Chemical reactions in batteries, fuel cells, and electrolysis occur at electrode interfaces. Although in situ methods for analyzing electrode interfaces are limited compared to surface analysis under vacuum conditions, new techniques and improvements have been contributing to discoveries and elucidation of mechanisms of electrochemical reactions. In this special issue, we will introduce the latest analytical methods for solid-liquid and solid-solid interfaces.

The local imaging technique, which is able to directly visualize the catalytic activity originating from the structural heterogeneity of the catalyst, is significantly required to provide the guidelines for the development of the new catalyst with an ability of efficient activity. Scanning electrochemical cell microscopy (SECCM) is one of the promising candidates because of the ability to obtain the distribution of redox activity on the sub-microscale by forming the meniscus on the sample surface using a nanopipette. We have so far developed single-barrel SECCM to further improve its spatial resolution. In addition, we have applied this technique to various catalysts, for instance, MoS₂ and SnS₂ nanosheets and TiO₂ nanotube. Here, we present the principle and setup of single-barrel SECCM and its applications to the chemistry of catalysis.

Shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), one of the application modes of surface-enhanced Raman spectroscopy (SERS), shows wide applicability in the materials and morphology of substrate being examined, and thus, is a powerful analytic technique that allows in situ/operando measurements at a solid-liquid interface, acquiring intrinsic vibrational spectra (fingerprint information) of materials at a molecular scale. This article provides an overview of the development of Raman spectroscopy suitable for in situ/operando interfacial analysis and introduces some recent technical developments of the amplifiers in SHINERS techniques.

The solid-liquid interface forms an electric double layer that enables the functioning of electronic devices and, thus, represents an important area of electrochemical research. As ionic liquids are becoming prominent candidates for new high-performing electrolytes, their interface with solid substrates (e.g., metal electrodes or organic semiconductors) attracts substantial attention. To investigate the buried interface between ionic liquids and substrates, attenuated total reflectance ultraviolet-visible (ATR-UV-Vis) spectroscopy is a powerful method due to its short penetration depth and strong absorbance of materials in the UV and visible regions. Additionally, a newly developed electrochemical setup combined with ATR-UV-Vis (Electrochemical ATR-UV-Vis) has allowed the analysis of the interfacial area under the application of varying electric potential. The application of this new method to ionic liquid/organic semiconductor interfaces achieved simultaneous analysis of the organic semiconductor film and interfacial ionic liquids during transistor operation. In response to the applied gate voltage, the spectral peaks of the organic semiconductor shifted and bleached, correlating with the drain current. The potential dependence of ATR spectra of the ionic liquids on the metal electrode surface was also detected.

Synchrotron radiation plays a crucial role in the analysis of valence states which play an important role in the function of materials. In this paper, the development and application of the analysis using synchrotron-radiation-based soft X-rays are outlined, and the research expected in the next generation synchrotron radiation NanoTerasu is commented.

All-solid-state batteries (ASSBs) with solid electrolytes are expected to be promising energy storage devices. However, the large charge-transfer resistance at the electrode/electrode homo-interface and/or electrode/solid-electrolyte hetero-interface needs to be solved for practical applications. To overcome this problem, it is necessary to understand how the ions are transferred during battery operation. Here, we used operando electron energy-loss spectroscopy (EELS) using a scanning transmission electron microscope (STEM) to directly visualize the Li-ion movement in the cathode materials in bulk-type ASSBs. We show that the ion movement is strongly related to the crystal orientations of the cathode particles, and that Li-ions move even in the open-circuit conditions of the batteries.

Elemental analysis with muonic x-rays is one of the analytical methods to identify elements non-destructively. Since the energy of the muonic x-rays is almost 200 times higher than one of the fluorescent x-rays, light elements such as lithium can be identified. We have applied this technique to detection of metallic lithium deposition in a Li-ion battery, which prevents from promoting the recovery and reuse of used batteries. Since the capture ratio of muon by lithium in metallic lithium is enormously higher than one in anode, it is possible to detect metallic lithium with high sensitivity by the technique. This technique is being developed to be also used in factories and yards during the industrial processes with portable accelerators for muons, which will promote the recovery and reuse of Li-ion batteries in the future.