Karki Khim, Wu Lijun, Ma Ying, Armstrong Mark J, Holmes Justin D, Garofalini Stephen H, Zhu Yimei, Stach Eric A, Wang Feng
Sustainable Energy Technologies Department , Brookhaven National Laboratory , Upton , New York 11973 , United States.
Department of Condensed Matter Physics and Materials Science , Brookhaven National Laboratory , Upton , New York 11973 , United States.
J Am Chem Soc. 2018 Dec 26;140(51):17915-17922. doi: 10.1021/jacs.8b07740. Epub 2018 Dec 12.
Intercalation-type electrodes have now been commonly employed in today's batteries as such materials are capable of storing and releasing lithium reversibly via topotactic transformation, conducive to small structural change, but they have limited interstitial sites to hold Li. In contrast, conversion electrodes feature high Li-storage capacity, but often undergo large structural change during (de)lithiation, resulting in cycling instability. One exception is iron fluoride (FeF), a conversion-type cathode that exhibits both high capacity and high cycling stability. Herein, we report a lithiation-driven topotactic transformation in a single crystal of FeF, unveiled by in situ visualization of the spatial and crystallographic correlation between the parent and converted phases. Specifically, conversion in FeF resembles the intercalation process but involves transport of both Li and Fe ions within the F-anion array, leading to formation of Fe preferentially along specific crystallographic orientations of FeF. Throughout the process, the F-anion framework is retained, creating a checkerboard-like structure, within which the volume change is largely compensated, thereby enabling the high cyclability in FeF. Findings from this study, with unique insights into conversion reaction mechanisms, may help to pave the way for designing conversion-type electrodes for the next-generation high energy lithium batteries.
嵌入型电极如今已普遍应用于当今的电池中，因为这类材料能够通过拓扑化学转变可逆地存储和释放锂，有利于产生较小的结构变化，但它们容纳锂的间隙位点有限。相比之下，转换型电极具有高锂存储容量，但在（脱）锂过程中往往会发生较大的结构变化，导致循环稳定性较差。一个例外是氟化铁（FeF），一种兼具高容量和高循环稳定性的转换型阴极。在此，我们报道了FeF单晶中锂化驱动的拓扑化学转变，通过原位可视化母相和转变相之间的空间和晶体学相关性得以揭示。具体而言，FeF中的转变类似于嵌入过程，但涉及Li和Fe离子在F - 阴离子阵列中的传输，导致优先沿着FeF的特定晶体学取向形成Fe。在整个过程中，F - 阴离子框架得以保留，形成类似棋盘的结构，其中体积变化在很大程度上得到补偿，从而使FeF具有高循环稳定性。这项研究的发现对转换反应机制有独特见解，可能有助于为下一代高能锂电池设计转换型电极铺平道路。
