Computer aided-molecular design of high performance nano-carbon materials: Na on graphenes

The electronic states of sodium ion (Na⁺) trapped on the model surfaces of amorphous carbon have been investigated by means of hybrid density functional theory (DFT) calculations to elucidate the nature of interaction between Na⁺/Na and the amorphous carbon surfaces. Also, direct molecular orbital-molecular dynamics (MO-MD) calculation [Tachikawa and Shimizu, J. Phys. Chem. B, 110 (2006) 20445] was applied to diffusion processes of the Na⁺ ion on the model surface of amorphous carbon. Seven models of graphene sheets (n = 7, 14, 19, 29, 37, 44 and 52, where n means numbers of rings in each carbon cluster) were considered in the present study. The B3LYP/LANL2MB calculations showed that the sodium ion is located at 2.24-2.26 Å from the graphene surfaces. The direct MO-MD calculations showed that the Na⁺ ion diffuses freely on the surface above 300 K. At higher temperature (1100 K), the Na⁺ ion moved from the center to edge region of the model surface. The nature of the interaction between Na⁺ and the amorphous carbon surfaces was discussed on the basis of theoretical results.     

Emission of core-excited autoionizing sodium atoms from NaCl induced by electron bombardment

In the time-of-flight spectrum of positive ions produced by electron bombardment of NaCl surface a delayed signal of Na⁺ is detected. This observation is attributed to production of core-excited autoionizing metastable Na^** atoms which have their characteristic lifetimes in the 100 ns range. Moreover, it is noticed that the populations of the various metastable Na^** states depend on the NaCl layer thickness. The lifetimes of some quartet metastable Na^** states have been determined as well.     

Direct intercalation of alkali metal ions in FeOCl

The direct intercalation of Li⁺, Na⁺, K⁺ and their crown ether complexes has been observed when FeOCl reacts with the respective methoxide salts of these ions. The lattice parameters of the compounds formed are reported. The intercalation process is interpreted in terms of oxidation of the methoxide ion accompanied by a partial reduction of the Fe(III) in the FeOCl, with the intercalation of the alkali metal ion in the lattice for charge compensaion.     

Interstitial Occupancy by Extrinsic Alkali Cations in Perovskites and Its Impact on Ion Migration

Recent success in achieving highly stable Rb‐containing organolead halide perovskites has indicated the possibility of incorporating small monovalent cations, which cannot fit in the lead‐halide cage with an appropriate tolerance factor, into the perovskite lattice while maintaining a pure stable “black” phase. In this study, through a combined experimental and theoretical investigation by density functional theory (DFT) calculations on the incorporation of extrinsic alkali cations (Rb⁺, K⁺, Na⁺, and Li⁺) in perovskite materials, the size‐dependent interstitial occupancy of these cations in the perovskite lattice is unambiguously revealed. Interestingly, DFT calculations predict the increased ion migration barriers in the lattice after the interstitial occupancy. To verify this prediction, ion migration behavior is characterized through hysteresis analysis of solar cells, electrical poling, temperature‐dependent conductivity, and time‐dependent photoluminescence measurements. The results collectively point to the suppression of ion migration after lattice interstitial occupancy by extrinsic alkali cations. The findings of this study provide new material design principles to manipulate the structural and ionic properties of multication perovskite materials.     

Penne‐Like MoS2/Carbon Nanocomposite as Anode for Sodium‐Ion‐Based Dual‐Ion Battery

The dual‐ion battery (DIB) system has attracted great attention owing to its merits of low cost, high energy, and environmental friendliness. However, the DIBs based on sodium‐ion electrolytes are seldom reported due to the lack of appropriate anode materials for reversible Na⁺ insertion/extraction. Herein, a new sodium‐ion based DIB named as MoS₂/C‐G DIB using penne‐like MoS₂/C nanotube as anode and expanded graphite as cathode is constructed and optimized for the first time. The hierarchical MoS₂/C nanotube provides expanded (002) interlayer spacing of 2H‐MoS₂, which facilitates fast Na⁺ insertion/extraction reaction kinetics, thus contributing to improved DIB performance. The MoS₂/C‐G DIB delivers a reversible capacity of 65 mA h g^?1 at 2 C in the voltage window of 1.0–4.0 V, with good cycling performance for 200 cycles and 85% capacity retention, indicating the feasibility of potential applications for sodium‐ion based DIBs.     

