Li intercalation in 2D iron phosphate synthesized from the partial dehydration and deprotonation of vivianite.

This study focuses on the cation intercalation of structurally unique compounds synthesized from the partial dehydration and deprotonation of coordinated water molecules in hydrous materials. Partial dehydration can potentially result in hydrous materials with a porous nature, which maintains the parent structure of the material, and deprotonation causes oxidation in the hydrous materials. Li-intercalation experiments were conducted on the hydrous iron(II) phosphate mineral, vivianite (Fe2+3(PO4)2·8H2O), and its oxidized and partially dehydrated product, santabarbaraite. Vivianite comprises two-dimensional Fe3(PO4)2 sheets and coordinated water molecules. The oxidation progress of the Fe2+ of vivianite increased cathodic capacities up to 156 mA h g-1. The Li-intercalation reaction rate increased significantly owing to dehydration because the partial dehydration of vivianite created structural space for the diffusion of Li+. Furthermore, X-ray diffraction measurements revealed that Li intercalation did not cause the formation of byproducts.

Photoinduced Nonvolatile Displacive Transformation and Optical Switching in MnTe Semiconductors.

MnTe is considered a promising candidate for next-generation phase change materials owing to the reversible and nonvolatile phase transformation between its α and ß' phases by irradiation of a nanosecond laser or application of a pulse voltage. In this work, for a faster phase control of MnTe, the response of metastable ß-MnTe thin films to femtosecond (fs) laser irradiation was investigated. Using ultrafast optical spectroscopy, we inferred transient phase transformation. Moreover, with an increase in laser-excitation fluence, a nonvolatile structural change from the ß to α phase was experimentally observed by Raman spectroscopy and transmission electron microscopy without ablation damage on the sample. The observation results strongly suggest that the fs-laser-induced ß â†’ α phase transformation proceeds through the nucleation and growth mode without a large temperature increase.

Optimizing LiMn1.5M0.5O4 cathode materials for aqueous photo-rechargeable batteries.

Spinel oxides are promising for high-potential cathode materials of photo-rechargeable batteries. However, LiMn1.5M0.5O4 (M = Mn) shows a rapid degradation during charge/discharge under the illumination of UV-visible light. Here, we investigate various spinel-oxide materials by modifying the composition (M = Fe, Co, Ni, Zn) to demonstrate photocharging in a water-in-salt aqueous electrolyte. LiMn1.5Fe0.5O4 exhibited a substantially higher discharge capacity compared to that of LiMn2O4 after long-term photocharging owing to enhanced stability under illumination. This work provides fundamental design guidelines of spinel-oxide cathode materials for the development of photo-rechargeable batteries.

Ultraporous, Ultrasmall MgMn2O4 Spinel Cathode for a Room-Temperature Magnesium Rechargeable Battery.

Magnesium rechargeable batteries (MRBs) promise to be the next post lithium-ion batteries that can help meet the increasing demand for high-energy, cost-effective, high-safety energy storage devices. Early prototype MRBs that use molybdenum-sulfide cathodes have low terminal voltages, requiring the development of oxide-based cathodes capable of overcoming the sulfide's low Mg2+ conductivity. Here, we fabricate an ultraporous (>500 m2 g-1) and ultrasmall (<2.5 nm) cubic spinel MgMn2O4 (MMO) by a freeze-dry assisted room-temperature alcohol reduction process. While the as-fabricated MMO exhibits a discharge capacity of 160 mAh g-1, the removal of its surface hydroxy groups by heat-treatment activates it without structural change, improving its discharge capacity to 270 mAh g-1─the theoretical capacity at room temperature. These results are made possible by the ultraporous, ultrasmall particles that stabilize the metastable cubic spinel phase, promoting both the Mg2+ insertion/deintercalation in the MMO and the reversible transformation between the cubic spinel and cubic rock-salt phases.

Examining Electrolyte Compatibility on Polymorphic MnO2 Cathodes for Room-Temperature Rechargeable Magnesium Batteries.

Rechargeable magnesium batteries are promising candidates for post-lithium-ion batteries, owing to the large source abundance and high theoretical energy density. However, there remain few reports on constructing practical cells with oxide cathodes and Mg anodes at room temperature. In this work, we compare the reaction behavior of various MnO2 polymorph cathodes in two representative electrolytes: Mg[TFSA]2/G3 and Mg[Al(hfip)4]2/G3. In Mg[TFSA]2/G3, discharge capacities of the MnO2 cathodes are well consistent with the changes in Mg composition, where nanorod-like α-MnO2 and λ-MnO2 show the capacities of about 100 mA h g-1 at room temperature. However, this electrolyte has the disadvantage that the Mg anodes are easily passivated. In contrast, Mg[Al(hfip)4]2/G3 allows highly reversible deposition/dissolution of Mg anodes, whereas the discharge process of the MnO2 cathodes involves a large part of side reactions, in which the MnO2 active material takes part in some reductive reaction together with electrolyte species instead of the expected Mg2+ intercalation. Such an unstable electrode/electrolyte interface would lead to continuous degradation on/near the cathode surface. Thus, the interfacial stability between the oxide cathodes and the electrolytes must be improved for practical applications.

