Influence of Stacking on H⁺ Intercalation in Layered <i>A</i>CoO₂ (<i>A</i> = Li, Na) Cathode Materials and Implications for Aqueous Li-Ion Batteries: A First-Principles Investigation

Li- and Na-ion batteries are effective energy storage technologies. Nonetheless, currently used organic-electrolyte batteries present well-known safety problems. Therefore, the research community is intensively looking for potential alternatives. Aqueous batteries based on low-cost salts in water could be an interesting choice since they are safe and environmentally benign. However, working with aqueous electrolytes brings new detrimental mechanisms such as proton intercalation. Understanding the (de)­intercalation chemistry of protons and alkali is one of the keys for designing cathode materials in such aqueous electrochemical cells. In this work, we carry out density functional theory calculations to investigate the H+/alkali exchange in layered LiCoO2 and NaCoO2 materials. By computing the grand potential phase diagram and voltage–composition plots, we determine the relative stability of several orderings of protons, alkali, and vacancies. The fully protonated CoO2 lattice (CoO­(OH)) is revealed to be the most stable insertion product due to the formation of interlayer hydrogen bonds. Our computations demonstrate the key role of layer stacking: H+ insertion is favored in prismatic (P) stacking, while Li favors octahedral (O) stacking. While the fully protonated layered cobalt oxide is the thermodynamically favored product when protons and alkali compete, we show that mixing protons and lithium is energetically disfavored because of the different stacking preferences. We suggest that the kinetic difficulty in nucleating fully protonated phases in the layered oxide prevents proton insertion when cycling LiCoO2 in an aqueous electrolyte. The good cyclability and lack of proton insertion in LiCoO2 are, therefore, a result of the slow kinetics of protonation in partially lithiated cobalt oxide. On the other hand, we demonstrate that NaCoO2 is prone to proton and alkali mixing due to the different stacking preferences for sodium. We hypothesize that this could lead to proton intercalation and poor performances in aqueous batteries for NaCoO2 cathodes

Effect of Aqueous Electrolytes on LiCoO₂ Surfaces: Role of Proton Adsorption on Oxygen Vacancy Formation

Aqueous electrolytes are a safer, greener, and cheaper solution for energy storage applications. However, aqueous Li-ion batteries (ALIBs) suffer from faster degradation and poorer cyclability. The presence of H+ and O loss have often been claimed to deteriorate electrode materials in aqueous electrolytes. Understanding the surface reactivity of the commercial LiCoO2 cathode with respect to aqueous electrolytes and O loss is essential for designing cathode materials in such aqueous electrochemical cells. In this work, we use density functional theory calculations to investigate the stability and structure of several low-index surfaces of layered Li1–xCoO2 (0 ≤ x ≤ 0.5) before and after H+ adsorption. We compute the binding energies of H+ from low to full coverage regimes. By employing ab initio atomistic thermodynamics, we determine the stability of O vacancies on protonated and nonprotonated layered LiCoO2 surfaces. Our computations demonstrate that O loss is energetically favorable on the lowest energy surfaces, i.e., on the most exposed surface terminations. We suggest that the O vacancy formation is directly related to the transition metal (Co) coordination. Finally, the role of H+ on O loss is investigated, showing that H+ can facilitate the generation of O vacancies in some surface terminations

Electronic Properties of Hybrid Zinc Oxide–Oligothiophene Nanostructures

Using density functional theory in combination with model potential molecular dynamics, we study hybrid systems consisting of oligothiophene molecules with increasing chain length (two, four, and six rings) adsorbed onto a ZnO nanoparticle model. We investigate the energetics of adhesion and the morphological features at the curved interface. We compute the energy-level alignment taking many body effects into account within the ΔSCF approach. Our results show that, as a consequence of the local curvature of the interface, the electronic coupling between the organic and inorganic component affects the energy-level alignment in all systems, making it less favorable for charge separation. In particular, the energy-level alignment for sexithiophene on the ZnO curved nanoparticle does not lead to a type-II junction with staggered band gaps, contrary to what was recently found for sexithiophene on a flat (101̅0) ZnO surface. Although the limited size (and hence the large curvature) of the nanoparticle does not allow us to make a general statement, this indicates a trend that is valid for systems in which quantum confinement effects are important. As a side result of our study, we propose a simple practical model to predict the energy-level alignment in hybrid systems, which gives consistent results compared to ΔSCF

Searching for Materials with High Refractive Index and Wide Band Gap: A First-Principles High-Throughput Study

Materials combining both a high refractive index and a wide band gap are of great interest for optoelectronic and sensor applications. However, these two properties are typically described by an inverse correlation with high refractive index appearing in small gap materials and vice-versa. Here, we conduct a first-principles high-throughput study on more than 4000 semiconductors (with a special focus on oxides). Our data confirm the general inverse trend between refractive index and band gap but interesting outliers are also identified. The data are then analyzed through a simple model involving two main descriptors: the average optical gap and the effective frequency. The former can be determined directly from the electronic structure of the compounds, but the latter cannot. This calls for further analysis in order to obtain a predictive model. Nonetheless, it turns out that the negative effect of a large band gap on the refractive index can counterbalanced in two ways: (i) by limiting the difference between the direct band gap and the average optical gap which can be realized by a narrow distribution in energy of the optical transitions and (ii) by increasing the effective frequency which can be achieved through either a high number of transitions from the top of the valence band to the bottom of the conduction or a high average probability for these transitions. Focusing on oxides, we use our data to investigate how the chemistry influences this inverse relationship and rationalize why certain classes of materials would perform better. Our findings can be used to search for new compounds in many optical applications both in the linear and non-linear regime (waveguides, optical modulators, laser, frequency converter, etc.)

