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Owing to their high theoretical capacity, metal fluorides have attracted significant interest as materials for fabricating the cathode of lithium secondary batteries. In the present study, a nanocomposite of LiF and FeF2 is prepared by a fluorolytic sol–gel method in an ethanol solution, for use as the cathode material of a lithium secondary battery. The produced LiF/FeF2 composite is characterized by broad X-ray diffraction peaks attributed to the nanosized (∼10 nm) LiF and FeF2 crystals, a large Brunauer–Emmett–Teller surface area of 119 m2 g-1, and adsorption–desorption hysteresis, attributed to the presence of mesopores. The results of charge–discharge tests indicates an initial discharge capacity of 225 mAh (g-LiF/FeF2)−1 through reversal conversion at a current rate of 10 mA (g-LiF/FeF2)−1. Based on a combination of galvanostatic intermittent titration, X-ray absorption, and X-ray diffraction investigations, a new reaction mechanism is developed, namely, the conversion of the local environment of an Fe atom from a rutile-type FeF2 structure to a rhenium trioxide-type FeF3 structure during charging, with the subsequent discharge resulting in the insertion of Li+ into the rhenium trioxide-type FeF3 structure, followed by the conversion reaction to LiF and FeF2.

Inhomogeneous reaction distributions have been observed in composite electrodes of active materials with poor electrical conductivity. Localized electronic pathways in the electrode are one cause of these in-plane reaction distributions. The influence of the carbon content and surface modification of the current collector on the inhomogeneous reaction was examined for the LiFePO4 cathodes of a lithium-ion battery. The temperature and rate dependences of the reaction distribution were investigated at 300, 313, and 333 K and discharge rates of 0.2 and 2.0C by in situ X-ray absorption near edge structure imaging for test cells containing electrolyte solutions with concentrations of 0.1, 0.5, and 1.0 M. The chemical state maps and histograms demonstrated that a higher discharge rate and higher temperature promoted the inhomogeneous reaction at the LiFePO4 cathode. It was revealed that the reaction spots sharpened owing to IR drop in the reaction channels at the higher discharge rate, while the limited ionic diffusion affected the in-plane inhomogeneous reaction at room temperature and lower electrolyte concentrations.

The abundance of sodium resources has sparked interest in the development of sodium-ion batteries for large-scale energy storage systems, amplifying the need for high-performance negative electrodes. Although transition metal phosphide electrodes have shown remarkable performance and great versatility for both lithium and sodium batteries, their electrochemical mechanisms in sodium batteries, particularly vanadium phosphides, remain largely elusive. Herein, we delineate the performance of VP2 as a negative electrode alongside ionic liquids in sodium-ion batteries. The polycrystalline VP2 is synthesized via one-step high energy ball-milling and characterized using X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Electrochemical tests ascertained improved performance at intermediate temperatures, where the initial cycle was conducted at 100 mA g−1 yielded a significantly higher discharge capacity of 243 mAh g−1 at 90 °C compared to the limited capacity of 49 mAh g−1 at 25 °C. Enhanced rate and cycle performance are also achieved at 90 °C. Electrochemical impedance spectroscopy and scanning electron microscopy further reveal a reduced charge transfer resistance at 90 °C and the formation of a uniform and stable solid electrolyte interface (SEI) layer after cycling. X-ray diffraction and nuclear magnetic resonance spectroscopy are used to confirm the conversion-based mechanism forming Na3P after charging.

It has been found that carbonaceous materials containing a transition metal coordinated by 4 nitrogens in the square-planar configuration (metal–N4 site) on the surface possessed a catalytic activity for various electrochemical reactions related to energy conversion and storage; i.e., oxygen reduction, hydrogen evolution, and quite recently, the electrode reactions in vanadium redox flow batteries (VRFB). The catalyst for the VRFB positive electrode discharge reaction, i.e., the dioxovanadium ion reduction, was formed by coating the surface of cup-stack carbon nanotubes with a carbonaceous thin film with the Fe–N4 site generated by the sublimation, deposition, and pyrolysis of iron phthalocyanine. In this study, the influence of the physical properties of the catalyst on the electrochemical reactions was investigated to optimize the coating. With an increase in the coating, the specific surface area increased, whereas the pore size decreased. The surface Fe concentration was increased in spite of the Fe aggregation inside the carbon matrix. The catalytic activity enhancement was achieved due to the increase in the specific surface area and the surface Fe concentration, but was lowered due to the decrease in the pore size, which was disadvantageous for the penetration of the electrolyte and the mass transfer.

