title: Structures of delithiated and degraded LiFeBO(3), and their distinct changes upon electrochemical cycling.
abstract: Lithium iron borate (LiFeBO3) has a high theoretical specific capacity (220 mAh/g), which is competitive with leading cathode candidates for next-generation lithium-ion batteries. However, a major factor making it difficult to fully access this capacity is a competing oxidative process that leads to degradation of the LiFeBO3 structure. The pristine, delithiated, and degraded phases of LiFeBO3 share a common framework with a cell volume that varies by less than 2%, making it difficult to resolve the nature of the delithiation and degradation mechanisms by conventional X-ray powder diffraction studies. A comprehensive study of the structural evolution of LiFeBO3 during (de)lithiation and degradation was therefore carried out using a wide array of bulk and local structural characterization techniques, both in situ and ex situ, with complementary electrochemical studies. Delithiation of LiFeBO3 starts with the production of LitFeBO3 (t ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Lit-xFeBO3 (x < 0.5). However, the large overpotential needed to drive the initial two-phase delithiation reaction results in the simultaneous observation of further delithiated solid-solution products of Lit-xFeBO3 under normal conditions of electrochemical cycling. The degradation of LiFeBO3 also results in oxidation to produce a Li-deficient phase D-LidFeBO3 (d ≈ 0.5, based on the observed Fe valence of ∼2.5+). However, it is shown through synchrotron X-ray diffraction, neutron diffraction, and high-resolution transmission electron microscopy studies that the degradation process results in an irreversible disordering of Fe onto the Li site, resulting in the formation of a distinct degraded phase, which cannot be electrochemically converted back to LiFeBO3 at room temperature. The Li-containing degraded phase cannot be fully delithiated, but it can reversibly cycle Li (D-Lid+yFeBO3) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V).
authors.affiliation: Chemistry Department, Stony Brook University , Stony Brook, New York 11794, United States.
authors.forename: Shou-Hang
authors.initials: SH
authors.lastname: Bo
authors.forename: Kyung-Wan
authors.initials: KW
authors.lastname: Nam
authors.forename: Olaf J
authors.initials: OJ
authors.lastname: Borkiewicz
authors.forename: Yan-Yan
authors.initials: YY
authors.lastname: Hu
authors.forename: Xiao-Qing
authors.initials: XQ
authors.lastname: Yang
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Chupas
authors.forename: Karena W
authors.initials: KW
authors.lastname: Chapman
authors.forename: Lijun
authors.initials: L
authors.lastname: Wu
authors.forename: Lihua
authors.initials: L
authors.lastname: Zhang
authors.forename: Feng
authors.initials: F
authors.lastname: Wang
authors.forename: Clare P
authors.initials: CP
authors.lastname: Grey
authors.forename: Peter G
authors.initials: PG
authors.lastname: Khalifah