Temperature Driven Phase Evolution of FeF₃as Active Material in Lithium-Ion Batteries

Abstract

Iron trifluoride as an active material for a lithium-ion battery positive electrode has attracted interest owing to its high theoretical capacity (713 mAh g −1 ). However, the large volume expansion during cycling and the poor electrical conductivity of the fluoride hinder the reversible performance of FeF 3 . Moreover, the electrochemical mechanism during reduction and oxidation is not resolved yet, involving complex phase transitions. Recent findings about iron fluorides indicated improved reversibility when the active material is operated with solid polymer electrolytes at temperatures above 60 °C. We herein investigate the electrochemical mechanism of such assembly using operando X-ray absorption spectroscopy along with ex-situ X-ray diffraction for samples with matching liquid electrolytes at several temperatures and selected voltages. We show that the electrochemical mechanism is closely related to the applied temperature, involving distinct reaction pathways between room temperature cells and cells at the temperature required for solid polymer electrolytes. We show that these differences in phases formed at different temperatures are particularly linked to the metastability of the Li x FeF 3 insertion phase, undergoing a conversion reaction to form FeF 2 + LiF at temperatures above 40 °C. This study highlights the need for an in-depth comprehension of the electrochemical mechanism of such materials, notably for understanding the high-temperature performance improvement for FeF 3 with solid polymer electrolytes.