Electronic structure changes of sulfurs in amorphous TiS3 and MoS3 for positive electrodes of all-solid-state lithium batteries are examined by X-ray photoelectron spectroscopy (XPS) and the X-ray absorption near edge structure (XANES). The all-solid-state cell with amorphous TiS3 electrode shows the reversible capacity of about 510 mAh g−1 for 10 cycles with sulfur-redox in amorphous TiS3 during charge-discharge process. On the other hand, the cell with amorphous MoS3 shows the 1st reversible capacity of about 720 mAh g−1. The obtained capacity is based on the redox of both sulfur and molybdenum in amorphous MoS3. The irreversible capacity of about 50 mAh g−1 is observed at the 1st cycle, which is attributed to the irreversible electronic structure change of sulfur during the 1st cycle. The electronic structure of sulfur in amorphous MoS3 after the 10th charge is similar to that after the 1st charge. Therefore, the all-solid-state cell with amorphous MoS3 electrode shows relatively good cyclability after the 1st cycle.

The aluminum rechargeable battery is a desirable device for large-scale energy storage owing to the high capacity derived from the properties of the aluminum metal anode. The development of cathode materials is needed to compose battery systems. However, the design principles of the cathode materials have not been determined. We focus on the high capacity FeS2 cathode materials and investigate the discharge/charge reaction mechanisms in chloroaluminate ionic liquids as the electrolyte at 55°C. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) measurements are performed for the discharged and charged samples. S 3p-orbitals are shown to play an important role in the redox reactions from the results of the S and Fe K-edge XANES spectra. As a result of the redox reaction, FeS2 is transformed into low crystalline FeS and amorphous Al2S3, as shown by the XRD and S, Al, and Fe K-edge XANES spectra. This reaction mechanism is different from the reaction observed with lithium ion.

LiFePO 4 is a practically used cathode material for lithium-ion batteries due to a high theoretical capacity, high cycle capability and the high-rate performance. The metastable Li x FePO 4 (L x FP) phase with an intermediate composition appears in the non-equilibrium state at high-rate condition. However, the formation process of the metastable L x FP phase and its impact to the electrochemical property are still unclear. In order to elucidate these points, we directly observed the phase transition behavior by applying operando XRD during 10C charge-discharge. L x FP phase does not form in charge reaction but preferentially forms in discharge reaction. The phase transition from L x FP to Li-rich phase is less likely to proceed in the end of discharge reaction. The asymmetric phase transition between LiFePO 4 and FePO 4 results in decreasing the discharge capacity and increasing the irreversible capacity at high-rate conditions.

The reversible redox reaction of oxygen ion was directly observed by soft X-ray absorption spectroscopy for Li1.16Ni0.15Co0.19Mn0.50O2, the Li-rich manganese layered oxide. The O K-edge spectra indicated that a peroxide phase (namely the formation of O22−) appeared and disappeared reversibly by charging and discharging. It was simultaneously observed both in the total electron yield mode and in the partial fluorescence X-ray yield mode, indicating that the peroxide phase exists stably in the crystalline network. These species should play an important role in the high capacity positive electrode.

The electrochemical performance of a Na 2 FeP 2 O 7 positive electrode has been evaluated in an inorganic ionic liquid NaFSA–KFSA (FSA = bis(fluorosulfonyl)amide) at 363 K. The electrode delivers a reversible capacity of 91 mAh g −1 with excellent rate capability (59 mAh g −1 at 2000 mA g −1 ) and a capacity retention of 91% over 1000 cycles, which facilitates the development of low-cost and high-safety sodium secondary batteries for large-scale energy storage applications. The average oxidation state of iron increases upon sodium extraction, as evidenced by the edge shift of an X-ray absorption spectroscopy analysis. According to an extended X-ray absorption fine structure analysis, the sodium extraction is accompanied by a shortening of Fe–O bonds.

