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Green needlelike single crystals of Na2MnP2S6were grown by crystallization after melting a mixture of Na2S, Mn, and P2S5. The crystal structure of the new compound was determined from single-crystal X-ray diffraction data. The chemical composition was confirmed by energy-dispersive X-ray spectroscopy, and the ionic conductivity was measured using electrochemical impedance spectroscopy. Na2MnP2S6crystallizes in the orthorhombic system, space group Pbcn(no. 60) with a= 7.3850(6) Å, b= 21.6682(13) Å, c= 6.2495(4) Å, V= 1000.04(12) Å3, and Z= 4. The Na and Mn atoms are octahedrally coordinated to the S atoms. They form [Na2MnS6]8–honeycomb-like structure within which P–P dimer units are located. Two of the three sodium atomic positions are partially occupied. The group-subgroup transformations emphasized a strong relationship with the P3̅m1-Li4P2S6structure. Indeed, 4Li atoms were replaced by 2Na, Mn, and □ (□: vacancy), and half of the P–P dimer units have the same distribution within the tunnels. The new cationic ordering is at the origin of the trigonal to orthorhombic symmetry transformation (P3̅m1 to Pbcn). The ionic conductivity of Na2MnP2S6is 6.2 × 10–7S cm–1(Ea = 0.41 eV) at 31 °C.

Sulfide-based solid electrolytes have attracted considerable attention for application in solid-state lithium batteries because of their high ionic conductivities, suitable mechanical properties, and successful operation with various active anode and cathode materials. However, sulfides react with traces of moisture to generate toxic H2S gas. This undesirable degradation reaction reduces lithium ionic conductivity, which is crucial to solid-state batteries. To understand the effect of moisture degradation on solid electrolyte particles, impedance spectroscopic measurements at high frequencies (up to 100 MHz) were performed at various temperatures. From the spectral analysis results, we separated the impedance components into grain boundaries and the internal bulk of the solid electrolyte particles before and after exposure to moist air. By calculating the activation energy of lithium ionic conduction from the temperature dependence of each impedance component (Arrhenius plot), we determined that the impedance and activation energy of the grain boundaries between the solid electrolyte particles increased. This indicates that lithium ionic conduction between the solid electrolyte particles was inhibited during the initial stage of exposure to moist air.

In all-solid-state lithium-ion battery research, sulfide-based solid electrolytes (SEs) have been intensively studied because of their attractive properties, such as high lithium ionic conductivity, plasticity, and thermal stability. The most significant problem with SEs is the toxic hydrogen sulfide (H2S) gas generation upon their exposure to moisture. While many studies have developed new materials to improve the ionic conductivity and moisture durability of SEs, only a few have elucidated the degradation mechanism and surface state by moisture exposure at the molecular and atomic levels. In this study, we investigated the moisture-exposed argyrodite-structured sulfide-based Li6–xPS5–xClxSE by 1H magic angle spinning nuclear magnetic resonance spectroscopy to analyze the hydrogen species generated from the undesirable surface degradation reactions in the air with a dew point of −20 °C (0.9 g m–3of H2O). The spectra showed peaks from surface hydrogen components such as LiOH, LiOH·xH2O, thiol groups, and unreacted H2O cluster adsorbates. Additionally, we conducted temperature-programmed desorption mass spectrometry measurements on moisture-exposed SE samples to confirm the desorption of H2O clusters upon heating.

Li metal is a promising negative electrode material for the development of all-solid-state batteries with high energy densities. However, the short-circuiting of batteries owing to Li dendrite formation is challenging. To solve this issue, it is crucial to form a suitable interface between the Li metal and solid electrolyte. This study demonstrates that the incorporation of a Sn interlayer at the interface between the Li metal and Li3PS4(LPS) electrolyte improved the Li stripping/plating performance of all-solid-state Li symmetrical cells. The cycling performance was further enhanced by replacing pure Li metal with a Li–Mg alloy. Galvanostatic cycling tests were conducted on a symmetrical cell of Li–Mg/Sn/LPS/Sn/Li–Mg at a current density of 1.0 mA cm–2and a temperature of 100 °C. The operation of the cell was stable for 5000 h without short-circuiting. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed that Sn film incorporation into the Li or Li–Mg/LPS interface suppressed the reductive LPS decomposition and maintained a stable interface. These findings will facilitate the development of interfacial modifications between Li metal and sulfide solid electrolytes to enhance the cycling performance of all-solid-state Li metal batteries.

