Pieremanuele Canepa

Odyssey of Multivalent Cathode Materials: Open Questions and Future Challenges

The rapidly expanding field of nonaqueous multivalent intercalation batteries offers a promising way to overcome safety, cost, and energy density limitations of state-of-the-art Li-ion battery technology. We present a critical and rigorous analysis of the increasing volume of multivalent battery research, focusing on a wide range of intercalation cathode materials and the mechanisms of multivalent ion insertion and migration within those frameworks. The present analysis covers a wide variety of material chemistries, including chalcogenides, oxides, and polyanions, highlighting merits and challenges of each class of materials as multivalent cathodes. The review underscores the overlap of experiments and theory, ranging from charting the design metrics useful for developing the next generation of MV-cathodes to targeted in-depth studies rationalizing complex experimental results. On the basis of our critical review of the literature, we provide suggestions for future multivalent cathode studies, including a strong emphasis on the unambiguous characterization of the intercalation mechanisms.

Diffusion of small molecules in metal organic framework materials.

Ab initio simulations are combined with in situ infrared spectroscopy to unveil the molecular transport of H2, CO2, and H2O in the metal organic framework MOF-74-Mg. Our study uncovers--at the atomistic level--the major factors governing the transport mechanism of these small molecules. In particular, we identify four key diffusion mechanisms and calculate the corresponding diffusion barriers, which are nicely confirmed by time-resolved infrared experiments. We also answer a long-standing question about the existence of secondary adsorption sites for the guest molecules, and we show how those sites affect the macroscopic diffusion properties. Our findings are important to gain a fundamental understanding of the diffusion processes in these nanoporous materials, with direct implications for the usability of MOFs in gas sequestration and storage applications.

Role of Structural H2O in Intercalation Electrodes: The Case of Mg in Nanocrystalline Xerogel-V2O5

Cointercalation is a potential approach to influence the voltage and mobility with which cations insert in electrodes for energy storage devices. Combining a robust thermodynamic model with first-principles calculations, we present a detailed investigation revealing the important role of H2O during ion intercalation in nanomaterials. We examine the scenario of Mg(2+) and H2O cointercalation in nanocrystalline Xerogel-V2O5, a potential cathode material to achieve energy density greater than Li-ion batteries. Water cointercalation in cathode materials could broadly impact an electrochemical system by influencing its voltages or causing passivation at the anode. The analysis of the stable phases of Mg-Xerogel V2O5 and voltages at different electrolytic conditions reveals a range of concentrations for Mg in the Xerogel and H2O in the electrolyte where there is no thermodynamic driving force for H2O to shuttle with Mg during electrochemical cycling. Also, we demonstrate that H2O shuttling with the Mg(2+) ions in wet electrolytes yields higher voltages than in dry electrolytes. The thermodynamic framework used to study water and Mg(2+) cointercalation in this work opens the door for studying the general phenomenon of solvent cointercalation observed in other complex solvent-electrode pairs used in the Li- and Na-ion chemical spaces.

The Intercalation Phase Diagram of Mg in V2O5 from First-Principles

We have investigated Mg intercalation into orthorhombic V2O5, one of only three cathodes known to reversibly intercalate Mg ions. By calculating the ground-state MgxV2O5 configurations and by developing a cluster expansion for the configurational disorder in δ-V2O5, a full temperature–composition phase diagram is derived. Our calculations indicate an equilibrium phase-separating behavior between fully demagnesiated α-V2O5 and fully magnesiated δ-V2O5, but also motivate the existence of potentially metastable solid solution transformation paths in both phases. We find significantly better mobility for Mg in the δ polymorph, suggesting that better performance can be achieved by cycling Mg in the δ phase.

Water Cluster Confinement and Methane Adsorption in the Hydrophobic Cavities of a Fluorinated Metal–Organic Framework

Water cluster formation and methane adsorption within a hydrophobic porous metal organic framework is studied by in situ vibrational spectroscopy, adsorption isotherms, and first-principle DFT calculations (using vdW-DF). Specifically, the formation and stability of H2O clusters in the hydrophobic cavities of a fluorinated metal-organic framework (FMOF-1) is examined. Although the isotherms of water show no measurable uptake (see Yang et al. J. Am. Chem. Soc. 2011 , 133 , 18094 ), the large dipole of the water internal modes makes it possible to detect low water concentrations using IR spectroscopy in pores in the vicinity of the surface of the solid framework. The results indicate that, even in the low pressure regime (100 mTorr to 3 Torr), water molecules preferentially occupy the large cavities, in which hydrogen bonding and wall hydrophobicity foster water cluster formation. We identify the formation of pentameric water clusters at pressures lower than 3 Torr and larger clusters beyond that pressure. The binding energy of the water species to the walls is negligible, as suggested by DFT computational findings and corroborated by IR absorption data. Consequently, intermolecular hydrogen bonding dominates, enhancing water cluster stability as the size of the cluster increases. The formation of water clusters with negligible perturbation from the host may allow a quantitative comparison with experimental environmental studies on larger clusters that are in low concentrations in the atmosphere. The stability of the water clusters was studied as a function of pressure reduction and in the presence of methane gas. Methane adsorption isotherms for activated FMOF-1 attained volumetric adsorption capacities ranging from 67 V(STP)/V at 288 K and 31 bar to 133 V(STP)/V at 173 K and 5 bar, with an isosteric heat of adsorption of ca. 14 kJ/mol in the high temperature range (288-318 K). Overall, the experimental and computational data suggest high preferential uptake for methane gas relative to water vapor within FMOF-1 pores with ease of desorption and high framework stability under operative temperature and moisture conditions.

