Effects of Zn²⁺ and H⁺ Association with Naphthalene Diimide Electrodes for Aqueous Zn-Ion Batteries

Organic electrodes have been extensively developed to enhance cycling performance in aqueous zinc (Zn)-ion cells. However, little is known about an ion-association process, which caused insufficient diagnoses of various cyclability results. Protons (H+) are charge carriers alongside Zn2+ ions in mildly acidic electrolyte solutions and preferentially participate in the storage process. In addition, dissociation of water can supply the additional H+, and the increased pH yields the precipitate of zinc hydroxy sulfate. Here, we demonstrated the critical effect of the H+ for cycling stability of Zn-ion cells, using a 1,4,5,8-naphthalene diimide (NDI) electrode. Stepwise electron transfer of the NDI electrode proceeded via surface association of the charge carriers and subsequent solid-state diffusion. The H+ intercalation to the NDI electrode became pronounced with an increasing current rate and led to the dissolution of the NDI molecules. We exhibited stable 1000 cycles with a capacity retention of 93.8% by suppression of the H+ impact and suggested a solution-mediated NDI reassembly mechanism as the core reason for the capacity fading.