Laser-induced breakdown spectroscopy to obtain quantitative three-dimensional hydrogen mapping in a nickel-metal-hydride battery cathode for interpreting its reaction distribution.

We present a method for obtaining a three-dimensional quantitative hydrogen distribution in a Ni-MH battery cathode using laser-induced breakdown spectroscopy (LIBS) and demonstrate that the reaction distribution in the cathode can be interpreted based on a state-of-charge (SOC) distribution converted from the hydrogen distribution. In this method, we measured the hydrogen emission-line intensities at 656.28 nm for a model cathode cycled five times at 2.3 mA cm-2 and a commercial Ni-MH battery cathode cycled 1000 times at 1C under a 3000 Pa helium atmosphere. Our results show that the average SOC in the SOC distributions of the cathodes agreed with those evaluated from X-ray diffraction and charge-discharge curves and that the overcharged areas exhibited SOC values above 100%. The present LIBS method will allow us to understand the deterioration mechanism of a Ni-MH battery and improve its cycle life and capacity.

Light-induced Li extraction from LiMn2O4/TiO2 in a water-in-salt electrolyte for photo-rechargeable batteries.

Photocharging of high-potential spinel LiMn2O4 is demonstrated by using a water-in-salt electrolyte and TiO2 nanoparticles. In a developed half-cell system with an electron acceptor, Li extraction from LiMn2O4 proceeds under the illumination of UV-visible light at an estimated rate of ∼23 mA g-1. This work paves the way for high-potential cathode materials in photo-rechargeable batteries.

Excellently balanced water-intercalation-type heat-storage oxide.

Importance of heat storage materials has recently been increasing. Although various types of heat storage materials have been reported to date, there are few well-balanced energy storage materials in terms of long lifetime, reversibility, energy density, reasonably fast charge/discharge capability, and treatability. Here we report an interesting discovery that a commonly known substance, birnessite-type layered manganese dioxide with crystal water (δ-type K0.33MnO2 ⋅ nH2O), exhibits a water-intercalation mechanism and can be an excellently balanced heat storage material, from the above views, that can be operated in a solid state with water as a working pair. The volumetric energy density exceeds 1000 MJ m-3 (at n ~ 0.5), which is close to the ideally maximum value and the best among phase-change materials. The driving force for the water intercalation is also validated by the ab initio calculations. The proposed mechanism would provide an optimal solution for a heat-storage strategy towards low-grade waste-heat applications.

Electrochemically Induced Strain Evolution in Pt-Ni Alloy Nanoparticles Observed by Bragg Coherent Diffraction Imaging.

Strain is known to enhance the activity of the oxygen reduction reaction in catalytic platinum alloy nanoparticles, whose inactivity is the primary impediment to efficient fuel cells and metal-air batteries. Bragg coherent diffraction imaging (BCDI) was employed to reveal the strain evolution during the voltammetric cycling in Pt-Ni alloy nanoparticles composed of Pt2Ni3, Pt1Ni1, and Pt3Ni2. Analysis of the 3D strain images using a core-shell model shows that the strain as large as 5% is induced on Pt-rich shells due to Ni dissolution. The composition dependency of the strain on the shells is in excellent agreement with that of the catalytic activity. The present study demonstrates that BCDI enables quantitative determination of the strain on alloy nanoparticles during electrochemical reactions, which provides a means to exploit surface strain to design a wide range of electrocatalysts.

Structure Design of Long-Life Spinel-Oxide Cathode Materials for Magnesium Rechargeable Batteries.

Development of metal-anode rechargeable batteries is a challenging issue. Especially, magnesium rechargeable batteries are promising in that Mg metal can be free from dendrite formation upon charging. However, in case of oxide cathode materials, inserted magnesium tends to form MgO-like rocksalt clusters in a parent phase even with another structure, which causes poor cyclability. Here, a design concept of high-performance cathode materials is shown, based on: i) selecting an element to destabilize the rocksalt-type structure and ii) utilizing the defect-spinel-type structure both to avoid the spinel-to-rocksalt reaction and to secure the migration path of Mg cations. This theoretical and experimental work substantiates that a defect-spinel-type ZnMnO3 meets the above criteria and shows excellent cycle performance exceeding 100 cycles upon Mg insertion/extraction with high potential (≈2.5 V vs Mg2+ /Mg) and capacity (≈100 mAh g-1 ). Thus, this work would provide a design guideline of cathode materials for various multivalent rechargeable batteries.

Circumventing huge volume strain in alloy anodes of lithium batteries.

