A heterogeneous phase/structure distribution in the bulk of spinel lithium nickel manganese oxides (LNMOs) is the key to maximizing the performance and stability of the cathode materials of lithium-ion batteries. Herein, we report the use of two-dimensional ptychographic X-ray absorption fine structure (XAFS) to visualize the density and valence maps of manganese and nickel in as-prepared LNMO particles and unsupervised learning to classify the three-phase group in terms of different elemental compositions and chemical states. The described approach may increase the supply of information for nanoscale characterization and promote the design of suitable structural domains to maximize the performance and stability of batteries.

The development of efficient electrochemical energy storage devices is key to foster the global market for sustainable technologies, such as electric vehicles and smart grids. However, the energy density of state-of-the-art lithium-ion batteries is not yet sufficient for their rapid deployment due to the performance limitations of positive-electrode materials. The development of large-capacity or high-voltage positive-electrode materials has attracted significant research attention; however, their use in commercial lithium-ion batteries remains a challenge from the viewpoint of cycle life, safety, and cost. In this review, after summarizing the limitation issues associated with large-capacity/high-voltage positive electrodes and already attempted technical solutions, a machine-learning technique is applied to analyze the reported dataset to hierarchize various technical solutions by their effectiveness in improving performance. The proposed study highlights the importance of integrating systematic experimental data collection with modern data analysis techniques for rational development of large-capacity/high-voltage positive electrodes. The scope is extended to important technical issues with other cell components, such as electrolytes and additives, binders, conductive carbon, current collectors, and impurity control for total optimization.

Development of high-performance aqueous batteries is an important goal for energy sustainability owing to their environmental benignity and low fabrication costs. Although a layered vanadyl phosphate is one of the most-studied host materials for intercalation electrodes with organic electrolytes, little attention has been paid to its use in aqueous Li+ systems because of its excessive dissolution in water. Herein, by controlling the water concentration, we demonstrate the stable operation of a layered vanadyl phosphate electrode in an aqueous Li+ electrolyte. The combination of experimental analyses and density functional theory calculations reveals that reversible (de)lithiation occurs between dehydrated phases, which can only exist in an optimal water concentration.

Reversibility of an electrode reaction is important for energy-efficient rechargeable batteries with a long battery life. Additional oxygen-redox reactions have become an intensive area of research to achieve a larger specific capacity of the positive electrode materials. However, most oxygen-redox electrodes exhibit a large voltage hysteresis >0.5V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O-2(2-) dimers during the oxygen-redox reaction. Therefore, avoiding O-O dimer formation is an essential challenge to overcome. Here, we focus on Na2-xMn3O7, which we recently identified to exhibit a large reversible oxygen-redox capacity with an extremely small polarization of 0.04V. Using spectroscopic and magnetic measurements, the existence of stable O-center dot was identified in Na2-xMn3O7. Computations reveal that O-center dot is thermodynamically favorable over the peroxide-like O-2(2-) dimer as a result of hole stabilization through a (sigma+) multiorbital Mn-O bond. Majority of oxygen-redox electrodes exhibit a large voltage hysteresis but its origin is not fully understood. Here, the authors use combined RIXS and magnetic measurements to provide insights into the origin of the typically large voltage hysteresis observed upon oxygen redox.

MXene electrodes in electrochemical capacitors have a distinctive behavior that is both capacitive and pseudocapacitive depending on the electrolyte. In this work, to better understand their electrochemical mechanism, first-principles calculations based on the density functional theory combined with the implicit solvation model are used (termed as 3D reference-interaction-site model). From the viewpoint of their electronic states, the hydration shell prevents orbital coupling between MXene and the intercalated ions, which leads to the formation of an electric-double layer and capacitive behavior. However, once the cations are partially dehydrated and adsorbed onto the MXene surface, because of orbital coupling of the cation states with the MXene states, particularly for surface-termination groups, charge transfer occurs and results in a pseudocapacitive behavior.

The oxygen-redox chemistry of lithium-rich layered transition-metal oxides has recently paved a path for the development of large-capacity positive electrode materials. Although oxygen-redox activity typically arises from the labile 2p states of undercoordinated oxide ions, the instability of layered structures after extracting excess Li ions often leads to transformation to a spinel structure, causing capacity fading and voltage decay after charge/discharge cycles. In this work, we perform density functional theory calculations to determine the oxygen-redox activity of spinel LiMg0.5Mn1.5O4 in an attempt to discover a robust three-dimensional framework for oxygen-redox reactions. The presence of ionic Mg2+ in the framework results in labile O 2p states for oxygen-redox activity. However, in contrast with the dominant Mn-O interactions that could stabilize oxidized oxide ions in a typical lithium-rich layered oxide Li2MnO3, spinel LiMg as Mn16O4 is dominated by O-O interactions near the Fermi level, which are less able to stabilize holes, leading to decomposition reactions that include release of oxygen gas.

High-energy-density batteries have been a long-standing target toward sustainability, but the energy density of state-of-the-art lithium-ion batteries is limited in part by the small capacity of the positive electrode materials. Although employing the additional oxygen-redox reaction of Li-excess transition-metal oxides is an attractive approach to increase the capacity, an atomic-level understanding of the reaction mechanism has not been established so far. Here, using bulk-sensitive resonant inelastic X-ray scattering spectroscopy combined with ab initio computations, we demonstrate the presence of a localized oxygen 2p orbital weakly hybridized with transition metal t(2g) orbitals that was theoretically predicted to play a key role in oxygen-redox reactions. After oxygen oxidation, the hole in the oxygen 2p orbital is stabilized by the generation of either a (sigma + pi) multiorbital bond through strong pi back-donation or peroxide O-2(2-) through oxygen dimerization. The multiorbital bond formation with sigma-accepting and pi-donating transition metals can thus lead to reversible oxygen-redox reaction.

Identifying high-voltage cathode materials is critically important for increasing the energy density of Na ion batteries. Through a comprehensive density-functional survey, we demonstrate that oxygen redox in R (3) over bar (ilmenite structure) Na1MO3 generates high operating voltage upon extraction and insertion of a Na ion. In the R (3) over bar structure, O ions are undercoordinated by two M and two Na ions and two vacant sites, creating unhybridized O 2p states with a nonbonding character that are lifted closer to the Fermi level. Since O 2p and M t(2g), states do not significantly overlap at the top of the valence band, the redox reaction is mainly borne on O ions. We also show that, in general, higher covalent bonding between the transition metal and oxygen results in higher voltage in this class of materials in which O redox is dominant. Furthermore, a thorough examination of the phase stability of R (3) over bar Na1MO3 compounds reveals that Na1VO3 is an economical high-voltage (5.907 V) cathode with robust cyclability for Na ion batteries. Finally, although the crystal overlap Hamilton population does not indicate any significant bonding between oxidized O ions upon desodiation in NaxMO3 compounds, we predict that gaseous O-2 may still develop through thermodynamic decomposition of Na1MO3 to Na1MO2 in some compounds.

Drastic electronic-structure changes in an Fe2O3 thin film anode for a Li-ion battery during discharge (lithiation) and charge (delithiation) processes were observed using operando Fe 2p soft X-ray emission spectroscopy (XES). The conversion reaction forming metallic iron due to the lithiation reaction was confirmed by operando XES in combination with the analysis using full-multiplet calculation. The valence of Fe at the open-circuit voltage (OCV) before the second cycle was not Fe3+, but Fe2+ with a weak p-d hybridization, suggesting a considerable irreversibility upon the first discharge-charge cycle and a weakened Fe-O bond after the first cycle. Moreover, we revealed that the Fe 3d electronic-structure change during the second cycle was to some extent reversible as Fe2+ (2.7 V vs. Li/Li+: open circuit voltage) -> Fe-0 (0.1 V vs. Li/Li+: discharged) -> Fe(2+delta)+ (3.0 V vs. Li/Li+: charged). This operando Fe 2p XES in combination with the full-multiplet calculation provides detailed information for redox chemistry during a discharge-charge operation that cannot be obtained by other methods such as crystal-structure and morphology analyses. XES is thus very powerful for investigating the origin and limitation of the lithiation function of anodes involving conversion reactions.

We report Na2Fe(HPO3)(2) as the first HPO32--based iron(ii) cathode material for sodium-ion batteries, which delivers a reversible capacity of approximately 100 mA h g(-1) at an average reaction voltage of 3.1 V. In situ X-ray diffraction and ex situ(57)Fe Mossbauer spectroscopy clarify reversible (de)sodiation associated with the Fe3+/Fe2+ redox reaction.

An aqueous supercapacitor is a prospective energy storage device that achieves affordable clean-energy supply. Although recent intensive research on concentrated electrolytes paved the way to improve its low energy density by expanding the narrow electrochemical potential window of water, it was compromised by slow rate capability with respect to the capacitor standard. In this work, to reveal the rate-determining ion-transport mechanism that limits the reaction kinetics of aqueous capacitor electrodes, electrochemical impedance spectroscopy was conducted on layered titanium carbide (MXene) electrodes with concentrated aqueous electrolytes (water-in-salt and hydrate melt). With increasing salt concentration, the dissociation of contact-ion-pair becomes dominant to ion transport both in the bulk electrolyte and at the electrode-electrolyte interface. (C) 2019 The Electrochemical Society.

A new iron fluorophosphate compound, Fe(H2PO4)(2)F, was synthesized by a solvent-less method at low temperatures. Its structure was determined from single crystal X-ray diffraction. Fe(H2PO4)(2)F was found to have a 3-dimensional structure built up by parallel chains of corner-sharing octahedra. This novel compound crystallizes in a tetragonal 14/mcm space group with the following lattice parameters: a = b = 9.01870(10) angstrom and c = 7.8058(2) angstrom. Further characterization of the material was conducted by TGA and Mossbauer spectroscopy. Proton conductivity measurements under dry condition revealed activation energies of E-a = 0.43 eV (along c-axis) and E-a = 0.41 eV (along a-axis) with corresponding room-temperature conductivities 2.6 x 10(-7) S cm(-1) and 5.8 x 10(-8) S cm(-1).

