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

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 pi-pi stack is perpendicular to the substrate surface. The alternative dominant texture is face-on dominance, where the pi-pi 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.