Pressure-induced spin crossover in a Prussian Blue analogue

The structural and chemical versatility of functional molecular materials, such as molecular magnets and metal-organic frameworks (MOFs), underlie important technological, industrial, and environmental applications. The extensive structural complexities now well-documented for these systems are likely to be associated with unprecedented pressure-induced behavior compared with the traditional solid state materials more commonly explored under high pressure conditions.1 Furthermore, the typically open (low density, often porous) nature of these materials is likely to induce such phenomena at more moderate pressures, such as may be routinely encountered in practical applications.2,3 Here we report pressure-induced spin-state switching in the Prussian Blue analogue, FePt(CN)6, including in situ Synchrotron (17-BM, Advanced Photon Source) and Neutron (SNAP, Spallation Neutron Source) powder diffraction studies. Work done at Argonne and use of the Advanced Photon Source (APS) was supported by the U.S. Department of Energy under Contract No. DE-AC02-06CH11357. Research at Oak Ridge National Laboratory's Spallation Neutron Source (SNS) was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy.

In situ PDF and solid-state NMR studies of antimony anodes for Na-ion batteries

Sodium-ion batteries have attracted attention in recent years because of the natural abundance of sodium compared to lithium, making them particularly attractive in applications such as large-scale grid storage where low cost and sustainability, rather than light weight is the key issue [1]. Several materials have been suggested as cathodes but far fewer studies have been done on anode materials and, because of the reluctance of sodium to intercalate into graphite, the anode material of choice in commercial lithium-ion batteries, the anode represents a significant challenge to this technology. Materials which form alloys with sodium, particularly tin and antimony, have been suggested as anode materials; their ability to react with multiple sodium ions per metal-atom give potential for high gravimetric capacities[2]. However, relatively little is known about the reaction mechanism in the battery, primarily due to drastic reduction in crystallinity during (dis)charging conditions, but also because the structures formed on electrochemical cycling may not be alloys known to exist under ambient conditions. In this study, we present a study of antimony as an anode in sodium-ion batteries, using in situ pair distribution function (PDF) analysis combined with ex situ solid-state nuclear magnetic resonance studies. PDF experiments were performed at 11-ID-B, APS using the AMPIX electrochemical cell [3], cycling against sodium metal. Inclusion of diffuse scattering in analysis is able to circumvent some of the issues of crystallinity loss, and gain information about the local structure in all regions, independent of the presence of long-range order in the material. This approach has been used to probe local correlations in previously uncharacterised regions of the electrochemical profile and analyse phase progression over the full charge cycle. This analysis has been linked with ex situ 23Na solid-state NMR experiments to examine the local environment of the sodium; these show evidence of known NaxSb phases but indicate additional metastable phases may be present at partial discharge.

Phase transition of nanoparticulate LiFePO₄ during high rate cycling

A fundamental understanding of an electrode material requires the elucidation of its phase transformation mechanism during charge and discharge. Ex situ methods, which are carried out under equilibrium condition, have been widely used in charactering the thermodynamic phases at different states of charge, from which a thermodynamic phase transformation pathway can be constructed. However, ex situ measurements do not always reflect the process occurred in an operating battery as the non-equilibrium operating condition might result in deviations from the thermodynamic process, especially for high-rate materials, such as LiFePO4, which is predicted to exhibit a fundamentally different phase transformation process at high rates [1,2]. To probe the process at high rate, an in situ method with reasonable temporal resolution must be employed. In this work, the high rate galvanostatic cycling process of LiFePO4 nanoparticle electrode in a customised AMPIX cell [3] was investigated in situ by time-resolved synchrotron X-ray powder diffraction. Formation of continuous non-equilibrium solid solution phases between LiFePO4 and FePO4 was observed at 10 C rate. The in situ diffraction patterns were analysed by a refinement strategy that accounts for the asymmetrical diffraction peak profiles due to Li composition variations.

Operando studies of nanoscale olivine cathodes for Li-ion batteries

Compounds of interest for ion storage in advanced batteries frequently exhibit phase transformations, driven by large and variable electrochemical driving forces inherent to practical use. Understanding how materials variables (e.g. composition, nanoscale-crystallite size and dynamic electrochemical conditions) affect the phase transition is of vital importance for practical applications as the reversibility and stability of these structural transformations determine the energy, power, and lifetime of the system. Due to its outstanding power, safety and cycle-life olivine LiFePO4 (LFP) has during the past decade become a widely used, and one of the most well-studied, lithium ion battery cathode materials. It is well-established that for LiFePO4 the storage/release of lithium is accompanied by a first-order phase transition between lithiated and delithiated states. However, it would be a mistake to conclude that the behavior of pure LFP is representative of all olivines, in particular the vast range of doped and mixed-metal olivines that are also of interest for their advantageous electrochemical properties.1,2 Utilizing operando synchrotron radiation powder X-ray diffraction (SR-PXD), we demonstrate here, by systematic screening of the electrochemical driven phase transitions in a series of LiMnyFe1-yPO4 (y =0.1-0.8) powders, a completely different phase transformation mode dominated by formation of metastable solid solutions for nanoscale LMFP compared to the binary lithiation states within the extremely well-studied case of LFP. Through Rietveld refinement the misfit strains during phase transformations are examined, revealing small elastic misfits between phases within the extended solid solution regime. On the basis of the time- and state-of-charge dependence of the olivine structure parameters, we propose a coherent transformation mechanism, and finally, we bring evidence that the observed metastability is enabled by particle size reduction to the nanoscale.

