Correlation between the Coherence Length and Ionic Conductivity in LiNbOCl₄ via the Anion Stoichiometry

LiNbOCl₄ 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-xNbO_1-xCl_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 LiNbOCl₄ are highly dependent on the O^2-/Cl^- anion ratio in the material. Specifically, we show that, while an amorphous phase fraction of LiNbOCl₄ 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 [NbOCl₄]_∞^- 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 LiNbOCl₄ can be maximized by controlling the maximum coherence length in the material through the anion ratio. Notably, we achieved high ionic conductivities for LiNbOCl₄ 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 LiNbOCl₄ 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 LiNbOCl₄ 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 LiNbOCl₄ to optimize its properties and highlights the potential of LiNbOCl₄ for use as a catholyte in solid-state batteries.