title: Structure Stabilization by Mixed Anions in Oxyfluoride Cathodes for High-Energy Lithium Batteries.
abstract: Mixed-anion oxyfluorides (i.e., FeOxF2-x) are an appealing alternative to pure fluorides as high-capacity cathodes in lithium batteries, with enhanced cyclability via oxygen substitution. However, it is still unclear how the mixed anions impact the local phase transformation and structural stability of oxyfluorides during cycling due to the complexity of electrochemical reactions, involving both lithium intercalation and conversion. Herein, we investigated the local chemical and structural ordering in FeO0.7F1.3 at length scales spanning from single particles to the bulk electrode, via a combination of electron spectrum-imaging, magnetization, electrochemistry, and synchrotron X-ray measurements. The FeO0.7F1.3 nanoparticles retain a FeF2-like rutile structure but chemically heterogeneous, with an F-rich core covered by thin O-rich shell. Upon lithiation the O-rich rutile phase is transformed into Li-Fe-O(-F) rocksalt that has high lattice coherency with converted metallic Fe, a feature that may facilitate the local electronic and ionic transport. The O-rich rocksalt is highly stable over lithiation/delithiation and thus advantageous to maintain the integrity of the particle, and due to its predominant distribution on the surface, it is expected to prevent the catalytic interaction of Fe with electrolyte. Our findings of the structural origin of cycling stability in oxyfluorides may provide insights into developing viable high-energy electrodes for lithium batteries.
authors.affiliation: Brookhaven National Laboratory , Upton, New York 11973, United States.|Nuclear Fuel Cycle Process Development Group, Korea Atomic Energy Research Institute , Daejeon 34057, Republic of Korea.
authors.forename: Sung-Wook
authors.initials: SW
authors.lastname: Kim
authors.affiliation: Department of Materials Science and Engineering, Rutgers University , North Brunswick, New Jersey 08902, United States.
authors.forename: Nathalie
authors.initials: N
authors.lastname: Pereira
authors.affiliation: Institute for Materials Research and Department of Chemistry, State University of New York at Binghamton , Binghamton, New York 13902, United States.
authors.forename: Natasha A
authors.initials: NA
authors.lastname: Chernova
authors.affiliation: Institute for Materials Research and Department of Chemistry, State University of New York at Binghamton , Binghamton, New York 13902, United States.
authors.forename: Fredrick
authors.initials: F
authors.lastname: Omenya
authors.affiliation: Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Peng
authors.initials: P
authors.lastname: Gao
authors.affiliation: Institute for Materials Research and Department of Chemistry, State University of New York at Binghamton , Binghamton, New York 13902, United States.
authors.forename: M Stanley
authors.initials: MS
authors.lastname: Whittingham
authors.affiliation: Department of Materials Science and Engineering, Rutgers University , North Brunswick, New Jersey 08902, United States.
authors.forename: Glenn G
authors.initials: GG
authors.lastname: Amatucci
authors.affiliation: Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Dong
authors.initials: D
authors.lastname: Su
authors.affiliation: Brookhaven National Laboratory , Upton, New York 11973, United States.
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
keywords: electron energy loss spectroscopy (EELS); iron oxyfluoride; lithium batteries; mixed-anion cathodes; scanning transmission electron microscopy (STEM)