A Structurally Flexible Halide Solid Electrolyte with High Ionic Conductivity and Air Processability

In this work, a structurally revivable, chloride-ion conducting solid electrolyte (SE), CsSn$_{0.9}$In$_{0.067}$Cl$_3$, with a high ionic conductivity of 3.45 × 10$^{−4}$ S cm$^{−1}$ at 25 °C is investigated. The impedance spectroscopy, density functional theory, solid-state $^{35}$Cl NMR, and electron paramagnetic resonance studies collectively reveal that the high Cl$^−$ ionic mobility originates in the flexibility of the structural building blocks, Sn/InCl$_6$ octahedra. The vacancy-dominated Cl$^−$ ion diffusion encompasses co-ordinated Sn/In(Cl) site displacements that depend on the exact stoichiometry, and are accompanied by changes in the local magnetic moments. Owing to these promising properties, the suitability of the CsSn$_{0.9}$In$_{0.067}$Cl$_3$, as an electrolyte is demonstrated by designing all-solid-state batteries, with different anodes and cathodes. The comparative investigation of interphases with Li, Li–In, Mg, and Ca anodes reveals different levels of reactivity and interphase formation. The CsSn$_{0.9}$In$_{0.067}$Cl$_3$ demonstrates an excellent humidity tolerance (up to 50% relative humidity) in ambient air, maintaining high structural integrity without compromises in ionic conductivity, which stands in contrast to commercial halide-based lithium conductors. The discovery of a halide perovskite conductor, with air processability and structure revival ability paves the way for the development of advanced air processable SEs, for next-generation batteries

Fluorine-Substituted Halide Solid Electrolytes with Enhanced Stability toward the Lithium Metal

The high ionic conductivity and good oxidation stability of halide-based solid electrolytes evoke strong interest in this class of materials. Nonetheless, the superior oxidative stability compared to sulfides comes at the expense of limited stability toward reduction and instability against metallic lithium anodes, which hinders their practical use. In this context, the gradual fluorination of Li$_2$ZrCl$_{6–x}$F$_x$ (0 ≤ x ≤ 1.2) is proposed to enhance the stability toward lithium-metal anodes. The mechanochemically synthesized fluorine-substituted compounds show the expected distorted local structure (M2–M3 site disorder) and significant change in the overall Li-ion migration barrier. Theoretical calculations reveal an approximate minimum energy path for Li$_2$ZrCl$_{6–x}$F$_x$ (x = 0 and 0.5) with an increase in the Li+ migration energy barrier for Li$_2$ZrCl$_{5.5}$F$_{0.5}$ in comparison to Li$_2$ZrCl$_6$. However, it is found that the fluorine-substituted compound exhibits substantially lower polarization after 800 h of lithium stripping and plating owing to enhanced interfacial stability against the lithium metal, as revealed by density functional theory and ex situ X-ray photoelectron spectroscopy, thanks to the formation of a fluorine-rich passivating interphase