Superionic Lithium Intercalation through 2 x 2 nm(2) Columns in the Crystallographic Shear Phase Nb18W8O69

Nb18W8O69 (9Nb2O5·8WO3) is the tungsten-rich end-member of the Wadsley–Roth crystallographic shear (cs) structures within the Nb2O5–WO3 series. It has the largest block size of any known, stable Wadsley–Roth phase, comprising 5 × 5 units of corner-shared MO6 octahedra between the shear planes, giving rise to 2 × 2 nm2 blocks. Rapid lithium intercalation is observed in this new candidate battery material and 7Li pulsed field gradient nuclear magnetic resonance spectroscopy—measured in a battery electrode for the first time at room temperature—reveals superionic lithium conductivity with Li diffusivities at 298 K predominantly between 10–10 and 10–12 m2·s–1. In addition to its promising rate capability, Nb18W8O69 adds to our understanding of the large family of high-performance Wadsley–Roth complex metal oxides

Cation Disorder and Lithium Insertion Mechanism of Wadsley-Roth Crystallographic Shear Phases from First Principles.

Wadsley-Roth crystallographic shear phases form a family of compounds that have attracted attention due to their excellent performance as lithium-ion battery electrodes. The complex crystallographic structure of these materials poses a challenge for first-principles computational modeling and hinders the understanding of their structural, electronic and dynamic properties. In this article, we study three different niobium-tungsten oxide crystallographic shear phases (Nb12WO33, Nb14W3O44, Nb16W5O55) using an enumeration-based approach and first-principles density-functional theory calculations. We report common principles governing the cation disorder, lithium insertion mechanism, and electronic structure of these materials. Tungsten preferentially occupies tetrahedral and block-central sites within the block-type crystal structures, and the local structure of the materials depends on the cation configuration. The lithium insertion proceeds via a three-step mechanism, associated with an anisotropic evolution of the host lattice. Our calculations reveal an important connection between long-range and local structural changes: in the second step of the mechanism, the removal of local structural distortions leads to the contraction of the lattice along specific crystallographic directions, buffering the volume expansion of the material. Niobium-tungsten oxide shear structures host small amounts of localized electrons during initial lithium insertion due to the confining effect of the blocks, but quickly become metallic upon further lithiation. We argue that the combination of local, long-range, and electronic structural evolution over the course of lithiation is beneficial to the performance of these materials as battery electrodes. The mechanistic principles we establish arise from the compound-independent crystallographic shear structure and are therefore likely to apply to niobium-titanium oxide or pure niobium oxide crystallographic shear phases.We acknowledge the use of Athena at HPC Midlands+, which was funded by the EPSRC on grant EP/P020232/1, in this research via the EPSRC RAP call of spring 2018. C.P.K. thanks the Winton Programme for the Physics of Sustainability and EPSRC for financial support. K.J.G. thanks the Winston Churchill Foundation of the United States and the Herchel Smith Foundation. K.J.G. and C.P.G. also thank the EPSRC for funding under a program grant (EP/M009521/1)

First-Principles Study of Localised and Delocalised Electronic States in Crystallographic Shear Phases of Niobium Oxide

Crystallographic shear phases of niobium oxide form an interesting family of compounds that have received attention both for their unusual electronic and magnetic properties, as well as their performance as intercalation electrode materials for lithium-ion batteries. Here, we present a first-principles density-functional theory study of the electronic structure and magnetism of H-Nb$_2$O$_5$, Nb$_{25}$O$_{62}$, Nb$_{47}$O$_{116}$, Nb$_{22}$O$_{54}$, and Nb$_{12}$O$_{29}$. These compounds feature blocks of niobium-oxygen octahedra as structural units, and we show that this block structure leads to a coexistence of flat and dispersive energy bands, corresponding to localised and delocalised electronic states. Electrons localise in orbitals spanning multiple niobium sites in the plane of the blocks. Localised and delocalised electronic states are both effectively one-dimensional and are partitioned between different types of niobium sites. Flat bands associated with localised electrons are present even at the GGA level, but a correct description of the localisation requires the use of GGA+U or hybrid functionals. We discuss the experimentally observed electrical and magnetic properties of niobium suboxides in light of our results, and argue that their behaviour is similar to that of $n$-doped semiconductors, but with a limited capacity for localised electrons. When a threshold of one electron per block is exceeded, metallic electrons are added to existing localised electrons. We propose that this behaviour of shear phases is general for any type of $n$-doping, and should transfer to doping by alkali metal (lithium) ions during operation of niobium oxide-based battery electrodes. Future directions for theory and experiment on mixed-metal shear phases are suggested

Lithium Diffusion in Niobium Tungsten Oxide Shear Structures.

