Redox Potential and Crystal Chemistry of Hexanuclear Cluster Compounds.

Most of TM6-cluster compounds (TM = transition metal) are soluble in polar solvents, in which the cluster units commonly remain intact, preserving the same atomic arrangement as in solids. Consequently, the redox potential is often used to characterize structural and electronic features of respective solids. Although a high lability and variety of ligands allow for tuning of redox potential and of the related spectroscopic properties in wide ranges, the mechanism of this tuning is still unclear. Crystal chemistry approach was applied for the first time to clarify this mechanism. It was shown that there are two factors affecting redox potential of a given metal couple: Lever's electrochemical parameters of the ligands and the effective ionic charge of TM, which in cluster compounds differs effectively from the formal value due to the bond strains around TM atoms. Calculations of the effective ionic charge of TMs were performed in the framework of bond valence model, which relates the valence of a bond to its length by simple Pauling relationship. It was also shown that due to the bond strains the charge depends mainly on the atomic size of the inner ligands.

Metal-Metal Bond in the Light of Pauling's Rules.

About 70 years ago, in the framework of his theory of chemical bonding, Pauling proposed an empirical correlation between the bond valences (or effective bond orders (BOs)) and the bond lengths. Till now, this simple correlation, basic in the bond valence model (BVM), is widely used in crystal chemistry, but it was considered irrelevant for metal-metal bonds. An extensive analysis of the quantum chemistry data computed in the last years confirms very well the validity of Pauling's correlation for both localized and delocalized interactions. This paper briefly summarizes advances in the application of the BVM for compounds with TM-TM bonds (TM = transition metal) and provides further convincing examples. In particular, the BVM model allows for very simple but precise calculations of the effective BOs of the TM-TM interactions. Based on the comparison between formal and effective BOs, we can easily describe steric and electrostatic effects. A possible influence of these effects on materials stability is discussed.

A revisit of the bond valence model makes it universal.

The use of simple, intuitive equations to correlate the geometry of crystal structures with electron descriptors of chemical bonds and material structural stability is a great advantage of the Bond Valence Model (BVM), which is based on Pauling's principles of bond order (BO) conservation and exponential BO/bond length relationship. However, the high potential of BVM to be used as an important analytical tool was overlooked in recent inorganic chemistry due to its empirical character and serious restrictions for its application. Recent quantum chemistry data (BOs and electron densities at the bond critical points, ρc) enable us to establish the validity of the BVM to any type of chemical bonds, as well as a direct BO/ρc relationship. Such a BVM revisit overcomes most of the limitations anticipated previously for the model and thus makes it universal. This Perspective highlights the advance in model development, in particular its application to compounds with metal-metal bonds, which allows us to establish (i) a linear correlation between BOs and stretching force constants used as a measure of the bond strength and (ii) a quantitative description of the steric/electrostatic effects in cluster compounds enabling us to understand their nature and the influence of such effects on structural stability. Thus, using interatomic distances, the simple Pauling equation and empirical constants, it is possible to calculate effective BOs and predict stretching force constants and electron densities at the bond critical points in any complex compound, all of this at zero cost!

Assessing the Strength of Metal-Metal Interactions.

A proper comparison between bond strengths of different atom pairs is relevant only for the same formal bond order (BO) of atomic interactions, e.g., for single bonds, because it is clear that the higher is the BO or the number of the electron pairs responsible of bonding, the stronger is the bond. For the metal-metal interactions, such a comparison of the bond strengths is especially problematic because formal BOs may differ from the effective ones by more than 1 v.u. (valence units). In this paper, we investigated the strength of bonding and its correlation to structural parameters (bond length/BO) for 24 metal-metal pairs. A simple way of the BO and bond-strength analysis was proposed and verified on the transition metal dimers. In contrast to the previous studies, the effective BOs were not calculated from spectroscopic data or related to reference compounds, but determined independently based on the Pauling model and bond valence parameters obtained from recent quantum chemistry data. To characterize the bond strength, we used the force constants based on the available experimental or DFT data of stretching frequencies. A linear correlation between the effective BOs and force constants of the metal-metal bonds, which was confirmed in this work, allows for prediction of the stretching frequencies according to the effective BOs (and/or the bond lengths) and vice versa.

Bond Order Conservation Principle and Peculiarities of the Metal-Metal Bonding.

Pauling's principles developed later in the bond valence model (BVM) are fundamental in description of bonding in ionic solids and surface phenomena on metals, but applicability of these principles to the metal-metal bonds in the bulk compounds was demonstrated only recently, with a spotlight on the bond valence-bond length correlations. This work is focused on the bond order conservation in cluster compounds and determination of empiric bond valence parameters for the metal-metal bonds, which ensure very simple and reasonably accurate bonding analysis, with zero cost, in any complex cluster compound. Such peculiarities of cluster compounds as matrix effect and nonuniform distribution of the ionic charges (bond valence sums) on the ligands around metal clusters, as well as other important examples of the BVM application to compounds with metal-metal bonds, are discussed.

