Highly Doped Conjugated Polymers for Electrochemical Energy Storage

Electrodes composed of conjugated polymers (CPs) offer the potential of high-capacity, high-energy density electrochemical energy storage. CP electrodes are conductive, have high Coulombic efficiency, and are synthetically versatile. These materials store charge through electrochemical reduction and oxidation, otherwise known as de-doping and doping, respectively. Commonly explored CP electrodes, such as polyaniline or polypyrrole, can only achieve doping levels of 0.3-0.5, which result in only modest capacities and energies. Here, several routes to achieve doping levels as high as 0.9 are presented, wherein the result is a CP electrode that bears a capacity 90% of its theoretical value. It should be noted that even higher doping levels are possible with the addition of redox-active side chains. In the first route, aniline is synthesized in the presence of a polyacid to achieve a water-dispersible polyaniline:polyacid colloid. The polyacid acts as the dopant and provides non-covalent interactions that stabilize polyaniline in its most highly doped state, pernigraniline salt. Because of this stability, polyaniline can be reversibly reduced and oxidized up to 4.5 V vs. Li/Li⁺. The implementation of polyaniline:polyacid in a layer-by-layer electrode is also presented. The second route to high doping levels focuses upon electron-rich polymers, such as poly(dithienopyrroles), or poly(DTPs). Side chains are used to modify the poly(DTP)’s interaction with electrolyte, the backbone’s planarity, and the resultant conjugation. In the long term, high-capacity, high-energy CP electrodes potentially enable the formation of flexible energy and power for use in flexible electronics and other applications requiring unconventional shapes and form factors.