In-situ transmission electron microscopy imaging of formation and evolution of Li_xWO_3 during lithiation of WO_3 nanowires

摘要

The phase transition from monoclinic WO_3 to cubic Li_xWO_3 during lithiation of WO_3 is one of the key features for tungsten oxide as the most used electrochromic material. Conventionally, the lithium intercalation of WO_3 has been studied by building generic layered electrochromic device combining with structural characterization and electrochemistry measurement at macro scale. In-situ transmission electron microscopy (in-situ TEM) has been proposed as a method for revealing the detailed mechanism of structural, physical, and chemical properties. Here, we use in-situ TEM method to investigate the formation and evolution of Li_xWO_3 in real-time during the electrochemical lithiation of WO_3 nanowires. The dynamic lithiation process is recorded by TEM imaging, diffraction, and electron energy loss spectroscopy. The WO_3-Li_xWO_3 phase boundary of reaction front has been observed at high resolution. The timeliness of crystallinity of Li_xWO_3 and the intercalation channels for Li ions are also identified. Moreover, the co-existence of both polycrystalline Li-poor area and amorphous Li-rich phases of Li_xWO_3 was found. Our results provide an insight into the basic lithiation process of WO_3, which is significantly important for understanding the electrochromic mechanism of tungsten oxide.