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Understanding the differences in the structures and defects in the stable crystalline phase and metastable phase is important for increasing the ionic conductivities of a solid electrolyte. The metastable phase often has higher conductivity than the stable phase. In this study, metastable lithium thiogallate, Li 5 GaS 4 , was synthesized via mechanochemistry and stable Li 5 GaS 4 was obtained by heating the metastable phase. The metastable Li 5 GaS 4 sample was found to have an antifluorite-type crystal structure with cationic disorder, while the stable phase was found to have a monoclinic crystal structure, similar to that of another solid electrolyte, Li 5 AlS 4 . In both the structures, the Ga 3+ cations were surrounded by four S 2- anions in tetrahedral coordination. The conductivity of the metastable phase was determined to be 2.1 × 10 -5 S cm -1 at 25 °C, which is 1000 times greater than that of the monoclinic phase. The high conductivity of the metastable phase was achieved owing to cation disorder in the crystal structure., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The dynamic changes of ionic conduction path in the cross-sectional graphite composite electrodes of bulk-type all-solid-state lithium batteries have been monitored using operando confocal microscopic observations for color changes of graphite in response to their stage structures. The ionic conduction path decreased in the cross-sectional direction as cycle numbers increased, with simultaneous capacity degradation. The local reactivity of lithiation and delithiation was evaluated by image analysis considering state-of-charge (SOC) values. Electrode thickness changes were examined from the confocal microscope images obtained in the operando observations. The results revealed that voids and cracks were formed during cycle tests and that the thickness gradually increased. These cracks and voids were one of the main contributors to the limitation of ionic conduction paths in the depth direction. Operando microscopic observation and subsequent image analysis elucidated not only the morphological changes of active materials but also the differences in local SOC changes in the electrode.

A new crystalline lithium-ion conducting material, Li 4 SnS 4 with an ortho-composition, was prepared by a mechanochemical technique and subsequent heat treatment. Synchrotron X-ray powder diffraction was used to analyze the crystal structure, revealing a space group of P6 3 / mmc and cell parameters of a = 4.01254(4) Å and c = 6.39076(8) Å. Analysis of a heat-treated hexagonal Li 4 SnS 4 sample revealed that both lithium and tin occupied either of two adjacent tetrahedral sites, resulting in fractional occupation of the tetrahedral site (Li, 0.375; Sn, 0.125). The heat-treated hexagonal Li 4 SnS 4 had an ionic conductivity of 1.1 × 10 -4 S cm -1 at room temperature and a conduction activation energy of 32 kJ mol -1 . Moreover, the heat-treated Li 4 SnS 4 exhibited a higher chemical stability in air than the Li 3 PS 4 glass-ceramic.

All-solid-state sodium batteries using Na 3 Zr 2 Si 2 PO 12 (NASICON) solid electrolytes are promising candidates for safe and low-cost advanced rechargeable battery systems. Although NASICON electrolytes have intrinsically high sodium-ion conductivities, their high sintering temperatures interfere with the immediate development of high-performance batteries. In this work, sintering-free NASICON-based composites with Na 3 PS 4 (NPS) glass ceramics were prepared to combine the high grain-bulk conductivity of NASICON and the interfacial formation ability of NPS. Before the composite preparation, the NASICON/NPS interfacial resistance was investigated by modeling the interface between the NASICON sintered ceramic and the NPS glass thin film. The interfacial ion-transfer resistance was very small above room temperature; the area-specific resistances at 25 and 100 °C were 15.8 and 0.40 Ω cm 2 , respectively. On the basis of this smooth ion transfer, NASICON-rich (70-90 wt %) NASICON-NPS composite powders were prepared by ball-milling fine powders of each component. The composite powders were well-densified by pressing at room temperature. Scanning electron microscopy observation showed highly dispersed sub-micrometer NASICON grains in a dense NPS matrix to form closed interfaces between the oxide and sulfide solid electrolytes. The composite green (unfired) compacts with 70 and 80 wt % NASICON exhibited high total conductivities at 100 °C of 1.1 × 10 -3 and 6.8 × 10 -4 S cm -1 , respectively. An all-solid-state Na 15 Sn 4 /TiS 2 cell was constructed using the 70 wt % NASICON composite electrolyte by the uniaxial pressing of the powder materials, and its discharge properties were evaluated at 100 °C. The cell showed the reversible capacities of about 120 mAh g -1 under the current density of 640 μA cm -2 . The prepared oxide-based composite electrolytes were thus successfully applied in all-solid-state sodium rechargeable batteries without sintering.

Solid electrolytes with compositions of (100 - x )(0.75Li 2 S·0.25P 2 S 5 )· x LiBH 4 (mol %, 0 ≤ x ≤ 100) were mechanochemically prepared from the 75Li 2 S·25P 2 S 5 (mol %) glass and LiBH 4 crystal. The samples with x ≥ 43 have crystalline phases and those with x ≤ 33 formed a glassy phase. The crystalline phase was identified as argyrodite Li 6 PS 5 (BH 4 ). The x = 50 sample formed a crystalline phase and demonstrated a high lithium-ion conductivity of 1.8 × 10 -3 S cm -1 at 25 °C with an activation energy of 16 kJ·mol -1 . The argyrodite-type crystal with a BH 4 - anion that occupies the halide site is a novel and promising solid electrolyte., Competing Interests: The authors declare no competing financial interest.