Sol-Gel Based Synthesis of LiNiFeF6 and Its Electrochemical Characterization

Abstract

Lithium transition metal oxides are commonly used as cathode materials in modern mobile and stationary power supplies. Lithium transition metal fluorides are an interesting new class of materials for lithium ion batteries featuring a higher voltage due to substitution of oxygen by the more electronegative fluorine. A sol-gel based process with trifluoroacetic acid as fluorine source was used to synthesize LiNiFeF6. Ball-milling with carbon and binder was applied to obtain an electrochemical active LiNiFeF6/carbon/binder nano composite. In this study we report on the first electrochemical characterization of a quaternary lithium transition metal fluoride as positive electrode for lithium ion batteries, containing two different transition metals. After 20 cycles of galvanostatic cycling a reversible specific capacity of 88 mAh/g, which is 92% of the initial specific capacity, was retained. In a rate performance test with rates of up to 1C a reversible capacity of 53 mAh/g was obtained. The electrochemically active redox couple Fe3+/Fe2+ was identified by Mössbauer spectroscopy and cyclic voltammetry.