Hydrochloric acid-free synthesis of LiNbOCl4 superionic conductor for all-solid-state Li batteries

Abstract

Bulk-type all-solid-state Li batteries (ASLBs) employing inorganic solid electrolytes are considered a next-generation energy storage system due to their potentials to overcome the limitations of current lithium-ion batteries (LIBs) such as a safety concern and narrow operating temperature. Inorganic solid electrolytes (SEs) with high ionic conductivity, good chemical- and electrochemical stability are crucial for high-performance ASLBs. Among them, halide SEs have gained attention for their high-voltage stability, high ionic conductivity, and potentially lower cost compared to sulfide counterparts. Notably, the recently reported LiNbOCl₄ exhibiting high ionic conductivity (≥ 10 mS cm⁻¹) can be a promising candidate. However, in the literature, LiNbOCl₄ was prepared by the reaction of LiOH and NbCl₅, producing caustic HCl as a by-product. This is problematic for large-scale production and may hinder potential improvement through compositional modification. In this work, we demonstrate an alternative hydrochloric acid-free synthesis route using NbOCl₃ that can yield LiNbOCl₄ with the same crystal structure and high ionic conductivity of 8.4 mS cm⁻¹ at 25 °C. To confirm its feasibility for the bulk-type ASLB application, its electrochemical properties and dry room stability were also investigated.