L. Andrew Wray

Rhombohedral Prussian White as Cathode for Rechargeable Sodium-Ion Batteries

A novel air-stable sodium iron hexacyanoferrate (R-Na1.92Fe[Fe(CN)6]) with rhombohedral structure is demonstrated to be a scalable, low-cost cathode material for sodium-ion batteries exhibiting high capacity, long cycle life, and good rate capability. The cycling mechanism of the iron redox is clarified and understood through synchrotron-based soft X-ray absorption spectroscopy, which also reveals the correlation between the physical properties and the cell performance of this novel material. More importantly, successful preparation of a dehydrated iron hexacyanoferrate with high sodium-ion concentration enables the fabrication of a discharged sodium-ion battery with a non-sodium metal anode, and the manufacturing feasibility of low cost sodium-ion batteries with existing lithium-ion battery infrastructures has been tested.

Manganese–cobalt hexacyanoferrate cathodes for sodium-ion batteries

Prussian Blue analogues (PBAs) have shown promise as electrode materials for grid-scale batteries because of their high cycle life and rapid kinetics in aqueous-based electrolytes. However, these materials suffer from relatively low specific capacity, which may limit their practical applications. Here, we investigate strategies to improve the specific capacity of these materials while maintaining their cycling stability and elucidate mechanisms that enhance their electrochemical properties. In particular, we have studied the electrochemical and structural properties of manganese hexacyanoferrate (MnHCFe) and cobalt hexacyanoferrate (CoHCFe) in an aqueous, sodium-ion electrolyte. We also studied manganese–cobalt hexacyanoferrate (Mn–CoHCFe) solid solutions with different Mn/Co ratios that combine properties of both MnHCFe and CoHCFe. The materials have the characteristic open-framework crystal structure of PBAs, and their specific capacities can be significantly improved by electrochemically cycling (oxidizing and reducing) both the carbon-coordinated Fe and the nitrogen-coordinated Co or Mn ions. In situ synchrotron X-ray diffraction studies and ex situ soft X-ray absorption spectroscopy combined with an in-depth electrochemical characterization provide insight into the different electrochemical properties associated with the Fe, Co, and Mn redox couples. We show that cycling the C-coordinated Fe preserves the crystal structure and enables the outstanding kinetics and cycle life previously displayed by PBAs in aqueous electrolytes. On the other hand, the N-coordinated Co and Mn ions exhibit a slower kinetic regime due to structural distortions resulting from the weak N-coordinated crystal field, but they still contribute significantly towards increasing the specific capacity of the materials. These results provide the understanding needed to drive future development of PBAs for grid-scale applications that require extremely high cycle life and kinetics.

Quantitative probe of the transition metal redox in battery electrodes through soft x-ray absorption spectroscopy

Most battery positive electrodes operate with a 3d transition-metal (TM) reaction centre. A direct and quantitative probe of the TM states upon electrochemical cycling is valuable for understanding the detailed cycling mechanism and charge diffusion in the electrodes, which is related with many practical parameters of a battery. This review includes a comprehensive summary of our recent demonstrations of five different types of quantitative analysis of the TM states in battery electrodes based on soft x-ray absorption spectroscopy and multiplet calculations. In LiFePO4, a system of a well-known two-phase transformation type, the TM redox could be strictly determined through a simple linear combination of the two end-members. In Mn-based compounds, the Mn states could also be quantitatively evaluated, but a set of reference spectra with all the three possible Mn valences needs to be deliberately selected and considered in the fitting. Although the fluorescence signals suffer the self-absorption distortion, the multiplet calculations could consider the distortion effect, which allows a quantitative determination of the overall Ni oxidation state in the bulk. With the aid of multiplet calculations, one could also achieve a quasi-quantitative analysis of the Co redox evolution in LiCoO2 based on the energy position of the spectroscopic peak. The benefit of multiplet calculations is more important for studying electrode materials with TMs of mixed spin states, as exemplified by the quantitative analysis of the mixed spin Na2−x Fe2(CN)6 system. At the end, we showcase that such quantitative analysis could provide valuable information for optimizing the electrochemical performance of Na0.44MnO2 electrodes for Na-ion batteries. The methodology summarized in this review could be extended to other energy application systems with TM redox centre for detailed analysis, for example, fuel cell and catalytic materials.

