Pair Distribution Function Analysis Research Articles

Unravelling the convoluted and dynamic interphasial mechanisms on Li metal anodes.

Accurate understanding of the chemistry of solid-electrolyte interphase (SEI) is key to developing new electrolytes for high-energy batteries using lithium metal (Li0) anodes1. SEI is generally believed to be formed by the reactions between Li0 and electrolyte2,3. However, our new study shows this is not the whole story. Through synchrotron-based X-ray diffraction and pair distribution function analysis, we reveal a much more convoluted formation mechanism of SEI, which receives considerable contributions from electrolyte, cathode, moisture and native surface species on Li0, with highly dynamic nature during cycling. Using isotope labelling, we traced the origin of LiH to electrolyte solvent, moisture and a new source: the native surface species (LiOH) on pristine Li0. When lithium accessibility is very limited as in the case of anode-free cells, LiOH develops into plate-shaped large crystals during cycling. Alternatively, when the lithium source is abundant, as in the case of Li||NMC811 cells, LiOH reacts with Li0 to form LiH and Li2O. While the desired anion-derived LiF-rich SEI is typically found in the concentrated electrolytes or their derivatives, we found it can also be formed in low-concentration electrolyte via the crosstalk effect, emphasizing the importance of formation cycle protocol and opening up opportunities for low-cost electrolyte development.

Etched Silicon Planar CRL Optics for the High-Energy X-ray Diffraction Beamlines 11-ID-B and 11-ID-C at the APS

The focusing of high-energy X-rays is most commonly achieved by commercially available compound refractive lenses (CRL), which are added together in larger structures. There is a need for high-energy X-ray beamlines to enable focusing in only one dimension down to a few micrometers to study layered materials and thin films. High-energy X-rays above 50 keV require often 100 lenses or more, therefore it is more efficient to etch the complete arrays of lenses into a monolithic substrate like a silicon wafer. Custom lens arrays were fabricated for the high-energy x-ray beamlines 11-ID-B, and 11-ID-C at the Argonne National Laboratory. The fabrication required tuning parameters of the Bosch process to optimize the etching of deep vertical sidewalls at a reasonable speed without overheating. The beamline 11-ID-B is equipped with lens arrays of 85-110 lenses for 59 keV and 190-250 lenses for 87 keV photon energy, focusing the beam vertically below 5.0 µm at the sample positions between 1600 mm to 2100 mm. 11-ID-C has received arrays of 435 lenses that focus 106 keV X-rays to a vertical height of 2.0 µm at the sample position. In both cases, the flux density gain in the focal spot is larger than a factor of 20, relative to using slits to cut the beam size. The lens setup is permanently installed and can be moved in and out of the beam during the experiment without further alignment. The lenses have enabled operando diffraction studies on interfaces in batteries and the development of pair distribution function (PDF) analysis using grazing incidence geometry.

Designing Glass and Crystalline Phases of Metal-Bis(acetamide) Networks to Promote High Optical Contrast.

Owing to their high tunability and predictable structures, metal-organic materials offer a powerful platform to study glass formation and crystallization processes and to design glasses with unique properties. Here, we report a novel series of glass-forming metal-ethylenebis(acetamide) networks that undergo reversible glass and crystallization transitions below 200 °C. The glass-transition temperatures, crystallization kinetics, and glass stability of these materials are readily tunable, either by synthetic modification or by liquid-phase blending, to form binary glasses. Pair distribution function (PDF) analysis reveals extended structural correlations in both single and binary metal-bis(acetamide) glasses and highlights the important role of metal-metal correlations during structural evolution across glass-crystal transitions. Notably, the glass and crystalline phases of a Co-ethylenebis(acetamide) binary network feature a large reflectivity contrast ratio of 4.8 that results from changes in the local coordination environment around Co centers. These results provide new insights into glass-crystal transitions in metal-organic materials and have exciting implications for optical switching, rewritable data storage, and functional glass ceramics.

Enhanced Catalytic Performance of a Ce/V Oxo Cluster through Confinement in Mesoporous SBA-15.