Do the basic crystal chemistry principles agree with a plethora of recent quantum chemistry data?

The main descriptors of chemical bonding such as bond order (BO) and electron density at the bond critical point, ρc, are customarily used to understand the crystal and electronic structure of materials, as well as to predict their reactivity and stability. They can be obtained in the framework of crystal chemistry and quantum chemistry approaches, which are mostly applied as alternatives to each other. This paper verifies the convergence of the two approaches by analyzing a plethora of quantum chemistry data available in the literature. The exponential correlation between the electron descriptors [BO ij and ρc(ij)] and the length of chemical bonds, Rij , which is basic in crystal chemistry, was confirmed for 72 atom pairs, regardless of the nature of their interactions (ionic/covalent, metal-metal, etc.). The difference between the BO ij  (Rij ) correlations obtained in this work and those accepted in crystal chemistry for the same atomic pairs does not exceed the dispersion of quantum chemistry data, confirming the qualitative validity of the BO conservation principle. Various examples are presented to show that knowledge of the exponential parameters ensures a surprisingly simple determination of two basic electron descriptors in any complex compound with known interatomic distances. In particular, the BO analysis for 20 Re6-cluster complexes illustrates the BO conservation for systems with delocalized electrons. Despite the significant transfer of electron density from the Re-Re to the Re-ligand bonds, the total number of Re valence electrons used in bonding remains close to the formal value of seven electrons.

Understanding the influence of Mg doping for the stabilization of capacity and higher discharge voltage of Li- and Mn-rich cathodes for Li-ion batteries.

Although Li- and Mn-rich layered cathodes exhibit high specific capacity, the cathode materials of the general formula Li1+x[NiyMnzCow]O2 (x + y + z + w = 1) suffer from capacity fading and discharge-voltage decay during prolonged cycling, due to the layered-to-spinel transformation upon cycling to potentials higher than 4.5 V vs. Li. In this paper, we study the effect of Mg doping (by partial replacement of Mn ions) on the electrochemical performance of Li- and Mn-rich cathodes in terms of specific capacity, capacity retention and discharge voltage upon cycling. Mg-doped Li- and Mn-rich Li1.2Ni0.16Mn0.54Mg0.02Co0.08O2 and Li1.2Ni0.16Mn0.51Mg0.05Co0.08O2 cathode materials were synthesized by a self-combustion reaction (SCR), and their electrochemical performance in Li-ion batteries was tested. The replacement of a small amount of Mn ions by Mg ions in these materials results in a decrease in their specific capacity. The doping of a small amount of Mg (x = 0.02) resulted only in the stabilization of the capacity, whereas a greater amount (x = 0.05) resulted in improved capacity retention and discharge voltage upon cycling. Li1.2Ni0.16Mn0.51Mg0.05Co0.08O2 exhibits a low specific capacity of about 160 mA h g-1, which increases and then stabilizes at about 230 mA h g-1, and finally decreases to 210 mA h g-1 during 100 cycles. The substitution of Mg for Mn (x = 0.05) results in a higher discharge voltage than the other two cathode materials examined in this study. Structural analysis of the cycled electrodes suggests that Mg suppresses the activation of Li2MnO3 during the initial cycling, and hence, partially prevents layered-to-spinel transformation, resulting in a better electrochemical performance of the Mg-doped cathode material as compared to the undoped material.

Single-Wall Carbon Nanotube Doping in Lead-Acid Batteries: A New Horizon.

The addition of single-wall carbon nanotubes (SWCNT) to lead-acid battery electrodes is the most efficient suppresser of uncontrolled sulfation processes. Due to the cost of SWCNT, we studied the optimization loading of SWCNT in lead-acid battery electrodes. We optimized the SWCNT loading concentrations in both the positive and negative plates, separately. Loadings of 0.01% and 0.001% in the positive and negative active masses were studied, respectively. Two volts of lead-acid laboratory cells with sulfuric acid, containing silica gel-type electrolytes, were cycled in a 25% and 50% depth-of-discharge (DOD) cycling with a charging rate of C and 2C, respectively, and discharge rates of C/2 and C, respectively. All tests successfully demonstrated an excellent service life up to about 1700 and 1400 cycles for 25% and 50% DOD operations, respectively, at a low loading level of SWCNT. This performance was compared with CNT-free cells and cells with a multiwall carbon nanotube (MWCNT) additive. The outstanding performance of the lead-acid cells with the SWCNT additive is due to the oxidative stability of the positive plates during charging and the efficient reduction in sulfation in both plates while forming conducting active-material matrices.

