Structures of Delithiated and Degraded LiFeBO3, and Their Distinct Changes upon Electrochemical Cycling

• Published in: Inorganic chemistry Vol. 53; no. 13; pp. 6585 - 6595
• Main Authors: Bo, Shou-Hang, Nam, Kyung-Wan, Borkiewicz, Olaf J, Hu, Yan-Yan, Yang, Xiao-Qing, Chupas, Peter J, Chapman, Karena W, Wu, Lijun, Zhang, Lihua, Wang, Feng, Grey, Clare P, Khalifah, Peter G
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07.07.2014
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic500169g

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 Li t FeBO3 (t ≈ 0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Li t–x FeBO3 (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 Li t–x FeBO3 under normal conditions of electrochemical cycling. The degradation of LiFeBO3 also results in oxidation to produce a Li-deficient phase D-Li d FeBO3 (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-Li d+y FeBO3) at a thermodynamic potential of ∼1.8 V that is substantially reduced relative to the pristine phase (∼2.8 V).