Optimizing Interfacial Process of Mg Metal Anode by Porphyrin Adsorption Layer in Cl-Free Conventional Electrolyte

Abstract

The viability of Mg metal batteries is severely threatened by the passivating instinct of Mg metal in Cl-free conventional electrolytes. To realize the reversible Mg stripping/plating in the conventional electrolyte, 5,10,15,20-tetraphenylporphyrin (TPP) with planar structure and strong adsorption ability is added into electrolyte to optimize the interfacial process by constructing a dynamic molecular adsorption layer on Mg anode, which is different from the method of tailoring artificial solid electrolyte interphases to facilitate Mg2+ transport. On one side, TPP molecules served as a protective layer to avoid the direct contact between Mg anode and passivation-inducing factors from electrolyte, alleviating passivation effectively. On the other side, a locally high concentration of TPP is created near the Mg surface for Mg2+ solvation sheath reorganization. Theoretical calculations verify that [MgTPP(DME)2] in TPP-containing electrolyte possesses higher stability and a lower energy barrier than [Mg(DME)3]2+ during desolvation, further alleviating passivation and reducing the overpotential of Mg metal. Consequently, the Mg symmetrical battery exhibits a reduced polarization voltage of 0.6 V from ~2 V and a longer cycling life of over 500 h. With TPP-containing electrolyte, the Mg||V2O5 full battery presents a higher initial capacity of 186.5 mAh g-1, which confirms the practicability of this additive strategy.

Keywords: Mg metal anode, Cl-free conventional electrolyte, Adsorption layer, Mg2+ solvation optimization.