Nathalie Pereira

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.

Failure Mechanism and Improvement of the Elevated Temperature Cycling of LiMn2O4 Compounds through the Use of the LiAlxMn2-xO4-zFz Solid Solution

Physiochemical and electrochemical means were used to characterize the failure mechanisms of the lithium manganese oxide based solid solutions utilized as positive electrodes for Li batteries. Resultant data supports a theory in which the 22 and 55°C failure mechanisms are one in the same. The poor performance is exacerbated during cycling due to a mechanism linked to the symbiotic relationship between physical destruction induced by a surface Jahn-Teller distortion and the classic failure mechanisms presented for the chemical failure of the spinel at elevated temperature storage. Based on these findings, a spinel, LiAl x Mn 2-x O 4-z F z , was fabricated which exhibits enhanced stability at elevated temperature.

The Electrochemistry of Zn3 N 2 and LiZnN A Lithium Reaction Mechanism for Metal Nitride Electrodes

LiZnN has been isolated by way of an electrochemical conversion reaction of Zn 3 N 2 with Li. We show that Zn 3 N 2 reversibly reacts with lithium electrochemically, exhibiting a large reduction capacity of 1325 mAh/g corresponding to the insertion of 3.7 Li per Zn. Of this initial capacity, 555 mAh/g were found to be reversible. Through the use of extensive in situ and ex situ X-ray diffraction, the reaction mechanism with lithium was identified as a conversion reaction of Zn 3 N 2 into LiZn and a matrix of βLi 3 N, the high pressure form of Li 3 N. Upon oxidation, LiZn transformed into metallic Zn, while βLi 3 N contributed to the transformation into LiZnN. This is the first identification of a reversible Li 3 N conversion mechanism. The formation of LiZnN as the new end member of the electrochemical reaction with lithium was identified as the cause of the irreversible loss observed during the first cycle. The βLi 3 N and LiZn oxidation into LiZnN was found to be reversible upon subsequent cycles. Poor cycle life was mainly attributed to the electromechanical grinding of the Li-Zn alloying reaction. Cu 3 N is also introduced as a material utilizing a similar conversion reaction but exhibiting improved cycle life.

Electrochemistry of Cu3N with lithium: A complex system with parallel processes

Cu 3 N was examined as a candidate negative electrode material for rechargeable Li-ion batteries. Cu 3 N electrodes exhibited good cycle life and excellent rate capabilities. The investigation of the materials electrochemical reaction mechanism revealed that the electrochemistry of Cu 3 N is rich with several parallel processes. In addition to a reversible lithium/copper nitride conversion process and the formation/decomposition of an organic layer at the surface of the nanocomposite, large cycle number and elevated temperature were found to promote a reversible lithium/copper oxide conversion process. Although the lithium/metal nitride conversion process was found to exhibit poor cycling stability, it constituted a fundamental step in the electrode chemistry as it generated highly active Cu nanoparticles which may have activated the formation of an organic layer and the formation of copper oxide. The oxidation of Cu metal into Cu 2+ to form CuO and its reduction contributed to the increase in capacity with cycle number. However, the maximum capacity obtained at high rate and elevated temperature far exceeded the theoretical capacity associated to the reduction of pure Cu 2+ into Cu metal These results suggest that the formation/decomposition of an electrolyte interface layer, which may become more substantial with cycling, and other reaction processes, such as a Li-Cu alloying reaction, may provide the additional capacity during cycling.

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.

The Electrochemistry of Germanium Nitride with Lithium

Ge 3 N 4 was investigated for its electrochemical activity with lithium as a possible negative electrode material for Li-ion batteries. Ge 3 N 4 was found to reversibly react with Li, exhibiting high capacity, 500 mAh/g, and maintaining good cycling stability. The reaction mechanism of Ge 3 N 4 with lithium was investigated in detail using in situ and ex situ X-ray diffraction (XRD) in reflection, in situ XRD in transmission, ex situ transmission electron microscopy, and selected-area electron diffraction (SAED). The two phases, α- and β-Ge 3 N 4 , of the electrode material mostly maintained their respective crystalline microstructure during cycling. A substantial integrated intensity decrease in the XRD Bragg reflections observed during the first lithiation and the concurrent emergence of diffuse rings in SAED suggest the reaction of Ge 3 N 4 with lithium may be limited thereby converting only the outermost shell of the Ge 3 N 4 crystal. The identification of α-Li 3 N and Ge at the end of the first delithiation using SAED supports a lithium/metal nitride conversion reaction process. The formation of the Li 3 N matrix was found to be consistent with a 50% irreversible capacity loss in the first cycle.

Enhancement of the electrochemical properties of Li1Mn2O4 through chemical substitution

The link between room temperature (RT) cycling failure for Li1Mn2O4-type spinels and elevated temperature (ET) failure of Li1.05Mn1.95O4 materials was investigated by physical and electrochemical characterization. Failure for both ET and RT cycling occurred at the end of discharge. Substantial evidence suggesting a link based on the cooperative Jahn–Teller distortion was found. Based on this knowledge, LiAlxMn2−xO4−δFz materials were fabricated. These novel compounds were found to offer much improved capacity and ET performance than present generation materials. Three hundred cycles at 55°C resulted in 15% capacity loss. Storage in charged and discharged state for 4 days at 70°C revealed less than 1.6% irreversible capacity loss.

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.