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

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.