J-ICE: a new Jmol interface for handling and visualizing crystallographic and electronic properties

The growth in complexity of quantum mechanical software packages for modelling the physicochemical properties of crystalline materials may hinder their usability by the vast majority of non-specialized users. Consequently, a free operating-system-independent graphical user interface (GUI) has been developed to drive the most common simulation packages for treating both molecules and solids. In order to maintain maximum portability and graphical efficiency, the popular molecular graphics engine Jmol, written in the portable Java language, has been combined with a specialized GUI encoded in HTML and JavaScript. This framework, called J-ICE, allows users to visualize, build and manipulate complex input or output results (derived from modelling) entirely via a web server, i.e. without the burden of installing complex packages. This solution also dramatically speeds up both the development procedure and bug fixing. Among the range of software appropriate for modelling condensed matter, the focus of J-ICE is currently only on CRYSTAL09 and VASP.

First-principles evaluation of multi-valent cation insertion into orthorhombic V2O5

A systematic first-principles evaluation of the insertion behavior of multi-valent cations in orthorhombic V2O5 is performed. Layer spacing, voltage, phase stability, and ion mobility are computed for Li(+), Mg(2+), Zn(2+), Ca(2+), and Al(3+) intercalation in the α and δ polymorphs.

When metal organic frameworks turn into linear magnets

(Received 3 December 2012; revised manuscript received 9 February 2013; published 7 March 2013)WeinvestigatetheexistenceoflinearmagnetisminthemetalorganicframeworkmaterialsMOF-74-Fe,MOF-74-Co, and MOF-74-Ni, using first-principles density functional theory. MOF-74 displays regular quasilinearchains of open-shell transition-metal atoms, which are well separated. Our results show that within these chains,forallthreematerials,ferromagneticcouplingofsignificantstrengthoccurs.Inaddition,thecouplinginbetweenchains is at least one order of magnitude smaller, making these materials almost perfect one-dimensional(1D) magnets at low temperature. The interchain coupling is found to be antiferromagnetic, in agreementwith experiments. While some quasi-1D materials exist that exhibit linear magnetism, mostly complex oxides,polymers, and a few other rare materials, they are typically very difficult to synthesize. The significance of ourfinding is that MOF-74 is very easy to synthesize and it is likely the simplest realization of the 1D Ising modelin nature. MOF-74 could thus be used in future experiments to study 1D magnetism at low temperature.DOI: 10.1103/PhysRevB.87.094407 PACS number(s): 75

Understanding the Initial Stages of Reversible Mg Deposition and Stripping in Inorganic Nonaqueous Electrolytes

Multivalent (MV) battery architectures based on pairing a Mg metal anode with a high-voltage (∼3 V) intercalation cathode offer a realistic design pathway toward significantly surpassing the energy storage performance of traditional Li-ion-based batteries, but there are currently only few electrolyte systems that support reversible Mg deposition. Using both static first-principles calculations and ab initio molecular dynamics, we perform a comprehensive adsorption study of several salt and solvent species at the interface of Mg metal with an electrolyte of Mg2+ and Cl– dissolved in liquid tetrahydrofuran (THF). Our findings not only provide a picture of the stable species at the interface but also explain how this system can support reversible Mg deposition, and as such, we provide insights in how to design other electrolytes for Mg plating and stripping. The active depositing species are identified to be (MgCl)+ monomers coordinated by THF, which exhibit preferential adsorption on Mg compared to possible...

Spectroscopic characterization of van der Waals interactions in a metal organic framework with unsaturated metal centers: MOF-74-Mg.

The adsorption energies of small molecules in nanoporous materials are often determined by isotherm measurements. The nature of the interaction and the response of the host material, however, can best be studied by spectroscopic methods. We show here that infrared absorption and Raman spectroscopy measurements together with density functional theory calculations, utilizing the novel van der Waals density functional vdW-DF, constitute a powerful approach to studying the weak van der Waals interactions associated with the incorporation of small molecules in these materials. In particular, we show how vdW-DF assists the interpretation of the vibrational spectroscopy data to uncover the binding sites and energies of these molecules, including the subtle dependence on loading of the IR asymmetric stretch mode of CO(2) when adsorbed in MOF-74-Mg. To gain a better understanding of the adsorption mechanism of CO(2) in MOF-74-Mg, the results are compared with CO within MOF-74-Mg.