RESUMO
Fluoride-ion batteries (FIBs) have received significant attention as promising alternatives to conventional lithium-ion batteries, but a reversible redox reaction has not been confirmed yet for liquid-electrolyte-type FIBs. We conducted ex situ X-ray diffraction and energy dispersive X-ray analyses for a conventional full-cell assembly of FIBs, in which BiF3, a Pb plate (or Pb powder), and tetraethylammonium fluoride dissolved in propylene carbonate were used as the positive electrode, negative electrode, and liquid electrolyte, respectively. A FIB using a Pb plate exhibited a flat operating voltage at â¼0.29 V during the discharge reaction with a discharge capacity of â¼105 mA h g-1. The reversible electrochemical reaction was, however, attained when the discharge and charge capacities were controlled to be less than 20 mA h g-1. In a such capacity-limited cycle test, Bi and PbF2 phases were formed during the discharge reaction, while BiF3 and Pb phases were generated during the charge reaction. Therefore, a reversible movement of F- ions between the BiF3 and Pb electrodes, i.e., reversible redox reaction was firstly confirmed for the liquid-electrolyte-type FIB. We also attempted to improve the reversibility at the first cycle by replacing the Pb plate with Pb powder electrodes, and consequently, the FIB using an annealed Pb powder indicated the best electrochemical performance.
RESUMO
Calcium disilicide (CaSi2) possesses a layered structure composed of alternating monolayers of silicene (MLSi) and calcium. Here the mechanism by which fluorine (F) diffusion into CaSi2 leads to a phase transformation from MLSi to bilayer silicene (BLSi) was investigated. Disorder in intra-layer atomic arrangements and F aggregation were observed using HAADF-STEM in areas of low F concentration. Transformation of MLSi to BLSi in CaSi2Fx was predicted to occur at x = 0.63 based on cluster expansion (CE) and density functional theory (DFT) analyses, and these results agreed well with HAADF-STEM observations. The occurrence of F aggregation at low concentrations was also confirmed by Monte Carlo simulations using the interaction parameters obtained in CE analysis. Bader charge analysis, DFT calculations of charged states, and ab initio molecular dynamics simulations indicated that the aggregated F atoms withdrew electrons from MLSi, destabilizing the buckled honeycomb structure of MLSi in CaSi2. This charge imbalance caused the transformation of MLSi to the covalent-like BLSi.
RESUMO
Silicene, a two-dimensional honeycomb network of silicon atoms like graphene, holds great potential as a key material in the next generation of electronics; however, its use in more demanding applications is prevented because of its instability under ambient conditions. Here we report three types of bilayer silicenes that form after treating calcium-intercalated monolayer silicene (CaSi2) with a BF4(-) -based ionic liquid. The bilayer silicenes that are obtained are sandwiched between planar crystals of CaF2 and/or CaSi2, with one of the bilayer silicenes being a new allotrope of silicon, containing four-, five- and six-membered sp(3) silicon rings. The number of unsaturated silicon bonds in the structure is reduced compared with monolayer silicene. Additionally, the bandgap opens to 1.08 eV and is indirect; this is in contrast to monolayer silicene which is a zero-gap semiconductor.
RESUMO
Calcium-intercalated multilayer silicene CaSi2 exhibits a massless Dirac-cone π-electron-band dispersion like graphene, while the Dirac point is about 2 eV away from the Fermi level due to diiimide-based charge transfer from the Ca atoms to the silicene layers. This indicates that the graphene-like electronic structure with a massless Dirac cone is stably formed in the metal-intercalated multilayer silicene.
RESUMO
The electrical conduction mechanism in La3Ta0.5Ga5.3Al0.2O14 (LTGA) single crystals was studied by nonstoichiometric defect formation during crystal growth. Since stoichiometric LTGA is not congruent, the single crystal grown from the stoichiometric melt was Ta-poor and Al-rich, where Al atoms were substituted not only in Ga sites but also in Ta sites. The population of the substitutional Al in Ta sites increased with increasing oxygen partial pressure during growth (growth-pO2) in the range from 0.01 to 1 atm. Below 600 °C, substitutional Al atoms in Ta sites were ionized to yield holes, and thus the electrical conductivity of the LTGA crystal depended on temperature and the growth-pO2. The dependence of the electrical conductivity on the growth-pO2 decreased as temperature increased. The temperature rise increases ionic conductivity, for which the dominant carriers are oxygen defects formed by the anion Frenkel reaction.
RESUMO
La(3)Ta(0.5)Ga(5.5)O(14) (LTG) single crystals, which have no phase transition up to the melting point, were heat-treated in air at temperatures from 1000°C to 1450°C for 10 h. LaTaO(4) (LT) and LaGaO(3) (LG), which coexist with LTG in the three-phase region on the Ga-poor side, precipitated on the surface of the crystal for heat treatments above 1300°C because of Ga evaporation during the heat treatment. The Ga-poor state near the surface of the 1450°C heat-treated specimen was confirmed by electron probe micro-analysis measurements. The electrical resistivity of LTG single crystals decreased by heat treatment in the range of 1000°C to 1200°C for 10 h in air, where no precipitation was observed, whereas the resistivity increased with heat treatment over 1400°C for 10 h in air. The electrical resistivity of the Ga-poor surface region was higher than that of the interior.
