Impact of Stacking Faults and Li Substitution in LixMnO3 (0 ≤ x ≤ 2) Structural Transformations upon Delithiation.

Lithium-rich layered oxides appear in most roadmaps as next generation Li-ion cathode materials owing to their superior capacity. Within this family, Li2MnO3 represents the archetype material and is often taken as model compound to better understand the complex structural modifications occurring in the first charging cycle. In this work, density functional theory (DFT) calculations have been used to understand the impact of stacking faults in the structural transformations occurring in Li2MnO3 upon delithiation, which are found to hinder the phase transformations leading to structural degradation. The formation energies of both ideal and defective LixMnO3 compositions and the analysis of the encountered ground states have been used to rationalize the predicted differences in terms of structural evolution. From the understanding of the origin in the O1 phase transformation, Mg substitution is proposed as alternative strategy to improve the structural stability in this family of materials.

DFT-Assisted Solid-State NMR Characterization of Defects in Li2MnO3.

The complete description of defective structures and their impact on materials behavior is a great challenge due to difficulties associated with their reliable characterization in the nanoscale. In this paper, density functional theory (DFT) calculations are used to elucidate the solid-state nuclear magnetic resonance (NMR) spectra of Li2MnO3 which, combined with X-ray diffraction (XRD), provide a full description of disorder in this compound. While XRD allows accurate quantification of planar defects, the use of solid-state NMR reveals limited vacancy concentrations that were undetected by XRD as NMR is highly sensitive to the atomic local environments. The combination of these methods is here proved highly effective in overcoming the challenges of describing in great detail limited concentrations of disorder in transition metal oxides, providing information about structural variables that are essential to their application.

Enhanced electrochemical performance of Li-rich cathode materials through microstructural control.

The microstructural complexity of Li-rich cathode materials has so far hampered understanding the critical link between size, morphology and structural defects with both capacity and voltage fadings that this family of materials exhibits. Li2MnO3 is used here as a model material to extract reliable structure-property relationships that can be further exploited for the development of high-performing and long-lasting Li-rich oxides. A series of samples with microstructural variability have been prepared and thoroughly characterized using the FAULTS software, which allows quantification of planar defects and extraction of average crystallite sizes. Together with transmission electron microscopy (TEM) and density functional theory (DFT) results, the successful application of FAULTS analysis to Li2MnO3 has allowed rationalizing the synthesis conditions and identifying the individual impact of concurrent microstructural features on both voltage and capacity fadings, a necessary step for the development of high-capacity Li-ion cathode materials with enhanced cycle life.

Capture and dissociation in the complex-forming CH(v = 0,1) + D2 → CHD + D, CD2 + H, CD + HD reactions and comparison with CH(v = 0,1) + H2.

Rate coefficients for the CH(v = 0,1) + D(2) reaction have been determined for all possible channels (T: 200-1200 K), using the quasiclassical trajectory method and a suitable treatment of the zero point energy. Calculations have also been performed on the CH(v = 1) + H(2) reaction and the CH(v = 1) + D(2) → CH(v = 0) + D(2) process. Most of the results can be understood considering the key role played by the deep minimum of the potential energy surface (PES), the barrierless character of the PES, the energy of the reaction channels, and the kinematics. The good agreement found between theory and experiment for the rate coefficients of the capture process of CH(v = 0) + D(2), the total reactivity of CH(v = 1) + D(2), H(2), as well as the good agreement observed for the related CH(v = 0) + H(2) system (capture and abstraction), gives confidence on the theoretical rate coefficients obtained for the capture processes of CH(v = 1) + D(2), H(2), the individual reactive processes of CH(v = 1) + D(2), H(2), the abstraction and abstraction-exchange reactions for CH(v = 0) + D(2), and the inelastic process mentioned above, for which there are no experimental data available, and that can be useful in combustion chemistry and astrochemistry.

Capture and dissociation in the complex-forming CH + H2 → CH2 + H, CH + H2 reactions.

The rate coefficients for the capture process CH + H(2)→ CH(3) and the reactions CH + H(2)→ CH(2) + H (abstraction), CH + H(2) (exchange) have been calculated in the 200-800 K temperature range, using the quasiclassical trajectory (QCT) method and the most recent global potential energy surface. The reactions, which are of interest in combustion and in astrochemistry, proceed via the formation of long-lived CH(3) collision complexes, and the three H atoms become equivalent. QCT rate coefficients for capture are in quite good agreement with experiments. However, an important zero point energy (ZPE) leakage problem occurs in the QCT calculations for the abstraction, exchange and inelastic exit channels. To account for this issue, a pragmatic but accurate approach has been applied, leading to a good agreement with experimental abstraction rate coefficients. Exchange rate coefficients have also been calculated using this approach. Finally, calculations employing QCT capture/phase space theory (PST) models have been carried out, leading to similar values for the abstraction rate coefficients as the QCT and previous quantum mechanical capture/PST methods. This suggests that QCT capture/PST models are a good alternative to the QCT method for this and similar systems.

Effect of the total angular momentum on the dynamics of the H2 + H2 system.

