Alleviating zinc dendrite growth by versatile sodium carboxymethyl cellulose electrolyte additive to boost long-life aqueous Zn ion capacitors

Abstract

Zn ion capacitors (ZICs) have become a strong candidate for energy storage equipment because the advantages of environment friendly, high safety, low cost. However, the Zn anode suffers from severe dendrite growth and irreversible side reactions, resulting in a short cycle life of ZICs. In this work, a green and low-cost sodium carboxymethyl cellulose (CMC-Na) is used as an additive in ZnSO4 electrolyte. The density functional theory calculations and molecular dynamics simulations demonstrate that CMC-Na can effectively optimize the electrolyte environment and regulate the electrode-electrolyte interface. Among them, Na+ has a lower redox site compared to Zn2+, Na+ can be reduced preferentially to Zn2+, the formation of the electrostatic protection layer can inhibit the formation of differential electric field and promote the uniform distribution of the electric field, which is conducive to the homogenization of the nucleation sites of the Zn deposition layer. The organic molecules contain abundant hydrophilic groups (OH, COO−), which can effectively optimize the structure of Zn2+ solvation sheath by destroying the hydrogen bonding network in the environment of ZnSO4 electrolyte, thus reducing the rate of side reactions and the random growth of dendrites will be suppressed. And the Zn//Zn cells with ZnSO4+CMC-Na(0.13) electrolyte enable to perform a stable cycle over 1600 h (at 4 mA cm−2 with a capacity of 1 mAh cm−2), which is vastly superior that of without additive (only 70 h). The results confirm that the import of CMC-Na additive into the ZnSO4 electrolyte can effectively prevent side reaction and achieve uniform deposition of Zn dendrites. The low-cost electrolyte additive provides an innovative strategy or the protection of Zn metal electrode in aqueous electrochemical energy storage devices.