To increase catalytic efficiency, mesoporous supports have been widely applied to immobilize well-defined metal oxide clusters due to their ability to stabilize highly dispersed clusters. Herein, a redox-active heterometallic Ce12V6-oxo cluster (CeV) was first presynthesized and then incorporated into mesoporous silica, SBA-15, via a straightforward impregnation method. Scanning transmission electron microscopy (STEM) and Fourier transform infrared spectroscopy (FTIR), in concert with scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS), verified the successful introduction of the CeV cluster inside the pore of SBA-15. The 51V magic angle spinning solid-state nuclear magnetic resonance (51V MAS NMR) spectroscopy and differential pair distribution function (dPDF) analysis confirmed the structural integrity of the CeV cluster inside the SBA-15. The composite was then benchmarked for liquid-phase oxidation of 2-chloroethyl ethyl sulfide (CEES) under mild conditions and gas-phase oxidative dehydrogenation (ODH) of propane under high temperatures (up to 550 °C). The catalytic reactivity results demonstrated 8- and 14-fold increase in turnover frequency (TOF) values of the composite (CeV@10SBA-2) than the bulk CeV cluster under the same conditions for CEES oxidation and ODH, respectively. These results highlight the improved reactivity of the catalytically active CeV cluster as attributed to the higher dispersion of the discrete cluster upon immobilization within the SBA-15 support.

Size and Strain of Zinc Sulfide Nanoparticles Altered by Interaction with Organic Molecules.

Nanosized zinc sulfides (nano-ZnS) have size-dependent and tunable physical and chemical properties that make them useful for a variety of technological applications. For example, structural changes, especially caused by strain, are pronounced in nano-ZnS < 5 nm in size, the size range typical of incidental nano-ZnS that form in the environment. Previous research has shown how natural organic matter impacts the physical properties of nano-ZnS but was mostly focused on their aggregation state. However, the specific organic molecules and the type of functional groups that are most important for controlling the nano-ZnS size and strain remain unclear. This study examined the size-dependent strain of nano-ZnS synthesized in the presence of serine, cysteine, glutathione, histidine, and acetate. Synchrotron total scattering pair distribution function analysis was used to determine the average crystallite size and strain. Among the different organic molecules tested, those containing a thiol group were shown to affect the particle size and size-induced strain most strongly when added during synthesis but significantly reduced the particle strain when added to as-formed nano-ZnS. The same effects are useful to understand the properties and behavior of natural nano-ZnS formed as products of microbial activity, for example, in reducing environments, or of incidental nano-ZnS formed in organic wastes.

Mechanochemical Synthesis and Study of the Local Structure of NaGaS2 Glass and Glass-Ceramics.

NaGaS2 is a newly discovered compound that has already shown great promise for a variety of applications because of its layered structure and ion exchange properties. In this work, crystalline NaGaS2 has been synthesized by an alternative method to what has been previously published, namely, by mechanochemistry, either by a direct one-step process or by a two-step process. In the one-step process, crystalline NaGaS2 is directly formed by milling sodium sulfide Na2S and gallium(III) sulfide Ga2S3. However, an amorphous material is present in majority together with the crystalline phase. In the two-step process, amorphous NaGaS2 is first obtained by mechanical milling and then heated above its glass transition temperature to obtain a glass-ceramic mainly composed of crystalline NaGaS2. For the two-step process, changes of the local atomic-level structure in amorphous NaGaS2 and after crystallization were analyzed by high-field solid-state nuclear magnetic resonance (NMR) spectroscopy as well as by X-ray total scattering and pair distribution function (PDF) analysis. Based on quantitative analysis on the 23Na NMR spectra, modifying the annealing treatment can promote the formation of the crystalline phase up to a molar fraction of 83.8%.

