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

Abstract

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.