Since the launch of lithium-ion batteries, elements (such as silicon, tin, or aluminum) that can be alloyed with lithium have been expected as anode materials, owing to larger capacity. However, their successful application has not been accomplished because of drastic structural degradation caused by cyclic large volume change during battery reactions. To prolong lifetime of alloy anodes, we must circumvent the huge volume strain accompanied by insertion/extraction of lithium. Here we report that by using aluminum-foil anodes, the volume expansion during lithiation can be confined to the normal direction to the foil and, consequently, the electrode cyclability can be markedly enhanced. Such a unidirectional volume-strain circumvention requires an appropriate hardness of the matrix and a certain tolerance to off-stoichiometry of the resulting intermetallic compound, which drive interdiffusion of matrix component and lithium along the normal-plane direction. This metallurgical concept would invoke a paradigm shift to future alloy-anode battery technologies.

Electrochemical phase transformation accompanied with Mg extraction and insertion in a spinel MgMn2O4 cathode material.

One of the key challenges when developing magnesium rechargeable batteries (MRB) is to develop Mg-intercalation cathodes exhibiting higher redox potentials with larger specific capacities. Although Mg-transition-metal spinel oxides have been shown to be excellent candidates as MRB cathode materials by utilizing the valence change from trivalent to divalent of transition metals starting from Mg insertion, there is no clear evidence to date that Mg can be indeed extracted from the initial spinel hosts by utilizing the change from trivalent to quadrivalent. In this work, we clearly present various experimental evidences of the electrochemical extraction of Mg from spinel MgMn2O4. The present electrochemical charge, i.e., extraction treatment of Mg, was performed in an ionic liquid at 150 °C to ensure Mg hopping in the spinel host. Our analyses show that Mg can be extracted from Mg1-xMn2O4 up to x = 0.4 and, afterwards, successively be inserted into the Mg-extracted (demagnesiated) host via a two-phase reaction between tetragonal and cubic spinels. Finally, we also discuss the difference in electrochemical features between LiMn2O4 and MgMn2O4.

Solvation-Structure Modification by Concentrating Mg(TFSA)2-MgCl2-Triglyme Ternary Electrolyte.

Mg(TFSA)2/triglyme(G3)-based electrolytes (TFSA: bis (trifluoromethanesulfonyl) amide) are one of candidates for magnesium rechargeable batteries, but the passivation of Mg-metal anode due to the TFSA anion is fatal in practical use. In this work we show that at elevated temperatures around 150 °C a comparable amount of MgCl2 salt can be dissolved in concentrated Mg(TFSA)2/G3 solutions, and the passivation of Mg metal is markedly suppressed in such highly concentrated solutions (1 ≤ G3/Mg-salts ≤ 2) in comparison with in the dilute solutions (G3/Mg-salts ≫ 2). By decreasing the amount of G3 solvent, the solvation structure of Mg2+ ions is modified in that free TFSA anions are drastically lowered, which would consequently decrease the reactivity of TFSA anions. We also demonstrate that a full-cell using MgCo2O4 cathode with the electrolyte of Mg(TFSA)2/MgCl2/G3 1:1:2 at 150 °C delivers a cell voltage of ∼2 V versus Mg-metal anode.

Two distinct crystallization processes in supercooled liquid.

Using molecular dynamics simulations we show that two distinct crystallization processes, depending on the temperature at which crystallization occurs, appear in a supercooled liquid. As a model for glass-forming materials, an Al2O3 model system, in which both the glass transition and crystallization from the supercooled liquid can be well reproduced, is employed. Simulations in the framework of an isothermal-isobaric ensemble indicate that the calculated time-temperature-transformation curve for the crystallization to γ(defect spinel)-Al2O3 exhibited a typical nose shape, as experimentally observed in various glass materials. During annealing above the nose temperature, the structure of the supercooled liquid does not change before the crystallization, because of the high atomic mobility (material transport). Thus, the crystallization is governed by the abrupt crystal nucleation, which results in the formation of a stable crystal structure. In contrast, during annealing below the nose temperature, the structure of the supercooled liquid gradually changes before the crystallization, and the formed crystal structure is less stable than that formed above the nose temperature, because of the restricted material transport.

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.

Bulk-nanoporous-silicon negative electrode with extremely high cyclability for lithium-ion batteries prepared using a top-down process.

We synthesized freestanding bulk three-dimensional nanoporous Si using dealloying in a metallic melt, a top-down process. Using this nanoporous Si, we fabricated negative electrodes with high lithium capacity, nearing their theoretical limits, and greatly extended cycle lifetimes, considerably improving the battery performance compared with those using electrodes made from silicon nanoparticles. By operating the electrodes below the accommodation volume limit of their pores, we prolonged their cycle lifetime.

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.

Elastic constant measurement of Ni-base superalloy with the RUS and mode selective EMAR methods.

This paper reports the elastic constants of the Ni-base single crystal superalloy (TMS-26) with a rafted (lamellar) structure having tetragonal symmetry. The elastic constants have been measured at room temperature with the resonance ultrasound spectroscopy method and the mode-selective electromagnetic acoustic resonance method. The value of the elastic constant C33 (250.4 GPa) is almost equal to that of c11 (252.5 GPa), which indicates that the rafted structure virtually has the elastic anisotropy of cubic system.