Elastic properties and lattice dynamics of alkali chalcogenide compounds Na2S,Na2Se and Na2Te

We report on first principles study of the elastic, vibrational and dielectric properties of the alkali chalcogenide compounds Na₂S, Na₂Se and Na₂Te using the pseudopotential method within the local density approximation and linear response theory. The calculated lattice constants for the studied compounds are in good agreement with available experimental data as well as with other theoretical results. The phonon dispersion curves and phonon density of states are calculated by using density functional perturbation theory. For Na₂S the experimental features of lattice dynamics data are well reproduced by our calculations. The width of the acoustic frequency range decreases with increasing chalcogen atomic number. The phonon densities of states show that Na⁺ ions are involved in the lower frequency modes in Na₂S. However, the inverse occurs in the case of Na₂Se and Na₂Te. The mean-square ionic displacements show that Na⁺ ions perform large thermal vibrations even at temperatures well below the melting point. The Born effective charge increases, while the IR oscillator strength decreases with anion atomic number.     

Size‐, Water‐, and Defect‐Regulated Potassium Manganese Hexacyanoferrate with Superior Cycling Stability and Rate Capability for Low‐Cost Sodium‐Ion Batteries

Potassium manganese hexacyanoferrate (KMHCF) is a low‐cost Prussian blue analogue (PBA) having a rigid and open framework that can accommodate large alkali ions. Herein, the synthesis of KMHCF and its application as a high‐performance cathode in sodium‐ion batteries (NIBs) is reported. High‐quality KMHCF with low amounts of crystal water and defects and with homogeneous microstructure is obtained by controlling the nucleation and grain growth by using a high‐concentration citrate solution as a precipitation medium. The obtained KMHCF exhibits superior cycling and rate performance as a NIB cathode, showing 80% capacity retention after 1000 cycles at 1 C and a high capacity of 95 mA h g^?1 at 20 C. Unlike conventional single‐cation batteries, the hybrid NIB with KMHCF as cathode and Na as anode in Na‐ion electrolyte displays three reversible plateaus that involve stepwise insertion/extraction of both K⁺ and Na⁺ in the PBA framework. In later cycling, the K⁺–Na⁺ cointercalated phase is partially converted into a cubic sodium manganese hexacyanoferrate (NaMHCF) phase due to the increasing replacement of Na⁺ for K⁺.     

Heat capacity and Na+ ion disorder in Nasicon-type solid electrolytes Na3M2P3O12 (M2= Fe2, Cr2, ZrMg) in the temperature range 10 to 300 K

Heat-capacity measurements on Nasicon-type solid electrolytes have been performed by adiabatic calorimetry. The lattice contribution is estimated from the usual model using spectroscopic data and the excess heat capacities are discussed in terms of low-temperature magnetic transitions in iron and chromium derivatives and of Na⁺ ion disorder in the three compounds at higher temperatures. The entropy excess at 300 K is related to the relative ionic conductivity and the occupancy factors of Na⁺-available sites.     

O3‐Type Layered Ni‐Rich Oxide: A High‐Capacity and Superior‐Rate Cathode for Sodium‐Ion Batteries

Inspired by its high‐active and open layered framework for fast Li⁺ extraction/insertion reactions, layered Ni‐rich oxide is proposed as an outstanding Na‐intercalated cathode for high‐performance sodium‐ion batteries. An O3‐type Na_0.75Ni_0.82Co_0.12Mn_0.06O₂ is achieved through a facile electrochemical ion‐exchange strategy in which Li⁺ ions are first extracted from the LiNi_0.82Co_0.12Mn_0.06O₂ cathode and Na⁺ ions are then inserted into a layered oxide framework. Furthermore, the reaction mechanism of layered Ni‐rich oxide during Na⁺ extraction/insertion is investigated in detail by combining ex situ X‐ray diffraction, X‐ray photoelectron spectroscopy, and electron energy loss spectroscopy. As an excellent cathode for Na‐ion batteries, O3‐type Na_0.75Ni_0.82Co_0.12Mn_0.06O₂ delivers a high reversible capacity of 171 mAh g^?1 and a remarkably stable discharge voltage of 2.8 V during long‐term cycling. In addition, the fast Na⁺ transport in the cathode enables high rate capability with 89 mAh g^?1 at 9 C. The as‐prepared Ni‐rich oxide cathode is expected to significantly break through the limited performance of current sodium‐ion batteries.     

A Microfluidic Ion Sensor Array

A balanced concentration of ions is essential for biological processes to occur. For example, [H⁺] gradients power adenosine triphosphate synthesis, dynamic changes in [K⁺] and [Na⁺] create action potentials in neuronal communication, and [Cl^?] contributes to maintaining appropriate cell membrane voltage. Sensing ionic concentration is thus important for monitoring and regulating many biological processes. This work demonstrates an ion‐selective microelectrode array that simultaneously and independently senses [K⁺], [Na⁺], and [Cl^?] in electrolyte solutions. To obtain ion specificity, the required ion‐selective membranes are patterned using microfluidics. As a proof of concept, the change in ionic concentration is monitored during cell proliferation in a cell culture medium. This microelectrode array can easily be integrated in lab‐on‐a‐chip approaches to physiology and biological research and applications.     