Partial breakdown of translation symmetry at a structural quantum critical point associated with a ferroelectric soft mode

We report that complete suppression of a phonon-driven structural phase transition causes partial breakdown of a three-dimensional translation symmetry in a well-defined sublattice. This state is revealed for a dielectric compound, ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}$, that comprises an ${\mathrm{AlO}}_{4}$ network incorporated into a hexagonal Ba(Sr) sublattice. Pair distribution function analyses and inelastic neutron scattering experiments provide clear-cut evidence of the ${\mathrm{AlO}}_{4}$ network forming a continuum of Al-O short-range correlations similar to glasses, whereas the Ba(Sr) sublattice preserves the original translational symmetry. This glassy network significantly dampens the phonon spectrum and transforms it into the broad one resembling those typically observed in glass materials.

Structural and thermodynamic evolution of an amorphous SiOC ceramic after swift heavy ion irradiation

Radiation induced structural and energetic changes have been widely studied in crystalline materials, but studied to a much lesser extent in amorphous solids. Using a combination of neutron pair distribution function (PDF) analysis and high temperature oxide melt solution calorimetry, we obtained critical insights into the structural and energetic evolution in a swift heavy ion irradiated amorphous SiOC polymer-derived ceramic. The radiation modified SiOC structure becomes energetically less stable by 24.0±1.6 kJ/mol compared to the unirradiated structure. This destabilization is related to decreased connectivity of the Si-O-C network and destruction of free carbon. In comparison with unirradiated SiOC, the irradiated structure is more likely to become phase separated during subsequent thermal annealing. Our study has important implications for evaluating amorphous SiOC ceramic as a possible radiation resistant structure for nuclear applications.

Mapping structural heterogeneity at the nanoscale with scanning nano-structure electron microscopy (SNEM)

Here we explore the use of scanning electron diffraction (also known as 4D-STEM) coupled with electron atomic pair distribution function analysis (ePDF) to understand the local order (structure and chemistry) as a function of position in a complex multicomponent system, a hot rolled, Ni-encapsulated, Zr65Cu17.5Ni10Al7.5 bulk metallic glass (BMG), with a spatial resolution of 3 nm. We show that it is possible to gain insight into the chemistry and chemical clustering/ordering tendency in different regions of the sample, including in the vicinity of nano-scale crystallites that are identified from virtual dark field images and in heavily deformed regions at the edge of the BMG. In addition to simpler analysis, unsupervised machine learning was used to extract partial PDFs from the material, modeled as a quasi-binary alloy, and map them in space. These maps allowed key insights not only into the local average composition, as validated by EELS, but also a unique insight into chemical short-range ordering tendencies in different regions of the sample during formation. The experiments are straightforward and rapid and, unlike spectroscopic measurements, don’t require energy filters on the instrument. We spatially map different quantities of interest (QoI’s), defined as scalars that can be computed directly from positions and widths of ePDF peaks or parameters refined from fits to the patterns. We developed a flexible and rapid data reduction and analysis software framework that allows experimenters to rapidly explore images of the sample on the basis of different QoI’s. The power and flexibility of this approach are explored and described in detail. Because of the fact that we are getting spatially resolved images of the nanoscale structure obtained from ePDFs we call this approach scanning nano-structure electron microscopy (SNEM), and we believe that it will be powerful and useful extension of current 4D-STEM methods.

Characterization of the Local Atomic Structure of ALD Coated Interfaces

To accelerate technological innovations using atomic layer deposition (ALD) coatings, we need to establish better understanding of structure-property relationships for ALD films. Historically, the ALD community has had difficulty connecting the atomic structure of ALD films with their performance, largely because of the significant challenge in determining the atomic-scale structure of ultrathin ALD films that are often amorphous, polycrystalline, or defective. In this work, we describe a series of recent efforts that collectively aim to improve understanding of the atomic structure of ALD coatings and inform structure-property understanding. These efforts employ experimental measurements including (a) inert-transfer XPS, (b) in-situ synchrotron high energy X-ray diffraction, and (c) cryogenic electron diffraction. Of particular focus in this talk is the use of X-ray and electron diffraction measurements in combination with pair distribution function (PDF) analysis and reverse Monte Carlo (RMC) structural modeling. We describe a newly developed approach employing alternating RMC and molecular statics steps (RMC-MS) that improves the physical accuracy of model structures derived from experimental diffraction data with minimal additional computational cost over conventional RMC modeling. We also describe efforts using localized cryogenic electron diffraction and PDF analysis (cryo-ePDF) with a ≤ 5 nm spot size to measure atomic structure at ALD interfaces. Together, these advances allow us to quantify differences in the atomic structure as a function of position through the depth of ALD films. A simple two-phase model comprised of bulk and interfacial layers with distinct atomic structure features is consistent with our measurements for both aluminum oxide and zinc oxide ALD coatings. The approaches we report yield atomic structure models that can be used to inform computational studies of the properties if ALD films and will aid in the selection and modification of ALD coating chemistries to address technological needs.

