Thermal decomposition pathways of bulk electrolytes on vanadium oxide nanocrystals

The thermal stability of electrolytes at an elevated temperature induced by battery charge-discharge cycling is critical for the long cycling performance of a rechargeable battery. For many multivalent systems, such as rechargeable magnesium batteries, which offer great potential for high energy density and utilize earth-abundant resources, electrolyte instability and electrode surface passivation, arising from electrolyte decomposition, remain as major roadblocks. Understanding the electrolyte decomposition pathways at the electrode-electrolyte interface is essential to provide guidance in overcoming this challenge. In this work, in situ ¹³C magic angle spinning nuclear magnetic resonance (MAS NMR) and first-principles calculations were used to investigate the thermal decomposition of the electrolyte in a system consisting of MgV₂O₄, a novel cathode for magnesium batteries, mixed with a bulk electrolyte consisting of magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)₂) in diglyme (G2). We show that significant electrolyte decomposition is observed in bulk 1.0 M Mg(TFSI)₂ in G2 mixed with nanometer sized MgV₂O₄ powder at elevated temperature. This observation is to mimic the possible thermal decomposition that might happen during battery cycling. We demonstrate that the MgV₂O₄ surface is covered by a layer of decomposed G2 products. We conclude that the dominant reaction pathway for electrolyte decomposition is the thermal decomposition of the pure electrolytes at elevated temperatures, followed by adsorption of G2 decomposition products to the MgV₂O₄ surface. The activation energy for the major decomposition pathway is obtained. This work highlights the importance of studying thermal decomposition of electrolytes for overall system stability and explores electrolyte stability at significantly elevated temperatures.