The future of green mobility depends on continued research into alternative energy storage such as batteries, fuel cells and super-capacitors. However, to exceed the demands of current hybrid, plug-in hybrid and electric vehicles, new battery systems with high energy density are required. Magnesium (Mg) is an attractive alternative to current lithium-ion technologies because of the transfer of 2 electrons per magnesium-ion, higher volumetric capacity of magnesium metal compared to lithium metal (3833 mAh/cm³ Mg vs 2061 mAh/cm³ Li), and greater natural abundance [1]. Many challenges must be met to promote magnesium batteries as a practical candidate for the next generation of batteries. One of the pivotal issues is a complete understanding of the Mg deposition and dissolution interface.

Recently, advances in electrolyte synthesis has encouraged continued research into high energy density materials for magnesium batteries. Our group introduced a new family of electrolytes for Mg battery based on anions incorporating the B-H motif [2]. Among them, magnesium borohydrides (Mg(BH₄)₂) provides a segue between classic electrolytes based on Grignard reagents and the new electrolytes, such as magnesium monocarborane (MMC), which stands out as the first simple-salt magnesium compound stable to Mg metal, providing high anodic stability, non-corrosive electrochemistry, and chemically robustness [3]. However, the mechanism of deposition and dissolution of magnesium remains a complex process.

Operando electrochemical cells coupled with spectroscopic and microscopic tools are powerful techniques to understand the dynamic processes in batteries. Previously, we have used Operando electrochemical/X-ray absorption to identify the active species in organohaloaluminate electroltyes [4-5], and here we extend that research for (Mg(BH₄)₂) electrolytes. In combination with electrochemical impedance spectroscopy, we identify the presence of an active interphase required for magnesium deposition and dissolution. For MMC, we have determined that magnesium deposition and dissolution to be able to sustain high-current (10 mA/cm²), high-charge (5 mAh) and cold temperature (0^oC) without the presence of magnesium dendrites. We utilize electron microscopy and X-ray photoelectron spectroscopy to elucidate the chemistry and mechanism of the promising anode / electrolyte interphase.

[1] Muldoon, J.; Buccur, C.; Gregory, T. Chem. Rev. 2014, 114, 11683.

[2] Tutusaus O.; Mohtadi, R.; ChemElectroChem 2015, 2, 51.

[3] Tutusaus, O.; Mohtadi, R.; Arthur, T.S.; Mizuno, F.; Nelson, E.G.; Sevryugina, Y.V. Angew. Chem. Int. Ed. 2015, 54, 7900.

[4] Arthur et al., Electrochem. Comm., 2012, 24, 43.