Electrochemical Conversion: Beyond the First Cycle

Conversion based electrode materials offer increased energy storage compared to conventional intercalation materials due to the multiple electrons that reacts per metal ion. However, loss in capacities upon repeated cycling has limited the development of this technology for commercial application. Most structural studies focus on the first discharge-charge cycle [1,2,3]. To understand the loss in capacities with repeated cycling, studies must be extended beyond the first cycle. In conversion reactions, large structural transformations occur such that the electrode is reduced to the nanoscale. Pair distribution function (PDF) analysis is well suited to characterize the structural changes occurring in such nanomaterials. Conversion based iron fluorides (FeF3, FeF2, and FeOF) have been a focus of both structural and mechanistic studies [1,2,3]. An in-depth PDF analysis of what happens beyond the first cycle will be presented for these.

Non-crystalline samples under high-pressure conditions

Taking advantage of synchrotron x-ray diffraction, PDF and tomographic techniques, the P-V curve of non-crystalline samples were studied under high-pressure conditions. Two element and several metallic glass cases were performed. The procedure of crystallization of amorphous Se upon compression at room temperature, which was studied in diamond anvil cell combined synchrotron x-ray PDF and 3D imaging techniques; the melting and solidification procedure of Ga in large volume press at room and high temperature; and complicated crystallization, re-rystalization, melting behavior of Ce-based metallic glass, will be presented to show the capability of revealing structure and dynamics behaviors in P-V-T-t domains using these advanced techniques.

Materials for energy storage and conversion: Probing functionality with PDF

In the last decade, the potential of the pair distribution function (PDF) method as a versatile tool for materials characterization has expanded enormously, driven by accelerated data acquisition (from hours to sub-second) and the advent of dedicated PDF instruments, such as 11-ID-B at the Advanced Photon Source. New time-resolved, in-situ/operando, parametric, and combined experimental capabilities coupled with innovative model-independent approaches to data analysis are being developed to harness the growing potential of this methodology. For example, while the complex multicomponent architecture of batteries and their coupled electronic, chemical and structural transformations complicate investigations of functionality, through the development of new insitu PDF measurement capabilities and analytical approaches, we have been able to gain insight into the structure and reactivity of these electrochemical energy storage systems.[1] This presentation will describe recent studies of electrode reactions during cycling and the atomic structure of electrolytes.[2]

From atoms to electrodes: Mesoscale effects in electrochemical conversion

Batteries are complex multicomponent devices wherein mesoscale phenomena-the nanoscale structure and chemistry of different components, and interactions thereof-drive functionality and performance. For example, electron/ion transport within the composite electrodes relies on bi-continuous nanostructuring to form electrically and ionicly conductive paths. Electrochemical conversion of different salts of a given metal yields a common and ostensibly identical product: the zero valent metal. For example, maximal lithiation of iron-based electrodes produces metallic iron nanoparticles for oxide, fluoride, and oxyfluoride electrodes alike. Accordingly, these provide an opportunity to explore the coupling of nanostructure development and anion chemistry, and correlate these with electrochemical performance. We combine synchrotron-based small angle X-ray scattering (SAXS) and pair distribution function (PDF) measurements to probe metallic iron formed by electrochemical conversion of different iron compounds across multiple length-scales and decouple the influence of anion chemistry and reaction temperature on the atomic structure and nanoscale morphology.

Simultaneous infrared spectroscopy and X-ray PDF measurements

Combining insights from diffuse reflection infrared Fourier transform spectroscopy (DRIFTS) and X-ray pair distribution function (PDF) analysis has the potential to provide deeper insight into complex materials systems under reactive conditions, including heterogeneous catalysts and host-guest systems. We have developed instrumentation and non-ambient reaction cells that enable combined PDF-IR studies without compromise to either measurement. Through careful selection of the IR spectrometer and optics, the IR instrument and reaction cell were adapted to allow angular dispersive X-ray measurements without change to the active IR components, resulting in IR data that are entirely uncompromised. The associated PDF and diffraction data are shown to be of comparable quality and resolution to standard geometry PDF/diffraction measurements. We have demonstrated this approach through the study of desorption of coordinated and non-coordinated guests from the nanoporous Prussian blue analogue MnII3[CoIII(CN)6]2(H2O)6·x{H2O}. The combined data shows how the release of guests from different sites is coupled to structural relaxation of the framework. We will also discus recent results where we have extended the approach to the study of zeolite catalysts.