Fadwa Badway

Carbon-Metal Fluoride Nanocomposites Structure and Electrochemistry of FeF 3 : C

The practical electroactivity of electrically insulating iron fluoride was enabled through the use of carbon-metal fluoride nanocomposites (CMFNCs). The nanocomposites were fabricated through the use of high energy mechanical milling and resulted in nanodomains of FeF, on the order of 1-20 nm encompassed in a matrix of carbon as characterized by transmission electron microscopy and X-ray diffraction (XRD) Electrochemical characterization of CMFNCs composed of 85/15 wt % FeF 3 /C resulted in a nanocomposite specific capacity as high as 200 mAh/g (235 mAh/(g of FeF 3 ) with the electrochemical activity associated with the Fe 3+ → Fe 2+ occurring in the region of 2.8-3.5 V. The CMFNCs revealed encouraging rate capability and cycle life with <10% fade after 50 cycles. Structural evolution during the first lithiation reaction was investigated with the use of ex situ and in situ XRD. Initial results suggest that x from 0 to 0.5 in Li x FeF 3 proceeds in a two-phase reaction resulting in a phase with significant redistribution of the Fe atoms within a structure very similar to the base FeF 3 . FeF 3 -based CMFNCs also exhibited a very high specific capacity of 600 mAh/g at 70°C due to a reversible reaction at approximately 2 V.

Carbon Metal Fluoride Nanocomposites High-Capacity Reversible Metal Fluoride Conversion Materials as Rechargeable Positive Electrodes for Li Batteries

The structure and electrochemistry of FeF 3 :C-based carbon metal fluoride nanocomposites (CMFNCs) was investigated in detail from 4.5 to 1.5 V, revealing a reversible metal fluoride conversion process. These are the first reported examples of a high-capacity reversible conversion process for positive electrodes. A reversible specific capacity of approximately 600 mAh/g of CMFNCs was realized at 70°C. Approximately one-third of the capacity evolved in a reaction between 3.5 and 2.8 V related to the cathodic reduction reaction of Fe 3+ to Fe 2+ . The remainder of the specific capacity occurred in a two-phase conversion reaction at 2 V resulting in the formation of a finer Fe:LiF nanocomposite. Upon oxidation, selective area electron diffraction characterization revealed the reformation of a metal fluoride. Evidence presented suggested that the metal fluoride is related to FeF 2 in structure. A pseudocapacitive reaction is proposed as a possible mechanism for the subsequent Fe 2+ → Fe 3+ oxidation reaction. Preliminary results of FeF 2 , NiF 2 , and CoF 2 CMFNCs were used in the discussion of the electrochemical properties of the reconverted metal fluoride.

Conversion Reaction Mechanisms in Lithium Ion Batteries: Study of the Binary Metal Fluoride Electrodes

Materials that undergo a conversion reaction with lithium (e.g., metal fluorides MF(2): M = Fe, Cu, ...) often accommodate more than one Li atom per transition-metal cation, and are promising candidates for high-capacity cathodes for lithium ion batteries. However, little is known about the mechanisms involved in the conversion process, the origins of the large polarization during electrochemical cycling, and why some materials are reversible (e.g., FeF(2)) while others are not (e.g., CuF(2)). In this study, we investigated the conversion reaction of binary metal fluorides, FeF(2) and CuF(2), using a series of local and bulk probes to better understand the mechanisms underlying their contrasting electrochemical behavior. X-ray pair-distribution-function and magnetization measurements were used to determine changes in short-range ordering, particle size and microstructure, while high-resolution transmission electron microscopy (TEM) and electron energy-loss spectroscopy (EELS) were used to measure the atomic-level structure of individual particles and map the phase distribution in the initial and fully lithiated electrodes. Both FeF(2) and CuF(2) react with lithium via a direct conversion process with no intercalation step, but there are differences in the conversion process and final phase distribution. During the reaction of Li(+) with FeF(2), small metallic iron nanoparticles (<5 nm in diameter) nucleate in close proximity to the converted LiF phase, as a result of the low diffusivity of iron. The iron nanoparticles are interconnected and form a bicontinuous network, which provides a pathway for local electron transport through the insulating LiF phase. In addition, the massive interface formed between nanoscale solid phases provides a pathway for ionic transport during the conversion process. These results offer the first experimental evidence explaining the origins of the high lithium reversibility in FeF(2). In contrast to FeF(2), no continuous Cu network was observed in the lithiated CuF(2); rather, the converted Cu segregates to large particles (5-12 nm in diameter) during the first discharge, which may be partially responsible for the lack of reversibility in the CuF(2) electrode.

