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Solutions made of tetraglyme (G4) containing Ca(TFSI)2 have been studied as models to understand the solvation structure and the conductivity properties of multivalent ions in low dielectric constant ethereal electrolytes. These solutions have been characterised using electrochemical impedance spectroscopy, rheological measurement, and Raman spectroscopy. The ionic conductivity of these electrolytes shows an intriguing non-monotonic behaviour with temperature which deviates from the semi-empirical Vogel–Tammann–Fulcher equation at a critical temperature. This behaviour is observed for both Mg(TFSI)2 and Ca(TFSI)2, but not LiTFSI, indicating a difference in the solvation structure and the thermodynamic properties of divalent ions compared to Li+. The origin of this peculiar behaviour is demystified using temperature-controlled Raman spectroscopy and first-principles calculations combined with a thermodynamic analysis of the chemical equilibrium of Ca2+ ion-pairing versus solvation. As long-range electrostatic interactions are critical in solutions based on low dielectric ethereal solvents, a periodic approach is here proposed to capture their impact on the solvation structure of the electrolyte at different salt concentrations. The obtained results reveal that the thermodynamic and transport properties of Ca(TFSI)2/G4 solutions stem from a competition between enthalpic (ionic strength) and entropic factors that are directly controlled by the solution concentration and temperature, respectively. At high salt concentrations, the ionic strength of the solution favours the existence of free ions thanks to the strong solvation energy of the polydentate G4 solvent conjugated with the weak complexation ability of TFSI−. At elevated temperatures, the configurational entropy associated with the release of a coordinated G4 favours the formation of contact ion-pairs due to its flat potential energy surface (weak strain energy), offering a large configuration space. Such a balance between ion-pair association and dissociation not only rationalises the ionic conductivity behaviour observed for Ca(TFSI)2/G4 solutions, but also provides valuable information to extrapolate the ionic transport properties of other electrolytes with different M(TFSI)n salts dissolved in longer-chain glymes or even poly(ethylene oxide). These findings are essential for the understanding of solvation structures and ionic transport in low-dielectric media, which can further be used to design new electrolytes for Li-ion and post Li-ion batteries as well as electrocatalysts. [ABSTRACT FROM AUTHOR]

While lithium metal is considered an ideal anode for the next generation of high-energy-density batteries, some major issues such as huge volume change and continuous dendrite formation during lithium plating have hindered its practical applications. Zinc oxide (ZnO) modification of surfaces has shown great potential for inducing a homogeneous Li plating to attain dendrite-free lithium metal anodes. Although considerable improvements in electrochemical performance have been achieved, the detailed mechanism of the evolution of Li nucleation and growth morphology remains elusive. Here, we combine experimental and theoretical calculations to study the Li deposition behaviour during and after the initial nucleation on a thin and uniform layer of ZnO-coated 3D nickel foam. Upon lithiation of the ZnO layer, Li2O and LiZn are formed through a conversion reaction; this composite layer provides specific properties ensuring a homogeneous Li plating. The results showed that dendrite growth not only leads to the formation of cracks on the surface but also provokes the breakoff of some parts of the converted layers from the bulk surface. In addition, no new nucleation occurs upon continued Li deposition, with Li plating mainly taking place on the initial nuclei underneath the protective layer. As a result, large granular Li particles grow at the site of the initial Li nucleation centre, leading to the improvement of electrochemical performances. A deeper understanding of the mechanism of Li nucleation and growth and the morphology of the formed dendrites can help with the development of lithium metal batteries. [ABSTRACT FROM AUTHOR]

Mastering the metal–electrolyte interface is mandatory for the development of reliable rechargeable magnesium batteries. Nevertheless, most of the current electrolytes contain chloride species to bypass the surface passivation of magnesium, making them corrosive to other cell components and potentially irrelevant for industrial application. Here, we demonstrate a novel approach to bypass the use of such electrolytes via the mediation of an alloy-type interface prepared by coating the surface of a magnesium electrode with liquid gallium. Chemical alloying induces the formation of a surface layer, mainly composed of intermetallic Mg2Ga5, enabling significantly improved electrochemical performance with a simple chloride-free Mg(TFSI)2/DME electrolyte. Sensibly less-polarized and more stable plating/stripping is observed with symmetric cells at a current density of 0.1 mA cm−2, and longer cycle life is achieved in full cells with positive electrodes based on sulphur and organic composites. This proof-of-concept offers room for improvement in the coating protocol and could be tuned with other liquid metals. More importantly, it opens the door to electrolytes previously considered as non-compatible with magnesium metal, and consequently paves the way for the application of metal electrodes in practical magnesium batteries. [ABSTRACT FROM AUTHOR]

This paper presents a grand canonical formalism to treat electrochemical effects at interfaces. This general formalism is linked with the classical chemical hydrogen electrode (CHE) approximation and an improved approximation is proposed. This new approximation including a higher order correction that (i) keeps the low computational cost of classical CHE approach, (ii) does not require to know the type of reaction (electrochemical/not electrochemical) and (iii) should give better estimates in many problematic cases. Beyond the applicability domain of this new approximation, the homogeneous background method (HBM) which is a potential dependent density functional theory method is then presented. HBM allows computing and extracting, at ab initio level, electrochemical properties of molecules either adsorbed or in the double layer. In particular, quantitative redox potential and number of exchanged electrons can be computed giving access to non-integer electron exchange or decoupled electron/proton transfer reactions. Tools for rationalizing electrochemical reactivity consisting of potential dependant projected density of states, Fukui function and metallicity index are defined. The methodology and tools are applied to examples relevant to the energy domain in order the compare reactivity in the outer Helmholtz plane and at the surface. Then, the combination of HBM and reactivity create a toolbox usable to predict and investigate the different redox, degradation and ageing processes occurring at an electrochemical interface such as the one found in energy materials but also in all electrochemical applications. [ABSTRACT FROM AUTHOR]

Novel p-type SrSn1−xAlxO3 (x = 0, 0.2, 0.5) perovskites are presented as potential candidates for electro-optical applications. A combined experimental and theoretical study reveals that chemical substitutions can be used as a lever to stabilize oxygen holes in the valence band. [ABSTRACT FROM AUTHOR]