Melamine induced co-regulation of solvation structure and interface engineering to achieve dendrite-free Zn-ion hybrid capacitors

Aqueous zinc (Zn)-based energy storage devices have attracted great attention due to their inherent low cost, intrinsic safety, and environmental friendliness. However, the growth of Zn dendrite and side reactions from water decomposition have limited the lifespan of Zn-ion energy storage. Herein, a green and multifunctional melamine (M) molecules is proposed as electrolyte additive and incorporated into the ZnSO4 electrolyte to modulate the deposition/stripping environment of Zn2+ and stabilize the Zn anode. The results of experiments coupled with density-functional theory (DFT) calculations and molecular dynamics (MD) simulations show that M molecules are able to optimize the electrolyte environment and regulate the electrode-electrolyte surface. More specifically, the hydrophilic group (-NH2) in the M molecule can effectively break the electrolyte sheath structure and reconstruct the hydrogen bonding network, because the M molecule has a stronger binding energy compared to H2O. Meanwhile, the -N= groups in M can anchor Zn2+ and cooperate with them to form a dynamic electrostatic protective layer, change the electric field distribution around the Zn deposition layer, provide more nucleation sites for Zn2+ and promote the deposition of Zn along the (002) crystal plane, thus inhibiting the random growth of Zn dendrite and the occurrence of side reactions. Consequently, the Zn//Zn symmetric cell assembled with the ZnSO4+M electrolyte presents boost the stable lifespan over 1,100 hours at a current density of 2 mA cm−2, 1 mAh cm−2, and also up to 1900 stable cycles with almost 100 % Coulomb efficiency for Zn//Cu asymmetric cell.