Pyrene Diimide Based π-Conjugated Copolymer and Single-Walled Carbon Nanotube Composites for Lithium-Ion Batteries

A π-conjugated copolymer based on a pyrene diimide unit (P(PyDI-T2)) was synthesized using the Suzuki–Miyaura cross-coupling reaction between 4,5,9,10-pyrene diimide-2,7-diboronic ester and 2,2′-dibromo-5,5′-bithiophene for use as an active material in a Li-ion battery. Usually, diimide molecules are known to demonstrate reversible redox processes with a maximum of a two-electron insertion per unit. The unique structure of pyrene diimide, consisting of a pyrene core bearing two imide functions lying on formal double bonds, was anticipated to potentially demonstrate reversible redox processes involving four electrons per unit via aromatic stabilization. Also, the 2- and 7-positions are much less sterically hindered than similar relative positions in other diimides and hence should permit good electron mobility in the resulting polymer. Unfortunately, we were unable to observe any reduction phenomena in a half-cell (with a Li anode), likely due to negligible electronic conductivity. Subsequently, P(PyDI-T2) was blended with SWCNTs for dispersion in order to enhance both conductivity and surface area. High concentration dispersion was filtered to obtain a freestanding film of P(PyDI-T2)/SWCNT, with an intrinsic conductivity of 30 S/cm, which was used directly as a cathode in the half-cell. Reversible redox pointing to two one-electron phenomena was observed with 40.6 mA h/g at a galvanostatic slow rate of C/50 with a high mass loading of 3.2 mg/cm2. Density functional theory calculations were performed on pyrene diimide units to elucidate poor mobility and to compare Gibbs free energies for the second reduction of each site.