Characterization of sub-stoichiometric tungsten trioxide (WO3−X) using impedance spectroscopy

Tungsten oxide electroceramic materials are routinely used in metal oxide semiconductor (MOS) gas sensors and photoelectric devices due to their gasochromic and photoelectrochromic properties. Hexagonal tungsten oxide bronze phases are ideal intercalation hosts for Li+ cations and are therefore utilised as electrode materials in lithium ion batteries. Previous research has been largely concentrated on thin and thick films due to these applications. This work aims to fill the voids in the electrical characterization of dense non-stoichiometric WO3−X polycrystalline materials. The electrical properties of which are mainly attributed to grain boundary phases and internal oxygen concentration gradients (OCGs) across grain boundaries and inter-connecting pores. The sintering regime of WO3−X was optimised to produce fully sintered pellets with varying grain sizes and oxygen stoichiometry. Impedance spectroscopy (IS) was used to electrically characterize the microstructure, in addition to conventional electron microscopy characterization. This technique is especially valuable since it allows the deconvolution of the total impedance response of materials into their constituent components, e.g., Zbulk, Zgb, and Zel, according to their characteristic time constants.