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.

The practical electroactivity of electrically insulating iron fluoride was enabled through the use of carbon-metal fluoride nanocomposites ͑CMFNCs͒. The nanocomposites were fabricated through the use of high energy mechanical milling and resulted in nanodomains of FeF 3 on the order of 1-20 nm encompassed in a matrix of carbon as characterized by transmission electron microscopy and X-ray diffraction ͑XRD͒. 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 3ϩ → Fe 2ϩ occurring in the region of 2.8-3.5 V. The CMFNCs revealed encouraging rate capability and cycle life with Ͻ10% 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.

The structure and electrochemistry of FeF 3 :C-based carbon metal fluoride nanocomposites ͑CMFNCs͒ 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 3ϩ to Fe 2ϩ . 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 2ϩ → Fe 3ϩ 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.

Expectations for the next generation of lithium batteries include greater energy and power densities along with a substantial increase in both calendar and cycle life. Developing new materials to meet these goals requires a better understanding of how electrodes function by tracking physical and chemical changes of active components in a working electrode. Here we develop a new, simple in-situ electrochemical cell for the transmission electron microscope and use it to track lithium transport and conversion in FeF 2 nanoparticles by nanoscale imaging, diffraction and spectroscopy. In this system, lithium conversion is initiated at the surface, sweeping rapidly across the FeF 2 particles, followed by a gradual phase transformation in the bulk, resulting in 1-3 nm iron crystallites mixed with amorphous LiF. The real-time imaging reveals a surprisingly fast conversion process in individual particles (complete in a few minutes), with a morphological evolution resembling spinodal decomposition. This work provides new insights into the inter-and intra-particle lithium transport and kinetics of lithium conversion reactions, and may help to pave the way to develop highenergy conversion electrodes for lithium-ion batteries.

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