中西 康次

Silicon has been considered to be one of the most promising anode active materials for next-generation lithium-ion batteries due to its large theoretical capacity (4200 mA h g-1, Li22Si5). However, silicon anodes suffer from degradation due to large volume expansion and contraction. To control the ideal particle morphology, an experimental method is required to analyze anisotropic diffusion and surface reaction phenomena. This study investigates the anisotropy of the silicon-lithium alloying reaction using electrochemical measurements and Si K-edge X-ray absorption spectroscopy on silicon single crystals. During the electrochemical reduction process in lithium-ion battery systems, the continuous formation of solid electrolyte interphase (SEI) films prevents the achievement of steady-state conditions. Instead, the physical contact between silicon single crystals and lithium metals can prevent the effect of SEI formation. The apparent diffusion coefficient and the surface reaction coefficient are determined from the progress of the alloying reaction analyzed by X-ray absorption spectroscopy. While the apparent diffusion coefficients show no clear anisotropy, the apparent surface reaction coefficient of Si (100) is more significant than that of Si (111). This finding indicates that the surface reaction of silicon governs the anisotropy of practical lithium alloying reaction for silicon anodes.

All-solid-state fluoride-ion batteries (FIBs) that use fluoride ions as carrier ions offer a new horizon for next-generation energy storage devices owing to their high specific capacities. Materials that utilize topochemical insertion and desorption reactions of fluoride ions have been proposed as cathodes for FIBs; among them, Ruddlesden-Popper-type perovskite-related compounds are promising cathode materials owing to reversible fluoride-ion (de)intercalations with low volume expansion compared to conversion-type cathode materials. Although it is essential to improve the power density of the compounds for practical application, the relationship between the structure and power density is still not clearly understood. In this study, we synthesized chemically fluorinated Ruddlesden-Popper compounds, LaSrMnO4 and apical-site-substituted oxyfluoride Sr2MnO3F, and examined the correlations between their structures and electrochemical properties; Sr2MnO3F showed better power density. Open-circuit voltage measurements, X-ray absorption spectroscopy, and synchrotron X-ray diffraction revealed that electrochemical F- insertion into LaSrMnO4 proceeds via a two-phase reaction with relatively high volume expansion, whereas that into Sr2MnO3F proceeds via a solid-solution reaction with relatively low volume expansion. The substitution of oxygen in the apical sites with fluorine suppressed phase transitions with large volume changes, resulting in improved power density.

Improving the rate performance of lithium-ion batteries is important for the widespread utilization of electric vehicles and energy grids. Because the rate-determining step for cathode materials with a two-phase reaction is nucleation reaction at the material surface, surface modification is a promising approach for achieving this goal. However, the cause of this improvement in the reaction rate at the interface between the surface-modified cathode and the electrolyte is not clearly understood. In this study, we prepared a surface-nitrided LiFePO4 thin film and investigated its electrochemical properties. In addition, we examined its surface structure using surface-sensitive X-ray absorption spectroscopy measurements and first-principles calculations, and discussed the correlation between the rate performance and the interfacial reaction. The experiments revealed the formation of a new energy level and the increase of the Fe-O bond distance at the surface of LiFePO4 due to nitrogen doping. The electronic and local structural changes accelerated lithium ion diffusion at the interface between surface-nitrided LiFePO4 and the electrolyte, improving the rate performance.

All-solid-state fluoride-ion batteries (FIBs) are regarded as promising energy storage devices; however, currently proposed cathodes fail to meet the requirements for practical applications in terms of high energy density and high rate capability. Herein, the first use of stable and low-cost cuprous oxide (Cu2O) as a cathode material for all-solid-state FIBs with reversible and fast (de)fluorination behavior is reported. A phase-transition reaction mechanism involving Cu+/Cu2+ redox for charge compensation is confirmed, using the combination of electrochemical methods and X-ray absorption spectroscopy. The first discharge capacity is approximately 220 mAh g(-1), and fast capacity fading is observed in the first five cycles, which is ascribed to partial structural amorphization. Compared with those of simple metal/metal fluoride systems, the material shows a superior rate capability, with a first discharge capacity of 110 mAh g(-1) at 1 C. The rate-determining step and probable structural evolutions are investigated as well. It is believed that the comprehensive investigations of Cu2O as a cathode material described in this work can lead to an improved understanding of all-solid-state FIBs.

