Prolonging lifespan of Prussian blue electrochromic films by an acid-free bulky-anion potassium organic electrolyte

Abstract

A novel KCF 3 SO 3 -TEP electrolyte is developed to support long-term color switching of PB electrochromic films, taking advantages of the synergistic benefits from the suitable size K + cation, bulky CF 3 SO 3 − anion and TEP molecule. • A novel KCF 3 SO 3 -TEP electrolyte is developed to prolong the lifespan of PB films. • This electrolyte simultaneously suppresses mechanical degradation and “ion trapping” effect. • The beneficial influence is attributed to the suitable size K + , bulky CF 3 SO 3 − and TEP molecule. • This electrolyte shows excellent fire retardance and electrochemical stability. Electrochromic (EC) technology may reform the smart windows, information displays, and optical camouflage fields by their controllably changeable optical properties. With an open-framework lattice consisting of common elements, Prussian blue (PB) is recognized as an important EC material, thanks to its multifaceted merits including low cost, proper redox potential, high electrochemical activity, and fast reaction kinetics. This material is also a popular electrode material of secondary batteries. Nevertheless, establishing durable PB-based EC devices and batteries remains still substantially challenging, due to the “ion-trapping” effect and structural degradation during cyclic color-switching/charge–discharge. In this article, we report an acid-free and safe KCF 3 SO 3 -TEP (KCF 3 SO 3 in Triethyl phosphate) organic electrolyte to significantly prolong the cycling lifespan of PB films. Comparative experiments and systematic characterization attribute the prolonged lifespan to the bulky CF 3 SO 3 − anion and large TEP solvent molecule of the electrolyte, which are both resistive to co-insertion into the PB lattice. This rationally-designed electrolyte enables an impressive cyclic color-switching stability of 1000 times without any detectable EC performance or mechanical degradation, overwhelmingly outperforming the well-established acidized KCl-H 2 O and LiClO 4 -PC electrolytes. Finally, as a demonstration of its application, a bifunctional PB/Zn EC battery is fabricated based on a K + /Zn 2+ -CF 3 SO 3 -TEP dual-cation electrolyte. These findings may shed light on the in-depth understanding of PB’s electrochemical degradation, as well as the electrolyte design of robust PB-based electrochemical devices.