Facile intercalation of alkali ions in WO3 for modulated electronic and optical properties: Implications for artificial synapses and chromogenic application

Tungsten oxides (WO3) are widely recognized as multifunctional systems owing to the existence of rich polymorphs. These diverse phases exhibit distinct octahedra-tilting patterns, generating substantial tunnels that are ideally suited for iontronics. However, a quantitative comprehension regarding the impact of distinct phases on the kinetics of intercalated conducting ions remains lacking. Herein, we employ first-principles calculations to explore the spatial and orientational correlations of ion transport in γ- and h-WO3, shedding light on the relationship between diffusion barriers and the size of the conducting ions. Our findings reveal that different types and concentrations of alkali-metals induce distinct and continuous lattice distortions in WO3 polymorphs. Specifically, γ-WO3 is more appropriate to accommodate Li+ ions, exhibiting a diffusion barrier and coefficient of 0.25 eV and 9.31×10−8 cm2 s−1, respectively. Conversely, h-WO3 features unidirectional and sizeable tunnels that facilitate the transport of K+ ions with an even lower barrier and a high coefficient of 0.11 eV and 2.12×10−5 cm2 s−1, respectively. Furthermore, the introduction of alkali-metal into WO3 tunnels tends to introduce n-type conductivity by contributing s-electrons to the unoccupied W 5d states, resulting in enhanced conductivity and tunable electronic structures. These alkali metals in WO3 tunnels are prone to charge transfer, forming small polaronic states and modulating the light absorption in the visible and near-infrared regions. These tunable electronic and optical properties, combined with the high diffusion coefficient, underscore the potential of WO3 in applications such as artificial synapses and chromogenic devices.