arXiv subjects

Michael J. Zdilla

Publications and source records attributed to Michael J. Zdilla.

3 recordsLinked to original sources

arXiv2026

Nanoconfined Grain Boundaries Increase Conductivity of Polycrystalline Molecular Crystals

Soft-solid molecular crystals consist of crystalline grains and fluid grain boundaries (GB) that enhance grain binding and transport of Li+ ions between the grains. The total ionic conductivity consists of ion migration in both the grains and GBs. To unravel these contributions in adiponitrile (Adpn)-LiPF6 molecular crystals, the GB volume fraction was varied by changing the size of the crystals and the Adpn-LiPF6 molar ratio. Molecular dynamic (MD) simulations indicate that ion motion was sub-diffusive in the grains and well-diffusive in the GBs, with GBs characterized as disordered nano-confined regions of higher charge carrier concentration (1M) than in saturated Adpn-LiPF6 solutions (0.04M), and Li+ ions predominantly solvated by cyano groups with few contact ion pairs. The diffusivity in the GBs is at least an order of magnitude higher than in the crystalline grains. The emergent picture is the grains as a reservoir of ions that migrate to the faster-conducting GBs.

cond-mat.mtrl-sci↗

arXiv2025

Enhanced activity in layered-metal-oxide-based oxygen evolution catalysts by layer-by-layer modulation of metal ion identity

Few-layered potassium nickel and cobalt oxides show drastic differences in catalytic activity based on metal ion preorganization. Uniform compositions $[(\mathrm{CoO}_2/\mathrm{K})_6$ or $(\mathrm{NiO}_2/\mathrm{K})_6]$ show limited activity, while homogenously-mixed-metal cobalt/nickel oxides $[(\mathrm{Co}_n\mathrm{Ni}_{1-n}\mathrm{O}_2/\mathrm{K})_6]$ display moderate improvement. However, a layer-by-layer arrangement of cobalt and nickel oxide sheets [e.g., $(\mathrm{CoO}_2/\mathrm{K}/\mathrm{NiO}_2/\mathrm{K})$] provides superior catalytic performance, reducing the oxygen evolution overpotential by more than 400 mV. Density functional theory simulations provide an illustration of the electronic properties (density of states and localization of orbitals) that promote catalysis in the layer-segregated materials over those of homogeneous composition. This study reveals that atomic preorganization of metal ions within layered catalysts plays a more crucial role than overall metal composition in enhancing catalytic efficiency for oxygen evolution.

cond-mat.mtrl-sci↗

arXiv2024

Comparing Meta-GGAs, +U Corrections, and Hybrid Functionals for Polaronic Point Defects in Layered MnO$_2$, NiO$_2$, and KCoO$_2$

Defects in a material can significantly tune properties and enhance utility. Hybrid functionals like HSE06 are often used to describe solids with such defects. However, geometry optimization (including accounting for effects such as Jahn-Teller distortion) using hybrid functionals is challenging for the large supercells needed for defect study. The proposed r$^2$SCAN+rVV10+U+$\mathrm{U_d}$ method, which is computationally much cheaper and faster than hybrid functionals, can successfully describe defects in materials with the proper choice of U (for the d orbitals of the host atom) and $\mathrm{U_d}$ (for those of the defect atom), as shown here for small polaron defects in layered transition-metal oxides. For a range of U and $\mathrm{U_d}$ around literature values (from solid-state reaction energies) for a given transition-metal ion and its oxidation state, we find that this approach predicts localized polaronic states in band gaps, as hybrid functionals do. The layered materials birnessite ($\mathrm{K_nMnO_2}, n=0.03 $) and $\mathrm{K_nNiO_2},n=0.03$, with one K atom intercalated between layers in a supercell, are found to have one localized occupied $\mathrm{e_g}$ polaronic state on the transition metal ion reduced by the insertion of the K atom, when the geometry is calculated as above using published U values. The expected Jahn-Teller distortion is not observed when U=$\mathrm{U_d}$=0. Layered cobalt oxide with additional potassium ions intercalated ($\mathrm{K_nCoO_2},n=1.03$) is different, due to a dramatic difference in electronic configuration of the defected Co(II) ion: A single extra K atom in the supercell leads to four localized electrons in the band gap, using standard U values, and even for U=$\mathrm{U_d}$=0.

physics.comp-ph↗