Kunimitsu Kataoka

Lithium-ion dynamics of garnet-type Li 6.5 La 3 Zr 1.5 Ta 0.5 O 12 single crystals were investigated using pulsed field gradient (PFG) NMR and impedance spectroscopy over a wide temperature range up to 700 K. The diffusion coefficient measured by PFG-NMR exhibits non-Arrhenius behavior at elevated temperatures ( > 400 K ). The conductivity diffusion coefficient is consistently higher than the PFG-NMR diffusion coefficient across the entire temperature range, with a Haven ratio of 0.3–0.4, indicating that the motion of mobile Li + ions is correlated. Remarkably, the Haven ratio exhibits little temperature dependence despite the non-Arrhenius behavior, representing the first experimental observation in garnet-type electrolytes and consistent with the molecular dynamics simulations. This indicates that ion-ion correlations remain essentially unchanged over a wide temperature range. Combined with the local Li + ion dynamics obtained from spin-lattice relaxation analysis, these results suggest that long-range diffusion is suppressed at low temperatures due to short-range ordering of Li + ions.

Three-electrode evaluation is essential for designing all-solid-state S/Li–Si full cells that combine high energy density with strong resistance to Li short-circuiting.

Understanding and controlling Li-ion transport in garnet-type oxides is central to advancing solid electrolytes for all-solid-state energy storage systems. Garnet electrolytes have long been recognized for their high ionic conductivity, and the availability of large single crystals now enables a direct, frequency-resolved view of Li-ion dynamics over a broad time window. In such structurally and chemically homogeneous systems, responses from electrical and nuclear magnetic resonance measurements are expected to align, yielding a consistent picture of frequency-dependent Li-ion hopping processes. Here, we investigate single-crystalline Li6.5La3Zr1.5Ta0.5O12 and probe ion dynamics from the kHz to GHz range using a combination of 7Li nuclear spin relaxation (NSR) and electrical conductivity spectroscopy. Long-range transport (5 × 10–4 S cm–1 at 293 K) is consistently described by activation energies in the range of 0.41 to 0.47 eV, whereas localized ion dynamics, observed, e.g., by laboratory-frame NSR, are associated with much lower barriers of approximately 0.22 eV. In comparison with single-crystalline Li6La3ZrTaO12, which exhibits reduced ionic mobility, we propose that changes in ionic mobility are compensated by a higher effective charge-carrier concentration Nc in systems with lower Li contents. For Li6.5La3Zr1.5Ta0.5O12, the Li+ mobility is higher by approximately 1 order of magnitude; however, the resulting conductivity only slightly exceeds that of Li6La3ZrTaO12, presumably due to a lower density of mobile charge carriers. This finding highlights that Nc is a key parameter to consider in these systems that indeed sensitively governs the practical ionic conductivity.

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Hexagonal bronze oxides are represented by the chemical formula A_xMO₃, where A is an electropositive element such as alkali metal or alkaline earth metal and M is a transition metal such as Mo, W, or Re. We report the first synthesis of a tellurium-containing rhenium bronze oxide, Te_0.30ReO₃. The compound has an orthorhombic crystal structure, with tellurium located within the ab plane along with rhenium and oxygen, and planarly coordinated to three oxygen atoms. Electron localization function (ELF) analysis revealed that tellurium has two ELF attractors above and below the TeO₃ plane and that the ELF structure around tellurium is similar to that of ClF₃. Based on valence shell electron pair repulsion theory, two equatorial positions in the coordination structure of the trigonal bipyramid are occupied by lone pairs and the remaining three positions are occupied by oxygen. Electron density calculations demonstrated that the unique coordination structure of the tellurium arises from the hybridization of the 5s and 5p orbitals of tellurium with the 2p orbitals of oxygen.

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