All-solid-state lithium batteries that use lithium metal as the anode have extremely high energy densities. However, for lithium metal anodes to be used, lithium dendrite formation must be addressed. Recently, the addition of lithium iodide (LiI) to sulfide solid electrolytes was found to suppress lithium dendrite formation. It is unclear whether the cause of this suppression is the improvement of the ionic conductivity of the solid electrolyte itself or the electrochemical properties of the lithium metal/solid electrolyte interface. In this study, the cause of the suppression was quantitatively elucidated. The effect of the interphase on the dendrite growth of doping LiI into Li3PS4 was determined using Xray absorption spectroscopy and X-ray computed tomography measurements. The results revealed that LiI-doped Li3PS4 suppressed the dendrite formation by maintaining the interface due to inhibition of the reductive decomposition of Li3PS4. In addition, annealed LiI-doped Li3PS4 showed a greater dendrite suppression ability as the ionic conductivity increased. From these results, we not only found that the physical properties of the lithium metal/solid electrolyte interface and the bulk ionic conductivity contribute to lithium dendrite suppression but also quantitatively determined the proportions of the contributions of these two factors.

All-solid-state fluoride-ion batteries (FIBs) are regarded as attractive alternatives to traditional energy storage systems because of their high energy density; however, they are not applicable at room temperature owing to sluggish ion transport in both the electrolyte and electrode. In this study, a rational design of a Cu-Pb nanocomposite is reported, which was tested as a room-temperature cathode material for all-solid-state FIBs. Following electrochemical pretreatment, self-generated PbF2 could act as a fast fluoride-ion conductor and consequently enhance the kinetics of the Cu/CuF2 phase transition process upon cycling. The detailed reaction mechanism and phase transition process were verified using the X-ray absorption near edge structure. The Cu-Pb nanocomposite could realize reversible (de)fluorination at room temperature with high performance and good cyclability.

Voltage and capacity decay problems of Li2MnO3-based lithium rich layered cathode materials are still unsolved, which are the major barriers for its practical application. Intensive investigations of their mechanisms have been proceeded in recent years, which clarify cation ordered arrangement and spinel deterioration are culprits in structural aspect. Herein, we conduct various analyses to a rocksalt type Li2Nb0.15Mn0.85O3, whose first charge process is along with primary oxygen redox as Li2MnO3, but subsequently shows much better capacity retention. This material suffers from severe voltage decay and overpotential during the cycle but hard X-ray diffraction indicates it can remain rocksalt structure without any tendency of spinel deterioration or cation ordering even with massive lithium vacancies. XAS results further show there is no evident oxygen redox activity from the second cycle, while the redox range of Mn nearly has no change during the cycle. Our results demonstrate that disordered lithium rich cathode materials could be promising to exhibit highly reversible capacity upon long cycling, and their voltage decay problem could be easier to be solved for different mechanism from layered one. (C) 2020 Elsevier B.V. All rights reserved.

Fluoride ion batteries (FIBs) are regarded as promising energy storage devices, and it is important and urgent to develop cathode materials with high energy densities for use in FIBs. However, systematic investigations of 3d transition metal/metal fluorides have been rarely reported thus far because of the restricted reversibility and unfavorable interfacial compatibility of 3d transition metal/metal fluorides with solid-state electrolytes. Herein, 3d transition metals are investigated by utilizing thin-film cells with LaF3 substrates. The highly reversible (de)fluorinations of Cu, Co, and Ni are validated at various temperatures. High capacity utilizations of 79.5%, 100%, and 90.5% are obtained during the initial cycle at 150 °C. By combining results from X-ray absorption spectroscopy (XAS) and electrochemical characterization, the electrochemical behaviors of Cu, Co, and Ni, as well as experimental evidence of the two-phase transition mechanism during the M/MF2 reaction are reported for the first time. This provides new insights required for future cathode designs for use in all-solid-state FIBs.