High-energy-resolution soft X-ray emission spectroscopy (XES) was applied to understand the changes in the electronic structure of LiMn2O4 upon Li-ion extraction/insertion. Mn 2p-3d-2p resonant XES spectra were analyzed by configuration-interaction full-multiplet (CIFM) calculations, which reproduced both dd and charge-transfer (CT) excitations. From the resonant XES spectra it is found that Mn3+ and Mn4+ coexist in the initial state, while this changes into Mn4+ in the charged-state. For the discharged-state, the Mn3+ component appears again although the dd excitations are slightly modified from those for the initial state. Furthermore, negative CT energy is expected for the Mn4+ configuration, which suggests very strong hybridization between the Mn 3d and O 2p orbitals. The large difference in the CT effect between the Mn4+ and Mn3+ states should give mechanical stress to the Mn-O bond during charge-discharge cycling, leading to capacity fading.

Phase-pure Mo2AlB2 with a single Al layer, a possible precursor for MBenes, was synthesized topochemically by the removal of an Al layer from MoAlB. Ab initio calculations predicted the sequential staging transformation from MoAlB through Mo4Al3B4 (stage II) to Mo2AlB2 (stage I).

Electrochemical double-layer (EDL) capacitors operating at high charge/discharge rates are an important class of electrochemical energy storage devices. Aqueous EDL capacitors show great potential for use as inexpensive devices with much higher power; however, their energy density is severely limited by the narrow electrochemical window of water (1.23 V) and the small specific capacity of the electrodes. Here, we develop a high voltage aqueous supercapacitor based on a highly concentrated Li+ aqueous electrolyte (hydrate melt) and a two-dimensional titanium carbide MXene electrode. Experimental and theoretical analyses reveal the existence of dense hydrated Li+ in the interlayer space of the deeply charged MXene, which is realized by the wide electrochemical window of a hydrate-melt electrolyte. The hydrate-melt electrolyte together with the large capacitance MXene Ti2CTx improves the performance of an aqueous lithium-ion supercapacitor, offering a promising strategy for advanced aqueous capacitors.

Efficient energy storage in the form of batteries contributes to building sustainable society. As advanced batteries need positive electrode materials capable of larger capacity, higher voltage, and lower cost, it is important to search for novel electrode materials. Among various inorganic/organic materials, cyanido-bridged coordination compounds are promising candidates for battery electrodes due to their ability to undergo solid-state redox reaction associated with ion (de)intercalation. In this review, recent results about the thermodynamic and kinetic aspects of the solid-state electrochemistry of cyanido-bridged coordination compounds are summarized, providing a fundamental basis toward developing cyanide electrodes for advanced batteries. (C) 2019 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as cation migration, layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Here we demonstrate a self-repairing phenomenon of stacking faults upon desodiation from an oxygen-redox layered oxide Na2RuO3, realizing much better reversibility of the electrode reaction. The phase transformations upon charging A(2)MO(3) (A: alkali metal) can be dominated by three-dimensional Coulombic attractive interactions driven by the existence of ordered alkali-metal vacancies, leading to counterintuitive self-repairing of stacking faults and progressive ordering upon charging. The cooperatively ordered vacancy in lithium-/sodium-rich layered transition-metal oxides is shown to play an essential role, not only in generating the electro-active nonbonding 2p orbital of neighbouring oxygen but also in stabilizing the phase transformation for highly reversible oxygen-redox reactions.

Owing to developments in theoretical chemistry and computer power, the combination of calculations and experiments is now standard practice in understanding and developing new materials for battery systems. Here, we briefly review our recent combined studies based on density functional theory and molecular dynamics calculations for electrode and electrolyte materials for sodium-ion batteries. These findings represent case studies of successful combinations of experimental and theoretical methods.

A spin transition between high-spin (HS) and low-spin (LS) states in a solid can occur when the energies of two spin configurations intersect, which is usually caused by external perturbations such as temperature, pressure, and magnetic fields, with substantial influence to its physical and chemical properties. Here, we discover the electrochemical "redox reaction" as a new driving force to induce reversible HS-LS spin transition. Although reversible solid-state redox reaction has been thoroughly investigated as the fundamental process in battery electrode materials, coupling between redox reactions and spin transitions has not been explored. Using density functional theory calculations, we predicted the existence of redox-driven spin transition occurring exclusively for the Co3+/Co2+ redox couple in layered transition-metal oxides, leading to a colossal potential hysteresis (>1 V) between the cathodic (LS Co3+ to LS Co2+) and anodic (HS Co2+ to HS Co3+) reactions. The predicted potential hysteresis associated with the spin transition of Co was experimentally verified for NxTi0.5Co0.5O2 by monitoring the electrochemical potential, local coordination structure, electronic structure, and magnetic moment.

Electric double-layer capacitors are efficient energy storage devices that have the potential to account for uneven power demand in sustainable energy systems. Earlier attempts to improve an unsatisfactory capacitance of electric double-layer capacitors have focused on meso- or nanostructuring to increase the accessible surface area and minimize the distance between the adsorbed ions and the electrode. However, the dielectric constant of the electrolyte solvent embedded between adsorbed ions and the electrode surface, which also governs the capacitance, has not been previously exploited to manipulate the capacitance. Here we show that the capacitance of electric double-layer capacitor electrodes can be enlarged when the water molecules are strongly confined into the two-dimensional slits of titanium carbide MXene nanosheets. Using electrochemical methods and theoretical modeling, we find that dipolar polarization of strongly confined water resonantly overscreens an external electric field and enhances capacitance with a characteristically negative dielectric constant of a water molecule.

Resorting to oxygen redox in addition to that of transition metal in oxide cathode materials can increase the capacity of rechargeable Na ions batteries. Through comprehensive density functional calculations, we demonstrate dominant oxygen participation in the redox reaction in cation disordered hexagonal and ordered monoclinic polymorphs of Na2-xRuO3 (0 <= x <= 0.75). In both polymorphs, when O ions are coordinated by more than three Na ions, unhybridized orphaned O 2p states are lifted closer to the Fermi level and therefore become accessible for the redox reaction. Moreover, high Ru-O covalency promotes greater 2p-4d hybridization at the top of the valence band further increasing the density of states of the electrochemically labile O 2p orbitals near the Fermi level. Consequently, throughout cycling, the contribution of the O 2p states to the charge compensation mechanism becomes nearly as twice as that of Ru 4d states in both polymorphs. Due to broader dispersion of the O 2p states near the Fermi level, the cation disordered polymorph, nonetheless, has a higher voltage of 2.458 V compared to the voltage of the cation ordered polymorph of 2.243 V. (C) The Author(s) 2019. Published by ECS.

O3-type lithium-rich transition-metal oxides have been attracting interest because of their large specific capacity achieved by additional oxygen-redox reactions. However, their practical application is precluded in part by continuous capacity and voltage fading upon cycling, which mainly originates from layer-to-spinel phase transformation in an O3-type oxide-ion array. In this work, as an attempt to suppress these degradations, we report O2-type cobalt-free lithium-rich layered oxides (Li[Li1/4Mn3/4]O-2-Li[Ni1/3Mn2/3]O-2 solid solution) which deliver large capacities of 220-250 mAh/g without substantial capacity or voltage fading upon cycling. Our study demonstrates that controlling the O2/O3 polymorphism is a promising approach toward designing large capacity positive electrode materials. (C) 2018 The Electrochemical Society.

Increasing the energy density of rechargeable batteries is of paramount importance toward achieving a sustainable society. The present limitation of the energy density is owing to the small capacity of cathode materials, in which the (de)intercalation of ions is charge-compensated by transition-metal redox reactions. Although additional oxygen-redox reactions of oxide cathodes have been recognized as an effective way to overcome this capacity limit, irreversible structural changes that occur during charge/discharge cause voltage drops and cycle degradation. Here, a highly reversible oxygen-redox capacity of Na2Mn3O7 that possesses inherent Mn vacancies in a layered structure is found. The cross validation of theoretical predictions and experimental observations demonstrates that the nonbonding 2p orbitals of oxygens neighboring the Mn vacancies contribute to the oxygen-redox capacity without making the Mn-O bond labile, highlighting the critical role of transition-metal vacancies for the design of reversible oxygen-redox cathodes.

We have investigated the nanostructuring effects on the local structure of V2O5 cathode material by means of temperature dependent V K-edge X-ray absorption fine structure measurements. We have found that the nanostructuring largely affects V-O and V-V bond characteristics with a general softening of the local V-O and V-V bonds. The obtained bond strengths correlate with the specific capacity shown by the different systems, with higher capacity corresponding to softer atomic pairs. The present study suggests the key role of local atomic displacements in the diffusion and storage of ions in cathodes for batteries, providing important information for designing new functional materials.

Using a scorpionate-based complex, [Fe-III(Tp)(CN)(3)](-), as a building block, a new cyanide-based molecular material [{Fe-III(Tp)(CN)(3)}(2)Ni-II(H2O)(2)]4H(2)O (1), which can be viewed as a lower dimensional model of Prussian blue analogues, was investigated as a lithium-ion storage host.

