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
Chemistry Department, Stony Brook University , Stony Brook, New York 11794, United States.
Inorg Chem. 2014 Jul 7;53(13):6585-95. doi: 10.1021/ic500169g. Epub 2014 Jun 10.
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).
硼酸铁锂（LiFeBO₃）具有较高的理论比容量（220 mAh/g），与下一代锂离子电池的领先正极候选材料相比具有竞争力。然而，一个使其难以充分发挥这种容量的主要因素是一个竞争性的氧化过程，该过程会导致LiFeBO₃结构的降解。LiFeBO₃的原始、脱锂和降解相具有共同的框架，其晶胞体积变化小于2%，这使得通过传统的X射线粉末衍射研究难以解析脱锂和降解机制的本质。因此，利用多种体相和局部结构表征技术，包括原位和非原位技术，并结合互补的电化学研究，对LiFeBO₃在（脱）锂和降解过程中的结构演变进行了全面研究。LiFeBO₃的脱锂始于通过两相反应生成LitFeBO₃（t≈0.5），随后该相进一步脱锂形成Lit-xFeBO₃（x<0.5）。然而，驱动初始两相脱锂反应所需的大过电位导致在电化学循环的正常条件下同时观察到Lit-xFeBO₃的进一步脱锂固溶体产物。LiFeBO₃的降解也会导致氧化，产生锂缺陷相D-LidFeBO₃（d≈0.5，基于观察到的铁价态约为2.5+）。然而，通过同步辐射X射线衍射、中子衍射和高分辨率透射电子显微镜研究表明，降解过程导致铁在锂位点上发生不可逆的无序排列，从而形成一个独特的降解相，该相在室温下不能电化学转化回LiFeBO₃。含锂的降解相不能完全脱锂，但它可以在相对于原始相（约2.8 V）大幅降低的热力学电位约1.8 V下可逆地循环锂（D-Lid+yFeBO₃）。