Investigation of yttrium and polyvalent ion intercalation into nanocrystalline vanadium oxide

The electrochemical reactivity of cations such as Ca 2+ , Mg 2+ , and Y 3+ into crystalline V 2 O 5 materials was investigated. The ionic diffusion constant of Li + and Y 3+ into microcrystalline and nanocrystalline V 2 O 5 was measured by the galvanostatic intermittent titration technique. The Y 3+ ion diffusion constant into a 500 nm crystalline V 2 O 5 was found to be approximately two orders of magnitude lower than for the Li + ion. In order to enable practical intercalation of Y 3+ , a nanocrystalline V 2 O 5 was fabricated through a combustion flame synthesis technique, For the first time, reversible electrochemical intercalation of Y 3+ into a host structure was shown to be feasible. An asymmetric hybrid cell configuration was utilized in order to provide a reversible counter electrode during intercalation, Preliminary data indicates Y 3+ can be reversibly intercalated into V 2 O 5 with apparent gravimetric capacities exceeding that of Ca 2 +, Mg2 + , or Li + over the limited voltage range of 2.5 to 4.2 V (Li/Li + ). The concept of polyvalent intercalation is discussed relative to intercalation, pseudocapacitance, apparent specific capacity, and practical energy storage systems.

Iron Oxyfluorides as High Capacity Cathode Materials for Lithium Batteries

Nanostructure iron oxyfluoride compounds of tunable oxygen content were fabricated by a solution process utilizing iron metal and fluorosilicic acid solutions as precursors. Simple adjustment of the synthesis atmosphere, temperature, and time enabled the fabrication of iron oxyfluoride materials with compositions spanning over the entire range from pure FeF 2 to FeOF. Nanocomposites were fabricated from iron oxyfluorides of various oxygen content and activated carbon in order, for the first time, to evaluate the impact of oxygen on the iron fluoride electrochemical performance. The introduction of oxygen proved beneficial to cycling stability but at the expense of reversible capacity. A combination of electrochemical and structural characterization analyses was performed to identify the iron oxyfluoride reaction mechanism.

Structure and Electrochemistry of Carbon-Metal Fluoride Nanocomposites Fabricated by Solid-State Redox Conversion Reaction

Utilizing a solid-state redox-driven conversion reaction enabled by mechanochemistry, conductive C:FeF 3 nanocomposites were fabricated from insulative CF 1 :FeF 2 precursors. All reactions were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. The latter provided insights to the progression of the CF and C phases and the metal fluoride during the course of the reaction. Such nanocomposites resulted in a four order of magnitude increase in electrical conductivity and enabled excellent specific capacity approaching 500 mAh/g vs. Li with good reversibility, although at slow rates. Utilizing the theoretical basis of the technique, other couples were examined to experimentally isolate the oxidative power of CF 1 . In the process, we have also shown that a composite of CF 1 :CrF 2 can be easily converted to C:CrF 3 . The resulting nanocomposite exhibited a specific capacity of 682 mAh/g at an average voltage of approximately 1.9 V. The technique is also a powerful method for the fabrication of single phase metal fluoride solid solutions, as demonstrated with the fabrication of Cr 0 . 5 Fe 0 . 5 F 3 .

Investigation of the Lithiation and Delithiation Conversion Mechanisms of Bismuth Fluoride Nanocomposites

and LiF during lithiation and the reformation of BiF3 during delithiation. It has been shown that only the high-pressure tysonite phase of BiF3 reforms during the oxidation sweep and that no bismuth fluoride compound with an oxidation state of the bismuth lower than 3 is formed as intermediate during the lithiation or delithiation reactions. Finally, it has been demonstrated that the different plateaus or pseudo plateaus observed on the lithiation and delithiation voltage profiles stem from polarization changes brought about by the dramatic structural changes occurring in the nanocomposite upon cycling. A model, based on the variation of the electronic and ionic transport mechanisms as a function of the state of completion of the conversion and reconversion reactions, is proposed to explain those polarization changes.