Fluoride ion batteries (FIBs) are regarded as promising energy storage devices, and it is important and urgent to develop cathode materials with high energy densities for use in FIBs. However, systematic investigations of 3d transition metal/metal fluorides have been rarely reported thus far because of the restricted reversibility and unfavorable interfacial compatibility of 3d transition metal/metal fluorides with solid-state electrolytes. Herein, 3d transition metals are investigated by utilizing thin-film cells with LaF3 substrates. The highly reversible (de)fluorinations of Cu, Co, and Ni are validated at various temperatures. High capacity utilizations of 79.5%, 100%, and 90.5% are obtained during the initial cycle at 150 degrees C. By combining results from X-ray absorption spectroscopy (XAS) and electrochemical characterization, the electrochemical behaviors of Cu, Co, and Ni, as well as experimental evidence of the two-phase transition mechanism during the M/MF2 reaction are reported for the first time. This provides new insights required for future cathode designs for use in all-solid-state FIBs.

<p>New concepts for electrochemical energy storage devices are required to handle the physicochemical energy density limit that Li-ion batteries are approaching. All-solid-state fluoride-ion batteries (FIBs), in which monovalent fluoride anions...</p>

With the increasing development of electric vehicles and portable devices, there is a strong requirement for high-energy batteries. To improve battery energy, multielectron transfer electrode reactions can be applied. Previously, batteries based on fluoride-ion shuttle (F- ion shuttle batteries, FiBs) have been reported, utilizing electrodes with multielectron transfer reactions. Although these FiBs exhibit high theoretical energy densities, reported capacities are significantly less than theoretical values. Moreover, charge-discharge mechanisms are not clarified. In this study, the feasibility of FiBs as extremely high-energy batteries has been demonstrated using a model cell with a Cu cathode and a LaF3 anode. By conducting experiments under an atmosphere without impurities, the Cu/LaF3 battery has been successfully operated with almost theoretical capacity. The Cu/LaF3 battery has been exhibited a superior cycle life at 80 degrees C, with feasibility for room- temperature operation.

Nanostructured LiMnO2 integrated with Li3PO4 was successfully synthesized by the mechanical milling route and examined as a new series of positive electrode materials for rechargeable lithium batteries. Although uniform mixing at the atomic scale between LiMnO2 and Li3PO4 was not anticipated because of the noncompatibility of crystal structures for both phases, our study reveals that phosphorus ions with excess lithium ions dissolve into nanosize crystalline LiMnO2 as first evidenced by elemental mapping using STEM-EELS combined with total X-ray scattering, solid-state NMR spectroscopy, and a theoretical ab initio study. The integrated phase features a low-crystallinity metastable phase with a unique nanostructure; the phosphorus ion located at the tetrahedral site shares faces with adjacent lithium ions at slightly distorted octahedral sites. This phase delivers a large reversible capacity of ∼320 mA h g-1 as a high-energy positive electrode material in Li cells. The large reversible capacity originated from the contribution from the anionic redox of oxygen coupled with the cationic redox of Mn ions, as evidenced by operando soft XAS spectroscopy, and the superior reversibility of the anionic redox and the suppression of oxygen loss were also found by online electrochemical mass spectroscopy. The improved reversibility of the anionic redox originates from the presence of phosphorus ions associated with the suppression of oxygen dimerization, as supported by a theoretical study. From these results, the mechanistic foundations of nanostructured high-capacity positive electrode materials were established, and further chemical and physical optimization may lead to the development of next-generation electrochemical devices.

