Capacity fading mechanism of conversion-type FeF₃ electrode: Investigation by electrochemical operando nuclear magnetic resonance spectroscopy

Abstract

FeF₃ has attracted considerable attention as a positive electrode material for next-generation rechargeable lithium ion batteries, because of its low cost, low risk, and high energy density, which facilitate a conversion-type lithiation/delithiation reaction. However, the conversion reaction of the FeF₃ electrode is known to suffer from capacity fading during repeated discharge–charge cycles. Herein, we find an interesting correlation between capacity fading behavior and spectral evolutions in electrochemical operando nuclear magnetic resonance (NMR) measurements. The operando ⁷Li NMR spectra demonstrate the reversible formation of metallic Fe by the conversion process during the early discharge–charge cycles. However, it is gradually suppressed after repeated cycles. Moreover, LiF is augmented at the fully charged states, indicating that FeF₃ is no longer recovered after repeated cycles. The active material can converge into FeF₂ and LiF in the degraded electrode. Another factor associated with capacity degradation is the electrolyte decomposition occurring at high voltages, which results in a resistive film coating the electrode surface. We therefore conclude that the film accumulation on repeated discharging inhibits the conversion reaction to metallic Fe and LiF, leading to a characteristic capacity fading behavior of FeF₃.