Reaction Mechanism of Li2MnO3 Electrodes in an All-Solid-State Thin-Film Battery Analyzed by Operando Hard X-ray Photoelectron Spectroscopy.

Li2MnO3 is a promising cathode candidate for Li-ion batteries because of its high discharge capacity; however, its reaction mechanism during cycling has not been sufficiently explicated. Observations of Mn and O binding energy shifts in operando hard X-ray photoelectron spectroscopy measurements enabled us to determine the charge-compensation mechanism of Li2MnO3. The O 1s peak splits at an early stage during the first charge, and the concentration of lower-valence O changes reversibly with cycling, indicating the formation of a low-valence O species that intrinsically participates in the redox reaction. The O 1s peak-splitting behavior, which indicates the number of valences of O in Li2MnO3, is supported by the computational results for an O3 to O1 structural transition. This is in agreement with the results of our previous study, wherein we confirmed this O3 to O1 transition based on in situ surface X-ray diffraction analysis, X-ray photoelectron spectroscopy, and first-principles formation energy calculations.

Operando analysis of electronic band structure in an all-solid-state thin-film battery.

Material characterization that informs research and development of batteries is generally based on well-established ex situ and in situ experimental methods that do not consider the band structure. This is because experimental extraction of structural information for liquid-electrolyte batteries is extremely challenging. However, this hole in the available experimental data negatively affects the development of new battery systems. Herein, we determined the entire band structure of a model thin-film solid-state battery with respect to an absolute potential using operando hard X-ray photoelectron spectroscopy by treating the battery as a semiconductor device. We confirmed drastic changes in the band structure during charging, such as interfacial band bending, and determined the electrolyte potential window and overpotential location at high voltage. This enabled us to identify possible interfacial side reactions, for example, the formation of the decomposition layer and the space charge layer. Notably, this information can only be obtained by evaluating the battery band structure during operation. The obtained insights deepen our understanding of battery reactions and provide a novel protocol for battery design.

Sequential delithiation behavior and structural rearrangement of a nanoscale composite-structured Li1.2Ni0.2Mn0.6O2 during charge-discharge cycles.

Lithium- and manganese-rich layered oxides (LMRs) are promising positive electrode materials for next-generation rechargeable lithium-ion batteries. Herein, the structural evolution of Li1.2Ni0.2Mn0.6O2 during the initial charge-discharge cycle was examined using synchrotron-radiation X-ray diffraction, X-ray absorption spectroscopy, and nuclear magnetic resonance spectroscopy to elucidate the unique delithiation behavior. The pristine material contained a composite layered structure composed of Ni-free and Ni-doped Li2MnO3 and LiMO2 (M = Ni, Mn) nanoscale domains, and Li ions were sequentially and inhomogeneously extracted from the composite structure. Delithiation from the LiMO2 domain was observed in the potential slope region associated with the Ni2+/Ni4+ redox couple. Li ions were then extracted from the Li2MnO3 domain during the potential plateau and remained mostly in the Ni-doped Li2MnO3 domain at 4.8 V. In addition, structural transformation into a spinel-like phase was partly observed, which is associated with oxygen loss and cation migration within the Li2MnO3 domain. During Li intercalation, cation remigration and mixing resulted in a domainless layered structure with a chemical composition similar to that of LiNi0.25Mn0.75O2. After the structural activation, the Li ions were reversibly extracted from the newly formed domainless structure.

Application of Anomalous X-ray Scattering Method to Liquid Electrolytes Used in a Battery: Local Structural Analysis around a Dilute Metallic Ion.

In liquid electrolytes used for a battery, various metal complexes are formed as a result of ion-solvent and ion-ion interactions, which strongly influence the properties of the electrolyte and thus the performance of the battery. Therefore, the structural characterization of such complexes is of great importance. In this study, the anomalous X-ray scattering (AXS) technique was applied to the potassium hydroxide solution including ∼0.3 mol % zinc, which is widely used in various batteries such as the alkaline battery. In spite of the small amount of the metallic ions, we have successfully extracted a local structure around zinc after careful data analysis. The obtained pair distribution function exhibited not only the short-range correlation corresponding to the Zn-O bond within the zincate anion but also a medium-range correlation above 3.5 Å. The present study demonstrates the capability of the AXS technique to detect local structures around dilute metallic ions in liquid electrolytes, which will largely extend the applicable range of this technique, especially to the field related to batteries.

