As the main ion supplier of the electrochromic process, the electrolyte layer is one of the most important components for electrochromic devices. Currently, researchers mainly focus on composite electrolyte and limit concentration. There are few studies about the relationship between electrolyte concentration and electrochromic performance, especially cycle stability. Therefore, we aim to investigate the electrochromic performance of tungsten oxide films before and after the cyclic voltammetry-based cycling test in electrolytes with varied lithium perchlorate (LiClO₄) concentrations (0.1, 0.5, 1.0, and 2.0 mol/L). The results show that the film using the 1.0 mol/L electrolyte possesses the shortest coloring and fading time. The charge capacity is as high as 25.2 mC?cm^-2, and the decay rate of film at 1.0 mol/L electrolyte is only 25.4% after 6000 cycles. We reveal the influence of electrolyte concentration on electrochromic performance, which is of significance for the development and design of tungsten oxide-based electrochromic devices.

Tungsten oxide films are prepared on ITO bases by the radio frequency (RF) magnetron sputtering method. The thicknesses of the films are measured by a step meter (Bruker-DektakXT), and the scanning electron microscopy (SEM) images are conducted on Zeiss-Sigma 500 at a voltage of 15 kV. The crystalline structure of tungsten oxide films is examined on an X-ray diffractometer (Philips-X'Pert) by Cu Ka radiation. The LiClO₄ is dissolved in propylene carbonate (PC) solution to prepare LiClO₄-PC electrolytes with varied LiClO₄ concentrations (0.1, 0.5, 1.0, and 2.0 mol/L). The response time, cycle performance, and diffusion coefficient of the tungsten oxide film are evaluated by chronoamperometry and cyclic voltammetry tests at a CHI760E electrochemical workstation. The modulation rate is characterized by ultraviolet-visible spectrophotometry (Shimadzu-UV3150).

The prepared amorphous tungsten oxide films exhibit a nano-size peak-like surface structure at a constant thickness of about 500 nm (Fig. 1). The electrochromic properties and cyclic stability of these tungsten oxide films at different concentrations of LiClO₄-PC electrolyte (0.1, 0.5, 1.0, and 2.0 mol/L) are evaluated. After 6000 CV cycles, the films in 0.1 mol/L and 1.0 mol/L electrolytes demonstrate a higher optical modulation rate (Figs. 2 and 5). In terms of response time, the film in 1.0 mol/L electrolyte shows the shortest coloring and bleaching time both before and after the cyclic voltammetry test (Figs. 3 and 6). Additionally, the film in the 1.0 mol/L electrolyte exhibits an initial charge capacity of 25.2 mC?cm^-2 (Fig. 4). After 6000 CV cycles, its charge capacity is still as high as 18.8 mC?cm^-2 with the lowest decay rate of 25.4%, which is superior to the films in the electrolytes at other concentrations (Fig. 7). Meanwhile, the film in the 1.0 mol/L electrolyte shows the weakest ion stacking effect and the highest ion diffusion coefficients (Figs. 7 and 8). The SEM results also demonstrate that it has the best integrity after 6000 CV cycles (Fig. 9).

We explore the influence of electrolyte concentration on the electrochromic performance of tungsten oxide films. The results reveal that there is no significant difference in the optical modulation rate at various LiClO₄ electrolyte concentrations. However, the film using the 1.0 mol/L electrolyte possesses the shortest response time and the best cyclic stability. Its charge capacity decreases from 25.2 to 18.8 mC?cm^-2 after 6000 CV cycles, with a lower decay rate of 25.4%. The best film integrity of the tungsten oxide after 6000 CV cycles further proves the improved cyclic stability of the 1.0 mol/L electrolyte. In conclusion, in 1.0 mol/L LiClO₄-PC electrolyte, the tungsten oxide film shows optimal electrochromic performance, especially long-term cyclic stability. This could be attributed to the fact that the diffusion coefficients of lithium ions of the tungsten oxide film in the 1.0 mol/L electrolyte are significantly higher than those of other concentrations, weakening the lithium ion stacking effect. The proposed concentration-performance relationship of tungsten oxide films is significant for the mechanistic study and future development of electrochromic devices.