The Role of Surface Adsorbed Cl^-Complexes in Rechargeable Magnesium Batteries

Most electrolyte solutions supporting highly reversible Mg deposition/dissolution contain chlorides. These come as charged or neutral complex species, frequently as Mg-based complex ions, and Lewis-base anions. The electroactive species are usually comprised also of solvent molecules and chlorides as ligands. Numerous studies had shown the critical role of Cl-based species on the electrochemical reaction kinetics, deposition and dissolution overpotentials, electrocrystallization morphology, and many other imperative aspects of the electrochemical responses. Only very few, quite exotic and not fully studied, Cl-free electrolyte solutions have been reported to maintain reversible Mg deposition. Over the years, several compelling theories have been proposed to explain the decisive role of the Cl-containing species on the electrochemical reaction mechanisms during magnesium deposition and dissolution. The role of chlorine-based species in intercalation processes of Mg ions into crystalline hosts was seldom pursued, though. Interestingly, it appears that the Chevrel phase (CP) MgxMo6S8, the main benchmarking Mg intercalation compound, does not intercalate Mg in Cl-free solutions at room temperature (RT). This was established over and again during past decades and recently had been corroborated over a large set of experiments in our lab. Herein, we reveal, via experimental work, that Cl-based species also have a critical role in the electrochemical processes of intercalation. In this work, we studied the function of these during the course of the intercalation process of Mg ions into CP electrodes. We demonstrate that the intercalation process is extremely sluggish, to the point of nonexistent, from Cl-free electrolyte solutions. Quite remarkably, the addition of chlorine-comprising species opens the door to facile magnesium intercalation. On the basis of the numerous experimental results, we suggest a detailed Mg intercalation mechanism into CP. The proposed mechanism emphasizes the importance of the Cl-based species on the charge transfer process across the solid/solution interface. We propose that surface absorbed Cl-containing complexes reduce the activation energy for the interfacial charge transfer stage involving the transport of Mg ions from the solution to the crystalline phase and vice versa. We also propose a relatively simple manner to alleviate the intercalation impediment of Cl-free electrolyte solutions by the cathode's chemical or electrochemical pretreatment. This project has both scientific and applicative significance and may open a hatch for the future development of practical rechargeable Mg batteries (RMBs).