Chemical-state distributions in charged LiCoO2 cathode particles visualized by soft X-ray spectromicroscopy.

Lithium-ion deintercalation/intercalation during charge/discharge processes is one of the essential reactions that occur in the layered cathodes of lithium-ion batteries, and the performance of the cathode can be expressed as the sum of the reactions that occur in the local area of the individual cathode particles. In this study, the spatial distributions of the chemical states present in prototypical layered LiCoO2 cathode particles were determined at different charging conditions using scanning transmission X-ray microscopy (STXM) with a spatial resolution of approximately 100 nm. The Co L3- and O K-edge X-ray absorption spectroscopy (XAS) spectra, extracted from the same area of the corresponding STXM images, at the initial state as well as after charging to 4.5 V demonstrate the spatial distribution of the chemical state changes depending on individual particles. In addition to the Co L3-edge XAS spectra, the O K-edge XAS spectra of the initial and charged LiCoO2 particles are different, indicating that both the Co and O sites participate in charge compensation during the charging process possibly through the hybridization between the Co 3d and O 2p orbitals. Furthermore, the element maps of both the Co and O sites, derived from the STXM stack images, reveal the spatial distribution of the chemical states inside individual particles after charging to 4.5 V. The element mapping analysis suggests that inhomogeneous reactions occur on the active particles and confirm the existence of non-active particles. The results of this study demonstrate that an STXM-based spatially resolved electronic structural analysis method is useful for understanding the charging and discharging of battery materials.

Redox Reaction in Ti-Mn Redox Flow Battery Studied by X-ray Absorption Spectroscopy.

We performed X-ray absorption studies for the electrolytes of a Ti-Mn redox flow battery (RFB) to understand the redox reaction of the Ti/Mn ions and formation of precipitates in charged catholyte, because suppression of the disproportionation reaction is a key to improve the cyclability of Ti-Mn RFB and enhance the energy density. Hard X-ray absorption spectroscopy with a high transmittance and soft X-ray absorption spectroscopy to directly observe the 3d orbitals were complementarily employed. Moreover, the Ti/Mn 3d electronic structure for each precipitate and solution in the charged catholyte was investigated by using scanning transmission X-ray microscopy: the valence of Mn in the precipitate is mostly attributed to 4+, and the solution includes only Mn2+ . This charge disproportionation reaction should occur after the Mn ions in the catholyte should be oxidized from Mn2+ to Mn3+ by charge.

Operando resonant soft X-ray emission spectroscopy of the LiMn2O4 cathode using an aqueous electrolyte solution.

The Mn 3d electronic-structure change of the LiMn2O4 cathode during Li-ion extraction/insertion in an aqueous electrolyte solution was studied by operando resonant soft X-ray emission spectroscopy (RXES). The Mn L3 RXES spectra for the charged state revealed the Mn4+ state with strong charge-transfer from the O 2p to Mn 3d orbitals dominates, while for the open-circuit-voltage and discharged states it is ascribed to the mixture of sites with Mn3+ and Mn4+ states. The degree of charge transfer is significantly different between the Mn3+ and Mn4+ states, indicating that the redox reaction takes place on the strongly-hybridized Mn 3d-O 2p orbital rather than the localized Mn 3d orbital.

Oxygen Redox Versus Oxygen Evolution in Aqueous Electrolytes: Critical Influence of Transition Metals.

Aqueous lithium-ion batteries are promising electrochemical energy storage devices owing to their sustainable nature, low cost, high level of safety, and environmental benignity. The recent development of a high-salt-concentration strategy for aqueous electrolytes, which significantly expands their electrochemical potential window, has created attractive opportunities to explore high-performance electrode materials for aqueous lithium-ion batteries. This study evaluates the compatibility of large-capacity oxygen-redox cathodes with hydrate-melt electrolytes. Using conventional oxygen-redox cathode materials (Li2 RuO3 , Li1.2 Ni0.13 Co0.13 Mn0.54 O2 , and Li1.2 Ni0.2 Mn0.6 O2 ), it is determined that avoiding the use of transition metals with high catalytic activity for the oxygen evolution reaction is the key to ensuring the stable progress of the oxygen redox reaction in concentrated aqueous electrolytes.

Nonpolarizing oxygen-redox capacity without O-O dimerization in Na2Mn3O7.

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 >0.5 V upon charge/discharge, and hence possess unacceptably poor energy efficiency. The hysteresis is thought to originate from the formation of peroxide-like O22- 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 0.04 V. Using spectroscopic and magnetic measurements, the existence of stable O-• was identified in Na2-xMn3O7. Computations reveal that O-• is thermodynamically favorable over the peroxide-like O22- dimer as a result of hole stabilization through a (σ + π) multiorbital Mn-O bond.