Sulfur is one of the promising next-generation cathode materials because of its low cost and high theoretical gravimetric capacity. However, the reaction mechanism of the sulfur cathode is largely influenced by the electrolyte and the intermediate sulfur species during the first discharge process has not been quantitatively explored in different electrolytes. In this study, we elucidated the reaction mechanism of sulfide cathodes by using three different electrolyte systems, viz., a conventional liquid electrolyte [LiPF6/ethylene carbonate (EC)/ethylene-methyl carbonate (EMC)], a concentrated liquid electrolyte [lithium bis(trifluorosulfonyl)amide (LiTFSA)/tetraglyme (G4):1,1,2,2-tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE)], and a solid-state electrolyte (Li3PS4). Soft X-ray absorption spectroscopy was used to examine the reaction mechanism of the sulfur cathode in the liquid and solid-state electrolytes during the first discharge process. In the conventional electrolyte, the sulfur cathode was reduced to long-chain polysulfide (S-6(2-)) during the first discharge process, and the polysulfide subsequently dissolved into the electrolyte. In the concentrated electrolyte, the sulfur cathode was reduced to midchain polysulfide (S-4(2-)) at the initial stage of the first discharge process and then reduced to short-chain polysulfide (S-2(2-)) and Li2S, followed by the formation of long-chain polysulfide (S-6(2-)). In the solid-state electrolyte, the sulfur cathode was reduced to long-chain polysulfide (S-6(2-)) at the initial stage of the first discharge process and was gradually reduced to mid-chain polysulfide (S-4(2-)), short-chain polysulfide (S-2(2-)), and Li2S. The differences in these reaction pathways govern electrochemical properties such as the difference in discharge voltage.

To clarify the effects of anion species and solvents on the Coulombic efficiency and polarization of magnesium deposition/dissolution reactions, the anode/electrolyte interfacial behavior of magnesium tetrakis(hexafluoroisopropyloxy) borate (Mg[B(HFIP)(4)](2)) and magnesium bis(trifluoromethanesulfony)amide (Mg(TFSA) 2 ) was investigated and compared in triglyme and 2-methlytetrahydrofuran (2-MeTHF). When using triglyme, which has strong interaction with magnesium ions, decomposition of [B(HFIP)(4)](-) in Mg[B(HFIP)(4)](2)/triglyme was hard to occur because of the high reduction stability of the uncoordinated [B(HFIP)(4)](-) anion, resulting in significantly higher Coulombic efficiency and smaller polarization than Mg(TFSA)(2)/triglyme. When 2-MeTHF was used as the solvent, magnesium deposition/dissolution reactions occurred in the Mg[B(HFIP)(4)](2)/2-MeTHF electrolyte but not in the Mg[TFSA](2)/2-MeTHF electrolyte. This is because the coordinated [B(HFIP)(4)](-) anion in Mg[B(HFIP)(4)](2) /2-MeTHF is stable at the magnesium deposition potential. However, the reductive stability of the coordinated [B(HFIP)(4)](-) anion is inferior to that of the uncoordinated [B(HFIP)(4)](-) anion, resulting in the Mg[B(HFIP)(4)](2)/2-MeTHF Coulombic efficiency being lower than that of Mg[B(HFIP)(4)](2)/triglyme. Our results indicate that solvents that could not be used with Mg(TFSA) 2 are suitable in weakly coordinating anion electrolytes, such as Mg[B(HFIP)(4)](2). Controlling the interaction between magnesium ions and anions by selecting suitable anions and solvents is essential for designing new electrolytes for magnesium rechargeable batteries.

