Electrochemical energy storage (EES) has become increasingly important with the rise of renewable energy platforms such as wind and solar. Redox-active polymers, especially pi-conjugated polymers, are of interest for use in EES due to their conductivity, tunability, sustainability, and non-toxicity. Pi-conjugated polymers store energy via a doping mechanism, in which ions compensate the charge generated on the pi-conjugated backbone as a result of oxidation or reduction. Here, we investigate n-type redox-active poly(fluorene-alt-naphthalene diimide) (PFNDI) as an electrode for organic EES. The naphthalene diimide (NDI) unit is particularly redox-active and the polyfluorene (PF) unit provides π-conjugation and some redox activity. However, the nature and mechanism of energy storage for PFNDI is not well-described. Further, n-type polymers in general have high impedance in the doped state and stability issues. We hypothesized that the synergy between the PF and NDI units would address these issues. Using cyclic voltammetry, galvanostatic cycling, and impedance spectroscopy, we demonstrate that PFNDI stores charge by cathodic doping with Li+ ions. We show that PFNDI can be stably and reversibly doped with 86% capacity retention up to 500 cycles while maintaining 99.8% coulombic efficiency. With added carbon, the PFNDI composite electrode showed an improved rate performance and capacity over pure PFNDI. This work demonstrates that PFNDI can be used as an energy storage material in Li-ion batteries and has potential for modifications to improve the performance in the anode. Enhanced stability is attained by charge storage and resonance at the NDI unit, along with pi-conjugation along the backbone.

Poly(fluorene-alt-naphthalene diimide) energy storage electrodes

2019 Spring National Meeting

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