Design of High-Performance Solid Electrolytes Guided By Crystal Structure Characterization and Understanding

Solid electrolyte is the key component in all-solid-state-batteries that is currently limiting the commercialization of this technology. An ideal solid electrolyte should have high room temperature ionic conductivity, low electronic conductivity, good compatibility with both cathode and anode, good mechanical properties, high air and moisture stability and low manufacture cost. Among these requirements, the improvement of ionic conductivity is prioritized as the conductivity of most existing solid electrolyte is still much lower than that of conventional liquid electrolyte.The improvement of ionic conductivity and design and development of solid electrolyte materials are closely related to our understanding on the ionic diffusion mechanism in solids and the structure-property relationship. We believe that rational design of high-performance solid electrolyte should start from careful characterization and good understanding of the crystal structure. Here we report the crystal structure characterization on sulfides and halide solid electrolytes and the design and development of novel solid electrolytes based on our findings in structural characterizations. Ex situ high resolution synchrotron X-ray and neutron diffraction and pair distribution function analysis are used to understand the crystal structures in great details. In situ X-ray diffraction for different synthesis methods is coupled with variable temperature electrochemical impedance spectroscopy to understand the structure-property relationship in the solid electrolytes. The design, synthesis and electrochemical evaluation of several solid electrolytes will be presented and discussed.

Electrochemical and Structural Study on PVDF-Based Polymer Electrolytes for Solid-State Batteries

Lithium-ion batteries are widely used today in powering devices from portable electronics to electric vehicles. Despite their great success, this battery chemistry relies on organic solvent-based liquid electrolytes which are highly flammable, leading to major safety concerns. In contrast, solid-state batteries, which are based on solid-state electrolytes, are regarded to have much better safety characteristics and potentially higher energy density than the conventional lithium-ion batteries. Solid-state electrolytes usually include ceramics, polymers, gels, and composites. Among them, polymer materials have attracted considerable attention due to their great interfacial contact, flexibility, and easy fabrication. In particular, polyvinylidene fluoride (PVDF) polymer electrolyte is a promising candidate as it can potentially provide a high voltage window and enable high energy density solid-state batteries. However, the current PVDF-based polymer electrolyte is still not compatible with high voltage cathodes, such as layered lithium transition metal oxides and the interaction between the salt and PVDF polymer has not been fully elucidated.We have systematically studied the PVDF-based electrolytes with different salts and salt combinations, including lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). The optimized electrolyte delivers excellent performance for solid-state Li||LiFePO4 cells, achieving a specific capacity of over 150 mAh/g and lasting more than 50 cycles with over 99% capacity retention. Similar tests have also been applied to lithium nickel manganese cobalt oxides (NMC), and the preliminary results show promising cycling stability and capacity retention. In addition to electrochemical study, we employed a range of synchrotron-based X-ray techniques, including diffraction, pair distribution function analysis, and absorption spectroscopy, to investigate the interactions between the polymer and lithium salt in the polymer electrolytes. This knowledge will provide valuable information for designing new polymer electrolyte systems.Acknowledgments: The work at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. Department of Energy through the Advanced Battery Materials Research (BMR) Program under contract DE-SC0012704. This research used beamline 23-ID-2 and 28-ID-2 of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Brookhaven National Laboratory under Contract No. DE-SC0012704.