Extended full-dimensional quasiclassical trajectory calculations have been performed for the H(a)H(b) (v(ab) = 10, 11, 12, 13, 14, j(ab) = 0) + H(c)H(d) (v(cd) = 0, j(cd) = 0) collisions at values of the translational energy ranging from threshold to 1.5 eV and values of the total angular momentum quantum number J varying from zero to very large ones. Collision-induced dissociation, four-center exchange reaction, and single exchange process probabilities have been calculated. Full-dimensional classical calculations were found to reproduce well the corresponding (J = 0) quantum results, including the thresholds. In contrast, the agreement of full-dimensional classical calculations with the corresponding both quantum and classical reduced dimensionality ones was found to be poor. The effect of varying J on the efficiency of the various processes has also been investigated. Four-center reactions were found to be favored by low values of J, whereas dissociation processes were found to be favored by higher values of J, as expected from the fact that energy exchange takes place at longer range than mass exchange. To evaluate to what extent the J = 0 full-dimensional calculations represent the unconstrained dynamics of the system, J-shift model classical results were compared with the all-J ones. Product vibrational distributions for both partially dissociative and exchange processes were also found to depend significantly on the value of J.

A detailed comparison of centrifugal sudden and J-shift estimates of the reactive properties of the N + N2 reaction.

An extended comparison of the reactive properties of the N + N(2) exchange reaction calculated on a non-collinear dominant potential energy surface using both a centrifugal sudden and a J-shift quantum method is reported. The choice of carrying out such an investigation for N + N(2) is motivated by the fact that the best available (and currently used for spacecraft re-entry simulations) computed set of kinetic data has been worked out using the low level J-shift approximation though based on exact quantum zero total angular momentum probabilities. The fact that our investigation is carried out for a heavy system and a potential energy surface free of wells in the strong interaction region minimizes the occurrence of tunnel, resonance and interference effects which would make the rationalization of the result difficult and the centrifugal sudden treatment less accurate. The study has provided evidence of two important limits of the J-shift approximation: the wrong determination of the maximum value of the total angular momentum quantum number J contributing to reactivity and the lack of deformation of the partial reactive probability dependence on energy at fixed J value. Accordingly, it has been found that the J-shift state-specific cross sections underestimate the corresponding CS values when the initial diatomic rotational energy is low while the situation reverses when the initial diatomic rotational energy is high.

A comparison of the quantum state-specific efficiency of N + N2 reaction computed on different potential energy surfaces.

State-to-state exact quantum probabilities of the N + N2 exchange reaction have been calculated on the recently proposed L4 potential energy surface fitted to high level ab initio points using full-dimensional time-independent quantum techniques. Thermal rate coefficient values calculated on L4 were found not to differ from those calculated on a previously proposed potential energy surface. On the contrary, state-specific reaction probabilities calculated on the two surfaces are shown to differ significantly. Arguments for attributing the difference to specific features of the considered potential energy surfaces are provided.

Quantum mechanical capture/phase space theory calculation of the rate constants for the complex-forming CH + H(2) reaction.

Six-dimensional wave packet calculations on an accurate potential energy surface are used to obtain the quantum mechanical capture (QM C) probabilities for CH + H(2) corresponding to a variety of total angular momenta and internal reactant states. Rate constant calculations are made feasible by employing a Monte Carlo based sampling procedure. The QM C probabilities alone are also used to estimate the high pressure CH + H(2) rate constants corresponding to stabilization or CH(3) formation. The rate constants for CH + H(2) --> CH(2) + H reaction in the low pressure limit are obtained by combining the QM C probabilities with a phase space theory (PST) approximation for product formation from the complex. Our results are compared with the experimental results of Brownsword et al. (J. Chem. Phys. 1997, 106, 7662), as well as with purely classical PST calculations. The QM C probabilities are shown to be highly dependent on the initial rotational states of the reactants corresponding to orientational restrictions on complex formation. Consistent with this, our QM C high pressure rate constants for CH(3) formation are lower than the purely classical PST rate constants. These QM C rate constants also are in reasonable accord with experiment. A similar but somewhat more subtle picture emerges regarding the QM C/PST rate constants for CH(2) + H formation.

Modeling the global potential energy surface of the N + N2 reaction from ab initio data.

A new global potential energy surface for the N + N2 exchange reaction has been built from ab initio data. To overcome the difficulty of carrying out ab initio calculations for a large set of geometries the alternative strategy of fitting the minimum energy paths of the surface and their angular dependence using a modified LAGROBO functional form has been adopted. In this way we have been able to reproduce all the main features of the potential using a fairly small set of ab initio values.

Theoretical study of the complex-forming CH + H2 --> CH2 + H reaction.

The complex-forming CH + H2 --> CH2 + H reaction is studied employing a recently developed global potential energy function. The reaction probability in the total angular momentum J = 0 limit is estimated with a four-atom quantum wave packet method and compared with classical trajectory and statistical theory results. The formation of complexes from different reactant internal states is also determined with wave packet calculations. While there is no barrier to reaction along the minimum energy path, we find that there are angular constraints to complex formation. Trajectory-based estimates of the low-pressure rate constants are made and compared with experimental results. We find that zero-point energy violation in the trajectories is a particularly severe problem for this reaction.