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.

Wenda Hu

Appearances