A Dive into the Electrodeposition Kinetics and Mechanisms in Nonaqueous Magnesium Electrolytes

The fundamental complexity of the divalent chemistry of Mg²⁺ necessitates in situ isolation to disentangle electron transfer and chemical steps during electrochemical deposition of Mg. To investigate the electron transfer mechanism of Mg²⁺ battery electrolyte, two drastically different electrolytes were examined: an all phenyl complex (APC) and a Mg(TFSI)₂ using an in situ rotating ring-disk electrochemical technique supported with simulation. Three key mechanistic steps were found to be analogous in both the APC and Mg(TFSI)₂ electrolyte during electrochemical reduction that are a rate-limiting Mg²⁺ desolvation/adsorption step followed by two sequential electron transfers. For the first time, the first electron transfer is shown to form an electrochemically captured transient Mg^·+ intermediate, and the second electron transfer forms plated Mg°. Upon oxidation of Mg°, an adsorbed Mg^·+ intermediate is proposed to exist at the disk electrode where a sluggish desorption allows a second electron transfer to form Mg²⁺ before it is detected at the ring. While APC demonstrated a more facile redox mechanism, regardless of the solvation environment of the Mg²⁺ species, both Cl-complexed and TFSI^- coordinated Mg²⁺ undergo the same electron transfer mechanism, giving broader than anticipated insight into the underlying working mechanism of electrolytes for rechargeable Mg metal battery applications.