Challenging metal-ion rocking-chair and zinc-ion mechanisms in mild acidic to neutral aqueous electrolytes

The utilization of 2,5-ditertbutyl hydroquinone (DTBHQ) as a potential active material in aqueous zinc-ion batteries (AZIBs) was studied for the first time. Thanks to its two hydrophobic groups, DTBHQ demonstrates a maximum solubility of 629 µM/L, significantly lower than that of other reported small quinone derivatives for AZIBs. This property translates to remarkable long-term capacity retention, even at a high electrolyte to active mass ratio. Significantly, our findings provide unequivocal support for a proton insertion mechanism as the main electrochemical process in the solid state in mono- and divalent aqueous electrolytes, thereby challenging the prevailing notion of metal-ion rocking-chair and zinc-ion mechanisms widely reported for aqueous batteries. This novel insight that holds for electrolytes based on metal- (K+, Zn2+ and Mg2+) and non-metal cations (NH4+ and NH3OH+) has profound implications for the understanding and development of aqueous batteries since the operating reduction potential was found to be directly proportional to the pKa values of the predominant acid/base couple of the tested aqueous electrolytes.