Masashi Okubo

Pseudocapacitance of MXene nanosheets for high-power sodium-ion hybrid capacitors.

High-power Na-ion batteries have tremendous potential in various large-scale applications. However, conventional charge storage through ion intercalation or double-layer formation cannot satisfy the requirements of such applications owing to the slow kinetics of ion intercalation and the small capacitance of the double layer. The present work demonstrates that the pseudocapacitance of the nanosheet compound MXene Ti2C achieves a higher specific capacity relative to double-layer capacitor electrodes and a higher rate capability relative to ion intercalation electrodes. By utilizing the pseudocapacitance as a negative electrode, the prototype Na-ion full cell consisting of an alluaudite Na2Fe2(SO4)3 positive electrode and an MXene Ti2C negative electrode operates at a relatively high voltage of 2.4 V and delivers 90 and 40 mAh g−1 at 1.0 and 5.0 A g−1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

Fast Li-Ion Insertion into Nanosized LiMn2O4 without Domain Boundaries

The effect of crystallite size on Li-ion insertion in electrode materials is of great interest recently because of the need for nanoelectrodes in higher-power Li-ion rechargeable batteries. We present a systematic study of the effect of size on the electrochemical properties of LiMn(2)O(4). Accurate size control of nanocrystalline LiMn(2)O(4), which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn(2)O(4) with extremely small crystallite size of 15 nm cannot accommodate domain boundaries between Li-rich and Li-poor phases due to interface energy, and therefore lithiation proceeds via solid solution state without domain boundaries, enabling fast Li-ion insertion during the entire discharge process.

Synthesis of Triaxial LiFePO4 Nanowire with a VGCF Core Column and a Carbon Shell through the Electrospinning Method

A triaxial LiFePO4 nanowire with a multi wall carbon nanotube (VGCF:Vapor-grown carbon fiber) core column and an outer shell of amorphous carbon was successfully synthesized through the electrospinning method. The carbon nanotube core oriented in the direction of the wire played an important role in the conduction of electrons during the charge-discharge process, whereas the outer amorphous carbon shell suppressed the oxidation of Fe2+. An electrode with uniformly dispersed carbon and active materials was easily fabricated via a single process by heating after the electrospinning method is applied. Mossbauer spectroscopy for the nanowire showed a broadening of the line width, indicating a disordered coordination environment of the Fe ion near the surface. The electrospinning method was proven to be suitable for the fabrication of a triaxial nanostructure.

Bimetallic cyanide-bridged coordination polymers as lithium ion cathode materials: core@shell nanoparticles with enhanced cyclability.

Prussian blue analogues (PBAs) have recently been proposed as electrode materials for low-cost, long-cycle-life, and high-power batteries. However, high-capacity bimetallic examples show poor cycle stability due to surface instabilities of the reduced states. The present work demonstrates that, relative to single-component materials, higher capacity and longer cycle stability are achieved when using Prussian blue analogue core@shell particle heterostructures as the cathode material for Li-ion storage. Particle heterostructures with a size dispersion centered at 210 nm composed of a high-capacity K(0.1)Cu[Fe(CN)(6)](0.7)·3.8H(2)O (CuFe-PBA) core and lower capacity but highly stable shell of K(0.1)Ni[Fe(CN)(6)](0.7)·4.1H(2)O have been prepared and characterized. The heterostructures lead to the coexistence of both high capacity and long cycle stability because the shell protects the otherwise reactive surface of the highly reduced state of the CuFe-PBA core. Furthermore, interfacial coupling to the shell suppresses a known structural phase transition in the CuFe-PBA core, providing further evidence of synergy between the core and shell. The structure and chemical state of the heterostructure during electrochemical cycling have been monitored with ex situ X-ray diffraction and X-ray absorption experiments and compared to the behavior of the individual components.

Sodium-Ion Intercalation Mechanism in MXene Nanosheets

MXene, a family of layered compounds consisting of nanosheets, is emerging as an electrode material for various electrochemical energy storage devices including supercapacitors, lithium-ion batteries, and sodium-ion batteries. However, the mechanism of its electrochemical reaction is not yet fully understood. Herein, using solid-state (23)Na magic angle spinning NMR and density functional theory calculation, we reveal that MXene Ti3C2Tx in a nonaqueous Na(+) electrolyte exhibits reversible Na(+) intercalation/deintercalation into the interlayer space. Detailed analyses demonstrate that Ti3C2Tx undergoes expansion of the interlayer distance during the first sodiation, whereby desolvated Na(+) is intercalated/deintercalated reversibly. The interlayer distance is maintained during the whole sodiation/desodiation process due to the pillaring effect of trapped Na(+) and the swelling effect of penetrated solvent molecules between the Ti3C2Tx sheets. Since Na(+) intercalation/deintercalation during the electrochemical reaction is not accompanied by any substantial structural change, Ti3C2Tx shows good capacity retention over 100 cycles as well as excellent rate capability.

