Electrochemical Performance of Carbon-Coated Cauliflower-like WO3 for Li-Ion Batteries

Metal oxides have been considered and investigated as alternative anodes for use in lithium ion batteries. The research focus is on two different mechanism groups: intercalation–deintercalation reaction and conversion (redox) reaction. The intercalation–deintercalation mechanism is that transition metal oxides and other compounds with a multidimensional layer structure can reversibly intercalate lithium ions into the lattice without destroying their crystal structure. While conversion mechanism usually produces metallic nanoparticles embedded in an insulating Li2O matrix and the shows generally much higher theoretical capacity than that of already-commercialized graphite. Among the metal oxides, tungsten oxides (WO2, WO3, and WO3-x H2O) as one of the important metal oxides with respect to physicochemical properties, have been extensively investigated for such applications as gas sensors, photocatalysis, electrochromic devices, electronic devices, dye-sensitized solar cells, supercapacitors, and lithium ion batteries. Interestingly, the tungsten oxides have better electronic conductivity (10–10-6 S cm-1) than some of the other oxides; in addition, they also rely on designing a nanostructured particle with a small radius and right crystalline phase for fast lithium-ion insertion and superior electrochemical performance. From this point of view, a wide variety of approaches have been pursued over the years for the synthesis of tungsten oxides, such as nanoparticles, nanowires, nanorods, and nanoflowers using various approaches which include a hydrothermal reaction, combustion process, sol–gel process, solution-phase reaction, template directed method, mechanochemical activation, and hot-wire chemical vapor deposition (HWCVD) method. However, most of the previous approaches led to the formation of monoclinic W18O49, hexagonal WO3, and WO3-xH2O nanostructures. There have been very few studies on nanostructured monoclinic WO3 (m-WO3). In this study, we will present a facile hydrothermal process that obtains cauliflower-like carbon-coated WO3 oxide with a high rate-capability and good cycling performance for lithium-ion batteries. The motivation in this study is to explore the possibility of using m-WO3 as an anode material for lithium-ion batteries and to elucidate its reaction mechanism with lithium during the charge–discharge process.