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

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• 作者：Shou-Hang Bo ; Kyung-Wan Nam ; Olaf J. Borkiewicz ; Yan-Yan Hu ; Xiao-Qing Yang ; Peter J. Chupas ; Karena W. Chapman ; Lijun Wu ; Lihua Zhang ; Feng Wang ; Clare P. Grey ; Peter G. Khalifah
• 刊名：Inorganic Chemistry
• 出版年：2014
• 出版时间：July 7, 2014
• 年：2014
• 卷：53
• 期：13
• 页码：6585-6595
• 全文大小：614K
• ISSN：1520-510X

文摘

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_tFeBO₃ (t 鈮?0.5) through a two-phase reaction, and the subsequent delithiation of this phase to form Li_t鈥?i>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 Li_t鈥?i>xFeBO₃ under normal conditions of electrochemical cycling. The degradation of LiFeBO₃ also results in oxidation to produce a Li-deficient phase D-Li_dFeBO₃ (d 鈮?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+yFeBO₃) at a thermodynamic potential of 1.8 V that is substantially reduced relative to the pristine phase (2.8 V).