The role of surface termination on phonon-mediated relaxation of an excited electron in quantum dots was investigated using first-principles simulations. The surface terminations of a silicon quantum dot with hydrogen and fluorine atoms lead to distinctively different relaxation behaviors, and the fluorine termination shows a nontrivial relaxation process. The quantum confined electronic states are significantly affected by the surface of the quantum dot, and we find that a particular electronic state dictates the relaxation behavior through its infrequent coupling to neighboring electronic states. Dynamical fluctuation of this electronic state results in a slow shuttling behavior within the manifold of unoccupied electronic states, controlling the overall dynamics of the excited electron with its characteristic frequency of this shuttling behavior. The present work revealed a unique role of surface termination, dictating the hot electron relaxation process in quantum-confined systems in the way that has not been considered previously.

The Na-ion battery is recognized as a possible alternative to Li-ion battery for applications where power and cost override energy density performances. However, the increasing instability of their electrolyte with temperature is still problematic. Thus, a central question remains how to design Na-based electrolytes. Here, we report discovery of a Na-based electrolyte formulation which enlists four additives (vinylene carbonate (VC), succinonitrile (SN), 1, 3-propane sultone (PS) and sodium difluoro(oxalate)borate (NaODFB) in proper quantities that synergistically combined their positive attributes to lead a stable solid electrolyte interphase (SEI) at both negative and positive electrodes surface at 55 °C. Moreover, we rationalized the role of each additive that consists in producing specific NaF coatings, thin elastomers, sulfate-based deposits and so on via combined impedance (EIS) and X-ray photoelectron spectroscopy (XPS). We demonstrated that empirical electrolyte design rules previously established for Li-ion technology together with theoretical guidance is a vital strategy in the quest for better Na-based electrolytes that can be extended to other chemistries. Overall, this finding, which we implement to practical 18650 cells, widens the route to the rapid development of the Na-ion technology based on the Na 3 V 2 (PO 4 ) 2 F 3 /C chemistry.

Atomistic calculations of the electronic stopping power in liquid water for protons and α-particles from first principles are demonstrated without relying on linear response theory. The computational approach is based on non-equilibrium simulation of the electronic response using real-time time-dependent density functional theory. By quantifying the velocity-dependence of the steady-state charge of the projectile proton and αparticle from non-equilibrium electron densities, we examine the extent to which linear response theory is applicable. We further assess the influence of the exchange-correlation approximation in real-time timedependent density functional theory on the stopping power with range-separated and regular hybrid functionals with exact exchange.

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Searches for new electrode materials for batteries must comply on financial and environmental costs to be useful in practical devices. The sol-gel chemistry has been widely used to design and implemented new concepts for the emergence of advanced materials such as hydride organic-inorganic composites. Here, we show that the simple reaction system including titanium alkoxide and water can be used to stabilize a new class of electrode materials. By investigating the crystallization path of anatase TiO 2 , an X-ray amorphous intermediate phase has been identified whose local structure probed by the pair distribution function, 1 H solid-state NMR and DFT calculations, consists of a layered-type structure as found in the lepidocrocite. This phase presents the following general formula Ti 2-x x O 4-4x (OH) 4x .nH 2 O (x ∼ 0.5) where the substitution of oxide by hydroxide anions leads to the formation of titanium vacancies () and H 2 O molecules are located in interlayers. Solid-state 1 H NMR has enabled to characterize three main hydroxide environments that are Ti-OH, Ti 2 2-OH and Ti 3-OH and layered H 2 O molecules. The electrochemical properties of this phase were further investigated versus lithium and is shown to be very promising with reversible capacities of around 200 mAh.g-1 and an operating voltage of 1.55 V. We further showed that the lithium intercalation proceeds via a solidsolution mechanism. 7 Li solid-state NMR and DFT calculations allowed to identify lithium host sites that are located at the titanium vacancies and interlayer space with lithium being solvated by structural water molecules. The easy fabrication, the absence of lithium and easier recycling and the encouraging properties makes this class of materials very attractive for competitive electrodes for batteries. We thus demonstrate that the revisit of an "old" chemistry with advanced characterization tools allows discovering new materials of technological relevance.

Proton chemistry is a fascinating field with both fundamental and applied aspects. The development of solid-state proton conductors relying on abundant elements could help bring these two aspects. In this scope, we synthesized a disordered structure which, as revealed by the real-space refinement of the pair distribution function, has been identified to be the trititanate arrangement. The layered structure is stabilized by the presence of hydronium ions and water molecules located in the interlayer space. This compound displays a high ionic conductivity of 4·10–2 S/m with an activation energy of 0.24 eV, assigned to H+ mobility as shown by broadband dielectric spectroscopy. Proton mobility was further evidenced by solid-state proton nuclear magnetic resonance. Density functional theory calculations revealed that proton transfer occurs both within the interlayer space and with terminal oxide of the titanate framework through a Grotthuss-based mechanism rationalizing the high conductivity measured experimentally. Finally, we investigated the electrochemical properties with respect to the proton as a charge carrier using proton-free (KCl) and proton-donor (buffer acetic acid) electrolytes. The results showed that the structure can reversibly intercalate protons at a very high rate opening existing perspectives in the development of negative electrode materials for aqueous proton batteries. Overall, this study helps better understand the proton transfer mechanism occurring in a confined layered-type structure.

Molecular behaviour of liquid water under proton irradiation is of great importance to a number of technological and medical applications. The highly energetic proton generates a time-varying field that is highly localized and heterogeneous at the molecular scale, and massive electronic excitations are produced as a result of the field-matter interaction. Using first-principles quantum dynamics simulations, we reveal details of how electrons are dynamically excited through non-equilibrium energy transfer from highly energetic protons in liquid water on the atto/femto-second time scale. Water molecules along the path of the energetic proton undergo ionization at individual molecular level, and the excitation primarily derives from lone pair electrons on the oxygen atom of water molecules. A reduced charge state on the energetic proton in the condensed phase of water results in the strongly suppressed electronic response when compared to water molecules in the gas phase. These molecular-level findings provide important insights into understanding the water radiolysis process under proton irradiation.

A lamellar lepidocrocite-type titanate structure with ~25% Ti 4+ vacancies was recently synthesized, and it showed potential for use as an electrode in rechargeable lithium-ion batteries.In addition to lithium, we explore this material's ability to accommodate other monovalent ions with greater natural abundance (e.g. sodium and potassium) in order to develop lower-cost alternatives to lithium-ion batteries constructed from more widely available elements.Galvanostatic discharge/charge curves for the lepidocrocite material indicate that increasing the ionic radius of the monovalent ion results in a deteriorating performance of the electrode. Using first-principles electronic structure calculations, we identify the relaxed geometries of the structure for various positions of the ion in the structure. We then use these geometries to compute the energy of formations. Additionally, we determine that all ions are favorable in the structure, but interlayer positions are preferred compared to vacancy positions. We also conclude that the exchange between the interlayer and vacancy positions is a process which involves the interaction between interlayer water and surface hydroxyl groups next to the titanate layer. We observe a cooperative effect between structural water and OH groups to assist alkali-ions to move from the interlayer to the vacancy site. Thus, the as-synthesized lepidocrocite serves as a prototypical structure to investigate both the migration mechanism of ions within a confined space along with the interaction between water molecules and the titanate framework.