Nikolaos Kateris

The drag force on planar structures of arbitrary shape is derived in free molecular flow using gas kinetic theory. The theory is formulated by considering the anisotropic intermolecular potential between the particle and gas molecules, in the limits of specular and diffuse scatterings. The drag force theory formulated by Dahneke [J. Aerosol Sci. 4, 147 (1973)10.1016/0021-8502(73)90066-9] for disklike bodies in free-molecular flow is shown to be a special case of the theory derived herein. The Einstein-Smoluchowski relationship is generalized with anisotropic drag to structures of arbitrary shape. Binary diffusion coefficients of planar molecules (benzene, naphthalene, phenanthrene, pyrene, coronene, and ovalene) in nitrogen are calculated using gas kinetic theory. The diffusion coefficients are in close agreement with available experimental data and with molecular dynamics simulations.

Lithium-sulfur (Li-S) battery is a promising battery technology, thanks to their high energy density and the use of inexpensive materials. A Li-S battery typically consists of a Li anode, a sulfur cathode or sulfur-based catholyte, and an organic solvent for lithium polysulfides (Li₂S _x ). Upon discharge, Li₂S _x chains accept Li and undergo successive chain shortening events, with the final discharge product being Li₂S. Li₂S is however insoluble in typical organic solvents and its irreversible precipitation leads to the impairment of Li-S batteries. The addition of phosphorus pentasulfide (P₂S₅) to a Li₂S _x catholyte has been experimentally and theoretically shown to form complexes with sulfur chains, which can accommodate Li₂S, thus overcome the problem of Li₂S precipitation and improving battery cyclability. [1] Quantum chemistry and specifically density functional theory (DFT) calculations have been used to predict molecular structures and electrochemical potentials for Li-S batteries, as well as other electrochemical systems. Here we present a series of DFT calculations (uB3LYP/6-31+G(2df,p)) using the P₂S₅-Li₂S₈ system as a case study to highlight considerations critical to reliable electrochemical predictions. Here, we focus on entropy, atomic structure of metal anode surface sites, and the level of DFT theory. The level of theory used throughout this study (uB3LYP/6-31+G(2df,p)) was chosen after careful comparison with other levels of theory, with varying methods and functionals (HF, B3LYP, M06, M11) and basis sets (3-21G, 6-31G(d), 6-31+G(2df,p), 6-311G(d), 6-311+G(2df,p), cc-pVTZ). The B3LYP functional with the 6-31+G(2df,p) or cc-pVTZ basis set performed best in predicting electrochemical potentials in agreement with the cyclic voltammetry experiments, as well as the nuclear magnetic resonance spectra. In addition, different solvents, all with similar dielectric constants, were considered and the resulting impact on E ⁰ was found to be negligible. The standard electrochemical potential for a full-cell reaction (E ⁰) is directly related to the Gibbs free energy of the reaction (ΔG ⁰): ΔG ⁰ = –nFE ⁰, where n is the number of electrons transferred and F is the Faraday constant. Gibbs free energy comprises an electronic energy (E _elec), a temperature dependent sensible enthalpy (H _sens) and a temperature dependent entropy term (–TS). Often, the sensible enthalpy and entropy terms are neglected and reaction Gibbs free energy is approximated as the difference of electronic energy between products and reactants. Our results for reactions involving lithium polysulfide chains and sulfur chains anchored on phosphorus groups indicate that, even at room temperature, the entropy term has a significant contribution to E ⁰ (of 0.2-0.3 V). Importantly, without considering this contribution, theoretically predicted electrochemical potentials do not match experimental results from cyclic voltammetry. Additionally, atomic structures of different surface sites impact the evaluation of E ⁰. Several surface sites, as shown in Fig. 1a, were simulated for the atom to be reacted (shown transparent in Fig. 1a), while keeping the rest of the atoms stationary. The Gibbs free energy of reaction was calculated for each site and its effect on E ⁰ was compared with that of the standard thermodynamic result for bulk Li(s), as shown in Fig. 1b. We found that as pexted, Li atoms in the ‘lone’ or ‘pair’ configuration react more readily. In the case of the ‘lone’ Li, E ⁰ increases by 0.35 V. In summary, DFT was used to identify the complexation and reaction mechanisms governing P₂S₅-Li₂S₈ catholyte batteries. In the calculation of electrochemical potentials for this system, we identified that special care should be taken to the temperature dependence of Gibbs free energy of reactions, the variation of local surface sites for the metal anode, and the level of DFT theory used. These considerations will aid the understanding of Li-S and other battery chemistry. [1] Wang, P., Kateris, N., Li, B., Zhang, Y., Luo, J., Wang, C., Zhang, Y., Jayaraman, A.S., Hu, X., Wang, H. and Li, W., 2023. High-Performance Lithium–Sulfur Batteries via Molecular Complexation. Journal of the American Chemical Society, 145 (2023) 18865–18876. Figure 1

Beyond lithium-ion technologies, lithium-sulfur batteries stand out because of their multielectron redox reactions and high theoretical specific energy (2500 Wh kg^-1). However, the intrinsic irreversible transformation of soluble lithium polysulfides to solid short-chain sulfur species (Li₂S₂ and Li₂S) and the associated large volume change of electrode materials significantly impair the long-term stability of the battery. Here we present a liquid sulfur electrode consisting of lithium thiophosphate complexes dissolved in organic solvents that enable the bonding and storage of discharge reaction products without precipitation. Insights garnered from coupled spectroscopic and density functional theory studies guide the complex molecular design, complexation mechanism, and associated electrochemical reaction mechanism. With the novel complexes as cathode materials, high specific capacity (1425 mAh g^-1 at 0.2 C) and excellent cycling stability (80% retention after 400 cycles at 0.5 C) are achieved at room temperature. Moreover, the highly reversible all-liquid electrochemical conversion enables excellent low-temperature battery operability (>400 mAh g^-1 at -40 °C and >200 mAh g^-1 at -60 °C). This work opens new avenues to design and tailor the sulfur electrode for enhanced electrochemical performance across a wide operating temperature range.

Understanding nanodiamond structures is of great scientific and practical interest. It has been a long-standing challenge to unravel the complexity underlying nanodiamond structures and to resolve the controversies surrounding their polymorphic forms. Here, we use transmission electron microscopy with high-resolution imaging, electron diffraction, multislice simulations, and other supplementary techniques to study the impacts of small sizes and defects on cubic diamond nanostructures. The experimental results show that common cubic diamond nanoparticles display the (200) forbidden reflections in their electron diffraction patterns, which makes them indistinguishable from new diamond (n-diamond). The multislice simulations demonstrate that cubic nanodiamonds smaller than 5 nm can present the d -spacing at 1.78 Å corresponding to the (200) forbidden reflections, and the relative intensity of these reflections increases as the particle size decreases. Our simulation results also reveal that defects, such as surface distortions, internal dislocations, and grain boundaries can also make the (200) forbidden reflections visible. These findings provide valuable insights into the diamond structural complexity at nanoscale, the impact of defects on nanodiamond structures, and the discovery of novel diamond structures.