Sodium solid electrolytes are expected to be safer and more durable than lithium solid electrolytes, which affect the properties of all-solid-state lithium-ion batteries nearing commercialization. The heat treatment of Na3PS4glass ceramics, a strong candidate for sodium solid electrolytes, results in ionic conductivity exceeding 10–4S cm–1at ambient temperature, depending on the heat-treatment conditions. First-principles calculations have suggested that vacancies at Na sites promote ionic conduction; however, the existence of vacancies has not been confirmed through a conventional average structure analysis of X-ray and neutron diffraction profiles or pair distribution functions. In this study, local structure analysis of Na3PS4glass ceramics using reverse Monte Carlo modeling was performed on the pair distribution function obtained from neutron total scattering measurements during and after heat treatment to identify the vacancy locations that differ from the Na occupation sites. The vacancies in Na3PS4glass ceramics after heat treatment revealed a more disordered atomic arrangement than Na, suggesting that the vacancies connecting Na occupation sites are most likely responsible for the promotion of Na ion conduction. The control of vacancies in solid electrolytes is essential for the design of practical all-solid-state sodium batteries.

All-solid-state batteries are promising energy storage devices owing to their high safety and energy density. Sulfide-based solid electrolytes have high ionic conductivities and are ductile. Although sulfide electrolytes are candidates for all-solid-state batteries, they are moisture-sensitive. Li4SnS4electrolytes can overcome some of these weaknesses and show the suppressed evolution of H2S gas in a humid environment. However, the suppression mechanism is unclear, making the design of moisture-resistant sulfide electrolytes challenging. Therefore, we investigated the structural changes of Li4SnS4in humid N2. X-ray structural analysis revealed that Li4SnS4was hydrated to Li4SnS4·4H2O without generating H2S gas. Furthermore, the hydrate could be reversibly dehydrated by heat treatment to the original sulfide, suggesting that the formation of stable hydrates suppresses the generation of H2S gas. These findings will aid in the design of moisture-resistant sulfide materials.

We tried to prepare solid-state batteries consisting of TiO2anodes and a Li3BO3–Li2SO4–Li2CO3solid electrolyte only by room-temperature pressing without any sintering process. The solid-state batteries successfully operated at 90 °C and showed reversible capacity in the range of 278–331 mA h g–1in the first cycles at 0.01C. In addition to this, the rate capability was remarkably improved by the doping of Nb or Ta into TiO2: the doped TiO2electrodes exhibited a capacity of 140 mA h g–1at 0.1C, whereas the undoped TiO2showed only 10 mA h g–1. The charge-transfer resistance of TiO2was reduced by the doping of Nb or Ta. The improvement in the anode performance was attributed to the improved electronic conductivity and the expanded interlayer distance by doping. These results demonstrated for the first time that the oxide anodes can operate in bulk-type oxide-based solid-state batteries prepared only by room-temperature pressing.

Sulfide solid electrolytes (SSEs) show a higher ionic conductivity than oxide solid electrolytes. However, SSEs generally exhibit a moisture-sensitive nature. This study focused on the degradation mechanisms of SSEs in a humid N2atmosphere, and Na3PS4(NPS) was selected as the SSE. The structural changes during exposure to humidified N2were monitored using a system comprising an X-ray diffractometer, a humidity regulator, and an H2S sensor. Structural evaluations revealed that the exposed NPS was a hydrate, and it was recovered to the NPS crystal phase by the removal of adsorbed/hydrated water molecules using regenerating treatments, such as heating at 200 °C or drying under reduced pressure with/without heating. The regenerated NPS showed an ionic conductivity of 5.3 × 10–5S cm–1at 25 °C, which is 40 times that of the exposed NPS. These results suggest that exposed NPS can be structurally and conductively regenerated to the NPS crystal phase using regeneration treatments.