To clarify the origin of the polarization of magnesium deposition/dissolution reactions, we combined electrochemical measurement, operando soft X-ray absorption spectroscopy (operando SXAS), Raman, and density functional theory (DFT) techniques to three different electrolytes: magnesium bis(trifluoromethanesulfonyl)amide (Mg(TFSA)2)/triglyme, magnesium borohydride (Mg(BH4)2)/tetrahydrofuran (THF), and Mg(TFSA)2/2-methyltetrahydrofuran (2-MeTHF). Cyclic voltammetry revealed that magnesium deposition/dissolution reactions occur in Mg(TFSA)2/triglyme and Mg(BH4)2/THF, while the reactions do not occur in Mg(TFSA)2/2-MeTHF. Raman spectroscopy shows that the [TFSA]- in the Mg(TFSA)2/triglyme electrolyte largely does not coordinate to the magnesium ions, while all of the [TFSA]- in Mg(TFSA)2/2-MeTHF and [BH4]- in Mg(BH4)2/THF coordinate to the magnesium ions. In operando SXAS measurements, the intermediate, such as the Mg+ ion, was not observed at potentials above the magnesium deposition potential, and the local structure distortion around the magnesium ions increases in all of the electrolytes at the magnesium electrode|electrolyte interface during the cathodic polarization. Our DFT calculation and X-ray photoelectron spectroscopy results indicate that the [TFSA]-, strongly bound to the magnesium ion in the Mg(TFSA)2/2-MeTHF electrolyte, undergoes reduction decomposition easily, instead of deposition of magnesium metal, which makes the electrolyte inactive electrochemically. In the Mg(BH4)2/THF electrolyte, because the [BH4]- coordinated to the magnesium ions is stable even under the potential of the magnesium deposition, the magnesium deposition is not inhibited by the decomposition of [BH4]-. Conversely, because [TFSA]- is weakly bound to the magnesium ion in Mg(TFSA)2/triglyme, the reduction decomposition occurs relatively slowly, which allows the magnesium deposition in the electrolyte.

The high anodic stability of electrolytes for rechargeable magnesium batteries enables the use of new positive electrodes, which can contribute to an increase in energy density. In this study, novel Ph3COMgCl-, Ph3SiOMgCl-, and B(OMgCl)3-based electrolytes were prepared with AlCl3 in triglyme. The Ph3COMgCl-based electrolyte showed anodic stability over 3.0 V vs. Mg but was chemically unstable, whereas the Ph3SiOMgCl-based electrolyte was chemically stable but featured lower anodic stability than the Ph3COMgCl-based electrolyte. Advantageously, the B(OMgCl)3-based electrolyte showed both anodic stability over 3.0 V vs. Mg (possibly due to the Lewis acidic nature of B in B(OMgCl)3) and chemical stability (possibly due to the hard acid character of B(OMgCl)3). B(OMgCl)3, which was prepared by reacting boric acid with a Grignard reagent, was characterized by nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared spectroscopy (FTIR), and X-ray absorption spectroscopy (XAS). The above analyses showed that B(OMgCl)3 has a complex structure featuring coordinated tetrahydrofuran molecules. 27Al NMR spectroscopy and Al K-edge XAS showed that when B(OMgCl)3 was present in the electrolyte, AlCl3 and AlCl2+ species were converted to AlCl4-. Mg K-edge XAS showed that the Mg species in B(OMgCl)3-based electrolytes are electrochemically positive. As a rechargeable magnesium battery, the full cell using the B(OMgCl)3-based electrolyte and a Mo6S8 Chevrel phase cathode showed stable charge-discharge cycles. Thus, B(OMgCl)3-based electrolytes, the anodic stability of which can be increased to ~3 V by the use of appropriate battery materials, are well suited for the development of practical Mg battery cathodes.

