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

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 nanopartides, 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.