Atsushi Sakuda

All-solid-state sodium batteries are attracting attention as next-generation energy devices because of their high safety, low cost, and high energy density. Na ₅ FeS ₄ , an earth-abundant sodium iron sulfide, is a promising candidate positive electrode active material with high capacity and reversibility. In this study, Al-substituted Na ₅ FeS ₄ positive electrode active materials were synthesized, and the effect of Al substitution on the electrode properties was investigated. It was found that Al substitution increases the apparent ionic conductivity of Na ₅ FeS ₄ . Moreover, all-solid-state cells using Na ₅ Fe _0.95 Al _0.05 S ₄ positive electrode active material exhibited an initial capacity of approximately 500 mAh g ^− 1 , exceeding that of Na ₅ FeS ₄ , and operated reversibly for more than 70 cycles. Na ₅ Fe _{1− x} Al _x S ₄ active materials with x ≤ 0.5 maintained sufficient apparent ionic diffusivity throughout the charge process, contributing to their high capacities. This study revealed three key findings: the formation of a solid solution between Na ₅ FeS ₄ and Na ₅ AlS ₄ over the entire composition range, the enhancement of apparent ionic diffusivity and capacity of Na ₅ FeS ₄ through slight Al substitution, and unexpectedly high electrode performance even up to 50% Al substitution. These findings contribute to the development of environmentally friendly, high-capacity sodium iron sulfide-based active materials for all-solid-state sodium batteries.

α-Li₃PS₄ is a promising solid-state electrolyte with the highest ionic conductivity among its polymorphs. However, its formation presents a thermodynamic paradox: the α-phase is the equilibrium phase at high temperature and transforms to the stable γ-Li₃PS₄ polymorph when cooled to room temperature; however, α-Li₃PS₄ can be synthesized and quenched in a metastable state via rapid heating at relatively low temperatures. The origin of this synthesizability and anomalous stability has remained elusive. Here, we address this paradox by presenting an experimental and computational time-temperature-transformation (TTT) diagram, constructed from a temperature-size phase diagram and experimental high-time-resolution isothermal measurements. Our density functional theory calculations reveal that at the nanoscale, the α-phase is stabilized by its low surface energy, which drastically lowers its nucleation barrier across a wide temperature range. This size-dependent stabilization is directly visualized using in-situ sub-second synchrotron X-ray diffraction and electron microscopy, capturing the rapid nucleation of nano-sized α-Li₃PS₄ and its subsequent slow transformation. This work presents a generalizable framework that integrates thermodynamic and kinetic factors for understanding nucleation and phase transformation mechanisms, providing a rational strategy for the targeted synthesis of functional metastable materials.

Aluminum (Al) is a promising alloy-type anode material for all-solid-state rechargeable lithium (Li) batteries (SSBs) owing to its high theoretical specific capacity, abundance, low cost, and recyclability. However, mechanical degradation during lithiation/delithiation and limited Li diffusion kinetics remain critical challenges in the application of Li–Al alloy (LixAl: 0 < x < 1 at room temperature) in SSBs. This study explores the use of nanoporous aluminum (NPAl) sheets as a potential approach to mitigate these challenges. NPAl sheets were fabricated via a galvanic replacement reaction method, resulting in bundles of Al filaments with an interfilament spacing of ∼200 nm. Compressed NPAl sheets (∼13 μm in thickness and with 17% porosity) were used as anodes and assembled into SSBs utilizing a Li6PS5Cl (LPSCl) solid electrolyte and a Li–In alloy counter electrode. Scanning electron microscopy observations confirmed that the nanoporous structure accommodated volume expansion during initial lithiation and suppressed crack formation upon delithiation. Backscattered electron images suggested that fast surface diffusion promoted long-range Li diffusion within NPAl anodes. Also, the NPAl anode reduced part of the interfacial resistance compared with the dense Al foil anode. Notably, the NPAl anode exhibited a capacity of 618 mAh g–1 (1.97 mAh cm–2) at 0.1 C after 50 cycles at 25 °C. The results suggest that nanoporous architecture leads to distinct mechanical and Li transport behaviors, offering an alternative design strategy for Li–Al alloy anodes for SSBs.