<i>h</i>‑MBenes (M/B = 1:1) as Promising Electrocatalysts for Nitrogen Reduction Reaction: A Theoretical Study

MAB phases and their two-dimensional (2D) derivatives MBenes have attracted increasing attention in electrochemical catalysis because of their unique structures and inherent electronic properties. Since the first hexagonal MAB (h-MAB) phase Ti2InB2 and 2D TiB h-MBene were discovered in 2019, the family of h-MBenes shows a promising perspective in electrochemical applications. In this work, the electrochemical nitrogen reduction reaction (eNRR) properties of discovered h-MBenes are studied theoretically for the first time. A volcano-shaped relationship between the limiting potential (UL) and the adsorption energy of the **NNH group (ΔENNH) is established. Moreover, it is found that the catalytic activity can be engineered by the bimetallic alloying effect, which applies to both in-plane ordered h-M′2/3M″1/3B phases and h-MBs with a second transition metal alloyed. Remarkably, guided by the revealed volcano-shaped relationship, Rh-alloyed hexagonal 2D WB and NbB with UL as small as −0.34 and −0.56 V, respectively, are designed. Finally, the transition metal alloying is revealed to regulate the orbital energy redistribution, consequently adjusting the binding strength of N-containing intermediates with h-MBene surfaces to an appropriate range. This work unravels the promise of h-MBenes as eNRR catalysts and can shed light on the potential for h-MBenes in extensive electrochemical applications

JaGeo/PaulingPublication: "The Limited Predictive Power of the Pauling Rules"

This is the version corresponding to the final publication "The Limited Predictive Power of the Pauling Rules".</p

Electronic Transport Properties of 1,1′-Ferrocene Dicarboxylic Acid Linked to Al(111) Electrodes

The electronic transport properties of the 1,1′-ferrocene dicarboxylic acid sandwiched between Al(111) electrodes are studied using first-principles methods. The transmission spectra and the current−voltage characteristics are computed for various two-terminal device models and their relation with the electronic structure of the molecule is thoroughly discussed. The current−voltage characteristics are asymmetric, spin-independent, and vary with the anchoring structure of the molecule to the electrodes. A fine-tuning of the molecular conductance can be easily achieved by applying a gate potential, which is included in our simulations. Interestingly, a spin-polarized current can emerge as a consequence of the gate potential with the relative contribution of the two spin channels varying with the bias

Surface Enhanced Infrared Absorption mechanism and modification of the plasmonic response

Surface Enhanced Infrared Absorption (SEIRA) is an experimental method where trace amount of a compound can be detected with high sensibility. This high detection sensibility is the result of the interaction of the molecules with a localized plasmon, usually from a metallic nano-particle. In this study we numerically investigate by discrete dipole approximation the origin of the Fano-like response of the system, including the induced transparency when the plasmon resonance and the molecular vibrational mode coincide. The detailed analysis of the localization of the absorption show that the modification of the absorption cross-section when the molecule is present comes from a change of the plasmonic resonance, not from the direct molecular response which is negligible. This sheds a new light on the SEIRA mechanism. In particular, it demonstrates that the sensibility is associated with the influence of the molecule on the plasmon resonance rather than with the local field enhancement itself

Data corresponding to the publication "The limited predictive power of the Pauling Rules"

This is the data set corrresponding to the publication "The limited predictive power of the Pauling rules". This data can be reproduced with the following code: https://doi.org/10.5281/zenodo.3654428</p

Limits to Hole Mobility and Doping in Copper Iodide

Over one hundred years have passed since the discovery of the p-type transparent conducting material copper iodide, predating the concept of the “electron–hole” itself. Supercentenarian status notwithstanding, little is understood about the charge transport mechanisms in CuI. Herein, a variety of modeling techniques are used to investigate the charge transport properties of CuI, and limitations to the hole mobility over experimentally achievable carrier concentrations are discussed. Poor dielectric response is responsible for extensive scattering from ionized impurities at degenerately doped carrier concentrations, while phonon scattering is found to dominate at lower carrier concentrations. A phonon-limited hole mobility of 162 cm2 V–1 s–1 is predicted at room temperature. The simulated charge transport properties for CuI are compared to existing experimental data, and the implications for future device performance are discussed. In addition to charge transport calculations, the defect chemistry of CuI is investigated with hybrid functionals, revealing that reasonably localized holes from the copper vacancy are the predominant source of charge carriers. The chalcogens S and Se are investigated as extrinsic dopants, where it is found that despite relatively low defect formation energies, they are unlikely to act as efficient electron acceptors due to the strong localization of holes and subsequent deep transition levels