Trade-off between H₂O-rich and H₂O-poor electric double layers enables highly reversible Zn anodes in aqueous Zn-ion batteries

Although constructing an H2O-poor electric double layer (EDL) to replace an initial H2O-rich EDL at the Zn/electrolyte interface has been employed to realize highly reversible Zn anodes in aqueous Zn-ion batteries, the essential functional mechanisms among them still require investigation. Herein, a series of EDL structures from H2O-rich to H2O-poor have been constructed to comprehensively investigate its impacts on Zn plating/stripping processes. For the first time, we reveal the undesirable dead Zn hidden behind the H2O-poor EDL due to the solvation of Zn2+ with H2O which is impeded by the poor electrode-solvent contact during the stripping process, which is usually neglected. Meanwhile, dendrites and side reactions relevant to the H2O-rich EDL can be gradually restrained by enlarging the Zn nucleation overpotential and diminishing direct contact between Zn and H2O together with the decrease of H2O in the EDL. Therefore, we propose an innovative concept of a trade-off between H2O-rich and H2O-poor EDLs to balance above effects which synergistically determine cycling stability, then to construct a moderate degree of H2O-poor EDL enabled by the use of 1 mM aspartame (APM) additive for realizing a highly reversible Zn anode. As a proof-of-concept, an outstanding cycling life of over 4500 h and a high average Coulombic efficiency of 99.67% over 1050 cycles can be achieved. The design criteria suggested in this study will provide new insight to guide future H2O-poor EDL regulation additives for the Zn anodes.