Growth Temperature and Electrochemical Performance in Vapor-Deposited Poly(3,4-ethylenedioxythiophene) Thin Films for High-Rate Electrochemical Energy Storage

Abstract

Poly(ethylene 3,4-dioxythiophene (PEDOT) films synthesized by oxidative chemical vapor deposition (oCVD) display strong electrochemical activity in the region from 2 to 4.2 V vs Li/Li+. By contrast, the more commonly studied PEDOT:polystyrenesulfonate (PSS) films have negligible electrochemical activity in this region. For the oCVD films, its small dopant anions (Cl–) that can easily enter and exit the polymer structure allow exchange with the Li+ counterion in solution, while for PEDOT:PSS, the poly(styrenesulfonate) dopant is a large macromolecule having substantially lower mobility. Here, we seek to elucidate the relationship between the structural characteristics of oCVD PEDOT thin films and their electrochemical properties, particularly in Li-ion electrolyte systems. Specifically, we seek to rationally design the thin-film properties of oCVD PEDOT for high-rate performance and cycle life by varying the film growth temperature. We observe that the dominant effect of increasing growth temperature is an in situ reorganization to an edge-on film texture. In this case, the π–π stack is perpendicular to the substrate surface. The alternative dominant texture is face-on dominance, where the π–π stack is parallel to the substrate surface. For the first time, we show that edge-on dominant films provide higher specific capacities for a given charge/discharge rate. Furthermore, Raman spectroscopy demonstrates that edge-on dominant films are less susceptible to oxidative damage after long-term cycling. This also enables edge-on dominant films to maintain lower charge-transfer resistances compared to identically cycled face-on films. Edge-on oCVD PEDOT is paired with molybdenum disulfide to demonstrate thick, optimized oCVD PEDOT thin films in asymmetric devices for high-rate electrochemical energy storage.