Formation, dynamics, and implication of solid electrolyte interphase in high voltage reversible conversion fluoride nanocomposites

Metal fluoride nanocomposites are uniquely suited as an alternative pathway to provide very high energy density cathodes for lithium batteries. Contrasted with modern intercalation compounds, they undergo conversion upon discharge into nanodomains of lithium fluoride and highly active metal. The nanosized metal formed during the discharge process along with the dynamic nature of the crystal structure may have considerable impact on the stability of any solid state interphase formed through reaction with the electrolyte. This is in contrast to the more macrocrystalline and stable crystal structure of traditional intercalation compounds. It has been found that the cyclic carbonates are susceptible to decomposition on the nanometal surfaces at potentials as high as 2.00 V vs. Li, and the products have been identified with Field Emission Scanning Electron Microscopy (FESEM), Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and X-ray Photoelectron Spectroscopy (XPS) as lithium carbonate species. Of greater importance is the impact of these decomposition products on the reversible cycling of the metal fluoride. Through a series of potentiodynamic and galvanostatic cycling trials, a clear relationship has been developed for the bismuth fluoride nanocomposites, the decomposition of the electrolyte solvent, and the cycle life. Acyclic organic carbonate solvents have been found to have minimal interaction and exhibited better long-term cycling performance than cyclic solvents.

EELS spectroscopy of iron fluorides and FeFx/C nanocomposite electrodes used in Li-ion batteries

A new type of positive electrode for Li-ion batteries has been developed recently based on FeF3/C and FeF2/C nanocomposites. The microstructural and redox evolution during discharge and recharge processes was followed by electron energy loss spectroscopy (EELS) to determine the valence state of Fe by measuring the Fe L3 line energy shift and from Fe L3/L2 line intensity ratios. In addition, transition metal fluorides were found to be electron beam sensitive, and the effect of beam exposure on EELS spectra was also investigated. The EELS results indicate that for both FeF3/C and FeF2/C nanocomposite systems, a complete reduction of iron to FeO is observed upon discharge to 1.5 V with the formation of a finer FeO/LiF subnanocomposite ( approximately 7 nm). Upon complete recharging to 4.5 V, EELS data reveal a reoxidation process to a Fe2+ state with the formation of a carbon metal fluoride nanocomposite related to the FeF2 structure.

Stoichiometric, Morphological, and Electrochemical Impact of the Phase Stability of Li x CoO2

The effect of stoichiometry, heat-treatment, and resulting bulk and surface properties on the electrochemical cycling stability of native Li x CoO 2 under a 35% depth-of-discharge protocol was investigated. The materials were fabricated from mixtures of Li 2 CO 3 and Co 3 O 4 with Li/Co ratios spanning from 0.95 to 1.20. The single-phase stoichiometric sample exhibited the highest electrochemical performance under the applied protocol. A combination of X-ray diffraction, Fourier transform infrared, transmission electron microscopy, and thermogravimetric (TGA) analyses revealed residual Co 3 O 4 and Li 2 CO 3 existed in the materials fabricated from all the nonstoichiometric mixtures with Li/Co 1, respectively. The use of TGA was found to be by far the most effective and sensitive tool for the detection and quantification of Li 2 CO 3 . Using various surface characterization techniques, we showed at least part of the residual Li 2 CO 3 phase forms a layer at the surface of the LiCoO 2 particles fabricated from lithium excess mixtures. The Li 1 + x CoO 2 samples, which cycle poorly at room temperature and exhibit bulk crystallographic properties that differ from stoichiometric LiCoO 2 , were found to cycle well after Li 2 CO 3 removal. The surface phase Li 2 CO 3 is the root of the poor room temperature cycling for the overstoichiometric Li 1 + x CoO 2 samples.