The charge/discharge capacity of current lithium-ion battery cathode materials is limited by the charge compensation of transition-metal redox during the charge/discharge processes. Recently, the use of oxide ion redox for charge compensation has been proposed to realize a higher charge/discharge capacity than that observed for transition-metal redox. Different stabilization mechanisms of the reversible oxide ion redox have been proposed. To clarify the mechanism, analysis of the electronic and local structures around oxygen is required. Because of the high-voltage region in which the oxide ion redox occurs, several reactions such as oxygen gas evolution and/or electrolyte oxidation are often included. Thus, operando measurements are required to directly prove this concept and generalize the understanding of the oxide ion redox. This study employs operando soft/hard X-ray absorption spectroscopy combined with X-ray diffraction spectroscopy for four lithium-excess electrode materials with different chemical bond natures. The experimental data together with online analysis of the generated on-charge gas reveal two extreme cases: Significantly enhanced covalent or ionic characters in the metal-oxygen chemical bonds, which are necessary conditions to stabilize the oxidation of the oxide ions. This finding provides new insights with exciting possibilities for designing high energy density cathode materials based on anion redox.

<p>The reaction mechanism of the sulfur cathode in the microporous carbon during discharge was observed by <italic>operando</italic> XAS.</p>

Sulfurized alcohol composite (SAC), a new type of organosulfur material, is applied to an all-solid-state cell with a sulfur-based solid electrolyte (SE), Li3PS4. The cell shows high specific capacity (600-800 mAh g(-1)), and the initial coulomb efficiency is improved by mechanically milling the cathode layer (SAC + SE). The improvement is attributed to the partial lithiation of the SAC sample by milling with SE, which is confirmed by S K-edge X-ray absorption fine structure measurements. Such prelithiation is advantageous for fabricating high-energy-density all-solid-state cells.

Li-rich layered oxides are promising candidates for next-generation cathode materials for Li-ion batteries because of their high capacity. However, a low cycle durability accompanied by structural changes poses a serious problem in their practical use. In this study, we focused on transition metal (TM) ion migration, which induces structural changes, and investigated the mechanism that triggers the degradation of the Li1.16Ni0.19Co0.19Mn0.46O2 structure, using X-ray diffraction spectroscopy (XDS), soft X-ray absorption spectroscopy (S-XAS), and first-principles calculations. XDS revealed that the Mn and Ni ion contents in the Li layer increase after the charge-discharge cycles. S-XAS showed that Mn, Co, and Ni become divalent after the charge-discharge cycles, suggesting that the divalent TM ions are constituents of the rock-salt-type metal monooxides at the surface. First-principles calculations indicated that the potential energy of the structure in which the TM ions exist in the Li layer is lower than that of the structure in which the TM ions exist in the TM layer with oxygen defects at the charging state. It was concluded that the mechanism of structural degradation involved the Mn and Ni ions that migrated to the Li layer during charging. Even during discharging, the migrated TM ions remain and accumulate at the surface, where oxygen defects could be easily formed. The structures in which the TM ions exist in the Li layer are rock-salt-type metal monooxides, which degrade the electrochemical properties by decreasing the potential of discharging. Thus, the oxygen defects are considered to trigger the structural degradation.

Opernado軟X線XAFS測定によりLiNi_⅓Co_⅓Mn_⅓O₂正極の初期充電過程における電荷補償機構が研究された．蛍光収量にて検出された軟X線XAFSスペクトルより，LiNi_⅓Co_⅓Mn_⅓O₂を構成する元素の価数はそれぞれNi²⁺，Co³⁺，Mn⁴⁺，O^2-である．初期充電過程において，Ni²⁺はNi⁴⁺へと酸化し，Mn⁴⁺は変化が見られなかった．O^2-はO K吸収端XAFSスペクトルに新規プリエッジピークが出現することから，O 2pから電子が脱離してホールが生成することで酸化する．Co³⁺はoperando Co L₃吸収端XAFSスペクトルにおいて充電初期から高エネルギー側へエネルギーシフトすることが観測された．LiNi_⅓Co_⅓Mn_⅓O₂電極の初期充電過程においてNiイオンが主に電荷補償を担い，Mnイオンは寄与しない．また，Oイオン，Coイオンもわずかではあるが電荷補償に寄与することが明らかとなった．

For an attempt to improve the cycle capability of the Li8FeS5 cells, we have prepared LiI-doped Li8FeS5 composite positive electrode materials. The obtained Li8FeS5 center dot xLiI sample cells showed the improved cycle capability, though the initial capacity value decreased with proportional to the mass of LiI. The improved cycle performance was attributable to the suppressed resistance rise of the cells, due probably to the suppression of high-resistive surface precipitates formed by the reaction between the active material and the electrolyte. The dopant LiI would stabilize the local structure against Li extraction/insertion reactions, as well as suppress the reaction with the electrolyte, leading to the improved cycle performance. (C) The Author(s) 2018. Published by ECS.

