Earth-Abundant Magnesium Metal Anode with Chemically Activated for Reversible Cycling in Simple Salt Electrolytes

Lithium (Li)-based electrochemistry has been widely used in battery markets, from portable devices to electric vehicles. However, Li has limited reserves in the Earth’s crust (20 ppm), uneven distribution, and highly reactive properties, which raise significant economic, geopolitical, and safety concerns. This drives the research for alternative electrochemical systems that utilize more earth-abundant ions.
Rechargeable magnesium batteries (RMBs) are a promising “beyond Li” technology, offering advantages in safety and cost compared to Li-based systems. Magnesium (Mg) metal provides a higher volumetric energy density (3833 mAh cm^–3) than Li metal (2062 mAh cm^–3), is abundant in the Earth’s crust (23,300 ppm), and is relatively stable with a low reduction potential (−2.37 V vs SHE). However, a key challenge in developing viable RMBs is the passivation of the Mg surface in conventional electrolytes, which hinders the reversibility and kinetics of Mg deposition/stripping. Unlike the ion-conducting solid-electrolyte interphase (SEI) prevalent in Li-ion batteries, SEI formed on Mg metal blocks Mg ion transport.
To address this issue, substantial efforts have focused on formulating electrolytes that enable reversible Mg deposition/stripping. As a prominent example, organomagnesium haloaluminate complexes and chloride (Cl)-based electrolytes have shown promise by preventing ion-insulating interphase formation on the Mg surface. However, their toxicity, limited oxidative stability, and high reactivity with cathodes and other cell components degrade the practical energy density of RMBs. Additionally, forming an artificial SEI on the Mg surface has shown potential for achieving reversible Mg chemistry. An artificial SEI composed of thermal-cyclized polyacrylonitrile with Mg triflate and Mg fluoride (MgF₂) improved Coulombic efficiency (CE) during Mg cycling. However, the effectiveness of these artificial SEI was evaluated at a rate lower than the expected rate capability of conventional batteries. Furthermore, the MgF₂ layer proved effective in all phenyl complex (APC) electrolytes, a highly corrosive electrolyte system that already exhibits a high CE without surface modifications. As such, despite the recent advancements, there has been no realistic option for constructing RMBs that exhibit comparable performances to commercial batteries, necessitating further evaluation in simpler salt electrolytes.
Herein, we suggest the surface modification strategy that chemically activates the Mg metal to facilitate Mg deposition/stripping in Cl additive-free simple Mg(TFSI)₂/diglyme electrolyte. Combined surface analyses proved the formation of an artificial interphase comprised of ether-complexed alkyl magnesium halides, accompanied by spontaneous surface nanostructuring upon the chemical activation. The resulting activated Mg anode exhibits unparalleled performances in terms of CE (>99.5%) and polarization (<0.26 V) over 1000 cycles in Mg||SS asymmetric cells. By regulating the activation conditions that induce divergent surface transformations, we further revealed the interplay of the factors that control the electrochemical behavior of Mg metal anodes. Owing to the high reversibility of the activated Mg anode, efficient cycling of the Mg full cell in the simple electrolyte is achieved by coupling with Mo₆S₈, sulfur, and anthraquinone-1,5-disulfonic acid disodium (AQDS) cathodes. Our activation strategy enabled the efficient cycling of Mg full-cell candidates using commercially available electrolytes, thereby offering possibilities for building practical Mg batteries.