Predicting room-temperature conductivity of Na-ion super ionic conductors with the minimal number of easily-accessible descriptors

Given the vast compositional possibilities Na$_nM_mM_{m'}$Si$_{3-p-a}$P$_p$As$_a$O$_{12}$, Na-ion superionic conductors (NASICON) are attractive but complicate for designing materials with enhanced room-temperature Na-ion conductivity $\sigma_{\rm Na,300K}$. We propose an explicit regression model for $\sigma_{\rm Na,300K}$ with easily-accessible descriptors, by exploiting density functional theory molecular dynamics (DFT-MD). Initially, we demonstrate that two primary descriptors, the bottleneck width along Na-ion diffusion paths $d_1$ and the average Na-Na distance $\langle d_{\rm Na-Na} \rangle$, modulate room-temperature Na-ion self-diffusion coefficient $D_{\rm Na,300K}$. Then, we introduce two secondary easily-accessible descriptors: Na-ion content n, which influences $d_1$, $\langle d_{\rm Na-Na} \rangle$, and Na-ion density $\rho_{\rm Na}$; and the average ionic radius $\langle r_M \rangle$ of metal ions, which impacts $d_1$ and $\langle d_{\rm Na-Na} \rangle$. These secondary descriptors enable the development of a regression model for $\sigma_{\rm Na,300K}$ with $n$ and $\langle r_M \rangle$ only. Subsequently, this model identifies a promising yet unexplored stable composition, Na$_{2.75}$Zr$_{1.75}$Nb$_{0.25}$Si$_2$PO$_{12}$, which, upon DFT-MD calculations, indeed exhibits $\sigma_{\rm Na,300K} > 10^{^3}$ S$\cdot$cm$^{-1}$. Furthermore, the adjusted version effectively fits $140$ experimental values with $R^2=0.718$

Observations of High Energy Cosmic-Ray Electrons from 30 GeV to 3 TeV with Emulsion Chambers

We have performed a series of cosmic-ray electron observations using the balloon-borne emulsion chambers since 1968. While we previously reported the results from subsets of the exposures, the final results of the total exposures up to 2001 are presented here. Our successive experiments have yielded the total exposure of 8.19 m^2 sr day at the altitudes of 4.0 - 9.4 g/cm^2. The performance of the emulsion chambers was examined by accelerator beam tests and Monte-Carlo simulations, and the on-board calibrations were carried out by using the flight data. In this work we present the cosmic-ray electron spectrum in the energy range from 30 GeV to 3 TeV at the top of the atmosphere, which is well represented by a power-law function with an index of -3.28+-0.10. The observed data can be also interpreted in terms of diffusive propagation models. The evidence of cosmic-ray electrons up to 3 TeV suggests the existence of cosmic-ray electron sources at distances within ~1 kpc and times within ~1x10^5 yr ago.Comment: 38 pages, 10 figures, 3 tables, Accepted for publication in Ap

Tuning the electronic, ion transport, and stability properties of Li-rich Manganese-based oxide materials with oxide perovskite coatings: a first-principles computational study

Lithium-rich manganese-based oxides (LRMO) are regarded as promising cathode materials for powering electric applications due to their high capacity (250 mAh g–1) and energy density (~900 Wh kg–1). However, poor cycle stability and capacity fading have impeded the commercialization of this family of materials as battery components. Surface modification based on coating has proven successful in mitigating some of these problems, but a microscopic understanding of how such improvements are attained is still lacking, thus impeding systematic and rational design of LRMO-based cathodes. In this work, first-principles density functional theory (DFT) calculations are carried out to fill out such a knowledge gap and to propose a promising LRMO-coating material. It is found that SrTiO3 (STO), an archetypal and highly stable oxide perovskite, represents an excellent coating material for Li1.2Ni0.2Mn0.6O2 (LNMO), a prototypical member of the LRMO family. An accomplished atomistic model is constructed to theoretically estimate the structural, electronic, oxygen vacancy formation energy, and lithium-transport properties of the LNMO/STO interface system, thus providing insightful comparisons with the two integrating bulk materials. It is found that (i) electronic transport in the LNMO cathode is enhanced due to partial closure of the LNMO band gap (~0.4 eV) and (ii) the lithium ions can easily diffuse near the LNMO/STO interface and within STO due to the small size of the involved ion-hopping energy barriers. Furthermore, the formation energy of oxygen vacancies notably increases close to the LNMO/STO interface, thus indicating a reduction in oxygen loss at the cathode surface and a potential inhibition of undesirable structural phase transitions. This theoretical work therefore opens up new routes for the practical improvement of cost-affordable lithium-rich cathode materials based on highly stable oxide perovskite coatings.Peer ReviewedPostprint (published version

High energy electron observation by Polar Patrol Balloon flight in Antarctica

We accomplished a balloon observation of the high-energy cosmic-ray electrons in 10-1000GeV to reveal the origin and the acceleration mechanism. The observation was carried out for 13 days at an average altitude of 35km by the Polar Patrol Balloon (PPB) around Antarctica in January 2004. The detector is an imaging calorimeter composed of scintillating-fiber belts and plastic scintillation counters sandwiched between lead plates. The geometrical factor is about 600cm^2sr, and the total thickness of lead absorber is 9 radiation lengths. The performance of the detector has been confirmed by a test flight at the Sanriku Balloon Center and by an accelerator beam test using the CERN-SPS (Super Proton Synchrotron at CERN). The new telemetry system using the Iridium satellite, the power system supplied by solar panels and the automatic flight level control operated successfully during the flight. We collected 5.7×10^3 events over 100GeV, and selected the electron candidates by a preliminary data analysis of the shower images. We report here an outline of both detector and observation, and the first result of the electron energy spectrum over 100GeV obtained by an electronic counter

