Lithium fluoride/iron difluoride composite prepared by a fluorolytic sol–gel method: Its electrochemical behavior and charge–discharge mechanism as a cathode material for lithium secondary batteries

Abstract

Owing to their high theoretical capacity, metal fluorides have attracted significant interest as materials for fabricating the cathode of lithium secondary batteries. In the present study, a nanocomposite of LiF and FeF2 is prepared by a fluorolytic solgel method in an ethanol solution, for use as the cathode material of a lithium secondary battery. The produced LiF/FeF2 composite is characterized by broad X-ray diffraction peaks attributed to the nanosized (~10 nm) LiF and FeF2 crystals, a large Brunauer-Emmett-Teller surface area of 119 m 2 g −1 , and adsorption-desorption hysteresis, attributed to the presence of mesopores. The results of charge-discharge tests indicates an initial discharge capacity of 225 mAh (g-LiF/FeF2) −1 through reversal conversion at a current rate of 10 mA (g-LiF/FeF2) −1. Based on a combination of galvanostatic intermittent titration, X-ray absorption, and X-ray diffraction investigations, a new reaction mechanism is developed, namely, the conversion of the local environment of an Fe atom from a rutile-type FeF2 structure to a rhenium trioxide-type FeF3 structure during charging, with the subsequent discharge resulting in the insertion of Li + into the rhenium trioxide-type FeF3 structure, followed by the conversion reaction to LiF and FeF2.

3D Honeycomb Architecture Enables a High‐Rate and Long‐Life Iron (III) Fluoride–Lithium Battery

“…The BF‐STEM images exhibit significant changes when comparing pristine and cycled FeF 3 @C samples (Figure a–c), showing visible decomposition of original nanoparticles into much smaller nanoparticles with increasing cycles, which indicates the occurrence of a conversion reaction upon one‐electron reaction in the high voltage region. The ex situ XRD results in Figure k display that the FeF 3 nanoparticles are converted into FeF 2 and LiF after discharging to 2.0 V, which is consistent with the previous experimental and DFT calculation results . In order to get a better understanding of the behavior of the one‐electron reaction, cyclic voltammetry (CV) curves were performed at different C rates.…”

“…Corresponding to two pairs of redox peaks in CV, the discharge curve in Figure S6 (Supporting Information) is also composed of two parts with different slopes, typical for a phase change processes. XRD, CV results, and previous reports demonstrate that FeF 3 turns to Li 0.5 FeF 3 (tri‐rutile) first, and then decomposes to FeF 2 (rutile) and LiF during discharge (lithiation) at high voltages between 4.5 and 2.0 V. After 400 cycles, the pristine FeF 3 nanoparticles with a size of 10–50 nm convert into ultrasmall nanodots with few nanometers (Figure c–e). EELS elemental maps show the C, F, and Fe distribution within cycled FeF 3 @C (after 400 cycles, in lithiated state).…”

“…However, most of the previously reported metal fluoride particles did not yield uniform particles with a mean size of less than 100 nm and with controlled morphology . Hence, rate capability and cycle life were limited (only 100 cycles achievable in total, small active mass loading of ≈1.0 mg cm −2 or no indication of mass loadings) and these reports were far‐off from the standard of a practical application.…”

“…For example, via one‐electron reaction in the high voltage region, FeF 3 ·0.33H 2 O hierarchical microspheres prepared by self‐assembly offered a promising cycling performance in 100 cycles at 0.1C rate with a discharge capacity of ≈170 mAh g −1 , and the LiF/FeF 2 nanocomposite prepared by a fluorolytic sol–gel method only lasted for 20 cycles at ≈C/20 rate . However, most of the previously reported metal fluoride particles did not yield uniform particles with a mean size of less than 100 nm and with controlled morphology . Hence, rate capability and cycle life were limited (only 100 cycles achievable in total, small active mass loading of ≈1.0 mg cm −2 or no indication of mass loadings) and these reports were far‐off from the standard of a practical application.…”

“…have been used to synthesize metal fluoride nanoparticles or metal fluoride–carbon composites . For example, via one‐electron reaction in the high voltage region, FeF 3 ·0.33H 2 O hierarchical microspheres prepared by self‐assembly offered a promising cycling performance in 100 cycles at 0.1C rate with a discharge capacity of ≈170 mAh g −1 , and the LiF/FeF 2 nanocomposite prepared by a fluorolytic sol–gel method only lasted for 20 cycles at ≈C/20 rate . However, most of the previously reported metal fluoride particles did not yield uniform particles with a mean size of less than 100 nm and with controlled morphology .…”