Effects of Ion Valence States and Radii on the Performance of Solid-State PEDOT:PSS Electrochromic Devices

Ion transport is a critical factor influencing the performance of electrochromic devices (ECDs). In this work, the effects of the valence state and ionic radius of six cations, Li⁺, Na⁺, K⁺, Mg²⁺, Zn²⁺, and Al³⁺, on the performance of poly(3,4-ethylenedioxythiophene)-polystyrene sulfonic acid (PEDOT:PSS)-based ECDs are investigated. Both ion migration and diffusion are discussed as processes that were analyzed, with results indicating that the primary contributor to the response of ECDs is ion migration. The response time decreases with increasing ionic valence and ionic radius, with the valence state having the predominant effect on response speed. Multivalent ions, due to strong electric field forces, migrate more rapidly and intercalate fewer ions at equivalent reaction charges, thereby facilitating a faster response. ECDs with larger-radius ions, which exhibit slower diffusion rates, also achieve faster response speeds. Among the tested ECDs with various electrolytes, the Al³⁺-based ECD demonstrates the fastest colored/bleached response time of 0.36/0.4 s. In addition, ions with larger radii introduce greater steric hindrance and structural damage during insertion and extraction, leading to ion retention issues and reduced cycling stability. After 500 cycles (2025 s) under a square wave voltage of ±1.5 V, the peak current of ECDs with Li⁺ (0.60 Å) electrolyte decreased by only 5.07/5.66%, whereas for K⁺ (1.33 Å) electrolyte, the decrease was 11.55/12.37%. ECDs utilizing small-radius, higher-valence ions, such as Al³⁺, achieved both fast response and high cycling stability, with peak current reductions of only 6.98/5.01%. Additionally, the charge storage dynamics were examined by analyzing the contributions from surface-controlled and diffusion-controlled charges, further confirming that ion migration dominates in ECDs. The high contribution of surface-controlled charges (80%) supports the fast response of ECDs and contributes to the high coloration efficiency (592.6 cm²/C for Al³⁺-based devices).