Article pubs.acs.org/JPCC
Thermodynamics and Kinetics of the Li/FeF3 Reaction by Electrochemical Analysis Ping Liu,* John J. Vajo, John S. Wang, Wen Li, and Jun Liu HRL Laboratories, LLC, 3011 Malibu Canyon Road, Malibu, California 90265, United States 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.
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INTRODUCTION Lithium ion batteries are leading candidates for powering electric vehicles and renewable energy storage. Intensive research effort continues to search for battery electrode materials with higher energy densities, which can be realized by higher capacities and/or high potentials. High potential systems, e.g., >4.5 V vs Li/Li+, challenge the stability of state-ofthe-art electrolytes.1,2 However, materials with higher capacities do not suffer from this limitation and may be more desirable. Most state-of-the-art cathode materials operate based on lithium insertion and removal. These reactions induce small to moderate amounts of crystal lattice distortion but do not usually cause structure collapse and phase segregation. This mechanism leads to material stability giving rise to long cycle life and fast kinetics. However, this usually limits the capacity of electrode materials. For example, in lithium metal oxides with the general formula of LiMO2 with M being a first row transition metal, the maximum capacity corresponds to less than one mole of lithium per mole of M. Very often, the capacity is substantially lower, e.g., the capacity of LiCoO2 is only ∼0.5 Li/Co. In contrast to insertion-based reactions, a new class of cathode materials, as exemplified by FeF3, operates as conversion reactions.3 The complete reaction for FeF3 can be described as
and nitrides, which are more suitable for use as anode materials in lithium ion batteries due to their low operating potentials.3,7−14 A recent review covers conversion reactions in general and provides an excellent assessment of our current understanding.15 Because of the promise of high capacity for FeF3, significant experimental and theoretical work has been performed to understand its reaction mechanisms. The most notable feature of the Li/FeF3 reaction, and conversion reactions in general, is the large (>1 V) voltage hysteresis between charge and discharge. This hysteresis makes use of FeF3-based batteries impractical. To understand the hysteresis, Ceder and coworkers16 used a first-principles calculation to examine the effect of particle size of the Fe that precipitates when the intermediate LixFeF3 reacts with Li. They found that when 1 nm Fe particles form, the potential for this reaction is considerably reduced from its bulk value. Furthermore, they presented a model that accounted for the significant hysteresis. Nonequilibrium paths derived by assuming much faster diffusion of Li than Fe gave reasonable agreement with experimental profiles. In this mechanism, the hysteresis is intrinsic and therefore not simply dependent on the slow reaction kinetics. The proposed kinetic model also explains why upon extraction of Li from a 3/1 mixture of LiF and Fe, a rutile FeF2-like structure can form, even when iron should be oxidized to Fe3+ by extraction of three Li+ per Fe.
3Li + FeF3 ↔ 3LiF + Fe
This reaction corresponds to 3 mols of Li per mole of Fe metal, leading to specific capacities 3−5 times those of state-ofthe-art oxides and phosphates.4−6 Similar conversion reactions have been reported for hydrides, oxides, sulfides, phosphides, © 2012 American Chemical Society
Received: December 11, 2011 Revised: February 13, 2012 Published: February 16, 2012 6467
dx.doi.org/10.1021/jp211927g | J. Phys. Chem. C 2012, 116, 6467−6473
The Journal of Physical Chemistry C
Article
Experimentally, Grey and co-workers,17 following the pioneering work of Amatucci et al.,4−6,18 used solid state NMR, X-ray diffraction (XRD), and pair density function (PDF) analysis to study the reaction mechanisms. They concluded that FeF3 first undergoes insertion up to Li0.5FeF3 with a rutile like structure, followed by extrusion of LiF and insertion of more lithium up to LiFeF3 and finally forming a mixture of α-Fe and LiF. The lithium removal process goes directly to a lithiated rutile phase and at the end of charge at 4 V, forms Li0.5FeF3. This observation is consistent with the firstprinciples calculations. In a more recent report on the Li/FeF2 reaction,19 high resolution TEM shows the formation of iron nanoparticles that remain interconnected to form electrical conduction pathways. These nanoparticles also maintain a large interfacial area with the LiF. This composite structure leads to its electrochemical reversibility. In order for the FeF3, and other conversion reactions in general, to be considered as practical battery materials, the large hysteresis between charge and discharge must be overcome.15 Various approaches in creating nanostructured FeF3 by either mechanical milling or solution synthesis have been shown effective in raising the rate capability and enhancing cycling stabilities.5,6,20,21 However, if indeed the hysteresis is intrinsic as suggested by the first-principles calculations, further development effort might not be fruitful in reducing it. The strongest experimental evidence for the hysteresis to be intrinsic comes from charge−discharge profiles obtained at C/100 to C/200 rates.17 Indeed, data obtained at such low rates are often considered as safe approximations of the equilibrium potential of a reaction. However, it is obvious that FeF3 reaction involves the migration of heavy atoms such as Fe and F. In addition, the phase nucleation and growth can be extremely slow. Both of these phenomena can lead to extremely low reaction kinetics, which could cast doubt on the validity of using C/100 data as an approximation for equilibrium. We note that this hysteresis is distinct from the relatively small (
