Role of the voltage window on the capacity retention of P2-Na2/3[Fe1/2Mn1/2]O2 cathode material for rechargeable sodium-ion batteries.

P2-Na2/3[Fe1/2Mn1/2]O2 layered oxide is a promising high energy density cathode material for sodium-ion batteries. However, one of its drawbacks is the poor long-term stability in the operating voltage window of 1.5-4.25 V vs Na+/Na that prevents its commercialization. In this work, additional light is shed on the origin of capacity fading, which has been analyzed using a combination of experimental techniques and theoretical methods. Electrochemical impedance spectroscopy has been performed on P2-Na2/3[Fe1/2Mn1/2]O2 half-cells operating in two different working voltage windows, one allowing and one preventing the high voltage phase transition occurring in P2-Na2/3[Fe1/2Mn1/2]O2 above 4.0 V vs Na+/Na; so as to unveil the transport properties at different states of charge and correlate them with the existing phases in P2-Na2/3[Fe1/2Mn1/2]O2. Supporting X-ray photoelectron spectroscopy experiments to elucidate the surface properties along with theoretical calculations have concluded that the formed electrode-electrolyte interphase is very thin and stable, mainly composed by inorganic species, and reveal that the structural phase transition at high voltage from P2- to "Z"/OP4-oxygen stacking is associated with a drastic increased in the bulk electronic resistance of P2-Na2/3[Fe1/2Mn1/2]O2 electrodes which is one of the causes of the observed capacity fading.

Suppressed Mobility of Negative Charges in Polymer Electrolytes with an Ether-Functionalized Anion.

Suppressing the mobility of anionic species in polymer electrolytes (PEs) is essential for mitigating the concentration gradient and internal cell polarization, and thereby improving the stability and cycle life of rechargeable alkali metal batteries. Now, an ether-functionalized anion (EFA) is used as a counter-charge in a lithium salt. As the salt component in PEs, it achieves low anionic diffusivity but sufficient Li-ion conductivity. The ethylene oxide unit in EFA endows nanosized self-agglomeration of anions and trapping interactions between the anions and its structurally homologous matrix, poly(ethylene oxide), thus suppressing the mobility of negative charges. In contrast to previous strategies of using anion traps or tethering anions to a polymer/inorganic backbone, this work offers a facile and elegant methodology on accessing selective and efficient Li-ion transport in PEs and related electrolyte materials (for example, composites and hybrid electrolytes).

Searching for Hidden Descriptors in the Metal-Ligand Bond through Statistical Analysis of Density Functional Theory (DFT) Results.

A statistical treatment of the DFT-computed heterolytic bond dissociation energies (BDE) between a diverse variety of metal fragments and ligands leads to the identification of five hidden descriptors that best characterize the bonding ability per moiety, and of a simple mathematical formula able to obtain from these hidden descriptors a BDE estimation within a few kcal/mol from the DFT value. A simple extension of this treatment beyond the original set of metal fragments and ligands is also presented. The first two hidden descriptors can be associated with the well-known concepts of σ-donation and π-effects, with the next two associated with cis influence and degree of covalency. The procedure can be easily extended to additional ligands and metal fragments, and it opens the way to an improved understanding of fundamental concepts of chemical bonding.

A computational study of radical initiated protein backbone homolytic dissociation on all natural amino acids.

Hydroxyl radical (˙OH) is known to be one of the most reactive species. In this work, the hydrogen abstraction by ˙OH from Cα and Cß atoms of all amino acids is studied in the framework of density functional theory as this is the most favorable reaction mechanism when this kind of radical attacks a protein. From the myriad routes that the oxidation of a protein by a ˙OH radical may follow, fragmentation of the protein is one of the most damaging ones as it hampers the normal function of the protein. Therefore, cleavages of the Cα-C and Cα-N backbone bonds have been investigated as the second step of the mechanism. To the best of our knowledge, this is the first time that this reaction pathway has been systematically studied for all natural amino acids. The study includes the effects that the solvent dielectrics or the conformation of the peptide model employed has on the reaction. Interestingly, the results indicate that the nature of the side chain has little effect on the H abstraction reaction, and that for most of amino acids the attack at the Cα atom is favored over the attack at the Cß atom. The origin of this preference relies on the larger capability of the formed radical intermediate to delocalize the unpaired electron, thus maximizing the captodative effect. Moreover, the reaction is more favorable when the reactant presents a ß-sheet conformation, with a completely planar peptide backbone. With respect to the homolytic splitting of the Cα-C and Cα-N bonds, the former is favorable for almost all amino acids, whereas Ser and Thr are the only amino acids favoring the latter. These results agree with previous investigations but an accurate description of the electronic density analysis performed indicates that the origin of the different reaction pathway preferences relies on a large stabilization of the product rather than bond weakening at the radical intermediate.

Quantum chemical study of the reactions between Pd+/Pt+ and H2O/H2S.

The study of the reactions of water and hydrogen sulfide with palladium and platinum cations has been completed in this work, in both low- and high-spin states. Our calculations predict that only the formation of platinum sulfide is exothermic (in both spin states), whereas for the remaining species the oxides and sulfides are found to be more reactive than their corresponding bare metal cations. An in-depth analysis of the reaction paths leading to metal oxide and sulfide species is given, including various minima, and several important transition states. All results have been compared with existing experimental and theoretical data, and earlier works covering the reaction of nickel cation with water and hydrogen sulfide to observe the trends for the group 10 transition metals.

Quantum chemical study of the reaction between Ni+ and H2S.

The reaction between the Ni(+) cation and H(2)S is studied by considering both the doublet ground state and the lowest-lying quartet state. For the doublet state the reaction is endothermic, whereas it is exothermic for the quartet state. Both CCSD(T)//B3LYP and B3LYP levels of theory, combined with the triple-zeta quality TZVP++G(3df,2p), predict that there are three spin crossings along the characterized reaction path. The first one is located after the first transition state, and the second and third ones before and after the second transition state. On the quartet potential energy surface, both transition states are close in energy to the reactants, while on the doublet surface both lie quite higher in energy. The doublet and quartet states of the HNiSH(+) four-membered intermediate lie very close in energy and their corresponding electronic configurations are connected by a single electron flip. This suggests that the -SH ligand would not prevent a facile intersystem crossing at this intermediate molecule, in contrast to the larger protection provided by the more electronegative -OH ligand.