Identifying the charge storage mechanism in polyimide anodes for Na-ion aqueous batteries by impedance spectroscopy

Polyimides such as poly(naphthalene four formyl ethylenediamine) (PNFE) demonstrate promising electrochemical properties as anode materials for aqueous sodium-ion batteries. They display high specific capacities and good cyclability. Still, an accurate physical model describing the exact charge storage mechanism in these electroactive polymers needs to be explored in more detail. This work aims to address this gap through an impedance study to elucidate the mechanisms of reduction and oxidation processes of polyimide electrodes. An electrochemical impedance model is also proposed, which enables good fitting of the impedance data at all potentials. This model confirms the commonly accepted two-stage enolization reaction as the main reduction path and also unveils a previously unrecognized single-electron pathway. A complete model was identified by including this novel pathway, which enabled a better understanding of these materials. This work proposes a new electrochemical impedance model for the redox mechanism of polyimide anodes in aqueous Na-ion batteries. It provides key insights into the discrepancy between experimental and theoretical specific capacities.