Tetragonal Distortion of a BaTiO3/Bi0.5Na0.5TiO3 Nanocomposite Responsible for Anomalous Piezoelectric and Ferroelectric Behaviors.

Ferroelectric mesocrystalline nanocomposites are functional materials with improved ferroelectricity via lattice strain engineering. In this study, X-ray diffraction (XRD) and soft X-ray absorption spectroscopy (XAS) are performed to determine the tetragonal distortion of Bi0.5Na0.5TiO3 (BNT) in a ferroelectric mesocrystalline BaTiO3 (BT)/BNT nanocomposite. The XRD results demonstrate the expansion of the BNT lattice in the BT/BNT nanocomposite. Using Williamson-Hall analysis, the tensile strain of BNT in BT/BNT-700 is confirmed. Shift and splitting of the eg orbital are observed for BNT in the BT/BNT nanocomposite in Ti L 3-edge XAS, suggesting the lower symmetry of the TiO6 octahedron in BNT, which is ascribed to a significant tetragonal distortion of BNT in the BT/BNT nanocomposite caused by the lattice mismatch between BNT and BT. It is found that the tetragonally distorted BNT in BT/BNT is responsible for the anomalous ferroelectric response of the mesocrystalline BT/BNT nanocomposite.

Operando soft X-ray emission spectroscopy of the Fe2O3 anode to observe the conversion reaction.

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) → Fe0 (0.1 V vs. Li/Li+: discharged) → Fe(2+δ)+ (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.

Stabilization of a 4.5 V Cr4+/Cr3+ redox reaction in NASICON-type Na3Cr2(PO4)3 by Ti substitution.

The development of high-voltage cathode materials composed of abundant metals for rechargeable batteries is a crucial task to realize higher energy density in large-scale electrical energy storage systems. Here we report a reversible Cr4+/Cr3+ redox reaction at 4.5 V vs. Na/Na+ in NASICON-type Na2CrTi(PO4)3 (NCTP). An unstable Cr4+/Cr3+ redox in Na3Cr2(PO4)3 is successfully stabilized by the substitution of Ti with Cr. The charge/discharge mechanism of NCTP was studied by powder X-ray diffraction and soft X-ray absorption spectroscopy.

Microscopic photoelectron analysis of single crystalline LiCoO2 particles during the charge-discharge in an all solid-state lithium ion battery.

We report synchrotron-based operando soft X-ray microscopic photoelectron spectroscopy under charge-discharge control of single crystalline LiCoO2 (LCO) particles as an active electrode material for an all solid-state lithium-ion battery (LIB). Photoelectron mapping and the photoelectron spectrum of a selected microscopic region are obtained by a customized operando cell for LIBs. During the charge process, a more effective Li extraction from a side facet of the single crystalline LCO particle than from the central part is observed, which ensures the reliability of the system as an operando microscopic photoelectron analyzer that can track changes in the electronic structure of a selected part of the active particle. Based on these assessments, the no drastic change in the Co 2p XPS spectra during charge-discharge of LCO supports that the charge-polarization may occur at the oxygen side by strong hybridization between Co 3d and O 2p orbitals. The success of tracking the electronic-structure change at each facet of a single crystalline electrode material during charge-discharge is a major step toward the fabrication of innovative active electrode materials for LIBs.

Mn 2p resonant X-ray emission clarifies the redox reaction and charge-transfer effects in LiMn2O4.

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.

Large Charge-Transfer Energy in LiFePO4 Revealed by Full-Multiplet Calculation for the Fe L3 -edge Soft X-ray Emission Spectra.

We analyzed the Fe 3d electronic structure in LiFePO4 /FePO4 (LFP/FP) nanowire with a high cyclability by using soft X-ray emission spectroscopy (XES) combined with configuration-interaction full-multiplet (CIFM) calculation. The ex situ Fe L2,3 -edge resonant XES (RXES) spectra for LFP and FP are ascribed to oxidation states of Fe2+ and Fe3+ , respectively. CIFM calculations for Fe2+ and Fe3+ states reproduced the Fe L3 RXES spectra for LFP and FP, respectively. In the calculations for both states, the charge-transfer energy was considerably larger than those for typical iron oxides, indicating very little electron transfer from the O 2p to Fe 3d orbitals and a weak hybridization on the Fe-O bond during the charge-discharge reactions.

Investigation of the relationship between the cycle performance and the electronic structure in LiAlxMn2-xO4 (x = 0 and 0.2) using soft X-ray spectroscopy.