Modular soft x-ray spectrometer for applications in energy sciences and quantum materials

Over the past decade, the advances in grating-based soft X-ray spectrometers have revolutionized the soft X-ray spectroscopies in materials research. However, these novel spectrometers are mostly dedicated designs, which cannot be easily adopted for applications with diverging demands. Here we present a versatile spectrometer design concept based on the Hettrick-Underwood optical scheme that uses modular mechanical components. The spectrometers optics chamber can be used with gratings operated in either inside or outside orders, and the detector assembly can be reconfigured accordingly. The spectrometer can be designed to have high spectral resolution, exceeding 10 000 resolving power when using small source (∼1μm) and detector pixels (∼5μm) with high line density gratings (∼3000 lines/mm), or high throughput at moderate resolution. We report two such spectrometers with slightly different design goals and optical parameters in this paper. We show that the spectrometer with high throughput and large energy window is particularly useful for studying the sustainable energy materials. We demonstrate that the extensive resonant inelastic X-ray scattering (RIXS) map of battery cathode material LiNi1/3Co1/3Mn1/3O2 can be produced in few hours using such a spectrometer. Unlike analyzing only a handful of RIXS spectra taken at selected excitation photon energies across the elemental absorption edges to determine various spectral features like the localized dd excitations and non-resonant fluorescence emissions, these features can be easily identified in the RIXS maps. Studying such RIXS maps could reveal novel transition metal redox in battery compounds that are sometimes hard to be unambiguously identified in X-ray absorption and emission spectra. We propose that this modular spectrometer design can serve as the platform for further customization to meet specific scientific demands.

Spectroscopic Determination of the Atomic f-Electron Symmetry Underlying Hidden Order in URu2Si2.

The low-temperature hidden-order state of URu2Si2 has long been a subject of intense speculation, and is thought to represent an as-yet-undetermined many-body quantum state not realized by other known materials. Here, x-ray absorption spectroscopy and high-resolution resonant inelastic x-ray scattering are used to observe electronic excitation spectra of URu2Si2, as a means to identify the degrees of freedom available to constitute the hidden-order wave function. Excitations are shown to have symmetries that derive from a correlated 5f(2) atomic multiplet basis that is modified by itinerancy. The features, amplitude, and temperature dependence of linear dichroism are in agreement with ground states that closely resemble the doublet Γ5 crystal field state of uranium.

Disorder enabled band structure engineering of a topological insulator surface

Three-dimensional topological insulators are bulk insulators with Z2 topological electronic order that gives rise to conducting light-like surface states. These surface electrons are exceptionally resistant to localization by non-magnetic disorder, and have been adopted as the basis for a wide range of proposals to achieve new quasiparticle species and device functionality. Recent studies have yielded a surprise by showing that in spite of resisting localization, topological insulator surface electrons can be reshaped by defects into distinctive resonance states. Here we use numerical simulations and scanning tunnelling microscopy data to show that these resonance states have significance well beyond the localized regime usually associated with impurity bands. At native densities in the model Bi2X3 (X=Bi, Te) compounds, defect resonance states are predicted to generate a new quantum basis for an emergent electron gas that supports diffusive electrical transport.

Multiplet resonance lifetimes in resonant inelastic x-ray scattering involving shallow core levels

Resonant inelastic X-ray scattering (RIXS) spectra of model copper- and nickel-based transition metal oxides are measured over a wide range of energies near the M-edge (h

Modification of Transition-Metal Redox by Interstitial Water in Hexacyanometalate Electrodes for Sodium-Ion Batteries

\nu

Selective interlayer ferromagnetic coupling between the Cu spins in YBa2Cu3O7-x grown on top of La0.7Ca0.3MnO3

=60-80eV) to better understand the properties of resonant scattering involving shallow core levels. Standard multiplet RIXS calculations are found to deviate significantly from the observed spectra. However, by incorporating the self consistently calculated decay lifetime for each intermediate resonance state within a given resonance edge, we obtain dramatically improved agreement between data and theory. Our results suggest that these textured lifetime corrections can enable a quantitative correspondence between first principles predictions and RIXS data on model multiplet systems. This accurate model is also used to analyze resonant elastic scattering, which displays the elastic Fano effect and provides a rough upper bound for the core hole shake-up response time.

Monovalent manganese based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries

A sodium-ion battery (SIB) solution is attractive for grid-scale electrical energy storage. Low-cost hexacyanometalate is a promising electrode material for SIBs because of its easy synthesis and open framework. Most hexacyanometalate-based SIBs work with aqueous electrolyte, and interstitial water in the material has been found to strongly affect the electrochemical profile, but the mechanism remains elusive. Here we provide a comparative study of the transition-metal redox in hexacyanometalate electrodes with and without interstitial water based on soft X-ray absorption spectroscopy and theoretical calculations. We found distinct transition-metal redox sequences in hydrated and anhydrated NaxMnFe(CN)6·zH2O. The Fe and Mn redox in hydrated electrodes are separated and are at different potentials, leading to two voltage plateaus. On the contrary, mixed Fe and Mn redox in the same potential range is found in the anhydrated system. This work reveals for the first time how transition-metal redox in batteries is strongly affected by interstitial molecules that are seemingly spectators. The results suggest a fundamental mechanism based on three competing factors that determine the transition-metal redox potentials. Because most hexacyanometalate electrodes contain water, this work directly reveals the mechanism of how interstitial molecules could define the electrochemical profile, especially for electrodes based on transition-metal redox with well-defined spin states.