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The sulfide solid electrolyte Li4SnS4 has gained attention owing to its high moisture durability. In this study, we quantitatively investigated the changes in the electrochemical properties and chemical/physical states of Li4SnS4 resulted from moisture exposure using the XRD, Raman spectroscopy, and high-frequency electrochemical impedance spectroscopy (HF-EIS). Li4SnS4 was subjected to Ar gas flow at a dew point ranging from −20 °C to 0 °C for 1 h, and sulfide hydrolysis generated only a minute amount of H2S. The XRD patterns and Raman spectra revealed the formation of Li4SnS4·4H2O with increasing dew point. The HF-EIS analysis, which was conducted to clarify the spatial distribution of the hydrate within the particle, revealed a significant decrease in the ionic conductivity of Li4SnS4; this result can be attributed to the increased grain-boundary (SE/SE particle contact) resistance due to the formation of Li4SnS4·4H2O at the particle surface, despite the generation of a minute amount of H2S. By combining these multifaceted analytical methods, we demonstrated that the thermodynamically stable surface hydrate Li4SnS4·4H2O reduced the lithium-ion conductivity without H2S generation owing to the hydrolysis of sulfide. Thus, we chemically, spatially, and quantitatively verified the mechanism underlying the observed decrease in the ionic conductivity.

To enhance the energy density of all-solid-state batteries, polysulfide positive electrodes have a great advantage of their high capacity. In this study, we developed Li _x VS _y ( x = 5–9, y = 4–6) comprised Li _2 S and LiVS _2 . Although Li _2 S is an insulator, Li _x VS _y shows a high electronic conductivity (∼10 ^−1 –10 ^−2 S cm ^−1 ) because it contains LiVS _2 with a high electronic conductivity. The theoretical capacity of Li _x VS _y is 626–789 mAh g ^−1 when all the Li in Li _x VS _y reacts. Li _x VS _y positive electrodes achieve a high energy density because they show high capacity with no conductive additives and high loading of Li _x VS _y .

In situ/operando techniques for electrochemical systems are useful for understanding the electrochemical reactions, as we presented in Part 1. Here we present a series of in situ/operando techniques for battery applications. Now the in situ/operando techniques presented in this paper has become powerful tools for the development of advanced battery systems such as Li-ion batteries, solid-state batteries, and other beyond Li-ion batteries. In the present paper we introduce the in situ/operando cell design of each measurement technique and discuss how we apply each technique for in the advanced battery materials development.

Spectroscopic and microscopic techniques are complementary to electrochemical studies because electrochemical data consists of current, voltage and time, and has no direct information concerning the chemical structure of active species. Hence electrochemical in situ/operando spectroscopy and microscopy become powerful tools for identification of the electrochemically active species during the electrochemical reactions. The present comprehensive paper provides the fundamental theory, cell design concepts, and measurement tips of various spectroscopic and microscopic techniques. X-ray absorption spectroscopy, infrared spectroscopy, surface plasmon resonance, Raman spectroscopy, confocal microscopy and electron microscopy are covered in the present paper. The introduced cell design becomes the critical part for chemists and materials scientists to start these measurements. Several advanced techniques from recent studies are also introduced.