Magnetic field strength controlled liquid phase syntheses of ferromagnetic metal nanowire.

The effect of the external magnetic field strength on the morphology of metal nickel (Ni) nanowires synthesized via electroless deposition is investigated. Under strong magnetic fields, Ni nanoparticles, precursors of Ni nanowires, easily align along the magnetic field to form straight nanowires with smooth surfaces. In contrast, under weak magnetic fields, branched Ni nanowires with rough surfaces are formed. By leveraging this magnetic effect on the morphology of Ni nanowires, simply changing the external magnetic field can synthesize various types of Ni nanowire nonwovens. Furthermore, different forms of Ni nanowires were grown on the sheet by the similar electroless deposition reaction on a non-magnetic copper metal sheet. The macroscopic morphology of this composite is closely correlated with the microstructures of Ni nanowires.

Two-Phase Reaction Mechanism for Fluorination and Defluorination in Fluoride-Shuttle Batteries: A First-Principles Study.

Fluoride-shuttle batteries (FSBs), which are based on fluoride-ion transfer, have attracted attention because of their high theoretical energy densities. The fluorination and defluorination reactions at the electrodes are the possible rate-determining steps in FSBs, and understanding the mechanism is important to achieve smooth charge/discharge. In this study, we discuss the thermodynamically favored pathways for the fluorination and defluorination reactions and compare the reactions through the solid-solution and two-phase-coexistent states by density functional theory (DFT) calculations. The free energies of the solid-solution and two-phase states approximate the energies calculated by DFT, and their accuracy was validated by comparison with experimental formation enthalpies and free energies. The relative formation enthalpies of typical, transition, and relativistic metal (Tl, Pb, and Bi) fluorides are well reproduced by DFT calculations within 0.1, 0.2, and 0.4 eV, respectively. We also show that the reaction pathway can be determined by comparing the formation enthalpies of the metal fluoride H, a fluorine vacancy HV, and an interstitial fluorine defect HI from the simple selection rule. The enthalpy relation of HI > H > -HV observed in all the calculations strongly suggests that fluorination and defluorination in FSB electrodes occur by a two-phase reaction. This fluorination and defluorination mechanism will be useful to clarify the rate-determining step in FSBs.

Degradation Mechanism of Conversion-Type Iron Trifluoride: Toward Improvement of Cycle Performance.

Conversion-type iron trifluoride (FeF3) has attracted considerable attention as a positive electrode material for lithium secondary batteries due to its high energy density and low cost. However, the conversion process through which FeF3 operates leads it to suffer from capacity degradation upon repeated cycling. To improve the cycle performance, in this study we investigated the degradation mechanism of conversion-type FeF3 electrode material. Bulk analyses of FeF3 upon cycling reveal incomplete oxidation to Fe3+ concomitant with the aggregation of LiF at the charged state. In addition, surface analyses of FeF3 reveal that a film covered the electrode surface after 10 cycles, which leads to a remarkable increase in resistance. We show that the choice of the electrolyte formulation is crucial in preventing the formation of the film on the electrode surface; thus, FeF3 shows better performance in an electrolyte comprising LiBF4 solute in cyclic carbonate solvents than in chain carbonate-containing LiPF6 as the electrolyte. This study underpins that a careful selection of solvent, rather than solute, is significantly essential to improve the cycle performance of the FeF3 electrode.

Structural characterization of an amorphous VS4 and its lithiation/delithiation behavior studied by solid-state NMR spectroscopy.

Vanadium sulfide (VS4) is one of the promising positive electrode materials for next-generation rechargeable lithium-ion batteries because of its high theoretical capacity (1196 mA h g-1). Crystalline VS4 has a unique structure, in which the Peierls-distorted one-dimensional chains of V-V bonds along the c axis are loosely connected to each other through van der Waals interactions. In this study, an amorphous VS4 is prepared by mechanical milling of the crystalline material, and its lithiation/delithiation behavior is investigated by solid-state nuclear magnetic resonance (NMR) spectroscopy. The amorphous VS4 shows a chain structure similar to that of crystalline VS4. The amorphous host structure is found to change drastically during the lithiation process to form Li3VS4: the V ions become tetrahedrally coordinated by S ions, in which the valence states of V and S ions simultaneously change from V4+ to V5+ and S- to S2-, respectively. When the Li insertion proceeds further, the valence state of V ions is reduced. After the 1st cycle, the amorphous VS4 recovers to the chain-like structure although it is highly disordered. No conversion to elemental V is observed, and a high capacity of 700 mA h g-1 is reversibly delivered between 1.5 and 2.6 V.