Al doping into LiMn2O4 is one of the well-known methods to improve the cycle performance of the LiMn2O4 cathode. We carried out soft X-ray emission spectroscopy (XES) for LiMn2O4 and LiAl0.2Mn1.8O4 to elucidate the relationship between the Mn 3d electronic structures and cycle performances. After the first cycle, the XES spectra of LiAl0.2Mn1.8O4 are almost unchanged compared to the initial state. In contrast, charge-transfer excitation for the XES of LiMn2O4 is significantly reduced, indicating that the Mn 3d-O 2p hybridization in LiMn2O4 should be easily weakened by charge-discharge. In LiAl0.2Mn1.8O4, the Mn-O bond becomes more stable due to the decrease of Mn3+ ions with Jahn-Teller distortion by Al3+ doping, resulting in the improved cycle performance.

Correlation between the O 2p Orbital and Redox Reaction in LiMn0.6 Fe0.4 PO4 Nanowires Studied by Soft X-ray Absorption.

The changes in the electronic structure of LiMn0.6 Fe0.4 PO4 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 charge-transfer effect from O 2p to Mn 3d orbitals should be considerably small, unlike that from the O 2p to Fe 3d orbitals. Instead, the O K-edge X-ray absorption spectrum shows a clear spectral change during the discharge process, suggesting that the hybridization of O 2p orbitals with Fe 3d orbitals contributes essentially to the reduction.

Electrochemical Li-Ion Intercalation in Octacyanotungstate-Bridged Coordination Polymer with Evidence of Three Magnetic Regimes.

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.

Anisotropic charge-transfer effects in the asymmetric Fe(CN)5NO octahedron of sodium nitroprusside: a soft X-ray absorption spectroscopy study.

The electronic structure of Na2[Fe(CN)5NO]·2H2O (sodium nitroprusside: SNP) was investigated by using soft X-ray absorption (XA) spectroscopy. The Fe L2,3-edge XA spectrum of SNP exhibited distinct and very large satellite peaks for L3 and L2 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 C4v symmetry were taken into account, revealed the Fe(2+) low-spin state with very strong effects of metal-to-ligand charge-transfer from the Fe 3d to NO 2p orbitals.

Distinguishing between High- and Low-Spin States for Divalent Mn in Mn-Based Prussian Blue Analogue by High-Resolution Soft X-ray Emission Spectroscopy.

We combine Mn L2,3-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 Mn(2+) low-spin state separately from that of the Mn(2+) 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 Mn(2+) low spin state having a stronger ligand-to-metal charge-transfer effect between Mn t2g and CN π orbitals than the Mn(2+) high spin state. The d-d-excitation peak nearest to the elastic scattering was ascribed to the Mn(2+) LS state by the CIFM calculation, indicating that the Mn(2+) LS state with a hole on the t2g orbital locates near the Fermi level.

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.

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.

Precise electrochemical control of ferromagnetism in a cyanide-bridged bimetallic coordination polymer.

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). K(0.2)Ni[Fe(CN)(6)](0.7)·4.7H(2)O (NiFe-PBA) shows Li ion insertion/extraction reversibly accompanied with reversible Fe(3+)/Fe(2+) reduction/oxidation. When Li ion is inserted quantitatively into NiFe-PBA, the ferromagnetic transition temperature T(C) gradually decreases due to reduction of paramagnetic Fe(3+) to diamagnetic Fe(2+), 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 Fe(2+) to paramagnetic Fe(3+), 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.

High-energy 'composite' layered manganese-rich cathode materials via controlling Li2MnO3 phase activation for lithium-ion batteries.

The 'composite' layered materials for lithium-ion batteries have recently attracted great attention owing to their large discharge capacities. Here, the 0.5Li(2)MnO(3)·0.5LiMn(0.42)Ni(0.42)Co(0.16)O(2)'composite' layered manganese-rich material is prepared and characterized by the synchrotron X-ray powder diffraction (SXPD). The relationship between its electrochemical performance and its 'composite' components, the Li(2)MnO(3) phase activation process during cycling and the cycle stability of this material at room temperature are elucidated based on its kinetic controlled electrochemical properties, dQ/dV curves and Raman scattering spectroscopies associated with different initial charge-discharge current densities (5 mA g(-1), 20 mA g(-1) and 50 mA g(-1)), cut-off voltages (4.6 V and 4.8 V) and cycle numbers (50 cycles and 150 cycles). Furthermore, its reaction pathways are tracked via a firstly introduced integrated compositional phase diagram of four components, Li(2)MnO(3), LiMn(0.42)Ni(0.42)Co(0.16)O(2), MO(2) (M = Mn(1-α-ß)Ni(α)Co(ß); 0 ≤α≤ 5/12, 0 ≤ß≤ 1/6) and LiMnO(2), which turns out to be a very important guiding tool for understanding and utilizing this 'composite' material.