Mg batteries have been proposed as potential alternatives to lithium-ion batteries because of their lower cost, higher safety, and enhanced charge density. However, the Mg metal readily oxidizes when exposed to an oxidizer to form a thin MgO passivation surface layer that blocks the transport of Mg2+ across the solid electrode-electrolyte interface (SEI). In this work, the adsorption and thermal decomposition of diglyme (G2) and electrolytes containing Mg(TFSI)2 in G2 on 10 nm-sized MgO particles are evaluated by a combination of in situ 13C single-pulse, surface-sensitive 1H-13C cross-polarization (CP) magic-angle spinning (MAS) nuclear magnetic resonance, and quantum chemistry calculations. At 180 °C, neat G2 decomposes on MgO to form surface-adsorbed -OCH3 groups that are captured as a distinctive peak located at about 50 ppm in the CP/MAS spectrum. At low Mg(TFSI)2 salt concentration, the main solvation structure in this electrolyte is solvent-separated ion pairs without extensive Mg-TFSI contact ion pairs. G2, likely including a small amount of G2-solvated Mg2+, adsorbs onto the MgO surface. At high Mg(TFSI)2 salt concentrations, contact ion pairs between Mg and TFSI are formed extensively in the solution with the first solvation shell containing one pair of Mg-TFSI and two G2 molecules and the second solvation shell containing up to six G2 molecules, namely, MgTFSI(G2)2(G2)6+. In the presence of MgO, MgTFSI(G2)2(G2)6+ adsorbs onto the MgO surface. At 180 °C, the MgO surface stimulates a desolvation process converting MgTFSI(G2)2(G2)6+ to MgTFSI(G2)2+ and releasing G2 molecules from the second solvation shell of the MgTFSI(G2)2(G2)6+ cluster into the solution. MgTFSI(G2)2+ and MgTFSI(G2)2(G2)6+ tightly adsorb onto the MgO surface and are observed by 1H-13C CP/MAS experiments. The results contained herein show that electrolyte composition has a directing role in the species present on the electrode surface, which has implications on the structures and constituents of the solid-electrolyte interface on working electrodes and can be used to better understand its formation and the failure modes of batteries.
Keywords: 13C NMR; Mg(TFSI)2; and DFT NMR calculations; diglyme; electrolytes; magnesium battery.