A Reversible Rocksalt to Amorphous Phase Transition Involving Anion Redox.

The charge-discharge capacity of lithium secondary batteries is dependent on how many lithium ions can be reversibly extracted from (charge) and inserted into (discharge) the electrode active materials. In contrast, large structural changes during charging/discharging are unavoidable for electrode materials with large capacities, and thus there is great demand for developing materials with reversible structures. Herein, we demonstrate a reversible rocksalt to amorphous phase transition involving anion redox in a Li2TiS3 electrode active material with NaCl-type structure. We revealed that the lithium extraction during charging involves a change in site of the sulfur atom and the formation of S-S disulfide bonds, leading to a decrease in the crystallinity. Our results show great promise for the development of long-life lithium insertion/extraction materials, because the structural change clarified here is somewhat similar to that of optical phase-change materials used in DVD-RW discs, which exhibit excellent reversibility of the transition between crystalline and amorphous phase.

Amorphous Metal Polysulfides: Electrode Materials with Unique Insertion/Extraction Reactions.

A unique charge/discharge mechanism of amorphous TiS4 is reported. Amorphous transition metal polysulfide electrodes exhibit anomalous charge/discharge performance and should have a unique charge/discharge mechanism: neither the typical intercalation/deintercalation mechanism nor the conversion-type one, but a mixture of the two. Analyzing the mechanism of such electrodes has been a challenge because fewer tools are available to examine the "amorphous" structure. It is revealed that the electrode undergoes two distinct structural changes: (i) the deformation and formation of S-S disulfide bonds and (ii) changes in the coordination number of titanium. These structural changes proceed continuously and concertedly for Li insertion/extraction. The results of this study provide a novel and unique model of amorphous electrode materials with significantly larger capacities.

Site- and phase-selective x-ray absorption spectroscopy based on phase-retrieval calculation.

Understanding the chemical state of a particular element with multiple crystallographic sites and/or phases is essential to unlocking the origin of material properties. To this end, resonant x-ray diffraction spectroscopy (RXDS) achieved through a combination of x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) techniques can allow for the measurement of diffraction anomalous fine structure (DAFS). This is expected to provide a peerless tool for electronic/local structural analyses of materials with complicated structures thanks to its capability to extract spectroscopic information about a given element at each crystallographic site and/or phase. At present, one of the major challenges for the practical application of RXDS is the rigorous determination of resonant terms from observed DAFS, as this requires somehow determining the phase change in the elastic scattering around the absorption edge from the scattering intensity. This is widely known in the field of XRD as the phase problem. The present review describes the basics of this problem, including the relevant background and theory for DAFS and a guide to a newly-developed phase-retrieval method based on the logarithmic dispersion relation that makes it possible to analyze DAFS without suffering from the intrinsic ambiguities of conventional iterative-fitting. Several matters relating to data collection and correction of RXDS are also covered, with a final emphasis on the great potential of powder-sample-based RXDS (P-RXDS) to be used in various applications relevant to practical materials, including antisite-defect-type electrode materials for lithium-ion batteries.

One-pot synthesis of silica-coated copper nanoparticles with high chemical and thermal stability.

With the recent development of nanotechnology, enhancement of the stability of nanomaterials is becoming ever more important for their practical applications. We studied the silica-coating of Cu nanoparticles and the enhanced stability of silica-coated Cu nanoparticles to oxidation. The metallic nanoparticles are easily oxidized and agglomerated compared with the bulk metals because the nanoparticles possess large specific surfaces. The Cu nanoparticle is one of the most difficult nanoparticles to be handled due to its absence of the oxidation stability. In the synthesis of silica-coated Cu nanoparticles via a sol-gel process using tetraethyl orthosilicate, the addition of NH3 as a catalyst of sol-gel reaction yielded homogeneous silica-coating. However, a large amount of Cu nanoparticles is instantly dissolved by forming complex ions in a NH3 solution during and before the silica-coating process. This is the difficulty in the silica-coating of Cu nanoparticles. In the present work, the dissolution behavior of Cu nanoparticles was electrochemically examined. This electrochemistry-based optimization of reducing power of a reaction bath enabled us to synthesize the silica-coated Cu nanoparticle via a consecutive liquid-phase reaction which requires only basic equipment and involves no separate centrifuging or extraction step. Cu nanoparticles coated by silica shells had the remarkable stability even in the presence of a strong oxidizing agent. Furthermore, we demonstrated that the highly stable Cu nanoparticles can be applied to a red pigment using a unique red color of Cu nanoparticles because of its surface plasmon resonance.