CONSPECTUS: The development of efficient electrochemical energy storage (EES) devices is an important sustainability issue to realize green electrical grids. Charge storage mechanisms in present EES devices, such as ion (de)intercalation in lithium-ion batteries and electric double layer formation in capacitors, provide insufficient efficiency and performance for grid use. Intercalation pseudocapacitance (or redox capacitance) has emerged as an alternative chemistry for advanced EES devices. Intercalation pseudocapacitance occurs through bulk redox reactions with ultrafast ion diffusion. In particular, the metal carbide/nitride nanosheets termed MXene discovered in 2011 are a promising class of intercalation pseudocapacitor electrode materials because of their compositional versatility for materials exploration (e.g., Ti2CTx, Ti3C2Tx, V2CTx, and Nb2CTx, where T is a surface termination group such as F, Cl, O, or OH), high electrical conductivity for high current charge, and a layered structure of stacked nanosheets for ultrafast ion intercalation. Various MXene electrodes have been reported to exhibit complementary battery performance, such as large specific capacity at high charge/discharge rates. However, general design strategies of MXenes for EES applications have not been established because of the limited understanding of the electrochemical mechanisms of MXenes. This Account describes current knowledge of the fundamental electrochemical properties of MXenes and attempts to clarify where intercalation capacitance ends and intercalation pseudocapacitance begins.MXene electrodes in aqueous electrolytes exhibit intercalation of hydrated cations. The hydrated cations form an electric double layer in the interlayer space to give a conventional capacitance within the narrow potential window of aqueous electrolytes. When nonaqueous electrolytes are used, although solvated cations are intercalated into the interlayer space during the initial stage of charging, the confined solvation shell should gradually collapse because of the large inner potential difference in the interlayer space. Upon further charging, desolvated ions solely intercalate, and the atomic orbitals of the desolvated cations overlap with the orbitals of MXene to form a donor band. The formation of the donor band induces the reduction of MXene, giving rise to an intercalation pseudocapacitance through charge transfer from the ions to MXene sheets. Differences in the electrochemical reaction mechanisms lead to variation of the electrochemical responses of MXenes (e.g., cyclic voltammetry curves, specific capacitance), highlighting the importance of establishing a comprehensive grasp of the electrochemical reactions of MXenes at an atomic level. Because of their better charge storage kinetics compared with those of typical materials used in present EES devices, aqueous/nonaqueous asymmetric capacitors using titanium carbide MXene electrodes are capable of efficient operation at high charge/discharge rates. Therefore, the further development of novel MXene electrodes for advanced EES applications is warranted.

Through comprehensive density functional calculations, we demonstrate oxygen's significant participation in the redox reaction in a Na excess NaxRuO3 cathode material. The availability of O electrons for the redox reaction originates from the local coordination environment. For high sodium content (x approximate to 2), O ions in the layered hexagonal Na2RuO3 compound are coordinated by four Na ions and consequently have their 2p electrons lifted closer to the Fermi level. For lower Na content (x approximate to 1), Na1RuO3 adopts an ilmenite type R-3 structure in which O ions are coordinated by two Ru and two Na ions. In this case, O undercoordination further elevates O 2p states closer to the Fermi level. In both cases, high O electronic population near the Fermi level facilitates continuous participation of O in the redox reaction over a wide range of Na concentrations. Based on this concept, we also predict that Na1NbO3 with an ilmenite framework is a suitable and economical candidate for high voltage and high capacity cathodes for Na ion batteries.

Lithium-ion batteries are key energy-storage devices for a sustainable society. The most widely used positive electrode materials are LiMO2 (M: transition metal), in which a redox reaction of M occurs in association with Li+ (de)intercalation. Recent developments of Li-excess transition-metal oxides, which deliver a large capacity of more than 200 mAh/g using an extra redox reaction of oxygen, introduce new possibilities for designing higher energy density lithium-ion batteries. For better engineering using this fascinating new chemistry, it is necessary to achieve a full understanding of the reaction mechanism by gaining knowledge on the chemical state of oxygen. In this review, a summary of the recent advances in oxygen-redox battery electrodes is provided, followed by a systematic demonstration of the overall electronic structures based on molecular orbitals with a focus on the local coordination environment around oxygen. We show that a 7r-type molecular orbital plays an important role in stabilizing the oxidized oxygen that emerges upon the charging process. Molecular orbital principles are convenient for an atomic-level understanding of how reversible oxygen-redox reactions occur in bulk, providing a solid foundation toward improved oxygen-redox positive electrode materials for high energy-density batteries.

Capacitive energy storage at the electrochemical double layer formed on a particle surface can enable efficient devices that deliver high power and exhibit excellent reversibility. However, even with state of the art nanocarbons with highly controlled morphology to maximize the specific surface area, the available energy density remains far below that of existing rechargeable batteries. Utilizing nanoparticles of transition metal oxides is a viable option to alleviate the conflict between energy and power densities by accommodating additional electrons around the surface transition metal sites, called "pseudocapacitance". However, an understanding of pseudocapacitive surfaces has been limited due to a lack of suitable analysis methodology. Here, we focus on the RuO2/water interface and elaborate on a reaction scheme including charge transfer into related surface orbitals using density functional theory calculations based on interfacial structures determined under a given electrode potential at a fixed pH of 0. The extensive contributions of the surface oxygen atoms and their surface-site dependence are revealed through the Ru-O orbital hybridization and O-H bond breaking/formation, largely deviating from the general explanation based only on the nominal valence states (penta-, tetra-, or trivalent) of Ru atoms.

Redox-responsive hydrogels have the potential for application in various fields, including biomedical science. We have developed a redox-responsive star block copolymer hydrogel based on iron ion cross-linking. The addition of the ferric ion (Fe3+) induced gelation of the star block copolymer solution within a few seconds, whereas the addition of the ferrous ion (Fe2+) did not. The resulting hydrogels cross-linked using Fe3+ showed storage moduli (G') of 26-1400 Pa and were stable under physiological conditions for as long as 1 month. The cross-linking between the star arms produced by the addition of Fe3+ enabled a fast, redox-responsive sol-gel/gel-sol transition. Furthermore, the hydrogel showed excellent injectability and biocompatibility in vivo, resulting in a rapid sol-gel/gel-sol transition in subcutaneous tissues in response to redox stimuli, such as the administration of ascorbic acid or hydrogen peroxide.

We demonstrate that Cd-113 NMR is a potent technique to monitor the local electronic and structural states of the Prussian blue electrode during Li+ intercalation, providing an atomic-scale insight into the reaction mechanism.

Pseudocapacitance is a key charge storage mechanism to advanced electrochemical energy storage devices distinguished by the simultaneous achievement of high capacitance and a high charge/discharge rate by using surface redox chemistries. MXene, a family of layered compounds, is a pseudocapacitor-like electrode material which exhibits charge storage through exceptionally fast ion accessibility to redox sites. Here, the authors demonstrate steric chloride termination in MXene Ti2CTx (T-x: surface termination groups) to open the interlayer space between the individual 2D Ti2CTx units. The open interlayer space significantly enhances Li-ion accessibility, leading to high gravimetric and volumetric capacitances (300 F g(-1) and 130 F cm(-3)) with less diffusion limitation. A Li-ion hybrid capacitor consisting of the Ti2CTx negative electrode and the LiNi1/3Co1/3Mn1/3O2 positive electrode displays an unprecedented specific energy density of 160 W h kg(-1) at 220 W kg(-1) based on the total weight of positive and negative active materials.

The changes in the electronic structure of LiMn0.6Fe0.4PO4 nanowires during discharge processes were investigated by using ex situ soft X-ray absorption spectroscopy. The Fe L-edge X-ray absorption spectrum attributes the potential plateau at 3.45 V versus Li/Li+ of the discharge curve to a reduction of Fe3+ to Fe2+. The Mn L-edge X-ray absorption spectra exhibit the Mn2+ multiplet structure throughout the discharge process, and the crystal-field splitting was slightly enhanced upon full discharge. The configuration-interaction full-multiplet calculation for the X-ray absorption spectra reveals that the chargetransfer effect from O2p to Mn3d orbitals should be considerably small, unlike that from the O2p to Fe3d orbitals. Instead, the O K-edge X-ray absorption spectrum shows a clear spectral change during the discharge process, suggesting that the hybridization of O2p orbitals with Fe3d orbitals contributes essentially to the reduction.

Discovery of novel compounds capable of electrochemical ion intercalation is a primary step toward development of advanced electrochemical devices such as batteries. Although cyano-bridged coordination polymers including Prussian blue analogues have been intensively investigated as ion intercalation materials, the solid-state electrochemistry of the octacyanotungstate-bridged coordination polymer has not been investigated. Here, we demonstrate that an octacyanotungstate-bridged coordination polymer Tb(H2O)(5)[W(CN)(8)] operates as a Li+-ion intercalation electrode material. The detailed magnetic measurements reveal that the tunable amount of intercalated Li+ ion in the solid-state redox reaction between paramagnetic [W-V(CN)(8)](3-) and diamagnetic [W-IV(CN)(8)](4-) in the framework enables the electrochemical control of different magnetic regimes. While the initial ferromagnetic long-range ordering is irreversibly lost upon lithium insertion, electrochemical switching between paramagnetic and short-range ordering regimes can be achieved.

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]O-2 slabs delivers a capacity of 180 mAhg(-1) (1.3-electron reaction), whereas disordered Na2RuO3 only delivers 135 mAhg(-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.

Sodium-ion batteries potentially provide the opportunities to realize the energy storage system beyond the state-of-the-art lithium-ion batteries, though at present their performance is limited partly due to lack of suitable positive electrode materials. NASICON-type Na3V2(PO4)(3) is a promising candidate for the positive electrode materials because of its high capacity and high operating potential, however, the electrode reaction of Na1+2xV2(PO4)(3) (0 <= x <= 1) including a biphasic region is not yet fully understood. Here, in order to clarify the microscopic mechanism of the biphasic reaction, the reaction entropy of the electrochemical cell including the Na1+2xV2(PO4)(3) positive electrode was measured using the potentiometric method. The temperature-dependent open-circuit voltage reveals that the reaction entropy is almost constant for 0.1 <= x <= 0.9. The constant reaction entropy of the electrochemical cell suggests that the electrode reaction proceeds through the boundary migration between the Na-rich and-poor phases without substantial change in the configurational entropy. (C) The Electrochemical Society of Japan, All rights reserved.