K2NaYCl6and KNa2YCl6were prepared by a mechanochemical synthesis route from NaCl, KCl, and YCl3as starting materials. The crystal structures and properties were characterized by X-ray powder diffraction, thermogravimetric and differential thermal analyses, energy-dispersive X-ray spectroscopy, and electrochemical impedance spectroscopy. At room temperature, both samples are isostructural to P21/c-Ba2LaRuO6. At high temperature (T< 500 °C), only K2NaYCl6exhibits a phase transition from the room-temperature form, which crystallizes in the monoclinic system, space group P21/c(No. 14) with a= 7.2447(4) Å, b= 7.5518(4) Å, c= 12.6454(8) Å, β = 124.557(3)°, V= 569.76(6) Å3, and Z= 2 to the high-temperature form, which crystallizes in the cubic system, space group Fm3̅m(No. 225) with a= 10.6903(1) Å, V= 1221.72(1) Å3, and Z= 4. The HT-K2NaYCl6(cF40) polymorph is isostructural to K2NaAlF6. The structures are monoclinic and cubic double perovskites A2BB′X6in which the octahedrally coordinated Na and Y atoms are well-ordered at the B and B′ sites, respectively. The A site is occupied by K and K/Na atoms in K2NaYCl6and KNa2YCl6, respectively. The P21/c⇄ Fm3̅mphase transition mechanism was accurately determined and is illustrated in a video. K2NaYCl6and KNa2YCl6exhibit ionic conductivities of 8 × 10–8and 4.9 × 10–9S cm–1at 75 °C, respectively.

Studies on local conduction paths in composite electrodes are essential to the realization of high-performance sulfide all-solid-state lithium batteries. Here, we directly evaluate the electrical properties of individual LiNi1/3Mn1/3Co1/3O2(NMC) electrode active material particles in composite positive electrodes by scanning probe microscopy (SPM) techniques. Kelvin probe force microscopy (KPFM) and scanning spreading resistance microscopy (SSRM) are combined. The results indicate that all NMC particles exhibit a charged state with increasing potential, but low electronic conduction paths exist at point of contacts of some NMC particles. Furthermore, the I–Vcharacteristics measured by conductive atomic force microscopy (C-AFM) suggest that these specific NMC particles show low charge–discharge reactivity. The results of the SPM techniques indicate that poor conduction locally limits the charge–discharge reactivity of electrode active materials, leading to the degradation of battery performance. Such an SPM combination accelerates the morphological optimization of composite electrodes by facilitating the investigation of the intrinsic electrical properties of the electrodes.

In sheet-type all-solid-state lithium-ion batteries with the sulfide-based solid electrolyte, composite electrodes consist of active material, solid electrolyte, conductive additive material, and binder. Thus, they form a three-dimensional ionic and electronic conduction pass. In composite electrodes, the reaction inhomogeneity derived from their morphology exerts a remarkable effect on battery performance. In this study, we prepared sheet-type composite electrodes for all-solid-state lithium-ion batteries with the sulfide-based solid electrolyte using different binder materials with different solvents and investigated the reaction distribution within the electrodes using the 2D-imaging X-ray absorption spectroscopy. Thus, we demonstrated that the dominant factor of the reaction distribution formation is the ionic conduction, depending on the structure of the composite electrode, and that the structure is influenced by the combination between the binder and the solvent used in the preparation of the sheet-type composite electrode.