Roles of transition metals interchanging with lithium in electrode materials.

Roles of antisite transition metals interchanging with Li atoms in electrode materials of Li transition-metal complex oxides were clarified using a newly developed direct labeling method, termed powder diffraction anomalous fine structure (P-DAFS) near the Ni K-edge. We site-selectively investigated the valence states and local structures of Ni in Li0.89Ni1.11O2, where Ni atoms occupy mainly the NiO2 host-layer sites and partially the interlayer Li sites in-between the host layers, during electrochemical Li insertion/extraction in a lithium-ion battery (LIB). The site-selective X-ray near edge structure evaluated via the P-DAFS method revealed that the interlayer Ni atoms exhibited much lower electrochemical activity as compared to those at the host-layer site. Furthermore, the present analyses of site-selective extended X-ray absorption fine structure performed using the P-DAFS method indicates local structural changes around the residual Ni atoms at the interlayer space during the initial charge; it tends to gather to form rock-salt NiO-like domains around the interlayer Ni. The presence of the NiO-like domains in the interlayer space locally diminishes the interlayer distance and would yield strain energy because of the lattice mismatch, which retards the subsequent Li insertion both thermodynamically and kinetically. Such restrictions on the Li insertion inevitably make the NiO-like domains electrochemically inactive, resulting in an appreciable irreversible capacity after the initial charge but an achievement of robust linkage of neighboring NiO2 layers that tend to be dissociated without the Li occupation. The P-DAFS characterization of antisite transition metals interchanging with Li atoms complements the understanding of the detailed charge-compensation and degradation mechanisms in the electrode materials.

Intercalation and Push-Out Process with Spinel-to-Rocksalt Transition on Mg Insertion into Spinel Oxides in Magnesium Batteries.

On the basis of the similarity between spinel and rocksalt structures, it is shown that some spinel oxides (e.g., MgCo2O4, etc) can be cathode materials for Mg rechargeable batteries around 150 °C. The Mg insertion into spinel lattices occurs via "intercalation and push-out" process to form a rocksalt phase in the spinel mother phase. For example, by utilizing the valence change from Co(III) to Co(II) in MgCo2O4, Mg insertion occurs at a considerably high potential of about 2.9 V vs. Mg2+/Mg, and similarly it occurs around 2.3 V vs. Mg2+/Mg with the valence change from Mn(III) to Mn(II) in MgMn2O4, being comparable to the ab initio calculation. The feasibility of Mg insertion would depend on the phase stability of the counterpart rocksalt XO of MgO in Mg2X2O4 or MgX3O4 (X = Co, Fe, Mn, and Cr). In addition, the normal spinel MgMn2O4 and MgCr2O4 can be demagnesiated to some extent owing to the robust host structure of Mg1-xX2O4, where the Mg extraction/insertion potentials for MgMn2O4 and MgCr2O4 are both about 3.4 V vs. Mg2+/Mg. Especially, the former "intercalation and push-out" process would provide a safe and stable design of cathode materials for polyvalent cations.

Time-resolved coherent diffraction of ultrafast structural dynamics in a single nanowire.

The continuing effort to utilize the unique properties present in a number of strongly correlated transition metal oxides for novel device applications has led to intense study of their transitional phase state behavior. Here we report on time-resolved coherent X-ray diffraction measurements on a single vanadium dioxide nanocrystal undergoing a solid-solid phase transition, using the SACLA X-ray Free Electron Laser (XFEL) facility. We observe an ultrafast transition from monoclinic to tetragonal crystal structure in a single vanadium dioxide nanocrystal. Our findings demonstrate that the structural change occurs in a number of distinct stages attributed to differing expansion modes of vanadium atom pairs.