Niobium tungsten oxides with crystallographic shear structures form a promising class of high-rate Li-ion anode materials. Lithium diffusion within these materials is studied in this work using density functional theory calculations, specifically nudged elastic band calculations and ab initio molecular dynamics simulations. Lithium diffusion is found to occur through jumps between 4-fold coordinated window sites with low activation barriers (80-300 meV) and is constrained to be effectively one-dimensional by the crystallographic shear planes of the structures. We identify a number of other processes, including rattling motions with barriers on the order of the thermal energy at room temperature, and intermediate barrier hops between 4-fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO3-like block units of the structures, cavities at the corners and edges host more isolated diffusion tunnels than those in the interior. Diffusion coefficients are found to be in the range of 10-12 to 10-11 m2 s-1 for lithium concentrations of 0.5 Li/TM. Overall, the results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure-property relationships identified in this work can be generalized to the entire family of crystallographic shear phases.Winton Programme for the Physics of Sustainability Winston Churchill Foundation Herchel Smith Foundatio

Natural abundance solid-state 33S NMR study of NbS3: applications for battery conversion electrodes.

We report ultra-wideline, high-field natural abundance solid-state 33S NMR spectra of the Li-ion battery conversion electrode NbS3, the first 33S NMR study of a compound containing disulfide (S22-) units. The large quadrupolar coupling parameters (CQ ≈ 31 MHz) are consistent with values obtained from DFT calculations, and the spectra provide evidence for the linear Peierls distortion that doubles the number of 33S sites

Niobium tungsten oxides for high-rate lithium-ion energy storage.

The maximum power output and minimum charging time of a lithium-ion battery depend on both ionic and electronic transport. Ionic diffusion within the electrochemically active particles generally represents a fundamental limitation to the rate at which a battery can be charged and discharged. To compensate for the relatively slow solid-state ionic diffusion and to enable high power and rapid charging, the active particles are frequently reduced to nanometre dimensions, to the detriment of volumetric packing density, cost, stability and sustainability. As an alternative to nanoscaling, here we show that two complex niobium tungsten oxides-Nb16W5O55 and Nb18W16O93, which adopt crystallographic shear and bronze-like structures, respectively-can intercalate large quantities of lithium at high rates, even when the sizes of the niobium tungsten oxide particles are of the order of micrometres. Measurements of lithium-ion diffusion coefficients in both structures reveal room-temperature values that are several orders of magnitude higher than those in typical electrode materials such as Li4Ti5O12 and LiMn2O4. Multielectron redox, buffered volume expansion, topologically frustrated niobium/tungsten polyhedral arrangements and rapid solid-state lithium transport lead to extremely high volumetric capacities and rate performance. Unconventional materials and mechanisms that enable lithiation of micrometre-sized particles in minutes have implications for high-power applications, fast-charging devices, all-solid-state energy storage systems, electrode design and material discovery.K.J.G. gratefully acknowledges support from The Winston Churchill Foundation of the United States, the Herchel Smith Scholarship, and the Science and Technology Facilities Council Futures Early Career Award. K.J.G and C.P.G thank the EPSRC via the LIBATT grant (EP/P003532/1). L.E.M. was funded by the European Union’s Horizon 2020 – European Union research and innovation program under the Marie Skłodowska–Curie grant agreement No. 750294. We thank Dr. Ieuan Seymour, University of Cambridge, and Prof. Bruce Dunn, University of California, Los Angeles, for fruitful discussions. We thank Drs. Jeremy Skepper and Heather Greer, University of Cambridge, for assistance with the electron microscopy and Dr. Maxim Avdeev, Bragg Institute, for his bond valence sum mapping program. We thank Dr. Olaf Borkiewicz, Advanced Photon Source, Argonne National Laboratory and Alisha Kasam, University of Cambridge for diffraction data reduction scripts. We thank Diamond Light Source for access to beamline B18 (SP14956, SP16387, SP17913) that contributed to the results presented here. This research used resources of the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE- AC02-06CH11357

