Structural phase transformation and Fe valence evolution in FeO x F 2x /C nanocomposite electrodes during lithiation and de-lithiation processes M. Sina, a K.-W. Nam, b D. Su, c N. Pereira, ad X.-Q. Yang, b G. G. Amatucci ad and F. Cosandey * a In this study, the structural changes of FeO x F 2x /C during the first discharge and recharge cycles were studied by ex situ electron microscopy techniques including annular dark field scanning transmission electron microscopy (DF-STEM) imaging, selected area electron diffraction (SAED) and electron energy loss spectroscopy (EELS) as well as by in situ X-ray absorption spectroscopy (XAS). The evolution of the valence state of Fe was determined by combined EELS using the Fe-L edge and XAS using the Fe-K edge. The results of this investigation show that the conversion reaction path during 1 st lithiation is very different from the re-conversion path during 1 st delithiation. During lithiation, intercalation is first observed followed by conversion into a lithiated rocksalt (Li–Fe–O–F) structure, and metallic Fe and LiF phases. During delithiation, the rocksalt phase does not disappear, but co-exists with an amorphous (rutile type) phase formed initially by the reaction of LiF and Fe. However, a de-intercalation stage is still observed at the end of reconversion similar to a single phase process despite the coexistence of these two (rocksalt and amorphous) phases. 1 Introduction Lithium ion batteries are now widely used as rechargeable energy storage devices for a variety of electronic applications ranging from appliances to laptops and cell phones. At the present time, intercalation compounds are used almost exclu- sively for the positive electrodes but despite their success, their practical capacity remains limited in the range of 120 to 200 mA hg 1 . 1–3 This limitation is caused in part by the intercalation process which limits the charge transfer to one electron per transition metal. In order to improve the capacity of positive electrodes, conversion materials are being explored where all the valence states of the transition metal are used, leading to higher theoretical capacities. In particular, reversible conver- sion materials based on metal uorides have been considered due to their high theoretical capacities (500 to 750 mA h g 1 ) at intermediate voltages (2–3 V). 4–9 Conversion materials have been explored since the 70s as primary battery electrodes but their more recent resurgence has been enabled through the use of metal uoride/carbon nanocomposites. 8 Numerous systems have now been explored such as FeF 2 , 5,10,11 NiF 2 , 5 CoF 2 , 5 TiF 3 , 6,12 FeF 3 , 4–7,10,13–16 BiF 3 , 17–19 and CuF 2 . 11,16,20,21 In these conversion systems, the cycling performance is strongly dependent on their synthesis method, uoride chemistry, cycling rate and temperature. For FeF 2 , direct two phase conversion to metallic Fe (3 nm in size) and LiF has been observed with the possible formation of a bi-continuous network, thus allowing for a high conductivity path. 11 In contrast, the poor reversibility of CuF 2 was attributed to Cu 1+ dissolution upon charge. 22 For a system with possible multiple valence states (Fe 0 , Fe 2+ and Fe 3+ ) such as for the FeF 3 system, more complex phase transformation reac- tions occur involving rst intercalation with possible phase transformation from an initial rhombohedral to a defective tri- rutile phase followed by conversion into metallic Fe and LiF. 23,24 In the search for conversion materials with improved capacity and cycle life, iron oxyuoride nanocomposites have been explored due to the high theoretical specic capacity of about 885 mA h g 1 for Fe 3+ OF aer full reduction of Fe 0 to its metallic state. 25 This system is also of interest as FeOF has the same rutile (p4 2 /mnm) structure as FeF 2 but with a higher Fe +3 valence state. In addition, it has been shown recently that single phase FeO x F 2x with a range of oxygen content x from 0 to 0.7 can be synthesized by a single synthesis processing path. 25 Studying the structural transformations that accompanied lithiation and delithiation is important in order to optimize the performance of the compound and to understand phenomena such as voltage hysteresis and transformation kinetics. For a Materials Science and Engineering, Rutgers University, Piscataway, NJ 08854, USA. E-mail: cosandey@rci.rutgers.edu; mahsa20@eden.rutgers.edu; npereira@rci. rutgers.edu; gamatucc@rci.rutgers.edu b BNL-Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: xyang@bnl.gov; knam@bnl.gov c BNL-CFN, Brookhaven National Laboratory, Upton, NY 11973, USA. E-mail: dsu@bnl. gov d Energy Storage Research Group (ESRG), Rutgers University, North Brunswick, NJ 08902, USA Cite this: DOI: 10.1039/c3ta12109g Received 29th May 2013 Accepted 24th July 2013 DOI: 10.1039/c3ta12109g www.rsc.org/MaterialsA This journal is ª The Royal Society of Chemistry 2013 J. Mater. Chem. A Journal of Materials Chemistry A PAPER Published on 15 August 2013. Downloaded by RUTGERS STATE UNIVERSITY on 15/08/2013 16:06:33. View Article Online View Journal