© 2018, The Author(s). The charge-discharge capacity of lithium secondary batteries is dependent on how many lithium ions can be reversibly extracted from (charge) and inserted into (discharge) the electrode active materials. In contrast, large structural changes during charging/discharging are unavoidable for electrode materials with large capacities, and thus there is great demand for developing materials with reversible structures. Herein, we demonstrate a reversible rocksalt to amorphous phase transition involving anion redox in a Li2TiS3 electrode active material with NaCl-type structure. We revealed that the lithium extraction during charging involves a change in site of the sulfur atom and the formation of S−S disulfide bonds, leading to a decrease in the crystallinity. Our results show great promise for the development of long-life lithium insertion/extraction materials, because the structural change clarified here is somewhat similar to that of optical phase-change materials used in DVD-RW discs, which exhibit excellent reversibility of the transition between crystalline and amorphous phase.

© 2018 Elsevier B.V. Among transition metal sulfides, VS4is a promising candidate material for the positive electrode in rechargeable Li/metal-sulfide batteries, due to its long, flat plateau at about 2.0 V and its high theoretical capacity (1195 mAh g−1). In this study, we prepared VS4positive electrode material by heating in a sealed tube, and studied local structural changes during charge/discharge cycles via X-ray absorption and scattering spectroscopies, focusing on the reversibility of VS4. The findings reveal that a VS4-like local structure was formed in the first cycle, and that subsequent cycles showed reversible changes.

Li-rich type manganese oxides are one of the most promising cathodes for lithium-ion batteries in recent years; thanks to their high energy density. In these cathodes, partial substitution of manganese by other transition metals such as nickel and cobalt has been proposed and shown to be effective in improving the performance; however, the role of such metals in the battery performance has not been clarified. We examined Ni-substituted Li2MnO3 as a model of Li2MeO3 solid-solution cathodes to understand the effect of the substituted Ni on the electrode performances by using a combination of resonant X-ray diffraction spectroscopy (RXDS) and operando X-ray absorption spectroscopy. The capacity and cyclability were improved by substituting Ni into the Li2MnO3 phase, which suggests its important roles in the cathodes. The change in the oxidation state and transbilayer migration of the transition metals as a function of the operating potential during the first charge-discharge processes were revealed by the site-selective analysis of RXDS. We discuss the influence of the irreversible and reversible migration of Ni and Mn ions on the electrode performance.

© 2018 Elsevier B.V. The structure of Fe-substituted Li2S-based positive electrode material Li8FeS5was analyzed using high-energy X-ray total scattering measurements. Pair distribution function (PDF) analyses indicated that the mechanically milled Li8FeS5sample could best be described as having an anti-fluorite structure in which Fe ions partially occupy Li sites in the Fm3¯m unit cell. The electrochemical properties of a cell utilizing Li8FeS5as the positive electrode were also consistent with this structural model.

Disulfide, which is a sub-group of organosulfides, has been studied as a promising cathode active material. Herein, to improve battery performance, we inserted disulfide into Cu-based metal organic framework (MOF) as a ligand. As a result, the disulfide inserted MOF exhibited a high capacity based on dual redox reactions of Cu ions and disulfide ligands, and a stable cycle performance. The S K-edge X-ray absorption fine structure analyses revealed a reversible cleavage and formation of S-S bond in the MOF during discharge and charge process.

