Batteries are a key technology in today’s society. Since the “lithium-ion” configuration has been widely accepted, significant efforts have been devoted to attain high energy and power densities to produce an excellent energy storage system without any safety issue. To improve the reliability and power characteristics of batteries, deep insights into the reactions at the electrode/electrolyte interface are necessary. It is rather difficult to detect the reaction at the electrochemical interface using polycrystalline electrode materials, which are used for practical battery systems. The model systems with epitaxial thin-film electrodes might be suitable for understanding these reactions, because the film electrodes provide flat surface, which makes easier to detect the reactions by in situ scattering techniques. The in situ techniques for directly observing surface structural changes of the electrodes have been developed for surface X-ray scattering and neutron reflectivity techniques. These techniques are reviewed and future studies on the interfacial reaction in batteries will be discussed.
In situ X-ray absorption spectroscopy (XAS) at transition-metal K-edges has been most widely used to analyze the redox-reaction during battery operations by monitoring valence change that is determined by absorption edge shift. However, current analysis of XAS spectrum is simply to compare the absorption edge position with those of reference materials, and thus, we could not always monitor true valence changes of the materials. To overcome such issue, we newly proposed to apply combined XAS analysis method of K-edge X-ray absorption spectroscopy and first principals calculation.
We developed operando soft X-ray emission spectroscopy (XES) techniques to accurately investigate the redox reactions and electronic structures of cathode materials of lithium-ion batteries. The operando cell consists of a special electrode tip with Si3N4 membrane window, Li-metal counter electrode and electrolyte solution. Operando Mn 2p XES for LiMn2O4 thin film with electrochemical charge/discharge reactions was performed using ultrahigh-resolution XES spectrometer at BL07LSU of SPring-8. The XES spectra showed a reversible Mn 3d electronic-structure change during the charge-discharge process. A redox reaction of Mn3+ ⇔ Mn4+ by charge/discharge was revealed. We also found a strong charge-transfer effect between the Mn 3d and O 2p orbitals for the Mn4+ state. The change of the charge-transfer effect between the Mn3+ and Mn4+ states suggests the Mn-O bond strength should change during charge-discharge process, which would be related to degradation of the cycle performance.
In order to design electro-catalytic system with high activity, we should know the structures at electrode/electrolyte interfaces in situ with both high spatial and time resolutions. Here, we introduced in situ in real time investigation of potential induced structure and its change at Au(111) and Au(100) single crystal electrodes/sulfuric acid solution interfaces by surface X-ray scattering (SXS).
We have developed direct liquid fuel anion exchange membrane fuel cell vehicles to deal with the global warming. Non-platinum group metals (PGM) catalyst has been researched to apply for both anode and cathode electrodes. A test driving was carried out for the fuel cell vehicle equipped with no precious metals as catalysts at SPring-8 in 2013. Here we introduce our results of advanced analysis for reaction mechanism and active site of non-PGM catalyst using synchrotron radiation X-rays at SPring-8.
Solid oxide fuel cell (SOFC), which is one type of fuel cells having a potential of high energy conversion efficiency, has been commercialized, for instance since 2011 in Japan. For further wide-spread commercialization of SOFC, improvements of the cell performance are highly required. Since electrode performance often limits the cell performance, it is important to improve the electrode kinetics based on the knowledge of physical/chemical states of the electrodes. However, SOFC operates at high temperature under specific atmospheric condition. Thus it is not so easy to directly observe the states of the electrodes under operation. In this paper, our experimental trials of direct observation of SOFC electrodes by means of X-ray absorption fine structure (XAFS) are reviewed.