Hajime Tanida

Direct Observation of a Metastable Crystal Phase of LixFePO4 under Electrochemical Phase Transition

The phase transition between LiFePO4 and FePO4 during nonequilibrium battery operation was tracked in real time using time-resolved X-ray diffraction. In conjunction with increasing current density, a metastable crystal phase appears in addition to the thermodynamically stable LiFePO4 and FePO4 phases. The metastable phase gradually diminishes under open-circuit conditions following electrochemical cycling. We propose a phase transition path that passes through the metastable phase and posit the new phases role in decreasing the nucleation energy, accounting for the excellent rate capability of LiFePO4. This study is the first to report the measurement of a metastable crystal phase during the electrochemical phase transition of LixFePO4.

First In Situ Observation of the LiCoO2 Electrode/Electrolyte Interface by Total-Reflection X-ray Absorption Spectroscopy†

Rechargeable lithium-ion batteries (LIBs) are widely used as electrical energy storage devices for technologies such as portable electronics and electric and hybrid vehicles, and they are considered to be serious power storage candidates for smart-grid electricity systems. Traditionally, research in the field has focused on battery improvement through a selective use of new or existing materials for positive and negative electrodes, as the bulk properties of electrodes primarily limit charge capacity and power. However, the durability of LIBs is largely rooted in side reactions that occur at the electrode/ electrolyte interface, especially those at the positive electrode. Thus, controlling the chemical stability of any electrode material with respect to the operating liquid electrolyte medium, which requires a control of the electrode/electrolyte interface through surface chemistry, is as important as designing new materials. The scale of such an interfacial region is speculated to be on the order of a few nanometers, which shall be deemed as approximately the Debye length. This scale indicates that structural and chemical information should be tracked with a resolution of a few nanometers to reveal the phenomena of the electrode/ electrolyte interface. Previous research has focused on a detailed examination of the interfacial reactions at the positive electrode surface by using methods such as X-ray photoelectron spectroscopy (XPS) and surface X-ray diffraction (SXRD). However, characterization of the electrode surface at the nanoscale under conditions of an operating battery remains insufficient because of the lack of suitable observation techniques. A proposed degradation mechanism for electrodes has been extrapolated from indirect information obtained from analysis of disassembled, deteriorated electrodes. To obtain concise and meaningful surface data, a technique that enables high-resolution analysis of chemical information at the solid electrode surface is required. X-ray absorption spectroscopy (XAS), which makes it possible to identify the electronic and local structures of a certain atom, is a potent and versatile technique to resolve the chemical states of a lithium-ion electrode material independently of its crystallinity. To extract information about the interfacial phenomena by XAS, total-reflection fluorescence XAS (TRF-XAS), which integrates the fluorescence yield obtained under total reflection, can be applied. A recent study has shown that polycrystalline thin films are preferred relative to epitaxial thin films (that are strongly influenced by the substrate) to simulate the conditions of applied composite electrodes. We herein use polycrystalline LiCoO2 thin films prepared by pulsed laser deposition (PLD) as the model electrodes; these electrodes are flat at the nanoscale and have structural properties similar to those of the applied composite electrode (see section S1 in the Supporting Information). Figure 1 shows the charge/discharge cycle dependencies of cyclic voltammograms (CVs) and electrochemical impedance spectra (EIS) of the LiCoO2 thin films used in this study (see section S1 in the Supporting Information). Typical CVs

RISING beamline (BL28XU) for rechargeable battery analysis

The BL28XU beamline, dedicated to rechargeable battery analysis, is described.

X-ray absorption spectroscopy of diluted system by undulator photon source and multi-element solid-state detector

Abstract In order to measure the extended X-ray absorption fine structure (EXAFS) spectrum of an ultra-diluted system, an optics and detector control system for a synchrotron radiation beamline is developed. The undulator gap width is continuously tuned to obtain the maximum X-ray photon flux during the energy scan for the EXAFS measurement. A piezoelectric translator optimizes the parallelism of the double crystal in a monochromator at each measurement point to compensate for mechanical errors of the monochromator, resulting in a smooth and intense X-ray photon flux during the measurement. For a detection of a weak fluorescence signal from diluted samples, a 19-element solid-state detector and digital signal processor are used. A K-edge EXAFS spectrum of iron in a myoglobin aqueous solution with a concentration of 5.58 parts per million was obtained by this system.

In situ two-dimensional imaging quick-scanning XAFS with pixel array detector

Two-dimensional imaging quick-scanning XAFS measurements were performed using a pixel array detector.

Oxidation behaviour of lattice oxygen in Li-rich manganese-based layered oxide studied by hard X-ray photoelectron spectroscopy

The oxidation/reduction behaviours of lattice oxygen and transition metals in a Li-rich manganese-based layered oxide Li[Li0.25Ni0.20Mn0.55]O1.93 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 O2 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.

Phase transition kinetics of LiNi0.5Mn1.5O4 analyzed by temperature-controlled operando X-ray absorption spectroscopy

LiNi0.5Mn1.5O4 (LNMO) is a promising positive electrode material for lithium ion batteries because it shows a high potential of 4.7 V vs. Li/Li(+). Its charge-discharge reaction includes two consecutive phase transitions between LiNi0.5Mn1.5O4 (Li1) ↔ Li0.5Ni0.5Mn1.5O4 (Li0.5) and Li0.5 ↔ Ni0.5Mn1.5O4 (Li0) and the complex transition kinetics that governs the rate capability of LNMO can hardly be analyzed by simple electrochemical techniques. Herein, we apply temperature-controlled operando X-ray absorption spectroscopy to directly capture the reacting phases from -20 °C to 40 °C under potential step (chronoamperometric) conditions and evaluate the phase transition kinetics using the apparent first-order rate constants at various temperatures. The constant for the Li1 ↔ Li0.5 transition (process 1) is larger than that for the Li0.5 ↔ Li0 transition (process 2) at all the measured temperatures, and the corresponding activation energies are 29 and 46 kJ mol(-1) for processes 1 and 2, respectively. The results obtained are discussed to elucidate the limiting factor in this system as well as in other electrode systems.

In situ two-dimensional micro-imaging XAFS with CCD detector

In situ two-dimensional (2D) micro-imaging X-ray absorption fine structure (XAFS) measurements were performed in transmission mode using a charge coupled device (CCD) detector, phosphor screen, and magnifying lens. This method makes it possible to display a 2D image with a spatial resolution of around 2 μm at each energy point in a XAFS spectrum. The method was applied to in situ transmission micro-imaging XAFS measurement with a quick scanning technique.

Standard X-ray mirror systems for SPring-8 beamlines

Abstract More than 20 X-ray mirror systems have been constructed in the SPring-8 beamlines since 1997. To simplify the design and reduce the construction, alignment, and maintenance time, the X-ray mirror systems have been standardized as far as possible. An overview of the design, alignment method, and performance of the standard X-ray mirror systems in SPring-8 is given here.

New configuration of photoconductive‐type diamond detector head for X‐ray beam position monitors

We designed and fabricated new diamond detector head for an X‐ray beam position monitor (XBPM). This monitor operates in photoconductive mode, and is shaped into a blade in order to reduce heat load. A pair of aluminum electrodes is formed on both sides of the diamond blade. The profile of the detection efficiency inside the diamond detector head was measured. The signal current is generated only between the pair of electrodes. The bias voltage dependence of signal current along a section of the detector head is also measured. The results show that the detector head operates in photoconductive mode. We demonstrated that this detector head is feasible for the XBPM.