An electrochemical cell was developed for operando soft X-ray absorption spectroscopic (XAS) study of an all-solid-state lithium-ion battery. Operando XAS experiments were performed for a battery using LiMn2O4 as a cathode material and a NASICON-type lithium conductive glass ceramic sheet (LICGC) as a solid electrolyte. O K-edge, Mn L-edge and Ti L-edge XAS spectra were taken during the charging process up to 2.2V. Detailed analysis of the XAS spectra revealed that the valence change from Mn3+ to Mn4+ occurred during charge with simultaneous change in the spectrum of O K-pre-edge region. Ti L-edge spectra revealed a partial change from Ti4+ to Ti3+ in the LICGC at the anode side, indicating a rather dispersed anode formation. (c) The Electrochemical Society of Japan, All rights reserved.

Clarification of the interaction between the electrode and the electrolyte is crucial for further improvement of the performance of lithium-ion batteries. We have investigated the structural change at the interface between the surface of a 104-oriented epitaxial thin film of LiCoO2 (LiCoO2(104)), which is one of the stable surfaces of LiCoO2, and an electrolyte prepared using a carbonate solvent (1 M LiClO4 in ethylene carbonate and dimethyl carbonate) by in situ neutron reflectivity measurements. Owing to the decomposition of the organic solvent, a new interface layer was formed after contact of LiCoO2(104) with the electrolyte. The composition and thickness of the interface layer changed during Li+ extraction/insertion. During Li+ extraction, the thickness of the interface layer increased and the addition of an inorganic species is suggested. The thickness of the interface layer decreased during Li+ insertion. We discuss the relationship between battery performance and the dynamic behavior at the interface.

© 2015 Elsevier B.V. All rights reserved. For an attempt to incorporate phosphorous ions into the Fe-containing Li2S, we have prepared Li2S-FePS3composite positive electrode materials using the combination process of the thermal heating and the mechanical milling. The XRD results showed that the Li2S-FePS3composite samples consisted of low-crystalline Li2S and small amounts of FeP as impurity. The electrochemical tests demonstrated that the Li2S-rich composite sample cells (Li2S:FePS3= 4:1 mol) showed relatively high initial discharge capacity (ca. 780 mAh·g- 1) without any pre-cycling treatments. This makes a clear contrast to the Fe-containing Li2S (Li2S-FeSxcomposite) sample cells, which showed the initial discharge capacity of ca. 330 mAh·g- 1and it was enlarged to ca. 730 mAh·g-1after the stepwise pre-cycling treatment. Ex-situ XRD and XAFS measurements showed the reversible changes of the peaks ascribed to Li2S and the local structures around S atoms during cycling. The incorporation of phosphorous ions into the Fe-containing Li2S was effective for improving the structural reversibility against Li extraction/insertion reactions, resulting in the improved electrochemical performance of the cells.

The oxidation/reduction behaviours of lattice oxygen and transition metals in a Li-rich manganese-based layered oxide Li[Li0.25Ni0.20Mn0.55]O-1.93 are investigated by using hard X-ray photoelectron spectroscopy (HAX-PES). By making use of its deeper probing depth rather than in-house XPS analyses, we clearly confirm the formation of O- ions as bulk oxygen species in the active material. They are formed on the 1st charging process as a charge compensation mechanism for delithiation and decrease on discharging. In particular, the cation-anion dual charge compensation involving Ni and O ions is suggested during the voltage slope region of the charging process. The Ni ions in the material are considered to increase the capacity delivered by a reversible anion redox reaction with the suppression of O-2 gas release. On the other hand, we found structural deterioration in the cycled material. The O- species are still observed but are electrochemically inactive during the 5th charge-discharge cycle. Also, the oxidation state of Ni ions is divalent and inactive, although that of Mn ions changes reversibly. We believe that this is associated with the structural rearrangement occurring after the activation process during the 1st charging, leading to the formation of spinel-or rocksalt-like domains over the sub-surface region of the particles.