Key features of organic electrolyte molecules in lithium ion battery

The lithium (Li) complexes of organic electrolyte solvents are theoretically investigated using the long-range correction for density functional theory in order to figure out the cause for the high performance of cyclic carbonate electrolytes in lithium ion batteries (LIBs). Calculations of the Li complexes with ethylene carbonate solvent molecules prove that ten ligand molecules should be incorporated to obtain near-degenerate four- and five-coordination optimum structures and dramatically improved orbital energies. The geometry optimizations of the Li complexes with thirteen types of organic solvent molecules give four-coordination neutral and five-coordination cation complexes for many solvent molecules. The five-coordination Li complexes are considered to use Berry pseudorotation to approach the electrodes from the Li atom. The calculated Koopmans, vertical and adiabatic ionization potentials and electron affinities show that near-degeneracy and structural deformation effects play significant roles in the electronic states of the Li complexes. Mulliken charge and dipole moment analyses indicate that the Li complexes of cyclic carbonates construct a deep electric double layer near electrodes due to the electron-donating ability of the ligand molecules. Molecular orbital analyses also explain that the Li complexes of cyclic carbonates easily construct a solid electrolyte interface, which contributes to Li ion conductance, by localizing the accepted electron to one ligand molecule. In conclusion, the Li complexes of cyclic carbonates have three main features: preference of five-coordination structures, high electron-donating ability of ligand molecules, and localization of the accepted electron to one ligand molecule

Global search for low-lying crystal structures using the artificial force induced reaction method : A case study on carbon

We propose an approach to perform the global search for low-lying crystal structures from first principles, by combining the artificial force induced reaction (AFIR) method and the periodic boundary conditions (PBCs). The AFIR method has been applied extensively to molecular systems to elucidate the mechanism of chemical reactions such as homogeneous catalysis. The present PBC/AFIR approach found 274 local minima for carbon crystals in the C-8 unit cell described by the generalized gradient approximation-Perdew-Burke-Ernzerhof functional. Among many newly predicted structures, three low-lying structures, which exhibit somewhat higher energy compared with those previously predicted, such as Cco-C-8 (Z-carbon) and M-carbon, are further discussed with calculations of phonon and band dispersion curves. Furthermore, approaches to systematically explore two- or one-dimensional periodic structures are proposed and applied to the C-8 unit cell with the slab model. These results suggest that the present approach is highly promising for predicting crystal structures

First-Principles Microkinetic Analysis of NO + CO Reactions on Rh(111) Surface toward Understanding NO<i>_x</i> Reduction Pathways

In the NO + CO catalytic reaction on Rh(111), it is known from experiments that N₂O and N₂ are formed at low and high reaction temperatures, respectively, although the mechanism has not been fully understood. Here, we clarified its detailed mechanism using ab initio density functional theory (DFT) and microkinetic analysis. We considered that the catalytic cycle consists of following steps: NO dissociation, N₂O formation, N₂ formation (via N–N recombination or N₂O decomposition), and CO₂ formation. Their reaction energies and activation barriers were evaluated by DFT calculations and were then employed for the microkinetics and reactor simulation. We then demonstrated that N₂O and N₂ are mainly formed at low and high temperatures, respectively, in agreement with experiments. This is because (i) N₂O formation has a lower activation barrier than that of N₂ formation and thus has a faster rate at low temperature, whereas N₂ formation is dominant at high temperature because of the large exothermicity, and (ii) at a higher temperature, NO dissociation occurs more and thus sufficient amount of surface N atom is provided, accelerating N + N → N₂. This study demonstrated that to analyze the catalytic reactions in a wide temperature range the combination of the DFT calculation, surface microkinetics, and reactor simulation plays a crucial role

Nonequilibrium molecular dynamics for accelerated computation of ion–ion correlated conductivity beyond Nernst–Einstein limitation

Abstract Condensed matters with high ionic conductivities are crucial in various solid devices such as solid-state batteries. The conduction is characterized by the cooperative ionic motion associated with the high carrier density. However, the high cost of computing correlated ionic conductivities has forced almost all ab initio molecular dynamics (MD) to rely on the Nernst–Einstein dilute-solution approximation, which ignores the cross-correlation effect. Here we develop a chemical color-diffusion nonequilibrium MD (CCD-NEMD) method, which enables to calculate the correlated conductivities with fewer sampling steps than the conventional MD. This CCD-NEMD is demonstrated to well evaluate the conductivities in the representative solid electrolyte bulk Li10GeP2S12 and Li7La3Zr2O12. We also applied CCD-NEMD to the grain boundary of Li7La3Zr2O12 and demonstrated its applicability for calculating interfacial local conductivities, which is essential for investigating grain boundaries and composite electrolytes. CCD-NEMD can provide further accurate understanding of ionics with ionic correlations and promote developing solid devices