Dendritic Magnesium Formation in Conventional Electrolytes: Local Adsorption of Amine Solvents at Inner Helmholtz Plane Matters

Abstract Simple salts electrolyte is imperative for developing commercially feasible rechargeable magnesium (Mg) metal batteries. However, Mg electro‐plating/stripping reversibility in these electrolytes is far from satisfactory, which is mainly caused by interfacial passivation and dendrite growth of Mg‐metal anode. Therefore, it is of great urgent to develop a molecule‐level mechanistic understanding on electrode–electrolyte interfacial reactions. In this work, the transformation pathways of solvents at the inner Helmholtz plane are revealed to play a key role in reversibility deterioration and cell failure of Mg‐metal anodes. A benzylamine co‐solvent is introduced into simple salts electrolyte, which is closely related to gas evolution, MgH 2 formation and possible electrochemical active species formation on surface of Mg‐metal anodes. The uneven adsorption of active species leads to a multiplicative effect on local current density, which causes dendrite growth. Interestingly, benzylamine solvents are demonstrated to do not directly participate in the first Mg 2+ solvation sheath structures, instead they exhibit specific adsorption at the inner Helmholtz plane attributed to significant steric hindrance. As a result, visual bottle‐type asymmetric cells display a fair cycle life of above 500 cycles, while coin‐type assembly exhibits serious soft short circuit and recovery behavior during cycling.