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.

Visualization of Structural Heterogeneities in Particles of Lithium Nickel Manganese Oxide Cathode Materials by Ptychographic X-ray Absorption Fine Structure.

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.

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.

Preparation of bioplastic using soy protein.

Soybean, one of the most abundant plants, has been cultivated around the world as a familiar crop. Especially, most of the soybean is globally used as a crop to obtain the oil. The degreased soybean contains a lot of protein in it. The part of the degreased soybean is used for the food of human consumption and livestock feed, however most of this are discarded as industrial waste throughout the world. Therefore, we demonstrated the preparation of bioplastics consisting of soy protein. Although the soy protein without the cross-linking reaction by formaldehyde (HCHO) was collapsed in water, bioplastics were stable in water. Additionally, the bending strength of the bioplastic increased with the HCHO concentration and showed the maximum value of approximately 35 MPa at a 1% HCHO concentration. Surprisingly, this bending strength value was the same as that of polyethylene. In contrast, the infrared spectra indicated the formation of methylene cross-linking between the basic amino acids, such as lysine and arginine. Finally, we estimated the biodegradable property of the bioplastic by pronase, one of the proteolytic enzymes. As a result, this bioplastic showed the weight loss of approximately 30% after the incubation time of 6 days. These results suggested that the bioplastic consisting of soy protein possesses a biodegradable property. Therefore, the bioplastic consisting of soybean may have the potential to be used as a biodegradable material, such as agricultural materials, industrial parts, and disposable items.

Nanostructured liquid-crystalline Li-ion conductors with high oxidation resistance: molecular design strategy towards safe and high-voltage-operation Li-ion batteries.

Nanostructured, uncharged liquid-crystalline (LC) electrolyte molecules having bicyclohexyl and cyclic carbonate moieties have been developed for application in Li-ion batteries as quasi-solid electrolytes, which suppress leakage and combustion. Towards the development of safe and high performance Li-ion batteries, we have designed Li-ion conductive LC materials with high oxidation resistance using density functional theory (DFT) calculation. The DFT calculation suggests that a mesogen with a bicyclohexyl moiety is suitable for the high-oxidation-resistance LC electrolytes compared to a mesogen containing phenylene moieties. A tri(oxyethylene) chain introduced between the cyclic carbonate and the bicyclohexyl moiety in the core part tunes the viscosity and the miscibility with Li salts. The designed Li-ion conductive LC molecules exhibit smectic LC phases over a wide temperature range, and they are miscible with added lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt up to 5 : 5 in molar ratio in their smectic phases. The resulting LC mixtures with LiTFSI show oxidation resistance above 4.0 V vs. Li/Li+ in cyclic voltammetry measurements. The enhanced oxidation resistance improves the performance of Li half-cells containing LC electrolytes.

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.

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.

Noncovalent Approach to Liquid-Crystalline Ion Conductors: High-Rate Performances and Room-Temperature Operation for Li-Ion Batteries.

We report advanced liquid-crystalline (LC) electrolytes for use in lithium-ion batteries (LIBs). We evaluated the potential of LC electrolytes with a half cell composed of Li metal and LiFePO4 which is a conventional positive electrode for LIBs. Low-molecular-weight carbonates of ethylene carbonate or propylene carbonate were incorporated into the two-dimensional (2D) nanostructured electrolyte composed of mesogen-containing carbonate and lithium bis(trifluoromethylsulfonyl)imide. The incorporation of low-molecular-weight carbonates increased the ionic conductivity with maintaining 2D nanostructures in the LC state. High-power performances at relatively high current densities induced by higher ionic conductivities have been achieved by LC electrolytes with low-molecular-weight carbonates. Furthermore, room-temperature operation of LIBs using LC electrolytes is reported for the first time. In the research field of electrolytes for LIBs, we demonstrate the progress of a new category of LC electrolytes.

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.

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

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

Phase transitions in a LiMn2O4 nanowire battery observed by operando electron microscopy.

Fast charge-discharge process has been reported to give a high capacity loss. A nanobattery consisting of a single LiMn2O4 nanowire cathode, ionic liquid electrolyte and lithium titanium oxide anode was developed for in situ transmission electron microscopy. When it was fully charged or discharged within a range of 4 V in less than half an hour (corresponding average C rate: 2.5C), Li-rich and Li-poor phases were observed to be separated by a transition region, and coexisted during whole process. The phase transition region moved reversibly along the nanowire axis which corresponds to the [011] direction, allowing the volume fraction of both phases to change. In the electron diffraction patterns, the Li-rich phase was seen to have the (100) orientation with respect to the incident electron beam, while the Li-poor phase had the (111̅) orientation. The orientation was changed as the transition region moved. However, the nanowire did not fracture. This suggests that a LiMn2O4 nanowire has the advantage of preventing capacity fading at high charge rates.

Assembly of Na3V2(PO4)3 nanoparticles confined in a one-dimensional carbon sheath for enhanced sodium-ion cathode properties.

Structural and morphological control is an effective approach for improvement of electrochemical properties in rechargeable batteries. One-dimensionally assembled structure composed of NASICON-type Na3 V2 (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 Na3 V2 (PO4 )3 within a carbon sheath. The mesostructure comprising NASICON-type Na3 V2 (PO4 )3 and carbon give a short sodium-ion transport pass and an efficient electron conduction pass. Electrochemical properties of NASICON-type Na3 V2 (PO4 )3 are improved on the basis of one-dimensional nanostructures designed in the present study.

Fabrication of porous cubic architecture of ZnO using Zn-terephthalate MOFs with characteristic microstructures.

A method for synthesizing porous cubic-shaped ZnO particles a few tens of micrometers in size is described on the basis of a pyrolytic conversion of Zn-terephthalate metal-organic frameworks (MOFs). MOF crystals were initially grown in solutions containing Zn(NO3)2·6H2O and terephthalic acid as solutes and N,N-dimethylformamide (DMF) or N,N-diethylformamide (DEF) as a solvent under a solvothermal condition. It was the key to controlling the microstructure of MOF cuboids for their use as an intermediate compound for ZnO. Actually, many cracks were formed and hence the cubic microstructure was somewhat destroyed in the pyrolytic conversion from dense MOF crystals (grown in the DMF solution) to ZnO. In contrast, mesocrystal-like MOF cuboids (grown in the DEF solution) could maintain their shape during the pyrolysis because of the relaxation against a MOF-to-ZnO volume change. The resultant ZnO with a highly porous cubic structure showed intense visible photoluminescence upon irradiation with ultraviolet light.

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.

Formation of nanostructured MnO/Co/solid-electrolyte interphase ternary composites as a durable anode material for lithium-ion batteries.

Nanoporous MnO frameworks with highly dispersed Co nanoparticles were produced from MnCO3 precursors prepared in a gel matrix. The MnO frameworks that contain 20 mol% Co exhibited excellent cycle performance as an anode material for Li-ion batteries. The solid-electrolyte interphase (SEI) formed in the frameworks through the electrochemical reaction mediates the active materials, such as MnO, Mn, and Li2O, during the conversion reaction in the charge-discharge cycle. The Co nanoparticles and SEI provide the electron and Li-ion conductive networks, respectively. The ternary nanocomposites of the MnO framework, metallic Co nanoparticles, and embedded SEI are categorized as durable anode materials for Li-ion batteries.