(Invited) Synergy of Carbonyl and Azo Chemistries for Wide-Temperature-Range Rechargeable Aluminum Organic Batteries

Developing low-cost, sustainable, and all-climate energy storage devices as alternatives to Li-ion batteries (LIBs) is critical for the global application of electric vehicles, grid-scale electrical energy storages. Among various emerging battery systems beyond LIBs, rechargeable aluminum organic batteries (RAOBs) stand out because of the high theoretical capacity, low cost, abundance, high sustainability, and high safety of Al and organic resources. However, the large ion size of Al complex ions, such as AlCl4 -, AlCl2 + and AlCl2+, or the strong Coulombic interaction between Al3+ ions and active materials, as well as the high diffusion energy barrier of Al3+ ion, lead to poor cyclic stability and sluggish reaction kinetics. Moreover, the reaction mechanism, interphase structure, and the impact of multi-functional groups in redox-active organic materials to the electrochemical performance of RAOBs remains elusive. To address these challenges, we designed and synthesized a redox-active polymer bearing carbonyl and azo groups as the cathode to achieve high-performance and wide-temperature-range RAOBs. The polymeric cathode exhibits a high reversible specific capacity, superior cyclic stability, fast charging capability, and a wide operation temperature range (-40oC to 100oC). X-ray photoelectron spectroscopy (XPS), pair distribution function (PDF) analysis, and soft X-ray absorption near edge structure (XANES) were employed to gain fundamental insight into the carbonyl and azo chemistries in RAOBs, as well as the cathode electrolyte interphase (CEI) structure. We demonstrated a step-by-step alumination/de-alumination reaction for carbonyl and azo groups in the polymer cathode and unravel a Al2O3- and AlN-rich CEI, which is critical for the impressive performance of RAOBs.

(Invited) Identification of Lithium Hydride and Nanocrystalline Lithium Fluoride in the SEI of Lithium Metal Anodes and the Stabilization of High Ni Layered Structure at Ultra-High Voltage through Cathode Electrolyte Interphase Engineering

Solid-electrolyte interphase (SEI) plays a pivotal role in the performance of lithium metal anode, which is promising for high energy batteries. Understanding SEI composition is important but very challenging. Here we report on the use of synchrotron-based X-ray diffraction and pair distribution function analysis to identify and differentiate the two elusive components, LiH and LiF, in the SEI of lithium metal anodes. LiH is identified as a component of SEI in high abundance, and the possibility of its misidentification as LiF in the literature is discussed. LiF in the SEI is found to have different structural features from LiF in bulk phase, including larger lattice parameter and smaller grain size (<3 nm). These characteristics favor Li+ transport, and explain why an ionic insulator, like LiF, has been found to be a favored component for SEI. PDF analysis of the SEI reveals key amorphous components related to solvent or anion decomposition.Nickel-rich layered cathode materials with high energy density are very promising for next generation batteries when coupled with lithium metal anode. However, the practical capacities accessible are far less than the theoretical value due to their structural instability during cycling, especially when charged at high voltages. Such challenge has been attributed to the concomitant electrolyte instability and cathode structural and morphological degradation. Therefore, various approaches have been developed through electrolyte engineering and bulk cathode material modifications. In this presentation, we will report a simple interphase engineering approach by using an ionic electrolyte additive, lithium difluorophosphate (LiDFP). Through theoretical calculation, we found that self-decomposition product PO2F of LiDFP additive is adsorbed strongly and uniformly on the Ni-rich cathode, which is further decomposed to Li3PO4 and LiF through the catalytic effect of transition metals. The new interphasial chemistry contributed from LiDFP stabilizes the bulk structure of Ni-rich cathode materials and prevents the performance degradation by suppressing the dissolution of transition metals and undesired structural changes. Even after 200 cycles with charge limit as high as 4.8 V, no rock-salt phase can be detected in LiNi0.76Co0.10Mn0.14O2 (NMC76), and the cell consisting of NMC76 cathode and lithium metal anode retains 97% of the initial capacity (235 mAh/g). Various advanced characterizations were used, on both atomic and electrode levels, to reveal the mechanism how a new interphase stabilizes the layer structures in bulk electrode. The dissolution of nickel was identified to be most serious at high voltage, rather than the widely believed manganese dissolution. The transition metal dissolution can be effectively suppressed by LiDFP additive. Furthermore, the cathode surface protected by LiDFP-derived interphase is shown to regulate a more uniform lithium distribution within the bulk cathode particles and effectively mitigate the strain and crack formation. Machine-learning assisted nano-tomography discovered that such interphase-bulk interaction is most effective for those small-sized, spherically-shaped particles.Acknowledgement:The work done at Brookhaven National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Vehicle Technology Office of the U.S. DOE through the Advanced Battery Materials Research (BMR) Program, (Battery500 Consortium), and Applied Battery Research for Transportation (ABRT) program under contract No. DE-SC0012704. This work used the resources of the Center for Functional Nanomaterials, a U.S. DOE office of Science User Facility, at Brookhaven National Laboratory, and beamlines 5-ID, 7-BM, 23-ID-2, and 28-ID-2 of the National Synchrotron Light Source II, a US DOE Office of Science user facility operated for the DOE Office of Science by Brookhaven National Laboratory under contract no. DE-SC0012704. The work done at Pacific Northwest National Laboratory was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies of the US Department of Energy through the Advanced Battery Materials Research (BMR) Program (Battery500 Consortium) under contract No. DE-AC02-05CH11231. The work at ARL was performed under JCESR, an Energy Research Hub funded by Basic Energy Science, US Department of Energy.