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 Na-23 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.

We have investigated the local structure of differently charged NaxCoO2, cathode material as a function of temperature by Co K-edge X-ray absorption fine structure (EXAFS) measurements. We have found that the Charge/discharge process has a direct effect on the bond characteristics of the cathode in the Na-ion batteries. The results reveal that the local Co-O bonds get softer, while the Co Co bonds hardly show any change during discharge (sodiation). The present study underlines the key role of local atomic displacements in diffusion and the reversibility of ions in cathodes for batteries and points toward the feasibility of NaxCoO2 to be used as a cathode material. The results are discussed in comparison with the lithiation/delithiation of LixCoO2 battery materials.

Raising the operating potential of the cathode materials in sodium-ion batteries is a crucial challenge if they are to outperform state-of-the-art lithium-ion batteries. Although the layered transition metal oxides, NaMO2 (M: transition metal), are the most promising cathode materials owing to their high theoretical capacity with much more stable nature than Li1-xMO2 system, factors influencing the redox potential have not yet been fully understood. Here, we identify redox potential paradox, E(Ni3+/Ni2+) > E(Ni4+/Ni3+), in an identical structural framework, namely, NaTi0.54+Ni0.52+O2 and NaFe0.53+Ni0.53+O2, which is induced by transition of the oxides from Mott-Hubbard to negative charge-transfer regimes. The origin of the unusually low E(Ni4+/Ni3+) is the surprisingly large contribution (over 80%) of oxygen orbital to the redox reaction, of which the primary effect on the electrochemical property is demonstrated for the first time, providing a firm platform to design better cathodes for advanced sodium-ion batteries.

An alluaudite-type sodium iron sulfate has recently been discovered as a 3.8 V cathode material for low-cost, high-power, and efficient sodium-ion batteries. To optimize the composition of the alluaudite phase and to explore further compounds, we have carefully surveyed the Na2SO4-FeSO4 binary system. Solid-state reactions at a moderate temperature of 623 K produce two stable phases: 1)vanthoffite-structured Na6Fe(SO4)(4) and 2)alluaudite-type Na2+2xFe2-x(SO4)(3) with a certain non-stoichiometry. The possible compositional and structural flexibilities demonstrated in this work inspire further improvement of the alluaudite-type sodium metal sulfates for advanced sodium-ion batteries.

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 2.4V and delivers 90 and 40 mAh g(-1) at 1.0 and 5.0 Ag-1 (based on the weight of the negative electrode), respectively, which are not attainable by conventional electrochemical energy storage systems.

The magnetic properties of a series of core-shell particles based on the Prussian blue analogues K0.1Cu[Fe(CN)(6)](0.7).3.5H(2)O and K0.1Ni[Fe(CN)(6)](0.7).4.4H(2)O (CuFe-PBA@NiFe-PBA) are investigated as a function of electrochemical titration and lithium ion insertion. The particles, with average size 305 +/- 50 nm, are reduced using the galvanostatic intermittent titration technique to prepare 10 samples of Lix(CuFe-PBA@NiFe-PBA) with 0 <= x <= 1.0. Magnetization as a function of temperature for each member of the series reveals the ferromagnetic ordering of the individual components, NiFe-PBA (T-c similar to 24 K) and CuFe-PBA (T-c similar to 18 K). The magnetic ordering of each component is suppressed upon reduction and Li+ incorporation, but in a stepwise fashion with the CuFe-PBA core reduced before the NiFe-PBA shell. The separate reductions of the core and shell are also seen in magnetization vs field measurements at low temperature. By introducing a lattice-gas model, the enthalpy changes (Delta H-i) associated with each redox couple after Li-ion insertion were calculated and applied to the mean field approximation to reproduce the magnetic transition temperatures. The results are significant as the CuFe-PBA@NiFe-PBA particles had previously been shown to exhibit superior performance over the individual components as cathode materials for lithium ion batteries, although the stepwise reduction had not previously been discerned. Furthermore, the report is the first showing the systematic control of magnetism in core-shell coordination polymer heterostructures by electrochemical guest ion insertion.

A new desodiated derivative compound, Na0.89Fe1.8(SO4)(3), was prepared by the chemical oxidation of alluaudite Na2.4Fe1.8(SO4)(3) Phase using NOBF4 as oxidant. The structure and valency of Fe were characterized by X-ray diffraction (XRD) and Fe-57 Mossbauer spectroscopy. Intercalation behavior of lithium ions in the structure of Na0.89Fe1.8(SO4)(3) was gauged by electrochemical analyses and ex-situ X-ray diffraction. A high capacity of 110 mAh g(-1) at 0.1 C was obtained with a good rate kinetics within a range of 0.1-10 C(1 C = 118 mAh g-1) involving a high Fe3+/Fe2+ redox potential of 3.75 V (vs. Li/Li+). These results confirmed that the Na2.4-delta Fe1.8(SO4)(3) framework was stable even after oxidation and forms a new competitive cathode for the reversible intercalation of lithium ions. (C) 2014 Elsevier B.V. All rights reserved.

We developed an electrochemical in situ cell for soft x-ray emission spectroscopy (XES) to accurately investigate the redox reaction and electronic structure of transition metals in the cathode materials for Li-ion battery. The in situ cell consists of a Li-metal counter electrode, an organic electrolyte solution, and a cathode on a membrane window which separates the liquid electrolyte from high vacuum and can pass the incoming and emitted photons. In this study, the Mn 3d electronic structure of LiMn204 thin-film electrode was clarified by the operand XES. At the charged state, the XES spectrum changed significantly from the open-circuit-voltage (OCV) state, suggesting oxidation of the Mn3+ component through Li-ion extraction. Upon discharge up to 3.0 V vs. Li/Li+, the XES spectrum almost returned to its profile at the OCV state with small difference, indicating the valence change of Mn: Mn3.6+ -> Mn4+ -> Mn33+ corresponding to the OCV, charged, and discharged states. (C) 2014 Elsevier B.V. All rights reserved.

Discovery of a new class of ion intercalation compounds is highly desirable due to its relevance to various electrochemical devices, such as batteries. Herein, we present a new iron-oxalato open framework, which showed reversible Na+ intercalation/extraction. The hydrothermally synthesized K4Na2[Fe(C2O4)(2)](3)2H(2)O possesses one-dimensional open channels in the oxalato-bridged network, providing ion accessibility up to two Na+ per the formula unit. The detailed studies on the structural and electronic states revealed that the framework exhibited a solid solution state almost entirely during Na+ intercalation/extraction associated with the reversible redox of Fe. The present work demonstrates possibilities of the oxalato frameworks as tunable and robust ion intercalation electrode materials for various device applications.

We combine Mn L-2,L-3-edge X-ray absorption, high resolution Mn 2p-3d-2p resonant X-ray emission, and configuration interaction full-multiplet (CIFM) calculation to analyze the electronic structure of Mn-based Prussian blue analogue. We clarified the Mn 3d energy diagram for the Mn2+ low-spin state separately from that of the Mn2+ high-spin state by tuning the excitation energy for the X-ray emission measurement. The obtained X-ray emission spectra are generally reproduced by the CIFM calculation for the Mn2+ low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t(2g) and CN pi orbitals than the Mn2+ high spin state. The d- d-excitation peak nearest to the elastic scattering was ascribed to the Mn2+ IS state by the CIFM calculation, indicating that the Mn2+ LS state with a hole on the t(2g) orbital locates near the Fermi level.

There is currently much interest in advanced energy storage systems because of their potential relevance to power grid storage media. Both earth abundance and high energy density of divalent Mg2+ ion make Mg batteries attractive as alternatives to Li-ion batteries, but the number of host materials for reversible intercalation is very limited due to the strong coulombic interaction induced in solid matrix. In this work, we report the electrochemical properties of FePO4 in aqueous Mg2+ electrolyte. The charge/discharge experiments showed that FePO4 delivers the first discharge capacity of 90 mAh g(-1) with subsequent partial reversibility. The ex-situ Mossbauer spectroscopy confirmed reversible redox of Fe3+/Fe2+ during the discharge and charge processes, while little change was observed in the ex-situ X-ray diffraction patterns. Activation energy for Mg2+ diffusion in FePO4 lattice was calculated to over three times larger than that of Lit. It is postulated that the electrochemical reaction of FePO4 in aqueous Mg2+ electrolyte proceeds in part by non-topochemical Mg2+ (de)intercalation accompanied by the irreversible transformation from the crystalline state to the amorphous state in the vicinity of particle surface. (C) The Electrochemical Society of Japan, All rights reserved.

Structural and morphological control is an effective approach for improvement of electrochemical properties in rechargeable batteries. One-dimensionally assembled ,structure composed of NASICON-type Na3V2(PO4)(3) nanoparticles were fabricated through an electrospinning method to meet the requirements for the development of efficient electrode materials in Na-ion batteries. High-temperature treatment of electrospun precursor fibers under an argon flow provides a nonwoven fabric of nanowires comprising crystallographically oriented nanoparticles of NASICON-type Na3V2(PO4)(3) within a carbon sheath. The mesostructure comprising NASICON-type Na3V2(PO4)(3) and carbon give a short sodium-ion transport pass and an efficient electron conduction pass. Electrochemical properties of NASICON-type Na3V2(PO4)(3) are improved on the basis of one-dimensional nanostructures designed in the present study.