All-solid-state lithium-ion batteries with flame-retardant inorganic solid electrolytes are promising because of their safety. Oxide-based solid electrolytes are attractive, owing to their high chemical stability and lithium-ion conductivity. The development of new oxide-based solid electrolytes with high ionic conductivity and ductility is crucial for improving the overall performance of all-solid-state batteries. Recently, the synthesis of amorphous solid electrolytes using mechanochemical processes was explored. Among them, a Li2O–LiI amorphous solid electrolyte has been shown to have high ionic conductivity (10–5S cm–1at 25 °C) and ductility comparable to those of sulfide solid electrolytes. However, the detailed structure and the ionic conduction mechanism of the Li2O–LiI electrolyte are not well understood. In this study, structural analysis of the Li2O–LiI electrolyte was conducted to elucidate its structure and conduction mechanism using neutron diffraction, transmission electron microscopy, 7Li magic-angle spinning nuclear magnetic resonance, and X-ray photoelectron spectroscopy. These structural analyses suggested that the Li2O–LiI electrolyte was primarily composed of amorphous components, mainly iodide and oxide ions, with high ionic mobility. The results of this study are useful for the further development of oxide-based solid electrolytes.

The Li2S–V2S3–LiI cathode materials doped with different LiI amounts developed in this study presented a high charge/discharge capacity without solid electrolyte and carbon in the composite electrode. The reaction mechanism of the cathode materials was elucidated using a combination of analytical methods. Potential step measurements revealed that LiI doping amounts above 10 mol % largely increased the ionic conductivity of the Li2S–V2S3–LiI cathodes during charging. The X-ray computed tomography and cross-sectional SEM–EDX mapping results demonstrated that the LiI-rich domain formed from the Li2S–V2S3–LiI cathode during charging because the lithium content of the cathode material changed. These results indicated that the self-forming LiI-rich domain served as an ionic conduction pathway in the cathode matrix during the first charge cycle and improved the electrochemical performance of the cathode materials. This material design strategy for composite electrodes with self-formed ionic conduction pathways is useful for designing high packing density electrodes for all-solid-state batteries, because the capacity per composite electrode is larger than that of conventional composite electrodes consisting of solid electrolyte and electrode active material.

To realize all-solid-state Li metal batteries, developing a technology that effectively joins Li metal with inorganic solid electrolytes is important. The ultrasonic-assisted fusion welding process, a novel process for joining Li metal and solid electrolytes, has the potential to offer advantages that cannot be obtained by the conventional pressing process of solid–solid joining because it uses molten Li. In this study, to investigate the applicability of this process to the joining of Li metal and sulfide solid electrolytes, amorphous Li3PS4was used as an electrolyte, and the interface conditions and electrochemical properties were evaluated. Molten Li filled the roughness of the surface of the pelletized electrolyte, and the fabricated Li–Li3PS4interface was in close contact without voids. Impedance analysis, SEM observations, and XPS depth profile analysis did not detect high-resistive layers. In the Li symmetric cells prepared by this process, electrochemical properties such as critical current density and repeated Li stripping/plating for 100 h were comparable or superior to the cells prepared by the conventional cold isostatic pressing process. Furthermore, this process achieved the joining of Li to the Li3PS4thin-film sheet without a short circuit. Repeated charge–discharge cycles have been successfully demonstrated in the Li|Li3PS4|Li4Ti5O12battery configuration.

Sulfide-based solid electrolytes (SEs) have been extensively studied because of their high lithium-ion conductivities. Among the solid electrolytes, glass or glass-ceramic SEs have suitable thermoplastic characteristics for battery manufacturing processes. However, sulfide-based SEs react strongly with moisture, resulting in toxic H2S gas generation and reduced lithium-ion conductivity. Although the effects of moisture exposure on sulfide-based SEs have been investigated by some researchers, related studies are scarce and further research is required. In this study, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were performed to investigate the structural changes in a glass-ceramic sulfide-based SE, viz., Li3PS4-0.2LiI before and after exposure to moisture, focusing on the changes in the surface chemical species, adsorbed H2O, and related species. In addition, SE samples exposed to moist air were subjected to recovery treatment by vacuum heating. XPS and DRIFTS analyses revealed that the main reason for the decrease in the lithium-ion conductivity of the glass-ceramic SE after moisture exposure is the adsorption of water onto the SE surface and/or its hydration. In particular, the H2O layer adsorbed to the SE surface physically and/or chemically decreases the lithium-ion conductivity at the grain boundaries (i.e., SE/SE interface), although the bulk conduction is maintained.