Remarkably Improved Electrochemical Performance of Li- and Mn-Rich Cathodes upon Substitution of Mn with Ni.

Li- and Mn-rich transition-metal oxides of layered structure are promising cathodes for Li-ion batteries because of their high capacity values, ≥250 mAh g-1. These cathodes suffer from capacity fading and discharge voltage decay upon prolonged cycling to potential higher than 4.5 V. Most of these Li- and Mn-rich cathodes contain Ni in a 2+ oxidation state. The fine details of the composition of these materials may be critically important in determining their performance. In the present study, we used Li1.2Ni0.13Mn0.54Co0.13O2 as the reference cathode composition in which Mn ions are substituted by Ni ions so that their average oxidation state in Li1.2Ni0.27Mn0.4Co0.13O2 could change from 2+ to 3+. Upon substitution of Mn with Ni, the specific capacity decreases but, in turn, an impressive stability was gained, about 95% capacity retention after 150 cycles, compared to 77% capacity retention for Li1.2Ni0.13Mn0.54Co0.13O2 cathodes when cycled at a C/5 rate. Also, a higher average discharge voltage of 3.7 V is obtained for Li1.2Ni0.27Mn0.4Co0.13O2 cathodes, which decreases to 3.5 V after 150 cycles, while the voltage fading of cathodes comprising the reference material is more pronounced. The Li1.2Ni0.27Mn0.4Co0.13O2 cathodes also demonstrate higher rate capability compared to the reference Li1.2Ni0.13Mn0.54Co0.13O2 cathodes. These results clearly indicate the importance of the fine composition of cathode materials containing the five elements Li, Mn, Ni, Co, and O. The present study should encourage rigorous optimization efforts related to the fine composition of these cathode materials, before external means such as doping and coating are applied.

High-Capacity Layered-Spinel Cathodes for Li-Ion Batteries.

Li and Mn-rich layered oxides with the general structure x Li2 MnO3 ⋅(1-x) LiMO2 (M=Ni, Mn, Co) are promising cathode materials for Li-ion batteries because of their high specific capacity, which may be greater than 250 mA h g(-1) . However, these materials suffer from high first-cycle irreversible capacity, gradual capacity fading, limited rate capability and discharge voltage decay upon cycling, which prevent their commercialization. The decrease in average discharge voltage is a major issue, which is ascribed to a structural layered-to-spinel transformation upon cycling of these oxide cathodes in wide potential ranges with an upper limit higher than 4.5 V and a lower limit below 3 V versus Li. By using four elements systems (Li, Mn, Ni, O) with appropriate stoichiometry, it is possible to prepare high capacity composite cathode materials that contain LiMn1.5 Ni0.5 O4 and Lix Mny Niz O2 components. The Li and Mn-rich layered-spinel cathode materials studied herein exhibit a high specific capacity (≥200 mA h g(-1) ) with good capacity retention upon cycling in a wide potential domain (2.4-4.9 V). The effect of constituent phases on their electrochemical performance, such as specific capacity, cycling stability, average discharge voltage, and rate capability, are explored here. This family of materials can provide high specific capacity, high rate capability, and promising cycle life. Using Co-free cathode materials is also an obvious advantage of these systems.

Exceptionally Active and Stable Spinel Nickel Manganese Oxide Electrocatalysts for Urea Oxidation Reaction.

Spinel nickel manganese oxides, widely used materials in the lithium ion battery high voltage cathode, were studied in urea oxidation catalysis. NiMn2O4, Ni1.5Mn1.5O4, and MnNi2O4 were synthesized by a simple template-free hydrothermal route followed by a thermal treatment in air at 800 °C. Rietveld analysis performed on nonstoichiometric nickel manganese oxide-Ni1.5Mn1.5O4 revealed the presence of three mixed phases: two spinel phases with different lattice parameters and NiO unlike the other two spinels NiMn2O4 and MnNi2O4. The electroactivity of nickel manganese oxide materials toward the oxidation of urea in alkaline solution is evaluated using cyclic voltammetric measurements. Ni1.5Mn1.5O4 exhibits excellent redox characteristics and lower charge transfer resistances in comparison with other compositions of nickel manganese oxides and nickel oxide prepared under similar conditions.The Ni1.5Mn1.5O4modified electrode oxidizes urea at 0.29 V versus Ag/AgCl with a corresponding current density of 6.9 mA cm(-2). At a low catalyst loading of 50 µg cm(-2), the urea oxidation current density of Ni1.5Mn1.5O4 in alkaline solution is 7 times higher than that of nickel oxide and 4 times higher than that of NiMn2O4 and MnNi2O4, respectively.

Is it True That the Normal Valence-Length Correlation Is Irrelevant for Metal-Metal Bonds?