The particle-size effects on the thermodynamic properties and kinetic behavior of a LixFePO4 electrode have a direct influence on the electrode properties. Thus, the development of high-performance Li-ion batteries containing a LixFePO4 cathode requires a complete understanding of the reaction mechanism at the atomic/nano/meso scale. In this work, we report electrochemical calorimetric and potentiometric studies on LixFePO4 electrodes with different particle sizes and clarify the particle-size effect on the reaction mechanism based on the entropy change of (de)lithiation. Electrochemical calorimetry results show that a reduction in particle size shrinks the miscibility gap of LixFePO4 while potentiometric measurements demonstrate that the LixFePO4 particles equilibrate into either a kinetically metastable state or a thermodynamically stable state depending on the particle size.

Electrospinning enables fabrication of nanowires (NWs) of various materials from a polymer solution. Nevertheless, few reports have described single crystallization of oxide and polyanion NWs. Its mechanism remains unknown. This report presents transmission electron microscopy observations of conversion from electrospun amorphous NWs to single-crystalline olivine lithium phosphate NWs. After nucleation and grain growth, single crystallization is achieved by the attachment of adjacent crystal grains with common crystallographic orientations in an amorphous phase confined to self-forming carbon shells. The present NW axes have no specific orientation. These results imply that self-forming shells play a key role in achieving single-crystalline NWs in electrospinning.

Phase separation and transformation induced by electrochemical ion insertion are key processes in achieving efficient energy storage. Exploration of novel insertion electrode materials/reactions is particularly important to unravel the atomic/molecular-level mechanism and improve the electrochemical properties. Here, we report the unconventional phase separation of a cyanide-bridged coordination polymer, Eu[Fe(CN)(6)]center dot 4H(2)O, under electrochemical Na-ion insertion. Detailed structural analyses performed during the electrochemical reaction revealed that, in contrast to conventional electrochemical phase separation induced by the elastic interaction between nearest neighbors, the phase separation of NaxEu[Fe(CN)(6)]center dot 4H(2)O is due to a long-range interaction, namely, cooperative rotation ordering of hexacyanoferrates. Kolmogorov-Johnson-Mehl-Avrami analysis showed that the activation energy for the phase boundary migration in NaxEu[Fe(CN)(6)]center dot 4H(2)O is lower than that in other conventional electrode materials such as Li1-xFePO4.

Core-sheath composite nanowires of LiMnxFe1-xPO4 (x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0) with vapor grown carbon fiber (VGCF) were synthesized by an electrospinning method. X-ray diffraction, SEM and TEM revealed that the electrospinning method could successfully fabricate a VGCF core at the nanowire-structured LiMnxFe1-xPO4 sheath covered with an amorphous carbon layer. Furthermore, the spot patterns by selected area electron diffraction confirmed that LiMnxFe1-xPO4 sheath consists of bundles of single-crystalline nanowire. The nanowire-structured electrode materials with uniformly dispersed carbon and highly crystalline active materials provided excellent rate capability. (C) 2013 Elsevier B.V. All rights reserved.

Na-ion batteries have been the subjects of intensive studies for grid-scale energy storage recently. O3-type NaFeO2 is a promising candidate for the Na-ion cathode materials, though the irreversibility during Na-ion extraction/insertion seriously hinders its practical application. The present work demonstrates that partial replacement of Fe in O-3-NaFeO2 with Ni leads to the significant improvement of the electrochemical properties. The Fe-57 Mossbauer and X-ray absorption spectra show that O3-type NaFeO2 NaNiO2 solid solution forms hybridized frontier orbital of a Fe-O-Ni bond via ligand-to-metal charge transfer, which plays a dominant role in the charge discharge process. The resulting O3-NaFe0.3Ni0.7O2 delivers an initial discharge capacity of 135 mA.h.g(-1), most of which is in a high-voltage region of 2.5-3.8 V, with a high initial Coulombic efficiency of 93%, and shows enhanced cycle stability.

The first iron complexes of high-spin iron(II) species directly coordinated to verdazyl radicals, [Fe-11(vdCOO)(2)(H2O)(2)].2H(2)O (1; vdCOO(-) = 1,5-dimethyl-6-oxo-verdazyl-3-carboxylate) and [Fe-11(vdCOO)(2)(D2O)(2)].2D(2)O (2), were synthesized. The crystal structure of 1 was investigated by single-crystal X-ray diffraction at room temperature and at 90 K. The compound crystallizes in the P1 space group with no phase transition between 300 and 90 K. The crystals are composed of discrete [Fe-11(vdCOO)(2)(H2O)(2)] complexes and crystallization water molecules. In the complex, two vdCOO(-) ligands coordinate to the iron(II) ion in a head-to-tail arrangement and two water molecules complete the coordination sphere. The Fe-X (X = O, N) distances vary in the 2.069-2.213 A range at 300 K and in the 2.0679-2.2111 A range at 90 K, indicating that the iron(II) ion is in its high-spin (HS) state at both temperatures. At 300 K, one of the coordinated water molecules is H-bonded to two crystallization water molecules whereas the second one appears as loosely H-bonded to the two oxygen atoms of the carboxylate group of two neighboring complexes. At 90 K, the former H-bonds remain essentially the same whereas the second coordinated water molecule reveals a complicated behavior appearing simultaneously as tightly H- bonded to two oxygen atoms and non-H-bonded. The Fe-57 Mossbauer spectra, recorded between 300 K and 10 K, give a clue to this situation. They show two sets of doublets typical of HS iron(II) species whose intensity ratio varies smoothly with temperature. It demonstrates the existence of an equilibrium between the high temperature and low temperature forms of the compounds. The solid-state magic angle spinning H-2 NMR spectra of 2 were recorded between 310 K and 193 K. The spectra suggest the existence of a strongly temperature- dependent motion of one of the coordinated water molecules in the whole temperature range. Variable- temperature magnetic susceptibility measurements indicate an antiferromagnetic interaction (J(Fe-vd) = -27.1 cm(-1); H = -J(ij)S(i)S(j)) of the HS iron(II) ion and the radical spins with high g(Fe) and D-Fe values (g(Fe) = 2.25, D-Fe = +3.37 cm(-1)) for the HS iron(II) ion. Moreover, the radicals are strongly antiferromagnetically coupled through the iron(II) center (J(vd-vd) = -42.8 cm(-1)). These last results are analysed based on the framework of the magnetic orbitals formalism.

The influence of particle size on the electrochemical properties of guest-ion storage materials has attracted much attention because of the extensive need for long cycle-life, high energy density, and high power batteries. The present work describes a systematic study of the effect of particle size on the guest-ion storage capabilities of a cyanide-bridged coordination polymer. A series of nickel hexacyanoferrate particles ranging from approximately 40 to 400 nm were synthesized by a co-precipitation method and were used as the cathode material for both Li-ion and Na-ion insertion/extraction experiments using organic electrolyte. A large polarization was observed for the largest particles during Li-ion cycling, indicating a heterogeneous ion concentration within the lattice. As a consequence, the available capacity of Li-ion intercalation at high rates is significantly improved by reducing the particle size. On the other hand, Na-ion intercalation shows excellent rate capability regardless of the particle size.

The electronic structure of Na-2[ Fe(CN)(5)NO]center dot 2H(2)O (sodium nitroprusside: SNP) was investigated by using soft X-ray absorption (XA) spectroscopy. The Fe L-2,L-3-edge XA spectrum of SNP exhibited distinct and very large satellite peaks for L-3 and L-2 regions, which is different from the spectra of hexacyanoferrates and the other iron compounds. A configuration-interaction full-multiplet calculation, in which the ligand molecular orbitals for the C-4v symmetry were taken into account, revealed the Fe2+ low-spin state with very strong effects of metal-to-ligand charge-transfer from the Fe 3d to NO 2p orbitals.

We demonstrate that core- shell nanoparticles consisting of two different Prussian blue analogues, one high capacity and the other robust, can provide enhanced rate capability as cathode materials in sodium-ion batteries.

A combination of high resolution x-ray absorption and x-ray emission spectroscopy is used to investigate electronic properties of LiCoO2 nanoparticles with various sizes down to 8 nm. We find that the nanostructuring has direct influence on the electronic structure of the title system, similar to the effect of delithiation. In particular, an abrupt reduction (increase) of the intersite (intrasite) 4p-3d hybridization is observed for nanoparticles with size smaller than 15 nm. Since the electrochemical properties are known to degrade in nanoparticles below the critical size of 15 nm, the results indicate a direct relationship between the intersite-intrasite correlations and the cathode efficiency, limiting the use of LiCoO2 nanoparticles. © 2013 AIP Publishing LLC.

Na-ion batteries have attracted much interest recently due to strong industrial demands for inexpensive and efficient energy storage. Although layered NaxMO2 (0 < x < 1, M: transition metal) is the subject of intense studies as cathode materials for the Na-ion batteries, the electrochemical properties of layered Na2MO3 have not been clarified to date. The present work demonstrates that Na2RuO3 can show reversible Na-ion insertion/extraction leading to the specific capacity of 140 mA h g(-1) at the average potential of 2.8 V vs. Na/Na+. Both the capacity retention and Coulombic efficiency are kept at almost 100% during 20 cycles, which suggests high reversibility of the electrochemical reaction. Due to the fast Na-ion diffusion and metallic conduction, Na2RuO3 can show the excellent rate capability, where 53% of the maximum discharge capacity is retained even at a high discharge rate of 5C. (c) 2013 Elsevier B.V. All rights reserved.

The development of high-performance Na-ion intercalation electrodes has been required recently because Na-ion batteries hold much promise for inexpensive and efficient energy storage, which can be deployed in a power grid. For both optimization and better understanding of the electrode materials, it is indispensable to clarify the relationship between the electronic state and electrochemical properties systematically. In this work, we studied the electrochemical properties of P2-Na(2/3)MnyCo(1-y)O(2) in detail. A series of the P2 phases was successfully synthesized by the conventional solid-state reaction. The solid solution P2 compounds showed that the redox potential of Co4+/Co3+ and Mn4+/Mn3+ shifts systematically by the transition-metal substitution. The charge discharge cycle tests revealed that with increasing y the initial specific capacity increases while the cycle stability degrades. The origin for the cycle degradation was analyzed by the electrochemical impedance spectroscopy, which evidenced that the substitution of Co for Mn accelerates the formation of the passivating layer at the electrode surface.

