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 Li1–xNbO1–xCl4+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 O2–/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 O2– 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 O2– anions cannot exist on the terminal sites of the [NbOCl4]∞– chains in the material, even when it is made with an excess of O2– 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 O2– 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–/O2– sites, even in the stoichiometric material, which were previously thought to not exist. Finally, we show that slightly reducing the Li+ and O2– 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.