Theoretically, Mg batteries can reach high energy density with cost-effective materials, but no such device to date has performance competitive with Li-ion technologies. One of the major barriers is the need for oxide cathodes that combine high capacity and voltage. Nanocrystals of MgV2O4 have been demonstrated to have the potential to reach high capacity for Mg2+ deintercalation, but the electro-chemical cycling performance is quite limited primarily due to the decomposition of electrolytes on the surface of MgV2O4. To gain insight into the active sites that catalyze the electrolyte decomposition, in this study we used a combination of in situ MAS NMR, XPS and computational modeling to study electrolyte (Mg(TFSI)2 in diglyme (G2)) decomposition by nanocrystal sized MgV2O4 powers. Here, in situ 13C MAS NMR at elevated temperature of 163 °C was employed to continuously monitoring electrolyte decomposition on the MgV2O4 surface by surface sensitive 1H-13C cross-polarization MAS NMR, the decomposed products leaked into the electrolytes by non-biased quantitative 13C single pulse MAS NMR. The pristine MgV2O4 materials and the spent MgV2O4 samples at the end of in situ MAS NMR experiments were analyzed by XPS. Quantum chemistry calculation of the observed chemical shifts combined with molecular dynamics modeling (MD) were used for identifying the surface active sites, the adsorption mechanisms of the electrolytes and the decomposed products on MgV2O4 surface. The following major are obtained. XPS shows the existence of residual MgO at the MgV2O4 surface of both pristine MgV2O4 materials and the spent MgV2O4 samples. In situ 1H-13C cross-polarization MAS NMR unambiguously detects a surface adsorbed -OCH3 group on MgV2O4 with a signature peak located at 50.2 ppm. This result indicates that the residual surface MgO on MgV2O4 surface plays a significant role of catalyzing G2 decomposition in this system. MD simulation reveals that it is highly possible to decompose G2 by cleavage of either -CH3 or -OCH3 but the cleavage of -OCH3 from G2 is more energetically favorable. The results indicate that searching for a stable electrolytes would depend on the choice of the solvents used in the electrolytes in the case of Mg(TFSI)2 salt for MgV2O4 cathode.

Solvation structures and thermal decomposition of electrolytes on novel divalent cathode MgV2O4 by in situ MAS NMR and computational modeling

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