Interfacial charge transfer is one of the most important fundamental steps in the charge and discharge processes of intercalation compounds for rechargeable batteries. In this study, temperature-dependent electrochemical impedance spectroscopy was carried out to clarify the origin of the high power output of aqueous batteries with Prussian blue analog electrodes. The activation energy for the interfacial charge transfer, E-a, was estimated from the temperature dependence of the interfacial charge transfer resistance. The E-a values with Li+ and Na+ aqueous electrolytes were considerably smaller than those with organic electrolytes. The small E-a values with aqueous electrolytes could result from the fact that the Coulombic repulsion at the interface is largely suppressed by the screening effect of hydration.

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)]center dot 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.

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 K0.1Cu[Fe(CN)(6)](0.7)center dot 3.8H(2)O (CuFe-PBA) core and lower capacity but highly stable shell of K0.1Ni[Fe(CN)(6)](0.7)center dot 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.

Temperature dependent Co K-edge extended X-ray absorption fine structure is used to investigate local disorder in LiCoO2 nanoparticles. We find that the nanostructuring has direct influence on the bondlength characteristics. The results reveal a substantial decrease in the force constant of Co-O bonds (the Co-O bonds becoming more flexible), while that for the Co-Co bonds showing hardly any change (or increases slightly) in LiCoO2 nanoparticles with respect to the bulk. Therefore, both random disorder and Co-O bondlength flexibility should be the factors to limit the battery characteristics of the LiCoO2 nanoparticles.

Hollow wires with thin nanowalls constructed from two-dimensionally connected, highly crystalline nanoparticles were fabricated by electrospinning to create electrode active materials of LiNi0.5Mn1.5O4 (5 V spinel) and 0.5Li(2)MnO(3)-0.5LiNi(1/3)Co(1/3)Mn(1/3) O-2 (solid-solution type). The resultant materials show large-energy-density properties suitable for use as cathode materials in Li-ion batteries.

Vapor Grown Carbon Fiber (VGCF)-core@LiMn0.4Fe0.6PO4-sheath heterostructure nanowire was successfully fabricated by an electrospinning method for high rate Li-ion batteries. The fabricated heterostructure has both the electronic conduction path (VGCF-core) and large reaction surface area with high crystallinity (LiMn0.4Fe0.6PO4-sheath), which enables the high charge-discharge rate capability.

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 Fe-57 Mossbauer spectroscopy and X-ray absorption near edge structure revealed redox of both Cu and Fe.

Ternary metal Prussian blue analogue nanoparticles were applied as cathode materials for Li-ion batteries. Heterometal substitution suppressed phase separation induced by over-lithiation, leading to both a long cycle life and a high rate capability.

High power Na-ion rechargeable batteries have attracted much interest recently. In particular, the development of nanostructured electrode materials is essentially required, because the large surface area and short Na-ion diffusion length could provide the high power density. In this paper, we report on the fabrication of single-crystalline Na0.44MnO2 nanowire, and the application to Na-ion rechargeable batteries. The single phase Na0.44MnO2 was successfully synthesized by the hydrothermal method. The SEM and TEM experiments proved that hydrothermally synthesized Na0.44MnO2 has single-crystalline nanowire morphology. The single-crystalline Na0.44MnO2 nanowire electrode in Na-ion batteries showed both the excellent cycle stability and high-charge/discharge-rate capability. (C) 2012 Elsevier B.V. All rights reserved.

To understand the electronic-structure changes of electrode materials during the charge/discharge processes is one of the most important fundamental aspects to improve the battery performance. Soft X-ray, absorption spectroscopy (XAS) was used to study a bimetallic NiFe Prussian blue analogue electrode. XA spectra were obtained during the charge/discharge and were analyzed by the configuration interaction full-multiplet (CIEM) calculation, in which the strong charge transfer due to the sigma/pi-donation and back donation of cyanide was taken into account. The CIFM calculation revealed that the metal-to-ligand charge transfer (MLCT) played an important role in the electronic state of Ni-N bond: The Fe3+-FC bond in the charged state is dominated by both the MLCT and ligand-to-metal charge transfer (LMCT), whereas only the MLCT strongly affects the Fe2+-C bond in the discharged state.

Extending the pyrophosphate chemistry for rechargeable Na-ion batteries, here we report the synthesis and electrochemical characterization of Na2FeP2O7, a novel Fe-based cathode material for sodium batteries. Prepared by conventional solid-state as well as solution-combustion synthesis (at 600 degrees C), the Na2FeP2O7 adopts a triclinic structure (space group: P-1) with three-dimensional channels running along [100], [- 110] and [01-1] directions. With no further optimization, the as-synthesized Na2FeP2O7 cathode was found to be electrochemically active, delivering a reversible capacity of 82 mAh.g(-1) with the Fe3+/Fe2+ redox potential centered around 3 V (vs. Na/Na+). With its theoretical capacity of similar to 100 mAh.g(-1) and potential high ratecapability, Na2FeP2O7 forms a promising novel cathode material, with the distinction of being the first ever pyrophosphate-class of cathode for sodium-ion batteries. (C) 2012 Elsevier B.V. All rights reserved.

Magnetic coordination polymers can exhibit controllable magnetism by introducing responsiveness to external stimuli. This report describes the precise control of magnetism of a cyanide-bridged bimetallic coordination polymer (Prussian blue analogue: PBA) through use of an electrochemical quantitative Li ion titration technique, i.e., the galvanostatic intermittent titration technique (GITT). K0.2Ni[Fe(CN)(6)](0.7)center dot 4.7H(2)O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe3+/Fe2+ reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature T-C gradually decreases due to reduction of paramagnetic Fe3+ to diamagnetic Fe2+, and the ferromagnetic transition is completely suppressed for Li-0.6(NiFe-PBA). On the other hand, T-C increases continuously as Li ion is extracted due to oxidation of diamagnetic Fe2+ to paramagnetic Fe3+, and the ferromagnetic transition is nearly recovered for Li-0(NiFe-PBA). Furthermore, the plots of T-C as a function of the amount of inserted/extracted Li ion x are well consistent with the theoretical values calculated by the molecular-field approximation.

We have studied the local structure of LiCoO 2 nanoparticles by Co K-edge x-ray absorption spectroscopy as a function of particle size. Extended x-ray absorption fine structure data reveal substantial changes in the near neighbor distances and the associated mean square relative displacements with decreasing particle size. X-ray absorption near edge structure spectra show clear local geometrical changes with decreasing particle size, similar to those that appear in the charging (delithiation) process. The results suggest that the LiCoO 2 nanoparticles are characterized by a large atomic disorder confined to the Co-O octahedra, similar to the distortions generated during the delithiation, and this disorder should be the primary limiting factor for a reversible diffusion of Li ions when nanoparticles of LiCoO 2 are used as cathode material in rechargeable Li ion batteries. © 2012 IOP Publishing Ltd.

An application of the Li-ion batteries to advanced transportation systems essentially requires the enhancement of the rate capability; thus, the fabrication of nanostructured cathode materials with the large surface area and short Li-ion diffusion length is particularly important. In this study, an electrospinning method was adopted for the synthesis of wire-structured LiCoO2. The diameter of the as-spun fiber obtained from the precursor solution with multiwalled carbon nanotubes (vapor-grown carbon fiber, VGCF) was thinner than that of as-spun fiber obtained from the solution without VGCF. After the heat treatment, wire-structured LiCoO2 was successfully obtained regardless of the existence of dispersed VGCF in the precursor solution, although the particle size LiCoO2 fabricated with VGCF was smaller than that of LiCoO2 fabricated without VGCF. The charge/discharge and rate-capability experiments revealed that both resulting materials show the reversible Li-ion insertion/extraction reaction. However, due to the existence of a small irreversible capacity at the initial cycles, the Interfacial resistance increases, resulting in the poor cyclability and lower charge/discharge rate capability, especially for nanowire LiCoO2 fabricated with VGCF.

Host frameworks with the ability to store guest ions are very important in a wide range of applications including electrode materials for Li-ion batteries. In this report, we demonstrate that the ion storage ability of the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) can be enhanced by suppressing vacancy formation. K-ions in the vacancy-suppressed PBA framework K1.72Mn[Mn(CN)(6)](0.93)center dot square(0.07).0.65H(2)O (square: a [Mn(CN)(6)](4-) defect) were electrochemically extracted. The open circuit voltages (OCVs) during K-ion extraction exhibited two specific plateaus at 3.0 and 3.7 V vs Li/Li+. Ex situ X-ray diffraction and IR spectroscopy revealed drastic structural and electronic changes during K-ion extraction. Furthermore, after K-ion extraction, the vacancy-suppressed PBA framework was applied to the cathode material for a Li-ion battery. The charge/discharge experiments revealed that the framework can accommodate a large amount of Li-ions.

