Top-Down Synthesis of Open Framework Fluoride for Lithium and Sodium Batteries

Abstract

Open framework materials as battery electrodes are drawing much more attention recently in view of the requirement on further improving energy storage and power densities, as well as use of a nonlithium cation (such as Na+ and Mg2+) transport. In this work, we show a novel top-down approach to prepare fluoride-based open framework (HTB-type FeF3·0.33H2O) as Li- and Na-storage cathode material, characterized by a topotactically solid–solid phase transformation and particle nanosizing with a 100% conversion yield. During this process, the typical Fe-based octahedral chains which are isolated in FeF3·3H2O precursor are well preserved but are connected with each other, triggered by the alleviated release of hydration water in an ionic liquid (IL) ambience. A comprehensive IL-based scenario to synthesize nanostructured open framework fluorides involving dissolution–precipitation, ionothermal and solid–solid breakdown methods is discussed, mainly depending on how the FeF6 octahedra are produced and linked.