AMOEBA Polarizable Force Field for Molecular Dynamics Simulations of Glyme Solvents.

Classical molecular dynamics (MD) simulations of electrolyte systems are important to gain insight into the atom-scale properties that determine the battery-relevant performance. The recent Tinker-HP software release enables efficient and accurate MD simulations with the AMOEBA polarizable force field. In this work, we developed a procedure to construct a universal AMOEBA model for the solvent family of glymes (glycol methyl ethers), which involves a refinement scheme for valence parameters by fitting the AMOEBA-derived atomic forces to those computed at the DFT level. The refined AMOEBA model provides a good description of both local and nonlocal properties in terms of the spectroscopic response of glyme molecules, as well as the liquid glyme density and dielectric constant. In addition, the complexation energies of alkali and alkaline-earth metal cations with tetraglyme molecules obtained from AMOEBA calculations are in good agreement with DFT results, demonstrating the suitability of the developed AMOEBA model for an accurate simulation of glyme-based battery electrolytes. We also expect the procedure to be transferable to the development of AMOEBA models for other battery electrolyte systems.

The Ir-OOOO-Ir transition state and the mechanism of the oxygen evolution reaction on IrO2(110).

Carefully assessing the energetics along the pathway of the oxygen evolution reaction (OER), our computational study reveals that the "classical" OER mechanism on the (110) surface of iridium dioxide (IrO2) must be reconsidered. We find that the OER follows a bi-nuclear mechanism with adjacent top surface oxygen atoms as fixed adsorption sites, whereas the iridium atoms underneath play an indirect role and maintain their saturated 6-fold oxygen coordination at all stages of the reaction. The oxygen molecule is formed, via an Ir-OOOO-Ir transition state, by association of the outer oxygen atoms of two adjacent Ir-OO surface entities, leaving two intact Ir-O entities at the surface behind. This is drastically different from the commonly considered mono-nuclear mechanism where the O2 molecule evolves by splitting of the Ir-O bond in an Ir-OO entity. We regard the rather weak reducibility of crystalline IrO2 as the reason for favoring the novel pathway, which allows the Ir-O bonds to remain stable and explains the outstanding stability of IrO2 under OER conditions. The establishment of surface oxygen atoms as fixed electrocatalytically active sites on a transition-metal oxide represents a paradigm shift for the understanding of water oxidation electrocatalysis, and it reconciles the theoretical understanding of the OER mechanism on iridium oxide with recently reported experimental results from operando X-ray spectroscopy. The novel mechanism provides an efficient OER pathway on a weakly reducible oxide, defining a new strategy towards the design of advanced OER catalysts with combined activity and stability.

Investigating the abnormal conductivity behaviour of divalent cations in low dielectric constant tetraglyme-based electrolytes.

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.

Advancement of the Homogeneous Background Method for the Computational Simulation of Electrochemical Interfaces.

Computational studies of electrochemical interfaces based on density-functional theory (DFT) play an increasingly important role in the present research on electrochemical processes for energy conversion and storage. The homogeneous background method (HBM) offers a straightforward approach to charge the electrochemical system within DFT simulations, but it typically requires the specification of the active fraction of excess electrons based on a certain choice of the electrode-electrolyte boundary location, which can be difficult in the presence of electrode-surface adsorbates or explicit solvent molecules. In this work, we present a methodological advancement of the HBM, both facilitating and extending its applicability. The advanced version requires neither energy corrections nor the specification of the active fraction of excess electrons, providing a versatile and readily available method for the simulation of charged interfaces when adsorbates or explicit solvent molecules are present. Our computational DFT results for Pt(111), Au(111), and Li(100) metal electrodes in high-dielectric-constant solvents demonstrate an excellent agreement in the interfacial charging characteristics obtained from simulations with the advanced HBM in comparison with the (linearized) Poisson-Boltzmann model (PBM).

Stacking Versatility in Alkali-Mixed Honeycomb Layered NaKNi2TeO6.