Interfacial charge transfer is a fundamental issue in both science and technology of the batteries. In this report, interfacial alkali-ion transfer between the cyanide-bridged coordination polymer (Prussian blue analogue, PBA) electrode and organic electrolytes was investigated. Electrochemical impedance spectroscopy (EIS) suggested that alkali-ion transfer at the K0.1Mn[Fe(CN)(6)](0.7)center dot 3.6H(2)O (MnFe-PBA) electrode-electrolyte interface involves two processes. One process could be interpreted as the ion transfer between the Outer Helmholtz Plane (OHP) and Inner Helmholtz Plane (IHP) including the solvation/desolvation process, the other could be interpreted as that between the IHP and electrode, including ad-ion diffusion on the electrode surface. Temperature dependence of the charge transfer resistances gave the activation energy for each process. The activation energy for Li-ion transfer between the OHP and IHP in propylene carbonate (PC) electrolyte is almost constant at the composition range of 0.1 < x < 0.6 in LixMnFe-PBA, which is comparable to that in ethylene carbonate (EC)-diethyl carbonate (DEC) electrolyte. In contrast, the activation energy for Li-ion transfer between the IHP and electrode depends largely on the Li-ion concentration in the PBA electrode. However, the averaged value for Li-ion transfer is higher than that for Na-ion transfer. This result indicated that Li-ion on the PBA surface diffuses with higher potential barrier than Na-ion. Furthermore, the effect of the interfacial charge transfer resistance was evaluated by the high charge/discharge rate experiments. (C) 2011 Elsevier Ltd. All rights reserved.

The electronic structure change during the reversible Li-ion storage reaction in a bimetallic MnFe-Prussian blue analogue (Li(x)K(0.14)Mn(1.43)[Fe(CN)(6)] center dot 6H(2)O) was investigated by soft x-ray absorption spectroscopy. The Mn L(2,3)-edgespectra revealed the unchanged Mn(2+) high-spin state regardless of Li-ion concentration (x). On the other hand, the Fe L(2,3)-edge spectra clearly revealed a reversible redox behavior as Fe(3+) <-> Fe(2+) states with Li-ion insertion/extraction. Experimental findings suggested strong metal-to-ligand charge transfer in conjunction with the ligand-to-metal one. The resulting charge delocalization between the Fe and CN is considered to contribute to the high reversibility of the Li-ion storage process.

The discovery of a new electrode material that provides a reversible Li ion insertion/extraction reaction is of primary importance for Li ion batteries. In this report, electrochemical Li ion insertion/extraction in valence tautomeric Prussian blue analogues AxMny[Fe(CN)6] (A: K, Rb) was investigated. Ex situ X-ray diffraction experiments revealed that the K salt without the valence tautomerism exhibits the Li ion insertion/extraction with a redox process of an Fe ion, while the Rb salt with the valence tautomerism exhibits that with a redox process of a Mn ion. Regardless of the redox-active metal ions, highly reversible Li ion storage was achieved. The electronic structure changes during the Li ion insertion/extraction are confirmed by XPS experiments. © 2010 American Chemical Society.

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 LiMn2O4, Accurate size control of nanocrystalline LiMn2O4, which is realized by a hydrothermal method, significantly alters the phase diagram as well as Li-ion insertion voltage. Nanocrystalline LiMn2O4 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.

Previously, we reported the fabrication of Na0.44MnO2 and LiMn2O4 single crystalline nanowire structure. Moreover, these electrodes showed good high rate property as lithium ion battery, because the nanostructure electrode is suitable for high rate lithium ion battery. Especially, the fabrication of LiMn2O4 single crystalline nanowire was very interesting results because the synthesis of 1-dimesional single crystal structure of LiMn2O4 is very difficult based on cubic crystal structure without anisotropic structure. The LiMn2O4 single crystalline nanowire was obtained thorough the self template method using Na0.44MnO2 nanowire. In this paper, we report the fabrication of Na0.44MnO2 and LiMn2O4 single crystalline nanowire structure and the property of lithium ion battery as review paper.

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.

Increasing industrial needs for rechargeable batteries delivering higher power has encouraged advanced research on nanosized electrochemically active compounds. Although nanosized electrode materials are expected to possess much higher power output due to short diffusion length, the nanosize effects on electrodes have not been understood well to date. Here, we report. that nanosized LiCoO2 as suggested as electrode material for Li-ion rechargeable batteries has anisotropic surface properties affecting electronic structures, which is evidenced by electron energy loss spectroscopy (EELS). EELS spectroscopy reveals the reduced valence state of the cobalt near the surface, especially along the stacking direction of CoO2 layers. The observed anisotropic surface property explains the nanosize effect on the electrochemical properties,

Nano-sizing of cathode materials for higher power Li-ion rechargeable batteries is an effective method to shorten a Li-ion diffusion path and achieve fast charge transfer. Nanocrystalline LiCoO2 was synthesized through a combination of rapid thermal annealing method and a sol-gel method assisted with a triblock copolymer surfactant, and the electrochemical properties including the Li-ion chemical diffusion coefficient was investigated. Li-ion deintercalation/intercalation experiments suggested an extreme small amount of cation mixing between Li+ and Co3+ within a layered structure of LiCoO2, which was not observable in the Raman spectroscopy. The analysis based on the solution for the diffusion equation of the cylinder model revealed that the cation mixing strongly decelerates the Li-ion diffusion in LiCoO2. © 2008 Elsevier B.V. All rights reserved.

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.

Iron mixed-valence complex, (n-C3H7)(4)N[(FeFeIII)-Fe-II(dto)(3)](dto = C2O2S2), shows a spin entropy-driven phase transition called charge transfer phase transition in [(FeFeIII)-Fe-II(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 [(FeFeIII)-Fe-II(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)[(FeFeIII)-Fe-II(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 [(FeFeIII)-Fe-II(dto)(3)](infinity)(-) and the photoisomerization of spiropyran is realized.

A systematic investigation on nanocrystalline LiCoO2 has been carried out using Raman spectroscopy. We synthesized nanocrystalline LiCoO 2 (ca. 20-50nm) through a combination of rapid thermal annealing at various annealing temperatures and a sol-gel method assisted with a triblock copolymer surfactant. Powder X-ray diffraction measurements revealed the formation of LiCoO2. The crystallite size of LiCoO2 from the Scherrer equation strongly depended on the annealing temperature. The crystallite size was confirmed by SEM and TEM measurements. Raman shifts of the A1g and Eg modes for nanocrystalline LiCoO2 exhibited a broadening and a frequency shift according to the crystallite size. While the frequency shift could be ascribed to a structural strain at the surface, the broadening was due to the phonon confinement effect produced by narrow crystal boundaries. © 2008 Elsevier Ltd. All rights reserved.

We experimentally show that the magnetocaloric properties of molecule-based Prussian blue analogues can be adjusted by controlling during the synthesis the amount of intrinsic vacancies. For CsxNi4II[Cr-III(CN)(6)]((8+x)/3), we find indeed that the ferromagnetic phase transition induces significantly large magnetic entropy changes, whose maxima shift from similar to 68 to similar to 95K by varying the number of [Cr-III(CN)(6)](3-) vacancies, offering a unique tunability of the magnetocaloric effect in this complex. (c) 2007 Elsevier B.V. All rights reserved.

Recently, battery technology has come to require a higher rate capability. The main difficulty in high-rate charge-discharge experiments is kinetic problems due to the slow diffusion of Li-ions in electrodes. Nanosizing is a popular way to achieve a higher surface area and shorter Li-ion diffusion length for fast diffusion. However, while various nanoelectrodes that provide excellent high-rate capability have been synthesized, a size-controlled synthesis and a systematic study of nanocrystalline LiCoO2 have not been carried out because of the difficulty in controlling the size. We have established the size-controlled synthesis of nanocrystalline LiCoO2 through a hydrothermal reaction and, for the first time, clarified the structural and electrochemical properties of this intercalation cathode material. Lattice expansion in nanocrystalline LiCoO2 was found from powder X-ray diffraction measurements and Raman spectroscopy. Electrochemical measurements and theoretical analyses on nanocrystalline LiCoO2 revealed that extreme size reduction below 15 nm was not favorable for most applications. An excellent high-rate capability (65% of the 1 C rate capability at 100 C) was observed in nanocrystalline LiCoO2 with an appropriate particle size of 17 nm.

We report on the magnetocaloric properties of two molecule-based hexacyanochromate Prussian blue analogs, nominally CsNiII[Cr-III(CN)(6)]center dot(H2O) and Cr-3(II)[Cr-III(CN)(6)](2)center dot 12(H2O). The former orders ferromagnetically below T-C similar or equal to 90 K, whereas the latter is a ferrimagnet below T-C similar or equal to 230 K. For both, we find significantly large magnetic entropy changes Delta S-m associated with the magnetic phase transitions. Notably, our studies represent an attempt to look at molecule-based materials in terms of the magnetocaloric effect for temperatures well above the liquid helium range.

