WO_(3)电致变色薄膜离子传输动力过程及其循环稳定性

Dynamic Process of Ions Transport and Cyclic Stability of WO_(3) Electrochromic Film

电致变色WO_(3)的离子传输动力学过程对其变色性能和循环稳定性具有重要的影响。离子传输过程涉及到WO_(3)电极的结构变形、相转变等复杂过程,导致通过传统的电化学阻抗谱很难进行有效研究。计时电位法是通过施加电流,测量电极材料响应电位的一种电化学表征方法。与其它电化学表征方法(阻抗谱法和伏安法)相比,该技术能够直接探测溶液–电极系统中不同状态下的电压分布,并经常被用于研究电极系统中的物质传输动力学行为,例如电极表面附近的质子吸附和传输现象。本工作采用计时电位技术研究和调控WO_(3)薄膜中的离子传输行为,结果表明:大的Li^(+)离子插入通量可拓宽WO_(3)/电解质界面处离子的传输通道,有助于离子传输动力和光响应速度的提升。然而,反复的离子插入/抽出行为会通过“离子球磨效应”减小WO_(3)/电解质界面处WO_(3)晶粒的尺寸,使得WO_(3)薄膜的致密性增强,阻碍离子传输和电解质渗透,导致插入的Li^(+)及反应产生的Li_(x)WO_(3)在WO_(3)结构中不可逆积累,薄膜的光学调制幅度和电致变色活性明显下降。该工作为电极材料中离子传输动力学分析和离子传输行为控制提供了一种有效的方法。

The dynamic process of ions transport in electrochromic WO_(3) film is usually studied by electrochemical impedance spectroscopy.However,the detailed features are hidden since the ions insertion into WO_(3) is a very complex process including structural deformation and phase transformations.Chronopotentiometry is an electrochemical characterization method that measures the response potential of a system under an imposed current.Compared to other dynamic characterization methods(impedance spectroscopy and CV),it allows direct access to the voltage contributions in different states of the solution-electrode system and has frequently been used to investigate kinetic effects such as adsorption and transport phenomena near electrode surface.In this study,chronopotentiometry is creatively applied to study ion transport kinetics and control ions insertion behavior in electrochromic WO_(3) film.The results suggest that a large ions insertion flux at the interface of WO_(3)/electrolyte could broaden ions transport channels due to the fierce lattice expansion during Li+ions insertion process,which further improves the ions transportation kinetics and gifts a fast switching speed of optical performance.However,the repeating ions insertion/extraction behaviors at the interface of WO_(3)/electrolyte for the long-term cycle process can reduce the size of WO_(3) grains as a“ball mill effect”.Especially,a large ions transport flux can aggravate the“ball mill effect”.Consequently,the structure of the WO_(3) film becomes very dense,which is unfavorable for ions transport and electrolyte permeation.This dense structure also leads to an irreversible accumulation of Li+ions and Li_(x)WO_(3) in the WO_(3) host structure,resulting in a decay of optical modulation ability and electrochromic activity.This work offers an efficient method to analyze ion transport kinetics in intercalation materials and a new understanding of the relationship between ion transport behavior and cyclic stability of electrochromic materials.