Photoluminescence and low-voltage cathodoluminescence characteristics of blue phosphor, ZnGa2O4:M (M = Na, Ag)

Photoluminescence and low-voltage cathodoluminescence characteristics of ZnGa₂O₄ phosphor doped with monovalent ions has been studied. Monovalent ions such as Na⁺ and Ag⁺ are incorporated into ZnGa₂O₄ lattices in order to increase the concentration of oxygen vacancies in the spinel lattice. By doping low concentrations of monovalent ions (Na⁺, Ag⁺) into ZnGa₂O₄, the self-activated blue luminescence originated from oxygen vacancies is enhanced. Also, the blue luminescence intensity is enhanced more along with a good color purity by annealing ZnGa₂O₄:Na⁺ in a reducing atmosphere, which is due to increasing the concentration of oxygen vacancies even more. The luminescence band at the UV region (λ_max=360 nm) does not become the major luminescence band by introducing Na⁺ ion into the ZnGa₂O₄ lattice, while the UV luminescence band becomes the major one by annealing the undoped ZnGa₂O₄ in a reducing atmosphere.     

Insulator-conductor transition in 12CaO·7Al2O3 films: On the stability of the crystal lattice under Ar bombardment

Recent experimental studies suggest that thin films of a nano-porous complex oxide 12CaO·7Al₂O₃ (C12A7) are unusually stable under bombardment with energetic Ar⁺ ions [M. Miyakawa et al., J. Appl. Phys. 97 (2005) 023510.]. We employ classical Molecular Dynamics to study the processes induced by knock-on collisions of Ar⁺ with the C12A7 lattice. Our results suggest that such collisions predominantly affect the anion sublattice and form Frenkel defect pairs. We find that most of these Frenkel pairs are unstable and readily recombine with the extra-framework O²⁻ ions present in the lattice at stoichiometric concentrations of ∼ 1.2 × 10²¹ cm^− 3. This process is further facilitated by fast diffusion of the extra-framework O²⁻ ions. These results provide a useful insight in the design of new radiation-resistant materials.     

3D Graphene Networks Encapsulated with Ultrathin SnS Nanosheets@Hollow Mesoporous Carbon Spheres Nanocomposite with Pseudocapacitance‐Enhanced Lithium and Sodium Storage Kinetics

The lithium and sodium storage performances of SnS anode often undergo rapid capacity decay and poor rate capability owing to its huge volume fluctuation and structural instability upon the repeated charge/discharge processes. Herein, a novel and versatile method is described for in situ synthesis of ultrathin SnS nanosheets inside and outside hollow mesoporous carbon spheres crosslinked reduced graphene oxide networks. Thus, 3D honeycomb‐like network architecture is formed. Systematic electrochemical studies manifest that this nanocomposite as anode material for lithium‐ion batteries delivers a high charge capacity of 1027 mAh g^?1 at 0.2 A g^?1 after 100 cycles. Meanwhile, the as‐developed nanocomposite still retains a charge capacity of 524 mAh g^?1 at 0.1 A g^?1 after 100 cycles for sodium‐ion batteries. In addition, the electrochemical kinetics analysis verifies the basic principles of enhanced rate capacity. The appealing electrochemical performance for both lithium‐ion batteries and sodium‐ion batteries can be mainly related to the porous 3D interconnected architecture, in which the nanoscale SnS nanosheets not only offer decreased ion diffusion pathways and fast Li⁺/Na⁺ transport kinetics, but also the 3D interconnected conductive networks constructed from the hollow mesoporous carbon spheres and reduced graphene oxide enhance the conductivity and ensure the structural integrity.     

Isostructural and Multivalent Anion Substitution toward Improved Phosphate Cathode Materials for Sodium‐Ion Batteries

Polyanion‐type phosphate materials are highly promising cathode candidates for next‐generation batteries due to their excellent structural stability during cycling; however, their poor conductivity has impeded their development. Isostructural and multivalent anion substitution combined with carbon coating is proposed to greatly improve the electrochemical properties of phosphate cathode in sodium‐ion batteries (SIBs). Specifically, multivalent tetrahedral SiO₄^4? substitute for PO₄^3? in Na₃V₂(PO₄)₃ (NVP) lattice, preparing the optimal Na_3.1V₂(PO₄)_2.9(SiO₄)_0.1 with high‐rate capability (delivering a high capacity of 82.5 mAh g^?1 even at 20 C) and outstanding cyclic stability (≈98% capacity retention after 500 cycles at 1 C). Theoretical calculation and experimental analyses reveal that the anion‐substituted Na_3.1V₂(PO₄)_2.9(SiO₄)_0.1 reduces the bandgap of NVP lattice and enhanced its structural stability, Na⁺‐diffusion kinetics and electronic conductivity. This strategy of multivalent and isostructural anion substitution chemistry provides a new insight to develop advanced phosphate cathodes.     