Cation Short-Range Ordering of MgAl2O4 and NiAl2O4 Spinel Oxides at High Temperatures via In Situ Neutron Total Scattering.

Complex oxides that adopt the isometric spinel structure (AB2O4) are important for numerous technological applications and are relevant for certain geological processes, which involve exposure to extreme environments such as high pressures and temperatures. Recent studies have shown that the changes to the spinel structure caused by these environments are complex and depend on the material length scale under consideration. In this study, we have expanded this approach to the behavior of spinels under high temperatures. In situ neutron total scattering experiments, coupled with pair distribution function analysis, performed on two spinel compositions with various levels of pre-existing disorder (MgAl2O4 and NiAl2O4) revealed that both compositions trend to a state of maximum disorder where the A and B cations are randomly distributed among the two available sites. Temperature-induced cation inversion, conventionally understood as an exchange of cations on the A and B sites, is locally expressed as an atomic rearrangement to a tetragonal symmetry, a correlation that is retained up to the maximum temperature studied (1000 °C). A complex thermal expansion behavior is revealed wherein the oxide materials expand heterogeneously at the level of coordination polyhedra with an apparent dependence on bond strength.

Reaction Mechanism of Na-Ion Deintercalation in Na2CoSiO4

Sodium transition metal silicates are potential candidate electrode materials to enable two-electron redox per transition metal ion center. Yet, the electrochemical reaction mechanism remains elusive despite the widely reported electrochemical activity for this class of materials as intercalation cathodes for Na-ion batteries. Adopting monoclinic Na2CoSiO4 as a model compound, we used high-resolution synchrotron X-ray diffraction (XRD) and X-ray pair distribution function (PDF) analysis to elucidate the structure of the partially desodiated Na2-xCoSiO4 phases for the Co3+/Co2+ redox couple. The appearance of satellite reflections in the intermediate Na1.5CoSiO4 and NaCoSiO4 phases manifests the formation of modulated structures, which are induced by Na+/vacancy and Co2+/Co3+ charge orderings. Accounting for these structural orderings is important to understand the function and performance of sodium transition metal silicate electrodes.

Extracting structural motifs from pair distribution function data of nanostructures using explainable machine learning

Characterization of material structure with X-ray or neutron scattering using e.g. Pair Distribution Function (PDF) analysis most often rely on refining a structure model against an experimental dataset. However, identifying a suitable model is often a bottleneck. Recently, automated approaches have made it possible to test thousands of models for each dataset, but these methods are computationally expensive and analysing the output, i.e. extracting structural information from the resulting fits in a meaningful way, is challenging. Our Machine Learning based Motif Extractor (ML-MotEx) trains an ML algorithm on thousands of fits, and uses SHAP (SHapley Additive exPlanation) values to identify which model features are important for the fit quality. We use the method for 4 different chemical systems, including disordered nanomaterials and clusters. ML-MotEx opens for a type of modelling where each feature in a model is assigned an importance value for the fit quality based on explainable ML.

