Poly(fluorene-alt-naphthalene diimide) as n-Type Polymer Electrodes for Energy Storage

Redox-active polymers, especially conjugated polymers, are of interest for use in battery electrodes due to their conductivity, synthetic versatility, and processability. Conjugated polymers store energy via a doping mechanism, in which counterions compensate the charge generated on the conjugated backbone as a result of oxidation or reduction. Here, n-type redox-active poly(fluorene-alt-naphthalene diimide) is investigated as an organic battery electrode. The naphthalene diimide (NDI) unit is redox-active, and the polyfluorene (PF) unit provides π-conjugation and redox activity. However, the nature and mechanism of energy storage for PFNDI are not well described. In addition, n-type polymers in general have high impedance in the doped state and stability issues. It is hypothesized here that the synergy between the PF and NDI units may address these issues. Cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy are utilized to demonstrate that PFNDI stores charge by cathodic doping with Li+ ions. PFNDI is reversibly doped with 86% capacity retention up to 500 cycles while maintaining 99.8% Coulombic efficiency. The relative stability is attributed to the resonance at the NDI unit, along with π-conjugation along the backbone. With added carbon, a PFNDI composite electrode showed an improved rate performance and capacity over pure PFNDI. The highest capacity recorded for the composite electrode was 39.8 mAh/g at 0.5 C, corresponding to an n-doping level of 1.6 Li+ ions/repeat unit. This work demonstrates that PFNDI is a candidate n-type polymer for reversible energy storage.