The reaction between P2-type honeycomb layered oxides Na2Ni2TeO6 and K2Ni2TeO6 enables the formation of NaKNi2TeO6. The compound is characterized by X-ray diffraction and 23Na solid-state nuclear magnetic resonance spectroscopy, and the structure is discussed through density functional theory calculations. In addition to the honeycomb Ni/Te cationic ordering, NaKNi2TeO6 exhibits a unique example of alternation of sodium and potassium layers instead of a random alkali-mixed occupancy. Stacking fault simulations underline the impact of the successive position of the Ni/Te honeycomb layers and validate the presence of multiple stacking sequences within the powder material, in proportions that evolve with the synthesis conditions. In a broader context, this work contributes to a better understanding of the alkali-mixed layered compounds.

Activation of anionic redox in d0 transition metal chalcogenides by anion doping.

Expanding the chemical space for designing novel anionic redox materials from oxides to sulfides has enabled to better apprehend fundamental aspects dealing with cationic-anionic relative band positioning. Pursuing with chalcogenides, but deviating from cationic substitution, we here present another twist to our band positioning strategy that relies on mixed ligands with the synthesis of the Li2TiS3-xSex solid solution series. Through the series the electrochemical activity displays a bell shape variation that peaks at 260 mAh/g for the composition x = 0.6 with barely no capacity for the x = 0 and x = 3 end members. We show that this capacity results from cumulated anionic (Se2-/Sen-) and (S2-/Sn-) and cationic Ti3+/Ti4+ redox processes and provide evidence for a metal-ligand charge transfer by temperature-driven electron localization. Moreover, DFT calculations reveal that an anionic redox process cannot take place without the dynamic involvement of the transition metal electronic states. These insights can guide the rational synthesis of other Li-rich chalcogenides that are of interest for the development of solid-state batteries.

Correlating ligand-to-metal charge transfer with voltage hysteresis in a Li-rich rock-salt compound exhibiting anionic redox.

Anionic redox is a double-edged sword for Li-ion cathodes because it offers a transformational increase in energy density that is also negated by several detrimental drawbacks to its practical implementation. Among them, voltage hysteresis is the most troublesome because its origin is still unclear and under debate. Herein, we tackle this issue by designing a prototypical Li-rich cation-disordered rock-salt compound Li1.17Ti0.33Fe0.5O2 that shows anionic redox activity and exceptionally large voltage hysteresis while exhibiting a partially reversible Fe migration between octahedral and tetrahedral sites. Through combined in situ and ex situ spectroscopic techniques, we demonstrate the existence of a non-equilibrium (adiabatic) redox pathway enlisting Fe3+/Fe4+ and O redox as opposed to the equilibrium (non-adiabatic) redox pathway involving sole O redox. We further show that the charge transfer from O(2p) lone pair states to Fe(3d) states involving sluggish structural distortion is responsible for voltage hysteresis. This study provides a general understanding of various voltage hysteresis signatures in the large family of Li-rich rock-salt compounds.

Unlocking anionic redox activity in O3-type sodium 3d layered oxides via Li substitution.

Sodium ion batteries, because of their sustainability attributes, could be an attractive alternative to Li-ion technology for specific applications. However, it remains challenging to design high energy density and moisture stable Na-based positive electrodes. Here, we report an O3-type NaLi1/3Mn2/3O2 phase showing anionic redox activity, obtained through a ceramic process by carefully adjusting synthesis conditions and stoichiometry. This phase shows a sustained reversible capacity of 190 mAh g-1 that is rooted in cumulative oxygen and manganese redox processes as deduced by combined spectroscopy techniques. Unlike many other anionic redox layered oxides so far reported, O3-NaLi1/3Mn2/3O2 electrodes do not show discernible voltage fade on cycling. This finding, rationalized by density functional theory, sheds light on the role of inter- versus intralayer 3d cationic migration in ruling voltage fade in anionic redox electrodes. Another practical asset of this material stems from its moisture stability, hence facilitating its handling and electrode processing. Overall, this work offers future directions towards designing highly performing sodium electrodes for advanced Na-ion batteries.

Unveiling Pseudocapacitive Charge Storage Behavior in FeWO4 Electrode Material by Operando X-Ray Absorption Spectroscopy.