Quantifying the Anomalous Local and Nanostructure Evolutions Induced by Lattice Oxygen Redox in Lithium-Rich Cathodes.

Due to their accessible lattice oxygen redox (l-OR) at high voltages, Li-rich layered transition metal (TM) oxides have shown promising potential as candidate cathodes for high-energy-density Li-ion batteries. However, this l-OR process is also associated with unusual electrochemical issues such as voltage hysteresis and long-term voltage decay. The structure response mechanism to the l-OR behavior also remains unclear, hindering rational structure optimizations that would enable practical Li-rich cathodes. Here, this study reveals a strong coupling between l-OR and structure dynamic evolutions, as well as their effects on the electrochemical properties. Using the technique of neutron total scattering with pair distribution function analysis and small-angle neutron scattering, this study quantifies the local TM migration and formation of nanopores that accompany the l-OR. These experiments demonstrate the causal relationships among l-OR, the local/nanostructure evolutions, and the unusual electrochemistry. The TM migration triggered by the l-OR can change local oxygen coordination environments, which results in voltage hysteresis. Coupled with formed oxygen vacancies, it will accelerate the formation of nanopores, inducing a phase transition, and leading to irreversible capacity and long-cycling voltage fade.

Structure and Surface Relaxation of CeO2 Nanoparticles Unveiled by Combining Real and Reciprocal Space Total Scattering Analysis.

We present a combined real and reciprocal space structural and microstructural characterization of CeO2 nanoparticles (NPs) exhibiting different crystallite sizes; ~3 nm CeO2 NPs were produced by an inverse micellae wet synthetic path and then annealed at different temperatures. X-ray total scattering data were analyzed by combining real-space-based Pair Distribution Function analysis and the reciprocal-space-based Debye Scattering Equation method with atomistic models. Subtle atomic-scale relaxations occur at the nanocrystal surface. The structural analysis was corroborated by ab initio DFT and force field calculations; micro-Raman and electron spin resonance added important insights to the NPs’ defective structure. The combination of the above techniques suggests a core-shell like structure of ultrasmall NPs. These exhibit an expanded outer shell having a defective fluorite structure, while the inner shell is similar to the bulk structure. The presence of partially reduced species testifies to the high surface activity of the NPs. On increasing the annealing temperature, the particle dimensions increase, limiting disorder as a consequence of the progressive surface-to-volume ratio reduction.

Influence of the Cation on the Reaction Mechanism of Sodium Uptake and Release in Bivalent Transition Metal Thiophosphate Anodes: A Case Study of Fe2P2S6

AbstractThe layered active material Fe2P2S6 was examined as anode material in sodium‐ion batteries (SIBs) and compared to previously investigated Ni2P2S6. A reversible specific capacity of 540 mAh g−1 was achieved after 250 cycles, depicting similar electrochemical performance as observed for Ni2P2S6. The rate capability and long‐term behavior of these two materials are also very similar. Another objective was to elucidate the reaction mechanism during discharging and charging by applying several techniques such as X‐ray diffraction, pair distribution function analysis as well as X‐ray absorption and solid state NMR spectroscopy. The results clearly demonstrate that the majority of Fe2+ is reduced to elemental Fe during the uptake of 5 Na/f.u., while an amorphous intermediate is generated, which was identified as Na4P2S6 by solid state NMR spectroscopy. Completely discharging against a Na metal counter electrode leads to the formation of nanocrystalline Na2S and indications of the formation of polymeric phosphorus were found. In sum, the Na uptake reaction process observed for Fe2P2S6 coincides with the previously unraveled reaction pathway of Ni2P2S6. We therefore conclude that a universal reaction takes places for bivalent transition metal thiophosphate (M2P2S6) electrodes in SIBs.