Phenomena at electrode/electrolyte interface of LiNi 0.5 Mn 1.5 O 4 are studied by in situ total-reflection fluorescence X-ray absorption spectroscopy (TRF-XAS), ex situ X-ray photoelectron spectroscopy (XPS), and electrochemical tests. Flat and well-defined thin films of LiNi 0.5 Mn 1.5 O 4 prepared by pulsed laser deposition (PLD) are used as model electrodes to facilitate the observation of the interface. The thin-film LiNi 0.5 Mn 1.5 O 4 electrode showed good cycling characteristics at around 4.7 V vs. Li/Li + . The TRF-XAS measurements reveal that nickel and manganese species at the surface have almost the same chemical states and local environments as those in the bulk when in contact with organic electrolyte solutions (1 mol dm −3 LiClO 4 in a 1:1 volumetric mixture of ethylene carbonate and diethyl carbonate). This is in sharp contrast to the behavior of a LiCoO 2 electrode, in which the surface cobalt species is irreversibly reduced by soaking to the organic electrolyte solutions, leading to gradual material deterioration during the delithiation/lithiation cycling (D. Takamatsu et al., Angew. Chem. Int. Edit., 51 (2012) 11597). It is suggested that the electrolyte decomposition products detected by XPS form a protective layer to restrict the reduction of the surface species of LiNi 0.5 Mn 1.5 O 4 , leading to good cycling characteristics of LiNi 0.5 Mn 1.5 O 4 in spite of its high operating potential.

The redox reaction of Li1.16Ni0.15Co0.19Mn0.50O2 positive electrode material during the charging and discharging processes was investigated by using spectroscopic methods, i.e. in situ hard X-ray absorption near edge structure (XANES) at transition metal K-edges and ex situ soft XANES at oxygen K- and transition metal L-edges. The spectral changes of constituent elements during the initial charging to 4.5 V vs. Li/Li+ are quite similar to those of conventional layer-structured positive materials, such as LiNi1/3Mn1/3Co1/3O2. Ni2+ and Co3+ ions are fully oxidized to Ni4+ and Co4+, while Mn4+ remains unchanged. Ligand oxygen ions also take part in charge compensation. In the process of charging to 4.8 V via the plateau voltage region, no significant spectral change appears except partial reduction of Ni and Co ions in spite of lithium extraction. By discharging to 2.0 V the spectra of each element return to those of the pristine material.

Solid electrolyte interface (SEI) films formed on Li1−xCoO2 electrodes were observed with hard X-ray photoelectron spectroscopy (HX-PES). This paper particularly focuses on film thickness estimation using HX-PES with theoretical calculation. The validity of the calculation was proven by experiments using model SEI films. The native film formed on a LiCoO2 composite electrode was estimated to be LiF with its thickness of 5 nm. Formation of Co (II) species on top of LiCoO2 was also indicated. Storage of the electrode at 60 °C brought about considerable film growth (30–40 nm) with carbonate compounds formation. SEI film changes during charging of the LiCoO2 electrode were also examined. The main component in the film was deduced to be LiF or a kind of fluorite, with its thickness decreased during charging. The SEI formation mechanisms are also elucidated.

The performance of a Li-ion battery (LIB) is mainly governed by the diffusion capabilities of lithium in the electrodes. Thus, for LIB improvement it is essential to characterize the lithium distribution. Most of the traditionally used techniques for lithium characterization give information about the local scale or in the macroscopic scale. However, the lithium behavior at the local scale is not mirrored at the macroscopic scale. Therefore, the lithium characterization in the mesoscopic scale would be of help to understand and to connect the mechanisms taking place in the two spatial scales. In this paper, we show a general description of the capabilities and limitations of ion beam analysis techniques to study the distributions of lithium and other elements present in the electrodes in the mesoscopic scale. The potential of the 7Li(p,α0)4He nuclear reaction to non-invasively examine the lithium distribution as a function of depth is illustrated. The lithium spatial distribution is characterized using particle induced γ-ray (μ-PIGE) spectroscopy. This technique allows estimating the density of the active particles in the electrode effectively contributing to the Li intercalation and/or de-intercalation. The advantages of the use of ion beam analysis techniques in comparison to more traditional techniques for electrode characterization are discussed.