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

The high ionic conductivityand good oxidation stability of halide-basedsolid electrolytes evoke strong interest in this class of materials.Nonetheless, the superior oxidative stability compared to sulfidescomes at the expense of limited stability toward reduction and instabilityagainst metallic lithium anodes, which hinders their practical use.In this context, the gradual fluorination of Li2ZrCl6-x F x (0 & LE; x & LE; 1.2) is proposed to enhance thestability toward lithium-metal anodes. The mechanochemically synthesizedfluorine-substituted compounds show the expected distorted local structure(M2-M3 site disorder) and significant change in the overallLi-ion migration barrier. Theoretical calculations reveal an approximateminimum energy path for Li2ZrCl6-x F x (x = 0 and0.5) with an increase in the Li+ migration energy barrierfor Li2ZrCl5.5F0.5 in comparisonto Li2ZrCl6. However, it is found that the fluorine-substitutedcompound exhibits substantially lower polarization after 800 h oflithium stripping and plating owing to enhanced interfacial stabilityagainst the lithium metal, as revealed by density functional theoryand ex situ X-ray photoelectron spectroscopy, thanks to the formationof a fluorine-rich passivating interphase.