Effects of argon-ion bombardment on the properties of the surface layer in spinel crystals of different composition

Ion-induced photon emission (IPE) during bombardment of magnesium aluminate spinel crystals MgO·nAl₂O₃ by 20 keV Ar⁺ ions was studied. The dependence of the yield of particles in specific excited states on the fluence of incident ions in the range of (0.1–1.6)×10¹⁷ ions/cm² was measured. It was shown that yield of magnesium and aluminum atoms and ions in most excited states do not depend (or slightly depend) on the fluence of ion bombardment. An exception was found for yields of Mg⁺ ions in the 4s ²S excited state and Al atoms in the 5p ²P⁰ excited state leading to emission lines at 292.8 and 669.6 nm, respectively. The yield of particles in these excited states drastically decreases at the start of ion bombardment. Analysis of these results and published data on the bombardment-induced surface modification of spinel crystals allows to elucidate the role of crystal structure and chemical bonding in the formation of some excited states. The dependence of excited state yield (except of Mg and Al indicated above) from spinel crystals of different composition MgO·nAl₂O₃ (n=1.0, 1.5, 2.0, and 2.5) does not reflect quantitatively the variation of the calculated bulk concentration of constituent atoms in these targets.     

Comparison of the low-energy decay mechanisms of C70 and C70

Low-energy multiple-collisional-excitation experiments have been performed on C₇₀⁺ and C₇₀⁻ at the ClusterTrap apparatus. The ions are stored in a Penning trap where they are excited via radial dipolar excitation before undergoing collisions with neutral argon atoms. The dominant decay mechanism for C₇₀⁺ (sequential loss of C₂-units) is compared with the dominant decay process of C₇₀⁻ (thermionic electron emission). A simple model based on the decay rates of the clusters is found to be in reasonable agreement with the experimental data obtained for the fragmentation process of the cationic fullerenes. The same model, when applied to the anions is observed to be in less agreement with the experimental results.     

Photoluminescence properties and photocatalytic reactivities of Cu/zeolite and Ag/zeolite catalysts prepared by the ion-exchange method

Transition metal ions (Cu⁺, Ag⁺) incorporated within the cavities of zeolites by an ion-exchange method exhibit unique photoluminescence under UV irradiation due to the inner shell type electronic transition (d⁹s¹ → d¹⁰). Detailed photoluminescence investigations revealed that the transition metal ions exist in highly dispersed state with linear 2 coordination sphere and interact with NO_x (NO and N₂O) in their photoexcited states. In fact, Cu⁺ and Ag⁺ ions within zeolites show an efficient and unique photocatalytic performance for the decomposition of NO into N₂ and O₂ at ambient temperature. Detailed studies of the interaction of NO_x with the excited states of these metal ions indicated that an electron transfer from the s orbital of the excited state of Cu⁺ or Ag⁺ ions into the π^* antibonding orbital of NO_x initiates the decomposition of NO_x into N₂ and O₂.     

Optical characterization of Bi doped SrS nanophosphors

Optical properties of Bi³⁺ doped SrS nanophosphors synthesized by solid state diffusion method in the presence of sodium thiosulfate as a flux have been reported. UV-vis absorption and photoluminescence (PL) spectra of SrS phosphors doped with trivalent Bi³⁺ ions either alone or in combination with charge compensating ions, were also studied. These studies reflect that the incorporation of Bi³⁺ into host lattice is facilitated by the charge compensating Na⁺ ions. PL emission for SrS:Bi shows a peak at 481 nm at an excitation wavelength of 430 nm, which is attributed to the transition from the ³P₁ to ¹S₀ states of Bi³⁺. We have also investigated the effect of different dopant concentrations on PL emission intensity.     

Surface layers in polycrystalline sodium \-alumina

X-ray powder diffractometry using CrK radiation has revealed the existence of surface layers 5m deep in samples of high-density polycrystalline sodium -alumina. The surface layer is prominent by virtue of its crystal lattice distortion in which the c-axis is elongated by 1%. Its presence is due to ion exchange between Na⁺ and H₃O⁺ that occurs on exposure to air.     

Effect of radiation damage on ion-induced electron emission from highly oriented pyrolytic graphite

The dependences of ion-electron emission yields γ for highly oriented pyrolytic graphite (HOPG) under 30 keV N₂⁺ ion irradiation on target temperature at different ion incidence angles (0-80°) have been measured. The fluences were 10¹⁸-10¹⁹ ion/cm² and irradiation temperatures were varied from room temperature to 400 °C. At normal ion incidence a step-like increase of yield, analogous to the γ-behaviour for polygranular graphites, has been found at an annealing temperature T_a. This effect and the changes of γ(T) with ion incidence angle are discussed in terms of the change of electron path length and the transparency of the lattice for ion beams with increasing degree of lattice order.