To mitigate the energy consumption in architectures, electrochromic smart windows are designed to dynamically modulate the solar irradiance under the stimulus of external potentials. The use of niobium(V) oxide nanocrystals (NCs) in electrochromic smart windows has recently drawn considerable attention due to their dual-mode electrochromism, where the absorbance of the materials in the visible and near-infrared (NIR) regions can be modulated independently. This phenomenon has shown the promise of allowing the smart windows made of niobium(V) oxide NCs to better adjust to various weather conditions, but still the underlying mechanism has not been extensively explored. In niobium(V) oxides, the main electrochromic mechanism enabling the change of their optical properties, known as the polaronic model, is governed by a faradaic process involving the redox of niobium ions accompanied by the insertion/de-insertion of lithium ions from the electrolyte. It is known that the crystal structure of niobium(V) oxides can greatly impact their lithium insertion/de-insertion kinetics and reversibility, and the local atomic structure surrounding the sites where the lithium inserted into often determines the spectral range and efficiency of the polaronic absorption. In this regard, we have developed the synthesis of niobium(V) oxide NCs with controllable crystal structure to investigate their structure-dependent electrochromism. By extending the NCs in different crystalline orientations, the local crystal structure surrounding the lithiated sites can be controlled. We aimed at investigating the influence of the structure of niobium(V) oxide NCs on their dual-mode electrochromic properties as a method to improve the performance of NC-based electrochromic smart windows.
