Thermodynamics and Kinetics of the Li/FeF₃ Reaction by Electrochemical Analysis

Abstract

FeF3 is a promising cathode material for lithium batteries with a very high energy density due to its complete reduction to a mixture of LiF and Fe. The material is not yet practical due to a greater than 1 V hysteresis during charge and discharge. Previous work has suggested that this hysteresis might be intrinsic due to different reaction pathways. We employ galvanostatic intermittent titration (GITT), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) to study the reaction thermodynamics and kinetics. GITT experiments, when the electrode is allowed to rest for 72 h following a discharge or charge step, show that the hysteresis is 280 mV, in contrast to >1 V observed previously with slow rate charge and discharge experiments. CV results obtained in different potential ranges indicate that the apparent hysteresis is mostly due to the large overpotential needed to overcome the energy barrier for the nucleation of the LiF/Fe composite phases. EIS results are consistent with the formation of the nanocomposite. Further, EIS results indicate that extended rest of the electrode under open circuit appears to result in coalescence of the Fe nanoparticles, which reduces the Fe/LiF interfacial area. This hypothesis is also consistent with our observation that the charge–discharge overpotentials at high rates are smaller than what the Ohmic law would dictate. On the basis of these results, a reaction mechanism for the reaction is presented. The mechanism also points to potential approaches to mitigate the hysteresis through nanoengineering of the material.

Electrochemical Cycling and Lithium Insertion in Nanostructured FeF3Cathodes

“…The voltage hysteresis remained around 0.7 V to the 100th cycle. The voltage hysteresis during cycling with a "shallow discharge" was much smaller than for the "deep discharge", consistent with previous cyclic voltammetry results 16 which are discussed in the next section.…”

“…Based on the capacity values obtained above, and consistent with the proposal of a topotactic lithiation mechanism, 5 Stage 1 lithiation occurs when FeF 3 /Li is discharged to approximately 2 V: 6,11,16 FeF 3 + xLi → Li x FeF 3 , 0 < x < 1 (Stage 1), which we studied as "shallow discharge". Most previous studies on FeF 3 used "deep discharge" that included both Stage 1 and Stage 2 With deep discharge, galvanostatic cycling gives a high capacity, but shows two issues: a large voltage hysteresis and poor cycle life.…”

“…Most previous studies on FeF 3 used "deep discharge" that included both Stage 1 and Stage 2 With deep discharge, galvanostatic cycling gives a high capacity, but shows two issues: a large voltage hysteresis and poor cycle life. The hysteresis was systematically studied using cyclic voltammetry and galvanostatic cycling under quasi-static conditions by Liu et al 16 In this work, the cyclic voltammograms with a 2 V lower potential cutoff showed only about 0.25 V difference between the two redox peaks. Whereas in the case of 1 V lower potential cutoff, the second redox reaction corresponding to the Stage 2 exhibits around 1 V difference between the oxidizing and reducing peaks.…”

“…However, the work to date indicates that the majority of the extended capacity for FeF 3 is realized at lower potentials than for oxide cathodes, requiring a lower practical cutoff voltage of 1 V and displaying a large hysteresis of approximately 1 V between charge and discharge cycles. 16,17 In addition, the performance of this material over a large number of cycles remains unclear.…”

Iron(III) fluoride synthesized by a fluorolysis method and its electrochemical properties as a positive electrode material for lithium secondary batteries

“…A new positive electrode material with large capacity is needed for these devices, because current positive electrode materials utilize insertion reactions with intrinsically limited capacities based on one-(or less) electron reaction per formula unit (140 mAh g −1 for LiCoO 2 (0.5 Li) [4,5] and 170 mAh g −1 for LiFePO 4 (1 Li) [6]). Thus, instead of such insertion materials, iron(III) fluoride (FeF 3 ) has been receiving attention as a positive electrode material with a high theoretical capacity of 712 mAh g −1 based on the three-electron reaction, reasonably high average operating potential of 2.7 V vs. Li + /Li, in addition to abundant resources of iron [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26].…”