The most intriguing feature of metal-metal bonds in inorganic compounds is an apparent lack of correlation between the bond order and the bond length. In this study, we combine a variety of literature data obtained by quantum chemistry and our results based on the empirical bond valence model (BVM), to confirm for the first time the existence of a normal exponential correlation between the effective bond order (EBO) and the length of the metal-metal bonds. The difference between the EBO and the formal bond order is attributed to steric conflict between the (TM)n cluster (TM=transition metal) and its environment. This conflict, affected mainly by structural type, should cause high lattice strains, but electron redistribution around TM atoms, evident from the BVM calculations, results in a full or partial strain relaxation.

Bond-valence model for metal cluster compounds. I. Common lattice strains.

The bond-valence model was commonly considered as inappropriate to metal cluster compounds, but recently it was shown that the model provides unique information on the lattice strains and stabilization mechanisms in (TM)6-chalcohalides (TM = transition metal in the cluster). The previous study was mainly devoted to the non-uniform distribution of the anion valences (bond-valence sums) around clusters. This and the following paper focuses on two additional phenomena: (i) a steric conflict between counter-cations and the cluster-ligand framework resulting in `common' lattice strains (this paper), and (ii) steric conflict between the small (TM)6-cluster and the large coordination polyhedron around the cluster or so-called matrix effect [the next paper; Levi et al. (2013), Acta Cryst. B69, 426-438]. It was shown that both phenomena can be well described by changes in the bond-valence parameters. The calculations were based on the structural data known to date for a variety of (TM)6-cluster compounds, Mx(TM)6Ly (TM = Nb, Mo, W and Re; M = various additional cations, L = the chalcogen and/or halogen ligands). The results were used to explain the structural peculiarities of these compounds with remarkable physical properties and the mechanisms of their stabilization.

Bond-valence model for metal cluster compounds. II. Matrix effect.

The bond-valence model was commonly considered as inappropriate to metal cluster compounds, but recently it was shown that the model provides unique information on the lattice strains and stabilization mechanisms in (TM)6-chalcohalides, Mx(TM)6Ly (TM = transition metal, L = the chalcogen and/or halogen ligands; M = counter-cation). The previous study was mainly devoted to the non-uniform distribution of the anion valences (bond-valence sums) around clusters. This and the previous paper are focused on two additional phenomena: (i) a steric conflict between counter-cations and the cluster-ligand framework resulting in `common' lattice strains [previous paper: Levi et al. (2013). Acta Cryst. B69, 419-425], and (ii) steric conflict between the small (TM)6-cluster and the large coordination polyhedron around the cluster or so-called matrix effect (this paper). It was shown that both phenomena can be well described by changes in the bond-valence parameters. This paper demonstrates that the matrix effect results in high strains in the TM-L bonds in most of the (TM)6-chalcohalides (TM = Nb, Mo, W and Re). In spite of this, the violations for the total TM valence are minimal, because the cluster stretching is fully or partially compensated by compression of the TM-L bonds. As a result, the influence of the matrix effect on the material stability is rather positive: it decreases the volume of the structural units and in many cases ensures a more favorable distribution of the bond valences around TM atoms, stabilizing the cluster compound.

Nonaqueous magnesium electrochemistry and its application in secondary batteries.

A revolution in modern electronics has led to the miniaturization and evolution of many portable devices, such as cellular telephones and laptop computers, since the 1980s. This has led to an increasing demand for new and compatible energy storage technologies. Furthermore, a growing awareness of pollution issues has provided a strong impetus for the science and technology community to develop alternatives with ever-higher energy densities, with the ultimate goal of being able to propel electric vehicles. Magnesium's thermodynamic properties make this metal a natural candidate for utilization as an anode in high-energy-density, rechargeable battery systems. We report herein on the results of extensive studies on magnesium anodes and magnesium insertion electrodes in nonaqueous electrolyte solutions. Novel, rechargeable nonaqueous magnesium battery systems were developed based on the research. This work had two major challenges: one was to develop electrolyte solutions with especially high anodic stability in which magnesium anodes can function at a high level of cycling efficiency; the other was to develop a cathode that can reversibly intercalate Mg ions in these electrolyte systems. The new magnesium batteries consist of Mg metal anodes, an electrolyte with a general structure of Mg(AlX(3-n)R(n)R')(2) (R',R = alkyl groups, X = halide) in ethereal solutions (e.g., tetrahydrofuran, polyethers of the "glyme" family), and Chevrel phases of MgMo(3)S(4) stoichiometry as highly reversible cathodes. With their practical energy density expected to be >60 Wh/Kg, the battery systems can be cycled thousands of times with almost no capacity fading. The batteries are an environmentally friendly alternative to lead-acid and nickel-cadmium batteries and are composed of abundant, inexpensive, and nonpoisonous materials. The batteries are expected to provide superior results in large devices that require high-energy density, high cycle life, a high degree of safety, and low-cost components. Further developments in this field are in active progress.