近年、自動車へ搭載可能な高出力型リチウムイオン2次電池の開発が強く望まれている。本研究では、電極材料のナノ構造化による高出力特性の向上を目指し、エレクトロスピニング法を用いたLiMn_0.6Fe_0.4PO₄ナノワイヤーの合成を行った。得られたLiMn_0.6Fe_0.4PO₄はカーボンによってコーティングされたワイヤー状の形態であり、これを用いて電気化学的評価を行った。

現行のリチウムイオン電池は研究開発の進展とともに性能の限界に近づきつつあり、新しい電極反応機構に立脚した革新的蓄電デバイスの開発が望まれている。特に、リチウムイオンの挿入脱離反応に伴う遷移金属イオンの酸化還元反応だけでは、未来社会における要求性能をエネルギー密度の観点で満たすことは難しい。そこで、本研究は、革新的蓄電デバイスの開発を目指し、アニオンを基盤とする固体電気化学反応を開拓する。
2019年度においては、酸化物イオンを酸化還元中心とする電極材料の解析技術を新たに開発した。すなわち、放射光軟X線発光分光スペクトルを用いることで、酸化物イオンの酸化状態、遷移金属との結合状態を精密に評価することが可能であることを見出した。特に、酸化還元が可能な酸化物イオンの軌道がバンド構造から孤立していることを直接観察することに成功し、また、充電に伴ってこの軌道が酸化され、バンドへ混成して正孔を安定化していることが分かった。更に、電子状態計算から、酸化物イオンの孤立した2p軌道が実際に存在していること、酸化に伴い遷移金属との軌道混成が著しく強まり、バンドへ混成することが分かり、軟X線発光分光により観測された状態変化を確認することにも成功した。以上の成果は、今後様々な酸化物材料系への展開が可能であり、波及効果の高いものである。

本研究は、MXeneと総称される層状化合物を、酸化還元を示すレドックスキャパシタ電極として応用する研究開発である。様々な電解液と組み合わせたキャパシタの性能評価を主眼に据えた研究計画であり、平成27年度から29年度において、Ti2CTx（Txは末端基：-F, -Cl, -OH, -O）の組成を持つMXeneをLi+キャパシタ、Na+キャパシタに応用した。その結果、200 mAh/g程度の充放電容量を示すこと、作成したキャパシタが高出力・高エネルギー密度を両立することを確認し、当初の目的をほぼ達成した。

初年度に我々が初めて発見したリチウム塩のハイドレートメルトとその異常な電気化学安定性について、昨年度は様々なリチウム塩で実現可能なことを見いだした。また、ナトリウム塩やカリウム塩のハイドレートメルトを新たに合成することにも合わせて成功した。これにより、アルカリ塩のハイドレートメルトが多様性を有し、これまでの常識を遙かに上回る広い電位窓と特異な輸送特性を実現する新たな電解質機能物質“群”として位置づけられることが示された。この特徴を生かし、通常の水溶液中では機能しない様々な電極材料が作動することを実証した上で、新規な電池デバイスを複数構築した。これら特殊な溶液物性の起源は溶液中でアルカリイオンや陰イオンと強く相互作用しつつも、水素結合ネットワークは一切形成しない孤立した特殊な水分子である。
可逆的電荷貯蔵機能を担う固体電極材料において、新たに格子欠陥を導入した材料設計を提示した。これまで格子欠陥は、可逆性や安定性を阻害する負の因子として捉えられてきたが、本研究では近接する酸素の電子軌道の孤立化と可逆的酸化還元反応への寄与を発現させる目的で、遷移金属欠陥を積極的に導入する材料設計を行った。その結果、欠陥周辺で孤立した酸素軌道による巨大可逆容量が出現することを見いだし、その安定性がこれまでの報告例を遙かに凌駕することがわかった。
昨年度報告した、Mxene層間化合物に閉じ込められた水分子の負性誘電率とキャパシタンス増強効果について、Graphene層間における同様の挙動を理論的に実証することに成功し、現象の一般性に対する端緒を得た。

層状ナノシート化合物MXeneの電極特性を明らかにするために、様々な組成のMXene合成に取り組んだ。例えば、Nb2CTxの組成を持つMXeneを、Nb2AlC組成のMAX相からAl層をエッチング除去することで合成した。得られたMXene Nb2CTxについて電気化学特性を評価したところ、エッチング剤に大きく依存した電極活性を示すことが分かった。すなわち、塩化物イオンを含むエッチング剤を使用した場合、ナトリウムイオンとの反応活性が著しく向上することが分かった。また、Mo2CTxの組成をもつMXeneについて、Mo2Ga2Cの組成のMAX相からGa2層をエッチング除去することで得た。このエッチングは非常に反応速度が遅く困難であったが、条件の最適化により完全なMXeneを合成することに成功した。得られたMXene Mo2CTxについて電極活性を調べたところ、アルカリ金属イオンの種類に依存したキャパシタンスを示すことが分かった。この結果は、Ti2CTxやTi3C2Txなどの組成を持つMXeneにおいても普遍的に観測される傾向であった。そこで、電子状態計算、溶媒和構造計算、および電位計算を行うことでMXeneの電化貯蔵メカニズムを検討した結果、溶媒和構造が特異的な誘電応答を示すことでキャパシタンスの特異的挙動が発現することが分かった。以上の知見を基にすることで、更なるキャパシタンスを実現することが可能となった。今後は、組成の更なる最適化、表面官能基の最適化を行うことで、より優れた電極特性をMXeneで実現することを狙う。

低炭素・省エネルギー社会の実現に貢献できる高性能蓄電デバイスの開発を研究目的として、酸化物イオンを可動イオンとして利用する全く新しいコンセプトの蓄電デバイスの基礎原理について研究を行った。特に、酸化物イオンを可動イオンとする固体電気化学反応、電極反応は未開拓であるため、様々な化合物について検討を行い、全く新しい材料系で酸化物イオンを可動イオンとして電極反応を示す系を見出すことに成功した。

イオン拡散チャンネルと電子伝導経路を併せ持つ多孔性配位錯体は、固体電気化学的イオン脱挿入に伴うホスト構造の酸化還元反応と、それに伴う磁性を含めた様々な物性のスイッチング現象を示すことが期待される。本研究では、イオンと電子の混合伝導性を示す多孔性配位錯体の開発を行い、磁性制御の実現を目指した。
まず、混合伝導性が良く知られているシアノ架橋配位高分子であるプルシアンブルー類似体について、精密な電流量制御によるリチウムイオン挿入量の制御を行うことで、連続的なキュリー温度の制御に成功した。
次に、プルシアンブルー類似体における電極－電解液界面の不安定性を改良するために、コアシェル構造を有するプルシアンブルー類似体を合成し、電極特性の耐久性を大幅に向上させることが可能であることを明らかにした。
更に、オクタシアノ錯体を構造形成分子として用いることで、３次元の電子伝導ネットワークと、２次元のイオン拡散チャンネルを有する配位高分子の合成に成功し、得られた配位高分子が実際の電極反応が可能であることを実証した。また、モリブデンの酸化還元により強磁性－常磁性のスイッチングに成功した。
最後に、プルシアンブルー類似体の多孔性を利用し、２価のカチオンであるマグネシウムイオンが可逆的に挿入脱離可能であることを示した。特に、メスバウアー分光、放射光硬X線吸収分光の詳細な電子状態の解析により、１価のカチオンでは観察されないヘテロ遷移金属の同時還元が確認され、２価のカチオン挿入の特異的な電子状態を明らかにすることに成功した。

本研究では、研究テーマとして「リチウム電池用電極材料の二相共存反応における核発生と相境界移動メカニズムの解明」を設定し、界面が核発生・成長メカニズムに与える影響を明らかにすることにより、高速 Li 脱挿入を可能とする材料設計の指針を確立することを目指して研究活動を行った。核発生、相成長のメカニズムを電気化学的に探るためにモデルの構築を行い、2相共存状態におけるリチウムイオン挿入は核発生・成長に支配されるというモデルの下で、電気化学応答関数を導出することに成功した。代表的な 2 相共存の電極材料である LiFePO4 について様々な粒子形態、過電圧印可条件下での過渡電流応答を測定しモデルによる解析を行ったところ、実験結果を再現することができた。解析の結果、初期の核発生頻度は過電圧に大きく依存し、過渡電流の振る舞いに大きな影響を及ぼすことが明らかになった。一方、大きな過電圧化においては、速度論効果によるモデルからの逸脱が生じ、更なるモデルの最適化が必要であることが分かった。

リチウムイオン2次電池の高性能化のために電極材料のナノ化を行い、表面積の増加に伴う電気化学特性の変化を解明した。LiCoO_2については、ナノ化に伴い表面近傍のコバルトが還元されており、15nm以下のナノ化は容量・電位の低下により電極特性の劣化が著しいことが分かった。LiMn_2O_4については、ナノ化に伴い構造変化が緩和し、バルク材料では利用できない酸化還元反応についても利用可能になることを明らかにした。

電気伝導性・磁性の光制御を目指した光応答性有機・無機複合錯体の開発を目指し、具体的に3つの系について合成・物性解析・光応答物性探索を行った。
まず、電気伝導性の光制御を目指し、電気伝導性を持つ有機分子BEDT-TTF(bis(ethylenedithio)tetrathiafulvalene)と、光応答性異性化反応を起こすことで知られるニトロシルルテニウム錯体[RuCl_5(NO)]^<2->を組み合わせた有機・無機ハイブリッド錯体を合成した。構造解析の結果、電気伝導層であるBEDT-TTFレイヤーは、有機超伝導を与える可能性が大きいκ型の構造を形成していることを明らかにした。電気伝導度・磁化率測定により、この系はトランスファー積分が小さく、オンサイトクーロン反発の強いモット絶縁体であることが判明した。また、この系に特異的なこととして、顕微ラマン分光測定により、BEDT-TTFからニトロシル基への電子移動が起こっていることを明らかにした。すなわち、ニトロシル錯体が有機ドナー分子とカウンターイオンとして相互作用することにより、ホールドープといった物性の制御に繋がる可能性を持つことを示した。
次に、磁性の光制御という目標に対して、光異性化分子ジアリールエテンをカチオン化し、面内強磁性相互作用を示すペロブスカイト型銅ハロゲン化錯体CuCl_4のカウンターカチオンとして導入したハイブリッド錯体を合成した。ジアリールエテンの開環形・閉環形を持つ錯体それぞれについて磁性測定を行った結果、開環形を対イオンとする錯体においては3.4Kで反強磁性転移するのに対し、閉環形を対イオンとする錯体においては1.8Kまで磁性転移が認められなかった。この現象は、ジアリールエテンの構造変化に対応した面内ヤーンテーラー歪みの増大に伴った磁気的相互作用の減少で説明され、同時に、ジアリールエテンの開環・閉環により磁気転移を制御することに成功したと言える。
最後に、相安定性を持つFe(II)錯体を光応答性部位として、ベルダジルラジカルスピンを配位させた錯体を新規に合成し、その物性を調べた。磁気測定の結果、ベルダジルラジカルと鉄スピンは反強磁性的相互作用をすることを磁気測定から明らかにした。