Three-dimensional nanoelectrode by metal nanowire nonwoven clothes.

Metal nanowire nonwoven cloth (MNNC) is a metal sheet that has resulted from intertwined metal nanowires 100 nm in diameter with several dozen micrometers of length. Thus, it is a new metallic material having both a flexibility of the metal sheet and a large specific surface area of the nanowires. As an application that utilizes these properties, we propose a high-cyclability electrode for Li storage batteries, in which an active material is deposited or coated on MNNC. The proposed electrode can work without any binders, conductive additives, and current collectors, which might largely improve a practical gravimetric energy density. Huge electrode surfaces provide efficient ion/electron transports, and sufficient interspaces between the respective nanowires accommodate large volume expansions of the active material. To demonstrate these advantages, we have fabricated a NiO-covered nickel nanowire nonwoven cloth (NNNC) by electroless deposition under a magnetic field and annealing in air. The adequately annealed NNNC was shown to be an excellent conversion-type electrode that exhibits a quite high cyclability, 500 mAh/g at 1 C after 300 cycles, compared to that of a composite electrode consisting of NiO nanoparticles. Thus, the present design concept will contribute to a game-changing technology in future lithium ion battery (LIB) electrodes.

Direct observation of a metastable crystal phase of Li(x)FePO4 under electrochemical phase transition.

The phase transition between LiFePO4 and FePO4 during nonequilibrium battery operation was tracked in real time using time-resolved X-ray diffraction. In conjunction with increasing current density, a metastable crystal phase appears in addition to the thermodynamically stable LiFePO4 and FePO4 phases. The metastable phase gradually diminishes under open-circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase and posit the new phase's role in decreasing the nucleation energy, accounting for the excellent rate capability of LiFePO4. This study is the first to report the measurement of a metastable crystal phase during the electrochemical phase transition of LixFePO4.

In situ two-dimensional imaging quick-scanning XAFS with pixel array detector.

Quick-scanning X-ray absorption fine structure (XAFS) measurements were performed in transmission mode using a PILATUS 100K pixel array detector (PAD). The method can display a two-dimensional image for a large area of the order of a centimetre with a spatial resolution of 0.2 mm at each energy point in the XAFS spectrum. The time resolution of the quick-scanning method ranged from 10 s to 1 min per spectrum depending on the energy range. The PAD has a wide dynamic range and low noise, so the obtained spectra have a good signal-to-noise ratio.

Change in local environment upon quasicrystallization of Zr-Cu glassy alloys by addition of Pd and Pt.

The effects of Pd and Pt, which are known quasicrystal (QC)-forming elements, on the local atomic structure in Zr(70)Cu(30) glassy alloys are investigated. A QC phase precipitates from a glassy phase above a certain temperature by a cooperative-like motion of icosahedral clusters. Quasicrystallization is accompanied by a significant change in the local environment around the Zr atoms and a slight change around the noble metal. However, the local environment around the Cu atoms remains almost the same during QC formation. It is suggested that two types of icosahedral polyhedra exist in the glassy state: one has a relatively perfect icosahedral structure formed around the Zr atoms. The other is in a distorted state around the Cu atoms. We speculate that the medium-range order (i.e. QC nucleus) has a Zr-centered icosahedral cluster as its core, and the QC grows via aggregation of possible clusters in the initial stage. Pd or Pt atoms stabilize and/or connect individual Zr-centered icosahedral clusters, facilitating the formation of the nucleus and growth of the QC phase.

Three-dimensional electron density mapping of shape-controlled nanoparticle by focused hard X-ray diffraction microscopy.

Coherent diffraction microscopy using highly focused hard X-ray beams allows us to three-dimensionally observe thick objects with a high spatial resolution, also providing us with unique structural information, i.e., electron density distribution, not obtained by X-ray tomography with lenses, atom probe microscopy, or electron tomography. We measured high-contrast coherent X-ray diffraction patterns of a shape-controlled Au/Ag nanoparticle and successfully reconstructed a projection and a three-dimensional image of the nanoparticle with a single pixel (or a voxel) size of 4.2 nm in each dimension. The small pits on the surface and a hollow interior were clearly visible. The Au-rich regions were identified based on the electron density distribution, which provided insight into the formation of Au/Ag nanoboxes.