Correlation between the Coherence Length and Ionic Conductivity in $LiNbOCl_4$ via the Anion Stoichiometry

Abstract

$LiNbOCl_4$ is a recently reported material with high $Li^+$ conductivities of ∼10 $mS·cm^{–1}$ at room temperature. Here, we explore how changing the anion ratio and the $Li^+$ content in the $Li_{1–x}NbO_{1–x}Cl_{4+x}$ series (−0.4 ≤ x ≤ 0.2) affects the ionic conductivity of the material. In doing so, we find that the maximum coherence length and ionic conductivity of $LiNbOCl_4$ are highly dependent on the $O^{2–}$/$Cl^–$ anion ratio in the material. Specifically, we show that, while an amorphous phase fraction of $LiNbOCl_4$ remains constant throughout the substitution series, any excess of $O^{2–}$ results in a rapid decrease in the maximum coherence length of the crystaline fraction in each sample. Through a combination of diffraction and spectroscopic techniques, we show that this occurs because the $O^{2–}$ anions cannot exist on the terminal sites of the $[NbOCl_4]_∞^{–}$ chains in the material, even when it is made with an excess of $O^{2–}$ resulting in a shortening of those chains. In contrast, it was observed that $Cl^–$ can occupy the bridging sites resulting in a dependence of the coherence length to the anion ratio. As such, the ionic conductivity of $LiNbOCl_4$ can be maximized by controlling the maximum coherence length in the material through the anion ratio. Notably, we achieved high ionic conductivities for $LiNbOCl_4$ consistent with literature reports only when the material was slightly $Li^+$ and $O^{2–}$ deficient, suggesting that the literature samples may also have been off-stoichiometry. In addition, we highlight the features missing from the current structural models of $LiNbOCl_4$ including the presence of mixed $Cl^–$/$O^{2–}$ sites, even in the stoichiometric material, which were previously thought to not exist. Finally, we show that slightly reducing the $Li^+$ and $O^{2–}$ contents in $LiNbOCl_4$ also translates to higher capacities when it is used as a catholyte in solid-state batteries. These findings show the importance of careful control of the stoichiometry in $LiNbOCl_4$ to optimize its properties and highlights the potential of $LiNbOCl_4$ for use as a catholyte in solid-state batteries