Toyoki Okumura

Solid electrolytes are expected to improve safety compared to liquid electrolytes. Among them, oxide-based solid electrolytes are attractive because of their higher chemical stability than sulfide- and chloride-based counterparts. In particular, pyrochlore-type materials such as Li_1.25La_0.58Nb₂O₆F are promising lithium-ion conductors because they exhibit high ionic conductivity (>10 mS cm^–1). However, there are currently no effective strategies for further enhancing their ionic conductivities. This study investigated Sr²⁺ doping as a potential approach for improving the ionic conductivity. Structural analysis confirmed the successful substitution of Sr into the oxyfluoride pyrochlore structure of Li_1.25La_0.58Nb₂O₆F within a specific composition range, resulting in lattice expansion. However, the ionic conductivities of the Sr-doped samples decreased to approximately 0.17–6.3 mS cm^–1 at 25 °C. This may be related to the incorporation of Sr²⁺, which could potentially fill lithium vacancies and hinder lithium-ion migration. Although Sr doping did not yield the expected improvement in ionic conductivity, this work provides critical insights into structural constraints and compositional optimization, which serve as guidelines for the design of pyrochlore-type oxyfluoride solid electrolytes with high ionic conductivity.

All-solid-state batteries (ASSBs) are next-generation energy storage systems with potential advantages in energy density and safety. However, loss of physical contact at the cathode−electrolyte interface during cycling leads to capacity fading. Here, we demonstrate that a supercooled, low-melting lithium salt enables a strategy for stabilizing cathode−electrolyte interfaces in sulfide-based ASSBs. The lithium salt, referred to as a single-cation ionic liquid (SCIL), exhibits a melting point of 76 °C and maintains a supercooled liquid state over periods. Through solvent-free processing at 90 °C, molten SCIL infiltrates interparticle voids without undesirable decomposition reactions, maintaining an ionic conductivity of 1.0 mS cm−1. When applied to composite cathodes, SCIL improves the initial Coulombic efficiency (from 76.8 to 85.2%), capacity retention (from 89.8 to 94.9% after 100 cycles), and rate capability (from 70 to 104 mAh g−1 at 1C). These findings demonstrate that supercooled lithium salts provide an interfacial stabilization strategy for sulfide-based ASSBs.

Proof-of-concept quasi-solid electrolytes (QSEs) combining Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ and a single-cation ionic liquid (SCIL) composed of lithium bis(fluorosulfonyl)amide and lithium (fluorosulfonyl)(trifluoromethanesulfonyl)amide are demonstrated without inducing concentration gradients. The QSEs exhibited ionic conductivities comparable to the SCIL without high-temperature sintering. These results demonstrate the potential of SCIL-based QSEs as model systems for investigating Li⁺ transport under concentration-gradient-free conditions in solid-state battery systems.

Magnesium and calcium batteries offer promising energy storage solutions characterised by cost-effectiveness, safety, and high energy density. However, the scarcity of viable electrode and electrolyte materials vastly hinders their advancement. This study utilises solid-state metathetical reactions involving predominantly chalcogen- and pnictogen-based honeycomb layered oxides with alkaline-earth halides/nitrates to synthesise Mg- and Ca-based materials previously achievable only under high-temperature/high-pressure conditions, as well as new metastable materials with unique crystal versatility. Particularly, we employ metathetical reactions involving Li4MgTeO6, Na2Mg2TeO6, and Na4MgTeO6 with MgCl/Mg(NO3)2 or Ca(NO3)2 at temperatures not exceeding 773 K to produce Mg3TeO6 polymorphs, ilmenite-type CaMg2TeO6 (Mg2CaTeO6), and double perovskite Ca2MgTeO6. Thus, we demonstrate that these materials, conventionally requiring gigascale pressures and high temperatures (>1273 K) for their proper synthesis, are now readily accessible at ambient pressure and considerably lower temperatures. Meanwhile, despite sub-optimal pellet densities, the synthesised ilmenite-type Mg3TeO6 and double perovskite Ca2MTeO6 (M=Mg,Ca,Zn) materials exhibit remarkable bulk ionic conductivity at room temperature, marking them as promising compositional spaces for exploring novel Mg2+ and Ca2+ conductors. Furthermore, this study extends the applicability of metathetical reactions to attain Mg- or Ca-based antimonates, bismuthates, ruthenates, tungstates, titanates, phosphates, and silicates, thus opening avenues to novel multifunctional nanomaterial platforms with utility in energy storage and beyond.

Direct visualization of the spatial distribution of oxygen redox activity in single particles of Li-rich layered oxide (LLO) is essential for a deeper understanding of its reaction mechanism. In this study, we propose a real-space mapping approach for identifying oxygen redox-active domains using scanning transmission electron microscopy (STEM)-based electron energy-loss spectroscopy (EELS). Electron-released oxygen (oxidized oxygen) was clearly detected by EELS and soft X-ray absorption spectroscopy (S-XAS) in the charged state of Li1+x(Ni0.3Mn0.7)1-xO2 (0 < x < 1/3). STEM-EELS spectrum imaging clarified that the fluctuations in oxygen redox activity closely correlate with the Ni distribution within LLO particles. The heterogeneous distribution of transition metals leads to non-uniform activation of oxygen in the particles, with Ni-rich domains suppressing oxygen redox activity. This real-space mapping of oxygen activity domains at the LLO single-particle level contributes to understanding the material’s mechanism.

All-solid-state (ASS) lithium-sulfur batteries are promising power sources with the potential for high capacity and safety. Lithium metal polysulfide cathodes can address issues arising from the low electronic conductivity of Li2S and S. This study synthesized lithium vanadium polysulfides (Lix VSy ) by the mechanochemical treatment of Li2S and V2S3. The Lix VSy system contains nanocomposites of Li2S and LiVS2 in an amorphous matrix; lithiation and delithiation occur in both Li2S and LiVS2 during charging and discharging. LiVS2 enhances the electronic conductivity of Lix VSy (~10-1-10-2 S cm-1) and the reversibility of charge-discharge reactions because of its high electronic conductivity and layered structure. Therefore, ASS batteries with Lix VSy show high capacity (~650 mAh g-1), even without conductive additives. Here, ASS full cells with high loading assembled using a composite cathode comprising Li8VS5.5 and a solid electrolyte in a 80:20 (wt.%) ratio (33 mg cm-2) and a composite Si anode (10.4 mg cm-2) exhibited a high areal capacity of 15 mAh cm-2, resulting in calculation of high energy densities of 853 Wh L-1 and 515 Wh kg-1 when assuming the cells were enlarged and stacked. This study is expected to expedite research on the development of high-performance ASS batteries.