In nanosized FeWO4 electrode material, both Fe and W metal cations are suspected to be involved in the fast and reversible Faradaic surface reactions giving rise to its pseudocapacitive signature. In order to fully understand the charge storage mechanism, a deeper insight into the involvement of the electroactive cations still has to be provided. The present paper illustrates how operando X-ray absorption spectroscopy is successfully used to collect data of unprecedented quality allowing to elucidate the complex electrochemical behavior of this multicationic pseudocapacitive material. Moreover, these in-depth experiments are obtained in real time upon cycling the electrode, which allows investigating the reactions occurring in the material within a realistic timescale, which is compatible with electrochemical capacitors practical operation. Both Fe K-edge and W L3 -edge measurements point out the involvement of the Fe3+ /Fe2+ redox couple in the charge storage while W6+ acts as a spectator cation. The result of this study enables to unambiguously discriminate between the Faradaic and capacitive behavior of FeWO4 . Beside these valuable insights toward the full description of the charge storage mechanism in FeWO4 , this paper demonstrates the potential of operando X-ray absorption spectroscopy to enable a better material engineering for new high capacitance pseudocapacitive materials.

New p-type Al-substituted SrSnO3 perovskites for TCO applications?

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.

Electrolyte Reactivity in the Double Layer in Mg Batteries: An Interface Potential-Dependent DFT Study.

The electrochemical degradation of two solvent-based electrolytes for Mg-metal batteries is investigated through a grand canonical density functional theory (DFT) approach. Both electrolytes are highly reactive in the double layer region where the solvated species have no direct contact with the Mg-surface, hence emphasizing that surface reactions are not the only phenomena responsible for electrolyte degradation. Applied to dimethoxyethane (DME) and ethylene carbonate (EC), the present methodology shows that both solvents should thermodynamically decompose in the double layer prior to the Mg2+/Mg0 reduction, leading to electrochemically inactive reaction products. Based on thermodynamic considerations, Mg0 deposition should not be possible, which contrasts with experiments, at least for DME-based electrolytes. This apparent contradiction is here addressed through the rationalization of the electrochemical mechanism underlying solvent electroactivation. An extended operation potential window (OPW) is extracted, in which the Mg2+/Mg0 reduction can compete with electrolyte decomposition, thus enabling battery operation beyond the solvated species thermodynamic stability. The chemical study of the degradation products is in excellent agreement with experiments and offers rationale for the Mg-battery failure in EC electrolyte and capacity fade in DME electrolyte. The potential-dependent approach proposed herein is thus able to successfully tackle the challenging problem of interface electrochemistry. Being fully transferable to any other electrochemical systems, this methodology should provide rational guidelines for the development of viable electrolytes for multivalent batteries and, more generally, energy conversion and storage devices.

Author Correction: Unified picture of anionic redox in Li/Na-ion batteries.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Charge Transfer Band Gap as an Indicator of Hysteresis in Li-Disordered Rock Salt Cathodes for Li-Ion Batteries.

Disordered rock salt cathodes showing both anionic and cationic redox are being extensively studied for their very high energy storage capacity. Mn-based disordered rock salt compounds show much higher energy efficiency compared to the Ni-based materials as a result of the different voltage hysteresis, 0.5 and 2 V, respectively. To understand the origin of this difference, we herein report the design of two model compounds, Li1.3Ni0.27Ta0.43O2 and Li1.3Mn0.4Ta0.3O2, and study their charge compensation mechanism through the uptake and removal of Li via an arsenal of analytical techniques. We show that the different voltage hysteresis with Ni or Mn substitution is due to the different reduction potential for anionic redox. We rationalized such a finding by DFT calculations and propose this phenomenon to be nested in the smaller charge transfer band gap of the Ni-based compounds compared to that of the Mn ones. Altogether, these findings provide vital guidelines for designing high-capacity disordered rock salt cathode materials based on anionic redox activity for the next generation of Li ion batteries.

Unified picture of anionic redox in Li/Na-ion batteries.

Anionic redox in Li-rich and Na-rich transition metal oxides (A-rich-TMOs) has emerged as a new paradigm to increase the energy density of rechargeable batteries. Ever since, numerous electrodes delivering extra anionic capacity beyond the theoretical cationic capacity have been reported. Unfortunately, most often the anionic capacity achieved in charge is partly irreversible in discharge. A unified picture of anionic redox in A-rich-TMOs is designed here to identify the electronic origin of this irreversibility and to propose new directions to improve the cycling performance of the electrodes. The electron localization function is introduced as a holistic tool to unambiguously locate the oxygen lone pairs in the structure and follow their participation in the redox activity of A-rich-TMOs. The charge-transfer gap of transition metal oxides is proposed as the pertinent observable to quantify the amount of extra capacity achievable in charge and its reversibility in discharge, irrespective of the material chemical composition. From this generalized approach, we conclude that the reversibility of the anionic capacity is limited to a critical number of O holes per oxygen, hO ≤ 1/3.