Electrochemical Mg2+ intercalation into a bimetallic CuFe Prussian blue analog in aqueous electrolytes

Mg2+ intercalation/deintercalation is achieved by using aqueous electrolytes and Prussian blue analog electrodes. Ex situ X-ray diffraction evidenced the solid solution process of Mg2+ intercalation/deintercalation, while the 57Fe Mossbauer spectroscopy and X-ray absorption near edge structure revealed redox of both Cu and Fe.

Control of charge transfer phase transition and ferromagnetism by photoisomerization of spiropyran for an organic-inorganic hybrid system, (SP)[Fe(II)Fe(III)(dto)3] (SP = spiropyran, dto = C2O2S2).

Iron mixed-valence complex, (n-C(3)H(7))(4)N[Fe(II)Fe(III)(dto)(3)](dto = C(2)O(2)S(2)), shows a spin entropy-driven phase transition called charge transfer phase transition in [Fe(II)Fe(III)(dto)(3)](-)(infinity) around 120 K and a ferromagnetic transition at 7 K. These phase transitions remarkably depend on the hexagonal ring size in the two-dimensional honeycomb network structure of [Fe(II)Fe(III)(dto)(3)](-)(infinity). In order to control the magnetic properties and the electronic state in the dto-bridged iron mixed-valence system by means of photoirradiation, we have synthesized a photosensitive organic-inorganic hybrid system, (SP)[Fe(II)Fe(III)(dto)(3)](SP = spiropyran), and investigated the photoinduced effect on the magnetic properties. Upon UV irradiation at 350 nm, a broad absorption band between 500 and 600 nm appears and continuously increases with the photoirradiation time, which implies that the UV irradiation changes the structure of spiropyran from the closed form to the open one in solid state. The photochromism in spiropyran changes the ferromagnetic transition temperature from 5 to 22 K and the coercive force from 1400 to 6000 Oe at 2 K. In this process, the concerted phenomenon coupled with the charge transfer phase transition in [Fe(II)Fe(III)(dto)(3)](-)(infinity) and the photoisomerization of spiropyran is realized.

Intermediate honeycomb ordering to trigger oxygen redox chemistry in layered battery electrode

Sodium-ion batteries are attractive energy storage media owing to the abundance of sodium, but the low capacities of available cathode materials make them impractical. Sodium-excess metal oxides Na2MO3 (M: transition metal) are appealing cathode materials that may realize large capacities through additional oxygen redox reaction. However, the general strategies for enhancing the capacity of Na2MO3 are poorly established. Here using two polymorphs of Na2RuO3, we demonstrate the critical role of honeycomb-type cation ordering in Na2MO3. Ordered Na2RuO3 with honeycomb-ordered [Na1/3Ru2/3]O2 slabs delivers a capacity of 180 mAh g−1 (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAh g−1 (1.0-electron reaction). We clarify that the large extra capacity of ordered Na2RuO3 is enabled by a spontaneously ordered intermediate Na1RuO3 phase with ilmenite O1 structure, which induces frontier orbital reorganization to trigger the oxygen redox reaction, unveiling a general requisite for the stable oxygen redox reaction in high-capacity Na2MO3 cathodes.

Determination of activation energy for Li ion diffusion in electrodes.

Higher power Li ion rechargeable batteries are important in many practical applications. Higher power output requires faster charge transfer reactions in the charge/discharge process. Because lower activation energy directly correlates to faster Li ion diffusion, the activation energy for ionic diffusion throughout the electrode materials is of primary importance. In this study, we demonstrate a simple, versatile electrochemical method to determine the activation energy for ionic diffusion in electrode materials via temperature dependent capacitometry. A generalized form of the temperature dependence of the discharge capacity was derived from the diffusion equation. This method yielded activation energy values for Li ion diffusion in LiCoO2 comparable to those obtained from ab initio calculations.

Reversible solid state redox of an octacyanometallate-bridged coordination polymer by electrochemical ion insertion/extraction.

Coordination polymers have significant potential for new functionality paradigms due to the intrinsic tunability of both their electronic and structural properties. In particular, octacyanometallate-bridged coordination polymers have the extended structural and magnetic diversity to achieve novel functionalities. We demonstrate that [Mn(H2O)][Mn(HCOO)(2/3)(H2O)(2/3)](3/4)[Mo(CN)8]·H2O can exhibit electrochemical alkali-ion insertion/extraction with high durability. The high durability is explained by the small lattice change of less than 1% during the reaction, as evidenced by ex situ X-ray diffraction analysis. The ex situ X-ray absorption spectroscopy revealed reversible redox of the octacyanometallate. Furthermore, the solid state redox of the paramagnetic [Mo(V)(CN)8](3-)/diamagnetic[Mo(IV)(CN)8](4-) couple realizes magnetic switching.