O7 by incorporation of Li+ ions

Abstract: Y2Ti2O7:Er3+/Yb3+ (EYYTO) phosphors co-doped with Li+ ions were synthesized by a conventional solid-state ceramic method. X-ray diffraction studies show that all the Li+ co-doped EYYTO samples are highly crystalline in nature with pyrochlore face centred cubic structure. X-ray photon spectroscopy studies reveal that the incorporation of Li+ ions creates the defects and/or vacancies associated with the sample surface. The effect of Li+ ions on the photoluminescence up-conversion intensity of EYYTO was studied in detail. The up-conversion study under ∼976 nm excitation for different concentrations of Li+ ions showed that the green and red band intensities were significantly enhanced. The 2 at% Li+ ion co-doped EYYTO samples showed nearly 15- and 8-fold enhancements in green and red band up-converted intensities compared to Li+ ion free EYYTO. The process involved in the up-conversion emission was evaluated in detail by pump power dependence, the energy level diagram, and decay analysis. The incorporation of Li+ ions modified the crystal field around the Er3+ ions, thus improving the up-conversion intensity. To investigate the sensing application of the synthesized phosphor materials, temperature-sensing performance was evaluated using the fluorescence intensity ratio technique. Appreciable temperature sensitivity was obtained using the synthesized phosphor material, indicating its applicability as a high-temperature-sensing probe. The maximum sensitivity was found to be 0.0067 K−1 at 363 K.

Coulomb heating behavior of fast light diclusters thorough the Si ⟨ 110 ⟩ direction: influence of the mean charge state

Abstract: In this work we report on the results for the Coulomb heating of H+ 2, B+ 2 and C+ 2 diclusters traveling in Si 〈 110 ⟩ direction covering an energy range from 200 keV/ion to 2400 keV/ion. Those results were obtained by combining the Rutherford backscattering spectrometry (RBS) and the particle induced X-ray emission (PIXE) techniques. By comparing the present results to those obtained previously for ions traveling in the narrower Si ⟨ 100 ⟩ channel, several common features are observed for the Coulomb heating values; especially, they follow a linear relationship as a function of the stored potential per ion. However, at variance with previous results, it is shown that the use of a Dirac-Hartree-Fock-Slater (DHFS) potential based on the ion mean charge states in amorphous targets leads to a considerable disagreement between the Coulomb heating values and the expected potential energies stored in the dicluster prior to the Coulomb explosion. In order to investigate this problem, a numerical procedure was developed in order to calculate the mean charge state values for ions traveling under channeling conditions. The use of the resulting charge states led to a linear relationship between the Coulomb heating values and the stored potential energy per ion of the diclusters. Moreover, the Coulomb heating/stored potential energy ratio amounts to about 2/3, which is in full agreement with those results obtained for the Si ⟨ 100 ⟩ direction.

Local Structure and Dynamics in the Na Ion Battery Positive Electrode Material Na3V2(PO4)2F3

Abstract: Na3V2(PO4)2F3 is a novel electrode material that can be used in both Li ion and Na ion batteries (LIBs and NIBs). The long- and short-range structural changes and ionic and electronic mobility of Na3V2(PO4)2F3 as a positive electrode in a NIB have been investigated with electrochemical analysis, X-ray diffraction (XRD), and high-resolution 23 Na and 31 P solid-state nuclear magnetic resonance (NMR). The 23 Na NMR spectra and XRD refinements show that the Na ions are removed non- selectively from the two distinct Na sites, the fully occupied Na1 site and the partially occupied Na2 site, at least at the beginning of charge. Anisotropic changes in lattice parameters of the cycled Na3V2(PO4)2F3 electrode upon charge have been observed, where a (= b) continues to increase and c decreases, indicative of solid-solution processes. A noticeable decrease in the cell volume between 0.6 Na and 1 Na is observed along with a discontinuity in the 23 Na hyperfine shift between 0.9 and 1.0 Na extraction, which we suggest is due to a rearrangement of unpaired electrons within the vanadium t2g orbitals. The Na ion mobility increases steadily on charging as more Na vacancies are formed, and coalescence of the resonances from the two Na sites is observed when 0.9 Na is removed, indicating a Na1−Na2 hopping (two-site exchange) rate of ≥4.6 kHz. This rapid Na motion must in part be responsible for the good rate performance of this electrode material. The 31 P NMR spectra are complex, the shifts of the two crystallograpically distinct sites being sensitive to both local Na cation ordering on the Na2 site in the as-synthesized material, the presence of oxidized (V 4+ ) defects in the structure, and the changes of cation and electronic mobility on Na extraction. This study shows how NMR spectroscopy complemented by XRD can be used to provide insight into the mechanism of Na extraction from Na3V2(PO4)2F3 when used in a NIB.

Structural enhancement of Na3V2(PO4)(3)/C composite cathode materials by pillar ion doping for high power and long cycle life sodium-ion batteries

Abstract: Structurally stabilized Na3V2(PO4)3/C composite cathode materials with excellent electrochemical performance can be obtained by incorporating functional pillar ions into the structure. As pillar ions, K-ions have a larger ionic radius compared to Na-ions, and play an important role in enlarging the Na-ion diffusion pathway and in increasing the lattice volume by elongating the c-axis, thereby improving the rate performance. Furthermore, since the incorporated K-ions rarely participate in the electrochemical extraction/insertion reactions, they can stabilize the Na3V2(PO4)3 structure by suppressing significant lattice volume changes or structural distortion, even in a wide range of voltage windows accompanying multiple transitions of V ions and phase distortions. We investigated how the K-ion doping level affected the crystal structure and electrochemical properties of Na3V2(PO4)3 cathode materials for Na-ion batteries.

Simulations and theory of ion injection at non-relativistic collisionless shocks

Abstract: We use kinetic hybrid simulations (kinetic ions-fluid electrons) to characterize the fraction of ions that are accelerated to non-thermal energies at non-relativistic collisionless shocks. We investigate the properties of the shock discontinuity and show that shocks propagating almost along the background magnetic field (quasi-parallel shocks) reform quasi-periodically on ion cyclotron scales. Ions that impinge on the shock when the discontinuity is the steepest are specularly reflected. This is a necessary condition for being injected, but it is not sufficient. Also, by following the trajectories of reflected ions, we calculate the minimum energy needed for injection into diffusive shock acceleration, as a function of the shock inclination. We construct a minimal model that accounts for the ion reflection from quasi-periodic shock barrier, for the fraction of injected ions, and for the ion spectrum throughout the transition from thermal to non-thermal energies. This model captures the physics relevant for ion injection at non-relativistic astrophysical shocks with arbitrary strengths and magnetic inclinations, and represents a crucial ingredient for understanding the diffusive shock acceleration of cosmic rays.