All-solid-state potassium batteries have attracted considerable attention as energy storage devices. The development of suitable ionic conductors has received significant attention for the commercialization of the batteries. Here, we report the potassium-ion conductors KTaCl6 prepared using the mechanochemical method and then heat treatment. The mechanochemically prepared sample comprises the monoclinic crystalline phase embedded within the amorphous component, as revealed by X-ray diffraction and transmission electron microscopy. The mechanochemically prepared sample exhibits an ionic conductivity of 1.8 × 10−5 S cm−1 at 30 °C. Upon heat treatment, the sample undergoes crystallization, leading to a decrease in ionic conductivity. It is suggested that the amorphous phase plays a key role in the ion conduction of the KTaCl6 chlorides.

The development of solid electrolytes with a high ionic conductivity is crucial for realizing all-solid-state sodium batteries. Chloride solid electrolytes, particularly NaTaCl6, have garnered significant attention because of their high conductivity, favorable formability, and good electrochemical stability at high potentials. Mechanochemically synthesized NaTaCl6 consists of both amorphous and crystalline phases, with the former being closely associated with its high ionic conductivity. In this study, xZrO2·NaTaCl6 (x = 0–2) samples were prepared by the mechanochemical method, in which the addition of ZrO2 is intended to facilitate the amorphization of the NaTaCl6 crystalline phase. This approach yielded NaTaCl6 with nanocrystalline ZrO2 particles dispersed in the amorphous phase. The 1.1ZrO2·NaTaCl6 sample exhibited an ionic conductivity of 1.3 × 10–3 S cm–1 at 25 °C and an electrochemical stability window of approximately 1.4–4.1 V versus Na+/Na. The amorphization of NaTaCl6, induced by the addition of ZrO2, contributed to its high ionic conductivity and improved electrochemical stability. This study demonstrates that the amorphization of NaTaCl6-based solid electrolytes leads to enhanced ionic conductivity.

The formation and long-term retention of intimate solid–solid interfaces are key requirements for high-performance all-solid-state batteries (ASSBs). In practical cells, interfacial resistance and mechanical degradation are strongly governed by how electrolyte powders consolidate, how cracks initiate, and how contact evolves under stack pressure during assembly and cycling. Sulfide-based solid electrolytes are particularly attractive because they exhibit a “room-temperature pressure sintering” phenomenon ¹, in which particles densify simply by applying pressure at room temperature. This processing route can reduce process energy compared with thermal sintering.

Li6PS5Cl argyrodite sulfide electrolytes exhibit high ionic conductivity, making them promising candidate electrolyte materials for all-solid-state batteries. Although water is an environmentally friendly and attractive solvent, its use in the synthesis of Li6PS5Cl has been limited due to the low moisture stability of Li6PS5Cl. In this study, Li6PS5Cl argyrodite sulfide electrolytes were synthesized via liquid-phase synthesis using water as the main solvent through optimization of both the synthesis conditions and the composition of the Li3PS4·Li2S·LiCl precursor. In this process, the precursor is formed via the aqueous phase, followed by heat treatment that drives the crystallization of Li6PS5Cl and improves its crystallinity. A series of electrolyte samples derived from precursors with varying Li3PS4 contents, xLi3PS4·Li2S·LiCl (x = 1.0–1.4), were systematically prepared and characterized, revealing their structural and electrochemical properties. The electrolyte derived from the optimal precursor composition (x = 1.2) exhibited a high ionic conductivity of 1.2 × 10–3 S cm–1 at 25 °C in a green compact, and the all-solid-state cells assembled using this electrolyte demonstrated reversible charge–discharge behavior over 100 cycles at room temperature. These results demonstrate that water can be successfully used as the main solvent to synthesize argyrodite-type sulfide electrolytes and provide a versatile strategy for sustainable production of high-performance sulfide solid electrolytes suitable for practical use.

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The development of a mass production process for sulfide electrolytes is crucial for the advancement of all-solid-state batteries. In this context, liquid-phase synthesis has gained attention as a practical and scalable approach for the synthesis of these materials. However, conventional liquid-phase synthesis typically employs organic solvents, which pose both environmental and safety concerns. In this study, a novel liquid-phase synthetic strategy is presented for the preparation of sulfide electrolytes using an aqueous solution, which eliminates the requirement for organic solvents. Consequently, a Li₁₀GeP₂S₁₂ electrolyte was successfully synthesized, and was demonstrated to exhibit an ionic conductivity of 1.6 × 10^-3 S cm^-1 at 25 °C in a powder compact. Additionally, an all-solid-state Li-In/LiNi_1/3Co_1/3Mn_1/3O₂ cell incorporating the developed electrolyte demonstrated stable and reversible cycle performances over 500 cycles. Overall, this work demonstrates the feasibility of using water as an environmentally friendly solvent for synthesizing high-performance sulfide electrolytes.