Sodium Intercalation Mechanism of 3.8 v Class Alluaudite Sodium Iron Sulfate

Alluaudite sodium iron sulfate Na$_{2+2x}$Fe$_{2−x}$(SO$_4$)$_3$ is one of the most promising candidates for a Na-ion battery cathode material with earth-abundant elements; it exhibits the highest potential among any Fe$^{3+}$/Fe$^{2+}$ redox reactions (3.8 V vs Na/Na$^+$ ), good cycle performance, and high rate capability. However, the reaction mechanism during electrochemical charging/discharging processes is still not understood. Here, we surveyed the intercalation mechanism via synchrotron X-ray diffraction (XRD), $^{23}$Na nuclear magnetic resonance (NMR), density functional theory (DFT) calculations, X-ray absorption near edge structure (XANES), and Mössbauer spectroscopy. Throughout charging/discharging processes, the structure undergoes a reversible, single-phase (solid solution) reaction based on a Fe$^{3+}$/Fe$^{2+}$ redox reaction with a small volume change of ca. 3.5% after an initial structural rearrangement upon the first charging process, where a small amount of Fe irreversibly migrates from the original site to a Na site. Sodium extraction occurs in a sequential manner at various Na sites in the structure at their specific voltage regions.The present work was financially supported from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) under the “Element Strategy Initiative for Catalysts & Batteries” (ESICB) project. The synchrotron XRD experiments were performed under KEK-PF User Program (No. 2013G670). Crystal structures and the Fourier difference maps were drawn by VESTA.65 G.O. acknowledges financial support from JSPS Research Fellowships under “Materials Education Program for the Future Leaders in Research, Industry, and Technology” (MERIT) project. This project has received funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 655444 (O.P.). R.P. gratefully acknowledges financial support through the Marie Curie Actions People Program of the EU’s Seventh Frame work Program (FP7/2007-2013), under the grant agreement n.317127, the ‘pNMR project’. K.J.G. gratefully acknowledges funding from The Winston Churchill Foundation of the United States and the Herchel Smith Scholarship. This work made use of the facilities of the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886.This is the final version of the article. It first appeared from American Chemical Society via http://dx.doi.org/10.1021/acs.chemmater.6b0109

The Role of Ionic Liquid Breakdown in the Electrochemical Metallization of VO2: An NMR Study of Gating Mechanisms and VO2 Reduction.

Metallization of initially insulating VO2 via ionic liquid electrolytes, otherwise known as electrolyte gating, has recently been a topic of much interest for possible applications such as Mott transistors and memory devices. It is clear that the metallization takes place electrochemically, and, in particular, there has previously been extensive evidence for the removal of small amounts of oxygen during ionic liquid gating. Hydrogen intercalation has also been proposed, but the source of the hydrogen has remained unclear. In this work, solid-state magic angle spinning NMR spectroscopy (1H, 2H, 17O, and 51V) is used to investigate the thermal metal-insulator transition in VO2, before progressing to catalytically hydrogenated VO2 and electrochemically metallized VO2. In these experiments electrochemical metallization of bulk VO2 particles is shown to be associated with intercalation of hydrogen, the degree of which can be measured with quantitative 1H NMR spectroscopy. Possible sources of the hydrogen are explored, and by using a selectively deuterated ionic liquid, it is revealed that the hydrogenation is due to deprotonation of the ionic liquid; specifically, for the commonly used dialkylimidazolium-based ionic liquids, it is the "carbene" proton that is responsible. Increasing the temperature of the electrochemistry is shown to increase the degree of hydrogenation, forming first a less hydrogenated metallic orthorhombic phase then a more hydrogenated insulating Curie-Weiss paramagnetic orthorhombic phase, both of which were also observed for catalytically hydrogenated VO2. The NMR results are supported by magnetic susceptibility measurements, which corroborate the degree of Pauli and Curie-Weiss paramagnetism. Finally, NMR spectroscopy is used to identify the presence of hydrogen in an electrolyte gated thin film of VO2, suggesting that electrolyte breakdown, proton intercalation, and reactions with decomposition products within the electrolyte should not be ignored when interpreting the electronic and structural changes observed in electrochemical gating experiments.Oppenheimer Foundation The Winston Churchill Foundation of the United States Herchel Smith Scholarship EPSRC (EP/MO09521/1) EU H2020 program “Phase Change Switch” Alexander von Humboldt Foundatio