Chemical Activity of the Peroxide/Oxide Redox Couple: Case Study of Ba5Ru2O11 in Aqueous and Organic Solvents.

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba5Ru2O11, which contains peroxide O22- groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty σ* antibonding O-O states limits the reversibility of the electrochemical reactions when the O22-/O2- redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O22- states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O-O bond order lower than 1 would be preferred for these applications.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode ß-Li2IrO3.

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g-1. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a ß-Li2IrO3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e- per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (Mn+) and anionic (O2)n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, ß-Li2IrO3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir4+ at potentials as low as 3.4 V versus Li+/Li0, as equivalently observed in the layered α-Li2IrO3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Visualization of O-O peroxo-like dimers in high-capacity layered oxides for Li-ion batteries.

Lithium-ion (Li-ion) batteries that rely on cationic redox reactions are the primary energy source for portable electronics. One pathway toward greater energy density is through the use of Li-rich layered oxides. The capacity of this class of materials (>270 milliampere hours per gram) has been shown to be nested in anionic redox reactions, which are thought to form peroxo-like species. However, the oxygen-oxygen (O-O) bonding pattern has not been observed in previous studies, nor has there been a satisfactory explanation for the irreversible changes that occur during first delithiation. By using Li2IrO3 as a model compound, we visualize the O-O dimers via transmission electron microscopy and neutron diffraction. Our findings establish the fundamental relation between the anionic redox process and the evolution of the O-O bonding in layered oxides.

An oxysulfate Fe2O(SO4)2 electrode for sustainable Li-based batteries.

High-performing Fe-based electrodes for Li-based batteries are eagerly pursued because of the abundance and environmental benignity of iron, with especially great interest in polyanionic compounds because of their flexibility in tuning the Fe(3+)/Fe(2+) redox potential. We report herein the synthesis and structure of a new Fe-based oxysulfate phase, Fe2O(SO4)2, made at low temperature from abundant elements, which electrochemically reacts with nearly 1.6 Li atoms at an average voltage of 3.0 V versus Li(+)/Li, leading to a sustained reversible capacity of ≈125 mAh/g. The Li insertion-deinsertion process, the first ever reported in any oxysulfate, entails complex phase transformations associated with the position of iron within the FeO6 octahedra. This finding opens a new path worth exploring in the quest for new positive electrode materials.

Palladium-silver mesowires for the extended detection of H2.

Palladium-silver mesowires are prepared by electrochemical decoration of graphite step-edges with a good control of the alloy composition and wire diameter. As-prepared arrays are used for hydrogen sensing and demonstrate extended detection capabilities up to the whole concentration range of H(2) depending on the alloy composition. At low silver content, low hydrogen concentration is detected but the sensing window is narrow because of sensor saturation. The sensing window is advantageously extended to higher hydrogen concentrations for quantitative measurements up to pure H(2) flows with Ag-rich alloys. The mechanism responsible for these behaviors implies the statistical distribution in surface composition rather than the structural characteristics and stability domains of the corresponding hydride phases.

Updated references for the structural, electronic, and vibrational properties of TiO2(B) bulk using first-principles density functional theory calculations.

To get updated references on the structural, electronic, and vibration properties of the metastable TiO(2)(B) compound, infrared and Raman spectra of TiO(2)(B) are computed within the density functional theory framework and all active modes are assigned. Phonons and their possible coupling with the macroscopic electric fields resulting from the long-range interactions of instantaneous local dipoles (due to nuclear vibrations) in polar solids are taken into account through supercell calculations and longitudinal optical-transversal optical splitting corrections. Full structural relaxations using conventional density functional theory and hybrid functionals with localized Gaussian-type orbitals or plane-wave basis sets reveal a similar deviation of the local Ti environment compared to the TiO(2)(B) structural refinements reported so far. Such deviations are shown to be significant from those computed for anatase using the same method, thus yielding distinguishable spectroscopic responses for the two polymorphs.