Tungsten Bronzes ΑxWO3 (A=Li, Na or K) As Electrodes for Fast Aqueous Batteries

Abstract

Aqueous batteries, asymmetric supercapacitors and hybrid devices using oxides as electrodes have been envisioned for stationary applications due to their low environmental impact and the easiness to recycle oxides when operated in a water based electrolyte [1]. This is especially true when neutral pH electrolytes are used. Quite a lot of positive electrode oxides have been proposed up to now but it seems more difficult to combine high capacity and low potential electrochemical window for negative electrodes [2]. In the present study we propose tungsten oxide WO3 as a potential candidate for negative electrode of mild aqueous electrolyte energy storage device. More specifically the metastable phase of hexagonal bronze (h-WO3) was targeted. This phase cannot be obtained by purely structural transformation upon temperature [3]. This structure possesses a rigid tungsten-oxygen framework built up of layers containing corner-shared WO6 octahedra that are arranged in six-membered rings. The layers are stacked along the [00l] axis, giving rise to one-dimensional tunnels being able to host different alkali cations such as Li+, Na+, and K+ [4]. This arrangement can result in interesting electrochemical properties when used as electrode materials for energy storage devices [5].Hexagonal tungsten oxides with the general formula AxWO3 (A= Li, Na, or K and 0 < x < 0.33) were successfully synthesized by hydrothermal synthesis. The materials were characterized by X-ray Diffraction, Scanning and Transmission Electron Microscopy, Energy dispersive X-ray spectroscopy, and BET specific surface area measurements. Their electrochemical behaviors in different aqueous electrolytes such as LiNO3 (5M and 1M), Li2SO4, Na2SO4, and K2SO4. (0.5 M) were investigated and specific capacities up to ~80 C g-1 were measured. In this communication we will detail the electrochemical response of the different tungsten bronzes and we will give some more fundamental insights about the charge storage mechanisms using advanced characterization techniques.