Monodisperse FeF2 Nanorods in Ionic Liquid Electrolytes: A Comprehensive Study of the Conversion Mechanism

Abstract

Transition metal fluorides display the combination of high capacity and high electrode potential that are essential to boosting the energy density of lithium ion batteries. However, their application as cathode materials has been hindered by an incomplete understanding of their electrochemical capabilities and limitations. Herein, we present a system of single crystalline, monodisperse iron (II) fluoride nanorods and detail how their morphology and uniformity make them ideal for mechanistic study. High-resolution analytical transmission electron microscopy reveals intricate morphological features, lattice orientation relationships, and oxidation state changes that redefine the conversion reaction with unprecedented spatial resolution. We first introduce the presence of surface specific reactions and examine how they critically influence phase evolution, diffusion kinetics and cell failure. Next, we examine how the reversibility of the conversion reaction is governed by topotactic cation diffusion through an invariant lattice of fluoride anions and the nucleation of metallic particles on semi-coherent interfaces. We will focus on how each mechanistic feature establishes new principles for improving reaction hysteresis, kinetics and reversibility and how this new holistic understanding can inform the more effective application of metal fluorides. To ensure a pertinent result, ex-situ data was extracted from coin cells that deliver near theoretical capacity (570 mAh g-1) and extraordinary cycling stability (>90 % capacity retention after 100 cycles). This exceptional performance was enabled solely through the use of an ionic liquid electrolyte. Using a combination of impedance spectroscopy, TEM, and X-ray photoelectron spectroscopy, we explain how the solid electrolyte interphase forming ability of this electrolyte precludes the major failure mechanisms associated with transition metal fluorides. We conclude this talk by examining the complementary nature of this cathode electrolyte pairing, particularly with regards to high-temperature cycling. Figure 1